vinjamuri sai sudhir [ id no] 2008b3a4475h

8/7/2019 VINJAMURI SAI SUDHIR [ ID NO] 2008B3A4475H

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A REPORT

ON

MOTION AND POWER ANALYSIS OF ROBOTS

Submitted in fulfillment of theStudy Projects BITS C323

BY

VINJAMURI SAI SUDHIR (2008B3A4475H)

UNDER THE SUPERVISION OF

Dr.Y.V.D.RAOAssociate Professor, Mechanical Department

BIRLA INSTITUTE OF TECHNOLOGY & SCIENCE, PILANI

HYDERABAD CAMPUS

October, 2010


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CERTIFICATE

This is to certify that the report entitled -Motion and Power analysis of robots and submitted by

Vinjamuri Sai Sudhir ID No 2008B3A4475H in partial fulfillment of the requirements of BITS

C323 Study Projects embodies the work done by him/her under my supervision.

Signature of student Signature of faculty

Date: Date:


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ACKNOWLEDGEMENTS

I would like to express my deepest gratitude to all those who helped me with their continuous

guidance and constant encouragement at all the stages of the work.

I would like to thank DrY.V.D.Rao [Mechanical department] for designing our course in the

best possible way by keeping our interests in mind. We truly appreciate all his efforts to make us

feel comfortable by his willingness to help in any possible way.

No work can be accomplished alone. It needs the shades of various people to give it a final stage.

I would like to take this opportunity to extend my warm thoughts to those people who helped me

to make this course a wonderful experience.

I would also like to thank my friends Mr.Chinmay gore and Mr.Santosh Kanbargi for helping me

out at all the stages.


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ABSTRACT

This is a brief mid-sem report for the Study oriented project [SOP] of the course BITSC323.

Here, we discuss about the issues related with motion and power in designing a robot.

This report includes the issues related to motion such as the degrees of freedom, motion planning

etc.It is also discusses about the different motors that can be used to power up the motor for the

movement of the robot and its components. Here ,in some cases, we specifically discuss about

the design of a robotic arm and discussions related to it.


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TABLE OF CONTENTS

ACKNOWLEDGEMENTS.[4]

ABSTRACT.[5]

INTRODUCTION....[7]

MOTION PLANNING AND SPACES....[9]

ACTUATORS ..[12]

MOTORS..[13]

SENSORS.[19]

INDUSTRIAL ROBOT[25]

ROBOTIC JOINTS AND TYPES[26]

ROBOT CLASSIFICATION[31]

CARTESIAN CONFIGURATION...[33]

CYLINDRICAL CONFIGURATION..[36]


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Definition

Motion planning (a.k.a., the "navigation problem", the "piano mover's problem") is a

term used in robotics for the process of detailing a task into discrete motions.

Example

For example, consider navigating a mobile robot inside a building to a distant waypoint. It

should execute this task while avoiding walls and not falling down stairs. A motion planning

algorithm would take a description of these tasks as input, and produce the speed and turning

commands sent to the robot's wheels.

Concepts

A basic motion planning problem is to produce a continuous motion that connects a start

configuration S and a goal configuration G, while avoiding collision with known obstacles. The

robot and obstacle geometry is described in a 2D or 3D workspace, while the motion is

represented as a path in (possibly higher-dimensional) configuration space.

Figure 1 example for motion planning


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Configuration space A configuration describes the pose of the robot, and the configuration

space C is the set of all possible configurations.

Free space The set of configurations that avoids collision with obstacles is called the free spaceCfree. The complement of Cfree in C is called the obstacle or forbidden region.

Figure 2 configuration space of a point size robot white = c free , grey = obstacle

ACTUATORS

Common robotic actuators utilize combinations of different electro-

mechanical devices

Synchronous motor

Stepper motor

AC servo motor

Brushless DC servo motor Brushed DC servo motor


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Direction Changing which direction a DC-motor turns is very simple: simply reverse the

polarity. Both pairs of switches ( (S1A,S1B) and (S2A, S2B) ) will always switch together. This

ircuit is called an H-bridge. In a real design the switches can be several different components.

Figure 4 H Bridge

Speed Speed is a little bit more complicated .This can be done by trying to slow down a motor

by reducing its voltage with a variable resistor or Other ways . This will lead to reduction in

motor speed, motor strength while also consuming a lot of electricity as large amounts of heat

are generated by the resistor.

