Cascade Loop Specifications

ssylee · 2010-03-22T00:59:16+00:00

In a cascade control architecture with an inner and outer loop,assuming the reference and disturbances are step-like signals, is itfairly common for the inner loop to be running much faster than theouter loop? If so, why is this the case? Thanks in advance.

Mar 22, 2010 22 Replies

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pnachtwey 16 years ago

JCH you are beat again but no one cares. It wasn't worth my time. I know that it is easy to beat your responses because they are limited by your target filter.

No one bothered to ask why the actual position is leading the target position in the sine wave simulation. ( The answer is zeros in the outer loop). I have shown my gains. It does take much to translate what I did to Scilab script to verify my gains.

I did discover a few things. The double integrator make simulating this system, using state space, prone to integration errors. The response appears to change a lot when the sample time in changed. The response is much slower at low sample times because of the integrating error. I wonder if using RK4 would be better. I will try that out. I have alway wondered which would be better,

I get exactly the same response when I don't use just a single loop when I place the closed loop poles at -10 IF I only have the P gain in the forward path. Otherwise there is a difference in the zero locations which adversely affect the sine wave significantly.

The response you see is the slowest. If I move the inner closed loop poles to -30 then the response is very quick.

Peter Nachtwey

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pnachtwey 16 years ago

Now I *PLONK* the JCH and the whole news group that gave opinions instead of hard facts and calculations.

What's wrong JCH? I haven't got a reply. Are you ever going to admit defeat and go away?

Tim, your statement "So if your "faster" means "higher bandwidth", then yes"

This is right most of the time but not all of the time. It is the times where it isn't right that bother me.

Mr Vladifilter man "Inner loop -> local feedback -> less phase shift -> higher bandwidth" This doesn't make sense and you can't justify it?

JCH "The inner loop must have faster dynamics and is used to pre-control the system using an auxiliary process value that leads to improved performance. Disturbance d2 is instantly feed-back controlled. "

And you you posted a link to a page that said it wasn't necessarily true. I pointed that out and even then you ignored it. Apparently you to read and understand the documents you post links too.

Dr Ebert just wants to sell his e-book. It is too bad Dr Ebert didn't post something I could challenge. I love picking on PhDs.

George "To avoid interaction between the loops, the inner control loop should respond AT LEAST 3 times faster than the outer loop. " George just wants to sell expertune but I wonder where the expert in the tuning is. George, did you work through JCH's problem? It is obvious that your are wrong about the 3/1 ratio. What you say may apply to process control application but you should not say the inner loop must be faster than the outer by 3/1 as a general case. I can prove that in some cases the ratio must be greater than 10/1. It all depends on the system and the choices in how it is going to be controlled. In JCH's problem the inner loop can be slower than the outer loop.

Consider this. A hydraulic system has a outer position loop control and an inner acceleration/force loop control. The inner loop is two derivatives from the postion so don't you think the acceleation loop would be MUCH higher that 3/1?

ssylee shoud be careful about who he listens too.

The whole point of this 'thumb in the news groups eye is' that too often people repeat what they have heard or read on the web. Too often people think that their experience is true for all cases when it it not. Too often people provide opinions instead of facts.

Do the math guys. If you do it right you will find that if the inner loop for JCH's problem can have two inner loop poles at -30 then the outer loop can have just one pole at -30 and one the one pole outer loop at -30 will be faster than the two pole inner loop poles at -30.

JCH may still be saved. At least he is still willing to put some time into his posts and do some math. He only needs to listen.

Peter Nachtwey

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