ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
275  

Figure 15: Before (Left) and after addition of Braycote leak path closures (Right) 

Working in a Dirty World 

As part of the final preparations for launch and closeout of the Verification and Validation matrices, the testing performed on all of the Rover pyros was summarized in order to ensure the Rover had been properly and completely tested. Although the pin puller used in the Differential Restraint had been qualified and testing had been completed at the sub-system and system level tests over temperature one concern was raised that the pin puller had never been fired in a dirty environment. None of the rover release pyros had been tested in dirt because they are fired shortly after the rover enters the Mars atmosphere while the rover is still connected to the Descent Stage and approximately one-hundred feet above the surface. The Differential Restraint pin puller, however, is fired much later in the deploy sequence, when the rover is approximately 2 seconds from final touchdown. Not only is there dust expected in the atmosphere at the level of the firing of the Differential Restraint pin puller, but the Descent Stage rocket engines are also expected to kick up significant quantities of Mars dirt into the atmosphere. The tight clearances between the Horizontal Swingarm Bracket bore and the pin puller pin (nominal 0.0015” diametral clearance) created a concern that small particulates of sand/dust could get stuck between the two surfaces and jam the joint, thereby preventing retraction of the pin. Additionally, there was a concern that if debris were to enter the pin puller device itself the retraction of the pin could fail.   Two tests were conducted to demonstrate the ability of the Differential Restraint Pin Puller to successfully retract in the presence of large quantities of dirt. Hardware which mimicked the Flight hardware clearances and tolerances was used to perform the test. This hardware had previously been used to qualify the pin puller for release while subjected to large lateral loads over temperature and varying percentages of NASA Standard Initiator charge. An acrylic chamber was built to sit around the Differential Restraint mock hardware and a GN2 line was setup with two inlets to the container to create a simulated dust storm. In total, 700 ml of dust simulant was added to the chamber, including 600 ml of cohesionless fine grain sand, which was used in traverse testing characterization for the MSL rover, 50 ml of JSC-1 lunar soil simulant and 50 ml of BP-1 dust (see Figure 16). GN2 was pumped into the chamber at  345 kPa (50 psi) for 30 seconds prior to firing the pin puller to create the simulated dust storm.
276   Figure 16: Dirty Pin Puller Test Setup: Pretest (Left), Posttest (Right) 

The first test involved a dual firing of 100% charged NSI while the second test involved only a single NSI charged to 100%. Both tests included the application of the flight limit lateral load applied to the pin puller pin through the use of a strain gaged bolt connected to the Linkage hardware simulator. Both tests resulted in successful retraction of the pin puller. Upon inspection of the pin puller pin, Linkage monoball, and Horizontal Swingarm Bracket bore after the test no unusual wear marks or scoring were found, indicating that the fine grains of dust and sand had not jammed or impeded the retraction of the pin. The monoball in the Linkage, did however, have a significant increase in drag due to the presence of dirt in the lubricant on the monoball increasing the rolling friction between the ball and race. The increased drag in the monoball is not significant, however, since it did not provide enough resistance to jam the pin and prevent it from retracting.    Key Findings:   Consider operational environments during the design of hardware (ex. how particulate  contaminants affect hardware clearances and failure modes)   Pin Pullers can successfully operate in the presence of fine grain particulates  

Summary 

The design of the Differential Restraint for the MSL mobility system involved striking a difficult balance between strict load and deflection requirements with the tight design space available. In total, three major design iterations were required to get the Differential Restraint from concept to the launch pad, and along the way some interesting lessons were learned. The first redesign effort was required when the loads model drastically increased the predicted maximum loads. The failure to consider the shimming procedure and misalignments allowed for by piece part clearances and tolerances was a costly mistake which led to the failure of the design in a major system level test causing the entire design to be subjected to major scrutiny and redesigned for a second time. The tolerances and clearances, when considered, allowed for over four times the assumed misalignment between the axis of the Spring Plunger and the Linkage which proportionally increased the bending stress in the Spring Plunger. The absence of a proper thread relief callout on the Spring Plunger drawing provided an easy target for scrutiny of the design, but was also easily corrected in the redesign efforts. The leaking of Braycote during vacuum bake should not have been a surprise, but the failure to assess the meaning of the viscosity of the lubricant at high temperatures led to the requirement of yet another design Band-Aid, the implementation of which would have neglected the effects of depressurization and rules of thumb for venting ratios except for a last minute catch of the newly created issue. The development of the Differential Restraint was not a linear or
279 Mars Science Laboratory’s Dust Removal Tool 

