Space Station operations and maintenance are expected to make unprecedented demands on astronaut EVA. With Space Station expected to operate with an 8 to 10 psi atmosphere (4 psi for Shuttle operations), the effectivness of pressurized gloves is called into doubt at the same time that EVA activity levels are to be increased. To address the need for more frequent and complex EVA missions and also to extend the dexterity, duration, and safety of EVA astronauts, NASA Ames and Stanford University have an ongoing cooperative agreement to explore and compare alternatives. This is the final Stanford/Ames report on manually powered Prehensors, each of which consists of a shroud forming a pressure enclosure around the astronaut's hand, and a linkage system to transfer the motions and forces of the hand to mechanical digits attached to the shroud. All prehensors are intended for attachment to a standard wrist coupling, as found on the AX-5 hard suit prototype, so that realistic tests can be performed under normal and reduced gravity as simulated by water flotation

Aldrich, J.

Leblanc, M.

Leifer, Larry J.

Sabelman, E.

Schwandt, D.

English

NASA Technical Reports Server

DESIGN, DEVELOPMENT and EVALUATION / ~ - 5  q--3? 
of STANFORD/AMES EVA PREHENSORS 13 7 ,;I:? 
I 
Final Report 
submitted to: 
National Aeronautics and Space Administration 
Ames Research Center 
Moffet Field, CA 94035 
(NASA-CR-1826 66) DESIGN, DEVELOPRENT A1 D 
EVALUATION OF STANFORD/AHES EVA PBEHEYSOBS 
Final Report [Stanford U n i v . )  10 pCSCL 05H 
N 88- 2 25 40 
Unclas  
G3/54 0139969 
by: 
Professor Larry J. Leifer 
in collaboration with : 
J. Aldrich, M. LeBlanc, E. Sabelman & D. Schwandt 
Center for Design Research 
Stanford University 
Department of Mechanical Engineering 
Stanford, CA 94305 
May, 1988 
1 
https://ntrs.nasa.gov/search.jsp?R=19880013156 2020-03-20T06:15:45+00:00Z
Design. Development and Evaluation of Stanford/Ames EVA Prehensors 
SUMMARY 
Space Station operations and maintenance are expected to  make unprecedented demands on 
astronaut Extra-Vehicular Activity (EVA). With the Space Station expected to operate with a 
8-10 psi atmosphere (versus 4 psi for Shuttle operations), the effectiveness of pressurized gloves is 
called into doubt at  the same time tha t  EVA activity levels must be increased. To address the need 
for more frequent and more complex EVA missions and also to extend the dexterity, duration and 
safety of EVA astronauts, NASA-Ames Research Center and Stanford University have an ongoing 
cooperative agreement to explore and compare alternative solutions. 
This document plus the accompanying computer tape constitute the final report of design and 
development of a series of Stanford/Ames manually-powered EVA Prehensors. Each prehensor 
consists of a shroud forming a pressure enclosure around the astronaut’s hand, and a linkagesystem 
to transfer the motions and forces of the hand to mechanical digits connected to the shroud. All 
prehensors are intended for attachment to a standard wrist coupling, as found on the AX-5 hard suit 
prototype, so that realistic tests can be performed under normal and reduced gravity as simulated 
by water flotation. A collateral t a sk  is the development and implementation of a uPerformance 
Assessment Work Station” for evaluating the prehensor designs. 
Work described herein includes final design modifications to  two types of manually powered 
prehensors for compatibility with existing wrist connections, production of detail part drawings 
and specifications in CADAM format, preparation of specification and supplier lists for purchased 
components, and submission of plans for testing and comparison with conventional gloves under 
realistic conditions. 
PROBLEM STATEMENT and GOALS 
In its most general form, the intent of this design effort is to determine whether the function of the 
gloved hand may be adequately replaced or improved upon by a mechanical gripper powered by 
the muscular movement of the user’s hand. The motivation stems from the present and foreseen 
incapacity of the EVA Suit glove t o  accommodate the extremes of required function: from low-load, 
