A mass and vibration optimised solution for 6D precision heavy instrument alignment and its application to METIS

Hexapods are general solutions that provide movement with six degrees of freedom for instrument positioning, alignment, and support. In the case of the METIS instrument, the hexapod must satisfy the following stringent requirements: a) support the 11-ton weight of an instrument; b) allow alignment and provide position stability to the instrument to within a tenth of a millimeter; c) provide an adjustment range of about 20 cm; d) support the instrument allowing for accelerations of over 3 g in all directions; e) have the lowest mass possible. Commercial linear actuators that are generally used in such cases are designed for extended movement, include a complete set of bearings that constrain each actuator lateral displacements and a sophisticated central screw that defines only the longitudinal movement. These solutions tend to be heavy and costly if roller screws are used to avoid backslash. They encompass ranges that are a major fraction of the total length and are designed for fast movement. Both these characteristics exceed the requirements of the METIS application. We present an optimized design for the hexapod which includes a different, lightweight, sturdy, small-range, highprecision, no backslash, earthquake-proof actuator. The design of the hexapod is such that it can be used, in general, as a mass and vibration optimized solution for precision heavy instrument alignment.