Lecture3 Actuators

8/20/2019 Lecture3 Actuators

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Mobile Robotics:

3. Actuators

Dr.Brian

Mac

Namee(w

ww.com

p.dit.ie/bmacna

m

ee)

http://www.comp.dit.ie/bmacnamee


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2 Ac!nowled"ments

#$ese notes are based ($ea%il&)

on t$ose pro%ided b& t$e aut$ors

to accompan& 'ntroduction to

Autonomous Mobile Robots b&

Roland *ie"wart and lla$ R.Nourba!$s$

More information about the book is available at:http://autonomousmobilerobots.ep.ch/

The book can be bought at:

The MIT Press and Amazon.com

http://autonomousmobilerobots.epfl.ch/http://mitpress.mit.edu/catalog/item/default.asp?ttype=2&tid=10138http://www.amazon.com/Introduction-Autonomous-Mobile-Intelligent-Robotics/dp/026219502Xhttp://mitpress.mit.edu/images/products/books/026219502X-f30.jpghttp://www.amazon.com/Introduction-Autonomous-Mobile-Intelligent-Robotics/dp/026219502Xhttp://mitpress.mit.edu/catalog/item/default.asp?ttype=2&tid=10138http://autonomousmobilerobots.epfl.ch/http://autonomousmobilerobots.epfl.ch/http://autonomousmobilerobots.epfl.ch/


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+3ntroduction

A robot must be able to interact p$&sicall& wit$

t$e en%ironment in w$ic$ it is operatin"

Actuators are t$e components of a robot t$at

enable it to affect t$e en%ironment, sa&, b&

e-ertin" forces upon it or mo%in" t$rou"$ it

ell ta!e a loo! at:

0 1lectric motors

0 Artificial muscles

0 neumatics $&draulics


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+3Robot 4oints

Robot 5oints can be eit$er rotary (also !nown

as re%olute) or prismatic (telescopin")


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+3Robot 4oints (cont6)

rismatic 7artesian

robot

Rotar& *7ARArobot


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+3Robot 4oints (cont6)


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+3 Actuator 7ontrol

Robots are classified b& control met$od into

servo and non-servo robots

Nonser%o robots are essentiall& openloop

de%ices w$ose mo%ements are limited to

predetermined mec$anical stops

*er%o robots use closedloop computer control

to determine t$eir motion


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+37losed =oop 7ontroller

A closed-loop controller uses feedbac! to

control states or outputs of a d&namical s&stem

rocess inputs $a%e an effect on t$e processoutputs, w$ic$ is measured wit$ sensors and

processed b& t$e controllerC t$e result is used

as input to t$e process, closin" t$e loop

Controller Motor OutputInput Output

Measurement

Feedback


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+37losed =oop 7ontroller

7losedloop controllers $a%e t$e followin"

ad%anta"es o%er openloop controllers:

0 Disturbance re5ection (suc$ as unmeasured

friction in a motor)

0 uaranteed performance e%en wit$ modeluncertainties, w$en t$e model structure does not

matc$ perfectl& t$e real process and t$e model

parameters are not e-act

0 nstable processes can be stabiliEed

0 Reduced sensiti%it& to parameter %ariations

0 mpro%ed reference trac!in" performance


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+3#&pes of Actuators

*ome of t$e most common actuators are:

0 Electric motors, t$e most common actuators in

mobile robots, used bot$ to pro%ide location b&

powerin" w$eels or le"s, and for manipulation

b& actuatin" robot arms 0 Artificial muscles of %arious t&pes, none of

w$ic$ are %er& "ood appro-imations of li%in"

muscles

0 Pneumatic and hydraulic actuators, used in

industr& for lar"e manipulation tas!s but seldom

for mobile robots


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+31lectric Motors

1lectric motors are t$e most common sourceof torFue for mobilit& and/or manipulation inrobotics

#$e p$&sical principle of all electric motors is

t$at w$en an electric current is passed t$rou"$a conductor (usuall& a coil of wire) placedwit$in a ma"netic field, a force is e-erted ont$e wire causin" it to mo%e


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+3Gow Do 1lectric Motors or!H

#$e classic D7 motor $as a rotatin" armature in t$e

form of an electroma"net

A rotar& switc$ called a commutator re%erses t$e

direction of t$e electric current twice e%er& c&cle, to

flow t$rou"$ t$e armature so t$at t$e poles of t$eelectroma"net pus$ and pull a"ainst t$e permanent

ma"nets on t$e outside of t$e motor

As t$e poles of t$e armature electroma"net pass t$e

poles of t$e permanent ma"nets, t$e commutatorre%erses t$e polarit& of t$e armature electroma"net.

