Noise

30 Nov 2019 at 21:14 // under Simple Saturdays Process Control

What is noise? Depends on what subject you are talking about. If you are talking about what your ears sense, then it is a bunch of pressure waves that move through the air. But in the case of this post, I am actually talking about noise in a process control.

When we measure something, it generally doesn't stay exactly the same. If you do have a reading that never moves, you should check that it is actually reading what it is supposed to be reading and has not gotten stuck.

Noise is the movement that is not real. If we are measuring the flow in a pipe, we want to know how much fluid is actually passing through the pipe. But what we see on the control system tends to bounce around. Some of this will be because the process is genuinely changing, some will be because the signal picks up interference in the line back to the control system. Some ways of measuring properties just tend to create noisy signals that bounce around a lot.

These unwanted movements can cause problems with control. You want the control system to react to genuine movements in the property. If the temperature drops below the target, increase the amount of heating. If the flow increases above the target, close the valve. This is fine if the temperature genuinely did drop or the flow genuinely increased. But what if it was just noise? What if there was a jump in the flow reading even if the real flow stayed the same? Then the control system would close the valve unnecessarily, reduce the real flow, measure that change, then have to open back up again. We would have introduced a real disturbance to the process that wouldn't have been there if the system was in manual.

Automated systems rely on measuring what is going on. If there are false signals, the automatic system is unlikely to notice and will react as if they are real.

There are some things that can be done to help reduce the problem ¹, but the best thing is to avoid the noise in the first place.

1. Which I hope to cover in a future post. ↩

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