A MOVABLE SENSOR HEAD

TECHNICAL FIELD

This invention relates to a detector system with a sensor head supported by, and able to be moved independently of, a supporting body.

INCORPORATION BY REFERENCE

This patent application claims priority from:

- Australian Provisional Patent Application No 2010900242 titled "Metal detector improvements for robotic applications" filed 22 January 2010.

The entire content of this application is hereby incorporated by reference.

BACKGROUND

The detection of landmines, explosive remnants of war and improvised explosive devices is a difficult and dangerous activity. Currently, most of this activity is done by trained operators and soldiers who sweep the areas of interest with hand-held or push-cart metal detectors, magnetometers or ground penetrating radars. An alternative method, at least for relatively flat areas with good road access, is to use blast-resistant vehicles or robotic vehicles (remotely controlled or autonomous) with detectors and other sensors mounted on them.

Given that such vehicles contain large amounts of metal, it is likely that the operation of the sensitive detection system is hampered by the presence of the vehicle. Relative movement between the sensors and the vehicle is particularly troublesome, and the problem is exacerbated if the sensor head must follow the ground contour. So far, the only means available to counteract this problem is to reduce sensitivity or increase the separation between the sensor head and the metal parts of the vehicle and /or to limit their relative movement through rigid construction. The detection of metal objects is generally done with electromagnetic methods, with frequencies ranging from 0 (DC) to several GHz: magnetometers, pulse-induction (PI) or continuous-wave (CW) metal detectors and ground penetrating radars (GPR). Magnetometers are passive devices (they sense changes produced by ferromagnetic objects in the magnetic field of the Earth), while the other detection systems are active (they transmit a time-varying magnetic field (the primary field) or a transmit signal, and receive the secondary field produced by the objects under the influence of the transmit magnetic field). The object of this invention is to describe methods to mitigate the effect of a metallic vehicle on the detection equipment, in effect allowing the sensor head to be mounted closer to and /or allowed more movement relative to, the vehicle. BRIEF SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provided an apparatus for detecting a target including: a supporting body;

' a metal detector system including a sensor head, supported by and able to be moved independently of the supporting body, to transmit a transmit signal and to receive a first signal, the first signal includes signals due to the target and the supporting body;

a sensor for producing a second signal indicative of movement or position of the sensor head relative to the supporting body; and

a processor for processing the first signal, the processing includes substantially cancelling the signals due to the supporting body from the first signal using the second signal, to produce an indicator signal indicating the presence of the target.

In one form, the second signal is multiplied by a predetermined coefficient prior to being subtracted from the first signal.

In one form, the sensor includes one or more receivers positioned at the supporting body, the one or more receivers measure the strength of the transmit signal to produce the second signal. In one form, the one or more receivers are one or more coils. In one form, the one or more coils are three mutually orthogonal coils.

In one form, the metal detector system is a continuous wave (CW) metal detector.

In one form, the metal detector system is a pulse induction (PI) metal detector.

In one form, the metal detector system is a magnetometer.

In one form, the metal detector system is a ground penetrating radar (GPR) system. In one form, the metal detector system is a combination of a continuous wave (CW) metal detector and a ground penetrating radar (GPR) system. In one form, the metal detector system is a combination of a pulse induction (PI) metal detector and a ground penetrating radar (GPR).

According to a second aspect of the present invention, there is provided method for detecting a target using the apparatus defined in the first aspect, the method including:

receiving a first signal due to the target and the supporting body;

producing a second signal indicative of movement or position of the sensor head relative to the supporting body; and

processing the first signal to substantially cancel the signals due to the supporting body from the first signal using the second signal, to produce an indicator signal indicating the presence of the target.

Throughout this specification and the claims that follow, unless the context requires otherwise, the words 'comprise' and 'include' and variations such as 'comprising' and 'including' will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

The reference to any background or prior art in this specification is not, and should not be taken as, an acknowledgment or any form of suggestion that such background or prior art forms part of the common general knowledge.

Specific embodiments of the invention will now be described in further detail with reference to and as illustrated in the accompanying figures. These embodiments are illustrative, and not meant to be restrictive of the scope of the invention. Suggestions and descriptions of other embodiments may be included within the scope of the invention but they may not be illustrated in the accompanying figures or alternatively features of the invention may be shown in the figures but not described in the specification.

