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Submitted by: | Chetan swaroop B.tech. ME

H.C.S.T. Farah, Mathura

STEERING MECHANISM OF AGV’S

RRoobboottss:: The term robot comes from the Czech word ―robota”, generally

translated as "forced labor." This describes the majority of robots fairly

well. Most robots in the world are designed for heavy, repetitive

manufacturing work. They handle tasks that are difficult, dangerous or

boring to human beings.

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Controller: A controller acts like a brain of robot. It performs the action

of storing and sequencing the data in memory, initiation and stopping of the

motions of the manipulator and interacting with the environment.

Effectors: The effectors are the tools, a sort of gripper, which directly

interacts with the job. These are design to handle a wide variety of job

effectively.

Sensors: Sensors are to sense the working environment like obstacles

temperature, torque etc. to provide an automatic control of manipulator for a

wide range of working variables. These are necessary for an intelligent

robot.

Energy sources: It is require to cause the motion of manipulator arm

and its linkages. It may take energy in the form electrical, hydraulic, or

pneumatic devices.

Manipulator geometry: Manipulator is a fancy name of the

mechanical arm. It is an assembly of segments and joints. That can be

conveniently divided into three sections:

a. The arm- consisting of one or more segments or joints.

b. Wrist-consisting one to three segments and joints.

c. Grippers- mean of attaching or grasping.

Positioning, orienting, and how many degrees of

freedom: The arm and wrist of a

manipulator perform two separate functions –

positioning and orienting. Each joint provides

one degree of freedom of motion.

Theoretically the minimum no. of dof to

reach at any location in work envelope. And

orient the gripper in any orientation is six-

three for location and three for orientation. In

other words there must be at least three

bending or extending motions to get position and three twisting and rotating

motion to get orientation. The three twisting motion that gives orientation

are libeled as – pitch , roll and yaw for tilting , twisting and bending left and

right. There is no easy labeling system for the arm itself since there are many

ways to achieve gross positioning using extended segments and pivoted or

twisted joints.

Robotic arm: The most common manufacturing robot is the robotic

arm. A typical robotic arm is made up of seven metal segments, joined by

six joints. The computer controls the robot by rotating individual step motors

connected to each joint (some larger arms use hydraulics or pneumatics).

Unlike ordinary motors, step motors move in exact increments. This allows

the computer to move the arm very precisely, repeating exactly the same

movement over and over again. The robot uses motion sensors to make sure

it moves just the right amount.

An industrial robot with six joints closely resembles a human arm -it

has the equivalent of a shoulder, an elbow and a wrist. Typically, the

shoulder is mounted to a stationary base structure rather than to a movable

body. This type of robot has six degrees of freedom, meaning it can pivot in

six different ways. A human arm, by comparison, has seven degrees of

freedom.

Arm geometries: The three general layouts for three dof are called

Cartesian, cylindrical, and polar. They are named for the shape of the

volume that the manipulator can reach and orient the gripper into any

position- the work envelope. Some of them

use all sliding motion, some use only

pivoting joints some both.

Pivoting joints have a drawback of

preventing the manipulator from reaching

every cubic centimeter of the work

envelope. Because the elbow can’t fold back

completely on itself. This creates dead

space- place where the arm can’t reach. That

is inside the gross work volume. On a robot

it is frequently required for the manipulator

to fold very compactly.

Cartesian or rectangular arm: A Cartesian robot arm uses three

linear motions, to move around a cube shaped working envelope. This

geometry is just like three dimensional XYZ

coordinate system, in fact it is how it is

controlled and how the working end moves

around the work envelope. There are two basic

layouts based on how the arm segments are

supported, gantry and cantilevered.

Mounted on the front of a robot, the first

two dof of a cantilever Cartesian manipulator can

move left/right, up/down. The Y-axis is not

necessarily needed in a mobile robot because the

robot move back/ forward. It has the benefit of

requiring a very simple control algorithm.

