Abstract:
A system and method for switching power sources for an aircraft electric brake system is disclosed. The method removes battery power from electric brake actuator controls during flight until landing gear extension occurs. The method utilizes a logic circuit to switch between available power sources based on a plurality of control signals. The method minimizes the total power drawn during flight, and saves battery power if the aircraft calls for operating on battery power only.

Description:
TECHNICAL FIELD 
     Embodiments of the present invention relate generally to aircraft power systems, and more particularly to aircraft electrical brake control power systems. 
     BACKGROUND 
     Historically aircraft braking control has been operated via direct cable or hydraulic connection. Cable and hydraulic control connections suffered from weight, performance and reliability issues. Many of these issues have been improved upon by using electrically actuated and controlled brake systems. Electrically actuated and controlled brake systems are colloquially referred to as “brake by wire” systems. 
     A brake by wire system is usually electrically powered by both the aircraft system power and a backup battery. An electric brake actuation unit (EBAC) is a high power subsystem of a brake by wire system. The EBAC and other loads are connected to the battery during flight. The battery supplies backup power to its connected loads so if a loss of active power occurs in flight, the battery can support those loads that are fed by it. The battery is connected to the loads by a switch that is usually on in flight. 
     Because braking is not required during flight, it is desirable to remove the power from the EBAC so that power is saved in flight for use by other loads. Other desirable features and characteristics of embodiments of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background. 
     BRIEF SUMMARY 
     A system and method for switching power for an aircraft electric brake system is disclosed. The method receives control signals from the aircraft electric brake system and utilizes a logic circuit to switch between active power supply units and a battery power supply unit based upon the control signals. The method removes the battery power from the EBACs during flight thereby minimizing total power drawn on the battery power supply unit, and saving the battery power unless the aircraft operation calls for operating on battery power only. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the present invention may be derived by referring to the detailed description and claims when considered in conjunction with the following figures, wherein like reference numbers refer to similar elements throughout the figures. 
         FIG. 1  is a schematic representation of a power switching system for an aircraft electric brake system; 
         FIG. 2  is a flow chart illustrating a process for switching power for an aircraft electric brake system; and 
         FIG. 3  illustrates an example embodiment of a logic circuit for switching power for an aircraft electric brake system. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is merely illustrative in nature and is not intended to limit the embodiments of the invention or the application and uses of such embodiments. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description. 
     Embodiments of the invention may be described herein in terms of functional and/or logical block components and various processing steps. It should be appreciated that such block components may be realized by any number of hardware, software, and/or firmware components configured to perform the specified functions. For example, an embodiment of the invention may employ various electric brake actuators, integrated circuit components, e.g., memory elements, digital signal processing elements, logic elements, look-up tables, or the like, which may carry out a variety of functions under the control of one or more microprocessors or other control devices. In addition, those skilled in the art will appreciate that embodiments of the present invention may be practiced in conjunction with any number of digital data transmission protocols and/or aircraft configurations, and that the system described herein is merely one example embodiment of the invention. 
     For the sake of brevity, conventional techniques and components related to signal processing, aircraft braking, braking control, and other functional aspects of the systems and the individual operating components of the systems may not be described in detail herein. Furthermore, the connecting lines shown in the various figures contained herein are intended to represent example functional relationships and/or physical couplings between the various elements. It should be noted that many alternative or additional functional relationships or physical connections may be present in an embodiment of the invention. 
     The following description refers to elements or nodes or features being “connected” or “coupled” together. As used herein, unless expressly stated otherwise, “connected” means that one element/node/feature is directly joined to or directly communicates with another element/node/feature, and not necessarily mechanically. Likewise, unless expressly stated otherwise, “coupled” means that one element/node/feature is directly or indirectly joined to or directly or indirectly communicates with another element/node/feature, and not necessarily mechanically. Thus, although the schematics shown in the figures depict example arrangements of elements, additional intervening elements, devices, features, or components may be present in an embodiment of the invention (assuming that the functionality of the system is not adversely affected). 
     Embodiments of the invention are described herein in the context of one practical application, namely, a power switching system for an aircraft braking system. In this context, the example technique is applicable to provide redundancy and avoid inadvertent brake application on an aircraft. Embodiments of the invention, however, are not limited to such aircraft applications, and the techniques described herein may also be utilized in other applications. 
