IAM Stepper Motor Basics

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A stepper motor is an electromechanical device which converts electrical pulses into

discrete mechanical movements. The shaft or spindle of a stepper motor rotates indiscrete step increments when electrical command pulses are applied to it in the proper sequence. The motors rotation has several direct relationships to these applied input pulses. The sequence of the applied pulses is directly related to the direction of motor shafts rotation. The speed of themotor shafts rotation is directly related tothe frequency of the input pulses and thelength of rotation is directly related to thenumber of input pulses applied.

Stepper Motor Advantagesand Disadvantages

Advantages

1. The rotation angle of the motor isproportional to the input pulse.

2. The motor has full torque at stand-still (if the windings are energized)

3. Precise positioning and repeat-ability of movement since good

stepper motors have an accuracy of 3 – 5% of a step and this error isnon cumulative from one step tothe next.

4. Excellent response to starting/stopping/reversing.

5. Very reliable since there are no con-tact brushes in the motor.Therefore the life of the motor issimply dependant on the life of thebearing.

6. The motors response to digital

input pulses provides open-loopcontrol, making the motor simplerand less costly to control.

7. It is possible to achieve very lowspeed synchronous rotation with aload that is directly coupled to theshaft.

8. A wide range of rotational speedscan be realized as the speed isproportional to the frequency of theinput pulses.

Disadvantages

1. Resonances can occur if notproperly controlled.

2. Not easy to operate at extremelyhigh speeds.

Open Loop OperationOne of the most significant advantagesof a stepper motor is its ability to beaccurately controlled in an open loopsystem. Open loop control means nofeedback information about position isneeded. This type of controleliminates the need for expensivesensing and feedback devices such asoptical encoders. Your position isknown simply by keeping track of theinput step pulses.

Stepper Motor TypesThere are three basic stepper motortypes. They are :

• Variable-reluctance

• Permanent-magnet

• Hybrid

Variable-reluctance (VR)This type of stepper motor has beenaround for a long time. It is probablythe easiest to understand from astructural point of view. Figure 1shows a cross section of a typical V.R.stepper motor. This type of motorconsists of a soft iron multi-toothedrotor and a wound stator. When thestator windings are energized with DCcurrent the poles become magnetized.Rotation occurs when the rotor teethare attracted to the energized statorpoles.

Permanent Magnet (PM)Often referred to as a “tin can” or“canstock” motor the permanentmagnet step motor is a low cost andlow resolution type motor with typicalstep angles of 7.5° to 15°. (48 – 24steps/revolution) PM motors as the

Figure 1. Cross-section of a variable-

reluctance (VR) motor.

Industrial Circuits Application Note

Stepper Motor Basics

Figure 2. Principle of a PM or tin-can stepper motor.

Figure 3. Cross-section of a hybrid stepper motor.

15°

A

B

C

D

A '

B'

C '

D '

1

6

5

4

3

2

N SSN NN

S N

S

N

NN

S

S

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name implies have permanentmagnets added to the motor structure.The rotor no longer has teeth as withthe VR motor. Instead the rotor ismagnetized with alternating northand south poles situated in a straightline parallel to the rotor shaft. Thesemagnetized rotor poles provide anincreased magnetic flux intensity andbecause of this the PM motor exhibits

improved torque characteristics whencompared with the VR type.

Hybrid (HB)The hybrid stepper motor is moreexpensive than the PM stepper motorbut provides better performance withrespect to step resolution, torque andspeed. Typical step angles for the HBstepper motor range from 3.6° to 0.9°(100 – 400 steps per revolution). Thehybrid stepper motor combines thebest features of both the PM and VRtype stepper motors. The rotor is

multi-toothed like the VR motor andcontains an axially magnetized con-centric magnet around its shaft. Theteeth on the rotor provide an evenbetter path which helps guide themagnetic flux to preferred locations inthe airgap. This further increases thedetent, holding and dynamic torquecharacteristics of the motor when com-pared with both the VR and PMtypes.

The two most commonly used typesof stepper motors are the permanent

magnet and the hybrid types. If adesigner is not sure which type willbest fit his applications requirementshe should first evaluate the PM type asit is normally several times less expen-sive. If not then the hybrid motor maybe the right choice.

