Abstract:
A method is provided for arranging the components and power connection points within an electromechanical braking system architecture in order to better maintain isolation of the power busses, and thereby improve overall integrity of the system, while still meeting system redundancy, performance and safety requirements as in the past. In addition, a method is provided for connecting and efficiently using available power in emergency braking and parking modes.

Description:
TECHNICAL FIELD 
     The present invention relates generally to braking systems for vehicles, and more particularly to a method for connecting and distributing power to an electromechanical braking system in an aircraft. 
     BACKGROUND OF THE INVENTION 
     Various types of braking systems are known. For example, hydraulic, pneumatic and electromechanical braking systems have been developed for different applications. The aerospace industry presents unique operational and safety issues with regard to many braking systems. For example, the need for system redundancy in case of a system or component failure is particularly germane to braking operations of an aircraft. 
     Brake system architectures for aircraft have been developed previously which meet different redundancy, performance and safety requirements. Such architectures include, for example, redundant digital brake control units (BSCUs) which carry out the brake control and antiskid processing functions. In addition, such architectures include, for example, redundant electromechanical actuator controllers (EMACs) which convert commands from the BSCUs to brake actuator forces. Each EMAC provides electrical power to electromechanical brake actuators included within the brakes for the wheels of the aircraft. 
     FIG. 1 represents such a brake system architecture which has been developed in the past. The architecture, generally designated in FIG. 1 as braking system  30 , includes the aforementioned BSCUs and EMACs which are represented collectively as an electromechanical braking controller  60 . The controller  60  receives as its primary inputs i) the brake command signals from pilot brake pedal transducers  46  located in the cockpit of the aircraft, and ii) the outputs of torque and wheel speed sensors  62  included as part of a brake  34  on each wheel  36  of the aircraft. 
     The braking system  30  receives power from three primary power busses and a secondary power buss included within the aircraft. As is known, an aircraft oftentimes will include multiple power busses. In the exemplary embodiment, the aircraft includes primary power busses PWR 1 , PWR 2  and PWRess. Each power buss preferably is independent of one or more of the other power busses to provide a level of redundancy. For example, the power buss PWR 1  consists of an alternating-current (AC) power source AC 1  and a commonly generated direct-current (DC) power source DC 1 . Similarly, the power buss PWR 2  consists of an AC power source AC 2  and a commonly generated DC power source DC 2 ; and the power buss PWRess consists of an AC power source ACess and commonly generated DC power source DCess. 
     The power buss PWR 1  (i.e., AC 1  and DC 1 ) may be derived from power generated by the left wing engine in the aircraft, for example. Similarly, the power buss PWR 2  (i.e., AC 2  and DC 2 ) may be derived from power generated by the right wing engine. In this manner, if the left wing engine or the right wing engine fails, power is still available to the system  30  via the power buss corresponding to the other engine. 
     The power buss PWRess (i.e., ACess and DCess) may be derived from power generated by the parallel combination of the left wing engine and the right wing engine. In such manner, power from the power buss PWRess will still be available even if one of the engines fail. In addition, DCess can be powered by a battery in case of total loss of the aircraft engines. 
     More particularly, the aircraft further includes a DC power buss supplied by a battery on board the aircraft. This power is represented by a DChot power source. The battery may be charged via power from one of the other power sources, or may be charged separately on the ground. The DChot power source is configured to provide battery power to the DCess power buss in the event of loss of AC power. 
     Various circumstances can arise where power from one or more of the power busses may become unavailable. For example, the left wing engine or the right wing engine could fail causing the PWR 1  (AC 1 /DC 1 ) and PWR 2  (AC 2 /DC 2 ) power sources to go down, respectively. Alternatively, power generating equipment such as a generator, inverter, or other form of power converter could fail on one of the respective power busses resulting in the AC 1 /DC 1 , AC 2 /DC 2  and/or ACess/DCess power sources becoming unavailable. In addition, a failure can occur in the cabling providing the power from the respective power sources to the system  30 , thus effectively causing the respective power source to no longer be available. For this reason, the routing of the power cables for the different busses preferably occurs along different routes throughout the plane to avoid catastrophic failure on all the power buss cables at the same time. 
