Abstract:
A method for controlling the speed of a vehicle having a regenerative or active braking capacity when the vehicle is traveling downhill using a cruise control system on the vehicle. When brake pedal is applied and the cruise control is set, the vehicle is put into a controlled braking mode. The cruise control system controls are then used inversely to the normal operation where decreases in vehicle speed are provided by applying motoring torque. For the inversed cruise control, increasing the vehicle speed is prohibited by applying more regenerative braking torque.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates generally to a method for controlling the speed of a vehicle traveling downhill that has a regenerative braking capability and, more particularly, to a method for controlling the speed of a vehicle traveling downhill that has a regenerative braking capability by using a cruise control system of the vehicle. 
         [0003]    2. Discussion of the Related Art 
         [0004]    Fuel cell, battery electric and hybrid vehicles employ a supplemental power source, such as a high voltage DC battery, an ultracapacitor and/or a flywheel. The supplemental power source provides supplemental power for the various vehicle auxiliary loads, for system start-up and during high power demands when the main power source is unable to provide the desired power. A fuel cell stack or an engine with a generator provides power to an electric traction system through a DC high voltage electrical bus for vehicle operation. The battery or ultracapacitor provides supplemental power to the electrical bus during those times when additional power is needed beyond what the continuous electric power source can provide, such as during heavy acceleration. For example, a fuel cell stack may provide 70 kW of power. However, vehicle acceleration may require 100 kW of power. 
         [0005]    The regenerative power available from an electric traction system during regenerative braking is also used to recharge the battery or ultracapacitor. The ancillary loads of the vehicle and in the case of hybrids and fuel cell vehicles, the engine or fuel cell system, may act as an additional energy sink in case the battery is fully charged. The brake system and the propulsion system may have a brake blending function, which will distribute on driver brake pedal demand the available regenerative energy in an optimum of efficiency and system reliability between the brakes, the battery and the electric loads. Besides alternative propulsion systems, the brake system will have more and more active functions, and if such an active brake in a conventional engine driven vehicle allows brake control without applying the brake pedal, the functions of inversed cruise control can apply also. 
         [0006]    An electric vehicle typically is able to provide regenerative braking where the electric traction motor on the vehicle operates as a generator that generates current in response to a driving force on the motor. The regenerative braking energy can be used to recharge the battery or ultracapacitor, or operate system loads. 
         [0007]    When a vehicle is traveling downhill, it will typically gain speed. In order to maintain a constant speed and prevent the vehicle from increasing its speed, the vehicle operator may have to operate the vehicle&#39;s brakes to prevent the increase in speed. For a conventional vehicle, the vehicle operator can put the vehicle into a lower gear to allow the engine to prevent or reduce an increase in the vehicle speed. Using the brake system has disadvantages because of over-dimensioning the brakes and/or providing additional wear and tear on the brakes. For an electric vehicle, lower gears may not be provided or the electric drive motor may not have an intrinsic characteristic of braking torque. Thus, putting the vehicle in a lower gear to reduce the vehicle speed when traveling downhill may not be an option. 
       SUMMARY OF THE INVENTION 
       [0008]    In accordance with the teachings of the present invention, a method for controlling the speed of a vehicle having a regenerative or active braking capacity when the vehicle is traveling downhill using a cruise control system on the vehicle is disclosed. When brake pedal is applied and the cruise control is set, the vehicle is put into a controlled braking mode. The cruise control system controls are then used inversely to the normal operation where decreases in vehicle speed are provided by applying motoring torque. For the inversed cruise control, increasing the vehicle speed is prohibited by applying more regenerative braking torque. 
         [0009]    Additional features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is an illustration of a control interface for a vehicle cruise control system; 
           [0011]      FIG. 2  is a graph with time on the horizontal axis and vehicle speed on the vertical axis showing the normal motoring cruise control for a vehicle; and 
           [0012]      FIG. 3  is a graph with time on the horizontal axis and speed on the vertical axis showing a process for controlling vehicle speed using a braking cruise control method. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0013]    The following discussion of the embodiments of the invention directed to a method for controlling the speed of an electric vehicle traveling downhill having a regenerative braking capability using a cruise control system on the vehicle is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses. 
