Patent Description:
Exemplary embodiments pertain to the art of fire suppression and, more particularly, to a fiber Bragg grating-based pressure sensor for a pressure bottle.

A pressure bottle stores substances at high pressures. In an aircraft fire protection system, for example, a pressure bottle stores fire suppressants such as liquified halon and nitrogen gas at pressures that are about <NUM> to <NUM> pounds per square inch (PSI). Monitoring the pressure of the pressure bottle is performed to obtain information such as whether discharge or leakage of the contents of the pressure bottle has occurred. <CIT> describes various sensor designs, <CIT> relates to a fire extinguisher, <CIT> shows fiber Bragg grating sensors, <CIT> describes fiber optic transducers for use in a well, <CIT> relates to a usage monitoring system of a gas tank, and <CIT> shows an optical sensor based on an FDML wavelength swept laser.

The present invention provides a system as claimed in claim <NUM>.

The two or more FBGs may include a same periodic variation in refractive index.

Additionally or alternatively, in this or other embodiments, one of the two or more FBGs is affixed to a center of the diaphragm seal.

Additionally or alternatively, in this or other embodiments, another of the two or more FBGs is affixed at a radial distance r from the center of the diaphragm seal that has a radius R, and the radial distance r is less than <MAT>.

Additionally or alternatively, in this or other embodiments, the two or more FBGs are in different locations of one optical fiber.

Additionally or alternatively, in this or other embodiments, the two or more FBGs are in two or more optical fibers.

Additionally or alternatively, in this or other embodiments, the light source is a laser to output white light as the incident light to one or more optical fibers that include the two or more FBGs.

Additionally or alternatively, in this or other embodiments, the processing circuitry determines a baseline wavelength for each of the two or more FBGs based on the reflected light resulting from each of the two or more FBGs without any strain on the diaphragm seal.

Additionally or alternatively, in this or other embodiments, the baseline wavelength for each of the two or more FBGs is a same wavelength.

Additionally or alternatively, in this or other embodiments, the processing circuitry determines a shift from the baseline wavelength for each of the two or more FBGs based on the reflected light resulting from each of the two or more FBGs when the diaphragm seal is under strain, and determines a pressure change in the pressure bottle based on a difference in the shift for each of the two or more FBGs.

The present invention also provides a method of assembling a pressure sensor to sense pressure in a pressure bottle as claimed in claim <NUM>.

The affixing the two or more FBGs may include adhesive bonding the two or more FBGs.

Additionally or alternatively, in this or other embodiments, the affixing the two or more FBGs includes affixing two or more FBGs with a same periodic variation in refractive index.

Additionally or alternatively, in this or other embodiments, the affixing the two or more FBGs includes affixing one of the two or more FBGs to a center of the diaphragm seal.

Additionally or alternatively, in this or other embodiments, the affixing the two or more FBGs includes affixing another of the two or more FBGs at a radial distance r from the center of the diaphragm seal that has a radius R, the radial distance r being less than <MAT>.

Additionally or alternatively, in this or other embodiments, the affixing the two or more FBGs includes affixing one optical fiber with the two or more FBGs at different locations of the one optical fiber.

Additionally or alternatively, in this or other embodiments, the affixing the two or more FBGs includes affixing two or more optical fibers with each of the two or more FBGs in each of the two or more optical fibers.

Additionally or alternatively, in this or other embodiments, the method also includes configuring the light source to output white light as the incident light to one or more optical fibers that include the two or more FBGs.

Additionally or alternatively, in this or other embodiments, the configuring the processing circuitry includes configuring the processing circuitry to determine a baseline wavelength for each of the two or more FBGs based on the reflected light resulting from each of the two or more FBGs without any strain on the diaphragm seal.

Additionally or alternatively, in this or other embodiments, the configuring the processing circuitry includes configuring the processing circuitry to determine a shift from the baseline wavelength for each of the two or more FBGs based on the reflected light resulting from each of the two or more FBGs when the diaphragm seal is under strain, and to determine a pressure change in the pressure bottle based on a difference in the shift for each of the two or more FBGs.