Durin" t$at instant of switc$in" polarit&, inertia !eeps

t$e motor "oin" in t$e proper direction


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+3Gow Do 1lectric Motors or!H (cont6)

A simple DC electric motor: when the coil is

powered, a magnetic field is generated around

the armature. The left side of the armature is

pushed away from the left magnet and drawn

toward the right, causing rotation


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The armature continues to rotate


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When the armature becomes horizontally

aligned, the commutator reverses the direction of

current through the coil, reversing the magnetic

field. The process then repeats.


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+31lectric Motors

1lectric motors usuall& $a%e a small ratin",

ran"in" up to a few $orsepower

#$e& are used in small appliances, batter&

operated %e$icles, for medical purposes and in

ot$er medical eFuipment li!e -ra& mac$ines

1lectric motors are also used in to&s, and in

automobiles as au-iliar& motors for t$e

purposes of seat ad5ustment, power windows,

sunroof, mirror ad5ustment, blower motors,

en"ine coolin" fans and t$e li!e


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+3*tepper Motors

$en incremental rotar& motion is reFuired in

a robot, it is possible to use stepper motors

A stepper motor possesses t$e abilit& to mo%e

a specified number of re%olutions or fraction of

a re%olution in order to ac$ie%e a fi-ed and

consistent an"ular mo%ement

#$is is ac$ie%ed b& increasin" t$e numbers of

poles on bot$ rotor and stator

Additionall&, soft ma"netic material wit$ man&

teet$ on t$e rotor and stator c$eapl& multiplies

t$e number of poles (reluctance motor)


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+3*tepper Motors

#$is fi"ure illustrates t$e desi"n

of a stepper motor, arran"ed

wit$ four ma"netic poles

arran"ed around a central rotor

Note t$at t$e teet$ on t$e rotor

$a%e a sli"$tl& ti"$ter spacin"

to t$ose on t$e stator, t$is ensures t$at t$e two

sets of teet$ are close to eac$ ot$er but notFuite ali"ned t$rou"$out


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+3*tepper Motors (cont6)

Mo%ement is ac$ie%ed w$enpower is applied for s$ortperiods to successi%e ma"nets

$ere pairs of teet$ are least

offset, t$e electroma"neticpulse causes ali"nment and asmall rotation is ac$ie%ed, t&picall& @2o


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+3Gow Does A *tepper Motor or!H

The top electromagnet (1) is charged, attracting thetopmost four teeth of a sprocket.


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+3Gow Does A *tepper Motor or!H (cont6)

The top electromagnet (1) is turned off, and theright electromagnet (2) is charged, pulling the

nearest four teeth to the right. This results in a

rotation of 3.6°

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The bottom electromagnet (3) is charged; another3.6° rotation occurs.

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The left electromagnet (4) is enabled, rotating again by3.6°. When the top electromagnet (1) is again charged, the

teeth in the sprocket will have rotated by one tooth

position; since there are 25 teeth, it will take 100 steps to

make a full rotation.

3


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+3*tepper Motor

*tepper motors $a%e se%eral ad%anta"es:

0 #$eir control is directl& compatible wit$ di"ital

tec$nolo"&

0 #$e& can be operated open loop b& countin"

steps, wit$ an accurac& of ±@ step. 0 #$e& can be used as $oldin" de%ices, since t$e&

e-$ibit a $i"$ $oldin" torFue w$en t$e rotor is

stationar&

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+31lectric Motors: Mountin"

$en used wit$ rotar& 5oint s&stems, motors

can produce torFue b& bein" mounted directl&

on t$e 5oints or b& pullin" on cables

#$e cables can be t$ou"$t of as tendons t$at

connect t$e actuator (muscle) to t$e lin! bein"

mo%ed

*ince cables can appl& force onl& w$en pulled,

it is necessar& to use a pair of cables to obtainbidirectional motion around a 5oint, t$is implies

mec$anical comple-it&

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+31lectric Motors: Mountin" (cont6)

Mountin" motors directl& on 5oints allows for

bidirectional rotation, but suc$ mountin" ma&

increase t$e p$&sical siEe and wei"$t of t$e

5oint, and t$is ma& be undesirable in some

applications

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+31lectric Motors: =inear Mo%ement

#$e fact t$at electric motors produce rotational

motion raises an issue wit$ re"ard to t$eir use

in robots

?or linear translation it is necessar& to

translate rotational to linear motion

0 ?or e-ample, prismatic 5oints reFuire linear

translation rat$er t$an rotation from t$e motor

=eadscrews, beltandpulle& s&stems, rac!andpinion s&stems, or "ears and c$ains are

t&picall& used to transform rotational to

translational motion

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+3 Artificial Muscles

Durin" t$e past fort& &ears a

number of attempts $a%e beenmade to build artificial muscles

Muscles contract w$en

acti%ated, since t$e& areattac$ed to bones on twosides of a 5oint, t$e lon"itudinals$ortenin" produces 5oint rotation

Bilateral motion reFuires pairs ofmuscles attac$ed on opposite sides of a

5oint are reFuired to produce

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+3 Artificial Muscles: McIibben #&pe

