Abstract:
A tilt sensor having a printed circuit board disposed in a direction normal to a reference plane for measuring a tilt angle with a pair of electrodes formed on a first surface of the printed circuit board. A cover is affixed to the printed circuit board and creates a cavity between the cover and the printed circuit board with a predetermined gap between the cover and the pair of electrodes, the cover acting as a common electrode to the pair of electrodes to create a pair of capacitors. An inert liquid is contained in the cavity, the surface level of the inert fluid varying according to the tilt angle of the reference plane. A signal processing circuit is formed on a second surface of the printed circuit board that produces an output signal corresponding to a difference in capacitance between the two capacitors as a tilt angle detection output.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to sensors, and more particularly, to an integrated tilt sensor.  
       BACKGROUND OF THE INVENTION  
       [0002]     Liquid levels, such as a carpenter level, utilize the interplay between a surface of a liquid and gravity to measure whether a surface is level. The principles of a liquid level can be applied to detecting angular motion. Angle detection devices provide an output voltage proportional to tilt angle where tilting the sensor off level will cause an unbalanced electrical signal. Initial commercial electric liquid tilt sensors were electrolytic tilt sensors. These devices provided an output voltage proportional to tilt angle where tilting the sensor caused an unbalanced resistance to a common electrode. Liquid angle sensors using similar principles but with differential capacitive (as opposed to resistive) sensing have also gained popularity in use.  
         [0003]     In current sensor designs, the signal conditioning electronics, and the sense elements, are typically located on two separate ceramic substrates. Due to this separation, interconnections between the signal conditioning electronics and the sense elements are cumbersome and unreliable. In addition, the requirements of the interconnections create numerous packaging constraints. The construction also suffers from poor temperature tracking between the electronics and the sense elements.  
         [0004]     Accordingly, there is a need to overcome the long felt problems noted above.  
       SUMMARY OF THE PREFERRED EMBODIMENTS  
       [0005]     The present invention provides a tilt sensor that is integrated to provide both the sensing element as well as the signal processing circuitry to generate high-level signals appropriate for use by downstream circuitry.  
         [0006]     It is a preferred object of the present invention to provide a tilt sensor that includes a circuit board having a first side and a second side, with a circuit located on the first side, and a pair of capacitors coupled to the circuit. The pair of capacitors includes a pair of electrodes on the second side, a metal ring surrounding the pair of electrodes on the second side, and, a cover sealed to the metal ring and spaced from the pair of electrodes to act as a common electrode for the pair of electrodes to form the pair of capacitors, the cover having a cavity, wherein the pair of electrodes and the metal ring are electrically coupled to the circuit.  
         [0007]     It is also a preferred object of the present invention to provide a method for assembling a tilt sensor using a circuit board. The method includes the steps of forming a pair of differential electrodes on a first side of the circuit board and forming an electrical circuit on a second side of the circuit board. The method also includes the steps of creating a plurality of through holes in the differential electrodes, forming a metal ring around the pair of differential electrodes and coupling the electrical circuit to the pair of differential electrodes and the metal ring. The method further includes the steps of attaching a cover to the metal ring to form a cavity, filling the cavity with a nonconductive liquid, and sealing the plurality of through holes.  
         [0008]     It is yet another preferred object of the present invention to provide a tilt sensor having a printed circuit board disposed in a direction normal to a reference plane for measuring a tilt angle, with a pair of electrodes formed on a first surface of the printed circuit board and a cover affixed to the printed circuit board and creating a cavity between the cover and the first surface of the printed circuit board, the cover being adjacent to the pair of electrodes with a predetermined gap between the cover and the pair of electrodes, the cover acting as a common electrode to the pair of electrodes to create a pair of capacitors. The tilt sensor also includes an inert liquid contained in the cavity, the surface level of the inert liquid varies according to the tilt angle of the reference plane, and a signal processing circuit formed on a second surface of the printed circuit board, the signal processing circuit section producing an output signal corresponding to a difference in capacitance between the two capacitors as a tilt angle detection output.  
