SYSTEM AND METHOD FOR CONTROLLING A BRAKE SYSTEM IN A VEHICLE

Abstract

A method for controlling a brake system in a vehicle includes using a first brake pedal map when the vehicle has a first load; the first brake pedal map allows a first predetermined non-friction braking torque to be reached. The method further includes using a second brake pedal map allowing a second predetermined non-friction braking torque, lower than the first predetermined non-friction braking torque, to be reached. The second brake pedal map is used when the vehicle has a second load lower than the first load.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional patent application No. 61/643,669 filed 7 May 2012, which is hereby incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a system and method for controlling a brake system in a vehicle.

BACKGROUND

Vehicles today are increasingly equipped with electric drive motors, which, in addition to propelling the vehicle, can capture braking energy to charge a battery. Depending on how the vehicle powertrain is configured, this process, known as “regenerative braking”, can occur at the front axle, the rear axle, or both. There are other kinds of non-friction braking, for example, engine braking, that occur when the compression of the engine provides a negative torque to the vehicle drive wheels. Where the engine is only connected to one axle, as in a two-wheel-drive vehicle, or where the regenerative braking is only available at one axle, there may be competing interests between trying to brake in such a way as to maximize non-friction braking, for example, to maximize energy capture in a regenerative brake system, and more evenly distributing braking torque between the front and rear wheels to provide better vehicle handling.

Adding complexity to the braking control system is consideration of the vehicle carrying load. This may be of particular concern with commercial vehicles where the difference between the loaded weight and unloaded weight is significant. If, for example, a brake system is configured to maximize non-friction braking at the rear axle for the fully loaded vehicle, the brake system may over brake at the rear wheels when the vehicle is unloaded. In addition, if the brake pedal travel is mapped the same for the loaded and unloaded conditions, the brake pedal may be “too sensitive” when the vehicle is in the unloaded condition—i.e., a very hard braking may occur for a very small amount of pedal travel. Conversely, if the brake system is configured to maximize non-friction braking at the rear axle for the unloaded vehicle, the brake system may not utilize all of the available non-friction braking—e.g., it may not capture all of the possible regenerative braking—when the vehicle is loaded. This may be due, in part, to the lack of sensitivity of the brake pedal, which now may be depressed so far as to engage the vehicle's friction brakes before all of the available non-friction braking energy is utilized.

SUMMARY

At least some embodiments of the invention include a method for controlling a brake system in a vehicle. The method includes using a first brake pedal map allowing a first predetermined non-friction braking torque to be reached when the vehicle has a first load, and using a second brake pedal map allowing a second predetermined non-friction braking torque lower than the first predetermined non-friction braking torque to be reached when the vehicle has a second load lower than the first load.

At least some embodiments of the invention include a method for controlling a brake system in a vehicle. The method includes braking the vehicle with at least some non-friction braking until a first non-friction braking torque is reached when the vehicle has a first load, and braking the vehicle with at least some non-friction braking until a second non-friction braking torque, lower than the first non-friction braking torque, is reached when the vehicle has a second load lower than the first load.

At least some embodiments of the invention include a control system for controlling a brake system in a vehicle. The control system includes a controller configured to brake the vehicle with at least some non-friction braking until a first predetermined non-friction braking torque is reached when the vehicle has a first load, and to brake the vehicle until a second predetermined non-friction braking torque, lower than the first predetermined non-friction braking torque, is reached when the vehicle has a second load lower than the first load.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a simplified schematic diagram of a vehicle having a control system in accordance with embodiments of the present invention;

FIG. 2 shows a brake distribution chart for a vehicle in a fully loaded condition;

FIG. 3 shows a brake distribution chart for the vehicle in an unloaded condition using the same braking torque control as shown in FIG. 2;

FIG. 4 shows a brake distribution chart for the vehicle in the unloaded condition using a different braking torque control;

FIG. 5 shows a brake distribution chart for the vehicle in the loaded condition using the same braking torque control as shown in FIG. 4;

FIG. 6 shows a flowchart illustrating a method in accordance with embodiments of the present invention;

FIG. 7 shows a chart illustrating the relationship between vehicle braking torque and pedal input in accordance with embodiments of the present invention; and

FIG. 8 shows additional details of the brake system shown in FIG. 1.

