Abstract:
An energy impulse generator for transferring an energy pulse to a surface. The generator exchanges messages with a computing means through a communication interface. The generator comprises a housing. The generator further comprises an impact assembly movably mounted within the housing between at least a resting position, a latched position and an impact position. In the impact position, the impact assembly transfers the energy pulse to the surface. The generator also includes an energy storage means attached to the impact assembly. In the latched position, the energy storage means is capable of releasing a specified amount of energy to the impact assembly such that upon release of the specified amount of energy the impact assembly moves from the latched position to the impact position then returns to the resting position. The computing means controls the release of the specified amount of energy to the impact assembly.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    This application claims the benefit of priority under 35 U.S.C § 119 from Canadian Patent Application no. 2,366,030 filed on Dec. 20, 2001, the disclosure of which is incorporated by reference as if set forth in full in this document.  
           [0002]    1. Field of the Invention  
           [0003]    The present invention relates to the field of non-intrusive testing of a medium located under a surface. More specifically, the present invention is concerned with an intelligent profiling system permitting the mechanical characterization of a medium under a surface.  
         BACKGROUND OF THE INVENTION  
         [0004]    In the field of geophysical exploration for example, non-intrusive techniques have been sought and developed as a supplement or an alternative to conventional in-situ testing techniques involving boring because these techniques are non-destructive. In some cases where boring is not feasible, for example in granular soils, such non-intrusive techniques are the only way to explore the underground. Also, they generally are more cost-effective.  
           [0005]    Non-intrusive techniques are also used for exploring a medium situated under a surface in various other fields, for example, for assessing the wear conditions of roads, of bridges, of bar joints in buildings, of concrete walls, etc, or for detecting subsurface pockets in mining or military applications.  
           [0006]    Interestingly, surface waves, and especially Rayleigh waves, are very useful in the field of non-intrusive testing. One of the well known method in the art is Spectral Analysis of Surface Wave (“SASW”), for instance, which makes use of surface waves for determining shear velocity profiles of the underground without intrusion. This method involves a pair of sensors, at least one source of impulses, and a signal processing system.  
           [0007]    Although such a technique using surface waves permits exploration of a broad range of thickness of soils, by changing the distance between the two sensors and by using different sources of impulses, in the case of SASW discussed hereinabove for instance, its operation generally requires actions from a highly skilled worker expert in the field in order to obtain useful information on the subsurface medium under investigation,  
           [0008]    Therefore, in spite of the efforts in the field, there is still a need for a system allowing profiling of a medium under a surface, comprising sensors, a generator of impulses and a user computing interface, and permitting collecting, analyzing, and processing the data for display and use by a non-expert.  
         OBJECTS OF THE INVENTION  
         [0009]    An object of the present invention is therefore to provide an improved profiling system.  
         SUMMARY OF THE INVENTION  
         [0010]    In one of its embodiments, the present invention relates to an energy impulse generator for transferring an energy pulse to a surface. The generator exchanges messages with a computing means through a communication interface. The generator comprises a housing. The generator further comprises an impact assembly movably mounted within the housing between at least a resting position, a latched position and an impact position. In the impact position, the impact assembly transfers the energy pulse to the surface. The generator also includes an energy storage means attached to the impact assembly. In the latched position, the energy storage means is capable of releasing a specified a mount of energy to the impact assembly such that upon release of the specified amount of energy the impact assembly moves from the latched position to the impact position then returns to the resting position. The computing means controls the release of the specified amount of energy to the impact assembly. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    In the appended drawings:  
         [0012]    [0012]FIG. 1 is a schematic representation of a profiling system according to an embodiment of the present invention;  
         [0013]    [0013]FIG. 2 is a perspective view of a displacement sensor used in the profiling system of FIG. 1;  
         [0014]    [0014]FIG. 3 is a top plan view of the displacement sensor of FIG. 2;  
         [0015]    [0015]FIG. 4 is a sectional view taken along the line  4 - 4  of FIG. 3;  
         [0016]    [0016]FIG. 5 is a top view of the substrate of the displacement sensor of FIG. 2;  
         [0017]    [0017]FIG. 6 is a diagram of a circuit equivalent to the displacement sensor of FIG. 5;  
         [0018]    [0018]FIG. 7 is a schematic sectional view of an energy impulsion generator used in the profiling system of FIG. 1; and  
         [0019]    [0019]FIG. 8 is a block diagram of a sensor in accordance with another embodiment of the invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0020]    Generally stated, the system of the present invention enables a non-intrusive physical analysis of mechanical characteristics of a medium located under a surface, and a display of the results thereof.  
