Variable Stiffness Robotic Hand Driven by Twisted-Coiled Polymer Actuators

This letter proposes a variable stiffness robotic hand driven by twisted-coiled polymer actuators (TCAs) connected in series with a servomotor. Unlike slow conventional TCA systems, the grasping motion of the proposed hand is realized by the servomotor, which improves the speed of actuation significantly. It was shown that by using the servomotor, the rise time and settling time in set-point regulation control were $\text{0.1}\;{\mathrm {s}}$ and $\text{0.33}\;{\mathrm {s}}$, respectively, which were significantly improved compared with those results of 3.26 s and 12.40 s using TCAs only. Since the stiffness of TCAs can be changed by varying their temperature, the stiffness of the hand can be realized by changing the temperature of the TCA. By applying model predictive control (MPC) and active cooling, the temperature of the TCA can be controlled to follow a sinusoidal waveform at a frequency of 0.1 Hz with an average steady-state error of 0.9 Celsius degrees. The proposed robotic hand can grasp a load of 80 g and approximately achieve a 5-fold variable stiffness. In addition, the fingers of the hand with soft joints are fully made using flexible thermoplastic polyurethane (TPU), therefore reducing the need for complex assembly processes.