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Design Ideas |
The design in 
Figure
1 allows full direction and step control of a 12V, four-phase
stepper motor from a 5V, TTL/CMOS-compatible logic controller. The circuit uses
a Philips SAA 1027 control IC to generate the correct step sequences from the
count input. The IC also has a reset function that allows you to temporarily
stop the motor and reset the count and a mode control that specifies direction.
The TIL199 optoisolator isolates the control signals. Although this IC is a
relatively low-speed isolator, it's fast enough for the drive speeds the stepper
motor accommodates. Isolating the control lines, rather than the motor-pulse
sequence, reduces the number of optoisolators required.
You exercise on-off control by simply halting the count sequence. The motor automatically stops on removal of the control-logic signals. A 5W dc/dc converter (NMXS0512U) steps the supply voltage to as high as 12V and isolates the motor and controller power supply from the logic lines. Complete isolation of the motor from the logic-line controller reduces the motor noise that could otherwise reach the logic supply. Measurements show only 25 mV of ripple with the motor operating at a 244-Hz count rate. The inverters at the optoisolator inputs, in conjunction with the inversion inherent in the optoisolators themselves, yield signals to the controller IC of the same polarity as those applied to the inverter inputs.
An external resistor sets the current supply to each phase. In this design, 120[ohm] produces 300 mA per phase. The dc/dc converter can produce 5W of power, and the controller IC can sink 500 mA, so the same circuit can drive a higher power motor. The circuit is easy to interface to the UART port of any microcontroller. It provides a simple method to control stepper motors without the problems of separate supply lines and motor-noise interference in the logic circuits. (DI #1916)