Method for operating a vehicle braking system

Abstract

A method for operating a vehicle braking system that has both friction and regenerative braking capabilities. According to one embodiment, the method monitors wheel slip at a number of vehicle wheels, evaluates individual wheel slip at a single wheel as well as collective wheel slip affecting multiple wheels, compares the wheel slip with one or more speed-based thresholds based on the vehicle speed, and uses the comparisons to assist in distinguishing between those situations that justify disabling regenerative braking and those that do not.

Description

TECHNICAL AREA

[0001] The present invention relates generally to a vehicle braking system and in particular to a vehicle braking system capable of both friction and regenerative braking operation. BACKGROUND

[0002] Vehicle braking systems that possess both friction and regenerative braking capabilities will sometimes terminate regenerative braking upon detecting the first sign of potential vehicle instability. For example, some vehicle braking systems will abruptly terminate regenerative braking if the vehicle travels over a small pothole and experiences temporary, isolated wheel slip at a single wheel. In this situation, an abrupt and immediate termination of regenerative braking may not be justified or necessary to maintain vehicle stability, and it can result in reduced fuel economy, increased wear on the friction brake components, and a loss of deceleration while the friction braking torque builds up to replace the aborted regenerative braking torque.

[0003] German patent application DE 198 46 848 A1 discloses a method for vehicle stabilization in which only the brake pressure of the rear axle wheels is controllable. During braking while cornering, an oversteering or understeering driving condition is detected, and by influencing the brake pressure of at least one wheel on the rear axle, a moment counteracting this driving condition is generated.

[0004] In US patent application 6,231,134 B1, a vehicle braking system is disclosed that is capable of both friction braking and regenerative braking. If the total braking torque from regenerative and friction braking at a wheel exceeds an upper limit corresponding to the coefficient of friction of the road on which the vehicle is traveling, the regenerative braking torque is reduced to zero.

[0005] The German patent application DE 10 2008 017 480 A1 discloses a method for operating a vehicle braking system in which, to avoid overbraking on a rear axle of the vehicle, the regenerative braking torque on the rear axle is reduced so that a slip occurring on at least one vehicle wheel of the rear axle does not exceed a slip threshold assigned to this rear wheel or only to an insignificant extent.

[0006] Patent application US 2008 / 0 100 132 A1 discloses a method for controlling regenerative braking in an electric vehicle, in which wheel slip is detected during regenerative braking and, if this wheel slip increases, the regenerative braking torque is reduced. If the wheel slip exceeds a set value, a brake anti-lock braking system is activated and the regenerative braking torque is continuously reduced.

[0007] The object of the invention is to prevent, as far as possible, the unnecessary interruption of a regenerative braking process in order to maintain vehicle stability.

[0008] This problem is solved by methods having the features of independent claims 1 and 9. SUMMARY

[0009] According to one embodiment, a method for operating a vehicle braking system is provided. The method may include the steps of: (a) determining wheel slip; (b) determining a vehicle speed; (c) comparing the wheel slip with a speed-based threshold that is based on the vehicle speed; and (d) then, if the wheel slip exceeds the speed-based threshold, disabling regenerative braking.

[0010] According to a further embodiment, a method for operating a vehicle braking system is provided. The method may include the steps of: (a) determining a first wheel slip and a second wheel slip; (b) comparing the first wheel slip with a first threshold and comparing the second wheel slip with a second threshold, and confirming that the first threshold differs from the second threshold; and (c) then, if either the first wheel slip exceeds the first threshold or the second wheel slip exceeds the second threshold, switching off regenerative braking. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Preferred exemplary embodiments are described below in conjunction with the accompanying drawings, where the same reference numerals denote the same elements and where: Fig.1 is a block diagram of an exemplary vehicle braking system that exhibits both friction and regenerative braking capabilities; and Fig. 2. A flowchart of an exemplary procedure that can be used to design a vehicle braking system, such as the one used in Fig. As shown in 1, to operate. DETAILED DESCRIPTION OF THE EXECUTION FORM

[0012] The method described below evaluates one or more braking conditions and, based on this evaluation, distinguishes between situations that justify disabling regenerative braking and those that do not. Disabling regenerative braking typically reduces the vehicle's fuel economy, and therefore this method seeks to reduce the number of instances in which regenerative braking is unnecessarily or prematurely deactivated. According to an exemplary embodiment, this method monitors wheel slip at a number of vehicle wheels, evaluates individual wheel slip at a single wheel as well as collective wheel slip at two or more wheels, and compares the wheel slip to one or more speed-based thresholds derived from a vehicle speed.These techniques can improve the ability of the present procedure to delimit or differentiate between those situations in which switching off regenerative braking is correct and those in which it is premature or unnecessary.

