A Small Parallel Manipulator for the Active Alignment and Focusing of the Secondary Mirror of the VLTI ATS

摘要

The paper describes the mechanism made for the 140-mm secondary mirror of the VLTI Auxiliary Telescopes. This mechanism consists of an hexapod parallel manipulator, capable of orienting its mobile base along all six degrees of freedom by means of six independent linear actuators. The six linear actuators are attached to a base plate along the vertices of a regular hexagon. Each actuator pushes/pulls from the base plate an oblique leg to which it is linked by a flexible pivot. At the other end, the six legs are linked pairwise by flexible pivots to three points of the mirror cell, which can therefore be displaced to any position and angle by the combined motion of the six actuators. An advantage of this configuration is that the vertical motion (used to procure the focus adjustment of the telescope) is obtained by a direct 1:1 action of all six actuators, without any coupling nor intervening lateral forces. Each actuator consists of a closely integrated system comprising a motor, a resolver (for the angular incremental measurement), a high precision screw with planetary rollers, and a bearing. A LVDT position detector provides the absolute position of each actuator. A standalone multi-axis digital controller board controls the hexapod.