Abstract:
An integrated fluxgate-induction sensor is formed of a combined fluxgate sensor and induction sensor using a common core. The sensor may be in serial operation where it switches between a fluxgate mode for measuring static magnetic fields and an induction mode for measuring alternating magnetic fields. Additionally, the sensor may be used in an interleaved operation where the sensor operates from the fluxgate mode during the transition period where its core is changing from a high permeability state to a low permeability state or vice versa, while the sensor operates in the induction mode when the core is in its high permeability state. The resulting sensor provides for a compact magnetic sensor system capable of sensing magnetic fields which oscillate from zero frequency to 10 kHz and higher.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present invention claims the benefit of U.S. provisional application No. 60/607,301 entitled “Integrated fluxgate-induction sensor” filed Sep. 7, 2004. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention generally pertains to the art of sensors for measuring magnetic fields. More particularly, this invention relates to a sensor for measuring both static and oscillating magnetic fields.  
         [0004]     2. Discussion of the Prior Art  
         [0005]     Typically, magnetic sensors are divided into two categories. The first category includes those sensors that are designed to measure static magnetic fields. The second category includes those sensors designed to measure oscillating magnetic fields. For example, the Earth&#39;s main magnetic field is quasi-static, while those magnetic fields produced by alternating current electricity are oscillating fields. Sensors designed to measure static or low frequency fields have an upper frequency response generally around a few thousand hertz. For example, fluxgate and optically pumped magnetometers fall into this category. The oscillating magnetic field sensor category is dominated by magnetic induction sensors, which typically operate from 10 Hz to 100 kHz or higher. The exceptions to this rule include the superconducting quantum interface devices (SQUID). SQUID sensors can measure from a static field to a field oscillating at approximately 1 MHz. Magnetoresistive sensors can also measure static and oscillating magnetic fields. However, SQUIDS are difficult to use in practical applications and magnetoresistive sensors lack sensitivity adequate for many applications.  
         [0006]     There are a number of emerging applications that require magnetic measurements from a static field to one oscillating in the order of 20 kHz, for which the overall system size and weight are important criteria. For these systems, using separate static and oscillating magnetic sensors is unfavorable. For example, future advanced detection systems for unexploded ordnance will require a combination of static and oscillating magnetic measurements, preferably three-axis vector signals, to provide characterization of target shape and reduce false alarm rate. Present oscillating magnetic field sensors used for unexploded ordnance detection are predominately based on using induction coils. To measure the static magnetic field, a second sensor, usually an optically pumped magnetometer or a fluxgate magnetometer, is required. The need for independent alternating static and magnetic sensors increases system size, weight, and cost, while preventing a rigorously co-located measurement of the target response. Another example of sensors which need a wide frequency response range is sensors used for atmospheric and planetary magnetic fields. The overall size and weight of such sensors are critical factors. In these fields, the largest possible upper operating frequency is typically desired. A further example is sensors used as part of electromagnetic surveillance systems.  
         [0007]     In all of these examples, measurements of multiple components of the magnetic field are generally desired. Further, there is a concern that when separate sensors are used, i.e., one sensor for static fields and another sensor for oscillating magnetic fields, the metallic or magnetically permeable components of one magnetic sensor will disturb the field measured by the other sensor. In particular, a static magnetometer, such as a fluxgate magnetometer, cannot generally be put in close proximity to an induction sensor that uses a high permeability core because the signal detected by the fluxgate magnetometer will be affected by the distortion of the magnetic field caused by the induction sensor core.  
         [0008]     Accordingly, there exists a need in the art for a compact magnetic sensor system capable of sensing magnetic fields that oscillate from a frequency of zero to 100 kHz and higher. Further, since using separate static and oscillating magnetic sensors is not favorable, a single compact sensor with a capability to operate in the entire frequency range is desired.  
       SUMMARY OF THE INVENTION  
       [0009]     The present invention combines a fluxgate sensor with an induction sensor using the same high-permeability material for both operational modes. The magnetic field sensor constitutes a low-noise sensor and is able to operate in both a fluxgate mode to measure a static magnetic field and an induction mode to measure an oscillating magnetic field. With this structure, potential crosstalk problems with the core are removed verses employing two separate sensors wherein one sensor would affect the response of the other. The present sensor evinces an advantageous combination of bandwidth sensitivity, size, and cost. Further, the present invention makes formation of a multi-axis system easier by minimizing the size of each combination static and oscillating sensor channel.  
