National Instruments 371685C-01 Network Card User Manual
Contents
1. e Tee eset 2 4 Trapezoidal 2 5 The Autotunimg Algorithm oi Ete P E Oen eet 2 5 Tuning ineo teet da teet ad dens 2 6 Chapter 3 Using the PID Control Toolkit Desigmng a Control Strategy er tee pe rp mne 3 1 Setting Timing eee UG ERE Le re PETERE LEER e TR tastes 3 1 Tuning Controllers Manually sese 3 2 Usingthe PID Tei Dray t cene e gc et de f UR 3 4 PUD Controller e tete rte t eu 3 4 Using PID with 3 6 Using PID with Gain Scheduling eere 3 7 Using PID with Lead Laterne ee aa aea e 3 8 Using PID with Setpoint Profiling 2 3 8 Using Ramp 3 9 National Instruments Corporation V LabWindows CVI PID Control Toolkit User Manual Contents Converting between Percentage of Full Scale and Engineering Units 3 9 Using PID on Real Time RT Targets eee 3 10 Using PID with DAQ eene eene 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 Conventions The LabWindows CVI PID Control Toolkit User Manual describes the PID Control Toolkit for LabWindows CVI The manual describes the features functions and operatio2. Using the PID Control Toolkit functions you can develop the following control applications based on PID controllers e Proportional P proportional integral PI proportional derivative PD and proportional integral derivative PID algorithms e Gain scheduled PID e PID autotuning e Precise PID e Lead lag compensation e Setpoint profile generation e Multiloop cascade control e Feedforward control e Override minimum maximum selector control e Ratio bias control Refer to the LabWindows CVI PID Control Toolkit Help which you can access by selecting Start Programs National Instruments PID Control Toolkit for CVI LabWindows CVI PID Help for more information about the functions National Instruments Corporation 1 3 LabWindows CVI PID Control Toolkit User Manual 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 u t K cope ni If the error and the controller output have the same range 100 to 100 controller gain is the reciprocal of proportional band 7 is the integral time in minutes also called the reset time and Tis the derivative time in minutes also called the rate time The fol
3. PID 0 25 0 5 0 1127 Table 2 3 Tuning Formula under P Only Control Slow Controller K Ti 0 13K PI 0 13K 0 87 PID 0 15K 0 57 0 1127 Table 2 4 Tuning Formula under or PID Control Fast Controller K T Ta P PI 0 97T 1 3 331 PID LATA 2 01 0 5 Table 2 5 Tuning Formula under or PID Control Normal Controller K 0 447 0 47 5 331 PID 0 537 4 0 0 81 National Instruments Corporation 2 7 LabWindows CVI PID Control Toolkit User Manual Chapter 2 PID Algorithms Table 2 6 Tuning Formula under PI or PID Control Slow Controller K T Ta P 0 26T 1 PI 0 24 5 33 PID 0 327 4 0 0 81 3 Note During tuning the process remains under closed loop PID control It is not necessary to switch off the existing controller and perform the experiment under open loop conditions In the setpoint relay experiment the SP signal mirrors the SP for the PID controller LabWindows CVI PID Control Toolkit User Manual 2 8 ni com Using the PID Control Toolkit This chapter contains the basic information you need to design a control strategy using the PID Control Toolkit functions Designing a Control Strategy When you design a control strategy sketch a flowchart that includes the physical process and control elements such as valves and measurements Add fe
4. 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 Chapter 1 Overview of the PID Control Toolkit 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
5. 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 PLATFORM
6. a full year after purchase after which you may renew to continue your benefits For information about other technical support options in your area visit ni com services or contact your local office at ni com contact Training and Certification Visit ni com training for self paced training eLearning virtual classrooms interactive CDs and Certification program information You also can register for instructor led hands on courses at locations around the world System Integration If you have time constraints limited in house technical resources or other project challenges National Instruments Alliance Partner members can help To learn more call your local NI office or visit ni com alliance B 1 LabWindows CVI PID Control Toolkit User Manual Appendix B Technical Support and Professional Services If you searched ni com and could not find the answers you need contact your local office or NI corporate headquarters Phone numbers for our worldwide offices are listed at the front of this manual You also can visit the Worldwide Offices section of ni com niglobal to access the branch office Web sites which provide up to date contact information support phone numbers email addresses and current events LabWindows CVI PID Control Toolkit User Manual 2 ni com Glossary A algorithm autotuning Autotuning Wizard bias bumpless transfer C cascade control closed loop controller controller outpu
7. can set the component to use the value you have supplied to the pidAttrDeltaT attribute by setting pidAttrUseInternalTimer to 0 Use the pi dAttrDeltaT attribute for fast loops including instances in which you use acquisition hardware to time the controller input National Instruments Corporation 3 1 LabWindows CVI PID Control Toolkit User Manual Chapter 3 Using the PID Control Toolkit Tuning Controllers Manually 5 The following controller tuning procedures are based on the work of Ziegler and Nichols the developers of the Quarter Decay Ratio tuning techniques derived from a combination of theory and empirical observations Corripio 1990 Experiment with these techniques and the process control simulation examples to compare them For different processes one method might be easier or more accurate than another For example some techniques that work best when used with online controllers cannot stand the large upsets described here To perform these tests set up your control strategy with the PV SP and output displayed on a large strip chart with the axes showing the values versus time Refer to the Closed Loop Ultimate Gain Tuning Procedure and Open Loop Step Test Tuning Procedure sections for more information about disturbing the loop and determining the response from the graph Refer to Tuning of Industrial Control Systems as listed in Appendix A References for more information about these procedures Closed Loop Ultima
