Design and optimization of a decoupled RP flexure joint for an adjustable-motion-direction compliant mechanism

Abstract

Callus deformations, including amplitudes and directions, greatly impact bone healing. Many existing devices can hardly adjust the callus-deformed directions since their unexpected deformations are not constrained properly. This study presents a way to adjust the callus-deformed direction by suppressing unexpected callus deformations. This is realized by designing a decoupled revolute-prismatic(RP) flexure joint and integrating it into an adjustable-motion-direction compliant mechanism (AMD-CM). This decoupled RP flexure joint is designed by serially combining a 3-leaf cross flexure joint and a parallelogram flexure joint according to their eigenstructures. The decoupled property eliminates the rotation axis misalignment and suppresses unexpected callus rotational deformations. First, eigencompliance analysis of decoupled RP flexure joints is performed and shows that the rotation axis misalignment is eliminated. Second, the AMD-CM’s pseudo-rigid-body model is developed, and the constant translational stiffness condition is found. Then, optimization is performed to obtain large compliance ratios and constant translational stiffness. Finite element analysis shows that by using the optimized decoupled RP flexure joint, the callus-deformed direction can be adjusted. The unexpected callus rotational deformation is also reduced greatly compared to previously proposed devices. Finally, experiments show that the optimized decoupled RP flexure joint can realize an adjustable callus-deformed direction.