Abstract:
A normally open thermal switch ( 200 ) having a bimetallic disk ( 18 ) is configured for operational testing in its installed position when exposed to a changing temperature by a test box ( 400 ) having a power source ( 400   a ). The in-place testing advantageously confirms triggering action of the switch by an event indicator ( 400   c ) at the operational temperatures designed into the switch ( 200 ). The temperature of the triggering action is presented on a temperature display ( 400   b ) and recorded by a data recorder ( 400   d ) of the test box ( 400 ). The switch ( 200 ) incorporates a heating element ( 24   c ) to heat changing the bimetallic disk ( 18 ) to snap activate at the operative temperatures. The thermal switch ( 200 ) is coupled with the test box ( 400 ) to confirm its operation without having to remove the switch from its installed location.

Description:
BACKGROUND OF THE INVENTION 
   Thermal switches are used in a variety of applications where it is desirable to activate and/or deactivate equipment as a function of sensed temperature. Such applications may include: rocket motors and thrusters, battery charge rate control, temperature control for fuel systems, environmental controls, overheat protection as well as many others. In several thermal switch applications, it is desirable to know when the switch has been activated and at what temperature. For example, it is desirable to know that the switch is functioning correctly when the switch is part of a safety system or is part of a control system used to protect equipment. Snap-action thermal switches are utilized in a number of applications, such as temperature control and overheat detection of mechanical devices such as motors and bearings. In some applications, multiple thermal switches are located at different positions around the equipment. For example, in some aircraft wing, fuselage, and cowling overheat detection applications, multiple thermal switches are located just behind the leading edge flap, while other thermal switches are spaced along the length of each wing. Additional thermal switches are located in the engine pylon and where the wing attaches to the fuselage. In this example, the multiple thermal switches are connected electrically in parallel, such that just two wires are used to interface between all of the switches on each wing and an instrument that monitors the temperature of the aircraft&#39;s wing, fuselage, and cowling. 
   Current snap-action thermal switch designs typically provide open and closed functions only. Typically, all of the thermal switches in the aircraft wing, fuselage, and cowling overheat detection applications are operated in the normally open state. The thermal switches are thus all in the “open” state until an overheat condition is detected, at which time one or more of the switches change to the “closed” state, thereby completing the circuit causing a “right wing,” “left wing” or “fuselage” overheat indication to appear in the cockpit. The pilot then follows the appropriate procedure to reduce the overheat condition. 
   Current snap-action thermal switches used in parallel operation, multiple thermal switch overheat detection systems suffer from various drawbacks. The integrity of the wire harness between the cockpit and the wing tip cannot be assured because the circuit is always open under normal operating conditions. If a switch connector is not engaged or the wire harness contains a broken lead wire, a malfunction indication will not occur, but neither will the overheat detection system operate during an actual in-flight overheat condition. Furthermore, if an overheat condition does occur, current snap-action thermal switches are not equipped to provide information describing the exact location of the overheat. In both instances, flight safety is compromised, and later correction of the problem that caused the overheat condition is made more difficult because of the inability to pinpoint the overheat fault. 
   One application for thermal switches that clearly illustrates the disadvantages of prior art devices is duct leak overheat detection systems. The duct leak overheat detection system is part of the aircraft deicing system. In this type of deicing system, hot air is forced pneumatically through a tube along the leading edge of the wing. Thermal switches located along this duct, indicate overheating, which could otherwise lead to structure failure and other system failures. When a thermal switch is tripped, a light illuminates in the cockpit indicating a “right” or “left” wing overheat condition. If, after shutting the system down on the appropriate wing, the switch does not reset, the airplane must divert to an emergency landing. Upon landing, the airplane maintenance personnel have no way of knowing which particular switch has been activated, because there exist multiple thermal switches linked to a particular cockpit light. The existing airplane systems have only provided the crew with an indication of the particular wing semispan along which a thermal switch was tripped. If the switch has reset, there is no indication to the maintenance personnel that it was tripped by the overheat condition. This dearth of information requires the crew to physically access and inspect the entire system along the appropriate wing semispan. Even in applications where only one temperature probe indicated an alarm temperature in-flight, extensive and expensive troubleshooting is sometimes necessary. For example, an airborne alert from a temperature probe in aircraft turbine bleed air ductwork may require engine run-up and monitoring on the ground to determine whether the probe and/or the bleed air system is faulty. 
