Abstract:
A method and system for sensing magnetic fields using an AC current carrying wire which vibrates or moves in the presence of a magnetic field. The system uses the wire movement to determine one or more characteristics of the field. The wire is connected at a first and second location to the base with an portion between the connection locations having an apex spaced apart from the base. The monitoring system detects movement of the wire at the portion while an electrical current having the same frequency as the mechanical resonant frequency of the wire is sent through the wire and a magnetic field surrounds the portion. The reporting system determines one or more characteristics of the magnetic field based upon the detected movements.

Description:
This application claims the benefit of U.S. Provisional Patent Application Serial No. 60/283,925 filed on Apr. 17, 2001, which is herein incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to magnetic field sensors and, more particularly, to a system and method that use an AC current carrying elastic element (e.g., wire) which vibrates or moves in the presence of a magnetic field, and using the wire movement to determine one or more characteristics of the magnetic field. The frequency of the AC current is matched to the mechanical resonant frequency of the elastic element. 
     BACKGROUND OF THE INVENTION 
     Typical magnetic field sensors in use today include compass needles, Hall probes and super-conductive quantum interference devices (“SQUIDs”). The compass needle was discovered about 2,000 years ago by the Chinese, and the Hall probes and SQUIDs were developed fairly recently. The particular sensor one might use in a given situation often depends upon several factors such as the required accuracy, sensitivity and economic constraints. For example, a compass needle is easy to use, inexpensive and does not require electric power or circuits, but it can only indicate the direction of a field. Hall probes are more robust and can measure fields over a large range of field strengths, but their accuracy is compromised by temperature changes and they have problems related to baseline drift, noisy signals and fail in high radiation environments. The most sophisticated magnetic field sensors are the SQUIDs, which can measure magnetic fields with extremely high precision. SQUIDs require liquid helium and complex circuitry to operate, however, making them expensive and impractical. 
     SUMMARY OF THE INVENTION 
     A system for sensing fields in accordance with embodiments of the present invention includes a wire having an interior wire portion and an exterior wire portion, the wire connected at a first and a second location to a base with an interior wire portion between the connection locations having an apex spaced apart from the base. A monitoring system detects movement of the wire at least at the interior wire portion while an electrical current is sent through the wire and a magnetic field surrounds at least the interior wire portion. A reporting system determines one or more characteristics of the magnetic field based upon the detected movement of the interior wire portion. 
     In one embodiment, the interior wire portion may have a circular or loop configuration. Alternatively, the interior wire portion may include a square configuration. In another embodiment, the monitoring system includes a set of electrodes spaced apart to form a capacitor. The interior wire movement in between the electrodes alters a capacitance value across the electrodes. The reporting system may determine the magnetic field characteristics based upon the altered capacitance values. The monitoring system alternatively may include a photo emitter that emits light towards an inlet in a photo detector. In this example, the interior wire movement deflects at least a portion of the light away from the photo detector inlet. Here, the reporting system may determine the magnetic field characteristics based upon an amount of light deflected away from the inlet. 
     A method for sensing fields in accordance with embodiments of the present invention includes detecting movement in a wire while sending an electrical current through the wire and a magnetic field surrounds the wire, and determining one or more characteristics of the magnetic field based upon the detected movement of the wire. 
