pid tuning rslogix 5000

[stu]

hi guys, i have a machine which the heaters pid was set up when the machine came in, but now the heaters are taking along time to heat up and when the temp drops it drops about 25 C and then heats up very slow, i am not to sure what to adjust in the tunings tab any ideas.thanks stu

 

[rdrast]

Wait ... It was fine, and now (either gradually or suddenly) is having trouble?

If that's the case, I'd look elsewhere before starting to re-tune what was a working PID loop.

 

[stu]

I Have asked the people that run the machine and they said it has been doing this for a long time may be from when it was set up, i have checked the tc there are all ok. heaters are all fine. any ideas?

 

[daba]

It is always difficult to explain a PID tuning without knowing the application, what are the heating response times, thermal loading, TC placement in relation to the heat source, etc, etc.

But in general, a heating application usually calls for a relatively high P gain, so you have heat fully on until near the setpoint, then an integral term to bring the PV up to SP. Most times there is little need for a D gain.

I would suggest you start with I and D at zero, and push the P gain up until you achieve a steady state (note : this will settle with PV below the setpoint). Then add a small amount of integral gain, but don't attempt to use I to get the PV up to SP at this time, because on the next heat-up cycle the I term will have been active from start-up, so will have already boosted the output accordingly.

Observe the response (a good way to do this is to create a Trend in RSLogix5000 - tags PID.SP, PID.PV, and PID.OP). Try to obtain a fast approach to SP with no overshoot, by increasing the I term by small amounts, then observing again the next heat-up from "cold".

A certain gentleman I once knew from Eurotherm in the UK just watched the process working in "sloppy tuned" mode, made copious notes, referred to his watch constantly, then spent a pleasant evening in his hotel working out the PID terms to use. Absolutely spot-on first time - I don't know how he did it but he was brilliant!.

Anyway, most times PID tuning is by trial and error, but don't try to do too much too soon or the "error" could be costly. Tread carefully, but always reflect the change in response to what you altered, and by how much.

All the badly tuned PID loops I have ever seen have had too much I and D gains, obviously attempting to compensate for too little P gain.

One last point, you will get nowhere if the PID is making calculations based on wrong data, and the most important thing it needs to know is how much time has elapsed since it calculated last, so make sure the PID instruction is either triggered by a timer, or resides in a Periodic Task. Set the PID tag's Update time (PID.UPD) to the same period as the instruction execution rate.

HTHY

 

[504bloke]

A certain gentleman I once knew from Eurotherm in the UK just watched the process working in "sloppy tuned" mode, made copious notes, referred to his watch constantly, then spent a pleasant evening in his hotel working out the PID terms to use. Absolutely spot-on first time - I don't know how he did it but he was brilliant!.

And his name was ?

Did you work there ?

 

[504bloke]

hi guys, i have a machine which the heaters pid was set up when the machine came in, but now the heaters are taking along time to heat up and when the temp drops it drops about 25 C and then heats up very slow, i am not to sure what to adjust in the tunings tab any ideas.thanks stu

Double check the heaters, wiring and tc's etc

Make sure 3 phase heaters have all three phases of the heater working (ie one phase hasnt gone open circuit)

Some excellent posts by Ron

1,2,3
& 4

 

[Roy Matson]

Good advice Daba, I would add start with Proportional gain of 1, if it starts to cycle back off until it settles down then back of another 10-25% before applying Integral. You can calculate the integral Gain from the cycle time as per Seigler and Nichols
Roy

 

[Ron Beaufort]

if it's possible, post your entire program ... in MOST cases when the PID won't "work right" it simply hasn't been programmed correctly ... usually the "timing/triggering" or the "scaling" is at fault ...

notice that I'm NOT saying that you won't eventually need to "retune" the system ... I'm just saying that you should make sure that everything else is correct first ...

 

[Adam303]

I would first look at the heaters.Once a pid loop is set and working properly than you should not have a problem there. Ohm out the heaters check amp readings and compare to new herter if you dont have the data.

 

[johnfing]

How to tune your RSLogix PID loop

I would use a tool like FasTune - its a PID tuner that automatically logs your PID loop data via RSLinx, and based on a single manual bump will calculate your gain and reset. It will also allow you to verify the results easily by running your loop in auto and visually verifying the reaction to the new tuning parameters. See it at https://processsoftwaresolutions.com