System and method for limiting regenerative braking

Abstract

A method (100) for controlling application of regenerative braking torque to a plurality of wheels (30, 32, 62, 64) of at least one of a hybrid electric vehicle (10) or an electric vehicle to avoid braking instability, the method (100) comprising: Detecting an angle of a steering wheel of the vehicle (10); Detecting a speed of the vehicle (10); Detecting a brake pedal position when a driver of the vehicle (10) actuates a brake pedal of the vehicle (10); detecting wheel slip of each of a pair of front wheels (62, 64) of the vehicle (10); detecting wheel slip of each of a pair of rear wheels (30, 32) of the vehicle; Controlling the application of the regenerative braking torque based on the sensed wheel slip relative to at least one predetermined wheel slip limit; characterized by Determining a requested lateral acceleration representing a steady-state lateral acceleration that the vehicle (10) would achieve at an actual vehicle speed and with a currently sensed steering wheel angle; and wherein the at least one predetermined wheel slip limit is determined at least partially based on the determined required lateral acceleration, wherein a rate of change of a maximum allowable regenerative braking torque during states of decreasing and increasing the regenerative braking torque is individually controlled such that: a first rate of change is permitted as the regenerative braking torque decreases, thus helping to avoid braking instability; and a second rate of change, which is smaller than the first rate of change, is permitted when the regenerative braking torque increases so as to improve the drivability of the vehicle (10).

Description

AREA

[0001] The present disclosure relates to regenerative braking systems for use with electrically powered vehicles, and more particularly to a system and method for controlling regenerative braking in a manner that controls limiting of regenerative braking to keep the vehicle more stable during braking maneuvers. BACKGROUND

[0002] The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.

[0003] In a vehicle powered entirely or partially by an electric motor, kinetic energy during braking can be converted into electrical energy and stored in a battery. This is called "regenerative braking." Another term for this type of energy conversion is "recuperative braking."

[0004] In a hybrid electric vehicle (HEV), regenerative braking provides by far the greatest fuel savings compared to other typical HEV techniques (e.g., stopping an internal combustion engine when not in use / needed, engine load shifting, etc.). In a battery electric vehicle (BEV), regenerative braking extends the vehicle's range. If the vehicle has a standard braking system (e.g., with anti-lock braking system (ABS), TCS, and ESP), regenerative braking, as requested by the driver's braking effort and possibly modulated by the vehicle's brake control system, is added to braking via the base braking system.

[0005] DE 10 2010 003 076 A1 relates to a method for controlling wheel brake slip for a vehicle with an electric drive, comprising the following steps: detecting a brake signal by means of a slip control device, wherein the slip control device generates an electric drive signal and a friction brake signal for controlling a predetermined slip value of a wheel of the vehicle, transmitting the electric drive signal to an electric drive control device and transmitting the friction brake signal to a friction brake control device, wherein the friction brake control device actuates a friction brake of the wheel in accordance with the friction brake signal to generate a friction brake torque and the electric drive control device actuates the electric drive in accordance with the electric drive signal to generate an electric drive torque.

[0006] WO 2007 / 118 588 A1 discloses a method for influencing the handling of a vehicle. For this purpose, a cornering variable is determined that represents the current cornering of the vehicle, and the wheel contact force on at least one vehicle wheel is influenced according to a functional relationship depending on the determined cornering variable. When a predetermined driving or operating state of the vehicle is present or reached, the functional relationship is modified, and the wheel contact force is influenced according to the modified functional relationship depending on the cornering variable. The device according to the invention relates to a device configured to carry out the method according to the invention.

[0007] DE 10 2013 214 806 A1 discloses a method for operating a braking system for a motor vehicle with at least two axles, in which the wheels of a first axle are connected to a regenerative brake and the wheels of at least one axle are equipped with wheel brakes, wherein all wheels of the motor vehicle have wheel speed sensors. The target braking torque of the regenerative brake is limited if a slip value, which is weighted with a measure of the actual braking torque of at least one wheel of the first axle, exceeds an upper threshold value.

[0008] A key objective for any HEV is to maximize regenerative braking while maintaining vehicle stability during braking maneuvers. Avoiding rear "over-braking," i.e., excessive regenerative braking at the vehicle's rear wheels, is particularly important. This has been a significant challenge for various state-of-the-art systems.

[0009] Thus, it can be considered an object of the invention to provide an improved brake control method and system. SUMMARY

[0010] This problem is solved by the subject matter of the respective independent claims.

