Abstract:
The brake lining and wear estimation system operates using extant vehicle sensors such as those used to implement height and stability control systems or anti-lock brake systems. This minimizes, or eliminates, the need to modify the brake units themselves. Compressed air systems used on heavy duty vehicles to support the operation of various systems, including air brakes, provide pressure monitoring features in various pneumatic circuits, which can be monitored to estimate brake lining application pressure or vehicle loading. Vehicle speed and brake actuation periods are derived from other commonly employed sensors. Data collected from the sensors is supplied to a central, programmable computer over a data bus to generate the heat and wear estimates.

Description:
BACKGROUND OF THE INVENTION 
   1. Technical Field 
   The invention relates to a system for brake lining wear and temperature estimation and more particularly to such a system implemented using extant sensing elements of an air brake system, of an air suspension system, or both. 
   2. Description of the Problem 
   Effective brakes are essential to safe motor vehicle operation. Primary brake systems dissipate vehicle kinetic energy through brake friction linings/pads as heat. These brake pads have a relatively short life and require regular replacement. Heavy vehicles have historically exhibited problems with brake overheating and fade, especially when the vehicles are descending along long grades. Overheating reduces stopping ability and accelerates brake pad wear. 
   Inspection of the brake system has traditionally involved disassembly of the wheel mechanism and visual examination of the pads. It has long been recognized that it would be desirable to incorporate some kind of sensor into the brake pads that monitor wear of the pads without the need to disassemble the brake system. Were the same sensor used to monitor brake temperature the addition to vehicle complexity would be minimized. 
   Various brake lining wear detection systems and brake temperature measurement systems are known in the art. Indirect wear or temperature estimation systems which minimize, or avoid alteration of the brake system are less usual. In principal, brake wear has been related to three variables in brake operation: (1) the pressure at which the brakes are actuated; (2) the length of time that the brakes are actuated; and (3) the rotational speed of the brake disc or drum relative to the wearable friction elements. These factors were applied in a system for determining lining condition as described in German Laid Open Application DE 34 07 716 A1 to Maurer, et al. The apparatus described by Maurer, et al. provides a pressure sensor and a timer disposed in operational cooperation with at least one of the brake lining installations for a vehicle. The rotational velocity of the disc (or brake drum) is provided by a velocity sensor. Beginning with a new installation of brake linings the signals generated by the various sensors are supplied to a plotting device which in turn generated a wear index (tracking an internal model) for display to the vehicle operator. Estimation of the brake lining wear was indirect, but the system still relied on additional sensors placed in close proximity to the brake lining. 
   In European Patent Application 0 594 113, temperature calculation was made fully indirect. The signal generated by a pressure sensor such as might be placed on a brake control valve was used as an indicator of brake pressure. This signal and a speedometer signal were used as inputs to an incremental brake temperature calculation algorithm as long as the signal of the pressure sensor was high. Brake temperature was incrementally decremented when the pressure sensor signal indicated that the brake was off. The algorithm operated cyclically. 
   SUMMARY OF THE INVENTION 
   According to the invention there is provided a motor vehicle brake lining temperature and wear estimation system. The brake lining and wear estimation system operates using extant vehicle sensors such as those used to implement height and stability control systems or anti-lock brake systems, minimizing, or eliminating, the need to modify the brake units themselves. Compressed air systems used on heavy duty vehicles to support the operation of various systems, including air brakes, and provide pressure monitoring features in various pneumatic circuits, which can be monitored to estimate brake lining application pressure. Wheel speed, which in turn can be equated to brake rotor speed, is used to determine the area of the rotor swept by the brake lining. Duration of application of the brakes is supplied by the duration of anti-lock brake system actuation signals. All of this data is supplied over a vehicle integrated data bus, preferably a bus conforming to the SAE J1939 standard, to a programmable body computer which operates on the data to produce instantaneous estimates of brake lining wear and brake lining temperature. Similarly, a height controller using air springs can monitor pressure in the air springs to determine vehicle axle loading. Axle loading can be used in combination with wheel or vehicle speed sensing to determine changes in kinetic energy of the vehicle. Further correlated to brake actuation signals, the portion of the kinetic energy dissipated through the brakes can be estimated to generate estimates of wear and temperature for the brake linings. 
   Additional effects, features and advantages will be apparent in the written description that follows. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself however, as well as a preferred mode of use, further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
