Abstract:
In a powertrain that includes wheels for driving a vehicle, an engine including a crankshaft, a machine driveably connected to the crankshaft and able to operate alternately as an electric motor and electric generator, a transmission including an input clutch driveably connected to the crankshaft and an output driveably connected to at least two of the wheels, and an electric storage battery having a variable state of charge and electrically connected to the machine, a method for controlling vehicle creep including adjusting a torque capacity of the input clutch to a desired magnitude of input clutch torque transmitted to the wheels, determining a desired change in torque produced by the machine such that a speed of the crankshaft is controlled to a desired idle speed, using the magnitude of input clutch torque capacity and the desired change in torque produced by the machine to determine a desired magnitude machine torque, and using the machine to produce said desired magnitude of machine torque.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates generally to an apparatus and method for controlling vehicle creep in a hybrid electric vehicle (HEV). 
         [0003]    2. Description of the Prior Art 
         [0004]    A powershift transmission is a geared mechanism that includes no torque converter, but instead employs two input clutches driveably connected to an engine crankshaft. The transmission produces multiple gear ratios in forward and reverse drive and transmits power continuously using synchronized clutch-to-clutch shifts. 
         [0005]    The transmission incorporates gearing arranged in a dual layshaft configuration between the transmission input and its output. One input clutch transmits torque between the input and a first layshaft associated with even-numbered gears; the other input clutch transmits torque between the transmission input and a second layshaft associated with odd-numbered gears. The transmission produces gear ratio changes by alternately engaging a first input clutch and running in a current gear, disengaging the second input clutch, preparing a power path in the transmission for operation in the target gear, disengaging the first clutch, engaging the second clutch and preparing another power path in the transmission for operation in the next gear. 
         [0006]    In a vehicle powertrain that provides a continuous drive connection between a power source and the vehicle wheels, creep is the tendency for a vehicle, when at a stand-still or relatively low speed, to move slowly in either a forward or reverse direction, depending on the position of the gear selector, when neither the accelerator pedal nor brake pedal is depressed. Generally, a vehicle operator expects the vehicle to creep (1) from a standstill as the driver releases the brake pedal with no accelerator pedal input, and (2) from a coast down condition as the vehicle decelerates to a lower speed with little or no wheel brake or accelerator pedal input. 
         [0007]    The vehicle will creep to a unique terminal speed, i.e., a relatively low speed, for a given road load and vehicle load. The creep speed declines with increasing road grade until it reaches zero and vehicle roll-back becomes imminent. The engine should never stall during a vehicle creep condition. The vehicle creep speed is a defined and specified vehicle requirement. 
         [0008]    For a vehicle with a conventional automatic transmission, vehicle creep is automatically provided as a result of the torque transfer provided through the torque converter&#39;s fluid coupling. In a vehicle having a powershift transmission, the vehicle creep torque is provided by controlling the clutch torque capacity while slipping the clutch in order to prevent engine stall. 
         [0009]    During a vehicle creep condition, the transmission clutch torque capacity is further increased as the driver releases the brake pedal. The increase in clutch torque capacity disturbs the control of the engine idle speed because the increase in clutch torque capacity loads the engine. Therefore, engine idle speed control must be coordinated with any increase in the clutch torque capacity in order to avoid poor engine idle speed control due to delayed engine torque response due to manifold filling as clutch torque capacity is increased, and potential engine stall if too much clutch torque capacity is provided while the engine torque has not increased accordingly. 
         [0010]    Unlike a conventional vehicle powertrain having a powershift transmission, a hybrid electric vehicle with a powershift transmission includes multiple propulsion paths and “active” torque actuators, i.e., an engine and electric machines, which can be used during a vehicle creep condition. Therefore, a more sophisticated vehicle creep control system is needed to deal with the complexities and added powertrain operating modes of a HEV. 
