Sampling Period

Many closed-loop systems behave normally on the bench, then become shaky, slow, overshooting, or unstable in the field.

The gains did not change. The algorithm did not change. The actuator did not change. The difference may come from an ordinary-looking detail: the sampling period.

A controller does not observe the world continuously. An embedded device usually repeats this sequence:

read sensor
-> compute error
-> update controller
-> write actuator output
-> wait for the next period

That period decides how often the controller sees the system, how often it corrects output, how much delay exists, whether noise is amplified, and what integral and derivative action actually mean.

When a control system first becomes closed loop, the first problem is often not “it does not move.” It is “it keeps moving back and forth.”

Temperature overshoots and undershoots. Motor speed swings around the target. Liquid level control keeps opening and closing a valve. Pressure rises too high, then falls too low. Smaller parameters make the response too slow. Larger parameters make it oscillate.

It is easy to summarize this as: the PID is not tuned well.