2.2ComponentSelection(140318)

Robotics Exp. 1

Dept. of Robot Engineering

Yeungnam Univ.

2. Mechanical Design

Table of Contents

1. Introduction


3. Circuit Design

4. Sensor Control

5. Motor Control

6. Communication

7. Navigation

8. Mapping

9. SLAM

10. Multi robots

2.1 Selection of a robot

2.2 Component Selection

2.2.0 Overview

2.2.1 Motors

2.2.2 Batteries

2.2.3 Wheels

2.2.4 Miscellaneous

How to build a mobile robot

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2.2 Component Selection

2.2.0 Overview

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Robot

Interior Outside

Wheel Articular

6- Articular 4- Articular 4 - Wheel 2 - Wheel

Laser Ultrasonic

MIC

IMU

Zigbee GPS Touch

Encoder Vision Bluetooth Compass

Example of component selection

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Example)

USB

USB USB

18 Motors

Laser

Sensor

Tilt Sensor,

gyro sensor

6 Switches

Compass

Motor

interface

board

sensor

interface

board SSD

LAN

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2.2.1 Motors

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The flow chart seen left will

guide you to the

recommended drive

technology.

Source: Parker co.

(1996/1997 Compumotor)

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Too many robotics projects fail because the motors were too junky and

didn't work, or generated too much electrical noise, or were too weak.

Does the robot need to move fast or slow and what kind of payload (how

heavy) does it need to carry?

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There is no ideal drive configuration that simultaneously maximizes

stability, maneuverability and controllability.

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Motor Types and Their Applications: It should be stressed that there is a wide range of applications which can be

equally well met by more than one motor type, and the choice will tend to

be dictated by customer preference, previous experience or compatibility

with existing equipment. => Motor sizing and Selection software

package

High torque, Low speed: continuous duty applications are appropriate to the step motor.

At low speeds it is very efficient in terms of torque output relative to both

size and input power.

Microstepping can be used to improve smoothness in low-speed

applications such as a metering pump drive for very accurate flow control.

High torque, High speed: continuous duty applications suit the servo motor, and in fact a step motor

should be avoided in such applications because the high-speed losses can

cause excessive motor heating.

SERVO VERSUS STEPPER...

WHAT YOU NEED TO KNOW

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Short, rapid, repetitive moves: are the natural domain of the stepper due to its high torque at low speeds, good

torque-to-inertia ratio and lack of commutation problems. The brushes of the

DC motor can limit its potential for frequent starts, stops and direction changes.

Low speed, high smoothness applications: are appropriate for microstepping or direct drive servos.

Applications in hazardous environments: or in a vacuum may not be able to use a brushed motor.

Either a stepper or a brushless motor is called for, depending on the demands of

the load.

Bear in mind that heat dissipation may be a problem in a vacuum when the loads

are excessive.

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Motor selection

start

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Determination of driving mechanism

Determination of motion pattern of mechanical parts

Calculation of load torque

Calculation of moment of inertia

Selection of motors and drives

Calculation of acc. and dece. Torque

Calculation of driving torque

Final determination of the motor

stop

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For an optimal robot choosing motors would involve calculations of

– weight, gearing ratios, desired terrain, desired velocity and

acceleration, voltage, power consumption, and controllability, etc.

Bigger the motors, shorter the battery life, and more expensive and

complicated the motor control circuitry.

Example) Voltage: 5V-8V

Torque: double what you think you will need

Type: servos.

– Servos are much easier to control, but they generally are less energy efficient,

have explicit voltage requirements, and are less intuitive.

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Motor selection (Selection procedure)

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How to calculate moment of inertia

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Checking of load torque

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Motor and load inertia

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How to calculate moment of inertia

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Service factor

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1) Speed suitable for use

Fig. 1 shows the typical torque curve, input dissipation curve and vibration curve.

In Fig. 1, the motor shows variations of 1100 to 1800 [min–1] according to the load.

The speed most suitable for the load of the equipment is as follows:

1200 to 1250 [min–1] for 50 Hz

1500 to 1550 [min–1] for 60 Hz

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In this speed range, as can be seen from Fig. 1, the

input dissipation becomes minimum, which means

that the temperature rise of the motor is reduced

accordingly. => the life of the motor, the

insulation life, ball bearing grease life, etc. in

particular, is prolonged. Also the vibration is

minimized: in particular the gear noise caused when

a gear head is used is reduced optimally.

