Abstract:
A method for controlling a hybrid vehicle having a traction motor and a torque converter between an engine and a step ratio automatic transmission during a traction control event. The method includes reducing motor torque, and subsequently controlling the torque converter or the step ratio automatic transmission while maintaining engine torque constant during a wheel slip condition of the traction control event to lower driving force transmitted from a driving wheel to a road surface.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part of U.S. application Ser. No. 13/465,407 filed May 7, 2012, which is incorporated by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to a traction control system for a hybrid vehicle. 
       BACKGROUND 
       [0003]    A hybrid vehicle powertrain includes an engine and an electric motor. Torque, which is produced by the engine and/or by the motor, may be transferred to the vehicle drive wheels through a transmission. A fraction battery connected to the motor supplies energy to the motor for the motor to produce motor torque. The motor may provide a negative motor torque to the transmission (for example, during regenerative braking) Under such conditions, the motor acts as a generator to the battery. 
         [0004]    A hybrid vehicle may have a parallel configuration, a series configuration, or a combination thereof. In a parallel configuration (i.e., a modular hybrid transmission (“MHT”) configuration), the engine is connectable to the motor by a disconnect clutch and the motor is connected to the transmission. The motor may be connected to the transmission via a torque converter having a torque converter clutch. The engine, the disconnect clutch, the motor, the torque converter, and the transmission are connected sequentially in series. 
       SUMMARY 
       [0005]    Embodiments of the present invention are directed to a controller and a control strategy for a hybrid electric vehicle having an engine, an electric motor, a torque converter with a torque converter clutch, and a transmission. The controller and the control strategy control the motor to lower a driving force transmitted from one or more driving wheels to a road surface during a traction control event. The driving force may be lowered by reducing the torque of the motor in response to a traction control event. Further, if there is any electric motor limitation due to battery state of charge (SOC), the torque converter torque ratio and/or gear ratio can be controlled to support the torque reduction request for the traction event, while maintaining the engine torque at the current level. Such a control strategy can be executed to reduce and/or minimize the engine torque disruption to improve overall vehicle drivability and fuel economy. 
         [0006]    Advantageously, the controller and the control strategy can be utilized as a traction control mechanism. Typically, a traction control event occurs when the available traction force is suddenly reduced due to a change of the friction coefficient between the driving wheels and the road, resulting in excessive wheel slip. According to a conventional system, the vehicle quickly reduces the engine torque, and under certain circumstances, the vehicle additionally applies brake torque, to reduce the wheel speed to regain the appropriate traction force. Once the wheel speed slows down to regain sufficient traction force and the tire/road friction returns to normal, the engine torque can be increased to the driver demand level to resume normal driving. 
         [0007]    Certain disadvantages may be encountered by quickly reducing engine torque. This quick reduction is usually accomplished by utilizing a spark retard. The spark retard process negatively impacts fuel economy and emission, and may destabilize the combustion process. Alternatively, an air/fuel path can be utilized to reduce the engine torque. However, this process is slower and it also takes a relatively long time to raise the engine torque back up to meet the driver demand after the traction control event concludes. 
         [0008]    In contrast to the typical operation occurring as a result of quickly reducing engine torque for traction control, a controller and the control strategy in accordance with embodiments of the present invention maintain the engine torque at a substantially constant torque while using the electrical motor to convert a portion of the torque output from an engine into current to charge a battery in response to a traction control event. This is an option because traction control events are typically short-lived, and therefore, the system can go into a battery charging mode that it would not otherwise be operating in. As a result of substantially maintaining engine torque, a reduction in fuel emissions can be realized. Also, charging the battery by using the motor improves fuel economy. Further, the operation of the controller and the control strategy may reduce driveline disturbances during the traction control event. For instance, better quality torque control is achieved during the traction control event by virtue of the faster response characteristics of the electric machine, thereby improving performance while entering and exiting a traction control event, and during the traction control event. 
         [0009]    In at least one embodiment, brake torque applied as the result of fraction control can also come from regenerative braking Further, the controller and the control strategy of embodiments of the present invention can be used in addition to conventional engine and/or braking systems for traction control. 
