Abstract:
Rotatable target wheels mount plastic ring magnets with increased magnetic flux for driving either digital or variable reluctance, magnetic wheel rotation sensors. The ring magnets are adhesively secured and optionally captured on a ring support member to provide dependable operation in vehicle wheel speed sensing and other applications.

Description:
TECHNICAL FIELD  
         [0001]    This invention relates to magnetic target wheels or encoders for use in providing a source of magnetic flux for sensing applications.  
         BACKGROUND OF THE INVENTION  
         [0002]    It is known in the art to provide a magnetic target wheel or encoder in the form of a rotatable disc having a molded rubber magnetic coating around its outer rim. The magnetic flux strength which these encoders are capable of generating is enough for digital applications, but is not sufficient for variable reluctance (VR) applications. Both economic and application advantages could be obtained if an improved target wheel was available with sufficient magnetic flux strength for use in both digital and VR applications.  
         SUMMARY OF THE INVENTION  
         [0003]    The present invention provides embodiments of magnetic target wheels carrying magnets with increased magnetic flux strength suitable for use in both digital and VR applications and providing improved utility and a reduced manufacturing cost. In a generic form, the target wheel includes a support member having a support ring including a connector for mounting the wheel on a rotatable member. The support ring carries a molded plastic ring magnet generating a magnetic flux adequate for both digital and variable reluctance sensing applications. The encoder may include a radial flange and optionally an annular rim extending axially from the flange. In one embodiment, a plastic ring magnet is adhesively fixed directly to the radial flange and positioned coaxially around the axis of rotation of the wheel. In another embodiment the plastic ring magnet is fixed to an interior surface of the annular rim and is preferably secured thereto by a suitable adhesive. The supporting portions including the support ring, connector and radial flange, if provided, may be formed as a unitary stamping formed with a magnetic steel. Optionally for suitable applications the support ring may be made of a nonmagnetic metal or as a molded plastic member.  
           [0004]    These and other features and advantages of the invention will be more fully understood from the following description of certain specific embodiments of the invention taken together with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]    In the drawings:  
         [0006]    [0006]FIG. 1 is a plan view of a first embodiment of magnetic target wheel according to the invention;  
         [0007]    [0007]FIG. 2 is a cross sectional view from line  2 - 2  of FIG. 1;  
         [0008]    [0008]FIG. 3 is an enlarged view of the portion in circle  3  of FIG. 2;  
         [0009]    [0009]FIG. 4 is a pictorial view of the target wheel of FIGS.  1 - 3 ;  
         [0010]    [0010]FIG. 5 is a plan view of an alternative embodiment of target wheel according to the invention;  
         [0011]    [0011]FIG. 6 is a cross sectional view from the line  6 - 6  of FIG. 5;  
         [0012]    [0012]FIG. 7 is an enlarged view of the portion in circle  7  of FIG. 6; and  
         [0013]    [0013]FIG. 8 is a pictorial view of the target wheel of FIGS.  5 - 7 . 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0014]    Referring first to FIGS.  1 - 4  of the drawings in detail, numeral  10  generally indicates a first embodiment of target wheel in accordance with the invention. Wheel  10  includes a stamped support member  12  which includes a hub  14 , a radial flange  16  carried by the hub and an annular rim  18  extending axially from the outer periphery of the annular rim. The support member forms a wheel that is rotatable about a central axis  20 .  
         [0015]    Hub  14  acts as a connector for connecting the target wheel to a rotatable member such as a vehicle wheel. The connector could take other forms as needed. For example, the radial flange could have fastener openings through an inner portion for attaching the target wheel to a rotating body.  
         [0016]    The radial flange  16  connects the hub  14  radially with the annular rim  18 . A molded plastic ring magnet  22  is carried on an inner surface  24  of the annular rim  18  and is secured in place by a suitable adhesive  26  such as high temperature glue or epoxy. The plastic ring magnet is molded and carries magnetic particles having an overall flux strength adequate to provide sufficient flux for variable reluctance (VR) as well as digital applications. The support member  12  is preferably stamped from sheet magnetic steel, but may be formed from any suitable nonmagnetic metal or, alternatively, a suitable plastic material. FIG. 4 is a pictorial view of the completed target wheel  10 .  
         [0017]    Referring now to FIGS.  5 - 8 , numeral  30  generally indicates an alternative embodiment of magnetic target wheel in accordance with the invention. Target wheel  30  includes a support member  32  including a hub  34  and a radial flange  36  supported by the hub. The hub and flange forming the support member are preferably made as a stamping from a magnetic steel sheet. If desired, a nonmagnetic metal or a suitable plastic material could be used to form the support member. The support member is concentric about a central axis  38  which forms a rotational axis for the wheel. The hub  34  acts a connector for attaching the support member to a rotatable member such as a vehicle wheel. Other suitable forms of connectors could be used if desired as in the first embodiment.  
         [0018]    The radial flange  36  directly carries a plastic ring magnet  40  which is attached by a suitable adhesive  42 , such as high temperature glue or epoxy, to a radial face  44  of the radial flange  36 . The ring magnet  40  is positioned concentrically on the radial face so that it is rotatable about the axis  38  in a concentric and balanced fashion that avoids the application of any shear forces to the adhesive  42  holding the ring magnet  40  to the radial flange  36 .  
         [0019]    This second embodiment of FIGS.  5 - 8  differs from the first embodiment by the manner of mounting of the ring magnet on the support member. In the first embodiment, the magnet  22  is contained within the annular rim  18  so that centrifugal forces of the magnet are directly applied against the rim  18  and the adhesive  26  is subject only to compressive forces. In the second embodiment of FIGS.  5 - 8 , the magnet  40  is mounted to a radial face  44  so that the adhesive could be subject to shear forces if the plastic ring magnet is caused to expand due to centrifugal force or temperature changes. However, use of a proper adhesive will maintain the magnet in place under all conditions. Thus the selection of a particular embodiment for use in a sensing application is primarily determined by the location of the sensor which reacts to the magnetic flux in order to determine wheel speed or rotational speed of another member to which the target wheel  10  or  30  may be attached.  
         [0020]    Both embodiments illustrated, as well as other variations referred to, provide easily manufactured target wheels capable of universal use for both digital and VR applications.  
         [0021]    While the invention has been described by reference to certain preferred embodiments, it should be understood that numerous changes could be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the disclosed embodiments, but that it have the full scope permitted by the language of the following claims.