Cylindrical arm: The body of this type is such that the robotic arm can

move up and down along a vertical member. The arm can rotate about that

vertical axis and the arm can also extend or contract. This construction

makes the manipulator able to work in a cylindrical space. The dimensions

of the cylindrical space are defined as, radius by the extent of the arm and

z

y

x

height by the movement along the vertical member.

The cylindrical manipulator base body has one

revolute joint at the fixed frame, one cylindrical joint

about the axis of rotation and one prismatic joint in the

arm of the manipulator.

The position of the end is defined by the

extension of the arm, height of the arm and rotation of

the main body axis. These are the three variables to be controlled to position

the end effectors of a cylindrical base robot. In other words this type of

structure forms a cylindrical coordinate system and be controlled the same

way.

Introduction to Autonomous vehicles: Autonomous

vehicles or automatic guided vehicles AGV are robotic vehicle which can

perform desired tasks in unstructured environments without continuous

human guidance. Many kinds of robots have some degree of autonomy.

Different robots can be autonomous in different ways. A high degree of

autonomy is particularly desirable in fields such as space exploration,

cleaning floors, mowing lawns, and waste water treatment.

Some modern factory robots are autonomous within the strict confines of

their direct environment. It may not be that every degree of freedom exists in

their surrounding environment but the factory robot's workplace is

challenging and can often contain chaotic, unpredicted variables. The exact

orientation and position of the next object of work and.

A fully autonomous robot has the ability to

Gain information about the environment.

Work for an extended period without human intervention.

Move either all or part of itself throughout its operating environment

without human assistance.

Avoid situations that are harmful to people, property, or itself unless

those are part of its design specifications.

Omnidirectional vehicle and its need: An omni directional

vehicle is one which can move in any direction from any position without

changing its orientation. Due to the less floor area and narrow path space the

vehicle has to be holomomic, in order to quickly respond when commanded.

Motion control classification of AGV’S: It can be classified as

open loop and the closed loop control system. In open loop system it not

required to measure the output of the system and perform any error

correcting step. While in close loop system there is a requirement of one or

more feedback sensors that measures and respond to the error in output

variables.

Close loop system: In close loop control system there is a feedback

loop that continuously compares the system’s response with input

commands or setting to correct error in motor and/or load speed, load

position or motor torque. They are also called servo systems.

ALTERNATIVES

Ex:

MEASURMENT MEARURMENT

MOTION

CONTROLLER

AMPLIFIER

MOTOR

LOAD

FEEDBACK

SENSOR MEASURMENT

MOTION

CONTROLLER

(velocity)

AMPLIF

IER

MOTOR

LOAD

FEED BACK

SENSOR

(tachometer)

Velocity

command

Open loop control system: In a typical open loop motion control

system it includes a stepper motor with a programmable indexer or pulse

generator and a motor driver, as shown in the figure the system does not

need any feedback sensor because load position and velocity are control by

the predetermined number and the direction of input of input digital pulse

sent to the motor driver from the controller.

An open loop control system is that which have no feedback sensors and

hence load positioning is lower and position errors ( commonly called step

errors) accumulate over time. For these reasons the open loop systems are

most often specified in applications where the load remain constant and load

motion is simple and low position speed is acceptable.

Issues of wheel size and number of wheels in designing a

vehicle

Wheel size: In general larger the wheel, the larger the obstacle the

vehicle can get over. In most suspension and derivetrain system, a wheel

will be able to roll itself over an obstacle that is about one third the diameter

of the wheel. In a well designed four wheel vehicle it can be increased a

little, but the limit in the most suspension is something less than the half the

diameter of the wheel.

Three wheels are the minimum required for the static stability, the

three wheel robots are most common. Mobility and complexity is increased

by further adding of the more wheels. Let’s take a look on the wheel

vehicles in through order. The most basic vehicle will have minimum

MOTION

CONTROLER AMP

LIER

Step

motor

LOAD

number of the wheels. It also possible to make a one wheel vehicle but with

a limited mobility.

