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BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The subject invention generally pertains to industrial doors having a pliable door curtain, and more specifically to a system responsive to a door impact. 
     2. Description of Related Art 
     Industrial doors in which the door itself is made of pliable material such as fabric are used in a variety of applications, typically for the purpose of separating areas within a building, or closing off building doorways that lead outside. Examples of such pliable doors are planar doors, overhead-storing doors, concertina doors and roll-up doors. Planar doors include frame members on which the fabric comprising the door is disposed. This plane of material is then movable between a doorway blocking position and a storage position, wherein the plane of material and associated frame members are disposed above the doorway. The frame typically includes extensions extending past either side of the door, and which are receivable within guide tracks to guide the door through its vertical movement. These extensions may include wheels or trolleys. An overhead-storing door is similar in that the fabric door is maintained on frame members and is movable between doorway blocking and storage positions. In this door, however, the storage position is overhead, as in a typical garage door. Accordingly, the guide members associated with such a door will curve between the vertical and horizontal. A concertina door includes a fabric panel supported by spaced-apart ribs or stays that are guided for movement along a track. As the ribs travel along the track, the fabric panel folds and unfolds between the ribs to respectively open and close the door. A typical roll-up door comprises a roll-up panel or fabric curtain that is wound about a roller journalled for rotation above the doorway. To close the door, the roller pays out the curtain as two vertical tracks disposed along either side edge of the doorway guide the side edges of the curtain generally along a vertical plane across the doorway. The rotation of the roller is reversed to open the door. Roll-up doors are typically either powered open and closed, or are powered open and allowed to fall closed by gravity. As the invention herein is envisioned for use primarily with roll-up doors, it will be described with reference thereto. However, the invention may also be used in combination with other such pliable industrial doors. 
     Some roll-up doors have a rigid leading edge provided by a rigid or semi-rigid bar disposed along a lower portion of the curtain. The rigidity of the bar helps keep the curtain within the side tracks and helps the curtain resist wind and other air pressure differentials that may develop across opposite sides of the door. 
     Other roll-up doors, however, have a curtain with a relatively soft leading edge. To help keep such a curtain within its guide tracks, as well as keep the curtain taut and square to the doorway, opposite ends of the bottom portion of the curtain can be held in tension by two opposing carriages or trolleys that are constrained to travel along the tracks: one in each track. However, the door&#39;s lower leading edge does not necessarily have to be held in tension, especially when the door is not subject to significant pressure differentials. 
     Industrial doors are commonly installed in warehouses, where the doors are very susceptible to being struck by forklifts or other vehicles. To protect the door and the vehicle from damage and to protect personnel in the vicinity of the collision from injury, often some type of breakaway or compliant feature is added to the door. For a door having a rigid reinforcing bar along its leading edge, the bar may be provided with sufficient flexibility and resilience to safely pop out of its track when struck. Alternatively, a hard edge door may have its bottom bar connected at either end to carriages engageable with the tracks such that the bottom bar breaks away from the carriages for an impact. Doors having a relatively soft leading edge may have sufficient flexibility to absorb an impact, or a bottom portion of the door&#39;s curtain can be coupled to its two guide carriages by way of a breakaway coupling. The coupling releases the curtain from the carriage upon being subjected to a predetermined breakaway force, thereby limiting the impact force to a predetermined safe level. More information on break away couplings can be found in U.S. Pat. No. 5,638,883, which is specifically incorporated by reference herein. 
     A collision can also occur when a door accidentally closes upon an obstacle in its path, such as an object or a person. To protect the door and obstacle from damage or injury, often some type of switch is installed generally along the lower portion of the door to detect when an obstacle has been encountered. An example of such a switch would be an elongated bumper switch, tape-switch or some other elongated switch extending along the lower, leading edge of the roll-up panel. In reaction to sensing the obstacle upon impact, a set of electrical contacts of the switch typically close to stop or reverse the motor that drives roller. 
