WO2014012610A1

WO2014012610A1 - Method and device for contactless distance measurement

Info

Publication number: WO2014012610A1
Application number: PCT/EP2013/001720
Authority: WO
Inventors: Bostjan Podobnik; Frank POVŠE; Klemen ŽBONTAR
Original assignee: Lpkf Laser & Elektronika D.O.O.
Priority date: 2012-07-20
Filing date: 2013-06-12
Publication date: 2014-01-23
Also published as: EP2874778A1; DE102012106613B3

Abstract

The invention relates to a method and device (1) for contactless distance measurement in which method a distance (a) of an object (2) from a reference point (3) is determined by a laser beam (4) in an illustrated measuring cycle. During the measuring cycle the measuring point (5) is moved relative to the object (2) to be measured along a predetermined course of a closed circumferential contour (7) on the object (2) at a constant relative speed. Because distance measurement is implemented in this dynamic way, the laser energy is focussed only on the respective measuring point (5) of the circumferential contour (7) for an extremely brief period and therefore the energy input acting on a respective point on the surface of the object (2) is comparatively small.

Description

Method and device for non-contact distance measurement

The invention relates to a method, in particular triangulation measuring method, for contactless distance measurement, in which the distance of an object from a laser source or a stationary reference point determined by means of a processing for laser processing of an object processing laser by detecting a reflected in a measuring point of the object beam by means of an optical sensor becomes. Furthermore, the invention relates to a device for carrying out the method.

Such a measuring method according to the principle of triangulation is a method of optical distance measurement taking into account the trigonometric trigonometric functions, in which a laser beam is focused, for example, on an object whose distance to the measuring device is to be determined.

As the distance of the object from the optical sensor changes, so does the angle at which the reflected beam is observed, and thus the position of its image on the sensor. From the position change, the distance of the object from the processing laser or a reference point is calculated by means of the trigonometric trigonometric functions.

From the position of the reflected light in the image plane, the distance between the optical sensor and the object can be calculated. Typically, the triangulation measurement method is used in a measuring range of 0.01 mm to about 1000 mm.

The optical sensor is a photosensitive element, by which the position of the light spot in the image plane can be determined. For example, a CCD cell, a CMOS camera or an optical position sensor, for example

CONFIRMATION COPY PSD, to be used. For measuring direct or reflected light, it is also possible to use simple optical components such as photodiodes or phototransistors.

In addition to visible light, the signal can also be generated by other radiations such as microwaves, UV radiation, IR radiation, X-rays, sound waves or radioactive radiation.

It is also already known to project a pattern, such as a line or a striped pattern, onto the object so that the distance information on all points of the pattern can be calculated with a single camera image.

For example, EP 2 418 040 A1 describes a method for controlling a joining device with a processing laser, the scanner optics being equipped with at least one external projector. The projector serves to project measuring light in the form of measuring structures, at least one line, onto the workpiece to be machined.

It has also been thought to use as a light source usable for material processing laser so that it can be used in addition to its processing function either simultaneously or alternatively for distance measurement.

DE 10 2006 004 919 A1 describes a laser beam welding head with a welding beam and means for optically detecting the position of a weld at a leading measuring position, wherein a correction signal generated based on a deviation of the weld from a desired position can be used directly to correct the position of the welding beam.

EP 2 062 674 A1 describes a method for preparing a laser welding process, with a sensor device detecting the position of a joint on a workpiece in a measuring region leading to a laser beam position.

From DE 10 2006 040 612 A1 a distance measuring device, in particular for detecting the altitude of a machining tool relative to the surface of a workpiece is known.

Furthermore, DE 10 2006 030 061 A1 relates to a laser beam welding method, in which the processing laser beam is also used as a measuring laser beam at the same time Position detection of the joint to realize. A distance measurement, however, is not revealed.

A disadvantage for the accuracy of the measurement proves in practice a well-known under the name "Speckies" effect, in which the light is scattered the stronger the shorter the wavelength. Especially with glossy surfaces, the light is not reflected in the direction of the optical sensor, so dropouts can not be reliably excluded.