[0013] Although the exemplary method is described here in the context of a brake-by-wire system, such as an electro-hydraulic braking system (EHB system) or an electromechanical braking system (EMB system), it should be noted that the method can also be used with any number of other braking systems and is not limited to the disclosed embodiment. For example, the present method can be used with other brake-by-wire or non-brake-by-wire systems, regenerative braking systems (e.g., those found in hybrid vehicles, battery electric vehicles, etc.), and other braking systems that use different technology types (e.g., disc brakes, drum brakes, or a combination thereof). These are only some of the possibilities, as the present method can be used with other vehicle braking systems and is not limited to use with any particular type.

[0014] With reference to Fig.Figure 1 shows a block diagram of an exemplary vehicle braking system 10, which has both friction and regenerative braking capabilities and generally includes a brake pedal sensor 12, wheel speed sensors 14-20, a vehicle speed sensor 22, an electronic module 30, a friction braking system 32, and a regenerative braking system 34. The vehicle braking system 10 can be part of a hybrid vehicle or a battery electric vehicle (BEV), which, for example, uses a combination of friction and regenerative braking to control the vehicle's speed and to generate electrical energy. In such an arrangement, a friction braking torque is generated in a conventional manner and acts against the vehicle's forward torque through frictional resistance generated by disc brakes, drum brakes, etc. The regenerative braking torque, on the other hand, is generated by a generator (e.g., a battery).an electric motor operated in reverse direction) generates and acts against the forward torque of the vehicle through electromagnetically derived forces that act against the forward rotation of the vehicle wheels and / or other drivetrain components (this process also charges a battery that can later be used to power the vehicle).

[0015] The brake pedal sensor 12 supplies the vehicle braking system 10 with a brake pedal signal containing information regarding the position, movement, applied force, braking intent, and / or general state of the brake pedal (hereinafter collectively referred to as the "brake status"). Any number of different types of brake pedal sensors can be used; these include non-contact sensors (e.g., optical sensors, electromagnetic sensors, etc.), contact sensors (e.g., potentiometers, contact switches, etc.), and those that measure the force applied by the driver to the brake pedal, to name a few. In a brake-by-wire application, the brake pedal sensor 12 can be integrated into a brake pedal simulator or emulator, which transmits the expected mechanical feel of the brake pedal to the driver. The brake pedal simulator can also incorporate other mechanical and / or electronic components, including sensors, etc.contain.

[0016] The wheel speed sensors 14-20 supply the brake system 10 with wheel speed signals containing information regarding the position, rotational speed, acceleration, slip, and / or general condition of the wheels (hereinafter collectively referred to as "wheel status"). According to one embodiment, individual speed sensors are coupled to each of the vehicle's four wheels and separately report the rotational speed of the four wheels. The wheel speed sensors 14-20 can incorporate electromagnetic elements, optical elements, or any other technology known in the art.

[0017] The vehicle speed sensor 22 supplies the vehicle braking system 10 with a vehicle speed signal, which generally represents the vehicle's speed or velocity. A variety of different vehicle speed sensors and sensing techniques can be used, including those that utilize wheel speed, ground speed, accelerator pedal position, clutch pedal position, gear selector lever position, vehicle acceleration, engine speed, engine torque, and throttle valve position, to name just a few. In one embodiment, the vehicle speed sensor 22 derives or calculates a vehicle speed signal from the wheel speed signals described above (in which case it may be possible to assemble or combine the vehicle speed sensor 22 with one or more of the wheel speed sensors 14-20).In another embodiment, the vehicle speed sensor 22 determines the vehicle's speed relative to the ground by directing radar, laser, or other signals onto the ground and analyzing the reflected signals. In yet another embodiment, the vehicle speed sensor 22 is coupled to specific parts of the vehicle and determines the vehicle's speed accordingly. For example, a vehicle speed sensor can be coupled to a transmission output shaft, behind the speedometer, or even to individual wheel speed sensors 14-20 as mentioned above. The vehicle speed sensor 22 can comprise electromagnetic elements, optical elements, or any other technology known in the field.It is also possible that the vehicle speed signal is supplied to the vehicle braking system 10 from another component, module and / or system in the vehicle, such as an engine control module (ECM).