As described above, an optimum speed should be

considered in selecting a motor.

Calculation of motor capacity

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2) Examination of load of equipment

Examine the torque required for the load regarding the following three

items.

Minimum required torque at starting of the equipment

Maximum load torque at load variations of the equipment

Load torque at stable rotation

When the load torque is (1) to (4) in Fig. 2, the starting torque for (1), the stalling

torque for (2) both the starting torque and stalling torque for (3) and (4) should be

considered.

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3) Calculation of required torque

When the load of the equipment is (1), (3) or (4) in Fig. 2

Calculate the approximate value of the required starting torque Ts. In Fig. 3

(Conveyor), for example, calculate the required force F from “T = Fr”. Then select

suitable motors from catalog or the attached S-T data and check the minimum

starting voltage, the minimum stable voltage and the speed in stable rotation. In

accordance with the equipment load status calculated based on the above-mentioned

examination, select a motor with the most suitable S-T curve.

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4) Measurement of minimum starting voltage

Couple the motor to the load to be measured and connect a variable

transformer and voltmeter as shown in the figure to the right. Increase the

voltage continuously from 0 volt at the rate of 3 V/sec with this variable

transformer and measure the when the rotating part of the equipment starts

and gets ready for acceleration.

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5)Measurement of minimum stable voltage

Drive the equipment in a stable state.

Using the above-mentioned variable transformer, decrease the voltage

gradually.

Measure the voltage at the limit of the motor speed allowing the equipment

to function, that is, when the equipment begins to stop.

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6) Measurement of motor with gear head

When a motor alone is coupled to equipment, the speed is measured at

output shaft section using a strobe light etc. In the case of a motor with a

gear head, the speed is calculated from the following formula.

n = i x n1

n : Motor speed (min–1)

n1 : Speed of gear output shaft or pulley etc. attached to it

i : Reduction ratio of gear head (e.g. i = 30 for 1/30)

When measuring the speed of a gear output shaft having a large reduction

ratio, do not measure the number of revolutions per minute, but measure

the time taken for the gear output shaft to rotate 100 turns using a

stopwatch after putting a mark on the shaft. Then calculate the number of

revolutions per minute from the measured time.

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7) Example of motor selection

Application : Driving of conveyor

Voltage : 100 V

Speed : 30 min–1

Working condition : Continuous

Frequency : 60 Hz

Select a motor that meet the above.

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(1) Speed suitable for specifications Because the required speed is 30 min–1, the gear ratio that realizes a rated

motor speed (60 Hzarea) of 1500 to 1550 min–1 is 1500/30 to 1550/30

= 50 to 51.67.

Therefore use a gear ratio of 1/50.

(2) Calculation of required torque Measure the approximate load with a spring balance etc. Assume that it is

2.65 N·m (375.27 oz-in).

After referring to catalog, select a gear a reduction gear.

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(3) Actual measurement of minimum starting voltage, minimum stable

voltage and speed Assume that the following are obtained as a result of actual measurement.

Minimum starting voltage: 75 V

Minimum stable voltage: 55 V

Speed: 1700 min–1

(4) From speed-torque curve of 4-pole 25 W induction motor Ts : Starting torque Ts = 0.16 N·m (22.66 oz-in)

Tm : Stalling torque Tm = 0.25 N·m (35.4 oz-in)

The torque is proportional to the square of the voltage and the following values

are obtained.

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From the above, it can be seen that this application is a constant torque load and that

the 4-pole 25 W induction motor still has a more than sufficient

capacity.

In addition, as is evident from the S-T curve of the attached S-T data, Ts and Tm of the

4-pole 15 W induction motor are as follows:

Ts = 0.1 N·m (14.16 oz-in)

Tm = 0.15 N·m (21.24 oz-in)

Considering the voltage drop and variation when used for conveyors, Ts and Tm of the

4-pole 15 W induction motor at 90 V are assumed to be as follows:

Ts = 0.08 N·m (11.33 oz-in)

Tm = 0.12 N·m (16.99 oz-in)

When the voltage drop and variation or load variation is thought to be insignificant, the

4-pole 15 W induction motor and gear head MX7G50B can be used.

When the voltage variation or load variation is significant, the 4-pole 25 W induction

motor should be used.

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Battery power selection for motor driving

Recommendation:

Ex) When we need high torque: DC 24 V

In general: > 250 W : DC 12 V (High current)

< 250 W : DC 24 V (Low current)

(cf: Serial connection of DC 12 V batteries for DC 24 V motor)

In general, around 40% over spec. in torque of the motor.

