Motor Control Basic

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Industrial Circuits Application Note Stepper Motor Basics A stepper motor is an electromechanical device which converts electrical pulses into discrete mechanical movements. The shaft or spindle of a stepper motor rotates in discrete 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 rota
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  1  A stepper motor is an electromechanical device which converts electrical pulses intodiscrete 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 goodstepper 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 digitalinput 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 Operation One 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 Types There are three basic stepper motortypes. They are :ãVariable-reluctanceãPermanent-magnetãHybrid 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        NSS N N NSNS NNNSS  2name 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 exhibitsimproved 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 ismulti-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 permanentmagnet 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 typehas 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 Power In 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 froma size/output power/cost point of view.  When to Use a StepperMotor A 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 placein many different applications. Someof these include printers, plotters,highend office equipment, hard diskdrives, medical equipment, faxmachines, automotive and many more. The Rotating Magnetic Field When 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 Generation The torque produced by a steppermotor depends on several factors.ã The step rateã The drive current in the windingsã The drive design or typeIn 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. NNNNSSS  3 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) ÷ lwhere:N=The number of winding turnsi=currentH=Magnetic field intensityl=Magnetic flux path lengthThis 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 Angles Usually 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 orunipolar mode.A pole can be defined as one of theregions 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 toincrease 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 whichthe 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 ÷ (N Ph × Ph)=360/NN Ph =Number of equivalent poles perphase = number of rotor polesPh=Number of phasesN=Total number of poles for allphases togetherIf the rotor and stator tooth pitch isunequal, a more-complicated relation-ship exists. Stepping Modes The following are the most commondrive modes.ã Wave Drive (1 phase on)ã Full Step Drive (2 phases on)ã Half Step Drive (1 & 2 phases on)ã 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 I B Phase APhase BStator AStator B NS 12345678 NS Rotor   Rotor I A I B Phase APhase BStator AStator B 12345678 NS Phase A NS Phase B NS Rotorotor V M V M Rotor I A I B Phase APhase BStator AStator B NNSS 12345678 NS Rotor   Rotor motors with the same winding param-eters this excitation mode would resultin 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  4 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 → A B → AB → A B and the rotor steps from position1 → 3 → 5 → 7 . Full step moderesults 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 theother steps one phase on each stator.The stator is energized according tothe sequence AB → B → A B → A → AB →   B → A B → 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 ÷ 2 π ) ×  sin (T a ÷ T h )where:Z =rotor tooth pitchT a =Load torqueT h =Motors rated holding torqueX=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 2 to Q 1 . Step Angle Accuracy One 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 TorqueAngleT H T a  ABC StablePointUnstablePointStablePointUnstableRegionO a O TorqueAngle   T Load T H1 T H2 O O 12 O NormalWave Drive full step Half-step drivePhase 1 2 3 4 1 2 3 4 1 2 3 4 5 6 7 8A ã ã ã ã ã ã B ã ã ã ã ã ã A ã ã ã ã ã ã B ã ã ã ã ã ã 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. Moreinformation on microstepping can befound in the microstepping chapter. Torque vs, AngleCharacteristics The 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 torquevs 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 a to the motor shaft you inessence create an angulardisplacement, Θ a . This angulardisplacement, Θ a , is referred to as alead or lag angle depending on wetherthe motor is actively accelerating ordecelerating. When the rotor stopswith an applied load it will come torest at the position defined by thisdisplacement angle. The motordevelops a torque, T a , in opposition tothe 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 themotor. Once T h is exceeded the motorenters an unstable region. In thisregion a torque is the oppositedirection is created and the rotorjumps over the unstable point to thenext stable point.
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