You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to door openers, and more particularly, but not by way of limitation, to a door opener having a gas-powered control system. 
     2. Description of the Related Art 
     Environmental concerns have created a demand for motor vehicles powered by a fuel other than gasoline. One such alternative fuel for motor vehicles is natural gas. However, natural gas is a safety hazard because it forms a highly explosive mixture with air. In an enclosed structure, such as a garage, this mixture is susceptible to ignition. Therefore, it is highly desirable to eliminate ignition sources in enclosed structures to prevent the mixture from exploding. 
     In garages, ignition sources include electric garage door openers, which may spark, thereby igniting flammable gases in the air. One solution to eliminate this ignition source requires using a garage door opener with an explosion proof motor. However, due to their cost, many garage owners cannot afford such a motor. 
     Another solution replaces the electric door motor with a pneumatic door motor. One such door opener design is disclosed in U.S. Pat. No. 4,891,908, issued Jan. 9, 1990, to Aquilina. Aquilina uses a door opener with a pneumatic motor, however, electrical components stop or reverse the door if objects jam underneath the door during closing. 
     Another door opener with a pneumatic motor is disclosed in U.S. Pat. No. 4,417,418, issued Nov. 29, 1983, to Warning. The Warning motor includes two pistons that are housed within respective cylinders and moved by pressurized air to raise and lower the door. A back pressure within the cylinders slows the door at the end of its travel during opening and closing. Nevertheless, an electrical system controls the operation of the garage door. 
     Still another pneumatic door opener is disclosed in U.S. Pat. No. 3,921,335, issued Nov. 25, 1975, to Hewitt et al. The pneumatic door opener has a pneumatic control scheme that activates a back pressure that slows the door&#39;s opening and closing. A pneumatic sensing means opens a valve that releases back pressure depending upon the position of the door. However, the disclosed pneumatic control fails to stop and reverse the door if the door encounters an obstruction during closing. 
     Still yet another pneumatic door opener is disclosed in U.S. Pat. No. 5,937,579, issued Aug. 17, 1999, to Baczewski, et al. Baczewski, et al. provides a pneumatic door opener with a reversing and stopping function, however, a less complicated and more cost effective design would be desirable. 
     Accordingly, a simple and more cost effective pneumatic door opener having a pneumatic control system with door stopping and reversing capabilities improves operability and safety over conventional pneumatic door openers. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a simplified pneumatic control system provides door control operations for opening, closing, stopping and reversing a door. The door control system includes a gas source, a pneumatic motor, a first pilot valve interposed between the gas source and the motor, and pressurized manual check valves as control inputs. The control system further includes a second pilot valve and limit triggers to stop the door travel at a desired limit point, a braking system and a manual override function for manual movement of the door. The reversing function activates when an obstruction in encountered while the door is closing. Additionally, provisions are provided for pressurized leading edge buttons and remote activation of the control system. 
     A second embodiment includes a biasing return in the first pilot valve to force the system into a hold to open and hold to close arrangement. In this embodiment, the door only travels while a pilot valve is vented. This embodiment further includes a manual override function. Use of this type of arrangement reduces the complexity of the system and further provides for remote activation. 
     Associated methods for controlling the door are also provided. 
     It is therefore an object of the present invention to provide a simplified constantly pressurized pneumatic door control system. 
     It is a further object of the present invention to provide a hold to open pneumatic control system. 
     It is still further an object of the present invention to provide a remotely activated pneumatic control system. 
     It is still yet further an object of the present invention to provide a pneumatic control system that includes a leading edge function. 
     Still other objects, features, and advantages of the present invention will become evident to those of ordinary skill in the art in light of the following. Also, it should be understood that the scope of this invention is intended to be broad, and any combination of any subset of the features, elements, or steps described herein is part of the intended scope of the invention. 
    
    
     
       BRIEF DESCRIPTIONS OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating the main components of a pneumatic door opener of the present invention. 
         FIG. 2  is a diagram illustrating the components of the pneumatic door opener. 
         FIG. 3  is a block diagram illustrating a control system of the pneumatic door opener according to the preferred embodiment. 
         FIG. 4  is a block diagram illustrating a control system for a second embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As illustrated in  FIGS. 1-3 , a pneumatic door opener  5  includes a filter-regulator-lubricator  20 , a control system  100 , and a motor  300 . A first or main air source  10  supplies air that passes through the filter  20  en route to the control system  100 . Although in this preferred embodiment one air source  10  is utilized, multiple air sources may be used. Furthermore, gases other than air may be used. The filter-regulator-lubricator  20  filters the air, regulates the pressure, and adds oil that lubricates the valves of the control system  100 . The filter-regulator-lubricator  20  removes particles and moisture that may cause the valves of the control system  100  to stick. The air enters the control system  100  to initiate and then control door operation by powering the motor  300 , which opens, closes and reverses the direction of the door. 
     As shown in  FIG. 2 , the pneumatic door opener  5  includes a motor  300 , a gearbox  308 , a manual override bar  304 , a brake  240 , an open limit trigger  217 , a close limit trigger  218 , and the control system  100 . The air from the main air source  10  enters the motor  300  and turns the vanes that communicate with the gearbox  308 . The gearbox  308 , in turn, is in communication with the pulley operated overhead door. One of ordinary skill in the art will readily recognize that this connection can be designed for any kind of door. The open limit trigger  217  and the close limit trigger  218  stop the opening and closing of the door at each end of its travel (described herein). The open limit trigger  217  and the close limit trigger  218  may actuate any suitable limit notification device, including a limit lever  220 , a pneumatic button, or the like, to terminate a sequence. 
