Design of robotic finger using twisted string actuator with modular passive return rotational joints to achieve high grasping force: Application to wearable sixth finger

In this paper, a new type of robotic finger is introduced that uses a twisted string actuator (TSA) and modular passive return rotational (PPR) joints. The design is intended to be simple, compact, lightweight, and energy-efficient while producing high grasping force with a relatively small motor. The PPR joints are based on the beam-buckling principle and are designed to match the non-linear TSA force profile, resulting in high grasping force throughout the finger's full flexion motion and passive finger extension. To evaluate the performance of the robotic finger, we fabricated a prototype and conducted experiments to assess its object grasping cycle, passive finger extension, grasping force, stable grasping condition, shape adaptability, and energy consumption. The finger weighs 170 grams and achieved a high force throughout the flexion motion, producing a maximum grasping force of 43.3 N at full flexion using a stall torque of 32 mNm. The modularity of the PPR joint allows for scalability and adaptability to handle different objects. We also demonstrated the finger's potential as a wearable sixth robotic finger (SRF), evaluating its object grasping competency, shape adaptability, and wearability. The finger was able to grasp various objects with a maximum payload of 1.0 kg and a hanging payload of up to 5 kg. Overall, the proposed robotic finger has the potential to be used as an SRF to compensate for arm disorders’ grasping capability.