Everything You Need To Know About Stepper Motors

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Everything You Need to Know AboutStepper MotorsThe Powerful Ability of Highly Reliable Stepper MotorsStepper motors are often misconceived asthe lesser of servo motors, but as a matter of fact,they are highly reliable just like servo motors. Themotor operates by accurately synchronizing withthe pulse signal output from the controller to thedriver, achieving highly accurate positioning andspeed control. Stepper motors feature high torqueand low vibration at low-speeds, ideal forapplications requiring quick positioning in shortdistance.Stepper MotorsExample: 0.72 Stepper Motor and DriverPackages RK II SeriesConsists of a high efficiency 0.72 steppermotor and digitally controlled driver.Pulse Input orBuilt-in ControllerKey Points: Ease-of-Use, Simple Operationsand Low Cost“Stepper motors? Servo motorsmust have better performance." This is atypical response when asked about steppermotors. Obviously there is a majormisconception about stepper motors. Infact, stepper motors have been used invarious types of applications such asadvanced equipment and accessibleautomated instruments. The reasons whystepper motors have been continuouslychosen are explained in this article.Some readers may say that theyhave never seen a stepper motor before.Stepper motors have been used in manyapplications and industries as the motorsolution for drive systems requiring highaccuracy control, such as factoryautomation (FA), manufacturing equipmentfor semiconductor, FPD and solar panel,medical devices, analytical instruments,precision stage, financial systems, foodpackaging machines, and aperturediaphragm adjustments for cameras.According to the survey of steppermotors users, many favor stepper motorsfor their "ease-of-use," "simple operations",and "low cost" derived from the structureand system configuration. It makes sensethat many users find such positive aspectsin stepper motors, thanks to the simplestructure and system configuration.However, some readers may beskeptical about the actual performance ofthe motor in terms of its accuracy andtorque. It is not easy to fully grasp thewhole idea unless there are comparisonexamples against other control motors suchas servo motors. By knowing thecharacteristics and taking on differentapproaches based on required operations,stepper motors certainly can reduce thecost of the equipment.The characteristics and technicalinformation of stepper motors are explainedbelow:Impressive "Stopping Accuracy." MovesQuickly in the "Low / Mid-Speed Range"Why do you use a stepper motor?Easy to use: 34%Inexpensive: 17%Simple operations:16%No need for tuning: 12%Other: 21%Stepper motors have remarkablestopping accuracy, and accurate controlwith open-loop is possible. For example,when using the RK II Series for positioningof a rotating table, its stopping accuracy iswithin 0.05 degrees (with no load).*# of questionees: 258 (multiple answers allowed)/researched by Oriental Motor1www.orientalmotor.com

Because stopping position errors do notaccumulate between steps, high accuracypositioning is possible. The structure of thestepper motor, which requires no encoder,allows for the simple drive system and lowcost.torque within the required speed range forshort-stroke operations (less rotations), thusthey are suitable for selecting the desiredstep angle for multiple rotation tables andinching applications. This is due to theshort positioning time for the short-strokeoperation, thus the motor decelerates andstops before reaching the maximum speed.In other words, high-speed characteristicsare usually not required.Point1Fantastic Stopping Accuracy!High Responsiveness and ExcellentSynchronizationFor example, when converting stoppingaccuracy 0.05 of a stepper motor to the ballscrew mechanism:The third remarkable feature ofstepper motors is responsiveness. Theopen-loop control, which sends one-waycommands to the motor, has a high followup mechanism toward commands. Whileservo motors, which wait for feedback fromthe encoder, tend to have "delays" withcommands, stepper motors operatesynchronously with a pulse. Therefore,there are very few "delays," resulting inexcellent response. For this reason, steppermotors are suitable for applications thatrequire synchronous operations of multiplemotors. One example is a boardtransferring application that requires twoconveyors, with one motor mountedrespectively, to transfer boards in betweenthe two conveyors.Operating Conditions: Motor: RK II Series Lead of ball screw: 10mmStopping Accuracy: 1.4μmGenerally, accuracy of a ground ball screwtype is 10μm. When using a rolled ball screwtype, its accuracy declines to 20μm,indicating that the stopping accuracy of astepper motor is much higher than that of ballscrew types. 0.05 1.4μmPoint2Positioning of RotatingTablePositioning UsingPositioning Ball ScrewExcellent Low / Mid-Speed Range!Example: Torque of a motor frame size 85 mmis equivalent to a rated torque of a 400 Wservo motor when 1000 r/min.High torque within the low /mid-speed range is another great feature ofstepper motors. One of the main features ofservo motors is to generate flat torque frommid to high speed ranges. Servo motors aresuitable for long-stroke operations (manyrotations). On the other hand, torquecharacteristics of stepper motors are notflat. The torque curve within the low /mid-speed range tends to become veryhigh, while becoming very low in the highspeed range. Besides the stable rotations inthe low speed range, which servo motorsstruggle with, stepper motors can offer highTorque in an even lower speed rangecan be up to 5 times higher. For a shortdistance positioning, having high torque in thelow / mid-speed range is essential.2www.orientalmotor.com