   SUMMARY OF THE EMBODIMENTS 
   Embodiments provide a thermal switch test system that provides a ready indication that the thermal switch has experienced temperatures that triggered operation of the switch. Particular embodiments include a thermal switch with a heating element and a test box that is able to be coupled to the thermal switch at the installed position of the thermal switch so that temperature responsive actuator testing of the thermal switch may be conducted in situ, i.e., at the installed position of the thermal switch. The in situ testing of the thermal switch permits the advantageous testing without incurring the cost and inconvenience of thermal switch removal. 
   A particular embodiment includes a thermal switch having two pairs of four contacts in communication with a test box having an electrical power source, a temperature display, an event indicator, and a data recorder. The event indicator and temperature display communicates with the data recorder. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings. 
       FIG. 1  is a top plan view of one alternative embodiment of the thermal switch with self-test feature embodied as a snap-action thermal switch having leads to a heating element; 
       FIG. 2  is a cross-sectional side view of the snap-action thermal switch with self-test feature showing the leads coupled with the heating element; 
       FIG. 3  is a top plan view of another alternative embodiment of the thermal switch with self-test feature embodied as a snap-action thermal switch having leads to a heating element and leads to a temperature sensing thermalcouple; 
       FIG. 4  is a cross-sectional side view of the snap-action thermal switch with self-test feature showing the leads coupled with the heating element and leads to the temperature sensing thermalcouple; 
       FIG. 5  is a pictorial presentation of one test box embodiment coupled with a housing having one embodiment of thermal switch with self-test feature; 
       FIG. 6  is a pictorial presentation of another test box coupled with a housing having another embodiment of thermal switch with self-test feature; 
       FIG. 7  is a pictorial presentation of a coupling schematic of the one test box embodiment coupled with one embodiment of the thermal switch with self-test feature; 
       FIG. 8  is a pictorial presentation of another coupling schematic of the other test box embodiment coupled with another embodiment of thermal switch with self-test feature; and 
       FIG. 9  is a pictorial presentation of the one and another test box embodiments ready for coupling to installed one and other embodiments of the thermal switch with self-test features located on an aircraft. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  is a top plan view of one embodiment of a thermal switch  200  embodied as a snap-action thermal switch having leads to a heating element.  FIG. 2  is a cross-sectional side view of the snap-action thermal switch  200  showing the leads coupled with the heating element. The thermal switch  200  depicted in  FIGS. 1 and 2  is configured in a normally open position. A switch configuration that is normally in the closed is also within the scope of this one embodiment. The thermal switch  200  has two additional leads  24   a  and  24   b  which are electrically isolated from a header  33 . The leads  24   a  and  24   b  are coupled to a heating element  24   c . Circumscribing the terminals  20  and  22  are glass insulators  28 . The insulators  28  separate the terminals  20 ,  22  from the header  33 . 
   The thermal switch  200  includes a pair of electrical contacts  14 ,  16   b  that are mounted on the ends of a pair of spaced-apart, electrically conductive terminals  20  and  22 . The electrical contacts  14 ,  16   b  are moveable relative to one another between an open and a closed state under the control of a thermally responsive actuator  18 . The contact  16   b  is moveable via an armature spring  16 . The spring  16  is attached to the terminal  22 . The contact  14  is non-moveable or fixed. When the contact  16   b  touches the contact  14 , a closed circuit exists. Whenever the contact  16   b  is spaced from or otherwise does not touch the contact  14 , an open circuit exists. 