         [0009]     Other objects, features and advantages of the present invention will become apparent to those skilled in the art from the following detailed description. It is to be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the present invention, are given by way of illustration and not limitation. Many changes and modifications within the scope of the present invention may be made without departing from the spirit thereof, and the invention includes all such modifications.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     The invention may be more readily understood by referring to the accompanying drawings in which:  
         [0011]      FIG. 1  is a perspective view of a tilt sensor configured in accordance to a preferred embodiment of the present invention;  
         [0012]      FIG. 2  is an exploded perspective view of the tilt sensor of  FIG. 1 .  
         [0013]      FIG. 3  is a front elevational view of the tilt sensor of  FIG. 1 .  
         [0014]      FIG. 4  is a cross-sectional view of the tilt sensor of  FIG. 1 , taken along line  4 - 4 .  
         [0015]      FIG. 5  is a back elevational view of a circuit board of the tilt sensor of  FIG. 1 .  
         [0016]      FIG. 6  is a front elevational view of the circuit board of the tilt sensor of  FIG. 1 .  
         [0017]      FIG. 7  is a bottom plan view of the tilt sensor of  FIG. 1 .  
         [0018]      FIG. 8  is a perspective view of a tilt sensor configured in accordance to another preferred embodiment of the present invention;  
         [0019]      FIG. 9  is an exploded perspective view of the tilt sensor of  FIG. 8 .  
         [0020]      FIG. 10  is a front elevational view of the tilt sensor of  FIG. 8 .  
         [0021]      FIG. 11  is a cross-sectional view of the tilt sensor of  FIG. 8 , taken along line  11 - 11 .  
         [0022]      FIG. 12  is a front elevational view of a circuit board of the tilt sensor of  FIG. 8 .  
         [0023]      FIG. 13  is a side elevational view of the circuit board of the tilt sensor of  FIG. 8 .  
         [0024]      FIG. 14  is a back elevational view of a circuit board of the tilt sensor of  FIG. 8 .  
         [0025]      FIG. 15  is a side elevational view of the tilt sensor of  FIG. 8 .  
         [0026]      FIG. 16  is a back elevational view of the tilt sensor of  FIG. 8 .  
         [0027]      FIG. 17  is a chart of the operational characteristics of the tilt sensor of  FIG. 1 . 
     
    
       [0028]     Like numerals refer to like parts throughout the several views of the drawings.  
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0029]     The present invention is an integrated tilt sensor that can measure the tilt angle of a structure in which the sensor is mounted with respect to a reference axis. The integrated tilt sensor generates a signal proportional to the angle of tilt of the sensor. The sensor can be used to form a part of a closed loop control system, or as an indicator. In one embodiment, the sensor incorporates a sense element and signal conditioning electronics on the same structure.  
         [0030]      FIG. 1  is a perspective view of a tilt sensor  100  configured in accordance to a preferred embodiment of the present invention, including a sensor cover  102  having a cable feed-through  122  from which a wiring harness  118  for connecting tilt sensor  100  extends, and a pair of mounting holes  120  for mounting to a location of interest, such as a firewall of a vehicle. It should be noted that tilt sensor  100  can be mounted on any plane whose tilt angle is to be detected, and the use of the sensor is not necessarily limited to any particular application unless otherwise stated herein. Wiring harness  118  includes a group of sensor circuit assembly connectors  124  to connect to a ribbon cable  104 . Wiring harness  118  also includes a group of sensor wires  126  for connecting tilt sensor  100  to an external circuit.  
         [0031]     As illustrated in  FIG. 2 ,  FIG. 4  and  FIG. 7 , a printed circuit board (PCB) assembly  114  contains a sense element  142  and electronic circuitry  140  necessary for generating a high-level signal proportional to a tilt angle measured by sense element  142 . PCB assembly  114  is a dual-sided PCB, with electronic circuitry  140  being integrated on one surface of PCB assembly  114  and covered by a feed-through plate  110 . Sense element  142  is disposed on an opposite side of PCB assembly  114 , as described herein.  
