Abstract:
A speed sensor assembly is provided that isolates the internal components of the speed sensor from detrimental external environmental influences. Further provided is a new seal that seals between a housing and an overmold of a speed sensor and is capable of use in speed sensors of varying types and of all different sizes and shapes. Provided is a seal in which a bead of glue cured by ultraviolet light is applied around part of the exterior surface of a housing, after which an injected molded plastic overmold is applied over at least that part of the housing having the applied ultraviolet cured glue. In this way, once the housing is overmolded, the ultraviolet cured glue forms a gasket-like seal between the housing and the overmold preventing the penetration of water or other contaminants into the internals of the speed sensor. Also provided is a mounting fixture for use during a process of applying the seal to the housing.

Description:
CROSS-REFERENCE APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 60/092,915, filed Jul. 15, 1998. This application is also a divisional of co-pending U.S. Application Ser. No. 09/157,117, filed on Sep. 18, 1998. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to a sensor for detecting the angular velocity of a rotating element, and more particularly, to a speed sensor for use in a vehicle anti-lock braking system or transmission. Specifically, the invention relates to a new seal that protects the internal components of a sensor from exposure to external environmental elements, as well as a new method for applying the seal to a sensor. 
     BACKGROUND OF THE INVENTION 
     It is known in the art to mount a speed sensor in the transmission or in the bearing or wheel end assembly of a vehicle such as a truck or an automobile. The sensor measures the angular velocity of a rotating element within the transmission or bearing assembly. Typically, sensors of this type utilize variable reluctance, a form of electromagnetic sensing, generally known and understood by those skilled in the art. Such sensors are used in transmissions to assist in electronic shifting and in vehicle anti-lock braking (ABS) systems to detect wheel speed. 
     Generally, sensors of this type include a coil mounted on a bobbin which is inserted into a housing. A wheel speed sensor for use in an ABS application typically is connected or bolted into a hub assembly which is located by a brake, usually a front brake, of a vehicle. A tone wheel rotates with or around an axle, typically a front axle, and induces a voltage signal within the wheel speed sensor. The wheel sensor communicates with an on-board controller or computer of the vehicle when any of the vehicle&#39;s four wheels lock-up in a panic breaking situation. The sensor sends a voltage signal to the computer and the computer determines if a correct voltage signal has been sent. Based on the signal received, if the computer determines that any of the wheels of the vehicle has locked-up, the vehicle computer automatically takes over and controls the vehicle braking system, pumping each brake for a safer controlled, skid-free stop. 
     The environment in which sensors of this type are subjected can be extremely harsh and, as a result, the sensors must be capable of withstanding a significant amount of repeated temperature variations over a wide range of temperatures (thermal cycling), intense vibration and exposure to corrosive elements. One such wheel speed sensor is described in commonly assigned U.S. Pat. No. 5,629,618, which is hereby incorporated by reference. In the &#39;618 patent, a wheel speed sensor is described such that in order to seal the sensor from the environment, a bobbin is connected to a housing and the combination is overmolded with an injected molded plastic cover, i.e., an overmold, to provide a molded seal between the bobbin and the housing. 
     SUMMARY OF THE INVENTION 
     Speed sensors can be of many different shapes and sizes. The overall configuration of a speed sensor is largely dependant on the type and make of vehicle in which the speed sensor will be placed. 
     One problem with known sensors is that if the bobbin and the housing do not form a mechanical seal between one another, the potential exists for water or other outside influences to flow into the interior of the speed sensor and damage the coil. The &#39;618 patent describes a speed sensor having a housing that provides a sealed environment for a bobbin and a coil. 
     Another problem, similar to the problem described above, is that if an overmold does not provide a proper seal between a housing and a bobbin/terminal area, water or other contaminants can leak into the interior of the speed sensor and damage the coil or other internal components. Without a mechanical bond or seal between the overmold and the housing, there is not a hermetic seal, i.e., airtight and impervious to external influences. Further, the extreme temperatures and other external influences to which the sensor is subjected can cause additional degradation of the seal. One solution to the problems described above is to use an O-ring external seal between an overmold and a cooperating housing. Another solution to this problem is to use an externally applied ultraviolet cured glue to seal a housing to a surface of an overmold. However, both of these solutions require speed sensor assemblies that are relatively large in shape. For example, if an external O-ring is used, the overmold must be of sufficient size to allow for the placement of the O-ring between the overmold and the housing. The diameter of the overmold near the location where an O-ring is positioned must be about 5.5 mm greater than the diameter of the housing, or 2.75 mm per side, in order to provide sufficient space for an externally applied O-ring. In the case of externally applied ultraviolet cured glue, the surface upon which the glue is dispensed must be large enough to accommodate the applied glue. Similar to the requirements necessary for use of an external O-ring, the surfaces upon which externally applied glue is applied require a total of about 5.5 mm of space available to which glue may be applied. The diameter of an overmold according to the present invention is only about 3.5 mm larger than the diameter of the housing near that location where a conventional O-ring or externally applied ultraviolet glue would be used. Also the diameter of the overmold of the present invention is offset in its relationship to the housing by about 0.9 mm near the location where a conventional O-ring or externally applied ultraviolet glue might be considered. Thus, these prior sealing techniques are not capable of use in a speed sensor of the present invention. 