   According to one embodiment of the invention, the thermally responsive actuator  18  is a snap-action bimetallic disc that inverts with a snap-action as a function of a predetermined temperature between two bi-stable oppositely concave and convex states. The movement of the actuator  18  is conveyed to the moveable contact  16   b  via an intermediary striker pin  19 . The striker pin  19  is configured to transfer force or otherwise engage with the actuator  18  and the armature spring  16 . It also provides electrical isolation beneath the switch and the expandable case. 
   In a first state, the bimetallic disc actuator  18  is convex relative to the relatively moveable electrical contacts  14 ,  16   b , whereby the electrical contacts  14 ,  16   b  are moved apart such that they form an open circuit. In a second state, the bimetallic disc actuator  18  is concave relative to the relatively moveable electrical contacts  14 ,  16   b , whereby the electrical contacts  14 ,  16   b  are moved together such that they form a closed circuit. 
     FIG. 3  is a top plan view of one alternative embodiment of a thermal switch  300  having leads  24   a  and  24   b  to a heating element  24   c  and leads  26   a  and  26   b  to a temperature sensor  26   c .  FIG. 4  is a cross-sectional side view of the snap-action thermal switch  300  showing the leads  24   a  and  25   b  coupled with the heating element  24   c  and the leads  26   a  and  26   b  coupled with temperature sensor  26   c . The thermal switch  300  depicted in  FIGS. 3 and 4  is configured in a normally open position, but can be implemented in a normally closed position. Circumscribing the terminals  20  and  22  are glass insulators  28 . The insulators  28  separate the terminals  20 ,  22  from the header  33 . 
     FIG. 5  is a pictorial presentation of one test box  400  for use with the thermal switch  200  shown in  FIGS. 1 and 2 . The thermal switch  200  is included in a housing  220 . In one embodiment, the test box  400  includes a female coupling with ports that connect to pins in the housing  220  that electrically connect to the leads  24   a  and  24   b  and to the posts  20  and  22 . A wire harness or other cabling means may serve to connect the test box  400  to the installed housing  220 . For example, the thermal switch  200  is fixed within the housing  220  that in turn is installed in a bleed air duct of an aircraft. In one embodiment, the test box  400  includes a power source  400   a  (such as an adjustable power source), a display  400   b , an event indicator  400   c , a data storage device  400   d , and a processing component  402 . The processing component  402  is coupled to the power source  400   a , the display  400   b , the event indicator  400   c , and the data storage device  400   d . The processing component  402  may be a microprocessor configured to process temperature-related and time-related signals associated with the operational status of the thermal switch  200 . This is described in more detail below in  FIG. 6 . 
     FIG. 6  is a pictorial presentation of a coupling schematic of the test box  400 . The test box  400  is designed to display a signal indicating a change of contact status between the leads  20 ,  22 . The change in contact status may be from a normally open position to a closed position, or a normally closed position to an open position between the leads  20 ,  22 . The test box  400  also displays the temperatures at which the change in contact status occurred. 
   A power source  400   a  controlled by a processing component  402  delivers electrical current to the heating element  24   c  via the leads  24   a ,  24   b . The power source  400   a  can be adjustable via a mechanically turnable knob, adjusted by keyboard entry or by some other means. Depending on the electrical power delivered to the heating element  24   c  and duration of the delivered power, a temperature value is determined by the processing component  402  and sent to the display  400   b  for presentation. The temperature value includes a movement-generating temperature that causes the actuator to move. For example, when the actuator is in the form of a bimetallic disk  18 , the bimetallic disk  18  snaps or toggles. The snapping of the bimetallic disk  18  causes the contact  16   b  to close and touch the fixed contact  14 . A current signal is then sent via the terminals  20 ,  22  to the event indicator  400   c  and an event is signaled by the indicator  400   c  either visually or audibly. The processing component  402  records the temperature value of the movement-generating temperature at the time the switch  200  toggles and stores it in the storage device  400   d . The test box  400  may be wirelessly or hard-wire linked to another device for extracting the information recorded on the storage device  400   d.    
