# Patent

## Title
Three-dimensional scanner with external tactical probe and illuminated guidance

## Abstract

An assembly that includes a projector and camera is used with a processor to determine three-dimensional (3D) coordinates of an object surface. The processor fits collected 3D coordinates to a mathematical representation provided for a shape of a surface feature. The processor fits the measured 3D coordinates to the shape and, if the goodness of fit is not acceptable, selects and performs at least one of: changing a pose of the assembly, changing an illumination level of the light source, changing a pattern of the transmitted With the changes in place, another scan is made to obtain 3D coordinates.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
The present Application is a nonprovisional application of U.S. Provisional Application 61/791,797 entitled “Three-Dimensional Coordinate Scanner and Method of Operation” filed on Mar. 15, 2013, the contents of which are incorporated by reference in its entirety. This present application is also a Continuation In Part of U.S. patent application Ser. No. 13/443,946, filed on Apr. 11, 2012. U.S. patent application Ser. No. 13/443,946 is a nonprovisional application of U.S. Provisional Application 61/592,049 filed on Jan. 30, 2012 and U.S. Provisional Application 61/475,703 filed on Apr. 15, 2011.


BACKGROUND OF THE INVENTION
The subject matter disclosed herein relates to a three-dimensional coordinate scanner and in particular to a triangulation-type scanner having multiple modalities of data acquisition.
The acquisition of three-dimensional coordinates of an object or an environment is known. Various techniques may be used, such as time-of-flight or triangulation methods for example. A time-of-flight systems such as a laser tracker, total station, or time-of-flight scanner may direct a beam of light such as a laser beam toward a retroreflector target or a spot on the surface of the object. An absolute distance meter is used to determine the distance to the target or spot based on length of time it takes the light to travel to the target or spot and return. By moving the laser beam or the target over the surface of the object, the coordinates of the object may be ascertained. Time-of-flight systems have advantages in having relatively high accuracy, but in some cases may be slower than some other systems since time-of-flight systems must usually measure each point on the surface individually.
In contrast, a scanner that uses triangulation to measure three-dimensional coordinates projects onto a surface either a pattern of light in a line (e.g. a laser line from a laser line probe) or a pattern of light covering an area (e.g. structured light) onto the surface. A camera is coupled to the projector in a fixed relationship, by attaching a camera and the projector to a common frame for example. The light emitted from the projector is reflected off of the surface and detected by the camera. Since the camera and projector are arranged in a fixed relationship, the distance to the object may be determined using trigonometric principles. Compared to coordinate measurement devices that use tactile probes, triangulation systems provide advantages in quickly acquiring coordinate data over a large area. As used herein, the resulting collection of three-dimensional coordinate values provided by the triangulation system is referred to as a point cloud data or simply a point cloud.
A number of issues may interfere with the acquisition of high accuracy point cloud data when using a laser scanner. These include, but are not limited to: variations in the level of light received over the camera image plane as a result in variations in the reflectance of the object surface or variations in the angle of incidence of the surface relative to the projected light; low resolution near edge, such as edges of holes; and multipath interference for example. In some cases, the operator may be unaware of or unable to eliminate a problem. In these cases, missing or faulty point cloud data is the result.
Accordingly, while existing scanners are suitable for their intended purpose the need for improvement remains, particularly in providing a scanner that can adapt to undesirable conditions and provide improved data point acquisition.
BRIEF DESCRIPTION OF THE INVENTION
According to one aspect of the invention, a method of determining three-dimensional coordinates of points on a surface of an object is provided, the method including: providing an assembly that includes a first projector and a first camera, the first projector and the first camera being fixed in relation to one another, there being a baseline distance between the first projector and the first camera, the first projector having a light source, the first camera having a lens and a photosensitive array; providing a processor electrically coupled to the first projector and the first camera; providing a mathematical representation of a shape of a feature on the surface; providing a value for an acceptable goodness of fit; sending a first transmitted light from the first projector onto the object; acquiring by the first camera a first reflected light and sending a first signal to the processor in response, the first reflected light being a portion of the first transmitted light reflected from the surface; determining by the processor a first measured set of three-dimensional (3D) coordinates of first points on the surface, the first measured set based at least in part on the first transmitted light, the first signal and the baseline distance; determining by the processor a first measured subset of points, the first measured subset of points being a subset of the first points on the surface, the first measured subset of points being measured points corresponding to the feature; fitting by the processor 3D coordinates of the first measured subset of points to the provided mathematical representation of the shape of the feature, the fitting including comparing the 3D coordinates of the first measured subset of points to 3D coordinates of a first derived subset of points to obtain a collection of residual errors, the first derived subset of points being a collection of points lying on the shape of the feature, each of the residual errors from the collection of residual errors being a measure of a separation of corresponding 3D coordinates from the first measured subset and the first derived subset, the fitting further consisting of mathematically adjusting position and orientation of the shape to minimize the collection of residual errors according to a minimization rule; determining by the processor a measured goodness of fit, the measured goodness of fit being a mathematically derived quantity obtained from the collection of residual errors; determining by the processor whether the 3D coordinates of the first measured subset of points are acceptable based at least in part on a comparison of the measured goodness of fit to the acceptable goodness of fit; determining by the processor whether the first measured set of 3D coordinates is acceptable based at least in part on whether the 3D coordinates of the first measured subset of points are acceptable; if the first measured set of 3D coordinates is acceptable, storing the first measured set of 3D coordinates; if the first measured set of 3D coordinates is not acceptable, taking steps (a)-(e): (a) selecting by the processor at least one action to be taken and taking the action, the at least one action selected from the group consisting of: changing a pose of the assembly, changing an illumination level of the light source, changing a pattern of the transmitted light, and measuring the feature by illuminating a mechanical probe and imaging spots of light on the probe with the first camera; (b) sending a second transmitted light from a first projector onto the object or illuminating spots of light on the mechanical probe held in contact with the object; (c) acquiring by the first camera a second reflected light and sending a second signal to the processor in response, the second reflected light being a portion of the second transmitted light reflected from the surface or the mechanical probe; (d) determining by the processor a second measured set of 3D coordinates of second points on the surface, the second measured set of 3D coordinates based at least in part on the second transmitted light, the second signal and the baseline distance; and (e) storing the second measured set of 3D coordinates.
According to another aspect of the invention, a method is provided of determining three-dimensional coordinates of points on a surface of an object, the method comprising: providing an assembly that includes a first projector, a first camera, and a second camera, the first projector, the first camera, and the second camera being fixed in relation to one another, there being a first baseline distance between the first projector and the first camera, there being a second baseline distance between the first projector and the second camera, the first projector having a light source, the first camera having a first lens and a first photosensitive array, the second camera having a second lens and a second photosensitive array; providing a processor electrically coupled to the first projector, the first camera, and the second camera; providing a mathematical representation of a shape of a feature on the surface; providing a value for an acceptable goodness of fit; sending a first transmitted light from the first projector onto the object; acquiring by the first camera a first reflected light and sending a first signal to the processor in response, the first reflected light being a portion of the first transmitted light reflected from the surface; determining by the processor a first measured set of three-dimensional (3D) coordinates of first points on the surface, the first measured set based at least in part on the first transmitted light, the first signal and the first baseline distance; determining by the processor a first measured subset of points, the first measured subset of points being a subset of the first points on the surface, the first measured subset of points being measured points corresponding to the feature; fitting by the processor 3D coordinates of the first measured subset of points to the provided mathematical representation of the shape of the feature, the fitting including comparing the 3D coordinates of the first measured subset of points to 3D coordinates of a first derived subset of points to obtain a collection of residual errors, the first derived subset of points being a collection of points lying on the shape of the feature, each of the residual errors from the collection of residual errors being a measure of a separation of corresponding 3D coordinates from the first measured subset and the first derived subset, the fitting further consisting of mathematically adjusting position and orientation of the shape to minimize the collection of residual errors according to a minimization rule; determining by the processor a measured goodness of fit, the measured goodness of fit being a mathematically derived quantity obtained from the collection of residual errors; determining by the processor whether the 3D coordinates of the first measured subset of points are acceptable based at least in part on a comparison of the measured goodness of fit to the acceptable goodness of fit; determining by the processor whether the first measured set of 3D coordinates is acceptable based at least in part on whether the 3D coordinates of the first measured subset of points are acceptable; if the first measured set of 3D coordinates is acceptable, storing the first measured set of 3D coordinates; if the first measured set of 3D coordinates is not acceptable, taking steps (a)-(e): (a) selecting by the processor at least one action to be taken and taking the action, the at least one action selected from the group consisting of: changing an illuminated field-of-view of the assembly, changing a pose of the assembly, changing an illumination level of the light source, changing a pattern of the transmitted light, and measuring the feature by illuminating a mechanical probe and imaging spots of light on the probe with the first camera; (b) sending a second transmitted light from the first projector onto the object or illuminating spots of light on the mechanical probe held in contact with the object; (c) acquiring by an imager a second reflected light and sending a second signal to the processor in response, the second reflected light being a portion of the second transmitted light reflected from the surface or the mechanical probe, the imager being the first camera if the at least one action does not include changing an illuminated field-of-view of the assembly, the imager being the second camera if the at least one action includes changing an illuminated field-of-view of the assembly; (d) determining by the processor a second measured set of 3D coordinates of second points on the surface, the second measured set of 3D coordinates based at least in part on the second transmitted light and the second signal; and (e) storing the second measured set of 3D coordinates.
According to another aspect of the invention, a method is provided for determining three-dimensional coordinates of points on a surface of an object, the method including: providing an assembly that includes a first projector, a first camera, a second projector, and a second camera, the first projector, the first camera, the second projector, and the second camera being fixed in relation to one another, there being a first baseline distance between the first projector and the first camera, there being a second baseline distance between the second projector and the second camera, the first projector having a first light source, the first camera having a first lens and a first photosensitive array, the second projector having a second light source, the second camera having a second lens and a second photosensitive array; providing a processor electrically coupled to the first projector, the first camera, the second projector, and the second camera; providing a mathematical representation of a shape of a feature on the surface; providing a value for an acceptable goodness of fit; sending a first transmitted light from the first projector onto the object; acquiring by the first camera a first reflected light and sending a first signal to the processor in response, the first reflected light being a portion of the first transmitted light reflected from the surface; determining by the processor a first measured set of three-dimensional (3D) coordinates of first points on the surface, the first measured set based at least in part on the first transmitted light, the first signal and the first baseline distance; determining by the processor a first measured subset of points, the first measured subset of points being a subset of the first points on the surface, the first measured subset of points being measured points corresponding to the feature; fitting by the processor 3D coordinates of the first measured subset of points to the provided mathematical representation of the shape of the feature, the fitting including comparing the 3D coordinates of the first measured subset of points to 3D coordinates of a first derived subset of points to obtain a collection of residual errors, the first derived subset of points being a collection of points lying on the shape of the feature, the residual errors being a measure of the separation of corresponding 3D coordinates of the first measured subset and the first derived subset, the fitting further consisting of mathematically adjusting a pose of the shape to minimize the residual errors according to a minimization rule; determining by the processor a measured goodness of fit, the measured goodness of fit being a mathematically derived quantity obtained from the collection of residual errors; determining by the processor whether the first set is acceptable based on a comparison of the measured goodness of fit to the acceptable goodness of fit; if the first set for the measured feature is acceptable, storing the first set of 3D coordinates; if the first set for the measured feature is not acceptable, taking steps (a)-(e): (a) selecting by the processor at least one action to be taken and taking the action, the at least one action selected from the group consisting of: changing an illuminated field-of-view of the assembly, changing a pose of the assembly, changing an illumination level of the light source, changing a pattern of the transmitted light, and measuring the feature by illuminating a mechanical probe and imaging spots of light on the probe with the first camera; (b) sending a second transmitted light from a light transmitter onto the object or illuminating spots of light on the mechanical probe held in contact with the object, the light transmitter being the first projector if the at least one action does not include changing an illuminated field-of-view of the assembly, the light transmitter being the second projector if the at least one action includes changing an illuminated field-of-view of the assembly; (c) acquiring by an imager a second reflected light and sending a second signal to the processor in response, the second reflected light being a portion of the second transmitted light reflected from the surface or the mechanical probe, the imager being the first camera if the at least one action does not include changing an illuminated field-of-view of the assembly, the imager being the second camera if the at least one action includes changing an illuminated field-of-view of the assembly; (d) determining by the processor a second measured set of 3D coordinates of second points on the surface, the second measured set of 3D coordinates based at least in part on the second transmitted light and the second signal; and (e) storing the second measured set of 3D coordinates.
These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings.



