DETECTOR

Abstract: The panel is set in vibration by an excitor (22) and changes in the pattern of vibration due to the loading or damping of the panel by an attached device are detected. The input from piezoelectric detectors is passed by amplifier (12) and analog-to-digital converter (13) to a microprocessor (14). Under control of programs held by EPROM (16) the inputs from successive test cycles are analyzed and compared to determine whether a significant change in the condition of the panel has recurred, in which case an alrm is given by an indicator (25).

Description: DETECTOR The present invention concerns equipment for detecting a device, such as a car bomb, attached to a panel of a vehicle or the like. Car bombs are the most efficient way which terrorists have devised to kill or maim their victims. It is a method which is used frequently in Northern Ireland. Except on relatively rare occasions these devices have been attached to the vehicle by quite powerful permanent magnets. Several systems relying on the detection of the magnetic fields emitted by the magnets have been devised to detect these bombs. Clearly, these systems can only detect those devices which have been attached magnetically and are thus extremely limited in scope. The present invention seeks to overcome this disadvantage. It senses the change in the mechanical properties of the car panel to which the bomb is attached. It does not depend on the magnetic properties of the explosive device. In accordance with the invention there is provided equipment for detecting the attachment of a device to a panel of a vehicle or the like, comprising means for exciting vibrations in the panel, means for detecting the vibrations, control and analysis means for controlling the exciting and detecting means, for recording a normal vibration pattern, and for detecting departures from that pattern resulting from the attachment of a device, and an alarm triggered by the control and analysis means when the presence of a device is detected. PRINCIPLE OF OPERATION When a membrane such as a metal plate is caused to vibrate certain frequencies will predominate. The amplitude and frequency of the displacements depend on the tension in, and the mass per unit area of the membrane. If an extra mass is attached to the membrane both these quantities will change. If, on the other hand, an external force is applied to the membrane, caused perhaps by an object being pressed against it, the tension in the membrane will change, also the vibrations will be differently damped. The vibrational pattern will be altered as a result. The invention utilises these principles by considering the floor pan of the vehicle as a membrane under tension. The floor-pan is forced into vibration. The pattern of vibrations is detected, analysed and stored in microprocessor memory. This process is repeated at certain predetermined intervals and the vibration patterns correlated with one another. A sudden change in this correlation indicates a change in the mass, the tension, or vibrational damping of the vehicle's floor-pan. This is interpreted and a warning given if suitable criteria are met. An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which Fig. 1 is a block diagram of the electronic circuitry of a car-bomb detector, and Fig. 2 is a flow chart of the operations performed by the circuitry. The structure of the vehicle can be set into vibration either mechanically or by piezo-electric crystals. A set of receiving piezo-electric crystals detect the resulting frequency pattern. This signal is amplified by a variable gain amplifier and filtered to remove unwanted frequencies. It is then routed to an analogue to digital converter and thence to a microprocessor and other digital hardware as shown in the block diagram of Fig.l. The block diagram shows the equipment for one channel with a single piezo-electric detector which is connected by way of pre-amplifier 10, filter 11, amplifier 12, and analog to digital converter 13 to the digital electronic circutry. Normally a separate detector is required for each panel and has its own amplifiers 10, 12 and filters 11. The analog to digital converter 13 and the remainder of the circuit, in particular a micro-processor 14, RAM 15, EPROM 16 and associated digital integrated circuits, are common to all the channels. The operation of the equipment, which is summarized in the flow chart of Fig. 2, will now be described in detail. When the vehicle's ignition is switched off power is supplied to the equipment. A timing circuit controls the power-up sequence. The equipment's random access memory RAM 15 is tested and a suitable delay period follows to allow the driver and passengers to alight. Upon the termination of this delay the program variables are initialised. The microprocessor 14 now energises the excitation system. This causes the floor pan etc. of the vehicle to vibrate. The frequency content and amplitude of these vibrations will be characteristic of the vehicle's condition at the time of the occupants' exit. They are detected by the receiving piezo-electric crystals and the resultant signals sent to the analogue to digital converter 13 and hence to the microprocessor for analysis. The signals are first examined to determine if the environmental noise level is too high for a satisfactory analysis to be made. If this should prove to be the case the system is instructed to wait until the noise level decreases suitably. A signal analysis is now performed, the result is stored in RAM in an array called say B, for each piezo-electric detector. The content of each of these arrays is then indicative of the vehicle's condition immediately upon its vacation. As soon as this analysis has been completed the excitors are once again energised in an identical manner as previously. This sequence can be repeated several times if desired and the average values are entered in the array. The excitors are energised again, the signals received are subjected to the same signal analysis but this time the results are sent to a different array, say S. A figure of merit is obtained to express the dissimilarity between the contents of the pairs of arrays obtained, namely arrays B and S. In effect it describes how closely consecutive excitations of the vehicle's structure resemble one another. This figure is termed BC in the accompanying flowchart. Ideally BC should of course be zero. As soon as this analysis is complete and the value of BC obtained, the contents of array S are zeroed. The delay subroutine is entered once again but this time to give a pre-deter ined delay of approximately one minute. The excitation procedure is initiated again, the signal analysis performed as before but this time a new figure of merit CO, which describes the similarity of the contents of B and the new S, is obtained. CO is now compared with BC and the result of this comparison is stored as a correlation figure. The magnitude of this correlation number is a measure of how similar the contents of new array S are to those of array B. Those in array B describe the vibrational pattern of the vehicle immediately upon its vacation. BC tells how closely a typical set of values in array S might resemble those in B when the vehicle has been undisturbed whilst CO tells how closely the most recently acquired set in S resembles that in B. If for three consecutive trials, the result of the comparison between BC and CO is greater than a certain figure stored as an initialised variable called say DEC1, then an alarm is given. The figure of merit stored in DEC1 can be termed the decision figure. Alternatively sequential signal testing can be employed to test when sufficient data has been recorded for a decision to be made. This is particularly useful if a pseudo-random noise generator is used to excite vibrations in the panel. Referring now to Fig.l, this shows an example of a circuit which may be used for carrying out the operations described. A relay 17 is coupled to the vehicle ignition system so that the vehicle's 12 volt supply is connected to a voltage regulator 18 when the ignition is switched off. The voltage regulator 18 provides a 5 volt supply to power the microprocessor 14 and associated circuits which are interconnected by way of an address bus 19 and data bus 20. The EPROM 16 holds the programs for carrying out the operations shown in the flow chart, while the RAM 15 acts as a store for the Pages B and S, A decoder 21 decodes the addresses. An exciter circuit 22 delivers the required excitation voltage to the piezo-electric crystals to vibrate the vehicle. This is controlled by an output latch 23 and output buffer 24 which also control an indicator or alarm 25 in accordance with the decisions resulting from each test cycle as shown at the bottom of the flow chart of Fig.2. Because the vibrational pattern of the car may change slowly, for example as it dries or mechanical strains are relieved by environmental temperature alterations, the content of array B is updated. This updating is only carried out when the ratio of CO to BC exceeds a threshold level representing a close match between the values in array S and those in array B and only when the ratio of CO to BC is less than a predetermined level. The updating can be effected by replacing a percentage of each value in array B with a corresponding percentage of the value in array S. An interrupt program is entered if a car door is opened. This program retains the setting of the warning indicator but indicates by an interrupt flag that some person has entered or attempted to enter the vehicle during the owner's absence, perhaps to place a device. It only becomes effective after the interrupt flag has been cleared to allow the driver and passengers to alight from the vehicle. If a device is attached to the vehicle body without direct contact it is still possible by analysis of the vibration pattern to detect changes in loading or damping. Alternatively the vehicle panel can be equipped with elements which are pressed against the panel when a device is attached.