         [0010]    In one embodiment, a method for controlling a hybrid vehicle having a traction motor and a torque converter between an engine and a step ratio automatic transmission during a traction control event is disclosed. The method includes reducing motor torque, and subsequently controlling the torque converter or the step ratio automatic transmission while maintaining engine torque constant during a wheel slip condition of the traction control event to lower driving force transmitted from a driving wheel to a road surface. The controlling step may be initiated upon receiving a signal that the motor has reached a motor torque reduction limit. The motor torque reduction limit may be the battery state of charge (SOC) top limit. The motor torque reduction limit may be battery charging availability diminishment. The controlling step may include controlling the torque converter. The controlling step may include controlling the torque ratio of the torque converter by modulating the torque converter to produce a variable magnitude of slip. The controlling step may include controlling the step ratio automatic transmission. The controlling step includes controlling the gear ratio of the step ratio automatic transmission. 
         [0011]    In another embodiment, a system for controlling a hybrid electric vehicle having a traction motor and a torque converter between an engine and a transmission is disclosed. The system includes a controller configured to enter a traction control event, and lower a driving force transmitted from a driving wheel to a road surface by reducing traction motor torque and subsequently controlling the torque converter or the transmission before reducing engine torque during a wheel slip condition of the traction control event. The controller may be further configured to initiate the controlling step upon receiving a signal that the motor has reached a motor torque reduction limit. The motor torque reduction limit may be the battery state of charge (SOC) top limit. The motor torque reduction limit may be battery charging availability diminishment. The controller may be further configured to control the torque converter during the traction control event. The controller may be further configured to control the transmission during the traction control event. 
         [0012]    In yet another embodiment, a hybrid electric vehicle including an engine, an electric traction motor selectively coupled to the engine by a clutch, a torque converter, a transmission, and a controller is disclosed. The controller is configured to reduce motor torque and subsequently controlling the transmission or the torque converter while maintaining engine torque constant during a wheel slip condition of a fraction control event. The controller may be further configured to initiate the controlling step upon receiving a signal that the motor has reached a motor torque reduction limit. The motor torque reduction limit may be a battery state of charge (SOC) top limit. The motor torque reduction limit may be a battery charging availability diminishment. The controller may be further configured to control the torque converter during the traction control event. The controller may be further configured to control the transmission during the traction control event. 
         [0013]    Additional objects, features, and advantages of embodiments of the present invention will become more readily apparent from the following detailed description when taken in conjunction with the drawings, wherein like reference numerals refer to corresponding parts. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  illustrates a block diagram of an exemplary hybrid vehicle powertrain in accordance with an embodiment of the present invention; and 
           [0015]      FIG. 2  illustrates a flowchart describing operation of a control strategy for controlling the motor to lower a driving force transmitted for the driving wheels to a road surface with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    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. 
         [0017]    Referring now to  FIG. 1 , a block diagram of an exemplary powertrain system  100  for a hybrid electric vehicle in accordance with one or more embodiments is shown. Powertrain system  100  includes an engine  102 , an electric machine such as an electric motor and generator  104  (otherwise referred to as a “motor”), a traction battery  106 , a disconnect clutch  108 , a torque converter  110 , and a multiple-ratio automatic transmission  112 . 
         [0018]    Engine  102  and motor  104  are drive sources for the vehicle. Engine  102  is connectable to motor  104  through a disconnect clutch  108  whereby engine  102  and motor  104  are connected in series. Motor  104  is connected to torque converter  110 . Torque converter  110  is connected to engine  102  via motor  104  when engine  102  is connected to motor  104  via disconnect clutch  108 . Transmission  112  is connected to the drive wheels  114  of the vehicle. The driving force applied from engine  102  and/or motor  104  is transmitted through torque converter  110  and transmission  112  to drive wheels  114  thereby propelling the vehicle. 
         [0019]    Torque converter  110  includes an impeller rotor fixed to output shaft  116  of motor  104  and a turbine rotor fixed to the input shaft  118  of transmission  112 . The turbine of torque converter  110  can be driven hydro-dynamically by the impeller of torque converter  110 . Thus, torque converter  110  may provide a “hydraulic coupling” between output shaft  116  of motor  104  and the input shaft  118  of transmission  112 . 
