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Abstract
In this paper, we fabricate a transmissive fluorescent temperature sensor (TFTS) that based on Er3+/Yb3+/Mo6+ tri-doped tellurite fiber, which has the advantages of compactness and simplicity, corrosion resistance, high stability and anti-electromagnetic interference. The doping of Mo6+ ions will enhance the up-conversion (UC) fluorescence emission efficiency of Er3+ ions, thus improving the signal-to-noise ratio of TFTS. Using the fluorescence intensity ratio (FIR) technique, the real-time thermal monitoring performance of TFTS is evaluated experimentally. Apart from good stability, its maximum relative sensitivity is 0.01068 K−1 at 274 K in the measured temperature range. In addition, it is successfully used to monitor the temperature variation of the stator core and stator winding of the motor in actual operation. The results show that the maximum error between the FIR-demodulated temperature and the reference temperature is less than 1.2 K, which fully confirms the effectiveness of the TFTS for temperature monitoring. Finally, the FIR-based TFTS in this work is expected to provide a new solution for accurate and real-time thermal monitoring of motors and the like.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Motors play an extremely critical role in maintaining the proper operation of various mechanical systems, ranging from large aircraft and ships to small handheld fans and shavers [1–4]. The long-term running of motors usually results in an accumulation of heat, which may lead to serious damage of internal structures if temperature goes too high. For instance, the heat may damage the stator core insulation, causing a short circuit between the pieces, thereby exacerbating the core local eddy current loss, and in serious cases may lead to stator winding damage. Similarly, when overheated, the effect of the stator winding insulation will be reduced, or even break down at high temperatures. The normal temperature range for the rated operation of the motor is between 333 K and 353 K. Beyond this threshold, an imminent motor failure lurks, and immediate shut down is usually necessary. Therefore, it is important to monitor temperature changes of the stator core and stator winding in real time. However, in order to prevent external disruptions, the motors are generally enclosed in a plastic shell or iron shell. In addition, the motor's winding is made up of copper coils, which generates a magnetic field during operation. These factors pose great difficulty for the thermal detection of motors. There is an urgent need for developing temperature sensors that can be encapsulated inside the motors and which are not subject to electromagnetic interference.
Compared with traditional electronic and thermocouple temperature sensors, fluorescent temperature sensors (FTSs) are of great interest because of their compactness and simplicity, corrosion resistance, high stability and electromagnetic interference resistance [5–7]. Generally, FTSs can be divided into fluorescence intensity type [8], fluorescence intensity ratio (FIR) type [9–11], and fluorescence lifetime type [12,13]. Particularly, FTSs drawing on FIR technique, which is based on the ratio of the fluorescence intensity between two thermally coupled energy levels, are popularized by many researchers. The core materials of FTSs are rare earth ions, and the matrix materials of them are to provide a suitable lattice structure for generating fluorescence. Among the many rare earth ions, Er3+ ions are considered to be a good activator in FTSs, while Yb3+ and Mo6+ ions are used as sensitizers to increase the absorption cross-sectional area of Er3+ ions. Among them, it becomes possible to decrease the transition temperature and enhance the initial crystallization temperature of tellurite glasses by incorporating an appropriate concentration of Mo6+ ions [14,15]. This leads to an improved thermal stability of the glasses, making the process of fiber drawing more efficient. In recent years, tellurite glasses have been widely used as matrix materials because of their loose lattice structure, high refractive index, and low phonon energy [16–18]. Numerous studies have shown that temperature measurement can be realized based on the up-conversion (UC) fluorescence emission by doping rare-earth ions into tellurite glasses [19–22]. Among them, a portable luminescent thermometer based on green upconversion emission of Er3+/Yb3+ co-doped tellurite glass has been proposed by Manzani et al. in 2017 [23]. After that, they made a Nd3+-doped fluoroborontellurite glass as a near-infrared optical thermometer in 2022, which further improves the performance of the temperature sensor [24]. With the help of FTSs and based on the FIR technique, thermal monitoring in various fields also has been realized, such as the recently reported thermal detection of chips [25], vivo brain [26], and miniature winding coils [27], etc. However, the application in ratiometric optical temperature measurement of motors is still lacking so far.
