Interferometric Encoder Cyclic Error Compensation
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Solution Overview
Problem
Interferometric encoder systems face significant errors due to non-harmonic and axis-dependent cyclic errors, which conventional electronic compensation methods cannot correct, especially at low speeds and during alignment processes.
Innovation Solution
The system generates complex prototype signals to represent non-harmonic and axis-dependent cyclic errors, calculates corresponding coefficients, and compensates interference signals using these signals to reduce sideband frequencies, allowing for effective error correction even at low speeds.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional electronic compensation methods are used, then the system is simple to operate, but non-harmonic and axis-dependent cyclic errors cannot be corrected
Solution Approach 1:
The compensation method segments the cyclic error correction into distinct components: non-harmonic cyclic error compensation through complex prototype signals, and axis-dependent cyclic error compensation through multi-axis signal processing. Each error type is addressed with specialized processing tailored to its characteristics, enabling precise correction without requiring complete system redesign.
Solution Approach 2:
The system performs preliminary characterization of cyclic errors by measuring and storing error coefficients at multiple positions before actual measurement operations. This pre-characterization data is then used during operation to rapidly compensate errors without real-time complex calculations, improving both accuracy and operational speed.
2Measurement precision
If filtering is used to correct cyclic errors, then errors can be reduced during high-speed motion, but errors cannot be corrected during low-speed operations such as alignment
Solution Approach 1:
The compensation system dynamically adjusts processing parameters based on operational conditions. During high-speed motion, filtering parameters are optimized for speed-related cyclic errors. During low-speed operations and alignment, the system switches to coefficient-based compensation methods that remain effective at all speeds, ensuring continuous error correction across the full operational range.
Solution Approach 2:
The system transitions from static filtering approaches to dynamic compensation that adapts to varying operational speeds. By using pre-characterized error coefficients and complex prototype signals that model cyclic errors across different motion states, the system maintains correction effectiveness whether the measurement object is stationary, moving slowly during alignment, or moving rapidly during measurement.
3Ease of manufacture
If the grating period of the encoder scale is increased, then the system is easier to manufacture, but position error increases due to reduced contrast and larger cyclic error effects
Solution Approach 1:
The system creates complex prototype signals that are mathematical copies of the expected cyclic error patterns. These prototype signals model the non-harmonic and axis-dependent cyclic errors based on pre-characterized coefficients, allowing the system to replicate and subtract error patterns without physically modifying the encoder scale. This enables use of coarser, easier-to-manufacture gratings while maintaining precision through software-based error cancellation.
Solution Approach 2:
The patent replaces mechanical precision requirements with electronic signal processing. Instead of relying on perfectly manufactured fine-pitch gratings, the system uses electronic compensation algorithms that process interferometric signals to remove cyclic errors. This substitution allows use of mechanically simpler, coarser encoder scales while achieving high measurement precision through digital signal processing.
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 the accuracy of interferometric encoder systems by compensating for cyclic errors, improving tolerance to optical, mechanical, and electronic imperfections, and reducing operating costs through improved performance.
Implementation Method 1
an interferometer generates the optical interference signal by overlapping and interfering a 'measurement beam' that interacts with (e.g., reflects from) from the measurement object with a second beam, sometimes called a 'reference beam' that does not interact with the measurement object
Implementation Method 2
The cyclic errors can be produced as a result of 'beam mixing' (where a portion of an input beam that nominally forms the reference beam propagates along a measurement path and/or vice versa)
Implementation Method 3
These cyclic errors have a frequency shift (i.e., the 'Doppler' frequency) that is a non-integer multiple of the frequency difference between the components of the original input beam
Data Source
AI summary
A method includes obtaining, from a detector of an interferometry system, an interference signal based on a combination of a first beam and a reference beam, subsequent to the first beam being diffracted by an encoder scale, obtaining, through an electronic processor, an error compensation signal based on a non-harmonic cyclic error that modifies the interference signal, and outputting information about a change in a position of the encoder scale relative to an optical assembly of the interferometry system based on the interference signal and the error compensation signal.