ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
242 

Figure 1. Microvibrations  measurement setup: payload (green), piezoelectric  dynamometer (dark blue),  stone table (gr ey). An outer dome is used for noise protection.   The achievable measurement frequency range is typically from 1 Hz to 350 Hz. The lower limit is  defined  by the natural frequency of the vibration- insulated stone table, while the upper limit is determined by the  natural frequency of the system consisting of the dynamometer itself together with the object being  measured.  

Correct mounting of the measured object to  the dynamometer is critical in order to obtain a good  measurement result. The dynamometer and the measured object must be fastened with a sufficient  number  of bolts to ensure a proper  mechanical coupling.  

Last, but not least, s ound and electromagnetic interference (EMI) should be avoided in or near the  measurement setup. The dynamometer is connected to the  charge amplifier with a special high- insulation  cable. Data recording is handled via  a laptop and an analog- to-digital  converter.  

New Measuring Trends  

As discussed previously, t he maximum measurabl e frequency for microvibrations is currently in the range  of about 350 Hz. However , higher cut -off frequencies are now increasingly required in order to allow for  the measurement of l arger objects. Unfortunately , the standard dynamometer design itself will be a  limiting factor in trying to achieve these requirements.  Recent  standard dynamometers are comprised of   four three axis force sensors sandwiched between a top and base plate, each made of steel ( as shown in  Fig. 2).  

Figure 2. Structure of a piezoelectric dynamometer to measure microvibrations with four   3-component sensors
243 The dynamometer behaves like a second- order spring/mass system with a dominant n atural frequency  and so measurements must be taken well below this  natural frequency. Addition of any further mass  lowers the natural frequency  even more. Therefore,  if the dynamometer itself is small, simply mounting a  heavy object to investigate it  can itself have a major influence and lower the natural frequency of the  system  (as illustrated in Fig. 3).  

Figure 3. Natural frequency of dynamometer with and without additional mass  

So, i f measurements up to 500 Hz are required, the dynamometer should have a natural frequency of  >1500 Hz , otherwise resonance will exert too much influence on the measurement signal. Therefore, a  high natural frequency is a must . 

The dynamometer’s size and stiffness ha ve a considerable effect on t he natural frequency. The larger the  dynamometer, the heavier the top plate will be –  thereby reducing the natural frequency. This effect  cannot , unfortunately,  be entirely compensated for by increasing the stiffness of the sensors.  

However, r ecent  advanc ements in dynamometer design mean that higher natural frequencies are now  possible. Th e consequence of this is that it becomes possible to  easily isolate microvibrations and further  reduce their causes.  

In addition to this , new testing requirements are also demanding an increase in the size of the  dynamometers in order to test complete s ubsystems and whole small satellites. Comb ined with the higher  frequency  requirement , this means , with known materials , the design limits have already been reached  and any further improvements could only have been  incremental while requiring enormous time and cost   to achieve.  Hence new materials for the dynamometer top plate were considered.  

Ceramic Top Plate Dynamometer  

As previously  described, the dynamometer size has a critical impact on natural frequency of the unit , and  while they help add stiffness ; heavy top plates are especially unfavorable. A s earch for new materials to  be considered for the dynamometer top plate showed that ceramic s offer  highly advantageous properties  (see Table 1).
244 Table 1.  Steel and ceramic –  material properties    Steel 17- 4 PH  Ceramic  (Al2O3)  Specific gravity  7.8 3.84  Modulus of elasticity  (Young´s modulus)  190,000 MPa 370,000 MPa  Tensile  strength  1200  MPa 300 MPa  Thermal expansion  10.8 µm/m -K 5.7 µm/m -K 

The l ow specific gravity and a high modulus of elasticity offered by ceramics  show a clear benefit ,  however , the low tensile strength and low thermal expansion would be drawbacks.  

As seen  in Fig.  4, finite element method (FEM) calculations  show ed that natural frequencies can be  increased by 40% if ceramic  top plates with similar  dimensions to steel  were used. This would lead to  significant improvements  in microvibration detection.   

