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FIELD OF THE INVENTION 
     The field of the invention generally relates to methods and devices for controlling movable barrier operators. More specifically, the invention relates to detecting obstructions in the pathways of moveable barriers. 
     BACKGROUND OF THE INVENTION 
     Barrier movement operators are automated systems which are used to move a barrier with respect to an opening. Examples of barriers to be moved include garage doors, gates, fire doors and rolling shutters. A number of barrier movement operators have been sold over the years most of which include a head unit containing a motor connected to a transmission. The transmission, which may include, for example, a belt drive, a chain drive, a screw drive or extendible arm is then coupled to the barrier for opening and closing. 
     Such barrier movement operators also typically include a wall control unit, which is connected to send signals to the head unit thereby causing the head unit to open and close the barrier. In addition, these operators often include a receiver unit at the head unit to receive wireless transmissions from a hand-held code transmitter or from a keypad transmitter, which may be affixed to the outside of the area closed by the barrier or other structure. 
     Obstructions may exist or may enter the pathway of the moveable barrier. Previous systems have allowed the barrier operator systems to determine if an obstruction has been encountered and to either stop or reverse the direction of the travel of the barrier once this determination has been made. For instance, some previous systems measured the force applied to the barrier by the motor. The systems then compared the measured force to an expected value plus a fixed cushion value. If the comparison indicated that the measurement value exceeded the expected value plus the cushion value (together, a threshold value), then the downward barrier movement was reversed. These systems typically determined the force by measuring the barrier speed or current in the motor and then calculated the force using these measurements. 
     In some previous systems, capacitive sensor arrangements were used to detect the obstruction in the pathway of the door by detecting the capacitance between a conductive element and the ground. However, in these previous systems, the sensed capacitance between a conductive element and the ground increases above a threshold as the sensor approaches the ground thereby indicating that an “obstruction” existed. Consequently, as the door neared the ground, the capacitance reading would have to be adjusted or discarded because the reading would indicate an obstruction existed, when, in fact, the ground was the “obstruction” being detected. Thus, these previous systems could not be used to detect an obstruction unless the readings were sufficiently adjusted to take into account the detection of the ground or other reference. 
     SUMMARY OF THE INVENTION 
     A system and method for determining whether an obstruction is present in the pathway of a movable barrier avoids false detections of obstructions. As a conductive member on the edge of the moveable barrier approaches the ground, a post or some other reference object, the capacitance between the conductive member and its ground is measured. This measured value is maintained at a substantially constant value or held below a threshold as the conductive element approaches the reference. The approach is easy and inexpensive to implement on current moveable barrier systems. 
     In many of these embodiments, a conductive member is positioned in a holder, which is attached to the moveable barrier. The capacitance is measured between the conductive member and an electrical reference, such as the ground or a fence post. A sensor circuit is coupled to the conductor and determines whether an obstruction exists in the path of the barrier by analyzing the measured capacitance. The conductive member and holder are maintained at a fixed distance from the reference as the conductive member approaches the reference such that the sensed capacitance between the conductive member and its ground remains below a predetermined threshold and/or remains substantially constant. 
     In one example, the holder and conductive member may move or pivot away from the reference as the holder and conductive member combination approaches the reference. Thus, the sensor circuit does not determine an obstruction exists when the reference is approached. 
     The movable barrier may be any type of movable barrier, for example, an overhead door, a sliding gate or a swinging gate. The conductive member may be any type of conductive material or object, for example, a wire or rod. The holder may be any type of arrangement that is capable of supporting or holding the conductive member. For example, the holder may be fabricated from a stiff plastic or rubber. Other examples of holders are possible. 
     Thus, a system and method is provided whereby the capacitance sensed between the conductive member and its ground is maintained at a substantially constant value. Since the capacitance as measured between the conductive member and its ground remains substantially constant (in the absence of a real obstruction), false obstruction readings are avoided. In addition, the sensed capacitance does not have to be altered by the system to take into account the increase in capacitance as the conductive member approaches a reference, such as the physical ground. Furthermore, the system can be easily and inexpensively fabricated and applied to current moveable barrier systems. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a system using a capacitive door edge sensor according to the present invention; 
         FIG. 2  is a diagram of a capacitive door edge sensor according to the present invention; 
         FIGS. 3   a - b  are diagrams of a capacitive door edge sensor in a garage door arrangement according to the present invention; 
         FIGS. 4   a - b  are diagrams of a capacitive door edge sensor in a garage door system according to the present invention; and 
         FIGS. 5   a - c  are diagrams of a capacitive door edge sensor in a swing or sliding door system according to the present invention. 
     
    
    
     Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of the various embodiments of the present invention. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     For illustrative purposes, the following description refers to a moveable barrier that is a garage door. However, it will be understood by those skilled in the art that the moveable barrier may not only be a garage door but may be any type of barrier such as a fire door, shutter, window, gate. Other examples of barriers are possible. 