DETAILED DESCRIPTION

As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention.

FIG. 1 shows a simplified schematic diagram of a portion of a vehicle 10. The vehicle 10 includes a friction brake system 12, controlled by a brake controller 14, and a non-friction, regenerative brake system 16, which is part of the vehicle powertrain. The regenerative brake system 16 includes one or more electric machines, such as electric motors, which are operable to provide regenerative braking for the vehicle 10. The regenerative brake system 16 is controlled by a control system, or vehicle system controller (VSC) 18, which communicates with the brake controller 14, for example, through a controller area network (CAN). The VSC 18 may include other controllers, such as a powertrain control module (PCM), and in some embodiments, the brake controller 14 may be integrated into the VSC 18. Thus, a control system in accordance with embodiments of the present invention may control various systems within the vehicle 10 by using a single controller, separate software controllers within a single hardware device, or a combination of separate software and hardware controllers.

The brake controller 14 receives vehicle operator inputs from a brake pedal 20, and the VSC 18 receives operator inputs from an accelerator pedal 22. A brake sensor 24 (which can be more than one sensor), is configured to detect the position of the brake pedal 20, and send one or more signals to the brake controller 14. Similarly, an accelerator pedal sensor 26 (which can also be more than one sensor), is configured to detect the position of the accelerator pedal 22, and send one or more signals to the VSC 18. The VSC 18 and the brake controller 14 use various inputs, including the inputs from the sensors 24, 26, to decide how to control the friction brake system 12 and the regenerative brake system 16. The friction brake system 12 operates to slow the speed of rear vehicle wheels 28 and the front wheels (not shown) through the application of one or more friction elements in accordance with methods known in the art. The regenerative brake system 16 is also operable to reduce the speed of the rear vehicle wheels 28 by having at least one electric motor produce a negative torque which is transferred through the powertrain to the rear vehicle wheels 28.

The friction brake system 12 includes one or more sensors, represented in FIG. 1 by a single sensor 30. The sensor 30 is configured to send signals to the brake controller 14 related to various conditions within the friction brake system 12. For example, if the friction brake system 12 should experience reduced braking capability, perhaps due to a loss of boost or the loss of a hydraulic circuit, the sensor 30 can communicate this condition to the brake controller 14, which in turn communicates with the VSC 18. Similarly, the regenerative brake system 16 has one or more sensors, represented in FIG. 1 by the sensor 32. The sensor 32 may detect such conditions as motor speed, motor torque, power, etc. The sensor 32 communicates directly with the VSC 18, which can use these inputs in combination with the other inputs to control the brake systems 12, 16.

The vehicle 10 also includes a body/chassis system 34. The body/chassis system 34 includes structural elements of the vehicle 10, including such things as a vehicle suspension system. The vehicle wheels 28, shown separately in FIG. 1, may be considered a part of the larger body/chassis system 34. One or more sensors, shown in FIG. 1 as a single sensor 36, are configured to detect various conditions of the body/chassis system 34, and to communicate with the VSC 18. The sensor 36 may detect such conditions as the deflection of, or the load on, various elements of the body/chassis system 34, as well as load distribution. Similarly, a sensor 38, which represents one or more sensors, is configured to detect conditions of the vehicle wheels 28, including the wheel speed. The sensor 38 is shown in FIG. 1 communicating with the larger body/chassis system 34, which in turn communicates with the VSC 18. Alternatively, the sensor 38 can be directly connected to the VSC 18.

In the embodiment shown in FIG. 1, the regenerative brake system 16 is a rear-axle regenerative brake system, configured to capture braking energy from the rear wheels 28 only. Although embodiments of the invention are described and illustrated in conjunction with a rear axle, regenerative brake system, other embodiments may include other types of non-friction braking, such as engine braking, and may also include front axle or four-wheel non-friction brake systems. With a regenerative brake system, it is often desirable to capture as much braking energy as possible, while not allowing too great a difference in braking distribution between the front and rear brakes so as to affect vehicle handling. Toward that end, a controller, such as the VSC 18, can be programmed to perform a number of steps in accordance with embodiments of the present invention. Initially, a first predetermined, non-friction braking torque, which in this embodiment is a maximum desired regenerative braking torque, for the vehicle 10 can be provided when the vehicle 10 is carrying a first load, which, for example, may be a maximum capacity load conveniently identified by the vehicle's “gross vehicle weight” (GVW). This value of the braking torque can be “provided” to the VSC 18 by direct programming, or information can be provided to the VSC 18 and it can perform an internal calculation.

