Abstract:
A method for operating a powertrain system of a vehicle includes determining an initial creep torque command in an operator-selected direction of travel, adjusting the initial creep torque command responsive to an operator braking request and responsive to a change in direction of vehicle speed relative to the operator-selected direction of travel, and operating the hybrid powertrain to generate axle torque in response to the adjusted creep torque command.

Description:
TECHNICAL FIELD 
     This disclosure is related to control systems for hybrid powertrain systems. 
     BACKGROUND 
     The statements in this section merely provide background information related to the present disclosure. Accordingly, such statements are not intended to constitute an admission of prior art. 
     Powertrain architectures for vehicles include hybrid powertrain systems that employ multiple torque-generative devices including internal combustion engines and non-combustion torque machines that transmit mechanical torque either directly or via a transmission device to a driveline for use as propulsion torque. Known internal combustion engines can also generate torque which may be transmitted to a torque machine to generate power that is storable as potential energy in an on-board storage device. Internal combustion engines include multi-cylinder heat engines that convert stored fuel to mechanical power through combustion processes, and non-combustion torque machines include multiphase electric motors that transform electric power to mechanical power. An electrical energy storage device, e.g., a battery, stores DC electrical power that can be transferred and converted to AC electric power using an inverter device to operate the multiphase electric machine to generate mechanical power to achieve work. Hybrid powertrain systems generate mechanical power that is transferred to a vehicle driveline responsive to an output torque request. Power outputs from the engine and the electric machine(s) are controlled to be responsive to the output torque request. 
     SUMMARY 
     A method for operating a powertrain system of a vehicle includes determining an initial creep torque command in an operator-selected direction of travel, adjusting the initial creep torque command responsive to an operator braking request and responsive to a change in direction of vehicle speed relative to the operator-selected direction of travel, and operating the hybrid powertrain to generate axle torque in response to the adjusted creep torque command. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       One or more embodiments will now be described, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  illustrates a vehicle including an exemplary hybrid powertrain system coupled to a driveline and controlled by a control system, in accordance with the disclosure; 
         FIGS. 2-1 and 2-2  illustrate an axle creep torque control routine for controlling the hybrid powertrain system of  FIG. 1 , in accordance with the disclosure; 
         FIG. 3  illustrates operation of the exemplary hybrid powertrain system operating without benefit of the axle creep torque control routine when the vehicle is stopped or moving slowly in an uphill direction, in accordance with the disclosure; and 
         FIG. 4  illustrates operation of the exemplary hybrid powertrain system operating with the axle torque control routine in a vehicle stopped at a stoplight and facing an uphill direction, in accordance with the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings, wherein the showings are for the purpose of illustrating certain exemplary embodiments only and not for the purpose of limiting the same,  FIG. 1  schematically illustrates a vehicle including an embodiment of a hybrid powertrain system  20  coupled to a driveline  60  and controlled by a control system  10 . The exemplary vehicle is configured as a hybrid-electric vehicle employing an internal combustion engine (engine)  40 , transmission  50  and first and second electrically-powered torque machines  35  and  36 , respectively. Other hybrid-electric powertrain configurations are within the scope of this disclosure, including a powertrain configuration that includes a single electrically-powered torque machine arranged in series with the engine and transmission, and other powertrain systems that monitor rotational direction and speed of an output member of the powertrain system. Like numerals refer to like elements throughout the description. 
     The vehicle may be configured to operate in one of an electric vehicle (EV) mode and a hybrid vehicle (HV) mode. Operating the vehicle in the EV mode includes generating all propulsion torque from one or more of the torque machines  35 ,  36  driven by electric power. Operating the vehicle in the HV mode includes generating the propulsion torque from the engine  40  and the first and second torque machines  35 ,  36 . The engine  40  may execute autostart and autostop control schemes during operation in the HV mode. The HV mode may include one or more operating states wherein all the propulsion torque is generated by the engine  40 . 