Two wheel vehicles: There are two obvious

layouts of two wheels, wheels side by side, wheels aft

and fore. A common bicycle is an example of this, but

for robot it yet difficult to use because of it is not

inherently stable. The side by side wheels is also not

inherently stable but easy to control at low speed.

Dean kamen developed the segway two wheel

balancing vehicle, proving it is proving it is possible,

and is actually fairly mobile. But it can’t get over

bumps much higher than one quarter a wheel height.

Three wheel layout: there are five possible layouts of a three

wheeled vehicle the most common and easiest to implement, but with,

perhaps the least mobility is presented by a kid’s tricycle. But powering only

one of the three wheels results in the lowering of the net traction, which

further lowers the motive force. In order to

improve the mobility the three wheels all

terrain cycle (ATC) was developed. In this the

rear two wheels are powered through

differential, and the front steer.

Increasing the mobility of three wheel vehicle

can also be accomplished by reversing the

layout, putting the two wheels in front. This

layout works fine for the low speed, but the

geometry is difficult to control at higher speed

as the force on the rear wheel tends to make the

vehicle turn more sharply.

Steering with the front wheels on the reverse tricycle removes the

steering problem. But adds the complexity of steering and driving both

wheels. This layout allows the placing the more weight on the passive rear

wheel, significantly reducing the flipping over tendencies and mobility is

moderate. The layout is still dragging around the passive wheel , however,

and mobility is further enhanced if the wheel is powered.

By various combinations of

steering and driving each wheel

different layouts are obtained

The most complicated and the highest mobility three wheel layout is

one where the all three wheels are powered and steered as well. This layout

is extremely versatile, providing motion in any direction without need of

moving the vehicle, this is called holonomic motion and is very useful for

robotic motion.

Four wheeled vehicles: The most basic four wheel vehicle actually

doesn’t use a differential. It has two wheels on each side that are coupled

together and is steered just like a differential steered tricycle. Since the

wheels are inline on each side and do not turn when a corner is commanded,

they slide as the vehicle turns. The sliding action gives the steering method

its name skid steering.

The problem with skid steering non suspended drive is that as the

vehicle goes over bumps, one wheel necessarily come off ground, this

problem doesn’t exist in two or three wheeled vehicles, but is more major to

deal with on vehicle with more than three wheels.

Five wheel layout: This is basically the tricycle layout, but with an

extra pair of wheels in the back to increase traction and ground contact area.

The front wheel is not normally powered and is only for steering. This is a

fairly simple layout relative to its mobility, especially if the side wheel

pairs are driven together through a simple chain or belt drive. Although

the front wheels must be pushed over obstacles, there is ample traction

from all that rubber on the four rear wheels.

Six wheel layout: The most basic six-wheeled vehicle, shown in

Figure 4-21, is the skid steered non-suspended design. This is very much

like the four-wheeled design with improved mobility simply because there is

more traction and less ground pressure because of the third wheel on each

side. The wheels

can be driven with chains, belts, or bevel gearboxes in a simple way,

making for a robust system. An advantage of the third wheel in the skid-

steer layout is that the middle wheel on each side can be mounted slightly

lower than the other two, reducing the weight the front and rear wheel pairs

carry. The lower weight reduces the forces needed to skid them around when

turning,

reducing turning power. The offset center axle can make the vehicle

Wobble a bit. Careful planning of the location of the center of gravity is

required to minimize this problem.

An even trickier layout adds two pairs of four-bar mechanisms

supporting the front and rear wheel pairs. These mechanisms are moved by

linear actuators, which raise and lower the wheels at each corner

independently. This semi-walking mechanism allows the vehicle to negotiate

obstacles that are taller than the wheels, and can aid in traversing other

difficult terrain by actively controlling the weight on each wheel. Skid

steering can be improved by adding a steering mechanism to the front pair of

wheels, and grouping the rear pair more closely together. The main problem

with these simple layouts is that when one wheel is up on a bump, the lack

of suspension lifts the other wheels up, drastically reducing traction and

mobility.

