Abstract:
The present invention discloses a magnetoresistive gear tooth sensor, which includes a magnetoresistive sensor chip and a permanent magnet. The magnetic sensor chip is comprised of at least one magnetoresistive sensor bridge, and each arm of the sensor bridge has at least one MTJ element group. The magnetoresistive gear tooth sensor has good temperature stability, high sensitivity, low power consumption, good linearity, wide linear range, and a simple structure. Additionally, the magnetoresistive gear tooth sensor has a concave soft ferromagnetic flux concentrator, which can be used to reduce the component of the magnetic field generated by the permanent magnet along the sensing direction of the MTJ sensor elements, enabling a wide linear range. Because it is arranged as a gradiometer, the magnetoresistive gear tooth sensor bridge is not affected by stray magnetic field; it is only affected by the gradient magnetic field generated by gear teeth in response to the permanent magnet bias. The magnetoresistive gear tooth sensor of the present invention is able to detect the position of a specific tooth or a missing tooth of a gear. This magnetoresistive gear tooth sensor is also capable of determining the speed and direction of motion of a gear.

Description:
CROSS-REFERENCE TO A RELATED APPLICATION 
       [0001]    This application is a 35 U.S.C. §371 national phase application of PCT/CN2013/076707, filed on Jun. 4, 2013, which claims a priority to a Chinese Patent Application No. CN 201210180465.0, filed on Jun. 4, 2012, incorporated herein by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to magnetic position and speed detection sensors, and particularly relates to the use of MTJ elements configured as a magnetoresistive gear tooth sensor. 
       BACKGROUND 
       [0003]    Gear tooth sensors are mainly used in automatic control systems to measure the speed and direction of rotation of a gear. Presently, the most commonly used gear tooth sensors utilize optical or magnetic sensing. In rotating mechanical systems harsh conditions such as vibration, shock, oil, and etc. are present which are not well tolerated by optical sensing, and do not affect magnetic sensors, so magnetic sensors have advantages over optical sensors in these systems. There are many different types of magnetic sensors used in prior art as the magnetic sensor element of a magnetic gear tooth sensor, including Hall (Hall) effect sensors, anisotropic magnetoresistance (AMR) sensors, and giant magnetoresistance (GMR) sensors. 
         [0004]    Hall Effect sensors have very low sensitivity, typically requiring the use of a flux concentrator to increase sensitivity of the sensor which also increases size and weight. In addition, the Hall sensor element is a sensor has high power consumption and concentrators can exhibit poor linearity. AMR elements have higher sensitivity than Hall Effect elements, but suffer from much narrower linear working range. AMR element&#39;s also require a ‘set/reset’ coil used to reduce hysteresis, which not only leads to a more complex manufacturing process, hut also increases AMR sensor size and power consumption. GMR sensors have higher sensitivity than AMR elements, but also suffer from narrow linear working range. Further, the response curve of a multilayer element is unipolar, and multilayer GMR elements cannot measure the polarity of the magnetic field. 
         [0005]    In recent years, a new type of magnetoresistive sensor, known as a magnetic tunnel junction (MTJ) have begun to find acceptance for use as magnetic sensors in industrial applications. The working principle of an element is based on the use of the tunneling magnetoresistance effect (TMR) in magnetic multilayer films. MTJ elements show much higher magnetoresistance than AMR or GMR elements. Compared with the Hall Effect, an MTJ sensor has better temperature stability, higher sensitivity, lower power consumption and better linearity, and requires no extra flux concentrator structure to improve sensitivity. Compared with AMR sensors, MTJ sensors have better temperature stability, higher sensitivity, wider linear operating region, and they do not require the extra ‘set/reset’ coil structure. Compared with GIVIR sensors. MTJ sensors have improved temperature stability, higher sensitivity, lower power consumption, and a wider linear operating range. 
         [0006]    Magnetic gear tooth sensors typically use a printed circuit board (PCB) based structure to support the components. PCB based gear sensor is usually comprised of a magnetic sensor chip, some circuitry, and a permanent magnet. The permanent magnet produces an applied magnetic field H apply , which produces a change in the presence of a gar tooth that the magnetic sensor chips detect and then output a proportional voltage signal; the peripheral circuit is used for signal processing and conversion of the sensor output into an appropriate signal. The applied field generated by the permanent magnets produces a weak H apply  at the physical location of the magnetic sensor chip along the sensing direction, which limits the amount of field it can be designed to deliver. As a result, for the PCB type gear tooth sensor, improved suppression of external interference and increased H apply  are technical challenges that remain to be solved. 
         [0007]    Although MTJ elements have very high sensitivity, they have the following issues: 
         [0008]    (1) External magnetic field generated by the permanent magnets along the sensitive direction of the MTJ element H apply  is too large, causing the MTJ element to exhibit nonlinear performance, or worse still saturating the MTJ element; 
         [0009]    (2) The magnetic field at the physical location of the sensor chips produced by the permanent magnets H apply  and the external magnetic field can change, making the MTJ elements vulnerable to outside magnetic field interference in addition to drift in H apply  produced by the permanent magnet; 
         [0010]    (3) Inability to determine the position of a gear tooth or the existence of missing gear teeth; 
         [0011]    (4) No method to determine the direction of movement of the gear; 
         [0012]    (5) Low cost mass production has not yet been achieved. 
         [0013]    Therefore, a need to improve magnetic gear tooth sensor technology to accurately sense the motion and health of gears. 
       SUMMARY OF THE INVENTION 
       [0014]    The object of the present invention is to provide a magnetoresistive gear tooth sensor. 
         [0000]    Accordingly the invention describes a magnetoresistive gear tooth sensor including a magnetic sensor chip, a first permanent magnet, wherein the magnetic sensor chip includes at least one bridge, and each arm of the bridge includes at least one MTJ element group, and an MTJ element group comprises at least one MTJ sensor element. 
