Patent Publication Number: US-11047441-B2

Title: Brake component prognosis

Description:
INTRODUCTION 
     The present invention relates generally to the field of vehicles and, more specifically, to a system and method for estimating the remaining life of a brake component as it wears from use. 
     The brake rotor is an integral component of braking systems across multiple types of motor vehicles. The disc brake is an energy conversion device which converts mechanical energy to heat. Disc braking systems consist of a non-rotating friction material and application sub-systems, as well as a brake rotor that rotates with the wheel. To stop or slow the vehicle the friction material subsystem is engaged with the braking surfaces (rotor cheeks) of the brake rotor to generate heat due to friction, thereby converting mechanical energy to heat, and thereby slowing the rotation of the wheel. 
     The performance of the braking system, in general, and the brake rotor in particular, is determined to a large extent by the condition of the surface finish of the rotor cheeks. The normal course of operation of a brake system involves production of high levels of friction, which, in turn, generates high temperatures on the rotor cheek surfaces. Environmental effects caused by exposure to corrosive agents, such as road salt and water, exacerbate corrosion of the rotor. These effects, either singly or in combination, can result in pedal pulsation or a corroded braking surface. 
     The braking system is an aggressive environment for corrosion and high temperature oxidation of the brake rotors, which are typically made of cast iron. The oxides produced can preferentially spall during normal brake applications. Oxide spalling produces local high spots, which form deep grooves or scoring of the rotor cheek surfaces. These surface features can produce pedal pulsation during braking. 
     Further, in some vehicles use of open style wheel designs leave the rotor braking surfaces visible to onlookers. Surface corrosion that would normally be inconsequential with respect to the operation of the braking system becomes an issue due to the perception of this oxidation. 
     Corrosion causes the thickness of the rotor to reduce over time, and below certain thickness levels, the rotor operation may be sub-optimal, for example the vehicle may not stop as urgently. Accordingly, it is desirable to determine and indicate a status of the brake rotor, so that the brake system can be serviced and the rotors replaced in a timely manner. 
     SUMMARY 
     Embodiments according to the present disclosure provide a number of advantages. For example, embodiments according to the present disclosure enable real-time determination of brake rotor fatigue for each braking event and accumulation of the brake rotor fatigue to determine a brake rotor life expectancy. 
     In one aspect, a method for determining a remaining life expectancy of a brake component includes the following steps of providing vehicle parameters that identify operating conditions of a vehicle, using the vehicle parameters to determine work done by a brake of the vehicle as brake work, using the brake work to determine brake rotor temperature, using the brake rotor temperature to determine a fatigue damage of the brake component, accumulating the fatigue damage to determine a cumulative fatigue damage of the brake component, and comparing the cumulative fatigue damage to a damage limit to provide an estimation of the remaining life expectancy of the brake component. 
     In some aspects, the brake component is a vehicle brake rotor and the fatigue damage is a thermal and mechanical energy-based damage calculation. In some aspects, the method further includes using the brake rotor temperature to determine an energy-based brake rotor wear, accumulating the brake rotor wear to provide an estimation of the thickness of the vehicle brake rotor, computing an oxidative brake rotor wear based on a time of exposure of the vehicle brake rotor, computing the brake rotor wear by summing the oxidative brake rotor wear and the energy-based brake rotor wear, and using the brake rotor wear to determine the cumulative fatigue damage of the vehicle brake rotor. 
     In some aspects, the oxidative brake rotor wear is computed by scaling the time of exposure of the vehicle brake rotor by a predetermined oxidative wear parameter. 
     In some aspects, the predetermined oxidative wear parameter is specific to a material of the vehicle brake rotor. 
     In some aspects, providing vehicle parameters includes providing brake rotor friction material, brake rotor cooling rate, vehicle mass, road grade, dynamic brake proportioning, vehicle weight distribution, vehicle speed, wheel speed and brake pressure. 
     In some aspects, determining the brake work includes subtracting resistance values from total work done by the brake of the vehicle where the resistance values are forces that reduce the total work. 
     In some aspects, determining the brake work includes determining braking force as brake pressure times brake rotor area times brake rotor coefficient of friction. 
     In some aspects, determining the brake work includes determining braking power as torque produced by the vehicle times velocity of the vehicle divided by rolling radius of a vehicle wheel. 
