WO2001007866A1

WO2001007866A1 - System for scanning of the geometry of large objects

Info

Publication number: WO2001007866A1
Application number: PCT/NO2000/000235
Authority: WO
Inventors: Alf Pettersen
Original assignee: Metronor Asa
Priority date: 1999-07-13
Filing date: 2000-07-10
Publication date: 2001-02-01
Also published as: AU7459900A; JP2003505682A; EP1200798A1; NO993446L; NO313113B1; NO993446D0

Abstract

System for detection of the surface geometry of an object (6), comprising a sensor unit (1) with apparatus (2) for local, point by point detection of surface geometry, and a robot unit (4) for moving the sensor unit (1), in the sensor unit (1) there is included an optical scanner unit (2) for non-touch probing and detection of the surface geometry of the object, and a position measuring unit (3) designed to determine the position of the sensor unit (1) in a global coordinate system defined by a network (8) of reference points (9) in known positions. A computing unit (5) is provided and designed for collection of data from the scanner unit (2) and the position measuring unit (3) and for transformation of the data from the scanner unit (2) to relate them to the global coordinate system. Further, there is present a method for detection of the surface geometry of an object (6), and a method for calibration of a sensor unit (1).

Description

SYSTEM FOR SCANNING OF THE GEOMETRY OF LARGE OBJECTS

The present invention relates to a system as well as a method to scan the geometry of objects as described in the introduction to claims 1 and 11

A number of laser based scanners for measurement of surfaces exist These are characterized by having a limited working volume, or limited measurement volume, requiring a specific offset from the surface, and requiring a specific orientation relative to the surface Scanners are therefore often mounted on accurate coordinate measurement machines, such that the scanner can be moved step-wise across the surface to scan region after region. A coordinate measurement machine is complex, inflexible and costly

There is a need for more flexible, portable solutions. An alternative is attaching a scanner to a robot, in such a way that the robot moves the scanner across the object. For each position, a part ofthe surface is scanned. This measurement is registered relative to the position ofthe scanner, and is transformed to a global coordinate system provided by the robot.

Most robots have low accuracy. Transformation ofthe data based on the robot's coordinate system will therefore not produce a sufficiently accurate description ofthe object's total geometry. Calibration ofthe robot to better describe its movements will help, but due to effects like wear and temperature variations, this is still not good enough

The present invention combines a solution comprising a robot and scanner with use of a position measuring device as described in Norwegian Patent No 303 595 The scanner and the position measuring device are integrated into one sensor unit In this solution, the position measuring device provides information on the exact position of the scanner The robot is only used to move the sensor unit

The characteristic features ofthe system and method are set forth in claims 1 and 1 1, respectively further embodiments are set forth in the dependent claims

Figure 1 shows the system solution

Figure 2 shows an alternative configuration Figure 3 shows as an example a scanner unit, a triangulation sensor

Figure 4 shows an example of a position measuring device as described in Norwegian patent No 303 595

Figure 5 shows a method for determining the internal geometry ofthe sensor unit

Figure 1 shows an example of a configuration ofthe system. It consists of principally two units, a sensor 1 and a robot 4. The robot is used to position the sensor in relation to the current area 12 on the object 6 The sensor 1 registers the local geometry ofthe object, and measures its own position relative to a global coordinate system 13

The sensor unit 1 consists of a scanner unit 2 for local scanning of geometry plus position measuring unit 3 to determine the sensor unit's own position relative to the global coordinate system 13. The scanner unit 2, e.g. a laser scanner, emits a laser beam 11 which scans a limited region 12 The position measuring unit 3 registers its own position, and thereby the position ofthe sensor unit 1, relative to a network 8 of reference points 9 The positions ofthe reference points are known relative to the global coordinate system 13

The system also includes a computing unit 5 which collects the data from scanner unit 2 and position measuring unit 3 and transforms all information from the scanner unit to the same global coordinate system 13. The computing unit 5 also sends data to the robot 4 to control the robot's movement relative to the object.

