Unit 7 Actuators NIKU Parted 2

7/23/2019 Unit 7 Actuators NIKU Parted 2

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UNIT 7:--- pared 2

Actuators (Saeed B Niku)

Hydraulic actuators proportional feedback control Pp272

(Ref: Fi !"#$ % Proportional Hydraulic

&er'o al'e )it* Feedback +oop,

Pp2#

.lectric /otor Funda/entals

0ifferences %1 and 01 Type otors

Pp2#

01 /otors Pp2#

%1 /otors (Re'ersible, Pp23

4rus*less 01 /otors Pp2!

&tepper /otors:- &tructure

principle of operation

&peed-tor5ue c*aracteristics

Pp2

Pp26$

Pp26

HYDRAULIC ACTUATORS

Hydrauic syste!s and actuators offer

• a "i#" po$er%to%$ei#"t ratio ar#e &orces at o$ speeds (bot* linear and rotary

actuation,

• co/patibility )it* /icroprocessor and electronic controls and

• Tolerance of e8tre/e *a9ardous en'iron/ents"

• any lare robots of t*e past decade /ostly used in auto/obile production

• 0ue to leakae proble/s )*ic* are al/ost ine'itable in *ydraulic syste/s and due to

t*eir po)er unit )ei*t and cost t*ey are not used any /ore"

• No)adays /ost robots are eectric"

•T*ere are still /any robots in i'dustry t*at *a'e *ydraulic actuators" %dditionally for special applications suc* as 'ery lare robot and cii serice robots *ydraulic actuators

/ay be t*e appropriate c*oice"

T*e total force t*at a linear cylinder can deli'er can be tre/endously lare for its si9e"

% *ydraulic cylinder can deli'er a force of

F = p x A lb )*ere A is t*e effecti'e area of t*e piston or ra/ and p is t*e )orkin pressure"

i#ure *+, % Rotary Hydraulic Actuator " T*is actuator can be directly attac*ed to a re'olute oint

)it*out any need for ear reduction"

For e8a/ple for a pressure of ;$$$ psi e'ery s5uare inc* of t*e cylinder de'elops ;$$$-lb of

force" In rotary cylinders t*e sa/e principle is true e8cept t*at t*e output is a tor5ue )*ere:



(!"<,

)*ere p is t*e fluid pressure

t is t*e t*ickness or )idt* of t*e rotary cylinder and

r1 and r2 are t*e inner and outer dia/eters of t*erotary cylinder as s*o)n in Fiure !"#"

T*e flo) rate and 'olu/e of oil needed in a *ydraulic syste/ are

(!"3,

(!"!,

)*ere

dx is t*e desired displace/ent and

x is t*e desired 'elocity of t*e piston"

%s you can see by co'troi'# t*e 'olu/e of t*e fluid oin i'to t*e cylinder t*e tota

dispace!e't can be co'troed" 4y controllin t*e rate in )*ic* t*e fluid is sent to t*e cylinder

t*e eocity can be co'troed" T*is is done t*rou* a ser'o 'al'e t*at controls t*e 'olu/e of t*e

fluid as )ell its rate"

% "ydrauic syste! enerally consists of t*e follo)in parts:

• Hydraulic linear or rotary cylinders and ra/s" T*ese pro'ide t*e force or tor5ue needed to

/o'e t*e oints and are controlled by t*e ser'o 'al'es or /anual 'al'es"

• % *ydraulic pu/p )*ic* is a *i*-pressure pu/p t*at pro'ides *i*-pressure fluid to tile

syste/"

• .lectric (or ot*ers suc* as diesel enine, /otor )*ic* operates t*e *ydraulic pu/p"

• 1oolin syste/ )*ic* rids t*e syste/ of t*e *eat enerated" In so/e syste/s in addition

to coolin fans radiators and cooled air are used"

•

Reser'oir )*ic* keeps t*e fluid supply a'ailable to t*e syste/" &ince t*e pu/p isconstantly supplyin pressure to t*e syste/ )*et*er or not t*e syste/ is usin it all t*e

e8tra pressuri9ed fluid as )ell as all t*e returned fluid fro/ t*e cylinders flo) back into

t*e reser'oir"

• &er'o 'al'e is a 'ery sensiti'e 'al'e t*at controls t*e a/ount and t*e rate of fluid to t*e

cylinders" T*e ser'o 'al'e is enerally dri'en by a *ydraulic ser'o/otor"

• &afety c*eck 'al'es *oldin 'al'es and ot*er safety 'al'es t*rou*out t*e syste/"

• 1onnectin *oses )*ic* are used to transport t*e pressuri9ed fluid to t*e cylinders and

back to t*e reser'oir"

• &ensors )*ic* are used to control t*e /otion of t*e cylinders" T*ey include position

