Abstract:
The invention discloses a method and apparatus for determining the rotational status of a gear wheel whether or not it is actually turning. A key feature is the magnetic angle sensor that is used. Said sensor comprises a bridge structure of four MR devices in a square array. The direction of the pinned reference layer is the same for all four devices and lies along one of the diagonals of said square array. A single wafer process is used to manufacture the invented device.

Description:
This application is related to application No. 12/082,257, now U. S. Pat. No. 7,619,407 and herein incorporated, by reference, in its entirety. 
     FIELD OF THE INVENTION 
     The invention relates to the use of magnetic field angle sensors for determining the rotational status of a gear wheel. 
     BACKGROUND OF THE INVENTION 
     As shown in  FIG. 1 , 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 . 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. 
     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. 
     A GMR based gear tooth sensor has also been described 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 , 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. 
     If two or more permanent magnets are used to generate the bias field, it becomes possible to locate the GMR sensors within an area in which the magnetic field is zero when the sensor is opposite a gap between teeth, rising to its maximum value when opposite a tooth. 
     References 
     1. Infineon application note “Dynamic Differential Hall Effect Sensor IC TLE 4923” 
     2. NVE application note “Precision Gear Tooth and Encoder Sensors” 
     A routine search of the prior art was performed with the following references of interest being found: 
     In U.S. Patent Application 2006/0261801, Busch teaches four AMR elements arranged in a Wheatstone bridge to form a gear tooth sensor. U.S. Pat. No. 7,112,957 (Bicking) discloses MR sensors in a Wheatstone bridge to sense gear teeth in various positions. A permanent magnet is also disclosed. 
     U.S. Pat. No. 5,351,028 (Krahn) shows a gear tooth sensor using a permanent magnet and MR elements 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 magneto-restive sensor while, in U.S. Pat. No. 7,138,793, (Bailey shows that a gear tooth sensor can be a Hall effect sensor or a GMR sensor. 
     SUMMARY OF THE INVENTION 
     It has been an object of at least one embodiment of the present invention to provide a method for determining the rotational status of a gear wheel, whether or not said wheel is rotating. 
     Another object of at least one embodiment of the present invention has been to provide a detector based on said method. 
     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. 
     These objects have been achieved by displacing the rotation sensor so that it overhangs only part of the wheel&#39;s rim, extending a distance away from the wheel&#39;s edge. The field sensor uses a bridge structure made up of four MR devices that form a square array. The direction of the pinned reference layer is the same for all four devices and lies along one of the diagonals of said square. 
     The device senses the field associated with each tooth of the wheel (said field going to zero in the gap between teeth). So, the signal that the sensor outputs can be used to determine the precise rotational status of the wheel, enabling, for example, two gear wheels to be precisely aligned at the time that they are brought together. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1 . Prior art device using a Hall effect IC based gear tooth sensor. 
         FIGS. 2   a - d . Prior art device showing the sequence as a gear moves past a GMR based gear tooth sensor. 
         FIG. 3 . Shows the basic apparatus of the invention. 
         FIGS. 4   a - 4   b . These illustrate the spatial relationship between the sensor and the rotational state of the toothed wheel. 
         FIG. 5 . Compares the in-plane magnetic field as a function of the offset distance of the MR sensor from the gear wheel edge for when a tooth goes by and when a gap goes by. 
         FIGS. 6   a - 6   d . Shows four embodiments of the invention that differ from one another in the direction given to the pinned reference field. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Throughout the discussion that now follows, we will use the terms ‘toothed wheel’ and ‘gear wheel’ interchangeably.  FIG. 3  shows two views of gear wheel  31  (full face at left and edge-on at right). As can be seen, sensor  32 , along with its associated permanent magnet  33  (or multiple magnets) has been located a distance d from rim  36  of wheel  31 . Also seen in  FIG. 3  is a key feature of the invention namely the positioning of sensor  32  so that its centerline  34  is displaced away from edge  35  of the gear wheel by an amount δ. 
     Acceptable values for d are in the range of from 0.1 to 10 mm, with from 1 to 6 mm being preferred, while acceptable values for δ are in the range of from 0 to 5 mm, with from 1 to 3 mm being preferred. 
     The consequences of the geometrical arrangement shown in  FIG. 3  are schematically illustrated in  FIG. 4 . Since the sensor is no longer directly above the wheel the vertical field that it senses when a tooth moves directly opposite to it is less than would be the case if the sensor were directly above the wheel (as in prior art arrangements). On the other hand, it can now sense the horizontal field that is associated with each tooth. In particular, said horizontal field lies in the plane of the MR sensor&#39;s free layer, the plane in which it has the greatest influence of the free layer. This is the case that is illustrated in  FIG. 4   a.    
     The tooth&#39;s horizontal field moves away from the sensor very rapidly as the gap between teeth arrives opposite the sensor. This is the case illustrated in  FIG. 4   b  where it can be seen that, while the vertical (i.e. out-of-plane) field, corresponding to the device&#39;s anisotropy direction, remains essentially unchanged, the horizontal (i.e. in-plane) field, along the devices easy axis, has changed significantly. As a result, when the sensor is used to measure gear wheel speed, it experiences a readily detected alternating field within the MR film plane 
       FIG. 5  shows simulated results for the MR signal as function of the offset distance from the MR sensor to gear wheel edge (designated δ in  FIG. 3 .) The width of gear tooth was assumed to be 6 mm and the sensor to be at a vertical distance of 3 mm from a front tooth. Thick solid curve  51  represents the horizontal magnetic field experienced by the MR sensor as a tooth goes by, while broken curve  52  represents the horizontal magnetic field experienced by the MR sensor as a gap goes by. Thus, as the gear toothed wheel turns, the MR sensor experiences a periodic in-plane (or horizontal) field, and generates an output signal whose polarity changes at the same rate as going from tooth to gap. 