BRIEF DESCRIPTION OF THE DRAWING
The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:
FIG. 1 is a top schematic view of a scanner in accordance with an embodiment of the invention;
FIG. 2 is a flow chart showing a method of operating the scanner of FIG. 1;
FIG. 3 is a top schematic view of a scanner in accordance with another embodiment of the invention;
FIG. 4 is a flow chart showing a method of operating the scanner of FIG. 3;
FIG. 5A is a schematic view of elements within a laser scanner according to an embodiment;
FIG. 5B is a flow chart showing a method of operating a scanner according to an embodiment;
FIG. 6 is a top schematic view of a scanner in accordance with another embodiment of the invention;
FIG. 7 is a flow chart showing a method of operating the scanner according to an embodiment;
FIGS. 8A and 8B are perspective views of a scanner used in conjunction with a remote probe device in accordance with an embodiment of the invention;
FIG. 9 is a flow chart showing a method of operating the scanner of FIG. 5;
FIG. 10 is top schematic view of a scanner according to an embodiment;
FIG. 11 is a flow chart showing a method of operating the scanner of FIG. 10;
FIG. 12 is a flow chart showing a diagnostic method according to an embodiment; and
FIG. 13 is a flow chart showing a procedure carried out automatically be a processor for eliminating problems encountered in measurements made with triangulation scanners.



