A Single-Actuated Cable-Driven Robotic Hand Designed for Adaptive Grasps

Developing a dexterous robotic hand that mimics the human hand natural movements is challenging due to the complicated hand anatomy. To design a practical robotic hand several requirements should be addressed, including affordability, portability, light weight, self-containment (meaning the hand itself contains all the mechanical and electrical components), dexterity, and the ability to provide a safe, powerful, and robust grasp. These are often conflicting requirements forcing the designer to prioritize the main design characteristics for a given application. Therefore, in the existing designs the requirements are only partially satisfied, leading to complicated, bulky, and expensive solutions. To address this gap, a novel single-actuated, cable-driven, and self-contained robotic hand is presented in this work, which satisfies the abovementioned practical requirements.

In the proposed design, all the requirements are satisfied using anthropomorphic, underactuated, and compliant design. An anthropomorphic design is presented by focusing on the joint and tendon structure of the hand anatomy. The robotic hand covers 19 degrees of freedom (DOFs) out of the 20 DOFs for the human hand. The designed hand is cable-driven, wherein the cable configuration for the thumb and fingers is inspired by the tendon structure of the human hand. Combined with the specialized phalanges’ structure, this cable configuration ensures that the robotic hand can reproduce natural finger movements.

The proposed design is highly underactuated, where only one motor is used to actuate the robotic hand. A combination of differential mechanisms is designed to distribute the actuation force evenly between the engaged joints, which significantly reduces the number of required actuators. Moreover, the proposed mechanism can provide an adaptive grasp, which means the fingers can complete the grasp regardless of the object’s geometry. More specifically, if one of the fingers is blocked by an object, others can continue bending until the grasp is completed. Also, a slider-crank linkage mechanism is utilized to actuate the thumb and synchronize its movement with the fingers. While a single-actuated mechanism does not allow for each joint or finger to be controlled separately, it provides the opportunity to place all the components in the palm of the robotic hand and present a compact, lightweight, and self-contained design. Furthermore, an important goal is to ensure a stable grasp with minimum gripping force at the fingertips, which is achieved by the proposed design.

In addition to the above considerations, a compliant design is achieved by using passive elastic elements, which are added to ensure a robust and safe interaction with the environment. These elements are used instead of mechanical joints to keep the phalanges together and provide a simpler design. Moreover, the elastic elements are mounted on the dorsal side of the fingers to passively drive the extension movement of the thumb and fingers.

The functionality of the proposed design is assessed using a Simscape (Simulink/MATLAB) simulation and a 3D printed prototype. Power, pinch, and tripod grasps are the most common grasp postures, which are successfully evaluated through the simulation and the experiments. Moreover, using the Simscape simulation, the adaptive behavior of the proposed mechanism is evaluated by blocking each finger and studying other fingers’ movement. It is shown that the robotic hand can provide a robust grasp for a wide range of object geometries despite of its simple design. Furthermore, the simple design and small number of parts help with easier and faster fabrication and assembly process.