50 Hexapod Systems for ALMA Largest Ground-Based Telescope Array

13 Billion Light Years in the Spotlight

50 antennas of the ALMA observatory use high-precision hexapods from PI. Under extreme ambient conditions of the Atacama Desert, the hexapods align the subreflectors to the large main reflectors of the radio telescopes.

The ALMA (Atacama Large Millimeter Array) observatory currently is the most advanced telescope in the world; its antennas receive electromagnetic radiation at millimeter and submillimeter wavelength. ALMA provides imaging with up to now unachieved resolution and sensitivity.

It gives insights into the birth of new galaxies, stars and planets, but also into star systems of the early universe that are more than 13 billion light years away.

Technological demands here are high: Low air pressure, temperature differences of up to 50°C, strong winds, dust and rain are additional challenges in 5000 meters above sea level that have to be faced.

Hexapod for Aligning the Subreflectors

Here PI uses its technological knowledge and long-term experience in micro- and nanopositioning technology.

To keep the optical path at its optimum, the subreflectors even out external impacts on the mechanical system. Deviations can occur, for example, with antenna tracking that compensates for global rotation, with bending of telescope components due to gravitation, or through thermal effects or wind load.

The hexapod systems with six degrees of freedom in motion are installed behind the subreflectors and allow for 6-D positioning with resolution in the submicrometer and arcsecond range. The positioning system's parallel kinematics structure is considerably more compact and stiffer than serially stacked multi-axis systems and leads to a very high resonant frequency. Since only a single platform is actuated, the moved mass is significantly smaller. This results in improved dynamics with considerably faster response.

For the hexapods in the ALMA antennas, PI developed and manufactured highly stiff and robust joints, suited for operation also in extreme ambient conditions. These hexapods can, therefore, adjust the position of the subreflectors within a travel range of several millimeters with a precision in the micrometer range.

High-Performance Controller and High-Resolution Position Detection

The digital controller's components for position control of the hexapods have been adapted especially to reduced atmospheric pressure.

With this high-performance digital control technology, that combines incremental position sensors and optical reference sensors in the hexapod's individual struts, PI provides a high-resolution measurement and control system for the ALMA observatory.

Successful Start-Up

Before start-up of the observatory, PI could prove the reliability and accuracy of its positioning systems in the ALMA VertexRSI test antenna, provided by Vertex Antennentechnik GmbH, a General Dynamics company. Also the technological predecessor of ALMA, the radio telescope of the Atacama Pathfinder Experiment (APEX) in Chile, uses the high-performance six-axis positioning system.

The measurement data collected up to now and the insights deduced are of great scientific importance and prove the performance of the radio telescopes and their precision components.

About ALMA

ALMA stands for Atacama Large Millimeter/submillimeter Array and is located on the Chajnantor plateau in the Atacama desert in the north Chilean Andes. It is an international astronomical partnership between Europe, North America and East Asia in cooperation with the Republic of Chile. The construction and operation of the antennas for Europe is monitored by the ESO (European Southern Observatory). On March 13, 2013, ALMA officially started operation.

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