Abstract. Acronyms. Introduction

Transcription

1 Performance Testing of a less Extravehicular Activity Hand Tool on the Reduced-Gravity Flying Laboratory Peter Molina * and Jennifer Probst * Department of Aerospace Engineering, University of Colorado-Boulder Boulder, CO Abstract The Hubble Space Telescope (HST) Servicing Missions have demonstrated that extravehicular activity (EVA) power tools need a hand (non-powered) ratcheting capability to accomplish delicate and unforeseen tasks. Depending on the pitch, or spacing of the teeth on a conventional ratcheting hand socket wrench, sometimes more then /th of a turn in the backward direction can be required to engage the next tooth. This minimum motion requirement of the ratcheting device causes the wrench to work inefficiently in confined spaces. Furthermore, the bulkiness of an astronaut s space suit and gloves causes additional range-of-motion problems when ratcheting wrench tools are used for EVA tasks. Therefore, removing the ratchet from the design can increase the usability of future EVA tools. No successful ratchet-less wrench has been developed to date. However, the NASA Goddard Space Flight Center (GSFC) has developed and patented a special three dimensional (-D) roller locking technology that, when used in place of the traditional ratcheting mechanism, results in a device with infinite indexibility and infinitely small range-of-motion. This unique geometry permits construction of compact locking mechanisms that can withstand large loads because the s are very small but have large contact radii and therefore, low contact stress. During the summer of, GSFC developed and tested the use of this -D sprag technology in commercial hand ratcheting tools to create a commercial ratchetless wrench. The Space Systems Laboratory (SSL) at the University of Maryland is extending this commercial wrench development to EVA tools. In order for the mechanism to become an EVA tool it must first be tested in a weightless environment to evaluate performance when used for certain tasks. During the two week period from March st to April th a team of undergraduates from the University of Colorado at Boulder had the opportunity to fly an experiment on the * AIAA Student Members NASA KC--A Vomit Comet to performance test the new EVA wrench. This experiment was performed in coordination with the NASA Reduced Gravity Student Flight Opportunities Program. The data collected from this study suggests that an EVA Roller wrench generally seems to require less of a mental effort than the existing NASA EVA Wrench. Specifically for tasks with a limited range-ofmotion, degrees or less, the NASA EVA Wrench requires a greater mental effort than the roller wrench. Acronyms µ micro ( - ) -D three-dimensional EVA extravehicular activity GSFC Goddard Space Flight Center ISS International Space Station NASA National Aeronautics and Space Admin. RPCM Removable Power Control Module SSL Space Systems Laboratory Introduction Experience with extravehicular activity (EVA) has shown that hand ratcheting tools are necessary to accomplish delicate and unforeseen tasks. Depending on the pitch, or spacing of the teeth on a conventional ratcheting hand socket wrench, sometimes more then /th of a turn in the backward direction can be required to engage the next tooth. This minimum motion requirement of the ratcheting device causes the wrench to work inefficiently in confined spaces. Furthermore, the bulkiness of an astronaut's spacesuit and gloves causes additional range-of-motion problems when ratcheting wrench tools are used for EVA tasks. Therefore, removing the ratchet from the design can increase the usability of future EVA tools. No successful ratchetless wrench has been developed to date. However, the National Aeronautics and Space Administration s (NASA) Goddard Space Flight Center (GSFC) has developed and patented a special threedimensional (-D) sprag/roller technology that, when used in place of the traditional ratcheting mechanism, results in a device with infinite indexibility and an infinitely small range-of-motion. Locking occurs because of the wedging action between the tapered periphery of the -D sprag and a grooved race, as shown in Figure. This unique geometry permits construction of compact locking mechanisms that can

