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Linear Motors Application Guide www.aerotech.com Dedicated to the Science of Motion
LinearMotorsApplication Guide
www.aerotech.com
Dedicated to the Science of Motion
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Let’s take a trip through time, back toAugust 29, 1831, arriving at MichaelFaraday’s workshop. The greatscientist and father of electricalengineering has just discovered that acopper disk, spinning within ahorseshoe magnet, generateselectricity in a wire. This “induction” isfundamental to all electro-technologythat will follow. Mr. Faraday hascreated the first-ever generator.
We approach Mr. Faraday and askthe question, “Sir, do you think that oneday your discovery will be capable ofpositioning nine-micron-thick opticalfibers, end-to-end, at acceleration ratesof ten meters per second, at resolutionsmeasured consistently in nanometers?”We can only guess what his replymight have been.
However, the linear motors of today,which are capable of breathtakingspeeds and accuracies, are foundedon the same basic principles thatFaraday discovered. It is by examiningthese principles, together with somepractical hints and formulae, that wewill remove any mystery about theconstruction and application of direct-drive linear motors.
A little historyContentsA little history 2
Back to basics 4
Types of linear motors 6
The benefits of linear motors 12
Applying linear motors as components 16
Commutation of linear motors 24
Linear motor quick selection guide 33
Specialty and custom motors 34
Copyright © 2010, Aerotech, Inc. Information in thisbrochure is subject to change without notice.
WORLD HEADQUARTERSAerotech, Inc.101 Zeta DrivePittsburgh, PA 15238Ph: 412-963-7470Fax: 412-963-7459Email: [email protected]
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Those that know anything about linearmotors can be forgiven for immediatelythinking of maglev trains, superguns, oreven futuristic elevator systems. Youmight even recall Major Boothroydusing the linear motor of one of Q’stoys to propel a tray at decapitatingspeed in the 1977 Bond movie, “TheSpy Who Loved Me.” All havecaptured public attention and itsimagination in recent years. The linearmotor has really come of age througha dramatic increase in practical andbeneficial industrial applications.
The linear motor was invented byProfessor Eric Laithwaite, the Britishelectrical engineer who died onDecember 6, 1997 at age 76. Itprojected a shuttle across a weavingloom using a linear motor. ProfessorLaithwaite was fascinated with theweaving process ever since hisboyhood spent in Lancashire, the UK’shome of textile manufacture.
Professor Laithwaite described hisinvention as “no more than anordinary electric motor, spread out.”The principle created magnetic fieldson which an object rested andtravelled without being slowed by
friction. This magnetic levitation hadlong been understood, but it wasLaithwaite who pioneered thecommercial development of the firstpractical applications, developingdirect linear drives for both machineryand transport.
Linear motors have evolved in severalguises but perhaps the most commonlyencountered are tubular, flat, or “U-channel” types, which are findingincreasing use thanks to their lowprofiles and high output. For all intentsand purposes, and for the purposes ofthis discussion, we can assume mostlinear motors for motion control usebrushless technology.
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of the sun, apart from getting warmfeet, Mr. Arbuckle might weigh near4,500 kg (nearly 10,000 lb)!
There... we’ve already fallen into thetrap. As far as SI units are concerned,it is mass that is measured in kilograms.The unit of weight is named after goodold Sir Isaac and is, of course, thenewton. Weighing machines arescaled in kilograms for convenience,but really should be marked innewtons. Take our Hollywood pal Mr.Arbuckle to the moon and he weighsone sixth of what he does on Earth. Hismass has not changed, but the forceacting on it has. A trip to the moon is agreat way to lose weight, but doesnothing for the waistline!
Take Newton’s first law of motion: amass continues in a state of rest, or ofmotion at uniform velocity, unless aforce acts upon it. OK, so let’s go backto space and give Mr. Arbuckle agentle shove. He now weighs nothingand we watch him float across thespacecraft.
Now, if we give him a harder push, heflies across and bangs into thebulkhead, enabling us to witness
Back to basicsIf you’ve never puzzled whether or notto put gravity (g) into an equation, andhave never struggled to state thedifference between weight and mass,then you may skip this chapter.
For the rest of us mere mortals, here isa simple reminder of what we’redealing with when considering motorspecification. All we, as engineers,really need to consider are the lawsaccording to Faraday’s predecessor,Sir Isaac Newton.
Let’s start with mass andweight. Mass is the
unchanging quality of abody (Fatty Arbuckle
had a largemass) whileweight is the
force that massexerts in a
gravitational field(don’t let Mr. Arbucklefall on you). However,
the weight variesaccording to gravity. Forinstance, in outer spaceyou could throw Mr.Arbuckle a long way,
whereas on the surface
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Newton’s second law, which states: therate of change of velocity (acceleration)of a mass is proportional to the appliedforce and occurs in the direction of theapplied force.
However, when we gave spacemanArbuckle a hard shove, we also flewbackwards at the same rate. Thisoccurrence is described by Newton’sthird law: action and reaction areequal and opposite.
So, what has all this got to do withspecifying and using linear motors?Well, we are all interested in motionand that means considering the massand the acceleration. What we needto know is the dimension and directionof the force required to make a loadmove how and where we want it to.That force is calculated as the mass xacceleration (and that means anyacceleration including gravity). This isvery important when making linearmotor assessments because thefrictional resistance is normally verylow, which can be a disadvantagewhen the motor is in the verticalposition.
Newton’s laws indicate that once amoving mass has been accelerated, itshould remain at a constant velocitywithout the need for further force. Yeahright! As any engineer knows there area lot of forces preventing that scenario:friction, bearing resistance, airresistance, even lubricants and gravityall conspire against us as engineers.
Sir Isaac Newton
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track have no mechanical connection– i.e., no brushes. Unlike rotary motors,where the rotor spins and the stator isheld fixed, a linear motor system canhave either the forcer or the magnettrack move.