     However, switches are impractical for use on a door having a relatively soft leading edge, because the normal flexing of the door curtain could trip the switch prematurely. This can happen regardless of whether the soft leading edge of the curtain is held taut or left relatively loose. Therefore, some doors with a soft leading edge instead include a switch with closed biased contacts that are held open by the tension in the leading edge of the curtain. When an impact forces the leading edge of the curtain to break away from its guide tracks, the resulting release of tension within the curtain allows the switch&#39;s contacts to close, The closed contacts provide a signal that can be conveyed to the door&#39;s control circuit or an alarm circuit by way of a wire or battery powered radio transmission. Alternatively, a sensing mechanism may be associated with the guide carriages or trolleys associated with the soft edge. This sensing mechanism has a first state when the breakaway connection to the leading edge is intact, and a second state upon breakaway. This change to this second state is detected to stop or reverse the door. 
     In hard edged doors with a tape switch or other elongated switch, such elongated switches are typically inserted into a sheath attached to the curtain or incorporated within the curtain itself to allow a more durable or suitable sealing member to be installed just below the switch. This allows the very bottom or leading edge of the roll-up panel to be provided with a more compliant sealing material that can effectively conform to seal against the floor beneath the doorway when the door is closed. However, installing switches in such a manner, makes them rather inaccessible for servicing. Serviceability is particularly important, as the switch itself, being disposed along the lower portion of the roll-up panel, places the switch&#39;s electrical contacts and other electrical parts in a vulnerable position where they are subject to repeated impacts that could eventually damage the switch. 
     Further, when such a switch is used on a door having a breakaway coupling, wiring connecting the switch to a terminal associated with the motor&#39;s control needs to accommodate the separation of the coupling. That is often accomplished by running a separate coiled wire (i.e., multi-conductor cable) along the outside of the track and extending the wire from the terminal to the switch. Such a wire is usually coiled so it can stretch to accommodate the up and down motion of the door panel as well as the motion of the panel upon breaking away from its carriage. However, an exposed coiled wire can be unsightly, especially when it becomes permanently stretched out from use and begins to sag. As the wire sags, it becomes prone to snagging adjacent parts of the door or other items nearby. 
     SUMMARY OF THE INVENTION 
     In order to more effectively synthesize a safety switch with a breakaway coupling of a roll-up door, there is provided a breakaway coupling that includes at least one electrical contact that remains coupled to a guide carriage of the door even after the coupling disengages the door&#39;s roll-up panel from the carriage. 
     This eliminates the need for externally running a separate coiled or otherwise flexible wire out to the roll-up panel. 
     It also positions the electrical contacts of the switch at a more serviceable location and at a location that is beyond the impact-vulnerable central portion of the roll-up panel&#39;s leading edge. 
     In some embodiments, the electrical contacts of the switch are an integral part of the breakaway coupling itself, which is relatively more rugged than the small delicate electrical contacts of a conventional electrical switch. 
     By integrating a safety switch with an omni-directional breakaway coupling, the switch also becomes omni-directional in that it is responsive to an impact from any direction. 
     There is also provided an impact detection system wherein the sensing circuit includes a conductor that extends across the width of the doorway. For normal door operation, the conductor conducts electricity as part of the sensing circuit. For an impact, however, the conductor is no longer a conductive part of the circuit. This change can be detected and interpreted as an impact having occurred. 
     There is also provided a breakaway coupling wherein a member associated with a door guide track (e.g., a trolley or guide carriage) and a conductor are in electrical, conductive contact for normal door operation, and are not in conductive contact for a breakaway condition. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a front view of one embodiment with a cut-away portion showing a breakaway coupling. 
     FIG. 2 is a cross-sectional top view taken along line  2 — 2  of FIG. 1, but with the roll-up panel and wiring leading to the breakaway coupling omitted for clarity. 
     FIG. 3 is the same view as FIG. 1, but with one of the breakaway couplings disengaged. 
