Politecnico di Milano

Scuola di Ingegneria Industriale e dell’Informazione

Corso di laurea magistrale in:

Mechatronics and Robotics

Development of a Mechanical

Intra-Row Weeder System for an

Autonomous Electric Vehicle

Relatore: Luca Bascetta

Tesi di laurea di:

Portioli Michele

Matr: 837790

Anno Accademico 2015/2016

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Contents

List of figures ...................................................................................................................................... 5

List of tables ........................................................................................................................................ 8

Sommario ............................................................................................................................................ 9

Abstract ............................................................................................................................................. 10

General introduction........................................................................................................................ 11

Weed management overview........................................................................................................... 12

Biological weeding ......................................................................................................................... 12

Manual weeding ............................................................................................................................. 12

Chemical weeding .......................................................................................................................... 13

Mechanical weeding ...................................................................................................................... 16 Cultivating tillage ....................................................................................................................... 17 Cutting and mowing ................................................................................................................... 24

Pulling ........................................................................................................................................ 24

Thermal weeding ............................................................................................................................ 25

Guidance system ............................................................................................................................ 25

Automated intra-row weeder technologies .................................................................................... 26

Unmanned vehicle with automated weed system ........................................................................... 29

Comparison between weed control techniques .............................................................................. 31

Intra-row weeding design ................................................................................................................ 33

Choice motivations ......................................................................................................................... 33

Design constraints.......................................................................................................................... 36

Intra-row mechanism overview ...................................................................................................... 38

Mathematical model for soil resistance prediction........................................................................ 39

Kinematics ...................................................................................................................................... 43

Simulation results ........................................................................................................................... 51 Forces ......................................................................................................................................... 51 Torque ........................................................................................................................................ 60 Power ......................................................................................................................................... 61

Gear motors choice .......................................................................................................................... 62

Tool ................................................................................................................................................ 62

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Slider .............................................................................................................................................. 64

Dynamic simulation ......................................................................................................................... 66

Tool ................................................................................................................................................ 66

Slider .............................................................................................................................................. 69

Motor wheel ................................................................................................................................... 72

Conclusions .................................................................................................................................... 75

Prototype design ............................................................................................................................... 78

Static assessment .............................................................................................................................. 80

Tool ................................................................................................................................................ 80

Slider .............................................................................................................................................. 81 Translation unit .......................................................................................................................... 81 Section bars ................................................................................................................................ 83 Linear guides .............................................................................................................................. 83

FEM results ................................................................................................................................ 83

Dynamic assessment ......................................................................................................................... 88

Batteries comparisons ...................................................................................................................... 89

Costs analysis .................................................................................................................................... 93

Conclusions ....................................................................................................................................... 94

Future developments........................................................................................................................ 95

References ......................................................................................................................................... 96

Acknowledgments .......................................................................................................................... 100

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List of figures Figure 1 - Hand weeding worker into a strawberry field in Oxnard, California [8] .......................... 12

Figure 2 - On-the-row weeding through plane [12] ........................................................................... 14

Figure 3 - Inter-row chemical weeder [13] ........................................................................................ 14

Figure 4 - Intra-row chemical weeder with fixed nozzles [14] .......................................................... 15

Figure 5 - Detection of seed plants (blue cross) and weed (red circle) for visual system [14] .......... 15

Figure 6 Inter-row hoeing machine with miller cutter [18] ............................................................... 16

Figure 7 - Schematic diagram of the rolling harrow: (A) frame; (B) front axle equipped with spike

discs; (C) rear axle equipped with cage rolls; (D) chain drive; (E) three-point linkage. (Drawn by

Andrea Peruzzi.) [6] ........................................................................................................................... 17

Figure 8 – Possibilities and machine setting for weed control in crop rows for small seed crops, tightly

spaced (on top) and relative explanation (on bottom) ........................................................................ 18

Figure 9 - Flex-tine harrow in silage maize [6] ................................................................................. 19

Figure 10 – Rotary hoeing machine [19] ........................................................................................... 19

Figure 11 – Vertical axis brush weeder [6] ........................................................................................ 20

Figure 12 – Vertical axis rotary cultivators [6] .................................................................................. 21

Figure 13 – Basket weeder [20] ......................................................................................................... 21

Figure 14 – Finger weeder [21] .......................................................................................................... 22

Figure 15 – Torsion weeder [22] ........................................................................................................ 23

Figure 16 – Pneumatic weeder “Pneumat” [23] ................................................................................. 23

Figure 17 – Weed puller for wild mustard from soybean fields [6] .................................................. 24

Figure 18 - The autonomous and GPS-based system for intra-row mechanical weed control in

operation from behind (1) and from the side (2). (a) side-shift and cycloid hoe GPS antenna pole, (b)

wheel for height adjustment, (c) front passive frame, (d) rear active frame (side-shifting), (e) tilt

sensor, (f) wheel for control of cycloid hoe tillage depth, (g) soil-engaging disc for resisting

counteracting forces from side-shift, (h) parallelogram, hydraulic motor and reduction gear, (j) digital

video camera, (k) tine-rotor housing, (l) rotary solenoid, (m) tine spindles, (n) sigmoid-shaped tines,

(o) white plastic sticks serving as artificial plants [25] ...................................................................... 26

Figure 19 – Robovator [26] ................................................................................................................ 27

Figure 20 – Robocrop [27] ................................................................................................................. 27

Figure 21 – Remoweed, the whole system on top and the detail of weeding operation on bottom [29]

............................................................................................................................................................ 28

Figure 22 – IC Weeder of Steketee [30] ............................................................................................ 29

Figure 23 – Carre Anatis [33] ............................................................................................................ 30

Figure 24 – IBEX agribot [34] ........................................................................................................... 30

Figure 25 – Hortibot [35] ................................................................................................................... 31

Figure 26 – Machine developed by A Kielhorn with rotary hoes [38] .............................................. 35

Figure 27 – Machine developed by Dedousis [25] ............................................................................ 35

Figure 28 – Unmanned electric vehicle developed by Politecnico di Milano ................................... 36

Figure 29 – The four configurations (A,B,C,D) of the unmanned electric vehicle design by Politecnico

di Milano ............................................................................................................................................ 37

Figure 30 – Lateral view of the vehicle showing some possible configurations the structure can

assumes .............................................................................................................................................. 38

Figure 31 – Schematic representation of soil failure plane with relative parameters [45], [46] ....... 39

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Figure 32 – Experimental relationship between rupture distance ratio (m=f/d) and tine rake angle 𝜶

[41] ..................................................................................................................................................... 42

Figure 33 – Scheme of weeding kinematics ...................................................................................... 42

Figure 34 – Region covered by the tool with 3 tines at 350 rpm tool rotation, 0.8 km/h cart speed per

1 second.............................................................................................................................................. 44

Figure 35 – Region covered by the tool with 3 tines at 350 rpm tool rotation, 0.8 km/h cart speed per

0.2 second........................................................................................................................................... 44

Figure 36 - Region of soil covered by the tool when is approaching the crop plant ......................... 45

Figure 37 - Region of soil covered by the tool when is approaching three crop plants. Their disposition

refers to Consorzio Cremasco Pomodori conference ......................................................................... 46

Figure 38 - Region of soil covered by the tool when is approaching three crop plants. Alternative

trajectory ............................................................................................................................................ 47

Figure 39 - Region of soil covered by the tool when it is approaching two crop plants ................... 48

Figure 40 - Region of soil covered by the tool when it is approaching two crop plants. Alternative

solution ............................................................................................................................................... 48

Figure 41 - Cubic profile of slider translation movement. On the top the displacement, in the middle

the velocity and on the bottom the acceleration ................................................................................. 49

Figure 42 – Cycloidal profile of slider translation movement. On the top the displacement, in the

middle the velocity and on the bottom the acceleration ..................................................................... 50

Figure 43 – Displacement profile on cubic (solid line) and cycloidal (dashed line) motion laws ..... 51

Figure 44 – Soil resistance function of failure angle 𝜷 for frictional soil ......................................... 53

Figure 45 – Horizontal forces comparison for frictional soil. The forces are function of the number of

tines (one or three), of the rotation speed of the tool (200 rpm, 350 rpm, 500 rpm) and of the cart

speed................................................................................................................................................... 54

Figure 46 - Horizontal force for three-tine tool for frictional soil, function of tool rotation speed and

cart velocity ........................................................................................................................................ 55

Figure 47 – Soil resistance function of failure angle 𝜷 for cohesive soil .......................................... 57

Figure 48 – Horizontal forces comparison for cohesive soil. The forces are function of the number of

tines (one or three), of the rotation speed of the tool (200 rpm, 350 rpm, 500 rpm) and of the cart

speed................................................................................................................................................... 58

Figure 49 - Horizontal force for three-tine tool, function of tool rotation speed and cart velocity.

Comparison between frictional soil and cohesive soil ....................................................................... 59

Figure 50 – Torque required for weeding, function of soil type (frictional, cohesive), rotation tool

speed (200 rpm, 350 rpm, 500 rpm) and cart speed ........................................................................... 60

Figure 51 – Power required for weeding, function of soil type (frictional, cohesive), rotation tool

speed (200 rpm, 350 rpm, 500 rpm) and cart speed ........................................................................... 61

Figure 52 – BMU Driver and AC Brushless motor with reducer ...................................................... 62

Figure 53 – Characteristic curve of BMU 300W AC Brushless motor ............................................. 63

Figure 54 – AC Brushless motor SMV 38I to the left, and the related planetary gear LPB+120-1S-3

(alpha Riduttori S.p.A.) to the right ................................................................................................... 64

Figure 55 – Characteristic curve of slider gear-motor (in red), load curve (in blue) ......................... 65

Figure 56 – Root locus of the system 𝑮𝒗, 𝒕𝒐𝒐𝒍 ∙ 𝑹𝒗, 𝒕𝒐𝒐𝒍. The x marker are the poles, the hollow

circle marker the zeros, the solid circle the poles considering the gain ............................................. 67

Figure 57 – Bode plot for the tool gear-motor system ....................................................................... 67

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Figure 58 – System response to the reference in velocity (on the top), zoom at second 10 when the

tool starts working (on the bottom) .................................................................................................... 68

Figure 59 – Voltage (top left), current (top right) and power (on the bottom) of the tool system ..... 69

Figure 60 – Root locus of the system 𝑮𝒔𝒍𝒊𝒅𝒆𝒓 ∙ 𝑹𝒔𝒍𝒊𝒅𝒆𝒓. The x marker are the poles, the hollow

circle marker the zeros, the solid circle the poles considering the gain ............................................. 70

Figure 61 – Bode plot for the slider gear-motor system .................................................................... 70

Figure 62 – System response to the reference in position (on the top), zoom at second 10 when the

tool starts working (on the bottom) .................................................................................................... 71

Figure 63 – Voltage (top left), current (top right) and power (on the bottom) of the tool system ..... 72

Figure 64 – Root locus of the system 𝑮𝒎𝒐𝒕𝒐𝒓 𝒘𝒉𝒆𝒆𝒍 ∙ 𝑹𝒎𝒐𝒕𝒐𝒓 𝒘𝒉𝒆𝒆𝒍. The x marker are the poles,

the hollow circle marker the zeros, the solid circle the poles considering the gain ........................... 73

Figure 65 – Bode plot for the slider gear-motor system .................................................................... 73

Figure 66 – System response to the reference in position (on the top), zoom at second 10 when the

tool starts working (on the bottom) .................................................................................................... 74

Figure 67 – Voltage (top left), current (top right) and power (on the bottom) of the tool system ..... 75

Figure 68 – Simulink block diagram of the whole system: motor-wheels, tool and slider ............... 76

Figure 69 – Current consumption of the whole system made up by four motor-wheels, slider motor

and tool motor .................................................................................................................................... 76

Figure 70 – Power consumption of the whole system made up by four motor-wheels, slider motor and

tool motor ........................................................................................................................................... 77

Figure 71 – CAD representation of prototype ................................................................................... 78

Figure 72 – CAD representation of prototype, frontal view .............................................................. 78

Figure 73 - CAD representation of prototype, lateral view ............................................................... 79

Figure 74 – Tool representation ......................................................................................................... 81

Figure 75 – ITEM translation unit: photo to the left, drawing with dimension to the right .............. 81

Figure 76 – Translation unit representation with relative moment vectors (on the left), guide section

to the right .......................................................................................................................................... 82

Figure 77 – Linear Guide, from ITEM website ................................................................................. 83

Figure 78- CAD view of the specific component studied (slider support), to the left; stress analysis to

the right .............................................................................................................................................. 84

Figure 79 – Analysis results: safety factor to the left, displacement to the right ............................... 84

Figure 80 – FEM analysis results. On top Von Mises stresses of the weeding system; in the center

stresses distribution with a reduced scale; on the bottom safety factor distribution .......................... 86

Figure 81 – FEM analysis results: displacement of weeding system components ............................ 87

Figure 82 – First natural mode of weeding system ............................................................................ 88

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List of tables Table 1 – Weed time for different vegetables [10] ............................................................................ 13

Table 2 – Comparison of different intra-row weed control technologies [3] .................................... 32

Table 3 – Motor-wheel characteristics ............................................................................................... 36

Table 4 – Vehicle configurations ....................................................................................................... 37

Table 5 – Summary of soil properties [41] ........................................................................................ 40

Table 6 – Parameters of weeding operation used for representation of Figure 36 ............................ 45

Table 7 – Summary of frictional soil properties [41] and process data ............................................. 52

Table 8 – Results obtained for frictional soil ..................................................................................... 55

Table 9 - Summary of cohesive soil properties [41] and process data .............................................. 56

Table 10 – Results obtained for cohesive soil .................................................................................... 59

Table 11 – Summary of the mechanical parameters having as assumption the soil reaction force equal

to 70 N ................................................................................................................................................ 60

Table 12 – Tool gear-motor and driver characteristics ...................................................................... 62

Table 13 – Slider gear-motor characteristics ..................................................................................... 64

Table 14 – Dynamic simulation data ................................................................................................. 66

Table 15 – General characteristics of tool dimensions and resistance ............................................... 80

Table 16 – General characteristics of tine dimensions and resistance ............................................... 80

Table 17 – General characteristics of translation unit KLE 6 60x60 dimensions and resistance ...... 81

Table 18 – General characteristics of linear guides dimensions and resistance ................................ 83

Table 19 – Batteries comparison ........................................................................................................ 89

Table 20 – Battery pack mounted on the vehicle, composed by 3 acid lead batteries ....................... 90

Table 21 – Battery pack mounted performances compared with different batteries types ................ 91

Table 22 – Cycle durability for different battery type [49] ................................................................ 92

Table 23 – BOM of mechanical weeding system .............................................................................. 93

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Sommario Il controllo meccanico delle infestanti è oggi la tecnologia più promettente per il miglioramento della

resa delle coltivazioni. Il diserbo chimico è oggi ritenuto nocivo per l’ambiente, pericoloso per la

salute dell'uomo e degli animali, e la causa della formazione di colture resistenti agli erbicidi. Anche

il lavoro manuale non è considerabile la strategia migliore poiché è causa di logoramento per il corpo

umano ed è costoso.

Il progresso dell'informatica e dell'elettronica negli ultimi anni consente alla tecnologia per la

rimozione di infestanti, in particolare quella intra-fila, di essere la migliore tra quelle utilizzate

oggigiorno per tale obiettivo. Infatti, la combinazione di un sistema meccanico, di uno di visione e di

una scheda di controllo consente la rimozione di erbacce non solo tra due righe (diserbo interfilare),

ma anche nello spazio tra una pianta e la successiva lungo la linea della semina (diserbo intra-fila).