2 withstand large loads because the sprags are very small but have large contact radii and therefore, low contact stress. Thus, a wrench incorporating -D sprags is a logical step in EVA tool evolution. Figure : GSFC -D Roller Locking Geometry For the last three years, GSFC has been developing and testing the use of this -D sprag/roller technology in commercial hand ratcheting tools to create a commercial ratchetless -D roller wrench. The Space Systems Laboratory (SSL) at the University of Maryland is extending this commercial roller wrench development to EVA tools. One simulation often used in the development of new EVA tools is to evaluate its performance in a weightless environment when used for certain tasks. During the two week period from March st to April th, a team of undergraduates from the University of Colorado at Boulder had the opportunity to fly an experiment on the NASA KC-A Vomit Comet to performance test the SSL s prototype EVA -D roller wrench (referred to an EVA Roller wrench for the rest of the paper). This experiment was performed as part of the NASA Reduced Gravity Student Flight Opportunities Program. The program is funded by NASA, administered by the Texas Space Grant Consortium, and sponsored by the Reduced Gravity Office at Ellington Field, near the Johnson Space Center. This program provides a unique opportunity for teams of undergraduates to experience first hand the complications and joys of working in a weightless environment. Test Objectives The objective of the experiment was to compare the efficiency of an EVA Roller wrench with a current EVA ratchet tool while accomplishing various tasks. This was done using a subjective evaluation of the perceived mental workload required for a task using a modified Cooper-Harper rating scale. A statistical analysis and comparison of the Cooper-Harper data for the NASA EVA Wrench (an Essex-style wrench) and EVA Roller wrench was then performed. Additional data on the backdrive torque, or the torque exerted while resetting the wrench, of both wrenches was collected with the use of a torque sensor. These tests were not conducted to verify the -D sprag and roller technology, but simply to evaluate the performance and efficiency of the EVA Roller wrench compared to current EVA ratcheting tools. The data obtained from these tests in a weightless environment, along with a battery of other tests, will be used in the development of a space-qualified version of the EVA Roller wrench in the coming years. The KC- Environment The experiment was performed aboard the NASA KC- Vomit Comet. The aircraft presents a unique test environment by performing repeated parabolic maneuvers. These maneuvers provide - seconds of weightlessness, during which the test is performed. The test fixture for the experiment was mounted in the rear of the aircraft on the starboard side. The experiment was carried out on two flights, one on April st and one on April nd,, with the team s journalist flying on the second day. Only two experimenters were allowed on each flight. Figure shows the layout of the KC- Aircraft. Figure : KC- Internal Layout The flight profile for the test begins with taxi and take off. The aircraft then cruises to a point off the coast of Texas and proceeds up to, ft altitude. Once there, the aircraft begins to fly a parabolic flight plan. The aircraft begins with a high angle climb, during which. g s are experienced. After climb, the aircraft noses over the top and it is at this point that µ-g is experienced. The period of weightlesness lasts for ~

3 seconds. The aircraft then noses down and once again. g s are experienced on pullout. Figure illustrates the aircraft flight profile. Figure : KC- Flight Profile The aircraft repeats this parabolic flight path for ten parabolas. A parabola is one complete cycle from pullup to pull-up. After ten parabolas the aircraft turns -degrees and begins the cycle of parabolas again. There are a total number of forty parabolas, or weightless periods, per flight. All testing was performed during the seconds of weightlessness. Test Description Testing was performed using two different wrenches, the EVA Roller ratchetless wrench and the NASA EVA wrench, currently used on Space Shuttle missions. Both wrenches operate in a manner similar to a standard ratchet wrench at a -degree angle to a bolt. The test subject had their feet constrained at the base of the test fixture to accurately emulate an astronaut performing an EVA task with a hand held tool. Figure illustrates the test fixture configuration. Figure : Test Fixture Configuration The first type of test performed was a range-of-motion test, using a bolt located in the center of the test fixture. The range-of-motion through which the wrench may move was constrained by pegs spaced at -, -, - and -degree increments. The task began as soon as a weightless environment had been established. The test subject would rise to the fixture and mate the wrench with the bolt. The prototype EVA Roller wrench was constructed to perform loosening tasks only, therefore all test evaluations were loosening tasks. Once the wrench was mated with the bolt and the handle was properly inserted between the constraint pins, the test subject began to loosen the bolt. The test subject continued loosening the bolt until the weightless period ended. The test subject then used the period between parabolas to evaluate the task they had just performed using the Cooper-Harper evaluation method as depicted in Table. Once the test subject had selected the appropriate number they informed the test conductor, standing to the side of the test fixture, who recorded the information in the test notebook.