"Logic," as used here in, includes but is not limited to hardware, firmware, software, and/or combinations of each to perform a function(s) or an action(s), and/or to cause a function or action from another component. For example, based on a desired application or needs, logic may include a software controlled microprocessor, discrete logic such as an application specific integrated circuit (ASIC), or other programs embodied in a logic device. Logic may also be fully embodied as software.

"Software," as used here in, includes but is not limited to, one or more computer readable and/or executable instructions that cause a computer or other electronic device to perform functions, actions, and/or behave in a desired manner. The instructions may be embodied in various forms such as routines, algorithms, modules or programs including separate applications or code from dynamically linked libraries. Software may also be implemented in various forms such as a stand-alone program, a function call, a servlet, an applet, instructions stored in a memory, part of an operating system or other type of executable instructions. It will be appreciated by one of ordinary skilled in the art that the form of software is dependent on, for example, requirements of a desired application, the environment it runs on, and/or the desires of a designer/programmer or the like. Those of skill in the art would understand that information and signals may be represented using any of a variety of technologies and techniques. For example, data, instructions, commands, information, signals, bits, symbols, and chips may be referenced throughout the above description may be represented by voltages, currents, electromagnetic waves, magnetic fields or particles, optical fields or particles, or any combination thereof.

Those of skill in the art would further appreciate that the various illustrative logical blocks, modules, circuits, and algorithm steps described in connection with the embodiments disclosed herein may be implemented as electronic hardware, computer software, or combinations of both. To clearly illustrate this functional equivalence of hardware and software, various illustrative components, blocks, modules, circuits, and steps have been described above generally in terms of their functionality. Whether such functionality is ^' implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the present invention.

The steps of a method or algorithm described in connection with the embodiments disclosed herein may be embodied directly in hardware, in a software module executed by a processor, or in a combination of the two. For a hardware implementation, processing may be implemented within one or more application specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), field programmable gate arrays (FPGAs), processors, controllers, microcontrollers, microprocessors, other electronic units designed to perform the functions described herein, or a combination thereof. Software modules, also known as computer programs, computer codes, or instructions, may contain a number a number of source code or object code segments or instructions, and may reside in any computer readable medium such as a RAM memory, flash memory, ROM memory, EPROM memory, registers, hard disk, a removable disk, a CD-ROM, a DVD-ROM or any other form of computer readable medium. In the alternative, the computer readable medium may be integral to the processor. The processor and the computer readable medium may reside in an ASIC or related device. The software codes may be stored in a memory unit and executed by a processor. The memory unit may be implemented within the processor or external to the processor, in which case it can be communicatively coupled to the processor via various means as is known in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 depicts a top view of a general form of a vehicle-based detector.

Figure 2 depicts the processing of the first signal from the sensor head with the second signal ^' from a sensor.

Figure 3 depicts one embodiment of the sensors.

Figure 4 depicts one embodiment of the sensor head and the sensor.

Figure 5 depicts an example of the first signal, the second signal and the cancellation of the second signal from the first signal.

Figure 6 depicts functional block diagrams of one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

This invention relates to an apparatus for detecting a target object with a sensor head supported by, and able to be moved independently of, a supporting body supporting the sensor head. In particular, the invention detects the relative position and/or the relative movement of the sensor head with respect to the supporting body in order to cancel signals due to the supporting body so that such signals will not affect the performance of the detector system.

In the following description, the principle of the invention is explained using a metal detector as an illustration, but the invention is not limited to any specific type of metal detector, and variations to suit other types of sensors are described. For example, a GPR can function as a metal detector but it can also detect material such as plastics, and it is envisaged that the invention can be applied to a detector system using a GPR.

The invention can also be applied to any application deemed appropriate by a person skilled in the art. To better describe the invention, examples of vehicle-based detector with one or more sensor heads movable relative to the body of the vehicle are described herein. Such examples are in no way limiting the invention as claimed. Other examples include sensor heads attached to a robotic arm movable relative to the base of the robotic arm or retractable sensor heads along a conveyer belt. Figure 1 depicts a top view of a general form of a vehicle-based detector. A typical metal detector operates by transmitting electromagnetic energy (transmit signal) into the ground through one or more transmitters within sensor head 2 where the energy interacts with a buried object, which in turn generates an electromagnetic energy. The sensor head 2 also includes one or more receivers to receive and measure the electromagnetic energy from the buried object, which differs from an electromagnetic energy returned by the ground.

