Patent Publication Number: US-2022212642-A1

Title: Fault-tolerant brake load alleviation

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority from U.S. Provisional Patent Application No. 63/133,838 entitled “FAULT-TOLERANT BRAKE LOAD ALLEVIATION,” filed Jan. 5, 2021, the contents of which are incorporated by reference in their entirety. 
    
    
     FIELD OF THE DISCLOSURE 
     The present disclosure is generally related to brake load alleviation systems and methods. In particular, the present disclosure relates to fault-tolerant brake load alleviation systems and methods. 
     BACKGROUND 
     Other factors being equal, lighter weight vehicles tend to be more efficient than heavier vehicles. Accordingly, vehicle designers, manufacturers, and users may prefer to decrease structural weight of a vehicle; however, the option to lower the weight of many vehicle structures is limited due to material and design considerations. For example, a vehicle that is braking heavily may experience significant loads. The magnitude of loads expected due to heavy braking can be large enough to drive the design of the structures for the vehicle, which may result in increased vehicle weight. 
     SUMMARY 
     In a particular implementation, a brake system control unit includes one or more sensor interfaces configured to receive a brake torque signal from a brake torque sensor. The brake system control unit also includes a torque estimator configured to generate an estimated brake torque signal based, at least in part, on a brake model and a brake actuator command. The brake system control unit further includes control circuitry configured to generate the brake actuator command to actuate a brake actuator of a brake system. The brake actuator command is generated based on a brake pedal command and a load alleviation command. The load alleviation command is based on the brake torque signal or the estimated brake torque signal, depending on whether a sensor fault condition associated with the brake torque sensor is detected. 
     In another particular implementation, a method includes receiving, at a brake system control unit, a brake pedal command. The method also includes determining, at the brake system control unit, whether a sensor fault condition is detected based on a brake torque signal from a brake torque sensor. The method further includes, in response to detecting the sensor fault condition, accessing a brake model from a memory accessible to the brake system control unit, generating an estimated brake torque signal based on the brake model and a brake actuator command, and generating a load alleviation command based on the estimated brake torque signal. 
     In another particular implementation, a vehicle includes one or more wheels coupled to a structure and one or more brake systems. Each brake system includes one or more sensors and one or more brake actuators. The vehicle also includes one or more brake system control units. Each brake system control unit includes one or more sensor interfaces configured to receive a brake torque signal from a brake torque sensor. Each brake system control unit also includes a torque estimator configured to generate an estimated brake torque signal based, at least in part, on a brake model and a brake actuator command. Each brake system control unit further includes control circuitry configured to generate the brake actuator command to actuate a brake actuator of the one or more brake actuators. The brake actuator command is generated based on a brake pedal command and a load alleviation command. The load alleviation command is based on the brake torque signal or the estimated brake torque signal, depending on whether a sensor fault condition associated with the brake torque sensor is detected. 
     The features, functions, and advantages described herein can be achieved independently in various implementations or may be combined in yet other implementations, further details of which can be found with reference to the following description and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a brake system configured to provide fault-tolerant brake load alleviation according to a particular implementation. 
         FIG. 2  is a block diagram of a vehicle that includes the brake system of  FIG. 1  coupled to one or more structures according to a particular implementation. 
         FIG. 3  is a diagram illustrating aspects of the brake system of  FIG. 1  according to a particular implementation. 
         FIG. 4  is a diagram illustrating aspects of the brake system of  FIG. 1  according to a particular implementation. 
         FIG. 5  is an example of a table including historical brake command and brake torque data according to a particular implementation of the brake system of  FIG. 1 . 
         FIG. 6  is another example of a table including historical brake command and brake torque data according to a particular implementation of the brake system of  FIG. 1 . 
         FIG. 7  is an example of a data structure including historical brake command and brake torque data according to a particular implementation of the brake system of  FIG. 1 . 
         FIG. 8  is a graph illustrating aspects of a brake model according to a particular implementation of the brake system of  FIG. 1 . 
         FIG. 9  is a flowchart of a method implemented by the brake system of  FIG. 1  according to a particular implementation. 
         FIG. 10  is a flowchart of another method implemented by the brake system of  FIG. 1  according to a particular implementation. 
         FIG. 11  is a flowchart illustrating a life cycle of a vehicle that includes the brake system control unit of  FIG. 1 . 
         FIG. 12  is a diagram illustrating a particular example of a vehicle that includes the brake system control unit of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     The weight of structural elements coupled to brake systems of some vehicles can be reduced by using a brake load alleviation system. A brake load alleviation system protects the structural elements of the vehicle by limiting loads applied to the structural elements during braking. For example, a closed-loop brake load alleviation system uses sensor feedback data to indicate brake force or brake torque applied during braking and limits the brake force or brake torque to some specified threshold to protect the structural elements of the vehicle. 
     The sensors that provide the sensor feedback data in such systems are generally located at or near wheels of the vehicle. As such, these sensors may be exposed to harsh environments, which can lead to sensor failure. When sensor feedback data is not available due to sensor failure, the brake load alleviation system is either bypassed or operates in an open-loop mode. 
     When a brake load alleviation system is bypassed or in open-loop mode, load limits enforced by the brake load alleviation system may be exceeded unless other operational limits are imposed on the vehicle. For example, an aircraft may be required to operate with a reduced take-off weight limit to ensure that structural load limits are not exceeded. As another example, braking distances of the vehicle may be increased to reduce peak brake force. In an aircraft example, increased braking distances may require the aircraft to use a longer runway, which may delay dispatch of the aircraft if no such runway is available or is overburdened. 
     Aspects disclosed herein present systems and methods for fault-tolerant brake load alleviation. The disclosed systems and methods enable improved operation of a brake load alleviation system when a sensor fault is present. For example, the disclosed systems and methods may enable operation of the brake load alleviation system even with a sensor fault that would prevent a traditional brake load alleviation system from performing its desired function. The fault-tolerant brake load alleviation systems and methods disclosed use a vehicle-specific (or even axle-specific or wheel-specific) brake model that is generated during closed-loop operations to generate an estimated feedback sensor signal when the feedback signal is not available or is not reliable (e.g., due to a sensor fault). Because the brake model is custom-built for the particular vehicle (and perhaps for a specific axle or wheel of the vehicle) and frequently updated, the estimated feedback signal reliably limits the loads to which vehicles structures are subjected, enabling operation of the vehicle without imposing additional operational limits (e.g., operational weight limits or braking distance limits). 
     The figures and the following description illustrate specific exemplary embodiments. It will be appreciated that those skilled in the art will be able to devise various arrangements that, although not explicitly described or shown herein, embody the principles described herein and are included within the scope of the claims that follow this description. Furthermore, any examples described herein are intended to aid in understanding the principles of the disclosure and are to be construed as being without limitation. As a result, this disclosure is not limited to the specific embodiments or examples described below, but by the claims and their equivalents. 