As previously noted, the pressure of a pressure bottle is monitored. In an aircraft fire suppression application, for example, this monitoring ensures that proper pressure is maintained during storage and confirms discharge or indicates leakage. Embodiments of the systems and methods detailed herein relate to a fiber Bragg grating-based pressure sensor for a pressure bottle. As detailed, two or more fiber Bragg gratings (FBGs) may be attached to different areas of the diaphragm at the opening of the pressure bottle. The FBGs may be positioned to measure strain distribution on the diaphragm. Prior pressure sensors involve a piezo-resistive strain gauge coupled with a temperature sensor to result in a temperature-compensated pressure transducer (TCPT) with a compensated temperature range of -<NUM> degrees Celsius to <NUM> degrees Celsius. The strain gauge is adhered to a diaphragm seal covering an opening of the pressure bottle. The pressure in the pressure bottle pushes the diaphragm against the opening such that a measurement of the strain on the diaphragm indicates the pressure in the pressure bottle. Because this strain measurement is affected by temperature, the separate temperature sensor is needed.

On the other hand, the FBG-based pressure sensor has built-in temperature compensation such that it does not require an additional temperature sensor. The temperature effect on each of the FBGs is the same. Thus, determining pressure by combining a strain measurement of each of the FBGs arranged on the diaphragm makes the pressure determination temperature-independent. As such, an external temperature measurement is not required. Additionally, the FBG-based pressure sensor for a pressure bottle, according to one or more embodiments, is immune to electromagnetic interference, moisture ingress and gasses, lightning, and fatigue.

<FIG> shows an exemplary pressure bottle <NUM> with an FBG-based pressure sensor <NUM> according to one or more embodiments. The fire suppression system of an aircraft may include two or more pressure bottles <NUM> that discharge successively in high-rate discharge and low-rate discharge phases. The pressure bottles <NUM> may be interconnected according to exemplary arrangements. Those known configurations and arrangements are not detailed herein. Generally, each pressure bottle <NUM> includes a relief valve <NUM> and one more discharge assemblies 130a, 130b (generally referred to as <NUM>). Each discharge assembly <NUM> may include an explosive cartridge or squib that is triggered (e.g., by a fire detection system) to initiate discharge of material in the interior volume <NUM> via a discharge port of the discharge assembly <NUM>. The pressure sensor <NUM> is further detailed with reference to <FIG>. As discussed with reference to <FIG>, a circular diaphragm <NUM> at an opening in the pressure bottle <NUM> represents an interface between the interior volume <NUM> and the pressure sensor <NUM>.

<FIG> is a block diagram of aspects of an FBG-based pressure sensor <NUM> according to an exemplary embodiment. An optical fiber <NUM> is shown. A light source <NUM> transmits incident light <NUM> into the optical fiber <NUM>. The light source <NUM> may be a continuous or pulsed laser that transmits white light (i.e., light including many wavelengths), for example. The incident light <NUM> is reflected back as reflected light <NUM>, which is shown as having components of reflected light 245a and 245b. A circulator <NUM> may be used to direct incident light <NUM> from the light source <NUM> into the optical fiber <NUM> and to direct reflected light <NUM> from the optical fiber <NUM> into the photodetector <NUM>. The photodetector <NUM> detects characteristics (i.e., intensity and wavelength) of this reflected light <NUM>. Processing circuitry <NUM> processes the detections of the photodetector <NUM> to determine temperature-compensated pressure in the pressurized bottle <NUM>, as discussed with reference to <FIG>. The processing circuitry <NUM> may include memory and one or more processors.

The optical fiber <NUM> includes a core <NUM> made of glass, for example. The optical fiber <NUM> also includes a protective outer coating <NUM> with a cladding <NUM> concentrically between the core <NUM> and the outer coating <NUM>. The refractive index of the cladding <NUM> is lower than the refractive index of the core <NUM>. This causes internal refraction of incident light <NUM> at the border of the core <NUM> and cladding <NUM> and prevents loss of incident light <NUM> at the sides of the optical fiber <NUM>. Instead, incident light <NUM> transmitted into the optical fiber <NUM> is returned as reflected light <NUM>.

Two FBGs 220a, 220b (referred to generally as <NUM>) are shown along the optical fiber <NUM>. Each FBG <NUM> is an area of the core <NUM> in which the refractive index has been altered. In the exemplary case shown in <FIG>, the same periodic variation in the refractive index is written into the core <NUM> as each FBG <NUM>. When incident light <NUM> with a range of wavelengths encounters an FBG <NUM>, only a specific wavelength is reflected. This specific wavelength is referred to as the Bragg wavelength for the FBG <NUM> and is dictated by the refractive index variation of the FBG <NUM>. All other wavelengths are transmitted.

