LIGHT VEHICLE RANGE DISCRIMINATOR

Abstract: An intrusion detection apparatus (10) gives a signal when a light vehicle approaches a geophone (12), and a heavy veh­ icle discriminator (15) disables the detection apparatus when the approaching vehicle is a heavy one.

Description: This invention relates to geophone apparatus for use with land mines. Occasion sometimes arises to interdict the movement of vehicles in an outdoor area. This may be done by distributing mines, throughout the area, which function upon firing to disable a vehicle within an effective range from the mine. Mines are accordingly equipped with geophone apparatus for detecting the approach of targets within the range thereof. It is desirable that such mines be not readily perceptible, and thus avoidable, by approaching vehicles: passive vehicle detecting arrangements are accordingly preferred. In this respect, seismic devices such as geophones, supported on or buried in the ground and responsive to vibrations in the earth caused by the vehicles, have been found quite satisfactory, as is taught in US-A- 4,468,763. A difficulty arises, however, since in such arrangements the seismic signal due to a light wheeled vehicle near to the geophone is not readily distinguishable from the seismic signal due to a heavy tracked vehicle more remote from the geophone. Some land mines are also known to be effective against light vehicles, such as jeeps, but to be relatively ineffective against heavier vehicles, such as armored tanks. It is inefficient to cause a mine to fire when its target is too remote for the mine to be effective, or when the target is one against which the mine is ineffective, since mines are expensive, high-technology devices. US-A 35 85 851 describes a seismic sensing apparatus, but having separate high frequency and low frequency sensors. This sensing apparatus is used to distinguish between seismic signals generated by a man walking into a particular area and signals generated by a vehicle such as a tank approaching that area. A counter is connected to the high frequency sensor for counting high frequency seismic pulses generated by a person walking into said area. The second sensor responds to low frequency seismic signals generated by a vehicle. An intrusion of the area by walking people is indicated when a predetermined number of high frequency pulses are received in the absence of low frequency signals. For conditioning the output signal of each of the sensors an amplifier, a rectifier and a filter is connected to the output of each sensor. An apparatus has been developed for receiving seismic vibrations from an approaching vehicle, distinguishing whether the seismic source is a heavy vehicle or a light vehicle, and giving a control signal if the source is a heavy vehicle. One arrangement for accomplishing this is described in prior patent application EP-A-0 209 782 published 28.1.1987, another one in DE-A-33 06 155. The present invention discloses an improvement over EP-A-0 209 782 and permits detecting the approach of a light vehicle within a predetermined range from the sensor. The present invention comprises seismically responsive means for detecting the presence of approaching vehicles, means distinguishing seismic signals due to heavy vehicles, means giving a control signal when a light vehicle is within a predetermined range, and means inhibiting the control signal if the seismic signal source is determined to be a heavy vehicle. Further embodiments of the invention are pointed out in the dependent claims. For a better understanding of the invention, its advantages, and objects attained by its use, reference should be had to the drawing in which there is illustrated and described a preferred embodiment of the invention. BRIEF DESCRIPTION OF THE DRAWING In the drawing, FIGURE 1 is a block diagram of the combination of elements making up the invention, and FIGURE 2 is a more detailed showing of the apparatus of FIGURE 1. DESCRIPTION OF THE PREFERRED EMBODIMENT Referring first to FIGURE 1, an intrusion detecting device 10 is connected at 11 to a geophone 12, and gives an output 13 when an intruding vehicle is detected by the geophone. Also connected to geophone 12, at 14, is a heavy vehicle discriminator 15, which gives output 16 when a heavy vehicle is detected. Outputs 13 and 16 are connected to a logical AND 18 so that a control signal is supplied at 19 when a light vehicle at proper range is detected, but is inhibited if the seismic source is a heavy vehicle. Signal 19 may be used to control the firing of a mine with which the geophone is associated, or for any other desired purpose. Turning now to FIGURE 2, one form of an intrusion detector 10 is shown to comprise a low frequency channel 111 including a preamplifier 112 connected at 11 to geophone 12 and supplying a signal through a bandpass filter 114 having a pass band of about 1.5 to 8.5 Hertz, an isolation amplifier 116, and a full wave rectifier 118 as an input 119 to a short-term averager 122 having a time constant of about 4 seconds and an input 123 to a long-term averager 124, having a time constant of about 12 seconds. The averager outputs 126 and 128 respectively are supplied to a comparator 120, together with the output 130 of an offset device 132, and comparator 120 gives signal 13 when a light vehicle is within a predetermined range of the geophone. FIGURE 2 also shows that in discriminator 15 geophone signal 14 is supplied to a wide band preamplifier 214, which energizes bandpass filters 216 and 218 with signals 220 and 222. Filter 216 is a part of a high frequency branch 223, and preferably has a pass band of about 20 Hertz to 150 Hertz. Filter 218 is part off a low frequency branch 224, and preferably has a pass band of about 1.5 Hertz to 8.5 Hertz. Branch 224 has a considerably higher gain than channel 111 of detector 10. The output 225 from filter 216 is fed to a full wave rectifier 226 which supplies inputs 228 and 230 respectively to a first low pass filter 232, acting as a short-term averager, and a second low pass filter 234, acting as a long-term averager. Filter 232 preferably has a time constant of about 8 seconds, and filter 234 preferably has a time constant of about 16 seconds. The outputs 236 and 238 respectively of filter 232 and 234 are supplied to a comparator 240, together with the output 242 of an offset device 244. The output 246 from filter 218 is fed to a full wave rectifier 248 which supplies inputs 250 and 252 respectively to a first low pass filter 254, acting