SIMATIC. Standard Controller. Preface, Contents Function Blocks for Standard Controllers. Standard Controller. Product Overview

Transcription

1 Preface, Contents Function Blocks for s SIMATIC User Manual Product Overview Designing Digital Controllers 2 Configuring and Starting the 3 Signal Processing in SP/PV Channels and PIG Controller Functions 4 The Continuous Controller (PID_C) 5 The Step Controller (PID_S) 6 The Loop Scheduler and Examples of Controller Configurations 7 Technical Data and Block Diagrams 8 Parameter Lists of the 9 Configuration Configuration Tool Product Overview Working with the Configuration Tool Appendix Appendix A A Glossary, Index C79-G776-C95-2

2 Safety Guidelines! This manual contains notices which you should observe to ensure your own personal safety, as well as to protect the product and connected equipment. These notices are highlighted in the manual by a warning triangle and are marked as follows according to the level of danger: Danger indicates that death, severe personal injury or substantial property damage will result if proper precautions are not taken.! Warning indicates that death, severe personal injury or substantial property damage can result if proper precautions are not taken.! Caution indicates that minor personal injury or property damage can result if proper precautions are not taken. Note draws your attention to particularly important information on the product, handling the product, or to a particular part of the documentation. Qualified Personnel The device/system may only be set up and operated in conjunction with this manual. Only qualified personnel should be allowed to install and work on this equipment. Qualified persons are 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. Correct Usage Note the following:! Warning 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. Trademarks SIMATIC and SINEC are registered trademarks of SIEMENS AG. Third parties using for their own purposes any other names in this document which refer to trademarks might infringe upon the rights of the trademark owners. Copyright Siemens AG 995 All rights reserved The reproduction, transmission or use of this document or its contents is not permitted without express written authority. Offenders will be liable for damages. All rights, including rights created by patent grant or registration of a utility model or design, are reserved. Siemens AG Automation Group Industrial Automation Systems Postfach 4848, D-9327 Nürnberg Siemens Aktiengesellschaft Disclaimer of Liability We have checked the contents of this manual for agreement with the hardware and software described. Since deviations cannot be precluded entirely, we cannot guarantee full agreement. However, the data in this manual are reviewed regularly and any necessary corrections included in subsequent editions. Suggestions for improvement are welcomed. Siemens AG 995 Technical data subject to change. Order No. 6ES7 83-2AA2-8BG ii C79 G776 C95 2

3 Preface Purpose of the Manual This manual will help you when selecting, configuring, and assigning parameters to a controller block for your control task. The manual introduces you to the functions of the configuration tool and explains how you use it. How the Manual Fits into the Into the Package Standard Controller S7-3/4 Standard Controller S7-3/4 Function Blocks for Standard Controller Configuration Standard Controller Manual Standard Controller The S7-3/4 software product includes three separate products: The product consists essentially of the two controller blocks PID_C and PID_S. The product Configuration consists essentially of the tools for configuring the controller blocks. This product is referred to simply as configuration tool in this manual. The manual is a separate product and describes both the product and the product Configuration Standard Controller. C79-G776-C95-2 iii

4 Preface The Standard Controller Package The software package provides a comprehensive concept for implementing control functions in SIMATIC S7 programmable logic controllers. The controller is completely programmed with its full range of functions and features for signal processing. To adapt a controller to your process, you simply select the subfunctions you require from the complete range of functions. The time and effort required for configuration is therefore reduced to omitting functions you do not require. In all these tasks, you are supported by the configuration tool. Since configuration is restricted to selecting or, in some cases, extending basic functions, the concept of the standard controller is easy to learn. Even users with only limited knowledge of control systems will be in a position to create high-quality controls. Contents of the Manual Chap. provides you with an overview of the standard controller Chap. 2 explains the structure and the functions of the standard controller Chap. 3 helps you to design and start up a standard controller Chap. 4 explains the signal processing in the setpoint/process variable channel Chap. 5 explains the signal processing in the continuous controller Chap. 6 explains the signal processing in the step controller Chap. 7 shows you how to work with the loop scheduler and introduces examples of controller structures Chap. 8 contains technical data and block diagrams Chap. 9 contains parameter lists for the standard controller Chap. provides you with an overview of the configuration tool iv C79-G776-C95-2