     In some aspects, the method further includes notifying of the remaining life expectancy estimation of the brake component using telematics. 
     In another aspect, a vehicle brake system for determining a remaining life expectancy of a brake component includes the brake component and a processor. In some aspects, the processor is configured to receive vehicle parameters that identify operating conditions of a vehicle, determine work done by a brake system of the vehicle as brake work by using the vehicle parameters, determine a brake rotor temperature using the brake work, determine a fatigue damage of the brake component using the brake rotor temperature, determine a cumulative fatigue damage of the brake component by accumulating the fatigue damage, and compare the cumulative fatigue damage to a damage limit to provide an estimation of the remaining life expectancy of the brake component. 
     In some aspects, the brake component is a vehicle brake rotor and the fatigue damage is a thermal and mechanical energy-based damage calculation, and the processor is further configured to determine an energy-based brake rotor wear using the brake rotor temperature, accumulate the brake rotor wear to provide an estimation of the thickness of the vehicle brake rotor, compute an oxidative brake rotor wear based on a time of exposure of the brake rotor, compute the brake rotor wear by summing the oxidative brake rotor wear and the energy-based brake rotor wear, and determine the cumulative fatigue damage of the vehicle brake rotor using the brake rotor wear. 
     In some aspects, the oxidative brake rotor wear is computed by scaling the time of exposure of the brake rotor by a predetermined oxidative wear parameter. 
     In some aspects, the predetermined oxidative wear parameter is specific to a material of the brake rotor. 
     In some aspects, the vehicle parameters include brake rotor friction material, brake rotor cooling rate, vehicle mass, road grade, dynamic brake proportioning, vehicle weight distribution, vehicle speed, wheel speed and brake pressure. 
     In some aspects, the processor is further configured to notify the remaining life expectancy estimation of the brake component using telematics. 
     In yet another aspect, a computer program product includes a non-transitory computer readable medium having computer executable instructions, and the computer executable instructions cause a processing unit to determine a remaining life expectancy of a brake component. In some aspects, the instructions include the steps of computing work done by a brake system of a vehicle based on vehicle parameters that identify operating conditions of a vehicle, determining a brake rotor temperature based on the work done by the brake system, determining a fatigue damage of the brake component based on the brake rotor temperature, determining a cumulative fatigue damage of the brake component by accumulating the fatigue damage, and comparing the cumulative fatigue damage to a damage limit to provide an estimation of the remaining life expectancy of the brake component. 
     In some aspects, the brake component is a vehicle brake rotor and the fatigue damage is a thermal and mechanical energy-based damage calculation. In some aspects, determining the remaining life expectancy of the brake component further includes determining an energy-based brake rotor wear using the brake rotor temperature, accumulating the brake rotor wear to provide an estimation of the thickness of the vehicle brake rotor, determining an oxidative brake rotor wear based on a time of exposure of the vehicle brake rotor, computing the brake rotor wear by summing the oxidative brake rotor wear and the energy-based brake rotor wear, and using the brake rotor wear to determine the cumulative fatigue damage of the vehicle brake rotor. 
     In some aspects, the oxidative brake rotor wear is computed by scaling the time of exposure of the vehicle brake rotor by a predetermined oxidative wear parameter, the predetermined oxidative wear parameter being specific to a material of the vehicle brake rotor. 
     In some aspects, the vehicle parameters include brake rotor friction material, brake rotor cooling rate, vehicle mass, road grade, dynamic brake proportioning, vehicle weight distribution, vehicle speed, wheel speed and brake pressure. 
     In some aspects, the computer executable instructions further cause the processing unit to notify the remaining life expectancy estimation of the brake component via telematics of the vehicle. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure will be described in conjunction with the following figures, wherein like numerals denote like elements. 
         FIG. 1  depicts example components of a disc brake system of a vehicle according to an embodiment. 
         FIG. 2  is a block diagram of a brake component life expectancy estimation system, according to one or more embodiments. 
         FIG. 3  is an example block diagram of a model and data fusion processor according to one or more embodiments. 
         FIG. 4  is a flowchart of an example method for estimating a life expectancy of a brake component, according to one or more embodiments. 
         FIG. 5  is a flowchart of an example method for notifying the vehicle operator of the estimated brake component life expectancy, according to one or more embodiments. 
     