Kiel Davis*, Jason Herman*, Mike Maksymuk*, Jack Wilson*, Philip Chu*1,  Kevin Burke**, Louise Jandura** and Kyle Brown** 

Abstract 

The Dust Removal Tool (DRT) is designed to expose the natural surfaces of Martian rocks obscured by layers of dust deposited by aeolian processes. The DRT, contained within a cylinder 154-mm long and 102-mm in diameter, has a mass of 925 grams. Using a single brushless DC motor, the DRT removes dust from an area 45 mm in diameter. During the dust removal process, a set of brushes articulate to maintain surface contact as they rotate at high speed. The DRT belongs to a special class of aerospace mechanisms designed to interact with unstructured extraterrestrial surface objects and environments. The wide range of rock surface characteristics along with severe resource constraints makes the DRT solution non-trivial. The mechanism features a high reduction single-stage planetary gear box and pivoting brushes that both offered lessons learned. The flight unit DRT was integrated with the MSL rover in early 2011 and is currently on track to begin surface operations at Mars’ Gale Crater in August 2012.   Introduction 

The Dust Removal Tool (DRT) is a critical component of Mars Science Laboratory’s (MSL) Sample Acquisition, Sample Processing and Handling (SA/SPaH) subsystem. The aeolian-deposited reddish iron oxide dust that covers ever ything on the surface of Mars masks many characteristics of rocks and makes  it difficult for scientists to identify optimal rock targets for further interrogation and possible sample acquisition. For instance, as shown in Figure 1, dust layers only microns deep can obscure many visual clues to a rock’s origin (e.g., color or emissivity , cracks and inclusions) from instruments like the MSL’s  MastCam and Mars Hand Lens Imager [1]. MSL’s Alpha-particle X-ray spectrometer (APXS), a key source of information about a rock’s elemental composition, is effectively blinded by layers of dust as little as 5 microns deep [2]. It is therefore critically important to remove dust from the surface of rocks. 

Figure 1. Left: MER Spirit Pancam false-color image of Mazatzal on Sol 86 after numerous Rock Abrasion  Tool brushing and grinding operations; Right: MER Spirit  Microscopic Imager false-color image of Mazatzal  on Sol 79 after a RAT brushing operation. Image credit: NASA/JPL-Caltech/Cornell                                                    * Honeybee Robotics Spacecraft Mechanisms Corp., New York, NY  ** Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 

Proceedings of the Aerospace Mechanisms Symposium, Jet Propulsion Laboratory, May 16-18, 2012
280 Planetary scientists may have first appreciated the dust’s complicating effect during the Mars Pathfinder  mission where APXS data were contaminated by dust and meaningful inferences could only be drawn through a process that assumed an amount of dust that had to be subtracted from the spectra [1]. Unlike Mars Pathfinder, the Mars Exploration Mission rovers Spirit and Opportunity both carried a tool for doing away with the dust a rock’s surface. This tool, developed by Honeybee Robotics, is called the Rock Abrasion Tool (RAT) [4]. The RAT’s primary purpose is to remove the rock’s weathered rind by grinding an area 45 mm in diameter to a depth of 5 mm. By accomplishing its primary objective it naturally has to also remove surface dust. The RAT is equipped with brushes for sweeping away cuttings produced during the grinding process – see Figure 2. During the initial stages of RAT development, it was never envisioned or required that the RAT’s brushes be used to strictly brush away surface dust layers. But through experimentation before and during the MER mission, it was eventually found that the RAT’s high-speed brush, which protruded several millimeters beyond the grinding wheel’s reach, is particularly effective at removing even fine layers of dust from all manner of rocks including those with pitted, vesicular surface textures.   The RAT can also accommodate a large degree of surface topography variation and robotic arm (IDD) positioning error due to its design architecture and 3 degrees of freedom. Prior to brushing or grinding, the IDD preloads the RAT against the surface of the rock via the RAT’s butterfly mechanism (see Figure 2). The RAT has three actuators including a Z-axis actuator which moves the grinding wheel and brushes linearly toward and away from the rock surface. Once preloaded, a RAT software algorithm employing all three actuators, detects the rock’s local surface position in the Z-axis reference frame. The RAT grind brush is then positioned with respect to the surface such that its bristles are engaged and the grinding wheel is not engaged. In this way, the arm positioning error is rendered more or less moot and successful brushing is achieved on a wide range of uneven surfaces.  