5 Preface Chap. shows you how to work with the configuration tool Audience This manual is intended for the following readers: S7 programmers programmers of control systems operators service personnel Conventions in the Text Further Information Further Support To make it easier for you to find information in the manual, certain conventions have been used: First glance through the titles in the left margin to get an idea of the content of a section. Sections dealing with a specific topic either answer a question about the functions of the tool or provide information about necessary or recommended courses of action. References to further information dealing with a topic are indicated by (see Chapter or Section x.y). References to other manuals and documentation are indicated by numbers in slashes /.../. These numbers refer to the titles of manuals listed in the Appendix. Instructions for you to follow are marked by a black dot. Sequences of activities are numbered or explained as explicit steps. Alternative courses of action or decisions you need to take are indicated by a dash. This manual is intended as a reference work that provides you with the information you will require to work with the standard controller. You do, however, require a broader scope of information that is available in the following manuals: /7/, /7/, //, //, /23/, /232/, /234/, and /352/. If you have questions about using the controller block or the configuration tool, please contact your Siemens SIMATIC distributor or representative. You will find the addresses in the appendix SIEMENS Companies and Representatives in the S7-3 Programmable Controller, Hardware and Installation Manual. C79-G776-C95-2 v

6 Preface If you have questions or comments relating to the manual itself, please complete the Remarks Form at the end of the manual and return it to the appropriate address. We would also appreciate your appraisal of the manual on the Remarks Form. To help you to become familiar with SIMATIC S7 programmable logic controllers, we provide training courses. Please contact your regional training center or the main training center in D-9327 Nürnberg, Germany Tel vi C79-G776-C95-2

7 Contents Function Blocks for s Product Overview The Product The Software Product Environment and Applications Designing Digital Controllers Process Characteristics and Control Identifying Process Characteristics Feedforward Control Multi-Loop Controls Structure and Mode of Operation of the Signal Flow Diagrams Configuring and Starting the Defining the Control Task Configuring a Project (Checklist) Assigning Parameters for the The Sampling Time CYCLE How the is Called Range of Values and Signal Adaptation (Normalization) Signal Processing in the Setpoint/Process Variable Channels and PID Controller Functions Signal Processing in the Setpoint Branch Setpoint Generator (SP_GEN) Ramp Soak (RMP_SOAK) Normalization of the External Setpoint (SP_NORM) FC Call in the Setpoint Branch (SPFC) Limiting the Rate of Change of the Setpoint (SP_ROC) Limiting the Absolute Value of the Setpoint (SP_LIMIT) Setpoint Adjustment Using the Configuration Tool C79-G776-C95-2 vii

8 Contents 4.2 Signal Processing in the Process Variable Branch Entering the Process Variable in the Peripheral Format (CRP_IN) Normalizing the Process Variable Input in the Floating Point Format (PV_NORM) Damping the Process Variable (LAGST) Extracting the Square Root (SQRT) FC Call in the Process Variable Branch (PVFC) Monitoring the Process Variable Limits (PV_ALARM) Monitoring the Rate of Change of the Process Variable (ROCALARM) Changing the Manipulated Variable Using the Configuration Tool Processing the Error Signal Filtering the Signal with DEADBAND Function Monitoring the Error Signal Limit Values (ER_ALARM) The PID Controller Functions Signal Processing in the PID Controller Algorithm Integrator (INT) Derivative Unit (DIF) The Continuous Controller (PID_C) Control Functions of the Continuous PID Controller Processing the Manipulated Variable Signal Modes affecting the Manipulated Variable Signal Manual Value Generator (MAN_GEN) FC Call in the Manipulated Variable Branch (LMNFC) Limiting the Rate of Change of the Manipulated Value (LMN_ROC) Limiting the Absolute Value of the Manipulated Variable (LMNLIMIT) Normalization of the Manipulated Variable to the Format of a Physical Variable (LMN_NORM) Manipulated Value Output in the Peripheral Format (CRP_OUT) Influencing the Manipulated Value With the Configuration Tool Continuous Controller in Cascade Control Pulse Generator (PULSEGEN) The Step Controller (PID_S) Control Functions of the PID Step Controller Manipulated Variable Processing on the Step Controller With Position Feedback Signal Modes of the Step Controller Influencing the Manipulated Variable With the Configuration Tool Limiting the Absolute Value of the Manipulated Variable (LMNLIMIT) Processing the Position Feedback Signal (LMNR) Generating the Actuating Signals (QLMNUP/QLMNDN) Manipulated Variable Processing on the Step Controller Without Position Feedback Signal Step Controllers in Cascade Controls viii C79-G776-C95-2