         [0020]    Torque converter  110  further includes a torque converter clutch (e.g., a bypass clutch). The torque converter clutch is controllable across a range between an engaged position (e.g., a lock-up position, an applied position, etc.) and a disengaged position (e.g. an unlocked position, etc.). In the engaged position, the converter clutch mechanically connects the impeller and the turbine of torque converter  110  thereby substantially discounting the hydraulic coupling between these components. In the disengaged position, the converter clutch permits the hydraulic coupling between the impeller and the turbine of torque converter  110 . 
         [0021]    When the torque converter clutch is disengaged, the hydraulic coupling between the impeller and the turbine of torque converter  110  absorbs and attenuates unacceptable vibrations and other disturbances in the powertrain. The source of such disturbances includes the engine torque applied from engine  102  for propelling the vehicle. However, fuel economy of the vehicle is reduced when the converter clutch is disengaged. Thus, it is desired that the converter clutch be engaged when possible. 
         [0022]    The torque converter clutch may be controlled through operation of a clutch valve. In response to a control signal, clutch valve pressurizes and vents the converter clutch to engage and disengage. The operation of torque converter  110  can be controlled such that converter clutch is neither fully engaged nor fully disengaged and instead is modulated to produce a variable magnitude of slip in torque converter  110 . The slip of torque converter  110  corresponds to the difference in the speeds of the impeller and the turbine of torque converter  110 . The slip of torque converter  110  approaches zero as converter clutch  110  approaches the fully engaged position. Conversely, the magnitude of the slip of torque converter  110  becomes larger as the converter clutch moves toward the disengaged position. 
         [0023]    When operated to produce a variable magnitude of slip, torque converter  110  can be used to absorb vibrations (for example, when gear ratio changes are being made, when the driver releases pressure from the accelerator pedal, etc.) by increasing the slip, thus causing a greater portion of the engine torque to be passed from the impeller to the turbine of torque converter  110  through hydro-dynamic action. When chance of objectionable vibration and disturbance is absent, the converter clutch can be more fully engaged so that fuel economy is enhanced. However, again, as noted above, it is desired that the converter clutch be engaged when possible as the fuel economy of the vehicle is increased when the converter clutch is engaged. 
         [0024]    As indicated above, engine  102  is connectable to motor  104  through disconnect clutch  108 . In particular, engine  102  has an engine shaft  122  connectable to an input shaft  124  of motor  104  through disconnect clutch  108 . As further indicated above, output shaft  116  of motor  104  is connected to the impeller of torque converter  110 . The turbine of torque converter  110  is connected to the input shaft of transmission  112 . 
         [0025]    Transmission  112  includes multiple gear ratios. Transmission  112  includes an output shaft  126  that is connected to a differential  128 . Drive wheels  114  are connected to differential  128  through respective axles  130 . With this arrangement, transmission  112  transmits a powertrain output torque  132  to drive wheels  114 . 
         [0026]    Engine  102  is a primary source of power for powertrain system  100 . Engine  102  is an internal combustion engine such as a gasoline, diesel, or natural gas powered engine. Engine  102  generates an engine torque  134  that is supplied to motor  104  when engine  102  and motor  104  are connected via disconnect clutch  108 . To drive the vehicle with engine  102 , at least a portion of engine torque  134  passes from engine  102  through disconnect clutch  108  to motor  104  and then from motor  104  through torque converter  110  to transmission  112 . 
         [0027]    Traction battery  106  is a secondary source of power for powertrain system  100 . Motor  104  is linked to battery  106  through wiring  136 . Depending on the particular operating mode of the vehicle, motor  104  either converts electric energy stored in battery  106  into a motor torque  138  or sends power to battery  106  through wiring  136 . To drive the vehicle with motor  104 , motor torque  138  is also sent through torque converter  110  to transmission  112 . When generating electrical power for storage in battery  106 , motor  104  obtains power either from engine  102  in a driving mode or from the inertia in the vehicle as motor  104  acts as a brake in what is referred to as a regenerative braking mode. 
