FIBER BRAGG GRATING-BASED ADVANCED PNEUMATIC FIRE / OVERHEATING DETECTOR

Description

BACKGROUND

[0001] This disclosure relates generally to aircraft fire and overheat detection systems. More particularly, this disclosure relates to optical signals used in aircraft system overheat and fire detection systems.

[0002] During operation of an aircraft, numerous on-board components and sub-systems are continuously or periodically monitored. Various methods for monitoring these components and sub-systems of the aircraft have been used. For example, sensors and / or transducers can be affixed to an aircraft at specific locations so as to produce signals indicative of various physical phenomena experienced at those specific locations. These signals can then be transmitted to an analyzer that interprets the signals received by the analyzer. These signals can be processed to generate parametric data that can be correlated to measurements of physical phenomena. Some of the specific locations where it would be desirable to affix a sensor and / or transducer might be locations that have harsh environments. For example, some such locations might expose any affixed sensor to high temperatures, high pressures, high levels of exposure to electromagnetic interference, etc.

[0003] In particular, advance pneumatic detectors are used to detect fire / overheat events. An advance pneumatic detector (APD) is a tube sealed at one end with a movable diaphragm. The diaphragm forms one electrical contact side of an electrical alarm switch. When the gas inside the tube is heated it expands, pressing out the diaphragm, which in turn closes the alarm circuit, signaling a fire / overheat event to the crew. When the tube leaks gas, a second, similar switch opens, signaling a failure to the crew. Current mechanical systems with APDs can be complicated to manufacture and / or assemble, and the switch components can be contaminating during assembly, leading to potential latent failures in the field.

[0004] Optical transducers have found use in many of harsh environment locations. Optical sensors and / or transducers can produce optical signals indicative of various physical phenomena. For example, optical transducers can produce optical signals indicative of stress, strain, temperature, tilt, rotation, vibration, pressure, etc. Various sensors and / or transducers employ various types of technologies. For example, some sensors use Fabry-Pérot Interferometry (FPI), while others use fiber Bragg grating (FBG) technologies. Some of these technologies and techniques produce optical signals having a spectrum that is indicative of the measured parameter. Spectrum analysis and / or spectral measurement of such signals is performed to determine a measure of the physical phenomena causing the specific spectrum of the optical signal.

[0005] EP 3 096 117 A1 discloses a temperature sensor that has a pressure sensor, the distal end of which is inserted in a sealed chamber filled with a liquid. The pressure sensor has a light guide, a cavity at a distal end of the light guide, a diaphragm forming a wall of the cavity and being configured to deflect with applied pressure, and a detector to detect changes in light reflection due to deflection of the diaphragm. The liquid changes volume in response to temperature changes and this volume change is sufficient to change the pressure applied on the diaphragm.

[0006] US2014 / 266746A1 discloses a pneumatic sensing apparatus for use in an overheat or fire alarm system. The apparatus comprises sensing means containing a pressurized gas, coupled to an optical pressure sensor. Different types of optical pressure sensors may be used, such as intensity based, Fabry-Perot based and Fiber Bragg Grating based.SUMMARY

[0007] In a first aspect of the present invention, there is provided a system for detecting a fire or overheating event according to claim 1.

[0008] In a second aspect of the present invention, there is provided a method for detecting an overheat condition according to claim 12.BRIEF DESCRIPTION OF THE DRAWINGS

[0009] FIG. 1 is a perspective view of a representative fire / overheat detection system with a fiber Bragg grating (FBG) coupled to an advance pneumatic detector. FIG. 2 is a perspective view of an FBG. FIG. 3 is flow chart of a representative method for using a system described herein to detect a fire / overheat event. DETAILED DESCRIPTION

[0010] Engine fire / overheat events are critical failures in an aircraft and therefore are carefully monitored. Current systems employed include APDs connected to electrical alarm circuits. This physical connection makes the system expensive to manufacture, complicated to assemble, and susceptible to latent failures. A single FBG can be used to replace the electrical alarm switch and fault switch, making it more reliable, and less complicated to manufacture and assemble.

[0011] As described herein, an FBG is coupled to the metal diaphragm of a pneumatic tube that is filled with a pressurized gas. Light is transmitted through the FBG and a wavelength of light is reflected back to a light sensor. When a fire / overheat event occurs the gas expands, pressing out the diaphragm, which compresses the FBG, resulting a wavelength shift. Conversely, if a failure in the pneumatic tube occurs the pressure reduces, collapsing the diaphragm, which in turn stretches the FBG or releases compression, resulting in a different wavelength shift. Fire / overheat events and faults in the detector can be detected by determining the direction and magnitude of the shift in the wavelength of reflected light. The resulting sensor is more accurate, lighter, smaller, and less prone to latent failures.

