Design and experiment of a compact wrist mechanism with high torque density

• A pitch–yaw wrist mechanism with high torque density and human-like appearance
• Two linear motors actuate a spherical mechanism through respective slider-cranks.
• Joint and link strengths have been verified to withstand high torque output.
• Torque density of 7.20 Nm/kg can be achieved with experiment validation.
• Control experiments showed that the wrist has a speed comparable to a human wrist.

This paper proposes a new wrist mechanism for robot manipulation. To develop multi-DOF wrist mechanisms that can emulate human wrists, compactness and high torque density are the major challenges. Traditional approaches use a series of rotary motors that require gearing to amplify the output torque. This often results in an oversized wrist. Alternatively, large linear force can be easily attained in a compact space by using leadscrew motors. Inspired by the muscle–tendon actuation pattern, the proposed mechanism consists of two parallel placed linear motors. Their translational motions are transmitted to two perpendicular axes of rotation through a spherical mechanism and two slider crank mechanisms. A high torque density can be achieved. Static and dynamic models are developed to design and analyze the wrist mechanism. Experiments of a wrist prototype are presented and the results are discussed. The novel mechanism is lightweight and has a humanlike appearance. It is expected to serve as a basis to develop humanoid manipulators for human-friendly interactions.