ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
58   Figure 5.  Wire race bearing and dynamic feed -through  of the PSDDS . 

Dynamic Feed- through  At the beginning of the ExoMars project , a test campaign has been performed in which different possible  rotating shaft sealing technologies have been investigated under a representative environment.   

The sealing types examined first were O -rings or other polymeric seals supported by springs.  These kind  of seals were assessed to be not appropriate because of the inherent strong parasitic torque that is created during shaft rotation and the strict material requirements of the mission. The second sealing  type that was subjected to these tests was  breakable seals. However, this option had to be discarded  because,  after the unavoidable breakage of the seal, the volume where the instrument measurements  take place woul d have been exposed to the contamination generated by other components  of the  Rover. Also, the possibility of using magnetic feed- throughs  has been investigated but they failed the  qualitative assessment because the envelope and mass that was required to transfer the torque would  have exceeded the allowed limits.    These preliminary tests  showed that dynamic seals, consisting of a grease barrier which is kept in place  by polyme ric seals or a so- called grease labyrinth provide a leakage rate (even after motion) of   6E-7 mbar*l(He)/s by a seal length of 170 mm and a friction torque not excee ding 950 mNm over a  temperature range between room temperature (20°C) and - 60°C, which is the worst case operational  temperature.    These preliminary tests determined that the dynamic sealing is the option that  is to be implemented into  the SPDS  and practically evaluated in detail . The realization of this concept for the PSDDS can be seen  in Figure 5. The sealing function is realized by Braycote 601EF grease that is placed in a labyrinth  between the moving and the fixed part of the housing. It is kept in position by one flexible C-shaped  sealing ring (which is preloaded  by an internal spring)  to avoid having the grease pushed out of the  sealing over time when the differential pressure of 0.1 bar is applied for a long period or 0.25  bar for a  short period (proof pressure) . The sealing has a very low sti ffness to minimize its frictional  torque. This is  possible because it does not need to fulfill any sealing func tion itself.
60 The integration procedure for the wire race bearing in PSHS and PSDDS is identical and is performed in  the following order:   1. The two bottom rings and the upper outer ring are placed on its dedicated positions   2. The structure is put in position on spacers   3. The cage is inserted   4. All balls are inserted into the pockets of the cage   5. The upper inner ring is inserted   6. The shim and load is inserted   7. The screws are inserted   8. The spacer is removed   9. The screws  are tightened 

Table 1.  Results of the Positioning and Performance T ests    Ambient  Mars -like  Mechanism play   Min play [µm]  33.3 77.6  Mean play [µm]  63.6 108.7  Max play [µm]  98.5 137.4  Peak -to-Peak value [µm]  65.2 59.8  Hard Stop accuracy   Hard -Stop accuracy ± [µm] 8.8 5.9  Max [µm]  -0.3 27.6  Mean [µm]  -8.7 20.4  Min [µm]  -17.9 15.8  Repeatability   Repeatability CW ± [µm] 4.3 3  Repeatability CCW ± [µm] 1.1 1.9  Relative accuracy   Relative accuracy 1deg step ± [µm] 24 15.3  Relative accuracy 0.1deg step ± [µm] 8.4 6  Relative accuracy 0.01deg step  ± [µm] 7.6 8.6  Absolute accuracy   Max value [µm]  127.6 103.6  Mean [µm]  43 -5.3  Min [µm]  -25.5 -60.6  Peak -to-Peak [µm]  153.1 164.2  Absolute accuracy only CW   Max [µm]  84.2 0.1  Mean [µm]  39.3 -25.5  Min [µm]  -25.5 -60.6  Peak -to-Peak [µm]  109.7 60.7    The bearing is not self -holding until all the steps are complete d. After the integration of the bearing,  the  rotational torque of bearing preload has to be measured.    The measure ment of  the bearing torque has to be measured without any other additional torque  contribution to achieve a direct conclusion on the torq ue. The force that is needed to turn the bearing has  been measured along a circumference (see Fig ure 6).
61  

