Patent Publication Number: US-2007108838-A1

Title: Regenerative braking control system and method

Description:
TECHNICAL FIELD  
      The present invention relates generally to a system and method for operating a hybrid electric vehicle, and in particular to controlling regenerative braking for a hybrid electric vehicle.  
     BACKGROUND  
      Regenerative braking systems seek to recover the kinetic energy of a vehicle, which is normally dissipated as heat by conventional hydraulic friction braking systems. The recovery of the kinetic energy occurs during braking via an electric motor that operates as a generator to restore power to a battery or other energy storage device. As commonly known, vehicles equipped with regenerative braking systems may also have anti-lock braking systems (ABS) that improve vehicle control and stability in the event of wheel slip. However, when a wheel slip condition occurs, the anti-lock brake system customarily causes disengagement of the regenerative braking system. Consequently, the vehicle operator experiences a lunge forward feeling due to the instantaneous loss of braking torque and deceleration. This sudden loss of deceleration is undesirable to the vehicle operator.  
      Thus, the present invention was conceived in view of these and other disadvantages of regenerative braking systems.  
     SUMMARY  
      The present invention includes a system and method for controlling a regenerative braking system of a vehicle having multiple wheels. The method includes applying a regenerative braking torque to at least one wheel. The method includes determining whether a second wheel of a vehicle is experiencing an anti-lock braking system (ABS) event. The method also includes compensating, at a determined rate, the regenerative braking torque applied to the one wheel upon determining that the second wheel of the vehicle is experiencing the ABS event.  
      The system includes a vehicle having multiple wheels and a regenerative braking system. The vehicle is configured to apply regenerative braking torque to at least one wheel. The vehicle is configured to determine that at least a second wheel is experiencing an ABS event. Additionally, the vehicle is configured to compensate, at a predetermined rate, the regenerative braking torque applied to the one wheel upon determining that the second wheel of the vehicle is experiencing the ABS event.  
      The above embodiments and other embodiments, features and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The features of the present invention, which are believed to be novel, are set forth with particularity in the appended claims. The present invention, both as to its organization and manner of operation, together with further objectives and advantages thereof, may be best understood with reference to the following description, taken in connection with the accompanying drawings in which:  
       FIG. 1  illustrates a vehicle having a regenerative braking system according to an embodiment of the present invention; and  
       FIG. 2  illustrates a flow diagram for a method for controlling a regenerative braking system in accordance with an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION  
      By way of example, a system and method for implementing the present invention is described below. The system and methodology may be adapted, modified or rearranged to best fit a particular implementation without departing from the scope of the present invention.  
       FIG. 1  illustrates a schematic representation of a vehicle  10  in accordance with one embodiment of the present invention. The vehicle  10  includes an engine  12  and an electric machine, or generator  14 . The engine  12  and the generator  14  are connected through a power transfer unit, which in this embodiment is a planetary gear set  16 . Of course, other types of power transfer units, including other gear sets and transmissions, may be used to connect the engine  12  to the generator  14 . The planetary gear set includes a ring gear  18 , a carrier  20 , planet gears  22 , and a sun gear  24 .  
      The generator  14  can also be used as a motor, outputting torque to a shaft  26  connected to the sun gear  24 . Similarly, the engine  12  outputs torque to a shaft  28  connected to the carrier  20 .  
      A brake  30 , may be, but not necessarily provided for stopping rotation of the shaft  26 , thereby locking the sun gear  24  in place. Because this configuration allows torque to be transferred from the generator  14  to the engine  12 , a one-way clutch  32  may be provided so that the shaft  28  rotates in only one direction. Having the generator  14  operatively connected to the engine  12 , as shown in  FIG. 1 , allows the speed of the engine  12  to be controlled by the generator  14 . It is recognized that alternative embodiments may not include brake  30  and/or clutch  32 .  
      The ring gear  18  is connected to a shaft  34 , which is connected to rear vehicle drive wheels  36  through a second gear set  38 . Additionally, the vehicle  10  includes a set of front wheels  35  that may be directly coupled to engine  12 . The vehicle  10  includes a second electric machine, or motor  40 , which can be used to output torque to a shaft  42 . Other vehicles within the scope of the present invention may have different electric machine arrangements, such as more or less than two electric machines. In the embodiment shown in  FIG. 1 , the motor  40  and the generator  14  can both be used as motors to output torque. Alternatively, each can also be used as a generator, outputting electrical power to a high voltage bus  44  and to an energy storage device, or battery  46 .  