FIG. 2 shows a brake distribution chart 40 for the vehicle 10 at the vehicle's GVW, which, for example, may be 3000 kilograms (kg). The chart 40 illustrates a rear deceleration for the vehicle 10 along the vertical axis, and a front deceleration along the horizontal axis. The sum of these two decelerations is the total deceleration for the vehicle 10, which can be easily converted into a vehicle braking force or a vehicle braking torque because there is a known relationship between each of these values. Therefore, providing a vehicle with a desired non-friction braking torque as discussed above can also be described and illustrated in terms of a rear deceleration as shown in the chart 40 in FIG. 2.

The chart 40 shows a number of curves, including an ideal brake distribution curve 42. The ideal brake distribution curve 42 illustrates a theoretical line along which the front and rear brakes would lock-up simultaneously. An equal pressure curve 44 is also illustrated in the chart 40, and represents a line along which equal pressure is applied to both of the front and the rear brakes. The ideal brake distribution curve 42 is not coincident with the equal pressure curve 44, because in practice, a vehicle does not have an equal weight distribution between the front and rear wheels. As shown in FIG. 2, the lines 42, 44 cross at point Z₁, which may be different for different vehicles and different loading conditions of the same vehicle. A number of equal deceleration lines 46 are also illustrated in the chart 40, and indicate lines along which the front and rear wheels of the vehicle 10 are decelerating equally.

As described above, it may be desirable to optimize regenerative braking—i.e., to capture as much energy as possible—while at the same time ensuring that there is not an undesirable impact on vehicle handling. For any given vehicle, and vehicle loading condition, the “optimum” amount of regenerative braking that can be captured from the front, rear or both pairs of wheels of a vehicle can be estimated. Using the vehicle 10 at GVW as an example, a maximum desired regenerative braking torque (in this example for the rear regenerative brake system) is shown in the chart 40 by the line 48, which generally illustrates the rear regenerative braking balance for the vehicle 10 at GVW. In the chart 40, the maximum non-friction braking torque is shown as a rear deceleration of −2 meters per second squared (m/ŝ2). For the vehicle 10, this level of deceleration can be translated into a deceleration torque of approximately 1700 Newton-meters (Nm). After reaching this maximum value, the line 48 slopes downward and toward the right of the chart 40, indicating a combination of front and rear braking, until the equal pressure curve 44 is reached.

The slope of the line 48 is generally less than the slope of the equal deceleration lines 46, and is brought below the ideal brake distribution curve 42 somewhere at or before the intersection point Z₁. The specific way in which the maximum rear braking torque (in this case −2 m/ŝ2) is chosen, and how the rest of the brake balance line (or curve) is determined, can be based on any number of factors a brake system designer wishes to consider. In the examples of embodiments of the present invention described herein, the optimum non-friction braking torque—which in this case coincides with the optimum regenerative braking torque—is chosen to provide a “maximum desired” amount of regenerative braking while still providing a required level of vehicle handling. Although the first part of the curve 48 is vertical, indicating exclusive use of the regenerative (rear) brakes until a deceleration of 2 m/ŝ2 is reached, the initial deceleration may also include some front braking, as indicated by the line 48′, which intersects the sloping part of the line 48 and follows its path from there. As braking occurs along the line 48′, and along the sloped portion of line 48, it may be a combination of friction and non-friction braking, or, in the case where non-friction braking is available at both axles, it may be exclusively non-friction braking even though both sets of wheels are braking.

As discussed above, embodiments of a method of the present invention may be executed, for example, by the VSC 18. Using information, for example, from the chart 40 in FIG. 2, the method may include braking the vehicle 10 with at least some non-friction braking (such as regenerative braking) until the first predetermined non-friction braking torque (in this case −2 m/ŝ2) is reached. This does not mean that non-friction braking ceases once the first non-friction braking torque is reached; rather, it means that non-friction braking is controlled to not exceed this level (as discussed above, the sloping portion of the line 48 may include non-friction braking, but along this portion of the line 48, the amount of rear axle braking is being reduced). The first predetermined non-friction braking torque is based on the vehicle 10 at a first load, which as described above, is its GVW. One of the reasons that the chosen non-friction braking torque is load dependent, is because braking conditions change with a vehicle when it is loaded versus when it is unloaded.

