Abstract:
The invention discloses a rotation sensor suitable for gear wheels. MR (magneto-resistive) sensors are placed inside a zero field region generated by at least two permanent magnets. Said sensors are divided into two groups that are immersed in different locally generated magnetic environments. A differential signal taken between the two groups then senses the movement of the wheel&#39;s teeth. A single wafer method for manufacturing the device is also briefly described.

Description:
RELATED APPLICATIONS 
       [0001]    application Ser. No. 11/788,912, filed Apr. 23, 2007, describes a magnetic detector that may be used to measure electric currents without being affected by local temperature fluctuations and/or stray fields. application Ser. No. 11/904,668, filed Sep. 28, 2007, describes simultaneous setting of exchange pinning field magnetization in more than one direction for several thin film structures on a single substrate. Both applications are herein incorporated, by reference, in their entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to the general field of tachometry, specifically toothed wheels, through use of magnetic detectors based on GMR (Giant Magneto-resistive) or MTJ (Magnetic Tunnel Junction) devices 
       BACKGROUND OF THE INVENTION 
       [0003]    A conventional gear tooth sensor [ 1 ] consists of an IC (integrated circuit)  11  that includes Hall effect sensors  12  together with a single hard magnet  13 , as shown in  FIG.1 . The IC supports two Hall sensors, which sense the magnetic profile of the ferrous target simultaneously, but at different points, thereby generating a differential internal analog voltage that is further processed for precise switching of the digital output signal. To achieve a high differential signal output, the two Hall probes (or sensors) are spaced at a certain distance so that one Hall sensor faces field concentrating tooth  14  and the other Hall sensor faces gap  15  in the toothed wheel. A permanent magnet mounted with one pole on the rear side of the IC produces a constant magnetic bias field. 
         [0004]    If one Hall sensor momentarily faces a tooth while the other faces a gap between teeth, the gear tooth acts as a flux concentrator. It increases the flux density through the Hall probe and a differential signal is produced. As the gear wheel turns, the differential signal changes its polarity at the same rate of change as from the tooth to the gap. An integrated highpass filter regulates the differential signal to zero by means of a time constant that can be set with an external capacitor. In this way only those differences that changed at a minimum rate are evaluated. The output signal is not defined when in the steady state. 
         [0005]    A GMR based gear tooth sensor has also been proposed in which the sensing structure is similar to traditional Hall IC based gear tooth sensor except that the two Hall probes are replaced by two GMR sensors [ 2 ], as shown in  FIGS. 2   a - d.  Due to the high sensitivity of GMR devices, the GMR based gear tooth sensors provide a very large output signal which is stable over the rated temperature and voltage range. As a result, GMR based gear tooth sensors feature excellent air-gap performance and an extremely stable operating envelops as well as robust reliability characteristics. 
         [0006]    As shown in  FIGS. 2   a - d,  the magnetic field generated by the bias magnet is influenced by the moving gear tooth, the GMR sensors serving to detect the variation of the magnetic field component within the GMR film plane. The signal output is then generated from differential signals from two GMR sensors or a GMR bridge. Since the permanent magnet is mounted with either pole on the rear side of the GMR sensors (as in the Hall IC based gear tooth sensor) the magnetic field is essentially perpendicular to the GMR films. So any variation in the mechanical placement of the permanent magnet or any tilting of the GMR sensors during assembly could result in a large offset field in the GMR film plane. As a result, degradation may occur. Also, in this prior art design, the two GMR sensors have to be placed a certain distance apart (˜0.5 mm to 5 mm), depending on the particular gear tooth dimensions, so they cannot be initially formed on the same wafer. Instead, the individual GMR sensors must first be formed separately and then assembled along with other components of the IC. This is expensive and prone to causing additional variations from one installation to the next. 
       REFERENCES 
       [0000]    
       
         
           
             1. Infineon application note “Dynamic Differential Hall Effect Sensor IC TLE 4923” 
             2. NVE application note “Precision Gear Tooth and Encoder Sensors” 
             3. Taras Pokhil, et., “Exchange Anisotropy and Micromagnetic Properties of PtMn/NiFe bilayers,” J. Appl. Phys. 89, 6588 (2001) 
           
         
       