Sensor head 2 is supported by supporting body 1 , which is the body of a vehicle. In one embodiment, the supporting body 1 supports the sensor head through a use of a beam or an arm 5. The sensor head 2 can be smaller or bigger than the supporting body 1 , but supporting body 1 can support the sensor head 2 above the ground during operation.

Depending on the applications, the sensor head 2 can also be located at either side of the vehicle or behind the vehicle, or in any other place deemed appropriate by a person skilled in the art. For example, in certain operation, it may be desirable to detect objects at the side of the vehicle.

The sensor head 2 is movable relative to the supporting body 1. For example, sensor head 2 may move side-to-side during operation. The movement path of the sensor head may form an arc as may be implemented by the arm 5 or other device attached between the arm 5 and the sensor head 2. It is also possible to have the beam 5 being lengthened /shortened such that the sensor head 2 may move away and towards the supporting body 1. The arm may be made from non-metallic material and parts of the supporting body may be made from non-metallic material to minimise their influence on the performance of the detector apparatus.

A sensor of motion or relative position, not shown in Figure 1 but depicted pictorially in Figure 2 as sensor 3, monitors the movement of the sensor head 2 relative to the supporting body 1. Examples of relative movement/position sensor are described later in the specification.

During operation, it is expected that the sensor head 2 moves relative to the supporting body 1 , for example, during a sweeping action, the sensor head 2 may move from side-to-side relative to the supporting body 1. Accordingly, the position of the sensor head 2, with respect to the supporting body 1 , changes during operation. Further, if the terrain the vehicle travels on is uneven rather than perfectly flat, the movement of the sensor head 2 with reference to the supporting body 1 may be increased and be different from that intended.

The supporting body 1 , and in some cases the arm 5, will include metallic parts. Often, the outer of the supporting body 1 is entirely metallic. The electromagnetic energy transmitted from the sensor head 2 might interact with those metallic parts. As the sensor head 2 moves relative to the supporting body 1 , signals due to the presence of those metallic parts may elicit indication of those metallic parts as targets. Even if the metallic parts are not registered as desirable target, signals due to the presence of the metallic parts can interfere in many ways with the signals from sought targets.

By using a relative movement/position sensor, it is possible to take into consideration the effect of supporting body 1 more accurately than by just approximating the distance and/or motion of the sensor head 2 from and with respect to the supporting body 1.

Figure 2 depicts the processing of the first signal 1 1 from the sensor head 2 and the second signal 12 from a relative movement/position sensor 3 within a processor 14 which performs further processing to produce an indicator signal 13, indicating the presence of a target. The location of the sensor 3 can be different in different embodiments. For example, it is possible to position the relative movement/position sensor on or within the supporting body 1 , on or within the sensor head 2, or on or within the beam 5.

The metal detector used as an example is most sensitive to metal objects passed under the sensing coils. With reference to figure 2, ideally the metal detector produces a first signal 1 1 , from sensor head 2, that includes the combined response from the targets and their environment, for example signals from magnetic ground. Processor 14 may be co-located with the sensor head or be mounted on/in the supporting body or arm. The processor 14 removes those components of the first signal 1 1 due to the environment, leaving only signals from metallic targets for detection and classification, in order to produce an indicator signal 13.

In one embodiment, the first signal 1 1 is demodulated or sampled by processor 14, prior to processing, to cancel the signals due to the supporting body 1 from the demodulated or sampled first signal. The first signal 1 1 may also be filtered prior to that processing.