Un veicolo senza pilota equipaggiato da un sistema automatico di controllo delle infestanti potrebbe

essere il miglior sistema sia in termini di efficienza che per economicità.

Un attuatore meccanico è stato progettato per il diserbo intra-fila tale da costituire uno dispositivo

versatile, adatto a quasi tutti i tipi di colture orticole. È costituito da un utensile a tre lame montate su

un disco rotante; l'utensile viene trascinato, perpendicolarmente rispetto alla direzione di marcia, da

un sistema di trasmissione a cinghia che è montato direttamente sul veicolo; il veicolo è elettrico,

alimentato da quattro ruote motrici elettriche.

Le simulazioni effettuate indicano che, per una velocità del veicolo 0,8 km / h la forza di reazione del

terreno per un singolo dente è di circa 70 N, la forza necessaria per spostare l'intero sistema 450 W,

la corrente necessaria 10 A

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Abstract The mechanical control of weeds is nowadays the most promising technology for the enhancement

of crop yielding. The chemical weeding is discovered not to be environmentally focused, dangerous

for humans and animals’ health and cause of herbicide-resistant foods. The development of

informatics and electronics technologies in the last decade allows the intra-row weed technology to

be the best solution in weed management. In fact, the combination of a mechanical weeder, a camera

vision system and a control board permit the removal of weeds not only between two rows (inter-row

weeding) but also in the gap between one plant and the following along the seeds line (intra-row

weeding). Moreover, since the hand labour is cause of many injuries for human body and it is

expensive, an automatic weed control system arming an unmanned vehicle could be the best system

for the task and economically feasible.

A mechanical weeding actuator is designed for intra-row weeding purpose. It is thought to be

applicable to almost every kind of vegetable crops. It is constituted by a three tines tool mounted on

a rotating disc; the tool is dragged, perpendicularly respect the direction of travel, by a belt drive

system mounted directly on the unmanned vehicle; the vehicle is electric, powered by four electric

drive wheels.

The simulations made indicate that, for a vehicle speed of 0.8 km/h, the reaction force of the soil

during weed uprooting for one single tine is around 70 N, the power necessary to move the entire

system 450 W, the current needed 10 A.

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General introduction From the birth of agriculture the weed removal has been a fundamental operation for vegetable

harvesting. In many quotations from ancient sources, gathered in Farm Weeds of Canada, it is

illustrated the respect for weed prevention even before the medieval period [1]. The growing of the

plant is always accompanied by the growing of the weed; both organisms are in need of the same

nutrients, extracted from the terrain, and of the sun. The sharing of the resources may lead to the

survival of the weed and to the death of the vegetable, in a vital competition for nutrients and

resources [2].

The phenomenon is emphasized if the unwelcome plant is particularly aggressive, with a high

growing speed or if it has large and thick leaves able to shade the cultivation.

Hence, in order to increase the cultivation yield, the weed management has to be used. It concretizes

in weed prevention. Prevention is the exclusion of weed from the interested part of field not yet

infested; it is the most cost effective operation that growers can take. It is a technique suitable for any

crop dimensions, from a small vegetable crop to a large property. Prevention operation, for the vast

majority of crops, must be done during the first four to six weeks from the seed planting in order to

prevent yield losses due to early season weed competition [3].

In the past the weed prevention has had exclusively the form of manual labour. A large proportion of

the work force in the world is involved in agriculture or related occupations [4].

According to a study conducted in the northern of India about farm hand tools injuries [5], a

significant number of farm injuries (58%) are caused by hand tool, involving a very high number of

female farm workers (65%). The causes are due to slippages of tool from hand because of the presence

on handle of sweat or paint, hitting a hard surface, in impact of the spade in the soil that provoke a

bouncing back of hand tool, unpredictable soil conditions, inappropriate diameter and length of

handle, inappropriate material and texture of handle, inappropriate clearance for hand in handles and

mismatch of anthropometric dimension and tool handles. An improper handle dimensions can bring

to a reduction of the force required, demanding higher impact velocities for cutting, loose grip,

slippage and inaccurate direction of applied force.

The manual weed management was outdated in the last century and substituted by a chemical weed

management through the use of synthetic herbicides, largely available and considered of high

effective and selective. The decade after the World War II constituted the watershed between non-

chemical and chemical weed management; the last one becomes the predominant option for weed

control; however the ecological and social consequences of herbicide use have been disregarded or

downplayed. The widespread reliance on herbicides cause the development of herbicide resistant

weeds, inducing negative effect on human health and on environment.

Later on the impact of chemical pesticides became an important global issue for the sustainability of

our food production system. More and more pesticide-free food produced in organic farming becomes

popular all over the world. The alternative weed control technologies, considered uneconomic or

unfeasible in the past, are revisited and developed [6].

Purely mechanical weeding actuators are reconsidered and thanks to mechatronics progresses of last

years the mechanical system is accompanied by an integrated control logic and electronic devices;

from this combination of technologies it was possible to compute such complicate weed processes

that in the past were possible just through manual work.

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Weed management overview

Biological weeding Biological weed control uses specialized insects (e.g. rye, sorghum, mustards, velvetbean, black

walnut) that reduce weeds growth or eventually kill them. These plant species area known to release

chemicals which can influence weed species either directly by influencing their growth and/or seed

germination, or indirectly by affecting soil biology (e.g. by inhibiting mycorrhizal inoculation

potential) [7]. This phenomenon is called allelopathy and it could be used to destroy or suppress

weeds.

This environmentally friendly option offers several advantages respect other weed control. Tillage

damage the soil structure, or make the soil vulnerable to erosion. No synthetic herbicides are added

to the environment and the control results more durable and less expensive in the long run.

Weed control using this option in steep or rough terrains, or environmentally sensitive areas (e.g.

river or lakeside) is easier and/or more acceptable than mechanical and chemical options. It offers

interesting potential for weed management systems, but more research is needed before this technique

can be fully exploited under field conditions.

Manual weeding Manual weeding is the first weed control practiced by humans in history. The first weeding was made

by hands, ripping weeds from the terrain, and then dropped to the soil or gathered in baskets.

Gradually the management evolved to hand tool weeding, like hoe, that reduce farmers effort.

Sometimes manual weeding is still used today in little farms, if labour is available at low wages, if

farm cannot afford machineries or if has poor resources, like it happens for vegetable gardens. Among

advantages the absence of noise, the absence of chemical herbicides and the reliability of weed

elimination.

Figure 1 - Hand weeding worker into a strawberry field in Oxnard, California [8]

The disadvantages are the time-consuming, the labor-intensive and the difficulties of the weeding if

the concentration of moisture in the terrain is low, if the soil surface is not loose, if the grass to remove

are at early stages. Moreover there is the risk of weeds survival if they are dropped into standing water

[9].

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The hand labour necessary for intra-row weeding is expensive, time consuming and difficult to

organise. In the Netherlands, the average time required for manual weed in organic row crops is about

45 h ha−1 for planted vegetables, but increases to more than 175 h ha−1 for direct-sown under field

conditions; it may vary from 24 h ha−1 for transplanted lettuce (Lactuca sativa L.) to 162 h ha−1 for

sown fennel (Foeniculum vulgare L.) in Italy, while 100–300 h ha−1 for hand weeding in onions and

carrots in Denmark and Sweden [10]. Table 1 – Weed time for different vegetables [10]

The repetitive and heavy hand weed labour, replicated for several hour per days provokes permanent

back injuries to workers. According to the Medical Unit of the state Division of Occupational Safety

and Health, hand weeding is much harder on the back, shoulders and wrists, and could expose workers

to pesticides.

California was the first state to restrict the hand weeding of crops. The ban took place in the 1975.

The overcoming of hand weeding will take place in more and more countries in the world, causing a

higher request of alternative weed machineries.

Growers and farmers argue that in many cases there are no reasonable alternatives to hand weeding

because long-handled tools are too imprecise and would damage the crop. They say hand weeding

reduces the use of often-criticized herbicides.

However, hand weed is considered a human-rights abuse from the Rural Legal Assistance

Foundation, because it claimed it exists the hand weed alternative, the long-handled hoe, safer and

available but workers are not permitted to use it [11].

Chemical weeding The introduction of chemical weeding brought to a relevant enhancement in crop yield respect manual

weeding. It has the advantage to be automatable and increase weeding throughput many times respect

manual labour.

The herbicides are sprayed from nozzles located on a tractor trailer or on a plane or helicopter. The

weed control could be either on the row, inter-row or intra-row. The on-the-row weeding involve the

biggest quantity of herbicide, usually metazachlor, spayed from flying vehicles or big tractors,

without distinguish between tillage plants and weeds. These applications pose potentially significant

ecotoxicological risks to non-target plants and associated pollinators.

Crop Planting method Hours ha−1

Onion Sown 177

Carrot Sown 152

Sugar beet Sown

Planted

82

28

Vegetables Planted 46

Cereals Sown 12

Potato Sown 9

14

Figure 2 - On-the-row weeding through plane [12]

The inter-row weeding permit a reduction of herbicide, generally glyphosate, respect on-the-row

weeding, acting more selectively on weeds. Specific shield could be present to further reduce the

contamination with plants.

Figure 3 - Inter-row chemical weeder [13]

The intra-row weeding instead is the most selective weed control in which the herbicide is sprayed

from a moving nozzle directly on weeds, avoiding to hit plants; it is the most expensive technique

and most technological advanced one. Another chemical weeder system consists on fixed nozzles

whose ejection takes place only when the nozzle is over the weed Figure 4.

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Figure 4 - Intra-row chemical weeder with fixed nozzles [14]

Both intra-row systems require a visual system able to detect weeds and to distinguish them from

seed plants.

Figure 5 - Detection of seed plants (blue cross) and weed (red circle) for visual system [14]

The advantage of intra-row chemical weeding respect other is the possibility of a relevant reduction

in the quantity of herbicide that constitute a reduction in weed costs (less herbicide bought from the

farmer) and a less quantitative of substances, toxic for human and animal health, that penetrate in the

terrain. In fact some studies reveal the presence of metolachlor, carbetamide and propyzamide

contaminant surface water and ground [15].

Metolachlor in particular has been detected in concentrations ranging from 0.08 to 4.5 parts per billion

(ppb) throughout the United States and it is classified as a Category C pesticide by the United States

Environmental Protection Agency (USEPA), which indicates limited evidence of

carcinogenicity. Evidence of the bioaccumulation of metolachlor in edible species of fish as well as

its adverse effect on the growth and development raise concerns on its effects on human health [16].

Many multinational agriculture factories and research centres like NIAB TAG, SRUC, Organic

Research Centre Elm Farm, Monsanto, Micron Sprayer Ltd, Tillett and Hague Technology Ltd,

Garfords Farm Machinery and John Deere etc. are investing in new alternative weeding approach.

In more and more countries, consumer aversion towards pesticides and their negative environmental

impacts have caused government to introduce restrictions on herbicide availability and use. For the

European Union (EU) the “EU Agricultural Pesticides Directive 91/414/EEC” is presented.

According to Dirk A.G Kurstjens [17], despite the large availability of many different products,

present herbicides exploit only 15–20 different modes of action, whereas only one new target site has

16

been commercialised in the last 20 years. In all known target sites, herbicide resistance has occurred,

including 310 weed biotypes, involving 10 glyphosate-resistant weeds. It is estimated that in the last

decade 100 new resistant biotypes weed became resistant and it is attributable to the use of

monoculture. Recurring use of few modes of action promotes weed resistance development, and thus

narrows the effective of chemical weed control. Weed communities have shifted the ban of glyphosate

within 5–8 years, so that other herbicides are required to control weeds. However these alternative

are very rarely discovered and, moreover, the time between synthesis and sale of a new agrochemical

is on average 9 years or more.

Mechanical weeding European legislation (e.g. the revision of 91/414 EEC and the Water Framework Directive) impose

to farms the reduction of herbicide reliance. Herbicides does not cover all the weed spectrum and for

some weed species there is, or soon will be, no means of control. Mechanical weed control is now

more widely practiced; there are however some circumstances when some mechanical weeder

technologies are unsatisfactory: in wet weather and for control of perennial weeds and species with a

strong tap root. Among mechanical weeding it is possible to cite flame and steam weeding: they are

rarely used because they damage invertebrates and consume large amounts of energy [14].

Figure 6 Inter-row hoeing machine with miller cutter [18]

Mechanical weed management consists of three main techniques: the use of tillage, cutting weeds

and pulling weeds.

Tillage refers to the changing of soil conditions for enhancement of crop production. During primary

tillage the objective is to reduce soil strength, burying a portion of weed seeds at depths from which

they are not able to emerge or bringing seeds up to the soil surface so they are exposed directly to

cold or warm temperatures desiccation conditions. The tools used to perform primary tillage are

mouldboard ploughs.

In secondary tillage the soil is pulverized and mixed with fertilizer, lime, manure and pesticides,

levelled and firmed. The equipment used for secondary tillage is composed by harrow (disc, spring

tine, radial blade and rolling) and power take-off (PTO) machines. The tines vibrate with the forward

movement of the tractor, which helps in incorporating crop and weed residues into the soil. The

17

seedbed is worked at a 15-20 cm depth with rigid or flexible tines; the effect is to uproot the weed

flora and control the vegetative and reproductive structures of perennial species that are brought to

the soil surface where they may be exposed to the elements. Generally speaking, any cultivator

passage has a weeding action, but it might also stimulate weed seed germination and emergence.

Another technique, that is included in secondary tillage, is the tillage in darkness that consists of

doing the seedbed operation during the night or covering the tillage with an opaque material that

prevents light from reaching the tilled soil, exploiting the reliance, for many weed species, on sun

rays for germination. This techniques decreases of about 30% the weed at the ground.

In order to compute these techniques is possible to use a rolling harrow developed by researchers at

the University of Pisa, Italy, in 2005, shown in Figure 7.

Figure 7 - Schematic diagram of the rolling harrow: (A) frame; (B) front axle equipped with spike discs;

(C) rear axle equipped with cage rolls; (D) chain drive; (E) three-point linkage. (Drawn by Andrea Peruzzi.) [6]

Cultivating tillage The cultivating tillage is a technique useful to carry out shallow tillage to loosen the soil and to control

weeds. It is used after crop planting. Soil loosening has been proved to improve crop yield, this

because it breaks soil crust, simplifying crop development and growth; it also breaks soil capillaries

avoiding water evaporation under warm and dry growing situations, it enhances mineralization of

organic matter and improves water infiltration in the soil.

Cultivating tillage can destroy weeds in several different ways: complete or partial burial of weeds,

uprooting and breakage of the weed roots, mechanical tearing, breaking, and cutting. Cultivation is

more effective in dry soils because weeds often die by desiccation, meanwhile when the soil is too

wet cultivation could damage the soil structure and spread perennial weeds.

Cultivators can be classified according to the effect of weed management (primary or secondary

tillage as seen before), or according to where they are used in a crop. These last category involve:

broadcast cultivators, which are passed both on and between the crop rows; inter-row

cultivators, which are only used between crop rows; and finally, intra-row cultivators, which are used

to remove weeds from the crop rows.

Information concerning which harrow or other equipment to choose for various crop growth stages

has been compiled in Figure 8.

