Drives & Controls May 2022

48 n SCIENTIFIC May 2022 www.drivesncontrols.com Robot-mounted hexapod helps to position telescope mirrors precisely O n Christmas Day last year, the largest and most powerful telescope ever launched into space blasted into orbit from a launchpad in French Guiana. The $10bn James Webb Space Telescope (JWST), the successor to the Hubble telescope launched in 1990, will be able to capture events that occurred a couple of hundred million years after the Big Bang. Orbiting around one million miles from Earth, the new telescope has a lightweight primary mirror segmented into 18 hexagonal pieces mounted on a hinged surface more than 6.5m in diameter. It covers an area similar to a tennis court. By comparison, the Hubble mirror was just 2.4m in diameter. The telescope was launched with the mirrors in a folded position. Once in orbit, a delicate and intricate unfolding operation began to create the large mirror. This took several weeks to complete, but initial results suggest that everything has gone to plan. The telescope was first assembled in a massive clean room at Nasa’s Goddard Space Flight Center. Its segments were pieced together using a precision robotic arm known as the primary mirror alignment and integration fixture. This lifted and lowered each of the 18 mirror segments to their locations on the telescope structure. A custom-built hexapod (known as a Stewart platform) was mounted on the end of the arm robot to position the mirror segments precisely during assembly. The hexapod, supplied by PI (Physik Instrumente), provided precisely controlled movement with six degrees of freedom. Hexapods with their programmable centre of rotation (pivot point) are ideal for precise multi-axis alignment tasks, such as this. The hexapod used in the telescope application had a 200kg load capacity and positioned the parts with sub- micrometre resolution. “In order for the combination of mirror segments to function as a single mirror they must be placed within a fewmillimetres of one another, to fraction-of-a-millimetre accuracy,” explains Eric Smith, director of the JWT programme.“A human operator cannot place the mirrors that accurately, so we developed the robotic system to do the assembly.” The armmoved in six directions to manoeuvre over the telescope structure. While one team of engineers controlled the arm, another took measurements using lasers to ensure that each mirror segment was placed, bolted and glued perfectly in position before moving on to the next. The engineers used reference points known as“spherically mounted retroreflectors”to accomplish this positioning. A laser tracker, similar to the ones used by surveyors, used the reference points to determine precisely where the mirror segments should go. “Instead of using a measuring tape, a laser is used to measure distance very precisely,” explains the telescope’s assembly integration and test director, Gary Matthews.“Based off of those measurements, a coordinate system is used to place each of the primary mirror segments. The engineers can move the mirror into its precise location on the telescope structure to within the thickness of a piece of paper.” The James Webb Space Telescope is an international project led by Nasa with the European Space Agency and the Canadian Space Agency as partners. n A key element of the recently launched James Webb space telescope is a 6.5m-diameter mirror formed from 18 precisely- positioned hexagonal pieces. To assemble this structure with sub- micrometre accuracy, Nasa engineers used a special robot arm with a six-axis hexapod stage mounted on its end. One of the first test images received form the James Webb space telescope following its deployment earlier this year The robotic arm with its attached hexapod stage lowers one of the primary mirror segments for the James Webb space telescope onto a test backplane at Nasa’s Goddard Space Flight Center. Image: Nasa/Chris Gunn