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One of the essential advantages of the QTSculptor system is its simple
configurability for various measurement extents and object sizes.
The connection between object size, measurement extent, number of
required range images, and measurement precision are explained in the following
paragraphs.
Sensor head and tripod
The sensor head for the stripe projection method consists of a white light
LCD projector and a CCD camera. Since there are no mechanically moving parts
in either the camera or the projector, a high accuracy of reproduction in
measurements and the longevity of all components is guaranteed. With the projector
various stripe patterns are projected in quick succession on the object and are
recorded by the CCD camera. The geometry of the object is measured through
mathematical combinations of the stripe patterns.
Measurement extent
The measurement extent of the system can be configured
by the user with minimal effort. The lower bound
on the measurement extent is essentially controlled by the lens used.
In the standard configuration the smallest measurement extent is
approximately 100mm x 100mm. The measurement extent's upper limit is
constrained only by the obtainable contrast of the projector on the
object surface.
In the standard configuration, measurement extents of 1000mm x 1000mm
and even higher are attainable under favorable lighting conditions.
For large measurement extents it is recommended that the ambient
light in the environment is reduced. For measurement extents smaller
than 100mm an exchange of the sensor's lens is recommended.
This allows for the measurement extent to be shrunken to 60mm x 60mm.
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Standard lens
(150mm)
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Alternate lens
(100mm)
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Measurement extent [mm]²
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Measurement distance [mm]
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Measurement extent [mm]²
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Measurement distance [mm]
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100 x 100
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500
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60 x 60
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260
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200 x 200
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702
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100 x 100
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312
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300 x 300
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944
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150 x 150
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379
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400 x 400
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1186
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200 x 200
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445
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500 x 500
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1460
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250 x 250
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511
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750 x 750
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2033
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300 x 300
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577
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1000 x 1000
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2683
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350 x 350
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643
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1500 x 1500
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3850
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400 x 400
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710
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2000 x 2000
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5060
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500 x 500
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842
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Table 1: Measurement extent for
standard and alternate lenses.
Measurement precision
The measurement precision of the 3D sensor scales in
correspondence with the measurement extent.
One must however differentiate between the lateral resolution and the
depth resolution. The lateral resolution is a direct result of the
resolution of the camera and affects the spacing of the measured points along the
object surface.
The depth resolution is a result of the measuring method and is
approximately 5 times the lateral resolution. A camera with a
1000 pixel resolution results in a relative precision of the
3D sensor of approximately 1:5000.
For the smallest measurement extent (100mm x 100mm) a precision of
0.02mm in depth and a lateral point spacing of 0.1mm results.
For a measurement extent of 1000mm x 1000mm a precision of 0.2mm in
depth and a lateral point spacing of 1mm results.
The connection between camera resolution, measurement extent, and point
spacing is tabulated in table 2.
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Characteristics of the sensor head
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Resolution for
100 / 500 / 1000 mm picture field
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sensor head
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resolution
grid size
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shot speed in seconds
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lateral resolution in [mm]
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depth resolution in
[mm]
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High-Res
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1280 x 960
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4.8 sec.
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0.08 / 0.39 / 0.78
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0.015 / 0.078 / 0.15
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Standard
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1024 x 768
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2.4 sec
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0.10 / 0.49 / 0.98
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0.020 / 0.098 / 0.20
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Fast
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780 x 580
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1.2 sec.
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0.13 / 0.64 / 0.41
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0.026 / 0.128 / 0.26
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Ultra-Fast
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640 x 480
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0.6 sec.
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0.16 / 0.78 / 1.56
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0.031 / 0.156 / 0.31
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Table 2: Resolution and measurement accuracy
of the sensor head.
According to the characteristics of
the object surface the actual precision may vary slightly.
With QTSculptor the precision can be controlled
during registration.
Object size
The object size can be smaller or larger than the
measurement area. Small objects can be measured with the
precision cited for the smallest possible measurable area 100mm x 100mm
(with the standard lens).
For larger objects the complexity of the complete acquisition of an
object increases according to the number of necessary range images
that must be taken.
The standard configuration is suitable for objects between
30mm and 2000mm.
Number of range images
The number of range images that must be taken
is affected by the size and complexity of the object in
question.
For objects whose size fits within the measurement extent, between
15 and 30 range images are typically necessary for the complete
acquisition of the object. This number of course also depends
on the complexity of the object. For instance, 8 views at an angle
from above, 8 views at an angle from underneath, 1 from directly
above, and 1 from directly below might suffice for a typical object.
For larger objects the number of views grows accordingly. Very
large datasets consisting of several hundred views can be processed
by QTSculptor without a problem.
Measurement time
The minimal time required to acquire a range image
is defined by the sensor head. The minimal times are given in table 2.
In order to measure very difficult surfaces (e.g. dark) the time required
for a shot may increase. This maximizes the measurable area and depending
on the nature of the surface can improve the data quality.
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