     There are other problems with these prior art techniques that prevent widespread use of such prior sealing means in all applications. The O-ring sealing technique requires an extremely detailed and complex injection molding fixture in order to provide an annular grove in the overmold for placement of an O-ring between the housing and the overmold to seal the connecting surfaces of these two pieces. Once molded, further assembly is required wherein the O-ring must be installed in the annular groove after the injection molding process. This additional assembly adds delay and cost to the overall manufacturing process. 
     When using the externally applied ultraviolet cured glue sealing technique, the part receiving the glue must have two edges large enough for the glue to rest upon. Not all speed sensors are designed to include these two edges or even large enough edges. As noted, the overall configuration of a speed sensor is largely dependent on the type and make of vehicle in which the speed sensor will be placed. Moreover, it has been observed that externally applied ultraviolet cured glue is susceptible to thermal breakdown as a result of the harsh environments to which the sensors are subjected. Another problem with externally applied ultraviolet cured glue is that the shape of such an applied glue is only controlled by the applicator. If not properly or evenly applied, the glue may cure leaving a gap between the parts to be sealed. Also, an uneven glue application may create a glue seal that is thin at various locations around the sealed part thereby leaving a weak seal point. In such situations, water or some other contaminant may find its way to the core of the speed sensor and short out the electrical function of the sensor. 
     Thus, what is needed is a speed sensor assembly that provides a sealed environment to isolate the internal components of the sensor from detrimental external forces. What is further needed is a new seal that seals between a housing and an overmold of a speed sensor, and yet is capable of use in a speed sensor of all different sizes and shapes. In other words, what is needed is a seal means that does not require a speed sensor to be of a particular size and shape such as those needed for conventional sealing techniques. What is also needed is a new method of applying an ultraviolet cured glue to a speed sensor that eliminates application problems of conventional glue sealing techniques. 
     The solution to providing a speed sensor assembly that seals and protects the internal parts of the speed sensor from harsh environmental conditions resides in providing a seal in which a bead of glue cured by ultraviolet light is applied around part of the exterior surface of a housing, after which an injected molded plastic overmold is applied over at least that part of the housing having the applied ultraviolet cured glue. In this way, once the housing is overmolded, the ultraviolet cured glue forms a gasket-like seal between the housing and the overmold preventing the penetration of water or other contaminants into the internals of the speed sensor. 
     The solution to evenly distributing a glue seal to a speed sensor housing resides in providing a fixture, made from a material in which ultraviolet cured glue will not adhere, such that the housing is vertically located upon the fixture wherein a groove is provided between the housing and the fixture so that the bead can be applied in proper shape and form around the outside surface of the housing. 
     A principal feature of the invention is the provision of a sensor having a housing, an overmold and an ultraviolet cured glue seal between the housing and the overmold such that a seal is created to prevent the introduction of water or other contaminants into the interior of the sensor. 
     Yet another feature of the invention is the provision of a seal means that is capable of use in all appropriate sensors and which is not dependent on the size of the sensor. 
     A further feature of the invention is the provision of a method to control the size and shape of an applied seal placed between a housing and an overmold of a sensor. 
     Yet a further feature of the invention is the provision of a fixture adapted to receive a housing of a sensor and which locates the housing in an appropriate position as a seal means is applied around part of the external surface of the housing such that the seal is evenly applied around the surface. 
     Other features and advantages of the invention will become apparent to those skilled in the art upon review of the following detailed description, claims, and drawings. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a speed sensor embodying the invention. 
     FIG. 2 is an exploded perspective view of a bobbin and a can of a speed sensor similar to that shown in FIG.  1 . 
     FIG. 3 is a side elevational view of the speed sensor of FIG.  2 . 
     FIG. 4 is a central cross-section of the speed sensor as shown in FIG.  3 . 
