ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
526 Mechanisms t esting  

Cleanliness assessment   The whole build plate was thoroughly cleaned after SLM in order to remove unfused metal particles and  other potential contaminants before the HIP treatment.  During the cleaning process performed in ultrasonic  (US) bath, cavitation was visibly very homogenous, which indicates optimum US exposure . A significant   amount of metallic particles was collected. In total 0.55 g for a total build mass  of 1251  kg, representing  0.044%.  

This process was repeated after HIP to see if more unfused particles could be removed. Here, only few  particles have been collected for a total mass of 0.03 gram. They can be classified in four catergories: raw  powder, round dark particles, flakes (probably contaminants during HIP process) and a few bigger particles  which should be partially melted powder agregates , as shown in Figure 11.  

Figure 11. Collected particles after HIP.  

Metrology   A combination of 2D optical metrology and 3D laser scan has been performed in order to evaluate the  global build plate deformations  which are mainly due to the stress generated during the SLM and thermal  post-process es (HIP, solution annealing and age hardening). The  metrology has been performed first after  SLM, then after HIP and finaly after SA -AH (still to be done at the time of writing). The comparisons are  done first with the 3D CAD model , then between the step before and the actual state.  The first re sults are  shown in Figure 12. More work is ongoing to assess the impact of these deformation on performance.  

       Figure 12. Left: global deformation of the build plate after SLM. Right: overview of the deformation for one  mechanism. Scale in mm.
527 Next Steps  

The next step of  the COMAM project is the final metrology measurements following the thermal treatments  and the machining.  This will be followed by performance measurements, vibration, shock and thermal  cycling testing.  The performance measurements will be compared with the simulation results to validate the  design and ensure the compliance with the requirements.   

Conclusions  

We have described CSEM’ s methodology developed to design, optimize and verify the development of  an  innovative compliant mechanism made by additive manufacturing. We have sur passed challenges and  pushed this technology  forward to implement innovative solutions.  

The ESA COMAM project is ongoing. The next steps are the manufacturing of two Elegant Breadboard  Models followed by  the test campoaign ; performance, vibration, shocks, thermal cycles and lifetime. In  parallel, the testing of the characterization samples is  in progress.  

CSEM continues to work on the ultimate goal to have a global tool for the optimization of compliant  mechanisms.  In parallel, functionalization of AM parts has been demonstrated and further projects will bring  new examples of adding electric al, thermal and optical features.  

References  

1. Merriam, Ezekiel G., Stiffness Reduction Strategies for Additively Manufactured Compliant  Mechanisms, All Theses and Dissertations. Paper 5873, 2016.  

2. Saudan, H., Vaideeswaran, K.,  Kiener, L.  & Dadras,  M. (2016 ). Additive  Manufactured Metallic Flexible  Structures, a focus on Manufacturing Strategies, Material Analysis and Fatigue Verification. In Proc.  European Conference on Spacecraft Structures , Materials and Environmental Testing, Toulouse,  France,  27-30 Sept. 2016.  

3. Saudan, H., Kiener, L., Perruchoud, G., Vaideeswaran, K. & Dadras,  M. (2017). Additively  manufactured and topologically optimized compliant mechanisms: technological assessment  approach, latest achievements and current work in progress.   In Proc. 17th European Space Mechanisms & Tribology Symposium, Hatfield, United Kingdom, 20- 22  Sept. 2017.  

4. Saudan, H., Kiener, L., Perruchoud, G., Vaideeswaran, K., Dadras,  M. & Cochet, F. (2018).  Compliant mechanisms and space grade product redesign based on Additive Manufacturing.   In Proc. SPIE 10706, Advances in Optical and Mechanical Technologies for Telesc opes and  Instrumentation III, 107062T .
529 Flexible Waveguides for RF  Transmission across PSP HGA Rotary Actuator  

Deva Ponnusamy*, Weilun Cheng* , Ted Hartka* , Devin Hahne *, Calvin Kee *,  Mike Marley * and David Napolillo * 

