Optical Encoder Grating Rotation for Signal Noise Reduction
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Conventional optical encoders face challenges in preventing unwanted diffracted orders of light from reaching the detector, leading to signal errors and noise, particularly when using spatial filtering techniques, as some interfering light components align with desired diffraction orders, making it difficult to block them effectively.
Innovation Solution
The optical encoder configuration includes a scale grating, an illumination source with a diffraction grating, and a detector portion, where the illumination source grating is rotated relative to the scale grating, and the detector is oriented to align with desired interference fringes, ensuring that undesired interfering light components are misaligned and their signal contributions can be treated as constant, allowing for effective removal through signal processing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If spatial filtering techniques are used to block unwanted diffracted orders, then signal noise is reduced, but some interfering light components align with desired diffraction orders making it difficult to block them effectively
Solution Approach 1:
The patent introduces asymmetric angular offsets between the illumination source grating and scale grating, and between the detector and scale grating. This asymmetric configuration causes unwanted diffracted orders to diverge angularly from desired orders, enabling effective spatial filtering without blocking aligned interfering components.
2Measurement precision
If an aperture is incorporated in a telecentric imaging configuration to provide spatial filtering, then certain unwanted orders of diffracted light are blocked, but other unwanted diffraction orders give rise to interfering light components that are aligned with the interfering light components arising from desired diffraction orders at the spatial filtering aperture
Solution Approach 1:
The patent extends the filtering approach from a single spatial dimension to multiple dimensions by introducing angular offsets in both the illumination path and detection path. This multi-dimensional angular separation ensures that unwanted diffraction orders are separated from desired orders in angular space, preventing alignment at the aperture plane.
3Device complexity
If the illumination source light diffraction grating and scale grating are aligned, then the imaging system is simpler, but unwanted diffracted orders align with desired orders at the detector causing signal errors
Solution Approach 1:
The patent changes the angular orientation parameters of the gratings relative to each other. By introducing a specific angular offset between the illumination source grating and scale grating, the system transforms the alignment condition to achieve separation of desired and unwanted diffracted orders while maintaining system simplicity.
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 significantly reduces signal errors and noise by misaligning undesired interference fringes with the detector, enabling high-accuracy displacement measurements by separating desired and undesired light components, thereby improving the encoder's precision.
Implementation Method 1
an illumination source light diffraction grating that inputs the collimated light and outputs diffracted light components (DLC) to the scale grating
Implementation Method 2
respective sets of interference fringes formed by different respective sets of interfering light components
Implementation Method 3
a first lens positioned to receive desired and undesired interfering light components from the scale grating, the first lens having an optical axis and a focal length F that defines a focal point located along the optical axis
Implementation Method 4
an aperture element positioned along the optical axis approximately at the focal length F between the first lens and the detector portion
Implementation Method 5
the movement or position of the displaced scale pattern image is detected with a photodetector arrangement
Data Source
AI summary
A device for measuring the relative displacement between two members comprises a scale grating, an illumination source, a detector portion, and an imaging portion. The illumination source outputs diffracted light components (DLC) to the scale grating. The DLC comprises desired interfering light components comprising +1 and −1 order DLC and undesired interfering light components comprising diffraction orders that are not the +1 and −1 order DLC. The detector portion comprises at least a first array of periodically arranged optical detector sensing areas. The illumination source light diffraction grating is positioned proximate to the scale grating and oriented relative to the scale grating such that respective sets of interference fringes formed by different respective sets of interfering light components are differently rotated about the optical axis. The detector portion is oriented such that the optical detector sensing areas are aligned with interference fringes formed by the desired interfering light components.