METHOD FOR OPERATING A VEHICLE BRAKE SYSTEM

Abstract

A vehicle braking system and method designed to improve the driver's braking perception by compensating for the effects that humidity and temperature can have on braking performance. According to one embodiment, the method determines whether a braking event is currently occurring and, if so, measures the humidity and brake temperature. With this information, the method is capable of compensating for assumed changes in the coefficient of friction (µ) of one or more brake components, such as brake pads or rotors, and providing a modified brake command signal accordingly.

Description

AREA

[0001] The present invention generally relates to a vehicle braking system and, more particularly, to a method according to the preamble of claim 1 for operating a vehicle braking system under various temperature conditions, humidity conditions, and / or other conditions. A generic method is essentially disclosed in DE 199 11 902 C1. Regarding the further prior art, reference is made to DE 44 27 170 C1. BACKGROUND

[0002] There are numerous types of vehicle braking systems, including those that incorporate hydraulically actuated disc brakes. In such a system, a driver's depressing of a brake pedal causes a hydraulic piston to actuate a caliper on the outside of the wheel, which clamps a rotor between a pair of brake pads. The caliper supports the brake pads, which frictionally engage the rotating rotor and slow it down according to a number of factors, including the coefficient of friction (µ) of the materials.

[0003] The coefficient of friction (µ) of a brake pad and / or rotor can be affected by the system temperature as well as the humidity of the surrounding atmosphere. When a vehicle is stationary for extended periods of high humidity, the wetness can cause rotor corrosion or alter the properties of the transfer layer, which can increase the coefficient of friction (µ) and cause the brake pads to grip the rotor more aggressively; this condition is sometimes referred to as brake lock. When the brake pad and / or rotor are dry due to low wetness or low humidity conditions, the coefficient of friction can decrease, causing the brakes to respond less aggressively. In some cases, the increased and / or decreased coefficients of friction are noticeable to the driver and are therefore undesirable. SUMMARY

[0004] According to the invention, a method for operating a vehicle brake system is provided, which is characterized by the features of claim 1.

[0005] Further, a vehicle braking system is described. The vehicle braking system may include: a humidity sensor that provides humidity readings; an electronics module coupled to the humidity sensor and receiving the humidity readings, the electronics module configured to use the humidity readings to generate modified brake command signals; and a friction braking system coupled to the electronics module and receiving the modified brake command signals. The modified brake command signals control the friction braking system in a manner that compensates for wetness in the vehicle braking system. DRAWINGS

[0006] Preferred exemplary embodiments are described hereinafter in conjunction with the accompanying drawings, in which like reference numerals designate like elements, and in which: Fig. 1 is a block diagram of an exemplary vehicle braking system having both regenerative braking capabilities and friction braking capabilities; Fig. 2 is a flowchart of an exemplary method associated with a vehicle braking system, such as that shown in Fig. 1 shown; and Fig. Figure 3 is a graph showing an example recording illustrating relationships between humidity, temperature and a correction factor. DESCRIPTION

[0007] The vehicle braking system and method described herein are designed to improve the driver's braking perception by compensating for the impact that humidity and temperature can have on braking performance. According to one embodiment, the present method determines whether a braking event is currently occurring and, if so, measures one or more braking conditions, such as humidity and temperature. With this information, the method is able to adjust or compensate for changes in the coefficient of friction (µ) that may occur in braking components, such as brake pads or rotors, and provide a modified braking command signal accordingly. This results in improved braking perception, especially during the first few friction braking events, which follow an extended period in which the vehicle is stationary and moisture accumulates on the braking components.

[0008] Although the exemplary method is described herein in the context of a brake-by-wire system, such as an electro-hydraulic braking (EHB) system or an electro-mechanical braking (EMB) system, it should be appreciated that the method may also be used with any number of other braking systems and is not limited to the disclosed embodiment. For example, the present method may be used with other brake-by-wire systems, regenerative braking systems (e.g., those found in hybrid vehicles, battery electric vehicles, etc.), as well as other braking systems that utilize other types of technology (e.g., disc brakes, drum brakes, or a combination thereof). These are just a few of the possibilities, as the present method may be used with any vehicle braking system that utilizes friction braking.

