stepper working

8/3/2019 Stepper Working

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StepperMot or TheoryofOperation

Stepper motors provide a mea ns forprecisepositioning and speed control without the use offeedbacksensors. The basicoperationofa steppermotor allows the shaft to move a precisenumberofdegrees eachtime a pulseofelectricity issent to the motor. Since the shaft ofthe motor moves only the number ofdegreesthat it was designedforwhen each pulseisdelivered, you can control the pulses that are sent and control the

positioning and speed. The rotor ofthe motorproduces torque from the interactionbetween the magneticfieldin the stator and rotor. The strength ofthe magneticfieldsisproportional to the amount ofcurrent sent to the

stator and the numb erofturns in the windings.

The steppermotor uses the theory ofoperationformagne ts to make the motor shaft turn a precisedistancewhen a pulseofelectricity isprovided. You lea rnedpreviously that likepolesofa magnet repel and unlike

polesattract. Figure 1 shows a typicalcross -sectional view ofthe rotor and statorofa steppermotor. Fromthisdiagram you can see that the stator ( stationary winding) has eightpoles, and the rotor has sixpoles(threecompletemagnet s). The rotor will require 24pulsesofelectricity to move the 24 steps to make onecomplete revolution. Another way to say thisis that the rotor will move precisely 15 foreach pulseofelectricity that the motor receives. The number ofdegrees the rotor will turn when a pulseofelectricity isdelivered to the motor can be calculated by dividing the numb erofdegrees in one revolutionofthe shaft(360) by the numberofpoles (north and south) in the rotor. In thisstepper motor 360 isdivided by 24 to get15.

When no power isapplied to the motor, the residualmagnetismin the rotor magnets willcause the rotor todetent oralign one set ofitsmagneticpoles with the magneticpolesofone ofthe stator magnets. Thismeansthat the rotor will have 24 possibledetentpositions. When the rotor isin a detent position,it will have enoughmagneticforce to keep the shaft frommoving to the next position. Thisis what makes the rotor fee llikeit isclickingfrom one position to the next as you rotate the rotor by hand with no power applied.

Fig 1. Diagram that shows the position of the six-pole rotor and eight -pole stator of a typical steppermotor.

When power isapplied,it isdirected to only one ofthe stator pairsof windings, whichwillcause that windingpair tobecome a magnet. One ofthe coilsforthe pairwillbecome the North Pole, and the other willbecomethe South Pole. When thisoccurs, the stator coil that is the North Polewill attract the closest rotor tooth thathas the oppositepolarity, and the statorcoil that is the South Polewill attract the closest rotor tooth that hasthe oppositepolarity. When current isflowing through thesepoles, the rotor willnow have a much strongerattraction to the stator winding, and the increased torque iscalledholding torque.


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By changing the current flow to the next stator w inding, the magneticfieldwillbe changed 45. The rotor willonly move 15beforeitsmagneticfieldswillagainalign with the change in the stator field. The magneticfieldin the stator iscontinually changed as the rotormoves through the 24 steps to move a totalof360. Figure2shows the positionofthe rotorchangingas the current supp lied to the stator changes.

FIGURE 2. Movement of the steppermotor rotor as curr ent ispu lsed to the stator. (a) Curr ent isapplied to the A and A windings, so the A winding isnorth, (b) Current i sapplied to B and Bwindings,so the B winding isnorth, (c) Current isapplied to the C and C windings,so the C wind ingisnorth, (d) Curr ent isapplied to the D and D windings so the D winding isnorth. (e) Curr ent isapplied to the A and A windings, so the A winding isno rth.