18

Figure 8 – Possibilities and machine setting for weed control in crop rows for small seed crops, tightly spaced (on top) and

relative explanation (on bottom)

Broadcast cultivators

An example of broadcast cultivator is reported in Figure 9, that represents a flex-tine harrow

cultivator, the most commonly used in Europe. It has fine and flexible tines that destroy weeds by

vibrating in all directions, and rigid tine harrows, best for heavy soils, consisting of several sets of

spikes or rigid blades angled at the tip. It is a kind treatment that destroys only weeds that are at the

white-thread stage (weeds that have germinated but not emerged), dicotyledonous weed seedlings

before the two leaf stage, and monocotyledonous weeds at the one-leaf stage without damaging the

well rooted crop. Harrowing weeds in their earlier stages of development (e.g. until the first true

leaves are visible), can result in excellent levels of control; the disadvantage is that it might have to

19

be repeated several times to maintain acceptable weed control levels during the growing season.

Harrows can be used post-emergence in cereals, maize, potatoes, peas, beans, many planted

vegetables and relatively sensitive crops such as sugarbeet. Speed cultivation could reach 12 km/h.

Figure 9 - Flex-tine harrow in silage maize [6]

An alternative broadcast technique is the use of rotary hoes in which a harrow with two gangs of

hoe wheels roll on the ground. The hoe wheels have many rigid and curved teeth that penetrate in

the soil, between 2 to 5cm, and lift it, so young weed seedlings emerge. The system is selective

because the crop seeds, being deeper than then working depth and better rooted, are not affected by

hoes action. Rotary hoes usage for weed control is quite a cheap and quick technique, the velocity

of operation range is in between 8 to 24 km/h.

Figure 10 – Rotary hoeing machine [19]

Inter-row cultivators

Inter-row cultivator is the earliest and the most widespread type of cultivator used in row crops. Tools

with three to five shanks, called gangs, are mounted on a toolbar, one gang per inter-row. The weed

action should not damage the crop; some accidents can occur if the working is very close to the crop

row or if the tractor speed is high. The gangs mounted on the toolbar can either have rigid or vibrating

20

shanks to which various types of tool can be attached; the most common are shovels, sweeps and

weed knives. The typical working depth is 4 cm, if it is superior crop root pruning can occur.

Discs

Between tools arming inter-row cultivators discs are used to throw soil towards the crop row or to

remove soil and weeds away from the row. Generally they are used in combination with other

weeding tools.

Brush weeders

There are many types of brush weeders, horizontal-axel or vertical-axes, generally made of fiberglass.

They are either driven by tractor’s power take-off (PTO) shaft, or electric motors or hydraulic motors.

In the case the brushes are rear-mounted, a second operator might be necessary to steer the brushes

so to cultivate as close as possible to the crop without injury it.

They work very superficially uprooting, burying or braking weeds. If the soil is too hard the brush

weeder would remove only the upper part of weeds and they would readily regrow.

Figure 11 – Vertical axis brush weeder [6]

Rotary cultivators

Rotary cultivators are cultivators with multi-heads tool (one per inter-row) that rotate along their axis

at high speed. The difference with brush weeder is that the inter-row gangs are composed of blades,

points of knives instead of brushes. They are designed for shallow tillage, remaining close to crop

row. They are very effective in controlling weeds.

21

Figure 12 – Vertical axis rotary cultivators [6]

Basket weeders

Basket weeders have cylindrical tools, in general two, ground-driven that rotate with horizontal axis.

The front and rear tool are connected to one another through a drive chain with transmission ratio

equal to two. The front set of baskets loosens the soil, the second pulverizes it, uprooting young

seedlings.

Figure 13 – Basket weeder [20]

Intra-row weeding

Finger weeders

Two truncated steel cones are ground-driven. Each cone has rubber spikes of fingers that sink in the

soil. The crop row is between the two counter-rotating cones. The fingers connect together in the row

and have the scope of pulling out small weeds. The distance between the pair of cone could be

increased in order to avoid crop plant damage. This type of cultivator is effective against young weed

22

seedlings and it is gentle to crop that is well rooted. Compared to harrows, finger weeder system has

the necessity to be driven by an accurate steering system to work as close as possible to crop row,

otherwise their working capacity would be very low. The effect of finger weeder work is the uprooting

of weeds and to push them away from crop row. The disadvantage is that the weeds must be small

and/or easy to uproot.

If finger weeders are used for a post-emergence, selective, gentle and very precise weeding treatment

in order to realize weed control in only one passage, a precision cultivator equipment with guidance

system is added so to reach the wanted precision.

This tool can be used for beans, spring-seeded rape, seeded onions (from two-leaf stage and beyond),

red beet and sugarbeet (from 2–4 leaves), carrots (two-leaf stage and beyond) and strawberries.

Figure 14 – Finger weeder [21]

Torsion weeders

Torsion weeders are constituted by a pair of spring tines connected to a rigid frame with the possibility

of bending downward and back toward crop row so that just a short segment of the spring (few

centimetres long) work very closely to the crop. The tips flex with the soil contours and around the

plant, uprooting young weeds within the row. The tine diameter is between 5 to 10 mm depending on

the aggressiveness wanted.

Torsion weeder reduce the weed density to 60-80% of the previous weed population. However it

requires a very precise guidance system in order to be close to crop row.

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Figure 15 – Torsion weeder [22]

Pneumatic weeder

The compressed air, till 1 MPa, is used to blow small weeds out of the row. A nozzle is placed on

each side of crop row. This weed control is effective with widely spaced rows, such tulips.

Figure 16 – Pneumatic weeder “Pneumat” [23]

Intelligent weeder

In order to increase the selectivity of the weed control and reduce the crop plants damage intelligent

weeders are needed. The simplest systems have crop detection system based on light interception that

pilot hoes to move in and out of the crop row around the crop plants. They are very effective but their

speed is limited. For the case of Sarl Radis from France, one of the first commercially available

intelligent weeder, the speed is limited at 3 km/h.

Currently some researchers, cooperating with several small companies, are developing other

intelligent weeders systems using vision to detect crop plants.

24

Cutting and mowing These methods promote crop formation, control weed size and seed production, and minimize

competition between weeds and crop.

Cutting reduces leaf area of weeds, slows their growth and decreases or prevents seed production.

Mowing is most effective against annual or perennial weeds, like dicotyledonous weeds, more

vulnerable than monocotyledonous ones.

Cutting and mowing are not sufficient to a total weed control; they must be combined with other weed

management techniques, like the ones seen before.

They are weed management methods commonly used in turf, in vineyards, in orchards, in pastures

and in forage crops.

Between conventional cutting and mowing tools, laser cutting is considered a potential energy-

efficient alternative to non-chemical weed control; it delays the weeds’ growth, their competitiveness

and it kill them.

Some developments of the technology are, however, required; regrowth appears after laser cutting,

most of all if the cut is not done at the base of weed plant stem.

Another efficient weed control in development is water-jet cutting, using water at very high pressure

(2000–3000 bar) and using 5–25 l/min.

Pulling Pulling systems remove weeds that grow taller than the crop. They are constituted by rubber tyres

rolling in opposite directions, pulling the stems of weeds, breaking them or even uprooting the whole

weed plant.

This technique is faster than manual weed removal but it has not to be used as primarily weed control,

but rather as the last weed control techniques, useful to eradicate weed plants that escape from other

weed control techniques.

Figure 17 – Weed puller for wild mustard from soybean fields [6]

25

Thermal weeding Many techniques using thermal power are used for weed control. Between them it is possible to

classify: fire, directed flaming, hot water, steam, microwave, infrared, ultraviolet radiation,

electrocution and freezing. The effect of heat is the coagulation of proteins and the bursting of

protoplasm due to expansion, which consequently kills the tissue. Weeds can also be killed by very

low temperature effect, e.g. by freezing terrestrial weeds using dry ice or liquid nitrogen , by exposing

aquatic weeds to low air temperature or by removing water from a pond or lake. Some of these

possibilities are currently used commercially (e.g. flaming and steam); before their adaptation to

commercial agriculture some of them need further research and technology development, and some

appear to be impractical at present (e.g. lasers and electrocution).

The advantages of thermal weed are several. It won’t affect buried seeds, leaves dead biomass on the

soil surface which provides protection against erosion and moisture loss, may kill some insect pests

and pathogens and, most significantly, do not contaminate the environment with synthetic herbicides.

However, due to their high costs, lack of outstanding control, risks of fire ignition and in general other

safety concerns, their use is limited. Another observation is that some of these thermal weeding

options are occasionally encouraged as highly environmentally friendly but they implicate the use of

a large amount of fossil-fuel energy, gasoline or diesel, and moreover they are not entirely non-

polluting.

Guidance system Guidance systems, that could be mechanical or electronic, allow many weed control (chemical or

mechanical) to be done at greater speeds and reduce the risk of crop damage. The yield of the weeding

increase consistently; it is proved that, in onions planted in 25 cm spaced rows, hoeing 1 cm closer to

the row will keep an additional 6.5% of the field clean of weeds. This will save between 10 and 30

hours of weeding per hectare in organically grown onions.

The guidance system for horticultural crops, it is required to be very accurate for operations near to

the row; for this reason a second person, beside the tractor driver, seats on rear-mounted cultivator to

steer it, like it is shown in Figure 11.

The steering system could be mechanical, in which weeding equipment mounted rigidly to the front

of the tractor responds with an amplified movement following the driver’s steering correction.

Consequently this makes the crop row approach precise but the steering inaccurate. This system does

not permit to achieve an optimum effect in the case of differences in soil structure, uneven soil or the

presence of ruts.

The steering system could alternatively be autonomous. This is now possible thanks to the progress

in cameras and software useful to process images acquired live and processed in real-time. These

systems consist of a camera which detects the crop row(s) a few meters in front of the cultivator. The

camera is mounted on a toolbar that can shift laterally thanks to hydraulic actuators. Software uses

the image acquired to calculate the row position; it sends then the information to actuators control

that corrects the position of the inter-row cultivators and so of the camera. Systems that recognize the

individual crop plant are under development from big farmer industries but yet developed by some

research centers.

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Automated intra-row weeder technologies Weed control, in particular intra-row one, benefits greatly from automation technology; automation

allows to optimize energy consumption, increase crop yield and reduce weed control costs reducing

the number of operators needed. It allows to combine the precision of manual weed with the power

of the mechanical one.

Some factories and research centres have developed inter-row weeder systems, now commercially

available.

According to M. Norremark [24], the uprooting and/or reburial effect provided by this kind of

mechanism account for 47–61% weed control. The expected intra-row weed control effect of

uprooting and burial is 47–61 and 1–17% reductions, respectively, provided by a single treatment,

with best control at low soil moisture.

Dedousis projects an unmanned, self-propelled intra-row weeding system, shown in Figure 18 [25].

This autonomous machine is able to operate weed control thanks to a rotating tool, composed by eight

sharp tines.

The machine follows a predefined track parallel to crop rows and makes turns at the end of rows, the

tractor that tows the machine provides hydraulic and electrical power to equipment (constituted by

the weeding system, rotating tool and lifting system), and pulls, lifts and lowers the equipment itself

at predefined waypoints.

Figure 18 - The autonomous and GPS-based system for intra-row mechanical weed control in operation from behind (1) and

from the side (2). (a) side-shift and cycloid hoe GPS antenna pole, (b) wheel for height adjustment, (c) front passive frame, (d)

rear active frame (side-shifting), (e) tilt sensor, (f) wheel for control of cycloid hoe tillage depth, (g) soil-engaging disc for

resisting counteracting forces from side-shift, (h) parallelogram, hydraulic motor and reduction gear, (j) digital video

camera, (k) tine-rotor housing, (l) rotary solenoid, (m) tine spindles, (n) sigmoid-shaped tines, (o) white plastic sticks serving

as artificial plants [25]

M. Norremark concludes that combining intra-row and inter-row tillage using the automatic weed

control system, like the Dedousis machine, the field is covered between 88 and 91% with a single

treatment compared to 82% of an inter-row treatment alone.

The F. Poulsen Engineering is specialized in advanced technical solution in agricultural field; it

produces Robovator, a vision based hoeing machine for controlling weed in row crops. The robot is

provided with a plant detection camera for each row, a mechanical tool powered hydraulically that

moves in and out of the row when a crop plant is passing. Robovator is able to work with 2cm to 30

cm plant size, a distance between the rows from 25 cm to 40 cm, speed between 1-4 km/h [26].

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Figure 19 – Robovator [26]

Garford Farm produces farming technology, among which Robocrop Guided Hoes that is an

automated weeding system provided of a hydraulic sideshift, image camera, wheel speed sensors and

a control console. The vision system distinguishes crop plants from weeds if they present more foliage

than weeds and if they present a colour closer to the centre of the green band (540 nm) or red band

(620 nm) when working in red mode.

The tool is a metal tine that rotates vertically around the axis of motor reducer. The tool thanks to

rotation removes weeds between one crop plant to the other along the row line [27].

Figure 20 – Robocrop [27]

Costruzioni Meccaniche Ferrari produces Remoweed, an automatic inter-row and inter-plant hoe

system able to scan crop soil, with an infrared optical system, and remove weeds using a hydraulic

pistons to move a pair of cutting blades in and out the row. The minimum distance between two rows

is 27 cm, plant dimension can vary between 2 to 15 cm, 12000-14000 plants/h/row, pressure of

hydraulic system between 20 to 60 bar. The weeding system could be self-propelled [28].

28

Figure 21 – Remoweed, the whole system on top and the detail of weeding operation on bottom [29]

IC Weeder, produced by Steketee factory, is an automatic hoeing machine using image camera to hoe

around the crop plants for both green plants and red plants (like red lettuce). It can be armed with

hoeing blades and tines, torsion weeders, finger weeders, harrow weeders or also it is possible to

equip the machine with a chemical spray system [30]. To see it in action please check the video [31]

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Figure 22 – IC Weeder of Steketee [30]

Unmanned vehicle with automated weed system The mechanical tools for tillage are improving ever more and the current trend is the development of

an automatic guidance system. This approach will result in self-guided, self-propelled and

autonomous machines that will cultivate crops with minimal operator intervention. To this scope,

real-time images acquisition and analysis will be performed using one or several video cameras; a

GPS-based positioning system will be used to map crop and weed locations. Cultivator system will

be able to selectively control weeds within the crop row thanks to the assist of sensors that will enable

the machine to discriminate crop plants from weeds so to selectively destroy the latter.

Simon Blackmore, of Harper Adams University College, sustains that the 80% of the energy going

into cultivation is implied in damages on crop caused by big tractors maneuvers and their crossing on

the crop [32]. The solution proposed is the use for agriculture works little and light machines, like

unmanned vehicle, self-propelled, autonomous, low powered. The use of this robots for agricultural

tasks, like weed control, reduces costs devolved for field operations, even after considering the fixed

costs of machinery and maintenance. Blackmore cites a Danish study about organic farming that

concludes that these agricultural robot, called agribots, could reduce the cost of weeding by half.

Among agribots, Carre Anatis, produced by Agri Machinery, provides inter-row weeding using hoes

fixed to a frame; three hoes in total, one per row. The particularity of this machine, respect the ones

previously presented, is to be unmanned and electrically powered by 3 batteries (with 4 hour of

autonomy and 4 hour of charging time). It is also available the hybrid version with a generator [33].

