WO1996026413A1

WO1996026413A1 - Optical method for determining of three-dimensional form of an object

Info

Publication number: WO1996026413A1
Application number: PCT/FI1995/000100
Authority: WO
Inventors: Asko PERTTILÄ; Pertti Puumalainen
Original assignee: Mäntsälän Saha Ky
Priority date: 1995-02-24
Filing date: 1995-02-24
Publication date: 1996-08-29
Also published as: AU1812895A

Abstract

The present invention is related to an optical method of shape determination for a three-dimensional object. Conventional methods have been found clumsy and costly. In the method according to the invention, a surface of light-diffusing material (1) is placed at one side of the measurement point of the cross-sectional periphery of said object (3), on which surface by means of a scanning laser beam (10) is formed a light source (9) sweeping in the plane of the cross section to be measured, and that at the other side of said object are located a plurality of sensors (6, 7, 8) correspondingly aligned toward the object in the plane of said cross section to be measured, said sensors serving to detect rays emitted by said scanning light source (9). Each shadowing or unshadowing of the rays from the light source as detected by each of the sensors (6, 7, 8) is indicative of a tangential point on the cross-sectional periphery of the object (3), whereby, provided that the position of the light source is known as a function of time, a reconstruction of the peripheral cross section of the object (3) can be made from the detected tangential points of its periphery. Further, by moving the object in relation to the measurement means in a direction perpendicular to that of the cross section measurement, a reconstruction of the three-dimensional peripheral shape of the object can be made from the succession of thus reconstructed peripheral cross sections.

Description

OPTICAL METHOD FOR DBTHRMIN1NG OF THREE-DIMENSIONAL FORM OF AN OBJECT

The present invention is related to a method of shape determination for a three-dimensional object.

Conventionally, a plurality of methods are used based on imaging in which the periphery of the object, in the plane imaged by the camera, is obtained by direct meas- urement from the image, while the height of the object is typically determined from the length of the object shadow cast by side illumination and measured by means of a sep¬ arate camera. The cameras may be of the line sensor type, whereby the objects must be moved in order to obtain a two-dimensional image, or matrix cameras, whereby a three-dimensional image can be obtained directly. Dimensional measurement of wood objects, particularly logs, has been implemented using an imaging technique in which a moving object is imaged by means of a matrix camera from its side. When an illuminating laser beam, for instance, is converted into a fanned beam aimed obliquely on the log surface within the imaging area of the camera at 45 angle, for instance, the image will contain a curved line, which corresponds to the dimen- sional variations of the object in the imaging direction of the camera. By taking such images from two or three directions, the cross section of the object such as a log can be determined. A three-dimensional image of the ob¬ ject is then formed by moving the object in its longitu- dinal direction past the cameras.

The above-described methods are often found rather expen¬ sive and typically require extremely powerful computer processing systems, resulting in a high price which is a major shortcoming of conventional commercial techniques. It is an object of the present invention to provide a method suitable for three-dimensional shape determination of an object, said method being capable of overcoming the drawbacks of conventional techniques. It is a particular object of the present invention to provide a method char¬ acterized by low price combined with simple and reliable function.

The goal of the invention is achieved by virtue of a method characterized in the annexed claims.

In the method according to the invention, a surface of light-diffusing material is placed at one side of the measurement point of the cross-sectional periphery of the object, on which surface by means of a scanning laser beam is formed a light source sweeping in the plane of the cross section to be measured, and at the other side of said object are located a plurality of sensors corre¬ spondingly aligned toward the object in the plane of said cross section to be measured, said sensors serving to detect rays emitted by said scanning light source so that each shadowing or unshadowing of the rays from the light source as detected by each of the sensors is indicative of a tangential point on the cross-sectional periphery of the object permitting, whereby, provided that the posi¬ tion of the light source is known as a function of time, a reconstruction of the peripheral cross section of the object can be made from the detected tangential points of its periphery, and further, by moving the object in rela- tion to the measurement means in a direction perpendic¬ ular to that of the cross-section measurement, a recon¬ struction of the three-dimensional peripheral shape of the object can be made from the succession of thus reconstructed peripheral cross sections. The method is simple, cost-effective and reliable in service. The method disclosed herein is best suited for extremely rapid measurement of cross sections of relatively small objects with simple peripheries when the object is moved in a direction perpendicular to the plane of measured cross section. Thence, object motion actuated by means of, e.g., a belt conveyor can be employed to form a three-dimensional image of the object dimensions. Multi¬ ple applications can be found for the method in, e.g., dimensional control of boards and planks in wood indus¬ try, chip size monitoring and averaging measurement thereof in paper and pulp industry, and broadly, the flawless quality of produced objects travelling on a conveyor belt.

In the following, the invention will be examined in more detail with reference to the attached drawing illustrating the operating principle and a practical embodiment of the method according to the invention.