This is illustrated in FIG. 3 where a braking distribution chart 50 for the vehicle 10 is shown when it has a second load lower than the first load; in this case the vehicle 10 is unloaded—i.e., it is not carrying any payload. Thus, in FIG. 3, the weight of the vehicle 10 is equal to its “curb weight”. Although the fully loaded GVW weight and the unloaded curb weight are used in the examples described and illustrated herein, it is understood that embodiments of the invention may be applied to any or all of the various loading conditions that may exist between these two extremes. In the chart 50, the equal deceleration lines 46 and equal pressure curve 44 are the same as in FIG. 2, while the brake balance curve 52 and ideal brake distribution curve 54 are different from their counterparts 48, 42 shown in FIG. 2.

If the same level of braking torque is applied to the vehicle 10 at its curb weight as was applied at GVW (1700 Nm, see above), the result is a greater rear deceleration as shown by the brake balance curve 52 in the chart 50 in FIG. 3. In this example, the rear deceleration has increased from −2 m/ŝ2 to −-2.8 m/ŝ2, as indicated by the label “Overbraking Rear Axle”. As discussed above, this level of rear braking may be undesirable; therefore, embodiments of the present invention may utilize different non-friction (in this embodiment, rear) braking torques for different loading conditions of the same vehicle. This is illustrated in FIG. 4, which shows a braking distribution chart 56 for the vehicle 10 at the second loading condition, which is its curb weight. In this example, a different optimum rear braking torque has been chosen so as to provide the desired vehicle handling throughout the braking event; this is indicated by the brake balance curve 58.

As shown in the chart 56 in FIG. 4, the maximum rear deceleration is −1.3 m/ŝ2, which translates into a rear braking torque of approximately 900 Nm. Therefore, a system and/or method in accordance with embodiments of the present invention may provide a second predetermined non-friction braking torque lower than the first predetermined non-friction braking torque when the vehicle has a second load lower than the first load. The vehicle 10 is then braked exclusively with the rear brakes 28 using regenerative braking until the first predetermined non-friction braking torque of 1700 Nm is reached when the vehicle 10 is at GVW; however, when the vehicle 10 is at its curb weight, it is braked exclusively with the rear brakes 28 using regenerative braking only until a second predetermined non-friction braking torque of 900 Nm is reached. As described above, the first and second predetermined non-fiction braking torques of 1700 Nm and 900 Nm represent first and second desired regenerative braking torques for the vehicle 10 for the two different loading conditions, each of which may be maximum desired values for the respective loading conditions. Although the examples above rely on exclusive use of the rear brakes until the desired non-friction braking torque levels are reached, different embodiments may use a combination of front and rear brakes, such as described above in conjunction with the braking curve 48′ shown in FIG. 2. Moreover, some non-friction braking may still occur along the sloping portion of the line 58, but non-friction braking is controlled to not exceed the second predetermined non-friction braking torque.

As described above, embodiments of the present invention provide two different maximum desired regenerative braking torques for two different loading conditions of a vehicle, such as the vehicle 10. Using the maximum desired regenerative (in this case, rear) braking torque from a fully loaded vehicle for the same vehicle at a lower load resulted in the undesirable effect of overbraking the rear axle, which was illustrated and described in conjunction with FIG. 3. It is similarly undesirable to use the maximum desired regenerative braking torque provided for the unloaded condition—such as illustrated and described in FIG. 4—when the vehicle has a higher payload. This is illustrated in FIG. 5, where a region of lost regenerative braking energy is labeled “Lost Regen at GVW”. This results from abandoning regenerative braking too soon—i.e., at a braking torque level that is below the maximum desired level.