     As mentioned above, such previously developed systems have been shown to satisfy system redundancy, performance and safety requirements associated within an aircraft braking system. Nevertheless, there is a desire to improve the capabilities of such braking systems with respect to other possible failures within an aircraft or other vehicle. For example, there is a strong need in the art for a method for partitioning the power buss(es) within the braking system to reduce the risk of impairing or failing a power buss or supply as a consequence of a system or component failure. Moreover, there is a strong need in the art for a method for further maintaining brake control in an emergency or parking mode despite loss of a power buss and or BSCU, for example. 
     SUMMARY OF THE INVENTION 
     The present invention provides a manner for arranging the components and power connection points within a braking system architecture in order to better maintain isolation of the power busses, and thereby improve overall integrity of the system, while still meeting system redundancy, performance and safety requirements as in the past. In addition, the invention provides a manner for connecting and efficiently using available power in emergency braking and parking modes. 
     In accordance with one particular aspect of the invention, a method is provided for distributing power to an electromechanical braking system. The braking system includes a plurality of brake actuators for effecting a braking torque on wheels of a vehicle, a plurality of electromechanical actuator controllers (EMACs) for providing drive control of the brake actuators in response to brake command signals, and at least one brake control unit (BSCU) for converting an input brake command signal into the brake command signals which are provided to the EMACs. The method includes the steps of configuring at least two of the plurality of EMACs to function redundantly in providing drive control to the brake actuators in response to the brake command signals; and providing power to the at least two EMACs via respective power busses having different power sources. 
     According to another aspect of the invention, a method for distributing power to an electromechanical braking system are provided in which the system includes a plurality of brake actuators for effecting a braking torque on wheels of a vehicle, at least one electromechanical actuator controller (EMAC) for providing drive control of the brake actuators in response to brake command signals, and a plurality of brake control units (BSCUs) for converting an input brake command signal into the brake command signals which are provided to the at least one EMAC. The method includes the steps of configuring at least two of the plurality of BSCUs to function redundantly in providing brake command signals to the at least one EMAC in response to the input brake command signal; and providing power to the at least two BSCUs via respective power busses having different power sources. 
     In accordance with yet another aspect of the invention, a method for controlling braking in an electromechanical braking system comprising at least one brake actuator for effecting a braking torque on a wheel of a vehicle, at least one electromechanical actuator controller (EMAC) for providing drive control of the brake actuator in response to brake command signals, and at least one brake control unit (BSCU) for converting an input brake command signal into the brake command signals which are provided to the EMAC, the BSCU providing antiskid operations in relation to the input brake command signal, is provided. The method includes the steps of under predefined normal braking conditions, inputting the input brake command signal to the BSCU to obtain a brake command signal which is provided to the EMAC to implement braking; and under predefined emergency or parking conditions, inputting the input brake command signal directly to the EMAC so as to bypass the BSCU and implement braking. 
     To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a general block diagram representative of known electromechanical braking systems; 
     FIG. 2 is a detailed block diagram of the electromechanical braking system which incorporates the features of the present invention; 
     FIG. 3A is a timing diagram illustrating operation of the electromechanical braking system in a first alternate braking mode in which a primary AC power source has failed; 
     FIG. 3B is a timing diagram illustrating operation of the electromechanical braking system in a second alternate braking mode in which an essential primary AC power source has failed; 
     FIG. 3C is a timing diagram illustrating operation of the electromechanical braking system in an emergency braking mode in which all primary power sources have failed; 
     FIG. 3D is a timing diagram illustrating operation of the electromechanical braking system in a park (ultimate) braking mode in which all primary power sources are unavailable; 
     FIG. 4A is a timing diagram illustrating operation of the electromechanical braking system during failure of a brake system control unit; and 
     FIG. 4B is a timing diagram illustrating operation of the electromechanical braking system during failure of an electromechanical actuator controller. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention will now be described with reference to the drawings, wherein like reference labels are used to refer to like elements throughout. 
     Referring initially to FIG. 2, an electromechanical braking system  30  is shown. As will be explained in more detail below, the system  30  utilizes power buss partitioning in accordance with the present invention in order to reduce and/or eliminate the risk of impairing or failing a power buss or supply as the consequence of a system or component failure. In addition, the invention present provides a method for connecting and efficiently using the available power in the system  30  in parking and emergency modes. 
     The braking system  30  as shown in FIG. 2 has an exemplary architecture for satisfying typical redundancy, performance and safety requirements within an aircraft. Such architecture is presented by way of example to illustrate the context in which the principles of the present invention may be employed. It will be appreciated, however, that the present invention has utility with other architectures and is not limited to the particular architecture shown. The manner in which the present invention provides for power buss partitioning and efficient braking in the parking and emergency modes can be applied to other architectures as well. 
