Hexapod systems from PI (Physik Instrumente) for ALMA, largest ground-based telescope array in the world

13 billion light years in the spotlight

Physik Instrumente (PI): 50 antennas of the ALMA observatory use high-precision Hex-apods from PI. Under extreme ambient conditions of the Ata-cama Desert, the Hexapods align the sub-reflectors 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 sub-millimeter wave-length. ALMA provides imaging with up to now unachieved resolu-tion 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, tempera-ture differences of up to 50°C, strong winds, dust and rain are addi-tional challenges in 5000 meters above sea level that have to be faced.

Hexapod for aligning the sub-reflectors

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

To keep the optical path at its optimum, the sub-reflectors 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 sub-reflectors and allow for 6D positioning with resolution in the submicrometer and arcsecond range. The position-ing 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 manu-factured highly stiff and robust joints, suited for operation also in extreme ambient conditions. The Hexapods can adjust the position of the sub-reflectors precisely to several millimeters.

High-performance controller and high-resolution position detection

The digital controller's components for position control of the Hexa-pods have been adapted especially to reduced atmospheric pres-sure. 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 de-duced are of great scientific importance and prove the performance of the radio telescopes and their precision components.

PI in Brief
Well-known for the high quality of their products, PI is one of the leading players in the global market for precision positioning technology. PI has been developing and manufacturing standard and OEM products with piezo or motor drives for 40 years now. With four German factories and ten subsidiaries and sales offices abroad, the PI group is represented internationally.