Patent Application
20160076881
1. Field of the Invention
The present disclosure generally relates to a measurement apparatus and an adjusting method thereof.
2. Description of the Related Art
There has been a measurement apparatus of a pattern projection system that projects a light pattern on an object, picks up an image of the object, and measures a three-dimensional shape of the object based on an image pickup result.
The measurement apparatus is provided with a head including a projection unit that projects a light pattern, and an image pickup unit that picks up an image of an object. A calculation unit is provided inside or outside the measurement apparatus. An image of the object on which a known light pattern is projected from a projection lens of the projection unit is picked up by a sensor through an image pickup lens of an image pickup unit. The calculation unit performs calculation based on triangulation using the image pickup result, and obtains a distance to the object, and the shape of the object.
Mechanism tolerance, and image distortion caused by the optical system occur in the image pickup unit and the projection unit used in the measurement apparatus, which produce measurement errors. Therefore, measurement accuracy is improved by acquiring a calibration value for calibrating a measurement result by measuring a known reference, and correcting the measurement result so as to remove the measurement error resulting from the mechanism and optical system (Japanese Patent Laid-Open No. 2008-170280).
Japanese Patent Laid-Open No. 2008-170280 discloses the following: a calibration chart as a reference is measured while changing a distance from a projection unit. In a state in which a relative angle between an optical axis of the image pickup lens and a sensor is inclined from a vertical angle, defocusing distribution of an image occurs on a sensor plane. Image distortion depending on the distance from the projection unit changes due to the combination that defocusing distribution changes depending on the distance to the calibration chart, and aberration of image pickup lens.
Data of image distortion depending on the distance from the projection unit is measured by measuring a calibration chart, while changing the distance from the projection unit, the data is stored as calibration data and the image data is calibrated using the calibration data.
It has been found out that image distortion depending on distance is caused by inclination of an image pickup lens in a direction crossing an epipolar plane during triangulation to cause inclination of an angle between a sensor plane and an optical axis of the image pickup lens from a vertical angle. Similarly, a tilt angle of a projection lens and a light pattern forming unit also causes distortion of a light pattern to be projected.
Japanese Patent Laid-Open No. 2008-170280 does not describe the tilt angle, and does not adjust the image pickup unit in consideration of the tilt angle. In the method of Japanese Patent Laid-Open No. 2008-170280, if calibration data quantity is large, calculation for calibration in actual shape measurement takes time.
According to an aspect of the present disclosure, a measurement apparatus for measuring an object based on an image of the object, includes a projection unit including a pattern forming unit configured to form a light pattern, and a projection optical system configured to project the formed light pattern on the object; and an image pickup unit including an image pickup portion provided with a light-receiving surface and configured to receive light from the object on the light-receiving surface and pick up an image of the object, and an image-forming optical system configured to guide light from the object on which the light pattern is projected to the light-receiving surface, wherein the image pickup unit includes an adjustment unit that makes an angle of the image-forming optical system with respect to the light-receiving surface adjustable, and the adjustment unit makes a tilt angle of the image-forming optical system in a direction crossing an epipolar plane defined by the projection unit and the image pickup unit adjustable.
Further features of the present disclosure will become apparent from the following description of exemplary embodiments with reference to the attached drawings.
Hereinafter, preferred embodiments of the present disclosure will be described in detail with reference to the attached drawings.
A measurement apparatus of the present embodiment is described with reference to
The projection unit 1 includes a projection pattern setting element (a pattern forming unit) 6, a housing 5, and a projection optical system 4. The projection unit 1 applies light, emitted from a light source, to a projection pattern setting element 6 that forms the light pattern, and projects a pattern set in a measurement space by the projection optical system 4. In the present embodiment, as illustrated in
The image pickup unit 2 includes a sensor element 9 (a photoelectric conversion element, such as a CMOS and a CCD) having a light-receiving surface, an image pickup lens (an image-forming optical system) 7 that guides light from the object to the light-receiving surface, and a sensor housing 8. The sensor element 9 functions as an image pickup portion that receives light from the object on the light-receiving surface and acquires an image of the object.