      The battery  46  is a high voltage battery that is capable of outputting electrical power to operate the motor  40  and the generator  14 . Other types of energy storage devices and/or output devices can be used with a vehicle, such as the vehicle  10 . For example, a device such as a capacitor can be used, which, like a high voltage battery, is capable of both storing and outputting electrical energy. Alternatively, a device such as a fuel cell may be used in conjunction with a battery and/or capacitor to provide electrical power for the vehicle  10 .  
      As shown in  FIG. 1 , the motor  40 , the generator  14 , the planetary gear set  16 , and a portion of the second gear set  38  may generally be referred to as a transaxle  48 . The transaxle  48  is analogous to a transmission in a conventional vehicle. Thus, when a driver selects a particular gear, the transaxle  48  is appropriately controlled to operate according to the gear selection. To control the engine  12  and the components of the transaxle  48 —e.g., the generator  14  and motor  40 —a control system, including a first controller  50 , is provided. As shown in  FIG. 1 , the controller  50  is a combination vehicle system controller and powertrain control module (VSC/PCM). Although it is shown as a single hardware device, it may include multiple controllers in the form of multiple hardware devices, or multiple software controllers within one or more hardware devices. The controller  50  logic, including logic associated with other controllers (e.g., TCM  56 ) may be partitioned in any number of ways without imposing any limitation on the claimed invention.  
      A controller area network (CAN)  52  allows the controller  50  to communicate with the transaxle  48  and a battery control module (BCM)  54 . Just as the battery  46  has the BCM  54 , other devices controlled by the controller  50  may have their own controllers. For example, an engine control unit (ECU) may communicate with the controller  50  and may perform control functions on the engine  12 . In addition, the transaxle  48  may include one or more controllers, such as a transaxle control module (TCM)  56 , configured to control specific components within the transaxle  48 , such as the generator  14  and/or the motor  40 . Accordingly, as shown in  FIG. 1 , the TCM  56  communicates with a generator inverter  45  and a motor inverter  41 . In one embodiment, the generator inverter  45  and the motor inverter  41  are coupled to a control module  47  and a control module  43 , respectively. Control modules  43  and  47  are capable of converting raw vehicle sensor data readings to a format compatible with the TCM  56  and sending those readings to the TCM  56 .  
      Although the vehicle  10 , shown in  FIG. 1 , is a HEV, it is understood that the present invention contemplates the use of other types of vehicles. In addition, although the vehicle  10  shown in  FIG. 1  is a parallel-series HEV, the present invention is not limited to HEV&#39;s having such a “powersplit” configuration. Furthermore, although the vehicle  10  is illustrated having a single motor (i.e., motor  40 ), other embodiments may include additional motors without departing from the scope of the present invention. Thus, the present invention is applicable to an alternative embodiment of vehicle  10  having a motor, such as motor  40 , coupled directly to a front axle (not shown) of front wheels  35 . Additionally, in alternative embodiments vehicle  10  may be a fuel-cell vehicle without departing from the scope of the present invention.  
      As shown, vehicle  10  further includes friction brakes  37 . Brakes  37  include a brake disc  37   a , a caliper  37   b , and a speed sensor  49  that communicates with an anti-lock braking system (ABS) module  39 . Caliper  37   b  is operable with brake disc  37   a  for slowing and/or stopping vehicle  12 . ABS module  39  is operable with a pressure adjustment unit  51 . In response to a brake request from a brake pedal  55 , pressure adjustment unit  51  is configured to enable proper distribution of braking fluid pressure to brakes  37  through the use of liquid pressure passages  53 . Although the embodiment shown in  FIG. 1  illustrates a braking system that utilizes hydraulics, it is recognized that the friction braking system of  FIG. 1  may be a pure brake-by-wire (BBW) system, an electro-mechanical braking system, an electro-hydraulic braking system, or a hydro-mechanical braking system without departing from the scope of the present invention. In either embodiment, ABS module  39  is operable with controller  50  and TCM  56  for monitoring and controlling the performance of the generator  14  and the motor  40 .  
      In the event wheels  35  enter ABS control via ABS module  39 , during active regenerative braking, the torque generated by the motor  40  and/or the generator  14  is compensated. Compensation of the braking torque occurs in a manner so as to minimize the driver&#39;s perception of loss of deceleration. In one embodiment, the torque generated by the motor  40  and the generator  14  is reduced in a controlled manner at a determined rate. Accordingly, the reduction in torque mitigates any “lunge forward” feeling experienced by vehicle occupants when wheels  35  enter ABS mode and regenerative braking is reduced. In one embodiment, the ABS module  39  and the speed sensor  49  detect a potentially locking wheel slip event experienced by the front wheels  35 . It is recognized that the term “wheel slip” herein refers to any condition which causes the engagement/activation of the ABS.  
