What is a Control Loop?

21 Sep 2019 at 21:20 // under Simple Saturdays

The way we manage to get our factories to make the things we want them to is by getting liquids to flow various pipes at the right speeds, by making sure they are at the correct temperature. We need to ensure tanks don’t overflow and the pressure inside is not too high or too low.

In other words, we need to make sure that we control our process. This is done by lots of control loops. Loops to control the flow of feed rates into the reactors, another set of loops controlling the various temperatures and pressures within the reactors. We need to control all these things to make sure we actually make the product we want and that we do it in a safe way. After all, you won’t make very much product if you melt your reactor.

This article will look at the basic building block of the control system: the control loop.

Before going much further, we should be careful about the terms used. If we are not careful, we could end up with a sentence like:

The controller controls the outlet temperature by controlling the position of steam control valve.

A control loop: * Measures something about the process. * Compares the measurement with the value it wants * Adjusts something in the process.

Hopefully the adjustment will mean that whatever is being measured moves closer to the value that it wants. This process is repeated regularly. Perhaps it does this every second. If the control system is on a rocket, it will be much more often than that, but in most cases, once a second is fast enough.

The thing that is measured is often known as the process value or PV. The usual things that are measured are temperature, pressure, flow or level. There can be many other things that are measured using analysers, but they are not used as often.

The value that the process should be at is known as the Setpoint or SP. This is usually entered by the operator.

The thing that is adjusted is usually a control valve, though it could be the speed of a pump or a fan. This is known as the Output or OP.

There is much more detail that can be added, how the different algorithms decide how much to adjust the control valve or the motor. There are ways to combine multiple measurements or multiple outputs. But the simple loop just looks at one thing, compares it with what it should be and changes something that will affect the process move the measurement back to where it should be.

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