GB2159995A - Vehicle detection system - Google Patents

Abstract

A seismic vehicle detection system which determines the passage of vehicles from phase cancellation between responses of two geophones G1, G2. A magnetic sensor M having a relatively limited range excludes all seismic responses from remote sources, these being known to produce confusing and indeterminate results. <IMAGE>

Description

SPECIFICATION Vehicle detection system This invention relates to a vehicle detection system employing seismic sensing means.lt has been previously proposed to detect the passage and sometimes classification of vehicles by means of seismic detectors positioned adjacent a vehicle route. Figure 1 shows one such known arrangement in which seismic detectors G1 and G2 are spaced apart in the direction of vehicle travel and about five metres, say, from the vehicle carriageway. The seismic detectors are buried together with processing circuitry 'S' to which the two sensing units are connected. Each seismic unit contains a geophone and low frequency amplifier while the processing circuitry 'S' contains essentially low frequency signal processing and logic circuits. The system is powered by a battery.For a vehicle approaching from the left the resultant ground vibrations at G2 lag in phase those at G1, by an angle determined by the spacing 'D' (typically, one metre) and the surface wave velocity. At 20 Hz this phase angle is typically 20 . When the vehicle is symmetrically opposite the mid-point of G1 and G2 the phase lag is zero and as the vehicle continues towards the right the phase lag reverses and the signals at G1 lag those at G2. Circuits in the unit 'S' detect this phase transition which should thus indicate a vehicle on the line of symmetry between the seismic detectors.

The seismic energy generated and the consequent amplitude of the seismic signals received provide an indication of vehicle classification, i.e., an indication of tracked or wheeled vehicle.

It has been found that detection errors occur in some circumstances particularly in the detection of military tracked vehicles slowly approaching the detection system. A phase transition may be detected and the passage of a vehicle signalled when the real vehicle is in fact still several hundred metres from the detection system. The cause of such false detections is not fully understood. Errors can also arise from seismic effects of low flying aircraft and also from battlefield noise.There is thus a considerable degree of uncertainty in the response of the basic arrangement.

We have considered improving the selectivity of the system by employing infra-red detectors in conjunction with the seismic detection. A major disadvantage of this arrangement is however, that the infra-red equipment is visible to an enemy and is also subject to damage and interference by vehicles, animals etc.

An object of the present invention is therefore to nullify the effect of seismic sources remote from the detection system in a manner which lends itself to security and durability.

According to the present invention, a vehicle detection system employing seismic sensing means responsive to vehicle movement within a predetermined range, further comprises magnetic sensing means responsive to magnetic anomaly within a range significantly smaller than said predetermined range, the two sensing means being arranged to cooperate so that a positive vehicle detection requires a response from both sensing means.

Preferably, said seismic sensing means is responsive to the direction of vehicle movement past the seismic sensing means. The system may comprise two seismic sensing units spaced apart in a direction of vehicle travel and means for detecting the relative phase of seismic signals received simultaneously at the two sensing units.

Preferably, said seismic sensing means is responsive to the seismic energy of the vehicle to produce an indication thereof.

Said magnetic sensing means may be a flux-gate magnetometer coupled to said seismic sensing means to inhibit an output therefrom except in the presence of a response from the magnetometer.

According to another aspect of the invention in a method of detecting vehicle movement, seismic signals arising within a predetermined range are sensed, magnetic signals arising within a significantly smaller range are sensed, and the seismic signals are accepted as an indication of vehicle movement only in the presence of the magnetic signals.

A vehicle detection system employing seismic and magentic sensing means in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, of which: Figure 1 is a diagram of a known seismic detection system; Figure 2 is a diagram of a similar system incorporating a magnetic sensing means; Figure 3 is a block diagram of the logic circuit of the system; and Figure 4 is a flow chart of the system operation.

Figure 1 shows a known system briefly described above. The circuitry 'S' provides an indication of vehicle direction and also of vehicle classification in accordance with its generation of seismic energy.

Figure 2 shows a similar system in which two seismic sensors, geophones G1 and G2, are buried close, within five metres say, to a vehicle route 1, the geophones being spaced about a metre apart in the direction of vehicle movement. Buried symmetrically between the two geophones is a fluxgate magnetometer M arranged to sense in a vertical plane. Also buried with the equipment is processing circuitry S. The magnetometer M can typically detect perturbations in ambient flux density of less than one part in 10,000. Such a perturbation results when a typical vehicle passes within a range of about 12 metres.Since the perturbation magnitude is approximately inversely proportional to the cube of the vehicle range only very close vehicles are detected by the magnetometer despite its sensitivity within that range.As shown in Figure 2 the area of influence, 2, of the magnetometer extends over the local road area.lt will not detect vehicles, even large tracked vehicles, at any remote location.