SERVO MOTORS

Servo mechanism

A servo mechanism , orservo is an automatic device that uses error-sensing feedback to

correct the performance of a mechanism. The term correctly applies only to systems where the

feedback or error-correction signals help control mechanical position or other parameters.

Servo motorServo motors are perfect control motors, they are totally self contained. They

contain a motor, Gear box and driver electronics, meaning they can be controlled directly from a

microcontroller, without the need for interface electronics.


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Figure 5 servo motor

STEPPER MOTOR

A stepper motor (orstep motor) is a brushless, synchronous electric motor that can divide a

full rotation into a large number of steps. The motor's position can be controlled precisely

without any feedback mechanism as long as the motor is carefully sized to the application.

Stepper motors are similar to switched reluctance motors (which are very large stepping

motors with a reduced pole count, and generally are closed-loop commutated.)


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Figure 6 Stepper motor

Working

Stepper motors work in a similar way to dc motors, but where dc motors have 1

electromagnetic coil to produce movement, stepper motors contain many. Stepper motors are

controlled by turning each coil on and off in a sequence. Every time a new coil is energized, the

motor rotates a few degrees, called the step angle. Repeating the sequence causes the motor to

move a few more degrees and so on, resulting in a constant rotation of the motor shaft.

y The diagram below shows how a stepper motor works. The magnet in the middle of the

arrangement is connected to the motor shaft and produces the rotation. The 4 magnets

around the outside represent each coil of the stepper motor. As different coils are

energized the central magnet is pulled in different directions. By applying the correct

sequence of pulses to the coils the motor can be made to rotate.


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operation with finer position resolution then stepper motors, but both require one or before

feedback sensors in closed loops, adding to system cost and complexity. Brush-type permanent-

magnet (PM) DC servomotors have wound Armatures or rotors that rotate within the magnetic

field produced by a PM stator. As the rotor turns, current is applied sequentially to the

appropriate Armature windings by a mechanical commutator consisting of two

Or more brushes sliding on a ring of insulated copper segments. These Motors are quite mature,

and modern versions can provide very high performance For very low cost.

There are variations of the brush-type DC servomotor with its iron core rotor that permit more

rapid acceleration and deceleration because of their low-inertia, lightweight cup- or disk-type

armatures. The disk-type armature of the pancake-frame motor, for example, has its mass

concentrated close to the motors faceplate permitting a short, flat cylindrical housing. Thisconfiguration makes the motor suitable for faceplate mounting in restricted space, a feature

particularly useful in industrial robots or other applications where space does not permit the

installation of brackets for mounting a motor with a longer length dimension. The brush-type

DC motor with a cup-type armature also offers lower weight and inertia than conventional DC

servomotors. However, the tradeoff in the use of these motors is the restriction on their duty

cycles because the epoxy-encapsulated armatures are unable to dissipate heat buildup as easily as

iron-core armatures and are therefore subject to damage or destruction if overheated.

However, any servomotor with brush commutation can be unsuitable for some applications due

to the electromagnetic interference (EMI) caused by brush arcing or the possibility that the

arcing can ignite nearby flammable fluids, airborne dust, or vapor, posing a fire or explosion

hazard. The EMI generated can adversely affect nearby electronic circuitry. In addition, motor

brushes wear down and leave a gritty residue that can contaminate nearby sensitive instruments

or precisely ground surfaces. Thus brush-type motors must be cleaned constantly to prevent the

spread of the residue from the motor. Also, brushes must be replaced periodically, causing

unproductive downtime.


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applications are also limited because of their inability to dissipate heat as readily as rotary motors

with metal frames and cooling fins, and the exposed magnetic fields of some models can attract

loose ferrous objects, creating a safety hazard.

stepping PM Brush PM Brushless

Cost Low Medium High

Smoothness Low to Good to excellent Good to excellent

Speed range 0-1500 rpm(typical) 0-6000 rpm 0-10000 rpm

Torque High-(falls of with

speed)

Medium High

Required feedback None Position velocity Commutation and

position velocity

Maintenance None Yes None

Cleanliness Excellent Brush dust Excellent

Factors

Factors that must be considered:-

Positioning accuracy

Reliability

Speed of operation

Cost

Other factors


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SENSORS

A sensor is an electronic device that transfers a physical phenomenon

(temperature, pressure, humidity, etc.) into an electrical signal.Sensors in Robotics are used for both internal feedback control and external interaction with the

outside environment.