     FIG. 5 is a partial, enlarged cross-section showing in more detail the seal between the can and the overmold. 
     FIG. 6 is a perspective view with portions broken-away of a mounting fixture embodying the invention. The mounting fixture is employed in applying the ultraviolet cured glue to the can. 
     FIG. 7 is a partial, enlarged cross-section showing in greater detail the cooperation between the can and the mounting fixture during the creation of the seal of the present invention. 
    
    
     Before one embodiment of the invention is explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and to arrangements of components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and is capable of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Shown in FIG. 1 of the drawings is a speed sensor  10  embodying the invention. As is described below, a new seal  100  according to the present invention and shown, for example in FIGS. 3-5, is provided within the speed sensor  10  to isolate the internal components of the speed sensor from external influences. 
     The speed sensor  10  detects angular velocity of a rotating member such as a rotating bearing element of a bearing assembly in a vehicle wheel or a rotating element in the transmission of a vehicle such as an automobile or truck. While such sensors are commonly used as both transmission speed or wheel speed sensors, the sensor  10  is a wheel speed sensor for use in a vehicle anti-lock brake system. Sensor  10  is connected to a hub assembly  12  which is located by a brake (not shown), usually a front brake, of a vehicle. A tone wheel  14  rotates with a front axle  16  and induces an alternating voltage signal within the wheel speed sensor. Such wheel speed sensors and their operation are generally known to those skilled in the art. 
     As shown in FIGS. 2-4, the sensor includes a bobbin or bobbin assembly  18  that fits within a housing or can  20 . While the can  20  shown in the drawings is generally cylindrical (i.e., circular in cross section), it should be understood that housings or cans having different cross-sectional configurations (e.g., oval, rectangular, pentagonal, hexagonal, etc.) are appropriate. The combination of the can  20  and bobbin  18  is overmolded to form the finished product, as shown in FIG.  1 . 
     The bobbin  18  is generally a cylindrical injection molded plastic shell. Referring to FIGS. 2 and 4, the bobbin  18  has opposite ends  24  and  26 . The bobbin  18  includes a top portion  30 , a middle portion  32 , a bottom portion  34 , and an axis  28 . The top portion  30  has an annular groove  36 , and an O-ring  37  is typically placed within the annular groove  36  in order to assist in creating a seal between the can  20  and bobbin  18 . Top portion  30  also includes an annular recess  38 , and terminal  44  is positioned in recess  38 . The top portion  30  also includes a shoulder  40  between the annular groove  36  and annular recess  38 , and a shoulder  42  adjacent the annular recess  38 . Shoulder  42  defines one end of the top portion  30 . 
     Still referring to FIGS. 2 and 4, middle portion  32  includes an annular recess  46  located adjacent the shoulder  42  of the top portion  30  of the bobbin  18 . An induction coil  48  mounts within the annular recess  46 . The coil  48  is an electrical conductor such as copper wire that is wound around bobbin  18  and within the recess  46 . The electrical properties of the coil  48  may vary depending upon the number of turns of the electrical conductor and the thickness or gauge of the conductor. These variables are dictated by the particular application in which the sensor is being used. A piece of electrically insulating tape  50  is wound around the coil  48  to secure the coil  48  in place in the recess  46 . Middle portion  32  includes a shoulder  52 . As the top of recess  46  is bordered by shoulder  42 , the bottom of recess  46  is bordered by shoulder  52 . Middle portion  32  has an annular groove  54 , and an O-ring  56  is typically placed within the annular groove  54  to further assist is maintaining a seal between the can  20  and the bobbin  18 . 
     FIG. 4 best shows a pair of electrically conductive leads  58 , steel pole pieces  60  and permanent magnets  68  molded into bobbin  18 . Each lead  58  includes opposite ends  62  and  64 . One of the ends  62  defines the terminal  44  positioned in the annular recess  38 . The other ends  64  extend beyond the top portion  30  of the bobbin  18 . The ends  64  provide an electrical connection with a cable  66 , shown best in FIGS. 2-4, Cable  66  is shown in FIGS. 3 and 4 connected to ends  64  of leads  58 . Cable  66  has electrical wires  86  and  88  protruding out from an end  90  of cable  66 . The electrical wires  86  and  88  are surrounded by plastic sheaths  92  to protect the wires from damage. The ends  96  and  98  of wires  86  and  88 , respectively, are soldered to ends  64  of leads  58  to make an electrical connection. The cable  66  transmits the voltage signal produced in the sensor to the vehicle computer. 
