WO2006114216A1

WO2006114216A1 - Method and device for scanning an object using robot manipulated non-contact scannering means and separate position and orientation detection means

Info

Publication number: WO2006114216A1
Application number: PCT/EP2006/003518
Authority: WO
Inventors: Bart Van Coppenolle; Lieven De Jonge
Original assignee: Metris N.V.
Priority date: 2005-04-25
Filing date: 2006-04-18
Publication date: 2006-11-02

Abstract

The present invention relates to a method and system for measuring an object, comprising a non-contact scanning means (NCSM) attached to the effector end of a robot, wherein the position and orientation of the NCSM is determined using a non-contact position and orientation detection means (NCPODM). The position and orientation of the NCSM is measured continuously by said NCPODM. By doing so the precise position of the NCSM is determined independently of the robot, so the error owing to inherent movement inaccuracies of industrial robots can be diminished.

Description

METHOD AND DEVICE FOR SCANNING AN OBJECT USING ROBOT MANIPULATED NON-CONTACT SCANNERING MEANS AND SEPARATE POSITION AND ORIENTATION DETECTION MEANS.

FIELD OF THE INVENTION

The invention relates to the use of a robot to position and to orientate a non-contact scanning means for measuring an object, while the position and orientation of the non- contact scanning means is measured continuously by a non-contact measurement device while the robot is moving the scanning means.

BACKGROUND TO THE INVENTION

Non-contact scanners are common in industrial metrology for the measurement of parts, for example in the engineering industries such as the automotive and aircraft manufacturing. Most non-contact scanners have a relatively small measurement range because of the high requirements on measurement accuracy. To enable non-contact scanners to measure a large object such as a car, three different solutions are practiced depending on the type of non-contact scanner.

The first solution is to mount the scanner on a tripod and to measure the full object by relocating the scanner in a manual way in different positions around the object. A moire fringe projector, for example, is used in combination with this technique. However, the accuracy of combining the different measurements taken in the different positions into one reference coordinate system is limited. Furthermore, the time taken to scan an object depends on the speed of repositioning and setting up the scanner between readings.

A better accuracy is obtained in the second solution: the scanner is mounted on a Coordinate Measuring Machine (CMM) that moves the scanner over the object during the scanning process, and the CMM is moved by a computer in an automatic, controlled way. A laser line scanner, for example, is used in combination with this technique. The position and the orientation of the scanner are continuously recorded during scanning with a high accuracy, due to the high accuracy of the CMM. Being a precision device, however, CMM has itself only a limited measurement volume.

For measuring larger volumes, larger CMMs or localizers are available. These instruments are heavily engineered so that, despite their large size, the location and orientation of the measurement probe can be derived internally from angles adopted by and movements of the localizer. This equipment, being precision quality, is usually very expensive and necessitates qualified workers to operate it in order to maintain the required accuracy. An example such equipment is a robot localizer as disclosed in EP 0 963 816. This describes a precision robot localizer that measures accurately the position of the robot itself from internal sensors. It is calibrated using an external camera system to monitor the robot arms, comparing the actual arm positions to a robot model. However, such calibration adds considerable time to the measurement, requiring least squares fitting after every movement. Furthermore, the robot and the external sensor must be in the same reference frame, Furthermore, significant effort is required in EP 0 963 816 to transpose a calibration method from one robot type to another type. In particular, optimisation algorithms must be adapted. Therefore, such systems are expensive, high-maintenance and slow.

A third solution is using the scanner in a manual, handheld way. Large objects can be measured with such a handheld scanner but it is only operated in a manual way. Manual movements are problematic because areas can be accidentally excluded from scanning. Furthermore, the scanner is generally swept at irregular speed, so the resolution of the map so obtained is uneven. Furthermore, the size of the object is limited by the reach of the person, consequently hard-to-reach areas or very large objects cannot be easily scanned. Furthermore, there is nothing to prevent accidental collision between the scanner and the object.

The present invention provides a system and method for scanning an object which overcomes the problems of the prior art.

SUMMARY OF SOME EMBODIMENTS OF THE INVENTION

One embodiment of the present invention is a system for the measurement of an object comprising:

- a non-contact scanning means, NCSM, attached to the effector end of a robot,

- a non-contact position and orientation detection means, NCPODM, - said NCPODM configured to determine the position and orientation of the NCSM during measurement.

