Experimental results are presented to show that SMA actuators are able to control forces both rapidly and precisely. An antagonistic pair of constrained actuators is shown to be capable of rapidly changing the set-point throughout a /spl plusmn/7.00 N range. Although the system entered into a limit cycle at the set-point, the limit cycle magnitude was small. The peak to peak amplitude of the limit cycles was only 0.07 N with a worst case average offset of 0.016 N at high force rates up to 75 N.s/sup -1/. Simulation results also indicate that accuracy could be improved with an increased sampling rate. The SMA actuator pair was also shown to be capable of accurate tracking. The maximum tracking error for a f=0.500 Kz, 2.00 N sine wave was 0.04 N. For the 2 Hz tracking, the maximum error was approximately equal to the boundary layer width which was set at 0.30 N.

Further development of a three level switching controller (Grant and Hayward, 1995) is presented. The controller was originally designed to drive 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. While the original controller performed satisfactorily, it was hampered by the presence of limit cycles at higher gains. By allowing the upper switching level to be proportional to the velocity if is possible to achieve a damped response, analogous to PD control for linear systems, effectively quenching the limit cycle. With this damping, it is possible to decrease the rise time by almost half and maintain the same steady state accuracy.

A simulator is developed to model and design high strain shape memory alloy (SMA) tension actuators. The simulator may be used predict characteristics of a given actuator, or to design its geometry under specifications such as force, speed, stroke and size. The accuracy of the model is verified experimentally in reference to an existing NiTi shape memory alloy prototype actuator. Having developed some confidence in the model, the performance of the proposed actuation mechanism is compared to other existing technologies. In particular, the force-displacement and speed characteristics of a micro-solenoid electro-magnetic actuator and a muscle-size pneumatic actuator are compared to those of the SMA actuators with same dimensions.

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.