       SUMMARY OF THE INVENTION 
       [0011]    In a powertrain that includes wheels for driving a vehicle, an engine including a crankshaft, a first electric machine driveably connected to the crankshaft and able to operate alternately as an electric motor and electric generator, a second electric machine driveably connected to at least two of the wheels, a transmission including an input clutch driveably connected to the crankshaft and an output driveably connected to at least two of the wheels, and an electric storage battery having a variable state of charge and electrically connected to both electric machines, a method for controlling vehicle creep including adjusting the torque produced the second electric machine to provide vehicle creep torque to the wheels, adjusting a torque capacity of the input clutch to a desired magnitude of input clutch torque transmitted to the wheels, determining a desired change in torque produced by the first machine such that a speed of the crankshaft is controlled to a desired idle speed, using the magnitude of input clutch torque capacity and the desired change in torque produced by the first machine to determine a desired magnitude machine torque, and using the first machine to produce said desired magnitude of machine torque. 
         [0012]    An advantage of the vehicle creep control system is its use of the additional propulsion paths and power sources to improve vehicle creep performance and account for the problems and deficiencies found in a conventional vehicle with a powershift transmission. The control strategy supports multiple HEV powertrain operating modes during vehicle creep conditions by blending torque produced by multiple power sources which are transmitted through multiple propulsion paths during vehicle creep conditions. 
         [0013]    The vehicle creep control enhances powershift transmission control by providing coordinated clutch torque capacity control when vehicle creep is provided or assisted by the additional electric machines. The control is robust and provides responsive engine idle speed control due to electric machine responsiveness when controlling clutch torque capacity during vehicle creep. It automatically operates similar to a conventional vehicle with a powershift transmission when the electric power sources are not used. It is applicable to any HEV powertrain that includes either a powershift transmission having wet or dry input clutches, or an automatic transmission having no torque converter. 
         [0014]    The scope of applicability of the preferred embodiment will become apparent from the following detailed description, claims and drawings. It should be understood, that the description and specific examples, although indicating preferred embodiments of the invention, are given by way of illustration only. Various changes and modifications to the described embodiments and examples will become apparent to those skilled in the art. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0015]    The invention will be more readily understood by reference to the following description, taken with the accompanying drawings, in which: 
           [0016]      FIG. 1  is a schematic diagram showing an automotive vehicle powertrain of a hybrid electric vehicle that includes a powershift transmission; 
           [0017]      FIG. 2  is a schematic diagram showing propulsion and power flow of the vehicle powertrain of  FIG. 1 ; 
           [0018]      FIG. 3  is a schematic diagram of a vehicle creep controller; 
           [0019]      FIGS. 4A-4G  are graphs of various powertrain and vehicle parameters before, during and following a vehicle creep condition in which torque blending is not used; 
           [0020]      FIGS. 5A-5G  are graphs of various powertrain and vehicle parameters before, during and following a vehicle creep condition in which torque blending occurs; and 
           [0021]      FIG. 6  is a schematic diagram showing details of a powershift transmission. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0022]    Referring first to  FIGS. 1 and 2 , the powertrain  10  configuration includes a first power source such as an internal combustion engine  12 , a diesel engine or a gasoline engine; a power transmission  14  driveably for producing multiple forward and reverse gear ratios, such as a wet-clutch powershift transmission; an electric machine  16  driveably connected to the engine crankshaft and transmission input  18 , such as a crankshaft integrated starter/generator (CISG) for providing starter/generator capability; and an additional electric machine  20  driveably connected to the rear axles  22 , 23 , such as a electric rear axle drive (ERAD), for providing additional propulsion capability in either an electric drive or hybrid drive mode. The transmission output  24  is connected through a final drive unit and differential mechanism  26  to the front axles  28 ,  30 , which drive the front wheels  32 ,  33 , respectively. ERAD  20  drives the rear wheels  34 ,  35  through ERAD gearing  48 , a differential mechanism  36 , rear axles  22 ,  23  and wheels  34 ,  35 . 
         [0023]    An electronic engine control module (ECM)  24  controls operation of engine  12 . An electronic transmission control module (TCM)  26  controls operation of transmission  14  and the input clutches  38 ,  39 . An integrated starter controller (ISC)  40  controls operation of CISG  16 , ERAD  20  and the system for charging an electric storage battery  42 , which is electrically coupled to the electric machines  16 ,  20 . 
         [0024]      FIG. 2  shows the power and energy flow paths from the power sources  12 ,  16 ,  20  to the load at the vehicle wheels  32 - 35 . Power produced by engine  12  and power produced by CISG  16  is combined at  44  and is transmitted to the transmission input  18 . Electric power produced by both electric machines  16 ,  20  is combinable at  46  for charging the battery  42 , or is transmitted from the battery to the electric machines. Mechanical power produced by ERAD  20  is transmitted through ERAD gearing  48  to the load at the rear wheels  34 ,  35  through the rear final drive  36 . 
