ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
110 Anoma ly Investigation   

Despite a successful testing campaign of the first and second EM iteration s of the clamshell mechanism,   an anomaly  of the V3- position flight clamshell  occurred immediately following protoflight vibration  testing.   The unit  failed to actuate; the proper v oltage was sent to the TiNi Pinpuller , a faint click was heard,  but no  movement was observed. The electrical ground support equipment ( EGSE)  was checked, determined to  be functional, and v oltage was applied to the redundant (backup) circuit of the pinpuller,  yielding the same  result, but with no additional audible click .  

The frozen clamshell was carefully photographed in this state, and a course of action was devised. It was  unknown at that time if the failure was contained to only the pinpuller, or if was problem at a higher -level  assembly  and something in the rest of the mechanism was bound. As the mounting screws were being  carefully (no shocks or bumps) removed from the top mount to the titanium hinge housing, the clamshell  deployed. From this event, it was theorized that perhaps the distortions in the housing caused by  the screws  prevented the cup from sliding down a perfectly straight bore. This  also immediately indicat ed that the  problem was  not isolated to the pinpuller, which was maintaining a constant 44-N (10-lbf) pull force, and  that there were external effects acting on the greater  mechanism. Despite the deployment, a fish scale was  attached to the back of the pinpuller to measure the extraction for ce necessary to remove the sliding cup  and pinpull er assembly  (Figure 11) . 36 N ( 8 lbf) was measured;  it should have been nearly zero. With the  addition of the predicted 18-N (4 lbf) disengagement force necessary to actuate the hinge legs, the 53-N  (12-lbf) combined force is outside the oper ational capabilities of the P10. Along with  the extraction of the  cup, dark, powder -like FOD was observed to come out  as well. This FOD was collected for chemical  analysis, and i t was determined t o be comprised primarily  tungsten disulfide powder, with a combination of   trace amounts of  titanium. This was  FOD  produced from the mechanism itself , and not from an external  source. This was not an anomaly that was caused by mishandling or a dirty environment.               

 Figure 11: Removal of the sliding cup and pinpuller assembly, immediately after deployment anomaly
111 An extensive investigation effort was initiated to determine the root cause of the  V3 clamshell anomaly,   focusing on the following aspects: the design of the mechanism, the manufacturing process, the testing  environment, and the selected material s. A fishbone diagram (Figure 12)  was created, and each potential  contributor was subsequently investigated.  The theory that  the housing distorted enough to bind the cup   was quickly discounted through additional FEA analysis.  

Figure 12: Completed c lamshell  Failure Review Board investigation fishbone diagram   

Upon removal from the housing, the pinpuller was evaluated electrically and mechanically for functionality.  It was quickly determined that it was destroyed; it physically  could be reset and maintained a constant  spring force, but it could not be actuated via either circuit . Though initially suspect, t he damaged Pinpuller   was determined not the root cause of the f ailure, but a side effect that resulted. TiNi Pinpuller s are designed  to automatically disconnect and shut off  the flow of power  upon actuati on. If the Pinpuller  were physically  prevented from actuation, as was the case in this situation, the electrical contacts will remain closed ,  overheating the actuation element . The nature of the EGSE being used to conduct this deployment  (power  supply with push button) resulted in power being applied to the devices  applied for much longer period of  time than designed.  Typical Pinpuller  actuation times are 30 to 80 msec  (dependent on device  temperature)2; so an uncontrolled button press of  approximately 1 second is an order of magnitude too  long, resulting in irreversible damage to the nickel titanium shape memory actuation wire. One of the  lessons learned, and immediate changes made as a result of this anomaly, was t o implement EGSE and  on-board SC  commands that provide a  pre-determined pulse of current, instead of a simple button  press .  This has the effect of protecting the Pinpuller from electrical damage in the case of a frozen mechanism,  both during testing, and in space.  

