ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
Figure 1. MIL-PRF-39023-style pot, HGAG pot, and WSA pot, from left to right  2) A conductive plastic disk bonded to the cup. This disk contains two annular tracks: a <360"  resistive track (the resistance element that the wipers rub) and a 360" conductive track (or slip ring)  co-molded into the Diallyl Phthalate disk substrate.  3) External electrical connection via three brass terminals potted into the cup on the MIL-PRF-39023-  style pots, or three 28 AWG pigtails provided on the WSA and HGAG pots.  4) Internal electrical connections made by welding, soldering or bonding with conductive epoxy, and  employing solid, flat ribbon wire conductors connecting:  a) the end termination junctions of the resistance element to the "CW" and "CCW" input  terminals or input pigtails, and  b) the junction on the conductive track (slip ring) to the "W" wiper terminal or wiper pigtail.  5) Series connected wipers mounted to, but insulated from, the rotating shaft. Sweeping over both  tracks, they pick up the voltage from the resistive track and send it through the conductive track (slip  ring) to the wiper terminal. Wipers were comprised of multiple, thin, cantilevered contact fingers  made of precious metal alloys with contact forces controlled to approximately 209.  During calibration, each resistance element was manually trimmed to a specified linearity tolerance by  scratching the surface of the conductive plastic disk with a scribe, and/or by placing dots of conductive  silver paint on its surface. All resistance elements were 6250 * 20% Ohm and rated for 1 watt; all wiper  circuits were rated to carry 10 mA. The MER avionics energized the resistance elements with 5 VDC for a  power dissipation of <5 mW and drew only 50 nanoAmps through the pot wiper circuits.  Qualification Testing  To expedite matters, the MER project designated one mechanical engineer to be cognizant of all pot  technical issues and qualification testing. For validation purposes non-flight Engineering Model (EM) pots  were ordered and received well before the flight pots. To reduce cost and lead time, the EM pots were  subjected to a reduced set of acceptance tests by the manufacturer. When the EM pots arrived at JPL  they were distributed to the end users without careful inspection or electrical testing.  Most of the qualification testing on the flight pots was done by the manufacturer per a JPL-approved  Acceptance Test Procedure (ATP). However, since the manufacturer did not have thermal-vacuum test  chambers capable of the required -120" to +llOo C range, the thermal-vacuum qualification tests were  performed at JPL. When the flight pots were received at JPL, each was given a room temperature  functional test. First the total resistance of each pot was measured. Then 5 VDC was applied across the  input terminals of each MIL-PRF-39023-style pot to energize the resistance element. While its bearing-  supported shaft was manually rotated, the output voltage of each pot was plotted on a strip chart recorder.  304
Figure 3. Arc damage to fingers on wipers, and to conductive plastic disks on S/N 1035 and 1041  Immediately after documenting the extensive damage in these pots, I began investigating probable  causes. As I was not present during the original testing, I had to perform some detective work. I  discovered that the manufacturer's rating for the current through the wiper circuit, though never specified  in any of the MER pot specs, was only 10 mA. Whenever a wiper was positioned near the "CCW" end of  the resistance element, a low resistance ( cc 500 Ohms ) circuit existed between the "CCW" and "W"  terminals (Figure 4). If 5 VDC was applied between the "CCW" and "W" terminals when the wiper was  positioned near the "CCW" end of the resistance element, a current well in excess of the manufacturer's  rating would have flowed through the wiper circuit, damaging the pot.  ccw TERM1 N AL (t W TERMINAL  C W TERM1 NAL  Figure 4. The wiper position shown forms a low resistance circuit between CCW and W terminals  306
