WO2012160578A1

WO2012160578A1 - Device for detecting the vibration characteristics of buried objects using acoustic techniques

Info

Publication number: WO2012160578A1
Application number: PCT/IT2011/000169
Authority: WO
Inventors: Sallustio QUARANTA; Andrea BULLETTI; Lorenzo Capineri; Giovani BORGIOLI; Marco Calzolai
Original assignee: SPACE SOFTWARE ITALIA SpA
Priority date: 2011-05-20
Filing date: 2011-05-20
Publication date: 2012-11-29
Also published as: EP2710409A1

Abstract

Device for detecting the vibrational features of objects buried at a shallow depth comprising a transducer ( 3 ) for transforming electric signals in time variable forces, at least a rigid rod ( 1 ) able to penetrate the soil configured in such a way that it transmits by contact (or by interposition of small layers of soils) the time variable forces exerted by said transducer to the object whose vibrational features are intended to be detected and detects the vibrations of the same object, and at least a sensor ( 2 ) for detecting the vibrations transmitted by said object to the device through at least one of said rigid rods, characterized in that said transducer is an electromagnetic actuator.

Description

DEVICE FOR DETECTING THE VIBRATION CHARACTERISTICS OF BURIED OBJECTS USING ACOUSTIC TECHNIQUES DESCRIPTION

The present invention relates to a device for detecting the vibrational features of objects buried at a shallow depth by means of acoustic techniques .

The detection of vibrational features of objects buried at a shallow depth in the soil is useful in various applications, and mainly in discriminating if an object individuated in the soil is for example a mine, a piece of wood, a plastic box, a metal fragment, a stone, without difficult digging operations .

It is known that a solid body, excited by a force containing a determined frequency spectrum, vibrates according to a combination of "vibrating modes" peculiar to the body itself. The vibrating modes excited by the force depend on the application point of the force and on the frequencies contained in the same force. It is thus possible to detect by means of suitable sensors the vibration induced in the solid body in the same point where the force is applied, or in other points of the body itself and by means of analysis carried out by using various mathematical methods known at the state of the art, to define the frequency response of the solid body by comparison with the force. The comparison of this frequency response with the typical frequency response of the kind of objects to be individuated, for example anti-personnel mines, allows to determine if there is an anti-personnel mine or a different kind of object .

At the state of the art, there are known various devices which use this principle to detect the vibrational features of bodies buried at a shallow depth thus discriminating the presence of mines. In "A study on prodding detection of antipersonnel Landmine Using active Sensing Prodder" by Jun Ishikawa and Atsushi Lino, presented at the international symposium about the humanitarian mine removal held in Croatia, 27-29 April 2010, it was presented a device made up of a penetrating rod, about 20 cm long on which a piezoelectric actuator able to apply a vibration on the same rod and a spring are installed in series. An accelerometer is rigidly constrained on the rod, the accelerometer detecting the vibration of the rod in contact to the object whose vibrational features are to be determined.

Fig. 1 shows the mathematical model with concentrated parameters proposed by the authors of said study, which schematizes the features of the whole rod-object system by means of parameters of mass, elasticity and damping. With the device described, the authors carried out a series of tests on known objects, among which also objects simulating the anti-personnel mines, by exciting them by means of a vibration applied by the piezoelectric crystal with frequency signals between 100 Hz and 10kHz and by detecting the system vibration by means of the accelerometer . The authors concluded that an object simulating an anti-personnel mine can be discriminated with respect to other solid metal objects for variable frequencies between 3 kHz and 5 kHz.

Another device known at the state of the art is described in the application PCT/CA98/00973, with priority in a Canadian patent application of DEW Engineering and Development, 16/10/1997. In this document it is described and claimed a device, shown in fig. 2 of the present patent application, comprising an acoustically coherent solid body having an end provided for detection and the other one for coupling to the means (the rod (10)), an acoustic transducer (11) used to generate a vibration starting from an electric signal and at the same time to convert an input vibration in an output electric signal, and a housing (12) for coupling said elements to the means in an efficient acoustic way (without distorting the input signal). In the dependent claims there are specified some embodiments of the housing and of the rod, and it is claimed the addition of a force sensor to signal to the operator that he is applying an excessive pressure. Referring to the transducers, at page 3 line 25, there are mentioned only piezoelectric transducers. It is suitable to underline that in the description, the device is referred as "ultrasonic probe". That is to say that the used frequencies are ultrasonic, therefore greater than 20 kHz.

In no device known at the state of the art, there are mentioned transducers other than the piezoelectric crystals. The use of the piezoelectric crystals is connected to some limits in terms of amplitude of the applied oscillation, which directed the authors of the first study to the use of a spring with low elastic constant, equal to about 2300 N/m, to obtain a compression of 5 mm applying a force of 11.5 N on the target obj ect .

