CN106004462B

CN106004462B - System and method for controlling regenerative braking in a vehicle

Info

Publication number: CN106004462B
Application number: CN201610190934.5A
Authority: CN
Inventors: 丹尼尔·A·高博; 戴尔·斯科特·克劳姆贝兹
Original assignee: Ford Global Technologies LLC
Current assignee: Ford Global Technologies LLC
Priority date: 2015-03-30
Filing date: 2016-03-30
Publication date: 2020-10-13
Anticipated expiration: 2036-03-30
Also published as: DE102016105399A1; CN106004462A

Abstract

A system and method for controlling regenerative braking in a vehicle is disclosed. Vehicles and methods for controlling regenerative braking may utilize a maximum available regenerative braking torque for a period of time during a braking event. As vehicle speed and/or driveline torque decreases, the regenerative braking torque is controlled to deviate from a maximum value. The point at which the regenerative braking torque deviates from the maximum value is selected based on the level of vehicle deceleration. The regenerative braking torque then drifts out smoothly until it reaches zero. Regenerative braking torque reaches zero at very low vehicle speeds, eliminating inefficiencies associated with operating the motor at very low speeds.

Description

System and method for controlling regenerative braking in a vehicle

Technical Field

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

Background

Electric vehicles, Hybrid Electric Vehicles (HEVs), and virtually any vehicle that utilizes an electric machine, such as an electric motor, may be configured to provide regenerative braking using the electric machine to at least assist with parking. Furthermore, non-electric vehicles may also be configured to provide regenerative braking, for example, through the use of a hydraulic system. Regenerative braking provides a number of advantages over the use of only a friction braking system. For example, the use of regenerative braking (providing negative torque to the vehicle wheels via the electric motor) reduces wear on the friction elements of the friction braking system. Further, during regenerative braking, the motor may act as a generator to generate electrical power that may be used directly or stored in a storage device, such as a battery.

Because of the advantages associated with regenerative braking, some regenerative braking control systems may attempt to apply maximum regenerative braking torque to maximize overall vehicle efficiency. However, it may be desirable to avoid this strategy when the vehicle (and thus the motor) is running at very low speeds. This is because although the motor is capable of producing very high torque at very low speeds, it does so with very low efficiency. Therefore, the regenerative braking torque may be controlled to gradually decrease to zero at low vehicle speeds. While this strategy may provide efficiency, under certain conditions the vehicle operator may experience inconsistent braking feel when the friction brakes are engaged. This may occur, for example, if the regenerative braking is ramped down rapidly and the friction brakes are engaged for a very short period of time. This can occur particularly realistically when the friction braking is cold.

Disclosure of Invention

Embodiments of the present invention may provide a vehicle and method for controlling regenerative braking by starting or ending regenerative braking using different points, thereby accommodating a wide variety of vehicle conditions (such as different levels of vehicle deceleration and reduced performance friction braking). This may provide a number of advantages over vehicles and methods for controlling regenerative braking based on a single set point that is not responsive to different vehicle conditions.

Embodiments of the present invention may also provide a method for controlling regenerative braking in a vehicle having a regenerative braking system. The method may include determining a first vehicle condition while the vehicle is braking. A second vehicle condition may also be determined and the regenerative braking torque reduced to zero. The reduction of the regenerative braking torque may be initiated when the second vehicle condition reaches the first predetermined value. The first predetermined value may be based on a first vehicle condition.

Embodiments of the present invention may also provide a method for controlling a vehicle having a regenerative braking system. The method may include determining when a vehicle operator commands vehicle braking. At least regenerative braking may be used to reduce vehicle speed when an operator commands vehicle braking. A first vehicle condition may be determined when the vehicle is braking. A second vehicle condition may be determined, and the regenerative braking torque may be reduced when the second vehicle condition reaches the first predetermined value. The first predetermined value may be based on a first vehicle condition.

Embodiments of the present invention may also provide a vehicle including an electric machine operable to provide regenerative braking to the vehicle. The at least one sensor may be configured to detect corresponding vehicle conditions and output at least one signal related to each corresponding detected vehicle condition. The controller may be in communication with the motor and the at least one sensor. The controller may be configured to determine the first vehicle condition and the second vehicle condition based on signals received from the at least one sensor when the vehicle is braking. The controller may be further configured to command the electric machine to reduce the regenerative braking torque to zero. The reduction of the regenerative braking torque may be initiated when the second vehicle condition reaches the first predetermined value. The first predetermined value may be based on a first vehicle condition.

