Verifying an all fused silica miniature optical fiber tip pressure sensor performance with turbine engine field test Xingwei Wang *a, Juncheng Xu a, Yizheng Zhu a, Bing Yu a, Ming Han a, Zhuang Wang a, Kristie L. Cooper a, Gary R. Pickrell a, Anbo Wang a, Aditya Ringshia b, and Wing Ng b a Center for Photonics Technology, The Bradley Department of Electrical and Computer Engineering, b Department of Mechanical Engineering, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061 USA ABSTRACT Pressure sensors are the key elements for industrial monitoring and control systems to lower equipment maintenance cost, improve fuel economy, reduce atmospheric pollution, and provide a safer workplace. However, the testing environment is usually harsh. For example, inside the turbine engine, temperatures might exceed 600 C and pressures might exceed 100psi (690kPa), where most current available sensors cannot survive. Moreover, due to the restricted space for installation, miniature size of the sensor is highly desirable. To meet these requirements, a novel type of all fused silica optic fiber tip pressure sensor with a 125µm diameter was developed. It is a diaphragm based pressure sensor in which a Fabry-Pérot interferometer is constructed by the end face of an optical fiber and the surface of a diaphragm connected by a short piece of hollow fiber. The FP cavity length and the interference pattern will change according to ambient pressure variation. Its main improvement with respect to previously developed optical sensors, such as those utilizing techniques of wet etching, anodic bonding and sol-gel bonding, is the fact that no chemical method is needed during the cavity fabrication. Its dynamic pressure performance was verified in a turbine engine field test, demonstrating not only that it can safely and reliably function near the fan of a turbine engine for more than two hours, but also that its performance is consistent with that of a commercial Kulite sensor. Key words: Fiber optic sensors; miniature; pressure sensor; turbine engine 1. INTRODUCTION Pressure measurements are very important for industrial monitoring and control systems. They can help to lower equipment maintenance cost, improve fuel economy, reduce atmospheric pollution, and provide a safer workplace. In the industrial community, miniature pressure sensors, which can sustain extreme high temperature, high pressure and corrosive environments, are in great demand. For example, pressure testing is commonly required in oil well downholes, jet engines, or power generation equipments, where the space for sensor installation is limited and the environment is harsh 1-2. However, most of the current pressure sensors, which are electrical-based, suffer from many inherent problems 3-4, such as high temperature dependence, electromagnetic interference, bulky size, poor performance in mechanical properties, temperature endurance, accuracy, reliability, long time stability, repeatability, etc. For instance, industry primarily relies on semiconductor pressure sensors for pressure measurement at elevated temperatures, but their limited maximum operating temperature is only 482 C, which can easily be exceeded in many industrial environments. Inside the turbine engine, temperatures might exceed 600 C and pressures might exceed 100psi (690kPa), where most current available sensors cannot survive 5-7. In addition, they exhibit very short lifetimes and rather poor reliability in corrosive environments. Last but not least, thermal expansion mismatch between different materials can cause changes in * xiwang@vt.edu; phone: 1 540 951-3488; fax: 1 540 231-2158; http://www.ee.vt.edu/~photonics Sensors for Harsh Environments II, edited by Anbo Wang, Proc. of SPIE Vol. 5998, 59980L, (2005) 0277-786X/05/$15 doi: 10.1117/12.631050 Proc. of SPIE Vol. 5998 59980L-1 AC Pressure Sensor
