Abstract:
Various embodiments of an apparatus and method for monitoring a brake operation are disclosed. In accordance with one aspect, the brake operation monitoring system comprises a plurality of wheel speed sensors, a brake demand sensor; a plurality of stability sensors and a controller. The controller comprises wheel speed ports; a brake demand port; stability sensor ports; a communication port for receiving a plurality of messages; and a processing unit comprising control logic. The control logic receives a brake demand signal, at least one stability signal indicative of the cornering of the vehicle, and individual wheel speeds. The control logic calculates a master value to compare to individual wheel speed signals if the brake demand signal indicates no braking.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a continuation of application Ser. No. 14/533,149 “Method, Controller and system for Monitoring Brake Operation” filed Nov. 5, 2014. The entire disclosure of the aforementioned application is herein expressly incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    The present invention relates to an apparatus and method for monitoring brake operation on a commercial vehicle. Commercial vehicles, such as tractor-trailers and buses, are generally equipped with an anti-lock braking or stability control system. These systems control vehicle braking in the event of wheel slip or instability of the vehicle by monitoring a variety of sensors on the vehicle. Some vehicles are equipped with brake wear sensing devices that indicate the wear of each brake lining. A warning is typically given to the operator when the wear exceeds a predetermined threshold. However, the brake wear sensors do not measure brake drag, which can occur when the brake shoe or brake pads are not completely released at the completion of a service brake application. The brake wear sensors do not measure an underperforming brake, which can occur when the brake at a particular wheel end does not apply as quickly or as fully as the other wheel end brakes on the vehicle. Information from the wear devices and sensors associated with the stability control system are typically evaluated instantaneously during vehicle operation, which does not indicate any long-term trends in the brake operation. These brake wear systems are typically separate from the anti-lock braking or stability control systems so that any information learned from the brake wear system is not used to improve braking performance. 
       SUMMARY 
       [0003]    Various aspects of a brake operation monitoring system are disclosed. In accordance with one aspect, the brake operation monitoring system comprises a plurality of wheel speed sensors, each of the plurality of wheel speed sensors correlated with a specific wheel location; a brake demand sensor; a plurality of stability sensors for receiving stability signals; and a controller. The controller comprises a plurality of wheel speed inputs for receiving individual wheel speed sensor signals; a brake demand input; a plurality of stability sensor inputs for receiving stability signals; a communication port for transmitting and receiving a plurality of messages; and a processing unit comprising control logic, wherein the processing unit is in communication with the plurality of wheel speed inputs and the communication port. The control logic is capable of receiving a brake demand signal indicative of a brake demand; receiving at least one stability signal indicative of the cornering of the vehicle; receiving signals indicative of individual wheel speeds; determining a master wheel speed signal from the individual wheel speed signals if the brake demand signal indicates no braking; determining a baseline calibration of the individual wheel speed signals based on the difference of each wheel speed signal from the master wheel speed signal if the at least one stability signal indicates no cornering; and storing the differences in the processing unit. 
         [0004]    In accordance with another aspect, a method for monitoring brake operation on a vehicle comprises receiving a brake demand signal indicative of a brake demand; receiving at least one stability signal indicative of the stability of the vehicle; receiving a plurality of wheel speed signals from individual wheel speed sensors associated with individual wheel ends of the vehicle; determining the velocity of the vehicle is at least as great as a predetermined minimum velocity; calculating a master value from the plurality of wheel speed signals wherein the brake demand signal indicates no braking and wherein the at least one stability signal indicates no cornering; determining a baseline calibration of each of the individual wheel speed signals; and storing the baseline calibration in the processing unit. 