    
    
     The foregoing and other features of the present disclosure will become more fully apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. Understanding that these drawings depict only several embodiments in accordance with the disclosure and are not to be considered limiting of its scope, the disclosure will be described with additional specificity and detail through the use of the accompanying drawings. Any dimensions disclosed in the drawings or elsewhere herein are for the purpose of illustration only. 
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure are described herein. It is to be understood, however, that the disclosed embodiments are merely examples and other embodiments can take various and alternative forms. The figures are not necessarily to scale; some features could be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. As those of ordinary skill in the art will understand, various features illustrated and described with reference to any one of the figures can be combined with features illustrated in one or more other figures to produce embodiments that are not explicitly illustrated or described. The combinations of features illustrated provide representative embodiments for typical applications. Various combinations and modifications of the features consistent with the teachings of this disclosure, however, could be desired for particular applications or implementations. 
     Certain terminology may be used in the following description for the purpose of reference only, and thus are not intended to be limiting. For example, terms such as “above” and “below” refer to directions in the drawings to which reference is made. Terms such as “front,” “back,” “Left,” “right,” “rear,” and “side” describe the orientation and/or location of portions of the components or elements within a consistent but arbitrary frame of reference which is made clear by reference to the text and the associated drawings describing the components or elements under discussion. Moreover, terms such as “first,” “second,”; “third,” and so on may be used to describe separate components. Such terminology may include the words specifically mentioned above, derivatives thereof and words of similar import. 
     Vehicle brake components, such as the brake rotor and brake caliper, are stress-intensive components subject to fatigue damage based on use case. For example and without limitation, rotors used on race cars can exhibit fatigue cracking after hard race track use. Autonomous vehicles undergo heavy duty cycle use and may be subject to periodic mandatory inspections. Additionally, the rotors/drums of heavy duty towing and commercial vehicles can crack due to high loads and intense duty cycles. The methods discussed herein combine thermal models of rotor wear, fatigue damage models, and customer use load data to create a prognostic model that may be executed in vehicle controllers and, in some embodiments, be used to notify the vehicle operator of a remaining life expectancy of a vehicle brake component, such as, for example and without limitation, a brake rotor. 
       FIG. 1  depicts example components of a disc brake system  51  of a vehicle according to one or more embodiments. It should be noted that in other examples, the disc brake system  51  may include additional, fewer, and/or different components than those illustrated here. The disc brake system  51  includes a rotor  53  and pads  55 . In one or more examples, the rotor  53  is also referred to as a disc. The brake system  51  further includes a brake caliper assembly  57 . In one or more examples, the rotor  53  has pad-contacting surfaces (cheeks)  66 ,  67 . The brake caliper  57  is typically fixed to the vehicle and fits around a sector of the rotor  53 . When a vehicle operator engages a brake pedal of the vehicle, hydraulic fluid is pressurized in a brake hose connected to the brake caliper  57  and forces the friction material of the pads  55  against both surfaces  66 ,  67  of the rotor  53 , which is rotating with a wheel of the vehicle. The frictional engagement between the pads  55  and the rotating rotor  53  serves to slow, and possibly stop, the vehicle wheel. 
     It should be noted that while solid rotors may be used, medium to large rotors used on heavier or higher performance vehicles might employ vented rotors. Vented rotors incorporate radial cooling passages located between the brake pad-contacting rotor surfaces  66 ,  67 , which, under rotor rotation, function as air pumps to circulate air from the rotor center to the outside of the rotor. The location of the cooling outlets of the rotor may vary depending on the type of rotor. 
     The technical solutions described herein facilitate using sensor information, driver braking information, and driver brake models to predict or estimate brake rotor thickness, which is one input used to generate a real-time rotor fatigue damage calculation and a cumulative fatigue damage calculation and provide an indication of remaining brake rotor life, such as in remaining miles or percentage of brake rotor thickness. As will be discussed in detail below, the rotor fatigue damage algorithm and cumulative fatigue damage algorithm use various parameters and sensor signals to generate the models, including, but not limited to, brake rotor material properties, brake rotor cooling rate, brake temperature, vehicle mass, road grade, dynamic brake proportioning, vehicle weight distribution, brake pressure applied, braking energy, braking torque, fatigue damage, cooling coefficients, cumulative rotor damage threshold, etc. 
     The fatigue and cumulative fatigue damage calculations discussed herein incorporate, in some embodiments, a rotor wear model disclosed in U.S. patent application Ser. No. 15/651,475, filed Jul. 17, 2017, titled “BRAKE ROTOR PROGNOSIS”, which is hereby incorporated by reference in its entirety. Additionally, the fatigue and cumulative fatigue damage calculations discussed herein incorporate a rotor temperature model disclosed in U.S. Pat. No. 7,694,555, titled “BRAKE PAD PROGNOSIS SYSTEM”, which is hereby incorporated by reference in its entirety. 
       FIG. 2  is a block diagram of a brake component life expectancy estimation system  10 , according to one or more embodiments. The system  10  includes a brake controller  12  and a powertrain controller  14 . Further, the system  10  includes a model and data fusion processor  16 , a brake rotor wear estimation processor  18 , and a brake component life expectancy estimation processor  20 . 
     The brake controller  12  includes a brake rotor temperature estimation processor  26 , wheel speed sensors  28 , a brake pressure processor  30  and a brake power calculation processor  32 . Additionally, the brake controller  12  includes a processor  34  providing brake system dynamics, a processor  36  providing brake cooling rate and a processor  38  that provides signals from an anti-lock brake system (ABS), traction control system (TCS), electronic control system (ECS) and an active cornering control (ACC). All or most of the calculations done in the processors  26 - 38  are currently available on vehicles and/or are well known to those skilled in the art. 
     The power train controller  14  includes a vehicle mass estimation processor  40 , a road grade estimation processor  42  and a road roughness estimation processor  44 . Additional signals may be available for estimating the brake rotor thickness, such as vehicle odometer  46 , global positioning system (GPS) signals  48 , map information  50 , vehicle telematics  52 , and an ambient temperature sensor  54 . All of the signals provided by these processors and devices are also typically available on a vehicle, and would be readily available to those skilled in the art. 
     The brake rotor temperature estimation processor  26  estimates the temperature of the brake rotor  53 . Depending on the brake rotor material, the brake rotor wear will depend on the temperature of the brake rotor  53 . The cooling rate of the brake rotor helps determine the brake rotor temperature, and is dependent on the vehicle design, vehicle speed, wheel speed, ambient temperature, altitude, etc. As the vehicle is operated, the air flowing around the brake rotor  53  will determine how fast the rotor  53  is cooled from the previous braking event. in one or more examples, the brake rotor temperature is measured with sensors. However, such sensors are expensive, and thus, an algorithm estimating the brake rotor temperature is used in one or more examples. 
     The braking power processor  30  estimates the braking energy dissipation in the brake system  51 . This calculation uses various inputs, such as stopping distance, stopping time, brake rotor temperature, etc. The brake pressure processor  30  can use the master cylinder pressure of the braking system  51 , the weight distribution in the vehicle and the dynamic brake proportioning for the proportional brake pressure at each wheel to determine the brake pressure. The dynamic brake proportioning is based on where the weight in the vehicle is distributed, and is a known calculation. 
     The vehicle mass estimation processor  40  estimates the vehicle mass, typically based on engine torque, and is a process well known to those skilled in the art. The mass of the vehicle may change as a result of the number of passengers, load in the trunk, fuel capacity, etc. Further, it is known in the art to estimate the road grade in the estimation processor  42  in combination with the estimation of the vehicle mass. 
     The processor  16  can calculate the braking energy using equation (1) below. The braking energy is the work done by the brakes to decelerate the vehicle, and is the total work minus the rolling resistance, the aerodynamic drag, the engine braking and the road grade. The brake work can be used to calculate the power dissipated by the brakes, where power equals work/time. The power can be calculated at predetermined time intervals, for example, every 10 ms, during the braking event.
 