     The manual override bar  304  includes a handle  306  that terminates communication between the pneumatic door opener  5  and the overhead door, thereby permitting manual door operation. Releasing the handle  306  reestablishes communication between the pneumatic door opener  5  and the door. 
     The control system  100  includes a first pilot valve  120 , a second pilot valve  130 , a third pilot valve  140 , a fourth pilot valve  126 , and a shuttle check valve  127 . The pilot valves may be of any suitable manufacturer. The first pilot valve  120  is a two position, double differential pilot valve with no springs. As such, the valve  120  will stay in a shifted position until forcibly returned to a neutral or centered position. The second pilot valve  130  is a two position, double pilot, spring centered, pressure pilot valve. The third pilot valve  140  is a two position, double differential pilot valve without springs. The fourth pilot valve  126  is a pressure pilot valve. The shuttle check valve  127  automatically selects the highest pressure side. 
     The control system  100  further includes a first return air cylinder  134 , a second return air cylinder  135 , a brake air cylinder  128 , a limiter valve  195 , and a cam operated valve  150 . The first return air cylinder  134  is coupled to the first pilot valve  120 , such that it recenters a piston in the valve  120  from a first position associated with opening of the door. The second return air cylinder  135  is likewise coupled to the first pilot valve  120  to center the piston of the pilot valve  120 , thereby going from a second position associated with closing the door to a neutral or centered position. The brake air cylinder  128  is used to release the brake  240  in the gearbox  308  of the motor  300 . The limiter valve  195  is used to remove the leading edge control system from the control circuit, thereby allowing the door to close fully. The cam valve  150  is a manually operated valve used to stop the operation of the motor  300 . 
     The control system  100  still further includes an open button  160 , a close button  170 , a stop button  180 , and a leading edge reverse button  190 . The buttons are manually opened check valves. In a normal state, the pneumatic control system  100  is pressurized. A continuously pressurized control system  100  allows for multiple activation buttons on a single pilot valve, and remote activation through an extended line. Once a bleeder button is activated, that particular side of a pilot valve is vented, thereby creating a pressure differential within the pilot valve. A piston in the valve then shifts from the higher pressure side to the lower pressure side, thereby aligning passages to alternate pressure lines. The buttons, either remotely or mounted to the pilot valves, are used to activate operations such as opening the door, closing the door, stopping the door, and reversing the direction of the door. 
     As shown in  FIG. 3 , the main air source  10  of the control system  100  is connected to lines  102   a  and  102   b . The line  102   a  branches off to a first end  260  of supply lines  103  and a first end  262  of a line  104 . The line  102   b  branches off to a first end  264  of a line  105  and a first end  266  of a line  106 . A second end  261  of line  103  is connected to a first inlet port  201  of the first pilot valve  120 . The first pilot valve  120  includes the first inlet port  201 , a second inlet port  224 , a third inlet port  225 , a first button port  202 , a second button port  203 , a first outlet port  204 , and a second outlet port  205 . The first button port  202  is connected to a line  122  that is, in turn, connected to an inlet port  171  of the close button  170 . The buttons may be connected remotely or directly to the button ports. The second button port  203  is connected to a line  121  that is connected to an inlet port  161  of the open button  160 , either remotely or directly. Lines  122  and  121  may represent passages within the first pilot valve  120  if the buttons  160  and  170  are directly connected to the first and second button ports  202  and  203  of the first pilot valve  120 . The second inlet port  224  and the third inlet port  225  are open to atmosphere. A first end  268  of a line  107  is connected to the first outlet port  204 . 
     Line  107  further branches off into a first end  278  of a line  109 , a first end  280  of a line  111  and a first end  282  of a line  113 . A first end  270  of a line  108  is connected to the second outlet port  205  and further branches off to a first end  272  of a line  110 , a first end  274  of a line  112 , and a first end  276  of a line  114 . 
     The first air cylinder  134  is attached to a first end  251  of a valve bore  401  of the first pilot valve  120  such that the piston  400  in the valve  120  is returned from an open position to a neutral position when the first air cylinder  134  is pressurized. Similarly, the second air cylinder  135  is attached to a second end  252  of the valve bore  401  such that the piston  400  of the pilot valve  120  is returned from a close position when the second air cylinder  135  is pressurized. The piston  400  further includes two passages per position. An open or first position includes an “open” passage  402  and a “first exit” passage  405 . A close or second position includes a “close” passage  406  and a “second exit” passage  407 . 
     A second end  279  of the line  109  and a second end  273  of the line  110  connect to a first port  124  and a second port  125  of the motor  300 , respectively. The first and second ports  124  and  125  are used as both inlets and outlets for gases, depending on the desired direction of rotation of the vanes in the motor  300 . 
     A second end  281  of the line  111  connects to a first chamber port  227  of the fourth pilot valve  126 . A second chamber port  228  is connected to a second end  275  of the line  112 . The fourth pilot valve  126  further includes an inlet port  286 , an outlet port  287 , a piston  288 , and a spring  289 . The inlet port  286  is connected to a first end  284  of a line  118 . The outlet port  287  is open to atmosphere. The spring  289  is located adjacent to the piston  288 . The piston  288  includes a passage  420 . A second end  285  of the line  118  is connected to the open button pressure circuit of the first pilot valve  120 . 