Figure 4. Natural frequency F x,y direction in relation to dynamometer size  

In such a case, lower strength could  be accept able considering the very small forces and loads i nvolved  in microvibration measurement s. In order to still allow  for a correct mounting of the dynamometer, the  steel base plate was retained,  as it has no effect on the natural frequency of the dynamometer.  

Measurements of the natural frequency in the z -direction show ed a very well -defined peak at about  2570 Hz (Fig. 5). In the shear directions  (Fx,y), the natural frequency was  about 1950 Hz. For  comparison, dynamometers with steel top plates reach about 1400 Hz in the shear di rection.
245 

Figure 5. Natural frequency in F z direction, Ceramic top plate dynamometer type Z21492  

However, after further investigation , the low thermal expansion proved to be a problem;  despite the FEM  calculation, a full validation of this behavior  was not possible. So, an extended investigation with  experimental specimens was undertaken to ensure that the difference in thermal expansion,  between a  steel base plate and a n Al2O3 top plate,  would not lead to fractures in the latter because of its special  structural design.  

Effects of Dynamometer mounting   The dynamometer must be mounted on a stiff bas e, ideally a stone table or steel table (min 10x more  mass than dynamometer with test object).  

Essential is also proper mounting. The dynamometer must be screwed with four screws on the stiff , flat  and clean bas e. Incorrect mounting affects natural frequency heavily. The effect of different incorrect  mounting was measured at Kistler in our  test lab. The dynamometer was mounted on a stiff bas e plate  and excited with an impulse hammer in Z –direction.
247 Characterization of the Optical Calibration System for Sentinel 4 UVN Satellite 

The CSL required a natural frequency greater than 1500 Hz to characterize the optical calibration system  of the Sentinel 4 UVN satellite. Kistler’s dynamometer type Z21492 , with a ceramic top plate,  was  selected due to its large dimensions and high natural resonance frequency.  

The dynamometer was tested on an insulated stone table and in a sound- insulated area by Kistler . The  next step was the validation measurements of subassemblies for the Sentine l 4 satellite. After completion  of this validation, CSL will perform further calibration measurements so that they will be able to offer the  space community a superior test facility for characterization of microvibration measurements , down to a  noise floor  of 0.01 mN (Narrow band noise ΔF=1 Hz).  

S4/UVN Calibration Assembly  

The Sentinel 4 mission covers the needs for continuous monitoring of Earth atmospheric composition and  air pollution using a high- resolution Ultraviolet/Visible/Near -Infrared (UVN) sounder instrument to be  deployed on two geostationary MTG -S satellites.  The radiometric accuracy of the UVN instrument relies  on periodic in- orbit re -calibrations using the UVN Calibration Assembly (UVN CAA). This mechanism has  been designed, built and qualified by CSL.  The mechanism is composed of a multi -functional wheel with  optical diffusers and a mirror that are successively placed in front of the camera during the calibration.  The rotation is activated by a stepper motor and controlled by a resolver.  

Figure 9. S4/UVN CAA   MTG -S being an Earth observation satellite with accurate pointing requirements, the exported torques  and micro- vibrations that are generated by subsystems can degrade the performance of the MTG  instruments. Therefore, the micro- vibrations emitted by the Calibration Assembly during the motion of the  mechanism shall be reduced to the minimum achievable.  

In this goal, the micro- vibrations and exported torques of the Calibration Assembly were es timated using  a model and then characteri zed using the Kistler micro -vibrations dynamometer.  

Micro -Vibrations Modelling  

To evaluate the micro- vibration levels in the moving mechanism a Simulink model has been created that  incorporated the elements that generate the vibrations as well as the main components that influence the
248 transmission of vibrations in the mechanism.  The elements that generate the micro- vibrations are:   • The stepper motor   • The bearing   • The friction   The main elements that influence the transmission of the vibrations are:   • The stepper motor controller   • The transfer function from the motor to the spacecraft interface;   • The lever effect from the spacecraft interface to the spacecraft Cent er of Gravity ( CoG). 