         [0005]    In accordance with another aspect, a controller for detecting a brake system deficiency of a vehicle comprises a plurality of wheel speed inputs for receiving individual wheel speed sensor signals, each of the plurality of wheel speed inputs correlated with a specific wheel location; a brake demand input; a plurality of stability sensor inputs for receiving stability signals; a communication port for transmitting and receiving a plurality of messages; and a processing unit comprising control logic, wherein the processing unit is in communication with the plurality of wheel speed inputs and the communication port. The control logic is capable of receiving a brake demand signal indicative of a brake demand; receiving at least one stability signal indicative of the cornering of the vehicle; receiving signals indicative of individual wheel speeds; calculating a master value if the brake demand signal indicates no braking; determining a baseline calibration of the individual wheel speed signals based on the difference of each wheel speed signal from the master value if the at least one stability signal indicates no cornering; and storing the differences in the processing unit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    In the accompanying drawings which are incorporated in and constitute a part of the specification, embodiments of the invention are illustrated, which, together with a general description of the invention given above, and the detailed description given below, serve to exemplify the embodiments of this invention. 
           [0007]      FIG. 1  illustrates a schematic representation of a vehicle having a tractor portion and a trailer portion, according to an example of the present invention. 
           [0008]      FIG. 2  illustrates a method of implementing the brake monitoring system, according to an example of the present invention. 
           [0009]      FIG. 3  illustrates another method of implementing the brake monitoring system, according to an example of the present invention. 
           [0010]      FIG. 4  illustrates another method of implementing the brake monitoring system, according to an example of the present invention. 
           [0011]      FIG. 5  illustrates another method of implementing the brake monitoring system, according to an example of the present invention. 
           [0012]      FIG. 6  illustrates another method of implementing the brake monitoring system, according to an example of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    With reference to  FIG. 1 , a vehicle  10  with a tractor portion  12  and a trailer portion  13  is illustrated, according to an example of the present invention. Both the tractor portion  12  and the trailer portion  13  can be equipped with the brake monitoring function, or only one portion may be equipped with the brake monitoring function. 
         [0014]    The tractor portion  12  of vehicle  10  generally has at least six wheel locations. The wheel locations of the tractor portion  12  include right front, right mid, right rear, left front, left mid and left rear. The trailer portion  13  of vehicle  10  generally has at least four wheel locations. The wheel locations of the trailer portion  13  include right front, right rear, left front and left rear. 
         [0015]    In the example shown in  FIG. 1 , the tractor portion  12  of vehicle  10  includes four wheel speed sensors  14   a ,  14   b ,  14   c ,  14   d . The trailer portion  13  of the vehicle  10  includes two wheel speed sensors  14   e ,  14   f . Each wheel speed sensor  14   a ,  14   b ,  14   c ,  14   d ,  14   e ,  14   f  generates a wheel speed signal for the particular wheel location when the tires are rotating. There can be greater than six or fewer than six wheel speed sensors on the vehicle  10 . 
         [0016]    The tractor portion  12  of vehicle  10  includes a brake control device  34   a ,  34   b ,  34   c ,  34   d  at each wheel location. Each brake control device  34   a ,  34   b ,  34   c ,  34   d  is pneumatically connected to control valves  35   a ,  35   b  and used for anti-lock braking and/or stability control. In other examples, there can be a single brake control device per vehicle axle or a brake control device for each wheel location. In one example, the brake control devices  34   a ,  34   b ,  34   c ,  34   d  are antilock brake control modulators. 
         [0017]    The tractor portion  12  of vehicle  10  includes a brake pedal  24 . Two brake demand sensors  26   a ,  26   b  are mounted on or proximate to the brake pedal  24 . In one example, the brake demand sensors  26   a ,  26   b  are pressure sensors. 
         [0018]    Since the tractor portion  12  of the vehicle  10  is equipped with a stability control system, such as the Bendix® ESP® Full Stability System, the tractor portion  12  includes a combination lateral acceleration sensor and yaw rate sensor  26 . The tractor portion  12  of the vehicle  10  includes a steering angle sensor  30 , which is also used in stability control of the vehicle  10 . 
         [0019]    The tractor portion  12  of the vehicle  10  includes a radar sensor  32  for use in an adaptive cruise with braking system, such as the Bendix Wingman® Advanced ACB. The information transmitted by the radar sensor  48  typically includes automated deceleration requests. A deceleration signal is created in response to the automated deceleration request when the ACB system determines the vehicle  10  needs to decelerate in order to maintain a certain following distance between the vehicle  10  and a target vehicle. 