The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings.
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention provide advantages increasing the reliability and accuracy of three-dimensional coordinates of a data point cloud acquired by a scanner. Embodiments of the invention provide advantages in detecting anomalies in acquired data and automatically adjusting the operation of the scanner to acquire the desired results. Embodiments of the invention provide advantages in detecting anomalies in the acquired data and providing indication to the operator of areas where additional data acquisition is needed. Still further embodiments of the invention provide advantages in detecting anomalies in the acquired data and providing indication to the operator where additional data acquisition may be acquired with a remote probe.
Scanner devices acquire three-dimensional coordinate data of objects. In one embodiment, a scanner 20 shown in FIG. 1 has a housing 22 that includes a first camera 24, a second camera 26 and a projector 28. The projector 28 emits light 30 onto a surface 32 of an object 34. In the exemplary embodiment, the projector 28 uses a visible light source that illuminates a pattern generator. The visible light source may be a laser, a superluminescent diode, an incandescent light, a Xenon lamp, a light emitting diode (LED), or other light emitting device for example. In one embodiment, the pattern generator is a chrome-on-glass slide having a structured light pattern etched thereon. The slide may have a single pattern or multiple patterns that move in and out of position as needed. The slide may be manually or automatically installed in the operating position. In other embodiments, the source pattern may be light reflected off or transmitted by a digital micro-mirror device (DMD) such as a digital light projector (DLP) manufactured by Texas Instruments Corporation, a liquid crystal device (LCD), a liquid crystal on silicon (LCOS) device, or a similar device used in transmission mode rather than reflection mode. The projector 28 may further include a lens system 36 that alters the outgoing light to cover the desired area.
In this embodiment, the projector 28 is configurable to emit a structured light over an area 37. As used herein, the term “structured light” refers to a two-dimensional pattern of light projected onto an area of an object that conveys information which may be used to determine coordinates of points on the object. In one embodiment, a structured light pattern will contain at least three non-collinear pattern elements disposed within the area. Each of the three non-collinear pattern elements conveys information which may be used to determine the point coordinates. In another embodiment, a projector is provided that is configurable to project both an area pattern as well as a line pattern. In one embodiment, the projector is a digital micromirror device (DMD), which is configured to switch back and forth between the two. In one embodiment, the DMD projector may also sweep a line or to sweep a point in a raster pattern.
In general, there are two types of structured light patterns, a coded light pattern and an uncoded light pattern. As used herein a coded light pattern is one in which the three dimensional coordinates of an illuminated surface of the object are found by acquiring a single image. With a coded light pattern, it is possible to obtain and register point cloud data while the projecting device is moving relative to the object. One type of coded light pattern contains a set of elements (e.g. geometric shapes) arranged in lines where at least three of the elements are non-collinear. Such pattern elements are recognizable because of their arrangement.
In contrast, an uncoded structured light pattern as used herein is a pattern that does not allow measurement through a single pattern. A series of uncoded light patterns may be projected and imaged sequentially. For this case, it is usually necessary to hold the projector fixed relative to the object.
It should be appreciated that the scanner 20 may use either coded or uncoded structured light patterns. The structured light pattern may include the patterns disclosed in the journal article “DLP-Based Structured Light 3D Imaging Technologies and Applications” by Jason Geng published in the Proceedings of SPIE, Vol. 7932, which is incorporated herein by reference. In addition, in some embodiments described herein below, the projector 28 transmits a pattern formed a swept line of light or a swept point of light. Swept lines and points of light provide advantages over areas of light in identifying some types of anomalies such as multipath interference. Sweeping the line automatically while the scanner is held stationary also has advantages in providing a more uniform sampling of surface points.
The first camera 24 includes a photosensitive sensor 44 which generates a digital image/representation of the area 48 within the sensor's field of view. The sensor may be charged-coupled device (CCD) type sensor or a complementary metal-oxide-semiconductor (CMOS) type sensor for example having an array of pixels. The first camera 24 may further include other components, such as but not limited to lens 46 and other optical devices for example. The lens 46 has an associated first focal length. The sensor 44 and lens 46 cooperate to define a first field of view “X”. In the exemplary embodiment, the first field of view “X” is 16 degrees (0.28 inch per inch).
Similarly, the second camera 26 includes a photosensitive sensor 38 which generates a digital image/representation of the area 40 within the sensor's field of view. The sensor may be charged-coupled device (CCD) type sensor or a complementary metal-oxide-semiconductor (CMOS) type sensor for example having an array of pixels. The second camera 26 may further include other components, such as but not limited to lens 42 and other optical devices for example. The lens 42 has an associated second focal length, the second focal length being different than the first focal length. The sensor 38 and lens 42 cooperate to define a second field of view “Y”. In the exemplary embodiment, the second field of view “Y” is 50 degrees (0.85 inch per inch). The second field of view Y is larger than the first field of view X. Similarly, the area 40 is larger than the area 48. It should be appreciated that a larger field of view allows acquired a given region of the object surface 32 to be measured faster; however, if the photosensitive arrays 44 and 38 have the same number of pixels, a smaller field of view will provide higher resolution.
In the exemplary embodiment, the projector 28 and the first camera 24 are arranged in a fixed relationship at an angle such that the sensor 44 may receive light reflected from the surface of the object 34. Similarly, the projector 28 and the second camera 26 are arranged in a fixed relationship at an angle such that the sensor 38 may receive light reflected from the surface 32 of object 34. Since the projector 28, first camera 24 and second camera 26 have fixed geometric relationships, the distance and the coordinates of points on the surface may be determined by their trigonometric relationships. Although the fields-of-view (FOVs) of the cameras 24 and 26 are shown not to overlap in FIG. 1, the FOVs may partially overlap or totally overlap.
The projector 28 and cameras 24, 26 are electrically coupled to a controller 50 disposed within the housing 22. The controller 50 may include one or more microprocessors, digital signal processors, memory and signal conditioning circuits. The scanner 20 may further include actuators (not shown) which may be manually activated by the operator to initiate operation and data capture by the scanner 20. In one embodiment, the image processing to determine the X, Y, Z coordinate data of the point cloud representing the surface 32 of object 34 is performed by the controller 50. The coordinate data may be stored locally such as in a volatile or nonvolatile memory 54 for example. The memory may be removable, such as a flash drive or a memory card for example. In other embodiments, the scanner 20 has a communications circuit 52 that allows the scanner 20 to transmit the coordinate data to a remote processing system 56. The communications medium 58 between the scanner 20 and the remote processing system 56 may be wired (e.g. Ethernet) or wireless (e.g. Bluetooth, IEEE 802.11). In one embodiment, the coordinate data is determined by the remote processing system 56 based on acquired images transmitted by the scanner 20 over the communications medium 58.
A relative motion is possible between the object surface 32 and the scanner 20, as indicated by the bidirectional arrow 47. There are several ways in which such relative motion may be provided. In an embodiment, the scanner is a handheld scanner and the object 34 is fixed. Relative motion is provided by moving the scanner over the object surface. In another embodiment, the scanner is attached to a robotic end effector. Relative motion is provided by the robot as it moves the scanner over the object surface. In another embodiment, either the scanner 20 or the object 34 is attached to a moving mechanical mechanism, for example, a gantry coordinate measurement machine or an articulated arm CMM. Relative motion is provided by the moving mechanical mechanism as it moves the scanner 20 over the object surface. In some embodiments, motion is provided by the action of an operator and in other embodiments, motion is provided by a mechanism that is under computer control.
Referring now to FIG. 2, the operation of the scanner 20 according to a method 1260 is described. As shown in block 1262, the projector 28 first emits a structured light pattern onto the area 37 of surface 32 of the object 34. The light 30 from projector 28 is reflected from the surface 32 as reflected light 62 received by the second camera 26. The three-dimensional profile of the surface 32 affects the image of the pattern captured by the photosensitive array 38 within the second camera 26. Using information collected from one or more images of the pattern or patterns, the controller 50 or the remote processing system 56 determines a one to one correspondence between the pixels of the photosensitive array 38 and pattern of light emitted by the projector 28. Using this one-to-one correspondence, triangulation principals are used to determine the three-dimensional coordinates of points on the surface 32. This acquisition of three-dimensional coordinate data (point cloud data) is shown in block 1264. By moving the scanner 20 over the surface 32, a point cloud may be created of the entire object 34.
During the scanning process, the controller 50 or remote processing system 56 may detect an undesirable condition or problem in the point cloud data, as shown in block 1266. Methods for detecting such a problem are discussed hereinbelow with regard to FIG. 12. The detected problem may be an error in or absence of point cloud data in a particular area for example. This error in or absence of data may be caused by too little or too much light reflected from that area. Too little or too much reflected light may result from a difference in reflectance over the object surface, for example, as a result of high or variable angles of incidence of the light 30 on the object surface 32 or as a result of low reflectance (black or transparent) materials or shiny surfaces. Certain points on the object may be angled in such as way as to produce a very bright specular reflectance known as a glint.
Another possible reason for an error in or absence of point cloud data is a lack of resolution in regions having fine features, sharp edges, or rapid changes in depth. Such lack of resolution may be the result of a hole, for example.
Another possible reason for an error in or an absence of point cloud data is multipath interference. Ordinarily a ray of light from the projector 28 strikes a point on the surface 32 and is scattered over a range of angles. The scattered light is imaged by the lens 42 of camera 26 onto a small spot on the photosensitive array 38. Similarly, the scattered light may be imaged by the lens 46 of camera 24 onto a small spot on the photosensitive array 44. Multipath interference occurs when the light reaching the point on the surface 32 does not come only from the ray of light from the projector 28 but in addition, from secondary light is reflected off another portion of the surface 32. Such secondary light may compromise the pattern of light received by the photosensitive array 38, 44, thereby preventing accurate determination of three-dimensional coordinates of the point. Methods for identifying the presence of multipath interference are described in the present application with regard to FIG. 12.
If the controller determines that the point cloud is all right in block 1266, the procedure is finished. Otherwise, a determination is made in block 1268 of whether the scanner is used in a manual or automated mode. If the mode is manual, the operator is directed in block 1270 to move the scanner into the desired position.
There are many ways that the movement desired by the operator may be indicated. In an embodiment, indicator lights on the scanner body indicate the desired direction of movement. In another embodiment, a light is projected onto the surface indicating the direction over which the operator is to move. In addition, a color of the projected light may indicate whether the scanner is too close or too far from the object. In another embodiment, an indication is made on display of the region to which the operator is to project the light. Such a display may be a graphical representation of point cloud data, a CAD model, or a combination of the two. The display may be presented on a computer monitor or on a display built into the scanning device.
In any of these embodiments, a method of determining the approximate position of the scanner is desired. In one case, the scanner may be attached to an articulated arm CMM that uses angular encoders in its joints to determine the position and orientation of the scanner attached to its end. In another case, the scanner includes inertial sensors placed within the device. Inertial sensors may include gyroscopes, accelerometers, and magnetometers, for example. Another method of determining the approximate position of the scanner is to illuminate photogrammetric dots placed on or around the object as marker points. In this way, the wide FOV camera in the scanner can determine the approximate position of the scanner in relation to the object.
In another embodiment, a CAD model on a computer screen indicates the regions where additional measurements are desired, and the operator moves the scanner according by matching the features on the object to the features on the scanner. By updating the CAD model on the screen as a scan is taken, the operator may be given rapid feedback whether the desired regions of the part have been measured.
After the operator has moved the scanner into position, a measurement is made in block 1272 with the small FOV camera 24. By viewing a relatively smaller region in block 1272, the resolution of the resulting three-dimensional coordinates is improved and better capability is provided to characterize features such as holes and edges.
Because the narrow FOV camera views a relatively smaller region than the wide FOV camera, the projector 28 may illuminate a relatively smaller region. This has advantages in eliminating multipath interference since there is relatively fewer illuminated points on the object that can reflect light back onto the object. Having a smaller illuminated region may also make it easier to control exposure to obtain the optimum amount of light for a given reflectance and angle of incidence of the object under test. In the block 1274, if all points have been collected, the procedure ends at block 1276; otherwise it continues.
In an embodiment where the mode from block 1268 is automated, then in block 1278 the automated mechanism moves the scanner into the desired position. In some embodiments, the automated mechanism will have sensors to provide information about the relative position of the scanner and object under test. For an embodiment in which the automated mechanism is a robot, angular transducers within the robot joints provide information about the position and orientation of the robot end effector used to hold the scanner. For an embodiment in which the object is moved by another type of automated mechanism, linear encoders or a variety of other sensors may provide information on the relative position of the object and the scanner.
After the automated mechanism has moved the scanner or object into position, then in block 1280 three-dimensional measurements are made with the small FOV camera. Such measurements are repeated by means of block 1282 until all measurements are completed and the procedure finishes at block 1284.
