Abstract:
A sensor for measurement of shaft angle values of a rotatable magnetized shaft employs a plurality of bridge circuits with corresponding magnetoresistive sensing elements disposed spaced-apart from a magnet disposed in an end of the shaft. The bridge circuits are held by a base which includes a cavity for receipt of the end of the shaft having the magnet. A housing secures the shaft and positions the shaft for rotation within the cavity. The base is formed of polybutalene teraphthallate which is permeable to magnetic lines of force, thereby permitting a coupling of the magnetic field of the shaft to the magnetoresistive sensors for measurement of an angle of the shaft about the shaft axis.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a shaft angle sensor suitable for use in an automotive environment, and more particularly, to a shaft angle sensor employing magneto-resistive material for reduced sensitivity to spacing between rotary and stationary elements of the sensor. 
     In the use of electrical and electromechanical systems in an automobile, it is often necessary to employ an angle sensor to provide an indication of an angle of rotation of a rotatable component relative to a fixed component in the automobile. By way of examples, electrical circuitry is employed: (1) in the operation of a throttle body and/or carburetor with respect to the positioning of a butterfly valve, (2) in conjunction with the positioning of a throttle pedal for feeding fuel to an engine, (3) in conjunction with measurement of rotational angle of shaft used in rotary chassis height sensor equipment, (4) in an electronic throttle body sensor for regulating fuel to an engine, as well as in (5) operation of brake, clutch and positioning of a gear selection, by way of example. It is desirable to have such sensing of rotational angle to be accomplished precisely for optimum operation of the foregoing components of the automobile. However, the automobile represents a hostile environment to the employment of such sensors because of vibration, dirt, and temperature variations, as well as to mechanical concerns of tolerance, spacing, displacement, shaft and bearing wear. A further difficulty arises in the fabrication of the sensor for the automotive environment wherein there is need for excessive precision in the assembly of the sensor. 
     SUMMARY OF THE INVENTION 
     The aforementioned problems are overcome and other advantages are provided by a shaft-angle sensor which, in accordance with the invention, comprises a rotatable magnet shaft placed adjacent a bridge circuit having a magnetoresistive (MR) sensing element which determines an angle of the flux of the magnetic shaft. Preferably, a double bridge circuit with a pair of the MR sensing elements is employed, wherein the two bridge circuits are rotated 45 degrees relative to each other to provide the equivalent of sine and cosine components of a measure of the magnetic field produced by a magnet in the end of the magnet shaft. A characteristic of the MR sensor is its ability to measure magnetic field angle while being tolerant to displacement between the rotary and the fixed portions of the sensor. This tolerance to displacement is attained by a fabrication of the sensor with a base of plastic material which is essentially transparent to the magnetic field, wherein the base provides a support for the bridge circuits, and also forms a cavity for receiving a magnetized end of the magnet shaft. Use of the plastic base permits the sensor to be fabricated of circuit components which are later assembled and, wherein, a slight displacement from optimum placement of the components has no more than a negligible error in the measurement due to the tolerance of the MR sensing element to displacement between the rotary and the fixed parts of the sensor. In addition, the plastic material is resistant to moisture and is temperature stable. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The aforementioned aspects and other features of the invention are explained in the following description, taken in connection with the accompanying drawing figures wherein: 
     FIG. 1 is an isometric view of an angle sensor configured for use of a rotary chassis height sensor; 
     FIG. 2 is an exploded view of the angle sensor of FIG. 1; 
     FIG. 3 is a further exploded view of the angle sensor of FIG. 1; 
     FIG. 4 is an exploded view, with the components shown in side-by-side arrangement, of a further embodiment of the angle sensor suitable for use as a throttle position sensor; 
     FIG. 5 is an exterior view of the angle sensor of FIG. 4; 
     FIGS. 6 and 7 show top and bottom views of a further embodiment of the invention, suitable for use as an electronic throttle body sensor; and 
     FIGS. 8 and 9 are graphs showing output signals of the MR sensors. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to FIG. 1, there is shown a sensor  20  suitable for use as a rotary chassis height sensor (RCHS) in connection with a linkage employed in a connection of an automobile body to the wheels of the automobile. The sensor comprises a housing  22  and a lever arm  24  extending from a shaft  26  (shown in FIG. 2) for rotation about an axis of the shaft  26  relative to the housing  22 . At the end of the arm  24 , there is located a ball joint  28  for pivotally connecting with an arm of the support linkage (not shown) of the automobile, such that changes in a vertical displacement of the vehicle body relative to the ground result in a pivoting of the arm  24  relative to the housing  22 . A socket  30  provides for receipt of the plug of an electrical cable (not shown) for activating electrical circuitry within the sensor  20 . The housing  22  is provided with bores  32  for receipt of mounting lugs (not shown) by which the sensor  20  is secured to a mounting support, such as a frame of the vehicle body (not shown). 