FIG. 6 shows a flowchart 60 summarizing a method and system in accordance with embodiments of the present invention. At step 62, the process is started, and at step 64 a determination is made as to load estimation for a vehicle, such as the vehicle 10. This load estimation comes from inputs 66, for example, to the VSC 18, that may provide information on the load level and “load quality”. Information such as this can come from, for example, a sensor or sensors such as the sensor 36 shown in FIG. 1. A sensor that detects deflection levels of a suspension system is one example of a load detection sensor. The “load quality” factor may be provided to give an indication of the accuracy of the sensor itself, or the accuracy of the particular measurement as it relates to the vehicle payload—i.e., a weight sensor may provide a higher quality measurement than a deflection sensor, which must be used in a calculation to estimate the actual load.

Next, at step 68, a front-to-back distribution of vehicle load is determined based on inputs 70 providing a front-to-back load distribution detection and distribution quality. When a vehicle load is distributed toward a front of the vehicle, which may be defined, for example, as in front of the rear axle, or in front of a center of gravity for the vehicle, it may not be possible to provide a desired level of braking torque at the rear axle without having an impact on vehicle handling. Therefore, a system and method in accordance with embodiments of the present invention may choose an initial value for the first predetermined non-friction braking torque, such as illustrated and described in FIG. 2 for the rear wheels, and may also choose an initial value for the second predetermined non-friction braking torque, such as illustrated and described in FIG. 4 (also for the rear wheels). Then, if it is determined that the first or second loads are distributed toward a front of the vehicle, the first and second predetermined non-friction braking torques can be modified such that they are reduced to a somewhat lower level to account for the low distribution. Although the “second load” illustrated and described above was considered a zero payload for the vehicle 10, the center of gravity of the vehicle at curb weight may be distributed toward a front of the vehicle, and even in an unloaded condition, the load distribution may be a factor to consider.

At step 72, a determination of brake torque level is made based on brake pedal input, load, and load distribution. To make such a determination, a controller, such as the VSC 18, may receive a brake pedal input indicated at 74, for example, from a brake pedal 20 and sensor 24 shown in FIG. 1. Other inputs are shown at 76 in FIG. 6 which are brake level versus pedal input maps, described below in conjunction with FIG. 7. The process shown in FIG. 6 is ended at 78.

FIG. 7 shows a graph 80, which shows brake level versus pedal input maps for a vehicle, such as the vehicle 10 under two different loading conditions: a GVW loading with good distribution and a minimum loading with poor distribution. The first condition is illustrated by a first brake pedal map, shown as a first curve 82, while the second condition is illustrated by a second brake pedal map, shown as a second curve 84. Although the vertical axis of the graph 80 is labeled as “vehicle braking torque” it is understood that it could be labeled in terms of braking force or deceleration as described above. Along the horizontal axis is “pedal input” which relates to the travel of a brake pedal, such as the brake pedal 20 shown in FIG. 1; thus, points on the curves 82, 84 at different positions along the pedal input axis represent different brake pedal positions.

The curves 82, 84 respectively represent the vehicle braking torque as first and second functions of the brake pedal position for the vehicle having first and second loads, such as described above. These functions can be programmed into the VSC 18 as formulas, if they can be defined that way, or as data tables that can be accessed, with certain values output based on certain inputs received. Thus, the VSC 18 may output vehicle braking torque as functions of brake pedal position. These functions, and the curves that represent them, such as the curves 82, 84, can be chosen by a brake system specialist so that different values of vehicle braking torque are achieved for different pedal inputs. A curve representing a maximum load with a poor distribution might appear below the curve 82, while a curve representing a minimum load with a good distribution might appear above the curve 84, but below the second, lower maximum load curve, and these would represent alternative pedal maps. As shown in FIG. 7, the second brake pedal map 84 corresponds to a lower set of vehicle braking torques than the first brake pedal map 82 over a range of brake pedal positions or pedal inputs.

As discussed above in conjunction with FIG. 2, the total vehicle braking torque can be obtained by adding the front and rear decelerations and converting this sum to a braking torque, which is shown on the vertical axis in the graph 80 in FIG. 7. Because the level of pedal input for a particular vehicle braking torque or deceleration has an impact on driver expectations, it may be desirable to control the level of pedal input for any given vehicle braking torque. In addition, as discussed below in conjunction with FIG. 8, some vehicles may engage friction brakes when the pedal has traveled a certain distance or angle; therefore, controlling pedal input versus vehicle braking torque may be important to ensure that the maximum desired non-friction braking is achieved before the friction brake system engages or becomes the exclusive braking mechanism.