[0012] FIG. 1 is a perspective view of a representative fire / overheat detection system 100 with a fiber Bragg grating (FBG) coupled to an APD. FIG. 1 shows fire / overheat detection system 100 having pneumatic tube 102, diaphragm 104, FBG 106, light source / detector 108, inert gas 110, and enclosure 112. Pneumatic tube 102 is filled with inert gas 110 under pressure. Pneumatic tube 102 is sealed at one end with diaphragm 104. FBG 106 is surrounded by enclosure 112 and attached to diaphragm 104 at one end and secured at the opposite end. Light source / detector 108 is configured to transmit light through FBG 106 and receive the reflected light.

[0013] Pneumatic tube 102 is a thin elongated tube which is hermetically sealed. The tube can be, for example, between 1 foot and 80 feet, between 3 feet and 60 feet, or between 5 feet and 40 feet. The circumference of the tube can be, for example, between 0.040 inches to 0.1 inches, between 0.05 inches and 0.08 inches, or between 0.06 and 0.07 inches. The tube can be made from, for example, stainless steel, nickel alloys, other metal alloys, fiberglass, ceramic, polymer, or a combination thereof. The pneumatic tube is filled with inert gas 110. The gas in the pneumatic tube is under pressure. The pressure can be, for example, between 15 PSI and 450 PSI, between 20 PSI and 425 PSI, or between 30 PSI and 400 PSI. The pneumatic tube in placed in an area where a fire / overheat event could occur, for example an engine.

[0014] Pneumatic tube 102 is sealed at one end with diaphragm 104. Diaphragm 104 is a thin membrane of a flexible material. The membrane can have, for example, a thickness of between 0.25 mm and 1.5 mm, between 0.30 mm and 1.25 mm, or between 0.5 mm and 1 mm. The flexible material is a material which can suitably move under the change in inert gas pressure enough to induce measurable stress on the FBG. When the temperature rises, for example in a fire / overheat event, gas 110 expands and the pressure increases. In these cases, diaphragm 104 can deform outward to create a measurable pressure on the FBG. When pneumatic tube 102 is damaged, the pressure of inert gas 110 will decrease due to leakage of the gas out of pneumatic tube 102, and as a result, diaphragm 104 can collapse, resulting in a detectable change of pressure on the FBG.

[0015] Diaphragm 104 is connected to FBG 106. FBG 106 is connected to diaphragm 104 and light source / detector 108 in such a way that a deformation of diaphragm 104 will change the amount of strain on FBG 106. FBG 106 can be fixedly connected to diaphragm 104 and / or light source / detector 108. FBG 106 can be held in place at diaphragm 104 and / or detector 108 by a pressure or friction fit, such that collapse of diaphragm decreases strain. As shown in FIG. 2, FBG 106 is a short segment of optical fiber that reflects particular wavelengths of light and transmits others. The optical fiber can be made of, for example, glass, polymer, or a combination thereof. Grating inclusions 202 in the refractive index of the fiber core of FBG 106 generates a wavelength-specific dielectric mirror. Variation in the refractive index can be accomplished through grating inclusions.. Distance 204 between grating inclusions 202 when FBG 106 is uninstalled and at standard temperature can be, for example, between 3 nm and 8 nm, between 4 nm and 7 nm, or between 5 nm and 6 nm. When FBG 106 is installed in the APD and the system is at standard temperature and pressure it has an initial strain. When the strain on FBG 106 increases or decreases relative to the initial strain due to deformation of diaphragm 104 the distance between variations 202 changes, correspondingly changing the wavelength reflected by variations 202. The total length of FBG 106 can be. between 1 mm and 7 mm, between 1.5 mm and 6 mm, or between 2 mm and 5 mm. FBG 106 is contained in enclosure 112. Enclosure 112 can be thermally and / or hermetically sealed.

[0016] Light is transmitted through FBG 106 by light source / detector 108. Light source / detector 108 can be one component designed to both transmit and detect wavelengths of light, or it can be multiple components which collectively transmit and detect wavelengths of light. The light source can be a continuous or intermittent light source. In some embodiments light source 108 is, for example, an optical pulse generator or broad spectrum laser. In some embodiments, light source 108 can transmit a continuous or pulsed electromagnetic spectrum in the UV range of, for example, between 5 nm and 400 nm, between 7.5 nm and 350 nm, or between 10 nm and 300 nm. In other embodiments, light source 108 can transmit discrete wavelengths of light. In some embodiments, light detector 108 is, for example, a photodetector. Incident light from light source 108 is transmitted through FBG 106. Light is reflected back to light detector 108. The wavelengths of light reflected are received and the information is transmitted to a processor for analysis and possible alarm.