Claims: CLAIMS : 1. Equipment for detecting the placement of a device against a panel of a vehicle or the like, comprising means for exciting vibrations in the panel, means for detecting the vibrations, control and analysis means for controlling the exciting and detecting means, for recording a normal vibration pattern, and for detecting departures from that pattern resulting from the placing of a device, and an alarm triggered by the control and analysis means when the presence of a device is detected. 2. Equipment as claimed in claim 1 in which the control and analysis means are constructed to analyze and record the frequency and amplitude of the vibrations from one or more detectors. 3. Equipment as claimed in claim 1 or 2 in which the control and analysis means executes repeated test cycles of excitation and analysis. 4. Equipment as claimed in claim 3 in which in a first series of test cycles an average base level or normal pattern of vibration is established and in subsequent cycles a specimen pattern of vibration is determined and compared with the normal pattern. 5. Equipment as claimed in claim 4 in which by comparison of each specimen pattern with the normal pattern a figure of merit is established representing the dissimilarity between the two patterns and the extent of any change in this figure of merit from one test cycle to another is used to decide on the triggering of the alarm. 6. Equipment as claimed in claim 4 or 5 in which the stored values for the normal pattern are up-dated by reference to the specimen values when these diverge from the stored values without reaching a level which results in triggering of the alarm. 7. Equipment as claimed in any of the preceding claims in which the exciting means comprise a piezo-electric device. 8. Equipment as claimed in any of the preceding claims in which the detecting means are piezo-electric crystals.

Inventors: BYRNE FREDERICK NICHOLAS, BYRNE, FREDERICK, NICHOLAS
Applicants: BYRNE FREDERICK NICHOLAS, BYRNE, FREDERICK, NICHOLAS
Application year: 1993
Main classification: G01V1
IPC codes: B60R21, G01V1