Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02346278 2001-05-04
PORTABLE APPARATUS FOR 3-DIMENSIONAL SCANNING
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to 3-
dimensional scanning, and, more particularly, to a
portable 3-dimensional scanning apparatus for hand-held
operations.
2. Description of the Prior Art
In the field of information sensing, machine
vision technologies provide valuable information about
the environment and about specific objects of interest
through close inspection. Known 3-D data acquisition
systems have been provided using 3-D sensors based on the
active triangulation principle. In such systems, a
specific known and fixed pattern of illumination (i.e.
structure illumination) is projected from a laser and
optical arrangement on an object to be measured, and the
intersection of that emitted pattern is observed from a
known and fixed oblique angle by a digital camera, such
as a charged coupled device (CCD) array, whereby the
position of the illuminated points on the object
translate to positions on the camera array and the
position of the illuminated points on the object can be
computed trigonometrically.
For instance, one of these systems is referred
to as a laser profilometer, wherein as indicated, a laser
beam is used for illumination. Such profilometers
analyze deformations of a laser line on an object such as
to evaluate, for instance, the depth (Z-axis) as well as
the horizontal position (X-axis) of the object.
Generally, the translation of either one of the
profilometer and the object to be scanned with the help
of a translation mechanism allows to obtain the missing
vertical position (Y-axis). Consequently, a 3-D profile
of the object is scanned.
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The above described system is widely used in
industrial environments whereat the objects to be scanned
are conveyed, whereby no translation mechanism is
required with the profilometer. However, this system is
not as convenient when, for instance, the object to be
scanned is idle and/or hard to displace. In such cases,
it is necessary to move the profilometer. In the event
where the piece is large and/or defines a complex shape,
it may be complicated to move the profilometer with the
help of a simple translation mechanism. Thus, the use of
a robot is often required, thereby entailing an increase
in costs and often a decrease in precision.
A versatile 3-D data acquisition system would
allow to digitize without contact the 3-D shape of an
object while computing the absolute position and
orientation of its scanned points, thus giving an
operator, in different instances, the freedom to
manipulate the system as if he was painting the surface
of this object. The gathered 3-D data could, for
instance, be used offline for the update of a work site
model or for close and specific inspection of the shape
integrity of objects compared to their CAD models.
SUMMARY OF THE INVENTION
It is therefore an aim of the present invention
to provide a profilometer trackable in a volume space for
3-D data acquisition.
It is a further aim of the present invention to
provide a 3-D data acquisition system having a compact
and portable scanner portion.
It is still a further aim of the present
invention to provide an improved method for 3-D scanning
of objects.
Therefore, in accordance with the present
invention, there is provided an apparatus for three-
dimensional scanning, said apparatus being manually
maneuverable and comprising a profilometer including a
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light beam projector, an objective and a light detector,
said profilometer configured for obtaining a two-
dimensional profile of an object by active triangulation;
and a positioning device being trackable in a volume
space for providing six degrees-of-freedom of said
apparatus; whereby a three-dimensional profile is
calculatable by relating the two-dimensional profile with
time-corresponding positions and orientations of said
apparatus.
Also in accordance with the present invention,
there is provided a system for three-dimensional
scanning, comprising said above described apparatus, and
further comprising a three-dimensional profile calculator
remote from said apparatus for tracking said apparatus in
said volume space and relating positions and orientations
of said apparatus with a time-corresponding two-
dimensional profile of the object for calculating a
three-dimensional profile thereof and for referring the
object to a static position and orientation.
Further in accordance with the present
invention, there is provided a method for three-
dimensional scanning, comprising the steps of (i)
scanning a two-dimensional profile of an object with a
profilometer projecting a light beam on the object and
using active triangulation; (ii) tracking said
profilometer in a volume space for obtaining positions
and orientations thereof; and (iii) calculating a three-
dimensional profile of the object by relating time-
corresponding two-dimensional profile and positions and
orientations.
