ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
Each of these bearings has 1130-N static load capability, which is assumed to be sufficient taking into  account that the bearing adds damping due to friction (2% damping assumed). The low temperature forces the choice of a solid film lubrication. In order to avoid sensitivity to moisture, gold is selected. Gold  lubrication shows a temperature and humidity independent behavior and is acceptable for this application  due to the low number of revolutions. Axial fit: To keep the preload constant over a temperature range of 300K, the bearings and the distance  rings (which adjust the preload) have to be made out of the same material (or at least material with the same CTE). These distance rings are shimmed with precision shims to adjust a friction torque below  5 mN-m.    Radial fit:  The housing material and all other parts of the PACS structure and mirrors are made of  aluminum AA6061 in order to have an instrument that can be aligned at 293K and operated at 4K. The fitting of the bearing diameter therefore has to take into account the relative shrinkage between stainless  steel and aluminum in the temperature range between 290K and 4K. The shrinkage can be approximated  as a 5 thorder polynomial equation.     Therefore, the fitting diameter of the ball bearing housing is 26 μm larger in order to compensate for the shrinkage. Position Device To ensure powerless positioning at the filter positions, the Filter Wheel is equipped with a magnetic  position device. The setup of the magnetic position device (90°) is sketched in Figure 3.  21
Figure3. Locking system schematic By design, the Filter Wheel is balanced. Therefore, under gravity there are only extremely low torques   (<1 mN-m) due to backdriving moments due to eccentricity or residual magnetic fields. In space, there will  be almost no backdriving moment at all. The bearing friction cannot be assumed as a powerless positioning device. Therefore, a magnetic ratchet system was implemented. The torque of the actuator is linear to the current but the Ohmic power consumption goes with the second power of the current. Therefore, the ratchet torque is set to a low level (10 mN-m which is roughly twice the max friction). This  value adds the functionality of automatic centering (without ECSS margins). The implemented solution is  a set of 3 SmCo magnets in series where the middle one is mounted to the rotor. This configuration is  repeated in a 90° pattern. To achieve the requested torque, SmCo magnets with a diameter of 1.5 mm and a length of 3 mm were selected. Position Sensor In order to keep the dissipation in the system low and to increase reliability, the position sensor is  implemented using contactless technology. The position sensing is performed by Hall effect sensors that detect the magnetic field of magnets mounted on the rotor. Figure 4. Decoder system schematic The selection of the sensor is based on the following parameters:  operating temperature range, low  dissipation, and linearity. To work at the temperature range of 4.2K the Infineon ion-implanted GaAs hall generator KSY 10 was selected. The hall current was set from 7 mA to 0.75 mA, which reduces the hall  voltage from 200 mV to ~20 mV. This is sufficient to detect the position with an accuracy better 10 arcmin 22
at the instrument level but at the same time reduces the power dissipation by a factor of 10. The  dissipated power at 4.2K for a 180° switch is then:  0.85 mW * 2 (positions nominal)*5 sec (switching time)= 8.5 mWs. For this application, the SmCo magnets from the position device (dia.1.5 mm*3 mm) are reused. Note:  For the actual 90° ratchet system (double magnets) the torque has to be doubled.   Figure 5. Absolute value of the Hall sensor si gnal and the restoring torque for a 180° magnetic  ratchet system Actuator In this application, the driving parameter for the sizing of the motor is not the motorization (as usual in space mechanisms), but power/heat dissipation since the lifetime of the mission is dependent on the  dissipation inside the cryostat. In general, dissipation is minimized when the efficiency of the motor is high, friction is minimized, and the motor inertia and the actuated inertia are approximately equal. The Filter Wheel rotating parts have an inertia of ~0.1 g-m²; this fits into an alr eady existing Cryotor quer  developed by the FU Berlin (C 84)**with an inertia of 0.12 g-m². This Cryotorquer has also a high  efficiency for slow rotating torque applications at low temperature. In addition, this Cryotorquer has heritage from several low temperature space projects. Therefore, this Cryotorquer was selected. The pancake-shaped electronically commutated torque motor C84 developed by the FU Berlin is  described in Figure 6.  The rotor consists of a package of alternating SmCo magnets on both sides of the coils (stator) that allows a comparable high air gap (0.5 mm) without decreasing performance. The magnetic loops are closed by means of the yoke rings made of good permeability material for low temperatures. The stator is composed of two redundant coil packages and each consists of one set of coils for phase 1 and a second set for phase 2. In normal operation, the two coil sets for each phase are interconnected. The current for each phase is supplied by the corresponding power source of the electronic control unit. 23