         [0015]    Preferably, the magnetoresistive gear tooth sensor further comprises a soft ferromagnetic flux concentrator with a U-shaped slot placed between the first permanent magnet and the sensor chip such that the slot is located on the side of the soft ferromagnetic flux concentrator facing the sensor chip. 
         [0016]    Preferably, each sensor arm comprises a multiplicity of MTJ element groups interconnected in series, parallel, or a combination of series and parallel. 
         [0017]    Preferably, the multiplicity of series, parallel or combination of series and parallel interconnected MTJ elements in each MTJ element group has the same sensing direction. 
         [0018]    Preferably, the MTJ element groups within each arm of the sensor bridge have the same sensing direction. 
         [0019]    Preferably, the sensor chip comprises a half-bridge, full-bridge, or two full bridges wired as a gradiometer. 
         [0020]    Preferably, each MTJ group comprises a plurality of MTJ elements connected in series, parallel, or a combination thereof. 
         [0021]    Preferably, each MTJ element group comprises a series, parallel, or a combination of series and parallel connected elements having the same sensitive direction. 
         [0022]    Preferably, each of the MTJ element is comprised of a multilayered structure with the following sequence: a pinning layer, a ferromagnetic pinned layer, a tunnel barrier, and a ferromagnetic free layer. 
         [0023]    Preferably, the magnetoresistive gear tooth sensor, further comprising a pair of second permanent magnets positioned adjacent to each MTJ element group in order to provide a bias magnetic field for each MTJ element group and perpendicular to the sensing direction of the MTJ element groups. 
         [0024]    Alternatively, the magnetoresistive gear tooth sensor, further comprises a pair of second permanent magnets positioned on both of the sides of each MTJ element group obliquely with respect to the sensing direction of the corresponding MTJ element group such that the pair of second permanent magnets provides a bias magnetic field cancelling the Neel coupling generated by of the MTJ elements within the MTJ element group. 
         [0025]    Preferably, each of the second permanent magnets has a magnetization that is aligned at an angle with respect to the sensitive direction of the MTJ element group in order to better cancel Neel coupling. 
         [0026]    Preferably, each of the MTJ elements is a multilayer structure comprising a sequence of layers that are sequentially deposited, including a pinned layer, a tunnel barrier layer, a free layer and a magnetic bias layer. 
         [0027]    More preferably, each of the MTJ element groups further comprises an isolation layer between the magnetic bias layer and the ferromagnetic freelayer. 
         [0028]    Preferably, the magnetoresistive gear tooth sensor further comprises a control circuit electrically connected to the magnetic sensor chip. 
         [0029]    Preferably, the control circuitry is used to determine the position of a gear tooth according to a relationship between the position of the gear tooth and the voltage signals of the magnetoresistive gear tooth sensor bridge. 
         [0030]    Preferably, the magnetic sensor chip includes two full bridge sensors, wherein each arm of the two full bridge sensors includes an MTJ element group, and the microcontroller is used to determine the movement direction of a gear using the voltage output of the gear tooth sensor bridges. 
         [0031]    Preferably, the magnetoresistive gear tooth sensor further comprises an outer casing. 
         [0032]    The present invention has the following advantages: 
         [0033]    (1) the sensor element is a MTJ element, which when compared with a Hall Effect sensor, AMR sensor, or GMR element sensor has better temperature stability of the sensor, higher sensitivity, lower power consumption, better linearity, wider linear region, and simpler structure; 
         [0034]    (2) the sensor is provided with a concave soft ferromagnetic flux concentrator, that reduces the magnetic field parallel to the sensitive direction of the MTJ element, at the position of the magnetic sensor chip to ensure the MTJ element operates in its linear region, greatly improving sensor performance; 
         [0035]    (3) the sensor is a magnetic sensor chip using a gradiometer bridge, so that the sensor is not susceptible to interference from external magnetic interference field in addition to the unwanted sensitive direction biasing effect of the permanent magnet bias field; 
         [0036]    (4) In one preferred embodiment the magnetoresistive gear tooth sensor, further comprises a pair of second permanent magnets positioned adjacent to each MTJ element group in order to provide a bias magnetic field for each of the MTJ element groups and perpendicular to the sensing direction of the MTJ element groups. By varying the magnetic bias field on the MD element in this perpendicular direction, it is possible to adjust the saturation field of the MTJ element, thereby obtaining a high sensitivity sensor with sensitivity that can be tuned according to specific needs; 
         [0037]    (5) In one preferred embodiment, the MTJ element group is provided with a pair of inclined permanent magnets wherein the MTJ element sits between the pair of permanent magnets, and the magnetic field generated by the permanent magnets is inclined along the sensitive direction of the MTJ elements in order to eliminate the Neel coupling field of the MTJ elements, thereby ensuring the magnetic operating point of the MTJ element is in its linear region, which improves the linearity of the sensor; 
         [0038]    (6) In another preferred embodiment, the magnetic bias field is provided by a magnetic layer built on top of the MTJ element free layer, wherein the bias layer provides a magnetic field in the freelayer in a direction perpendicular to the MTJ element sensitive direction. The magnetic bias field from the bias layer on the MTJ element can be varied in order to adjust the saturation field of the sensor, thereby obtaining a high sensitivity sensor wherein the sensitivity can be tuned according to specific needs; 
         [0039]    (7) The sensor can be provided with control circuitry used to determine the location of a tooth gear, existence of missing gear teeth, and the location of missing gear teeth; 
         [0040]    (8) Moreover, the sensor when so outfitted with appropriate control circuitry is able to determine the velocity of the gear and the direction of movement of the gear; 
         [0041]    (9) It may be used for both linear and circular gears; 
         [0042]    (10) It may be mass produced at low cost. 
     