     The present invention provides a convenient, efficient and inexpensive system and method for sensing magnetic fields. Additionally, the present invention enables magnetic fields to be sensed in environments where conventional devices have difficulty or are unable to function. Further, the present invention is easily manufactured and does not require expensive materials for assembly. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a system for sensing magnetic fields in accordance with one embodiment of the present invention; 
     FIG. 2 is a partial front view of the system for sensing magnetic fields shown in FIG. 1; 
     FIG. 3 is a partial cross-sectional top view of the system for sensing magnetic fields shown in FIG. 1; 
     FIG. 4A is a partial cross-sectional side view taken along the line  4 — 4  of the system for sensing magnetic fields shown in FIGS. 1-2; 
     FIG. 4B is a partial cross-sectional side view taken along the line  4 — 4  of the system for sensing magnetic fields shown in FIGS. 1-2 showing the wire moving; 
     FIG. 5 is a flow chart of a process for sensing magnetic fields in accordance with another embodiment of the present invention; 
     FIG. 6 is a flow chart of a process for calibrating a magnetic field sensor system in accordance with another embodiment of the present invention; 
     FIG. 7 is a graph showing the relationship between an amount of wire displacement and an electrical signal output from the detection system; 
     FIG. 8 is a graph showing an amplitude of an electrical signal output from the detection system when an electrical current is applied to the wire at various frequencies; 
     FIG. 9 is a graph showing the relationship between a Hall probe measurement taken at various magnetic field intensities and an electrical signal output from the detection system; 
     FIG. 10 is a graph showing an electrical signal output from the detection system as a function of a magnetic field measured with a Hall probe; 
     FIG. 11 is a graph showing a residual of measured fit for the magnetic sensor system in accordance with one or more embodiments and a Hall probe; 
     FIG. 12 is a partial perspective view of a system for sensing magnetic fields in accordance with another embodiment of the present invention; and 
     FIG. 13 is a partial top view of the system for sensing magnetic fields shown in FIG.  12 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A system  10  for sensing magnetic fields in accordance with embodiments of the present invention is shown in FIGS. 1-4B. The system  10  includes sensor device  12 , which has a movement detection system  22  coupled to a reporting system  28 . A method in accordance with embodiments of the present invention includes the movement detection system  22  in device  12  detecting movement in the wire  16  while sending an electrical current through the wire  16  and a magnetic field surrounds the wire  16 , the reporting system  28  determining one or more characteristics of the magnetic field based upon the detected movement of the wire  16 . The present invention provides a convenient, efficient and inexpensive system  10  and method for sensing magnetic fields. Additionally, the present invention enables magnetic fields to be sensed in environments where conventional devices have difficulty or are unable to function. Further, the present invention is easily manufactured and does not require expensive materials to be assembled. 
     The sensor device  12  includes base  14 , wire  16 , connectors  18 , power source  20  and movement detection system  22 . In this embodiment, the base  14  is made of a non-conductive insulating material such as plastic, or any type of conductive material (i.e., metals) having a non-conductive coating, for example. 
     The wire  16  is connected to the base  14  by connectors  18  made of solder or glue, for example. The wire  16  is made of copper, although a number of other conductive materials such as gold or platinum may be used. Further, the wire  16  has a low damping rate corresponding to a Q value of about 200, although a damping rate corresponding to a Q value of about 1000 may be obtained where the wire  16  is enclosed in a vacuum by an optional enclosure  34  as will be described further herein below. In this embodiment, an interior portion of the wire  16  in between connectors  18  has a substantially looped configuration formed by looping that section of wire onto itself as shown in FIGS. 1-4B. By way of example only, the wire  16  has a diameter (“d”) of about 0.030 mm. As shown in FIG. 2, the distance between the apex A of the loop in the looped section of wire  16  and the lowest point of a horizontal section of the wire  16  over the base  14  from the looped portion and through the connectors is illustrated by line H which in this example is about 4.0 mm. Further, the length of the wire  16  illustrated by the line L measured from a first end of the horizontal section of the wire  16  to the substantial center of the looped portion is about 3.125 mm so that 2L is about 6.24 mm in this example. 
     Although exemplary diameters and dimensions have been provided above, a number of other values may be used depending upon the particular application of device  12 , the magnetic field intensity desired to be sensed or the types of material used to form the wire  16 . Additionally, the wire  16  and other components of the device  12  may be formed to have much smaller diameters and dimensions using a variety of manufacturing methods such as nano-fabrication. The low damping rate of wire  16  mentioned earlier, the diameters and dimensions of wire  16  combined with the loop configuration formed in the interior portion of wire  16  enables at least a section nearest the apex A of the looped portion of wire  16  to be displaced and move back and forth about an a angle formed between the looped portion of the wire  16  and the Z axis to produce a substantially vibrating motion as shown in FIGS. 4A-4B for sensing magnetic signals as will described in further detail herein below. In other embodiments, the designations of the axis may be different. 
