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# Using SIMULINK to work backwards?

## Using SIMULINK to work backwards?

(OP)
Hello folks,

I am a student of mechanical engineering and am analyzing some data. The data is collected by a tachometer from a flexible shaft, whose torsional damping coefficient (and stiffness) (in addition to the damping coefficient and stiffness of several torsional bearings in the system) I am to determine. Certainly after having drawn the free body diagram, I have a differential equation to model the motion, which I can then put into SIMULINK and produce a theoretical model of the vibration.

The only problem?

One of the terms in this equation is an unknown, so I don't have a value to put into one of the GAIN boxes in my free-response SIMULINK model.

But I know what the graph SHOULD look like! The SIMULINK model only serves to be a theoretical model and should match my experimental data, which I have! I could show you a photo of it, but it's just like any exponentially decaying sinusoidal function. Heck, I have the peaks and the period and can find all attributes of that graph.

So my question is, can I use both SIMULINK and the data from the experiment to work backwards and determine what this unknown damping constant is?! I fiddled around with the model by varying the gain and checking the scope to see if the model mimics my experimental values, but it's just not a good enough way. There *must* be another way.

Any help on this is appreciated.

### RE: Using SIMULINK to work backwards?

Student postings are not allowed. However, the short answer should be yes. There are brute force approaches, such as adjusting the unknown until it matches, but you should already understand regression and minimizing least squares, but there are others called system identification that are used to determine even more complicated configurations.

TTFN
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Of course I can. I can do anything. I can do absolutely anything. I'm an expert!

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