Patent Number: 048641464
Section: description

DETAILED DESCRIPTION OF THE DISCLOSURE Use of the disclosed Universal Fire Simulator methods and apparatus described herein, provides simulation for at least three situations for which operational fire sensing systems are designed: a fire, for example a fuel fire; a munitions ignition without an ensuing fire; and a munitions ignition followed by a fire. For purposes of the following discussion, the term "visible spectral region" refers to radiation at wavelengths of about 0.35 to 1.0 micrometers. The term "ultraviolet spectral region" refers to radiation at wavelengths of about 0.2 to 0.35 micrometers. The term "infrared spectral region refers to radiation at wavelengths of about 1.0 to and beyond 6.0 micrometers. The following discussion can be better understood by referring to FIG. 1 in conjunction with FIGS. 2 and 3. Referring to FIG. 1, the various components of a preferred arrangement of a Universal Fire Simulator can be seen in block diagram. The fire simulator system 10 includes a blackbody transmitter 100 which generates and radiates blackbody radiant energy 42, which radiant energy also contains superimposed carbon dioxide and water vapor emissions in the infrared spectral region. The blackbody transmitter 100 includes a "white-noise" generator 20 of a type generally available commercially, a first power amplifier 30, of a type generally available commercially, and a blackbody source 40. An alternate type of "white noise" generator 20 may be obtained by amplifying a current passed through a zener diode. Construction of a blackbody source is generally known by persons practiced in the art. Such a source may be constructed from heating wire coils mounted on an electrical insulator with appropriate heat-sinking and installed in an infrared-reflecting cavity. A further understanding of the arrangement of the blackbody source 40 may be had by referring to FIGS. 5 and 6 and the further description, infra. The white-noise generator 20 is a source of random electrical signals 22 in the range from 0 Hertz (direct current) to about 100 Hertz. The signals 22 are amplified by the first power amplifier 30 and appropriately filtered by an electrical filter, producing amplified first signal currents 32 which heat the blackbody source 40. Heating excites the blackbody source 40 causing it to radiate energy in the visible and infrared spectral regions and varying in amplitude generally according to the waveform of the first signal current 32. The first power amplifier 30 gain may be set to produce a current 32 which when modulated will heat the blackbody source 40 to a temperature of, for example, 1200 to 1700 degrees Kelvin. A curve of radiation versus wave length for a 1600 degrees Kelvin blackbody is included in FIG. 2. Radiation from the blackbody source 40 at temperatures near 1600.degree. K. simulates one attribute of a hydrocarbon fire. Still referring to FIG. 1, the varying amplitude of the first signal current 32 simulates fire flickering. The amplitude of the radiative output 42 from the blackbody source 40 versus its frequency can be made to simulate that of a fire such as the hydrocarbon fire depicted in curve A of FIG. 3 by appropriate choices in blackbody source 40 construction and filtering of first amplifier 30 output. Referring to FIG. 3 there are shown two curves depicting the approximate flicker of a hydrocarbon pan fire. The curves were obtained from experimental data made in 1983 by K. Shamordola of Santa Barbara Research Center. Curve A describes the variation of relative amplitude of long-wave length radiation with frequency. Such wave lengths are generally in the near-infrared to far infrared spectral regions. Curve B describes the same phenomena for short-wave length radiation. Short wave lengths are generally those in the visible and ultraviolet spectral regions. The first and second signal currents 32, 52 are made to follow the relative amplitude versus frequency of such long wavelength curve and such short wavelength curve, respectively. Referring again to FIG. 1, the blackbody source 40 is filled with a mixture of gases which include nitrogen and carbon dioxide. Water vapor may also be added to the mixture. These gases are heated within the source 40 and radiate energy at characteristic emission bands, for example, 2.7 and 4.3 micrometers. Such carbon dioxide and water vapor emission is another attribute of a hydrocarbon fire. An on-off control circuit 12, of conventional design, controls the operation and selection of the blackbody transmitter 100. A human operator normally would select and activate the entire system 10 by means of the on-off control circuit 12, although automatic operation may be provided. The system 10 further includes an ultraviolet transmitter 200 which generates and radiates energy in the ultraviolet spectrum. The white-noise electrical signals 22 generated by the white-noise generator 20 are amplified by a second power amplifier 50 and appropriately filtered by an electrical filter producing amplified second signal currents 52. The second signal currents 52 are used to drive an ultraviolet source 60 which primarily emits ultraviolet radiant energy 62, varying generally in accordance with the waveform of the amplified second signal currents 52. In conjunction with the waveform of the amplified second signal currents, the thermal mass