Multi-track Absolute Encoder Yaw Sensitivity Reduction
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Solution Overview
Problem
Existing absolute encoders with separate incremental and absolute tracks are sensitive to yaw and require complex optical systems, making them impractical for high accuracy applications due to increased size and cost, as well as sensitivity to scale defects and contamination.
Innovation Solution
A multi-track scale with separate narrow incremental and absolute tracks allows for the use of a periodic diffractive optic to filter incremental information, reducing yaw sensitivity and enabling accurate optical fringes, while detectors can view the entire scale width to extract both incremental and absolute information effectively.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If separate incremental and absolute tracks are used, then absolute position information can be obtained, but the system becomes sensitive to yaw and requires complex optical systems
Solution Approach 1:
The patent combines multiple incremental tracks and absolute tracks into a single multi-track scale structure. The narrow tracks are arranged in groups where incremental tracks provide high-resolution position information and absolute tracks provide coarse position information. This merging approach allows a single optical system to read all tracks simultaneously, reducing the complexity compared to separate optical systems for each track type.
Solution Approach 2:
The scale is segmented into multiple narrow tracks with distinct functions - some tracks are dedicated to incremental positioning while others are dedicated to absolute positioning. This segmentation allows the optical system to process different types of position information from separate physical tracks, enabling the use of a periodic diffractive optic to filter incremental information while maintaining absolute position capability.
2Area of stationary object
If imaging system is used to view entire scale width, then field of view increases, but depth of focus decreases and spherical aberration increases
Solution Approach 1:
The patent transitions from a single wide track to multiple narrow tracks arranged in the dimension perpendicular to the direction of travel. This dimensional change allows the optical system to view the entire scale width by distributing tracks across the field, while each narrow track maintains sufficient depth of focus for accurate measurement. The narrow track width in the perpendicular dimension enables the optical system to achieve both wide field of view and adequate depth of focus.
3Measurement precision
If periodic diffractive optic is used to filter incremental information, then optical fringes are improved, but the system becomes sensitive to standoff changes
Solution Approach 1:
The patent segments the optical measurement function by using separate tracks for incremental and absolute positioning. The periodic diffractive optic is applied specifically to the incremental tracks to generate high-precision optical fringes, while the absolute tracks provide standoff-insensitive position information. This segmentation allows the system to tolerate standoff changes because the absolute tracks continue to provide accurate coarse position data even when optical fringe quality degrades.
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 provides robust alignment tolerances, reduced system complexity, and improved accuracy by separating and filtering incremental and absolute signals, leading to more reliable position tracking with reduced mechanical and optical complexities.
Implementation Method 1
a periodic diffractive optic to optically filter the incremental scale position information, which is done in many high accuracy encoder systems
Implementation Method 2
a read head with an optical detector to measure relative movement between the read head and scale
Data Source
Figure 1~3
Figure 2
AI summary
An optical position encoder includes a scale having multiple tracks separated in a direction perpendicular to travel, the tracks including an incremental track and an absolute track, the scale interacting with an incident first light beam to generate a second light beam having components carrying respective optical patterns produced by the incremental track and absolute track respectively. The encoder further includes a set of optical detectors including at least first and second detector arrays of differing properties to detect the respective optical patterns produced by the incremental track and absolute track respectively, each of the first and second detector arrays spanning multiple tracks of the scale and configured to respond to a respective detector-specific component of the second light beam more strongly than to another component of the second beam specific to another of the detector arrays.