It is also possible to perform a cancellation of the signals, due to the supporting body 1 , prior to any further processing by processor 14. The first signal 1 1 , thus processed, can be further processed to produce, for example, classification of the type of the metal object detected, e.g. coins, landmines,

If the metal detector is supported by a supporting body I and the sensor head 2 moves with respect to the metal in the vehicle, the metal detector may also detect signals resulting from the proximity of metal on/in or adjacent the arm/supporting body and may not distinguish between the signals produced by the sought targets and those produced by the supporting body 1 (when the targets and the supporting body are both under the influence of the transmission), based only on the first signal 1 1 . Generating a second signal 12, that is indicative of the relative movement between the sensing coils and the vehicle, enables differentiation of target signals and signals due to the supporting body 1 , This differentiation allows cancellation of those signals due to the supporting body 1 , from the first signal using the second signal. There is a number of means that could be used to produce the second signal 12. Some examples include a receive coil, mounted so as to be substantially fixed with respect to the supporting body 1 , that detects the transmitted field, a time-varying target-like circuit mounted on the vehicle, a transmitter similarly mounted transmitting a unique reference signal detectable by the receive coils of the sensor head, or a sensor of mechanical movement.

If the first and the second signals (1 1 , 12) are strongly correlated, in the sense that their relationship is linear with no delay between them and minimal noise, the effect of the vehicle could be removed by applying the following calculation for each signal in the data streams: Corrected First Signal = First Signal - x Second Signal

In this formula, the second signal 12 is multiplied by a correlation coefficient and subtracted from the first signal, to give a corrected first signal. The correlation coefficient K can be measured experimentally by considering the situation where there are no targets, the sensing coils are moved within the allowable or expected range and the first and the second signals ( 1 1 , 12) are recorded and compared. In principle, it is possible to calculate theoretically the correlation coefficient K and so the decision to use an experimental or a calculated correlation coefficient is a matter of practicality. In some embodiments, the relationship between the first signal 1 1 and the second signal 12 could be non-linear, there could be time delays between them and either could be affected by noise. If the relationship between the first signal 1 1 and the second signal 12 is non-linear, then known methods to map from the second signal 12 to the first signal 1 1 can be applied. For example, one could use interpolation between tabulated values, or transform one of the first 1 1 or second 12 signals, using a certain type or types of non-linear functions, to linearise it with respect to the other signal, could use an inherently non-linear processing method such as neural networks to model the transfer function between the two signals, or to use a look-up table.

If there is a fixed time delay between the first 1 1 and the second 12 signals, an appropriate delay can be introduced in the path of the advanced signal. However, if the time delay is frequency-dependent or additional delays are not acceptable, another type of filter, for example a Kalman filter, can be used to predict the delayed signal and to reduce the effect of noise on the output signal.

The generation of the second signal 12 depends, to some extent, on the technology employed by relative movement/position sensor 3.

In one embodiment, as illustrated in Figure 3, the sensor head 2 is supported on arm/beam 5, connected to the supporting body 1 , which can rotate through a range of angles with mechanical sensors, for example, rotary encoders or gyroscopes, generating the second signal 12. The mechanical sensors can be located at joints 31 and 32 to detect and report the relative movement/position of the sensor head 2 with respect to the supporting body 1 to the processor 14 through second signal 12. Based on the second signal 12, processor 14 processes the first signal using one of the techniques described above.

For metal detectors (either CW or PI), sensor 3 can be one or more receive coils affixed to the supporting body 1. One embodiment is as illustrated in Figure 4 where the sensor head 2 includes at least one transmit coil 39 and an array of receive coils 42. Depending upon the task to which the metal detector is applied, the detector can be equipped with just one transmit coil 39 and one receive coil 42 in its sensor head.

In one embodiment, the array of receive coils 42 within the sensor head 2 provides the first signal 1 1 to the processor 14. Another receive coil 41 , which may or may not be similar to one of the receive coils 42, is attached to the supporting body 1 as sensor 3. When the receive coil 41 is away from the detection area of interest, the second signal 12 generated by the receive coil 42 is substantially free of any signal from a target. An example of such a set of signals is illustrated in Figure 5. In Figure 5, waveform 51 represents an example of the first signal 1 1 after demodulation, and waveform 53 represents an example of the second signal 12 after demodulation. For simplicity of the example, coefficient K is assumed to be 1 , and the waveform 53 can be subtracted from waveform 51 to obtain waveform 55, which is substantially free from the signal due to the supporting body 1. The peak 56 obviously indicates a target in waveform 55, but in 51 it might not be differentiated from the signals due to the relative movement between the sensor head 2 and the supporting body 1.