[0011] In one aspect, the present disclosure relates to a method for controlling application of regenerative braking torque to a plurality of wheels of at least one of a hybrid electric vehicle or an electric vehicle to avoid braking instability. The method may include sensing an angle of a steering wheel of the vehicle; sensing a speed of the vehicle; sensing a brake pedal position when a driver of the vehicle applies a brake pedal of the vehicle; sensing wheel slip of each of a pair of front wheels of the vehicle; and sensing wheel slip of each of a pair of rear wheels of the vehicle. A demanded lateral acceleration may be determined, representing a steady-state lateral acceleration that the vehicle would achieve at an actual vehicle speed and with a currently sensed steering wheel angle.The application of regenerative braking torque may then be controlled based on the sensed wheel slip values ​​relative to at least one predetermined wheel slip limit. At least one predetermined wheel slip limit is determined based at least in part on the determined requested lateral acceleration. A rate of change of a maximum allowable regenerative braking torque is individually controlled during states of decreasing and increasing regenerative braking torque such that a first rate of change is allowed when the regenerative braking torque decreases, thus helping to avoid braking instability. Furthermore, a second rate of change, smaller than the first rate of change, is allowed when the regenerative braking torque increases, thus improving vehicle drivability.

[0012] In another aspect, the present disclosure relates to a method for controlling application of regenerative braking torque to a plurality of wheels of at least one of a hybrid electric vehicle or an electric vehicle to avoid braking instability. The method may include sensing a brake pedal position when a driver of the vehicle applies a brake pedal of the vehicle; determining a requested lateral acceleration representing a steady-state lateral acceleration that the vehicle would achieve at an actual vehicle speed and with a currently sensed steering wheel angle. The method may also include sensing wheel slip of each of a pair of front wheels of the vehicle and determining a minimum front wheel slip for the two front wheels.Wheel slip of each of a pair of rear wheels of the vehicle may also be sensed and used to determine a maximum rear wheel slip for the two rear wheels. The application of regenerative braking torque may be controlled such that the regenerative braking torque cannot be increased in response to brake pedal movement, but is instead held constant in a holding condition when one of the following conditions occurs: the maximum rear wheel slip exceeds a first predetermined limit; or the maximum rear wheel slip exceeds the minimum front wheel slip by a second predetermined limit. The first and second predetermined limits are determined based at least in part on the determined requested lateral acceleration.

[0013] In another aspect, the present disclosure relates to a system for controlling application of regenerative braking torque to a plurality of wheels of at least one of a plurality of a hybrid electric vehicle or an electric vehicle to avoid braking instability, the system comprising: a steering control sensor adapted to detect an angle of a steering wheel of the vehicle; a sensor for detecting a speed of the vehicle; a brake controller adapted to detect a brake pedal rate when a driver of the vehicle depresses a brake pedal of the vehicle; an inertial measurement system adapted to detect wheel slip of each of a pair of front wheels of the vehicle and to detect wheel slip of each of a pair of rear wheels of the vehicle;a processor-based hybrid control system adapted to: determine a requested lateral acceleration representing a steady-state lateral acceleration the vehicle would achieve at an actual vehicle speed and with a currently sensed steering wheel angle; control the application of the regenerative braking torque based on the sensed wheel slip relative to at least one predetermined wheel slip limit; and wherein the at least one predetermined wheel slip limit is determined based at least in part on the determined requested lateral acceleration. The processor-based hybrid control system is further configured to detect wheel slip by: analyzing wheel slip of each of a pair of front wheels of the vehicle and determining therefrom a minimum front wheel slip for the two front wheels (r_FrtBrkSlip);and analyzing a wheel slip of each of a pair of rear wheels of the vehicle comprises determining a maximum rear wheel slip for the two rear wheels therefrom (r_RrBrkSlip);