In the exemplary case of FBG 220a and FBG 220b being the same, the wavelength reflected by each FBG <NUM> (i.e., the Bragg wavelength) is the same. That is, the first FBG 220a that is encountered by the incident light <NUM> reflects the Bragg wavelength in reflected light 245a and transmits the rest through to the second FBG 220b. Any light at the Bragg wavelength remaining in the transmitted incident light <NUM> is reflected by the second FBG 220b as reflected light 245b. The processing circuitry <NUM> can distinguish the reflected light 245a, 245b associated with each FBG 220a, 220b based on timing. That is, reflected light 245a resulting from FBG 220a will be received at the photodetector <NUM> before reflected light 245b resulting from FBG 220b, because the FBG 220a will be encountered first. When the Bragg wavelength in the reflected light 245a and the reflected light 245b is the same because the FBGs 220a and 220b are the same, the difference in the wavelength of the reflected light 245a and 245b is <NUM>.

While the FBGs 220a, 220b are shown as being part of the same optical fiber <NUM>, they may be part of two different optical fibers <NUM>, as shown in <FIG>. In that case, a splitter <NUM> may be used at the output of the light source <NUM> to input incident light <NUM> into both optical fibers <NUM>. In addition, a multiplexer may be used to input reflected light <NUM> from both optical fibers <NUM> into the circulator <NUM> to be provided to the photodetector <NUM>. Further, while two FBGs <NUM> are discussed for explanatory purposes, any number of FBGs <NUM> may be used.

When the FBGs <NUM> remain unchanged, the Bragg wavelength that is reflected by each also remains unchanged. However, if the optical fiber <NUM> is stretched or compressed (i.e., strained) at an area corresponding with one or both of the FBGs <NUM>, the refractive index variation of the corresponding FBG <NUM> is changed, and, consequently, the corresponding reflected Bragg wavelength is changed (i.e., shifted). The shift in the Bragg wavelength of one or both of the reflected signals 245a, 245b indicates strain. Thus, by attaching the FBGs <NUM> to a diaphragm <NUM> within the pressure bottle <NUM>, this strain detection for the diaphragm <NUM> is used to sense the pressure in the pressure bottle <NUM>, as detailed with reference to <FIG>.

<FIG> shows a diaphragm <NUM> used by the FBG-based pressure sensor <NUM> according to one or more embodiments. As noted with reference to <FIG>, the diaphragm <NUM> is at an opening in the pressure bottle <NUM>. When the pressure bottle <NUM> is filled with pressurized material, the pressure in the interior volume <NUM> of the pressure bottle <NUM> pushes the diaphragm <NUM> against the opening and maintains the seal on the pressure bottle <NUM>. In <FIG>, the diaphragm <NUM> is shown flat (i.e., no pressure pushing on the diaphragm <NUM>) with two FBGs <NUM> affixed to two different radial locations on the diaphragm <NUM>. Each FBG <NUM> is affixed to the diaphragm <NUM> by adhesive bonding, for example. In the exemplary case shown in <FIG>, the FBGs <NUM> are associated with different optical fibers <NUM> such that one FBG 220a is part of one optical fiber 210a, and the other FBG 220b is part of another optical fiber 210b. The splitter <NUM> that would split the incident light <NUM> from the light source <NUM> is indicated, but the other components are not shown again in <FIG>. The positioning of the FBGs <NUM> on the diaphragm <NUM> is further discussed with reference to <FIG>, and the determination of pressure change in the pressure bottle <NUM> using the FBGs <NUM> is discussed with reference to <FIG>.

<FIG> illustrates strain distribution on the diaphragm <NUM> under different levels of strain ε. Curve 410a corresponds to tangential strain in the circular diaphragm <NUM>, while curve 410b is the radial strain in the diaphragm <NUM>. The FBGs 220a and 220b are both radially positioned, as indicated. Thus, the FBGs 220a, 220b only undergo radial straining. Radial straining is tensile and maximum at the center and reduces gradually as radius increases, becoming compressive beyond a particular radius. The diaphragm <NUM> undergoes tension up to a radial distance <MAT>, where R is the radius of the diaphragm <NUM>. By attaching the FBG 220a in the center (at r = <NUM>) and the FBG 220b at a radius <MAT>, the sensitivity of the FBG-based pressure sensor is improved because both FBGs <NUM> will undergo tension but of different magnitudes.

<FIG> illustrates exemplary reflected signals <NUM> used by the FBG-based pressure sensor <NUM> according to one or more embodiments. The initial reflected signals 245a, 245b that are received when the diaphragm <NUM> is flat are shown. As indicated and previously discussed, the reflected signals 245a, 245b are the same because the FBGs 220a, 220b are the same and, thus, their Bragg wavelengths are the same. When, for example, pressurized material is put in the interior volume <NUM> of the pressure bottle <NUM> and that pressure is exerted on the diaphragm <NUM>, the diaphragm <NUM> will stretch. In this position of the diaphragm <NUM>, reflected signals 245a' and 245b' are received.

