Set Point Force Control of Shape Memory Alloy Actuator Using Fuzzy Controller

Shape memory alloy (SMA) has a property that when the temperature of SMA is stabilized, the fraction of Martensite phase keeps the same, and output force of SMA actuator also keeps constant. Based on that, a control strategy for set point force control of SMA actuators is proposed that during the process of driving, to stabilize output forces of SMA actuator, some voltages called balance voltages which can stabilize the temperature of SMA at the point will be input to SMA actuator. Besides, in order to shorten force rising times, before output force reaches the desired force, the max control voltage will be input to SMA actuator. Those balance voltages are related to output forces, therefore, its impossible to get all balance voltages. In this thesis, some balance voltages at different output forces are acquired by experiments. Based on this data, a series of fuzzy rules are used to fit the curve of all balance voltages. Since the balance voltages line is not accurate, balance voltages are needed to be slightly adjusted near the fitting curve according to force errors. Based on the control strategy, a fuzzy controller, with two input variables-desired force and force error and one output-control voltage, is designed. Finally, for testing the performance of the proposed controller, an experimental prototype is implemented. Experiment results show that the proposed controller is successfully applied to SMA actuator, and have a better control performance than conventional PI controller.