     A second end  283  of the line  113  attaches to a first port  333  of the shuttle check valve  127 . A second end  277  of the line  114  attaches to a second port  334  of the shuttle check valve  127 . A first end  336  of a line  115  attaches to a third port  335  of the shuttle check valve  127 . The shuttle check valve  127  includes a shuttle  244  that moves away from a higher pressure. The line  115  connects the shuttle check valve  127  to the brake air cylinder  128 . A second end  307  of the line  115  connects to an inlet port  338  of the brake air cylinder  128 . The brake air cylinder  128  is coupled to the brake  240 . 
     Additionally, a limiter valve  195  and a leading edge button  190  also attach to the open button pressure circuit of the first pilot valve  120 . In this embodiment, the leading edge button  190  is a remote button connected to the open button pressure circuit. Activation of the leading edge button  190  forces the control system  100  into the door opening sequence. A first end  292  of a line  119  attaches to the open pressure circuit of the first pilot valve  120 . A second end  293  connects to an inlet port  296  of the limiter valve  195 . An outlet port  297  of the limiter valve  195  connects to a first end  294  of a line  116 . A second end  295  of the line  116  connects to an inlet port  298  of the leading edge button  190 . The limiter valve  195  further includes a limit cutout  299  and a two position piston  421 . The piston  421  further includes a passage  422 , such that gas may pass through the piston  422  when the limiter valve  195  is in a first position and not pass through in a second position. 
     A second end  263  of the line  104  connects to a first inlet port  133  of the second pilot valve  130 . The second pilot valve  130  is in communication with the limit lever  220  that is in communication with the close limit trigger  218  and the open limit trigger  217 . The second pilot valve  130  includes a second inlet port  136 , a third inlet port  137 , a first outlet port  138 , and a second outlet port  139 . The first outlet port  138  is coupled to a first end  339  of a line  131 . A second end  340  of the line  131  is coupled to an inlet port  341  of the first return air cylinder  134 . The second outlet port  139  is coupled to a first end  342  of a line  132 . A second end  343  of the line  132  is coupled to an inlet port  344  of the second return air cylinder  135 . 
     The second pilot valve  130  further includes a bore  415  and a piston  403 . The bore  415  includes a first end  236  and a second end  237 . The piston includes two sets of passages, “pass through” passages  408  and  409  and “stop” passages  404  and  405 . 
     A second end  265  of the line  105  is coupled to a first inlet port  345  of the third pilot valve  140 . The third pilot valve  140  is a two position differential pressure valve without springs. The third pilot valve  140  includes a return air cylinder  143  in alignment with a bore  418 , and a mechanical stop  349 . The third pilot valve  140  still further includes a piston  419 , a second inlet port  346 , and a third inlet port  347 . The piston  419  includes a “pass through” passage  416  and a “manual” passage  417 . The third pilot valve  140  still even further includes a button port  348 , a first outlet port  352 , a second outlet port  327 , and a differential port  441  coupled with the air source  10 . A first end  355  of a line  142  is coupled to the button port  348 . A second end  356  of the line  142  is coupled to an inlet port  181  of the stop button  180 . A first end  353  of a line  141  is coupled to the first outlet port  352  of the third pilot valve  140 . A second end  354  of the line  141  is divided into two lines and connected to the second inlet port  136  and the third inlet port  137  of the second pilot valve  130 . 
     A second end  267  of the line  106  is coupled to an inlet port  154  of the cam operated valve  150 . A second port  155  of the cam operated valve  150  is coupled to a first end  153  of a line  151 . A second end  152  of the line  151  is coupled to the second inlet port  346  of the third pilot valve  140 . The cam operated valve  150  further includes an actuator  156  and a piston  431  with an “exhaust” passage  433 , a “stop” passage  432  and an outlet port  162 . In this preferred embodiment, the actuator  156  is a manually operated handle. 
     In operation, the control system  100  is continuously pressurized, and is capable of opening the door  350 , closing the door  350 , reversing the direction of the door  350 , and stopping the movement of the door  350 . The control system  100  further includes provisions for manually stopping the motor  300  and overriding the leading edge function. 
     In an equilibrium state, the main air source  10  pressurizes the lines  102   a  and  102   b , thereby pressurizing the lines  103 ,  104 ,  105  and  106 . The line  103  terminates at the inlet port  201  of the first pilot valve  120 . In a non-biased condition, the piston  400  of the first pilot valve  120  is located in the center of the bore  401 . As there are no passages in the center of the piston  400 , no air passes through the piston  400  to the outlet ports  204  or  205  and the lines  107  and  108 . If the open button  160  is depressed, the first end  408  of the valve bore  401  adjacent to the open button  160  loses pressure due to the venting by the open button  160 , and the piston  400  is forced to a first position nearest the open button end of the bore  401  by the pressure differential. In the shifted position, the “open” passage  402  lines up with the inlet port  201  and the first outlet port  204  to allow gas into the line  107 . As the line  107  is pressurized, the connected lines  109 ,  111 , and  113  are similarly pressurized. 
     Gas in the line  113  pressurizes the first side  242  of the shuttle check valve  127 , thereby forcing the shuttle  244  to block the second port  334  of the shuttle check valve  127 . The shuttle check valve  127  then builds pressure to further increase the flow of gas through the motor  300 . As the gas pressure builds in the shuttle check valve  127 , gas is able to exit the shuttle check valve  127  through the third port  335 , thereby entering line  115  and pressurizing the brake air cylinder  128  used as a brake  240  release. Pressurizing the brake air cylinder  128  releases the brake  240  in the gearbox  308  of the motor  300 , so that the motor  300  components are able to rotate when gas flows through the motor  300 . The brake  240  is normally spring loaded in the engaged position. 