     In one embodiment, an electric brake actuator control (EBAC) is a high power device that consumes 4 kW peak power. This power consumption necessitates the EBAC being actively cooled by forced air in the airplane. Critical avionics must be able to survive a loss of cooling system event until landing is accomplished. By turning off the EBAC during flight until landing gear extension, most of the time is eliminated when an EBAC would need to withstand the loss of cooling event. Also, by removing the EBAC power from the power system until landing gear extension, total power draw on the battery is minimized for certain operational modes, such as when the airplane operates only from battery power. Additionally, by turning power to the EBAC completely off, an EBAC will not draw any power in what would be a low power sleep mode which could be used during aircraft towing, and will save battery power unless braking is commanded. In one embodiment, the power switching function is performed by electric brake power supply units (EBPSUs) as explained in detail in the context of  FIGS. 1-3  below. 
       FIG. 1  is a schematic representation of a power switching system  100  suitable for use with an aircraft electrical braking system. As shown in  FIG. 1 , the example power switching system includes a left side power switching configuration  102  configured to switch power for the left side electrical braking subsystem, and a right side electric power switching configuration  104  configured to switch power for the right side electrical braking subsystem. In this regard, having separate EBPSUs improves system availability and reliability. 
     The system described herein can be applied to any number of power switching configurations for an aircraft, and system  100  for switching power for an aircraft electric brake system is depicted in a generic manner to illustrate its deployment flexibility. In this example, the system  100  may include a left side power switching configuration  102  and a right side power switching configuration  104 . The terms “left” and “right” as used herein refer to the port and starboard of the aircraft respectively relative to the center line of the plane. These terms are used herein for convenience of description and are not intended to limit or restrict the scope or application of the invention in any way. In practice, the two architectures may be independently controlled in the manner described below. In operation, each power switching system can independently switch power. 
     The left side power switching configuration  102  may generally include: at least one left brake system control unit (“BSCU”)  126 , at least one left EBAC  110 / 118 , at least one left EBPSU  114 / 122  which includes at least one left power switching logic circuit  116 / 124 , at least one active power supply unit  112 / 120 , and a battery power supply unit  128 . This configuration  102  may be adapted to receive, transmit, exchange, or otherwise process a number of data and/or control signals. These signals may include, without limitation: at least one active power signal  134 / 142 , a battery power signal  136 , at least one left BSCU battery power enable/disable signal  146 / 150 , at least one left braking event signal  132 / 140 , at least one left braking power signal  133 / 141 , a battery ON/OFF switch signal (not shown in  FIG. 1 ), and at least one power distribution signal (reference numbers  130 / 144 / 138 / 148 ). 
     An embodiment may use any number of BSCUs but the example described below uses only one left side BSCU  126 . The left BSCU  126  is an electronic control unit that has embedded software to digitally compute the braking command. The electrical/software implementation allows further optimization and customization of braking performance and feel. The left BSCU  126  may be generally realized by a microcontroller, which includes suitable processing logic and software that is configured to carry out the left BSCU  126  operations described herein. The microcontroller may be a computer such as, without limitation, a PowerPC  555  that hosts software and provides external interfaces for the software. The left BSCU  126  monitors various airplane inputs to provide control functions such as, without limitation, pedal braking, parking braking, autobrake and gear retract braking for the left side electrical braking subsystem. In addition, the left BSCU  126  blends the antiskid command (which could be generated internal or external from the BSCU provide optimal control of braking). The left BSCU  126  obtains pedal control signals and wheel data such as wheel speed, rotational direction value for the wheels, and tire pressure. The left BSCU  126  processes its input signals and generates one or more left BSCU  126  output signals that are used as input to the left EBACs  110 / 118 . The left BSCU  126  can generate independent output signals for use by the left EBACs  110 / 118  under its control. The left BSCU  126  may be coupled to one or more left EBACs  110 / 118 . 
     In connection with the power switching technique described herein, the left BSCU is configured to generate at least one left BSCU battery power enable/disable signal  146 / 150 , wherein the at least one left BSCU battery power enable/disable signal  146 / 150  is configured to switch the battery power off to disconnect power from the at least one left EBAC  110 / 118  and/or the left BSCU  126 . 
