Method and apparatus for non-contact measuring of object surfaces

• US 5,135,309 A
• Filed: 03/08/1991
• Issued: 08/04/1992
• Est. Priority Date: 03/09/1990
• Status: Expired due to Term

• Current Assignee: Carl-Zeiss-Stiftung
• Original Assignee: Carl-Zeiss-Stiftung
• Inventors: Hof, Albrecht, Kuchel, Michael
• Primary Examiner(s): Rosenberger, Richard A.
• Application Number:
  US07/666,247
• Time in Patent Office: 515 Days
• Field of Search: 356/376, 250/237 G
• US Class Current: 356/604
• CPC Class Codes: G01B 11/2531 using several gratings, projected with variable angle of incidence on the object, and one detection device
• CPC Class Codes: G06T 7/521 from laser ranging, e.g. using interferometry; from the projection of structured light

Abstract

Several bar patterns are projected in sequence on the object (O) to be measured by time-division multiplexing, and images of the bar patterns are recorded by a camera (K). The phases of each bar pattern, as distorted by the object, are calculated for preselected image points by a computer connected with the camera. For each image point, the calculated phases for one of said bar patterns are compared to the phases calculated for at least one other of said bar patterns, thereby producing a beat frequency which can be used to determine height measurements in the direction of the camera axis (z). In order to increase the range of the height measurements, at least two beat frequencies of quite different effective wavelengths are generated and evaluated. Different systems are disclosed for generating the different beat frequencies. In one embodiment, the bar patterns are projected by three different projectors (P₁, P₂, P₃) which are inclined at different angles relative to each other (α₁, α₂). In a second embodiment, only two projectors are used, but each projector has two gratings, the respective periods of which differ from each other only slightly.

Claims

1. In apparatus for the non-contact measurement of object surfaces, said apparatus having:

at least two projectors for projecting bar patterns onto the surface of the object to be measured;

a video camera (K) having an axis aligned with the object to be measured for recording images of bar patterns reflected from a plurality of points on the surface of the object; and

computer means for evaluating said recorded images to determine height information (z) for said plurality of points;

the improvement wherein said projectors project at least two pairs of gratings (G₁₁ /G₁₃, G₁₂ /G₁₄), the respective gratings of each pair being projected by a respective one of said projectors, the gratings of each pair having the same period but the periods of the two pairs of gratings being slightly different so that the periods of said two gratings pairs differ from each other by an amount which is small when compared to the period of each grating.

2. The apparatus according to claim 1 further comprising color control elements (F₁₁ F₁₂, F₁₃ F₁₄) for causing said bar patterns to be projected in different colors, and wherein said camera (K) is a color camera.

3. In a method for the non-contact measurement of object surfaces by projecting bar patterns on the object surface, detecting reflections of said patterns with a camera, and evaluating the camera images of said reflections to calculate height measurements for a plurality of points on said object surface, the improvement comprising the steps of:

projecting several bar patterns on the object surface, said patterns having the same period or slightly different periods;

comparing the detected reflections of said bar patterns; and

creating at least two beat frequencies from said reflected patterns so that each said beat frequency has a different effective wavelength.

4. The method according to claim 3 comprising the further steps of:

separately recording the image of each bar pattern at each of said plurality of points;

calculating the bar phase (δ

₁, δ

₂, δ

₃) of each reflected bar pattern for each said point; and

computing the differences (Δ

₁, Δ

₂, Δ

₃) between the bar phases calculated for each bar pattern for each said point and using said computed bar phase differences to determine the height measurement (z) for each point in the direction of the camera axis.

5. The method according to claim 3 wherein:

said bar patterns all have the same period; and

each said bar pattern is projected through a respective projection center so that, for each said point on the object surface, the respective projections through said respective projection centers and said point are inclined at different angles with respect to each other.

6. The method according to claim 5 wherein said bar patterns are created by illuminating three gratings (G₁, G₂, G₃) positioned on a common plane.

7. The method according to claim 3 wherein said bar patterns are created by illuminating two pairs of gratings (G₁₁ /G₁₂, G₁₃ /G₁₄), each pair of gratings having the same period, but the periods of the two pairs of gratings being slightly different so that the periods of said two grating pairs different from each other by an amount which is small when compared to the period of each pair.

8. The method according to claim 7 wherein at least two of said bar patterns (G₁₁, G₁₂, G₁₃, G₁₄) are projected separately using illumination of different colors or different polarization.

9. Apparatus for the non-contact measurement of object surfaces, comprising:

at least two projectors for projecting respective bar patterns onto the surface of the object to be measured, each projector having a respective projection center;

camera means for recording images of bar patterns reflected from a plurality of points on the surface of the object, said camera means having a lens with a lens center and an axis aligned with the object to be measured;

said projectors being arranged so that projections through said respective projection centers and each said point on the object surface are inclined, respectively, at different angles to axes through each said point and said lens center; and

means for evaluating the phase differences between said projected patterns at each of said plurality of points.

10. The apparatus of claim 9 further comprising at least one additional projector (P₁), each projector having a respective grating (G₁, G₂, G₃) for projecting a respective one of said bar patterns, and wherein at least two of said gratings having the same periodicity and all of said gratings are positioned on a common support (W).

11. The apparatus of claim 10 wherein all of said gratings (G₁, G₂, G₃) are positioned in the same plane and wherein said common support is movable relative to the optical axis of the camera in the plane of said gratings.