Design of shape memory alloy actuator with high strain and variable structure control

A novel shape memory alloy (SMA) actuator consisting of a number of thin NiTi fibers woven in a counter rotating helical pattern around supporting disks is first described. This structure can be viewed as a parallel mechanism used to accomplish a highly efficient transformation between force and displacement. The actuator overcomes the main mechanical drawback of shape memory alloys, that being limited strain. Two variable structure controllers are applied to a pair of antagonist actuators. The first involves a switching control input creating a sliding mode in conjunction with a linear control activated within a boundary layer in the vicinity of the set point. The second involves a multistage switching control that simplifies amplifier construction. Experimental performance results in the time domain are discussed