     The hybrid powertrain system  20  employs communications paths  55 , mechanical power paths  57 , and high-voltage electric power paths  59 . The mechanical power paths  57  mechanically couple elements that generate, use, and/or transfer torque, including the engine  40 , the first and second torque machines  35 ,  36 , transmission  50  and driveline  60 . The high-voltage electric power paths  59  electrically connect elements that generate, use, and/or transfer high-voltage electric power, including the energy storage system  25 , an inverter module  30 , and the first and second torque machines  35 ,  36 . The high-voltage electric power paths  59  include a high-voltage DC bus  29 . The communications path  55  includes high-speed data transfer lines to effect communications between various elements of the vehicle, and may include one or more of a direct connection, a local area network bus, and a serial peripheral interface bus, and include a high-speed communications bus  18 . 
     The exemplary energy storage system  25  is a high-voltage battery fabricated from a plurality of lithium-ion cells. It is appreciated that the energy storage system  25  may include a plurality of electrical cells, ultracapacitors, and other devices configured to store electric energy on-vehicle. 
     The engine  40  is preferably a multi-cylinder direct fuel injection internal combustion engine that converts fuel to mechanical power through a combustion process. The engine  40  is equipped with a plurality of sensing devices and actuators configured to monitor operation and deliver fuel to form a combustion charge to generate torque. The engine  40  may be configured to execute autostart and autostop control schemes and fuel cutoff (FCO) control schemes during ongoing operation of the vehicle. The engine  40  is considered to be in an OFF state when it is not being fueled and is not spinning. The engine  40  is considered to be in an FCO state when it is spinning but is not being fueled. 
     The first and second torque machines  35 ,  36  preferably include multi-phase electric motor/generators electrically connected to the inverter module  30  that are configured to convert electric energy to mechanical power and convert mechanical power to electric energy that may be stored in the energy storage system  25 . The first and second torque machines  35 ,  36  have limitations in power outputs in the form of minimum and maximum torques and rotational speeds. 
     The inverter module  30  includes first and second inverters  32  and  33  that electrically connect to the first and second torque machines  35 ,  36 , respectively. The first and second torque machines  35 ,  36  interact with the respective first and second inverters  32  and  33  to convert stored electric energy to mechanical power and convert mechanical power to electric energy that may be stored in the energy storage system  25 . The first and second electric power inverters  32  and  33  are operative to transform high-voltage DC electric power to high-voltage AC electric power and also operative to transform high-voltage AC electric power to high-voltage DC electric power. The energy storage system  25  electrically connects via the high-voltage bus  29  to the inverter module  30  that connects to the first and second torque machines  35 ,  36  to transfer electric power therebetween. In one embodiment, an external connector  26  electrically connects to the energy storage system  25  and is connectable to an external AC power source to provide electric power for charging the energy storage system  25  during vehicle static periods. 
     The transmission  50  preferably includes one or more differential gear sets and controllable clutch components to effect torque transfer over a range of speeds between the engine  40 , the first and second torque machines  35 ,  36 , and an output member  62  that couples to the driveline  60 . In one embodiment the transmission  50  is a two-mode transmission device configurable to transfer torque in one of an input-split mode and a compound-split mode. Operating parameters associated with mechanical power transfer include power between the engine  40  via the transmission  50  indicated by input torque and input speed, and power between the transmission  50  and the driveline  60  indicated by output torque and output speed. The driveline  60  may include a differential gear device  65  that mechanically couples to an axle  64  or half-shaft that mechanically couples to a ground-engaging wheel  66  in one embodiment. The wheel  66  includes a controllable friction brake that operatively couples to a brake controller. The differential gear device  65  is coupled to the output member  62  of the hybrid powertrain system  20 , and transfers output power therebetween. The driveline  60  transfers propulsion power between the transmission  50  and a road surface. A vehicle speed sensor  61  is configured to monitor rotation of the output member  62  and provides data including rotational position, speed, and direction of rotation to the control system  10 . The data from the vehicle speed sensor  61  is employed to determine a magnitude and direction of vehicle speed in one embodiment. 