Figure 6.  Test setup to measure the preload  torque  

The nominal values have been determined considering the following requirements :  • The preload is required to be high enough to ensure that the bearing balls do not start moving  during launch vibration that  would result in a gap between balls and bearing wires. This can  cause damages on the bearing race wires , reducing accuracy and lifetime of the b earing.   • The torque resulting from  the preload is required to be as small as possible to ensure the  movement of the bearing. This needs to be guaranteed over the whole operational temperature  range.    In a next step,  the shim under the preload frame has to be selected,  such  that the given preload value is  met. During the integration,  it was observed that the recommended step width between the different  shims  of 20 µm was too high to meet the required torque. To achieve the  defined torque values  the shim  step width  was reduced to 10 µm. A second observation during the integration was that the tightening  torque of the screws has a big impact on the preload.  This means that the full preload has to be applied to  the screws for every test  in order to achieve representative results .   An important positioning requirement for both PSDDS and PSHS is the relative radial position of  the  dosing output funnel (PSDDS) with respect to the P yrolysis O ven (installed on the PSHS) and radial  position of the P yrolysis O ven (PSHS) with respect to the O ven Tapping Stations (which is a n Instrument  mechanism  that when operated closes the Oven to facilitate analysis with a Gas Chromatographer . It  contains the G as Chromatographer  inlet head and a separable connector to the side of the O ven to  supply power to the O ven heaters and read the achieved temperature to control the pyrolysis process.    To verify the Oven position performance during the PSHS test campaign , that due to PSDDS -PSHS  mechanisms similarity is directly linked to the Dosing Station positioning  performance, the PSHS  roundness and evenness  have been measured.  Their  deviations are caused  only by the manufacturing  tolerances of the structure parts and the precision of the wire race bearing.  It has to be mentioned  that  since the bearing only runs on thin and therefore flexible wires, the structure can be considered as part of  the bearing and has also an influence on the performance of the bearing. PSHS roundness and evenness  were measured via distance laser sensors . The test parameter s under which the test has been conducted  are the following:
62 • 4x ambient, no tilt , velocity 2  deg/s   • 6x Mars -like(-60°C,  6 mbar CO 2), no tilt , velocity 2 deg/s   • 4x ambient, tilt , velocity 2 deg/s   • 4x Mars -like(-60°C, 6 mbar CO 2), tilt, velocity 2  deg/s  

The tilt measurements  (see Figure 7) has been performed under an inclination  of 10° to the horizontal  position, which is the maximal incline under which the measurements will be performed on Mars.  

Figure 7.  Test setup for roughness and evenness (left), measurement lines for roughness   and evenness (right) 

Table 2.  Results of the Roundness and Evenness M easurements   Repetition  1 2 3 4 5 6 Maximum   Roundness  [µm]          ambient, no tilt  0.152 0.099 0.133 0.094   0.152  Mars -like, no tilt  0.143 0.089 0.106 0.147 0.122 0.110 0.147  ambient, tilt  0.098 0.120 0.118 0.102   0.120  Mars -like, no tilt  0.107 0.185 0.107 0.164   0.185  Evenness  [µm]         ambient, no tilt  0.083 0.112 0.079 0.117   0.117  Mars -like, no tilt  0.124 0.144 0.122 0.144 0.115 0.154 0.154  ambient, tilt  0.156 0.158 0.158 0.166   0.166  Mars -like, no tilt  0.130 0.118 0.118 0.126   0.130   The results are showing that the maxim um radial  difference of all measured points along a circle is   0.185 mm (requirement , peak -to-peak 150 µm) and the maxim um difference of all measured points  along  a circle in  plane is 0.166 mm (requirement , peak -to-peak 200 µm). The s ame performances  are expected  on the PSDDS, this allows  precise positioning of D osing Stations output funnels on the O vens  receptacles  for an optimal dosing performance.
63 Dynamic Feed- through (grease seal)   PSDDS and PSHS both have the same type of dynamic feed- through sealing  but they are implemented in  different way s. While the PSDDS has on one side a n axial plastic seal and a labyrinth on the other sid e to  keep the grease in place (see Figure 5), the PSHS has two radial plastic sealings  with grease in bet ween  (see Figure 8 [2]). 

