Rockwell PID Execution Time: the Difference Between Continuous and Periodic Tasks
Have you ever seen an Integral Term on a PID controller that seems an order of magnitude larger or smaller than what should be used? Often, the issue stems from the method in which logic is executed on the Process Automation Controller (PAC). More specifically, when the PID block is executed in the PAC it needs to match the update time configured in the PID block. For instance, if the PID Block’s update time is set to 500 milliseconds, the rung on which the PID block is located also needs to execute at 500 milliseconds. If the PID block is executed significantly faster than its configured update time during a PID controller tuning session, it can be expected that the outcome will leave something to be desired from a controller performance standpoint. What’s more, since the issue stems from a data mismatch, commercial PID loop tuning software won’t be able to provide a better answer.
The root of the problem is frequently linked to the way in which the ControlLogix PAC’s ladder logic is executed. It’s not uncommon for the PID block to be configured to run as a continuous task versus a periodic task. While the two modes are similar, there are important differences that directly affect how the PID functions and how it can be tuned for improved performance. Read on to learn more about these two types of tasks and what is considered best-practice for loop tuning.
Continuous Tasks
Periodic Tasks
It’s important for practitioners who use the ControlLogix PAC to be aware of the different types of tasks and their use. That’s especially true if they intend to tune PID control loops using software or any model based tuning method.
If your plant utilizes the ControlLogix PAC from Rockwell Automation, click here to learn more about the problem and the solution.
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