LMD18245_06 Datasheet, PDF (17/21 Pages) National Semiconductor (TI) – 3A, 55V DMOS Full-Bridge Motor Driver

The Typical Application (Continued)
HALF STEP DRIVE WITH TORQUE COMPENSATION
To make the motor take half steps, the windings can also be
energized with sinusoidal currents (Figure 12). 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, micro-
stepping reduces both full step oscillations and resonances
that occur as the motor and load combination is driven at its
natural 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 13 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
microstep. α, therefore, increases from 0 to 315˚ in incre-
ments of 45˚. Each full 360˚ cycle comprises eight half steps.
Rounding |cosα| to four bits gives D A, the decimal equiva-
lent of the binary number applied at M4 A through M1 A.
DIRECTION A controls the polarity of the current in winding
A. Figure 12 shows the sinusoidal winding currents.
Top Trace: Phase A Winding Current at 1A/div
Bottom Trace: Phase B Winding Current at 1A/div
Horizontal: 2 ms/div
*500 steps/second
01187824
BRAKE A = BRAKE B = 0
01187844
90˚ ELECTRICAL/FULL STEP ÷ 2 MICROSTEPS/FULL STEP = 45˚ ELECTRICAL/MICROSTEP
FIGURE 12. Winding Currents and Digital Control Signals for Half Step Drive with Torque Compensation
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