     Power source  20  is coupled to the wire  16  for supplying the wire  16  with an electrical current. In this embodiment, power source  20  may be an alternating current (“AC”) power source. In this embodiment, the power source  20  may supply the wire  16  with an electrical current at a predetermined amplitude (e.g., about 85 mA) and at a predetermined frequency (e.g., about 259 Hz), these amplitudes and frequencies being determined during a calibration process described further herein below in connection with steps  120 - 160  in FIG.  6 . 
     The movement detection system  22  includes a photo emitter  24  and a photo detector  26  mounted on the base  14 . In this embodiment, the photo emitter  24  may be any device that can send light signals from an outlet  25  such as a laser emitting device or a LED device. The photo detector  26  may be any device that can detect and quantify an amount of light signals being received at an inlet  27  such as a photo diode, photo transistor or photovoltaic cell. Furthermore, the emitter  24  and detector  26  may also be an off-the-shelf type device such as an opto-electronic assembly model H21A1 manufactured by Newark Electronics Inc., for example, and described in Newark Electronics 2000, Catalog 118, p. 586, which is herein incorporated by reference in its entirety. In embodiments of the present invention, the photo emitter  24  is oriented towards the photo detector  26  and has a substantially clear line of site with respect to the detector  26  to enable the emitter  24  to send light signals towards the inlet  27 . Moreover, most of the light signals may travel unobstructed along the X axis towards the detector  26 , but some of the signals may strike a surface of the wire  16  in the section nearest the apex A of the looped portion when the wire  16  is in a resting state (i.e., the loop portion is not vibrating or moving) and still be reflected towards the detector  26  although the light signals may travel unobstructed along the X axis and along the periphery of the wire  16  without striking the wire. 
     The reporting system  28  includes a processor  30  and a display device  32 , although the system  28  may comprise other components, other numbers of the components, and other combinations of the components. The processor  30  may include an analog volt meter or a digital oscillator and is coupled to the photo detector  26 . In embodiments of the present invention, the processor  30  receives electrical signals from the detector  26  representing the amount of light being received by the detector  26 . The processor  30  processes the electrical signals and may display a value representing the amount of light signals being received by the detector  26  using display device  32 , including changes in the amount of light received. 
     Processor  30  may also include a central processing unit, memory, user input interface and an input/output interface, which are coupled together by a bus system or other link. The processor  30  may execute a program of stored instructions for a process of sensing magnetic fields, and to display the results using the display device  32  in accordance with at least one embodiment of the present invention as described herein and illustrated in FIGS. 5-6. Further, the programmed instructions are stored in the memory of the processor  30 , although some or all could be stored and retrieved from other locations. A variety of different types of memory storage devices, such as a random access memory (“RAM”) or read only memory (“ROM”), or a floppy disk, hard disk, CD ROM. or other computer readable medium that is read from and/or written to by a magnetic, optical, or other reading and/or writing system that is coupled to the processor  30 , can be used for memory. The user input device enables an operator to generate and transmit signals or commands to the processor  30  during the calibration process described further herein below in connection with FIGS. 6-13. A variety of different types of user input devices can be used, such as a keyboard or computer mouse. 
     The system  10  optionally includes an enclosure  34  that substantially surrounds the device  12 , but at least surrounds the looped portion of the wire  16 . Further, the enclosure  34  may be coupled to a vacuum device  36  that evacuates air from the space within the enclosure  34  to create a vacuum within the enclosure  34 . The vacuum eliminates any potential drag or friction that may be imposed upon the loop portion of the wire  16  that may otherwise hamper the ability of the wire  16  to vibrate and ultimately reduce the sensitivity of the of the device  12 . 
