ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
12 Thus, s chedule pressure should not be permitted  to result in cut ting corners in the design cycle. If by cutting  corners in the design cycle, you don’t get it right, there is likely not time to do it again. If major changes to  the initial configuration are needed,  it could result in  the need for an additional  hardware iteration before  the flight build. It could also result in sticking with  a troublesome architecture and suffering with performance  issues throughout the development  and beyond, adding to cost and schedule. Cutting corners in the initial  design cycle is a risk not likely to be worth taking. Performing the necessary engineering analys es in the  initial design cycle  is of key importance in successfully developing hardware efficiently .  

Conclusion  

The team was very motivated to be tasked with meet ing requi rements not faced before. In the past, w e  have developed pointing mechan isms and scanners requiring arc second levels of pointing, but we never  before developed hardware to meet sub- arcsecond  levels  of pointing.  Pointing requirement s were met with  margin (Figure 1 1). If we used the BSM to point a laser from NASA/ ARC to San Francisco International   Airport, a distance of 40  km, we could keep the center of that beam on a dime.  

Various lessons learned have been d iscussed . Most of the lessons learned dealt with problems that  appeared and were in short order, understood and solved. However, throughout the BSM development, verification of pointing was a major focus. In particular, verification of systemati c error throughout mission  life, full range of motion, and full range of temperature,  was extremely  challenging. The details of the  methodology used for pointing verification go  well beyond the scope of this paper, requiring a paper solely  on this topic.     Being challenged with each issue that arose, understanding the problem, and finding the solution, is what  fulfills us  as engineers  as each team member can attest. The team is very proud of the BSM development  and was recognized by upper management for the outstanding achievements of this very challenging task .  

The BSM Flight Model was integrated into ATLAS as shown in Figure 12 . The BSM location on ATLAS is  identified in Figure 13 . 

Acknowledgements  

Willie E. Barber, Gregory A.  Bowers, William G. Bowers, Thomas L. Capon, Gene G. Gochar, Andrea O.  Poulin, Joseph P. Schepis, and David S. Schwinger from NASA Goddard Space Flight Center, Eric L. Holt   from Stinger Ghaffarian Technologies, Inc., Greenbelt, MD , and Doug las B. Leviton  from Leviton Metrology  Solutions, Inc., Boulder, CO .
16 Introduction  

In LISA Pathfinder , some of the critical technologies developed for the in- space detection of gravitational  waves will be tested [1] [2]. Scope of the mission is  to set a 2-kg gold- coated AuPt test mass (TM) into a  purely geodesic trajectory, i.e. , bring it to free- fall condition inside the spacecraft by reducing any force  other than gravity under the level of 10 fN/Hz1/2 in the measurement bandwidth ( 1-30 mHz).  Such a  challenging requirement drove the design of the whole mission. In particular, the  Grav itational Reference  Sensor  hosting the proof mass is internally gold coated and large gaps are present in order to limit the  stray forces produced by local sources (i.e. , charge patches). The Caging and Vent Mechanism  secures  the TM during the launch [3], while the Grabbing Positioning and Release Mechanism (GPRM) handles it  in orbit and injects it into the geodesic trajectory (Figure 1) [4] [5].  

Figure 1 .  The Test Mass held by the two opposed GPRMs with detail of the release tip  