In this paper, a transmissive fluorescent temperature sensor (TFTS) of a sandwich structure that based on Er3+/Yb3+/Mo6+ tri-doped tellurite fiber is proposed for real-time thermal monitoring of motors using FIR technique. First, tellurite glasses of different molar ratios of Mo6+ ions are prepared using the melting-quenching technique, and draw the glass with the optimal UC fluorescence emission concentration into optical fiber. Then, this tellurite fiber is coupled between two multimode fibers (MMF) to form the proposed TFTS. Experiments are conducted to evaluate its temperature sensing characteristics. And taking advantage of its simple structure and small size, TFTS is mounted on the surface of the stator core and stator winding, and realize the real-time thermal monitoring of motors, which fully confirms the practicality of TFTS for accurate and real-time thermal monitoring of motors and the like.
2. Principle of FIR
It has been repeated reported that the 2H11/2, 4S3/2 energy of Er3+ ions are thermally coupled, and the corresponding FIR follows Boltzmann's principle as a function of temperature [28,29].
where FIR is defined as the ratio of I531 and I545, both of which are the intergrated UC fluorescence intensities of Er3+ ions, ΔE is the energy gap between these two energy states, KB is the Boltzmann constant, C is a constant, and T is the absolute temperature of thermal equilibrium.According to the FIR technique, the relative sensitivity (Sr), which represents the sensing capability of the sensor, can be defined as [30,31]:
3. Experiments and results
3.1 Design of TFTS
Compared with Er3+ and Yb3+ ions co-doped tellurite glasses, tellurite glasses triple-doped with Er3+, Yb3+ and Mo6+ ions exhibit enhanced UC fluorescence emission intensity [32]. Thereby, we adopt (77.3-x) TeO2 + 5ZnO + 5Li2CO3 + 12Bi2O3 + 0.2Er2O3 + 0.5Yb2O3 + xMoO3 (x = 0, 2, 4, 6, 8,10 mol%) to fabricate TFTS. In addition, in order to acquire satisfactory fluorescence emission, we have prepared Er3+/Yb3+/Mo6+ tri-doped tellurite glasses (TZLB) with different molar ratios of Mo6+ ions using the melt annealing method. Excited by a 980 nm Laser Diode (LD), the UC fluorescence spectra of these Er3+/Yb3+/Mo6+ tri-doped TZLB glasses are investigated under the same optical density. Figure 1(a) shows the recorded spectra in the range of 500-650 nm. We can see that there are two distinct emission peaks, which are located around the wavelength of 531 nm and 545 nm, respectively. The stronger emission band near 545 nm corresponds to the energy transition 4F9/2 to 4I15/2 of Er3+ ions, while the sub-emission band near 531 nm corresponds to the energy state 2H11/2 to 4I15/2 [33,34]. It should be noted that with the increase of the molar ratio of Mo6+ ions, the position of the fluorescence peak hardly changes, only the emission intensity becomes stronger. The strongest fluorescence intensity is observed at 8 mol% of Mo6+ ions. Beyond it, the fluorescence intensity starts to decrease due to the burst effect. Therefore, the Er3+/Yb3+/Mo6+ tri-doped TZLB glass with the best emission concentration (8 mol% of Mo6+) is drawn to fiber and used as the sensing unit. The produced Er3+/Yb3+/Mo6+ tri-doped TZLB fiber has its either end connected to an MMF to form a sandwich structure, completing the fabrication of TFTS, as shown in Fig. 1(c). Finally, the TFTS was used for the following optical density selection and thermal monitoring experiments.
Fig. 1. (a) UC fluorescence spectra of Er3+/Yb3+/Mo6+ tri-doped TZLB glasses with different molar ratios of Mo6+ ions. (b) The schematic diagram of the energy level of Er3+/Yb3+/Mo6+ tri-doped TZLB glass. (c) The structural diagram of TFTS.
When pumping the fabricated TFTS with different optical densities, it is found that the integral center and spectral shape of the output UC fluorescence spectrum remain basically unchanged, but the intensity increases continuously with the increasing optical density, as shown in Fig. 2(a). This indicates a close relationship between UC fluorescence intensity and optical density. At 1.523 W/cm2, the fluorescence intensity of Er3+ ions at 545 nm reaches about 10000 counts of magnitude, as shown by Fig. 2(b). Since this moderate exciting optical density produces fluorescence emission of sufficient intensity while inducing relatively small thermal effects, it is chosen for the subsequent experiments on real-time temperature monitoring of motors.
