Book Analog Interfacing to Embedded Microprocessors
enabled.
The circuit shown in Figure 7.17 illustrates the concept. In practice,
the comparator reference voltage could be fixed, or it could come
from a microprocessor-controlled DAC. This would permit software control
of the current and therefore the torque. This would allow a stepper motor
to be used in an application with varying loads, as long as the microprocessor
knows approximately what the load is. It could also be used to com-
pensate for the torque variation due between single-coil and two-coil
drive when half-stepping, or to generate the varying signals needed for
microstepping.
The chopping frequency has to be high enough to be significantly greater
than the maximum step rate of the motor, but low enough that the transis-
tors can respond. If the chopping frequency is too high, the drive transistors
will spend too much time in the linear region (during the turn-on and turn-
annaabi.ee 
