Logical design of sequence controllers for machine operations
2014-04-01T11:34:04Z (GMT) by
Recent developments in switching theory have not been adequately exploited for industrial control applications. Consequently, the design of most industrial control systems is carried out on an intuitive basis. The aim of this study was to develop systematic techniques for the logical design of sequence controllers, which control the repetitive occurrences of a number of machine operations taking place in a definite order. Machine operations, for this study, were broadly divided into two classes, namely, pulse mode operations involving switching on or off of various control elements, and level mode operations for which the control output is maintained to hold the controlled position of the control element (like a spring return actuator). The design of a sequence controller for a large number of actuators has been simplified by an approach in which the controller is realised by a number of subsystems. Two standard circuits, namely, a standard actuator control circuit and an actuator control module have been developed -for level mode operations; the realisation of the controller using these circuits is straightforward and does not require an expertise in switching circuit theory. Two different circuit modules, a 'combinational module' and a 'sequential module', have been developed for cascade realisation of sequence controllers for pulse mode operations. A generalised model of programmable sequence controllers (PSC's), which are suitable for industries requiring frequent changes in the sequence of operations, has been proposed. The basic tape realisation of the PSC, in which the controller reads information from a tape and executes the corresponding operation, has been extended to control a larger number of operations. A systematic study has been carried out for an asynchronous sequential circuit realisation of a PSC using read-only memories (ROIiP s). A considerable reduction in the memory capacity of the ROM has been achieved by generating the present state variables by a counter. Control systems were constructed for the experimental verifications of some of the theoretical results; the machine operations for testing the systems were simulated by electronic circuits. The study provides a general basis to systematise the design of sequence controllers for industrial automation.