National Instruments 371685C-01 User Manual

LabWindowsTM/CVI

PID Control Toolkit User Manual

LabWindows/CVI PID Control Toolkit User Manual

May 2008 371685C-01

Support

Worldwide Technical Support and Product Information

ni.com

National Instruments Corporate Headquarters

11500 North Mopac Expressway Austin, Texas 78759-3504 USA Tel: 512 683 0100

Worldwide Offices

Australia 1800 300 800, Austria 43 662 457990-0, Belgium 32 (0) 2 757 0020, Brazil 55 11 3262 3599, Canada 800 433 3488, China 86 21 5050 9800, Czech Republic 420 224 235 774, Denmark 45 45 76 26 00, Finland 358 (0) 9 725 72511, France 01 57 66 24 24, Germany 49 89 7413130, India 91 80 41190000, Israel 972 3 6393737, Italy 39 02 41309277, Japan 0120-527196, Korea 82 02 3451 3400, Lebanon 961 (0) 1 33 28 28, Malaysia 1800 887710, Mexico 01 800 010 0793, Netherlands 31 (0) 348 433 466, New Zealand 0800 553 322, Norway 47 (0) 66 90 76 60, Poland 48 22 3390150, Portugal 351 210 311 210, Russia 7 495 783 6851, Singapore 1800 226 5886, Slovenia 386 3 425 42 00, South Africa 27 0 11 805 8197, Spain 34 91 640 0085, Sweden 46 (0) 8 587 895 00, Switzerland 41 56 2005151, Taiwan 886 02 2377 2222, Thailand 662 278 6777, Turkey 90 212 279 3031, United Kingdom 44 (0) 1635 523545

For further support information, refer to the Technical Support and Professional Services appendix. To comment on National Instruments documentation, refer to the National Instruments Web site at the info code

feedback.

ni.com/info and enter

Important Information

Warranty

The media on which you receive National Instruments software are warranted not to fail to execute programming instructions, due to defects in materials and workmanship, for a period of 90 days from date of shipment, as evidenced by receipts or other documentation. National Instruments will, at its option, repair or replace software media that do not execute programming instructions if National Instruments receives notice of such defects during the warranty period. National Instruments does not warrant that the operation of the software shall be uninterrupted or error free.

A Return Material Authorization (RMA) number must be obtained from the factory and clearly marked on the outside of the package before any equipment will be accepted for warranty work. National Instruments will pay the shipping costs of returning to the owner parts which are covered by warranty.

National Instruments believes that the information in this document is accurate. The document has been carefully reviewed for technical accuracy. In the event that technical or typographical errors exist, National Instruments reserves the right to make changes to subsequent editions of this document without prior notice to holders of this edition. The reader should consult National Instruments if errors are suspected. In no event shall National Instruments be liable for any damages arising out of or related to this document or the information contained in it.

XCEPT AS SPECIFIED HEREIN, NATIONAL INSTRUMENTS MAKES NO WARRANTIES, EXPRESS OR IMPLIED, AND SPECIFICALLY DISCLAIMS ANY WARRANTY OF

MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. CUSTOMER’S RIGHT TO RECOVER DAMAGES CAUSED BY FAULT OR NEGLIGENCE ON THE PART OF NATIONAL

NSTRUMENTS SHALL BE LIMITED TO THE AMOUNT THERETOFORE PAID BY THE CUSTOMER. NATIONAL INSTRUMENTS WILL NOT BE LIABLE FOR DAMAGES RESULTING

FROM LOSS OF DATA, PROFITS, USE OF PRODUCTS, OR INCIDENTAL OR CONSEQUENTIAL DAMAGES, EVEN IF ADVISED OF THE POSSIBILITY THEREOF. This limitation of

the liability of National Instruments will apply regardless of the form of action, whether in contract or tort, including negligence. Any action against National Instruments must be brought within one year after the cause of action accrues. National Instruments shall not be liable for any delay in performance due to causes beyond its reasonable control. The warranty provided herein does not cover damages, defects, malfunctions, or service failures caused by owner’s failure to follow the National Instruments installation, operation, or maintenance instructions; owner’s modification of the product; owner’s abuse, misuse, or negligent acts; and power failure or surges, fire, flood, accident, actions of third parties, or other events outside reasonable control.

