Learn how to setup and tune a PID loop ion a GS4 Variable Frequency Drive (VFD). Part 1: Setup and System Test; Part 2: Sensor signal filtering and Trial and Error tuning; Part 3: System Linearization and Formula method for tuning; Part 4: Loose ends
where we back up and cover things we glossed over in the previous three videos.
In this video we’ll setup the drive and get familiar with our system. We’ll then walk through two different ways to tune the PID parameters. There is a LOT to cover in those videos, so to keep things moving, I’ll won’t explain every little detail. Instead I’ll use this last video to back up and take a closer look at the things we glossed over. For this demo we’ll maintain the pressure of a pumping system regardless of how much demand is placed on it. This is a perfect application for the GS4 Drives built in PID function Here’s a little demo version of that system for us to play with. We have a GS4 drive controlling this jet pump motor which is driving this centrifugal pump. The pump sends water to these valves, which return the water to the reservoir so we don’t have to keep refilling it. This 15-psi pressure sensor is providing pressure values back to the GS4 drive as a 0 to 10-volt signal, where 10 volts is 15 psi. I’m going to reset this drive to factory default so if you are following along we are both starting at the exact same place and then I’ll setup my motor parameters. OK, these are the parameters I just entered for my motor and drive controls, yours will probably be different. Notice that because I am using this 3600 RPM Jet Pump Motor, I had to set the number of Poles to 2 before I could set the RPM above 1800. I love that the GS4 is smart like that and won’t let me screw it up, but if you don’t know to look for the poles parameter you might wonder why you can’t change the RPM above 1800. So, set the poles parameter first if you are using a high-speed motor. I also zeroed out the acceleration and deceleration ramps because they will interfere with the PID function. We need to tell the drive that we are using analog input 1 as a PID feedback signal and that it will be used as a 0 to 10 Volt signal. We also need to make sure the terminal board switch for analog input 1 is set to volts and not current. Great, we’re done with the basic drive setup. Before doing ANY PID stuff, you need to fundamentally understand what your system is capable of, how it reacts to loads, if your feedback sensor is working, and if that sensors range is appropriate. Let’s fire this thing up and see what we have. Suppose we need to maintain a system pressure of 6 psi. So, I’ll set the frequency to something low - maybe 20 Hz as a starting point, open the first valve and start the drive. We can monitor the feedback signal on analog input 1 right on the main by display using parameter 8.0 and setting it to an 11. That shows us the sensor signal as a percentage. The sensors range is 0 to 15 psi. The 6 psi we want is 40% of that range, so, let’s increase the drive frequency until we get the 6 psi – or 40% we want. So, with one valve open, we need this much drive frequency to get 6 psi. If we open the 2nd valve, we see the pressure drop to this, so we increase the drive frequency to get our 6 psi back and add that to the chart. Let’s also add how much we had to change the frequency to get our pressure back. Open the third valve, pressure drops … up the drive frequency to get it back … and add that to the chart. Open the fourth valve, system pressure drops, add more drive speed to get the pressure back … and add that to the chart. So, we just manually did what we want the drive to do automatically, right? Every time the pressure dropped we manually added more drive frequency to compensate. That’s exactly what we want the drive’s PID function to do for us. Before we implement that let’s check a couple things. First, we see that since we are only using roughly 30 to 40Hz of the drive’s 60 Hz frequency range, we now know the drive has more than enough frequency range to compensate for the pressure drop even when all valves are open. That’s really important to know – if you can’t get your system to work manually without PID, then PID isn’t going to magically make it better. PID ca n only automate what the system is already capable of doing. If we plot the frequencies we needed to compensate for each vale opened, we see our system isn’t really linear, is it? We won’t worry about that in this video, but we WILL cover how to correct for that in the next video because it is important if you want to get the best possible results. We know our sensor is working and that when the motor speeds up, the sensor value goes up. We call that a forward acting system. A reverse acting system would be something like a cooling system where increasing the motor speed reduces the temperature sensors output. We also know that the sensor is appropriately sized for the job. We’re operating in the middle of this sensor’s 15 psi range. If we were using a 100-psi sensor, and we only needed around 6 psi, then we would be using a very small portion of the sensors range. We CAN make that work, but it is much harder to setup PID and the lack of resolution will make it more difficult for PID to maintain an accurate output. So, make sure your sensors range is appropriate for your feedback signal. Great. We have setup the drive’s basic parameters, and we know enough about how our system operates to begin setting up PID. The next two videos will show you two different ways to tune a PID system. The first one is very common, but it requires a lot of trial and error guess work that can take your system down for a long time which costs you money. So why does everyone use it? Because it is conceptually easy. The second method produces a solid result every time without all the tedious time-consuming trial and error guesswork. You just take a couple open loop measurements, and use those to generate the PID closed loop numbers. This method is conceptually more difficult, but in practice is quicker, easier, more reliable and minimizes system down time which saves you money. Now beware, we’re going to gloss over a lot of low level details and skip over some things in these videos to avoid getting bogged down, so we’ll have one final video that goes back explains those things in more detail. Click on the information bar that just appeared at the top of the screen to go directly to the next video. Click here to learn more about the GS4 drive. Click here to learn about AutomationDirect’s free award-winning support options and click here to subscribe to AutomationDirect’s YouTube channel so you will be notified when we publish new videos.