[0009] With reference to Fig. 1, a block diagram of an exemplary vehicle braking system 10 is shown, having both friction braking capabilities and regenerative braking capabilities, and generally includes a brake pedal sensor 14, brake temperature sensors 16-22, a humidity sensor 24, an electronics module 30, a friction braking system 32, and a regenerative braking system 34. The vehicle braking system 10 and the method described below may be used with a wide variety of vehicles, including, for example, a standard internal combustion engine (ICE) vehicle, a hybrid electric vehicle (HEV), a plug-in hybrid electric vehicle (PHEV), an extended-range electric vehicle (EREV), or a battery electric vehicle (BEV), and utilizes a combination of friction braking and regenerative braking to control the vehicle's speed and to generate electrical energy.In such an arrangement, frictional braking torque can be generated in a conventional manner, and this acts against the forward thrust of the vehicle through frictional resistance created by disc brakes, drum brakes, etc. Regenerative braking torque, on the other hand, can be generated by operating an electric motor in a reverse direction so that it acts like a generator, which in turn produces an electromagnetically derived torque that acts against the forward thrust of the vehicle (this process also charges a battery, which can later be used to propel / power the vehicle).As mentioned above, it is not necessary that the vehicle braking system and method be used with a vehicle having both friction braking capabilities and regenerative braking capabilities, as they can be used with a vehicle having only friction braking or another braking arrangement.

[0010] The brake pedal 12 is a pedal or lever operated by the driver and, in accordance with this particular embodiment, monitored by a brake pedal sensor 14. In a brake-by-wire application such as the exemplary EHB system shown here, the brake pedal 12 may be mechanically coupled to a brake pedal simulator or emulator by means of a spring mechanism or the like to convey the expected mechanical feel to the driver. The brake pedal simulator may also include other mechanical and / or electronic components including sensors, etc. Those skilled in the art will appreciate that the brake pedal 12 may also be mechanically coupled, for example, to a master cylinder, which acts as a mechanical backup in the event of any malfunction in the brake-by-wire system.

[0011] Any number of different sensors, components, devices, modules, subsystems, systems, etc., may provide the vehicle braking system 10 with information or inputs that may be used with the present method. These include, for example, the Fig. 1, as well as others known in the art but not shown here. It should be appreciated that the brake pedal sensor 14, the brake temperature sensors 16-22, the humidity sensor 24, and any other sensor disposed in and / or utilized by the vehicle braking system 10 may be embodied in hardware, software, firmware, or a combination thereof. These sensors may directly sense or evaluate the conditions for which they are designed, or they may indirectly evaluate these conditions based on information provided by other sensors, components, devices, modules, subsystems, systems, etc. Furthermore, these sensors may be directly coupled to the electronics module 30, indirectly coupled via other electronic devices, via a vehicle communications bus, a network, etc.or coupled according to another arrangement known in the art. Furthermore, these sensors may be incorporated into a vehicle component, device, module, subsystem, system, etc. (e.g., sensors provided in an engine control module, an energy management system, an anti-lock brake system (ABS), etc.), they may be stand-alone components (as in . Fig. 1 is shown schematically) or they may be provided according to another arrangement. It is possible that any of the sensor readings described below may be provided by an engine control module, a transmission control module, a brake control module, a driveline control module, or other component, device, module, subsystem, system, etc., rather than being provided directly by an actual sensing element. In some cases, multiple sensors may be used to sense a single parameter (e.g., as a means of providing signal redundancy). These are just a few of the possibilities, as any type of suitable sensor or sensor arrangement known in the art may also be used.