     Particular implementations are described herein with reference to the drawings. In the description, common features are designated by common reference numbers throughout the drawings. Some features described herein are singular in some implementations and plural in other implementations. To illustrate,  FIG. 2  depicts a vehicle  200  that includes one or more brake systems  100  (“brake system(s)” in  FIG. 2 ), which indicates that in some implementations the vehicle  200  includes a single brake system  100 , and in other implementations the vehicle  200  includes multiple brake systems  100 . For ease of reference herein, such features are generally introduced as “one or more” features and are subsequently referred to as optionally plural, which is indicated by “(s)” following a term, as in “brake system(s)”  100  of  FIG. 2 . Such features may also be referred to in the singular when a representative of such features is being described. 
     As used herein, various terminology is used for the purpose of describing particular implementations only and is not intended to be limiting. For example, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. The terms “comprise,” “comprises,” and “comprising” are used interchangeably with “include,” “includes,” or “including.” Additionally, the term “wherein” is used interchangeably with the term “where.” As used herein, “exemplary” indicates an example, an implementation, and/or an aspect, and should not be construed as limiting or as indicating a preference or a preferred implementation. As used herein, an ordinal term (e.g., “first,” “second,” “third,” etc.) used to modify an element, such as a structure, a component, an operation, etc., does not by itself indicate any priority or order of the element with respect to another element, but rather merely distinguishes the element from another element having a same name (but for use of the ordinal term). As used herein, the term “set” refers to a grouping of one or more elements, and the term “plurality” refers to multiple elements. 
     As used herein, “generating,” “calculating,” “using,” “selecting,” “accessing,” and “determining” are interchangeable unless context indicates otherwise. For example, “generating,” “calculating,” or “determining” a parameter (or a signal) can refer to actively generating, calculating, or determining the parameter (or the signal) or can refer to using, selecting, or accessing the parameter (or signal) that is already generated, such as by another component or device. As used herein, “coupled” can include “communicatively coupled,” “electrically coupled,” or “physically coupled,” and can also (or alternatively) include any combinations thereof. Two devices (or components) can be coupled (e.g., communicatively coupled, electrically coupled, or physically coupled) directly or indirectly via one or more other devices, components, wires, buses, networks (e.g., a wired network, a wireless network, or a combination thereof), etc. Two devices (or components) that are electrically coupled can be included in the same device or in different devices and can be connected via electronics, one or more connectors, or inductive coupling, as illustrative, non-limiting examples. In some implementations, two devices (or components) that are communicatively coupled, such as in electrical communication, can send and receive electrical signals (digital signals or analog signals) directly or indirectly, such as via one or more wires, buses, networks, etc. As used herein, “directly coupled” is used to describe two devices that are coupled (e.g., communicatively coupled, electrically coupled, or physically coupled) without intervening components. 
     The following discussion frequently refers to brake torque as an indication of load applied to structures of a vehicle. It is noted at the outset that brake force or load may be used instead of or in addition to brake torque to indicate load applied to the structures of the vehicle. For convenience of description (e.g., rather than constantly making reference to “brake torque or brake force”), brake torque is used throughout the following description. However, it is understood that brake force can be substituted for brake torque throughout the following description with corresponding calculation changes where needed (e.g., converting force to torque by use of data descriptive of the configuration of the brake system and attachments to the structures). 
       FIG. 1  depicts an example of a brake system  100  according to a particular implementation. The brake system  100  includes a brake load alleviation system  134  configured to limit load applied to a structure of a vehicle during vehicle braking. The target brake load of brake load alleviation system  134  may vary as a function of time to further limit dynamic braking loads. As a result, the structure can be designed to withstand smaller braking loads than may be encountered without the brake load alleviation system  134 . The brake load alleviation system  134  uses feedback (e.g., a brake torque signal  128 ) from one or more brake torque sensors  126  to limit the braking load. For example, the brake torque signal  128  indicates load that is applied to the structure of the vehicle during a braking operation, and, during normal operation (e.g., when no sensor fault is detected), the brake load alleviation system  134  determines a load alleviation command  136  based on the brake torque signal  128 . The load alleviation command  136  is used to limit a brake actuator command  118  sent to a brake actuation system  120  responsive to a brake pedal command  106 . 
     The brake system  100  further includes a fault-tolerant torque system  130 . The fault-tolerant torque system  130  is configured to detect whether the brake torque sensor(s)  126  is experiencing a fault condition. If the fault-tolerant torque system  130  detects a fault condition associated with the brake torque sensor(s)  126 , the fault-tolerant torque system  130  provides an estimated brake torque signal  132  (rather than the brake torque signal  128 ) to the brake load alleviation system  134 . The estimated brake torque signal  132  is generated based on a model of the brake system  100 . In a particular aspect, during braking operations in which the fault-tolerant torque system  130  does not detect a fault condition, the fault-tolerant torque system  130  updates the model of the brake system  100 . As a result the model is regularly updated, and the fault-tolerant torque system  130  is able to generate values of the estimated brake torque signal  132  that closely approximates values of the brake torque signal  128  that would be present if no sensor fault condition were present. 
     The brake system  100  includes a pedal system  104  configured to generate the brake pedal command  106  based on input from a user  102 . The brake pedal command  106  is combined, at a first node  108 , with the load alleviation command  136  to generate a brake load alleviation compensated brake pedal command  110 . The load alleviation command  136  limits the brake pedal command  106  to prevent a braking operation from exceeding specified load limits associated with a structure. 
     In some implementations, the brake system  100  includes or is coupled to brake automation system(s)  112  that provide brake commands (e.g., brake automation system command(s)  114 ) that are combined, at a second node  116 , with the brake load alleviation compensated brake pedal command  110  to generate the brake actuator command  118 . In implementations that do not include the brake automation system(s)  112 , brake load alleviation compensated brake pedal command  110  is used as the brake actuator command  118 . 
     The brake actuator command  118  is provided to the brake actuation system  120  which actuates the brake  124  responsive to the brake actuator command  118 . The brake  124  performs a braking operation which decreases the speed of the vehicle and applies a resulting load to structures of the vehicle. If the brake torque sensor(s)  126  are operating properly, the brake torque sensor(s)  126  send the brake torque signal  128  to the fault-tolerant torque system  130 . The brake torque signal  128  indicates a measured brake torque generated due to the braking operation. If the brake torque sensor(s)  126  are experiencing a fault condition, the brake torque sensor(s)  126  either do not generate the brake torque signal  128  or generate a brake torque signal  128  that is outside an expected range. 
     The fault-tolerant torque system  130  evaluates the brake torque signal  128  to determine whether the brake torque sensor(s)  126  are experiencing a fault condition. The fault-tolerant torque system  130  outputs the brake torque signal  128  to the brake load alleviation system  134  if no fault condition is detected. If a fault condition is detected, the fault-tolerant torque system  130  outputs the estimated brake torque signal  132 . The estimated brake torque signal  132  is generated based on the model of the brake system  100  and the brake actuator command  118 , as described further below. 