In one embodiment, the projector 28 changes the structured light pattern when the scanner switches from acquiring data with the second camera 26 to the first camera 24. In another embodiment, the same structured light pattern is used with both cameras 24, 26. In still another embodiment, the projector 28 emits a pattern formed by a swept line or point when the data is acquired by the first camera 24. After acquiring data with the first camera 24, the process continues scanning using the second camera 26. This process continues until the operator has either scanned the desired area of the part.
It should be appreciated that while the process of FIG. 2 is shown as a linear or sequential process, in other embodiments one or more of the steps shown may be executed in parallel. In the method shown in FIG. 2, the method involved measuring the entire object first and then carrying out further detailed measurements according to an assessment of the acquired point cloud data. An alternative using the scanner 20 is to begin by measuring detailed or critical regions using the camera 24 having the small FOV.
It should also be appreciated that it is common practice in existing scanning systems to provide a way of changing the camera lens or projector lens as a way of changing the FOV of the camera or of projector in the scanning system. However, such changes are time consuming and typically require an additional compensation step in which an artifact such as a dot plate is placed in front of the camera or projector to determine the aberration correction parameters for the camera or projector system. Hence a scanning system that provides two cameras having different FOVs, such as the cameras 24, 26 of FIG. 1, provides a significant advantage in measurement speed and in enablement of the scanner for a fully automated mode.
Another embodiment is shown in FIG. 3 of a scanner 20 having a housing 22 that includes a first coordinate acquisition system 76 and a second coordinate acquisition system 78. The first coordinate acquisition system 76 includes a first projector 80 and a first camera 82. Similar to the embodiment of FIG. 1, the projector 80 emits light 84 onto a surface 32 of an object 34. In the exemplary embodiment, the projector 80 uses a visible light source that illuminates a pattern generator. The visible light source may be a laser, a superluminescent diode, an incandescent light, a light emitting diode (LED), or other light emitting device. In one embodiment, the pattern generator is a chrome-on-glass slide having a structured light pattern etched thereon. The slide may have a single pattern or multiple patterns that move in and out of position as needed. The slide may be manually or automatically installed in the operating position. In other embodiments, the source pattern may be light reflected off or transmitted by a digital micro-mirror device (DMD) such as a digital light projector (DLP) manufactured by Texas Instruments Corporation, a liquid crystal device (LCD), a liquid crystal on silicon (LCOS) device, or a similar device used in transmission mode rather than reflection mode. The projector 80 may further include a lens system 86 that alters the outgoing light to have the desired focal characteristics.
The first camera 82 includes a photosensitive array sensor 88 which generates a digital image/representation of the area 90 within the sensor's field of view. The sensor may be charged-coupled device (CCD) type sensor or a complementary metal-oxide-semiconductor (CMOS) type sensor for example having an array of pixels. The first camera 82 may further include other components, such as but not limited to lens 92 and other optical devices for example. The first projector 80 and first camera 82 are arranged at an angle in a fixed relationship such that the first camera 82 may detect light 85 from the first projector 80 reflected off of the surface 32 of object 34. It should be appreciated that since the first camera 92 and first projector 80 are arranged in a fixed relationship, the trigonometric principals discussed above may be used to determine coordinates of points on the surface 32 within the area 90. Although for clarity FIG. 3 is depicted as having the first camera 82 near to the first projector 80, it should be appreciated that the camera could be placed nearer the other side of the housing 22. By spacing the first camera 82 and first projector 80 farther apart, accuracy of 3D measurement is expected to improve.
The second coordinate acquisition system 78 includes a second projector 94 and a second camera 96. The projector 94 has a light source that may comprise a laser, a light emitting diode (LED), a superluminescent diode (SLED), a Xenon bulb, or some other suitable type of light source. In an embodiment, a lens 98 is used to focus the light received from the laser light source into a line of light 100 and may comprise one or more cylindrical lenses, or lenses of a variety of other shapes. The lens is also referred to herein as a “lens system” because it may include one or more individual lenses or a collection of lenses. The line of light is substantially straight, i.e., the maximum deviation from a line will be less than about 1% of its length. One type of lens that may be utilized by an embodiment is a rod lens. Rod lenses are typically in the shape of a full cylinder made of glass or plastic polished on the circumference and ground on both ends. Such lenses convert collimated light passing through the diameter of the rod into a line. Another type of lens that may be used is a cylindrical lens. A cylindrical lens is a lens that has the shape of a partial cylinder. For example, one surface of a cylindrical lens may be flat, while the opposing surface is cylindrical in form.
In another embodiment, the projector 94 generates a two-dimensional pattern of light that covers an area of the surface 32. The resulting coordinate acquisition system 78 is then referred to as a structured light scanner.
The second camera 96 includes a sensor 102 such as a charge-coupled device (CCD) type sensor or a complementary metal-oxide-semiconductor (CMOS) type sensor for example. The second camera 96 may further include other components, such as but not limited to lens 104 and other optical devices for example. The second projector 94 and second camera 96 are arranged at an angle such that the second camera 96 may detect light 106 from the second projector 94 reflected off of the object 34. It should be appreciated that since the second projector 94 and the second camera 96 are arranged in a fixed relationship, the trigonometric principles discussed above may be used to determine coordinates of points on the surface 32 on the line formed by light 100. It should also be appreciated that the camera 96 and the projector 94 may be located on opposite sides of the housing 22 to increase 3D measurement accuracy.
In another embodiment, the second coordinate acquisition system is configured to project a variety of patterns, which may include not only a fixed line of light but also a swept line of light, a swept point of light, a coded pattern of light (covering an area), or a sequential pattern of light (covering an area). Each type of projection pattern has different advantages such as speed, accuracy, and immunity to multipath interference. By evaluating the performance requirements for each particular measurements and/or by reviewing the characteristics of the returned data or of the anticipated object shape (from CAD models or from a 3D reconstruction based on collected scan data), it is possible to select the type of projected pattern that optimizes performance.
In another embodiment, the distance from the second coordinate acquisition system 78 and the object surface 32 is different than the distance from the first coordinate acquisition system 76 and the object surface 32. For example, the camera 96 may be positioned closer to the object 32 than the camera 88. In this way, the resolution and accuracy of the second coordinate acquisition system 78 can be improved relative to that of the first coordinate acquisition system 76. In many cases, it is helpful to quickly scan a relatively large and smooth object with a lower resolution system 76 and then scan details including edges and holes with a higher resolution system 78.
A scanner 20 may be used in a manual mode or in an automated mode. In a manual mode, an operator is prompted to move the scanner nearer or farther from the object surface according to the acquisition system that is being used. Furthermore, the scanner 20 may project a beam or pattern of light indicating to the operator the direction in which the scanner is to be moved. Alternatively, indicator lights on the device may indicate the direction in which the scanner should be moved. In an automated mode, the scanner 20 or object 34 may be automatically moved relative to one another according to the measurement requirements.
Similar to the embodiment of FIG. 1, the first coordinate acquisition system 76 and the second coordinate acquisition system 78 are electrically coupled to a controller 50 disposed within the housing 22. The controller 50 may include one or more microprocessors, digital signal processors, memory and signal conditioning circuits. The scanner 20 may further include actuators (not shown) which may be manually activated by the operator to initiate operation and data capture by the scanner 20. In one embodiment, the image processing to determine the X, Y, Z coordinate data of the point cloud representing the surface 32 of object 34 is performed by the controller 50. The coordinate data may be stored locally such as in a volatile or nonvolatile memory 54 for example. The memory may be removable, such as a flash drive or a memory card for example. In other embodiments, the scanner 20 has a communications circuit 52 that allows the scanner 20 to transmit the coordinate data to a remote processing system 56. The communications medium 58 between the scanner 20 and the remote processing system 56 may be wired (e.g. Ethernet) or wireless (e.g. Bluetooth, IEEE 802.11). In one embodiment, the coordinate data is determined by the remote processing system 56 and the scanner 20 transmits acquired images on the communications medium 58.
Referring now to FIG. 4, the method 1400 of operating the scanner 20 of FIG. 3 will be described. In block 1402, the first projector 80 of the first coordinate acquisition system 76 of scanner 20 emits a structured light pattern onto the area 90 of surface 32 of the object 34. The light 84 from projector 80 is reflected from the surface 32 and the reflected light 85 is received by the first camera 82. As discussed above, the variations in the surface profile of the surface 32 create distortions in the imaged pattern of light received by the first photosensitive array 88. Since the pattern is formed by structured light, a line or light, or a point of light, it is possible in some instances for the controller 50 or the remote processing system 56 to determine a one to one correspondence between points on the surface 32 and the pixels in the photosensitive array 88. This enables triangulation principles discussed above to be used in block 1404 to obtain point cloud data, which is to say to determine X, Y, Z coordinates of points on the surface 32. By moving the scanner 20 relative to the surface 32, a point cloud may be created of the entire object 34.
In block 1406, the controller 50 or remote processing system 56 determines whether the point cloud data possesses the desired data quality attributes or has a potential problem. The types of problems that may occur were discussed hereinabove in reference to FIG. 2 and this discussion is not repeated here. If the controller determines that the point cloud has the desired data quality attributes in block 1406, the procedure is finished. Otherwise, a determination is made in block 1408 of whether the scanner is used in a manual or automated mode. If the mode is manual, the operator is directed in block 1410 to move the scanner to the desired position.
There are several ways of indicating the desired movement by the operator as described hereinabove with reference to FIG. 2. This discussion is not repeated here.
To direct the operator in obtaining the desired movement, a method of determining the approximate position of the scanner is needed. As explained with reference to FIG. 2, methods may include attachment of the scanner 20 to an articulated arm CMM, use of inertial sensors within the scanner 20, illumination of photogrammetric dots, or matching of features to a displayed image.
After the operator has moved the scanner into position, a measurement is made with the second coordinate acquisition system 78 in block 1412. By using the second coordinate acquisition system, resolution and accuracy may be improved or problems may be eliminated. In block 1414, if all points have been collected, the procedure ends at block 1416; otherwise it continues.
If the mode of operation from block 1408 is automated, then in block 1418 the automated mechanism moves the scanner into the desired position. In most cases, an automated mechanism will have sensors to provide information about the relative position of the scanner and object under test. For the case in which the automated mechanism is a robot, angular transducers within the robot joints provide information about the position and orientation of the robot end effector used to hold the scanner. For other types of automated mechanisms, linear encoders or a variety of other sensors may provide information on the relative position of the object and the scanner.
After the automated mechanism has moved the scanner or object into position, then in block 1420 three-dimensional measurements are made with the second coordinate acquisition system 78. Such measurements are repeated by means of block 1422 until all measurements are completed. The procedure finishes at block 1424.
It should be appreciated that while the process of FIG. 4 is shown as a linear or sequential process, in other embodiments one or more of the steps shown may be executed in parallel. In the method shown in FIG. 4, the method involved measuring the entire object first and then carrying out further detailed measurements according to an assessment of the acquired point cloud data. An alternative using scanner 20 is to begin by measuring detailed or critical regions using the second coordinate acquisition system 78.
It should also be appreciated that it is common practice in existing scanning systems to provide a way of changing the camera lens or projector lens as a way of changing the FOV of the camera or of projector in the scanning system. However, such changes are time consuming and typically require an additional compensation step in which an artifact such as a dot plate is placed in front of the camera or projector to determine the aberration correction parameters for the camera or projector system. Hence a system that provides two different coordinate acquisition systems such as the scanning system 20 of FIG. 3 provides a significant advantage in measurement speed and in enablement of the scanner for a fully automated mode.
An error may occur in making scanner measurements as a result of multipath interference. The origin of multipath interference is now discussed, and a first method for eliminating or reducing multipath interference is described.
The case of multipath interference occurs when the some of the light that strikes the object surface is first scattered off another surface of the object before returning to the camera. For the point on the object that receives this scattered light, the light sent to the photosensitive array then corresponds not only to the light directly projected from the projector but also to the light sent to a different point on the projector and scattered off the object. The result of multipath interference, especially for the case of scanners that project two-dimensional (structured) light, may be to cause the distance calculated from the projector to the object surface at that point to be inaccurate.
An instance of multipath interference is illustrated in reference to FIG. 5A, in this embodiment a scanner 4570 projects a line of light 4525 onto the surface 4510A of an object. The line of light 4525 is perpendicular to the plane of the paper. In an embodiment, the rows of a photosensitive array are parallel to the plane of the paper and the columns are perpendicular to the plane of the paper. Each row represents one point on the projected line 4525 in the direction perpendicular to the plane of the paper. The distance from the projector to the object for that point on the line is found by first calculating the centroid for each row. For the surface point 4526, the centroid on the photosensitive array 4541 is represented by the point 4546. The position 4546 of the centroid on the photosensitive array can be used to calculate the distance from the camera perspective center 4544 to the object point 4526. This calculation is based on trigonometric relationships according to the principles of triangulation. To perform these calculations, the baseline distance D from the camera perspective center 4544 to the projector perspective center 4523 is required. In addition, knowledge of the relative orientation of the projector system 4520 to the camera system 4540 is required.