We focus here on the latter phase performed by the GPRM, in which the TM is gently held in the center  of  the Gravitational Reference Sensor  electrode housing by two opposed release- dedicate d tips and then is  left in free fall after their quick retraction. The critical requirement of this phase, determined by the  capability of the capacitive control system to catch and re- center  the TM after release, is the TM residual  velocity, which must be below 5 μm/s. The nominal  design of the two opposed GPRM release  mechanisms provides a symmetric action on both sides of the TM, therefore perfect cancelation of forces and zero residual velocity. However, the ground testing of the release syst em highlighted some  asymmetries.  First, adhesive bonds are produced at both TM- release tip contacts  [6], whose strength is  affected by the surface topography at the microscopic scale, which is not controlled by the conventional   machining processes. This converts into a low-repeatable adhesion behavior and high probability of noncancelation of i ts effect on the two opposed TM sides  (Figure 2 right). Second, some asymmetry is  present also on the motion of the tips , which are commanded by two different actuators  (for instance, if  one of the tips is actuated with a time lag and/or moves with a different velocity , Figure 2 center ). Even in  the absence of adhesion this asymmetry makes the contact force time history different at the two opposed contacts, with a net contribution of their time integral, i.e. , developed impulse.  Referring to  Figure 2 left, the net impulse is represented by the area enclosed by the two force- time histories, which  quantifies the overall level of asymmetry of the two contacts.  

Figure 2 .  Asymmetry of contact forces on the TM (positive=pushing), pushing actions (preload )  and pulling actions (adhesion)   The test approach is twofold.  The effect of pushing forces (Figure 2 center) is ruled by the behavior of the  mechanism (the holding force control level, the repeatability of the retraction) and is quantified by analysis
17 of the tests of retraction of the piezo and the holding force accuracy level.  The effect of pulling forces  (Figure 2 right) is ruled by the behavior of adhesion and is quantified by the tests performed with the  Transferred Momentum Measurement Facility ( TMMF , Figure 3)  [7] [8] [9] [10] [11 ]. The in- flight  environment is reproduced by suspending the TM mock up as a pendulum inside a vacuum chamber, and the release experiment consists in the approach and retraction of the tip with subsequent measurement of  the swing oscillation produced (i.e., the transferred momentum ) [12]. In order  to maximize the  representativeness of the experiment, in the latest test configuration of the TMMF the EQM of the GPRM  is integrated to perform the release phase of the TM.  

The two effects may be added since in flight there is no effect of the first effect to the second. This is  motivated by the fact that the net velocity produced by the pushing actions (acting first) is negligible with respect to the velocity of retraction of the t ip (about three orders of magnitude).   

In-Flight Release Dynamics  

Tests performed with the previous set -ups and contact mechanics models show that the tip- TM contact  persists with relative displacements (from initial penetration under 0.3 N to final elongation at the  detachments) of a few microns. By choosing a reference displacement value for the detachment (say   2 μm), it is possible  to quantify the statistic distribution of the time required to obtain the separation of the  tip from the T M. If we consider the tests performed by RUAG on the retraction quickness of the tip –   unloaded – by restricting the results to the 24 tests performed on the flight models (FM1, FM2, FM3, FM4,  nominal and redundant piezos  [13]), we obtain a representative dispersion of the time to 2 μm as shown   in Figure 4. The mean and standard deviation of the time to 2 μm are 0.04 ms and 0.005 ms respectively.
18  

Figure 3 .  The Transferred Momentum Measurement Facility  

Figure 4.  Time histories of the FM tip retraction (left) - Time to  2μm, statistical distribution (right)  

Since the pushing force asymmetry is produced by the non- repeatability of the mechanism, we calcula te  the statistical distribution  of the differences among times to 2 μm, which is shown in Figure 5. The  standard deviation is about 7 μs.
19  

Figure 5.  Statistical distribution of the differences of time to 2 μm 

Measurements performed at MAGNA show that the voltage drops 120  V – 0 V commanded by the Caging   Control Unit  to the two piezo stacks have a fair synchronization but are characterized by a different slope  (in particular during the incipient motion), as shown in Figure 6. 