     The brake load alleviation system  134  generates the load alleviation command  136  based on the brake torque signal  128  or the estimated brake torque signal  132 . Thus, the brake load alleviation system  134  is able to operate reliably when a sensor fault condition is detected. 
       FIG. 2  depicts an example of a vehicle  200  that includes one or more wheels  204  coupled to one or more structures  202 . The vehicle  200  also includes one or more brake systems  100 . The brake system(s)  100  are configured to limit loads applied to the structure(s)  202  during braking. As a result, the structure(s)  202  can be designed to withstand smaller braking loads than may be encountered without the described brake system(s)  100 . Ensuring that the structure(s)  202  are subjected to smaller braking loads enables vehicle designers to reduce the overall weight of the vehicle  200 . The vehicle  200  can include or correspond to any wheeled vehicle with onboard brakes. For example, the vehicle  200  may be a land vehicle, such as a truck, a train, or a car. As another example, the vehicle  200  may be an aircraft, in which case the wheel(s)  204  correspond to those installed on a landing gear. Further, the vehicle  200  may be moved by an onboard engine or motor, or the vehicle  200  may be moved by an offboard source. For example, the vehicle  200  may include a trailer or train car with an onboard brake system. 
     In a particular implementation, each wheel  204  is associated with a brake system  100 . In some implementations, one brake system  100  is associated with two or more of the wheels  204  (e.g., multiple wheels on a common axle). In the example illustrated in  FIG. 2 , each brake system  100  includes a brake actuation system  120 , such as a pneumatic, electrical, or hydraulic power source that provides power to actuate one or more brake actuators  236  of the brake system  100 . In other implementations, two or more brake systems  100  share a brake actuation system  120 . For example, a single hydraulic system may be coupled to two or more brake systems  100  of the vehicle  200 . 
     Many implementations use friction-based braking. In such implementations, each brake actuator  236  is coupled indirectly to one of the wheels  204  via a pair of friction surfaces. For example, a wheel  204  is coupled to a rotor or drum that includes a first friction surface that turns with the wheel  204 . In this example, the brake actuator  236  associated with the wheel  204  is coupled to a brake stator, brake pad, or brake shoe that includes a second friction surface. The brake actuator  236  moves the second friction surface into contact with or away from contact with the first friction surface. To illustrate, during braking, the brake actuator  236  presses the second friction surface into contact with the first friction surface to decrease a rate of rotation of the wheel  204 . In other implementations, the brake system  100  uses another mechanism, in addition to or instead of friction, to decrease a rate of rotation of the wheel  204 . One example of a non-friction-based brake mechanism is regenerative braking in which electromotive forces are used to decrease a rate of rotation of the wheel  204 . Other examples include compression braking or hydraulic braking in which braking causes compression of or induces drag in a fluid to decrease a rate of rotation of the wheel  204 . 
     In the example illustrated in  FIG. 2 , the brake system(s)  100  include or are associated with the brake automation system(s)  112 , such as an autobrake system  212 , and/or an antiskid system  214 . The brake system(s)  100  also include one or more brake system control units  220 . Each brake system control unit  220  includes control circuitry  222 , one or more sensor interfaces  224 , and the brake load alleviation system  134 . In some implementations, the brake automation system(s)  112  are integrated within the brake system control unit  220 . In still other implementations, the brake automation system(s)  112  are omitted. 
     The sensor interface(s)  224  of a brake system control unit  220  are configured to receive sensor data and/or signals from sensor(s)  230  of the brake system(s)  100 . For example, the sensor(s)  230  may include one or more brake torque sensors  126  that are configured to provide one or more brake torque signals to the brake system control unit(s)  220  via the sensor interface(s)  224 . In some implementations, one or more brake load sensors may be used instead of or in addition to the brake torque sensor(s)  126 . As another example, the sensor(s)  230  may include one or more brake operating environment sensors  234  that are configured to provide one or more brake operating environment signals to the brake system control unit(s)  220  via the sensor interface(s)  224 . The brake operating environment sensor(s)  234  measure conditions such as vehicle speed, ground speed, wheel speed, brake temperature, wheel temperature, or other braking-related conditions. 
     The control circuitry  222  is configured to generate brake actuator command(s) to actuate the brake actuator(s)  236  responsive to one or more brake input signals (e.g., the brake pedal command  106  of  FIG. 1 , the brake automation system command(s)  114  of  FIG. 1 , or both). The brake load alleviation system  134  is configured to provide the control circuitry  222  with the load alleviation command  136  of  FIG. 1  to limit the brake actuation signal(s) such that such the load applied to a portion of the structure(s)  202  during braking is less than specified load limit(s)  254 . 
     For example, during operation, the brake system  100  receives the brake input signal(s) (e.g., the brake pedal command  106  of  FIG. 1 , the brake automation system command(s)  114  of  FIG. 1 , or both). The brake system control unit  220  provides the brake actuator command  118  of  FIG. 1  to the brake actuation system  120  based on the brake input signal(s) and based on the load alleviation command  136  from the brake load alleviation system  134 . During normal operation, the load alleviation command  136  is based on the brake torque signal  128  from the brake torque sensor(s)  126 . However, when a sensor monitor  240  of the fault-tolerant torque system  130  detects a sensor fault condition associated with the brake torque sensor(s)  126 , the load alleviation command  136  from the brake load alleviation system  134  is determined based on one or more brake models  256  of the fault-tolerant torque system  130 . 
     The sensor monitor  240  is configured to detect fault conditions associated with the sensor(s)  230 . For example, the sensor monitor  240  may compare a measured brake torque value (indicted by the brake torque signal  128  of  FIG. 1 ) to one or more fault criteria  242 . In this example, the fault criteria  242  indicate an expected range of brake torque values, and the sensor monitor  240  indicates that a sensor fault condition is detected if the measured brake torque value is outside of the expected range of brake torque values. In some implementations, the expected range of brake torque values is based on brake torque values stored in a memory during periods of operation of the brake system  100  when no fault condition was detected. Additionally, or alternatively, in some implementations, the expected range of brake torque values is based on default values, such as brake torque values determined during testing of the vehicle  200  or other similar vehicles. 
     In some implementations, at least one of the fault criteria  242  is based on historical brake torque values and one or more brake actuator command values associated with the one or more historical brake torque values. The brake actuator command values correspond to values indicated by the brake actuator command  118  of  FIG. 1 . For example, the brake actuator command  118  can indicate a relative magnitude of a braking operation, such as a percentage of an operational range of the brake system  100 . To illustrate, a brake actuator command value of fifty percent (50%) indicates that the braking operation should be approximately half as aggressive as a braking operation performed responsive to a one hundred percent (100%) brake actuator command value.  FIG. 5  illustrates an example of a table  500  indicating threshold values  508  associated with the fault criteria  242  according to a particular implementation. In  FIG. 5 , the table  500  includes a set of initial default brake torque values  504  and a set of subsequent historical brake torque values  506 , where each brake torque value corresponds to a particular value of the brake actuator command. In some implementations, the table  500  represents values for particular brake operating environment conditions, such as a particular brake temperature, wheel temperature, wheel speed, ground speed, or a combination thereof that can have an effect on brake torque. In such implementations, other tables may be used to represent values for other brake operating environment conditions. 