Figure 6. Schematic cross section of the Cryotorquer C 84 developed by FU Berlin Figure 7.  Position of the coil-sets of the two phase indicating the 15° angle shift of the two sets  and an example for the current vectors of the two phases. The torque capability of the motor at 250 mA is 142 N-mm. The maximum torque needed is 5 mN-m for  the friction and 10 mN-m for the positioning device. 24
Development of the Coating In order to achieve the demanding stray light reduction requirements, a new coating had to be developed  since there is no coating commercially available that reduces the stray light at a wavelength of  57 to 200 μm significantly. Therefore, KT has developed a coating (KT-72) which is indeed highly effective with respect to stray light reduction but at the same time bears some risks for the mechanisms. In order to have reasonable stray light reduction, the effect of black paint in the optical path in the coating has to be of the order of magnitude of the wavelength or higher. The thickness of the paint inside the PACS instrument was therefore set in the range of 400 to 500 μm, which is far away from the thickness of standard paints. In addition, the coating includes glass beads in order to change the direction of the light path into the plane of the coating.     Figure 8. Cut through a KT-72 sample. During the development of the coating, two effects drove the development, the delta CTE of the materials  and the aerodynamic configuration. The delta CTE of Paint (~40e-6) / Glass (6-) and Aluminum (14) (Housing) and a temperature gradient of  400K (bake out) to 4K leads to high stresses in the paint and between the coating and the base material,  25
which is at the limit of feasibility. Therefore, the connection between the different layers had to be  optimized. One major parameter is time. The time-dependent oxidation and the contamination between the layers avoid a perfect adhesion. ALODINE is known as a very good surface to apply coatings. But for  our coating, this turned out to be only sufficient when the coating was applied within 48 hours after  ALODINE application. After this period, a reactivation of the surface became necessary.  First tests were made with flat samples. But this turned out to be not sufficient since the aerodynamic  configuration of the parts have an influence to the distribution between large and heavy particles and paint μ-particles. Therefore the result varies with the geometrical setup. The complex setup of the instrument did not allow a flat configuration of coating application. Therefore, KT did not change the source of the contamination but the effect. The surfaces that had a “bad shape” and contained loose particles were “brushed” with a conventional  electrical toothbrush. All parts were cleaned with a clean room vacuum cleaner.   Test Results The following development tests were performed by the FU Berlin:  - Vibration tests at 4.2K (sine search, sine, random) - Functional and performance tests at 4.2K after vibration tests For the tests at 4.2K, the Filter Wheel was placed in a sealed container.  Figure 9. PACS FW in the sealed vibration adapter In Figure 10, the test results before and after the vibration tests are shown.  26
Figure 10. Current (winding 1+2) and torque of  the two redundant coils before vibration The functional test of the Filter Wheel shows very smooth rotation with 1.8 mN-m average torque. The  high spikes (±12 mN-m) are due to the magnetic ratchet system   Figure 11. Torque after vibration tests The first graph in Figure 11 shows the torque after the 0.5g sine search between 5 and 2000 Hz at 4.2K.  The torque increased to 4 mN-m with a ripple of 1 mN-m. The second graph in Figure 11 shows the  torque after the full sine vibration between 5 and 100 Hz with 19.8g max acceleration. The torque is  27
