Abstract:
In one embodiment, an electric field sensor is provided including an elongated conductor; a circuit including an input and an output connected across the elongated conductor wherein said circuit includes a DC to AC signal converter; wherein said elongated conductor is operative to impose a DC signal on said circuit input in response to being exposed to an electric field and broadcast an AC signal converted from said DC signal in response to said electric field being greater than a threshold level.

Description:
This is a Divisional application of U.S. Ser. No. 12/272,667, filed on Nov. 17, 2008, which is issued as U.S. Pat. No. 7,944,361. 
    
    
     TECHNICAL FIELD 
     The disclosure relates to electric field sensors and more particularly to an electric field sensor that is combined with an antenna that operates to emit a signal (whistles) at a predetermined frequency in response to a high electric field, such as immediately prior to a lightning strike. 
     BACKGROUND 
     Lightning strikes are desirable to protect against, including with respect to aircraft. One method of warning of an imminent lightning strike includes measuring the strength of an electric field. 
     One approach in the prior art has included using electric field mills which include a spinning set of fan blades that alternately cover and uncover an anode plate that is alternately charged by the electric field when uncovered and the charge state determined followed by discharge of the anode plate when covered. 
     While an electric field mill gives a good absolute measure of an electric field, an electric field mill includes moving parts, such as a high rpm motor (e.g., 1650 rpm) and two metallic coaxial disks separated by a fixed distance, and therefore has the drawbacks of being bulky, complex, and sensitive to stress and wear, requiring frequent servicing. 
     Accordingly there is a need for an electric field sensor that overcomes at least some of the drawbacks of prior art field sensors e.g., is smaller, has few or no moving parts, and requires less or infrequent servicing. 
     SUMMARY 
     An electric field sensor (whistle) is provided that may be used to warn of an imminent lightning strike. Some action may then be taken in response. The electric field sensor may be included on an aircraft. 
     In one exemplary embodiment, an electric field sensor is provided including an elongated conductor; a circuit including an input and an output connected across the elongated conductor wherein the circuit includes a DC to AC signal converter; wherein the elongated conductor is operative to impose a DC signal on the circuit input in response to being exposed to an electric field and broadcast an AC signal converted from the DC signal in response to the electric field being greater than a threshold level. 
     In another exemplary embodiment, a lightning strike warning system is provided including one or more electric field sensors mounted on or within a skin of a vehicle, the electric field sensors each including: an elongated conductor; a circuit including an input and an output connected across the elongated conductor wherein the circuit includes a DC to AC signal converter; wherein the elongated conductor is operative to impose a DC signal on the circuit input in response to being exposed to an electric field and broadcast a radio signal converted from the DC signal in response to the electric field being greater than a threshold level; and, one or more radio signal receivers on the vehicle adapted to receive the radio signal from the one or more electric field sensors. 
     In another exemplary embodiment, a method of sensing and warning of an electric field is provided, the method including providing an elongated conductor; providing a circuit including an input and an output connected across the elongated conductor wherein the circuit includes a DC to AC signal converter; generating a DC signal in response to exposure of the elongated conductor to an electric field; and, broadcasting an AC signal converted from the DC signal in response to the electric field being greater than a threshold level. 
     In another exemplary embodiment, a method of warning of a lightning strike near a vehicle in provided, the method including providing one or more electric field sensors mounted on or within a vehicle skin, the electric field sensors each including: an elongated conductor; a circuit including an input and an output connected across the elongated conductor wherein the circuit includes a DC to AC signal converter; wherein the elongated conductor is operative to impose a DC signal on the input in response to being exposed to an electric field and broadcast a radio signal converted from the DC signal in response to the electric field being greater than a threshold level; and, providing one or more radio signal receivers on the vehicle, the one or more radio signal receivers receiving the radio signal from the one or more electric field sensors. 
    
    
     
       BRIEF DESCRIPTION OF THE ILLUSTRATIONS 
       Exemplary embodiments of the invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1A  is a representative implementation of the electric field whistle according to an exemplary embodiment. 
         FIGS. 1B-1E  show exemplary antenna shapes according to exemplary embodiments. 
         FIG. 2A  is a representative implementation of the electric field whistle according to an exemplary embodiment. 
         FIG. 2B  is a representative implementation of the electric field whistle according to an exemplary embodiment. 
         FIG. 3  is a representative implementation of a plurality of electric field whistles according to an exemplary embodiment. 