[0018] The electronic module 30 is an electronic device or unit arranged in the vehicle and includes an electronic processing unit 50 and a storage unit 52. The electronic processing unit 50 can control certain operational aspects of the vehicle's braking system 10 by executing various electronic instructions, including those of the present method. Some examples of suitable electronic processing units include digital and analog microprocessors, microcontrollers, application-specific integrated circuits (ASICs), or any other processing unit known in the field.The storage device 52 can comprise any type of electronic working memory and can be used to store some of the electronic instructions that constitute the present method, as well as sensor read values, threshold values, lookup tables, and any other data structure that may contain data used by the method described herein. According to one embodiment, the electronic module 30 is an electronic brake control module (EBCM) that controls a combination of friction and regenerative braking operation. In such an arrangement, it can control the friction braking system 32 and / or the regenerative braking system 34 via command signals sent to these systems.The electronic brake control module (EBCM) 30 can be a separate, stand-alone electronic module, or it can be integrated into a larger module or system, such as a traction control system, a brake anti-lock braking system (ABS), or an integrated vehicle control module (VICM), to name a few possibilities.

[0019] It should be noted that each of the components described here—including the brake pedal sensor 12, the wheel speed sensors 14–20, the vehicle speed sensor 22, and the electronic module 30—can contain any combination of hardware and / or software components and is not limited to a specific form or embodiment. Furthermore, these components can be combined with, integrated into, or otherwise contained within other components, modules, and / or systems arranged throughout the vehicle, and they need not be separate, discrete components as schematically depicted in Fig. 1 is shown.

[0020] The friction braking system 32 is shown here as an electro-hydraulic braking system (EHB system), although it could be an electromechanical or other type of braking system capable of generating a friction braking torque in a conventional manner. According to this exemplary embodiment, the friction braking system 32 comprises a hydraulic unit 60 hydraulically coupled to brake units 72-78 located at each vehicle wheel or corner. Although the hydraulic unit 60 is shown schematically to have a separate hydraulic control unit 62, an accumulator 64, a master cylinder 66, and one or more actuators 68 arranged together, it should be noted that any combination of these and / or other devices could instead be provided according to a number of other arrangements known in the field.For example, the hydraulic control unit 62 can be integrated into the EBCM 30 or another module and connected to the actuators 68 via electrical connections. The hydraulic control unit 62 can interact with the EBCM and act as an intermediary or driver for the various electromechanical actuators and devices in the friction braking 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 actuators 68 so that the fluid pressure in hydraulic lines 70 is maintained at a set pressure. In the case of a disc brake configuration, the fluid pressure drives brake pistons in brake units 72-78 and controls the braking force and braking torque exerted by them.Experts will recognize that the hydraulic control unit 62 can perform any number of different tasks and execute a variety of different instructions, including those of the present procedure. Since the general structure and operation of the accumulators, master cylinders, actuators, and other components of the hydraulic unit 60 are generally known, further description has been omitted.

[0021] The brake units 72-78 are each arranged at a corner of the vehicle and, according to one embodiment, each comprise a rotor 80, a brake caliper 82, a brake piston 88, and (not shown) brake pads, and can be part of an electro-hydraulic braking system (EHB system) or another type of system. As a person skilled in the art will recognize, a tire-wheel assembly (not shown) is attached to a hub with several wheel nuts, so that the tire, the wheel, the hub, and the rotor 80 all rotate together. The brake caliper 82 spans the rotor 80 and carries brake pistons 88, so that during a braking event, a pressure and friction braking force can be applied by brake pads to opposite sides of the rotor. The friction braking forces slow down the rotation of the rotor 80 and thus the rotation of the tire-wheel assembly and ultimately the vehicle.The brake pistons for each of the different wheels or each of the different corners can be: controlled all together, controlled on a wheel-by-wheel basis, controlled in groups (for example, 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 here are not limited to use with disc brake systems and can be used with other brake systems and arrangements, including drum brake systems.