Fig. 2. (a) UC fluorescence spectra of TFTS (8 mol% of Mo6+) at different optical densities. (b) UC fluorescence spectrum of TFTS (8 mol% of Mo6+) at the optical density of 1.523 W/cm2.
Placing TFTS in a constant temperature and humidity chamber and adjusting temperature within the range of 274-373 K, the output UC fluorescence spectra are recorded in the wavelength range of 500-575 nm under the optical density of 1.523 W/cm2, as shown by Fig. 3(a). It can be observed that the spectral positions and shapes remain relatively stable, but the fluorescence intensities tend to decrease with the increase of temperature. Detailed information about the fluorescence intensities at wavelengths of 531 and 545 nm are depicted in Fig. 3(b). Notably, slight intensity increase occurs at 320 K and 360 K, respectively, which may be attributed to the wind force generated by the air conditioner of the chamber during the spectra recording. Nevertheless, this irregularity has extremely limited effect on the characterization of temperature sensing performance of TFTS.
Fig. 3. (a) UC fluorescence spectra of TFTS in the temperature range of 274-373 K. (b) Fluorescence intensities with temperature variation at UC spectral wavelengths of 531 and 545 nm.
3.2 Temperature sensing characteristics
Repeatability is an important feature reflecting the static characteristics of a temperature sensor. Generally, better repeatability indicates more reliable sensing performence. In this work, six sets of repeated temperature measurement experiments are conducted in a constant temperature and humidity chamber to evaluate the TFTS's repeatability. Additionally, an electronic thermometer with an accuracy of 0.1 is used to calibrate the temperature. Each experiment lasts one day, and each measurement point is held for 1 minute to guarantee its accuracy. The FIR values of these six sets of experiments exhibit high consistency, indicating that TFTS has good repeatability (see Fig. 4(a)). Based on it, the FIR mean values and error bars are calculated for each temperature point. We can find that relatively large errors occur at 353.9 K (0.0052) and 373.2 K (0.0042), while those of the remaining 16 temperature points do not exceed 0.004, which further verifies the repeatability of TFTS. A nonlinear fitting curve (R2 = 0.99916) of FIR versus temperature is obtained based on Eq. (1), establishing an exponential function between the FIR values and temperature to be measured. In addition, the relative sensitivity of the TFTS is calculated using Eq. (2). As shown in Fig. 4(c), the relative sensitivity gradually decreases with the temperature increase, the maximum value being 0.01068 K−1 at 274 K. Table 1 shows a comparison of the relative sensitivities of TFTS and other temperature sensors of tellurite matrix materials doped with different rare earth ions. The results show that TFTS has some advantages in terms of sensitivity. Although it has a narrower temperature detection range, it is able to meet the requirements of temperature monitoring of the motor as discussed in this study. If there is a need for higher temperature detection, we can simply use a constant temperature and humidity chamber to raise the temperature and recalibrate the temperature detection range of the TFTS.
Fig. 4. (a) FIR values with temperature variation during the repeatability tests. (b) Averaged FIR values with error bars as a function of temperature. (c) Relative sensitivity curve using FIR. (d) FIR values at fixed temperatures (288.1 K and 348.8 K) during 90 repeated tests.
Table 1. Relative sensitivity of different tellurite glasses as temperature sensors.
Finally, in order to verify whether TFTS can work stably at a fixed temperature for a long time, 90 repeated thermal monitoring tests are performed at 293.4 K and 316.9 K, respectively. Each time after recording the UC fluorescence spectra, the 980 nm LD is turned off immediately to reduce the interference of its thermal effect on the results, and a time interval of 1 min is inserted between two successive tests. As shown in Fig. 4(d), the maximum fluctuations of FIR values are controlled within 0.01551 at 293.4 K, and within 0.01908 at 316.9 K throughout the 90 repetition tests. This verifies that TFTS has good stability at a fixed temperature and can be well applied to the following thermal monitoring experiments of the motor.
