Abstract:
A method for attaching a sensing device to a substrate at a pre-determined distance from the substrate is provided. The method preferably includes providing a compliant adhesive and depositing a first layer of the compliant adhesive onto the substrate, curing the first layer of the compliant adhesive, depositing a second layer of the compliant adhesive directly onto the first layer of the compliant adhesive, inserting the sensing device into the second layer of the compliant adhesive and curing the second layer of the compliant adhesive.

Description:
TECHNICAL FIELD OF THE INVENTION  
         [0001]    The present invention generally relates to a method of attaching a sensing device to a substrate and, more specifically, a method of attaching a sensing device to a substrate to allow relative movement between the sensing device and the substrate after the sensing device is attached.  
         BACKGROUND  
         [0002]    Low pressure sensors such as fuel tank pressure transducers are typically mounted to the substrate to which they are attached in a way that will allow slight relative movement between the sensor and the substrate. This is due to the fact that sensors of this type are very sensitive to stresses and strains which can affect the accuracy of the sensor, or cause damage to the sensor. Additionally, in order for the sensor to operate correctly, the sensor must be mounted at a specific distance from the substrate. Conventional methods of mounting such sensors to a substrate include using complex equipment to maintain the sensor at a distance while the adhesive being used to mount the sensor cures, or by using time and pressure placement solutions which have low capability. One specific conventional method involves placing glass beads within a thermal adhesive to maintain a standoff distance between the sensor and the substrate during curing of the adhesive, however, there is a need in the industry for an improved method of attaching a sensing device, such as a fuel tank pressure transducer, to a substrate, such as the interior of an automobile gas tank, at a specific distance from the substrate.  
         SUMMARY OF THE INVENTION  
         [0003]    In accordance with an aspect of the present invention, the method for attaching a sensing device to a substrate at a pre-determined distance from the substrate preferably includes providing a compliant adhesive, depositing a first layer of the compliant adhesive onto the substrate, curing the first layer of the compliant adhesive, depositing a second layer of the compliant adhesive directly onto the first layer of the compliant adhesive, inserting the sensing device into the second layer of the compliant adhesive, and curing the second layer of the compliant adhesive. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0004]    [0004]FIG. 1 is a top view of a sensing device of the present invention;  
         [0005]    [0005]FIG. 2 is a side view of the sensing device of the present invention; and  
         [0006]    [0006]FIG. 3 is a detailed view of a portion of the sensing device shown in FIG. 2. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0007]    The following description of the preferred embodiment of the invention is not intended to limit the scope of the invention to this preferred embodiment, but rather to enable any person skilled in the art to make and use the invention.  
         [0008]    Referring to FIGS. 1 and 2, a sensing device of the present invention is shown generally at  10 . A sensor  20  is attached at a pre-determined distance  12  from a substrate  14  with an compliant adhesive  16 . A first layer  18  of the compliant adhesive  16  is attached to the substrate  14 . Referring to FIG. 2, the first layer  18  of the compliant adhesive  16  has a thickness approximately equal to the pre-determined distance  12 . The sensor  20  is attached to a top surface  22  of the first layer  18  of the compliant adhesive  16  by a second layer  24  of the compliant adhesive  16 . Referring to FIG. 3, the second layer  24  of the compliant adhesive  16  is preferably thinner than the first layer  18  of compliant adhesive  16  and is positioned between the sensor  20  and the first layer  18  of compliant adhesive  16  to secure the sensor  20  to the top surface  22  of the first layer  18  of the compliant adhesive  16 .  
         [0009]    The compliant adhesive  16  can be any generally used adhesive that cures to a compliant, but solid composition. It is preferable for the adhesive  16  to be compliant due to the sensitivities of the sensors that may be mounted to the substrate  14 . Pressure sensors and strain sensors are very sensitive to stresses and must be able to flex with respect to the substrate  14  or there is a risk of damaging the sensors  20 .  
         [0010]    In the preferred embodiment, the sensor  20  is a fuel tank pressure transducer which is mounted within the gas tank of an automobile. The fuel tank pressure transducer is mounted directly over a vapor port  26  to gage the pressure of the gasoline vapors in the tank. Due to the environment, the preferred compliant adhesive  16  is a silicon based, gasoline resistant, adhesive. This type of compliant adhesive  16  is common in the industry, and any suitable adhesive based upon the environment and the type of sensor  20  being used is appropriate.  
         [0011]    Before the sensor  20  is applied to the substrate  14 , an appropriate adhesive  16  is selected. As described above, compliant adhesives  16  are common in the industry, and an appropriate adhesive must be selected based upon the environment into which the sensor  20  is being mounted, and the type of sensor  20 . The first layer  18  of the compliant adhesive  16  is deposited onto the substrate  14  and cured in place. In the preferred embodiment, the first layer  18  of the compliant adhesive  16  is deposited onto the substrate  14  through a screen printing process. A screen printing process is preferred because the screen printing process will allow the compliant adhesive  16  to be deposited onto the substrate  14  in a controlled manner. The screen printing process will also make it possible to control the thickness at which the first layer  18  of compliant adhesive  16  is applied, and will also allow the first layer  18  of compliant adhesive  16  to be deposited onto the substrate  14  in a specific pattern that is suitable for the particular sensor  20  and application. It is to be understood, however, that any method of depositing the compliant adhesive  16  that will allow the thickness of the layer being deposited to be controlled and will allow the adhesive  16  to be deposited in the desired pattern would be suitable.  
