ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
41 Regarding the random vibration test , the preliminary levels for whi ch the mechanism was analyzed to were   10 grms (20- 2000 Hz span with a 50 to 800 Hz 0.08 g²/Hz plateau). Later in the project, when the updated  and refined DSOC full system vibration simulations were performed, the specified levels at PAM interface  location  had to be increased.  The final random vibration levels are much higher and follow the power  spectral density (PSD)  shown in Table 5.  

Table 5. Final PSD for PAM Random Vibration Test  X -Axis Test Spec  Y-Axis Test Spec  Z-Axis Test Spec   FREQ(Hz)  ASD(g²/Hz)  FREQ(Hz)  ASD(g²/Hz)  FREQ(Hz)  ASD(g²/Hz)   20 0.100 20 0.1 20 0.1  50 3.500 50 3.5 50 3.5  150 3.500 395 3.5 340 3.5  2000  0.010 2000  0.01 2000  0.01  Grms = 28.03  Grms = 42.0  Grms = 39.5  

Updated mechanism simulation showed that th ese new levels remain acceptable, regarding mechanical  stress in the parts. The cables and wires, however, we re more affected, raising concerns with  the updated  vibration levels. Wire and cable epoxy staking upgrade s are being implemented to mitigate the  risk of wire  damage, mainly by reducing the free length between tie- down locations .  

   Figure 18. Overview of SG Wires, Initial Configuration ( Left) and R einforced Cabling ( Right) 

At the time of paper publication , full level vibration tests are planned on a representative model.  

Mechanism Delivery Status   At the time of the  paper  publication , the engineering model (EM) has been delivered. The EQM vibration  test is still to be performed for closure of design verification. FM2 acceptance test series has started with  thermal vacuum cycling. FM1 was unfortunately irreparably damaged during a subcontracted bake- out  failure (decompressive explosion, not implosion, something not expected in a vacuum test), and a new  model will be assembled using spare parts and tested.  

Conclusion  

In this paper, the development, procurement, integration and test ing of a novel double tilt PAM mechanism  is presented.  The mirror integration verification method is explained and the comparison with actual  measurements indicate that the results were quite reliable for this case.  

The test results now available  are presented and indicate that the PAM mechanism is working as intended.  Some measurement artifacts on the OP Thermal Vacuum Cycle (thermal hysteresis) were investigated and  some improvements will be implemented for incoming FM2 tests.
42   The upcoming tests are the random vibration and shock with increased levels. The cable wiring is being  reinforced in order to ensure the mechanism survival.  

Acknowledgment  

The authors want to thank all partners involved, among others: Marshall Bernklow and Richard Aigbaeken  from L3- Harris -SSG , as well as Dan McDonald and Joseph Kovalik from JPL  Optical  Communication  Laboratory  for their support during the project.  

References  

1. R. Le Letty, F. Barillot, H. Fabbro, F. Claeyssen, Ph. Guay, L. Cadiergues, Miniature Piezo Mechanisms  for Optical and Space applications Proc ACTUATOR Conf, Pub. Messe Bremen (G), June 2004, pp  177-180  2. E. Prevost, A. Weickman, S. Belmana, F. Bourgain, O. Sosnicki, F. Claeyssen, Beam Steering  Mechanism For Earthcare Atmospheric Lidar Instrument Atlid – An Ultra -Stable Piezoelectric Tip Tilt  Mechanism, Proc. ICSO, Biarritz, Oct. 2016   3. F. Claeyssen, T. Maillard, O. Sosnicki, F. Barillot, A.Pages, C.Belly, A.Bataille, M.Logeais, G.Aigouy,  T.Porchez, F. Bourgain,  Beam Steering Mirrors from space applications to optronic solutions , Proc.  OPTRO Conf, Paris , Feb. 2018   4. F. Claeyssen, K . Benoit, G . Aigouy, T .Maillard, M. Fournier, Olivier Sosnicki, Large- Stroke Fast Steering  Mirror For Space Free -Space Optical Communication, Proc. OPTRO Conf, Paris, Feb. 2020   5. F. Bourgain, O. Sosnicki , C. Belly, F. Barillot, F. Claeyssen, An Improved Accurate Beam Steering  Piezoelectric Mechanism for ATLID Instrument , Proc. Actuator 2014 Vol. pp. 293-296  6. T. Mower ., Degradation of titanium 6Al -4V fatigue strength due to electrical discharge machining ,  International Journal of Fatigue, vol.64, pp 84- 96
43 Design, Development and Verification of the METimage Scanner and   Derotator Mechanisms 

