Digital position control system

Abstract

The aim of this project is to develop a Digital Closed Loop Control System, which can be used for 1 a Machine Tool, with special mention to direct digital transducers and transistor pulse-width modulated controlled d.c. Motor. Although one considers digital Control system as distinct from the analogue equivalent many of such systems are in fact a hybrid of digital and analogue circuitry. The principal reasons for this are threefold. a) Output transducer requires an analogue signal, e.g. d. c. motor; b) Control loop stabilization problems are more readily solved using analogue techniques; c) Input transducers provide an analogue s1gnal e.g. dynamo tachometer. Obviously one can do nothing about (a) and (b) but the advent of digital transducers, especially digital encoders, have eliminated reason (c) but we shall discuss this later in detail. In the now far past digital controllers have tended to be considerably more expensive than the equivalent analogue system but with the introduction of integrated circuits into industrial equipment this situation has changed drastically. This factor, together with increasing pressure from industry for more sophisticated methods of control has led to a substantial increase in applications over a very wide field. What are the advantages of Digital Control? There are perhaps three principal reasons. a) With digital Control it is possible to achieve considerably higher accuracies and degrees of resolution than those obtainable using analogue techniques. For example, as we will see later a position c0ntrol system requiring a resolution of 0.001 mmm over a range of 1m can be readily designed using digital techniques, whereas to achieve the same resolution using analogue techniques one would require several stages in cascade. b) The second reason for using a digital Control system is the frequent requirement for a means of storing information for further use. Such storage is readily provided for information in digital form on the other hand to achieve the same capability using analogue techniques 1the use of serve-set potentiometers, sample and hold amplifiers or other complex devices are required. c) Another main reason is that unlike an analogue controller, the dynamic range of a digital controller is not directly related to its accuracy. For example, if an analogue system having a full-scale range is limited to 10V because of the amplifier capability, the smallest increment detected might be 10mV. Hence this would limit the dynamic range to 1000 to 1. A digital controller, however, is only limited in range by the capacity of the logic and hence the range may be extended by adding logic without reducing the accuracy of the system. With all these advantages a hand, Digital Control Systems are taking over in many different applications one of the most important being machine tool control.