Position feedback is the most common requirement in closed-loop motion control systems, and

the most popular sensor for providing this information is the rotary optical encoder. The axial

shafts of these encoders are mechanically coupled to the drive shafts of the motor. They

generate either sine waves or pulses that can be counted by the motion controller to determine

the motor or load position and direction of travel at any time to permit precise positioning.

Analog encoders produce sine waves that must be conditioned by external circuitry for counting,

but digital encoders include circuitry for translating sine waves into pulses. Absolute rotary

optical encoders produce binary words for the motion controller that provide precise position

information. If they are stopped accidentally due to power failure, these encoders preserve the

binary word because the last position of the encoder code wheel acts as a memory.

Linear optical encoders, by contrast, produce pulses that are proportional to the actual lineardistance of load movement. They work on the same principles as the rotary encoders, but the

graduations are engraved on a stationary glass or metal scale while the read head moves along

the scale.

Tachometers are generators that provide analog signals that are directly proportional to motor

shaft speed. They are mechanically coupled to the motor shaft and can be located within the

motor frame. After tachometer output is converted to a digital format by the motion controller,

a feedback signal is generated for the driver to keep motor speed within preset limits.

Other common feedback sensors include resolvers, linear variable differential transformers

(LVDTs), Inductosyns, and potentiometers. Less common are the more accurate laser

interferometers. Feedback sensor selection is based on an evaluation of the sensors accuracy,


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repeatability , ruggedness, temperature limits, size, weight, mounting requirement s, and cost,

with the relative importance of each determined by the application.

Desirable features of sensors

Accuracy

Precision

Operating range

Speed of response

Calibration

Reliability Cost Ease of operation

Sensors 1

Human senses: sight, sound, touch, taste, and smell provide us vital information to function

and survive

Robot sensors: measure robot configuration/condition and its environment and send such

information to robot controller as electronic signals (e.g., arm position, presence of toxic gas)

Robots often need information that is beyond 5 human senses (e.g., ability to: see in the dark,detect tiny amounts of invisible radiation, measure movement that is too small or fast for the

human eye to see)

Figure 8 Flexi force sensor


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\

Sensors 2

Vision Sensor: e.g., to pick bins, perform inspection, etc

Part-Picking: Robot can handle work pieces that are randomly piled by using 3-D vision sensor.

Since alignment operation, a special parts feeder, and an alignment pallete are not required, an

automatic system can be constructed at low cost.

Figure 9 3-D vision sensor

Figure 10 insight vision

Sensors 3Force Sensor: e.g., parts fitting and insertion, force feedback in robotic surgery.

Figure 11 Force sensors


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Parts fitting and insertion: Robots can do precise fitting and insertion of machine parts by using force

sensor. A robot can insert parts that have the phases after matching their phases in addition to simply inserting

them. It can automate high-skill jobs.

Figure 12 precise fitting sensor

Sensors 4

6 ultrasonic sonar transducers to explore wide, open areas

Obstacle detection over a wide range from 15cm to 3m

16 built-in infrared proximity sensors (range 5-20cm) Infrared sensors act as a virtual bumper and allow for negotiating tight spaces

Figure 13 kuala robot


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Link: A rigid piece of material connecting joints in a robot.

Joint: The device which allows relative motion between two adjoining links in a robot.

y A robot joint is a mechanism that permits relative movement between parts of a robot

arm. The joints of a robot are designed to enable the robot to move its end-effectors

along a path from one position to another as desired.

Robotic movements

The basic movements required for a desired motion of most industrial robots are:

y Rotational movement: This enables the robot to place its arm in any direction on a

horizontal plane.

y Radial movement: this enables the robot to move its end-effectors radially to reachdistant points.

y Vertical movement: This enables the robot to take its end-effector to different

heights.

THE ROBOTIC JOINTS

Types of joints1) Linear Joint (L)

2) Rotational joint (R)

3) Twisting Joint (T)

4) Revolving Joint (V)

Linear joints: are also known as sliding as well asPrismatic joints (L) . They are called

prismatic because the cross section of the joint is considered as a generalized

prism. They permit links to move in a linear relationship.

Revolute joints:

permit only angular motion between links. Their variations include:

Rotational joint (R)


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Twisting joint (T)

Revolving joint (V)

Rotational joint(R): is identified by its motion, rotation about an axis perpendicular to the

adjoining links.

Twisting joint (T): is also a rotational joint, where the rotation takes place about an axis

that is parallel to both adjoining links.

