Siemens S7-300, S7-400 User Guide

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Preface, Contents

SIMATIC

Standard Software for S7-300 and S7-400 PID Control

User Manual

Introduction

Parameter Assignment

Function Blocks

References Glossary, Index

1 2 3

C79000-G7076-C516-01

Safety Guidelines

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Qualified Personnel

Correct Usage

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defined as persons who are authorized to commission, to ground, and to tag circuits, equipment, and systems in accordance with established safety practices and standards.

Note the following:

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The reproduction, transmission or use of this document or its contents is not permitted without express written authority. Of fenders will be liable for damages. All rights, including rights created by patent grant or registration of a utility model or design, are reserved.

This device and its components may only be used for the applications described in the catalog or the technical description, and only in connection with devices or components from other manufacturers which have been approved or recommended by Siemens.

This product can only function correctly and safely if it is transported, stored, set up, and installed correctly, and operated and maintained as recommended.

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Siemens AG Automation Group Industrial Automation Systems Postfach 4848, D-90327 Nürnberg

Siemens Aktiengesellschaft C79000-G7076-C516

Standard Software for S7-300 and S7-400 – PID Control

T echnical data subject to change.

E Siemens AG 1996

Preface

Purpose

Audience

Structure of “PID Control”

This manual supports you when working with the controller blocks of PID Control.

The manual introduces you to the functions of the controller blocks and familiarizes you with the user interface for assigning parameters to the blocks. This user interface includes online help, that further supports you when assigning parameters to the blocks.

This manual is intended for the following readers:

– S7 programmers – Programmers of closed-loop control systems – Operators – Service personnel

PID Control S7-300 and S7-400

Function blocks PID Control

The “PID Control” software package includes the following components:

– The function blocks CONT_C, CONT_S and PULSEGEN. – The parameter assignment user interface for configuring the controller

blocks.

– The manual consisting mainly of a description of the function blocks.

Standard Software for S7-300 and S7-400 – PID Control C79000-G7076-C516-01

Parameter assignment user interface PID Control

Electronic manual PID Control

iii

Preface

Content of the Manual

Chapter

Provides you with an overview of PID Control

Further Information

Additional Assistance

Chapter 2

Chapter 3

Explains how to call the parameter assignment user interface

Describes the function blocks FB 41 “CONT_C”, FB 42 “CONT_S” and FB 43 “PULSEGEN”

This manual is designed as a reference work providing you with the information you require about PID Control. Depending on your experience, you may well need further information that can be found in the manuals /70/, /71/, /100/, /101/, /231/, /232/, /234/.

If you have any questions regarding the use or application of PID Control, please contact the Siemens representative in your area. You will find a list of addresses in the Appendix “SIEMENS Worldwide“ in the “S7-400 Programmable Controller, Hardware and Installation” manual.

If you have any questions or comments on this manual, please fill out the remarks form at the end of the manual and return it to the address shown on the form. We would be grateful if you could also take the time to answer the questions giving your personal opinion of the manual.

Siemens also offers a number of training courses to introduce you to the SIMATIC S7 automation system. Please contact your regional training center or the central training center in Nuremberg, Germany for details. T el. +49-911-985-3154.

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1 Introduction 1-1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2 Parameter Assignment 2-1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3 Function Blocks 3-1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3.1 Continuous Control with FB41 “CONT_C” 3-2. . . . . . . . . . . . . . . . . . . . . . . . . . .

3.2 Step Control with FB42 “CONT_S” 3-9. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3.3 Pulse Generation with FB43 “PULSEGEN” 3-15. . . . . . . . . . . . . . . . . . . . . . . . . .

3.4 Example of Using PULSEGEN 3-24. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

A References A-1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Index Index-1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Introduction

The Concept of PID Control

Basic Functions

Applications

The function blocks (FBs) of the PID Control package consist of controller blocks for continuous control (CONT_C), for step control (CONT_S), and the FB for pulse duration modulation (PULSEGEN).

The controller blocks implement a purely software controller with the block providing the entire functionality of the controller. The data required for cyclic calculation is stored in data blocks assigned to the FB. This allows the FBs to be called as often as necessary.

FB PULSEGEN is used in conjunction with FB CONT_C to implement a controller with a pulse output for proportional actuators.

A controller created with the FBs consists of a series of subfunctions that you can activate or deactivate. In addition to the actual controller with its PID algorithm, integrated functions are also available for processing the setpoint and process variable and for adapting the calculated manipulated variable.

A controller implemented with the two controller blocks is not restricted to any particular application. The performance of the controller and its processing speed is only dependent on the performance of the CPU being used. With any given CPU, a compromise must be made between the number of controllers and the frequency at which the individual controllers are processed. The speed at which the control loops must be processed, in other words, the more often the manipulated variables must be calculated per unit of time, determines the number of controllers that can be installed (faster loops mean less controllers). There are no restrictions in terms of the type of process that can be controlled. Both slow processes (temperatures, tank levels etc.) and very fast processes (flow rate, motor speed etc.) can be controlled.

Process Analysis

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Note

The static behavior (gain) and the dynamic characteristics (time lag, dead time, reset time etc.) of the process to be controlled have a significant influence on the structuring and design of the controller and on the selection of the dimensions of its static (P component) and its dynamic (I and D components) parameters.

Precise knowledge of the type and characteristic data of the process to be controlled is essential.

1-1

Introduction

Choice of Controller

Creating the Controller

Online Help

Further Information

Note

The characteristics of control loops are decided by the given physical characteristics of the process or machine being controlled and can only be modified in minor ways. Good control quality is only possible if you choose the controller type most suited to your situation and adapt it to the time response of the process.

You can create a controller (structuring, parameter assignment, and call in the system program) largely without programming. Knowledge of STEP 7 is, however, necessary.