       FIG. 1  depicts a tandem trailer/tractor combination such as might provide an environment for the vehicle. 
       FIG. 2  is a block diagram of a vehicle electronic control system. 
       FIG. 3  is a block diagram illustration of an air brake control system. 
       FIG. 4  is a simplified data flow diagram illustrating a portion of the invention. 
       FIG. 5  is a layout for a tractor air suspension system. 
       FIG. 6  is a perspective view of a drum brake such as the present invention might be used to estimate the condition of. 
       FIG. 7  is a high level flow chart illustrating a temperature and wear estimation algorithm. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention is equally applicable to either drum brakes or disc brakes, although it is expected to find applicability primarily with drum brakes used in air brake systems on heavy duty trucks. The present invention is also applicable to pneumatically and hydraulically actuated brakes, although it is described with reference to a preferred pneumatic system. Referring now in particular to  FIG. 1 , a tandem trailer/tractor combination  10  equipped with air brake system  24  is illustrated as a possible environment of application for a preferred embodiment of the present invention. Tandem trailer/tractor combination  10  includes a tractor  26  and two trailers,  28  and  29 , respectively. Tractor  26  and trailers  28 , 29  are supported on wheels  12 , 14 ,  26 ,  32  and  33 , the rotation of which may be slowed or stopped using air pressure actuated brakes  36 . Air brake system  24  may be considered as including an air pressurization and storage subsystem including a compressor  16 , storage tanks  18  and air lines  20 ,  40  and  38 . The details of air brake system  24  are otherwise conventional. 
     FIG. 2  illustrates a vehicle electrical control system  40  having a controller area network (CAN) bus  42  as its backbone. CAN bus  42  conforms to the SAE J1939 protocol and provides for data transmission between a plurality of controllers connected to the bus including, an electrical system controller (ESC)/body computer  44 , an engine control module  45 , a stability and height controller  46 , a transmission controller  48 , a gauge controller  50 , an auxiliary gauge controller  52  and an anti-lock brake system (ABS) controller  54 . These controllers are generally attached to one or more devices the status of which may be transmitted over CAN bus  42  by its associated controller for the use of other controllers. For example, engine controller  45  is generally connected to a plurality of engine sensors  58  including, among other sensors, an engine air intake temperature sensor. The value generated by engine air intake temperature sensor  58 B may be used as a proxy value for ambient air temperature. The signal is exogenetic to the brake units. Transmission controller  48  controls a transmission  60  the output of which is measured by a drive shaft tachometer  62 . This signal may be used to generate a measurement of vehicle speed. A stability and height controller  46  controls the inflation of a plurality of air springs forming part of the vehicle suspension and represents an exogenous user of compressed air and which may be utilized to determine axle loading. Gauge controller  50  and auxiliary gauge controllers  52  are often used to collect and format telemetry from vehicle sensors. Here various pressure sensors  70 , which is described below include a compressed air tank pressure sensor  71  and individual brake line pressure sensors  304 , are shown as monitored by pressure data transmitters  68  connected to the auxiliary gauge controller  52 . Under some circumstances air pressure may be reported directly to the engine control module  45  or to body computer  44 . Auxiliary gauge controller  52  also provides an intermediary connector for a trailer attachment sensor  451 . ABS controller  54  provides control over brake valves  72  responsive to signals received from body computer  44  and a plurality of wheel speed sensors  74  directly connected to the ABS controller. Body computer  44 , which operates as a coordinator between other controllers and is programmable, is typically connected to a brake pedal switch  56  and accordingly determines the brake pedal&#39;s position. The various connections between sensors and controllers is somewhat flexible. For example, drive shaft tachometer  62  may be connected to report its output directly to engine control module  45  instead of to transmission controller  48 . 
     FIG. 3  is a schematic illustration of a set of controllers for an air brake system  80  involved in implementing a preferred embodiment of the invention. A vehicle engine  74  is mechanically linked to drive a compressor  70  which supplies air by a check valve  19  to compressed air storage tanks  18 . The representation of the air tanks  18  is simplified by not showing such components of a conventional air system such as an air dryer, and not distinguishing between dry tanks and wet tanks. As is conventional, the system is designed to maintain air pressure in a range of 100 to 125 psi. Air pressure is maintained by using a compressor  70  under the control of a governor  72 , which is responsive to air pressure communicated on a line to one of compressed air tanks  18 , usually a wet tank. Air pressure signals developed by air pressure sensor  71  are passed to body computer  44 , typically over CAN bus  42 . Air pressure sensor  71  is typically in communication with one of the dry tanks. Where two dry tanks are used either tank may be selected. Body computer  44  receives an engine tachometer signal from engine control module  45 , which is derived from an engine crankshaft position sensor  58 A. Brake pedal position sensor  56  functions as described above. Vehicle speed  84  may be supplied from a transmission controller (not shown) or the engine control module  45 , which generate the signal from the drive shaft tachometer. Preferably, individual wheel speed signals generated by a plurality of wheel speed sensors  74  associated with the ABS controller  54  provide accurate wheel speeds for each wheel. ABS controller  54  and body computer  44  may exchange data relating to air brake system operation  80 . 