[0014] Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The drawings described herein are for illustrative purposes only and are not intended to limit the scope of the present disclosure in any way. Fig. 1A is a high-level block diagram of typical major components of a hybrid electric vehicle (HEV) in which the regenerative braking system and method of the present disclosure is implemented; The Fig. 1B to 1C are an activity flow diagram illustrating the operation of the present system and method; Fig. 2 is a table showing a summary of test cases and test conditions under which specific tests were performed using a system and method according to the present disclosure; Fig. 3A is a graph of torque of the rear wheels and front wheels versus time under a straight-line traveling condition without the use of the system and method of the present disclosure, and wherein the vehicle has lost its grip and has begun to spin; Fig. Figure 3B is a graph of the slip angle of the vehicle in degrees with respect to time under the same conditions as above for Fig. 3A described; Fig.Figure 3C shows graphs of the vehicle’s forward speed (VelocityForward) and the accelerator pedal position (AccelPdPosn) versus time, with the vehicle under the same conditions as described above for Fig. 3A described; The Fig. 4A to 4C show diagrams relating to the Fig. 3A to 3C, but wherein the system and method of the present disclosure are applied to control regenerative braking; Fig. Figure 5 shows a table of test conditions for tests carried out on a vehicle while coasting during cornering; The Fig. 6A to 6C illustrate the performance parameters described above according to the Fig. 3A-3C, but without using the system and method of the present disclosure, and when the vehicle has lost its grip and is spinning while coasting during a cornering maneuver; and The Fig.7A to 7C illustrate representations respectively with respect to the Fig. 6A to 6C, but wherein the system and method of the present invention are applied to control regenerative braking. DETAILED DESCRIPTION

[0016] The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. It should be understood that throughout the drawings, corresponding reference characters designate like or corresponding parts and features.

[0017] Referring to Fig.1A shows a high-level block diagram of various components of a hybrid electric vehicle (HEV) 10 including a regenerative braking system according to the present disclosure. The HEV 10 in this example may include an electric motor control subsystem 12 with a torque control subsystem 14 and an inverter 16. An output of the inverter 16 may be supplied to an electric motor 18. A speed sensor 20 may be used to monitor the speed of an output shaft 22 of the electric motor. The output shaft 22 may apply an input drive signal to a rear differential 24. The rear differential 24 has axles 26 and 28 used to drive the right rear wheel (RRW) 30 and the left rear wheel (RLW) 32, respectively. The speed sensors 34 and 36 are used to detect the speed of each wheel 30 and 32, respectively.

[0018] The output signals of speed sensors 34 and 36 are transmitted to a brake control subsystem 38. The brake control subsystem 38 transmits the speed signals (wwfl, wwfr, ωwrl, and ωwrr) measured by sensors 34 and 36, along with a brake pressure signal (PBrk) and a brake pedal position signal (αBrkPed), to a vehicle CAN (Controller Area Network) bus 40. A high-voltage (HV) battery and control subsystem 42 and a processor-based hybrid control system 44 are also connected to the CAN bus 40. The hybrid control system 44 can receive the following inputs: Initial inputs for regenerative braking brake pressure P Brk Brake pedal position α BrkPed Accelerator pedal position α SW Steering wheel angle α SW Wheel angular velocity FL oh wfl Wheel angular velocity FR oh wfr Wheel angular velocity RL oh wrl Wheel angular velocity RR oh wrr Vehicle speed vVeh

[0019] The hybrid control system 44 uses the inputs listed above to generate a torque request signal (T mreq) which is input to the torque control subsystem. The Tmreq signal represents an internal signal on the CAN bus 40. T mreg is the target for torque control. The torque control controls the three-phase alternating current to achieve the target torque at the axle input of the electric motor 18 to the differential 24.

[0020] The CAN bus 40 also receives inputs from an inertial measurement system 46, a steering control subsystem 48, and an internal combustion engine (ICE) control subsystem 50. The ICE control system 50 is operatively associated with an internal combustion engine (ICE) 52 of the HEV 10 and is capable of receiving inputs from sensors associated with the ICE 52 and applying signals to various electronic and / or electromechanical components associated with the ICE 52.

[0021] Referring to Fig.1A, the ICE 52 includes an output shaft 54 ​​that drives a front transmission / differential subsystem 56. The transmission / differential subsystem 56, in turn, applies torque to each of the drive axles 58 and 60 associated with the right front wheel (FRW) 62 and the left front wheel (FLW) 64. Speed ​​sensors 66 and 68 detect the angular velocities of the FRW 62 and the FLW 64, respectively, and send electrical signals in accordance therewith to the brake control subsystem 38. Regenerative braking algorithm