Figure 6.  Caging Control Unit voltages  commanded to the two stacks (left, courtesy of MAG NA) -  Approximated resulting pushing force profiles (right)    As a consequence, the time lag between the two force time histories is assumed negligible and their behavior is assumed linear up to the zero level force (Figure 6  right). The net impulse given by such time  histories is calculated and the TM velocity produced by the pushing forces can be calculated as follows:  

 tmFv TMF ∆=0 21 0   (1) 

where F0 is the initial holding force (about 0.3 N), mTM is the flight TM mass and Δ t is the difference  between time t2  and t1 as in Figure 6 right.  The resulting statistical distribution of the TM velocity  produced by the pushing actions is plotted in Figure 7. The mean is zero (each difference is calculated  also with inverted order of the two velocities), while the standard deviation is 0.5 μm/s.
20   Figure 7.  Statistical distribution of the TM velocity produced by the pushing forces  

If we add the contribution of adhesion to the effect of the pushing forces,  the total in- flight velocity is given  by the following formula:  

    ∆∆+∆= RT TM TMflightvU mtmFv1 0 21  (2) 

where ΔU is the adhesion energy  [14], vRT is the velocity of the release tip at the detachment from the TM,  Δ(ΔU/ v RT) is the difference between the above quantities  relative to the two contacts. The object of the  TMMF ground testing is the quantification of the second term in E quation 2.  

On-Ground Release Testing  

The Transferred Momentum Measurement Facility   The past t esting activities showed that adhesion occurring at the contacting surfaces produces a pull on  the TM due to the retraction of the release- dedicated tip, up to bond failure. In the tests, care must be  taken to minimize all environmental effects related to the laboratory, which may affect the adhesion  dynamic failure and the developed pulling impulse. In the test set -up of the TMMF , the representativeness  of the in- flight TM release conditions is based on the following:   • integration of the GPRM EQM - Z inside the vacuum chamber   • representative vacuum level (around 10-7 mbar)   • adoption of representative material of the TM bulk (Au- Pt alloy), machining process (Kugler) and  gold coating (Selex Galileo)   • adoption of the same contact load ( 0.3 N).  The following differences between the test configuration and the f light configuration are present:   • mass of the TM mock -up. Based on the past test campaigns, the intermediate mass configuration  (TM mock -up of 0.0883 kg) is chosen. With respect to the heavy (0.844 kg) and  light (0.0096 kg)  configuration, this allowed for a more detectable adhesion contribution   • cleanliness. The TM mock -up surface has been cleaned by means of isoprop yl alcohol with ultra - sound bath. A mild baking procedure has  been performed in vacuum to enhance  outgassing.  The  whole experiment is located inside an ISO6 (class 1000)  clean room   • one sided release. This configuration has been chosen since the early stages of the test activity.  The main advantage consists in the absence of cancelation of the two opposed adhesion  impulses, maximizing the sensitivity of the experiment to the release velocity   • gold-coated release tip. The GPRM EQM release tip is made of gold- coated TiAlV  alloy.
21  

Figure 8.  Old test configuration (before GPRM integration)  

Figure 9.  Test configuration with GPRM EQM – Z 

In Figure 8 the TMMF configuration before the integration of the GPRM EQM is shown. In Figure 9 the  test configuration with GPRM EQM is shown: the axis of retraction of the release tip is horizontal, engaging the TM mock -up in the center . The TM x displacement is measured by a laser interferometer,  while the TM pitch and yaw angles  are measured by a position sensing device through an optical lever  system. The GPRM attitude (both pitch and yaw angles ) can be adjusted to explore different directions of  retraction of the tip. This is used both to search the maximum release velocity and to characterize its statistical distribution when the retraction is repeated with a representative misalignment of that occurring in flight . Pictures of TMMF and GPRM current setup are shown in Figure 1 0.
22     

Figure 10.  The TMMF in the clean room  (left) – Blocking system , suspended TM  and GPRM in the  vacuum chamber (right) 

