Optics: measuring and testing – Shape or surface configuration
Reexamination Certificate
1999-07-20
2003-04-01
Font, Frank G. (Department: 2877)
Optics: measuring and testing
Shape or surface configuration
Reexamination Certificate
active
06542249
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the measurement of the shape of an object or scene, and more particularly, to a method and apparatus to generate three-dimensional (3-D) coordinates of points of a surface, as well as a 3-D model of an object or scene, by a freely moving sensor whose position is not measured by any position tracking device, and which performs the measurement without contacting the object. The object and sensor are permitted free movement relative to one another, and therefore either the object or the sensor or both may be moving freely simultaneously.
2. Description of the Prior Art
Range sensors or scanners are commonly used for non-contacting measurement of the surface geometry of an object or scene (multiple objects). Typically the range-sensor consists of a range-sensor head and a mechanical positioning device, as shown schematically in
FIG. 1
(prior art), for the particular case where the range-sensor head consists of a laser source connected to a camera. Usually, the range-sensor head acquires profiles of 3-D coordinates of the surface being measured. In order to acquire sufficient data to describe an object surface, the range-sensor head must be translated or rotated relative to the object surface to acquire numerous profiles, which will later be integrated into a single object model. The object may also be translated or rotated relative to a stationary or moving range-sensor head. In all cases, the movement of the moving body or bodies, the range-sensor head, the object, or both, is carried out by mechanical positioning devices which move the range-sensor head or object, either by predefined increments or to unknown positions which are measured by a position sensor (not shown) on the positioning device, during the data acquisition. Accurate information of the incremental movement of the moving bodies is required in order to integrate all profiles collected from the same position of the range-sensor apparatus.
However, for large objects, such as car-body panels, engines, machines, or statues, whose dimensions exceed the working dimensions of the rotation or translation stages, or for immovable objects, or objects which are difficult or not desirable to manipulate, such as human bodies and archaeological artifacts, the method of moving the object relative to a stationary range-sensor head cannot be used. Furthermore, the scanning of such objects by moving the range-sensor head, cannot be carried out with the mechanical positioning device at a single position. The mechanical positioning devices are limited in their range of movement, and cannot move the range-sensor head completely around the object.
One approach to measure such objects is to first take one series of scans across the object (usually moving along only one translation axis), as shown in
FIG. 2
a
(prior art) and then reposition the entire scanning apparatus (the mechanical positioning device which holds the range-sensor head) several times to new positions or viewpoints to acquire a series of profiles or a range-image at each viewpoint, as shown in
FIG. 2
b
(prior art). The different range images or views, which consist of many profiles each and have large movement between viewpoints, can then be integrated into a single reference frame if there is sufficient overlap between range images, as shown in
FIG. 3
a
-
3
c
(prior art). However, one problem with this method is that estimates of the movement between viewpoints must be obtained before or during the data acquisition process, as these estimates are necessary to carry out the integration of the range-views. Another problem with this method is that very complex planning is needed to measure the full object surface and ensure that some overlap between views is obtained. Furthermore, for many environments, it is not possible to move translation or rotation devices to the site of the object, or to several different viewpoints around the object. The use of mechanical positioning devices is also not possible in measuring confined interior surfaces of complex shaped objects.
There have been several approaches to measure the surface geometry of objects without mechanical translation and rotary stages. These employ a hand-held range-sensor head, which would be continuously swept over the object surface to perform a scanning operation.
One device sold by 3D Scanners Ltd., of London, England, employs a range-sensor head attached to mechanical linkages or arms which are instrumented with rotary position sensors. The range-sensor head is hand-held and its motion is controlled by the user. The device allows a more continuous scanning and a more free access to the surface of an object than with translation and rotation stages by permitting simultaneous translations and rotations of the range-sensor head in all six degrees of freedom (three translations, three rotations). However, the device is highly limited by the lengths and possible orientations of the mechanical arms, and can therefore only accommodate small objects, or the apparatus requires repositioning and complex planning for large objects. The mechanical linkages and position sensors also add to the cost.
Another apparatus employs a non-mechanical magnetic-field tracking device to determine the position of the sensor-head, (see Fisher, R. B., Fitzgibbon, A., Gionis, A., Wright, M. and Eggert, D. “A Hand-Held Optical Surface Scanner for Environmental Modeling and Virtual Reality”,
Virtual Reality World '
96
Conference,
pp. 1-16, Stuttgart, Germany, 1996; and New Zealand Patent 293713 (Polhemus)), and if necessary, the target object (see U.S. Pat. No. 5,886,775, “Noncontact digitizing imaging system”) during scanning. The device has eliminated the more bulky mechanical positioning and position sensing devices, and permits continuous sensor-head movement by hand to facilitate the range data acquisition process. However, because of the use of magnetic-field sensors, the system is restricted to non-ferrous environments. It is also costly to include the position tracking devices. Furthermore, in the case where the object is moving, contact must be made with the object to fix a position sensor to it. The magnetic-field position tracking system also has a limited range.
A third apparatus uses a non-mechanical optical position sensor to track the position of the range-sensing head during scanning (see U.S. Pat. No. Re. 035,816, “Method and apparatus for three-dimensional non-contact shape sensing”, and U.S. Pat. No. 5,198,877 of the same title). Three point light sources (referred to as pilot lights) located on the range-sensing head emit light one at a time, using strobing. The emitted light is received by three multiplexed light sensors (operated one at a time) to determine the position of the range-sensor head. A major drawback of the apparatus is that it requires unobstructed lines of sight between the pilot lights located on the range-sensor head, and the pilot-light sensors. This would be restrictive or prohibitive, in manipulation of the range-sensor head when scanning completely around objects, or inside confined cavities. Another drawback of this apparatus is the use of components used to generate and detect light points on a surface for the purposes of sensor-head position tracking only. These components are separate from the range-sensing light source and receiver and add considerable cost and complexity to the system.
A fourth device and method is the only one known to the inventors to involve a fully unconstrained sensor without positioning devices [see Hebert P., and Rioux, M. “Toward a hand-held laser range scanner: integrating observation-based motion compensation”,
Proc Three
-
Dimensional Image Capture and Applications: SPIE
-3313, pp. 2-13, 1998; and international patent application no. PCT/CA98/00324, publication no. WO 98/45817). The method requires two separate scans of the same region by the range-sensor head, to acquire two sets of profiles having an angle with respect to one another, and thus to obtain a colle
Knopf George K.
Kofman Jonathan D.
Armstrong Craig R.
Borden Ladner Gervais LLP
Font Frank G.
Kinsman Leslie Anne
Merlino Amanda
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