Copyright

Under the copyright laws, this publication may not be reproduced or transmitted in any form, electronic or mechanical, including photocopying, recording, storing in an information retrieval system, or translating, in whole or in part, without the prior written consent of National Instruments Corporation.

National Instruments respects the intellectual property of others, and we ask our users to do the same. NI software is protected by copyright and other intellectual property laws. Where NI software may be used to reproduce software or other materials belonging to others, you may use NI software only to reproduce materials that you may reproduce in accordance with the terms of any applicable license or other legal restriction.

Trademarks

National Instruments, NI, ni.com, and LabVIEW are trademarks of National Instruments Corporation. Refer to the Terms of Use section on ni.com/legal for more information about National Instruments trademarks.

The mark LabWindows is used under a license from Microsoft Corporation. Windows is a registered trademark of Microsoft Corporation in the United States and other countries. Other product and company names mentioned herein are trademarks or trade names of their respective companies.

Patents

For patents covering National Instruments products, refer to the appropriate location: Help»Patents in your software, the patents.txt file on your media, or ni.com/patents.

WARNING REGARDING USE OF NATIONAL INSTRUMENTS PRODUCTS

(1) NATIONAL INSTRUMENTS PRODUCTS ARE NOT DESIGNED WITH COMPONENTS AND TESTING FOR A LEVEL OF RELIABILITY SUITABLE FOR USE IN OR IN CONNECTION WITH SURGICAL IMPLANTS OR AS CRITICAL COMPONENTS IN ANY LIFE SUPPORT SYSTEMS WHOSE FAILURE TO PERFORM CAN REASONABLY BE EXPECTED TO CAUSE SIGNIFICANT INJURY TO A HUMAN.

(2) IN ANY APPLICATION, INCLUDING THE ABOVE, RELIABILITY OF OPERATION OF THE SOFTWARE PRODUCTS CAN BE IMPAIRED BY ADVERSE FACTORS, INCLUDING BUT NOT LIMITED TO FLUCTUATIONS IN ELECTRICAL POWER SUPPLY, COMPUTER HARDWARE MALFUNCTIONS, COMPUTER OPERATING SYSTEM SOFTWARE FITNESS, FITNESS OF COMPILERS AND DEVELOPMENT SOFTWARE USED TO DEVELOP AN APPLICATION, INSTALLATION ERRORS, SOFTWARE AND HARDWARE COMPATIBILITY PROBLEMS, MALFUNCTIONS OR FAILURES OF ELECTRONIC MONITORING OR CONTROL DEVICES, TRANSIENT FAILURES OF ELECTRONIC SYSTEMS (HARDWARE AND/OR SOFTWARE), UNANTICIPATED USES OR MISUSES, OR ERRORS ON THE PART OF THE USER OR APPLICATIONS DESIGNER (ADVERSE FACTORS SUCH AS THESE ARE HEREAFTER COLLECTIVELY TERMED “SYSTEM FAILURES”). ANY APPLICATION WHERE A SYSTEM FAILURE WOULD CREATE A RISK OF HARM TO PROPERTY OR PERSONS (INCLUDING THE RISK OF BODILY INJURY AND DEATH) SHOULD NOT BE RELIANT SOLELY UPON ONE FORM OF ELECTRONIC SYSTEM DUE TO THE RISK OF SYSTEM FAILURE. TO AVOID DAMAGE, INJURY, OR DEATH, THE USER OR APPLICATION DESIGNER MUST TAKE REASONABLY PRUDENT STEPS TO PROTECT AGAINST SYSTEM FAILURES, INCLUDING BUT NOT LIMITED TO BACK-UP OR SHUT DOWN MECHANISMS. BECAUSE EACH END-USER SYSTEM IS CUSTOMIZED AND DIFFERS FROM NATIONAL INSTRUMENTS' TESTING PLATFORMS AND BECAUSE A USER OR APPLICATION DESIGNER MAY USE NATIONAL INSTRUMENTS PRODUCTS IN COMBINATION WITH OTHER PRODUCTS IN A MANNER NOT EVALUATED OR CONTEMPLATED BY NATIONAL INSTRUMENTS, THE USER OR APPLICATION DESIGNER IS ULTIMATELY RESPONSIBLE FOR VERIFYING AND VALIDATING THE SUITABILITY OF NATIONAL INSTRUMENTS PRODUCTS WHENEVER NATIONAL INSTRUMENTS PRODUCTS ARE INCORPORATED IN A SYSTEM OR APPLICATION, INCLUDING, WITHOUT LIMITATION, THE APPROPRIATE DESIGN, PROCESS AND SAFETY LEVEL OF SUCH SYSTEM OR APPLICATION.