     As shown in FIGS. 2 and 3, the housing  22  includes a base  34  which is provided with a cavity  36  at a bottom thereof for receiving a printed circuit board  38  having magneto-resistive MR circuitry  40  thereon. Electrical connection with the MR circuitry  40 , which circuitry may be constructed in the form of a bridge, by way of example, is made via the socket  30 . The top portion of the base  34  defines a cup  42  in the form of a cylindrical cavity for receiving a bottom end of the shaft  26 . A permanent magnet  44  is secured to the bottom end of the shaft  26 , and is received within the cup  42  upon insertion of the shaft  26  into the cup  42 . Both the permanent magnet  44  and the bridge circuitry  40  are aligned with the axis of the shaft  26 , upon insertion of the shaft  26  within the cup  42 , so that the bridge circuitry  40  is located directly beneath the magnet  44 , and is separated therefrom by a relatively small distance, or gap, defined by the thickness of the floor of the cavity  36 . Both the cup  42  and the shaft  26  are provided with circular cylindrical surfaces so as to permit rotation of the shaft  26  within the cup  42 . 
     The lever arm  24  is constructed with a base socket  46  having a circular cylindrical cavity  48  for receipt of a cover housing  50  which also has a circular cylindrical shape. The cover housing  50  has a cavity  52  for receiving the top end of the shaft  26  upon the interposition of an O-ring  54  about the top portion of the shaft  26  and in abutment with a flange  56  of the shaft  26 . Encircling the midpoint of the shaft  26  is a sleeve serving as a slide bearing  58  enabling rotation of the shaft  26  within the cover housing  50 . The top end of the shaft  26  is fashioned with a flat surface to form a key  60  in the configuration of the letter D. The key  60  mates with interior surface of the cavity  48  at a base terminus of the lever arm  24 , whereby rotation of the arm  24  is transmitted to the shaft  26 . 
     With reference to FIGS. 4-5, an alternative embodiment of the invention is shown as sensor  62 . The sensor  62  includes a base  64  which supports the electrical connection socket  30  and includes the mounting bores  32 . The base  64  includes the cavity  36 , at the bottom side thereof, for receipt of the MR bridge circuitry  40 , and includes also the cup  42  at the top side thereof for receipt of and for mating with the keyway of a magnet carrying shaft  66 . The shaft  66  is a relatively short shaft, or stub shaft, and includes a top recess  68  with a key  70  for receipt of an external drive shaft (not shown). The bottom side of the shaft  66  carries the permanent magnet  44  and, upon emplacement of the shaft  66  within the cup  42 , positions the magnet  44  in alignment with the MR bridge circuitry  40 . A cover  72  is secured to the top of the base  64 . Upon assembly of the sensor  62 , the shaft  66  is placed within the cup  42 , a circular spring  74  is disposed around the shaft  66  and rests upon a flange  76  of the shaft  66 , and the cover  72  is placed above the shaft  66  allowing an end of the shaft  66  to protrude through an aperture  78  of the cover  72 . Tabs  80  and  82  extend in an axial direction from the spring  74  to engage, respectively, a slot  84  in the cover  72  and a notch  86  in the flange  76  to provide a limitation on the amount of rotation which can be applied to the shaft  66 . The spring  74  pushes against the interior of the cover  72  and against the flange  76  of the shaft  66  to insure contact of the magnet  44  with the floor of the cup  42 . 
     With reference to FIGS. 6 and 7, there is shown an alternative embodiment of the sensor, indicated as sensor  88 . The sensor  88  includes a base  90  which supports the socket  30  for making connection with an external circuit. The sensor  88  has the feature of being only partially assembled, thereby to allow an equipment manufacturer to provide for final assembly of the components of the sensor  88  upon the occasion of construction of a device, such as a carburetor, which incorporates the sensor  88  as part of the overall equipment assembly. In the ensuring description, reference will be made to the carburetor for convenience, it being understood that the description applies equally to a throttle body. Tabs  92 ,  93  and  94  extend from the base  90  to provide for further connection of electric lines, such as power lines, by way of example, to circuitry supported by the base  90 . In particular, such circuitry includes the MR circuitry  40  located within a cavity  96  located within a back side of the base  90 . 
     With reference to the example of the connection of the sensor to a carburetor, it is the practice in construction of certain forms of carburetors to employ an electric motor (not shown) for positioning a valve element within the carburetor. A mounting region  98  is provided on the base  90  and serves for holding the electric motor. Upon a fixing of the base  90  to a frame element of the carburetor, the electric motor is positioned for mechanical engagement with a drive shaft assembly of the carburetor valve. A magnet carrying shaft  100  is supported within a suitable housing  102 , indicated in phantom view, for positioning the shaft  100  in alignment with the MR circuitry  40 . Due to the tolerance of MR sensor circuitry to positioning of the magnet, as will be described herein, great precision in the locating of the magnet carrying shaft  100  relative to the MR circuitry  40  is not required such that the housing  102  can be connected also to a frame element of the carburetor. This avoids the necessity of having the housing  102  and the base  90  being connected together prior to final assembly of the carburetor and its ancillary equipment. 