BrakingEnergy=½ M ( V   I   2   −V   F   2 )− E   RR   −E   G   −E   E   Equation (1)
 
     In equation (1), M is the mass of the vehicle, E RR  (RR=Rolling Resistance) is the energy required to roll the vehicle on a flat grade, which is a known value and can be represented as a function of vehicle speed, for example E RR =aV 2 +bV+C, where “V” is vehicle velocity and “a”, “b”, “c” are predetermined coefficients. Further, in equation (1), E G  (G=Grade) is the energy required to roll the vehicle as a result of the grade of the road, which is also a known value, E E  (E=Engine) is the braking provided by the engine itself, and is also a known value, V I  is the velocity of the vehicle at the beginning of the braking event and V F  is the velocity of the vehicle at the end of the braking event. In an alternate embodiment, vehicle deceleration can be used instead of the vehicle speed V, and can be provided by a longitudinal acceleration sensor. 
     The braking power dissipated by the braking event can also be estimated as power=force X velocity. Braking force can be calculated by the processor  16  as:
 
BrakingForce=Pressure·Area·μ  Equation (2)
 
     In Equation (2) μ is the friction coefficient of the brake rotor  53 , which is a function of the rotor temperature, and area is the surface area of the brake rotor  53 . 
     Alternately, the braking power can be calculated by the processor  16  as: 
     
       
         
           
             
               
                 
                   BrakingForce 
                   = 
                   
                     
                       Torque 
                       RollingRadius 
                     
                     · 
                     Velocity 
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     3 
                     ) 
                   
                 
               
             
           
         
       
     
     In Equation (3), The torque is calculated for both the front and the rear of the vehicle and is a function of the brake pressure and the dynamic brake proportioning. The RollingRadius is the rolling radius of the wheel and velocity is the vehicle velocity. 
     In one or more examples, the integration of the applied braking force is input into a physical thermal model for first order dynamics to determine an estimate of the brake temperature. Brake rotor dynamometer tests can be used to obtain the brake pad friction coefficient as a function of temperature. The tests are used to determine the amount of wear expected at different rotor temperatures, and the thermal model is configured accordingly. 
     Further, the force required to stop the vehicle can be estimated by the processor  16  as:
 
Force=Mass·Acceleration  Equation (4)
 