         [0020]    The tractor portion  12  of the vehicle  10  includes a battery  40 . The battery  40 , or battery pack, powers the entire vehicle  10 . The tractor portion  12  of the vehicle  10  includes an ignition switch  38 . The ignition switch  38  provides power whenever the driver has engaged the ignition of the vehicle  10 . 
         [0021]    The tractor portion  12  of the vehicle  10  includes a stop lamp  36 . Power is transmitted through a stop lamp switch (not shown) to the light the stop lamp  36  whenever the driver depresses the brake pedal  24  and an approximate brake pressure of six pounds per square inch (psi) is reached or exceeded. Power is also provided to the trailer portion  13  of the vehicle  10  at the same time the stop lamp switch on the tractor is activated. 
         [0022]    The tractor portion  12  of the vehicle  10  includes a serial communications bus  42 . The serial communications bus  42  carries messages in an SAE J1939 format or a proprietary format among controllers connected to the serial communications bus  42  on the tractor portion  12 . 
         [0023]    The tractor portion  12  of the vehicle  10  includes a display  44  so that the driver can see the status of the vehicle braking system, including the brake monitoring operation. The display  44  may be connected to the serial communications bus  42  or directly to a tractor controller  22 . 
         [0024]    Each wheel location on tractor portion  12  includes a tire sensor, illustrated by tire sensors  46   a ,  46   b ,  46   c ,  46   d ,  46   e ,  46   f . Each wheel location on trailer portion  13  includes a tire sensor, illustrated by tire sensors  46   g ,  46   h ,  46   i ,  46   j . The tire sensors  46   a ,  46   b ,  46   c ,  46   d ,  46   e ,  46   f ,  46   g ,  46   h ,  46   i ,  46   j  monitor tire characteristic information, such as the tire pressure, tire temperature, sensor battery voltage, vehicle load and tire vibration. The tire sensors can include an accelerometer or centrifugal switch to indicate a motion value, such as rotation of the wheel. The tire sensors  46   a ,  46   b ,  46   c ,  46   d ,  46   e ,  46   f ,  46   g ,  46   h ,  46   i ,  46   j  can be tire pressure sensors used in the Smartire® Tire Pressure Monitoring System from Bendix Commercial Vehicle Systems LLC, of Elyria, Ohio, for example. Each tire sensor  46   a ,  46   b ,  46   c ,  46   d ,  46   e ,  46   f ,  46   g ,  46   h ,  46   i ,  46   j  comprises a wireless transmitter that periodically transmits signals containing the tire related information and a unique sensor identification code (ID) in a selected data transmission format. 
         [0025]    The tractor portion  12  of vehicle  10  includes a tractor controller  22 . The tractor controller  22  can be a stand-alone controller or include functionality for controlling the anti-lock braking, stability control, or active cruise control with braking system, in addition to brake operation monitoring. 
         [0026]    The trailer portion  13  of vehicle  10  includes a trailer controller  23 . The trailer controller  23  can be a stand-alone controller or include functionality for controlling the anti-lock braking or stability control system in addition to brake operation monitoring. The trailer controller  23  receives power from the tractor portion  12  via a battery connection or via the connection for powering the stop lamp  36 . 
         [0027]    The tractor controller  22  includes control logic  21  for performing the brake monitoring function. The control logic  21  may also perform anti-lock braking, stability control or active cruise with braking functions. The control logic  21  may include volatile, non-volatile memory, solid state memory, flash memory, random-access memory (RAM), read-only memory (ROM), electronic erasable programmable read-only memory (EEPROM), variants of the foregoing memory types, combinations thereof, and/or any other type(s) of memory suitable for providing the described functionality and/or storing computer-executable instructions for execution by the control logic  21 . Values determined during operation of the brake monitoring methods may also be stored in the memory of the control logic  21 . 
         [0028]    The tractor controller  22  communicates with the serial communications bus  42 . The tractor controller  22  communicates with the display  44 . The display  44  informs the vehicle operator of any issues with the braking system that may be determined by the tractor controller  22 . The tractor controller  22  may also send information regarding brake operation to the serial communications bus  42 . 
