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
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, K , Tt und Tb , 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:
Heater ON, cooler OFF (heating)
Heater OFF, cooler OFF
Heater OFF, cooler ON (cooling)
The switching states can also be referred to as 1, 0 and -1.