ACQUISITION OF TOPOGRAPHIES OF OBJECTS HAVING ARBITRARY GEOMETRIES

• US 20090284756A1
• Filed: 05/15/2009
• Published: 11/19/2009
• Est. Priority Date: 05/16/2008
• Status: Active Grant

• Current Assignee: Ultra Electronics Forensic Technology, Inc. (Ultra Electronics Holdings PLC)
• Original Assignee: Forensic Technology WAI, Inc. (Ultra Electronics Holdings PLC)
• Inventors: ROBERGE, Danny, BEAUCHAMP, Alain
• Granted Patent: US 8,139,230 B2
• Time in Patent Office: Days
• US Class Current: 356/601
• CPC Class Codes: F42B 35/00 Testing or checking of ammunition apparatus for measuring the energy of projectiles G01L5/14
• CPC Class Codes: G01B 11/24 for measuring contours or curvatures
• CPC Class Codes: G01B 11/26 for measuring angles or tapers; for testing the alignment of axes

Abstract

There is described a method for positioning an object on an optical sensor system for acquiring a surface thereof, the sensor system having a set of motors for rotating the object around a motor axis perpendicular to an optical axis of the sensor system and for translating the object in X, Y and Z directions, the method comprising: (a) acquiring a relief map of an area in a field of view of the sensor system; (b) computing a normal representative of a topography of the relief map of the area; (c) determining an angle difference between the normal and the optical axis of the sensor system; (d) comparing the angle difference to a threshold angle to determine if the surface of the area is perpendicular to the sensor axis; (e) if the angle difference is greater than a threshold angle, rotating the object to obtain a new difference angle less than the threshold angle; and (f) translating the object to reposition the area in the field of view after the rotating has displaced the area.

Claims

1. A method for positioning an object on an optical sensor system for acquiring a surface thereof, the sensor system having a set of motors for rotating the object around a motor axis perpendicular to an optical axis of the sensor system and for translating the object in X, Y and Z directions, the method comprising:

(a) acquiring a relief map of an area in a field of view of the sensor system;

(b) computing a normal representative of a topography of the relief map of the area;

(c) determining an angle difference between the normal and the optical axis of the sensor system;

(d) comparing the angle difference to a threshold angle to determine if the surface of the area is perpendicular to the sensor axis;

(e) if the angle difference is greater than a threshold angle, rotating the object to obtain a new difference angle less than said threshold angle; and

(f) translating said object to reposition said area in said field of view after said rotating has displaced said area.

2. The method of claim 1, further comprising repeating steps (a) through (f) for said relief map of said area until said new difference angle is less than said threshold angle after said translating.

3. The method of claim 1, further comprising:

moving at least one of said object and a sensor to provide a new surface area in said field of view of said sensor system; and

repeating steps (a) through (f) for multiple areas of said surface and merging said multiple areas to create a mosaic of said surface of said object.

4. The method of claim 1, further comprising:

setting said set of motors to an initial position for an initial acquisition of said surface and a final position for a final acquisition of said surface; and

determining an actual position of said motor axis relative to a position reference in a reference coordinate system.

5. The method of claim 4, wherein said determining an actual position of said motor axis comprises:

(g) assuming an actual position of (0,

     0) for said motor axis;

(h) acquiring a first relief map of an area in a field of view of the sensor system with said set of motors at said initial position;

(i) performing a rotation of said surface about said motor axis by a small θ

value;

(j) acquiring a subsequent relief map of an area in the field of view of the sensor system;

(k) measuring a relative rotation and shift between the first relief map and the subsequent relief map;

(l) computing a temporary motor axis position using only the first relief map and the subsequent relief map;

(m) computing a confidence weight describing a confidence with the temporary motor axis position;

(n) updating the actual position of the motor axis position using the confidence weight and the temporary motor axis position;

(o) testing for convergence of said actual position; and

(p) repeating steps (i) to (o) until convergence is achieved.

6. The method of claim 4, wherein said setting said set of motors to an initial position comprises:

(q) acquiring a first relief map of an area in a field of view of the sensor system;

(r) computing a normal representative of a topography of the relief map of the area;

(s) measuring an angle between the normal and the optical axis and comparing with a threshold angle;

(t) if the measured angle is greater than the threshold angle, rotating the object by a small value to obtain a new angle;

(u) acquiring a second relief map of an area within the field of view after the rotating;

(v) determining a relative shift between common data from the first relief map and the second relief map;

(w) translating the object by the relative shift and repeating steps (q) through (w) until the measured angle is less than said threshold angle; and

(x) if the measured angle is less than said threshold angle, storing a current position of said set of motors as said initial position.

7. The method of claim 6, wherein said (r) computing a normal representative of a topography of the relief map of the area comprises:

defining an average profile along a Y direction describing a general shape of the topography; and

computing the normal of the average profile.

8. The method of claim 3, wherein said moving at least one of said object and a sensor to provide a new surface area in said field of view comprises having an overlap between successive areas that is greater than 50%.

9. The method of claim 8, wherein said moving comprises:

determining a position of a point of a current patch which is consistent with a predetermined fixed overlap between successive acquired areas;

identifying a normal at that point;

rotating the object in order to bring the normal of that point parallel to the optical axis;

translating the surface by Shift_y in order to bring that point in the center of the field of view; and

updating the motor axis position to R_y-update=R_y+Shift_y.

10. The method of claim 3, wherein said moving at least one of said object and a sensor to provide a new surface area in said field of view comprises having an overlap between successive areas that is less than 50%.