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Figure 23 – Carre Anatis [33]

Another project of a fully autonomous robot is the IBEX project. Co-funded by Innovate UK, the

IBEX is tasked with recognising and eliminate using sprayed herbicide encroaching weeds on remote

hillsides that are inefficient to spray manually or too dangerous to drive on with a tractor or quad

bike. The achievable terrain slope is up to 45 degrees; it can travel through mud and thick vegetation,

including bracken. It uses a combination of sensors and Bayesian machine learning software to be

fully aware of its surroundings. It is skilled of independent navigation, covering a pre-set user targeted

area or optimize routes using his own data acquired. It is provided of a camera and data link, always

active, that allows a supervisor to intervene if required [34].

Figure 24 – IBEX agribot [34]

Hortibot was developed from the Danish Institute of Agricultural Sciences, a section of Aarhus

University, DK. It is an unmanned vehicle able to detect weeds with a visual system and then

selectively spray herbicide over them [35].

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Figure 25 – Hortibot [35]

The machines presented in this thesis section are unmanned vehicles that provide intra-row weed

control using spays; the Carre Anatis is the only machine that provide mechanical weed control but

it is an inter-row control type. Instead machines able to compute an intra-row weed control are not

developed yet.

Comparison between weed control techniques From an economic point of view, mechanical weed management is more profitable respect chemical

one. Additional costs for chemical weeding, that are not generally considered, are the ones of

decontamination of the environment from herbicides or from their metabolites.

Mechanical weed control has some negative effects on agro-ecosystems. It breaks and exposes the

soil, making it vulnerable to water erosion and wind, reduces soil moisture and organic matter

contents, provokes seed movement in the worked soil area, induces germination of some weed seeds,

it may damage soil structure and crop roots, and compact the subsoil.

However, mechanical weed controls are continuously in development, pointing more and more on

automatic vision systems that improve precision in weed removal and hence crop yield.

In Table 2 it is reported the comparison between intra-row weed control technologies presented

previously; among them manual, chemical weeding, finger, torsion, brush and flame weeder [3]. It is

possible to notice that manual labour is the most expensive, while the chemical weeding is the

cheapest; these costs are referred on an hourly labour cost of $12. Farmers prefer chemical control

respect manual also because the weeding efficiency can reach also the 90%. Among mechanical

weeders, torsion one is the cheapest and with the highest yield.

32

Table 2 – Comparison of different intra-row weed control technologies [3]

Method Cost

(USD/acre)

Work rate

(ha/h)

Operating

Speed

(km/h)

Operating depth

(mm)

Weed

control (%)

Manual

weeding 312 0,01 NA 0-50 65-85

Chemical

weeding 15 2.9-5.9 4.8-9.6 On surface 80-90

Torsion

weeder 22 0.1-1.4 6.4-8.1 0-25 60-80

Finger

weeder 38 0.3-0.6 4.8-9.6 10-40 55-60

Brush weeder 74 0.1-0.3 1.6-4.8 25-50 60-80

Flame weeder 70-90 0.1-0.5 1.6-6.4 On surface 80-90

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Intra-row weeding design

Choice motivations Nonetheless tillage weeding is considered one of the most effective techniques for weed control,

individual techniques will rarely be enough to provide season-long weed control. Rather it is a

combination of weed control techniques, with cropping management systems, which will provide

acceptable levels of weed control during a growing season.

However, a choice respect all the weed techniques available could be done and it will fall to the

technology that is worth it, i.e. to the most efficient and most adapt for a future development of

agricultural sector, the one that points to the progress of food quality and the health of environment

and species habiting the world.

The final choice and, hence, the thesis design objective is the development of a mechanical intra-row

weeding machine that uses the most advanced technologies in weed control. Among all the kinds of

weed management technologies the most suitable for the project constraints and the one most

promising for the future of weed technology is the intra-row weed control computed by an unmanned

electric vehicle.

Respect other technologies it present the following benefits

1. Time savings

The intra-row weeding is much more effective than the inter-row, it required less passages

along the crop for the weed control.

2. Savings in pesticides

The use of mechanical force to destroy weeds permits to avoid the use of pesticides and hence

the contamination of the soil, water, food and avoid the formation of herbicide resistant weeds.

3. Environment friendly

Beside the avoidance of pesticides, the use of electric propulsion reduces air pollution, with

fewer 𝐶𝑂2 emissions. The damages provoke to the soil by big tractor tires, during the

manoeuvres actions, are avoided.

4. Reduced dimensions

The reduced dimensions imply a great manoeuvrability also in little spaces, agility both during

weeding and in the turning at the end of the row; it increases the consumption of the soil area

as consequence of good manoeuvrability. There are lot inaccessible swathes land in poor

economic areas that can be reached with a small vehicle and, if brought under control, could

be made useful for tillage.

5. Reduced weight

Mechatronic systems exploit the combination between mechanical and electronic

technologies to enhance machines performances; by an electric vehicle a large use of electric

motors allows to reduce the number of mechanical components for motion transmission, like

shafts, gear boxes, joints, pulleys and belts, and hence reduce the total weight.

6. Mechanical flexibility

The machine could operate in almost every kind of crop soil, from the moister to dryer ones,

in plane and in pendency; the reduced dimension gives the possibility to work in narrowed

crops, with very little distance between rows, with every kind of dimension and shape: the

crop row is not necessary to be a straight line. The distance, along the row, between one crop

plant and the following could be not constant.

34

7. Electronic flexibility

The use of a control board allows to implement whatever control logic, decide to optimize

precision in trajectory tracking or power consumption; it allows to monitor in real time

resources, save and send data track from GPS, sensors measurements.

8. Reduced costs

The fixed costs of machine are less than the ones needed for a tractor-powered weed control,

thanks to littler dimensions, weight, higher efficiency; the use of unmanned vehicle reduces

the costs for workers.

9. Avoid manual labour

The precision in weed control, that an autonomous vehicle could provide, is near to the one

granted by manual ability, consenting the updating of manual labour, extremely stressful for

human back and inefficient. Moreover, in last decades, workers willing to manual labour are

rarer.

10. Safety

It avoids thermal weed control, flame and laser, avoids the risk of ignition fire. Moreover the

use of electrical propulsion instead of hydraulic one for actuators exclude the risk of oil losses.

Once decided that the intra-row system to design is a mechanical one, another choice about the kind

of mechanical system must be done. The following intra-row mechanical weed technologies are now

compared and evaluated according to [36]: brush weeder, torsion weeder, and tactile hoes (very

similar to basket weeder).

Mechanical control of intra-row small weeds can be carried out successfully with brushes and torsion

weeder when the weather during and after the treatment is dry. The tactile hoe can control larger

weeds as well, but weeds close to crop plants need to be controlled by spot spraying.

Hence the best way to compute a mechanical weed is through the use of torsion weeder that have a

high capacity of weed control at a relatively low cost and hence can compete with chemical weed

control, in contrast to brushes and tactile hoes. However, wet periods with abundant weed growth

make weed control by torsion weeder problematic.

In conclusion, combining the positive effects of brush and torsion weeder together it is possible to

obtain the best solution for weed control. For these reasons, the chosen tool is a rotary hoes

mechanism: it combines the efficiency in weeds removal of brushes with the capacity of uproot, bury

and cut any kind of weeds of torsion weeder.

The weeding effect of the hoe tines is similar to that of the summation of single hoe effect and is

mainly due to uprooting, weed soil coverage and root cutting [25]. This mechanism has been declared

the best one for weed control also from Kouwenhoven [36], Mohd Taufik Ahmad [3], M. Norremark

[37], A. Kielhorn [38] and P. Dedousis [25].

In Figure 26 and Figure 27 they are reported two machines between the ones just cited.

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Figure 26 – Machine developed by A Kielhorn with rotary hoes [38]

Figure 27 – Machine developed by Dedousis [25]

According to Mohd Taufik Ahmad [3] rotary hoes, called flexible tines, are the best weed technology

between saw teeth, flat blades and nylon brushes, because it has the capacity to cut, uproot, bury

weeds, it allows an easy manoeuvrability. Respect nylon brushes, that can cut, uproot and bury weeds

too, flexible tines create less dusts during weed operations.

The final choice will fall to a mechanical intra-row weeder made up by a rotary disc as tool, provided

by a series of tines disposed circumferentially.

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Design constraints The intra-row weeding system with a rotary tines tool, designed in this thesis, must be mounted on

an unmanned electric vehicle that is previously developed from Politecnico di Milano.

Figure 28 – Unmanned electric vehicle developed by Politecnico di Milano

The vehicle is powered by four motor-wheels that have two functions:

1. allow the vehicle to move forward if all four wheel are actuated

2. allow rotation along the vehicle vertical axis if the left wheels are actuated with one direction

of rotation and the right wheels are actuated with the opposite direction

The motor-wheels model is HUB10GL of UU Motor seller [39]. They have the following mechanical

and electrical characteristics:

Table 3 – Motor-wheel characteristics

Mechanical specifics Unit of measure Value

Diameter cm 25.4

Total mass kg 5.7

Motor mass kg 3.5

Power W 500

Voltage V 36

Maximum velocity Km/h 36

Electric specifics

Resistance phase-phase mΩ 160

Inductance phase-phase mH 0.76

Number of polar couples 10

𝐾𝑒 V/(rad/s) 0.36

𝐾𝑡 Nm/A 0.36

37

The motor type is a brushless tri-phase, sinusoidal motor, the supply voltage of hall sensor is of 5 V.

The structure of the vehicle is made up by ITEM profiles [40] that results particularly versatile. In

fact ITEM components give modularity to the assembly and allow the structure to be modified easily,

fundamental requirement for a prototype construction.

The structure is thought to assume a wide range of configurations; among them, four representative

configurations are shown:

Table 4 – Vehicle configurations

Configuration Alpha [deg] Wheelbase [mm] Clearance [mm] Track [mm]

A 30 750 250 750

B 30 750 250 1000

C 45 750 300 750

D 45 600 250 750

Figure 29 – The four configurations (A,B,C,D) of the unmanned electric vehicle design by Politecnico di Milano

In Table 4 and in Figure 29 are reported respectively the characteristics and the images of vehicle

configurations. Depending on the task the vehicle must compute, the characteristic of the soil or, in

general, the surrounding environment, one can choose the most suitable configuration. In Table 4 the

parameter Alpha quantify the angle of the wheel support respect the horizontal frame to which it is

jointed, Wheelbase is the distance between the wheel along longitudinal direction, Clearance is the

distance of the horizontal frame from the soil and Track is the distance between two wheels along

transversal direction.

A B

C D

38

For our purpose, depending on the width or height of crop plant or the distance between rows, it is

possible to change these parameters, providing great flexibility to the machine that becomes adaptable

to a large variety of vegetable crops. With such flexible system the vehicle could be saleable to a

multiplicity of farmers.

However the available configurations are more than four: the system can be arrange in an infinity of

configurations as shown in Figure 30.

Figure 30 – Lateral view of the vehicle showing some possible configurations the structure can assumes

The battery box, 48 V and 27 Ah, is constituted by three lead batteries, 6.42 kg each, with a total

weight of the battery box of 19.26 kg.

The rover will be equipped with the control system, the visual guidance system and the weeding

system. It will be presented in next paragraphs the weeding system design with his relative control

logic; the visual guidance system will not be designed, it will be just presented.

Intra-row mechanism overview The intra-row mechanism is composed by:

1. Tines: metal based, they can be set with different slopes

2. Tool: a rotating metal disc at which the tines are jointed

3. Tool shaft: connect the tool to the gear motor. It can be set to different length

4. Tool gear motor: jointed to the tool shaft and supported by the a translating system

5. Slider: the translating system made up by pulley-belt transmission that translate the tool gear

motor, and a pair of guides used as support

6. Frame: sustains the whole weeding mechanism and connect it with the vehicle frame

7. Encoders used for speed measurement of tool shaft and of the pulleys rotation

The mechanism is made up by two actuation

8. Gear motor driving the tool axis rotation

9. Gear motor driving the pulley of the slider transmission

The rotating tines are devoted to the weeding operation: they are sunk partially in the soil and they

break it. They realize both inter-row and intra-row weeding. The slider mechanism intervenes for the

movement of the tool inside and outside the crop row, computing a translation perpendicular respect

the vehicle trajectory. From an external observer each tine compute a rototranslation movement.

39

Mathematical model for soil resistance prediction The forces returned by the soil during a hoeing operation have been the objective of several studies

[41], [42], [43], [44].

The P. N. Wheeler work [41] provides a force prediction model that can be used for single and

multiple tines that compute a hoeing action. The tines high speed obliges inertia force to be considered

in the model. The soil is supposed to work according to Mohr-Coulomb criterion.

The soil inertia should be considered if the speed of the tine working the soil is greater than the critical

speed, reported in ( 1 ).

√5𝑔𝑤 ( 1 )

In this equation g corresponds to gravitational force and w to the width of tine in meters.

For the specific case of narrow tines the critical speed is redefined in order to consider the volume of

disturbed soil and hence the working depth; the formulation according to Godwin [43] of critical

speed transforms in equation ( 2 ) in which d stands for working depth.

√5𝑔(𝑤 + 0.6𝑑) ( 2 )

For a cylindrical tine the soil failure occurs upward, forwards and sideways.

Using the mechanics of the equilibrium, soil resistance can be expressed as a function of failure angle

𝛽, soil characteristics, and tool parameters.

Figure 31 – Schematic representation of soil failure plane with relative parameters [45], [46]

A simplified form of this soil resistance is given by the following equation

𝑃 = (0.5 𝛾 ∙ 𝑑 ∙ 𝑟 (1 +

2 𝑠

3 𝑤 ) + 𝑞 ∙ 𝑟 (1 +

𝑠

𝑤) sin(𝛽 + 𝜙) + 𝑐 ∙ 𝑑 ∙

cos(𝜙)

sin(𝛽) (1 +

𝑠

𝑤)

− 𝑐𝑎 ∙ 𝑑 ∙cos(𝛼 + 𝛽 + 𝜙)

sin(𝛼)) ∙

𝑤

sin(𝛼 + 𝛽 + 𝛿 + 𝜙) ( 3 )

Where

𝑟 = 𝑑 ∙ (cot(𝛼) + cot(𝛽)) ( 4 )

40

𝑠 = 𝑑 ∙ √cot(𝛽)2 + 2 ∙ cot(𝛼) ∙ cot(𝛽)

In the equation ( 3 ) and ( 4 ) the following terms are present: 𝛾 [𝑘𝑁/𝑚2] is the bulk unit weight, d

[m] is the depth of the blade in the soil, w [m] is the width of the tine, 𝜙 [rad] internal friction angle,

𝑐𝑎 [𝑘𝑁/𝑚2] is the soil interface adhesion, 𝛼 [rad] rake angle, 𝛿 [rad] soil metal friction.

Before calculating the soil resistance, it is necessary to determine the shape of soil failure. In order to

predict the soil cutting resistance, it is not appropriate to assume, a priori, a failure shape because the

soil cutting resistance is dependent on the failure shape itself. According to the passive earth pressure

theory, the rupture angle 𝛽, that rules the soil failure, should be obtained by minimizing the soil

cutting resistance. The minimum value can be obtain by taking the derivative of P respect 𝛽 equal to

zero. The value of the failure angle 𝛽 is then used to determine the other dimensionless factors and

the final soil resistance.

Because the soil type is unknown, being the weeder machine designed for a large variety of tillage,

the parameters referring to the soil are unknown too. However it is possible to assume the values of

the parameters to be equal to the parameters referred to a specimen soil. More precisely, the

parameters refer or to a completely frictional or to a completely cohesive one, hence studying two

opposite cases. In this way it is possible to obtain a range of values that the reaction force belong to.

Whatever soil the machine will work, the force of reaction would be inside the range found.

