Apparatus for Wide-Area Fire Detection

This invention relates to wide-area fire detection and especially to the detection of forest fires.

Wide-area fire detector apparatus as known, e.g., from European Patent Document EP-A1-0298182, serves for the localization of infrared radiation emitted by objects at a temperature in a range of approximately 300.degree. to 1500.degree. C. in a surveillance area extending several kilometers. Such apparatus is particularly suited for the detection of forest fires from a central observation point in a large forested area. Included in such apparatus is a scanning device with azimuthal freedom of movement and with an optical focusing device, e.g., a reflector, for directing infrared radiation from forest fires in a number of detection areas onto a corresponding number of detector elements. Such detector elements are arranged closely spaced in a row perpendicular to the reflector axis. When the detector apparatus is rotated or panned azimuthally and approximately horizontally about an approximately vertical axis, a number of concentric detection areas result which have different elevation or inclination to the horizontal, and which are periodically scanned as the apparatus turns. If a detector apparatus is installed at an elevated location, e.g., on a mountaintop or on a tall mast, an area extending several kilometers can be monitored by a single detector apparatus for infrared radiation originating from forest fires. The site of a fire can be determined and reported by means of a suitable evaluation circuit.

It is a drawback of such known apparatus that the sensitivity of detection decreases with increasing distance, i.e., with decreasing elevation or inclination of a detection area from the horizontal. In other words, detection of a fire is more difficult at a distance than at close range. In accordance with German Patent Document DE-A1-3710265, this disadvantage is avoided when a detector apparatus moves not only azimuthally, but also by periodic variation of the angle of elevation. During such vertical panning movement, the focal length of the focusing device is automatically adjusted, as a function of the angle of elevation, to maintain approximately constant infrared detector resolution in the entire surveillance area. This requires complicated and failure-prone control means with additional movable components. As a result, long-term operation at a remote location is virtually impossible, as the apparatus requires frequent service.

A further disadvantage of such known forest-fire detectors lies in their susceptibility to parasitic infrared radiation from extraneous sources, especially to direct or reflected solar radiation. While the intensity peak of solar radiation lies in the range of visible light, solar intensity in the infrared range, i.e., in the range of thermal radiation from a forest fire, can be strong enough to erroneously trigger a fire alarm signal. Even diffuse light can have such strong infrared component triggering a false alarm.

The invention provides wide-area fire detector apparatus with reduced dependence of the detector sensitivity on the distance to a fire site, and with reduced likelihood of malfunction due to parasitic radiation having a radiation maximum in another spectral range. For the elimination of parasitic radiation, the infrared-sensitive detector elements are arranged pair-wise in differential circuits. And, alone or in combination with such pair-wise arrangement, additional, light-sensitive detector elements are included with corresponding infrared-sensitive detector elements in an inhibition circuit for eliminating solar radiation.

Preferably, the detector elements are formed and/or arranged such that detector sensitivity does not decrease significantly with decreasing angle of inclination from the horizontal, of the detection areas formed by the detector elements and the optical focusing apparatus.

Particularly advantageous are provisions for increased detector sensitivity, as a function of decreasing angle of elevation (and thus of increasing distance), including increased detector receiving areas or an increased number of equal-area detectors for detection at greater distances. Advantageous further, for achieving distance-independent sensitivity, is the provision of different degrees of amplification in the evaluation circuits for different detector elements, as a function of the angle of elevation of the corresponding detection regions.

Advantageously, several groups of detector elements are combined into an optical assembly on a common support in a row perpendicular to the optical axis of the assembly, with groups close to the optical axis (and serving for long-range detection) having a lesser vertical extent, a lesser receiving area, or a lesser number of detector elements as compared with groups at a greater distance from the optical axis (and serving for close-range detection).

Preferably included with the infrared-sensitive detector elements are light-sensitive detector elements in differential circuits for screening out solar radiation. Preferably, the former are sensitive to radiation in the spectral range of approximately 3-5 micrometers, and the latter to radiation in the spectral range of approximately 0.6-1 micrometer, i.e., in the visible and near infrared range. When such light-sensitive detector elements are connected to the infrared-sensitive detector elements in inhibition circuits, alarm signals are blocked when the light-sensitive detector elements receive optical radiation of at least a predetermined intensity. Thus, high-intensity optical radiation will not be reported as from a fire.

