31 – Control systems

Publisher Summary The purpose of a control system on a refrigeration or air-conditioning plant is to provide automatic operation to avoid the cost of attendant labour or for where control is too complex for manual operation. A control system maintains the controlled conditions closer than could be achieved by manual operation, and it provides maximum efficiency and economy of operation. The control system also ensures safe operation at all times. The control system will consist of a loop, with detector (sensor), controller and controlled device. There are two types of detector: two-position (on–off) and proportional. A two-position detector can be used to operate a floating control. Proportional detectors measure the process condition, which can then be compared by the controller with the required value. If a controller is used with an on–off detector, it functions only as an amplifier to transmit the detector signal to the controlled device. It can modify the speed of this action by a bias or by a slow-speed operating motor, as in the floating control. Controllers generally for use with proportional detectors will measure the displacement of the signal from a pre-set value and transmit a proportional signal to the controlling device. Controlled devices commonly consist of an actuator, which accepts the signal from the controller and works the final element. Control systems can quickly escalate into unmanageable complexity, and the initial approach to the design of a suitable control system should examine the purpose of each item, and the effect on others, to eliminate those that are not essential.