Method and sensor for opto-electronic angle measurements

• US 5,196,900 A
• Filed: 10/09/1990
• Issued: 03/23/1993
• Est. Priority Date: 04/12/1988
• Status: Expired due to Term

• Current Assignee: Metronor AS
• Original Assignee: Metronor AS
• Inventors: Pettersen, Alf
• Primary Examiner(s): Buczinski, Stephen C.
• Application Number:
  US07/582,936
• Time in Patent Office: 896 Days
• Field of Search: 356/141, 356/152, 356/1, 356/5, 356/375, 356/376
• US Class Current: 356/141.4
• CPC Class Codes: G01B 11/00 Measuring arrangements characterised by the use of optical techniques
• CPC Class Codes: G01C 1/00 Measuring angles
• CPC Class Codes: G01C 11/02 Picture taking arrangements specially adapted for photogrammetry or photographic surveying, e.g. controlling overlapping of pictures

Abstract

A camera comprising a lens forms images onto a two-dimensional array of photo-sensitive elements, e.g. a CCD or CID sensor. The lens has a well defined center of rotational symmetry, giving an unambiguous definition of spatial directions. Such cameras form angle sensors to measure spatial directions towards light point sources or reflecting points illuminated by active light sources. The angle sensor is accurately calibrated using a high precision angle reference. The angle sensors are applied in various system configurations for spatial position and geometry measurements, including a system configured for simultaneous measurements of the three-dimensional coordinates of a plurality of points on a surface. That system comprises a subsystem for projection of multiple point sized light spots onto a surface, and a minimum of two angle sensors.

Claims

1. An opto-electronic angle sensor measuring spatial directions towards point sized light sources or reflecting points illuminated by a light source, said sensor comprising a spherical lens and a two dimensional array of photosensitive elements, and said sensor further comprising:

means for computing a spatial direction of said light sources or illuminated points from the position of the image of said light sources or illuminated points as registered in the local coordinates of said array of photosensitive elements, said means for computing further using a two dimensional calibration table created by a single calibration that allows the angle sensor to be used in various locations without requiring any recalibration, said calibration resulting from a determination of the center of rotational symmetry of the angle sensor lens, and a determination of the relation between the spatial directions and the position of the image on the photosensitive array by the use of a high precision angle reference wherein said calibration table is obtained using light sources having a well defined and known spectral distribution; and

means for performing a statistical analysis of the intensity values registered by multiple neighboring photosensitive elements wherein the resolution and accuracy of the angle measurements are increased to fractions of the size of said photosensitive elements.

2. In a method relating to an opto-electronic angle sensor for measuring spatial directions towards point sized light sources or reflecting points illuminated by a light source, said sensor comprising a spherical lens and a two-dimensional array of photosensitive elements, said method including a single calibration that allows the angle sensor to be used in various locations without requiring any recalibration, said calibration comprising the steps of:

determining the center of rotational symmetry of the angle sensor lens by the steps of;

correcting the position of an adjustable mounting fixture attached to the angle sensor to a position corresponding to the rotational axes of said sensor, the lens optical axis being defined as the x axis, the z axis of the angle sensor being defined by the vertical symmetry axis of said mounting fixture and the y axis being defined by its orthogonality to said x and z axes;

defining rotations around the z axis as horizontal angles and rotations around the y axis as vertical angles;

mounting the angle sensor onto the top of a rotary table and leveling to ensure that the optical axis is horizontal;

mounting a minimum of two light sources at approximately the same level as the optical axis such that a straight line can be drawn through the two light sources and the rotational axis of the rotary table; and

adjusting the mount of the angle sensor to the rotary table until the images of the two light sources are overlapping, independent of the rotary position of the angle sensor, and clamping said mounting fixture to the angle sensor in this position;

calibrating said angle sensor for measurement of angles relative to said two rotational axes using a high precision rotary tale and a substantially vertically mounted linear device comprising light source means in the form of at least one of an array of light sources or illuminated reflecting points and an illuminated string or slit, said linear device having a length corresponding to the field of view of the angle sensor in one dimension, said calibrating step comprising the steps of;

mounting the angle sensor onto the top of said rotary table and leveling such that one of its rotational axes is exactly vertical, parallel and coincides with the rotational axis of the rotary table;

mounting said linear device parallel to said rotational axis;

step-wise rotating the angle sensor while the image of the linear device and the corresponding rotary table angle are simultaneously registered for every step to perform a determination of the relation between the angle and the position of the image on the photosensitive array;

repeating the calibrating steps for the second rotational axis of the angle sensor; and

processing all registered data to establish a two-dimensional calibration table relating image coordinates to spatial directions given as horizontal and vertical angles;

wherein said calibration is based on the use of light source means having a well defined and known spectral distribution, and values of intensity registered by multiple neighboring photosensitive elements are statistically analyzed to increase the resolution and accuracy of the angle measurements to fractions of the size of said photosensitive elements.

