Slanted Code Slits for Lower Harmonic Distortion in Optical Encoders
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Solution Overview
Problem
Optical encoding systems suffer from total harmonic distortion in photocurrents, leading to degraded positioning accuracy and error signals due to imperfect 90-degree phase shift in differential quadrature signals and optical magnification mismatch.
Innovation Solution
The optical encoding system employs slanted code slits and photodiodes relative to the radial direction of the code disk to reduce total harmonic distortion, using a slant angle to improve positioning accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional code slits and photodiodes are arranged radially, then the structure is simple and easy to manufacture, but total harmonic distortion occurs degrading positioning accuracy
Solution Approach 1:
The code slits are arranged at a slant angle relative to the radial direction, breaking the symmetric radial arrangement. This asymmetric configuration causes the photocurrents from adjacent photodiodes to have different harmonic distortion characteristics, allowing for distortion compensation and improved positioning accuracy.
Solution Approach 2:
The arrangement parameters of the code slits are changed by introducing a slant angle θ relative to the radial direction. This parameter change transforms the conventional radial arrangement into a slanted arrangement that reduces total harmonic distortion in the photocurrents, thereby improving measurement precision.
2Measurement precision
If differential index is used to confirm absolute position, then positioning capability is achieved, but error index signal is generated due to optical magnification mismatch
Solution Approach 1:
The slanted arrangement of code slits creates asymmetric optical path lengths for different photodiodes, which compensates for optical magnification mismatch. This asymmetric geometry ensures that the differential index method can accurately determine absolute position without generating error index signals.
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 solution effectively reduces total harmonic distortion from 3.63% to 0.122%, enhancing positioning accuracy and minimizing error signals.
Implementation Method 1
The photodiode array receives modulated light from the code disk to output differential quadrature signals
Data Source
AI summary
There is provided an optical encoding system including a photodiode array and a code disk opposite to each other. The code disk is arranged with multiple code slits at a ring area corresponding to the photodiode array. A length direction of each photodiode of the photodiode array has at least one deviation angle with respect to a length direction of the multiple code slits to reduce the total harmonic distortion in photocurrents.