The procedure that covered the functional testing of these pots read:  Set multimeter to 10 kOhm - 20 kOhm range ...  Perform the following steps for each potentiometer:  1.  2.  3.  4.  5. Remove each potentiometer from its shipping container.  Record the resistance from the CW to the CCW terminals in the table at the end of this  procedure. Resistance should be between 5 kOhm to 7.5 kOhm.  Remove lead from CW terminal and reattach lead onto W terminal.  Using a chart recorder, power supply set to 5V, and multi-meter, rotate the rotor of each  potentiometer through its entire electrical range by hand. Attempt to rotate the rotor at a constant  rate. Look for discontinuities or noise in the signal. Record the potentiometer part and serial  number on each strip chart, and append the strip charts to this procedure.  Replace each potentiometer into its shipping container.  No electrical schematic was given and no polarities were mentioned. Note that step 3 left the pot's "CCW"  and "W" terminals connected to test leads and implied rather than mandated the switching of the  multimeter from the "Ohms" to "DCV" range. Step 4 connected a power supply but did not specify how,  and though the power supply could deliver 3 amps, there was no instruction to set a current limit. Then  rotating the pot ensured that the wiper would pass over the "CCW" end of the resistance element.  The same set-up was used to test all pots. Had the power supply been wired across every pot's "CCW"  and "W" terminals, all pots would have been damaged identically. However, the first seven pots tested  using this set-up passed. SIN 1035 was the eighth pot tested but the first one damaged. S/N 1041 was the  twenty-sixth pot tested, but only the second one damaged. Therefore, it can be concluded that nominally  the power supply and instrumentation were wired correctly and that the power supply was not intentionally  connected across every pot's "CCW" and "W" terminals.  Close examination of the internal damage indicated that the wipers were stationary when the problem  occurred. Note that Pomona Electronics brand "Minigrabber" test clips were used to connect to the pot  terminals (Figure 5). When depressed, a beryllium-copper hook projects from the end of these test clips.  As the procedure does not mention turning off the power supply outputs, the pots were likely hooked up to  live (powered) test clips. The terminals on the MIL-PRF-39023 style pots are only 6mm apart. When  attaching the powered clip meant for the "CW" terminal, it would have been very easy to bump the "W"  terminal with the metal hook, or even clip it to the "W" terminal outright. If the wiper happened to be  positioned near the "CCW" end of the resistance element, a high current could have flowed through the  wiper circuit and caused the damage observed.  Figure 5. The live, exposed hook on the test clip is a danger to pots with closely spaced terminals  307
Figure 9. X-ray image of undamaged wiper and slip ring brushes in HGAG pot  After plotting and examining the data, I hypothesized that a high resistance leakage path between the  “CCW” end termination of the resistance element and the slip ring could explain the anomaly. A schematic  of a pot with an internal leakage resistance was drawn (Figure 10) and an equivalent resistance equation  was written as a function of shaft angle. This mathematical model was iterated with different values of  leakage resistance until the model predictions matched the measured total resistance data. Note that the  51.9 kOhm leakage resistance found in this way was 8.3 times the pot’s nominal total resistance.  -I  0 50 100 150 200 250 300 350  Rotor Angle, CCW cw  Figure 10. Measurements match model of HGAG pot with internal leakage resistance RL =51 kOhm  The prediction of the model fits the data extremely well, supporting the hypothesis. The extremely linear  output of this pot supports the conclusion that this leakage resistance was present all along but never  discovered (because per a strict reading of the ATP instructions, no one was looking for it). Any non-  linearity caused by the leakage resistance was likely removed by adding scratches and/or drops of silver  paint when the pot’s linearity was adjusted by the manufacturer.  True confirmation of the hypothesis requires direct measurement and so the pot was disassembled by  removing the rotor from the stator. In this condition the leakage resistance could be accurately measured  because the slip ring and the resistance element were no longer bridged by the brushes. With the pot  disassembled, the leakage resistance measured 51.4 kOhms to the “CCW” lead, and 57.6 kOhms to the  “CW” lead. Note that 57.6 - 51.4 = 6.2 kOhm, the nominal total resistance.  The resistance element showed a very large number of linearizing marks, with all the silver paint dots  grouped in the middle of the element and all the scratches at the ends near the junctions; exactly what  one would expect if the pot had been adjusted to get rid of a manufactured-in leakage resistance.  It is reasonable to ask what caused the leakage resistance. There was a concern that it could have  formed from conductive debris worn off the surface of the resistance element or slip ring by a rough wiper,  31 4