     Gas in the line  109  then enters the first port  124  of the motor  300  and forces the vanes to rotate, thereby rotating the motor driveshaft  351  and opening the door  350 . The gas passing through the motor  300  then exits the second port  125  of the motor and pressurizes the lines  110  and  108 . As the gas fills the line  108 , it is forced to exit through the second outlet port  205  of the first pilot valve  120 . Gas entering the first pilot valve  120  through the second outlet port  205  then passes through the “first exit” passage  405  of the piston  400  and exits the third inlet port  225  of the first pilot valve  120 . As such, the gas passing through the motor  300  is unrestricted and the motor  300  will continue to open the door  350  until the opening limit trigger  217  is activated. 
     Once the door  350  fully opens, the open limit trigger  217  depresses the limit lever  220 , thereby moving the piston  403  in the second pilot valve  130  out of a non-biased position. Normally, the piston  403  is centered with spring returns, such that no gas is able to pass from the line  104  to the outlet ports  138  and  139 . When the limit lever  220  is depressed, the piston  403  is forced from the centered position to a position aligning the inlet port  133  with the “stop” passage. As such, pressurized gas from the line  104  passes through the inlet port  133 , through the “stop” passage  404  in the piston  403 , and through the outlet port  139  to the line  132 . Since the line  132  is connected to the first return air cylinder  134 , the first return air cylinder  134  is also pressurized. The first return air cylinder  134  extends to move the piston  400  of the first pilot valve  120  back to a centered position, thereby shutting off the gas flow through the first pilot valve  120 . Accordingly, the motor  300  no longer rotates the driveshaft  351  and the door  350  stops opening. The second pilot valve  130  remains in the shifted position as long as the door  350  is at the ends of its travel, however, the piston  400  in the first pilot valve  120  is free to move to close the door  350 . 
     In the equilibrium state, the lines  102   a  and  103  are pressurized by the main air source  10 , and the piston  400  is in a non-biased condition, substantially centered in the bore  401  of the first pilot valve  120 . As there are no passages in the center of the piston, no gas passes through the piston to the outlet ports  204  and  205  or the lines  107  and  108 . If the close button  170  is depressed in this condition, the second end  252  of the valve bore  401  loses pressure due to the venting by the close button  170 , and the piston  400  is forced to a second position nearest the second end  252  by the pressure differential. In the shifted position, the “close” passage  406  lines up with the inlet port  201  and the second outlet port  205  to allow gas into the line  108 . As the line  108  is pressurized, the connected lines  110 ,  112 , and  114  are also pressurized, thereby forcing the shuttle  244  in the shuttle check valve  127  to block the first port  333 , therein pressurizing the line  115  and thus the brake air cylinder  128  to effect the release of the brake  240 . 
     Gas in the line  110  then enters the second port  125  of the motor  300  and forces the vanes to rotate, thereby rotating the motor driveshaft  351  and closing the door  350 . The gas passing through the motor  300  then exits the first port  124  and pressurizes the lines  109  and  107 . As the gas fills the line  107 , it is forced to exit through the first outlet port  204  of the first pilot valve  120 . Gas entering through the first pilot valve  120  through the first outlet port  204  then passes through the “second exit” passage  407  of the piston  400  and exits the second inlet port  224  of the first pilot valve  120 . As such, the gas passing through the motor  300  is unrestricted and the motor  300  will continue to close the door  350  until the close limit switch  218  is activated. 
     Once the door  350  fully closes, the close limit trigger  218  depresses the limit lever  220 , thereby moving the piston  403  in the second pilot valve  130  out of a non-biased position. Normally, the piston is centered with spring returns, such that no gas is able to pass from the line  104  to the outlet ports  138  and  139 . When the limit lever  220  is depressed, the piston  403  is forced from the centered position to a position aligning the inlet port  133  with the “stop” passage  405 . As such, pressurized gas from the line  104  passes through the inlet port  133 , through the “stop” passage  405  in the piston  403 , and through the outlet port  138  to the line  131 . Since the line  131  is connected to the second return air cylinder  135 , the second return air cylinder  135  is also pressurized. The second return air cylinder  135  extends and moves the piston  400  of the first pilot valve  120  back to a centered position. The flow of gas through the first pilot valve  120  is now shut off. Accordingly, the motor  300  no longer rotates and the door  350  stops closing. The second pilot valve  130  remains in the shifted position as long as the door  350  is at the ends of its travel, however, the piston  400  in the first pilot valve  120  is free to move to open the door  350 . 
     Upon depression of a stop button  180 , the control system  100  stops the movement of the door. Normally, the line  105  is constantly pressurized by the main air source  10 , and the piston of the third pilot valve  140  resides in a neutral position. In the neutral position, the piston does not allow pressurized gas to pass from the first inlet port  345  to the first outlet port  352 , however, it does allow gas to pass from the first outlet port  352  through the piston to the second inlet port  346 . The third inlet port  347  and the second outlet port  327  are unused in this preferred embodiment. 
