National Instruments PID Control Toolkit 371685C-01 - Manual (Page 21)

Chapter 3 Using the PID Control Toolkit
LabWindows/CVI PID Control Toolkit User Manual 3-4 ni.com
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. Use PidSetAttribute 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 pidAttrOutputMin 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.
• Fast PID algorithm ( pidFastPidAlgorithm )
• Precise PID algorithm ( pidPrecisePidAlgorithm )
Use the pidAttrAlgorithm attribute, which you can set using PidSetAttribute ,
to specify the algorithm to use. pidFastPidAlgorithm is the default value.