Method for controlling regenerative braking in a vehicle

1. Method for controlling braking in a vehicle including a friction braking system and a powertrain system including a torque machine operative to react tractive torque input from a wheel of the vehicle, a microprocessor performs the following steps:

monitoring operation of the powertrain system;

determining a driver intended total brake torque;

determining a regenerative braking capacity based upon the operation of the powertrain system;

determining a regenerative braking request based upon a time-rate change in the regenerative braking capacity comprising determining the regenerative braking request based upon the regenerative braking capacity modified using a first slope regulating function when the time-rate change in the regenerative braking capacity exceeds a threshold and determining the regenerative braking request based upon the regenerative braking capacity modified using a second slope regulating function when the time-rate change in the regenerative braking capacity is within the threshold; and

determining a motor torque command for the torque machine based upon the regenerative braking request; wherein determining the regenerative braking request based upon the regenerative braking capacity modified using the first slope regulating function when the time-rate change in the regenerative braking capacity exceeds the threshold comprises applying a three-term Weibull function in a forward-fitting large curve fit operation to maximize the regenerative braking request based upon the regenerative braking capacity; and

wherein determining the regenerative braking request based upon the regenerative braking capacity modified using the second slope regulating function when the time-rate change in the regenerative braking capacity is within the threshold comprises applying a three-term Weibull function in a forward fitting small curve fit operation to maximize the regenerative braking request based upon the regenerative braking capacity.

2. The method of claim 1 , further comprising determining a braking torque command to the friction brakes based upon the driver intended total brake torque and the regenerative braking request.

3. The method of claim 1 , wherein determining the regenerative braking request based upon the regenerative braking capacity modified using the first slope regulating function further comprises determining the regenerative braking request based upon the regenerative braking capacity modified using a slope regulating function determined based upon the time-rate change in the regenerative braking capacity.

4. The method of claim 3 , wherein the slope regulating function includes a first curve portion whereat the regenerative braking request increases exponentially with increasing regenerative braking capacity, a second curve portion in which the regenerative braking request increases at a substantially constant rate with increasing regenerative braking capacity near an inflection point, and a third curve portion in which the regenerative braking request logarithmically increases with increasing regenerative braking capacity.

5. The method of claim 3 , wherein the slope regulating function includes a first curve portion whereat the regenerative braking request decreases exponentially with decreasing regenerative braking capacity, a second curve portion in which the regenerative braking request decreases at a substantially constant rate with decreasing regenerative braking capacity near an inflection point, and a third curve portion in which the regenerative braking request logarithmically decreases with decreasing regenerative braking capacity.

6. Method for controlling braking in a vehicle including a friction braking system and a powertrain system including a plurality of torque machines operative to react tractive torque input from a driveline, a microprocessor performs the following steps:

monitoring operation of the powertrain system;

determining a driver intended total brake torque;

determining a regenerative braking capacity based upon the operation of the powertrain system;

determining a regenerative braking request based upon the regenerative braking capacity modified using a slope regulating function comprising determining the regenerative braking request based upon the regenerative braking capacity modified using a first slope regulating function when the time-rate change in the regenerative braking capacity exceeds a threshold and determining the regenerative braking request based upon the regenerative braking capacity modified using a second slope regulating function when the time-rate change in the regenerative braking capacity is less than the threshold;

determining motor torque commands for the first and second torque machines based upon the regenerative braking request, and

determining a braking torque command to the friction brakes based upon the driver intended total brake torque and the regenerative braking request.

7. The method of claim 6 , further comprising determining the regenerative braking request based upon the regenerative braking capacity modified using the slope regulating function determined based upon the time-rate change in the regenerative braking capacity.

8. Method for controlling braking in a vehicle including a friction braking system and a powertrain system including a torque machine operative to react tractive torque input from a driveline, a microprocessor performs the following steps:

determining a regenerative braking capacity based upon the operation of the powertrain system;

determining a regenerative braking request based upon the regenerative braking capacity modified using a slope regulating function, wherein the slope regulating function comprises a first slope regulating function when the time-rate change in the regenerative braking capacity exceeds a threshold and the slope regulating function comprises a second slope regulating function when the time-rate change in the regenerative braking capacity is less than the threshold;

determining a motor torque command for the torque machine based upon the regenerative braking request, and

determining a braking torque command to the friction brakes based upon the driver intended total brake torque and the regenerative braking request; wherein determining the regenerative braking request based upon the regenerative braking capacity modified using the first slope regulating function comprises applying a three-term Weibull function in a forward-fitting large curve fit operation to maximize the regenerative braking request based upon the regenerative braking capacity; and

wherein determining the regenerative braking request based upon the regenerative braking capacity modified using the second slope regulating function comprises applying a three-term Weibull function in a forward-fitting small curve fit operation to maximize the regenerative braking request based upon the regenerative braking capacity.