Abstract:
A method is presented for detecting an alarm condition with an optical fiber sensing system. An interrogator with a light source, a spectrometer, and a data processor is used to conduct a fast scan of a plurality of fiber optic sensing elements. First environmental parameter values are calculated for each fiber optic sensing element from spectrographic data collected by the interrogator during the first scan, and compared with a first threshold value. If the first environmental parameter value exceeds the first threshold value for any fiber optic sensing element, the fast scan is interrupted to perform a high resolution slow scan of that fiber optic sensing element. The optical fiber sensing system reports an alert if this high resolution slow scan indicates the alarm condition.

Description:
BACKGROUND 
       [0001]    The present invention relates generally to an optical fiber sensing system, and more particularly to a zoned fire and overheat detection system using optical fiber sensing elements. 
         [0002]    Fiber optic sensors are currently used to measure a wide range of parameters in distributed systems ranging from construction sites to aircraft wings. Some such sensors include pressure, strain, and temperature sensors, but fiber optics may generally be used to measure many quantities that can be tied to a physical state of a fiber optic sensing element. Some fiber optic temperature sensors, for instance, operate by detecting thermal expansion of a fiber optic strand, or of a surrounding sheath around or gap between strand segments, with an interferometer. Others sensors detect changes in parameters such as temperature and pressure from Raman backscatter. A data processor correlates interferometer readings to changes in the physical state of the fiber optic sensing element. 
         [0003]    Most fiber optic temperature sensors comprise a fiber optic sensing element and an interrogator with a light source, a spectrometer, and a data processor. The sensing element consists of a fiber optic strand that extends from the interrogator into a sensing region. During operation, the interrogator emits light down the fiber optic sensing element. Changes in temperature alter the physical state of the sensing element, and thus its optical characteristics. The spectrometer and data processor assess these differences to identify changes in temperature. 
         [0004]    Modern temperature sensors utilize a wide range of spectroscopy and interferometry techniques. These techniques generally fall into two categories: point and quasi distributed sensing based on Fiber Bragg Gratings (FBGs), and fully distributed sensors based on Raman, Brillouin, or Rayleigh scattering. The particular construction of fiber optic sensing elements varies depending on the type of spectroscopy used by the sensor system, but all fiber optic sensors operate by sensing changes in the physical state of the fiber optic sensing element. FBG sensors, for instance, determine a change in temperature (ΔT) by sensing a relative shift in Bragg wavelength (λ B /λ B ): 
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         [0000]    where p c  is the strain optic coefficient, ε is the applied strain, α Λ  is the thermal expansion coefficient of the optical fiber, and α n  is its thermo-optic coefficient. 
         [0005]    Fiber optic sensing elements are inexpensive, durable, and easily installed relative to conventional electrical temperature sensors. The most expensive element of most fiber optic temperature sensors, therefore, is the interrogator. To reduce costs, some sensor systems attach a plurality of sensing elements to each interrogator via a switch which periodically cycles through each sensing element, allowing a single interrogator to service many separate sensing elements, which may be situated in a number of different detection areas. 
         [0006]    Switching fiber optic sensor systems are not without drawbacks. Rapid switching necessitates high interrogator scan rates that limit the spatial and/or temperature resolution achievable by the system. Conversely, systems that switch only slowly between sensing elements visit each sensing element infrequently, and may allow dangerous heat conditions to go unnoticed for tens of seconds which may be critical to fire and heat control. 
       SUMMARY 
       [0007]    The present invention is directed toward a system and method for detecting an alarm condition with an optical fiber sensing system. An interrogator with a light source, a spectrometer, and a data processor is used to conduct a fast scan of a plurality of fiber optic sensing elements. First environmental parameter values are calculated for each fiber optic sensing element from spectrographic data collected by the interrogator during the first scan, and compared with a first threshold value. If the first environmental parameter value exceeds the first threshold value for any fiber optic sensing element, the fast scan is interrupted to perform a high resolution slow scan of that fiber optic sensing element. The optical fiber sensing system reports an alert if this high resolution slow scan indicates the alarm condition. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a schematic block diagram of an optical fiber overheat sensing system according to the present invention. 
