Process control actions

The use of controller and the nature of controller are very much application specific. The various factors which influence the controller candidature are plant and operating conditions, safety, cost, availability, weight, size, accuracy and reliability. An industrial automatic controller consists of an error detector, ampligfier, controller, processor, control and measuring elements, feedback amplifier etc. A comparator can be used as an error detector. A comparator generates a signal proportional to the deviation. Measuring element responds to the signal from the detecting element and gives measured value of the controlled condition. The value of the controlled condition to which the automatic controller mechanism is set, is known as set value. Deviation is the difference between the set value and the measured value. Depending upon the deviation, a controlling element sends the control signal to the correcting unit. Automatic controller is a device which compares the signal and reduces the deviation. A sustained deviation which occurs by virtue of inherent characteristic of proportional controller action is known as the offset.