Next, a measurement method using the measurement apparatus is described. The light pattern 12 consists of white and black binary stripes. While the projection unit 1 projects a plurality of kinds of light patterns, the image pickup unit 2 picks up an image for each pattern, and a signal (i.e., data) of the picked up image is stored in the calculation unit 3. The measurement space is divided based on the set value of the stripes and data of the image pickup result for each set value. Then a pixel position of the projection pattern setting element 6 of the projection unit 1 and a pixel position of the sensor element 9 of the image pickup unit 7 are correlated to an object point on a surface of the object 11 in the measurement space. Since the relationship in the position and the posture between the projection unit 1 and the image pickup unit 2 is known in advance, a distance (a position) of point on the surface of the object 11 is calculated by the calculation unit 3 in accordance with the triangulation. This process is carried out for a plurality of points on the surface of the object 11 on which the pattern is projected, whereby the shape of the object 11 is obtained. Here, the pattern is set with white and black binary stripes, but multi-values a plurality of colors may also be used.
Details of the image pickup unit 2 are illustrated in
As magnification of lenses becomes higher due to an influence of size reduction in measurement apparatuses, required precision of the relative angle between the sensor element 9 and the sensor side fixing surface 23 becomes higher and higher and, at the same time, cost reduction is required. This causes a manufacturing error that produces a sensor tilted out of required precision. In that case, if the lens side fixing surface 21 and the sensor side fixing surface 23 are made to abut each other in their entire circumferences and fixed to each other with nothing inserted therebetween, the optical axis of the image pickup lens 7 and the light-receiving surface of the sensor element 9 are tilted from the vertical angle.
In the image pickup unit 2 of the present embodiment, it is possible to provide a larger gap than a thickness of a shim previously prepared at a part of the circumferential direction of the optical axis between the lens side fixing surface 21 and the sensor side fixing surface 23. Specifically, an inner diameter and a root diameter of the female screw 24 with respect to a root diameter and outer diameter of the male screw 22 of mount so that the image pickup lens 7 may be fixed in a tilted manner even if a shim of the maximum thickness is inserted. The maximum thickness of the shim to prepare can be estimated from the manufacturing error of the relative angle between the sensor element 9 and the sensor side fixing surface 23. A space larger than the shim 27 is provided where the shim 27 is able to be inserted. Thus a sensor stand 25 and the like does not interfere with the shim 27 near a positioning portion 26 of the sensor side fixing surface 23.
The shim 27 is illustrated in
The shim 27 is inserted in a gap between the lens side fixing surface 21 and the sensor side fixing surface 23 with the shim 27 aligned with the positioning portion 26, the image pickup lens 7 (i.e., the male screw) is rotated, and the lens side fixing surface 21 and the sensor side fixing surface 23 are partially brought into contact, whereby the image pickup lens 7 is fixed. Since the lens side fixing surface 21 and the sensor side fixing surface 23 are fixed in a tilted manner, the angle θ between the optical axis of the image pickup lens 7 and the light-receiving surface of the sensor element 9 may be adjustable to a predetermined angle. The positioning portion 26 of the sensor side fixing surface 23 is provided in the direction (e.g., the vertical direction) to cross a certain epipolar plane with respect to the optical axis of the image pickup lens 7. Therefore, the adjustment unit 14 makes the tilt angle of the image pickup lens 7 in a direction to cross the epipolar plane defined by the projection unit 1 and the image pickup unit 2 adjustable and may attach the image pickup lens 7 at a predetermined angle. Here, the epipolar plane is a plane including the object side principal point of the projection lens 4, the image side principal point of the image pickup lens 7, and the object point. Since the object has a certain magnitude, the epipolar plane may be defined about one object point of the object. The epipolar plane is parallel to an xz plane of
Although not illustrated in
According to the present embodiment, the measurement error may be reduced by adjusting the image pickup unit so as to reduce image distortion. The adjustment unit has a simple configuration in which the shim is simply inserted and fixed. The adjustment unit has a single-axis configuration in the adjustment direction, which is simpler than a multi-axis configuration.
Next, an adjusting method of the measurement apparatus is described.