'elocity /anetic touc* and ot*er sensors"

i#ure *+- is a sc*e/atic dra)in of a typical *ydraulic syste/"



i#ure *+- &c*e/atic of a *ydraulic syste/ and its co/ponents"

i#ure *+. is a sc*e/atic dra)in of a position control pilot 'al'e for a *ydraulic cylinder also

called a spool 'al'e"

i#ure *+. &c*e/atic of a spoo ae ill neutral position"

T*is is a balanced 'al'e )*ic* /eans t*at t*e pressures on t*e t)o sides of t*e spool are e5ual"

T*us it takes 'ery little force to /o'e t*e spool e'en t*ou* it /ay be under 'ery *i* pressures"

=*en a ser'o/otor is attac*ed to t*e spool 'al'e to operate it a ser'o 'al'e is created" T"e sero

ae and t*e cyi'der toet*er for/ a "ydrauic sero!otor" %s t*e spool /o'es up or do)n it

opens t*e supply and return ports t*rou* )*ic* t*e fluid tra'els to t*e cylinder or is returned to

t*e reser'oir" 0ependin on t*e si9e of t*e openin of t*e port t*e supply fluid flo) rate is

controlled and so is t*e eocity of t*e cylinder" 0ependin on t*e lent* of ti/e t*at t*e port is

kept open t*e total a/ount of t*e fluid to t*e cylinder and t*us its total tra'el is controlled" T*iscan be )ritten as

(!"7,

and

(!",

)*ere q is t*e flo) rate

C is a constant

x is t*e spool>s displace/ent A is t*e area of t*e piston and

y is piston>s displace/ent"

1o/binin .5uations (!"7, and (!", and desinatin d/dt as D )e *a'e



and

(!"6,

=*ic* s*o)s t*at t*e *ydraulic ser'o/otor is an interator"

T*e co!!a'd to t*e ser'o/otor controllin t*e spool 'al'e co/es fro/ t*e co'troer"

• T*e controller sets t*e current to tile ser'o/otor as )ell as t*e duration t*e current is

applied )*ic* in turn controls t*e position of t*e spool"

• T*us for a robot )*en t*e controller *as calculated *o) /uc* and *o) fast a oint /ust

/o'e it sets t*e current and its duration to t*e ser'o/otor )*ic* in turn controls t*e

position and rate of /o'e/ent of t*e spool 'al'e )*ic* in turn controls t*e flo) of t*e

fluid and its rate to t*e cylinder )*ic* /o'es t*e oint"

•T*e sensors pro'ide &eed/ack to t*e controller for accurate and continued control"i#ure *+* s*o)s t*e flo) of t*e fluid as t*e spool 'al'e /o'es up or do)n" %s you can

see a si/ple /otion of t*e spool controls t*e /otion of t*e cylinder"

To pro'ide feedback to t*e ser'o 'al'e eit*er eectro'ic or !ec"a'ica feedback can be added to

t*e 'al'e (?t*er)ise it )ill 'ot /e a sero ae but a /anual spool 'al'e," i#ure *+0 s*o)s a

si!pe !ec"a'ica &eed/ack oop"

• &ince t*is is a 'ery si/ple syste/ and can be easily understood it is /entioned *ere"

• a si/ple le'er is added bet)een t*e output and t*e input to pro'ide an error sinal to t*e

syste/"

• %s t*e desired position for t*e load is set by t*e set-point le'er say up t*e spool 'al'e is

opened )*ic* )ill operate t*e cylinder"

• Ho)e'er in reality t*is le'er ar/ is pro'idin an error sinal to t*e syste/ and t*us t*e

syste/ pro'ides a sinal (fluid pressure, to t*e cylinder"

• T*e error sinal is interated by t*e interator (in t*is case t*e cylinder, and as t*e error

approac*es 9ero t*e sinal to t*e syste/ oes to 9ero"

• %s t*e cylinder starts to /o'e in t*e direction of t*e set desired position (in t*is case

up)ard, t*e error sinal beco/es increasinly s/aller )*ic* in turn reduces t*e output

sinal by closin t*e spool 'al'e"

• 4y t*e ti/e t*e load is at t*e desired position t*e spool 'al'e is closed and t*e load stops"

i#ure *+1 s*o)s t*e sc*e/atic of t*e block diara/ for t*is feedback loop



Fiure !"! &c*e/atic of a spool 'al'e in open position" 0ependin on )*ic* ports are opened t*e

direction of /otion of t*e piston )ill c*ane

• T*ere are /any intricate and s/all passae)ays inside a *ydraulic 'al'e-especially in

ser'o'al'es )it* feedback control interated into t*e 'al'e"

• T*is is )*y t*ese syste/s are so susceptible to dirt or iscosity c*anes due to

te!perature"• T*e s/allest of &orei#' /odies can affect t*e ser'o'al'e by restrictin its passae)ays or

ports"