     The operation of the system  10  for sensing magnetic fields in accordance with embodiments of the present invention will now be described with reference to FIGS. 1-13. Referring to FIG.  5  and beginning at step  100 , the device  12  is calibrated and if the optional enclosure  34  and vacuum device  36  are used, the vacuum device  36  is operated to create a vacuum within the enclosure  34 . 
     Referring to FIG. 6, a sensor device  12  calibration process will now be described in accordance with an embodiment of the present invention. Beginning at step  120 , the photo emitter  24  is operated to send light signals towards the photo detector  26 . A device such as a micro-screw is used to precisely move the loop portion of the wire along the Z axis in very small increments (e.g., 0.001 mm), although other devices may be used to move the loop portion in larger increments. As the wire  16  moves in the path of the light signals, the amount of light received by the photo detector  26  changes and these changes are reflected in the electrical signal output of the detector  26 . The varying displacement of the wire  16  along the Z axis is mapped or associated with the electrical signal output of the photo detector  26 , and these associations are stored in the memory of the processor  30 . By way of example only, the graph in FIG. 7 shows that as the amount of the loop portion displacement measured in mm units increases the detector output signal increases in mV units. The solid circles in the graph show that in the range of about a 0.7-1.1 mm displacement of the loop portion from its initial resting position (i.e., 0.0 mm displacement), the signal output of detector  26  is substantially proportional to the displacement of the loop portion. This relationship can be expressed as:            δ                   U        [   mV   ]           δ                   z        [   mm   ]           =   421                          
     Here, δz represents the change in the position of the loop portion, and δU is the change in the detector signal. A least-squares fit of the straight line shown in the graph through the solid circles results in a relationship of U[mV]=m 0 +m 1 z[mm] with m 0 =−110 and m 1 =421. Further, δU=1 mV corresponds to a current of δ/I=δU/R=0.16 μA when an external circuit resistor having a value of R=6.35 kΩ is used on the detector  26  output signal. 
     Next at step  140 , the fundamental resonance frequency (“f 1 ”) and quality factor (“Q”) values of the wire  16  are determined. The f 1  value is unique to wire  16  and may depend upon a number of factors including the type of material, diameter and dimensions used to form the wire  16 , and ambient temperature surrounding the wire  16 , for example. In this example, power source  20  supplies the wire  16  with an AC electrical current having an amplitude of about 170 mA, although a number of other amplitudes may be used depending upon the application of sensor device  12  and the intensity of the magnetic fields desired to be sensed. The frequency of the AC electrical current may be gradually increased from an initial minimum frequency (e.g., 0 Hz) up to a maximum frequency (e.g., 262 Hz), although the frequency may be decreased from a maximum frequency to a minimum frequency, for example. The amplitude of the electrical signal output from photo detector  26  is measured for each frequency that the AC electrical current supplied to wire  16  is adjusted to. 
     Referring to the graph in FIG. 8, the value of the amplitude of the electrical signal output from photo detector  26 , which represents the mechanical resonance of the wire  16 , is indicated by a solid circle for each frequency value. The exemplary data obtained from the graph (i.e., solid circles) may be used to determine the f 1  and Q values for wire  16  by applying the data to the following equation:        A   =       A   0             (       f   2     -     f   1   2       )     2     +       f   2            f   1   2     /     Q   2                                      
     In the equation above, A 0  is a constant, and the equation is derived from the following equation:          I               2        α            t   2           =         -     (       π                 S                   d   2         16                 L       )          α     -     K             α          t         +       T   l          (   t   )                                
     Thus, the f 1  value for wire  16  in this example is about 259 Hz, and the Q factor is about 198. These values are stored in the memory of the processor  30  for later retrieval and processing as described further herein. 
