Source: https://patents.google.com/patent/US8792707
Timestamp: 2018-06-23 13:59:06
Document Index: 701761529

Matched Legal Cases: ['Application No. 2010', 'Application No. 2010', 'Application No. 2010', 'Application No. 201080040329', 'Application No. 08', 'Application No. 200880111247', 'Application No. 200880112194', 'Application No. 2010', 'Application No. 2012', 'Application No. 12', 'Application No. 2010', 'Application No. 2010']

US8792707B2 - Phase analysis measurement apparatus and method - Google Patents
Phase analysis measurement apparatus and method Download PDF
US8792707B2
US8792707B2 US12733022 US73302208A US8792707B2 US 8792707 B2 US8792707 B2 US 8792707B2 US 12733022 US12733022 US 12733022 US 73302208 A US73302208 A US 73302208A US 8792707 B2 US8792707 B2 US 8792707B2
US12733022
US20100158322A1 (en )
Timothy Charles Featherstone
A non-contact method and apparatus for inspecting an object via phase analysis. A projector projects an optical pattern onto the surface of an object to be inspected. At least first and second images of the surface on which the optical pattern is projected are then obtained. The phase of the optical pattern at the surface is changed between the first and second image by moving the projector relative to the object.
This invention relates to a method and apparatus for measuring an object without contacting the object and in particular to a method and apparatus in which an object's surface topography can be determined by analysing the phase of an optical pattern projected on an object.
Non-contact optical measuring systems are known for measuring the topography of a surface via phase analysis of a periodic optical pattern on an object. These may typically consist of a projector which projects a structured light pattern onto a surface and a camera, set at an angle to the projector, which detects the structured light pattern on the surface. Height variation on the surface causes a distortion in the pattern. From this distortion the geometry of the surface can be calculated. Such systems are commonly known as structured light analysis, phase profilometry, phase-shift analysis or fringe analysis systems.
U.S. Pat. No. 6,100,984 discloses a projector for use in such a system. A laser beam is incident on a lens which diverges the beam on to a liquid crystal system to generate at least one fringe pattern on the surface to be measured. A computer is used to control the pitch and phase of the fringe pattern generated by the liquid crystal system. Photographic equipment is positioned to take an image of the fringe pattern on the surface. The computer and the liquid crystal system then perform a phase-shifting technique and another picture is taken of the new image. Using these two images it is possible to obtain an accurate map of the topography of the surface. The use of a liquid crystal system requires complex interfacing resulting in relatively high power consumption and subsequent heat generation. Such a system can be expensive.
WO 0151887 also discloses a structured light analysis system which has a fringe projector comprising an internal refractor which can be manipulated to change the position of the projected fringe on the object and hence the phase of the fringe at the object's surface, and also discloses moving the object to reposition the fringe on the object.
The invention describes a method of phase shifting an optical pattern projected on an object to be inspected by phase analysis, in which the phase is shifted by moving the optical pattern source relative to the object.
A non-contact method for inspecting an object via phase analysis, comprising: i) a projector projecting an optical pattern onto the surface of an object to be inspected; ii) obtaining at least first and second images of the surface on which the optical pattern is projected, in which the phase of the optical pattern at the surface is changed between the first and second image by moving the projector relative to the object.
It is an advantage of the present invention that the phase of the optical pattern at the object can be displaced by moving the projector. In certain circumstances this can avoid the need for expensive and/or complex equipment to be provided in the projector in order to obtain a change in position of the optical pattern on the object. For example, it could be possible to provide the projector without any internal moving parts. As the projector is moved, large and/or heavy objects can be easily measured. Furthermore it can allow the in-situ measurement of an object during machining so that re-datuming in order to continue machining is not required. As will be understood, the optical pattern as projected by the projector could be the same for the at least first and second phase-shifted images.
Preferably, the optical pattern extends in two dimensions. This enables the determination of the topology of the surface of an object in two dimensions from a single image of the optical pattern on the object. The optical pattern can be a substantially full-field optical pattern. A substantially full-field optical pattern can be one in which the pattern extends over at least 50% of the field of view of an imaging device for obtaining at least one of the at least first and second images, at a reference plane (described in more detail below), more preferably over at least 75%, especially preferably over at least 95%, for example substantially over the entire field of view of the imaging device at a reference plane. The reference plane can be a plane that is a known distance away from the imaging device. Optionally, the reference plane can be a plane which contains the point at which the projector's and imaging device's optical axes intersect. The reference plane can extend perpendicular to the imaging device's optical axis.
Preferably the optical pattern is a substantially periodic optical pattern. As will be understood, a periodic optical pattern can be a pattern which repeats after a certain finite distance. The minimum distance between repetitions can be the period of the pattern. Preferably the optical pattern is periodic in at least one dimension. Optionally, the optical pattern can be periodic in at least two dimensions. The at least two dimensions can be perpendicular to each other.
Preferably the optical pattern as imaged in the at least first and second images is projected over an area of the object. Preferably the pattern extends over an area of the object so as to facilitate the measurement of a plurality of points of the object over the area using the method of the present invention.