• &i/ilarly 'iscosity c*anes can c*ane t*e response of t*e 'al'e as t*e fluid beco/es less

or /ore 'iscous"

i#ure *+0 &c*e/atic of a si/ple control de'ice )it* proportional feedback

Fiure !" 4lock diara/ of t*e *ydraulic syste/ )it* proportional feedback control sc*e/e

N3U4ATIC D35IC3S

• Pneu/atic de'ices are principally 'ery si!iar to "ydrauic syste!s"

• % source of pressuri9ed air is used to po)er and dri'e linear or rotary cylinders controlled

by /anual or electrically controlled solenoid 'al'es"

• &ince t*e source of pressuri9ed air is separate fro/ t*e /o'in actuators t*ese syste/s

*a'e lo)er inertial loads"

• a pneu/atic de'ices operate at a /uc* lo)er air pressure usually up to ;$$-;2$ psi t*eir

po)er-to-)ei*t ratio is /uc* lo)er t*an *ydraulic syste/s

•

T*e /aor proble/ )it* pneu/atic de'ices is t*at air is co/pressible and t*us itco/presses and defor/s under load"



• %s a result pneu/atic cylinders are usually only used for insertion purposes )*ere t*e

actuator is all t*e )ay for)ard or all t*e )ay back)ard or t*ey are used )it* ;@2-deree-

of-freedo/ oints t*at are fully on or fully off"

• 1ontrollin t*e e8act position of pneu/atic cylinders is 'ery difficult"

• ?ne )ay to control t*e displace/ent of t*e pneu/atic cylinders is called differential

dit*erin"

• In t*is syste/ t*e e8act location of t*e piston is sensed by a feedback sensor suc* as alinear encoder or potentio/eter"

• T*is infor/ation is used in a controller t*at t*e air pressure on t*e t)o sides of t*e

cylinder t*rou* a ser'o 'al'e to control t*e e8act position"

*+* 3L3CTRIC 4OTORS

• =*en a )ire carryin a current is placed )it*in a /anetic field it e8periences a force

nor/al to t*e plane for/ed by t*e /anetic field and t*e current as F= I x B"

• If t*e )ire is attac*ed to a center of rotation t*e resultin tor5ue )ill cause it to rotate

about t*e center of rotation"

• 1*anin t*e direction of t*e /anetic field or t*e current causes t*e )ire to continuously

rotate about t*e center of rotation as s*o)n in Figure 6.13"

Fiure !";# =*en a )ire carryin a current is placed )it*in a /anetic field it )ill e8perience a

force in a direction nor/al to a plane for/ed by t*e current and t*e field

•

In practice to acco/plis* t*is c*ane in t*e current eit*er a set of co//utators and brus*es are used for 01 /otors"

• T*e current is electronically s)itc*ed for 01 brus*less /otors or %1 current is used for

%1 /otors"

T*is is t*e /asic pri'cipe be*ind all electric /otors"

T*ere are /any types of electric /otors t*at are used in robotics" T*ey include t*e follo)in:

• DC !otors

• Reersi/e AC !otors

• Brus"ess DC !otors

•

Stepper !otors

Sero!otor



• 36cept &or stepper !otors all ot*er types of /otors can be used as a sero!otor"

• In eac* case t*e tor7ue or po$er output of t*e /otor is a function of t*e strent* of t*e

/anetic fields and t*e current in t*e )indins"

• &o/e /otors *a'e per!a'e't !a#'ets (4s," T*ese /otors enerate less *eat since t*e

&ied is a$ays prese't and 'o curre't is needed to build t*e/" er!a'e't !a#'et

/otors t*e field is co'sta't" %dditionally under certain conditions it is possible t*at t*e

per!a'e't !a#'et /ay et da!a#ed and lose its field strent* in )*ic* case t*e /otor beco/es useless" T*is is because t*e iron /ass around t*e /anet *olds t*e field intact

until t*ey are separated" Aou s*ould ne'er take a /otor apart as t*e per/anent /anet

)ill beco/e sinificantly )eaker" To increase t*e strent* of t*e per/anent /anets in

/otors /ost /anufacturers /aneti9e t*e /anets after asse/blin t*e /otor" otors

)it*out per/anent /anets do not *a'e t*is proble/

• ?t*ers *a'e a so&t iro' core and $i'di'#s )*ere an electric curre't creates t*e

/anetic field" In t*is case /ore "eat is #e'erated but )*en needed t*e !a#'etic &ied

can be aried by c*anin t*e current""

01 /otor %1 /otor

Fiure !";< Heat dissipation pat* of /otors

?ne i/portant issue in t*e desi#' a'd operatio' of all /otors is t"e dissipatio' o& "eat"