     The front/rear brake proportioning information and the cornering information available from the brake controller  12  is used to determine the power distribution on each axis and corner. The vehicle mass estimation is available from the powertrain controller  14 , and is also used in these equations. From the braking energy or the braking power, the brake rotor temperature can be determined as a proportional value, and from the brake rotor temperature, the brake rotor wear can be determined as a proportional value. For example, one or more look-up tables in the estimation processor  18  facilitate determining the proportional values. The look-up table(s) are populated based on the relationship between the braking energy and the brake rotor temperature and the brake rotor temperature and the brake rotor wear based on the calculations discussed above and the properties of the brake rotor. Each time the system  10  calculates the wear of the brake rotor, it is added to the previous calculations of wear over time, and can then be extrapolated from the vehicle mileage to determine the remaining mileage for each brake rotor. Alternatively, or in addition, instead of using look-up tables, in one or more examples, the estimation processor  18  determines the rotor wear dynamically using a predetermined computation formula that is based on the relationship between the braking energy and the brake rotor temperature and the brake rotor temperature and the brake rotor wear. 
     The processor  16  uses a combination of the above available information to estimate oxidation of the rotor  53 . For example, the processor  16  uses an oxidative wear model for the material of the rotor  53  to determine how much the rotor  53  has worn based on the combination of the above available information. For example, the oxidative wear model uses a time of exposure of the rotor  53  and an oxidative wear parameter that is a predetermined configurable value. 
     In one or more examples, the oxidative wear parameter is based on a type of the rotor  53 , for example, material, shape, dimensions, and other parameters of the rotor  53 . The oxidative wear parameter represents a rate of penetration of oxides into the rotor material. In one or more examples, the time of exposure of the rotor  53  is monitored from the time since manufacture of the vehicle and/or since the rotor  53  is serviced. For example, the processor measures a time associated with each braking event, for example amount of time the vehicle operator holds the brake pedal down to cause the vehicle to slow down. The measured time is accumulated to provide the time of exposure of the rotor  53 . Alternatively, or in addition, the time of exposure is time measured since the rotor  53  is installed on the vehicle. The time of exposure is reset when the rotor  53  is replaced. In one or more examples, a vehicle subsystem, such as a vehicle computer keeps track of the time since the rotor in installed on the vehicle and provides access to the value when requested. 
     Scaling the time of exposure with the oxidative wear parameter provides an amount (for example, in millimeters or micrometers) of corrosion of the rotor  53 . Alternatively, in one or more examples, the processor  16  uses a look-up table to determine the amount of corrosion of the rotor based on the time of exposure, the look-up table including corrosion values based on the oxidative wear parameter. The oxidative wear parameter is the rate of corrosion for the rotor. In one or more examples, the oxidative wear parameter is adjusted according to a vehicle location, for example which are available from a global positioning system (GPS) of the vehicle  10 . For example, the processor  16  uses a look-up table to determine the oxidative wear parameter to use based on location coordinates received from the GPS. The location coordinates may be used to identify a geographic region, such as a city, a state, or the like, and an oxidative wear parameter corresponding to the geographic region is then used for determining the corrosion of the rotor  53 . 
     The wear estimate generated by the wear estimation processor  18  is, in some embodiments, used by the processor  16  to determine a brake component fatigue damage. In some embodiments, the brake component is a brake rotor. As discussed in greater detail herein, the brake component fatigue damage is a real-time calculation of the fatigue damage to a brake component based on a combination of the information discussed above. The processor  16  further determines a cumulative brake component fatigue damage. The brake component life expectancy estimation processor  20  compares the cumulative brake component fatigue damage to a damage limit to estimate a remaining life expectancy of the brake component, as discussed in greater detail herein. 
       FIG. 3  depicts an example block diagram of the model and data fusion processor  16  according to one or more embodiments. The rotor wear estimation processor  18  uses a combination of the above available information to estimate the thickness of the brake rotor  53 . The processor  16  uses a rotor temperature model  310  to determine a temperature of the rotor  53 . The processor  16  further uses a rotor energy to determine braking energy that is absorbed by the rotor  53 . Based on the rotor temperature and the rotor braking energy absorbed, the processor  16  determines a corrosion or wear of the rotor  53  according to an energy-based wear model  320 . The processor  16  uses a wear volume per unit of energy absorbed by the rotor to determine how much the rotor  53  wears based on the model  320 . For example, the processor  16  determines an energy-based wear of the rotor  53  according to a volume of wear per energy units absorbed at the determined temperature. For example, the processor  16  computes the energy units absorbed by the rotor  53  at a braking event at the determined temperature using the equations described herein. Further, a volume of wear of the rotor  53  is computed by multiplying the computed energy-based wear by a surface area of the rotor  53 . 
     Further, the processor  16  uses a rotor oxidative wear model  330  to compute an oxidative wear of the rotor  53 . For example, the processor  16  computes the oxidative wear of the rotor  53  based on the oxidative wear parameter and the time of exposure of the rotor  53 . 
     The processor  16  computes the energy based wear and the oxidative wear periodically at a predetermined frequency, such as 10 ms, 20 ms, or any other predetermined frequency. Alternatively, or in addition, the processor  16  computes the wear every time a braking event occurs. 
     The processor  16  forwards the computed wear to the rotor thickness estimation processor  18 . The rotor thickness estimation processor  18  accrues the wear of rotor  53  over time as the vehicle is being operated. The rotor thickness estimation processor  18  uses the accrued wear to determine an estimated thickness of the rotor  53 . The rotor thickness estimation processor  18  further uses the thickness of the rotor  53  to estimate life of the rotor  53 , for example in terms of time, miles, or any other parameter, for example, using a corrosion model specific to the rotor  53 . 
     The processor  16  also uses a rotor fatigue model  340  to compute rotor damage based on brake torque and speed/power and the rotor temperature. For example, the processor  16  uses the rotor temperature, as well as the braking torque and rotational speed in a thermal- and mechanical-based surface stress model determine the real-time fatigue damage to the rotor. Fatigue damage to the rotor for a single braking event, in some embodiments, is modeled as a proportional ratio of the product of heat flux into the braking surface and the square root of the braking time, to the strength of the rotor material at the peak temperature it reaches during the braking event. Proportionality is a function of relevant thermo-mechanical properties of the rotor material, such as Young&#39;s Modulus, density, thermal conductivity, and Poisson&#39;s ratio, for example and without limitation. 
