Let's Talk About PID Loops

PID control loops have been the subject of many a “what the heck is that” conversations over the years. I'm here to pull back the curtain and reveal the wizard for what it really is.

We’re going to have a little discussion about what these loops are, how they affect your building automation system, and some of the steps you can take as building engineers to tune them to how they’re actually supposed to be tuned. Hopefully by the end you can have a better understanding of some of the mystery surrounding PID loops and, at the very least, you will be able to ask more informed questions the next time it’s service contract renewal season.

So, what is a PID control loop?

Proportional Integral Derivative Loops. It’s a mouthful and the name doesn’t mean a whole lot to very many people, unless you have a degree in mathematics. But they’re essential to the building automation system and are the main process loop for which the building controls are based on. A properly tuned control loop is critical to the proper function of the original sequence of operations.

At its most basic function, the PID loop ensures that the proper output is synced to the proper setpoint in terms of airflow.

What you really need to know is that as time goes on, as with anything, building automation PID loops can drift out of setpoints and become…sloppy. When this happens, the building becomes less efficient and no longer operates as originally programmed, which in turn causes energy inefficiencies and the dreaded hot/cold call.

What do I need to do about it?

I’m sure all building engineers, facility managers, and property managers, have had to go through the dreaded service contract renewal period. This is the time of year when the pesky controls salesperson comes around and goes through all of their service offerings, and the subject of control loop tuning comes up. They might mention that they have a proprietary tool that only they can use, and that it takes hours and hours to do, and if you don’t do it, your building will fall apart and never run efficiently again!

The good news is: I’m going to share with you a few quick steps that you can take to tune the buildings control loops yourself!

1. First, find a room/zone that you’d like to tune and adjust the setpoint down 2 or 3 degrees. We’re going to find out if the loops are in proportion to the setpoints. Ideally you will see a spike in the output value and for discharge air, which is usually 20 degrees.
2. Take that 20-degree value and multiply by the error, which in this case is going to be the setpoint value of 2 or 3 degrees. That’ll give you the output percentage of 40 to 60. So as the setpoint goes up or down, those proportions will all stay the same.
3. Now the tricky part: you want to figure out how quickly your loops are reacting to your setpoint. Generally, it should take less than two minutes for that percentage to get to 100. You’re going to want to set the “I” value to something low and see how long it takes to get to 40-100 percentage.

We want our loops to react quickly. The quicker they can react, the more efficient we can hit our setpoints. Ultimately, we’re going to take our “I” value, increase it by .05 when the controller is in hand, and continue to increase it until we achieve the speed we want our loop to react.

It takes time to figure this out, and even if you can’t, that’s okay! Ideally, you want to find a controls contractor that doesn’t just sell you a service; you want to find a company that works hand-in-hand with you to help create a better built environment.

Contact LONG today for additional help with PID loops or other building automation needs!