     According to the exemplary architecture, the system  30  includes two BSCUs  40  designated BSCU 1  and BSCU 2 , respectively. BSCU 1  and BSCU 2  are redundant and are both configured to provide an input/output interface to the aircraft electronics within the cockpit, for example, via a bus  70 . In addition, BSCU 1  and BSCU 2  each contain circuitry for performing top level brake control and antiskid algorithm processing functions. BSCU 1  and BSCU 2  each receive proportional brake command signals from the transducers  46  via cable  48 . 
     BSCU 1  and BSCU 2  are each designed to receive the proportional brake command signals from the transducers  46  and process the signals based on the aforementioned brake control and antiskid algorithms to produce a brake command signal which is provided to EMACs  44 . The particular brake control and antiskid algorithms employed by the BSCUs  40  can be conventional, and hence further detail based thereon is largely omitted in the present description for sake of brevity. 
     BSCU 1  and BSCU 2  each provide brake commands and otherwise communicate with the EMACs  44  via a communication bus  50 . The system  30  includes four redundant EMACs  44  respectively labeled EMAC Left 1 , EMAC Left 2 , EMAC Right 1  and EMAC Right 2 . As shown in FIG. 2, each EMAC  44  is coupled to the communication bus  50  so as to be able to receive brake commands from each of the BSCUs  40  and otherwise communicate with the other devices coupled to the bus  50 . The EMACs  44  receive left and right brake commands from the BSCUs  40  and provide control signals to actuator modules within the brakes  34  to drive the actuator modules to their commanded position. In this manner, controlled braking may be effected. 
     Each brake  34  included in the system  30  includes, for example, four separate actuator modules (designated by numerals 1-4). Each actuator module  1 - 4  includes an electric motor and actuator (not shown) which is driven in response to electrical control signals provided by a respective EMAC  44  to exert mechanical braking torque on a respective wheel  36 . Each EMAC  44  controls half of the actuator modules  1 - 4  for the wheels  36  on either the left wing landing gear or the right wing landing gear. Thus, EMAC Left 1  provides control to actuator modules  1  and  3  of each of the wheels  36  in the left side landing gear (representing the left brakes) via cable  52 . Similarly, EMAC Left 2  has its output coupled to the remaining actuator modules  2  and  4  of the wheels  36  in the left side landing gear via cable  52 . EMAC Right 1  similarly provides power to the actuator modules  1  and  3  for the wheels  36  in the right side landing gear (representing the right brakes), and EMAC Right 2  provides power to the remaining actuator modules  2  and  4  in the right side landing gear via another cable  52 . 
     Thus, when the system  30  is fully operational (i.e., during normal operation) each of the EMACs  44  receives brake commands from BSCU 1  and BSCU 2  which will be generally redundant. Nevertheless, the EMACs  44  may be configured to give commands provided by BSCU 1  priority or vice versa. In the event commands are not received from one of the BSCUs  40 , the EMACs  44  are configured to default to the other BSCU  40 . During normal operation, all four actuator modules  1 - 4  will receive brake control signals from their respective EMAC  44  to provide full braking. 
     Although not shown in FIG. 2, the outputs of the wheel speed and torque sensors  62  for each brake  34  are coupled to the respective EMACs  44  via the cables  54  (FIG.  1 ). The EMACs  44  are configured to condition the signals and provide the measured wheel speed and torque to the BSCUs  40  via the communication bus  50 . The BSCUs  40  in turn use such information in a conventional manner for carrying out brake control and antiskid processing. 
     According to the present invention, EMAC Left 2  and EMAC Right 2  differ from the remaining EMACs in that they also receive left and right proportional brake commands directly from the transducers  46  via a separate cable  72 . As is discussed in more detail below, such direct input of the brake commands from the transducers  46  is used during emergency braking operations. Also, EMAC Left 2  and EMAC Right 2  receive a parking brake control signal from a switch located in the cockpit via the cable  72  for carrying out a parking brake operation as described below. 
