Gentle Handling of Strawberries Using a Suction Device

b i o s y s t em s e n g i n e e r i n g 1 0 9 ( 2 0 1 1 ) 3 4 8e3 5 6

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Research Paper

Gentle handling of strawberries using a suction device

Shigehiko Hayashi a,*, Kohei Takahashi b, Satoshi Yamamoto a, Sadafumi Saito a,Takashi Komeda b

a Institute of Agricultural Machinery, Bio-oriented Technology Research Advancement Institution, National Agriculture and Food Research

Organization, 1-40-2 Nisshin, Kita, Saitama 331-8537, Japanb Shibaura Institute of Technology, 307 Fukasaku, Minuma, Saitama 337-8570, Japan

a r t i c l e i n f o

Article history:

Received 16 January 2011

Received in revised form

22 April 2011

Accepted 26 April 2011

Published online 15 June 2011

* Corresponding author. Tel./fax: þ81 48 654E-mail address: [email protected] (S. Hay

1537-5110/$ e see front matter ª 2011 IAgrEdoi:10.1016/j.biosystemseng.2011.04.014

This study investigated the feasibility of gentle handling strawberries using a suction device.

This picking-up method, in which the fruit itself is moved towards the suction device by

a suctioning airflow, is proposed to prevent damage to the pericarp. The picking-up equip-

ment comprises a Cartesian coordinatemanipulator, suction device,machine vision system,

belt conveyor, and control unit. The suction device has a tapered tubewith an inner diameter

of 25 mm, and generates a suction airflow of approximately 45 l min�1. The machine vision

systemassesses theorientation of the fruit, and the suction device approaches the fruit along

the lineof fruit orientation.An investigationof theeffective space for suctioning revealed that

the smaller the fruit, the larger the effective space. Its height was about equal to or slightly

greater than half the fruit diameter; however, the permissible distance in the transverse

direction was small. Because the inclination of the suctioned fruit varied considerably, our

proposed picking-up method was not always able to hold the fruit in a constant posture. In

the approach position (80� fromthe vertical), the suction device required a suction forcemore

than double that required in the vertical position. In picking-up performance tests, success

rates for four cultivars weremore than 95%without dropping the fruit at an approach height

of 16mm;however, the ratedecreased to71.9% for the long-tapered ‘Deco rouge’ at aheight of

19 mm. The time required to pick and transfer a fruit was 8.9 s.

ª 2011 IAgrE. Published by Elsevier Ltd. All rights reserved.

1. Introduction the pericarp, the harvested fruits are precooled in storage

The strawberry (Fragaria� ananassa Duch.), which has gained

worldwide popularity, is one of the most difficult crops to

handle because of its soft pericarp. In Japan in 2008, 190,700 t

were produced under a cultivated area of 6471 ha (MAFF, 2007,

178), mostly for fresh consumption. In forcing culture,

strawberry plants are transplanted into greenhouses in

September and harvested from November to April the

following year. Nearly all operations, especially harvesting,

sorting, and packing, are performed manually. To harden

7137.ashi).. Published by Elsevier Lt

before packing. They are sorted using a grading standard

(size and shape) that differs according to producing area.

The strawberries are methodically laid in plastic boxes in

two layers to protect them from vibration during transport

to the market and to provide visual appeal. This two-layer

packing style is the commonest method used in Japan. The

use of single-layer packing methods that employ soft sheets

with hollows is, however, becoming common.

A number of studies on the mechanisation of strawberry

grading have been pursued over the years. A grade

d. All rights reserved.

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Fig. 1 e Photo of strawberry picking-up equipment.

b i o s y s t em s e ng i n e e r i n g 1 0 9 ( 2 0 1 1 ) 3 4 8e3 5 6 349

classification technique based on the geometric features of

a projected image of the fruit was proposed (Nagata, Bato,

Mitarai, Cao, & Kitahara, 2000; Nagata, Kinoshita, Asano, Cao,

& Hiyoshi, 1996). Imou, Kaizu, Morita, and Yokoyama (2006)

developed a 3D shape measurement system using a volume

intersection method. Grading systems for strawberries based

on two or three characteristics have also been designed

(Bato, Nagata, Cao, Hiyoshi, & Kitahara, 2000; Liming &

Yanchao, 2010). Other non-destructive methods based on

firmness have been reported. An estimation technique for

fruit firmness using hyperspectral imaging has been

proposed (Tallada, Nagata, & Kobayashi, 2006), and

relationships between strawberry fruit firmness and the

content of alcohol-insoluble solids and hydrochloric acid-

soluble pectin have been elucidated (Kashiwazaki, Nagasue,

Soutome, Nakajima, & Omori, 2007). These previous studies

were fundamental findings on grading but could not be

incorporated into the mechanical handling of packing.

