University of Twente Student Theses


Design, implementation and validation of a viscoelastically overconstrained flexure mechanism for increased dynamic performance

Stevens, F.H. (2023) Design, implementation and validation of a viscoelastically overconstrained flexure mechanism for increased dynamic performance.

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Abstract:Due to their beneficial properties, flexure-based setups are typically used for high-precision applications. Benefits include a lack of hysteresis, friction and play, leading to excellent repeatability. Typically, these systems are designed to be exactly constrained, since overconstraining degrees of freedom can cause unwanted system behaviour due to stresses in the system. Misalignments, differences in thermal expansion coefficients, play and tolerances can cause these stresses. In theory, if additional flexures can be added without the introduction of internal stresses, better system performance can be obtained in the form of symmetric designs and higher support stiffnesses. In the case of complex systems, it is near impossible to assemble a system in a manner where little to no stress is introduced, which causes significant performance degradation. To provide a simple yet effective solution to eliminating the undesired effects of internal stress, a viscoelastic material is added to the base of flexures. This allows for the overconstrained design of setups without the requirement of perfect assembly. Viscoelastic materials provide high support stiffness at the parasitic frequency and allow for the relaxation of the stresses introduced in the assembly of the system. The performance of a viscoelastically overconstrained system can be assessed in two ways. The support stiffness at the parasitic frequency and the tolerance for misalignment. Based on the geometry of the elastomer, these performance indicators can be adjusted for the requirements. Optimization of these parameters is performed using the flexible multibody software SPACAR and is compared to exactly constrained and overconstrained setups to assess the performance. Simulations are verified using an experimental setup. Differences in critical buckling misalignment and overall lower parasitic frequencies are found. These can be explained due to parasitic compliances and SPACAR model simplifications. Better modelling of the parasitic frequency can improve the model even further since the chosen rubber was optimized for a stiffer setup. With these differences taken into account, the behaviour of the system is improved by adding a viscoelastic material and it shows benefits compared to both exactly constrained and overconstrained designs. The parasitic frequency using an elastomer is increased by 5.4% compared to an exactly constrained setup. This is measured to be 96.7% of the parasitic frequency in case of one overconstraint. The critical misalignment could not be measured for the elastomer, which means that it is at least a range of -50 to 50 mrad, compared to -8 to 8 mrad for the overconstrained setup.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:52 mechanical engineering
Programme:Mechanical Engineering MSc (60439)
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