Abstract:
A process for controlling the speed of a vehicle travelling on a slope is disclosed. The vehicle is equipped with an electronic system which, through active brake intervention, adjusts the speed of the vehicle to a predetermined constant hill descent speed upon actuation of a hill descent control mode. The process comprises generating in the electronic system a continuously variable desired speed, and upon actuation of said hill descent control mode, the desired speed is continuously adjusted until it is equal to the predetermined constant hill descent speed. While the desired speed is being adjusted, the vehicle speed is continuously adjusted through active brake intervention until it is equal to the desired speed, so that after a transition period, the vehicle speed is equal to the predetermined constant hill descent speed.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a process for controlling the speed of a vehicle travelling on a slope by means of active braking intervention. 
     Road vehicles having systems that maintain the vehicle at a constant speed as it travels on a gradient by automatically applying the brakes (&#34;active braking&#34;) are known. Such vehicles are, as a rule, equipped with an anti-locking brake system (ABS), as well as with an automatic slip control (ASR). The components of the ABS and ASR systems, such as the electronic control units, wheel speed sensors, solenoid regulating valves, etc., are used to effect the abovementioned speed regulation on a gradient. Using the known devices, the driver is relieved from having to actuate the brake pedal constantly when the vehicle is travelling on a gradient. Even with a changing incline, uniform speed is maintained. 
     In DE-A-24 19 242, an electro-pneumatic brake force control circuit is disclosed that maintains a constant vehicle speed as the vehicle travels on a gradient through active braking so long as neither the gas pedal nor the brake pedal is actuated. The speed regulation on a gradient is switched off immediately when either the brake pedal or the gas pedal is actuated, or as soon as the superimposed ABS becomes activated. 
     Equipping a vehicle with a &#34;hill descent control&#34; mode (referred to hereinafter as &#34;HDC&#34;) is also known from International Patent Application WO 96/11826 A1. This type of speed regulation on a gradient can be switched on manually by the driver with the activation of a switch. This type of regulation is able to maintain the vehicle at a constant low speed on a steep slope by means of active regulated braking of the vehicle without the driver having to actuate the brake. This system is particularly suitable for off-road vehicles driving on a slope that is steep so that the engine braking effect is no longer sufficient to decelerate the vehicle, even when the vehicle is in the lowest gear. 
     It is the object of the present invention to provide a process for controlling the speed of a vehicle travelling on a slope whereby the transition between normal driving and the state with active braking state is rendered particularly comfortable. 
     SUMMARY OF THE INVENTION 
     This object is achieved by a process for controlling the speed of a vehicle travelling on a slope, wherein the vehicle is equipped with an electronic system which, through active brake intervention, adjusts the speed of the vehicle to a predetermined constant hill descent speed upon actuation of a hill descent control mode. The process comprises generating in the electronic system a continuously variable desired speed, and upon actuation of the hill descent control mode, the desired speed is continuously adjusted in the electronic system until it is equal to the predetermined constant hill descent speed. While said desired speed is being adjusted, the vehicle speed is continuously adjusted through active brake intervention until it is equal to the changing desired speed, so that after a transition period, the vehicle speed is equal to the predetermined constant hill descent speed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a vehicle on a slope, 
     FIG. 2 shows a schematic representation of the pneumatic and electronic brake system of a vehicle. 
     FIGS. 3 to 7 are diagrams of the HDC regulating process according to the present invention under different situations. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The vehicle illustrated in FIG. 1 has two axles (1) and (2), of which the front axle (1) is considered the lower or downhill axle, and rear axle (2) is considered the higher or uphill axle. The vehicle may travel forward or backwards on the gradient. 
     FIG. 2 schematically shows the electronic and pneumatic systems of the vehicle illustrated in FIG. 1. In another embodiment, the pneumatic braking system shown here is replaced by a hydraulic or mixed braking system. 
     The vehicle shown has the two axles (1) and (2). The pressure medium for the braking system is supplied to the axles (1) and (2) by two pressure supply tanks (3, 4), one for each axle. The pressure medium is conveyed to the wheel brake cylinders (7) via a brake valve (5) which is actuated by a brake pedal (6). Disposed before the brake cylinders (7) are the ABS/ASR control solenoid valves (8). The brake valve (5) is provided with an ASR solenoid valve (5&#39;), as brake pressure is applied to the individual wheels without the brake pedal (6) being pressed when the ASR is activated. 