     Both BSCU 1  and BSCU 2  are designed to operate on DC power. According to the present invention, however, BSCU 1  is coupled to the DC 1  power source and BSCU 2  is coupled to a different power source, namely the DC 2  power source. Thus, different power busses (e.g., PWR 1  and PWR 2 ) are used to supply operating power to the respective BSCUs  40 . Similarly, EMAC Left 1  and EMAC Right 1  are designed to operate on power from the different power busses PWR 1  and PWR 2 , respectively. Specifically, EMAC Left 1  receives AC operating power from the AC 1  source and DC operating power from the DC 1  source. EMAC Right 1  receives AC operating power from the AC 2  source and DC operating power from the DC 2  source. 
     Also according to the present invention, EMAC Left 2  and EMAC Right 2  are configured to operate on power from the PWRess power buss. Specifically, both EMAC Left 2  and EMAC Right 2  receive AC operating power from the ACess source and DC operating power from the DCess source. In addition, EMAC Left 2  and EMAC Right 2  are designed to operate in an emergency mode based on power provided by the DChot bus as discussed below. 
     The system  30  is designed to carry out built-in testing among the EMACs  44  to detect the loss of power from any of the primary power busses PWR 1 , PWR 2  and PWRess. Such built-in testing can be carried out by configuring the EMACs  44  to poll each other via the communication bus  50 , for example. If an EMAC  44  fails to respond to polling by another, for example, it can be assumed that power from the particular power buss servicing the EMAC  44  is unavailable or that the EMAC  44  itself has failed. The polling EMACs  44  then communicate such information to the BSCUs  40  via the bus  50 . The BSCUs  40  in turn command the functioning EMACs  44  to revert to an alternate mode of braking. Other techniques for detecting the loss of power on one of the power busses or the failure of one of the components can be used as will be appreciated. 
     Braking Modes 
     The braking system  30  includes five primary operating modes including a normal mode, alternate mode 1, alternate mode 2, emergency mode and park (ultimate) mode. In each mode braking is available despite failure of a power buss, etc., as will now be explained with reference to FIGS. 3A-3D and  4 A- 4 B. 
     FIGS. 3A-3D and  4 A- 4 B illustrate the state of respective power busses and components within the system  30  with respect to time during different failure modes. A line level “A” in the figures indicates that the power buss or component is available and operational. A line level “IN” indicates that the power buss or component is inactive or unavailable. With respect to a line level between “A” and “IN”, this indicates that the brakes or components are partially available or operational as will be further described below. 
     Normal Mode 
     Normal mode operation is defined as operation during which power from all the primary power busses PWR 1 , PWR 2  and PWRess is available, and the BSCUs  40  and EMACs  44  are functional. Referring initially to FIG. 3A, normal mode operation is shown at a time prior to a failure time tf. As is shown, all of the power busses are available, the BSCUs  40  and EMACs  44  are receiving power and are operational. Moreover, each of the actuator modules  1 - 4  in the left brakes and right brakes are powered and operational. 
     Alternate Mode 1 
     Alternate mode 1 is defined as operation during which the power buss PWR 1  or PWR 2  is unavailable due to failure, for example, but the power buss PWRess remains available. 
     FIG. 3A illustrates a particular example where, at a failure time tf, the power buss PWR 1  (AC 1 /DC 1 ) fails. As noted above, such failure may occur due to engine failure, power converter failure, broken power cable, etc. Since BSCU 1  is powered by the power buss PWR 1 , BSCU 1  will stop functioning at time tf as represented in FIG.  3 A. However, since BSCU 1  and BSCU 2  are redundant and BSCU 2  receives operating power from the power buss PWR 2  (AC 2 /DC 2 ), brake control operation and antiskid processing may still be carried out. 
     Since BSCU 2  receives operating power from power buss PWR 2  and therefore does not require power from power buss PWR 1 , BSCU 2  is isolated from power buss PWR 1  as well as BSCU 1 . Thus, a failure of power buss PWR 1  and/or BSCU 1  will not produce a consequential failure of power buss PWR 2 . For example, a short circuit or breakdown of the power buss PWR 1  and/or BSCU 1  will not result in a catastrophic failure of power buss PWR 2 . Of course, the same is true with respect to the reverse situation if BSCU 2  and/or PWR 2  were to experience a failure. Power bus PWR 1  would remain available to BSCU 1  as it is isolated within the braking system  30  from the failed BSCU 2  and/or power buss PWR. 
