Abstract:
A fuel system sensor has an acetal substrate that does not corrode in fuel, and the substrate bears conductors that are connected to terminals partially embedded in the substrate. Various methods for forming the conductor paths on the acetal are disclosed. The terminals can be formed with thermal relief structure.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to sensors that can be used in corrosive liquids such as gasoline or ethanol-based fuels. 
       BACKGROUND OF THE INVENTION 
       [0002]    Sensors such as fluid level sensors that are used in vehicle fuel systems typically include electrically conductive elements for generating and/or sensing electric or magnetic fields. Electrically conductive elements in fuel systems may also be used as electromagnetic field shields. 
         [0003]    As understood herein, the electrically conductive elements typically are supported on a substrate, often under fairly precise tolerance constraints regarding spacing between elements, etc. Furthermore, the substrate typically is exposed to the same corrosive environment such as engine fuel as are the conductors. The present invention is directed to both of the above two sometimes competing design considerations, i.e., providing conductor substrates that can withstand prolonged exposure to corrosive liquids such as engine fuel while facilitating relatively precise disposition of the conductors on the substrates. 
       SUMMARY OF THE INVENTION 
       [0004]    A vehicle system holds a corrosive liquid such as gasoline, and a sensor is in fluid communication with the vehicle system. The sensor has a polymer substrate such as acetal that bears one or more electrical conductors that can be used as sensing elements and/or that can send signals to, e.g., an engine control module. 
         [0005]    In some applications, the sensing elements are protected by their own polymer coating, which case the sensor need not be additionally coated with a polymer. Also, electrical terminals may be embedded in the substrate and can be placed in electrical contact with the conductor, with the terminals being formed with thermal relief structure. The substrate may be formed with chamfered holes through which the conductors engage respective terminals. 
         [0006]    In another aspect, fuel system sensor includes a substrate made of acetal and having at least one hole formed therein. One or more electrical conductors are provided, and respective terminals are embedded in the substrate to communicate with the conductor through the hole. 
         [0007]    In still another aspect, a method for making a corrosive liquid system sensor includes providing an acetal substrate, and, using electroless plating, forming at least one electrical conductor on the substrate. 
         [0008]    The details of the present invention, both as to its structure and operation, can best be understood in reference to the accompanying drawings, in which like reference numerals refer to like parts, and in which: 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a block diagram of a non-limiting system in accordance with present principles; 
           [0010]      FIG. 2  is a perspective view of a sensor made in accordance with present principles; 
           [0011]      FIG. 3  is a perspective view of a terminal with thermal relief structure; 
           [0012]      FIG. 4  is a detail perspective view showing a chamfered terminal hole; 
           [0013]      FIG. 5  is a flow chart of a first method of making the sensor using a photo resist layer; 
           [0014]      FIG. 6  is a flow chart of a second method of making the sensor using two molding steps with a carrier plastic; and 
           [0015]      FIG. 7  is a flow chart of a third method of making the sensor using laser etching. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0016]    Referring initially to  FIG. 1 , a system is shown, generally designated  10 , which includes a vehicle fuel tank  12  that is cooperatively engaged with various fuel system components  14  such as injection systems, fuel pumps, etc. A sensor  16  in accordance with present principles is engaged with the fuel tank  12  and/or fuel system components  14  and can be immersed in fuel to sense, e.g., its percentage of ethanol concentration, level in the tank  12 , etc. The sensor  16  can send signals to a computer such as an engine control module (ECM)  18  that in turn can use the signals to drive an output  20  such as a fuel tank level gage or fuel tank low level lamp or fuel pressure gage or other appropriate output device. While “sensor” is the term used herein to describe the structure shown in  FIGS. 2-4  and made in accordance with  FIGS. 5-7 , it is to be understood that the term “sensor” also includes structures used for shielding. It is to be further understood that while the non-limiting system  10  assumes a fuel (gasoline or diesel) application, the sensor  16  may be used in other corrosive liquids such as alcohol and oil. For present purposes water is not considered “corrosive”. 
         [0017]      FIG. 2  shows that the sensor  16  includes a polymer substrate  22  preferably made of acetal such as the acetal marketed under the trade name “Delrin.” The substrate  22  may be parallelepiped-shaped as shown. One or more electrical conductors may be disposed on the substrate  22 . 
         [0018]    Each conductor  24 ,  26  is electrically connected to one or more respective metal terminals  28  that, in the embodiment shown in  FIG. 2 , are partially embedded in the substrate  22  and partially emerge from an end thereof. More particularly, an electrical path is established from a conductor  24 / 26  to a respective terminal  28  through a respective hole  30  that is formed in the substrate  22  in accordance with principles below. 