    
     
       BRIEF DESCRIPTION 
         [0043]      FIG. 1  is a schematic block diagram of an MTJ element; 
           [0044]      FIG. 2  shows an idealized transfer curve response of the resistance of a MTJ element as a function of H apply , where the external magnetic field H apply  is applied along the sensitive direction of the MTJ element; 
           [0045]      FIG. 3  shows a more realistic plot of the resistance of an MTJ element as a function of external magnetic field H apply , when the external magnetic field H apply  is aligned along the sensitive direction of the MTJ element; 
           [0046]      FIG. 4  illustrates one manner in which MTJ elements may be interconnected in series; 
           [0047]      FIG. 5  is a schematic diagram of a set of permanent magnets at the sides of a MTJ element; 
           [0048]      FIG. 6  is a cross-sectional view of a pair of permanent magnets showing the magnetic field distribution around the MTJ element; 
           [0049]      FIG. 7  shows a pair of inclined permanent magnets around an MTJ element; 
           [0050]      FIG. 8  is a top-down view showing the physical location of the half-bridge sensor elements; 
           [0051]      FIG. 9  is an equivalent circuit diagram of the half-bridge as shown in  FIG. 8 ; 
           [0052]      FIG. 10  is a top view of the physical location of the sensors in full-bridge; 
           [0053]      FIG. 11  is an equivalent circuit diagram of the full bridge of  FIG. 10 ; 
           [0054]      FIG. 12  shows the measured output voltage signal of a full-bridge magnetic field sensor; 
           [0055]      FIG. 13  shows the physical location of the sensor elements of a double full-bridge sensor; 
           [0056]      FIG. 14  shows an equivalent circuit diagram of the double full-bridge sensor; 
           [0057]      FIG. 15  shows an exemplary construction of a magnetoresistive gear tooth sensor; 
           [0058]      FIG. 16  is a schematic diagram of the sine wave voltage signal of the magnetoresistive gear tooth sensor in Example 1; 
           [0059]      FIG. 17  is a schematic drawing showing the effect of missing gear teeth on the output voltage waveform of the magnetoresistive gear tooth sensor; 
           [0060]      FIG. 18  is a schematic diagram of the output waveforms of a magnetoresistive gear tooth sensor with dual full bridge; 
           [0061]      FIG. 19  is a second MTJ element embodiment including a magnetic biasing layer on top of the freelayer, 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0062]    The present invention will be further described along with the appended figures implementations that are presented below. 
       First Implementation: 
       [0063]      FIG. 1  shows a schematic diagram of a first embodiment of an MTJ element  11 . The first embodiment of an MTJ element  11  is a multilayer film structure, shown in  FIG. 1 , which comprises sequentially deposited on the substrate  111 , an insulating layer  112 , a bottom electrode layer  113 , a pinning layer  114 , a pinned layer  115 , a tunnel barrier layer  116 , a ferromagnetic free layer  117 , and a top electrode layer  118 . Pinned magnetic layer  115  and ferromagnetic free layer  117  are comprised of ferromagnetic metals and alloys including Fe, Co, Ni, FeCo, FeNi, FeCoB, or FeCoNi. The pinned layer  115  may be a trilayer ferromagnetic layer, in which two ferromagnetic layer are separated by a Ru layer, for example, a FeCo/Ru/FeCo trilayer. Pinning layer  114  and pinned layer  115  are exchange coupled so that the direction of the magnetic moment  1151  of the pinned layer  115  is rigidly fixed in one direction, and the presence of external magnetic field H apply  does not change the direction of magnetic moment  1151 . Pinning antiferromagnetic layer  114 , may be comprised of various materials including PtMn, IrMn or FeMn. The tunnel barrier layer  116  material may include MgO or Al 2 O 3 . The direction of the magnetic moment  1171  of free layer  117  can change as the direction or magnitude of the external magnetic field H apply  changes. Under the influence of an external magnetic field H apply , the direction of the magnetic moment  1171  of the free layer  117  can be parallel to the pinned layer  115  magnetic moment  1151  and it can be gradually rotated into to the direction antiparallel to the magnetic moment  1151  of the pinned layer  115 , and vice versa. In the present embodiment, the direction of the magnetic moment  1171  of the free layer  117  is defined as the sensitive direction of the first MTJ element  11 . Top electrode layer  118  and the bottom electrode layer  113  are generally composed of non-magnetic conductive materials. The substrate  111  material is typically silicon, quartz, pyrex, GaAs, or AlTiC. The area covered by the insulating layer  112  is larger than the area of the bottom electrode layer  113 . Top electrode layer  118  and bottom electrode layer  113  are used for electrically connecting MTJ elements to each other and to other components. In the present embodiment, the top electrode layer  118  and the bottom electrode layer  113  are electrically connected to an ohmmeter  12  to measure the resistance of the first MTJ element  11 . 
         [0064]    The magnetoresistance of the first MTJ element  11  depends on the relative orientation magnetic moments of the free layer  117  and the pinned layer  115 . When the direction of the magnetic moment  1171  of the free layer  117  is parallel to the direction of the magnetic moment  1151  of the pinned layer  115 , the resistance value of the first MTJ element  11  is at its minimum, and the first MTJ element  11  is in the low resistance state; when the magnetic moment  1171  of the free layer  117  is antiparallel to the direction of the magnetic moment  1151  of the pinned layer  115 , the resistance value of the first MTJ element  11  is maximum, and first MTJ element  11  is in a high resistance state. Known methods may be used to achieve linear behavior of the first MTJ element&#39;s  11  resistance as a function of an external magnetic field H apply  response between the high resistance state and the low resistance state. 
         [0065]    An idealized drawing of the external magnetic field H apply  dependence of the resistance of MTJ element  11  is shown in  FIG. 2 , where the external magnetic field H apply  is applied along the sensitive direction of first MTJ element  11 . When the first MTJ element  11  is in the low resistance state or a high resistance state, the response curve reaches saturation. The MTJ element  11  resistance value in the low resistance state is denoted as R L ; the resistance value in a high resistance state of the MTJ element  11  is denoted as R H . Between the high resistance state and the low resistance state, the resistance of MTJ element  11  denoted R, changes linearly with the external magnetic field H apply . The slope of the resistance curve of the first MTJ element  11  R as a function of magnetic field H apply , that is the rate of change of the resistance value R of the first MTJ element  11  with the external magnetic field H apply  is called the sensitivity of the MTJ element  11 .  FIG. 2  shows the first MTJ resistance response curve of the external magnetic field H apply  element  11  is not on the straight line H apply =0 and not axisymmetric, but about H apply =H o . It is offset by amount H O . H O  is often called the Neel coupling field. Typically, Ho is the range of 1-40 Oe. 
         [0066]    The resistance R in the linear region of the response curve shown in  FIG. 2  of the first MTJ element  11  can be approximated as: 
         [0000]    
       