         [0025]    In the HEV powertrain  10 , power can be transmitted to the wheels  32 - 35  during a vehicle creep condition solely in an electric drive mode by the electric machine  20 , independently of the engine  12  and transmission  14 , or in a parallel drive mode by a combination of engine  12 , transmission  14  and the electric machines  20 ,  16 . Two propulsion paths, mechanical and electrical, can be used to meet a given propulsion demand request. The engine  12  and CISG  16  can provide power to the wheels by transmitting torque through the transmission  14  in the mechanical propulsion path to the front axles  28 ,  30 , and the ERAD motor  20  can provide power directly in the electrical propulsion path to the rear axles  22 ,  23 . 
         [0026]    Referring now to  FIG. 3 , a HEV creep control system includes a controller  50 , which includes an electronic microprocessor, accessible to electronic memory containing stored functions, variables, and control algorithms and electronic signals produced by various sensors representing operating parameters and variables of the vehicle, engine  12 , CISG  16 , ERAD  20 , transmission  14 , input clutches  38 ,  39 , ERAD gearing  48  and final drive  26 , front and rear differentials  26 ,  36 , such as CISG and ERAD speed and temperature sensors, a vehicle speed sensor, brake pressure sensor. The microprocessor executes the algorithms and produces control commands to which the engine  12 , CISG  16  and ERAD  20  respond by producing torque, and the transmission  14  responds by engage and disengaging input clutches  38 ,  39  and alternately engaging a forward gear and reverse gear. 
         [0027]    The vehicle operator&#39;s demand for wheel torque is represented by the degree to which the brake pedal  62  is depressed. An electronic signal representing the brake pedal position  62  produced by a sensor, an electronic signal representing the brake pressure  64  produced by a sensor in response to depressing the brake pedal  62 , and an electronic signal  68  representing the current vehicle speed produced by a shaft speed sensor are received as input by a driver demanded wheel torque function  70 . The accelerator pedal is not depressed when vehicle creep is being controlled. Function  70  accesses in electronic memory a first function  72 , which produces a desired wheel torque when indexed by vehicle speed  68  and accelerator pedal position, and a second function  74 , which produces a desired wheel torque indexed by vehicle speed and brake pedal displacement or brake pressure  64 . At  76 , the magnitude of the desired wheel torque T W     —     DES  required to meet the driver&#39;s propulsion request while vehicle creep control is operative is produced from the output produced by functions  72  and  74 . 
         [0028]    At  80 , the wheel torque to be produced at the front wheels T W     —     FA    32 ,  33  and rear wheels T W     —     RA    34 ,  35  is determined such that the sum of the distributed propulsion torques equals the desired wheel torque determined from function  70 . The strategy for propulsion distribution accounts for vehicle stability and dynamics constraints, energy management and efficiency criteria, the torque capabilities of the engine  12 , CISG  40 , ERAD  20 , and transmission  14  and the state of charge (SOC) of battery  42 . 
         [0029]    At  82 , the desired ERAD torque T ERAD     —     DES  is determined based on the front axle wheel torque T W     —     RA  to be produced at the rear wheels  34 ,  35  and the gear ratios produced by the final drive  36  and ERAD gearing  48 . At  84 , controller  50  issues a command for ERAD  20  to produced the desired ERAD torque. 
         [0030]    At  86 , the desired transmission output torque T O     —     FA  is determined based on the front axle torque T W     —     FA  to be produced at the front wheels  32 ,  33  and the gear ratios produced by the final drive  26  and transmission  14 . 
         [0031]    If the magnitude of desired transmission output torque T O     —     FA  is greater than a reference torque magnitude, the gear selector  88  is in the Drive position, and the vehicle speed  68  is less than a reference vehicle speed, indicating that the transmission propulsion path will be used for vehicle creep, a vehicle creep control algorithm is entered and executed at  90 . If these conditions are absent, control passes to  92  where the transmission  14  is maintained in neutral, with no torque at the transmission output  24 . The input clutches  38 ,  39  are stroked, i.e., clearance spaces between components within in the clutches are closed such that the clutches have no current torque transmitting capacity but have imminent torque capacity potential. 