as a short-term averager, and a second low pass filter 256, acting as a long-term averager. Filter 254 preferably has a time constant of about 4 seconds, and filter 256 preferably has a time constant of about 12 seconds. The outputs 258 and 260 respectively of filters 254 and 256 are supplied to a comparator 262, together with the output 264 of an offset device 266. The output 268 of comparator 240 is fed through an inverter 270 to comprise a first input 272 to an OR circuit 274, having a second input 276 from an oscillator 278 and a feedback input 280 comprising the output of a counter 282 energized at 284 from OR circuit 274. The output of counter 282 is also supplied at 286 to an inverter 288 which supplies a first input 290 to an AND circuit 292. A second input 294 to AND circuit 292 is supplied by comparator 262, and the output 296 of AND 292 is supplied through a latch 298 and an inverter 300 to constitute input 16 to AND circuit 18. In computer terminology the outputs of counter 282, AND circuit 292, latch 298, and comparators 120, 240, and 262 are normally LOW, and those of inverters 288 and 300, OR circuit 274, and inverter 270 are HIGH. OPERATION A moving heavy vehicle acts as a source of seismic vibrations of two types, identified respectively as Rayleigh waves and pressure waves. The Rayleigh waves occur in a high frequency band and a low frequency band, while the pressure waves occur only in a low frequency band, which is the same however as that of the low frequency Rayleigh waves. Pressure waves are more subject to attenuation than are Rayleigh waves, so that for an approaching heavy vehicle the Rayleigh waves are detected the sooner. A moving light vehicle acts as a source of seismic vibrations of the same types, except that there is no significant low frequency band of Rayleigh waves. The amplitudes of the light vehicle signals are also smaller, so that they are detected after a greater delay than the heavy vehicle signals. When a geophone is coupled to the ground, there is always a seismic signal at a background level. When this is rectified and sent to averaging circuits such as 122 and 124, the outputs of these circuits continue generally at constant levels. By suitably selecting the components of offset apparatus such as 132, the inputs to a comparator such as 120 may be so set that in the absence of any signal from an approaching vehicle, the long-term average 128 with offset 130 is always greater than the short-term average 126, so that comparator 120 produces no output 13. In the same manner, offset devices 244 and 266 can be set so that in the absence of any signal from an approaching vehicle, long-term averages 238 and 260 with offsets 242 and 264 are always greater than short-term averages 236 and 258, and comparators 240 and 262 provide no outputs. When any vehicle approaches a geophone , the seismic signals in general increase as the vehicle moves closer. If a light vehicle approaches the geophone, the initial low frequency seismic signal consists of pressure waves. If a heavy vehicle approaches the geophone, the initial low frequency seismic signal consists of Rayleigh waves. Because of lesser soil attenuation, low frequency Rayleigh waves travel much further than low frequency pressure waves. The low frequency seismic component to channel 111, whether Rayleigh or pressure waves, causes the signal produced by rectifier 118 to increase, so that outputs 126 and 128 both increase. However, because of the difference in time constants in circuits 122 and 124, the output of the former can increase more rapidly than the output of the latter until the relation of the signals to comparator 120 reverses, and the comparator now gives an output 13. The distance, from the geophone to the vehicle, at which output 13 is given may be adjusted by offset device 132, which is set so that the distance is small enough for a light vehicle approaching the mine to be incapacitated. If the approaching vehicle is a heavy one, the initial seismic input contains low Rayleigh waves, and because of their lesser attenuation, the output 13 from comparator 120 occurs at an earlier time, when the heavy vehicle is far from the geophone which renders ineffective mine operation. To prevent this, output 13 is supplied to the mine through AND circuit 18, which does not give its output 19 in response thereto unless discriminator signal 16 is also present, identifying the approaching vehicle as a light one, as will now be described. If the approaching vehicle is a heavy one, the low frequency Rayleigh waves act in channel 224 to cause comparator 262 to go HIGH at a time t₁, and the high frequency Rayleigh waves act in channel 223 to cause comparator 240 to go HIGH at a time t₂. It has been determined that, for heavy vehicles, time t₁ may occur before or after time t₂, but that time t₁ always occurs before a time t₃ which is later than t₂ by about 4.3 seconds. If t₁ precedes t₂, the output 294 of comparator 262 can pass through AND 292 to comprise a HIGH output at 296, which passes through and is held in latch 298, disabling AND 18 through inverter 300. If t₂ precedes t₁, the output of comparator 240 goes HIGH, that of inverter 270 goes LOW, and pulses from oscillator 278 may now pass through OR 274 to counter 282. After a predetermined count is reached, corresponding to 4.3 seconds, counter outputs 280 and 286 go HIGH, the former interrupting further counting and the latter acting through inverter 288 to disable AND 292, and thus prevent any later operation of comparator 262 from affecting latch 298. An approaching light vehicle will not act through discriminator 15 to disable AND 18 because for light vehicles comparator 262 does not give its output 294 until after the 4.3 second delay in counter 282 has disabled AND 292. From the above it will be evident that branches 223 and 224 of channel 15 cooperate to supply signals which actuate latch 298 at a value which disables AND 18 if an approaching vehicle is a heavy one. Channel 111 supplies a signal at 13 to AND 18 whether the vehicle is heavy or light. Since channels 224 and 111 have much the same characteristics, it is desired that channel 224 operate sooner than channel 111, which can be accomplished by giving it a higher gain. From the above it will be evident that the invention comprises intrusion detection apparatus for giving a signal when a light vehicle approaches a geophone, together with apparatus for disabling the detection apparatus when the approaching vehicle is a heavy one.