Simple holders, for example, have been used to transport

fruit (Nagata, Kinoshita, Asano, Cao, & Hiyoshi, 1997). As an

attempt to achieve mechanical packing, Konya and Omori

(2010) proposed a peduncle-grip-type tray and handled the

strawberry fruit by grasping its peduncle. Nevertheless,

a peduncle-grip-type tray of this type has not yet been

accepted by the distribution business. Kitazawa, Ishikawa,

Nakamura, and Shiina (2008) also proposed a new packing

style in which the fruit is held in a vertical position, and

confirmed less damage during transport than seen with the

conventional two-layer packing style. However, this method

requires manual labour. In a few producing areas, a weight-

sorting machine or non-destructive sorter that grades

according to size, weight, sugar content or acid content

(Yamada, Tanaka, & Takada, 2009) is used for practical

reasons. The picked fruits are placed individually on

a conveyor or a transport pan by workers and then sorted

automatically; however, the packing task is done manually.

Gentle handling techniques are clearly crucial to achieve the

mechanisation of strawberry fruit packing.

Pneumatic or electromechanical grippers (e.g. Tedford, 1990)

and suction techniques arewidely utilised for handling delicate

agricultural material. For strawberries, force feedback control

using mechanical grippers seems to be acceptable; however,

the control system risks being complicated and expensive. It

would also be difficult to narrow the space between fruits in

a package due to contact of fingers with neighbouring fruits.

On the other hand, the suction technique is also utilised to

harvest or transport crops, including cherry tomato (Subrata

et al., 1998), tomato (Monta et al., 1998), mushroom (Reed &

Tillett, 1994), and deciduous fruits (Ishii, Toita, Kondo, &

Tahara, 2003). The suction technique has certain features that

allow the measurement of positional error for target objects

and deal with different sizes of agricultural products, but

there is no literature that mentions the application of suction

techniques to the handling of strawberry fruit.

This study examines the gentle handling of strawberry

fruit using suction airflow, with the ultimate aim of mecha-

nisation of packing. The suction device is employed to suction

the calyx side to avoid damage and ease the pick-up motion

from a conveyor, since the calyx side of the fruit is signifi-

cantly firmer than the equatorial portion or apex side (Konya,

Omori, & Hayashi, 2007). Amachine vision system is also used

to detect the calyx side before picking-up. A Cartesian

coordinate manipulator with a rotary actuator is used to

manoeuvre the end-effector (the suction device), since this

type of manipulator shows high-precision positioning, high-

speed motion and easy control, and recently, much easier

and cheaper (Windows-based) control software has

improved the operational capability of the manipulator. The

end-effector can also be positioned at arbitrary angles with

a stepper motor, allowing the suction device to approach the

fruit from any direction.

The objectives of this study were: (1) to develop a suction

device for handling strawberry fruit; (2) to devise a gentle

handling method using the suction device; and (3) to conduct

picking-up tests using several cultivars that differ in shape.

The study endeavoured to obtain an effective space to suction

the fruit by examining the positional data between the fruit

and the suction device, such as depth, height, angle, and

transverse distance.

2. Materials and methods

2.1. Strawberry picking-up setup

The strawberry picking-up equipment is composed of

amanipulator, a suctiondevice, amachine vision system, a belt

conveyor for fruits, and a control unit as shown in Fig. 1. A block

diagram of the system is illustrated in Fig. 2. The Personal

Computer (PC) can control all the components except the

Light Emitting Diode (LED) light. Although three belt conveyors

for trays were incorporated in advance for a further study of

sorting, this study did not use them. Moreover, the aim of this

mechanical handling technique is to combine it with a large-

scale sorting system such as a non-destructive sorter (Yamada

et al., 2009), so the picking-up equipment is designed for fruits

that are manually supplied on a belt conveyor.