     Each of the left EBACs  110 / 118  may be realized as a microcontroller which includes suitable processing logic and software that is configured to carry out the EBAC operations described herein. The microcontroller may be a computer such as, without limitation, a PowerPC  555  that hosts software and provides external interfaces for the software. Each EBAC  110 / 118  obtains BSCU output signals, processes those signals, and generates the actuator signals that are used to control the brake mechanisms for landing gear wheels. 
     The at least one left EBPSU  114 / 122  is coupled to at least one left EBAC  110 / 118  and to left BSCU  126 . The at least one left EBPSU  114 / 122  is configured to supply power to the left BSCU  126 , and to the at least one left EBAC  110 / 118 . The left EBPSUs  114 / 122  supply 28 volt power to the left BSCU  126  and the left EBACs  110 / 118  via the power distribution signals (reference numbers  130 / 144 / 138 / 148 ). Each of the EPBSUs  114 / 122  includes and/or communicates with the at least one left power switching logic circuit  116 / 124 . 
     The at least one left power switching logic circuit  116 / 124  is configured to switch (connect/disconnect) the battery power supply unit  128 , and the active power supply units  112 / 120  for the left EBACs  110 / 118  and the left BSCU  126  as needed in the manner described in more detail in the context of  FIGS. 2-3  below. 
     The battery power supply unit  128  is configured to supply power to the at least one left EBPSU. In this example, there is only one battery power supply unit  128  powering the left electric brake system components. Usually during the flight, the battery power supply unit  128  supplies power to the loads that are connected to it, so that if a loss of active power occurs in flight, the battery can support those loads that are fed by it. In this regard, when the active power sources are invalid, the battery will be supplying power continually. However, battery power supply unit  128  can supply power to the aircraft for a few minutes without the aircraft engines running. After the few minutes, the active power sources (powered by a ram air turbine that extends out of the aircraft) power the aircraft loads. 
     The at least one left active power supply unit  112 / 120  is coupled to the at least one left EBPSU  114 / 122  and is configured to supply active power for the at least one left EBAC  110 / 118 . The active power supply unit  112 / 120 , may be supplied/controlled, without limitation, for example by a transmitter/rectifier unit (TRU). Each individual left EBAC  110 / 118  may switch to obtain power from the battery power supply unit  128  or from the left active power supply unit  112 / 120 , unless a failure is indicated in one of the left active power supply units  112 / 120  in which case the left EBAC  110 / 118  gets power from the battery power supply unit  128 . The failure/validity of the left active power supply unit  112 / 120  may be determined by examining the left active power signal  134 / 142 . 
     The at least one left active power signal  134 / 142  is generated by the at least one left active power supply unit  112 / 120  and is indicative of validity of the at least one left active power supply unit  134 / 142 . The active power signal  134 / 142  may be, for example, about 28 volts. 
     The left battery power signal  136  is generated by the battery power supply unit  128  and is indicative of validity of the battery power supply unit  128 . The left battery power signal  136  may be, for example, about 28 volts. The battery power signal may be turned on or off by the battery ON/OFF switch. 
     The battery ON/OFF switch signal (not shown in  FIG. 1 ) is configured to initiate connecting/disconnecting the battery power supply unit  128  to/from the at least one left EBAC  110 / 118 . The battery ON/OFF switch is controlled externally by a battery switch (not shown in  FIG. 1 ). The battery switch may be located in a cockpit of the aircraft and is usually on during the flight. In this regard, when the active power sources are invalid, the battery will be supplying power for a limited time, as mentioned above, after which the active power sources power the aircraft loads. 
     The at least one left BSCU battery power enable/disable signal  146 / 150  is generated by the left BSCU  126  and is configured to disconnect/connect the battery power supply unit  128  from/to the at least one left EBAC  110 / 118 . The left BSCU battery power enable/disable signal  146 / 150  is used by the left BSCU  126  to keep the power from the battery power supply unit  128  on to the left side power switching configuration  102 . The left BSCU battery power enable/disable signal  146 / 150  is fed into a power switching logic circuit, as explained below, so that when the battery power switch is on, the left BSCU  126  can use the same left BSCU battery power enable/disable signal  146 / 150  to turn the EBAC  110 / 118  on and off without removing power from itself. If the battery switch is off, the BSCU removing this signal would cause the battery power to be removed from both itself and the EBACs. In this regard, since the EBACs are powered off during most of the flight, the reliability of the EBACs is improved. The left BSCU battery power enable/disable signal  146 / 150  may be, for example, a discrete signal that is either open or connected to ground. 