9 Contents 7 The Loop Scheduler and Examples of Controller Configurations The Loop Scheduler (LP_SCHED) APP_: Step Controller (Fixed Setpoint Controller) With Process Simulation APP_2: Continuous Controller With Process Simulation APP_3: Multi-Loop Ratio Control APP_4: Blending Control APP_5: Cascade Control Technical Data and Block Diagrams Technical Data: Function Blocks Block Diagrams of the Parameter Lists of the Parameters of the PID_C Function Block Parameters of the PID_S Function Block Parameters of the PULSEGEN Function Block Parameter of the LP_SCHED Function Configuration Configuration Tool - Product Overview Purpose of the Configuration Tool Functions Program and Data Structure Working with the Configuration Tool Preparations General Operating Instructions Configuring and Assigning Parameters Curve Recorder Loop Monitor Process Identification Final Jobs Appendix A References A- Glossary Index Glossary- Index- C79-G776-C95-2 ix

10 Contents Figures - Overview of the Functions of the Continuous Controller Software Block Contents of the Software Package Environment of the S7-3 Software Package Step Response of a Self-Regulating Process (first order) Step Response of a Self-Regulating Process with Dead Time Step Response of a Non Self-Regulating Process (I Process) Compensating a Disturbance Affecting Process Input (signal names of the standard controller) Ratio Control with Two Loops (APP_3) Blending Control for Three Components (APP_4) Two-Loop Cascade Control (APP_5) Sampling Controller of the Standard controller in the Closed Loop Sequence of Functions of the (continuous controller) Manipulated Variable Branch of the Step Controller with Position Feedback Signal Manipulated Value Branch of the Step Controller without Position Feedback Signal Signal Flow Diagram of Setpoint Processing Signal Flow Diagram of the Process Variable and Error Signal Processing Signal Flow Diagram of the Control Function Signal Flow Diagram of Actuating Signal Generation with the Continuous Controller Signal Flow Diagram of Actuating Signal Generation on the Step Controller with Position Feedback Signal (LMNR_ON = TRUE) Block Diagram of Manipulated Variable Generation on the Step Controller without Position Feedback Signal (LMNR_ON = FALSE) Types of Process that can be Controlled with the Connecting the Process Signals to the Actuating Signal Outputs of the the Configuration of the Setpoint Branch of the (checklist points 4 and 5) Configuration of the Process Variable and Error Signal Branch of the (checklist points 6, 7 and 8) Configuration of the Manipulated Value Branch (checklist point 8) Configuration of the Controller Functions PID_C and PID_S (checklist point 9) Determining the Equivalent System Time Constant TE Connecting the Start-Up Blocks with the Sample APP_ Calling a Controller with the Loop Scheduler LP_SCHED Example of the Adaptation of the Temperature Range 2 to +85_ C Converted to. to. % Changing the Setpoint by Setting SPUP x C79-G776-C95-2