•

%s )it* t*e *eat ererated in /any ot*er de'ices t*e enerated *eat in /otors e'entually beco/es t*e decidi'# &actor about its si8e a'd po$er"

• T*e *eat is enerated pri/arily fro/ t*e resistance of t*e )irin to electric current (load

related, but includes *eat due to iron losses includin eddy current losses and *ysteresis

losses friction losses brus* losses and s*ort-out circuit losses (speed related, as )ell"

• T*e *i*er t*e current t*e /ore *eat is enerated as W = RI 2"

• T*icker )ires enerate less *eat but are /ore e8pensi'e are *ea'ier (/ore i'ertia, and

re5uire /ore space"

• %ll /otors enerate so/e *eat" Ho)e'er )*at is i/portant is t*e pat* t*at t*e *eat /ust

take to lea'e t*e /otor since if t*e *eat is dissipated faster /ore enerated *eat can be

dissipated before da/ae occurs"

i#ure *+9- s*o)s t*e *eat leakae pat* to t*e en'iron/ent for an AC%type /otor and a DC%type

/otor"

In DC%type /otors

• t*e rotor contains t*e )indin and carries t*e current and t*us *eat is enerated in t*e

rotor"

• T*is *eat /ust o fro/ t*e rotor t*rou* t*e air ap t*rou* t*e per/anent /anets

t*rou* t*e /otor>s body and be dissipated into t*e en'iron/ent" (It /ay also o t*rou*

t*e s*aft to t*e bearins and out,"

• &ince air is a 'ery ood isolator"

• T*us t*e total *eat transfer coefficient for t*e DC !otor is relati'ely lo)"01 /otors are

• 'ery co//on in industry and



• *a'e been used for a lon ti/e"

• %s a result t*ey are reia/e sturdy and relati'ely po$er&u"

In 01 /otors

• t*e stator is a set of fi8ed per/anent /anets creatin a fi8ed /anetic field

• )*ile t*e rotor carries a current"

• T*rou* brus*es and co//utators t*e direction of current is c*aned continuously

causin t*e rotor to rotate continuously"• 1on'ersely if t*e rotor is rotated )it*in t*e /anetic field a 01 current )ill de'elop and

t*e /otor )ill act as a enerator" (T*e output is 01 but not constant,"

?n t*e ot*er *and

In an AC type /otor

• t*e rotor is a Per/anent anet and t*e )indin is in t*e stator

• T*e enerated *eat in t*e stator is dissipated to t*e air by conduction t*rou* t*e /otor>s

body"

• %s a result t*e total *eat transfer coefficient is relati'ely *i* especially because no air

ap e8ists"• %s a result AC%type /otors can be e8posed to relati'ely *i*er currents )it*out da/ae

and t*us t*ey are enerally /ore po)erful for t*e sa/e si9e"

• &tepper /otors alt*ou* not %1 /otors *a'e a si/ilar constructionB

• T*e rotor is a per/anent /anet and t*e stator contains t*e )indins" T*us stepper

/otors *a'e ood *eat dissipation capability"

• ?f course anot*er /aor factor in t*e difference bet)een /rus"ed and /rus"ess /otors

is t*e i&e of t*e brus*es and co//utators as )ell as t*e p*ysical li/itation of /ec*anical

s)itc*in by brus*es"

• Brus"ess DC !otors AC !otors and stepper !otors are all /rus"ess and t*us t*ey

and are sturdy and enerally *a'e o'# i&e (only li/ited by t*e life of rotor bearins,

If per/anent /anets are used to enerate t*e /anetic field t*e output tor5ue T M is proportional

to t*e /anetic flu8 and t*e current in t*e rotor )indins I rtr " T*en

(!";$,

)*ere ! t is a constant" &ince in per/anent /anets t*e flu8 is constant t*e output tor5ue beco/es

a function of I rtr and to control t*e output to5ue I (or correspondin 'oltaes, /ust be c*aned"

If instead of per/anent /anets soft iron cores )it* )indins are used for t*e stator as )ell t*en

t*e output tor5ue is a function of currents in bot* t*e rotor and t*e stator )indins:

(!";;,

Here bot* ! t and ! " are constants"

T*rou* t*e use of po)erful /anets /ade of rare eart* /aterials and alloys t*e perfor/ance of

/otors *as been i/pro'ed sinificantly" %s a result t*e po)er to-)ei*t ratio of /otors is /uc*

better t*an before and t*ey *a'e replaced al/ost all ot*er types of actuators"

AC 4otors



• .lectric %1 /otors are si/ilar to 01 /otors e8cept t*at t*e rotor is per/anent /anet

t*e stator *ouses t*e )indins and all co//utators and brus*es are eli/inated"

• T*is is possible because t*e c*anin flu8 is pro'ided by t*e %1 current not by

co//utation"