Embodiments of the invention may include a method for controlling regenerative braking in a vehicle. The method may include the step of reducing regenerative braking to zero starting at a first vehicle speed when the amount of dissipated braking energy is greater than the braking energy limit. The method may further include the step of reducing regenerative braking to zero starting at a second vehicle speed greater than the first vehicle speed when the amount of dissipated braking energy is not greater than the braking energy limit.

Embodiments of the invention may include a method for controlling regenerative braking in a vehicle, the method comprising the steps of: based on the first vehicle condition satisfying at least one criterion, beginning to reduce regenerative braking to zero when the vehicle speed reaches the first vehicle speed during braking; based on the first vehicle condition not satisfying the at least one criterion, reducing regenerative braking to zero is initiated when the vehicle speed reaches a second vehicle speed greater than the first vehicle speed during braking.

According to one embodiment of the invention, the first vehicle condition is an amount of dissipated braking energy, and the first vehicle condition satisfying the at least one criterion occurs when the amount of dissipated braking energy is greater than the braking energy limit.

According to one embodiment of the invention, the amount of dissipated braking energy is based on at least an amount of friction torque of at least one wheel of the vehicle and a rotational speed of at least one wheel of the vehicle.

According to an embodiment of the invention, the amount of dissipated braking energy is further based on at least a previously calculated amount of dissipated braking energy and a time since a previous calculation.

According to one embodiment of the invention, the difference between the first vehicle speed and the second vehicle speed is a function of the amount of brake energy dissipated.

According to one embodiment of the invention, the difference between the first vehicle speed and the second vehicle speed decreases with increasing amount of dissipated brake energy.

According to one embodiment of the invention, the braking energy limit is based on at least a vehicle braking system temperature and a vehicle ignition system status.

According to one embodiment of the invention, the vehicle ignition system state comprises how long the vehicle has been in the ignition switch off state.

Embodiments of the invention may include a system for controlling regenerative braking in a vehicle. The system may include a control system including at least one controller configured to: when the amount of dissipated braking energy is not greater than a braking energy limit (which is based at least on the vehicle braking system temperature and the vehicle ignition system state), increasing the vehicle speed at which regenerative braking begins to decrease to zero.

According to one embodiment of the present disclosure, the vehicle ignition system state includes how long the vehicle has been in the ignition switch off state.

According to one embodiment of the disclosure, the amount of dissipated braking energy is based on at least an amount of friction torque of at least one wheel of the vehicle and a rotational speed of at least one wheel of the vehicle.

According to an embodiment of the present disclosure, the amount of dissipated braking energy is further based on at least a sum of previously calculated amounts of dissipated braking energy and a time since a previous calculation.

According to one embodiment of the present disclosure, the amount of vehicle speed increase where regenerative braking begins to decrease to zero is a function of the dissipated braking energy.

Drawings

FIG. 1 shows a simplified schematic diagram of a vehicle in accordance with at least some embodiments of the invention;

FIG. 2 shows a flow diagram illustrating a method in accordance with at least some embodiments of the invention;

FIG. 3 illustrates a plurality of torque curves for controlling regenerative braking in accordance with at least some embodiments of the present disclosure;

FIG. 4 illustrates different sets of torque curves for controlling regenerative braking in accordance with at least some embodiments of the present disclosure;

FIG. 5 shows a regenerative torque curve and a friction torque curve controlled together to establish a constant total torque curve;

FIG. 6 shows a regenerative torque curve and a derated friction torque curve, which combine to create an inconsistent total torque curve;

FIG. 7 illustrates a regenerative torque curve combined with a derated friction torque curve to produce a smoothed total torque curve;

FIG. 8 shows a flow diagram illustrating a method in accordance with at least some embodiments of the invention;

FIG. 9 illustrates a plurality of torque curves for controlling regenerative braking in accordance with at least some embodiments of the present disclosure;

FIG. 10 illustrates different sets of torque curves for controlling regenerative braking in accordance with at least some embodiments of the present invention.

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 according to the present invention. The vehicle 10 includes a friction braking system 12 controlled by a brake controller 14. The vehicle 10 also includes a regenerative braking system 16 as part of the vehicle driveline. In particular, the regenerative braking system 16 includes one or more electric machines (e.g., electric motors) operable to provide regenerative braking to the vehicle 10. The regenerative braking system 16 is controlled by a control system, such as a Vehicle System Controller (VSC)18, comprised of one or more controllers. VSC18 may include other controllers, such as a Powertrain Control Module (PCM). In fact, the brake controller 14, shown in FIG. 1 as a separate controller, may be integrated into the VSC18, or may communicate with the VSC18, PCM and other controllers via a controller area network or other communication system. Thus, the various systems within vehicle 10 (including regenerative braking system 16 and friction braking system 12) may be controlled by 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 input from a brake pedal 20, and the VSC18 receives operator input from an accelerator pedal 22. In particular, the brake sensor 24 (which may 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 may 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 VSC18 and the brake controller 14 use various inputs, including inputs from the

sensors

24, 26, to determine how to control the friction braking system 12 and the regenerative braking system 16. The friction braking system 12 operates to reduce the rotational speed of the wheels 28 through the application of one or more friction elements according to methods well known in the art. Similarly, the regenerative braking system 16 is operable to reduce the rotational speed of the vehicle wheels 28 by causing the at least one electric motor to generate negative torque that is transmitted to the vehicle wheels 28 via the driveline.

