Abstract:
An apparatus, system, and method for solid state spark detection are disclosed. A semiconducting photodetector has a pair of electrical conductors for carrying an electrical current generated when the semiconducting photodetector receives light energy generated by sparks. An electrical current detector is operably coupled with the electrical conductors for sensing the electrical current generated to signal receipt of the light energy generated by sparks. The present invention can be coupled to a monitoring device, such as an oscilloscope, for monitoring intensity and/or timing of spark phenomena. A plurality of semiconducting photodetectors and associated electrical current detectors may be used in concert if desired. The semiconducting photodetectors are suitably adapted to respond to light energy of different magnitudes or wavelengths to more particularly study sparks of different natures.

Description:
FIELD OF THE INVENTION 
   This invention relates generally to detection of light energy and, more specifically, to detecting light energy caused by the generation of sparks. 
   BACKGROUND OF THE INVENTION 
   Unwanted sparks or arcs of electrical energy can present a problem. At a minimum, generation of sparks can interfere with light sensitive applications or ruin photosensitive emulsions. On another hand, of perhaps greatest concern is the generation of sparks in the presence of flammable or combustible materials, which can result in a fire or an explosion. It is for this latter reason that chemical producers, petroleum companies, and aerospace companies, just to name a few, study spark generation phenomena to prevent harms that might result from sparks or arcs caused by lightning, discharge of static electricity, or other sources. 
   To name one example, in the aerospace industry, it is a concern that lightning striking an aircraft could result in the generation of sparks where fuel-vapor mixtures might exist. The Federal Aviation Authority (FAA) recognizes that, at a minimum, spark energy of 200 uJ is capable of igniting a fuel-vapor mixture. As is understood in the art, although lightning strikes pass right through aluminum aircraft en route to a ground source and harm virtually never results, manufacturers remain vigilant in evaluating new materials and components to ensure unacceptable sparks are not produced. To that end, manufacturers test aircraft materials and components by simulating lightning strikes and monitoring to determine if sparks are produced. 
   Spark detection tests are conducted in light-sealed chambers. Simulated lightning is generated by discharging a bank of high voltage capacitors. The simulated lightning is applied to an object of study, such as a “coupon” of aircraft fuselage material, or joined sections of aircraft hull material, which is coupled to ground. Whether the electrical charge carried by the simulated lightning strike passes through to ground or results in generation of spurious sparks is conventionally monitored photographically. Photographs of the object of study are taken by 35 millimeter or Polaroid® cameras. The shutter of the camera is opened before the simulated lightning strike is applied and closed before the light-sealed chamber is opened to study the object under test. As is known, ASA 1000 35 millimeter film or ASA 3000 Polaroid® film will reveal the emission of a 200 uJ or greater spark. Studying the resulting photographs, therefore, allows researchers to determine if sparks resulted from simulated lightning strikes and, specifically, where on the object of study sparks originated. 
   Unfortunately, photographic methods do not provide information as detailed or refined as researchers would like. Because the response of photographic film to light is highly nonlinear, the intensity of sparks generated must be estimated from the brightness of the image of the spark or the cross-sectional area on the film spanned by the spark&#39;s image. Film also may not be able to perceive light in the infrared spectrum or even the deep red of the visual spectrum. Thus, events of possible concern could be missed. Also, film cannot indicate exactly when the spark actually was generated during the interval the shutter was open. Further, film cannot indicated how long the event lasted or whether multiple sparks that might appear on a photograph occurred simultaneously or at different times. Finally, particularly with 35 millimeter film, researchers also must wait while the film is developed to begin their studies. 