[0022] The regenerative braking system 34 uses an 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 operation, including aspects of the present procedure, and it may interact with the EBCM 30, the hydraulic control unit 62, and / or another component, device, module, system, etc., in the vehicle. The motor / generator 94 may be electromagnetically coupled to one or more powertrain components, including output shafts, axles, vehicle wheels, etc., and it uses the rotation of the powertrain component(s) to decelerate the vehicle and to generate electrical energy to charge a battery (not shown). Fig. While Figure 1 schematically depicts the motor / generator 94 as a single combined unit, the motor and generator can be separate and provided as two distinct units, or multiple motors / generators can be provided (e.g., separate motors / generators for the front and rear wheels, separate motors / generators for each wheel, separate motors / generators for different functions, etc.), to name just a few possibilities. Therefore, the following description refers only to a single combined motor / generator unit 94, although other embodiments with a regenerative braking system can be used instead.

[0023] It should be noted again that the preceding descriptions of the vehicle braking system 10 and its various components and devices are exemplary and provided for illustrative purposes only. The method described here can be used with any number of different vehicle braking systems and is not limited to these exemplary embodiments.

[0024] With reference to now Fig.Figure 2 shows an exemplary method 100 for operating a vehicle braking system and, in particular, for accurately determining when it is appropriate to disable regenerative braking due to potential vehicle instability. Starting with step 102, the method collects various pieces of information from the vehicle, including, but certainly not limited to, wheel speeds, vehicle speed, and brake status. In one embodiment, the wheel speed sensors 14-20 provide wheel speed signals, the vehicle speed sensor 22 provides a vehicle speed signal, and the brake pedal sensor 12 provides a brake status signal that includes a driver braking intention (also known as a driver-requested braking torque). The information collected in step 102 can be obtained from components, modules, and systems other than those shown in Figure 2. Fig.The data can be supplied by the exemplary sensors shown in Figure 1 (for example, wheel speed can be processed and provided by a stability control system or a brake anti-lock braking system (ABS)); they can be derived or calculated from any other information (for example, vehicle speed can represent the speed of the vehicle relative to the ground and be the result of a complex algorithm based on wheel speed); they can be filtered, converted, or otherwise processed before their content is evaluated (for example, wheel speed signals can be obtained in the time domain and converted to the frequency domain, or obtained as analog signals and converted to digital signals), etc.It should also be noted that wheel speed, vehicle speed and brake status are only some of the potential vehicle operating conditions that can be collected and used by the present procedure, as accelerator pedal position, clutch pedal position, gear selector lever selection, vehicle acceleration, engine speed, engine torque, throttle valve position and / or any other suitable vehicle operating condition can be used instead of or in addition to the parameters mentioned herein.

[0025] Next, step 104 uses the information gathered in the previous step to determine the wheel slip for one or more vehicle wheels. Wheel slip can result from a number of different factors, including road defects (e.g., potholes or uneven road joints), road conditions (e.g., wet or gravel surfaces), and vehicle conditions (e.g., worn tires), to name a few. In one embodiment, step 104 determines a separate wheel slip value for each vehicle wheel by comparing each wheel rotational speed to a common vehicle speed and looking for deviations (the rotational speed of the wheel rotation can be converted to the vehicle speed relative to the ground, or vice versa, for this comparison).Wheel slip values ​​can take various forms, including speed differences, percentages, ratios, etc. In one embodiment, the wheel speed for each wheel (W. s ) of the vehicle speed (V s ) subtracted to obtain a velocity delta (Δ s ) to determine (Equation 1). The velocity delta (Δ s ) can then be determined by the vehicle speed (V s ) be divided to obtain a velocity delta percentage (Δ s% ) to arrive at (equation 2). Speed ​​delta (Δs) = vehicle speed (Vs) − wheel speed (Vw) Speed ​​delta percentage (Δs%) = Speed ​​delta (Δs) / Vehicle speed (Vs)

[0026] A “wheel slip,” as used here, can include speed delta values, speed delta percentage values, and / or any other values ​​or information—regardless of their form—related to wheel slippage at one or more of the vehicle's wheels. This information may be calculated by the EBCM 30 or provided by another unit, module, or system in the vehicle, and this information may relate to individual wheel slip or collective wheel slip, as discussed in more detail below. One reason for determining the speed delta percentage is that it takes the vehicle's speed into account.

