The Skeptic Tank

History

Nitinol is interesting to us, however, because of its reaction to changes in temperature. Normally, metals expand (slightly) when heated and contract (slightly) when cooled. That's why jar lids open more easily after running them under hot water. Nitinol however, shows the reverse property; shrinking when heated. Once cooled, it can be stretched back to its original length.

Since nitinol has some electrical resistance, simply running a current through it causes it to heat, and thus to shrink. This mimics the musculature of animals, which require an electrical signal to flex - hence the name 'muscle wire'.

Mark Tilden was an early advocate of using muscle wire in robotics, specifically those that rely on legs rather than on wheels.

Each manufacturer's brand of nitinol (Flexinol™ is a common one) is composed of a slightly different mixture (or admixture, if you will) of nickel and titanium. A common mixture is called 55-nitinol and is composed of 55% nickel and 45% titanium. Each type shrinks and expands at different temperatures, so you can select a type that actuates in a cold environment, such as on a satellite in outer space, or a warm environment such as the human body.

Unfortunately, the percentage of contraction is typically limited to between 4% and 8%, and the wire must cool before it is able to be stretched back into place and then heated and shrunk again. This means any robot that has its mobility dependent on muscle wire will likely move rather slowly compared to its motor-driven brethren.

Nitinol is not able to convert electricity into motion as well as motors can, but is much lighter than even a small motor and is appropriate for small, light-weight robots.

Many companies now fabricate nitinol for use in robotics and medical devices. The latter is probably the best venue for nitinol-based robotic devices, such as prosthetics, stents, shunts, or other small valves. Because it is non-corrosive, nitinol is an appropriate material to use within the human body.

Problems

One thing about muscle wires is that if you drop a piece, you're unlikely to find it. You're more likely to search on the floor and pick up one of Yan-yan's hairs, only to hook it up, put voltage to it and see smoke. Yan-yan's hairs do not make good actuators.

Another issue is that in every schematic I've seen the nitinol does double duty as actuator and current-bearing wire. The problem here is that they can't be insulated since the insulation would hinder their contraction and stretching. So we had to be very careful to not allow the exposed wires to touch any other metal contacts.

Problem #3 has to do with the extrememly specific power requirements of nitinol. It was easy to get a single wire to contract off of a 9V battery, but trying to power multiple wires did not result in less contraction, it resulted in zilch. Similarly, after we ran out of 0.1mm wire, we were unable to get any of the 0.05mm or 0.15mm wire to do anything. I couldn't tell how much the volatge mattered, but thinner wire can't tolerate much above 80 milliamps, and thicker wire needs at least 300. We tried putting 9V batteries in series and again in parallel in order to try higher voltages and currents but got nowhere. We may have toasted the 050 and not had enough for the 150. DC motors, those gentle souls, are so adaptive when it come to variability of current and voltage. But muscle wire is high-strung.

Which leads to p4: since the wire only has about 5% contraction, they must be taut to have any noticeable effect. Compounding that, they do not spring back when cooled unless a force acts on them to stretch them back to their original length. So we had to discover an entirely new art form of stringing nitinol wire to be under the precise tension to keep them taut at rest and under tension when flexed, yet not under so much tension that they are unable to perform. That sounds like a metephor for giving technical presentations.

The bottom line is that this tempered metal is temperamental (tempura-mental?) and although I imagine many future ITP students struggling in the exact same way we did, I don't think we'll be playing with it much.