and scattered between them. However that proposed mechanism was eliminated when upon disassembly  the resistance element and slip ring showed no signs of wear, scraping or dusting.  The conductive plastic disk was made of carbon particles in an insulating matrix, co-cured at 160" C. It is  believed that if some conductive carbon particles in the junction area were displaced from the resistance  element and co-cured in place, a high resistance leakage path between the element and the slip ring  would have resulted.  While the discovery of the varying total resistance was significant, S/N 1017 has always been within  tolerance. Testing verified its linearity, and though it varied, the total resistance was within the 6250 * 20%  (5000 - 7500) Ohm allowable range. Of course, this leakage resistance does waste a bit of power.  Based on the preceding failure analysis, this problem was attributed to manufacturing error due to a faulty  production process. Pot S/N 1017 was labeled "Limited Use (Non-Flight Part)" and was not used for flight.  LESSON 19. Being within tolerance does not mean a pot is problem free.  LESSON 20. Unique behavior is always worth investigating until it is understood, to make sure that  the problem does not affect other pots in the lot.  WSA Pots  During functional testing of the WSA pots before the three thermal cycles, the total resistance of WSA  Potentiometer SIN 1039 measured 7400 Ohms. After the three thermal cycles, the total resistance  dropped to 6900 Ohms. Compared to the other WSA pots in the lot, this change was a full order of  magnitude larger than expected.  After duplicating the original functional test setup and now taking the proper precautions (see Lessons  1 - 6), the problem was verified.  Next the build documentation was examined to see if it held any clues to this anomaly. The spec called for  a total resistance of 6250 Ohms * 20%. The in-process data sheet filled out by the pot manufacturer listed  the total resistance as 7391 Ohms. After environmental testing, the in-process data sheet listed the total  resistance as 7404 Ohm. While S/N 1039 was in spec, all the other WSA potentiometers supplied to JPL  had much lower total resistances. A histogram of WSA total resistance values (Figure 11) clearly shows  SIN 1039 to be an outlier.  Total Resistance, Ohms  Figure 11. Histogram of Total Resistance values showing the lone outlier, S/N 1039  SIN 1039 was still a flight pot, so I was in no rush to tear it apart. Besides, FeinFocus X-ray images of S/N  1039 revealed no internal damage.  Instead I elected to do more testing. The pot was powered with 1.0 VDC across the input wires and the  wiper voltage was measured and recorded as the rotor was rotated through 360 degrees in 5 degree  31 5
Mechanism Development, Testing, and Lessons Learned for the  Advanced Resistive Exercise Device  Christopher D. ~amoreaux* and Mark E. Landeck*  Abstract  The Advanced Resistive Exercise Device (ARED) (Figure 1) has been developed at NASA Johnson  Space Center, for the International Space Station (ISS) program. ARED is a multi-exercise, high-load  resistive exercise device, designed for long duration, human space missions. ARED will enable  astronauts to effectively maintain their muscle strength and bone mass in the micro-gravity environment  more effectively than any other existing devices. ARED's resistance is provided via two, 20.3 cm (8 in)  diameter vacuum cylinders, which provide a nearly constant resistance source. ARED also has a means  to simulate the inertia that is felt during a 1-G exercise routine via the flywheel subassembly, which is  directly tied to the motion of the ARED cylinders. ARED is scheduled to fly on flight ULF 2 to the ISS and  will be located in Node 1. Presently, ARED is in the middle of its qualification and acceptance test  program. An extensive testing program and engineering evaluation has increased the reliability of ARED  by bringing potential design issues to light before flight production. Some of those design issues,  resolutions, and design details will be discussed in this paper.  Figure 1. Test subject performing a squat on ARED  NASA Johnson Space Center, Houston, TX  Proceedings of the 3@ Aerospace Mechanisms Symposium, Langley Research Center, May 17-19,2006
Framc  I  Main Arm/  Figure 2. ARED - Man in the Loop Testing Unit - Profile and Front View  General ARED Mechanism Overview  ARED has seven main subsystems (Figure 2); vacuum cylinders, flywheels, frame, platform, arm base,  cable-pulley, and the main armllift bar. A motion schematic for a squat on ARED is shown in Figure 3.  The vacuum cylinders are the main generators of resistive force for ARED. The details of the cylinders will  be discussed fully in the next section. The piston rods are attached to the arm base assembly.  Motion I$  Figure 3. Schematic of ARED showing motion and simple free body diagram