         [0032]    If these conditions are present, at  94 , the desired transmission output torque T O     —     FA  and the current transmission gear are used to determine the clutch torque capacity T CL     —     CAP     —     CRP  of the input clutch  38 ,  39  that is associated with the current gear during vehicle creep. At  96 , a desired clutch torque capacity T CL     —     CAP     —     DES  of the subject input clutch is commanded by controller  50  according to the creep clutch torque capacity T CL     —     CAP     —     CRP  produced at  94 . The torque capacity of the subject clutch is produced in response to the desired clutch torque command T CL     —     CAP     —     DES , and a signal representing the clutch torque capacity T CL     —     CAP     —     CRP  during vehicle creep is transmitted to a summing junction  98 . The subject clutch is always slipping when vehicle creep is being controlled by controller  50 . 
         [0033]    If the SOC of battery  42  is less than a reference SOC, at  100 , controller  50  determines the battery charge torque T QBAT     —     CHG  and at  102  issues an engine torque command T ENG     —     DES , the increase being substantially equal to the engine torque required to charge the battery. If the actual SOC of battery  42  is greater than the reference SOC, engine torque is controlled at  102  to zero brake torque since CISG  16  will control idle speed. A signal representing the battery charge torque T QBAT     —     CHG  is also transmitted to summing junction  98 . 
         [0034]    A crankshaft idle speed closed-loop control algorithm  104  is used to determine a desired change in CISG torque ΔT CISG     —     CL  based on a crankshaft speed feedback error  108  between the desired idle speed  110  and actual crankshaft speed  112  using a PID closed-loop controller  106  or a comparable controller. The desired change in CISG torque ΔT CISG     —     CL  produced by closed-loop controller  104  is also transmitted to summing junction  98 . 
         [0035]    At summing junction  98 , the desired change in torque produced by CISG  16  ΔT CISG     —     CL  and the creep clutch torque capacity T CL     —     CAP     —     CRP  and the battery charge torque T QBAT     —     CHG  are added. The desired change in torque ΔT CISG     —     CL  represents a closed-loop CISG torque required to maintain idle speed control, and the sum of battery charge torque T QBAT     —     CHG  and creep clutch torque capacity T CL     —     CAP     —     CRP  represent an open-loop feed-forward CISG torque with which to maintain idle speed control. The battery charge torque T QBAT     —     CHG  is a negative value and reduces the feed-forward CISG torque since an increase in battery charge torque would cause an increase in idle speed. The creep clutch torque capacity T CL     —     CAP     —     CRP  is a positive feed-forward CISG torque since an increase in clutch torque would cause a decrease in idle speed. At  114 , controller  50  issues a command to produce the overall desired CISG torque T CISG DES according to the output of summing junction  98  which includes both the closed-loop and feed-forward CISG torque commands. 
         [0036]    Controller  50  determines if the creep mode control algorithm  90  should be exited based on the current operating conditions. If the vehicle operator tips into the accelerator pedal and a sufficient increase in the desired transmission output torque occurs, and vehicle speed is above a reference speed, or if the gear selector  88  is moved to the Neutral/Park position, then the vehicle creep mode control is exited. 
         [0037]    If these conditions are absent, control returns to  86 . Clutch torque capacity is controlled according to either a vehicle launch condition upon the occurrence of a heavy accelerator pedal tip-in, or a transmission disengagement condition. 
         [0038]      FIGS. 4A-4G  are graphs of various powertrain and vehicle parameters before, during and following a vehicle creep condition in which torque blending is not used.  FIG. 4A  shows that the gear selector  88 , i.e. shift lever position, may be in the N or neutral position during period A, thereafter it is moved to the D or drive position at the beginning of period B before vehicle creep control begins. The brake pedal  62  is depressed during periods A and B, and is released at the beginning of period C, where vehicle creep control begins at  120 , and continues to be released until the beginning of period D. The accelerator pedal displacement  121  is zero throughout periods A-D and is depressed gradually during period E, which terminates the vehicle creep control. 
