There are two sample times, T, used in process controller design and tuning.
One is the control loop sample time that specifies how often the controller samples the measured process variable (PV) and then computes and transmits a new controller output (CO) signal.
The other is the rate at which CO and PV data are sampled and recorded during a bump test of our process. Bump test data is used to design and tune our controller prior to implementation.
In both cases, sampling too slow will have a negative impact on controller performance. Sampling faster will not necessarily provide better performance, but it is a safer direction to move if we have any doubts.
Fast and slow are relative terms defined by the process time constant, Tp. Best practice for both control loop sample time and bump test data collection are the same:
Sample time should be 10 times per process time constant or faster (T ≤ 0.1Tp).
Yet perhaps we can gain an appreciation for how sample time impacts controller design and tuning with this thought experiment:
Based on this thought experiment, sampling too slow is problematic and sampling faster is generally better.
Keep in mind the "T ≤ 0.1Tp" rule as we study PID control. This applies both to sampling during data collection, and the "measure and act" loop sample time when we implement our controller.
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1 comments to "Sample Time Impacts Controller Performance"
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