The friction braking system 12 includes one or more sensors, represented in FIG. 1 by a single sensor 30. The sensors 30 are configured to send signals to the controller 14 related to various conditions within the friction braking system 12. For example, if the friction braking system 12 is to experience reduced braking capability that may result from loss of boost (loss of boost) or loss of hydraulic circuit, the sensor 30 may communicate this to the brake controller 14, which in turn communicates with the VSC 18. Similarly, the regenerative braking system 16 has one or more sensors, represented in FIG. 1 by sensor 32. The sensors 32 may detect conditions such as motor speed, motor torque, power, etc. The sensors 32 communicate directly with the VSC18, which the VSC18 can use in conjunction with other inputs to control the

braking 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 elements such as a vehicle suspension system. The wheels 28 shown separately in fig. 1 may be considered part of a larger body/chassis system 34. One or more sensors (shown as a single sensor 36 in fig. 1) are configured to detect various conditions of the body/chassis system 34 and communicate with the VSC 18. The sensors 36 may detect conditions such as deflection (deflection) of various elements of the body/chassis system 34 or loads on various elements. Similarly, a sensor 38, representative of one or more sensors, is configured to detect a condition of the wheel 28 that includes a wheel speed. The sensors 38 are shown in fig. 1 in communication with the larger body/chassis system 34, which body/chassis system 34 in turn communicates with the VSC 18. Alternatively, the sensor 38 may be directly connected to the VSC 18.

Fig. 2 shows a flow chart 40 illustrating a method according to the invention. The method begins at step 42 and determines an overall braking demand of the vehicle 10 based on driver input at step 44. These inputs may include brake pedal position and accelerator pedal position as sensed by

sensors

24 and 26. An initial determination is made at step 46 in which it is determined whether a braking event is in progress. If the input indicates that a braking event is not in progress, the process ends. Conversely, if it is determined that a braking event is in progress, a first vehicle condition is determined at step 48. The first vehicle condition may be any one of a number of different vehicle conditions, such as friction braking performance, vehicle deceleration, total braking torque (including both friction braking and regenerative braking), total braking power, total braking force, brake pedal position, suspension load, and suspension position.

In step 49, a first predetermined value is determined (PV 1). As explained in more detail below in connection with fig. 3-7, the first predetermined value is based on the first vehicle condition determined in step 48. Thus, the first predetermined value may be different for different vehicle conditions. This provides a number of advantages over systems and methods that reduce regenerative braking torque based on a single set point that is not responsive to a plurality of vehicle conditions.

At step 50, a second vehicle condition is determined, although this step occurs sequentially after

steps

48 and 49 in flowchart 40, in fact it may occur simultaneously with

steps

48 and 49 or before

steps

48 and 49. In practice, the determination of the first and second vehicle conditions may be continuous such that the VSC18 receives periodic updates of the vehicle conditions at some predetermined frequency.

The second vehicle condition determined in step 50 may include a condition such as the speed of the vehicle 10, the driveline torque, or a combination of vehicle speed and driveline torque. The second vehicle condition is then compared to the first predetermined value at step 52. For example, if the second vehicle condition is vehicle speed, the speed of the vehicle 10 will be monitored to determine when it reaches some predetermined speed. Since the braking event is ongoing, the speed of the vehicle 10 is decreasing. Thus, the condition in step 52 is satisfied when the VSC18 determines that the vehicle speed is at or below the predetermined speed. Therefore, in order to satisfy the condition in step 52, the second vehicle condition need not be in full agreement with the first predetermined value.

As shown in the flowchart 40, if the second vehicle condition has not reached the first predetermined value, the method loops back before step 50 and determines the second vehicle condition again. If the condition is satisfied in step 52, the regenerative braking torque is reduced beginning at step 54. As described in detail below, if the braking event continues, the regenerative braking torque will be reduced to zero. The process then ends, as shown in block 56, if it is determined that a braking event is not in progress (at step 46), which will also occur.