     All embodiments of the shaft angle sensor share certain common structural features. The printed circuit board of the MR circuitry includes a pair of bridge circuits having sensing elements comprising permalloy magnetoresistive material for sensing the magnetic field. The bridge circuits are rotated 45° relative to each other to provide the equivalent of sine and cosine measures of the magnetic field produced by the magnet in the bottom of the magnet shaft. This magnet is a rectangular shaped magnet having a single north pole and a single south pole. The magnet of the magnet shaft contacts the bottom of the cylindrical cavity, or cup, of the base to slide thereon upon rotation of the shaft relative to the cavity. The base is fabricated of a material known as Valox, this being a plastic material having the chemical name polybutalene teraphthallate mixed with fibrous glass. This plastic is manufactured by General Electric and is moisture resistant and temperature stable, ideal for the automobile environment. The cover of the housing is fabricated of polybutylene teraphallate, the shaft comprises Teflon (a fluorinated hydrocarbon) impregnated with nylon and glass fiber, and there is an ultrasonic weld between the cover and the base, which weld permits rotation of the shaft relative to the base. A feature in the use of this plastic material is the fact that magnetic field lines of the magnet of the magnet shaft can go straight through the floor of the cylindrical cavity to permeate the magnetoresistive material in the sensing elements. 
     The advantage of using the angle sensor of the invention, as compared to other sensors of the prior art, may be understood in terms of the theory of operation of the angle sensor of the invention. This may be explained by comparing operation of a magnetoresistive sensor to a Hall-effect magnetic field sensor. There is an important distinction between the use of a Hall-effect magnetic field sensor and the magnetoresistive sensor. The Hall-effect sensor outputs a signal having an amplitude which is very sensitive to the amplitude of the magnetic field impinging upon the sensor. The signal outputted by the magnetoresistive sensor is dependent primarily upon the direction of the magnetic field and is only slightly affected by the magnitude of the impinging magnetic field. 
     As a result, location of the Hall-effect sensor relative to the source of the magnetic field is critical for obtaining a proper measure of the field strength. Any deviation from anticipated location, as might occur with normal mechanical tolerances or vibration in the use of the automotive vehicle would introduce an error in the measurement of the field strength. This error, in terms of a fraction of the entire field strength, is sufficient to introduce an excessively large error in the measurement of the angle of the shaft which holds the magnet. In contrast, use of the magnetoresistive material of the present invention provides essentially the same accuracy to measurement of shaft angle irrespectively of whether the magnetoresistive material be offset from a desired location, either in terms of displacement in a direction along the shaft axis, or in a direction transverse to the shaft axis. 
     The double bridge circuit of the magneto-resistive material is available commercially and is made by Philips. The total permitted rotation of the magnet shaft, by way of example is limited to 90° of total travel by the stops on the magnet shaft and the projections of the spring in the embodiments of the throttle position sensor. The spring also serves the function of providing pre-stressing of the magnet shaft in terms of rotational angle to insure that there is no lash during an imparting of rotation by an external driver shaft to the magnet shaft. With respect to the relative insensitivity of the magnetoresistive sensor to position relative to the source of a magnetic field, it is noted that a nominal thickness of the plastic floor of the cylindrical cavity does not interfere with the operation of the magnetoresistive field sensor. In fact, it may be possible to have a thickness as large as one-quarter inch, this thickness constituting the gap between the sensor and the magnet. With respect to measurement errors experienced by the MR sensor, the error in degrees is proportional to the square of an offset in the central axis of an MR array from the desired location of the axis, and inversely proportional to the square of the diameter or diagonal of the array. 
     With respect to FIGS. 8 and 9, it is noted that the preferred embodiment of the invention employs a pair of MR bridge circuits which serve as redundant sensors. FIG. 8 represents the output voltage of a single bridge circuit as a function of angle for the situations of three different temperature environments, namely room temperature (RT), −40 degrees Celsius, and +100 degrees Celsius. The three graphs for the three different temperatures virtually overlap due to the temperature stability of the sensor circuit. 
     FIG. 9 demonstrates the case for the two sensor bridge circuits, wherein one of the circuits is portrayed as channel a (cha) and the other of the two sensor bridge circuits is portrayed as channel b (chb). The outputs of the two bridge circuits are inverted to facilitate a showing of the relationship of voltage versus angle for such channel. Three different temperature ranges are also indicated, namely +25° C., +135° C., and −45° C., wherein the graphs virtually overlap for each channel due to the temperature stability of the MR bridge circuits.