         [0032]     In one embodiment, tilt sensor  100  uses an inert, nonconductive liquid or fluid  148  such as silicone oil and measures the change of the capacitance based on the movement of the fluid to determine an angle of tilt of the sensor. As such, it is impervious to the magnitude of gravity and mainly measures the angle of the gravity vector. In other embodiments, any non-conductive fluid with a dielectric constant greater than or equal to air may be used. Inert fluid  148  is sealed in a cavity formed by a PCB cover  116  and PCB assembly  114 . The cavity is partially filled with inert fluid  148  and sealed. In one embodiment, approximately 50% of the volume is filled. This will allow the sensor to be tilted by 90 degrees in either direction. In another embodiment, a fill level of within +/−5% of 50% of the volume is tolerable for most applications. Further, in one embodiment, a solder mask or glass passivation is used to cover PCB assembly  114  to prevent soaking of inert fluid  148  into the circuit board. In another embodiment, PCB assembly  114  may be coated with another protective barrier, such as a non-conductive coating such as one made from Teflon(™) or silicon.  
         [0033]     Referring to  FIG. 5 , sense element  142  includes a pair of capacitative electrodes  144  and  146  etched out on the back side of PCB assembly  114 , each forming one of the electrodes in two separate capacitor sense elements, referred to herein as Cr and Cp, respectively. In one embodiment, PCB cover  116  is a conductive metal cover, and functions as a common electrode to pair of capacitative electrodes  144  and  146  such that the two electrodes form two capacitor sense elements. When tilt sensor  100  is tilted from the 0 angle, inert fluid  148  is redistributed, creating an imbalance of capacitance between Cp and Cr. This capacitance change is detected, and amplified to a high level signal by an applications specific integrated circuit (ASIC) contained in electronic circuitry  140 . The operating characteristics of sense element  142  is shown in  FIG. 17 , below. In one embodiment, metallization of PCB assembly  114  is used to form the sensing element (pair of capacitative electrodes  144  and  146 ), as well as provide a shield for the on-board electronics (electronic circuitry  140 ). For example, a ring  148  is located on the side of PCB assembly  114  that is enclosed by PCB cover  116 . In one embodiment, PCB cover  116  is sealed to PCB assembly  114  by attaching PCB cover  116  to ring  148  via metal reflow techniques such as soldering. Feed-through plate  110 , PCB cover  116 , and the metallization layer on PCB assembly  114  forms the shield and protects tilt sensor  100  from electromagnetic interference.  
         [0034]     Once PCB cover  116  is attached to PCB assembly  114 , the cavity formed by PCB cover  116  and PCB assembly  114  is partially filled with inert fluid  148  through a pair of through or fill holes  138 . Pair of fill holes  138  also facilitates the filling, of the fluid—one hole is used for filling, and the other hole is used as vent during the filling. Once cavity has been filled with inert fluid  148 , pair of fill holes  138  is sealed. Pair of fill holes  138  may be sealed with any suitable mechanical or chemical seal. In one embodiment, the seal is effected by a pair of solder plugs  252 . In other embodiments, glue (e.g., epoxy) or some other mechanical plug may be used to seal pair of fill holes  138 . In one embodiment, the pair of fill holes  138  are plated with a conductive interior surfaces to provide the electrical connection from pair of capacitative electrodes  144  and  146 , on one side of PCB assembly  114 , to electronic circuitry  140 , on the other side of PCB assembly  114 . Then, similar to how PCB cover  116  is attached to PCB assembly  114 , feed-through plate  110  is attached to PCB assembly  114  by being soldered to a ring  132 .  