11 Contents 4-2 Functions and Parameters of the Setpoint Generator Ramp Soak with Seven Coordinates ( to 6) Counting the Coordinates and Time Slices Influencing the Ramp Soak with the Default Signal DFRMP_ON The Effect of the Hold Signal RMP_HOLD on the Ramp Soak How the RMP_HOLD Hold Signal and the CONT_ON Continue Signal Affect the Ramp Soak Functions and Parameters of the Ramp Soak Functions and Parameters for Normalizing the External Setpoint Calling an FC in the Setpoint Branch Limiting the Rate of Change of the Setpoint SP(t) Functions and Parameters for Limiting the Rate of Setpoint Change Limits for the Absolute Values of the Setpoint SP (t) Functions and Parameters of the Absolute Value Limits of the Setpoint Intervention in the Setpoint Branch Using the Configuration Tool Functions and Parameters for Normalizing Physical Process Variables Functions and Parameters of the Absolute Value Limitation of the Setpoint Functions and Parameters for Extracting the Square Root of the Process Variable Signals Calling an FC Block in the Process Variable Branch Process Variable PV Monitoring the Limit Values Functions and Parameters of the Process Variable Limit Value Monitoring Monitoring the Rate of Change (Slope) of the Process Variable PV(t) Functions and Parameters of the Rate of Change Monitoring of the Process Variable PV(t) Intervening in the Process Variable Branch Using an Operator Panel Filtering Noise Affecting the Error Signal ER using a DEADBAND Functions and Parameters of the DEADBAND Function in the Error Signal Channel Monitoring the Limit Values of the Error Signal ER Functions and Parameters of the Error Difference ER Limit Value Monitoring Control Algorithm of the (Parallel Structure) Control Algorithm with the P and D Actions in the Feedback Path P Controller with Operating Point Setting Step Response of the P Controller Step Response of the PI Controller PI Controller with Smooth Switchover: Manual Automatic Step Response of the PD Controller Step Response of the PID Controller Modes of the Integrator in the PI/PID Controller Functions and Parameters of the Integrator C79-G776-C95-2 xi

12 Contents 4-39 Functions and Parameters of the Derivative Unit Block Diagram of the Controller with Continuous Actuating Signal ( Software Package) Manual Value Generation with the Changing the Manipulated Variable by Setting MANUP Functions and Parameters of the Manual Manipulated Value Generator Calling an FC in the Manipulated Variable Branch Limitation of the Rate of Change of the Manipulated Variable LMN(t) Functions and Parameters of the Manipulated Value Rate of Change Limits Absolute Value Limits of the Manipulated Variable LMN(t) = OUTV (t) Functions and Parameters of the Absolute Value Limits of the Manipulated Variable Functions and Parameters for Manipulated Value Normalization to a Physical Value Functions and Parameters of Manipulated Variable Conversion to the Peripheral Format Intervening in the Manipulated Variable Branch From an Operator Panel Two-Loop Cascade Control System Connecting a Cascade With Two Slave Control Loops Pulse Duration Modulation Synchronization of the Start of the Period How the Pulse Output Switches On and Off Symmetrical Curve of the Three-Step Controller (Ratio Factor = ) Asymmetrical Curve of the Three-Step Controller (Ratio Factor =.5) Two-Step Controller With Bipolar Range ( % to %) Two-Step Controller With a Monopolar Range (% to %) Functions and Parameters of the Pulse Generator Functions of the Step Controller With a Position Algorithm Functions of the Step Controller Without Position Feedback Signal Step Controller With Position Feedback Signal Modes and Generating Manual Values on the Step Controller With a Position Feedback Signal Interventions in the Manipulated Variable Branch Using an Operator Panel (OP) Absolute Value Limits of the Manipulated Variable LMN(t) = OUTV (t) Functions and Parameters of the Absolute Value Limitation of the Manipulated Value Processing the Position Feedback Signal With the Step Controller Functions and Parameters of the Peripheral Value Conversion for the Position Feedback Signal Generating the Binary Actuating Signal on the Step Controller With Position Feedback Signal xii C79-G776-C95-2