      Upon detection of the wheel slip event (i.e., activation of ABS control) by front wheels  35 , the ABS module  39  generates a signal for the TCM  56  that indicates the occurrence of ABS activation. As such, the TCM  56  is configured to generate signals for a controlled reduction of regenerative braking torque being applied to rear wheels  36  by the motor  40  and the generator  14 . Thus, when the rear wheels  36  reach the road surface location where the front wheels  35  experienced the wheel slip condition, the torque has been reduced in a manner that mitigates the “lunge forward” feeling that is caused by conventional regenerative braking systems. Consequently, when the ABS system is activated for the rear wheels  36 , any subsequent reduction in regenerative applied torque is less noticeable to the vehicle occupants.  
      In the event the ABS system is not activated within a determined time period for the rear wheels  36 , the amount of regenerative braking torque allowed at the rear wheels  36  may be increased. Furthermore, in the event the vehicle  10  stops or begins accelerating, the TCM  56  generates signals for the generator  14  and the motor  50  to enable the application of an additional amount of torque to the rear wheels  36 . In one aspect of the present invention, the amount of added regenerative braking torque is equivalent to the original unreduced amount of regenerative braking torque.  
      Now, referring to  FIG. 2 , a flow diagram is shown that illustrates a method for controlling the application of regenerative braking torque generated by the generator  14  and the motor  50 . As described above, the torque generated by the generator  14  and/or the motor  50  provides motive force to the vehicle. Accordingly, block  70  is the entry point for the method. As depicted by block  71 , the method includes applying regenerative braking torque at a desired level. As described in the foregoing, the generator and/or motor of the vehicle are capable of providing regenerative braking torque. Block  72  depicts the determination of whether the ABS system has been engaged for the front wheels of the vehicle. If the ABS system has been activated, a timer is set as shown by block  73 . It is recognized that in some instances it is possible for the front wheels of the vehicle to experience an ABS event, while the rear wheels do not experience an ABS event. As such, the time for which the timer is set may be dependent upon the vehicle wheel base and speed of the vehicle. In an alternative embodiment, the time for which the timer is set may be a predetermined time period, including, but not limited to one minute or less. In yet another embodiment the predetermined time period may be greater than one minute.  
      As shown by block  74 , the method reduces the regenerative braking torque applied to the rear wheels at a determined rate. Block  76  depicts the determination of whether the ABS system has been activated at the rear wheels. If the ABS system has not been activated, block  80  occurs. At block  80 , the method determines whether the vehicle has received an acceleration command via the vehicle&#39;s accelerator pedal. If the vehicle has not received an acceleration command, block  82  occurs. At block  82 , the method determines whether the vehicle has stopped. If the vehicle has not stopped, step  84  occurs, wherein the method determines whether the timer originally set at block  73 , has expired. If the timer has expired, the method determines whether the regenerative braking torque is at a desired level as shown by block  86 . If the regenerative braking torque is not at a desired level, block  78  occurs. At block  78 , the regenerative braking torque is increased at a determined rate. At block  78 , the increase in regenerative braking torque occurs in a manner that is minimally noticeable, if not completely unnoticeable by vehicle occupants. If the regenerative braking torque is at a desired level, the method returns to block  71 . Referring back to block  80 , if the vehicle acceleration has been commanded, the method returns to block  71 . Referring to block  82 , if the vehicle has stopped, the method also returns to block  71 . Referring to block  84 , if the timer has not expired, the method returns to block  74 .  
      Now, referring back to block  72 , if the ABS system has not been activated at the front wheels, block  88  occurs. At block  88 , the method determines whether the ABS has been activated at the rear wheels. If the ABS system has not been activated at the rear wheels, the method returns to block  71 . In the event the ABS has been activated at the rear wheels, block  89  occurs. At block  89 , the timer may be set. In one aspect, if the timer was set at block  73  and has not yet expired, the timer may be re-initialized at block  89 . In one embodiment, the time for which the timer is set may be a predetermined time period, including, but not limited to one minute or less. In yet another embodiment the predetermined time period may be greater than one minute. At block  90 , the method reduces the regenerative braking torque applied to the rear wheels for ABS control. Additionally, referring to block  76 , if the ABS is active at the rear wheels, the method sets the timer as depicted by block  89 . Accordingly, block  90  occurs wherein the method reduces the regenerative braking on the rear wheels for ABS control. Block  92  depicts the performance of ABS control on the rear wheels. As shown by block  94 , the method determines whether the ABS event is over. If the ABS event has not ended, the method returns to block  92 . If the ABS event has ended, the method returns to block  80 . In alternative embodiments, the regenerative braking torque may be increased (e.g., block  78 ) without waiting for the timer to expire in the event the rear wheels have experienced an ABS event that has ended (as determined at block  94 ).  
      While the best mode for carrying out the invention has been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention as defined by the following claims.