Another embodiment of the present invention is a system for the measurement of an object comprising: - a non-contact scanning means, NCSM₁ attached to the effector end of a robot,

- a non-contact position and orientation detection means, NCPODM, - said NCPODM configured to determine the position and orientation of the NCSM during measurement,

- said system configured to determine the measurement of the object from data obtained from the NCSM and NCPODM.

Another embodiment of the present invention is a system as described above wherein the NCSM is configured to measure the object in a three-dimensional co-ordinate system, defined in fixed relation to the NCSM.

Another embodiment of the present invention is a system as described above wherein the NCPODM is configured to determine the position and orientation of NCSM in a three- dimensional co-ordinate system, defined in fixed relation to the object.

Another embodiment of the present invention is a system as described above, wherein the position of the NCPODM is fixed relative to the object.

Another embodiment of the present invention is a system as described above, further comprising a processing means configured to transform data measured by the NCSM into a three-dimensional co-ordinates defined in fixed relation to the object.

Another embodiment of the present invention is a system as described above, further comprising a processing means configured to determine the next translational and/or rotational movements of the effector end, using data from the NCSM.

Another embodiment of the present invention is a system as described above, wherein said NCSM is an optical scanner.

Another embodiment of the present invention is a system as described above, wherein said NCSM is a scanner using laser light for measurement

Another embodiment of the present invention is a system as described above, wherein said NCSM comprises two or more light sources.

Another embodiment of the present invention is a system as described above, wherein said NCPODM comprises two or more CCDs. Another embodiment of the present invention is a method for measuring an object using an NCSM attached to an effector end of a robot arm comprising the steps of: a) measuring the object using said NCSM, b) determining, from a NCPODM, the three-dimensional position and orientation of the NCSM, c) using the position and orientation information of step b) to transform the measurements of step a) into a three dimensional measurement in the co-ordinate system of the object, and d) obtaining a measurement of the object.

Another embodiment of the present invention is a method as described above, wherein the measurement of step a) is in a three-dimensional co-ordinate system, defined in fixed relation to the NCSM.

Another embodiment of the present invention is a method as described above, wherein the position and orientation determined in step b) is in a three-dimensional co-ordinate system, defined in fixed relation to the object.

Another embodiment of the present invention is a method as described above, wherein the position of the NCPODM is fixed relative to the object.

Another embodiment of the present invention is a method as described above, wherein the measurement of step a) are used to determine the next translational and/or rotational movements of the effector end.

Another embodiment of the present invention is a method as described above, wherein said NCSM is an optical scanner.

Another embodiment of the present invention is a method as described above, wherein said NCSM is a scanner using laser light for measurement

Another embodiment of the present invention is a method as described above, wherein said NCSM comprises two or more light sources.

Another embodiment of the present invention is a method as described above, wherein said NCPODM comprises two or more CCDs. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 : Illustrates a system according to the present invention comprising a base 6 mounted robot 1 , which arm provides an effector 5 capable of rotational movements and translations in three dimensions. Attached to the effector is a non-contact scanning means (NCSM) 5 for scanning the surface of the object 4. Separately mounted from the moving parts of the robot/NCSM is a non-contact position and orientation detection means (NCPODM) 3, which detects the position and orientation of the NSCM 2.

Figure 2: Illustrates a NCSM 2, provided on the surface with a recognition means which is a pattern of LEDs 7.

DESCRIPTION OF THE INVENTION

The present invention relates to a method and system for measuring an object, comprising a non-contact scanning means (NCSM) attached to the effector end of a robot, wherein the position and orientation of the NCSM is determined using a non-contact position and orientation detection means (NCPODM). The position and orientation of the NCSM is measured continuously by said NCPODM. By doing so the precise position of the NCSM is determined independently of the robot, so errors owing to inherent movement inaccuracies of industrial robots can be diminished. The system thus determines the measurement of the object from data obtained from the NCSM and NCPODM.

The measurement of the object may result in a numerical representation of part or all of the object. The measurement may be represented, for example, as a cloud point, a table of co-ordinates, a list of location of points, or as any desired format depending on the intended use.

The articles "a" and "an" are used herein to refer to one or to more than one, i.e. to at least one, the grammatical object of the article. By way of example, "an object" means one object or more than one object.

The description below is made in reference to Figures 1 and 2. The Figures represent only a single embodiment of the invention. The intention is to provide clarity to the description by reference to the Figures, but by no means to limit the invention.

