Siemens S7-300, S7-400 User Guide

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

A

C79000-G7076-C516-01

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Correct Usage

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Disclaimer of LiabilityCopyright E Siemens AG 1996 All rights reserved

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subsequent editions. Suggestions for improvement are welcomed.

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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
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Parameter
assignment user
interface
PID Control

Electronic
manual
PID Control

iii

Preface

Content of the
Manual

Chapter

1

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. [phone].

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Contents

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

1

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

2

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

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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.

3

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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