Figure 2 shows an alternative configuration The reference points 9 are attached to the object 6 The position measuring unit 3 is placed to view these reference points.

The scanner unit 2 can e g be one ofthe following types, but is not limited to this

• Laser distance meter which measures the distance between the scanner unit and the object in a single point,

• Triangulation sensor based on single-axis scanning laser or laser line projection combined with camera (e.g CCD sensor) Such a sensor scans one line from each sensor position

• Triangulation sensor based on dual-axis scanning laser or laser raster projection combined with camera (e g CCD sensor) • Sensor based on projection of pattern combined with one or more cameras (e g CCD sensor)

Figure 3 shows the principle for a triangulation sensor It contains a laser 14 which emits a laser beam 11 or a laser plane (line projection) The laser projects a point 15 or a line on the object 6 The point 15 is imaged through a lens 16 onto a sensor 17, e g a CCD array The scanner unit 2 is calibrated in such a way that it measures the position ofthe point 15 relative to an internal coordinate system 18 In an alternative implementation, the laser beam can be aimed at the surface through a two-axis, movable mirror In this way, a region ofthe surface can be scanned from each position ofthe triangulating sensor

The significant characteristic ofthe scanner is that it registers the local geometry of an object relative to the internal coordinate system ofthe scanner For each scanner position, a registration can be made of a point, points along a line, or points in a two- dimensional pattern

In a preferred embodiment the position measuring device 3 is of a type described in Norwegian patent No 303 595, as shown in figure 4 Essentially, this comprises one or more cameras mounted together in a unit. Each camera sees a reference pattern in the form of points, lines or other easily recognizable objects. For each position ofthe sensor unit 1, the points 9 in the reference pattern 8 are imaged through the lens 19 onto the sensor 20 The data is transferred to the computing unit 5 Software in the computing unit calculates the position and orientation ofthe position measuring unit 3 relative to the reference pattern 8 The position measuring device in figure 4 is shown with carrying handle 21 and activation switch 22 for manual operation. Also shown is an illumination source 23 for illumination ofthe reference pattern 8, and a mechanical probe 24 for point by point measurement by touching an object, as described in Norwegian patent No 303 595

It will be advantageous if the reference pattern 8 is known in the coordinate system of the object, or is a part ofthe object itself as depicted in figure 2 This can be achieved if the object has holes that can be recognized by the position measuring device, or if the reference pattern is attached to the object, e g by mounting easily recognizable targets into holes or depressions in the object These targets may e g be purely passive markers, light sources, light reflectors or similar It is significant that the geometrical relationship between the scanner unit 2 and the unit 3 for measuring position be known and stable This can partly be achieved through a stable, precise and known mechanical construction, and through separate calibration as described below

The sole purpose ofthe robot 4 is to position the sensor unit 1 in the correct position and oπentation relative to the object 6 Several types of robot pπnciples may be used, e g arm robots, Cartesian robots, and robots with one, two, three or more degrees of freedom The robot may be controlled by a predefined program, or by using the measured position ofthe sensor unit relative to the object and feed the robot instructions for relative movement in relation to the current position

For each position, the data from sensor unit 1 must be evaluated in the same global coordinate system 13 This requires that the relationship between the coordinate system Xs, Ys, Z_s, ofthe scanner unit and the coordinate system ofthe unit for measuring position Xc, Yc, Zc is known Figure 5 illustrates a method for determining this relationship The sensor unit 1 is positioned such that at least three ofthe reference points 9 are inside its measurement volume By simultaneously registering the position ofthe reference points relative to the coordinate system ofthe scanner unit, and the position ofthe unit for measuring position relative to the reference points, the information required to calculate the transform between these two coordinates is acquired

Claims

P a t e n t C l a i m s

1

System for detection ofthe surface geometry of an object (6), comprising a sensor unit (1) with apparatus (2) for local, point by point detection of surface geometry, and a robot unit (4) for moving the sensor unit (1), characterized in that the sensor unit (1) includes an optical scanner unit (2) for non-touch probing and detection ofthe surface geometry ofthe object, and a position measuring unit (3) designed to determine the position ofthe sensor unit (1) in a global coordinate system defined by a network (8) of reference points (9) in known positions, and that a computing unit (5) is provided and designed for collection of data from the scanner unit (2) and the position measuring unit (3) and for transformation ofthe data from the scanner unit (2) to relate them to the global coordinate system.