The parameters 𝜙, 𝑐𝑎 and 𝛿 can, hence, assume two different value depending if the soil is a frictional

or a cohesive soil.

The values of the parameters necessary to determine the soil resistance are listed in Table 5; they are

extracted from [41].

Table 5 – Summary of soil properties [41]

Soil property Units Cottenham soil series

(Frictional soil)

Evesham soil series

(Cohesive soil)

Moisture content (dry base) % 8.7 36

Bulk unit weight, 𝛾 (dry base) 𝑘𝑁/𝑚3 14.9 16.7

Internal friction angle, 𝜙 deg 30 8.6

Cohesion, c 𝑘𝑁/𝑚3 10 16

Soil interface adhesion, 𝑐𝑎 𝑘𝑁/𝑚3 2.6 3.1

Soil metal friction, 𝛿 deg 15 8.6

Sand % 73 17

Silt % 10 28

Clay % 17 55

Once the soil resistance is determined, the horizontal draft force H, required to move the tool, and the

vertical force V can be calculated as follows:

𝐻 = 𝑃 sin(𝛼 + 𝛿) + 𝑐𝑎 ∙ 𝑑 ∙ 𝑤 ∙ cot(𝛼)

𝑉 = 𝑃 cos(𝛼 + 𝛿) − 𝑐𝑎 ∙ 𝑑 ∙ 𝑤

( 5 )

In order to include the speed effect, the equation ( 6 ) [41] is necessary. It is valid for wide tines and

is based upon inertial forces. The inertia is calculated considering a wedge of soil of width and depth

equal tine width and depth respectively.

41

𝑁𝑎 =

tan(𝛽) + cot(𝛽 + 𝜙)

cos(𝛼 + 𝛿) + sin(𝛼 + 𝛿) ∙ cot(𝛽 + 𝜙) ∙ (1 + tan(𝛽) tan(𝛼))

�̅� = 𝐻 + 𝛾 ∙ 𝑣2 ∙ 𝑁𝑎 ∙ 𝑑 ∙ (𝑤 + 0.6 𝑑) sin(𝛼 + 𝛿)

�̅� = 𝑉 + 𝛾 ∙ 𝑣2 ∙ 𝑁𝑎 ∙ 𝑑 ∙ (𝑤 + 0.6 𝑑) cos(𝛼 + 𝛿)

𝑣 = 𝑣𝑐𝑎𝑟𝑡 + 𝜔 ∙ 𝑟𝑡𝑜𝑜𝑙

( 6 )

The new reaction forces �̅� and �̅� are calculated starting from H and V previously calculated and

adding a term that is function of 𝑁𝑎 parameter. The speed of the tine is designated with v; it is the

composition of the longitudinal velocity due to the movement of the cart and the tangential velocity

that has the tines mounted along the circumference of the circular tool.

The force acting on the tine, extracted from the mathematical model, is the reaction of the soil to a

tine working along a straight line. The weeding tool designed in this publication usees rotating tines

that follow a trajectory similar to a cycloid and not to a straight line; but, for the purpose of this

project, it is supposed that the force due to the action of tine along a straight line is equal to the one

along the cycloid, hypothesis confirmed by Mohod Taufik Ahmad [3].

The tines force provokes a torque acting along tool axis that could be quantified according to equation

( 7 ).

𝑇 = �̅� ∙ 𝑁𝑡𝑖𝑛𝑒𝑠 ∙ 𝑟𝑡𝑜𝑜𝑙 ( 7 )

𝑁𝑡𝑖𝑛𝑒𝑠 is the number of tines. It is possible to calculate the power necessary to maintain the torque T

at the speed of rotation 𝜔:

�̇�𝑡𝑜𝑜𝑙 = 𝑇 ∙ 𝜔 ( 8 )

The power just presented is the one necessary to the gear-motor that is jointed with the tool.

The weeding tool, respect a global observer, compute a rotation along his axis and a translation along

a guide in order to shift the tool itself inside the row and outside the row crop. The reaction of the soil

during translation of the slider, supposing that the tool is not moving, can be considered identical to

the reaction previously calculated for the rotating tool.

The final soil reaction would be the composition to the force �̅� due to the rotation of the tool and to

the translation of the slider. Hence, the maximum force applied on tine would be equal to 2 ∙ �̅�.

The power of the slider can be defined as:

�̇�𝑠𝑙𝑖𝑑𝑒𝑟 = �̅� ∙ 𝑁𝑡𝑖𝑛𝑒𝑠 ∙ 𝑣𝑠𝑙𝑖𝑑𝑒𝑟 ( 9 )

Where 𝑣𝑠𝑙𝑖𝑑𝑒𝑟 is the translating velocity of the slider.

In order to have confirm of the goodness of the used approach, an alternative method is used.

According to McKyes [41] the value of 𝛽 can be approximate to the following expression:

𝛽 = arctan (

1

𝑚 − 𝑐𝑜𝑡(𝛼)) ( 10 )

The value of m can be found from the following chart:

42

Figure 32 – Experimental relationship between rupture distance ratio (m=f/d) and tine rake angle 𝜶 [41]

The kinematics of the weeding mechanism should provide a correct trajectory for the weed control,

in order to avoid the contact between tines and crop plant, and obtain the largest weed control area as

possible, bringing the tines as close as possible to crop plant

In the Figure 33 it is reported the scheme of weed mechanism: it is sketched the tool, with radius

𝑟𝑡𝑜𝑜𝑙; the canopy, with radius 𝑟𝑐𝑎𝑛𝑜𝑝𝑦; the dashed circumference represents the position of the tool in

the instant in which it is closest to the canopy. They are also reported the velocity of the cart 𝑣𝑐𝑎𝑟𝑡

(of the vehicle), with S the forward motion distance travelled from the tool, with y the lateral distance,

with 𝜃 the angle of departure from cart travel direction.

Figure 33 – Scheme of weeding kinematics

𝑂𝐵̅̅ ̅̅ =

𝑑

2+ 𝑟𝑐𝑎𝑛𝑜𝑝𝑦

𝜃 = acos (

𝑂𝐵̅̅ ̅̅

𝑟𝑐𝑎𝑛𝑜𝑝𝑦 + 𝑟𝑡𝑜𝑜𝑙)

𝑦 = (𝑟𝑡𝑜𝑜𝑙 + 𝑟𝑐𝑎𝑛𝑜𝑝𝑦) sin(𝜃)

43

𝑠 =

𝑑

2+ 𝑟𝑡𝑜𝑜𝑙

𝑇𝑜𝑡𝑎𝑙 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 = √𝑦2 + 𝑠2

𝑡𝑖𝑚𝑒 =𝑠

𝑣𝑐𝑎𝑟𝑡

𝑣𝑦 = 𝑣𝑐𝑎𝑟𝑡 ∙ tan (𝑦

𝑠)

( 11 )

With equation ( 11 ) it is possible to calculate the time necessary to travel the distance s and the

velocity of the slider as function of the velocity of the cart.

However, the procedure illustrated in equation ( 11 ) is difficulty applicable; in fact, when the tool

reaches the distance d, it suddenly should assume the velocity 𝑣𝑦 and, hence, have ann infinite

acceleration. An alternative kinematic approach bases on the cycloidal motion law: when the tool is

approaching the canopy area, the system starts moving with a cycloidal motion law that allows to

have finite acceleration (and finite jerk) providing a soft velocity transition. The trajectory would be

a sinusoidal curve.

Kinematics The main objective of the design is working with efficiency the widest portion of soil possible. For

this purpose the rotation of the tool must be synchronized with longitudinal movement of cart.

According to [3], a good coverage of soil area is obtained for rotation speed of the tool that follow

the relation:

𝜔 [𝑟𝑝𝑚] =

350

0.8∙ 𝑣𝑐𝑎𝑟𝑡 [

𝑘𝑚

ℎ] ( 12 )

The relation is linear; the graphic representation of the soil coverage with a rotating speed of 350 rpm,

traveling speed of the cart of 0.8 km/h, is shown in Figure 34, that refers to 1 second of working. The

three different colored lines refers each of the three tines.

44

Figure 34 – Region covered by the tool with 3 tines at 350 rpm tool rotation, 0.8 km/h cart speed per 1 second

Figure 35 – Region covered by the tool with 3 tines at 350 rpm tool rotation, 0.8 km/h cart speed per 0.2 second

In Figure 35 it is shown the coverage of the area in the first 0.2 seconds. In the initial stage, the

maximum distance between the kerfs of two tines is 7.4 cm. When it has passed at least 0.7 seconds

the new kerfs start to intersect with the ones computed in first instants; in this condition the maximum

distance between two kerfs is 6.96 cm, considering a width of each tine of 0.5 cm, the actual maximum

distance of kerfs results 6.46 cm.

The trajectory of each tine is a cycloidal curve that respects the following relations:

𝑥 = 𝑟𝑡𝑜𝑜𝑙 ∙ cos (2 𝜋 𝑓 𝑡 +

2

3𝜋𝑘) + 𝑣𝑐𝑎𝑟𝑡 ∙ 𝑡

45

𝑦 = 𝑟𝑡𝑜𝑜𝑙 ∙ 𝑠𝑖𝑛 (2 𝜋 𝑓 𝑡 +

2

3𝜋𝑘)

𝑘 = 0,1,2

𝑡 = 𝑡𝑖𝑚𝑒 ( 13 )

The trajectory made from the tool during the weeding is shown in the Figure 36 that shows the

trajectories of each tine (in this case three). The movement is divided in three phases: before the

approaching to the crop plant (to the left), during approaching (in the center), and after (to the right).

The direction of vehicle is marked with the dashed line.

The data of tool and canopy used for the figure are the following:

Table 6 – Parameters of weeding operation used for representation of Figure 36

Parameter Unit of measure value

𝑟𝑐𝑎𝑛𝑜𝑝𝑦 cm 10 cm

𝑟𝑡𝑜𝑜𝑙 cm 6.35 cm

Stroke cm 16.35 cm

𝜔 rpm 350 rpm

Cart speed km/h 0.8 km/h

Slider speed cm/s 6.06 cm/s

The radius of tool, 6.35 cm, corresponds to the one considered most effective from the research study

of Ahmad [3]; the canopy radius is chosen arbitrarily. From tool and canopy radii depends the stroke

length, equal to the sum of two radii.

Figure 36 - Region of soil covered by the tool when is approaching the crop plant

It is possible to notice from Figure 36 that the weed control takes place only to one side of the crop

row, being the weeding system provided of just one tool. In order to compute a correct weed operation

46

it is hence necessary that the vehicle passes two times on the same row so to compute the weeding

also to the other side of crop row. This additional operation double the weeding time, making the

weed control longer, less productive, more difficult to manage. The alternative is the use an intra-row

weeding system provided of two tools instead of one; in this manner the system would became more

complex, heavier, it would requires more power of motors and battery.

The system designed in this thesis is provided of just one tool being the vehicle just a prototype, with

the main task the design of a good mechanism more than the optimization of weeding performances.

It is now presented the kinematics of weed operation in real situation, in which the system must react

to interaction with the crop plants that are disposed along the row at a certain inter-plant distance.

For this purpose it is presented, as example, the disposition of tomatoes crop plants adopted by

Consorzio Cremasco Pomodori, a potential purchaser of the unmanned electric vehicle object of this

thesis. These data are declared during a conference held by the consortium in 2016.

The disposition of the tomato plants is the following:

Distance between two row: 140 cm

Distance between plants along the row: 60 cm

Radius of canopy: from 0 to 30 cm or even 40 cm

Considering to work with continuity of motion this specific type of crop field, the trajectory follows

by the tool could be the one shown in Figure 37.

Figure 37 - Region of soil covered by the tool when is approaching three crop plants. Their disposition refers to Consorzio

Cremasco Pomodori conference

The speed necessary to the slider to compute the required stroke is 6.06 cm/s.

The governing equations of tines trajectories are the followings:

𝑥 = 𝑟𝑡𝑜𝑜𝑙 ∙ cos (2 𝜋 𝑓 𝑡 +

2

3𝜋𝑘) + 𝑣𝑐𝑎𝑟𝑡 ∙ 𝑡

𝑦 = 𝑟𝑡𝑜𝑜𝑙 ∙ 𝑠𝑖𝑛 (2 𝜋 𝑓 𝑡 +2

3𝜋𝑘) + 𝑠𝑡𝑟𝑜𝑘𝑒 ∙ sin (

𝑣𝑐𝑎𝑟𝑡

1.2𝑚∙ 2𝜋𝑡)

𝑘 = 0,1,2

( 14 )

47

𝑡 = 𝑡𝑖𝑚𝑒

An alternative trajectory con be the one shown in Figure 38.

Figure 38 - Region of soil covered by the tool when is approaching three crop plants. Alternative trajectory

It provides a weeding operation just to one side to the crop row. The disadvantage of this trajectory

is the abrupt deceleration and acceleration that the gear motor of the slider is forced to execute;

moreover the non-negligible inertia of the slider system impede, practically, to instantaneously stop

the slider and restart it. Hence the inversion of direction must be preceded and followed by a

deceleration and acceleration phase respectively; this additional phase would worse the weed

efficiency because the tool would follows a smoothed trajectory, remaining farther to the crop row

and, thus, missing to remove weeds present along the row.

However this alternative trajectory is the only suitable solution if the canopy radius is bigger than the

semi-distance between two following crop plants. In this situation the weeding would be limited to

an inter-row weeding and to follow the canopy region boundaries. But the intra-row system in

objective would lose sense and a classic inter-weeding system can be used more efficiently.

The governing equations of the trajectories are the following:

𝑥 = 𝑟𝑡𝑜𝑜𝑙 ∙ cos (2 𝜋 𝑓 𝑡 +

2

3𝜋𝑘) + 𝑣𝑐𝑎𝑟𝑡 ∙ 𝑡

𝑦 = 𝑟𝑡𝑜𝑜𝑙 ∙ 𝑠𝑖𝑛 (2 𝜋 𝑓 𝑡 +

2

3𝜋𝑘) + |𝑠𝑡𝑟𝑜𝑘𝑒 ∙ sin (

𝑣𝑐𝑎𝑟𝑡

1.2𝑚∙ 2𝜋𝑡)|

𝑘 = 0,1,2

𝑡 = 𝑡𝑖𝑚𝑒 ( 15 )

Summarizing, the preferential trajectory is the one depicted in Figure 37 because gives continuity of

movement, avoids abrupt change in directions, provides a good coverage of the canopy area; hence

it is the one adopted for the thesis project. The alternative trajectory (Figure 38) is instead abandoned.

If the distance between two crop plants is higher, it is possible to distinguish two trajectories patterns.

The one reported in Figure 39 involves the tool to alternatively move once to the left of crop plant

and once to the right. The same considerations made for the trajectory of Figure 37 are still valid.

48

Figure 39 - Region of soil covered by the tool when it is approaching two crop plants

In Figure 40 it is reported an alternative solution of tool trajectory in which the tool remains always

to just one side of crop row.

Figure 40 - Region of soil covered by the tool when it is approaching two crop plants. Alternative solution

When the tool is approaching the crop plant the gear motor of the slider is actioned and the established

movement is computed.

The translational velocity of the slider depends on the battery box power mounted on the vehicle, the

fastest the movement the better the weeding operation because the tool removes weeds closer, and

hence more harmful for the crop plant.

49

The dimensioning of the tool gear motor is relatively simple because the rotation of the tool is

constant; hence it is necessary to choice a motor that supplies the wanted torque at the wanted angular

velocity.