     In the example of FIG. 3A, since EMAC Left 1  receives power from the power buss PWR 1  it also becomes unavailable at time tf. Because EMAC Left 1  becomes unavailable, the actuator modules  1  and  3  controlled by the EMAC in the left brakes are disabled. Nevertheless, each of the remaining EMACs  44  remain operational. Accordingly, two of the four actuator modules (i.e.,  2  and  4 ) remain available for braking as controlled by the EMAC Left 2 . Ordinarily this would result in a loss of 50% of the total available braking force on the left wheels  36 . However, the EMACs  44  are designed to increase the upper force limit exerted by the respective actuator modules  1 - 4  in the alternate mode. 
     The risk that the power buss PWR 2  may become disabled as a consequence of the failure of power buss PWR 1  (or the failure of EMAC Left 1  itself) is avoided in accordance with the present invention. The remaining EMACs  44  and the power provided thereto are isolated within the system  30  from the power buss PWR 1 . 
     Similar operation to that shown in FIG. 3A would occur if the power buss PWR 2  (AC 2 /DC 2 ) failed rather than the power buss PWR 1 . In such case, however, BSCU 1  would remain operational and BSCU 2  would stop functioning. Similarly, EMAC Right 1  would stop functioning and the remaining EMACs  44  would continue to operate. The actuator modules  1  and  3  in the right brakes would be disabled, but the EMAC Right 2  would increase the maximum force limit of the actuator modules  2  and  4  similar to that previously described. 
     Alternate Mode 2 
     Alternate mode 2 is defined as operation during which the power buss PWRess is unavailable due to failure, for example, but the power busses PWR 1  and PWR 2  remain available. 
     For example, FIG. 3B illustrates how the power buss PWRess fails at time tf while power busses PWR 1  and PWR 2  remain active. In such case, EMAC Left 2  and EMAC Right 2  are considered unavailable by the system  30  as shown. Although EMAC Left 2  and EMAC Right 2  receive power via the DChot bus, such power is utilized only in the emergency mode discussed below. 
     Since EMAC Left 2  and EMAC Right 2  are not operational, the actuator modules  2  and  4  for each of the brakes  34  for the left and right wheels  36  are disabled. In this case, only 50% of the actuator modules  1 - 4  are active for each of the brakes  34 . Nevertheless, failure of the PWRess is detected and the BSCUs  40  instruct the remaining EMAC Left 1  and EMAC Right 1  to increase the force limits of the active actuator modules  1  and  3  so as to provide at least a majority of the normal braking force. Again, this reduced braking function in the left and right brakes is reflected in FIG.  4 B. 
     It will again be appreciated that according to the present invention, failure of the power buss PWRess and/or EMAC Left 2  or EMAC Right 2  will not result in a consequential failure of the power buss PWR 1  or PWR 2  or the remaining EMACs since the power from power buss PWRess is provided separately to the EMAC Left 2  and EMAC Right 2 . The power to the EMACs Left 1  and Right 1  is provided separately by the other power busses, and hence avoids consequential failure. Again, the reverse is also true. 
     Emergency Mode 
     The emergency mode is defined as failure of all the primary power sources PWR 1 , PWR 2  and PWRess. Only the DCess power source remains available by virtue of battery power provided via the DChot power. 
     FIG. 3C illustrates the emergency mode where all the primary power sources PWR 1 , PWR 2  and PWRess fail at or before time tf. In such case, both BSCUs  40  become disabled as does EMAC Left 1  and EMAC Right 1 . Only EMAC Left 2  and EMAC Right 2  remain active on a limited basis by virtue of the DCess power source. EMAC Left 2  and EMAC Right 2  are configured to recognize such condition and are designed to operate under condition on the brake commands provided directed thereto from the transducers  46  via cable  72 . 
     Under such condition, only actuator modules  2  and  4  remain active in each brake  34 . According to the present invention, EMAC Left 2  and EMAC Right 2  are designed to use the pedal input commands received directly from the transducers  46  to achieve proportional brake force application using the actuator modules  2  and  4  in each brake  34 . Such pedal input commands may derive power from the DCess source via connecting cables  72  and  48 , and the system  30  preferably is designed to provide the most direct electrical path between the transducers  46  and the brakes  34  to minimize the number of intermediate components, and hence decrease the possibility of component failure in that path. 
     Since only actuator modules  2  and  4  remain active in each brake, it is preferable that EMAC Left 2  and EMAC Right 2  be configured to increase the upper force limit of each actuator module under such condition. However, care should be taken to maximize the amount of braking achievable in view of the limited amount of power available via the DCess source. It is noted that in the emergency mode, both BSCUs  40  are disabled, and hence antiskid protection is not available. 