About This Manual

Conventions ...................................................................................................................vii

Related Documentation..................................................................................................vii

Chapter 1 Overview of the PID Control Toolkit

System Requirements ....................................................................................................1-1

Installation Instructions..................................................................................................1-1

Activation Instructions...................................................................................................1-2

PID Control Toolkit Applications..................................................................................1-2

PID Control....................................................................................................................1-3

Chapter 2 PID Algorithms

The PID Algorithm ........................................................................................................2-1

Implementing the PID Algorithm with the PID Functions .............................2-2

Gain Scheduling ..............................................................................................2-3

The Precise PID Algorithm............................................................................................2-4

Error Calculation .............................................................................................2-4

Proportional Action .........................................................................................2-4

Trapezoidal Integration ...................................................................................2-5

The Autotuning Algorithm ............................................................................................2-5

Tuning Formulas .............................................................................................2-6

Chapter 3 Using the PID Control Toolkit

Designing a Control Strategy.........................................................................................3-1

Setting Timing.................................................................................................3-1

Tuning Controllers Manually ..........................................................................3-2

Using the PID Library....................................................................................................3-4

PID Controller .................................................................................................3-4

Using PID with Autotuning.............................................................................3-6

Using PID with Gain Scheduling ....................................................................3-7

Using PID with Lead-Lag ...............................................................................3-8

Using PID with Setpoint Profiling ..................................................................3-8

Using Ramp Generators ..................................................................................3-9

Contents

Converting between Percentage of Full Scale and Engineering Units ...........3-9

Using PID on Real-Time (RT) Targets........................................................... 3-10

Using PID with DAQ Devices ........................................................................ 3-10

Appendix A References

Appendix B Technical Support and Professional Services

Glossary

Index

LabWindows/CVI PID Control Toolkit User Manual vi ni.com

About This Manual

The LabWindows/CVI PID Control Toolkit User Manual describes the PID Control Toolkit for LabWindows features, functions, and operation of the toolkit. To use this manual, you need a basic understanding of process control strategies and algorithms.

™

/CVI™. The manual describes the

Conventions

The following conventions appear in this manual:

» The » symbol leads you through nested menu items and dialog box options

to a final action. The sequence File»Page Setup»Options directs you to pull down the File menu, select the Page Setup item, and select Options from the last dialog box.

This icon denotes a note, which alerts you to important information.

bold Bold text denotes items that you must select or click in the software, such

as menu items and dialog box options. Bold text also denotes parameter names.

italic Italic text denotes variables, emphasis, a cross-reference, or an introduction

to a key concept. Italic text also denotes text that is a placeholder for a word or value that you must supply.

monospace Text in this font denotes text or characters that you should enter from the

keyboard, sections of code, programming examples, and syntax examples. This font is also used for the proper names of disk drives, paths, directories, programs, subprograms, subroutines, device names, functions, operations, variables, filenames, and extensions.

System Requirements

Your computer must meet the following minimum system requirements to run the PID Control Toolkit:

• LabWindows/CVI 7.x or later

• Windows Vista/XP/2000

Installation Instructions

If you already have an earlier version of the PID Control Toolkit installed on your computer, you must uninstall it before installing this version of the PID Control Toolkit.

