3. Controllers and controller types

3.4 Discontinuous controllers

3.4.1 Two-point controller

The two-point controller is one of the simplest controller types. For example, it is used as a thermostat in ovens and refrigeration systems.

Two-point controllers are switches, which can adopt only two output states depending on the input signal:

(on: y = 100% and off: y = 0%)

In general, the two-point controller switches with a certain hysteresis at

This results in the progression of control and manipulating variables shown. In a steady state condition, the manipulating variable y is pulse width modulated and has a period T of

Two-point controller

Time response for two-point controller

The setpoint α of the controller is given by the duty ratio

These two characteristic variables for the two-point controller depend on the set hysteresis on the one hand and on the parameters of the control loop, Ks , Tt und Tgt , on the other.

The amplitude x0 of the switching oscillation is approximately calculated as

3.4.2 Three-point controller

Another type of discontinuous controller is the three-point controller. Essentially, it consists of two connected two-point controllers, each of which provides a separate output. The switching thresholds of the two two-point controllers are separated from one another by a dead zone.

Switching behaviour of a three-point controller, e.g. in temperature control

Three-point controllers are often used in motor drives, for example for control valves. In a drive, they can determine the drive direction (OPEN or CLOSED) and stopping.

Other applications can be found in temperature control. In this case, there are three switching states:

  1. Heater ON, cooler OFF (heating)

  2. Heater OFF, cooler OFF

  3. Heater OFF, cooler ON (cooling)

The switching states can also be referred to as 1, 0 and -1.