         
           
             
               
                 
                   R 
                   = 
                   
                     
                       
                         
                           
                             R 
                             H 
                           
                           - 
                           
                             R 
                             L 
                           
                         
                         
                           2 
                            
                           
                             H 
                             s 
                           
                         
                       
                        
                       
                         ( 
                         
                           H 
                           - 
                           
                             H 
                             0 
                           
                         
                         ) 
                       
                     
                     + 
                     
                       
                         
                           R 
                           H 
                         
                         - 
                         
                           R 
                           L 
                         
                       
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0067]    In formula (1), H s  represents the saturation field, H s  is defined as follows: assuming the Neel coupling field H o =0 then the tangent line of the resistance vs, H apply  curve of the MTJ element  11  intersects the saturated resistance at the positive saturation field and the negative resistance saturation value at the negative saturation field value. In an ideal state, the variation of R of the first MTJ element  11  with the external magnetic field H apply  is perfectly linear, and there is no hysteresis. In practice however, an MTJ element  11  usually has some small hysteresis, and the resistance response curve as a function of H apply  of the first MTJ element  11  is more closely approximated by a curve as shown in  FIG. 3 . This is due to the magnetic material, and the resistive response to the external magnetic field of the first MT′ element  11  is more curved, 
         [0068]    In many applications, there will be a plurality of magnetoresistive MTJ elements  11  connected in series, parallel, or a combination of series and parallel as an MTJ element group. In the present embodiment, the MTJ element group  13  is composed of six MTJ elements  11  connected in series, as shown in  FIG. 4 , and within the MTJ element group  13 , the set of six MTJ elements  11  have the same sensing direction  1171 . MTJ element group  13  is electrically connected to other components, such as for example an ohmmeter  12 . A current  131  flows through the MTJ element group  13 , and the path of the current  131  is shown in  FIG. 4 . Typically, the direction of current  131  does not affect the resistance value of the Mil element group  13 . The resistance value may be changed by changing the number of MTJ element MTJ elements  11  in the MTJ element group  13 . A single MTJ element group  13  alone may serve as a bridge arm of a sensor bridge, or a plurality of MTJ element groups  13  may be interconnected within the same arm in series, parallel, or a combination thereof to form a bridge arm with appropriate resistance, 
         [0069]    In order to provide the element  11  or MTJ element group  13  with a bias magnetic field H cross , and eliminate its Neel coupling field H o , permanent magnets  14  can be set on both sides of an MTJ element  11  or group  13 , and the permanent magnets  14  may be tilted. In the present embodiment, as shown in  FIG. 5  permanent magnets  14  are placed on both sides of the first MTJ element group  13 , and the permanent magnets  14  are tilted relative to the sensitive direction  1171  of the MTJ element group  13 . The magnetic field distribution of the permanent magnet  14 , in the vicinity of the first MTJ element group  13  is shown in  FIG. 6 . In the present embodiment, the permanent magnet  14  is a rectangular parallelepiped shape. As shown in  FIG. 7 , there is an angle between the permanent magnets long side  14  and the sensitive axis  1171  of the MTJ element  13 , and this angle is denoted θ sns . Each permanent magnet  14  has a specific length L, width W, and thickness t, and there is gap G between the two permanent magnets  14 , and these physical metrics are used to set the bias field value. 
         [0070]    The magnetic field H mag  between the two permanent magnets  14  in the gap can be considered to be due to virtual magnetic charges (ρ s ) that form at the edge of the permanent magnet plates, and H mag  also depends on the shape and orientation of the plates. As shown in  FIG. 7 , the angle defined between the permanent magnets&#39; remnant magnetization M r    141  and the sensitive direction of the of the MTJ element group  11  is tilted at an angle θ mag . ρ s  magnetic charge density forms at the edge of the permanent magnet  14  as a function of the magnitude of remanent magnetization M r    141 , the orientation angle θ mag    141 , as well as the inclination angle θ sns  of the permanent magnet  14 . At the edge of the permanent magnet  14  magnetic charge density ρ s  can be expressed as: 
         [0000]      ρ s   =M   γ  cos(θ mag +θ sns )  (2)
 