Note When you install the PID Control Toolkit, your user account must have

administrator privileges.

Complete the following steps to install the PID Control Toolkit:

1. Insert the PID Control Toolkit CD into the CD drive. If the CD does not run automatically, open Windows Explorer, right-click the CD drive icon, and select AutoPlay.

2. On installation startup, the National Instruments PID Control Toolkit screen appears. Click Install Toolkit.

3. In the User Information panel, enter your name and organization and the serial number found on your Certificate of Ownership card. LabWindows/CVI uses this serial number when you run the NI Activation Wizard.

4. Follow the instructions on the screen. When the PID Control Toolkit has been successfully installed, click Finish.

Chapter 1 Overview of the PID Control Toolkit

Activation Instructions

The first time you launch LabWindows/CVI after installing the PID Control Toolkit, you are prompted to activate the toolkit. Complete the following steps to activate the PID Control Toolkit.

1. Click Activate Products.

2. Select the Automatically activate through a secure Internet connection option and click Next. Your computer must be connected to the Internet for this option to work. If you do not have Internet access on your computer, refer to the LabWindows/CVI Release Notes.

3. Enter the serial number with the number found on your Certificate of Ownership card. Click Next.

4. Fill in the necessary information and click Next.

5. Check the option if you would like to receive a confirmation email of your activation and click Next.

6. After a brief moment, you should receive a message indicating whether the PID Control Toolkit has been activated or not. Click Next.

Note If your activation was not successful, you can update the serial number, get help

from National Instruments, or evaluate the toolkit.

7. Continue to follow the instructions on the screen.

8. When you successfully activate, click Finish. LabWindows/CVI displays a window indicating when this license expires.

For more information about activation, refer to the LabWindows/CVI Release Notes.

PID Control Toolkit Applications

The PID Control Toolkit contains functions you can use to develop LabWindows/CVI control applications. For more information about the types of applications you can develop, refer to the example programs that are installed with the toolkit.

LabWindows/CVI PID Control Toolkit User Manual 1-2 ni.com

PID Control

Currently, the PID algorithm is the most common control algorithm used in industry. Often, PID is used to control processes that include heating and cooling systems, fluid level monitoring, flow control, and pressure control. When using PID control, you must specify a process variable and a setpoint. The process variable is the system parameter you want to control, such as temperature, pressure, or flow rate. The setpoint is the desired value for the parameter you are controlling. A PID controller determines a controller output value, such as the heater power or valve position. When applied to the system, the controller output value drives the process variable toward the setpoint value.

You can use the PID Control Toolkit functions with National Instruments hardware to develop LabWindows/CVI control applications. Use I/O hardware, such as DAQ devices, FieldPoint I/O modules, or GPIB boards, to connect your PC to the system you want to control. You can use the LabWindows/CVI I/O functions with the PID Control Toolkit to develop a control application or modify the examples provided with the toolkit.

Using the PID Control Toolkit functions, you can develop the following control applications based on PID controllers:

• Proportional (P), proportional-integral (PI), proportional-derivative (PD), and proportional-integral-derivative (PID) algorithms

• Gain-scheduled PID

• PID autotuning

• Precise PID

• Lead-lag compensation

• Setpoint profile generation

• Multiloop cascade control

• Feedforward control

• Override (minimum/maximum selector) control

• Ratio/bias control

Chapter 1 Overview of the PID Control Toolkit

Refer to the LabWindows/CVI PID Control Toolkit Help, which you can access by selecting

Start»All Programs»National Instruments»PID Control Toolkit for CVI»LabWindows CVI PID Help, for more information about the functions.

PID Algorithms

This chapter explains the fast PID, precise PID, and autotuning algorithms.

The PID Algorithm

The PID controller compares the setpoint (SP) to the process variable (PV) to obtain the error (e), as follows:

e = SP – PV

Then the PID controller calculates the controller action, u(t), as follows. In this equation, K is the controller gain.