         [0014]      FIG. 1  is an illustration of a vehicle steering wheel  10  showing a control interface  12  for a vehicle cruise control system. The control interface  12  includes a cruise control on/off button  14  for turning the cruise control system on and off, a resume/accelerate button  16  for increasing the vehicle speed or resuming the cruise control operation and a set/decelerate button  18  for setting the cruise control speed or decelerating the cruise control speed. The operation of a vehicle cruise control system with these control buttons is well understood to those skilled in the art. A brief discussion of this operation will be given below with reference to  FIG. 2 . 
         [0015]      FIG. 2  is a graph with time on the horizontal axis and speed on the vertical axis showing the normal motoring operation of a cruise control system for an electric or non-electric vehicle. Line  22  represents the situation where the vehicle is coasting and the vehicle speed is decreasing, line  24  represents a situation where the vehicle is traveling downhill and the vehicle speed is increasing and line  26  represents the situation where gas is being applied and the vehicle speed is increasing. For any of these situations, the set button  18  may be pushed to set a particular cruise control speed at point  28 , where the set speed is identified by line  30 . At a hold point  32  an action is performed where the cruise control is disengaged by some operation, such as pressing a hold button, applying the vehicle brakes, etc. The hold button can be an additional button on the control interface  12 , or can be a control of one of the existing buttons. The vehicle speed may be changed with the cruise control is disengaged, such as by providing gas at line  34 , coasting at line  36  or applying the vehicle brakes at line  38 . If gas is applied at the hold point  32 , such as to pass a vehicle, then the vehicle operator may lift off of the gas pedal at point  40  and coast at line  42  back to the cruise control mode to resume the controlled speed. If the vehicle is coasting at the line  36  and the resume button  16  is pressed at point  44 , then the control will return to the controlled speed line  30  at point  46 . If the brake has been applied at the line  38  and the resume button  16  is pressed at point  48 , then the cruise control speed will return at point  50 . While in the control mode for the cruise control system, the resume/accelerate button  16  can be pushed at point  52  to increase the vehicle speed and the set/decelerate button  18  can be pushed at point  54  to decrease the vehicle speed. 
         [0016]    The normal operation of a cruise control system as discussed above can be inverted in an electric vehicle having regenerative braking to be in a regenerative braking cruise control mode to control the vehicle speed using regenerative braking when the vehicle is traveling downhill so that the vehicle does not increase or change its speed.  FIG. 3  is a graph with time on the horizontal axis and speed on the vertical axis showing the operation of the cruise control system discussed above for the regenerative braking cruise control mode. In the motoring cruise control mode described above, the cruise control was set at point  28  when the vehicle was coasting or gas was being applied at the lines  22 ,  24  and  26 . For the invention, the cruise control system will know that it is to be put in the braking cruise control mode to control the vehicle downhill speed when the vehicle brake pedal is being applied at line  60  and the cruise control speed is set by the set button  18  at point  62  to set the downhill speed at line  64 . 
         [0017]    In this design, the cruise control system is using the braking provided by the electric traction motor to control the vehicle speed instead of the throttle to maintain the vehicle speed as the vehicle is going downhill. In other words, now that the vehicle is in the braking cruise control mode, as the vehicle speed increases, more regenerative braking is applied and as the vehicle speed decreases, less regenerative braking is applied as the incline of the hill changes so that the speed of the vehicle is maintained constant. 
         [0018]    At point  66 , the braking cruise control mode is disengaged by a hold operation, such as applying gas at line  68 , applying brake at line  70  or pressing a hold button so the vehicle coasts at line  72 . For the gas line  68 , throttle is not actually being applied, but braking is being withdrawn so that the speed of the vehicle increases. At point  74 , the resume button  16  is pressed so that braking is again applied and the vehicle speed is returned to the selected control speed of the line  64  at point  76 . If the vehicle speed has been increasing by the removal of the braking on the line  72  where the vehicle is coasting and the resume button  16  is pressed at point  78 , the braking is reapplied and the selected control speed is resumed at point  80 . If the vehicle brakes have been applied at the line  70  and then the brakes are disengaged at point  82 , then the vehicle coasts at line  84  by the removal of the brakes and braking control is again provided at point  86  to bring the vehicle back to its set control speed. As above, the accelerate button  16  can be applied at point  88  to increase the controlled vehicle speed and the decelerate button  18  can be applied at point  90  to decrease the controlled speed of the vehicle by providing more braking. 
         [0019]    The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.