National Instruments PID Control Toolkit 371685C-01 - Manual (Page 24)

Chapter 3 Using the PID Control Toolkit
© National Instruments Corporation 3-7 LabWindows/CVI PID Control Toolkit User Manual
If you are using LabWindows/CVI 8.0 and later, click Add Additional Module in the
Drivers & 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 attribute 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:
• Setpoint
• Process variable
• Controller output
• 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:
• Proportional gain (K
c
)
• Integral time (T
i
)
• Derivative time (T
d
)
• 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.