Most applications for linear motors, atleast in positioning systems, use amoving forcer and static track, butlinear motors can also be used with amoving track and static forcer. With amoving forcer motor, the forcer weightis small compared to the load.However, this arrangement requires acable management system with high-flex cable since the cable has to followthe moving forcer. With a moving trackarrangement, the motor must move theload plus the mass of the magnet track.However, there is the advantage thatno cable management system isrequired.
Similar electromechanical principlesapply whether the motor is rotary orlinear. The same electromagnetic forcethat creates torque in a rotary motoralso does so in the linear counterpart.Hence, the linear motor uses the samecontrols and programmable
Types of linear motorsWe’ve already heard ProfessorLaithwaite’s description of a linearmotor as a rotary motor rolled-out flat.The forcer (rotor) is made-up of coils ofwires encapsulated in epoxy and thetrack is constructed by placingmagnets on steel. The forcer of themotor contains the windings, Halleffect board, thermistor, and theelectrical connections. In rotary motors,the rotor and stator require rotarybearings to support the rotor andmaintain the air-gap between themoving parts. In the same way, linearmotors require linear guide rails thatwill maintain the position of the forcerin the magnetic field of the magnettrack. Rotary servomotors haveencoders mounted to them to givepositional feedback of the shaft. Linearmotors need positional feedback in thelinear direction and there are manydifferent linear encoders on the markettoday. By using a linear encoder, theposition of the load is measureddirectly which increases the accuracyof the position measurement.
The control for linear motors isidentical to rotary motors. Like abrushless rotary motor, the forcer and
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positioning as a rotary motor. In arotary motor, torque is measured inNm (lb-ft) and for the linear motorsforce is N (lb). Velocity is measured inrev/min for the rotary and m/s (ft/s)for linear motors. Duty cycles aremeasured in the same way for bothtypes of motor.
Looking at the various motor types, wesee that a linear motor directlyconverts electrical energy to linearmechanical force, and is directlycoupled to the load. There is nocompliance or windup, and higheraccuracy and unlimited travel areachieved. Today, linear motorstypically reach speeds of 5 m/s, withhigh accelerations of 5 g in practice.Theoretically, motors can reach over20 g with 40 m/s velocity. However,bearings and required motionparameters de-rate this performancesomewhat. There is no wear, nolubrication, and therefore minimal orno maintenance cost for linear motors.Finally, there is higher systembandwidth and stiffness, giving betterpositional repeatability and accuracyas well as higher speeds.
A linear motor can be flat, U-channel,or tubular in shape. The configurationthat is most appropriate for aparticular application depends on thespecifications and operatingenvironment.
Cylindrical movingmagnet linear motors
In these motors, the forcer is cylindricalin construction and moves up anddown a cylindrical bar that houses themagnets. These motors were amongthe first to find commercial application,but do not exploit all of the spacesaving characteristics of their flat and U-channel counterparts.
The magnetic circuit of the cylindricalmoving magnet linear motor is similarto that of a moving magnet actuator.The difference is that the coils arereplicated to increase the stroke. Thecoil winding typically consists of threephases, with brushless commutationusing Hall effect devices.
The forcer is circular and moves upand down the magnetic rod. This rod isnot suitable for applications sensitive to
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The forcers are ironless, which meansthere is no attractive force and nodisturbance forces generated betweenforcer and magnet track. The ironlesscoil assembly has low mass, allowingfor very high acceleration.
Typically thecoil winding isthree phase,with brushlesscommutation.Increased performance can beachieved by adding air cooling to themotor. This linear motor design is bettersuited to reduced magnetic fluxleakage due to the magnets facingeach other and being housed in a U-shaped channel. This also minimizesthe injury risks of fingers being trappedby powerful magnets.
magnetic flux leakage and care mustbe taken to make sure that fingers donot get trapped between the magneticrod and an attracted surface. A majorproblem with the design of tubularmotors is shown when the length oftravel increases. Due to the fact thatthe motor is completely circular andtravels up and down the rod, the onlypoint of support for this design is at theends. This means that there will alwaysbe a limit to length before thedeflection in the bar causes themagnets to contact the forcer.
U Channel Linear motor
This type of linear motor has twoparallel magnet tracks facing eachother with the forcer between theplates. The forcer is supported in themagnet track by a bearing system.
Coil
Epoxy
Aerotech U-channellinear motors
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Due to the design of the magnet track,they can be added together toincrease the length of travel, with theonly limit to operating length being thelength of cable management system,encoder length available, and theability to machine large, flat structures.
Flat-type linear motors
There are three designs of thesemotors: slotless ironless, slotless iron,and slotted iron. Again, all types arebrushless. To choose between thesetypes of motors requires anunderstanding of the application. Thefollowing is a list of the maincharacteristics of each type of motor.
Slotless ironless flat motors
The slotless ironless flat motor is aseries of coils mounted to an aluminumbase. Due to the lack of iron in the
forcer, the motor has no attractive forceor cogging (the same as U-channelmotors). This will help with bearing lifein certain applications. Forcers can bemounted from the top or sides to suitmost applications.
Ideal for smooth velocity control suchas scanning applications, this type ofdesign yields the lowest force output offlat-track designs. Generally, flatmagnet tracks have high magnetic fluxleakage and care should be takenwhile handling these to prevent injuryfrom magnets trapping you betweenthem and other attracted materials.
Aerotech BLMFflat linear motor
Aluminum base
CoilEpoxy
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forcer to the magnet track as they willattract each other and may causeinjury. This motor design producesmore force than the ironless designs.
Slotted iron flat motors
In this type of linear motor, the coilwindings are inserted into a steelstructure to create the coil assembly.The iron core significantly increases theforce output of the motor due tofocusing the magnetic field created bythe winding. There is a strong attractiveforce between the iron-core armatureand the magnet track, which can beused advantageously as a preload foran air-bearing system. However, theseforces can also cause increasedbearing wear at the same time. Therewill also be cogging forces, which canbe reduced by skewing the magnets.
