Abstract:
A method of making an encoder for use in a speed sensor includes providing a metal tube. A material containing ferrite is then extruded over the metal tube to produce an encoder tube. The encoder tube is cut to a given length to form an encoder. The encoder is then magnetized within a magnetic field.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to speed sensors and more particularly to a speed sensor encoder wheel and a method of making same.  
       BACKGROUND OF THE INVENTION  
       [0002]     Encoder wheels are used in speed sensor devices to determine the rotational speed of a component. For example, in motor vehicle applications, an encoder wheel often forms part of an anti-lock braking system for determining the rotational speed of the wheels. Other applications include coupling to engine crankshafts and within transmissions.  
         [0003]     The encoder wheel typically includes an outer surface having magnetized ferrite therein. The ferrite is magnetized to form alternating north and south poles around the circumference of the encoder wheel. As the encoder wheel rotates, a magnetic sensor measures the change in the magnetic field of the alternating poles. From this alternating magnetic field, the rotational speed of the encoder wheel may then be calculated, and in turn the rotational speed of the component to which the encoder wheel is coupled is known. Alternatively, hall effect sensors, which do not use magnets, can also be utilized for detecting the rotational speed.  
         [0004]     A conventional encoder wheel is produced from a tube of stamped steel with a ferrite loaded rubber compression molded thereon. However, in order to retain the ferrite loaded rubber, the stamped steel must have flanges or features to engage the rubber. This in turn enlarges the size of the encoder wheel. Moreover, such features increase the cost of manufacturing the encoder wheel and increase the costs of packaging. Finally, each encoder wheel is made separately and individually. This can lead to non-uniformity between encoders. Accordingly, it is an object of the present invention to provide an encoder wheel and method of making that addresses these limitations.  
       SUMMARY OF THE INVENTION  
       [0005]     A method of making an encoder for use in a speed sensor includes providing a metal tube. A material containing ferrite is then extruded over the metal tube to produce an encoder tube. The encoder tube is cut to a given length to form an encoder wheel. The encoder wheel is then magnetized within a magnetic field.  
         [0006]     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0008]      FIG. 1  is a schematic view of an exemplary motor vehicle having an encoder constructed according to the principles of the present invention;  
         [0009]      FIG. 1   a  is an enlarged schematic view of a portion of an encoder wheel illustrating the magnetization on the encoder wheel;  
         [0010]      FIG. 2  is a front perspective view illustrating the cutting of the encoder wheels of the present invention from an encoder tube; and  
         [0011]      FIG. 3  is a schematic view of an exemplary extrusion apparatus for constructing the encoder tube of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0012]     With reference to  FIG. 1  of the drawings, an encoder wheel  10  constructed according to the principles of the present invention is shown in operative association with an exemplary motor vehicle  12 . In the particular example provided, the encoder wheel  10  forms part of an anti-lock braking system  14 . The anti-lock braking system  14  further includes a magnetic field sensor  16  in communication with a vehicle control module  18 . The encoder wheel  10  is preferably coupled to an axle shaft  20  that rotatingly drives a pair of wheels  22 . With brief reference to  FIG. 1A , the encoder wheel  10  is magnetized such that it includes a plurality of alternating north and south magnetic poles around its circumference. As the wheel  22  and axle shaft  20  rotate, the encoder wheel  10  in turn rotates. The magnetic sensor  16  senses the magnetic field emitted from the encoder wheel  10  as the magnetic field alternates between positive and negative magnetic fields as the encoder wheel  10  rotates. The vehicle control module  18  may then use this information to determine the rotational speed and/or rotational position of the encoder wheel  10 , and therefore of the axle shaft  20  and the wheel  22 . It should be appreciated that the encoder wheel  10  may be part of any magnetic speed sensing system, for example, as part of an engine speed sensing system where the encoder wheel  10  is mounted on a crankshaft or camshaft (not shown) so that the rotational speed and/or rotational position of the crankshaft or camshaft can be accurately determined for use in an engine control system.  
         [0013]     Turning to  FIG. 2 , a plurality of encoder wheels  10  are illustrated, each being identical to another. The encoder wheels  10  are cut from a continuous length of encoder tubing  28 , the manufacturing of which will be described in greater detail below. The encoder  10  includes an inner tube  24  and an outer tube  26  extruded overtop the inner tube  24 . The inner tube  24  is preferably steel formed by an extrusion method. However, various other metals may be employed for the inner tube  24  and various other methods of making, such as stamping, may be employed.  
         [0014]     As noted above, the outer tube  26  is an extruded material that encases the inner tube  24 . The outer tube  26  is preferably rubber loaded with ferrite, although any extrudable material that can be loaded with ferrite may be employed, such as, for example, a plastic or thermoplastic material. In the preferred embodiment, the encoder wheel  10  includes an inner diameter of 44.45 mm and an outer diameter of 40.45 mm with a width of 20 mm. However, it should be appreciated that the encoder  10  may be of virtually any size without departing from the scope of the invention.  
         [0015]     With reference to  FIG. 3 , the continuous length of encoder tubing  28  is formed using an extrusion process on the inner tube  24 . For example, the inner tube  24  is pre-fabricated and fed into a mold  30 . The mold  30  encases the inner tube  24  and defines a mold cavity  32 . The mold cavity  32  in turn will define the outer surface of the outer tube  26  ( FIG. 2 ). The mold  30  is coupled to an extrusion apparatus  34 .  
         [0016]     The extrusion apparatus  34  may take many forms without departing from the scope of the invention. Moreover, more than one extrusion apparatus  34  may be used with the mold  30  (e.g., an extrusion apparatus  34  at 120 degree intervals around the mold  30 ). In the particular example provided, a mixture of plastic granules and ferrite  36  are fed into a hopper  38 . The hopper  38  feeds the plastic granules and ferrite  36  into a cylinder  40 . The cylinder  40  is heated at an elevated temperature by heating elements  42  surrounding the cylinder  40 . A screw mechanism  44  is located within the cylinder  40  and is rotatingly driven by a motor (not shown).  
         [0017]     As the plastic granules  36  enter the cylinder  40 , they are heated by the heating elements  42  and become semi-molten/molten plastic, generally indicated by reference numeral  46 . The screw mechanism  44  as it rotates forces the mixed semi-molten plastic and ferrite  46  into the mold  30 , thereby filling the mold cavity  32  surrounding the inner tube  24 . The semi-molten/molten plastic and ferrite  46  then cools and forms the outer tube  26  ( FIG. 2 ). The mold  30  may then be opened and the continuous encoder tube  28  removed therefrom. Alternatively, the inner tube  24  may be fed through the mold  30  in a continuous process without opening the mold  30 . Once the encoder tube  28  has been formed, any number of encoders  10  may be cut from the encoder tube  28  at any desired width. Magnetization of the ferrite within the outer tube  26  to form alternating poles (see  FIG. 1A ) may be accomplished either before or after cutting of the encoder tube  28  to form the encoder wheels  10 . The ferrite within the outer tube  26  is magnetized to preferably include 32 pole pairs all of equal widths, though any number of pole pairs may be used and may include asymmetrical widths.  
         [0018]     By extruding the outer tube  26  onto the inner tube  24 , the encoder wheel  10  finishes with a low profile (i.e., no flanges or bumps) that allows the encoder  10  to fit within small areas, including driveshafts. Moreover, extrusion of the outer tube  26  onto the inner tube  24  allows a large number of encoders  10  to be cut from a single encoder tube  28 , thereby saving on manufacturing costs and increasing uniformity among encoders  10 .  
         [0019]     The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.