Abstract:
A speed target wheel includes an annular body portion and a plurality of gear teeth. The annular body portion consists of a plastic material and the plurality of gear teeth project from the annular body portion. Each of the plurality of gear teeth includes a base portion adjacent the annular body portion and a tip portion, where the base portion is integrally formed with the annular body portion and substantially comprises the plastic material, and where the tip portion includes a ferrous region integrally encompassed by the plastic material.

Description:
FIELD 
       [0001]    The present disclosure relates to target wheels for sensors as used in mechanical devices, and more particularly to plastic target wheels incorporating ferrous material. 
       BACKGROUND 
       [0002]    The statements in this section merely provide background information related to the present disclosure and may or may not constitute prior art. 
         [0003]    In mechanical devices such as power transmissions, engines or other machines, efficient control of the operation of the device often relies on a controller receiving data from sensors within the device. A transmission controller, for example, may require the rotational speed of an internal gear, shaft or other rotating member. Typically, this is accomplished using a speed target wheel rotationally secured to such a shaft or member and a sensor targeting the speed target wheel. The speed target wheel is generally ferrous and includes target teeth that are detected by the sensor as they rotate past the sensor. The sensor is positioned to target the radial surface of the teeth of the speed target wheel. Metal target wheels are often heavy, relatively expensive, and may require complex cutting or shaping procedures to create a suitable target. 
         [0004]    Accordingly, there is a need for a lightweight and relatively inexpensive speed target wheel. 
       SUMMARY 
       [0005]    A speed target wheel includes an annular body portion and a plurality of gear teeth. The annular body portion consists of a plastic material and the plurality of gear teeth project from the annular body portion. Each of the plurality of gear teeth includes a base portion adjacent the annular body portion and a tip portion, where the base portion is integrally formed with the annular body portion and substantially comprises the plastic material, and where the tip portion includes a ferrous region integrally encompassed by the plastic material. 
         [0006]    In another example of the present invention, the ferrous region of the plurality of gear teeth is disposed adjacent an outer radial surface of the tip portion of the plurality of gear teeth. 
         [0007]    In yet another example of the present invention, the ferrous region includes a ferrous member. 
         [0008]    In yet another example of the present invention, the plurality of gear teeth have a predefined width and the ferrous member has a length that is substantially similar to the predefined width. 
         [0009]    In yet another example of the present invention, the ferrous member is substantially cylindrically shaped. 
         [0010]    In yet another example of the present invention, the ferrous member is substantially the size of the tip portion of each of the plurality of gear teeth. 
         [0011]    In yet another example of the present invention, an outer surface of each ferrous member is partially exposed at an outer radial portion of the speed target wheel at the tip portion of each of the plurality of gear teeth. 
         [0012]    In yet another example of the present invention, the ferrous region includes a plurality of ferrous particles interspersed in the plastic material of the plurality of gear teeth. 
         [0013]    In yet another example of the present invention, at least a portion of the plurality of ferrous particles is exposed along an outer surface of the gear teeth. 
         [0014]    In yet another example of the present invention, the body portion includes a smooth inner surface for press fitting on a shaft. 
         [0015]    In yet another example of the present invention, the ferrous region is included in each of the gear teeth in an amount and is disposed at a location that is effective to generate a signal in a Hall effect sensor disposed for reading the movement of the ferrous region. 
         [0016]    In yet another example of the present invention a method of manufacturing a speed target wheel is provided. The method includes providing an injection molding form having a body form portion and a tooth form portion for creating an annular speed target wheel, inserting ferrous material into the tooth form portion of the injection molding form, injecting plastic into the injection molding form to create an annular plastic target wheel having gear teeth with embedded ferrous material, and removing the annular plastic target wheel from the injection molding form. 
         [0017]    In yet another example of the present invention the method further includes removing an outer portion of the gear teeth to expose the embedded ferrous material. 
         [0018]    In yet another example of the present invention the method further includes inserting the ferrous material into the tooth form portion of the injection molding form at an outer radial portion of the injection molding form at the tooth form portion. 
         [0019]    Further aspects and advantages of the present disclosure will become apparent by reference to the following description and appended drawings wherein like reference numbers refer to the same component, element or feature. 
     
    
     
