WO1999019684A1

WO1999019684A1 - A mine detector and a method for detecting mines

Info

Publication number: WO1999019684A1
Application number: PCT/DK1998/000450
Authority: WO
Inventors: Lars Christensen; Michael Jensen
Original assignee: Minedetektor Aps
Priority date: 1997-10-15
Filing date: 1998-10-15
Publication date: 1999-04-22
Also published as: AU9621198A

Abstract

A mine detector (1) serving for, by means of ultrasound, locating and identifying a mine (10) buried in the ground (9). The mine detector (1) comprises an ultrasonic transmitter (13) for transmitting ultrasonic signals (11) into the ground (9), at least three ultrasonic receivers (14; 15) for receiving reflected signals from the emitted ultrasonic signals (11), and at least one computer (19) connected to the at least one ultrasonic receiver (14; 15) for processing the signals. The three ultrasonic receivers (14) are placed at a distance both from the ultrasonic transmitter (13) and from each other. Between the computer (19) and the ultrasonic receivers (14; 15) is inserted a microprocessor (20, 21), and the computer (19) is programmed to, on the basis of the signals received from the microprocessor (20, 21), compute data representative of an image of the locality and configuration of the searched mine (10). The mine detector (1) is functioning with a high degree of security and can also be used for identifying mines (10) without metallic part, such as plastic and/or glass mines.

Description

A mine detector and a method for detecting mines

The invention relates to a portable detector for, by means of ultrasound, locating and identifying an object, such as a mine, buried in the ground, and comprising at least one ultrasonic transmitter for transmitting ultrasonic signals into the ground, at least one ultrasonic receiver for receiving reflected signals from the emitted ultrasonic signals, and at least one computer connected to the at least one ultrasonic receiver for processing the signals .

Such a detector is largely used for detecting mines buried in the ground. The problem of mines has existed for many years. However, it is only during recent years that the problem has been given a lot of attention in the public and the media. By now this attention has been joined by a global political desire to solve the many problems related to the mine laying. In defiance of this political desire, new mine types are still being manufactured and developed that the existing mine detecting and clearing equipment have more and more trouble locating and neutralising.

It is believed that at present more than 110 million mines have been laid spread over about 70 countries. These kill or maim about 25,000 people every year, mainly in the third world. In addition to this, it is estimated that there are about twice as many mines in various places in the world that are ready to be laid.

The reason for the "popularity" of the mines is that they are an effective defensive weapon, they are inexpensive and easily accessible, it as an effective way to hinder "the enemy" in supplying its troops, and when they are laid, they are generally destructive to a country's infrastructure and agriculture. Even if a war ended, the mines would still be a large problem to the country in question and to its inhabitants. It costs between $3 and $4 to manufacture a simple land mine, but it costs between about $240 and $960 to remove it again. It is estimated that for each removed mine, 20 new ones are laid. In 1996, about 100,000 mines were removed while nearly 2,000,000 mines were laid according to the UN. The UN believes that it will take up to 1,100 years to remove the already laid mines with the present mine clearing methods.

The problems of mine spreading can be divided into primary and secondary categories.

The primary problems are that, as mentioned earlier, about 25,000 people are killed or maimed by mines every year. In addition to this, the laying of mines has negative consequences for the food supply of a number of countries as the mines are typically laid in rice fields or other crop fields. This is especially the case in those countries that already have a low degree of self-sufficiency of food so that the consequences of the presence of mines can be malnutrition and famine.

Of secondary problems, the heavy costs of medicine and an increased risk of dangers of blood-related infections can among others be mentioned. Persons who are maimed by mines typically require from two to six times more blood transfused than injured in other accidents. As blood is short-supply goods in the poor countries, there is a high risk of the blood used being infected by e.g. HIV-virus .

Where the mine laying inflict great social and economical problems on the third world, the West is only economically affected.

The costs of the third world relating to treatment of maimed, shortage of food and medicine etc. are mainly covered by westerly financed co-operation organisations and bilateral development aid. It is for instance estimated that about $750 million are spent alone on the 250,000 people who have lost a part of the body in connection with land mine accidents. These and other means could advantageously be spent on improving the many other problems of the third world if the mine problem was solved.

There are many different types of land mines. The two main groups are antipersonnel mines and antitank mines. In the following, antipersonnel mines will mainly be mentioned.