[0027] For example, a velocity delta value (Δ sA speed difference (Δ) of 3 mph (4.8 km / h) is a bigger problem for a vehicle traveling at 10 mph (16 km / h) than for the same vehicle traveling at 70 mph (112.6 km / h). Using the same characteristic, a speed delta percentage (Δ s% A 10% slippage rate might be more problematic for the vehicle traveling at 70 mph (112.6 km / h) than for the vehicle traveling at 10 mph (16 km / h). Therefore, step 104 can determine any combination of suitable wheel slip information.

[0028] Next, step 106 evaluates the wheel slip information from the previous step and determines whether or not a condition of excessive wheel slip exists. There are several different ways to perform this determination. According to one possibility, step 106 can compare wheel slip to a speed-based threshold that is dependent on the vehicle speed. This differs from techniques that compare wheel slip to a threshold that is not related to the vehicle speed. As mentioned above, a speed delta value (Δ) can be used. sA speed-based threshold of 3 mph (4.8 km / h) may be more problematic for a vehicle traveling at 10 mph (16 km / h) than for the same vehicle traveling at 70 mph (112.6 km / h); consequently, a speed-based threshold can account for this relationship. In accordance with another possibility, step 106 can compare a common wheel slip with a common threshold, where the common wheel slip accounts for the individual wheel slip of two or more wheels. The common threshold is preferably a speed-based common threshold, but this is not necessary. To illustrate the use of the common wheel slip, consider the example of the vehicle driving over a small pothole located on the left side of the vehicle (typically, this situation does not justify disabling regenerative braking).Although the individual wheel slip of the left front wheel may exceed an individual threshold (suggesting the existence of an excessive wheel slip condition), step 106 can consider the combined wheel slip of the two front wheels (which includes the right front wheel that did not encounter the pothole) and conclude that the combined wheel slip does not exceed a common threshold and that no excessive wheel slip condition exists. Any number of wheel groups can be used to determine and evaluate a collective wheel slip (for example, wheels mounted on the same vehicle axle can be grouped together (front wheels, rear wheels, etc.), wheels mounted on the same side of the vehicle can be grouped together (left-side wheels, right-side wheels, etc.)).), diagonally or obliquely opposite wheels can be grouped together, etc.). By using common wheel slip / thresholds in addition to or instead of individual wheel slip / thresholds, step 106 may be able to obtain a more accurate picture of the overall wheel slip situation facing the vehicle and therefore make a better decision regarding the disabling of regenerative braking.

[0029] Step 106 can perform these assessments, calculations, determinations, etc., using lookup tables or other data structures. For example, one or more lookup tables can be used to carry speed-based thresholds, with the vehicle speed as the input and the wheel slip threshold as the output.According to one exemplary embodiment, four separate two-dimensional lookup tables are used to carry different speed-based thresholds: a first lookup table carries individual wheel slip thresholds for the front wheels (in this case, the front wheels use the same threshold), a second lookup table carries common wheel slip thresholds for the front wheels, a third lookup table carries individual wheel slip thresholds for the rear wheels (in this case, the rear wheels use the same threshold), and a fourth lookup table carries common wheel slip thresholds for the rear wheels. It is possible that the individual and / or common wheel slip thresholds for the front wheels are the same, that they are different, or that other lookup tables are provided containing wheel slip thresholds for other wheel groups (e.g.,(left wheels, right wheels, drive wheels, etc.). The various threshold values ​​may have been empirically derived from tests during the development of the vehicle (e.g., static information that generally does not change) or during the vehicle's service life (e.g., dynamic information that is updated, modified, changed, etc.), and they may take into account factors such as vehicle speed and conditions, including weather conditions (e.g., rain, snow, ice, etc.), drivetrain conditions (e.g., front-wheel drive, rear-wheel drive, all-wheel drive), vehicle conditions (e.g., weight, load, tire pressure, etc.), and be used to populate the various lookup tables.