[0022] The present disclosure focuses not only on defining regenerative braking torque based on a driver request (i.e., accelerator and brake pedal position), but rather on more effectively limiting a request for regenerative braking torque to better avoid instability during vehicle braking. The present disclosure further relates to how regenerative braking may be aborted in the event that the margin for braking instability is determined to be too low. In one aspect, the regenerative braking control methodology of the present disclosure effectively utilizes the requested lateral acceleration to help limit regenerative braking. Referring to the Fig.1B through 1C, an activity diagram 100 is shown illustrating an algorithm (i.e., a methodology) by which the system operates. In summary, and with reference to the activity diagram 100, the present disclosure includes monitoring and / or controlling: the required lateral acceleration, i.e. the steady-state lateral acceleration that the vehicle would achieve at the actual vehicle speed and steering wheel angle, is monitored (101); the maximum permissible regenerative braking torque (102), which is a function of the required lateral acceleration in order to improve braking stability during cornering; is limited to individual calibrations for increasing (103a) and decreasing (103b) regenerative braking scenarios to provide the final maximum allowable regenerative braking torque (103c). This maximum allowable torque is the limit when no rear brake instability has been detected. Typically, a rapid decrease rate is enabled to avoid braking instability, and a slow increase rate is used to improve drivability; The stability of the rear brake is estimated by monitoring the rear wheel slip (max. from left / right) (105a) and the front wheel slip (min. from left / right) (105b); The driver-requested regenerative braking torque (113) shall not increase (e.g., as a function of the brake pedal) to provide a limited driver-requested regenerative braking torque (106a) when rear wheel slip exceeds a limit (R_RrBrkSlipHoldLim, 106b) or when rear wheel slip exceeds front wheel slip by another limit (R_RrFrtBrkSlipDiffHoldLim) (106c).

[0023] Thus, any condition where the rear slip exceeds R_RrBrkSlipHoldLim, or a rear / front difference exceeds R_RrFrtBrkSlipDiffHoldLim, may result in a "hold condition" (i.e., when the regenerative braking torque may not be increased); the hold condition (109a) is locked until the driver releases the brake pedal and applies some degree of accelerator input.

[0024] The potentially restricted driver demand due to the stopping condition is then limited with the final maximum allowable regenerative braking torque (111).

[0025] The maximum positive rate of change (104) of the limited regeneration request (114) is a function of the brake pedal rate. The rate is obtained from the brake pedal position via time derivative and low-pass filtering; A low brake pedal rate => low rate of change of regenerative braking torque. This allows an increased maximum allowable regenerative braking torque to be provided smoothly when the driver holds the brake pedal stationary. On the other hand, when a change in brake pedal rate is detected, the request is followed by a higher response and thus improved controllability. Regenerative braking is deactivated (108a) if the rear wheel slip (max. left / right) exceeds a limit (R_RrBrkSlipAlwdLim) (112a) or if the maximum rear wheel slip exceeds the front wheel slip (min. left / right) by another limit (R_RrFrtBrkSlipDiffAlwdLim) (112b) → Deactivate regeneration (108b); The deactivation conditions (109b) are locked until the driver releases the brake pedal and applies a certain amount of accelerator pedal input; The rate of deactivation of regenerative braking torque (110b) is a function of the severity of the braking instability. The measure of braking instability is the slip error (110a) (i.e., how much the rear slip or rear / front slip difference has changed after the regeneration hold condition (110a). If regenerative torque is not deactivated, a high negative rate (110c) is permitted; Permissible regenerative braking torque is reduced (107a) with the increase of the base braking torque (107b) that occurs after the holding condition (109a); The limits R_RrBrkSlipHoldLim, R_RrFrtBrkSlipDiffHoldLim, R_RrBrkSlipAlwdLim and R_RrFrtBrkSlipDiffAlwdLim are all a function of the required lateral acceleration (in the Fig. 1B to 1C not shown) to improve braking stability during cornering. List of inputs, outputs, and internal states