         [0039]    In  FIG. 4B , the overall desired wheel torque T W     —     DES    123  and desired front axle wheel torque T W     —     FA  increase at the beginning of creep control at  120  as the brake pedal  62  is released during period C and remain constant during period D until creep control terminates at  124 . The desired front axle wheel torque T W     —     FA  is equal to the overall desired wheel torque T W     —     DES  since no rear axle wheel torque T W     —     RA  is provided by the ERAD  20 . At a point during period D, the vehicle reaches a steady-state creep speed once the wheel torque is equal to the road load  122 . 
         [0040]    In  FIG. 4C , the input clutch torque capacity is zero until it begins to ramp-up at the beginning of creep control  120  to the desired clutch torque capacity  126  since there is an increase in the desired front axle wheel torque T W     —     FA  During period D, the desired clutch torque capacity  126  remains constant since the desired front axle wheel torque T W     —     FA  is also constant until creep control terminates at  124 . 
         [0041]    In  FIG. 4D , vehicle speed is zero until it ramps-up at the beginning of creep control  120  as input clutch torque transmits the current crankshaft (engine and/or CISG) torque to the wheels. Vehicle speed reaches a controlled steady vehicle creep speed  128  once the wheel torque equals the road load  122 , which remains constant until creep control terminates at  124 . 
         [0042]    In  FIG. 4E , the speed  130  at gear box side (i.e. clutch output) of the input clutch  38 ,  39  is zero until it ramps-up at the beginning of creep control  120  as the input clutch gains torque capacity. Clutch speed  130  is smaller than the crankshaft idle speed and remains constant until creep control terminates at  124 . The speed  132  of crankshaft  18  is controlled to a constant desired crankshaft idle speed  134  until creep control terminates at  124 . 
         [0043]    In  FIG. 4F , engine brake torque  136  is positive and constant while battery  42  is being charged to the reference SOC required to supply electric energy to the CISG  16 . Engine brake torque  136  decreases following the battery charge and remains constant until creep control terminates at  124  unless the battery SOC falls below the reference SOC. CISG torque  138  is negative during the battery charging period A &amp; B, and ramps-up to a positive torque when vehicle creep control begins at  120  due to the increase in clutch torque capacity. During period D, CISG torque  138  remains constant and positive until creep control terminates at  124 . 
         [0044]    In  FIG. 4G , torque  140  produced by ERAD  20  is zero since only front axle wheel torque T W     —     FA , is desired. The transmission output torque  142  is zero until it ramps-up at the beginning of creep control  120  as the input clutch  38 ,  39  gains torque capacity, and remains constant during period D until creep control terminates at  124 . 
         [0045]      FIGS. 5A-5G  are graphs of various powertrain and vehicle parameters before, during and following a vehicle creep condition in which torque blending occurs.  FIG. 5A  shows that the gear selector  88  may be in the N or neutral position during period A, thereafter it is moved to the D or drive position at the beginning of period B before vehicle creep control begins. The brake pedal  62  is depressed during periods A and B, is released at the beginning of period C, where vehicle creep control begins  120 , and continues to be released until the beginning of period D. The accelerator pedal displacement  121  is zero throughout periods A-D and is depressed gradually during period E, which terminates the vehicle creep control. 
         [0046]    In  FIG. 5B , the overall desired wheel torque T W     —     DES    144  is initially all provided by ERAD  20  to the rear wheels during period C following the beginning of creep control at  120  until the period D at which the desired front axle torque  146  T W     —     FA  ramps-up to meet the desired wheel torque. ERAD torque is reduced to zero as the front axle torque  146  produced by engine  12  and the CISG  16  is blended in and remains constant until creep control terminates at  124 . 
         [0047]    In  FIG. 5C , the input clutch torque capacity  148  is zero until period D when it begins to ramp-up since there&#39;s an increase in the desired front axle wheel torque T W     —     FA  after a brief period C. The input clutch torque capacity is zero during period C since vehicle creep is solely provided by ERAD  20 . During period E, the input clutch torque capacity  148  remains constant until creep control terminates at  124 . 
         [0048]    In  FIG. 5D , vehicle speed  128  is zero until it ramps-up at the beginning of creep control  120  as ERAD torque drives the rear wheels  34 ,  35 . Vehicle speed reaches a controlled steady vehicle creep speed at a point during period E once the wheel torque equals the road load  122 , and remains constant until creep control terminates at  124 . 