         [0071]    The magnetic field H mag  emanating from the edge of the permanent magnet  14  can be expressed as: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         H 
                         -&gt; 
                       
                       mag 
                     
                      
                     
                       ( 
                       
                         r 
                         -&gt; 
                       
                       ) 
                     
                   
                   = 
                   
                     4 
                      
                     
                         
                     
                      
                     π 
                      
                     
                       
                         ∫ 
                         Surface 
                       
                        
                       
                         
                           
                             ρ 
                             s 
                           
                           
                             
                               ( 
                               
                                 
                                   r 
                                   -&gt; 
                                 
                                 - 
                                 
                                   
                                     r 
                                     -&gt; 
                                   
                                   ′ 
                                 
                               
                               ) 
                             
                             2 
                           
                         
                          
                         
                             
                         
                          
                         
                            
                           
                             S 
                             ′ 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0072]    As shown in  FIG. 7 , the magnetic emanating from the edge of the permanent magnet  14 , H mag  has a component  13  perpendicular to the sensitive axis direction  1171 , and it is defined as a bias magnetic field H cross  of the MTJ element group. When θ mag =θ sns =π/2, the bias magnetic field H cross  can be expressed as: 
         [0000]    
       
         
           
             
               
                 
                   
                     H 
                     cross 
                   
                   = 
                   
                     
                       - 
                       8 
                     
                      
                     
                       
                         M 
                         r 
                       
                       ( 
                       
                         
                           α 
                            
                           
                               
                           
                            
                           
                             tan 
                             ( 
                             
                               Lt 
                               
                                 
                                   ( 
                                   
                                     
                                       W 
                                       2 
                                     
                                     - 
                                     
                                       G 
                                       2 
                                     
                                   
                                   ) 
                                 
                                  
                                 
                                   
                                     
                                       L 
                                       2 
                                     
                                      
                                     
                                       
                                         
                                           t 
                                           2 
                                         
                                          
                                         
                                           ( 
                                           
                                             
                                               W 
                                               2 
                                             
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                                               G 
                                               2 
                                             
                                           
                                           ) 
                                         
                                       
                                       2 
                                     
                                   
                                 
                               
                             
                             ) 
                           
                         
                         + 
                         
                           α 
                            
                           
                               
                           
                            
                           
                             tan 
                             ( 
                             
                               Lt 
                               
                                 
                                   ( 
                                   
                                     
                                       W 
                                       2 
                                     
                                     + 
                                     
                                       G 
                                       2 
                                     
                                   
                                   ) 
                                 
                                  
                                 
                                   
                                     
                                       L 
                                       2 
                                     
                                      
                                     
                                       
                                         
                                           t 
                                           2 
                                         
                                          
                                         
                                           ( 
                                           
                                             
                                               W 
                                               2 
                                             
                                             + 
                                             
                                               G 
                                               2 
                                             
                                           
                                           ) 
                                         
                                       
                                       2 
                                     
                                   
                                 
                               
                             
                             ) 
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
         [0073]    Using formula (4) it can be seen that by adjusting the shape of the two permanent magnets  14 , the gap size G between the two, and the orientation and size of the remanent magnetization M r    141 , it is possible to change the bias magnetic field H cross  at the location of the MTJ element group  13 . By varying the bias magnetic field H cross  it is possible to adjust the saturation field of the MTJ element group  13 , thereby setting the sensitivity of the MTJ element group  13 . 
         [0074]    The bias magnetic field H cross  can also be expressed as: 
         [0000]        H   cross   =H   mag  sin θ mag   (5)
 
         [0075]    The magnetic field emanating from the edge of permanent magnet  14  generates a magnetic field component H off  along the sensitive direction  1171  of the MTJ element group  13 . The H off  component  1171  can be expressed as: 
         [0000]        H   off   =H   mag  cos θ mag   (6)
 