Thus, with regard to the examples described above, a controller, such as the VSC 18, may provide the first predetermined non-friction braking torque, or first maximum desired regenerative braking torque, at a first position of a brake pedal, such as the brake pedal 20 shown in FIG. 1. The first maximum desired regenerative braking torque is indicated by point (a) on the curve 82, which corresponds to a pedal position of (d₁). This point may be chosen, for example, to ensure that the first maximum desired regenerative braking torque is reached before the friction brakes are engaged. This is illustrated in FIG. 8, which shows a gap 86 between the brake pedal 20 and the point of engagement of the friction brake system 12. Although FIG. 8 is a simplified schematic drawing, it does illustrate the general relationship between brake pedal travel and friction brake system engagement that exists in some vehicles. Therefore, one factor to consider in providing a brake pedal travel of a certain distance for a certain vehicle loading condition is the distance allowed before the friction brake system is engaged.

The VSC 18 may also provide the second maximum desired regenerative braking torque, at a second position of the brake pedal. If it is desired to have the sensitivity of the brake pedal at the same level or perhaps slightly more sensitive when the vehicle is in the unloaded condition, the second position of the brake pedal may be equal to or less than the first position of the brake pedal. This is illustrated in the graph 80 where the second maximum desired regenerative braking torque is shown as point (b₁) on the curve 84, which also corresponds to the first pedal position (d₁). If, however, a somewhat more sensitive brake pedal is desired for lower loading conditions, the curve 84 could be adjusted to include the point (b₀) such that the corresponding pedal position was (d₀). If the first pedal position (d₁) is chosen to be the same or nearly equal to the width of the gap 86, then it may not be possible to make the brake pedal less sensitive for a lower loading condition, otherwise, the brake pedal may engage the friction brake system before the maximum desired regenerative braking is achieved.

As shown in FIG. 7, the curves 82, 84 are not parallel over most of their range. They begin to approach parallelism at higher pedal input positions. This is because when the brake pedal is at or near a bottoming out position, front and rear wheels are braking simultaneously and the difference between braking during a loaded and unloaded condition is negligible. Although the curves 82, 84 could be made to be parallel over their entire range, having them nonparallel as shown in FIG. 7 provides some advantages. For example, at lower levels of pedal travel, the curve 82 rises more steeply than the curve 84. This means that for the same change in pedal travel, a greater vehicle braking torque is applied when the brake system is controlled in accordance with curve 82. This may result in similar deceleration regardless of the load; however, providing substantially similar curves at the higher level may provide tactile response (vehicle braking torque v. brake pedal travel) that may be desirable as it provides feedback to the operator of the load that is being carried.

As described above, braking control according to the curve 82 is applied when a vehicle has a large load with good distribution. Therefore, it may be desirable to have a smaller amount of pedal travel provide a greater increase in braking torque, as this might be expected by a vehicle operator when it is known that the vehicle has a large payload. The shapes of the curves 82, 84, and their underlying functions, can be adjusted to provide different vehicle braking torque outputs for different pedal inputs as desired. In each case, however, the curves 82, 84 provide brake pedal maps that allow the maximum desired regenerative braking torque to be achieved before the friction brakes are engaged, or at least, before the friction brakes are exclusively engaged, which could cause less than the maximum desired regenerative braking to be achieved. Where the inputs are based on actual measurements and accurate information, the curves can be more aggressive; whereas, the use of estimates and the absence of information may require a more conservative braking control.

While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the invention.

Claims

1. A method for controlling a brake system in a vehicle comprising:

using a first brake pedal map allowing a first predetermined non-friction braking torque to be reached when the vehicle has a first load; and

using a second brake pedal map allowing a second predetermined non-friction braking torque lower than the first predetermined non-friction braking torque to be reached when the vehicle has a second load lower than the first load.

2. The method of claim 1, wherein the first and second brake pedal maps are defined by vehicle braking torque versus brake pedal input.

3. The method of claim 2, wherein the second brake pedal map corresponds to a lower set of vehicle braking torques than the first brake pedal map over a range of brake pedal inputs.

