used to control the action of a final control element in such a way as to tend to reduce the deviation to zero" 2Likewise,a feedback control system is a system which tends to maintain a prescribed relationship of one system variable to another by comparing functions of these variables and using the difference as a means of control. The advanced type of automation that revolutionized manufacturing,aircraft, communications and other industries,is feedback control,which is usually continuous and involves taking measurements using a sensor and making calculated adjustments to keep the measured variable within a set range.The theoretical basis of closed loop automation is control theory. A thermostat is a simple negative feedback controller:when the temperature(the "process variable"or PV)goes below a set point(SP),the heater is switched on. Another example could be a pressure switch on an air compressor.When the pressure(PV)drops below the threshold(SP),the pump is powered.Refrigerators and vacuum pumps contain similar mechanisms operating in reverse,but still providing negative feedback to correct errors. s Simple on-off feedback control systems like these are cheap and effective.In some cases,like the simple compressor example,they may represent a good design choice. In most applications of on-off feedback control,some consideration needs to be given to other costs,such as wear and tear of control valves and perhaps other start- up costs when power is reapplied each time the PV drops.Therefore,practical on-off control systems are designed to include hysteresis which acts as a deadband,a region around the set point value in which no control action occurs.The width of deadband may be adjustable or programmable. Unit4 Automatic Control 69used to control the action of a final control element in such a way as to tend to reduce the deviation to zero " . 12 Likewise, a feedback control system is a system which tends to maintain a prescribed relation of one system variable to another by comparing functions of these variables and using the difference as a means of control. B The advanced type of automation that revolutionized manufacturing , aircraft , communications and other industries , is feedback control, which is usually continuous and involves taking measurements using a sensor and making calculated adjustments to keep the measured variable within a set range. The theoretical basis of closed loop automation is control theory. J4 A thermostat is a simple negative feedback controller: when the temperature (the "process variable" or PV) goes below a set point (SP), the beater is switched on. Another example could be a pressure switch on an compressor. When the pressure(PV) drops below the threshold (SP), the pump is powered. Refrigerators and vacuum pumps contain similar mechanisms operating in reverse , but still providing negative feedback to correct errors. 15 Simple on-off feedback control systems like these are cheap and effective. In some cases, Like the simple compressor example, they may represent a good design choice. 16 In most appLications of on-off feedback control , some consideration needs to be ven to other costs, such as wear and tear of control valves and perhaps other start￾up costs when power is reapplied each time tbe PY drops. Therefore, practical on-off control systems are designed to include hysteresis which acts as a deadband, a region around the set point value in which no control action occurs. The width of deadband may be adjustable or programmable . Unit 4 Automatic Control 6 9