Figure 10. Micro -vibrations Simulink model   The stepper motor controller is a Simulink block that takes in input the motion that the motor should follow  as well as the micro- stepping parameters and transforms it into the sine wave that will be the current input  of the stepper motor.  The stepper motor is another Simulink block that takes in input the current of the  two phases and that generates the output torque of th e motor. The modelling also takes into account the  detent torque of the stepper motor.  

The next block of the simulation is the output shaft. This block simulates the behavior of the driving shaft  by modelling the resistive torque on the shaft. That includes the friction in the bearing and the rotation  inertia of the multi -functional wheel. This block can also take into account some cyclic resistive torque  generated by im perfect balls or track.  

The last block is the exportation of the results in the MATLAB  workspace. The results of the Simulink  computation are composed of the torques and forces generated by the assembly at the motor location.  After the simulation, a Matlab script is r an in the workspace to take into account for the mechanical  transfer funct ion from the motor interface to the Calibration Assembly interface.  

The input transfer functions were previously recovered from the FEM analysis. Once the temporal results  are obtained in MATLAB, a frequency analysis is carried out to identify the exported torques and microvibrations in different frequency bands. The final result of the analysis is a temporal response of the  injected micro- vibrations at the spacecraft CoG.  

Micro -Vibrations Optimization  

Using the produced model, an optimi zation an alysis was carried out to reduce the micro- vibrations  exported to the spacecraft. The driving of the stepper motor can be tuned by multiple parameters. The  main parameters are the maximum current injected in the motor and the motion profile.  These parameters  have been analy zed to reduce the micro- vibrations to the minimum achievable.
249 The first optimi zation performed was determining the impact of the input current as well as the sensitivity  to the detent torque of the stepper motor. It was found that the mi cro-vibrations created by the stepper  motor were primarily affected by the ratio between the holding torque (proportional to input current) and  the detent torque. The higher the ratio, the lower the micro- vibrations level.  

The second analysis performed was determining the impact of the driving profile on the generated microvibrations.  A micro -stepping strategy was introduced to reduce the exported micro- vibrations but even  with a perfect input signal, the micro- vibrations induced by the stepper motor cannot be erased. It was  evaluated that having more than 64 µsteps / step does not improve the micro- vibrations behavior.  

A specific input current profile was also suggested to, in theory;  remove the non -uniformity of the output  torque and so to provide a constant output torque during the motion. This profile was working well in  simulations even in open- loop conditions with external perturbations. But it was evaluated incompatible  with the actual electronic control system due to cross use of table lookup with other subsystems.   Following the rejection of an advanced current profile, more common profiles were evaluated including a  constant velocity profile, a constant acceleration profile and a jerk profile.  

For a jerk -based profile, a full motion is divided in 7 parts:  4 transitions phases (Tt), 2 constant  acceleration phases (Ta) and 1 constant velocity phase (Tv).  By tweaking the transition and acceleration  phases, the motion profile could be tuned and optimi zed to reduce the exported micro- vibrations during a   complete motion.  It was found that the best profile to reduce the micro- vibrations is a jerk profile that  tends to a constant velocity profile. Tuning the acceleration part of the jerk profile allowed reducing the  micro -vibrations during the transient par t of the curve.  

Figure 11. Jerk and Acceleration definition  

Micro -Vibrations Setup  

Once the sensitivity analysis and optimi zation performed on the Simulink model  was complete, it was  required to compare the results with physical measurements.  Therefore,  a test campaign has been  performed using the Kistler dynamometer developed for this application. Fig. 12  shows the CAA mounted  on the micro- vibrations table.
250 

Figure 12. UVN CAA mounted on µVib table   The table is capable of recor ding the forces in three axes as well as the torques around the three axes  with a high sensitivity as described in the first part of this article.  