     In  FIG. 5 , the threshold values  508  indicate a lower value for a valid brake torque sensor reading for each brake actuator command value. To illustrate, for a brake actuator command value  502  of ten percent (10%), a brake torque sensor reading of 0 ft-lb (foot-pounds) or more is considered valid based on the threshold values  508 ; however, for a brake actuator command value  502  of fifty percent (50%), a brake torque sensor reading of 24200 ft-lb or more is considered valid. In some implementations, the table  500  may also indicate an upper threshold for one or more the brake actuator command values  502 . 
     In some implementations, the threshold values  508  are determined based on the historical brake torque values  506 . For example, the threshold value  508  for the brake actuator command value of fifty percent (50%) may be determined based on a statistical analysis of the historical brake torque values  506  corresponding to the brake actuator command value of fifty percent (50%). To illustrate, the threshold value  508  for the brake actuator command value of fifty percent (50%) may be set based on a multiple (e.g., 2×) of a standard deviation of the historical brake torque values  506  corresponding to the brake actuator command value of fifty percent (50%). In other illustrative examples, other statistical analyses can be used to determine a lower (or upper) bound of a valid sensor reading based on the historical brake torque values  506 . 
     Returning to the example of  FIG. 2 , the fault-tolerant torque system  130  includes a torque estimator  250  that is configured to generate the estimated brake torque signal  132  of  FIG. 1  when the sensor monitor  240  detects a sensor fault condition. The estimated brake torque signal  132  is based, at least in part, on the one or more brake models  256  and the brake actuator command  118  provided to the brake actuation system,  120 . The brake model(s)  256  relate the brake actuator command  118  (and possibly other data, such as brake operating environment data) to historical brake torque values measured during periods of operation when no sensor fault was detected. As an example, a brake model  256  may include parameters of a gain-based torque estimation function, such as in Equation 1: 
       τ estimate   =B×G    Equation 1
 
     where τ estimate  is an estimated brake torque value, B is a value indicating a magnitude of the braking operation (e.g., a value of the brake actuator command), and G is a brake gain value based on historical brake torque measurements during periods when no sensor fault was detected. In some implementations, the brake gain value, G, has different values depending on brake operating environment values, such as wheel speed, ground speed, brake temperature, or wheel temperature. 
     Additionally, or alternatively, the value of the brake gain value, G, may be valid for a particular range of brake actuator command values. For example, a matrix data structure may include brake gain values, G, for various combinations of brake actuator command values, wheel speeds, ground speeds, brake temperatures, wheel temperatures, or other braking-related values. In some circumstances, the brake gain value, G, used to calculate the estimated brake torque value, τ estimate , may be determined by interpolation between available brake actuator command values and brake gain values. In this example, the estimated brake torque value, τ estimate , is determined by selecting a brake gain value, G, based on the brake actuator command value, B, values of the brake operating environment signal(s), or both, and multiplying the selected brake gain value, G, by the brake actuator command value, B. 
     As another example, a brake model  256  may include one or more tables or other data structures or knowledge representations that store measured brake torque values when particular brake actuator commands  118  were provided to the brake actuation system  120  during historical braking operations when no sensor fault was detected. In this example, if a sensor fault is detect during braking, a value of the brake actuator command  118  sent to the brake actuation system  120  is used to look up, retrieve, and/or calculate an estimated brake torque value from the brake model(s)  256 . If the value of the brake actuator command  118  does not correspond exactly to a brake actuator command value of the brake model(s)  256 , the estimated brake torque value may be estimated by interpolation between two or more values in the brake model(s)  256 . Alternatively, the brake model(s)  256  may include both the brake gain value, G, from Equation 1 and values from one or more tables. In this example, a coarse estimate of the brake torque value may be determined from the one or more tables and subsequently be refined using the brake gain function. 
       FIG. 6  illustrates an example of a table  600  indicating estimated brake torque values  602  for various brake actuator command values  502  according to a particular implementation. In  FIG. 6 , the table  600  includes the set of initial default brake torque values  504  and the set of historical brake torque values  506 , described with reference to  FIG. 5 . In the example illustrated in  FIG. 6 , the estimated brake torque value  602  associated with each brake actuator command value  502  is an average of the historical brake torque values  506  for the brake actuator command value  502 . For example, for the brake actuator command value  502  of thirty percent (30%), the estimated brake torque value  602  is 30756, which is an average determined based on the historical brake torque values  506 . In some implementations, a sliding average value is used. For example, after a particular number of historical brake torque values  506  are stored in the table  600 , a new historical brake torque value  506  added to the table  600  replaces the oldest historical brake torque value  506  for the same brake actuator command value  502 , and the average of the historical brake torque values  506  is recalculated to determine the estimated brake torque value  602  for the brake actuator command value  502 . The initial default brake torque values  504  are used to determine the estimated brake torque value  602  if sufficient historical brake torque values  506  are not available. 
     In some implementations, the brake load alleviation system  134  includes different brake models  256  for different brake operating environments. In such implementations, the specific brake model  256  used in a particular situation is selected based on a brake operating environment value from the brake operating environment sensor(s)  234 . For example,  FIG. 7  illustrates an example of a set of tables  700  of the brake model(s)  256  according to a particular implementation. In the example of  FIG. 7 , the table  600  is a first table associated with a first brake operating environment value, such as a first brake temperature value, T 1 . In this example, the set of tables  700  also includes one or more additional tables associated with other brake operating environment value, such as a second table  704  associated with a second brake temperature value, T 2 , and a third table  706  associated with a third brake temperature value, T 3 . In this example, an estimated brake torque value may be interpolated between tables  600 ,  704 ,  706 , between brake actuator command values  502 , or both. To illustrate, an estimated brake torque value for a brake actuator command value  502  of forty-five percent (45%) at a brake temperature value between T 2  and T 1  may be determined by interpolation between the estimated brake torque values associated with the brake actuator command values  502  of forty percent (40%) and fifty percent (50%) for the brake temperature T 1  and the estimated brake torque values associated with the brake actuator command values  502  of forty percent (40%) and fifty percent (50%) for the brake temperature T 2 . Although brake temperature is used as an example in  FIG. 7 , in other examples, other brake operating environment values are used in addition to or instead of the brake temperature. 
       FIG. 8  illustrates an example of the brake model(s)  256  represented as a surface  812  in a feature space  802  according to another particular implementation. In  FIG. 8 , the feature space  802  has a brake actuator command dimension  806 , a brake torque dimension  808 , and one or more brake operating environment dimensions  804 . For ease of illustration, only one brake operating environment dimension  804  is illustrated in  FIG. 8 ; however, in some implementations, the feature space  802  includes more than one brake operating environment dimension  804 . For example, the feature space  802  may include a wheel temperature dimension and a wheel speed dimension, or some other combination of brake operating environment dimensions. 
