feedback control of Shape Memory Alloys

[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?

[Admin]

So the SMA, I cannot divulge any info on it. But lets just say it is a new type of SMA being developed that operates just like current SMA, but is stronger/faster/more efficient.

While some smart phd dudes are making it, my job is to get it to do useful stuff in a robot fish. There are two versions being developed. One works by electrical voltage, while the other works by applying heat. I am not sure what measurable changes occur with a length change, just looking for stuff other people have done, or at least ideas, so I can look for these things. The SMA's are still in prototype stage, so there isnt anything I can test at the moment.

The simulation guys tell me that if I get an SMA array (at least 5 of them) to follow a certain motion pattern, I can get the fish to do cool stuff . . . like swim n stuff . . . if the pattern is wrong, it wont work very well, or at all.

So I just need to somehow get a position controller on them that is very small, efficient, and easy to make - mainly cause SMA is very not efficient, and I have a lot to control simultaneously . . .

I would need the same position control as in that link, but with a different method. Not entirely sure if another way is possible . . .

[2Hektik]

attach a slotted card to it and count positions as it moves past via a photo transister setup simiar to that of an optical encoder. Or colour code it and try and recieve differnt values for each colour (position)?

[2Hektik]

any leads yet?

 

[Admin]

Hmmm the slotted card thing gave me an idea. But I havnt looked into it yet. Was thinking of possibly a tiny digital phototransistor array setup that through electronics outputed a single analog value. It may be awhile before my boss requires me to do this, I just want a solution before she asks . . . Thanks for the idea!

[sdk32285]

HI i might be working with SMA's for a project. Was their ever a resolution on how to due the feedback. Note: i have similar constraints mm accuracy and small size.

[Admin]

The people developing the SMA's are having this strange issue where the SMA twists instead of contracts . . . until they solve that I have no way of hooking it up to a device . . . Been 14+ months since they had this problem . . . will probably be another half a year . . .