         [0012]    Referring again to FIG. 1, in the preferred embodiment, the first layer  18  of compliant adhesive  16  is placed onto the substrate  14  in a pattern consisting of eight oblong segments  28 . Referring to FIG. 2, each of the eight oblong segments  28  has a specific height, which is approximately equal to the pre-determined distance  12  at which the sensor  20  must be spaced from the substrate  14 .  
         [0013]    The adhesive  16  is preferably applied in a viscous state. After the first layer  18  of complaint adhesive  16  is deposited to the substrate  14 , the compliant adhesive  16  is then cured. Once the adhesive  16  is cured, the first layer  18  of complaint adhesive  16  becomes solid. Although solid, the adhesive  16  remain compliant, in that the adhesive  16  will accommodate slight relative movement between the sensor  20  and the substrate  14  to prevent stresses from transferring from the substrate  14  to the sensor  20  and causing damage to the sensor  20 .  
         [0014]    It is to be understood, that the first layer  18  of the compliant adhesive  16  does not need to be completely cured before placing the sensor  20  on top of the first layer  18 . When the compliant adhesive  16  is completely cured, the first layer  18  is solid throughout. The first layer  18  of compliant adhesive  16  need be cured only to a point where enough of the first layer  18  has solidified to support the pressure exerted upon the first layer  18  when the sensor  20  is attached thereto. It is not necessary that the first layer  18  is completely cured, only that the first layer  18  has cured to a point that will adequately support the sensor  20  and maintain the pre-determined distance  12 .  
         [0015]    After the first layer  18  of compliant adhesive  16  has been substantially cured, a second layer  24  of compliant adhesive  16  is applied to the top surface  22  of the cured first layer  18  of the compliant adhesive  16 . Preferably, the second layer  24  is thinner than the first layer  18 , as the purpose for the second layer  18  is only to provide an attachment between the sensor  20  and the first layer  18 . The second layer  24  is preferably deposited to the top surface  22  of the first layer  18  by the same screen printing process used to deposit the first layer  18 . Once again, the screen printing process allows the thickness of the second layer  24  to be controlled, and allows the second layer  24  to be deposited in a pattern that matches exactly the pattern of the first layer  18 . In the preferred embodiment, the second layer  24  is deposited with the same screen printing process as was used to deposit the first layer  18 . This is preferred because using the same process reduces the number of different components needed to manufacture the device  10 , and also insures that the pattern of the second layer  24  will match the pattern of the first layer  18 . However, just as with the first layer  18 , any method of depositing the compliant adhesive  16  that will allow the thickness of the second layer  24  to be controlled and will allow the adhesive  16  to be deposited in the desired pattern would be suitable.  
         [0016]    Referring again to FIG. 3, immediately after the second layer  24  has been deposited to the top surface  22  of the first layer  18 , and before the second layer  24  is cured, the sensor  20  is inserted into the second layer  24  of compliant adhesive  16 . The sensor  20  is pressed into the second layer  24  until the sensor  20  rests essentially upon the top surface  22  of the first layer  18 . As described above, the first layer  18  is deposited at a thickness approximately equal to a pre-determined distance  12  at which the sensor  20  must remain from the substrate  14 .  
         [0017]    When the sensor  20  is pressed into the second layer  24  of compliant adhesive  16 , only a very thin portion  30  of the second layer  24  of compliant adhesive  16  remains between the sensor  20  and the top surface  22  of the first layer  18  of compliant adhesive  16 . This very thin portion  30  of the second layer  24  that remains between the first layer  18  and the sensor  20  attaches the sensor  20  to the first layer  18 . Although the sensor  20  does not rest directly on the top surface  22  of the first layer  18  of compliant adhesive  16 , the thickness of the very thin portion  30  of the second layer  24  of compliant adhesive  16  is minimal to the pre-determined distance  12  that the sensor  20  must be spaced from the substrate  14 . In other words, the thickness of the second layer  24  of compliant adhesive  16  is preferably not sufficient to affect the performance of the sensor  20 . When the sensor  20  is attached onto the top surface  22  of the first layer  18  of compliant adhesive  16 , the sensor  20  will automatically be spaced the appropriate distance  12  from the substrate  14 . This allows the sensor  20  to be placed to the substrate  14  and insures that the appropriate distance between the substrate  14  and the sensor  20  is maintained without the use of any complicated gauging or measuring devices to insure accuracy. The screen print process insures accurate thickness of the first layer  18  of the compliant adhesive  16 , and once the first layer  18  cures, the distance  12  between the top surface  22  of the first layer  18  and the substrate  14  is established, making the placement of the sensor  20  much less complicated and more accurate.  
         [0018]    After the sensor  20  has been inserted into the second layer  24  of compliant adhesive  16  and rests upon the top surface  22  of the first layer  18  of compliant adhesive  16 , the second layer  24  is then cured. Once the second layer  24  is cured, the sensor  20  is held firmly to the top surface  22  of the first layer  18 , at the appropriate distance  12 . Both the first layer  18  and the second layer  24  will allow slight relative movement between the sensor  20  and the substrate  14  to prevent any stresses from being transferred to the sensor  20  from the substrate  14 .  
         [0019]    The foregoing discussion discloses and describes one preferred embodiment of the invention. One skilled in the art will readily recognize from such discussion, and from the accompanying drawings and claims, that changes and modifications can be made to the invention without departing from the true spirit and fair scope of the invention as defined in the following claims. The invention has been described in an illustrative manner, and it is to be understood that the terminology which has been used is intended to be in the nature of words of description rather than of limitation.