Precise Control for Intrinsically Sensing Soft Robotic Tentacle With Free-Stroke TCA

Twisted and Coiled Actuators (TCAs) are promising in soft robotics for their high energy density, light weight, and low voltage. However, current TCA-based soft robots face challenges of limited deformation and control precision, mainly due to the preloading requirement of TCAs and the lack of suitable intrinsic sensing capabilities. To address these issues, we designed a TCA with high load capacity, free stroke, and self-sensing capabilities, proposed flexible optical fiber-based posture and tactile sensing methods, and developed a multi-loop feedback controller. Collectively, these enable millimeter-level tracking accuracy in a soft tentacle robot. The TCA, with an optimized manufacturing process, achieves a 30% free stroke without preloading, a 32% improvement in ultimate stress, and temperature self-sensing capabilities with a maximum error of less than 6%. Combining the TCAs with compliant macro-bend optical fibers and soft optical waveguides, we created a soft robotic tentacle with intrinsic posture and tactile sensing, and designed a multi-input multi-output closed-loop and feedforward controller. Experiments demonstrate that the model-based feedforward significantly improves the control performance, reducing the rise time by 15.3%. The trajectory tracking error remains within the millimeter range, and the repetitive positioning error for hexagonal trajectories reaches sub-millimeter precision. The tactile sensor of the robot enables real-time perception of the object's modulus and pressing states. These findings highlight the soft robotic tentacle's potential for various applications, including underwater exploration, detection, and sampling.