     The control system  10  includes a control module  12  that signally connects to an operator interface  13 . The operator interface  13  includes a plurality of human/machine interface devices through which the vehicle operator commands and controls operation of the vehicle, including an operator acceleration request via an accelerator pedal  17 , an operator braking request via a brake pedal  16 , a transmission range selection via a PRNDL lever  15  or another suitable device, a vehicle speed request, e.g., through a cruise control system  14 , and vehicle operation control via an ignition key. Although the control module  12  and operator interface  13  are shown as individual discrete elements, such an illustration is for ease of description. The functions described as being performed by the control module  12  may be combined into one or more devices, e.g., implemented in software, hardware, and/or application-specific integrated circuitry (ASIC) and ancillary circuits that are separate and distinct from the control module  12 . Information transfer to and from the control module  12  may be accomplished using the communications paths  55 , including, e.g., communications bus  18 . The control module  12  preferably signally and operatively connects to individual elements of the hybrid powertrain system  20  via the communications bus  18 . The control module  12  signally and/or operatively connects to the sensing devices of each of the energy storage system  25 , the inverter module  30 , the first and second torque machines  35 ,  36 , the engine  40  and the transmission  50  to monitor and control operation and determine parameters thereof. 
     Control module, module, control, controller, control unit, processor and similar terms mean any one or various combinations of one or more of Application Specific Integrated Circuit(s) (ASIC), electronic circuit(s), central processing unit(s) (preferably microprocessor(s)) and associated memory and storage (read only, programmable read only, random access, hard drive, etc.) executing one or more software or firmware programs or routines, combinational logic circuit(s), input/output circuit(s) and devices, appropriate signal conditioning and buffer circuitry, and other components to provide the described functionality. Software, firmware, programs, instructions, routines, code, algorithms and similar terms mean any instruction sets including calibrations and look-up tables. The control module has a set of control routines executed to provide the desired functions. Routines are executed, such as by a central processing unit, and are operable to monitor inputs from sensing devices and other networked control modules, and execute control and diagnostic routines to control operation of actuators. Routines may be executed at regular intervals, for example each 100 microseconds, 3.125, 6.25, 12.5, 25 and 100 milliseconds during ongoing engine and vehicle operation. Alternatively, routines may be executed in response to occurrence of an event 
     The control module  12  executes control routines  11  to control operation of the engine  40  in coordination with the first and second torque machines  35 ,  36  and the transmission  50  to generate an output torque and output speed at the output member  62  to transfer mechanical power to the driveline  60  to generate axle torque in response to the operator acceleration request and the operator braking request. The axle torque is either a positive (propulsion) torque or a negative (regenerative braking) torque. One of the control routines  11  referred to herein as creep operation controls operation of the torque-generative devices, i.e., the engine  40  and the first and second torque machines  35 ,  36  to generate a magnitude of axle torque referred to as a creep torque, which is a magnitude of axle torque that is commanded when the operator acceleration request  17  is zero, the transmission range selection  15  is one of the drive ranges, e.g., drive (D) or reverse (R), and the vehicle speed is less than a speed threshold associated with creep operation, which is at or near 5 MPH (8 km/h) for example. 
       FIGS. 2-1 and 2-2  schematically illustrate an axle creep torque control routine for controlling operation of a hybrid powertrain system, e.g., as described with reference to  FIG. 1 . The axle creep torque control routine is employed to control a hybrid powertrain system to generate propulsion torque in a manner that achieves a magnitude of creep torque, limits vehicle rollback when operating the vehicle on an incline, and limits vehicle roll-forward acceleration when operating the vehicle on a decline, all while minimizing energy loss caused by axle torque cancellation initiated by the operator braking request. Overall, the axle creep torque control routine monitors and controls vehicle operation under conditions when the vehicle speed is less than a threshold with no operator input to the accelerator pedal, taking into account the magnitude and direction of vehicle speed in relation to a commanded and expected direction of vehicle speed. This operation permits vehicle operation that achieves a creep torque while minimizing vehicle rollback, thus minimizing need for operator braking force that works against the creep torque generated by the powertrain system. Table 1 is provided as a key to  FIG. 2-1  wherein the numerically labeled blocks and the corresponding functions are set forth as follows. 
     
