Method for aligning a laser projector and laser projector arrangement for projecting a laser image

Abstract

A method for aligning a laser projector for projecting a laser image onto a workpiece surface, comprising the steps of: Providing a laser projector assembly having a laser source for projecting a laser image onto a workpiece surface, a secondary light source for illuminating the workpiece surface, a photogrammetry device for generating an image of the workpiece surface, and a laser sensor for detecting a laser beam, wherein the laser source, the laser sensor, the secondary light source, and the photogrammetry device are enclosed in an assembly housing; Attaching reflective targets to the workpiece surface; Transmitting light from the secondary light source to the workpiece surface; Receiving light at the photogrammetry device from the reflective targets; Detecting a pattern of the reflective targets on the workpiece surface in a three-dimensional coordinate system; and after detecting the pattern of the reflective targets on the workpiece surface in the three-dimensional coordinate system, scanning the targets with a laser beam generated by the laser source and guided by the identified pattern of reflected targets to reflect the laser beam against the laser sensor, and calculating a precise location of the targets from the reflected laser beam to direct the laser projector where the laser image is to be projected onto the workpiece surface, wherein the photogrammetry device has a single camera.

Description

EARLIER APPLICATIONSThe present application claims priority to U.S. Provisional Patent Application No. 62 / 408,944, filed Oct. 17, 2016, the contents of which are incorporated herein by reference.TECHNICAL FIELDThe present invention relates generally to an improved method of projecting reticles. More particularly, the present invention relates to an improved method of aligning a laser projector with a three-dimensional workpiece surface upon which a laser template is projected.BACKGROUNDEver-higher manufacturing tolerances require improvements in manufacturing techniques. One such improvement is the projection of reticles onto a workpiece surface to direct a manufacturing process. This technique allowed the production of products with tolerances not previously attainable. Examples of laser scanning systems for measuring object surfaces are known from the documents US 2014 / 0 293 023 A1, U.S. Pat. No. 9,410,793 B2, US 2005 / 0 121 422 A1, DE 10 2014 001 045 A1, US 2010 / 0 134 598 A1 and US 2013 / 0 253 682 A1. However, limitations of existing technology have limited wider use of laser-projected images in industrial applications. For example, projecting an original onto a three-dimensional surface has proven difficult because it is not possible to quickly identify the three-dimensional workpiece surface and focus the laser beam onto the three-dimensional workpiece surface in a precise manner while operating in a manufacturing environment.Accurate projection of an original pattern onto a three-dimensional workpiece surface requires precise calibration of the relative position between the workpiece surface and the laser projector. Initially, this has been accomplished by locating reflective targets on the workpiece surface, measuring the target coordinates relative to a three-dimensional coordinate system of the workpiece surface, and then locating the position of the projector relative to the workpiece surface using a process of calculating the position from the projector, wherein known laser projections to the targets pass through known three-dimensional target coordinates. Periodically, the document scanning sequence is stopped and a target is located to check variation in the projected pattern location due to variation in the position of the projector relative to the tool or to compensate for other factors such as drift due to, for example, ambient temperature variations. When a fluctuation is detected, the targets are moved, a new document scanning sequence is calculated and is transmitted again by the laser projector.The time involved in scanning target positions using a conventional laser projector has proven to be slow and inefficient. As a result, evaluation of projection drift was performed only intermittently, and correction of the projected patterns resulted in marked interruptions of the visible template.Therefore, it would be desirable to develop a more efficient method for locating a three-dimensional workpiece surface relative to a laser projector to improve the precision and quality of a reticle projection.SUMMARYA method of aligning a laser projector to project a laser image onto a workpiece surface is disclosed. A laser projector arrangement is provided having a laser source for projecting the laser image onto the workpiece surface. A secondary light source illuminates the workpiece surface and a photogrammetry apparatus generates an image from the workpiece surface. Reflective targets are attached to the workpiece surface. Light transmitted from the secondary light source toward the workpiece surface is reflected toward the photogrammetry apparatus to determine a location of the workpiece surface in a three-dimensional coordinate system. After determining a location from the workpiece surface in the three-dimensional coordinate system, the targets are scanned with a laser beam generated by the laser source to reflect the laser beam against a laser sensor. The laser sensor signals a processor that calculates a location for projecting the laser image onto the workpiece surface from the reflected laser beam.The