     FIG. 4 is a cross-sectional top view taken along line  4 — 4  of FIG. 3, but with the roll-up panel and wiring leading to the breakaway coupling omitted for clarity. 
     FIG. 5 is a front view of another embodiment with a cut-away portion showing a breakaway coupling. 
     FIG. 6 is a cross-sectional top view taken along line  6 — 6  of FIG. 5, but with the roll-up panel and wiring leading to the breakaway coupling omitted for clarity. 
     FIG. 7 is the same view as FIG. 5, but with one of the breakaway couplings disengaged. 
     FIG. 8 is a cross-sectional top view taken along line  8 — 8  of FIG. 7, but with the roll-up panel and wiring leading to the breakaway coupling omitted for clarity. 
     FIG. 9 is a front view of another embodiment with a cut-away portion showing a breakaway coupling. 
     FIG. 10 is a cross-sectional top view taken along line  10 — 10  of FIG. 9, but with the roll-up panel and wiring leading to the breakaway coupling omitted for clarity. 
     FIG. 11 is the same view as FIG. 9, but with both of the breakaway couplings disengaged. 
     FIG. 12 is a cross-sectional top view taken along line  12 — 12  of FIG. 11, but with the roll-up panel and wiring leading to the breakaway coupling omitted for clarity. 
     FIG. 13 is a front view of another embodiment with a cut-away portion showing a breakaway coupling. 
     FIG. 14 is a cross-sectional top view taken along line  14 — 14  of FIG. 13, but with the roll-up panel and wiring leading to the breakaway coupling omitted for clarity. 
     FIG. 15 is the same view as FIG. 13, but with one of the breakaway couplings disengaged. 
     FIG. 16 is a cross-sectional top view taken along line  16 — 16  of FIG. 15, but with the roll-up panel and wiring leading to the breakaway coupling omitted for clarity. 
     FIG. 17 is a front view of another embodiment with a cut-away portion showing a breakaway coupling. 
     FIG. 18 is a cross-sectional top view taken along line  18 — 18  of FIG. 17, but with the roll-up panel and wiring leading to the breakaway coupling omitted for clarity. 
     FIG. 19 is the same view as FIG. 17, but with one of the breakaway couplings disengaged. 
     FIG. 20 is a cross-sectional top view taken along line  20 — 20  of FIG. 19, but with the roll-up panel and wiring leading to the breakaway coupling omitted for clarity. 
     FIG. 21 is a front view of another embodiment with a cut-away portion showing a breakaway coupling. 
     FIG. 22 is a cross-sectional top view taken along line  22 — 22  of FIG. 21, but with the roll-up panel and wiring leading to the breakaway coupling omitted for clarity. 
     FIG. 23 is the same view as FIG. 21, but with one of the breakaway couplings disengaged. 
     FIG. 24 is a cross-sectional top view taken along line  24 — 24  of FIG. 23, but with the roll-up panel and wiring leading to the breakaway coupling omitted for clarity. . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     To provide a more durable and readily accessible elongated switch for use along a lower portion of a roll-up door panel releasably held by a breakaway coupling, the embodiment of FIGS. 1-4 detects electrical continuity through the coupling itself. Referring to FIG. 1, a roll-up door  10  includes a pair of vertically extending members such as vertical side frames  12  that supports a roller  14  upon which a flexible roll-up panel, such as a fabric curtain  16 , is wound and unwound to respectively open and close the door. In this example, a motor drive unit  18  drives roller  14  to feed panel  16  up and down as vertical slits  20  in frame  12  guide side edges  22  of curtain  16  generally along a vertical plane across the doorway. A lower portion  24  of curtain  16  includes a compliant sealing member  26  at the very bottom or leading edge  28  of the curtain to ensure that the curtain seals against the floor when door  10  is closed. 