[0026] The following is a detailed listing of inputs, outputs, and internal states used by a regenerative braking algorithm (i.e., methodology) implemented by the present disclosure: Transformations to initial inputs to generate inputs for the activity flow diagram from Fig. 1A M_DrvRegReq = f (α SW , v Veh ) r_RrBrkSlip = f (ω wrl , ω wrr , in Veh ,) r_FrtBrkSlip = f (ω wfl , oh wfr , v Veh , a SW ) a_LatCmd = f (v Veh , α Sw ) Inputs M_DrvRegReq regenerative braking torque requested by the driver r_RrBrkSlip rear brake slip (maximum left and right rear brake slip) r_FrtBrkSlip front brake slip (minimum left and right front brake slip) a_LatCmd required lateral acceleration. Steady-state lateral acceleration would reach the vehicle at the actual vehicle speed and steering wheel angle. r_BrkPed brake pedal position r_AccPed Accelerator pedal position P_Brk_Press master cylinder brake pressure output M_Req Req Request for the final regenerative braking torque AAM Internal states of the regenerative braking torque algorithm Internal variables r_RrBrkSlipHoldLim Limit of the rear brake longitudinal slip. This is defined as a base calibration multiplied by one or more factors that are functions of, for example, a_LatCmd r_RrFrtBrkSlipDiffHoldLim Rear / front brake slip differential hold limit. Defined as the base calibration multiplied by one or more factors that are functions of, for example, a_LatCmd r_RrBrkSlipAlwdLim permissible rear wheel longitudinal slip. Is defined as a base calibration multiplied by one or more factors that are functions of, for example, a_LatCmd r_RrFrtBrkSlipDiffAlwdLim permissible rear / front longitudinal slip difference. Is defined as a base calibration multiplied by one or more factors that are functions of, for example, a_LatCmd r_RrBrkSlipErr rear brake slip error b_RegHold logical state that r_RrBrkSlip or r_RrBrkSlip-r_FrtBrkSlip exceeds a hold level (downward) b_RegDsbl logical state that r_RrBrkSlip or r_RrBrkSlip-r_FrtBrkSlip exceeds a permissible degree (downward) M_RegMaxAlldw maximum permissible regenerative torque. A function of the required lateral acceleration M_DrvRegLim limited due to the stop condition M_DrvRegReq M_RegRegLim final limited regeneration request before reduction, with increase of mechanical braking after the holding condition M_DrvRegReqHold M_DrvRegReq, sampled and stored in the b_RegHold condition M_RrMechBrk mechanical rear wheel braking torque = K*brake pressure, where K depends on the rear wheel brake cylinder diameter, the effective rear wheel braking radius, and the friction coefficient of the rear wheel brake pad M_RrMechBrkHold M_RrMechBrk, sampled at the b_RegHold condition M_RrMechBrkInc maximum increase of the mechanical rear braking torque since the b_RegHold condition

[0027] In the following description of the regenerative braking algorithm of the present disclosure, the brake slip and regenerative braking torque are positive during braking. This may not be the case in the actual implementation and should not limit this application.

[0028] The maximum allowable regenerative torque (M_RegMaxAlldw, 102) is set as a function of the requested lateral acceleration (a_latCmd, 101). The rate of change is limited separately for increasing (103a) and decreasing (103b) (calibration) to provide the final M_RegMaxAlldw (103c). This maximum allowable torque is the limit when no rear brake instability has been detected.

[0029] The brake slip of the front and rear wheels is continuously monitored to determine the maximum (i.e., the larger value) of the left and right front brake slip (r_FrtBrkSlip, 105a) and the maximum of the left and right rear brake slip (r_RrBrkSlip, 105b).

[0030] If r_RrBrkSlip is higher than r_RrBrkSlipHoldLim (106b) or r_RrBrkSlip-r_FrtBrkSlip is higher than RfrtrBrkSlipDiffHoldLim (106c), then b_RegHold is set = true, b_RegHold is locked for the active braking cycle (109a), i.e., until the brake pedal is released and an accelerator pedal is applied (removing the coast-to-deceleration braking request).

[0031] The driver requested regenerative torque (113) may not be increased if b_RegHold is true, which provides a limited driver requested regenerative braking torque (M_Drv_RegLim, 106a).

[0032] M_Drv_RegLim is still limited by M_RegMaxAlldw (111).

[0033] If regeneration is not disabled by b_RegDsbl (109b) as described below, M_Drv_RegLim is then rate limited, providing M_RegReqLim (114).

[0034] The allowable rate of change for the increasing torque (104) is a function of the brake pedal rate. A low pedal rate → low rate of change of the regenerative braking torque.

[0035] If regeneration is not disabled, the allowable decreasing torque rate is a constant high negative rate (calibration).

[0036] The mechanical torque of the rear braking torque (M_RrMechBrk, 107d) is continuously calculated from the brake pressure (P_Brk_Press).

[0037] When the hold condition b_RegHold becomes true, M_RrMechBrk is sampled in M_RrMechBrkHold (107c).

[0038] If the driver now brakes harder, M_RrMechBrk increases after b_RegHold. The increase in mechanical braking M_RrMechBrklnc (107b) is tracked according to: ((n) is used to indicate the sample number n) M_RrMechBrklnc (n) = max (M_RrMechBrkInc (n-1), M_RrMechBrk (n) - M_RrMechBrkHold).