Revolving joint (V):

is another rotational joint, where the rotation takes place perpendicular to

one another at this kind of joint. The rotation involves revolution of one link about another.


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WRIST MOVEMENT

The Wrist movement is designed to enable the robot to orient the end effector properly with

respect to the task to be performed.Eg. Human hand

To solve the orientation problem, the wrist is normally provided with upto 3 DOF.

1) Wrist Roll- which involves rotation of the wrist mechanism about the arm axis. Also

called as wrist swivel.

2) Wrist Pitch- If the wrist roll is in its center position , the pitch would involve the up and

down rotation of the wrist. Also called as wrist bend.

3) Wrist Yaw- If the wrist roll is in its center position of its range, wrist yaw would involve

the right or left rotation of the wrist.


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Figure 18 Repeatability

Joint notation scheme

Physical configuration of the robot manipulator can be described by means of a joint notation

scheme .

Considering the arm and body joints first, the letters can be used to designate the particular robot

configuration starting with the joint closest to the base and proceeding to the joint configuration

that connects to the wrist.

Robot configuration Symbol

Polar configuration TRL

Cylindrical configuration TLL,LTL,LVL

Cartesian configuration LLL

Joint arm configuration TRR, VVR


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ROBOT CLASSIFICATION

1) Depending on Configuration

2) Depending on type of control system

3) Fixed or variable sequence robot

4) Depending upon generation

5) Servo / non servo robots

6) Point to Point or continuous controlling robots

Classification based on Physical configuration:1. Cartesian configuration

2. Cylindrical configuration

3. Polar configuration

4. Joint-arm configuration

5. SCARA

CARTESIAN CONFIGURATION:

Robots with Cartesian configurations consists of links connected by linear joints (L). Gantry

robots are Cartesian robots (LLL).

Figure 19 Cartesian configuration


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Cartesian robots

A robot with 3 prismatic joints the axes consistent with a Cartesian coordinate system

Commonly used for:

pick and place work

assembly operations

handling machine tools

arc welding

Figure 20 Cartesian robots

Advantages

ability to do straight line insertions into furnaces.

easy computation and programming.

most rigid structure for given length.


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linear axes is hard to seal

wont reach around obstacles

exposed drives are difficult to cover from dust and liquids

POLAR CONFIGURATION

Polar robots have a work space ofspherical shape. Generally, the arm is connected to

the base with a twisting (T) joint and rotatory (R) and linear (L) joints follow.

Figure 22 Polar configuration

The designation of the arm for this configuration can be TRL or TRR.

Robots with the designation TRL are also called spherical robots. Those with the

designation TRR are also called articulated robots. An articulated robot more closely

resembles the human arm.


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The CP robot is capable of performing movements along the controlled path. With CP

from one control, the robot can stop at any specified point along the controlled path.

All the points along the path must be stored explicitly in the robot's control memory.

Applications Straight-line motion is the simplest example for this type of robot. Some

continuous-path controlled robots also have the capability to follow a smooth curve path

that has been defined by the programmer. In such cases the programmer manually moves

the robot arm through the desired path and the controller unit stores a large number of

individual point locations along the path in memory (teach-in).

CONTINUOS- PATH CONTROL ROBOT [CP]

Typical applications include:

spray painting

finishing

gluing

arc welding operations

CONTROLLED-PATH ROBOT

In controlled-path robots, the control equipment can generate paths of different geometry

such as straight lines, circles, and interpolated curves with a high degree of accuracy.

Good accuracy can be obtained at any point along the specified path.

Only the start and finish points and the path definition function must be stored in the

robot's control memory. It is important to mention that all controlled-path robots have a

servo capability to correct their path.

NEW ROBOTIC ARM DESIGN BASED ON ELEPHANTS TRUNK

The Bionic Handling Assistant (BHA) from Festo of Esslingen, Germany is a working

concept inspired by the anatomy of an elephant's trunk. Festo's goal of developing the


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BHA was to study and improve the interaction between humans and robots. Perhaps the

impression of elephants as being gentle giants was a big factor in the early stages of

development.

Figure 24 Arm design based on elephant's trunk

ROBOT SPECIFICATIONS

Characteristics Units

No of Axes Numbers(eg 1,2)

Max speed/cycle time mm/sec

Load carrying capacity (pay

load)

Kg

Reach & stroke mm


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Total orientation Degrees

Repeatability --

Precision & Accuracy mm

CALCULATION OF VELOCITY

Every time the wheel rotates an entire revolution, the robot travels the distance equal to the

circumference of the wheel. So multiply the circumference by the number of rotations per

minute, and we then get the distance the robot travels in a minute.