combination of the secondary light source flashing light against the workpiece surface with a workpiece and laser reflecting targets attached to the workpiece enhances the ability to quickly identify an accurate location for scanning a reticle on the workpiece surface. This method improves the quality of reticles by significantly reducing the amount of time required to relocate the workpiece surface in the event of drift or dynamic motion. In addition, the photogrammetry apparatus signals the processor a general location of the targets attached to the workpiece while simultaneously identifying a three-dimensional configuration from the workpiece surface. This step eliminates the need for the laser scanner to independently locate the targets, thereby further reducing the alignment time.BRIEF DESCRIPTION OF THE DRAWINGSOther advantages of the present invention will become readily apparent when better understood by reference to the following detailed description when considered in connection with the accompanying drawings. FIG. 1 is a schematic view of relevant parts of the laser projector of the present invention; FIG. 2 shows a secondary light source that transmits light to a workpiece; FIG. 3 shows light from the secondary light source reflected to a photogrammetry array by a laser projector; FIG. 4 shows a laser beam projected by a laser projector against reflective targets attached to the workpiece; FIG. 5 shows the laser beam reflected from the reflecting targets attached to the workpiece toward the laser projector; and FIG. 6 is a perspective view of the workpiece onto which a laser template is projected from the laser projector assembly.DETAILED DESCRIPTIONA schematic of a laser projector assembly used to carry out the method of the present invention is shown generally at 10 in Figure 1. the assembly 10 includes a laser source 12 which generates a laser beam 14 in a known manner. The laser beam 14 is projected by a focusing lens 16 against a beam splitter 18, and the beam splitter 18 redirects the laser beam 14 against a galvanometer assembly 20. The beam splitter 18 allows a portion of the laser beam 14 to pass through to a light sensor 22.The light sensor 22 provides reliable power output control through closed loop processing. As such, the light sensor 22 is connected to a processor 24 via an analog circuit for generating a power control loop. The processor directs necessary power adjustments to the laser source 12 based on input from the light sensor 22 to maintain a desired laser image resolution during execution. In this manner, the beam splitter 18 directs the laser beam 14 against the galvanometer assembly 20 at a desired laser power.The galvo assembly 20 includes a first galvo motor 30 and a second galvo motor 32. the first galvo motor 30 provides pivotal movement to a first galvo mirror 34 and the second galvo motor 32 provides pivotal movement to a second galvo mirror 36.The first galvo mirror 34 and the second galvo mirror 36 redirect the laser beam 14 through the exit aperture 26 against a workpiece 38 (FIGS. 1-4 ), as will be further explained below. The first galvo motor 30 and the second galvo motor 32 are electronically connected to the processor 24 such that the processor 24 continuously calculates the orientation of the first galvo mirror 34 and the second galvo mirror 36 to identify a direction in which the laser beam 14 is projected through the output aperture 26.The first galvo mirror 34 and the second galvo mirror 36 redirect a reflected laser beam 40 through the beam splitter 18 to a reflection laser sensor 42. The reflective laser sensor 42 is also electronically connected to the processor 24 such that the processor 24 calculates an orientation of the first galvo mirror 34 and the second galvo mirror 36 at the time when the reflected laser beam 40 contacts the reflective laser sensor 42. In this manner, the processor 24 determines a direction from which the reflected laser beam 40 originates, as will be discussed further below.A photogrammetry assembly 44 includes a first camera 46 for generating an image of a workpiece surface 48 of the workpiece 38. the first camera 46 is electronically coupled to the processor 24 for transmitting an image of the workpiece 38. In an alternative embodiment, a second camera 50 is also electronically connected to the processor 24 to generate a stereo image of the workpiece surface 48. In this embodiment, the first camera 46 and the second camera 50 are enclosed in the array housing 28 such that the complete laser array 10 is stand-alone as a single module. It should be appreciated, however, that the photogrammetry assembly 44, whether there is one camera 46 or two cameras 46, 50, need not be mounted within the assembly housing 28, but may be separately disposed. However, it is desirable that the photogrammetry assembly 44 be located at a known location relative to the laser projector 28.A secondary light source 52 adjacent to the laser source 12 provides secondary illumination 54 for the workpiece 38 and for the workpiece surface 48. In one embodiment, the secondary light source 52 is an LED strobe array located near the first camera 46 and a second camera 50. However, it is not critical that the secondary light source 52 be disposed proximate one of the cameras 46, 50. Further, locating the cameras 46, 50 on a rigid frame 56 relative to the galvanometer assembly 20 reduces the need to accurately identify the relative location between the cameras 46, 50 and the laser assembly 10 by other methods as disclosed in U.S. Pat. No. 9,200,899 B2, the contents of which are incorporated herein