FEATURE COVER STORY
PI HEXAPODS CHOSEN FOR CRITICAL Scientific Space Mission
The much anticipated launch of the LISA Pathfinder space mission will mark the birth of the next phase of an incredible project that will one-day help scientists look at the structure of galaxies and our early universe. We might even get a preview into the structure and nature of spacetime itself. Hexapods from Physik Instrumente(PI)were the ideal choice for the micro precision measurement
L
ISA Pathfinder will be a huge step towards a spacebornre gravitational wave observatory, which is planned to address the European Space Agency ‘L3’ science theme. LISA Pathfinder will place two test masses in space and then allow a near perfect gravitational free-fall whilst measuring their relative positions with an unprecedented level of accuracy. In their free-fall they will be untouched, but shielded by the spacecraft. The two test masses: 46mm3 cubes made of gold-platinum, will each be suspended in their own vacuum chamber with capacitive sensors to measure their relative positions, and an optical readout for the critical axis. When one of the test-masses moves away from its null position, a signal is sent to the control system which activates the micro- propulsion thrusters of the spacecraft to enable it to remain centred on this test mass. The second test mass is made to follow the first by control forces applied via a mixture of Micro-Newton (cold gas) thrusters and capacitive actuation. The primary objective is to measure
deviations from geodesic motion. The science of LISA Pathfinder consists of measuring and creating an experimentally- anchored physical model from all spurious effects and stray forces. The ability to measure the miniscule motions of test particles in perfect free-fall is needed to eventually construct a mission, such as eLISA, to detect gravitational waves. eLISA is a candidate large-scale space
mission (scheduled to launch in 2034) designed to detect one of the most elusive phenomena in astronomy - gravitational waves. With eLISA we will be able to observe the entire universe directly with gravitational waves, learn about the formation of galaxies, stellar evolution, and possibly the structure and nature of spacetime. LISA Pathfinder is a European Space
Agency (ESA) project with many of the leading European astronomy research centres involved including Scientists at the Institute for Gravitational Research from the School of Physics and Astronomy at Glasgow University.
12 WINTER 2015 | MICROMATTERS
to measure the position and angle of the test masses providing an accurate monitoring of their separation at the picometre (10-12m) level. In eLISA, measuring the relative positions of similar test masses will enable us to detect gravitational waves passing through the system.”
TEST PLATFORM FOR PRECISION LASER BEAM Dr Killow and his colleagues needed to create a test platform to allow a laser beam target to be precision- manipulated during construction of the flight-hardware optical bench interferometer, and selected PI’s M- 824 Hexapod, a 6-axis micropositioning system. “Components of the optical bench
Dr Christian Killow, a SUPA (Scottish
Universities Physics Alliance) Advanced Research Fellow, explains: “Gravitational waves can pass through matter virtually unaffected, allowing us to detect and measure things, for example black holes coalescing, which are hidden to the electromagnetic spectrum. LISA Pathfinder, a precursor to a spaceborne gravitational wave observatory, is designed to test the technology needed to detect gravitational waves in space for the first time. [The Institute for Gravitational Research] has developed a compact, on-board optical bench interferometer which uses laser beams
Figure 1:
LISA Pathfinder launch composite at IABGs space test centre
(Image by Esa)
Figure 2: Optical bench (Image courtesy of Glasgow University)
Figure 3: Hexapods from PI
must be precision-aligned to the sub- micron level and the system must be sufficiently robust to cope with the testing environment of launch into space. We selected the M-824 Hexapod because PI supplies quality instruments that we were confident would perform as expected, and we’ve been very happy with our choice. Having successfully used the hexpods to build the optical bench for LISA Pathfinder, we are now working to automate and industrialise the precision-placement techniques, where again the hexapods are proving invaluable”, Dr Christian Killow concluded. PI 6-axis Hexapods are increasingly used in simulation/test environments due to their versatility, speed, precision and load capacity. Whether creating a multi- directional rapid vibration to simulate the unsteady hand of an amateur photographer, or a large solid aerodynamic object being performance tested against varying wind directions, the level of control and precision positioning with PI’s 6-axis hexapods provides unrivalled results for both design and test phases of a precision project.
Physik Instrumente
www.physikinstrumente.co.uk T: 01234 756 360
/ MICROMATTERS
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