13 Contents 6- Functions of the Three-Step Element THREE_ST Functions of the Pulse Generator PULSEOUT Functions and Parameters for Generating Actuating Signals on the Step Controller Step Controller Without Position Feedback Signal Manual Mode With the Step Controller Without Position Feedback Signal Generating the Binary Actuating Signal on the Step Controller Without Position Feedback Signal Functions of the Three-Step Element THREE_ST Simulation of the Position Feedback Signal Functions and Parameters for Generating Actuating Signals on the Step Controller Without Position Feedback Signal Two-Loop Cascade Control With a Step Controller Connecting a Cascade With Two Secondary Control Loops and a Step Controller Principle of the Controller Call Using the Loop Scheduler LP_SCHED Call Sequence of Four Loops Called at Different Intervals Block Diagram and Parameters of the LP_SCHED Function Example APP_, Control Loop Structure and Parameters of the Process Block PROC_S Blocks for Example : Connection and Call FC (APP_), Connections and Call Functions and Parameters of the PROC_S Process Model Control Loop With Step Controller Following a Step Change in the Setpoint Example APP_2, Control Loop Structure and Parameters of the Process Block PROC_C Blocks of Example 2: Connection and Call Connecting and Calling FC (APP_2) Functions and Parameters of the Process Model PROC_C Controlling With a Continuous Controller and Setpoint Step Changes Over the Entire Measuring Range Ratio Control With Two Loops (APP_3) Blocks for Example 3: Connection and Call Circuit Diagram and Parameters for the FC Block APP_ Blending Control for Three Components (APP_4) Blocks for Example 4: Connection and Call Block Diagram and Parameters of the FC Block APP_ Two-Loop Cascade Control System (APP_5) Blocks of Example 4: Connection and Call Block Diagram and Parameters of the FC Block APP_ Block Diagram of the Continuous Controller: PID_C Block Diagram of the Step Controller: PID_S (with position feedback signal LMNR = TRUE ) Block Diagram of the Step Controller: PID_S (without position feedback signal LMNR = FALSE ) LP_SCHED Function Program Structure Data Structure Open Control Loop C79-G776-C95-2 xiii

14 Contents -2 Closed Control Loop A- Transfer Function of a Self-Regulating Third Order Process..... Glossary- Tables 3- Manipulated Variable, Type of Control and Required Controller Blocks Modes of the Ramp Soak (RMP_SOAK) Shared Data Block (DB_NBR) with the Start Point and Four Time Slices Assigned Selecting the Controller Structure Modes of the Continuous Controller Modes of the Step Controller Modes of the Step Controller Without Position Feedback Signal Shared Data Block DB_LOOP for the Controller Call Blocks for Example Parameters of the Process Block PROC_S (DB: FB) Blocks for Example Parameters of the Process Block PROC_C (DB: FB) Blocks for Example Blocks for Example Blocks of Example Input Parameters of PID_C (continuous controller) Output Parameters of PID_C (continuous controller) Static Local Data of PID_C (inputs) Static Local Data of PID_C (outputs) Static Local Data used by the Configuration Tool PID_C Input Parameters of PID_S (step controller) Output Parameters of PID_S (step controller) Static Local Data of PID_S (inputs) Static Local Data of PID_S (outputs) Static Local Data used by the Configuration Tool (step controller PID_S) RMP_SOAK Function (PID_C and PID_S): Shared Data Block (DB_NBR), with Default of Start Point and Four Time Slices Input Parameters of PULSEGEN Output Parameters of PULSEGEN Input Parameters of LP_SCHED Global Data Area DB_NBR Criteria for PI and PID Controllers xiv C79-G776-C95-2

15 Product Overview What Does this Chapter Describe? This chapter describes the following: The concept of the standard controller The software packages of the standard controller The environment in which you can use the controller and its applications C79-G776-C95-2 -

