Electronic instrumentation for the characterization of a rotary thermoelectric motor driven by shape memory alloy springs

Abstract

The possibility of using Shape Memory Alloys (SMA) has been currently studied in development of actuators and thermoelectric motors. Generally, these devices use the SMA characteristics of high strain or force capability to produce the displacement and, in this case, the displacement is produced by supplying heat to the actuator element. Their applications are still limited to low speed, but, with the possibility of high torques. When they are used as a thermoelectric motor, the development of a suitable instrumentation becomes indispensable to drive and measure the electrical parameters and the motor motion. Based on that need, this paper presents the electronic instrumentation to characterize a thermoelectric motor grounded on shape changes in shape memory alloys. The prototype of the motor has been built using mini SMA springs as active elements. The geometry of the motor and the spring support base have been developed in ABS (Acrylonitrile Butadiene Styrene) polymer. A current amplifier circuit was designed to generate heat and, consequently, produce the motor displacement. The circuit was switched for pulse width modulation (PWM), which has produced the heating by Joule effect. The image technique has been used with a camera placed on the rotor shaft and the SMA actuator, for the strain and displacement measurement. A load cell has been used for the force measurement and a thermocouple for the temperature measurement, both properly conditioned. The electrical current and the potential difference have been measured on it, and so, the electrical resistance variation of the actuator was obtained. Thus, in order to consolidate the thermoelectric motor characterization, results of electrical resistance, force and displacement are presented as a function of heating temperature on the actuators. Furthermore, results of force related to strain and the motor shaft rotation in the time domain are also shown.