Achieving high-quality and large-stroke constant torque by axial force release

Abstract

Compliant constant-torque mechanisms (CCTMs) maintain constant-torque without the need for complex closed-loop feedback systems, broadening their applications in rehabilitation devices, surgical tools, and cooperative robotic arms. However, CCTMs present considerable design challenges due to the pronounced nonlinearities that arise due to large deflections and multi-axial loadings. Traditional CCTM design strategies focus on managing post-buckling phenomena, often leading to increased stresses and an imbalance in positive and negative stiffness, compromising torque consistency and stroke capacity. This study introduces a novel CCTM that effectively decouples the multi-axial loadings and releases axial forces, isolating beam bending forces. This decoupling is achieved by incorporating a parallel-guided compliant mechanism at the fixed end of the beam, which reduces stress and enhances torque stability throughout the operational range. Through the partical swarm optimization of geometric design parameters using the chained beam constraint model, this research has produced a CCTM capable of maintaining torque fluctuations below 0.39% over a rotational range of 18° to 68°. Experimental validations confirm the design’s superiority in providing an extended constant torque stroke and improved consistency, distinguishing it from conventional straight-beam CCTMs.