⎛⎞

ut() K

If the error and the controller output have the same range, –100% to 100%, controller gain is the reciprocal of proportional band. T time, and T represents the proportional action.

The following formula represents the integral action.

The following formula represents the derivative action.

is the derivative time in minutes, also called the rate time. The following formula

⎜⎟

⎜⎟ ⎝⎠

is the integral time in minutes, also called the reset

uDt() KcTd

----

t() Kce=

-------

t() =

etd

∫

etd

∫

-------

------

Chapter 2 PID Algorithms

Implementing the PID Algorithm with the PID Functions

This section describes how the PID Control Toolkit functions implement the fast (positional) PID algorithm. The fast PID algorithm is the default algorithm used in the PID Control Toolkit.

Error Calculation

The following formula represents the current error used in calculating proportional, integral, and derivative action, where PV

is the filtered process variable.

e( k) = (SP PV

)–

Proportional Action

Proportional action is the controller gain times the error, as shown in the following formula:

k()= Kc* ek()()

Trapezoidal Integration

Trapezoidal integration is used to avoid sharp changes in integral action when there is a sudden change in the PV or SP. Use nonlinear adjustment of the integral action to counteract overshoot. The following formula represents the trapezoidal integration action.

------

k()=

∑

i 1=

ei() ei 1–()+

----------------------------------

tΔ

Partial Derivative Action

Because of abrupt changes in the SP, apply derivative action to only the PV, not to the error (e), to avoid derivative kick. The following formula represents the partial derivative action.

uDk() = Kc

-----

Δt

k()PVfk 1–()–()–

Controller Output

Controller output is the summation of the proportional, integral, and derivative action, as shown in the following formula:

uk() u

LabWindows/CVI PID Control Toolkit User Manual 2-2 ni.com

k() uIk() u+Dk()+=

Chapter 2 PID Algorithms

Output Limiting

The actual controller output is limited to the range specified for control output, as follows:

if uk() u

then uk() u

max

=≥

max

and

if uk() u

then uk() u

min

=≤

min

The following formula shows the practical model of the PID controller.

ut() K

SP PV–()

----

(SP PV)dt T

∫

dPV

------------

––

The PID functions use an integral sum correction algorithm that facilitates anti-windup and bumpless manual-to-automatic transfers. Windup occurs at the upper limit of the controller output, for example, 100%. When the error (e) decreases, the controller output decreases, moving out of the windup area. The integral sum correction algorithm prevents abrupt controller output changes when you switch from manual to automatic mode or change any other parameters.

The default ranges for the SP, PV, and output parameters correspond to percentage values; however, you can use actual engineering units. If you use engineering units, you must adjust the corresponding ranges accordingly. The T

and Td parameters are specified in minutes.

In manual mode, you can change the manual input to increase or decrease the output.

All the PID control functions are reentrant. Multiple calls from high-level functions use separate and distinct data.

Note As a general rule, manually drive the PV until it meets or comes close to the SP

before you perform the manual-to-automatic transfer.

Gain Scheduling

Gain scheduling refers to a system in which you change controller parameters based on measured operating conditions. For example, the scheduling variable can be the SP, the PV, a controller output, or an external signal. For historical reasons, the term gain scheduling is used even if other parameters, such as the derivative time or integral time parameters, change. Gain scheduling effectively controls a system whose dynamics change with the operating conditions.

+ 27 hidden pages

National Instruments 371685C-01 User Manual

Specifications and Main Features

Frequently Asked Questions

User Manual

PID Control Toolkit User Manual

Support

Worldwide Technical Support and Product Information

National Instruments Corporate Headquarters

Worldwide Offices

Important Information

Warranty

Copyright

Trademarks

Patents

WARNING REGARDING USE OF NATIONAL INSTRUMENTS PRODUCTS

Contents

About This Manual

Conventions

Related Documentation

System Requirements

Installation Instructions

Activation Instructions

PID Control Toolkit Applications

PID Control

The PID Algorithm

Implementing the PID Algorithm with the PID Functions

Gain Scheduling