Before the advent of practical andaffordable linear motors, all linearmovement had to be created from
Slotless iron flat motors
The slotless iron flat motor is similar inconstruction to the slotless ironlessmotor except the coils are mounted toiron laminations and then to thealuminum base. Iron laminations areused to direct the magnetic field andincrease the force.
Due to the iron laminations in theforcer, an attractive force is nowpresent between the forcer and thetrack and is proportional to the forceproduced by the motor. As a result ofthe laminations, a cogging force isnow present on the motor. Care mustalso be taken when presenting the
Steel laminations withaluminum base
Epoxy Coil
Skewed magnets
Aerotech BLMFS5 iron-core flat linear motor
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rotary machines by using ball or rollerscrews or belts and pulleys. For manyapplications, such as where high loadsare encountered and where the drivenaxis is in the vertical plane, thesemethods remain the best solution.However, linear motors offer manydistinct advantages over mechanicalsystems, including very high and verylow speeds, high acceleration, almostzero maintenance (there are nocontacting parts), and high accuracywithout backlash.
Achieving linear motion with a motorthat needs no gears, couplings, orpulleys makes sense for manyapplications where unnecessarycomponents that diminish performanceand reduce the life of a machine canbe removed.
In the following sections we comparethe performance and cost of varioustranslational mechanics including beltand pulley, rack and pinion, and leadscrew, to a U-channel brushless linearmotor.
Steel laminations
Coil Slot
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and not the actual load position, alsocontributes to inaccuracy. A secondlinear encoder could be used tomeasure the actual load position, butthis would add more cost and requirea special servo setup so that positioncan be achieved quickly .
Settling time is also a problem with beltsystems. Even the best reinforced beltshave some compliance whenpositioning ±1 encoder counts. Thiscompliance will cause a ringing, orsettling delay, at the end of a veryquick move, making it impossible topush the machine to a higherthroughput. This problem worsens withlonger belts.
The best that can be achieved in a beltand pulley system in terms ofpositioning repeatability is around 25to 50 μm. Since both speed andrepeatability is the name of the gamewhen it comes to servo mechanisms,the belt and pulley system is not agood choice for high speed, highaccuracy machines.
On the other hand, a linear system canreach speeds of 10 m/s and positionthe load to within 0.1 μm or better.
The benefits of linear motorsLinear motor versusbelt and pulley
A popular way to produce linearmotion from a rotary motor, the beltand pulley system typically has itsthrust force capability limited to thetensile strength of the belt. At the sametime, accuracy and repeatability sufferfrom the inherent limitations of the belttravel system.
For example:
A belt and pulley system comprising a100 mm diameter pulley and a 5:1gearbox could produce 3.14 m/s oflinear motion, with the motor’s inputspeed at 3000 rev/min. Thetheoretical resolution of this system witha 10,000 PPR (pulses per revolution)encoder through the gearbox wouldbe 6.3 μm.
However, positioning a load on a beltthrough a 5:1 gearbox to 6.3 μm inany repeatable manner is practicallyimpossible. Mechanical windup,backlash, and belt stretching would allcontribute to inaccuracies in thesystem. The fact that the measuringdevice (rotary encoder) is reallymeasuring the motor shaft position,
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Only the resolution of the linearencoder used and the stability ofmechanics limit the performance. Sincethere is no backlash or windup, adirect-drive linear motor system willhave repeatability to one encodercount over and over again.
Settling time is also unchallenged,since the load is directly connected tothe moving forcer coil and there is noinherent backlash in the linear motorsystem. The encoder is also directlyconnected to the load to keep thepositioning accuracy where it reallymatters. All this adds up to the shortestsettling times achievable and highperformance within an encoder count.
Even in long travel linear motorsystems, performance and accuracyremain undiminished since magnettracks are stackable and the loadremains directly connected to theforcer. At the same time, with thrustlimited for the belt and pulley systems,loads have to be light. Conversely, atypical linear motor can produceseveral thousand newtons of thrustforce and still not compromiseperformance.
Linear motor versusrack and pinion
The rack and pinion system ismechanically stiffer than a belt andpulley, but the same translationalequations apply. So a 100 mm piniongear through a 5:1 gearbox couldproduce a 3.14 m/s linear speed at3000 rpm, although rack and pinionprovides more thrust capability. Onceagain, the lack of accuracy andrepeatability is the major drawback.The gearbox and pinion gear will havebi-directional inaccuracies and, overtime, wear will increase the problem.
As with the belt and pulley system,backlash in the system prevents theencoder on the motor from detectingthe actual load position. The backlashin the gears not only leads toinaccuracy but also causes instabilityin the servo system, forcing lower gainsand slower overall performance.
Linear motors do not encounter suchsystem limitations and can push amachine to greater speeds. Even as themechanics wear over time, the direct-coupled linear motor and encoder willalways provide the most accuratepositioning.
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efficient at converting rotary motion tolinear motion, at typically 90% of theoutput. This type of screw systemoutperforms the lead screw for highduty cycles. A precision-ground ballscrew will improve accuracy, but iscostly and, over time, will still wearand result in reduced accuracy andrepeatability. Either way, whether leadscrew or ball screw, the basic screwsystem cannot achieve high linearspeeds without a compromise onsystem resolution. It is possible toincrease the speeds of a ball screw byincreasing the pitch (e.g., 25 mm/rev),but this directly affects the positionalresolution of the screw. Also, too higha rotational speed can cause a screwto whip or hit a resonant frequency,causing wild instability and vibration.This problem is magnified as the lengthof the screw increases. This obviouslylimits the ability to increase amachine’s throughput, or increasetravel while maintaining positionalresolution.
When compared with a screw, thelinear motor system does not introduceany backlash or positioning problemswith the feedback device, as the linearslide bearing is its only friction point.
Linear motor versus screw systems
Probably the most common type ofrotary-to-linear translational mechanicsis the screw, which includes both leadscrews and ball screws.