Claims: A light vehicle detector comprising: a) a single geophone (12) adapted to respond to seismic vibrations in the ground; b) two signal processing channels (216, 226, 232 to 240; 218, 248, 254 to 262) connected to said single geophone (12); whereby c) the first channel comprises a first band pass filter (218) having a low frequency pass band and the second channel includes a second band pass filter (216) having a higher frequency pass band; d) a full wave rectifier (248, 226) is connected to the output (246, 225) of each band pass filter; e) two low pass filters (254, 256; 232, 234) with different time constants are connected to the output of each rectifier (248, 226) with the low pass filter (254, 232) having the shorter time constant forming a short term averager and the low pass filter (256, 234) having the longer time constant forming a long term averager; f) a comparator (262; 240) in each channel receives the output signals (258, 260; 238, 236) of its two associated low pass filters (254, 256; 232, 234) together with an offset signal (264, 242) compensating background noise signals; g) the output (294) of the comparator (262) in the channel including the low frequency band pass filter (218) is connected to one input of a first AND-gate (292); h) the output (268) of the comparator (240) in the channel including the higher frequency band pass filter (216) is in communication with a first input (272) of an OR-gate (274); i) a second input (276) of said OR-gate (274) is connected to an output of an oscillator (278); j) the output of said OR-gate (274) is connected to a counter (282) providing an output signal as soon as a predetermined count is reached; k) the output of said counter (282) is in communication with a second input (290) of said first AND-gate (292) and with a third input (280) of said OR-gate (274); l) a latch circuit (298) is connected to the output (296) of said first AND-gate (292) inhibiting said output (100) signal (296) of said first AND-gate (292) when said first signal (268) precedes said second signal (294) by more than a predetermined interval; m) a third signal processing channel (111, 10) is also connected to said single geophone (12); n) said third channel is a low frequency channel (111) and comprises: n1) a third band pass filter (114) and a third full wave rectifier (118); n2) a short-term averager (122) and a long-term averager (124) both connected to the output of said third full wave rectifier (118); n3) a third comparator (120) receiving the output signals of the two averagers together with an offset signal (130), said third comparator giving an output signal when a light vehicle is within a predetermined range of the geophone; o) said latch circuit (298) is connected between said first AND-gate (292) and one input of a second AND-gate (18) having its other input (13) connected to the output of the third comparator (120) in the third channel (10) and delivering at its output (19) a signal if the vehicle is a light one within a predetermined range from the geophone. The detector of claim 1, characterized in that a) the geophone (12) gives first signals in response to Rayleigh waves in a higher frequency band, and gives further signals in response to seismic vibrations in a lower frequency band; b) the third signal processing channel (10) connected to receive said further signals gives an output when the long term average thereof exceeds the short term average thereof by a predetermined amount; and c) the output signal of said latch (298) disables the output of said third channel when the long term average of said first signals exceeds the short term average thereof at a time before or within a predetermined interval after the time when the long term average of said first signals exceeds the short term average thereof by a predetermined amount. The detector of claim 1 or 2, characterized in that a) the third band pass filter (114) has a pass band of about 1,5 to 8,5 Hz; b) the short-term averager (122) has a time constant of about 4 sec.; c) the long-term averager (124) has a time constant of about 12 sec.; d) the first band pass filter (218) has a pass band of about 1,5 to 8,5 Hz; e) the second band pass filter (216) has a pass band of about 20 to 150 Hz; f) the low pass filters (254, 256) in the first channel have time constants of 4 and 12 sec., respectively; and g) the low pass filters (232, 234) in the second channel have time constants of 8 and 16 sec., respectively.

Inventors: BRAUNLING, RUSSELL D., DR.
Applicants: HONEYWELL INC.
Application year: 1986
Main classification: G01V1
IPC codes: F42C13, G01V1, G08B13