         [0029]    The tractor controller  22  receives a signal indicative of brake demand. The brake demand signal may come directly from the pressure sensors  26   a ,  26   b  or from another source in the service braking circuit. The tractor controller  22  includes at least one input for receiving signals indicative of the stability of the vehicle  10 . The tractor controller  22  may receive yaw rate signals, lateral acceleration signals and/or steering angle sensor signals. The tractor controller  22  includes individual inputs for the wheel speed sensor signals from the wheel speed sensors  14   a ,  14   b ,  14   c ,  14   d . Each of these signals may be directly connected to the tractor controller  22  as shown in  FIG. 1  or may be received via other means, such as via the serial communications bus  42 . 
         [0030]    The tractor controller  22  is connected to the battery  40  for powering the tractor controller  22 . The tractor controller  22  also includes input from the ignition switch  38 . The tractor controller  22  receives power at the ignition switch input only when the ignition switch  38  is engaged. 
         [0031]    The tractor controller  22  communicates with the brake control devices  34   a ,  34   b ,  34   c ,  34   d  and control valves  35   a ,  35   b . The brake control devices  34   a ,  34   b ,  34   c ,  34   d  receive an air supply from either an application of the brake pedal  24  by the driver or from the control valves  35   a ,  35   b . The control valves  35   a ,  35   b  are independently controlled by the tractor controller  22  to provide supply pressure independent of the driver&#39;s brake application. 
         [0032]    The control logic  21  of the tractor controller  22  uses the each of the sensors and devices described above to monitor brake operation at a wheel end. For example, brake drag can occur when the pressure applied to a brake is not fully released at the end of the service brake application. A cause of brake drag includes a misaligned caliper that holds the brake pad in contact with the rotor at the end of the service brake application. Brake drag is monitored after a braking event while the vehicle  10  is moving. The control logic  21  of the tractor controller  22  also identifies an under-performing brake at a wheel end. An under-performing brake is one that does not apply the brake to the same pressure level as the other wheel end brakes. Some causes of under-performing brakes include out of adjustment brakes or over worn brake pads. Under-performing brakes are monitored during a braking event. 
         [0033]    A series of flowcharts for implementing methods of monitoring brake operation according to examples of the present invention are shown in  FIGS. 2-6 . 
         [0034]    In  FIG. 2 , the master value determination method  100  is initiated in step  102 . The control logic  21  determines if the vehicle  10  is braking in step  106  by determining if there is a brake demand signal from the pressure sensors  26   a ,  26   b . If there is no brake demand signal, the method  100  proceeds to step  108 . If there is a brake demand signal, the method  100  returns to step  102 . In step  108 , the control logic  21  determines if the vehicle  10  is accelerating, either by using the signals from the wheel speed sensors  14   a ,  14   b ,  14   c ,  14   d  or other indicators of acceleration, such as acceleration messages on the serial communications bus  42 . If the vehicle  10  is not accelerating, the method  100  continues to step  110 . If the vehicle  10  is accelerating, the method  100  returns to step  102 . In step  110 , the control logic  21  determines if the vehicle  10  is cornering, either by using the signal from the steering angle sensor  30  or other indicators of cornering. Cornering includes hard left and right turns, as well as turning to exit a highway via an off ramp. A change in the steering angle due to a normal lane change on the highway would not be considered cornering. If the vehicle  10  is not cornering, the method  100  continues to step  112 . If the vehicle  10  is cornering, the method  100  returns to step  102 . 
         [0035]    In step  112 , the control logic  21  receives each of the wheel speed signals from wheel speed sensors  14   a ,  14   b ,  14   c ,  14   d . In step  114 , the wheel speed signals are monitored for an initiation time period. In one example, the initiation time period is about thirty (30) seconds. In another example, the initiation time period is about sixty (60) seconds. If the initiation time period has not elapsed, the method  100  continues to collect the wheel speed signals as in step  112 . 