The object 4 as used herein is an object to be scanned or part thereof. The method and system may be suitable for use with large objects, such as car panels, aircraft panels, with other large objects whose dimensions are sought, and are not amenable for measurement by method of the art, such as CMM or manual means. The system and method can provide measurements of the object in a three-dimensional coordinate system in fixed in relation to said object.

The robot 1 moves the NCSM over part or all of the surface of the object during the measurement. A robot, according to the invention, is a device comprising an effector end 5 which position is capable of rotational and translational movement in two or three dimensions. The robot can be a standard industrial robot. It is preferably a non-measuring robot. The range of translational movement of a robot effector end 5 is normally greater than that of a CMM. The former does not accurately determine the position of the effector end, unlike a CMM whose joint movements are precisely known and accurately controlled, making a CMM suitable for use in measuring small objects. Industrial robots, on the other hand comprise heavy arms and, while being suitable for performing repetitive with tasks on heavy objects, their absolute accuracy of movement is very poor. Owing to this inherent inaccuracy, robots are not regarded as useful for the automatic, accurate measurement of objects.

The present invention is able to utilize the large range of movement associated with a inexpensive standard robots, and overcomes the inherent inaccuracies of robot movement by measuring externally the position and orientation of the NCSM, using a NCPODM.

There is no requirement to calibrate the industrial robot since the positions and orientations of the robots arms are irrelevant; only the position and orientation of the

NCSM.

The robot 1 may be controllable by a processing means. The robot may be programmed to scan the object automatically. The base 6 of the robot may be fixed in relation to the object 4.

A further advantage of using a robot is that parts of the surface of an object cannot be accidentally excluded from scanning, as the live feedback regarding unscanned patches can be used to control the effector movements. Furthermore, the resolution of the map may be even, or of partly higher resolution where required. Furthermore, a robot is able to access hard-to-reach areas or very large objects. Furthermore, the robot can be programmed to prevent accidental collision. Furthermore the robot enables the scanning process to be entirely automatic. The NCSM 2 is coupled to the effector end 5 of the robot. An NCSM is well known in the art. It generally comprises one or more emitter source(s), sending (e.g. light) waves to the object of interest, and one or more receivers, which capture the part of the object that is hit by the emitter source. Data from the NCSM detector is converted into an accurate 3D point in relation to three dimensional coordinate system of the NCSM. The NCSM may be a scanner using laser light for measurement, an optical scanner or a laser line scanner.

Data from the NCSM 2 may also be also used to control the movement of the robot, for example, in determining the next translational/rotational movement and preventing collision with the object. The movements of the robot and the NCSM attached thereto, which movements cover the region of the object to be scanned can be provided by well known scanning methods and routines.

The NCSM 2 further comprises a recognition means, suitable for detection by the NCPODM 3. The recognition means may be simply the shape of the NCSM 2 itself which can be tracked by the. NCPODM. For example, an irregular shape of NCSM which has a different appearance at several orientations can be detected by a NCPODM configured with a shape recognition system. Alternatively, a recognition means may be an irregular shaped artifact attached to the NCSM for detection by the NCPODM. Alternatively, it may be a reflective strip disposed at one or more positions on the surface of the NCSM. Preferably, the NCSM is provided with one or more visible or infrared light sources, such as light emitting diodes (LEDs) 7 or electroluminescent sources. These are positioned and arranged in such pattern to allow the location and orientation of the NCSM to be detected by the NCPODM. It is an embodiment of the invention that the recognition means is a pattern comprising one or more light sources (e.g. 1 , 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 , 12, 13, 14, 15, 16, 17, 18, 19, 20 or more, or a number between any two of the aforementioned values) which emit light at the same wavelength. It is another embodiment of the invention that the recognition means is a pattern comprising two or more light sources (e.g. 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 , 12, 13, 14, 15, 16, 17, 18, 19, 20 or more, or a number between any two of the aforementioned values) wherein at least two light sources emit light at the different wavelengths. It is another embodiment of the invention that the recognition means is a pattern comprising two or more light sources (e.g. 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 , 12, 13, 14, 15, 16, 17, 18, 19, 20 or more, or a number between any two of the aforementioned values) wherein at least two light sources pulsate at a different frequencies (e.g. at 0, 1 , 2, 3, 4, 5, 6, 7 ,8, 9, 10, 15, 20, 30, 40, or 50 Hz, or at a frequency between any two of the aforementioned values). It is another embodiment of the invention that the recognition means comprises a pattern of a combination of steady, pulsating, and/or different coloured light sources.

