AB PLC 5/60 PID output sticking at 0%

[not4u2c]

Can someone please help explain why during normal operation the PID output percentage goes to zero and stays this way until it is switched from auto to manual then back to auto. As far as I know, the PID will be controlling as it should but the output will suddenly go to zero.

 

[Ken Roach]

Just from that screenshot, no.

Do a thorough cross-reference on the PID control block to start. Is there an HMI that might be writing to the controller ?

What sort of logic switches from Manual to Auto and back ? Or is an HMI writing the Manual and Auto modes directly ?

I'd have to see the whole program to begin to diagnose the problem. The PLC-5 PID function has been mature for decades so I don't think you're going to find a firmware issue. Post the Series and Revision of the controller just for completeness.

 

[rguimond]

I'll bet the problem is one of the following:

1. You have a rung that's MOVing a value of "0" to the address assigned to the PID output
2. You have a HMI that's writing to the address
3. You're reading a value from another PLC into that address
4. You're writing into that address from another PLC

Not sure about PLC 5/60 address scheme, but in a SLC 500, the PID output is Nx:11 if the PID control block is Nx:0

 

[OkiePC]

I'll bet the problem is one of the following:

1. You have a rung that's MOVing a value of "0" to the address assigned to the PID output
2. You have a HMI that's writing to the address
3. You're reading a value from another PLC into that address
4. You're writing into that address from another PLC

Not sure about PLC 5/60 address scheme, but in a SLC 500, the PID output is Nx:11 if the PID control block is Nx:0

5. The PID control in the PLC is making use of the Auto/Manual feature
6. The pId is saturated:

a) because the CVV direction is inverted (unlikely in a previously working scenario)
b) because the intgrator hasn't yet unwind, and the CV is off the chart temporarily​

7. the feedback signal is missing, mis-configured, or out of calibration

Someone else wanna do 8 through 14 for us?

Paul