Take the Pain out of PIDs

In our experience teaching PID loops to student's the number one factor that determines if a PID loop works or doesn't is the proportional constant.

Now we do realize that a lot of controls manufacturers have different ways of calculating the proportional constant.

Let's focus in on a common equation.

Proportional constant = Throttle Range / Sensor Span

The proportional constant is the value that when multiplied against error (the difference between sensed value and setpoint) determines output on a PID Loop (not factoring in I or D).

In theory if you have your P value set right then you should be at 100% output at full error (of your sensed range).

And this is where people mess up, they just throw in whatever P value they memorized.

It's really simple though. Look at your loop, what is the controlled range? Not the sensed range but the controlled range. Just because you have a 0-5 in w.c. pressure sensor does not mean you control 5 in w.c.

In reality you probably control 0.5+/- in w.c. which would give you a value of 1. This would give you a P value of 100. Now there are thoughts on including the negative side of a sensor span. After all do you really need to be driving an output after it drops below setpoint?

There's much more to this topic and you can watch a free video on the topic right here.