by reference. However, these methods may also be incorporated into the method of alignment of the present application for additional dimensional verification if desired.Referring now to Figures 2 through 5, the method for accurately projecting the reticle 56 onto the workpiece surface 48 will be discussed. Reflective targets 58 are attached to the workpiece surface 48 of the workpiece 38. In one embodiment, the targets 58 are attached to a relevant reference point of a three-dimensional workpiece surface 48 such that three-dimensional features of the workpiece surface 48 can be accurately calculated from a location of the target 58. A plurality of targets 58 may be mounted at spaced locations on the workpiece surface 48. In one embodiment, four targets provide sufficient reflective information to accurately calculate three-dimensional contours of the workpiece surface 48. More or fewer targets 58 may be selected based on a particular application.At the beginning of an alignment cycle, the secondary light source 52 transmits the secondary light 54 to the workpiece 38. The photogrammetry assembly 44 receives the secondary light reflected from the workpiece surface 48 of the workpiece 38 and also the secondary light reflected from the targets. Locating the targets 58 in a known position relative to the workpiece surface 48, such as on the reference point, allows the photogrammetry assembly 44 to use the target configuration 58 to locate the three-dimensional configuration of the workpiece 38 to ultimately determine a location of the three-dimensional surface 48 in a three-dimensional coordinate system. In this manner, the photogrammetry assembly 44 signals the processor 24 to calculate changes in contour that define the three-dimensional workpiece surface 48.As set forth above, the photogrammetry assembly 44 also detects the secondary light 54 reflected from the targets 58, and the processor 24 also determines a general position of the targets 58 in the three-dimensional coordinate system when signaled by the photogrammetry assembly 44. Based on the target coordinates 58 from the secondary light 54, the galvo motors 30, 32 direct the laser beam 14 generated by the laser source 12 to directly scan the targets 58 with the laser beam 14. As such, the processor 24 recognizes a target 54 pattern and calculates the location required to scan the targets 58 with the laser beam 14 to calculate an accurate location of the reticle 56 on the workpiece surface 48.Once the targets 58 are computed coordinates, the laser beam 14 is projected by the laser source 12 onto the targets 58, as shown in Figure 4. FIG. 5 shows the laser beam 14 reflected from the targets 58 through the exit apertures back to the projector assembly 10. By means of retro-reflection, the return laser beam 40 is redirected by the first galvo mirror 34 and the second galvo mirror 36 through the beam splitter 18 to the reflection laser sensor 42. At this time, the reflected laser sensor 42 receives the reflected laser beam 40, the first galvo motor 30, and the second galvo motor 32 signal to the processor a location from which the return laser beam 40 originates. Using the orientation of the galvo motor 30, 32, the processor 24 calculates an accurate location of the targets 58 and is therefore able to accurately project the reticle 56 as shown in FIG. 6.Each camera 46, 50 includes a CMOS sensor or, alternatively, a CCD sensor, depending upon the requirements of a specific application. The sensors in one embodiment include a multi-megapixel sensor electronically connected to the processor 24. In one embodiment, a five megapixel sensor provides sufficient image quality. Each camera 46, 50, whether used alone or in stereo, has a viewing angle between about 60 degrees and 80 degrees to provide a broad optical field of view. However, alternative viewing angles may be desirable depending on a size of the workpiece 38 or a distance between the assembly 10 and the workpiece 38. Specifically, it is assumed that the field of view is 75 degrees in a horizontal direction and less in a vertical direction. It is also within the scope of this invention that the laser beam 14 and the secondary light 54 contain a same or similar wavelength. However, in alternative embodiments, the laser beam 14 and the secondary light 54 may include different wavelengths. For example, it is further contemplated that the secondary light 54 may be infrared or other invisible light detectable only by the photogrammetry assembly 44.The projector assembly 10 of the present invention may also identify dynamic motion or motion between the workpiece 38 and the assembly 10, as disclosed in co-pending U.S. patent application Ser. No. 61 / 757,412, the contents of which are incorporated herein by reference. Interrupted flashes by the secondary light source 52, however, provide monitoring of the location of the targets 58 and the workpiece surface 48 allows the assembly to identify drift of either the workpiece 38, the assembly 10, or even the laser beam 14. When a drift is detected, the processor 24 re-initiates the sequence to identify a location of the workpiece surface relative to the laser projector 10.The invention has been described in an illustrative manner, and it is to be understood that the terminology has been used in the nature of description rather than of limitation. Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that throughout the specification, the reference numerals are for convenience only and are not intended to be limiting in any way, as the invention may be practiced otherwise than as specifically described.

