HALF STEP DRIVE WITH TORQUE COMPENSATION

To make the motor take half steps, the windings can also be

energized with sinusoidal currents (

Figure 11). Controlling

the winding currents in the fashion shown doubles the step

resolution without the significant torque ripple of the prior

drive technique. The motor takes one half step each time the

level of either winding current changes. Half step drive with

torque compensation is microstepping drive. Along with the

obvious advantage of increased step resolution, microstep-

ping reduces both full step oscillations and resonances that

occur as the motor and load combination is driven at its natu-

ral resonant frequency or subharmonics thereof. Both of

these advantages are obtained by replacing full steps with

bursts of microsteps. When compared to full step drive, the

motor runs smoother and quieter.

Figure 12 shows the lookup table for this application of the

typical application circuit. Dividing 90˚electrical per full step

by two microsteps per full step yields 45˚ electrical per mi-

crostep.

α, therefore, increases from 0 to 315˚ in increments

of 45˚. Each full 360˚ cycle comprises eight half steps.

Rounding |cos

α| to four bits gives D A, the decimal equiva-

lent of the binary number applied at M4 A through M1 A. DI-

RECTION A controls the polarity of the current in winding A.

Figure 11 shows the sinusoidal winding currents.

DS011878-22

Top Trace: Phase A Winding Current at 1A/div

Bottom Trace: Phase B Winding Current at 1A/div

Horizontal: 1 ms/div

*500 steps/second

DS011878-23

BRAKE A = BRAKE B = 0

FIGURE 10. Winding Currents and Digital Control Signals for Half Step Drive without Torque Compensation

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