         [0028]    As described, engine  102 , disconnect clutch  108 , motor  104 , torque converter  110 , and transmission  112  are connectable sequentially in series as illustrated in  FIG. 1 . As such, powertrain system  100  represents a parallel or modular hybrid transmission (“MHT”) configuration in which engine  102  is connected to motor  104  by disconnect clutch  108  with motor  104  being connected to transmission  112  through torque converter  110 . 
         [0029]    Depending on whether disconnect clutch  108  is engaged or disengaged determines which input torques  134  and  138  are transferred to transmission  112 . For example, if disconnect clutch  108  is disengaged, then only motor torque  138  is supplied to transmission  112 . If disconnect clutch is engaged, then both engine torque  134  and motor torque  138  are supplied to transmission  112 . Of course, if only engine torque  134  is desired for transmission  112 , disconnect clutch  108  is engaged, but motor  104  is not energized such that engine torque  134  is only supplied to transmission  112 . 
         [0030]    Transmission  112  includes planetary gear sets (not shown) that are selectively placed in different gear ratios by selective engagement of friction elements (not shown) in order to establish the desired multiple drive ratios. The friction elements are controllable through a shift schedule that connects and disconnects certain elements of the planetary gear sets to control the ratio between the transmission output and the transmission input. Transmission  112  is automatically shifted from one ratio to another based on the needs of the vehicle. Transmission  112  then provides powertrain output torque  132  to output shaft  126  which ultimately drives drive wheels  114 . The kinetic details of transmission  112  can be established by a wide range of transmission arrangements. Transmission  112  is an example of a transmission arrangement for use with embodiments of the present invention. Any multiple ratio transmission that accepts input torque(s) from an engine and/or a motor and then provides torque to an output shaft at the different ratios is acceptable for use with embodiments of the present invention. 
         [0031]    Powertrain system  100  further includes a powertrain control unit  142 . Control unit  142  constitutes a vehicle system controller. Based on repositioning an accelerator pedal, the driver of the vehicle provides a total drive command when the driver wants to propel the vehicle. The more the driver depresses pedal, the more drive command is requested. Conversely, the less the driver depresses pedal, the less drive command is requested. When the driver releases the pedal, the vehicle begins to coast. 
         [0032]    Control unit  142  apportions the total drive command between an engine torque signal (which represents the amount of engine torque  134  to be provided from engine  102  to transmission  112 ) and a motor torque signal  146  (which represents the amount of motor torque  138  to be provided from motor  104  to transmission  112 ). In turn, engine  102  generates engine torque  134  and motor generates motor torque  138  for transmission  112  in order to propel the vehicle. Such engine torque  134  and motor torque  138  for propelling the vehicle are “positive” torques. However, both engine  102  and motor  104  may generate “negative” torques for transmission  112  in order to brake the vehicle. 
         [0033]    Control unit  142  is further configured to control clutch valve in order to control operation of the torque converter clutch of torque converter  110 . Control unit  142  controls the operation of torque converter  110  such that the converter clutch is modulated across a range between the engaged and disengaged positions to produce a variable magnitude of slip in torque converter  110 . Again, the slip of torque converter  110  corresponds to the difference between the input rotational speed and the output rotational speed of torque converter  110 . The output rotational speed approaches the input rotational speed as the converter clutch approaches the engaged position such that the slip is zero when the converter clutch is in the fully engaged position. Conversely, the output rotational speed lags the input rotational speed as the converter clutch approaches the disengaged position such that the magnitude of the slip becomes larger. A rotation sensor is configured to sense the slip of torque converter  110  and provide information indicative of the slip to control unit  142 . 
         [0034]    Referring now to  FIG. 2 , with continual reference to  FIG. 1 , a flowchart  200  describing operation of a control strategy for traction control in accordance with an embodiment of the present invention is shown. 
         [0035]    In block  202 , the vehicle is operating in a normal driving mode. In decision block  204 , the controller queries whether or not a traction control start has been requested. The traction control event can be detected by sensing an acceleration slip of one or more driving wheels above a certain value. The controller may recognize a wheel slip condition in one or more of the driving wheels. The traction control event may also be signaled by another module or software process on board the vehicle. If a traction control start is requested, then the control strategy proceeds to decision block  206 . If a traction control start is not requested, then the control strategy loops back to block  202 . 