In spite of the much focus on the mine problem in the recent years, the various existing mine detection and clearance technologies are still insufficient in a large number of fields. Research is done in a number of new technologies but they are often very costly. In reality, this means that the manual metal detector still is the most widespread mine detection technology.

The size of an antipersonnel mine varies from about 4 cm to 13 cm in circumference. Its content of explosives varies from a few grams to half a kilo or more. When buried, the mine type can not immediately be established with the present detection technologies, and the extent of the subsequent detonation is an unknown and dangerous factor in mine clearance. Furthermore, the traditional metal detector technologies have difficulty in differentiating mines from other metal objects and in estimating how deep the mine is buried.

However, the biggest problem of the traditional mine detectors is the spreading of plastic mines. Earlier, the mines usually contained metal, but mine manufacturing of plastic is becoming more and more widespread. Thereby, the most widespread, the least expensive, and the fastest detection technology - the metal detector - is put out of action. This is a serious threat to the life and health of the deminer, which has resulted in the fact that the detection and clearance process is slower today than merely ten years ago.

It is a known fact that mines made of e.g. plastic and/or glass and not containing any metallic parts can be located by means of mine detectors using ultrasound. For this purpose, an ultrasonic transmitter is used that transmits ultrasonic signals into the ground during the mine detection. The reflected signals are detected by one or more ultrasonic receivers and put in a computer for processing the signals and for outputting the result for assessment of the character of possibly located objects. However, it has turned out that these mine detectors are not functioning with the necessary degree of security.

The object of the invention is to provide a portable detector of the kind mentioned in the opening paragraph that is functioning more safely during operation than known so far, and that also can be used for identifying mines without metallic parts.

The novel and unique features according to the invention, whereby this is achieved, is the fact that the mine detector has at least three ultrasonic receivers placed at a distance from both the ultrasonic transmitter and from each other, and that the computer is programmed to, on the basis of the reflected signals, compute data that are representative of a three- dimensional image of the locality and configuration of the searched object.

The data computed in this way can advantageously be used for showing a three-dimensional looking image of a detected object on a display which is part of the detector. Thereby, the deminer is enabled to determine exactly if the object is a mine which is to be neutralised or if it is an object which is harmless in itself .

The computer can also have a database with reference values of known mines, and it can furthermore be programmed to compare these reference values with the computed data and thereby reveal if the detected object is exactly one of these known mines. Thereby, the occupational safety of the miner is increased substantially.

Besides the at least three ultrasonic receivers placed at a distance from both the ultrasonic transmitter and from each other, there can, near the ultrasonic transmitter, be placed a second ultrasonic receiver for receiving the part of the emitted ultrasonic signal that is reflected directly from the ground surface. This signal, the offset signal, can be used for determining the immediate distance between ground and ultrasonic transmitter and ultrasonic receiver respectively so that the computations are made as if the ultrasonic transmitter and the ultrasonic receiver always are directly above the ground surface.

The at least one microprocessor can advantageously consist of a master microprocessor and one slave microprocessor for each of the ultrasonic receivers.

In this connection, it is an advantage when the ultrasonic transmitter and the ultrasonic receiver are mounted mainly on level on a shared printed circuit board.

To increase the extent of the area that the detector covers at a given moment, several of these printed circuit boards can furthermore be placed side by side on the same detector whereby the computer is programmed to put the data received from each printed circuit board together with data which are representative of a three-dimensional image of the locality and configuration of the searched object.

The ultrasound source can typically be a crystal that is made to oscillate by means of an applied voltage. However, the natural oscillation frequency of the crystal is too high to be applicable in the present context, and a binary counter/divider can therefore be inserted between the crystal and the ultrasonic transmitter for dividing the oscillation frequency of the crystal to e . g . 40 kHz.

In order to avoid disturbing interference between the signals reflected by ultrasonic signals emitted in succession after each other, it is important that new ultrasonic signals are not emitted until the microprocessors have finished processing the reflection signals which it already has received.

The separation of the signals as regards time can take place by means of an electronic switch connected to one of the ultrasonic receiver, the controlling ultrasonic receiver, or the related slave microprocessor. The switch breaks the connection between the ultrasound source and the ultrasonic transmitter when the microprocessor is in operation, and closes this connection when the microprocessor is not in operation.

