Force Sensor port connections confusion

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Jeff
Jeff on 12 May 2017
Commented: Jeff on 31 May 2017
I have constructed 2 SIMSCAPE models of a 3 DOF system. One is with a force sensor connected in parallel with a 3 degree of freedom mass/spring/damper system & the other is in series. The connection schemes in SIMSCAPE is a bit confusing; nonetheless, I have attached an image of how I interpret the 2 configurations in a conventional mechanical scheme.
In the case with the series connection there is a node that remains free. Hence, I would not expect them to contribute to the response of the system. However, if I employ the zpk or tf commands listed below I find a singular REAL pole locate @ c1/b1. So my interpretation of the connections must not be correct. Can anyone provide a correct schematic interpretation of my 2 models? My parallel connection schematic I believe is correct.
I have included both .slx models along with the necessary input data to allow you to experiment with the results. Let me know if anything is missing & I will provide it.
zpk(SIMULINKbode.values)
tf(SIMULINKbode.values)
Thanks for any assistance

Accepted Answer

Ed Marquez
Ed Marquez on 18 May 2017
Hi Jeff,
this is definitely an interesting application - and thanks for attaching these nice models. I think the doc link below can provide some helpful information for your measurements in Simscape:
https://www.mathworks.com/help/physmod/simscape/ug/basic-principles-of-modeling-physical-networks.html#bq89sba-3
I would suggest sticking to the force measurement in series. Simscape works with through and accross variables and the product of these two equals power in that domain (P = V*I in electrical, P = F*V in mechanical translation, or P = T*w in mechanical rotation). The through variables are always measured in series, and the across variable measured in parallel. In your models, if you want to place a gauge in parallel with the elements, then you should be using an ideal motion sensor (for x and v).
The controls part of the system (zpk and tf) heavily depends on the measurements being correct, so I would start by making sure that the response measurements yield good numbers and then re-visit the controls part after that. I hope this help and good luck!
  2 Comments
Jeff
Jeff on 31 May 2017
Hi Ed,
Your direction did help. I was able to construct an accurate mathematical model based on the schematic below. The results concur with the force sensor in parallel. However, there was a perplexing discrepancy which I have posted @ https://www.mathworks.com/matlabcentral/answers/340167-reconciling-zpk-output
If you have a working knowledge of the zpk command maybe you can provide some insight on the discrepancy.

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