The lead screw system, thoughinexpensive, is an inefficient way ofproducing linear motion, which istypically less than 50% of the output. Itis also not a good choice for high-duty-cycle applications because the nut thatrides the screw suffers from wear dueto the friction interface. Furthermore,positional accuracy and repeatabilityare a problem because the screw is
typically not precision-made and hasinherent inaccuracies. The resultinghigh friction may minimize backlashbut produces heat and wear, reducingaccuracy and repeatability. The ball-screw system uses a ball nut on thescrew and is therefore much more
Aerotech ATS1000 linear stage
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As with all the other translation systemsdiscussed, the positioning of the loadin a screw system is made with a rotaryencoder mounted on the motor. Thecontroller never really closes a loop atthe load. In a linear motor system theencoder is at the load, reading actualload position.
Consider the application
As with any technology, there arealways limitations and caution must beused to employ the correct solution inany application. While cost was oncea limitation in selecting linear motors,improved manufacturing methods andincreasing volume have combined tomake the expense of a linear motorsolution comparable with a typicalscrew and motor alternative. Indeed,when cost of ownership is taken intoaccount, a linear motor system will,over time, prove to be considerablyless expensive than the traditionalscrew alternative. Today the superiorperformance of linear motors alsohelps meet the more exacting demandsfrom OEMs for higher productivity.
A disadvantage with linear motors isthey are not inherently suitable for usein a vertical axis. Due to its noncontact
operation, if the motor is shut downany load that has been held verticallywould be allowed to fall. There arealso no failsafe mechanical brakes forlinear motors at present. The onlysolution that some manufacturers haveachieved is the use of an aircounterbalance.
Environmental conditions must also beconsidered. Although the motor itself isquite robust, it cannot be readilysealed to the same degree as a rotarymotor. In addition, linear encoders areoften employed as feedback devicesand therefore care must be taken toensure that the encoder is also suitedto the environment. That said, linearmotors have been successfullyemployed in ceramic cutting, anenvironment where highly abrasiveceramic dust has led to the downfall ofmany supposedly more robustsolutions. Again, the motor suppliershould be familiar with all of theoptions and offer advice in each case.
In conclusion, where loads are notexcessive and the driven axis ishorizontal, the linear motor has manyadvantages over traditionaltranslational mechanical systems.
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However, while linear motors can bereadily purchased as components andbuilt into customized assemblies, thereis little information available to help thedesigner decide how best to integrateall of the components for optimumresults. The following is just a briefoverview of some of the considerationswhen purchasing and using linearmotors as components. It suggestswhat other components might berequired and how they can be utilizedwithin the overall design of thepositioning system.
What other componentsto consider
The basic additional elements requiredfor brushless linear-motor-drivenpositioning systems are the servo driveor amplifier with its commutationsystem, the feedback system, and thelinear bearings that support the load tobe driven and to maintain the precisionof travel. To complete the design,motor power and encoder cablemanagement is essential. Additionally,mechanical protection devices such ashard stops may also be desirable asare over-travel limit switches and datum
Applying linear motorsas componentsAs previously discussed, brushlesslinear servomotors offer numerousadvantages over lead screw, ballscrew, or belt-driven transmissionsystems for linear positioning. Withhigher bandwidth and bettersmoothness in speed control from verylow to very high speeds, higheracceleration, zero backlash, andincredibly long life with almost nomaintenance, linear motors are beingspecified for more and moreproduction, test, and researchapplications.
Aerotech manufactures a wide rangeof linear motors available ascomponents, or already built intocomplete positioning systems forapplications from medium accuracypick-and-place machines to ultra-highprecision systems used forsemiconductor production and sub-micron laser processing. With all of thecomponents necessary to support andposition the load, provide position andspeed feedback, advanced motioncontrols, and complete cabling andsafety features, these systems areready for integration into theapplication.
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or home switches, although thesefunctions are sometimes handledthrough the linear position feedback.
As linear motors are friction-free andthe linear bearing system is normally alow friction device, braking may berequired for power-loss or power-offconditions. For those verticalpositioning applications where it isdesirable to use a linear motor,designers will almost certainly need toconsider a counterbalance and abraking system. As we covered in theprevious section, in many cases it maybe preferable to stay with traditionallead screw or ball-screw designs forvertical axes where their designreduces back-driving under power-offconditions.
Consideration should also be given tomotor cooling, which may be requiredto increase performance or improvethermal stability for the application.Both air cooling and water-jacketcooling systems can be supplied.
Choice of motor and which drivetechnology to use
The choice of linear motor that bestsuits the application will clearlydetermine the design requirements forthe rest of the system. As discussed inprevious chapters, linear motors aregenerally divided into three groups:cylindrical moving magnet, U-channel,and flat. Within these three motortypes there are significant variations inconstruction and performance, and theresulting specifications are availablefrom each manufacturer.
A note of caution: When choosing thebest linear motor for the job, careshould be taken when comparingmanufacturers’ specifications to ensurea legitimate “apples-to-apples”
Aerotech Ndrive seriesincludes both PWMand linear models
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reduced but the exceptionally smoothcog-free performance is more suited tohigh-precision positioning. However,for many applications overallperformance (acceleration,deceleration, and settling time) isdetermined by a combination of thelimitations of the bearings, feedbackdevice, and cable managementsystem.
Servo drives or amplifiers can bedivided into two main types: PWM(Pulse Width Modulation) or linear.Both require commutation to delivercurrent to the motor (commutation iscovered in depth in a followingchapter of this guide). The PWMamplifier uses high frequency switchingto force current into the motor, whereasthe linear amplifier uses a smoother butless dynamic analog voltage control.
As a general guide to the performancefor slotless linear motors with theseamplifiers, PWM power stages deliverhigher speeds and produce more loadcarrying motor force than linearamplifier stages, and are used for themajority of applications. While thePWM stage provides a high level ofsmoothness with excellent in-position
comparison. Aerotech linear motorspecifications are clearly defined inboth the model specifications data aswell as in our Engineering Referencethat defines the metrology method aswell as each specification term.
