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288 ROBOT MOVEMENT WITH SHAPE MEMORY ALLOY

weight. Though the material may be very thin (a typical thickness is 0.006″ (six mil)—slightly
wider than a strand of human hair), it’s exceptionally strong.
In fact, the tensile strength of SMA rivals that of stainless steel: the breaking point of just
one slender wire is a whopping 6 pounds. Even under this much weight, SMA stretches little.
In addition to its strength, SMA also shares the corrosion resistance of stainless steel.
Shape memory alloys change their internal crystal structure when exposed to certain
higher- than- normal temperatures, and this includes the induced temperatures caused by pass-
ing an electrical current through the wire. The structure changes again when the alloy is
allowed to cool.
More specifically, during manufacture the SMA wire is heated to a very high temperature,
which embosses or “memorizes” a certain crystal structure. The wire is then cooled and
stretched to its practical limits. When the wire is reheated, it contracts because it is returning
to the memorized state. That’s why SMA is often referred to as “memory wire.”
Shape memory alloys have an electrical resistance of about 1 ohm per inch. That’s far
more than ordinary hookup wire, so SMAs will heat up more rapidly when an electrical cur-
rent is passed through them. The more current that passes through, the hotter the wire
becomes and the more contracted the strand.
Under normal conditions, a 2″ to 3″ length of SMA is actuated with a current of about
450 milliamps. That creates an internally generated temperature of about 100 to 130°C;
90°C is required to achieve the shape memory change. Most SMAs can be manufactured to
change shape at most any temperature, but 90°C (194°F) is a standard value for off- the- shelf
material.
Excessive current should be avoided. Why? Extra current causes the wire to overheat,
which can greatly degrade its shape memory characteristics. For best results, current should
be as low as necessary to achieve the contraction desired. SMAs will contract by 2 to 4 per-
cent of their length, depending on the amount of current applied. The maximum contraction
of typical SMA material is 8 percent, but that requires heavy current that can, over a period
of just a few seconds, damage the wire.

Using Shape Memory Alloy

Shape memory alloys need little support paraphernalia. Besides the wire itself, you need some
type of terminating system (the hardest part!), a bias force, and an actuating circuit. We’ll
discuss each of these in the following sections.

TERMINATING SYSTEM

Because SMA can’t be readily soldered to anything, the ends have to be mechanically
terminated— not the “Are you Sarah Conner?” type of terminating, but the kind involving
crimp- on lugs and other kinds of solderless connectors. Because SMAs physically contract,
using glue or another adhesive will not secure the wire to the mechanism. These and other
crimp terminators are available from companies that sell shape memory alloy wire.
You can make your own crimp- on connectors using 18- gauge or smaller solderless crimp
connectors— the smaller, the better. Although these connectors are rather large for the typical
thin 0.006″ SMA, you can achieve a fairly secure termination by folding the wire in the con-
nector and pressing firmly with a suitable crimp tool, as shown in Figure 25- 1. Be sure to
completely flatten the connector. If necessary, place the connector in a vise to squish it all the
way shut.

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