Turning to FIG. 3, the method shown in FIG. 2 is described in detail. The torque curve abc represents the maximum available regenerative braking torque (or regeneration limit) for a vehicle such as the vehicle 10. Since curve abc represents braking torque, it is always negative. Thus, as the maximum available regenerative braking torque increases, the negative value of the curve abc becomes larger.

From the graph in fig. 3, it is clear that the available amount of regenerative braking torque increases as the vehicle speed decreases, and the available amount of regenerative braking torque reaches a maximum at a certain relatively low vehicle speed. As discussed above, it is inefficient to run the electric motor at very low speeds despite having a large amount of available torque. Therefore, the regenerative braking control system may blend out (blend out) the regenerative braking torque from a certain value to zero to deviate from the regenerative limit curve, thereby reducing inefficiencies. Thus, there is a conflict between the desire to use the maximum available amount of regenerative braking torque and the desire to reduce motor inefficiencies and provide a smooth braking experience for the vehicle operator. The present invention balances these conflicting goals by examining various vehicle conditions and adjusting the point at which the blend-out of the regenerative braking torque begins (i.e., adjusting the point at which the regenerative braking torque deviates from the regeneration limit curve).

In addition to the regeneration limit curve abc, four additional torque curves are shown in FIG. 3: curve adhi, curve aehi, curve afhi, and curve aghi. The VSC18 is configured to control regenerative braking torque on the vehicle 10 according to torque curves similar to these curves. Of course, the torque curves shown in FIG. 3 represent only four possible torque curves selected among the myriad of possible torque curves for purposes of illustration. Each torque curve in FIG. 3 corresponds to a particular first vehicle condition, such as deceleration or total brake torque.

Specifically, the curve adhi is used when the vehicle deceleration is about 0.8g or the total brake torque is about 5000 Nm. Of course, the actual braking torque depends on a number of factors including vehicle mass; therefore, a value of 5000Nm is used for illustration purposes only. Similarly, curves aehi and afhi are used when vehicle deceleration is approximately 0.6g and 0.4g or when total brake torque is approximately 3750Nm and 2500Nm, respectively. The aghi curve is used when the vehicle deceleration is below 0.2g or the total braking torque is below 1250 Nm. For other vehicle decelerations, or other total braking torque levels, a torque curve appropriate for the respective deceleration or total braking torque will be used.

It is noted that the torque curve corresponding to a vehicle deceleration of 0.8g need not coincide with the torque curve corresponding to a total braking torque of 5000 Nm. The same is true for the other three torque curves. The double labeling of deceleration and total braking torque is for illustrative purposes only and does not necessarily imply a consistent relationship between a particular deceleration and a particular total braking torque value. For convenience, most of the following description of fig. 3 and 4 relates exclusively to deceleration; however, it should be understood that the same description applies to the total braking torque.

Referring back to the flowchart in fig. 2, the vehicle deceleration is indicative of the first vehicle condition determined in step 48. As described above, other vehicle conditions may be used in place of vehicle deceleration. For example, a plurality of torque curves to be used to control regenerative braking may be established based on different levels of total braking power. Similarly, the first vehicle condition may be any one of a number of different vehicle conditions (including total braking force, brake pedal position, suspension load, suspension position, or friction braking performance). As described above in connection with fig. 1, one or more of the various sensors associated with each of the vehicle systems may send signals to the VSC18 to provide information regarding selected vehicle conditions. The VSC18 can then use one or more torque curves (such as those shown in fig. 3) to control regenerative braking of the vehicle 10.

Returning to FIG. 3, it can be seen that for high values of driveline torque and/or vehicle speed, control of regenerative braking follows a regenerative limit curve. At some point, control of regenerative braking begins to deviate from the regenerative limit and the regenerative braking torque merges from a point on the regenerative limit curve and drops to zero. Each of the points at which one of the torque curves deviates from the regeneration limit curve (i.e., points d, e, f, and g) represents the first predetermined value used in step 52 in fig. 2. As clearly shown in fig. 3, the point at which the torque curve deviates from the regeneration-limiting curve (i.e., the first predetermined value) is based on the first vehicle condition (in this example, the vehicle deceleration). Thus, unlike some regenerative braking control systems, the present invention uses a different point to begin blending out regenerative braking torque.

As shown in fig. 3, the point at which the fusion on the torque curve starts is also the point of maximum regenerative braking torque. Because the regenerative braking torque is melted out more quickly for higher levels of vehicle deceleration, there is an inverse relationship between vehicle deceleration and maximum regenerative braking torque. Conversely, because higher levels of vehicle deceleration blend out more quickly, there is a direct relationship between the determined deceleration value at the beginning of the blend-out and the driveline torque and/or vehicle speed.

