Optical fiber type tip timing sensor based on APD array imaging method

Abstract

The invention discloses an optical fiber type tip timing sensor based on an APD array imaging method. The optical fiber type tip timing sensor comprises N transmitting end couplers, a transmitting optical fiber, a transmitting optical fiber bundle cable, a receiving and transmitting end beam combiner, a receiving and transmitting optical fiber cable, a probe, a receiving optical fiber bundle cable, a beam combiner, a receiving optical fiber and N receiving end couplers. N > = 5; the N transmitting end couplers are respectively connected into the laser transmitters with different wavelengths, and are respectively connected with the beam combiner at the transmitting end through N transmitting optical fibers; the N receiving end couplers are respectively connected with the beam combiner of the receiving end through N receiving optical fibers; the beam combiner of the transmitting end and the beam combiner of the receiving end are respectively connected with the input end of the beam combiner of the receiving and transmitting end through a transmitting optical fiber bundle cable and a receiving optical fiber bundle cable; the output end of the transceiving end beam combiner is connected with a probe through a transceiving optical fiber cable; and a to-be-detected blade is arranged in front of the probe.

Description

Technical Field

[0001] The present invention belongs to the field of sensors, and particularly relates to an optical fiber tip-timing sensor based on an APD array imaging method. Background Art

[0002] Major rotating machinery such as steam turbines and aero-engines is a core component of key equipment such as industrial generator sets, steam units, various aircraft, and ships. As the core component for a rotating machinery to do work, the parameter changes of the working state of the blade directly affect the working efficiency and operation safety of the rotating machinery. Blade vibration is one of the main reasons for the failure of rotating machinery. Therefore, the on-line measurement of blade vibration parameters is an important part of the state monitoring and full-life cycle health management of rotating machinery, and is an important guarantee for the safe, efficient, and stable operation of the unit. Due to the rapid development trend of modern aero-engines in recent years, the requirements for high thrust, high performance, and high speed have placed the rotor blades in a more complex and harsh working condition. The blade will generate various vibration modes including bending vibration, torsional vibration, and bending-torsional combined vibration under complex external excitations. The high-order vibration and small cracks in the blade will directly be reflected in the small changes of the blade vibration parameters. It is calculated that the amplitude of the high-order bending vibration of the blade is only dozens of micrometers, and the vibration displacement change caused by small cracks in the blade is also between ten and dozens of micrometers. Therefore, the measurement accuracy of the vibration displacement of the rotor blade must be high enough to accurately measure the small amplitude, and then accurately monitor the working state of the rotor blade.

[0003] The tip-timing technology is the main means in the field of non-contact on-line measurement of blade vibration parameters at home and abroad. Among them, the optical fiber tip-timing sensor has the advantages of small structure, fast response speed, and high measurement accuracy, and is especially suitable for measurement environments such as narrow spaces and high speeds. The existing optical fiber tip-timing sensor adopts a fiber bundle structure, which consists of multiple multimode receiving fibers closely surrounding a single-mode transmitting fiber. The transmitting fiber and the receiving fibers are independent of each other, so a high signal-to-noise ratio can be obtained. To measure the blade vibration, multiple detection holes need to be opened on the casing for installing the sensor probe. However, due to the limitations of the casing opening of the aero-engine, the number of measurement points using the existing tip-timing sensor is very limited, and at the same time, the intervention of the sensor will affect the overall aerodynamic model of the aero-engine. In addition, the spot radius of the probe of the existing optical fiber tip-timing sensor irradiated on the blade end face is relatively large, and the measured accuracy is low when the blade moves at high speed; the distance between the measurement points of the traditional optical fiber sensor is relatively far, and it is difficult to solve the high-order vibration with a small calculated amplitude. Summary of the Invention

[0004] The object of the present invention is to overcome the deficiencies in the prior art, achieve the measurement of blade vibration information with a single probe, improve the resolution of blade vibration identification, and provide an optical fiber tip-timing sensor based on the APD array imaging method. This sensor reduces the overall size of the front end of the tip-timing system through the structural layout of the combination of the optical fiber array and the microlens array, realizes the measurement of blade vibration with a single probe, and achieves a lower system intervention degree and a higher timing accuracy tip-timing sensor.

