SMART MATERIALS - SHAPE MEMORY ALLOY (SMA)

Page Two

MUSCLE WIRE

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Muscle wire is also a nickel and titanium alloy. At room temperature it can be stretched by a small force. However, when a small current is passed through the wire it returns to a much harder form and to its original length with a reasonable force.
When in use a muscle wire can be stretched up to 8 percent of its length and still recover. However, this can only be done a few times until it breaks or stops returning to its original length. Its life cycle can be extended dramatically if it is stretched to between 3 to 5 percent of its overall length. Within this range it will go through the stretching and return cycle millions of times.

The diagram above shows a battery and switch connected to muscle wire. A small weight stretches the muscle wire approximately 3 to 5 percent of its length. However, when a current is applied to the wire, it shortens lifting the weight. This cycle of turning on and off the current has the effect of lifting and then lowering the weight.

A clever use of muscle wire and a PIC micro-controller circuit is seen below. A robotic hand has ‘stretched muscle wires’ attached to the base of each finger. When current is applied to the muscle wire it contracts to its ‘natural’ length, pulling on the ordinary wire ,making the fingers look as if they are moving.
A PIC micro-controller can be programmed so that outputs are switched ON or OFF. When switched ON the muscle wire contracts (shrinks) to its original length. In the example, five of the outputs have been programmed to switch on and off, making the fingers of the hand move.

Below is a model railway signal. When current is passed through the ‘stretched’ muscle wire it returns to its original length, lifting the signal.