         [0035]      FIG. 6  illustrates the components in electronic circuitry  140  as laid out on a circuit  156 . Electronic circuitry  140  includes a group of I/O pads  158  for connection to wiring harness  118 , including a C-EXT pad used as a digital download I/O to download data from an external computer to communicate with electronic circuitry  140 . A custom ASIC  160  is used to control the operation of sensor  100  as well as detect the tilt through an analysis of the detected differential capacitance values. A ground pad  162  is used as a ground for electronic circuitry  140 .  
         [0036]     The electrical coupling of wiring harness  118  to electronic circuitry  140  in accordance with one embodiment of the present invention is described as follows. First, a set of lead wires  112  is soldered to group of I/O pads  158  at a group of attachment points  154  to provide connectivity to electrical circuitry  140 . Lead wires  112  passes through a set of pass-through holes  150  in feed-through plate  110  and a set of openings  136  in a feed-through capacitor  108 . Feed-through plate  110  is attached to PCB board assembly at this point. Feed-through capacitor  108  is used to provide immunity to electromagnetic interference (EMI). Feed-through capacitor  108  is not necessary if the sensor is used in an environment with low interference. Lead wires  112  is then soldered to a ribbon cable  104  with a set of solder joints  130 . Group of sensor circuit assembly connectors  124  of wiring harness  118  is connected to ribbon cable  104  via a set of mounting receptors  128  on ribbon cable  104 . Ribbon cable  104  allows tilt sensor  100  to be more easily assembled as PCB board assembly  114  and its associated covers can be placed into the cavity of sensor cover  102 . A spacer  106  is used to maintain proper distance between the top surface of feed-through plate  110  and the interior of sensor cover  102 .  
         [0037]      FIG. 8  illustrates a tilt sensor  200  configured in accordance to a second embodiment of the present invention that unlike to tilt\sensor  100 , does not include a tilt sensor cover. Tilt sensor  200  includes a set of wires  218  for connecting tilt sensor  200  to an external circuit. Tilt sensor  200  also includes a pair of mounting holes  220  for mounting to a location of interest, such as a firewall of a vehicle. Set of wires  218  includes a group of sensor circuit assembly connectors  224  to connect to a set of lead wires  212 . Set of wires  218  also includes a group of sensor wires  226  for connecting tilt sensor  200  to an external circuit.  
         [0038]     As shown in  FIG. 9  and  FIG. 11 , a printed circuit board (PCB) assembly  214  contains a sense element  242  and electronic circuitry  240  necessary for generating a high-level signal proportional to a tilt angle measured by sense element  242 . PCB assembly  214  is a dual-sided PCB, with electronic circuitry  240  being integrated on one surface of PCB assembly  214  and covered by a feed-through plate  210 . Sense element  242  is disposed on an opposite side of PCB assembly  214 , as described herein.  
         [0039]     In one embodiment, tilt sensor  200  uses an inert, nonconductive liquid or fluid  248  similar to inert nonconductive fluid  148  and measures the change of the capacitance based on the movement of the fluid to determine an angle of tilt of the sensor. Inert fluid  248  is sealed in a cavity formed by a PCB cover  216  and PCB assembly  214 . The cavity is only partially filled with inert fluid  248  and sealed with re-flowing of metals. Specifically, in one embodiment, inert fluid  248  is filled into one hole of pair of fill holes  138 , and the other hole is used as a vent during the filling. Similar to PCB assembly  114 , a solder mask or glass passivation is used to cover PCB assembly  214  to prevent soaking of inert fluid  248  into the circuit board. Also, other protective barriers may be used to coat PCB assembly  214 . Metalization of PCB assembly  214  is used to form parts of the sensing elements as well as provide a shield for the on-board electronics (e.g., electronic circuitry  240 ).  
