Abstract:
A method for manufacturing an electrolytic tilt sensor comprises: a) forming sensing electrodes on a generally planar surface of a dielectric substrate; b) forming a reference electrode on the surface; c) mounting a housing to the substrate so that the sensing electrodes and the reference electrode are contiguous to a volume defined between the housing and the substrate; d) forming a fluid tight seal between the housing and the substrate; e) injecting an electrolytic fluid into the volume; f) sealing the electrolytic fluid in the volume; and g) forming an electrical circuit on the substrate for generating an output signal representing the angle of the dielectric substrate with respect to a gravitational field, wherein the electrical circuit includes an oscillator mounted on the surface.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of application Ser. No. 09/878,504 filed 11 Jun. 2001, now U.S. Pat. No. 6,625,896. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention generally relates to tilt sensors, and more particularly to a an electrolytic tilt sensor that is manufactured using standard printed circuit board fabrication techniques. 
     Traditional tilt sensors generally use some mechanism that is influenced by the local gravitational field in order to determine the level of tilt from some horizontal reference position. One type of sensor uses a weighted, rotating pendulum that is attached to a potentiometer or variable capacitor. Accuracy of this type of sensor is limited by the design and cost of the shaft and bearing about which the pendulum swings. For many applications, a pendulum type tilt sensor is too large, heavy, and expensive. A second type of tilt sensor measures the gravitational force on a conventional or micro-machined weighted beam. Although these types of tilt sensors can be small and relatively inexpensive, the electrical output varies as the sine/cosine of the tilt angle whereupon the relation between tilt angle and electrical output varies considerably throughout the range of the sensor. A third type of tilt sensor uses variations of the traditional “bubble level.” If the fluid in this type of sensor is electrically conductive, electrodes placed in contact with the fluid result in an electrical response related to the tilt angle. The changing electrical impedance between the contacts can be converted to a useable output with proper signal conditioning electronics. 
     Historically, most of these types of sensors have been hand assembled using precious metal electrode wires, glass housings, and lead wires that have been sealed and attached by hand. 
     Thus, these types of sensors require skilled labor assembly, which tends to be costly. More recently, glass housed tilt sensors have been made by machine, which lowers their fabrication costs, but are only available in limited configurations. Even with lowered manufacturing costs, tilt sensors that have glass housings are very fragile and still expensive to mount in an instrument housing. Some manufacturers sell sensors mounted in machined metal housings. However, sensors mounted in metal housings are relatively expensive, and too large for many applications. 
     A need therefore exists for a tilt sensor that is inexpensive to manufacture, small, and easily mounted. A further need exists for a tilt sensor that can be massed produced with very repeatable performance characteristics. 
     SUMMARY OF THE INVENTION 
     A method for manufacturing an electrolytic tilt sensor comprises: a) forming sensing electrodes on a generally planar surface of a dielectric substrate; b) forming a reference electrode on the surface; c) mounting a housing to the substrate so that the sensing electrodes and the reference electrode are contiguous to a volume defined between the housing and the substrate; d) forming a fluid tight seal between the housing and the substrate; e) injecting an electrolytic fluid into the volume; f) sealing the electrolytic fluid in the volume; and g) forming an electrical circuit on the substrate for generating an output signal representing the angle of the dielectric substrate with respect to a gravitational field, wherein the electrical circuit includes an oscillator mounted on the surface. 
     In another aspect of the invention, an electrolytic tilt sensor, comprises: a) a dielectric substrate having a first planar surface; b) a first sensing electrode affixed to the dielectric substrate and having a second planar surface entirely in contact with the first planar surface; c) a second sensing electrode affixed to the dielectric substrate and having a third planar surface entirely in contact with the first planar surface; d) a reference electrode affixed to the dielectric substrate and having a fourth planar surface entirely in contact with the first planar surface; e) a housing mounted to the dielectric substrate so that the first and second sensing electrodes and the reference electrode are contiguous to a volume defined between the housing and the dielectric substrate; f) a fluid tight seal formed between the housing and the dielectric substrate; g) an electrolytic fluid contained within the volume; and h) electrical circuitry mounted on the dielectric substrate and electrically coupled to the first and second sensing electrodes, and to the reference electrode for generating an electrical signal representing an angular displacement of the electrolytic fluid with respect to the dielectric substrate, wherein the electrical circuitry includes an oscillator mounted on the first planar surface. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates an electrolytic tilt sensor that is manufactured in accordance with methods embodying various features of the present invention. 
     FIG. 2 illustrates a view showing one side of the printed circuit board of the electrolytic tilt sensor of FIG.  1 . 
     FIG. 3 is a cross-sectional view of the electrolytic tilt sensor of FIG. 1 taken along reference line  3 — 3 . 
     FIG. 4 illustrates a view showing a second side of the printed circuit board of the electrolytic tilt sensor of FIG.  1 . 
     FIG. 5 is an example of electrical circuitry of the electrolytic tilt sensor shown in FIG.  1 . 
