Brake pad life prognosis system for regenerative braking vehicles

A system for determining thickness of a brake pad includes a controller configured to provide a total braking energy of a vehicle with a regenerative energy braking system, and a rotor braking system that includes a plurality of brake pads. The system controller determines the work done by the brake pads as a combination of front brake pad work and rear brake pad work, then accumulates the brake pad wear to provide an estimation of the brake pad thickness using the front brake pad work and the rear brake pad work. The controller outputs a message indicative of the estimation of the brake pad thickness via an output system in communication with the controller

INTRODUCTION

The subject disclosure relates to a system for estimating brake pad thickness as it wears from use, and more particularly to a system for brake pad life prognosis for regenerative braking vehicles.

Vehicle brake pads typically last between 20,000 and 80,000 miles depending on the type of driving, i.e., city, highway, rural, etc., where the average brake pad life is about 50,000 miles. The thickness of the brake pad gradually decreases as a result of wear as it is used. When the thickness of the brake pad becomes sufficiently small, a mechanical scraper may make contact with the brake pad. The mechanical scraper makes an annoying high frequency noise, which is an unfriendly reminder that the brake pad needs to be replaced. Although the noise does alert the vehicle operator that the brake pad is worn out, it does not give the vehicle operator advanced warning, or a continuous determination of the lining thickness, only that the brake pad has worn down to a low level. Therefore, for example, if a long trip is planned, there is no indication that the brake pads may not last the journey.

It is known in the art to provide a sensor that determines brake pad thickness as it wears. For example, sensors are known that include one or more wires extending across the brake pad at certain thickness levels so that when the wire breaks, the sensor will provide an indication that the brake pad thickness has been reduced a certain amount. However, such sensors are typically expensive, and do not provide a continuous indication of brake pad thickness through the life of the brake pad.

Other systems provide an estimation of brake pad thickness based on operating conditions of the vehicle, such as brake work performed by the brake system. But existing systems are not compatible with regenerative braking systems.

Accordingly, it is desirable to provide a brake life prognosis system that combines an energy partitioning model with braking energy parameters to accurately predict brake pad thickness. It is also desirable for the prognosis system to output real-time brake life information that is useful to an end user.

SUMMARY

In one exemplary embodiment a method for determining thickness of a brake pad is described. The method includes providing, via a processor, a total braking energy of a vehicle with a regenerative energy braking system and a rotor braking system comprising a plurality of brake pads. The processor then determines the work done by the brake pads as a combination of front brake pad work and rear brake pad work. The processor accumulates brake pad wear to provide an estimation of the brake pad thickness using the front brake pad work and the rear brake pad work and outputs, via an output system in communication with the processor, a message indicative of the estimation of the brake pad thickness.

In another exemplary embodiment, a system for determining thickness of a brake pad includes a controller configured to provide a total braking energy of a vehicle with a regenerative energy braking system, and a rotor braking system that includes a plurality of brake pads. The system controller determines the work done by the brake pads as a combination of front brake pad work and rear brake pad work, then accumulates the brake pad wear to provide an estimation of the brake pad thickness using the front brake pad work and the rear brake pad work. The controller outputs a message indicative of the estimation of the brake pad thickness via an output system in communication with the controller.

In another exemplary embodiment, a non-transitory computer-readable medium storing instructions is described. The computer-readable medium includes instructions executable by a processor for performing a method for determining thickness of a brake pad. The method includes providing, via the processor, a total braking energy of a vehicle with a regenerative energy braking system and a rotor braking system comprising a plurality of brake pads. The processor then determines the work done by the brake pads as a combination of front brake pad work and rear brake pad work. The processor accumulates brake pad wear to provide an estimation of the brake pad thickness using the front brake pad work and the rear brake pad work and outputs, via an output system in communication with the processor, a message indicative of the estimation of the brake pad thickness.

In yet another exemplary embodiment, determining the work done by the brake pads includes providing deceleration parameters.

In another exemplary embodiment, determining the work done by the brake pads includes providing regenerative blending signals indicative of a relative portion of overall brake energy.

In yet another exemplary embodiment, determining the work done by the brake pads further includes providing motor data indicative of regenerative brake energy harvested by a motor generator.

In another exemplary embodiment, accumulating the brake pad wear includes providing brake pad friction material, brake pad cooling rate, vehicle mass, road grade, dynamic brake proportioning, vehicle weight distribution, vehicle speed, wheel speed and brake pressure.

In yet another exemplary embodiment, determining the front brake pad work and the rear brake pad work includes determining brake pad work at a particular wheel of the vehicle.

