Low-Coherence Interferometry for Tool-to-Surface Distance Sensing
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Solution Overview
Problem
Current methods for determining the separation distance between a processing tool and a material, especially in laser processing, are inaccurate and cumbersome, particularly when dealing with non-metallic materials and complex surface geometries, as they rely on capacitive sensors that are sensitive to the material's shape and processing conditions.
Innovation Solution
The implementation of low coherence optical interferometry techniques, which use a measurement optical path integrated into the machine tool and a reference optical path to determine the separation distance by detecting interference fringes, providing accurate and robust measurements independent of the material's morphology and processing conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If capacitive sensors are used to determine separation distance, then measurement can be performed, but measurement precision deteriorates due to sensitivity to material shape and processing conditions
Solution Approach 1:
The patent replaces capacitive sensors (electrical field-based) with optical interferometry systems (optical field-based). The optical measurement system uses low coherence light sources and interferometric detection to measure separation distance, eliminating the sensitivity issues of capacitive sensors to material shape and processing conditions while maintaining measurement capability across diverse materials including metals and non-metals.
Solution Approach 2:
The patent changes the measurement parameter from electrical capacitance to optical interference patterns. By using low coherence optical interferometry, the system measures separation distance through optical path difference detection, which is independent of material electrical properties and geometric shape, thereby improving both precision and adaptability.
2Measurement precision
If optical interferometry is used to measure separation distance, then measurement precision improves, but device complexity increases due to optical path requirements
Solution Approach 1:
The patent extracts and eliminates the complex reference optical path from the interferometry system. By using only a measurement optical path that travels to the workpiece and back, the system achieves accurate separation distance measurement without requiring a separate reference path, thereby reducing device complexity while maintaining measurement precision.
Solution Approach 2:
The measurement optical path serves dual functions: it measures both the separation distance and the workpiece surface position simultaneously. The optical path configuration uses the workpiece itself as the measurement target, eliminating the need for additional reference components and simplifying the overall system architecture.
3Length of stationary object
If sub-sampling distortion is present in interference fringe detection, then measurement range increases, but measurement reliability deteriorates due to aliasing effects
Solution Approach 1:
The patent converts the harmful aliasing effect of sub-sampling into a beneficial feature. By intentionally operating in the sub-sampling regime and using low coherence light sources, the system transforms the aliasing distortion into an extended measurement range capability. The low coherence length ensures that interference fringes only form within a specific depth range, allowing the system to accurately measure larger separations that would otherwise cause aliasing in coherent systems.
Solution Approach 2:
The patent changes the coherence parameter of the light source from coherent to low coherence. This parameter change fundamentally alters the interference behavior, allowing the system to operate in sub-sampling conditions without aliasing errors. The low coherence length acts as a natural gate that limits interference to the desired measurement range, converting what would be a reliability issue into an extended measurement capability.
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 allows for precise and accurate measurement of separation distances over a wide range, unaffected by the material's shape or processing conditions, enabling reliable control of laser processing operations such as cutting, drilling, and welding, and is applicable to both metallic and non-metallic materials.
Implementation Method 1
A measurement optical beam M is generated by a low coherence optical radiation source, and a reference optical beam R is generated by the same source and led towards a sensor arrangement in a second direction of incidence, at a predetermined angle of incidence with respect to the first direction of incidence of the measurement beam M. The measurement beam M and the reference beam R are superimposed, forming a pattern of interference fringes F.
Data Source
AI summary
Method and system for determining separation distance between an object and a processing or measuring tool involve generating a measurement beam of low coherence optical radiation, leading the measurement beam towards the object and the reflected measurement beam towards an optical interferometric sensor assembly in a first direction of incidence, generating a reference beam of low coherence optical radiation, and leading the reference beam towards the optical interferometric sensor assembly in a second direction of incidence, superimposing the measurement and reference beams on a common region of incidence, detecting position of a pattern of interference fringes between the measurement and reference beams on the region of incidence, and determining difference in optical length between a measurement optical path and a reference optical path on position of the pattern of interference fringes along an illumination axis to determine current separation distance between the processing or measuring tool and the object.