Abstract. Acronyms. Introduction

Performance Testing of a Ratchetless

Extravehicular Activity Hand Tool on the

Reduced-Gravity Flying Laboratory

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 1/8th 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 (3-D) roller locking Sprag 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 Sprags are very small but have large contact radii and therefore, low contact stress.

During the summer of 1997, GSFC developed and tested the use of this 3-D sprag technology in commercial hand “ratcheting” tools to create a commercial “ratchetless” Sprag wrench. The Space Systems Laboratory (SSL) at the University of Maryland is extending this commercial Sprag wrench development to EVA tools. In order for the Sprag 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 21st to April 4th 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-135-A “Vomit Comet” to performance test the new EVA wrench. This experiment was performed in coordination with the 1998 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 Ratchet Wrench. Specifically for tasks with a limited range-of-motion, 30 degrees or less, the NASA EVA Ratchet Wrench requires a greater mental effort than the roller wrench.

Acronyms µ micro (10-6)

3-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 1/8th 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 three-dimensional (3-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 3-D sprag and a grooved race, as shown in Figure 1. This unique geometry permits construction of compact locking mechanisms that can

withstand large loads because the sprags are very small but have large contact radii and therefore, low contact stress. Thus, a wrench incorporating 3-D sprags is a logical step in EVA tool evolution.

Figure 1: GSFC 3-D Roller Locking Sprag Geometry

For the last three years, GSFC has been developing and testing the use of this 3-D sprag/roller technology in commercial hand “ratcheting” tools to create a commercial “ratchetless” 3-D roller wrench.1 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 21st to April 4th, a team of undergraduates from the University of Colorado at Boulder had the opportunity to fly an experiment on the NASA KC-135A “Vomit Comet” to performance test the SSL’s prototype EVA 3-D roller wrench (referred to an EVA Roller wrench for the rest of the paper). This experiment was performed as part of the 1998 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.2 A statistical analysis and comparison of the Cooper-Harper data for

the NASA EVA Ratchet Wrench (an Essex-style wrench)3 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 3-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-135 Environment

The experiment was performed aboard the NASA KC-135 “Vomit Comet.” The aircraft presents a unique test environment by performing repeated parabolic maneuvers. These maneuvers provide 20-25 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 1st and one on April 2nd, 1998, with the team’s journalist flying on the second day. Only two experimenters were allowed on each flight. Figure 2 shows the layout of the KC-135 Aircraft.

Figure 2: KC-135 Internal Layout4

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 25,000 ft altitude. Once there, the aircraft begins to fly a parabolic flight plan. The aircraft begins with a high angle climb, during which 1.8 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 ~25

seconds. The aircraft then noses down and once again 1.8 g’s are experienced on pullout. Figure 3 illustrates the aircraft flight profile.

Figure 3: KC-135 Flight Profile4

The aircraft repeats this parabolic flight path for ten parabolas. A parabola is one complete cycle from pull-up to pull-pull-up. After ten parabolas the aircraft turns 180-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 25 seconds of weightlessness.

Test Description

Testing was performed using two different wrenches, the EVA Roller “ratchetless” wrench and the NASA EVA Ratchet wrench, currently used on Space Shuttle missions. Both wrenches operate in a manner similar to a standard ratchet wrench at a 90-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 4 illustrates the test fixture configuration.

Figure 4: 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 15-, 30-, 45-and 60-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 1. 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.

Table 1: Modified Cooper-Harper Rating2

Very easy, highly desirable

Operator mental effort is minimal and desired performance is easily obtainable 1 Easy, desirable Operator mental effort is low and desired

performance is attainable 2 Fair, mild difficulty Acceptable operator mental effort is required

to attain adequate system performance 3 Minor but annoying

difficulty

Moderately high operator mental effort is required to attain adequate system performance

4

Moderately objectionable difficulty

High operator mental effort is required to attain adequate system performance 5 Very objectionable but

tolerable difficulty

Maximum operator mental effort is required to attain adequate system performance 6 Major difficulty Maximum operator mental effort is required to

bring errors to moderate level 7 Major difficulty Maximum operator mental effort is required to

avoid large or numerous errors 8 Major difficulty Intense operator mental effort is required to

accomplish task, but frequent of numerous errors persist

9

Impossible Instructed task cannot be accomplished

reliably 10

Cooper- Harper Modified Test Evaluation Method Note: Evaluation procedure flows from left to right.

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)

Errors are small and inconsequential

Instructed task can be accomplished most of the time

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 2 details the order in which all three tests were performed and Figure 5 shows a test in progress. In this picture, the test subject (left) loosens the RPCM bolt while the test conductor (right) tightens the range-of-motion bolt for the upcoming test.