[0012] The brake pedal sensor 14 provides the vehicle braking system 10 with a braking signal including braking readings that generally represent the position, movement, applied force, and / or state of the brake pedal. Therefore, the braking readings are generally representative of a driver braking request or driver braking intent. Any number of different types of braking sensors may be used, including non-contact sensors (e.g., optical sensors, electromagnetic sensors, etc.), contact sensors (e.g., potentiometers, contact switches, etc.), and those that measure the force the driver applies to the brake pedal, to name a few. In a brake-by-wire application, the brake sensor 14 may be incorporated into a brake pedal simulator or emulator, which conveys the expected mechanical feel of the brake pedal to the driver as well as provides a braking signal.

[0013] The brake temperature sensors 16-22 provide the vehicle braking system 10 with brake temperature signals containing brake readings generally representing the temperature or temperature condition of one or more brake components. In one embodiment, each of the brake temperature sensors 16-22 determines a brake rotor temperature and may do so in a number of different ways. For example, the brake temperature sensors 16-22 may be implemented in software such that they indirectly determine or calculate the assumed brake rotor temperature using a temperature model and various inputs such as wheel speed, duration of a braking event, brake force, etc. This approach does not require a physical temperature sensor, as the sensor is actually a collection of electronic instructions and / or data structures, such as lookup tables and the like.In another example, the brake temperature sensors 16-22 include actual sensing elements, such as embedded thermal probes, infrared sensors, etc., located proximate the brake assemblies and directly sensing the temperature of the brake rotor, brake pad, lining, and / or other suitable brake component. In another example, the brake temperature sensors 16-22 are part of another component, device, system, etc., within the vehicle, such as a brake anti-lock system (ABS) or a traction control system. Any suitable approach may be used to provide the vehicle braking system 10 with information regarding the actual or estimated temperature of one or more brake components.

[0014] Humidity sensor 24 provides vehicle braking system 10 with a humidity signal containing humidity readings representative of the moisture, wetness, and / or precipitation in the atmosphere surrounding the vehicle. Some examples of how humidity sensor 24 may determine and provide humidity readings include: directly measuring the humidity of the atmosphere outside the vehicle; indirectly determining humidity by collecting readings from other components, devices, modules, systems, etc., located on the vehicle; indirectly determining humidity by collecting readings from other nearby vehicles via vehicle-to-vehicle communication; or indirectly determining humidity by receiving radio transmissions containing weather reports, forecasts, etc., from a weather service.According to an exemplary embodiment, humidity sensor 24 is an existing component mounted within an engine intake, such as one in fluid communication with an intake manifold, and already provides humidity information to an engine control module (ECM). Other techniques and sensors are also possible.

[0015] The electronics module 30 may include any combination of electronic processing devices, storage devices, input / output (I / O) devices, and / or other known components, and may perform various functions related to control and / or communication. In an exemplary embodiment, the electronics module 30 includes an electronic storage device 50 and an electronic processing device 52. These modules may be embodied in hardware, software, or a combination thereof. Depending on the particular embodiment, the electronics module 30 may be a single, standalone unit or module; it may be incorporated into or contained within one or more other electronic modules or systems; it may be part of a larger network or system (e.g.,an anti-lock braking system (ABS), a stability control system, a traction control system, an integrated vehicle control module (VICM), a motion and energy control module (MEC module), a hybrid control module, etc.), or it may embody a combination of these arrangements, to name a few possibilities. According to one embodiment, the electronics module 30 is an electronic brake control module (EBCM) that controls a combination of friction braking operations and regenerative braking operations. In such an arrangement, it may control the friction braking system 32 and / or the regenerative braking system 34 via brake command signals sent to these systems. The present method is not limited to any particular embodiment.

