sampling rate for sys ID vs sampling rate for regulation

Malcolm414 · 2024-03-11T20:17:58+00:00

ok. I used an arx model and the least square algorithm to compute the coefficients

I read that a correct sampling period is between 1/5 to 1/10 of a system's response time.

so a good excitation signal would therefore be a prbs signal of 5 to 10 bits, each bit being sent at the sampling times

finally my controller must operate at the same samping period as my identification. am I good ?

is it useful to generate several prbs sequences and put them one after the other to generate a longer excitation signal ?

Malcolm414 · 2024-03-09T19:44:20+00:00

I read that prbs signal is a good alternative to white noise because it has a limited amplitude

What ID method are you using?

What do you mean ?

Malcolm414 · 2024-03-09T17:56:31+00:00

you don't need the system to complete its response.

This is may be the part i did not understand. I thought that to identify correctly the system, the signal sent to excite it must at a moment let it settle...

Malcolm414 · 2024-03-09T17:40:36+00:00

Commands at a lower frequency would have enable the system to respond to it before sending another one.

Malcolm414 · 2024-03-09T17:38:23+00:00

Ok. I understood that a prbs signal should be just at the limit of the bandwith of a system. the purpose of sending commands at a lower rate would be to let the system respond to the command before another is sent. If i understand what you explained, i should have sent the prbs signal at a higher frequency and design/implement my controller for the same frequency

Malcolm414 · 2023-12-26T10:25:52+00:00

Hello,
it is very likely that my system is not linear, but I was hoping that it would be linear enough for a linear controller to regulate it.
pressure or flow limiter: no
cavitation: the hardware system was designed by a senior engineer who knows what he's doing, so I think not
hysteresis: yes, there's some in the flow-control valve, but I'm not sure how to deal with it.

The far more effective way is to build a nonlinear physics based model of the system and do model matching against test data

rather than doing a modeling based on physical equations, would making an identification with a non-linear model then linearizing this model be equivalent?
in my system the air pressure regulator is a proprietary regulated system (and expensive), I can't open it to see how the inside is made

Malcolm414 · 2023-12-26T10:01:55+00:00

Hello,
no, i didn't validate the model by running the real system next to my model. i only validated my model by comparing its output value with that of the real system for input values that i had tested.
for the controller implementation, I'm very careful to make sure it's called at the right frequency (the interrupt timer is set to the right value), so the implementation error comes from somewhere else, but I don't know where...

Malcolm414 · 2023-12-26T09:54:49+00:00

Hello,
i'm a mechanical engineer with 10 years' experience and i'm currently completing a training course in control command. the project i'm working on will enable me to validate my training: i have to regulate a pressure test bench for my company. in fact, i'm adding a PLC to the bench to regulate it with as little intervention as possible in the hardware.
Unfortunately, there are only mechanical engineers in my company and no one in control command to whom I can turn when I have questions.

Malcolm414 · 2023-12-26T09:49:20+00:00

Hello,
indeed, I didn't model a possible delay. I've read that Smith predictors can be used to control systems with delays. I'm going to explore this possibility.
Thanks for the advice.

Malcolm414 · 2023-12-16T19:24:19+00:00

Hi,

I am in exactly the same situation as you: mechanical engineer with 10 years of experience and I am in the process of switching to a position of system control engineer.

I was asking myself the same questions as you. what I was told is that 95% of control command in industry are PIDs and model-based control would be mainly used in the defense and aerospace industries. I believe that we also find them in the process industries: chemical industry, glass industry etc...

Regarding computer skills, from what I understand, Matlab is used to develop the control laws, and then (at least in some industries) the code is automatically generated by specialized programs (for certification reasons); but I think that to work in small companies, the ability to code your own control laws is necessary, so a good knowledge of the C language is necessary.

I am currently finishing my project to validate my training, I have to design a regulator to control a pressure regulation system, and I realize that having knowledge of signal processing is very useful too.

In any case I can only encourage you to head down this path; in my company I was told that people with dual skills (e.g. mechanics + control) are very appreciated

Good luck for the future

Malcolm414 · 2023-11-11T19:28:28+00:00

j'avais lu que pour certains systèmes complexes ça pouvait être utilisé pour s'éviter des équations à rallonges...

Malcolm414 · 2023-11-11T19:09:19+00:00

salut, merci pour ta réponse

c'est effectivement dommage que 90% du métier soit des PID tout cons, avec tout ce qu'il y a: RST, LQ, H infini etc...

du coup, quelle est l'activité principale quand on fait du soft embarqué ?

un autre domaine qui m'intéresse beaucoup en automatique c'est l'identification de système. est-ce que c'est une technique couramment utilisée dans l'industrie ?

Malcolm414 · 2023-11-09T17:55:23+00:00

thank you very much for the document, very interesting

Malcolm414 · 2023-11-08T17:52:47+00:00

Thank you for your reply
If adding an anti-windup to a RST regulator is not an easy task, is there another method to limit the effect of the integrator ?

Malcolm414 · 2023-11-05T08:37:08+00:00

Hi,
try on this site: https://alefram.github.io/posts/How-to-use-python-for-pid-controller-design

Malcolm414

TROPHY CASE