Suitable optical patterns for use with the present invention include patterns of concentric circles, patterns of lines of varying colour, shades, and/or tones. The colour, shades and/or tones could alternate between two or more different values. Optionally, the colour, shade and/or tones could vary between a plurality of discrete values. Preferably, the colour, shade and/or tones varies continuously across the optical pattern. Preferably, the periodic optical pattern is a fringe pattern. For example, the periodic optical pattern is a set of sinusoidal fringes. In this case, the method will comprise obtaining a plurality of fringe-shifted images.
The optical pattern can be in the infrared to ultraviolet range. Preferably, the optical pattern is a visible optical pattern. As will be understood, an optical pattern for use in methods such as that of the present invention are also commonly referred to as a structured light pattern.
Suitable projectors for the optical pattern include a digital light projector configured to project an image input from a processor device. Such a projector enables the pattern projected to be changed. Suitable projectors could comprise a light source and one or more diffraction gratings arranged to produce the optical pattern. The diffraction gating(s) could be moveable so as to enable the pattern projected by the projector to be changed. For instance, the diffraction grating(s) can be mounted on a piezoelectric transducer. Optionally, the diffraction gratings could be fixed such that the optical pattern projected by the projector cannot be changed. Optionally the projector could comprise a light source and a hologram. Further, the projector could comprise a light source and a patterned slide. Further still, the projector could comprise two mutually coherent light sources. The coherent light sources could be moveable so as to enable the pattern projected by the projector to be changed. For instance, the coherent light sources can be mounted on a piezoelectric transducer. Optionally, the coherent light sources could be fixed such that the optical pattern projected by the projector cannot be changed.
The method can comprise obtaining at least a third phase-shifted image of the optical pattern on the surface. The more images obtained then the more images that are available for analysis in order to calculate the topographical data. This accuracy and reliability of the topographical data can increase with the number of images obtained.
The projector can be moved by any amount which provides a change in the position of the projected optical pattern relative to the object. Preferably the projector is moved such that the position of the pattern on the object is at least nominally moved by a non-integral multiple of the period of the pattern. For instance, when the optical pattern is a fringe pattern, the projector can be moved such that the position of the pattern on the object is at least nominally moved by a non-integral multiple of the fringe period. For example, the projector can be moved such that the position of the pattern on the object is at least nominally moved by a ¼ of the fringe period. As will be understood, the actual distance the projector is to be moved relative to obtain such a shift in the pattern on the object can depend on a number of factors including the period of the periodic optical pattern projected and the distance between the object and the projector.
As will be understood, moving the projector will cause a change in the position of the optical pattern on the object. However, it may appear from images of the optical pattern on the object taken before and after the movement that the optical pattern has not moved. This can be referred to as nominal movement. Whether or not the movement is nominal or actual will depend on a number of factors including the form of the optical pattern projected, and the shape and/or orientation of the surface of the object relative to the projector. For instance, the change in position of the optical pattern on a surface for a given movement will be different for differently shaped and oriented surfaces. It might be that due to the shape and/or orientation of the surface that it would appear that the optical pattern has not changed position, when it fact it has moved and that that movement would have been apparent on a differently shaped or positioned object. What is important is that it is known that the movement is such that it would cause a change in the position of the optical pattern on a reference surface of a known shape and orientation relative to the projector. Accordingly, it is possible to determine the shape and orientation of the surface by effectively determining how the position of the optical pattern as imaged differs from the known reference.
The at least first and second images can be obtained by at least one suitable imaging device. Suitable imaging devices can comprise at least one image sensor. For example, suitable imaging devices can comprise an optical electromagnetic radiation (EMR) sensitive detector, such as a charge-coupled device (CCD), a complementary metal-oxide-semiconductor (CMOS). Suitable imaging devices can be optically configured to focus light at the image plane. As will be understood, the image plane can be defined by the image sensor. For example, suitable imaging devices can comprise at least one optical component configured to focus optical EMR at the image plane. Optionally, the at least one optical component comprises a lens.
The at least first and second images can be obtained by an imaging device unit comprising at least one imaging device. The imaging device unit could comprise a single imaging device. The at least one first and at least one second images can be obtained by a single imaging device. The single imaging device can comprise a single image sensor. Accordingly, the first and second images can be obtained by a single image sensor.
Step ii) can comprise moving the projector and imaging device relative to the object. This is especially the case when the imaging device and the projector are in a fixed spatial relationship relative to each other. This might be the case, for instance, when the imaging device and projector are provided as a single unit. For example, the imaging device and projector could be provided as a single probe device.
When the object and imaging device are moved relative to each other, then the amount of relative movement should be sufficiently small such that the perspective of the object obtained by the imaging device in each of the images is substantially the same. In particular, preferably the movement is sufficiently small such that the images image substantially the same points on the object. For instance, the images obtained in step ii) can overlap by at least 50%, preferably by at least 75%, more preferably by at least 90%, especially preferably by at least 95%, for example by at least 97%. It can be preferred that the movement is sufficiently small such that the perspective of the object obtained by the image sensor in each of the images is substantially the same such that that any change in the perspective between the plurality of images can be compensated for in the step of analysing the plurality images (described in more detail below).
As will be understood, a perspective can be a particular view point of the object.
A perspective can be defined by the position and/or orientation of the image sensor relative to the object.