There also excist some specialstepper motor designs. One is the discmagnet motor. Here the rotor isdesigned sa a disc with rare earthmagnets, See fig. 5 . This motor type

has some advantages such as very lowinertia and a optimized magnetic flowpath with no coupling between thetwo stator windings. These qualitiesare essential in some applications.

Size and PowerIn addition to being classified by theirstep angle stepper motors are alsoclassified according to frame sizeswhich correspond to the diameter of the body of the motor. For instance asize 11 stepper motor has a body di-ameter of approximately 1.1 inches.Likewise a size 23 stepper motor has abody diameter of 2.3 inches (58 mm),etc. The body length may however,vary from motor to motor within thesame frame size classification. As ageneral rule the available torque out-

put from a motor of a particular framesize will increase with increased bodylength.

Power levels for IC-driven steppermotors typically range from below awatt for very small motors up to 10 –20 watts for larger motors. The maxi-mum power dissipation level orthermal limits of the motor are seldomclearly stated in the motor manu-facturers data. To determine this wemust apply the relationship P=V ×I.For example, a size 23 step motor maybe rated at 6V and 1A per phase.

Therefore, with two phases energizedthe motor has a rated power dissipa-tion of 12 watts. It is normal practiceto rate a stepper motor at the powerdissipation level where the motor caserises 65°C above the ambient in stillair. Therefore, if the motor can bemounted to a heatsink it is oftenpossible to increase the allowablepower dissipation level. This isimportant as the motor is designed tobe and should be used at its maximumpower dissipation ,to be efficient from

a size/output power/cost point of view.

When to Use a StepperMotorA stepper motor can be a good choicewhenever controlled movement isrequired. They can be used to advan-tage in applications where you need tocontrol rotation angle, speed, positionand synchronism. Because of the in-herent advantages listed previously,stepper motors have found their place

in many different applications. Someof these include printers, plotters,highend office equipment, hard diskdrives, medical equipment, faxmachines, automotive and many more.

The Rotating Magnetic FieldWhen a phase winding of a steppermotor is energized with current amagnetic flux is developed in thestator. The direction of this flux isdetermined by the “Right HandRule” which states:

“If the coil is grasped in the righthand with the fingers pointing in thedirection of the current in the winding(the thumb is extended at a 90° angleto the fingers), then the thumb willpoint in the direction of the magneticfield.”

Figure 5 shows the magnetic fluxpath developed when phase B is ener-gized with winding current in thedirection shown. The rotor then alignsitself so that the flux opposition isminimized. In this case the motorwould rotate clockwise so that itssouth pole aligns with the north poleof the stator B at position 2 and itsnorth pole aligns with the south poleof stator B at position 6. To get themotor to rotate we can now see thatwe must provide a sequence of energizing the stator windings in sucha fashion that provides a rotatingmagnetic flux field which the rotorfollows due to magnetic attraction.

Torque GenerationThe torque produced by a steppermotor depends on several factors.

• The step rate

• The drive current in the windings

• The drive design or type

In a stepper motor a torque is devel-oped when the magnetic fluxes of therotor and stator are displaced fromeach other. The stator is made up of ahigh permeability magnetic material.The presence of this high permeabilitymaterial causes the magnetic flux tobe confined for the most part to thepaths defined by the stator structurein the same fashion that currents areconfined to the conductors of an elec-tronic circuit. This serves to concen-trate the flux at the stator poles. The

Figure 4. Principle of a disc magnet motor developed by Portescap.

NN NN

S SS




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Figure 5. Magnetic flux path through atwo-pole stepper motor with a lag betweenthe rotor and stator.

Figure 6. Unipolar and bipolar wound stepper motors.

torque output produced by the motoris proportional to the intensity of themagnetic flux generated when thewinding is energized.

The basic relationship whichdefines the intensity of the magneticflux is defined by:

H = (N×

i)÷

l where:N = The number of winding turns

i = current

H = Magnetic field intensity

l = Magnetic flux path length

This relationship shows that themagnetic flux intensity and conse-quently the torque is proportional tothe number of winding turns and thecurrent and inversely proportional tothe length of the magnetic flux path.

From this basic relationship one cansee that the same frame size steppermotor could have very different torqueoutput capabilities simply by chang-ing the winding parameters. Moredetailed information on how thewinding parameters affect the outputcapability of the motor can be foundin the application note entitled “DriveCircuit Basics”.