The present invention is therefore based on the object to provide a method and a device for non-contact distance measurement, which overcome the disadvantages mentioned above and inexpensive to manufacture and are universally applicable.

The first object is achieved by a method according to the features of claim 1. The further embodiment of the invention can be found in the dependent claims.

According to the invention, therefore, a method is provided in which, during the distance measurement, the measuring point of the processing laser is moved relative to the object to be measured along a predetermined course on the object. According to the invention, instead of deflecting a light spot or a strongly focused light spot and detecting the beam reflected by the object, the processing laser beam is dynamically deflected onto a multiplicity of successive light spots on the surface of the object, an increase in accuracy compared to mere reflection could be achieved in a surprisingly simple manner of a spot of light or spot on the surface of the object. It has been found that the laser beam produced in this way during the measurement produces a significantly better detectable sharp contour, which in practice leads to significantly lower error influences. In addition, the laser power is concentrated only for an extremely short period of time to a respective measuring point and thus effectively reduces the energy input of the processing laser acting on a respective surface point of the object. In particular, the laser spot or laser focus is moved on the object along the predetermined course of the peripheral contour, so that an irreversible action of the laser on the object due to the short residence time can be excluded. In other words, therefore, a processing laser that causes a permanent change in the object during its processing cycle without a change in its parameter setting during of the measuring cycle can be used without irreversible change in the properties of the surface point acted upon by the laser energy during the measuring cycle. Furthermore, the effects of the unwanted speckle effect due to the dynamic course of the laser spot on the object during the distance measurement are far less pronounced than is the case with static measuring methods according to the prior art.

The dynamic deflection of the measuring point could for example be done by two intersecting lines. On the other hand, a modification of the invention in which the processing laser is deflected along a closed circumferential contour, which is preferably free of points of discontinuity and therefore enables a stationary change of the measuring point on the surface, is particularly promising. In addition, by avoiding intersecting lines and the associated additional energy input into a certain surface point, the maximum energy input relative to a single measurement point can be substantially reduced.

It also proves to be particularly practical if the processing laser of the peripheral contour is tracked with a steady, especially constant speed. As a result, a continuous action of the laser energy with respect to the affected surface points of the object is ensured. If it comes to a thermal energy input due to the laser energy, so unwanted material stresses can be avoided by a uniform distribution of the acted upon by the laser energy surface points. For example, the circumferential contour can run concentrically around a corresponding center, which corresponds for example to the centroid or a geometric center of the object.

In this case, an increase in the accuracy of the measuring method can be achieved in a simple manner in that the measuring point of the processing laser is performed several times along the predetermined course. Thus, by repeatedly tracking the measurement point on the predetermined course or the contour on the object, the accuracy of the measurement can be increased in a simple manner, without requiring an enlargement of the detection range of the optical sensor for this purpose.

The predetermined course of the measuring point could follow a continuous or broken line. In addition, it is particularly expedient if the contour corresponds to a geometric shape or a figure. By following the measurement line of a mathematically easily determinable shape or figure, possible deviations can occur or errors occurring comparatively easily detected and the results of a logical test or control are subjected.

In another, also particularly promising modification in which the contour corresponds to the detection of a relative orientation of the object relative to the reference point of a non-rotationally symmetric, in particular polygonal shape, in addition to the distance measurement at the same time a relative angular position of the object relative to the reference point can be determined so as to determine the orientation of the object as well as its orientation.

Preferably, in connection with simple geometric shapes, only individual measuring points are detected in the course of the contour in order to be able to carry out a quality control with little effort in this way.

In this case, a further modification according to the invention proves to be particularly useful in which only a fulfillment or non-compliance is determined for individual, predetermined measuring points, so that the optical sensor is used only for monitoring these discrete measuring points and their compliance. Thus, different values for the distance of the object from the reference point are not detected for a particular measuring point, but rather only the fulfillment of a value determined by the detection range of the sensor is controlled. As a result, the optical sensor as well as the controller can be significantly simplified. To this end, for example, so-called pinholes which detect the exposure of a certain point by the light reflected from the measuring point have proven to be suitable. For example, a simple photodiode allows the detection of compliance with certain forms or limits.