Essentially, U-channel and flat linearmotors will require bearings tocomplete the design, whereas thecylindrical type can replace orcomplement the bearing because itsconstruction often includes a ball-bushing-type bearing that can supporta small load. However, caution shouldbe observed when considering thecylindrical linear motor as thesupporting bearing because themagnetic rod will increasingly sag ordeflect in proportion to its length and,if unsupported, can make contact withthe forcer with catastrophic results.
Aerotech’s range of linear motors isdivided between flat and U-channeltypes that include slotted and slotless,iron core, and ironless designs. Iron-core linear motors can provide moreforce, but a slight cogging effect isalways present that may provedetrimental to very high positionstability. In the ironless design, force is
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stability for micron-level positioning, forthose applications where ultra-highprecision positioning is required, andposition stability into the nanometerrange is paramount, linear amplifiersshould be considered. This type ofamplifier also has a much reducedEMC noise footprint (almost negligible)and its high bandwidth and zerocrossover distortion is also ofsignificant benefit for ultra-highprecision synchronized multi-axispositioning applications.
Feedback System
Linear encoder feedback andcommutation systems are required forall closed-loop brushless servo linearmotors, although there may be somebrushless linear motor applicationswhere position feedback could beprovided by other devices such aslinear potentiometers or magnet-basedsystems. However, the principal ofdesigning-in these feedback devices issimilar to the linear encoder.
Most linear motor stages use opticalscale and reading-head-type encoders,although it is also possible to usemagnetic encoder systems that are
extremely robust and useful for hostileenvironment applications. For precisionapplications, linear optical encodersare capable of higher levels ofaccuracy than their magneticcounterparts due to their finer scalepitch. Whereas magnetic linearencoders can provide accuracies toaround ±3 μm over a one meterlength, the best linear optical scalesexceed ±1 μm per meter. With the useof suitable amplified sinusoidalencoders, resolutions withmultiplication for magnetic encoderscan be as low as 0.05 μm, while theoptical designs can provide 0.010 μm(10 nm) and beyond.
Aerotech AGS15000 linear motor gantry uses optical encoders
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smaller cyclic or sub-divisional errorcaused by the interpolation method.And while this error is non-accumulating, it can have a significanteffect on the servo control loop andcan cause hunting, temperature rise,and generally reduce the dynamicsystem accuracy, so it will need to beconsidered.
It is possible to map an encoder-basedsingle-axis (or dual-axis) setup with aninterferometer and calibrate theaccuracy with actual positionalinformation at a point of reference. Thisinformation is then used within themotion controller as a look-up table todynamically correct position duringmotion. This error correction method isincreasingly specified for highprecision systems such as high-accuracy laser machining where theresulting calibrated positioninformation can be included toprecisely trigger laser pulse delivery.
Bearing Technology
The bearing technology used in alinear motor application should becapable of supporting the load as wellas maintaining the air gap between
One of the major benefits of using alinear encoder system in a linear motorsetup is that the reading head andscale can be placed very close to or atthe bearing centerline, where it willmeasure the linear position at its mostaccurate point – i.e., free from Abbeerrors that are caused by very smallangular deviations in straightness andflatness being amplified by thedistance between the bearing and thepoint of measurement.
Optical linear encoders can take theform of glass scales or tapes. Glass orother specialized low thermalcoefficient materials used as scales arenormally the more expensive choiceand have superior temperaturestability, but tapes are much easier toinstall and are more durable.
In most precision applications whereoptical or magnetic encoders are used,the actual scale pitch is presented as asine wave and interpolated to yieldmuch finer resolution, but this does notimprove the overall linear accuracy.The accuracy of the full scale can beseen as a gradient with increasingerror over distance. This dominantscale error is accompanied by a
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platen and moving forcer, which istypically less than 0.75 mm for the flatmotor. As we are dealing withprecision systems, only high-precisionbearing systems are considered hereand in this category the main choicesare air bearings, crossed-rollerbearings, and linear motion guidebearings. Aerotech’s own range ofmedium to ultra-high precisionpositioning stages with direct-drivelinear servomotors use all of thesebearing styles to suit performance andcost requirements for the application.
Air bearings are normally used forvery high-precision applications wherethe friction-free characteristics of thebearing perfectly complement thenoncontact linear motor and feedbackdevice design, allowing the very bestpositioning performance. Air bearingstypically rely upon a very small airgap of just a few microns with
pressurized air forming a cushionbetween the moving and fixed parts.
Most linear motion guide, ball, andcrossed-roller-based bearing designsinclude cages to separate the actualball or roller, and a top to eliminatebearing-to-bearing contact duringmotion. This also helps to reduceskewing and noise, improvelubrication, and generally to optimizeacceleration and high-speedperformance while providing a muchlonger life potential. The choice of ballor roller bearings is dictated by thetype of loading that the mechanicalsystem is subjected to, and linearmotion guides are also supplied in achoice of preload to suit loadrequirements. Roller-based bearingsprovide much higher load, shock, andimpact load characteristics due toincreased contact area betweenbearing surfaces and the bearingways, but they increase friction and insome high-precision positioningapplications, this factor may hinder in-position performance and low-speedstability. Lubrication is another criticalfactor for high-precision applications,where the wrong lubricant may
Aerotech ABL1500air-bearing stage
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Linear motion guide bearings may bespecified for demanding applicationssuch as cleanroom, vacuum, corrosionresistant, high speed, and low/hightemperature. These bearings usuallyinclude sealing strips as standard andprovision for special lubrication. Evenfine movement applications can beaddressed with modified standardproducts that include speciallubrication.
During the design phase, all of thedirect and offset loading requirements,the speeds and duty cycle, and lifecalculations of the complete positioningsystem must be carefully determined toensure that the bearing system selectionis suited to the application. When acomplete positioning stage is selected,these calculations have already beenmade and all the specification detailsare available from the manufacturer. Inthe case of purchased components thechoice of bearing system and its designmust be determined and suitable cablemanagement pre-selected before thelinear motor and feedback system aresized and selected.
introduce stick-slip (stiction) and rendervery short duration, micron-level moveserratic or otherwise difficult to achieve.