         FIG. 4  is a representative implementation of a method of using the electric field whistle according to an exemplary embodiment. 
         FIG. 5  is a representative implementation of an aircraft using the electric field whistle according to an exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is merely exemplary (illustrative) in nature and is not intended to limit the described embodiments or the application and the uses of the described embodiments. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the disclosure. 
     Referring to  FIG. 1A  is shown an exemplary embodiment of an electric field whistle (electric field sensor) generally indicated by reference numeral  10 . The electric field sensor includes an antenna  14  and a circuit  20  connected in series where an output of the antenna  14  is connected to an input of the circuit  20  and an output of the circuit  20  is connected to an input of the antenna  14 . In an exemplary embodiment, the antenna  14  may include a predetermined length of an elongated conductor, such as a rod or square shaped length of metal e.g., of metal wire, or where the conductor is cylindrical or hollow at the center. 
     It will be appreciated that the elongated conductor may be embedded in a sheath of insulating material or that the elongated conductor may be conductor lines deposited onto a substrate, such as an insulating substrate. The elongated conductor may include any width, but in an exemplary embodiment, has a width (or diameter) of about 0.5 mm to about 10 mm. 
     The antenna  14  between two antenna ends  14 A and  14 B need not be straight. For example, the antenna  14  is preferably substantially straight as shown (e.g., rod or cylinder shaped) but it will be appreciated that the antenna  14  may include other geometrical shapes, such as wave shaped (e.g.,  FIG. 1B ), coil shaped (e.g.,  FIGS. 1C ,  1 E), and arc shaped (e.g.,  FIG. 1D ). It will be further be appreciated that the length of the antenna  14  will at least in part determine the frequency at which a radio frequency signal is emitted (whistles) from the antenna. 
     In one exemplary embodiment, the antenna may have a length from about 1 cm to about 50 cm, more preferably from about 10 cm to about 30 cm, and emits a signal (whistles) at a frequency that may range from about 140 MHz to about 7 GHz, preferably from about 235 MHz to about 700 MHz. 
     Still referring to  FIG. 1A , a circuit  20  is electrically connected across the antenna  14  (e.g., across each end of the antenna  14 ). A first portion (e.g. first end) of the antenna  14 A may be connected e.g., wiring  15 A to an input  22 A of the circuit  20  where the input is connected to a voltage dependent resistance device  22  which may be a varistor, zener diode or tranzorb (also referred to as Transzorb™) which has a high resistance at low voltage but rapidly loses resistance at high voltage. Preferably, the voltage dependent resistance device  22  includes a threshold voltage that an imposed voltage must be above in order for electrical conduction to occur. 
     More preferably, the voltage dependent resistance device  22  may be a tranzorb that conducts electricity in response to receiving a voltage signal having a magnitude greater than the breakdown voltage of the tranzorb. For example, the tranzorb may clamp an output voltage to its breakdown voltage. In an exemplary embodiment, the voltage dependent resistance device  22  may have a threshold voltage of from about 15 to 50 volts, more preferably from about 25 to about 35 volts. 
     The output from the voltage dependent resistance device  22  is output to DC to AC converter  24 , which may be any conventional DC to AC converter. An output  24 B from circuit  20  is then connected e.g., wiring  15 B, to a second portion (e.g. second end  14 B) of the antenna  14 . Preferably, an output AC (radio) signal frequency of the DC to AC converter may be tuned (prior to use) to match the antenna, e.g., output a radio signal at or near the resonant frequency of the antenna, which may at least in part be determined by the length of the antenna  14 . The antenna, in turn emits (broadcasts) the radio signal (whistles) at or near the frequency output by the DC to AC converter. It will be appreciated that the amplitude of the emitted radio frequency may be proportional to a magnitude of the sensed electric field (e.g., voltage at input of the circuit  20 ). It will further be appreciated that the antenna  14  provides the power source for the electric field whistle by virtue of its presence in a changing electric field. The antenna  14  together with circuit  20  makes up the electric field whistle (sensor)  10  according to an exemplary embodiment. 