8. in integral action when there is a sudden change in the PV or SP The following formula represents the trapezoidal integration action for the precise PID algorithm Use nonlinear adjustment of integral action to counteract the overshoot The larger the error the smaller the integral action as shown in the following formula and in Figure 2 1 K GEIL 1 h EU c 2 p N SHUT rng 0 9 0 8 0 7 0 6 0 5 0 4 0 3 0 2 01 02 4000 250 500 250 00 250 500 750 1000 Error Figure 2 1 Nonlinear Multiple for Integral Action SP ng 100 The Autotuning Algorithm Use autotuning to improve performance Often many controllers are poorly tuned As a result some controllers are too aggressive and some controllers are too sluggish PID controllers are difficult to tune when you do not know the process dynamics or disturbances In this case use autotuning Before you begin autotuning you must establish a stable controller even if you cannot properly tune the controller on your own National Instruments Corporation 2 5 LabWindows CVI PID Control Toolkit User Manual Chapter 2 PID Algorithms Figure 2 2 illustrates the autotuning procedure excited by the setpoint relay experiment which connects a relay and an extra feedback signal with the SP Notice that the PID Library autotuning functions directly implement this process The existing controller remains in the loop PV P I Cont
9. is 100 to 100 which corresponds to values specified in terms of percentage of full scale However you can change this range so that the controller gain relates engineering units to engineering units instead of percentage to percentage The lead lag compensator coerces the controller output to the specified range Using PID with Setpoint Profiling Using the setpoint profiling feature you can generate a profile of setpoint values over time for a ramp and soak type PID application For example you might want to ramp the setpoint temperature of an oven control system over time and then hold or soak the setpoint at a certain temperature for another period of time Use this feature to implement any arbitrary combination of ramp hold and step functions Provide setpoint time pairs to the setpoint profile The setpoint profile maintains the setpoint specified in each pair for the corresponding times specified in the time array LabWindows CVI PID Control Toolkit User Manual 3 8 ni com Chapter 3 Using the PID Control Toolkit You can use the setpoint profiler as follows 1 Call PidSetpointProfileCreate to create a setpoint profile Use a pair of time and setpoint value arrays to specify the setpoint profile with the time values in ascending order 2 UsePidSetSetpointProfileAttribute to set the setpoint profile attributes Use PidSetpointProfileNextSetpoint to obtain the setpoint from the profile in a loop and provide this setpoint to
10. to the distribution The version of LabWindows CVI you are using determines how you add the merge module to the distribution If you are using LabWindows CVI 7 x create a file group in the distribution kit named MSMs Include CVIPIDRuntime msm in that file group Build the distribution kit and CVIPIDRunt ime msm will be seamlessly merged in as an MSI merge module instead of just being included as a file LabWindows CVI PID Control Toolkit User Manual 3 6 ni com Chapter 3 Using the PID Control Toolkit If you are using LabWindows CVI 8 0 and later click Add Additional Module in the Drivers amp Components tab of the Edit Installer dialog box In the Select Merge Module dialog box browse to and select CVIPIDRuntime msm For additional information about distributing LabWindows CVI applications refer to the LabWindows CVI Help Using PID with Gain Scheduling Most processes are non linear Therefore PID parameters that produce a desired response at one operating point might not produce a satisfactory response at another operating point Using the gain scheduling feature you can apply different sets of PID parameters for different regions of controller operation The gain scheduler selects and outputs one set of PID gains from a gain schedule based on the current gain scheduling value The gain scheduling value input can be anything and is based on the gain scheduling criteria that you set Use the pidGSAttrGainScheduleCriteria attrib
11. Help National Instruments Corporation Vii LabWindows CVI PID Control Toolkit User Manual Overview of the PID Control Toolkit This chapter describes how to install the toolkit and describes Proportional Integral Derivative PID control applications System Requirements Your computer must meet the following minimum system requirements to run the PID Control Toolkit e LabWindows CVI 7 x or later e 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 Oninstallation startup the National Instruments PID Control Toolkit screen appears Click Install Toolkit 3 Inthe 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 National Instruments Corp
12. LabWindows CVI PID Control Toolkit User Manual May 2008 NATIONAL 371685C 01 gt INSTRUMENTS 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 ni com info and enter the info code feedback 2005 2008 National Instruments Corpo
13. S 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 Contents About This Manual GOTHOS vii Related vii Chapter 1 Overview of the PID Control Toolkit System Requirements re e aep Lee Vds deua eH t 1 1 Installation Instructions ccccccccccssssscecceeesssceccceessseeecccessssceeceesssseeeceesesseececeseseeeeeess 1 1 Activation e e ENSE rege 1 2 PID Control Toolkit 1 2 PID Gontrol 4 eroe RES Ie 1 3 Chapter 2 PID Algorithms tita fee ee ree e o eee teer e pe Dre edes 2 1 Implementing the PID Algorithm with the PID Functions 2 2 Gain Scheduling eee eto epe e te ORE Cnr 2 3 The Precise PND Algorithm eere tite ee eR Ire e eene 2 4 Error Calculation iode e e bete 2 4 Proportional ACh OM e ert eee
