In Part 1, we found out that our system is capable of holding 6 psi regardless of
how many valves we have opened, that we had plenty of extra motor speed to help PID accelerate quickly,
and which motor speeds correspond to the min and max sensor readings.
Now we just enter the PID-related parameters and turn it on – right?
Well, in theory - yes. But there are a lot of parameters to keep track of
which can make PID seem overwhelming. The good news is, I’ve sifted through
all the related parameters and pulled out the bare minimum you’ll need to get up and running quickly,
and arranged them into an excel spreadsheet configuration tool that looks like this.
It’s just your basic feedback diagram where we take the set point and the process variable,
take the difference to produce an error signal, run that through the PID algorithm,
and send the control output to the motor. The parameters associated with each of
those are grouped around each item plus the motor control group, the drive control group,
and the display configuration parameters. This legend tells you what the colors mean.
All of the blue parameters are factory defaults and the gray cells are read-only, so all we have
to do is take one group at a time and fill in the yellow blanks. When we do those turn white.
Before we get started please note that this worksheet applies only to the WEG
CFW500 drive’s built-in PID. It doesn’t apply to the internal or external PLC application
versions of PID that the CFW500 also has. We’ll do a separate version for those.
And this is a summary of how this system is set up.
I’m using this drive exactly as we left it in Part 1
where we had done a factory reset so you know exactly where I am starting from,
we entered the motor parameters you see here in white and the display parameters shown in white.
Let’s enter the min and max drive frequency parameters we found in the previous
video just to finish that section. Great, all the yellow cells are gone so that section is done.
When you press the up-down arrow keys on the keypad the result gets converted
to engineering units and stored here. It’s gray to remind us it is read only.
This is the value we want to show on the display. The raw set point is in percent and is stored
in Parameter 525. We want 6 psi, which is 40% of our sensor's 15 psi range, so we put a 40 here.
The filter is for when you use an analog input for your set point. We’re entering the set point from
the HMI keypad, so filtering doesn’t matter. We’ll leave it at the blue default value.
If you see a little red triangle in the cell, then there is a hover note explaining what I did
or listing relevant options. For most applications,
you will find that these blue default PID coefficients will be a great starting
point – especially for monitoring pressure in pumping applications. The manual has this
awesome chart that gives you good values to start with for other types of applications like flow,
temperature, etc., so I highly recommend starting with the values in this table.
If you want to tweak those, check out this video that shows you how to tune a PID loop on
a CFW300 drive. The techniques shown in that video apply exactly the same way to the CFW500 drive.
Let’s set up the drive control. To keep things simple, we are going to set this drive up for
remote mode only. That way we don’t have to remember to make sure we are in the right mode. Again the
little red triangle reminds us there is a note that has all the relevant
options listed out. Looks like we want a 1 for remote only so I’ll enter that here.
We’ll use the HMI for the speed reference or set point. The drive defaults to having a
digital input control run stop in remote mode, so we’ll keep that blue default value too.
Digital Input 1 is the default for run stop in remote mode so that’s good too. And we’ll use
Digital Input 2 to control the manual auto operation of the PID loop, which is a 22.
That means Digital Input 2 controls this guy. When we flip that switch to Auto, the PID loop
runs normally. When we flip him to Manual, it stops the PID process and allows us to manually
enter whatever value we want to appear at the control output. We enter that value in Parameter
121 in Hertz. We’ll set that to zero so we don’t get any surprises when we turn this thing on.
Bumpless mode prevents large changes or large “bumps” in the control output when you switch
from manual to automatic mode. Suppose you are messing with the system in manual mode, and
then you switch to automatic mode. But because you were messing with the control output, the process
variable could now be nowhere near the set point. That creates a large difference which can create a
large sudden change in the output which could be hard on your system or possibly even dangerous.
If you enable bumpless mode, and you are using the HMI for the set point, the drive simply copies the
current process variable into the set point which guarantees there won’t be a large difference.
Once PID is up and running you can then re-adjust the set point back to where you want it.