On ground release dynamics   During the engagement, the TM mock -up is blocked on its rear side by three needles, which are actuated  to adjust their attitude and allow engaging the TM with a minimal variation of its pitch and yaw angles with  respect to the equilibrium . According to the tested GPRM handover procedure from the grabbing plunger  to the release tip, the tip to TM contact force before the release is set to 0.3 N. This residual holding force  is balanced by the blocking needles, which are not  retracted during the release and produce an elastic  push on th e TM when the load is recovered at the retraction of the tip. As a consequence, the measured  velocities are affected by this systematic contribution, which following the nominal in- flight procedure  (nearly synchronous two- sided release) will be much smaller.  The force- time histories plotted in Figure 2  left in the test configuration are modified as shown in Figure 1 1. The force applied by the release tip starts  from F0 and quickly drops to zero, exerts the adhesive pull and vanishes in a time frame of 0.04 ms, while  the push of the blocking needles produce a cosine- like force profile which relaxes in a time frame of about  0.4 ms. 

Figure 11.  Force- time histories on the TM in the on -ground configuration    The stiffness of the blocking system is about 6· 10 5 N/m, which is on the same order of magnitude of the  axial stiffness of the GPRM. This means that the tested configuration is representative of an in- flight onesided release. If the measured velocities are rescaled by the flight TM to mock -up ratio (about  20), they  describe the TM velocity produced by a release performed by the retraction of just one tip.  The TM  velocity produced by the experiment can be calculated as follows:
23  RT mockup needles mockuptestvU m k mFv∆+ =1 0  (3) 

where mmockup  is the mass of the TM mock -up and k needles  is the stiffness of the blocking needles. The   control uncertainty of the force F0 is not negligible and the dispersion of the produced velocities is  affected also by this variable. From the analysis of the force signal, we assume for the holding for ce F0 a  Gaussian distribution with mean 0.3 N and standard deviation 0.05  N. This is likely to be a worst case  assumption.  If we consider that the experiment is repeated, the dispersion of the measured velocity (i.e. ,  the differences between pairs of measured velocities) is given both by the dispersion of the applied  preload F0 and the dispersion of the behavior of adhesion:  

    ∆∆+∆=∆ RT mockup needles mockuptestvU m k mFv1 0  (4) 

Here the term Δ (ΔU/v RT) is related to the difference of the behav ior of adhesion at the same side between  different tests , which is assumed descriptive of the difference of the behavior of adhesion between the  two opposed sides. As a worst case, we assume that all the dispersion of the measured velocities is due  to adhesion:  

    ∆∆≈∆ RT mockuptestvU mv1  (5) 

Therefore we can calculate:  

 test mockup RTv mvU∆=   ∆∆  (6) 

By substituting Equation 6 into Equation 2 we get:  

 test TMmockup TMflight vmmtmFv ∆+∆=0 21  (7) 

Basically, the differences between pairs of measured velocities may be rescaled by the mass ratio and  added to the velocity produced by the pushing forces.  

Test Results  

During every test, three signals are acquired: TM x -displacement, pitch and yaw angles (Figure 12). At  the release, after a quick acceleration phase the TM mock -up moves with almost constant linear and  angular (both pitch and yaw ) velocities. Angular components of velocity are associated to misalignments  of the release tip and the resultant of the forces applied by the three blocking needles with respect t o the  TM center of mass. However, t he amount of kinetic energy due to angular velocities (pitch and yaw) after  the release is negligible (0.5%) compared to that associated to the linear velocity.  In Figure 13, the TM  displacement  and linear velocity are shown. The velocity signal is obtained as the discrete time derivative  of the sampled displacement , and it is used to estimate the release time interval, after which the velocity  can be considered constant. The final velocity is therefore estimated as the sl ope of the displacement
24 signal after the release, through a linear least square fit. Fit uncertainty is very small compared to the  estimated final velocity.  

