ORIGIN OF THE INVENTION 
     The invention described herein was made in the performance of official duties by an employee of the Department of the Navy and may be manufactured, used, licensed by or for the Government for any governmental purpose without payment of any royalties thereon. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to magnetic sensors, and more particularly to a magnetic anomaly sensing system having a precision synchronized transceiver that directly measures magnetic field strength for improved detection and/or discrimination of targets. 
     BACKGROUND OF THE INVENTION 
     The basic construction of a prior art eddy-current-based active magnetic anomaly sensor system includes a transmitter and a receiver. The transmitter induces anomalous magnetic induction fields in an electrically conductive or magnetic target located in the sensor detection space. The receiver detects/discriminates the anomalous magnetic induction fields propagating from the target. The transmitter typically consists of electronic circuitry that drives a time dependent electrical current through an induction coil to generate a time and vector distance dependent magnetic induction field. The induction coil can be driven by a continuous wave or pulsed signal. When the generated magnetic induction field interacts with a target, anomalous magnetic moments are induced with in the target which, in turn, cause anomalous magnetic fields to propagate from the target. The sensor system&#39;s receiver typically consists of an induction or “search coil” sensor coupled to signal amplification and processing circuitry to condition and process the magnetic fields detected by the search coil. Through Faraday induction, the search coil generates a voltage proportional to the time derivative of the target&#39;s magnetic anomaly fields lying along the search coil&#39;s axis. Such sensor systems have a variety of shortcomings. 
     For portable active sensor systems having a transmitter and receiver in close proximity to one another, the spatial variation between the actively induced magnetic anomaly field and its time derivative at the receiver decreases with the inverse 6-th power of target-to-receiver distance. Accordingly, to double the detection range of a sensor system, transmitter amplitude or receiver sensitivity must be increased by a large factor, i.e., a factor of 25 6  or 64. Also, relying on the time derivative of the magnetic anomaly field limits the sensor&#39;s time discrimination capability, receiver bandwidth and low frequency sensitivity. 
     Another shortcoming of prior art active magnetic anomaly sensing systems is the interference generated by the transmitter at the receiver. Since the transmitter drive fields are many orders of magnitude larger than the target&#39;s induced magnetic anomaly fields, the transmitted signal has a tendency to overwhelm or jam the reception of the much smaller magnetic anomaly fields. Even with specialized transmitter-receiver geometries, systems that use a continuous wave transmitter drive signal tend to lose much of a target&#39;s transient response. To combat this problem, pulsed transmitters are used and operate on the theory that reception occurs when the transmitter is off. While this works to a certain degree, time constant or transient effects of a typical transmitter coil last for tens of microseconds. Unfortunately, it is in this time frame that the strongest target-signature-related magnetic anomaly field transients are generated by the target. Thus, even though the transmitter coil is deactivated, coil transients tend to jam reception of the strongest magnetic anomaly fields. This problem precludes the use of prior art active magnetic anomaly sensing systems in the detection of non-conductive plastic mines in a conductive media (e.g., seawater) since plastic mines have an extremely short transient response. 
     Still another shortcoming of prior art active magnetic anomaly sensor systems stems from the use of inductive search coils as the magnetic anomaly field sensing element. Specifically, this type of sensing element responds primarily to the time derivative of magnetic flux change components that are parallel to the coil&#39;s axis. Therefore, the sensing element has limited spatial direction sensing capabilities for resolving the direction and magnitude of the three-dimensional vector components that comprise the magnetic anomaly field caused by the target. The lack of three-dimensional resolution limits the system&#39;s target localization and classification capabilities. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide an active magnetic anomaly sensing method and system for sensing magnetic anomalies associated with a target when the target is subjected to a magnetic induction field. 
     Still another object of the present invention is to provide an active magnetic anomaly sensing system capable of directly detecting induced magnetic anomaly fields associated with a target. 
     Yet another object of the present invention is to provide an active magnetic anomaly sensing system that minimizes interference between the transmitter and receiver portions thereof. 
     A further object of the present invention is to provide an active magnetic anomaly sensing system capable of resolving a target-generated magnetic anomaly field in three dimensions. 
     A still further object of the present invention is to provide an active magnetic anomaly sensing system capable of detection, localization and classification of electrically non-conductive targets such as plastic mines immersed in conductive media such as seawater. 
     Other objects and advantages of the present invention will become more obvious hereinafter in the specification and drawings. 