The manipulator is a Cartesian coordinate type with

a rotary actuator, designed specifically for high-speed perfor-

mance and ease of control. The actuator ismounted ona linear

slide along the z-axis and rotates around this axis, giving it four



PC

Cartesian coordinate manipulator (x-, y-, and z-axis)

Rotary actuator (roll)

Mot

ion

cont

rol

Boar

d

Suction deviceStepper motor for tilting

suction deviceMotor controller

Solenoid valves(suction and purge)

Vacuum sensor

Relay

Conveyor for fruit

Belt conveyorDIO

boa

rd

Machine vision

IEEE

1394

CCD colour camera

LED light

RS2

32C

Manipulator (4 DOF)

Fig. 2 e Block diagram of strawberry picking-up

equipment.

Fig. 4 e Schematic diagram of internal structure of suction

device to pick up strawberry fruit.


degreesof freedom(DOFs). Fig. 3 showsa schematic diagramof

themanipulator. The ranges and speeds of the three slides are

1300 mm and 240 mm s�1 for the x-axis, 600 mm and

180 mm s�1 for the y-axis, and 300 mm and 150 mm s�1 for

the z-axis. The actuator rotates 360� at up to 150 � s�1, so the

suction device can approach a target fruit on the conveyor

belt from all directions. The manipulator picks up the fruit,

transports it, and places it in a hole in the tray.

An air-suction method to handle strawberry fruits is

proposed in this study because it can compensate for errors

caused by machine vision. As shown in Fig. 4, the developed

Fig. 3 e Schematic diagr

suction device is composed of a tapered tube, an ejector, and

a pressure sensor. The ejector installed at the bottom

generates a suction airflow of approximately 45 l min�1 when

compressed air is supplied at 0.6 MPa. The tip of the tapered

tube has an inner diameter of 25 mm and is covered with

cushioning material that contacts the pericarp of the

strawberry fruit. It can be tilted from �80 to þ80� by a stepper

motor, with a maximum speed of 100 � s�1. The pressure

sensor checks whether the device has successfully suctioned

a fruit. Purge air can be supplied while releasing the fruit.

am of manipulator.




The machine vision system consists of a Charge Coupled

Device (CCD) colour camera (1024 � 768 pixels) and two LED

light sources (3.6 W each). The distance between the camera

and the conveyor is approximately 500 mm, and the illumi-

nance around the conveyor was 130 Lx. The machine vision

software was developed using the Halcon image-processing

tool to assess fruit orientation.

The conveyor has a 100mm-wide belt and can supply fruits

at a speed of 100 mm s�1. Cushioning material is attached to

its surface to prevent bruising. In Fig. 3, the conveyor conveys

the fruit from right to left. When the fruit arrives under the

machine vision system, the conveyor stops on receiving

a digital signal from the computer.

2.2. Machine vision system

2.2.1. Machine vision algorithmDetecting the orientation of the calyx and fruit is an essential

technique for suctioning the strawberry fruit from the calyx

side when the suction device picks it up from the conveyor.

The machine vision extracts three parts of a strawberry fruit:

the whole fruit, the red fraction, and the calyx fraction. Fig. 5

shows a sample picture of image processing. First, the

machine vision captures the colour image of the fruit, and

inter-picture operation is applied to obtain the grey images.

Second, the whole fruit, red fraction, and calyx fraction are

respectively segmented using the following equations.

48R� 45G > Tw (1)

2ðR� G� BÞ > Tr (2)

2ðG� RÞ > Tc (3)

where R, G are B image frames of the colour camera, and Tw, Tr

and Tc are the threshold values for detecting each fraction.

Third, the centroids of the each fraction are calculated, and

the line connecting the centroids of the calyx and red fruit

fractions is defined as the fruit’s orientation. The positional

data (rows and columns on the image frame) are transformed

from camera coordinates to manipulator coordinates using

a previously calibrated homogeneous transform matrix.