When motor speed changes in operation, we need a motor with twice

torque of normal operation case for the operation in low speed.

Watt of motor is proportional to torque (current).

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Example of motor spec.

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2.2.2 Battery selection

What would be a good battery for the robot?

Decide on how long we want our robot to run before a recharge,

Determine (by using datasheets and experiments with a multimeter) how

much current your electronics and motors draw.

Learn about the different types of batteries.

The 3 most commonly used batteries.

Alkaline batteries

NiMH (Nickel Metal Hydride) batteries

NiCad (Nickel Cadmium) batteries

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Desirable properties of the battery

Rechargeable

High mAh (energy capacity)

Consider putting batteries in both parallel and in series to vary/control

total voltage and mAh

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a) Alkaline batteries

an alkaline electrolyte of potassium hydroxide.

The most common, easiest to get, and cheapest.

Low power capacities, heavy, have trouble supplying large amounts of

current in short time periods, and get expensive to constantly replace.

Used in many household items such as MP3 players, CD players,

digital cameras, pagers, toys, lights, and radios, to name a few.

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b) NiMH (Nickel Metal Hydride) batteries

Cell phone batteries are often NiMH (=> Lithium).

Rechargeable as much as you want

Good current output

Highest energy capacity.

More expensive than the other two batteries.

Good for small size robots and for powering circuits.

Note) NiMH batteries usually take like 5-10 hours to fully recharge

depending on various factors.

Ni-MH Batteries

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c) NiCad (Nickel Cadmium) batteries

Good for small to medium size range robots.

The highest current output

More affordable than NiMH's

Recharged within one or two hours.

Over many charges, it can only store less and less energy after each

recharge. To prevent memory effect, whenever you wish to recharge

your NiCad, you must first fully discharge it.

http://upload.wikimedia.org/wikipedia/commons/4/41/NiCd_various.jpg

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2.2.3 Wheels

Wheel selection(Size)

Big wheels:

Let the robot move faster.

The robot has less torque to carry a heavy payload

Fine position control is harder

The sensors often cannot keep up with fast changes in position.

Wheel size is a design tradeoff to decide.

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Wheel selection(type)

The four basic wheel types

a) Standard wheel:

rotation around the

(motorized) wheel axle

and the contact point.

b) Castor wheel: rotation

around the wheel axle,

the contact point and the

castor axle.

c) Swedish (or mecanum)

wheel: rotation around the

wheel axle, around the rollers

and around the contact point.

d) Ball or spherical

wheel.

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Omni wheel (poly wheel)

The ability to move in all directions.

The Omniwheels design is based upon the use of a series of free turning

barrel-shaped rollers, which are mounted, in a staggered pattern around the

periphery of a larger diameter main wheel.

The combination of these two rolling elements provides a compact and

inexpensive unit for moving heavy loads in any direction along a plane,

doing so smoothly and with minimum effort.

They are often used in small robots. (ex: Robocup) .

Omniwheels combined with conventional wheels:

Ex a six wheel vehicle employing two conventional wheels on a center

axle and four omniwheels on front and rear axles.

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Driving methods of Omni-Wheel

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2.2.4 Miscellaneous

Material for robot body

The light material

Soft material, ex) Plastic

Aluminium

Balsa

Carbon fiber

High Density

Polyethylene (HDPE)

Styrofoam

Vacuum molding

Velcro, Tape

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Solved Problems

Problem 2.2.1 Define the following terms.

a) Differential drive, b) synchronous speed

Ans)

Synchronous speed: The speed at which an induction motor will operate depends

on the input power frequency and the number of electrical magnetic poles.

ω = 2 60 f / n

where

ω = pump shaft rotational speed (rev/min, rpm)

f = frequency (Hz, cycles/sec)

n = number of poles

in the motor.

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Problem 2.2.2 DC motors are cheaper than servos, so why use a servo?

Ans) They contain a full motor driver, a gear box, a position sensor, a built

in PID loop control algorithm, and only require a single wire hooked

directly up to your microcontroller for full motor control. DC motors

on the other hand cannot operate directly from a microcontroller.

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Question 2.2.1 List possible actuators for mobile robots and compare them.

Question 2.2.2 Describe alternative types of locomotion and where they are

used/interesting.

Review Questions

Documents

2.2ComponentSelection(140318)