         [0036]    Once the initiation time period has elapsed, the control logic  21  determines a master value in step  120 . The master value will be compared individually to each wheel speed signal in later steps. A master value can be determined in multiple different manners. In one example, the master value is set as the mode, or most frequently occurring, wheel speed signal. In another example, the master value is the mean, or average, of all of the wheel speed signals. In another example, the master value is set to match the two closest wheel speed signals. In another example, the master value is the vehicle velocity estimated by the tractor controller  22  using the wheel speed signals. In another example, the vehicle velocity is received from another controller on the vehicle  10 , such as an engine controller, a radar controller or a separate anti-lock braking system controller and received by the tractor controller  22  on the serial communications bus  42 . In another example, the master value is set as the wheel speed signal equal to or nearly equal to the overall vehicle velocity. Once the control logic  21  determines the master value, the master value is stored in the memory portion of the control logic  21  in step  122 . Once the master value is stored in step  122 , the brake monitoring operation can move sequentially into any one of short-term monitoring method  140 , drag monitoring method  180 , underperforming brake monitoring method  220 , and long-term monitoring method  240  or all monitoring methods  140 ,  180 ,  220 ,  240  can run simultaneously. 
         [0037]    In an example as shown in  FIG. 3 , a short-term monitoring method  140  is shown. In step  142 , the short-term monitoring method  140  is initiated. In step  144 , the velocity of the vehicle  10  is monitored to determine if the velocity is less than a predetermined minimum velocity. In one example, the predetermined minimum velocity is five (5) miles per hour and in another example, the predetermined minimum velocity is ten (10) miles per hour. If the vehicle  10  is moving equal to or greater than the predetermined velocity, the method  140  continues to step  146 . If the velocity of the vehicle  10  is less than the predetermined velocity, the method returns to step  142 . 
         [0038]    In step  146 , the control logic  21  determines if the vehicle  10  is being controlled by an active safety system, such as a stability control or active cruise with braking system. An active safety system will automatically control the vehicle brakes, such as with stability control interventions or active cruise with braking interventions. If the vehicle  10  is being controlled by an active safety system, the method  140  returns to step  142 . If the vehicle is not being controlled by an active safety system, the method  140  continues to step  148 . 
         [0039]    In step  148 , the control logic  21  determines if the brake demand signal from the pressure sensors  26   a ,  26   b  is greater than a predetermined maximum braking threshold or less than a predetermined minimum braking threshold. In one example, the predetermined minimum braking threshold is about ten pounds per square inch (10 psi) and the predetermined maximum predetermined braking threshold is about thirty pounds per square inch (30 psi). If the brake demand signal is greater than the predetermined maximum braking threshold or less than the predetermined minimum braking threshold, the method  140  returns to step  142 . If the brake demand signal is less than or equal to the predetermined maximum braking threshold or greater than or equal to the predetermined minimum braking threshold, the method  140  continues to step  149 . In this manner, the control logic  21  determines that the brakes have been applied on the vehicle by the driver in an attempt to decelerate the vehicle in a normal manner. 
         [0040]    In step  149 , the brake demand signal is monitored to determine if brake demand is still present. If the brake demand is still present, the method  140  returns to step  142 . If the brake demand is not present, the method  140  continues to step  150 . 
         [0041]    In step  150 , the control logic  21  selects the lowest velocity wheel speed signal of the individual wheel speed sensors  14   a ,  14   b ,  14   c ,  14   d  to compare to the master value, as determined by method  100 . The master value may be determined anew each time the short-term monitoring method  140  is initiated. The lowest velocity wheel speed signal is selected as it is more likely to be wheel speed sensor on the wheel end with a potential brake drag issue. In step  152 , the control logic  21  determines the difference of the lowest velocity wheel speed sensor signal from the master value. In step  154 , the difference of the lowest velocity wheel speed signal from the master value is integrated, meaning that the difference is accumulated over time. For example, to calculate an integrated difference, velocity V 1  at time t 1  is subtracted from the velocity V 2  at time t 2  and the difference is divided by the difference in time t 2 −t 1  to obtain an integrated velocity IV 1 . The integrated velocity IV 2  is then subtracted from the velocity V 3  at time t 3  and the difference is divided by the difference in t3−t2, and so on. In step  156 , the integrated difference is compared to a short-term threshold. If the difference is equal to or greater than the short-term threshold, the method  140  continues to step  158 . If the difference is less than the short-term threshold, the method  140  returns to step  150 . In one example, the short-term threshold is between 3 and 10. In another example, the short-term threshold is 4. 