The projector could be laterally displaced relative to the object in order to displace the optical pattern on the surface. Optionally, the projector is rotated relative to the object. In a preferred embodiment, the projector and imaging device are moved between images by rotating the projector and imaging device about the imaging device's perspective centre. It has been found that rotating about the imaging device's perspective centre makes it easier to process the images to compensate for any relative movement between the object and imaging device (discussed in more detail below). In particular it makes matching corresponding pixels across a number of images easier. For instance, matching corresponding pixels is possible using a coordinate transformation which is independent of the distance between the object and the imaging device. Accordingly, it is not necessary to know the distance between the object and imaging device in order to process the images to compensate for any relative movement between the object and imaging device.
Optionally, the method further comprises processing the phase-shifted images to obtain topographical surface data. Accordingly, the method can be used to obtain topographical data regarding a surface of the object. As will be understood, the object can be unknown. That is the object can be of unknown dimensions. As will be understood, the processing can be performed by a processor device that is separate to the device controlling the projector and/or imaging device.
The method can comprise determine topographical data across the entire of one of the first and second images. Optionally, the method can comprise determining the topographical data across only a part of one of the first and second images. In particular, the method comprises determining topographical data for a continuous section of the object on which the optical pattern is projected. A continuous section of the object can be a part of the object which is enclosed by a plurality of irregularities or discontinuities in the optical pattern as described in more detail below.
As will be understood, topographical surface data can be data indicating the topography of at least a part of the object's surface. The topographical data can be data indicating the height of the object's surface relative to the imaging device, at at least one point on the object, and preferably at a plurality of points on the object. The topographical data can be data indicating the gradient of the object's surface, at at least one point on the object, and preferably at a plurality of points on the object.
As will be understood, topographical data can be determined by effectively analysing the phase of the optical pattern on the surface. There are many known techniques for determining topographical data from a set of phase-shifted images and are often referred to as phase stepping algorithms. For instance suitable phase stepping algorithms are described in Creath, K. “Comparison of phase measurement algorithms” Proc. SPIE 680, 19-28 (1986).
Known phase-stepping algorithms can require that the corresponding points on the image correspond to the same point on an object. As will be understood, this will not be the case in embodiments in which the imaging device moves relative to the object. Accordingly, the method can comprise processing the at least first and second images to compensate for any relative movement between the object and imaging device. Once compensated, corresponding points on the plurality of images should represent the same point on the object. Processing the images can comprise identifying common image areas covered by the at least first and second images. Processing the images can comprise adjusting the image coordinates of at least one of the first and second images. Processing the images can comprise applying a coordinate transformation to at least one of the first and second images. The coordinate transformation can be a linear translation. This can have the effect of cropping the images. Optionally, the coordinate transformation can be a non-linear function that may depend on the camera internal parameters (including lens aberrations), the image coordinates of the points being transformed, the relative motion of the object and sensing device, the distance to the object and other system parameters. As will be understood, the most appropriate coordinate transformation will be the one that most accurately makes the position of an object in the transformed images invariant under the relative motion of object and imaging device.
However, it has been found that it is possible to process the at least first and second images to obtain topographical surface data without compensating for any relative movement between the object and the imaging device between the at least first and second images. This has been found advantageous as it enables the method to be performed even in situations in which the relative motion of the imaging device and the object may not be accurately compensated for. For instance, if the imaging device has been moved laterally and the standoff distance is not large compared to the depth of the measurement volume. For instance, the at least first and second images can be obtained in situations in which the ratio of the depth of field to standoff is less than 10:1, for example less than 5:1, for instance 1:1. Accordingly, it is possible that step iii) involves processing at least first and second images in which the corresponding points on the at least first and second images represent different points on the object. This can be achieved, for instance, by effectively analysing the change in phase of the optical pattern across corresponding points on the at least first and second images. In other words, this can be achieved, by effectively analysing the change in phase of the optical pattern across the same pixels on the at least first and second images. Suitable algorithms for processing the at least first and second images which have not been processed to compensate for relative movement include Carré algorithms which are well known and for example described in Carre, P. “Installation et utilisation du comparateur photoelectrique et interferential du Bureau International des Podis et Mesure” Metrologia 2 13-23 (1996), and also 5-frame algorithms as described in G. Stoilov, T. Dragostinov, “Phase-stepping interferometry: five-frame algorithm with an arbitrary step”, Opitcs and Lasers in Engineering 28, 61-69 (1997). As will be understood, processing phase-shifted images using a Carré algorithm provides modulation amplitude and phase-shift data as well as phase data.
Processing the at least first and second images can comprise calculating a phase map from the at least first and second images. As will be understood, a phase map is a map which contains the phase of a pattern projected onto the object's surface for a plurality of pixels in an image. In particular, this can comprise calculating a wrapped phase map from the at least first and second images. Accordingly, step iii) can comprise calculating a wrapped phase map using a phase stepping algorithm. Step iii) can further comprise unwrapping the wrapped phase map and obtaining the topographical data from the unwrapped phase map. The topographical data could be in the form of height data. As will be understood, height data can detail the position of a plurality of points on the surface relative to the imaging device.
As will be understood, when the object and imaging device are moved together relative to each other, then step iii) can comprise: a) processing at least one of the first and second images to compensate for movement between the object and imaging device; b) calculating an (e.g. wrapped) phase map using the compensated images. Step iii) can further comprise c) unwrapping a wrapped phase map and obtaining a topographical data regarding the surface of the object.
