16 PTR Mar/Apr 2011Special Feature By Mark Howard of Zettlex Limited.Accurate Measurementof Shaft SpeedMeasuring or controlling the speed of most rotating shafts isstraightforward but significant problems can arise as soon as we start totalk about accuracy. Mark Howard from Zettlex Ltd. examines the issuesinvolved and suggests some simple solutions to longstanding problems.There is a famous questionthat sits at the crossroads ofgeography, mathematics andphilosophy: ‘How long is thecoastline of Britain?’ At firstglance, a pretty straightforwardquestion – just think back to yourgeography textbook and dredgeup the correct number. Mostpeople say it’s around 11,000miles – estimated from theperimeter of a triangle formed byDover, John O’Groats and Land’sEnd with a ‘fudge factor’ addedon for Wales.For pub quizzes, this answermight be good enough, but is itaccurate? Well, you could get allthe ordnance survey maps andtrace the coastline with a pieceof string and eventually produce asecond estimate.Certainly, the answer will bedifferent to the first, but is itmore accurate? The snag is howdo you measure? Where do youtrace the string for an accuratemeasurement? How far up therivers do you go? Do you traceall the way to the source of theriver and then back along theother bank? Do you measure thecoastline at high tide or low tide?Do you trace in and out of everyrock? At an extreme you would bemeasuring around every pebbleand every grain of sand, to suchan extent that the answer is muchlarger than first expected. Sothe truly accurate answer to thequestion is – it depends on howyou measure it.The same kind of bizarrelogic applies to shaft speedmeasurement. If you simplyneed a rough idea of how manyrevolutions a shaft completesin one minute, it’s prettystraightforward and even thecrudest of measuring systemsshould deliver a fair answer.But what happens when youneed to know what the speedis, say, every millisecond andthen control the actual speed sothat this corresponds to a tightlytoleranced set point?To borrow some terminology fromthe old vinyl record turntables,speed variations can bedescribed as ‘wow’ and ‘flutter’.Wow typically refers to speedvariations over relatively longperiods and flutter refers to speedvariations over relatively shortperiods, typically less than onceper revolution. Often, both areimportant and the requirementsto tightly control both wow andflutter are common in manysectors of industry: CNC machinetool motion control; aircraft flightcontrols; radar antennae; andweapons control systems. Suchcontrol engineering issues donot just relate to complex motioncontrol but also to driving shaftsat a constant speed (especiallywhen there are variations in load).When one approaches the issueof a shaft that must rotate attruly constant speed, then thesame logic that has us measuringaround the grain of sand willquickly tell you that to get a shaftto rotate at a perfectly constantspeed is – in the extreme –impossible.One common problem is thatthe shaft speed is not measureddirectly but indirectly. A typicaldrive system will include a shaft,driven by a gearbox or pulleys,a motor and a motor encoder.Readings from the motor encoder– usually a stream of pulses –are used to calculate the speedof the shaft itself. In turn, theoutput from the encoder is fedback (in a servo loop) to themotion controller, which adjuststhe <strong>power</strong> fed to the motor toincrease or decrease speed.A significant problem is thatalthough the system may besomewhat mechanically coupled,what’s going on at the motor isnot the same as what’s going onat the shaft. For example, anygears in between the encoderand shaft will not be perfect andare subject to wear, backlash,thermal expansion/contraction,mechanical tolerances andclearances. Further effectsinclude mechanical friction(especially ‘stiction’ at lowerspeeds), variations in lubricantproperties, mechanical twist dueto torque, shaft bending, shaftconcentricity and so on.To determine what is actuallygoing on at the output shaftneeds an encoder on theactual shaft itself. In reality,this can prove both difficultand expensive, especially if theshaft is large or where spaceconstraints are tight. A furtherdifficulty arises when the shaftspeed is low, since accuratespeed control will dependon sufficient measurementinformation being producedper revolution, to give sensibleor timely control of the motor.A measuring device on theshaft with, say, 100 counts perrevolution is not going to permitaccurate