of ultraviolet source 60 is selected so that the amplitude of ultraviolet radiant energy 62 varies with frequency, for example as depicted in FIG. 3, curve B. The source 60 of ultraviolet radiant energy 62 may be an ultraviolet bulb similar to the type commercially available and used to erase EPROM (erasable-programmable) computer circuit ships. Other sources of ultraviolet radiation are a carbon arc or a fluorescent bulb without the fluorescent coating. The gain of the second power amplifier 50 is set such that the peak energy produced by the ultraviolet source 60 is in the same ratio to the peak energy transmitted by the blackbody source 40 as is the peak ultraviolet energy to peak blackbody energy characteristic of a fire. The operation of the ultraviolet transmitter 200 is selected and controlled by the on-off control circuit 12. The system 10 further includes an ignition flash transmitter 300 which includes a high voltage power source 70 of a type generally commercially available or easily constructed by one practiced in the art, a switch 90 and a discharge tube source 80. The high voltage source 70 produces a momentary surge of electrical current, depicted by the waveform 72 when the switch 90 is closed. The current will cause the discharge tube source 80 to flash, emitting light and heat energy 82. The discharge tube source 80 may be a commercially available electronic flash device commonly used for photography. Alternatively, a steady source, having a color temperature of about 3000 degrees Kelvin, quickly exposed by a shutter may be used. A steady source such as a tungsten filament lamp generating roughly 500 watts of total radiation could be exposed by a shutter which would produce a duration of less than 0.25 seconds of radiation having a rise time of one millisecond or less. Such a lamp source could be operated for just a few seconds during simulation to prevent overheating. The ignition flash transmitter 300 is selected and controlled by the on-off control circuit 12 which acts to close the switch 90 when an ignition flash is to be simulated. The simulation of munitions penetration of a vehicle compartment followed by a fire therein, may be obtained by regulating, through use of the on-off control circuit 12, the sequence and timing of selection and operation of the blackbody transmitter 100, ultraviolet transmitter 200 and ignition flash transmitter 300. It should be clear to one practiced in the art that the disclosed invention may be adjusted to simulate the radiative output of most fires in the spectral bands currently used by known fire sensing systems. Calibration of the system 10 may be accomplished by use of a radiometric transfer standard, for example a bank of detectors having a variety of spectral responses. The relative radiation outputs of the transmitters 100, 200, 300 may be adjusted to match the properties of the fire to be simulated. The disclosed invention has the potential for use with respect to fire sensing systems not yet designed. Referring now to FIG. 2, therein is shown the curve of approximate radiant emittance (fire excitance in watts/cm.sup.2 -micrometer) of a hydrocarbon fire after Linford, et al. The radiant emittance has the characteristics of a 1600 degree Kelvin blackbody with superimposed "spikes" of radiation at 0.31, 2.7 and 4.4 micrometers. This curve is typical of the distribution of radiant energy produced by a fire which the system 10 will simulate. Referring to FIG. 4, therein is depicted the measured radiant emittance of radiation 42 from an embodiment of the blackbody source 40. The basic curve is approximately that of a 500 degree Kelvin blackbody. CO.sub.2 and H.sub.2 O emissions at about 2.7 and 4.4 micrometers are prominently superimposed on the curve. Increasing the current supplied to the blackbody will raise its temperature changing the shape of the basic curve in accordance with the well-known Planck's Radiation Law. Referring now to FIGS. 5 and 6, in conjunction with FIG. 1, therein is illustrated a preferred arrangement of the disclosed Universal Fire Simulator apparatus which may be transported to the site of the fire sensing system being tested. The blackbody source 40, ultraviolet source 60 and discharge tube source 80 may be packaged together with several electrical circuit components into a radiation head 101 that will fit into the palm of the operators hand. The radiation head 101 includes a housing 102 which is a drawn or cast, elongated metal box closed at one end and having dimensions of approximately 11/2 inches high 11/2 inches wide and 3 inches long. To the open end of the housing 102 is attached a frame 172 into which is mounted an upper optical window 160 and a lower optical window 170. The upper window 160 is rectangular and is mounted with its long dimension along the width and its short dimension along the height of housing 102. It serves as a window for a blackbody cavity 120. The upper window 160 is made from material which is transparent to radiation in the visible and infrared spectral regions, for example, zinc selenide (ZnSe). The upper window 160 encloses about the upper five-sixths of the open end of housing 102. The lower optical window 170 is rectangular and is mounted below and parallel to the upper window 160 in the frame 172. It encloses the remaining one-sixth of the open end of housing 102. The lower window 170 serves as