Siemens Module B3 manual Preface Fundamentals

Automation and Drives - SCE

3.DISCONTINUOUS ACTION CONTROLLER AS TWO POSITION CONTROLLER 3.1 Function and Problem Description

A process value (for example, the level) is to be kept as constant as possible with a discontinuous action controller.

The output voltage at a digital output of the PLC generates the manipulated variable y which can be set either to "ON" (voltage =24V) or "OFF" (voltage =0V).

The process value represents the controlled variable x, which is suitably recorded with a measuring sensor (PEW XY).

It is the controller’s task to keep the controlled variable X constant at a specified setpoint, whereby the influence of disturbance variables z that can not be anticipated is to be eliminated. The PLC S7- 300 is used for this task as discontinuous action controller. The S7-300 is to solve the control problem by reading out a binary manipulated variable y depending on the respective setpoint/actual comparison w-x.

To prevent valve V1 from constantly being switched on and off when the controlled variable x has reached the setpoint w, a switching hysteresis is installed for discontinuous action controllers. Because of the switching hysteresis, the controlled variable x oscillates between a high response value Xo and a low response value Xu. The difference between the high response value Xo and the low response value Xu is called differential Xs = Xo - Xu.

Xs is often specified depending on the amount of setpoint w. For example, 10% of setpoint w: Xs = w/10.

In this program, a two position controller can be switched on with the button “Start“ to fill a tank. With the button “Stop“, the controller can be switched off. The tank is filled by means of a pump that can be triggered digitally. The setpoint is specified via a potentiometer at the analog input "AI_Fill_Setpoint“.

In a subroutine, the analog value for the process variable Level is to be entered, and normalized to the physical variable "Liter“. The normalized value is made available as floating point number in MD20.

For a level of 10 liters, the level sensor returns 0V, for 100 liters 10V.

The analog value for the setpoint is also to be entered here, and made available as floating point number in MD24.

If the controller is switched on, the lamp "Display_ON“ is to light up.

Below, the structogram for the program of the step controller is provided.

	Preface Fundamentals	Discontinuous Action Controllerr	Controller Block (S)FB41	Setting the System Appendix
	T I A Training Document	Page 31 of 64		Module
				B3
Issued: 02/2008				Control Engineering with STEP 7