BRIEF DESCRIPTION OF THE DRAWINGS
Having thus generally described the nature of
the invention, reference will now be made to the
accompanying drawings, showing by way of illustration a
preferred embodiment thereof, and in which:
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Fig. 1 is a block diagram illustrating an
apparatus for 3-dimensional data acquisition in
accordance with the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to Fig. 1, a 3-D data acquisition
(3-D DA system) is generally shown at 10. The 3-D DA
system 10 comprises a portable and compact 3-D scanning
apparatus illustrated at 12, which comprises a
profilometer 14 and a positioning device 16. The
profilometer 14 transmits a 2-D profile of an object it
scans to a 3-D profile calculator unit 18, which will be
described hereinafter. The positioning device 16,
integrally joined to the profilometer 14 to form the 3-D
scanning apparatus 10, is trackable in a volume space by
the 3-D profile calculator unit 18 such that its 6
degrees-of-freedom (DOF) are known. The profilometer and
positioning device relation data is stored by the 3-D
profile calculator unit 18 and is generally shown at 20,
such that the scanned 2-D profile of the object is
positioned and oriented in space by knowing the relation
of the profilometer 14 and the positioning device 16,
whereby a 3-D profile thereof is calculatable. The 3-D
profile may be transmitted from the 3-D profile
calculator unit 18 to a 3-D polygonal model generator 26
which will transform the 3-D profiles into a 3-D
polygonal model, or from the 3-D profile calculator unit
18 to a CAD system 24 as raw cloud data. The 3-D
polygonal model is then transmitted from the a 3-D
polygonal model generator 26 to the CAD system 24. A
user interface 22 is provided for commanding and
controlling the 3-D profile calculator unit 18, the 3-D
scanning apparatus 12 and the 3-D polygonal model
generator 26.
The present application of a 3-D scanning
apparatus 12 as an unattached hand-held scanner requires
that the field of view be produced by the movement of a
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light pattern along the surface of an object to be
digitized, as typically known of laser profilometers.
The instantaneous field of view would be the length of
the line of the structured light seen on a detector
portion of the profilometer 14 and the field of view in
the direction of the scan is defined by the path length
of the scan.
For close range applications, the measurement
performance and task requires a medium to high range
resolution. The range resolution is usually evaluated in
terms of the working range. As an example, a typical
value for the range resolution is about 1/1000 of the
working range taking into account the sub-pixel accuracy
on the detector portion. This parameter is a function of
the characteristics of the range measurement technique.
Other important parameters are mentioned below
as examples and are not intended to limit the scope of
the present invention. One such important parameter is
the scanning speed. The time required for the recording
of the image of the line is a function of the scanning
speed and the maximum tolerable displacement of the line
on the surface. If we assume that the scanning apparatus
will be moved manually with a maximum speed of 100 mm/s,
the integration time should be around 1 ms for a
displacement of about 0.1 mm corresponding approximately
to the line width of the focused laser beam on the
surface of the object.
Another important parameter is the sampling
rate, which is defined by the detector portion refresh
rate. Actual standards CCD detectors can operate at 30
frames/sec. At this frame rate, the sampling interval in
the scanning direction is 3.3 mm assuming a maximum
scanning speed of 100 mm/s. The sampling interval
perpendicular to the scanning direction would be the
angular field of view divided by the number of pixels in
a row. Typical parameters of a scanning device may
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include a stand-off of 100 mm and a working range of
100 mm.
Active profilometers offer the advantage of
having their own illumination, being independent of the
background radiation. For example, the use of a laser
source as structured light may be specified in terms of
wavelength (detector spectral response), power of energy
(detector sensitivity) and power of consumption. Among
the laser light sources available on the market, diode
lasers are compact, reliable and are available in a broad
range of power and wavelength. For close range operation
and compact system requirement, this light source offers
an obvious advantage, whereby a laser profilometer
(illustrated at 14) using the above described active
triangulation technique is proposed as optical range
sensor. The profilometer 14 produces a light beam (i.e.
laser) resulting in a line on the object to be scanned.
The profilometer 14 also comprises an objective and a
detector portion (e. g. CCD detector, CMOS), which
measures the location of the image of the illuminated
line on the object surface.
The 3-D scanning apparatus 12 requires a
compact design of the profilometer 14 in order to be able
to hold it with one hand. Accordingly, the laser
profilometer preferably comprises a progressive scan
miniature camera, an objective lens and a structured
light laser projector. The casing of the above described
profilometer 14 is comparable in size to that of a
commercial camescope. It is pointed out that the
detector must be tilted according to the Scheimpflug
condition ("Optical Range Imaging Sensors, Machine Vision
and Applications", by P.J. Besl, published in 1988,
pp. 127-152).