Another limit connected to the use of an actuator of piezoelectric kind is that the force is transmitted by contact to the exciting element, thus generating the acoustic coupling problems, to whose solution it is directed the yet mentioned patent application PCT/CA98/00973. If the force is transmitted by physical contact between the rod and the transducer acting as exciter, the acoustic coupling is an important problem.

In fact a very low contact force causes a bad transmission of vibrations, while a high contact force, function of the force applied by the end of the rod on the mine, can cause a detonation of the same mine.

For this reason it is advantageous a device which avoids the need that the force is transmitted by pressure between the rod and the transducer.

According to a first aim, the object of the present invention provides a device for detecting the vibrational features of objects buried at a shallow depth, by means of a sensorized penetrating rod where the actutator allows to apply forces of variable frequency, and allows the rod to be subjected to a significant shift upon application of modest forces, normally in the IN order.

A second aim of the device object of the present invention is to provide a device for detecting the vibrational features of objects buried at a shallow depth by means of a sensorized penetrating rod where an electromagnetic actuator exerts the force on the exciter element without direct physical contact, thus avoiding the problems connected to the need to transmit vibrations of every frequency of interest by means of a superficial coupling between different elements.

Another aim of the object of the present invention is to provide a device for detecting vibrational features of objects buried at a shallow depth, able to discriminate if the object under examination is a mine or not by applying vibrations of frequency sensibly lower than what is known at the state of the art.

According to another aim, the object of the present invention provides a device for mine removal comprising various kinds of sensors and a method for using such device. These and other advantages will be clear from the following description which refers to figures 3 to 5.

Figure 3 shows the sensorized rod;

Figure 4 shows the application scheme of the use of the rod;

Figure 5: shows the basic scheme of an electromagnetic actuator (for example of the "voice coil" kind) .

As it is shown in figure 3, according to a preferred embodiment, the device object of the present invention comprises a rod (1) functioning as exciter of the object under examination and as receiver of the vibration of the same object, an accelerometer (2) to detect the vibration of the object and an electromagnetic actuator (3) to exert, through the rod (1), a force on the object whose vibrational features are intended to be determined. The use of the device shown in fig. 2 by an operator is similar to the use of devices known at the state of the art.

The point of the rod (1) is put in contact to the object (6) under examination. If the object of interest is buried in the soil (7) at a shallow depth, normally few cm, the rod can conveniently approach the object by penetrating in the same soil according to -an angle of 90° with respect to the horizontal .

An electric signal generated by means of common devices and therefore not shown in figure is converted by the electromagnetic actuator (3) in a mechanical force coaxial with the rod (1), which transmits it to the object (6).

The object (6) is then put in vibration, and the vibrations thereof are transmitted by means of the same rod (1) to the accelerometer (2) which converts the accelerations in an electric signal, which can be sampled and analyzed by means of common signal sampling and analysis methods. The accelerometer used can be both of the uniaxial and triaxial kind.

By way of example, the wave shape of the force signal can be a sinusoidal sweep of time variable frequency comprised in the whole frequency range of interest. Such range can, by way of not limiting example, cover the whole range of frequencies between 0 Hz and 2000 Hz. Alternatively, as signals there can be used series of cycles of modulated signals with controlled spectrum (for example cycles of sinusoidal or square wave signals) with frequency chosen to excite resonance frequencies useful to discriminate objects of the anti^¬ personnel mine kind from other kind of objects.

The detected signal processing and its comparison with the input signal can occur both in the time domain and in the frequencies domain, by means of mathematical methods aiming at determining the frequency response of a vibrating system, which are widely known in literature and therefore not described in more detail here. It is then possible to use characteristic parameters extracted by the processing carried out on the detected signals on various kinds of objects to provide a database of vibrational features of the objects. This allows an automatic classification in one of the known kinds of the unknown objects on which the measure of the vibrational features is carried out. In this way, by using an equivalent model of the kind reported in fig. 1, it is possible to determine an electronic signature of the objects of the mine kind and of objects of other kind.