[0016] The electronic storage device 50 may include any suitable electronic storage medium and may store a variety of data, information, and / or electronic instructions. These may include, for example, sensed vehicle conditions (e.g., those provided by sensors 14-24), lookup tables and other data structures, algorithms (e.g., electronic instructions used to implement the method described below), vehicle component characteristics, and background information (e.g., operating settings, etc., for the various vehicle components), or others. The method described below—as well as any combination of electronic instructions and information needed to perform this algorithm—may be stored or otherwise maintained in the storage device 50.

[0017] The electronic processing device 52 may include any type of suitable electronic processor (e.g., a microprocessor, a microcontroller, an application-specific integrated circuit (ASIC), etc.) that executes electronic instructions for software, firmware, programs, algorithms, scripts, etc. The example processing device 52 is not limited to any one type of component or device. The electronics module 30 may be electronically connected to other vehicle devices, modules, systems, etc., via a suitable connection and may interact with them as needed. Of course, these are only a few of the possible arrangements, functions, and capabilities of the electronics module, as others are certainly possible.

[0018] The friction brake system 32 is shown here as an electro-hydraulic brake (EHB) system, although it may be an electromechanical or other type of brake system that generates friction braking torque in a conventional manner. According to this exemplary embodiment, the friction brake system 32 includes a hydraulic unit 60 hydraulically coupled to the brake units 72-78 located outboard of each of the vehicle wheels or corners. Although the hydraulic unit 60 is schematically shown as including a separate hydraulic control unit 62, an accumulator 64, a master cylinder 66, and one or more actuators 68 arranged together, it should be appreciated that any combination of these and / or other devices may instead be provided according to a number of other arrangements known in the art.For example, the hydraulic control unit 62 may be integrated into the EBCM 30 or another module and connected to the actuator(s) 68 via electrical connections. The hydraulic control unit 62 may interact with the EBCM 30 and act as an intermediary or driver for the various electromechanical actuators and devices in the friction brake system 32. In one example, the hydraulic control unit 62 receives brake command signals from the EBCM 30, processes these signals, and uses them to operate the actuator(s) 68 so that the fluid pressure in hydraulic lines 70 is maintained at a desired pressure. In the case of a disc brake embodiment, the fluid pressure drives brake pistons in the brake units 72-78 and controls the braking force and torque applied by them.Those skilled in the art will appreciate that hydraulic control unit 62 may perform any number of other tasks and execute a variety of other instructions, including those of the present method. Since the general structure and operation of accumulators, master cylinders, actuators, and other components of hydraulic unit 60 are well known, further description has been omitted.

[0019] The brake units 72-78 may be part of any suitable vehicle braking system, including systems employing disc brakes, drum brakes, electro-hydraulic braking, electromechanical braking, regenerative braking, brake-by-wire, etc. In an exemplary embodiment, the brake units 72-78 are each located outboard of a vehicle corner and each generally includes a brake rotor 80, a brake caliper 82, a piston 88, and brake pads (not shown) and is part of an electro-hydraulic braking (EHB) system. As will be appreciated by those skilled in the art, a tire-wheel assembly (not shown) is attached to a hub with multiple lug nuts so that the tire, wheel, hub, and rotor 80 all rotate together.The brake caliper 82 grips the rotor 80 and supports the brake piston 88 so that compression and frictional braking force can be applied by brake pads to opposite sides of the rotor during a braking event. The frictional braking forces slow the rotation of the rotor 80 and, in turn, the rotation of the tire-wheel assembly and, ultimately, the vehicle. The brake pistons for each of the various wheels or corners can be controlled all together, controlled on a wheel-by-wheel basis, controlled in groups (e.g., the front wheels are controlled separately from the rear wheels), or controlled according to another known method.It should be noted that the method and system described herein are not limited to use with disc brake systems and may be used with other braking systems and assemblies including electromechanical brakes (EMB), electric brake calipers (e-calipers) and / or drum brake systems.