Tracked vehicles: Tracked vehicles uses tracks in place of the wheel to

overcome the limitations of the wheeled vehicles. The basic track formed by

a drive sprocket, idler, and road wheels. Tracks simplify the problem

somewhat and can climb stairs more smoothly than wheeled drivetrains,

allowing higher speeds, but they have difficulty staying aligned with the

stairs. Tracked vehicles uses the differential steering to steer the vehicle.

Advantage of tracked vehicles over the wheel vehicles: There are some drawbacks of a wheeled vehicles such as it can’t get through

the soft terrains, and obstacles of a size that can be jammed between the two

wheels, and tracked vehicle facilitates solution of the same. There are a few

obstacles and terrains which would stop a six wheeled rocker bogie vehicle,

but not stop a similar sized tracked vehicle. They are-

• Very soft terrain: loose sand, deep mud, and soft powder snow

• Obstacles of a size that can get jammed between wheels

• Crevasses

Tracked get this higher mobility at a cost of greater complexity and lower

drive efficiency, so tracks are better for these situations, but not inherently

better overall.

Disadvantages of tracked vehicles:

Most types have many more moving parts than a wheeled layout,

all of which tend to increase rolling friction.

The greater number of moving parts also increase complexity, and

one

of the major problems of track design is preventing the track from

being thrown off the suspension system.

General description of vehicle movement: The figure

shown below demonstrates the forces acting on a vehicle while moving up a

grade. The tractive effort , Ft in the contact area between tires and driven

wheels and the road surface propels the vehicle forward. While the vehicle is

moving , there is resistance that tries to stop it’s movement. The resistance

generally includes rolling resistance, aerodynamic drag, and uphill

resistance. According to Newton’s second law, vehicle acceleration can be

written as 𝑑𝑉

𝑑𝑡= Ft − Ftr

𝛿𝑀

Where V is vehicle’s speed , Ft is total tractive effort of the vehicle,

Ftr, is total resistance , M is the total mass of vehicle, and 𝛿 is the mass

factor, which is an effect of rotating components in the power train.

The above equation indicates that speed and acceleration depends upon the

tractive effort, resistance and the vehicle’s mass.

Vehicle resistance: As shown in figure given below vehicle resistance

opposite it’s movement includes rolling resistance of tires, appearing in

figure a as rolling resistance torque Trf and Trr aerodynamics drag F , and

grading resistance( the term Mvg 𝑠𝑖𝑛𝛼 in figure ).

Rolling resistance: The rolling resistance of tires on hard surfaces is

primarily caused due to hysteresis in the tires materials this is due to the

deflection of the carcasses while the tire is rolling. The hysteresis causes the

unsymmetrical distribution of ground reaction forces. The pressure in the

leading half of contact area is larger than that in trailing half, as shown in

figure this phenomenon result in ground reaction force shifting forward. This

forwardly shifting ground reaction force, with the normal load acting on

wheel centre creates a moment , that opposes the rolling of wheel. On soft

surfaces, the rolling resistance is primarily caused by deformation of ground

as shown in figure b. the ground reaction force almost completely shifts to

the leading half.

FIGURE 2: Tire deflection and rolling resistance on a. hard b. soft road surface

The moment produced by the forward shift of the resultant ground reaction

force is called the rolling resistant moment, as shown in figure 2.a and can

be expressed as

Tr = Pa .

To keep the wheel rolling, a force F acting on the centre of the wheels, is

require to balance this rolling resistance moment. This force is expressed by

F = 𝐓𝐫

𝐫𝐝=

𝐏𝐚

𝐫𝐝 = Pfr’

Where rd is the effective radius of the tire and fr = a/rd is called the rolling

resistance coefficient. In this way the rolling resistance can be replaced

equivalently by a horizontal force acting on wheel centre in the opposite

direction of the moment of wheel. This equivalent force is called rolling

resistance with a magnitude of

Fr = P frr

Where P is the normal load acting on the centre of the rolling wheel.