Figure 2. Left: MER Spirit RAT on Mars prior to its first operation on Adirondack rock; Right: MER  Opportunity RAT on Mars after many operations. Image credit: NASA/JPL-Caltech/Cornell   The original plan for MSL was to include a next generation RAT with all the same functionality as the MER RAT but designed to last longer and penetrate stronger rocks [5]. However, that tool was eventually descoped by the JPL flight project office in 2007 to meet project budget constraints. This left the MSL science team without a method for clearing the blinding dust from rock surfaces. So later in 2007 a new effort was initiated to develop a tool, the DRT, designed solely for the purposes of removing dust.  Like other MSL mechanisms [6], the DRT is designed for very long life in very harsh conditions. But the DRT is unique and interesting for at least two reasons. First, as a device that directly, physically engages
281 an extraterrestrial object (rock) so as to manipulate this object’s characteristics in some way, the DRT  belongs to a special class of aerospace mechanisms. In this context, it is interesting and informative to future missions to understand how the DRT design solves the difficult problem of adequately removing micron-scale particles from a rock’s uneven and often pitted surface. Second, from a pure mechanisms perspective, the DRT offers a valuable data point for planetary gear box designers along with other lessons. Due to extreme volume constraints, there was just enough room for a single stage planetary gear reduction necessary for meeting torque margin requirements. Practical limits of planetary stage reductions are considered 3:1 (lowest, planets become very small) to 10:1 (highest, sun becomes very small). A ratio closer to 5:1 is the most balanced with the highest performance rating and is therefore more common in space mechanisms designs. The DRT design incorporated a 10.4:1 single stage planetary gear reduction which pushes the limits of what is conventionally considered practical.  This paper will provide an overview of the DRT including requirements, design, manufacturing and qualification testing. It will present some of the key trades made during the development process including brush articulation kinematics and bristle geometry to maximize the reachable workspace. A significant part of the paper will deal specifically with brush pivot torque margin problems, the planetary gear design and related test results.    DRT Overview   The DRT is mounted on the MSL robotic arm (RA) turret as shown in Figure 3. Once placed on a rock by the RA, the DRT’s primary functional requirement is to clear dust layers up to 2 mm deep from an area on the rock’s surface no less than 45 mm in diameter – this diameter is driven by the APXS field of view and RA positioning accuracy. Like the MER RAT, the DRT is expected to face all kinds of rocks with diverse surface textures and topographies. Unlike the MER RAT however, the DRT only has a single actuator and is not preloaded against the surface. The design must therefore accommodate robotic arm positioning error in addition to varying local rock surface topography. These new design and operational constraints were challenging and required a substantial departure from the heritage RAT brush design.  

Figure 3. Left: MSL rover; Right: SA/SPaH turret with DRT.  Image credit NASA\ JPL-Caltech [6]    To deploy the DRT, the RA first confirms the rock surface location using various contact sensors on the turret and then positions the DRT at a safe stand-off distance (not in contact with the rock). The DRT motor is then energized such that the bristles spin at relatively high speed and the RA then moves the DRT toward the rock surface to a position where the bristles are theoretically engaged. The DRT engagement position and angle may be off by as much as 10 mm and 15°, respectively, in any direction due to RA positioning error. The DRT motor current and encoder are monitored for a stall condition, but otherwise it is an open loop operation where the DRT motor is left energized for a period of time
282 (~60 seconds) to sweep the rock. The RA then pulls the DRT back away from the rock while the bristles  are spinning at low speed so as not to inadvertently foul the cleared area with a sudden stop and dragging of the bristles (an undesirable effect observed during testing). The DRT motor is then de-energized while in free-space.  In addition to the single actuator and RA operational constraints, the other major constraints included the allowable volume and mass for the DRT and the fact that the DRT had to employ a particular JPL-supplied motor model. The DRT was to fit in a cylindrical envelope approximately 134-mm long and  141 mm in diameter. The allowable mass was 950 grams which included the mass of the motor (350 grams). The DRT design was to assume that the motor could provide 28 mN-m of torque at 10,000 rpm.  The DRT engineering challenge could then be boiled down to designing a set of brushes with the reach and compliance to meet the primary functional requirement (i.e., sweep a 45-mm diameter) across the range of surface topography and texture scenarios while not exceeding the tool volume, mass and motor limits. The resulting DRT fit within a cylindrical envelope 154-mm long and 102 mm in diameter and weighed 925 grams. The mechanism is comprised of a JPL-supplied motor integrated with a Honey bee  custom-designed planetary gearbox which drove a sub-assembly called the brush block. The brush block consists of an asymmetric set of brushes each on spring loaded pivot (hinge) in order to accommodate the wide variation in surface geometry. A resistive strip heater is bonded to the outside of the planetary gearbox. A rotating post at the center of the brush block guards against overloading the tool’s brushes against the rock.   