The dimensioning of the slider gear motor, instead, has to take into account of the inertia of the system

that depends on the associated acceleration. The motion law defines the trend of the acceleration. The

most suitable motion law for the purpose of the thesis is the one that minimizes the power because

small power consumption corresponds to less motor weight, less cart weight and hence more battery

autonomy.

The motion law chosen is the cubic law because it minimizes the power; Their equations are reported

in ( 16 ).

�̈� = 𝑐𝑎

ℎ

𝑡𝑎2

(1 −2

𝑡𝑎𝑡)

�̇� = 𝑐𝑎

ℎ

𝑡𝑎2

𝑡 (1 −𝑡

𝑡𝑎)

𝑦 = 𝑐𝑎

ℎ

𝑡𝑎2

𝑡2 (1

2−

2

3

𝑡

𝑡𝑎)

𝑐𝑎 = 6

ℎ = 1𝑚 𝑡𝑜𝑡𝑎𝑙 𝑟𝑖𝑠𝑒

𝑡𝑎 = 1𝑠 𝑎𝑐𝑡𝑢𝑎𝑡𝑖𝑜𝑛 𝑡𝑖𝑚𝑒 ( 16 )

The total rise is 1 meter so to give high versatility to the machine and weed a large variety of crop

field. The actuation time it is supposed to be 1 second; it should be considered as a superior limit.

The gear motor can reach such speed but it is not advisable to use it because the battery autonomy

would be very low. However, thinking of a possible future strengthening of the battery package, it is

now presented the slider kinematics with such value of slider actuation time.

Figure 41 - Cubic profile of slider translation movement. On the top the displacement, in the middle the velocity and on the

bottom the acceleration

50

In order to obtain a translation of the slider of 1 meter the gear motor must compute 40 rad of rotation,

being the radius of a pulley, not defined yet, equal to 25 cm. It is reported the cycloidal profile too,

that is the one to which figures from Figure 36 to Figure 40 refer to.

Figure 42 – Cycloidal profile of slider translation movement. On the top the displacement, in the middle the velocity and on

the bottom the acceleration

It is possible to notice that the difference in position curve between cubic motion law and cycloidal

motion law is very similar. Hence the cover area studied for the cycloidal curve is approximately the

same of cubic one, so it is possible to adopt the same conclusion and assume the area covered is the

same.

51

Figure 43 – Displacement profile on cubic (solid line) and cycloidal (dashed line) motion laws

Simulation results The design process for weed system is an iterative process. The amperes necessary to move the two

gear motors and the four motor-wheels must guarantee an acceptable battery autonomy, considering

a battery consumption of 27 Ah.

From the weeding-efficacy point of view, the tool rotating speed, translating slider speed and cart

speed are related by precise relationships that must be satisfied in order to provide a correct weed

control.

Using a parameterized calculation procedure it is possible to easily forecast forces, torques, powers,

currents and voltages of similar systems with different battery box. In fact, if a more powerful battery

box is available the motors velocities could be increased, increasing in this way the throughput of

weed control.

The slider stroke depends on the kind of crop the machine is working on, the life stage of crop plants,

the distance between crop plants along the row. This parameter should be designed every time the

machine starts to work a new tillage.

All the calculations made for soil force prediction and the estimation of velocities, trajectories,

powers, torques and other mathematical and physical entities are all elaborated and computed through

the software Matlab, with the Student Edition released from Politecnico di Milano.

Forces The forces that born in the contact between soil and tine depend on the typology of soil. Different

results can be obtain considering a frictional or a cohesive soil.

Frictional soil

The forces that born in the contact between soil and tine depend on the typology of soil. Different

results can be obtain considering a frictional or a cohesive soil.

52

The values used for forces prediction are reported in Table 7.

Table 7 – Summary of frictional soil properties [41] and process data

Soil property Units Frictional soil

Bulk unit weight, 𝛾 (dry base) kN/m3 14.9

Internal friction angle, 𝜙 deg 30

Cohesion, c kN/m3 10

Soil interface adhesion, 𝑐𝑎 kN/m3 2.6

Soil metal friction, 𝛿 deg 15

Rake angle, 𝛼 deg 75

Surcharge pressure, q kPa 0

Working depth m 0.0254

Rotation speed rpm 168

Cart speed km/h 0.8

𝑟𝑡𝑜𝑜𝑙 m 0.0635

Width tine mm 5

Number of tines - 3

The data of frictional soil, bulk unit weight, internal friction angle, cohesion factor, soil interface

adhesion, soil metal friction refer to [41]. Rake angle is chosen with a trial and error method; the

surcharge pressure is unknown, it is taken equal to zero (his contribution inside equations is very

little); the working depth chosen is the value that allows to have good performance in terms of weed

control with a restrained current consumption; the tool rotational speed is the lowest admitted for a

speed cart of 0.8 km/h; the cart speed is chosen with the intention of minimize power required and

current consumption; the choice of the number of tines has been made between five and three tines,

and finally three tines are selected in order to decrease weed forces; 𝑟𝑡𝑜𝑜𝑙 is chosen considering

Ahmad work [3] that provides experimental results about the efficacy of the reported tool radius.

Moreover the study confirms that with such working depth, rotation speed, cart speed, width tine and

number of tines the weed control is effective.

In this conditions the critical speed results equal to 0.9964 𝑚/𝑠, far exceeded by the working

velocities.

The soil resistance as seen in chapter “Mathematical model for soil resistance prediction”, depends

on failure angle 𝛽. In particular, the failure angle that characterizes the soil fracture will be the one

that minimizes the soil resistance. The Figure 44 shows that the minimum soil resistance, considering

one tine working the soil, corresponds to a value of 𝛽 near to 39 deg.

53

Figure 44 – Soil resistance function of failure angle 𝜷 for frictional soil

The resulting horizontal forces for one and three tines are reported in Figure 45, in which they are put

into relation with the angular speed and with cart velocity.

54

Figure 45 – Horizontal forces comparison for frictional soil. The forces are function of the number of tines (one or three), of

the rotation speed of the tool (200 rpm, 350 rpm, 500 rpm) and of the cart speed

55

The force increase with the increase of the cart speed and with the increase of the rotation speed of

the tool. The dependence of the force with respect the rotation speed is best appreciated in Figure 46;

it represents the force of three tines working.

Figure 46 - Horizontal force for three-tine tool for frictional soil, function of tool rotation speed and cart velocity

The graphs reported in Figure 45 and Figure 46 are useful to give a quick overview of forces playing,

of the feasibility of a new weed machine with a different cart speed, rotation speed or number of tines.

The weeding mechanical system is thought to be the more flexible and adaptable as possible, both to

soil characteristics and to crop plant dimensions; hence the design of the machine for a new purchaser

is supposed to be the fastest as possible and the most repeatable as possible. The choice of gear

motors, which depends from forces playing during weeding, is advisable to be accurate and done

quickly.

Considering the project constraints, previously presented in “Design constraints” section, and the

design data chosen consequently (reported in Table 7), the following results can be reported.

Table 8 – Results obtained for frictional soil

Frictional soil

Horizontal draft force for one tine N 42.98

Vertical force for one tine (positive force downwards) N -0.33

Maximum speed m/s 2.34

Wheeler-Godwin model

Horizontal draft force for one tine considering speed N 75.98

Vertical force for one tine considering speed (positive force

downwards) N -0.33

McKyes model

Horizontal draft force for one tine considering speed N 123.33

Vertical force for one tine considering speed N -0.33

Torque necessary for three tines Nm 14.47

56

Power necessary for tool motor W 254.63

Power necessary to slider motor W 128.94

The firsts three results in Table 8 refer to the calculation of horizontal and vertical force neglecting

the inertial contribution of the force due to disturbed volume. Below these results they are presented

the forces calculated with Wheeler-Godwin and McKyes model. The forces provided by the McKyes

model are higher and they are assumed to be less accurate because they are based on the graphical

interpolation of the rupture distance ratio that is obtained empirically for a reference soil.

The lasts three results of Table 8 refer to torque and power necessary for the tool to compute the weed

operation and to the power needed for the slider for the same purpose.

Cohesive soil

In the case the target soil is more similar to a cohesive one, using the mathematical procedure for

force prediction and the data reported in Table 9, it is possible to find the relation between cart speed,

rotation speed and force necessary to uproot weeds.

Table 9 - Summary of cohesive soil properties [41] and process data

Soil property Units Cohesive soil

Bulk unit weight, 𝛾 (dry base) 𝑘𝑁/𝑚3 16.7

Internal friction angle, 𝜙 deg 8.6

Cohesion, c 𝑘𝑁/𝑚3 16

Soil interface adhesion, 𝑐𝑎 𝑘𝑁/𝑚3 3.1

Soil metal friction, 𝛿 deg 8.6

Rake angle, 𝛼 deg 75

Surcharge pressure, q kPa 0

Working depth m 0.0254

Rotation speed rpm 168

Cart speed km/h 0.8

𝑟𝑡𝑜𝑜𝑙 m 0.0635

Width tine mm 5

Number of tines - 3

The same considerations about data of Table 7 can be done also about Table 9.

The behavior of soil resistance function of the failure angle 𝛽 is reported in Figure 47.

57

Figure 47 – Soil resistance function of failure angle 𝜷 for cohesive soil

It is possible to notice the minimum of soil resistance in correspondence of a failure angle near to

60°.

It is now possible to calculate the horizontal force for different cart speed, tool rotation speed and for

a tool with one tine and one with three tines; results are summarized in Figure 48.

58

Figure 48 – Horizontal forces comparison for cohesive soil. The forces are function of the number of tines (one or three), of

the rotation speed of the tool (200 rpm, 350 rpm, 500 rpm) and of the cart speed

59

Summing up the results obtained for three-tine tool for frictional and cohesive soil, it is possible to

notice that the force of soil reaction obtained for frictional soil is greater than for cohesive soil.

Figure 49 - Horizontal force for three-tine tool, function of tool rotation speed and cart velocity. Comparison between

frictional soil and cohesive soil

Table 10 – Results obtained for cohesive soil

Cohesive soil

Horizontal draft force for one tine N 26.07

Vertical force for one tine (positive force downwards) N 2.52

Maximum speed m/s 2.3394

Wheeler-Godwin model

Horizontal draft force for one tine considering speed N 59.1

Vertical force for one tine considering speed (positive force

downwards) N 6.23

McKyes model

Horizontal draft force for one tine considering speed N 82.5

Vertical force for one tine considering speed N 8.85

Torque necessary for three tines Nm 11.26

Power necessary for tool motor W 198.18

Power necessary to slider motor W 78.21

As expected the values of forces, torque and powers needed for cohesive soil are lower than ones of

frictional soil.

Considerations

The final values of the forces that will be considered are in between the frictional and cohesive soil.

In order to determine the precise values of soil reactions it is necessary to compute soil analysis on

the target crop field. The soil properties that should be found experimentally are:

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Bulk unit weight

Internal friction angle

Cohesion

Soil metal friction

Soil interface adhesion

In absence of these parameters, it is estimated a one tine reaction force of the soil of 70 N at 176 rpm.

This value is chosen considering the Wheeler-Godwin model, the most accurate. Tool torque and

slider force are hence derived and presented in Table 11:

Table 11 – Summary of the mechanical parameters having as assumption the soil reaction force equal to 70 N

Mechanical parameters summary

Force for one tine N 70

Force for three tines N 210

Torque of tool gear-motor Nm 11.56

Power tool gear-motor W 213

Torque slider gear-motor Nm 5.25

Force acting on sled N 246.9

Power slider W 111.45

Torque The toque necessary to the gear motor driving the tool is reported in Figure 50; it represents the

behaviour of the torque function of the kind of soil (frictional or cohesive), rotational speed of tool

(200 rpm, 350 rpm, 500 rpm) and the traveling speed of the cart.

Figure 50 – Torque required for weeding, function of soil type (frictional, cohesive), rotation tool speed (200 rpm, 350 rpm,

500 rpm) and cart speed

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Power The power necessary for the tool and slider gear motor, considering a speed of 1m/s, is reported in

Figure 51; it represents the behaviour of the power function of the kind of soil (frictional or cohesive),

rotational speed of tool (200 rpm, 350 rpm, 500 rpm) and the traveling speed of the cart.

Figure 51 – Power required for weeding, function of soil type (frictional, cohesive), rotation tool speed (200 rpm, 350 rpm,

500 rpm) and cart speed

62

Gear motors choice

Tool The tool is driven by a gear motor, constituted by an alternate current brushless motor, and,

considering the high torque and the low rotational speed necessary for the weed control, a planetary

gear that reduces the motor speed and increase the torque.

The AC brushless motors, although they are more expensive than DC motors, provide more flexibility

to the motor control. They can be easily controlled with an inverter that allows to regulate voltage

without any constraints and easily step them up or down.

DC motors performances depend on the efficiency that is function of the speed value; moreover the

efficiency information generally are not provided from the motor producer, increasing the complexity

of control design.

In DC motors the brushes change commutator segments constantly, causing sparking and wear.

AC motors instead use slip-rings and the brushes last much longer, making the machines much more

reliable. Frequently the brushes last until the bearings wear out while DC motors have to have brushes

replaced fairly often.

The motor chosen is produced by BMU Brushless Motor and has the characteristics reported in Table

12 [47].

Figure 52 – BMU Driver and AC Brushless motor with reducer

Table 12 – Tool gear-motor and driver characteristics

Brushless AC motor and driver package BMU Series

Motor

Name BLM 6300 SP-AS

Power W 300

Rated voltage VAC Single-Phase 200-240 /

Three-Phase 200-240

Permissible voltage range -15%; +10%

Frequency Hz 50 / 60

Permissible frequency range -5%; +5%

Rated input current A Single-Phase 3.4 /

Three-Phase 2.1

63

Maximum input current A Single-Phase 7.8 /

Three-Phase 4.7

Rated speed rpm 3000

Rated torque Nm 0.955

Maximum instantaneous torque Nm 1.43

Rotor inertia J: ∙ 10−4𝑘𝑔 ∙ 𝑚2 0.67

Round shaft permissible inertia J: ∙ 10−4𝑘𝑔 ∙ 𝑚2 12

Speed control range rpm 80~4000 (speed ratio 1:50)

Reducer

Reducer type Parallel shaft gearhead

Transmission ratio 15

Rotation speed (with 3000 rpm motor

speed) rpm 200

Torque at 80~3000 rpm Nm 12.9

Torque at 4000 rpm Nm 9.7

Motor speed with transmission ratio 15 rmp 3000

Permissible radial load at 3000 rpm N 800

Permissible axial load at 3000 rpm N 200

Permissible load inertia J: X 10−4𝑘𝑔 ∙ 𝑚2 450-1000

Total weight (motor + reducer) kg 5.2

Cost € 370

Driver

Name BMUD300-C

Type Single-Phase, Three-Phase 200-

240 VAC

Cost € 165

Figure 53 – Characteristic curve of BMU 300W AC Brushless motor

64

Slider The gear motor required to slider is relatively more stressed respect the one of the tool; it has to win

soil forces and the inertia of whole translating system. The forces of inertia are considered the ones

that born using a cubic motion law, as presented in “Kinematics” chapter. The moving mass are

constituted by the sled connected with the transmission belts, two additional belts that act as support,

the tool gear motor, the tool itself.

The motor chosen is AC brushless motor from SMV Series of Servotecnica producer, the reducer is

a planetary gear of Alpha Riduttori S.p.A. The relative characteristics are reported in Table 13.