Abstract  

The High G ain Antenna ( HGA ) on the Parker Solar Probe ( PSP) spacecraft was mounted on a s ingle axis  rotary actuator  with a  range of motion of ±45 degree. During the early phase of the program, a trade was  performed to select the appropriate technique to manage the Radio -Frequency  (RF) trans mission across  the rotary joint . The Flexible WaveG uide (FWG) option seemed attractive, due  to its low mass and good  RF characteristics . However , the performance of these waveguides across rotary joints under repeated  articulation was not well understood. So a development program was conducted and flight -like FWG  assemblies were s ubjected to various test s. The successful performance of the waveguide resulted in the  selection of the  FWG for the PSP HGA assembly. As the flight design progressed, additiona l structural and  thermal analyses were performed on the HGA assembly to evaluate the use of these waveguides . The  flight batch of waveguides  were subjected to a comprehensive batch qualification and screening program.  Special tools and procedures  were developed for installation of the flight waveguides on to the HGA  assembly . The PSP spacecraft was launched in August 2018 and the FWG s have been performing well as  planned. The paper discusses the development, qualification and flight activities and the key lessons  learned along the way.  

Introduction  

The PSP spacecr aft is equipped with a Ka-band Cassegrain- type High Gain Antenna as shown in   Figure 1.  The main reflector is a 0.6-meter  diameter composite dish with a notched area on the edge for  increased dynamic clearance to the spacecraft in stowed configur ation  

Figure 1. PSP HGA Assembly  

* Johns Hopkins University Applied Physics Laboratory, Laurel, MD  

Proceedings of the 45th Aerospace Mechanisms Symposium, NASA Johnson Space Center, 2020
532 performance tests, nondestructive examination and visual inspection. Following is a brief description of the  various inspection and test techniques employed in the test program.   

Resistance Torque and Flex Life Cycle   Resistance torque was measured over the range of motion on a torque test rig as shown in Figure 3. The  maximum resistance torque was less than 30 mN -m and did not have an influ ence on the torque margin.  The resistance torque was also considered as a good m easure of the uniformity of the waveguide  thicknesses and was viewed as an indicator  of consistency and process control.  

Figure 3. Resistance Torque and Life Cycle Test  

The stresses induced due to the ± 45-degree motion wer e well below the elastic limit, yet life tests were  performed to ensure that cyclic loading  does not cause any damage or performance degradation. The  cycling test was performed at ambien t, hot and cold temperatures, in  ambient GN 2 atmosphere (Figure 4) .  The test article was flexed insi de a thermal chamber by a  drive system made up of a motor and a torque  sensor . The flight batch qualification unit was subjected to 330 cycles, equally divided between ambient,  +170°C and -105°C. The resistance torque was mo nitored to observe any trend.  

Structural Evaluation   The key structural feature of the FWG is the thin, corrugated Ni -Co wall of the waveguide that  allows it to  flex. The waveguide length was chosen to minimize the elastic stresses due to the cyclic motion . The long   unsupported length also meant lower resonance frequencies and higher load due to launch environments.  One particular area of interest was the natural frequency of  the waveguide in its l aunch configuration. The  intent was to not have the waveguide resonances couple with the spacecraft frequencies. In the launch  configuration, the high gain antenna is stowed at 45°, which put the waveguides in a gentle curvature  (Figure 5).
533   Figure 4. Life Cycle Test at Temperature Extremes  

During the development phase, a set of six waveguides with soldered flanges were subjected to low -level  sine sweep  from 5 Hz to 500 Hz, to determine the natural frequencies in the off -axial directions. The  waveguides were mounted on test blocks that flexed the wavegu ides by 45 degree s flange- to-flange, as in  launch configuration. A miniature, single -axis teardrop accelerometer was attached to the middle of each  waveguide, aligned in the direction of the test axis. There was a concern that the accelerometer attachment  might by itself influence the resonance of the waveguide. To address this, the test was repeated on one of  the waveguides with a non -contact laser vibrometer. There was no difference in the resonance values or  amplification measured by the accelerometer and the vibrometer. All subsequent tests only used an  accelerometer.  