Other Application Examples:RK II Series 85 mm Motor and Output400W/750W Servo MotorRKS596AC - RKS599AC - RKS5913AC - Torque [N・m]8Low Rigidity MechanismServo Motor 400 W(Rated Torque)Servo Motor 750 W(Rated Torque)6420200800400600Speed [r/min]1000Frequent Starting and StoppingPoint3High Responsiveness!【Stepper Motor】Fluctuation in LoadSpeedSpeed【AC Servo Motor】Advantage of Great FeaturesTimeSetting TimeTimePositioning TimePositioning Time(Open-Loop Control)Operates synchronouslywith operation command(Closed-Loop Control)Operates against operationcommand with delayBesides cost reduction, steppermotors have many advantages in terms ofperformance. The following chart shows theconverting torque of the RKII Seriesexample to that of typical servo wattageranges. On the following pages, detailedinformation of stepper motors, such asbasic structure, system, and exampleapplications, is introduced for moreinformation on stepper motors.How many W are there when converting the torque ofstepper motors to output?Board Transfer ConveyorMotorFrame SizeSuitable Applications!42 mmOther than an inching applicationwith frequent starting and stopping, steppermotors are suitable for positioning of imagecheck processors that dislike vibrations,cam drives that would be difficult to adjustwith servo motors, and low rigiditymechanisms such as a belt drive.Furthermore, cost is reduced significantlyby replacing a ball screw drive to a beltdrive.60 mm85 mmProductNameList PriceRKS543 461.00RKS544 462.00RKS545 469.00RKS564 506.00RKS566 511.00RKS569 516.00RKS596 566.00RKS599 606.00RKS5913 649.00Motor Output(Reference)Equivalent to 50〜100 WEquivalent to 100〜200 WEquivalent to 200〜400 WEquivalent to 400〜750 W*Calculated with RK II SeriesFor a 42mm motor frame size, its output is equivalent to that of a servo motorwith 50 to 100W. Also, the output of a 85mm frame size motor is equivalent tothat of a servo motor with 400 to 700W. It may vary depending on a series andtype, but this chart can be used during a product selection.3www.orientalmotor.com