Sebastian Rieger * and Armin Jago* 

Abstract  

This paper presents  the major design choices and lessons learned from the development and verification  of the METimage Scanner and Derotator mechanisms. The modification of a standard drive unit design to  project -specific requirements is described . This includes a presentati on of design modifications that were  implemented to meet  long-term storage requirements.  Tests with notable lessons learned are explained  and the derived lessons learned are discussed.  Details on the design and development of the optical  components , the METimage Solar Calibration Device mechanism,  and the control electronics are not  presented in the paper . 

Introduction  

METimage is a cross -purpose, medium resolution, multi -spectral optical imaging radiometer for  meteorological applications onboard the MetOp- SG satellites. It is capable of measuring thermal radiance  emitted by the Earth and solar backscattered radiation in 20 spectral bands from 443 to 13, 345 nm [1]. The  instrument is developed by Airbus Defence and Space on behalf of the German Space Administration . 

The METimage instrument is based on th ree key optical assemblies which include mechanisms. These are  the Scanner Assembly , the Derotator Assembly , and the Solar Calibration Device (see Figure 1). All three  mechanisms are developed by Airbus and the mechanisms team is embedded in the instrument team. The  optical elements and the mechanism control electronics are developed by external partner s. This paper  focusses on the Scanner mechanism and the Derotator mechanism.  

Figure 1. Scanner, Derotator, Solar Calibration Device (left to right , not to scale)  

As shown in Figure 2, the Scanner mechanism is located at the Nadir -side entrance of the optical head of  the instrument . The Scanner mirror  is tilted by 45 degrees and reflects the optical beam into the telescope  at an angle of 90  degrees. The Derotator is located inside the instrument, between the telescope and the  detectors. It rotates at exactly half the speed of the Scanner in order to achieve a regular imaging geome try  in the focal  plane . During sun calibration phases, the Solar Calibration Device mechanism rotates one of  its diffusors such that it is exposed to sunlight; the sunlight is reflected into the field of view of the scanner . 

* Airbus Defence and Space GmbH, Friedrichshafen, Germany   sebastian.rieger@airbus.com / armin.jago@airbus.com  

Proceedings of the 45th Aerospace Mechanisms Symposium, NASA Johnson Space Center, 2020
44  

Figure 2. METimage Scanner and Derotator embedded in the optical head of the instrument  

Design Overview  

The Scanner mechanism and the Derotator mechanism consist of a drive unit (blue/cyan in Figure 1) and  the scanner mirror and derotator mirror assembly , respectively. The Scanner drive unit design comprises  two sets of angular contact ball bearings (hyperstatic layout), whereas the Derotat or drive unit comes with  a single bearing pair (isostatic layout). The motor and encoder concept is identical to the standard drive  unit of Airbus Defence and Space for both mechanisms  (Figure 3). This design  has been flown on several  missions, such as MHS (NOAA, MetOp)  and the FY3 satellites;  it also  forms the design baseline for the  MWI, ICI and MWS scanning mechanisms for MetOp- SG. The standard design was adapted to me et the  needs of the respective project.  

Figure 3. Standard drive uni t concept (left) and c ross-sectional views of Scanner ( center ) and   Derotator (right) drive unit s 

Telescope  assemblyOptical beam Scanner assembly Warm optical  assembly Cryogenic  subsystem Derotator  assemblyInstrument support  structureAperture stopNadir baffle Angular -Contact Ball Bearing Pair Brushless DC Motor Optical EncoderOptics Interface Optics Interface Angular -Contact Ball Bearing PairBrushless  DC Motor Optical Encoder Stationary interfaceAngular-Contact Ball Bearing PairRotating interface Rotating interfaceStationary interfaceBrushless  DC Motor Angular-Contact Ball Bearing Pair Scanner Derotator
45 Adaptation  of Standard Design  to METimage  Requirements  

In order to reduce programmatic and technical risk, existing (flight -proven) concepts, designs and  components were selected wherever feasible. This came with the following benefits:   • Use of high TRLs was  made and, hence, qualification effort  was reduced   • Common procurement with other projects  could  be performed   • A well- advanced design was available early in the project   • Testing of flight -representative mechanisms/components was  possible at an early stage   • Flight -grade components of early models could  be re- used for flight models   The METimage Scanner and Derotator  designs are based on the existing standard drive unit concept  but  were modified in order to meet the specific METimage needs.  