The convergence of the regenerative braking torque is shown in fig. 3 as a straight line defined by two points on the torque curve. For example, torque curve adhi includes a first curve segment dh defined by a maximum regenerative braking torque at point d and a zero regenerative braking torque at point h. The VSC18 can determine when these points are reached because each of these points corresponds to a vehicle condition (such as powertrain torque or vehicle speed). Thus, for a determined deceleration of 0.8g, the VSC18 may control the regenerative braking torque to begin a blend-out at a vehicle speed of 25 miles per hour (mph) and to end the blend-out at a vehicle speed of 5 mph. Similarly, if the determined vehicle deceleration is 0.4g, the VSC18 may begin to blend out regenerative braking at a vehicle speed of 15mph, and still end blending out at a vehicle speed of 5 mph.

Instead of using vehicle speed as the second vehicle condition, the VSC18 may also rely on driveline torque to determine when to begin to fuse out regenerative braking torque. For example, if the determined deceleration is 0.8g and the vehicle speed is 25mph, the VSC18 may also rely on the powertrain torque to determine whether to begin to blend out the regenerative braking torque. The blend-out of regenerative braking torque may be delayed if the driveline torque is only 1000Nm at a vehicle speed of 25 mph. However, if the powertrain torque vehicle speed is 2000Nm at 25mph, the VSC18 may control the regenerative braking torque to begin to blend out.

Although the blend of regenerative braking torque is shown as linear in FIG. 3, it may be non-linear. For example, the blend-out of regenerative braking torque may be based on a predetermined percentage of the maximum available regenerative braking torque. In this case, the blend-out will occur as a function of the regeneration limit curve, and thus, the blend-out will be non-linear for the non-linear portion of the regeneration limit curve.

In the example described in connection with FIG. 3, the VSC18 controls regenerative braking torque according to a single torque curve. Since vehicle conditions change during a braking event, the VSC18 can be configured to control regenerative braking according to more than one curve, even for a single braking event. When vehicle deceleration is used as the first vehicle condition, one method of using more than one curve to control regenerative braking in a single braking event includes using more than one curve only when vehicle deceleration is increased during the braking event. Thus, if the vehicle deceleration remains constant or decreases during a braking event, the VSC18 will control regenerative braking according to a single curve (such as the curve afhi in fig. 3). However, if the vehicle deceleration increases from 0.4g to 0.8g during a braking event, the VSC18 may begin controlling the regenerative braking torque according to the curve afhi, and then complete the rollout based on the curve adhi.

As shown in FIG. 3, each of the segments of the fused curves defining regenerative braking torque (i.e., segments dh, eh, fh, and gh) represents a change in braking torque with some change in driveline torque and/or vehicle speed. Alternatively, each of the curve segments dh, eh, fh, and gh may be represented by a period of time, since the blend-out of regenerative braking torque occurs over a period of time during a braking event. One way in which the regenerative braking torque may be controlled for different vehicle conditions, such as different vehicle decelerations, is to use a single, constant period of time (to fuse out the regenerative braking torque during that period of time). For example, regardless of the determined vehicle deceleration, a period of 6 seconds may be selected as the value during which the regenerative braking torque is to be brought down. Thus, points d, e, f, and g may be determined such that each of curve segments dh, eh, fh, and gh completes within a 6 second time period.

Another way of controlling the blend-out of regenerative braking torque is shown in fig. 4. In fig. 4, only one of the torque curves follows the regeneration limit curve (i.e., the torque curve for vehicle deceleration less than 0.2g or total brake torque less than 1250 Nm). At higher vehicle deceleration (or total brake torque), the entire torque curve moves up so that it is constant at the point where the blend-out of regenerative torque begins, regardless of vehicle deceleration (or total brake torque). Although the curves shown in fig. 3 and 4 are based on different levels of vehicle deceleration (or total brake torque), similar curves may be generated based on different vehicle conditions, such as brake pedal position. The brake pedal position may be used directly, or may be related to other vehicle conditions such as total braking power or total braking force. Similarly, different torque curves for different suspension loads and/or suspension positions may be used to generate the torque curves.

As described above, the first vehicle condition determined by the VSC18 can also be friction braking performance. Fig. 5 shows three torque curves: a regenerative torque curve (similar to the torque curves shown in fig. 3 and 4), a friction torque curve (representing a torque curve for a friction braking system such as friction braking system 12), and a constant total torque curve (representing the sum of the regenerative torque curve and the friction torque curve).