14. This book provides detailed tuning procedures The following material is referenced in this manual Corripio A B 1990 Tuning of Industrial Control Systems Raleigh North Carolina ISA Ziegler J and N B Nichols 1942 Optimum Settings for Automatic Controllers Trans ASME 64 759 68 National Instruments Corporation A 1 LabWindows CVI PID Control Toolkit User Manual Technical Support and Professional Services Visit the following sections of the award winning National Instruments Web site at ni com for technical support and professional services National Instruments Corporation Support Technical support resources at ni com support include the following Self Help Technical Resources For answers and solutions visit ni com support for software drivers and updates searchable KnowledgeBase product manuals step by step troubleshooting wizards thousands of example programs tutorials application notes instrument drivers and so on Registered users also receive access to the NI Discussion Forums at ni com forums NI Applications Engineers make sure every question submitted online receives an answer Standard Service Program Membership This program entitles members to direct access to NI Applications Engineers via phone and email for one to one technical support as well as exclusive access to on demand training modules via the Services Resource Center NI offers complementary membership for
15. action up k PV k PV k 1 Controller Output Controller output is the summation of the proportional integral and derivative action as shown in the following formula u k up k uj k u p k LabWindows CVI PID Control Toolkit User Manual 2 2 ni com Chapter 2 PID Algorithms Output Limiting The actual controller output is limited to the range specified for control output as follows if u k gt u max then u k t max and If u K lt u then u k umin The following formula shows the practical model of the PID controller t dPV u t k sr Pns GP Pra r 1 10 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 and parameters are specified in minutes In manual mode you can change the manual input to increase or decrease the output All the PID control funct
16. ale 3 9 error calculation 2 2 examples NI resources B 1 F fast PID algorithm 3 4 G gain schedule contents 3 7 gain scheduling 2 3 3 7 H help file accessing 1 3 help technical support B 1 LabWindows CVI PID Control Toolkit User Manual Index installation instructions 1 1 instrument drivers NI resources B 1 integral action 2 1 integral time 2 1 K KnowledgeBase B 1 L lag compensation 3 8 lead compensation 3 8 lead lag compensator 3 8 National Instruments support and services B 1 Nichols N B A 1 nonlinear adjustment of integral action 2 2 2 5 0 open loop tuning procedure 3 3 output limiting 2 3 output rate limiting 3 5 P partial derivative action 2 2 percentage of full scale converting from engineering units 3 0 PID algorithm 2 1 to 2 3 autotuning algorithm 2 5 to 2 8 calculating controller action 2 1 controller output 2 2 error calculation 2 2 LabWindows CVI PID Control Toolkit User Manual nonlinear adjustment of integral action 2 2 output limiting 2 3 partial derivative action 2 2 proportional action 2 2 trapezoidal integration 2 2 gain scheduling 2 3 PID controller 1 3 typical use 3 4 PID Library 3 4 pidAttrAlgorithm 3 4 pidAttrLimitOutputRate 3 5 pidAttrUseInternalTimer 3 1 pidGSAttrGainScheduleCriteria 3 7 precise PID algorithm 2 4 3 4 calculating controller action nonlinear adjustment of integral action 2 5 proporti
17. apter 3 Using the PID Control Toolkit Open Loop Step Test Tuning Procedure The open loop step test tuning procedure assumes that you can model any process as a first order lag and a pure deadtime This method requires more analysis than the closed loop tuning procedure but the process does not need to reach sustained oscillation Therefore the open loop tuning procedure might be quicker and more reliable for many processes Observe the output and the PV on a strip chart that shows time on the x axis Complete the following steps to perform the open loop tuning procedure Putthe controller in manual mode set the output to a nominal operating value and allow the PV to settle completely Record the PV and output values 2 Make a step change in the output Record the new output value 3 Wait for the PV to settle From the chart determine the values as derived from the sample displayed in Figure 3 1 The variables represent the following values e Td Deadtime in minutes T Time constant in minutes e K Process gain change in PV change in output Ma lt 552 24 2 ELTE 63 2 Max Min ae Min y i Output a i 1 Figure 3 1 Output and Process Variable Strip Chart 4 Multiply the measured values by the factors shown in Table 3 2 and enter the new tuning parameters into your controller Table 3 2 provides the proper values for a qua
18. d and classic autotuning 1 Call PidCreateWithAutotune to create the controller and obtain the PID handle that identifies that controller in subsequent function calls The PID Library invokes the callback function provided to PidCreateWithAutotune when the following PID events occur e pidNoiseEstimateEvent Noise estimation is complete e pidRelayCycleEvent A setpoint relay cycle is complete e pidAutotuneEvent Autotuning is complete When you use wizard based autotuning the library invokes the callback function only when the autotuning is complete 2 Provide the PV to the controller in a loop and obtain the controller output which is again applied on the system 3 While the PID control loop is being run call PidAutotuneShowDialog if you want to use wizard based autotuning This function launches the Autotuning Wizard To use classic autotuning call the functions in the Autotuning class 4 Once the control loop ends call PidDiscard to discard the PID controller and release its resources Distributing Applications That Use Wizard Based Autotuning Use the LabWindows CVI application distribution feature to deploy applications you create using the PID Control Toolkit The PID Control Toolkit installs CVIPIDRuntime msmin the C Program Files Common Files Merge Modules directory This file installs CVIPIDAtUI d11 in the system directory If you deploy applications that use wizard based autotuning you must add this merge module