Figure 8.  Wire race bearing and dynamic feed -through of the PSHS   

To fully evaluate the dynamic feed -through,  the integration and preload measurement of the wire  race  bearing was completed. For this the following integration steps were performed:   1. Open bearing and remove all bearing parts   2. Fill the plastic seal/seals  with grease   3. Put the sealing’s/sealing in there dedicated positions   4. Fill the space between the sealings or between sealings  and labyrinths  with grease  5. Reintegrate the bearing as described in the section before   The respective torque of both the bearing and the dynamic feed- through has  been measured after these  integration steps . As the  friction  torque generated by the bearing was measured in the integration step  before,  the additional  torque resulting from the dynamic seal s and the grease can be easily determined.   These tests were only performed during ambient conditions (see Figure 6). 

Table 3.  Results of the torque measurements   PSDDS  Max torque  [Nm]   Total  0.85  Preload  bearing  0.27  Feed -through  0.58      PSHS  Max torque  [Nm]   Total  0.86  Preload  bearing  0.11  Feed -through  0.75   Compared with the PSHS, t he preload in the PSDDS b earing is higher be cause the step range for the  shimming was chosen too  high and the required torque of 0.13 Nm could not be met, with either of the  shim s that have  been used . As result,  it was decided in co- operation with the supplier of the bearing that  the combination with the higher  torque will be  used because it provides  more safety during the vibration  test. The only drawback  of this  solution is that a higher torque has to be provided by the drive- train but
64 since the feed- through creates les s friction torque this effect can be compensated. The lower friction  torque of the dynamic sealing can be explained with the fact that  in the PSDD S there is  only one PTFE  sealing,  instead of two in the PSHS.  

During the PSHS test campaign the leakage of the mechanism  has been measured with a H elium leak  detector. As the PSHS  end-effector  reaches into the ALD,  the pressure must be applied on the outside of  the mechanism.  To achieve this, the setup was built in a way that the chamber and the inside of the  mechanism are connected with each other  and through this it wa s possible to create a vacuum  on both  sides at the same time with an additional vacuum pump. After  this, the connection needs to  be separated  and the chamber is  filled with 100 -mbar  Helium. When this condition is reached,  the leakage  measurement instrument is switched on and the measurement  starts . This paper only describes  the  leakage measurements that  were  performed after the vibration and shock tests.  The leakage  measurements  were done during qualification thermal cyc ling of the mechanism . In total , 8 thermal cycles  were conducted  and during the first, fourth and eight h cycle  the leakage measurements were performed.   For each cyc le (1st, 4th, and 8th), leakage m easurement s were performed under  the following condition s:  1. Ambient temperature, 100 mbar   2. Ambient temperature, 200 mbar   3. Ambient temperature, 100 mbar   4. 70°C, 100 mbar   5. 70°C, 200 mbar   6. 70°C, 100 mbar   7. -60°C, 100 mbar   8. -60°C, 200 mbar   9. -60°C,  100 mbar    The test results are shown in the T able 4. 

Table 4.  Results of the PSHS Leak Rate M easurements  Cycle  Enviro nment  Leak Rate [mbar*l/s]     Temperatur e [°C] at 100  mbar  after motion and 200  mbar   1st 20 5.6E-7 8.6E-7  1st -60 2.8E-7 5.0E-7  1st 70 1.1E-6 1.6E-6  4th 20 3.7E-7 6.3E-7  4th 70 4.7E-7 9.8E-7  4th -60 3.1E-6 3.2E-6  8th 20 7.5E-7 7.9E-7  8th 70 2.0E-6 2.0E-6  8th -60 1.2E-6 1.4E-6 

During these measurements,  leakage peaks could be detected which were not present during the  characterization  test of the chamber without the mechanism. Therefore,  it can  be concluded that the  peaks are caused by  the dynamic seal of the PSHS. The peaks can be explained with the following  physical effects:   • Helium absorbed or trapped inside the grease   • Helium penetration through the grease (e.g. , bubble penetration)    Nevertheless , the peaks are a valid measurement and had to be taken into account for the overall   leakage  rate. Rates  given  in this paper  are average rates over a measurement time of 30 min including   peaks.
65 Conclusions and Outlook  

