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BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a proximity sensor. In particular, the invention relates to a non-contact strip sensor for identifying the presence of proximate objects from variations in an electric field. 
   2. Description of the Related Art 
   Proximity sensors are widely used in the automotive industry to automate the control of power accessories. For instance, proximity sensors are often used in power window controllers to detect the presence of obstructions in the window frame when a window panel is being directed to the closed position. Such sensors can also be used to detect the presence of obstructions in other types of automotive closures such as sunroofs, side doors, sliding doors, lift gates, and deck lids. 
   A variety of capacitor-based proximity sensors are known in the art. For example, U.S. Pat. No. 6,377,009 discloses a system for preventing the pinching or trapping of a foreign object by a closing panel (such as a window) through the use of a sensing electrode or plate. This electrode is a metal strip or wire which is embedded in a plastic or rubber molding strip placed behind a piece of fascia or other trim part. The metal strip or wire and the chassis of the vehicle collectively form the two plates of a sensing capacitor. A foreign object placed between these two electrodes changes the dielectric constant and thus varies the amount of charge stored by the sensing capacitor over a given period of time. The charge stored by the sensor capacitor is transferred to a reference capacitor in order to detect the presence of a foreign object. Similar capacitive sensing applications are known from DE 4036465A, DE 4416803A, DE 3513051A1, and DE 4004353A. 
   One of the problems with the known prior art capacitive sensing systems relates to the distance between the electrode(s) and the sheet metal or chassis ground of the automobile, which is in relative close proximity to the sensor electrode(s). Each of these systems incorporates a large background capacitance into the measurement. For example, in the U.S. Pat. No. 6,377,009 reference, the electrode may be only 5-8 mm away from the sheet metal of the automobile. Accordingly, it becomes relatively more difficult to sense small objects near the electrode because of the dominance of the background capacitance, and special measures have to be taken to filter out or eliminate this capacitance from the decision-making process. 
   SUMMARY OF THE INVENTION 
   According to one aspect of the invention, a proximity sensor is provided which is mountable adjacent to an aperture of a metallic motor vehicle for determining the presence of an object in the path of or proximate to a closure panel that moves between a fully open position and a closed position. The sensor includes first and second switches encased in a non-conductive casing mountable on the vehicle. The two electrodes define a capacitance of CE 1 / 2  therebetween, a parasitic capacitance CE 1 / 2  between the first electrode and chassis ground, and a parasitic capacitance CE 2  between the second electrode and chassis ground. A first switch selectively connects the first electrode to a reference capacitor (C 1 ) or to chassis ground and a second switch selectively connects the second electrode to a first voltage reference source (V ref1 ) or to chassis ground. The first and second switches are controlled in order to periodically charge the capacitance CE 1 / 2  and transfer the charge stored thereon to the reference capacitor without transferring substantially any charge stored on the capacitances CE 1  and CE 2  to the reference capacitor. As described in great detail below, this is made possible because the sensor circuit essentially short-circuits the parasitic capacitances during the charge transfer cycle. 
   The reference capacitor (C 1 ) preferably forms part of a charge integrator circuit connectable to the first switch. More particularly, the first switch is connected to the negative input terminal of the integrator, the positive input terminal being grounded. 
   An obstruction may be sensed by transferring charge from the sensing capacitance CE 1 / 2  to the reference capacitor for a fixed number of cycles and then comparing the measured capacitance against a baseline reference. Alternatively, the number of cycles required to charge the reference capacitor (over a fixed time period) until it reaches a predetermined voltage level can be measured and compared against a baseline reference. 