     The at least one left braking event signal  132  is generated by the left BSCU  126  and is indicative of occurrence of a braking event at the at least one left EBAC  110 / 118 . As mentioned above, the BSCU monitors various airplane inputs to provide control functions such as, without limitation, pedal braking, parking braking, autobrake, and gear retract braking. In this regard, the left BSCU  126  transmits the brake command to the left EBACs  110 / 118  via the at least one left braking event signal  132 / 140 . If the left braking event signal  132 / 140  indicates occurrence of a braking activity (such as landing) the left brake system reverts to getting power from the battery power supply unit  128  as explained in the context of  FIGS. 2-3  below. 
     The at least one left braking power signal  133 / 141  may be, for example, a signal of about 130 volts. In this regard, an open/ground discrete signal is used for power switching, and then the actual power is sent from the EBPSU to the EBACs. 
     The right side power switching configuration  104  has a structure that is similar to the left side power switching configuration  102 . Accordingly, the configuration and operation of these components will not be redundantly described herein. As shown in  FIG. 1 , the right side power switching configuration  104  may generally include: at least one right BSCU  168 , at least one right EBAC  152 / 160 , at least one right EBPSU  156 / 164  which includes at least one right power switching logic circuit  158 / 166 , at least one active power supply unit  154 / 162 , and the battery power supply unit  128 . This configuration  104  may be adapted to receive, transmit, exchange, or otherwise process a number of data and/or control signals. These signals may include, without limitation: at least one active power signal  174 / 182 , a battery power signal  176 , at least one right BSCU battery power enable/disable signal  186 / 190 , at least one right braking event signal  172 / 180 , at least one braking power signal  173 / 181 , a battery ON/OFF switch signal (not shown in  FIG. 1 ) and at least one right power distribution signal (reference numbers  170 / 178 / 184 / 188 ). 
       FIG. 2  is a flow chart illustrating a process for switching power sources for an aircraft electric brake system according to an example embodiment of the invention. Process  200  receives control signals from the BSCUs and the aircraft power supply units and removes the battery power from EBACs and/or the BSCUs during flight. Additionally, process  200  switches between the power supply units based upon the received control signals. The various tasks performed in connection with process  200  may be performed by software, hardware, firmware, or any combination thereof. For illustrative purposes, the following description of process  200  may refer to elements mentioned above in connection with  FIG. 1 . In practical embodiments, portions of process  200  may be performed by different elements of a system, e.g., at least one BSCU, at least one EBAC, at least one EBPSU, at least one power switching logic circuit  116 / 124 , at least one active power supply unit, or a battery power supply unit. 
     Process  200  may begin by inquiring whether a BSCU battery power enable/disable signal is disabled (inquiry task  202 ). If the BSCU battery power enable/disable signal is not disabled, process  200  continues to check if a braking event occurred. If the battery power enable/disable signal is disabled (inquiry task  202 ), process  200  inquires, whether the battery switch and the towing switch is OFF (inquiry task  204 ). If the battery switch and the towing switch is OFF, process  200  removes/disconnects the battery power from the BSCUs and the EBACs (task  210 ) during the flight. In other words, process  200  switches out the battery power supply unit such that it no longer powers the electric brake system. If either the battery switch or towing switch is on, process  200  disconnects the battery power supply unit and active power supply unit only from the EBACs (task  206 ) during flight. This allows the one signal to do two different functions based upon the state of the battery switch, and removes the need for having two signals between the BSCU and the EBPSU. If the battery switch or the towing switch is ON (inquiry task  204 ), then process  200  disconnects the battery power supply unit and the active power supply unit from the EBACs (task  206 ) during the flight. 
     As a result of either task  206  or task  210 , the power remains disconnected until the BSCU battery power enable/disable signal is not disabled. At that point, whether a braking event has occurred is checked by inquiry task  212 . The braking event, as explained above, may be pedal braking, parking braking, auto braking, gear retract braking, or the like, and the braking event is indicated by a braking event signal (such as a 130 volt control signal). 
