Unipolar Stepper Motor Control Circuit

This circuit controls a small, four-phase, five-wire, unipolar stepper motor, commonly designated the `KP4M4-001. ` This type of motor was used in many 5 1/4` floppy disk drives in older computers. Now obsolete, such disk drives are often available on the surplus market for a small fraction of the motor`s original cost. The stepper motors are easy

to extract from the drives, and are ideal for many applications. This arrangement was used in the scale model of a RADAR set to control the position of a miniature parabolic antenna. (Note that some 5 1/4" floppy disk drives used a four-wire "bipolar" motor, which is not compatible with this circuit. ) Referring to the schematic, U1 contains a relaxation oscillator and a binary ripple counter. U1`s Q5 output provides a 100 Hertz square wave for the clock inputs of "D" flip-flops U3A and U3B. U1`s Q12 output provides a level that alternates between "low" and "high" logic levels every 64 cycles of Q5. This signal is applied to the inputs of exclusive-or gates U2A and U2B. The flip-flops and gates form a quadrature waveform generator with complimentary outputs. These waveforms are applied to U4, a set of Darlington drivers capable of providing the required sink current for the motor. The waveforms and their relationships are illustrated in the following figure. The motor rotates one step, or 3. 6 degrees, for each positive clock transition. Rotation occurs in one direction for 230. 4 degrees (64 clock cycles x 3. 6 degrees), and then reverses. Clockwise rotation occurs when U1`s Q12 output is a logic "high" level, and counterclockwise rotation occurs when it is "low. " If desired, pins 2 and 5 of gates U2A and U2B may be disconnected from the counter, and alternatively provided with an external direction control signal. The...

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