It was appar ent however , during c lose inspection of the sliding cup that was forcefully extracted from the  housing that there were very small ( 0.1 mm, 0.0039 in) areas of damage at the bottom of the cup. These  three areas were spaced about 120 degrees apart, radially, on the circumference of the sliding cup. There  were corresponding areas of similar damage on the housing as well.  Instead of  material  buildups like on  the cup, the housing damage was manifested as shallow pits. It was obvious that there was a transfer of  material, and that t hese build -up locations ( Figure 13) on the cup were substantial  enough to close the gap  between the sliding cup and the housing; effectively seizing the cup in place. These areas were  photographed and documented with the aid of  a microscope.
112  

Figure 13: Locations and micrograph of damaged area on the circumference of the sliding cup   

Dimensional Analysis  

A detailed investigation of the dimensional clearance between the cup and the housing began, to determine  what the realized clearances between the two parts were, and if they were designed to be too tight. All  previous clamshell mechanisms (EM1, EM2, and F light) were disassembled and each housing and cup  part measured. Up to this point, each iteration of both the housing ID and the sliding cup OD were only spot  checked; only one measurement of each were taken of one part  in the lot. In addition to the full postinspection, it was determined that the profile, or  effective  shape of the parts should be measured and  ascertained as well . This was accomplished by taking multiple diameter measurements of each part, in  the  critical areas  of part interaction. As depi cted in F igure 15 these diameter measurements were made in  1.27mm ( 0.050 in) step increments  along the length of each part .         

Figure 14: Snippets of original flight drawings of critical diameter dimensions and tolerances
113  

Figure 15: Detailed measurement s and shape of all sliding cups fabricated up to the time of the anomaly         This inspection process was illuminating, as it became immediately clear that the specific sliding cup  that  seized was out of family from all of  the other sliding cup parts that were fabricated during the entire  mechanism development effort. In addition to the effects of a larger outside diameter sl iding cup, which  effectively reduced the diametrical gap between the parts to half (.022 mm, .0009  in), (Figure 16)  from the  average nominal (.045 mm, .0018 in), but the back half of the sliding cup was  tapered over  .0254 mm, ( .001  in) diametrically over the distance of approximately 8 mm (.32 in). The spots where the damage occurred  corresponded exactly where the diameter was the largest.  The resulting tapered- shape adds credence to  the theory that the affected sliding cup was contacting with the housing on a line profile only,  as opposed  to the distributed as -designed surface contact  of a correctly fabricated cylinder. A line contact would  effectively magnify the contact stresses to  the reduced areas of contact during the vibration testing that was  performed immediately prior.   .
114   Figure 16: Relationship of all housing/slider fits EM1 through flight (dimensions in inches)  

Material Discussion  

With the dimensional issues with the anomalous part in revealed, the investigative team focused on the  selected materials as a contributor to the root cause of the problem.  A mentioned earlier,  both the sliding  cup and the housing were coated and dry film lubricated 6Al -4V titanium; a  decision  made to eliminate any   potential CTE effects within the mechanism. The tendency for bare titanium to w ear and gall  to itself is well  documented,  particularly for space applications,  where solid- phase welding can occur at areas of high  friction and elevated pressure despite temperatures well below the melting point2. The effects of  this  material phenomenon were considered to be mitigated in this mechanism through proper selection of  surface coatings and lubrication; a position supported by t he lack of any other example of titanium- titanium  galling or  during the clamshell development . Despite the presence of those coatings , it is theorized  that the   increas ed contact pressure that resulted from the tapered- cup line contacts, was enough to transfer titanium  material from the housing to the sliding cup and lightly cold- weld the two together.  This deposition filled the  reduced gap between the p arts and increased the sliding force beyond the capabilities of the 44-N (10-lbf)  pinpuller.  The combined effects of a singular out of tolerance part  and localized titanium -titanium galling  was determined to the root cause of this anomaly.  These effects were further exacerbated by control issues  observed during testing; some clamshell configurations experienced higher dynamic loads than others, but  that is a topic worthy of a different paper.    