         [0076]    As can be seen from formula (6), by adjusting the shape, size, residual magnetization M r , and the inclination angle θ mag    141  of the permanent magnets  14 , it is possible to change the magnetic field H mag  generated by the permanent magnets  14  at the location of the MTJ element  13  along the sensitive direction to produce component H off  in order to eliminate the Neel coupling field H o  of the MTJ element  11  to ensure that the operating point of the MTJ element  11  is in its linear region. 
         [0077]      FIG. 8  is a top view of the physical location of the half-bridge  15  sensor arms in the XY plane.  FIG. 9  is an equivalent circuit diagram of half bridge  15 . Half-bridge  15  includes two bridge arms  151  and  152 , the two bridge arms may be comprised of MTJ element groups  13 , and the resistance of the two bridge arms, respectively, may be denoted, R 1  and R 2 , Bridge arm  151  and arm  152  have sensitive direction along the same sensitive direction  1171 . As shown in  FIG. 8 , at bridge arm  151  and bridge arm  152  the external magnetic field strength H apply  for both sensors is applied along the same sensitive direction  1171  and it can have different magnitude at each arm. The permanent magnets  14  on both sides of the arm  151  and the arm  152  are tilted. Two input terminals IN1 and 1N 2  are provided half-bridge  15 , and input terminal IN2 may be grounded. The half-bridge  15  is denoted as OUT1. When a steady bias voltage V bias  is applied between the input terminal IN1 and the input terminal IN2, then when the field on arms  151  and  152  changes, the resistance value of the resistance values of R 1  and R 2  change differently, producing an output voltage signal of the output terminal V OUT1 =V 1 . 
         [0078]      FIG. 10  is a top view showing the physical location of sensor arms in a full-bridge  16 .  FIG. 11  shows the equivalent circuit diagram for the full bridge  16 . Full-bridge  16  comprises four arms  161 ,  162 ,  163  and  164 , and each of the arms may comprise MTJ element group  13 , the resistance values of the four arms of the bridge are denoted as R 3 , R 4 , R 5 , and R 6 . Bridge arm  161 , arm  162 , arm  163  and arm  164  are sensitive along the same direction  1171 . Application of a gradient magnetic field H apply  along the sensitive direction  1171  changes the output. As shown in  FIG. 10  different values of magnetic field strength of the external magnetic field H apply  may exist at the location of bridge arms  161 ,  162 ,  163 ,  164 . Each bridge arm  161 ,  162 ,  163  and  164  is respectively provided with a pair of inclined permanent magnets  14 . Two input terminals of the full bridge  16 , are denoted respectively IN3 and IN4, where input terminal IN4 may be connected to ground. The full-bridge  16  has two outputs OUT2 and OUT3 respectively. When applying a steady bias voltage of value V bias  to the bridge, bridge arm resistance R 3  of arm  161  or R 4  of arm  162  change, and bridge arm resistance R 5  of arm  163  or bridge arm resistance R 6  of arm  164  thus have different values, such that output OUT2 and output OUT3 produce voltages V 2  and V 3  respectively, and the full-bridge  16  will produce a differential output voltage signal of V OUT2 =(V 3 −V 2 ). 
         [0079]    Ideally, full-bridge  16  output signal V OUT2  has no response to a common mode magnetic field H cM , but large response to differential-mode magnetic field H dM . In the presence of a common mode magnetic field H cM , arm  161 , arm  162 , arm  163 , and arm  164  have the same resistance value, so the full-bridge  16  outputs zero voltage signal. Ideally, the resistance values of the four full-bridge  16  arms in the absence of an applied magnetic field are equal to R, i.e., R 3 =R 4 =R 5 =R 6 =R, and four full-bridge  16  arms have equal sensitivity of S, that is, S R3 =S R4 =S R5 =S R6 =S R , then: 
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         [0080]    As can be seen from the formula (9), full-bridge  16  outputs a voltage signal only in the presence of a differential magnetic field H dM , but not to a common mode magnetic field H cM . Therefore, the full-bridge  16  has a strong ability to reject common-mode magnetic interference. The typical output response of a full-bridge sensor  16  is shown in  FIG. 12 . 
         [0081]    In practical applications, the sensor can use two full-bridges, known as a double full-bridge.  FIG. 13  shows a top view of the physical location of the sensor arms in a double full-bridge.  FIG. 14  shows an equivalent circuit diagram of the double full-bridge  17 . Double full bridge  17  includes eight bridge arms  171 ,  172 ,  173 ,  174 ,  175 ,  176 ,  177  and  178 , which may for example comprise comprising at least one MTJ element  111  or a plurality or MTJ elements electrically connected in series, parallel, or a combination of series and parallel, wherein the resistance of the eight bridge arms have values R 7 , R 8 , R 9 , R 10 , R 11 , R 12 , R 13  and R 14 , respectively.  FIG. 14 , shows an example where arms  171 ,  172 ,  173 , and  174  form one full bridge, and bridge arms  175 ,  176 ,  177 , and  178  form a second full bridge. Preferably, the sensitive directions of the 8 bridge arms in the double full-bridge  17  are all the same and aligned along the MTJ element sensitive direction  1171 . A gradient magnetic field can produce changes in H apply  along the sensitive direction  1171 . As shown in  FIG. 13 , the strength of the external magnetic field H apply  at the physical location of the arm  171  and  172  and H apply  at the position of the arm  173  and  174  may produce different H apply  at those physical locations; and also different magnetic field strength H apply  may occur at the location of arms  175  and  176  and at the location of arms  177  and  178 , Each arm within the double full-bridge  17  is located between a pair of inclined permanent magnets  14 . Two input terminals of the double full bridge  17  respectively are IN5 and IN6, where for example, the input terminal IN6 may be grounded. The four outputs of the double full bridge  17  are OUT4, OUT5, OUT6 and OUT7. Steady V bias  voltage is applied between the input terminals IN5 and IN6, The bridge arm resistance pairs R 7  of arm  171 /R 8  of arm  172  and R 9  of arm  173 /R 10  of arm  174  may have different sizes, likewise bridge arm resistance pairs R 11  of arm  175 /R 12  of arm  176  and R 13  of arm  177 /R 14  of arm  178  may have different values s, and then bridge outputs OUT4, OUT5, OUT6, and OUT7 respectively will have different voltages V 4 , V 5 , V 6 , and V 7 . The double full bridge  17  output voltage signals are then defined as V OUT4 =(V 5 −V 4 ) and V OUT5 =(V 7 −V 6 ). 