• %s t*e flu8 enerated by t*e %1 current c*anes t*e rotor follo)s it and rotates"

• %s a result %1 /otors *a'e fi8ed no/inal speeds a function of t*e nu/ber of poles on

t*eir rotor and t*e line fre5uency (!$ H9,"• &ince %1 /otors can dissipate *eat /ore fa'orably t*an 01 /otors t*ey can be /ore

po)erful"

• T*e sa/e principles of back-e/f *old for %1 /otors" T*ere are also re'ersible %1 /otors

a'ailable"

Brus"ess DC 4otors

• 4rus*less 01 /otors are a "y/rid bet)een %1 /otors and 01 /otors"

• %lt*ou* not e8actly t*e sa/e t*eir construction is 'ery si/ilar to an %1 /otor"

•T*e /aor difference is t*at brus*less 01 /otors are operated )it* an electronicallys)itc*ed 01 )a'efor/ t*at is si/ilar to an %1 current (eit*er sine )a'e or trape9oidal

)a'efor/, but is not necessarily at !$ H9"

• %s a result unlike %1 /otors 01 brus*less /otors can be operated at any speeds

includin 'ery lo) speeds"

• To operate a feedback sinal is necessary to deter/ine )*en to s)itc* t*e direction of t*e

current" In practice a resol'er an optical encoder or a Hall .ffect sensor attac*ed to t*e

rotor sends a sinal to a controller )*ic* s)itc*es t*e current to t*e rotor"

• For s/oot* operation and al/ost constant tor5ue t*e rotor usually *as t*ree p*ases in it"

T*us t*ree currents )it* ;2$C p*ase s*ift are fed into t*e rotor"

•

4rus*less 01 /otors are operated by a co'troer circuit" T*ey )ill not operate if youconnect t*e/ directly to a 01 po)er source

Direct%Drie 3ectric 4otors

• 0irect-dri'e electric /otors are 'ery si/ilar in construction to brus*less 01 /otors or

steppers"

• T*e /aor difference is t*at t*ey are desined to deli'er a 'ery lare tor5ue at 'ery lo)

speeds and )it* 'ery *i* resolution"

• T*ese /otors are intended to be used directly )it* a oint )it*out any ear reduction"

• 0irect-dri'e /otors are still 'ery e6pe'sie and 'ery "eay but *a'e i/pressi'e

c*aracteristics" % <$-D /otor produces a continuous tor5ue of ;3$ N/ at a /a8i/u/ #

rps )it* a resolution of #$ arc-sec"

Sero!otors

• %n i/portant issue in all electric /otors is t*e back electro/oti'e force or back e/f"

• a )ire carryin a current )it*in a /anetic field )ill e8perience a force )*ic* causes it to

/o'e"

• &i/ilarly if a )ire (conductor, /o'es )it*in a /anetic field suc* t*at it )ill cross t*e

field lines a current )ill be induced into t*e conductor"

• T*is is t*e basic principle of electric po)er eneration"



• Ho)e'er it also /eans t*at )*en t*e )ires of t*e )indins in a /otor are rotatin in t*e

/anetic field of t*e /anets a current )ill be induced in t*e/ in t*e opposite direction

of t*e input current"

• T*is current is called back-e/f and it tends to reduce t*e effecti'e current of t*e /otor"

• T*e faster t*e /otor rotates t*e larer t*e back-e/f is"

• 4ack-e/f current is usually e8pressed as a function of rotor speed:

(!";2,

Here # $ is typically i'en in 'olts per ;$$$ rp/" %s t*e /otor approac*es its no/inal no-load

speed t*e back-e/f is lare enou* t*at t*e /otor speed )ill stabili9e at t*e no/inal no-load

speed )it* its correspondin effecti'e current" Ho)e'er at t*is no/inal speed t*e output tor5ue

of t*e /otor is essentially 9ero" T*e /otor>s 'elocity is o'erned by

(!";#,

)*ere R is t*e )indins> resistance"

• If a oad is applied to t*e /otor it )ill slo) do)n resultin in s/aller back-e/f larer

effecti'e current and conse5uently a positi'e net tor5ue"

• T*e ar#er t"e oad t*e slo)er t*e /otor )ill rotate in order to de'elop t*e ar#er

tor7ue"

• If t*e load beco/es increasinly larer t*ere co/es a ti/e )*en t*e !otor stas t*ere is

no back-e/f t*e effecti'e current is at its /a8i/u/ and t*e tor5ue is at its /a8i/u/"

• Unfortunately in eac* case )*en t*e back e/f is s/aller alt*ou* t*e output tor5ue is

larer since t*e net current is larer so is t*e enerated "eat"

•Under sta or 'ear%sta conditions t*e enerated "eat /ay be lare enou* to da!a#et"e !otor"

