Precision Moves With Magnetostriction

Magnetostrictive actuators originally developed for sonar may prove useful for producing movements measured in micrometers. A cross-sectional diagram of a typical Terfenol-D actuator reveals the Terfenol-D rod, permanent magnets, and coil driver. Magnets bias the Terfenol-D magnetic domains for ac use. Versions biased for dc use would substitute other coils for the magnets. The Terfenol-D rod pushes against a spring-loaded output rod to generate displacements in the range of hundreds of micrometers. Typical operating range for magnetostrictive applications Terfenol-D has a higher energy density than piezoceramic materials often used for magnetostrictive actuation. In the 1970s, the U.S. Navy wanted to improve its sonar technology. It needed a material able to transmit powerful acoustic signals to detect enemy submarines. Engineers at the Naval Ordinance Laboratory in Washington, D.C., solved the problem by developing a giant-magnetostrictive material called Terfenol-D. Magnetostriction is the property that causes ferromagnetic materials to change shape in a magnetic field. It is the source of the 60-cycle buzz you hear from electrical transformers. Terfenol-D is an alloy of terbium, dysprosium, and iron. It is said to produce "giant" magnetostriction, strain much greater than any of the other magnetostrictive materials. Simply, this gets accomplished by energizing an electrical coil that surrounds a piece of Terfenol-D. Electrical current flowing through the coil induces a magnetic field which acts to realign the magnetic domains in the Terfenol-D. As the domains rotate, they distort the atomic structure, causing the material to grow and contract as the current is applied and removed, efficiently converting electricity into motion.