Functional joint mechanisms with constant-torque outputs

This paper presents a type of functional joint mechanism with constant-torque outputs. Unlike torsional springs, whose torque increases as rotation increases, a constant-torque joint mechanism (CTJM) provides a nearly constant torque over a specific rotation interval. Instead of using sensorized control, CTJMs passively maintain a constant torque. Potential applications include dynamic and static balancing of machines, human joint rehabilitative devices, and human mobility-assisting devices. To meet practical needs, a CTJM should have a large constant-torque region with sufficient flatness. We propose lumped-compliance models and distributed-compliance models for designing a CTJM. For both models, design formulations are given, with results discussed and compared. The prototypes are fabricated based on the distributed-compliance models and are verified by comparing with finite element methods. Effects of modeling, dimension, and material variations on CTJMs are investigated. Guidelines are given for designing CTJMs of various sizes and torque magnitude. Experiments study the torque-to-rotation curves of using different materials. Their resistances to hysteresis and stress relaxation are compared.

► Torque is passively maintained at a constant level without using control. ► Various lumped-compliance models are first presented and analyzed. ► Using distributed-compliance model, a 70° constant-torque region is obtained. ► Constant-torque curve is insensitive to material modulus and dimension variation. ► Certain polymer materials are suggested to resist hysteresis and stress relaxation.