[0039] To avoid even higher rear brake slip, the regenerative braking torque M_ReqReqLim is then decreased (but cannot become negative) with the increase of M_RrMechBrk, providing the final limited regeneration request M_RegReq (115): M_ReqReq=max(0, M_DrvRegReqLim−M_RrMechBrkInc).

[0040] Normally this limits the rear brake slip well, but if r_RfBrkSlip exceeds r_RrBrkSlipAlwdLim or if r_RrBrkSlip - r_FrtBrkSlip exceeds r - RFrtrBrkSlipDiffAlwd, a regeneration diable condition b_RegDsbl (108a) is set, B_RegDsbl is locked for the active braking cycle, ie until the brake pedal is released and some throttle is applied (removing the coast-down braking request) (109b).

[0041] At b_RegDsbl, regenerative braking is canceled by outputting the M_RegReqLim (114) at a rate defined by the calibration and with a scaling factor that is a function of the rear brake slip error (r_RrBrkSlipErr, 110a): r_RrBrkSlipErr=max(r_RrBrkSlip−r_RrBrkSlipHold,r_RrBrkSlip−r_FrtBrkSlip−r_RFrtrBrkSlipDiffHold). A high r_RrBrkSlipErr means that r_RrBrkSlip or r_RrBrkSlip - r_FrtBrkSlip exceeds its holding level by a significant margin. A low r_RrBrkSlipErr means that M_RegReq can be increased slowly for better comfort. A high r_RrBrkSlipErr is required for a rapid increase in braking stability.

[0042] Fig. Figure 2 shows a summary table of test results using the system and algorithm of the present disclosure for regenerative braking during a system straight-ahead coasting condition, and with Rblim off. In the following discussion, the acronym "Rblim" represents the use of the algorithm (i.e., methodology) of the present disclosure for limiting braking torque. Fig.Figures 3A to 3C are graphs illustrating the front and rear torque (3A), slip angle (3B), and forward speed relative to the accelerator pedal position (AccelPdlPosn) (3C) when a hybrid vehicle traveling straight ahead on a low-µ surface is decelerating with an electric idle torque of 750 Nm, with Rblim off. The vehicle slip angle indicates whether the vehicle is rotating during this test. Fig.Figures 4A to 4C illustrate graphs of the same performance parameters, but with Rblim on, a maximum body slip angle of less than 1 degree, and the vehicle on a low-p (0.4) surface. In this example, the vehicle remains stable. Note that the body slip angle is defined as the angle between the vehicle velocity vector and the forward direction of the vehicle body. For straight-line travel, the vehicle velocity must be in the forward direction (i.e., the slip angle is close to 0).

[0043] Fig. Figure 5 shows a table of test results for regenerative idle braking during cornering. Fig. 6A and 6C respectively show the rear torque and the front torque, each plotted against time ( Fig. 6A), where the slip angle and StW_Angl are each plotted against time ( Fig.6B) and VelocityForward and AccelPdPosn are both plotted against time ( Fig. 6C), with an electric idle torque of 750 Nm in a curve starting at 40 km / h, with Rblim. The body slip angle indicates that the vehicle is rotating during this test.

[0044] The Fig. 7A to 7C illustrate the performance change from the graphs of the Fig. 6A to 6C when Rblim is activated. The vehicle remains stable with a body slip angle of less than 5 degrees.

