Mid-IR Interferometry Caliper for Non-Contact Sheet Thickness
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Solution Overview
Problem
Traditional methods for measuring the thickness of moving sheets like paper and plastic are inaccurate and prone to drift due to physical contact, which causes marking and dirt accumulation, and are sensitive to scattering properties, making it difficult to achieve sub-micron accuracy in dynamic environments.
Innovation Solution
The use of mid-IR interferometry with a laser beam in the 3-50 micron range to measure web thickness by scanning and analyzing the interference pattern between the top and bottom surfaces, allowing for non-contact, accurate thickness measurement using regression analysis and interference minima detection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If traditional contacting measurement methods are used, then the measurement can be performed, but the sheet is marked and dirt accumulates on measuring heads causing measurement drift
Solution Approach 1:
The patent replaces mechanical contact-based measurement systems with optical interferometry. A laser beam is directed at the sheet surface, and interference patterns are analyzed to determine thickness without physical contact, thereby eliminating sheet marking and dirt accumulation on measuring heads while maintaining measurement accuracy.
2Reliability
If laser triangulation or confocal microscopy techniques are used, then non-contact measurement is achieved, but the measuring head still contacts one side of the web
Solution Approach 1:
The patent uses optical interferometry to eliminate mechanical contact requirements entirely. By analyzing interference patterns of reflected laser light, the system determines sheet thickness without the measuring head needing to contact or be positioned relative to either surface, enabling fully non-contact measurement.
3Adaptability or versatility
If prior art optical techniques are used, then measurement can be performed, but they are very sensitive to scattering properties making them unsuitable for all paper products
Solution Approach 1:
The patent employs mid-infrared laser radiation (wavelengths of 3-50 microns) instead of visible or near-infrared light. This wavelength change reduces sensitivity to scattering properties while maintaining the ability to measure thickness through interference patterns, thereby extending adaptability to various paper and web materials.
4Measurement precision
If difference between two independent distance measurements is used, then thickness can be calculated, but both measurements must be stable which is difficult to achieve in dynamic environments
Solution Approach 1:
The patent replaces the method of calculating thickness from the difference of two independent distance measurements with direct interferometric measurement. By analyzing the interference pattern of reflected laser light, the system directly determines thickness without requiring two separate stable measurements, thereby achieving sub-micron accuracy in dynamic production environments.
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 method provides reliable, non-contact thickness measurement with better than 1 micron accuracy, resistant to scattering and environmental fluctuations, suitable for a wide range of web materials without the need for physical contact, enabling precise online monitoring during production.
Implementation Method 1
measuring the intensity of an interference pattern that forms from the superposition of radiation that is reflected from the exposed outer surface and from the inner surface
Implementation Method 2
interference pattern which forms by interference between first radiation reflected from the exposed outer surface and second radiation reflected from an inner surface
Implementation Method 3
mid-IR interferometry with a laser beam in the 3-50 micron range to measure web thickness by scanning and analyzing the interference pattern
Data Source
AI summary
Non-contacting caliper measurements of free standing sheets such as porous polymer and paper detect mid-IR interferometric fringes created by the reflection of light from the top and bottom surfaces of the sheet. The technique includes directing a laser beam at a selected angle of incidence onto a single spot on the exposed outer surface wherein the laser beam comprises radiation having a wavelength in the 3-50 micron range and scanning the laser beam through a selected angle range as the laser beam is directed onto the exposed outer surface and measuring the intensity of an interference pattern that forms from the superposition of radiation that is reflected from the exposed outer surface and from the inner surface. Thickness can be extracted from the fringe separation in the interference pattern. Rotating and focusing elements ensure that the spot position on the sheet remains the same while varying the incident angle.