     When the stop button  180  is depressed, the piston  419  moves toward the stop button  180  due to the pressure differential in the bore  418 . Once shifted, the piston  419  allows the pressurized gas to flow from the first inlet port  345  to the first outlet port  352 , thereby pressurizing the line  141 . Once the line  141  is pressurized, the gas flows into the second and third inlet ports  136  and  137  of the second pilot valve  130 , through the “pass through” passages  408  and  409 , and into the outlet ports  138  and  139 . As such, the lines  131  and  132 , and subsequently, the first and second return air cylinders  134  and  135  are pressurized. Once the first and second return air cylinders  134  and  135  are pressurized, the piston  400  of the first pilot valve  120  is centered, therein ceasing gas flow to the motor  300  and stopping the motion of the door. After the motor  300  has stopped, compressed gas is allowed behind the piston in the return air cylinder  143 , which causes the return air cylinder  143  to extend. Upon extending, the return air cylinder  143  forces the piston of the third pilot valve  140  to return to a centered position. The stop  349  prevents the piston from moving past the centered position. 
     The control system  100  further includes a manual override circuit. The manual override circuit allows an operator to manually depress the cam-operated valve  150  to stop the motor  300 , and pull a cable to release the brake. This particular segment of the control system is fed off of the line  106 . In this preferred embodiment, the line  106  is substantially continuously pressurized by the main air source. However, the cam operated valve  150  does not allow the pressurized air to enter the system in the normal position. 
     In the normal position, the piston  431  in the cam operated valve  150  is situated such that it allows gas to pass from the second port  155 , through the “exhaust” passage  433  to the outlet port  162 . As such, the line  151  serves as an exhaust line for the control system  100 . When the handle  156  is depressed, the piston  431  is shifted such that the “stop” passage  432  is aligned with the first port  154  and the second port  155 , therein pressurizing the lines  151  and  141 . The pressurized gas continues through the system, illustratively through the second pilot valve  130  to the lines  131  and  132 , and the first and second return air cylinders  134  and  135 . As the first and second return air cylinders  134  and  135  are pressurized, the piston of the first pilot valve  120  is returned to the centered position, thereby ceasing the flow of pressurized gas to the motor  300 . 
     At this point, the operator may release the brake  240  by pulling a cable. Once the brake  240  is released, the operator may manually reposition the door. Once manual operations are completed, the handle  156  may be moved to the normal position, and the brake  240  may be reengaged. 
     The reversing function of the control system  100  reverses the direction of door movement when the door  350  experiences an obstruction in the door&#39;s path. The reversing function is controlled by the fourth pilot valve  126 . In operation, the pressures experienced in the lines  111  and  112  directly reflect the pressures in the ends of the passage in the fourth pilot valve  126 . If the motor  300  is receiving gas from the gas source  10 , then there is an imbalance between the two sides of the fourth pilot valve  126 . In the normal position, the piston  288  of the fourth pilot valve  126  is situated such that it is free to move in the fourth pilot valve  126 . The piston  288  is connected to a biasing mechanism. In this preferred embodiment, the biasing mechanism is a spring  289 . In use, the pressure differential must overcome the spring  289  force to move the piston  288  towards the first chamber port  227 . This motion is associated with the closing operation. 
     As the control system  100  conducts a closing operation, gas is pressurizing the lines  108 ,  110 ,  112 , and  114 , thereby creating an imbalance situation in the fourth pilot valve  126 . The increased pressure in the line  112  forces the piston in the fourth pilot valve  126  to move toward the first chamber port  227 . The distance moved can be ascertained by knowing the spring rate of the spring  289 , the area of the piston and the pressure differential between the two sides of the fourth pilot valve  126 . Movement to compress the spring  289  is acceptable, however, extreme differential pressures will cause the spring  289  to compress to a point where the “vent” passage  420  in the piston  288  aligns with the outlet port  287 . Extreme pressure differentials are experienced when the door encounters an obstruction, as the compressed gas experiences a resistance due to the mechanical resistance experienced by the door. Alignment of the “vent” passage  420  with the inlet port  286  and the outlet port  287  allows the pressure in the line  118  and the open button pressure circuit to drop, thereby activating the open sequence of the control system  100  and reversing the direction of the door  350 . The door  350  then reverses direction, and opens as previously described. 
     The leading edge portion of the control system  100  is designed to sense obstructions under a leading edge of the closing door. The leading edge function requires that leading edge buttons  190  be remotely placed along the leading edge of the door, such that a button  190  would be compressed when an obstruction is encountered. The leading edge buttons  190  are remotely connected to the pressure circuit for the open button  160 , generally, the line  121 . Should a leading edge button  190  be activated, a low pressure develops in the open button  160  pressure circuit. The low pressure experienced in the open button  160  pressure circuit is enough to cause the piston in the first pilot valve  120  to move and align the “open” passage  402  with the entry port  201  to commence the flow of gas associated with opening the door  350 . It should be clear to one skilled in the art that the quantity and placement of the leading edge buttons  190  may vary depending upon door sizes and spacing between the leading edge buttons  190 . Further, it should be noted that the leading edge buttons  190  may be fixtured together to form a bar and cover virtually the entire leading edge, thereby minimizing the hardware and expense. 
     In the normal position, the piston  421  in the limiter valve  195  is aligned such that the passage  422  is connected to the inlet port  296  and the outlet port  297 . Under normal operations with the door  350  in a non-closed position, gas is allowed to pass through the limiter valve  195 . In this position, the leading edge buttons  190  are pressurized and functional. When a door  350  moving downward engages an obstruction, a leading edge button  190  is depressed. The activation of the leading edge button  190  causes a low pressure in the open button  160  pressure circuit, thereby causing an imbalance in the first pilot valve  120 . The piston  400  in the first pilot valve  120  moves to align the “open” passage  402  with the pressurized line  103  to commence the opening of the door  350 . The door  350  will reverse direction and open fully. 