     
         [0010]    A routine search of the prior art was performed with the following references of interest being found: 
         [0011]    U.S. Patent Application 2003/0107366 (Busch et al) teaches that a sensing element can be one or two Hall elements or four magnetoresistive elements arranged in a Wheatstone bridge. In U.S. Pat. No. 7,195,211, Kande et al. teach that a gear tooth sensor can comprise a Hall effect sensor or a magnetorestive sensor. U.S. Pat. No. 7,138,793 (Bailey) also shows that a gear tooth sensor can be a Hall effect sensor or a GMR sensor. 
       SUMMARY OF THE INVENTION 
       [0012]    It has been an object of at least one embodiment of the present invention to provide a method for measuring rotation speeds of gear wheels that include some ferromagnetic material. 
         [0013]    Another object of at least one embodiment of the present invention has been to provide a detector based on said method. 
         [0014]    Still another object of at least one embodiment of the present invention has been to describe a process whereby said detector may be manufactured on a single chip or wafer without needing to add separately manufactured sub-assemblies or parts. 
         [0015]    A further object of at least one embodiment of the present invention has been that said detector be relatively insensitive to its exact orientation relative to the toothed wheel that it is sensing. 
         [0016]    These objects have been achieved by providing a design for a magneto-resistive gear tooth sensor based on single wafer processes together with new and improved methods for performing gear tooth sensor assembly. The result is a complete single chip solution with high accuracy and high signal output suitable for application to a wide range of gear tooth dimensions. 
         [0017]    The MR sensor is located inside the near field region between the two (or more) permanent magnets for minimizing offset field effects generated during assembly. In prior art sensors, the offset field and variations thereof are unavoidably large, and at least two individual MR chips have had to be used and located at certain precise distances from the gear toothed wheel. 
         [0018]    The invention teaches how to achieve MR-based measurements by using a single MR sensor having a better assembly margin. The same single MR sensor can be used over a range of different gear toothed wheel dimensions. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1 . Prior art device using a Hall effect IC based gear tooth sensor. 
           [0020]      FIGS. 2   a - d.  Prior art device showing sequence as gear moves past a GMR based gear tooth sensor. 
           [0021]      FIG. 3 . The basic design of the invention&#39;s MR sensor assembly. 
           [0022]      FIG. 4 . Plots the MR signal that is generated as the toothed wheel rotates. 
           [0023]      FIG. 5 . Illustrates a first embodiment of the invention. 
           [0024]      FIG. 6 . Illustrates a second embodiment of the invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0025]    An important feature of the present invention is the use of multiple permanent magnets in conjunction with MR (GMR or MTJ) based gear tooth sensors. As seen in  FIG. 3 , sensor  31  (comprising two MR sensing elements or one MR Wheatstone bridge) whose magneto-resistive thin films  32  faces rotating gear  11  while two or more permanent magnets  33 , with the same poles facing gear  11 , are symmetrically arranged with respect to MR sensor  31  so that neither of them directly faces MR sensor  31 . 
         [0026]    As also shown in  FIG. 3 , the magnetic field at the MR sensor (as directly generated by a permanent magnet) depends on geometry as well as on the magnet&#39;s strength. For example, the right side magnet in  FIG. 3  produces a vertical field as well as a horizontal field in the MR film plane. The vertical field is proportional to (αx-β)M, where α and β are the angles subtended by the two opposing poles of magnet  33  at the MR sensor and M is the remnant magnetization of the magnet. 
         [0027]    Arranging these two angles to be equal to each other reduces the vertical field to zero. Furthermore, since there are two magnets symmetrically located at opposite sides of MR sensor  32 , the horizontal field is also cancelled out. When used for gear tooth wheel speed detection, the sensor assembly is brought close to and facing the gear wheel. As the ferrous teeth move, the magnetic flux generated by the permanent magnets changes and the MR sensor experiences a changing field in the MR film plane. 
         [0028]      FIG. 4  shows simulated data for the MR signal as the toothed wheel rotates. The pitch of the gear tooth was assumed to be 6 mm while the sensor was specified to be 1.5 mm from a tooth at closest approach. Curve  41  represents the horizontal magnetic field experienced by the MR sensor. Since there is always some tilting of the MR sensor relative to the magnets, the offset field due to the tilting was also calculated. In the disclosed design, since the vertical field is zero (or close to zero) because of the special arrangement of the magnets, the offset field remains very small. This is reflected in curve  42 , while for a design of the prior art the offset field could be as large as 60% of the horizontal field even for a small (3-degree) tilt, as shown by curve  43 . An offset field of this magnitude is sufficient to cause an unacceptably large detection error. 