     In the example illustrated in  FIG. 8 , the coordinate location in the feature space  802  of a particular point on the surface  812  indicates an expected brake torque value for given brake operating environment value and brake actuator command values. To illustrate, in  FIG. 8 , a point  810  on the surface  812  can be identified based on a brake operating environment coordinate (e.g., approximately E 3  in  FIG. 8 ) and a brake actuator command value coordinate (e.g., approximately 75% in  FIG. 8 ). The brake operating environment coordinate and the brake actuator command coordinate together specify a unique location on the surface  812 , and an estimated brake torque value for the unique location (e.g., the point  810 ) is specified by a brake torque coordinate of the point  810 . 
     In  FIG. 2 , the fault-tolerant torque system  130  also includes a model updater  252 . The model updater  252  is configured to update the brake model(s)  256  when no sensor fault condition associated with the brake torque sensor(s)  126  is detected. For example, the model updater  252  may store model update data associating a value from the brake actuator command  118  with a brake torque value from the brake torque signal  128 . As another example, the model updater  252  may verify or update parameters of the brake gain function (such as Equation 1) based on the value of the brake actuator command  118  and the brake torque value. In some implementations, the model update data also associate values of the brake operating environment data with the brake actuator command value and the brake torque signal. 
     Thus, the fault-tolerant torque system  130  enables fault tolerant and reliable operation of the brake load alleviation system  134  when a sensor fault condition is detected. To illustrate, on an aircraft, the fault-tolerant torque system  130  determines the force or torque gain for each brake on a given landing gear and stores the information in a memory of the brake system control unit  220  to generate the brake model(s)  256 . The brake model(s)  256  are regularly or periodically updated (by the model updater  252 ) to account for changes in the brake system(s)  100  or other portions of the vehicle  200 . During an initial learning phase (e.g., before sufficient actual operational data is available to generate a brake model  256  that is customized to the brake system(s)  100 ), the brake model(s)  256  use default values (e.g., based on testing or certification data of the vehicle or based on conservative engineering estimates). To illustrate, during the initial learning phase, the fault-tolerant torque system  130  determines the estimated brake torque signal  132  using initial parameters of the brake model  256  and subsequently uses updated parameters generated by the model updater  252 . 
     Although the sensor interface(s)  224 , the control circuitry  222 , the fault-tolerant torque system  130 , and the brake load alleviation system  134  are depicted as separate components in  FIG. 2 , in other implementations the described functionality of two or more of the sensor interface(s)  224 , the control circuitry  222 , the fault-tolerant torque system  130 , and the brake load alleviation system  134  can be performed by a single component. In some implementations, each of the sensor interface(s)  224 , the control circuitry  222 , the fault-tolerant torque system  130 , and the brake load alleviation system  134  correspond to or include hardware, such as an application-specific integrated circuit (ASIC) or a field-programmable gate array (FPGA), or the operations described with reference to the sensor interface(s)  224 , the control circuitry  222 , the fault-tolerant torque system  130 , and the brake load alleviation system  134  may be performed by a processor executing computer-readable instructions. 
       FIG. 3  is a diagram illustrating aspects of the brake system  100  of  FIGS. 1 and 2  according to a particular implementation. The diagram illustrated in  FIG. 3  illustrates a single wheel  204  and associated components (e.g., one or more strut(s)  312 , a brake actuator  236 , an actuator sensor  238 , brake clamps  314 , one or more rotor(s)  316 , one or more brake torque sensor(s)  126 , and one or more brake operating environment sensor(s)  234 ). When more than one wheel  204  of the vehicle  200  has brakes, the control circuitry  222 , the brake load alleviation system  134 , the fault-tolerant torque system  130 , and the components associated with the wheel  204  may be replicated for each braked wheel. 
     In  FIG. 3 , a brake pedal  332  is coupled to a brake pedal sensor  302 . The brake pedal sensor  302  generates a brake pedal input signal  304  based on a position of the brake pedal  332 . Brake pedal command logic  306  generates the brake pedal command  106 , which is provided to the control circuitry  222  and to the fault-tolerant torque system  130 . In the implementation illustrated in  FIG. 3 , the fault-tolerant torque system  130  uses the brake pedal command  106  to determine a brake mode, which assists with detecting sensor faults, as described with reference to  FIG. 4 . 
     The first node  108  of the control circuitry  222  determines the brake load alleviation compensated brake pedal command  110  based on a difference between the brake pedal command  106  and the load alleviation command  136  from the brake load alleviation system  134 . The load alleviation command  136  is based on either the brake torque signal  128  or the estimated brake torque signal  132  from the fault-tolerant torque system  130 . 
     In the example of  FIG. 3 , the second node  116  determines the brake actuator command  118  based on the brake load alleviation compensated brake pedal command  110  and one or more brake automation system commands  114  from the brake automation system(s)  112 . In this example, the brake automation system(s)  112  include systems or components that provide brake input via mechanisms other than the brake pedal  332 . The autobrake system  212  and the antiskid system  214  of  FIG. 2  are examples of such brake automation system(s)  112 . Although  FIG. 3  illustrates the first node  108  and the second node  116  as summing nodes, in other implementations, more complex control logic may be used. 
     In the example of  FIG. 3 , the brake actuator command  118  is provided to the fault-tolerant torque system  130 . The brake actuator command  118  is also provided to the brake actuation system  120  of  FIGS. 1 and 2 , which in  FIG. 3  includes a third node  308 , the brake actuator  236 , the actuator sensor  238 , and a compensator  322 . The third node  308  generates a brake actuator signal  310  based on the brake actuator command  118  and an actuator compensation command  324  from the compensator  322 . The compensator  322  generates the actuator compensation command  324  based on an actuator sensor signal  320  from the actuator sensor  238  associated with the brake actuator  236 . For example, in  FIG. 3 , the compensator  322  may compensate for response characteristics of a servo or valve of the brake actuator  236 . In another example, the compensator  322  may be coupled to multiple brake actuators  236  and actuator sensors  238 . In this example, the compensator  322  may be used to adjust the brake actuator signals  310  sent to the multiple brake actuators  236  based on the brake actuator command  118  to even out braking among the multiple brake actuators  236 . 
     In the particular example illustrated in  FIG. 3 , responsive to the brake actuator signal  310 , the brake actuator  236  causes the brake clamps  314  to contact or press on the rotor(s)  316  thereby braking the wheel(s)  204  and applying a torque on the structure(s)  202  due to braking force coupled to the structure(s)  202  via the strut(s)  312 . The brake torque sensor(s)  126  are coupled to the strut(s)  312 , the structure(s)  202 , or both, to generate the brake torque signal  128  indicative of the braking torque applied to the structure(s)  202 . In some implementations, the dimensions of the strut(s)  312  are known and the braking torque may be indicated by measuring braking force applied by the brake clamps  314 . In such implementations, a force sensor rather than a torque sensor can be used to measure a value that is indicative of the brake torque. 