     The real-time fatigue damage computed by the processor  16  using the rotor fatigue model  340  is used by a cumulative fatigue damage model  350  to accumulate or sum the fatigue damage to the rotor caused by each individual application of the brake. Cumulative damage to the rotor over multiple braking events may be computed using Miner&#39;s Rule, previously generated knowledge of the Stress or Strain vs. Fatigue Life behavior of the rotor, for example and without limitation. The stress versus fatigue life characterization is generated, in some embodiments, via physical testing and is characterized by the slope of a fitted curve on log-scale data known to those skilled in the art as an “S-N” curve. Fatigue life is estimated, in some embodiments, by comparing accumulated damage from all of the braking events the part participated in since the rotor was installed to a validated rotor damage level or damage limit. In some embodiments, the damage limit or validated rotor damage level generally relates to the expected life of the rotor subject to severe use in the field. In some embodiments, the cumulative fatigue damage model incorporates the rotor wear determined by the rotor thickness estimation processor  18  to refine the calculation of the cumulative fatigue damage to the brake rotor. In such an embodiment, stress and corresponding fatigue damage would be calculated at the rotor surface and also at discrete levels below the rotor surface. A risk of brake rotor failure is predicted when the cumulative damage to a rotor layer exceeds a predetermined maximum threshold. In some embodiments, the stress is calculated using a one-dimensional thermal-mechanical stress model. 
     The processor  16  computes the real-time fatigue damage of the brake component and the cumulative fatigue damage periodically at a predetermined frequency, such as 10 ms, 20 ms, or any other predetermined frequency. Alternatively, or in addition, the processor  16  computes the real-time fatigue damage and cumulative fatigue damage every time a braking event occurs. 
     The processor  16  forwards the cumulative fatigue damage estimation to a brake component life expectancy estimation processor  20 . The brake component life expectancy estimation processor  20  accrues the cumulative fatigue damage of the brake component over time as the vehicle is operated. The accrued cumulative fatigue damage of the brake component is compared to a damage limit of the brake component. The damage limit of the brake component is, in some embodiments, a limit determined through testing and validation of the brake component, adjusted, in some embodiments, by a predetermined safety factor. 
     The brake component life expectancy estimation processor  20  generates a brake component life expectancy estimation signal. In some embodiments, the life expectancy estimation signal is used alone or in combination with a wear model, such as the rotor wear models  320 ,  330  to generate a refined rotor life expectancy estimate. The life expectancy estimation processor  20  estimates the life expectancy of a vehicle brake rotor, in one embodiment. In some embodiments, the estimated life expectancy is expressed as a percentage of remaining life of the brake component. 
     The proportioning of the information can be calibrated for different applications and different vehicles. An estimate of the amount of material of the brake wear lost together with the mileage that the vehicle has been driven since the rotors were last changed are recorded on the vehicle. The current estimation can be stored in separate modules on the vehicle. This is used to store the information should a fault occur on one of the modules causing it to be replaced. An estimate of the remaining vehicle mileage can be obtained in a number of ways, such as from driver braking characteristics, linear interpolation or look-up tables of the mileage and the brake rotor thickness estimation. The estimate of the remaining life or life expectancy of the brake component is recorded on the vehicle and may be stored in separate modules on the vehicle for use in future calculations of the estimates of the total wear of the brake component. 
     If the system  10  includes a brake rotor sensor that provides signals indicative of actual brake rotor thickness, the signals are used to gradually ramp out any differences between the estimation of the brake rotor thickness and the actual thickness over the remaining rotor thickness and life of the rotor  53 . For example, the sensor may be set so that one or more wires break at a specific rotor thickness. If a significant difference exists between the estimate and the actual thickness, as determined when the sensor wires break contact, then this will be used to gradually adjust the estimate so that when the rotor  53  is near the replacement period, or the next sensor measurement, the total system accuracy will be as high as possible. For example, the estimate of the rotor life remaining is increased or decreased at a rate different from that observed so that the end of the life of the rotor  53  will be accurately determined. 
     It should be noted that although the brake sensor discussed above employs wires that break to give an indication of rotor thickness, other types of brake sensors can be used in other examples of the system  10 . For example, an indirect sensor or sensing mechanism can be used to infer the brake rotor thickness. Suitable examples include brake fluid level sensors or measuring the displacement of the brake calipers, such as in an electro-mechanical or brake-by-wire system. 
       FIG. 4  depicts a flowchart of an exemplary method for determining a remaining life expectancy of a brake component, such as, for example and without limitation, a vehicle brake rotor. The method shown in  FIG. 4  may be used with the brake component life expectancy system discussed herein. The method includes receiving and collecting various vehicle signals, such as brake pressure, wheel speeds, vehicle speed, longitudinal acceleration, dynamic brake proportioning, brake being applied, etc., as shown at  410 . The method further includes obtaining system estimates from the power train controller  14 , such as the vehicle mass, road grade, amount of engine braking, rolling resistance, rotor surface area etc., as shown at  415 . The method further includes obtaining system estimates from the brake controller  12 , such as the brake temperature, as shown at  420 . The method further includes computing the brake work from braking energy, as shown at  425 . For example, the braking energy is computed as per the equation (1). The braking energy can be calculated for any one of the several brake rotors on the vehicle or can be one calculation per vehicle axle. 
     Additionally, or alternately, the method includes determining the brake work using braking power from, for example, equations (2) and (3), as shown at  430 . In this calculation, the brake work is determined by braking power and pressure, such as provided by equation (2). Errors can sometimes occur when determining the mass of the vehicle for the braking energy calculation and the friction coefficient value μ can include errors in the brake power estimation. Therefore, a more accurate determination of the brake work may be provided by combining the two work calculations. 
     The method further includes determining the brake rotor temperature, as shown at  435 , and determining the brake rotor wear, as shown at  440  in the manner discussed above. Determining the brake rotor wear, at  440 , includes computing the energy-based wear and the oxidation wear, and the total rotor wear is a sum of the two wear computations. The brake rotor wear is determined for each braking event, and is added to the accumulated value, as shown at  445  to determine the remaining brake rotor thickness. 
