4 Final Report - Emits - ESA

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