NCPODM 3 are known in the art, and comprise at least one detector, and optionally source(s) of electromagnetic radiation such as IR or visible light. A NCPODM 3 is capable of recording the position and orientation of the recognition means in three dimensional space, in relation to three dimensional coordinate system of the object. Thus, the coordinate system used by the NCPODM 3 can be fixed relative to the object. The NCPODM 3 is not part of the moving robot/NCSM assembly. It may be located remotely from the object and robot/NCSM assembly. According to an aspect of the invention, the position of the NCPODM 3 is fixed relative to the object. According to an aspect of the invention, the position of the NCPODM 3 is absolutely fixed; the NCSM and/or the object (see below) may move. It is generally located in a position such that it is capable of detecting the movements of the recognition means.

The recognition means discussed above, is detected by a NCPODM 3 suitably configured. For example, where the recognition means is a reflective strip, the NCPODM 3 may comprise one or more light source(s) and electronic camera(s). Alternatively, where it is a shape, the NCPODM 3 may comprise one or more electronic cameras and optional light source(s). Alternatively, where it is an arrangement of light sources, the NCPODM 3 may comprise one or more electronic camera(s) suitably configured. Preferably the NCPODM 3 comprises two or more (e.g. 2, 3, 4 or 5) electronic cameras such as CCD cameras, for the detection of light source.

Data from the NCPODM 3 is used to calculate the position and orientation of the NCSM 2 relative to the object. Such data is used to transform the readings from the NCSM 2 into the co-ordinate system of the object, so obtaining a measurement of the object. It may also used to correct inaccuracies in the movements of the robot. For example, the speed, stop and start signals to the robot may be modified based on data received from the NCPODM 3.

By separately monitoring the position of the NCSM 2, large robotic devices, not intended for use in measuring can be employed by manufacturers. Thus manufactures, typically having large robots already on site for construction of parts may quickly adapt them for use for measurement without the expense of precision CMM-type components. Furthermore, industrial robots are easy to operate and maintain, requiring less specialist skills compared with a CMM or robot localizer. The present invention is generic and works on all types of robots without additional effort from the user.

Another advantage of the present invention is that the robot itself does not require calibration. Calibration is necessary using a robot localizer because the measurements obtained from the localizer are influences by aging, wear and stress. Calibration can require an operator to find a common reference frame of the robot-sensor combination and to update a model of the robot before the scanning process begins (e.g. as described in EP 0963 816). By contrast, in the present invention no calibration is necessary because the location of the robot arms and hand are not needed, just the position and orientation of the NCSM as captured externally. As a result, there is thus a gain in user time and a gain in accuracy.

It is a further aspect of the invention that the NCPODM also monitors the position and orientation of the object during measurement. The position of an object can change during measurement, particularly if the object is lightweight and the area to be measured is large. Therefore, there is a need to also track any movements of the object during measurement. In this embodiment, the object is monitored by NCPODM by way of a recognition means as elaborated above for the NCSM. For example, the object may be provided with one or more LEDs. It is an aspect of the invention that the recognition means comprises LEDs in which one or more of the pattern, colour, pulsing frequency or the combination of these is different compared the recognition means of the NCSM. Where the object moves during measurement, the position and orientation data of the object recorded by the NCPODM may be used to correct the measurement of the object.

A processing means may be used by the invention to perform one or more of the steps below: a) converting data from NCSM 2 into a three dimensional point in relation to three dimensional coordinate system of the NCSM, b) determining the next translation and/or rotational movements of the robot using data from the NCSM 2, c) determining, from data from the NCPODM 3, the three dimensional position and orientation of the NCSM 2, d) transforming the three dimensional point determined in step a) into a three dimensional point in the co-ordinate system of the object, e) optionally using data from the NCPODM 3 to change the movements of the robot 1 arm during measurement. A processing means may be built into one or more devices such as the robot 1 , NCSM 2 or NCPODM 3. For example, a processor may be built into the NCSM 2 to compute the next movement of the effector 5. Alternatively, a processing means may be located in a computer, such as a desktop PC. The processing means present in one or more devices and a computer. Alternatively, all the processing means may reside in a computer or collection of computers. The configuration of the processing means within the system and method will depend on the equipment available, the capability of processing means already therein and other operational factors.

