ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
289 Figure 11. Cross Section of Gearbox Assembly 

Analysis predicted that a 10.4:1 reduction ratio gave a torque margin of 140% and a thermal dissipation margin of 475% which exceeded the 125% requirement for a CDR-level design. Reducing the ratio to 6:1 decreases those margins to 41% and 99%, respectively. The gearbox single-stage ratio selection of 10.4:1 is somewhat unconventional when considering planetary gearbox rules of thumb which limit the highest practical reduction ratio to 10:1 due to decreased pinion size and increased sliding within the gear mesh (i.e. decreased efficiency and life).   

Figure 12. Actuator margin analysis methodology    The specific sliding ratio (SSR) is defined as the sliding velocity divided by the rolling velocity at the gear mesh. The SSR varies along each gear mesh. A higher SSR means that a gear mesh design has relatively more sliding action compared to a lower SSR and would therefore have increased friction and decreased efficiency and life. Therefore it is desirable to design a reduction stage to minimize the SSR within the constraints of other driving requirements. The rate of change of the SSR along the line of action should also be minimized for similar reasons.
290 The DRT required a high single-stage ratio in order to maintain torque margin on the motor as previously  discussed. This in combination with a pre-defined 15T pinion fixed the gear proportions. Figure 13 shows   how the SSR at the sun-planet gear mesh for the 10.4:1 design changes during a single mesh cycle. Looking at the lowest and highest points of single tooth contact (LPSTC and HPSTC respectively), i.e., the critical stress jump points, the SSR does not exceed a magnitude of 1.0. The worst-case SSR value occurs on the dedendum of the planet during recess action with a magnitude greater than 3.0. Because the SSR is less than 1.0 at both the LPSTC and HPSTC, the DRT gear design was deemed acceptable.    Other analyses included gear life analysis based on the American Gear Manufacturers Association (AGMA) standards 2001-D04 and 908-B89, backlash analysis, and momentary overload analysis. Backlash analysis included contributions from manufacturing tolerances as well as the operational thermal range to ensure that backlash existed under worst-case stack-up conditions. This was validated via accurate involute profile simulations. Momentary overload calculations included a combination of AGMA 2001-D04 and finite-element analysis (FEA) depending on the gear material selected.    

Figure 13. Sun-planet gear mesh specific sliding ratio   To validate the above analyses and verify that the DRT gearbox efficiency was acceptable, the DRT was subjected to output axis dynamometer testing across the operational temperature range. Dynamometer data was acquired at temperature set points (+70°C, +23°C, -55°C, & -70°C) at multiple motor voltages (6, 10, 16 and 22 volts). Speed-torque and current-torque profiles were created. The motor torque constant (K t) values calculated from the dynamometer data, as shown in Figure 14, match up well with  the K t values measured for the A300 motor prior to integration with the DRT gearbox. This data shows the  capability to design efficient planetary gearboxes with relatively high reduction ratios given careful gear tooth profile selection and analysis.    

Figure 14. DRT-integrated A300 Kt values across the protoflight thermal range are shown on the  left.  A300 Kt values (not integrated with DRT are shown on the right.     6 Volts 10 Volts 16 Volts 22 Volts 16 Volts 21 Volts Temperature CCW CCW CCW CCW CCW CCW +70°C 18.0 19.4 20.0 19.2 20.0 20.7 +23°C 18.8 20.4 19.8 20.1 21.1 21.0 -55°C 19.9 20.2 20.5 20.3 N/A N/A -70°C 21.4 22.0 20.3 20.7 23.1 23.1DRT-integrated A300 Kt values A300 Kt values
292 Acknowledgements 

The DRT team at Honeybee Robotics would like to acknowledge and thank the team of scientists,  engineers and managers at NASA JPL that offered their input during the DRT development process. The DRT marks the third time that Honeybee has developed a flight qualified robotic end-effector for a landed Mars mission through close collaboration and teamwork with JPL.   References 