16 Product Overview. The Product Concept of the Standard Controller The software product essentially consists of two function blocks (FBs) containing the algorithms for creating the control and signal processing functions for a continuous or step controller. This is therefore purely a software controller in which the standard function blocks implement all the functions of the controller. The response of the controller itself and the characteristics of the functions in the measuring channel and control channel are implemented or simulated by the numeric algorithms of the function block. The data required for these cyclic calculations are located in data blocks for each specific control loop. An FB is only required once even if you want to create several controllers. Each controller is represented by an instance DB which is created for the particular application. When using the Configuration tool, the DB is created implicitly. This means that designing a specific controller is restricted to specifying the structure and the values of the parameters in the dialogs of the user interface software. The configuration tool creates the instance DB. The algorithms for a particular controller are calculated in the processor of the S7 programmable logic controller (PLC) at the selected sampling times. The results of the calculations, in other words, the updated values of the input and output variables (measured variable and manipulated variable) and status messages (limit values) are saved in the instance DB or transferred to the process peripherals. When operating a large number of control loops, it is usually the case that some loops must be processed more often than others although each loop itself must be processed at equidistant intervals. For this situation, there is a loop scheduler (LP_SCHED) available with which extensive plant control systems can be configured clearly and simply. This also ensures that the load on the CPU is spread out. Overview of the Basic Functions In many control tasks, the classic PID controller that influences the process is not the sole important element but great demands are also made on signal processing. A controller created with the software package therefore consists of a series of subfunctions for which you can select parameter values separately. In addition to the actual controller with the PID algorithm, functions are also available for processing the setpoint and process variable and for adapting the calculated manipulated variable. The package also includes display and monitoring functions (not illustrated in the schematic overview of the functions). -2 C79-G776-C95-2

17 Product Overview Setpoint SP PV Error signal PID algorithm Process variable Manual value Manipulated value LMN MAN Figure - Overview of the Functions of the Continuous Controller Software Block Creating the Controller Using the S7-3/4 software package, you can configure a controller for a particular control task. You can configure the controller with a restricted range of functions. Using so-called structure switches, you can activate or deactivate subfunctions or disable entire branches. Once you have reduced the structure, you then only need to select parameter values for the remaining functions. All the phases in the creation of a controller, such as structuring the controller, assigning parameters, and calling the controller in the system program can be achieved largely without programming. Knowledge of STEP 7 is necessary. The structure of the instance DB is explained in Chapter 9 of this manual. A line is reserved for each structure or parameter value. By editing the entries, you can specify both the structure and the required characteristics of the controller. This method is, however, complicated and is not recommended. The configuration tool designed specially for the standard controller simplifies this job considerably. Note The configuration tool cannot be used for assigning parameters to the PULSEGEN and LP_SCHED blocks. The functions of these blocks can only be defined by entries in the corresponding data blocks. C79-G776-C

18 Product Overview.2 The Software Product Structure of the Standard Controller Product The diskettes of the package contain five working examples, three standard function blocks (SFBs) as well as one standard function (SFC) and a database for important application configurations. Instance DB Instance DB Instance DB Shared DB Standard FB PID_C Standard FB PID_S Standard FB PULSEGEN Standard FC LP_SCHED Example APP_ (fixed setpoint controller with discontinuous output) Example APP_2 (fixed setpoint controller with continuous output) Example Example Example APP_3 APP_4 APP_5 (multi loop (blending (cascade ratio controller) controller) controller) Setup On-line help Figure -2 Contents of the Software Package The PID_C standard function block contains all the control functions of a continuous PID controller. The PID_S standard function block contains all the control functions of a PID controller with a three-step output. The PULSEGEN standard function block for pulse duration modulation of analog actuating signals is used in conjunction with the PID_C standard function block to create a controller with a pulse output for proportional actuators. The LP_SCHED schedules the controller calls in a cyclic interrupt priority class in applications requiring a lot of control loops. The block is also responsible for initializing the controller structure when the CPU or PLC is started up. The software package also contains a setup program for installing the on a programming device or personal computer and an on-line help system that provides information about subfunctions and parameters while working with the package. -4 C79-G776-C95-2

19 Product Overview Preconfigured Application Structures The package also contains five data structures (instance DBs) for the most commonly used controller types and for the most important multi-loop controllers. These sample structures (APP_ to APP_5) are complete and ready for operation and you can use them if creating a new controller from the very start would be too time consuming or if you want to avoid errors creating connected controller structures. The following sample structures are available: Name Functions Comment APP_ Fixed setpoint controller with discontinuous output for integrating actuators (for example motor drives) PID step controller with three-step action APP_2 Fixed setpoint controller with continuous output for proportional actuators Analog PID controller APP_3 Multi-loop ratio controller The ratio between two process variables is kept constant APP_4 Blending controller Components to be mixed are kept to a constant percentage and the total amount is controlled APP_5 Cascade controller Improvement of the controller performance by including process variables in secondary control loops Configuration of the Standard Controller The functions of the Configuration software package are described in Chapter of this manual. C79-G776-C

