Scaling up shape memory alloy actuators using a recruitment control architecture

This paper presents new experimental results from a human-size robotic forearm, created to demonstrate the effectiveness of recruitment-based control architectures for large actuators made from shape memory alloys (SMA) and other active materials. The robot arm is actuated antagonistically by two actuators made up of 60 small SMA springs arranged in parallel, which are activated in an on/off fashion using Joule heating. The force and stiffness of each actuator is controlled by recruiting a desired number of springs to contract. The joint position is then controlled using equilibrium point servo control. The results presented in this paper show that the combination of equilibrium point control of the arm joint and recruitment-based control of each actuator's stiffness solve some of the major problems of scalability and response speed often associated with active material actuators.