Direct digital control of dc machines: The use of z-transform analysis, simulation and compensation techniques for the practical implementation of a DDC system for dc machines

Over the past few years, digital computers have played an ever-increasing role in the field of process control, in a supervising as well as a direct controlling capacity. Direct digital control applications have not been common in electrical engineering applications, owing to the fast sampling rates required. The use of minicomputers specifically for direct digital control purposes may well change this situation, particularly as the cycle time and price of such computers is decreased. This paper describes an actual example of the use of a minicomputer to effect the speed control of a separately excited dc machine. Digital data input and output facilities were used, with a hard-wired interface to effect the phase control of a thyristor bridge, and a digital tachometer. The system was analysed using z-transform methods, and a degree of compensation, based on the sampled-data nature of the system, introduced. The algorithms developed were found to be economical of computer time, because of the use of the hard-wired interface. Two machines were controlled on a time-sharing basis, indicating that this type of application could be considerably extended to use one computer to perform a number of control functions.