     In exemplary operation of the electric field whistle, the antenna  14  senses an electric field present over the length of the antenna, such as in the presence of an electric field surrounding a vehicle (e.g. aircraft  94 ) such as immediately preceding a lightning strike (St. Elmo&#39;s fire). The presence of the electric field causes a DC voltage to be generated and input to the circuit  20  including tranzorb  22  which only allows a signal to pass to the DC to AC converter  24  if the voltage is above a predetermined threshold value. If the DC voltage is above the threshold value, the DC to AC converter  24  then outputs an AC radio frequency signal to the antenna  14  at a frequency matching the antenna  14 , which then whistles by broadcasting a radio frequency signal. The radio signal may then be received by a radio receiver e.g.,  30 , which may be included in some other part of the aircraft  94  such as included in electrical systems of the aircraft, e.g.,  104 , shown in  FIG. 5 , or by some other radio receiver. 
     In response, a controller e.g.,  32  may send a signal shutting down selected electrical system  34  in the aircraft, or issue commands causing other aircraft systems to take other action including at least one of prior to, during or following sensing of the high electric field. It will also be appreciated that individual aircraft systems may include a radio receiver/controller capability programmed to respond without the necessity of a separate receiver  30  and controller  32 . 
     In exemplary operation, the electric field whistle is preferably able to respond e.g., sense an electric field circuit and send a radio signal on the time scale of the order of milliseconds or less, allowing a warning to be received on the aircraft within from about 1 millisecond to about 2 seconds prior to a lightning strike. 
     Referring to  FIG. 2A , in an exemplary embodiment, the antenna  14  and/or associated circuit  20  may be embedded within an aircraft skin  200 , which may include a polymer composite material such as carbon fiber reinforced polymer. For example, the antenna  14  may be made about flush with the surface of the aircraft skin to optimize sensitivity to an electric field outside the aircraft, e.g., such as a high electric field (St. Elmo&#39;s Fire) present immediately prior to a lightning strike. It will be appreciated that the antenna  14  and/or associated circuit  20  may be mounted on a separate substrate, which may in turn be mounted on or embedded within the aircraft skin. 
     In another exemplary embodiment, Referring to  FIG. 2B , the electric field whistle may have the antenna  14  and associated circuit  20  arranged such that the antenna  14  protrudes outward with respect to the aircraft skin  200 , e.g., makes an angle theta θ of 90 degrees or less with respect to the surface of the aircraft skin. For example, the antenna may include wiring to a protruding end by including a sheath conductor insulated from the antenna  14  as wiring, similar to coaxial cable, where the core conductor is the antenna. 
     In another exemplary embodiment, referring to  FIG. 3 , the aircraft may include a plurality of electric field whistles e.g.,  31 A,  31 B,  31 C,  31 D in different parts of the aircraft  94  that are in radio communication (via whistling RF shown by arrows) with a receiver/controller  33  that may be programmed to predict a particular location e.g.,  35  where a lightning strike may occur with respect to the aircraft. 
     For example, by comparing the relative amplitudes of a plurality of electric field whistles, each of which emit an RF signal at a different frequency (e.g., have a different length) in response to a sensed electric field, and where the amplitude of the emitted RF signals are proportional to the magnitude of the sensed electric field, receiver/controller  33  may be programmed to determine properties of the electric field e.g., where the electric field is most intense, and thereby predict a location where lightning strike is most likely. In response to a predicted location of a lightning strike, e.g.,  35 , action may then be taken by receiver/controller  33 , such as shutting down sensitive electrical systems e.g.,  37  in a particular area of an aircraft proximate to the predicted location of a lightning strike. 
     Referring to  FIG. 4 , is shown an exemplary method of operation of the electric field sensors. In step  401 , one or more electric field whistles are provided near the surface of an aircraft skin. In step  403 , the one or more electric field whistles are exposed to a high electric field, such as that present from an imminent lightning strike, and generate an electric field in response. In step  405 , in response, each of the one or more electric field whistles emits (whistles at) a respective RF signal. In step  407 , in response, one or more RF signal receivers on the aircraft receive the RF signal. In step  409 , in response, action is taken in at least one of before, during, and following a lightning strike. 
     As shown in  FIG. 5 , the aircraft  94  may include an airframe  98  with a plurality of systems  96  and an interior  100 . Examples of high-level systems  96  include one or more of a propulsion system  102 , an electrical system  104 , a hydraulic system  106 , and an environmental system  108 . Any number of other systems may be included. Although an aerospace example is shown, the principles of the invention may be applied to other industries, such as the automotive industry. 
     Although the embodiments of this disclosure have been described with respect to certain exemplary embodiments, it is to be understood that the specific embodiments are for purposes of illustration and not limitation, as other variations will occur to those of skill in the art.