   Upon generation of a braking demand signal with displacement of the brake pedal as detected by brake pedal position sensor  56  a braking demand signal is supplied from body computer  44  to ABS controller  54 , which meters the application of air pressure to each of a plurality of actuators  306  by the control of a plurality of valves  302 . One each of valves  302  is associated with a particular brake assembly  410 , such as the brake drum assembly depicted in  FIG. 4 , and brake actuator  306 . The air pressure applied to each brake actuator  306  is measured using an air pressure sensor  304 . This air pressure signal is responsive to the brake actuation signals generated by the ABS controller  54 . ABS controller  54  reports time duration of the actuation signals for each brake actuator  306 , the pressure in the air brake line to each brake actuator from pressure sensors  304 , and, if available, the wheel speed signal from wheel speed sensors  74 . Illustration of the present invention with an anti-lock brake system does not limit the invention to use with such a system. 
   Referring to  FIG. 4 , the source and flow of data used to determine kinetic energy dissipated during braking is illustrated. Depending vehicle configuration, one or two methods of determining kinetic energy may be implemented. The stability and height controller  46  generates axle loading estimations  340  which may be summed to find weight or mass. This result is broadcast over the CAN bus  42 . Transmission controller  48  (or body computer  44 ) reads the weight data/mass data and combines it with locally generated vehicle speed (or a proxy therefore) from the transmission tachometer  62  to generate a kinetic energy calculation  342 . Where the process is initiated by engagement of the brakes, a change in kinetic energy during the period of brake engagement is readily determined. Brake status is provided by body computer  44 . 
   Heavy duty trucks frequently incorporate engine brakes, the use of which forces correction of the change in kinetic energy calculation. The correction calculation may be carried out by the body computer  44  or the engine control module  45  from the raw kinetic energy calculation  342  and additional data used to determine energy expended by the vehicle&#39;s engine operating as a brake. This data includes vehicle speed (V), the current gear selection from the transmission controller  48  and indication of operation of the engine as a brake from the engine control module  45 . Combination of these factors allows determination of a corrected energy data  346 . This energy calculation figure is available for vehicles equipped with either hydraulic or pneumatic brake actuation systems. 
   Where pneumatic brakes are used, a second, preferred method of determination of the dissipation of energy through the friction brakes is readily available. Brake application pressure, derived possibly from the auxiliary gauge controller  52  pressure readings, is combined with changes in vehicle speed (V) measurement to produce energy estimate  348 . Where corrected energy data  346  and energy estimate  348  are both available, they may be supplied to a comparator operation  350  executed by the body computer  44  for a fault determination. 
   Referring to  FIG. 5 , a tractor  26  illustrates individual height leveling valves  452  which are provided for each air spring  44 . Pressure sensors  422  are connected into the pneumatic circuit for each air spring fed by a height leveling valve  452  which allow for axle load determination. Air springs  444  are fitted between frame side rails  48  and axles  450 . Air lines  462  connect compressed air tank  18 , which is installed on tractor  26  between side frame rails  48 , to the valves  452 . An engine  470  provides motive power for tractor  26 , driving a propeller shaft  476  by a transmission  72 . A tachometer  62  may be coupled to propeller shaft  76  to determine the average rotational velocity of the drive wheels and thereby allow vehicle speed to be approximated. Where an air suspension is used, height of the vehicle may be held constant by varying air pressure to each of the air springs  444  under the control of stability and height controller  46 . Pressure sensor  422  readings may then be equated to vehicle weight as is known in the art. 
   Referring now to  FIG. 6 , a brake drum  413  has an inner brake surface  414  for frictionally engaging the brake lining  411 . An actuator such as an S-cam arrangement displaces the brake shoes  415  outwardly towards the inner brake surface  414  bringing brake lining  411  into contact with the inner brake surface. Brake linings  411  are mounted to the brake shoe  415  to frictionally engage the brake drum  13  and thus provide braking force. The generic brake drum arrangement  410  and actuation mechanism is completely conventional and is well known in the art. 
   Brake lining wear is a function of contact area between the lining/pad and a brake rotor or drum, applied pressure, the coefficient of dynamic friction and the distance the rotor or drum travels over the lining while pressure is applied. This can be represented as:
 