     If a braking event occurs (inquiry task  212 ) process  200  checks whether the active power is valid (inquiry task  220 ). If the active power is valid (inquiry task  220 ), then the EBACs and the BSCUs switch to get power from the active power supply units (task  226 ), and process  200  leads back to task  202 . However, if the braking event has occurred (inquiry task  212 ) and the active power is not valid then process  200  checks whether battery power is valid (inquiry task  222 ). If the battery power is valid, then process  200  reconnects the battery power supply unit (task  224 ) to the EBACs and BSCUs and remains connected until the battery power is not valid (inquiry task  222 ). In this regard, task  224  leads back to inquiry task  222  and keeps checking the validity of the battery power supply unit. The EBACs and the BSCUs remain connected to the battery power while braking is commanded. When getting power from the battery power supply unit, process  200  does not allow switching back to the active power unless the battery power supply until is invalid (inquiry task  222 ). In other words, if the active power (TRU) comes back during the braking event, process  200  does not switch to the TRU until braking is over. This will constrain the switching to only one transition from the TRU to the battery power and prevents possible power transients to the BSCUs. If the battery power supply unit is invalid (inquiry task  222 ) and the active power is valid (inquiry task  223 ) then EBACs and the BSCUs connect to active power supply (task  226 ). If the braking event occurs (inquiry task  212 ), and the active power is not valid (inquiry tasks  220  and  223 ) and the battery power is also not valid (inquiry task  222 ), process  200  leads back to task  202  and no power switching occurs. 
     If a braking event does not occur (inquiry task  212 ), and the active power is valid (inquiry task  216 ), then the EBACs and the BSCUs switch to get power from the active power supply units (task  218 ), and process  200  leads back to task  202 . If the braking event does not occur (inquiry task  212 ), and the active power is also not valid (inquiry task  216 ), process  200  connects to the battery power supply unit (task  217 ) and leads back to task  202 . 
     The process  200  may be performed by one or more suitably configured power switching logic circuits  300  as explained below. The power switching logic circuit  300  receives the control signals from the BSCU and the power supply units, and switches (connects/disconnects) power sources for the EBACs, and/or for the BSCUs based upon the control signals as explained below. In one embodiment, the power switching logic circuit  300  switches the power for the EBACs off during the flight. In this regard, power is saved. 
       FIG. 3  illustrates a power switching logic circuit  300  that is suitable for use with an aircraft electrical braking system for switching power according to an example embodiment of the invention. Power switching logic circuit  300  may include: an active power/battery decision circuit  329 , an active power (TRU 28 volt) switching circuit  331 , and a battery power (BATTERY 28 volt) switching circuit  333 . In practice, these elements may be coupled together in the illustrated arrangement using any suitable interconnection architecture. The system  300  described herein can be applied to any number of power switching logic circuit configurations for an aircraft, and circuit  300  is depicted to illustrate one of many possible examples. 
     Active power/battery decision circuit  329 , determines whether the TRU or the battery provide power to the brakes; the active power switching circuit  331 , determines additional conditions for when active power is switched to the brakes; and the power switching circuit  333 , determines additional conditions for when battery power is switched to the brakes. 
     The active power/battery decision circuit  329  determines whether the TRU or the battery should provide power to the brakes by processing received control signals indicating the status of aircraft systems. In this example embodiment, the TRU/battery decision circuit  329  receives and/or processes a plurality of control signals which may include: an active power signal  316  (which is a logic high value when the TRU voltage is greater than 24 volts and is otherwise a logic low value); a battery power signal  312  (which is a logic high value when the battery voltage is greater than 22 volts and is otherwise a logic low value); a braking event signal  302  (which is a logic high value when the 130 volt BSCU enable/disable signal is enabled and is otherwise a logic low value). The TRU/battery decision circuit  329  may include: a flip flop circuit  326  and a plurality of logic gates  308 / 320 / 324 . 
     The flip flop circuit  326  includes: a set input  328 , a reset input  330 , and a Q output  332 . The flip flop circuit is any standard flip flop circuit and is configured to prevent toggling from the battery power to the active power supply as explained below. The reset input  330  is configured to receive the active power signal  316 . When the active power signal  316  is low, the Q output  332  is set low and the set input  328  is ignored. As will be explained below, the Q output  332  set low indicates the TRU voltage is not greater than 24 V and the TRU should not be used. When the active power signal  316  is high, the Q output  332  is controlled by the set input  328 . Those skilled in the art are familiar with flip flop truth table, the flip flop circuits, and the general manner in which they are controlled, and such known aspects will not be described in detail here. 
