C-FBG Thermal Sensing for Sub-Surface L-PBF Temperature Mapping
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Solution Overview
Problem
Existing additive manufacturing techniques, particularly laser powder bed fusion (L-PBF), face challenges in achieving consistent production of defect-free parts due to the lack of high spatial and temporal resolution thermal measurement capabilities, especially for sub-surface thermal data, which is crucial for understanding microstructure evolution and enabling closed-loop control systems.
Innovation Solution
A thermal measurement system utilizing chirped fiber Bragg grating (C-FBG) sensors combined with machine learning signal processing, capable of achieving micrometer-level spatial resolution and high temporal resolution, integrated with a calibration subsystem and data acquisition system to provide accurate sub-surface thermal data during L-PBF processes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If traditional fiber optic sensors (OFDR, FBGs) are used for thermal measurement, then measurement capability is provided, but spatial resolution is limited to millimeter-level which is insufficient for capturing steep thermal gradients
Solution Approach 1:
The fiber Bragg grating is divided into multiple segments along its length, with each segment having a distinct Bragg wavelength. This segmentation allows different portions of the fiber to sense temperature at different locations simultaneously, achieving high spatial resolution (micrometer-level) by distributing measurement points along the fiber rather than using a single point sensor
Solution Approach 2:
The patent transitions from traditional point-based or low-resolution distributed sensing to a multi-dimensional approach by encoding spatial information in the wavelength domain. Each spatial position along the fiber corresponds to a specific wavelength, creating a wavelength-position mapping that enables simultaneous multi-point measurement with micrometer-level resolution
2Measurement precision
If high-speed infrared cameras are used for surface monitoring, then high spatial and temporal resolution is achieved, but sub-surface thermal data cannot be obtained
Solution Approach 1:
The patent uses an optical fiber as an intermediary medium to access sub-surface temperature information. The fiber is embedded within the material being measured, allowing it to sense thermal conditions at depth rather than only at the surface. This intermediary approach enables non-contact, electrically isolated temperature measurement inside the workpiece
Solution Approach 2:
The patent replaces mechanical contact-based temperature sensors (like thermocouples) with an optical sensing system. By substituting electrical/mechanical sensing with optical sensing through the fiber, the system achieves both sub-surface measurement capability and immunity to electromagnetic interference, while maintaining high spatial and temporal resolution
3Measurement precision
If thermocouple arrays are used for thermal measurement, then temperature data is obtained, but spatial resolution is insufficient and mechanical discontinuities are created when embedded
Solution Approach 1:
The patent replaces mechanical contact sensors (thermocouples) with an optical sensing system based on fiber Bragg gratings. The optical fiber is thin and flexible, creating minimal mechanical disruption when embedded, while the distributed sensing capability along the fiber provides superior spatial resolution compared to discrete thermocouple arrays
Solution Approach 2:
The optical fiber acts as a thin, flexible sensing element that can be embedded within the material with minimal mechanical disruption. The fiber's flexibility and thin profile allow it to conform to the surrounding material structure, reducing mechanical discontinuities while maintaining measurement capability
4Measurement precision
If existing fiber optic solutions are used, then thermal measurement is enabled, but spatial resolution is limited to millimeter-level which cannot capture steep thermal gradients
Solution Approach 1:
The fiber is segmented into multiple sensing regions along its length, with each segment providing independent temperature measurement at a specific location. This segmentation enables the system to capture steep thermal gradients by providing multiple measurement points distributed along the fiber, with spacing small enough to resolve rapid temperature changes
Solution Approach 2:
The patent changes the operational parameters of the fiber optic sensor by using broadband light sources and spectrometer detection to measure Bragg wavelength shifts across multiple grating segments simultaneously. This parameter change from narrowband to broadband operation enables high spatial resolution distributed sensing
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
Enables direct measurement of reheating and re-melting phenomena in L-PBF processes, providing experimental validation data for understanding microstructure evolution and enabling closed-loop control systems, with spatial resolution of 28.8 μm per pixel and temporal resolution of up to 10 kHz.
Implementation Method 1
a chirped fiber Bragg grating (C-FBG) sensor configured to encode spatial temperature information in a reflection spectrum
Implementation Method 2
a machine learning subsystem may include a neural network model configured to demodulate complex reflection spectra from the C-FBG sensor to generate spatial temperature profiles
Implementation Method 3
The thermal measurement system also includes an optical sensing subsystem may include a chirped fiber Bragg grating (C-FBG) sensor configured to encode spatial temperature information in a reflection spectrum
Data Source
AI summary
A thermal measurement system enables high-resolution sub-surface temperature monitoring during additive manufacturing processes through machine learning demodulation of chirped fiber Bragg grating (C-FBG) sensors. An optical sensing subsystem includes a C-FBG sensor that encodes spatial temperature information in wavelength for high-temperature operation. A neural network model transforms complex reflection spectra into spatial temperature profiles with micrometer-scale resolution, overcoming limitations of traditional demodulation methods. A calibration subsystem generates synchronized spectral and thermal imaging data for training the neural network using controlled thermal profiles. The system captures steep thermal gradients and rapid cooling rates during laser powder bed fusion operations. A fiber embedding technique maintains the sensor in a strain-free condition at controlled sub-surface depths. The integration of high-temperature C-FBG sensors with machine learning signal processing achieves significant improvement in spatial resolution compared to traditional fiber optic thermal measurement approaches.