National Instruments 371685C-01 User Manual

LabWindowsTM/CVI

PID Control Toolkit User Manual

LabWindows/CVI PID Control Toolkit User Manual
TM
May 2008 371685C-01

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Contents

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
© 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........................................................... 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.

Related Documentation

The following documents contain information that you may find helpful as you read this manual:
LabWindows/CVI PID Control Toolkit Help
LabWindows/CVI Help
NI-DAQmx Help
Traditional NI-DAQ (Legacy) C Function Reference 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:
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.
1
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.
© National Instruments Corporation 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. 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.
© 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:
2
e = SPPV
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
d
⎜⎟
=
c
⎜⎟ ⎝⎠
is the integral time in minutes, also called the reset
i
u
p
u
I
uDt() KcTd
t
1
----
e
++
T
i
t() Kce=
K
-------
t() =
T
=
T
etd
0
t
c
etd
i
0
de
-------
dt
de
------
d
dt
c
© 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.
f
e( k) = (SP PV
)
f
Proportional Action
Proportional action is the controller gain times the error, as shown in the following formula:
u
k()= Kc* ek()()
P
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
K
------
u
k()=
I
T
c
i
i 1=
ei() ei 1()+
----------------------------------
2
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.
T
d
uDk() = Kc
-----
Δt
PV
k()PVfk 1()()
f
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()+=
P
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()
c
t
1
----
+=
(SP PV)dt T
T
i
0
dPV
f
------------
d
dt
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.
i
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.
© National Instruments Corporation 2-3 LabWindows/CVI PID Control Toolkit User Manual
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