       
         
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 BLOCK 
                 BLOCK CONTENTS 
               
               
                   
               
             
             
               
                 202 
                 Monitor magnitude and direction of Vss, Tacclr, 
               
               
                   
                 Tbrk, PRNDL 
               
               
                 204 
                 Is Tacclr = 0, PRNDL = R, D, L, AND Vss less 
               
               
                   
                 than threshold? 
               
               
                 206 
                 Determine preferred axle creep torque 
               
               
                 208 
                 Control powertrain system to achieve preferred 
               
               
                   
                 axle creep torque 
               
               
                 210 
                 End 
               
               
                   
               
             
          
         
       
     
     The axle creep torque control routine periodically executes during ongoing vehicle operation. Various vehicle and operator parameters are monitored, including a magnitude and direction of vehicle speed (Vss), a transmission range selection (PRNDL), an operator acceleration request (Tacclr) and an operator braking request (Tbrk) ( 202 ). When the operator acceleration request is zero, i.e., the operator is not depressing the accelerator pedal, the vehicle speed is less than a speed threshold associated with creep operation, and the transmission range selector is in one of the drive ranges, e.g., drive (D) or reverse (R) ( 204 )( 1 ), an axle creep torque determination routine ( 206 ) is executed to determine a preferred axle creep torque, and operation of the powertrain system is controlled to achieve the preferred axle creep torque ( 208 ). When these conditions are not met or are no longer met ( 204 )( 0 ), this iteration of the axle creep torque control routine ends ( 210 ). 
       FIG. 2-2  schematically shows the axle creep torque determination routine  206  of  FIG. 2-1 , employing monitored inputs including an initial creep torque command  214 , a magnitude and direction (signed) vehicle speed  212 , an operator braking request  216 , and a transmission range selection  215 . The initial creep torque command  214  is a calibrated value for axle torque that is determined in relation to vehicle speed when the operator acceleration request is zero. The initial creep torque command  214  is a magnitude of axle torque that achieves a vehicle speed that is an order of magnitude of 5-8 km/h (3-5 mph) when the vehicle is operating on a flat surface. The signed vehicle speed  212  includes magnitude and direction of the vehicle speed. The transmission range selection  215  indicates operator-selected direction of travel, i.e., one of drive (D), reverse (R) or neutral (N). 
     The initial creep torque command  214  is compared to a threshold torque  218  ( 220 ) and a logic signal  221 , i.e., either 0 or 1, is generated. The logic signal  221  is set equal to “1” when the initial creep torque command  214  is greater than the threshold torque  218 , and the logic signal  221  is set equal to “0” when the initial creep torque command  214  is less than the threshold torque  218 . The logic signal  221  is employed by a subsequent logic operator  230  to select either the initial creep torque command  214  or a second creep torque command  229  as a braking-modified creep torque command  232 . 
     The operator braking request  216  is input to calibration table  222 , which selects a first numerical multiplier  223  that is within a range between 0 and 1 (or 0% and 100%) in direct relation to the magnitude of the operator braking request  216  (0% —no braking torque request and 100% —maximum braking torque request). The first numerical multiplier  223  is employed to adjust the initial creep torque command  214  by numerical multiplication ( 224 ) to determine a braking-modified creep torque command  225 . This operation decreases the magnitude of axle torque that is generated responsive to the initial creep torque command  214  in response to an increase in the operator braking request  216 . 
     The signed vehicle speed  212  is input to calibration table  226 , which selects a second numerical multiplier  227  based upon the magnitude and direction of the vehicle speed and the operator-selected direction of travel. By way of example, the second numerical multiplier  227  is set equal to 1 when the vehicle speed is zero. The second numerical multiplier  227  is set according to an inverse relationship with the minimum multiplier from calibration  222  when the vehicle speed is less than zero and the transmission range selection  215  indicates the operator-selected direction of travel is drive (D), or when the vehicle speed is greater than zero and the transmission range selection  215  indicates the operator-selected direction of travel is reverse (R). By way of example, when the minimum multiplier from calibration  222  is 0.04, the maximum value at a negative vehicle speed is 1/0.04, or 25. The second numerical multiplier  227  is set according to a pass-through relationship when the vehicle speed is greater than zero and the transmission range selection  215  indicates the operator-selected direction of travel is drive (D), or when the vehicle speed is less than zero and the transmission range selection  215  indicates the operator-selected direction of travel is reverse (R). By way of example, when the signed vehicle speed  212  is positive, the calibration table  226  provides a multiplication value of 1.0 for second numerical multiplier  227 . The braking-modified creep torque command  225  is multiplied by the second numerical multiplier  227  ( 228 ) to determine the second creep torque command  229 . 