     An important advantage of the arrangement disclosed above is that there is a high tolerance margin for the offset distance of the MR sensor from the gear wheel edge. Said offset distance can vary by up to 1.5 mm without changing the sensor&#39;s output signal by more than 10%. Additionally, the difference in the horizontal field between when a tooth is directly opposite the sensor and when a gap is directly opposite is substantial, easily reaching values of 20 Oe or more. 
     Another important advantage of the invention is that it lends itself to being manufactured through a wafer-level process (see reference incorporated earlier above) 
     Referring now to  FIG. 6   a , shown there, as a first embodiment, is a Wheatstone bridge consisting of four identical MR elements (stripes), R 1 , R 2 , R 3  and R 4 . To fabricate these four sensing elements, a full MR film is first deposited (including pinned and pinning layers, a transition layer, a free layer, and a capping layer). This is followed by thermal annealing in the presence of a large external field in order to set (pin) the reference magnetization along a specific direction. Photolithography is then used to pattern the MR film into four separate rectangular shapes that all have the same large aspect ratio (typically between 1.5 and 1000) but with different orientations in the plane. 
     The long axes of R 1  and R 4  have the same orientation, shown as solid arrows and angled 45 degree away from the direction of (above-mentioned) reference magnetization  61 . R 2  and R 3  are also oriented to have their long axes pointing 45 degree away from magnetic reference pinning direction  61 , but in the perpendicular direction to R 1  and R 4  so that R 2  and R 3  end up at right angles to R 1  and R 4 . R 1  and R 2  are series connected to form one branch of Wheatstone bridge, while R 3  and R 4  are series connected to form the other. The long axes of R 2  and R 3  are parallel to the axis of the toothed wheel which is also the in-plane field direction of their free layers (as shown earlier in  FIG. 4   a ) while the long axes of R 1  and R 4  are perpendicular to the axis of the toothed wheel, i.e. perpendicular to the in-plane field direction of their free layers. 
     A constant voltage is applied between the junction of R 1  and R 3  and the junction of R 2  and R 4 . Output voltage Vout 1  is taken at the junction of R 1  and R 2 ) while output voltage Vout 2  is taken at the junction of R 3  and R 4 . As the toothed wheel rotates, the influence of gear field  62  on the bridge elements changes. Field  62  has a constant direction, but it alternates in magnitude at the MR bridge&#39; location. The free magnetizations (FM) in MR elements R 1  and R 4  rotate away from their longitudinal direction as they respond to the field, being shown in the figure as broken lines, while the free magnetizations in MR elements R 2  and R 3  do not change since the gear field is in the same direction as their initial magnetization direction in the longitudinal direction. The value of Vout 1 −Vout 2  is thus proportional to gear field  62  and so can be amplified and processed to provide a value for the speed of rotation of the toothed wheel. 
     Some additional (though not necessarily all) possible embodiments are shown in  FIGS. 6   b, c , and  d . All follow similar arrangements to that seen in  FIG. 6   a  except that the direction of reference magnetization  61  is different in each case. In  FIG. 6   b , R 1  and R 4  have their long axes pointing 135 degree away from magnetic reference direction  61 , while R 2  and R 3  have long axes pointing −135 degree away from magnetic reference direction  61 . In  FIG. 6   c , R 1  and R 4  have long axes pointing −45 degree away from direction  61 , while R 2  and R 3  have long axes that point 45 degree away from  61 . In  FIG. 6   d , R 1  and R 4  have long axes pointing −135 degree away from  61 , while R 2  and R 3  have long axes pointing 135 degree away from  61 . 
     The MR elements may be either GMR (Giant Magneto-resistance) or MTJ (Magnetic Tunnel Junction) devices. It should also be noted that, although one of the main principles of operation is that the resistances of the R 2  and R 3  elements are unaffected by the size of the gear field, they should not be replaced by fixed resistors because, during operation, some heating of the system is inevitable. Since fixed resistors will have different TCRs (temperature coefficients of resistance) from the MR devices, such heating would introduce errors into the bridge&#39;s output. 
     Regardless of which type of MR device is employed, the invented system has a number of additional applications beyond measuring rotation speeds, these include: 
     (a) The general principles governing the operation of the invention make it capable of application to any structure, which is subject to cyclic motion, for the purpose of determining where in its cycle the structure is. 
     (b) There is no requirement for rotation to be taking place while the precise positions of teeth and gaps are being sensed. 
     (c) The position of a tooth in a first gear wheel relative to a second gear wheel, or to a cam, can be precisely determined, whether or not either or both parts are rotating. This type of TPOS (True Power On State) function, allows the first wheel to ‘know’, at power-up time, whether it is facing a tooth or a gap before the two (gear wheels or wheel and cam) are brought together. 
     (d) As can be seen in the right hand portion of  FIG. 3 , the out-of plane field will vary, depending on the value of δ. Although it is a feature of the invention that this variation is normally small, it can, if need be, be made more sensitive by (for example) reducing the aspect ratios of the MR elements. Such a modification enables the system to detect any wobble that may be occurring as the wheel rotates well before it manifests itself mechanically. If only wobble is to be measured, the wheel need not be toothed as long as it has a ferromagnetic portion extending inwards from the outer edge (rim). If a toothed wheel is used then the same sensor could be used to measure both rotation and wobble simultaneously.