I've been really interested in Flexinol lately (aka Nitinol Muscle Wires). I've come to a nice stopping point where I've put together some basic actuators and a driver circuit so I figured it was a good time to post.
The embedded video shows two projects. One is linear, moving in a straight line like a solenoid. The other rotates on a pivot point like a servo.
For those unfamiliar, Nitinol is a shape memory alloy. When made into Muscle Wire it will respond to heat and contract when it reaches a specific temperature. In these circuits the heat is generated by running an electric current through the wire.
The control circuit for both devices is built around a PIC16F690 microcontroller and a ULN2003A Darlington array. All the micro does for this demonstration is toggle power to the Darlingtons at intervals of about two seconds. Flexinol actually needs a good bit of juice to do its magic- how much varies by the gauge of the wire. Here I'm providing 200ma to the rotating actuator and 320ma to the linear actuator.
Nitinol is interesting stuff. It's been around since the early 1960s and even though the potential for robotics is fairly obvious, you don't see it used in too many projects. I've seen Nitinol described both as "a solution looking for a problem" and "a revolution waiting to happen."
Flexinol has some very attractive characteristics. Since the Nitinol shape memory effect does not rely on magnetism for movement, there is none of the electrical noise you get with servos and other motors. Flexinol is also very strong but adds virtually no weight to a project. The largest gauge available is .020" (.51mm) which can lift 7.85 pounds (3560 grams). These projects use .004" Flexinol which will lift .31 pounds and .005" Flexinol which will lift .49 pounds.
Working with Flexinol does present some significant challenges. Solder or glue won't hold for long so you have to crimp it. I'm using #2-56 machine screws and nuts as adjustable crimps and they seem to be working out pretty well. They're also good general fasteners and electrical conductors so I'm using them for all the attachments. Regardless of how it is crimped, ultimate failure at the crimp joint is a chronic problem with Flexinol projects. Another big problem is cycle time. Nitinol needs to cool off in between contractions and with the heavier gauges that can take many seconds. Cycle time is not as much of an issue with these actuators because of the relatively small gauge of wire.
Although it contracts with a lot of force, Flexinol doesn't move very far. Depending on the configuration it will usually shorten by about 4-5%. Both of these devices apply mechanical advantage to convert some of the available force to movement over a greater distance. The leverage applied in the rotating actuator is probably familiar to most people. The linear actuator uses a less common technique called an angled pull to trade some force for greater distance.
One significant challenge in building even simple Flexinol projects is that the wire does not return automatically to its elongated shape when cooled, but needs to be stretched back to the original length. This stretching is usually accomplished with some type of spring or bias weight, or even another piece of Flexinol. Finding the right spring for an application can be tricky. It needs to be strong enough to pull the Flexinol back into shape, but not so strong that the Flexinol can't move to begin with. There's some science to the selection, but when working with found parts or designing yourself, there's also some trial and error. The linear actuator shown here uses a compression spring that was salvaged from a ball point pen which is performing quite well. The rotating actuator uses a crude extension spring that I fashioned from K&S .020" music wire. You'll see a lot of music wire springs in Flexinol projects. Like the machine screws, music wire is conductive and in this actuator is part of the circuit.
The main structures of the actuators are made from Plastruct styrene plastic. Styrene is available from hobby shops and mail order. It's very generic stuff usually marketed to scratch model builders, model train builders etc. For these projects I used .080" sheets and strips, 3/16" diameter tubes, and .100" diameter rods.