         [0021]    Such a medium separated from direct exploration by a surface can be the underground, the thickness of a concrete wall, the thickness of a joint bar and the like. For illustration purposes, the present invention will be described using an embodiment dealing with geophysical testing. Therefore, in the following, the medium to be studied is a subsurface region of the underground, through the surface thereof.  
         [0022]    More precisely, the system of the present invention makes use of sensors that detect the velocity of shear waves induced in the subsurface region under test by means of an excitation generated by a generator of impulses.  
         [0023]    Turning now to FIG. 1 of the appended drawings, the system according to an embodiment of the present invention will be described.  
         [0024]    Basically speaking, the system  10  comprises three units or system components: a sensing assembly  12 ; an energy impulse generator  14 , (referred to in the following as EIG); and a user-computing interface  16 , (referred to in the following as UCI).  
         [0025]    As can be seen in FIG. 1, the sensing assembly  12  comprises displacement sensors  18  placed at various locations on a surface  20 . The sensing assembly  12  may comprise a number of sensors  18  comprised between one sensor  18 , which is located successively at various locations, and a plurality of sensors  18 . In a specific embodiment of a system  10  according to the present invention, the sensing assembly  12  comprises four sensors  18 . Obviously, other sensor quantities are possible as well. The role of the sensors  18  is to detect a movement in response to bursts of impacts generated by the energy impulse generator  14  on the surface  20 .  
         [0026]    Each one of the displacement sensors  18  of the sensing assembly  12 , and the energy impulse generator  14 , are connected to the user-computing interface  16  by means of an communication interface  21 . Many different techniques may be used to interconnect the sensors  18  to the user-computing interface  16 . For example, the communication interface  21  may include fiber optics cables, coaxial cables, multi-conductor cables, an optical link, a RF link, shown in FIG. 1 under label  22 . Alternatively, multiplexing means may be considered for the interface of communication  21 . Communication interface  21  is used to relay messages, comprising instructions and/or data, between system components.  
         [0027]    As displayed in FIGS.  2  to  4 , the sensor  18  is protected within a housing. The housing may include a plate  27  and a casing  24  closed by a top cover  25  Provided the surface  20  is not too hard, the displacement sensor  18  is attached to the surface  20  by means of a thread attachment  26  mounted on a plate  27  on which the casing  24  can be inserted and secured by edges  28  of the plate  27  (see FIG. 2 a ). Alternatively, in the case where the surface  20  is too hard, the plate  27  can be fixed thereto by means of an adhesive  29  (see FIG. 2 b ), or even simply deposited on the surface  20 .  
         [0028]    The casing  24  is provided with a communication connector  30  (see FIG. 3) for connection to the user-computing interface  16  by means of a connection  22  of the interface of communication  21  (see FIG. 1).  
         [0029]    It is to be noted that the top cover  25  also supports a shock absorbing element  32  and a damping element  34 , which are symmetrically located relative to a shock absorbing element  32 ′ and a damping element  34 ′ attached to the casing  24 , and may support an optional communication antenna  36  or an optical diffuser (not shown).  
         [0030]    A semiconductor substrate  42  is protected within the casing  24 , as shown in the cross section of FIG. 4. A mass  38  is supported within an opening  40  of the semiconductor substrate  42  supporting strain gauges  44  and resistors  46  and  48  (shown in FIG. 5). The mass  38  is allowed to move in response to acceleration. As will easily be understood by one skilled in the art, movement of the mass  38  induced by shear waves generated in the subsurface region under surface  20  cause strains on the semiconductor substrate  42 .  
         [0031]    A person skilled in the art will understand that the semiconductor substrate  42  and the elements which it supports (mass  38 , strain gauges  44 , resistors  46 , etc.) may be broadly referred to as an accelerometer or an accelerometer assembly or unit. An accelerometer may be broadly defined as a device whose response is linearly proportional to the acceleration of the material (e.g., in this case, a surface) with which it is in contact. A person skilled in the art will understand that the accelerometer or sensor  18  need not be in direct contact with the surface. Contact via other intermediary elements or media is also considered.  