Accordingly, step a) can comprise identifying common image areas covered by the plurality of images and step b) can comprise calculating a phase map using the common image areas only. In particular step a) can comprise adjusting the image coordinates to compensate for relative movement between the object and the imaging device.
Analysing the at least first and second image can comprise determining the gradient of the surface. This can be the gradient relative to the imaging device. Determining the gradient of the surface can comprise calculating a phase shift map from the plurality of images. There are many suitable algorithms for generating a phase shift map from the plurality of images. For example, a Carré algorithm can be used to generate the phase shift map. Determining the gradient of the surface relative can further obtaining a gradient map based on the phase shift map. The gradient map can be obtained by converting the value of each of the points on a phase shift map to a gradient value. The value of a point in a phase shift map can be converted to a gradient value using a predetermined mapping procedure. As will be understood, a phase shift map can detail the phase shift for a plurality of points on the surface due to the change in position of projected fringes on the object's surface. The phase shift can be bound in a range of 360 degrees. A gradient map can detail the surface gradient of a plurality of points on the surface.
The method can further comprise integrating the gradient map to obtain height data. As explained above, height data can detail the position of a plurality of points on the surface relative to the imaging device.
The projector and the imaging device can be mounted on a coordinate positioning apparatus. This enables accurate position information regarding the location and/or orientation of the projector and imaging device to be obtained.
The object can be located in a measurement space and the method can further comprise determining the three-dimensional coordinates of the topographical data within the measurement space.
As mentioned above, preferably the at least first and second images are obtained from a same first perspective. Accordingly, the method comprises obtaining a first set of a plurality of images. The method can further comprise the imaging device obtaining at least a second set of a plurality of images of the object from at least a second perspective that is different to the first perspective. The method can then further comprise identifying at least one target feature on the object to be measured from the first and at least second sets of a plurality of images, and then determining the position of the target feature on the object relative to the imaging device. Details of a method and apparatus for identifying topographical data of a surface of an object as well as identifying and determining the position of target features on an object are disclosed in the co-pending PCT application filed on the same day as the present application with the title NON-CONTACT PROBE and having the applicant's reference number 743/WO/0 and claiming priority from UK Patent Application nos. 0716080.7, 0716088.0, 0716109.4. Subject matter that is disclosed in that application is incorporated in the specification of the present application by this reference.
Preferably, the image analyser is configured to identify at least one irregularity in the optical pattern as imaged in the at least one first and second images as the at least one target feature. This is advantageous as target features can be identified without the use of markers placed on the object. This has been found to enable highly accurate measurements of the object to be taken quickly. It has also been found that the method of the invention can require less processing resources to identify points on complex shaped objects than by other known image processing techniques. Details of a method of identifying an irregularity in an optical pattern in each of at least one first and second images as a target feature are disclosed in the co-pending PCT application filed on the same day as the present application with the title NON-CONTACT MEASUREMENT APPARATUS AND METHOD and having the applicant's reference number 741/WO/0 and claiming priority from UK Patent Application nos. 0716080.7, 0716088.0, 0716109.4. Subject matter that is disclosed in that application is incorporated in the specification of the present application by this reference.
An irregularity in the optical pattern can be a deformation of the optical pattern caused by a discontinuous feature on the object. Such a deformation of the optical pattern can be caused at the boundary between two continuous sections of an object. For example, the boundary could be the edge of a cube at which two faces of the cube meet. Accordingly, a discontinuous feature on the object can be where the gradient of the surface of the object changes significantly. The greater the gradient of the surface, the greater the deformation of the optical pattern at that point on the surface. Accordingly, a discontinuity could be identified by identifying those points on the object at which the optical pattern is deformed by more than a predetermined threshold. This predetermined threshold will depend on a number of factors, including the size and shape of the object to be measured. The predetermined threshold can be determined and set prior to operation by a user based on the knowledge of the object to be measured.
In embodiments in which the optical pattern is a fringe pattern, an irregularity can be identified by identifying those points on the object at which the phase of the fringe pattern changes above a predetermined threshold.
According to a second aspect of the invention there is provided an apparatus for inspecting an object via phase analysis, the apparatus comprising: a projector configured to project a optical pattern onto the surface of an object to be measured, the projector being moveable relative to the object; an imaging device configured to obtain a plurality of images of the object on which the optical pattern is projected; and in which the projector is configured to be moved relative to the object between obtaining the phase-shifted images to cause a change in phase of the optical pattern on the object. As will be understood, the apparatus can further comprise an image analyser configured to analyse the images to obtain topographical surface data.
According to a third aspect of the invention there is provided a non-contact method for inspecting an object via phase analysis, comprising in any suitable order: i) a projector projecting an optical pattern onto the surface of an object to be inspected; ii) an imaging device obtaining a plurality of phase-shifted images of the optical pattern on the surface, in which the projector and imaging device are in a fixed spatial relationship relative to each other and in which the position of the optical pattern on the object is moved between images by relative movement between the projector relative to the object about the imaging device's perspective centre. Rotating about the perspective centre can be advantageous as the imaging device's perspective of the object does not change so points on the object hidden due to occlusion before the rotation will also be hidden due to occlusion after the rotation. It has been found that rotating about the imaging device's perspective centre makes it easier to process the images to compensate for any relative movement between the object and imaging device. In particular it makes matching corresponding pixels across a number of images easier. For example, matching corresponding pixels is possible using a coordinate transformation which is independent of the distance between the object and the imaging device. Accordingly, it is not necessary to know the distance between the object and imaging device in order to process the images to compensate for any relative movement between the object and imaging device.