   In an attempt to improve on photographic techniques, photomultiplier tubes (PMTs) also have been used to monitor sparks. For example, PMTs allow for determining precisely when sparks are generated. Also, the output of a PMT can be used to measure intensity of sparks detected. However, although PMTs are very sensitive and can detect even single-photon events, they also have disadvantages. For example, because of the unpredictability of the output of the photomultiplying process, the output may not accurately represent intensity of sparks recorded. Also, PMTs are expensive to manufacture and are extremely fragile. In addition, PMTs are very sensitive to noise common in high energy environments such as lightning testing chambers. PMTs also must be kept away from light sources, receiving light signals through expensive fiber-optic lines. 
   Therefore, there is an unmet need in the art for a better way to measure emission of potential sparks and to more precisely determine the timing and intensity of sparks emitted. At the same time, it is desired to have a spark detector that is inexpensive, durable, and that yields quick results. 
   SUMMARY OF THE INVENTION 
   The present invention provides a system for detecting sparks by using a solid state device for detecting the light energy generated by the sparks. Using a solid state device, such as a semiconducting photodetector, offers advantages over highly nonlinear photographic techniques in being able to measure spark intensity and to report the exact time at which sparks were emitted. The present invention also provides advantages over PMT detection of sparks because semiconducting photodetectors are less expensive, more durable, and more easily protected from expected noise phenomena than are PMT detectors. 
   An exemplary embodiment of the present invention employs a semiconducting photodetector having a pair of electrical conductors for carrying an electrical current generated when the semiconducting photodetector receives light energy generated by sparks. An electrical current detector is operably coupled with the electrical conductors for sensing the electrical current generated to signal receipt of the light energy generated by sparks. 
   The present invention can be coupled to a monitoring device, such as an oscilloscope, for monitoring intensity and/or timing of spark phenomena. Further, a plurality of semiconducting photodetectors and associated electrical current detectors may be used in concert if desired. The individual semiconducting photodetectors can be suitably adapted to respond to light energy of different magnitudes or wavelengths to more particularly study sparks of different natures. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings. 
       FIG. 1  is a schematic view of a semiconducting photodetector circuit employed in an embodiment of the present invention; 
       FIG. 2  is a disassembled, perspective view of a semiconducting photodetector device used in an embodiment of the present invention; 
       FIG. 3  is a block diagram of a spark testing apparatus employing an embodiment of a solid state photodetector of the present invention; and 
       FIG. 4  is a flowchart of a method for detecting sparks of an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  is a schematic diagram of an exemplary solid state spark detecting circuit  100  used in an embodiment of the present invention. In general, the circuit  100  employs a solid state, semiconducting photodetector  104  to detect sparks by generating an electric current in response to receiving light energy from the sparks. The electric current generated by the semiconducting photodetector  104  is amplified by a first operational amplifier (op amp)  112  and a second op amp  116 . The amplified electrical current is monitored and studied with a monitoring device  136  to determine the presence and intensity of generated sparks as detected by the solid state spark detecting circuit  100 . 
   More specifically, the semiconducting photodetector  104  shown in  FIG. 1  is suitably a P-N junction photodiode, such as a Vishay-Telefunken BPW46. The BPW46 is a high speed, high sensitivity photodetector with a photon-receiving area of 7.5 square millimeters, a 5 nanoampere dark current, a response time of 100 nanoseconds, and a spectral bandwidth of 600 to 1150 nanometers. The BPW46 is capable of detecting sparks at and below the 200 uJ energy limit acknowledged by the FAA as the minimum energy spark capable of igniting a fuel-vapor mixture. 
   In addition to P-N junction photodiodes like the BPW46, other semi-conducting photodiodes can be used. For example, P-i-N photodiodes can be used as the semiconducting photodetector  104  in the spark detecting circuit  100 . Advantageously, P-i-N photodiodes feature an intrinsic layer of pure silicon between the acceptor P layer and the donor N layer to extend the width and thus the sensitivity of the photosensitive junction, also. Also, a highly biased avalanche P-N junction photodiode can be used. Avalanche P-N junction photodiodes are highly sensitive to light energy and are able to react to even single photons received. 