2.2.2. Measurement accuracy of fruit orientationThe machine vision algorithm was examined in terms of its

assessment of fruit orientation. A strawberry fruit was placed

manually under the camera to align the fruit vertically in the

camera frame. In this position, the actual fruit orientationwas

regarded as 0�, and themachine vision softwaremeasured the

Fig. 5 e Image-processing process: (a) captured image, (b) enhan

grey image, and (e) resulting image; arrow indicates orientation

fruit orientation. Next, the camera rotated clockwise in steps

of 30� up to 330� and the software measured the fruit orien-

tation at each position. The fruit orientation angle as

measured by the software was compared with the actual

orientation. Forty-seven samples of the cultivar ‘Beni-hoppe’

with a maturity level of more than 70% were used in this test.

Preliminary tests showed the measurement error for fruit

orientation to be 5.3� with a standard deviation of 4.2�. Thisappeared to be caused chiefly by misdetection of the calyx; its

measured position deviated from the actual position when

relatively large calyx leaves were bent or clustered together on

one side, even though the red fruit fraction was almost

perfectly detected. In other words, much of the measurement

error consists of rotational rather than deviational error.

2.3. Lying angle of strawberry fruit and approach angleof suction device

The angle of approach of the suction device appears to be an

important factor for picking-up the strawberry fruit lying on

the conveyor. Thus, the inclination of a line connecting the

fruit apex and the centre of calyx to the horizontal (conveyor

belt) was defined as the lying angle, as shown in Fig. 6.

Measurement of 55 samples of the cultivar ‘Beni-hoppe’

revealed the lying angle to be 21.8� with a standard

deviation of 3.4�.Given that the fruit lies at angle of about 20�, an appro-

priate approach angle was investigated. The suction device

was maintained at the position shown in Fig. 6, and the

success rate of suction was calculated for 20 samples

(average weight: 10.4 g). The approach angle was set to 10�,20� and 30�, and then the height was set at 15 mm and

20 mm. The relationship between approach angle and

success rate is shown in Fig. 7. The success rate was above

70%, except for an approach angle of 30� and a height of

15 mm. Moreover, visual observation revealed that the upper

tip of the suction device sometimes pushed the fruit at an

approach angle of 20 or 30�, raising concerns about the risk

of bruising. We concluded that an approach angle of 10� was

adequate, although the optimal approach angle was not

clear. The following experiments therefore used an

approach angle of 10� to investigate the performance of the

suction device.

2.4. Picking-up operation

Fig. 8 shows a flowchart of the picking-up operation. The fruit

conveyor is started and the fruits are placedmanually upon it,

ced R-G grey image, (c) R-G-B grey image, (d) enhanced G-R

of fruit.



Fig. 6 e Approaching motion of suction device and definition of approach angle, lying angle, height, depth and transverse

distance.

Start

Move fruit conveyor

Stop conveyor when fruit in frame


after the trays are set. At the same time, the camera starts

capturing images. When a fruit comes into an image frame,

the conveyor stops moving and the above-described

machine vision algorithm is executed. The suction device is

managed and positioned at the calyx side in a tilted posture

(approach angle: 10�) as shown in Fig. 6, and is advanced

towards the fruit along the line of the fruit orientation as

assessed by the algorithm. Preliminary tests found that

bruises or abrasions on the pericarp occurred due to surface

friction with the conveyor belt when the tip of the device

pushed the fruit. To reduce these types of damage,

a picking-up method was adopted in which the fruit itself

moves towards the tip of the suction device as a result of

suction airflow: the suction device approaches the fruit

slowly until a negative pressure is generated, with the

maximum approach depth set at 20 mm. The suction device

lifts the fruit, transports it vertically, and deposits it in the

hole of the appropriate tray using purge air.

100

80

90

100

50

60

70

20

30

40

Succ

ess

rate

, %

0

10

20

10 20 3010 20 30Approach angle, °

Fig. 7 e Effect of approach angle on suction success rate:,,

15 mm high of suction device; B, 20 mm high of suction

device.

2.5. Functional tests

2.5.1. Effect of approach direction of suction deviceThe direction of approach of the suction device affects the

success rate. A suction test was therefore conducted to clarify

both the effective space and adequate space for suctioning the

Image processing

Control suction device

Suction and approach fruit

Successful suction ?