Basics of Stepper MotorsOperation & Structure Useful for Speed Control and PositionControlA stepper motor rotates with a fixed step angle, justlike the second hand of a clock. Highly accuratepositioning can be performed with open-loop controlthanks to the mechanical structure within the motor.When pulses are input to a driver through a pulsegenerator, stepper motors position according to thenumber of input pulses. The basic step angle of5-phase stepper motors is 0.72 and 1.8 for 2-phasestepper motors. The rotating speed of the steppermotor is determined by the speed of the pulsefrequency (Hz) given to the driver, and it is possibleto freely change the motor rotation by simplychanging the number of input pulses or frequenciesto the driver. Stepper motors not only serve asposition control motors, but also as speed controlmotors with high synchronization. Accurate Positioning (Number of Steps)While having full control of rotation and speed, thesimple structure of stepper motors is achievedwithout using electrical components, such as anencoder within the motor. For this reason, steppermotors are very robust and have high reliability withvery few failures. As for stopping accuracy, 0.05 (without cumulative pitch errors) is very accurate.Because positioning of stepper motors is performedby open-loop control and operated by the magnetized stator and magnetic rotor with small teeth,stepper motors have a higher follow-up mechanismtoward commands than that of servo motors. Also, nohunting occurs when stopping stepper motors. Theyare also excellent in belt drives, which have lowrigidity.No electronic parts,such as a sensor,are used.StatorShaft Stepper Motors Uses:・High frequency, repetitive positioning of fixed step angles・Positioning that requires long stopping time due to widthadjustment, etc.・Fluctuating loads and changing rigidity・Positioning that divides 1 cycle・Motor shafts that requires synchronous operationPositioning Accuracy 0.05 A-PhaseRotorB-Phase0.72 C-Phase0.72 1.44 D-Phase 0.05 E-PhaseOperating Systemdriver is recommended. Cost is reduced byeliminating controllers, such as a pulse generatorand PLC positioning modules. Simple Control With No Sensor or Feedback Built-in Sensor Closed-Loop TypeBecause it is possible to perform accurate positioningand position control while synchronizing with thenumber of command pulses and speed, there is noneed for devices, such as a sensor, for positioning.Therefore, the entire system is simple to build. Ifadvanced control, such as an interpolation operation,is not required, the built-in controller function typeNumber of pulses proportional to therotation amount of the motor5-Phase2-Phase1 Pulse0.72 1.8 10 Pulses7.2 18 125 Pulses90 225 Pulse SignalDriverPositioningCPUPower SupplyThe amount of rotationand speed of the motorcan be easily controlvia pulse signals.Although high accuracy positioning is possible withopen-loop control, what would happen if a problemoccured? In order to avoid such pitfalls, an encodertype or built-in sensor closed-loop control type motor(AR Series) can be used.Power SupplyRotor PositionDetection SensorMotorAR seriesPLC Module4www.orientalmotor.com

Can the Cost be Further Reduced?The common issue among design engineers is cost reduction. Is there really no way to further reduce cost? Tofind out a cost reduction test, with specification enhancements, was conducted based on the ball screwmechanism. The following explains the details of the test:PlanProblemsFurther increase speed!Further reduce cost!AC Servo Motor50 W7 kgLead: 12 mm・Max. Speed: 600 mm/sec・Stroke: 1000 mmChange the Mechanism to Belt Pulley Ball screw if trying to increase speed Belt mechanism may be moresuitable 1000mm/sec to 1500mm/sec is possible with the belt mechanism.Change to belt if there is no issue with positioning accuracy. Reduce cost significantly if changing to belt is possible Belt is inexpensivebut its low rigidity may affect stability of servo motor operation, even withautomatic tuning.① Difference in Stopping Accuracy between Screw and Belt. How Much Stopping Accuracy IsRequired?② Impact of Low Rigidity. Impact on Settling Time, Avoiding Tuning Problem① Better stopping accuracy with the screw. No problem to change to the belt? The application's required stopping accuracy is 0.05 〜 0.1mm, which is not as accurateas the one for the screw. Therefore, it should be okay to replace with the belt.② If changing to the belt, rigidity on the mechanism gets low, thus the servo motormovements becomes unstable. Among positioning motors, stepper motors do not have a built-in encoder. For this reason,they require no adjustment and are strong against low rigidity. Their movements are stableregardless of fluctuating loads. If the output is the same, consider stepper motors.Mechanism: Belt Pulley Motor: Try with Stepper MotorGeneral Servo Motor (50W) Ball ScrewList PriceEstimationLinear Motion Mechanism① Further Increase Speed ② Further Reduce Cost【Conditions of the Originally- Planned Equipment】Mechanism: Ball Screw Servo MotorConditions such as a load, speed, and lead, shown onthe right, are determined based on the servo motorattached with ball screws and steel plate.General Servo MotorBall Screw, Guide, etc.Total5 Phase Stepper Package RKII Series Belt Pulley (1/2 Reduce) 850.00 320.00 Motor 530.00 Driver 165.00 Rolled Ball Screw 130.00 Guide 1,145.00List PriceMissionRKII SeriesRKS569AA-3 438.60* Web Price 35.00 Belts *2 pcsBelt Pullet, Guide, etc. 109.00 Pulleys *3 pcs Idler *1 pc 130.00 GuidesTotal 712.60Belt Lead: 50mm Transportable Mass - Max. permissible load 7kg Traveling Speed - Improved to 800 mm/ secMotor By changing from stepper motor to servo motor, reduced cost by 50%!Mechanism By changing from ball screw to belt mechanism, reduced cost by 7%!ResultsThere Was Much Room for Cost Reduction!By conducting a zero-based review of the mechanism as well as the motor selection based oncharacteristics, we managed to increase specifications and reduce cost, even with the motorsize became slightly larger. In the past, motor selection was done based on its ease-of-useor familiarity. After this exercise, differences of operations between servo motors and steppermotors became clear. It was surprising that stepper motors are more affordable thanexpected. There must be room for cost reduction of other devices using this method. Thisexercise reacknowledged that the well-balanced selection between motor specifications andcost, while maximizing motor characteristics is the key.5www.orientalmotor.com