Velocity = circumference * rpm

Velocity = diameter * pi * rpm OR Velocity = 2 * radius * pi * rpm

MOTOR TORQUE AND FORCE

High force is required to push other robots around, or to go up hills and rough terrain, or have

high acceleration. As can be calculated with statics, just by knowing the wheel diameter and

motor torque, we can determine the force.


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Torque = Distance * Force

Distance = Wheel Radius

Force = Torque / Wheel Radius

CALCULATING ADDITIONAL ACCELARATION FOR INCLINEDSURFACE

acceleration for inclines = 32 ft/s^2 * sin((angle of inclination * pi) / 180)


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REVOLUTE JOINT

Revolute joints constitute purely rotational motion along the joint axis. Revolute joints are the

most commonly found joint in industrial and research robots.

SCREW JOINT

Screw joints are really just a combination of the first two types of joints. They constitute a

simultaneous rotation and linear motion along a joint axis. Some mathematicians claim that this

is the only joint necessary to understand the kinematics of robots. True screw joints have very

few applications. Screws are more often used in tools for a robot end effector rather than a joint

of motion for a robot.

JOINT MOTION

Individually the different joints are very simple. Their motions are easy to understand andvisualize. In the Joint Motion Example on the left a prismatic and revolute joint are used to move

a simple robot end effector. Only one joint moves at a time so that you can easily see the

individual movements provided by the prismatic joint (linear motion of the yellow member along

the red member) and the revolute joint (rotational motion of the red member about the base).

Figure 28 screw joint

Figure 27 revolute joint


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In practice , a good wrist design should possess the following characteristics:

Three degrees of freedom

Spherical motion

Large workspace[ i.e large angular orientation range]

Remote drive capability

Compact size, light weight and low inertia.

High accuracy and repeatability

High mechanical stiffness.

Low manufacturing costs.

Rugged and reliable deisgn.


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Figure 30 robotic arm using servo motor

BALL AND SOCKET JOINT

Ball-and-socket joint in engineering, mechanical connection used between parts that must be allowed some relative

angular motion in nearly all directions. As the name implies, the joint consists essentially of a spherical knob at the

end of a shaft, with the knob fitting securely into a mating socket. Like other mechanical joints, a ball-and-socket

joint must have some provision for lubrication and is normally provided with a seal to prevent loss of the lubricant.

Joints of this type are commonly used in mounting the front wheels of automobiles, allowing these wheels

movement sufficient for steering. In this application they are usually called ball joints.


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Figure 31 ball socket joints

A joint, such as the shoulder or hip joint, in which a spherical knob or knoblike part of

one bone fits into a cavity or socket of another, so that some degree of rotary motion is

possible in every direction.

A mechanical device consisting of a spherical knob at the end of a shaft that fits securely

into a socket. Ball-and-socket joints are used to connect parts of a machine that require

rotary movement in nearly all directions. Ball-and-socket joints allow the front wheels of

a car to be turned by the steering mechanism.

A ball-and-socket joint, for example for use with an articulated support in an

automobile, includes an essentially ring-shaped or cup-shaped joint housing and a bearing shell

disposed in an interior space of the joint housing. The ball of a ball pin is slidingly received in

the bearing shell. The inside contour of the joint housing and the outside contour of the bearing

shell are essentially shaped as a truncated cone. The bearing shell can be essentially freely

positioned in the joint housing in an axial direction of the ball pin. The position at which the

bearing shell is anchored in the joint housing can then be precisely and reproducibly adjusted byway of the truncated conical surfaces. The service life, robustness and quality of ball-and-socket

joints can be significantly improved during manufacture, while simplifying their fabrication and

reducing their manufacturing cost.


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POWER ANALYSIS FOR A SKID STEERED ROBOT

Mobile robots are increasingly being developed for outdoor missions that demand an extended

degree of autonomy, such as search and rescue, disaster response, agriculture, military,forestry, mining, and planetary exploration. A key aspect of vehicle autonomy is power

consumption, which has become particularly relevant in applications with critically limited

power sources. However, this issue has not been usually perceived as a major problem because

most mobile robots use pure rolling wheels, which are regarded as an efficient traction

system.

In general, power efficiency has been considered at different levels in the robot system design.

