Method and apparatus for non-contact three-dimensional surface measurement

• US 7,286,246 B2
• Filed: 03/22/2004
• Issued: 10/23/2007
• Est. Priority Date: 03/31/2003
• Status: Active Grant

• Current Assignee: Mitutoyo Corporation
• Original Assignee: Mitutoyo Corporation
• Inventors: Yoshida, Hiroyuki
• Primary Examiner(s): Nguyen; Sang H.
• Application Number:
  US10/805,435
• Publication Number:
  US 20040246496A1
• Time in Patent Office: 1,310 Days
• Field of Search: 356600-625, 250/237.G, 250/559.22, 382/286, 382/149, 382/141
• US Class Current: 356/605
• CPC Class Codes: G01B 11/2527 with phase change by in-plane movement of the patern
• CPC Class Codes: G01B 11/254 Projection of a pattern, viewing through a pattern, e.g. moiré

Abstract

A non-contact three-dimensional surface measurement method is provided in which a grating pattern projected onto an object being measured, while the phase of the pattern is being shifted, is observed in a different direction from a projection direction to analyze the contrast of a grating image deformed in accordance with the shape of the object and thereby obtain the shape thereof. The method enables measurement of a three-dimensional shape over a large measurement range in a short time in a non-contact manner by successively shifting the focus on the projection and the imaging sides to enlarge the measurement range in the direction of depth.

Claims

1. A non-contact three-dimensional surface measurement method comprising the steps of:

observing a grating pattern projected onto an object being measured, while a phase of the pattern is being shifted, in a different direction from a projection direction;

analyzing contrast of a grating image deformed in accordance with a shape of the object and thereby obtaining the shape thereof;

shifting focuses of the projection and imaging sides back and forth continuously, by altering a position of a grating filter and an imagine device with respect to a projection lens and an imaging lens, respectively, to enlarge a measurement range in a direction of depth;

successively entering image data of the object while the focus and the phase are being shifted at a constant speed; and

preparing a three-dimensional map by processing the entered data in a computer, by determining an order of a fringe from a focal center and unwrapping the phase to calculate an absolute value of the phase, and calculating a three-dimensional position of a point of intersection of a grating plane of the fringe order and an epipolar line of an imaging point.

2. The non-contact three-dimensional surface measurement method according to claim 1, wherein the step of preparing the three-dimensional map further comprises the steps of:

extracting a intensity waveform at image coordinates;

eliminating an offset component and a fluctuating component of the intensity waveform;

calculating a focal center; and

calculating a phase of the focal center.

3. The non-contact three-dimensional surface measurement method according to claim 2, wherein the step of eliminating the offset component and the fluctuating component of the intensity waveform is carried out to calculate a differential waveform of the intensity waveform.

4. The non-contact three-dimensional surface measurement method according to claim 2, wherein the step of eliminating the offset component and the fluctuating component of the intensity waveform is carried out by averaging intensity value in the vicinities plus or minus π

at one phase shift value of the intensity waveform.

5. The non-contact three-dimensional surface measurement method according to claim 2, wherein the step of calculating the focal center is carried out such that a frequency average for each of squared waveforms of curves indicative of every n-th variations in intensity is determined being weighted corresponding to a height of a crest of an extracted waveform.

6. The non-contact three-dimensional surface measurement method according to claim 2, wherein the step of calculating the phase of the focal center is carried out to determine the phase through multiplication by a mother waveform and by Fourier integration.

7. A non-contact three-dimensional surface measurement apparatus in which a grating pattern projected onto an object being measured, while a phase of the pattern is being shifted, is observed in a direction different from a projection direction to analyze contrast of a grating image deformed in accordance with a shape of the object being measured and thereby obtaining the shape thereof, the apparatus comprising:

means for projecting a grating pattern onto the object being measured while shifting a focus back and forth continuously, by altering a position of a grating filter with respect to a projection lens, and shifting a phase;

means for entering image data of the pattern projected onto the object being measured while the focus is being shifted back and forth continuously, by altering a position of an imaging device with respect to an imaging lens; and

means for preparing a three-dimensional map by processing the image data entered, wherein the means for preparing the three-dimensional map further comprises;

means for determining an order of a fringe from a focal center and unwrapping the phase to calculate an absolute value of the phase, and

means for calculating a three-dimensional position of a point of intersection of a grating plane of the grating order and an epipolar line of an imaging point.

8. A non-contact three-dimensional surface measurement apparatus in which a grating pattern projected onto an object being measured, while a phase of the pattern is being shifted, is observed in a direction different from a projection direction to analyze contrast of a grating image deformed in accordance with a shape of the object being measured and thereby obtaining the shape thereof, the apparatus comprising:

means for projecting a grating pattern onto the object being measured while shifting a focus back and forth continuously, by altering a position of a grating filter with respect to a projection lens, and shifting a phase;

means for entering image data of the pattern projected onto the object being measured while the focus is being shifted back and forth continuously, by altering a position of an imaging device with respect to an imaging lens; and

means for preparing a three-dimensional map by processing the image data entered, wherein the means for preparing the three-dimensional map comprises a computer, the computer comprising;

a frame grabber for capturing an input image from an imaging device of a measurement head,

a motion control board for controlling a shift mechanism of the measurement head,

software for controlling the measurement head, the software having a function of entering a sequence of images while shifting a focus and a phase at the same time to transfer the images to a main memory of the computer, a function of displaying online an image corresponding to a measurement range of the measurement head, and a function of setting a range of a measurement depth, and

software for preparing the three-dimensional map, the software having a function of calculating a focal center from a intensity waveform having a varying amplitude resulting from a focus shift, a function of calculating a phase of the intensity waveform near the focal center, a function of unwrapping the phase using focal center data, a function of correcting coordinates on an image considering distortion of an optical system, a function of determining a point of intersection of an epipolar line starting from a pixel in the imaging device and a grating plane of a projected grating pattern to calculate a three-dimensional position of the pattern projected onto a pixel, a function of reproducing the sequence of entered images to check for the image data having been properly entered, a function of displaying the three-dimensional map prepared, and a function of storing data on a group of points of the three-dimensional map in a CAD format.