Smart and small

Get ready for actuators that can fit into smaller spaces, react more quickly, and save weight thanks to smart materials. “We’ve come to the determination that there’s no limit to how lazy people can be,” an auto-industry insider said recently. Drivers and passengers want more control at their fingertips. Intelligent, agile material-based actuators are giving it to them and even anticipating their desires. Although shape-memory alloys (SMAs), piezoelectrics, and magnetostrictive materials have been in practical use since the middle of the 20th century, engineers are finding new ways to work these smart actuators into automobile systems, aerospace, and manufacturing. “We’re systematically looking at all the things that are actuated in a car to see if we can replace them with a shape-memoryalloy wire instead of an electric motor. And then we’re looking at all the things that aren’t actuated because it was previously too expensive or too difficult to place an actuator in that spot,” said Jan Aase, head of General Motors’ Vehicle Development Research Lab. The capabilities of SMA actuators vary with wire composition, actuator geometry, and material processing. For applications like GM’s, the wire is usually prestrained up to 5% in its more malleable low-temperature state. The amount of prestrain and the wire’s length determine the actuator’s stroke. When heated above its transition temperature, the material’s crystal structure shifts into a higher-stiffness form, contracting the wire and relieving the applied strain. (See box below.) Stroke speed is determined by how fast the wire heats above its transition temperature and cools below it. Most applications use an electrical current of 4 A or less to heat the wire. Thinner wires and higher amperages get the actuator to its transition temperature faster. The alloy formulation can also be tweaked to set the transition temperature anywhere from –200 to above 300°C. Once the heat source is removed, the wire must cool below its transition temperature to return to malleability. Opposing pairs of wires, springs, or weights must be used as bias loads to stretch the cooled wire back