Phases, Poles and Stepping AnglesUsually stepper motors have twophases, but three- and five-phasemotors also exist.

A bipolar motor with two phaseshas one winding/phase and a unipolarmotor has one winding, with a centertap per phase. Sometimes the unipolarstepper motor is referred to as a “four-phase motor”, even though it only hastwo phases.

Motors that have two separatewindings per phase also exist—thesecan be driven in either bipolar or

unipolar mode.A pole can be defined as one of the

regions in a magnetized body wherethe magnetic flux density is con-centrated. Both the rotor and thestator of a step motor have poles.Figure 2 contains a simplified pictureof a two-phase stepper motor having 2poles (or 1 pole pairs) for each phaseon the stator, and 2 poles (one polepair) on the rotor. In reality severalmore poles are added to both the rotorand stator structure in order to

increase the number of steps perrevolution of the motor, or in otherwords to provide a smaller basic (fullstep) stepping angle. The permanentmagnet stepper motor contains anequal number of rotor and stator polepairs. Typically the PM motor has 12pole pairs. The stator has 12 pole pairsper phase. The hybrid type steppermotor has a rotor with teeth. Therotor is split into two parts, separatedby a permanant magnet—making half of the teeth south poles and half northpoles.The number of pole pairs isequal to the number of teeth on one of the rotor halves. The stator of a hybridmotor also has teeth to build up ahigher number of equivalent poles(smaller pole pitch, number of equivalent poles = 360/teeth pitch)compared to the main poles, on which

the winding coils are wound. Usually4 main poles are used for 3.6 hybridsand 8 for 1.8- and 0.9-degree types.

It is the relationship between thenumber of rotor poles and the equival-ent stator poles, and the number thenumber of phases that determines thefull-step angle of a stepper motor.

Step angle=360 ÷ (NPh× Ph)=360/N

NPh

= Number of equivalent poles perphase = number of rotor poles

Ph = Number of phases

N = Total number of poles for allphases together

If the rotor and stator tooth pitch isunequal, a more-complicated relation-ship exists.

Stepping ModesThe following are the most commondrive modes.

• Wave Drive (1 phase on)

• Full Step Drive (2 phases on)

• Half Step Drive (1 & 2 phases on)

• Microstepping (Continuouslyvarying motor currents)

For the following discussions pleaserefer to the figure 6.

In Wave Drive only one winding isenergized at any given time. Thestator is energized according to thesequence A → B → A → B and therotor steps from position 8 → 2 → 4→ 6. For unipolar and bipolar wound

IB

Phase A

Phase B

Stator A

Stator B

NS

1

2

345

6

78

N

S

RotorRotor

IA

IB

Phase A

Phase B

Stator A

Stator B

1

2

345

6

78

N

S

Phase A

N

S

Phase B

NS RotorotorVM

VM

Rotor

IA

IB

Phase A

Phase B

Stator A

Stator B

N

NS

S

1

2

345

6

78

N

S

Rotor

Rotor

motors with the same winding param-eters this excitation mode would result

in the same mechanical position. Thedisadvantage of this drive mode is thatin the unipolar wound motor you areonly using 25% and in the bipolarmotor only 50% of the total motorwinding at any given time. Thismeans that you are not getting themaximum torque output from themotor




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Table 1. Excitation sequences for different drive modes

Figure 7. Torque vs. rotor angular position.

Figure 8. Torque vs. rotor angle position at different holding torque.

In Full Step Drive you are ener-gizing two phases at any given time.The stator is energized according tothe sequence AB → AB → AB →

AB and the rotor steps from position1 → 3 → 5 → 7 . Full step mode

results in the same angular movementas 1 phase on drive but the mechanicalposition is offset by one half of a fullstep. The torque output of theunipolar wound motor is lower thanthe bipolar motor (for motors with thesame winding parameters) since theunipolar motor uses only 50% of theavailable winding while the bipolarmotor uses the entire winding.