         [0029]    An additional feature of the invention is the methodology that we now disclose that we have developed in order to be able to employ a wafer-level process for manufacturing the MR (GMR or MTJ) based gear tooth sensors along with their associated multiple permanent magnets. In the sensor, two MR sensing elements or one MR Wheatstone bridge are used to sense the magnetic field. 
         [0030]    A first embodiment of the invention is illustrated in  FIG. 5 . Seen there is Wheatstone bridge  56  which comprises four identical MR elements (stripes),  51 ,  52 ,  53  and  54 . A constant voltage is applied across both branches ( 51 ,  52 ) and ( 53 ,  54 ). Output voltage Vout 1  is taken from the mid-point of the first branch ( 51 , 52 ) and output voltage Vout 2  is taken from the mid-point of the second branch ( 53 ,  54 ). Each MR element has the same uniaxial anisotropy along its longitudinal direction, while the pinned magnetization (PM) within the MR is perpendicular to its longitudinal direction. Also shown is arrow  57  which represents the gear field. 
         [0031]    A key feature of a first embodiment of the invention is magnetic shield  55  which comprises a layer of soft ferromagnetic material located either underneath or above elements  51  and  54  alone. The presence of the shield means that opposite arms of the bridge are immersed in different magnetic environments. When the invention is in use, as the toothed wheel rotates, an alternating magnetic field (gear field) is produced at the MR bridge and the free magnetizations (FM) in MR elements  52  and  53  rotate in response to the field, while the free magnetizations in the MR elements  51  and  54  do not change because of the shielding effect. The output signal (Vout 1 -Vout 2 ) is generated as a function of the local gear field. Said output signal is amplified and processed in a suitable circuit to determine the rotational speed of the toothed wheel. The fact that, in each case, one of the MR elements is magnetically shielded while the other is not, results in a larger differential signal than would be obtained if all elements were immersed in the same magnetic environment. 
         [0032]    In  FIG. 6  we illustrate a second embodiment of the invention. Shown there is MR bridge  66  and single turn coil  67 . The MR Wheatstone bridge consists of four identical MR elements (stripes),  61 ,  62 ,  63  and  64 . A constant voltage is applied across both branches ( 61 ,  62 ) and ( 63 ,  64 ). Output voltage Vout 1  is from the mid-point of the first branch ( 61 ,  62 ) and output voltage Vout 2  is from the mid-point of the second branch ( 63 ,  64 ). Each MR element has the same uniaxial anisotropy along its longitudinal direction, while the pinned magnetization (PM) within the MR is also along its longitudinal direction. 
         [0033]    Coil  67 , made of a highly conductive material, is located underneath or above all elements and generates magnetic fields at  61  and  64  along their first longitudinal direction, as well as magnetic fields at  62  and  63  along their second longitudinal direction (antiparallel to the field direction seen by elements  61  and  64 ). Thus, as in the first embodiment, MR sensors  61  and  64  see different magnetic environments than do sensors  62  and  63 , resulting, as before, in an enhanced differential signal. 
         [0034]    During operation of the device, prior to each reading, a large electric current pulse (symbolized by arrows  68 ) is passed through coil  67 , to pre-set the magnetization of the free layers in MR elements  61  and  64  along the first longitudinal direction, as well as to pre-set the free layer magnetizations in other two MR elements,  62  and  63  along the second longitudinal direction. As the toothed wheel rotates, an alternating magnetic field (gear field) is produced on the MR bridge, to which the magnetizations in MR elements  62  and  63  respond by rotating so as to decrease their resistance values, while magnetizations in MR elements  61  and  64  respond by rotating so as to increase their resistance values. The output signal (Vout 1 -Vout 2 ) is generated as function of the local gear field. Said output voltage is amplified and processed in a suitable circuit for determining the rotational speed of the toothed wheel. The technology disclosed in application Ser. No. 11/904,668 (incorporated above by reference) was used to fabricate the structures that constitute the present invention in a single wafer process. Briefly, the various sub-structures are simultaneously magnetized into different directions through use of exchange pinning fields; all sub-structures were first formed in their required orientations. A layer of hard magnetic material was then deposited, suitably patterned to control the direction of its flux, and then magnetized through a single exposure to a strong magnetic field. The assemblage was then thermally annealed (in the absence of any applied field) at a temperature higher than the AFM material&#39;s blocking temperature, following which the thin film structures became magnetically pinned in the intended directions. 
         [0035]    The poles of the two permanent magnets (corresponding to magnets  33  in  FIG. 3 ) typically had cross-sectional areas in the range of from 0.5 to 50 sq. mm and heights (pole separation) in the range of from 0.5 to 20 mm. Following exposure to an external field in the range of from 1,000 to 20,000 Oe, these permanent magnets acquired a magnetization in the range of from 0.1 to 1.0 Tesla. Suitable materials for the permanent magnets included (but were not limited to) Alnico, Samarium cobalt, Neodymium-Iron-Boron and other ceramic magnets. 
         [0036]    For both embodiments, the MR elements may be either GMR or MTJ devices.