         [0042]    In step  158 , a brake drag error counter is incremented by one increment when the integrated difference of the lowest velocity wheel speed signal is greater than or equal to the short-term threshold. 
         [0043]    In step  160 , the brake drag error counter is compared to an error count threshold. If the accumulated brake drag error counter is less than the error count threshold, the method  140  continues to step  162 . If the brake drag error counter is greater than or equal to the error count threshold, the method  140  continues to step  168  and a short-term fault is logged by the control logic  21 . In one example, the error count threshold is between 3 and 10. In another example, the error count threshold is 5. 
         [0044]    If the control logic  21  logs a short-term brake drag fault, the control logic  21  can send an indicator signal to the display  44  to indicate to the driver that there is likely a brake drag occurring at the wheel end with the fault. 
         [0045]    If the brake drag error counter has not equaled or exceeded the error count threshold in step  160 , the control logic  21  continues to step  162  and determines if short-term time period from the initiation of the short-term monitoring in step  142  has elapsed. If the short-term time period has elapsed, the brake drag error counter is reset to zero in step  164 . This reset prevents noise on a wheel speed signal from causing errors in the measurement or integration. If the short-term time period has not elapsed, the method  140  returns directly to step  150  where the comparison of the lowest velocity wheel speed signal to the master value begins anew. In one example, the short-term time period ranges from about five (5) minutes to about ten (10) minutes. 
         [0046]    In the example in  FIG. 4 , a drag monitoring method  180  is shown. In step  182 , the drag monitoring method  180  is initiated. In step  184 , the control logic  21  monitors the velocity of the vehicle  10  to determine if the velocity is greater than or equal to a predetermined minimum velocity. In one example, the predetermined minimum velocity is five miles per hour. In another example, the predetermined minimum velocity is ten miles per hour. If the vehicle  10  is moving greater than or equal to the predetermined minimum velocity, the method  180  continues to step  186 . If the velocity of the vehicle  10  is less than the predetermined minimum velocity, the method  180  returns to step  182 . 
         [0047]    In step  186 , the control logic  21  determines if the vehicle  10  is cornering, either by using the signal from the steering angle sensor  30  or other indicators of cornering. If the vehicle  10  is not cornering, the method  180  continues to step  188 . If the vehicle  10  is cornering, the method  180  returns to step  182 . 
         [0048]    In step  188 , the control logic  21  determines if the brake demand signal is less than a predetermined maximum braking threshold. In one example, the predetermined maximum braking threshold is about twenty pounds per square inch. In another example, the predetermined maximum braking threshold is about thirty pounds per square inch. If the brake demand signal is less than the predetermined braking threshold, the method  180  continues to step  190 . If the brake demand signal is greater than or equal to the predetermined maximum braking threshold, the method returns to step  182 . 
         [0049]    In step  190 , the control logic  21  compares all of the wheel speed signals of wheel speed sensors  14   a ,  14   b ,  14   c ,  14   d  individually to the master value. In step  192 , the control logic  21  determines the difference of each wheel speed sensor signal from the master value. The master value may be determined anew each time the drag monitoring method  180  is initiated. 
         [0050]    In step  194 , in order to determine if a brake drag exists at any wheel, the difference of each wheel speed signal from the master value is integrated, meaning that the difference is accumulated over time, similar to step  154  of the short-term monitoring method  140 . In step  195 , the integrated difference of each of the wheel speed sensor signals is compared to a drag threshold. If the difference is equal to or greater than the drag threshold, the method  180  continues to step  196 . If the difference is less than the drag threshold, the method  180  returns to step  190 . In one example, the drag threshold is between 8 and 15. In another example, the drag threshold is 10. 
         [0051]    In step  196 , the drag error counter is incremented. The drag error counter is not the same as the brake drag error counter of the short-term monitoring method  140 . In step  198 , the drag error counter is compared against a drag error threshold. If the drag error counter is less than the drag error threshold, the method  180  continues to step  200 . If the drag error counter is greater than or equal to the drag error threshold, the method  180  continues to step  204 . 