The invention will be explained in greater detail below, describing, by way of example, an embodiment with reference to the drawing, in which

Fig. 1 is a perspective view of an ultrasonic mine detector according to the invention,

Fig. 2 is a schematic view of a deminer searching for mines with the mine detector of fig. 1,

Fig. 3 is on a larger scale a schematic plane view of three printed circuit boards placed side by side for the mine detector of fig. 1 and 2,

Fig. 4 is a side view of the printed circuit boards in fig. 3 in operation over a piece of land with a buried mine,

Fig. 5 is a graphic picture showing the emergence in time of the reflected ultrasonic signal, and

Fig. 6 is a block diagram of the mine detector.

The mine detector 1 in fig. 1 consists mainly of a scanner 2, a colour cartridge 3, software (not shown), a display 4, interface (not shown) , one or more batteries (not shown) , a control stick 5, a control handle 6, and a sling 7. Fig. 2 shows how a deminer 8 is scanning a piece of land 9 for mines. In the piece of land is buried a mine 10 which is detected by means of ultrasonic waves 11.

The different components of the mine detector 1 are functioning in the following way :

The scanner 2 reads the placing of the mine 10 in the ground 9. It furthermore registers how deep the mine is buried, what type of mine it is, its size and explosive force, whether the mine is connected to other mines or not, and if there are special circumstances which makes it especially difficult to eliminate the mine .

The colour cartridge 3 is activated over the detected mine 10 and thereby renders its placing visible.

The software analyses the signals of the scanner 2 and translates them to a graphic image.

The graphic image reproduces the exact placing and type of the mine 10 on the display.

Interface is used for activating the functions of the mine detector.

A rechargeable battery is used for supplying power to scanner, display, and interface.

The control stick 5 is the backbone of the mine detector, to which all the parts of the mine detector is secured.

The mounted control handle 6 gives the deminer 8 complete control over the mine detector.

The sling 7 makes it possible to use the mine detector for a long time without the deminer 8 getting tired. The deminer scans an area with the mine detector, and when a mine 10 is registered, its type is determined, and its exact placing is marked. The necessary precautions are subsequently taken after which the mine can be dismantled.

As shown in fig. 3 and 4, there is on the scanner 2 in this case fitted three identical printed circuit boards 12 each with one ultrasonic sender 13 of the kind used at gynaecological departments, and nine ultrasonic receivers 14 of the kind used for locating air holes.

There will always be air holes in antipersonnel and antitank mines, and there will also be air traps above and below the mine. Contrary to ordinary ultrasonic receivers for locating specific materials, it is, with the ultrasonic receivers 14 and software according to the invention that are used in this case, possible to determine whether the existence of an air trap is due to a mine or whether it is a natural air trap e.g. made in frozen ground.

The printed circuit boards 12 are fitted so closely side by side on the scanner 2 that they cover 100%, but they must not be fitted so that they overlap each other as disturbing interference can thereby be generated between emitted and reflected signals from the adjacent printed circuit boards.

It is important that the timing is in order between the transmission and the reception of the signal as this is how the size and depth of air traps is determined.

Each ultrasonic receiver has seven different signals. No signal means that no air trap has been located, and the six control cables each have a depth indication from 0 to 50 cm. These indications are gathered from all ultrasonic receivers via a parallel port (not shown) . Thereby, a 3D image of the ground beneath the ultrasonic receivers can be formed on the display 4 - actually a 2D image but with depth indications. The signals are gathered in a parallel port (not shown) by means of a converter (not shown) which can take 255 control cables (six from each ultrasonic receiver) and gather them to a digital signal before it, via eight control cables, is sent on into the parallel port. Here, the software takes the signals and divides them into 255 individual signals again.

The software now sees, on the basis of the signals, to visualise on the display 4 how the dispersion of air holes and their depth is beneath the ultrasonic receivers. The image can thereby be maintained and studied more closely before the deminer at the screen decides whether it is a mine or not. The recognition process can also be left to the software but as mines often are homemade explosive devices, visual recognition is preferred.

The software includes the following steps:

1. The 8 signals from the parallel port are divided into 255 individual signals .

2. The signals are assigned to the respective ultrasonic receivers.

3. On the display is recorded when the scanner has contact with an object, and this record shows which scanner it is and its position.

4. The depth of the object can be seen from the signals, and this is done for each ultrasonic receiver on the same line/position without crossing existing points . The size is best determined by checking if the holes occur regularly, continuously or not.

