Patent Application
20140192187
The present disclosure relates to a coordinate measuring machine and, more particularly, to a non-contact measurement device of a portable articulated coordinate measuring machine.
Portable articulated arm coordinate measuring machines (AACMMs) have found widespread use in the manufacturing or production of parts where there is a need to rapidly and accurately verify the dimensions of the part during various stages of the manufacturing (e.g. machining) or production of the part. Portable AACMMs represent a vast improvement over known stationary or fixed, cost-intensive, and relatively difficult to use measurement installations, particularly in the amount of time it takes to perform dimensional measurements of relatively complex parts. Typically a user of a portable AACMM simply guides a probe along the surface of the part or object to be measured. The measurement data are then recorded and provided to the user. In some cases, the data are provided to the user in visual form, for example, three dimensional (3-D) form on a computer screen. Alternatively, the data may be provided to the user in numeric form, for example, when measuring the diameter of a hole, the text “Diameter=” is displayed on a computer screen.
Three-dimensional surfaces may be measured using non-contact techniques as well. One type of non-contact device, sometimes referred to as a laser line probe or laser line scanner, emits a laser light either on a spot, or along a line. An imaging device, such as a charge-coupled device (CCD) for example, is positioned adjacent the laser. The laser is arranged to emit a line of light which is reflected off of the surface. The surface of the object being measured causes a diffuse reflection which is captured by the imaging device. The image of the reflected line on the sensor will change as the distance between the sensor and the surface changes. By knowing the relationship between the imaging sensor and the laser and the position of the laser image on the sensor, triangulation methods may be used to measure three-dimensional coordinates of points on the surface. One issue that arises with laser line probes, is that the density of measured points may vary depending on the speed at which the laser line probe is moved across the surface of the object. The faster the laser line probe is moved, the greater the distance between the points and a lower point density. With a structured light scanner, the point spacing is typically uniform in each of the two dimensions, thereby generally providing uniform measurement of workpiece surface points.
The amount of data produced by a non-contact device is determined by the pixel resolution and the frame rate of the imaging device. It is desirable to scan at fast frame rates with high resolution cameras, because this reduces the amount of time required to accurately perform a part scan. However, the amount of information capable of being transmitted from the camera to a processing device is limited by the data transfer rates of current communication technology.
According to one embodiment of the invention, a portable coordinate measuring machine is provided for measuring three-dimensional coordinates of an object in space. The coordinate measuring machine includes a manually positionable articulated arm portion having opposed first and second ends, the arm portion including a plurality of connected arm segments, each arm segment including at least one position transducer for producing a position signal; a base section connected to the second end; a probe assembly connected to the first end, the probe assembly including a housing, a first processor, a projector, and a camera: the first processor disposed within the housing; the projector disposed within the housing, the projector having a light source configured to emit a first light onto a surface of the object, the projector being configured to project the first light to form a line on a plane arranged perpendicular to the direction of propagation of the first light; the camera arranged within the housing, the camera including a lens and an image sensor, the image sensor having an array of pixels on a sensor plane, the lens configured to receive a second light and to image the second light onto the image sensor, the second light being a reflection of the first light from the surface, the image sensor further configured to send a first electrical signal to the first processor in response to receiving the second light, the first processor coupled to the image sensor and configured to determine a plurality of centroids based at least in part on the first electrical signal, there being a baseline distance between the projector and the camera; and a second processor, external to the housing, configured to receive the position signals from the transducers and to receive the plurality of centroids from the first processor, the second processor further configured to determine and store, or transmit to an external device, the three-dimensional coordinates a plurality of points on the surface, the three-dimensional coordinates based at least part on the position signals, the received centroid data, and the baseline distance.