To understand the error caused by multipath interference, consider the point 4527. Light reflected or scattered from this point is imaged by the lens 4542 onto the point 4548 on the photosensitive array 4541. However, in addition to the light received directly from the projector and scattered off the point 4527, additional light is reflected off the point 4526 onto the point 4527 before being imaged onto the photosensitive array. The light will mostly likely be scattered to an unexpected position and cause two centroids to be formed in a given row. Consequently observation of two centroids on a given row is a good indicator of the presence of multipath interference.
For the case of structured light projected onto an area of the object surface, a secondary reflection from a point such as 4527 is not usually as obvious as for light projected onto a line and hence is more likely to create an error in the measured 3D surface coordinates.
By using a projector having an adjustable pattern of illumination on a display element 4521, it is possible to vary the pattern of illumination. The display element 4521 might be a digital micromechanical mirror (DMM) such as a digital light projector (DLP). Such devices contain multiple small mirrors that are rapidly adjustable by means of an electrical signal to rapidly adjust a pattern of illumination. Other devices that can produce an electrically adjustable display pattern include an LCD (liquid crystal display) and an LCOS (liquid crystal on silicon) display.
A way of checking for multipath interference in a system that projects structured light over an area is to change the display to project a line of light. The presence of multiple centroids in a row will indicate that multipath interference is present. By sweeping the line of light, an area can be covered without requiring that the probe be moved by an operator.
The line of light can be set to any desired angle by an electrically adjustable display. By changing the direction of the projected line of light, multipath interference can, in many cases, be eliminated.
For surfaces having many fold and steep angles so that reflections are hard to avoid, the electrically adjustable display can be used to sweep a point of light. In some cases, a secondary reflection may be produced from a single point of light, but it is usually relatively easy to determine which of the reflected spots of light is valid.
An electrically adjustable display can also be used to quickly switch between a coded and an uncoded pattern. In most cases, a coded pattern is used to make a 3D measurement based on a single frame of camera information. On the other hand, multiple patterns (sequential or uncoded patterns) may be used to obtain greater accuracy in the measured 3D coordinate values.
In the past, electrically adjustable displays have been used to project each of a series of patterns within a sequential pattern—for example, a series of gray scale line patterns followed by a sequence of sinusoidal patterns, each having a different phase.
The present inventive method provides advantages over earlier methods in selecting those methods that identify or eliminate problems such as multipath interference and that indicate whether a single-shot pattern (for example, coded pattern) or a multiple-shot pattern is preferred to obtain the required accuracy as quickly as possible.
For the case of a line scanner, there is often a way to determine the presence of multipath interference. When multipath interference is not present, the light reflected by a point on the object surface is imaged in a single row onto a region of contiguous pixels. If there are two or more regions of a row receive a significant amount of light, multipath interference is indicated. An example of such a multipath interference condition and the resulting extra region of illumination on the photosensitive array are shown in FIG. 5A. The surface 4510A now has a greater curvature near the point of intersection 4526. The surface normal at the point of intersection is the line 4528, and the angle of incidence is 4531. The direction of the reflected line of light 4529 is found from the angle of reflection 4532, which is equal to the angle of incidence. As stated hereinabove, the line of light 4529 actually represents an overall direction for light that scatters over a range of angles. The center of the scattered light strikes the surface 4510A at the point 4527, which is imaged by the lens 4544 at the point 4548 on the photosensitive array. The unexpectedly high amount of light received in the vicinity of point 4548 indicates that multipath interference is probably present. For a line scanner, the main concern with multipath interference is not for the case shown in FIG. 5A, where the two spots 4546 and 4527 are separated by a considerable distance and can be analyzed separately but rather for the case in which the two spots overlap or smear together. In this case, it may not be possible to determine the centroid corresponding to the desired point, which in FIG. 15E corresponds to the point 4546. The problem is made worse for the case of a scanner that projects light over a two-dimensional area as can be understood by again referring to FIG. 5A. If all of the light imaged onto the photosensitive array 4541 were needed to determine two-dimensional coordinates, then it is clear that the light at the point 4527 would correspond to the desired pattern of light directly projected from the projector as well as the unwanted light reflected off the object surface to the point 4527. As a result, in this case, the wrong three dimensional coordinates would likely be calculated for the point 4527 for light projected over an area.
For a projected line of light, in many cases, it is possible to eliminate multipath interference by changing the direction of the line. One possibility is to make a line scanner using a projector having inherent two-dimensional capability, thereby enabling the line to be swept or to be automatically rotated to different directions. An example of such a projector is one that makes use of a digital micromirror (DMD), as discussed hereinabove. For example, if multipath interference were suspected in a particular scan obtained with structured light, a measurement system could be automatically configured to switch to a measurement method using a swept line of light.
Another method to reduce, minimize or eliminate multipath interference is to sweep a point of light, rather than a line of light or an area of light, over those regions for which multipath interference has been indicated. By illuminating a single point of light, any light scattered from a secondary reflection can usually be readily identified.
The determination of the desired pattern projected by an electrical adjustable display benefits from a diagnostic analysis, as described with respect to FIG. 12 below.
Besides its use in diagnosing and correcting multipath interference, changing the pattern of projected light also provides advantages in obtaining a required accuracy and resolution in a minimum amount of time. In an embodiment, a measurement is first performed by projecting a coded pattern of light onto an object in a single shot. The three-dimensional coordinates of the surface are determined using the collected data, and the results analyzed to determine whether some regions have holes, edges, or features that require more detailed analysis. Such detailed analysis might be performed for example by using the narrow FOV camera 24 in FIG. 1 or the high resolution scanner system 78 in FIG. 3.
The coordinates may also be analyzed to determine the approximate distance to the target, thereby providing a starting distance for a more accurate measurement method such as a method that sequentially projects sinusoidal phase-shifted patterns of light onto a surface, as discussed hereinabove. Obtaining a starting distance for each point on the surface using the coded light pattern eliminates the need to obtain this information by vary the pitch in multiple sinusoidal phase-shifted scans, thereby saving considerable time.
Referring now to FIG. 5B, an embodiment is shown for overcoming anomalies or improving accuracy in coordinate data acquired by scanner 20. The process 211 starts in block 212 by scanning an object, such as object 34, with a scanner 20. The scanner 20 may be a scanner such as those described in the embodiments of FIGS. 1, 3, 5 and FIG. 7 for example having at least one projector and a camera. In this embodiment, the scanner 20 projects a first light pattern onto the object in block 212. In one embodiment, this first light pattern is a coded structured light pattern. The process 211 acquires and determines the three-dimensional coordinate data in block 214. The coordinate data is analyzed in query block 216 to determine if there are any anomalies, such as the aforementioned multipath interference, low resolution around an element, or an absence of data due to surface angles or surface reflectance changes. When an anomaly is detected, the process 211 proceeds to block 218 where the light pattern emitted by the projector is changed to a second light pattern. In an embodiment, the second light pattern is a swept line of light.
After projecting the second light pattern, the process 211 proceeds to block 220 where the three-dimensional coordinate data is acquired and determined for the area where the anomaly was detected. The process 211 loops back to query block 216 where it is determined if the anomaly has been resolved. If the query block 216 still detects an anomaly or lack or accuracy or resolution, the process loops back to block 218 and switches to a third light pattern. In an embodiment, the third light pattern is a sequential sinusoidal phase shift pattern. In another embodiment, the third light pattern is a swept point of light. This iterative procedure continues until the anomaly has been resolved. Once coordinate data from the area of the anomaly has been determined, the process 211 proceeds to block 222 where the emitted pattern is switched back to the first structured light pattern and the scanning process is continued. The process 211 continues until the operator has scanned the desired area of the object. In the event that the scanning information obtained using the method of FIG. 11 is not satisfactory, a problem of measuring with a tactile probe, as discussed herein, may be used.
Referring now to FIG. 6, another embodiment of a scanner 20 is shown mounted to a movable apparatus 120. The scanner 20 has at least one projector 122 and at least one camera 124 that are arranged in a fixed geometric relationship such that trigonometric principles may be used to determine the three-dimensional coordinates of points on the surface 32. The scanner 20 may be the same scanner as described in reference to FIG. 1 or FIG. 3 for example. In one embodiment, the scanner is the same as the scanner of FIG. 10 having a tactile probe. However the scanner used in the embodiment of FIG. 6 may be any scanner such as a structured light or line scanner, for example, a scanner disclosed in commonly owned U.S. Pat. No. 7,246,030 entitled “Portable Coordinate Measurement Machine with Integrated Line Laser Scanner” filed on 18 Jan. 2006 which is incorporated by reference herein. In another embodiment, the scanner used in the embodiment of FIG. 6 is a structured light scanner that projects light over an area on an object.
In the exemplary embodiment, the moveable apparatus 120 is a robotic apparatus that provides automated movements by means of arm segments 126, 128 that are connected by pivot and swivel joints 130 to allow the arm segments 126, 128 to be moved, resulting in the scanner 20 moving from a first position to a second position (as indicated in dashed line in FIG. 6). The moveable apparatus 120 may include actuators, such as motors (not shown), for example, that are coupled to the arm segments 126, 128 to move the arm segments 126, 128 from the first position to the second position. It should be appreciated that a movable apparatus 120 having articulated arms is for exemplary purposes and the claimed invention should not be so limited. In other embodiments, the scanner 20 may be mounted to a movable apparatus that moves the scanner 20 via rails, wheels, tracks, belts or cables or a combination of the foregoing for example. In other embodiments, the robot has a different number of arm segments.
In one embodiment, the movable apparatus is an articulated arm coordinate measurement machine (AACMM) such as that described in commonly owned U.S. patent application Ser. No. 13/491,176 filed on Jan. 20, 2010. In this embodiment, the movement of the scanner 20 from the first position to the second position may involve the operator manually moving the arm segments 126, 128.
For an embodiment having an automated apparatus, the moveable apparatus 120 further includes a controller 132 that is configured to energize the actuators to move the arm segments 126, 128. In one embodiment, the controller 132 communicates with a controller 134. As will be discussed in more detail below, this arrangement allows the controller 132 to move the scanner 20 in response to an anomaly in the acquired data. It should be appreciated that the controllers 132, 134 may be incorporated into a single processing unit or the functionality may be distributed among several processing units.
By carrying out an analysis with reference to FIG. 12, it is possible to position and orient the scanner 20 to obtain the desired measurement results. In some embodiments, a feature being measured may benefit from a desired direction of the scanner. For example, measurement of the diameter of a hole may be improved by orienting the scanner camera 124 to be approximately perpendicular to the hole. In other embodiments, a scanner may be positioned so as to reduce or minimize the possibility of multipath interference. Such an analysis may be based on a CAD model available as a part of the diagnostic procedure or it may be based on data collected by the scanner in an initial position prior to a secondary movement of the scanner 20 by the apparatus 120.
Referring now to FIG. 7, the operation of the scanner 20 and movable apparatus 120 will be described. The process starts in block 136 with scanning the object 34 with the scanner 20 in the first position. The scanner 20 acquires and determines coordinate data for points on the surface 32 of the object 34 in block 138. It should be appreciated that the movable apparatus 120 may move the scanner 20 to acquire data on surface points in a desired area. In query block 140, it is determined whether there is an anomaly in the coordinate data at point 142, such as multipath interference for example, or whether there is a need to change direction to obtain improved resolution or measurement accuracy. It should be appreciated that the point 142 of FIG. 6 may represent a single point, a line of points or an area on the surface 32. If an anomaly or need for improved accuracy is detected, the process continues to block 144 where the movable apparatus 120 moves the position of the scanner 20, such as from the first position to the second position, and rescans the area of interest in block 146 to acquire three-dimensional coordinate data. The process loops back to query block 140 where it is determined whether there is still an anomaly in the coordinate data or if an improvement measurement accuracy is desired. If these cases, the scanner 20 is moved again and the process continues until the measurement results achieve a desired level. Once the coordinate data is obtained, the process proceeds from query block 140 to block 148 where the scanning process continues until the desired area has been scanned.
In embodiments where the scanner 20 includes a tactile probe (FIG. 10), the movement of the scanner from the first position to the second position may be arranged to contact the areas of interest with the tactile probe. Since the position of the scanner, and thus the tactile probe, may be determined from the position and orientation of the arm segments 126, 128 the three-dimensional coordinates of the point on the surface 32 may be determined.
In some embodiments, measurement results obtained by the scanner 20 of FIGS. 8A, 8B may be corrupted by multipath interference. In other cases, measurement results may not provide the desired resolution or accuracy to properly measure some characteristics of the surface 32, especially edges, holes, or intricate features. In these cases, it may be desirable to have an operator use a remote probe 152 to interrogate points or areas on the surface 32. In one embodiment shown in FIGS. 8A, 8B, the scanner 20 includes a projector 156 and cameras 154, 155 arranged on an angle relative to the projector 156 such that light emitted by the projector 156 is reflected off of the surface 32 and received by one or both of the cameras 154, 155. The projector 156 and cameras 154, 156 are arranged in a fixed geometric relationship so that trigonometric principles may be used to determine the three-dimensional coordinates of points on the surface 32.
In one embodiment, the projector 156 is configured to emit a visible light 157 onto an area of interest 159 on the surface 32 of object 34 as shown in FIG. 8A. The three-dimensional coordinates of the illuminated area of interest 159 may be confirmed using the image of the illuminated region 159 on one or both of the cameras 154, 155.