         [0036]    In decision block  206 , the controller queries whether the powertrain system is in hybrid mode or EV mode. If the powertrain system is in EV mode, the control strategy proceeds to block  208 . If the powertrain system is in hybrid mode, the control strategy proceeds to block  210 . 
         [0037]    In block  208 , the electrical motor torque is reduced to make the total powertrain torque meet the traction control request. The traction control request is a request for reduced torque so that the wheel speed is reduced to eliminate the wheel slip condition and may initiate from another control module or software process on the vehicle. 
         [0038]    In decision block  212 , the controller queries whether or not a traction control end has been requested. The end of the traction control event occurs when the wheel speed has been reduced a sufficient amount to eliminate the wheel slip condition. If the traction control has ended, then the control strategy proceeds to block  208 . If the traction control has not ended, then the control strategy loops back to block  214 , and the reduction of the electrical motor torque continues until the traction control event ends. 
         [0039]    In block  210 , the engine torque is kept at substantially constant torque while the electrical motor torque is reduced to make the total powertrain torque meet the traction control request. The reduction in electrical motor torque can be carried out by applying a negative torque to the electrical motor. During such mode of operation, the electrical motor acts as a generator that converts a portion of the torque output by the engine into current stored by the battery. After block  210 , the control strategy proceeds to decision block  216 . 
         [0040]    In decision block  216 , the controller queries whether or not a traction control end has been requested. The end of the traction control event occurs when the wheel speed has been reduced a sufficient amount to eliminate the wheel slip condition. If the traction control has ended, then the control strategy proceeds to block  214 . If the traction control has not ended, then the control strategy proceeds to decision block  218 . 
         [0041]    In decision block  218 , the controller queries whether the battery state of charge is at a top limit or the battery charging availability is diminishing in a relatively short time period. If either of these conditions is present, then the control strategy proceeds to block  220 . If neither condition is present, then the control strategy loops back to block  210 . 
         [0042]    In block  222 , the torque ratio or gear ratio of the torque converter is controlled to make the total the total powertrain torque meet the traction control request. While performing this control step, the engine torque is kept at steady state and the motor torque is maintained. This control strategy reduces and/or minimizes engine torque disruption to improve overall vehicle drivability and fuel economy. 
         [0043]    In decision block  224 , the controller queries whether or not a traction control end has been requested. The end of the traction control event occurs when the wheel speed has been reduced a sufficient amount to eliminate the wheel slip condition. If the traction control has ended, then the control strategy proceeds to block  230 . If the traction control event has not ended, then the control strategy proceeds to block  226 . 
         [0044]    In block  226 , the engine torque is reduced through the air/fuel path to balance the negative electrical motor torque limitation due to the battery status discussed above. The use of this additional engine torque reduction mechanism allows the total powertrain torque to meet the traction control request. The engine torque may be set lower based on a negative torque limitation of the motor due to battery charging limits. After block  226 , the control strategy proceeds to decision block  228 . 
         [0045]    In decision block  228 , the controller queries whether or not a fraction control end has been requested. The end of the traction control event occurs when the wheel speed has been reduced a sufficient amount to eliminate the wheel slip condition. If the traction control has ended, then the control strategy proceeds to block  230 . If the traction control has not ended, then the control strategy proceeds to block  226 . 
         [0046]    In block  230 , the control strategy recognizes that the traction control event has ended. As such, the electrical motor torque is increased and/or the engine torque is increased through the air/fuel path. These increases are done to make the total powertrain torque meet the drive demand under normal operating conditions. 
         [0047]    As shown in block  232 , the contribution of the engine torque level and the motor torque level is optimized based on the total powertrain torque required. 
         [0048]    Moving back to block  214 , the electrical motor torque is increased to make the total powertrain torque meet the drive demand. This increase is done to make the total powertrain torque meet the drive demand under normal operating conditions. After block  214 , the control strategy proceeds to block  232 . 
         [0049]    In one or more embodiments, a traction control module or software process may transmit a torque request signal to a module or software process responsible for adjusting the motor and/or engine torque. 
         [0050]    While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the present 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 present invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the present invention.