The natural frequencies in the narrow section direction ranged from 51 Hz to 60 Hz, and the wide section  direction ranged from 60 Hz to 65 Hz. Because of the curvature of the waveguide in the launch  configuration, the waveguide is somewhat preloaded and therefore stiffer in the wide direction, which led  to a higher natural frequency when compared to the narrow direction. The variation in natural frequencies   between waveguides w as attributed to minute variations in wall thickness due to the electroforming process   control . Regardless, the minimum resonant frequency was 51 Hz, which was well separated from the  spacecraft frequencies, and would pose no problems with over -amplification of its acceleration responses  during launch vibration.  

Figure 5. Vibration  Test in 45 Degree Bend Configuration
535 to sunlight , depending on HGA position. A t certain times in the mission the sun exposure could occur at 0.7  AU, which would effectively double the solar flux on the waveguide. This was especially concerning given  the optical properties of the nickel -plated exterior surface of the waveguide. Metals  inherently have low  emissivity, and do not radiate he at easily to space. M etals can get  very hot in the sun if not properly sunk  to another boundary temperature. The typical values for  absorptivity -to-emissivity ratio for nickel from  various sources was  around 10 (0.4 / 0.04). This optical property, coupled with the high solar flux an d  relatively high r esistance to the flange, would result in high temperatures in the center of the waveguide.  

The other key thermal issue with the waveguide was the temperature of the flanges.  The HGA itself was  allowed to run relatively hot with a desi gn limit of 160°C and this caused waveguide flange temperatures  as high at 160°C .  

Thermal Analysis   Analysis of the flex waveguide in Thermal Desktop software showed very high temperatures for the middle  portions of the waveguide due to high thermal resistance between the middle of the waveguide and the  flanges. A detailed model of the waveguide was created to estimate the temperature of  the middle section  with higher fidelity (Figure 7) . This model showed temperatures around 400°C depending on the diss ipation  estimates. The temperature at the middle of the waveguide for the given flange boundary conditions and a  high heat dissipation of 1.7 W was 416°C . Despite the high temperature at the middle, the model  still  showed the flange areas at reasonable tem peratures. The high temperature was not of particular concern  from the waveguide material point of view, but could cause severe degradation or possible loss of the silver  coating at 425°C. Hence it was decided to perform a test  to verify the high temperature phenomenon.  

Figure 7. RF Induced Waveguide Temperature -  Prediction  

Thermal Test   A test was developed using a development  waveguide in a vacuum chamber. This test used a Direct  Current ( DC) voltage to apply dissipation across the waveguide. During different configurations of the test ,  the temperature was measured with thermocouples and with an Infrared ( IR) camera. Both methods were  prone to high errors, the thermocouples at high temperature, and the IR camera on a low emissivity surface.  However , the test was more to gain a general understanding of the issue , not necessarily to completely  quantify the temperatures.  The setup for  the test in a thermal vacuum chamber is shown in Figure 8. In t his  setup a thermocouple (T C) was placed in the middle of the FWG  to get a direct measurement of the  temperature. In another set up  this TC was removed and photos were taken with an IR camera. The red  and black wires attached to the flanges of the waveguide were connected to a power supply and used to  run current through the waveguide and simulate the dissipation from an RF signal.
536 The test showed results that were somewhat consist ent with the analysis. Temperatures in the testing were  not quite as high, however there was a concern that TC adhesion to the WG could have been affected at  the high test temperature and resu lts might not have been completely  accurate. The highest observed  temperature of the waveguide TC during testing was 276°C. Temperature measurements were also done  with the IR camera and the IR image is shown in Figure 8. This clearly showed the relatively cold  temperature at the flanges  and increase in temperature towards the middle.  The testing was considered  successful since it confirmed high temperatures predicted by analysis and also showed that the predictions  were conservative relative to the test. The test also demonstrated that flange temperatures would stay close  to their interface temper atures and would not overheat due to the dissipation in the waveguide. RF testing  after the thermal vacuum test showed no degradation  to the waveguide characteristics.      Despite the fact that the test showed generally positive results, concern remained that additional heat on  the waveguide from the sun could still result in excessive temperatures. For this reason,  sun shields and a  sun visor were added to the HGA assembly to prevent  direct exposure of the waveguides to the sun , without  interferin g with the waveguide or the swept volume. Another takeaway from the test was the significance  of the RF dissipation and the desire to  run RF power through the HGA assembly during the HGA level  thermal balance testing. This testing was completed but is beyond the scope of this discussion.  