Performance requirements   The performance drift error (PDE) of the Scanner and, to a lesser  extent, of the Derotator is one of the key  performance parameters of the METimage instrument. The PDE has a direct impact on the image quality  as it affects the co- registration of the instru ment . The Scanner PDE during earth view shall not exceed  25 µrad over a period of 10  ms; the Derotator PDE shall not exceed 100 µrad.  The PDE is the maximum  pointing error in a window of 10 ms, relative to the pointing error at the beginning of this window . 

Significant effort was spent on both mechanisms  and control electronics in order to obtain designs compliant  to this requirement . The major mechanism contributors to the PDE are   • Bearing friction variations   • Motor disturbance torques   • Encoder measurement accuracy  Bearing friction and friction variation were minimized by reducing the diameter and the preload of the  bearings. Consequently, different bearing dimensions were selected for Scanner and Derotator,  respectively; the large diameter of the Derotator s haft and the corresponding Derotator bearing size would  have resulted in too high friction for the Scanner mechanism.  

The disturbance torques of the motor occur mainly at the following frequencies (per motor revolution):   • Rotor pole number: caused by magnets passing an imperfection on the stator   • Stator slot number: caused by imperfections  on the rotor passing the stator slots   • Product of rotor pole number and stator slot number, divided by their greatest common divisor :  motor cogging, caused by the fact  that the magnetic field is dependent on the distance between  magnets and stator slots  

The brushless DC motor design of the standard drive unit comes with a keyway at the stator outer diameter   (Figure 4). This keyway was  introduced as a positioning feature for projects where drive electronics without  modifiable commutation angle offset are used. For these cases, the motor needs to be integrated into the  mechanism in a p re-defined orientation. This positioning feature is neither needed for the METimage  mechanisms, nor can the corresponding motor disturbances be accepted. Therefore, the keyway was  removed for the METimage motor design.  

Figure 4. Motor stator  positioning keyway  that was removed in the METimage design
47 Power consumption requirement s  The scan profile of the Scanner and Derotator was optimized for integration time per pixel during earth  view. The scan period of 1.728 seconds  is defined by the satellite orbit and the spati al resolution of 500  m  at Nadir.  Consequently, a dynamic scan profile with a low velocity during earth view  and sun calibration and  a high- acceleration/deceleration phase  outside this range was defined  (refer to Figure 6). The need to  maximize earth view duration was traded against power and exported torques requirements.  

Figure 6. Scan profile of Scanner and Derotator  

The combined peak power consumption of the Scanner and Derotator mechanisms had to be less than  40 W. The average power consumption over one revolution had to be less than  8.0 W and 6.0 W,  respectively, for the Scanner and Derotator mechanism.  A multitude of parameters affect the power  consumption of a mechanism. The major ones are:   • Bearing friction   • Motor losses ( mainly iron and copper  losses)   • Inertial moments during acceleration phases   • Encoder power consumption  In order to re- use existing designs as much as possible, t he encoder was not modified. Motor power could  have been reduced by approx imately  30% by using NdFeB magnets; however, motor magnets made from  SmCo were preferred for programmatic reasons (refer to section Long- term storage). Bearing friction was  minimized by sizing the bearings to the actual needed dimension, and by reducing the preload to the  minimum possible value that was  required to ensure that the mechanisms withstand the launch environment   (Scanner: 88- mm pitch diameter, Derotator: 124 mm) . In addition, the Derotator bearing concept was  modified for power as well as lifetime reasons : instead of two bearings in hyperstatic layout, one bearing in  an isostatic layout was chosen. This was found feasible in a trade of power against performance   requirements.  

Inertial moments that need to be overcome by the motor  occur during acceleration and deceleration phases.  They are dependent on the rotating inertia and the acceleration. The resulting inertial moment is overcome  by motor phase current incr ease and, hence, causes higher power consumption.  Together with the  instrument team, a scan profile was found that satisfied the instrument needs and for which the Scanner
48 and the Derotator were  compliant to the power requirements  with SmCo magnets . This permitted avoiding  NdFeB magnets.  