Referring to the Figure, the apparatus shown therein comprises a glass plate 1 with an opalescent surface over which the object 3 under measurement is moved, a laser 4, a rotatable multifaceted scanning mirror 5 and sensor units 6, 7, 8, which are placed to the opposite side of the object with respect to the surface of the glass plate 1. The method is based on sweeping a laser beam 10 over the opalescent-surfaced glass plate 1 so as to obtain a fast scanning light source 9 with an omnidirectional emission pattern. In the illustrated application, the light source 9 is implemented by aiming the beam 10 of the laser 4 onto a rotating multifaceted scanning mirror 5 that reflects the beam to sweep over the opalescent- surfaced glass plate in the direction indicated by the arrow marked by reference numeral 2 in the diagram. This arrangement provides a light source that moves over the opalescent-surfaced glass plate. The light source may be deviated in a plurality of alternative ways using, e.g., an acoustooptic deflector to avoid mechanically actuated components or in any other conventional manner. Having the scanning light source 9 implemented in the above-described manner, the object 3 under measurement is brought above the crosswise sweep of the scanning light source. In the illustrated case, the object is a board moving to a planer in a direction perpendicular to the plane of diagram, toward the viewer of the diagram. The practical purpose of such dimensional control is to ensure that the board is fully edged, that is, has sufficient height for planing on both edges.

Next, a situation is examined in which the light source is sweeping from its right limit position to its left limit position. Initially, all three light intensity sensors, formed by photodiodes 6, 7, 8, can detect the light. After the scanning light source has swept over a certain distance, the photodiode 6 will be shadowed from seen the light source. Now, a timer is started, and at the instant the photodiode 7 in turn is shadowed, the timer is stopped and time lapse t_x is stored in memory. Simultaneously the timer is restarted and stopped again when the photodiode 7 again detects the light, whereby time lapse t₂ (corresponding to the situation shown in the diagram) is stored in memory. Simultaneously, the timer is started for a third time lapse measurement ending when the photodiode 8 detects the light. The timer is stopped and time lapse t₃ is stored in memory. The photodiodes of the sensor units are located sufficiently far, that is, their distances from the board under meas¬ urement are substantially larger than the width of the board, and the sweep speed of the light source is kept constant. The sweep speed of the light source can be measured by placing photodiodes at the edges of the opalescent-surfaced glass plate and adjusting the sweep speed v to, e.g., 1 cm/ms. If the sensor units containing the photodiodes 6 and 8 are aimed at 45* angle toward the object, the right-side height of the board can be directly computed from the formula h_x = t₁ x v, the board width from the formula 1 = t₂ x v, and the left-side height from the formula h₂ = t₃ x v. If the speed of the light source is exactly 1 cm/ms, the time lapses measured in milliseconds give the board dimensions directly in centimeters. Dull edges are detected as follows: if the lower surface board is dull-edged at its left side, for instance, the photodiode 6 detects the light earlier than the photodiode 7. Correspondingly, if the upper surface of the board is dull-edged, it is obviously seen as a reduced value of h₂ or/and h₂.

The above-described embodiment utilizes three photodiodes as sensors for detecting three dimensions of the object's cross section, whereby the motion of the object permits determination of the three-dimensional peripheral shape of the object. Obviously, the number of the photodiode sensor units can be greater. Then, the cross section of the object can be measured in more detail as in principle each added photodiode sensor permits an additional facet to be determined in the cross section of the object.

Naturally, the disclosed method cannot measure the con¬ cavity of concave regions as the light rays always are limited by the tangential points, or boundaries, of such regions in the periphery of the object's cross section. In practice, the sweep plane of the laser light rays is slightly inclined away from the plane of imaging rays reaching the light intensity sensor units, thus avoiding laser rays transmitted directly through the opalescent- surfaced glass plate from reaching the sensor units.

Not limited by the exemplifying embodiment described above, the invention may be varied within the scope and spirit of the annexed claims. Accordingly, the invention may be generally applied to any dimensional surveillance of objects provided that such an object can be brought within the geometrical confines of the above-described measurement environment.

Claims

Claims :

1. An optical method of shape determination for a three- dimensional object, c h a r a c t e r i z e d in that a surface of light-diffusing material (1) is placed at one side of the measurement point of the cross-sectional periphery of said object (3), on which surface by means of a scanning laser beam (10) is formed a light source (9) sweeping in the plane of the cross section to be measured, and that at the other side of said object are located a plurality of sensors (6, 7, 8) correspondingly aligned toward the object in the plane of said cross section to be measured, said sensors serving to detect rays emitted by said scanning light source (9) so that each shadowing or unshadowing of the rays from the light source as detected by each of the sensors (6, 7, 8) is indicative of a tangential point on the cross-sectional periphery of the object (3), whereby, provided that the position of the light source is known as a function of time, a reconstruction of the peripheral cross section of the object (3) can be made from the detected tangential points of its periphery, and further, by moving the ob¬ ject in relation to the measurement means in a direction perpendicular to that of the cross-section measurement, a reconstruction of the three-dimensional peripheral shape of the object can be made from the succession of thus reconstructed peripheral cross sections.

2. A method as defined in claim 1, c h a r a c t e r - i z e d in that said scanning laser beam is formed by aiming the beam of a laser (4) via a rotating multi¬ faceted scanning mirror (5) onto said diffusing material

(1).

3. A method as defined in claim 1, c h a r a c t e r ¬ i z e d in that said point source of light is formed onto an opalescent surface (1) of a light-diffusing glass plate located directly under said object (3) under meas¬ urement.

4. A method as defined in claim 3, c h a r a c t e r - i z e d in that the sweep plane of the rays emitted by said laser is slightly inclined away from the plane of imaging rays reaching said light intensity sensors (6, 7, 8) , thus avoiding laser rays transmitted directly through the diffusing material (1) from reaching said light intensity sensors (6, 7, 8).