Reflective Encoder Polarization Filter Noise Reduction

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Solution Overview

Problem

Reflective encoders suffer from poor signal-to-noise ratios due to internal light reflections within the source-detector module, which complicates their use in compact mechanical systems where precise alignment is challenging, and they require less expensive assembly compared to transmissive encoders.

Innovation Solution

The encoder design incorporates a light source and photodetector encapsulated in a transparent medium with a gap, utilizing linearly polarized light and a polarization filter to reduce Fresnell-reflected light interference, enhancing the signal-to-noise ratio by blocking partially polarized background illumination.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Ease of manufacture

If reflective encoders use a single emitter-detector element with encapsulated light source and photodetector, then assembly cost is reduced and manufacturing is simplified, but signal-to-noise ratio deteriorates due to internal light reflections

Engineering Contradiction:

Improveassembly costVSAvoidsignal-to-noise ratio

Core Design Contradiction:

Ease of manufactureVSReliability

Solution Approach 1:

The patent converts the harmful effect of internal light reflections into a beneficial solution by introducing a polarization filter. The filter blocks the polarized reflected light from the encapsulation-air boundary while allowing the non-polarized or differently polarized signal light from the code wheel to pass through, thus transforming the noise problem into a solvable optical filtering application

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

Solution Approach 2:

The patent changes the optical parameter of light polarization to differentiate between signal and noise. By using a linearly polarized light source and corresponding polarization filter, the system exploits polarization state differences to separate the useful reflected light from the code wheel from the harmful internal reflections at the encapsulation boundary

Inventive Principle:

Principle #35Parameter changes

2Volume of moving object

If reflective encoders are used in compact mechanical systems, then space requirements are reduced, but alignment precision becomes more difficult to maintain

Engineering Contradiction:

Improveencoder sizeVSAvoidalignment precision

Core Design Contradiction:

Volume of moving objectVSManufacturing precision

Solution Approach 1:

The patent merges the light source and photodetector into a single integrated emitter-detector element with shared optical components. This consolidation reduces the number of separate components that need alignment, simplifying the mechanical structure and reducing the overall encoder size while maintaining measurement functionality

Inventive Principle:

Principle #5Merging (Combining)

Solution Approach 2:

The polarization filtering mechanism provides self-alignment tolerance because it operates optically rather than mechanically. The polarization filter automatically distinguishes between signal and noise based on polarization state, reducing sensitivity to mechanical misalignment between the code wheel and detector

Inventive Principle:

Principle #25Self-service

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 improves the signal-to-noise ratio by minimizing background light interference, allowing for more accurate position measurement in compact systems while maintaining cost-effective assembly.

Implementation Method 1

a light source that generates light and directs a portion of the generated light to the imaging element

Methodology Applied

Scientific EffectLight propagation: Light

Implementation Method 2

a photodetector that generates a signal representing an intensity of light received by the photodetector

Methodology Applied

Scientific EffectPhotoelectric effect: Photoelectric Effect

Implementation Method 3

Part of the light generated by the light source is reflected at the encapsulation-air boundary back toward the detector

Methodology Applied

Scientific EffectFresnel reflection: Reflection

Implementation Method 4

A polarization filter can also be included in the light source to further improve the rejection of the light reflected from the interface of the transparent medium and the gap

Methodology Applied

Scientific EffectPolarization filtering: Polarisation

Data Source

PatentUS7304294B2Reflective encoder with reduced background noise

Publication Date: 2007.12.04 AVAGO TECHNOLOGIES INTERNATIONAL SALES PTE LTD

AI summary

An encoder having a code scale and an emitter detector module is disclosed. The code scale includes alternating reflective and opaque stripes. The emitter-detector module includes a light source that generates light and directs a portion of the generated light to the imaging element and a photodetector that generates a signal representing an intensity of light having a linear polarization in a predetermined direction received by the photodetector, the light source and the photodetector are encapsulated in a transparent medium, wherein there is a gap between the transparent medium and the code scale. The predetermined direction is chosen to reduce the intensity of light received by the photodetector that is reflected from an interface between the transparent medium and the gap. A polarization filter can also be included in the light source to further improve the rejection of the light reflected from the interface of the transparent medium and the gap.