National Instruments 371685C-01 Using PID with Gain Scheduling

Chapter 3 Using the PID Control Toolkit

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 (Kc)

• Integral time (Ti)

• Derivative time (Td)

• 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.