Half Step Drive combines bothwave and full step (1&2 phases on)drive modes. Every second step onlyone phase is energized and during the

other steps one phase on each stator.The stator is energized according tothe sequence AB → B → AB → A

→ AB → B → AB → A and therotor steps from position 1 → 2 → 3→ 4 → 5 → 6 → 7 → 8. This resultsin angular movements that are half of those in 1- or 2-phases-on drivemodes. Half stepping can reduce aphenomena referred to as resonancewhich can be experienced in 1- or 2-phases-on drive modes.

The displacement angle is deter-mined by the following relationship:

X = (Z ÷ 2π) × sin(Ta÷ T

h) where:

Z = rotor tooth pitch

Ta

= Load torque

Th = Motors rated holding torque

X = Displacement angle.

Therefore if you have a problemwith the step angle error of the loadedmotor at rest you can improve this bychanging the “stiffness” of the motor.This is done by increasing the holdingtorque of the motor. We can see thiseffect shown in the figure 5.Increasing the holding torque for aconstant load causes a shift in the lagangle from Q

2to Q

1.

Step Angle AccuracyOne reason why the stepper motor hasachieved such popularity as a position-ing device is its accuracy and repeat-ability. Typically stepper motors willhave a step angle accuracy of 3 – 5%of one step. This error is also non-cumulative from step to step. Theaccuracy of the stepper motor ismainly a function of the mechanicalprecision of its parts and assembly.Figure 9 shows a typical plot of thepositional accuracy of a stepper motor.

Step Position Error

The maximum positive or negativeposition error caused when the motorhas rotated one step from the previousholding position.

Step position error = measured stepangle - theoretical angle

Positional Error The motor is stepped N times from aninitial position (N = 360°/step angle)

and the angle from the initial position

Torque

Angle

TH

Ta

A B C

Stable

Point

Unstable

Point

Stable

Point

UnstableRegion

Oa

O

Torque

Angle

TLoad

TH1

TH2

O O1 2O

NormalWave Drive full step Half-step drive

Phase 1 2 3 4 1 2 3 4 1 2 3 4 5 6 7 8A • • • • • •

B • • • • • •

A • • • • • •

B • • • • • •

The excitation sequences for theabove drive modes are summarized inTable 1.

In Microstepping Drive thecurrents in the windings arecontinuously varying to be able tobreak up one full step into manysmaller discrete steps. More

information on microstepping can befound in the microstepping chapter.

Torque vs, AngleCharacteristicsThe torque vs angle characteristics of a stepper motor are the relationshipbetween the displacement of the rotorand the torque which applied to therotor shaft when the stepper motor isenergized at its rated voltage. An idealstepper motor has a sinusoidal torque

vs displacement characteristic asshown in figure 8.

Positions A and C represent stableequilibrium points when no externalforce or load is applied to the rotorshaft. When you apply an externalforce T

ato the motor shaft you in

essence create an angulardisplacement,Θ

a. This angular

displacement, Θa, is referred to as a

lead or lag angle depending on wetherthe motor is actively accelerating ordecelerating. When the rotor stops

with an applied load it will come torest at the position defined by thisdisplacement angle. The motordevelops a torque, T

a, in opposition to

the applied external force in order tobalance the load. As the load isincreased the displacement angle alsoincreases until it reaches themaximum holding torque, T

h, of the

motor. Once Th

is exceeded the motorenters an unstable region. In thisregion a torque is the oppositedirection is created and the rotorjumps over the unstable point to the

next stable point.




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Figure 9. Positional accuracy of a stepper motor.

Figure 10. Torque vs. speed characteristicsof a stepper motor.

is measured at each step position. If the angle from the initial position tothe N-step position is Θ

Nand the

error is ∆ΘN

where:

∆ΘN

= ∆ΘN

- (step angle) × N.

The positional error is the differenceof the maximum and minimum but is

usually expressed with a ± sign. Thatis:

positional error = ±1 ⁄ 2(∆Θ

Max- ∆Θ

Min)

Hysteresis Positional Error The values obtained from the measure-ment of positional errors in bothdirections.

Mechanical Parameters,Load, Friction, InertiaThe performance of a stepper motorsystem (driver and motor) is alsohighly dependent on the mechanicalparameters of the load. The load isdefined as what the motor drives. It istypically frictional, inertial or acombination of the two.

