Lifting up … then down …

by lmctague

Flexinol wire can exhibit an astonishing grace when it lifts up with current and then relaxes without. I find this movement particularly elegant when the piece of fabric (with flexinol stitched in) is suspended. Then, the action of lifting up with current then slowly releasing without is most balletic.

I had initially imagined creating a series of hung pieces that, when activated, would nest one into another … much like the petals of a flower. However, the behavior of the longest length of wire (about 6.5″) was not as I expected. Rather than simply curling in a single arc, it curled round several times, much like a ringlet of hair. Quickly making adjustments, I then decided to aim for a simple see-sawing motion with two pieces that, in syncopated rhythm, would curl in this ringlet formation.

This short video demonstrates this:

The calculated resistances are as follows:
For switchPin1, the resistance in the wire itself measured 13.8 ohms. Adding to that the resistances in all the other strands of connecting wire, I got a total of 14.5 ohms. As I chose to use my computer to power the arduino, I calculated the current by dividing 5volts (the voltage coming out of my computer) by 14.5 ohms.

V = I * R

5 v =  I * 14.5ohms

I = 0.34 amps (just shy of the targeted range of 0.35 – 0.41 amps required to trigger the 0.006″ diameter nitinol wire to move)

For switchPin2, the resistance in the wire measured 10.3 ohms. As I would need a bit more resistance to lower the amount of current in the circuit, I added two 1 ohm resistors to the circuit. In total, I had 12.6 ohms. Therefore:

5 v = I * 12.6 ohms

I = 0.40 amps (perfectly on target!)

I placed the piece on a simple structure that elevates it off the ground such that the wire strips hang. When I first tested it, switchPin 2, or the left arm, activated while switchPin 1 did not. After checking all the connections for the unresponsive switch, I discovered a blocked connection. After fixing that, I tested the piece again and this time switchPin1 did indeed transform but then switchPin 2 seemed to have gone lame! I wonder if there is something incorrect about my wiring such that the current is directing itself via the shorted possible route. Just a guess … will have to test further to figure this out.