DETAILED DESCRIPTION

Referring now toFIG. 1, a vehicle10may include an engine11that drives a transmission13. The engine11may be a combustion engine as known in the art. In other aspects, the vehicle10may be fully-electric and may not include a combustion engine. The vehicle10further includes one or more motor generators21that selectively drive or are driven by the wheels22. For example, when vehicle10is operating in an electric motor-driven mode, one or more motor generators21drive the wheels22and are powered by a battery24. When the vehicle10is braking, the motor generators21are driven by the wheels22to slow rotation of the wheels22and to produce current that is used to charge the battery24. The production of current in this way is referred to in the art as regenerative braking. The battery24communicates with the motor generators21via a voltage regulator25. Some vehicles may include both of the engine11and the motor generators21, and as such, may include an electric mode and a separate engine drive mode. In the electric mode, the voltage regulator25regulates driving current from the battery24to the motor generators21. In the braking mode, the voltage regulator25regulates charging current from the motor generators21to the battery24. When in braking mode, the generation of power by the motor generators21also functions to slow the vehicle10.

A drive controller27communicates with the engine11, a power train controller14, a brake controller12, and the voltage regulator25. The drive controller27continuously monitors driving conditions to selectively power the vehicle10electrically and/or using the engine11. If the conditions are proper for electric drive, the drive controller27deactivates the engine11and drives the vehicle10with the motor generators21in the motor mode. When the brake is depressed, the drive controller27receives a brake signal. In response, the drive controller27operates the motor generators21in the generator mode to retard motion of the vehicle10and signals the voltage regulator25to charge the battery24using current generated by the motor generators21.

The vehicle10can include mechanical brake systems in addition to the regenerative braking system.FIG. 2depicts example components of a disc brake system18of a vehicle according to one or more embodiments. It should be noted that in other examples, the disc brake system18may include additional, fewer, and/or different components than those illustrated here. The disc brake system18includes a brake rotor53and brake pads55. In one or more examples, the rotor53is also referred to as a disc. The disc brake system18further includes a brake caliper assembly59. In one or more examples, the rotor53has brake pad-contacting surfaces66,67. When a vehicle operator steps on a brake pedal of the vehicle10, hydraulic fluid is pressurized in a brake hose connected to the brake caliper assembly59and forces the brake pads55of the brake caliper assembly59against both surfaces66,67of the rotor53, which is rotating with a wheel22of the vehicle. The frictional engagement between the brake pads55and the surfaces66,67of the rotating rotor53serves to slow, and possibly stop, the vehicle wheel. The brake pads55will wear down over time from the mechanical friction and heat generated by contact with the rotor53.

The technical solutions described herein facilitate using sensor information, driver braking information, and driver brake models to predict or estimate brake pad thickness, and provide an indication of remaining brake pad life. This may include determining remaining miles of life left in brake pad operation or a percentage of brake pad thickness. According to some embodiments, a system controller will report the remaining life of the brake pads to a vehicle operator. As will be discussed in detail below, the brake pad thickness estimation algorithm uses various parameters and sensor signals to provide the estimation, 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 power, and regenerative energy generation with respect to particular wheels and vehicle10portions (e.g., front and rear) etc.

The regenerative braking system disclosed herein uses displacement on demand (DOD) in an electric/electric-hybrid (such as vehicle10) to increase the current generated to recharge the battery24. Specifically, if the engine11is powering the vehicle10and braking is initiated by an operator, cylinders15of the engine11may be deactivated to reduce engine11braking. The motor generators21are able to absorb an increased amount of the vehicle's kinetic energy to produce current to charge the battery24.

FIG. 3is a block diagram of a brake pad life prognosis system60(hereafter “system60”), according to one or more embodiments. ConsideringFIG. 3in conjunction withFIG. 1, the system60includes the brake controller12and the power train controller14. Further, the system60includes a model and data fusion processor16and a brake pad estimation processor20. The brake pad estimation processor20outputs prognostic brake pad information to a display processor56. The display processor56outputs the messages to a display in the cabin of the vehicle10to inform a user.

The brake controller12includes a brake pad temperature estimation processor26, wheel speed sensors28, a brake pressure processor30and a brake power calculation processor32(shown inFIG. 1). Additionally, the brake controller12includes a brake system dynamics processor34providing brake system dynamics, a processor36providing brake cooling rate, and a multi-function processor38that provides signals from an anti-lock brake system (ABS), traction control system (TCS), electronic control system (ECS) and an active cruise control (ACC). All or most of the calculations done in the processors26-38are currently available on vehicles and/or are well known to those skilled in the art.