5. The line is copied to a file. 6. The scanning of a new area by the ultrasonic receivers is awaited. 7. The next line on the screen is passed on to, and step 1 is then proceeded with.

Each line must be wide enough to be able to read the depth but narrow enough to be able to determine what the object is there is beneath the scanner. Fig. 3 - 6 show more specifically how the mine detector according to the invention is arranged and functions.

As mentioned earlier, there is, on each of the three printed circuit boards 12, placed an ultrasonic transmitter 13 and nine ultrasonic receivers 14,15 which are placed in asymmetric order. One of the ultrasonic receivers 15 is placed near the ultrasonic transmitter 13. The ultrasonic receivers 14 are placed asymmetrically in relation to the ultrasonic transmitter 13 in order to thereby obtain reflection signals to optimally be able to compute and form a three-dimensional looking image of the located object.

On the printed circuit board 12, there are, with a mutual distance, furthermore placed two colour jets 16 that are supplied with colour from the colour cartridge 3.

During scanning, the scanned area is marked with colour spots

(not shown) that are sprayed out of the colour jets. A located mine or a suspicious object are similarly marked with a colour spot (not shown) .

During scanning, an ultrasonic signal 11 (see also fig. 2) is transmitted into the ground 9 from the ultrasonic transmitter 13. A part 17 of this signal 11 is reflected by the ground 9 to the centrally placed ultrasonic receiver 15 which is activated. The first reflection signal 17 is used as an offset signal for determining the immediate distance between the ground and the ultrasonic receivers. This part of the scanning process is important as the deminer is carrying the mine detector manually across an often uneven terrain. The mine detector can therefore not be carried in an exactly fixed distance from the ground.

The second part of the emitted signal 11 continues further down into the ground 9, and during this, it hits the mine 10 whereby it is reflected as a second reflection signal 18 to the other ultrasonic receivers 14 which convert the reflection signal 18 into signal strength and difference in received time in relation to offset.

Fig. 5 shows graphically how the first and the second reflection signal 17 and 18 are placed in a system of co-ordinates with time as abscissa and strength as ordinate . The first reflection signal 17, the offset signal, is only used for resetting the starting point of the computations while the second reflection signal 18 is used for generating an image on the display 4. An essential cause for this image appearing three-dimensionally on the display is that the ultrasonic receivers 14 are placed at a distance from the ultrasonic transmitter 13 and from each other.

The functional structure of the mine detector is illustrated by means of the block diagram 6 in which only three of the ultrasonic receivers 14 placed at a distance from the ultrasonic transmitter 13 are shown for the sake of simplicity.

The process is superiorly controlled by a computer 19 and a master microprocessor 20 which again controls one slave microprocessor 21 for each ultrasonic receiver 14. Between each slave 21 and the related ultrasonic receiver 13, an amplifier 22 is inserted for amplifying the signal emitted by the ultrasonic receiver. Similarly, an amplifier 22 is inserted between the ultrasonic receiver 15 and the master 20.

As shown, the ultrasonic transmitter 13 is connected to the master 20 and via an amplifier 23 and a 14-stage ripple-carry binary/divider (4060) 24 connected to a crystal 25 which is made to oscillate when it is applied a voltage.

The natural oscillation frequency of the crystal 25 is too high to be used immediately in the present context, and a 14-stage ripple-carry binary/divider (4060) 24 is therefore inserted to divide the oscillation frequency of the crystal to e.g. 40 kHz.

The computer 19 is connected to a display 26 for showing an image of the located object. The function of the master 20 is to control the slaves 21 and to communicate to and from the computer 19 and the slaves 21. Besides this, the master is arranged to start the transmitter 13 and to detect the offset signal 17 through the ultrasonic receiver 15 placed near the transmitter 13. When the offset signal has been generated, the master 20 activates the slaves 21 which thereby are ready to read the related ultrasonic receivers 14.

When the slaves 20 have reported back to the master 20, it gets ready for sending the measured signals to the computer 19 for further processing. The computer 19 then starts creating an image of the located mine and its location in relation to the scanner via a premade database.

When the computer 19 has finished the computations necessary for this purpose, it reports to the master 20 that the above process can be repeated. The process is then started by letting the ultrasonic transmitter 13 again transmit an ultrasonic wave 11 into the ground. This new ultrasound wave 11 is thus only transmitted when the reflection signals 18 from the previous ultrasonic wave are finished being computed by the computer. Thereby, the risk of disturbing interference being generated between the reflection signals from two or more successive ultrasound emissions is completely eliminated.