The processing laser can be assigned a scanner optics, by which the laser beam is deflected. Another, also particularly advantageous embodiment of the method is achieved in that the deflection is effected by a movement of a temporarily fixing the object, for example by means of a movable table in two axes.

Furthermore, the object is still achieved by a device for carrying out a triangulation measurement method, with a device for relative movement of the object to be measured fixing recording relative to a processing laser or a fixed reference point in a particular horizontal plane. The invention allows for various embodiments. To further clarify its basic principle, one of them is shown in the drawing and will be described below with reference to a schematic diagram. This shows a device 1 for contactless distance measurement, in which a distance a of an object 2 from a reference point 3 in a measuring cycle is determined by a laser beam 4, which is used as a processing laser during a processing cycle, not shown, for the material processing of the object 2. The determination of the distance a of the reference point 3 of the device 1 is carried out according to the principle of triangulation by detecting a reflected from the object 2 measuring point 5 by means of an optical sensor 6. In this case, the measuring point 5 during the measuring cycle relative to the measuring object 2 along a predetermined course of a closed circumferential contour 7, which is shown here only by way of example by two measuring points 5 ', 5 "on a circle on which object 2 moves at a constant relative speed for an extremely short period of time focused on the respective measuring point 5 of the circumferential contour 7, so that the energy input acting on a respective surface point of the object 2 during the measuring cycle compared to the duty cycle is substantially reduced .An unwanted irreversible change of the object 2 is therefore excluded during the measurement cycle. In a simplified mode, only compliance with a specific circumferential contour 7 is controlled by detecting the assignment of the reflected beam of one or more selected measuring points 5, 5 ', 5 "to a specific segment 8 of the optical sensor 6. In order to achieve the desired relative movement 2, the object 2 is moved relative to the reference point 3 in the direction of the X-axis and the Y-axis by means of a receptacle 9 temporarily fixing the object 2.

Claims

PATE NTANS PRINT HE

1. A method, in particular triangulation measuring method, for non-contact distance measurement, wherein the distance (a) of an object (2) from a laser source or a fixed reference point (3) by means of a material processing of an object (2) certain processing laser by detecting a in a measuring point (5) of the object (2) reflected beam is determined by means of an optical sensor (6), characterized in that during the distance measurement a plurality of measuring points (5 ', 5 ") of the processing laser are detected by the object to be measured (2) is moved along a predetermined course on the object (2) relative to the reference point (3) or the processing laser.

2. The method according to claim 1, characterized in that the processing laser along a closed peripheral contour (7) is deflected.

3. The method according to claims 1 or 2, characterized in that the processing laser is tracked the predetermined course with a steady, in particular constant speed.

4. The method according to at least one of the preceding claims, characterized in that the measuring point (5) of the processing laser is performed several times along the predetermined course, in particular the complete circumferential contour (7).

5. The method according to at least one of the preceding claims, characterized in that the peripheral contour (7) corresponds to a geometric shape or a figure.

6. The method according to at least one of the preceding claims, characterized in that the peripheral contour (7) for detecting a relative orientation of the object relative to the reference point (3) corresponds to a non-rotationally symmetrical, in particular polygonal shape.

7. The method according to at least one of the preceding claims, characterized in that along the predetermined course on the object (2) only individual measuring points (5 \ 5 ") are detected.

8. The method according to at least one of the preceding claims, characterized in that only individual discrete measuring points (5) are monitored by means of the optical sensor (6).

9. The method according to at least one of the preceding claims, characterized in that the deflection by a movement of the object (2) temporarily fixing receptacle (9).

10. Device (1) for carrying out a triangulation measurement method for contactless distance measurement according to at least one of the preceding claims, characterized by a device for relative movement of the object to be measured (2) fixing recording (9) against a processing laser or a fixed reference point (3) in a particular horizontal plane.