The main purpose of the arm base assembly (Figure 4) is to provide a mechanism to adjust the load  setting. A ball screw in the arm base provides this function. By turning the ball screw, the attachment  point of the piston rods moves along the ball screw. This changes the moment arm between the pivot  point and the cylinder load application point and allow for load adjustment at 1-lb increment. The arm  base assembly transfers the load from the cylinders to both the bar and cable exercise hardware. The  dual nature of the arm base minimizes the reconfiguration required to switch between the two types of  exercises, resulting in a more efficient exercise routine. Details of the arm base will be discussed later.  I  Figure 4. ARED Arm Base Assembly (Top Cover Removed)  The flywheel assembly is mounted to an end cap of the cylinder and directly meshes with the motion of  the piston by means of a gear and gear rack. The details of the flywheel assembly will be discussed in the  next section.  The main arm and lift bar assembly (Figure 2) transfers the load from the arm base to the exerciser. The  main arm assembly lifts the front end of the arm base using a contact surface. The lift bar portion of the  assembly allows for bar adjustment from 25.4 cm (10 in) above the platform to 183 cm (72 in) above the  platform. This wide range of adjustment allows for any bar exercise from a dead-lift to a squat and  accommodates a range of human subject from 5Ih-percentile Asian female to 95Ih-percentile American  male. As with free weights, the position of the bar can also be “racked using the upper stop mechanism.  The upper stop mechanism allows the crew to start the squat and heel-raise exercises from a standing  position.  The cable-pulley assembly also transfers the load from the arm base to the subject. The cable arms push  down on the rear portion of the arm base assembly (Figure 5). The cable-pulley assembly enables a  variety of cable exercises such as one-arm cable row, hip abductions, and one-arm curls. It employs a  series of pulleys, timing belts, and cables to achieve this function. One of the pulleys in the assembly is  cammed to compensate for geometry changes during the stroke in order to create a constant load at the  end of the exercise rope. The pulley ratios provide a maximum of 136 kgf (300 Ibf) and 183 cm (72 in) of  stroke.  320
Cable Arm to Arm  Base Interface  -1  Figure 5. Detail showing the cable arms pushing on the back of the arm base assembly  Figure 6. ARED in the folded configuration  Front view shows the details of the cable pulley mechanism.  The platform (Figure 7) assembly's main purpose is to provide an adequate exercise surface and to  provide containment for some of the pulleys for the cable-pulley mechanism. Its secondary purpose is to  house load cells and wiring for the instrumentation system. The platform and main arm also can be folded  up (Figure 6) to aid in storage and crew translation on ISS.  32 1
Figure 7. Picture of the platform CAD model with the footplates removed.  This shows the electronics inside the platform assembly.  Vacuum Cylinder Design Details  There are two vacuum cylinders (Figure 8) used on ARED. Each cylinder provides a constant 340-kgf  (750-lbf) load. The design uses a standard cylinder/piston concept with endplates and four tie rods  holding the assembly together.  The original cylinder shell was made of 6061-T6 AI, anodized on the outer surface, and left bare on a 16  RMS ID surface. Braycote 601 lubricant was applied to the entire ID of the cylinder shell. Early vacuum  cylinder tests indicated that the surface finish of the interior surface, measured in RMS, is the critical  parameter in maintaining a vacuum, while the piston is moving. The current flight design calls for a 20.3-  cm (841) ID with an 8 RMS or better surface finish, 4.8-mm (0.19-in) wall thickness, and is 38.1 cm (15 in)  in length. To achieve the best possible surface finish, three different manufacturing methods were tried;  ground and honed, electro polish, and hand polish. Results showed that the ground and honed process  provided a more consistent and controlled surface finish while maintaining the required roundness. To  date, the best surface finish achieved is less than 1 RMS on an aluminum 6061-T6 cylinders,  manufactured at the Micro-machining Department at NASA’s Glenn Research Center.  