         [0052]    The drag monitoring method  180  can cross check information with a tire sensors  46   a ,  46   b ,  46   c ,  46   d ,  46   e ,  46   f  on an individual wheel end. In step  204 , the tire temperature is analyzed from the tire sensor in the wheel end that has exceeded the drag error threshold. In step  206 , the tire temperature is compared to a threshold temperature. If the tire temperature exceeds the threshold temperature, a drag fault is logged in step  208 . Because a dragging brake can cause high temperatures at a wheel end, the high temperature will be captured by a tire sensor in the tire at that wheel end as well. Information about tire temperature can be exchanged over the serial communication bus  42  or the tractor controller  22  can receive the tire pressure information directly from tire sensors  46   a ,  46   b ,  46   c ,  46   d ,  46   e ,  46   f.    
         [0053]    If the control logic  21  logs a drag fault, the control logic  21  may indicate to the driver via indicator  44  that there is brake drag at the wheel end with the higher than drag threshold error counter and higher than temperature threshold at the wheel end. 
         [0054]    If the brake drag error counter has not equaled or exceeded the drag threshold in step  198 , the control logic  21  continues to step  200  and determines if drag time period from the initiation of the drag monitoring step  190  has elapsed. If drag time period has elapsed, the error counter is reset to zero in step  202 . This function compensates for noise on any wheel speed signal that may cause errors in the measurement or integration. If the drag time period has not elapsed, the method  180  returns directly to step  190  to continue to compare all wheel speed signals to the master value. In one example, the drag time period ranges from about ten (10) minutes to about fifteen (15) minutes. 
         [0055]    Once a short-term fault is determined using method  140  or a drag fault is determined in method  180 , the wheel end with brake drag can be given less brake pressure by the tractor controller  22  via the brake control device  34   a ,  34   b ,  34   c ,  34   d  in order to prevent the wheel end from becoming overbraked and affecting the stability of the vehicle. 
         [0056]    In another example shown in  FIG. 5 , an underperforming brake monitoring method  220  is shown. In step  222 , the underperforming brake monitoring method  220  is initiated. 
         [0057]    In step  224 , the brake demand signal is monitored to determine if a brake demand is present. If the brake demand is not present, the method  220  returns to step  222 . If the brake demand is present, the method  220  continues to step  224 . 
         [0058]    In step  224 , the control logic  21  compares all of the wheel speed signals of wheel speed sensors  14   a ,  14   b ,  14   c ,  14   d  individually to the master value. In step  226 , the control logic  21  determines the difference of each wheel speed sensor signal from the master value. The master value may be determined anew each time the underperforming brake monitoring method  220  is initiated. 
         [0059]    In step  228 , in order to determine if an underperforming brake exists at any wheel, an integrated difference of each wheel speed signal from the master value is calculated, meaning that the difference is accumulated over time. In step  230 , the integrated difference of each wheel speed sensor signal is compared to an underperforming brake threshold. If the difference is equal to or greater than the underperforming brake threshold, the method  220  continues to step  232 . If the difference is less than the underperforming brake threshold, the method  220  returns to step  222 . In one example, the underperforming brake threshold is between 12 and 20. In another example, the underperforming brake threshold is 15. 
         [0060]    In step  232 , the underperforming brake error counter is incremented by one. In step  234 , the underperforming brake error counter is compared against an underperforming brake error count threshold. If the underperforming brake error counter is less than the underperforming brake error threshold, the method  220  continues to step  238 . If the underperforming brake error counter is greater than or equal to the underperforming brake error threshold, the method  220  continues to step  236 , where a fault is logged. 
         [0061]    If the underperforming brake error counter has not exceeded the underperforming brake threshold in step  234 , the control logic  21  continues to step  238  and determines if an underperforming brake time period from the initiation of the underperforming brake monitoring step  222  has elapsed. If the underperforming brake monitoring time period has elapsed, the error counter is reset to zero in step  239 . This function compensates for noise on any wheel speed signal that may cause errors in the measurement or integration. If the underperforming brake time period has not elapsed, the method  220  returns directly to step  222  to continue to compare all wheel speed signals to the master value. In one example, the underperforming brake time period ranges from about ten (10) minutes to about fifteen (15) minutes. 