In the present embodiment, an angle between the light-receiving surface of the sensor element 9 and the sensor side fixing surface 23 (the image pickup portion side) is measured by an autocollimator 30 (S101). As illustrated in
Since a cover glass (not illustrated) exists generally before the sensor element 9, the reflected light spots of the substrate 31 and the sensor element 9 are extracted from three reflected light spots of the substrate 31, the sensor element 9, and cover glass. First, if no substrate 31 exists, no reflected light spot from the substrate 31 exists. Thus, the reflected light spot from the substrate 31 may be extracted based on a difference of measurement results of angles of the reflected light spots in the cases where the substrate 31 exists and where the substrate 31 does not exist. The reflected light spots from the sensor element 9 and the cover glass are described. Since light is reflected twice on the front and back surfaces of the cover glass and the light spots of the cover glass become brighter than the reflected light spots of the sensor element 9 by a wedge, the reflected light spots of the sensor element 9 may be extracted from the viewpoint of light quantity. By the method described above, the reflected light spots of the sensor element 9 and the reflected light spots of the substrate 31 at the same angle with the sensor side fixing surface 23 are extracted. The angle between the light-receiving surface of the sensor element 9 and the sensor side fixing surface 23 when the image pickup unit 2 is mounted on the head 10 is obtained based on the positions of the extracted reflected light spots. A tilt angle component with respect to the direction to cross the epipolar plane is extracted based on the angle. The extracted angle component corresponds to the tilt angle component of the image pickup lens 7 in the direction to cross the epipolar plane, or an angle component between a surface including the optical axis of the image pickup lens 7 and the object side principal point of the projection lens 4 and the light-receiving surface of the sensor element 9.
If the substrate 31 is not able to be disposed directly on the sensor side fixing surface 23 as illustrated in
Next, based on the angle measured by the above method, an angle adjustment amount by the adjustment unit is calculated (S102). This adjustment amount is the adjustment amount of the tilt angle of the image pickup lens 7 in the direction to cross the epipolar plane, or the angle between the surface including the optical axis of the image pickup lens 7 and the object side principal point of the projection lens 4 and the light-receiving surface of the sensor element 9, and corresponds to a thickness of the shim 27. A method for calculating the thickness of the shim 27 is described with reference to
With this adjusting method, the tilt angle of the image pickup lens 7 in the direction to cross the epipolar plane, or the angle between the surface including the optical axis of the image pickup lens 7 and the object side principal point of the projection lens 4 and the light-receiving surface of the sensor element 9 may be adjusted.
Next, another adjusting method of the measurement apparatus is described. In this Example, the angle between the light-receiving surface of the sensor element 9 and the sensor side fixing surface 23 is measured using a three-dimensional measuring machine.
First, as illustrated in
Next, the substrate 31 is retracted from above the sensor side fixing surface 23, a probe of a three-dimensional measuring machine is made to contact with the surface of the sensor element 9 to measure the positions of the plurality of points on the surface of the sensor element 9. Then planar fit is applied to a plurality of measurement points to measure the tilt angle of the sensor element 9. A cover glass exists generally before the sensor element 9. Suppose that the angle between the cover glass and the sensor element 9 is managed, and that the relative angle therebetween is known or sufficiently small.
Based on the measured tilt angle of the sensor element 9 and the tilt angle of the sensor side fixing surface 23, a relative angle of these angles is obtained. Among the obtained relative angles, a tilt angle component with respect to the direction to cross the epipolar plane is extracted. Subsequent processes are the same as those of Example 1.
The angle of the sensor side fixing surface 23 is measured by disposing the substrate 31 in the present embodiment. Alternatively, an adapter of which angle of a surface with respect to the angle of the sensor side fixing surface 23 is managed may be used, or the sensor side fixing surface 23 may be measured directly. Instead of the three-dimensional measuring machine, a height measurement machine that measures the height using a probe moving along a single-axis direction may be used.
Next, another adjusting method of the measurement apparatus is described. In the present embodiment, images of a reference chart (an evaluation pattern) 13 are picked up at a plurality of distances, and an adjustment amount by the adjustment unit is calculated based on an evaluation result obtained from the images of the chart.
A chart 13 is installed in a measurement space as illustrated in
Next, images of the chart 13 are picked up at a plurality of distances (in the z direction) using the measurement apparatus with no shim inserted between the lens side fixing surface 21 and the sensor side fixing surface 23 (S201). Based on the chart image at each distance, in-screen distortion of the sensor element 9 is evaluated as a characteristic of the chart image, and the in-screen distortion depending on the distance is obtained (S202). This process refers to as measurement A. The evaluation value here is the screen distortion that is the lateral shift amount of the reference coordinate. For example, the evaluation value of the chart 13a is the lateral shift amount of the center coordinates, and the evaluation value of the chart 13b is the lateral shift amount of the cross point coordinates.