slightly decreased to 3.8 mN-m with a ripple of 1 mN-m. Torque after random vibration with 7.3g RMS /g198 lower average torque but ~1.5 mN-m ripple.  The lifetime test at 4K (40000 start stop cycles) showed no increase in torque. A failure of the Filter Wheel mechanism occurred during ground-testing of the flight spare instrument.  While this failure (clamping) could be explained by exceeded manufacturing tolerances and recovered by shimming, it was nevertheless decided at instrument level to increase the motor current to gain a higher  safety margin for the on-orbit operation. Flight Data Over the whole mission (4 years in orbit and 2 years on ground) no failure occurred. The temperature  over the whole in-orbit lifetime was stable at 2.92K. The wheel was driven with constant current sine of  70 mA which generates a torque of 56 mN-m for the nominal windings, a value which is considered as  necessary and sufficient to overcome the bearing friction. The photometer Filter Wheel performed ~6000 rotations the spectrometer Filter Wheel ~3000. The flight  data we received for the evaluation are the Spectrometer Filter Wheel data from BOL (2009) and EOL(20013), which are shown hereafter. Figure 12. Current and ripple of  the nominal coils during flight The magnetic field readout of the motor hall sensors shows the smooth movement with no significant  change between BOL and EOL. Since the Filter Wheels operated inconspicuously there are only very few high-speed data records from the operation of the wheels.-5.0  -4.0  -3.0  -2.0  -1.0  0.0  1.0  2.0  3.0  4.0  5.0   -80.0 -60.0 -40.0 -20.0 0.0 20.0 40.0 60.0 80.0  28.7  29.2 29.7 30.2 30.7 31.2  31.7  32.2 32.7 33.2 Motor current A[mA]  Motor current B[mA]  Sinus B  Current Deviation B[mA]  28
Figure 13. Signal of motor Hall sensors during commissioning Figure 14. Signal of motor Hall sensors at end of life Conclusions and Lessons Learned The development of the PACS Filter Wheels was successful and the two flight models performed well in  the Herschel program. The usage of an already existing and proven cryotorquer eased the development.  Also the contactless ratchet system and the contactless sensors were very predictable and reliable. Only the bearing showed some degradation during the vibration tests. The torque ripple increased to 1.5 mN-m and the overall torque showed some unpredictable variation after the vibration tests. In order to  avoid such a variation, the complete bearing should be suspended in a setup where all CTE’s are -15000 -10000 -5000 0 5000 10000 15000  34.3  34.8 35.3  35.8  36.3  36.8  37.3 37.8 38.3 38.8 39.3 39.8 2009_HALLA [raw]  2009_HALLB [raw] acceleration  ratchet system  -15000 -10000 -5000 0 5000 10000 15000  28.2  28.7 29.2 29.7 30.2 30.7 31.2 31.7 32.2 32.7 33.2 33.7 2013_HALLA [raw]  2013_HALLB [raw] acceleration  ratchet system  29
matched to the bearing CTE. This homogeneous thermal shrinking setup was then isostatically mounted  against the housing.  In addition, a deviation from the specified manufacturing tolerances in the flight spare model has led to a  clamping in the flight spare mechanism that could be recovered by additional shimming.   The development of the paint was the highest risk for the function of the Filter Wheels since one of the 100-μm glass beads could destroy a ball bearing and thus stop the Filter Wheel. The huge effort that was  put into the development of the coating has paid off.   References 1. M. Schoele, U. Ruppert,   “CRYOGENIC MOTORS FOR HERSCHEL/PACS AND JAMES WEBB/MIRI AND NIRSPEC"  FUB (Freie Universität Berlin), Department of Physics, Low Temperature Laboratory, Arnimallee  14, 14195 Berlin, Germany 30
Table 1.  Contamination requirements for the SCM mechanism Contamination Type Unit Inside optical cavity Outside optical cavity Particulate (PAC) ppm 30 100 Molecular (MOC) ng/cm250 100 Table 2.  Positioning accuracy and repeatab ility requirements over 16.5-mm Z-displacement Direction Positioning accuracy Repeatability X translation ±10 μm ±5 μm Y translation ±50 μm ±3 μm Z translation ±80 μm ±30 μm Rotation around X ±5 arcmin ±30 arcsec Rotation around Y ±50 arcsec ±25 arcsecRotation around Z ±10 arcmin ±5 arcmin YXZ Metallic BellowsCompliant RodSlit CarrierSlit Wafer Lower BladeUpper Blade Figure 1.  Linear Slit Change Mechanism design 32