To be able to de'elop ar#er tor7ues $it"out so$i'# do$' t"e !otor on )ould *a'e to

increase t*e current (to t*e rotor t*e stator or bot* if soft iron core is used," In suc* a case

alt*ou* t*e /otor rotates at t*e sa/e speed and t*e back e!& is sti t*e sa/e t*e larer current

)ill increase t*e net effecti'e current and t*us t*e tor5ue" 4y aryi'# t"e curre't (or

correspondin 'oltae, t*e speed tor7ue balance can be /aintained as desired" T*is syste/ is

called a %er&'tr "



Fiure !";! &c*e/atic of a sero!otor controller" % sensor sends 'elocity and position sinals to

t*e controller )*ic* controls t*e output 'elocity and position of t*e ser'o/otor

T*e output tor5ue T de'eloped by t*e /otor can be e8pressed as a function of t*e tor5ue constant

# T typically in (.i)/*'p as in T = I # T " T*e opposin tor5ues are friction tor5ue T " 'iscous

da/pin tor5ue (as a function of speed, )# D and t*e opposin load T +" T*us t*e /otor>s tor5ue

is o'erned by

(!";<,

.5uations (!";2, (!";#, and (!";<, o'ern t*e /otions of t*e /otor"

% sero!otor is

• a 01

• %1

• brus*less or

• e'en stepper/otor )it* feedback t*at can be controlled to /o'e at a desired speed (and

conse5uently tor5ue, for a desired anle of rotation"

• To do t*is a feedback de'ice sends sinals to t*e controller circuit of t*e ser'o/otor

reportin its anular position and 'elocity"

• If as a result of *i*er loads t*e 'elocity is lo)er t*an desired set 'alue t*e current is

increased until t*e speed is e5ual to t*e desired 'alue"

• If t*e speed sinal s*o)s t*at t*e 'elocity is larer t*an desired t*e current is reduced

accordinly"

• If position feedback is used as )ell t*e position sinal is used to s*ut off t*e /otor as t*e

rotor approac*es t*e desired anular position"

• /any different types of sensors /ay be used for t*is purpose includin encoders

resol'ers potentio/eters and tac*o/eters"

• If a position sensor suc* as a potentio/eter or encoder is used its sinal can be

differentiated to produce a 'elocity sinal" Figure 6.16 is a sc*e/atic of a si/ple control

block diara/ for a ser'o/otor"

Stepper 4otors



• &tepper /otors are ersatie o'#%asti'# si!pe /otors t*at can be used in /any

applications"

• In /ost applications t*e stepper /otors are used $it"out &eed/ack " T*is is because

unless a step is /issed a stepper /otor steps a kno)n anle eac* ti/e it is /o'ed" T*us

its anular position is al)ays kno)n and 'o &eed/ack is necessary"

•&tepper /otors co/e in /any di&&ere't &or!s and pri'cipes o& operatio'"

• &tepper /otors can be used in di&&ere't !odes by $iri'# t*e/ differently"

• 1onnect a stepper /otor to po)er it )ill 'ot rotate and it rotates only )*en t*e /anetic

field is rotated t*rou* its different )indins"

• =*en 'ot po$ered steppers *a'e a residua tor7ue called detent tor5ue and !a6i!u!

tor7ue is de'eloped )*en t*ey do not turn"

• It re5uires an e8ternal tor5ue to turn a stepper /otor e'en )*en not po)ered

• %ll stepper /otors need a !icroprocessor or drier:co'troer- (i'de6er, circuit for

rotation"

• &tepper /otors are used e8tensi'ely in 'o'i'dustria robots and robotic deices as )ell

as in ot*er de'ices t*at are used in conunction )it* robots fro/ toolin /ac*ines to perip*eral de'ices and fro/ auto/atic /anufacturin to control de'ices"

Structure o& Stepper 4otors

%1 /otor Fiure !";< Heat dissipation pat* of /otors

?n t*e ot*er *and

• In an AC type /otor t*e rotor is a er!a'e't 4a#'et and t*e )indin is in t*e stator

• T*e enerated *eat in t*e stator is dissipated to t*e air by conduction t*rou* t*e /otor>s

body"

• %s a result t*e total *eat transfer coefficient is relati'ely *i* especially because no air

ap e8ists"

• %s a result AC%type /otors can be e8posed to relati'ely *i*er currents )it*out da/ae

and t*us t*ey are enerally /ore po)erful for t*e sa/e si9e"

• &tepper /otors alt*ou* 'ot AC !otors; *a'e a si!iar co'structio'B t*e rotor is aper!a'e't !a#'et and t*e stator contains t*e $i'di'#s"

• T*us stepper /otors *a'e ood *eat dissipation capability"

• &ince t*e *eat enerated in t*e coils can easily dissipate t*rou* t*e /otor>s body stepper