The method can further comprise iii) processing the phase-shifted images to obtain topographical surface data.
According to a fourth aspect of the invention there is provided computer program code comprising instructions which, when executed by a controller, causes the machine controller to control at least one projector, imaging device and image analyser in accordance with the above described methods.
According to a fifth aspect of the invention there is provided a computer readable medium, bearing computer program code as described above.
Accordingly, this application describes, a non-contact method of measuring an object, comprising in any suitable order the steps of: i) a projector projecting a structured light pattern onto the surface of an object to be measured; ii) an image sensor obtaining a plurality of images of the structured light pattern on the surface, and iii) obtaining topographical data regarding the surface by analysing the plurality of images, in which the method further comprises moving the object and projector relative to each other between obtaining each of the plurality of images. This application also describes, an apparatus for measuring an object located in a measurement space, the apparatus comprising: a projector configured to project a structured light pattern onto the surface of an object to be measured, in which the object and projector are moveable relative to each other such that the position of the structured light pattern on the surface can be changed; an image sensor configured to obtain a plurality of images of the structured light pattern on the surface; and an image analyser configured to obtain topographical data regarding the surface by analysing a plurality of images obtained by the image sensor in which the position of the structured light pattern on the surface is different in each of the plurality of images.
With reference to FIG. 13, the projector 40 comprises a laser diode 50 for producing a coherent source of light, a collimator 52 for collimating light emitted from the laser diode 50, a grating 54 for producing a sinusoidal set of fringes, and a lens assembly 56 for focussing the fringes at the reference plane 64. As will be understood, other types of projectors would be suitable for use with the present invention. For instance, the projector could comprise a light source and a mask to selectively block and transmit light emitted from the projector in a pattern.
Referring first to FIG. 4, the operation begins at step 100 when the operator turns the CMM 2 on. At step 102, the system is initialised. This includes loading the probe 4 onto the articulating head 16, positioning the object 28 to be measured on the base 10, sending the CMM's encoders to a home or reference position such that the position of the articulating head 16 relative to the CMM 2 is known, and also calibrating the CMM 2 and probe 4 such that the position of a reference point of the probe 4 relative to the CMM 2 is known.
However, this need not necessarily be the case, so long as the position of the fringes on the object is moved. For example, the fringe shifting could be achieved by rotating the probe 4. For instance, the probe 4 could be rotated about an axis extending perpendicular to the projector's image plane 60. Optionally the probe could be rotated about an axis extending perpendicular to the imaging device's 44 image plane. In another preferred embodiment the probe 4 can be rotated about the imaging device's 44 perspective centre. This is advantageous because this ensures that the perspective of the features captured by the imaging device 44 across the different images will be the same. It also enables any processing of the images to compensate for relative movement of the object and image sensor to be done without knowledge of the distance between the object and image sensor.
Reference is now made to FIG. 2. Row A shows the view of the object 28 at each of the three perspectives with no fringes projected onto it. Row B illustrates, for each of the first, second and third perspectives the image 1000 that will be obtained by the imaging device 44 at step 206 of the process for capturing a perspective image set 104. Schematically shown behind each of those images 1000 are the fringe shifted images 1002, 1004 and 1006 which are obtained during execution of steps 300 and 302 for each of the first, second and third perspectives. FIGS. 14( a) to 14(d) shows an example of the images 1000-1006 obtained for the first perspective. As shown, the relative position of the object and imaging device has moved slightly between obtaining each image in an image set for a perspective, and this needs to be taken into consideration and/or compensated for during processing of the images as described in more detail below (especially as described in connection with FIG. 8).
It is possible that the above process could result in false discontinuities being identified due to the phase map being wrapped. For example, adjacent pixels might have phase values of, for instance, close to 0degrees and 360 degrees respectively. If so, then it would appear as if there has been a large phase jump between those pixels and this would be identified as a discontinuity. However, the phase jump has merely been caused as a result of the wrapping around of the phase, rather than due to a discontinuity in the surface of the object being measured. An example of this can be seen in the first wrapped phase map 1010 for the first perspective at point 36 where the phase values jump from 360 degrees to 0 degrees (illustrated by the dark pixels and light pixels respectively). The phase value for adjacent pixels will jump significant at point 36 due to the phase map being wrapped.