The current flight piston design is made from 6061-T651 AI using a Nylon Molygard wear strip and a self  lubricating Carboxilated Nitrile U-ring seal from Parker Hannifin Corporation. The open end of the U-ring  seal is oriented toward the pressure side of the piston. Braycote 601 lubricant is applied to the wearstrip  and u-ring seal. Three different wearstriph-ring seal combinations were evaluated to achieve the most  efficient vacuum under dynamic conditions.  1. A single wearstrip with a single u-ring seal.  2. A single wearstrip with 2 u-ring seals.  3. A single wearstrip with a spring-energized u-ring seal.  Through a series of tests, it was determined that option 1 worked well as long as the manufacturing and  assembly tolerances (Figure 8) were controlled. With such a large diameter piston/cylinder and long  stroke, it was important to maintain perpendicularity and parallelism between the piston, piston shaft, and  cylinder wall. A jig was designed to assist in assembling the cylinder and piston to the required alignment.  322
Figure 8. Piston/Cylinder Design  The bottom end cap design, also 6061, includes a Linear Bearing packed with Rheolube 2000 grease.  This bearing interfaces with the piston shaft made of 15-5 PH stainless steel and heat treated to a H1025  condition. The shaft has a circular cross section the length of the piston stroke that interfaces with the  linear bearing. The remaining portion of the shaft is rectangular for attaching a gear rack used to drive the  flywheel. The bottom cap design includes a Fluorocarbon rubber bumper and a 3.2-mm (0.125-in) thick  polyester grade polyurethane foam filter. The foam filter was added to the bottom end cap design to  restrict airborne debris from being sucked into the cylinder. The rubber bumper prevents piston damage  in the event of bottoming out during assembly (Figure 8).  The head cap, on the vacuum side of the piston, is 6061 AI and is sealed with the cylinder shell using a  standard Butyle rubber O-ring seal. The head cap also includes a relief valve and a fluorocarbon rubber  bumper. To evacuate the cylinder, the relief valve is opened and the piston is pushed to the top of the  stroke and bottomed out against the rubber bumper. The valve is then closed (Figure 9), and a vacuum is  established.  I  I Venting/Evacuation  Figure 9. Assembled Cylinders showing top caps and valves for venting/evacuation  323
Design Details - Flywheel Assembly  The flywheel assembly is the component of ARED that simulates the inertial effect of weightlifting in 1-g  environment. When lifting free weights in 1-g, the total foot reaction force is dependent on the  acceleration of weights on the exerciser's shoulders. While performing a squat, this inertial effect creates  a spike in load at the bottom of the stroke and load relief at the top of the stroke. The exercise physiology  community speculates that this inertia spike plays a major role in increasing bone density in 1-g and, by  the same effect, slows the rate that bone density is lost in a zero-g environment.  -- . - I' I  Figure 10. Picture showing the over-center mechanisms, in the locked position, between the  flywheel and cylinder assembly  The flywheel assembly mounts to one end of the cylinder assembly. It attaches using a hinge and an  over-center mechanism (Figure 10). The purpose of this attachment method is to allow for the flywheel  mechanism to be engaged and disengaged depending on the exercise and the preference of the user. A  U-link (Figure 11) deforms during actuation and allows for the mechanism to go over-center. The design  of the U-link was challenging, because it had to elongate by 0.43 mm (0.017 in) and still be under the  allowed stress limits. After using the first prototype, it was obvious that the tolerance stack-up between  the hinge and the over-center mechanism made a significant difference in the over-center force. In order  to better control the over-center force, an eccentric spline bushing (Figure 11) was used in the over-  center handle. The hole in the bushing is 0.64 mm (0.025 in) off-set from the center of the bushing. The  offset allows for small adjustment of the over-center force.  4014171  4 01 7717 4015334  Figure 11. Detail of spline bushing and displacement analysis of the over-center u-link.  The heart of the flywheel assembly is a set of three gears and two inertial flywheels (Figure 12). The spur  gear meshes with a spur gear rack on the piston rod which makes the motion of the flywheels directly tied  to the motion of the cylinders of ARED. The flywheels provide the most load and rotate the fastest at the  higher load settings of ARED due to the longer cylinder stroke. This flywheel design uses a fixed gear  324