Digital control of shaft speed and position

Where speed accuracies within 0.1 percent are desired over a wide speed range, the digital approach has proved superior to the analog method. This article describes a typical hybrid digital/analog servo system used to control the speed of a dc motor. A vital part of the system is the comparator, which must combine the functions of frequency and phase discrimination and perform the transition smoothly. It is also shown how precise synchronism between two or more rotating systems, with respect to both speed and position, can be maintained, giving the effect of a ``synchronous link.´´