     To help keep curtain  16  within slits  20 , as well as help keep curtain  16  taut and square to the doorway under normal operation, and yet still release curtain  16  in the event of a collision, two breakaway couplings  32  releasably couple opposite ends  34  and  36  of lower portion  24  to two opposing carriages  38  or trolleys. In its broadest sense, only one breakaway coupling  32  is needed, but two is preferred. In some embodiments, curtain  16  is kept relatively taut by couplings  32  pulling an elongated member  56 , such as a steel cable, in tension. Other examples of elongated member  56  include, but are not limited to, a fabric strap or an integral fabric portion of curtain  16  itself. However, it should be noted that if desired, the leading edge of curtain  16  could be left relatively loose by not applying tension to member  56 . In such a case, member  56  would first be forced into tension by exertion of an external force upon the door as could by created by a collision. 
     To protect a door in the event of a collision, a breakaway feature can be provided by a variety of structures. For example, in this exemplary embodiment, breakaway couplings  32  are attached to first members such as carriages  38  that include rollers  40  attached to a bracket  42 . Rollers  40  and bracket  42  conform to the shape of frame  12  (see FIG. 2) to constrain carriage  38  to travel along tracks  44 , as door  10  opens and closes. In this example, tracks  44  are provided by the contour of frames  12 . Each breakaway coupling  32  includes an inner coupling member  46  that releasably engages an outer coupling member  48  to provide a breakaway connection therebetween. Under normal door operation, couplings  32  remain intact, i.e., their coupling members  46  and  48  remain connected to each other and move together. However, in the event of a collision creating a force sufficient to disconnect either breakaway coupling  32 , allowing independent relative movement between the members, the resulting separation of coupling members  46  and  48  protects the rest of the door (especially curtains  16 ) from damage. A disconnectable coupling or breakaway connection can be provided by any one of a wide variety of available mechanisms including, but not limited to, various fittings that mechanically snap together and apart. However, in some preferred embodiments, the disconnectable joint is provided by magnetic attraction between coupling members  46  and  48 . Of course, breakaway couplings may also be provided between trolleys and the rigid bars associated with hard edge doors. The teachings herein are intended to apply to such hard edge doors as well as the soft edge doors specifically described. 
     In this example, each outer coupling member  48  includes a magnet  50 , while each inner coupling member  46  is of a material that is attracted to magnet  50  (e.g., a ferromagnetic material, such as iron or an iron alloy). Magnet  50  is pivotally connected to bracket  42  by way of a hinge  52  that includes a torsional spring  54  that biases the position of magnet  50  generally away from the center of the doorway and towards side frame  12 . A similar arrangement is provided at both the right and left side of the doorway. Elongated member  56  connects the two inner coupling members  46  to each other. In this example, the elongated member is a conductor in the form of an electrically conductive steel cable  56  that runs through an elongated aperture  58  extending horizontally across curtain  16 . 
     Under normal operation, cable  56  is kept taut across the width of the doorway by a face  60  of each inner coupling member  46  being magnetically clamped to the magnet  50  of its respective outer coupling member  48 . However, when a collision occurs (i.e., the door strikes an obstacle or something strikes the door) that deflects cable  56  with sufficient force to overcome the magnetic attraction of either breakaway coupling  32 , the two halves of the coupling will separate, as shown near the left side of FIGS. 3 and 4. Note that outer coupling member  48  being restrained by side frame  12  enhances this action. When this occurs, usually part of the curtain pulls out of slit  20  as well. Also, for the magnet  50  that breaks away, the spring loaded hinge  52  urges the magnet to swing back and magnetically cling to the side of frame  12 , which prevents the disengaged trolley  38  from slamming to the floor. Further details of the construction, operation and various alternate embodiments of a magnetic breakaway coupling are disclosed in U.S. Pat. No. 5,638,883, which has already been incorporated by reference herein. 