   According to another aspect of the invention, a method is provided for sensing the presence of an object in the path of or proximate to a closure panel, mounted in a metallic structure, that moves in an aperture between a fully open position and a closed position. The method includes mounting first and second electrodes encased in a non-conductive casing on the structure near the closing edge of the aperture. The two electrodes define a capacitance CE 1 / 2  therebetween, a parasitic capacitance CE 1  between the first electrode and chassis ground, and a parasitic capacitance CE 2  between the second electrode and chassis ground. The method includes cyclically connecting (a) the second electrode to a voltage reference source (V ref1 ) and the first electrode to chassis ground, and (b) the second electrode to chassis ground and the first electrode to the reference capacitor. This periodically charges the capacitance CE 1 / 2  and transfers the charge stored thereon to the reference capacitor whilst short-circuiting the parasitic capacitances. Then, the charge on the reference capacitor, the time period required to charge the reference capacitor to a specified voltage, or a calculated value for CE 1 / 2  can be compared against a reference value in order to derive an obstruction signal. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other aspects of the invention will be better understood from the following detailed description of preferred embodiments of the invention in conjunction with the drawings thereof, wherein: 
       FIG. 1  is a diagram of an automotive door having a non-contact obstruction sensor mounted thereto; 
       FIG. 2  is a fragmentary perspective view of a trim piece which contains embedded strip electrodes according for use with the obstruction sensor according to one embodiment of the invention; 
       FIG. 3  is a circuit diagram of a non-contact obstruction sensor according to a preferred embodiment of the invention; 
       FIG. 4  is a timing diagram showing the waveforms of various signals present in the circuit shown in  FIG. 3 ; and 
       FIG. 5  is a cross-sectional view of a window frame sealing system incorporating embedded electrodes for use with the obstruction sensor, according to a preferred embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  illustrates a typical automotive door  12  that is comprised of sheet metal and includes an aperture  14 , structured as a window frame  40 , which may be closed by a window pane or glass panel  16 . The glass panel  16  is raised or lowered by a window regulator (not shown) which includes an electric motor  52  (see  FIG. 3 ) as the motive driving source, as well known in the art, per se. The motor  52  is controlled in part by a non-contact obstruction sensor or anti-pinch assembly  10 , the particulars of which are described in greater detail below. The anti-pinch assembly  10  prevents the glass panel  16  from pinching or crushing a foreign object such as a finger (not shown) that may be extending through the aperture  14  when the panel nears its closed position. It will be appreciated by those skilled in the art that the anti-pinch assembly  10  can be applied to any motorized or automated closure panel structure that moves between an open position and a closed position. For example, a non-exhaustive list of closure panels includes windowpanes, doors, lift gates, sunroofs, and the like. For ease of description, the remainder of this disclosure will focus on the windowpane and window frame combination. 
   The anti-pinch assembly  10  includes two strip electrodes  24   a  and  24   b  such as wires that are preferably embedded in a non-conductive casing  18  mounted near or on the upper part of window frame  40 . As shown in  FIG. 2 , the casing  18  is preferably formed from an oblong elastomeric trim piece  20  that has an integrally formed cavity  22 . The cavity  22  enables the trim piece  20  to more readily deform, thus enabling the distance or space between the two electrodes  24   a  and  24   b  to be more readily altered. The trim piece  20  can be part of the window water sealing system, i.e., form part of a seal, or can form part of the decorative fascia of the vehicle.  FIG. 5  shows a preferred embodiment of the trim piece  20  which forms part of a sealing system  37  of the window frame  40 . The sealing system  37  is preferably formed from a flexible, and/or low durometer compound, in a range of less than  40  to  50  short. The flexibility of the sealing system  37  can also be controlled by the cross-sectional configuration, including controlling the thickness of the arms and walls supporting the electrodes. In the embodiment illustrated in  FIG. 5 , the electrodes  24   a  and  24   b  are molded directly into the sealing system  37 . 
   Referring additionally to the circuit diagram of  FIG. 3 , note that a capacitance CE 1 / 2  exists between the two electrodes  24   a  and  24   b . In addition, a capacitance CE 1  exists between electrode  24   a  and the chassis ground of the vehicle, presented by the sheet metal  12 . A second capacitance CE 2  likewise exists between electrode  24   b  and the chassis ground. 
   The sensor circuit shown in  FIG. 3  comprises the following major components:
         charging circuitry  41  for charging the electrodes  24   a  and  24   b , including switches S 1  and S 2  and a voltage reference source V ref1 ;   a charge integrator  42 , including an analog integrator U 1 A, capacitor C 1 , and a switch S 3  for enabling the integrator  42  to be reset;   a signal amplifier  44 , comprising op amp U 1 B; resistors R 4 , R 5  and capacitor C 3 ;   a signal convertor  46 , comprising comparator U 2 B; and   a micro-controller  48 .       