     FIG. 6 is a view of another embodiment of an electrolytic tilt sensor manufactured in accordance with the methods of the present invention. 
     FIG. 7 is a cross-sectional view of the electrolytic tilt sensor of FIG. 6 taken along reference line  7 — 7 . 
     FIG. 8 is an example of another embodiment of an electrolytic tilt sensor for detecting extremes of angular displacement that embodies various features of the present invention. 
     FIG. 9 shows an aperture formed in the housing of FIG.  1 . 
     Throughout the several view, like elements are referenced using like references. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIGS. 1,  2 , and  3 , collectively, there is shown an electrolytic tilt sensor  10  that includes an electrically insulating substrate  14 , such as a printed circuit board, an electrolytic sensing element  11  fabricated on the substrate  14 , and an electrical circuit  13  that is also fabricated on the substrate and electrically interconnected to the electrolytic sensing element  11 . The electrical circuit  13  of sensor  10  generates an output signal  15  that is functionally related to the angular displacement of the sensing element  11 , and hence, the substrate  14 , with respect to a local gravitational field  23 . In general, gravitational field  23  is oriented in a plane parallel to the surface of substrate  14  on which the electrolytic sensing element  11  is mounted. 
     The sensing element  11  includes a first sensing electrode  16 , a second sensing electrode  18 , a reference electrode  20 , and a housing  22 . The electrodes  16 ,  18 , and  20  are formed on the substrate  14  using standard printed circuit fabrication techniques, and are enclosed within housing  22  which is mounted to substrate  14 . An electrolytic liquid  24  partially fills the volume  26  defined between the substrate  14  and interior of the housing  22 , whereupon liquid  24  provides a varying degree of electrical continuity between the electrodes  16 ,  18 ,  20 , depending on the tilt angle of sensor  10 , including the substrate  14  and sensing electrodes  16  and  18 , and reference electrode  18 , with respect to a gravitational field  23 . A bead of sealant  28 , such as epoxy, is formed adjacent to the housing  22  and substrate  14  to provide a fluid tight seal so that the electrolytic fluid  24  is retained, or secured within volume  26 . The housing  22  may be shaped such as a cup, hemisphere, or any other shape for defining a volume between the housing  22  and the substrate  14 . By way of example, housing  22  may be made of glass, metal, plastic, nylon, quartz, or any other non-conductive material that provides a relatively rigid, fluid tight structure that may be mounted to and sealed with respect to substrate  14  so as to define a volume  26  for retaining electrolytic fluid  24  within the volume. 
     As shown in FIG. 2, first sensing electrodes  16  and  18  may be semicircular in shape and concentrically opposed about reference electrode  20 . However, it is to be understood that electrodes  16  and  18  may have other shapes, as for example, where a particular function of voltage or impedance versus tilt angle is required to suit the needs of a specific application. Such function may be linear, non-linear, asymptotic, or some combination of any or more functional relations. 
     Referring to FIG. 3, apertures  30 ,  32 , and  34  may be formed in substrate  14  to facilitate electrical continuity between the generally parallel and opposed surfaces of  36  and  38  of insulating substrate  14 . Reference electrodes  16  and  18  extend from side  36  of substrate  14  through side  38  of substrate  14 , where sides  34  and  36  are generally parallel and opposite each other. Sensing electrodes  16  and  18 , and reference electrode  20  extend through apertures  30 ,  32 , and  34 , respectively. Also referring to FIG. 4, sensing electrode  16  is electrically connected to first electrically conductive sensing trace  17  formed on surface  38  of insulating substrate  14 . Sensing electrode  18  extends through aperture  32  and is electrically connected to second electrically conductive sensing trace  19  that is formed on surface  38 . Reference electrode  20  extends through aperture  34  and is electrically connected to electrically conductive reference trace  21 . As a result of standard plating processes, vias  42  are typically formed in each of sensing electrodes  18  and  20 , and in reference electrode  20  that extend from surface  38  to surface  36 , and through traces  17 ,  18 , and  21  that are formed on surface  38  of substrate  14  as shown in FIG.  4 . Traces  17 ,  18 , and  21  maybe formed using standard printed circuit board fabrication techniques. 
     Electrolytic fluid  24  is an electrically conductive fluid such as alcohol, ionized water, or other electrically conductive fluids. A predetermined volume of fluid  24  may be injected into volume  26 , as for example, by use of a syringe, not shown, through one of vias  42  to partially, but not completely fill volume  26 . After fluid  24  is inserted into volume  26 , dollops of sealant  40 , such as epoxy, may be placed over vias  42  to secure the electrolytic fluid  24  within volume  26 . 