11. The method of claim 10, wherein said moving comprises:

identifying a normal of a furthest point of a topography of a current patch at a boundary of the field of view;

determining an angle BETA between the normal and the optical axis;

rotating the object by (1+ALPHA)*BETA, where ALPHA is a positive parameter that is a decreasing function of the overlap which approaches zero when the overlap approaches 50%;

translating the object by Shift_y in order to bring the point beyond a center of the field of view in accordance with the overlap; and

updating the motor axis position to R_Y-update=R_Y+Shift_y

12. The method of claim 3, wherein said moving at least one of said object and a sensor to provide a new surface area in said field of view comprises having an overlap between successive areas that is about 50%.

13. The method of claim 12, wherein said moving comprises:

identifying a normal of a furthest point of a topography of a current patch at a boundary of the field of view;

rotating the object in order to bring the normal of the furthest point parallel to the optical axis;

translating the surface in order to bring the furthest point in a center of the field of view; and

updating the motor axis position to R_Y-update=R_Y+Shift_y.

14. A computer readable memory having recorded thereon statements and instructions for execution by a computer to carry out the method of claim 1.

15. An object positioning system for use with an optical sensor system for acquiring a surface of the object, the sensor system having a set of motors for rotating the object around a motor axis perpendicular to an optical axis of the sensor system and for translating the object in X, Y and Z directions, the system comprising:

a processor in a computer system;

a memory accessible by the processor; and

an application coupled to the processor, the application configured for;

(a) acquiring a relief map of an area in a field of view of the sensor system;

(b) computing a normal representative of a topography of the relief map of the area;

(c) determining an angle difference between the normal and the optical axis of the sensor system;

(d) comparing the angle difference to a threshold angle to determine if the surface of the area is perpendicular to the sensor axis;

(e) if the angle difference is greater than a threshold angle, rotating the object to obtain a new difference angle less than said threshold angle; and

(f) translating said object to reposition said area in said field of view after said rotating has displaced said area.

16. The system of claim 15, wherein said application is also configured for:

setting said set of motors to an initial position for an initial acquisition of said surface and a final position for a final acquisition of said surface; and

determining an actual position of said motor axis relative to a position reference in a reference coordinate system.

17. The system of claim 16, wherein said determining an actual position of said motor axis comprises:

(g) assuming an actual position of (0,

     0) for said motor axis;

(h) acquiring a first relief map of an area in a field of view of the sensor system with said set of motors at said initial position;

(i) performing a rotation of said surface about said motor axis by a small θ

value;

(j) acquiring a subsequent relief map of an area in the field of view of the sensor system;

(k) measuring a relative rotation and shift between the first relief map and the subsequent relief map;

(l) computing a temporary motor axis position using only the first relief map and the subsequent relief map;

(m) computing a confidence weight describing a confidence with the temporary motor axis position;

(n) updating the actual position of the motor axis position using the confidence weight and the temporary motor axis position;

(o) testing for convergence of said actual position; and

(p) repeating steps (i) to (o) until convergence is achieved.

18. The system of claim 16, wherein said setting said set of motors to an initial position comprises:

(q) acquiring a first relief map of an area in a field of view of the sensor system;

(r) computing a normal representative of a topography of the relief map of the area;

(s) measuring an angle between the normal and the optical axis and comparing with a threshold angle;

(t) if the measured angle is greater than the threshold angle, rotating the object by a small value to obtain a new angle;

(u) acquiring a second relief map of an area within the field of view after the rotating;

(v) determining a relative shift between common data from the first relief map and the second relief map;

(w) translating the object by the relative shift and repeating steps (q) through (w) until the measured angle is less than said threshold angle; and

(x) if the measured angle is less than said threshold angle, storing a current position of said set of motors as said initial position.

19. The system of claim 18, wherein said (r) computing a normal representative of a topography of the relief map of the area comprises:

defining an average profile along a Y direction describing a general shape of the topography; and

computing the normal of the average profile.

20. The system of any one of claim 15, wherein said application is further configured for:

moving at least one of said object and a sensor to provide a new surface area in said field of view of said sensor system; and

repeating steps (a) through (f) for multiple areas of said surface and merging said multiple areas to create a mosaic of said surface of said object.

21. The system of claim 20, wherein said moving at least one of said object and a sensor to provide a new surface area in said field of view comprises having an overlap between successive areas that is greater than 50%.

22. The system of claim 21, wherein said moving comprises:

determining a position of a point of a current patch which is consistent with a predetermined fixed overlap between successive acquired areas;

identifying a normal at that point;

rotating the object in order to bring the normal of that point parallel to the optical axis;

translating the surface by Shift_y in order to bring that point in the center of the field of view; and

updating the motor axis position to R_y-update=R_y+Shift_y.

23. The system of claim 20, wherein said moving at least one of said object and a sensor to provide a new surface area in said field of view comprises having an overlap between successive areas that is less than 50%.

24. The system of claim 23, wherein said moving comprises:

identifying a normal of a furthest point of a topography of a current patch at a boundary of the field of view;

determining an angle BETA between the normal and the optical axis;

rotating the object by (1+ALPHA)*BETA, where ALPHA is a positive parameter that is a decreasing function of the overlap which approaches zero when the overlap approaches 50%;

translating the object by Shift_y in order to bring the point beyond a center of the field of view in accordance with the overlap; and

updating the motor axis position to R_Y-update=R_Y+Shift_Y

25. The system of claim 20, wherein said moving at least one of said object and a sensor to provide a new surface area in said field of view comprises having an overlap between successive areas that is about 50%.

26. The system of claim 25, wherein said moving comprises:

identifying a normal of a furthest point of a topography of a current patch at a boundary of the field of view;

rotating the object in order to bring the normal of the furthest point parallel to the optical axis;

translating the surface in order to bring the furthest point in a center of the field of view; and

updating the motor axis position to R_Y-update=R_Y+Shift_y.