/otors are ess susceptible to "eat da!a#e and since t*ere are 'o /rus"es or

co!!utators t*ey *a'e o'# i&e"

• In different types of stepper /otors t*e per!a'e't !a#'et rotors are different"

• T*e rotor follo)s a /o'in /anetic field enerated by t*e coils" %s a result so/e)*at

si!iar to /ot" AC !otors and /rus"ess DC /otors a rotor follo)s a /o'in flu8

under t*e control of a controller or dri'er"

ri'cipe o& Operatio'

I/aine a stepper /otor )it* t$o cois in its stator and a per!a'e't !a#'et as its rotor

as s*o)n in i#ure *+90"



=*en eac* of t*e coils of t*e stator is eneri9ed t*e per/anently

/aneti9ed rotor (or soft iron in 'ariable reluctance /otors, )ill

rotate to ai#' itself )it* t*e stator /anetic field"

Fi" !";7 (a,

T*e rotor )ill stay at t*is position unless t*e field rotates" %s t*e

po)er to t*e present coil is disrupted and is directed to t*e ne8t

coil t*e rotor )ill rotate aain to alin it self )it* t*e field in t*e

ne) position"

Fi" !";7 (b,

.ac* rotation is e5ual to t*e %tep *)gle )*ic* /ay 'ary fro/ 1,- degree% to as little as a "r*ti) of

a degree" (In t*is e8a/ple it is 6$C,"

T*e first coil )ill once aain be turned on /ut i' t"e opposite

poarity )*ile t*e second is turned off" T*is )ill cause t*e rotor to

rotate anot*er step in t*e sa/e direction"

Fi" !";7 (c,

Ne8t coil )ill once aain be turned on /ut i' t"e opposite

poarity )*ile t*e first is turned off" T*is )ill cause t*e rotor to

rotate anot*er step in t*e sa/e direction

Fi" !";7 (d,

T*e process continues as one coil is turned off and an ot*er is turned on" % se5uence of four steps )ill

brin t*e rotor back to e8actly t*e sa/e state it )as at t*e beinnin of t*e se5uence"

No) i/aine t*at at t*e conclusion of t*e first step

• Instead of tur)i)g "" )e il and tur)i)g ) te %e)d i l t*at bt 0uld be tur)ed

)"



In t*at case t*e rotor )ould only rotate <3C to alin itself

to t*e pat* of least reluctance (Figure 6.1,"

+ater if t*e first coil is turned off )*ile t*e second

re/ains on t*e rotor )ill rotate anot*er <3C"

T*is is called *alfstep operation and includes a se5uence of eigt '&e'e)t%"

?f course )it* t*e opposite on-off se5uence t*e rotor )ill rotate in t*e opposite direction"

ost industrial steppers run bet)een 9+1 to 0+. derees at full steppin" ?b'iously to reduce t*e

si9e of t*e steps t*e nu/ber of poles /ay be increased" Ho)e'er t*ere is a p*ysical li/it to *o)

/any poles /ay be used" To furt*er increase t*e nu/ber of steps per re'olution different

nu/bers of teet* can be built into t*e stator and rotor creatin an effect si/ilar to a caliper"For instance 3$ teet* on t*e rotor and <$ teet* on t*e stator )ill result in a ;"-deree step

anle )it* 2$$ steps per re'olution"

U'ipoar; Bipoar; a'd Bi&iar Stepper 4otors

U'ipoar stepper !otors are desined to )ork )it* one po)er source" Eenerally it is desirable

to *a'e one po)er source po)erin bot* t*e /otor )indins as )ell as t*e dri'e circuits" Unless

ot*er tec*ni5ues of s)itc*in are used )it* one po)er source it is i/possible to si/ply c*ane

t*e polarity of t*e coils by c*anin t*e polarity of t*e po)er fro/ t*e po)er source as t*is

)ould ruin t*e electronic dri'e circuitry" &ince t*e polarity of t*e coils cannot be c*aned t*ese

/otors /ay not be run in *alf-step /ode" Ho)e'er t*ere is only one po)er source used )*ic*

reduces t*e cost and t*e /otor )ill de'elop it full po)er"

In /ipoar !otors t*e assu/ption is t*at t*e polarity of t*e po)er source can be c*aned" %s a

result t*ere )ill eit*er be t)o po)er sources )*ere one po)ers t*e /otor )indins and its

polarity can be s)itc*ed and one is used to po)er t*e dri'e circuitry or t*at /ore sop*isticated

s)itc*in is used )it* one po)er supply to not da/ae t*e circuits" In t*is case t*e /otor )ill

re5uire /ore po)er supplies or electronics but can be run eit*er at full or *alf-step /odes and

)ill de'elop its full po)er"