speed control of a shaftthat is rotating at 1 rpm, sincethe measurement will only beupdated every couple of seconds.To control shaft speed accuratelyat low speeds, the greater theneed for high resolution angleinformation at the output shaft.There are 3 main methods formeasuring shaft position orspeed: magnetic, optical andinductive. The most commonis magnetic – usually Halleffect – but this is not usedin high accuracy or low speedapplications since it lacks theresolution, whilst magnetichysteresis or temperature effectswill degrade measurementperformance.Optical encoders offer goodmeasurement performance butare delicate and unreliable inharsh conditions. Optical sensorsare typically rated to only modesttemperature ranges (-20 to +70Celsius is typical); they can failbecause of foreign matter andharsh mechanical vibration orshocks can damage the opticalgrating.Inductive devices such asresolvers and synchros arethe traditional choice for highreliability or harsh environments,including military, oil and gas,aerospace, and heavy industrialapplications. Whilst thereputation of inductive devicesfor reliability and accuracy iswell founded, they are bulky,heavy and expensive, especiallyin the larger sizes or ‘A class’measurement performance.A new generation of inductivetechnique now enables moreand more people to chooseinductive devices for mainstreamcontrol applications. Rather thanthe traditional wire windings orspools, this new generation ofdevices uses printed, laminarconstructions, which dramaticallyreduces the bulk, weight and costcompared to traditional devices.At the same time, accuracyincreases and the possibilityof a wide range of shapes andsizes of sensor opens up. Inparticular, large bore devicescan be provided without massiveincreases in cost. In turn, thismakes direct mounting to theshaft more practical and hencemore accurate. Furthermore, theneed for high precision gearboxesis eradicated and generally thegearbox can be derated – allowingadditional cost reductions.For reader enquiries, pleasemention PTR:Mark Howard,Zettlex UK LtdNewton Court,Town Street,Newton, CambridgeCB22 7PETel: +44 (0) 1223 874 444Fax: +44 (0) 1223 874 111Email: info@zettlex.comwww.zettlex.com
PTR ProductsNew from Micromech is the NANOZ ? novel air bearing lift stageJust launched with a patentpending is the Alio NANO Z? a novelair bearing lift stage with TRUENANO POSITIONING? and to helpengineers and scientist further,it has been designed with a largeopen centre.It uses linear servomotors andvariable counterbalance to exceedprecision application needs suchas wafer inspection, where heavyceramic wafer plates need electricaland vacuum lines fed though thepositioning stage.Other applications such as TRUENANO? metrology, optical inspectionand laser machining are performedat accuracy levels less than100nm without the concern ofballscrew thermal growth, wedgelifts tip and tilt, non-repeatabilityand inaccuracies of other legacydesigns.The NANO Z? clever design almosteliminates pitch, yaw and roll overthe 24mm travel range with betterthan 90nm of repeatability in thestandard unit. Optional extremeprecision Nano Z? will deliver anamazing 10nm of repeatability withbetter than 50nm of accuracy.The NANO Z? footprint is optimisedfor compact size due to the novelair-bearing and linear motor designthus it can easily sit on top orinverted on an XY motion system.For metrology or other processesthis means no increase in themotion system’s overall verticalfootprint or the need for specialmounting compared to other designswhere the Z axis lifts with motorsand screw that need four times thevertical distance.For reader enquiries, pleasemention PTR:Stirling MorleyMicromech Ltd,5-8 Chilford Court, Rayne Rd,Braintree, Essex CM7 2QSTel: 01376 333333Fax: 01376 551849Email: stirling@micromech.co.ukwww.micromech.co.ukvisit our website:rarodriguez.co.uk...to request ourcatalogueOur KHK range coversover 9300 gear types.Standard range includes:Spiral bevel gearsSpecials and modified gearsEven specials carry a short lead time.Hardened and stainless steel gears and racksInjection moulded spur and bevel gearsR. A. RODRIGUEZ (UK) Ltd.TURNING IDEAS INTOENGINEERING SOLUTIONSTel: 01462 670044 Fax: 01462 670880Email: info@raruk.comMar/Apr 2011 PTR 17Ex-stockPrecision in the ExtremeInductive angle encodersthat tick all the boxesAccurateAbsoluteRobustLarge boreCompactLightweightEconomicalVisit us on-line at www.zettlex.com
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