4. The method as claimed in claims 2 or 3 wherein:

said calibration step includes mounting said linear device such that only a small part of the vertical field of view is covered by the length of the device, and only a fraction of the field of view of the angle sensor can be calibrated having the angle sensor levelled; and

the rest of the vertical field of view of the angle sensor is calibrated using a mounting fixture allowing the angle sensor to be tilted step-wise around an axis which is normal to the optical axis, so that in each tilted position a part of the photosensitive array is calibrated, and the tilt angle is measured to be used for computation of the relation between the observed image coordinates and the spatial angle.

3. In a method relating to an opto-electronic angle sensor for measuring spatial directions towards point sized light sources or reflecting points illuminated by a light source, said sensor comprising a spherical lens and a two-dimensional array of photosensitive elements, said method including a single calibration that allows the angle sensor to be used in various locations without requiring any recalibration, said calibration comprising the steps of:

determining the center of rotational symmetry of the angle sensor lens by the steps of;

correcting the position of an adjustable mounting fixture attached to the angle sensor to a position corresponding to the rotational axes of said sensor, the lens optical axis being defined as the x axis, the z axis of the angle sensor being defined by the vertical symmetry axis of said mounting fixture and the y axis being defined by its orthogonality to said x and z axes;

defining rotations around the z axis as horizontal angles and rotations around the y axis as vertical angles;

mounting the angle sensor onto the top of a rotary table and leveling to ensure that the optical axis is horizontal;

mounting a minimum of two light sources at approximately the same level as the optical axis such that a straight line can be drawn through the two light sources and the rotational axis of the rotary table; and

adjusting the mount of the angle sensor to the rotary table until the images of the two light sources are overlapping, independent of the rotary position of the angle sensor, and clamping said mounting fixture to the angle sensor in this position;

calibrating said angle sensor for measurement of angles relative to said two rotational axes by the use of a substantially vertically mounted linear device comprising light source means in the form of at least one of an array of light sources or illuminated reflecting points and an illuminated string or slit, said linear device having a length corresponding to the field of view of the angle sensor in one dimension, said calibrating step comprising the steps of;

leveling the angle sensor such that one of its rotational axes is exactly vertical;

mounting said linear device parallel to said rotational axis;

step-wise moving said linear device in a known direction crossing the sensor optical axis;

registering simultaneously the horizontal position of said linear array of light sources or reflecting points and its corresponding image to determine the relation between the angle and the position of the image on the photosensitive array;

repeating the calculating step for the second rotational axis of the angle sensor; and

processing all registered data to establish a two-dimensional calibration table relating image coordinates to spatial directions given as horizontal and vertical angles;

wherein said calibration is based on the use of light source means having a well defined and known spectral distribution, and values of intensity registered by multiple neighboring photosensitive elements are statistically analyzed to increase the resolution and accuracy of the angle measurements to fractions of the size of said photosensitive elements.

5. A system for opto-electronic measurements of spatial coordinates of one or multiple light point sources or reflecting points illuminated by one or multiple light sources, the system comprising:

at least two angle sensors;

a plurality of dedicated image processing units, each processing unit for converting an image registered by each respective angle sensor to a two dimensional angular value corresponding to each light source or illuminated point; and

a data processor for computing the coordinates of each light source or illuminated point, said data processor including means for obtaining the relations between the internal coordinate system of the individual angle sensors and the global coordinate system by one of the methods of;

leveling said angle sensors located in known positions, and calculating their orientations from the measured spatial directions to a light source or an illuminated point which is located in a known position and thereby constituting a common reference point;

said data processor calculating the positions and orientations of the angle sensors on the basis of measured directions to at least three light point sources in known global coordinates; and

said data processor being capable of calculating the positions and orientations of the angle sensors on the basis of measured directions to a number of given light points, wherein the mutual separation distance is known for at least two thereof, and the position of a third point relative to said two define the orientation of the global coordinate system.