     Next at step  160 , power source  20  supplies a sinusoidal AC electrical current to the wire  16  having a peak amplitude of 85 mA and at the resonance frequency f 1  (i.e., 259 Hz) determined above. At substantially the same time, the photo emitter  24  is operated to send light signals towards inlet  27  in the photo detector  26 , if not already operating. The photo detector  26  receives the light signals and transmits electrical signals to the processor  30  representing the amount of light signals being received at the inlet  27 . A Hall probe is placed along side the device  12 . A magnetic force By is applied to the device  12  and the Hall probe by a small permanent calibration magnet. The calibration magnet is placed an initial distance away from the device  12  and the Hall probe (e.g., 0.00 mm), and the magnet is gradually moved away to change the intensity of the magnetic force By In moving the calibration magnet away from the device  12  and the probe, the intensity of the force B y  is gradually decreased from an initial maximum intensity to a minimum intensity. 
     Referring to the graph in FIG. 9, the magnetic field intensities measured by the Hall probe as the magnet is moved away from the probe is represented by the solid triangles, and the empty circles represent the amplitudes of the electrical signals output from the photo detector  26  for the magnetic field intensities measured by the Hall probe as indicated by the solid triangles. These relationships are stored in the memory of the processor  30  for further processing and retrieval as described herein. Referring to FIG. 8, the graph shows a plot of the electrical signal amplitudes output from the detector  26  compared with the magnetic field intensity measurements taken by the Hall probe and fitted in with linear dependence. This fit provides:            δ                   B        [   G   ]           δ                     U     A                 C            [   mV   ]           ≈   0.0518                          
     Thus, a 1 mV change of the electrical signal indicates about a 52 mG change of the magnetic field intensity. Since the size of the calibration magnet is much smaller than the distance between the device  12  and the calibration magnet and the Hall probe and the magnet, the dependence of the magnetic field intensity on a position p can be approximated by:          B        (   p   )       =       C   1     +       C   2         (     p   -     C   3       )     3                                
     Here, C 1 , is a parameter that represents either the background field or the zero drift for the Hall probe. The parameter C 2  is proportional to the calibration magnet magnetic moment, and the parameter C 3  is set by the location of the Hall probe in a coordinate system used to define the calibration magnet position. The measurements taken from the Hall probes and the device  12  are applied to the equation above using C 1 , C 2  and C 3  as free parameters. 
     Referring to the graph in FIG. 11, a residual of the measured fit for the device  12  and the Hall probe is shown. At each point (i.e., solid circles), the electrical signal from the photo detector  26  is measured one or more times. The bars extending vertically from each point in the graph represent lo errors found from a statistical analysis of the measurements. The diamonds in the graph represent the measurements taken by the Hall probe. Thus, for each electrically signal amplitude output from the detector  26 , a magnetic field intensity value is determined and is stored in the memory of the processor  30  for later retrieval and processing as described herein. The device  12  is now calibrated to sense magnetic signals as described further herein. 
     Referring back to FIG.  5  and next to step  200 , once the device  12  is calibrated as described above in connection with steps  100 - 160 , the device  12  may be operated to sense magnetic signals. Power source  20  supplies an AC electrical current to the wire  16  having a peak amplitude of 85 mA and at the resonance frequency f 1  (i.e., 259 Hz) determined above during calibration at step  140 . At substantially the same time, the photo emitter  24  is operated to send light signals towards inlet  27  in the photo detector  26 . The photo detector  26  receives the light signals and transmits electrical signals representing the amount of light signals being received at the inlet  27  to the processor  30 . 
     Next at step  300 , a vertical magnetic force B y  is applied to the device  12  so that the magnetic field substantially surrounds at least the loop portion of the wire  16 . 
     Next at step  400 , the loop portion of wire  16  begins to vibrate or move. The degree or amount of vibration or movement of the loop portion depends upon the intensity of the magnetic field B y  substantially surrounding the loop portion. 
