Chirp issue

[cobayatron]

Hi,

I use a vi to control a shaker, since it's important to control the initial/final position of the shaker I add an offset value which determines the position around which the shaker oscillates. When the chirp signal finishes, I want the shaker to move back to the offset position so I don't input any undesirable forces in the structure. It seems to work fine, I can offset the shaker where I want and it responds to the chirp signal, if I stop the vi the shaker comes to the offset position as expected. There's a problem though, if I let the chirp finish the shaker seems to go back to a position close to 0 and then goes to the offset value. It's strange because that 0 value is not in the range and that never happens if I stop the chirp before ending.

I'm feeding the shaker using a CDAQ9172 and a NI9263 card with an amplifier.

Thanks! any help would be very appreciated.

[spacedan]

Dear Cobayatron,

1) Could you please upload your VI with comments about each node (not the obvious ones), but basically outlining why you are coding as you do. 

2) How do you decide whether or not the "shaker" acts as it does, is it by looking at the Time Signal/output plot?

3) Could you explain what you are using the shaker for, and what type of shaker you are using?

4) What offset value do you use, and what amplitude?

5) What exactly do you mean that it goes back to a position close to 0, do you mean that the chirp oscillates around 0? Similarly, what is meant by that it goes back to the offset value. Feel free to provide a picture of these states (provided you are talking about the output plot).

Kind regards,

[cobayatron]

Hi Daniel,

Thanks for your post.

1-Not sure which nodes, I upload it with some comments with possible confusing areas. Let me know if you need any other clarification.

2- I monitor an accelerometer both in time and frequency.

3- Shaker is an air bearing APSdynamic shaker, the test is a modal analysis of a wing.

4- The offset and amplitude are related, once in the final configuration I will have a force sensor attached to the shaker and then I'll offset the shaker so I don't induce initial forces on the wing. The amplitude will be dependent on what harmonic force I want to produce on the structure.

5- I work with an amplifier so effectively for me ( 0V is fully backwards and 1 V is fully extended). When the chirp finishes and before the shaker goes back to the final offset position, I can see the moving part moving backwards -that's what I mean by going towards 0V-.

Hope this helps clarifying a little. 

[spacedan]

Hi Cobayatron,

It is strange indeed. Theories:

1) If I interpret you correctly, your chirp signal never crosses 0?

2) DAQmx stop task forces the shaker to 0, on the other hand, this would mean that it would act according to the following after the while loop: to 0, to offset, to 0. The last part would be because of the final DAQMX stop task, I assume that this does not happen?

3) It might be something strange going on due to the fact that you close the task directly after the while loop instead of leaving it open. In addition, you do not have a create task. Perhaps create task is required for that task to operate as expected. You could try to

A) Leave the task open and just write the offset

B) Add create task node. 

[cobayatron]

Thanks for the suggestions.

1- That's right in my cases Offset>Amplitude always.

2- Nope, once it reaches the final offset and it stops the task and the Vi the shaker stays in offset position.

3- I tried to get rid of the stop just after the loop but then an error pops up when I try to start task again (-200479 error code). I've also use the create task but I cannot see any difference.

On my side, I got rid of the accelerometer line, which effectively made the loop faster (I guess because it doe not have to calculate the power spectrum anymore) and the problem still happens but it goes way faster to offset value.

My last attempt was to create a waveform which effectively tells the shaker what to do the whole process so I appended the chirp signal and a waveform of offset  values (which I was convinced it could work :((((  hehehe ) but it effectively didn't go back to offset. (???) 

[cobayatron]

Hi again,

It seems is not a problem of the software (not entirely). As far as I've seen playing with frequencies and amplitudes it does seem an overshoot control problem:

The higher the frequency the larger the shaker moves backwards and for the same frequency the higher the amplitude the larger the movement too. I cannot see the movement for very low frequencies. it actually starts to be noticeable for >5Hz.

Maybe I need to do closed loop control, I'll see what I can do.

Thanks for your help.

[spacedan]

Hi again,

Were you able to verify that it was a hardware related issue?