Being very sensitive, a favorable environment is required to avoid impacting the measurements. The two  main noise contributors are the acoustic and the seismic environment. Since a characterization of Flight  Models was foreseen, it required working in ISO5 environment which creates additional acoustic noise  due to continuous airflow. However, CSL is equipped with vacuum chambers located in ISO5  environment which allows reducing the acoustic noise by closing the chamber during the measurement.  Furthermore, each vacuum chamber is equipped with a very stable optical bench that is decoupled from  environmental vi brations thanks to a heavy seismic mass.  

The environment noises  shown in Tables 3 and 4 were reached, with the instrument installed on the  dynamometer, in Focal 5 and Focal 2 that are two vacuum chambers at CSL. The measurements are  from 0 to 500Hz.   Table  3. RMS Noise in Focal 2   Axis  Noise Focal 2   Fx 2.9E-2 N rms   Fy 2.9E-2 N rms   Fz 3.7E-2 N rms   Mx 3.8E-3 Nm rms   My 3.2E-3 Nm rms   Mz 1.2E-3 Nm rms  

Table 4. RMS Noise in Focal 5   Axis  Noise Focal 5   Fx 3.5E-2 N rms   Fy 4.9E-2 N rms   Fz 1.7E-1 N rms   Mx 4.4E-3 Nm rms   My 2.1E-3 Nm rms   Mz 2.2E-3 Nm rms
251 Fig. 13 shows  an example of the spectral signature of the noise for the Mz axis.  

10-2100102 Frequency [Hz]10-810-610-4Amplitude [Nm]Spectrum of Mz

Figure 13. Noise spectral signature for Mz   It can be seen from the noise level that the goal of 0.01 N is not reached. This is attributed to the noisy  environment in the cleanrooms. Furthermore,  the local ISO5 airflows were activated during the test in  Focal 5 hence the higher noise level seen. On the spectral signature, a peak at 50 Hz is present as well  as harmonics. This peak has limited impact on the measurement regarding  the UVN mechanism because  its amplitude is much lower than the measured microvibrations.  

Micro -Vibrations Characterization  

Three models were characterized using the dynamometer. The first model to be tested was the Life Test  Model on which an extensive campaign was done to check the impact of the optimization parameters  identified during the modelling of the system. Later, two Flight Models were characterized to check that  the behavior is repeatable between the different models.  

The characterization is performed at the interface of the dynamometer by frequency bands for the six  degrees of freedom. Once the results are obtained, the response is rotated and translated to the  theoretical injection point of the mechanism. From this injection point, the impact of the mechanism on the  CoG of the spacecraft is determined.  While the forces at the injection point should be used to compute  the torques at the CoG, they are discarded in  the computation because the levels are within the noise of  the dynamometer and when multiplied by the lever arm, it becomes the main contributor to the torques  seen at the spacecraft CoG which is unrealistic.  

In Figures  14, 15 and 16, a comparison between the model and the measurement is shown for the torque  around the Z axis of the mechanism. The measurement is  shown for three frequency bands from 1.2 to  500 Hz. The measurements appeared to be pretty well correlated with the simulation results. The  simulation allows getting more accurate data at frequencies that cannot be measured, e.g. , very low  frequency range.
253 

Figure 16. Simulated (blue) vs measured (green) µVib   The Life Test Model  measurements showed that the Simulink model conclusions in terms of reduction of  microvibrations and exported torques were correct except for the impact of the holding torque to detent  torque ratio. It was observed that changing the drive current (to vary the holding torque)  didn’t visibly  change the emitted perturbations of the mechanism.  

Conclusion  

The dynamometer developed by Kistler for CSL’s UVN calibration mechanism is of an innovative design  that includes a ceramic top table in order to increase the global Eigen frequency  of the system and by this  way allows measurements into a larger bandwidth.  

The measurements performed by CSL for  the UVN project showed that the actual microvibrations and  exported torques have been correctly modelled by the Simulink model and it will allow using the model to  extrapolate the predications towards the very low frequencies (< 1 Hz).  