It is very important that the bearingsystem is fixed to a rigid and soundbase to ensure that bearingstraightness and flatness can beachieved and that the systemharmonics are minimized so as not tointerfere with dynamic positioning andmotion performance. It should benoted that unsupported linear bearingsare not inherently straight and must befixed to a datum surface with the aid ofcalibrated alignment equipment suchas an angle collimator or laser-basedsystem to ensure the desiredstraightness and flatness and pitch, roll,and yaw accuracy.
Aerotech ANT series nanopositionersuse direct-drive linear motors andcrossed-roller bearings
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Linear brakes that fit the form of linearmotion guide rails are available as aconvenient solution for preventingstage movement under power-offconditions.
Cable management
As linear motors and feedback devicesare essentially friction-free and linearbearings have a predictable life, theworking life of a complete positioningsystem and the frequency ofmaintenance downtime is oftendependent upon the cablemanagement system.
E-chain can be used for motor powerand encoder cables, as well as otherelectrical, fiber optic, pneumatic, oreven hydraulic services required for aparticular project. The first task is todefine the total cable requirement anddetermine the diameter, weight, andbend radius for each. Most cablemanufacturers specify cable bendradius but it is very important to use thedynamic rather than the static rating.The bend radius refers to the insidediameter of the cable and whereverpossible “chain rated” cable should bespecified. This cable is specifically
designed to withstand the rigors ofconstant flexing and high accelerationsto provide a much longer working lifethan standard cable. Consideration forthe cable performance in the correctenvironment should also be given –i.e., temperature, humidity, cleanliness,vibration, etc. all have an effect oncable performance and life. The cablechain orientation – horizontal, sidemounted, standing or hanging – is alsoof importance as is the space andclearance envelope, how the chainmay need supported, andrequirements for maintenance access.
As with the choice of linear bearing,the weight and drag from the E-chainsystem will influence linear motorsizing and will be required to correctlyselect the motor type and power forthe finished system design.
To start, lets consider the brushedmotor. When current is applied to themotor, the correct winding is energizedby virtue of the brushes being incontact with the commutator at thepoint where the winding terminates. Asthe motor moves, the next coil in thesequence will be energized. Inbrushless motors, because there is nofixed reference, the first thing a controlor amplifier must determine is whichphase needs to be energized. Thereare a number of ways that this can beachieved, but by far the most popularis by using Hall effect devices. Thereare three of these devices, one foreach phase, and they give a signalthat represents the magnetic fieldsgenerated by the magnet track. Byanalyzing these fields, it is possible todetermine which part of the magnet
Commutation of linear motorsWhat is commutation and how does itaffect the performance of the linearmotors? Commutation is the process ofswitching current in the phases in orderto generate motion. Most linear motordesigns today use a three-phasebrushless design. In brushed motors,commutation is easy to understand asbrushes contact a commutator andswitch the current as the motor moves.Brushless technology has no movingcontacting parts and therefore is morereliable. However, the electronicsrequired to control the current in themotor are a little more complex.
The method of commutation dependson the application of the motor, but it isimportant to understand how the motorcan be commutated and whatdisadvantages some methods have.
HallA
0 60° 120° 180° 240° 300°
Phase C
Hall sequence forbrushless motorand phase current
Phase A
Phase B
HallB
HallC
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track the forcer is in and thereforeenergize the correct phase sequence.
There are three different types ofcommutation currently available on themarket: trapezoidal, modified six step,and sinusoidal. Trapezoidalcommutation is the simplest form ofcommutation and requires that digitalHall devices are aligned 30ºelectrically from the zero crossing pointof the phase. At each point that a Hallsignal transition takes place, the phasecurrent sequence is changed, thuscommutation of the motor occurs. Thisis the cheapest form of commutationand the motor phase current looks likethe diagram shown above.
Modified six step commutation is verysimilar to trapezoidal commutation.The digital Hall devices are alignedwith the zero crossing point of thephase per the diagram showing theHall sequence of a brushless motor.Again, at each point that the Hallsignal translation is seen the phasecurrent is switched. However, with thismethod two current sensors are usedand it provides a commutationsequence that is closer to the idealsinusoidal phase current. This methodis slightly more costly than trapezoidalcommutation due to sensing twocurrent levels. Both of these Hall-basedmethods will cause disturbance forcesresulting in higher running temperatureand motion that is not smooth.
00
0.5
1
-0.5
-1
30 60
Commutation Waveforms
Six Step
Sinusoidal
Trapezoidal
Electrical Degrees
90 120 150 180 210 240 270 300 330 360
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Commutation is done by generating asin(θ) phase A command signal and asin(θ +120°) phase B command signaland multiplying this by the currentcommand.
This method of commutation gives thebest results, due to the same processorbeing used to control current, velocity,and position, and yields faster settlingtime and tighter servo loops. Also, thenoise on the digital Halls is mucheasier to filter-out, creating a morereliable system. When sinusoidalcommutation is used with linear motors,the motion is smooth and the motor isdriven more efficiently, causing lessheating.
Sizing up linear motors
So how do we take advantage of thelinear motor’s superior performanceand what are the correct procedureswhen sizing and applying a linearmotor?
The ideal means to drive anysinusoidally-wound brushless motor isby sinusoidal commutation. There aretwo ways that this is commonlyachieved. Analog Hall effect devicescan generate a sinusoidal signal as themotor passes over the magnetic polesof the magnet track. The signals, whichare correct for motor commutation, arethen combined with the demand signalto correctly commutate the motor.
This method is the lower cost of the twomethods, but noise can be picked upon the Hall devices, affectingcommutation.
Another more popular method ofsinusoidal commutation is by using theencoder. When a change of state isdetected in the digital Hall signal, theincremental encoder signals can thenbe used to digitally determine where inthe commutation cycle the motor is.