19. edback from the process and any required computations Then use the PID Control Toolkit functions to translate the flowchart into an application You can handle the inputs and outputs using DAQ devices FieldPoint I O modules GPIB boards or serial I O ports You can adjust polling rates in real time Potential polling rates are limited only by your hardware Setting Timing According to control theory a control system must sample a physical process at a rate that is approximately 10 times faster than the fastest time constant in the physical process For example a time constant of 60 s is typical for a temperature control loop in a small system In this case a cycle time of 6 s is sufficient Faster cycling offers no improvement in performance Corripio 1990 The PID control feature lead lag feature and setpoint profile feature in the PID Control Toolkit are time dependent A component can acquire the timing information either from a value you supply to the pidAttrDeltarT attribute or from the built in internal timer By default the pidAttrUseInternalTimer attribute is set to 1 so the component uses the internal timer Call PidSetAttribute and PidGetAttribute to set and get PID controller attributes The internal timer calculates new timing information each time PidNextOutput is called When the function is called the timer determines the time since the last call to PidNextOutput and uses that time difference in its calculations You
20. fore clearing the DAQ tasks the DAQ device outputs a 0 LabWindows CVI PID Control Toolkit User Manual 3 10 ni com Chapter 3 Using the PID Control Toolkit The control loops can be timed in the following ways e Software Timed In software timed control loops the timing is controlled by the software loop rate You can implement a software loop with a timer construct such as a timer control or asynchronous timer or a while loop with a delay or sleep operation at the end e Hardware Timed In hardware timed control loops the timing is controlled by the DAQ device The DAQ device is configured with the appropriate sample rate and the sample mode is set to hard ware timed For more information about DAQ refer to the NI DAQmx Help or Traditional NI DAQ Legacy C Function Reference Help depending on the DAQ API you are using Also refer to the DAQ example programs National Instruments Corporation 3 11 LabWindows CVI PID Control Toolkit User Manual References The Instrument Society of America ISA the organization that sets standards for process control instrumentation in the United States offers a catalog of books journals and training materials to teach you the basics of process control programming The Corripio 1990 publication is an ISA Independent Learning Module book It is organized as a self study program covering measurement and control techniques selection of controllers and advanced control techniques
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23. ions are reentrant Multiple calls from high level functions use separate and distinct data E 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 National Instruments Corporation 2 3 LabWindows CVI PID Control Toolkit User Manual Chapter 2 PID Algorithms The Precise PID Algorithm This section describes how the PID Control Toolkit functions implement the precise PID algorithm Error Calculation The current error used in calculating integral action for the precise PID algorithm is shown in the following formula pp Pr e k SP PV L 1 L range where is the range of the SP and L is the linearity factor that produces a nonlinear gain term in which the controller gain increases with the magnitude of the error If L is 1 the controller is linear A value of 0 1 makes the minimum gain of the controller 10 K Use of a nonlinear gain term is
24. l lead lag Feed forward control schemes often use this kind of algorithm as a dynamic compensator Using lead lag you can simulate inertia of motors slow settling times in pipes and so on Lead compensation stabilizes a closed loop by reacting to how fast something is changing rather than its current state This process speeds up the reaction Lag compensation stabilizes a closed loop by slowing down the reaction to the present value so that the correction is made more slowly and does not overshoot This process slows down the reaction The typical usage of the lead lag follows 1 Pass gains to PidLeadLagCreate to create a lead lag compensator PidLeadLagCreate returns a handle you can use to identify the lead lag compensator in subsequent function calls 2 Use PidSetLeadLagAttribute and PidGetLeadLagAttribute to set and get the lead lag compensator attributes such as time intervals and minimum maximum output values Provide the input to the compensator in a loop and use PidLeadLagNextOutput to obtain the output This output is either applied to the system or to the controller based on whether the lead lag is being used as an input or an output filter 3 Once the control loop ends call PidLeadLagDiscard to discard the lead lag compensator and release its resources Also call PidDiscard to discard the PID controller The lead lag compensator can be used either as an input or an output filter to the PID controller The default output range
25. lowing formula represents the proportional action u t The following formula represents the integral action t K u t 7 edt 0 The following formula represents the derivative action d 4 Ki National Instruments Corporation 2 1 LabWindows CVI PID Control Toolkit User Manual 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 up K K e k 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 u kat Y eres e i Dr izl Partial Derivative Action Because of abrupt changes in the SP apply derivative action to only the PV not to the error to avoid derivative kick The following formula represents the partial derivative