Corrective Action       Several Failure Review Boards were held as a result of this incident , with reviewers invited from outside  organizations. With the root cause identified, a mitigation plan was developed, and unanimously supported  by the board. Ultimately, a complete set of  new active hinge halves, housings and sliding cups and were  re-fabricated as a result of this anomaly.  In an additional risk mitigation step, an entire new set of TiNi P10  pinpullers were procured as well, because the previous set had uncontrolled- length current pulses applied  to them  from the EGSE . Each mechanism part  was fully inspected to the same level of scrutiny as the parts   measured during the investigation, to ensure a minimal amount of part taper and shape. The inspection  data was then used to match the cups and the housings to each other to guarantee a minimum .055- mm  (.0022- in) diametrical gap in each assembly. Strategic material changes were considered, like a leaded
118 The robotic system on the outside consists of a 5 degree- of-freedom (DOF) Robotic Arm (RA), similar to  the roboti c arm implemented on the Curiosity rover, and a Turret assembly mounted to the end of the RA .  The Turret assembly contains a new rotary -percussive drill ( also referred to as the Corer), components of  which are described in [2], [3], and [4]; a ground contact sensor; a gas  dust removal tool (gDRT) , using  puffs of nitrogen to remove dust from abraded surfaces ; and two instruments, PIXL (Planetary Instrument  for X -ray Lithochemistry) and SHERLOC  (Scanning Habitable Environments with Raman and  Luminescence  for Organics and Chemicals) , for in -situ science.  

The robotic system (i.e., ACA) on the inside of the Rover  consists of a 3- DOF Sample Handling Assembly  (SHA) that manipulates sample tubes, gloves, and covers . Sample tubes with collected Martian material   are processed within the ACA  by moving the tube between stations using the SHA to support the following  functions :  assess the volume  of sample material collected, image the sample tube (multiple instances) ,  dispense a hermetic seal  into the sample tube, activate the hermetic seal to preserve the collected sample,  store the sealed sample tube until ready to drop, and finally drop- off the sealed sample tube to the surface  of Mars at prescribed locations.  

Figure 2 illustrates the SCS architecture and how the two robotic systems interact . Sample tubes stored  inside the ACA are inserted through the lower Bit Carousel (BC) door into sampling bits stored in the BC .  The BC rotor rotates the bit/tube to the upper door where the RA docks the Corer to the BC and the Corer  acquires the bit/tube using its actuated degrees -of-freedom . The RA undocks the Corer and then places  and preloads the Corer on the intended target . The Corer acquires a sample into the sample tube inside  the bit . After sample acquisition, the RA docks the Corer with the BC and the Corer transfers the filled  sample tube (still in the bit) to the BC . The RA undocks the Corer, and the BC rotates to orient the filled  sample tube for removal from the bit . After  BC rotor motion,  the filled sample tube is removed from the bit  by the SHA within the ACA and is ready to be processed using the ACA stations  (Figure 3) . Once the  acquired sample has been imaged in the sample tube, its volume has been measured, the sample  tube  has been hermetically  sealed, and stored for future deposit on the Martian surface,  the process repeats for  the suite of sample tubes available within the ACA . The remainder of this paper will focus on the ACA   portion of the Sampling and Caching Syst em. 

Figure 1. Perseverance Rover with SCS components highlighted, and Turret Assembly details .
119   Figure 2. Sampling System Architecture.  

Adaptive Caching Assembl y Description  and Operations  

The ACA  consists of stations/components as identified in Figure 3 (CAD images) , and Figures 4  and 5 (asbuilt images) : 

Figure 3. ACA CAD image s with stations identified , rotated to a bottom up view for visibility .  Left image:  SHA stowed for launch , and bit carousel doors closed.   Right  image:  SHA extended for sample tube operations , and bit carousel doors open.
120  

Figure 4. As-built ACA – Bottom Up View.  