6. A system for opto-electronic measurements as claimed in claim 5 wherein each angle sensor measures spatial directions towards point sized light sources or reflecting points illuminated by a light source, each angle sensor having a spherical lens and a two dimensional array of photosensitive elements and further comprises:

means for computing a spatial direction of said light sources or illuminated points from the position of the image of said light sources or illuminated points as registered in the local coordinates of said array of photosensitive elements, said means for computing including a two dimensional calibration table that is generated by a single calibration that allows the angle sensor to be used in various locations without requiring any recalibration, said calibration resulting from a determination of the center of rotational symmetry of the angel sensor lens, and a determination of the relation between the spatial directions and the position of the image on the photosensitive array by the use of a high precision angle reference wherein said calibration table is obtained using light sources having a well defined and known spectral distribution; and

means for performing a statistical analysis of the intensity values registered by multiple neighboring photosensitive elements wherein the resolution and accuracy of the angel measurements are increased to fractions of the size of said photosensitive elements.

7. A system as claimed in claim 6, wherein the light point sources are movable and the data processor comprises means for computing the dynamic behavior of the coordinates of each light source or illuminated point relative to a ground fixed coordinate system, or relative to each other.

8. A system as claimed in any one of claims 5-7, wherein:

the system further comprises a number of point sized light sources and a power supply for said light sources, at least three of said light sources being attached to each of a number of objects such that the positions of said light sources are known in object oriented local coordinate systems; and

the data processor comprises means to compute the position and orientation of each object relative to a ground fixed coordinate system or relative to each other, based on the measured global coordinates of each light source.

9. A system as claimed in any one of claims 5-7, wherein:

the system comprises means to illuminate a surface point by point; and

the data processor includes means to store a set of coordinates corresponding to points on a surface, and means to use these coordinates to describe the geometry of the surface.

10. A system as claimed in any one of claims 5-7, wherein:

said system comprises a number of light sources pointing in fixed directions to illuminate a corresponding number of points on an object or a surface; and

the data processor includes means to calculate the coordinates of the illuminated points and to compare them to nominal values.

11. An opto-electronic system for measuring spatial coordinates of points located on a surface, the system comprising:

means for illuminating the surface point by point and comprising a directed light source for emitting a light beam, a diffraction grating, and focusing optics to obtain a pattern of well defined discrete point sized light spots forming a curved line across the surface, wherein the system includes a minimum of two angle sensors to register the location of the projected light spots as spatial directions relative to said angle sensors; and

means for computing spatial coordinates for each discrete light spot based on registered spatial directions relative to the angle sensors.

14. An opto-electronic system for measuring spatial coordinates of points located on a surface as claimed in any one of claims 11-13, wherein the system includes a data processor for describing the geometry of the surface on the basis of the measured coordinate values for a plurality of illuminated points.

15. An opto-electronic system for measuring spatial coordinates of points located on a surface as claimed in any one of claims 11-13, wherein the system includes a data processor for comparing the measured spatial coordinates of each illuminated point to nominal values.

12. An opto-electronic system for measuring spatial coordinates of points located on a surface, the system comprising:

means for illuminating the surface point by point and comprising a directed light source for emitting a light beam, two diffraction gratings, each grating dividing an incident light beam to multiple light beams and mounted together such that the longitudinal axes of the two gratings are orthogonal, and focusing optics to obtain a two-dimensional pattern of discrete point sized spots onto the surface, wherein the system includes a minimum of two angle sensors to register the location of the projected light spots as spatial directions relative to said angle sensors; and

means for computing spatial coordinates for each light spot based on registered spatial directions relative to the angle sensors.

13. An opto-electronic system for measuring spatial coordinates of points located on a surface, the system comprising:

means for illuminating the surface point by point and comprising a directed light source for emitting a light beam, a diffraction grating, and focusing optics to obtain a pattern of well defined discrete point sized light spots forming a curved line across the surface, wherein the system includes a minimum of two angle sensors to register the location of the projected light spots as spatial directions relative to said angle sensors; and

means for computing spatial coordinates for each light spot based on registered spatial directions relative to the angle sensors, wherein the system includes a single axis rotary mirror to move the line of projected light spots across the surface in a direction orthogonal to the direction of said line of light spots.