           [0009]      FIG. 2  is a flowchart describing a scanning method used by the optical fiber overheat sensing system of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0010]      FIG. 1  is a schematic block diagram of optical fiber sensing system  10 , comprising interrogator  12 , optical switch  14 , and sensing elements  16   a,    16   b,  and  16 N. Interrogator  12  further comprises broadband light source  18 , high speed spectrometer  20 , and data processor  22 . Optical fiber sensing system  10  may be used to sense fires or overheat conditions in a wide range of applications, including on aircraft and other vehicles. Although optical fiber sensing system  10  is described herein as a temperature sensing system, optical fiber sensing system  10  may be used to monitor other parameters such as strain or pressure in other embodiments of the present invention. 
         [0011]    Sensing elements  16   a,    16   b,  . . . ,  16 N are optical sensing elements that extend from optical switch  14  to sensing locations within zones Z 1 , Z 2 , . . . , ZM. For purposes of explanation, sensing elements  16   a,    16   b,  . . . ,  16 N will be described hereinafter as FBG elements, although other types of sensing elements may equivalently be used. Similarly, although sensing elements  16   a,    16   b,  . . . ,  16 N are depicted as single fiber optic strands connected to optical switch  14  at only one end (i.e. to measure refracted light), other embodiments may comprise multiple fiber optic strands for comparative interferometry, or may be connected to optical switch  14  at both ends in a closed loop (i.e. to measure transmitted light). In the FBG system shown, each sensing element  16   a,    16   b,  . . . ,  16 N has a plurality of closely spaced FBGs, each with a single characteristic Bragg wavelength λ 1 , λ 2 , . . . , λ M  which can be used to distinguish between signals from each zone, as explained in further detail below. 
         [0012]    Interrogator  12  is an FBG interrogator comprising broadband light source  18 , high speed spectrometer  20 , and data processor  22 . Broadband light source  18  may, for instance, be a Superluminescent Light Emitting Diode (SLED) source capable of producing light at several wavelengths. High speed spectrometer  20  is a spectrometer capable of rapidly assessing relative shift in Bragg wavelength (Δλ B /λ B ). The particular speed requirements of high speed spectrometer  20  will depend on the number of optical sensing elements  16   a,    16   b,  . . . ,  16 N, and on the sampling speed requirements of optical fiber sensing system  10 , which may in turn be determined by safety or fire-suppression requirements of the monitored regions or systems. Data processor  22  is a microprocessor or other logic-capable device configured to calculate temperature changes (ΔT) from relative shifts in Bragg wavelength (Δλ B /λ B ), and further configured to run scanning method  100  (described below with respect to  FIG. 2 .). Data processor  22  may be a programmable logic device such as a multi-function computer, or a fixed-function processor. 
         [0013]    Optical switch  14  is a 1×N optical switch capable of sequentially connecting high-speed spectrometer  20  to each of sensing elements  16   a,    16   b,  . . . ,  16 N. More particularly, optical switch  14  is an optical switch capable of sequentially switching between sensing elements  16   a ,  16   b,  . . . ,  16 N at varying rates dictated by data processor  22 . Although optical switch  14  is depicted as a separate schematic block from interrogator  12 , both interrogator  12  and optical switch  14  may in some embodiments be housed in a common enclosure or situated on a shared circuit board. 
         [0014]    In the depicted embodiment, each sensing element  16   a,    16   b,  . . . ,  16 N is configured to sense temperature changes in M distinct zones. As stated above, each sensing element  16   a,    16   b,  . . . ,  16 N is outfitted with FBG having a distinct Bragg wavelength λ B  in each zone Z 1 , Z 2 , ZM, thereby allowing high speed spectrometer  20  and data processor  22  to distinguish between temperature changes in each zone. According to this embodiment, high speed spectrometer  20  identifies M distinct Bragg wavelengths from each sensing element  16   a ,  16   b,  . . . ,  16 N, corresponding to zones Z 1 , Z 2 , ZM, and analyzes the relative shift in each (e.g. Δλ 1 /λ 1 , Δ 2 /λ 2 , . . . Δλ M /λ M ). Processor  22  may alternatively or additionally differentiate between each zone Z 1 , Z 2 , ZM based on time-of-flight from each zone to interrogator  12 . Some embodiments of the present invention may sense only one temperature (i.e. only one zone) per sensing element  16   a,    16   b,  . . . ,  16 N. 