[0020] The regenerative braking system 34 uses electromagnetically derived regenerative braking torque to counteract the forward rotation of the vehicle wheels and may include a regenerative unit 90 with a control unit 92 and a motor / generator 94. The regenerative control unit 92 may control or manage certain aspects of the regenerative braking operations, which include aspects of the present method, and may interact with the EBCM 30, the hydraulic control unit 62, and / or another component, device, module, system, etc., within the vehicle. The motor / generator 94 may include both a motor and a generator (called a "motor") to produce both positive torque (acceleration) and negative torque (braking). The motor / generator 94 may be coupled to one or more driveline components, including output shafts, axles, vehicle wheels, etc.and may use the rotation of the drivetrain component(s) to decelerate the vehicle and generate electrical energy to charge a battery (not shown). Although . Fig. 1 schematically illustrates the motor / generator 94 as a single combined device, the motor and generator may be separated and provided as two separate devices, or multiple motor / generators may be provided (e.g., separate motor / generators for the front and rear wheels, separate motor / generators for each wheel, separate motor / generators for different functions, etc.), to name a few possibilities. Therefore, the following description refers only to a single combined motor / generator unit 94, although other embodiments of the regenerative braking system may be used instead. The motor / generator 94 may be AC ​​motors (e.g., a three-phase AC induction motor), DC motors, brushed or brushless motors, permanent magnet motors, etc.and may include a variety of components, such as cooling features, sensors, control units, and / or any other suitable components known in the art.

[0021] Again, the above description of the exemplary vehicle braking system 10 and the drawing in Fig. 1 is intended only to illustrate one possible embodiment, as the following method is not limited to use with this system only. Any number of other system arrangements, combinations, and architectures, including those different from that shown in Fig. 1 shown, may be used instead.

[0022] With reference now to Fig. 2 shows an exemplary method 100 for operating a vehicle braking system, and more particularly, for improving driver braking perception by reducing the effects of overly "grippy" brakes that occur due to certain humidity and temperature conditions. When a vehicle is stationary for extended periods of high humidity, the wetness can cause rotor corrosion and increase the corresponding coefficient of friction (µ), causing the brakes to respond in an unexpectedly aggressive manner. After the friction brakes have been applied several times, the rotor corrosion due to friction is worn away; however, during these first few friction braking events, the brakes may apply in a more forceful manner than expected.For example, in the case of a hybrid electric vehicle (HEV) initially employing a regenerative braking system, the friction braking system may not be engaged until the vehicle has been traveling for a certain amount of time. Therefore, the exemplary method 100 is designed to address this potential problem, but does so in a manner that allows for optimal use of a fuel-efficient regenerative braking system. It is possible for the method 100 to run in the background of the electronics module 30 and / or other device whenever the friction braking system 32 is engaged, or the method may be executed whenever a change in humidity, brake temperature, and / or another reading is detected, to name just two possibilities.

[0023] Beginning with steps 110 and 120, the method collects brake signals and determines whether a braking request is present. According to an example of a manual braking request, step 110 collects brake signals from the brake pedal sensor 14 or other device and uses the readings to determine whether the driver is currently requesting a braking event by manually applying the brake pedal. In an example of an automated braking request, step 110 uses brake signals from an electronic brake control module (EBCM) 30 or the like and uses the brake readings to determine whether the vehicle is currently automatically requesting braking without driver intervention (e.g., in the case of an automatic lane change system or an active cruise control system, where the vehicle automatically brakes without driver command).

[0024] Of course, any suitable technique or method may be used to detect a braking request, not just the exemplary ones described above. If step 120 determines that a braking request is present, the method proceeds to step 130; otherwise, the method returns for continued monitoring.

[0025] Step 130 then collects humidity and brake temperature signals from one or more sensors located anywhere in the vehicle. For example, the method may retrieve humidity signals from a humidity sensor 24 located at an engine intake or other suitable location, as well as brake temperature signals from the brake temperature sensors 16-22. As explained in greater detail above, these signals may be collected or calculated directly or indirectly from any number of different sources in any number of different ways and are not limited to any particular embodiment. In one embodiment, the humidity sensor 24 and the brake temperature sensors 16-22 provide their respective readings to the electronics module 30 periodically (e.g., every 0.5 s - 1 s).