Yes

No

Transport into tray(210mm horizontal and 135mm vertical position)

Fruit supplyYes

End

No

Fig. 8 e Flowchart of picking-up operation.




fruit: effectivespace isdefined inspaceandrepresentsasuction

success rate of 80.0% or more, and adequate space is repre-

sented as a suction success rate of 66.7e80.0%. A fruit was

placed in front of the suction device, as shown in Fig. 6, and the

suction air volume was increased gradually with the device in

a fixed position. Subsequently, the suctioning state, i.e.,

whether the fruit was successfully suctioned or not, was

investigated. The depth d between the fruit and the suction

device was set at 0, 5, or 10 mm. The height h of the tip of the

tapered tube from the conveyor surface was set at 16, 19, 22,

or 25 mm. In addition, the transverse distance t was set at 0, 5,

or 10 mm. Sample fruits were divided into five groups based

on the real weight of the fruit w (in g): w < 10; 10 � w < 20;

20 � w < 30; 30 � w < 40; and 40 � w. There were 15 samples

of the strawberry cultivar ‘Beni-hoppe’ in each group.

2.5.2. Fruit posture in suction deviceThe posture of the fruit suctioned by the suction device may

vary in the forward-backward or left-right direction. There-

fore, the inclination angle of the suctioned fruit wasmeasured

to evaluate fruit posture. Photos of the front and side views of

a fruit were taken using a digital camera after the suction

device had picked it up automatically and stopped in the

vertical orientation during transportation. A human observer

measured the inclination angle from the photos. Thirty

samples each of the cultivars ‘Beni-hoppe,’ ‘Deco rouge,’ and

‘Natsuakari’ were used in this test.

2.5.3. Transportation ability of suction deviceTo evaluate the ability of the suction device to transport the

strawberry fruit, the minimum negative pressure for holding

a fruit was measured with the suction device in two configu-

rations: the vertical orientation and 80� to the vertical (posture

while picking-up). The device suctioned a fruit such that the

orientation of the fruit coincided with the direction of the

suction device. The suctioning air volume was then gradually

reduced, and the negative pressure when the fruit detached

from the suction device was recorded. Sixty samples of the

cultivar ‘Beni-hoppe’ were used in this test.

2.6. Picking-up performance test

The picking-up performance test was conducted to evaluate

system performance. Fruits were manually placed on the

conveyor at equal intervals of approximately 200 mm in

random orientations, and the picking-up operation was initi-

ated following the flowchart shown in Fig. 8. The success rates

of picking-up at different approach heights were investigated,

Table 1 e Mean sizes of tested fruits.

Cultivar Fruit number Weighta Lengtha: L (mm)

Natsuakari 47 14.1 (5.0) 34.4 (5.0)

Deco rouge 32 16.5 (2.9) 41.8 (4.8)

Tochiotome 50 13.6 (3.8) 35.2 (4.1)

Beni-hoppe 52 13.0 (3.3) 36.0 (3.8)

a Values in parentheses are standard deviations.

b Based on visual observation.

and the execution time from image capture to release of the

fruit into the tray was measured. In addition, bruises or

abrasions on the pericarp of the tested fruit were visually

inspected immediately after the test and one week later,

after keeping them in cold storage at about 10 �C.In this test, the cultivars ‘Natsuakari,’ ‘Deco rouge,’

‘Tochiotome,’ and ‘Beni-hoppe’ were used, and the approach

height was set at 16 mm. The cultivars ‘Natsuakari’ and ‘Deco

rouge,’ which have shapes that are distinctively different from

the others, were also tested at the approach height of 19 mm.

Table 1 shows the mean sizes of tested fruits for the four

cultivars. Among the four types of fruit shapedlong-tapered,

square, tapered, and rotund (Liming & Yanchao,

2010)d‘Natsuakari’ appeared to be rotund, ‘Deco rouge’ was

long-tapered, and the other two were tapered.

3. Results and discussion

3.1. Effective space for suction picking-up

The success rates with the suction device at different heights,

transverse distances, and depths are shown in Table 2. The

dark grey cells indicate the effective space representing

a suction success rate of 80.0% or more, whereas the light

grey cells indicate an adequate space representing a suction

success rate of 66.7e80.0%.

