Massachusetts Institute of Technology
A new approach to the design and control of shape memory alloy (SMA) actuators is presented. SMA wires are divided into many segments and their thermal states are controlled individually as a group of finite state machines. Instead of driving a current to the entire SMA wire and controlling the wire length based on the analogue strain-temperature characteristics, the new method controls the binary state (hot or cold) of individual segments and thereby controls the total displacement proportional to the number of the heated segments, i.e. austenite phase. Although the inherent property of SMA is highly nonlinear and uncertain with a prominent hysteresis, this Segmented Binary Control (SBC) is robust and stable, providing characteristics similar to a stepping motor. Three major aims of the projects are: An efficient method for improving speed of response and power consumption is developed by exploiting the inherent hysteresis of SMA. Instead of keeping high temperature continually, the temperature is pulled back to an intermediate "hold" temperature that is substantially lower than the Austenite Finish (Af) temperature but is high enough to keep the austenite state. Coordination of the multitude of segments having independent thermal states allows for fast response with zero latency time even for thick SMA wires. The segmented architecture of SMA wires is extended to a multi-axis actuator array by arranging them in a two-dimensional array. The multi-axis control is streamlined and coordinated using a two-dimensional segmentation method in order to activate multiple links of a robot mechanism in a coordinated manner. The method is applied to a five-fingered robotic hand capable of taking a variety of postures with a 10-axis SMA actuator array using Peltier effect thermoelectric devices for selective local heating and cooling of SMA wires. An inter-departmental research team of undergraduate students (from mechanical, electrical, and materials engineering departments) will be formed to conduct this interdisciplinary project. One doctoral student specializing in robotic actuators will bring the new actuator technology and biologically inspired robots to a new undergraduate robotics course.