While the measured noise levels were suff icient for the current project, it is expected to reach a better  performance with the micro -vibrations dynamometer. Indeed, the environment of the measurement set -up  could not be efficiently optimi zed because of the stringent cleanliness requirements for U VN and  therefore additional measurements will be performed inside and outside cleanrooms with improved  acoustic environments in order to reduce the noise level to the minimum achievable.  

Acknowledgement  

This activity has been performed for  a contract bet ween CSL and OHB -M for the design and development  of the UVN calibration assembly.
258 blue and purple region above and below the pitch line of the driven gear, indicating tribofilm formation.  Regardless of MAC or P -SiSO, the driven gear showed more signs of wear than the driver gear.     

Figure 3. Inspection of Stage 3 components. (a) MAC show visible lubricant degradation (confirmed by  EDS) on stage 3 separator disc. Microscope image indicate wear above and below pitch line. (b) No sign  of lubricant degradation in P -SiSO case. Worn area color shift  indicates formation of boundary film.    

Results III –  Surface micro scale and elemental  analysis   Figure 4 displays the surface topography of the driven gears seen in Figure 3 , together with a topography  map of an unworn tooth. In the case of MAC and P -SiSO, the measurement was made after cutting the  teeth so that the full size of the  gear could be scanned. Despite this, very little data is recorded below the  pitch line. Above the pitch line, the MAC shows an elliptic region that covers about 1/3 of the gear, whereas  the case of P -SiSO is limited to the edges of the gear. The MAC topography has likely also been severly  worn below the pitch line, but the large height differential over the gear profile makes it difficult to capture  the effect on surface roughness in this area.   

Figure 4.Gear surfaces as seen by 3d profilometry in 10x objective. Wear patterns of MAC and P -SiSO are  compared with New (unworn) surface. Dedendum is mostly out of range because of gear involute profile,  but tendency  of high wear  by MACis seen . Regions I and II are selected for 50x objective evaluation.  

Increasing the magnification provides insight to the active wear and damage mechanisms. In Figure 5(a)  the MAC surface show signs of scuffing, with adhered particles and abrasive marks. In contrast, Figure 5(b)
259 show a surface where the original sintered pores remain, but the load bearing patches between pores are  very smooth. A surface roughness profile (avoiding pores) reveal that the surface parameters are  significantly improved compared to the unworn refer ence, as well as the MAC or PFPE (not shown).  

Figure 5. Regions I -II of MAC and P -SiSO respectively, with roughness profiles and parameters  along x  profile.  (a) MAC surface show evidence of partial seizures  (scuffing) . (b) P -SiSO produce smooth contact  patches and retains the porous structure of the unworn sintered material.   

Region II (P -SiSO) was analy zed by SEM- EDS investigate the lubrication mechanism. A patchy tribofilm is  clearly visible when using the low voltage high c urrent detector (vCD). However, the EDS analysis could  not confirm the presence of silicate. When comparing to a SRV model tribotest, shown in the top right inset  of Figure 6(b), it is clear that the tribofilms have a similar visible appearance. The EDS spectra also show  similarities in terms of Fe/O/C proportions, but Si is lacking in the analy zed gear surface. Possibly, the gear  tribofilm is too thin to be detectable by  EDS, even at the low accelerating voltages of 3- 5 keV used.  

Figure 6. SEM -EDS analysis of P -SiSO driven gear Region II . (a) Overview indicate gear surface is covered  with tribofilm. (b) Gear tribofilm compared to P -SiSO tribofilm generated in ball -on-flat SRV tribotest.  

Results IV –  Sub-surface micro scale analysis   The analysis is limited to ~1 mm3 of the gear teeth loc ated at the x -coordinate corresponding to the center  of the worn region in Figure 4. The colormap show the frequency of detected edges in the y -z plane. The  gray scale refers to the density of the material; bright regions correspond to metal (dense) and dark regions  to pores (air). Addendum (i) and dedendum (ii) regions on the driven (N) and driver (R) side of the gear are  chosen for comparison. Figure 7(a) show adhered metal, confirm ing scuffing at region (N ii). Interstingly, in  the same region there are also sub- surface edges detected, which indicates risk of sub- surface cracks. The