ABSTRACT
During the operating lifetime of the Space Station the Mobile Servicing
System (MSS) will be subject to a variety of environmental stresses in
addition to cyclic loading. There is a risk of failure due to (unpredictable)
micrometeorite and orbital debris (M/OD) damage. A recent evaluation of
these effects by Canpolar East concluded that the most vulnerable component
of the MSS is the robotic manipulator (boom). The current MSS design does
not include a means of determining, in-situ, the mechanical integrity of
the boom. Further, there is no means to determine residual functional capacity
of a damaged boom.
The evaluation has concluded that an integrity monitoring
system (IMS) would both reduce the risk of critical failure and enhance
the functional lifetime of the MSS.
A subsequent project was undertaken to design and
test a full scale mockup which demonstrated the technical feasibility of
monitoring boom stiffness changes due to fatigue and impact damage. Loading
deflection and high order modal oscillations as small as 2µm were
resolved using an adaptation of a machine vision product (VE-262) which
Canpolar East is developing for terrestrial automation applications.
An IMS based on the VE-262 machine vision product
was supplied to the Canadian Space Agency, Space Mechanics Group, for further
experimental testing and verification.
The following work, covering the second in a series
of three separate contract's with the Canadian Space Agency, included collaborating
experts from C-CORE and Spar Aerospace Ltd.
BACKGROUND
The Problem
The Mobile Servicing System (MSS) is Canada's contribution to the international
space station project. This robot will incorporate two, carbon fiber reinforced,
robotic manipulator arm (Space Station Remote Manipulator System - SSRMS)
mounted on a mobile base and will be based, in part, on technology originally
developed for the space shuttle's "Canada Arm". The MSS will play a major
role in the assembly and operation of the space station and, as such, represents
a major leap forward for Canada's space program. CSA awarded Spar Aerospace
Ltd. the prime contract for the design and construction of the MSS.
In 1993 Canpolar East completed a project for the
Canadian Space Agency which identified an avoidable critical failure modality
for the SSRMS [1]. The project concluded that micrometeorite
and orbital debris (M/OD) are a serious threat to the SSRMS. A category-1
(catastophic) structural failure of the SSRMS is defined as a 5% loss in
stiffness [2]. Designers have calculated that the SSRMS will
be able to withstand significant M/OD damage without being compromised.
However, the long term fatigue characteristics of the SSRMS composite material
are relatively unknown. Fatigue damage in composites is a progressive phenomenon.
The combined effects of M/OD and fatigue damage are not known [3].
Spar Aerospace Ltd., ATS confirmed the validity of the analysis and has
worked with Canpolar East to identify an appropriate remediation strategy.
In fact the CSA placed M/OD protection and damage inspection as the number
2 item in its list of the top ten issues for the SSRMS Critical Design
Review [4].
Canpolar East's study suggested that, in order to
ensure that a category-1 failure does not occur it would be prudent to
monitor the structural integrity of the SSRMS. Advance knowledge of deterioration
or damage could be combined with stress monitoring and operational loading
control to provide for "no failure" operation during the MSS lifetime.
The MSS is one of a few systems which is essential to the operation of
the Space Station.
The Solution
As an operational remediation, an Integrity Monitoring System (IMS),
was proposed.
The concept sketch shown in Figure 1 illustrates
a generic design for an IMS based on optical deflection monitoring. Deflection
due to torsion or bending could be monitored through the change in
location of a laser spot on a machine vision detector.
In 1994 Canpolar East, in collaboration with Spar
Aerospace Ltd. and C-CORE, completed a project to provide a feasibility
demonstration for an IMS. The work included the design and implementation
of a full scale mockup which demonstrated the feasibility of monitoring
boom stiffness changes due to fatigue and impact damage.
FIGURE 1:
IMS FUNCTIONAL REQUIREMENTS
The following SSRMS boom segment data, provided in Table 1, was supplied
by Spar Aerospace Ltd, and is based on a boom segment distance of 3,000
mm.