3.3 Temperature detection of motor
After evaluating the sensitivity and stability of the TFTS, we apply it to the real-time temperature detection of a medium-sized fan motor. The operating temperature range of the motor is 293-343 K at room temperature, and the temperature monitoring range of the designed TFTS is 274-373 K, which is sufficient to realize the temperature monitoring of the motor. Here, we fabricate two similar TFTSs following the aforementioned precedent. Before conducting the experiment, the fan blades are disassembled to avoid interference with the measurement results. Remove the motor casing and tightly attach the sensing units of TFTSs to the motor, one on the surface of the stator core and the other on that of the stator winding. After that, remount the casing. Figure 5 depicts the experimental setup, and in order to show the temperature monitoring process more clearly, the figure doesn’t involve the motor casing. A 980 nm LD is used as the pump source, an ocean optical spectrometer (USB 2000+, Ocean Insight) is used to decompose the light of complex composition into spectral lines and a computer is used to record the fluorescence spectrum. In addition, an infrared thermometer is used to provide reference temperature.
Fig. 6. (a) FIR values and the corresponding temperature during the thermal monitoring of the stator core. The inset shows the physical layout. (b) FIR values and the corresponding temperature during the thermal monitoring of the stator winding. The inset shows the physical layout.
Thermal monitoring starts after the motor runs for 1 minute, and the fluorescence spectrum is collected every minute. The experimental data are collated to attain the FIR value and the corresponding temperature of the stator core and stator winding, as shown by Fig. 6(a) and (b). The reference temperature is plotted in the figures as well. It can be found that the maximum error between the FIR-demodulated temperature and the reference temperature is only 1.2 K for the stator core and 1.0 K for the stator winding. The error of the stator core is slightly higher, because TFTS attached to it is much closer to the thermally conductive copper wire than the infrared thermometer, resulting in a relatively larger disparity with the reference temperature.
The TFTS proposed in this work, to the best of our knowledge, is the first FTS ever reported to be applied to real-time thermal monitoring of motors. It involves a principle (namely, the FIR technique) simpler than that of interferometric and optical micro-cavity temperature sensors [39–41], and achieve stable demodulation of temperature. Compared to Fiber Bragg Grating temperature sensors with intrinsic strain sensitivity [42], it will not distort temperature readings despite the motor vibration. Therefore, there is no need to encapsulate the TFTS, and its sensing unit can be directly attached to the surface of the stator core and stator windings to achieve accurate temperature monitoring. One the other hand, compared with electronic and infrared thermometers, this TFTS has the advantages of compactness and simplicity, corrosion resistance and electromagnetic interference resistance. In addition, integrating TFTS inside the motor casing can not only reduce the impact on measurement results due to the insulation of the motor casing, but also achieve better monitoring of the operating condition of the motor.
4. Conclusion
In conclusion, Er3+/Yb3+/Mo6+ tri-doped TZLB glasses with different molar ratios of Mo6+ ions are experimentally prepared, and that with optimal UC fluorescence emission intensity is drawn into optical fiber. This tellurite fiber works as the sensing unit of TFTS for real-time thermal monitoring. Utilizing the FIR technique, the thermal monitoring performance of TFTS is evaluated in the temperature range of 274-373 K, establishing an excellent nonlinear relationship (R2 = 0.99916) between FIR and temperature. In addition to this, the maximum relative sensitivity of TFTS is 0.01068 K−1 at 274 K in the measured temperature range. Finally, it is used to monitor the temperature variation of the stator core and stator winding under actual operation. This FIR-based TFTS has the characteristics of compactness and simplicity, corrosion resistance, high stability and electromagnetic interference resistance. It is expected to provide a new solution for accurate thermal monitoring of motors and the like.
Funding
Natural Science Foundation of Liaoning Province (2022JH2/101300241); National Natural Science Foundation of China (62203090); Fundamental Research Funds for the Central Universities (N2104022); Basic and Applied Basic Research Foundation of Guangdong Province (2022A1515220086); Natural Science Foundation of Hebei Province (F2020501040); 111 Project (B16009).
Acknowledgments
The authors thank the Liao Ning Revitalization Talents Program.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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