According to a preferred embodiment, as electromagnetic actuator can be used an actuator of the "voice coil" kind, shown in fig. 6. It is an electromagnetic actuator known at the state of the art where the axial mechanical force generated by the output shaft (8) mechanically connected in the present device to the rod (1) by means of springs (4) and fixing cylinder (5) is proportional to the current circulating in the electric winding (9). Since the "voice coil" circuit has a proper electric resistance, it is clear that by piloting the "voice coil" with a variable tension it is possible to modulate the current in the winding and thus the force expressed on the output shaft connected to the rod (1). As it is clear, the electromagnetic actuator exerts on the rod (1) the force applied by the piloting current independently of the position of the same rod, therefore its use solves the actuator/rod acoustic coupling problems in an easier and more efficient way than what described and claimed in the documents known at the state of the art. Moreover, unless the rod is not arrived at the end of the stroke, an excessive pressure applied by the operator causes an axial shifting of the rod and no increase of the exerted force on the tested object, which remains limited to the maximum value of force produced by the "voice coil". This fact represents an important safety factor, considering that the object under testing can be a mine and that an excessive pressure can lead to its explosion.

However, the choice of using an electromagnetic actuator does not represent a mere substitution of a piezoelectric crystal with another kind of actuator. The structural light weight of electromagnets of this kind allows in fact to apply forces with high frequencies, which are however very lower than the frequencies reachable by means of piezoelectric crystals.

Another advantage of the here provided electromagnetic actuator is the possibility to adjust the current circulating on the exciting element so to obtain the control on the penetration force of the sensorized rod. Moreover, the use of an electrodynamic actuator allows to work with lower piloting tensions unlike a piezoelectric actuator which needs high working tensions at equal supplied power. The choice of an electromagnetic actuator is therefore possible only after an experimentation which allowed to understand how to discriminate anti-personnel mines from objects of other kind by applying forces with frequencies under 2000 Hz, unlike what happens in the study by Ishikawa et al., where it is concluded that the discriminating frequencies are between 3 kHz and 5 kHz, and in the document PCT/CA98/00973 where it is referred to an "ultrasonic probe", thus indicating the functioning frequencies over 20 kHz.

According to another embodiment the object of the present invention comprises two or more distinct rods for the application of the force to the object under examination and for the contemporaneous detection of the vibration induced in one or more points of the same object. On each of the various rods there can be installed only one actuator, only one uniaxial or triaxial accelerometer, or both in function of the dimensions and of the kind of object which is intended to be discriminated. It is particularly efficient, upon experimentation, the device configuration with two penetration rods, the one as exciter provided with an electromagnetic exciter of the yet described type, the other one as receiver provided with a triaxial accelerometer. According to what previously said, another aim of the present invention is to provide a system for mine removal comprising a plurality of sensors for detecting the mines. At the state of art there are known various technologies for detecting buried objects. The present research in the detection of buried objects indicates however that no sensor based on a single technology mounted on a moving detection unit is able to identify surely and at the same time efficiently the mines buried in mined fields, particularly if this detection unit is not guided by a man.

For this reason, the present invention aims at providing a system having three different sensors for the identification and discrimination of buried objects, and a particularly efficient method for carrying out the mine removal by using the same system. Of the sensors comprised in the system, two are known at the state of the art, and the third one is the sensorized penetrating rod according to what previously said. The two technologies known at the state of the art, which are integrated in the system are the Metal Detector (MD) and a sensor of "haptic probe" (HP) type.

The Metal Detector is widely used in operative scenarios and is based on a mature technology. Unfortunately, in the construction of modern mines, realized to escape the MD detection, the use of metals is strongly reduced. To face this problem, there exist Metal Detectors sensible to gram fractions of buried metal. This extreme sensitivity, even if detects almost completely non- metallic mines, results clearly in many false positives, above all in soils, which were fields of battles or even only anthropic activities.

The Haptic Probe (HP) is a motorized metal probe penetrating the soil automatically, until the force applied exceeds 7 kg or until the probe arrives at the maximum of its extension. A description of this kind of sensor, known at the state of the art, is in the Italian patent n° 0001379054 with title "Apparatus for detecting objects immersed in incoherent materials and method of detection". The value of this sensor, coupled with MD, is to confirm or exclude the presence of a solid object at the point where MD detects the presence of metal material. A sensor as the one just cited is however able to exclude only partially the false positives detected by the Metal Detector, since it is not able to give any information on the kind of solid object individuated.

As widely described, a sensor comprising a sensorized rod according to the present invention is able to discriminate if the solid object individuated is a mine or not, thus reducing the false positives remarkably and accelerating the mine removal operations; moreover the previously described system with rod allows also the possible integration with the Haptic Probe.

The system described in the following can be conveniently installed on UGV vehicles (Unmanned Ground Vehicle) which will carry out the mine removal atomically and with remarkable time saving, thanks to the reduction of false positives obtained by the combined analysis of the data provided by the three sensors.

