Photoelectric Encoder Harmonic Removal via Grating Shift

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Solution Overview

Problem

Existing three-grating type photoelectric encoders face challenges in removing harmonic components without increasing manufacturing costs, particularly struggling to eliminate odd-order harmonic components like the 3rd order, and existing solutions either require costly variations in light receiving element patterns or are limited in their effectiveness.

Innovation Solution

The solution involves shifting parts of the first grating by P/(2n) in the direction of the measurement axis to remove harmonic components of the nth order, with options including dividing or continuously varying the grating pattern, and optionally shifting the third grating to enhance harmonic removal.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If phase difference or line width modulation is provided to remove harmonic components, then measurement precision is improved, but device complexity and manufacturing costs increase due to varying light receiving element patterns

Engineering Contradiction:

Improveharmonic component removalVSAvoidlight receiving element pattern variation

Core Design Contradiction:

Measurement precisionVSDevice complexity

Solution Approach 1:

The invention changes the grating pattern parameters (line widths and spaces) of the first and third gratings to specific values that satisfy mathematical relationships (W1+S1=P and W3+S3=P where W1≠W3). This parameter modification allows the system to remove harmonic components including odd-order harmonics without requiring complex light receiving element configurations, thereby improving measurement precision while avoiding increased device complexity

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

Instead of modifying the light receiving element pattern to remove harmonics (conventional approach), the invention inverts the approach by modifying the grating patterns themselves. By making the first and third gratings have different line width and space configurations, the system generates interference fringes that inherently cancel out harmonic components, including the problematic 3rd order harmonics, without requiring complex detector configurations

Inventive Principle:

Principle #13The other way round (Inversion)

2Ease of manufacture

If light receiving array is used instead of third grating with phase difference, then manufacturing costs are reduced, but only even-order harmonic components can be removed while odd-order harmonics like 3rd order remain

Engineering Contradiction:

Improvelight receiving array simplicityVSAvoidharmonic component removal effectiveness

Core Design Contradiction:

Ease of manufactureVSMeasurement precision

Solution Approach 1:

The invention modifies the grating parameters (line widths W1, W3 and spaces S1, S3) to satisfy specific relationships that enable removal of both even and odd-order harmonic components. By setting W1+S1=P and W3+S3=P with W1≠W3, the system achieves effective harmonic suppression including the 3rd order, maintaining manufacturing simplicity while improving measurement precision

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The invention inverts the conventional approach by not using a light receiving array with phase difference, but instead using gratings with different line width and space parameters. This inversion allows the use of a simple light receiving array while achieving comprehensive harmonic removal including odd-order harmonics that were previously impossible to eliminate

Inventive Principle:

Principle #13The other way round (Inversion)

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 approach effectively removes harmonic components on the light receiving array, averaging interference fringes to improve signal stability without increasing manufacturing costs, and can handle multiple orders of harmonics, including the 3rd order, without needing complex light receiving array configurations.

Implementation Method 1

three optical gratings serve as a spatial filter

Methodology Applied

Scientific EffectDiffraction: Diffraction

Implementation Method 2

first and third gratings 22 and 24 serving as an index scale

Methodology Applied

Scientific EffectInterference: Interference

Implementation Method 3

stabilize an output signal by averaging effect of the light receiving array

Methodology Applied

Scientific EffectAveraging effect:

Data Source

PatentUS7485845B2Photoelectric encoder capable of effectively removing harmonic components

Publication Date: 2009.02.03 MITUTOYO CORP

AI summary

A three-grating type photoelectric encoder includes a second grating formed on a scale and first and third gratings disposed on a side of a detector. A part of at least the first grating is shifted in a direction of a measurement axis by P/(2n) (wherein P is a grating pitch, n is the order of a harmonic component to be removed) in order to remove a harmonic component of the nth order. This encoder can be improved with high accuracy by removing harmonic components without increasing manufacturing costs.