     The coil  48  terminates with a pair of electrically conductive lead wires (not shown). As will be understood by those skilled in the art, the lead wires are normally skeined (i.e., braided for increased strength), and extend from annular recess  46  to the respective terminals  44  of the electrical leads  58 , in a manner known to those skilled in the art. As also generally understood, the lead wires are soldered to the respective terminals  44  in order to make a secure electrical connection between the lead wires of the coil  48  and the electrically conductive leads  58  molded into the bobbin  18 . 
     As best shown in FIGS. 2 and 4, the bobbin  18  is molded around the permanent magnets  68  and steel poles  60  such that the magnet  68  and poles  60  extend from the bottom portion  34  of bobbin  18  through the middle portion  32  of bobbin  18 . The magnets  68  alternate between the pole pieces  60  such that, looking from left to right in FIG. 4, there is a first located pole piece, followed by a magnet, followed by a pole piece, followed by a magnet, and finally terminating with another pole piece. In this way, the magnets  68  and poles  60  are located within the interior  70  of bobbin  18  and portions of the magnet  68  and poles  60  are located in a channel  72  of bobbin  18  beneath coil  48 . The magnets  68  engage the poles  60  to provide a permanent source of magnetic flux. The contact between the poles  60  and the magnets  68  provides a low reluctance flux path for the magnetic field generated by the magnets  68 . 
     The can  20  of a typical sensor such as that shown in FIG. 1 is best shown in FIG.  2 . The can  20  has a cylindrical sidewall  74  having opposite ends  76  and  78 . The cylindrical sidewall  74  has a generally cylindrical inner surface  80  and outer surface  81 . The top end  76  has a circular inwardly facing lip  82 . The bottom end  78  has a circular outwardly facing serrated edge  84 . As shown in FIG. 2, can  20  is adapted to mount over bobbin  18 . FIGS. 3 and 4 show can  20  installed over and around bobbin  18 . 
     With reference to FIG. 4, the lip  82  of the can  20  rests on shoulder  55  of the middle portion  32  of bobbin  18  when can  20  is mounted on bobbin  18 . As also shown in FIG. 4, O-rings  37  and  56  provide a seal between the can  20  and bobbin  18  so as to prevent the intrusion of water or other contaminants between the contacting surfaces of the can  20  and bobbin  18  so as to assist in preventing the coil  48 , terminals  44 , or other noted internal components, from becoming damaged or creating a short in the electrical circuit. 
     Still referring to FIG.  4  and as also shown in FIG. 5, a seal  100  incorporating the principles of the invention is shown between the can  20  and an overmold  22 . Prior to mounting the can  20  on the bobbin  18 , a seal  100  is applied to the bottom of the can  20  at the serrated edge  84  as shown, for example, in FIGS. 6 and 7. 
     The overall assembly of a typical sensor is now described. Bobbin  18  is formed of injected molded plastic. Poles  60 , magnets  68 , and leads  58  are positioned such that as bobbin  18  is created, poles  60 , magnets  68  and leads  58  are molded into bobbin  18 . Following the bobbin creation step, O-rings  37  and  56  are inserted within annular groves  36  and  54 , respectively, and coil  48  is wrapped around bobbin  18  within annual recess  46 . Tape  50  is applied around coil  48  and the lead wires of coil  48  are connected to terminals  44  as previously outlined. 
     In a separate assembly, usually simultaneous with the bobbin assembly step, a generally cylindrical, hollow can  20  is created. Can  20  may be made from various materials but most preferably is made of stainless steel. The bottom of can  20  has a serrated edge  84  and the top of can  20  has a lip  82 . Around part of the exterior surface of can  20 , a bead of acrylic anaerobic glue  100  is applied directly adjacent the serrated edge  84  of can  20 . The method of applying the glue  100  to the can  20  will be further described below. Once the glue  100  has been applied to the can  20  and the glue has cured, can  20  is slidably placed over bobbin assembly  18 . Can  20  is positioned over bobbin  18  such that lip  82  of can  20  abuts shoulder  55  of bobbin  18 . 
     In a further separate assembly, usually simultaneous with the other separate assembly steps, cable  66  is assembled. Once can  20  has been positioned over bobbin  18 , ends  96  and  98  of wires  86  and  88 , respectively, of cable  66  are soldered to respective ends  64  of leads  58 . 
     After cable  66  has been attached to bobbin  18 , plastic overmold  22  is injected and molded around parts of the can  20 , bobbin  18 , and cable  66 . Overmold  22  may be of any number of suitable injected molded plastics but glass filled Nylon, available from Loctite Corporation of Rocky Hill, Conn., has been found to be particularly suitable for speed sensors described herein. After the overmold  22  has been applied to the sensor, the sensor goes through further final assembly steps, known to those skilled in the art and not pertinent to the subject invention, until a final sensor assembly, such as sensor  10  shown in FIG. 1 is ready for use. 