highly intricate tasks  exemplified by instrument repair and adjustment, to prolonged heavy-load 
activities such as truss assembly or satellite retrieval. Results of this study may also apply to 
interfaces for teleoperators having improved translation of human hand motion into simple linear 
and rotary movements for transmission to a remote device, and to more capable end effectors for 
self-powered robots as envisioned for Space Station assembly tasks1. 
Actual wearing and use of an EVA suit equipped with prehensors is the only definitive test of 
completeness of the design. Impact of a particular design upon visual field, excessive use of certain 
muscle groups causing accelerated fatigue, possible compression or abrasion of soft tissue or skin at 
pressure points, inefficient or impossible end-effector paths due to constraints on joint motion2, or 
dimensional incompatibility with certain suit-related tasks such as glare shield operation cannot be 
completely predicted. Thus, in order to achieve acceptance, the performance of prehensors must be 
clearly superior to gloves for a given task .  One method of comparison is to provide a glove for one 
hand and a prehensor for the other, which may in fact be the preferred mode of use. The prehensor 
design must be inherently safe and reliable and its usage self-evident from the outset. 
WORK ACCOMPLISHED 
Three different prehensors were designed, each representing an increasing degree of complexity3: 
the “1-DOF” or “Prosthetic Prehensor” having a detachable two- or three-fingered end-effector is 
intended to establish criteria for mechanical simplicity, compactness, comfort and ease of training 
by which more capable devices can be measured. The “2-DOF” or “Multi-Grasp Prehensor” has 
2 
Design. Development and Evaluation of Stanford/Ames EVA Prehensors 
a fixed thumb plus two fingers capable of pinching against each other and against the thumb 
independently; a second thumb can potentially be added to perform specialized tasks. The “6 
DOF” or “Direct-Link Prehensor” was completed earlier and was reported on elsewhere‘. 
Since the last progress report (April, 1987), the following tasks have been accomplished: 
1. Final design modifications to two types of manually powered prehensors for compatibility with 
2. Production of detail part drawings and specifications in CADAM format, 
3. Preparation of specification and supplier lists for purchased components (also in computer- 
4. Generating plans for testing and comparison with conventional gloves under realistic conditions. 
existing wrist connections, 
readable format) , and 
1. SINGLE DEGREEOF-FREEDOM PREHENSOR-1DOF 
The one degree-of-freedom, two-finger prosthetic prehensor provides a modified pinch grasp. This 
design is restricted to a single linear hand motion and is intended to use off-the-shelf prosthetic 
hardware wherever feasible. The constraint on mobility is intended to establish criteria for 
mechanical simplicity, compactness, comfort and ease of training by which more capable devices 
can be measured. 
As described in the progress report of April, 1987, a full-scale functional mock-up including shroud 
but omitting the wrist coupling was constructed of acrylic plastic; fitted with a single-moving-finger 
hook-shaped end-effector, it proved capable of reasonably precise pinch grasp, such as holding a 
pencil, but was not intended for high-load situations. 
This design was originally produced as a right-handed version using CADAM. Supplementary work 
consisted of 
a. Part and assembly CADAM drawings for both left- and right-hand versions on the 
assumption that tests are to be performed with the left-hand 1-DOF unit being 
simultaneously compared to right-hand versions of other prehensors or gloves. 
b. CADAM files copied to machine-readable tape. 
c. Designs for alternate end-effectors interchangable with the baseline two-fingered Roberts- 
linkage unit, including three-fingered and ratchet-wrench devices (Figure 1). 
2. TWO DEGREEOF-FREEDOM PREHENSOR-2DOF 
The two degree-of-freedom, three-finger prehensor with one finger (“thumb”) fixed, is intended 
to provide experimental data regarding the cost and benefit of incremental degrees of freedom. 
