RSLogix5000 PID Question

Austin1985

Member
Join Date
Mar 2016
Location
Indiana
Posts
20
Hello,

Question:

(Temp is whats being controlled here by 3 fans)

Using a PID; can I set my output (CV) to be 100% at a specific input (PV)? In other words; the set point is 80, and (PV) is at 80 (not pushing hard / trying to climb) then the (CV) would run at set min. But then (PV) starts to climb, I want to make sure the (CV) will be at 100% when (PV) hits 81.5.

If (CV) is at 100% for X time, I will start a second fan to help bring down the temp back to 80. But I don't want to get hotter than 81.5 for very long.

Thanks
 
You can do what you want, with a high gain purely P controller. You can set a PID so that the output is 100% with a 0.0001% difference between SP and PV if you want.

And you can use a simple compare or timer to switch on a second/third fan.

Perhaps you need to explain your application better.
 
This is a cooling tower, and the temp of the water needs to be 80 degrees. If the water gets above 80 for X time, the lead fan will start. If that fan cannot keep that temp between 80 and 81.5, the next fan will start. If 2 fans cannot keep the temp below 81.5, a third fan will start. I figured; if (CV) is at 100% for X time, start the next fan. But I don't want the temp to be 82 or 83 with the (CV) at 70% or 80% for the lead fan.

Thanks for the reply!
 
Do you have speed control of the fans? It sounds like (and as is typical) you're just turning more fans on the further you get from setpoint. I'd start with a proportional only controller. Then just use compare statements to turn on each fan as your .CO increases. Something like:
if(.CO>0%)then(fan1=ON)
if(.CO>33%)then(fan2=ON)
if(.CO>66%)then(fan3=ON)
You could also improve this scheme with some debounce timing.


If the length of time your .PV is above a certain threshold matters, then just add some integral gain to the controller.
 
There is analog control of the fans. Turning on and off the fans are not an issue. But as I program this, I would like to use the (CV) of the PID to turn on the other fans instead of using the temp to turn on the other fans.

My goal is to make sure 1 fan is running full speed (60hz) for 30 seconds before turning on fan 2.

If I use temp to turn on the fans:
80 to 81.5 = 1 fan
81.5+ for 30 sec = 2 fans
81.5+ for 60 sec = 3 fans

Then I could be running 1 fan at ~45hz (CV at 75%) because the PID has not pushed the output to 100%.

I'll be able to play with the system in a couple weeks when it gets warmer out. I was just wanting to learn more about the PID instruction. Rather than just playing with the (P),(I), and (D). I've never set one up before.

Thanks for the reply!
 
I was just wanting to learn more about the PID instruction. Rather than just playing with the (P),(I), and (D). I've never set one up before.
Welcome to the forum! I'd strongly recommend using "Search this Forum" and search for "PID tutorial", "PID fundamentals", etc. There have been many useful posts over the years, especially a set by Ron Beaufort back in 2003-2004 that are most helpful. This would be time well spend in the weeks prior to your commissioning.
 
I agree with AGill, Split the PID output range up for the three fans. Since you have speed control I would do something like this:
.CO = 0% = all fans off
.CO = 1-33% = lead fan 0-100%
.CO = 34-66% = 2nd fan 0-100%
.CO = 67-100% = 3rd fan 0-100%

This takes care of speed and staging all with the same temperature control. If you wait for the controller output to reach 100% to start a second fan, how is your controller going to call for more cooling if it is already at 100%?

I once did a 3 cell cooling tower with 2 speed fans, it worked like this:
.CO = 0% = all fans off
.CO = 1-16% = lead fan low
.CO = 17-33% = 2nd fan low
.CO = 34-50% = 3rd fan low
.CO = 51-67% = lead fan high
.CO = 68-84% = 2nd fan high
.CO = 85-100% = 3rd fan high

This worked fine. The small gaps are to prevent excess switching of fans. The response of the cooling tower temperature should not be all that fast. You will probably end up with just proportional control, but you may need a little integral as well. If it happens to settle at 66% output, you don't want to start the next fan just to shut it off a few minutes later. You may need more or less gap, experience with your system will determine it.
 
I agree with AGill as well! That does sound efficient.

I just got out of a meeting, and they want an operator to run the pumps manually. This just made it easier for me to control the fans. One pump on = one fan on, two pumps one = two fans on....

If they turn on the second pump because the water is too hot; with the extra flow of water and both fans running, as the water cools, the PID will slow down both fans.

Actually, I could probably still use AGill's suggestion. If they happen to turn on all three pumps, and the water is cooling down, then I can start shutting off the fans as the water cools as the PID is lowering its output.

I'm assuming running more fan than running more water is more efficient, maybe?!
 
Last edited:

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