Simple Pressure Sensor with Highly Customizable Sensitivity Based on Fiber Bragg Grating and Pill-Shaped 3D-Printed Structure

This study presents a simple pill-shaped pressure sensor fabricated by fused deposition modelling (FDM) technology. A ﬁ ber Bragg grating (FBG) sensor was embedded inside the structure during the 3D printing process. The dimension of the sensor was simulated by the ﬁ nite element method to explore the customizability of the pressure sensor. The proposed pill-shaped pressure sensor showed great linearity and achieved high sensitivity with a total Bragg-shifted wavelength of 1.44nm in the pressure range between 0 and 0.35MPa, corresponding to a pressure sensitivity of 4.11nm/MPa. The measured pressure change matched closely with ﬁ nite element simulation, and the sensitivity could easily increase ﬁ ve times by a thirty percent increase in structure size. The results showed that the pill-shaped pressure sensor had high linearity and great feasibility and could be easily customized to a wide pressure range for static and dynamic pressure measurements in industrial applications.


Introduction
In recent years, fiber Bragg grating (FBG) sensors have had a vigorous development because of their outstanding advantages such as compactness, light configuration, high stability, high-reliability multiplexing, and immunity to electromagnetic interference. FBG is commonly used to measure diverse industrial and structural parameters, such as strain, pressure, temperature, humidity, liquid level, and even detect specific chemical compounds [1][2][3]. In the field of pressure sensors, sensitivity is an important parameter because it determines the resolution and accuracy of the sensing system. The pressure sensitivity measured with bare FBG is 3:04 × 10 −3 nm/MPa [4]. There are many ways to improve the sensitivity of sensors in general and pressure sensors in particular, as shown in Figure 1. It can improve from internal factors as well as external factors.
About improving external factors, many studies have been published and proposed. First, improve the sensor's sensitivity without embedding the FBG, i.e., not directly affecting the FBG segment, but attaching it to a structure to increase the sensitivity of the FBG, as some studies have previously published. For example, the FBG put in a glass-bubble housing achieves a sensitivity of 0.0278 nm/MPa [5], or it was glued on a Bourdon tube with a sensitivity of 1.414 nm/MPa [6], etc. Second, the sensitivity is improved by embedding the bare FBG in another material with an elastic structure, i.e., acting directly on the FBG segment. This approach is more complicated and takes more time to prepare and fabricate the sensor. There are many published research papers such as embedding a single polymer FBG into a silicone rubber diaphragm [7] or embedding into an epoxy resin diaphragm [8], etc., and the sensitivity has also been improved such as the measured pressure sensitivity is found to be 0.08 nm/MPa by polymer-coated fiber Bragg grating [9], and the pressure sensitivity improved up to 39.6 nm/MPa by using polymer packaged fiber grating pressure sensor [10]. Next, the pressure sensitivity has been reported to be 2.0 nm/MPa [11] or a shielded polymercoated fiber Bragg grating increased the pressure sensitivity as high as 5.277 nm/MPa [12]. The FBG encapsulated in a polymer-half-filled metal cylinder was proposed to improve the pressure sensitivity of an FBG sensor up to 33.5 nm/MPa [13], and some other studies have been mentioned in [14][15][16][17].
Along with the development of fiber optic sensors, today's 3D printing technology has also evolved significantly and can now perform important roles in many applications. 3D printing is a flexible, automated, customizable, and low-cost manufacturing process. Fiber Bragg grating (FBG) sensors can be embedded in 3D-printed elements for low cost and customized sensing elements. Some studies used a 3D-printed structure to improve the sensitivity such as the FBG was putted in resin cylinder with a sensitivity of 2.08 nm/MPa [18], and several other FBG pressure sensors embedded in 3D-printed materials have also been proposed [19][20][21][22][23].
In order to overcome the sensitivity limitation of the previously proposed bare FBG sensors, this study made external factor changes to improve the sensor's sensitivity by creating a 3D-printed diaphragm structure. It showed that certain efficiency saves a lot of fabrication time. Specifically, the structure is fabricated based on the principle of the pressure difference between spatial regions in order to create a reaction on the polymer diaphragm leading to deformation in the sensor fiber and its named pill-shaped structure. Here, the pill-shaped sensor will be tested in the pressure chamber. Their application is in a medium and low pressure measuring range with good sensitivity.