Flexible Guiding System Design The flexible guiding system of the slit carrier is provided by a parallel blade arrangement illustrated in Figure 1. With no relative motion between contacting parts, the blade arrangement is free of any friction  or wear as well as need for lubrication. Consequently, it inherently prevents the generation of particles or  contaminants, one of the main design drivers with respect to the optical cavity cleanliness requirement defined in Table 1. The flexible blades are dimensioned such to limit the deformation stresses and  subsequently the creep impact, ensuring the repeatability of the positioning over the full stroke of the slit  carrier and operational lifetime.  In order to minimize thermo-elastic deformation-induced positioning inaccuracies over the wide operational thermal environment range of -30°C to +60°C of the SCM, all parts structurally supporting the  slits from the instrument interface are designed in the same material, which is a titanium alloy.  Each blade is cut out of a thin titanium sheet using a photo-etching process to ensure high precision  dimensions, manufacturing tolerances, and reproducibility. This is a key element in the precise guiding ofthis parallel flexure arrangement since it requires the assembly of several mechanical elements. The manufacturing tolerances of the blade support frame, slit carrier, and provided blade pin-alignment  features are additional contributors to the precision guiding functionality of this system. Symmetry, parallelism and alignment of the flexible blades being a major driver in the positioning precision over the relatively long stroke, the characterization of the flex-blade guiding performances over the full stroke has  been considered as the first step for the evaluation of the performance of the full mechanism. Initial characterization tests on a representative breadboard model have therefore been integrated in the validation test program of the SCM and are presented in this article.  Considering the importance of launch loads on space structural designs, the main drawback of such a  thin blade parallel arrangement is the lateral load carrying capability (X-Y plane). While the presented  design was compatible with the initial random specifications, their evolution have led to the integration of  slit carrier displacement limiters (snubbers) between the slit carrier and blade support frame. This additional contact-free guiding feature is only provided in the launch configuration (slit 2 active) in order to  limit the range of the provided tight clearance of 30 microns (in X & Y) over the axial direction Z. Their  dimensioning is such to limit contact stresses and avoid generation of wear particles. Figure 2.  Titanium alloy flexible blade - scale in cm 33
Compliant Rod Connection Design In order to meet the slit positioning accuracies for the complete SCM mechanism, the centering and  alignment of the linear actuator axis with respect to the flexible guiding system axis is essential over the full design stroke. With the aim of limiting the transfer of alignment perturbations between the linear  actuator and the flexible guiding system, a compliant rod connection has been integrated between these  mechanical systems. The optimization of the compliant rod design is essential for the minimization of the alignment and repeatability requirements to be imposed on the more complex design of the linear  actuator mechanism. For this reason, a sensitivity analysis of the flexible guiding system (including  connection rod) to actuator misalignment has been performed using FEM analyses during the initial  design phase. These analyses included the consideration of thermal distortion effects, linear and rotational misalignments, as well as residual torque input from the linear actuator and for each of the four nominal slit positioning conditions. The studied misalignment inputs and the combined output inaccuracies are presented in Table 3 and Table 4, respectively. Table 5 presents the minimum reduction  factors between the considered perturbations and its outputs on the slit positioning. Table 3.  Linear actu ator perturbation outputs Direction Source Perturbation X translation misalignment 10 μm Y translation misalignment 10 μm Rotation around X misalignment 1 arcmin Rotation around Y misalignment 1 arcminRotation around Z misalignment 30 arcmin Table 4.  Total inaccuracies of slit positioning due to actuator input pe rturbations and thermal  effects (absolute values) Direction Unit required Slit 1 Slit 2 activeSlit 3 Slit 4 X translation μm ±10 2.04 2.77 3.5 4.22 Y translation μm ±50 1.19 1.6 2.01 2.42 Z translation μm ±80 30.2 27.9 25.5 23.1Rotation around X arcmin ±5 0.26 0.26 0.26 0.26 Rotation around Y arcsec ±50 27 27 27 27 Rotation around Z arcmin ±10 0.05 0.06 0.07 0.08 Table 5.  Minimum reduction fact or on slit positioning accuracy due to actuator perturbations Direction Reduction factor X translation 2.37 Y translation 4.13 Rotation around X 3.87 Rotation around Y 2.2 Rotation around Z 392 34
