Page 320 - Robot Builder's Bonanza

USING SHAPE MEMORY ALLOY 289

Crimp connector
SMA wire

Crimp tight

2-56 screw

Side view

Figure 25- 1 Use small crimp- on connectors to terminate the Figure 25- 2 A bias spring or weight
ends of the shape memory alloy wire. Small tools like those for is required as a counterbalance on the
making jewelry are a help. SMA wire.

Figure 25- 1 also shows a variation using miniature 2- 56 size machine screws (available at
hobby stores). You still use the crimp- on connector, but you carefully loop the wire around the
machine screw.

Experience in making personal jewelry comes in handy when working with shape memory
alloy. Many of the tools, techniques, and findings used to make things like necklaces and
G earrings can be used with SMA, too. Findings are the little doodads used along with the jewel
in jewelry— metal chain, clasps, pins, and, of course, crimp- on connectors. Locate these tools
and parts in well- stocked hobby stores and online at places such as Fire Mountain Gems.

BIAS FORCE
Apply current to the ends of an SMA wire, and it just contracts in air. To be useful,

• The wire must be attached to one end of the moving mechanism.
• It must be biased at the other end, like that in Figure 25- 2. The bias can be a spring or even
an unmovable part.

Besides providing physical support, the bias offers the counteracting force that returns the
SMA wire to its limber condition once current is removed from the strand. You can also use a
second SMA wire that contracts in the other direction and restores the state of the first wire,
but this involves some very detailed mechanical construction.

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