Synthesis and Optimisation of Large Stroke Flexure Hinges

Abstract

Flexure hinges are advantageous for use in high-precision applications because of their lack of hysteresis, friction and backlash. However, their range of motion is limited due to increasing stresses and a decreasing support stiffness at large strokes. Currently available hinges are typically designed for strokes of up to $10^\circ$ and only a small number of hinges achieve a stroke of $40^\circ$. In this paper we present hinge concepts with a stroke up to $90^\circ$. On the one hand, the conceptual design of such hinges is addressed and, on the other hand, a method to optimise the design of such hinges is presented. The hinge performance is measured by evaluating the natural frequency of the second relevant eigenmode of the hinge with a specific load. For best performance the minimal value for this frequency throughout the full operating range is maximised. Analysing known hinge concepts in view of this criterion gives insight in the strengths and weaknesses of these hinges. This knowledge helps to improve the hinge layout. Also the possibility of stacking multiple hinges is considered. To find the optimal geometric parameters of a given design, a numerical optimisation routine is formulated and implemented, making use of the tools present in the ANSYS finite element program. Two types of conceptual hinges are synthesised and optimised for a range of ${\pm }45^\circ$. Their behaviour is studied and compared to a commonly used three flexure cross hinge that is also optimised for this large stroke. A significant performance gain is found for the new hinge concepts. The paper shows that it is possible to design flexure hinges for large strokes of up to $90^\circ$. And, more importantly, a method for analysing their performance and optimizing their geometry has been established. The method allows for a quick assessment and optimisation of future large stroke flexure hinges.