In bifilar /otors t*e coils are center tapped as )as pre'iously discussed" In t*is case t*e polarity

of t*e coils can be c*aned si/ply by flo)in t*e current in eac* *alf of t*e coil" T*us )it*



si/ple circuits and one po)er supply t*e /otor can be run in eit*er full or *alf-step /ode but

since only *alf t*e coil is eneri9ed t*e /otor only de'elops *alf of its full po)er" Figure 6.2 is

a sc*e/atic dra)in for u'ipoar and /ipoar dri'e circuits" T*e s)itc*es are connected to t*e

/icroprocessor ports and are turned on and off by t*e /icroco/puter"

Fiure !"27 &c*e/atic dra)in for unipolar and bipolar dri'e circuits"

ost /otors are )ired to be used in any of t*e t*ree /odes" &o/e /otors are not" Figure 6.2,

s*o)s t*e sc*e/atic of different possibilities of coil )irin co/binations in /otors" In ei*t-lead

confiuration you /ay )ire t*e /otor in any /ode" as t*e coils are co/pletely detac*ed fro/

eac* ot*er" In t*is case it is also easy to find )*ic* t)o )ires are connected to )*ic* coil" T*e

si8-lead /otors are connected suc* t*at eac* coil is center tapped but t*e t)o coils are detac*ed"

In fi'e-lead /otors t*e t)o center-taps are connected toet*er" Four-lead /otors /ay not be used

in /i&iar /ode" 4y /easurin t*e resistance bet)een different )ires one /ay find )*ic* )ire is

connected to )*ic* coil"

Fiure !"2 &tepper /otor lead confiurations"

Stepper 4otor Speed%Tor7ue C"aracteristics

• &tepper /otors are 'ery useful in /any applications" T*ey do not re5uire any feedback as

it is assu/ed t*at stepper /otors ad'ance a kno)n anle e'ery ti/e a sinal is sent" %s

lon as t*e oad o' t"e !otor is ess t*an t*e tor5ue it can deli'er t*e steps )ill 'ot /e

!issed and t*is assu!ptio' is correct"

• if t*e oad is too ar#e or if t*e speed is !ore t*an t*e /otor is capable of rotatin steps

/ay be !issed and since t*ere is 'o &eed/ack all subse5uent positions )ill be $ro'#"

• &tepper /otors de'elop t*eir !a6i!u! tor7ue called "odi'# tor7ue at 9ero anular

'elocity )*en t*e rotor is statio'ary" (T*e tor5ue de'eloped )it* N? P?=.R is called

0etent Tor5ue",

• %s t*e speed o& t"e !otor i'creases t*e tor7ue it de'elops reduces sinificantly" T*us

it is 'ery i/portant t*at t*e desi#'er c*eck t*e speed%tor7ue c"aracteristic of stepper



/otors fro/ /anufacturers before a c*oice is /ade" i#ure *+2< is a typical speed-tor5ue

c*aracteristic"

Fiure !"26 % typical speed-tor5ue

• ?ne reason for t*is poor perfor/ance at "i#"er speeds is t*at since at eac* step t*e rotor

/ust first acceerate cruise and t*en deceerate and stop at t*e ne8t step and t*at t*is

process /ust be repeated for e'ery sinle step stepper /otors cannot rotate fast"

• If t*e si#'as co/in to t*e /otor are too &ast t*e rotor )ill 'ot *a'e t*e ti!e to

acceerate:deceerate and )ill !iss steps"

• &o one reason is t*e i'ertia of t*e rotor"

• Ho)e'er e'en /ore i/portant is t*e ater'ati'# !a#'etic &ieds of t*e stator"

• .'ery ti/e a coil is turned off t*ere )ill be a c*anin (in t*is case a decayin, flu8"

• T*e )ires of t*e coils in t*e presence of a c"a'#i'# &u6 )ill de'elop a /ack%e!& t*at

)ill slo) do)n t*e decay of t*e flu8 as a ne) one builds up due to t*e /ack%e!& current"

• T*e enerated flu8 )ill tend to "odG t*e rotor fro/ rotatin and )ill slo) it do)n" %s a

result t*e rotor is pulled back and )ill not be able to rotate freely"

• To re!edy t*is proble/ and to pre'ent *i*-current sparkin across t*e s)itc*es a

free)*eelin diode /ay be added to t*e circuit" T*e diode )ill allo) t*e current to

continue flo)in t*rou* t*e coil and be dissipated t*rou* con'ersion to *eat in t*e

resistor" T*e effecti'eness of t*e process can be increased by addin a 9ener diode to t*e

circuit" It is i/portant to reali9e t*at t*e 9ener diode>s breakdo)n 'oltae /ust be near t*e

transistor>s breakdo)n 'oltae ratin" ?t*er)ise it )ill *a'e no effect"

Documents

Unit 7 Actuators NIKU Parted 2