Figure 12.  TM displacement , pitch and yaw signals  

Figure 13.  Example of TM displacement and velocity signal during the release phase ,   along (0°,0°) direction  

The tests have been performed starting from the nominally aligned direction, searching the maximum release velocity. Across the measured maximum, a release tip misalignment of ±1.6 mrad (±0.092°) has  been explored along both yaw and pitch directions. Ten repetitions have been done along each direction, and test of hypothesis has been performed to check the significance of the difference between the mean of a direction and its neighbors . Two directions are assumed to produce the same mean velocity when  the p- value of the test of hypothesis is larger than 50%.    Table 1 shows the 10 measured velocities and their mean value for a single direction. The uncertainty of every estimated velocity  is significantly smaller compared to the standard deviation of the direction,  confirming that the measurement precision is adequate [15]. The results of the overall test campaign are
25 reported in Table 2. According to the literature [16] [17], a direction  of retraction of t he tip which is  misaligned with respect to the local orthogonal to the surface produces a shear stress which reduces  adhesion force. As a consequence, a reduction of the release velocity is expected along the directions of retraction which are locally misaligned. The results show that the peak velocity is reached along two directions, namely ( -0.3°, 0°) and ( -0.3°,-0.092°). The velocities along the surrounding directions are  significantly smaller, such that the hypothesis test rejects the hypothesis of equa l mean values. Some of  these directions produce velocities that  are statistically consistent. The overall behavior of the release  velocity is fairly regular and shows just an absolute maximum. Since the absolute maximum is reached  along two directions, we assume that the worst -case misalignment of the release tip direction spans of  ±1.6 mrad (±0.092°) around each, covering the yaw interval between -0.392° and - 0.208° and pitch  interval between - 0.184° and + 0.092°. This set of directions covers 120 tests, i. e., 120 values of the  release velocity v test of Equation 3. The distribution of the rescaled ( i.e., multiplied by the mass ratio)  velocities is plotted in Figure 1 4: the mean is 37.7 μm/s and the standard deviation is 1.5 μm/s.      Table 1.  Estimated final velocities for one release direction (- 0.3°, +0.092°)   test final velocity  (μm/s)  fit uncertainty  (μm/s)   1 828.14  0.73  2 863.33  0.79  3 886.09  0.84  4 841.69  0.74  5 790.82  0.61  6 862.65  0.69  7 884.84  0.79  8 878.80  0.80  9 842.54  0.72  10 876.14  0.81  mean   standard deviation  855.50   30.13   

Table 2.  Mean final velocity ( μm/s) of release directions . Neighbor numbers in equal colors   are statistically consistent. Numbers in gray color have been measured while searching   the direction of maximum with a large step (0.3°)    Yaw (°)   Pitch (°)  -0.6 -0.392  -0.3 -0.208  0 +0.3  +0.3     772   +0.092   848 856 839    0 820 807 873 843 847 847  -0.092   837 874 847    -0.184   839 838 859    -0.3     808
26   Figure  14.  Rescaled v test release velocities of 120 tests across the peak direction, covering yaw  interval ( -0.392°,  -0.208°) and pitch interval (- 0.184°,  +0.092°)  

The distribution of velocities plotted in Figure 1 4 is representative of an in- flight one -sided release and are  heavily affected by the systematic push of the blocking needles. This contribution cancels out if the  differences Δvtest between pairs of velocities is calculated. The statistical distribution of  the rescaled Δ vtest  variable (second member of Equation 7) is plotted in Figure 15 . The mean is zero (similarly to Figure 7)  while the standard deviation is 2 μm/s. 

Figure 1.  Rescaled Δvtest release velocities  

Results synthesis   The results plotted in Figure 7  (effect of pushing forces) and Figure 15  (effect of adhesion) need to be  convolved to obtain the resulting release velocity in flight, v flight of Equation 7.  The statistical distribution of  the overall release velocity is shown in Figure 1 6. The mean is zero and the standard deviation is   2.2 μm/s. The probability of a compliant release (magnitude of vflight less than 5 μm/s) is 96% . This result  is compatible with the previous estimations of the release velocity [ 18] [19] .