Three-point controller
A three-point controller is a discontinuous operating controls tristate. Depending on whether the actual value is below the first setpoint , between the first and second setpoint, or above the second setpoint, the first, second or third initial state is assumed. Three-point controllers are used when the manipulated variable is not continuously variable.
An example of a three-point controller is a heating / cooling thermostat - below the minimum temperature it switches on a heating device, above the maximum temperature it activates a cooling device; in the range between minimum and maximum temperature, both are switched off.
However, a three-point controller can also control two-stage actuators, for example gas or oil burners with two power stages, the second stage being added when the lower setpoint is not reached.
Another application is position controllers, for example for the height of the brushes in a car wash (too low / too high / stop). A two-point controller would have the disadvantage that the brushes would continuously move up and down around the setpoint.
Three-point controllers can be implemented mechanically (e.g. bimetal ) or electronically. A three-position controller can also be implemented by combining two two-position controllers with changeover contacts or additional relays or logic functions.