Double Pass Interferometric Encoder Beam Shear Compensation
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Solution Overview
Problem
Interferometric encoder systems face measurement inaccuracies due to optical errors such as beam mixing and beam shear, which are not effectively addressed by existing technologies, leading to deviations in displacement change measurements.
Innovation Solution
A double pass interferometric encoder system is designed with specific optical components that direct and redirect beams at distinct angles, reducing interference from ghost beams and compensating for beam shear, thereby enhancing measurement accuracy by configuring the measurement beam to make two passes through the encoder scale.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a single pass interferometric encoder system is used, then the device complexity is low, but measurement precision deteriorates due to optical errors such as beam mixing and beam shear
Solution Approach 1:
The optical path is segmented into two separate passes through the encoder scale. The measurement beam traverses the encoder scale twice, allowing the system to compensate for optical errors by comparing phase information from both passes. This segmentation of the measurement process into multiple stages enables error cancellation while maintaining reasonable system complexity.
Solution Approach 2:
An intermediary optical component (such as a retroreflector or additional beam steering element) is introduced to redirect the measurement beam back through the encoder scale. This intermediary element enables the double-pass configuration by mediating the beam's return path, allowing error compensation without requiring a complete redesign of the entire optical system.
2Measurement precision
If the measurement beam is directed at a large angle to the encoder scale, then beam shear is reduced, but beam mixing increases and measurement precision deteriorates
Solution Approach 1:
The measurement beam performs periodic interaction with the encoder scale by making two passes through it. The first pass establishes a reference phase, and the second pass provides a corrected measurement. This periodic interaction allows the system to distinguish between actual displacement signals and beam mixing artifacts, improving measurement precision while operating at optimal beam angles.
Solution Approach 2:
The system changes the operational parameters of the beam by varying its angle of incidence and utilizing both forward and backward passes through the encoder scale. By adjusting these parameters and analyzing phase differences from multiple passes, the system can compensate for beam mixing effects and maintain high measurement precision without requiring extremely small beam angles.
3Measurement precision
If optical components are added to implement double pass configuration, then beam shear is compensated, but device complexity increases
Solution Approach 1:
Existing optical components in the encoder system are designed to serve multiple functions. The beam steering components not only direct the measurement beam but also inherently compensate for beam shear through their geometric arrangement. The retroreflector or additional steering element serves both to create the double-pass path and to correct angular deviations, reducing the need for separate compensation components and limiting the increase in overall system complexity.
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
The double pass configuration significantly reduces measurement errors caused by ghost beams and beam shear, improving the accuracy of displacement measurements and compensating for small changes in orientation and position, thus enhancing the overall precision of the interferometric encoder system.
Implementation Method 1
The encoder head directs a first incident beam to the diffractive encoder scale... receive a first return beam from the diffractive encoder scale... redirect the first return beam to the diffractive encoder scale as a second incident beam
Implementation Method 2
an interferometer generates the optical interference signal by overlapping and interfering a measurement beam reflected from the measurement object with a second beam, sometimes called a 'reference beam'... Changes in the relative position of the measurement object correspond to changes in the phase of the measured optical interference signal
Data Source
AI summary
An encoder head includes one or more components arranged to: i) direct a first incident beam to the diffractive encoder scale at a first incident angle with respect to the encoder scale; ii) receive a first return beam from the encoder scale at a first return angle, the first return angle being different from the first incident angle; iii) redirect the first return beam to the encoder scale as a second incident beam at a second incident angle; and iv) receive a second return beam back from the encoder scale at a second return angle, the second return angle being different from the second incident angle, in which a difference between the first incident angle and second incident angle is less than a difference between the first incident angle and the first return angle and less than a difference between the second incident angle and the second return angle.