     To control the above-mentioned solenoid valves, an ABS/ASR electronic control unit (9) is provided. The ABS/ASR electronic control unit (9) is connected by electrical connection lines to the above-mentioned solenoid valves (5, 5&#39;, 8). The electronic control unit (9) obtains behavioral information for each individual wheel from wheel sensors (10). An actuating switch (11) serves to switch on the active braking (HDC) function. In FIG. 2, pneumatic or hydraulic lines are shown by black/white lines while electrical lines are shown as thin continuous black lines. 
     The manner in which the system functions during controlled downhill travel is explained below in further detail. 
     The &#34;hill descent control&#34; function of the vehicle is recognized either automatically by the electronic control unit (9), or when the driver manually activates the switch (11). Upon activation of the hill descent control function, the ASR solenoid valve (5&#39;) becomes activated and direct braking pressure is brought to bear upon the brake cylinders (7) of the wheels of the front axle (1). At this stage, the electronic control unit (9) regulates the speed of the vehicle by actuating the solenoid valves (8) of the ABS system in such a manner that a predetermined speed is maintained independently of any changes in the slope. 
     If a wheel locks up during a controlled descent, the HDC function is of course replaced by the normal ABS function. The hill descent control is also superseded when the driver wishes to reduce a previously set constant speed or to increase it by pressing on the brake pedal or the gas pedal. 
     In order to avoid overheating the brakes during a long controlled hill descent, it is furthermore advantageous to provide for the monitoring of their temperature and to alert the driver by means of a warning system when a limit value has been exceeded. The driver then has the option of stopping the vehicle until the brakes have cooled off. Such monitoring of the brake temperature can be accomplished by means of temperature sensors (not shown) on the wheel brakes, or the brake temperature can be reproduced by an electronic model within the electronic system (9). Although the second method is less precise, it is also less expensive. 
     A more precise representation of the transition of the vehicle speed (V Fahr ) to the controlled, hill descent speed on the slope (HDC V min ) is shown in the diagram of FIG. 3. 
     The vehicle travels at first at a current vehicle speed V Fahr  (solid line). Within the electronic system (9), a desired speed (V soll ) (dotted line) is generated. V soll  is at first AV above the vehicle speed V Fahr . A typical value for ΔV in this case is 5 to 7 km/h. ΔV can also be made dependent on vehicle speed, whereby the value of ΔV increases with the vehicle speed. The vehicle speed V Fahr  in FIG. 3 is below a value HDC V max  which is the maximum permissible regulated HDC speed. HDC V max  in this case is approximately 50 km/h. Beyond this speed, no control is provided. 
     It is assumed that the driver actuates the actuating switch (11) at the point in time t 1  and thus switches on the HDC function. Thereupon, the desired speed V soll  is lowered in the electronic system (9) in such a manner as to achieve a soft transition to the value HDC V min . At the point in time t 2 , the curve of the desired speed V soll  intersects the curve of the vehicle speed V Fahr . The vehicle speed V Fahr  is adjusted at that time to the desired speed V soll , so that the two curves are now congruent. V soll  and V Fahr  together with it are thereafter lowered gradually to HDC V min . At the point in time t 3 , the final speed HDC V min  is reached. The vehicle is now maintained automatically at this constant descent speed until the actuating switch (11) is switched off. The time T 1  between the actuation of the actuating switch (11) at point in time t 1  and the intersection point of desired speed V soll  and vehicle speed V Fahr  (start of control to point in time t 2 ) depends on the prior vehicle behavior (e.g., accelerating or decelerating), the increase of V soll  and the value of ΔV. This time T 1  typically has a value of approximately 0.5 s. 
     FIG. 4 is a diagram showing the transition from the so-called stand-by operation to the HDC control operation. The actuating switch (11) must be switched on for this. The starting speed of the vehicle V Fahr  lies above the HDC control range, i.e., above the HDC V max  control speed of approximately 50 km/h. 
     As the diagram shows, the driver eases back on the gas pedal or the throttle DK from approximately 60% to 0% between time t 0  and t 1 . Thereupon the vehicle speed V Fahr  is reduced until it intersects the desired speed V soll  =HDC V max  at point in time t 2 . At this point in time, the adjustment of the vehicle speed to the decreasing desired speed V soll  predetermined by the electronic system (9) begins by means of active braking. At the point in time t 3 , the HDC speed HDC V min  is attained again and the vehicle is held at a constant speed of approximately 10 km/h with active braking. 