         [0032]    As shown in FIG. 6, the circuit equivalent to the displacement sensor  18  comprises four strain gauges  44  and two resistors  46  forming a Wheatstone bridge. One diagonal of the bridge is connected to a DC voltage source  50 , while the other diagonal of the bridge serves as an output of the strain sensitive circuit and is connected to an amplification unit  52 . As will be explained hereinbelow, the strain gauges  44  are used as transducers for transforming a mechanical deformation on the semiconductor substrate  42  into an electric signal (or other type of information bearing signal). The resistor  48  is used for calibration purposes, as will be described hereinbelow.  
         [0033]    The strain gauges  44  are used to record the movement of the subsurface region under test, transmitted to the displacement sensor  18  by the mass  38 . They are temperature-compensated by means of the matched resistors  46 . It is to be noted that the high symmetry of the sensing circuit of FIG. 5 also contributes to the temperature compensation by allowing balancing of the Wheatstone bridge over a range of temperature.  
         [0034]    The strain gauges  44  can be glued on top of the semiconductor substrate  42 , built up by deposit onto substrate  42 , directly etched thereto. The direct etching of the semiconductor substrate  42 , by techniques known in the art, ensures a perfect location of the strain gauges  44  together with a minimized temperature mismatch, therefore a minimized stress concentration, thus enabling the manufacture of a highly sensitive displacement sensor  18 .  
         [0035]    The displacement sensor  18  further comprises an interface board  53  (also referred to herein as an interface unit), shown in FIG. 4, which supports the required communication circuitry attached to the communication connector  30  and/or antenna  36 . One of the communication circuitry functions is to modulate the signal representative of the surface acceleration (obtained from, for example, the Wheatstone bridge). The modulation includes any transformation of a signal to prepare for transmission over the communication interface  21 . As seen in FIG. 6, displacement sensor  18  may further include an analog to digital converter  47 , a transmitting circuit  49  (also referred to as sensor communication means) and a control circuit  57 . The control circuit  57  is used for power management, to adjust the level of amplification of the amplification unit  52  and its offset, during calibration to a prefixed value. Frequency filtering means (not shown), compensation and linearization means (not shown) may be added on substrate  42  to alter the electrical signal from the Wheatstone bridge. In an embodiment of the invention, substrate  42  also includes memory means and processor (neither are shown in FIGS.  2 - 6 ) The control circuit  57  also allows setting the dynamic range of the analog to digital converter  47 .  
         [0036]    Of course, the type of circuitry depends in part on the type of communication  22  of the interface of communication  21  between the displacement sensors  18  and the user-computing interface  16 .  
         [0037]    The displacement sensor  18  can either be externally powered or internally powered by means of an integrated power source  54  such as batteries located underneath the semiconductor substrate  42  (see FIG. 4). Such batteries can be located inside the casing wherever convenient, or even in an extra casing outside the casing  24 . In another embodiment of the invention, sensor  18  may be powered externally by radio-frequency signals.  
         [0038]    As explained hereinabove in relation to FIGS. 2 a  and  2   b , each displacement sensor  18  may be simply deposited on the surface  20 , or secured thereto by means of an adhesive  29  (FIG. 2 b ), or fastened thereto by means of a thread attachment  26  (FIG. 2 a ).  
         [0039]    The damping element  34  attached to the top cover  25 , and the corresponding damping element  34 ′ attached to the casing  24 , may be made of elastic or gel-like material. By ensuring a constant absorption of energy over a range of temperature, and provided they are made of a material having resistance to fatigue such as neoprene or silicone for example, they optimize the damping factor and contribute in maximizing the quality of the signal.  
         [0040]    Indeed, the performance of the displacement sensor  18 , as assessed in terms of amplitude and phase distortion, depends primarily on the magnification factor and the damping factor of the device.  
         [0041]    The shock absorbing pads  32  and  32 ′ are efficient in protecting the displacement sensor  18  from excessive shock, for example during handling.  
         [0042]    Thermoplastic, elastic, sealing product or rubber joints  55  are provided between the cover  25  and the casing  24  for sealing the displacement sensor  18  and protection against adverse environment (see FIG. 4).  