According to one embodiment of the invention, a method is provided for determining three-dimensional coordinates of points on a surface on an object. The method includes providing a device that includes a manually positionable articulated arm portion, a base section, a probe assembly, and a second processor, the arm portion having opposed first and second ends, the arm portion including a plurality of connected arm segments, each arm segment including at least one position transducer for producing a position signal, the base section connected to the second end, the probe assembly connected to the first end, the probe assembly including a housing, a first processor, a projector, and a camera, the projector disposed within the housing, the projector having a light source configured to emit a first light onto a surface of the object, the projector being configured to project the first light to form a line on a plan arranged perpendicular to the direction of propagation of the light, the camera arranged within the housing, the camera including a lens and an image sensor, the image sensor having an array of pixels on a sensor plane, the first processor coupled to the image sensor, there being a baseline distance between the projector and the camera, the second processor external to the housing; emitting from the projector the first light onto the surface; receiving with the lens a second light, the second light being a reflection of the first light from the surface; imaging with the lens the second light onto the sensor plane and, in response, sending a first electrical signal to the first processor; determining with the first processor a plurality of centroids of the points on the surface, the plurality of centroids based at least in part on the first electrical signal; receiving with the second processor the plurality of centroids; determining with the second processor the three-dimensional coordinates of the points on the surface based at least in part on the position signals, the plurality of centroids, and the baseline distance; and storing the three-dimensional coordinates of the points on the surface.
Referring now to the drawings, exemplary embodiments are shown which should not be construed to be limiting regarding the entire scope of the disclosure, and wherein the elements are numbered alike in several FIGURES:
Laser scanners and laser line probes (LLP) are used in a variety of applications to determine surface point coordinates and a computer image of an object. Embodiments of the present invention provide advantages in improving the resolution and accuracy of the measurements. Embodiments of the present invention provide still further advantages in providing the non-contact measurement of an object. Embodiments of the present invention provide advantages in reducing the calculation time for determining coordinates values for surface points.
As used herein, the term “structured light” refers to a two-dimensional pattern of light projected onto a continuous and enclosed area of an object that conveys information which may be used to determine coordinates of points on the object. A structured light pattern will contain at least three non-collinear pattern elements disposed within the contiguous and enclosed area. Each of the three non-collinear pattern elements conveys information which may be used to determine the point coordinates.
In general, there are two types of structured light, 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 may be ascertained by the acquisition of a single image. In some cases, the projecting device may be moving relative to the object. In other words, for a coded light pattern there will be no significant temporal relationship between the projected pattern and the acquired image. Typically, a coded light pattern will contain a set of elements (e.g. geometric shapes) arranged so that at least three of the elements are non-collinear. In some cases, the set of elements may be arranged into collections of lines. Having at least three of the elements be non-collinear ensures that the pattern is not a simple line pattern as would be projected, for example, by an LLP. As a result, the pattern elements are recognizable because of the arrangement of the elements.
In contrast, an uncoded structured light pattern as used herein is a pattern that does not allow measurement through a single pattern when the projector is moving relative to the object. An example of an uncoded light pattern is one which utilizes a series of sequential patterns and thus the acquisition of a series of sequential images. Due to the temporal nature of the projection pattern and acquisition of the image, there should be no relative movement between the projector and the object.
It should be appreciated that structured light is different from light projected by a LLP or similar type of device that generates a line of light. To the extent that LLPs used with articulated arms today have irregularities or other aspects that may be regarded as features within the generated lines, these features are disposed in a collinear arrangement. Consequently such features within a single generated line are not considered to make the projected light into structured light.
Arranged within the housing 22 of the non-contact measurement device 20 is a pair of optical devices, such as a projector 40 and a camera 50 (
The camera 50 includes a photosensitive or image sensor 52 (
As illustrated schematically in
It should be appreciated that while embodiments herein refer to the device 20 as being a handheld device, this is for exemplary purposes and the claimed invention should not be so limited. In other embodiments, the non-contact measurement device 20 may be mounted to a fixture, such as a robot for example. In other embodiments, the device 20 may be stationary and the object being measured may move relative to the device, such as in a manufacturing inspection process or with a game controller for example.
In one embodiment, the external device 70 operably coupled to the controller 60 is a portable articulate arm coordinate measuring machine (AACMM), as illustrated in
The measurement probe 102 housing includes a detachably mounted handle, connected to the housing 102 by way of, for example, a quick connect interface. The handle may be replaced with another attachment, such as a bar code reader or paint sprayer for example to provide additional functionality to the AACMM. In one embodiment, the non-contact measurement device 20 is configured to couple to the probe housing 102 in place of the handle, such as with a quick connect interface for example.
