Flexure-based selectively compliant mechanisms with less than six degrees of freedom are capable of meeting the demanding requirements of ultra precision positioning and scanning systems. However, machining imperfections induce undesirable motion and limit the mechanisms precision capability. A spatial kinematics based kinetostatic model is presented here that not only enables determination of inherently spatial parasitic motion due to machining imperfections, but also offers critical geometric insight into the motion characteristics of flexure mechanisms. The analytical development reveals that the geometric errors induced by machining imperfections perturb the special screw systems of motion of ideal flexure mechanisms to their corresponding general screw systems. This insight leads to clearly defined metrics that can capture the non-ideal behavior using screw system theory and is applicable to all selectively compliant mechanisms. This result is illustrated using one and two DOF mechanisms as examples. In the case of rotational DOF flexure mechanisms, the pitch of twist of motion captures the difference between the special and general screw systems and represents the intrinsic parasitic motion. The machining imperfections are regarded as Gaussian random variables with known variance, and the model is used to determine the variance of the pitch of twist via Monte Carlo simulation, leading to determination of the precision capability of the flexure mechanisms. The modeling and analysis is illustrated using one and two DOF rotation flexure mechanisms. Finally, the details of a test setup built to determine the parasitic motion of the one DOF rotational mechanism are presented. Experimental results indicate that the one DOF flexure mechanism is indeed executing screw motion rather than pure rotation.

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