Figure 8. RF Induced Waveguide Heating -  Measurement  

The exterior finish on the flight waveguides was changed from Ni plating to black paint and this further  reduced the temperature of the middle section. The thermal cycling limits for the flight waveguides were  established based on the flange temperatures. The waveguides were supported in the free- state and RF  performance evaluated pre - and post - thermal cycling. As explained earlier, the upper temperature of the  development model was underestimated. But this was revised prior to the flight article batch tests as the  thermal design matured. Also due to the criticality of the thermal environment and the issues experienced  with soldered waveguides, the flight batch quali fication article was subjected to a conservative 200 thermal  cycles.  

The t hermal cycling limits of the various models were as follows:   Development tests:  6 cycles, - 125°C to +100 °C   Qualification tests :  200 cycles, - 105°C  to +170 °C  Acceptance test:  7 cycles, -105°C  to +170 °C 

Waveguide Status and Performance Monitoring   The condition of the waveguide was monitored at every stage of the test program to detect any degradation.  This was done by visual inspection of exterior , inspection of interior with borescope, CT -scan, and RF tests.
537 Visual Inspection:   All surfaces were visually inspected to keep track of any surface features or defects   (Figure 9) . The exterior surfaces were inspected under 10X and 80X magnification. The condition of the  silver plating on the interior had a significant influence on the RF performance and so the inside surfaces  were inspected with a miniature borescope with a 90° aperture (Figure 10) . The borescope was mounted  to a linear stage and the waveguide was supported on a stationary  platform that allowed fine alignment of  the waveguide with the bore scope. With this setup, the coating could be inspected without the risk of contact  or damage.  The wav eguides were screened by the manufacturer prior to delivery and no significant defects  were noticed on the exterior or the interior. Any minor blemishes observed were photographed and tracked  over the course of the test program.   

Figure 9. Visual Inspection of Exterior Surfaces  

Figure 10. Inspection of Silver Plating  

Radiography :  The flexible parts of the waveguide as well as the flange attachment was inspected by  Computerized Tomography  (CT) scanning. This inspection step was  performed at key points during the  test flow and at the end of the test program. This allowed close examination of the exact cross -section of  interest and was very e ffective in inspecting and ensuring the quality of the  flange attachments. No defect   of any kind was  observed on waveguides with electroformed flange attachments  at any point during the  test progra m. CT-scan images of the flexible parts and a flange attachment  are shown in Figure 11.
538   Figure 11. CT -Scan of Flexible Waveguide with Electroformed Flange  

RF Test:   RF performance was evaluated by measuring the insertion loss and Voltage Standing Wave Ratio  (VSWR ) across the entire WR -34 frequency range of 22 GHz to 33 GHz . This test was performed with the  waveguide held in the straight and 45 degree bent positions . Typical RF performance results are shown in  Figure 12.  

Figure 12. RF Characterization of Waveguide
540 3. Radiography   4. Vibration Testing  – sine sweep and random vibration (Acceptance)   5. Bend resistance measurement and 5 flex cycles, ±45 degrees  @ ambient t emperature   6. Visual Inspection (20X to 80X)   7. Radiography   8. RF performance   All waveguides  successfully passed the acceptance test program. Two of the waveguides were installed to  the flight HGA assembly.  