temperature to appear as changes in pressure. Since temperature can fluctuate in extreme environments, a pressure sensor for these extreme environments will ideally have negligible sensitivity to temperature. 8-9 These requirements impose severe constraints on the sensor design and material composition. Fortunately, the miniature optical fiber tip pressure sensor presented in this paper is a good candidate to solve the above problems. It can be reliably used in high temperature, high pressure, and corrosive environments. In addition, it is chemically inert, very small, insensitive to temperature changes, immune to electromagnetic interference, easily manufacturable and inexpensive. It is an all fused silica optic fiber tip pressure sensor with a 125µm diameter. The miniature optical fiber tip pressure sensor is a diaphragm-based extrinsic Fabry-Pérot interferometric (EFPI) sensor. It has a piece of hollow fiber tubing connecting a piece of standard optical fiber and a pressure-sensitive diaphragm. The fiber endface and diaphragm define an etalon cavity. The length of the etalon cavity changes under applied pressure. Thus, the interference pattern shifts and supplies the results. The fabrication includes fusion splicing a hollow tube to an endface of an optical fiber, and then cleaving the hollow tube and fusion splicing a diaphragm to the hollow tube. This method requires only a fusion splicer and cleaver for fabricating the pressure sensor. Optionally, the diaphragm thickness can be adjusted by exposing an exterior surface of the diaphragm to an etchant. Because the sensor is capable of operating under harsh environment such as high temperature and high pressure, using materials with very the same CTE (coefficient of thermal expansion) as that of optical fiber is very important to reduce thermal induced air-gap changes and failures. Ordinary single mode optical fibers are made of fused silica. It s very straightforward to use fused silica for the cavity and the diaphragm. Its main improvement with respect to previously developed optical sensors, such as those utilizing techniques of wet etching -10-11, is the fact that no chemical method is needed during the cavity fabrication. This ensures the fabrication is safe, simple and cost efficient. Without the need to deal with hazardous products and a fume hood, the maintenance of the lab is also much easier. Every one, even those without chemical process training, can fabricate the sensor with common tools found in a photonics lab, such as cleaver and splicer. After the sensors were fabricated, their dynamic pressure performance was verified in a turbine engine field test, demonstrating not only that it can safely and reliably function near the fan of a turbine engine for more than two hours, but also that its performance is consistent with that of a commercial Kulite sensor. This paper is organized as following: the principle of the miniature pressure sensor is presented in section 2; the fabrication steps are illustrated in section 3; the installation location in the field test is introduced in section 4; then section 5 presents the dynamic pressure measurement results in a turbine engine. 2. SENSOR PRINCIPLE The configuration of the sensor is shown as Figure 1. A single mode silica optical fiber transmits light from a laser diode to the sensor element through a 2x2 coupler. In the sensor head, the laser light is partially reflected and partially transmitted across the gap formed by the end of the input fiber and a diaphragm. The light from the first interface (the input fiber end) and the light reflected at the second interface (the endface of the diaphragm) interfere with each other to modulate the returned optical spectrum. A sinusoidal interference signal can be obtained at the photo-detector due to differential phase changes between the two light beams as a result of changes in the EFPI sensor cavity spacing. Proc. of SPIE Vol. 5998 59980L-2 AC Pressure Sensor
Laser Diode Coupler Sensor Index Matching Gel Detector Optical Fiber Hollow Fiber Diaphragm Light d Figure 1: Configuration of the miniature sensor The detected photodiode signal current can be shown as a function of the phase difference between the two reflected optical fields and is given by: 2 2 I = I + I + 2I I cos φ φ (1) ( ) 1 2 1 2 1 2 where I 1 and I 2 are the light intensities reflected at the fiber end and the diaphragm respectively, and φ 1 φ 2 is the relative phase difference between the two light signals. If we assume I 1 and I 2 to be equal, equation (1) can be rewritten as 4π d I = 2I0 1+ cos λ (2) 4π d = 2I0 1+ cos φ0 + λ Proc. of SPIE Vol. 5998 59980L-3 AC Pressure Sensor