The process for calculating a wrapped phase map 400 will now be described with reference to FIG. 8. Calculating a wrapped phase map comprises calculating the phase for each pixel for one of a set of fringe-shifted images. This can be done using various techniques, the selection of which can depend on various factors including the method by which the fringe-shifted images are obtained. Standard phase-shifting algorithms rely on that the relative position between the object and imaging device 44 is the same across all of the fringe-shifted images. However, if either of the methods described above (e.g. either moving the probe 4 laterally or rotating it about the imaging device's perspective centre) are used to obtain the fringe-shifted images then the imaging device 44 will have moved a small distance relative to the object. Accordingly, for each successive image in a perspective image set, a given pixel in each image will be identifying the intensity of a different point on the object. Accordingly, if standard phase-shifting algorithms are to be used it is necessary to identify across all of the fringe shifted images which pixels correspond to same point on the object, and to then compensate for this. One way of doing this when the imaging device 44 has moved laterally is to determine by how much and in what direction the imaging device 44 has travelled between each image, and by then cropping the images so that each image contains image data common to all of them. For example, if the movement of the imaging device 44 between two images means that a point on an object has shifted five pixels in one dimension, then the first image can be cropped to remove five pixel widths worth of data.
This can be seen more clearly with reference to FIG. 15 which schematically illustrates corresponding rows of pixels for each of the first 1000, second 1002, third 1004 and fourth 1006 images. As can be seen, due to relative movement of the imaging device 44 and the object 28 between the images, the same point on an object is imaged by different pixels in each image. For instance, point X on the object 28 is imaged by the 7th pixel from the left for the first image 1000, the 5th pixel from the left for the second image 1002, the 3rd pixel from the left for the third image 1004 and the 4th pixel from the left for the fourth image 1006. An effective way of compensating for the relative movement of the image sensor and object 28 is to crop the image data such that each image 1000-1006 contains a data representing a common region, such as that highlighted by window 51 in FIG. 15.
Cropping the images is one example of a coordinate transformation, where the transformation is a linear function. This can be most accurate in situations where the distance to the object is known, or, for instance, where the stand-off distance is large compared to the depth of the measuring volume. As will be understood, and with reference to FIG. 18, the stand-off distance is the distance from the imaging device's perspective centre 76 to the centre of the imaging device's measurement volume and the depth of field 65 or depth of measurement volume is the range over which images recorded by the device appear sharp. In other words, the stand-off distance is the nominal distance from the probe 4 to the object to be measured. For instance, if the ratio of stand-off distance to depth of measuring volume is around 10:1 then there can be an error of up to 10% in the compensation for some pixels. If either the stand-off distance is not large compared to the depth of the measuring volume, or if the relative motion is not a linear translation, then the most appropriate coordinate transformation to compensate for relative motion of the imaging device and the object can depend, in general on the distance to the object and the actual motion. However, it has been found that if the motion is rotation about the imaging device's 44 perspective centre then the coordinate transformation that best compensates for the motion is independent of the unknown distance to the object. This is due to the geometry of the system and the motion. Furthermore, this enables accurate compensation to be performed even if the stand-off distance is not large compared to the depth of the measuring volume, for instance in situations in which the ratio of stand-off distance to depth of measuring volume is less than 10:1, for example less than 5:1, for instance 1:1.
Accordingly, this enables measurement of an object to be performed even when the probe is located close to the object.
As an example, with reference to FIG. 16, consider an object point Xp, imaged at x in the camera plane. If the imaging device 44 is translated by some vector dX with respect the the plane, then the point imaged by the imaging device 44 will change, as show. For clarity, the projector 40 is omitted from the diagram, but it is to be understood that the imaging device 44 and projector 40 are fixed with respect to each other.
I k =A+B cos φk
φk=φk-1+Δφk≈φk+∇φk-1.δX k, k>0
As will be understood, the description of the specific embodiment also involves obtaining and processing images to obtain photogrammetrical target points by identifying discontinuities in the pattern projected onto the object. As will be understood, this need not necessarily be the case. For example, the system and method of the invention might not be configured to determine target points for photogrammetrical purposes at all. If it is, then target points can be identified using other known methods. For instance, target points can be identified by markers placed on the object or by projecting a marker onto the object.
Further, although the invention is described as a single probe containing a projector and imaging device, the projector and image sensor could be provided separately (e.g. so that they can be physically manipulated independently of each other). Furthermore, a plurality of imaging devices could be provided.
1. A non-contact method for inspecting an object via phase analysis, comprising in any suitable order:
i) a projector projecting an optical pattern onto the surface of an object to be inspected;
ii) at least one imaging device obtaining a set of phase-shifted images of the optical pattern on the surface, wherein
the projector and the at least one imaging device are in a fixed spatial relationship relative to each other and in which the position of the optical pattern on the object is moved between images in the set of the phase-shifted images by rotating the projector and the at least one imaging device relative to the object about the imaging device's perspective centre.
2. A method as claimed in claim 1, in which the optical pattern as projected by the projector is the same for images in the set of phase-shifted images.
iii) processing the set of phase-shifted images to obtain topographical surface data.
4. A method as claimed in claim 3, wherein processing the set of phase-shifted images comprises analysing the phase of the optical pattern on the surface.
5. A method as claimed in claim 3, wherein step iii) comprises processing at least one of the images in the set of phase-shifted images to compensate for any relative movement between the object and the imaging device.
6. A method as claimed in claim 3, wherein in step iii) corresponding points in the set of phase-shifted images represent different points on the object.
7. A method as claimed in claim 6, wherein step iii) comprises analysing the change in phase of the optical pattern across corresponding points in the set of phase-shifted images.
8. A method as claimed in claim 3, wherein determining the topographical surface data comprises calculating a phase map from the set of phase-shifted images.
