Applications of closed-loop stepping motor control in multi-motor systems

Stepping motors, as digital machines, are commonly used in electromechanical systems since they can easily be interfaced to and controlled by computers. Electrical and mechanical interaction in multiple-motor systems, however, can lead to severe control problems, and in intelligent systems such as programmable XYtables, rotor position feedback from each machine is necessary to implement effective control algorithms. This paper describes new methods of detecting rotor position in stepping motors with chopper drives. By monitoring the motor phase current waveforms and detecting the chopping current rise-times, motor phase excitation angle may be varied continuously over a step cycle, and performance may be maximized. Two industrial applications are described: accurate trajectory control in an XY table, where close position coordination between two motors is necessary; and velocity control in a two-motor system where mechanical interaction creates a variable inertia load for one motor.