Trajectory Optimization for Thermally-Actuated Soft Planar Robot Limbs

Practical use of robotic manipulators made from soft materials will require planning for complex motions. We present the first approach for generating trajectories of a thermally-actuated soft robotic manipulator. Based on simplified approximations of the soft arm and its shape-memory-alloy (SMA) wires, we justify a dynamics model of a discretized rigid manipulator with joint torques proportional to wire temperature. Then, we propose a method to calibrate this model from hardware data, and demonstrate that the simulation aligns well with a test trajectory. Finally, we use direct collocation trajectory optimization with the non-linear dynamics to derive open-loop controls for feasible trajectories that closely align with desired reference inputs. Two example trajectories are verified in hardware. The results show promise for both open-loop planning as well as for future applications with feedback.