CHARACTERISATION AND TRACKING OF MEMBRANE SURFACES
AT NASA LANGLEY RESEARCH CENTER
Mark R. Shortis', Stuart Robson’, Richard S. Pappa’, Thomas W. Jones* and William K. Goad*
Department of Geomatics, University of Melbourne, Parkville, Australia, m.shortis@unimelb.edu.au
? Department of Geomatic Engineering, University College London, United Kingdom
? Structural Dynamics Branch, NASA Langley Research Center, Hampton, Virginia, U.S.A.
^ Instrumentation Systems Development Branch, NASA Langley Research Center, Hampton, Virginia, U.S.A.
KEYWORDS: surface characterisation, target tracking, membrane surface
ABSTRACT
This paper describes in detail two applications of characterisation and tracking of membrane surfaces using artificial targets. The first
application discussed is the measurement of the membrane and spar wing surfaces of a micro-flight vehicle. The second application
discussed is the measurement of a one metre long Fresnel lens membrane used to concentrate light on solar collectors. In both cases the
aim was to investigate the structural dynamic characteristics of the surfaces under induced vibration.
1. INTRODUCTION
NASA Langley Research Center (LaRC) in Virginia is one of
several NASA research centres in the United States of America.
The research programs at LaRC specialise in aerospace
technology and atmospheric physics. Two of the principal
experimental programs at LaRC are the design and analysis of
aerospace structures for space deployment and the design and
analysis of aerospace models for civilian and military
organisations. Consequently there is a continual need for non-
intrusive, high data-rate measurement for laboratory testing of
aerospace structures and wind tunnel testing of aerospace
models. Close range photogrammetry has been used routinely
at LaRC as a measurement and tracking tool, using a wide
variety of systems based both on film (Shortis, 1989) and CCD
video (Shortis and Snow, 1997) cameras.
The characterisation and tracking of surfaces at LaRC has
typically been based on discrete targets to signalise points of
interest or define the surface to be measured. Passive,
internally illuminated and retro-reflective targets have all been
used to accurately define surface points in generally
unfavourable circumstances. For wind tunnel applications in
particular, the lack of control over ambient lighting is always a
factor (Childers et al, 1994; Shortis and Snow, 1997), however
the primary reason for artificial targets is model surfaces
lacking in any clearly defined features because they are thin
membranes or polished metal surfaces (Burner and Martinson,
1996; Graves and Burner, 2001).
Further, artificial targets are favoured for tracking applications
to ensure a high level of accuracy of the motion or change of
shape (Robson and Shortis, 1997). The absolute shape of the
surface, if required, may be derived from an initial, static test.
The emphasis is typically on the accuracy of the relative
changes in the surface to be tracked, in order to determine
modes of vibration or cyclic motion. Although it is feasible to
track natural surface features if they are available, it is generally
accepted that artificial circular targets will realise superior
accuracy.
This paper describes two cases of the surface characterisation
and tracking of membrane surfaces using artificial targets. In
-90—
both cases the aim was to investigate the structural dynamic
characteristics of the surfaces under induced vibration. A
secondary issue for one case was a comparison between
membranes of differing thickness. The targets were tracked
using synchronised CCD video cameras and offline processing
of the captured images. The results of the photogrammetric
measurement were three dimensional visualisations of the
trajectories of the surface targets. The paper will describe the
experimental set-ups, the photogrammetric geometry and image
quality factors, and the algorithms used to track the target
images.
2. WING SURFACES OF MICRO-FLIGHT VEHICLES
The first application of photogrammetric monitoring of
membrane surfaces is the measurement of the membrane and
spar wing surfaces of a micro-flight vehicle. These 250 mm
wing span vehicles are in the first stages of research and
development towards a surveillance role in military
engagements and civilian operations, as well as the subject of
research to advance aerospace components and materials. The
military aspect of the research and development is primarily in
response to strong support from the Defense Advanced
Research Projects Agency (DARPA) to develop micro aerial
vehicles with a wing span of less than six inches and a speed of
less than 25 miles per hour. Various types of wings are under
investigation, however weight considerations have generally
directed aerospace designers to transparent monofilm
membrane surfaces supported by graphite/epoxy spars or
battens.
The concept of the micro-flight vehicle is that it will be an
autonomous vehicle with a payload of a sensor and radio
transmitter. The sensor is most likely to be a vision system
with sensitivity in the visible or infra-red bands, however
sensors for radio communications, radiation counters or
biological weapons detectors are also likely. The vehicles will
be released onto the battlefield to return intelligence
information on the adversary. The perceived advantages of the
vehicles are that they are not easily detected, relatively
inexpensive and therefore expendable, yet capable of providing
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