Siemens Module B3 manual Types of Controllers Two Position Controllers

Automation and Drives - SCE

2.7 Types of Controllers

2.7.1 Two Position Controllers

The essential feature of two position controllers consists of their knowing only two modes: “On“ and “Off“ -which makes them the simplest type of controller. Two-position controllers are used primarily when adhering to a setpoint exactly is less important than to keep the control system as simple as possible; or, when the actuator or the final control element does not allow for a continuous control system. The heating system mentioned several times above is -with a control loop having a room thermometer and a mixer- a continuous control system. To keep the water temperature in the boiler loop constant, typically a two position controller is used since it can, on the one hand, fluctuate by a few degrees, and on the other hand it is clearly simpler to switch the burner on and off than to do an exact dosing of fuel to be added.

Since theoretically -to adhere to the setpoint exactly- it would be necessary to switch a system on and off infinitely fast, the two position controller has a so-called “hysteresis“. It represents a kind of “environment“ around the setpoint within which the actual value may fluctuate. That means, we specify a minimum value that is lower than the setpoint, and a maximum value that is a little higher than the setpoint. Only if the actual value exceeds the maximum value or drops below the minimum value does the control system react. In most cases, the minimum and the maximum value are distanced from the setpoint equally; that is, the hysteresis generates a symmetrical environment around the set point.

In the case of the boiler water temperature, the burner would be switched on, for example, when the water temperature drops below the specified setpoint by more than a certain value. The burner continues to run until a certain value that is above the setpoint is exceeded. Only then will the burner be switched off. Another typical example is cooling. Usually, a cooler also does not support a continuous control system, but only knows the states “On“ and “Off“. It is switched on when the actual temperature exceeds the setpoint temperature by a few degrees, and is switched off when the actual temperature is a few degrees too low.

It is therefore typical for the two position controller to periodically fluctuate around the setpoint whose amplitude is roughly that of the hysteresis. The selection of the hysteresis depends on how exactly the setpoint has to be adhered to. If we select a large hysteresis, the actual value can deviate more considerably from the setpoint. If we select a smaller one, the setpoint is adhered to more exactly, but the system would have to switch more frequently. This again has its disadvantages, such as a higher wear of the switching devices, and the actuator or the final control element.

	Preface	Fundamentals	Discontinuous Action Controller Controller Block (S)FB41	Setting the System Appendix
	T I A Training Document		Page 18 of 64	Module
				B3
Issued: 02/2008				Control Engineering with STEP 7