In the example shown in FIG. 5, the regenerative braking torque is controlled in a manner similar to that described in FIGS. 3 and 4; in addition, the friction braking torque is controlled to increase to match the reduced regenerative braking torque. This region is shown in fig. 5 by the "Regen Ramp Out" indicated by an oval. Matching the blend-out (blend out) of regenerative braking torque to the corresponding blend in (blend in) of frictional braking torque may be difficult or impossible when the frictional braking performance is degraded. For example, if the friction braking system experiences a loss of boost or a loss of hydraulic circuit, the friction control system may not be able to control the friction braking torque according to the curve shown in FIG. 5.

Fig. 6 shows a situation where the regenerative braking torque follows a curve similar to the curves shown in fig. 3 and 4, but the performance of the friction braking system is reduced and is no longer independently controllable. The normal regenerative braking torque and the derated friction torque add up to produce an inconsistent total torque. Such inconsistent total torque may require the vehicle operator to compensate, for example, by adjusting brake pedal pressure. The present invention provides a solution for this by biasing out the regenerative braking torque as described above.

Fig. 7 shows the case where the present invention is applied to the case where the friction braking torque performance is lowered. In this case, the first vehicle condition determined in step 48 of fig. 2 is friction braking performance. In this example, the second vehicle condition is the vehicle speed indicated by the abscissa in the graph of fig. 7. The VSC18 can begin reducing the regenerative braking torque to zero at a predetermined value of vehicle speed that is based on reduced friction braking performance (i.e., based on the first vehicle condition).

As shown in fig. 7, the regenerative braking torque is gradually and smoothly deviated so that the total braking torque is also smooth without sudden change. This provides a smooth, consistent feel to the vehicle operator and eliminates the need for the vehicle operator to react quickly to compensate for abrupt changes in braking torque. For the example shown in FIG. 7, the first predetermined value (i.e., vehicle speed at the beginning of the regenerative braking torque excursions) may be the same for any type of reduced friction braking performance. Alternatively, the VSC18 can be configured to use different values depending on the type and severity of the reduced friction braking performance.

As noted above, rapid taper regenerative braking may result in inconsistent feel for the vehicle operator, particularly when the friction brakes are cold. Accordingly, embodiments of the present invention employ a fade strategy that takes into account brake system conditions to determine how quickly regenerative braking should fade and at what rate regenerative braking should fade. FIG. 8 shows a high level flow chart 58 illustrating a method according to an embodiment of the present invention, particularly showing information regarding whether a warm-up strategy should be used at regenerative braking ramp-down.

Typically, the warm-up strategy will use a higher vehicle speed than would normally be used as a starting point for taper regenerative braking. Starting the taper-off process at a higher vehicle speed provides a greater amount of time for the friction brakes to take over before the vehicle speed is at or near zero. By providing additional time for regenerative braking fade and friction brake take-over, the inconsistent feel that a vehicle operator would experience when applying cold friction brakes may be reduced or eliminated.

A method, which may be performed by a control system such as that described above, begins at step 60 and proceeds to decision block 62 where a determination is made as to whether a braking event is in progress in block 62. If the braking event is not ongoing, the method loops back and continues to query until the determination is affirmative. After step 62, the method moves to decision block 64, where a determination is made in block 64 whether the amount of dissipated Braking Energy (BED) is greater than the Braking Energy Limit (BEL). What are the parameters "BED" and "BEL" and how they are determined will be described in more detail below.

If it is determined that the dissipated braking energy is greater than the braking energy limit, the method proceeds to step 66 and the warm-up strategy is not used. However, if the dissipated braking energy is not greater than the braking energy limit, then a warm-up strategy is used. In general, when the first vehicle condition meets at least one criterion, the regenerative braking may be initially reduced to zero when the vehicle speed reaches the first vehicle speed during braking. Conversely, when the first vehicle speed does not meet the at least one criterion, the regenerative braking may begin to be reduced to zero when the vehicle speed reaches a second vehicle speed greater than the first vehicle speed during braking.

In the implementation shown in fig. 8 and described above, the first vehicle condition is the amount of dissipated braking energy, and the first vehicle condition meeting the at least one criterion occurs when the amount of dissipated braking energy is greater than the braking energy limit. More specifically, when the amount of dissipated braking energy is greater than the braking energy limit, the regenerative braking may be reduced to zero (e.g., without using a warm-up strategy) at a first vehicle speed and when the amount of dissipated braking energy is not greater than the braking energy limit, the regenerative braking may be reduced to zero (e.g., using a warm-up strategy) at a second vehicle speed that is greater than the first vehicle speed. When BED is not greater than BEL, the vehicle speed at which regenerative braking begins to decrease to zero is increased.