The application of the system comprising the three sensors, integrated on an UGV, provides a first scanning, carried out through the Metal Detector, in order to identify the positions where it is the buried metal. Among all the positions individuated, there will be obviously both "positives" (points where there is effectively a buried mine) , and "false positives", i.e. points where the Metal Detector signal is caused by objects other than mines. A second scanning, by using Haptic Probe, is carried out in order to exclude all the coordinates individuated by the first scanning to which no buried solid object corresponds. A third scanning, by using the sensorized penetrating rod according to the present invention excludes further all the objects which do not give an elastic response (for example stones, roots, wood) .

The three scanning operations can occur conveniently in series or in parallel, in function of the configuration and kind of the soil object of mine removal. It is therefore possible to carry out the second scanning with the Haptic probe after carrying out the first scanning with the Metal Detector on the whole area of interest, and the third one at the end of the second one. It is also possible to use the Haptic probe during the first scanning with the Metal Detector, as soon as the metal object is detected and to use the sensorized rod if the Haptic Probe confirms the presence of a solid body, to evaluate by means of the body elastic response if it is a mine or not.

Claims

1. Device for · detecting the vibrational features of objects buried at a shallow depth comprising a transducer (3) for transforming electric signals in time variable forces, at least a rigid rod (1) able to penetrate the soil configured in such a way that it transmits by contact (or by interposition of small layers of soils) the time variable forces exerted by said transducer to the object (6) whose vibrational features are intended to be detected and detects the vibrations of the same object, and at least a sensor (2) for detecting the vibrations transmitted by said object (6) to the device through at least one of said rigid rods (1), characterized in that said transducer (3) is an electromagnetic actuator.

2. Device according to claim 1, characterized in that said electromagnetic actuators are of "voice coil" kind.

3. Device according to any one of claims 1 to 2, characterized in that said sensors (2) are accelerometers , integrally constrained to said rods (1) .

4. Device according to any one of claims 1 to 2, characterized in that said sensors (2) are triaxial accelerometers, integrally constrained, to said rods (1) .

5. Device according to any one of claims 1 to 4, comprising only one rod.

6. Device according to any one of the preceding claims, comprising two or more rigid rods configured in such a way that they can be put in contact in different points with the objects, whose vibrational features are intended to be detected.

7. Device according to claim 6, characterized in that the electromagnetic actuator acts on the first of said rods and the accelerometer detects the vibration transmitted by the object under examination to the other rods.

8. Method for discriminating if objects buried at a shallow depth are mine by using the device according to any one of the preceding claims, comprising the steps of:

approaching said rod to objects, which are expected to be mines, by penetrating the soil;

- applying, by means of the actuator and at least by means of a rod, a vibration containing a determined frequency range to said objects; - detecting by means of said sensors the vibrations transmitted by said objects to at least one of said rods;

analyzing the vibrational features of said objects;

- analyzing the frequency response of said objects and comparison with the response coming from a database where there are responses of objects of mine type buried at different depths in different kinds of soil.

9. Method according to claim 8, characterized in that said frequency range is between 0 Hz and 2000 Hz.

10. Method according to claim 8 or 9, characterized in that said vibration comprises cycles of modulated signals with controlled spectrum with limited time duration at one or more frequencies in the range between 0 Hz and 2000 Hz.

11. System for individuating the positions of the mines buried in a mined soil comprising a metal detector, a haptic probe and a device according to any one of claims 1 to 5.

12. System for individuating the positions of the mines buried in a mined soil comprising a metal detector, a haptic probe and a device according to any one of the claims 1 to 5 mounted on an unmanned ground vehicle of the Unmanned Ground Vehicle type.

13. Method for individuating the positions of the mines buried in a mined soil by using a system according to claims 11 or 12, comprising the steps of:

- carrying out a first scanning of the soil of interest by means of the metal detector in order to detect the presence of objects containing metal;

- carrying out a second scanning of the soil of interest using the Haptic Probe in the points individuated by the coordinates detected in the previous step, in order to verify if in said points there are buried solid objects or noncarrying out a third scanning of the soil of interest using the device according to any one of the claims 1 to 5 in the points where the Haptic Probe confirmed the presence of a solid object, in order to determine if said solid object is a mine or not .

14. Method for individuating the positions of the mines buried in a mined soil by using a system according to claims 11 or 12, comprising the steps of: - carrying out a first scanning of the soil of interest by means of the metal detector in order to detect the presence of objects containing metal;

- interrupting the scanning with the metal detector whenever it is detected the presence of an object containing metal and verifying by means of the Haptic Probe if in such point it is present a buried solid object or not;

- in case of positive result of the test at the preceding point, using the device according to any one of the claims 1 to 5, in order to determine if the solid object individuated is a mine;

- restarting the scanning with the metal detector and repeating the preceding steps until the whole area of interest is covered.