     The leading edge circuit further includes an automatic cutout. The automatic cutout isolates the pressure circuit associated with the leading edge buttons  190 . The leading edge buttons  190  are cut out of the pressure circuit at a predetermined closing height, preferably right before the leading edge buttons  190  are activated by the closing of the door  350 , to allow for the door  350  to be fully closed without activating the leading edge buttons  190 . Upon activation of the limit cutout  299 , the piston  421  in the limiter valve  195  is shifted such that the leading edge segment of the control system  100  is disconnected from the rest of the control system  100 . Therein, the door  350  may be completely closed without activating the leading edge buttons  190 . In the case of damage to the leading edge circuit, the limiter valve  195  may be depressed, and the control system  100  will then hold pressure, thereby ensuring control of the door  350 . 
     As illustrated in  FIG. 4 , a second embodiment of the control system  100  is used to control a motor  300  for a hold to open and a hold to close a door regime. A hold to open, in this disclosure, is defined as a control system, wherein the movement of the door  350  is restricted to occurring when a button is depressed. The motor  300  is identical to the motor  300  as disclosed in the first embodiment, and like parts of the control system  100  have been labeled with like numerals. The control system  100  includes a main gas source  10 , a first pilot valve  120 , a second pilot valve  130 , a shuttle check valve  127 , and a cam operated valve  150 . The first pilot valve  120  is a two position, double differential pilot valve with springs. As such, the valve  120  will return to a neutral or centered position when the button is released. The second pilot valve  130  is a two position, double pilot, spring centered, pressure pilot valve. The shuttle check valve  127  forces a higher pressure gas to pass through the shuttle check valve  127  to a brake release apparatus. The cam valve  150  is a manually opened valve used to stop the operation of the motor  300 . 
     The control system  100  further includes a first return air cylinder  134 , a second return air cylinder  135 , and a brake air cylinder  128 . The first return air cylinder  134  is coupled to the first pilot valve  120 , such that it recenters a piston in the valve  120  from a first position associated with opening of the door. The second return air cylinder  135  is likewise coupled to the first pilot valve  120  to center the piston of the pilot valve  120 , thereby going from a second position associated with closing the door to a neutral or centered position. The brake air cylinder  128  is used to release a brake  240  located in the gearbox  308  of the motor  300 . 
     The control system  100  still further includes an open button  160 , a close button  170 , an open limit trigger  217 , and a close limit trigger  218 . The buttons are manually opened check valves. In a normal state, the pneumatic control system is pressurized. A continuously pressurized control system  100  allows for multiple activation buttons on a single pilot valve, and remote activation through an extended line. Once a bleeder button is activated, that particular side of a pilot valve is vented, thereby creating a pressure differential within the pilot valve. A piston in the pilot valve then shifts from the higher pressure side to the lower pressure side, thereby aligning passages to alternate pressure lines. The buttons, either remotely or directly mounted to the pilot valves, are used to activate operations including opening and closing of the door. 
     As shown in  FIG. 4 , the main gas source  10  of the control system  100  is connected to the lines  102   a  and  102   b . The line  102   a  branches off to a first end  260  of a line  103  and a first end  262  of a line  104 . The line  102   b  is coupled to a first end  266  of a line  106 . A second end  261  of the line  103  is connected to a first inlet port  201  of the first pilot valve  120 . The first pilot valve  120  includes the first inlet port  201 , a second inlet port  224 , a third inlet port  225 , a first button port  202 , a second button port  203 , a first outlet port  204 , and a second outlet port  205 . The first button port  202  is connected to a line  122  that is, in turn, connected to an inlet port  171  of the close button  170 . The buttons may be connected remotely or directly to the button ports. The second button port  203  is connected to a line  121  that is connected to an inlet port  161  of the open button  160 , either remotely or directly. Lines  122  and  121  may represent passages within the first pilot valve  120  if the buttons  160  and  170  are directly connected to the first and second button ports  202  and  203  of the first pilot valve  120 . The second inlet port  224  and the third inlet port  225  are open to atmosphere. A first end  268  of a line  107  is connected to the first outlet port  204 . 
     Line  107  further branches off into a first end  278  of a line  109 , a first end  280  and a first end  282  of a line  113 . A first end  270  of a line  108  is connected to the second outlet port  205  and further branches off to a first end  272  of a line  110  and a first end  276  of a line  114 . 
     The first return air cylinder  134  is attached to a first end  251  of a valve bore  401  of the first pilot valve  120  such that the piston  400  in the valve  120  is returned from a first or open position to a neutral position when the first return air cylinder  134  is pressurized. Similarly, the second return air cylinder  135  is attached to a second end  252  of the valve bore  401  such that the pilot valve  120  is returned from a second or close position when the second return air cylinder  135  is pressurized. The piston further includes two passages per position. An open position includes an “open” passage  402  and a “first exit” passage  405 . A close position includes a “close” passage  406  and a “second exit” passage  407 . 
     A second end  279  of the line  109  and a second end  273  of the line  110  connect to a first port  124  and a second port  125  of the motor  300 , respectively. The first and second ports  124  and  125  are used as both inlets and outlets for gases, depending on the desired direction of rotation of the vanes in the motor  300 . 