[0030] The wheel slip threshold values ​​can be determined using the speed delta threshold (Δ s%) can be expressed as a percentage of the speed and can decrease as the vehicle's speed increases. As mentioned above, a speed delta percentage (Δ) can be expressed as a percentage of the speed. s% A wheel slip threshold of 10% for a vehicle traveling at 70 mph (112.6 km / h) may be more problematic than for the same vehicle traveling at 10 mph (16 km / h). Although the present description is in the context of an example lookup table that includes percentages, it should be noted that Step 106 may algorithmically determine or model the wheel slip threshold values ​​instead of obtaining them from lookup tables, and it may use wheel slip threshold values ​​other than percentages. Regardless of how the wheel slip threshold values ​​are determined, expressed, and / or used, they are generally intended to represent the maximum amount of wheel slip that the procedure will tolerate before determining that an excessive wheel slip condition exists.

[0031] Step 106 can use any combination of one or more comparisons to determine whether an excessive wheel slip event exists. In the preceding example, which uses four different lookup tables, Step 106 evaluates individual front wheel slip, combined front wheel slip, individual rear wheel slip, and combined rear wheel slip. If the measured wheel slip exceeds the appropriate threshold for any of these four lookup tables, then Step 106 can determine that an excessive wheel slip condition exists. Consider the example where a vehicle traveling at 10 mph(16 km / h), and the following wheel slip values ​​are determined: an individual front left (FL) wheel slip value of 9%, an individual front right (FR) wheel slip value of 8%, an individual rear left (RL) wheel slip value of 11%, and an individual rear right (RR) wheel slip value of 9%. Furthermore, it is assumed that the speed-based thresholds, based on a vehicle speed of 10 mph (16 km / h), are as follows: individual front and rear wheel slip threshold values ​​of 12% and combined front and rear wheel slip threshold values ​​of 10%. The parameters of this example are summarized in the table below. Table I VL(9%) VR(8%) HL(11%) HR(9%) GV(9.8%) GH (11.9%) Wheel slip result Individual front wheel slip threshold 12 % 12 % Incorrect Individual rear wheel slip threshold value 12 % 12 % Incorrect Common front wheel slip threshold 10 % Incorrect Common rear wheel slip threshold 10 % True

[0032] In this particular case, neither of the individual wheel slips for VL, VR, HL, and HR exceeds its corresponding threshold (9% < 12%, 8% < 12%, 11% < 12%, and 9% < 12%), nor does the common front wheel slip GV exceed its threshold (both 9% and 8% are < 10%). However, the common rear wheel slip GH exceeds its threshold because one of the two individual wheel slips—in this case, the individual rear slip HL (11%)—is higher than the common rear threshold (10%). For the common wheel slip to remain below its common threshold, both individual wheel slips must be less than the common threshold. Since this particular embodiment is set to fail, each measured wheel slip must remain below its corresponding threshold (i.e.,If a failure occurs (all conditions must be met), the overall result of this evaluation would be that a condition with excessive slip exists. As shown in the preceding example, the individual wheel slip thresholds are generally higher than the combined wheel slip thresholds (e.g., 20% higher). This is because the procedure will tolerate slightly more wheel slip from a single wheel than from a pair of wheels. In this example, both individual and combined wheel slips with their respective thresholds are compared and are part of the evaluation. In other embodiments, step 106 may evaluate only the individual wheel slip, only the combined wheel slip, or any other combination of comparisons.