     Referring now to the drawings and especially to  FIG. 1 , a movable barrier operator, which is a garage door operator, is generally shown therein and includes a head unit  12  mounted within a garage  14 . More specifically, the head unit  12  is mounted to the ceiling of the garage  14  and includes a rail  18  extending there from with a releasable trolley  20  attached having an arm  22  extending to a multiple paneled garage door  24  positioned for movement along a pair of door rails  26  and  28 . The system includes a hand-held transmitter unit  30  adapted to send signals to an antenna  32  positioned on the head unit  12  as will appear hereinafter. An external control pad  34  is positioned on the outside of the garage having a plurality of buttons thereon and communicates via radio frequency transmission with the antenna  32  of the head unit  12 . An optical emitter  42  is connected via a power and signal line  44  to the head unit. An optical detector  46  is connected via a wire  48  to the head unit  12 . The head unit  12  also includes a receiver unit  102 . The receiver unit  102  receives a wireless signal, which is used to actuate the garage door opener. 
     The door  24  has a conductive member  125  attached. The conductive member  125  may be a wire, rod or the like. The conductive member  125  is enclosed and held by a holder  126 . The conductive member  125  is coupled to a sensor circuit  127 . The sensor circuit  127  transmits indications of obstructions to the head unit  12 . If an obstruction is detected, the head unit  12  can reverse direction of the travel of the door  24 . 
     As the holder  126  enclosing the conductive member  125  approaches the ground, the conductive member  125  is lifted by the holder  126 . Thus, the capacitance as measured by the sensor circuit  127  between the conductive member  125  and the ground  126  does not exceed a predetermined threshold and/or remains substantially constant in the absence of an obstruction. Since the capacitance remains substantially constant, a false detection of an obstruction (i.e. the ground  24 ) is avoided. On the other hand, if a real obstruction were present, the obstruction would be detected and the movement of the door  24  may be halted or the direction of travel of the door  24  may be reversed. 
     The head unit  12  has the wall control panel  43  connected to it via a wire or line  43 A. The wall control panel  43  includes a decoder, which decodes closures of a lock switch  80 , a learn switch  82  and a command switch  84  in the wall circuit. The wall control panel  43  also includes a light emitting diode  86  connected by a resistor to the line  43  and to ground to indicate that the wall control panel  43  is energized by the head unit  12 . Switch closures are decoded by the decoder, which sends signals along lines  43 A to a control unit  200  coupled via control lines to an electric motor positioned within the head unit  12 . In other embodiments, analog signals may be exchanged between wall control  43  and head unit 
     The wall control panel  43  is placed in a position such that an operator can observe the garage door  24 . In this respect, the control panel  43  may be in a fixed position. However, it may also be moveable as well. The wall control panel  43  may also use a wirelessly coupled connection to the head unit  12  instead of the wire  43 A. If an obstruction is detected, the direction of travel of the door  24  may be reversed by the control unit  200 . 
     Referring now to FIG.  2 ,one example of a capacitive sensor arrangement is described. A sensing circuit  201  includes a transmitter  202 , a controller  204 , a capacitive measurement circuit  206 , and a battery  203 . The battery  203  supplies DC power and current to a conductive member  210 . 
     The device  201  is coupled to a movable barrier  208 . The device  201 , specifically the capacitive measurement circuit  206  within the device  201 , is coupled to the conductive member  210 . The capacitive measurement circuit  206  detects changes in capacitance and forwards the information to the controller  204 . If a change occurs such that an obstruction is detected, the controller  204  informs the moveable barrier operator by sending a message via the transmitter  202 . The moveable barrier operator may stop of reverse direction of the moveable barrier. 
     The conductive member  210  may be incorporated into a holder  209 . The purpose of the holder  209  is to support and retain the conductive member  210 . As the holder  209  approaches the ground, for example, the holder  209  pivots in an upward direction. As the holder  209  pivots, the conductive member  210  also pivots. As the conductive member  210  is moved out of the way of the physical ground (or reference), the capacitance between the conductive member  210  to the physical ground (or reference) is held below a predetermined threshold and/or substantially constant as determined by the capacitive measurement circuit  206 . Because the conductive member  210  is moved away from the physical ground (or reference), the capacitance as measured between the physical ground (or reference) and the conductive member  210  at the capacitive measurement circuit  206  remains substantially constant and the capacitive measurement circuit  206  does not falsely indicate to controller  204  that there is an obstruction in the way of the movable barrier  208 . No adjustment of readings between the conductive member  210  and ground at the capacitive measurement circuit  206  is required. If an obstruction were in the way of the movable barrier  208 , then the capacitive measurement circuit  206  would detect the obstruction and relay the detection to the controller  204 . The controller  204  would then take an appropriate action, for example stopping and reversing the direction of travel of the moveable barrier  208 . 
