Design and Optimization of High-Performance Compliant Revolute Joints

Compliant mechanisms are mechanical systems that transmit motion through the deformation of flexible elements. Their elastic nature offers significant advantages over traditional rigid-body mechanisms. Among the most important benefits are high precision and, therefore, position repeatability. A compliant mechanism can be synthesized from a rigid-body system by replacing its rigid kinematic joints with flexible elements. The resulting mechanical system performs the same task of the original rigid-body mechanism; however, its motion deviates from that executed by the original rigid-body system. It follows that, even though they are highly precise, compliant mechanisms are inherently inaccurate and the source of inaccuracy can be localized at the flexible joints. The aim of this thesis is to establish, both qualitatively and quantitatively, criteria to evaluate accuracy for compliant revolute joints and to apply these criteria to the design of high performance flexural pivots. Firstly, the determination of the accuracy of planar flexures is addressed by analyzing and comparing the available criteria presented in the literature, including a novel criterion. The criteria are then applied to the design of a high-accuracy compliant joint by modifying the geometry and the relative position of the flexible elements of a well-known compliant pivot, namely the ”cross-axis flexural pivot”. The kinetostatic performances are firstly evaluated considering a pure moment load, then a procedure for designing the optimized pivot when actuated by cable-driven systems is proposed. Starting from the analysis of the weaknesses of the optimized pivot, a novel family of compliant joints based on a three-flexure arrangement is designed. Their kinetostatic performances are compared with the ones of the traditional cross-axis flexural pivot, i.e. prior the geometrical modifications. Two different arrangements, determined by the constraints layout, are proposed and two configurations are used for the design of two compliant mechanisms. The position and rotation accuracy of the compliant systems implementing the triple-axis pivots are then compared with those of the same compliant systems implementing the traditional cross-axis flexural pivots. Throughout the study, the effects of variations of the geometrical parameters on the kinetostatic performances of the pivots are investigated using finite element analyses and the chained beam constraint model. Several design maps are generated to illustrate the behaviour of performance indexes for various combinations of the geometrical parameters. The design maps serve as key tools for the design process, providing insight into the antagonistic behaviour of different kinetostatic features and leading to the identification of optimal trade-off solutions that meet the application requirements. A global performance index, that concurrently takes into account accuracy, rotational stiffness and range of motion, is also introduced. Experimental campaigns were conducted to verify the performance improvement of the enhanced curved cross-axis pivot and the novel triple-axis pivots with respect to the traditional cross-axis pivot.