The STEP 7 online help also provides you with information about the various FBs.

PID Control is a subset of Standard Control. For further information about the standard controller, refer to /350/.

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Parameter Assignment

Calling the Parameter Assignment User Interface

Online Help

You call the parameter assignment user interface of PID Control under Windows 95 using the following menu options:

S Start " SIMATIC " STEP 7 V3 " PID Control Parameter

Assignment

In the first dialog, you can either open an existing instance data block (DB) for an FB41 “CONT_C” or FB42 “CONT_S” or create a new data block as the instance data block. If you create a new instance data block, you will be prompted to assign the instance DB to an FB.

FB43 “PULSEGEN” does not have a parameter assignment user interface. You must set its parameters with STEP 7 tools.

Note

Using the parameter assignment user interface of PID Control, you can also assign parameters for the integrated control of the CPU 314 IFM. In this case, you create instance DBs that you assign to SFB41 or SFB42.

Online help is available in the parameter assignment user interface to support you when you assign parameters to the controller blocks. You can call the online help in three ways:

S Using the menu option Help " Contents... S By pressing the F1 key S By clicking the Help button in the parameter assignment dialogs

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2-1

Parametrierung

2-2

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Function Blocks

Note

The function blocks described in this chapter (FB41 to FB43) have only been released for S7/C7 CPUs with cyclic interrupt levels.

Chapter Overview

Section Description Page

3.1 Continuous Control with FB41 “CONT_C” 3-2

3.2 Step Control with FB42 “CONT_S” 3-9

3.3 Pulse Generation with FB43 “PULSEGEN” 3-15

3.4 Example of Using PULSEGEN 3-24

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3-1

Function Blocks

3.1 Continuous Control with FB41 “CONT_C”

Introduction

Application

Description

FB “CONT_C” is used on SIMATIC S7 programmable controllers to control technical processes with continuous input and output variables. During parameter assignment, you can activate or deactivate subfunctions of the PID controller to adapt the controller to the process.

You can use the controller as a PID fixed setpoint controller or in multi-loop controls as a cascade, blending or ratio controller. The functions of the controller are based on the PID control algorithm of the sampling controller with an analog signal, if necessary extended by including a pulse generator stage to generate pulse duration modulated output signals for two or three step controllers with proportional actuators.

Apart from the functions in the setpoint and process value branches, the FB implements a complete PID controller with continuous manipulated variable output and the option of influencing the manipulated value manually. In the following, you will find a detailed description of the subfunctions:

Setpoint Branch The setpoint is entered in floating-point format at the SP_INT input.

Process Variable Branch

The process variable can be input in the peripheral (I/O) or floating-point format. The CRP_IN function converts the PV_PER peripheral value to a floating-point format of -100 to +100 % according to the following formula:

Output of CRP_IN = PV_PER 

100

27648

The PV_NORM function normalizes the output of CRP_IN according to the following formula:

Output of PV_NORM = (output of CRP_IN)  PV_FAC + PV_OFF

PV_FAC has a default of 1 and PV_OFF a default of 0.

Error Signal

The difference between the setpoint and process variable is the error signal. T o suppress a small constant oscillation due to the manipulated variable quantization (for example in pulse duration modulation with PULSEGEN), a dead band is applied to the error signal (DEADBAND). If DEADB_W = 0, the dead band is switched off.

PID Algorithm

The PID algorithm operates as a position algorithm. The proportional, integral (INT), and derivative (DIF) actions are connected in parallel and can be activated or deactivated individually. This allows P, PI, PD, and PID controllers to be configured. Pure I and D controllers are also possible.

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Function Blocks

Manual Value

It is possible to switch over between a manual and an automatic mode. In the manual mode, the manipulated variable is corrected to a manually selected value. The integrator (INT) is set internally to LMN - LMN_P - DISV and the derivative unit (DIF) to 0 and matched internally. This means that a switchover to the automatic mode does not cause any sudden change in the manipulated value.

Manipulated Value

The manipulated value can be limited to a selected value using the LMNLIMIT function. Signaling bits indicate when a limit is exceeded by the input variable. The LMN_NORM function normalizes the output of LMNLIMIT according to the following formula:

LMN = (output of LMNLIMIT)  LMN_FAC + LMN_OFF

LMN_FAC has the default 1 and LMN_OFF the default 0.

The manipulated value is also available in the peripheral format. The CRP_OUT function converts the floating-point value LMN to a peripheral value according to the following formula:

LMN_PER = LMN 

27648

100

Modes

Error Information

Feedforward Control A disturbance variable can be fed forward at the DISV input.

Complete Restart/Restart

FB41 “CONT_C” has a complete restart routine that is run through when the input parameter COM_RST = TRUE is set.

During startup, the integrator is set internally to the initialization value I_ITVAL. When it is called in a cyclic interrupt priority class, it then continues to work starting at this value.

All other outputs are set to their default values.

The block does not check for errors internally. The error output parameter RET_VAL is not used.

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3-3

Function Blocks

Block Diagram

SP_INT

PV_IN

PV_PER



CRP_IN

INT

TI, INT_HOLD, I_ITL_ON, I_ITLVAL

DIF

TD, TM_LAG

PV_NORM

PV_FAC, PV_OFF

0.0



PVPER_ON

P_SEL



I_SEL



D_SEL

DEADBAND



DEADB_W

LMN_P

DISV

LMN_I

LMN_D

GAIN

MAN

MAN_ON



LMNLIMIT



LMN_HLM, LMN_LLM

Figure 3-1 Block Diagram of CONT_C

3-4

QLMN_HLM QLMN_LLM

LMN

LMN_NORM

LMN_FAC, LMN_OFF

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CRP_OUT

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LMN_PER

Function Blocks

Input Parameters

T able 3-1 contains the description of the input parameters for FB41 “CONT_C”.

