Method and apparatus for operating gradiometers in multiple modes

Abstract

A magnetic screening system uses directional gradiometers with high resolution and accuracy to measure magnetic field signatures of target objects (e.g., gun, knife, cell phone, keys) in a volume of interest. The measured signatures can be compared to signatures of known objects stored in a local database. Various mathematical processes may be used to identify or classify target object signatures. In a network of magnetic screening systems, the magnetic screening systems can transmit signatures to a central signature database, and a management computer can share the central signature database with all of the magnetic screening systems on the network. The magnetic screening system can operate in multiple modes, such as a tracking mode, measurement mode, and self-test mode. Through use of unique processes and designs, the magnetic screening system can achieve a high rate of processing persons for target objects.

1 . A gradiometer, comprising:
 multiple magnetometers to measure a magnetic field gradient; and   a processor, in communication with each of the magnetometers, that causes the gradiometer to operate in one of multiple modes.   
     
     
         2 . The gradiometer according to  claim 1  wherein the gradiometer operates in a measurement mode and the processor causes the gradiometer to operate in at least one of the following other modes: background offset reduction mode or calibration mode. 
     
     
         3 . The gradiometer according to  claim 1  wherein the processor causes the gradiometer to operate in at least one of the following modes: self-test mode, automatic alignment mode, or diagnostic mode. 
     
     
         4 . The gradiometer according to  claim 1  wherein, in self-test mode, the processor causes components of the gradiometer to be in states to compare measured performance of the components in those states against specified performance of the components in those states to determine operational readiness. 
     
     
         5 . The gradiometer according to  claim 1  wherein, in automatic alignment mode, the processor captures and averages magnetic field strength over a long duration while compensating for background disturbances to calculate the alignment of the gradiometers relative to the earth's magnetic field. 
     
     
         6 . The gradiometer according to  claim 1  wherein the processor uses automatic alignment for each gradiometer to determine relative orientations of each gradiometer to at least one other gradiometer in the system. 
     
     
         7 . The gradiometer according to  claim 1  wherein, in diagnostic mode, the processor captures and outputs the measured field strengths of each magnetometer in an unaltered state. 
     
     
         8 . The gradiometer according to  claim 1  wherein the processor is substantially a digital processor. 
     
     
         9 . The gradiometer according to  claim 1  wherein the processor causes the gradiometer to switch between a measurement mode and a background offset reduction mode. 
     
     
         10 . The gradiometer according to  claim 1  wherein, in a measurement mode, the processor causes the gradiometer to switch at least once between the measurement mode and a calibration mode. 
     
     
         11 . The gradiometer according to  claim 10  wherein the processor causes the gradiometer to switch between the measurement mode and the calibration mode at a rate greater than 0.1 Hz. 
     
     
         12 . The gradiometer according to  claim 1  wherein, in a measurement mode, the processor causes the gradiometer to remove a background offset from measurements on a sample-by-sample basis. 
     
     
         13 . The gradiometer according to  claim 1  wherein, in a measurement mode, the processor causes the gradiometer to sample at a rate greater than 50 Hz. 
     
     
         14 . The gradiometer according to  claim 1  wherein the processor collects sampled data at its sampling rate and outputs the sampled data at a selectable rate different from the sampling rate. 
     
     
         15 . The gradiometer according to  claim 1  wherein, in a measurement mode, the processor causes a drive circuit to drive the magnetometers with a waveform having at least two non-zero levels and an offset corresponding to a level for removing background offset. 
     
     
         16 . The gradiometer according to  claim 1  wherein, during a background offset reduction mode, the gradiometer determines background offset sensed by the gradiometer and wherein the processor uses the background offset to remove the background offset from measurement data captured by the magnetometers. 
     
     
         17 . The gradiometer according to  claim 1  further including calibration circuits associated with respective magnetometers, and wherein the processor causes the calibration circuits to activate during a calibration mode, during which the processor captures response of the magnetometers to the calibration circuits. 
     
     
         18 . The gradiometer according to  claim 17  wherein, during the calibration mode, the processor (i) captures and averages field strengths over measurement periods at multiple different calibration levels and (ii) calculates a calibration curve. 
     
     
         19 . The gradiometer according to  claim 1  further including electrical excitation circuits that the processor causes to output electrical excitation signals to respective magnetometers, and wherein the processor causes the excitation circuits to vary the excitation signals to the magnetometers. 
     
     
         20 . A method of operating a gradiometer, comprising:
 controlling a mode of operation of a gradiometer including multiple magnetometers that measure a magnetic field gradient; and   operating the gradiometer in a selected mode of operation.   
     
     
         21 . The method according to  claim 20  wherein controlling a mode of operation includes causing the gradiometer to operate in measurement mode and at least one of the following modes: background offset reduction mode or calibration mode. 
     
     
         22 . The method according to  claim 20  wherein controlling a mode of operation includes causing the gradiometer to operated in at least one of the following modes: self-test mode, automatic alignment mode, or diagnostic mode. 
     
     
         23 . The method according to  claim 20  wherein, in self-test mode, further including comparing measured performance of the gradiometer in settable states against specified performance in those states to determine operational readiness. 
     
     
         24 . The method according to  claim 20  wherein, in automatic alignment mode, further including capturing and averaging magnetic field strength over a long duration while compensating for background disturbances; and
 based on results of the averaging, calculating alignment of the gradiometers relative to the earth's magnetic field. 
 
     
     
         25 . The method according to  claim 24  further including determining relative orientation of each gradiometer to at least one other gradiometer in the system. 
     
     
         26 . The method according to  claim 20  wherein, in a diagnostic mode, further including capturing and outputting the measured field strengths by each magnetometer in an unaltered state. 
     
     
         27 . The method according to  claim 20  wherein controlling the mode of operation is performed in a substantially digital manner. 
     
     
         28 . The method according to  claim 20  further including causing the gradiometers to switch between a measurement mode and a background offset reduction mode. 
     
     
         29 . The method according to  claim 20  wherein, in a measurement mode, causing the gradiometer to switch at least once between the measurement mode and a calibration mode. 
     
     
         30 . The method according to  claim 29  wherein switching between the measurement and calibration modes occurs at a rate greater than 0.1 Hz. 
     
     
         31 . The method according to  claim 20  wherein, in a measurement mode, reducing background offset from measurements on a sample-by-sample basis. 
     
     
         32 . The method according to  claim 20  wherein, in a measurement mode, sampling occurs at a rate greater than 50 Hz. 
     
     
         33 . The method according to  claim 32  further including outputting sampled data at a selectable rate different from the sampling rate. 
     
     
         34 . The method according to  claim 20  wherein, in a measurement mode, driving the magnetometers with a waveform having at least two levels offset from a zero level and an offset corresponding to a level for removing a background offset. 
     
     
         35 . The method according to  claim 20  wherein, during a background offset reduction mode, further including determining background offset sensed by the gradiometer and, in a post-processing manner, removing the background offset from measurement data. 
     
     
         36 . The method according to  claim 20  further including:
 calibrating the magnetometers; 
 activating calibration during a calibration mode; and 
 capturing response of the magnetometers due to the calibrating. 
 
     
     
         37 . The method according to  claim 36  further including:
 capturing and averaging field strengths over measurement periods at multiple different calibration levels; and 
 calculating a calibration curve. 
 
     
     
         38 . The method according to  claim 20  further including outputting excitation signals and varying the excitation signals to the magnetometers. 
     
     
         39 . A gradiometer, comprising:
 multiple magnetometers to measure a magnetic field gradient; and   means for causing the gradiometer to operate in one of multiple modes.