   The semiconducting photodetector  104  is reverse biased with a direct current power supply  108 . A 9-volt battery is suitably used as the power supply  108 , with its negative terminal connected to the anode of the semiconducting photodetector and its positive terminal coupled to ground. Alternatively, if an avalanche P-N junction photodiode is selected as the photoconducting semiconductor, a larger power supply  108  having an output of larger than 9 volts could be employed to suitably bias the semiconducting photodetector. 
   An output of the semiconducting photodetector  104  is coupled with a first input of the first op amp  112  used in a voltage-follower configuration. A node  113  between the semiconducting photodetector  104  and the first input of the first op amp  112  is coupled to ground with a high-resistance pull-down resistor  120 . In the embodiment shown in  FIG. 1 , a 15 mega ohm resistor is used. A second input of the first op amp  112  receives a feedback branch  124  from an output of the first op amp  112 , thereby completing the voltage follower configuration. 
   The first op amp  112  shown in  FIG. 1  is suitably a CA3140 BiMOS op amp. The CA3140 is a low noise op amp, which is well suited to responding to and amplifying small electrical currents that might be generated by the semiconducting photodetector  104 . The CA3140 has a high input resistance of 1.5 terraohms, a slew rate of 9 volts/microsecond, and drains only 3 milliamps of current, thus allowing for long battery life. Other op amps known in the art with suitable low noise characteristics can also be used in embodiments of the present invention. 
   The output of the first op amp  112  is applied to a second input of a second op amp  116 , which is configured as an inverting amplifier. More specifically, the output of the first op amp  112  is coupled to the second input of the second op amp  116  through a resistor  128 , such as without limitation an 850 ohm resistor. The second input of the second op amp  116  also receives a feedback loop  132  from an output of the second op amp  116 . A first input of the second op amp  116  is tied to ground, thereby completing the inverting amplifier configuration. 
   The output of the second op amp  116 , which is the output of the solid state spark detecting circuit  100 , is coupled to the monitoring device  136 , such as an oscilloscope. As is well known in the art, an oscilloscope can be variably configured to read voltage and current signals. An appropriately equipped oscilloscope can be used to read both an applied voltage signal, such as might be used to simulate a lightning strike of the object under study, and the output of the solid state spark detecting circuit  100  to study a relationship between the applied electrical current and the resulting sparks detected. 
     FIG. 2  shows an exemplary spark detecting device  200  that includes the solid state spark detecting circuit  100  (FIG.  1 ). The solid state spark detecting circuit  100  ( FIG. 1 ) suitably is implemented on a circuit board  202  on which are mounted the semiconducting photodetector  104 , the first op amp  112 , and the second op amp  116 , all previously described. The semiconducting photodetector  104 , the first op amp  112 , and the second op amp  116  are electrically connected as described above in connection with FIG.  1 . 
   In addition, two nine-volt batteries  108   a  and  108   b  are used as a power supply for the solid state spark detecting circuit. Twin nine-volt batteries  108   a  and  108   b  are chosen, as will be appreciated by one ordinarily skilled in the art, to supply not only ground but also to supply positive and negative nine-volt voltage sources required to power the op amps. The terminals of the batteries  108   a  and  108   b  are connected as shown to a dual-throw switch  250 . As shown, a first input  252  of the switch  250  is connected to a first output  254  of the switch  250  when the switch  250  is closed. Similarly, a second input  256  of the switch  250  is connected to a second output  258  of the switch  250  when the switch  250  is closed. Coupling a positive terminal of the first battery  108   a  to the first input  252  of the switch  250  and a negative terminal of the second battery  108   b  to the second input of the switch  250  couples those respective terminals to the circuit board  202  as shown when the switch  250  is closed. At the same time, commonly coupling a negative terminal of the first battery  108   a  and a positive terminal of the second battery  108   b  to the circuit board  202  at a ground terminal creates the positive and negative nine-volt power sources used. 
