Advanced Control Techniques for the Brushless PermanentMagnet AC Motor by Position Sensor Elimination

HARSHIT AGARWALM.tech (Electric Drives & Power Electronics, IIT ROORKEE)

AbstractA new approach to the position sensorelimination of the Brushless PermanentMagnet AC Motors is presented. Rotorposition feedback is developed by extractingefficient information from the motor back-EMF voltage waveforms.The essential back-EMF is obtained fromdirect measurement.

IntroductionRotor position measurement in brushlesspermanent magnet (PM) motor drives usingconventional discrete sensors presents severaldisadvantages because of the sensor's negativeimpact on drive cost, reliability, and motorlength. The need for additional leads tointerconnect the sensor and controls isparticularly unacceptable in specialapplications such as compressor drives whichrequire hermetic sealing of the motor insidethe compressor unit.

Equivalent rotor position information can bedeveloped without discrete position sensors byprocessing motor terminal voltage and/orcurrent waveform. Drives using PM motorswith trapezoidal magnet MMF distributions(also known as brushless DC motor drives)provide attractive candidates for such indirectsensing since only two of the three motorphases are excited at any time instant. As aresult, the back-EMF voltage in the unexcitedphase can be conveniently measured toprovide the basis for determining invertercommutation instants. At least three differentalgorithms have been reported to accomplishthis task, referred to here as the zero-crossing,phase-locked-loop, and back-EMF integrationapproaches.

The zero-crossing approach is the simplest ofthe three, and is based on detecting the instantat which the back-EMF in the unexcited phasecrosses zero. This zero crossing triggers atimer, which may be as simple as an R-C timeconstant, so that the next sequential invertercommutation occurs at the end to this timinginterval. The price for this simplicity tends tobe noise sensitivity in detecting the zerocrossing, and degraded performance over widespeed ranges unless the timing interval isprogrammed as a function of rotor speed.

An alternative approach uses phase-lockedloop (PLL) techniques to lock onto the back-EMF waveform in the unexcited phasewinding during each 60 degree excitationinterval in order to determine the properinstant for the next inverter switch event. Thisalgorithm is designed to automatically adjustto changes in motor speed.

The third algorithm, referred to as the .back-EMF integration approach, providessignificantly improved performance comparedto the basic zero-crossing algorithm introducedabove. Instead of using the zero-crossing pointof the back-EMF waveform to trigger a timer,the rectified back-EMF waveform is fed to anintegrator, whose output is compared to pre-setthreshold. The adoption of an integratorprovides dual advantages of reduced switchingnoise sensitivity and automatic adjustment ofthe inverter switching instants to changes inrotor speed.One of the special problems faced by anyindirect position sensing scheme using back-EMF waveforms is low-speed performance.The basis for this problem is easy to appreciatesince the back-EMF amplitude is proportional

to rotor speed, thereby dropping to zero atrotor standstill. Choice of pulse-width-modulation (PWM) technique to achieve thecurrent regulation plays an important role indetermining the minimum speed at which theindirect position sensing algorithm willfunction.

Controller OperationThe controller block in Fig. 1 performs onekey function in order to achieve the desiredtorque control. That is indirect rotor positionsensing using measured back-EMFwaveforms.

Fig. 1 Controller for Position Sensor(V signal is generated)

Indirect Rotor Position SensingRotor position sensing is accomplished usingthe back-EMF integrator algorithm. Since theback-EMF voltage amplitudes can be verylarge compared to the logic supply voltage, anexternal resistive divider circuit delivers scaledversions of the three motor phase voltages.The neutral voltage of the wye-connectedmotor windings is artificially generated inorder to develop measurements of the threephase-to-neutral voltages. The signal selectorblock shown in Fig. 1 is responsible forselecting the phase-to-neutral motor voltage ofthe unexcited winding. This selected phasevoltage equals the desired back-EMF voltageneeded for position sensing as soon as theresidual inductive current flowing in theunexcited winding immediately following theremoval of excitation decays to zero. Thecontroller includes special provision to insurethat the position sensing is unaffected by theseresidual currents.

Fig. 2 Key wave forms illustrating operation ofindirect positionn sensing including the

idealized back & EMF wave (Ea, Eb, and Ec),the integrator output (Vint , the commutation

instants (Com), and the reset interval (Rst).