�
�
�
Sin (θ+120°)
Sin (θ)
θ
M
Encoder
∅A
∅B
∅CSin (θ+240°) X
X
X
ICMD
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Force
f a=maf f=mgμfg=s in(θ )mgfp=fa+f f+ fg
Velocity
v=u+atv 2=u 2+2as
v= +u
Distance
s=ut+½at 2
s=
s=
Acceleration
a=
a=
a=
To start with, here is a list of useful formulae:
2st 2
v–ut
v 2–u 2
2ss–ut2t 2
(u–v) t2
(v 2–u 2)2a
Coil temperature
T= RT
Time
t=
t=
t=
v–ua
2sv–u2sa
RMS force
frms = t 1f 12+ t 2f 2
2+ t 3f 32+ . . . + t nf n
2
t 1+ t 2+ t 3+ . . . + t n+ tdwell
f rms
MC( )
tdwell
time to repeattime
frms
F
V
f1
t1
f2
t2
fn
tn
Where:
faacceleration force (N)
fffriction force (N)
fggravitational force(N)
fppeak force (N)
m mass (kg)
a acceleration (m/s2 )
g gravity (9.81 m/s2 )
μ coefficient of friction
θ angle from horizontal in
degrees (vertical = 90°)
v final velocity (m/s)
u initial velocity (m/s)
t time (s)
s distance (m)
frmsAverage force
T Temperature rise
RTThermal resistance (°C/W)
MCMotor constant (N/√W)
2
tdwell
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Based on trapezoidal motion, timetaken to accelerate is:
= 0.0833 s
We now determine the peak speedrequired to make the move. In thiscase, because the move is symmetricaland divided into 3, the followingformula is used:
v = = 3 m/s
Note that this formula only works witha trapezoidal move. If you have adesired acceleration rate, then you canwork out the speed using one of theformulae on the previous page. Theload cannot accelerateinstantaneously from 0 to 3 m/s and as already shown itwill take 0.0833 s to reach this speed.We now need to calculate theacceleration rate:
a = = = 36 m/s2 ≅ 3.6 g
As an example, we will consider a 50 kg load (mass), that is required tomove 500 mm in 250 ms, dwell for275 ms, and then repeat. In this casewe can calculate the required forcesand therefore find the size of linearmotor and amplifiers required.
The first thing to consider is the movecharacteristics: What is the peakspeed? How fast do we need toaccelerate? How long will the movetake? What dwell do we have whenthe move finishes? In general, whenyou are unsure of the move parametersand just want to move from point topoint, the basic profile is thetrapezoidal move. With this move, themove time is divided equally into threeparts. The first part is acceleration, thesecond constant velocity, and the thirdpart is deceleration. This should give abalance between speed andacceleration to give the best motorcombination. But please remember thatif you size the motor this way, then youshould program it this way.
50 kg 500 mm
0.25 s3
3s = 3 x 0.5 m2t = 2 x 0.25 s
V
t1=t2=t3
250 mstdwell =275 ms
v – ut
3 m – 00.0833 s
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If required, you can also calculate thedistance taken to accelerate the load:
s=ut +½at2 =½x36x0.08332=0.125 m
Newton’s equation finds the forcerequired for the acceleration:
fa = ma = 50 kg x 36 m/s2 = 1800 N
This is the peak rating needed from theprospective motor, derived only fromthe acceleration force. It does notaccount for friction, gravitational orother opposing forces. For example, aquality crossed-roller bearing used tocarry the load has a coefficient offriction of about 0.0005 to 0.003.When the 50 kg rides on thesebearings, the frictional force is:
ff = mgμ = 50x9.81x0.003 = 1.47 N
Because friction always opposesmotion, it adds to the driving forcerequired. Another force that becomesrelevant is the gravitational force. In thisexample the force is zero because theload is supported by the bearings, butshould the load be at an angle, thenthe following formula is used:
fg = sin(θ)mg = sin(0)x50x9.81 = 0 N
Calculating the peak force is simply acase of adding these numberstogether:
fp=fa+ff+fg=1800+1.47+0=1801.47 N
Care must be taken with this peakforce. Any other external forces suchas cable management systems shouldalso be added to the peak force total.Next, with a known total ofacceleration and friction forces, thenext step is to calculate the continuousforce requirement. The rms force is theaverage force from the motor andhelps determine the final temperaturethat the coil will reach. Based on theabove example using a trapezoidalprofile, the calculation will be:
RMS force
frms =
=
= 1015 N
The rms force of 1015 N together withthe peak force requirement can thenbe used to choose a specific size andmodel of motor that can apply this
0.0833x1801.472+0.0833x1.472+0.0833x1801.472
0.0833+0.0833+0.0833+0.275
t 1f 12+ t 2f 2
2+ t 3f 32+ . . . + t nf n
2
t 1+ t 2+ t 3+ . . . + t n+ tdwell
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Once selected the previous formulaewould be repeated with the weight ofthe forcer added onto the load. Forsimplicity the new values are:
fp 1962 Nfrms 1105 Nv 3 m/s
To determine the coil temperature risein this configuration we need tocalculate it. If we assume the ambient
force continuously. Adding air-coolingcan significantly increase the rmsoutput force of a particular motor,which allows a smaller forcer coil tomaximize stroke length.