Figure 4. Dust Removal Tool
283 Brush Block Subassembly 

The brush design process was heavily informed by a prototype testing program that lasted beyond the  Critical Design Review. Rocks and dust analogs we re supplied to Honeybee by JPL. A test and  measurement methodology was established which quantitatively assessed brush performance. Per its equipment specification, the DRT dust removal capability was to be demonstrated by clearing 70% of dust particles less than 500 microns by surface area from a natural surface. It was especially important to clear the center of the 45-mm diameter area. Images of the surface were captured via digital camera and analyzed in software to determine the size of the area cleared by the tool. Additionally, the DRT would be required to perform dust removal operations up to 150 times on Mars and therefore a 2x demonstration (300 operations) would need to be performed with the final brush life test model. So quantitative measurements of bristle wear and brush shape degradation were also made to project life.  Initially the team considered several variants of the very simple “brush-on-a-stick” concept (left most image in Figure 5) but soon realized that bristle compliance alone was not enough to compensate for large surface height variations without massive increases in parasitic drag on the motor. Furthermore, relying on bristle compliance also worked the bristles much harder causing wear and flexing that shortened bristle life. So the notion of allowing the brush bristles to pivot about a spring loaded hinge was quickly adopted. The brushes can pivot up to 30° so that large surface height variations (10-20 mm) can be accommodated. By using a soft spring with a flat spring rate to keep the bristles loaded against the rock surface, the bristle contact force and resulting motor torque changes very little.  

Figure 5. Brush design evolution 

Figure 6. Matrix showing the major brush related design/ performance drivers 

45 mm Diameter  Circle> 70% Clean Center ClearingAvoid Bristle  EntanglementVolume Envelope  Restrictions< 50 N Reaction  Load into Turret< 5 Micron Dust Brush Width Brush Length Wire Bend Brush Offset Distance  from Center Brush Separation  Distance Brush Symmetry Brush Pivot Axis Distance  from Center Brush Angle Spring Preload Brush Speed Approach Algorithm Retract Algorithm Engagement Distance Brush DurationBrush  Geometry Brush Holder  Geometry Software and  PlacementRequirements and Restrictions
285 and Phoenix Icy Soil Acquisition Device cutting bit shaft [4][7]. The shaft is spring-loaded against a hardstop by dual torsion springs. End cover-plates with integral spring arbors enclose the spring-bushing area. The pivot shaft-bushing-spring system is dry lubricated.  

Figure 7.  DRT Brush pivot 

Figure 8. DRT Flight Model brush contact force vs. surface height   Sizing the pivot’s springs was an exercise in threading the needle. On the one hand, the springs needed to be strong enough to keep the brushes in contact with the rock over the complete range of pivot motion (including at its relaxed hard stop). And on the other hand they needed to be soft enough such that the brush contact force at the fully compressed pivot position caused very little drag on the motor and the motor torque margin requirement would be met. It only took on the order of 3 N of bristle contact force to achieve the allowable motor current limit. So the springs were designed to deliver about 1.4 N of contact 

Applied Brush Force vs. Center Post Distance from Target Across Operational Temperature Range 0.00.51.01.52.0 0 5 10 15 20 25 Distance from Center Post to Target Surface (mm)Average Measured Force (N)-70°C +23°C +70°C
287 margin should still be comfortably positive (FOS > 1). It was clear that something was missing from the  model.  