     In accordance with the present invention, an active magnetic anomaly sensing system includes a transmitter for transmitting a magnetic field towards a target such that the magnetic field induces magnetic moments in the target which cause a magnetic anomaly field to propagate from the target. 
     A first (magnetoresistive) sensor is positioned a distance D from the target for directly sensing magnetic field strength and producing a first output indicative thereof. A second (magnetoresistive) sensor is positioned a distance (D+d) from the target for directly sensing magnetic field strength and producing a second output indicative thereof. A controllable power supply is coupled to the transmitter for selectively activating and deactivating the transmitter. The first output and second output are produced when the transmitter is deactivated. The second output is subtracted from the first output to generate a differential output indicative of the magnetic anomaly field propagating from the target. Means and methods are provided to synchronize the response characteristics of the sensors with one another, and to synchronize the transmitter with the sensors so that deactivation of the transmitter results in a near instantaneous detection of magnetic field transients by the sensors. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of an embodiment of an active magnetic anomaly sensing system in its transmit mode in accordance with the present invention; 
     FIG. 2A is a schematic diagram of a segmented transmitter coil connected in series; 
     FIG. 2B is a schematic diagram of a segmented transmitter coil connected in parallel; 
     FIG. 3 is a schematic diagram of the active magnetic anomaly sensing system in its reception mode; and 
     FIG. 4 is a schematic diagram of a construction configuration for the transmitter coil and sensing elements in the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, and more particularly to FIGS. 1 and 3, an embodiment of an active magnetic anomaly sensing system is shown and referenced generally by numeral  100 . System  100  is configured in its transmit mode in FIG.  1  and in its reception mode in FIG.  3 . Accordingly, like reference numerals are used for common elements in FIGS. 1 and 3. 
     System  100  can be used for the detection, localization and/or classification of a target  202  located in an area of interest  200 . To be detectable, target  202  must either be made at least partially from electrically conductive or ferromagnetic materials, or be made from materials having a conductivity that is significantly different than that of the surrounding media. This criteria will be assumed for purposes of describing the present invention. 
     System  100  includes an inductive transmitter coil  102  capable of transmitting a magnetic field  300  towards area of interest  200  in which target  202  resides. In its preferred embodiment, transmitter coil  102  is constructed to provide an optimal combination of coil inductance, capacitance and electrical voltage breakdown immunity so that when a drive pulse is removed from coil  102 , coil current drops to zero in less than one microsecond. Such a rapid current drop insures that the generated magnetic field  300  also falls to zero in less than one microsecond. As will be explained further below, this rapid cutoff response time minimizes interference at the receiver portion of system  100  and also allows for the detection of the rapid transient responses associated with, for example, plastic mines in seawater. 
     Transmitter coil  102  is driven by a power supply  104  capable of supplying a COIL CURRENT and a SWITCH CONTROL PULSE. Accordingly, power supply  104  is representative of one or more controllable current and/or voltage supply(ies) and associated electronics, specific designs of which are well known in the art and which are, therefore, not a limitation of the present invention. 
     In the transmit mode (FIG.  1 ), the COIL CURRENT from power supply  104  is supplied to transmitter coil  102  through closed serially connected solid state switches  106  and  108 . A variety of solid state switches could be used in the present invention. One example of such a switch is the high current handling MOSFET switch model number Si4880DY manufactured by Vishay Siliconix, Santa Clara, Calif. Switches  106  and  108  close and remain closed during the HIGH portion of the SWITCH CONTROL PULSE received from power supply  104 . 
     In the simplest approach to the problem of quickly cutting off magnetic field  300 , transmitter coil  102  is a low inductance drive coil driven by a low drive current. A simple transient suppression device (TSD)  103  is coupled across the leads to transmitter coil  102 . Solid state switches  106  and  108  must be able to withstand (with no leakage or breakdown current) the inductive “kick” voltage that is produced when the coil current is cut off. However, the switches may be limited in their ability to withstand the transients caused by the switching of large magnetic drive coils. Accordingly, for those applications requiring large magnetic moment drive coils, it may be necessary to use a multi-section transmitter coil configuration. Two such configurations are illustrated schematically in FIGS. 2A and 2B where the prefix numeral indicates the type of component (e.g., each  102  is indicative of a transmitter coil subsection, each  103  is a transient suppression device, etc.) and the suffix letter (e.g., A, B, etc.) indicates a subsection. In each case, the field outputs of the drive coil sections are additive thereby creating a transmit field similar to that of a single large drive coil. However, each coil subsection has its own set of switches with sufficient capacity to handle the subsections&#39;s switching transients. Transient suppression circuits and/or devices such as “snubbers” or varistors can be used to reduce the effect of inductive kick on the switches. The subsections could be connected in series (FIG. 2A) or in parallel (FIG.  2 B). In either case, the coil subsections are physically located on the same coil form with transient suppression devices and switches physically located in close proximity to their coil. 