     As outer coupling member  48  alternately engages and separates from inner coupling member  46 , their mating surfaces,  62  and  60 , respectively, can serve as electrical contacts of a switch, i.e., a device whose electrical conductivity changes in response to an action. The switch can be used to convey or interrupt an electrical signal in reaction to the breakaway coupling separating. The electrical signal, in turn, can be used to activate an alarm or inhibit continued normal operation of the door, until the separated coupling and the rest of the door are returned to normal, i.e., each coupling is connected and curtain  16  is properly within slits  20 . For the breakaway system of FIGS. 1-4, disabling the operation of door  10  can be carried out by any one of a variety of circuits. In FIG. 1, for example, an electrical power source  64  (e.g., 24 VAC) delivers current in series through a coil  66  of a relay  68 , a wire  70 , electrically conductive bracket  42 , electrically conductive hinge  52 , the left outer coupling member  48  (being electrically conductive itself), the left inner coupling member  46  (also being electrically conductive and while engaging magnet  50 ), cable  56  (or a conductive wire parallel thereto in the case of a nonconductive elongated member), the right inner coupling member  46 , the right outer coupling member  48  (while engaging the right inner coupling member  46 ), right hinge  52 , right bracket  42  and a wire  76 . Wire  76  leads back to power source  64  to complete a sensing circuit  78  when both breakaway couplings  32  are intact. The completed circuit energizes coil  66  to close relay contacts  80  to be used as desired. For example, in some embodiment, relay contacts  80  enable a motor control circuit  82 , such as a conventional reversing motor starter that controls the operation of motor  18 . When either coupling  32  breaks away, its corresponding coupling halves  46  and  48 , which in this example serve as electrical contacts, separate to interrupt the continuity of sensing circuit  78 . When this happens, coil  66  de-energizes to open relay contacts  80 , which in turn disables motor control circuit  82  to stop motor  18 . Stopping motor  18  avoids jamming the door and damaging curtain  16  by preventing roller  14  from attempting to forcibly raise or lower a curtain that is uncoupled from one or both of its carriages  38 . However, it should be appreciated by those skilled in the art, that sensing circuit  78  could be independent of the operation of motor control circuit  82 . For example, circuit  78  could be used simply to activate an audible or visual alarm, or increment a counter that indicates how often door  10  has been subjected to an impact that caused it to break away. 
     The system shown in FIGS. 1-4 thus senses the exertion of a force above a predetermined magnitude on the curtain. To achieve this, sensing circuit  78  is included, and a conductor (cable  56 ) forms a part of the circuit and extends across the width of the doorway. For normal door operation when no force above the predetermined magnitude is exerted thereon, the conductor is an electrically conductive part of the sensing circuit. When a force above the curtain magnitude is exerted on the curtain, however, the conductor no longer forms a conductive part of the circuit. Here, this is due to the fact that the coupling members separate, electrically isolating the conductor from the remainder of the circuit. 
     For the exemplary embodiment just described, it should be appreciated by those skilled in the art, that the wiring diagram of sensing circuit  78  and motor control  82  are schematically illustrated in FIGS. 1 and 3. Much of the circuit and curtain  16  are omitted in FIGS. 2 and 4 to more clearly show other components of the breakaway system. In FIGS. 1 and 3, a simple loop  84  is shown to depict that wires  70  and  76  flex within a flexible cable carrier (e.g., a Model 06-10-028, of IGUS, Inc. from Providence, R.I.) disposed within frame  12  to follow the vertical movement of carriages  38  along tracks  44 . However, the actual path along which the wires are laid; the actual positions of the circuit components; and the actual location of where the wiring connects to the components, including carriage  38  and coupling  32 , can vary widely depending on personal preference and design details of the specific roll-up door to which the breakaway system is applied. In some embodiments, for example, cable  56  can be replaced by a non-conductive fabric strap with an electrical wire connected parallel thereto that electrically couples the two inner coupling members to each other. 