     Referring to FIG. 5, sensing electrodes  16  and  18  are electrically coupled to electrical circuit  13  which includes an oscillator  60  that provides AC electrical power to reference electrode  20 . Electrically conductive fluid  24  (not shown in FIG. 5) provides electrical continuity between reference electrode  20  and sensing electrodes  16  and  18  to an extent determined by the angular displacement of electrolytic sensing element  11  with respect to a local gravitational field  23 . Changes in angular displacement of electrolytic sensing element  11  cause the relative impedances detected by signal lines  57  and  59  to vary. Sampling pulses are provided through the Q output of circuit  61  to circuits  62  and  64  via signal line  65 . Circuits  62  and  64  collectively provide a phase demodulation circuit that is connected via signal lines  72  and  74 , respectively, to the positive input of operational amplifier  66 . Operational amplifier  66  transforms input signals  72  and  74  into an amplified DC output signal  15  that represents the angular displacement of sensing element  11  with respect to gravitational field  23 . The example of electrical circuit  13  depicted in FIG. 8 is presented by way of example only. It is to be understood that the scope of the invention includes the manufacture of an electrolytic tilt sensor having other suitable electrical circuitry formed along with electrolytic sensing element  11  on a single substrate such as substrate  14 . 
     FIGS. 6 and 7 show another example of an electrolytic tilt sensor  50 . Sensor  50  includes an electrically insulating substrate  14 , such as a printed circuit board, a first sensing electrode  16 , a second sensing electrode  18 , a reference electrode  20 , and a housing  22 . The electrodes  16 ,  18 , and  20  preferably are formed onto surface  36  of substrate  14  using standard printed circuit fabrication techniques, and are partially enclosed within housing  22  which is mounted to substrate  14 . An electrolytic liquid  24  partially fills the volume  26  defined between the substrate  14  and interior of the housing  22 . A bead of sealant  28 , such as epoxy, is formed adjacent to the housing  22  and substrate  14  to provide a fluid tight seal so that the electrolytic fluid  24  is retained within volume  26 . 
     Still referring to FIGS. 6 and 7, first sensing electrodes  16  and  18  may be semicircular in shape and concentrically opposed about reference electrode  20 . Housing  22  encloses electrodes  16 ,  18 , and  20  except where electrodes  16 ,  18 , and  20  extend beyond the external perimeter  52  of housing  22 . It is to be noted that electrolytic fluid  24  provides electrical continuity between the surfaces of electrodes  16 ,  18 , and  20  within volume  26 . The degree of electrical continuity depends on the tilt angle of sensor  50  with respect to gravitational field  23 . The regions of electrodes  16 ,  18 , and  20  that extend beyond perimeter  52  of housing  22  maybe coated with an electrically insulating coating, or layer  44  to facilitate the routing of lead traces on substrate  14 , and to reduce the number of steps required to seal the vias  42 . 
     FIG. 8 shows an example of another embodiment of an electrolytic tilt sensor embodying various features of the invention. In FIG. 8, tilt sensor  45  is shown to include sensing electrodes  50  and  52  that are formed on surface  47  of insulating substrate  49 . Sensing electrodes are located along an arcuate segment α—α and separated by angular displacement  20  with respect to a point C representing the center of arcuate segment α—α having radius R. 
     Reference electrode  54  is positioned on surface  47  and centered at an angle θ along arcuate segment α—α midway between sensing electrodes  50  and  52 . A housing  56  is mounted to surface  47  of substrate  49  to define a volume  53  between the housing and the surface  47  over electrodes  50 ,  52 , and  54  for holding electrolytic fluid (not shown) within the volume. A characteristic of electrolytic tilt sensor  45  is that it may be used to indicate only tilt angles of sensor  45  that attain a predetermined angle of displacement, as for example, ±θ, but no tilt angles less than /θ/. 
     When sensor  10  is in a neutral, or horizontal position, electrolytic fluid  24  typically covers half of the reference electrode  20  and equal lengths of the sensing electrodes  16  and  18 . 
     As the electrolytic sensing element  11  of sensor  10  is angularly displaced, the relative degree to which fluid  24  covers sensing electrodes  16  and  18  changes. In other words, more of electrode  18  or  20  will be covered by fluid  24  than will the other sensing electrode, while fluid  24  always covers reference electrode  20 . Thus, the electrical impedances between the reference electrode  20  and each of the sensing electrodes  16  and  18  changes as the angular displacement of sensing element  11  changes. Electrical circuit  13  generates an output signal  15  that is functionally related to the angular displacement of the sensor  10 , i.e. a positive angular rotation or negative angular rotation with respect to the gravitational field  23 . 
     Obviously, many modifications and variations of the present invention are possible in light of the above teachings. For example, electrolytic fluid  24  may inserted through an aperture  80  formed through housing  22 , as shown in FIG. 9, or some location in substrate  14  other than as described above. 
     Although the electrolytic tilt sensor  10  has been described as having two sensing electrodes, the scope of the invention includes the use of any number of sensing electrodes required to suit the needs of a particular application. For example, sensing electrode  10  may include only one sensing electrode if the sensor is only required to sense tilt in one direction. Three or more sensing electrodes may be used in sensor  10  for application where detection of incremental changes in tilt is desired. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.