KB88407
Published: December 16, 2019
Revision: 1.0

When input is out of range, PID output may not go to configured value


Applies To:

Firmware for GRV-EPIC-PR1

Versions Affected:

1.4.2 through 1.4.4

Resolved In Version:

1.5.0


Symptoms:

If a PID loop is configured with the option “Force output when input is out of range (auto mode only)”, then if the input (process variable, PV) goes out of range, the output should go to the configured value. If, however, the change from the current PID output to the configured value is smaller than the minimum output change, the PID output will not be set to the configured value. The following example can help illustrate this issue:

A PID is configured as follows:

  • Output minimum change is 3.
  • Output maximum change is 25.
  • When input is out of under-range, force output to 15.
  • When input is out of over-range, force output to 85.

If the current output is 87 and then the input goes out of range on the high limit, the output should be forced to 85. However, the PID will calculate an output change of 2: 87 (current) - 85 (high limit) = 2. Since 2 is less than the output minimum change (which is 3), the output stays at 87.

Workaround:

Configure the PID for "Switch to manual mode when the input goes out of range" and add logic to the control strategy to look for manual PID mode (use PAC Control command GetPidMode(PID Loop)) and set the PID output to the desired value.

Resolution:

Opto 22 has resolved this issue.

Questions?

Contact: Opto 22 Product Support.
Phone: 800-835-6786 or 951-695-3080
Email: support@opto22.com


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