Turning to FIG. 9, the method illustrated in FIG. 8 is described in detail. The torque curve abc represents the regeneration limit (see also fig. 3 and additional description thereof). In addition to the regeneration limit curve (curve abc), four additional torque curves are shown in FIG. 9: curve adhi, curve aehi, curve afhi, and curve aghi. The VSC18 is configured to control regenerative braking torque on the vehicle 10 according to torque curves similar to these curves. Of course, the torque curves shown in FIG. 9 are only representative of four possible torque curves selected among the myriad of possible torque curves for purposes of illustration. Each torque curve in FIG. 9 corresponds to a particular first vehicle condition, such as a value of BED.

In particular, the curve adhi is used when BED is equal to zero (BED-0). The curve shows that the excursions of the regenerative braking begin to occur at point d (at least in this example, point d is the highest vehicle speed at which the excursions begin). Referring back to FIG. 8, and in particular in decision block 64, when BED equals zero, it is unlikely to be greater than BEL, and therefore a warm-up strategy is used, as shown in step 68. The other end of the example shown in fig. 9 is a curve aghi representing a case where BED is greater than BEL (BED > BEL). As shown in fig. 8, no warm-up strategy will be used in this case. Thus, the vehicle speed at which the regenerative braking begins to deviate is represented by point g, which may be the "first vehicle speed" mentioned above.

Similarly, the vehicle speed represented by point d may be the "second vehicle speed" mentioned above, or alternatively, points e and f may represent that speed. This is because both the point e and the point f represent vehicle speeds higher than the vehicle speed at the point g, and both the curves aehi and afhi show control using the warm-up strategy. In particular, curve aehi is used when the value of BED is greater than zero but less than a fraction of BEL (X% BEL > BED > 0). When BED is greater than a fraction of BEL but still less than the full value of BEL (BEL > BED > X% BEL), the curve afhi is used.

Another way of controlling the slip of regenerative braking torque is shown in fig. 10. Here, only one of the torque curves follows the regeneration limit curve, i.e., the torque curve used when BED is greater than BEL. When the BED is not greater than the BEL, the entire torque curve is shifted up so that the point at which the roll-off of the regeneration torque begins is constant regardless of the relative value of the BED. In this family of curves, the same vehicle speed is used as the point for starting the deviating process of the regenerative braking torque; however, as the curve moves upward from the regeneration limiting curve, less regenerative braking is used and therefore less excursions need to occur during the same reduction in vehicle speed. The upper curve (where BED equals zero) is just above the regeneration limit and therefore has less excursions to complete during the same reduction in vehicle speed.

Although different ways of calculating a braking energy limit, such as described above, are possible, in at least some implementations of the invention, the BEL is a function of, or is based on, at least the braking system temperature and the ignition system state. While the ignition system state used to determine the BEL may be represented by any of several parameters, in at least some implementations of the invention, the ignition system state includes how long the vehicle has been in the ignition switch off state. This provides at least some indication of how cold the friction brake may have become since the last use of the friction brake.

With respect to the temperature of the brake system, the temperature of the brake system may be calculated in any of a number of different manners, including through the use of an internal temperature sensor that communicates directly with a portion of the control system (such as the VSC18, where the signal may be used by one or more controllers interconnected using a CAN such as described above). Alternatively, one or more pressure sensors may be used to estimate the temperature of the brake system for use in the calculation of the BEL. In at least some implementations, the initial brake system temperature will be determined from data collected almost immediately after system start-up (e.g., within five seconds after start-up).

Using the brake system temperature and the ignition switch off time to determine the BEL may include the use of a data table of associated temperatures and energies. In at least some implementations, the temperature/energy table will have at least eight data points; the temperature value may have a range of-60 ℃ to 400 ℃ (resolution of 0.01), while the energy value may have a minimum range of 0 to 250000 joules (J) (resolution of 4). Such a data table may be pre-programmed into the control system and used as a look-up table based on the brake system temperature determined as described above. For temperature values between data points, linear or other types of interpolation schemes may be used to determine corresponding energy values.

The energy value determined from the lookup table may be used as the BEL if the ignition-switch-off time has exceeded some predetermined ignition-switch-off time (e.g., two hours). If the actual ignition switch off time does not exceed the predetermined ignition switch off time, BEL may be set to zero or some relatively low value, so that in all or most cases the dissipated braking energy will exceed BEL and the warm-up strategy is not used or used at a lower level. The value of BEL may be further limited, if desired, for example, based on the brake system temperature being above a predetermined temperature. In at least some implementations, regardless of the energy value determined from the lookup table, the value of BEL will be set to no more than 10000J for brake system temperatures above 20 ℃.