Figure 5. As-built ACA – Front View (mounted on test fixture).  

The Caching Component Mounting Deck  (CCMD)  is a monolithic titanium structure to which all ACA  stations interface, and interfaces to the  Rover via three bipod assemblies . All ACA stations are made from  titanium t o minimize thermally induced position errors for robotic operations using the SHA . While the A CA  is designed to fit within  the Rover  for launch through landing, surface operations on Mars require the SHA  to extend approximately  200 mm below the Rover’s bellypan. Therefore, an ejectable bellypan was  implemented directly below the ACA volume,  which is released after landing to provide the SHA with an  unobstructed volume  to extend into during operations . Surface features are assessed via Rover imaging  prior to SHA motion to prevent  contact with potential obstacles  below  the Rover .
121 The ACA consists of 7 active degrees of freedom via gear motor assemblies (planetary gearboxes mated  to brushless motor s with magnetic detent brakes ) described in [5] . The gear motors support actuation of   the Bit Carousel , SHA, End Effector ( EE), and Sealing Station. A significant number of passive ball lock  mechanisms are implemented throughout the ACA to support locking and unlocking of hardware from  various stations/c omponents via SHA interaction. Hardware using ball lock mechanisms/features include  the bits, sample tubes, gloves, covers, EE  tube gripper, and the seal dispenser . A set of  ball lock mechanism   design guidelines were developed across the ACA to ensure cons istency  for each use case and included  details such as recommended geometry and clearances for mechanism features . These guidelines  also  accounted for dust inclusion in the mechanisms, which is a real concern for proper functionality . 

ACA Operations   A high- level end- to-end flow of the sample collection, processing, storage, and tube drop- off for a single  collected sample, focused on ACA operations, is shown in Figure 6. Every step, with the exception of the  three Sample Collection steps inv olving the Corer, requires a robotic interaction between the SHA and an  ACA station utilizing appropriate force and position limits to safeguard the hardware. The entire operation  from Sample Collection through Sample Storage executes autonomously in a few  hours . If a fault were to  occur during the sequence, the ACA is capable of recovering autonomously in some cases; otherwise,  operations will stop and the Rover will “ phone home” for assistance before proceeding. All Sample  Processing steps identified as i maging or assessment based are for documentation purposes only (to  support sample return selection), and are not decisional for proceeding through the full sequence, meaning  operations will not stop based on the results of those activities unless a non- recoverable fault were to occur.  

Figure 6. End-to-end sequence for caching each sample tube on Mars  

Instead of dropping off each sample tube once it’s processed, the current expected strategy is to place the  processed sample tube into storage until enough samples are collected in a particular region of interest, at  which point a group of samples will be dropped off at a designated location for caching. The location of  drop- off will be meticulously documented to support potential future retrieval and return to Earth, since the  sample tubes could become covered with dust and not easily visible. More than one drop- off location for  groups of sample tubes may be designated during the mission based on the number of regions of interest  visited for sampling activities.
124   Figure 7. BC Docking Assembly.  

Internal to the BC is a rotor mechanism assembly which actuates the carousel ’s rotor structure  to a desired  position for either tube or bit exchange operations  (Figure 8). The rotor structure is the primary interface  within the BC for the bit holders with bits installed, as  well as a sample tube sheath with a sample tube and  glove installed . The rotor  mechanism assembly consists of a n electronically commutated gear motor,   harmonic drive, and dual speed resolver  from Ducommun  Inc., with a back -to-back set of angular contact  bearings spaced  apart to carry both launch and operational loads . Wet lubrication is used within the rotor  mechanism components , which  requires  heating for proper functionality  at cold temperatures on Mars . A  spring energized Teflon seal is used to prevent debris from entering the mechanism within the rotor  assembly . Additionally, spring energized Teflon seals are used to prevent wet lubrication from migrating  into the carousel , which would be a contamination concern for the bits . The rotor mechanism does not  contain a hard stop and is free to rotate clockwise  (CW)  or counterclockwise (CCW) indefinitely as  commanded. The gear motor contains no brake, and only relies on a magnetic detent for  holding position.  