         [0043]    It is to be pointed out that the fact that the displacement sensor  18  of the present invention comprises a semiconductor substrate  42  that has integrated strain gauges  44 , amplification means  52  and control circuitry  57 , permits reducing the noise to signal ratio and therefore the contamination of the signal during transmission to the UIC  16 . The possibility for the displacement sensor  18  to include an analog to digital inverter  47  in case one such item is needed also contributes to the reduction of the noise to signal ratio during transmission.  
         [0044]    Furthermore, people in the art will be aware that the use of semiconductor strain gauges  44  enables achieving a gain superior to that obtained by using conventional foil strain gauges.  
         [0045]    It is also to be underlined that the use of a mass  38  contributes to increase the responsiveness, and therefore, the measurement capability, of the strain gauges assembly.  
         [0046]    As is generally known in the art, the displacement sensor  18  according to the present disclosure operates as follows: when power is fed to the circuit in absence of acceleration, the substrate  42  is not strained and the resistance of the strain gauges  44  is maintained at its original level so that the output signal of the circuit is zero. As an acceleration occurs, an external force is applied on the mass  38 , which causes deformation of the substrate  42  resulting in a change of the electric resistance values of the resistance elements since the substrate  42  bends and deforms the gauges  44 . This deformation changes the nominal resistance of the gauges  44 , causing the equilibrium conditions of the Wheatstone bridge to be broken, giving rise to a voltage output of the circuit. One skilled in the art will understand that analysis of this output voltage enables to obtain the characteristics of the subsurface region under test.  
         [0047]    Turning to FIG. 7 of the appended drawings, the energy impulse generator  14  will now be described.  
         [0048]    The energy impulse generator  14  comprises a spring  60  that is set into compression by a motor assembly  62  so as to store energy and to pull on an impact head assembly  64  through a latch  66 . The impact head assembly  64  is released by activating a solenoid  68  that pulls the latch  66 , thus unlocking the impact head assembly  64 , allowing the extension of the spring  60 .  
         [0049]    Obviously, the power source for the spring  60 , here exemplified as the motor assembly  62 , could be a pneumatic, a hydraulic, electrical or a mechanical source.  
         [0050]    The spring  60  is used against the inertia of the impact head assembly  64  and gives impulsion at the time of an impact, and also as a means for holding back the impact head assembly  64  so as to prevent it from bouncing back after an impact.  
         [0051]    A strain gauge circuitry  63  (also referred to more broadly as a strain measuring device), located on latch  66  for example or an accelerometer  67  located impact head assembly  64 , is used to monitor the energy, which is stored into the spring  60 , by comparing it with the energy command transmitted by the UCI  16  to the EIG  14  control circuit.  
         [0052]    A damper  72  is provided to absorb the shock produced on the assembly, while the energy impulse generator  14  thus transmits a burst of energy by the impact of the impact head assembly  64  on an element to be analyzed.  
         [0053]    A control circuit (not shown) permits to monitor the amount of energy release and the overall operation of the EIG  14 . EIG  14  may also include other circuitry (not shown) such as a processor which operates and manages EIG  14 ; and memory means. Memory means includes various types of memory such as Random Access Memory (RAM), Read-Only Memory (ROM), Electrically Erasable Programmable ROM (EEPROM), etc. RAM is used during calculations, for data storage, and for time-stamp recording (from the processor  84  or a sensor unit and to be transmitted or relayed). ROM comprises initialization codes, start sequences, etc. EEPROM may comprise operation algorithms, tables, sensor identification, etc. EEPROM data may be received via communication interface  21 .  
         [0054]    A power pack  65  is provided for holding a battery. It also adds weight to the overall structure of the EIG  14 . Power pack  65  may include rechargeable batteries. The batteries may be recharged in a contact or contact-free (e.g., via RF) fashion. Power supply through direct cable feed is also an option.  
         [0055]    The EIG  14  is fastened to the surface  20  using threaded attachment  26  or other attachment means essentially similar to those used for the displacement sensor  18  and described hereinabove. Other examples of attachment means include a weight, magnetic material, adhesive material, a “Ramset” or explosive driven type anchoring means, etc. It is understood that these types of attachment means can be used for sensor  18  as well.  
         [0056]    The energy for activating the above described process of impact generation is released by means of a command issued from the user computing interface  16  (UCI) and transmitted to the EIG  14  through a cable, an optical signal or a radio frequency signal. This command triggers the loading and unloading of the mechanism and thus the delivery of an energy pulse by the EIG  14 .  
