Optical Position Encoder Spatial Filtering Diffraction Orders
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Solution Overview
Problem
Optical position encoders face accuracy issues due to undesired harmonic components in the light intensity profile, which introduce distortion and reduce the overall precision of measurements.
Innovation Solution
The optical position encoder employs spatial filtering to separate and suppress diffraction orders other than the +/−1st diffraction orders, using a configuration with first and second diffraction gratings and an optical mask to direct only the +/−1st orders to the detector, ensuring a sinusoidal interference pattern and improved accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If diffraction gratings are used to generate interference patterns, then measurement precision is improved, but unwanted diffraction orders introduce harmonic components that reduce accuracy
Solution Approach 1:
The patent extracts and removes unwanted diffraction orders (0th, +/−2nd, and higher orders) from the optical path using spatial filtering techniques. By placing apertures or masks at specific locations where these unwanted orders converge, only the desired +/−1st diffraction orders reach the detector, eliminating harmonic components and achieving purely sinusoidal intensity profiles for accurate measurements.
Solution Approach 2:
The patent introduces intermediate optical elements (apertures, masks, or additional diffraction gratings) between the primary diffraction grating and the detector. These intermediaries selectively transmit desired diffraction orders while blocking unwanted ones, acting as mediators that purify the optical signal before detection.
2Measurement precision
If wavefront compensators are used to generate diffraction orders, then interference patterns are created, but 0th and higher diffraction orders reach the scale and reduce encoder accuracy
Solution Approach 1:
The patent removes unwanted diffraction orders generated by the wavefront compensator before they can reach the scale and detector. Spatial filters are positioned to block 0th and higher orders while allowing only +/−1st orders to proceed, eliminating the source of interference pattern distortion.
Solution Approach 2:
The patent exploits the natural spatial separation of different diffraction orders as a beneficial feature. By strategically positioning filters at locations where unwanted orders converge, the harmful presence of multiple diffraction orders is converted into a useful spatial filtering mechanism that automatically rejects harmonics.
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 enhances intra-fringe accuracy and overall encoder precision by eliminating distortions caused by unwanted diffraction orders, resulting in higher measurement accuracy.
Implementation Method 1
first and second spaced-apart diffraction gratings between the light source and the scale, the first diffraction grating having a location at which the source light beam is incident. The diffraction gratings and scale are mutually configured and cooperative to spatially separate +/−1st diffraction orders from other diffraction orders
Implementation Method 2
The optical paths further converge from the respective locations on the second diffraction grating to an area of the scale adjacent to the optical detector, and then extend from the area of the scale to the optical detector to create an interference pattern at the optical detector
Data Source
AI summary
An optical position encoder employs spatial filtering of diffraction orders other than +/− 1st diffraction orders for greater accuracy. The encoder includes a light source, a scale including a diffractive scale pattern, an optical detector adjacent to the scale, and first and second spaced-apart diffraction gratings between the light source and the scale. The gratings and scale are configured to spatially separate +/− 1st diffraction orders from other diffraction orders and to direct the +/− 1st diffraction orders along respective optical paths. The optical paths diverge from the first grating to respective locations on the second grating that are separated by more than the beam width of the source light beam. The optical paths further converge from the locations on the second grating to an area of the scale adjacent to the detector, and then extend from the area of the scale to the detector to create an interference pattern as a function of the relative position between the scale and the light source. Preferably, higher diffraction orders are separated by total internal reflection at the first diffraction grating.