[0005] The object of the present invention is achieved through the following technical solutions:

[0006] An optical fiber tip-timing sensor based on the APD array imaging method, comprising: N transmitting end couplers, transmitting optical fibers, transmitting optical fiber bundle cables, transceiver beam combiners, transmitting and receiving optical fiber cables, probes, receiving optical fiber bundle cables, beam combiners, receiving optical fibers, and N receiving end couplers; N≥5;

[0007] The N transmitting end couplers are respectively connected to laser transmitters with different wavelengths, and are respectively connected to the beam combiner at the transmitting end through N transmitting optical fibers; the N receiving end couplers are respectively connected to the beam combiner at the receiving end through N receiving optical fibers;

[0008] The beam combiner at the transmitting end and the beam combiner at the receiving end are respectively connected to the input end of the transceiver beam combiner through the transmitting optical fiber bundle cable and the receiving optical fiber bundle cable;

[0009] The output end of the transceiver beam combiner is connected to the probe through the transmitting and receiving optical fiber cable, and a blade to be measured is arranged in front of the probe.

[0010] Further, the transmitting optical fiber bundle cable and the receiving optical fiber bundle cable are coated with tensile and compressive resistant cable materials to ensure that the transmitting optical fiber bundle cable and the receiving optical fiber bundle cable have the ability of anti-tensile and anti-trampling;

[0011] Further, filters corresponding to the laser wavelengths emitted by the transmitting ends are respectively installed on the N receiving end couplers to improve the anti-interference ability.

[0012] Further, the transmitting and receiving optical fiber cable contains N small numerical aperture single-mode transmitting optical fibers for transmitting the transmitted optical signal and N large numerical aperture multimode receiving optical fibers for transmitting the received optical signal, and the transmitting and receiving optical fiber cable also selects tensile and compressive resistant cable materials.

[0013] Further, a first beam splitter, a second beam splitter, a third beam splitter and a half lens are provided inside the probe. The first beam splitter divides the transmitting and receiving optical fiber cable into a transmitting optical fiber cable and a receiving optical fiber cable. The transmitting optical fiber cable is further divided into N single-mode transmitting optical fibers by the second beam splitter. The N single-mode transmitting optical fibers are arranged in a rectangular array by being fixed inside a glass tube. A microlens array arranged in the same way as the single-mode transmitting optical fibers is fused at the front ends of the N single-mode transmitting optical fibers to achieve the collimation function of the light emitted from the single-mode optical fibers.

[0014] At the receiving end of the detection light, the receiving optical fiber cable is divided into N multimode receiving optical fibers by the third beam splitter and fixed into the same array arrangement by the glass tube. A same microlens array is fused at the front ends of the N multimode receiving optical fibers to achieve the focusing during the reception of the detection light. The arrangement modes of the two microlens arrays are completely the same, but the end faces are perpendicular to each other. A half lens with an optical axis angle of 45° with the output end of the transmitting optical fiber cable is provided in front of each microlens array to change the optical path so that the detection light can smoothly enter the microlens array at the front end of the multimode receiving optical fiber. During the measurement, the laser is emitted from the transmitting end, irradiated onto the to-be-detected blade through the transmission surface of the half lens, and then the reflected light from the end face of the to-be-detected blade is reflected by the reflection surface of the half lens and incident on the receiving end.

[0015] Further, the protection structures of the transmitting optical fiber cable, the receiving optical fiber cable and the transmitting and receiving optical fiber cable are selected as a metal armored sheath or a non-metallic sheath.

[0016] Further, FC connectors are adopted for the transmitting end coupler and the receiving end coupler.

[0017] Compared with the prior art, the beneficial effects brought by the technical solution of the present invention are as follows:

[0018] The present invention has the following beneficial effects:

[0019] 1. The present invention realizes the integration of multiple single-mode transmitting optical fibers inside a single sensor probe, and arranges the multiple single-mode transmitting optical fibers in a rectangular array. A microlens array arranged in the same way as the single-mode transmitting optical fibers is fused at the front ends of the multiple single-mode transmitting optical fibers to achieve the collimation function of the light emitted from the single-mode optical fibers. While increasing the number of measurement points, the vibration of the blade can be detected by a single sensor, and the overall intrusion degree of the front end of the system can be reduced.

