Design And Optimization Compliant Large-stroke Constant-force Mechanisms Of XIDIAN UNIVERSITY

Unlike the traditional rigid mechanism,the compliant mechanism does not rely entirely on the joint movements to achieve all the movements.It mainly utilizes the deformation of a compliant structure's flexibility to achieve the transfer and transformation of motion,force,and energy.The use of a compliant mechanism in designing a constant force mechanism can effectively reduce the clearance,friction,and wear in the mechanism and improve the performance of the mechanism.At present,the insufficiency of constant-force mechanisms research is mainly reflected in issues such as too small constant-force region and excessive volume.In this paper,three kinds of large-stroke constant-force compliant mechanisms were designed for these problems: the constant-force compliant Sarrus mechanism,the stiffness-combination constant-force compliant mechanism and constant-torque compliant joint.The main work is as follows:(1)Introduce the designs and modeling methods of constant-force mechanisms.The designs are divided into four kinds of conventional spring-energy-storage constant-force mechanism and four types of flexural-beam-energy-storage constant-force mechanism.Their working principles,design methods,modeling methods,advantages,disadvantages and applications are introduced.When discussing the flexural-beam-energy-storage constant-force mechanism,two universally applicable constant-force mechanisms are discussed in detail,namely the stiffness-combination constant-force mechanism and the curved-beam constant-force mechanism.(2)The constant-force characteristics of the Sarrus mechanism using a new type of flexural pivot were studied.A rigid body replacement method is used to replace some(or all)rigid pivots in a rigid Sarrus mechanism with a new type of flexural pivot,and the constant-force output is achieved by the energy superposition of the flexural pivots.Firstly,a new cross-spring flexural pivot with large range of motion,small shaft drift,and strong bearing capacity was selected as the compliant component of the compliant Sarrus mechanism by comparing several kinds of flexural pivots;the cross-spring flexural pivot was modeled to be able to describe the nonlinear stiffness in the large-angle state,the empirical moment calculation formula is used;then the empirical formula is used for the kinetostatic modeling of the Sarrus mechanism,and the relationship between the output force and each structural parameter is obtained;the particle swarm optimization is used to optimize the structural parameters of the compliant Sarrus mechanism.Finally,the correctness of the theoretical model is verified by finite element simulation and experiments.(3)Based on the idea of stiffness superposition,an adjustable stiffness-combination compliant constant-force mechanism was designed.A compliant bistable mechanism based on a fixed-guide beam was chosen as a negative stiffness mechanism.It was modeled and optimized using Chained Beam Constraint Model.Then the configuration of a positive stiffness mechanism was determined.The modeling and optimization were carried out.After that,the positive and negative stiffness mechanisms were combined to obtain the adjustable constant-force mechanism.Finally,the correctness of the theoretical model is verified by finite element simulation and experiments.(4)A method of designing distributed-compliance constant-torque flexural pivots is proposed.Firstly,a kind of negative stiffness mechanism was designed utilizing the leaf flexures.Then a six-segment Chained Beam Constraint Model was used to model the negative stiffness flexural pivot.The structural parameters of the negative stiffness flexural pivot were optimized,and a set of structural parameters with better negative stiffness characteristics was obtained;A new type of cross-spring flexural pivot was used as a positive stiffness mechanism.The positive and negative stiffness flexural pivots were combined to obtain the constant-torque characteristics.After that,the finite element verification was performed on the positive stiffness flexural pivot and negative stiffness flexural pivot respectively.Finally,a prototype was fabricated and tested.Although the test results and the theoretical analysis results were different,the variation trend was basically consistent.