[0026] Next, step 140 uses the humidity and brake temperature readings to determine a correction factor. As used herein, a "correction factor" broadly includes any number, factor, multiplier, additive, offset, and / or other value used to adjust or otherwise modify a brake command signal to compensate for changes in a coefficient of friction (µ) due to humidity, temperature, etc.According to one example without limitation of step 140, the electronics module 30 uses the humidity and brake temperature readings from the previous step in conjunction with one or more lookup tables to generate a scalar correction factor or multiplier, which can then be used to determine a modified brake command signal to compensate for changes in the coefficient of friction (µ) of one or more of the brake components. An example of such a lookup table is shown in the example record 200 of FIG. Fig. 3, where the brake temperature (°C) is an input and is plotted on the x-axis, the specific humidity (%) is another input and is plotted on the y-axis, and the correction factor is the output and is plotted on the z-axis.

[0027] In this particular embodiment, the correction factor is a scalar value ranging from approximately 0.7 to 1.0, however, other numerical ranges may be used instead (e.g., from approximately 0.7 to 1.3). Correction factors near 0.7 represent more aggressive compensation where more significant changes in the friction coefficients (µ) of brake components are expected, while correction factors around 1.0 represent less or no compensation because the friction coefficients (µ) are not expected to have changed significantly. As can be seen in record 200, as humidity increases at a particular brake temperature reading, the correction factor generally decreases, moving away from 1.0.This is because higher humidity typically leads to wetness or corrosion on the brake components, which in turn usually increases their coefficient of friction (µ) and makes them more "grippy" or "grippy." Although some wetness on brake components can increase their coefficient of friction, one skilled in the art will appreciate that significant amounts of water on brake components (e.g., when it rains) usually decrease their coefficient of friction. As also illustrated in record 200, as the brake temperature decreases for a particular humidity reading, the correction factor generally decreases and results in a more aggressive correction factor. Warmer brake temperatures usually result in less aggressive correction factors that are closer to the value 1.0 for a particular humidity reading, thus modifying an underlying brake command signal in a less significant manner.

[0028] Consequently, the present method generally expects wet / cold brakes to have the highest friction coefficients (µ) and therefore assigns them more aggressive correction factors (e.g., closer to a value of 0.7), while hot / dry brakes are expected to have the lowest friction coefficients (µ) and are therefore given correction values ​​closer to 1.0. Even if the external humidity is quite high, if the brake temperature exceeds a certain amount (e.g., approximately 40°C), the method assumes that the friction brakes are currently engaged and that this will evaporate much of the moisture on the brake components; this explains why the correction factor is close to 1.0 at high temperatures coupled with high humidity. It can be seen that a correction value of 1.0 generally has no effect on brake command signals (a value multiplied by 1.0 is the same).The method can also adjust the slew rate up and down, which is the rate at which the correction factor is allowed to change.

[0029] In determining the correction factor, step 140 may further consider other factors and variables. For example, the vehicle type (e.g., heavy, light, etc.), the brake system type (e.g., discs, drums, etc.), the brake unit arrangement (e.g., front, rear, left, right, etc.), the current driving style (e.g., aggressive driving, passive driving, etc.), and / or other factors may further influence the correction value. In one example, the present method may use a correction factor that is in the range of approximately 0.7 - 1.3; the range is generally determined by the susceptibility of the brake pad and rotor to moisture, as in the example of Fig.3. In situations where the coefficient of friction (µ) is expected to be lower than normal (i.e., situations where the brake components are more slippery), the correction value may increase above 1.0. It is possible for the method to use one lookup table for front brake units and a different lookup table for rear brake units, or to generate different correction factors for each brake unit or wheel depending on their particular brake temperature readings. In yet another embodiment, the method may average the humidity and / or brake temperature readings before entering them into a lookup table, data structure, or algorithm.It is also possible for the method to generate the correction factor based only on humidity and not take brake temperature into account; in such a case, a one-dimensional lookup table with humidity readings as input and the correction factor as output can be used.