1. Bishop, J., et al., 2002, A model for formation of dust, soil, and rock coatings on Mars: Physical  and chemical processes on the Martian surface,  JOURNAL OF GEOPHYSICAL RESEARCH,  VOL. 107, NO. E11, 5097, doi:10.1029/2001JE001581.  2. MSL Science Corner: Alpha Particle X-ray Spectrometer, http://mslscicorner.jpl.nasa.gov/Instruments/APXS/  3. Rieder, R., et al., 1997, The chemical composition of the Martian soil and rocks returned by the  mobile Alpha Proton X-ray Spectrometer: Preliminary results from the X-ray mode, Science, 278: 1771-1774.  4. Myrick, T., et al., 2004, Rock Abrasion Tool, Proceedings of the 37 th Aerospace Mechanisms  Symposium, NASA Johnson Space Center, May 19-21, 2004  5. Herman, J., Davis, K., 2008, Evaluation of Perflouropolyether Lubricant Lifetime in the High  Stress and High Stress-Cycle Regime for Mars Applications, Proceedings of the 39th Aerospace  Mechanisms Symposium, NASA Marshall Spaceflight Center, May 7-9, 2008  6. Okon, A., 2010, Mars Science Laboratory Drill, Proceedings of the 40th Aerospace Mechanisms  Symposium, NASA Kennedy Space Center, May 12-14, 2010  7. Chu, P., et al., 2008, Icy Soil Acquisition Device for the 2007 Phoenix Mars Lander, Proceedings  of the 39th Aerospace Mechanisms Symposium, NASA Marshall Spaceflight Center, May 7-9,  2008
293 A Zoom Lens for the MSL Mast Cameras:  Mechanical Design and Development 

Daniel DiBiase*, Dr. Jason Bardis* and Rius Billing*  

Abstract 

The Mars Science Laboratory (MSL), scheduled to land on Mars in August of 2012, brings with it the most  advanced set of instruments yet to land on another planet. One of MSL's primary science instruments, Mastcam, was originally scoped to be the first zoom lens camera to operate on Mars. After descope and  an 11th-hour reinstatement, zoom lens assemblies were built and mechanically qualified for the MSL mission in less than 12 months. Although they passed mechanical functional and life testing, the zoom lenses did not demonstrate superior optical performance (in the limited time available for optimization) compared to fixed focal length (FFL) lenses built following the earlier zoom descope. As a result, the Mastcam instrument does not have zoom capability as initially planned; it is equipped instead with 34mm and 100mm FFL lenses. Nevertheless, the design, development and qualification of the zoom lens mechanisms resulted in several valuable lessons learned. The Mastcam zoom lens, designed to operate in the severe environment of the MSL mission for one Martian year (approximately 2 Earth years) with 2x margin on life, is described along with the issues that were encountered in fabrication, assembly and testing in an accelerated schedule.   Introduction 

Mastcam Instrument Description and Background  On November 26, 2011 NASA launched the Mars Science Laboratory (MSL) on its way to Gale Crater on  Mars to learn more about the Red Planet’s climate and geological history. On or around August 6, 2012 the MSL rover, built by the Jet Propulsion Laboratory (JPL) and named "Curiosity," will land and begin its 687-day mission to explore and gather scientific data to assess whether Mars ever had, or still has now, an environment able to support life 1. To carry out this mission, the Curiosity rover is outfitted with the most  sophisticated array of scientific instruments ever sent to land on another planet. A Mast Camera system, or Mastcam for short, is one of the primary scientific instruments on the rover.   

Figure 1: Mastcam on Curiosity, the Mars Science Laboratory Rover  Image Credit: NASA/JPL-Caltech                                                     * MDA Information Systems, Inc. - Space Division, Pasadena, CA  