The optimal height for suctioning a fruit increased roughly

in proportion to the weight of the fruit; the smaller the fruit,

the larger was the effective space. The optimal height was

about equal to or slightly greater than half the fruit diameter.

If the fruit was too small, it could not be suctioned, since the

wide gap between the fruit and the tip of the device prevented

the negative pressure from building up to a sufficiently high

level to produce suctioning airflow. In contrast, when the fruit

was too large, it could not be moved by the airflow.

The results showed that a suction device with an inner

diameter of 25mm could pick up fruits weighing less than 40 g

with a high success rate if they were located in a space that

was amenable for suctioning. For bigger fruits, a suction

device with a bigger inner diameter would bemore suitable. In

this study, the picking-up equipment was designed to deal

with several sizes of fruit to identify an effective space for

suction; however, the results suggested the potential for using

the parallel output lines seen in existing specialised suction

devices. Moreover, good picking-up performance is most

likely at a height of 16e19 mm for ‘Beni-hoppe.’ In addition,

Diametera: B (mm) Ratio: L/B Shape categoryb

32.4 (3.7) 1.06 rotund

31.6 (2.6) 1.32 long-tapered

31.1 (3.4) 1.13 tapered

30.6 (2.7) 1.18 tapered



Table 2 e Effect of height, traverse distance, and depth of suction device on suction success rate: cultivar ‘Beni-hoppe’.

Weight range: w (g) w < 10 10 � w < 20 20 � w < 30 30 � w < 40 40 � w

Weight: Avg. (SD) (g) 7.2 (1.3) 13.7 (2.5) 24.1 (2.2) 35.6 (3.4) 46.6 (3.6)

Diameter: Avg. (SD) (mm) 24.0 (1.2) 29.2 (2.3) 33.9 (1.5) 38.8 (2.2) 40.4 (2.2)

Length: Avg. (SD) (mm) 28.9 (3.6) 38.8 (3.5) 48.3 (3.1) 53.5 (3.9) 61.8 (2.9)

Suction success rate (%) ha (mm) ta (mm) da (mm) da (mm) da (mm) da (mm) da (mm)

0 5 10 0 5 10 0 5 10 0 5 10 0 5 10

16 0 100.0 93.3 20.0 100.0 80.0 13.3 100.0 66.7 0.0 26.7 6.7 0.0 0.0 0.0 0.0

5 100.0 93.3 13.3 100.0 86.7 13.3 60.0 6.7 0.0 20.0 0.0 0.0 0.0 0.0 0.0

10 86.7 60.0 0.0 53.3 33.3 0.0 0.0 6.7 0.0 0.0 0.0 0.0 0.0 0.0 0.0

19 0 86.7 80.0 6.7 100.0 73.3 13.3 100.0 66.7 0.0 73.3 13.3 0.0 6.7 0.0 0.0

5 93.3 73.3 0.0 100.0 66.7 13.3 73.3 6.7 0.0 53.3 0.0 0.0 0.0 0.0 0.0

10 66.7 20.0 0.0 40.0 0.0 0.0 6.7 0.0 0.0 13.3 0.0 0.0 0.0 0.0 0.0

22 0 20.0 26.7 0.0 93.3 53.3 6.7 93.3 13.3 0.0 93.3 6.7 0.0 6.7 0.0 0.0

5 26.7 6.7 0.0 80.0 46.7 6.7 46.7 6.7 0.0 53.3 6.7 0.0 6.7 0.0 0.0

10 13.3 0.0 0.0 13.3 6.7 0.0 6.7 0.0 0.0 6.7 0.0 0.0 0.0 0.0 0.0

25 0 0.0 0.0 0.0 26.7 6.7 6.7 20.0 6.7 0.0 73.3 0.0 0.0 13.3 0.0 0.0

5 0.0 0.0 0.0 13.3 13.3 0.0 6.7 6.7 0.0 33.3 0.0 0.0 13.3 0.0 0.0

10 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

a h is the height of the suction device; t, the transverse distance; and d, the depth distance between fruit and suction device illustrated in Fig. 6.


a transverse distance of 5 mm would be permissible for small

fruits weighing less than 20 g.