We’ll leave that off for this demo just to keep things simple, but most of the time,
you will probably want that enabled. Check out the Bumpless video to learn more about that feature.
Each drive does this a little differently, but the video will give you a better understanding
of the kinds of things to expect. The process variable is stored in
engineering units in Parameter 40. Again, it’s grayed out to remind us it’s read only.
We have a forward-acting system so we’ll leave that at the default. Forward or direct-acting
means the pressure goes up when the motor speed goes up. A reverse-acting system would be like
a cooling system where the temperature goes down when the motor speed goes up.
We’re using Analog Input 1 for our process variable. This also enables
PID. To disable PID set this to a zero. This guy creates a dead zone for small
analog signals. We don’t want that messing with our PID calculations so leave it inactive.
We don’t need to change the gain, we’re using the default 0 to 10V analog input,
and don’t need an offset so we’ll leave all of those at the default blue values.
I like to add a little filtering on my analog inputs to help reduce noise,
but you want to keep this as small as possible because it will slow down the PID response. Learn
more about Noise reduction in this video. Finally, we need to configure the display.
We want to end up with the set point in psi on the first line of the display,
because that’s the line that the keypads up-down arrow keys control.
The process variable on the 2nd line also in psi, and the motor's current on the bar
graph. The motor's RPM is always in the upper right so we get that for free.
I marked all the display locations in red on our spreadsheet.
Parameter 205 is what we want to show on the first line so we put the set point in
engineering units there. If we wanted the percent value, then we would put this guy in here.
We want a 15 psi scale with 1 decimal point so that’s a 150.
We want the process variable in psi on the 2nd line of the display,
so we put that here. And it uses the same scaling. Output current is the default for the bar graph
and this drive can output 4.3 amps so we’ll leave those at the default too.
Look! All the yellow cells are gone which means we are done configuring parameters.
I went ahead and entered all of those into the drive to save us some time.
Here’s our test station.
I’m going to zoom in on the display and switches so you can see what I’m doing.
Let’s start with PID in Manual mode – Remember, that’s where we bypass PID
and we manually force the control output to the motor to whatever value we want it to be.
I’ll switch to Run … and because the set point is currently zero the drive spins the motor
up to the min output frequency we specified in Parameter 133, which comes out at this many RPMs.
That min frequency we found in the previous video should give us close to zero psi
and it does. If we manually increase the motor speed, nothing happens until we get
to the minimum speed we put in Parameter 133, and then the RPMs and system pressure start going up.
Perfect. I’ll adjust the motor speed to get the system pressure to 6 psi. And sure enough, this
is the same RPM we found we needed in the previous video to maintain 6 psi with one valve open. I’ll
open another valve. The system pressure drops. I’ll increase the RPMs to get back to 6 psi.
Open another valve. System pressure drops, Increase the motor speed to get back to 6 psi.
Open a third valve, same thing. This is exactly what we did in the previous video,
isn’t it? We manually adjusted the motor speed to compensate for valves opening and closing. That’s
what manual mode does – it allows us to manually control the system after PID is all set up.
Let’s go back to 1 valve but I’m not going to adjust the speed to get back to 6 psi.
We see the pressure went way up. Instead, let’s simply switch to Automatic mode and let PID do
it for us. And yep, PID automatically adjusts the motor speed to get the system back to 6 psi.
Open a second valve – and PID automatically adjusts the motor speed to compensate almost
as fast as I can switch the valve! 3 valves, same thing. PID automatically compensates.
And 4 valves, same thing. What if I want to run the system at 5 psi.
No problem, just change the set point, and as quick as I can change it, PID automatically
adjusts the motor speed to compensate, regardless of how many valves I have open.
How about 7 psi. Same thing. PID automatically adjusts the motor speed
so that no matter how many valves I open or close, the system automatically maintains 7 psi.
So that’s it. You should now have a pretty good idea of how to get PID up and running
on a WEG CFW500 Variable Frequency Drive. You’ll find a link to this configuration
tool, wiring diagrams, etc., in the YouTube description below the video.
Meanwhile, click here to learn more about the WEG CFW500 family of variable frequency drives.
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