20 Product Overview.3 Environment and Applications Hardware Environment The controllers created with the S7-3/4 software package can be run on the target systems (CPU with floating point and cyclic interrupt) of the S7-3 and S7-4 family. PG/PC OS/OP STEP 7 (S7 TOP) Configuration Parameter assignment Testing Commissioning Operation Monitoring CPU CP S7-3 MPI LAN bus Figure -3 Environment of the S7-3/4 Software Package Software Environment The S7-3/4 software package is intended for use in the STEP 7 program group. Apart from STEP 7, the following software is also required: Microsoft Windows 95 The configuration software for standard controllers can be installed locally on a programming device or personal computer or on a central network drive in a network. -6 C79-G776-C95-2

21 Product Overview The System Framework Since the digital implementation of controller functions always involves operations with complicated calculations (word-oriented processing), it is important to have an idea of the load on the CPU right from the beginning. The following guidelines will help: Size of a function block (PID_C or PID_S): 6 Kbytes Data per controller.5 Kbytes Basic data about the minimum run times (processing times) of a PID controller are listed in Section 8. (Technical Data). The amount of user memory required and therefore the number of control loops that can theoretically be installed with the available memory (at 5% utilization of the working memory by controller tasks) is listed in the technical data (see Section 8.). There are no memory requirements for an L stack. Interrupts are not delayed by processing the controller FB. Loop Scheduler If a lot of controllers or controllers with large sampling times have to be called, the cyclic interrupt priority class is inadequate to deal with the many calls required. The loop scheduler (LP_SCHED) can then be used in one cyclic interrupt priority class to call several controllers each at equidistant intervals. The tasks of the loop scheduler are as follows: Controlling the calls of the individual controllers within a cyclic interrupt priority class. Calling the installed standard controllers when the CPU is first started up. Possible Applications and Limitations of the Standard Controller The control function implemented by processing an FB can basically be used for any application. The control performance and the speed in which actual control loops are processed only depends 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 have to be processed. The faster the control loops have to be processed, in other words the more often the manipulated variables must be calculated per unit of time, the less the number of controllers that can be installed. The standard function blocks PID_C and PID_S allow you to generate and operate software controllers based on the conventional PID algorithm of the standard controller. Special functions in terms of handling process signals on the controller are not included. There are no restrictions to the type of process that can be controlled. Both slow processes (temperatures, tank levels) and very fast processes (flow rate, motor speed) can be controlled. C79-G776-C

22 Product Overview Types of control that can be implemented with the standard controller: Fixed setpoint control with P, PI, PD, PID controller Fixed setpoint control with continuous P, PI, PD, PID controller Fixed setpoint control with feedforward control Cascade control (step controller only in secondary loop) Ratio control (two loops) Blending control. Range of Functions of the Standard Controller By configuring the functions contained in the product, you can create controllers with the following characteristics and modes: Adjustment of the setpoint by a ramp soak Limitation of the rate of change of the reference input and (with controllers with a continuous output) of the manipulated variable Limitation of the absolute values of the reference input and (with controllers with a continuous output) of the manipulated variable Suppression of noise in the process variable or setpoint branch by filtering the error signal Suppression of high frequency oscillations in the process variable branch by delaying the process variable signal Linearization of quadratic functions of the process variable (flow control with differential pressure sensors) Possibility of calling your own functions in the setpoint, process variable and/or manipulated variable branch Manual mode (controlling the manipulated variable from a programming device or OP/OS) Monitoring two upper and two lower limits for the process variable and/or error signal Monitoring of the rate of change of the process variable The option of including a P and D action in the feedback path of the controller -8 C79-G776-C95-2

23 Designing Digital Controllers 2 What Does this Chapter Describe? This chapter describes the following: The process characteristics and their general influence on control functions The identification of the process characteristics Feedforward control Multi-loop controllers The structure of the standard controller and how it works Signal flow diagrams C79-G776-C