3.2. Fruit posture by suction picking-up

Fig. 9 shows the inclinations of the suctioned fruit viewed from

the front and side. The inclinations for ‘Beni-hoppe’ and ‘Deco

rouge’ varied widely; the maximum angle of inclination was

approximately 33� for both views. In contrast, ‘Natsuakari’

was suctioned at smaller inclinations. For ‘Beni-hoppe’ and

‘Deco rouge’, errors in approach direction caused larger

inclinations than in ‘Natsuakari’ because of the former’s

elongated shapes. Moreover, these results show that

40

20

0

-20

Incl

inat

ion

from

sid

e vi

ew, °

-40-40 -20 0 20 40

Inclination from front view, °

Fig. 9 e Inclinations of suctioned fruit from front and side

views: n [ 30; B, Natsuakari; -, Deco rouge; , Beni-

hoppe.

a picking-up method using suctioning airflow is not always

able to maintain a stable fruit posture.

3.3. Ease of transportation

Of 60 samples, 12 could not be suctioned if the device was

oriented at 80� from the vertical; the weight range of unsuc-

cessfully suctioned fruits was 36e52 g. These failures were

caused by the fruit turning in this configuration or due to

roughness on the calyx side, both of which led to insufficient

air pressure.

For the 48 samples suctioned successfully, the relationship

between the weight of the fruit and the minimum negative

pressure is shown in Fig. 10. The required negative pressure

increased with weight: the heavier the fruit, the higher the

negative pressure needed. The slope with the suction device

oriented at 80� from the vertical was more than twice the

slope at the vertical, indicating that in this device, the

suction force in the approach position should be more than

double that in the vertical position. This implies that the

device should be able to transport a fruit in the vertical

orientation if it is successful in the approach position.

3.4. Picking-up performance

3.4.1. Execution timeThe mean execution time required for a picking-up operation

was 8.9 s; this included 2.5 s for approach, 0.9 s for suction,

3.9 s for transportation, and 1.6 s for placement and return to

the initial position. This study reveals a mechanisation

process for picking-up, transferring and release of the straw-

berries by suction airflow, and our results suggest a greater

potential for mechanical packing if control of fruit posture is

added after suctioning.

Manual packing of two-layer boxes is estimated to take

8.3 s per fruit in Japan (Konya&Omori, 2010). Our result of 8.9 s

is a little slower than for manual packing, and the execution

time does not include alignment of the fruit in the box. A

faster speed would be required for practical application of



100.0 a a

90.0

aa a

80.0

b70.0 Su

cces

s ra

te, %

50.0

60.0

Natsuakari Deco rouge Tochiotome Beni-hoppe

Fig. 11 e Success rates for each cultivar in picking-up test:

-, 16-mm fixed height of suction device; , 19-mm fixed

height of suction device; letters at the right top of bars

indicate that success rates with the same letters are not

significantly different at the 5% level according to the c2

test.

5.0

4.0

4.5

3.0

3.5

2.0

2.5

1.0

1.5 Neg

ativ

e Pr

essu

re, k

Pa

0.0

0.5

0 10 20 30 40 50 60Weight, g

Fig. 10 e Relationship between fruit weight and minimum

negative pressure inside suction device: cultivar ‘Beni-

hoppe’; n [ 48; B, suction device of 80� to vertical position,

y [ 0.0887x, R2 [ 0.8689; , suction device of vertical

position, y [ 0.0406x, R2 [ 0.8403.


this technique. Considering that the most time-consuming

process was the transportation, there is likely to be room for

improvement by speeding up the motions of the x-, y-, and

z-axis sliders of the manipulator, and by searching for an

optimal path for transportation.

3.4.2. Success rate of picking-upIn the picking-up performance test, fruits were not dropped

during transportation even when the suction device tilted

from the approach posture (80� from the vertical) to the

vertical. The success rate for each cultivar is shown in Fig. 11.

It was found that a success rate of more than 95% could be

obtained in all cultivars if the suction device approached at

the height of 16 mm. The high success rate noted for ‘Beni-

hoppe’ (100%) supports the results of the functional tests on

effective space, and demonstrates that the suction device

can be positioned within the effective space.