Table 3-1 Input Parameters (INPUT) for FB 41 “CONT_C”

Parameter

Data Type

COM_RST BOOL FALSE

MAN_ON BOOL TRUE

PVPER_ON BOOL FALSE

P_SEL BOOL TRUE

I_SEL BOOL TRUE

INT_HOLD BOOL FALSE

I_ITL_ON BOOL FALSE

D_SEL BOOL FALSE

CYCLE TIME

SP_INT REAL

PV_IN REAL

PV_PER WORD

Range of

Values

>= 1ms

-100.0...100. 0 (%)

or phys. value 1)

-100.0...100. 0 (%)

or phys. value 1)

Default Description

COMPLETE RESTART The block has a complete restart routine that is processed when the input “complete restart” is set.

MANUAL VALUE ON If the input “manual value on” is set, the control loop is interrupted. A manual value is set as the manipulated value.

PROCESS VARIABLE PERIPHERAL ON If the process variable is read from the I/Os, the input PV_PER must be connected to the I/Os and the input “process variable peripheral on” must be set.

PROPORTIONAL ACTION ON The PID actions can be activated or deactivated individually in the PID algorithm. The P action is on when the input “proportional action on” is set.

INTEGRAL ACTION ON The PID actions can be activated or deactivated individually in the PID algorithm. The I action is on when the input “integral action on” is set.

INTEGRAL ACTION HOLD The output of the integrator can be “frozen” by setting the input “integral action hold”.

INITIALIZATION OF THE INTEGRAL ACTION The output of the integrator can be connected to the input I_ITL_VAL by setting the input “initialization of the integral action on”.

DERIVATIVE ACTION ON The PID actions can be activated or deactivated individually in the PID algorithm. The D action is on when the input “derivative action on” is set.

SAMPLING TIME

T#1s

The time between the block calls must be constant. The “sampling time” input specifies the time between block calls.

0.0

INTERNAL SETPOINT The “internal setpoint” input is used to specify a setpoint.

PROCESS VARIABLE IN

0.0

An initialization value can be set at the “process variable in” input or an external process variable in floating point format can be connected.

W#16#00

PROCESS VARIABLE PERIPHERAL The process variable in the I/O format is connected to the controller at the “process variable peripheral” input.

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Function Blocks

Table 3-1 Input Parameters (INPUT) for FB 41 “CONT_C”, continued

Parameter DescriptionDefault

Data Type

MAN REAL

Range of

Values

-100.0...100. 0 (%)

or phys. value 2)

0.0

GAIN REAL 2.0

TI TIME

>= CYCLE

T#20s

TD TIME >= CYCLE T#10s

TM_LAG TIME >= CYCLE/2 T#2s

>= 0.0 (%)

DEADB_W REAL

LMN_HLM REAL

LMN_LLM REAL

or phys. value 1)

LMN_LLM

...100.0 (%)

or phys. value 2)

-100.0...

LMN_HLM

(%) or phys. value 2)

0.0

100.0

0.0

PV_FAC REAL 1.0

PV_OFF REAL 0.0

LMN_FAC REAL 1.0

LMN_OFF REAL 0.0

MANUAL VALUE The “manual value” input is used to set a manual value using the operator interface functions.

PROPORTIONAL GAIN The “proportional value” input specifies the controller gain.

RESET TIME The “reset time” input determines the time response of the integrator.

DERIVATIVE TIME The “derivative time” input determines the time response of the derivative unit.

TIME LAG OF THE DERIVATIVE ACTION The algorithm of the D action includes a time lag that can be

assigned at the “time lag of the derivative action” input. DEAD BAND WIDTH

A dead band is applied to the error. The “dead band width” input determines the size of the dead band.

MANIPULATED VALUE HIGH LIMIT The manipulated value is always limited by an upper and lower limit. The “manipulated value high limit”input specifies the upper limit.

MANIPULATED VALUE LOW LIMIT The manipulated value is always limited by an upper and lower limit. The “manipulated value low limit”input specifies the lower limit.

PROCESS VARIABLE FACTOR The “process variable factor” input is multiplied by the process variable. The input is used to adapt the process variable range.

PROCESS VARIABLE OFFSET The “process variable offset” input is added to the process variable. The input is used to adapt the process variable range.

MANIPULATED VALUE FACTOR The “manipulated value factor” input is multiplied by the manipulated value. The input is used to adapt the manipulated value range.

MANIPULATED VALUE OFFSET The “manipulated value offset” is added to the manipulated value. The input is used to adapt the manipulated value range.

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Table 3-1 Input Parameters (INPUT) for FB 41 “CONT_C”, continued

Function Blocks

Parameter DescriptionDefault

Data Type

I_ITLVAL REAL

DISV REAL

1) Parameters in the setpoint and process variable branches with the same unit

2) Parameters in the manipulated value branch with the same unit

Output Parameters

Range of

Values

-100.0...100. 0 (%)

or phys. value 2)

-100.0...100. 0 (%)

or phys. value 2)

0.0

INITIALIZATION VALUE OF THE INTEGRAL ACTION The output of the integrator can be set at input I_ITL_ON. The initialization value is applied to the input “initialization value of the integral action”.

DISTURBANCE VARIABLE For feedforward control, the disturbance variable is connected to input “disturbance variable”.

T able 3-2 contains the description of the output parameters for FB41 “CONT_C”.

Table 3-2 Output Parameters (OUTPUT) for FB 41 “CONT_C”

Parameter

Data Type

Range of

Values

Default Description

LMN REAL 0.0 MANIPULATED VALUE

The effective manipulated value is output in floating point format at the “manipulated value” output.

LMN_PER WORD W#16#0000 MANIPULATED VALUE PERIPHERAL

The manipulated value in the I/O format is connected to the controller at the “manipulated value peripheral” output.