   The micro-controller  48  is programmed to control the period of time that the electrode capacitances are charged and provides two control signals C 0  and C 1  that control the state of switches S 1  and S 2 . Referring additionally to the waveform diagrams of  FIG. 4 , in an initial state, t 0 , switch S 1  is connected to the chassis ground and switch S 2  is connected to the voltage reference source, V ref1 , as shown in stippled lines in  FIG. 3 . Consequently, electrode  24   b  is connected to the voltage reference source V ref1  and electrode  24   a  is connected to the chassis ground. Hence, the capacitance between the second electrode  24   b  to ground, CE 2 , charges to the level of the reference voltage V ref1 , and the mutual capacitance CE 1 / 2  between electrodes  24   a  and  24   b  is charged to the level of V ref1 . The capacitance of electrode  24   a  to ground, CE 1 , does not have any influence because the electrode  24   a  is grounded, thus short-circuiting CE 1 . 
   To start the measurement operation, the micro-controller  48  first resets the charge integrator  42 . This is accomplished by setting control signal C 1  to close switch S 3  for a predetermined period of time in order to essentially short the pins of the op amp composing integrator  42 , thus discharging capacitor C 1  through the grounded op amp input. Note that resistor R 3  is provided to limit the discharge current flow of capacitor C 1 . 
   After a suitable time period required to reset the charge integrator  42  (typically&lt;5 ms), the micro-controller  48  toggles control signal C 1  at time t 1  in order to open switch S 3 . At the same time, the micro-controller  48  toggles control signal C 0  in order to flip switches S 1  and S 2  to the position shown in solid lines in  FIG. 3 . This causes electrode  24   b  to be connected to chassis ground and electrode  24   a  to be connected to the negative input of op amp U 1 A. In the previous time period (between t 0  and t 1 ), no charge was stored on CE 1  so it does not contribute any signal to U 1 A. In addition, with switch S 2  connected to ground, the parasitic background capacitance CE 2  is short circuited and thus the charge stored thereon is discharged via switch S 2  to ground without contributing any signal to U 1 A. Accordingly, the parasitic capacitance induced by the sheet metal of the vehicle is not measured. However, the capacitance CE 1 / 2  between the two electrodes  24   a ,  24   b  will be received by the integrator  42 . This is because the charged side of capacitance CE 1 / 2  is now connected to GND via switch S 2  causing the polarity of the capacitance to reverse and forcing the other side of CE 1 / 2  to got to −V ref1  in a transitional period. A negative current flows to the negative input of UA 1 , forcing the integrator output to become positive and accumulate the same quantum of charge held by CE 1 / 2 . 
   At time t 2 , the micro-controller  48  toggles control signal C 0  in order to flip switches S 1  and S 2  to the position shown in stippled lines in  FIG. 3 , following which electrode  24   b  is connected to the voltage reference source V ref1  and electrode  24   a  is connected to the chassis ground. During this time period, the capacitance of electrode  24   a  to ground, CE 1 , does not have any influence because the electrode  24   a  is shorted to ground. In contrast, the capacitance between the second electrode  24   b  to ground, CE 2 , charges to the level of the reference voltage V ref1 , and the mutual capacitance CE 1 / 2  between electrodes  24   a  and  24   b  is charged to the level of V ref1 . 
