Temperature control

Abstract: Automatic control systems, wherein a variable quantity is made to conform to a predetermined level, have been in use for several centuries; but, as with much technological development, the most rapid advances have taken place during recent years. In principle, many variables can be controlled by humans, but in practice this may be impossible, difficult, costly, or undesirable because of the need for continuous operation regardless of environment, large forces, and a fast response. The human reaction time of about 0.3 seconds is too slow for many applications. Further examples of control systems are liquid level control by ball valve, temperature control by thermostat, and surface control such as a ship's rudder or aircraft flaps by hydraulic servo. However, all types of control system can be modeled for analysis purposes, irrespective of the operating mechanism. In the 1950s, the potential of multiple loop systems was investigated, and the introduction of computers opened the way for much greater complexity in control systems. Computer control is usually applied to industrial problems in one of the three ways: supervisory control that continually adjusts the plant to optimum operation conditions, direct digital control, and hierarchy control that integrates the plants operation at every level from management decisions through to valve settings. Computer control is aided by the ability to measure and convert into electrical signals a wide range of system parameters such as temperature, pressure, speed, level, weight, flow, conductivity, and thickness. A fast rate of progress has been maintained to the present day, particularly since the introduction of microprocessors, so that only an introduction to automatic control systems can be attempted in a text of this length.