     The method further includes determining a fatigue damage of the brake rotor using the brake rotor temperature and the operating conditions of the vehicle, as shown at  450 , in the manner discussed above. Determining the fatigue damage of the brake rotor, at  450 , includes calculating the fatigue damage based at least in part on brake torque, brake speed/power, and rotor temperature. The fatigue damage to the brake rotor is determined for each braking event, and is added to the accumulated value, as shown at  455 , to determine a cumulative fatigue damage estimate. As discussed herein, the cumulative fatigue damage estimate is a sum of the fatigue damage to the brake component, such as the brake rotor, and in some embodiments also incorporates the estimated brake rotor wear determined at  440 . 
     The cumulative fatigue damage estimate, determined at  455 , is used to calculate the remaining life expectancy estimate of the brake rotor, at  460 . The remaining life expectancy estimate of the brake component, such as the brake rotor, is determined as discussed above by comparing the accrued cumulative fatigue damage to a predetermined damage limit of the component. 
     The method includes sending the estimated thickness information to the vehicle operator using, for example, vehicle telematics, as shown at  465 . Additionally or alternatively, at  465 , the method includes sending the estimated remaining life expectancy to the vehicle operator using, for example, vehicle telematics. 
       FIG. 5  is a flowchart of an example method for notifying the vehicle operator of the estimated remaining life expectancy of the vehicle brake component, according to one or more embodiments. In some embodiments, the vehicle brake component is a brake rotor  53 . The method includes determining whether the remaining life expectancy of the vehicle brake component is greater than a first predetermined threshold, as shown at  505 . The remaining life expectancy of the vehicle brake component is determined based on the process discussed herein. The first predetermined threshold is a predetermined value at which replacing the brake component is recommended. For example, the replacement threshold may be a proportional value, such as 50% of original expected life. It should be noted that the above values are examples, and that different embodiments may use different threshold values than those above. 
     If the replacement threshold is reached, the vehicle operator is notified to replace the vehicle brake component, as shown at  515 . If, for example, the remaining life expectancy has not reached the replacement threshold, the method includes determining if the remaining life expectancy has reached a second predetermined threshold, as shown at  510 . The second predetermined threshold may be a predetermined value that is representative of an inspection threshold. For example, the replacement threshold may be a proportional value, such as 75% of original life expectancy. It should be noted that the above values are examples, and that different embodiments may use different threshold values than those above. If the inspection threshold is reached, the vehicle operator is indicated to have the vehicle brake component inspected, as shown at  525 . 
     In one or more examples, regardless of the relation between the remaining life expectancy and the threshold values, the vehicle operator is informed of the current estimated remaining life expectancy, as shown at  530 . Further, the method includes determining a life of the brake component left based on the estimated wear of the brake component, as shown at  520 . For example, the life of the brake component may be measured in terms of an estimated number of miles that the brake component can be used before the replacement threshold is reached. For example, the method includes informing the vehicle operator in miles using a linear interpolation based on vehicle operation to date as to the remaining life of the brake component, as shown at  530 . The method thus facilitates the vehicle operator to be notified in any suitable manner, and can be informed of the miles remaining based on the current wear of the brake component as to when the brake component should be replaced. 
     In one or more examples, the vehicle is an autonomous vehicle with the vehicle operator being a processor unit. In such cases, for example, the processor unit receives the estimated rotor thickness and/or the remaining rotor life estimate and/or the remaining life estimate of the vehicle brake component. Based on such input, the vehicle operator processor unit automatically drives the vehicle to a service station. For example and without limitation, if the rotor thickness falls below the inspection threshold, the processor unit causes the vehicle to be driven to the service station for the rotor inspection. Alternatively, or in addition, if the rotor thickness falls below the replacement threshold, the processor unit causes the vehicle to be driven to the service station for the rotor replacement. Other automatic actions may also be performed in response to the rotor thickness comparison, such as scheduling servicing of the vehicle. 
     Alternatively, or in addition, the rotor thickness estimates and/or the remaining life expectancy estimates of the vehicle brake component are used for limiting vehicle operation if one or more of the estimates falls below a predetermined value, such as the replacement threshold. For example, limiting the vehicle operation may include limiting a maximum speed of the vehicle, limiting torque produced by an engine of the vehicle, or imposing any other such limitations to maximize safety of the passenger(s) of the vehicle. 
     It should be noted that although the examples so far describe computing the rotor thickness and using the computed thickness to determine the life of a rotor, in one or more examples, the rotor thickness of all the rotors equipped in the vehicle are analyzed. Accordingly, the vehicle operator is informed of the rotor thickness and rotor life estimated for each rotor that is installed on the vehicle. Additionally, while many of the examples discussed herein describe analyzing a brake rotor, in other embodiments, the methods discussed herein are used to analyze other stress-intensive brake components. 
     The technical solutions described herein facilitate predicting rotor wear for a disc brake system by combining energy and oxidative wear based models. The technical solutions predict brake disc wear over a wide range of vehicle use and generate an electronic disc wear/disc remaining life signal. Additionally, the technical solutions described herein facilitate predicting a life expectancy of one or more vehicle brake components by analyzing the real-time fatigue damage to the component, accumulating the fatigue damage caused by each braking event, and comparing the accumulated fatigue damage to a predetermined damage limit. The disc wear and/or life remaining may be displayed to the vehicle operator and/or used in various control algorithms that are implemented by one or more electronic control units (ECU) in the vehicle. 
     The technical solutions can save a vehicle owner from costly repairs resulting from excessive wear of a vehicle brake component. The technical solutions can further help owners of fleets (such as autonomous vehicle fleets) monitor brake life (in combination with the pad wear monitoring) to plan when to service vehicles. 
     The present technical solutions may be a system, a method, and/or a computer program product at any possible technical detail level of integration. The computer program product may include a computer readable storage medium (or media) having computer readable program instructions thereon for causing a processor to carry out aspects of the present technical solutions. 