       DRAWINGS 
         [0020]    The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way; 
           [0021]      FIG. 1  is a schematic view of an example of a speed sensor assembly according to principles of the present invention; 
           [0022]      FIG. 2  is a side view of an example of a speed target wheel according to principles of the present invention; 
           [0023]      FIG. 3  is an enlarged view of a portion of an example of a speed target wheel according to principles of the present invention; 
           [0024]      FIG. 4  is an enlarged view of a portion of an example of a speed target wheel according to principles of the present invention; and 
           [0025]      FIG. 5  is a flow chart of an example of a method of manufacturing a speed target wheel according to principles of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. 
         [0027]    Referring to the drawings, wherein like reference numbers refer to like components, in  FIG. 1  an example of a speed sensor assembly  10  for determining the speed of a shaft  12  is illustrated according to principles of the present invention. The speed sensor assembly  10  includes a speed target wheel  14  rotationally secured to the shaft  12  and a sensor  18  disposed to read the speed target wheel  14 . The shaft  12  is generally a shaft, carrier, hub, or other rotating component of a vehicle transmission whose rotational speed is to be measured. The transmission may be automatic, manual, dual clutch, or other types of transmissions without departing from the scope of the present invention. It should be appreciated that the sensor assembly  10  may be used in Power Take Off Units (PTU&#39;s), Center &amp; Rear Differentials, and Vehicle Wheel Speed Sensors without departing from the scope of the present invention. 
         [0028]    Referring now to  FIGS. 2 and 3 , and with continued reference to  FIG. 1 , the speed target wheel  14  is shown in several views in accordance with principles of the present invention. The speed target wheel  14  generally has a plastic molded body portion  20  and a plurality of gear teeth  24 . An inner surface  22  of the body portion  20  is press fit onto the shaft  12  to cause the shaft  12  and the target wheel  14  to rotate at substantially the same speed. In alternative embodiments the inner surface  22  of the target wheel  14  includes splines (not shown) to engage with splines (not shown) formed on the shaft  12 . Each of the gear teeth  24  includes a ferrous object or ferrous member  25  and a target surface  26  that faces radially outward from the target wheel  14 . The ferrous member defines a ferrous region of each of the gear teeth  24 . The ferrous member  25  is at least partially integrally encompassed or surrounded by a plastic that forms the remainder of the gear tooth  24 . In the example provided the ferrous member  25  is a cylinder having a diameter of several millimeters and a length of several millimeters and is integrally encompassed by placement in an injection molding form during molding of the target wheel  14 , as will be described below. It should be appreciated that other shapes and sizes of objects may be used without departing from the scope of the present invention. The target surface  26  may include a portion of the outer surface of the ferrous member  25 . For example, the plastic outer diameter portion of the target wheel  14  may be removed by abrasion, cutting, or other methods to expose a portion of the outer surface of the ferrous member  25  after the target wheel  14  has been molded. In alternative embodiments the outer portion of each gear tooth  24  is retained, and the ferrous member  25  is large enough to alter the field of the sensor  18  through a layer of plastic. 
         [0029]    The sensor  18  generally detects changes in a magnetic field to determine when each gear tooth  24  passes by the sensor  18 . The sensor  18  is generally fixed or securely attached to a transmission housing (not shown). The sensor  18  actively detects the ferrous member  25  in the target to determine speed. The sensor  18  generates a signal induced by the rotation of the speed target wheel  14  and the ferrous member  25  embedded in each gear tooth  24 . The sensor  18  may be of various types and the magnetic field may be generated in various ways without departing from the scope of the present invention. In accordance with one example of the present invention, the sensor  18  is a Hall effect type sensor. 
         [0030]    The sensor outputs a signal through a wire  30  that indicates the speed of the target wheel  14 . The sensor  18  is positioned within close proximity of the target surface  26  of the target wheel  14 . The distance between the speed target wheel  14  and the sensor  18  creates an air gap. The continuous rotation of the speed target wheel  14 —and thus the passage of gear teeth  24  past the sensor  18 —causes the sensor  18  to detect a continuous change of the magnetic field. The change in the magnetic field in the sensor  18  induces a signal in the wire  30  that is indicative of the speed of the speed target wheel  14  and the shaft  12 . 
         [0031]    A controller  32  is in electronic communication with the wire  30  to detect the signal current from the sensor  18 . The controller  32  is preferably an electronic device having a preprogrammed digital computer or processor, control logic, memory used to store data, and at least one I/O peripheral. However, other types of controllers may be employed without departing from the scope of the present invention. 
         [0032]    Referring now to  FIG. 4 , a portion of an alternative embodiment of a target wheel  14 ′ is shown in accordance with principles of the present invention. The target wheel  14 ′ is substantially similar to the target wheel  14  and may be used with the shaft  12  and the sensor  18  as previously described. The target wheel  14 ′ includes a plastic body portion  20 ′ and a plurality of gear teeth  24 ′. The gear teeth  24 ′ each include a ferrous region or region of ferrous particles  25 ′. The ferrous particles  25 ′ are included near an outer diameter of the target wheel  14 ′ and are at least partially exposed on an outer surface  26 ′ of the gear teeth  24 ′. For example, the region of ferrous particles  25 ′ may be added to a tooth region of a mold for injection molding plastic. The outer surface  26 ′ is preferably ground to expose the ferrous particles  25 ′ after molding the target wheel  14 ′. The region of ferrous particles  25 ′ alter a magnetic field when passing the sensor  18 , which in turn generates a signal indicative of the speed of the target wheel  14 ′ and the shaft  12 . 
         [0033]    Referring now to  FIG. 5 , a method of manufacturing a target wheel is indicated by reference number  100 . At step  102  an injection molding form is provided. The injection molding form has a body form portion and a tooth form portion to create a speed target wheel. In step  104 , ferrous material is inserted into the tooth form portion of the injection molding form. For example, the ferrous member  25  or the ferrous particles  25 ′ may be inserted into the tooth form portion near an outer diameter of the injection molding form. In one example the ferrous material is positioned in the tooth form portion by magnetizing a portion of the mold. 
         [0034]    In step  106 , plastic is injected into the injection molding form to create a plastic target wheel. The injection creates a plastic target wheel that has gear teeth with embedded ferrous material. For example, the plastic target wheel may be one of the plastic target wheels  14 ,  14 ′ having the gear teeth  24 ,  24 ′, respectively. The plastic target wheel is then removed from the injection molding form in step  108 . 
         [0035]    In step  110  an outer portion of each gear tooth is removed to expose the ferrous material and create a suitable target for a speed sensor. For example, the outer portion of the gear teeth may be ground through turning or grinding to expose the ferrous material. In alternative embodiments the ferrous material is included in an amount that will suitably alter a sensor&#39;s magnetic field through an outer plastic layer of the gear teeth, and step  110  may be omitted. It should be appreciated that other considerations such as packaging space may influence whether step  110  is to be carried out. 
         [0036]    Accordingly, the present invention provides a simple yet effective target wheel that has low complexity, is low cost, and is lightweight. 
         [0037]    The description of the disclosure is merely exemplary in nature and variations that do not depart from the gist of the disclosure are intended to be within the scope of the disclosure. Such variations are not to be regarded as a departure from the spirit and scope of the disclosure.