Friction is the resistance to motiondue to the unevenness of surfaceswhich rub together. Friction isconstant with velocity. A minimumtorque level is required throughoutthe step in over to overcome this

friction ( at least equal to the friction).Increasing a frictional load lowers thetop speed, lowers the acceleration andincreases the positional error. Theconverse is true if the frictional load islowered

Inertia is the resistance to changesin speed. A high inertial load requiresa high inertial starting torque and thesame would apply for braking. In-creasing an inertial load will increasespeed stability, increase the amount of time it takes to reach a desired speedand decrease the maximum self start

pulse rate. The converse is again trueif the inertia is decreased.

The rotor oscillations of a steppermotor will vary with the amount of friction and inertia load. Because of this relationship unwanted rotor oscil-lations can be reduced by mechanicaldamping means however it is moreoften simpler to reduce theseunwanted oscillations by electricaldamping methods such as switch fromfull step drive to half step drive.

Torque vs, SpeedCharacteristicsThe torque vs speed characteristics arethe key to selecting the right motorand drive method for a specificapplication. These characteristics aredependent upon (change with) the

motor, excitation mode and type of driver or drive method. A typical“speed – torque curve” is shown infigure9.

To get a better understanding of this curve it is useful to define thedifferent aspect of this curve.

Holding torqueThe maximum torque produced by

the motor at standstill.

Pull-In CurveThe pull-in curve defines a area refered

to as the start stop region. This is themaximum frequency at which themotor can start/stop instantaneously,with a load applied, without loss of synchronism.

Maximum Start RateThe maximum starting step frequencywith no load applied.

Pull-Out CurveThe pull-out curve defines an arearefered to as the slew region. It defines

the maximum frequency at which themotor can operate without losing syn-chronism. Since this region is outsidethe pull-in area the motor mustramped (accelerated or decelerated)into this region.

Maximum Slew RateThe maximum operating frequency of the motor with no load applied.

The pull-in characteristics vary alsodepending on the load. The larger theload inertia the smaller the pull-inarea. We can see from the shape of the

curve that the step rate affects thetorque output capability of steppermotor The decreasing torque output asthe speed increases is caused by thefact that at high speeds the inductanceof the motor is the dominant circuitelement.

AngleDeviation

TheoreticalPosition

PositionalAccuracy

HysteresisError

Torque

SpeedP.P.S.

Start-Stop Region

Pull-inTorquecurve

SlewRegion

Holding TorquePull-outTorque

Curve

Max Start Rate

Max Slew Rate

The shape of the speed - torque

curve can change quite dramaticallydepending on the type of driver used.The bipolar chopper type driverswhich Ericsson Components produceswill maximum the speed - torqueperformance from a given motor. Mostmotor manufacturers provide thesespeed - torque curves for their motors.It is important to understand whatdriver type or drive method the motormanufacturer used in developing theircurves as the torque vs. speed charac-teristics of an given motor can varysignificantly depending on the drive

method used.




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Stepper motors can often exhibit aphenomena refered to as resonance atcertain step rates. This can be seen as asudden loss or drop in torque at cer-tain speeds which can result in missedsteps or loss of synchronism. It occurswhen the input step pulse rate coin-cides with the natural oscillationfrequency of the rotor. Often there is aresonance area around the 100 – 200pps region and also one in the highstep pulse rate region. The resonancephenomena of a stepper motor comesfrom its basic construction and there-fore it is not possible to eliminate itcompletely. It is also dependent uponthe load conditions. It can be reducedby driving the motor in half or micro-stepping modes.

Figure 11. Single step response vs. time.

Single Step Response andResonancesThe single-step response character-istics of a stepper motor is shown infigure 11.

When one step pulse is applied to astepper motor the rotor behaves in a

manner as defined by the above curve.The step time t is the time it takes themotor shaft to rotate one step angleonce the first step pulse is applied.This step time is highly dependent onthe ratio of torque to inertia (load) aswell as the type of driver used.

Since the torque is a function of thedisplacement it follows that the accel-eration will also be. Therefore, whenmoving in large step increments ahigh torque is developed andconsequently a high acceleration. Thiscan cause overshots and ringing as

shown. The settling time T is the timeit takes these oscillations or ringing tocease. In certain applications thisphenomena can be undesirable. It ispossible to reduce or eliminate thisbehaviour by microstepping thestepper motor. For more informationon microstepping please consult themicrostepping note.

Angle

t T

Time

O


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