Analytical modeling of selectively compliant mechanisms for quantifying the nano-scale parasitic motion is presented. Flexure-based compliant mechanisms are capable of meeting the demanding requirements of the partially constrained ultraprecision motion systems. However, the geometric errors induced by manufacturing tolerances can limit the precision capability. Understanding parasitic motion at the nano-scale necessitates a 3-D model even for mechanisms that are designed to be planar. A spatial kinematics based kinetostatic model is used here. This approach systematically accounts for the geometric errors, and enables estimation of the inherently spatial parasitic motion. Using insights from screw theory, the parasitic motion is classified into intrinsic mechanism errors, and errors that can be minimized by calibration procedures. A metric that quantifies the intrinsic parasitic motion and characterizes the precision capability of the mechanism is identified. Monte Carlo simulation is used to propagate the variance of the geometric errors through the model to determine the statistical moments of the chosen metric. To illustrate the approach, the modeling and analysis is applied to a classical four-bar mechanism with flexure joints. The model is further used to investigate the key system parameters that influence the intrinsic parasitic motion in the mechanism. The simulation results indicate more than 50% improvement in the precision capability of the four-bar mechanism by improved design of flexure joints, without changing the manufacturing tolerance limits.

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