   An output of the circuit board  202 , which constitutes the output of the second op amp  116  ( FIG. 1 ) is coupled to a connector  260 . The connector  260  in an embodiment of the present invention is suitably a BNC connector matching an input of a conventional oscilloscope for monitoring the output of the solid state spark detecting circuit  100  (FIG.  1 ). 
   The electronic components previously described are contained, as shown in  FIG. 2 , in a shielding housing  270 . The shielding housing  270 , in one embodiment, has layers of steel or another suitably magnetically shielding material to block magnetic effects in a high voltage environment, such as a lightning simulator, from interfering with the operation of the spark detecting device  200 . The shielding housing  270  also may have layers of copper, aluminum, or any other suitably electrically shielding material to shield the electronics from electrical interference which also attends such an environment. As shown in  FIG. 2 , the shielding housing  270  can be cylindrical, with the switch  250  and the connector  260  mounted on sides of the shielding housing  270 . In one embodiment of the present invention, a front face  274  of the shielding housing  270  is positioned to face an expected source of potential sparks. In the front face  274  an observing opening  278  is made in which the semiconducting photodetector  104  is positioned to be able to receive light energy generated by potential sparks. The device  200  can be enclosed by adding a back cover  282  to the shielding housing  270 . The back cover  282  also should be constructed of shielding materials as previously described. 
   More specifically, in one embodiment the shielding housing  270  is suitably constructed using a section of copper pipe for electrical shielding, the section of copper pipe having an internal diameter and a length large enough to accommodate the electronics previously described. Copper end caps are suitably used to form both the front face  274  of the shielding housing  270  and the back cover  282 . An observing opening  278  is sized to accommodate the semiconducting photodetector  104  drilled or cut into the end cap serving as the front face  274 . A section of steel pipe with an inner diameter larger than the outer diameter of the end caps and as long as the section of copper pipe is suitably installed around the copper pipe to provide magnetic shielding. Holes can be drilled in the sections of copper and steel pipe for mounting the switch  250  and the connector  260  and through which wires connecting the switch  250  and the connector  260  to the circuit board  202  can pass. 
   Using embodiments of the present invention, multiple solid state, photodetecting semiconductor devices could be used in a single region of interest. Multiple circuits, like the solid state spark detecting circuit  100  ( FIG. 1 ) could be housed in a single shielding housing  270  or in separate shielding housings, with each semiconducting photodetector  104  being adapted to receive different types of light. For example, the semiconducting photodetectors  104  could be filtered to each respond to different magnitudes of light energy or different wavelengths of light energy. Using multiple circuits allows more particularized study of different types of sparks to better understand the spark-generating phenomena in bodies under test. 
   The spark detecting device shown in  FIG. 2  can be deployed in a number of environments where sparks are to be detected.  FIG. 3  shows one exemplary environment, such as a lightning simulator  300  in which a body under test  304  is contained in a light-tight chamber  308  and subjected to simulated lightning strikes. The body under test  304  may be a coupon of a aircraft material of a single type, a joined section of different materials, a fuel or petroleum fitting, or another body. The body under test  304  is connected to ground and will receive simulated lightning strikes. The simulated lightning strikes are generated by a lightning generator  312  that is powered by an appropriate power supply  316 . The lightning generator delivers the simulated lightning strike via a probe  320  disposed in or adjacent to the light-tight chamber  308 . The light-tight chamber  308  shields the body under test  304  from ambient light so that light energy which is detected is light energy generated by sparks caused by the simulated lightning strikes. 