The rolling resistance coefficient is the function of the tire material, tire

structure, tire temperature, tire inflation pressure, tread geometry, road

roughness, road material and presence and absence of liquid on the road.

The typical values of rolling resistance on various roads are given in table

below

Conditions Rolling

resistance

Car tires on concrete or asphalt 0.0013

Car tires on rolled gravel 0.02

Tar macadam 0.025

Unpvaved road 0.05

Field 0.1 – 0.2

Truck tires on concrete or asphalt 0.006 – 0.01

Wheels on rail 0.001 -0.002

Aerodynamic drag: when a vehicle travels at a particular speed into

the air the air encounters a force resisting its motion. This force is called the

aerodynamic drag. It mainly results from two components- shape drag and

skin drag.

Shape drag: the forward motion of vehicle pushes the air in front of it

however the air can’t instantaneously move out of the way and its pressure is

thus increased, resulting in high air pressure. In addition the air behind the

vehicle can’t instantaneously fill the space left by the forward motion of the

vehicle. This creates a zone of low air pressure. The motion has therefore

created two zones of pressure that oppose the motion of the vehicle by

pushing it forward ( high pressure in front) and pulling it backward( low

pressure in back) as shown in figure. The resultant force on the vehicle is the

shape drag.

Skin friction: air close to the vehicle moves almost at the speed of the

vehicle while air far from the vehicle remains still. In between the air

molecules move at a wide range of speed. The difference in the speed of the

two air molecules produces a friction that results in second component of the

aerodynamic drag.

Aerodynamic drag is a function of the vehicle speed V, vehicle front

area Af shape of the vehicle and air density ρ. Aerodynamic drag is

expressed as

Fw = 1

2 𝜌 A𝑓 Cd (V + Vw)2

Where cD = aerodynamics drag coefficient that characterizes the shape

of the vehicle and Vw is the component of the wind speed on the vehicle’s

moving direction which has a positive sign when air component is opposite

to the vehicle speed and negative when in the direction of the vehicle speed.

Grading resistance: when a vehicle goes up or down a slope, its weight

produces a component, which is always directed toward the downward

direction. This component either opposes the forward motion (grade

climbing) or helps the forward motion (grade descending). In vehicle

performance analysis only uphills operation is considered. This grading

force is usually called the grading resistance.

Fg = Mg sin∝ To simplify the equation the angle ∝ is replaced by the grade value. When

the road angle is small as shown in figure the grade is defined as

i = 𝐻

𝐿= tan ∝ ≈ sin ∝

the tire rolling resistance and grade resistance together are called road

resistance

Frd = Ff + Fg

= Mg (Fr cos∝ + sin ∝)

When ∝ is small Frd = Ff + Fg

= Mg (Fr + i)

Methods of steering: When a vehicle is going straight the wheels or

tracks all point in the same direction and rotate at the same speed, but only if

they are all the same diameter. Turning requires some change in this system.

This can be obtain by many methods some popular methods are as follows

Differential steering: A differential wheeled robot is a mobile

robot whose movement is based on two separately

driven wheels placed on either side of the robot body.

It can thus change its direction by varying the relative

rate of rotation of its wheels and hence does not

require an additional steering motion.

If both the wheels are driven in the same direction

and speed, the robot will go in a straight line.

Otherwise, depending on the speed of rotation and its

direction, the centre of rotation may fall anywhere in

the line joining the two wheels. If both wheels are

turned with equal speed in opposite directions, as it is

clear from the diagram shown, the robot will rotate about the central point of

the axis.

