Gear tooth sensor (GTS) with magnetoresistive bridge

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.

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 inFIG. 1, a conventional gear tooth sensor [1] consists of an IC (integrated circuit)11that includes Hall effect sensors12together with a single hard magnet13. 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 tooth14and the other Hall sensor faces gap15in 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 inFIGS. 2a-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's rim, extending a distance away from the wheel'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.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Throughout the discussion that now follows, we will use the terms ‘toothed wheel’ and ‘gear wheel’ interchangeably.FIG. 3shows two views of gear wheel31(full face at left and edge-on at right). As can be seen, sensor32, along with its associated permanent magnet33(or multiple magnets) has been located a distance d from rim36of wheel31. Also seen inFIG. 3is a key feature of the invention namely the positioning of sensor32so that its centerline34is displaced away from edge35of 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 inFIG. 3are schematically illustrated inFIG. 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's free layer, the plane in which it has the greatest influence of the free layer. This is the case that is illustrated inFIG. 4a.

The tooth's horizontal field moves away from the sensor very rapidly as the gap between teeth arrives opposite the sensor. This is the case illustrated inFIG. 4bwhere it can be seen that, while the vertical (i.e. out-of-plane) field, corresponding to the device'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. 5shows simulated results for the MR signal as function of the offset distance from the MR sensor to gear wheel edge (designated δ inFIG. 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 curve51represents the horizontal magnetic field experienced by the MR sensor as a tooth goes by, while broken curve52represents 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'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 toFIG. 6a, shown there, as a first embodiment, is a Wheatstone bridge consisting of four identical MR elements (stripes), R1, R2, R3and R4. 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 R1and R4have the same orientation, shown as solid arrows and angled 45 degree away from the direction of (above-mentioned) reference magnetization61. R2and R3are also oriented to have their long axes pointing 45 degree away from magnetic reference pinning direction61, but in the perpendicular direction to R1and R4so that R2and R3end up at right angles to R1and R4. R1and R2are series connected to form one branch of Wheatstone bridge, while R3and R4are series connected to form the other. The long axes of R2and R3are parallel to the axis of the toothed wheel which is also the in-plane field direction of their free layers (as shown earlier inFIG. 4a) while the long axes of R1and R4are 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 R1and R3and the junction of R2and R4. Output voltage Vout1is taken at the junction of R1and R2) while output voltage Vout2is taken at the junction of R3and R4. As the toothed wheel rotates, the influence of gear field62on the bridge elements changes. Field62has a constant direction, but it alternates in magnitude at the MR bridge' location. The free magnetizations (FM) in MR elements R1and R4rotate 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 R2and R3do not change since the gear field is in the same direction as their initial magnetization direction in the longitudinal direction. The value of Vout1−Vout2is thus proportional to gear field62and 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 inFIGS. 6b, c, andd. All follow similar arrangements to that seen inFIG. 6aexcept that the direction of reference magnetization61is different in each case. InFIG. 6b, R1and R4have their long axes pointing 135 degree away from magnetic reference direction61, while R2and R3have long axes pointing −135 degree away from magnetic reference direction61. InFIG. 6c, R1and R4have long axes pointing −45 degree away from direction61, while R2and R3have long axes that point 45 degree away from61. InFIG. 6d, R1and R4have long axes pointing −135 degree away from61, while R2and R3have long axes pointing 135 degree away from61.

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 R2and R3elements 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'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 ofFIG. 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.