The Nonlinear stress based switching control for shape memory alloy actuators

Shape Memory Alloy (SMA) actuators are an effective way to reduce weight and to minimize the complexity of various systems including biomedical and more specifically. Prosthesis systems. As actuators, these materials have been used in a wide range of applications from thin film actuators to helicopter rotor actuators [1]. SMA actuators provide an interesting alternative to conventional actuation methods. Their advantages create a means to drastically reduce the size, weight, and complexity of assistive and rehabilitation devices. The actuation of shape memory alloys is due to phase transformation from Austenite to Martensite phase or vice versa. It is important to recognize the co-dependent relationship between stresses, phase transformation, and transformation temperature, for it is this complex nonlinear relationship that causes many difficulties with regards to control design for SMA actuators. Because of these nonlinear relationships, it is not expected that a linear controller such as a PID controller can demonstrate an acceptable control performance for SMA systems. Since the transformation temperatures of the SMA wire are stress dependent, the stress of the wire is a better set-point for the controller. In a rotary actuator, a sliding mode controller can be designed to calculate the desired stress of the SMA wire based on the desired angular position. Realistic dynamic model [2] and extended Kalman filter [3] have been used to estimate the stress of the wire for such a controller. Performance of the sliding mode controller has been compared to that of a PID controller in Figure 1. The simulation results of the PID controller shows that the actual angle reaches the desire angle quickly, but the controller cannot track the trajectory. The sliding mode control on the other hand has a longer reach time and a better trajectory following performance. The reason for the delay in time is that while the temperature of the wire is less than Martensite starting transformation temperature, no actuation takes place. Therefore, to shorten this delay time it is important to apply a high voltage first and when the transformation begins, the controller takes its role. Based on this observation, in this paper a nonlinear switching control is proposed which can reach the desired angle fast and also track the trajectory effectively. In the proposed controller a PID and a sliding mode controller are working together under a main switching criterion. This criterion is based on 'S' value (the sliding surface). When the system is …