The base 116 may include an attachment device or mounting device 120. The mounting device 120 allows the AACMM 100 to be removably mounted to a desired location, such as an inspection table, a machining center, a wall or the floor for example. In one embodiment, the base 116 includes a handle portion 122 that provides a convenient location for the operator to hold the base 116 as the AACMM 100 is being moved. In one embodiment, the base 116 further includes a movable cover portion 124 that folds down to reveal a user interface, such as a display screen for example. The base 116 of the portable AACMM 100 generally contains or houses an electronic data processing system that includes two primary components: a base processing system that processes the data from the various encoder systems within the AACMM 100 as well as data representing other arm parameters to support three-dimensional (3-D) positional calculations; and a user interface processing system that includes an on-board operating system, a touch screen display, and resident application software that allows for relatively complete metrology functions to be implemented within the AACMM 100 without the need for connection to an external computer. It should be appreciated that coupling the device 20 to the probe housing 102 provides advantages in that the position and orientation of the device 20 is known by the electronic data processing system 210, so that the location of the object 10 relative to the AACMM 100 may also be ascertained. In one embodiment, the external device 70 is integrated into the electronic data processing system contained in the AACMM 100.
Referring again to the non-contact measurement device 20 of
Referring now to
The projector 40 projects a line 500 (shown in the FIG. as projecting out of the plane of the paper) onto the surface 12 of an object 10, which may be located at a first position 502 or a second position 504. The line of light emitted by the projector 40 is defined by the line formed on a plane arranged generally perpendicular to the direction of propagation of the light. Light scattered from the object 10 at the first point 506 travels through a perspective center 55 of the lens system 54 to arrive at the photosensitive array of pixels 53 at position 510. Light scattered from the object 10 at the second position 508 travels through the perspective center 55 to arrive at position 512. By knowing the relative positions and orientations of the projector 40, the camera lens system 54, the photosensitive array 53, and the position 510 on the photosensitive array 53, it is possible to calculate the three dimensional coordinates of the point 506 on the object surface 12. Similarly, knowledge of the relative position of the point 512, rather than 510 will yield the three dimensional coordinate of the point 508. The photosensitive array 53 may be tilted at an angle to satisfy the Scheimpflug principle, thereby helping to keep the line of light on the object surface in focus on the array.
One of the calculations described herein above yields information about the distance of the object 10 from the measurement device 20, in other words, the distance in the z direction, as indicated by the coordinate system 520 of
Each image captured by the camera 50 depicts a laser line constituting pixels on the array 53 of sensor 52 at which the light ray 514, 516 is detected. A one to one correspondence exists between pixels of the emitted light beam 500 and pixels in the imaged laser line 514, 516. The points in the imaged laser line 514, 516 are located in the plane 51 of the sensor 52 and are used to determine corresponding points of the emitted light beam 500 based on calibration data for the non-contact measurement device 20. For example, the photosensitive sensor 52 (
The captured images are then processed by the processor 55 coupled to the sensor 52. Each image is used to determine the location of the measured object 10 with sub-pixel accuracy. This is possible because the profile across the laser line approximates a Gaussian function and extends across multiple rows of pixels on the photosensitive sensor 52 image plane. The processor 55 further analyzes the profile of the imaged laser line to determine a center of gravity (COG) thereof, which is the point that best represents the exact location of the line. In one embodiment, the processor 55 determines a COG for each column of pixels in the array 53. The COG is a weighted average calculated based on the intensity of light measured at each pixel in the imaged laser line. Consequently, pixels having a higher intensity are given more weight in the COG calculation because the emitted light beams 500, and therefore the imaged laser line, are brightest at a center. If the light ray 514, 516 reflected from the surface 12 of the object 10 towards the camera 50 does not have enough light, the processor 55 will not be able to calculate a COG from the imaged laser line. Similarly, if the image is overexposed, thereby including an excess of in-band light, the processor 55 will not be able to calculate a COG from the imaged laser line.
