December 3, 2018

Precise Satellite Positioning to the Nearest Micron


The IOU-PIT mounting tool developed and constructed by Airworks with HEIDENHAIN LIP 581 linear encoders is located at OHB Systems in Europe’s largest ISO 5 cleanroom on a granite plate weighing several tons.

Highly compact mounting thanks to linear encoders from HEIDENHAIN

OHB Systems AG is constructing a new hyperspectral satellite called EnMAP for terrestrial monitoring. Five mirrors, six prisms and three cameras as well as electronic components are being mounted in an exceedingly compact space—with frequently only fractions of a millimeter between the components. Any contact between these parts must be completely excluded during the mounting process because this would destroy the sensitive components. HEIDENHAIN LIP 581 linear encoders and the POSITIP 880 digital readout provide help.

As expressed so appropriately at the start of the Star Trek movies: “Space, the final frontier.” Gavin Staton, a Project Manager in OHB in the EnMAP satellite program and Dave Mussett, a responsible engineer, certainly desire the vast distances of space for their project, but “even though outer space offers infinite expanses we can’t unfortunately use these when constructing satellites. Instead of that our task is to minimize room and weight.” Consequently, each cubic millimeter of installation space has to be intelligently utilized because the satellite must have the lowest possible mass and highly compact dimensions for its journey through space and along its terrestrial orbit. The fact that the construction must also be extremely stable does not make the project any easier, as demonstrated by the labyrinthine inner workings of the EnMAP satellite.

Rods, braces, and knockouts—the challenges during assembly


The required accuracy and utilization in a cleanroom were decisive aspects for the high level of complexity during construction of the IOU-PIT and the use of HEIDENHAIN encoders.

OHB Systems is constructing the EnMAP satellite at its Wessling location to the west of Munich, Germany. Most drivers on the A96 highway rushing past the front of the unobtrusive office complex have no idea of the extent of high tech it contains. The outside of the building certainly offers no hint that it contains Europe’s largest ISO 5 cleanroom in which the EnMAP satellite is being assembled. Put more precisely, though, not the entire satellite is being assembled here, but initially only the unit accommodating the optical instruments that form the core of the project.

This core is located in its own housing, the chassis of which was milled from one solid aluminum block. The base, external walls, and internal struts form an extremely robust unit within which all other parts of the complex optoelectronics are fitted and wired. Even mounting the first component to be installed is a high-precision task although the work space is still empty, because accessing this space is possible from only one side. The highly angular arrangement of the struts also significantly limits the freedom of movement.

Room in the work space becomes increasingly compact with each additional component until the flexibility of movement is finally reduced to just a few hundredths of a millimeter. Despite this the optical components—the mirrors, prisms, and cameras—must never knock against either each other, the housing, or the tool, due to their extremely sensitive surfaces. Even the slightest scratch would influence the optical system so much that the results would be rendered unusable. “The incident light would no longer be correctly diffracted, would not be correctly focused, and would no longer be correctly guided to the cameras via the mirrors. This means the cameras would no longer receive the complete luminous intensity and light spectrum, and in the worst case no light at all would reach the cameras. These would then supply distorted or unusable images or no images at all—rendering the complete EnMAP mission a failure,” explained Gavin Staton with regard to the critical importance of error-free mounting of the optical units.

The mounting tool for micron-precise assembly


Absolute cleanness and a very fine touch were needed for mounting a lens in the satellite.

The OHB engineers receive support during the mounting process from a special mounting tool called IOU-PIT (Instrument Optics Unit-Precision Installation Tool) from the Italian company Airworks. Those seeing this powerful unit in the cleanroom for the first time are reminded more of a crane for heavy loads than a mounting tool for the highly accurate positioning of very compact and lightweight components weighing no more than 20 kg. The IOU-PIT is a formidable apparatus with dimensions of 3040 mm x 2510 mm x 2300 mm and a weight of 2.5 tons. This constructive complexity results from the special demands placed on the IOU-PIT, namely the required precision during mounting and utilization in the cleanroom. This on the one hand requires extremely high levels of mechanical rigidity, and on the other the avoidance of several materials not permitted in the cleanroom.

Airworks engineers in the Italian town of Monfalcone developed and constructed the IOU-PIT. Airworks is an independent development office for sophisticated high-tech projects including aerospace, and Stefano Picinich has responsibility for the IOU-PIT. “Actually, for the IOU-PIT main mechanisms, we discussed the design with several high-end suppliers of positioning systems. Unfortunately, the project requirements were so challenging that their interest in the contract rapidly cooled down. At the end of the day, we couldn’t find a partner who had the courage to transform our construction into a real tool,” reminisced Picinich with a wry smile about the development phase. “So we carried out the construction ourselves and luckily found a partner for the measuring technology with HEIDENHAIN who, like us, did not recoil from the targeted accuracies.”

HEIDENHAIN linear encoders and digital readouts—knowing the precise position of the tool tip