     Until the invention of the subject application, without a proper seal between an overmold package and a can, water or other contaminants would leak into a bobbin/terminal area of a speed sensor causing the sensor to malfunction. The present invention provides a seal between an overmold and a can to prevent this type of failure. 
     As shown in FIGS. 3-7 and best shown in FIG. 5, a gasket-like seal of acrylic anaerobic ultraviolet cured glue  100  is placed between an overmold  22  and a can  20 . Placing the ultraviolet cured glue  100  between the plastic overmold  22  and can  20  will effectively stop the penetration of water or other contaminants into the terminal area of bobbin  18 . 
     With particular reference to FIGS. 6 and 7, a method of applying an ultraviolet cured glue to a can of a speed sensor is described. 
     A mounting fixture  110  having a base  126 , a top surface  111  defining a locating post  112  to locate can  20  during a glue seal application process. The fixture  110  may have one locating post  112  or any number of locating posts  112 , depending on the size of the fixture  110 . In alternative embodiments, not shown, the top surface may simply include recesses formed in the top surface for locating the cans. 
     The locating post  112  is generally circular in shape and has an exterior surface  114 . It should be noted, however, that locating post  112  may be of any shape depending on the shape of the interior  80  of can  20 . The locating post  112  extends above the top surface of the fixture  110 . Within the fixture  110  and positioned around the locating post  112 , is an annular groove  116 . The annual groove  116  is adapted to receive part of can  20 . As can  20  is placed over locating post  112 , outside edges  118  of serrated edge  84  practically abut wall  120  of fixture  110 , wall  120  being defined by groove  116 . Opening  122  is defined between the sidewall  74  of can  20  and wall  120 . Once can  20  is positioned over locating post  112  and opening  122  is created, glue  100  is deposited within opening  122 . 
     Glue  100  is generally an acrylic anaerobic glue curable by exposure to ultraviolet rays. However, Loctite  352 , available from Loctite Corporation of Rocky Hill, Conn., has been found to be particularly suited for the subject invention. Glue  100  is deposited in opening  122  in any number of ways, generally known to those skilled in the art. For example, a glue applicator (not shown) having a glue dispensing needle nozzle (not shown) may travel around opening  122  depositing glue within the opening  122 . 
     Another method of filling opening  122  with glue  100  may be accomplished by utilizing a stationary glue applicator whereby the fixture  110  rotates around the glue applicator such that the opening  122  is filled with glue  100  via a fixed glue applying nozzle. Because of the properties of glue  100 , when applied, the glue will center itself and evenly fill the opening  122 . Thus, a substantially symmetrical bead of glue is applied around the can. 
     Once glue  100  is positioned within opening  122 , the glue is subjected to an ultraviolet light source to cure the glue in a manner generally understood by those skilled in the art. The normal cure time for acrylic anaerobic glue exposed to ultraviolet light is generally 10-30 seconds. After the glue  100  has been subjected to an ultraviolet light source, the can  20  is removed from locating post  112 . Because the fixture  110  is made of a material that does not bond with ultraviolet cured glue  100 , and because the serrated edge  84  provides a ledge upon which the glue  100  is placed, when the can  20  is removed from the locating post  112 , a substantially uniform ring of glue  100  is affixed to a part of can  20  (see FIG.  3 ). The fixture  110  may be composed of many different materials, but it has been observed that plastic nylon, sometimes referred to as Delrin, or Teflon coated material, Delrin and Teflon are registered Trademarks of E.I. Du Pont De Nemours Company, works well with the subject invention. 
     As shown in FIGS. 3-7, and best shown in FIG. 4, it can be seen that the amount of a glue  100  applied to can  20  does not substantially protrude beyond the outside edges  118  of the serrated edge  84 . Generally, the glue seal  100  can extend slightly greater than or less than one millimeter beyond the outside edge  118  of the serrated edge  84 . This is important so that when the overmold  22  is applied around can  20 , there remains sufficient overmold material  124  to prevent a weak spot in the overmold where the can  20  and overmold  22  seal together. This will help ensure that water will not find its way to the terminal area of bobbin  18  and potentially cause damage to the sensor. It should be noted that the ultraviolet cured glue seal positioned between a can and an overmold is located farther from the end of an overmold than where conventionally applied O-rings or externally applied glue are applied. Various features of the invention are set forth in the following claims.