Compliant Constant Output/ Input Force Mechanisms- Topology Optimization with Contact

Abstract

We synthesize monolithic topologies of constant output (CoFM) and input (CiFM) force mechanisms. During synthesis, we capture all possible aspects of member deformation including finite displacements, buckling, interaction between members, their interaction with external surfaces, and importantly, interaction of the mechanism with flexible workpieces to capture force transfer in true sense. Features of constant force characteristics, e.g., magnitude(s) of the desired force(s), range of input displacement over which slope of the force-displacement curve is near zero, and distance between workpiece and the mechanism are controlled individually via novel objectives proposed herein.Two of the constant output and constant input force mechanisms each, are synthesized using stochastic optimization ensuring ready manufacturability. We observe that presence of external surfaces may not be required for singlepiece mechanisms to attain constant force characteristics. However, interesting solutions are possible if mutual contact is permitted. We also note that desired force characteristics may not remain the same with alteration in the workpieces' shape and/or material properties. We finally fabricate and test the synthesized mechanisms and find that the desired constant force characteristics are by-and-large retained.CoFM henceforth), need for force feedback gets reduced/eliminated which makes it cost effective and easy to use, without the requirement of sensors and control systems. Some conventional (rigidlink with springs) and compliant CFMs are described in [19-21] and [22-27] respectively. Wang and Xu [28] provide a detailed survey, highlighting advantages and pitfalls of five types of conventional and compliant CFMs each. Compliant mechanisms are preferred over rigid-link ones due to inherent advantages of no friction, no backlash or need for lubrication, ease of assembly and miniaturization.Thus, compliant CFMs are desired over conventional ones. Two design approaches can be employed for monolithic CFMs, just as for generic compliant mechanisms -for instance, those in [1] and [9] are designed using the Pseudo-Rigid-Body Model (PRBM) approach whereas Topology Optimization (TO) is adopted in [2].Compliant constant input force mechanisms (CiFMs), e.g., [29] can also find a variety of applications, especially if the actuation force required is ideally zero over a range of input displacement.Design methods for, and realization of the latter class of CiFMs, termed statically balanced compliant mechanisms (or SBCMs), have gained significant attention in recent years [30][31][32][33][34][35][36][37][38][39]. Static balancing is achieved by ensuring that the (potential) energy stored within the deforming continuum is constant [32] so that both, the actuation force 1 and stiffness 2 are zero over a range of input displacement [40]. Synthesis approaches for SBCMs employ the Pseudo-Rigid-Body approach, e.g., [1], stiffness compensation using building blocks, e.g., [32] and topology optimization, e.g, [40]. A compliant constant o...