Note that the maximum displacement values presented in Table 4 are based on a single geometrical  configuration consisting of the alignment of slit 2 on the optical axis (active slit), such that the results  include the Z-offset of the other slits. Therefore, the lowest slit (slit 4) being the furthest away from the  active slit, it presents the maximum absolute inaccuracies values in a worst case approach. Based on the defined perturbation inputs from the linear actuator, the slit positioning combined responses  are all compliant to the required values. More detailed analyses of the FEM model deformation results  have shown that the major contribution to the X translation is a slight tilting of the whole blade support  frame that has been stiffened in the QM/FM design. The transmission of X and Y misalignment are reduced by a minimum factor of 2.37 from the actuator to the slit positioning as shown in Table 5. Z displacement inaccuracies are governed by the thermo-elastic behavior along this axis over the wide  operational temperature range and the structural path from the SCM mechanical interface to the active slit position for this axis-symmetric mechanism design. All the parts involved in this structural chain aredesigned in the same material, titanium, such that the Z displacement inaccuracies are easily temperature compensated by the linear actuator control system. Mainly for this purpose, the resolution of  the stepper motor driven linear actuator has been set to 5 microns; see the following paragraph for more detailed information on the linear actuator design. The design limitation for the compliant rod is its buckling stability during the dimensioning vibration loads, which provides the limit for the decoupling features of this compliant element. Figure 3.  SCM temperature induced displacements from FEM sensitivity analyses over 50K – [m]  35
Linear Actuator Design In order to drive the linear positioning of the suspended slit carrier, a space-qualified low-power and  reduced-thickness stepper motor has been selected and combined with a custom-designed satellite roller  screw. The soft-preloaded satellite roller screw has been designed by Rollvis SA in close collaboration with Almatech to allow a direct and compact integration of the motor and bearings on the roller screw shaft, targeting the compliance to the required positioning accuracies and repeatability over the design life of the mechanism.  The advantages of a satellite roller screws over ball screw are mainly a higher number of points of contact  that distributes the transmitted loads. This allows a higher static and dynamic load capacity, in particular  during the launch environment. Moreover, it implies lower wear and longer lifespan which is one of the  SCM’s main design challenges and selection criteria. Another main advantage of satellite roller screw over ball screw linear drives are that they allow a more compact design. Furthermore, the ability to select  at will the pitch of the planetary roller screw independently to available ball sizes has contributed to the  advantages of such a solution. Based on the 200 steps per revolutions design of the selected motor, thisfeature allowed to set the resolution of the linear actuator to 5 microns per motor step, or 1 mm per rotor revolution. Finally, the design of the satellite roller screw dedicated to the SCM allowed the integration of  custom mechanical end-stops directly on the non-rotating lead screw.   Figure 4.  Satellite screw design illu stration (Courtesy of Rollvis SA) With the aim of assessing the guiding accuracy of the lead screw, measurements were performed on  commercial off-the-shelf satellite roller screws directly by the manufacturer of this component. These tests  have shown that the oscillation of the lead screw around the nut axis is better than ±5 microns, confirmingthe selection of a satellite roller screw driving mechanism for a high precision positioning application such as the SCM. The off-axis error will be further reduced on custom-made systems especially designed for  high-precision applications and by the matching of manufactured components resulting in the best final  performances results. The SCM’s rotating assembly is supported by a pair of angular contact preloaded ball bearings in face-toface configuration to lower the angular stiffness of the assembly and allow the integration of the bearing  preloading systems on the outer rings.  In order to evaluate the guiding accuracy of the output lead screw of the fully assembled linear actuator and optimize the linear actuator design, a tolerance analysis has been performed. This study included the  consideration of the all duplex bearing assembly parameters (type, size, tolerance class, arrangement,  groove wobble, bearing seat coaxiality…) and all geometrical uncertainties due to the manufacturing of structural parts supporting the rotating equipment. Sensitivity analyses of these main parameters allowed  36