[0017] FIG. 3 is flow chart of a representative method for using a system described herein to detect a fire / overheat event. FIG. 3 shows that a FBG is coupled to an advance pneumatic detector, as discussed above. Light is propagated through the coupled FBG. As the light travels through the FBG, it is selectively reflected back to the detector as discussed above. The light detector receives the reflected light. The light detector or an associated processor determines the wavelengths of light received. Wavelengths of reflected light corresponding to fire / overheat events, detector failure events, and standard operating conditions are predetermined. Predetermination can include experimental references and / or individual detector calibration (nominal condition). The reflected wavelength is compared to the predetermined wavelengths. If the reflect wavelength is within the allowed range corresponding to standard operating conditions then monitoring continues. If the reflected wavelength corresponds to a fire / overheat event or a detector failure event a notification is triggered. The notification can include the magnitude of the shift, the direction of the shift, which event the wavelength corresponds to, or a combination thereof.

[0018] While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.

Claims

1. A system (100) for detecting a fire or overheating event, the system (100) comprising: an elongated pneumatic tube (102) filled with an inert gas (110) under pressure; a diaphragm (104) hermetically sealing the pneumatic tube (102) on one end; a fiber Bragg grating (106), FBG, connected to the diaphragm (104) in such a way that a deformation of the diaphragm (104) will change the amount of strain on the FBG (106), wherein in response to temperature rises, the inert gas (110) expands and pressure within the elongated pneumatic tube increases thereby deforming the diaphragm (104) outward to create a measurable pressure on the FBG (106) and in response to damage of the pneumatic tube (102), the pressure of inert gas (110) decreases due to leakage of the gas out of pneumatic tube (102) thereby collapsing the diaphragm (104) creating a detectable change of pressure on the FBG (106); a light source (108) configured to transmit light through the FBG (106); a photodetector (108) configured to receive a reflected optical signal reflected from the fiber Bragg grating (106); and a processing unit configured to determine wavelengths of the reflected optical signal, and, based on the wavelengths determined, whether a fire / overheat event has occurred, the system is operating under standard conditions, or if damage has occurred to the system.

2. The system (100) of claim 1, wherein the diaphragm (104) has an initial position and the initial position corresponds to a known safe operating temperature.

3. The system (100) of claim 2, wherein, at the known safe operating temperature, the pressure of the inert gas (110) inside the sealed chamber (102) is greater than a pressure outside of the sealed chamber (102).

4. The system (100) of any one of claims 1-3, wherein the diaphragm (104) is configured to shift to a displaced position when the pressure in the sealed chamber (102) changes, and the change from the initial position to the displaced position results in a shift in the reflected wavelength.

5. The system (100) of claim 4, wherein the processing unit is further configured to determine an extent of an overheat condition based upon wavelength shift information compared to the reflected wavelength with the diaphragm at the initial position.

6. The system (100) of any one of claims 1-5, wherein a triggering threshold is selected for a specific system and is a function of the reflected wavelength with the diaphragm at the initial position.

7. The system (100) of any preceding claim, further comprising an enclosure (112) which is hermetically-sealed and thermally insulated, wherein the FBG (106) is encapsulated within the enclosure (112).

8. The system (100) of any preceding claim, wherein the FBG (106) has a length between 1 mm and 7 mm.

9. The system (100) of any preceding claim, wherein the FBG (106) has a plurality of grating inclusions (202) with a distance (204) between each of the grating inclusions (202); optionally wherein the distance (204) between each of the grating inclusions (202) is between 3 nm and 8 nm.

10. The system (100) of any preceding claim, wherein the light source (108) is a laser and / or wherein the light source (108) emits an initial spectrum and the initial spectrum is a continuous electromagnetic spectrum between 5 nm and 400 nm.

11. The system (100) of any preceding claim, wherein the reflected optical signal is indicative of the pressure exerted on the fiber Bragg grating (106) by the diaphragm (104).

12. A method for detecting a fire or overheating event, the method comprising: filling an elongated pneumatic tube (102) with an inert gas (110) under pressure; hermetically sealing the pneumatic tube (102) on one end via a diaphragm (104); connecting a fiber Bragg grating FBG (106) to the diaphragm (104) in such a way that a deformation of the diaphragm (104) will change the amount of strain on the FBG (106), wherein in response to temperature rises, the inert gas (110) expands and pressure within the elongated pneumatic tube increases thereby deforming the diaphragm (104) outward to create a measurable pressure on the FBG (106) and in response to damage of the pneumatic tube (102), the pressure of inert gas (110) decreases due to leakage of the gas out of pneumatic tube (102) thereby collapsing the diaphragm (104) creating a detectable change of pressure on the FBG (106); emitting, via a light source (108), an optical signal through the FBG (106) ; a photodetector (108) configured to receive a reflected optical signal reflected from the FBG (106); and determining, via a processing unit, wavelengths of the reflected optical signal, and, based on the wavelengths determined, whether a fire / overheat event has occurred, the system is operating under standard conditions, or if damage has occurred to the system.