Calibration of a profile measuring system

• US 9,014,999 B2
• Filed: 07/02/2009
• Issued: 04/21/2015
• Est. Priority Date: 07/04/2008
• Status: Active Grant

• Current Assignee: Sick IVP AB (Sick)
• Original Assignee: Sick IVP AB (Sick)
• Inventors: Turbell, Henrik, Andersson, Robert
• Primary Examiner(s): Lau, Tung S
• Application Number:
  US13/002,387
• Publication Number:
  US 20110288806A1
• Time in Patent Office: 2,119 Days
• Field of Search: 702/104
• US Class Current: 702/104
• CPC Class Codes: G01B 11/2504 Calibration devices
• CPC Class Codes: G01B 21/042 Calibration or calibration artifacts G01B3/30, G01B9/02072 take precedence
• CPC Class Codes: G06T 5/80 Geometric correction

Abstract

A method for calibrating a measuring system, which system comprises a structured light source, optics and a sensor. The light source is adapted to produce a light plane or sheet and the optics is located between the light plane and the sensor. The method is performed in order to obtain a mapping from the sensor to the light plane. In the method the light source is switched on such that the light plane is produced. In order to account for distortions due to the optics, a mapping calibration profile is introduced in the light plane, wherein the mapping calibration profile comprises at least three points forming a straight line. A non-linear mapping from the sensor to the light plane is then computed by using the at least three points. Next, in order to account for perspective distortions, a homography calibration profile is introduced in the light plane, wherein the homography calibration profile comprises at least four points the relative distance between which are predetermined. A homography from the sensor to at the light plane based on these four points is then computed. A calibration object for using in such a method is also presented.

Claims

1. A method for calibrating a measuring system, which system comprises a light source, optics and a sensor, wherein said light source is adapted to produce a light plane and said optics is located between said light plane and said sensor, wherein the light plane is a plane of light, which light plane is formed by light rays that are confined in said light plane when emitted by the light source, wherein said method is performed in order to obtain a mapping from at least one point on said sensor to at least one point in said light plane, said method comprising the steps of:

switching on said light source such that said light plane is produced;

introducing a mapping calibration profile in a first mapping position in said light plane, wherein said mapping calibration profile comprises at least three points forming a straight line;

characterized in that the method further comprises the steps of;

computing a non-linear mapping from at least a first portion of said sensor to at least a first portion of said light plane by using said at least three points of said mapping calibration profile;

introducing a homography calibration profile in a first homography position in said light plane, wherein said homography calibration profile comprises at least four points the relative distance between which are predetermined, and

computing a homography from at least a first portion of said sensor to at least a first portion of said light plane based on said at least four points of said homography calibration profile.

2. The method according to claim 1, wherein said method further comprising the steps of:

altering the position of said mapping profile to successive mapping positions, and

using information as regards said at least three points of said mapping calibration profile from each one of said successive mapping positions in order to compute said non-linear mapping.

3. The method according to claim 2, wherein said step of altering the position of said mapping profile to said plurality of successive mapping positions is performed such that said successive mapping positions are selected randomly.

4. The method according to claim 1, wherein said method further comprising the steps of:

altering the position of said homography profile to successive homography positions, and

using information as regards said at least four points of said homography calibration profile from each one of said successive homography positions in order to compute said homography.

5. The method according to claim 4, wherein said step of altering the position of said homography profile to said plurality of successive homography positions is performed such that said successive homography positions are selected randomly.

6. The method according to claim 1, wherein said mapping calibration profile comprises a planar surface on which said at least three points are located.

7. The method according to claim 1, wherein said homography calibration profile comprises a saw-toothed portion with predetermined dimensions, wherein at least four points are located on said saw-toothed portion.

8. The method according to claim 1, wherein said homography calibration profile extends in a longitudinal and a vertical dimension forming an profile calibration plane, wherein said profile calibration plane is adapted to be parallel with said light plane when said homography profile is in a homography position, said homography calibration profile further extending in a transversal dimension being perpendicular to said profile calibration plane, said homography calibration profile further comprising at least two straight control lines, wherein each one of said control lines forms an angle with said transversal dimension, said method further comprising a step of determining a measure of a profile tilt of said profile calibration plane in relation to said light plane by utilizing said control lines.