26. mping dead time T derivative control action EGU F feedback control feedback loop G gain gain scheduling The progressive reduction or suppression of oscillation in a device or system The interval of time expressed in minutes between initiation of an input change or stimulus and the start of the resulting observable response Control response to the time rate of change of a variable Engineering units Control in which a measured variable is compared to its desired value to produce an actuating error signal that is acted upon in such a way as to reduce the magnitude of the error See closed loop For a linear system or element the ratio of the magnitude amplitude or a steady state sinusoidal output relative to the causal input the length of a phasor from the origin to a point of the transfer locus in a complex plane The process of applying different controller gains for different regions of operation of a controller Gain scheduling is most often used in controlling nonlinear physical processes Hertz Cycles per second LabWindows CVI PID Control Toolkit User Manual G 2 ni com Instrument Society of America ISA integral control action L lag linearity factor load disturbance noise National Instruments Corporation G 3 Glossary The organization that sets standards for process control instrumentation in the United States Control action in which the output is prop
27. ms Any applications that are targeted for RT must not include the following functions PidAutotuneShowDialog PidAutotuneCloseDialog However you can use other functions in the Autotuning class to tune the PID controller in applications targeted for RT platforms Using PID with DAQ Devices This section addresses several important issues you might encounter when you use the DAQ APIs to control actual processes Complete the following steps to use the PID Library with a DAQ device 1 Configure the DAQ device and channels for both input and output Also configure the sample clocks if necessary 2 Call Pidcreate to create the PID controller Then call PidSetAttribute to configure the controller attributes 3 Within the control loop complete the following steps a Read the input from the DAQ device b Modify manipulate the input so that it can be provided to the controller This step is optional and required only in cases in which the controller input is derived from the acquired DAQ input c Supply this input to the controller and obtain the controller output d Modify manipulate the controller output so that it can be used as the DAQ device output This step is optional and required only in cases in which the output on the DAQ device is derived from the controller output e Write the output to the DAQ device 4 Discard the controller to release its resources Also clear all the DAQ tasks In some cases just be
28. n of the toolkit To use this manual you need a basic understanding of process control strategies and algorithms 3 bold italic monospace 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 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 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 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 Related Documentation The following documents contain information that you may find helpful as you read this manual e LabWindows CVI PID Control Toolkit Help e LabWindows CVI Help e NI DAQmx Help e Traditional NI DAQ Legacy C Function Reference
29. nt Range which you can specify using PidSetAttribute The precise PID algorithm implements a bumpless manual to automatic transfer which ensures a smooth controller output during the transition from manual to automatic control mode Control Input Filter You can use the filtered PV to filter high frequency noise from the measured values in a control application For example you can use a filtered PV if you are measuring process variable values using a DAQ device To use a filtered PV set pidAttrUseFilteredPV to 1 By default this attribute is set to 0 You can use PidSet ProcessVariableFilter and PidGetProcessVariableFilter to set or get custom filters As discussed in the Setting Timing section the sampling rate of the control system should be at least 10 times faster than the fastest time constant of the physical system Therefore if correctly sampled any frequency components of the measured signal that are greater than one tenth of the sampling frequency are a result of noise in the measured signal Gains in the PID controller can amplify this noise and produce unnecessary wear on actuators and other system components The filtered PV uses a low pass fifth order Finite Impulse Response FIR filter to filter out unwanted noise from input signals The cutoff frequency of the low pass filter is one tenth of the sampling frequency regardless of the actual sampling frequency value Output Rate Limiting Sudden changes in control output are
30. onal action 2 4 trapezoidal integration 2 5 error calculation 2 4 proportional action 2 4 trapezoidal integration 2 5 process variable 1 3 programming examples NI resources B 1 proportional action 2 2 precise PID algorithm 2 4 R rate time 2 1 real time targets using PID on 3 10 related documentation vii requirements system 1 1 reset time 2 1 ni com S setpoint 1 3 relay experiment 2 6 software NI resources B 1 step test tuning procedure 3 3 support technical B 1 system requirements 1 1 T technical support B 1 timing information acquiring 3 1 timing setting 3 1 training and certification NI resources B 1 trapezoidal integration 2 2 2 5 troubleshooting NI resources B 1 tuning procedure closed loop 3 2 open loop 3 3 step test 3 3 ultimate gain 3 2 National Instruments Corporation l 3 Index U ultimate gain tuning procedure 3 2 W Web resources B 1 windup 2 3 wizard based autotuning 3 6 distributing applications 3 6 Z Ziegler J G A 1 LabWindows CVI PID Control Toolkit User Manual
31. oration 1 1 LabWindows CVI PID Control Toolkit User Manual 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 Fillin the necessary information and click Next 5 Check 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 Continue to follow the instructions on the screen 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