the window for the ultraviolet cavity 180 and is made from material which is transparent to radiation in the visible and ultraviolet spectral regions, for example, quartz. The blackbody cavity 120 is attached to the frame 172, behind the upper window 160 and extends into the interior space of the housing 102. The blackbody cavity 120 is roughly parabolic in cross section in the side elevation. In front elevation cross section, the cavity 120 is rectangular. The sides of the cavity 120 are closed by plates. The blackbody coil assembly 110 is mounted inside the cavity 120 at the approximate focus of the parabolic shape. The axis of the assembly 110 is disposed parallel to the long dimension of the cavity in the front elevation. In the side elevation, the parabolic cross section of the cavity 120 tends to gather the radiation from coil assembly 110 into a more or less parallel beam. The coil assembly 110 includes a coil of heating wire 115, for example nichrome wire, wound upon an insulating form. One or more coil assemblies 110 may be mounted with high-temperature cement within the cavity 120, depending on the energy required to be presented to the fire sensing system under test. A typical fire sensing system must react to a hydrocarbon fire contained in a 14-inch diameter pan at a distance of 60 inches. A blackbody source 40 having about one square centimeter emitting area and positioned at a distance of two centimeters from the fire sensing system's detector will simulate the irradiance from such a fire. A larger blackbody source 40 of, say 20 square centimeters emitting area such as depicted in FIGS. 5-6, could be positioned at a distance of about 4.5 centimeters from the fire sensing system's detector. The cavity 120 is hermetically sealed to the window 160 and frame 172 and is back-filled with a gas mixture 130 at room pressure. The mixture 130 is carbon dioxide, water vapor and nitrogen. Nitrogen is used instead of air as a basis of the gas mixture 130 to reduce the oxidation of the blackbody source 40 components. Electrical leads which supply current to the coil assembly 110 are passed through the wall of the cavity 120 by use of conventional, hermetically sealed, feed-through insulators. Commercial blackbody devices generally have high emissivity cavities for efficient operation. However, in the present embodiment, high emissivity is not required because the signal produced by the heating wire coil is sufficiently large at the short distances at which the apparatus is placed from the fire sensing system under test. Emissivity may be increased, if desired, by proper blackening of the inside of reflector 120, blackening of heater assembly 110, or both. A reflector 180 is mounted to the frame 172 behind the lower window 170 and extends into the interior space of the housing 102. Within the reflector are mounted a commercially obtainable ultraviolet bulb 140 and commercially obtainable gas discharge tube 150. The reflector 180 is smaller than the cavity 120 but has a similar parabolic profile in the side elevation. The reflector 180 is rectangular in cross section in the front elevation and has closed sides. The reflector 180 does not require hermetic sealing because the ultraviolet bulb 140 and gas discharge tube 150 are themselves sealed. Electrical leads which supply current to the ultraviolet bulb 140 and gas discharge tube 150 are fed through the reflector 180 and join the wires leading away from the cavity 120. The electrical leads are formed into a wire bundle 108 leading out of the housing 102 through a connector 106 at the closed end of the housing. A mode switch 104 which provides for selection of the sequence and timing of the transmitters 100, 200, 300, is mounted on the closed end of the housing 102 and electrical leads which connect the mode switch 104 to the on-off control circuit 12 also join the wire bundle 108. Still referring to FIGS. 5-6 in conjunction with FIG. 1, the preferred arrangement of the radiation head 101 contains a high-voltage storage capacitor 109 which is part of the high voltage source 70 which is used to "flash" the explosion-simulating discharge tube 150. The storage capacitor 109 is mounted inside the housing 102 in the space between the cavity 120 and the closed end of the housing 102. Leads from the storage capacitor 109 join the wire bundle 108. The wire bundle 108 is connected by suitable cabling to the power amplifiers 30, 50, the high voltage source 70, the low-frequency white noise generator 20 and the on-off control circuit 12 which are located remotely from the radiation head 101. In normal use, the radiation head 101 is disposed in the field of view of the fire sensing system under test and within a few centimeters of a detector window of such system. The gains of the first and second power amplifiers 30, 50 are preset to produce the desired ratio of respective radiation outputs 42, 62 from the blackbody source 40 and ultraviolet source 60. The mode switch 104 is set to select the desired sequencing of the sources 40, 60, 80 and the on-off control circuit 12 is energized. Although the invention has been described in detail with reference to a particular embodiment, persons possessing ordinary skill in the art to which this invention pertains will appreciate that various modifications and enhancements may be made without departing from the spirit and scope of the invention.