Siemens Module B3 manual Preface Fundamentals

Page 31

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

Image 31
Contents Module B3 Automation and Drives SCE Table of Contents Automation and Drives SCE Prerequisites To 3 days Module C To 3 days Module DFrequency Converter at Simatic S7 Learning ObjectivePLC Hardware and software requiredTasks of Control Engineering Fundamentals of Control EngineeringControlled Variable Components of a Control LoopFeedback Variable r Controlling Element Disturbance Variable zSetpoint Value w Comparing ElementControlled System ActuatorDead Time Characteristics Step Function for Examining Controlled Systems ForAutomation and Drives SCE Time constant Proportional Controlled System with a Time DelayTu Delay time Tg Transition time Proportional Controlled System with Two Time DelaysControllability of P-Tn systems Proportional Controlled System with n Time Delays Controlled Systems without Inherent Regulation Types of Controllers Two Position Controllers Hysteresis Switch-On Value Manipulated Variable TimeThree Position Controllers Basic Types of Continuous Controllers Proportional Controllers P-Controller Automation and Drives SCE Integral Action Controllers I- Controller WithController Layout PI ControllersPID Controllers Derivative Action Controllers D-ControllerObjectives for Controller Adjustment 450.85 Digital Controllers DAC Preface Fundamentals YES StructogramExercise Assignment List Symbol Address CommentNetwork Possible Solution for the PLC ProgramD24/AIFillSetpNorm/Norm.value for level setpoint Network 10 Title Task Definition for PID Standard Controller Function Diagram of the control system with a PID controller Use FB 41 ContcDescription Exercise Example Automation and Drives SCE Automation and Drives SCE Automation and Drives SCE Automation and Drives SCE Automation and Drives SCE Automation and Drives SCE Automation and Drives SCE Automation and Drives SCE SP INT PLC’ Automation and Drives SCE Automation and Drives SCE Automation and Drives SCE Tu-TgApproximation GeneralT g Approximation Setting the PI-Controller according to Ziegler-Nichols For setpoint characteristic MAN Solution of the PLC program Setpoint Setting the PI controller according to Ziegler-NicholsAppendix Diagram of the controller blockComrst Bool False Input Parameters Data Value Range Default Description TypeParameter Data Type Value Range Default Lmnhlm Real Lmnllm Output Parameters Parameter Data Value Range Default Description Type