Abstract:
A spark detector lens assembly is adapted to block energy from static electric discharges. The lens assembly includes a housing which receives a lens. The lens is adapted to pass infrared frequency energy therethrough. A conductive coating is disposed on one side of said lens, which is effective to block radio frequency energy but pass infrared frequency energy. The conductive coating may be electrically connected to an electrical ground path. The lens assembly is especially effective for preventing false alarms by shielding a spark detector from radio frequency energy radiated by static electric discharges.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Application No. 60/546,449, filed Feb. 20, 2004. 

   BACKGROUND OF THE INVENTION 
   This invention relates generally to fire detection systems and more particularly to a fire detection system using an optical spark detector. Fire detection systems are often used in textile mills or other industrial environments where flammable materials such as textile fibers are entrained in a moving air stream and thus conveyed at high speed through enclosed duct work between processing stations which perform various kinds of operations on the fibers. Infrared detectors are positioned in the duct work at intervals and are designed to detect the presence of embers or hot metal fragments in the moving air stream which could cause a fire or explosion. An infrared detector detecting a source of infrared energy in the moving air stream generates a signal which may used to activate a visual or audible alarm. The signal may also be sent to a control panel, where further signals are transmitted to instantly shut off the fiber processing equipment. Signals may also operate diverters, fire extinguishers or other equipment intended to protect life and property from a fire or explosion. 
   This type of system senses primarily the infrared (IR) energy generated by a glowing ember. However, the material flowing through the ducts protected by such systems generates significant static electricity which spontaneously discharges from time to time. Unfortunately, when these static electric discharges occur they radiate energy, primarily radio frequency (RF) energy, which can trigger the detector and cause an alarm. Existing spark detection systems can not distinguish between these false alarms and a true spark. 
   Therefore, it is an object of the invention to provide a spark detection system which is resistant to false alarms caused by static electric discharges. 
   It is another object of the invention to provide a detector lens which blocks radio frequency but passes infrared energy. 
   BRIEF SUMMARY OF THE INVENTION 
   These and other objects are met in one preferred embodiment by providing a spark detector lens assembly adapted to block energy from static electric discharges. The lens assembly includes a housing for receiving a lens; a lens disposed in the housing, which is adapted to pass infrared frequency energy therethrough; and a conductive coating disposed on one side of the lens, which is effective to block radio frequency energy but pass infrared frequency energy therethrough. Means are provided for electrically connecting the conductive coating to an electrical ground path. 
   According to another preferred embodiment of the invention, the housing is electrically conductive. 
   According to another preferred embodiment of the invention, the conductive coating is electrically connected to the housing. 
   According to another preferred embodiment of the invention, a conductive epoxy is disposed in contact with the conductive coating and the housing. 
   According to another preferred embodiment of the invention, the conductive coating is composed of indium-tin-oxide. 
   According to another preferred embodiment of the invention, the thickness of the coating is from about 1 micron to about 4 microns. 
   According to another preferred embodiment of the invention, the thickness of the coating is about 1 micron. 
   According to another preferred embodiment of the invention, a spark detector includes a spark sensor responsive to the impingement of radiation thereon; a lens adapted and positioned to pass infrared energy therethrough to the spark sensor; a conductive coating disposed on one side of the lens, the coating effective to block radio frequency energy but pass infrared energy therethrough; and an electrical ground path connected to the coating. 
   According to another preferred embodiment of the invention, the housing is mounted in a casing. 
   According to another preferred embodiment of the invention, the casing is electrically conductive and is connected to an electrical ground path. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The subject matter that is regarded as the invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which: 
       FIG. 1  is schematic view of a fire detection system constructed in accordance with the present invention; 
       FIG. 2  is a perspective view of a detector assembly; 
       FIG. 3  is a side cross-sectional view of the detector assembly of  FIG. 2 ; 
       FIG. 4  is an enlarged cross-sectional view of a lens assembly constructed in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,  FIG. 1  illustrates an exemplary fire detection system  10  constructed in accordance with the present invention. The system  10  includes one or more detector assemblies  12 . Each detector assembly  12  is mounted to a protected space  16  so that its field of view will cover the desired area. In the illustrated example the protected space  16  is defined by a suction system duct  18  of a textile machine. However, the present invention may be used to detect sparks in any kind of equipment. 
   The detector assemblies  12  may be connected to a central control unit  20  by wires  22 . The central control unit  20  may be a PC computer, programmable logic controller (PLC), or other known device. The central control unit  20  can perform known functions such as monitoring and self-testing the spark detector assemblies  12 , triggering audible and visual alarms, shutting down the protected equipment, alarm logging, etc. 