4 Instructed task can be accomplished most of the time Errors are small and inconsequential Table : Modified Cooper-Harper Rating Cooper- Harper Modified Test Evaluation Method Note: Evaluation procedure flows from left to right. Very easy, highly Operator mental effort is minimal and desired desirable performance is easily obtainable Mental workload is acceptable Mental workload is NOT acceptable (mental workload is high and should be reduced) Errors are NOT small and inconsequential (major deficiencies; system redesign is strongly recommended) Instructed task can NOT be accomplished most of the time (major deficiencies; system redesign is mandatory) Easy, desirable Fair, mild difficulty Minor but annoying difficulty Operator mental effort is low and desired performance is attainable Acceptable operator mental effort is required to attain adequate system performance Moderately high operator mental effort is required to attain adequate system performance Moderately High operator mental effort is required to objectionable difficulty attain adequate system performance Very objectionable but tolerable difficulty Major difficulty Major difficulty Major difficulty Impossible Maximum operator mental effort is required to attain adequate system performance Maximum operator mental effort is required to bring errors to moderate level Maximum operator mental effort is required to avoid large or numerous errors Intense operator mental effort is required to accomplish task, but frequent of numerous errors persist Instructed task cannot be accomplished reliably The second type of test performed was the simulated removal of a Removable Power Control Module (RPCM) from its fixture. This task is an actual task planned for the International Space Station (ISS). Again, once a weightless environment had been established, the test subject rose up to the test fixture and began the loosening task on the RPCM. While the test subject was loosening the RPCM, the test conductor used a small power drill with a socket extension to tighten the bolt from the range-of-motion test performed on the previous parabola. Once the weightless period ended the test subject again used the Cooper-Harper evaluation method. The third and final test performed was a backdrive torque measurement. At the beginning of the weightless period the test subject rose and mated the wrench with the torque sensor. The subject then turned the wrench in the clockwise direction measuring the torque necessary to return the wrench to the loosening position. A computer mounted at the bottom of the test fixture collected the data from the sensor. Once several data points had been taken and gravity again set in, the test subject detached the wrench from the sensor. Table : Flight Test Schedule Parabola Task Acclimatize to microgravity Acclimatize to microgravity degree range-of-motion test Space Station RPCM Task degree range-of-motion test Space Station RPCM Task degree range-of-motion test Back-drive torque measurement Switch wrenches degree range-of-motion test Space Station RPCM Task degree range-of-motion test Space Station RPCM Task degree range-of-motion test Backdrive torque measurement Switch test subject/ test conductor Switch test subject/ test conductor degree range-of-motion test Space Station RPCM Task degree range-of-motion test Space Station RPCM Task degree range-of-motion test Back-drive torque measurement Switch wrenches degree range-of-motion test Space Station RPCM Task degree range-of-motion test Space Station RPCM Task degree range-of-motion test Back-drive torque measurement - Free Table details the order in which all three tests were performed and Figure shows a test in progress. In this picture, the test subject (left) loosens the RPCM bolt while the test conductor (right) tightens the range-ofmotion bolt for the upcoming test. Figure : Test Fixture Configuration

5 Equipment Description Many pieces of equipment were used to perform the testing some of which are shown in Figure. The follow is a list of the more important pieces. Figure : Test Equipment Video Cameras Several video cameras were fixed to mounting poles to record the testing. Still Cameras Both a digital camera and a standard film camera were used to photograph the tests in progress. These cameras were either mounted to a fixed pole or used by the freefloating journalist. Wrenches Two different wrenches were used for comparison testing as shown in Figure. The first wrench was the NASA EVA wrench. It uses standard ratchet mechanisms and requires approximately a backward movement of -degrees to engage the next tooth. The second tool was the EVA Roller wrench in development by the SSL. Both wrenches have a / male drive. The roller wrench is silver and the EVA wrench is gold. Figure : EVA Wrenches Back Drive Sensor The back drive torque sensor was mounted slightly above the range-of-motion constraint bolt. Both the bolt and sensor were mounted at shoulder level on the main face of the test fixture. The sensor consisted of a full, four-arm Wheatstone Bridge circuit attached to / socket extension. Since the torque sensor had a / female socket, a / -to-/ socket adapter was needed to allow the wrench to mate with it. The gages were connected to a strain gage meter with built-in transducer excitation. The meter was connected to a computer that recorded the data using a program developed at the SSL. RPCM The RPCM is a white plastic mockup of a module for the ISS. The mockup is used for testing in the Neutral Buoyancy Laboratory and loaned to us for use on our experiment. The module was mounted to the main fixture using two C-clamps. Range-of-motion Restraints Metal pegs were used to set the limits for the range-ofmotion test. They were spaced at -, - - and - degree increments to limit the angle through which the wrench could rotate. Data Acquisition Computer A Macintosh laptop computer was mounted to the base of the test fixture to collect data from the torque sensor. Padding The test fixture was extensively padded with PVC pipe insulation foam to soften rough edges that could hurt a person during the sudden onset of gravity. Electric Drill An electric drill was used by the test conductor to return the bolt to a tight position for the next test. Test Notebook A small notebook was mounted to the floor next to the test fixture so the test conductor could record data from the tests and any additional comments about test procedures. Test Procedures A list of the test procedure detailing what tasks were to be performed was taped to the test fixture to help the test conductor and subject prepare for the upcoming task. The complete test procedures were discussed earlier and are shown in Table.