Table 2: Flight Test Schedule Parabola Task

1 Acclimatize to microgravity 2 Acclimatize to microgravity 3 15 degree range-of-motion test 4 Space Station RPCM Task 5 30 degree range-of-motion test 6 Space Station RPCM Task 7 45 degree range-of-motion test 8 Back-drive torque measurement 9 Switch wrenches

10 15 degree range-of-motion test 11 Space Station RPCM Task 12 30 degree range-of-motion test 13 Space Station RPCM Task 14 45 degree range-of-motion test 15 Backdrive torque measurement 16 Switch test subject/ test conductor 17 Switch test subject/ test conductor 18 15 degree range-of-motion test 19 Space Station RPCM Task 20 30 degree range-of-motion test 21 Space Station RPCM Task 22 45 degree range-of-motion test 23 Back-drive torque measurement 24 Switch wrenches

25 15 degree range-of-motion test 26 Space Station RPCM Task 27 30 degree range-of-motion test 28 Space Station RPCM Task 29 45 degree range-of-motion test 30 Back-drive torque measurement 31-40 Free

Equipment Description

Many pieces of equipment were used to perform the testing some of which are shown in Figure 6. The follow is a list of the more important pieces.

Figure 6: 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 free-floating journalist.

Wrenches

Two different wrenches were used for comparison testing as shown in Figure 7. The first wrench was the NASA EVA Ratchet wrench. It uses standard ratchet mechanisms and requires approximately a backward movement of 15-degrees to engage the next tooth. The second tool was the EVA Roller wrench in development by the SSL. Both wrenches have a 3/8” male drive. The roller wrench is silver and the EVA Ratchet wrench is gold.

Figure 7: 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 1/4” socket extension. Since the torque sensor had a 1/4” female socket, a 3/8”-to-1/4” 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-of-motion test. They were spaced at 15-, 30- 45- and 60-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 2.

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-135.

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.

15 Degree Range-of-Motion Results

The 15-degree range-of-motion test was the most difficult to perform. The NASA EVA Ratchet wrench is such that a 15 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 8. On the second flight, the

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.

0 1 2 3 4 5 6 7 8 9 10

Subject 1 Subject 2 Subject 3 Subject 4

Cooper-Harper Task Value 15 Degree ROM Test

Ratchet Sprag

Figure 8: 15 Degree Range-of-Motion Test Results

30 Degree Range-of-Motion Results

For the 30 degree range-of-motion test, the NASA EVA Ratchet 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 9 that the NASA EVA Ratchet wrench was more difficult to use. This is attributed to the teeth spacing in the NASA EVA Ratchet wrench. Through the 30 degrees of throw, the NASA EVA Ratchet wrench could only engage two teeth leaving a portion of the throw with no effect. The roller wrench, which has no teeth, used 100% of this range of throw to perform work and was more efficient.

0 1 2 3 4 5 6 7 8 9 10

Subject 1 Subject 2 Subject 3 Subject 4

Cooper-Harper Task Value

30 Degree ROM Test

Ratchet Sprag

Figure 9: 30 Degree Range-of-motion Test Results

45 Degree Range-of-Motion Results

The data for this test, shown in Figure 10, 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 Ratchet Wrench. The data from test subject two for the ratchet

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.

0 1 2 3 4 5 6 7 8 9 10

Subject 1 Subject 2 Subject 3 Subject 4

Cooper-Harper Task Value

45 Degree ROM Test

Ratchet Sprag

Figure 10: 45 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 11 illustrates that the roller wrench generally seems that it requires less mental effort than the EVA Ratchet Wrench. However, both tools were quite capable of performing the task.

0 2 4 6 8 10

Subject 1 Subject 2 Subject 3 Subject 4

Cooper-Harper Task Value RPCM Task Attempt 1

Ratchet Sprag

Figure 11: RPCM Task Attempt 1 Test Results

0 1 2 3 4 5 6 7 8 9 10

Subject 1 Subject 2 Subject 3 Subject 4

Cooper-Harper Task Value RPCM Task Attempt 2

Ratchet Sprag

Figure 12: RCPM Task Attempt 2 Test Results

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.

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.

1.8-g

Problem: One of the most difficult aspects of testing was the transition from 1.8g’s to µ-g. Because the KC-135 aircraft is not a space vehicle, the transition from 1.8-g to µ-g is difficult to adjust to. The test subject and conductor must prepare for the next task during the period of 1.8-g. However, moving one's head in this 1.8-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.

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-135 because when 1.8-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-135 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 Ratchet wrench. Specifically for tasks with a limited range-of-motion, 30 degrees or less, the NASA EVA Ratchet 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

We would also like to thank the SSL at the University of Maryland for their support. Additional information on 3-D sprags and rollers, and the development and additional testing of their “ratchetless” EVA wrench can be found at http://wrench.ssl.umd.edu/

Additional information on the “ratchetless” wrench KC-135 tests conducted can be found at

http://rtt.colorado.edu/~molinap/kc135/

References

[1] Roberts, B., Manufacturing and Testing Requirements for a Reversible Hand-Socket Wrench Using Three-Dimensional Rollers, NASA/CR-1998-206849, May 1998.

[2] Sheridan, Thomas, B., Telerobotics, Automation, and Human Supervisory Control, 1992.

[3] EVA Tools and Equipment Reference Book, JSC-20466, Rev, B, November 1993.

[4] NASA Johnson KC-135 Reference http://zerog.jsc.nasa.gov/