         [0019]    Details of non-limiting terminals  28  and holes  30  are shown in  FIGS. 3 and 4 . In preferred non-limiting embodiments each terminal  28  is formed with a terminal opening  32  with which an electrical lead or connector can be engaged by, e.g., soldering, and each terminal  28  is also formed with thermal relief structure. In the particular embodiment shown in  FIG. 3 , the thermal relief structure is a narrower segment  34  that is formed between two wider segments  36 ,  38  of the otherwise parallelepiped-shaped terminal  28 . The narrower segment  34  resists heat transfer from the solder in and around the terminal opening and the wider segment  36  to the opposed wider segment  38 , which is embedded in the substrate  22 . As understood herein, providing terminals with thermal relief structure permits soldering without unduly damaging the acetal substrate  22 . 
         [0020]      FIG. 4  shows that to facilitate smooth transition of plating between the conductors  24 ,  26  and respective terminals  28 , the walls  40  of the holes  30  may be chamfered. In the specific embodiment shown the walls  40  slope inwardly as shown from the top surface of the substrate  22 . Each hole  30  may have a square periphery as shown or a circular periphery or other suitably shaped periphery. 
         [0021]    A first method for making the sensor  16  is shown in  FIG. 5 . It is to be understood that in all embodiments the substrate  22  initially is made by overmolding the polymer onto the terminals  28 . The holes  30  can be formed concurrently with the desired conductor patterns during etching or, more preferably, during the overmolding of the terminals. An extra inner shielding element made of thin metallic stock may also be incorporated into the substrate by overmolding. The shielding element terminates in or is soldered to a terminal. Or, if inner shielding is required it may be established by using two acetal substrates, plating a face of one substrate and then thermally bonding the two substrates together so that the shield is between the substrates. 
         [0022]    Commencing at block  42 , a photoresist layer is deposited on the substrate  22  by, e.g., gluing the resist layer to the substrate  22 . Moving to block  44 , the substrate with photoresist layer are exposed to light, e.g., ultraviolet light, in the desired pattern of the conductors to be subsequently plated. Thus, the photoresist layer is a mask that establishes the negative of the desired shape of the electrodes. 
         [0023]    Proceeding to block  46 , the substrate with remaining photoresist is etched to form the desired patterns of the conductors  24 ,  26 , which establish anchor points for the conductors to be plated. The etchant may be the substance marketed under the trade name “Delrin Etch.” The etch can be accomplished by submerging the substrate in a solution of the etchant. The mask is removed in accordance with photoresist removal principles known in the art. At block  48  the conductors  24 ,  26  are deposited onto the patterns formed in the substrate  22 , preferably using electroless plating techniques known in the art. If desired, the conductors  24 ,  26  may be protected by plating them with tin or aluminum through an electroplating process, and when aluminum is used it may be anodized. 
         [0024]    A second method for making the sensor  16  is shown in  FIG. 6 . Commencing at block  50 , a polymer such as acetal is molded into the substrate  22  with the desired pattern of the conductors  24 ,  26  formed in the mold. Next, at block  52 , the substrate is overmolded with a carrier plastic, except for the parts of the substrate that form the desired pattern of the conductors. These portions are not overmolded. Preferably, the carrier plastic is not sensitive to the etchant, so that only the portions of the substrate  22  that form the desired patterns of the conductors are etched at block  56 . The conductors  24 ,  26  are then plated onto the substrate at block  58  in accordance with principles above. If desired, the carrier plastic may be removed. 
         [0025]    In an alternate embodiment of  FIG. 6 , the initially molded part may be acrylonitrile-butadiene-styrene (ABS) and acetal may be used for the overmolding. The etching is done so that only the ABS, not the acetal, is etched. Then the ABS is plated. 
         [0026]    In another alternate embodiment of  FIG. 6 , the acetal prior to molding at block  50  may be mixed with a metallization agent such as Palladium, and the carrier plastic used for overmolding at block  52  is non-metallized acetal, which promotes plating at block  56  after etching at block  54 . 
         [0027]      FIG. 7  shows that alternatively to etching using a solvent, the substrate  22  may be directly etched at block  58  using a laser to form the desired conductor patterns in the substrate. The etched substrate is then plated at block  60  in accordance with principles above. 
         [0028]    While the particular SENSOR WITH POLYMER SUBSTRATE FOR USE IN CORROSIVE LIQUIDS is herein shown and described in detail, it is to be understood that the subject matter which is encompassed by the present invention is limited only by the claims.