         [0015]    Optical fiber sensing system  10  scans the plurality of sensing elements  16   a,    16   b , . . . ,  16 N, each of which may service a plurality of zones Z 1 , Z 2 , ZM. Spectrometer  20  and data processor  22  can scan sensing elements  16   a,    16   b,  . . . ,  16 N at variable rates, as described below with respect to  FIG. 2 . For each scanned sensing element  16   a,    16   b,  . . . ,  16 N, and for each scanned zone Z 1 , Z 2 , ZM, data processor  22  determines a deviation in temperature ΔT according to Equation 1. ΔT represents a change in temperature from a known baseline temperature T baseline , such that a current temperature T=T baseline +ΔT. 
         [0016]      FIG. 2  depicts scanning method  100 , a method whereby data processor  22  controls optical switch  14  to scan sensing elements  16   a,    16   b,  . . . ,  16 N at variable rates. In many fire and overheat detection systems, impermissible delays in overheat or fire detection can result in dangerous conditions developing before fire suppression or extinguishing apparatus can be deployed. It is therefore essential that such systems be capable of a high interrogator scan rate, so as to minimize the time delay between subsequent checks of each sensing element  16   a,    16   b , . . . ,  16 N. It is well known in the art, however, that spatial and temperature resolution are inversely related to interrogator scan rate in distributed optical fiber sensing systems. Although a high (fast) scan rate is necessary to ensure that all monitored components are interrogated frequently, the greater resolution provided by slower scanning rates may be needed to identify and localize a fire or overheat condition. Scanning method  100  allows optical fiber sensing system  10  to provide high spatial and temperature resolution when necessary, while maintaining a high normal scan rate, as described below. 
         [0017]    Data processor  22  begins each scan of sensing elements  16   a,    16   b,  . . . ,  16 N by initializing an element number n=1 (Step S 1 ) corresponding to sensing element  16   a,  and commanding high speed spectrometer  20  to perform a fast scan of corresponding sensing element  16   a  (step S 2 ), e.g. by sending a light pulse from interrogator  12  through optical switch  14  into sensing element  16   a  and back at 5 Hz. Data processor  22  assesses temperature changes, and the position along sensing element  16   a  of any temperature changes, according to Equation 1, above. This fast scan may be too brief to provide high position or temperature accuracy, but provides a ballpark temperature value T. 
         [0018]    Data processor  22  next compares sensed temperature T with a predetermined threshold value T max  corresponding to a possible overheat condition (step S 3 ). In some embodiments, data processor  22  may also determine a change in sensed temperature since a last measurement from sensing element  16   a  (i.e. ΔT/Δt=(T−T previous )/&lt;timestep&gt;), and compare this change in sensed temperature to a second threshold value ΔT max . (Step S 4 ). If either quantity exceeds the corresponding threshold value, data processor  22  initiates a slow scan of sensing element  16   a,  as described in greater detail below with respect to step S 8 . Otherwise, data processor  22  increments n, commands optical switch  14  to switch to the next sensing element, and repeats the process described above for sensing elements  16   b  through  16 N, until n=N (steps S 5  and S 6 ). Upon performing fast scans of all sensing elements  16   a,    16   b,  . . . ,  16 N (corresponding to n=1 through n=N), data processor  22  reinitializes n=1 and repeats the entire method  100  from the beginning (step S 7 ). By testing each sensing element  16   a,    16   b,  . . . ,  16 N using fasts scans, optical fiber sensing system  10  is able to provide at least a rough determination of temperature across all N sensing elements and M zones on a short timescale, e.g. 5 seconds or less. 
