System Identification for control

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21 Aug 2021 08:34 #218314 by jang
Hello 
I'm student want to study CNCcontroller
I'm using LinuxCNC 2.8. on commercial gantry(H-type) CNC system was consist of dual servo motor

And I can run LinuxCNC with commercial gantry system 

but, I can just run it.
Now, I want know about my system Identification
So I have to run the CNC with sine wave with various sine wave.
But, I can't command sine wave to LinuxCNC

I think I can use siggen or G-code for direct command similar with sine wave 
How can I position or velocity command with sine wave to LinuxCNC ????????????

help me pleaseeeee


 

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21 Aug 2021 17:39 #218335 by aleksamc
As for me, your topic is unclear.
1. It doesn't matter what gantry property is..
2. what is system identification do you mean?

"So I have to run the CNC with sine wave with various sine wave."
3. Do you know how many sine waves you need? I even not ask for what purpose you need them.

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23 Aug 2021 06:45 - 23 Aug 2021 06:49 #218462 by jang
System Identification is required to see the dynamic characteristics of the system.

I want to get the ability to respond according to commands (for drawing bode plot)
 this is mean system identification

To do this, I have to change the frequency of the sine wave according to time and apply it to the velocity(or position) command
finally I can get response ability from data.
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Last edit: 23 Aug 2021 06:49 by jang.

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23 Aug 2021 12:47 #218505 by Aciera
This is really just a stab in the dark:
If you make a gcode that commands a circle to be cut then one axis moves according to the sine and the other to the cosine. If you need to adjust the frequency of that then you would need to adjust the feed rate. If you need to adjust the frequency dynamically then you can use feed override while the machine moves.

Amplitude would be adjusted by making the circle bigger or smaller.

How do you measure the response of your router?

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23 Aug 2021 18:47 #218534 by aleksamc
Aciera It seems to me that he wants to analyze mechanical devices by analyzing op amps method.

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23 Aug 2021 18:52 #218536 by aleksamc
I studied at university such subject as jang.
I must say, that with sine wave you will nothing see.
It should be anylized to the step responce.

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23 Aug 2021 19:21 - 23 Aug 2021 19:22 #218540 by PCW
Well theoretically they are interchangeable via a Fourier transform

I suspect the main issue will be that the response to either
Step or Bode plot will be dominated by nonlinearities
Last edit: 23 Aug 2021 19:22 by PCW.

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24 Aug 2021 10:14 #218607 by jang

How do you measure the response of your router?

I don't know router sorry,,, but I got data using ethercat. and so I can watch the data with halscope.

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25 Aug 2021 05:42 #218677 by jang
thank you for your reply
I got some data about step command already
but I think the data can't be used for bodeplot
because bode plot require dynamic command and response like sine wave(in various frequency)
If I get the bode plot then I can know resonance(?) frequency or response which reached at -3dB

I have dual servo gantry system
Can I use stepgen in my linuxcnc ?? Or can I give velocity command with siggen???

First, I'll try with sim.axis example

 

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25 Aug 2021 20:05 #218733 by aleksamc
As for me you will never get resonance freaquency in gantry system.
With increasing of the freaquency change, you will get decay of responce of your motor, it will overheet and may be, will give some smell. It all depends on your persistence.
Resonanse freaquency could be only in systems that has big PID regulator gains. You can increase this values - and resive resonance. I had such things. But if your gains in safe zone, you will never had problems with your gantry system because it's very inertial and has counterforces, if we spoke about cnc.

About stepgen I think it's witten in manual.
linuxcnc.org/docs/html/man/man9/stepgen.9.html
You need siggen set to the input of te stepgen, possibly.

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