Power-aware mechanical designs for the vehicle have been proposed based either on efficient

passive locomotion mechanisms or redundant locomotion configurations that require specific

control methodologies .On the other hand, in small and light robots, non-mechanical components

(e.g., sensing, communications or computations) may be responsible for most of the power

consumption, so efficient scheduling as well as specific energy conservation techniques can have

a major impact .

In the case of heavier robots, tractive energy has a major share in power cost. In wheeledvehicles with no slippage, the focus has been on minimizing energy objective functions

ultimately related to path length ,without explicitly addressing actual power consumption. In

fact, this has been the traditional goal of path planning in 2D space .More generally, information

about 3D orography and terrain types can be used to plan an optimal path to minimize the energy

expended due to gravity and friction .

Some researchers have pointed out that motor resistances are the main source of energy

dissipation in the traction system of wheeled robots, which can be minimized with an appropriate

velocity profile .In this sense, motion control methods have been proposed to reduce the loss of

kinetic energy and to minimize accelerations in potential field strategies .Moreover, an energy

model of motor losses has been considered to test the performance of three different

motion patterns in searching open areas of varying sizes .


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Power consumption has also been studied for alternative locomotion mechanisms, such as

wheeled vehicles with redundant drives , limbed robots ,or snake-like robots

.Nevertheless, we find that power efficiency of tracked robots has not received enough attention

in the literature.

Tracked locomotion offers a large contact area with the ground, which provides better traction

than wheels on natural terrains .However, power consumption is much higher than in wheeled

robots because of friction and slippage. In this kind of vehicle, power dissipation due to track-

soil interactions is considerably larger than that of motor resistances.

They analyzed power efficiency of tracked mobile robots both from theoretical andexperimental points of view. In particular, we have analyzed power consumption for Auriga-

, a 190 kg mobile robot with rubber tracks for skid steering on approximately flat terrains.

.

In spite of the kinematic equivalence at moderate speeds between skid-steered tracked mobile

robots and differential drive wheeled vehicles ,power requirements are essentially

different. In the latter, the consumption of one motor is almost independent of the speed

commanded to the other due to the pure rolling hypothesis. However, for skid-steered

tracked vehicles, energy consumption of one motor heavily depends on the speed of the other

track.

The rest of the paper is organized as follows. Next section is devoted to a general study of power

efficiency in tracked vehicles. Then, section III particularizes this analysis to the skid-steered

mobile robot Auriga-, with simulated and experimental results. Path planning implications for

this kind of vehicles are deduced in section IV, and an outdoor point-based navigation scheme

for Auriga- is presented accordingly. Following, some conclusions and future work are given.

Finally, acknowledgements and references complete the paper.


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POWER CONSUMPTION OF TRACKED VEHICLES

Three locomotion configurations have been proposed for tracked vehicles: articulated steering,curved track steering, and skid steering .The latter is the most widely used, since it is simpler

from the mechanical stand point and it achieves a faster response. Nevertheless, slippage makes

skid steering less power efficient than the other configurations. Another difference of skid

steering is that its turning radius is not bounded, but the maximum forward speed of the vehicle

is limited proportionally with curvature.

The skid steering principle is based on controlling the relative velocities of both tracks, much in

the same way as differential drive wheeled vehicles (see Fig. 1). The fastest track is considered

external to the turn, and the slowest track is internal. Both tracks can be regarded as external in

straight line motion or when turning around the vehicles center.

Lets now consider that track motion can be expressed as a function of the Instantaneous Centersof Rotation (ICRs) of track treads on the ground plane. These ICRs represent the position of

equivalent ideal wheel contact points as illustrated in Fig. 2. Both track ICRs lie on a line parallel

to the local X-axis, and they are outside of the track contact surface. The ICRs coordinates with

respect to the local frame of the vehicle are dynamics-dependent but remain within a bounded


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area. Optimized constant values for ICR positions can be obtained from experimental

identification. A more detailed discussion about track ICRs can be found .

If the center of mass of the vehicle does not lie on the Y axis, then the track that is closer to it

will slip less due to pressure. This circumstance results in track ICRs that are not symmetrical

with respect to the Y axis (see Fig).