The magnetometer M and geophones G1 and G2 are coupled to the processing circuitry S which, as will be explained, is such that a positive detection of a vehicle is only obtained when both magnetometer M and geophones G1 and G2 respond sufficiently to give a vehicle indication.

Referring to Figure 3, this shows, schematically, the apparatus of the system, and particularly of the processing circuitry S. The two geophone outputs are sampled periodically in block 13 and a phase comparison is made. A positive result of this comparison indicates a vehicle to the left, say, at the symmetrically opposite position 15 (the target positon) on the route and a negative result indicates a vehicle to the right. A sufficiently small phase difference between the two outputs indicates a vehicle on or close to the above target position 15, or would do, subject to the above mentioned anomalies of distant vehciles and extraneous vibration sources.

The amplitude of the G1 and G2 outputs is also assessed in block 13 to give an indication of the weight and perhaps speed of the vehicle.

The resulting two outputs, indicative of the phase difference and the amplitude of the seismic signals, are applied to linear analogue gates 17 and 19.

The sampling frequency of the geophone outputs is such that a number of samples are obtained while the vehicle is in the vicinity of the target position 15. As a vehicle passes through this position therefore, the polarity of the phase differences will change.

The successive output signals from gates 17 and 19 (when enabled) are applied to vehicle classification and direction circuitry 23. The vibration amplitude is here decoded as vehicle type, the two being fairly closely correlated.

The successive phase difference signals (polarity) are stored temporarily in the block 23 and on the occurrence of a reversal of polarity, the direction of the transition, positive to negative or negative to positive, is determined to provide an indication of the direction of vehicle travel.

The two signals, vehicle classification and travel direction, are applied to transmitter Tx for transmission to remote receiver and monitor equipment 25 where it is stored and displayed. The link between transmitter and receiver may be by radio or line.

Detection and assessment of a vehicle presence as above does depend upon the gates 17 and 19 being enabled.The enabling signal on line 21 is derived from an amplitude threshold detector circuit 27 which derives an input from the magnetic sensor M and measures it against a threshold corresponding to a predetermined range of about twelve metres, as indicated by the broken line 2 in Figure 2. This range is significantly smaller than the range of the seismic detectors G1 and G2. Consequently, the gates 17 and 19 will be disabled for all vehicles outside the twelve metre range. False detections, of distant vehicle sources and other extraneous seismic disturbances will in general be excluded.

Referring to Figure 4 this flow chart shows the effective logic decisions made by the circuitry. First a check is made for the detection of a seismic phase transition in block 3. This check is made on a periodic basis. If output is negative no further action is taken until the check is made again.lf the output is positive a check (5) is made for an output from the magnetometer. If the output from this check is negative no further action is taken and the seismic transition is ignored. If a sufficient magnetic output is obtained, indicating a vehicle in the near vicinity then the nature of this transition is determined in an assessment, 7. The seismic signal amplitude is determined in an assessment 9, and the two outputs provide information, 11, of vehicle direction and classification which may be transmitted by radio to a display monitor at a remote position. A cable link may be used alternatively.

It will be apparent that one or both of the vehicle direction and vehicle classification may be omitted, although clearly a more complete picture of the situation is obtained, especially for military purposes, if both indications are obtained.

Claims (8)

1. A vehicle detection system employing seismic sensing means responsive to vehicle movement within a predetermined range, the system also comprising magnetic sensing means responsive to magnetic anomaly within a range significantly smaller than se d predetermined range and the two sensing mean being arranged to cooperate so that a positive vehicle detection requires a response from both sensing means.

2. A system according to Claim 1, wherein said seismic sensing means is responsive to the direction of vehicle movement past the seismic sensing means.

3. A system according to Claim 1 or Claim 2, wherein said seismic sensing means is responsive to the seismic energy of the vehicle to produce an indication thereof.

4. A system according to Claim 2, comprising two seismic sensing units spaced apart in a direction of vehicle travel and means for detecting the relative phase of seismic signals received simultaneously at the two sensing units.

5. A system according to any preceding claim wherein said magnetic sensing means is a fluxgate magnetometer coupled to said seismic sensing means to inhibit an output therefrom except in the presence of a response from the magnetometer.

6. A method of detecting vehicle movement, in which seismic signals arising within a predetermined range are sensed, magnetic signals arising within a significantly smaller range are sensed, and the seismic signals are accepted as an indication of vehicle movement only in the presence of said magnetic signals.

7. A vehicle detection system substantially as hereinbefore described, with reference to Figures 2 & 3 of the accompanying drawings.

8. A method of detecting vehicle presence substantially as hereinbefore described, with reference to Figures 2 & 3 of the accompanying drawings.