Contactless Rotation Detection via Heterodyne Interferometry
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Solution Overview
Problem
Conventional methods for measuring rotation in precision measurements are limited by coupling with linear movements, leading to measurement errors and beam walkoff, making it difficult to accurately detect rotational movements without influencing linear displacement and refractive index fluctuations.
Innovation Solution
An interferometric system using a probe beam with two monochromatic wavelengths of opposite chirality and a quarter wave plate attached to the rotating object, which directs the beam through a neutral beam splitter and photodetectors to measure phase differences, isolating rotational movement from linear displacement and refractive index effects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional linear displacement measurement is used to measure rotation, then rotational speed can be detected, but linear displacement of the object influences the measurement and causes beam walkoff
Solution Approach 1:
The patent replaces conventional mechanical/linear displacement-based rotation measurement with an optical interferometric system that directly measures rotation. The system uses a probe beam with opposite circular polarizations that interacts with a quarter-wave plate on the rotating object, detecting rotation through optical phase differences rather than linear displacement, thereby eliminating the harmful coupling between linear and rotational movements.
Solution Approach 2:
The patent changes the measurement parameter from linear displacement to optical phase difference. By using a probe beam with two monochromatic wavelengths of opposite chirality and measuring the phase difference between them after interaction with the rotating quarter-wave plate, the system directly detects rotational movement without being affected by linear displacement or refractive index fluctuations.
2Measurement precision
If linear displacement measurement is used for rotation detection, then rotational movement can be measured, but beam walkoff on detectors occurs which limits maximum measurable angular rotation
Solution Approach 1:
The patent replaces the conventional linear measurement approach that suffers from beam walkoff with an optical interferometric system. The system maintains the probe beam coaxial with the rotation axis throughout the measurement process, using optical phase detection rather than spatial separation of linear measurements, thereby eliminating beam walkoff and its limitations on maximum measurable angular rotation.
3Measurement precision
If conventional interferometry is used to measure rotation, then measurement is possible, but the system is sensitive to refractive index fluctuations in the propagation medium
Solution Approach 1:
The patent changes the measurement approach to use a probe beam with two monochromatic wavelengths of opposite chirality. The rotation measurement is derived from the phase difference between these two wavelengths after they interact with the rotating quarter-wave plate. This differential measurement approach makes the system insensitive to refractive index fluctuations, as both wavelengths experience the same environmental conditions but opposite rotation-induced phase shifts.
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 method allows for improved resolution in measuring rotation independently of linear displacement and refractive index fluctuations, providing accurate rotational measurements without interference from object movement or environmental factors.
Implementation Method 1
providing the probe beam comprised of two monochromatic wavelengths with circular polarizations of opposite chirality, with a fixed frequency difference for providing a heterodyne probe beam
Implementation Method 2
optical interferometry for detecting an optical path difference between a reference beam and a test beam. A phase shift between the test beam and the reference beam leads to fringes, which can be counted, due to the principle of optical wave interference
Implementation Method 3
By a quarter wave plate the optical beam is converted into a beam with circular clockwise and counter clockwise polarizations
Implementation Method 4
The neutral beam splitter may split the beam into a 50% beam directed to a reference photodetector, and into a 50% beam directed to the rotational object to be measured
Implementation Method 5
directs a reflected beam via a polarizer filter towards a first photodetector and that directs a transmitted beam towards a quarter wave plate
Data Source
AI summary
The invention pertains to a contactless measurement method for detecting rotation of an object over an axis coinciding with an optical axis of a probe beam. The probe beam is comprised of two monochromatic wavelengths with circular polarizations of opposite chirality, having a frequency difference for providing a heterodyne probe beam. A neutral beam splitter is provided that directs a reflected beam via a polarizer filter towards a first photodetector and that directs a transmitted beam toward a quarter wave plate attached to a rotatable object. A mirror reflects the probe beam, via the same quarter wave plate, back into the neutral beam splitter, which directs the reflected beam via a polarizer filter toward a second photodetector. The rotation is derived from the relative phase difference between the first and second photodetector signals.