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Research Papers

A Closed-Form Nonlinear Model for the Constraint Characteristics of Symmetric Spatial Beams

[+] Author and Article Information
Shorya Awtar

e-mail: awtar@umich.edu
Precision Systems Design Lab,
Mechanical Engineering,
University of Michigan,
2350 Hayward Street,
Ann Arbor, MI 48109

Uniform implies a non-varying cross- section along the beam length. Symmetric implies equal moments of area of the beam cross-section about the Y and Z axes.

Slender generally implies a length to thickness ratio greater than 20 [12,13].

1Corresponding author.

Contributed by the Mechanisms and Robotics Committee of ASME for publication in the Journal of Mechanical Design. Manuscript received March 1, 2012; final manuscript received October 25, 2012; published online January 24, 2013. Assoc. Editor: Ashitava Ghosal.

J. Mech. Des 135(3), 031003 (Jan 24, 2013) (11 pages) Paper No: MD-12-1139; doi: 10.1115/1.4023157 History: Received March 01, 2012; Revised October 25, 2012

The constraint-based design of flexure mechanisms requires a qualitative and quantitative understanding of the constraint characteristics of flexure elements that serve as constraints. This paper presents the constraint characterization of a uniform and symmetric cross-section, slender, spatial beam—a basic flexure element commonly used in three-dimensional flexure mechanisms. The constraint characteristics of interest, namely stiffness and error motions, are determined from the nonlinear load–displacement relations at the beam end. Appropriate assumptions are made while formulating the strain and strain energy expressions for the spatial beam to retain relevant geometric nonlinearities. Using the principle of virtual work, nonlinear beam governing equations are derived and subsequently solved for general end loads. The resulting nonlinear load–displacement relations capture the constraint characteristics of the spatial beam in a compact, closed-form, and parametric manner. This constraint model is shown to be accurate using nonlinear finite element analysis, within a load and displacement range of practical interest. The utility of this model lies in the physical and analytical insight that it offers into the constraint behavior of a spatial beam flexure, its use in design and optimization of 3D flexure mechanism geometries, and its elucidation of fundamental performance tradeoffs in flexure mechanism design.

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Figures

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Fig. 1

Spatial beam flexure—undeformed and deformed

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Fig. 2

A 3 DOF patial flexure mechanism

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Fig. 3

Spatial kinematics of beam deformation

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Fig. 4

Elastic matrix [H1]

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Fig. 5

Kinematic matrices [H2], [H3], and [H7]

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Fig. 6

Elastokinematic matrix [H4]

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Fig.7

Elastokinematic matrix [H5]

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