         [0010]     The sensor may be controlled to perform in either a serial operation or an interleaved operation. In serial operation, a fluxgate mode is operated in durations in the order of one tenth of a second, with alternating induction modes. In the interleaved operation, the sensor is operating in a fluxgate mode during a transition period when the core is changing from a high permeability state to a low permeability state or changing from a low permeability state to a high permeability state, with the sensor operating in the induction mode when the core is at the high permeability state. In such an arrangement, the same coils are used for both the induction sensor and the fluxgate magnetometer. Additionally, other common parts, like the sensing coil and drive coil, are also shared. A specific coil is used as a drive coil for the fluxgate mode and an anti-pulsing coil for the induction mode. Composite cores made of multiple smaller elements are preferably employed for a high frequency operation and fast impulse response.  
         [0011]     Additional objects, features and advantages of the present invention will become more readily apparent from the following detailed description of a preferred embodiment when taken in conjunction with the drawings wherein like reference numerals refer to corresponding parts in the several views. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is a circuit diagram of an integrated fluxgate-induction sensor with an associated probe according to a preferred embodiment of the invention;  
         [0013]      FIG. 2  is a cross-sectional view of the sensor probe shown of  FIG. 1 ;  
         [0014]      FIG. 3  is a graph showing system response of the integrated fluxgate-induction sensor with induction and fluxgate modes performed in serial operation; and  
         [0015]      FIG. 4  is a graph showing system response of the integrated fluxgate-induction sensor with induction and fluxgate modes performed in interleaved operation.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0016]     With initial reference to  FIGS. 1 and 2 , there is shown a circuit diagram of an integrated fluxgate-induction magnetometer sensor  10  constructed in accordance with the present invention. As illustrated, a drive circuit  100  for sensor  10  includes an oscillator  110  whose signal output is coupled by way of a frequency divider  120  to a driving amplifier  130 . The signal is then amplified by driving amplifier  130  and the amplified signal is applied to a drive coil  140  of a probe  150  of sensor  10 . As best seen in  FIG. 2 , drive coil  140  of probe  150  is wrapped around first and second elongated bars  152  and  153 , each formed of high magnetic permeable material. Drive coil  140  is wrapped in the reverse direction for both bars  152  and  153 . Elongated bars  152  and  153  actually form a core  155  of probe  150 . Preferably, the material also has a low energy requirement for saturation. The materials used for elongated bars  152  and  153  could be alloys, such as Hypemik Mumetal and Permalloy, but a nanocrystalline alloy or a ferrite material are preferably employed. As shown, drive coil  140  wraps around both elongated bars  152  and  153  individually. Both elongated bars  152  and  153  are arranged within a support or cover  163 . A sensing coil  170 , which is wrapped around both elongated bars  152  and  153  of core  155 , has split windings  172  and  173  as illustrated in  FIG. 2 . As referenced in  FIG. 1 , probe  150  is coupled to a pre-amp  180  and an output of pre-amp  180  is filtered by a filter  190  to produce a filtered signal. The filtered signal is sent to a demodulator  195 , which also is controlled by oscillator  110 . The output from demodulator  195  is amplified by an amplifier  200  and sent to an analog-to-digital converter  205 .  
         [0017]     As can be seen from the above description, a fluxgate sensor has been combined with an induction sensor to establish sensor  10 , with the same core  155  of high permeability material being employed for both operation modes. The resulting sensor  10  has low noise characteristics and is able to selectively operate both in a fluxgate mode to measure a static magnetic field and in an induction mode to measure an oscillating magnetic field. Sensor  10  is compact in nature and can be produced at a low cost relative to separate fluxgate and induction sensors. Although a double core configuration is shown, a single-core sensor can also be used to form a fluxgate-induction sensor. However, due to the presence of a large unbalanced flux, its performance is not as good as the two-core configuration in detecting the static field.  