   The light-tight chamber may be equipped with a conventional camera  324 , such as a 35 millimeter or Polaroid® camera, along with the solid state spark detecting device  200  of the present invention. The camera  324  and the solid state spark detecting device  200  are mounted to observe the body under test. The camera  324  and the solid state spark detecting device  200  both can be mounted to observe the body under test through light-tight openings in the light-tight chamber  308 , or can be positioned within the light-tight chamber, provided there is a mechanism by which a shutter of the camera  324  can be controlled from outside the light-tight chamber  308 . To enable the camera  324  and the solid state spark detecting device  200  to be able to better observe and detect sparks which might be emitted on a far side of the body under test, mirrors  340  can be positioned inside the light-tight chamber  308  to reflect light energy from the opposite side of the body under test  304  toward the camera  324  and the solid state spark detecting device  200 . The output of the solid state spark detecting device  200 , optionally along with a monitoring output  350  of the lightning generator  312 , can be connected to an oscilloscope  360  for studying the output of the solid state spark detecting device  200 . 
   In addition to the lightning simulator  300  environment shown in  FIG. 3 , it will be appreciated that the solid state spark detecting device  200  can be used in any number of contexts. The solid state spark detecting device  200  could be deployed, for example, in working devices and equipment to detect the generation of sparks. Although the context would require a light-controlled or otherwise darkened environment, the solid state spark detecting device  200  could be deployed in fuel flow testing devices. The flow of fuels can statically charge vessels through which they flow, thereby resulting in the risk of sparks arcing through surrounding air. Testing may be performed to ensure that sparks are not being generated which could ignite a fuel-air vapor. Similarly, in a petroleum refinery or other body through which fuels flow, a solid state spark detecting device could be deployed to monitor for emission of sparks which may indicate failed grounding circuitry or other potential problems. 
     FIG. 4  is a flow-chart describing a method of detecting sparks with a solid state spark detecting device such as that previously described. At a block  404 , the semiconducting photodetector device is positioned where it can receive light energy generated by potential sparks of interest. At a block  408 , the semiconducting photodetector is energized and enabled to monitor light energy generated by sparks. At a decision block  412 , if no light energy has been received, the semiconducting photodetector device continues to monitor generation of light energy. If light energy is detected, at a decision block  416  it is determined if the semiconducting photodetector device is configured to monitor an intensity of the light energy detected. If so, the intensity is measured at block  420 . As previously described, the intensity of the light energy can be measured to determine the energy of sparks emitted by coupling the output of the semiconducting photodetector device to a monitoring device, such as an oscilloscope, and setting the monitoring device to measure the voltage output of the semiconducting photodetector device. At a decision block  424 , if the stimulus applied to generate the sparks is known, at a block  428  the voltage output of the semiconducting photodetector is compared to that of the stimulus to analyze what type of stimulus produces what nature of sparks. 
   If at the decision block  416  it is determined that intensity of the sparks is not being measured, or at the decision block  424  it is determined that either the stimulus is not known, or the stimulus is known and has been compared to the intensity of the voltage output of the semiconducting photodetector device, then at a block  432  it is determined whether timing of the sparks generated also is being measured. If so, at a block  436 , times at which the sparks were generated are measured. At a decision block  440  it is determined if timing of the stimulus causing the sparks is known. If so, timing of the stimulus relative to that of the sparks generated is determined at a block  444 . If the intensity and/or timing have been measured at the blocks  428  and  444 , respectively, or if either measurement has been disregarded at the decision blocks  416  and  432 , respectively, the method returns to the block  408  to monitor for further emission of light energy from sparks. 
   It will be appreciated that many additional steps could be added to this method and remain within the broad principles of the present invention. For one example, the output of the solid state spark detecting device might not only be monitored and measured, but also recorded for playback, manipulation, and/or analysis. For a second example, as shown in  FIG. 3 , use of a conventional camera  324  ( FIG. 3 ) could be used to determine the exact point in the body under study  304  from which sparks were generated. Using a photographic camera to also monitor for emission of sparks would proceed in parallel with the method previously described. Use of a photographic camera may entail positioning the camera, opening the shutter prior to the expected generation of sparks, closing the shutter after the end of the expected generation of sparks, rendering a photograph through developing or Polaroid® process, and studying the sparks detected. 
   While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.