Rise time based characterization of sub-millimeter SMA helical actuator for sensorless displacement estimation

Bio mimicking micro/miniature robots require design aesthetics, simplicity, low power, lower computational requirement, resilient operation and repeatability. The main choice of actuators in such systems are Shape Memory Alloys (SMA). The larger strain and reduced size of the SMA sub millimeter diameter helical springs make them a potential choice in such systems. Sensorless position/ displacement control of these type of SMA spring actuators have been studied extensively due to the difficulty in physically attaching feedback sensors, which are larger than SMA actuators. For position estimation, electrical parameters like Inductance and Resistance were considered in the past. This paper presents a new approach based on the pulse response i.e. Rise time variation along the actuation of SMA spring. Considering SMA spring as an RL element, a characterization technique is proposed for sensorless position estimation based on rise time variation. The proposed method is independent of ambient temperature variations. Investigations have shown that rise time based position control through PWM based current drives can be implemented for manipulating displacement without a sensor.