Figure 9. Seal Drag Model   Next a complete disassembly of the EM Inner Brush pivot was performed and documented. Unfortunately, other than very minor evidence of components rubbing (dark marks), there was no clear culprit explaining the problem with the EM Inner Brush pivot. The EM brush block was reassembled and the unit was sent to JPL for further testing.  At JPL, tests were performed at temperature on both the EM and FM to more directly measure the pivot’s drag and spring torque and thereby achieve a clear demonstration of torque margin. A general model of the pivot behavior is shown in Figure 10 and assumes drag torque is equal in both directions and of course opposes the direction of motion. So in one direction, the measured torque is the sum of the spring and drag torque. While in the other direction, the measured torque is the difference between the spring and drag torque.  

Figure 10. Pivot spring drag torque model   The JPL test results are shown in the columns labeled “0 rpm” in Table 1. At 0 RPM, the brush block pivots demonstrated a margin of 100% or more (FOS ≥  2) in only one instance for the EM and only one 012345678 -0.001 0.000 0.001 0.002 0.003 0.004 0.005 0.006Additional Diametral Interference (in)Seal Drag Torque (in-oz)EM Outer Brush & FM Inner/Outer Brush @ -70C (predict) EM Inner Brush @ -70C (predict) Knee in  the curveEM Outer Brush & FM Inner/Outer Brush @ 23C (measured) EM Inner Brush @  23C (measured)
288 instance for the FM. In many cases, negative torque margin was calculated. The JPL tests confirmed that  the DRT pivot design did not meet the torque margin requirement.   Table 1. DRT Pivot Torque Factor of Safety (FOS) Test Results (min. required FOS: 2.0) 

   Next calculations were repeated to determine the torque margin when taking into account the centripetal forces acting on the brushes when the motor is spinning the brush block at 900 rpm – this was the speed used at the end of each performance test brushing operation after the DRT had been retracted away from the rock. These calculations are shown in the columns labeled “900 rpm” in Table 1. It was determined that operating the DRT at 900 rpm produces a calculated centripetal force that will reliably return the brush pivots to their hard stops. The minimum FOS for the FM in this case is 1.64 (Outer Brush Test #1 at -70°C).  No further work was done to determine root cause and no hardware changes were called for as a flight operations rule that spins the DRT brush block at 900 rpm before each use was deemed an adequate solution by the review board.   Motor & Gearhead Subassembly 

A challenge in the DRT design was to overcome the torque margin constraints due to the required use of a predefined motor. Based on the motor performance specifications, torque amplification was necessary and a custom single-stage planetary gearbox was designed (ref. Figure 11). The pinion of the motor was predefined as a long-addendum spur gear. This was likely done to avoid undercutting and increase the load capacity and life of the small pinion, necessitating the use of modified profile planet and ring gears.    Actuator margin analysis (ref. Figure 12) was performed based on those performance parameters as well as thermal limitations provided by JPL based on heat-up analyses. Various parameters were also estimated including seal and bearing losses as well as operational torque. The latter was estimated based on data from brush development tests.     0  r p m9 0 0  r p m0  r p m9 0 0  r p m0  r p m9 0 0  r p m0  r p m9 0 0  r p m Test #1 2.00 6.96 1.24 4.64 1.51 5.59 1.33 4.69 Test #2 1.78 6.54 1.23 4.60 1.83 6.44 1.56 5.54 Test #1 1.30 5.03 1.58 4.85 1.27 4.79 1.87 4.85 Test #2 1.23 5.28 1.42 4.63 1.17 4.60 2.20 6.20 Test #1 0.67 3.34 0.61 2.27 0.96 3.61 1.05 3.28 Test #2 1.06 3.58 0.54 2.29 0.74 3.13 0.52 2.13 Test #1 0.76 2.22 0.47 1.50 0.43 1.87 0.51 1.64 Test #2 0.57 2.04 0.35 1.33 0.38 1.78 0.64 1.77Test #FM DRT EM DRT Inner Brush Outer Brush Inner Brush Outer Brush Temp (°C) +70 +20 -30 -70