An acrylic mock-up, excluding the shroud, was fabricated and demonstrated, as described in 
the progress report of April, 1987. Hand-made drawings of the 2-DOF pressure-suit compatible 
prototype were submitted for fabrication in a right-hand version only. 
The 2-DOF prototype was originally described in hand-made drawings in a right-hand version only. 
Supplementary work consisted of 
a. Part and assembly CADAM drawings of the right-hand version, checked for dimensional 
b. A CADAM design for an adaptor section to couple the prehensor shroud to the standard 
c. CADAM files transferred to machine-readable tape. 
consistency and compatibility. 
AX-5 wrist. 
3 
Design, Development and Evaluation of Stanford/Ames EVA Prehensors 
3. DIRECT-LINK P R E H E N S O R 4 D O F  
The six degree-of-freedom, semi-anthropomorphic prehensor with two degrees-of-freedom for each of 
three fingers, demonstrates some ability to manipulate objects as well as grasp them, and represents 
the current extreme of level of complexity, cost and performance. No additional work was performed 
on this concept under this agreement. 
4. PERFORMANCE ASSESSMENT WORK STATION-PAW 
The PAW consists of hardware to enable the suited subject to perform a series of activities 
closely resembling actual usage in EVA situations. The test series may be combined with precise 
biomechanical descriptions of the functional requirements, so that prehensors can be compared 
to  gloved and unemcumbered human hand performance, and with standard and specially-devised 
performance tests such as are used in rehabilitation assessment following injury to the hand. 
In addition to analytical tools developed at Stanford for robotic end-effectors5 and existing 
standardized human hand assessment tests and NASA in-house and contracted tests that may be 
used as is or modified for evaluating prehensor vs. gloved and un-encumbered hand performance, 
the following performance measurement protocols were identified in the progress report of April, 
1987, as applicable to prehensor tasks: 
1. Stanford Instrumented Grasp Strength Assessment.  
2. Stanford Functional Hand Task Assessment. 
3. University of Minnesota Tracking System. 
A JR3 6-degree-of-freedom force-moment sensor was obtained for a temporary trial, and was found 
to  be suitable for purposes of the PAW either as a stand-alone device or attached to another 
structure such as a rotary crank. 
Supplementary work consisted of continued progress toward the goal of objective measurement 
of the relative performance of each prehensor against the performance standards of (1) the high- 
pressure space suit gloved hand, (2) developmental glove designs, and (3) the un-encumbered human 
hand, in particular: 
a. Identify additional standardized human hand assessment tests that may  be used as is or 
modified for prehensor evaluation. 
One such assessment protocol is the Jebsen Hand Function Test6, intended to assay function 
as it relates to capability to perform well-defined activities of everyday living, evaluated 
by a “time-motion measurement” method. It consists of seven subtests: 
1. 
2. 
3. 
4. 
5. 
6. 
7. 
Writing, 
Card turning (simulated page turning), 
Manipulation of small common objects (bottle caps, coins, etc . ) ,  
Simulated feeding (pick up beans with a teaspoon), 
Stacking checkers, 
Handle large light objects (move and manipulate empty cans), and 
Handle heavy objects (move and manipulate full cans). 
b. Develop additional quantitative and qualitative performance test strategies, methodologies 
and equipment. (see item e., below). 
c. Investigate specific measurement instrumentation, for example: 
0 Handles with quick-release capability on a six degree-of-freedom force/moment 
transducer to simulate various types of grasp (Figure 2). 
4 
Design. Development and Evaluation of Stanford/Ames EVA Prehensors 
0 Accelerometers, load cells, or other force/motion analysis sensors to monitor 
performance during specific ‘tasks. 
While it is possible to place accelerometers on a body so as to record segmental motion 
(Figure 3)) it would be necessary to telemeter outputs to a stationary receiver to avoid 
