Self-sensing characteristics and analysis of the stiffness of the SMA spring actuator

This research work is aimed to study the relationship between electrical resistance and stiffness of shape memory alloy (SMA) spring actuator. Self-sensing refers to the measurement of resistance of SMA along with the actuation of actuator. It is used to calculate its stiffness directly, instead of using a stiffness sensor. Despite investing numerous efforts in creating a reliable, robust self-sensing actuator, not much success has been achieved till now. This paper proposes a self-sensing for SMA spring actuator for stiffness measurement and its analysis. Voltage across SMA spring and known series resistance is used to establish the stiffness. Experiments are conducted to investigate the relationship between electrical resistance and stiffness of SMA spring actuator. To evaluate the characteristics between stiffness and resistance an experimental set up comprising of a steel spring biased SMA spring actuator is used. The SMA spring actuator is heated electrically with varying activation current and frequencies and then the corresponding stiffness is calculated from force and displacement. The polyfit model facilitates to replace the additional sensor and electronics for stiffness sensor. A qualitative agreement between simulated and experimental stiffness is observed. It provide scope for sensor less control by simplifying the control system. To make SMA more applicable to small scale robotic manipulations, its stiffness control using accurate self - sensing is best alternative.