 W=K×C   f   ×D×P 
 
   In the above relationship K is a constant, C f  is the dynamic coefficient of friction, D is the distance the rotor or drum travels over the lining/pad and P is the pressure applied to the lining. Pressure applied to the lining is directly proportional to the brake application air pressure measured by the ABS system and reported to the body computer  44 . Any required conversion factor may be folded into K. 
   Brake drum or rotor travel distance is proportional to the integral of vehicle speed over the time period that braking is applied. For a brake rotor the travel distance is further proportional to the mean radius of the pad location as related to the center of the wheel axle. For a brake drum the travel distance can be directly related to lining area. Where V is vehicle speed and T is the time that pressure is applied then:
 
 W=K×C   f   ×P×V×T 
 
   In practical application it is useful to consider the algorithm as an estimator valid over the life of a brake pad. Brake application pressure is preferably directly measured by means of the pressure sensors shown. However, pressure may be derived by measurement of brake pedal movement, or estimated by use of the output of a brake application pressure switch which is used to operate a brake lamp circuit (not shown). This is an estimate based upon application knowledge of applied pressure. Sensed pressure provides optimum information. 
   Where vehicle weight is known, preferably from an on board determination of axle load, brake wear and temperature may be estimated by computation of energy dissipated by braking action, that is by the change in velocity (ΔV) of the vehicle. The total kinetic energy dissipated where M is equal to mass of the vehicle then is:
 
 ΔE   K =½ M×ΔV   2  
 
   The energy lost during braking is dissipated through the braking system, which occurs through the generation of heat. Both brake lining wear and temperature rise are proportional to the energy dissipated. Brake application pressure is no longer a required value. However, wear and temperature estimation both must take into the account the possibility of engine braking and downshifting. Usually, during engine braking, conventional braking is not used, there the algorithms reflect cooling since the brakes are not in use. If engine braking is employed, the energy absorbed by the engine would be calculated as a function of engine speed and the result subtracted from the total energy dissipated as indicated by the changes in velocity occurring while the brakes are in use. 
   Brake lining temperature rise is proportional to the integral of the wear estimator as a function of time. In particular, since convective air cooling is almost always present, a time constant is involved both for heating, when the brake is applied, and for cooling when the brake is released. Specifically for heating and cooling of the brakes, respectively,
 
 T=kp   a ω(1×e −v/T ) and
 
 T=kp   a ωe −v/T .
 
   In the above relationships k is a constant, p a  is pressure applied, T is wheel angular velocity, t is the time duration of application of the brakes and T is the time system constant. Ambient temperature is typically taken to be the same as air temperature taken from the engine air intake temperature sensor  58 B. It is the starting point for the algorithms for comparison to a maximum allowed brake lining temperature which is in turn taken from the manufacturer&#39;s rating for the brake. Brake lining temperature is determined by running a summation of the cooling and heating temperature rise for the lining. 
   Referring to  FIG. 7  a flow chart illustrating implementation of an algorithm for brake application pressure sensing is described. An algorithm for weight based system would modify only the heat increment and decrement steps, and substitute Kinetic energy calculations for pressure readings. On an initial start of the vehicle, or after a sufficiently long period of rest of the vehicle during which the brakes are likely to a reached ambient temperature, the process begins. With the initial application of the brakes (step  602 ) the process reads the ambient temperature (engine air intake temperature) (step  604 ) supplied by the engine controller  45 . The air brake compressed air tank pressure is read at step  606 . At step  608  the individual brake application pressures are read. Then, at step  610  the tank pressure drop is determined. A tank pressure drop that is less than expected may be taken as indication of a possible brake actuation valve failure (step  612 ), occurrence of (the YES branch from step  612 ) which results in the appropriate warning being issued (step  614 ). Following the NO branch from step  612  or following execution of step  614  processing advances to step  616  where vehicle speed is read. Next, the first recalculation of temperature is executed. (step  618 ). The result of this evaluation is compared to a do not exceed temperature (step  620 ). If the temperature is excessive, a warning is provided to the driver. (step  622  following the YES branch from step  620 ). Next, a test to see if the brakes are still applied is executed. (step  624 ). If the brakes remain actuated program execution returns to step  616  to update speed and to recalculate the incremental increase in temperature. 
   Release of the brakes requires determining the extent of cooling. Following the NO branch from step  624 , accumulated wear of the brake pads or lining is calculated (step  626 ). Next, at step  628  brake temperature is recalculated for cooling. Next, at step  630  it is determined if the brakes have been reapplied. If not, program execution returns to step  628  to again redetermine the downward change in temperature. If the brakes have been reapplied the program returns to step  606  to read compressed air tank air pressure and execution of the steps following. 
   While the invention is shown in only one of its forms, it is not thus limited but is susceptible to various changes and modifications without departing from the spirit and scope of the invention.