6 µ-g Indicator A figurine of Marvin the Martian was tethered to the test fixture and used as a µ-g indicator. Once a weightless environment had been established Marvin would begin to float. This makes it very clear in the test video and still pictures if the image was taken in a weightless environment. Tethers Several tethers were employed for testing. Both wrenches were tethered to the test subject's wrist to prevent the tool from drifting away. The µ-g indicator, power drill, and RPCM were tethered to the test fixture to prevent them from drifting should they become lose. Velcro Velcro was used extensively on both the test fixture and the tools to keep equipment from drifting out of reach during the weightless period. Foot Restraints Foot restraints were employed to anchor the test subject and conductor. Astronauts performing extravehicular activities would be restrained in some form to keep them from floating away from the task at hand. Test Fixture The test fixture was the structure to which all test equipment was mounted. The test fixture was bolted to the floor in accordance with the standard bolt pattern of the KC-. test subjects used the palm wheel to turn the ratchet to reset it, making the task difficult, but not impossible. The roller wrench was also difficult to use for this task; however, if the palm wheel was used correctly the task was easier to perform. Degree ROM Test Subject Subject Subject Subject Figure : Degree Range-of-Motion Test Results Degree Range-of-Motion Results For the degree range-of-motion test, the NASA EVA wrench was able to perform the task without extra help of turning the palm wheel, however it can be seen in the results in Figure that the NASA EVA wrench was more difficult to use. This is attributed to the teeth spacing in the NASA EVA wrench. Through the degrees of throw, the NASA EVA wrench could only engage two teeth leaving a portion of the throw with no effect. The roller wrench, which has no teeth, used % of this range of throw to perform work and was more efficient. Results Results from the Cooper-Harper tests of the various tasks are detailed below. Data from the backdrive torque tests are not included in this paper because a calibration was never run on the torque sensor after the flight (it was used for a Shuttle experiment after the flight). It should be noted that as a result of the testing on the first flight, the test subjects on the second flight were able to improve performance after discussing the difficulties encountered. Note: a high Cooper-Harper number indicates increasing difficulty. Degree Range-of-Motion Results The -degree range-of-motion test was the most difficult to perform. The NASA EVA wrench is such that a degrees movement in the backward direction is required to engage the next tooth, making it exceedingly difficult to perform this task as seen by the test results shown in Figure. On the second flight, the Degree ROM Test Subject Subject Subject Subject Figure : Degree Range-of-motion Test Results Degree Range-of-Motion Results The data for this test, shown in Figure, indicates that the task was not extremely difficult and that the roller wrench generally seemed that it required less mental effort to complete the task than the EVA Wrench. The data from test subject two for the ratchet

7 is not in the same general range as the other data. This is thought to be attributed to Tether Tangle, as will be discussed in the next section. It should be noted in the attempt two data that there is another deviation from the general range of data. It is thought that this is attributed again to Tether Tangle. Degree ROM Test Subject Subject Subject Subject Figure : Degree Range-of-Motion Test Results RPCM Task Results The RPCM task was a test of perceived mental workload in removing a bolt that held the module in its frame. The data shown in Figure illustrates that the roller wrench generally seems that it requires less mental effort than the EVA Wrench. However, both tools were quite capable of performing the task. RPCM Task Attempt Subject Subject Subject Subject Figure : RPCM Task Attempt Test Results RPCM Task Attempt Subject Subject Subject Subject Figure : RCPM Task Attempt Test Results Difficulties Encountered in µ-g and Lessons Learned The micro-g environment provides many interesting challenges in performing tasks that are often taken for granted here on Earth. Below is a list of the significant difficulties and possible steps for improvement..-g Problem: One of the most difficult aspects of testing was the transition from.g s to µ-g. Because the KC- aircraft is not a space vehicle, the transition from.-g to µ-g is difficult to adjust to. The test subject and conductor must prepare for the next task during the period of.-g. However, moving one's head in this.-g can cause nausea and disorientation. Solution: Some improvement can be gained by memorizing the test procedure and rating scale. This would prevent unnecessary movement to look at charts during high gravity periods. Data recording could have been done using a mini tape recorder so that the test conductor did not have to focus on writing in the test notebook. Wrench Mating Problem: When the µ-g period begins the test subject must have the wrench in hand and rise to mate it with the bolt for the current test. This task can become quite difficult if one pushes on the floor too hard, giving too much upward velocity, or pushing off too softly. This makes it difficult to mate the wrench with the bolt quickly to begin the task. Solution: There is no solution to this problem with the current test configuration. The test fixture could be redesigned to minimize the distance the test subject must cover between the floor and the fixture. Wrench Push-off Problem: One problem encountered occurred when the range-of-motion task was initiated. The test subject would attempt to loosen the bolt; however, to keep the wrench mated with the bolt they would push on the palm wheel of the wrench. This pushing caused a reaction whereby the test subject began to move backward from the test fixture interfering with the test.

