Optical Encoder Scale Grating Phase Offset
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Solution Overview
Problem
Existing optical displacement encoders face challenges in achieving a combination of high resolution, compact size, robustness, and cost-effectiveness while providing a range-to-resolution ratio, particularly in low power consumption applications.
Innovation Solution
The development of a doubly telecentric optical displacement encoder configuration that incorporates a scale element with absolute and incremental track patterns, utilizing spatial filtering and imaging principles, and a phase grating to achieve high resolution and compact design, allowing for shared manufacturing techniques and components.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional imaging systems are used to image the scale pattern, then the encoder can achieve basic measurement functionality, but the system cannot simultaneously provide high resolution, compact size, and robustness
Solution Approach 1:
The scale pattern is divided into multiple scale grating portions (first, second, and third portions) with different spatial phases. Each portion is imaged onto corresponding photodetector regions, allowing independent optimization of each imaging path while maintaining overall system compactness and robustness
Solution Approach 2:
The patent introduces a spatial phase offset dimension by positioning scale grating portions at different lateral locations with distinct phase shifts. This additional spatial dimension enables high resolution without increasing the axial length of the encoder, effectively decoupling resolution from system complexity
2Reliability
If the encoder uses multiple scale grating portions with different spatial phases, then measurement robustness improves, but manufacturing complexity increases
Solution Approach 1:
A single scale member is designed with multiple scale grating portions that can be manufactured using conventional lithography techniques. The same manufacturing process creates all grating portions, and a single lens images all portions onto the photodetector array, eliminating the need for multiple separate manufacturing processes and reducing overall complexity
3Measurement precision
If telecentric imaging is used to improve measurement accuracy, then resolution improves, but the device size increases
Solution Approach 1:
The telecentric imaging requirement is segmented and applied only to specific critical imaging paths (first and second scale grating portions) rather than the entire system. This selective application maintains measurement accuracy where needed while avoiding the size penalty across the whole device
Solution Approach 2:
The patent achieves telecentric-like measurement accuracy without full telecentric geometry by utilizing spatial phase offsets in the lateral dimension. The phase-shifted grating portions create measurement sensitivity through lateral spatial relationships rather than requiring complex angular telecentric optics, thereby reducing device volume
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 enables high-resolution measurements with improved range-to-resolution ratio, compact size, and cost-effectiveness, suitable for low power consumption applications by utilizing shared manufacturing techniques and components.
Implementation Method 1
A phase grating is configured to diffract the source light to provide diffracted structured light to the scale track patterns
Implementation Method 2
The scale track patterns spatially modulate the incident light intensity to provide track-specific spatially modulated light patterns
Implementation Method 3
An imaging system is configured to focus and transmit the spatially modulated light patterns from the scale to the detector
Data Source
AI summary
A flexible optical displacement encoder configuration uses a source grating to illuminate a scale with structured light such that light from the scale is modulated with a beat frequency envelope, which may have a relatively coarse pitch that matches a desired detector pitch. An imaging configuration provides spatial filtering to remove the high spatial frequencies from the modulation envelope to provide a clean signal in the detected fringe pattern. This combination of elements allows an incremental scale track pattern with a relatively finer pitch (e.g., 4, 5, 8 microns) to provide fringes with a coarser pitch (e.g., 20 microns) at a detector. Various scale resolutions can use a corresponding source grating such that all combinations can produce detector fringes that match the same economical detector component.