         [0049]    In  FIG. 5E , the speed  150  at the gear box side, i.e. the clutch output side of the input clutch  38 ,  39 , is zero until it ramps-up during period D as the input clutch gains torque capacity. During period E clutch speed  150  remains constant until creep control terminates at  124 . The speed  152  of crankshaft  18  is controlled to a constant desired crankshaft idle speed  154  until creep control terminates at  124 . 
         [0050]    In  FIG. 5F , engine brake torque  156  is positive and constant while battery  42  is being charged to the reference SOC required to supply electric energy to the CISG  16  and ERAD  20 . Engine brake torque  156  decreases following the battery charge and remains constant until creep control terminates at  124 . CISG torque  158  is negative during the battery charging period, and is controlled to a delta torque around zero during period C in order to maintain idle speed while ERAD  20  is providing all the wheel torque. In order to maintain idle speed control, CISG torque  158  ramps-up to a positive torque while input clutch torque capacity increases during period D, and remains constant and positive until creep control terminates at  124 . 
         [0051]    In  FIG. 5G , torque  160  produced by ERAD  20  increases during period C as the brake pedal  62  is released at the beginning of creep control at  120  and decreases to zero during period D while transmission output torque  162  increases as the input clutch torque capacity  146  increases. Initially, during period C vehicle creep is powered solely by ERAD torque at the rear wheels  34 ,  35 . After ERAD torque reaches zero at the end of period D, vehicle creep is powered solely by the engine and CISG torque at the front wheels  32 ,  33 . The transmission output torque  162  remains constant after reaching its steady state magnitude until creep control terminates at  124 . 
         [0052]    The vehicle creep control provides coordinated clutch torque capacity control when vehicle creep is provided or assisted by the additional electric machines, i.e., torque blending, and robust, responsive engine idle speed control as creep propulsion is provided through the transmission. By using the CISG  16  to directly account for the clutch torque capacity disturbance, engine manifold filling delays can be avoided, thus leading to an optimum idle speed control. 
         [0053]      FIG. 6  illustrates details of a powershift transmission  14  that includes the first input clutch  38 , which selective connects the input  18  of the transmission alternately to the even-numbered gears  42  associated with a first layshaft  244 , and a second input clutch  241 , which selective connects the input  20  alternately to the odd-numbered gears  243  associated with a second layshaft  249 . Layshaft  244  supports pinions  260 ,  262 ,  264 , which are each journalled on shaft  244 , and couplers  266 ,  268 , which are secured to shaft  244 . Pinions  260 ,  262 ,  264  are associated respectively with the second, fourth and sixth gears. Coupler  266  includes a sleeve  270 , which can be moved leftward to engage pinion  260  and driveably connect pinion  260  to shaft  244 . Coupler  268  includes a sleeve  272 , which can be moved leftward to engage pinion  262  and driveably connect pinion  262  to shaft  244  and can be moved rightward to engage pinion  264  and driveably connect pinion  264  to shaft  244 . 
         [0054]    Layshaft  249  supports pinions  274 ,  276 ,  278 , which are each journalled on shaft  249 , and couplers  280 ,  282 , which are secured to shaft  249 . Pinions  274 ,  276 ,  278  are associated respectively with the first, third and fifth gears. Coupler  280  includes a sleeve  284 , which can be moved leftward to engage pinion  274  and driveably connect pinion  274  to shaft  249 . Coupler  282  includes a sleeve  286 , which can be moved leftward to engage pinion  276  and driveably connect pinion  276  to shaft  249  and can be moved rightward to engage pinion  278  and driveably connect pinion  278  to shaft  249 . 
         [0055]    Transmission output  24  supports gears  288 ,  290 ,  292 , which are each secured to output shaft  24 . Gear  288  meshes with pinions  260  and  274 . Gear  290  meshes with pinions  262  and  276 . Gear  292  meshes with pinions  264  and  278 . 
         [0056]    Couplers  266 ,  268 ,  280  and  282  may be synchronizers, or dog clutches or a combination of these. 
         [0057]    In accordance with the provisions of the patent statutes, the preferred embodiment has been described. However, it should be noted that the alternate embodiments can be practiced otherwise than as specifically illustrated and described.