Claims

[1] A method (100) for controlling application of regenerative braking torque to a plurality of wheels (30, 32, 62, 64) of at least one of a hybrid electric vehicle (10) or an electric vehicle to avoid braking instability, the method (100) comprising: Detecting an angle of a steering wheel of the vehicle (10); Detecting a speed of the vehicle (10); Detecting a brake pedal position when a driver of the vehicle (10) actuates a brake pedal of the vehicle (10); detecting wheel slip of each of a pair of front wheels (62, 64) of the vehicle (10); detecting wheel slip of each of a pair of rear wheels (30, 32) of the vehicle; Controlling the application of the regenerative braking torque based on the sensed wheel slip relative to at least one predetermined wheel slip limit; characterized by Determining a requested lateral acceleration representing a steady-state lateral acceleration that the vehicle (10) would achieve at an actual vehicle speed and with a currently sensed steering wheel angle; and wherein the at least one predetermined wheel slip limit is determined at least partially based on the determined required lateral acceleration, wherein a rate of change of a maximum allowable regenerative braking torque during states of decreasing and increasing the regenerative braking torque is individually controlled such that: a first rate of change is permitted as the regenerative braking torque decreases, thus helping to avoid braking instability; and a second rate of change, which is smaller than the first rate of change, is permitted when the regenerative braking torque increases so as to improve the drivability of the vehicle (10). [2] The method (100) of claim 1, wherein: detecting a wheel slip of each of a pair of front wheels (62, 64) of the vehicle (10) comprises determining therefrom a minimum front wheel slip (r_FrtBrkSlip) for the two front wheels (62, 64); and detecting a wheel slip of each of a pair of rear wheels (30, 32) of the vehicle (10) comprises determining therefrom a maximum rear wheel slip (r_RrBrkSlip) for the two rear wheels (30, 32). [3] The method (100) of claim 2, wherein controlling the application of the regenerative braking torque comprises controlling the regenerative braking torque such that the regenerative braking torque is not allowed to increase in response to brake pedal movement, but is instead held constant in a holding condition when any of the following conditions occur: the maximum rear wheel slip (r_RrBrkSlip) exceeds a first predetermined limit (R_RrBrkSlipHoldLim); or the maximum rear wheel slip (r_RrBrkSlip) exceeds the minimum front wheel slip (r_FrtBrkSlip) by a second predetermined limit (R_RrFrtBrkSlipDiffHoldLim). [4] The method (100) of claim 3, further comprising disabling the application of regenerative braking when one of the following conditions exists: the maximum rear wheel slip (r_RrBrkSlip) exceeds a third predetermined limit (R_RrBrkSlipAlwdLim); or the maximum rear wheel slip (r_RrBrkSlip) exceeds the minimum front wheel slip (r_FrtBrkSlip) by a fourth predetermined limit (R_RrFrtBrkSlipDiffAlwdLim). [5] The method (100) of claim 3, wherein the first and second predetermined limits (R_RrBrkSlipHoldLim and R_RrFrtBrkSlipDiffHoldLim) are each a function of the requested lateral acceleration and are set with a goal of improving the braking stability of the vehicle (10) while the vehicle (10) is cornering. [6] The method (100) of claim 4, wherein the third and fourth predetermined limits (r_RrBrkSlipAlwdLim and r_RrFrtBrkSlipDiffAlwdLim) are each a function of the requested lateral acceleration and are set with an objective of improving braking stability during cornering. [7] The method (100) of claim 3, wherein the holding condition is locked until the driver of the vehicle (10) releases pressure from the brake pedal and applies pressure to an accelerator pedal of the vehicle (10). [8] The method (100) of claim 4, wherein the disabling of the application of regenerative braking is maintained until the driver of the vehicle (10) releases pressure from the brake pedal and applies pressure to an accelerator pedal of the vehicle (10). [9] A method (100) for controlling application of regenerative braking torque to a plurality of wheels (30, 32, 62, 64) of at least one of a hybrid electric vehicle (10) or an electric vehicle to avoid braking instability, the method (100) comprising: Detecting a brake pedal position when a driver of the vehicle (10) actuates a brake pedal of the vehicle (10); Determining a requested lateral acceleration representing a steady-state lateral acceleration that the vehicle (10) would achieve at an actual vehicle speed and with a currently sensed steering wheel angle; Detecting a wheel slip of each of a pair of front wheels (62, 64) of the vehicle (10) and determining therefrom a minimum front wheel slip for the two front wheels (62, 64) (r_FrtBrkSlip); Detecting a wheel slip of each of a pair of rear wheels (30, 32) of the vehicle (10) and determining therefrom a maximum rear wheel slip for the two rear wheels (30, 32) (r_RrBrkSlip); Controlling the application of regenerative braking torque such that the regenerative braking torque cannot be increased in response to brake pedal movement, but is instead held constant in a holding condition when any of the following conditions occur: the maximum