In Fig. 2a you can see that when current isapplied to the A and Astator windings, they willbecome a magnetwith the top part ofthe windingbeing the North Pole, and the bottom part ofthe windingbeing the South Pole.You shouldnotice that thiswill cause the rotor to move a smallamount so that one ofitssouthpolesisaligned with the north stator pole (at A), and the opposite end ofthe rotorpole, whichis the north pole, wil l

align with the south poleofthe stator (at A). A lineisplaced on the south-polepieceso that you can follow itsmovement as current is moved from one stator w inding to the next. In Fig. 2b current has been turned off tothe A and A windings, and current is now applied to the stator windingsshown at the B and Bsidesof themotor. When thisoccurs, the stator winding at the Bpositionwill have the polarity forthe south poleof thestator magnet, and the winding at the Bpositionwillhave the north-polepolarity.In thiscond ition, the nextrotorpole that willbe able to align with the stator magnets is the next polein the clockwiseposition to the

previouspole. Thismeans that the rotor willonly need to rotate 15 in the clockwisepositionfor thisset ofpoles to alignitselfso that it attra cts the stator poles.

In Fig. 2c you can see that the C and Cstator w indings are againenergized, but this time the C windingis the


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north poleofthe magneticfield and the C windingis the south pole. Thischange inmagneticfieldwill causethe rotor to again move 15 in the clockwisepositionuntilitspoleswillalign with the C and Cstator poles.You shouldnotice that the original rotorpole that was labeled 1 now moved three steps in the clockwise

position.

In Fig.2d you can see that the D and Dstator windings are energized, the winding at Dpositionis the northpole. Thischange inpolarity willcause the rotor to move anoth er15 in the clockwisedirection. You shou ldnotice that the rotor has moved foursteps of15 each, whichmeans the rotorhas moved a totalof60 fromitsoriginalposition. This can be verified by the positionofthe rotorpole that has the line on it, whichisnow

pointing at the stator w inding that islocatedin the 2 o'clockposition.

In Fig.2e you can see that the A and Astator windings are energized, the winding at Apositionis the southpole. Thischange inpolarity willcause the rotor to move anoth er15 in the clockwisedirection. You shou ldnotice that the rotor has moved foursteps of15 each, whichmeans the rotorhas moved a totalof75 fromitsoriginalposition. Thus the sequence ofenergizing ABCDA willmove the rotor in the clockwisedirection.Itcan be easily verified that forthe count erclockwisedirection the seque nce should be ADCBA.

Stepper Motor Switching Sequence

The steppermotor can be operated in three different steppingmodes, namely,full-step,half-step, and micro-step.

Full -Step

The steppermotor uses a four-stepswitchingsequence, whichiscalled a full-stepswitchingsequencewhich isalready describedabove.

Hal f-Step

Another switchingsequence forthe stepper motor iscalled an eight-step orhalf-stepsequence. The switchingdiagramforthe half-stepsequ ence isshown inFig. 3. The mainfea tureof thisswitchingsequence is that youcan doub le the resolutionofthe stepp ermotor by causing the rotor to move halfthe distanceit does when thefull-stepswitchingsequence isused. Thismea ns that a 200-step motor, whichhas a resolutionof1.8, wil lhave a resolutionof400 steps and 0.9. The half-stepswitchingseque nce requires a specialstepper motor

controller, but it can be used with a standard hybrid motor. The way the controllergets the motor to reach thehalf-stepis to energize bothphases at the same time withequalcurrent.

FIGURE 3 The switching sequ ence for the eigh t-stepinput (half- step mode).

In thissequence the first step has SW1 is on, and SW2,SW3 and SW4 are off. The sequence forthe first stepis the same as the full-stepsequence. The second step has SW1 and SW2 are on and allofthe remaining


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switches are off. Thisconfigurationofswitchescauses the rotor to move an additionalhalf-stepbecause it isacted upon by two equalmagneticforces and the rotor turns to the equil ibriumposition whichishalfa stepangle. The thirdstep has SW2 is on, and SW1, SW4 and SW3 are off, whichis the same as step 2 ofthe full-step sequence. The sequence continuesforeight steps and then repea ts. The maindifferencebetween thissequence and the full-stepsequence is that the energizingsequence forhalfstep is A AB B BC C CD D DA.

