Abstract:
A vehicle and a method of operation. The method of operating the vehicle may include a method of controlling a creep torque in a vehicle having a powertrain configured to provide variable creep torque. The method may comprise the steps of: detecting a vehicle creep torque condition, determining a brake torque, and adjusting a creep torque applied by the powertrain based on the brake torque.

Description:
BACKGROUND OF INVENTION 
     The present invention relates generally to a method for controlling creep torque in a vehicle. 
     Automotive vehicles typically have a vehicle technical specification that defines how fast the particular vehicle should accelerate from a stop when it is idling in gear and the driver applies no pressure to the brake pedal or the accelerator pedal. To accomplish this, the powertrain system is designed to apply a specific amount of torque when the vehicle is stopped, called creep torque. This creep torque, then, acts against the brake application by the driver while the vehicle is stopped. Upon release of the brake pedal, the vehicle will begin to move, even without pressing on the accelerator pedal. In a vehicle with a conventional internal combustion engine, automatic transmission and torque converter, applying a creep torque is relatively easy since the engine is operating all of the time—even while the vehicle is stopped at a traffic light. 
     For some newer types of vehicles where an engine is not operating all of the time, applying creep torque is more of a challenge. Such vehicles may be, for example, two-mode hybrid vehicles, battery electric vehicles, and fuel cell vehicles. For these types of vehicles, when the vehicle is stopped (while in gear) the electric motor is typically operated to supply a creep torque similar to conventional vehicles. In this way, drivers switching between driving conventional vehicles and these newer types of vehicles will get the same sensation when releasing the brake to start-up from a stop. However, the extra energy used by the motor to maintain the creep torque while the vehicle is stopped while in gear somewhat defeats the purpose of implementing these non-conventional powertrains in the new types of vehicles. 
     SUMMARY OF INVENTION 
     An embodiment contemplates a method of controlling a creep torque in a vehicle having a powertrain configured to provide variable creep torque, the method comprising the steps of: detecting a vehicle creep torque condition, determining an initial brake torque, and adjusting a creep torque applied by the powertrain based on the initial brake torque. 
     An embodiment contemplates a method of controlling a creep torque in a vehicle having a powertrain configured to provide variable creep torque, the method comprising the steps of: detecting a vehicle creep torque condition, determining a brake torque, determining a new brake torque, calculating a rate of change of brake torque based on the brake torque and the new brake torque, and adjusting a creep torque applied by the powertrain based on the new brake torque and the rate of change of brake torque. 
     An embodiment contemplates a vehicle including a brake system and a powertrain system. The brake system includes a plurality of brakes, a brake controller configured to determine an amount of brake torque to apply to the plurality of brakes, a user operable brake pedal, and a brake position sensor configured to detect a position of the brake pedal. The powertrain system is configured to provide variable creep torque and includes a powertrain controller configured to adjust a creep torque based on the amount of brake torque indicated by the brake controller. 
     An advantage of an embodiment is reduced energy usage since the energy needed for creep torque is minimized while the vehicle is stationary, but the desired creep torque feel is still provided to the driver when needed during vehicle start-up. Moreover, the creep torque feel may even be improved somewhat since, while stopped in a creep torque situation, the brakes are not unnecessarily working against a creep torque produced by the vehicle. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic view of a vehicle to which a method for controlling a creep torque may be employed. 
         FIG. 2  is a flow chart disclosing a method for controlling a creep torque in a vehicle. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , a vehicle, indicated generally at  12 , is shown. The vehicle  12  includes a powertrain system  14  having variable creep torque capability, such as, for example, two-mode hybrid, battery electric, and fuel cell vehicles. A driver operated accelerator pedal  16  may include a position sensor  18  that communicates the pedal position to a powertrain controller  20 . 
     The vehicle also includes a brake system  22  having a brake controller  24  that controls the braking pressure applied by a right-front wheel brake  26 , a left-front wheel brake  27 , a right-rear wheel brake  28  and a left-rear wheel brake  29 . A brake position sensor  32  detects the position of a driver operated brake pedal  34  and communicates the position to the brake controller  24 . The brake controller  24  may be separate from or a portion of another controller and may be any desired combination of hardware or software as is known to those skilled in the art. The brake pressure may be applied to the brakes  26 - 29  via a hydraulic, pneumatic or electric subsystem, as desired for the particular vehicle. 
       FIG. 2  is a flow chart showing a method for controlling creep torque in the vehicle of  FIG. 1 . A determination is made whether the vehicle is in a creep torque condition, block  100 . A determination that the vehicle  12  is in a creep torque condition may be based on, for example, the vehicle  12  being in a driving gear, moving at a slow or no speed, and the driver applying little or no force on the accelerator pedal  16 . Of course, other factors or combinations of factors may be employed to determine that the vehicle  12  is in a creep torque condition. 
     If the vehicle  12  is determined to be in a creep torque condition, then an initial brake torque is determined, block  102 . The initial brake torque may be determined by the brake controller  24  based on input from the brake position sensor  32 . The brake position sensor  32 , of course, reflects the force applied to the brake pedal  34  by the driver. A new brake torque is determined, block  104 . A new brake torque may be determined, for example, every fifty milliseconds (or at some other preferred time interval). The rate of change of the brake torque is calculated, block  106 . The rate of change of the brake torque may be determined by comparing the new brake torque with the initial brake torque (i.e., the immediately prior determined brake torque). Of course, after the next time interval when another new brake toque is determined, it would then be compared to the previous new brake torque (which would then become the initial brake torque). The rate of change may also be calculated over multiple consecutive time intervals instead, if so desired. 
     The amount of creep torque applied by the powertrain system  14  is adjusted, block  108 . The adjustment is based on the new brake torque and the rate of change of brake torque. A simplified version of a possible equation that may be employed is that the creep torque=an initial creep torque−(the new brake torque)/(rate of change of brake torque), with the rate of change being the initial brake torque minus the new brake torque. Alternatively, a look-up table based on these variables may be employed. And, of course, the creep torque can be overridden when accelerator pedal or other torque request is received by the powertrain controller. 
     This creep torque process, then, provides the creep torque desired by the driver when needed, without unnecessarily providing a creep torque when not needed. In general, the higher the brake force applied to the brake pedal  34 , the less creep torque is applied. For example, when the driver is pressing on the brake pedal  34  and maintaining this force sufficient to indicate a desire to stop or stay stopped, the creep torque will be reduced to zero, thus reducing the energy used by the powertrain system  14 . As the driver starts to release the brake pedal  34 , the amount of creep torque is ramped-up by the powertrain system  14  in a way that creates essentially the same feel for the driver as the creep torque generated by a conventional vehicle. And, by looking at the rate of change of the brake torque, this allows for greater creep torque to be applied when the brake pedal is released more quickly, which may even improve the feel of creep torque to the driver over that of a conventional vehicle. 
     While certain embodiments of the present invention have 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.