The overall  concept of the PSDDS presented in this paper and particular design decisions with regar ds to  the three main challenges  (pre-torque device, bearing concept, and dynamic feed- through) have proven  suitable . The evaluation of the design was supported by test r esults obtained with the PSHS, which  adopts the same design solutions  under identical  circumstances. The PSHS qualification tests were   finished successfully, with satisfying results in all  tests. Of special interest were the positioning  performance tests, the leakage tests and the results of the roundness and evenness measurement.  In the  context of the PSHS, a ll requirements were  fulfilled with the proposed design and material solutions , i.e.,  the PSHS mechanism  is ready for the manufacturing of the flight model.  Currently, a detailed qualification  campaign of the PSDDS mechanisms is being carried out . The results of the PSHS campaign and the  preliminary results of the PSDDS tests provide a strong indication that the implemented design choices  will lead to a  successful PSDDS qualification campaign fulfilling all specific  functionality and performance  requirements. The final results of the PSDDS qualification campai gn will be presented at the AMS.    The four SPDS mechanisms will be sent  to the mission prime TAS -I, where they  will be completely  disassembled and after the replacement of parts worn out by sample material, the extensive ultracleaning and sterilization process will be performed with a subsequent mechanism re- integration foll owed  by integration into the ALD -Qualification  model  with all integration activities taking place inside an ISO3  AMC -9 glove box respectively an ISO7 HC clean room for parts outside the UCZ. After finalization of the  integration process, the ALD system qua lification campaign is carried out, in which, besides the standard  test program (leakage, vibration/shock and thermal -vacuum), several blank samples are processed by the  entire SPDS and analyzed by the instruments to investigate the cleanliness and perform ance of the entire  system.  

Figure 9.  Final integrated PSDDS  

References  

1. Richter, L utz., et  al. “Progress Report on Development of the ExoMars Sample Processing and  Distribution System (SPDS) and Related OHB Sample Handling Studies. ” ASTRA 2015, ESTEC,  Noordwijk, The Netherlands, 11- 13 May 2015   2. Paul, Robert , et al. "Development and Testing of a „ Backlash- Free“ Gas-Tight High Precisio n Sample  Handling Mechanism for Combined Science on the ExoMars 2018 Rover, ESMATS 2 015, Bilbao,  Spain   3. Phillips, Robin., et  al. Development of Brushed and Brushless DC Motors for use in the ExoMars  Drilling and Sampling Mechanism. AMS 2012   4. Kannel, J. W. and D. Snediker. "Hidden Cause of Bearing Failure." Machine Design (7 April 1977), pp.  78-82.
68 and high accuracy  requires the dynamic range of the IFPM to be in the order of 107 [2]. Moreover, the  mirror is not allowed  to introduce more than 3 pm/ √Hz of optical path length (piston) noise.  

Design concept   In the IFPM design,  the rotation of the mirror is guided by  two Haberland hinges, which are part of a  single monolithic  TiAlV structure. Their axis of rotation coincides  with the mirror surface, in order to  minimize the cross -coupling between the angular motion and optical path length variations. The mirror is  connected to a translational  actuator module via a stiff lever through the mirror  rotation axis; this way the  actuation force acts parallel to the mirror surface, which minimizes surface distortions.  

Figure 1.  Operating principle  of the walking piezo actuator. Due to the ellipsoidal movement of the  tip of the legs, the ceramic rod moves in horizontal direction. The numbers indicate the phases;  same- phase piezos are fed with the same voltage. Image courtesy of PiezoMotor ®. 

The actuation possibili ties are limited for IFPM  because of eLISA mission constraints (there are no  electromagnetic  forces allowed on the optical bench) and the large required stroke. After some successful  breadboard tests [5], the Piezo LEGS walking actuator by PiezoMotor  Uppsala AB has been selected.  The operating principle of  this walking piezo actuator is illustrated in  Figure 1. Each actuator   encompasses two sets of legs (depicted are two times two legs, the used piezostepper has two times  three legs), where each leg consists of two pieces of piezoelectric  material with a ceramic tip. By applying  different  voltages to each piezo the legs will elongate and bend.  A proper choice of the four driving  voltages (phases) can then cause an ellipsoidal movement of the tips of the legs,  thereby creating a  walking motion. As a result, a ceramic  driving rod pushed against the legs wi th the right preload  will move  in the horizontal direction.
69 The piezostepper principle is friction- based and thus relatively  stiff, while providing tens of newtons of  holding force. The stroke is only limited by the length of the driving rod, and is thus principally infinite; the  step size is only limited by the resolution of the DAC generating the voltages for the legs, and can thus be  in the order of  0.1 nm or less (one actuator cycle is typically a few μ m). As such, the complete IFPM  assembly utilizes a hard actuator  concept, which is both accurate, and stiff enough to potentially  withstand launch loads.  