Figure 13. Waveguide Placeholders Locate Bulkhead Flange Relative to Feed  

HGA Assembly  Flight Installation  

The flex waveguides span the gap between the waveguide bulkhead  on the spacecraft side  and the antenna  feed assembly  on the HGA side, across the axis of the drive actuator. The antenna feed assembly is  installed at the center of the main reflector, which is mounted to the actuator. The bulkhead feedthrough  connects the HGA to the RF components  located on the inside of the spacecraft .  

Though FWG s have less stringent alignment requirement than a RF rotary joint, they should be subjected  to pure bending only and any extension or twisting must be avoided. Therefore,  the feed ass embly and the  bulkhead feedthrough must be aligned and the distance between them must be controlled precisely  to  match the length of  the waveguides. T he two flange interfaces were both positioned at equal distance from  the drive actuator axis. The alignment requirement was met by performing this installation on a tooling  platform using waveguide Placeholders. The flexible waveguides are installed to the HGA assembly with  the actuator at the 0-degree position where both waveguides are straight without bending. The HGA - actuator assembly and the waveguide bulkhead br acket were located on the tooling platform, and the two  waveguide Placeholders were installed to the feed assembly on the reflector. Shims were added under the  waveguide bulkhead as needed to allow proper mating to the Placeholders. The waveguide placeholders  were then replaced by the FWG one at a time (Figure 13). The placeholders were machined to high  precision to match the FWG length and bolt pattern. The pinned- halves design (Figure 14) allowed easy  removal of the Placeholders to be replaced with the ac tual waveguides .
541   Figure 14. Exploded  View of Waveguide Placeholders  

To maintain the relationship between the bulkhead waveguide and the reflector assembly, a handling fixture  was used during transportation and handling (Figure 15) . All mounting interfaces between the HGA  assembly and the tooling plate or handling fixture were designed with a nominal  shim that could be adjusted  as required during installation. The detailed integration and handling procedures  using the associated  tooling and fixtures ensured that the waveguides were installed with utmost care to avoid undesirable  loading.  

Figure 15. Shimming Opportunities  

Lessons Learned  

Several valuable technical, procedural and philosophical lessons were learned over the course of this task .  Following is a brief description of the key lesson learned:   • Electroformed flexible waveguide made out of Ni -Co alloy, is a viable alternative to RF rotary joints  and coaxial cables, under certain situations.   • End flange attachment technique has a significant influence on RF losses.   • All aspects of new product should be thoroughly examined, not just the characteristics of interest.  While our program was primarily focused on the flexibility and life cycle, the only issues we faced  were related to flange attachment.   • Early testing during development phase greatly enhances confidence.   • The t hermal cycling test is very critical to evaluate flexible waveguides. Detailed thermal analysis  can be very effective in predicting the effect of RF transmission on waveguide temperature.
542 • Superior technical options when available should be given due consideration and not be dismissed  just due to lack of heritage.   • Laser vibrometers and miniature teardrop accelerometers can be effectively used in vibration  testing of flexible parts without influencing dynamic performance.   • Bore scopes can be used to inspect silver coatings on the inside of waveguides; this was valida ted  by RF tests and destructive inspection.   • CT scan is an effective tool to inspect soldered flange joints. RF performance can be directly  correlated to defective joints observed by CT scanning.  

Figure 16. HGA Assembly on PSP Spacecraft  

Summary  

The PSP HGA  assembly successfully used two flexible waveguides for RF transmission across the actuator   rotary joint  (Figure 16) . The analysis and test programs were effective in proving the suitability of the FWG  for on- orbit cyclic o perations. They were also effective in detecting some lat ent process defects and resulted  in the use of an improved waveguide with superior RF performance. The PSP spacecraft was launched in  August 2018 and has completed three close encounters with the sun so far. All aspects of the HGA and the  RF system are performing well as planned.  

Acknowledgement  

The authors would like to thank the members of the JHUAPL Space Simulation Lab for their support of this  task over a period spanning several years , Perry Malouf of JHUAPL for performing RF test s, and finally  Clency Lee Yow  and his team at Custom Microw ave, Inc. for their support.