Displacement Detection Using Diffraction Grating Interferometry
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Solution Overview
Problem
Conventional displacement detecting devices face challenges in accurately measuring vertical displacement of surfaces due to sensitivity issues, susceptibility to surface tilt, and limitations in measuring rapidly oscillating objects, as well as errors caused by foreign matter and beam shape changes.
Innovation Solution
A displacement detecting device utilizing a light source, beam splitting section, diffraction grating, reflecting section, beam combining section, and light receiving section, where the diffraction grating is arranged perpendicular to the measured surface, and the optical path lengths of the first and second beams are set equal to maintain stability despite wavelength variations, eliminating the need for a conventional drive mechanism.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a laser and PSD method is used to detect displacement, then the measurement can be performed non-contactly, but the sensitivity is low and the measurement is susceptible to surface tilt influence
Solution Approach 1:
The patent introduces a diffraction grating as an intermediary element between the laser and the measured surface. The grating converts the laser beam into multiple diffracted beams that illuminate different areas of the surface, and the reflected light is collected by a CCD camera. This intermediary structure eliminates the sensitivity issues of direct laser-PSD methods and makes the measurement immune to surface tilt effects.
Solution Approach 2:
The patent divides the single laser beam into multiple beams through the diffraction grating, creating several measurement points simultaneously. This segmentation allows parallel measurement of multiple surface locations, improving both accuracy and robustness against surface tilt by averaging or selecting appropriate measurement points.
2Area of stationary object
If the measurement range is widened in the laser-PSD method, then more areas can be measured, but the measurement resolution is degraded
Solution Approach 1:
The diffraction grating segments the laser beam into multiple parallel beams that can cover a wider measurement area. Each beam maintains its individual resolution characteristics, allowing the system to achieve both wide measurement range and high resolution simultaneously by measuring multiple points in parallel across the expanded area.
3Area of stationary object
If a Michelson interferometer is used to achieve wide measurement range and good linearity, then the measurement range becomes wide, but the measurement is affected by variation in wavelength of the light source and variation in refractive index of the air
Solution Approach 1:
The diffraction grating serves as a mediator that creates a measurement system less sensitive to environmental variations. By using geometric optics principles through the grating equation rather than interferometric methods, the system achieves wide measurement range while being more robust against wavelength variations and air refractive index changes that plague interferometric systems.
4Measurement precision
If the objective lens is moved up and down driven by an actuator to perform focus error signal-based displacement detection, then the measurement can be performed with high resolution, but the mechanical response frequency limits the ability to measure rapidly oscillating objects
Solution Approach 1:
The patent replaces the mechanical actuator-driven objective lens system with a stationary optical system using a diffraction grating and CCD camera. This substitution eliminates the mechanical response limitations, allowing the system to capture rapid oscillations without the bandwidth constraints imposed by motor-driven focus mechanisms.
Solution Approach 2:
Instead of physically moving the objective lens to track surface oscillations, the system uses the diffraction grating to create multiple virtual measurement points that capture the surface dynamics. The CCD camera records the light intensity changes at these points, creating a temporal copy of the surface oscillation without requiring mechanical movement.
5Measurement precision
If the beam diameter is reduced to about 2 μm to improve detection accuracy, then the linear scale detection accuracy improves to several nm to several hundreds nm, but the measurement is more affected by foreign matter and beam-like minute shape changes
Solution Approach 1:
The diffraction grating divides the narrow laser beam into multiple beams that illuminate different areas of the surface simultaneously. This segmentation reduces the impact of foreign matter or localized surface irregularities on any single measurement point, as the system can use data from multiple beams to compensate or identify anomalies.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration enables accurate and stable high-speed displacement measurement without mechanical response frequency limitations, reducing errors from environmental changes and foreign matter, and improving measurement resolution.
Implementation Method 1
a beam splitting section for splitting light emitted from the light source into a first beam to be incident on the member-to-be-measured and a second beam
Implementation Method 2
a diffraction grating for diffracting the first beam split by the beam splitting section and reflected by a surface-to-be-measured of the member-to-be-measured
Implementation Method 3
a reflecting section for reflecting the second beam split by the beam splitting section
Implementation Method 4
a beam combining section for superimposing the first beam diffracted by the diffraction grating and reflected again by the surface-to-be-measured and the second beam reflected by the reflecting section on each other
Implementation Method 5
a light receiving section for receiving interfering light of the first beam and the second beam superimposed by the beam combining section
Data Source
Figure 1
Figure 2~3
Figure 4
AI summary
A displacement detecting device (1) includes a light source (2), a beam splitting section (3), a diffraction grating (4), a reflecting section (6), a beam combining section (3), a light receiving section (8), and a relative position information output section (10). The diffraction grating (4) is adapted to diffract a first beam (L1) reflected by a surface-to-be-measured of a member-to-be-measured (9), and cause the diffracted first beam (L1) to be incident again on the surface-to-be-measured. The reflecting section (6) is adapted to reflect a second beam (L2) split by the beam splitting section to the beam splitting section (3). The light receiving section (8) is adapted to receive interfering light of the first beam (L1) and the second beam (L2). The relative position information output section is adapted to output displacement information of the surface-to-be-measured in the height direction based on intensity of the received interfering light.