FIG. 1 is a schematic side view of fire detector apparatus in accordance with a preferred embodiment of the invention;

FIG. 2 is a schematic top view of the apparatus of FIG. 1 and of its area of surveillance;

FIG. 3 is a schematic front view of a scanner assembly of a preferred fire detector apparatus;

FIG. 4 is a cross section of the scanner apparatus of FIG. 3;

FIG. 5 is a front view of an assembly of detector elements in apparatus of FIG. 1 and 2;

FIG. 6A through 6D are interconnection circuit diagrams for infrared detectors included in the assembly of FIG. 5;

FIG. 7 is an interconnection circuit diagram for optical detectors included in the assembly of FIG. 5; and

FIG. 8 is a flow chart for an exemplary signal processor using signals from infrared and optical detector circuits.

The apparatus of FIG. 1, for the detection of forest fires in an area B having an extent of several kilometers, comprises a scanning device 1 disposed at an elevated location of the surveillance area, e.g., on a mountaintop, on an observation tower 2, or on a mast. The scanning device 1 rotates or pans continuously and azimuthally about its vertical axis, periodically covering the entire surveillance area, receiving infrared radiation from the surveillance area by means of an optical assembly 3, and directing the radiation onto a detector assembly 4 which is connected to a suitable evaluation circuit for triggering an alarm signal when the detector assembly receives infrared radiation from the surveillance area characteristic of a forest fire.

As can be appreciated with reference to FIG. 2, the optical assembly 3 and detector assembly 4 are constructed and mutually disposed such that a number of separate, adjoining detection areas R1, R2, . . . , R8 are formed, concentric with respect to the location of the detector or scanning device, and with different elevation angles b1, b2, . . . , b8 with the horizontal H. Infrared radiation is separately received from and evaluated for these detection areas, so that, by means of the evaluation circuit, the azimuth a and distance d of a forest fire F can be determined and reported.

FIG. 3 and 4 show the construction of the scanning device 1 in further detail. Included, for focusing of infrared radiation arriving from the detection areas, is a spherical or parabolic reflector 6 and a detector support 7 for a number of detector elements S1, S2, . . . , S8 disposed at least approximately in the focal plane of the reflector 6. The axis A of the reflector 6 is horizontal or at a slight tilt with the horizontal, corresponding to the maximum detection distance, i.e., to the angle of elevation of the detection area R1 farthest away. The detector support is disposed asymmetrically relative to the optical axis A and extends upward for a distance, approximately from the axis A, such that only radiation from areas below the horizontal H are practically detected.

A number of detector elements S1, S2, . . . , S8 are provided radially on the detector support, forming separate radiation-sensitive zones, chips or "flakes" (of lithium tantalate, for example) whose output signals (to be evaluated separately) correspond to the radiation from the different detection areas (having different angles of elevation). The detector support 7 is located behind a window which is substantially transparent to thermal radiation from objects having a temperature of approximately 300.degree. to 1500.degree. C., so that, advantageously, the detector assembly responds only to radiation characteristic of a forest fire. Preferably, the window serves as an optical bandpass filter for passing 3- to 5-micrometer infrared radiation.

The above-mentioned spectral window has proven particularly advantageous because air is substantially transparent in its range, so that infrared detection is feasible over long distances. This is in contrast to the range from 5 to 8 micrometers where atmospheric absorption is considerable, with radiation from remote areas much attenuated and of limited utility for evaluation, and with severely limited detector range. Radiation at yet-greater wavelengths is likely to be parasitic radiation from objects having a temperature which is only slightly elevated. For example, such radiation may originate with automobile engines or from field or forest areas heated by intense sunlight.