[0020] 2. The present invention uses the wavelength division technology. To solve the problems of dense distribution of detection points in the array detection and signal crosstalk between the detection points, a combination of a multi-wavelength laser source array and a filter is adopted. Each set of transmitting and receiving array elements corresponds to one wavelength, and a filter corresponding to the wavelength is added in the photoelectric conversion part to reduce noise and improve the signal-to-noise ratio of the tip timing signal.

[0021] 3. By reasonably setting the arrangement mode of the array and combining with the semi-lens to compress the optical path, the present invention can effectively reduce the spot diameter formed by the emitted light on the end face of the measured blade, realize the arrangement of multiple measuring points in the circumferential and axial directions of the rotor, and obtain the vibration information of the blade in the circumferential and axial directions. The simultaneous multi-dimensional measurement of the bending vibration and torsional vibration of the blade to be measured is realized, and the measurement accuracy of the tip-timing signal is improved.

[0022] 4. The present invention realizes the acquisition of high-order vibration signals with small amplitudes through a dense array distribution. The measuring points of the traditional tip-timing system are far apart, and the high-order vibration signals have the characteristics of high frequency and low amplitude. The sampling rate of such signals is seriously insufficient using traditional methods and cannot be identified. This sensor greatly reduces the distance between the measuring points, reduces the spot size at the same time, improves the timing resolution, and improves the sampling rate of high-frequency signals through the above measures, so it can be identified.

[0023] In summary, the present invention overcomes the disadvantages of the existing fiber optic tip-timing sensor, such as large emission spot, low timing resolution, high degree of measurement signal crosstalk when installed at close range, and inability to meet the requirements of high-precision measurement of vibration parameters of moving blades of modern high-speed aero-engines. This sensor has the characteristics of high measurement accuracy, strong anti-interference ability, etc., and is applicable to the detection of high-order vibrations with small amplitudes. Description of the Drawings

[0024] Figure 1 Shows a schematic diagram of a fiber optic tip-timing sensor based on the APD array imaging method in this embodiment and a cross-sectional view of the optical fiber cable involved in the sensor.

[0025] Figure 2a Shows a schematic cross-sectional view of the sensor probe side in this embodiment, Figure 2b Shows a schematic arrangement diagram of the microlens array.

[0026] Figure 3 Shows the optical path inside the sensor probe when the present invention is working.

[0027] Figure 4 Shows the distribution of the measuring points on the end face of the blade to be measured and the tip-timing signal schematic diagram when the present invention is working.

[0028] Reference Numerals: 1 - transmitting end coupler, 2 - transmitting optical fiber, 3 - transmitting optical fiber bundle cable, 4 - transceiver beam combiner, 5 - transceiver optical fiber cable, 6 - probe, 7 - blade to be measured, 8 - receiving end coupler, 9 - receiving optical fiber, 10 - beam combiner, 11 - receiving optical fiber bundle cable, 12 - housing, 13 - first beam splitter, 14 - transmitting optical fiber cable, 15 - receiving optical fiber cable, 16 - second beam splitter, 17 - single-mode transmitting optical fiber, 18 - microlens array, 19 - semi-lens, 20 - multimode receiving optical fiber, 21 - third beam splitter. Detailed Implementation Modes

[0029] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0030] To overcome the shortcomings of the existing fiber optic tip-timing measurement system, such as the need to open multiple holes on the casing, a single sensor cannot simultaneously measure the bending vibration and torsional vibration of the blade, there are fewer measurement points, and the timing resolution is not high, which cannot meet the high-precision measurement of vibration parameters of modern high-speed aircraft engines. This embodiment designs a fiber optic sensor based on the APD array imaging method. By using a multi-fiber array arrangement structure, combining a semi-lens compression optical path and a microlens array technology, it can effectively reduce the spot diameter formed by the emitted light on the end face of the measured blade, realize the distribution of multiple measurement points in the circumferential and axial directions of the rotor, and realize the simultaneous measurement of the 7 bending vibrations and torsional vibrations of the blade to be measured. To solve the problem of dense distribution of measurement points in the array detection and signal crosstalk between measurement points, a combination of a multi-wavelength laser source array and a filter is adopted. Each set of transceiver array elements corresponds to a wavelength, and a filter corresponding to the wavelength is added to the photoelectric conversion part to reduce noise.