Next, the image pickup lens 7 is tilted in the direction to cross the epipolar plane by a known angle amount by inserting the shim of known thickness TB between the lens side fixing surface 21 and the sensor side fixing surface 23. Then, in the same manner as in the measurement A, the images of the chart 13 are picked up at a plurality of distances, and in-screen distortion is measured from the chart 13 at each distance. This process refers to measurement B.
Based on the thickness TB of the inserted shim and the difference of the result of measurement A and the result of measurement B, a sensitivity coefficient C of the shim thickness and in-screen distortion is calculated by the following Expression: C=TB/{(distance-dependent in-screen distortion of measurement B)−(distance-dependent in-screen distortion of measurement A) (Expression 2). Since the change in the relative angle between the optical axis of the image pickup lens 7 and the light-receiving surface of the sensor element 9 is obtained from the shim thickness TB and Expression 1, the sensitivity coefficient C may be obtained as a relationship between the relative angle and in-screen distortion.
Next, using the evaluation result in measurement A, shim thickness necessary for the insertion is obtained by multiplying the sensitivity coefficient C by the distance-dependent in-screen distortion of the measurement A (S203). Alternatively, the relative angle between the optical axis of the image pickup lens 7 to be adjusted and the light-receiving surface of the sensor element 9 is obtained by Expression 1. Then the shim of obtained thickness is inserted, whereby the adjustment unit can perform adjustment (S204).
In the present embodiment, the shim of known shim thickness is inserted, distance-dependent in-screen distortion is measured, and sensitivity is obtained based on the measurement result. Alternatively, sensitivity of the evaluation value of the chart image may be obtained in advance by simulation based on the relative angle between the optical axis of the image pickup lens 7 and the light-receiving surface of sensor element 9, and lens aberration, then the adjustment amount may be calculated using the measurement result of the measurement A.
In the present embodiment, distance-dependent in-screen distortion (i.e., the lateral shift amount of the reference coordinate) is used as the evaluation value. Alternatively, the relative angle and necessary shim thickness may be obtained based on other evaluation values, such as blur quantity (i.e., a tilt amount in the black and white border (edge) in the cross sections 13A and 13B) and light quantity (height of white portion in the cross sections 13A and 13B). Alternatively, distance measurement of the chart may be carried out and the distance of the reference coordinate on each chart in a plurality of distances (a shift amount in the Z direction) may be evaluated.
Next, a measurement apparatus of a second embodiment is described. The present embodiment differs from the first embodiment in the adjustment unit. Description is omitted about the same portions as those of the first embodiment. In the present embodiment, an adjustment mechanism (an adjusting member) 140 that adjusts tilt of the image pickup lens 7 in the direction to cross the epipolar plane is provided between the lens side fixing surface 21 of the image pickup unit 2 and the sensor side fixing surface 23. The direction to cross the epipolar plane is, for example, the direction vertical to the epipolar plane.
The adjustment mechanism 140 is described with reference to
The measurement method of the relative angle between the sensor element 9 and the optical axis of the image pickup lens 7 is the same as that of the first embodiment and is therefore not described. Based on the measured relative angle, the angle θ to be adjusted is calculated by the adjustment mechanism 140 so that the relative angle becomes a predetermined angle. Then the feeding amount Lc of the adjustment screw is obtained from the angle θ to be adjusted, and the relative angle is adjusted by rotating the adjustment screw 43 of the feeding amount.
Next, a measurement apparatus of a third embodiment is described. The measurement apparatus of the present embodiment differs in the adjustment unit from those of the first and the second embodiments. Description is omitted about the same portions as those of the first embodiment. In the present embodiment, the adjustment mechanism 140 for adjusting tilt of the image pickup lens 7 is provided between the fixing surface 23 of the image pickup unit 2 and the image pickup lens 7.
In the present embodiment, as illustrated in
Next, the tilt angle of the sensor element 9 is measured. A relative angle of the sensor element 9 and the fixing surface 81 of the sensor housing 8 is measured. First, as illustrated in
An amount of angle adjustment by the adjustment mechanism 140 is obtained based on the obtained tilt angle of the light-receiving surface of the sensor element 9, whereby the angle of the optical axis of the image pickup lens 7 is adjusted. The adjusting method of the adjustment mechanism 140 is the same as that of the second embodiment.