In another embodiment, the relative movement/position of the sensor head 2 may be determined by measuring the strength of the transmission by transmitter 39 within the sensor head 2 using receive coil 41 .

Ln one embodiment, the plane of the receive coil 41 is orthogonal to the plane of the transmit coil 39. This has the benefits of reducing the coupling between the transmit coil 39 and the receive coil 41 , thereby improving the signal to noise ratio of the signal 12.

In another embodiment, receive coil 41 (which is acting as sensor 3) is substituted with three mutually orthogonal receive coils. This may improve the estimation of the relative movement/position of the sensor 2 with respect to the supporting body 1 as the receive signals from the three mutually orthogonal coils 41 may provide a good three-dimensional movement/position estimation. The advantage of this method is that the relative time delays between the first 1 1 and second 12 signals are minimal, their processing being almost identical. In the case of the PI metal detector, it is best if the receive coil, that acts as a relative movement/position sensor 3, and its signal processing path are capable of receiving during the on-time of the transmitter. If this is not the case, variable secondary fields from the currents induced in the metal of the vehicle could be used for the same purpose. An alternative way of generating the second signal 12 is to transmit a reference signal, from the supporting body 1 , in such a way that the strength of the received reference signal depends strongly on the relative positions of the sensor head 2 and the supporting body 1. The reference signal is received by the receive coils 42 of the sensor head 2. In other words, in this embodiment, the sensor 3 incorporates the receive coils 42. The reference signal can be separated from the signals from targets using various techniques known to a person skilled in the art. For example, the reference signal is of a frequency different to that of the transmission from the sensor head 2 and thus the signals from sought targets. Alternatively, the transmit reference signal could be synchronised to the magnetic field transmitted by the sensor head 2, but modulated with a frequency just greater than the stop-band of the low-pass filters used in the metal detector. Figure 6 shows one embodiment of the functional block diagram with receiver 71 (for example it may be one of receive coils 42) receiving signals. The received signals are amplified 73, demodulated 75, and filtered 77 such that the signals from 71 as received by processor 14 are without the reference signal. If the reference signal is at a frequency higher than the transmission from the sensor head 2, the signals from 71 are low-pass filtered at 77. In the event that the reference signal is required to determine the relative movement/position of the sensor head 2 with respect to the supporting body 1 , the signals from 71 may be bandpass filtered at 77.

Alternatively, a separate receive coil 61 may be used to receive the reference signal, and the reference signal is processed as for the other metal detector receive coils (amplified 63, demodulated 65, filtered 67 in this or a different ordering of operations).

In another embodiment, sensor 3 can be the combination of mechanical sensors (which provides, for example, the side-to-side sweep information) and electromagnetic sensors described above (e.g. a receive coil on the supporting body 1 or. a transmit coil on the supporting body 1 ) to generate second signal 12. The correlation coefficient can then be determined for the first signal 1 1 and the second signal 12, which can then be used to determine the corrected signal. For example, some previously described magnetic measurements can be made, with the angle between the support beam 5 and sensor head 2 being the mechanical measurement available (see Figure 3). However, the correlation coefficient relating the first signal 1 1 and the up/down movements of the sensor head may depend on the position of the sensor head 2, a function of the angle. By utilising signals from both the mechanical and electromagnetic measurements to produce the second signal 12, the correct correlation coefficient for the second signal 12 and the first signal can be determined.

To some extent, the same methods could be applied to correct the signals 12 from GPR or magnetometer or combinations of metal detector, GPR and magnetometer. They system might require additional sensors, e.g. magnetic compass to determine the orientation of the system in the magnetic field of the Earth.

A detailed description of one or more preferred embodiments of the invention is provided above along with accompanying figures that illustrate by way of example the principles of the invention. While the invention is described in connection with such embodiments, it should be understood that the invention is not limited to any embodiment. On the contrary, the scope of the invention is limited only by the appended claims and the invention encompasses numerous alternatives, modifications, and equivalents. For the purpose of example, numerous specific details are set forth in the description above in order to provide a thorough understanding of the present invention. The present invention may be practiced according to the claims without some or all of these specific details. For the purpose of clarity, technical material that is known in the technical fields related to the invention has not been described in detail so that the present invention is not unnecessarily obscured.