Turnings in skid-steering vehicles are the cause of high sprocket torques in order to compensate

friction with the ground. This resistance is due to the dominant longitudinal component of track

contact surfaces, which tends to straight line motion. Thus, the external track counteracts the

friction of the internal track. Moreover, most of the external track contact surface is also opposed

to the turn. These effects result in great losses of energy.In the case of hard and uniform surface soils, such as asphalt, concrete or pavement, a general

anisotropic friction model can be assumed. In this case, the power Ps drawn by a track due to

slippage can be approximated by

where is the contact surface of the track, p is the pressure under each point of , !z is the angular

velocity of the vehicle, y is the terrain dependent friction coefficient in the longitudinal (localY-axis) direction, ~r is the vector of each point of with respect to the origin of the local frame

and ~rICR is the vector to the ICR of the track (see Fig. 2). This formula means that

instantaneous power losses are proportional to angular velocity, so energy spent due to slippage

depends on the following factor:

During straight line motion (!z = 0) no slippage occurs and Ps is negligible. According to Eq. 1,tread length greatly increases friction losses, since track points in the longitudinal extremes of

the tread increase the term k~r ~rICRk. Thus, given a particular tread shape, Ps strongly

depends of the track ICR positions (through vectors ~rICR in Eq. 1). Consequently, farther ICRs

result in lesser energy efficiency. Based on this effect, an efficiency index can be defined

as the inverse of the normalized distance between the track ICRs:


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and xICRl and xICRr are the local X coordinates of the left and right ICRs, respectively. Index

is a positive real value less than one. This efficiency index mainly depends on terrain type and

vehicle design, especially on the position of the center of gravity and on the track type. In the

case of ideal differential drive wheels, _=1, and the integral part of Eq. 1 corresponds to the only

contact point, where ~r = ~rICR.

Fig. 3. The mobile robot Auriga-_ with a mini-helicopter on its self-stabilized landing platform.

It must be noted that instantaneous power consumption can depend on the turning direction

because of ICR asymmetries. Then, the efficiency index _ represents an average value for the

vehicle, assuming that chances that the mobile robot turns

to the left or to the right are the same. Apart from turning friction, the power consumption of a


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traction motor depends proportionally of the track speed and also of the internal locomotion

resistances: the belt around its track train, the motor windings and the gear reductions between

the sprocket and the motor. In fact, these factors become the main sources of consumption

when the vehicle is in straight line motion. Thus, the power drawn due to propulsion Pp can be

approximately modelled as follows:

where Vl and Vr are the speeds for the left and right tracks respectively and K represent a

proportional constant.

POWER CONSUMPTION OF AURIGA-_

The Auriga-_ Mobile Robot The Auriga-_ tracked mobile robot has been designed as a

member of a group of robots for fire extinction tasks (see Fig. 3).

Its dimensions are 0.7m width, 1.2m length, 0.96m height, and 190 kg weight. It is equipped

with a small self-stabilized landing platform for radio controlled mini helicopters (employed

for fire detection) and a small fire extinguisher. Skid-steering is based on two independent

brushless AC motors with resolvers for dead-reckoning. The track train contains two rollers in

the middle as well as an idler and a sprocket at each end (see Fig. 5). The rubber belt is 0.16m

width, with a longitudinal contact surface of 0.72m length. The maximum speed of each track is

0.86m/s. The distance between track centerlines is L = 0.42m.


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Apart from the resolvers, the sensorial system of this mobile robot is composed of the following

elements:

A complete inertial measurement unit that combines accelerometers, gyroscopes and

magnetometers.

A GPS receiver that accepts differential corrections from a wide-area radio source.

The computation system is based on a pentium IV personal computer and a cFieldPoint, which is

a flexible programmable logic controller. The computer acts as user-interface, whereas the

cFieldPoint is dedicated to interface with the motor drivers and sensors. In this way, the

cFieldPoint is equipped with several input-output modules and executes a LabView program

in real-time. The energy needed by the vehicle is provided by a 3.8 kW petrol-fed 220V AC

generator. Each motor is powered by a servo-drive which allows power regeneration during track

deceleration. However, some of this energy is lost due to friction in the mechanical system andsome is converted to heat due to losses in the motor windings, cabling and drive electronics.

Moreover, if the regenerated energy exceeds the internal capacitor bank, it must be shunted into

an external resistor.

Tab. I shows the distribution of maximum power consumption

for the different components of the mobile robot. Note that

in this kind of vehicles, the locomotion system is responsible

for most of the energy consumed from the power source (up

to 80% for Auriga-_).

EXPERIMENTAL ANALYSIS

The efficiency index of Auriga-_ has been experimentally estimated with the value _ = 0.5. This

means that the distance between ICRs on the local X axis is approximately twice the distance

between track centerlines. Figure 4 shows the actual power consumption of the locomotion

system of Auriga-_ for half range of possible track speed combinations on hard smooth and a bit

slippery terrain.