[0030] At step 150, the method uses the correction factor to generate modified brake command signals that account for the wetness in the vehicle braking system. The application of the correction factor may vary, but according to an exemplary embodiment, the method multiplies a base friction coefficient value (µ b ) with the correction factor to arrive at a modified friction coefficient value (µ m ) and then uses the modified friction coefficient value (µ m ) to generate the modified brake command signals. The base friction coefficient value (µ b) may be a static value stored in the storage device 50 at the time of vehicle manufacture, or it may be a more dynamic value that is calibrated and adjusted over time. There are numerous methods for determining and storing the base friction coefficient value (µ b ), any of which can be used here. The modified friction coefficient value (µ m ) may then be applied, combined, or otherwise used in conjunction with the brake readings from the brake pedal sensor 14 to generate the modified brake command signals designed to more accurately and consistently deliver the braking torque requested by the driver by compensating for wetness, humidity, temperature, etc., in the vehicle braking system 10.

[0031] Step 160 then controls one or more aspects of the vehicle braking system 10 in accordance with the modified brake command signals. By way of non-limiting example, the electronics module 30 sends the modified brake command signals to the friction braking system 32 (e.g., to the hydraulic unit 60) so that the friction brakes can be activated in a manner that compensates for wetness, humidity, temperature, etc., as discussed above. By compensating for the detected humidity, the present method is able to optimally utilize either the friction braking system 32 or the regenerative braking system 34, as opposed to simply shutting down the regenerative braking system until the friction braking components have warmed up and dried, as is done in some systems. This can result in improved fuel economy.

Claims

[1] A method for operating a vehicle braking system (10) comprising the steps of: (a) receiving humidity readings from a humidity sensor (24), wherein the humidity readings are received from the humidity sensor (24) and brake temperature readings are received from one or more brake temperature sensors; (b) the humidity readings are used to generate a modified brake command signal that compensates for wetness in the vehicle brake system (10), wherein the humidity readings and the brake temperature readings are used to determine a correction factor, and the correction factor is used to generate the modified brake command signal so that it compensates for changes in a friction coefficient (µ) of one or more brake components; and (c) controlling one or more aspects of the vehicle braking system (10) in accordance with the modified braking command signal; characterized by , that where the correction factor is a scalar correction factor between 0.7 and 1.3, which is correlated with a base friction coefficient value (µ b ) to obtain a modified friction coefficient value (µ m ) or that the correction factor for a particular brake temperature reading generally decreases away from the value 1.0 as humidity increases, and the correction factor for a particular brake temperature reading generally increases towards the value 1.0 as humidity decreases; or that the correction factor for a particular humidity reading generally decreases away from the value 1.0 as the brake temperature decreases, and the correction factor for a particular humidity reading generally increases towards the value 1.0 as the brake temperature increases; or that Step (b) further comprises using the humidity readings, the brake temperature readings, and at least one additional factor selected from the following list to determine the correction factor: the vehicle type, the brake system type, the brake unit layout, or the current driving style; or Step (b) further comprises adjusting a rate of increase or decrease, and the rate of increase is the rate at which the correction factor is allowed to change. [2] The method of claim 1, wherein step (b) further comprises using a lookup table having the humidity readings and the brake temperature readings as inputs and the correction factor as an output. [3] The method of claim 1, wherein the correction factor is generally most aggressive when the brakes are wet and cold, and the correction factor is generally least aggressive when the brakes are dry and hot. [4] The method of claim 1, further comprising the step of: brake readings are collected and the brake readings are used to determine if there is a brake request for a friction brake event, and step (c) is only performed if there is a brake request for a friction brake event.

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