How many of you have used cascaded loops?

boiler application is one use.
place the faster response Pid2 in the inner loop.
Pid1 as the master, primary or outer loop.
Pid2 is the slave, secondary or inner loop.

turn off PID1 when tuning PID2, When tuning PID1 have PID 2 in auto.

You also need to tune PID2 first

almost forgot to add PID2 in this application handles the flow of steam and PID1 handles the amount of flow

As far as personal experience just had the 1 boiler system that I had to work on and only had to fine tune the PID2 to get it retuned
 
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This is one of the better quick articles I had in my collection on the subject
http://www.engr.uconn.edu/control/pdf/isa04-1.pdf
Good article. The only thing I don't agree with is the part where the inner loop must be much faster than the outer loop. This is one of the "myths" I mentioned. Yes, in general the inner loop is faster but by how much depends
on the system and the type of PIDs being used.

Did you tune the system using trial and error or did you use an auto tuner?

Now I wonder how many people understand the article you posted or even care. Would anybody bother to read it in a magazine?
 
I think the inner loop should be faster than the outer loop, it just seems to make sense.
I have never used auto-tuning but then I'm a grumpy old **** who thinks if you can't tune a loop you shouldn't be in the business.
Roy

Geeze U cant even say F a r t here, how PC is that?
 
I manually tuned mine My company doesn't like spending money so did it on a trial and error basis after I researched several articles on the subject and I never had faith in autotuners
 
I think the inner loop should be faster than the outer loop, it just seems to make sense.
I have never used auto-tuning but then I'm a grumpy old **** who thinks if you can't tune a loop you shouldn't be in the business.
Roy

Geeze U cant even say **** here, how PC is that?
Roy,
Get PC: use the correct medical term "Flatulence" ;)
 
Started goingback through my database found this article that agrees with you Peter on the problem with the previous article

Under summary it states that the inner does not have to be faster than the outer loop. Another myth I came across from other sites is that you need a 6 to 1 ratio.

http://www.bgu.ac.il/chem_eng/pages/Courses/oren%20courses/Chapter_10.pdf
the other articles are I have are more basic and don't explain much
 
I would definitely read an article like that in a magazine. I just would rather not have the requisite five additional pages that tell me how the engineer was about the lose his house, wife and car before implementing cascade control and then went on to cure cancer using cascade control.

Keith
 
Roy, I am a grumpy old **** too and getting older and grumpier everyday.

That topic deserves another thread.

I have never used auto-tuning but then I'm a grumpy old **** who thinks if you can't tune a loop you shouldn't be in the business.
I agree but that would limit the amount of product that can be installed and maintained.

I now know, from doing the math, that it takes quite a bit of fiddling to get cascaded loops to tune right. Finding the optimal gains is not easy. It would take a lot of trial and error since now there are twice as many gains to tweak. It is better if one can auto tune the system by exciting the inner loop and then moving a slider bar to adjust the gains for both loops. The slider bar adjust the speed of the response at the outer loop.

Mordred said:
after I researched several articles on the subject and I never had faith in autotuners
That too is a topic for another thread but the short answer is that most people say they have an auto tuner just for marketing hype. Most will only tune one kind of system which is a first order plus dead time system (FOPDT). If your system isn't a a FOPDT system then the auto tuning will not work.
It is that simple.

Mordred said:
http://www.bgu.ac.il/chem_eng/pages/...Chapter_10.pdf
the other articles are I have are more basic and don't explain much
This is better stuff. I like to go this deep only my emphasis is on motion control with acceleration, velocity and position loops.

At least you understand the problem about the myths. Do you think anybody else cares enough to wade through information like you posted?
I will.

I am reading the other forum thread about control engineering and it doesn't look like anyone wants to read a 10 page novel let alone a 20 page one.
 
At least you understand the problem about the myths. Do you think anybody else cares enough to wade through information like you posted?


My experience has been the only time most ppl are willing to wade through it is when they encounter a dilemna that they cannot solve. Rarely do ppl think ahead and study in advance but again another subject for another post lol
 
How was the cascaded loop tuned?
What did the cascased loops control?
What myths or rules of thumb have you heard or used?


My rule of thumb is "*never* use cascaded loops".

This is not a hard and fast rule, but my experience has been that 97+ times out of a hundred the control can be done easier (if not better) without cascaded loops.

A similar example is boiler feedwater/drum level control. Some people swear by 3-element feedwater, but if done correctly, 2-element will *almost always* provide as good (and more times than not better) control than 3-element. In addition it is *100%* of the time easier to configure, tune, and maintain a 2-element system than a 3-element system.

Even if you don't know what I am talking about in regards to boiler feedwater, just understand that *less* is most often *more* Most of the time, complicated control is just that, complicated. It is rarely better.

I know these are generalizations, but this is how I've made my living for 25+ years and I could give you exceptions to my rules too.
 
I use them in process control. An example is DO cascaded to flow controlled by valve position. I tune manually by trial and error. Sensitivity and difficulty for operators to tune is one reason I don't use PID.
 

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