4. The method of claim 1, wherein the vehicle includes a rear-axle regenerative brake system, the first predetermined non-friction braking torque corresponding to a maximum desired regenerative braking torque when the vehicle has the first load, and the second predetermined non-friction braking torque corresponding to a maximum desired regenerative braking torque when the vehicle has the second load.

5. The method of claim 1, further comprising providing the first predetermined non-friction braking torque at a first position of a brake pedal, and providing the second predetermined non-friction braking torque at a second position of the brake pedal equal to or less than the first position of the brake pedal.

6. The method of claim 1, wherein the first brake pedal map is represented by a first curve, and the second brake pedal map is represented by a second curve, the first and second curves being nonparallel for at least a range of brake pedal positions.

7. The method of claim 1, further comprising choosing an initial value for the first predetermined non-friction braking torque, and modifying the initial value of the first predetermined non-friction braking torque based on a front-to-back distribution of the first load.

8. The method of claim 7, further comprising choosing an initial value for the second predetermined non-friction braking torque, and modifying the initial value of the second predetermined non-friction braking torque based on a front-to-back distribution of the second load.

9. The method of claim 8, wherein the steps of modifying the initial values of the first and second predetermined non-friction braking torques include reducing the first and second predetermined non-friction braking torques when the first and second loads are distributed toward a front of the vehicle.

10. A method for controlling a brake system in a vehicle comprising:

braking the vehicle with at least some non-friction braking until a first non-friction braking torque is reached when the vehicle has a first load; and

braking the vehicle with at least some non-friction braking until a second non-friction braking torque, lower than the first non-friction braking torque, is reached when the vehicle has a second load lower than the first load.

11. The method of claim 10, further comprising providing the first non-friction braking torque at a first position of a brake pedal, and providing the second non-friction braking torque at a second position of the brake pedal equal to or less than the first position of the brake pedal.

12. The method of claim 10, further comprising defining vehicle braking torque as a first function of brake pedal position when the vehicle has the first load, and defining the vehicle braking torque as a second function of the brake pedal position different from the first function when the vehicle has the second load.

13. The method of claim 12, wherein the first function is represented by a first curve, and the second function is represented by a second curve, the first and second curves being nonparallel for at least a range of brake pedal positions.

14. The method of claim 10, wherein the vehicle includes a rear-axle regenerative brake system, the first non-friction braking torque corresponding to a first desired regenerative braking torque when the vehicle has the first load, and the second non-friction braking torque corresponding to a second desired regenerative braking torque when the vehicle has the second load.

15. The method of claim 14, further comprising:

choosing respective initial values for the first and second desired regenerative braking torques;

reducing the initial value of the first desired regenerative braking torque from its initial value when the first load is distributed toward a front of the vehicle; and

reducing the initial value of the second desired regenerative braking torque from its initial value when the second load is distributed toward a front of the vehicle.

16. A control system for controlling a brake system in a vehicle comprising:

a controller configured to brake the vehicle with at least some non-friction braking until a first predetermined non-friction braking torque is reached when the vehicle has a first load, and to brake the vehicle until a second predetermined non-friction braking torque, lower than the first predetermined non-friction braking torque, is reached when the vehicle has a second load lower than the first load.

17. The control system of claim 16, wherein the vehicle includes a rear-axle regenerative brake system, the first predetermined non-friction braking torque corresponding to a maximum desired regenerative braking torque when the vehicle has the first load, and the second predetermined non-friction braking torque corresponding to a maximum desired regenerative braking torque when the vehicle has the second load.

18. The control system of claim 17, wherein the controller is further configured to receive inputs related to a front-to-back distribution of vehicle load, and to reduce the first and second predetermined non-friction braking torques when the first and second loads are distributed toward a front of the vehicle.

19. The control system of claim 16, wherein the control system is further configured to receive inputs corresponding to brake pedal position, and to control the brake system to: reach the first predetermined non-friction braking torque at a first position of the brake pedal, and reach the second predetermined non-friction braking torque at a second position of the brake pedal equal to or less than the first position of the brake pedal.

20. The control system of claim 16, wherein the control system is further configured to output vehicle braking torque: as a first function of brake pedal position when the vehicle has the first load, and as a second function of brake pedal position when the vehicle has the second load, the first function yielding higher vehicle braking torques than the second function over a range of pedal positions.