     System  100  also has receiver components that include identical magnetoresistive sensing elements  110  and  112 , an electronic signal differencer  114  and receiver electronics  116 . Magnetoresistive sensing element  110  detects a magnetic anomaly field  302  generated by target  202  as well as other background magnetic fields, while sensing element  112  is a reference sensor positioned to essentially only detect the background magnetic fields. As will be explained further below, sensing element  110  is located/positioned such that it will be closer to target  202  than sensing element  112 . 
     Each of magnetoresistive sensing elements  110  and  112  has at least one ferromagnetic thin film element disposed along an axis. This is known as the field sensing axis. One or more field sensing axes can be defined by each sensing element  110  and  112 . The resistance, and consequently the output voltage, of each thin-film element disposed along a field sensing axis changes as a function of magnetic field strength that is parallel thereto. When positioned in system  100 , the field sensing axis of sensing element  110  is aligned parallel to the field sensing axis of sensing element  112 . If each sensing element is a multi-axis sensing element, the corresponding field sensing axes of the two sensing elements are aligned parallel to one another. 
     A transceiver synchronization modality is provided by the following configuration. A polarity biasing coil  110 A and  112 A is either included with or coupled to sensing element  110  and  112 , respectively. When energized, each of polarity biasing coils  110 A and  112 A causes its respective sensing element  110  and  112  to magnetically saturate along each field sensing axis to a selected polarity. Once saturated to a polarity, sensing elements  110  and  112  are poised to operate at their greatest possible sensitivity level when the saturating field is removed in the receiving mode. In terms of the illustrated embodiment, polarity biasing coils  110 A and  112 A must be able to handle the current load supplied to transmitter coil  102 . This is because coils  110 A and  112 A are coupled in a series connection with transmitter coil  102  and power supply  104 . That is, the COIL CURRENT supplied to transmitter coil  102  is also supplied to coils  110 A and  112 A when switches  106  and  108  are closed for the transmit mode illustrated in FIG.  1 . Examples of magnetoresistive sensing elements that include high-current handling polarity biasing coils are the models HMC1001 and HMC1002 available from Honeywell Solid State Electronic Center, Plymouth, Minn. For the illustrated embodiment, if the polarity biasing coil does not exist or has low current handling capacity, auxiliary coils serving as coils  110 A and  112 A (or a circuit that can deliver a current within the coils capacity) would have to be provided. 
     The output signals generated by each of sensing elements  110  and  112  (in the reception mode) are supplied to signal differencer  114  to generate a difference signal. Accordingly, if sensing elements  110  and  112  are set to the same polarity (by their respective polarity biasing coil), signal differencer  114  is a subtraction circuit. Conversely, if sensing elements  110  and  112  are set to opposite polarities, signal differencer  114  could be a summing circuit. In either case, the difference signal is supplied to receiver electronics  116  which can include a variety of signal conditioning and processing elements (e.g., amplifiers, filters, A/D converters, processors, clocks, etc.). A variety of configurations of receiver electronics  116  would be well known to one of ordinary skill in the art and, as such, does not constitute a limitation of the present invention. 
     In operation, transmission of magnetic field  300  towards target  202  occurs when power supply  104  supplies the HIGH portion of the SWITCH CONTROL PULSE to switches  106  and  108 , and the COIL CURRENT activates transmitter coil  102  through closed switches  106  and  108  as illustrated in FIG.  1 . Simultaneously, the COIL CURRENT is supplied to polarity biasing coils  110 A and  112 A in order to synchronize the transceiver and set the polarity of sensing elements  110  and  112 , respectively, while transmitter coil  102  is activated. During the reception mode of system  100  illustrated in FIG. 2, the LOW portion (e.g., zero) of the SWITCH CONTROL PULSE causes switches  106  and  108  to open. The opening of switches  106  and  108  in the illustrated embodiment causes the simultaneous deactivation of transmitter coil  102  and removal of biasing current from each of polarity biasing coils  110 A and  112 A. Since each of sensing elements  110  and  112  was magnetically saturated when the biasing current was removed, each of sensing elements  110  and  112  is synchronized to the same point of its response curve as the system switches from the transmit to the receive mode. Further, since each of elements  110  and  112  is saturated when the bias current is removed, each element is operating at its highest possible reception sensitivity within nanoseconds after transmitter coil  102  is deactivated. The outputs of sensing elements  110  and  112  are differenced at signal differencer  114  (as described above) and then processed by receiver electronics  116 . 