9. A method as claimed in claim 3, wherein analysing the set of phase-shifted images comprises determining the gradient of the surface.
10. A method as claimed in claim 9, wherein determining the gradient of the surface comprises calculating a phase shift map from the set of phase-shifted images and obtaining a gradient map based on the phase shift map.
11. A method as claimed in claim 10, further comprising integrating the gradient map to obtain the topographical data.
12. A method as claimed in claim 1, wherein the projector is mounted on a coordinate positioning apparatus.
13. A method as claimed in claim 3, wherein the object is located in a measurement space and step iii) comprises determining the three-dimensional coordinates of the topographical data within the measurement space.
14. A method as claimed in claim 1, wherein the optical pattern is a periodic optical pattern.
15. An apparatus for inspecting an object via phase analysis, the apparatus comprising:
a projector configured to project an optical pattern onto the surface of an object to be measured, the projector being moveable relative to the object;
at least one imaging device in a fixed spatial relationship with the projector and configured to obtain a plurality of phase-shifted images of the object on which the optical pattern is projected, wherein
the projector and the object are configured to be moved relative to each other by rotating the projector and the at least one imaging device relative to the object about the imaging device's perspective centre between obtaining the phase-shifted images to cause a change in phase of the periodic optical pattern on the object.
16. A non-transitory computer-readable storage medium storing a computer program that, when executed by a controller, causes the controller to control at least one projector, imaging device and image analyser to execute a non-contract method for inspecting an object via phase analysis, the program comprising in any suitable order:
i) instructions for causing a projector to project an optical pattern onto the surface of an object to be inspected;
ii) instructions for causing at least one imaging device to obtain a set of phase-shifted images of the optical pattern on the surface, wherein
the projector and at least one imaging device are in a fixed spatial relationship relative to each other and in which the position of the optical pattern on the object is moved between images in the set of the phase-shifted images by rotating the projector and the at least one imaging device relative to the object about the imaging device's perspective centre.
US12733022 2007-08-17 2008-08-15 Phase analysis measurement apparatus and method Active 2030-06-15 US8792707B2 (en)
PCT/GB2008/002759 WO2009024757A1 (en) 2007-08-17 2008-08-15 Phase analysis measurement apparatus and method
US20100158322A1 true US20100158322A1 (en) 2010-06-24
US8792707B2 true US8792707B2 (en) 2014-07-29
US20050201611A1 (en) 2004-03-09 2005-09-15 Lloyd Thomas Watkins Jr. Non-contact measurement method and apparatus
US20080075328A1 (en) 2006-09-15 2008-03-27 Sciammarella Cesar A System and method for analyzing displacements and contouring of surfaces
US20110317879A1 (en) 2009-02-17 2011-12-29 Absolute Robotics Limited Measurement of Positional Information for a Robot Arm
CA2528791A1 (en) 2005-12-01 2007-06-01 Peirong Jia Full-field three-dimensional measurement method
JP4905013B2 (en) 2006-09-19 2012-03-28 株式会社デンソー Appearance inspection apparatus, the appearance inspection method and height measurement method and the circuit board manufacturing method
"3D Coordinate Measurement-Milling on digitized data; Casted Blanks," www.gom.com, obtained Aug. 7, 2007, GOM mbH.
"3D Coordinate Measurement—Milling on digitized data; Casted Blanks," www.gom.com, obtained Aug. 7, 2007, GOM mbH.
"3D-Digitizing of a Ford Focus—Interior/Exterior—Product Analysis," www.gom.com, obtained Oct. 6, 2008, GOM mbH.
"Application Example: 3D-Coordinate Measurement Mobile 3D Coordinate Measurement for Shipbuilding," 6 pages, GOM Optical Measuring Techniques, downloaded Sep. 6, 2012 from http://www.gom.com/fileadmin/user-upload/industries/shipbuilding-EN.pdf.
"Application Example: 3D-Coordinate Measurement Mobile 3D Coordinate Measurement for Shipbuilding," 6 pages, GOM Optical Measuring Techniques, downloaded Sep. 6, 2012 from http://www.gom.com/fileadmin/user—upload/industries/shipbuilding—EN.pdf.
"Application Notes-TRITOP," 1 page, GOM Optical Measuring Techniques, downloaded Sep. 6, 2012 from http://www.gom.com/industries/application-notes-tritop.html.
"Application Notes—TRITOP," 1 page, GOM Optical Measuring Techniques, downloaded Sep. 6, 2012 from http://www.gom.com/industries/application-notes-tritop.html.
"Measuring Systems-ATOS," http://www.gom.com/EN/measuring,systems/atos/system/system.html, obtained Oct. 6, 2008, GOM mbH.
"Measuring Systems—ATOS," http://www.gom.com/EN/measuring,systems/atos/system/system.html, obtained Oct. 6, 2008, GOM mbH.
"Measuring Systems—TRITOP," http://www.gom.com/EN/measuring.systems/tritop/system/system.html, obtained Aug. 7, 2007, GOM mbH.
"optoTOP-HE—The HighEnd 3D Digitising System," http://www.breuckmann.com/index.php?id=optotop-he&L=2, obtained Oct. 6, 2008, Breuckmann.