[0031] Specifically, this embodiment provides a fiber optic tip-timing sensor based on APD array imaging, such as Figure 1 shown, taking a 4x2 rectangular array arrangement of probes as an example. It includes 8 transmitting end couplers 1, transmitting optical fibers 2, transmitting optical fiber bundle cables 3, transceiver beam combiners 4, transmitting and receiving optical fiber cables 5, probes 6, receiving optical fiber bundle cables 11, beam combiners 10, receiving optical fibers 9, and 8 receiving end couplers 8. Inside the probe, a beam splitter and a glass tube fixing method are used to arrange the transmitting optical fiber bundle 14 and the receiving optical fiber bundle 15 in a 4x2 rectangular array, and a microlens array 18 is fused at the front end of the optical fiber to compress the spot diameter; a semi-lens 19 is used to compress the optical path and reduce the volume of the probe; then, based on lasers of different wavelengths and filters, interference between adjacent measurement points is avoided; by arranging the measurement points in the circumferential and axial directions of the rotor, the bending vibration and torsional vibration signals can be obtained simultaneously. Compared with the traditional fiber optic tip-timing sensor, the spot diameter irradiated on the blade end face is effectively reduced, more measurement points are obtained, and the timing resolution is improved. The invention will be further described below with reference to the accompanying drawings and embodiments.

[0032] Preferably, the 8 transmitting end couplers 1 can adopt ST connectors. Each transmitting end coupler 1 contains a single-mode transmitting optical fiber for transmitting optical signals, and the single-mode transmitting optical fiber is located inside the transmitting optical fiber 2 to ensure that the optical fiber is not affected by trampling, pulling, etc., and improve reliability; different wavelengths of lasers are connected to each coupler for distinction. The size parameters of the single-mode transmitting optical fiber are (core diameter / cladding / coating layer, μm) 9 / 125 / 150, and the numerical aperture NA is 0.11.

[0033] Preferably, the 8 receiving end couplers 8 also adopt ST connectors. Each receiving optical fiber 9 contains a multi-mode receiving optical fiber inside; in particular, to avoid signal crosstalk between different measuring points, a filter for the laser wavelength emitted by the corresponding transmitting end can be embedded inside the receiving end coupler 8 to improve its anti-interference ability. The size parameters of the multi-mode receiving optical fiber are (core diameter / cladding / coating layer, μm) 50 / 125 / 150, and the numerical aperture NA is 0.22.

[0034] Preferably, the transmitting optical fiber bundle cable 3 is connected to one end of the transmitting optical fiber 2 through the transmitting end beam combiner 10. The transmitting optical fiber bundle cable 3 contains 8 single-mode transmitting optical fibers inside. The size parameters of the single-mode transmitting optical fiber are (core diameter / cladding / coating layer, μm) 9 / 125 / 150, and the numerical aperture is 0.11. The single-mode transmitting optical fiber is wrapped by a tensile and compressive resistant cable to ensure that the optical fiber is not affected by stepping on, pulling, etc., and improve reliability.

[0035] Preferably, the receiving optical fiber bundle cable 11 is connected to one end of the receiving optical fiber 9 through the receiving end beam combiner 10. The receiving optical fiber bundle cable 11 contains 8 multi-mode optical fibers inside. The size parameters of the multi-mode optical fiber are (core diameter / cladding / coating layer, μm) 50 / 125 / 150, and the numerical aperture NA is 0.22. The optical fiber is also wrapped by a tensile and compressive resistant cable to ensure that the optical fiber is not affected by stepping on, pulling, etc., and improve reliability.

[0036] Preferably, in the present invention, the transmitting optical fiber bundle cable 3 and the receiving optical fiber bundle cable 11 are combined into a transceiver optical fiber cable 5 through the transceiver end beam combiner 4. It contains 8 single-mode transmitting optical fibers and 8 multi-mode receiving optical fibers inside. The parameters are as follows: the size parameters of the single-mode optical fiber are (core diameter / cladding / coating layer, μm) 9 / 125 / 150, and the numerical aperture is 0.11; the size parameters of the multi-mode optical fiber are (core diameter / cladding / coating layer, μm) 50 / 125 / 150, and the numerical aperture NA is 0.22; the outermost layer is wrapped by a tensile and compressive resistant cable to ensure that the optical fiber is not affected by stepping on, pulling, etc., and improve reliability.