#$e McIibben muscle was t$e

earliest attempt at constructin"

an artificial muscle

#$is de%ice consisted of a

rubber bladder surrounded b&

a slee%e made of n&lon fibers in a $elical

wea%e

$en acti%ated b& pressuriEed air, t$e slee%epre%ented it from e-pandin" len"t$wise, and

t$us t$e de%ice s$ortened li!e li%in" muscles

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n t$e @>8s t$ere were attempts to use

McIibben muscles to produce mo%ements in

mec$anical structures strapped to

nonfunctional arms of Fuadriple"ics

#$e reFuired compresses air was carried in a

tan! mounted on t$e persons w$eelc$air

#$ese e-periments were ne%er completel&

successful

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*ince t$e @>8s t$ere $as been

se%eral ot$er attempts to de%elopimpro%ed McIibben t&pe artificialmuscles:

0 (Broo!s, @>99) de%eloped an

artificial muscle for control of t$earms of t$e $umanoid torso 7o"

0 (ratt and illiamson @>>)de%eloped artificial muscles forcontrol of le" mo%ements in a bipedwal!in" robot

Gowe%er, it is fair to sa& t$at no artificial musclesde%eloped to date can matc$ t$e properties of animalmuscles

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+3 Artificial Muscles: *$ape Memor& Allo&s

!hape memory alloys (*MAs) $a%e unusual

mec$anical properties

#&picall&, t$e& contract w$en $eated, w$ic$ is

t$e opposite to w$at standard metals do w$en

$eated (e-pand)

?urt$ermore, t$e& produce t$ermal mo%ement

(contraction) one $undred times "reater t$an

t$at produced b& standard metals

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+3 Artificial Muscles: *$ape Memor& Allo&s

Because t$e& contract w$en $eated, *MApro%ide a source of actuation for robots

After contraction, t$e material "raduall& returnsto its ori"inal len"t$ w$en t$e source of

acti%ation is remo%ed and it is allowed to cool*MAs $a%e two ma5or problems w$en used asartificial muscles:

0 #$e& cannot "enerate %er& lar"e forces 0 #$e& cool slowl& and so reco%er t$eir ori"inallen"t$ slowl&, t$us reducin" t$e freFuenc&response of an& artificial muscle in w$ic$ t$e&

are emplo&ed

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+3Nort$eastern ni%ersit&s Robot =obster

A robot lobster de%eloped

at Nort$eastern ni%ersit&

used *MAs %er& cle%erl&

0 #$e force le%els reFuired

for t$e lobsters le"s arenot e-cessi%e for *MAs

0 Because t$e robot is used underwater coolin" is

supplied naturall& b& seawater

More on t$e robot lobster is a%ailable at: $ttp://www.neurotec$nolo"&.neu.edu

+@

http://www.neurotechnology.neu.edu/http://www.neurotechnology.neu.edu/http://www.neurotechnology.neu.edu/


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+3 Artificial Muscles: 1lectroacti%e ol&mers

=i!e *MAs, 1lectroacti%e ol&mers (1As)

also c$an"e t$eir s$ape w$en electricall&

stimulated

#$e ad%anta"es of 1As for robotics are t$at

t$e& are able to emulate biolo"ical muscleswit$ a $i"$ de"ree of tou"$ness, lar"e

actuation strain, and in$erent %ibration

dampin"nfortunatel&, t$e force actuation and

mec$anical ener"& densit& of 1As are

relati%el& low

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+31lectroacti%e ol&mer 1-amples

Robotic face de%eloped b& a "roup led

b& Da%id Ganson. More information is

a%ailable at:

www.$ansonrobotics.com

Robotic $and de%eloped b& a "roup

led b& ra$am $itele&. More

information is a%ailable at:

www.elumotion.com

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i G d li A

http://www.hansonrobotics.com/http://www.elumotion.com/http://www.elumotion.com/http://www.hansonrobotics.com/


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+3neumatic G&draulic Actuators

=ar"e manipulators in industr& freFuentl&emplo& $&draulic dri%es, since suc$ dri%espro%ide a $i"$er torFuetowei"$t ratio t$anelectric motors

Gowe%er, because of t$e maintenanceproblems associated wit$ pressuriEed oil(includin" lea!s), $&draulic motors are notused in smaller mobile robots

neumatic dri%es $a%e been used as actuatorsin t$e past but are not currentl& popular

Air is compressible, resultin" in nonlinear

be$a%ior of t$e actuator

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*


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+3*ummar&

Actuators are t$e components of a robot t$at

interact p$&sicall& wit$ t$e en%ironment in

w$ic$ it is operatin"

#$e !e& issues wit$ re"ard to actuators

include:

0 ReFuired power (torFue etc)

0 ower reFuired

0 ei"$t etc

0 *peed

Lecture3 Actuators

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