Iterative optimization to enhance effective compliance for design of large stroke constant torque mechanisms

Compliant constant torque mechanisms (CTMs) offer a novel approach to maintain torque levels with their nonlinear torque rotation relation in a monolithic design. Current CTM designs have limited stroke (20 °-70 °) and coefficient of variation (COV) (±4%). Complex geometry and ineffective compliant members together reduce stroke, limiting CTM performance. This study proposes an iterative optimization approach to enhance stroke by identifying and eliminating ineffective members having low relative strain energy. Using design methods from constant force mechanisms (CFMs), our methodology employs the graph method with nodal perturbation to efficiently introduce more slenderness and tortuosity into the geometry. Through iterative optimization and ineffective member elimination in CTM design, we achieved extended stroke ranges of 20 °-130 ° and maintained COV of ±2%, overcoming limitations of traditional CTMs. The experimental results show a 30 °-139 ° stroke range and ±1.6% COV. Successfully doubling stroke performance potentially broadens CTM application in fields requiring large stroke, such as rehabilitation devices and robotic joints.