32. ortional to the time integral of the input That is the rate of change of output is proportional to the input Process gain Controller gain A lowpass filter or integrating response with respect to time A value ranging from 0 to 1 used to specify the linearity of a calculation A value of 1 indicates a linear operation A value of 0 indicates a squared nonlinear operation The ability of a controller to compensate for changes in physical parameters of a controlled process while the setpoint value remains constant Milliseconds In process instrumentation an unwanted component of a signal or variable Noise may be expressed in units of the output or in percent of output span LabWindows CVI PID Control Toolkit User Manual Glossary 0 output limiting overshoot PD PI PID PID control PID controller process gain K process variable PV proportional action proportional band PB PSI LabWindows CVI PID Control Toolkit User Manual G 4 Preventing a controller s output from traveling beyond a desired maximum range The maximum excursion beyond the final steady state value of output as the result of an input change Proportional Proportional derivative Proportional integral Proportional integral derivative A common control strategy in which a process variable is measured and compared to a desired setpoint to determine an error signal A proportional gain P is applied to the error
33. pidAt trOutputMin and pidAttrOutputMax attributes to specify the range of the controller output The default range is 100 to 100 which corresponds to values specified in terms of percentage of full scale However you can change this range so that the controller gain relates engineering units to engineering units instead of percentage to percentage The PID controller coerces the controller output to the specified range In addition the PID controller implements integrator anti windup when the controller output is saturated at the specified minimum or maximum values Refer to Chapter 2 PID Algorithms for more information about anti windup PID Algorithms The PID controller can use the following types of PID algorithms to determine the controller output e Fast PID algorithm pidFastPidAlgorithm e Precise PID algorithm pidPrecisePidAlgorithm Use the pidAttrAlgorithm attribute which you can set using PidSetAttribute to specify the algorithm to use p idFastPidAlgorithm is the default value LabWindows CVI PID Control Toolkit User Manual 3 4 ni com Chapter 3 Using the PID Control Toolkit The fast PID algorithm is faster and simpler than the precise PID algorithm Use the fast algorithm in fast control loops The precise PID algorithm uses the Two Degree of Freedom algorithm to control the PV which gives better results than the fast PID algorithm The precise PID algorithm also uses extra parameters such as Beta Linearity and Setpoi
34. ration All rights reserved 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
35. referred to as a precise PID algorithm The error for calculating proportional action for the precise PID algorithm is shown in the following formula BSP PV L 1 L PSP PX range where f is the setpoint factor for the Two Degree of Freedom PID algorithm described in the Proportional Action section The formula used to calculate derivative action for the precise PID algorithm is the same formula used to calculate derivative action for the fast PID algorithm Proportional Action In applications SP changes are usually larger and faster than load disturbances while load disturbances appear as a slow departure of the controlled variable from the SP PID tuning for good load disturbance responses often results in SP responses with unacceptable oscillation However tuning for good SP responses often yields sluggish load disturbance responses when set to less than one reduces the SP response overshoot without affecting the load disturbance response indicating the use of a Two Degree of Freedom PID algorithm is an index of the SP response importance from zero to one For example if you consider load response the most important loop performance set to 0 0 Conversely if you want the PV to quickly follow the SP change set to 1 0 up k K eb k LabWindows CVI PID Control Toolkit User Manual 2 4 ni com Chapter 2 PID Algorithms Trapezoidal Integration Trapezoidal integration is used to avoid sharp changes
36. response term A exp t T or in one of the transform factors such as 1 57 A numerical integration in which the current value and the previous value are used to calculate the addition of the current value to the integral value Volts The time during which the controller output is saturated at the maximum or minimum value The integral action of a simple PID controller continues to increase wind up while the controller is in the windup area LabWindows CVI PID Control Toolkit User Manual Index A applications 1 3 autotuning 3 6 classic 3 6 procedure 3 6 wizard based 3 6 distributing applications 3 6 autotuning algorithm tuning formulas 2 6 PI control fast 2 7 PI control normal 2 7 PI control slow 2 8 P only control normal 2 7 P only control slow 2 7 bumpless transfer 2 3 C calculating controller action 2 1 classic autotuning 3 6 closed loop tuning procedure 3 2 control strategy designing 3 1 controller action 2 1 gain 2 1 output 2 2 PID 1 3 conventions used in the manual vii Corripio A B A 1 National Instruments Corporation I 1 D DAQ devices using PID with 3 10 derivative action 2 1 derivative time 2 1 diagnostic tools NI resources B 1 distributing applications 3 6 documentation conventions used in manual vii NI resources B 1 related documentation vii drivers NI resources B 1 E engineering units converting from percentage of full sc
