Modeling and electrical-resistance feedback control of SMA actuators adapting to load disturbance

Abstract

Ti-Ni based Shape Memory Alloy (SMA) actuators have been used for robots because of their high power-to-weight ratios, easiness of simple ON-OFF driving, and flexibility. In addition, SMA actuators enable simultaneous self-sensing and displacement control of themselves by feedbacking their electrical-resistance values. Modeling of SMA actuators for servocontrol is not easy due to their characteristics such as nonlinearity, hysteretic behavior, and effect of temperature and stress. Most of past studies have not considered minor-loops in the hysteresis or simultaneous variation of temperature and stress; both are necessary to be considered when achieving a robust robot control with SMA actuators. This study proposes a novel SMA model for electrical-resistance feedback control, which enables to adapt load disturbance and easy implementation. Especially, in order to consider the stress and temperature variation and minor-loops of hysteretic behavior in the relation of temperature and strain, electrical-resistance model and phase transformation models were improved by considering phase transformations between three crystalline structures: austenite, twined-martensite and detwined-martensite. Displacement, stress, temperature and volume function of each phase can be calculated from applied voltage and electrical-resistance value, which are easy to observe. Model parameters were identified by applying several general experiments to the actual system. Through the verification experiments, calculation results of the proposed model from observed electrical-resistance values were confirmed to agree with the experimental results under complex temperature and stress variation. Subsequently, an electrical-resistance feedback control system with the proposed SMA model was developed, and the system showed to control the displacement successfully to a constant value with load disturbance.