QLMN_HLM BOOL FALSE HIGH LIMIT OF MANIPULATED VALUE REACHED

The manipulated value is always limited to an upper and lower limit. The output “high limit of manipulated value reached” indicates that the upper limit has been exceeded.

QLMN_LLM BOOL FALSE LOW LIMIT OF MANIPULATED VALUE REACHED

The manipulated value is always limited to an upper and lower limit. The output “low limit of manipulated value reached” indicates that the lower limit has been exceeded.

LMN_P REAL 0.0 PROPORTIONAL COMPONENT

The “proportional component” output contains the proportional component of the manipulated variable.

LMN_I REAL 0.0 INTEGRAL COMPONENT

The “integral component” output contains the integral component of the manipulated value.

LMN_D REAL 0.0 DERIVATIVE COMPONENT

The “derivative component” output contains the derivative component of the manipulated value.

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3-7

Function Blocks

Table 3-2 Output Parameters (OUTPUT) for FB 41 “CONT_C”, continued

Parameter DescriptionDefault

PV REAL 0.0 PROCESS VARIABLE

ER REAL 0.0 ERROR SIGNAL

Data Type

Range of

Values

The effective process variable is output at the “process variable” output.

The effective error is output at the “error signal” output.

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3.2 Step Control with FB42 “CONT_S”

Function Blocks

Introduction

Application

Description

FB42 “CONT_S” is used on SIMATIC S7 programmable logic controllers to control technical processes with digital manipulated value output signals for integrating actuators. During parameter assignment, you can activate or deactivate subfunctions of the PI step controller to adapt the controller to the process.

You can use the controller as a PI fixed setpoint controller or in secondary control loops in cascade, blending or ratio controllers, however not as the primary controller. The functions of the controller are based on the PI control algorithm of the sampling controller supplemented by the functions for generating the binary output signal from the analog actuating signal.

Apart from the functions in the process value branch, the FB implements a complete PI controller with a digital manipulated value output and the option of influencing the manipulated value manually. The step controller operates without a position feedback signal. In the following you will find the description of the partial functions:

Setpoint Branch The setpoint is entered in floating-point format at the SP_INT input.

Process Variable Branch

Output of CRP_IN = PV_PER 

The PV_NORM function normalizes the output of CRP_IN according to the following formula:

Output of PV_NORM = (output of CRP_IN)  PV_FAC + PV_OFF

PV_FAC has a default of 1 and PV_OFF a default of 0.

Error Signal

The difference between the setpoint and process variable is the error signal. T o suppress a small constant oscillation due to the manipulated variable quantization (for example due to a limited resolution of the manipulated value by the actuator valve), a dead band is applied to the error signal (DEADBAND). If DEADB_W = 0, the dead band is switched off.

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27648

3-9

Function Blocks

PI Step Algorithm

The FB operates without a position feedback signal. The I action of the PI algorithm and the assumed position feedback signal are calculated in one integrator (INT) and compared with the remaining P action as a feedback value. The difference is applied to a three-step element (THREE_ST) and a pulse generator (PULSEOUT) that creates the pulses for the actuator. The switching frequency of the controller can be reduced by adapting the threshold on of the three-step element.

Feedforward Control A disturbance variable can be fed forward at the DISV input.

Modes

Error Information

Complete Restart/Restart

FB42 “CONT_S” has a complete restart routine that is run through when the input parameter COM_RST = TRUE is set.

All other outputs are set to their default values.

The block does not check for errors internally. The error output parameter RET_VAL is not used.

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Block Diagram

SP_INT

PV_IN

PV_PER

CRP_IN

PV_NORM

PV_FAC PV_OFF

PVPER_ON



Function Blocks

DEADBAND



–

DEADB_W

GAIN

LMNR_HS LMNR_LS

LMNUP

LMNDN

LMNS_ON

DISV



THREE_ST

adaptiv

–

MTR_TM

LMNS_ON



1/TI



0.0



Figure 3-2 Block Diagram of CONT_S

AN D

INT

PULSEOUT

PULSE_TM, BREAK_TM

INT

LMNLIMIT

LMNR_SIM



1/MTR_TM

100.0 ,

0.0



QLMNUP

QLMNDN

LMNRS_ON ,LMNRSVAL

AN D

100.0

0.0

–100.0

0.0



–

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Function Blocks

Input Parameters

T able 3-3 contains the description of the input parameters for FB42 “CONT_S”.

Table 3-3 Input Parameters (INPUT) for FB 42 “CONT_S”

Parameter

Data Type

COM_RST BOOL FALSE COMPLETE RESTART

LMNR_HS BOOL FALSE HIGH LIMIT OF POSITION FEEDBACK SIGNAL

LMNR_LS BOOL FALSE LOW LIMIT OF POSITION FEEDBACK SIGNAL

LMNS_ON BOOL FALSE MANUAL ACTUATING SIGNALS ON

LMNUP BOOL FALSE ACTUATING SIGNALS UP

LMNDN BOOL FALSE ACTUATING SIGNALS DOWN

PVPER_ON BOOL FALSE PROCESS VARIABLE PERIPHERAL ON

CYCLE TIME

SP_INT REAL

PV_IN REAL -100.0...100.0

PV_PER WORD W#16#0000 PROCESS VARIABLE PERIPHERAL

Range of

Values

>= 1ms

-100.0...100.0 (%)

or phys. value

(%)

or phys. value

Default Description

The block has a complete restart routine that is processed when the input “complete restart” is set.

The “actuator at upper limit stop” signal is connected to the “high limit of position feedback signal” input. LMNR_HS=TRUE means the actuator is at upper limit stop.

The “actuator at lower limit stop” signal is connected to the “low limit of position feedback signal” input. LMNR_LS=TRUE means the actuator is at lower limit stop.

