Abstract:
An interleaved parallel MR bridge array is optimized for sensing fine pitch ring magnets and can be utilized to reduce ring magnet target size for wheel speed sensing applications.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This Application Claim to Provisional Application No. 60/108,709 filed on Nov. 17, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to the field of magnetically sensitive sensors. The present invention relates particularly to a sensor having a magnetoresistive (MR) bridge array optimized for reading fine pitch ring magnets with high air gap at low gauss levels. 
     2. Description of the Related Art 
     MR bridges have been proposed before for sensing ring magnets. For example, see Durable and Multifunctional Magnetic Rotation Sensor for Rotary Encoders, Sei et al., IEEE “Transitions on Magnetics”, Vol. mag-23, No. 5, pages 2185-2187, (September 1987), and Integrated MR Sensors for Automobile, Akiyama et al., IEEE “Transactions on Magnetics”, Vol. 30, No. 6, pages 4617-4619, November 1994. 
     However, Applicant is unaware of any practical commercial application of MR bridge and ring magnet which has been optimized for sensing at low gauss and high air gap applications for the sensing of fine pitch, 20 to 50 pole pair, ring magnets. This scenario is particularly important for automobile wheel speed sensing which figures prominently in anti-lock brake systems, anti-slip drive systems, power-assists, and other stability systems on modern automobiles. 
     SUMMARY OF THE INVENTION 
     The present invention shows an anisotropic magnetoresistive bridge constructed and arranged of parallel, interleaved bridge elements with dimensions optimized to maximize sensitivity of the sensor at large air gap and low gauss signals in an economically producable package. This is accomplished in a preferred embodiment by matching magnet pole dimensions to the spacing between the array halves to yield a positive and negative going waveform from the MR bridge by passage of a single pole over the array. The sensor can then output a square wave, or single pulse duty cycle, for the passage of each pole over the array. The length, width, thickness and spacing of the individual resistive elements, or runners, as well as their number and arrangement is also disclosed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more fully and completely understood from a reading of the Description of the Preferred Embodiment in conjunction with the drawings, in which: 
     FIG. 1 is a top view of the MR bridge array according to the present invention. 
     FIG. 2 shows the electrical bridge configuration of the arrangement of FIG.  1 . 
     FIG. 3 is a schematic version of the portion of the bridge array utilized for explaining the dimensions thereof. 
     FIG. 4 is a schematic illustration of a portion of a ring magnet utilized in conjunction with the MR bridge array for wheel speed sensing. 
     FIG. 5 is an illustration of the interleaving arrangement of an array half having 3 runners per resistive element. 
     FIG. 6 is an illustration of the interleaving arrangement of an array half having 5 runners per resistive element. 
     FIG. 7 is an illustration of the interleaving arrangement of an array half having 6 runners per resistive element. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Throughout the Description of the Preferred Embodiment, like components will be identified by like reference numerals. 
     Referencing FIG. 1, an MR bridge  11  is shown to include first and second halves of the array  13  and  15  respectively. The first half  13  includes parallel interleaved bridge resistors R 1  and R 3 ,  17  and  19  respectively. Each resistor is made up of multiple runners, or longitudinal elements, shorted together at the ends thereof. The second half  15  is substantially symmetrical with the first half  13  and contains interleaved bridge resistors R 2  and R 4 ,  21  and  23  respectively. Each half of the preferred array as shown has an interleaving arrangement of x, 2 y,  2 x, 2 y, x; where x is a runner of R 4  or R 3 , and y is a runner of R 1  or R 2 . As shown in FIG. 5, where 6 runners are selected, ie three runners per resistive element, a preferred arrangement is x, 2 y, 2 x,y. As shown in FIG. 6, a five runner per resistive element array is suggested as x,  2 y,  2 x,  2 y,  2 x, y. As shown in FIG. 7, a six runner per resistive element array is suggested as x, 2 y, 2 x, 2 y, 2 x, 2 y,x. While various interleaving schemes may be used, eg. simple x,y interleaving of alternate resistive element runners routing of the resistive tracks may become increasingly complex and result in more difficult processing. Interleaving of the array runners will give a better signal averaging over the relatively large span of the array than a grouping of all one resistive element runners to one side of the array half. The number of runners is optimally selected at between 3 and 6 runners to make up a single resistive element. This results in an optimal array area for signal averaging of the target magnetic field in the fine pitch magnet as specified below. As illustrated with the labeling of each of the eight runners in the second half  15 , the four runners constituting R 4  consist of one outside runner on each side and two center runners for R 4 . Two pair of two adjacent runners comprising R 2  are interleaved with and surrounded on each side by R 4  runners. 
