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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$e wire causin" it to mo%e
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+37omponents
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+3Gow Do 1lectric Motors or!H
#$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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+3Gow Do 1lectric Motors or!H (cont6)
The armature continues to rotate
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+3Gow Do 1lectric Motors or!H (cont6)
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$e number of poles (reluctance motor)
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2+
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+3*tepper Motors
#$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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+3Gow Does A *tepper Motor or!H (cont6)
The bottom electromagnet (3) is charged; another3.6° rotation occurs.
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2>
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+3Gow Does A *tepper Motor or!H (cont6)
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.
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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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+3 Artificial Muscles: McIibben #&pe
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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+3 Artificial Muscles: McIibben #&pe
*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
+3
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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+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