[0037] Specifically, in the sensor probe 6, the APD imaging array detection is realized by adopting the structure shown in Figure 2.

[0038] The probe 6 includes a housing 12, a first beam splitter 13, a second beam splitter 16, and a third beam splitter 21. The transceiver fiber cable 5 is connected to the first beam splitter 13 to divide the original transceiver fiber cable 5 into a transmitting fiber cable 14 and a receiving fiber cable 15. The transmitting fiber cable 14 is further divided into 8 single-mode transmitting fibers 17 by the second beam splitter 16 and is arranged in a 4x2 rectangular array by being fixed in a glass tube. A 4x2 microlens array 18 is fused at the front end of the single-mode transmitting fiber to achieve the collimation function of the light emitted from the single-mode fiber. The arrangement of the microlens array 18 is exactly the same as that of the single-mode transmitting fiber. The single-mode transmitting fiber and the microlens are connected by a coreless fiber, and the length of the coreless fiber is the focal length of the microlens. Through this structure, the light-emitting end of the single-mode transmitting fiber is on the focal plane of the lens.

[0039] At the receiving end of the detection light, there is also the same third beam splitter 21 to divide the receiving fiber cable into 8 multimode receiving fibers 20, which are fixed in the same 4x2 array by a glass tube. A 4x2 microlens array 18 is fused at the front end of the multimode receiving fiber 20 to achieve the focusing of the detection light during reception. Through the same coreless fiber structure as the transmitting end, the front end (light-receiving end) of the receiving fiber is located on the focal plane of the lens. The arrangements of these two microlens arrays 18 are exactly the same, but the end faces are perpendicular to each other. There is a half mirror 19 inside the probe with an optical axis angle of 45° with the output end of the transmitting fiber cable, which is used to change the optical path so that the detection light can smoothly enter the receiving array. The above single-mode fiber parameters are preferably (core diameter / cladding / coating layer, μm) 9 / 125 / 150, and the numerical aperture is 0.11; the multimode receiving fiber parameters are (core diameter / cladding / coating layer, μm) 50 / 125 / 150, and the numerical aperture NA is 0.22.

[0040] Preferably, during the measurement, the laser is emitted from the transmitting end, irradiates the to-be-measured blade 7 through the transmission surface of the half mirror 19, and then the reflected light from the end face of the to-be-measured blade 7 is reflected by the reflection surface of the half mirror and enters the receiving end, as Figure 3 shown.

[0041] Preferably, in this embodiment, by arranging measurement points in the circumferential and axial directions of the rotor, the bending vibration and torsional vibration signals are obtained through the differences in the timing pulses of different measurement points to achieve simultaneous measurement, as Figure 4 shown;

[0042] Preferably, the cable protective sleeve of the sensor in this embodiment can be selected from stainless steel hoses, metal hoses, plastic-coated armored pipes, plastic protection pipes, etc.; the inner diameter of the sensor cable protective sleeve should be larger than the outer diameter of the 8-fiber assembly. For example, for the transmitting cable composed of 4 fibers, a plastic-coated armored cable with an outer diameter of 2 mm can be selected, and for the receiving cable composed of 4 fibers, a plastic-coated armored cable with an outer diameter of 6 mm can be selected; the lengths of the transmitting cable, receiving cable, and transceiver cable should meet the actual usage requirements. For example, the transmitting length is selected to be 0.5 m, the receiving cable length is 0.5 m, and the fiber transceiver cable length is 3 m.

[0043] Preferably, for the single-mode transmitting fiber 2, multi-mode receiving fiber 9, the fiber in the single-mode transmitting fiber cable, and the fiber in the multi-mode receiving fiber cable in this embodiment, a fiber structure with a thinner cladding can be used, such as all-quartz fiber.

[0044] The present invention is not limited to the embodiments described above. The above description of the specific embodiments is intended to describe and illustrate the technical solutions of the present invention. The above specific embodiments are merely illustrative and not restrictive. Without departing from the spirit of the present invention and the scope protected by the claims, those of ordinary skill in the art can make many specific transformations in various forms under the inspiration of the present invention, and these all fall within the protection scope of the present invention.