TABLE 1:
| Load
[in-lb] |
Deflection
[mm] |
Deflection 5%
Stiffness Loss [mm] |
Deflection Resolution Requirements
[mm] |
| Max: 33,000 | 6.8 | 0.34 | 0.034 |
| Daily: 16,000 | 3.6 | 0.18 | 0.018 |
| FMS: 4,000 | 0.7 | 0.036 | 0.0036 |
IMS specification:
Sub-pixel Techniques
Sub-pixel interpolation methods have been developed for a variety of
video camera based photogrametric and sensing applications. A review of
some technologies was prepared by C-CORE [9]. The authors concluded
that a bisectional algorithm could provide sub-pixel resolutions as high
as one part in 140. Bisectional algorithms are computationally intensive
compared with centroid analysis technique which can provide resolutions
as high as 1 part in 90 [10,11]. A lot of sub-pixel work has
been carried out at the National Aeronautics Establishment (NAE) in support
of the Canadian Space Vision System (SVS).
IMS PROTOTYPE TEST PLAN
Methodology
The apparatus, which is depicted in schematic detail in Figure 2, was
designed to enable development and testing of the sub-pixel and centroid
tracking algorithms and to test and verify the IMS prototype precision,
stability and frequency response. Separate VE components were been
used in this bench apparatus.
The hardware included the assembly of a uni-directional
mechanical micrometer with LCD readout, a laser projector assembly, a laser
displacement sensor, a CCD module from the VE262, a 100 kg optical base
with miscellaneous optical holding fixtures and a 486 desktop computer
with a frame grabber from the VE262 and an A/D board for sampling data
from the laser displacement sensor. All target tracking software was coded
using Microsoft C++.
The laser pointer was mounted on the micrometer
stage and pointed at the center of the VE262 camera module CCD array which
was located approximately 200mm from the face of the laser pointer emitting
surface.
The laser pointer assembly consisted of a 3mW laser
diode mounted on a mechanical micrometer (to enable fine adjustment for
focusing) and a 25mm lens. The laser diode/micrometer and lens were then
mounted on an aluminum base. A 1 kohm potentiometer was used to control
the laser output power.
The laser displacement sensor was located
approximately 20mm from the edge of the LCD micrometer/laser pointer assembly.
The laser displacement sensor would be used as a cross check against the
LCD micrometer.
A typical calibration sequence would involve positioning
the laser spot, from the laser pointer assembly, onto the CCD array near
the centre axis. The laser spot would then be translated across the face
of the CCD in 100µm and 2µm increments using LCD micrometer.
Care was taken to calibrate from one direction only since there may be
up to 8µm backlash associated with the LCD micrometer. During these
incremental movements, the data from the LCD readout, the laser displacement
sensor and VE centroid tracking software was recorded. At each increment
the VE software would sample 199 frames and average the resulting centroid
position of the laser spot. As well the laser displacement output was averaged
over 199 readings.
FIGURE 2:
IMS BENCH TEST RESULTS
Precision
A cross calibration between the mechanical stage and IMS is shown in
Figure 3. The linearity difference for 2µm increments is illustrated.
It appears that the VE262 linearity/precision errors over a short range
are less than ± 1µm, over a long range ± 2µm.
FIGURE 3:
IMS prototype calibration against micrometer (10µm).
On this scale there are apparent non-linearities which may be the stage
or the IMS prototype. The stage precision is only good to ± 1µm.
Stability
The IMS electronic stability over the short term is illustrated in
Figure 4. The horizontal standard deviation (sigma) is about 0.02 pixel
for a target subtending 60 pixels. This compares to a predicted value of
sigma = 0.028. For the IMS mock-up this corresponds
to a electronic noise limited resolution of ± 0.4µm at 2 sigma.
FIGURE 4:
IMS prototype electronic noise floor.
The electronic noise in the IMS prototype falls inside an envelope
of 0.1 pixel. The standard deviation of the noise is about 0.02 pixel.
DISCUSSION AND CONCLUSION
The electronic noise limited resolution of the IMS prototype was ±
0.4µm at 2 . Linearity was in the order of ±2µm
over a 3000 µm displacement range. Drift due to a laser power variation
of 10% was less than 0.1µm. Frequency response of the prototype
software was 8.3 frames per second. The combined uncertainties in precision
were in the order of ± 1 µm (± 0.1 pixel). The accuracy
uncertainties are in the same order as the calibration tools which were
used, i.e., ± 1µm - ± 2µm. The required IMS precision
of ± 3µm can be delivered. It appears that, with proper calibration,
the system can also deliver an accuracy of the same order over a 3 mm deflection
range. The data acquisition rate was 8.3 frames per second, slightly above
the required 7.1 frames per second. This frame rate can be maintained
while tracking multiple targets. It appears that the bench prototype of
the IMS meets all of the functional specifications set out.