Which Has Higher Stopping Accuracy - Stepper Motor or Servo Motor?Customer Inquiry: Looking for amotor with good stopping accuracy. Howmuch of a difference is there betweenstepper motors and servo motors?According to the comparisoncharts, stopping accuracy betweenstepper motors and AC servo motors isalmost the same ( 0.02 〜 0.03 ).Accuracy depends on the mechanicalprecision of the motor for stepper motors,thus if stop position can be done per 7.2 ,positioning is done by the same smallteeth on the rotor at all times, according tothe motor structure. This makes it possibleto further improve stopping accuracy.However, stepper motors maygenerate displacement angle dependingon the load torque value. Also, dependingon the mechanism condition, AC servomotors may have wider hunting width as aresponse to gain adjustments. For thesereasons, some caution is required.Assumption: The AC servo motorNX Series is equipped with a 20-bitencoder, thus it should have a fineresolution, and good stopping accuracy.First, it is necessary to clarify thedifference between resolution andstopping accuracy: Resolution is thenumber of steps per revolution and it isalso called a step angle for steppermotors. It is needed when consideringhow precise the required positioningneeds to be. Stopping accuracy is thedifference between the actual stopposition and theoretical stop position.ResolutionStoppingAccuracyUp to 200,000steps(Minimum stepangle of aconventionalstepper motor is0.0018 ) 0.05 (Stoppositionaccuracy)AC ServoMotorUp to 100,000steps(Minimumresolution of aconventionalservo motor is0.0036 ) Stopping Accuracy of Stepper Motor(Actual measured value)0.06Stop Position Accuracy [ ]StepperMotorComparison of Stopping AccuracyBetween Stepper Motors and ACServo MotorsWithin 1 pulse(Encoderresolution)Actual ability iswithin 0.02 0.040.020 0.02 0.04 0.06060120180240Rotation Angle [ ]300360 Stopping Accuracy of AC Servo Motor(Actual measured value)Stop Position Accuracy [ ]Does this mean that the AC servomotor equipped with a high accuracyencoder has better stopping accuracythan stepper motors?Not quite. In the past there was noissue with the concept of "stoppingaccuracy of servo motors being equal toencoder resolution within 1 pulse."However, recent servo motors areequipped with the 20 bit encoder(1,048,576 steps) which has a very fineresolution. Because of this, errors due tothe encoder installation accuracy have ahuge effect on stopping accuracy.Therefore, the concept of stoppingaccuracy has slightly started to change.0.06Actual ability iswithin 0.03 0.040.020 0.02 0.04 0.06060120180Rotation Angle [ ]240300360Stop Position Accuracy [ ] Stepper Motor 7.2 Stopping Accuracy perPositioning (Actual measured value)0.060.020 0.02 0.04 0.066Actual ability iswithin 0.006 0.04060120180240Rotation Angle [ ]300360www.orientalmotor.comFeb. 2016

Lead: 12 mm

Everything You Need to Know About Stepper Motors The Powerful Ability of Highly Reliable Stepper Motors Stepper motors are often misconceived as the lesser of servo motors, but as a matter of fact, they are highly reliable just like servo motors. The motor operates by accurately synchronizing with the pulse signal output from the controller to the

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