     The brake torque signal  128  is provided to the fault-tolerant torque system  130 . Additionally, in some implementations, the brake operating environment sensor(s)  234  provide one or more brake operating environment signals  330  as input to the fault-tolerant torque system  130 . The fault-tolerant torque system  130  evaluates the brake torque signal  128  (e.g., based on the fault criteria  242  of  FIG. 2 ) to determine whether a sensor fault condition is detected. If no sensor fault condition is detected, the fault-tolerant torque system  130  provides the brake torque signal  128  to the brake load alleviation system  134 . In this circumstance, the brake load alleviation system  134  generates the load alleviation command  136  based on the brake torque signal  128 . In some implementations, when no sensor fault is detected, the fault-tolerant torque system  130  also uses the brake torque signal  128 , the brake actuator command  118 , and the brake operating environment signal(s)  330  to generate model update data to update the brake model(s)  256 . 
     If the fault-tolerant torque system  130  detects a sensor fault, the fault-tolerant torque system  130  provides the estimated brake torque signal  132  to the brake load alleviation system  134 . In this circumstance, the brake load alleviation system  134  generates the load alleviation command  136  based on the estimated brake torque signal  132 . The fault-tolerant torque system  130  calculates or looks up an estimated brake torque value based on historical values of the brake torque responsive to a similar value of the brake actuator command  118  and under similar brake operating environment (as indicated by the brake operating environment signal(s)  330 ). Additional details regarding operation of the fault-tolerant torque system  130  are described below. 
       FIG. 4  is a diagram illustrating aspects of the brake system  100  of  FIGS. 1-3  according to a particular implementation. In particular,  FIG. 4  illustrates further details of the fault-tolerant torque system  130  and interactions between the fault-tolerant torque system  130  and other components of the vehicle  200  of  FIG. 2 . 
     In  FIG. 4 , the brake pedal sensor  302  sends the brake pedal input signal  304  to the brake pedal command logic  306 . The brake pedal command logic  306  generates a filtered position signal  404  by passing the brake pedal input signal  304  through an anti-aliasing filter  402 . The brake pedal command logic  306  applies a pedal gain  406  to the filtered position signal  404  to generate the brake pedal command  106 . The brake pedal command  106  is provided to the control circuitry  222 , which generates the brake actuator command  118  based in part on the brake pedal command  106 . In some implementations, such as in the example illustrated in  FIG. 3 , the brake pedal command  106  is also provided to the fault-tolerant torque system  130 . To illustrate, in such implementations, the brake pedal command  106  may be provided to a brake mode detector  424  to facilitate detection of sensor fault conditions, as discussed further below. In other implementations, such as in the example illustrated in  FIG. 4 , the brake actuator command  118  is used for brake mode detection. 
     In the example illustrated in  FIG. 4 , the brake actuator command  118  is sent to the brake actuation system  120 . The brake actuation system  120  generates a braking force (and corresponding braking torque) due to braking in response to the brake actuator command  118 . A brake gain  410  describes a relationship between a value of the brake actuator command  118  and a value of the braking force or braking torque. 
     The brake torque sensor(s)  126  generate the brake torque signal  128  indicative of the braking torque. The brake torque signal  128  is provided to the fault-tolerant torque system  130 . Additionally, in some implementations, the brake operating environment sensor(s)  234  provide brake operating environment signal(s)  330  to the fault-tolerant torque system  130 . In the example, illustrated in  FIG. 4 , the brake operating environment sensor(s)  234  include one or more of brake temperature sensor(s)  412 , wheel temperature sensor(s)  414 , wheel speed sensor(s)  416 , or ground speed sensor(s)  418 . 
     In the example of  FIG. 4 , the fault-tolerant torque system  130  uses a low pass filter  420  to remove high frequency components of the brake torque signal  128  to generate a filtered torque signal  422 . The filtered torque signal  422 , the brake operating environment signal(s)  330 , and the brake actuator command  118  (or the brake pedal command  106 ) are provided as input to the sensor monitor  240 . 
     In  FIG. 4 , the sensor monitor  240  includes a brake mode detector  424 . The brake mode detector  424  determines whether a braking operation should be evaluated by the fault-tolerant torque system  130 . Since a function of the brake load alleviation system  134  is to limit loads applied to particular structures of a vehicle during braking, factors that contribute to loading the particular structures are considered to determine whether the braking operation should be evaluated by the fault-tolerant torque system  130 . For example, the brake mode detector  424  may consider brake operating environment information, such as wheel speed or ground speed, determined from the brake operating environment signal(s)  330 . In this example, if the wheel speed or ground speed is less than a threshold, the brake mode detector  424  may determine that the fault-tolerant torque system  130  does not need to evaluate the braking operation, in which case, no further action is taken by the fault-tolerant torque system  130 . As another example, the brake mode detector  424  may consider the magnitude of a braking operation, as indicated by the brake pedal command  106  or the brake actuator command  118 . In this example, if the magnitude of a braking operation is less than a threshold, the brake mode detector  424  may determine that the fault-tolerant torque system  130  does not need to evaluate the braking operation, in which case, no further action is taken by the fault-tolerant torque system  130 . 
     If the brake mode detector  424  determines that the braking operation should be evaluated by the fault-tolerant torque system  130 , the sensor monitor  240  compares the brake torque signal  128 , the filtered torque signal  422 , or both, to the fault criteria  242 . In the example of  FIG. 4 , if no fault is detected, valid sensor data  426  is provided to the model updater  252 , and the brake torque signal  128  is provided to the brake load alleviation system  134  to generate the load alleviation command  136 . Alternatively in the example of  FIG. 4 , if a fault is detected, a fault indication signal  434  is provided to the torque estimator  250 , and the torque estimator  250  determines the estimated brake torque signal  132  based on model output  442 . The estimated brake torque signal  132  is provided to the brake load alleviation system  134  to generate the load alleviation command  136 . 
     The valid sensor data  426  includes data from the brake torque signal  128  and the brake actuator command  118 . In some implementations, the valid sensor data  426  also includes data from the brake operating environment signal(s)  330 . The model updater  252  uses the valid sensor data  426  to generate model update data  432  to update the brake model(s)  256 . As an example, the model updater  252  stores historical data  428  indicating historical values of the valid sensor data  426 , such as a valid historical brake torque signal value, a corresponding brake actuator command value, and a corresponding brake temperature value. The model updater  252  adds the valid sensor data  426  as one or more data entries in the historical data  428 , and a calculator  430  of the model updater  252  determines the model update data  432  based on the valid sensor data  426  and the historical data  428 . For example, as described with reference to  FIG. 6 , the calculator  430  may determine an average or moving average of the historical data  428 . As another example, the calculator  430  may shift or modify the surface  812  within the feature space  802  of  FIG. 8  to generate the model update data  432 . As yet another example, the calculator  430  may update a brake gain parameter used by a torque estimation function, such as Equation  1  above. In some implementations, the historical data  428 , including the valid sensor data  426 , can be used to update or modify the threshold values  508  used by the fault criteria  242 , as described with reference to  FIG. 5 . The model update data  432  is stored in a memory  436  to update one or more tables  438  of the brake model(s)  256 , to update one or more parameters  440  of the brake model(s)  256 , or both. 