FIG. 5 shows the detector support on an enlarged scale and in further detail. The detector elements S are in the form of closely spaced flakes which are grouped pair-wise into zones whose length increases from bottom to top. The bottom-most group or zone Z1 serves for remote detection and includes just two flakes S1 and S1' which are differentially connected, in a dual circuit shown in FIG. 6A, to the input terminal FET of a signal evaluation circuit. The same type of circuitry is provided for each of the adjacent groups Z2, Z3, Z4. The group Z5, on the other hand, includes two pairs of flakes, namely the four detector elements S, S', S" and S'" in a differential quad-circuit shown in FIG. 6B. The further groups Z6 and Z7 each include eight detector elements in a differential double-quad-circuit shown in FIG. 6C. The top-most group Z8, serving for close-range detection, has the greatest vertical extent and consists of fourteen flakes which are grouped into seven pairs which are connected in a differential circuit shown in FIG. 6D. These differential pair- or dual-circuits serve to eliminate environmental influences which affect the two sensor elements of a pair equally. This applies, e.g., to intense ambient light reaching a pyroelectric broad-band detector to a non-negligible degree with radiation in the passband range of 3 to 5 micrometers equally affecting the two paired detector elements. In contrast, radiation from a localized fire site is sensed at slightly different times during a panning sweep by these detector elements, so that the differential circuit produces a signal which includes a positive and a negative pulse (steady-state signal=zero).

Due to increased height of detector zones towards the upper end of the detector array Z1, Z2, . . . , Z8, each zone corresponds roughly to an equal distance range R. Furthermore, due to different parasitic capacitance in the different circuits for the detector elements of different zones (i.e., in the dual-, quad-, double-quad circuits, etc.), detection sensitivity is largely independent of distance, or may even increase with increasing distance, thereby providing compensation for increasing atmospheric radiation absorption.

Further features serve for the exclusion of direct or indirect solar radiation which, typically, is quite intense in areas subject to forest-fire danger, and which at times exceeds 10.sup.5 lux. Solar radiation in the range of infrared radiation used for fire detection, i.e., with wavelengths between 3 and 5 micrometers, can reach levels triggering an alarm even in the absence of fire. Accordingly, the prevention of false alarms due to parasitic solar radiation is called for. For this purpose, a row C of light-sensitive solar cells is provided parallel to the row S of infrared-sensitive detector elements. Preferably, the peak sensitivity of the solar cells is between 0.6 and 1 micrometer. Analogous to the infrared detector elements, the solar cells are also paired as C1, C1'; . . . ; C8, C8' in differential connection. For example, as shown in FIG. 7 for the light detectors in group Z5, two photodiodes C5 and two photodiodes C5' are connected in parallel, the photodiodes C5 having anodes at ground, the photodiodes C5' having cathodes at ground, and connections being provided to a resistor R1 and to the input terminal (-) of an operational amplifier 71 which is supplied with operating voltages V+ and V-. With additional resistors R2 and R3 connected as shown, the circuit is adapted to produce an output voltage U-out corresponding to a photocurrent I-in. Specific components may be chosen as follows, for example: Siemens photodiodes SFH 983-F260C, Texas Instruments operational amplifier TL064C, R1=12.7 kilo-ohm, R2=100 kilo-ohm, R3=33 kilo-ohm.

The pairs of solar cells C are connected with corresponding groups of infrared detector elements S in inhibition circuits for blocking an alarm signal when sufficiently strong parasitic radiation is detected from the corresponding detection region (i.e., when the intensity of parasitic radiation exceeds a predetermined threshold). An inhibition circuit may be realized by software for execution by a microprocessor with memory, included with fire detector apparatus. Such software may be as schematically represented by FIG. 8, where the following features are included: a resettable clock; an infrared-signal alarm threshold value, ts; a light-signal threshold value, tc; a sampling time interval, delta; and the number of samples to be taken per sweep, n. Actual sample amplitude values s and c are as obtained, respectively, from the infrared-detector circuit of FIG. 6B and the light-detector circuit of FIG. 7. A half-minute sweep (through 360.degree., for example) may involve taking n=2.sup.12 =4096 samples s and c, with delta=7.32 msec.

This feature provides for protection against unnecessary expense for fire-fighting measures due to false alarms. Even greater protection is provided when a controllable TV camera is installed at the location of observation, which, when a fire alarm signal is produced by the fire detector apparatus, is automatically aimed at the localized fire site for visual verification.

The invention described above for the detection of forest fires is further applicable for monitoring other extended areas or lots for sources of infrared radiation. Examples are the monitoring of fuel depot areas and of automobile parking lots.