The scanner 20 is configured to cooperate with the remote probe 152 so that an operator may bring a probe tip 166 into contact with the object surface 132 at the illuminated region of interest 159. In an embodiment, the remote probe 152 includes at least three non-collinear points of light 168. The points of light 168 may be spots of light produced, for example, by light emitting diodes (LED) or reflective dots of light illuminated by infrared or visible light source from the projector 156 or from another light source not depicted in FIG. 8B. The infrared or visible light source in this case may be attached to the scanner 20 or may be placed external to the scanner 20. By determining the three-dimensional coordinates of the spots of light 168 with the scanner and by using information on the geometry of the probe 152, the position of the probe tip 166 may be determined, thereby enabling the coordinates of the object surface 32 to be determined. A tactile probe used in this way eliminates potential problems from multipath interference and also enables relatively accurate measurement of holes, edges, and detailed features. In an embodiment, the probe 166 is a tactile probe, which may be activated by pressing of an actuator button (not shown) on the probe, or the probe 166 may be a touch-trigger probe activated by contact with the surface 32. In response to a signal produced by the actuator button or the touch trigger probe, a communications circuit (not shown) transmits a signal to the scanner 20. In an embodiment, the points of light 168 are replaced with geometrical patterns of light, which may include lines or curves.
Referring now to FIG. 9, a process is shown for acquiring coordinate data for points on the surface 32 of object 34 using a stationary scanner 20 of FIGS. 8A, 8B with a remote probe 152. The process starts in block 170 with the surface 32 of the object 34 being scanned. The process acquires and determines the three-dimensional coordinate data of the surface 32 in block 172. The process then determines in query block 174 whether there is an anomaly in the coordinate data of area 159 or whether there is a problem in accuracy or resolution of the area 159. An anomaly could be invalid data that is discarded due to multipath interference for example. An anomaly could also be missing data due to surface reflectance or a lack of resolution around a feature such as an opening or hole for example. Details on a diagnostic procedure for detecting (identifying) multipath interference and related problems in given in reference to FIG. 12.
Once the area 159 has been identified, the scanner 20 indicates in block 176 to the operator the coordinate data of area 159 may be acquired via the remote probe 152. This area 159 may be indicated by emitting a visible light 157 to illuminate the area 159. In one embodiment, the light 157 is emitted by the projector 156. The color of light 157 may be changed to inform the operator of the type of anomaly or problem. For example, where multipath interference occurs, the light 157 may be colored red, while a low resolution may be colored green. The area may further be indicated on a display having a graphical representation (e.g. a CAD model) of the object.
The process then proceeds to block 178 to acquire an image of the remote probe 152 when the sensor 166 touches the surface 32. The points of light 168, which may be LEDs or reflective targets, for example, are received by one of the cameras 154, 155. Using best-fit techniques well known to mathematicians, the scanner 20 determines in block 180 the three-dimensional coordinates of the probe center from which three-dimensional coordinates of the object surface 32 are determined in block 180. Once the points in the area 159 where the anomaly was detected have been acquired, the process may proceed to continue the scan of the object 34 in block 182 until the desired areas have been scanned.
Referring now to FIG. 10, another embodiment of the scanner 20 is shown that may be handheld by the operator during operation. In this embodiment, the housing 22 may include a handle 186 that allows the operator to hold the scanner 20 during operation. The housing 22 includes a projector 188 and a camera 190 arranged on an angle relative to each other such that the light 192 emitted by the projector is reflected off of the surface 32 and received by the camera 190. The scanner 20 of FIG. 10 operates in a manner substantially similar to the embodiments of FIG. 1 and FIG. 3 and acquires three-dimensional coordinate data of points on the surface 32 using trigonometric principles.
The scanner 20 further includes an integral probe member 184. The probe member 184 includes a sensor 194 on one end. The sensor 194 is a tactile probe that may respond to pressing of an actuator button (not shown) by an operator or it may be a touch trigger probe that responds to contact with the surface 32, for example. As will be discussed in more detail below, the probe member 184 allows the operator to acquire coordinates of points on the surface 32 by contacting the sensor 194 to the surface 32.
The projector 188, camera 190 and actuator circuit for the sensor 194 are electrically coupled to a controller 50 disposed within the housing 22. The controller 50 may include one or more microprocessors, digital signal processors, memory and signal conditioning circuits. The scanner 20 may further include actuators (not shown), such as on the handle 186 for example, which may be manually activated by the operator to initiate operation and data capture by the scanner 20. In one embodiment, the image processing to determine the X, Y, Z coordinate data of the point cloud representing the surface 32 of object 34 is performed by the controller 50. The coordinate data may be stored locally such as in a volatile or nonvolatile memory 54 for example. The memory may be removable, such as a flash drive or a memory card for example. In other embodiments, the scanner 20 has a communications circuit 52 that allows the scanner 20 to transmit the coordinate data to a remote processing system 56. The communications medium 58 between the scanner 20 and the remote processing system 56 may be wired (e.g. Ethernet) or wireless (e.g. Bluetooth, IEEE 802.11). In one embodiment, the coordinate data is determined by the remote processing system 56 and the scanner 20 transmits acquired images on the communications medium 58.
Referring now to FIG. 11, the operation of the scanner 20 of FIG. 10 will be described. The process begins in block 196 with the operator scanning the surface 32 of the object 34 by manually moving the scanner 20. The three-dimensional coordinates are determined and acquired in block 198. In query block 200, it is determined if an anomaly is present in the coordinate data or if improved accuracy is needed. As discussed above, anomalies may occur for a number of reasons such as multipath interference, surface reflectance changes or a low resolution of a feature. If an anomaly is present, the process proceeds to block 202 where the area 204 is indicated to the operator. The area 204 may be indicated by projecting a visible light 192 with the projector 188 onto the surface 32. In one embodiment, the light 192 is colored to notify the operator of the type of anomaly detected.
The operator then proceeds to move the scanner from a first position to a second position (indicated by the dashed lines) in block 206. In the second position, the sensor 194 contacts the surface 32. The position and orientation (to six degrees of freedom) of the scanner 20 in the second position may be determined using well known best-fit methods based on images acquired by the camera 190. Since the dimensions and arrangement of the sensor 194 are known in relation to the mechanical structure of the scanner 20, the three-dimensional coordinate data of the points in area 204 may be determined in block 208. The process then proceeds to block 210 where scanning of the object continues. The scanning process continues until the desired area has been scanned.
A general approach may be used to evaluate not only multipath interference but also quality in general, including resolution and effect of material type, surface quality, and geometry. Referring also to FIG. 12, in an embodiment, a method 4600 may be carried out automatically under computer control. A step 4602 is to determine whether information on three-dimensional coordinates of an object under test are available. A first type of three-dimensional information is CAD data. CAD data usually indicates nominal dimensions of an object under test. A second type of three-dimensional information is measured three-dimensional data—for example, data previously measured with a scanner or other device. In some cases, the step 4602 may include a further step of aligning the frame of reference of the coordinate measurement device, for example, laser tracker or six-DOF scanner accessory, with the frame of reference of the object. In an embodiment, this is done by measuring at least three points on the surface of the object with the laser tracker.
If step 4602 returns a positive and determines that the three-dimensional information is available, then, in a step 4604, the computer or processor is used to calculate the susceptibility of the object measurement to multipath interference. In an embodiment, this is done by projecting each ray of light emitted by the scanner projector, and calculating the angle or reflection for each case. The computer or software identifies each region of the object surface that is susceptible to error as a result of multipath interference. The step 4604 may also carry out an analysis of the susceptibility to multipath error for a variety of positions of the six-DOF probe relative to the object under test. In some cases, multipath interference may be avoided or minimized by selecting a suitable position and orientation of the six-DOF probe relative to the object under test, as described hereinabove. If step 4602 returns a negative, meaning that three-dimensional information is not available, then a step 4606 measures the three-dimensional coordinates of the object surface using any suitable measurement method. Following the calculation of multipath interference, a step 4608 may be carried out to evaluate other aspects of expected scan quality. One such quality factor is whether the resolution of the scan is sufficient for the features of the object under test. For example, if the resolution of a device is 3 mm, and there are sub-millimeter features for which valid scan data is desired, then these regions of the object should be noted for later corrective action. Another quality factor related partly to resolution is the ability to measure edges of the object and edges of holes. Knowledge of scanner performance will enable a determination of whether the scanner resolution is good enough for given edges. Another quality factor is the amount of light expected to be returned from a given feature. Little if any light may be expected to be returned to the scanner from inside a small hole, for example, or from a glancing angle. Also, little light may be expected from certain kinds and colors of materials. Certain types of materials may have a large depth of penetration for the light from the scanner, and in this case good measurement results would not be expected. In some cases, an automatic program may ask for user supplementary information. For example, if the process is carrying out steps 4604 and 4608 based on CAD data, the type of material being used or the surface characteristics of the object under test may not be known. In these embodiments, step 4608 may include a further step of obtaining material characteristics for the object under test.
Following the analysis of steps 4604 and 4608, the step 4610 determines whether further diagnostic procedures should be carried out. A first example of a possible diagnostic procedure is the step 4612 of projecting a stripe at a predetermined angle to determine whether multipath interference is observed. The general indications of multipath interference for a projected line stripe were discussed hereinabove with reference to FIG. 5. Another example of a diagnostic step is step 4614, which is to project a collection of lines aligned in the direction of epipolar lines on the source pattern of light, for example, the source pattern of light 30 from projector 36 in FIG. 1. For the case in which lines of light in the source pattern of light are aligned to the epipolar lines, then these lines will also appear as straight lines in the image plane on the photosensitive array. The use of epipolar lines is discussed in more detail in commonly owned U.S. patent application Ser. No. 13/443,946 filed Apr. 11, 2012 the contents of which are incorporated by reference herein. If these patterns on the photosensitive array are not straight lines or if the lines are blurred or noisy, then a problem is indicated, possibly as a result of multipath interference.
The step 4616 selects a combination of predetermined actions based on the analyses and diagnostic procedure performed. If speed in a measurement is a factor, a step 4618 of measuring using a 2D (structured) pattern of coded light may be preferred. If greater accuracy is desired, then a step 4620 of measuring using a 2D (structured) pattern of coded light using sequential patterns, for example, a sequence of sinusoidal patterns of varying phase and pitch, may be performed. If the method 4618 or 4620 is selected, then it may be desirable to also select a step 4628, which is to reposition the scanner, in other words to adjust the position and orientation of the scanner to the position that reduces or minimizes multipath interference and specular reflections (glints) as provided by the analysis of step 4604. Such indications may be provided to a user by illuminating problem regions with light from the scanner projector or by displaying such regions on a monitor display. In one embodiment, the next steps in the process may be automatically selected. If the desired scanner position does not eliminate multipath interference and glints, several options are available. In some embodiments, the measurement can be repeated with the scanner repositioned and the valid measurement results combined. In other embodiments, different measurement steps may be added to the procedure or performed instead of using structured light. As discussed previously, a step 4622 of scanning a stripe of light provides a convenient way of obtaining information over an area with reduced probability of having a problem from multipath interference. A step 4624 of sweeping a small spot of light over a region of interest further reduces the probability of problems from multipath interference. A step of measuring a region of an object surface with a tactile probe eliminates the possibility of multipath interference. A tactile probe provides a known resolution based on the size of the probe tip, and it eliminates issues with low reflectance of light or large optical penetration depth, which might be found in some objects under test.
The quality of the data collected in a combination of the steps 4618-4628 may be evaluated in a step 4630 based on the data obtained from the measurements, combined with the results of the analyses carried out previously. If the quality is found to be acceptable in a step 4632, the measurement is completed at a step 4634. Otherwise, the analysis resumes at the step 4604. In some cases, the 3D information may not have been as accurate as desired. In this case, repeating some of the earlier steps may be helpful.
A method that can be used to identify and correct a problem caused by insufficient resolution, insufficient accuracy, poor illumination, poor choice of patterns, or multipath interference is to scan a surface feature having known geometrical characteristics and fit resulting scan data to the surface feature. If the scanned shape does not closely resemble the geometrical characteristics or if the residual errors in fitting 3D collected data to the surface feature are large, one of the problems described above may be responsible. An example of a surface feature that may be selected is a cylindrical hole. Examples of geometrical features that may be compared to scan data include diameter, form error, and depth.
FIG. 13 shows steps performed according to the method 5000. In a step 5005, an assembly including a projector and camera are provided, and a processor connected to the assembly elements is also provided. The projector and camera are separated by a baseline distance and, together with the processor, constitute a triangulation scanner capable of determining 3D coordinates of points on a surface of an object.
In a step 5010, a mathematical representation is provided for a shape of a feature. In an embodiment, the mathematical representation is one of a circle, a cylindrical hole, a raised cylinder, a sphere, an intersection of two planes, and an intersection of three planes.
In a step 5015, an acceptable goodness of fit is provided. In an embodiment, a chi-square test statistic is used as a measure of goodness of fit.