37. roller gt Process Figure 2 2 Process under PID Control with Setpoint Relay For most systems the nonlinear relay characteristic generates a limiting cycle from which the autotuning algorithm identifies the relevant information needed for PID tuning If the existing controller is proportional only the autotuning algorithm identifies the ultimate gain K and ultimate period T If the existing model is PI or PID the autotuning algorithm identifies the dead time and time constant 7 which are two parameters in the integral plus deadtime model as follows Ts e GE Tuning Formulas This package uses Ziegler and Nichols heuristic methods for determining the parameters of a PID controller When you autotune select one of the following types of loop performance fast 1 4 damping ratio normal some overshoot or slow little overshoot Refer to the following tuning formula tables for each type of loop performance Table 2 1 Tuning Formula under P Only Control Fast Controller K T Ta P 0 5K PI 0 4K 0 8T PID 0 6 0 5 0 127 LabWindows CVI PID Control Toolkit User Manual 2 6 ni com Chapter 2 PID Algorithms Table 2 2 Tuning Formula under P Only Control Normal Controller K T Ta P 02K PI 0 18K 0 87
38. rter decay ratio If you want less overshoot reduce the gain K Table 3 2 Factors for Determining Tuning Parameter Values Open Loop Controller PB Percent Reset Minutes Rate Minutes P 100 PI 110 KT T 3 33 T PID 80 KT T 2 00 T 0 50 T National Instruments Corporation 3 3 LabWindows CVI PID Control Toolkit User Manual Chapter 3 Using the PID Control Toolkit Using the PID Library The following sections describe how to use the PID Library to implement a control strategy PID Controller The PID controller requires several inputs including SP PID gains timer interval in case the internal timer is not used PV and output range PID gains include proportional gain integral time and derivative time The following steps provide an overview of typical PID controller use 1 Provide the PID gains to PidCreate to create a PID controller PidCreate returns a handle that you can use to identify the PID controller in subsequent function calls 2 UsePidSetAttribute to set the PID controller attributes such as SP time interval minimum and maximum controller output values and so on 3 Provide the PV to the controller in a loop and use PidNextOutput to obtain the controller output which is again applied on the system 4 Once the control loop ends call PidDiscard to discard the PID controller and free its resources You can call PidSetAttribute with the
39. s volts for analog output You can use PidConvertEGUToPercentage to convert any input from real engineering units to percentage of full scale and PidConvert PercentageToEGU to convert the controller output from percentage to real engineering units PidConvert PercentageToEGU has an additional input parameter bCoerce The default value of bCoerce is TRUE which indicates that the output is coerced to the range Note PID Library functions do not use the setpoint range and output range information to convert values to percentages in the PID algorithm The controller gain relates the output in engineering units to the input in engineering units For example a gain National Instruments Corporation 3 9 LabWindows CVI PID Control Toolkit User Manual Chapter 3 Using the PID Control Toolkit value of 1 produces an output of 10 for a difference between the SP and PV of 10 regardless of the output range and setpoint range Using PID on Real Time RT Targets Some PID applications are deterministic and therefore cannot be run on desktop operating systems Because the PID Library is supported on real time RT targets you can use it to develop deterministic applications For more information about developing and running RT applications refer to the LabWindows CVI Real Time Module Help Because RT systems do not support user interfaces you cannot use wizard based autotuning in PID applications that are targeted for RT platfor
40. signal an integral gain is applied to the integral of the error signal and a derivative gain D is applied to the derivative of the error signal The controller output is a linear combination of the three resulting values A controller that produces proportional plus integral reset plus derivative rate control action For a linear process the ratio of the magnitudes of the measured process response to that of the manipulated variable The measured variable such as pressure or temperature in a process to be controlled Control response in which the output is proportional to the input The change in input required to produce a full range change in output due to proportional control action PB 100 K Pounds per square inch ni com ramp reentrant execution setpoint SP span T time constant T trapezoidal integration W windup area National Instruments Corporation G 5 Glossary The total transient plus steady state time response resulting from a sudden increase in the rate of change from zero to some finite value of input stimulus Mode in which calls to multiple instances of a function can execute in parallel with distinct and separate data storage Seconds An input variable that sets the desired value of the controlled process variable The algebraic difference between the upper and lower range values In process instrumentation the value T in minutes in an exponential
41. t cycle time National Instruments Corporation G 1 A prescribed set of well defined rules or processes for the solution of a problem in a finite number of steps Automatically testing a process under control to determine the controller gains that will provide the best controller performance An automated graphical user interface provided in the Autotuning functions The Autotuning Wizard gathers some information about the desired control from the user and then steps through the PID autotuning process You must specify to use wizard based autotuning in PidCreateWithAutotune to use this feature The offset added to a controller s output A process in which the next output always increments from the current output regardless of the current controller output value Therefore transfer from automatic to manual control is always bumpless Control in which the output of one controller is the setpoint for another controller A signal path that includes a forward path a feedback path and a summing point and that forms a closed circuit Also called a feedback loop Hardware and or software used to maintain parameters of a physical process at desired values A quantity or condition that is varied as a function of the actuating error signal so as to change the value of the directly controlled variable The time between samples in a discrete digital control system LabWindows CVI PID Control Toolkit User Manual Glossary D da