Proceedings of the 41st Aerospace Mechanisms Symposium, Jet Propulsion Laboratory, May 16-18, 2012
295 build the fixed focal length (FFL) lenses (34 mm and 100 mm), which were completed after a tight   18-month development and are currently in transit to Mars. The zoom lens design, meanwhile, was put on hold until January 2010 when James Cameron, a co-investigator for Mastcam, made a personal visit to NASA Administrator Charles Bolden to request the reinstatement of the Mastcam zoom lenses. Ultimately this request was granted and in late February, with funding in place, the zoom lens development effort was resumed. This effort resulted in three fully integrated zoom lens assemblies (2 flight units and  1 life-test unit, shown in Figure 2 at various stages of integration) being completed in December, 2010, and tested into March of 2011. Although these assemblies passed mechanical functional and life testing, the initial optical performance of the zoom lenses, while adequate over most of their focal range, was not as good as that of the FFL lenses. With little time for optical adjustments and further testing before the November 2011 launch date, the 34 and 100 mm lens assemblies, which did fulfill all science objectives for the Mastcam instrument, were chosen over the zoom lenses for the MSL mission.  The development of both the zoom and FFL lens assemblies for Mastcam was a collaboration between MSSS, the prime contractor for JPL and responsible for the optics and electronics design and assembly, and MDA Information Systems, Space Division, contracted by MSSS to design and build the lens mechanical assemblies. In this partnership, MSSS and MDA also developed the Mars Hand Lens Imager (MAHLI), another MSL science camera, which is mounted on the end of Curiosity's robotic arm and shares many features and mechanisms in common with the Mastcam mechanical lens designs 6.    Paper Scope  This paper presents the mechanical design of the Mastcam zoom lens, along with issues encountered during assembly, integration and testing. Although not part of the MSL mission, the Mastcam zoom lens is one of a few instruments with actuated optics designed to operate on the Martian surface and the only Mars instrument thus far to offer optical zoom capability with a design proven through life testing. Not covered in this paper is the Mastcam FFL lens assembly design, which contains a focus mechanism almost identical to the MAHLI focus mechanism described in reference 6.   Zoom Lens Design 

Design Overview  The Mastcam zoom lens, shown in Figure 3, packs both a zoom mechanism and focus mechanism within  a volume approximately the size of a 473 ml (16 oz) can of soda (a cylindrical volume roughly 6.5 cm in diameter by 18 cm long.) The zoom mechanism adjusts EFL by moving 3 lens groups, each through unique motion profiles, with one actuator. The focus mechanism independently positions a fourth lens group driven by a second actuator for active focus. A filter wheel mechanism, driven by a third actuator, and located aft of the zoom mechanism and in front of the CCD detector, selects 1 of 8 optical filters.
296   Figure 3: Zoom Lens Design Overview

The design maintains strict alignment requirements for the moving lens groups, mounted on linear bearings, over their range of travel and throughout operational environments without detriment from backlash or thermal distortions. The lens structure is aluminum, which has good specific strength and stiffness and good thermal conductivity to reduce distortion-inducing thermal gradients. Wherever possible, lens assembly parts are fabricated from this material. Flexural features are used where parts of differing coefficient of thermal expansion (CTE), such as the linear bearings, are fastened to the lens structure.  The following design constraints drive the Mastcam zoom lens mechanical assembly design:   687 days (1 Martian year) minimum lifetime with a 2x test demonstration factor  - 300,000 focus cycles - 1,300 zoom cycles   -55°C to +40°C operational temperature range   -135°C to +60°C survival temperature range  160G quasi-static launch load  900 gram maximum mass (including optical elements, not including electronics)  > 0.4 optical MTF at 68 lp/mm  Distortion > -10% at 6.2 mm EFL, < +3% at 100.4 mm EFL   Control positions of 21 optical elements (as tight as 0.005 mm decenter and total indicated runout  alignment of moving lens groups)  - 3 moving zoom lens groups with unique but coordinated motion profiles - 1 focus lens group with independent motion control - 2 stationary lens groups
297 Figure 4: Zoom Lens Optical Design 