     In some embodiments, some components such as bracket  42  and hinge  52  are relied upon as electrical conductors in lieu of wires or jumpers, such as optional redundant jumper wires  72  and  74 . However, when doing so, some precautions need to be taken. For example, when bracket  42  is relied upon as an electrical conductor to complete sensing circuit  78 , bracket  42  should be electrically insulated from side frame  12 . This can be done by maintaining an air gap  86  between bracket  42  and frame  12  as shown in FIGS. 2 and 4, or by using various electrically resistive plastic bearing pads and rollers to keep the conductive parts of bracket  42  from contacting frame  12  (i.e., shorting out). Jumper wires  72  and  74  are shown as optional conductors to complete circuit  78  in an embodiment where bracket  42  and hinge  52  are not relied upon to conduct electrical current. 
     If desired, a circuit breaker or resettable fuse (e.g., a Model MF-R020, of Bourns, Inc. of Riverside Calif.) can be used to protect circuit  78  in the event of an electrical short or current overload. This is particularly important, as magnet  50  short circuits circuit  78  to a grounded frame  12  whenever coupling  32  associated with the magnet breaks away. It should be further noted that while the conductor in this embodiment, which extends across the width of the doorway and selectively either forms or does not form a conductive part of the sensing circuit, is carried on the door curtain, this need not be so. Rather, the conductor could extend across the width of the doorway at other locations and still perform its conducting/non-conducting function. 
     The embodiment of FIGS. 5-8 is similar to the one just described, however, cable  56  is replaced by a two-conductor cable  88 . And each breakaway coupling  90  and  92  has two sets of electrical contacts for a total of eight contacts  94   a-h  with contacts  94   d  and  94   e  sharing a common node at magnet  50 . Contacts  94   a  and  94   h  are respectively provided by separate magnets  96  and  98  that are electrically conductive, but are insulated from hinge  52  and carriage  38  by way of a nonconductive shim  100 . Each inner coupling member  108  and  112  includes an electrically nonconductive core  101  that electrically separates its respective contacts  94   b  and  94   g  (coupling member  108 ) and contacts  94   c  and  94   f  (coupling member  112 ). This arrangement allows wires  102  and  104  to share a common cable carrier disposed inside just one side frame  12  (e.g., the left or right side of the doorway). 
     Referring to FIG. 5, under normal door operation, power source  64  delivers current in series through coil  66 , wire  104 , magnet  96 , a first contact  94   a  of a left outer coupling member  106 , a second contact  94   b  of a left inner coupling member  108 , a first wire  110  of cable  88 , a third contact  94   c  of a right inner coupling member  112 , a fourth contact  94   d  of a right outer coupling member  114 , magnet  50 , a fifth contact  94   e , a sixth contact  94   f , a second wire  116  of cable  88 , a seventh contact  94   g  of left inner coupling member  108 , magnet  98 , and wire  102 . Wire  102  leads back to power source  64  to complete a sensing circuit  119  when both breakaway couplings  90  and  92  are intact. The completed circuit energies coil  66  to close relay contacts  80 , which enable the operation of motor  18  to open or close the door. 
     When either coupling  90  or  92  breaks away in reaction to a collision, its corresponding coupling halves separate to interrupt the continuity of sensing circuit  119 . If coupling  92  on the right breaks away, as shown in FIGS. 7 and 8, contact  94   c  and  94   f  separate from the combined contacts  94   d  and  94   e  that are disposed on the face of magnet  50 . If coupling  90  on the left breaks away, contacts  94   a  and  94   b  separate, and so do contacts  94   g  and  94   h . If either coupling  90  or  92  separates, the continuity of circuit  119  is interrupted to disable the operation of motor  82 , thus stopping the opening or closing of the door. The door is reset to normal operation by placing curtain  16  back into slits  20  and reconnecting the two halves of each breakaway coupling  90  and  92  that may have separated. 
     Although inner coupling halves  108  and  112  are shown connected to each other by cable  88 , in some embodiments, another elongated member such as a fabric strap or an integral portion of the door curtain itself extends across the width of curtain  16  and generally parallel to cable  88  to hold the two halves  108  and  112  together, which thus relieves the tension in wires  110  and  116  of cable  88 . 