In general, the amount of brake energy dissipated may be a function of, or based on, several parameters. For example, in at least some implementations, the amount of brake energy dissipated may be based on at least an amount of friction torque of at least one wheel of the vehicle and a rotational speed of at least one wheel of the vehicle. In at least some implementations, the BED may also be based on a previously calculated amount of dissipated braking energy and a time since the previous calculation. More specifically, in at least some implementations, BEDs will be initialized with a zero value after system startup. The BED may then be calculated by estimating the braking energy input to one or more vehicle wheels based on the braking pressure and the wheel speed. The equation for BED in at least some implementations can be written in the form:

bed (J) dissipated braking energy Z1(J) + friction torque (Nm) rotational speed (rad/s) × control loop time(s)

Wherein: the dissipated braking energy Z1(J) is the BED calculated previously (for the first such calculation)

The value is zero), in joules;

friction torque (Nm) is the friction braking torque determined from the braking pressure, in newton-meters, which can be measured by sensors on each circuit, rather than on each wheel or chassis;

rotational speed (rad/s) is the rotational speed of one or more vehicle wheels, which may be, for example, average or may use a single wheel rotational speed in radians per second;

the control loop time(s) is the elapsed time in seconds since the last prior determination of the BED.

As described in detail above, for example, in connection with FIG. 9, the point at which regenerative braking begins to drift out of or taper to zero may change under different vehicle conditions. This point may be selected based on any number of factors, such as those described above, or by other factors. Embodiments of the present invention employing the warm-up strategy described hereinabove may begin at a predetermined point where regenerative braking begins to ramp down to zero. For example, this point may represent a particular vehicle speed such as that shown in fig. 9 and described in connection with fig. 9 (which may be referred to as a "maximum regenerative speed" because the speed is the highest vehicle speed (where 100% regenerative braking is allowed and no friction braking is allowed). Thus, in embodiments using a warm-up strategy, the "first vehicle speed" described above may be the vehicle speed at which regenerative braking begins to taper off when the warm-up strategy is not being used (e.g., point g in fig. 9). The "second vehicle speed" described above may be the vehicle speed at which regenerative braking begins to ramp down when the warm-up strategy is used (e.g., points d, e, or f).

As described above, the second vehicle speed is greater than the first vehicle speed, as this allows more time for the regenerative braking to be deviated and for the friction braking to take over, providing a smoother transition. In at least some implementations, the difference between the first vehicle speed and the second vehicle speed may be a function of, or based on, the amount of braking energy dissipated (i.e., BED). This Speed difference may be identified as a "Warm-up ramp Speed" (WRS), and may be defined in at least some implementations by the following equation.

Wherein: MWRS is a predetermined, calibratable value of the maximum warm-up ramp-down rate; in some implementations, the value may be set to 10 kilometers per hour (kph);

BEL and BED are as defined above;

BERR is a braking energy taper range, and is a predetermined, calibratable range of braking energy values suitable for taper regenerative braking; in some implementations, the range may be 5000-.

From the above equation, it can be seen that when BEL is greater than BED (meaning that the warm-up strategy will be used, as described above), the value of WRS will be positive, and thus the second vehicle speed will be greater than the first vehicle speed. Conversely, if the value of BED is greater than the value of BEL, the value according to equation WRS will be zero, which correlates to the case where no warm-up strategy is used. Another relationship that may be gleaned from analysis of the above equation is that, in at least some cases, the value of WRS decreases as the BED value increases. Thus, where a warm-up strategy is employed and the value of WRS is not zero, the difference between the first and second vehicle speeds generally decreases as the BED value increases, while the difference in vehicle speeds generally increases as the BED value decreases.

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. Furthermore, the features of the various implementing embodiments may be combined to form further embodiments of the invention.

Claims

1. A method for controlling regenerative braking in a vehicle, comprising:

when the amount of dissipated braking energy is greater than the braking energy limit, beginning to reduce regenerative braking to zero at a first vehicle speed;

when the amount of dissipated braking energy is not greater than the braking energy limit, reducing regenerative braking to zero is initiated at a second vehicle speed that is greater than the first vehicle speed.

2. The method of claim 1, wherein a difference between the first vehicle speed and the second vehicle speed is a function of an amount of brake energy dissipated.

3. The method of claim 2, wherein the difference between the first vehicle speed and the second vehicle speed decreases as the amount of brake energy dissipated increases.

4. The method of claim 1, wherein the amount of dissipated braking energy is based on at least an amount of friction torque of at least one wheel of the vehicle and a rotational speed of at least one wheel of the vehicle.

5. The method of claim 4, wherein the amount of dissipated braking energy is further based on a previously calculated amount of dissipated braking energy and a time since a previous calculation.

6. The method of claim 1, wherein the braking energy limit is based on at least a vehicle braking system temperature and a vehicle ignition system status.

7. The method of claim 6, wherein the vehicle ignition system state comprises how long a vehicle has been in an ignition off state.