A novel model-based approach for resistance estimation using rise time and sensorless position control of sub-millimetre shape memory alloy helical spring actuator

Shape memory alloy shows considerable strain during heating and cooling. This effect is due to its phase transformation with temperature. Due to this property, shape memory alloys can be deployed for physical actuation in place of conventional actuators in bio-medical and bio-mimicking robots. Sub-millimetre diameter shape memory alloy wires wound as helical springs are also used for this purpose. Due to their small size, it is difficult to use sensors for temperature or displacement measurements of shape memory alloy springs. This article attempts to demonstrate that the rise time of the current through a sub-millimetre diameter shape memory alloy helical spring is directly proportional to its displacement. To characterize the rise time-displacement hysteresis, a constant current drive with overcurrent protection is developed. The data generated are utilized to implement an open-loop sensorless control. A method to estimate the resistance from the rise time is proposed with which the temperature of the shape memory alloy during actuation can be obtained. The design avoids using an analogue-to-digital converter for the direct measurement of voltage and current for measuring the resistance variation in the shape memory alloy under actuation. This helps in the development of a new sensorless control using only the digital Input/Output pins of a microcontroller/microprocessor.