     The set input  328  is configured to receive an output signal  325  from a logic gate  324  which may be an OR gate that receives the output signals  321 / 323  and produces the output signal  325  depending on the received signals  321 / 323 . If either of signals  321  or  323  is logic high the output signal  325  is logic high. 
     The logic gates  308 / 320 / 324  each having a plurality of inputs and an output, and may be, without limitation, any standard logic gates designed to carry out the operation of the TRU/battery decision circuit  329  suitable for aircraft electric brake systems as explained below. The logic gate  308  is an AND gate configured to receive the inverted value of the braking event signal  302  and the active power signal  316 , and produce an output signal  323  depending on the received signals  302 / 316  . The logic gate  320  is an AND gate configured to receive the inverted value of the battery power signal  312  and the active power signal  316 , and produce an output signal  321  depending on the received signals  312 / 316 . When the active power signal  316  is low, both output signal  323  and output signal  321  are set low. When the active power signal  316  is high, the output signal  323  has the value of the inverted value of the braking event signal  302 , and the output signal  321  has the value of the inverted value of the battery power signal  312 . Therefore, if the TRU voltage is greater than 24 V and (either the battery voltage is not greater than 22 volts, or the 130 volt BSCU enable/disable signal is disabled), then the output signal  325  is set to logic high. Logic gate  324 , which is explained above, produces the output signal  325  to be fed to the flip flop circuit  326 . 
     The output signal  325  and the active power signal  316  are fed to the flip flop circuit  326 . As mentioned above, the flip flop circuit  326  is configured to prevent toggling from the battery power and the active power. For example, if the active power is operating intermittently, then Q output  332  may toggle between 0 and 1 and the switches may be opening and closing back and fourth. In this regard, the braking system may see many power transients. The flip flop circuit  326  prevents unintentional transition from the battery power supply unit to the active power supply unit. The flip flop circuit  326  makes sure once switched to getting power from the battery power source the BSCUs and/or the EBACs continue obtaining power from the battery power source unless the braking event signal  302  is invalid (braking event can not be detected) or the battery power source is invalid (battery voltage is less than 22 volts), and the active power source is valid (TRU voltage is greater than 28 volts). In this regard, the Q output  332  controls the active power switching circuit  331  and the battery power switching circuit  333  as explained below. 
     The active power switching circuit  331  determines additional conditions for when active power is switched to the brakes by receiving control signals. In this example embodiment, the active power switching circuit  331  receives and/or processes a plurality of control signals which may include: the Q output  332  and a BSCU power enable/disable signal  348 . The Q output  332  is explained above. The BSCU power enable/disable signal  348  is a logic high value when the 28 volt BSCU enable/disable signal is enabled and is otherwise a logic low value. 
     The active power switching circuit  331  may include: a logic gate  350 , a TRU-BSCU switch  334 , a TRU-EBAC switch  344 , a plurality of relays  338 , and a plurality of relay control signals  339 / 335 . The logic gate  350  is an AND gate and is configured to receive the control signals  332 / 348  and output the relay control signal  335  to control (close/open) the TRU-BSCU switch  334 . The TRU-BSCU switch  334  is controlled by the relay control signal  339  and is configured to open/close to connect/disconnect the active power supply from the BSCU. The TRU-EBAC switch  344  is controlled by the relay control signal  335  and is configured to open/close to connect/disconnect the active power supply form the EBACs. Notably, the switches  334 / 344  are shown in an open position (no signal flow) in  FIG. 3 . The relays  338  are configured to close/open the switches  334 / 344  using the relay control signals  339 / 335 . The relay control signal  339  is controlled by the Q output  332  and is configured to close/open the TRU-BSCU switch  334 . The relay control signal  335  is controlled by the logic gate  350  and is configured to close/open the TRU-EBAC switch  344 . 