Optics  The optical system for the Mastcam zoom lens, shown in Figure 4, consists of 20 powered optical elements, ranging in approximate diameter from 5 mm to 5 cm and made from a variety of Schott® glasses, as well as an optical filter changeable via the filter wheel mechanism. Total glass mass is approximately 80 grams. The powered elements are arranged in 6 lens groups (4 moving and 2 stationary groups) plus a single fixed element at the front of the lens assembly:    The fixed front element serves a double duty of closing out the zoom lens assembly, thereby  sealing the lens mechanisms from the dusty Martian environment.    Directly behind the front lens element lies the moving lens doublet of the Focus Group. The focus  mechanism actuates the Focus Group axially over a ~9 mm range of travel to provide focus at distances from 2 meters to infinity.   The lens quartet of Zoom Group 1 resides behind the Focus Group. It moves over ~38 mm of  travel and is one of three lens groups actuated on a unique profile by the zoom mechanism.    Next in the optical path, the stationary relay lens doublet, or Relay Group, is held by the inner  housing of the lens assembly.   The Zoom Group 2 lens doublet is located behind the Relay Group. Zoom Group 2 is actuated  axially by the zoom mechanism through a ~22 mm range of travel on its unique motion profile.   Behind Zoom Group 2, a lens sextet in Zoom Group 3 is also actuated by the zoom mechanism  along a third unique profile through ~22 mm of axial travel.   The Field Lens Group is a stationary doublet that resides in the aft end of the lens assembly, in  front of the filter wheel assembly.    Last in the optical path, before the Bayer pattern filtered CCD detector, is one of 8 optical filters  that can be changed by the filter wheel mechanism.
298 Each optical element and lens group is required to be positioned within prescribed tolerances. The  tightest and loosest tolerance required of the moving groups is provided in Table 1  for the listed degrees  of freedom (DOF). These tolerances were  drivers for the mechanism design.  

Table 1: Alignment Tolerance Ranges for Moving Lens Groups  DOF Name Tolerance Value Description  Decenter 0.005-0.030 mm The radial distance (in orthogonal directions, X and Y) between the nominal optical axis and the center of the element or group  Tilt 0.0002-0.0012 radian  The angular tilt of the element or group about X and Y axes  Despace 0.03-0.10 mm The axial distance (in Z direction) between an element or group and its nominal axial position   Drive System   The three zoom Mastcam mechanisms are powered by stepper motors with planetary gearhead speed reductions (256:1 for the zoom and focus mechanisms and 16:1 for the filter wheel mechanism). Like the MAHLI focus mechanism, the driving pinion in the zoom and focus mechanisms is supported on a duplex pair of angular contact ball bearings. These bearings are significantly larger than those within the motor gearhead and therefore can endure higher driving loads. Shown in Figure 5, a custom Oldham coupler connects the output shaft of the motor gearhead to the pinion gear. The pinion in the filter wheel mechanism, discussed later, is attached directly to the output shaft of the 16:1 gearhead because driving loads are low.   Figure 5: Oldham Motor Coupler   Zoom Drive System  The zoom mechanism, shown in Figure 6, moves the three lens groups simultaneously with a drive system resembling the MAHLI focus mechanism 6: 1) The stepper motor rotates a pinion gear through the  Oldham coupler; 2) the pinion meshes with a gear cut directly into the cam tube; and 3) cam tube rotation moves each zoom group axially along linear bearings following motion profiles prescribed by cam slots cut 295 degrees around the circumference of the cam tube. Each end of the cam slots serves as a hardstop for the zoom groups. Electroless nickel (eNi) coats the cam surface as well as the integral gear on the cam tube for wear resistance. Plating thickness encompasses the full depth of Hertzian contact-stress fields. The edges of the cam surface are rounded with fillets that both aid coating processes and reduce stress concentrations. Further detail and rationale for the cam design is discussed in the Lessons Learned section. A radial bearing serves as a cam follower with a twin bearing used for preload, as described later, in the cam slot.
299 Figure 6: Zoom Mechanism Drive System 

When the zoom mechanism is not used, the cam-follower bearings can be driven into launch restraints at the aft end of the cam slots. Figure 7 shows how the launch restraint, attached to the inside wall of the cam tube, raises the follower bearing off the cam surface, holding the follower bearing pivot on a ramp in such a way that the bearing is free from contact and therefore unloaded. Material for both the pivot and the restraint ramp was selected to minimize galling in this sliding interface. The launch restraint primarily protects against launch loads but can be used at other times, such as during rover driving operations.    Figure 7: Launch Restraint for Cam Follower   The cam tube rotates on a pair of bearings with raceways integral to the cam tube and the inner housing on which it rides. The rational for and development process of these integral bearings are described in the Lessons Learned section. Although this type of design poses higher wear- life risks compared to a  standard bearing set, machining raceways directly into the aluminum parts obviates steel parts and eliminates distortions due to CTE mismatch. With the exception of the cam tube, all integral raceways are treated with a hard anodic coating (Type III per MIL-A-8625) left unsealed for maximum wear resistance and polished for smooth finish. The raceways on the cam tube are coated with eNi along with the rest of this part. The anodic coating was not suitable for the cam or gear teeth, as explained in the Lessons Learned section, and applying both anodize and eNi (the coating needed for the cam and gear surfaces) on the cam tube part proved prohibitively difficult.
300 Figure 8: Integral Bearings 

