Doubly Telecentric Encoder Lens Aberration Correction
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Solution Overview
Problem
Existing telecentric encoder configurations fail to provide a balance of economy, robustness, accuracy, and compact dimensions due to lens aberration errors that cause destructive interference in the periodic scale image, leading to reduced signal-to-noise ratio and resolution.
Innovation Solution
The implementation of a doubly telecentric encoder configuration with complementary lenses that have similar optical aberrations and lens thickness deviations, ensuring symmetric optical paths for different orders of light, thereby suppressing destructive interference and enhancing image contrast.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If telecentric imaging systems are used to provide desirable encoder features, then measurement reliability is improved, but lens aberration errors cause destructive interference that reduces measurement precision
Solution Approach 1:
The patent uses symmetric arrangement of optical components (lenses and apertures) to create balanced optical paths. The first and second telecentric lenses are positioned symmetrically about the optical axis, and the first and second apertures are arranged symmetrically, ensuring that aberrations in one path are compensated by the other path, thereby eliminating destructive interference while maintaining telecentric functionality
Solution Approach 2:
The patent compensates for lens aberration errors by introducing complementary optical paths with opposing characteristics. The symmetric arrangement of lenses and apertures creates counterbalancing optical paths where aberrations in one path are offset by corresponding aberrations in the other path, effectively canceling out the harmful effects on measurement precision
2Illumination intensity
If multiple orders of light are present in the scale image, then more light contributes to the image, but different path lengths through aberrated lenses cause phase differences leading to destructive interference and reduced signal-to-noise ratio
Solution Approach 1:
The symmetric optical configuration ensures that all diffracted light orders (0th, ±1st, ±2nd, etc.) traverse equal total path lengths through the lens-aperture-lens system. This symmetry equalizes the phases of all light orders at the detector plane, causing constructive interference that enhances the signal-to-noise ratio while allowing multiple orders to contribute to image formation
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 significantly improves the signal-to-noise ratio and accuracy of displacement measurements by ensuring that all orders of light have similar optical path lengths and phases, resulting in a higher resolution and more robust displacement sensing.
Implementation Method 1
The various diffracted orders of light may follow different paths through the telecentric lenses
Implementation Method 2
The complementary lenses are configured such that the symmetric optical paths nominally include similar or identical optical aberrations or lens thickness deviations (in total, along the optical paths), and thus have similar optical path lengths
Implementation Method 3
the orders of light may destructively interfere at various regions of the photodetector that sense the periodic scale image, reducing its S/N ratio
Data Source
AI summary
A method and apparatus for suppressing loss of scale image contrast due to interference effects from optical path length differences, including lens profile aberrations, are provided in a displacement sensing optical encoder that uses a telecentric imaging configuration. According to the invention, the encoder is configured such that the image light that reaches the detector comprises symmetric ray bundles concentrated symmetrically on opposite sides of the optical axis center at the limiting aperture of the telecentric imaging configuration. Central ray bundles, and/or other ray bundles that have an optical path lengths significantly different than the operational symmetric ray bundles are prevented or blocked. As a result destructive interference is prevented and image contrast is improved.