Preferred embodiments of the present disclosure have been described, but the present disclosure is not limited to the same. Various modifications and changes may be made without departing from the scope of the present disclosure.
For example, the calculation unit 3 is disposed inside the measurement apparatus in the above embodiments, but the calculation unit 3 may be disposed outside the measurement apparatus. In the embodiment, the light pattern extending in the direction vertical to the base line connecting the principal point of the projection lens and the principal point of the image pickup lens is projected. However, if a light pattern tilted from that vertical direction and extending obliquely is projected, the adjustment unit may adjust in the direction vertical to the light pattern (the periodic direction of the pattern) (i.e., in the binary pattern, the direction in which white and black edges are arranged periodically). Although the adjustment unit of the image pickup unit is described in the above embodiments, the same adjustment unit may be applied to the adjustment of the relative angle between the projection lens and the projection pattern setting element regarding the projection unit. Also in the case of the projection unit, depending on the relative angle between the projection lens and the pattern setting element, distortion of the light pattern to be projected changes depending on the distance to the object in the same manner as in the image distortion of the image pickup unit. Therefore, a length measurement error occurs if the distance is calculated based on the picked up result. It is therefore necessary to adjust the relative angle between the projection lens and the projection pattern setting element. Thus, the same adjustment unit as that of the image pickup unit may be applied to the projection unit. In the case of the projection unit, the relative angle between the fixing surface to which the projection lens is mounted and fixed, and the projection pattern setting element is measured, and the adjustment unit may adjust so that the optical axis of the projection lens is directed to an angle to cross the projection pattern setting element. The adjustment unit may adjust the tilt angle of the projection lens in the direction to cross the epipolar plane, and may adjust the angle between the surface including the optical axis of the projection lens and the image side principal point of the image pickup lens, and the light-receiving surface of the sensor.
Although two optical units in the head are the image pickup unit and the projection unit in the above embodiments, the present disclosure may be applied also to a measurement apparatus of a stereo method in which both the optical units are the image pickup units. Specifically, the measurement apparatus includes a first image pickup unit including a first image pickup portion provided with a first light-receiving surface, and configured to receive light from the object on the first light-receiving surface and pick up an image of the object, and a first image-forming optical system configured to guide light from the object to the first light-receiving surface. The measurement apparatus also includes a second image pickup unit including a second image pickup portion provided with a second light-receiving surface, and configured to receive light from the object on the second light-receiving surface and pick up an image of the object, and a second image-forming optical system configured to guide light from the object to the second light-receiving surface, In the case of this measurement apparatus, adjustment by the adjustment unit is performed to at least one image pickup unit, and the tilt angle of the image pickup lens in the direction to cross the epipolar plane passing through the image side principal point and the object point of both the image pickup lenses is made adjustable. Further, the angle between the surface including the optical axis of one image pickup lens and the image side principal point of the other image pickup lens, and the light-receiving surface of the sensor is made adjustable.
In the adjustment mechanism 140 of the second and the third embodiments, a tool with known feeding amount may be used instead of the adjustment screw. Although the adjustment mechanism adjusts the tilt of the image pickup lens, an adjustment mechanism of the tilt of the sensor housing 8 may be provided instead of or in addition to the tilt adjustment of the image pickup lens.
A method for manufacturing an article in the present embodiment is used, for example, in manufacturing an article, such as a metal part and an optical element. The method for manufacturing an article of the present embodiment includes a process of measuring a shape of an object using the measurement apparatus, and a process of processing the object based on a measurement result in the measuring process. For example, the shape of the object is measured using the measurement apparatus, and the object is processed (i.e., manufactured) so that the shape of the object becomes a design value based on the measurement result. Since the shape of the object may be measured with high accuracy with the measurement apparatus, the method for manufacturing an article of the present embodiment is advantageous in at least one of performance, quality, productivity, and production cost of article as compared to the related art methods.
While the present disclosure has been described with reference to exemplary embodiments, it is to be understood that the disclosure is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.
This application claims the benefit of priority from Japanese Patent Application No. 2014-186878, filed Sep. 12, 2014, which is hereby incorporated by reference herein in its entirety.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2014-186878 | Sep 2014 | JP | national |