         [0082]    When prepared as a magnetoresistive gear tooth sensor, the half-bridge  15 , full-bridge  16 , or double full-bridge  17  may be deposited on the same substrate using the same deposition process step, and this is often referred to as a single-chip magnetoresistive gear tooth sensor, Alternatively the bridges may be formed from a plurality of dice containing MD element groups  13  after cutting a wafer into dice, and electrically interconnecting the dice via wire bonds to interconnect the MTJ elements  11  of the dice into a bridge. Through this technique, the MTJ element groups may be interconnected to form half bridge  15 , full bridge  16 , or double full bridge  17  sensors, They can then be packaged as a single magnetoresistive sensor chip and the magnetoresistive sensor can then be connected to an ASIC (Application Specific Integrated Circuit, ASIC) or to a lead frame package pin within a semiconductor package, 
         [0083]      FIG. 15  shows an embodiment of the magnetoresistive gear tooth sensor  18  including a magnetic sensor chip  181 , a permanent magnet  182 , a control circuit  183 , a soft ferromagnetic flux concentrator  184 , and a housing structure  185 . The magnetic sensor chip  181 , the permanent magnet  182 , the control circuit  183 , and the soft magnetic flux concentrator  184  are integrated into the housing  185 . The magnetic sensor chip  181  includes at least one bridge, the bridge may be a half-bridge  15 , full bridge  16  or a double full-bridge  17 , where half-bridge  15 , and each of the full-bridge  16 , or double full-bridge  17  comprises at least a first MTJ element group  13 , said MTJ element group  13  includes a plurality of MTJ elements  11  connected in series, parallel, or a combination of series and parallel. The magnetic sensor chip  181  is electrically connected to control circuit  183 . The soft ferromagnetic flux concentrator  184  is disposed between the magnetic sensor chip  181  and the permanent magnet  182 , such that the concave opening  184  of the soft ferromagnetic flux concentrator points in the direction of the magnetic sensor chip  181 . In the preferred version of the present embodiment, the magnetic sensor chip  181  includes a double full-bridge  17 , each bridge arm of the double full-bridge  17  includes at least one MTJ element group  13 . The permanent magnets  182  for generating an external magnetic field H apply  are used to magnetize the ferromagnetic gear. The soft ferromagnetic flux concentrator  184  reduces the magnetic field H apply  of permanent magnet  182  along the sensitive direction  1171  of the magnetic sensor chip  181  in order to ensure the MTJ elements  11  operate in their linear operating region. When relative movement occurs between the gear and the magnetic sensor chip  181 , the magnetic field strength of the external magnetic field H apply  for a position where the magnetic sensor chip  1811  will change. Changes in the magnetic field strength detected by the magnetic sensor chip  181  can be used to transform the external magnetic field R apply  into a control signal for the control circuit  183 . The control circuit  183  processes the output signal of the magnetic sensor chip  181 . In the present embodiment, the control circuit  183  can transform the sinusoidal voltage signal voltage of the magnetic sensor chip  181  into a square wave form. 
         [0084]    In the present embodiment, the magnetoresistive sensor gear tooth sensor  18  may be stationary, and sitting above a gear, as shown in  FIG. 15 . When different gear positions at points A, B, C, D, and E sequentially pass by the magneto-resistive gear tooth sensor, the magnetoresistive gear tooth sensor  18  outputs a sine wave voltage signal shown in  FIG. 116 . The magnetorestive gear tooth sensor  18  sine wave varies dependent on the location near the above described signal points, and the relationship can be used to determine the position of a gear tooth. When a missing gear tooth passes by the sensor, a sine wave voltage signal or a square wave voltage signal of the magnetoresistive gar tooth sensor  18  output gear may appear as shown in  FIG. 17 . Examination of the magnetoresistive gear tooth sensor  118  sine or square output voltage waveform shows it can be used to determine whether or not a gear tooth is missing. If a gear tooth is missing, its position can be determined according to the specific location of in the sinusoidal or square wave waveform of the magnetoresistive gear tooth signal where the signal transition length changes. Because the magnetoresistive gear tooth sensor  18  of the present embodiment can use double full-bridge  18  chips  181 , magnetoresistive gear tooth sensor  18  can have two voltage output signal V OUT4  and V OUT5 , as shown in  FIG. 18 . By using the phase difference of the voltage signals V OUT4  and V OUT5  it is possible to determine the direction of movement of the gear. In this implementation of a magnetoresistive gear tooth sensor  18 , at the position of the magnetic sensor chip  181 , permanent magnet  18  produces H apply  which can be considered as common-mode. Since the magnetic sensor chip  181  with double full-bridge  17  and other bridges have strong common mode suppression of magnetic interference, the resulting magnetoresistive gear tooth sensor  18  is not susceptible to interference from external magnetic field H apply  or the field generated by the biasing permanent magnets  182 . 
       Second Implementation: 
