38 Programmable Logic Controllers in iron caps which are cut to have teeth. The rotor sets itself in the minimum reluctance position in response to a pair of stator coils being energised. To drive a stepper motor, so that it proceeds step-by-step to provide rotation, requires each pair of stator coils to be switched on and off in the required sequence when the input is a sequence of pulses (Figure 2.37). Driver circuits are available to give the correct sequencing and Figure 2.38 shows an example, the SAA 1027 for a four-phase unipolar stepper. Motors are termed unipolar if they are wired so that the current can only flow in one direction through any particular motor terminal, bipolar if the current can flow in either direction through any particular motor terminal. The stepper motor will rotate through one step each time the trigger input goes from low to high. The motor runs clockwise when the rotation input is low and anticlockwise when high. When the set pin is made low the output resets. In a control system, these input pulses might be supplied by a microprocessor. Input pulses Time Inputs to coils Pulse for 1st coil Pulse for 2nd coil Pulse for 3rd coil Pulse for 4th coil Time
Figure 2.37 Input and outputs of the drive system Supply voltage +12 V
15
13 6
14 4
Trigger
8 3
SAA1027
Rotation 2 Set
5
12
9 11
Brown
Red
Black Green
Red
Yellow
Stepper motor with its four stator coils
Figure 2.38 Driver circuit connections with the integrated circuit SAA1027