     Next at step  500 , the loop portion of the wire  16  moves into the path of the light signals being sent from the photo emitter  24  towards the photo detector  26  and therefore deflects at least a portion of the light signals away from the detector  26 . The electrical signals being transmitted from the detector  26  to the processor  30  change to reflect the difference in the amount of light signals being received at the inlet  27  of the detector  26 . This movement or vibration is detected and quantified by the detection system  22  and the processor  30  in the reporting system  28  based upon the relationships determined during the calibration performed in steps  100 - 160  between the displacement of the wire  16 , detection system  22  output, and the magnetic field intensities. The processor  30  compares the value representing the amount of light signals being received by the detector  26  with the stored relationships in the memory of the processor  30  to determine the intensity of the magnetic field substantially surrounding the looped portion of the wire  16  at that instance. 
     Next at step  600 , the display device  32  of the reporting system  28  displays the determined magnetic field intensity. The device  12  is operated as described above in steps  200 - 600  until magnetic fields are no longer desired to be sensed. At this point, the electrical currents applied to the wire  16 , the light signals sent from the photo emitter  24  to the photo detector  26 , and the electrical signals representing the amount of light signals received by the photo detector  26  to the processor  30 , and the operation of the reporting system  28  as described above cease. 
     In an alternative embodiment of system  10 , instead of using a photo emitter  24  and a photo detector  26 , a pair of capacitor plates are used. The system  20  operates in the same manner in this embodiment as described above in connection with one or more embodiments, except the movement of the wire  16  in between the plates and within the capacitive field formed therein changes the capacitance value measured between the plates. However, these changes in the capacitance value are used in the same manner as described above with respect to using the changes in the electrical signal values output from the photo detector  26  as described above. 
     Referring to FIGS. 12-13, an alternative embodiment of system  10  will now be described. Like reference numbers in FIGS. 12-13 are identical to those in and described with reference to FIG.  1 . Further, the device  12  in this embodiment is the same as described above with reference to FIGS. 1-4 in connection with one or more embodiments, except in this embodiment an element  40  is used instead of the wire  16 . Further, the element  40  is the same as the wire  16  except an interior portion of the element  40  in between connectors  18  has a substantially square portion formed in a semi square configuration instead of the loop configuration. The square portion is created by bending or forming a vertical section in the interior portion of the element  40  after the connector  18 ( 1 ), bending or forming the vertical section into a horizontal section that continues from the vertical section, bending or forming the horizontal section into another vertical section, and then bending or forming the other vertical section to form another horizontal section that continues over the base  14  and through the connector  18 ( 2 ). In this embodiment, each of the vertical and horizontal sections in the square portion may have a length up to about 1 mm, although other dimensions may be used depending upon a number of factors such as the material used to form the element  40 , the particular application of the device  12 , or the intensities of the magnetic fields desired to be sensed. 
     The same steps are performed for sensing magnetic fields as described above in connection with steps  100 - 600  in FIGS. 5-6, except at steps  100 ,  120 - 180 , and  400 , the element  40  moves instead of the wire  16 . Furthermore, the apex A (i.e., the horizontal section) of the square portion of element  40  is displaced back and forth in a direction F z  to create a vibrating movement along the Z axis in response to AC current being supplied to the element  40  and a magnetic force of B y  being applied on the element  40 . 
     Having thus described the basic concept of the invention, it will be rather apparent to those skilled in the art that the foregoing detailed disclosure is intended to be presented by way of example only, and is not limiting. Various alterations, improvements, and modifications will occur and are intended to those skilled in the art, though not expressly stated herein. These alterations, improvements, and modifications are intended to be suggested hereby, and are within the spirit and scope of the invention. Further, the recited order of elements, steps or sequences, or the use of numbers, letters, or other designations therefor, is not intended to limit the claimed processes to any order except as may be explicitly specified in the claims. Accordingly, the invention is limited only by the following claims and equivalents thereto.