Sensor Design
2.1. Fabrication and Package. The optical fiber used in this study has an inner core diameter of 9.6 μm and an outer glass (SiO2) coating diameter of 124.9 μm, made by Fibercore Ltd. (Southampton, UK). Bragg gratings were written into the fiber using a pulsed KrF-laser system (Coherent Xantos, 248 nm wavelength, 12 mJ/cm 2 , Coherent, USA). The FBG was fabricated using the B7 masker, and a peak wavelength at 1541.705 nm is used to sense the pressure.
Sensor design has been implemented in many different ways; it depends on the material parameters, geometry, and thickness. The design in our test was fabricated via 3D printing using polylactic acid (PLA) material which was considered in the numerical analysis. The print material selected in the digital simulation is PLA with the following parameters: Young's modulus of 3.45 GPa, Poisson ratio of 0.35, and density of 1290 kg/cm 3 .
These parameters have also been included in the numerical simulation by the ANSYS software. In this study, the pill-shaped structure is fabricated using PLA material in the 3D printer, INFINITY X1E, with a coating density of 100% and a smooth thickness of 0.2 mm. The print head temperature is 215 degrees Celsius, and the bedplate temperature is 65 degrees Celsius, both within the allowable range. The modelling has a design shown in Figure 2. The 3Dprinted model was then coated with waterproof paint (Ming Shing spray paint, Taiwan) to prevent leaks. The optical fiber (with the original FBG in the middle, no coating) is attached to the structure and fixed with high-quality adhesive (MXBON super glue, Taiwan).

Analyze by Numerical
Methods. The pill-shaped sensor will be tested in the low-pressure range. Hence, only numerical simulations (e.g., using the finite element method) can accurately represent the strain distribution across the diaphragms and, more importantly, the strain transferred to the FBG. In this study, before the experiment's sensor structure was carried out, we used the ANSYS 2020R1 software to analyze the sensor's response with a pill-shaped structure made of PLA material. About the structural meshing, the sensor body made of 3D-printed material has meshed with finite elements of 0.5 mm for the entire structure. The finite element model has 153665 nodes and 89362 elements. In the sensor structure, the main deformation component under the action of pressure is the diaphragm on both sides of the structure. Both of these diaphragms are 1 mm thick. Figure 3 shows that, at an applied pressure of 0.35 MPa, the maximum axial strain (με) reached 1948 at the center of the diaphragm, and with each change in pressure, the strain in the diaphragm has a linear response to the static pressure, and with larger diaphragm size, it is obvious that the resulting axial strain is also larger. Thus, the basis for placing optical fiber in this area is completely reasonable. Based on this numerical analysis, we experiment with the fabricated sensor.
For an FBG with two ends fixed by two polymer diaphragms located opposite each other, so the axial strain along the FBG due to an applied pressure P is given by

Experiment Set-Up
The experimental layout for this study is shown in Figure 4. The sensor is placed in a well-controlled pressure chamber to test the pressure response. The pressure inside the chamber is changed using a compressor that pushes water into the chamber and is controlled with an accurate pressure gauge. The experimental temperature was also well controlled at laboratory temperature (25°C ± 0:5°C), and the effect of temperature was considered to be within control. The light from a diode laser source (Model DL-BP1-CS5169A) is launched into the FBG through the 1 × 2 optical coupler. The reflectance spectrum of the FBG is then transferred to an optical coupler and fed to the optical spectrum analyzer (OSA, Model-MS9740A, Anritsu). The connector is an FC/UPC connector (FC/UPC FC/UPC SM SX 9/125, 3.0 mm, 2 m). The pressure applied in the pressure chamber compresses the polymer diaphragm on either side where air pressure is available. This results in the two diaphragms being pulled on both sides, producing axial strain in the FBG. The test pressure was increased up to 0.35 MPa in steps of 0.01 MPa, and the corresponding change in the Bragg wavelength of the FBG was recorded using the OSA. Figure 5 shows the recorded OSA spectra of the Bragg wavelength shift of the FBG corresponding to the pressure from 0 to 0.35 MPa with a step of 0.05 MPa. In the first experiment, the applied pressure went from 0 to 0.35 MPa; but in the second, the pressure decreased from 0.35 to 0 MPa and then increased again to 0.35 MPa for the 3rd test. The total Bragg wavelength shift over the applied pressure range was recorded as 1.44 nm, corresponding to a pressure sensitivity of 4.11 nm/MPa.