EPILOGUE
An IMS prototype with a precision of ± 2µm at sampling
frequencies up to 8 Hz was developed and demonstrated. The technology was
based on a modified commercial machine vision system manufactured by Canpolar
East. Subsequently the design was modified so that IMS functions could
be implemented using the existing cameras and Artificial Vision Unit (AVU)
on the SSRMS. The modified design only requires installation of several
laser projectors-targets which can be viewed by the SSRMS camera. The laser
projector-targets are low intensity solar powered devices. No additional
power or wiring is required. A small supplementary software package will
be needed for the AVU. The modified IMS can provide a spatial resolution
better than ±100µm, enough for monitoring of sixth order modal
oscillations. Boom stiffness can be calculated from modal frequencies.
Impact analysis suggests that the IMS could be integrated into the existing
SSRMS flight modules at a cost in the order of $3 Million. In August of
1994, Canpolar East delivered a prototype IMS to the CSA Space Mechanics
Group, for further experimental testing and verification. The prototype
IMS was based upon Canpolar East's machine vision product VE-262, which
included a custom target tracking software package and several custom laser
projector's.
ACKNOWLEDGEMENTS
The following have contributed to the successful completion of the
overall project, including two related projects with the Canadian Space
Agency - the details of which were not covered by this paper.
Dr. P.N. Smith, P.Eng., from Memorial University of Newfoundland.
Dr. J. Guigne and J. Guzzwell, P.Eng., from the Centre for Cold Ocean
Resource Engineering (C-CORE).
Dr. R. Pawluczyk, from Institute National D'Optique (INO).
Dr. X. Maldague, from Laval University.
Dr. P. Cielo, from the Industrial Materials Institute (IMI).
Dr. S.K. Chang, from Spar Aerospace Ltd.
Dr. R. Gosine and P. Lefeuvre, from the Centre for Cold Ocean Resource
Engineering (C-CORE).
Dr. S. Kalaycioglu, from Canadian Space Agency - Directorate of Space
Mechanics.
REFERENCES
[1] Canpolar East Inc. Final Report: Feasibility Assessment
of MSS Remote Materials Inspection Techniques Volume 1.
Contract report prepared for the Canadian Space Agency, 1993.
[2] Chang, S.K. Protection of MSS Materials from the Low Earth Orbit Environment. Overheads from presentation given by S.K. Chang, Staff Member, Advanced Technology Assistance Group, Spar Aerospace Ltd., at the STEAR forum on Protection of Materials in Space, Ottawa, 25-26 May, 1992.
[3] Canpolar East Inc. Final Report: Feasibility Assessment of MSS Remote Materials Inspection Techniques Volume 1. Contract report prepared for the Canadian Space Agency, 1993.
[4] Canpolar East Inc. Final Report Volume 1: Development of an Integrity Monitoring System For The MSS Boom (s).Contract report prepared for the Canadian Space Agency, September 1994.
[5] National Aeronautical Establishment, National Research Council Canada, Canadian Space Agency. NRCC/NAE Developed Real-Time Video Photogrammetry Technology, Released under terms and conditions of DSS FILE NO. 02SW.31098-9- TD-4.
[6] MacLean, S.G., Pinkney, H.F.L. (June, 1993) Canadian Aeronautics and Space Journal, Vol. 39, No. 2, 63-77.
[7] Sklair, C., Hoff, W., Gatrell, L. (1991) Accuracy of Locating Circular Features Using Machine Vision, Cooperative Intelligent Robotics in Space II, SPIE Vol. 1612.
[8] Kalaycioglu, S., Seifu, S., (1991) Telerobotics for Space Station Freedom, Proceedings of the 1991 IEEE International Symposium on Intelligent Control, 13-15 August 1991, Arlington Virginia, USA.
[9] Lefeuvre, P. Gosine, R. (1994) A Review of High Resolution Video Techniques. Contract report for Canpolar East Inc., May 1994, C-CORE contract number 94-C8.
[10] Bales, J.W., Barker, L.K. (1981) Marking Parts to Aid Robot Vision, NASA Technical Paper 1819, April 1981.
[11] Haralick, R.M., Shapiro, L.G., (1993) Measuration
Quantizing Error, Computer and Robot Vision Vol II, Addison Wesley.
44 Austin Street, St. John's, NF, Canada
A1B 4C2
Phone: (709) 722-6067
Fax: (709) 722-1138
http://www.vetech.com/
e-mail: info@canpolar.com