     Referring to  FIG. 3   a , an example of a capacitive sensor system for the detection of obstruction is described. A movable barrier  302  rests against the physical ground (or reference)  308 . The movable barrier  302  has attached to it a holder  306  for holding in place a conductive member  304  the holder is attached to the movable barrier  302  by a pivoting arrangement  305 . In this case, the holder is a non-conductor material and the conductive member is also a wire. As shown in  FIG. 3   a , it can be seen that as the movable barrier  302  approaches and finally comes to rest against the physical ground (or reference)  308 , the conductive member  304  is held above the physical ground (or reference)  308  at a fixed distance. Because the conductive member  304  remains a fixed distance of the ground  308 , the capacitance sensed by the conductive member  304  remains below a predetermined threshold and/or substantially constant. A sensor circuit (not shown) coupled to the conductive member  304  avoids the determination that an obstruction is in the pathway of the moveable barrier  302  when, in fact, no obstruction exists. In addition, the measurements made between the conductive member  304  and the ground of the circuit do not have to be compensated to take into account when the conductive member approaches the physical ground (or reference)  308 , which is substantially electrically the same as the ground of the circuit. 
     Referring now to  FIG. 3   b , it is shown that as the barrier moves in an upward direction  310 , the holder  306  holding the conductive member  304  falls downward. Conversely, it can be seen that if the barrier  302  were to move in a downward direction  311 , a curved end portion  313  of the holder  306  would force the holder  306  into the position shown in  FIG. 3   a.    
     Referring now to  FIG. 4   a , another example of a capacitive sensor system is described. A movable barrier  402  rests against the ground  408 . The movable barrier  402  has attached to it a holder flap  406  for holding in place a conductive member  404 . The holder flap  406  is attached to the movable barrier  402  by a pivoting arrangement  405  such as a hinge or the like. In this case, the holder flap  406  is a solid rubber flap or similar arrangement that is used to hold the conductive member  404 . The conductive member  404  may be a wire, rod or other type of sensing arrangement. As shown in  FIG. 4   a , as the movable barrier  402  approaches and finally comes to rest against the physical ground (or reference)  408 , the conductive  404  is held above the physical ground (or reference)  408  at a fixed distance. Because the conductive member  404  never gets within the fixed distance of the physical ground (or reference)  408 , the capacitance sensed by a sensor (not shown) coupled to the conductive member  404  remains below a predetermined threshold and/or substantially constant and the sensor avoids an erroneous determination that an obstruction exists in the path of the moveable barrier  402 . In addition, the measurements of the capacitance between the conductive member  404  and the physical ground (or reference)  408  do not have to be altered to take into account when the conductive member approaches the physical ground (or reference)  408 . 
     Referring now to  FIG. 4   b , it is shown that as moveable barrier  402  moves in an upward direction  410 , the holder flap  406  falls downward. Conversely, it can be seen that if the moveable barrier  402  were to move in a downward direction  411 , the end portion  413  of the holder  406  would force the barrier into the position shown in  FIG. 4   a.    
     Referring now to  FIGS. 5   a ,  5   b  and  5   c  another example of a capacitive sensing system is described. A sliding barrier  502  includes an attached conductive member  504 . A post  508  also includes a holder/conductive member  506 . 
     Referring now specifically to  FIG. 5   b , the sliding barrier  502  moves in direction  511  toward the post  508 . The holder/conductive member  504  includes a conductive member  509  and holder  510 . Both components are attached to the end of the sliding barrier  502 . The holder/conductive member  506  includes a conductive member  512  and a holder  513 . 
     Referring now to  FIG. 5   c , the holder/conductive member  514  moves away from the post  508  in direction  514  and holder/conductive member  506  moves away from the sliding barrier  502 . Thus, the conductive members  512  and  509  are always maintained at a minimum distance away from the sliding barrier  502  and the post  508 . Because of this separation, the measured capacitance at a sensor (not shown) between the conductive members  509  and  512  and the post  508  and the gate  512  remains below a predetermined threshold and/or substantially constant, and a false detection of an obstruction with the path of the sliding barrier  502  is avoided. In addition, the sensor measurements do not have to be altered to take into account the approach of the conductive members to a reference. 
     In another approach, the sliding door  502  can be replaced with a swinging door or gate. In this approach, the other components of the system remain the same. In addition, the system described with respect to  FIGS. 5   a - c  may include only a single holder/conductive member arrangement instead of the twin conductive members as shown in these examples. 
     While there has been illustrated and described particular embodiments of the present invention, it will be appreciated that numerous changes and modifications will occur to those skilled in the art, and it is intended in the appended claims to cover all those changes and modifications which fall within the true scope of the present invention.

Summary:
A system and method for sensing capacitance at the edge of a movable barrier and for detecting an obstruction includes positioning a conductive member within or about a holder. A sensed capacitance between the conductive member and ground. A controller is coupled to the sensor. The controller determines whether an obstruction exists within a path of the barrier by analyzing the sensed capacitance. The conductive member and holder move away from a physical reference as the physical reference is approached such that the sensed capacitance between the conductive member and the physical reference remains below a predetermined threshold.