Stiffness Analysis and Verification of Hydraulic Supporting Units for In-Situ Optical Testing of a 500 mm-Diameter Mirror

Abstract

A hydraulic in-situ support system is commonly used in the optical testing of mirrors, since it is convenient to unload the gravity of a mirror to be measured without the risk of being turned over or moved to another place. The existing supporting structures have several disadvantages, such as the problem of the output force deviating from the axis, being sensitive to machining loads, its flexible components easily leading to fatigue damage by cyclic loads and so on. A new single-cylinder hydraulic supporting unit with a ball hinge was proposed, analyzed and verified in this paper. A finite simulation based on four structural parameters’ effect on the stiffness of the proposed hydraulic supporting unit showed that increasing the thickness, elastic modulus and convolution width of the rolling diaphragm and decreasing the height of the rolling diaphragm to some extent was beneficial to a high stiffness. Moreover, it could be concluded from experiments that, in order to decrease the stiffness dispersion, the air ratio should be as low as possible and the values of the initial pressure and press speed should be as high as possible. These results are conducive to maintain a high stiffness of HSU to bear the processing load and reduce the low-order aberrations of mirror which provide a reference for future hydraulic supporting unit designs.