     In a similar embodiment, shown in FIGS. 9-12, contacts  94   c,d,e,f  of FIGS. 5 and 7 are replaced by an electrical switch  118 . Switch  118  is disposed on a right inner coupling member  120  of a breakaway coupling  122  and includes open biased contacts  124  and  126  that are held closed during normal operation of the door. Magnet  50  of outer coupling member  114  at the right side of the door magnetically clings to ferromagnetic blocks  128  that are on inner coupling member  120 . As magnet  50  magnetically clamps against blocks  128 , magnet  50  also depresses a switch actuator  130  that closes contacts  124  and  126  of switch  118 . When closed, contacts  124  and  126  provide electrical continuity between wires  110  and  116 . That continuity was previously provided by contacts  94   c, d, e, f  of the embodiment of FIGS. 5-8. When coupling  122  breaks away, as shown in FIGS. 11 and 12, actuator  130  returns to its normally extended position to open contacts  124  and  126  (i.e., break their continuity). This interrupts the current to relay  68  to activate an alarm, or disable motor  18  to stop the door. 
     The left breakaway coupling  90  of FIGS. 9-12 is the same as the one in the embodiment of FIGS. 5-8. It might also be noted that in FIGS. 11 and 12, both breakaway couplings  90  and  122  are shown in their uncoupled state, as this could actually occur in some collisions. 
     In some applications, it might be beneficial to eliminate the need to extend an electrical conductor across the width of the door curtain. This is accomplished in the embodiment of FIGS. 13-16, wherein both breakaway couplings  131  are basically the same, and their outer coupling halves  106  are the same as the left outer one of FIGS. 9-12. Each outer coupling member  106  includes a pair of spaced-apart magnets  96  and  98  that are electrically insulated from the rest of the coupling member by way of electrically nonconductive shim  100  between hinge  52  and magnets  96  and  98 . Each pair of magnets  96  and  98  provide a corresponding pair of electrical contacts:  132  and  134  on the left and  136  and  138  on the right. Each pair of contacts are shorted out (i.e., electrically connected to each other) by an inner coupling member  46 , which is the same as those used in the embodiment of FIGS. 1-4. However, the two inner coupling halves  46  are connected to each other by an elongated member  140  that does not need to be electrically conductive, such as for example, a cable, strap, or an integral portion of the door curtain itself. 
     During normal door operation, power supply  64  delivers current in series through relay  68 , wire  104 , contacts  132 , left inner coupling member  46 , contacts  134 , a second wire  142  that leads up and over to the right breakaway coupling  131 , contacts  138 , right inner coupling member  46 , contacts  136  and wire  144 . Wire  144  leads back to power supply  64  to complete a sensing circuit  147  that energizes relay  68  to enable motor  18  to open or close the door. 
     When either of couplings  131  are forced to break away, the separation of an inner coupling member  46  from its corresponding outer coupling member  106  opens contacts  132  and  134  or  136  and  138 , accordingly. In the example shown in FIGS. 15 and 16, the left breakaway coupling  131  separates to interrupt the continuity of circuit  147 , which de-energizes relay  68  to disable the normal operation of the door. The door is returned to normal operation by placing curtain  16  back into slits  20  and reconnecting the two halves of the left breakaway coupling  131 . 
     Another breakaway system that eliminates the need for extending an electrical conductor across the width of the door curtain is shown in FIG. 17-20. In this example, switch  118  (described earlier in reference to FIGS. 9-12) is attached to each outer coupling member  148  of breakaway couplings  150 . Each switch  118  is disposed within or adjacent a magnet  152  with the switch&#39;s actuator  130  depressed by an inner coupling member  146  that is magnetically drawn up against magnet  152 , as shown in FIGS. 17 and  18 . The two inner coupling halves  146  are connected to each other by elongated member  140  that does not need to be electrically conductive, such as for example, a cable, strap, or an integral portion of the door curtain itself. 