     Park (Ultimate) Mode 
     In the park (ultimate) mode, only power from the DChot source may be available as represented in FIG.  3 D. This may be because the aircraft is on the ground with the remaining power systems shut down. Alternatively, all the primary power busses PWR 1 , PWR 2  and PWRess (including DCess) may be unavailable or have failed similar to the emergency mode discussed above. 
     For the same reasons discussed above in relation to FIG.  3 C and the emergency mode, only EMAC Left 2  and EMAC Right 2  remain active in the park (ultimate) mode. Moreover, these particular EMACs are only partially active in the sense that they are operating based on power from the DChot source. Operation differs from the emergency mode in the following respects. 
     The cockpit includes a parking brake switch selectively activated by the pilot. The parking brake switch is coupled to EMAC Left 2  and EMAC Right 2  via the cables  48  and  72 , for example. EMAC Left 2  and EMAC Right 2  are both configured to provide a predetermined fixed braking force via the enabled actuator modules  2  and  4  in each of the brakes  34  upon closing of the parking brake switch. Power from the DChot source is used only to actuate the actuator modules  2  and  4  into position. Thereafter, a mechanical holding device within the actuator module holds the actuator mechanism in place so as to no longer require power from the DChot source. In this manner, the park mode uses power only during activation or when the park brake is released in order to conserve power in the aircraft battery. 
     Release of the parking brake is implemented by removing the brake clamping force as a result of the EMAC Left 2  and EMAC Right 2  disabling the mechanical holding device and driving each actuator module  2  and  4  to a running clearance position. Specifically, the parking brake switch in the cockpit being moved to a release position causes the EMAC Left 2  and EMAC Right 2  to release the parking brake. 
     In the event the power buss PWRess is available, the system can be designed to operate on power from DCess in order not to discharge the aircraft battery serving as the DChot Source. 
     The park (ultimate) mode is considered to be a final means of applying brakes in an aircraft emergency situation in order to stop the aircraft. The EMACs are configured preferably such that the park mode overrides any normal braking commands unless the normal braking command torque level is higher than the park torque level. If the remainder of the system  30  fails due to the BSCUs  40  or the main power busses PWR 1 , PWR 2  and PWRess failing, for example, it is noted that operation of the park (ultimate) mode is neither prevented nor delayed. 
     Referring now to FIG. 4A, a case where one of the BSCUs  40  fails is illustrated. For example, FIG. 4A shows how BSCU 1  may fail at time tf due to component failure. Since BSCU 1  and BSCU 2  are redundant, the EMACs  44  will continue to receive brake commands from BSCU 2 . Hence, the system  30  will continue to operate in a normal mode. 
     The failure of the BSCU 1  may create a short circuit or other adverse condition which could cause the power buss PWR 1  to fail due to its connection to BSCU 1 . In accordance with the present invention, however, BSCU 2  and the power buss PWR 2  are isolated within the braking system  30 . Thus, failure of BSCU 1  and/or power buss PWR 1  will not result in a consequential failure of power buss PWR 2 . The same principles apply if BSCU 2  was to fail instead. 
     Although not shown, if BSCU 2  was also to fail for some reason (e.g., component failure), the EMACs  44  are configured to revert to emergency mode operation. More specifically, in the absence of commands from the BSCUs  40 , EMAC Left 2  and EMAC Right 2  are configured to operate proportionally in the emergency mode based on the direct inputs from the brake pedal transducers  46  as described above. 
     FIG. 4B illustrates how if EMAC Right 1  fails at time tf 1  due to component failure, for example, the remaining EMACs  44  continue to operate such that the right brakes continue to provide at least partial braking. If EMAC Left 1  were to then fail at time tf 2 , for example, partial braking would again still be available in the left brakes. Thus, the present invention provides protection against component failure much in the same way as protection against failure of the power systems. 
     As in the case of a failed BSCU, the failure of one of the EMACs could potentially produce a short circuit or other adverse condition which could cause its respective power buss connected thereto to fail. In accordance with the present invention, however, the remaining EMACs in addition to providing for redundancy, receive power from a power buss which is isolated from the failed power buss within the braking system  30 . Thus, a consequential failure of the remaining power buss(es) is avoided. 
     Although the invention has been shown and described with respect to certain preferred embodiments, it is obvious that equivalents and modifications will occur to others skilled in the art upon the reading and understanding of the specification. The present invention includes all such equivalents and modifications.