8 Solution: There is no solution to this problem with the current test configuration. One possibility is a redesign of the foot restraint. The current foot restraint did not allow the test subject to transmit any reaction forces into the test fixture, as would be the case in any true EVA task. A more rigid foot restraint design would greatly improve testing procedures. Tether Tangle Problem: The use of tethers in the µ-g environment is necessary to keep equipment from getting loose, especially in the KC- because when.-g is reestablished, the item can fall and injure someone. However the tethers used to attach the wrenches to the test subject's wrist were continuously getting tangled with the range-of-motion pegs. This interfered with the test subject's ability to perform the scheduled task. Solution: A shortened or elastic type tether would have been more desirable. This would allow the tether to perform the same function but not having excess length to get tangled on the test fixture. Test Subject and Test Conductor Drift Problem: During the µ-g periods it is important for the test subject and conductor to be restrained while the test is being performed. This more accurately simulates the circumstances astronauts face during EVA tool operation. Without the foot restraints the test subject can begin to free float and then has nothing to react against when using the wrench, making the entire task futile. Should the test conductor become unrestrained they may drift into the way of the test subject hampering the test or be unable to perform their necessary tasks. Solution: The important thing is to make sure the test subject and conductor are well restrained with a reliable system. Test Equipment Drift Problem: During the course of the tests, equipment not secured by tether or Velcro was able to get lose on the aircraft. These items could float in the way of the test conductor or subject interfering with the testing. In addition when gravity returned the object could fall and injure someone. Solution: Try to have all items in the aircraft secured. Note: with multiple test teams it is inevitable that items will become dislodged and float free. Camera Views Obscured by Floaters Problem: Because the KC- presents such a unique experience, people from the other experiments were sometimes flying about the cabin of the aircraft. At some points, these floaters obscured the camera views of the test. Solution: Very little can be done to mitigate this problem unless there is only one experimental team onboard the aircraft. Unfortunately, this is not possible for the NASA Reduced Gravity Student Flight Opportunities Program. Conclusions The improvement of EVA tools for astronauts is a very important task. The time an astronaut spends in orbit is precious. If manual tasks can be performed more efficiently with a lower degree of mental effort, it frees valuable time for other important tasks. The data collected from this study suggests that an EVA Roller wrench generally seems to require less of a mental effort than the existing NASA EVA wrench. Specifically for tasks with a limited range-of-motion, degrees or less, the NASA EVA wrench requires a greater mental effort than the roller wrench. Acknowledgements We would like to take this opportunity to thank the following individuals and organizations for their help in performing this experiment. Dr. Donna Gerren Brian Roberts NASA Goddard Space Flight Center Johnson Space Center Neutral Buoyancy Facility at JSC Physiological Training Office at JSC Reduced Gravity Office at Ellington Field University of Colorado Office of the Dean of Engineering Department of Aerospace Engineering Undergraduate Research Opportunities Program Office Colorado Center for Astrodynamics Research Texas Space Grant Consortium

9 We would also like to thank the SSL at the University of Maryland for their support. Additional information on -D sprags and rollers, and the development and additional testing of their ratchetless EVA wrench can be found at Additional information on the ratchetless wrench KC- tests conducted can be found at References [] Roberts, B., Manufacturing and Testing Requirements for a Reversible Hand-Socket Wrench Using Three-Dimensional Rollers, NASA/CR--, May. [] Sheridan, Thomas, B., Telerobotics, Automation, and Human Supervisory Control,. [] EVA Tools and Equipment Reference Book, JSC-, Rev, B, November. [] NASA Johnson KC- Reference