         [0062]    Once an underperforming brake fault is determined using method  220 , the wheel end with the underperforming brake can be given more brake pressure by the tractor controller  22  via the brake control device  34   a ,  34   b ,  34   c ,  34   d  in order to maintain the stability of the vehicle. 
         [0063]    In the example in  FIG. 6 , a long-term monitoring method  240  is shown. In step  242 , the baseline calibration is initiated. The control logic  21  determines if the vehicle  10  is braking in step  244  by determining if there is a brake demand signal from the brake pedal  24 . If there is no brake demand signal, the method  240  proceeds to step  246 . If there is a brake demand signal, the method  220  returns to step  242 . In step  246 , the control logic  21  determines if the vehicle  10  is accelerating, either by using the signals from the wheel speed sensors  14   a ,  14   b ,  14   c ,  14   d  or other indicators of acceleration on the serial communications bus  42 . If the vehicle  10  is not accelerating, the method  240  continues to step  248 . If the vehicle  10  is accelerating, the method  240  returns to step  242 . In step  248 , the control logic  21  determines if the vehicle  10  is cornering, either by using the signal from the steering angle sensor  30  or other indicators of cornering. If the vehicle  10  is not cornering, the method  220  continues to step  250 . If the vehicle  10  is cornering, the method  240  returns to step  242 . 
         [0064]    The method  240  continues to step  250  where the control logic  21  compares each of the individual wheel speed signals to the master value, already determined as per method  100 , for a calibration time period equal to the initiation time period for the master value as completed as in method  100 . The difference between each wheel speed and the master value is determined in step  252 . The difference of each wheel speed signal from the master value is stored as a series of numbers in memory for use as the baseline calibration in step  254 . 
         [0065]    Once the baseline calibration is stored in step  254 , the long-term monitoring method is initiated in step  256 . The control logic  21  determines if a long-term time period has elapsed in step  258 , meaning it has been a predetermined time since the last brake demand signal indicating braking of the vehicle  10  was received. If the long-term time period has elapsed, the method continues to step  260 . If the long-term time period has not elapsed, the method returns to step  256 . In one example, the long-term time period ranges from about 20 minutes to 60 minutes. In another example, the long-term time period is about 30 minutes. 
         [0066]    In step  260 , the control logic  21  determines if the vehicle  10  is accelerating, either by using the signals from the wheel speed sensors  14   a ,  14   b ,  14   c ,  14   d  or other indicators of acceleration on the serial communications bus  42 . If the vehicle  10  is not accelerating, the method  240  continues to step  262 . If the vehicle  10  is accelerating, the method  240  returns to step  258 . 
         [0067]    In step  262 , all of the wheel speed signals are collected for the same period of time as the calibration time period. In step  264 , the wheel speed signals are compared to the baseline calibration as stored in step  254 . If the difference at any point in the comparison of the baseline calibration to a wheel speed signal is greater than or equal to long-term threshold as in step  266 , the method continues to step  268 . If each of the differences between the baseline calibration and the wheel speed signal over the calibration time period is less than the long-term threshold, the method  240  returns to step  256 . In one example, the long-term threshold is about 10. In another example, the long-term threshold is about 20. 
         [0068]    In step  268 , a long-term fault is logged when the difference between the baseline calibration and any individual wheel speed signal is greater than the long-term threshold. 
         [0069]    A similar method for monitoring brake operation can be done in the trailer controller  23  if the trailer controller  23  is equipped with anti-lock braking and stability control functions. The trailer controller  23  would determine its own master value using a method similar to method  100  and also perform a short-term monitoring method, drag monitoring method, underperforming brake monitoring method and long-term monitoring method similar to those described above. 
         [0070]    While the present invention has been illustrated by the description of embodiments thereof, and while the embodiments have been described in considerable detail, it is not the intention of the applicants to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. Therefore, the invention, in its broader aspects, is not limited to the specific details, the representative apparatus, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of the applicant&#39;s general inventive concept.