4 Final Report - Emits - ESA

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4 Final Report 4.4.2 Microvibrations 4.4.2.1 Candidate mitigation actions The following strategies are defined to limit the impact of microvibration on LoS stability: • Stopping cryocoolers during PAN imaging. This solution is successfully used in orbit for high accuracy LEO Earth observation. Since switching on and off would not be acceptable in terms of number of electronic cycling, the cryocoolers are in fact kept on, but the amplitude of the engine is simply turned to 0 during imaging and then turned back to full power, without any ageing effect. The drawback of this technique is that thermal control of cold detector is effectively turned-off, and its temperature raises by less than 1 K/s. In the case of GEO-Oculus, since the PAN image acquisition is very short (0.4 s), the temperature raise shall be much less than 1K. These very small temperature cycles are deemed to be acceptable for the detector. • Elastomeric suspension to isolate the spacecraft from microvibrations generated by cryocoolers or conventional ball-bearing reaction wheels (BBW). Elastomeric mounts sustaining launch efforts without clamping have been developed and qualified for reaction wheel isolation and will be flight proven with Pleiades in 2009. With suspension frequency around 15 Hz, elastomeric mounts allow efficient attenuation of disturbances above ~50 Hz, where major BBW harmonic disturbances are reported. They are also efficient for high order harmonics which dominate cryocoolers disturbances when the main disturbance at cooler rate (40 to 50 Hz) is cancelled by design (backto-back Stirling coolers or pulse tube technology). This is therefore the most mature solution to drastically reduce the high-frequency components of BBW & cryocoolers disturbances. • Magnetic Bearing reaction Wheels (MBW) is a reaction wheel where no mechanical contact between moving parts is established during normal operation. This is achieved by magnetic levitation and position control of the rotor. The direction of the rotation axis can be actively controlled within certain limits by adjustment of the magnetic fields. This feature allows creating relatively high torques perpendicular to the wheel rotation axis. Hence, a MBW can be used for limited agile slewing manoeuvres. MBW are known to generate much less perturbations than their ball-bearing equivalent. With the availability of such equipments, the resulting high-frequency micro-vibration at instrument level should be reduced. In the following sections, the two reaction wheel options (MBW and BBW + elastomeric isolator) are compared in terms of microvibration disturbance levels and technology maturity. Cryocooler microvibrations are assumed to be mastered by the combination of elastomeric suspension and cryocooler stop during the most jitter-sensitive phase, PAN imaging. 4.4.2.2 Magnetic bearing wheel (MBW) evaluation for Geo-Oculus The magnetic bearing wheels envisaged for Geo-Oculus corresponds to the new design of Rockwell Collin’s Teldix (RCT) wheels. The currently existing MBW has the status of a technology demonstration, with drive and control electronics located outside of the wheel, only sensor electronics placed inside. In the flight design, the complete electronics equipment will be put inside the wheel. Table 4-3 compares the technical data of Teldix's 15 Nms BBW to the data of the prototype MBW and the foreseen data of two future flight MBW: MWI 30-400/37 is the basic model adequate for Geo- Page 4-44 Doc. No: GOC-ASG-RP-002 Issue: 2 Astrium GmbH Date: 13.05.2009
4 Final Report Oculus, whereas MWI 100-100/100 has a bigger rotor and different motor design. Prototype MBW Figure 4.4-1: MBW characteristics and photography of the prototype MBW (courtesy of RCT) The microvibration levels generated by the prototype MBW wheels have been characterised in 2007 by EADS Astrium GmbH in the frame of the DLR study “High Precision Attitude Control of Earth Observation Satellites”. The results of this study show that the MBW disturbance levels are lower by a factor of 10 to 240 (depending on frequency and wheel rotation rate) than typical BBW levels. However, it shall be noted that the comparison is supposing hard-mounted wheels, whereas a BBW mounted on an elastomeric suspension would be the actual competitor for a high accuracy pointing mission. Moreover, microvibrations is analysed at the source, whereas its impact on LoS at PLM level, largely dependent on structure transmission is the relevant parameter. It is undoubted that the microvibrations will be lower, but the above ratios shall not be taken as granted. The MBW appear in all cases as a good candidate for the Geo-Oculus mission, due to its inherent low microvibration content. The low maturity level (TRL ~4) and the associated development and technical risks shall also be accounted for. The pre-development needs to be actively pursued to reach TRL 5 at the beginning of phase C/D. 4.4.2.3 Ball Bearing Wheel (BBW) option A second option is to use standard ball bearing wheels mounted on elastomeric mounts developed for LEO observation missions. This option has been analysed in 2007 in the frame of the CNES study “Constraints for High resolution observation on GEO”. A microvibration analysis was performed for three different structural transmissions, without elastomeric suspension, with a 15 Hz suspension, and with a 30 Hz one. Microvibration levels measured on 8 flight models of Pleiades BBW (18 Nms Teldix RSI) are used as input to the structural model, typical of a GEO spacecraft equipped with an optical payload for Earth observation. The results are post-processed so that to show the peak-to-peak variation of the LoS over an integration time of 70 ms. The results of this study are therefore directly relevant for Geo-Oculus. Doc. No: GOC-ASG-RP-002 Page 4-45 Issue: 2 Date: 13.05.2009 Astrium GmbH
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Geo-Oculus: A Mission for Real-Time
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DL Final Distribution List Report Q
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4 Final Report - Emits - ESA

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