     The deposition of the bridge runners and surrounding electrical connections on to a silicon substrate, or the like, are considered within the ordinary skill of the art. For example, the permalloy runners may be deposited by the techniques as disclosed in U.S. Pat. No. 5,820,924 to Witcraft et al., of common ownership herewith. 
     Referencing FIG. 3, the major dimensions of the array will be explained. These dimensions are optimized to sense a matched ring magnet  25  such as shown in FIG.  4 . This ring magnet  25  would have a pole-pair spacing, or pitch, 7 of about 4.5 mm with a general range of +3.0 mm up to about 7.5 mm for pole pair pitch. This type of magnet would typically be used for wheel speed sensing in automotive applications and, eg., be a fine pitch magnet of about 30-70 mm diameter with about 20-50 pole-pairs included thereon. Because of the physical constraints of the mounting of this magnet for automotive wheel speed sensing applications, a large air-gap between the array and magnet of 3.0 to 3.5 mm is required, meaning that the sensor must switch at low gauss levels of approximately 7 to 8 gauss. For comparison purposes, Honeywell&#39;s current Hall sensor part number VF431PP operates with a maximum air-gap of about 1.5 mm at 50-75 gauss and Honeywell&#39;s current MR sensor 2SS52M operates at about 2-2.25 mm air-gap at approximately 25±10 gauss. 
     Returning to FIG. 3, the center-to-center spacing  1  between the first half  13  and second half  15  of the array is on the order of 0.75 to 1.25 mm. This spacing lis preferably selected to match the distance of about 0.5, or half, of a pole face so that a pole transition over the center line of the array will place the first and second array halves on opposite pole faces. In other words, when one array half is over a pole transition, the opposite array half is over the center of a pole face. Because this arrangement results in a full bridge output yielding a high and a low going wave form for each pole face the sensor is able to output a pulse, or duty cycle, for each magnetic pole face on the target. Thus, when compared to previous magnetically sensitive ring magnet sensors which output one sensor pulse per pair of pole faces one may shrink the ring magnet target by half and obtain the same number of output pulses. 
     The number of runners  2  indicated by dots at each side of the array is selected to be between four and eight runners per resistive element and is preferably six. Multiple runners are selected to gain a length of resistive element suitable to avoid excess current draw through the array. in keeping with optimization of the other dimensions. The runner width  3  is nominally 28 microns and selected to be between 25 and 35 microns. The runner thickness  4  is nominally 175 Å and selected to be between 150 and 250 angstroms. The spacing between runners within the arrays  5  is nominally 3 microns but could be selected to be up to 5 microns. The runner length  6  is nominally 0.76 mm selected to be between 0.5 to 1.0 mm. While all factors in these dimensions are interdependent, it is believed that the stated ranges are the practical limits to satisfy the three major practical constraints of the maximum sensitivity, producability, and reasonable cost for this type of sensor. 
     As an added benefit, when following the teachings of the present invention for the construction and arrangement of the preferred embodiment of MR sensor array, the present sensing system has an improved insensitivity to run-out, i.e. a condition where the sensor is located more toward the inside or outside diameter of the ring magnet. The present sensor is also less sensitive to large tilt tolerances, up to 10°. Tilt is a condition where the sensing plane of the resistive elements is not perfectly parallel to the face of the ring magnet. By way of comparison, other known types of ring magnet sensors have a tolerance variation for tilt of only 2-3°. Also, the present sensing arrangement is less sensitive to twist, i.e. a condition where the sensing plane is parallel to the ring magnet face, but is turned in that plane so that the bridge runners are not perfectly in parallel with the pole transitions of the ring magnet. 
     While the present invention has been shown and described with reference to preferred embodiments, many alternatives will become apparent to the ordinarily skilled artisan upon disclosure of the present invention. Therefore the present invention is only to be limited by the claims appended hereto.