         [0019]    Threshold values T max  and ΔT max  are selected to trigger a slow scan whenever overheat conditions might have occurred, based on the limited accuracy measurements made during the fast scan of Step S 2 . Not every occurrence of T or ΔT/Δt exceeding the corresponding threshold value will indicate an overheat or fire event. If and when comparison of sensed temperature T and/or sensed change in temperature ΔT/Δt exceeds a corresponding threshold value for any sensing element  16   a,    16   b,  . . . ,  16 N (see steps S 3  and S 4 ), data processor  22  interrupts scanning of sensing elements  16   a,    16   b,  . . . ,  16 N to command high speed spectrometer  20  to begin a slow scan of the corresponding sensing element  16   a,    16   b,  . . . ,  16 N (step S 8 ). This slow scan may take several seconds, and may involve considerably higher pulse frequency (e.g. 1000 Hz) than the fast scan of step S 2 , consuming both greater time and greater energy. The slow scan of step S 8  allows data processor  22  to determine temperature T (and/or change in temperature ΔT/Δt) with much greater accuracy than the fast scan of step S 2 . In addition, the slow scan of step S 8  allows for greater time-of-flight resolution of overheat or fire positions along the particular sensing element  16   a,    16   b,  . . . ,  16 N. Data processor  22  may evaluate several parameters, including temperature T and change in temperature ΔT/Δt as compared with expected values, to determine whether an overheat or fire condition has occurred (step S 9 ), and accordingly report an overheat or fire alert, as necessary, to appropriate fire suppression or alarm system (step S 10 ). 
         [0020]    Method  100  enables optical fiber sensing system  10  to dynamically switch between fast and slow scanning rates, thereby retaining high scanning speeds during normal operation while allowing for precise temperature and position measurement of overheat or fire events. The fast scan of step S 2  provides a low resolution temperature measurement that provides information applicable to general condition monitoring. Data processor  22  may be capable of identifying fire/overheat events over a certain magnitude based on this fast scan, but may be unable to accurately identify all overheat/fire conditions. The fast scan rate information will, however, provide an indication of the potential occurrence of all overheat/fire conditions. This may be seen as an increase in absolute temperature, or as an anomalous sharp increase in the rate of rise of temperature within the affected sensing element. When a potential fire or overheat condition is identified in any particular element the scan rate of the interrogator will be reduced and the optical switch configured to individually address this element (step S 8 ). The information received for the slower scan rate is then used to determine whether a genuine overheat or fire alarm condition exists. After this step is performed, data processor  22  again increases the scan rate and resumes sequentially monitoring of all elements. 
         [0021]    In some instances each sensing element  16   a,    16   b,  . . . ,  16 N may be subjected to slow scans (as described above with respect to step S 8 ) on a periodic basis, in addition to any slow scans triggered by the threshold tests of steps S 3  and S 4 . These periodic slow scans provide accurate assessments of the environment of each sensing element  16   a,    16   b,  . . . ,  16 N which may, for instance, for used for health monitoring and fire protection purposes. In one embodiment, each full cycle of method  100  will include a slow scan for one sensing element  16   a ,  16   b,  . . . ,  16 N. A first full cycle of method  100  might include a scheduled slow scan of element  16   a,  for instance, while a second full cycle of method  100  might include a scheduled slow scan of element  16   b,  with this pattern repeating once all N sensing elements have been subjected to a slow scan. 
         [0022]    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 and equivalents may be substituted for elements thereof without departing from the scope of the invention. In particular, although the present invention has been described with respect to temperature sensing, a person skilled in the art will understand that method  100  may analogously be applied to systems which measure pressure, strain, or other quantities for which optical fiber sensors are available. Although sensing elements  16   a,    16   b,  . . . ,  16 N have been described as FBG sensing elements, other types of sensing elements may alternatively be used, with corresponding changes in the mathematical models used by data processor  22 . 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.