Cleanliness and contamination  control   METimage is an optical instrument. The mechanisms carry optical components and, thus, had to meet  stringent cleanliness and contamination requirements.  In general, the CVCM  (Collected Volatile  Condensable Materials ) and RML  (Recovered Mass Loss ) limits  of Table 1 applied to the mechanisms .  Table 1. Particulate and molecular contamination requirements   Mass of material concerned  CVCM  [%] RML  [%]  > 100 g < 0.01  < 0.1   10 – 100 g  <0.05  <1  < 10 g  <0.1 <1 

The CVCM and RML limits were considered for selection of surface treatments (paints) and lubricants. It  was decided not to use polyurethane- based paints as these paints show high outgassing levels over long  time. Black Keronite a nd Acktar Frac talBlackTM were implemented for all thermo- optical coatings.  A surface  modification  process developed at Airbus which creates black surfaces with high solar absorptivity ( titanium:  0.98, aluminum: 0.84) and infrared emissivity  (titanium: 0.94, aluminum: 0.95) while at the same time  providing an excellent bonding pre- treatment ( e.g. for bonding heaters , thermistors ) [4] was not selected by  the time the decision for the coatings was made. This was due to the – by that t ime – unclear handling  constraints of the surfaces treated with the laser.  However, laser process es developed by Airbus were  applied for increasing friction coefficient s (refer to section Temperature ranges ) and for performing REACh - compliant  bonding pre- treatment  for motor bonding [11]. 

Figure 7. Blackening by surface modification  with laser  (left: laser -treated titanium prototype , right:  microscopic view of laser -treated surface)  [4] 

The heritage bearing lubricant for most scanning mechanisms at Airbus Friedrichshafen is grease Maplub  SH051 -a, in combination with phenolic resin cages that are impregnated with Nye 2001a.  These MAC - based lubricants were chosen due to their excellent lifetime (in terms of revolutions) which exceeds the  lifetime of PFPE -based lubricants by far [2]. However, according to current literature,  PFPE- based greases  and oils like Braycote 601EF and Fomblin Z25 have a better outgassing behavior; for instance, the vapor  pressure of Fomblin Z25 is reported to be one order of magnitude better than the one of Nye 2001a [3].
49 Table 2. Lubricant outgassing data   Lubricant  Vapor pressure  at 20°C [mbar]  Vapor pressure  at 100°C [mbar]  TML [%]  CVCM [%]  Reference   Requirement  N/A N/A 1 (RML)  0.1 Table 1  Fomblin Z25  1.60E -13 2.80E -09 0.04 0.01 [3]  9.35E -10 (25°C)  3.53E -09 0.17 N/A [10]  Nye 2001a  5.33E -12 1.33E -08 0.40 N/A [3]  N/A 1.80E -09 0.08 0.12(*)  Test [6]   (*) Correcting the CVCMs for bulk evaporation losses was not possible due to QCM saturation. Therefore, noncompliance (0.12% vs. specified <0.10 %) was accepted.   In order to assess the suitability of the heritage MAC -based lubricants for METimage, an outgassi ng test of  the batch of Nye 2001a foreseen for the METimage Scanner and Derotator was performed. This test yielded  that the actual outgassing behavior of Nye 2001a was far better than expected and, except for the CVCM,  was in the same order of magnitude as  the one of Fomblin Z25 [6]. It is not clear whether this observation  is valid only for the tested batch, or whether literature data on Nye 2001a are in general too conservative.  

Temperature ranges   The specified non-operating temperature range of the Scan ner and Derotator  mechanism is -35°C to +55°C  (design temperatures).  In order to ensure zero slippage, the following modifications of the standard drive  unit design were implemented:   • Iso-static mounts  added at the interface between mechanism and instrument panel  (Figure 8)  • Laser treatment of slippage -critical surfaces  performed  (Figure 9)  Iso-static mounts are widely used design features for optical instruments. They compensate thermal  expansion mismatches in radial direction.  This prevents slippage of interface bolts due to thermal loads.  

Figure 8. Iso-static mo unts of Scanner  

As mentioned in section Cleanliness and Contamination Control , a surface functionalization process that  increases friction between adjacent parts has been developed and qualified at Airbus. This process is based  on a laser treatment of the surfaces between two friction partners. Application of this process increases the  friction coefficient  significantly , which helps to prevent  bolt slippage. For instance, the friction coefficient for  laser -treated surfaces of titanium on titanium (Ti6Al4V)  is >0.5, the one for titanium on aluminum 7075 is  >0.8.
51 • All handling surfaces (outer diameter) have two continuous layers, i.e. any potential damage to  the outer Parylene layer (e.g. during AIT) could  be accepted   The entire stator was coated with Parylene after nickel plating and before wiring and potting.  