In a step 5020, the projector transmits light to the surface, the reflected light of which is captured as an image on a photosensitive array within the camera. The photosensitive array provides an electrical signal to the processor in response to the image. The processor determines a first set of 3D coordinates for points on the surface based at least in part on the projected light, the electrical signal, and the baseline distance.
In a step 5025, a subset of points is selected from the points measured on the surface, the subset of points including those points corresponding to the feature having the shape described by the mathematical representation of step 5010.
In a step 5030, the processor fits measured 3D coordinates of the subset of points to corresponding 3D coordinates derived from the mathematical model of the shape. The fitting is carried out in a sequence of steps. At each step, the shape is adjusted in position and orientation to reduce or minimize the collection of residual errors according to a minimization rule. In an embodiment, the minimization rule is to reduce or minimize the sum of the square of each residual error selected from among the collection of residual errors.
In a step 5035, a value is calculated for the measured goodness of fit. In an embodiment, the measured goodness of fit is based on chi-square test statistic using residual errors from the calculation of step 5030.
In a step 5035, the value of the measured goodness of fit is compared to the value of the acceptable goodness of fit from step 5015. If the measured goodness of fit value indicates that the fit is better than that required by the acceptable goodness of fit value, the first set of measured 3D coordinates obtained in step 5020 is saved.
If in the step 5040 the measured goodness of fit is unacceptable, at least one of the following actions may be selected and taken: changing the pose of the assembly in relation to the object surface; changing the illumination emitted by the projector; changing the pattern of light emitted by the projector; and measuring the feature with a mechanical probe.
As used herein, the pose of the assembly is defined as the six degrees of freedom of the assembly relative to the object. In other words, the pose includes the relative position (e.g., x, y, z) and relative orientation (e.g., pitch, roll, yaw angles) of the assembly and object. The pose of the assembly may be changed by moving the assembly while holding the object still or by moving the object while holding the assembly still. The relative movement between the assembly and object may be obtained by attaching one of the object and assembly to a mechanical movement structure such as a robot, an articulated arm coordinate measurement machine (CMM), a Cartesian CMM, or a machine tool. In an embodiment, the mechanical movement structure includes a means of determining the relative pose of the assembly and object. Such an means might be provided for example by encoders (scales), distance meters, or inertial measurement units such as accelerometers, gyroscopes, magnetometers, and elevation sensors. Such indicators may provide only approximate values for the pose, with more precise measurements made to the assembly and processor.
Changing the pose of an object may be useful in overcoming several potential error sources in the collecting of 3D coordinates. One type of error that may be overcome by changing the pose of an object is multipath interference, which is caused when light illuminated by the projector reflects onto another point on the object, the other point being visible to the camera. By changing the direction, the angle of incidence of the projected light on the object surface is changed, thereby causing the reflected light to be redirected, possibly missing the surface and eliminating multipath interference as a result.
Changing the pose of an object may also be useful in overcoming problems resulting from extremely low or extremely high light levels. If light from the scanner strikes a steeply angled surface, in other words, if the light striking a surface has a large angle of incidence (has a large angle relative to the surface normal), the light reflected (scattered) from the surface into the camera is likely to be small. In some instances, the amount of light captured by the camera for a point on the object can be dramatically increased by changing the pose of the assembly relative to the object surface. Changing the pose of an object may also help overcome problems with glints, which are high levels of light specularly reflected off an object surface into the camera. Glints occur only at particular angles and usually only over small regions. Changing the pose of the assembly in relation to the object can eliminate glints.
Changing the pose of an object may also be useful in improving resolution and accuracy. For example, to measure the diameter of a hole, it is desired to view the hole at near-normal incidence as this provides a relatively large circular region to be measured. On the other hand, to measure the cylindricity (form error) of the side walls (bore) of a cylindrical hole, it is necessary to direct the light at a slight angle to the side walls, taking several measurements with the pose changed slightly in each case. Changing the pose may also be useful in improving the resolution by bringing the object features into focus on the camera. If the camera is too close to or too far away from the object surface, the modulation transfer function (MTF) of the measured features will be reduces, thereby causing a reduction in resolution.
In each of the methods described herein with respect to step 5050, the chosen actions are based on a selection by the processor without additional assistance on the part of an operator or observer. The effect of the pose on each of the effects described above may be automatically evaluated by a processor. The method of using the shape of an object in relation to the pose of a scanner to predict the possibility of multipath interference was discussed hereinabove. Basically the method used is to project a ray of light from the projector onto the surface having a known surface profile (from the measurement carried out in step 5020) and to use geometrical optics to determine whether the reflected ray of light will strike the object at a position in view of the camera. When a multipath interference problem has been identified by the processor, the processor may then carry out a computational procedure in which the potential for multipath interference is evaluated with the multiple possible poses being considered.
Changing the illumination emitted by the projector may be useful in overcoming several potential error sources in the collecting of 3D coordinates. Here illumination means the level of optical power emitted by the projector in combination with the exposure time of the camera and the aperture of the camera. In other words, the illumination level in a pixel well is determined by the number of electrons produced in a pixel well. The illumination level is near at a desired level if the number of electrons is relatively large but still in the linear region of the pixel response. If the optical power reaching a point on the object surface is too small or the camera exposure too short, the light received by a camera pixel will be low and the resulting signal to noise ratio of the pixel poor, thereby resulting in low accuracy of the measured 3D coordinates. Alternatively, if the optical power reaching a point on the object surface is too high for the exposure, the light received by the camera may be outside the linear region of the camera, for example, overfilling the corresponding pixel well. This too may cause errors in the measured 3D coordinates.
Problems in matching optical power emitted by the projector to the surface being measured can arise in several ways. In a first instance, the object surface being measured may be composed of several different materials or colors, some of the materials or colors having high reflectance and others having low reflectance. In a second instance, the object surface may be steeply inclined in some locations and may have low angles of incidence at other locations, thereby resulting in differing amounts of reflected (scattered) light. Some parts of the surface may be adjacent to edges—for example, the sides of holes or the sides adjacent to straight edges. Such regions often reflect relatively little light.
There are two main relatively simple ways of eliminating or reducing errors caused by incorrect illumination. In the method 5000, the steps to be taken are automatically determined by a processor without the assistance of an operator or observer. In a first method, a region of the surface under test is illuminated multiple times by the projector, and a camera image obtained for each illumination. The 3D coordinates of each point on the object surface is determined based on one or more of the captured images. For a given point, images having light levels too low or too high are thrown out. The decision of whether to keep or throw out a particular image is based on the electrical readings obtained from the corresponding pixel of the photosensitive array. In some cases, the processor may elect to use more than one of the images to obtain 3D coordinates for a given point on the object surface.
In a second method of eliminating or reducing errors caused by incorrect illumination, light is projected onto the object a single time but the optical power level is varied over the region of the projected light. In some cases, such as projection of a coded, single-shot pattern, this method may be relatively simple to implement. Similarly the method may be relatively easy to implement with stripes of light or swept points of light. The method may be more difficult to implement with sequential methods such as projection of spatially modulated light having shifts in phase between successive illuminations. However, even in these cases, there is usually some possibility for varying the illumination level to improve accuracy. The processor evaluates the possibilities to determine desired levels of illumination, desired number of exposures, and desired exposure times.
Changing the pattern of light emitted by the projector may be useful in overcoming several potential error sources in the collecting of 3D coordinates. In a first case, it may be important to determine the accuracy of a feature to a relatively high accuracy. In this case, the processor may decide to switch from a first projected pattern being a coded, single shot pattern that provides relatively low accuracy to a multi-shot sequential pattern, such as a phase-shifted pattern, that is calculated on a point-by-point basis to higher accuracy. Similarly in most cases using a sequential pattern of light rather than a coded, single-shot pattern of light will provide higher resolution because a sequential pattern of light is evaluated on a point-by-point basis while the resolution of a coded pattern of light depends on the size of the individual elements of the coded pattern.
In a second case, a distortion in the shape of the object may indicate that multipath interference is a problem. This diagnosis may be confirmed by the processor by carrying out a ray tracing procedure as described hereinabove. In this case, the scanner may switch from a structured light pattern projected over an area to a line of light pattern. In this case, it may also be advantageous to project the line of light in a desired direction to reduce or minimize the effects of multipath interference. In this case, the processor will direct the assembly to change its pose as appropriate to project the line in the desired direction. In another embodiment, a swept point or spot of light is projected, which reduces the possibility for multipath interference.
Measuring the feature with a mechanical (tactile) probe is a method that can in most cases be used when optical scanning methods do not provide desired results. In many embodiments, use of such a probe involved operator assistance under direction of the processor. A mechanical probe has the ability to measure holes, edges, or other regions that difficult to view with a scanner. This method enables measurement of areas having low reflectance and areas susceptible to multi-path interference.
In an embodiment discussed hereinabove with reference to FIGS. 8A, 8B, the projector within the scanner illuminates a region to be measured with a mechanical probe, the region being determined by the processor. The operator then holds a probe tip 166 in contact with the object surface. A light source, which may be a projector within the assembly, or a disconnected light source (which need not project a pattern) can be used to illuminate reflective targets on the mechanical probe.
In another embodiment, the mechanical probe may contain points of light such as LEDs to provide illumination. The 3D points of light may be determined using two cameras on an assembly (stereo vision) or a single camera may be used with distance determined based on the relative scale of separation of the illuminated spots.
In a step 5050, the projector within the assembly transmits a second pattern of light, possibly different than the first pattern of light, possibly with the assembly at a different pose, possibly with different illumination, or possibly by capturing light at known spots on a mechanical (tactile) probe.
In another embodiment, an assembly is provided with a second camera in addition to the first camera. In this embodiment, the combination of the first projector and first camera has a different field of view (and different baseline distance) than the combination of the first projector and the second camera. In this case, the step 5050 of FIG. 13 is modified to provide one additional possible action, namely to change the illuminated field-of-view. The surface region of the object measured by a scanner is determined by the overlap of the projected light pattern and the angular field-of-view of the camera. Usually as a camera and projector are moved closer together, a relatively smaller area is both illuminated and viewed by the camera, therefore reducing the illuminated field-of-view. By choosing two cameras, each having about the same number of pixels, but one having a much smaller illuminated field-of-view than the other, the camera with the smaller illuminated field-of-view will likely have higher resolution and accuracy and will in addition be less likely to encounter multipath interference.
In another embodiment, an assembly is provided with a second projector and a second camera in addition to a first camera and a first projector. Such an arrangement has advantages similar to that of the embodiment containing two cameras and a single projector, and these will not be repeated here.
Technical effects and benefits of embodiments of the present invention include the detection and correction of errors in measurements made by an optical coordinate measurement device.
Aspects of the present invention are described herein with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer readable program instructions.
These computer readable program instructions may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. These computer readable program instructions may also be stored in a computer readable storage medium that can direct a computer, a programmable data processing apparatus, and/or other devices to function in a particular manner, such that the computer readable storage medium having instructions stored therein comprises an article of manufacture including instructions which implement aspects of the function/act specified in the flowchart and/or block diagram block or blocks.
The computer readable program instructions may also be loaded onto a computer, other programmable data processing apparatus, or other device to cause a series of operational steps to be performed on the computer, other programmable apparatus or other device to produce a computer implemented process, such that the instructions which execute on the computer, other programmable apparatus, or other device implement the functions/acts specified in the flowchart and/or block diagram block or blocks.
The flowchart and block diagrams in the Figures illustrate the architecture, functionality, and operation of possible implementations of systems, methods, and computer program products according to various embodiments of the present invention. In this regard, each block in the flowchart or block diagrams may represent a module, segment, or portion of instructions, which comprises one or more executable instructions for implementing the specified logical function(s). In some alternative implementations, the functions noted in the block may occur out of the order noted in the figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the block diagrams and/or flowchart illustration, and combinations of blocks in the block diagrams and/or flowchart illustration, can be implemented by special purpose hardware-based systems that perform the specified functions or acts or carry out combinations of special purpose hardware and computer instructions.
While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.