Deformation of the diaphragm <NUM> will result in changes in the FBGs <NUM>. The changes in the FBGs <NUM> will result in differences in the reflected signals 245a', 245b' received in the deformed state of the diaphragm <NUM> as compared to the reflected signals 245a, 245b received in the undeformed state of the diaphragm <NUM>. The change in one FBG 220a, which is shown at the center of the diaphragm <NUM> in <FIG>, is more extensive than the change in the other FBG 220b, which is shown radially off-center on the diaphragm <NUM> in <FIG>. As a result, the shift Δλ<NUM> in the Bragg wavelength of the reflected signal 245a' as compared with reflected signal 245a (i.e., the reflected signal <NUM> corresponding with the FBG 220a) is higher than the shift Δλ<NUM> in the Bragg wavelength of the reflected signal 245b' as compared with reflected signal 245b (i.e., the reflected signal <NUM> corresponding with the FBG 220b), as shown.

On the right, pressure in mega Pascals (MPa) on the diaphragm <NUM> is shown along the x axis and wavelength shift in picometers (pm) is shown along the y axis. The shift Δλ<NUM> in the Bragg wavelength of the reflected signal 245a' as compared with reflected signal 245a is shown to increase as pressure increases. Similarly, the shift Δλ<NUM> in the Bragg wavelength of the reflected signal 245b' as compared with reflected signal 245b is shown to increase as pressure increases. The difference between these shifts (i.e., Δλ<NUM> - Δλ<NUM>) indicates the change in pressure ΔP on the diaphragm <NUM>, which indicates the change in pressure in the interior volume <NUM> of the pressure bottle <NUM>. More specifically: <MAT> The Bragg wavelength of the reflected signals 245a, 245b (i.e., in the undeformed state of the diaphragm <NUM>) is λ, pe is the effective photo-elastic coefficient, µ is the Poisson's ratio of the material of the diaphragm <NUM>, r is the radial distance to the FBG 220b, E is the Young's modules of the material of the diaphragm <NUM>, and h is the thickness of the diaphragm <NUM>.

As previously noted, temperature changes affect both FBGs 220a, 220b in the same way. Thus, because the strain on the diaphragm <NUM> that is sensed by the FBGs <NUM> is combined to sense a pressure change on the diaphragm <NUM>, the temperature effect cancels out or, put another way, this FBG-based pressure sensing is temperature-independent. As discussed with reference to <FIG>, positioning one FBG 220a at the center of the diaphragm <NUM> and another FBGs 220b at a radial distance of <MAT> on the diaphragm <NUM> improves the sensitivity of the FBG-based pressure sensor <NUM>. The differential wavelength (i.e., Δλ<NUM> - Δλ<NUM>) may be less than about <NUM> pm per kilo Pascal of pressure difference, for example.

Once the pressure bottle <NUM> is filled with pressurized material (e.g., fire suppressants such as liquified halon and nitrogen gas), there will be a strain exerted on the diaphragm <NUM> such that the pressurized state of the pressure bottle <NUM> will result in a shift Δλ<NUM>, Δλ<NUM> in the Bragg wavelength of each of the FBGs 220a, 220b. A decrease in pressure in the interior volume <NUM> of the pressure bottle <NUM> due to a leak or discharge will result in the shifts Δλ<NUM>, Δλ<NUM> decreasing and approaching <NUM>, as the diaphragm <NUM> approaches a flat, unstrained state.

Claim 1:
A system comprising a pressure bottle (<NUM>) and a pressure sensor (<NUM>) to sense pressure in the pressure bottle, the sensor comprising:
two or more fiber Bragg gratings (FBGs) (220a, 220b), each affixed to a different radial location of a diaphragm seal (<NUM>) of the pressure bottle;
a light source (<NUM>) configured to provide incident light (<NUM>) to the two or more FBGs;
a photodetector (<NUM>) configured to detect reflected light (<NUM>, 245a, 245b) resulting from the two or more FBGs; and
processing circuitry (<NUM>) configured to determine a pressure change in the pressure bottle based on the reflected light resulting from each of the two or more FBGs; and the system characterised in that
the two or more FBGs are affixed such that a wavelength of the reflected light resulting from the two or more FBGs is shifted in a same direction relative to a baseline wavelength in response to the pressure change.