Mechanism involved: A differential

drive, usually but not necessarily employing gears,

capable of transmitting torque and rotation

through three shafts, almost always used in one of

two ways. In one way, it receives one input and

provides two outputs; this is found in most

automobiles. In the other way, it combines two

inputs to create an output that is the sum, difference, or average, of the

inputs.

Input torque is applied to the ring gear(blue),

which turns the entire carrier (blue), providing

torque to both side gears (red and yellow), which in

turns may drive the left and right wheels. If the

resistance at both wheels is equal, the planet gear

(green) does not rotate. And both wheel turns at

same rate. If the left side gear (red) encounters

resistance, the planet gear (green) rotates about the left side gear, in turn

applying extra rotation to the right side gear (yellow).

Multiple drives: another way to drive a differential

steering is simply employing two or more driving sources.

It is very easy in operation but causes complexity in

employing. Also the failure of any of the driving sourse

will cause the complete demolishing of the whole

mechanism. Hence the reliability is halved. Also the gross

weight of the vehicle is increased by a considerable

amount, which is not desired.Some such drive systems are

shown in the figure below

Skid steering: The most basic four-wheeled vehicle actually doesn’t

even use a differential. It has two wheels on each side that are coupled

together and is steered just like differential steered tricycles. Since the

wheels are in line on each side and do not turn when a corner is commanded,

they slide as the vehicle turns. This sliding action gives this steering method

its name—Skid Steer.

Fundamental equation of steering: when an automobile

takes a turn on road, all the wheel should make concentric circle to ensure

that they roll on road smoothly and there is a line contact between the

surface of the path. This is achieved by mounting the two front wheels on

two short excel, pin jointed with the main front axle, known as stub axle.

Two rear wheels as rigidly mounted on the rear axle. The steering if thus

achieved by use of front wheel.

When vehicle is making a

turn towards one side, the front

wheel of that side must swing about

the pin through a greater angle than

the wheel of other side. The ideal

relation between the swings of two

wheels would be if the axle of stub,

when produced, intersects at a point

I on the common axis of the two

rear wheels as shown in the figure.

In that case, all the wheels of the

vehicle will move about a vertical

axis through, minimizing the tendency of the wheel to skid. Point I is called

the instantaneous centre of motion of four wheels.

Let θ and φ = angle turn by stub axles

l = wheel base

w = distance between the pivots of front axles

Then,

cotφ = 𝑃𝑇

𝑇𝑙 and cot θ =

𝑄𝑇

𝑇𝑙

cotφ – cotθ = 𝑃𝑇−𝑄𝑇

𝑇𝑙 =

𝑃𝑄

𝑇𝑙 =

𝑤

𝑙

This is known as fundamental equation of correct gearing.

Ackermann steering gear: Ackerman steering mechanism,

RSAB is a four bar chain as shown in fig.1.50. Links RA and SB which are

equal in length are integral with the stub axles. These links are connected

with each other through track rod AB. When the vehicle is in straight ahead

position, links RA and SB make equal angles α with the center line of the

vehicle. The dotted lines in fig.1.50 indicate the position of the mechanism

when the vehicle is turning left.

Let AB=l, RA=SB=r; BSQARP ˆˆ and in the turned position,

11 ˆ&ˆ BSBARA . IE, the stub axles of inner and outer wheels turn by θ

and φ angles respectively.

Neglecting the obliquity of the track rod in the turned position, the

movements of A and B in the horizontal direction may be taken to be same

(x).

Then,

r

xd sin

and

r

xd sin

Adding, sin2

2sinsin

r

d

[1]

Angle α can be determined using the above equation. The values of θ and φ

to be taken in this equation are those found for correct steering using the

equation L

w cotcot

. [2]

This mechanism gives correct steering in only three positions. One, when θ

= 0 and other two each corresponding to the turn to right or left (at a fixed

turning angle, as determined by equation [1]).