         [0057]    The user-computing interface (UCI)  16 , exemplified in FIG. 1, comprises a number of subsystems: a user interface system, comprising a keyboard, power and function keys, a display screen and/or a touch screen and/or a voice recognition device, an equipment interface, which allows connection to other output devices such as printers (not shown); an interface system with the IEG; a signal collecting system for collecting data from the displacement sensors  18  of the sensing assembly  12 ; a processing system, which performs the computations and manages the various interfaces together; and a computer interface system that permits connection to other computers.  
         [0058]    Of course, the UCI  16  stores a program or algorithm that, for example, can control the energy impulse generator  14  and the displacement sensors  18 , and collects and stores data from the displacement sensors  18  of the sensing assembly  12 . Furthermore, this program may analyze the collected data to calculate some properties or features of the medium under a surface and display them.  
         [0059]    It is to be noticed that each one of the different assemblies can operate in an autonomous fashion, or powered by a central unit.  
         [0060]    Most interestingly, provided the program and software stored in the UCI  16  is adequate, the system of the present invention can be used in a variety of applications.  
         [0061]    For example, in the field of geotechnical testing, the system of the present invention can be used to detect pockets or faults in the underground, in the mining industry. As a further example, in the military field, the system of the present invention can be used in order to study the geological structure of a terrain for the purpose of effective explosive positioning or hideouts uncovering. The present system could be used to supply data to systems such as the so-called JTIDS (“Joint Tactical Information Distribution System”).  
         [0062]    Additionally, people, in the art will foresee the possibility of adding GPS or gyroscope systems to locate each displacement sensor  18  of the sensing assembly  12 , and the EIG  14 . One possible application is related to the identification of an underground cavity and the determination of its spatial coordinates. An algorithm can be introduced into the UCI  16  that maps, through the use of a global positioning system (GPS), volumes that can be used for underground concealed hideouts, facilities, etc. In military applications in particular, such an algorithm may also be able to detect any structural fault so as to allow planning accordingly strategic delivery of payload in order to maximize the damage to cavities or underground-concealed areas.  
         [0063]    Another field of possible applications where the system  10  of the present invention can be used, providing the adequate algorithm is included into the UCI  16 , is the communication field, taking advantage of the property of low frequency shear waves to propagate over long distances or great depths. Such a specific user-computing interface  16  may perform unidirectional or bi-directional communication, detect, identify and locate movements on the ground surface. In this kind of application, the system  10  uses as a transmitter an electromechanical device that induces energy at various frequencies in the ground, resulting in ground waves. As low frequency shear waves propagate deep into the ground and over long distances, while high frequency waves can travel only short distances, a communication signal consists of an energy signature modulated in frequency and relative amplitude that initiates, delivers, and ends a predetermined communication protocol. Due to various reflections caused by the complex geophysical environment, the transmitted signal is scrambled in time and frequency domain during its way therethrough. The sensing assembly  12 , used at the receiving end, in conjunction with the UCI  16 , unscrambles this signal so as to reconstruct the frequency domain and its variation over time. The high frequency content thereof is used as a means to securely position the source of the signal.  
         [0064]    Additionally, the distribution of displacement sensors  18  enables position triangulation of an emitter or of any signal sources generated by troops, moving vehicles or impacts on the ground. Reconstructing incoming signals, the UCI  16  may process pattern recognition database to match signatures and identify an emitting source.  
         [0065]    A sensor  80  is shown in FIG. 8 in accordance with another embodiment of the invention. Sensor  80  comprises an accelerometer  88  whose response is in relation to the acceleration of the surface  20  with which sensor  80  is in contact. Accelerometer  88  may comprise strain gauges, capacitors, or piezoelectrical devices. Accelerometer  88  can therefore be conventional type accelerometers, but other technologies such as Micro Electro Mechanical Systems (MEMS) or Nano Electrical Mechanical Systems (NEMS).  
         [0066]    The signal (electrical or other equivalent message carrying type of signal) representative of the acceleration produced by accelerometer  88  is fed to amplifier  90 . In an exemplary embodiment, amplifier  90  is an automatic gain amplifier, Amplifier  90  may therefore act to increase the dynamic range of sensor  80 . The gain of amplifier  90  is transmitted to processor  84  which will in turn the signal sent by RF communication circuit  102 .  