9. The method according to claim 8, wherein in said method further comprises a step of compensating for said profile tilt when computing said homography.

10. The method according to claim 1, wherein said optics comprises an optical axis and said sensor extends in a plane with a sensor normal direction and wherein said optical axis forms a Scheimpflug angle with said sensor normal direction, wherein said method further comprises a step compensating for said Scheimpflug angle when generating said mapping from at least one point on said sensor to at least one point in said light plane.

11. The method according to claim 1, wherein said light source is a laser source such that said light plane is a laser plane.

12. The method according to claim 1, wherein said mapping calibration profile and said homography calibration profile are located on a single calibration object.

13. Computer program comprising computer program code executable in a computer or a processor, said computer program being adapted to receive a signal from a sensor of a measuring system, said measuring system further comprising a light source and optics, said light source being adapted to produce a light plane and said optics being located between said light plane and said sensor, wherein the light plane is a plane of light, which light plane is formed by light rays that are confined in said light plane when emitted by the light source, said computer program being storable on a computer-readable storage medium, said computer program being adapted to, when executed in a computer or processor, implement the following steps:

storing a mapping image recorded by said sensor of a mapping calibration profile located in a first mapping position in said light plane, wherein said mapping calibration profile comprises at least three points forming a straight line;

storing a homography image recorded by said sensor of a homography calibration profile located in a first homography position in said light plane, wherein said homography calibration profile comprises at least four points the relative distance between which are predetermined;

computing a non-linear mapping from at least a first portion of said sensor to at least a first portion of said light plane by using said mapping image comprising at least three points of said mapping calibration profile, and

computing a homography from at least a first portion of said sensor to at least a first portion of said light plane by using said homography image comprising said at least four points of said homography calibration profile.

14. Electronic control unit comprising computer program code executable in a computer or a processor, said computer program being adapted to receive a signal from a sensor of a measuring system, said measuring system further comprising a light source and optics, said light source being adapted to produce a light plane and said optics being located between said light plane and said sensor, wherein the light plane is a plane of light, which light plane is formed by light rays that are confined in said light plane when emitted by the light source, said computer program being storable on a computer-readable storage medium, said computer program being adapted to, when executed in a computer or processor, implement the following steps:

storing a mapping image recorded by said sensor of a mapping calibration profile located in a first mapping position in said light plane, wherein said mapping calibration profile comprises at least three points forming a straight line;

storing a homography image recorded by said sensor of a homography calibration profile located in a first homography position in said light plane, wherein said homography calibration profile comprises at least four points the relative distance between which are predetermined;

computing a non-linear mapping from at least a first portion of said sensor to at least a first portion of said light plane by using said mapping image comprising at least three points of said mapping calibration profile, and

computing a homography from at least a first portion of said sensor to at least a first portion of said light plane by using said homography image comprising said at least four points of said homography calibration profile.

15. A measuring system comprising a light source, optics and a sensor, wherein said light source is adapted to produce a light plane and said optics is located between said light plane and said sensor, wherein the light plane is a plane of light, which plane is formed by light rays that are confined in said plane when emitted by the light source, characterized in that said measuring system comprises an electronic control unit comprising computer program code executable in a computer or a processor, said computer program being adapted to receive a signal from a sensor of a measuring system, said measuring system further comprising a light source and optics, said light source being adapted to produce a light plane and said optics being located between said light plane and said sensor, said computer program being storable on a computer-readable storage medium, said computer program being adapted to, when executed in a computer or processor, implement the following steps:

storing a mapping image recorded by said sensor of a mapping calibration profile located in a first mapping position in said light plane, wherein said mapping calibration profile comprises at least three points forming a straight line;

storing a homography image recorded by said sensor of a homography calibration profile located in a first homography position in said light plane, wherein said homography calibration profile comprises at least four points the relative distance between which are predetermined;

computing a non-linear mapping from at least a first portion of said sensor to at least a first portion of said light plane by using said mapping image comprising at least three points of said mapping calibration profile, and

computing a homography from at least a first portion of said sensor to at least a first portion of said light plane by using said homography image comprising said at least four points of said homography calibration profile.