Results and Discussions
To study the repeatability response of the sensor, the experiment was repeated in three tests under laboratory conditions (at room temperature). Figure 6 shows a comparison of three pressure tests against the rise and fall of the pressure, indicating that the pressure sensitivity is 4.25, 4.06, and 4.15 nm/MPa, respectively. A linear analysis showed that the linear values of the data obtained in the three experiments were 0.9935, 0.9996, and 0.9933, respectively, all of which exceeded 0.99. Therefore, the mean standard deviation of the sensitivity of the pressure sensor in the repeated experiment is 0.055 nm/ MPa. The smaller the standard deviation, the closer the value is to the mean sensitivity, and thus, higher accuracy and the corresponding Bragg wavelength shift of the FBGs were recorded. As a result, the sensor possesses outstanding linearity and cycle repeatability. It can be used as a highly stable pressure sensor.
Regarding the sensor's sensitivity, it can be seen from Table 1 that the pill-shaped sensor in this study has a much higher sensitivity than the bare FBG sensor. It is 1334 times more than the bare FBG sensor mentioned in [4], respectively; 125 times more than the FBG sensor mentioned in [5]; and 2.9 times, two times, 12 times, 108 times, and almost four times that of the sensitivity of FBG sensors mentioned in [6,18,[26][27][28], respectively. On the other hand, when compared with the FBG sensors embedded in other structures, the sensor in this study shows good and much higher sensitivity than some types of sensors, as indicated in the methodology above. Of course, it is not the highest; for example, the polymer-coated sensor mentioned in [10,12,13] (Table 1) is more sensitive than the sensor in this study. However, it is clear that it is easier and faster to fabricate the structure for 3D-printed sensors than the above-compared structures. The sensitivity can improve drastically by altering the structure size, shape, or material.
In addition, in this experiment, pressure is applied to the diaphragm on both sides, so it will be more efficient and achieve better sensitivity than the one-sided response on FBGs. The experimental sensitivity is also close to the theoretical calculation. With this new sensor structure, we can completely change the thickness of the diaphragm to adapt       to the required pressure ranges. We can also increase the safety of FBGs by creating a protective layer on the two sides of the pill-shaped structure. This could make it a bit more difficult to implant FBG fiber into the structure and will probably alter the pressure range.
A numerical analysis method is used to evaluate the sensor structure's customizability. Figure 7 shows that the applied pressure from 0 to 0.35 MPa results in a wavelength shift from 0 to 1.44 nm. It is pretty close to the numerical simulation analysis results. It demonstrates the high reliability and potential of the sensor in this study. For this, the two structural dimensions of the sensor are compared. The entire structure's size is 62 × 25 × 20 mm (L × H × W), and the larger size for comparison in numerical simulation is 80 × 25 × 44 mm. Figure 8 shows that when the sensor size was increased by 30%, the sensitivity can improve almost five times. It also demonstrates the outstanding feature of 3D printing technology: the flexibility, easy adjustment, and fabrication of different sizes and thicknesses of the sensor corresponding to the value to be measured. In other words, using 3D printing technology in the fabrication of the sensor's structure shows a very high degree of customization to the desired applied pressure range.

Conclusion
A highly sensitive and linear FBG pressure sensor is designed and demonstrated. Pressure sensitivity is enhanced by connecting the two ends of the FBGs to the two diaphragms in the 3D printer-printed structure, which operates on the principle of differential pressure resulting in a tensile response of the diaphragm. Therefore, it directly induces tensile strain in the optical fiber. The test results show that the designed sensor has high linearity and good repeatability in pressure measurements with negligible standard deviation. As mentioned above, this pill-shaped sensor has much better sensitivity than other sensors in its class. Although it is not the most sensitive sensor, it excels in its simplicity of construction, high customization, and saving time and economy manufacturing. This sensor can measure pressure in technological processes with a suitable pressure range and can be calibrated to accommodate lower or higher pressure ranges.

Data Availability
Data is available on request.

Conflicts of Interest
The authors declare no conflict of interests.