General Misc > Misc

feedback control of Shape Memory Alloys

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Admin:
Does anyone have recommendations on how to do a feedback control loop for SMA's? Does not matter which kind of SMA, I just need one that I can control length of. Must be accurate and repeatable. I am expecting low torques, but my sensor mechanism must be VERY compact and energy efficient.

My thoughts were that when length changes, I can measure a change in resistance? Maybe do an analog measurement of it . . .

Thoughts needed!

zamboniman60:
I guess you could attach a string to the end of the wire, then wrap it around a pot, and spring load it for when the wire expands...

2Hektik:
Hey,

First post!

At low torques, the pot would experience pretty small changes...

Have you considered strain gauges? They are small and relatively inexpensive (for the cheaper variants). They can be quite acccurate, you simply need to amplify the output using a wheatestone bridge. Knowing the resistance per unit strain you can easily find the change in load.

Or there is the humble thermocouple. You can find the change in length per degree temperature (resistance change).

Both are obviously analogue outputs and a fairly common components.

Admin:
My goal is to control length, not really force or anything else.

Temperature and length do not correlate that well, so that wont wont.

A strain gauge, in serial with the device, will only give me force control  :(

The best method I have seen so far is using color coding and an external camera:
http://www.star.t.u-tokyo.ac.jp/projects/popup/tech.htm

But that doesnt work so well on a mobile robot fish . . .

2Hektik:
Thanks for the link!

So how will the SMA be used? If you can divulge the info that would be helpful.
I'm not very familiar with the use and properties of SMAs. So I'd look for other changes to the device that I could measure reliably.

How does the the input change over a known change in length?

Wait... getting a thought... You're using the SMAs to pull either side of the driving fin in, aren't you? What about speed of the fish if you use a pressure sensor?

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