The power train controller14includes a vehicle mass estimation processor40, a road grade estimation processor42and a road roughness estimation processor44. Additional signals may be available for estimating the brake pad thickness, such as a vehicle odometer46, GPS signals48, map information50, vehicle telematics52and an ambient temperature sensor54. 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 pad temperature estimation processor26estimates the temperature of the brake rotor53. Depending on the brake rotor material, the brake rotor wear may be based on the temperature of the brake rotor53. The cooling rate of the brake rotor helps determine the brake rotor temperature, and may be based on the vehicle design, vehicle speed, wheel speed, ambient temperature, altitude, etc. As the vehicle is operated, the air flowing around the brake pads55and the brake rotor53will determine how fast the brake pads55are cooled from the previous braking event. In one or more examples, the brake rotor and/or brake pad temperature is measured with sensors. However, such sensors are expensive, and thus, an algorithm estimating the brake pad temperature is used in one or more embodiments.

The brake pressure processor30estimates the braking energy dissipation in the disc brake system18. This calculation uses various inputs, such as stopping distance, stopping time, brake rotor temperature, etc. To determine the brake pressure, the brake pressure processor30can use any one or more of a master cylinder pressure of the disc brake system18(not shown), a weight distribution in the vehicle, and a dynamic brake proportioning for the proportional brake pressure at each wheel. The dynamic brake proportioning is based on where the weight in the vehicle10is distributed, and is a known calculation.

The vehicle mass estimation processor40estimates the vehicle10mass, typically based on the engine11torque, and is a process well known to those skilled in the art. The mass of the vehicle10may 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 road grade estimation processor42in combination with the estimation of the vehicle10mass.

The processor20can calculate the braking energy using equation (14) 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 140 ms, during the braking event.
BrakingEnergy=½M(VI2−VF2)−ERR−EG−EEEquation (14)

In equation (14), M is the mass of the vehicle, Energy Rolling Resistance (ERR) is the energy required to roll the vehicle10on a flat grade, which is a known value and can be represented as a function of vehicle speed. For example ERR=aV2+bV+C, where V is vehicle velocity and a, b, c are predetermined coefficients. Further, in equation (14), EG(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, EE(E=Engine) is the braking provided by the engine itself, and is also a known value, VIis the velocity of the vehicle10at the beginning of the braking event and VFis the velocity of the vehicle at the end of the braking event. In an alternate embodiment, vehicle10deceleration 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*velocity. Braking force can be calculated by the processor20as:
BrakingForce=Pressure*Area*μ  Equation (2)

In Equation (2) μ is the friction coefficient of the brake rotor53, which is a function of the rotor temperature, and area is the surface area of the brake rotor53.

Alternately, the braking power can be calculated by the processor20as:

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 Rolling Radius 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 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 processor20as:
Force=Mass*Acceleration  Equation (4)

The front/rear brake proportioning information and the cornering information available from the brake system dynamics processor34is used to determine the power distribution on each axle and corner. The vehicle mass estimation is available from the power train controller14, 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 brake pad estimation processor20facilitate 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 system140calculates 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 brake pad estimation processor20determines 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.

Further, the processor20uses a combination of the above available information to estimate oxidation of the rotor53. For example, the processor20uses an oxidative wear model for the material of the rotor53to determine how much the rotor53has worn based on the combination of the above available information. For example, the oxidative wear model uses a time of exposure of the rotor53and 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 rotor53, for example, material, shape, dimensions, and other parameters of the rotor53. 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 rotor53is monitored from the time since manufacture of the vehicle and/or since the rotor53is serviced. The processor20measures a time associated with each braking event. For example, the processor20measures the amount of time the vehicle operator holds the brake pedal down to cause the vehicle to slow down. The measured time is accumulated by the processor20to provide the time of exposure of the rotor53. Alternatively, or in addition, the time of exposure is time measured since the rotor53is installed on the vehicle. The time of exposure is reset when the rotor53is 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 rotor53. Alternatively, in one or more examples, the model and data fusion processor16uses 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 brake rotor53. 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 vehicle10. For example, the model and data fusion processor16uses 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 rotor53.

FIG. 4depicts an example block diagram of the model and data fusion processor16according to one or more embodiments. The model and data fusion processor16(hereafter “processor16”) uses a combination of the above available information to estimate the thickness of the brake pads55. The processor16uses a total braking energy calculation57that accounts for energy that is re-captured by deceleration using the power generated by the motor generators21. For example, the total braking energy calculation is an input for a subtractive regenerative braking energy model58(hereafter “regen braking model58”).

