High Frequency Agonist-Antagonist Controller for Shape Memory Alloy Actuators

Soft robotics still requires fundamental advances in sensor and actuator technologies to achieve its full potential. One of the main challenges is the development of effective control strategies for shape memory alloy (SMA) antagonist systems, where coactivation of agonist and antagonist actuators can generate mechanical forces that compromise the integrity of the system. This study presents the Asymmetric Agonist-Antagonist (A3) controller, which is designed to improve safety by preventing the simultaneous heating of both SMAs. The A3 controller employs two activation functions: Pedestrian Safety, that sets an activation offset as a function of position error; and Adaptive Gradient, that attenuates control signals when they are opposite to their primary direction of motion. Comparative tests with passive recovery and traditional PID control show that the A3 controller reduces position error, achieves smoother transitions, and eliminates abrupt control switching between SMA actuators. By mitigating opposing forces, the A3 controller enables safer SMA operation at higher frequencies, allowing these actuators to be integrated into new types of soft robotic applications. This approach represents a critical step toward robust and scalable SMA control, enabling further research into soft actuators.