It can be observed that power consumption achieves maximum values when both tracks have

opposite top speeds (turn around vehicles center at maximum angular speed), whereas minimum

values of energy occur when both track speeds are the same (i.e., straigh line motion with !z = 0).


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Energy losses have been characterized experimentally during straight line motion, obtaining a

proportional value K = 418.25 for Eq. 4. With the vehicle hoisted, the value of K = 327.2

remains high, meaning that power losses are quite important even without soil friction.

POWER MODEL SIMULATION:

Total power consumption of Auriga-_ due to locomotion P

can be obtained by summing up the results of evaluating Eqs.

4 and 1:


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A simplified punctual model for pressure distribution p on the track contact area has been

adopted (see Fig. 5). This distribution, which is affected by the center of gravity, assumes that

pressure is concentrated at the wheels contact points of the track train. The response of this

power model under the same conditions presented in the previous subsection (with y = 0.14) is

shown in Fig. 6. It can be observed that modelled energy requirements are quite similar to

experimental data.

IMPLICATIONS FOR AUTONOMOUS NAVIGATION

In order to minimize energy consumption of a skid-steered tracked vehicle during navigation,

some general guidelines for the path planning strategy can be deduced from the previous

theoretical and experimental analysis. In particular, unlike vehicles with pure rolling wheels, the

following considerations should be made:

Path planning should minimize the total amount of steered angle as much as the robotic

task can admit.

Smooth trajectories are not necessary and turns can be concentrated in a sharp way.

According to these ideas, a power-aware outdoor point to point navigation sheme is proposed

and tested in Auriga-_. It is presented in the next subsections.

POINT-based navigation scheme

The navigation plan is composed of a list of different goal points that the mobile robot has to

visit. The line segments between consecutive goal points are assumed to be obstacle free. In this

way, the path planner aim is that the vehicle moves in straight line, which is the less power

expensive case, specially for long distances. Then, turns are concentrated at the goal

points.

To achieve these goals, a simple heading control of the vehicle is proposed. When traveling to agiven goal point, the vehicle moves at a task-depend contant longitudinal speed. Angular speed

!z, which is usually zero, is only modified in order to keep heading towards the goal point.

Note that the aim of motion control is not to follow exactly the line segments but to reach the

next goal in straight line motion by eliminating the heading error _e (see Fig. 7). Thus, the

demanded angular speed can be computed as follows:


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where G is the angular gain. When the mobile robot approaches the goal point, a new goal is

commanded, which provokes a concentrated turn to compensate for the new heading error.

IMPLEMENTATION IN AURIGA-

The above navigation scheme has been implemented as a Lab view program, which is composed

of several concurrent processes, running in the c Field Point of Auriga-. The main process

implements a finite state machine that initializes the robot and commutates between the different

operation modes(idle, manual operation and automatic tracking). The rest of processes interactwith the sensors and drive units, gathering information and updating movement references. The

DGPS system is used to provide vehicle heading, which is employed to compute the heading

error.


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REFERENCES

y http://www.societyofrobots.com/robot_arm_tutorial.shtml

y http://en.wikipedia.org/wiki/Servomechanismy http://en.wikipedia.org/wiki/Stepper_motor

y http://en.wikibooks.org/wiki/Robotics/Components/Actuation_Devices/Mot

ors

y http://www.electronicsteacher.com/robotics/robotics-tutorial/beginners-

robotics/motors.php

y http://www.robotbasics.com/sensors-in-robotics

y http://www.societyofrobots.com/robot_arm_tutorial.s

y http://medgadget.com/archives/2010/04/new_robotic_arm_design_based_on

_elephants_trunk.htmlhtml

y http://www.ehow.com/how_5262443_make-robotic-arm.html

y http://robozeal.blogspot.com/search?q=basic+arm+design

y http://digitalclass.org/Tech%20Ed/Challenges/Robots/Robot%20Arms.htm

y Research papers by Jes us Morales, Jorge L. Mart nez, Anthony Mandow,

Alfonso J. Garc a-Cerezo,

y Jes us M. G omez-Gabriel and Salvador Pedraza

y E.T.S. Ingenieros Industriales, Universidad de M alaga, 29013 M alaga,

Spain

vinjamuri sai sudhir [ id no] 2008b3a4475h

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