Inside the BC  resides a suite of  nine bits to support coring (6x), regolith collection  (1x), and abrading (2x)  operations . These bits are “locked” within bit holders when not in use to survive not only launch vibration  and Rover traverse loading, but both bit exchange with the Corer  and tube exchange with the SHA. The bit  holders contain both axial and radial cam rollers to align a bit within a holder and minimize tipping which  could be detrimental for tube- to-bit insertion . In addition, an axial s pring mechanism provides compliance  to accommodate misalignments during bit insertion and removal activities from a bit holder via the drill.  

The BC assembly contains an upper and lower opening in its structure. The lower entry point  allows for  sample tubes to be inserted into bits for sample acquisition, as well as removal from a bit once a sample  has been collected (Figure 8). The upper entry point  supports bit exchange operations  with the Corer  assembly . Both of these openings  utilize a one-time deploy ment  door to maintain cleanliness of the  hardware within the BC until release on the surface of Mars . Both doors consist of a passive spring- actuated  hinge mechanism held closed by a latch that is deployed via a release mechanism. The release mechanism  is a series of passive spring- actuated mechanisms initiated by a separation nut non- explosive actuator  (NEA) device from Ensign- Bickford  Aerospace & Defense Company .
125                          

Figure 8. Tube Exchange Configuration in the BC   (coring bit in tube exchange position at lower door opening, locked in a bit holder).  

When the doors are closed, a HEPA filter allows for venting during launch depressurization, and to minimize  contamination from entering the BC prior to door actuation on the surface of Mars. Once the doors are  deployed on Mars, crushable honeycomb absorbs the shock and determines  the final resting position of  the doors during surface operations on Mars . This position has been factored into the overall assembly to  ensure inadvertent contact does not occur with surrounding hardware , and sufficient clearance exists for  interfacing hardware, such as the Turret /Corer  during docking.  

Sample Handling Assembly   The SHA is a 3- DOF robotic arm approximately 0.5- meter long with a compliant, single DOF E nd Effector  (EE) mounted at the end of the arm (Figure 9) . The SHA is responsible for all sample tube, glove, and cover   manipulations between ACA stations . In order to  reach  the required operational work space in the ACA, the  SHA uses a prismatic joint , identified as the Z -stage,  which is actuated by an electronically -commutated  gear motor assembly ; and two revolute joints , identified as the shoulder and elbow joints , which are  actuated by electronically -commutated gear motors with harmonic drives and dual  speed absolute position  resolvers . The Z -stage is responsible for the  linear motion of the SHA , and is designed to allow the EE, or  any component (i.e., sample tube, glove, cover) on the EE, to interact with the ACA stations utilizing a  minimum preload capability of 350 N . This applies only during station interactions when the SHA is retracted  and not fully extended. A minimum clearance of 4 mm between the top of a hermetical ly sealed sample  tube attached to the EE and the lowest feature of each ACA station is accommodated when the SHA is  fully extended such that the sample tube can be manipulated safely in free space within the ACA volume.  

The Z -stage consists of an outer housing that directly interfaces  to the CCMD , and an internal translation  tube called the slide adapter that mounts to the shoulder  (Figure 10). A welded metal bellows between the  outer housing and the shoulder closes  out the Z-stage and protect s the internal components from Martian  dust. The slide adapter translates against the outer housing using two sets of eight track roller bearings:  one set is attached to the end of the outer housing and preloaded against the external surfaces of the slide  adapter and the other set is attached to the slide adapter and preloaded against the internal surfaces of the  outer housing . These track roller bearings are set sufficiently apart to carry the cross -axis moment and side  loads, and wit h the slide adapter and the bore of the outer housing having square cross -sections, the  Rotor  Mechanism