Figure 54 – AC Brushless motor SMV 38I to the left, and the related planetary gear LPB+120-1S-3 (alpha Riduttori S.p.A.)

to the right

Table 13 – Slider gear-motor characteristics

Brushless AC motor (SMV Servotecnica) and planetary (Alpha Riduttori)

Motor

Name SMV 38I

Power W 792

Rated voltage V 560

Rated torque Nm 2.5

Stall torque Nm 3

Peak torque Nm 13

Rated current A 1.8

Stall current A 3

Peak current A 9.7

Rated speed rpm 3210

Peak speed rpm 9000

Voltage constant Vmin/1000 90

Torque constant Nm/A 1.5

Winding resistance Ohm 9.8

Winding inductivity mH 26

Armature inertia kg cm2 1.6

65

Weight kg 4.6

Reducer

Reducer name LPB+120-1S-3

Planetary gear stages 1

Transmission ratio 3

Maximum acceleration torque Nm 200

Rated output torque Nm 100

Emergence torque Nm 480

Nominal input speed rpm 480

Maximum input speed rpm 4800

Weight kg 7.3

Inertia moment kg cm2 5.37

Using the data reported in Table 13 and considering that translation of the slider follows a cubic

motion law, it is possible to superimpose the characteristic curve of motor with the torque-speed curve

that refers to the slider load (Figure 55). The red line is the motor curve, the blue one the curve of the

slider load reduced to the motor shaft, the dashed blue line the root mean square (RMS) of the load

curve. From the plot it is possible to notice that the motor characteristic curve contains completely

the load curve and that the dashed blue line, load RMS, is below the rated gear-motor torque. This

guarantees the correct design of the gear-motor system.

Figure 55 – Characteristic curve of slider gear-motor (in red), load curve (in blue)

66

Dynamic simulation The weeding system design has been simulated with Matlab and Sumulink, and internal package of

Matlab software. By means of this last informatics support it is possible to study the transitory

behaviour of all the moving components of the machine, assess mechanical and electrical power

consumption during all phases of machine working, have the confirm of results obtained in previous

section “Mathematical model for soil resistance prediction”, take into account of the inertia forces

contributes and design a control logic for each electric motors.

For the dynamic simulation, the inertia moment of all moving parts are calculated.

Table 14 – Dynamic simulation data

Tool Mass kg 2

Radius m 0.0625

Moment of inertia kg m2 0.0039

Slider

Mass of tool gear motor kg 5.2

Mass of fastening system kg ~1

Mass of tool kg 2

Total translating mass kg 8.2

Radius pulley m 0.025

Mass pulley kg 0.4

Moment of inertia pulley kg m2 2.5e-4

Moment of inertia slider kg m2 0.0054

Motor-wheel Mass kg 5.7

Radius m 0.127

Moment of inertia wheel kg m2 0.046

Weeding system mass kg 15.4

Battery box mass kg 8.5

Estimate of the vehicle total weight kg 72.2

Moment of inertia of whole cart reduced

to one motor-wheel kg m2 0.1456

Tool The transfer function between angular speed of the tool and the torque is reported in equation ( 17 );

it is a first order system. In order to control the system in velocity the control transfer function reported

in equation ( 18 ) is proposed.

𝐺𝑣,𝑡𝑜𝑜𝑙 =

1

𝐽𝑡𝑜𝑜𝑙

𝜏𝑡𝑜𝑜𝑙2 𝑠

( 17 )

𝑅𝑣,𝑡𝑜𝑜𝑙 = 7.5

𝑠 + 80

𝑠2 + 1000𝑠

( 18 )

67

In order to control the tool in velocity a second order control transfer function, 𝑅𝑣,𝑡𝑜𝑜𝑙 is needed. The

root locus of the controlled system is the following:

Figure 56 – Root locus of the system 𝑮𝒗,𝒕𝒐𝒐𝒍 ∙ 𝑹𝒗,𝒕𝒐𝒐𝒍. The x marker are the poles, the hollow circle marker the zeros, the solid

circle the poles considering the gain

The pole at the left (𝑠 = −1000) has such high frequency in order to have a good response if there is

a torque input disturbance: in fact, being the tool very light weight, the impact with the soil would

cause the sudden stop of rotation. With an aggressive control logic the system can reject the torque

disturb provided by the soil.

Figure 57 – Bode plot for the tool gear-motor system

68

Using Simulink software it is now presented the study on the system transitory. The tool is accelerated

from 0 rad/s to 276.46 rad/s (that is the speed of the tool 176 rpm, multiplied for the transmission

ratio 𝜏 = 15). In order to limit the power needed to the system, the tool starts working the soil from

the second 10 forward, so that the power needed for system acceleration does not superimpose with

the weed effort.

In Figure 58 it is represented the tool response to the reference in speed (on top); the zoom on weeding

start, at second 10, on the bottom. The contact with the soil provokes a reduction in angular velocity

of the motor till 180 rad/s but then it comes back to the reference quickly.

Figure 58 – System response to the reference in velocity (on the top), zoom at second 10 when the tool starts working (on the

bottom)

The behaviours of the voltage, current and power are reported in Figure 59.

69

Figure 59 – Voltage (top left), current (top right) and power (on the bottom) of the tool system

Slider The transfer function between angular speed of the gear-motor slider and the torque is reported in

equations ( 19 ) and ( 17 ); it is a first order system. In order to control the system in velocity the

control transfer function reported in equation ( 20 ) is proposed.

𝐺𝑠𝑙𝑖𝑑𝑒𝑟 =

1

𝐽𝑠𝑙𝑖𝑑𝑒𝑟 + 𝐽𝑝𝑢𝑙𝑙𝑒𝑦𝑠

𝜏𝑠𝑙𝑖𝑑𝑒𝑟2 𝑠

( 19 )

𝑅𝑠𝑙𝑖𝑑𝑒𝑟 = 8

𝑠 + 10

𝑠 + 200

( 20 )

The control logic needed is a position control because in this case the position of the slider must be

controlled. Hence the control transfer function 𝑅𝑠𝑙𝑖𝑑𝑒𝑟 is a first order function. The root locus of the

controlled system is the following:

70

Figure 60 – Root locus of the system 𝑮𝒔𝒍𝒊𝒅𝒆𝒓 ∙ 𝑹𝒔𝒍𝒊𝒅𝒆𝒓. The x marker are the poles, the hollow circle marker the zeros, the solid

circle the poles considering the gain

The resulting bode plot is the following:

Figure 61 – Bode plot for the slider gear-motor system

Using Simulink software it is now presented the study on the system transitory. The slider follows

the motion law presented in “Kinematics” chapter.

71

Figure 62 – System response to the reference in position (on the top), zoom at second 10 when the tool starts working (on the

bottom)

The position variation of the system respect the reference is very little. Moreover, it is not required a

very accurate positioning of the slider because the weeding efficiency would not suffer any changes;

hence it is possible to adopt a soft control, reducing wear of mechanical component and power

consumption. The behaviours of the voltage, current and power are reported in Figure 63.

72

Figure 63 – Voltage (top left), current (top right) and power (on the bottom) of the tool system

The maximum variation of voltage before the start of weeding is in between ±140 V; after, between

±155 V. The current ±0.11 A before, ±1.53 A after. The power ±15 W before and between +195W

and -115 after. Must be noticed that the power foreseen by previous calculation was 111W; the power

provided by the Simulink simulation is higher for two reasons: electric power, due to product between

voltage and current, is always greater than the mechanical one because the electro-mechanical

conversion of energy does not have an efficiency equal to 1; in the moment in which the tool starts

working the actuator is particularly stressed because of the sudden load disturb, hence it tries to

recover the reference and requires an additional amount of power.

Motor wheel The transfer function between angular speed of the gear-motor slider and the torque is reported in

equation ( 21 ), ( 19 ) and ( 17 ); it is a first order system. In order to control the system in velocity

the control transfer function reported in equation ( 22 ) is proposed.

𝐺𝑚𝑜𝑡𝑜𝑟 𝑤ℎ𝑒𝑒𝑙 =

1

(𝐽𝑚𝑜𝑡𝑜𝑟 + 𝐽𝑙𝑜𝑎𝑑)𝑠

( 21 )

73

𝑅𝑚𝑜𝑡𝑜𝑟 𝑤ℎ𝑒𝑒𝑙 = 630

𝑠 + 10

𝑠 + 100

( 22 )

The term 𝐽𝑚𝑜𝑡𝑜𝑟 is the inertia of the motor, 𝐽𝑙𝑜𝑎𝑑 is the inertia of the whole cart reduced to one single

wheel. The root locus of the controlled system is the following:

Figure 64 – Root locus of the system 𝑮𝒎𝒐𝒕𝒐𝒓 𝒘𝒉𝒆𝒆𝒍 ∙ 𝑹𝒎𝒐𝒕𝒐𝒓 𝒘𝒉𝒆𝒆𝒍. The x marker are the poles, the hollow circle marker the

zeros, the solid circle the poles considering the gain

The resulting bode plot is the following:

Figure 65 – Bode plot for the slider gear-motor system

Using Simulink software it is now presented the study on the system transitory. The rotation speed

passes from 0 rad/s to 1.75 rad/s in 2 seconds.

74

Figure 66 – System response to the reference in position (on the top), zoom at second 10 when the tool starts working (on the

bottom)

The speed variation of the system respect the reference is very little. The angular speed decrement,

when weeding starts, is 0.89 rad/s that correspond to 0.4 km/h lost by the cart. In less than half second

the cart speed is recovered. The behaviours of the voltage, current and power are reported in Figure

67.

75

Figure 67 – Voltage (top left), current (top right) and power (on the bottom) of the tool system

The maximum voltage before the start of weeding is 6.3 V; after, 7.4 V. The current before, 0.05 A

and 1.85 A after. The power 0.27 W before and 13.64 W after.

Conclusions The performances of the vehicle motors are now compared.

As it is possible to see from Simulink block diagram in Figure 68, the total current and power

consumption are the sum of the contribution brought by each single block (motor-wheels, tool, and

slider); instead, the voltage is just compared between the one coming from each block.

76

Figure 68 – Simulink block diagram of the whole system: motor-wheels, tool and slider

The voltage coming from batteries and dedicated to each motor must be independent because each

motor requires a different tension. This result is reached by means of the use of an inverter. Hence,

nonetheless the battery is 48V, the voltage can be brought to 170V, for the tool, and to 100V, for the

slider. The inverter is a necessary component, which was already present among the vehicle devices,

because it transforms the direct current from batteries to alternate current necessary to motor-wheels.

The current, considering the system after the transitory and after the weeding start, foresees a mean

consumption of about 10.2 A. in this condition, considering a battery consumption of 27 Ah, the

vehicle autonomy would be 2.64 hours.

Figure 69 – Current consumption of the whole system made up by four motor-wheels, slider motor and tool motor

The power consumption of the whole system is reported in Figure 70. The average power during

weeding is around 450W.

77

Figure 70 – Power consumption of the whole system made up by four motor-wheels, slider motor and tool motor

78

Prototype design The vehicle prototype is represented with a CAD software in order to define the component

dimensions, verify the feasibility of parts’ movement. The software used is Inventor 2014.

Figure 71 – CAD representation of prototype

Figure 72 – CAD representation of prototype, frontal view

79

Figure 73 - CAD representation of prototype, lateral view

The system dimensions are restrained. Laterally the slider motor is not bulging too much allowing

the vehicle to remain nimble. The weeding system develop vertically, living adequate space to the

battery pack, vision system or other equipment.

The whole vehicle presents the maximum longitudinal length equal to 1213 mm, maximum width

equal to 908.5 mm and height 775 mm.

80

Static assessment

Tool The principal design requirement for the tool is a torsional momentum of 11.56 Nm and an axial

stress due to bending moment caused by 297 N force (√2 ∙ 2102). If the tool is realized by steel or

aluminium, and the tool shaft is a hollow shaft, the physical characteristics are the followings:

Table 15 – General characteristics of tool dimensions and resistance

Tool characteristics

External radius mm 24

Internal radius mm 22

Torque Nm 11.56

Extra torque Nm 10

Force N 297

Extra Force N 200

Yield strength: 𝑌𝑠 MPa 300

Torsional stress: 𝜏 MPa 15.84

Axial stress: 𝜎 MPa 177.3

Equivalent stress: 𝜎𝐺𝑇 MPa 180.1

Safety coefficient: 𝜂 1.67

The tines are subjected to the direct resistance of the soil. The tine can be approximated to a

parallelepiped, with following characteristics:

Table 16 – General characteristics of tine dimensions and resistance

Tine characteristics

Width mm 24

Depth mm 22

height mm 95

Force N 100

Extra force N 50

Stress: 𝜎 MPa 177.3

Safety coefficient: 𝜂 1.55

81

Figure 74 – Tool representation

Slider

Translation unit The most stressed component of the slider mechanism is the translation unit. The sled, inside it, must

sustain the weight of the tool and of the gear-motor, withstand to the weeding force and to the inertia

forces.

The translation unit is chosen from ITEM Catalogue in order to easily joint it, through standardized

parts, to cart frame, constituted by ITEM profiles and parts too.

The model chosen is the KLE 6 60x60 (Cod. art.: 0.0.605.07) [48]; it has a toothed belt transmission,

roller bearings and hardened supports for the sled, rolling bearings for pulleys.

Figure 75 – ITEM translation unit: photo to the left, drawing with dimension to the right

Table 17 – General characteristics of translation unit KLE 6 60x60 dimensions and resistance

Translation unity characteristics

82

Series number 6

Total rise: H mm 500

Share of referral: B mm 75

Safety length: s mm 26

Sled length: 𝐿𝑠 mm 190

Total length of the slider: L mm 900

Maximum speed m/s 10

Maximum acceleration m/s2 10

Maximum torque Nm 12

Inertia per meter of length 𝑘𝑔 ∙ 𝑚𝑚2/𝑚 100

Weight kg 5

Maximum load: x direction N 500

Maximum load: y direction N 750

Maximum load: z direction N 500

Maximum momentum: x direction Nm 25

Maximum momentum: y direction Nm 50

Maximum momentum: z direction Nm 100

ITEM profile characteristics

𝐼𝑦 𝑐𝑚4 44.32

𝐼𝑧 𝑐𝑚4 57.46

𝑊𝑦 𝑐𝑚3 13.08

𝑊𝑧 𝑐𝑚3 19.15

Figure 76 – Translation unit representation with relative moment vectors (on the left), guide section to the right

The force acting on the sled, comprehensive of the weeding load and the inertia force, is 246.9 N,

value minor than the allowable threshold of 500 N. The momentum 𝑀𝑥 is around 3 Nm, largely below

the threshold of 25 Nm. All the other mechanical characteristics are largely respected.

83

Section bars The section bars are ITEM components, 40x40 millimetre section and code 0.0.026.03.

The bar that sustains the linear guide is 798 mm long; please refer to CAD representation in next

chapter.

Considering a safety load of 1000 N, the deflection in the centre of the section bar, supposed

constrained at the two ends with fixed joints, is estimated to 0.39 mm; this value is considered

acceptable. All the bars are suitable for the playing forces.

Linear guides In order to support the forces and momentum discharging on the translation unit, two linear guides

are added: one above and one below. The principal internal action that these guides sustain is the

momentum respect x axis.

The linear guides chosen are again from ITEM catalogue. The model chosen is 8 80x40 D14 with

code 0.0.386.11.

