PID Faults the Processore

Hi Guys
I am trying to test a pid and its limit. Everytime i give a range(Use SCP instruction) on its Set Point (n7:12) it faults the processor on me with a error value (020H) wiht the following description in the status file of SLC 5/04

"A minor error bit is set at the end of the scan. refer to S5 error bits"..... PLEASE SEE ATTACHED THE FILE.


Thanks in advance

The cause of the fault is the SCP instruction on rung 003. The way you have the numbers entered,you will exceed 32767, and the overflow bit will be set, faulting the SLC.

I believe the setup of the SCP mentioned earlier is reversed. You should have the input min and max set for the PID scaling 0-16383, and the output min and max set for 0-100.

Rung 3 has the SCP parameters round the wrong way causing an s:5/0 minor fault bit to be set.

See Pics attached

edit: damn - beaten to it by ken!

A lot of programmers put an unconditional rung at the end of the program that has OTU of S:5/0 to keep a minor fault from faulting out the processor.

There have been discussions here about whether or not that is good programming but I will not go into that.

The sun is shining, I am on a job assignment up here in Penn. and I going to spend the day in Gettysburg.

Thanks guys
The thing is this was a test file that i was trying to use to find out where the problem lies in my main program file. Yesterday i was at the site and trying to figure it out why the processor is faulted on me all the time.......May be this is one of the problems. The thing is i am using two pids (SPeed and Bypass Control Loop) to control my system. The speed pid takesdifferent PVs and SPs at different states. Also the engineering units for these PV and SPs are changed.....like uptill state 8 speed pid loop used SPeed in RPM as its PV and SP and after state 8 it uses Pressure in KPa as its PV and SP.......... My question is do i need to take some extra caution before feeding my speed pid in terms of units and ranges .......If yes how can i effectively do that. Lets say my speed ranges from 0 to 2200 rpm and my pressure from inlet ranges from 0 to 689 and from outlet ranges from 0 to 13789.....thanks in advance.

You will need to use the SCP for all your varibles, so that the input to the PID instruction 0-16383, then use conditional logic to move the correct SCP outputs to the PID inputs. Getting a bumpless transfer from one set of varibles to another may be a bit challenging.

quick update (short on time) ... plcnovel called me by phone earlier this afternoon ... if I understand correctly, his analog input module and his analog output module are both “configurable” types ... if so, I recommended that he set both of these modules for their “Scaled for PID” configuration ...



specifically, I !think! that he should be able to take the 4-to-20mA analog input from the field device into the analog input module and have the module itself scale the data from 0-to-16383 and feed it directly into the PV of the PID ...



also I !think! that he should be able to take the 0-to-16383 CV data from the PID and feed it directly into the analog output module and obtain a 4-to-20mA signal for his field device (valve, etc.) ...



from my end of the pipe, I have ZERO field experience in setting up the type of application/machinery that plcnovel is working on ... personally, I’m fairly ok on the Allen-Bradley PID instruction, but if anyone has some nuts-and-bolts experience on the specific type of machinery in question here, please take a stab at this ...



my basic “next step” recommendation is to temporarily disable the PID rung ... then (carefully! - and safely!) manually enter values into the PID’s CV address ... a value of “0” should result in a 4mA output signal to the field device ... a value of “16383” should result in a 20mA output signal to the field device ...



while this operation is going on, monitor the PID’s PV address ... when the input field device is putting out a 4mA signal, the PV address should contain the value “0” ... when the input field device is putting out a 20mA signal, the PV address should contain the value “16383” ...



personally, I always start out a “PID-controlled” project this way ... specifically, I make CERTAIN that the field signals are properly scaled (from 0 to 16383) at the PV and CV locations BEFORE the PID is ever activated ...



reality check: most industrial equipment won’t let you safely force it to the minimum and maximum values of its operational limits ... so you might have to use a “process calibrator” to simulate and nail down the 4-to-20mA signals ... but at least do SOMETHING to make sure that the PV and CV signals are properly scaled and addressed before trying to configure and tune the PID loop ...



note to plcnovel ... you REALLY need to post your complete .RSS file ... the folks on the forum can give you MUCH better help if they know exactly what hardware you’re using ... and how it is configured ... case in point: your “configurable” type analog modules may either complicate or simplify your problems - depending on how you have them configured ... it’s hard for us to give detailed advice when we don’t have detailed information to work from ... personally I will be teaching non-stop for the next two weeks and will have VERY little time to help ... I wish you the best of luck ...