The waveforms sketched in Fig. 2 help toexplain the operation of the position sensingalgorithm. The integrator block in Fig. 1consists of an analog integrator which beginsto integrate the selected back-EMF voltage (ormore precisely, its absolute value) as soon asthe back-EMF crosses zero, developing thesignal shown in Fig. 2.The shape of this signal can beappreciated from the fact that theinstantaneous back-EMF voltage is varyingapproximately linearly with time in thevicinity of the zero-crossing, so that

( ) = ( )= ( )V = E t2k

Where k is the integrator gain constant. Theinstant of the next commutation event occurswhen reaches a preset fixed thresholdvoltage . Since the amplitude of the back-EMF ( in the above equation) isproportional to speed, the conduction intervalsautomatically scale inversely with speed with a

fixed threshold voltage . As shown in Fig.2, the integrator is reset by signal Rst. Thewidth of the Rst reset pulse is set to insure thatthe integrator can never start integrating untilthe residual current in the unexcited phase hasdecayed to zero.

The choice of threshold voltage andintegrator constant k for a given motordetermines the specific alignment of the phasecurrent excitation waveform with the back-EMF voltage. Varying or k has the effectOf varying this current-voltage waveformalignment, measured in terms on an advanceangle. If perfect alignment corresponds to zeroadvance angle, it has been shown previouslythat moderate increases in this advance angle(e.g., by reducing k in above Eqn.) can be usedto force faster phase current build-up in orderto develop extra torque at high speeds.However, it has been suggested that anadvance angle of approximately 10 elec.degrees provides a good compromise betweenhigh-speed torque production and low-speedtorque-per-Amp efficiency.

As mentioned earlier, low-speed operationrequires special provisions since the back-EMF drops to zero at standstill. For motorstart-up, an oscillator sequentially steps thecommutation state machine at a fixed rate inthe desired direction of motor rotation,energizing two of the three motor phasesduring each interval. As soon as the rotormoves in response to this open-loop steppingsequence, the integrator of the positiondetection block in Fig. 1 starts integrating theback-EMF voltage from the unexcited phase.When the motor speeds up sufficiently so thatthe integrator reaches its threshold level beforethe next open-loop step, the start-up oscillatoris automatically overridden so that the back-EMF sensing scheme smoothly takes overcontrol of the inverter switch commutationsequencing.

Since motor back-EMF amplitude variesdirectly with rotor speed, the indirect position

sensing scheme is particularly sensitive at lowspeeds to noise generated by inverterswitching during PWM current regulation. Inorder to minimize this sensitivity, a specialPWM technique has been implemented whichpermits good tracking of the rotor positiondown to speeds of a few round/min. Thepurpose of this technique is to extinguish thecurrent in the phase windings at the end ofeach 120 degree conduction interval as quicklyas possible. By doing so, the terminal voltageof the unexcited winding becomes useful forback-EMF sensing as soon as possiblefollowing the off-commutation of the phase.This objective is accomplished by shiftingresponsibility for PWM switching among thesix inverter switches in a specific sequence. Itis based on the fact that, at any time instantduring motoring operation, only one of the twoactive inverter switches must execute thePWM switching for current regulation whilethe second switch is held in its "on" state.

Fig.3 Commutating signals for the inverterswitches showing a preferred PWM sequencefor fast current decay in the off-going phase.(AT-Phase A upper switch, AB=Phase A lower

switch, etc.)

The preferred sequence for shifting this PWMresponsibility is sketched in Fig. 3 (Note thatthis technique applies only during motoringoperation, as discussed) Each of the sixswitches is active for an interval of 120electrical during each cycle (signified by eithera high or chopped logic level in Fig.3, and thisactive interval can be separated into two 60

degree intervals. As shown in Fig. 3, eachswitch is held in its "on" state during the firstof these two active intervals and executesPWM switching during the second interval, assignified by the high-frequency chopping. Thisscheme meets the criterion of one (and onlyone) active PWM switch at all times. Usingthis sequencing technique, the free-wheelingcurrent in the off-going phase is driven to zeromore quickly than if the new on-going phaseimmediately enters PWM operation. Thissequencing technique also has the beneficialeffect of distributing the PWM switchingoperation evenly among all six inverterswitches during the course of the cycle.

ConclusionsBrushless PMAC Motor can also be operatedproperly without using the rotor positionsensing.Such back-EMF sensing schemes fortrapezoidal PMAC machines are sufficientlymature and straight forward that they havebeen successfully implemented in integratedcircuits, some of which are now in commercialproduction.Application for this type of sensor less controlinclude computer disk drives, compact disk(CD) Players, blowers, and pumps.

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