   Shortly thereafter, at time t 3 , the micro-controller  48  toggles control signal C 0  in order to flip switches S 1  and S 2  to the position shown in solid lines in  FIG. 3 . As previously described, the charge held on capacitor CE 1 / 2  is injected into capacitor C 1 , which is significantly larger than CE 1 / 2 . The charge held by CE 2  is not measured, as previously described. The process of transferring charge from capacitor CE 1 / 2  to capacitor C 1  of integrator  42  is continued in a pulsed manner as shown in  FIG. 4 . The process continues until signal Vi, which represents the charge stored on capacitor C 1 , reaches a voltage level equal to a second voltage reference, V ref2 . The comparator  46  can be used for this comparison, or alternatively, if the micro-controller has an analog-to-digital input, the micro-controller can execute this function directly. The number of pulses, N, required for Vi to reach V ref2  is recorded by the micro-controller  48 . Consequently, the measured capacitance of CE 1 / 2  can be calculated as:
 
 CE 1/2=( C 1 *V   ref2 )/( N*V   ref1 )
 
   In practice, the number of charge and discharge cycles, N, required to charge capacitor C 1  to the second voltage reference level, will vary depending upon the presence of a foreign object or not and/or ambient conditions. The nominal number of cycles N nom  can be measured in known circumstances, and then compared with operational data to determine if an object is proximate to the electrodes or otherwise present in the aperture. If N increases above a predetermined threshold value, the micro-controller  48  signals a motor controller  50  to stop or reverse the window regulator motor  52 . 
   Preferably, the sensor signal, being a measure of the capacitance CE 1 / 2  or N, is measured before the window is actuated in order to establish a base level. The sensor signal is then calculated from that base level, i.e., signal=current level−base level. This compensates for shifts in capacitance caused by variations of temperature and humidity. 
   In preferred embodiments, the micro-controller is programmed to generate a reference map, which correlates the sensor signal with the varying position of the window panel, as determined by an encoder  54 . This correlation can be carried out every time the window is actuated, thus filtering out ambient effects, such as atmospheric humidity or the presence of a water film that may exist on the glass pane, which may affect the dielectric constant between the two electrodes  24   a  and  24   b . The technique also compensates for the wearing of the mechanical parts involved. Using this reference map, the sensor signal can be dynamically measured and the motor  52  stopped whenever the signal exceeds the reference value by a predetermined threshold for a given position of the window pane. The predetermined threshold may depend on the position of the window pane. For example, when the window is far from being closed, the threshold value may be quite large. As the window nears its closed position, the threshold value can be reduced in order to increase the sensitivity of the obstruction sensor. In this matter, the sensitivity of the sensor can be increased at a critical point to ensure that fingers are not pinched when the window closes. See WO 02/12669, the contents of which are incorporated herein by reference, for additional information regarding the manner in which a reference map may be employed to reduce the effects of varying ambient conditions. 
   Alternatively, because the trim piece  20  is capable of deforming, at some point as the windowpane approaches its closing position, the change in the capacitance of CE 1 / 2  arising out of the movement of the electrodes  24   a ,  24   b  can be significantly larger than the effect arising from the mere proximity of a foreign object. In this case, it may be desirable to decrease the sensitivity of the obstruction sensor within a certain zone. See more particularly WO 03/038220, the contents of which are incorporated herein by reference, for additional information regarding this technique. 
   The preferred embodiments have been described with some particularity for the purposes of description. Those skilled in the art will understand that a number of variations and modifications may be made to the embodiments disclosed herein without departing from the spirit of the invention. For example, rather than measuring the number of charges and discharge cycles, N, required for the reference capacitor C 1  to reach the level of the second voltage reference, N can be fixed such that the controller measures or calculates the capacitance of CE 1 / 2 , which can be used as a basis for controller the motor. Similarly, a variety of other modifications may be made without departing from the spirit of the invention.

Summary:
A proximity sensor for sensing an object in the path of or proximate to a closure panel such as a vehicle window. First and second electrodes encased in a non-conductive casing are mounted on the metallic structure near the closing edge of the aperture. The two electrodes define a capacitance CE 1/2  therebetween, and parasitic capacitances CE 1  and CE 2  between the first electrode and chassis ground and the second electrode and chassis ground, respectively. A controller cyclically connects (1) the second electrode to a voltage reference source (V ref1 ) and the first electrode to chassis ground and (2) the second electrode to chassis ground and the first electrode to the reference capacitor, thereby periodically charging the capacitance CE 1/2  and transferring the charge stored thereon to the reference capacitor whilst short-circuiting the parasitic capacitances. The charge on the reference capacitor, the time period required to charge the reference capacitor to a specified voltage, or a calculated value for CE 1/2  are then compared against a reference value in order to derive an obstruction signal.