The actuating signal processing is switched to manual at the “manual actuating signals on” input..

With manual actuating value signals, the output signal QLMNUP is set at the input “actuating signals up”.

With manual actuating value signals, the output signal QLMNDN is set at the input “actuating signals down”.

If the process variable is read in from the I/Os, the input PV_PER must be connected to the I/Os and the input “process variable peripheral on” must be set.

T#1s SAMPLING TIME

The time between the block calls must be constant. The “sampling time” input specifies the time between block calls.

0.0 INTERNAL SETPOINT The “internal setpoint” input is used to specify a setpoint.

0.0 PROCESS VARIABLE IN An initialization value can be set at the “process variable in”

input or an external process variable in floating point format can be connected.

The process variable in the I/O format is connected to the controller at the “process variable peripheral” input.

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Table 3-3 Input Parameters (INPUT) for FB 42 “CONT_S”, Fortsetzung

Function Blocks

Parameter DescriptionDefault

Data Type

Range of

Values

GAIN REAL 2.0 PROPORTIONAL GAIN

The “proportional gain” input sets the controller gain.

TI TIME

>= CYCLE

T#20s RESET TIME

The “reset time” input determines the time response of the integrator.

DEADB_W REAL 0.0...100.0

(%)

or phys. value

1.0

DEAD BAND WIDTH A dead band is applied to the error. The “dead band width”

input determines the size of the dead band.

PV_FAC REAL 1.0 PROCESS VARIABLE FACTOR

The “process variable factor” input is multiplied by the process variable. The input is used to adapt the process variable range.

PV_OFF REAL 0.0 PROCESS VARIABLE OFFSET

The “process variable offset” input is added to the process variable. The input is used to adapt the process variable range.

PULSE_TM TIME >= CYCLE T#3s MINIMUM PULSE TIME

A minimum pulse duration can be assigned with the parameter “minimum pulse time”.

BREAK_TMTIME >= CYCLE T#3s MINIMUM BREAK TIME

A minimum break duration can be assigned with the parameter “minimum break time”.

MTR_TM TIME >= CYCLE T#30s MOTOR ACTUATING TIME

The time required by the actuator to move from limit stop to limit stop is entered at the “motor actuating time” parameter.

DISV REAL -100.0...100.0

(%)

or phys. value

0.0 DISTURBANCE VARIABLE For feedforward control, the disturbance variable is

connected to input “disturbance variable”.

1) Parameters in the setpoint and process variable branches with the same unit

2) Parameters in the manipulated value branch with the same unit

Standard Software for S7-300 and S7-400 – PID Control C79000-G7076-C516-01

3-13

Function Blocks

Output Parameters

T able 3-4 contains the description of the output parameters for FB42 “CONT_S”.

Table 3-4 Output Parameters (OUTPUT) for FB42 “CONT_S”

Parameter

QLMNUP BOOL FALSE ACTUATING SIGNAL UP

QLMNDN BOOL FALSE ACTUATING SIGNAL DOWN

PV REAL 0.0 PROCESS VARIABLE

ER REAL 0.0 ERROR SIGNAL

Data Type

Range of

Values

Default Description

If the output “actuating signal up” is set, the actuating valve is opened.

If the output “actuating signal down” is set, the actuating valve is opened.

The effective process variable is output at the “process variable” output.

The effective error is output at the “error signal” output.

3-14

Standard Software for S7-300 and S7-400 – PID Control

C79000-G7076-C516-01

3.3 Pulse Generation with FB43 “PULSEGEN”

Function Blocks

Introduction

Application

Description

FB43 “PULSEGEN” is used to structure a PID controller with pulse output for proportional actuators

Using FB43 “PULSEGEN”, PID two or three step controllers with pulse duration modulation can be configured. The function is normally used in conjunction with the continuous controller ~CONT_C”.

CONT_C

LMN

PULSEGEN

INV

The PULSEGEN function transforms the input variable INV ( = manipulated value of the PID controller) by modulating the pulse duration into a pulse train with a constant period, corresponding to the cycle time at which the input variable is updated and which must be assigned in PER_TM.

The duration of a pulse per period is proportional to the input variable. The cycle assigned to PER_TM is not identical to the processing cycle of the FB “PULSEGEN”. The PER_TM cycle is made up of several processing cycles of FB “PULSEGEN”, whereby the number of FB “PULSEGEN” calls per PER_TM cycle is the yardstick for the accuracy of the pulse duration modulation.

INV

100

(LMN)

QPOS_P

Figure 3-3 Pulse Duration Modulation

Standard Software for S7-300 and S7-400 – PID Control C79000-G7076-C516-01

PULSEGEN cycle

PER_TM

(= CONT_C cycle)

3-15

Function Blocks

Block Diagram

An input variable of 30% and 10 FB “PULSEGEN” calls per PER_TM means the following:

– “One” at the QPOS output for the first three calls of FB

“PULSEGEN” (30% of 10 calls)

– “Zero”at the QPOS output for seven further calls of FB “PULSEGEN”

(70% of 10 calls)

POS_P_ON

NEG_P_ON

Accuracy of the Manipulated Value

SYN_ON,

STEP3_ON,

ST2BI_ON

INV

PER_TM, P_B_TM,

RATIOFAC

Figure 3-4 Block Diagram of PULSEGEN

MAN_ON



QPOS_P QNEG_P

With a “sampling ratio” of 1:10 (CONT_C calls to PULSEGEN calls) the accuracy of the manipulated value in this example is restricted to10%, in other words, set input values INV can only be simulated by a pulse duration at the QPOS output in steps of 10 %. The accuracy is increased as the number of FB “PULSEGEN” calls per CONT_C call is increased. If PULSEGEN is called, for example 100 times more often than CONT_C, a resolution of 1 % of the manipulated value range is achieved.