         [0067]    After amplifier  90 , the signal is sent to a low-pass filter  92 . Low-pass filter  92  eliminates spectral aliasing in the frequency domain and distortion in the time domain. Sample and hold device  94  then receives the signal, samples it and holds it for a period of time sufficient for analog to digital converter  96  to perform its conversion of the analog signal to a digital signal.  
         [0068]    A person skilled in the art will understand that one can add a second set of accelerometer (not shown), amplifier (not shown) and low-pass filter (not shown) in parallel to the first set heretofore described and feed sample and hold device  94 . Sensor  80  described above acts as a simple accelerometer. With a second accelerometer placed so as to pick up acceleration in an axis which is perpendicular to the axis of the accelerometer  88 , sensor  80  becomes an inclinometer. In yet another embodiment of the invention, sensor  80  may comprise a third set of accelerometer (not shown), amplifier (not shown) and low-pass filter (not shown) in parallel to the first and second sets heretofore described and feed sample and hold device  94 . With a third accelerometer placed so as to pick up acceleration in an axis which is perpendicular to the axis of both accelerometer  88  and the second accelerometer, sensor  80  becomes a gyroscope.  
         [0069]    Persons skilled in the art will understand that one may calculate speed from of the integral over time performed the signal representative of the acceleration output by accelerometer  88 . One may also calculate distance by integrating speed over time. These calculations may take place in processor  84 .  
         [0070]    Returning to the embodiment shown in FIG. 8, the signal from analog to digital converter  96  is sent to low-pass filter  98  which will remove undesired frequencies. Low-pass filter  98  may also be incorporated within analog to digital converter  96 . In order to remove any distortions (in amplitude or phase) the signal is then compensated and linearized in compensation and linearization device  100 . The compensation and linearization device  100  will linearize the signal in order to guarantee a uniform performance in regards of the frequency component, and linearization device  100  will also spread the frequency spectrum of the signal.  
         [0071]    The signal is finally sent to communication circuit  102 . Communication circuit  102  send and receives messages comprising instructions and/or data through a communication interface  21  to a UCI  16  or to other sensors  80  (not shown) in a sensing assembly similar to sensing assembly  12 . Typical instructions include: reset; initialization; download; new algorithms; linearization, compensation and identification parameters (transmit or download); calibration; transmission mode (e.g., direct, network); start of sampling; energy conservation; etc. In network mode, a communication protocol will establish the best path for transferring data from sensor to sensor and finally to UCI  16 . Sensor  80  may therefore act as a data relay. Communication circuit  102  may be protected against electromagnetic interference. In an embodiment of the invention, each sensor  80  has its own Internet Protocol (IP) address and is addressed accordingly.  
         [0072]    Sensor  80  also comprises a processor  84  which operates and performs management of sensor  80 . Sensor  80  comprises memory means  86 . Memory means  86  includes various types of memory such as Random Access Memory (RAM), Read-Only Memory (ROM), Electrically Erasable Programmable ROM (EEPROM), etc. RAM is used during calculations, for data storage, and for timestamp recording (from processor  84  or another sensor  80  and to be transmitted or relayed). ROM comprises initialization codes, start sequences, etc. EEPROM may comprise operation algorithms, tables, sensor identification, etc. EEPROM data may be received via communication interface  21 .  
         [0073]    Power to sensor  80  is provided by power source  82 . Power source  82  may include rechargeable batteries. The batteries may be recharged in a contact or contact-free (e.g., via RF) fashion. Power supply through direct cable feed is also an option.  
         [0074]    Optionally, sensor  80  may also include a positioning circuit (not shown) such as an electronic gyroscope or a Global Positioning System (GPS) receiver.  
         [0075]    In an embodiment of the invention, a housing  104  comprises three sections. Housing  104  is hermetically sealed to protect all of its components from external elements. A first section  106  holds the batteries and power regulation circuitry. A second section  108  holds items  84  to  100  as well as the positioning circuit (not shown). A third section  110  holds communication circuit  102 . Surface of section  110  is conductive thereby providing an electromagnetic barrier to protect communication circuit  102  from electromagnetic interference (EMI).  
         [0076]    Although the present invention has been described hereinabove by way of preferred embodiments thereof, it can be modified, without departing from the spirit and nature of the subject invention as defined in the appended claims.