       [0085]      FIG. 19  is a schematic diagram of an implementation of a second type of MTJ element  21 . The second MTJ element implementation  21  is a multilayer film structure, as shown in  FIG. 19 , which comprises sequentially deposited on the substrate  211 , an insulating layer  212 , a bottom electrode layer  213 , a pinning layer  214 , a pinned layer  215 , a tunnel barrier layer  216 , a magnetic free layer  217 , a biasing layer  218 , and a top electrode layer  219 . Pinned layer  215  and freelayer  217  are composed of ferromagnetic materials. Exemplary ferromagnetic materials for the pinned layer  215  and the free layer  217  include Fe, Co, Ni, FeCo, FeNi, FeCo or FeCoNi. The pinned layer  215  may be a single ferromagnetic layer, or a composite layer with a thin Ru layer, for example, a trilayer of FeCo/Ru/FeCo. The pinning antiferromagnetic layer  214 , may be composed of various materials including PtMn, IrMn or FeMn. Pinning layer  214  and the pinned layer  215  are exchange coupling such that the pinned layer  215  has its magnetic moment  2151  rigidly fixed in one direction, such that under the influence of an external magnetic field H apply    215  its orientation remains unchanged. The tunnel barrier layer  216  may be made of MgO or Al 2 O 3 , The direction of the magnetic moment  2171  of the free layer  217  can change as the external magnetic field H apply  direction changes. Under the influence of an external magnetic field H apply , the direction of the magnetic moment  2171  of the free layer  217  can be parallel to the direction of the magnetic moment  2151  of the pined layer  215 , and it can gradually change with respect to the magnetic moment  2151  of the pinned layer into antiparallel orientation, and vice versa. Bias layer  218  may be an antiferromagnetic layer or a permanent magnet layer. Biasing layer  218  can be exchange coupling to the magnetic free layer  217  so that the bias magnetic layer  218  can provide a bias field perpendicular to the sensitive direction of the MTJ element  21 , H cross    217 . By varying the bias magnetic field H cross  it is possible to adjust the saturation field of the MTJ element  21 , thereby adjusting the sensitivity of the MTJ element  21 . When the bias layer  218  is an antiferromagnetic layer plus spacer layer, the Blocking Temperature is lower than that of the pinning layer  214 . Between the free magnetic layer  217  and the biasing layer  218  an isolating spacer layer may be deposited in order to attenuate the bias magnetic field H cross  from bias layer  218 . By changing the thickness of the spacer layer it is possible to adjust the size of the magnetic bias field H cross . The isolation layer is generally a non-magnetic material such as Ta, Ru, or Cu. The top electrode layer  219  and the bottom electrode layer  213  are generally non-magnetic conductive material. The substrate  211  is typically made of silicon, quartz, pyrex, GaAs, or AlTiC. The area of the insulating layer  212  is greater than the area of the bottom electrode layer  213 . The top electrode layer  219  and the bottom electrode layer  213  are used for electrically connecting with the other elements. 
         [0086]    In actual applications, a plurality of the second MTJ elements  21  may be used, and said plurality may involve series, parallel, or a combination of series and parallel connections to a second MTJ element group  23 . In the present embodiment, the second MTJ element group  23 , comprises groups of four MTJ elements  21  connected in series, wherein each has the same sensitive direction. The group  23  of second type of MTJ elements  21  may be electrically connected ohmmeter  12  or with the other components. The second MTJ element group  23  may be used as an arm of a bridge, and it can also be interconnected with multiple second MTJ element groups  23  in series, parallel, or a combination of series and parallel to form abridge arm. When used in a bridge MIT element group  23 , does not need permanent magnets  14  disposed on the sides of the MTJ element groups  23 . In this embodiment, the half-bridge  15 , full bridge  16 , and the double full-bridge  17  the bridge arms do not need permanent magnets  14  disposed around the sides of the MTJ element group  23 . In this example, the half-bridge  15 , full-bridge  16 , and double-full bridge  17  use the second type of Mil element group  23 , 
         [0087]    The magnetoresistive gear tooth sensor  18  described with element group  23  can be implemented similarly to the first implementation. 
         [0088]    The magnetoresistive gear tooth sensor described above using MTJ element as the sensitive elements, when compared with those using Hall Effect, AMR, or GMR elements, has better temperature stability, higher sensitivity, lower power consumption, better linearity, wider linear working field range, and has a simpler structure. The disclosed sensor has a concave soft ferromagnetic flux concentrator, a permanent magnet to reduce the external magnetic field along the sensitive direction of MTJ elements ensuring the MTJ element in magnetic sensor chip has a wide liner working range and improving the performance of the sensor. The disclosed sensor&#39;s magnetic sensor chips are bridges, making the sensor insusceptible to common mode magnetic field of permanent magnet or magnetic field interference. In optimal implementation, the MTJ element are surrounded by a pair of tilted permanent magnets, where the magnetic field generated by the pair of tilted of the permanent magnet in the direction perpendicular to the MTJ element sensitive direction provides the MTJ element with a magnetic bias field. The magnitude of the bias field can be used to adjust the sensitivity of the MTJ element according to application needs. In one preferred embodiment, the MTJ elements are placed between a pair of inclined permanent magnets, the permanent magnets produce a magnetic field component along the sensitive direction in order to eliminate the Neel coupling field, thereby ensuring the MTJ element operates in its linear region, improving the linearity of the sensor. In another preferred embodiment, a bias layer is deposited above the free layer and the bias layer can provide a bias field perpendicular to the sensitive direction of the MTJ element&#39;s free layer. By varying this bias field, the MTJ element saturation field can be adjusted to optimize the sensor, thereby obtaining a sensor with high sensitivity, which can be tuned according to different needs. The magnetoresistive gear tooth sensor can determine the position of the teeth of a gear, or when a gear tooth is missing. It likewise can determine the velocity of motion of a gear, and it can determine the direction of movement of a gear. It may be used with linear or circular gears. The disclosed implementations are conducive to low-cost mass production. 
         [0089]    It should be understood that the detailed description of the present invention provides only preferred embodiments, which are intended to be illustrative and not restrictive. Those of ordinary skill in the art upon reading the present specification can produce modified embodiments based on the present technical details described herein, wherein a portion of the technical features may be modified or replaced, but such modifications or replacements do not deviate from the spirit and scope of the technical program.