         [0018]     When operating in a fluxgate mode, oscillator  110  generates an excitation current lexe having an oscillating voltage signal or waveform with a certain frequency. Preferably, the waveform is an oscillating sawtooth waveform or a conventional sine waveform. The primary frequency of the signal is divided by two in frequency divider  120  and used to drive the material of core  155  of elongated bars  152  and  153  of probe  150  into and out of a magnetically saturated state twice with each two cycles of the waveform. By changing the core permeability, the core field change induces a voltage or output signal in sensing coil  170  proportional to the component of the static magnetic field strength Hs that is parallel to the axis of the drive coil  140 . When a magnetic material is saturated, its permeability to further magnetization decreases. The changing core magnetization induces a large voltage in sensing coil  170 . Since the two opposing magnetic bars  152  and  153  are placed in the same sensing coil  170 , their magnetizations cancel. The only net flux change is that caused by the constant magnetic field Hs and the changing differential permeability. In the preferred embodiment of the invention, drive circuit  100  operates in a second harmonic mode and the driving field in coil  140  is in the order of 1 kHz. The output signal proportional to the magnetic field Hs is filtered by filter  190  and processed by demodulator  195  and amplifier  200  to produce an output signal that represents the magnitude of magnetic field Hs seen by core  155  of magnetic probe  150  due to the magnetic field Hs. Typically, the output signal is then digitized to provide a relatively high resolution digital signal that can be processed by circuitry (not shown) to provide a result displayed to a user or sent to some other processing system.  
         [0019]     When sensor  10  operates as an inductive coil, the drive signal applied to drive coil  140  is changed. Rather than providing a drive signal that pushes the high magnetic permeability elongated bars  152  and  153  into and out of saturation, either no signal is provided to core  155  or, alternatively, drive coil  140  is operated as an anti-pulse coil to cancel signals directly coupled into the sensor from a transmitter coil (not shown) in the event that such a transmitter is used to excite magnetic signals in nearby objects. In that case, the magnetic field generated by the anti-pulse current in core  153  needs to be in the same direction as the field generated in core  152 . To achieve that, a switch (not shown) is employed to reverse the current direction in one of coils  140  and  170  around cores  152  and  153 . In either case, an alternating magnetic field present around core  155  will produce an output signal in sensing coil  170  which is indicative of the change in magnetic field Hs. As is well known, when a circular loop of area encased by a coil is placed in a time changing magnetic field, a voltage signal is induced in the loop that is equal to the negative time rate of change of the magnetic flux passing through the loop. The voltage signal of course can be increased by increasing the number of loops in the sensing coil. Alternatively, a current flows in the coil that is proportional to the magnetic field, and this current can be amplified to produce an output signal.  
         [0020]     Turning now to  FIG. 3 , there is shown a graph indicating sensor response when sensor  10  is in serial operation. Note that the fluxgate and induction (static and oscillating) sensor modes,  210  and  220  respectively, may be operated independently of one another. Such an operation represents a significant advancement because it provides for more compact sensor geometry. Additionally, sensor  10  may be operated in a time-shared pattern so that, within a suitable period, both sensing modes  210  and  220  make an adequate measurement of both static and oscillating magnetic fields. Depending on the application, the static and oscillating modes  210  and  220  can be operated in serial operation, as shown in  FIG. 3 , or in interleaved operation, as shown in  FIG. 4 , with sensor  10  providing an output signal indicative of a magnetic field oscillation in a range of zero (DC) to 10 kHz.  
         [0021]     In the serial operation, when fluxgate mode  210  is driven at about 1 kHz, the stable response of sensor  10  can be obtained in the order of 10-100 cycles. Therefore, measurement durations in the order of 0.1 seconds are obtained. Even if an idle period of a few milliseconds is produced to allow core  155  to recover to the high-permeability state after periodic driving field  250  is turned off, the 0.1-second measurement duration still occurs. The repetition of induction mode  220  is preferably a harmonic of 60 Hz to reduce power line interference. For example, if 30 Hz is selected as a repetition rate, there are a total of 33 microseconds for each measurement. Considering fifteen averages, the operating time for induction sensor  10  is in the order of half of a second.  
         [0022]     In the interleaved operation, sensor  10  operates in fluxgate mode  210  during the transition period when core  155  is changing from the high-permeability state to the low-permeability state or changing from the low-permeability state to the high-permeability state. Sensor  10  operates in induction mode  220  when core  155  is in the high permeability state.  
         [0023]     Although described with reference to a preferred embodiment of the invention, it should be readily understood that various changes and/or modifications can be made to the invention without departing from the spirit thereof. For instance, the shape and overall sensor configuration could be changed. As an example, three magnetic sensors could be set orthogonally to one another and used in a group to obtain multi-axis measurements of the magnetic field under consideration, such as the three-axis vector required to detect unexploded ordinance. In general, the invention is only intended to be limited by the scope of the following claims.