LASER DOT MATRIX BASED DIGITAL SYSTEM FOR FOR MONITORING TEXTURELESS ROOF STRATA OF
MINES
R. Singh, D. P. Chapman and K. B. Atkinson
University College London
United Kingdom
Commission V, Working Group 3
KEY WORDS: CCD, Close Range, Triangulation, Orientation Parameters, Template matching, Textureless object. Figure
ABSTRACT :
Projection of a large number of target points through a tiny diffraction grating based diode laser is used to solve two close range
digital photogrammetric problems: conjugate point identification and targetting the object. Laboratory investigations showed
high spatial stability and repeatability of dot matrix projection through a diffraction grating. A measurement repeatability of
around 1/30th of a pixel was achieved during their image co-ordinate measurement by least squares based template matching.
Accurate calibration of the projector was done with the help of an autoreflecting spherical target and placing the projector
rigidly over the telescope of a geodimeter. Using standard bundle adjustment procedures of photogrammetry, a camera model |
of the projector was developed. The interior orientation and lens distortion parameters of the projector were found comparable |
with those of a real CCD camera which were derived using a standard three dimensional testfield. A CCD camera placed over
a motor driven turning and tilting system is combined with this arrangement of the projector to scan a large surface area and
automatic derivation of three dimensional information of a textureless and featureless object can be achieved.
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1 INTRODUCTION
Mining measurements and inspections are quite different
from those of other industries due to both time and space
constraints. These constraints force a measuring system to
be remote, portable, fast, robust and accurate. Further, due
to the inhomogeneous nature and anisotropic character of the
rock mass, there is a requirement for continuous measure-
ment in space. Both quantitative and qualitative (pictorial)
information are equally important for analysis purposes be-
cause the relationship between stress and strain of the rock
mass is not exactly known. CCD based digital systems have
vast potential for such measurements and, recently, a number
of attempts have been made to utilise such systems for under-
ground inspection [Keran & Hendricks, 1995], measurement
[Singh et al., 1991] and guidance of machines [Hurteau et al.,
1991]. The work discussed here is aimed at the development
of a suitable system to monitor the movement of massive
sandstone strata during partial (Figure 1) extraction of coal
under built-up surface structures. Here deformation monitor-
ing of hanging structures involves extensive measurement of
unapproachable areas to estimate the nature of stability of
overlying strata.
Figure
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Optimised gallery width
Optimised pillar size
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1.1 Problem of massive sandstone strata Increased gravitational loading over rock beam |
The monitoring of massive sandstone strata generally corre-
sponds to remote measurement of a nearly flat, textureless
and featureless surface (Figure 2). Due to the light weight
and small size of CCD imaging systems, a conventional two
camera system initially seemed to be feasible. However the
requirement for synchronised photography (unstable object)
and lack of features (for stereomatching) in the object space
restrict its application to this problem. Other approaches like
CCD based shadow profilometry [Maerz et al., 1990] face two
serious problems: (1) inaccessible and dangerous areas of the
Figure 1: Intact roof with partial coal extraction (plan and
sectional views).
provides a better solution for remote measurement in poor
and controlled light conditions underground but the accuracy
problem remain.
Figure
telesc
1.2 Laser dot matrix projection
1.3
mine can not be measured by this technique as a linear feature
has to be placed over the object and (2) subpixel measure-
ment of both X and Y co-ordinates of the image point is
very difficult for a continuous linear feature like the shadow
of a string. Projection of a laser line in the object space
540
A diffraction grating based dot matrix projector (Figure 3)
is an attractive option not only due to its small size, light
weight and simple mechanism but it provides an effective way
to place a large number of high contrast measuring targets
over the object. The known orientation of these laser dots
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B5. Vienna 1996
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