     FIG. 7  is a pictorial presentation of another test box  450  for use with the thermal switch  300  shown in  FIGS. 3 and 4 . The thermal switch  300  is included in a housing  240 . In one embodiment, the test box  450  includes a female coupling with ports that connect to the pins in the housing  240  that electrically connect to the leads  24   a  and  24   b , and  26   a  and  26   b  and to the posts  20  and  22 . A wire harness or other cabling means may serve to connect the test box  450  to the installed housing  240 . For example, the thermal switch  300  is fixed within the housing  240  that in turn is installed in a bleed air duct of an aircraft. The test box  450  similarly includes the multiple components of the test box  400  but configured differently as described below in  FIG. 8 . 
     FIG. 8  is a pictorial presentation of a coupling schematic of the test box  450  with the thermal switch  300 . Similar to the test box  400 , the test box  450  is designed to display a signal indicating a change of contact status between the leads  20 ,  22 . The change in contact status may be from a normally open position to a closed position, or a normally closed position to an open position between the leads  20 ,  22 . The test box  450  also displays the temperatures at which the change in contact status occurred. 
   The test box  450  includes a processing component  458  coupled to a power source  460 , a display  462 , an indicator  464 , and a storage device  466 . The power source  460  as controlled by the processing component  458  delivers electrical current to the heating element  24   c  via the leads  24   a ,  24   b . The power source  460  can be adjustable via a mechanically turnable knob, adjusted by keyboard entry or by some other means. The actual temperature experienced within the internal spacing of the thermal switch  300  is measured by the temperature sensor  26   c . The processing component  458  instructs the display  462  to present the measured temperature. When the bimetallic disk  18  snaps, the contact  16   b  closes and touches the plate  14 . A current signal is then sent via the leads posts  20 ,  22  to the indicator  464  and the event is signaled by the indicator  464  either visually or audibly. The processing component  458  records the temperature value at the time the switch  300  toggles and stores it in the storage device  466 . The test box  450  may be wirelessly or hard-wire linked to another device for extracting the information recorded on the storage device  466 . 
     FIG. 9  is a pictorial presentation of the test boxes  400  and  450  for use of the thermal switches  200  and  300  on an aircraft. An aircraft  500  is shown with a distribution of installed thermal switches  200  and  300  within a wing structure  504 . For example, multiple switches  200  are installed on the aft section of the  504  and multiple switches  300  are installed on a forward section of the wing  504 . The in situ or in-place testing of the installed switches  200  is achieved via the coupling and operation of the test box  400 . Similarly, the in situ or in-place testing of the installed switches  300  is achieved via the coupling and operation of the test box  450 . 
   The aircraft  500  includes left (L) and right (R) cockpit indicators  506  and  508 . The cockpit indicators  506  and  508  indicate when the switches  200  and  300  in the respective wing (left or right) have toggled. The test boxes  400  and  450  may be coupled to the respective cockpit indicator  506  and  508  at the cable end that is connected to the switch housing  220  or  240 . When cockpit lights are respectively on or off in accord with the event indicator  400   c  or  464 , then the operational integrity between the thermal switches  200 ,  300  and the cockpit indicators  506  or  508  is good. In the event the cockpit indicators do not light in accord with a signal sent from the event indicator  400   c  or  464  then the connection of the cabling between the cockpit indicators  506  or  508  and the switches  200 ,  300  is bad. 
   While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. For example, the test box  400  or the test box  450  may be configured without a processing component. In these test boxes the confirmation that the thermal switch operates as intended, that is, proving that a change in contact status between the leads  20 ,  22  has occurred at actuator movement-generating temperatures, is verified by a user directly viewing the event indicator at the moment of actuator movement or reviewing the event signal data stored by the data recorder. 
   Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.