2

System as defined in claim 1 , characterized in that the robot unit (4) is designed for stepwise movement ofthe sensor unit (1)

3 System as defined in claim 1 , characterized in that the position measuring unit (3) consists of a camera-based sensor (7) and that said unit (3) is designed for registering an image of a network (8) consisting of reference points (9) in known positions.

4

System as defined in claim 1, characterized in that the network (8) of reference points (9) is on the object, and that the position for each reference point in the network is known relative to a coordinate system related to the object

5

System as defined in claim 1, characterized in that the robot unit (4) is designed to move the sensor unit (1) step-wise over the object (6)

6

System as defined in one or more ofthe preceding claims, characterized in that the scanner unit (2) is chosen from the following group laser scanner, single-point distance meter, laser-based tπangulation sensor combined with camera, tπangulation sensor with two-axis scanning laser, tπangulation sensor with laser raster projection in combination with camera, sensor based on pattern projection combined with at least one camera

7

System as defined in one or more ofthe preceding claims, characteπzed in that the robot unit (4) is chosen from the following group arm-based robot, Cartesian robot, robot with one, two or more degrees of freedom, program controlled robot, real-time position-controlled robot based on registered position ofthe sensor unit (1) in relation to the object and instruction for movement relative to current position

8

System as described in one or more ofthe preceding claims, characteπzed in that said camera in the unit (3) for measuring position is a CCD camera

9 System as described in claim 4, characterized in that the reference points (9) are holes or depressions in the surface ofthe object (6)

10

System as described in claim 4 or 9, characteπzed in that the reference points (9) consist of so-called "targets" placed on the object (6) or in the mentioned holes or depressions in the surface ofthe object (6)

11

Method for detection ofthe surface geometry of an object (6), where one uses a sensor unit (1) comprising apparatus (2) for local, point by point detection ofthe surface geometry, a position measuring unit (3) to determine the position ofthe sensor unit in relation to a network (8) of reference points (9) in known positions relative to a global coordinate system, and a robot unit for moving the sensor unit, characteπzed in that the sensor unit (1) is positioned such that a region ofthe surface ofthe object (6) is inside the measurement volume ofthe scanner unit (2), and that this region is scanned , that the position measuring unit (3) simultaneously determines the position ofthe scanner unit (1) relative to the coordinate system ofthe network (8), and that the data from the scanner unit (2) are transferred to a computing unit (5) where they are transformed to the coordinate system ofthe network (8) and stored.

12.

Method as specified in claim 11, characterized in stepwise movement ofthe sensor unit (1) by the robot unit (4)

13.

Method for calibration of a sensor unit (1) which comprises apparatus (2) for local detection of a surface geometry, and a position measuring unit (3) to determine the position ofthe sensor unit in a global coordinate system relative to a network (8) of reference points (9) in known positions, and where the sensor unit (1) is mounted on a robot unit (4) for movement relative to an object (6) characterized in that the sensor unit (1) is positioned such that at least one ofthe reference points (9) is inside the measurement volume ofthe scanner unit (2), that the position ofthe reference point (9) relative to the scanner (2) is determined, that the position measuring unit (3) simultaneously determines the position ofthe sensor (1) relative to the coordinate system ofthe network (8), that this is repeated until the position of at least three reference points (9) have been determined relative to the coordinate system ofthe scanner unit, and that a transformation matrix is calculated based on the data registered by the skanner unit (2) and the position measuring unit (3) and which describes the relationship between the two units.