The computer 19 can, besides showing a three-dimensional looking image on the display 4,26, also have a database (not shown) with reference values of known mines whereby the computer is programmed to compare these reference values with the computed data and thereby reveal if the located object is exactly one of these known mines .

Claims

1. A portable detector (1) for, by means of ultrasound, locating and identifying an object (10) , such as a mine, buried in the ground (9), and comprising at least one ultrasonic transmitter (13) for transmitting ultrasonic signals (11) into the ground (9), at least one ultrasonic receiver (14; 15) for receiving reflected signals (17; 18) from the emitted ultrasonic signals, and at least one computer (19) connected to the at least one ultrasonic receiver for processing the signals, characterised in that there are at least three ultrasonic receivers (14) placed at a distance from both the ultrasonic transmitter (13) and from each other, and that the computer (19) is programmed to, on the basis of the reflected signals, compute data that are representative of an image of the locality and configuration of the object (10) .

2. A detector according to claim 1, characterised in that the data computed by the computer (19) are representative of a three-dimensional looking image of the locality and configuration of the object (10) .

3. A detector according to claim 1 or 2 , characterised in that the computer (19) has a database with reference values of known mines, and that it furthermore is programmed to compare these reference values with the computed data and thereby reveal if the located object is exactly one of these known mines .

4. A detector according to claim 1, 2, or 3 , characterised in that there, besides the at least three ultrasonic receivers (14) placed at a distance both from the ultrasonic transmitter (13) and from each other, is placed a second ultrasonic receiver (15) near the ultrasonic transmitter (13) for receiving the part (17) of the emitted ultrasonic signal (11) which is reflected directly from the ground surface .

5. A detector according to each of the claims 1 - 4, characterised in that the at least one microprocessor

(20,21) comprises a master microprocessor (20) and one slave microprocessor (21) for each of the ultrasonic receivers .

6. A detector according to each of the claims 1 - 5, characterised in that the ultrasonic transmitter (13) and the ultrasonic receivers (14; 15) are mounted mainly on level on a printed circuit board (12) .

7. A detector according to each of the claims 1 - 6, characterised in that the detector (1) has several printed circuit boards (12) placed side by side and each having an ultrasonic transmitter (13) and at least three ultrasonic receivers (14;15), and that the computer (19) is programmed to put the data received from each printed circuit board together with data which are representative of a three- dimensional image of the locality and configuration of the searched object (10) .

8. A detector according to each of the claims 1 - 7, characterised in that it comprises a crystal (25) which is made to oscillate by applying a voltage, and a binary counter/divider inserted between the crystal (25) and the ultrasonic transmitter (13) for dividing the oscillation frequency of the crystal to e.g. 40 kHz.

9. A detector according to each of the claims 1 - 8, characterised in that it comprised a switch for breaking the connection between the crystal (25) and the ultrasonic transmitter (13) when the microprocessor (20,21) is in operation, and for closing this connection when the microprocessor (20,21) is not in operation.

10. A method for, by means of ultrasound, locating and identifying an object (10) , such as a mine, buried in the ground (9) whereby ultrasonic signals (11) are transmitted into the ground (9) from at least one ultrasonic transmitter (13), at least one ultrasonic receiver (14; 15) receives reflected signals (17; 18) from the emitted ultrasonic signals (11), and at least one computer (19) connected to the at least one ultrasonic receiver (14; 15) processes the signals, characterised in that the first ultrasonic receiver (15) placed near the ultrasonic transmitter (13) receives the first reflection signal (17) directly from the ground surface for each emitted ultrasonic signal (11), that the first reflection signal

(17) only is used for determining the distance from the ultrasonic transmitter (13) and the ultrasonic receivers

(14; 15) to the ground surface, that at least three other ultrasonic receivers (14) placed at a distance both from the ultrasonic transmitter (13) and from each other each, from the same ultrasonic signal (11), receive a second reflection signal (18) from objects (10) if any in the ground (9) whereby the computer (19) receives input representative of distance to the ground surface determined by the first reflection signal (17), and input representative of the other reflection signals (18), and on the basis of this and of a premade database, computes data representative of a three-dimensional image of said objects (10) and their location in the ground (9) .