restriction of motion due to cable interconnections; also, because the AX-5 suit joints 
permit unrestricted rotation as well as bending, tracking of accelerometer axis vector 
is complicated. 
In general, the application of force-measurement sensors should be to  the structure 
or object being manipulated, rather than to the EVA suit or prehensor, for the same 
reasons given above. 
d. Investigate requirements for  data acquisition, analysis and display hardware/software. 
Commercially available telemetry and data-logging hardware is adequate for performance 
assessment needs. Processing of the data so obtained can be begun using commercial 
software, but will ultimately require custom programming, for example, to relate EVA suit 
position to simultaneous measurement of force applied to an object being manipulated. 
e. Recommend possible methods to  apply assessment tests in simulated microgravity (flotation 
tank) and zero-g/vacuum environments, such as: 
0 Orthogonal video image analysis, 
0 Measurement of grasp and manipulation ability at extremes of reach, 
e Time-to-completion of tasks such as coupling and uncoupling bayonet-type connectors 
or screwing together pipe sections. 
The environment of the flotation tank (“wet facility”) is an extremely difficult one 
in which to obtain precise positional information by optical means, due to restricted 
fields of view and to multiple light reflections. Electromagnetic transponders placed 
on each suit or hand segment to be tracked may be a better option, but still are 
subject to operation in an electrically conductive medium within a steel tank. A 
better approach for position determination in the flotation tank may be self-contained 
acoustic transponders which are time-gated and/or frequency encoded so as to give 
unambiguous distance from an array of fixed ultrasonic sources based on time-of-flight 
measurement. However, both optical and electromagnetic methods are applicable to 
use in space, whereas acoustic transponders are not. 
Our recommendation is to omit (for the time being) obtaining position of suit segments, 
and concentrate on measuring performance by completion of specific tasks. Simulated 
tasks could consist of push-and-turn fixtures (Figure 4, inset) which optionally have 
digital position and force encoders and variable resistance to motion. These would be 
located on a rack at the extremes of reach (Figure 4); the rack could also hold less 
abstract task apparatus, such as couplings to be connected or bolts to be turned. 
f. Ezplore the potential of real-time dynamic display of prehensor performance using animated 
graphic solid modeling analysis and demonstration of prehensor design and control. If 
implemented, this concept would complement development of the PAW by generating a 
computer model of each prehensor and its properties, with the potential for predicting 
performance rather than relying on post facto testing to refine a design. To the extent that 
objects in the user’s environment are included in the model, it could also be valuable as a 
training tool to facilitate the transfer from glove to prehensor. 
5 
Design. Development and Evaluation of Stanford/Ames EVA Prehensors 
BIBLIOGRAPHY 
1. 
2. 
3. 
4. 
5. 
6. 
Brooks, T.L. & Bejczy, A.K., Survey of hand controllers for teleoperation, JPL Invention Report 
NPO-16610/6105, R.S. Jamieson, ed., pp. 1-72, Jan., 1986. 
Gilad, I., “Motion pattern analysis for evaluation and design of a prosthetic hook”, Arch Phys 
Med Rehabil, v. 66, pp. 399-402, 1985. 
Leifer, L., Jameson, J., LeBlanc, M., Wilson, D., Sabelman, E. & Schwandt, D., “Design, 
development and evaluation of EVA prehensors,” Proc. of the Workshop on Space Telerobotics, 
Pasadena, CA, Jan. 20-22, 1987. 
Jameson, J., “Report on the Stanford/Ames direct-link space suit prehensor,“ Proc. of the 
Workshop on Space Telerobotics, Pasadena, CA, Jan. 20-22, 1987. 
Salisbury, J.K, Kinematic and Force Analysis of Articulated Hands, Ph.D. Dissertation, Dept. 
of Mechanical Engineering, Stanford University, May, 1982. 
Jebsen, R.H., et al., “An objective and standardized test of hand function”, Arch Phys Med 
Rehabil, v. 56,  pp. 311-319,1969. 
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