RSLogix 5000 PID Output % Locked at 100%

Hello, all.

This is my first time working with a ControlLogix and RSLogix 5000 and I am having some trouble with the PID ladder function. I was in the field today with the processor in run mode and I will be back there again tomorrow to try to resolve this.

The PID will be used to control the speed of VFD-driven pumps based on header pressure. I have configured the PV, SP, and basic parameters of the PID instruction.

Once I moved from checkout of the manual routine and the physical I/O, I noticed that the PV was not updating in my PID block. The PID is in a continuous task, so I found out quickly that I needed to include a timer to trigger the PID. Once I included the timer logic, the PID became "live" and the PV updated continuously. The timer and the loop update time are configured for the same time, 0.250 seconds.

At the exact moment that I accepted the timer edits and the PID went live, the "Output %" of the PID went to 100%. Mind you, the PV was much higher than the SP. Here are some additional notes:

- PID is running in Auto.
- None of the tuning constants have been configured yet (all zeros). I actually changed these (mainly the proportional) to some large values just to see if I could get the output to drop from 100%, but no go.
- Please see the (2) attached screenshots of additional configuration.
- The Set Output value is set to 100%. I tried to change this, but it goes right back to 100%. It seems that if I change the CV High Limit, the Output % changes to match it.

Can anyone help steer me in the right direction? I will be onsite again in the morning and I am concerned that it will be an all day affair trying to figure this out.


Thanks!

Justin

I recommend you to download the Logix5000 Controllers General Instructions Reference Manual:

http://literature.rockwellautomation.com/idc/groups/literature/documents/rm/1756-rm003_-en-p.pdf

Pages 507 to 526 explain how the PID instruction works and what you need to know about its configuration.

I think you have to change Configuration -> Control Action to PV - SP. Of course, you need at least a non zero Kp and Ki.

nhatsen said:

I recommend you to download the Logix5000 Controllers General Instructions Reference Manual:

http://literature.rockwellautomation.com/idc/groups/literature/documents/rm/1756-rm003_-en-p.pdf

Pages 507 to 526 explain how the PID instruction works and what you need to know about its configuration.

I think you have to change Configuration -> Control Action to PV - SP. Of course, you need at least a non zero Kp and Ki.

Click to expand...

Thanks, nhatsen. I have been working from the help topics within RSLogix which seems to have similar information, but I will take a look through the PDF, too.

I thought I had the control action correct, but maybe I don't. Essentially, if my pressure is higher than my setpoint, I want to decrease the speed of my pump. I am set up for independent gains, but could it be that I want dependent gains instead?

Please forget my statement about the control action; I work with oil pumps and when the inlet pressure goes higher than SP, the speed is increased.

In the manual you will find some ladder examples.

You can decide between dependent and independent gains according to your experience and the method you use to calculate the parameters. In my case, I am used to independent gains because is the common configuration in the installed applications.