where d denotes the length of the cavity, d is the airgap change, λ is the laser diode wavelength of operation in free space, and ф 0 is the phase constant related to initial airgap. When the applied pressure changes, the diaphragm will deform accordingly, resulting in an air-gap change 12 : 2 4 3(1 µ )R L= P (3) 3 16Eh where P is the pressure variation, R is the radius of the diaphragm, E is the Young s modulus for the material, µ is Poisson s ratio and h is the thickness of the diaphragm. Therefore the airgap change is linearly dependent on the applied pressure. Resolving the air-gap change from the spectral shift can thus provide information about pressure. Figure 2 gives the photograph of such a structure. Optical Fiber Hollow Fiber Diaphragm - - Figure 2: Photograph of the new sensor structure 3. SENSOR FABRICATION Simply speaking, the fabrication process can be summarized as the following steps: 1. Splice a piece of hollow fiber tube at the cleaved end of a standard fiber. 2. Cleave the hollow fiber tube near the junction according to the requirement of the cavity length. The cavity length can be cleaved down to the order of micrometers under the inspection of a microscope or CCD. 3. Connect the diaphragm at the other end of the hollow fiber tube. The method can be splicing, agglutinating, heating, etc. 4. Control the diaphragm s thickness to the order of sub-microns by dipping the fiber head into HF acid of suitable concentration. This step is optional according to the requirement. Cavity fabrication and diaphragm bonding are two main steps during fabrication. The cavity is formed by cleaving and splicing a piece of hollow fiber. This is one of the unique feature of this sensor. Table 1 illustrates the 5 steps to fabricate the sensor cavity. Proc. of SPIE Vol. 5998 59980L-4
Table 1 Illustration of Cavity Fabrication Description Schematics Photos a) Cleave a piece of standard optical fiber. SM Fiber b) Cleave a piece of hollow fiber. Hollow Fiber c) Align these two fibers with a splicer. d) Splice the standard optical fiber with the hollow fiber. Splicing point e) Cleave the other end of the hollow fiber. - 111 Proc. of SPIE Vol. 5998 59980L-5
A simple bonding method for diaphragm with the cavity is shown in Table 2. A pure fused silica fiber is spliced with the cavity and then cleaved, leaving a diaphragm on top of the cavity. Table 2 Illustration of diaphragm bonding Description Schematics Photos a) Complete cavity formation. b) Align the hollow tube with a piece of fiber. c) Splice the hollow tube with the optical fiber. d) Cleave the optical fiber to form the diaphragm. 4. INSTALLATION LOCATION IN THE FIELD TEST After the sensors were fabricated, we successfully installed and tested the miniature fiber tip pressure sensor near the fan of a turbo engine. The installation location of the sensors is shown in Figure 3-4. Figure 3 shows the five holes in the ring encircling the fan of the engine, which were used for sensor installation. The center hole was used for the installation of a commercial Kulite sensor, which acted as a reference during our test. The miniature fiber tip sensor was positioned 40mm away from the Kulite sensor, as shown in Figure 4. Proc. of SPIE Vol. 5998 59980L-6
Figure 3 Installation locations are highlighted by the red ellipse. Safety Plate Kulite Sensor Optical Miniature Sensor Figure 4 Outside view of the location with the installed sensors. 5. Performance Analysis Before the field test, the optical sensors and the commercial Kulite sensor were calibrated. Their response is about 15.7mV/psi and 33mV/psi, respectively. During more than two hours test, both sensors functioned reliably. Field test results show that both the optical sensor and the Kulite sensor have a high response to the fundamental frequency and its multiple frequencies, as shown in Figure Proc. of SPIE Vol. 5998 59980L-7