Mi cro StepMode

The full-step and half-stepmotors tend to be slightly jerky in theiroperationas the motor moves fromstep to

step. The amount of resolutionisalsolimited by the numb erofphysicalpoles that the rotor can have. Theamount of resolution (number ofsteps) can be in-creased by manipulating the current that the controllersends to the motor duringeach step. The current can be adjustedso that it lookssimilar to a sine wave.Figure 4 shows the waveformforthe current to each phase. From thisdiagram you can see that the currentsent to each ofthe foursetsof windingsis timedso that there isalways aphase difference witheach other.The fact that the current to each individualphase increases and decreases like a sine wave and that isalwaysout of time with the otherphase willallow the rotor to reach hundreds ofintermediatesteps. In fact it is

possibleforthe controller to reach as many as 500 microsteps fora full-stepseque nce, whichwillprovide100,000 steps foreach revolution.

FIGURE 4. Phase-currentdiagram for a steppermotor control lerin micro step mode.

Types ofStepper Motors

StepperMotorsOverview

A stepper, orstepp ing motor converts electronicpulsesintoproportionatemechanicalmovement. Eachrevolutionofthe stepper motor's shaft ismade up ofa seriesofdiscreteindividualsteps. A step isdefinedasthe angu lar rotationproducedby the output shaft each time the motor receives a step pulse.These types ofmotors are very popu larindigitalcontrolcircuits,such as robotics,because they are idea lly suitedforreceivingdigitalpulsesforstep control. Each step causes the shaft to rotate a certain number ofdegrees. Astep angle represents the rotationofthe output shaft caused by each step, measured indegrees. Figure5il lustrates a simpleapplicationfora steppermotor. Each time the controller receives an input signal, the paperisdriven a certainincrementaldistance.In addition to the paper drivemechanismin a printer,stepp ermotorsare alsopopularinmachine tools,process controlsystems, tape and diskdrivesystems, and programmable

controllers.


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Figure 5. Paper drive mechanism using steppermachine

The most populartypes ofstepper motors are permanent- magnet (PM) and variable reluctance(VR).

Permanent-magnet (PM) StepperMotors

The per manent-magn et steppermotor operates on the reactionbetween a permanent-magnet rotor and anelectromagneticfield.Figure 6 shows a basic two-pole PM stepp ermotor. The rotor shown inFigure 6(a) hasapermanent magnet mounted at each end. The statorisil lustratedinFigure 6(b). Both the stator and rotor

are shown as havingteeth. The teeth on the rotorsurface and the stator polefaces are offset so that there wil lbe only a limitednumberofrotor teeth aligning themselves with an energizedstatorpole. The numb erof tee thon the rotor and stator determine the step angle that will occur each time the polarity ofthe windingisreversed. Greater the number ofteeth, smaller the step angle.

Figure 6 Componen ts of a PM stepp er motor: (a) Rotor; (b) stator

When a PM stepper motor has a steady DC signalapplied to one stator winding, the rotor willovercome theresidual torque and line up w ith that stator field. The holding torque isdefinedas the amount of torquerequired to move the rotor one full step with the statorenergized. An important characteristicofthe PMsteppermotor is that it can maintain the holding torque indefinitely when the rotor isstopped. When no powerisapplied to the windings, a smallmagneticforceisdevelopedbetween the permanent magnet and the

stator. Thismagneticforceiscalled a residual, ordetent torque. The detent torque can be noticedbyturning a stepper motor by hand and isgenerally about one-tenth ofthe holding torque.

Figure 7(a) shows a permanent magnet steppermotor withfourstator windings. By givingpulses the statorcoilsin a desiredsequence, it isposs ible to control the speed and directionofthe motor. Figure 7(b) showsthe timingdiagramforthe pulses required to rotate the PM steppermotor il lustratedinFigure 7(a). Thissequenceofpositive and negativepulsescauses the motor shaft to rotate counterclockwisein 90 steps. ThewaveformsofFigure 7(c) illustrate how the pulses can be overlapped and the motor made to rotatecounterclockwiseat 45 intervals.


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Figure 7 (a) PM steppermotor; (b) 90 step; (c) 45 step.