         [0040]     Referring to  FIG. 14 , similar to sense element  142  of tilt sensor  100 , sense element  242  of tilt sensor  200  includes a pair of capacitative electrodes  244  and  246  etched out on the back side of PCB assembly  214 , each forming one of the electrodes in two separate capacitor sense elements, referred to herein as Cr and Cp, respectively. Similarly, PCB cover  216  is a metal cover, and functions as a common electrode to pair of capacitative electrodes  244  and  246  such that the two electrodes form two capacitor sense elements. When tilt sensor  200  is tilted from the 0 angle, inert fluid  248  is redistributed, creating an imbalance of capacitance between Cp and Cr. This capacitance-change is detected, and amplified to a high level signal by a custom ASIC contained in electronic circuitry  240 . As the operating fundamental of sense element  242  is similar to sense element  142 , the operating characteristics of sense element  242  is shown in  FIG. 17 . As discussed, metallization of PCB assembly  214  is used to form the sensing element as well as provide a shield for the on-board circuitry.  
         [0041]     PCB cover  216  of tilt sensor  200  is sealed in the same fashion as PCB cover  116  of tilt sensor  100 . Specifically, PCB cover  216  is sealed to PCB assembly  214  by attaching PCB cover  216  to a ring  248  via metal reflow techniques such as soldering. Similar to the process as described for tilt sensor  100 , once PCB cover  216  is attached to PCB assembly  214 , the cavity formed by PCB cover  116  and PCB assembly  214  is partially filled with inert fluid  248  through a pair of fill (through) holes  238 . Once cavity has been filled with inert fluid  248 , the pair of fill holes  238  is sealed with a pair of plugs  252 . The pair of fill holes  238  may be sealed with any suitable mechanical or chemical seal. In one embodiment, the sealing is done with solder. In another embodiment, the pair of fill holes  238  are sealed using glue (e.g., epoxy). In yet another embodiment, a screw or other mechanical plug may be used. Similar to how PCB cover  116  is attached to PCB assembly  214 , feed-through plate  210  is attached to PCB assembly  214  by being soldered to a ring  232 .  
         [0042]     The electrical coupling of set of wires  218  to electronic circuitry  240  in accordance with one embodiment of the present invention is described as follows. First, a set of lead wires  212  is soldered to PCB assembly  214  to provide connectivity to electrical circuitry  240 . Lead wires  212  passes through a set of pass-through holes  250  in feed-through plate  210  and a set of openings  236  in a feed-through capacitor  208 . Feed-through plate  210  is then attached to PCB board assembly through a reflow process. Similar to feed-through capacitor  108 , feed-through capacitor  208  is used to protect from EMI. Lead wires  212  is then soldered to a group of sensor circuit assembly connectors  224  on wiring harness  218  with a set of solder joints  230 .  
         [0043]      FIG. 17  is a graph illustrating the operational characteristics of tilt sensor  100 . The vertical axis represents the output voltage while the horizontal axis represents the detected tilt angle. In one preferred embodiment, a voltage range of 0.5 volts to 4.5 volts is used, the end points of the voltage range representing the minimum tilt angle to a maximum tilt angle for which the sensor is configured to detect, respectively, with a mid-point of 2.5 volts. Thus, where the sensor is configured to detect a maximum tilt angle ranging from −90 degrees to +90 degrees, the voltage will range from 0.5 volts for a detected angle of −90 degrees and 4.5 volts for a detected angle of +90 degrees. In one preferred embodiment, the range of tolerance is within 5%. In another preferred embodiment, the range of tolerance is within 10%. In other embodiments, the output voltages can be reversed such that when tilt sensor  100  detects a maximum tilt angle, it will output 0.5 volts and it will output 4.5 volts when it detects a minimum tilt angle. Other ranges of voltages or currents may be used to represent detected tilt angles.  
         [0044]     The embodiments described above are exemplary embodiments of a tilt sensor configured in accordance with the present invention. Those skilled in the art may now make numerous uses of, and departures from, the above-described embodiments without departing from the inventive concepts disclosed herein. Accordingly, the present invention is to be defined solely by the scope of the following claims.