LASER LINE PROBE HAVING IMPROVED HIGH DYNAMIC RANGE

• US 20150015898A1
• Filed: 07/11/2014
• Published: 01/15/2015
• Est. Priority Date: 07/15/2013
• Status: Active Grant

• Current Assignee: Faro Technologies Incorporated
• Original Assignee: Faro Technologies Incorporated
• Inventors: Atwell, Paul C., Macfarlane, Keith G.
• Granted Patent: US 9,267,784 B2
• Time in Patent Office: Days
• US Class Current: 356/625
• CPC Class Codes: G01B 11/005 coordinate measuring machines
• CPC Class Codes: G01B 11/03 by measuring coordinates of points
• CPC Class Codes: G01B 11/25 by projecting a pattern, e.g. one or more lines, moiré fringes on the object G01B11/255 takes precedence ; image analysis for depth or shape recovery G06T7/50
• CPC Class Codes: G01B 5/008 using coordinate measuring machines
• CPC Class Codes: H04N 25/589 with different integration times, e.g. short and long exposures

Abstract

A method for measuring three-dimensional coordinates of an object surface with a line scanner, the line scanner including a projector and a camera, the projector projecting onto the object surface a first line of light at a first time and a second line of light at a second time, the integrated energy of the second line of light different than the first line of light, the camera capturing the reflections of the first line of light and the second line of light, a processor processing the collected data after discarding portions of the image that are saturated or dominated by electrical noise, and determining three-dimensional coordinates of the object surface based at least in part on the processed data and on a baseline distance.

Claims

1. A method for measuring three-dimensional coordinates of a surface of an object, the method comprising:

providing a line scanner that includes a processor, a projector and a camera, the line scanning having a first frame of reference, the projector including a light source and a projector lens, the light source being configured to emit a line of light substantially straight when projected onto a plane perpendicular to a direction of propagation of the line of light, the projector having a projector perspective center, the camera including a photosensitive array and a camera lens, the camera having a camera perspective center, the camera lens being configured to form an image of a portion of the surface on the photosensitive array, the photosensitive array including an array of pixels, the array of pixels having M rows and N columns, where M and N are integers, each of the pixels in the array of pixels configured to convert an optical energy captured by each of the pixels into an electrical value corresponding to a digital value, the processor configured to receive the digital values, the line scanner having a baseline, the baseline being a straight line segment between the projector perspective center and the camera perspective center, the projector having a projector orientation with respect to the baseline, the camera having a camera orientation with respect to the baseline, the processor configured to control the light source and to receive the digital values;

generating a first line of light at a first time, the first line of light having a first optical power;

projecting the first line of light onto the surface;

reflecting the first line of light as a first reflected light from the surface;

receiving the first reflected light with the camera lens;

forming with the camera lens a first image of the first reflected light on the photosensitive array over a first integration time and generating in response a first optical energy for each of the pixels, the first optical energy for each of the pixels depending at least in part on a first integrated energy, the first integrated energy equal to an integral of the first optical power over the first integration time;

sending for each of the pixels a first digital value to the processor to obtain a first M×

N array of first digital values;

generating a second line of light at a second time, the second line of light having a second optical power;

projecting the second line of light onto the surface;

reflecting the second line of light as a second reflected light from the surface;

receiving the second reflected light with the camera lens;

forming with the camera lens a second image of the second reflected light on the photosensitive array over a second integration time and generating in response a second optical energy for each of the pixels, the second optical energy for each of the pixels depending at least in part on a second integrated energy, the second integrated energy equal to an integral of the second optical power over the second integration time, wherein the second integrated energy is different than the first integrated energy;

sending for each of the pixels a second digital value to the processor to obtain a second M×

N array of second digital values;

determining with the processor for each of the M rows of the first M×

N array of first digital values a first maximum digital value and a first center value, the first maximum digital value for the row equal to a maximum of the N first digital values of the row, the first center value based at least in part on at least one of the N first digital values of the row;

determining with the processor for each of the M rows of the second M×

N array of second digital values a second maximum digital value and a second center value, the second maximum digital value for the row equal to a maximum of the N second digital values of the row, the second center value based at least in part on at least one of the N second digital values of the row;

determining with the processor a composite center value for each of the M rows in which a first condition or a second condition is met, the first condition being that the first maximum digital value is less than a saturation level and exceeds a noise level, the second condition being that the second maximum digital value is less than the saturation level and exceeds the noise level, the composite center value based at least in part on the first center value and the second center value;

calculating with the processor for each of the M rows having a composite center value three-dimensional coordinates of a point on the surface, the three-dimensional coordinates calculated in the first frame of reference, the three-dimensional coordinates based at least in part on the composite center value, a length of the baseline, the projector orientation, and the camera orientation; and

storing the three-dimensional coordinates for each of the M rows having a composite center value.

2. The method of claim 1 wherein in the steps of generating a first line of light at a first time and generating a second line of light at a second time, the first optical power is different than the second optical power.

3. The method of claim 1 wherein in the steps of forming with the camera lens a first image and forming with the camera lens a second image, the first integration time is different than the second integration time.

4. The method of claim 1 wherein in the steps of generating a first line of light and of forming with the camera lens a first image, the first optical power is constant over the first integration time.

5. The method of claim 1 wherein, in steps of generating a second line of light at a second time and forming with the camera lens a second image, at least one of the second optical power and the second integration time is based at least in part on the M first maximum digital values.

6. The method of claim 1 wherein, in the step of providing a line scanner, the light source is a laser diode or a superluminescent diode and the projector lens is a cylinder lens or a Powell lens.

7. The method of claim 1 wherein, in the step of providing a line scanner, the projector further includes a microelectromechanical systems (MEMS) element.

8. The method of claim 1 wherein the line of light is substantially perpendicular to the baseline.

9. The method of claim 1 further including a step of calculating for each of the three-dimensional coordinates in the first frame of reference three-dimensional coordinates in a second frame of reference.

10. The method of claim 9 further including a step of attaching the line scanner to an end of an articulated arm CMM, the articulated arm CMM being in the second frame of reference, the articulated arm CMM determining a position and an orientation of the line scanner in the second frame of reference.

11. The method of claim 9 further including a step of measuring a position and an orientation of the line scanner with a laser tracker, the laser tracker being in the second frame of reference.

12. The method of claim 9 further including a step of measuring a position and orientation of the line scanner with two cameras, the two cameras having a separation distance, the two cameras being in the second frame of reference.

13. The method of claim 1 wherein in the step of determining with the processor for each of the M rows of the first M×

N array a first maximum digital value and a first center value, the first center value is further based at least in part on a centroid of the first digital values of the row.

14. The method of claim 13 wherein the centroid is calculated to sub-pixel resolution.

15. The method of claim 1 wherein in the step of determining with the processor for each of the M rows of the first M×

N array a first maximum digital value and a first center value, the first center value is further based at least in part on a value obtained using curve fitting.

16. The method of claim 15 wherein the curve fitting is a fitting to a Gaussian shape.

17. The method of claim 1 wherein in the step of determining with the processor a composite center value, the composite center value is selected as the first center value if the first maximum digital value exceeds the second maximum digital value and is selected as the second center value if the second maximum digital value exceeds the first maximum digital value.

18. The method of claim 1 wherein in the step of determining with the processor a composite center value, the composite center value is selected as a weighted average of the first center value and the second center value, wherein the weighting is based on relative magnitudes of the first maximum digital value and the second maximum digital value.