     During normal operation of the door, current from power supply  64  passes in series through relay  68 , a wire  154 , closed contacts  124  and  126  on the left breakaway coupling, a wire  156 , closed contacts  124  and  126  on the right breakaway coupling, and back to power supply  64  through a wire  158  to complete the continuity of a sensing circuit  160 . This energizes relay  68  to enable motor  18  to open or close the door. 
     When either of couplings  159  are forced to break away, the separation of an inner coupling member  146  from its corresponding outer coupling member  148  allows the switch actuator  130  associated with the separated coupling to open its contacts  124  and  126 . In the example shown in FIGS. 19 and 20, the left breakaway coupling  150  separates to interrupt the continuity of circuit  160 , which de-energizes relay  68  to disable the normal operation of the door. The door is returned to normal operation by placing curtain  16  back into slits  20  and reconnecting the two halves of the left breakaway coupling  150 . 
     FIGS. 21-24 illustrate another embodiment of a breakaway system that is very similar to the embodiment of FIG. 17-20. However, instead of switches  118  with normally open contacts held closed, the breakaway system employs switches  162  that have normally closed contacts. One switch  162  on a left breakaway coupling  164  has contacts  166  and  168 , and another switch  162  on the right breakaway coupling  164  has contacts  170  and  172 . Each breakaway coupling includes a magnet  174  on an outer coupling member  176  that magnetically clings to inner coupling member  146 . The two inner coupling halves  146  are connected to each other by elongated member  140  that does not need to be electrically conductive, such as the examples mentioned earlier. 
     During normal operation of the door, current from power supply  64  passes in series through relay  68 , a wire  178 , normally closed contacts  166  and  168  on the left breakaway coupling, a wire  180 , normally closed contacts  170  and  172  on the right breakaway coupling, and back to power supply  64  through a wire  182  to complete the continuity of a sensing circuit  184 . This energizes relay  68  to enable motor  18  to open or close the door. 
     When a coupling  164  breaks away, for example, the left breakaway coupling  164  of FIGS. 23 and 24, the coupling&#39;s spring-loaded hinge  52  swings its magnet  174  and its adjacent switch  162  up against the side of frame  12 . The side of frame  12  depresses the switch&#39;s actuator  130  to open contacts  166  and  168 , which interrupts the continuity of circuit  184 . This, in turn, de-energizes relay  68  to disable the normal operation of the door. Although frame  12  is the structure that actuates switch  162  as hinge  52  moves the switch, the actuation could be carried out by a variety of other structures in the vicinity, including but not limited to the hinge itself. The door is returned to normal operation by placing curtain  16  back into slits  20  and reconnecting the two halves of the left breakaway coupling  164 . 
     Although the invention is described with respect to preferred embodiments, modifications thereto will be apparent to those skilled in the art. For example, in providing a breakaway coupling that includes two coupling halves that are magnetically attracted to each other, either coupling member could be the magnet with the other coupling member being of a material attracted to the magnet. Also, one coupling member could be an integral component or extension of carriage  38  itself. For instance, it is well within the scope of the invention to eliminate hinge  52  and provide an inner coupling member with a magnet that clings directly to bracket  42  of carriage  38 . In such a case, the portion of bracket  42  that engages the magnet would serve as the outer coupling member. Since other modifications will be apparent to those skilled in the art, the scope of the invention is to be determined by reference to the claims, which follow.

Summary:
An impact sensing system for a powered roll-up door combines an electrical switch and a breakaway coupling. To avoid damage or injury resulting from the door&#39;s roll-up curtain accidentally closing upon an obstacle or something striking the curtain, the breakaway coupling responds to such a collision by breaking away, which releases a lower portion of the curtain from between its two vertical guide tracks. Each breakaway coupling includes a set of electrical contacts that make or break in response to the coupling breaking away. When the curtain&#39;s lower portion becomes effectively derailed from its guide track, the electrical contacts disable continued operation of the door to prevent the door&#39;s drive motor from jamming the curtain. In some embodiments, the breakaway coupling is releasably held together by way of magnetic attraction between two coupling segments, with one electrical contact on each segment to comprise one set of functional contacts.