Figure 77 – Linear Guide, from ITEM website

Table 18 – General characteristics of linear guides dimensions and resistance

Translation unity characteristics

Sled length: S mm 190

Total rise: H mm 614

Guide total length: L mm 900

FEM results Given the complexity of certain component, a FEM analysis is computed. The software is the FEM

environment inside Inventor 2014 package.

A Detailed analysis is computed on the slider support because it is supposed to be one of the most

stressed component. The maximum stress reported is around 3 MPa in correspondence of the

connection between the vertical support and the horizontal one at the base, Figure 78: the red area is

84

the one in which stresses are greater. In Figure 78 (to the left) the yellow narrows represent the forces

acting on the structure: the weeding force and the torque given from the tool gear-motor.

Figure 78- CAD view of the specific component studied (slider support), to the left; stress analysis to the right

The stress value is very little compared to the material resistance (that it is supposed to be metal).

According to the analysis, the safety factor along all structure is around 15, which is a confirm of the

design correctness and safety (Figure 79). The maximum displacement is in correspondence of the

encoder support and it results being 0.022 mm (Figure 79).

Figure 79 – Analysis results: safety factor to the left, displacement to the right

85

The analysis is also made considering all the translating system that is supposed to be constrained

rigidly to the vehicle frame and it is subjected to the weeding force and to the torque reaction of the

soil.

The analysis of entire structure consider a configuration more critical than the real one in order to be

more precautionary on the machine dimensioning. The structure supposed is deprived of the

translation unit; this is done for two main reasons: the FEM analysis would be faster and can be more

detailed with respect the most stressed components, and because in this way the load is distributed

only along the supports (upper and downer linear guides). The last motivation is intended to provide

a more robust design of the system: the entire load is considered to be supported by linear guides so

to allow to the translation unit to be dedicated exclusively to the system actuation and not to system

supporting. This assumption is reinforced by the constructive complexity of translation unit that cause

consistent plays, surely greater than linear guides’ plays; it results that the linear guides are more rigid

than translation unit and hence support a greater percentage of load.

The simulation is considered to have loads greater than the ones expected for weeding: the force

applied is 500N, the torque 20Nm. The additional load considered has the purpose to include the

eventuality of extra loads, due to soil areas more rigid, contact with stones, with a soil prominence or

with a thicker root.

86

Figure 80 – FEM analysis results. On top Von Mises stresses of the weeding system; in the center stresses distribution with a

reduced scale; on the bottom safety factor distribution

The FEM analysis shows that the maximum stress is focused on the tool stick top; in this particular

area the bending moment produces high axial stresses due to the length of the tool. The order of

magnitude of stresses are in agreement with previews analytical calculations

Another critical point is along the guides of the linear-guide component; the criticism is due to high

stresses caused by the high length of the guide and to the deformation induced to the guide. This last

aspect could create problem in sled roller bearing fluency: if the guide is too bent the sled could jam.

The displacements due to the system of forces applied to the tool is analyse during FEM simulation

too. The results are reported in Figure 81; they don’t show a particular criticism for the good working

of the system. The maximum displacement estimated, referred to a system softer than the real one, is

87

about 2.5mm. The weeding process success is not affected by a so little value of displacement; the

trajectory of each tine would change a bit but not consistently.

Figure 81 – FEM analysis results: displacement of weeding system components

88

Dynamic assessment The vibration analysis of the translation system has the purpose to put in evidence resonance problems

caused by tool and pulleys rotation, or translation of slider.

Analysing the spectrum of frequencies it is possible to compute a complete dynamic assessment. The

analysis is computed on translation system only because studying the entire vehicle, including frame,

wheels, battery back is computationally too heavy.

From simulation, using again the software Inventor 2014, it is possible to find the following modal

frequencies:

Modal frequencies

1𝑠𝑡 Hz 45.56

2𝑛𝑑 Hz 65.88

3𝑟𝑑 Hz 80.03

4𝑡ℎ Hz 92.19

5𝑡ℎ Hz 95.10

6𝑡ℎ Hz 119.18

7𝑡ℎ Hz 167.52

8𝑡ℎ Hz 190.59

The tool rotation speed is of 176 rpm, 2.93 Hz, while the maximum velocity of the slider gear-motor

is 3210 rpm, 53.5 Hz. This last rotation speed is the one that goes closer to the first natural frequency

of the system. It is possible to assume that when the motor reaches this rotation speed, vibration

phenomena can occur causing an enhancement of stresses, wear, tracking imprecision, loosening bolts

and damage of electric components. The first system mode is depicted in Figure 82. A slightly higher

first natural frequency is expected because the translation unit, absent in simulation, would give more

rigidity to the system. For this reason the first modal frequency can approaches the force frequency

induced by slider motor rotation.

Figure 82 – First natural mode of weeding system

However, it is recommended to better investigate the dynamic behaviour of the system through tests

on the final vehicle prototype, including cart frame, battery pack, vision system and other accessories.

89

Batteries comparisons The battery comparison, from [49] Table 19 – Batteries comparison

Cell chemistry Marketed Energy density Specific

power

Cost Self-

discharge

rate

by

mass

by

volume

year MJ/kg

Wh/kg

MJ/L

Wh/L

W/kg Wh/$

$/kWh

%/month

Lead-acid

(SLA, VRLA)

1881

0.11–

0.14

30–40

0.22–0.27

60–75

180

6.99–

17.98

56–143

3–20

Nickel-iron

(NiFe)

1901

0.07–

0.09

19–25

0.45

125

100

4.25–

5.67

176–235

20–30

Nickel-cadmium

(NiCd, NiCad) 1960

0.11

30

0.36

100

150–200 10

Nickel-hydrogen

(𝑵𝒊𝑯𝟐) 1975

0.16–

0.23

45–65

0.22

60

150–200

Nickel-metal

hydride

(NiMH, Ni-MH)

1990

0.36

100

1.44

401

250–1000

3.4

294

30

Low self-

discharge nickel-

metal hydride

2005

0.34

95

1.27

353

250–1000 0.42

90

(LSD NiMH)

Lithium cobalt

oxide

(𝑳𝒊𝑪𝒐𝑶𝟐)

1991

0.70

195

2.0

560

2.83

353

Lithium iron

phosphate

(𝑳𝒊𝑭𝒆𝑷𝑶𝟒)

1996

0.32–

0.47

90–130

1.20

333

200 4.5

Lithium

manganese oxide

(𝑳𝒊𝑴𝒏𝟐𝑶𝟒)

1999

0.54

150

1.5

420

2.83

353

Lithium nickel

cobalt aluminum

oxide

(𝑳𝒊𝑵𝒊𝑪𝒐𝑨𝒍𝑶𝟐)

1999

0.79

220

2.2

600

Lithium nickel

manganese cobalt

oxide

(𝑳𝒊𝑵𝒊𝑴𝒏𝑪𝒐𝑶𝟐)

2008

0.74

205

2.1

580

The costs reported in the table are in USD.

The batteries already mounted are acid lead batteries (code FIAM FGC22703), with the following

characteristics:

Table 20 – Battery pack mounted on the vehicle, composed by 3 acid lead batteries

Battery pack characteristics

Voltage V 36

Capacity Ah 27

Energy Wh 972

Volume dm3 (litres) 10.89375

Weight kg 25.5

91

Energy density Wh/l 89.2254

Energy density Wh/kg 38.11764

Maximum current A 270

Considering that the total volume occupied by the battery pack, comprehensive of three batteries, is

the parameter to maintain constant, it is possible to calculate the autonomy that the vehicle would

reach using different kind of batteries (Table 21), (the voltage is maintained equal to 36V).

The formula used to find the battery weight and life are the following:

𝐵𝑎𝑡𝑡𝑒𝑟𝑦 𝑤𝑒𝑖𝑔ℎ𝑡:𝑒𝑛𝑒𝑟𝑔𝑦 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 [

𝑊ℎ𝐿 ]

𝑒𝑛𝑒𝑟𝑔𝑦 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 [𝑊ℎ𝑘𝑔

]∙ 10.8938 [𝐿]

𝐵𝑎𝑡𝑡𝑒𝑟𝑦 𝑙𝑖𝑓𝑒: ℎ =𝑒𝑛𝑒𝑟𝑔𝑦 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 [

𝑊ℎ𝑘𝑔

]

𝑉 ∙ 𝐴∙ 𝐵𝑎𝑡𝑡𝑒𝑟𝑦 𝑤𝑒𝑖𝑔ℎ𝑡

=𝑒𝑛𝑒𝑟𝑔𝑦 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 [

𝑊ℎ𝑘𝑔

]

36𝑉 ∙ 10.2𝐴∙ 𝐵𝑎𝑡𝑡𝑒𝑟𝑦 𝑤𝑒𝑖𝑔ℎ𝑡

( 23 )

Table 21 – Battery pack mounted performances compared with different batteries types

Cell chemistry Energy density Weight Cost System

current

consumption

Battery

life

by mass by volume

Wh/kg Wh/L kg $/kWh A h

Lead-acid

(mounted)

38.12 89.23 25.5 56–

143 10.2 2.6472

Lithium iron

phosphate

(𝑳𝒊𝑭𝒆𝑷𝒐𝟒)

110 333 32.98 41-333 10.2 9.88

Lithium

manganese oxide

(𝑳𝒊𝑴𝒏𝟐𝑶𝟒)

150 420 30.5 353 10.2 12.4591

Lithium nickel

cobalt aluminum

oxide

220 420 20.8 ~700 10.2 12.4619

92

(𝑳𝒊𝑵𝒊𝑪𝒐𝑨𝒍𝑶𝟐)

Lithium nickel

manganese cobalt

oxide

(𝑳𝒊𝑵𝒊𝑴𝒏𝑪𝒐𝑶𝟐)

205 580 30.82 ~700 10.2 17.2

All lithium batteries provide much more autonomy respect the lead acid one, considering constant

the volume of battery pack.

The LiFePo4 have an excellent lifespan, high level of safety but low specific energy compared to

newer lithium batteries. This battery is commonly used in vehicle traction EV.

The LiNiCoAlO2 and LiNiCoAlO2 are used for many electric vehicle, included TESLA cars, and for

powering tools. This batteries are very powerful but have a dangerous chemistry and high costs. [50].

Depending on the available budget and on vehicle autonomy necessities, the solutions proposed in

Table 21 cover a wide range of choices.

The LiFePo4 could be a good trade-off between life cycle and costs. If the weeder vehicle is required

to work 24 h/day, the battery can be changed 3 times a day (being his autonomy greater than 8h), or

it is possible to have just two batteries and power the vehicle alternating them; during the use of the

first one the other can be recharged in time span.

The cycle durability of lithium cells is greater than the lead-acid ones; between lithium cells, the

lithium iron phosphate has a high durability, at the level of the more expensive lithium battery type;

it reach 1000-2000 cycles. Only lithium nickel manganese cobalt oxide provide a greater durability.

Table 22 – Cycle durability for different battery type [49]

Cell chemistry

Cycle durability

# cycles

Lead-acid

500 typical, 800 max

Lithium cobalt oxide

500–1000

Lithium iron phosphate

1000–2000

Lithium manganese oxide

300–700

Lithium nickel cobalt aluminum oxide

1000-1500

Lithium nickel manganese cobalt oxide

5000

93

Costs analysis The costs analysis includes the expenditures necessary for translating system realization. The cost of

vehicle frame and motor-wheels are already sunk.

The BOM presented in this section gives an order of magnitude about the investment necessary for

the weeding system realization. Some components’ price is already available from website, catalogue

or databases, others are estimated on the base of the price of similar components. The components

constituted by elementary geometries are, instead, supposed to be realizable using university

machines, like lathes or millers. For these last components it is reported just the cost of the steel

necessary for the realization, that it is supposed 480 €/ton [51].

Table 23 – BOM of mechanical weeding system

# price [€] price

estimated [€] code

weight

[kg]

BMU AC brushless motor (BLM 6300 SP-

AS) and reducer (parallel shaft gearhead) 1 370 € 5,2

BMU package driver (BMUD300-C) 1 165 €

Encoder: lika I40, compatto con albero

sporgente 2 unknown 300 €

ITEM Profilato 8 40x40, L=247 mm 2 21,7 € 2603 1,02

ITEM Profilato 8 40x40, L=300 mm 2 21,7 € 2603 1,235

ITEM Profilato 8 40x40, L=499 mm 2 21,7 € 2603 2,05

ITEM squadra 8 40x40 Zn Alluminio 14 3,13 € 41124 0,11

ITEM: componente angolare 2 9,92 € 38800 0,008

ITEM: Set di guide con bussole a sfere 8

80x80 D14 1

on

request 200 € 38611

ITEM: Snodo 8 40x40 4 11,28 € 60112 0,31

ITEM: Unità lineare KLE 6 60x60 1 919 € 60507 4,8

Reducer: LPB+120-1S-3 1 unknown 500 € 7,3

Slider support 2 1,2

SMV 38I_AC brushless motor 1 unknown 900 € 4,6

Support slider motor 1 0.42 € 0,865

Support tool motor 1 6.89 € 14,356

Tool stick 1 0.32 € 0,67

Vertical support 2 2.4 € 5

The total cost of weeding mechanism is about 3900€. The effective cost strongly depends on the

availability of the raw material and manufacturing machines.

94

Conclusions In the present thesis it is designed a mechanical intra-row weeding system, focused on the weed

control of vegetables crop production. The choice of the best weeding system typology, among the

ones available on the market and proposed by researchers’ studies, falls on the intra-row weeding

system that allows to eliminate efficiently the weed plants and increase crop yield. Mechanical

weeders induce progressive desertion of chemical weeding for a healthier, environmental friendly

one, as the presented system is.

One of the most studied system for mechanical intra-row weeding is the rotary hoeing one. It provides

at the same time weed plant uprooting, cutting and burying; the canopy area worked is bigger respect

the one provided by just one single hoe; the weeding efficiency is high compared with other weeding

mechanical system, and the intra-row system further increase productivity.

The power consumption is around 450 W and, with the already installed battery pack, the autonomy

reaches 2.64 h. Using 𝐿𝑖𝐹𝑒𝑃𝑜4 battery the life cycle can rise till 9.88 h.

The prototype is realized with many standardized components from ITEM catalogue that match each

other, and with component easy to be built with the most common manufacturing machines.

The stresses and strains analysis report the conformity of the system designed. These results and the

goodness of the system projected are confirmed by FEM analysis.

The costs amount of the weeding system are supposed to be around 3900€.

95

Future developments The intra-row weeding systems are the future of agricultural weed control, the unmanned vehicle the

future of transportation and the automatic machines the future of work. The machine designed in this

thesis has great potentialities and it has good opportunities to be developed to create a working

prototype salable in to the market.

Future developments necessary for this purpose are:

- Physical realization

- Performances studies (kinematics, power and current consumption, battery life cycle, weed

control yield)

- Choice of control board and implementing of the code for actuation

- Study of the integrated vision system for weed detection

- Soil characteristic analysis in order to enhance the accuracy of the forces prediction model

- Design and study of a prototype with two rotating tools that translate opposite to one another

96

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100

Acknowledgments

Address daily efforts to social purposes respect private ones is the starting point for change the world

we are living. Many people that helped me to realize and improve my work have in mind the same

concept of people cooperation and spontaneously offer me a sincere support.

Among these people, I want to remember my girlfriend Camilla, my family, my friends Lav, Gabu,

Edo, Kevin, Francesco; all the volunteers, children, parents, students of Spazio Mondi Migranti; and

my bands (Stranibanda and RG).