What is a PID loop?

Having been in a meeting (that i was actually chairing) i have found myself sitting among many engineers all talking about PID loops. i have no idea what these are and i find that the answers on this forum are so good that i thought i would again put out what could possibly be a stupid question.

we are using S7 plcs. S7 315 2DP to be exact.

My question is, what are they? what applications do they have? and do they require any other hardware to work?

Thanks in advance

Cheers for that, thats very interesting and really quite simple. is there a function for this within the PLC or do you just create it yourself iwth a combination of functions?

Free with some softwares, others sell their pid separatly. You can make your own, but that will probably take some effort.

90% of all VFD's also have this feature as well as most temp controllers

Dirt said:

Free with some softwares, others sell their pid separatly. You can make your own, but that will probably take some effort.

Click to expand...

Most of the applicatons I've met only need the P+I terms which are not too hard to implement yourself. If you are taking multiple readings per second you will need a PLC with a timed interrupt to make sure you update your loop at regular intervals.
The derivative term is complicated to implement yourself and if you need it, its usually more cost effective to buy a controller or add-on with it built in.

ToolGuyFred said:

Most of the applicatons I've met only need the P+I terms which are not too hard to implement yourself. If you are taking multiple readings per second you will need a PLC with a timed interrupt to make sure you update your loop at regular intervals.
The derivative term is complicated to implement yourself and if you need it, its usually more cost effective to buy a controller or add-on with it built in.

Click to expand...

Agreed. Have done a PI myself.

Edit: They often want a PID but really only needs a PI.

PID is a mathamatical approach to controlling something in the most perfect way all day everyday. The P represents proportionality. The bigger the difference (error) between what some parameter is and what it is suppose to be (setpoint), the larger the correction to cancel the error.

I represents Integration, or the area under the curve of the error, think of it as the product of time and error. The longer the error persists, the greater the I term in the correction.

D represents the Derivative, or rate of change of the process toward or away from the setpoint. This term works in opposition to the P and the I. If they they are both saying to go up and the process starts to go up too fast, then the D will say OK slow down a bit so as not to overshoot.

I prefer to write my own controllers which incorporate these features but which are accompanied with other rules to limit the tendancy to be tuned wrong as often as right, when something that influences the process changes like surrounding temperature, product density, or a million other things. Therefore, I recommend that you take my opening statement (aimed at the purists) with a grain of salt!

The traditional PID approach has been around for a very long time, is over used and by and large they are difficult to impliment correctly unless you use a very skilled approach. Most are wrong as often as they are right and I'll never use another one as long as I live. By that's just me, unconventional!

Best Regards,

Bob A.

I've done a ton of PID. In fact I probably do more process control, both regularoy and supervisor/advanced than anything else.

Very rarely have I not been able to make a vendor-provided algorithm work effectively.
I've seen some 'home brewed' versions and generally they are either re-inventing the wheel or just plain dont work as good.

That said, I also come across alot that are badly tuned. either they act so fast the loops in manual all the time or so slow that when theres a process upset nothing responds and again, the loop ends up in manual.

PID control tends to be one of those 'black magic' areas. with discrete if you tell a device to do something, it does it. On or off, real simple. PID has all sorts of shades of grey in how and when it works and under what circumstances.

I guess what I'm trying to say is that when used correctly PID is awesome but there are alot more ways to misuse it.

Bob A. said:

The traditional PID approach has been around for a very long time, is over used and by and large they are difficult to impliment correctly unless you use a very skilled approach. Most are wrong as often as they are right and I'll never use another one as long as I live.

Click to expand...

100% agree. Its often harder to get the tuning right than implement the loop in the first place. There are procedures to get you into the right ball park but there's no substitute for experience (i.e. getting it wrong and tweaking till it comes right...)

Fred,
I once worked in a research lab where we were curing the coatings on easy open can ends. There was a specially "filtered" pyrometer that measured the temperature of the ends and I was suppose to control the IR heating tubes in the oven to prevent the ends from charring.

I had four zones of heating tubes both top and bottom. The oven designer provided 8 PID controllers for this propose. I used to spend about half the day trying to get all those things to be right at the same time. Eventually, I put all put the last top section on a pot and then tweeked the last section to finish the cure at a safe temperature. That project lasted about 2 years, but it seemed to me that it took 10.

That was when I began to dislike PID. My deep seated hatred set in when I was asked to fix the gauge control system on a 4 Stand Cold Reduction Mill. The ratio of product width from wide to narrow was 2 to 1, and the thickness had a 10 to 1 ratio. Hardness was about 4 to 1, etc.

Some one had tried to impliment the only thing that they had ever heard about, PID. Most of the time, the operators ran it manually. I sat down with the operators and asked them exactly what they did. After about a dozen rules and about 3 months construction on the first PLC 5/40 that rolled off the line, I was credited with saving the company $771,000 in the first 6 months. And in a nut shell, the formula for becoming a prima donna, my crowning achievement!

I never tell someone else not to use PID, I just suggest that they call me if it doesn't work well enough. I have steady work!

Best Regards,

Bob A.