     If the gas pedal is not eased back completely to 0% at the point in time t 0 , the vehicle speed V Fahr  is maintained at a value which corresponds to the throttle position. In this case even a higher descending speed than HDC V min  =10 km/h can be set. In FIG. 4 for example, a descending speed of 20 km/h is shown. The relationship between throttle position and desired speed is calculated so that the engine (in the different gears different characteristic lines are used) cannot work against the brake. 
     In the diagram of FIG. 5, the HDC control is shown again for a transition from stand-by operation to controlled operation. In this case, the original speed of the vehicle (ca. 30 km/h) is between HDC V max  and HDC V min . 
     The HDC actuating switch (11) is now switched on. The vehicle is in the rising phase of a hill crest at the beginning of the diagram. Shortly before point in time t 0 , the driver eases up on the gas pedal since he is now at the apex of the hill crest. Before point in time t 0  and in accordance with the driver&#39;s wish (as indicated by the gas pedal position), the desired speed V soll  is set at HDC V max . As the gas pedal is released, V soll  rapidly drops to V Fahr  +ΔV and is then further reduced at a predetermined rate. Starting at point in time t 0 , the diagrams of FIGS. 5 and 3 are identical. This means that the desired speed V soll  continues to be reduced and that it intersects the vehicle speed V Fahr  at the point in time t 2 . From that point in time on, the vehicle speed V Fahr  is adjusted to the desired speed V soll  and after a period of approximately 7 seconds, at point in time t 3 , it reaches the constant hill descent speed HDC V min  of approximately 10 km/h. 
     FIG. 6 is a diagram showing a starting state wherein the vehicle is traveling downhill at a speed HDC V min  of approximately 10 km/h with active braking. At point in time t 1 , the driver reconsiders and decides to increase his vehicle speed V Fahr  by stepping on the gas pedal at point in time t 1 . The ABS/ASR electronic system (9) then also increases the desired speed V soll . Since the vehicle is adjusted to the changed desired speed V soll , the vehicle speed V Fahr  increases accordingly. The desired speed V soll  increases at a predetermined rate to a new constant value through electronic control intervention. A rate greater than the predetermined one is not permitted. A throttle position of 0 to 30% is assigned a desired speed of HDC V min  to HDC V max . From point in time t 2  until point in time t 3 , the driver removes his foot from the gas pedal again, so that the desired speed V soll  and the vehicle speed V Fahr  are again slowly adjusted to the starting value HDC V min . The latter is attained at point in time t 4 . 
     Immediate termination of control occurs when a desired speed greater than HDC V max  is called for by the throttle (gas pedal) position. This is the case only when the gas pedal is set at more than approximately 30%. 
     For safety reasons, the HDC speed control function is allowed only when either the first gear, the reverse gear or an off-the-road gear (transmission switched on) is used. 
     In FIG. 7 a transition from stand-by operation to HDC controlled operation is shown again. The original speed of approximately 4 km/h of the vehicle is however lower here than HDC V min . The HDC actuating switch (11) is switched on. The vehicle continues to travel straight on. 
     Starting at the point in time t 1 , the vehicle speed V Fahr  increases and reaches the desired speed V soll  at point in time t 2 . V soll  is above the previous vehicle speed, or above the vehicle speed which existed at the point in time when the actuating switch (1) was switched on, by the value ΔV1. Starting at point in time t 2 , the vehicle speed V Fahr  is again adjusted to the desired speed V soll . The desired speed V soll  is in turn changed within the electronic system (9) with a soft transition to the hill descending speed HDC V min . It is attained approximately at point in time t 3 . 
     As FIG. 7 shows, the desired speed V soll  can also be lower than the limit speed HDC V min . However, HDC V min  still amounts to V Fahr  +ΔV1. The desired speed V soll  increases at a predetermined rate only once it has been intersected by V Fahr . In this manner a controlled vehicle acceleration is possible even on extremely steep slopes. 
     The person skilled in the art recognizes that the embodiments described above and the above-mentioned possibilities do not exhaust the area of protection of the invention, but that all embodiments with characteristics as mentioned in the claims fall under their protection.