Automatic Synchronization

3-16

Note

The call frequency must be programmed by the user.

It is possible to synchronize the pulse output with the block that updates the input variable INV (for example CONT_C). This ensures that a change in the input variable is output as quickly as possible as a pulse. The pulse generator evaluates the input value INV at intervals corresponding to the period PER_TM and converts the value into a pulse signal of corresponding length.

Standard Software for S7-300 and S7-400 – PID Control

C79000-G7076-C516-01

Function Blocks

Since, however, INV is usually calculated in a slower cyclic interrupt class, the pulse generator should start the conversion of the discrete value into a pulse signal as soon as possible after the updating of INV.

T o allow this, the block can synchronize the start of the period using the following procedure: If INV changes and if the block call is not in the first or last two call cycles of a period, the synchronization is performed. The pulse duration is recalculated and in the next cycle is output with a new period (see Figure 3-5).

LMN = INV = 30.0 LMN = INV = 80.0 LMN = INV = 50.0

Processing of CONT_C

CYCLE of CONT_C

Period start

0000110 01111111110011

. . . .

CYCLE of PULSEGEN

PULSEGEN detects: INV has changed and the call is not in the first or in the last two cycles of a period.

Processing of PULSEGEN Processing of PULSEGEN in the first or in

Synchronization of period start

PER_TM PER_TM

No synchronization required

PULSEGEN detects: INV has changed to 80.0 or 50.0 and the call is in the first or in the last two cycles of the period.

the last two cycles of the period

Figure 3-5 Synchronization of the Period Start

The automatic synchronization can be disabled at the “SYN_ON” input (= FALSE).

Note

With the beginning of a new period, the old value of INV (in other words, of LMN) is simulated in the pulse signal more or less accurately following the synchronization.

Standard Software for S7-300 and S7-400 – PID Control C79000-G7076-C516-01

3-17

Function Blocks

Modes

Three-Step Control

Depending on the parameters assigned to the pulse generator, PID controllers with a three-step output or with a bipolar or monopolar two-step output can be configured. The following table illustrates the setting of the switch combinations for the possible modes.

Switch

Mode

Three-step control FALSE TRUE Any Two-step control with bipolar control

range (-100 % to +100 %) Two-step control with monopolar control

range (0 % ... 100 %) Manual mode TRUE Any Any

MAN_ON STEP3_ON ST2BI_ON

FALSE FALSE TRUE

FALSE FALSE FALSE

In the “three-step control” mode, the actuating signal can adopt three states. The values of the binary output signals QPOS_P and QNEG_P are assigned to the statuses of the actuator. The table shows the example of a temperature control:

Actuator Heat Off Cool

Output signal

QPOS_P TRUE F ALSE F ALSE QNEG_P F ALSE F ALSE TRUE

Based on the input variable, a characteristic curve is used to calculate a pulse duration. The form of the characteristic curve is defined by the minimum pulse or minimum break time and the ratio factor (see Figure 3-6). The normal value for the ratio factor is 1. The “doglegs” in the curves are caused by the minimum pulse or minimum break times.

Minimum Pulse or Minimum Break Time

A correctly assigned minimum pulse or minimum break time P_B_TM can prevent short on/off times that reduce the working life of switching elements and actuators.

Note

Small absolute values at the input variable LMN that could otherwise generate a pulse duration shorter than P_B_TM are suppressed. Large input values that would generate a pulse duration longer than (PER_TM P_B_TM) are set to 100 % or -100 %.

The positive and negative pulse duration is calculated by multiplying the input variable (in %) with the period time:

INV

Pulse duration =

100

 PER_TM

3-18

Standard Software for S7-300 and S7-400 – PID Control

C79000-G7076-C516-01

Function Blocks

Three-Step Control Asymmetrical

Duration of the

PER_TM

PER_TM - P_B_TM

-100 %

Figure 3-6 Symmetrical Characteristic Curve of the Three-Step Controller (Ratio

Factor = 1)

P_B_TM

positive pulse

Continuously OFF

Duration of the negative pulse

Continuously ON

100 %

Using the ratio factor RATIOFAC, the ratio of the duration of positive to negative pulses can be changed. In a thermal process, for example, this would allow different system time constants for heating and cooling. The ratio factor also influences the minimum pulse or minimum break time. A ratio factor < 1 means that the threshold value for negative pulses is multiplied by the ratio factor.

Ratio Factor < 1

The pulse duration at the negative pulse output calculated from the input variable multiplied by the period time is reduced by the ratio factor (see Figure 3-7).

Duration of the positive pulse =

Duration of the negative pulse =

INV

100

INV 100

 PER_TM

 PER_TM 

RATIOFAC

Standard Software for S7-300 and S7-400 – PID Control C79000-G7076-C516-01

3-19

Function Blocks

Duration of the positive pulse

PER_TM

PER_TM - P_B_TM

Two-Step Control

-100 %

Figure 3-7 Asymmetrical Characteristic Curve of the Three-Step Controller (Ratio

Factor = 0.5)

P_B_TM

0.5 * P_B_TM

0.5 * (PER_TM - P_B_TM)

0.5 * PER_TM

Duration of the negative pulse

100 %

Ratio Factor > 1

The pulse duration at the positive pulse output calculated from the input variable multiplied by the period time is reduced by the ratio factor.

INV

Duration of the negative pulse =

Duration of the positive pulse =

100

INV 100

 PER_TM

PER_TM



 RATIOFAC

In two-step control, only the positive pulse output QPOS_P of PULSEGEN is connected to the on/off actuator. Depending on the manipulated value range being used, the two-step controller has a bipolar or a monopolar manipulated value range (see Figures 3-8 and 3-9).

