Pid slc 500

oldnerd

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I have a PID in a SLC 500 that does not function as it should. Have deleted it remade it and still have a problem also moved it up and down in the ladder.
It reads a temperature sensor, the signal is good and has been calibrated.
I have a set point of 195, my process value is always on 2 no matter how you set the PID up. I have a cooling PID with the same Integer bit as the heat and it works as it should. These two PID's control a 3 way valve that calls for heat and cool of an oil heater. Has anyone ran into this before?
 
I have a cooling PID with the same Integer bit as the heat
Click to expand...

What exactly do you mean by "the same integer bit" ?

Did the loop ever work correctly ?

The most common error with PID in the SLC-500 is how you execute the PID instruction itself. Is yours executed in Timed mode, or in a STI interrupt routine ?

In general, the SLC PID instruction is on an unconditional rung; if you make the rung go false, the Integral and Derivative terms are zeroed out, which is generally undesirable.

Are you sure you have not set the control block mode to Manual (Bit 1 of Word 0 of the control block) ?

Which model of SLC controller are you running ?
 
Last edited:
I worked on a system that was similar. It had 2 PIDs one for heat and one for cool. Based on the setpoints and the system temperature only 1 PID actually ran at a time. the output was a 4-20 mA signal, 4-12 mA was Cooling and 12-20 mA was heating. 4 mA was max cooling, 11.99 was Min cooling, 12 mA nothing 12.01 was min heating 20 mA was max heating. it would be easier to see your code.
 
The Integer bit for the set point is the same for the cooling and heating in both PID loops.
I am not 100% sure it worked but was told it did, you know how that is. The mode is time.
It is in Auto and it is on a 1747-L452 .
 
oldnerd said:
The Integer bit for the set point is the same for the cooling and heating in both PID loops.
I am not 100% sure it worked but was told it did, you know how that is. The mode is time.
It is in Auto and it is on a 1747-L452 .
Click to expand...


I guessing you mean "an integer word" (a 16 bit word that holds the value of your setpoint , correct?)

Post your .RSS file if you can (zip it first) and someone may spot something.
 
Mickey,
AT work and the company security would prevent a post of the program.
Thanks for your reply. Yes it is an integer word
 
The process value comes to an analog card the raw value goes through a compute block and then moved into the PID as a set point value.. also to the HMI for display. Same thing for my cooling, it works as it should have a 195 in it also and a process variable of 193 with a 2 error but the heat pid only shows a 2 in the process variable no matter what I do.
 
Ken,
Thank you for the replay, trust me I have tried all I know on this one and have not ever seen this before. I deleted the pid and put it back and it still writes a 2 to the process value for the heating, the cooling is the same and it works.
 
oldnerd said:
The process value comes to an analog card the raw value goes through a compute block and then moved into the PID as a set point
Click to expand...

That sounds a little unusual.

Is the second PID in the same subroutine as the first?

I'm not sure we can help without seeing the program, just a lot of guessing. (not helpful to me).
 
Last edited:
Thank all for the input, I finally took off my blinders and found what it was.
Some one had put a 0 in the Smax. block.
 

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