42. te Gain Tuning Procedure Although the closed loop ultimate gain tuning procedure is very accurate you must put your process in steady state oscillation and observe the PV on a strip chart Complete the following steps to perform the closed loop tuning procedure 1 Setboth the derivative time and the integral time on your PID controller to 0 2 With the controller in automatic mode carefully increase the proportional gain K in small increments Make a small change in the SP to disturb the loop after each increment As you increase K the value of the PV should begin to oscillate Keep making changes until the oscillation is sustained neither growing nor decaying over time 3 Record the controller proportional band as a percent where PB 100 K 4 Record the period of oscillation 7 in minutes 5 Multiply the measured values by the factors shown in Table 3 1 and enter the new tuning parameters into your controller Table 3 1 provides the proper values for a quarter decay ratio If you want less overshoot increase the gain K Note Proportional gain is related to proportional band PB as follows K 100 PB Table 3 1 Factors for Determining Tuning Parameter Values Closed Loop Controller PB Percent Reset Minutes Rate Minutes P 2 00 PB PI 2 22 PB 0 83 PID 1 67 0 50 7 0 125 LabWindows CVI PID Control Toolkit User Manual 3 2 ni com Ch
43. the controller 4 the control loop ends call PidSetpointProfileDiscard to discard the setpoint profile and release its resources Also call PidDiscard to discard the PID controller At any time in the control loop you can use pidSetpointProfileAttrElapsedTime to get the elapsed time You also can check if the profile is complete using the pidSetpointProfileAttrProfileComplete attribute Using Ramp Generators A ramp generator is a simple component that you can use to generate a ramp output Typically you use a ramp generator as follows 1 Call PidRampCreate to create a ramp generator Specify the SP initial output and the rate at which the output of the ramp changes 2 Call PidSetRampAttribute to set the ramp generator attributes Use PidRampNextOutput to obtain the output of the ramp in a loop 4 Call PidRampDiscard to discard the ramp generator and release its resources Converting between Percentage of Full Scale and Engineering Units As described in the previous sections the default SP PV and output ranges for the PID Library functions correspond to a percentage of the full scale Proportional gain relates percentage of full scale output to percentage of full scale input This is the default behavior of many PID controllers used for process control applications To implement PID in this way you must scale all inputs to percentage of full scale and all controller outputs to actual engineering units such a
44. undesirable or even dangerous for many control applications For example a sudden large change in the SP can cause a very large change in controller output Although in theory this large change in controller output results in fast system response it may also cause unnecessary wear on actuators or sudden large power demands In addition the PID controller can amplify noise in the system which results in a constantly changing controller output You can use output rate limiting to avoid the problem of sudden changes in controller output To enable output rate limiting set pidAttrLimitOutputRate to 1 set pidAttrOutputRate and pidAttrInitialOutput to limit the rate of change of the controller output and specify the controller output value on the first iteration of the control loop respectively Call PidSetAttribute and PidGetAttribute to set and get these attributes National Instruments Corporation 3 5 LabWindows CVI PID Control Toolkit User Manual Chapter 3 Using the PID Control Toolkit Using PID with Autotuning You can use autotuning to improve controller performance There are two ways in which you can autotune a controller e Wizard Based Autotuning You can use the PID Autotuning Wizard to tune the parameters e Classic Autotuning You can use the functions in the Autotuning class to develop a custom autotuning user interface Complete the following steps to autotune a controller These steps explain both wizard base
45. ute to set the gain scheduling criteria Call PidSetGainScheduleAttribute and PidGetGainScheduleAttribute to set and get gain scheduling attributes The pidGSAttrGainScheduleCriteria attribute can take the following values e Setpoint Process variable e Controller output e Gain schedule variable provided by the user through the pidGSAttrUserGainScheduleVariable attribute The gain schedule is a list of gain sets A gain set consists of the following features e Proportional gain K Integral time 77 e Derivative time T j e Gain control value The PID Library uses the gain set that has the smallest control value that is greater than the value of the signal specified by the gain schedule criteria For example if three gain sets have control values equal to 10 20 and 30 and the value of the signal specified by the gain schedule criteria is 15 then the second gain set is used If the value of the signal specified by the gain schedule criteria is greater than the gain schedule s largest control value then the gain set with the largest control value is used You also can set pidGSAttrSelectionMode to pidManual to allow you to set the gain sets manually By default the mode is automatic National Instruments Corporation 3 7 LabWindows CVI PID Control Toolkit User Manual Chapter 3 Using the PID Control Toolkit Using PID with Lead Lag The lead lag compensator uses a positional algorithm that approximates a true exponentia
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