Once the image is processed, the image is discarded and the processed COG data is transferred to the controller processer 62 where the three dimensional coordinates are calculated. It should be appreciated that the communication bus 59 between the processor 55 coupled to the sensor 52 and the controller 60 has a limited bandwidth. It should further be appreciated that by determining the COG in the processor 55, advantages in processing speed are gained over prior art systems which transferred the acquired images (e.g. large data volume) to the controller 60.
Referring now to
The device first emits a structure light pattern with projector 40 having a projector plane which projects the pattern through a center 84 of the lens 46 and onto surface 12 of the object 10. The light from the pattern is reflected from the surface 12 and the reflected light 82 is received through the center 86 of lens 54 by a photosensitive array 53 of sensor 52 in the camera 50. Since the pattern is formed by structured light, it is possible in some instances for the processor 55 to determine a one to one correspondence between the pixels in the emitted pattern 80, such as pixel 88 for example, and the pixels in the imaged pattern, such as pixel 90 for example. This correspondence enables the processor 62 to use triangulation principals to determine the coordinates of each pixel in the imaged pattern. The collection of three-dimensional coordinates of points on the surface 12 is sometimes referred to as a point cloud. By moving the scanner 20 over the surface 12 (or moving the surface 12 past the scanner 20), a point cloud may be created of the entire object 10.
For each of the elements in the structured light pattern, at least one centroid or COG is determined. Similar to as described above, with reference to the laser light probe (LLP), the centroid values are calculated by the first processor 55 directly coupled to the sensor array 53. These centroid/COG values, rather than the images, are then transmitted via a wired or wireless bus 59 to the controller 60 where a second processor 62 determines the three-dimensional coordinates. However, if the pattern reflected from the surface 12 of the object 10 towards the camera 50 does not have enough light, the processor 55 will not be able to calculate a centroid from the imaged structured light pattern. Similarly, if the image is overexposed and includes an excess of in-band light, the processor 55 will not be able to calculate a centroid from the imaged structured light pattern.
The processor 62 decodes the centroids of the acquired image to determine the three-dimensional coordinates of the object 10. To determine the coordinates of a centroid, the angle of each projected ray of light 80 intersecting the object 10 at a point 75 is known to correspond to a projection angle phi (Φ), so that Φ information is encoded into the emitted pattern. In an embodiment, the system is configured to enable the Φ value corresponding to each pixel in the imaged pattern to be ascertained. Further, an angle omega (Ω) for each pixel in the camera 50 is known, as is the baseline distance “D” between the projector 40 and the camera 50. Since the two angles Ω, Φ and the distance D between the projector 40 and camera 50 are known, the distance Z to point 75 on the object 10 may be determined. With the distance Z known, the three-dimensional coordinates may be calculated for each surface point in the acquired image.
By including center of gravity/centroid processing functionality within the processor 55 of the sensor 52, the overall efficiency of the non-contact measurement device 20 is improved. Only processed center of gravity or centroid data, and not the acquired image, will be transmitted to the second processor 62 of controller 60. Because center of gravity or centroid data is much smaller and less complex than an image, the size and therefore the amount of time required to transmit the processed data to the controller over a conventional communication bus 59 is significantly reduced.
Embodiments of the LLP 500 have been described herein as being included within an accessory device or as an attachment to a portable AACMM 100. However, this is for exemplary purposes and the claimed invention should not be so limited. Other embodiments of the LLP 500 are contemplated by the present invention, in light of the teachings herein. For example, the LLP may be utilized in a fixed or non-articulated arm (i.e., non-moving) CMM. Other fixed inspection installations are contemplated as well. For example, a number of such LLPs 500 may be strategically placed in fixed locations for inspection or measurement purposes along some type of assembly or production line; for example, for automobiles.
While the invention has been described with reference to example embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. Moreover, the use of the terms first, second, etc. do not denote any order or importance, but rather the terms first, second, etc. are used to distinguish one element from another. Furthermore, the use of the terms a, an, etc. do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item.
This application claims the benefit of U.S. provisional patent application Ser. No. 61/750,124 filed Jan. 8, 2013, the entire contents of which are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61750124 | Jan 2013 | US |