Design improvements   The original IFPM design [5] had an actuator module in which three of such piezosteppers were  combined in a triangular  orientation, pushing against a triangular driving rod. The choice for three  steppers was partly motivated by launch load and redundancy considerations, and partly  to combine the  guidance of the driving rod and pr eloading of the piezosteppers in one single component. This actuator   module has been manufactured, but thorough testing showed that the nominal preload on the  piezosteppers  was too high and the distribution of the preload over the different legs was very u neven. It  turned out that the performance of the actuator is very sensitive to such preload deficiencies, causing  unreproducible irregular open- loop motion, which is why the original design never met its  expected  performance.  

Figure 2.  CAD model of the latest IFPM design . 

It has therefore been decided to loosen the launch  load and redundancy targets and focus on the proof - of-concept, by redesigning the actuator module to a two-stepper -concept. In this planar c onfiguration , two  piezosteppers are pushed on either side of a rectangular  driving rod; the preload distribution along the  legs is now  much better defined and is tunable via the bolts through the leaf springs  with which one of the  piezosteppers is  connected to the actuator module. Figure 2 shows a CAD  model of the updated design.  

Realization  The updated actuator module has been manufactured and assembled, and combined with the previously  realized mirror unit. A picture of the complete IFPM assembly  is shown in Figure 3. The setup has  been  tested under  normal laboratory conditions (room temperature, atmospheric  pressure), the results of which  are presented in this paper. In these tests , the mirror rotation h as been measured directly via a Renishaw  differential interferometer.  Both the reference and measurement beam are directed at the mirror; the  mirror angle is calculated (with nrad resolution) using the distance between the beams  and their optical  path length difference. The mechanism  is also equipped with a Micro- E incremental encoder and a 20-μm
70 pitch scale, located at a radius of 31. 06 mm from  the mirror rotation axis. This sensor has a 1. 2-nm  resolution,  which is thus equivalent to 38. 6 nrad of mirror  rotation. The encoder was originally meant for  beam acquisition purposes, but can also be used as a feedback  sensor (as an alternative to the external  interferometer).  

Figure 3.  Picture of the realized two -stepper IFPM. (Photo: TNO / Gert Witvoet) 

The piezosteppers are fed by four high- voltage space- qualified  analog amplifiers, one for each of the four   phases of the actuators. The voltage waveforms are generated by a dSpace data acquisition system with  a 16- bit D/A converter; t he encoder (via a 24- bit digital encoder  interface) and the interferometer (via a  16-bit A/D converter)  are connected to the same dSpace system. This  system offers a rapid prototyping  environment in MATLAB /Simulink, which provides great flexibility in meas urement  possibilities and  controller design.  

System Behavior  

The motion of the legs of the walking actuator is determined by the voltage distribution along the four  phases  as a function of time. Although the open- loop motion  will never be perfectly linear, the exact  shape of these voltage waveforms has a large influence on the velocity  variatio ns during an actuator  cycle [6]. For example, the horizontal and vertical motion of the first set of legs can be approximated by   𝑥𝑥𝑝𝑝1=𝑐𝑐1�𝑢𝑢1(𝑡𝑡)−𝑢𝑢2(𝑡𝑡)�  𝑦𝑦𝑝𝑝1=𝑐𝑐2(𝑢𝑢1(𝑡𝑡)+𝑢𝑢2(𝑡𝑡))  where 𝑐𝑐1 and 𝑐𝑐2 are motor coefficients and 𝑢𝑢 1(𝑡𝑡) and 𝑢𝑢2(𝑡𝑡) are the voltages applied to phase 1 and 2  respectively  [7]. Pure sinusoidal waveforms are known to exhibit  zero velocity at the transfer points (i.e. ,  when one set  of legs take over from the other), which is undesirable from a performance point -of-view.  Therefore so- called  asymmetric waveforms [ 7] have been used, where the  voltage of the first phase is  defined between 0 and the  maximum 𝐴𝐴 (here 48 V) as