## Claims

1. A method of determining three-dimensional coordinates of points on a surface of an object, the method comprising:
providing an assembly that includes a first projector and a first camera, the first projector and the first camera being fixed in relation to one another, there being a baseline distance between the first projector and the first camera, the first projector having a light source, the first camera having a lens and a photosensitive array;
providing a processor electrically coupled to the first projector and the first camera;
providing a mathematical representation of a shape of a feature on the surface;
providing a value for an acceptable goodness of fit;
sending a first transmitted light from the first projector onto the object;
acquiring by the first camera a first reflected light and sending a first signal to the processor in response, the first reflected light being a portion of the first transmitted light reflected from the surface;
determining by the processor a first measured set of three-dimensional (3D) coordinates of first points on the surface, the first measured set based at least in part on the first transmitted light, the first signal and the baseline distance;
determining by the processor a first measured subset of points, the first measured subset of points being a subset of the first points on the surface, the first measured subset of points being measured points corresponding to the feature;
fitting by the processor 3D coordinates of the first measured subset of points to the provided mathematical representation of the shape of the feature, the fitting including comparing the 3D coordinates of the first measured subset of points to 3D coordinates of a first derived subset of points to obtain a collection of residual errors, the first derived subset of points being a collection of points lying on the shape of the feature, each of the residual errors from the collection of residual errors being a measure of a separation of corresponding 3D coordinates from the first measured subset and the first derived subset, the fitting further consisting of mathematically adjusting position and orientation of the shape to minimize the collection of residual errors according to a minimization rule;
determining by the processor a measured goodness of fit, the measured goodness of fit being a mathematically derived quantity obtained from the collection of residual errors;
determining by the processor whether the 3D coordinates of the first measured subset of points are acceptable based at least in part on a comparison of the measured goodness of fit to the acceptable goodness of fit;
determining by the processor whether the first measured set of 3D coordinates is acceptable based at least in part on whether the 3D coordinates of the first measured subset of points are acceptable;
if the first measured set of 3D coordinates is acceptable, storing the first measured set of 3D coordinates;
if the first measured set of 3D coordinates is not acceptable, taking steps (a)-(e):
(a) selecting by the processor at least one action to be taken and taking the action, the at least one action selected from the group consisting of: changing a pose of the assembly, changing an illumination level of the light source, changing a pattern of the transmitted light, and measuring the feature by illuminating a mechanical probe and imaging spots of light on the probe with the first camera;
(b) sending a second transmitted light from a first projector onto the object or illuminating spots of light on the mechanical probe held in contact with the object;
(c) acquiring by the first camera a second reflected light and sending a second signal to the processor in response, the second reflected light being a portion of the second transmitted light reflected from the surface or the mechanical probe;
(d) determining by the processor a second measured set of 3D coordinates of second points on the surface, the second measured set of 3D coordinates based at least in part on the second transmitted light, the second signal and the baseline distance; and
(e) storing the second measured set of 3D coordinates.

2. The method of claim 1 wherein, in the step of providing a mathematical representation of a feature on the surface, the feature is selected from the group consisting of: a circle, a cylindrical hole, a raised cylinder, a sphere, an intersection of two planes, and an intersection of three planes.

3. The method of claim 1 wherein, in the step of determining by the processor a measured goodness of fit, the measured goodness of fit is based on a chi-square test statistic.

4. The method of claim 1 wherein, in the step of fitting by the processor 3D coordinates of the first measured subset of points, the minimization rule is to minimize the sum of the square of each residual error from among the collection of residual errors.

5. The method of claim 1 wherein, in the step of selecting by the processor at least one action, changing the pose of the assembly includes changing the relative positions of the assembly and the object.

6. The method of claim 1 wherein, in the step of selecting by the processor at least one action, changing the pose of the assembly includes changing the relative orientations of the assembly and the object.

7. The method of claim 1 further including a step of providing a movable structure, the movable structure configured for attachment to the apparatus or to the object.

8. The method of claim 1 wherein, in the step of selecting by the processor at least one action, the changing an illumination level of the light source includes increasing or decreasing the emitted light multiple times to obtain multiple images with the first camera.

9. The method of claim 8 wherein, in the step of selecting by the processor at least one action, the multiple images are combined to obtain a high dynamic range image.

10. The method of claim 1 wherein, in the step of selecting by the processor at least one action, the changing an illumination level of the light source includes changing the illumination levels non-uniformly over a projected region.

11. The method of claim 1 wherein, in the step of selecting by the processor at least one action, the changing a pattern of the transmitted light includes changing from a structured pattern of light projected over an area to a stripe of light projected in a line, the stripe of light being projected in a plane perpendicular to the baseline.

12. The method of claim 11 wherein, in the step of selecting by the processor at least one action, the orientation of the apparatus is changed to change the direction of the stripe of light on the object.

13. The method of claim 1 wherein, in the step of selecting by the processor at least one action, the changing a pattern of the transmitted light includes changing from a coded, single-shot pattern of structured light to an uncoded, sequential pattern of structured light, the single-shot pattern of structured light enabling the first measured set of 3D coordinates using a single pattern, the sequential pattern of light enabling determining by the processor of a second measured set of 3D coordinates.

14. The method of claim 1 wherein, in the changing a pattern of the transmitted light in step (a) and the sending a second transmitted light in step (b), the second transmitted light is a spot of light swept over the surface of the object.

15. The method of claim 1 wherein the measuring the feature by illuminating a mechanical probe in step (a) further includes transmitting an indicator light with the first projector, the indicator light illuminating a region of the object to be brought into contact with the probe tip.

16. The method of claim 1 wherein, in the step of selecting by the processor at least one action, three actions are selected including changing a pose of the assembly, changing an illumination level of the light source, and changing a pattern of the transmitted light.

17. A method of determining three-dimensional coordinates of points on a surface of an object, the method comprising:
providing an assembly that includes a first projector, a first camera, and a second camera, the first projector, the first camera, and the second camera being fixed in relation to one another, there being a first baseline distance between the first projector and the first camera, there being a second baseline distance between the first projector and the second camera, the first projector having a light source, the first camera having a first lens and a first photosensitive array, the second camera having a second lens and a second photosensitive array;
providing a processor electrically coupled to the first projector, the first camera, and the second camera;
providing a mathematical representation of a shape of a feature on the surface;
providing a value for an acceptable goodness of fit;
sending a first transmitted light from the first projector onto the object;
acquiring by the first camera a first reflected light and sending a first signal to the processor in response, the first reflected light being a portion of the first transmitted light reflected from the surface;
determining by the processor a first measured set of three-dimensional (3D) coordinates of first points on the surface, the first measured set based at least in part on the first transmitted light, the first signal and the first baseline distance;
determining by the processor a first measured subset of points, the first measured subset of points being a subset of the first points on the surface, the first measured subset of points being measured points corresponding to the feature;
fitting by the processor 3D coordinates of the first measured subset of points to the provided mathematical representation of the shape of the feature, the fitting including comparing the 3D coordinates of the first measured subset of points to 3D coordinates of a first derived subset of points to obtain a collection of residual errors, the first derived subset of points being a collection of points lying on the shape of the feature, each of the residual errors from the collection of residual errors being a measure of a separation of corresponding 3D coordinates from the first measured subset and the first derived subset, the fitting further consisting of mathematically adjusting position and orientation of the shape to minimize the collection of residual errors according to a minimization rule;
determining by the processor a measured goodness of fit, the measured goodness of fit being a mathematically derived quantity obtained from the collection of residual errors;
determining by the processor whether the 3D coordinates of the first measured subset of points are acceptable based at least in part on a comparison of the measured goodness of fit to the acceptable goodness of fit;
determining by the processor whether the first measured set of 3D coordinates is acceptable based at least in part on whether the 3D coordinates of the first measured subset of points are acceptable;
if the first measured set of 3D coordinates is acceptable, storing the first measured set of 3D coordinates;
if the first measured set of 3D coordinates is not acceptable, taking steps (a)-(e):
(a) selecting by the processor at least one action to be taken and taking the action, the at least one action selected from the group consisting of: changing an illuminated field-of-view of the assembly, changing a pose of the assembly, changing an illumination level of the light source, changing a pattern of the transmitted light, and measuring the feature by illuminating a mechanical probe and imaging spots of light on the probe with the first camera;
(b) sending a second transmitted light from the first projector onto the object or illuminating spots of light on the mechanical probe held in contact with the object;
(c) acquiring by an imager a second reflected light and sending a second signal to the processor in response, the second reflected light being a portion of the second transmitted light reflected from the surface or the mechanical probe, the imager being the first camera if the at least one action does not include changing an illuminated field-of-view of the assembly, the imager being the second camera if the at least one action includes changing an illuminated field-of-view of the assembly;
(d) determining by the processor a second measured set of 3D coordinates of second points on the surface, the second measured set of 3D coordinates based at least in part on the second transmitted light and the second signal; and
(e) storing the second measured set of 3D coordinates.

18. A method of determining three-dimensional coordinates of points on a surface of an object, the method comprising:
providing an assembly that includes a first projector, a first camera, a second projector, and a second camera, the first projector, the first camera, the second projector, and the second camera being fixed in relation to one another, there being a first baseline distance between the first projector and the first camera, there being a second baseline distance between the second projector and the second camera, the first projector having a first light source, the first camera having a first lens and a first photosensitive array, the second projector having a second light source, the second camera having a second lens and a second photosensitive array;
providing a processor electrically coupled to the first projector, the first camera, the second projector, and the second camera;
providing a mathematical representation of a shape of a feature on the surface;
providing a value for an acceptable goodness of fit;
sending a first transmitted light from the first projector onto the object;
acquiring by the first camera a first reflected light and sending a first signal to the processor in response, the first reflected light being a portion of the first transmitted light reflected from the surface;
determining by the processor a first measured set of three-dimensional (3D) coordinates of first points on the surface, the first measured set based at least in part on the first transmitted light, the first signal and the first baseline distance;
determining by the processor a first measured subset of points, the first measured subset of points being a subset of the first points on the surface, the first measured subset of points being measured points corresponding to the feature;
fitting by the processor 3D coordinates of the first measured subset of points to the provided mathematical representation of the shape of the feature, the fitting including comparing the 3D coordinates of the first measured subset of points to 3D coordinates of a first derived subset of points to obtain a collection of residual errors, the first derived subset of points being a collection of points lying on the shape of the feature, the residual errors being a measure of the separation of corresponding 3D coordinates of the first measured subset and the first derived subset, the fitting further consisting of mathematically adjusting a pose of the shape to minimize the residual errors according to a minimization rule;
determining by the processor a measured goodness of fit, the measured goodness of fit being a mathematically derived quantity obtained from the collection of residual errors;
determining by the processor whether the first set is acceptable based on a comparison of the measured goodness of fit to the acceptable goodness of fit;
if the first set for the measured feature is acceptable, storing the first set of 3D coordinates;
if the first set for the measured feature is not acceptable, taking steps (a)-(e):
(a) selecting by the processor at least one action to be taken and taking the action, the at least one action selected from the group consisting of: changing an illuminated field-of-view of the assembly, changing a pose of the assembly, changing an illumination level of the light source, changing a pattern of the transmitted light, and measuring the feature by illuminating a mechanical probe and imaging spots of light on the probe with the first camera;
(b) sending a second transmitted light from a light transmitter onto the object or illuminating spots of light on the mechanical probe held in contact with the object, the light transmitter being the first projector if the at least one action does not include changing an illuminated field-of-view of the assembly, the light transmitter being the second projector if the at least one action includes changing an illuminated field-of-view of the assembly;
(c) acquiring by an imager a second reflected light and sending a second signal to the processor in response, the second reflected light being a portion of the second transmitted light reflected from the surface or the mechanical probe, the imager being the first camera if the at least one action does not include changing an illuminated field-of-view of the assembly, the imager being the second camera if the at least one action includes changing an illuminated field-of-view of the assembly;
(d) determining by the processor a second measured set of 3D coordinates of second points on the surface, the second measured set of 3D coordinates based at least in part on the second transmitted light and the second signal; and
(e) storing the second measured set of 3D coordinates.