     A second end  283  of the line  113  attaches to a first port  333  of the shuttle check valve  127 . A second end  277  of the line  114  attaches to a second port  334  of the shuttle check valve  127 . A first end  336  of a line  115  attaches to a third port  335  of the shuttle check valve  127 . The shuttle check valve  127  includes a shuttle  244  that moves away from a higher pressure. The line  115  connects the shuttle check valve  127  to the brake air cylinder  128 . A second end  307  of the line  115  connects to an inlet port  338  of the brake air cylinder  128 . 
     A second end  263  of the line  104  connects to a first inlet port  133  of the second pilot valve  130 . The second pilot valve  130  is in communication with the close limit switch  218  and the open limit switch  217 . The second pilot valve  130  includes a second inlet port  136 , a third inlet port  137 , a first outlet port  138 , and a second outlet port  139 . The first outlet port  138  is coupled to a first end  339  of a line  131 . A second end  340  of the line  131  is coupled to an inlet port  341  of the first return air cylinder  134 . The second outlet port  139  is coupled to a first end  342  of a line  132 . A second end  343  of the line  132  is coupled to an inlet port  344  of the second return air cylinder  135 . 
     The second pilot valve  130  further includes a bore  415  and a piston  403 . The bore  415  includes a first end  236  and a second end  237 . The piston includes two sets of passages, “pass through” passages  408  and  409  and “stop” passages  404  and  405 . 
     A second end  267  of the line  106  is coupled to an inlet port  154  of the cam operated valve  150 . A second port  155  of the cam operated valve  150  is coupled to a first end  153  of a line  151 . A second end  152  of the line  151  is coupled to the second inlet port  136  and the third inlet port  137  of the second pilot valve  130 . The cam operated valve  150  further includes an outlet port  162 , an actuator  156  and a piston with an “exhaust” passage  433 , and a “stop” passage  432 . In this preferred embodiment, the actuator  156  is a manually operated handle. 
     In operation, the control system  100  is continuously pressurized, and is capable of opening and closing the door  350 . While this embodiment is similar in construction to the first embodiment disclosed, the addition of return springs to the first pilot valve  120  forces the operator to hold the open button  160  or the close button  170  when operating the control system  100 . The door  350  moves as long as one of the buttons is depressed. In this arrangement, a stop valve and a reversing function valve are no longer required for safe operation. 
     In an equilibrium state, the main air source  10  pressurizes the lines  102   a  and  102   b , thereby pressuring the lines  103 ,  104 , and  106 . In a non-biased condition, the piston  400  of the first pilot valve  120  is located in the center of the bore  401 . As there are no passages in the center of the piston  400 , no air passes through the piston to the outlet ports  204  or  205  and the lines  107  and  108 . If the open button  160  is depressed, the first end  408  of the valve bore  401  adjacent to the open button  160  loses pressure due to the venting by the open button  160 , and the piston  400  is forced toward the open button end of the bore  401  by the pressure differential. In the shifted position, the “open” passage  402  lines up with the inlet port  201  and the first outlet port  204  to allow air into the line  107 . As the line  107  is pressurized, the connected lines  109  and  113  are similarly pressurized. 
     Air in the line  113  pressurizes the first side of the shuttle check valve  127 , thereby forcing the shuttle  244  to block the second port  334  of the shuttle check valve  127 . The shuttle check valve  127  then builds pressure to further increase the flow of air through the motor  300 . As the air pressure builds in the shuttle check valve  127 , air is able to exit the shuttle check valve  127  through the third port  335 , thereby entering line  115  and pressurizing the brake air cylinder  128  used as a brake  240  release. Pressurizing the brake air cylinder  128  releases the brake  240  in the gearbox  308  of the motor  300 , so that the motor components are able to rotate when air flows through the motor  300 . The brake  240  is normally spring loaded in the engaged position. 
     Air in the line  109  then enters the first port  124  of the motor  300  and forces the vanes to rotate, thereby rotating the motor driveshaft  351  and opening the door  350 . The air passing through the motor  300  then exits the second port  125  of the motor and pressurizes the lines  110  and  108 . As the air fills the line  108 , it is forced to exit through the second outlet port  205  of the first pilot valve  120 . Air entering the first pilot valve  120  through the second outlet port  205  then passes through the “first exit” passage  405  of the piston  400  and exits the third inlet port  225  of the first pilot valve  120 . As such, the air passing through the motor  300  is unrestricted and the motor  300  will continue to open the door  350  until the open button  160  is released or the opening limit switch  217  is activated. If the open button  160  is released, the piston  400  in the first pilot valve  120  will return to the neutral or centered position by a spring. 
     Once the door fully opens, the open limit trigger  217  depresses the limit lever  220 , thereby moving the piston  403  in the second pilot valve  130  out of a non-biased position. Normally, the piston  403  is centered with spring returns, such that no air is able to pass from the line  104  to the outlet ports  138  and  139 . When the limit lever  220  is depressed, the piston  403  is forced from the centered position to a position aligning the inlet port  133  with the “stop” passage  404 . As such, pressurized air from the line  104  passes through the inlet port  133 , through the “stop” passage  404  in the piston  403 , and through the outlet port  139  to the line  132 . Since the line  132  is connected to the first return air cylinder  134 , the first return air cylinder  134  is also pressurized. The first return air cylinder  134  extends to move the piston  400  of the first pilot valve  120  back to a centered position, thereby shutting off the air flow through the first pilot valve  120  to the motor  300 . Accordingly, the motor  300  no longer rotates the second driveshaft  312  and the door  350  stops opening. The second pilot valve  130  remains in the shifted position as long as the door  350  is at the ends of its travel, however, the piston  400  in the first pilot valve  120  is free to move to close the door  350 . 
