CorrespondenceCLOSED-LOOP CONTROL OF STEPPINGMOTORS: PREDICTION AND REALISATIONOF OPTIMUM SWITCHING ANGLE

The authors of Paper 1929B [/EE Proc. B, Electr. PowerAppL, 1982,129, (4), pp. 211-216] are to be congratulatedfor providing a lucid account of the closed-loop controlrequirements of a stepping motor. Optimisation ofswitching angle is given particular prominence and amethod is described whereby this quantity may be syn-thesised by means of a phase locked loop operating onshaft encoder step position pulses.

However, there is no mention in the discussion of thistechnique of the loss of accuracy arising from the phaseerror between the encoder position signal, i.e. the input tothe phase locked loop, and the 'divide by m' signal, whichis derived from the oscillator output of the loop. That thephase locked loop can be controlled to suppress thiserror is perfectly true, but this can only be achieved sometime after motor acceleration and the consequent rise inposition pulse repetition rate has been initiated. [A]Phase error, in the interim period, is unavoidable andinaccuracy in the selection of switching angle will followas a result.

Two questions are asked of the authors. First, did theymeasure or otherwise evaluate the phase error, and sec-ondly, if so, how did its respective magnitude and dura-tion compare with the minimum required switching angleand the motor acceleration time.

C.L. HALSALL 26th February 1987

Department of Electrical and Electronic EngineeringPaisley College of TechnologyHigh StreetPaisleyRenfrewshire PA1 2BEUnited Kingdom

I would like to thank Dr. Halsall for his valuable andcomplimentary comments. It is inevitable that during

transient operation some error will arise in the synthesisof the frequency-multiplied signal obtained by using thephase-locked loop techniques described in the paper.Unfortunately no data on the size of the phase error areavailable. However, it would seem that such an errordoes not necessarily result in switching angle inaccuracy;provided that the switching angle is optimised, eithermanually using the methods described in Section 3.2.2 ofthe paper, or automatically using a microprocessor-basedcontroller [B], any repeatable errors in the synthesisedposition signal are compensated by the speed discrimi-nator setting.

Since the paper's publication five years ago the tech-nique has been adopted in several stepping and brushlessDC drives [C, D]. Recently its importance has beendiminished not only by the commercial availability ofhigh-resolution low-cost optical encoders, but also by theintroduction of reliable techniques for continuous moni-toring of rotor position using current waveform analysis[E].

P.P. ACARNLEY 24th March 1987

Department of Electrical and Electronic EngineeringUniversity of Newcastle upon TyneNewcastle upon Tyne NE1 7RUUnited Kingdom

References

A GARDNER, F.G.: 'Phaselock techniques' (John Wiley and Sons,1979, 2nd edn.)

B ACARNLEY, P.P., HILL, R.J., and HOOPER, C.W.: 'Closed-loopcontrol of stepping motors: optimisation of switching angle anddeceleration initiation'. IEE Conf. Publ. 234, 1984, pp. 365-368

C ACARNLEY, P.P: 'Stepping motors: a guide to modern theory andpractice', (Peter Peregrinus Ltd., 1984, 2nd edn.)

D DANBURY, R.: 'Improved method of controlling stepping motorswitching angle', Electron. Lett., 1985, 21, (10), pp. 432-434

E ACARNLEY, P.P., HILL, R.J., and HOOPER, C.W.: 'Detection ofrotor position in stepping and switched motors by monitoring ofcurrent waveforms', IEEE Trans., 1985, IE-32, (3), pp. 215-222

192 IEE PROCEEDINGS, Vol. 134, Pt. B, No. 4, JULY 1987