The correct values of φ, [φc] corresponding to different values of θ, for

correct steering can be determined using equation [2]. For the given

dimensions of the mechanism, actual values of φ, [φa] can be obtained for

different values of θ. T he difference between φc and φa will be very small

for small angles of θ, but the difference will be substantial, for larger values

of θ. Such a difference will reduce the life of tyres because of greater wear

on account of slipping.

But for larger values of θ, the automobile must take a sharp turn; hence is

will be moving at a slow speed. At low speeds, wear of the tyres is less.

Therefore, the greater difference between φc and φa larger values of θ ill not

matter.

As this mechanism employs only turning pairs, friction and wear in the

mechanism will be less. Hence its maintenance will be easier and is

commonly employed in automobiles

R S

A B

A'

B'd xx

d

c

P

Q

Fig.1.49

Tracked Vehicle Steering: In order to steer a tracked vehicle, it is

necessary to drive one track faster than the other, causing the vehicle to turn

toward the slower track. This is called "skid steering" or "differential

steering". While the theory is simple.

Drive Systems: Brakes were required in simplest version of the

tracked vehicles, the simplest way to steer a tracked system was simply steer by slowing one track by apply the brakes on one side.

Dual Drive: The simplest way to achieve this is to drive each track with a

separate power source. However, it has its drawbacks. First of all, it requires

two driving source. With all the attendant extra weight, complexity, and

maintenance headaches. In this case, two motors does not result in twice the

reliability, but half the reliability, as if either fails, the vehicle is effectively

immobilized and capable only of spinning in circles.

A second problem is that it becomes difficult to drive in a straight line.

Track speed is a function of power and ground drag ... while it is possible to

coordinate two driving source such that they produce the same amount of

power, it is highly unlikely that each track will experience the same amount

of drag, and as a result the tracks will turn at slightly different speeds

Omni directional vehicle: Omnidirectional is use to describe the

ability of a system to move instantaneously in any direction from any

configuration. Omni directional vehicle have vast advantage over a

conventional vehicle in terms of mobility in congested environment. Path

planning in general is a difficult task, especially when considering vehicle

dynamics and moving obstacles. Omnidirectional vehicles have some

desirable properties:

1. They are very maneuverable, able to navigate tight quarters,and

2. Have few constraints on path planning.

Caster type two wheels ODV: Guiding mobile robots along

desired trajectories is an important problem in mobile robot navigation the

typical differential drive mechanisms used by many mobile robots, the

current position and orientation can be easily estimated. Two wheels Caster

Type (TWCT) is specially serve as ODV. In this mechanism the tricycle is

powered by the two rear wheels and the differential steering is used to steer

the rear wheels. This type of the vehicle provides the holononic steering. So

all the three wheels can be rotate in a circle which centre is at the middle of

the vehicle. This can be easily obtain by simply rotating the two rear wheels

in opposite directions with a same rate.

Four wheel ODV: The most basic four-

wheeled vehicle actually doesn’t even use a

differential. It has two wheels on each side that are

coupled together and is steered just like differential

steered tricycles. Since the wheels are in line on

each side and do not turn when a corner is

commanded, they slide as the vehicle turns. This

sliding action gives this steering method its name—

Skid Steer. For turning the vehicle about its centre

we can use two methods- 1. By driving each wheel

individually and keeping the direction of the wheels along the tangential

direction.

2. or rotating the wheels with the same amount but the direction of the two

same side wheels ( one rear and other front) opposite to the direction of the

two of the different side as shown in figure.

Rear

1

Frt2

Introduction to Mecanum wheel:

Mecanum wheel is based on the principle of a

central wheel with a number of rollers placed at an

angle around the periphery of the wheel. Rollers

are mounted on angles as shown in Fig 1.The

sideview of the wheel is circular. This

configuration transmits a portion of the force in

the rotational direction of the wheel to a force

normal to the direction of the wheel.

The angled peripheral roller translates a portion of the force in the

rotational direction of the wheel to

force normal to the wheel directional.