     One of the initial creep torque command  214  and the second creep torque command  229  is selected as the braking-modified creep torque command  232  using the logic operator  230 . This means that the initial creep torque command  214  is selected as the braking-modified creep torque command  232  when the initial creep torque command  214  is less than the torque threshold  218 , and the second creep torque command  229  is selected as the braking-modified creep torque command  232  when the initial creep torque command  214  is greater than the torque threshold  218 . The braking-modified creep torque command  232  is compared with the initial creep torque command  214  and the minimum of the two values is selected as a preferred axle creep torque command  240 , which is employed to control operation of the hybrid powertrain system to generate axle torque. Operation of the hybrid powertrain system may include operation with the engine in the OFF state and the first and second torque machines controlled to achieve the preferred axle creep torque command  240 . 
     Thus, under low speed conditions when the operator is providing no input to the accelerator pedal and the vehicle is operating on a relatively flat surface, the preferred axle creep torque command  240  is equal to or less than the initial creep torque command  214  and decreases with an increase in the operator braking request  216  so long as the vehicle is moving in the forward direction and the operator-selected direction of travel is drive (D). Under low speed conditions when the operator is providing no input to the accelerator pedal and the vehicle is stopped or rolling backwards from the operator-selected direction of travel is drive (D), the preferred axle creep torque command  240  can increase up to a magnitude equal to the initial creep torque command  214  to prevent vehicle rollback responsive to an operator braking request. 
     The axle creep torque determination routine  206  enables calibration flexibility to minimize required magnitude of operator input to the brake pedal and minimize applied mechanical braking torque to counteract axle creep torque when the vehicle is operating on a flat road surface during an approach to a traffic sign, thus minimizing expenditure of electric energy through the mechanical vehicle brakes during vehicle creep. The axle creep torque determination routine  206  prevents a vehicle from accelerating backwards when a brake pedal is depressed by employing the signed vehicle speed as part of the torque cancellation logic. 
       FIG. 3  graphically shows operation of an exemplary hybrid powertrain system operating without benefit of the axle creep torque control routine when the vehicle is stopped or moving slowly in an uphill direction, such as at a stoplight. Plotted parameters include operator braking request (%)  316 , operator acceleration request  317  (%), signed vehicle speed (km/h)  314 , indicating both direction and magnitude of speed, and axle torque  310  (N-m), all shown on the vertical axis in relation to time  320  on the horizontal axis. At time  321 , the operator acceleration request  317  falls to zero, indicating the operator has removed their foot from the accelerator pedal. The axle torque  310  decreases to achieve a commanded creep torque and the vehicle speed  314  decreases in response. At time  322 , the operator braking request  316  is applied at a magnitude near 10%, thus cancelling the axle torque  310  at the commanded creep torque as indicated by the decrease in the axle torque  310 . The vehicle speed  314  continues to decrease and goes negative, i.e., the vehicle reverses its direction of travel and accelerates in the reverse direction. This may be vehicle rollback, such as may occur when the vehicle is stopped at a stoplight and facing an uphill direction. At time  323 , the operator braking request  316  is increased and the vehicle speed  314  (which is negative) increases towards zero, e.g., to stop vehicle rollback. The vehicle speed  314  reaches zero at time  324 . 
       FIG. 4  graphically shows operation of the exemplary hybrid powertrain system operating with the axle creep torque control routine in a vehicle stopped at a stoplight and facing an uphill direction. Plotted parameters include operator braking request (%)  416 , operator acceleration request  417  (%), signed vehicle speed (km/h)  414 , indicating both direction and magnitude of speed, and axle torque  410  (N-m), all shown on the vertical axis in relation to time  420  on the horizontal axis. Prior to time  421 , the operator acceleration request  417  falls to and remains at zero, indicating the operator has removed their foot from the accelerator pedal, thus initiating a request for creep torque. The operator braking request  416  is at or about 10%, and the axle torque  410  is at a commanded creep torque with the vehicle speed  414  at zero speed. The operator braking request  416  is released at time  421  and reduces to 0% at time  422 , at which point the vehicle speed  414  goes negative. The axle torque  410  increases in response to the negative vehicle speed. At time  423 , the operator braking request  416  increases, but does not cause cancellation of the axle torque  410 , as they work in concert to stop vehicle rollback. As the vehicle speed  414  approaches zero speed, the axle torque  410  tapers off towards a magnitude that was initially occurring. The vehicle speed  414  reaches zero at time  424 . 
     The disclosure has described certain preferred embodiments and modifications thereto. Further modifications and alterations may occur to others upon reading and understanding the specification. Therefore, it is intended that the disclosure not be limited to the particular embodiment(s) disclosed as the best mode contemplated for carrying out this disclosure, but that the disclosure will include all embodiments falling within the scope of the appended claims.