3-20

Two-Step Control with Bipolar Manipulated Variable Range (-100% to 100%)

Duration of the pos. pulse

PER_TM

PER_TM - P_B_TM

Continuously Off

P_B_TM

-100.0 %

Figure 3-8 Characteristic Curve with Bipolar Manipulated Value Range

(-100 % to 100 %)

0.0 %

Standard Software for S7-300 and S7-400 – PID Control

Continuously On

100.0 %

C79000-G7076-C516-01

Function Blocks

Two-Step Control with Monopolar Manipulated Variable Range (0% to 100%)

Duration of the positive pulse

PER_TM

PER_TM - P_B_TM

P_B_TM

0.0 %

Figure 3-9 Characteristic Curve with Monopolar Manipulated Value Range

(0 % to 100 %)

100.0 %

The negated output signal is available at QNEG_P if the connection of the two-step controller in the control loop requires a logically inverted binary signal for the actuating pulses.

Manual Mode in Two/Three-Step Control

Modes

Actuator

Pulse

QPOS_P TRUE F ALSE QNEG_P F ALSE TRUE

On Off

In the manual mode (MAN_ON = TRUE), the binary outputs of the three-step or two-step controller can be set using the signals POS_P_ON and NEG_P_ON regardless of INV.

POS_P_ON NEG_P_ON QPOS_P QNEG_P

Three-step control FALSE FALSE FALSE FALSE

TRUE FALSE TRUE FALSE

FALSE TRUE FALSE TRUE

TRUE TRUE FALSE FALSE

Two-step control FALSE Any FALSE TRUE

TRUE Any TRUE FALSE

Complete Restart/Restart

During a complete restart, all the signal outputs are set to 0.

Error Information

Standard Software for S7-300 and S7-400 – PID Control C79000-G7076-C516-01

The block does not check for errors internally. The error output parameter RET_VAL is not used.

3-21

Function Blocks

Input parameters

Table 3-5 Input Parameters (INPUT) for FB 43 “PULSEGEN”

Parameter

INV REAL -100.0...100.0 (%) 0.0 INPUT VARIABLE

PER_TM TIME >=20*CYCLE T#1s PERIOD TIME

P_B_TM TIME >= CYCLE T#50ms MINIMUM PULSE/BREAK TIME

RATIOFAC REAL 0.1 ...10.0 1.0 RATIO FACTOR

STEP3_ON BOOL TRUE THREE STEP CONTROL ON

ST2BI_ON BOOL FALSE TWO STEP CONTROL FOR BIPOLAR

MAN_ON BOOL FALSE MANUAL MODE ON

POS_P_ON BOOL FALSE POSITIVE PULSE ON

NEG_P_ON BOOL FALSE NEGATIVE PULSE ON

Data Type

Range of Values Default Description

An analog manipulated value is connected to the input parameter “input variable”.

The constant period of pulse duration modulation is input with the “period time” input parameter. This corresponds to the sampling time of the controller. The ratio between the sampling time of the pulse generator and the sampling time of the controller determines the accuracy of the pulse duration modulation.

A minimum pulse or minimum break time can be assigned at the input parameters “minimum pulse or minimum break time”.

The input parameter “ratio factor” can be used to change the ratio of the duration of negative to positive pulses. In a thermal process, this would, for example, allow different time constants for heating and cooling to be compensated (for example, in a process with electrical heating and water cooling).

The “three-step control on” input parameter activates this mode. In three-step control, both output signals are active.

MANIPULATED VALUE RANGE ON With the input parameter “two-step control for bipolar manipulated value range on” you can select between the modes “two-step control for bipolar manipulated value” and “two-step control for monopolar manipulated value range”. The parameter STEP3_ON = FALSE must be set.

By setting the input parameter “manual mode on”, the output signals can be set manually.

In the manual mode with three-step control, the output signal QPOS_P can be set at the input parameter “positive pulse on”. In the manual mode with two-step control, QNEG_P is always set inversely to QPOS_P.

In the manual mode with three-step control, the output signal QNEG_P can be set at the input parameter “negative pulse on”. In the manual mode with two-step control, QNEG_P is always set inversely to QPOS_P.

3-22

Standard Software for S7-300 and S7-400 – PID Control

C79000-G7076-C516-01

Table 3-5 Input Parameters (INPUT) for FB 43 “PULSEGEN”, continued

Function Blocks

Parameter DescriptionDefault Range of Values

SYN_ON BOOL TRUE SYNCHRONIZATION ON

COM_RST BOOL FALSE COMPLETE RESTART

CYCLE TIME >= 1ms T#10ms SAMPLING TIME

Data

Type

By setting the input parameter “synchronization on”, it is possible to synchronize automatically with the block that updates the input variable INV. This ensures that a changing input variable is output as quickly as possible as a pulse.

The block has a complete restart routine that is processed when the “complete restart” input is set.

The time between block calls must be constant. The “sampling time” input specifies the time between block calls.

Note

The values of the input parameters are not limited in the block. There is no parameter check.

Output Parameters

Table 3-6 Output Parameters (OUTPUT) for FB43 “PULSEGEN”

Parameter

QPOS_P BOOL FALSE OUTPUT POSITIVE PULSE

QNEG_P BOOL FALSE OUTPUT NEGATIVE PULSE

Data Type

Range

Values

Default Description

The output parameter “output positive pulse” is set when a pulse is to be output. In three-step control, this is always the positive pulse. In two-step control, QNEG_P is always set inversely to QPOS_P.

The output parameter “output negative pulse” is set when a pulse is to be output. In three-step control, this is always the negative pulse. In two-step control, QNEG_P is always set inversely to QPOS_P.

Standard Software for S7-300 and S7-400 – PID Control C79000-G7076-C516-01

3-23

Function Blocks

3.4 Example of Using PULSEGEN

Control Loop

PV_PER

Figure 3-10 Control Loop

Using the continuous controller CONT_C and the pulse generator PULSEGEN, you can implement of fixed setpoint controller with a switching output for proportional actuators. Figure 3-10 illustrates the basic signal sequence in the control loop.