Depending on each individual wheel

direction and speed, the resulting

combination of all these forces

produces a total force vector in any

desired direction thus allowing the

platform to move freely in direction of

resulting force vector, without

changing the direction of the wheel.

A demonstration of the different directional movement of

the vehicle is shown in the figure given below. Supposing

a situation when the vehicle have to move in a direction

perpendicularly right to the direction of movement of the

vehicle. To provide the vehicle such a moment the wheels

of the vehicle is driven in different manners as shown in

figure. The resultant component of the velocity vector is

in the direction of right side to that the movement of the

vehicle. In this case the magnitude of all the wheels

velocities is same, but in some other cases the magnitude

can also different. For different desired motion of vehicle direction of the

wheel motion is shown in fig. the velocities of the wheels are shown by the

length of the arrow.

Considerations in designing of a typical vehicle layout: the general consideration in the designing of a typical vehicle are

Purpose of the vehicle designing- number of wheels and wheel size,

chassis,

Working ambience- availability of the space, nature of the ground,

suitable materials for tires etc

Power requirements and the desired speed etc.

Resistances offered by the environment- rolling resistance,

aerodynamic drag, grading resistance etc.

Designing of an autonomous vehicle for the load of about 600kg including

the self mass of the vehicle and a velocity of the 1.5 m/s with moderate

acceleration and the concrete surface and the normal working temperature

and indoor purpose.

The specification are given below

Number of wheels = 6

Mass of vehicle = 300 kg

Load on the vehicle = 300 kg

Linear velocity of the vehicle = 1.5m/s

Time taken to achieve the velocity from zero = 15 sec.

Dia of the wheel = 400 mm

= 0.4 m

Designing of a vehicle for the desired purpose:For the indoor uses

the vehicle should be omnidtrectional because of less floor area available for

the turning, so we will design an omnidirectional vehicle. The working

temperature is normal so the tires can be car tires. The standard value of the

rolling resistance for the concrete and the car tires is 0.013, as the velocity of

the vehicle is very low, for the given purpose, the aerodynamic drag can

safely neglected. Also the working surface is horizontal enough to leave the

grading resistance. So we’ll design the motor power only for the rolling

resistance.

Net force available for the acceleration of the vehicle

Fnet = force produced by the motor – total rolling resistance

netF F F rolling -------------(1)

Calculation of rolling resistance

Load on each wheel = 𝑡𝑜𝑡𝑎𝑙 𝑙𝑜𝑎𝑑

𝑛𝑜 .𝑜𝑓 𝑤ℎ𝑒𝑒𝑙𝑠

P = 300+300

6

P = 100 kg

P = 100 ×9.81

P = 981 N

Rolling resistance on each wheel

Frolling = P × f r

Where f r is coefficient of rolling resistance, for car tires and concrete its

value is 0.013

Frolling = 981 × 0.013

= 12.75

Total rolling resistance on each wheel

𝐹rolling = no. of wheels × rolling resistance of each wheel

𝐹rolling = 6 × 12.75

=76.5 N

From the law of motion

Fnet = M . a

From equation (1)

F – Frolling = m. a

= 600× 1.5

15

= 60 N

F = 60 + Frolling

F = 60 + 76.5

=136.5N

Angular velocity of wheel

ω = 𝑣

𝑟

= 1.5

0.2

= 7.5 R/s

Power required by the motor

P = F × v

P = 136.5 × 1.5

= 204.75 W

= 205 (suppose) watts

Taking a factor of safety equal to the 1.5

P = 205×1.5

= 307.5 watts

Hence the power required to drive the motor will be approximately 308

watts.

A possible layout of the vehicle: a possible layout of the vehicle

can be as shown below in which the wheel no. 1 &2 are the driving wheels

and the other are supporting. In the proposed layout the two driving wheels

are mounted in front and aft configuration in order to ensure that the vehicle

should not leave the ground contact at the inclined or when any wheel goes

over an obstacle.

1 2