Shape Memory Alloy (SMA)

An alloy that springs back to its original shape when heated, useful as an artificial muscle when powered electrically.

A shape memory alloy is deformed at low temperature and then returns to its original shape once heated above its transformation temperature; the most common example is Nitinol (a nickel-titanium alloy discovered in the early 1960s at the US Naval Ordnance Laboratory). The mechanism is a transition between two crystal structures, martensite and austenite. Made into a thin wire and heated electrically, it contracts by a few percent and produces a pulling force, so it can serve as an actuator or “artificial muscle.” Its advantages are a high force-to-weight ratio, a simple structure, and silent operation; its disadvantages are a small range of motion, slow recovery because it depends on passive cooling, low energy efficiency, and significant hysteresis that makes precise control difficult. It's commonly used in miniature robots, biomimetic fingers, and soft robots.

ExampleA few electrically heated Nitinol wires pulling on tendons can curl a small biomimetic finger; once power is cut, it cools and a spring resets it to its original position.

Also called

Memory Alloy, Nitinol

Related

Artificial Muscle · Actuator · Soft Robot · Tendon-Driven Actuation · Dielectric Elastomer Actuator (DEA)

Sources

Shape-memory alloy - Wikipedia