     The fault indication signal  434  causes the torque estimator  250  to generate the estimated brake torque signal  132  based on the model output  442 , which is based on the brake actuator command  118  and the brake model(s)  256 . In some implementations, the torque estimator  250  generates the estimated brake torque signal  132  based further on the brake operating environment signal(s)  330 . As a first example, when the brake model(s)  256  include the tables  438  or other data structures (such as illustrated in  FIGS. 5-7 ) that store estimated brake torque values for particular brake actuator command values, the torque estimator  250  looks up a value of the estimated brake torque signal  132  from the table(s)  438  or other data structure(s) based on the value of the brake actuator command  118 . As a second example, when the brake model(s)  256  include parameters  440  of a function (such as the brake gain parameter of Equation 1), the torque estimator  250  calculates a value of the estimated brake torque signal  132  based on the value of the brake actuator command  118  and the brake gain parameter value. 
       FIG. 9  is a flowchart of an example of a method  900  implemented by the brake system  100  of  FIGS. 1-4  according to a particular implementation. For example, the method  900  may be initiated, performed, or controlled by the fault-tolerant torque system  130  or one or more components thereof. 
     The method  900  includes, at block  902 , receiving at least a brake actuator command. For example, the fault-tolerant torque system  130  of  FIGS. 1-4  receives the brake actuator command  118  from the control circuitry  222 . At block  902 , the method  900  may also include receiving other signals including a brake torque signal and/or one or more brake operating environment signal(s). For example, the fault-tolerant torque system  130  of  FIGS. 1-4  receives the brake torque signal  128  from the brake torque sensor(s)  126  and receives the brake operating environment signal(s)  330  from the brake operating environment sensor(s)  234 . In some situation, the brake torque sensor(s)  126  may experience a fault condition that results in the fault-tolerant torque system  130  not receiving the brake torque signal  128  when a brake torque signal  128  is expected (e.g., during a braking operation in which significant brake torque is generated). Such situations result in detection of a fault condition as discussed further below. 
     The method  900  includes, at block  904 , performing brake mode detection, which in the method  900  includes, at block  906 , determining whether brakes are applied. For example, a determination of whether the brakes are applied may be made based on the brake pedal command  106  or based on the brake actuator command  118 . If the determination at block  906  is that the brakes are not applied, the method  900  returns to block  902  to await receipt of subsequent signals. 
     If the determination at block  906  is that the brakes are applied, the method  900  proceeds, at block  908 , to determine whether a speed of the vehicle  200  is greater than a threshold. For example, a wheel speed value or a ground speed value from the brake operating environment signal(s)  330  may be compared to a threshold. If the determination at block  908  is that the speed of the vehicle  200  is less than (or less than or equal to) the threshold, the method  900  returns to block  902  to await receipt of subsequent signals. If the determination at block  908  is that the speed of the vehicle  200  is greater than the threshold, the method  900  determines, at  910 , whether a fault condition is detected. In implementations that do not use the brake operating environment sensor(s)  234  to generate the brake operating environment signal(s)  330 , the decision at block  908  is omitted. 
     If a brake torque signal  128  is received at block  902 , the determination, at block  910 , of whether a fault condition is detected includes comparing a value indicated by the brake torque signal  128  to the fault criteria  242  to determine whether the brake torque signal value is valid. If the value indicated by the brake torque signal  128  is valid (e.g., if the value of the brake torque signal is within a threshold range indicted by the fault criteria  242 ), block  910  indicates that no fault is detected, and the method  900  proceeds to block  912 . If the value indicated by the brake torque signal  128  is not valid (e.g., if the value of the brake torque signal  128  is outside the threshold range indicated by the fault criteria  242 ), block  910  indicates that a fault is detected, and the method  900  proceeds to block  914 . Additionally, in some implementations, if no brake torque signal  128  is received at block  902  when one is expected (e.g., when the brake mode detection of block  904  indicates that the brakes are applied and the vehicle is moving at a speed greater than a threshold), block  910  indicates that a fault is detected. 
     If the determination at block  910  is that no fault condition is detected, then the brake torque signal  128  is considered to include valid sensor data  426 , and the method  900  includes, at block  912 , updating a brake model based on the valid sensor data  426 . For example, the valid sensor data  426  may be provided to the model updater  252 , which may update the brake model(s)  256 . 
     If the determination at block  910  is that a fault condition is detected, the method  900  includes, at block  914 , generating an estimated brake torque signal  132  based on a brake model  256 . For example, the torque estimator  250  may use the brake model(s)  256  and the brake actuator command  118  to determine a value of the estimated brake torque signal  132 . 
     Additionally, if the determination at block  910  is that no fault condition is detected, the value indicated by the brake torque signal  128  is provided to the brake load alleviation system  134 . Alternatively, if the determination at block  910  is that a fault condition is detected, the estimated brake torque signal  132  is provided to the brake load alleviation system  134 . 
       FIG. 10  is a flowchart of another example of a method  1000  that is implemented by the brake system  100  of  FIGS. 1-4  according to a particular implementation. For example, the method  1000  may be initiated, performed, or controlled by the fault-tolerant torque system  130  or components thereof. 
     The method  1000  includes, at block  1002 , determining whether a sensor fault condition is detected based on a brake torque signal  128  from a brake torque sensor  126 . For example, the sensor monitor  240  of the fault-tolerant torque system  130  determines, based on the brake torque signal  128  from the brake torque sensor  126  whether a sensor fault condition is detected. In some implementations, the fault-tolerant torque system  130  also uses other data to determine whether a sensor fault condition is detected, such as a value of the brake pedal command  106 , a value of the brake actuator command  118 , a value of a brake operating environment signal  330 , or a combination thereof. 
     When a determination is made, at block  1004 , that no sensor fault condition is detected, the method  1000  includes, at block  1012 , generating the brake actuation signal based on the brake torque signal. For example, as shown in  FIG. 3 , the fault-tolerant torque system  130  provides the brake torque signal  128  to the brake load alleviation system  134  when no sensor fault condition is present, and the brake load alleviation system  134  generates that load alleviation command  136  based on the brake torque signal  128 . In this example, the control circuitry  222  uses the load alleviation command  136  and the brake pedal command  106  to generate the brake actuator command  118 . Thus, in this situation, the brake actuator command  118  is based on the brake torque signal  128 . 