CONT_C

LMN

INV QPOS_P

Loop with switching output

PULSEGEN

QNEG_P

The continuous controller CONT_C forms the manipulated value LMN, that is converted to pulse-break signals QPOS_P or QNEG_P by the pulse generator PULSEGEN.

Block call and Connection

3-24

The fixed setpoint controller with a switching output for proportional actuators PULS_CTR consists of the blocks CONT_C and PULSEGEN. The block call is implemented so that CONT_C is called every 2 seconds (=CYCLE*RED_FAC) and PULSEGEN every 10 ms (=CYCLE). The cycle time of OB35 is set to 10 ms. The connections can be seen in Figure 3-11.

During a complete restart, the block PULS_CTR is called in OB100 and input COM_RST is set to TRUE.

Standard Software for S7-300 and S7-400 – PID Control

C79000-G7076-C516-01

OB 100 (Complete restart) OB 35 (10ms)

Function Blocks

TRUE (OB 100)

FALSE (OB35)

T#10ms

COM_RST

CYCLE

RED_FAC

SP_INT

PV_PER

PULS_CTR, DPULS_CTR FB 50, DB 50

COM_RST

CYCLE

RED_FAC

200

PULS_CTR, DPULS_CTR FB 50, DB 50

COM_RST

CYCLE SP_INT

PV_PER

LMN

CONT_C call reduced by RED_FAC

COM_RST

CYCLE

PER_TM

INV

QPOS_P

Figure 3-11 Block Call and Interconnection

STL Program for FB PULS_CTR

Table 3-7 FB PULS_CTR

Address

0.0 in SP_INT REAL Setpoint

4.0 in PV_PER WORD Process variable in periph. format

6.0 in RED_FAC INT Call reducing factor

8.0 in COM_RST BOOL Complete restart

10.0 in CYCLE TIME Sampling time

14.0 out QPOS_P BOOL Actuating signal

16.0 stat DI_CONT_C FB-CONT_C Counter

142.0 stat DI_PULSEGEN FB-PULSEGEN Counter

176.0 stat sCount INT Counter

0.0 temp tCycCtr TIME Controller sampling time

Standard Software for S7-300 and S7-400 – PID Control C79000-G7076-C516-01

Declaration Name Type Comment

3-25

Function Blocks

Table 3-8 Network 1

STL Description

A #COM_RST JCN M001 L 0 T #sCount

M001: L #CYCLE

L #RED_FAC *D T #tCycCtr

L #sCount L 1

-I T #sCount L 0 <=I

JCN M002 CALL #DI_CONT_C COM_RST :=#COM_RST CYCLE :=#tCycCtr SP_INT :=#SP_INT PV_PER :=#PV_PER L #RED_FAC T #sCount

M002: L #DI_CONT_C.LMN

T #DI_PULSEGEN.INV CALL #DI_PULSEGEN PER_TM :=#tCycCtr COM_RST :=#COM_RST CYCLE :=#CYCLE QPOS_P :=#QPOS_P BE

//Complete restart routine

//Calculate controller sampling time

//Decrement counter and compare with zero

//Conditional block call and set counter

3-26

Standard Software for S7-300 and S7-400 – PID Control

C79000-G7076-C516-01

References

/70/ Manual: S7-300 Programmable Controller,

Hardware and Installation

/71/ Reference Manual: S7-300, M7-300 Programmable Controllers

Module Specifications

/100/ Manual: S7-400/M7-400 Programmable Controllers,

Hardware and Installation

/101/ Reference Manual: S7-400/M7-400 Programmable Contr ollers

Module Specifications

/231/ User Manual: Standard Softwar e for S7 and M7,

STEP 7

/232/ Manual: Statement List (STL) for S7-300 and S7-400,

Programming

/234/ Programming Manual: System Software for S7-300 and S7-400

Program Design

/350/ User Manual: SIMATIC S7,

Standard Control

Standard Software for S7-300 and S7-400 – PID Control C79000-G7076-C516-01

A-1

A-2

Standard Software for S7-300 and S7-400 – PID Control

C79000-G7076-C516-01

Index

Applications, 1-1

CONT_C, 3-2 CONT_S, 3-9 Continuous control, 1-1 Control

continuous control with SFB41, 3-2

step control with FB42, 3-9 Controller, choosing, 1-2 CPU 314 IFM, 2-1

FB41 “CONT_C”, block diagram, 3-4 FB41 CONT_C, 3-2

block diagram, 3-4 FB42 “CONT_S”, block diagram, 3-11 FB42 CONT_S, 3-9 FB43 “PULSEGEN”, block diagram, 3-16 FB43 PULSEGEN, 3-15

automatic synchronization, 3-16

three-step control, 3-18

three-step control asymmetrical, 3-19

two-step control, 3-20

Integrated control, 2-1

Online help, 1-2, 2-1

Parameter assignment user interface

calling, 2-1

online help, 2-1 PID Control, concept, 1-1 Process analysis, 1-1 Pulse duration modulation, 1-1, 3-15 PULSEGEN, 3-15

Step control, 1-1

Standard Software for S7-300 and S7-400 – PID Control C79000-G7076-C516-01

Index-1

Index

Index-2

Standard Software for S7-300 and S7-400 – PID Control

C79000-G7076-C516-01

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Standard Software for S7-300 and S7-400 – PID Control C79000-G7076-C516-01

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Standard Software for S7-300 and S7-400 – PID Control

C79000-G7076-C516-01

Siemens S7-300, S7-400 User Guide

Specifications and Main Features

Frequently Asked Questions

User Manual

Preface

Contents

Introduction

Parameter Assignment

Function Blocks

3.4 Example of Using PULSEGEN

References

Index