5.. Comparisons of the responses are given up to sixfold frequency; their frequency and corresponding signal amplitude are listed in Table 3. 0.8 0.8 Optical Sensor 0.4 0.2 L 0 20 Frequency (khz) 0.8 60 Kulite Sensor 80 0 0 10 20 30 40 60 Frequency (khz) 80 Figure 5. Signals for optical sensor and Kulite sensor Table 3 Comparison results between the optical sensor and the Kulite sensor Frequency (khz) Amplitude (psi) Optical Sensor Kulite Sensor Optical Sensor Kulite Sensor Frequency 1 5.985 5.985 0.4470 0.4491 Frequency 2 11.970 11.970 0.2520 0.3393 Frequency 3 17.960 17.955 0.2119 0.2208 Frequency 4 23.945 23.940 0.0918 0.1267 Frequency 5 29.940 29.930 0.0528 0.1378 Frequency 6 35.923 35.915 0.0427 0.0706 Table 3 shows little difference between the results of the fundamental frequency and its multiple frequencies obtained from these two kinds of sensors. This consistency between the optical sensor and the commercial Kulite sensor is a strong support for the validity of the optical sensor. Proc. of SPIE Vol. 5998 59980L-8
6. CONCLUSIONS In summary, this paper presents the principle, fabrication, and field test results of an all fused silica miniature optical fiber tip pressure sensor. For dynamic pressure measurements in a turbine engine, it showed comparable results with a semiconductor Kulite pressure sensor. The sensor is made of all fused silica material, which eliminates the thermal expansion mismatch problem. The miniature sensor has great potential to work at high temperature environments where conventional pressure sensors cannot survive. ACKNOWLEDGMENTS This research was sponsored by the U.S. Department of Energy under grant DE-FC36-01G011050. The authors would like to thank Micron Optics for the partial donation of the CTS used in this research. REFERENCES 1. D. Goustouridis, P. Normand, and D. Tsoukalas, Ultraminiature silicon capacitive pressure-sensing elements obtained by silicon fusion bonding, Sens. Actuators A, vol. A68, pp. 269-274, 1998. 2. P. Melva s, E. K lvesten, P. Enoksson, and G. Stemme, A free-hanging strain-gauge for ultraminiaturized pressure sensors, Sens. Acuators A, vol. 97-98, pp. 75-82, 2002. 3. Bing Yu, Dae Woong Kim, Jiangdong Deng, Hai Xiao, and Anbo Wang, Fiber Fabry-Perot Sensors for Detection of Partial Discharges in Power Transformers, Applied Optics-OT, vol. 42 Issue 16, pp. 3241-3250, Jun. 2003. 4. Z. Wang, F. Shen., X. Wang, and A. Wang, Smart Structures & Systems (SSS), An International Journal of Mechatronics, Sensors, Monitoring, Control, Diagnosis, and Maintenance. (to be published). 5. Gander, M.J., et al, Embedded micromachined fiber-optic Fabry-Perot pressure sensors in aerodynamics applications, IEEE Sensors Journal, vol. 3, pp. 102-107, Feb. 2003. 6. J. Xu, G. Pickrell, X. Wang, W. Peng, K. Cooper and A. Wang, A novel temperature-insensitive optical fiber pressure sensor for harsh environments, IEEE Photon. Tech. Lett., vol. 17, no. 4, Apr, 2005. 7. J. Xu, G. Pickrell, B. Yu, M. Han, Y. Zhu, X. Wang, K. Cooper, and A. Wang, Epoxy-free high-temperature fiber optic pressure sensors for gas turbine engine applications, in SPIE OpticsEast Sensors for Harsh Environments, Anbo Wang; Ed. Proc. SPIE Vol. 5590, p. 1-10, (2004). 8. X. Wang, J. Xu, Y. Zhu, B. Yu, M. Han, K. Cooper, G. Pickrell and A. Wang, An optical fiber tip sensor for pressure measurement in high temperature environments, Optics in the Southeast sponsored by OSA, Charlotte, NC, Nov. 4-5, 2004. 9. X. Wang, J. Xu, Y. Zhu, B. Yu, M. Han, K. Cooper, G. Pickrell and A. Wang, An Optical Fiber Tip Pressure Sensor for Medical Applications, Conference on Lasers and Electro-Optics / Quantum Electronics and Laser Science Conference (CLEO/QELS), 2005. 10. Y. Zhu, A. Wang, Miniature fiber-optic pressure sensor, IEEE Photon. Tech. Lett., vol. 17, no. 2, Feb, 2005. 11. Y. Zhu, G. Pickrell, X. Wang, J. Xu, B. Yu, M. Han, K. Cooper, A. Wang, A. Ringshia, and W. Ng, Miniature fiber optic pressure sensor for turbine engine, in SPIE OpticsEast Sensors for Harsh Environments, Anbo Wang; Ed. Proc. SPIE Vol. 5590, p. 11-18,(2004). 12. M. D. Giovanni, Flat and Corrugated Diaphragm Design Handbook. New York: Mercel Dekker, 1982. Proc. of SPIE Vol. 5998 59980L-9