Considering the effect of the approach height, the success

rate decreased to 71.9% for ‘Deco rouge’: a significant differ-

ence was obtained when the approach height of the suction

device was 19 mm. This is presumably because the suction

device approached from a position higher than the height of

effective space: as ‘Deco rouge’ has a long-tapered shape,

a lower approach height would be more suitable. On the other

hand, ‘Natsuakari’ showed a continuously high success rate.

Since ‘Natsuakari’ has a rotund shape and its diameter is

bigger, it appears that the height of effective space located is

higher in this cultivar than for ‘Deco rouge.’

It was observed that the cause of picking-up failure was

roughness of the calyx side rather than fruit orientation error.

The suction device was able to compensate for fruit orienta-

tion errors caused by the machine vision algorithm, and

successfully pick upmost of the fruit.When the calyx sidewas

dented or not uniformly curved, sufficient negative pressure

could not be obtained, resulting in picking-up failure. It was

also confirmed that the fruit itself moved or sometimes jum-

ped towards the tip of the suction device due to the suction

airflow. Thismovement appears to suppress the occurrence of

bruises or abrasions, and consequently no damage to the

pericarp was observed immediately after the picking-up test.

Moreover, no bruises were observed on the pericarp of the

calyx side one week after keeping in cold storage. A compar-

ison with non-suctioned fruits kept under the same condi-

tions showed there to be no difference in degree of damage,

although the colouring had progressed.

Our study clarified the importance of control of the suction

device with a height of effective space of close to or slightly

greater than half the fruit diameter. However, since there are

individual differences in fruit diameter, an additional sensor

to detect the fruit diameter or the fruit height, which has

a certain relationship with the fruit diameter, would improve

air picking-up performance by dealingwith differences in fruit

shape such as rotund, tapered and long-tapered types.

The next direction of this study will be mechanical single-

layer packing onto a soft sheet with hollows, in which fruit

alignment in the package will be an important technique.

Further studies should focus on detecting the inclination of

the strawberry fruit during transportation and on techniques

for correcting the fruit posture.

4. Conclusion

This study demonstrated the feasibility of gentle handling of

strawberry fruit by a suction device. The developed picking-up

equipment comprised a 4 DOF manipulator, a suction device,

a machine vision system, a belt conveyor, and a control unit.




The suction device had a tapered tube with an inner diameter

of 25 mm, and generated a suction airflow of approximately

45 l min�1. The fruit placed on the conveyor wasmoved under

the camera, and the machine vision system assessed the

fruit’s orientation. The suction device was advanced towards

the fruit along the line of the fruit’s orientation. A picking-up

method is proposed in this study in which the fruit itself is

moved towards the tip of the suction device by suctioning

airflow to prevent bruises or abrasions on the pericarp from

the device pushing on the fruit.

The effective space for suctioning was analysed. For the

cultivar ‘Beni-hoppe,’ the smaller the fruit, the larger was the

effective space. Itwas found that the optimalheightwasaround

or slightly greater than half the fruit diameter; however, the

permissible distance in the transverse directionwas small. The

inclinationsof thesuctionedfruit viewedfromthefrontandside

varied considerably; one reason seemed to be measurement

error in fruit orientation by the machine vision. This indicated

that the proposed picking-up method had some difficulty in

holding the fruit in a constant posture. In the approach position

(80� from the vertical), the suction device required a suction

force of more than twice that required in the vertical position.

In picking-up performance tests for ‘Natsuakari,’ ‘Deco

rouge,’ ‘Tochiotome,’ and ‘Beni-hoppe,’ when the approach

height of the suction devicewas fixed at 16mm, our picking-up

equipmentachievedapicking-upsuccessrateofmorethan95%

without dropping the fruit during transportation. The success

rate decreased to 71.9% for the long-tapered ‘Deco rouge,’ when

the approach height of the suction device was 19 mm. No

damage was observed to the tested fruits. The execution time

for picking-up a fruit and placing it into a traywas 8.9 s, and the

most time-consuming process was the transportation.

Acknowledgements

The authors gratefully thankMr. TakanobuYano for providing

test samples of ‘Natsuakari’ and ‘Deco rouge,’ and for fruitful

advice on the characteristics of Japanese cultivars.

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Gentle Handling of Strawberries Using a Suction Device

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