Claims

1. A fiber optic tip-timing sensor based on an APD array imaging method, characterized in that, Including: N transmitting-end couplers (1), transmitting optical fibers (2), a transmitting optical fiber bundle cable (3), a transceiver beam combiner (4), a transceiver optical fiber cable (5), a probe (6), a receiving optical fiber bundle cable (11), a beam combiner (10), receiving optical fibers (9), and N receiving-end couplers (8); N≥5; The N transmitting-end couplers (1) are respectively connected to laser transmitters of different wavelengths and are respectively connected to the beam combiner (10) at the transmitting end through N transmitting optical fibers (2); the N receiving-end couplers (8) are respectively connected to the beam combiner (10) at the receiving end through N receiving optical fibers; The beam combiner (10) at the transmitting end and the beam combiner (10) at the receiving end are respectively connected to the input end of the transceiver beam combiner (4) through the transmitting optical fiber bundle cable (3) and the receiving optical fiber bundle cable (11); The output end of the transceiver beam combiner (4) is connected to the probe through the transceiver optical fiber cable (5), and a to-be-measured blade is arranged in front of the probe.

2. The fiber optic tip-timing sensor based on the APD array imaging method according to claim 1, wherein The transmitting optical fiber bundle cable (3) and the receiving optical fiber bundle cable (11) are coated with tensile and compressive resistant cable materials to ensure that the transmitting optical fiber bundle cable (3) and the receiving optical fiber bundle cable (11) have the capabilities of anti-tensile and anti-stomping.

3. The fiber optic tip-timing sensor based on the APD array imaging method according to claim 1, wherein Filter films corresponding to the laser wavelengths emitted by the corresponding transmitting ends are respectively installed on the N receiving-end couplers (8) to improve the anti-interference ability.

4. The fiber optic tip-timing sensor based on the APD array imaging method according to claim 1, characterized in that The transceiver optical fiber cable (5) contains N small numerical aperture single-mode transmitting optical fibers for transmitting transmitted optical signals and N large numerical aperture multimode receiving optical fibers for transmitting received optical signals, and the transceiver optical fiber cable (5) also selects tensile and compressive resistant cable materials.

5. The fiber optic tip-timing sensor based on the APD array imaging method according to claim 1, wherein The probe (6) is internally provided with a first beam splitter (13), a second beam splitter (16), a third beam splitter (21), and a half mirror (19). The first beam splitter (13) divides the transceiver optical fiber cable (5) into a transmitting optical fiber cable (14) and a receiving optical fiber cable (15). The transmitting optical fiber cable (14) is further divided into N single-mode transmitting optical fibers (17) by the second beam splitter (16). The N single-mode transmitting optical fibers (17) are arranged in a rectangular array by being fixed in a glass tube. A microlens array (18) with the same arrangement as the single-mode transmitting optical fibers is fused at the front ends of the N single-mode transmitting optical fibers (17) to realize the collimation function of the light emitted from the single-mode optical fibers; At the receiving end of the detection light, the receiving optical fiber cable (15) is divided into N multimode receiving optical fibers (20) by a third beam splitter (21), and is fixed into an identical array arrangement by a glass tube. A same microlens array (18) is fused at the front ends of the N multimode receiving optical fibers (20) to achieve focusing during the reception of the detection light; the arrangement modes of the two microlens arrays (18) are completely the same, but the end faces are perpendicular to each other; a half mirror (19) with an optical axis angle of 45° with the output end of the transmitting optical fiber cable (14) is arranged in front of each microlens array (18) for changing the optical path so that the detection light can smoothly enter the microlens array (18) at the front end of the multimode receiving optical fiber (20); during the measurement, the laser is emitted from the transmitting end, irradiates on the to-be-measured blade through the transmission surface of the half mirror (19), and then the reflected light from the end face of the to-be-measured blade is reflected by the reflection surface of the half mirror (19) and enters the receiving end.

6. The fiber optic tip-timing sensor based on the APD array imaging method according to claim 5, wherein The protection structures of the transmitting optical fiber cable (14), the receiving optical fiber cable (15) and the transmitting and receiving optical fiber cable (5) are selected as a metal armored sheath or a non-metallic sheath.

7. The fiber optic tip-timing sensor based on the APD array imaging method according to claim 1, characterized in that, The transmitting end coupler (1) and the receiving end coupler (8) adopt FC connectors.

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