Displacement Detection Using Optical Path Length Control
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Solution Overview
Problem
Existing displacement detecting devices face challenges in accurately measuring high-speed vibrations and are prone to errors due to foreign objects or small shape changes, with limitations in measurement range and sensitivity, especially when the measured surface is inclined or has a small beam diameter.
Innovation Solution
A displacement detecting device that employs a first diffraction grating on the measured surface and a head with a light source, displacement detecting unit, and light receiving unit, utilizing a second diffraction grating and reference reflecting member to maintain constant optical path length and reduce the need for mechanical driving mechanisms, allowing for accurate measurement without mechanical response frequency limitations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If an actuator including a magnet and a coil is used to move the objective lens upward/downward, then the objective lens can be positioned accurately, but the mechanical response frequency is limited by the structure and weight of the actuator
Solution Approach 1:
The patent replaces the mechanical actuator system with an optical interference measurement system. Instead of mechanically moving the objective lens to measure displacement, the invention uses light interference patterns to detect displacement optically, eliminating mechanical response limitations entirely
Solution Approach 2:
The patent introduces light as an intermediary to transfer displacement information from the measured object to the detector. By using light waves to carry measurement information rather than direct mechanical contact, the system achieves high-speed measurement without mechanical inertia constraints
2Measurement precision
If the beam diameter is reduced to narrow down the detection point, then measurement precision improves, but errors increase due to influence by foreign objects or small shape changes
Solution Approach 1:
The patent transitions from one-dimensional point detection to two-dimensional area detection by analyzing interference patterns across multiple pixels. This dimensional expansion allows the system to maintain precision while reducing sensitivity to local disturbances like foreign objects or small shape changes
Solution Approach 2:
The patent creates multiple virtual images of the measurement point through the diffraction grating system, allowing simultaneous observation of multiple positions. This copying approach enables averaging or comparison of multiple measurements, reducing the impact of localized errors from foreign objects or surface irregularities
3Measurement precision
If a large numerical aperture objective lens is used to improve accuracy, then detection accuracy improves, but the beam diameter on the measured surface becomes small
Solution Approach 1:
The patent uses a diffraction grating to split the optical path into multiple beams that illuminate different areas simultaneously. This transforms a single-point measurement into a multi-point measurement, maintaining high accuracy while reducing sensitivity to foreign objects through spatial diversity
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
Enables high-accuracy, stable measurement of displacement in the height direction of a measured member at high speeds, reducing errors from foreign objects and small shape changes, and expanding the use conditions of the device.
Implementation Method 1
a first diffraction grating provided on a measured surface of a measured member
Implementation Method 2
the light receiving unit receives the second light flux and the first light flux that returns from the first diffraction grating
Data Source
AI summary
A displacement detecting device includes a first diffraction grating, a light source, a displacement detecting unit, and a light receiving unit. The displacement detecting unit includes a light flux dividing unit, a second diffraction grating, and a reference reflecting member. An incident angle of a first light flux to the first diffraction grating, a diffraction angle of the first diffraction grating, an incident angle of the first light flux to the second diffraction grating, and a diffraction angle of the second diffraction grating are angles at which a displacement amount in an optical path length of the first light flux from the light flux dividing unit to the first diffraction grating and a displacement amount in an optical path length of the first light flux from the first diffraction grating to the second diffraction grating become equal in a case where a measured member is displaced in a direction orthogonal to a measured surface.