Claims

A method of aligning a laser projector for projecting a laser image onto a workpiece surface, comprising the steps of: providing a laser projector assembly having a laser source for projecting a laser image onto a workpiece surface, a secondary light source for illuminating the workpiece surface, a photogrammetry apparatus for generating an image from the workpiece surface, and a laser sensor for detecting a laser beam, wherein the laser source, the laser sensor, the secondary light source, and the photogrammetry apparatus are enclosed in an assembly housing; mounting reflective targets on the workpiece surface; transmitting light from the secondary light source to the workpiece surface; receiving light at the photogrammetry apparatus from the reflective targets; detecting a pattern of the reflecting targets on the workpiece surface in a three-dimensional coordinate system; and after detecting the pattern of the reflecting targets on the workpiece surface in the three-dimensional coordinate system, scanning the targets with a laser beam generated by the laser source and directed by the identified pattern of the reflecting targets to reflect the laser beam against the laser sensor, and calculating a precise location of the targets from the reflected laser beam to direct the laser projector where the laser image is to be projected onto the workpiece surface, wherein the photogrammetry apparatus comprises a single camera.The method of claim 1, wherein the step of transmitting light from the secondary light source is further defined by transmitting intermittent flashes of light from the secondary light source.The method of claim 1, further comprising the step of the laser source and the secondary light source transmitting a light having the same wavelength.The method of claim 1, further including the step of the photogrammetry assembly detecting the position of the workpiece and a pattern of the targets disposed on the workpiece surface to direct the laser beam toward individual targets based on the fixed position of the photogrammetry assembly relative to the laser projector assembly.The method of claim 1, further comprising measuring a drift from the workpiece surface from a first position based on light from the secondary light source reflected from the targets against the photogrammetry assembly.The method of claim 1, further including the step of reflecting light from the secondary light source from the targets to the photogrammetry assembly to determine a position of the workpiece.The method of claim 1, wherein the step of mounting the reflective targets on the workpiece surface is further defined by mounting reflective targets on the workpiece surface at known positions relative to the workpiece surface.A laser projector assembly for projecting a laser image onto a workpiece surface, comprising: a laser source for projecting a laser image onto a workpiece surface, a secondary light source for illuminating the workpiece surface, a photogrammetry apparatus for generating an image from the workpiece surface, and a laser sensor for detecting a laser beam, wherein the laser source, the laser sensor, the secondary light source, and the photogrammetry apparatus are enclosed in an assembly housing; wherein the laser projector assembly is adapted to: transmit light from the secondary light source to the workpiece surface, wherein the workpiece surface comprises reflective targets; receive light at the photogrammetry apparatus, wherein the light is reflected from the reflective targets; detecting a pattern of the reflecting targets on the workpiece surface in a three-dimensional coordinate system; and after detecting the pattern of the reflecting targets on the workpiece surface in the three-dimensional coordinate system, scanning the targets with a laser beam generated by the laser source and directed by the identified pattern of the reflecting targets to reflect the laser beam against the laser sensor, and calculating a precise location of the targets from the reflected laser beam to direct the laser projector where the laser image is to be projected onto the workpiece surface, wherein the photogrammetry apparatus comprises a single camera.

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