rear wheel slip (r_RrBrkSlip) exceeds a first predetermined limit (R_RrBrkSlipHoldLim); or the maximum rear wheel slip (r_RrBrkSlip) exceeds the minimum front wheel slip (r_FrtBrkSlip) by a second predetermined limit (R_RrFrtBrkSlipDiffHoldLim); and wherein the first and second predetermined limits are determined based at least in part on the determined requested lateral acceleration. [10] The method (100) of claim 9, further comprising disabling the application of regenerative braking when one of the following conditions exists: the maximum rear wheel slip (r_RrBrkSlip) exceeds a third predetermined limit (R_RrBrkSlipAlwdLim); or the maximum rear wheel slip (r_RrBrkSlip) exceeds the minimum front wheel slip (r_FrtBrkSlip) by a fourth predetermined limit (R_RrFrtBrkSlipDiffAlwdLim). [11] The method (100) of claim 9, wherein when the regenerative braking torque is kept constant in the holding condition, the regenerative braking torque is further kept constant until a condition is detected in which the operator has released the brake pedal and has started to modulate an accelerator pedal of the vehicle (10). [12] The method (100) of claim 10, wherein, if the application of regenerative braking torque has been disabled, the application of regenerative braking torque remains disabled until a condition is detected in which the operator has released the brake pedal and has begun to modulate an accelerator pedal of the vehicle (10). [13] The method (100) of claim 10, wherein the third predetermined limit (R_RrBrkSlipAlwdLim) and the fourth predetermined limit (R_RrFrtBrkSlipDiffAlwdLim) are each a function of the requested lateral acceleration to improve braking stability during cornering. [14] The method (100) of claim 9, wherein a rate at which regenerative braking is deactivated is a function of a particular degree of braking instability. [15] The method (100) of claim 9, further comprising controlling the application of the regenerative braking torque such that an allowable level of the regenerative braking torque is decreased with an increase in the mechanical braking torque provided by the vehicle (10) occurring after the stop condition. [16] A system for controlling application of regenerative braking torque to a plurality of wheels (30, 32, 62, 64) of at least one of a plurality of a hybrid electric vehicle (10) or an electric vehicle to avoid braking instability, the system comprising: a steering control sensor (48) adapted to detect an angle of a steering wheel of the vehicle (10); a sensor for detecting a speed of the vehicle (10); a brake controller (38) adapted to detect a brake pedal rate when a driver of the vehicle (10) operates a brake pedal of the vehicle (10); an inertial measurement system (46) adapted to detect wheel slip of each of a pair of front wheels (62, 64) of the vehicle (10) and to detect wheel slip of each of a pair of rear wheels (30, 32) of the vehicle (10); characterized by a processor-based hybrid control system (44) adapted to: Determining a requested lateral acceleration representing a steady-state lateral acceleration that the vehicle (10) would achieve at an actual vehicle speed and with a currently sensed steering wheel angle; Controlling the application of the regenerative braking torque based on the sensed wheel slip relative to at least one predetermined wheel slip limit; and wherein the at least one predetermined wheel slip limit is determined based at least in part on the determined requested lateral acceleration; wherein the processor-based hybrid control system (44) is further configured to detect wheel slip by: Analyzing a wheel slip of each of a pair of front wheels (62, 64) of the vehicle (10) and determining therefrom a minimum front wheel slip for the two front wheels (62, 64) (r_FrtBrkSlip); and Analyzing a wheel slip of each of a pair of rear wheels (30, 32) of the vehicle (10) comprises determining a maximum rear wheel slip for the two rear wheels (30, 32) therefrom (r_RrBrkSlip). [17] The system of claim 16, wherein the hybrid control system (44) is further configured to control the application of regenerative braking torque such that the regenerative braking torque cannot be increased in response to brake pedal movement, but is instead held constant in a holding condition when any of the following conditions occurs: the maximum rear wheel slip (r_RrBrkSlip) exceeds a first predetermined limit (R_RrBrkSlipHoldLim); or the maximum rear wheel slip (r_RrBrkSlip) exceeds the minimum front wheel slip (r_FrtBrkSlip) by a second predetermined limit (R_RrFrtBrkSlipDiffHoldLim); and wherein the first and second predetermined limits are both determined based at least in part on the determined requested lateral acceleration. [18] The system of claim 16, wherein the hybrid control system (44) is further configured to disable regenerative braking when one of the following conditions exists: the maximum rear wheel slip (r_RrBrkSlip) exceeds a third predetermined limit (R_RrBrkSlipAlwdLim); or the maximum rear wheel slip (r_RrBrkSlip) exceeds the minimum front wheel slip (r_FrtBrkSlip) by a fourth predetermined limit (R_RrFrtBrkSlipDiffAlwdLim); and wherein the third and fourth predetermined limits are both determined based at least in part on the determined required lateral acceleration.

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