According to some embodiments, the regen braking model58provides vehicle parameters that includes deceleration data attributed to one or more of the motor generators21. Because weight distribution of the vehicle10can vary the amount of stopping friction (or regenerative energy transfer) per wheel, the regen braking model58includes parameters including a combination of the front brake pad work and the rear brake pad work. As an example, the front brake pad work can account for energy at the front wheels only. The rear brake pad work can account for the energy at the rear wheels only. According to one or more embodiments, the brake pad work respective to the front of the vehicle10and the rear of the vehicle10can be accumulated and subtracted from the energy calculation at the rotor temperature model62. Accordingly, the energy-based friction wear model64will be based on the true energy absorbed by the brake pads55and the rotor53, which would have been skewed absent the subtractive regenerative braking energy model58.

Accordingly, the regen braking model58subtracts an accumulation of the braking forces absorbed by the regenerative braking system, and feeds the adjusted energy into the rotor temperature model to determine a temperature of the rotor53and the brake pads55. The processor16further uses a rotor energy calculation to determine braking energy that is absorbed by the rotor53and the brake pads55.

Based on the rotor temperature and the rotor braking energy absorbed, the processor16determines a corrosion or wear of the brake pads55according to the energy-based wear model64. The processor16uses a wear volume per unit of energy absorbed by the rotor to determine how much the rotor53and the brake pads55have worn based on the model64. For example, the processor16determines an energy-based wear of the rotor53according to a volume of wear per energy units absorbed at the determined temperature. As another example, the processor16computes the energy units absorbed by the rotor53and the brake pads55at a braking event at the determined temperature using the equations described herein. Further, a volume of wear of the rotor53and the brake pads55is computed by multiplying the computed energy-based wear by a surface area of the rotor53.

The processor16computes the energy based wear periodically at a predetermined frequency, such as 140 ms, 20 ms, or any other predetermined frequency. Alternatively, or in addition, the processor16computes the wear every time a braking event occurs.

The processor16forwards the computed wear to the brake pad estimation processor20. The brake pad estimation processor20accrues the wear of brake pads55over time as the vehicle is being operated. The brake pad estimation processor20uses the accrued wear to determine an estimated thickness of the brake pads55. The brake pad estimation processor20further uses the thickness of the brake pads55to estimate life of the brake pads, for example in terms of time, miles, or any other parameter, for example, using a corrosion model specific to the brake pads55.

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.

If the system60includes a brake pad sensor that provides signals indicative of actual brake pad 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 brake pads55. 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 brake pads55are 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 brake pads55will 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 system60. 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. 5depicts a flowchart of an example method for estimating a brake pad thickness, according to one or more embodiments. 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 at68. The method further includes obtaining system estimates from the power train controller14, such as the vehicle mass, road grade, amount of engine braking, rolling resistance, rotor surface area etc., as shown at70. The method further includes obtaining system estimates from the brake controller12, such as the brake temperature, as shown at72. The method further includes computing the brake work from braking energy, as shown at74. For example, the braking energy is computed as per the equation (14). The braking energy can be calculated for any one of the several brake pads, every one of the several motor generators21, or can be one calculation per vehicle axle (e.g., front brake pad work and/or rear brake pad work).

Additionally, or alternately, the method includes determining the brake work using braking power from, for example, equations (2) and (3), as shown at76. 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 at78, and determining the brake pad wear, as shown at80in the manner discussed above. Determining the brake pad wear, at80, includes computing the deceleration parameters, and providing the regenerative blending signals observed from both of the front brake pads and the rear brake pads (e.g., per axle). The regenerative blending energy indicates the amount of the overall braking energy harvested by the regenerative brake system compared to the energy of the rotors and brake pads. The brake pad wear is determined for each braking event, and is added to the accumulated value. The method includes sending the estimated thickness information to the vehicle operator using, for example, the vehicle telematics52in communication with the display processor56.

It should be noted that although the examples so far describe computing the brake pad thickness and using the computed thickness to determine the life of a brake pad, in one or more examples, the brake pad thickness of all the brake pads equipped in the vehicle are analyzed. Accordingly, the vehicle operator is informed of the brake pad thickness and brake pad life estimated for each brake pad that is installed on the vehicle.

The technical solutions described herein facilitate predicting brake pad 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. 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 wearing through a brake disc. 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 bring vehicles in for service.

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.

To clarify the use of and to hereby provide notice to the public, the phrases “at least one of <A>, <B>, . . . and <N>” or “at least one of <A>, <B>, . . . <N>, or combinations thereof” or “<A>, <B>, . . . and/or <N>” 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.