     The battery power switching circuit  333  determines additional conditions for when battery power is switched to the brakes by receiving control signals. In this example embodiment, the battery power switching circuit  333  receives and/or processes a plurality of control signals which may include: the Q output  332 , the BSCU power enable/disable signal  348 , a battery ON/OFF switch signal  356  and a towing ON/OFF switch signal  358 . The battery power switching circuit  333  may include: a plurality of logic gates  368 / 384 / 360 , a BAT-BSCU switch  372 , a TRU-EBAC switch  378 , a plurality of relays  338 , and a plurality of relay control signals  337 / 341 . 
     BSCU battery power enable/disable signal  348  is explained above. The battery ON/OFF switch signal  356  is a logic high value when the battery is switched on and is a logic low value when the battery is switched off, and the towing ON/OFF switch signal  358  is a logic high value when the towing mode is switched on and is a logic low value when the towing mode is switched off. The battery ON/OFF switch and the towing ON/OFF switch are physical switches that a pilot can control to connect to the battery power supply unit to operate equipment in the two modes. 
     Each of the logic gates  360 / 368 / 384  has a plurality of inputs and an output, and each may be, without limitation, any standard logic gate designed to carry out the operation of the logic circuit  333  suitable for aircraft electric brake systems as explained below. 
     The logic gate  360  is a thee-input one-output OR gate and is configured to receive the battery ON/OFF switch signal  356 , the towing ON/OFF switch signal  358 , and the BSCU battery power enable/disable signal  348 . Output  362  of the logic gate  360  is fed to the logic gate  368 . 
     The logic gate  368  may be a two-input one-output AND gate and is configured to receive the Q output  332  and the output of the logic gate  360  (signal  362 ) and output the relay control signal  337  to control (close/open) the BAT-BSCU switch  372 . The BAT-BSCU switch  372 , is controlled by the relay signal  337  and is configured to open/close to connect/disconnect the battery power supply unit form the BSCU. 
     The logic gate  384  may be a two-input one-output AND gate and is configured to receive the Q output  332  and the BSCU power disable/enable signal  348  and output the relay control signal  341  to control (close/open) the BAT-EBAC switch  378 . The BAT-EBAC switch  378 , is controlled by the relay control signal  341  and is configured to open/close to connect/disconnect the active power supply form the EBACs. 
     Notably, the switches  372 / 378  are shown in an open position (no signal flow) in  FIG. 3 . The relays  338  are configured to close/open the switches  372 / 378  using the relay control signals  337 / 378 . The relay control signal  337  is controlled by the logic gate  337  and is configured to close/open the BAT-BSCU switch  372 . The relay control signal  341  is controlled by the logic gate  384  and is configured to close/open the BAT-EBAC switch  378 . 
     In one example embodiment, the power switching logic circuit  300  may operate as follows: If the active power signal  316  is valid (TRU voltage &gt;24 volt) and the battery power signal  312  is not valid (battery voltage &lt;22 volt) then Q output  332  is logic high. In this regard, the power switching logic circuit  300  reverts to obtaining power from active power source. In this example, the TRU-EBAC switch  344  and the BSCU-TRU switch  334  are both closed allowing the EBAC/BSCU to obtain power from the TRU. Switches  372 / 378  both being open thereby not allowing passage of the BSCU power enable/disable signal  348 , causes the battery power source to connect to EBAC/BSCU. 
     In another example embodiment, the power switching logic circuit  300  may operate as follows: When the battery switch signal  356  or the towing switch signal  358  (inputs to the logic gate  360 ) is on (logic high) and the Q output  332  is logic low or logic high, the BSCU can turn the EBAC on/off by enabling/disabling the BSCU power enable/disable signal  348 . For example, when the battery switch signal  356  is on, the BSCU disconnects the EBAC from the battery power by setting the BSCU enable/disable signal  348  to disable (logic low). In this regard, switch  372  closes (relay control signal  337  is logic high at the output of the logic gate  368 ) allowing the battery power supply unit to supply power to BSCU, and switch  378  opens (relay control signal  339  is logic low at the output of the logic gate  384 ) to prevent the battery power supply unit from supplying power to the EBAC. 
     While at least one example embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the example embodiment or embodiments described herein are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing the described embodiment or embodiments. It should be understood that various changes can be made in the function and arrangement of elements without departing from the scope of the invention, where the scope of the invention is defined by the claims, which includes known equivalents and foreseeable equivalents at the time of filing this patent application.