     When a determination is made, at block  1004 , that a sensor fault condition is detected, the method  1000  includes, at block  1006 , accessing a brake model from a memory accessible to the brake system control unit. For example, the brake system control unit  220  of  FIG. 2  accesses the brake model(s)  256 . In some implementations, such as illustrated in  FIGS. 5-7 , the brake model(s)  256  includes a plurality of data entries representing historical brake torque values  506  corresponding to various brake actuator command values  502 . In some such implementations, the plurality of data entries also include one or more default brake torque values  504 , where each of the default brake torque values  504  represents a brake torque that is used for a respective brake command value until a sufficient number of historical brake torque values  506  are accumulated. In other implementations, the brake model(s)  256  includes parameters  440  of a brake gain function, such as Equation  1 , and the values of the parameters  440  of the brake gain function are based on historical brake torque values and corresponding brake command values. In some such implementations, initial or default parameter values of the parameters  440  may be used until the parameters  440  are updated to generate update parameter values based on model update data  432 . 
     The method  1000  also includes, at block  1008 , generating an estimated brake torque signal  132  based on the brake model(s)  256  and a brake actuator command  118 . For example, the torque estimator  250  generates the estimated brake torque signal  132  based on the brake model  256  and based on the brake actuator command  118 . The brake actuator command  118  is generated in a manner that limits load applied to structures  202  of the vehicle  200  due to braking to less than a specified load limit. In some implementations, an estimated brake torque value of the estimated brake torque signal  132  is determined by interpolation between brake torque values from the brake model(s)  256 . 
     The method  1000  also includes, at block  1010 , generating a load alleviation command  136  based on the estimated brake torque signal  132 . For example, as shown in  FIGS. 1-4 , the brake load alleviation system  134  generates the load alleviation command  136  based on the estimated brake torque signal  132  when a sensor fault condition is detected. 
     The method  1000  thus enables operation of a brake load alleviation system to continue when a sensor is present. The fault-tolerant brake load alleviation systems and methods disclosed use a vehicle-specific brake model that is generated and/or updated when no sensor fault is present to generate an estimated brake torque signal  132  when the brake torque signal  128  is not available or is not reliable (e.g., due to a sensor fault). Because the brake model is custom-built for the particular vehicle, the estimated brake torque signal  132  reliably limits the loads to which vehicles structures as subjected, enabling operation of the vehicle without imposing additional operational limits (e.g., operational weight limits or braking distance limits). 
       FIG. 11  is a flowchart illustrating a method  1100  representing a life cycle of a vehicle that includes the structure(s)  202 , the sensor(s)  230 , and the brake system control unit  220  of  FIG. 2 . The vehicle can include an aircraft or a land craft. 
     During pre-production, the exemplary method  1100  includes, at  1102 , specification and design of a vehicle, such as the vehicle  200  of  FIG. 2 , or the aircraft  1200  of  FIG. 12 . During specification and design of the vehicle, the method  1100  may include specification and design of the structure(s)  202 , the sensor(s)  230 , and the brake system control unit  220 . At  1104 , the method  1100  includes material procurement, which may include procuring materials for the structure(s)  202 , the sensor(s)  230 , and the brake system control unit  220 . 
     During production, the method  1100  includes, at  1106 , component and subassembly manufacturing and, at  1108 , system integration of the vehicle. For example, the method  1100  may include component and subassembly manufacturing of and system integration of the structure(s)  202 , the sensor(s)  230 , and the brake system control unit  220 . At  1110 , the method  1100  includes certification and delivery of the vehicle and, at  1112 , placing the vehicle in service. Certification and delivery may include certification of the structure(s)  202 , the sensor(s)  230 , and the brake system control unit  220  to place the structure(s)  202 , the sensor(s)  230 , and the brake system control unit  220  in service. While in service by a customer, the vehicle may be scheduled for routine maintenance and service (which may also include modification, reconfiguration, refurbishment, and so on). At  1114 , the method  1100  includes performing maintenance and service on the vehicle, which may include performing maintenance and service on the structure(s)  202 , the sensor(s)  230 , and the brake system control unit  220 . 
     Each of the processes of the method  1100  may be performed or carried out by a system integrator, a third party, and/or an operator (e.g., a customer). For the purposes of this description, a system integrator may include without limitation any number of manufacturers and major-system subcontractors; a third party may include without limitation any number of venders, subcontractors, and suppliers; and an operator may be an airline, a leasing company, a military entity, a service organization, and so on. 
     Aspects of the disclosure can be described in the context of an example of a vehicle. A particular example of a vehicle is an aircraft  1200  as shown in  FIG. 12 . 
     In the example of  FIG. 12 , the aircraft  1200  includes an airframe  1250  that includes the structure(s)  202 . The aircraft  1200  also includes a plurality of systems  1220  and an interior  1252 . Examples of the plurality of systems  1220  include one or more of a propulsion system  1222 , an electrical system  1224 , an environmental system  1226 , a hydraulic system  1228 , and the brake system  100 . The brake system  100  includes the sensor(s)  230  and the brake system control unit  220  of  FIG. 2 . The aircraft  1200  may also include any number of other systems. 
     In some implementations, a non-transitory, computer readable medium stores instructions that, when executed by one or more processors, cause the one or more processors to initiate, perform, or control operations to perform part or all of the functionality described above. For example, the instructions may be executable to implement one or more of the operations or methods of  FIGS. 1-10 . In some implementations, part or all of one or more of the operations or methods of  FIGS. 1-10  may be implemented by one or more processors (e.g., one or more central processing units (CPUs), one or more graphics processing units (GPUs), one or more digital signal processors (DSPs)) executing instructions, by dedicated hardware circuitry, or any combination thereof. 
     The illustrations of the examples described herein are intended to provide a general understanding of the structure of the various implementations. The illustrations are not intended to serve as a complete description of all of the elements and features of apparatus and systems that utilize the structures or methods described herein. Many other implementations may be apparent to those of skill in the art upon reviewing the disclosure. Other implementations may be utilized and derived from the disclosure, such that structural and logical substitutions and changes may be made without departing from the scope of the disclosure. For example, method operations may be performed in a different order than shown in the figures or one or more method operations may be omitted. Accordingly, the disclosure and the figures are to be regarded as illustrative rather than restrictive. 
     Moreover, although specific examples have been illustrated and described herein, it should be appreciated that any subsequent arrangement designed to achieve the same or similar results may be substituted for the specific implementations shown. This disclosure is intended to cover any and all subsequent adaptations or variations of various implementations. Combinations of the above implementations, and other implementations not specifically described herein, will be apparent to those of skill in the art upon reviewing the description. 
     The Abstract of the Disclosure is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. In addition, in the foregoing Detailed Description, various features may be grouped together or described in a single implementation for the purpose of streamlining the disclosure. Examples described above illustrate but do not limit the disclosure. It should also be understood that numerous modifications and variations are possible in accordance with the principles of the present disclosure. As the following claims reflect, the claimed subject matter may be directed to less than all of the features of any of the disclosed examples. Accordingly, the scope of the disclosure is defined by the following claims and their equivalents.