     The computer readable storage medium can be a tangible device that can retain and store instructions for use by an instruction execution device. The computer readable storage medium may be, for example, but is not limited to, an electronic storage device, a magnetic storage device, an optical storage device, an electromagnetic storage device, a semiconductor storage device, or any suitable combination of the foregoing. A non-exhaustive list of more specific examples of the computer readable storage medium includes the following: a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), a static random access memory (SRAM), a portable compact disc read-only memory (CD-ROM), a digital versatile disk (DVD), a memory stick, and any suitable combination of the foregoing. A computer readable storage medium, as used herein, is not to be construed as being transitory signals per se, such as radio waves or other freely propagating electromagnetic waves, electromagnetic waves propagating through a waveguide or other transmission media (e.g., light pulses passing through a fiber-optic cable), or electrical signals transmitted through a wire. 
     Computer readable program instructions described herein can be downloaded to respective computing/processing devices from a computer readable storage medium or to an external computer or external storage device via a network, for example, the Internet, a local area network, a wide area network and/or a wireless network. The network may comprise copper transmission cables, optical transmission fibers, wireless transmission, routers, firewalls, switches, gateway computers and/or edge servers. A network adapter card or network interface in each computing/processing device receives computer readable program instructions from the network and forwards the computer readable program instructions for storage in a computer readable storage medium within the respective computing/processing device. 
     Computer readable program instructions for carrying out operations of the present technical solutions may be assembler instructions, instruction-set-architecture (ISA) instructions, machine instructions, machine dependent instructions, microcode, firmware instructions, state-setting data, configuration data for integrated circuitry, or either source code or object code written in any combination of one or more programming languages, including an object oriented programming language such as Smalltalk, C++, or the like, and procedural programming languages, such as the “C” programming language or similar programming languages. The computer readable program instructions may execute entirely on the user&#39;s computer, partly on the user&#39;s computer, as a stand-alone software package, partly on the user&#39;s computer and partly on a remote computer or entirely on the remote computer or server. In the latter scenario, the remote computer may be connected to the user&#39;s computer through any type of network, including a local area network (LAN) or a wide area network (WAN), or the connection may be made to an external computer (for example, through the Internet using an Internet Service Provider). In some embodiments, electronic circuitry including, for example, programmable logic circuitry, field-programmable gate arrays (FPGA), or programmable logic arrays (PLA) may execute the computer readable program instructions by utilizing state information of the computer readable program instructions to personalize the electronic circuitry, in order to perform aspects of the present technical solutions. 
     Aspects of the present technical solutions are described herein with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the technical solutions. It will be understood that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer readable program instructions. 
     These computer readable program instructions may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. These computer readable program instructions may also be stored in a computer readable storage medium that can direct a computer, a programmable data processing apparatus, and/or other devices to function in a particular manner, such that the computer readable storage medium having instructions stored therein comprises an article of manufacture including instructions which implement aspects of the function/act specified in the flowchart and/or block diagram block or blocks. 
     The computer readable program instructions may also be loaded onto a computer, other programmable data processing apparatus, or other device to cause a series of operational steps to be performed on the computer, other programmable apparatus or other device to produce a computer implemented process, such that the instructions which execute on the computer, other programmable apparatus, or other device implement the functions/acts specified in the flowchart and/or block diagram block or blocks. 
     The flowchart and block diagrams in the Figures illustrate the architecture, functionality, and operation of possible implementations of systems, methods, and computer program products according to various embodiments of the present technical solutions. In this regard, each block in the flowchart or block diagrams may represent a module, segment, or portion of instructions, which comprises one or more executable instructions for implementing the specified logical function(s). In some alternative implementations, the functions noted in the blocks may occur out of the order noted in the Figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the block diagrams and/or flowchart illustration, and combinations of blocks in the block diagrams and/or flowchart illustration, can be implemented by special purpose hardware-based systems that perform the specified functions or acts or carry out combinations of special purpose hardware and computer instructions. 
     A second action may be said to be “in response to” a first action independent of whether the second action results directly or indirectly from the first action. The second action may occur at a substantially later time than the first action and still be in response to the first action. Similarly, the second action may be said to be in response to the first action even if intervening actions take place between the first action and the second action, and even if one or more of the intervening actions directly cause the second action to be performed. For example, a second action may be in response to a first action if the first action sets a flag and a third action later initiates the second action whenever the flag is set. 
     To clarify the use of and to hereby provide notice to the public, the phrases “at least one of &lt;A&gt;, &lt;B&gt;, . . . and &lt;N&gt;” or “at least one of &lt;A&gt;, &lt;B&gt;, &lt;N&gt;, or combinations thereof” or “&lt;A&gt;, &lt;B&gt;, . . . and/or &lt;N&gt;” are to be construed in the broadest sense, superseding any other implied definitions hereinbefore or hereinafter unless expressly asserted to the contrary, to mean one or more elements selected from the group comprising A, B, . . . and N. In other words, the phrases mean any combination of one or more of the elements A, B, . . . or N including any one element alone or the one element in combination with one or more of the other elements which may also include, in combination, additional elements not listed. 
     It will also be appreciated that any module, unit, component, server, computer, terminal or device exemplified herein that executes instructions may include or otherwise have access to computer readable media such as storage media, computer storage media, or data storage devices (removable and/or non-removable) such as, for example, magnetic disks, optical disks, or tape. Computer storage media may include volatile and non-volatile, removable and non-removable media implemented in any method or technology for storage of information, such as computer readable instructions, data structures, program modules, or other data. Such computer storage media may be part of the device or accessible or connectable thereto. Any application or module herein described may be implemented using computer readable/executable instructions that may be stored or otherwise held by such computer readable media. 
     While the above disclosure has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from its scope. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the disclosure without departing from the essential scope thereof. Therefore, it is intended that the present disclosure not be limited to the particular embodiments disclosed, but will include all embodiments falling within the scope thereof.