Two-Dimensional Position Encoder Drift Compensation
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Solution Overview
Problem
Low-cost, low-resolution position encoders often suffer from measurement drift, low accuracy, and lack of repeatability, particularly in applications requiring sensitivity on the order of nanometers.
Innovation Solution
A position encoder system utilizing a grating coupled with an image sensor assembly, including a single illumination source, a wedge-shaped optical element, and two one-dimensional image sensors, which generates reference and measurement beams to monitor relative movement between objects, with a control system applying a drift compensation algorithm to correct for position drift.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If low-cost, low-resolution position encoders are used, then manufacturing cost is reduced, but measurement precision and reliability deteriorate
Solution Approach 1:
The image sensor is divided into multiple regions (first region and second region) with different functions. The first region captures reference beam information while the second region captures diffracted beam information, allowing the system to achieve high measurement precision using a standard, cost-effective image sensor without requiring specialized high-resolution sensors.
Solution Approach 2:
A diffractive optical element is introduced as an intermediary between the illumination source and the image sensor. This element diffracts the illumination beam into multiple orders, creating the necessary interference pattern that enables precise measurement of grating displacement, thereby improving measurement precision without increasing sensor cost.
2Ease of manufacture
If low-cost position encoders are used, then manufacturing cost is reduced, but measurement repeatability deteriorates
Solution Approach 1:
The system continuously monitors the interference pattern formed by the reference beam and diffracted beam as the grating moves. By analyzing the phase changes in the interference pattern captured by the image sensor, the system can reliably determine grating displacement and maintain consistent, repeatable measurements across multiple cycles, overcoming the repeatability issues of low-cost encoders.
3Measurement precision
If high sensitivity (0.1-0.2 nm) is required, then measurement precision is improved, but device complexity increases
Solution Approach 1:
The patent transitions from traditional one-dimensional linear encoders to a two-dimensional imaging approach. By capturing interference patterns across the entire image sensor array and analyzing phase variations in multiple dimensions, the system achieves high sensitivity (0.1-0.2 nm) without requiring complex mechanical or optical components, thus improving precision without proportionally increasing complexity.
4Measurement precision
If multiple image sensors are used to improve measurement accuracy, then measurement precision is improved, but device complexity increases
Solution Approach 1:
A single image sensor is designed to perform multiple functions: capturing both the reference beam (first region) and the diffracted measurement beam (second region), and enabling measurement in multiple dimensions (two degrees of freedom). This multi-functional approach achieves high measurement precision without requiring multiple separate sensors, thereby reducing device complexity.
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
Enables accurate, cost-effective measurement of position in two degrees of freedom at the nanometer level, improving sensitivity and repeatability while reducing measurement errors.
Implementation Method 1
a first reference beam that is reflected by the first optical surface and directed at the first image sensor, and a second reference beam that is reflected by the second optical surface and directed at the second image sensor
Implementation Method 2
a transmitted beam that is transmitted through the optical element and is directed at and impinges on the grating to create a first measurement beam that is diffracted by the grating and directed at the first image sensor, and a second measurement beam that is diffracted by the grating and directed at the second image sensor
Implementation Method 3
the first reference beam and the first measurement beam are interfered at the first image sensor to generate a first measurement signal; and the second reference beam and the second measurement beam are interfered at the second image sensor to generate a second measurement signal
Data Source
AI summary
A position encoder for monitoring relative movement between a first object and a second object includes a grating that is coupled to the first object, and an image sensor assembly that is coupled to the second object. The image sensor includes a first image sensor; a second image sensor that is spaced apart from the first image sensor; an optical element that includes a first optical surface and a second optical surface that is spaced apart from the first optical surface; and an illumination system. The illumination system directs an illumination beam at the optical element to create (i) a first reference beam that is reflected by the first optical surface and directed at the first image sensor, (ii) a second reference beam that is reflected by the second optical surface and directed at the second image sensor, and (iii) a transmitted beam that is transmitted through the optical element and is directed at and impinges on the grating to create a first measurement beam that is diffracted by the grating and directed at the first image sensor, and a second measurement beam that is diffracted by the grating and directed at the second image sensor.