A more recent development in PM stepper motor technology is the thin-di sk rotor. T his type ofstepp ermotordissipates much less power inlossessuch as heat than the cylindrical rotor and as a result,it isconsiderablymore efficient.Efficiency is a primary concern inindustrialcircuitssuch as robotics,because a highly efficientmotor will run cooler and produce more torque orspeed foritssize. Thin-disk rotor PM stepper motors arealsocapableofproducingalmost double the stepsper second ofa conventional PM steppermotor. Figure8shows the basicconstructionofa thin-disk rotor PM motor. The rotor isconstructed ofa special type ofcobalt-steel, and the statorpoles are offset by one-halfa rotorsegment.

Figure 8. Thin-diskrotor PM steppermotor.

Variable-reluctance (VR) StepperMotors

The variable-reluct ance (VR) steppermotor differsfrom the PM stepperin that it has no permanent-magnetrotor and no residual torque to hold the rotor at one position when turned off. When the stator coilsareenergized, the rotor teeth will align with the energizedstator poles. This type ofmotor operates on the

principleofminimizing the reluctancealong the path ofthe appliedmagneticfield. By alternating the windingsthat are energizedin the stator, the stator fieldchanges, and the rotor is moved to a new position.

The statorofa variable-reluctancestepper motor has a magnetic core constructed with a stackofsteel


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laminations. The rotor ismade ofunmagnetizedsoft stee l with teeth and slots. The relationship among stepangle, rotor teeth, and stator teeth isexpressed using the followingequation:

= Ns

Nr 360 -----------------------------(1)NsNr

Where = step angle in degrees

Ns = Number of teeth on stator core

Nr = Number of teeth on rotor core

Figure 9 shows a basic variable-reluctancesteppermotor. In thiscircuit, the rotor isshown withfewer tee ththan the stator. Thisensures that only one set ofstator and rotor teeth willalign at any giveninstant. The

statorcoils are energizedingroups referred to as phases. In Figure 9, the stator has six teeth and the rotorhasfourteeth. According to Eq. (1), the rotor will turn 30 each time a pulseisapplied.Figure 9 (a) shows the

positionofthe rotor when phase A isenergized.As longas phase A isenergized, the rotor willbe heldstationary. When phase A isswitchedoffand phase B isenergized, the rotor will turn 30 until twopolesofthe rotor are aligned under the north and south polesestablished bypha se B. The effect of turningoffphaseB and energizingphase C isshown inFigure 9 (c). In thiscircuit, the rotor has again moved 30 and isnowaligned under the north and south polescreatedbypha se C. After the rot orhas been displaced by 60 fromitsstartingpoint, the step sequ ence has completed one cycle.Figure 9 (d) shows the switchingsequence tocomplete a full 360 of rotationfora variable-reluctance motor withsix stator poles and four rotorpoles. Byrepeating thispattern, the motor will rotate in a clockwisedirection. The directionofthe motor ischanged byreversing the pattern of turning ON and OFF each phase.

Figure 9. Variable-reluctance stepper motor and switching sequence .


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The VRstepper motors mentioned up to thispoint are allsingle-stackmotors. That is,all the pha ses arearranged in a singlestack, orplane. The disadvantageof thisdesignfora steppermotor is that the steps aregenerally quitelarge (above 15). Multistackstepper motors can produce smallerstep sizesbecause themotor isdividedalongitsaxiallengthintomagnetically isolatedsections, orstacks. Each ofthese sectionsisexcited by a sepa rate w inding, orphase. In this type ofmotor, each stack corresponds to a phase, and thestator and rotor have the same tooth pitch.

HybridStepperMotors

The hybrid step motor consistsoftwopiecesofsoft iron, as wellas an axially magnetized, roundpermanent-magnet rotor. The term hybridisderivedfrom the fact that the motor isoperated under the combined

principlesofthe permanent magnet and variable-reluctancestepper motor s. The stator core structure ofahybridmotorisessentially the same as its VRcounterpart. The maindifferenceis that in the VR motor, onlyone ofthe twocoilsofone pha se is wound on one pole, while a typicalhybrid motor will have coilsoftwodifferentphases wound on one the same pole. The two coils at a pole are wound in a configuration known as a

bifilarconnection. Each poleofa hybrid motor iscovered withuniformly spaced teeth made ofsoft stee l. Thetee thon the two sectionsofeach pole are misaligned with each other by a half-toothpitch. Torque is createdin thehybrid motor by the interactionofthe magneticfieldofthe permanent magnet and the magnetic fieldproduced by the stator.

