<title>Development of a control system for a nonlinear Terfenol-D actuator</title>

A promising use of terfenol-D is as the actuating element in mechanical actuators. Actuators made from terfenol-D take advantage of the material's large displacement capabilities, high energy and relatively wide bandwidth. Terfenol-D actuators would be more useful if their distortion levels were lower. When operation of these actuators is restricted to strains where the material's behavior is linear, their performance is only comparable to standard actuators. Our objective was to design a control system to reduce distortion of a terfenol-D actuator. A terfenol-D actuator was tested to obtain operational data. Nonlinear system identification was performed with a fourth order Volterra expansion. The left inverse was estimated using piecewise linear functions. Feedforward compensation for a feedback controller was designed by representing the inverse/actuator model pair as a linear transfer function and using standard linear control design techniques. Because the inverse was found for operation at a single frequency, the out-of-band frequency response of the linear representation of the inverse/actuator was very severe. This made it impractical to close a feedback loop around the actuator. Instead, the inverse was used as a predistortion filter in open-loop control, which resulted in an 18 dB reduction in total harmonic distortion at the frequency of interest.