     As mentioned above, system  100  is configured such that sensing element  110  will be positioned closer to target  202  than sensing element  112 . A possible construction illustrating this requirement is depicted schematically in FIG. 4 where transmitter coil  102  is mounted on the end of a rigid rod  10 . Electrical connections between the elements are omitted for clarity of illustration. Mounted adjacent transmitter coil  102  (or within transmitter coil  102  for space saving reasons) is sensing element  110  such that a distance D separates sensing element  110  from target  202 . Also mounted on rod  10  is sensing element  112  located a distance (D+d) from target  202 . The field sensing axis(es) (not shown) of sensing element  112  is(are) aligned parallel to those of sensing element  110 . The distance d between sensing elements  110  and  112  must be such that sensing element  112  detects very little of magnetic anomaly field  302  as compared with sensing element  110 . That is, sensing element  110  detects magnetic anomaly field  302  along with all other surrounding magnetic fields, while sensing element  112  is positioned to detect only all other surrounding magnetic fields. In this way, when the outputs of sensing elements  110  and  112  are “differenced”, only the magnetic anomaly field  302  due to target  202  will be processed by receiver electronics  116 . The distance d between sensing elements  110  and  112  will vary depending on the active sensing application and sensitivity of the particular sensing element. Typically, the distance d will range from approximately 4 centimeters to approximately 1 meter. Also, note that solid state switches  106  and  108  can be mounted on rod  10  in close proximity to sensing elements  110  and  112 , respectively, in order to minimize current transients when switches  106  and  108  are to be opened. 
     The advantages of the present invention are numerous. The unique application of magnetic induction drive coil and receiver synchronization, combined with the direct field sensing capabilities of magnetoresistive field sensing elements, provides a better system and method of active magnetic signal detection, localization and/or classification. By magnetically saturating the sensing elements during the transmission mode, the sensing elements are placed at their highest sensitivity for the reception mode. Further, since the reception mode is started at the same time the transmission mode is ended, the present invention is sensitive to the strongest and highest target information content magnetic anomaly field transients propagating from a target. The present invention&#39;s high-speed, synchronous transceiver response capability makes it well-suited to the problem of detecting non-conductive plastic mines in seawater. 
     When the sensor&#39;s transceiver (i.e., transmitter coil  102  and sensing elements  110 / 112 ) is moved so that a target is within the sensor&#39;s detection range, a signal is generated that can be used by the operator to home in on the target. Depending on the level of sophistication built in to its receiver electronics  116 , the invention can be used as a relatively simple proximity detector that detects and localizes conductive objects. For more complex and critical tasks such as detection, localization and classification of explosive mines, the present invention&#39;s output can be used in a complex analysis of the temporal and spatial variations of target response in order to extract information about target geometry and composition. This information can be critical in the discrimination between real targets and background clutter. This present invention is applicable to a wide variety of commercial and military uses where its enhanced magneto-inductive target detection and discrimination capabilities can be used to provide more accurate and complete information about target characteristics. 
     Although the invention has been described relative to a specific embodiment thereof, there are numerous variations and modifications that will be readily apparent to those skilled in the art in light of the above teachings. For example, as mentioned above, the magnetoresistive field sensing elements could be constructed with thin-film elements disposed on multiple axes such as three orthogonal axes in order to make the sensing element sensitive to a magnetic anomaly field in three dimensions. Use of magnetoresistive sensing elements in this fashion is disclosed in U.S. Pat. No. 5,359,287. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described.
An active magnetic anomaly sensing system includes a transmitter for transmitting a magnetic field towards a target. The magnetic field induces magnetic moments in the target which cause a magnetic anomaly field to propagate from the target. A first sensor positioned a distance D from the target directly senses magnetic field strength and produces a first output. A second sensor positioned a distance (D+d) from the target directly senses magnetic field strength and produces a second output. A controllable power supply is coupled to the transmitter for selectively activating and deactivating the transmitter. The first and second outputs are produced when the transmitter is deactivated. The second output is subtracted from the first output to generate a differential output indicative of the magnetic anomaly field propagating from the target. Means and methods are provided to synchronize the response characteristics of the sensors with one another, and to synchronize the transmitter with the sensors so that deactivation of the transmitter results in a near instantaneous detection of magnetic field transients by the sensors.