     FIGS. 2 and 3  illustrate an exemplary detector assembly  12 . The detector assembly  12  includes a sheetmetal housing  24  having a front plate  26 , a removable back plate  28 , and a plurality of side walls  30 . A lens assembly  32  containing a lens  34  is installed into an opening in the front plate  26 . The lens  34  may be curved as depicted or may be flat. A printed circuit board (PCB)  36  containing a spark sensor  38  and sensor amplification and control circuits  40  of a known type is mounted in the housing  24 , for example with the illustrated studs  42 , standoffs  44 , and nuts  46 . The spark sensor  38  is mounted so that it faces through the lens  34 . In the illustrated example, the spark sensor  38  is located in the correct position by selection of the length of the sensor leads  48  that connect it to the PCB  36 . A plurality of input and output leads  50  run from the PCB  36  to a terminal block  52  which is mounted on a cover  54  that protects the PCB  36 . The housing  24 , lens assembly  32 , and internal components are connected to a ground path  56 . The particular structure of the detector assembly  12  is for illustration only and may be modified to suit a particular application. For example, the housing  24  could be constructed of a conductive plastic, high-temperature plastic, or composite material. Nonlimiting examples of suitable high-temperature plastics include polyphenylene oxide (PPO) resin or a combination of PPO and styrene which is available under the trade name NORYL. The housing may be electroplated to make it electrically conductive. The lens assembly  32  could be made an integral part of the housing  24 . 
     FIG. 4  illustrates the lens assembly  32  in more detail. The lens assembly  32  includes an electrically conductive, generally cylindrical barrel  58  having a lens end  60  and a mounting end  62 . The mounting end  62  includes suitable mounting means for securing the lens assembly  32  to the housing  24 , such as the illustrated threads  64 . The lens end  60  includes means for securing a lens  34 . In the illustrated example, the lens  34  is disposed against a shoulder  66  formed in the barrel  58  and held in place by a flange  68  which is crimped down against the lens  34 . 
   The lens  34  allows electromagnetic energy to pass to the spark sensor  38  while protecting the spark sensor  38  from debris and physical damage. The lens  34  is of a known type which functions as a low-pass filter transparent to energy corresponding to approximately the infrared band and lower frequencies, and may be constructed from a material such as glass or plastic. A conductive coating  70  is disposed on the interior surface  72  of the lens  34 . The coating  70  is electrically coupled to the barrel  58 , for example by solder or the drop of conductive epoxy  74  illustrated. 
   The coating  70  is a material that blocks RF energy while allowing IR energy to pass through it substantially unimpeded. Applicants have discovered that using an RF shielding coating prevents false alarms caused by static electric spark discharges. The coating  70  absorbs RF energy, and conducts any accumulated charge away from the spark sensor  38  though a ground path. In the illustrated example the coating  70  comprises a known indium tin oxide material, however any conductive coating which is transparent to IR energy may be used. Examples of other known conductive coating materials include nickel, gold, silver, and graphite. The thickness of the coating  70  is selected so that infrared energy can pass substantially unimpeded to the spark sensor  38 . The coating  70  may be relatively thin, as the energy level of any expected static electric discharge is quite low. Furthermore, as the coating thickness increases, it would undesirably block the transmission of IR energy. In the illustrated example the thickness of the coating  70  is about 1 micrometer (0.039 mil) to about 4 micrometers (0.156 mil), and preferably about 1 micrometer (0.039 mil). This results in an infrared attenuation of only about 0.1%. 
   The coating  70  may be applied to the lens  34  by any of a number of known processes including physical vapor deposition (PVD) and chemical vapor deposition (CVD). One preferred physical vapor deposition process for applying the coating  70  is sputtering. 
   In operation, the lens  34  and the coating  70  cooperate to allow the spark sensor  38  to detect sparks from glowing material flowing through the protected volume while significantly reducing or eliminating false alarms caused by static electric discharges. It is thought that the energy from static discharges occurs mostly in the RF and UV bands, and that the combination of the lens filtering effect and the RF barrier coating  70  prevent substantially all of the energy of static electric discharges from reaching the spark sensor  38 . In any case, it has been observed that the lens assembly  32  described above substantially reduces the incidence of false alarms attributable to static electric discharges. 
   The foregoing has described a spark detection system having a coated lens assembly. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention. Accordingly, the foregoing description of the preferred embodiment of the invention and the best mode for practicing the invention are provided for the purpose of illustration only and not for the purpose of limitation.