Stepper motors are rated interms ofthe numb erofsteps per second, the steppingangle, and loadcapacity inounce-inches and the pound-inchesoftorque that the motor can overcome. The numb erofsteps per second

isalso known as the stepping rate. The actualspeed ofa stepp ermotor isdependent on the step angleandstep rate and isfound using the followingequation:

(s /s)N =

6 -------------------------------- (2)

Where N = motor speed in RPM

= step angle in degreess/s = number of steps per second

Figure 10 shows a plot ofthe relationshipbetweenpull-in torque versuspulses per second fora typical

stepper motor. From this curve, it isapparent that torque isgreat est at zero steps per second and decreasesas the numberofsteps increases.

Figure 10. Torque versus steps per second for a steppermotor.

The directionof rotationisdetermined by applying the pulses to either the clockwise orcoun terclockwisedrivecircuits. Rotor displacement can be very accurately repeated with each succeedingpulse.Steppingmotorsare generally operated without feedback, whichsimplifies the controlcircuit considerably. One ofthe most


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common stepper motor drivecircuitsis the unipolar drive,shown inFigure 11. Thiscircuit uses bifilarwindings and four Darlington transistors to control the directionof rotation and the stepping rate ofthe motor.

Figure 11. Unipolar steppermotor drive.

Stepper motor drivers are availableinhalf-step or full-stepconfigurations.Full-stepdrivers are the simplest indesign and have a controlsequence oftwo on-timeperiodsfollowed by two off-timeperiods. The half-stepmode ofoperationprovides a smoother, quieterperformance withhigherspeed capabil ity and efficiency.Figure 12(a) shows the switchingsequence wave shapes ofa typicalsteppermotor. Each stepper motorwindingisenergized one in every fourinpu tpulses.Consequently, the pulse trainforeach windinghas a 25

percent duty cycle. The steppermotor output shown inFigure 12(b) has a step angleof30.

Figure 12. Switching sequence waveshapes.


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Stepper Motor App lica tions

Steppe r motors are used in a widevariety ofapplicationsinindustry,including computerperipherals,business machines,motioncontrol, and robotics, which are includedinprocess control and machine toolapplications. A completelistofapplicationsis shown below.

Computer Peripherals

App lication Use

Floppy Disc positionmagneticpickup

Printer carriagedrive

Printer rotate character whee l

Printer paperfeed

Printer ribbon wind/rewind

Printer positionmatrixprint head

Tape Reader index tape

Plotter X-Y-Zpositioning

Plotter paperfeed

Business Machines

App lication Use

Card Reader positioncards

Copy Machine paperfeed

BankingSystems credit card positioning

BankingSystems paperfeed

Typewriters (automatic) head positioning

Typewriters (automatic) paperfeed

Copy Machine lenspositioning

Card Sorter route card flow

Process Control

App lication Use

Carburetor Adjusting air-fuelmixtureadjust

ValveControl fluid gas metering

Conveyor maindrive

In-ProcessGaging parts positioning

Assembly Lines parts positioning

Sil iconProcess ing I. C. waferslicing

I. C. Bond ing chippositioning

Laser Trimming X-Ypositioning

LiquidGasketDispensing

valve coverpositioning

Mail HandlingSystems feeding and positioning


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letters

Machine Too l

App lication Use

Milling Machines X-Y-Z tablepositioning

Drilling Machines X-Y tablepositioning

GrindingMachines downfeedgrinding whee l

GrindingMachines automatic whee ldressingElectron Beam Welder X-Y-Zpositioning

LaserCutting X-Y-Zpositioning

Lathes X-Ypositioning

Sewing X-Y tablepositioning

stepper working

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