Two-point controller
A two-point controller is a discontinuous working controller with two output states. In the simplest case, depending on whether the actual value is above or below the setpoint , the upper or lower initial state is assumed. Two-point controllers are used when the manipulated variable is not continuously variable, but can only switch between two states, e.g. B. On / Off. The two-position controller reaches the steady state, but never comes to rest. In the event of major changes in the reference variable, however, it can correct system deviations more quickly than is possible with other control methods.
A well-known example of a two-point controller is the thermostat , which controls a heating or cooling device with a non-variable output. In contrast to this, a radiator thermostatic valve that controls the flow of radiators for space heating is a continuous controller .
In order to avoid rapid oscillations of the output, two-position controllers often have a built-in hysteresis , i.e. H. a certain dead zone around the threshold value by which the actual value must change in order to cause a switch to the opposite initial state. For example, a thermostat switches a heater off when it reaches 21 ° C, but only switches it on again when it falls below 18 ° C, which corresponds to a hysteresis of 3 K.
Both the controller itself and the hysteresis can be implemented mechanically (e.g. bimetal ) or electronically.
A closer look at the two-position controller can be found in the article Controller .