14 - Principles of control systems

Abstract

This chapter discusses the principles of simple open-loop and closed-loop control systems. A control system may be designed to maintain a speed, a voltage level, or a water level constant within specified limits. Such a system is known as a regulator, and the reference in it is constant, though it may be capable of adjustment. In regulator controls, an error is always present when the system is working normally. Control systems also exhibit a reaction or response time. This is the time interval which occurs between the error being detected and the system output making an appropriate adjustment. An open-loop system is the most rudimentary form of control. A very simple example of an open-loop device is a moving-coil ammeter. In such a system, the magnetic system is formed and stabilized, the moving coil is wound and pivoted within the magnetic field, restoring springs are fitted, and connecting wires are brought out to terminals. The ammeter is then calibrated, and then it is assumed that it would always correctly indicate to the level of the current flowing through it. True closed-loop control does away with human intervention; the human operator is replaced by some mechanical or electrical system that senses the difference between the reference level and the actual output level and utilizes this difference to bring the output level as close as possible.