24 Designing Digital Controllers 2. Process Characteristics and Control 2 Process Characteristics and the Controller The static behavior (gain) and the dynamic characteristics (time lag, dead time, reset times etc.) of the process to be controlled have a significant influence on the type and time response of the signal processing in the controller responsible for keeping the process stable or changing the process according to a selected time schedule. The process has a special significance among the components of the control loop. Its characteristics are fixed either by physical laws or by the machinery being used and can hardly be influenced. A good control result is therefore only possible by selecting the controller type best suited to the particular process and by adapting the controller to the time response of the process. Precise knowledge of the type and characteristic data of the process to be controlled is indispensable for structuring and designing the controller and for selecting the dimensions of its static (P mode) and dynamic (I and D modes) parameters. Process Analysis To design the controller, you require exact data from the process that you obtain by means of a transfer function following a step change in the setpoint. The (graphical) analysis of this (time) function allows you to draw conclusions about the selection of the most suitable controller function and the dimensions of the controller parameters to be set. The configuration tool supports you to a large extent during the phase of process analysis. Before describing the use of the Configuration tool the next sections briefly look at the most common processes involved in automation. You may possibly require this information to help you to decide the best procedure for the analysis and simulation of the process characteristics. 2-2 C79-G776-C95-2

25 Designing Digital Controllers Type and Characteristics of the Process The following processes will be analyzed in greater detail: Self-regulating process Self-regulating process with dead time Process with integral action Self-regulating Process Most processes are self-regulating, in other words, after a step change in the manipulated variable, the process (controlled) variable approaches a new steady-state value. The time response of the system can therefore be determined by plotting the curve of the process variable with respect to time PV(t) after a step change in the manipulated variable LMN by a value greater than.5% of its total range. 2 LMN LMN t PV T g K S = PV LMN PV Legend: K S transfer coefficient T u time lag T g settling time t T u Figure 2- Step Response of a Self-Regulating Process (first order) If the process response within the manipulated variable range is linear, the transfer coefficient K s indicates the gain of the control loop. From the ratio of the time lag to the settling time T u /T g, the controllability of the process can be estimated. The smaller this value is, in other words the smaller the time lag relative to the settling time, the better the process can be controlled. According to the values T u and T g, the time response of a process can be roughly classified as follows: T u <.2 min and T g < 2 min fast process T u >.5 min and T g > 5 min slow process The absolute value of the settling time therefore has a direct influence on the sampling time of the controller: The higher T g is, in other words the slower the process reaction, the higher the sampling time that can be selected. C79-G776-C

26 Designing Digital Controllers Self-Regulating Process with Dead Time Many processes involving transportation of materials or energy (pipes, conveyor belts etc.) have a time response similar to that shown in Figure 2-2. This includes a start-up time T a made up of the actual dead time and the time lag of the self-regulating process. In terms of controllability of the process it is extremely important that T t remains small relative to T g or in other words that the relationship T t /T g is maintained. 2 LMN LMN t PV T g PV Legend: T t dead time T u time lag Ta start-up time (= T t +T u ) T g settling time t T t T u T a Figure 2-2 Step Response of a Self-Regulating Process with Dead Time Since the controller does not receive any signal change from the transmitter during the dead time, its interventions are obviously delayed and the control quality is therefore reduced. When using a standard controller, such effects can be partly eliminated by choosing a new location for the measuring sensor. 2-4 C79-G776-C95-2

27 Designing Digital Controllers Process with Integral Action Here, the slope of the ramp of the process variable (PV) after changing the manipulated variable by a fixed amount is inversely proportional to the value of the integration time constant (reset time) TI. LMN LMN 2 PV LMN Steady-state condition Legend: TI reset time t TI t Figure 2-3 Step Response of a Non Self-Regulating Process (I Process) Processes with an I component are, for example liquid level processes in which the level can be raised or lowered at different rates depending on the opening of the final control element. Important processes involving the I action are also the commonly used motor drives with which the rate of change of a traversing movement is directly proportional to the speed of the drive. If no disturbance variables occur before the I element of a process with integral action (which is usually the case), a controller without I action should be used. The effects of a disturbance variable at the process input can usually be eliminated by feedforward control without using an I action in the controller. C79-G776-C