[0033] Another example is provided for a vehicle traveling at 70 mph (112.6 km / h) when it encounters a small, isolated pothole on the left side of the vehicle (hence the wheel slip values ​​for the left side are significantly higher than those for the right). Because these values ​​are based on a vehicle speed of 70 mph (112.6 km / h), the various wheel slip threshold values ​​are generally lower than in the previous example, which was based on 10 mph (16 km / h) (usually, less wheel slip is tolerated at higher speeds). This example is summarized in Table II, where none of the wheel slip conditions are met (i.e., none of the measured wheel slip values ​​exceeds its associated wheel slip threshold value), and no excessive wheel slip condition is found.Unlike the example in Table I, the individual front and rear wheels in Table II have different wheel slip threshold values. It should be noted that the examples in Tables I and II are only intended to illustrate some of the possible ways in which individual and collective wheel slip can be assessed, and that the present procedure is not limited to these specific assessments or these exemplary values. If step 106 determines that no excessive wheel slip condition exists, the procedure returns to step 102 for continued monitoring; if an excessive wheel slip condition exists, the procedure proceeds to the next step. Table II VL(3%) VR(1%) HL(3%) HR(0%) GV(3.1%) GH (3.0%) Wheel slip result Individual front wheel slip threshold 5 % 5 % Incorrect Individual rear wheel slip threshold value 4 % 4 % Incorrect Common front wheel slip threshold 4 % Incorrect Common rear wheel slip threshold 4 % Incorrect

[0034] Steps 108 and 110 are optional and are provided to prevent the regenerative braking process from being aborted due to momentary excessive wheel slip conditions that do not threaten vehicle stability. Any type of counter and / or timer arrangement can be used, including the exemplary embodiment where an excessive wheel slip counter is used to ensure that the excessive wheel slip condition exists for a specified number of process cycles. The excessive wheel slip counter can be incremented or decremented until it reaches some maturity at step 110. In one embodiment, the excessive wheel slip condition must persist for 5 process cycles, which, at an average of 7 ms per process cycle, results in approximately 35 ms during which the excessive wheel slip condition must exist.If the excessive wheel slip condition ceases to exist at any point, step 106 sends the procedure control to step 112, where the excessive wheel slip counter is reset and the procedure jumps back to the beginning. However, if the excessive wheel slip counter expires, the procedure proceeds to step 114. As noted above, steps 108-110 are optional, and other techniques can be used to ensure that excessive slip is not momentary.

[0035] Step 114 disables regenerative braking and can do so in a number of different ways. In an embodiment where the brakes are currently being applied (either manually by the driver or automatically by a device, module, system, etc.), Step 114 can disable regenerative braking by causing the EBCM 30 or other device to send appropriate command signals to the regenerative braking system 34 and the friction braking system 32, disabling regenerative braking while activating friction braking. These command signals may cause an abrupt disabling and activating of the regenerative and friction braking systems, respectively, or they may initiate a gradual disabling of regenerative braking.For example, step 114 can disable regenerative braking by progressively reducing the amount of regenerative braking torque over a number of steps or procedure iterations, as opposed to disabling regenerative braking in a single step. The progressive reduction of the amount of regenerative braking can be based on the magnitude and / or location (front to rear, right to left, etc.) of wheel slip, the magnitude of the braking intention or the braking torque requested by the driver, and / or some other factor. It should be noted that procedure 100 is not limited to the specific manner or technique used by step 114 to disable regenerative braking or to blend regenerative and friction braking, as these techniques may already exist in the vehicle and be handled by some other algorithm.The person skilled in the art will recognize that any number of different procedures and techniques can be used to determine the optimal blending or combination of frictional and regenerative braking forces. It is also possible for the EBCM 30 or another device to provide the output of Procedure 100 (i.e., the excessive wheel slip condition status) to other devices, modules, and / or systems in the vehicle, such as an Electronic Stability Control (ESC), Vehicle Stability Assist (VSA), Vehicle Stability Control (VSC), etc.

[0036] In an embodiment where the brakes are not currently being applied, step 114 can disable regenerative braking by setting a flag or initiating some other process that prevents regenerative braking in the near future. For example, if step 114 sets a flag to prevent regenerative braking due to an excessive wheel slip condition, and the driver were to engage the brakes 1.0 second later (assuming the condition still exists), then the regenerative braking system 34 may be prevented from operating, so that all braking responsibility rests with the friction braking system 32. This disabling or preventing of regenerative braking may continue until it is determined that the excessive wheel slip conditions no longer exist, at which point the flag may be reset.