In one embodiment of the invention, a system comprises a laser line scanner mounted on the effector end of a robot 1 , said scanner comprising two or more LEDs mounted on the scanner, and an arrangement of two or more CCD cameras not part of the moving robot/scanner assembly configured to determine the orientation and position of the scanner during the scanning.

In another embodiment of the invention, a system for a non-contact measurement device for determining the location of points on the surface of a three-dimensional object comprises a. a three-dimensional coordinate system defined in a fixed relation to said object, b. NCSM determining the location of points on the surface of said object, said location of said points determined in relation to a three-dimensional coordinate system in fixed relation to said NCSM, c. a robot with said NCSM means connected to its end effector, said robot moving said scanning means over said object. d. NCPODM remotely located from both said object and said NCSM for detecting the position and orientation of said NCSM in relation to said coordinate system in fixed relation to said object. e. computing means connected to said NCSM and to said NCPODM for determining the position of the measurements of said scanning means in relation to said coordinate system in fixed relation to said object, by combining said measurements of said NCSM with the respective measurements of said NCPODM in relation to said coordinate system in fixed relation to said object.

In another embodiment of the invention, a method for measuring an object using a NCSM 2 attached to an effector end of a robot 1 arm comprises the steps of a) measuring an object using said NCSM 2, b) determining, from a NCPODM 3, the three-dimensional position and orientation of the NCSM 2, c) using the position and orientation information of step c) to transform the measurements of step a) into a three dimensional measurement in the co-ordinate system of the object, d) obtaining a measurement of the object.

The scanning results can be used in numerous ways but mainly for dimensional quality control. The method and system of the present invention overcomes the limitations of the state-of-the-art. The NCSM 2 mounted on a robot 1 makes the scanner move in an automatic way in a large measurement volume. To avoid the inherent inaccuracies of the industrial robot the position and orientation of the scanner is independently determined by a second non-contact measurement device (NCPODM 3).

Claims

1. A system for the measurement of an object comprising:

- a non-contact scanning means, NCSM₁ attached to the effector end of a robot, - a non-contact position and orientation detection means, NCPODM,

- said NCPODM configured to determine the position and orientation of the NCSM during measurement,

2. A system according to claim 1 wherein the NCSM is configured to measure the object in a three-dimensional co-ordinate system, defined in fixed relation to the NCSM.

3. A system according to claims 1 or 2 wherein the NCPODM is configured to determine the position and orientation of NCSM in a three-dimensional co-ordinate system, defined in fixed relation to the object.

4. A system according to any of claims 1 to 3 wherein the position of the NCPODM is fixed relative to the object.

5. A system according to any of claims 1 to 4, further comprising a processing means configured to transform data measured by the NCSM into a three-dimensional coordinates defined in fixed relation to the object.

6. A system according to any of claims 1 to 5, further comprising a processing means configured to determine the next translational and/or rotational movements of the effector end, using data from the NCSM.

7. A system according to any of claims 1 to 6 wherein said NCSM is an optical scanner.

8. A system according to any of claims 1 to 6 wherein said NCSM is a scanner using laser light for measurement

9. A system according to any of claims 1 to 8 wherein said NCSM comprises two or more light sources.

10. A system according to any of claims 1 to 9 wherein said NCPODM comprises two or more CCDs.

11. A method for measuring an object using an NCSM attached to an effector end of a robot arm comprising the steps of: a) measuring the object using said NCSM, b) determining, from a NCPODM, the three-dimensional position and orientation of the NCSM, c) using the position and orientation information of step b) to transform the measurements of step a) into a three dimensional measurement in the coordinate system of the object, and d) obtaining a measurement of the object.

12. A method according to claim 11 wherein the measurement of step a) is in a three- dimensional co-ordinate system, defined in fixed relation to the NCSM.

13. A method according to claims 11 or 12 wherein the position and orientation determined in step b) is in a three-dimensional co-ordinate system, defined in fixed relation to the object.

14. A method according to any of claims 11 to 13 wherein the position of the NCPODM is fixed relative to the object.

15. A method according to any of claims 1 1 to 14, wherein the measurements of step a) are used to determine the next translational and/or rotational movements of the effector end.

16. A method according to any of claims 11 to 15 wherein said NCSM is an optical scanner.

17. A method according to any of claims 11 to 16 wherein said NCSM is a scanner using laser light for measurement

18. A method according to any of claims 11 to 17 wherein said NCSM comprises two or more light sources.

19. A method according to any of claims 1 1 to 18 wherein said NCPODM comprises two or more CCDs.