     In the equilibrium state, the lines  102   a  and  103  are pressurized by the main air source  10 , and the piston is in a non-biased condition, substantially centered in the bore of the first pilot valve  120 . As there are no passages in the center of the piston, no air passes through the piston to the outlet ports  204  and  205  or the lines  107  and  108 . If the close button  170  is depressed in this condition, the second end  252  of the valve bore loses pressure due to the venting by the close button  170 , and the piston is forced towards the second end  252  by the pressure differential. In the shifted position, the “close” passage lines up with the inlet port  201  and the second outlet port  205  to allow air into the line  108 . As the line  108  is pressurized, the connected lines  110  and  114  are also pressurized, thereby forcing the shuttle  244  in the shuttle check valve  127  to block the first port  333 , therein pressurizing the line  115  and thus the brake air cylinder  128  to effect the release of the brake  240  as previously disclosed. 
     Air in the line  110  then enters the second port  125  of the motor  300  and forces the vanes to rotate, thereby rotating the motor driveshaft  351  and closing the door  350 . The air passing through the motor  300  then exits the first port  124  and pressurizes the lines  109  and  107 . As the air fills the line  107 , it is forced to exit through the first outlet port  204  of the first pilot valve  120 . Air entering through the first pilot valve  120  through the first outlet port  204  then passes through the “second exit” passage  407  of the piston  400  and exits the second inlet port  224  of the first pilot valve  120 . As such, the air passing through the motor  300  is unrestricted and the motor will continue to close the door  350  until the close button  170  is released or the close limit trigger  218  is activated. If the close button  170  is released, the piston  400  in the first pilot valve  120  is moved to the neutral or centered position by the springs. 
     Once the door  350  fully closes, the close limit trigger  218  depresses the limit lever  220 , thereby moving the piston  403  in the second pilot valve  130  out of a non-biased position. Normally, the piston  403  is centered with spring returns, such that no air is able to pass from the line  104  to the outlet ports  138  and  139 . When the limit switch lever  220  is depressed, the piston  403  is forced from the centered position to a position aligning the inlet port  133  with the “stop” passage  405 . As such, pressurized air from the line  104  passes through the inlet port  133 , through the “stop” passage  405  in the piston  403 , and through the outlet port  138  to the line  131 . Since the line  131  is connected to the second return air cylinder  135 , the second return air cylinder  135  is also pressurized. The second return air cylinder  135  extends and moves the piston  400  of the first pilot valve  120  back to a centered position. The flow of air through the first pilot valve  120  is now shut off. Accordingly, the motor  300  no longer rotates and the door  350  stops closing. The second pilot valve  130  remains in the shifted position as long as the door  350  is at the ends of its travel, however, the piston  400  in the first pilot valve  120  is free to move to open the door  350 . 
     The control system  100  further includes a manual override circuit. The manual override circuit allows an operator to manually depress the cam-operated valve  150  to stop the motor  300 , and pull a cable to release the brake. This particular segment of the control system is fed off of the line  106 . In this preferred embodiment, the line  106  is substantially continuously pressurized by the main air source. However, the cam operated valve  150  does not allow the pressurized air to enter the system in the normal position. 
     In the normal position, the piston  431  in the cam operated valve  150  is situated such that it allows air to pass from the second port  155 , through the “exhaust” passage  433  to the outlet port  162 . As such, the line  151  serves as an exhaust line for the control system  100 . When the actuator  156  is depressed, the piston  431  is shifted such that the “stop” passage  432  is aligned with the first port  154  and the second port  155 , therein pressurizing the line  151 . The pressurized air continues through the system; illustratively through the second pilot valve  130  to the lines  131  and  132 , and the first and second return air cylinders  134  and  135 . As the first and second return air cylinders  134  and  135  are pressurized, the piston of the first pilot valve  120  is returned to the centered position, thereby ceasing the flow of pressurized gas to the motor  300 . 
     At this point, the operator may release the brake  240  by pulling a cable. Once the brake  240  is released, the operator may manually reposition the door. Once manual operations are completed, the actuator  156  may be moved to the normal position, and the brake  240  may be reengaged. 
     Although the present invention has been described in terms of the foregoing preferred embodiment, such description has been for exemplary purposes only and, as will be apparent to those of ordinary skill in the art, many alternatives, equivalents, and variations of varying degrees will fall within the scope of the present invention. That scope, accordingly, is not to be limited in any respect by the foregoing detailed description; rather, it is defined only by the claims that follow.

Summary:
A simplified pneumatic door control system provides door control operations for opening, closing, stopping and reversing a door. The door control system includes as control inputs a gas source, a pneumatic motor, a first pilot valve, and pressurized manual check valves. The pressurized manually opened check valves may be used to remotely activate the control system. The control system further includes a second pilot valve, limit triggers, and a braking system. A reversing function of the control system provides the ability to reverse the direction of the door should the door encounter an obstruction upon closing. A second embodiment provides a hold to open control scheme, wherein the door travels only while a pressurized check valve is vented. Corresponding methods for controlling the door are also provided.