"Picture Perfect Measurements, Do I need to use special targets with the system?," 1 page, Geodetic Systems Inc., downloaded Sep. 6, 2012 from http://www.geodetic.com/do-i-need-to-use-special-targets-with-the-system.aspx.
"Picture Perfect Measurements, The Basics of Photogrammetry," 14 pages, Geodetic Systems Inc., downloaded Sep. 6, 2012 from http://www.geodetic.com/v-stars/what-is-photogrammetry.aspx.
Aug. 16, 2013 Office Action issued in Japanese Patent Application No. 2010-521465 w/translation.
Aug. 9, 2013 Office Action issued in Japanese Patent Application No. 2010-521466 w/translation.
Aug. 9, 2013 Office Action issued in Japanese Patent Application No. 2010-521467 w/translation.
Carré, "Installation et utilisation du comparateur photoelectrique et interférential du Bureau International des Poids et Mesures," 1966, Metrologia, pp. 13-23, vol. 2, No. 1, France (with abstract).
Cooper at al., "Theory of close range photogrammetry," Close Range Photogrammeny and Machine Vision, 2001, pp. 9-51, Whittles Publishing.
Cuypers, W., et al., "Optical measurement techniques for mobile and large-scale dimensional metrology," Optics and Lasers in Engineering, 47, (2009), pp. 292-300.
Dec. 2, 2013 Chinese Office Action issued in Chinese Patent Application No. 201080040329.0 (with English-language translation).
European Office Action issued in European Patent Application No. 08 788 328.6 dated Feb. 27, 2014.
Feb. 16, 2013 Office Action issued in Chinese Application No. 200880111247.3 (with English translation).
Feb. 16, 2013 Office Action issued in Chinese Application No. 200880112194.7 (with English translation).
Feb. 25, 2014 Office Action issued in Japanese Patent Application No. 2010-521467 (with English translation).
Feb. 7, 2013 Office Action issued in U.S. Appl. No. 12/733,021.
Fryer, "Camera Calibration," Close Range Photogrammetty and Machine Vision, 1996, pp. 156-179, Whittles Publishing.
Geometrical Product Specifications (GPS)—Geometrical Features, British Standard, BS EN ISO 1466-1:2000.
Ishiyama et al., "Absolute phase measurements using geometric constraints between multiple cameras and projectors," Applied Optics 46 (17), pp. 3528-3538 (2007).
Japanese Office Action issued in Japanese Patent Application No. 2012-528441 dated Jan. 28, 2014 (w/ translation).
Kemper et al, "Quantitative determination of out-of-plane displacements by endoscopic electronic-speckle-pattern interferometry" Optics Communication 194 (2001), pp. 75-82, Jul. 1, 2001. *
Kowarschik et al., "Adaptive optical three-dimensional measurement with structured light," Optical Engineering 39 (1), pp. 150-158 (2000).
Notice of Allowance issued in U.S. Patent Application No. 12/733,021 dated Sep. 3, 2013.
Nov. 16, 2012 Office Action issued in Japanese Patent Application No. 2010-521466 (with English Translation).
Nov. 18, 7008 International Search Report issued in International Patent Application No. PCT/GB2008/002760.
Sansoni et al., "Three-dimensional vision based on a combination of gray-code and phase-shift light projection: analysis and compensation of the sytematic errors," Applied Optics, 1999, pp. 6565-6573, vol. 38, No. 31, Optical Society of America.
Scharstein et al. "High-Accuracy Stereo Depth Maps Using Structured Light" Proceedings of the 2003 IEEE Computer Society Conference on Computer Vision and Pattern Recognition (CVPR'03), Jun. 18, 2003. *
Sep. 19, 2012 Office Action issued in U.S. Appl. No. 12/733,021.
Sep. 21, 2012 Office Action issued in Japanese Patent Application No. 2010-521465 (with English Translation).
Stoilov et al., "Phase-stepping Interferometry, Five-frame Algorithm with an Arbitrary Step," Optics and Lasers in Engineering, 1997, pp. 61-69, vol. 28.
Takeda et al, "Fourier-transform method of fringe-pattern analysis for computer-based topography and interferometry", Optical Society of America, Jan. 1982, vol. 72, No. 1, pp. 156-160.
Translation of CN 101105393, previously submitted on Jan. 30, 2014.
Translation of JP-A-2007-24764 originally cited in an Information Disclosure Statement filed on Nov. 20, 2012.
U.S. Appl. No. 12/733,021, filed Feb. 3, 2010 in the name of Weston at al.
U.S. Appl. No. 13/392,710, filed on Feb. 27, 2012 in the name of Weston et al.
Wallace, Iain et al., "High-speed photogrammetry system for measuring the kinematics of insect wings," Applied Optics, vol. 45, No. 17, Jun. 10, 2006, pp. 4165-4173.
Wolfson, Wendy and Gordon, Stephen J., "Three-Dimensional Vision Technology Offers Real-Time Inspection Capability," Sensor Review, 1997, pp. 299-202, vol., 17, No. 4. MCB University Press.
USRE46012E1 (en) 2016-05-24 grant
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNORS:WESTON, NICHOLAS J.;HUDDART, YVONNE R.;MOORE, ANDREW J.;AND OTHERS;SIGNING DATES FROM 20080916 TO 20080923;REEL/FRAME:023925/0402