Figure 10. Bonding varnish protrusions  visible after Nickel plating  (left), c orrosion on nickel -plated stator  sample after humidity test ( top right ) and sample with P arylene layer  after humidity test  (bottom right ) 

Two types of magnet materials were considered during the design of the motor: NdFeB  and SmCo. By the  time the MetOp- SG projects were started, an alert had been raised on a previous NdFeB  type used on  MetOp (first generation) . The nickel -plating of the motors had delaminated and the magnets had corroded.   SmCo  is resistive to corrosion but is known to be brittle and its remanence is approx imately  30% lower than  the one of NdFeB , which causes higher power consumption of the mechanism.  

More corrosion- resistant  NdFeB  types had become available since the development of the MetOp satellites .  However, our magnet supplier recommended not using nickel plating due to several issues observed with  such platings  on magnets  in the past. Hence, the new type of NdFeB  and a coating or plating would have  had to be qualified for space and long- term storage.  

From an  accommodation point of view, motor designs with SmCo magnets as well as with NdFeB  magnets  were feasible. The higher power consumption of a SmCo magnet motor was traded against technical and  programmatic risk associated with a qualification program for the MWI, ICI, MWS and METimage projects ,  with two agencies, four instrument primes, and the satellite prime involved. After a scan profile had been  agreed that permitted the use of SmCo magnets from a technical point of view , the decision was made to  use SmCo magnets . It should be noted  that this resulted in some handling issues during AIT; even our most   experienced AIT personnel  struggled with the brittleness of the magnets and – despite being extremely  careful – damaged a couple of  magnets.
54 Accelerated testing   Accelerated testing of bearings lubricated with fluid lubricants (oils and/or greases) is not trivial. It is widely  accepted  that the lubrication regime should be mainta ined when accelerating a test in order to keep the  test representative.  For lubrication containing grease instead of pure oil, no tribological validation can be  performed beyond any tribological criticism. In addition, there is no practical  means to measure the oil film  thickness in an assembled mechanism [8, 9]. T he test data evaluation performed to select the METimage   life test temperatures  was based on the assumption that , when varying speed and temperature,  same  torque is an indication for same oil film thickness . Despite the  mentioned uncertainties, keeping the oil film  thickness constant was considered the most suitable and widely accepted approach when accelerating a  test with fluid lubricants.  In addition, this approach is considered conservative due to accelerated tribochemical degradation of lubricants at higher temperatures.  

The maximum possible acceleration factor was initially determined from bearing film thickness calculations.  The final value was confirmed by friction measurements of the bearings of the life test  models . The friction  measurement was performed with the bearings integrated into the mechanism; the motor was not mounted  as its resistive torque would have corrupted the bearing friction measurements . The friction measurement s  were performed at different temperatures and operating speeds in a dry nitrogen atmosphere. The results  of the Derotator bearings are shown in Figure 12. Subsequently, the f riction torque  ((3) in Figure 12) at the  nominal average speed of the mechanism (1) and at the nominal  average temperature (2) over one orbit  was determined. Assuming th at same  friction torque meant  same oil film thickness , the life test  temperatures  (4) for different speeds could directly be read from the plot. Respecting the 60°C limit this  yielded an acceleration factor of 3 for both mechanisms . 

Figure 12. Bearing friction measurement results and life test acceleration factor for Derotator   The MWS and MWI/ICI projects ran their life tests with acceleration factor s of 3.9 (MWS, discontinuous  scan profile like METimage) and 5 (MWI/ ICI, constant speed), respectively , and at temperatures between  40°C and 50°C . The life tests of the IASI -NG mechanisms developed by Airbus Toulouse for CNES were  performed with an acceleration factor of approx imately  11 at a temperature of 62 .5°C. This was possible  due to the lower operating temperature of IASI -NG. 

Life test profil e  The profile for the accelerated life test of the Scanner is shown in Figure 13. The major constraints were:   • The average velocity  was increased as described in the prev ious paragraphs.  0100200300400 0 10 20 30 40 50 60 70 80Torque [mNm] Speed [rpm]0°C 10°C 20°C 30°C 40°C 50°C 60°C (1)(3)(2) 32.5°C Nominal average  speed 17.5 rpmAcceleration x 2x3x4(4) 50°C (4) 60°C (4) 65-70°C