Hand-held operations assume that the 3-D
scanning apparatus 12 can be moved freely in space within
a given working volume, preferably without any mechanical
fixture. The object is scanned by simply moving the 3-D
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scanning apparatus 12 around continuously in any
convenient orientation and location in a scanning
session.
Full hand-held scanning requires that the
positioning device 16 provides the 6 DOF of the 3-D
scanning apparatus 12, whereby the instantaneous
orientation (roll, pitch, yaw) and location (X, Y, Z) of
the profilometer 14 can be measured in order to get its
actual position and orientation in space. This way, the
2-D profile scanned by the profilometer 14 can be
referenced in a static coordinate system which
corresponds to the positioning device 16.
In the context of specific inspection or work
site modeling, if the coordinate system of the
positioning device 16 is known relative to an origin of a
CAD model, then absolute measurements can be made and
transposed in this CAD model. The accuracy of the
positioning device 16 is important in the development of
a hand-held 3-D scanning apparatus. On the other hand,
the positioning system's accuracy has a major impact on
other aspects such as its weight, its size and its price.
In order to keep the hand-held 3-D scanning apparatus
compact, portable and of relatively low cost, options for
reducing the positioning system accuracy may be
considered. One of these options is to use a less
accurate positioning system that still allows the user to
get an adequate looking display feedback during the
scanning stage and use software techniques to improve the
positioning accuracy in a post-processing stage.
The sampling rate is a less but still important
issue. Once again, it should be higher or at least equal
to the profilometer sampling rate. The latency of the
positioning device 16 should be as small as possible but,
moreover, it shall be constant all along the scanning
session. Since the positioning system and profilometer
latencies are not necessarily the same, positioning and
range data can be acquired at different times by the 3-D
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profile calculator unit 18. It is imperative to ensure
that the registration of positioning and range data
occurs for time corresponding events.
Some positioning systems need a line-of-sight
which has to be maintained between emitters and
receivers. In the present invention, this limitation has
other constraints on the 3-D scanning apparatus working
volume as some positions are usually prohibited. The use
of no line-of-sight positioning systems, although not
restricted by the present invention, would be a very
interesting feature in the hand-held design of the 3-D
scanning apparatus 18.
Other positioning systems suffer from various
interferences. For instance, magnetic-based systems are
usually affected by the presence of metallic objects
inside or near the working volume. In the case of
positioning systems based on inertial technology, the
sensor output drifts over time.
Finally, if the positioning device 16 is to be
attached to the profilometer 14, the compactness and
portability features of the resulting 3-D scanning
apparatus 12 require that the weight and the size thereof
are as small as possible. The positioning device 16
shall not restrict the operation of the 3-D scanning
apparatus 12 because of its weight or its size.
The positioning device 16 may be based on
ultrasounds. Ultrasonic positioning devices determine
distance by measuring the elapsed time of flight of an
acoustic wave.
The ultrasonic positioning device, used in the
first implementation of the present invention, allows
full 6-DOF measurement in a volume space of approximately
1 m3 with an accuracy of about 2% of the emitter-receiver
distance. Although it has accuracy, line-of-sight and
space volume restrictions, its very low cost makes it an
attractive candidate for the development of the present
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invention even though the latter should not be limited by
the use of this specific ultrasonic positioning device.
The portable 3-D scanning apparatus 12 of the
present invention may be used with a dedicated
acquisition and visualization software, which is capable
of displaying and manipulating the points as a 3-D image
and to save it into a predetermined file format. Such a
system could convert raw 3-D profile data into polygonal
models and is embodied in Fig. 1 as a 3-D polygonal model
calculator 26.
The profilometer 16 of the present invention
may be calibrated using a special calibration test bench.
Translation stages and special acquisition and analysis
software are used to perform the calibration. A second
calibration process consists in verifying the absolute
offsets between the positioning device receiver
coordinate system and the laser profilometer coordinate
system. These offset values are used in the acquisition
and visualization software for coordinate system
transformation involved in producing the 3-D data points.
Accordingly, the present invention is based on
the combination of a laser profilometer and a positioning
device (ultrasonic or other) as it permits a more rapid
and intuitive digitization of the shape of a given
object. The applications of the present scanning
apparatus 12 are varied. They also include, for
instance, the creation of virtual catalogs on the
Internet for the retail market, and also the creation and
updating of virtual environments in the area of games or
simulators, CAD model update, artefacts scanning and
animation.
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