[0037] It is understood that the foregoing description is not a definition of the invention, but rather a description of one or more preferred exemplary embodiments of the invention. The invention is not limited to the specific embodiments disclosed herein, but is instead defined solely by the claims below. Furthermore, the statements contained in the foregoing description relate to specific embodiments and should not be construed as limitations on the scope of protection of the invention or on the definition of terms used in the claims, except where a term or phrase has been explicitly defined above. Various other embodiments and various changes and modifications to the disclosed embodiments will be apparent to the person skilled in the art.For example, the specific combination and sequence of steps is only one possibility, as the present method may include a combination of steps that has fewer, more, or different steps than those shown here. Furthermore, it is not necessary for the present method to be used with an electro-hydraulic braking system (EHB system), as it may instead be used with other braking systems, such as an electromechanical braking system (EMB system). All such further embodiments, changes, and modifications are said to be within the scope of the appended claims.

[0038] The expressions "for example," "e.g.," "such as," "like," and "as," and the verbs "comprehensive," "exhibiting," "containing," and their other verb forms, when used in this description and the claims, shall each be understood as open-ended when used in conjunction with an enumeration of one or more components or other items, meaning that the enumeration shall not be interpreted as excluding other, additional components or items. Other expressions shall be understood in such a way that their broadest reasonable meaning is used, unless they are used in a context that requires a different interpretation.

Claims

[1] Method for operating a vehicle braking system comprising the steps of: (a) an individual wheel slip for a single vehicle wheel and a collective wheel slip for multiple vehicle wheels are determined; (b) a vehicle speed is determined; (c) the individual wheel slip is compared to an individual speed-based threshold based on the vehicle speed, and the collective wheel slip is compared to a common speed-based threshold based on the vehicle speed, wherein the individual speed-based threshold changes depending on the vehicle speed and the individual speed-based threshold is higher than the common speed-based threshold at the same vehicle speed; and (d) when individual wheel slip exceeds the individual speed-based threshold or collective wheel slip exceeds the collective speed-based threshold, regenerative braking is switched off. [2] Method according to claim 1, wherein step (a) further comprises determining an individual wheel slip for each of a plurality of vehicle wheels. [3] Method according to claim 2, wherein step (a) further comprises determining an individual wheel slip for a first vehicle wheel, an individual wheel slip for a second vehicle wheel and a common wheel slip that takes into account the individual wheel slips for both the first and the second vehicle wheel; and wherein step (c) further comprises comparing the common wheel slip with the common speed-based threshold. [4] Method according to claim 3, wherein the first and second vehicle wheels are mounted on the same axle of the vehicle, so that the method can evaluate the wheel slip from side to side. [5] Method according to claim 3, wherein the first and second vehicle wheels are attached to the same side of the vehicle, so that the method can evaluate the wheel slip from front to rear. [6] Method according to claim 1, wherein step (a) further comprises determining the individual wheel slip and / or the common wheel slip over a period of time; and wherein step (c) further comprises comparing the individual wheel slip with the individual speed-based threshold and / or the common wheel slip with the common speed-based threshold over a period of time, such that instantaneous wheel slip does not necessarily lead to the deactivation of regenerative braking. [7] Method according to claim 1, wherein step (d) further comprises switching off the regenerative braking operation by progressively reducing the amount of regenerative braking over a plurality of steps, as opposed to terminating the regenerative braking operation in a single step. [8] Method according to claim 7, wherein the progressive decrease in the amount of regenerative braking is based on a magnitude and / or location of wheel slip, wherein the decrease in regenerative braking is more aggressive the greater the amount of wheel slip. [9] Method for operating a vehicle braking system comprising the steps that: (a) a first wheel slip and a second wheel slip are determined, the first wheel slip being an individual wheel slip for a single vehicle wheel and the second wheel slip being a common wheel slip for multiple vehicle wheels; (b) the first wheel slip is compared to a first threshold and the second wheel slip is compared to a second threshold, the first threshold being an individual threshold and the second threshold being a common threshold, and the first threshold being higher than the second threshold; and (c) when either the first wheel slip exceeds the first threshold or the second wheel slip exceeds the second threshold, regenerative braking is switched off.

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