For this application, an Aerotech motorthat suits the above requirement is theBLMX-502-B motor with air cooling.The specification of this motor is asfollows:
Parameter Unit Value
Continuous force @1.36 bar N 1063
Continuous force no air N 601
Peak Force N 4252
BEMF line-line V/m/s 54.79
Continuous current @1.36 bar Amp Peak 22.30
Continuous current no air Amp Peak 12.60
Force Constant, sin drive N/Amp Peak 47.67
Motor Constant N/√W 46.23
Thermal Resistance @1.36 bar ºC/W 0.12
Thermal Resistance no air ºC/W 0.39
Resistance 25ºC, line-line Ohms 1.1
Resistance 125ºC, line-line Ohms 1.8
Inductance, line-line mH 1.0
Max Terminal Voltage VDC 340
Magnetic pole pitch mm 30.00
Coil Weight kg 4.45
Coil Length mm 502.0
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temperature is 20ºC, then this shouldbe added to the coil to get the finalcoil temperature rise:
In this application final coiltemperature rise will be 88.56ºC.Typically temperatures over 100ºCshould be avoided. If you aredesigning a high accuracy system thenthe temperature that the coil reacheswill be significant. As the temperatureof the coils increase, so willsurrounding areas, and expansion willoccur changing the accuracy of thesystem.
Next we need to size an amplifier todrive this motor. As it is sinusoidallywound, a sinusoidal amplifier isrecommended and we have workedout the characteristics for one. First, weneed to check our current requirement.If we need to create 1962 N peakwith the BLMX-502-B, then thefollowing formula is used to calculatepeak current:
PeakCurrent (lp) = =
= 41.16 Apeak
frmsMC( )2 1105
46.23( )2
fpForce_Constant
196247.67
Continuous current is calculated in the same way, so for 1105 Ncontinuous:
ContCurrent (lrms) = =
= 23.18 Apeak cont
We also must check for the requiredbus voltage. To do this we need tomake sure that we have enoughvoltage to drive the peak currentacross the coil resistance, taking intoaccount the motor voltages beinggenerated. To do this we calculate asfollows:
Drive_Voltage_minimum =(lp x Coil_Resistance) + (v x BEMF) =(41.16 x 1.8) + (3 x 54.79) = 238.46 V
The amplifier required to drive thismotor for this application will have thefollowing specification:
Peak Current 41.16 ACont. Current 23.18 AMin Bus Voltage 238.46 V
frms
Force_Constant110547.67
T = RT = 0.12 x = 68.56°C
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Aerotech has a wealth of experiencein using linear motors and if there is any doubt about the applicationcharacteristics or any otherparameters, it is always best to ask.We are happy to work with you tosolve these uncertainties.
A suitable drive from Aerotech wouldbe the Soloist HPe50. With thiscontroller/drive, the maximum speedthat could be reached would be:
v =
= 4.85 m/s
Please note that in these calculationsthe resistance of the coil at 125ºC wasassumed as this was worst case.
There are many different types ofmove profile, including sinusoidalacceleration profiles. All of these willaffect the sizing of the linear motor.Aerotech has sizing software that will help you to size linear motors withmany of these combinations built in.The key issue to remember is toprogram the motor the same as thecalculated parameters. In the aboveexample, if we altered theacceleration rate, the force woulddramatically increase and coulddamage the motor coil.
Max_Bus_Voltage_Amp - (lp x Coil_Resistance)BEMF =
340 - (41.16 x 1.8)54.79
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Linear motor quickselection guide
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Linear motors are available with customwindings and magnet tracks, air andwater cooling options, and completemotor designs with minimal lead times.
Specialty and custom motorsAerotech’s unique in-house motor designand manufacturing capabilities allow foreasy changing of electrical andmechanical specifications.
Motor Model Units BLMFS2-200 BLMFS3-160 BLMFS3-220Performance Specifications
Continuous N 286.0 387.0 525.0Force, no air lb 64.3 87.0 118.0
Peak Force N 1144 1550 3080
lb 257 347 700
Attraction Force N 1600 2780 3860
lb 400 626 870
Electrical SpecificationsWinding
–A –A –ADesignation
BEMF, line-line V/m/s 59.4 41.6 63.0
V/in/s 1.51 1.06 1.60
Continuous Amppk 5.5 13.8 10.4Current, no air Amprms 3.9 9.8 7.4
N/Amppk 51.6 36.5 50.8Force Constant lb/Amppk 11.6 8.2 11.4
sine drive N/Amprms 72.7 51.6 71.8 lb/Amprms 16.4 11.6 16.1
Motor Constant N/√W 20 29.8 36.8
lb/√W 4.51 6.6 8.3
Thermal Resistance ˚C/W 0.49 0.45 0.36
Resistance, 25˚C ohms 6.2 1.52 2.3
Resistance, 125˚C ohms 8.7 2 3
Inductance mH 5.2 16.5 25
Max Terminal Voltage VDC 320 320 320
Mechanical Specifications
Coil Weight kg 2.4 5.4 7.5
lb 5.2 11.9 16.5
Coil Length mm 200.0 160.0 220.0
in 7.87 6.3 8.67
Heat Sink Area mm 300x300 250x250 250x250Thickness 12.7 mm (0.5 in) in 12x12 10x10 10x10
Magnet Track kg/m 4.8 6.8 6.8Weight lb/ft 3.2 4.6 4.6
Magnetic Pole mm 30.0 22.5 22.5
Pitch in 1.18 0.89 0.89
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Dedicated to the Science of Motion
AEROTECH is a world leader inpositioning and motion control, withoffices and subsidiaries in the UnitedStates, Europe, and Asia.
We are at the forefront of linear motortechnology, with a wide range oflinear motors and stages.
These form part of a comprehensiveline-up of class-leading standardproducts including motion controllers,amplifiers, and rotary motors.
We can also provide completeengineered systems for your specificapplication.
Our products and solutions are backedby a worldwide technical support andcustomer service network, dedicated toproviding outstanding life-cycle supportservices.
All Aerotech amplifiers are rated inAmppk; use force constant in Amppkwhen sizing.
All performance and electrical motor specifications ±10%.
Specifications at 125˚C operatingtemperature unless otherwise specified.
Aerotech Inc (U.S.A.)
Aerotech UK Aerotech Germany
Aerotech Japan Aerotech China
★ - Aerotech Headquarters ● - Direct Field Sales Office ▲ - Aerotech Subsidiary ■ - Representative
Dedicated to the Science of Motion
Aerotech’s Worldwide Sales and Service Locations
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