The integral bearings, shown in Figure 8, consist of a 4-point bearing at the aft end of the cam tube and a 3-point bearing at the forward end. The 4-point bearing establishes axial position of the cam tube. A set of small, axially-acting springs preloads one raceway. The springs are housed on the backside of this raceway in a radial pattern of blind holes with a precise depth to protect the springs from over-compression. This scheme allows for adjustment of the total spring force by varying the number of springs installed. The 3-point bearing provides moment stability for the cam tube while allowing for axial compliance. Custom ball separators, made from aluminum and coated with eNi, ensure smooth operation by preventing contact and scrubbing between neighboring stainless steel balls.   

Figure 9: Focus Mechanism Drive System   Focus Drive System  The focus mechanism, shown in Figure 9, moves the focus lens group with the following drive system: 1) the stepper motor rotates a pinion gear through the Oldham coupler; 2) the pinion gear drives a smaller spur gear, increasing speed to meet focus speed requirements, supported by a duplex pair of angular contact ball bearings; 3) the spur gear rotates a 2-start lead screw through a custom 2-start helical flexible coupling; and 4) the lead screw drives the focus group axially along a linear bearing via a lead screw nut captured within the focus lens cell subassembly. The lead screw is made from stainless steel; the lead screw nut is brass, which prevents lead screw wear. Wear debris from the brass nut, potentially
301 detrimental to optical performance, was shown in tests to be adequately contained within the lubricant  grease mixture. The focus group motion is limited by hardstops that contact the linear bearing slide at both ends of the travel range.  A snubber restraint pin limits launch and rover driving loads on the single linear bearing that supports the focus group. The pin, located on the focus lens cell opposite from the linear bearing, travels along an axial slot in the lens structure. The pin makes contact with the slot during high lateral loads, limiting the strain in the linear bearing. The helical coupler is protected from overstraining by stops that limit the axial displacement of the lead screw which would otherwise tend to over-extend or over-compress the coupler.   Filter Wheel Drive System  The drive system of the filter wheel mechanism consists of a spur gear set as shown in Figure 10. The pinion, mounted directly to the motor gearhead, drives an eNi-coated bull gear machined directly into the wheel holding the filter elements. This wheel rotates continuously to position any one of the 8 optical filters in the light path; the mechanism is balanced and does not have hardstops or launch restraints. The wheel is supported on a 4-point integral bearing similar to that used for the cam tube in the zoom mechanism.    Figure 10: Filter Wheel Mechanism Drive System   Lubrication  Although initially designed to operate with dry lubricant, the Mastcam mechanisms are lubricated with Castrol Braycote® Micronic 601EF grease due to a programmatic decision to reduce wear-life risk at the expense of additional power required for heaters. A perfluorinated polyether (PFPE) based lubricant, Braycote® exhibits low outgassing properties, important for minimizing optics contamination, and excellent cold temperature performance, enabling the mechanism to meet requirements at temperatures as low as -55°C and to provide limited operation down to -70°C. Braycote® 601 additionally contains a corrosion inhibitor useful for protecting the ferrous surfaces in the mechanisms during pre-launch operations. To reduce friction further, tribological surfaces are polished with lay direction aligned with wear and/or friction direction.   A Nyebar® fluorocarbon barrier coating (type Q), with a low surface-tension, non-wettable surface, stops lubrication migration by preventing capillary action of the Brayco® 815Z base oil on part surfaces. By holding the lubricant in place, the barrier coating serves a dual purpose of reducing contamination risk to the optics and extending lubricant wear life. Fears of an incompatibility between the fluorocarbon Nyebar® barrier coating and the PFPE based Braycote® lubricant 7,8,9 were allayed with a simple test that  showed over 4 years of active repulsion of the lubricant by the barrier coating when applied with a specific process.