Abstract:
The invention relates to a method for controlling a deceleration arrangement ( 10 ) of a motor vehicle with at least one regenerative braking device ( 12 ) which is designed to decelerate the motor vehicle by means of a variable braking torque and to generate electrical energy corresponding to the braking torque. A return speed of an operating element for driving the motor vehicle, in particular an accelerator pedal ( 22 ), is detected (S 2 ). A predefined braking torque is exerted (S 5 ) by the regenerative braking device ( 12 ) if the return speed and/or a variable correlating thereto exceed(s) or fall(s) below a predefined threshold value.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a method for controlling a deceleration arrangement of a motor vehicle, as well as a control device that is designed to carry out such a method. 
     It is already known for hybrid vehicles or purely electrically driven vehicles to recover kinetic energy during braking processes by generator mode operation of the electrical machine and to charge a battery with said energy (regenerative braking) Here the electrical machine produces a negative drive torque, i.e. it operates as a generator. In the case in which the electrical machine is neither producing a positive nor a negative driving torque, the motor vehicle is not driven forwards and is only accelerated or decelerated because of external influences (e.g. air resistance, rolling resistance, road gradient) (said phenomenon is referred to in the English literature as “freewheeling”). In the case of negative driving torque the regenerative braking torque can be regulated as the degree of deceleration of the vehicle, which is perceptible by the driver. 
     In modern motor vehicles various operating concepts are known in which the electrical machine is operated as a generator. E.g. vehicles are known in which recovery is carried out in steps of fixed specified sizes. If the gas pedal is released regenerative braking is initiated for a specified drive torque (this depends on e.g. the driving mode, which e.g. can be a sports, a comfort or an environmentally friendly driving mode). 
     Motor vehicles are also known, in which the regenerative braking torque can be manually adjusted by the driver using switches. The regenerative braking torque is set when releasing the gas pedal (if the brake pedal is not operated). Regenerative braking is only initiated if the gas pedal is fully released. In the event of brake pedal operation the regenerative torque is additionally increased until the predefined regeneration braking torque is reached. From a specified brake pedal travel, moreover, a hydraulic brake is activated, which further increases the braking torque (additively). 
     Finally, motor vehicles are known, with which the regenerative braking already starts before the complete release of the gas pedal. This causes the motor vehicle to coast at a certain pedal position (i.e. the electrical machine is neither generating a positive nor a negative drive torque), and the driver can regulate the degree of the regeneration intensity by means of adjusting smaller gas pedal positions. The exact gas pedal travel at which the motor vehicle coasts can be speed-dependent here. At higher speeds said point lies at a higher gas pedal position. The reason for this is that the maximum drive torques and regeneration torques vary depending on the revolution rate and hence depending on the speed. At higher speed, frictional losses in the electrical machine have to be increasingly compensated. The consequence for the driver is that he has a longer gas pedal travel available at higher speeds for regulating the level of the regenerative braking. 
     Because the highest possible efficiency is to be achieved during regenerative braking and when driving the motor vehicle by means of the electrical machine, the electrical-regenerative brake is connected to one of more axles of the motor vehicle. With such an arrangement minimal friction and hence the highest possible efficiency are achieved. In other words the electrical-regenerative brake only acts upon that axle, e.g. the front axle, which is also being driven by the electrical machine. Depending on the braking situation an unbalanced and hence non-ideal distribution of the braking torques on the individual axles can occur. For example, the braking torque can act on the respective axles in a ratio that is not proportional to the respective weights on the individual axles. An unbalanced brake force distribution in turn can adversely affect the driving dynamics or the controllability of the motor vehicle. For vehicles with a regenerative deceleration arrangement, particularly strong effects on driving stability are to be reckoned with because of the desire to recover the maximum amount of braking energy. In other words there is a conflict of aims between a very high level of energy recovery by regenerative braking on the one hand and a high level of driving stability during the braking process on the other hand. 
     SUMMARY OF THE INVENTION 
     According to the invention a method is provided for controlling a deceleration arrangement of a motor vehicle with at least one regenerative braking device. The regenerative braking device is designed to decelerate a motor vehicle by means of a variable braking torque and to generate electrical energy according to the braking torque. With the method according to the invention, the speed of return of an operating element for driving the motor vehicle, especially a gas pedal, is detected. With the method according to the invention, moreover, a predefined braking torque is exerted by the regenerative braking device if the speed of return and/or a variable correlated therewith exceeds or falls below a certain threshold value. 
     Furthermore, according to the invention a control device for carrying out such a method is provided. 
     The motor vehicle is especially an electric vehicle. The term “electric vehicle” refers within the wording of this text e.g. exclusively to vehicles driven by electrical energy as well as to hybrid vehicles, which comprise combinations of electric motors with internal combustion engines or fuel cells. The term “gas pedal” refers to an operating element that a driver operates and thereby can adjust the speed or the acceleration of the vehicle. 
     With the invention a maximum possible efficiency is achieved. The driver can initiate the regeneration mode by releasing the gas pedal and thus does not have to operate an additional operating element to activate the regeneration mode. Improved operating comfort can be provided in this way without an additional operating element. 
     According to one configuration of the method according to the invention, the predefined braking torque is determined according to a driving situation of the motor vehicle. 
     In this way the control or regulation algorithms of a control device according to the invention that is implementing the method can be optimally adapted to the specified boundary conditions or the specific driving situation of the motor vehicle. 
     With one embodiment of the method according to the invention the motor vehicle is driven by an electric motor. The electric motor can be operated as a generator as a component of the regenerative braking device for decelerating the motor vehicle. 
     This means that a single electrical machine can be operated as a drive unit for driving the motor vehicle or as a generator for recovery of energy. Said dual function requires no additional equipment and thus reduces the installation space and the costs. 
     With one embodiment a maximum braking torque is generated by the regenerative braking device if the speed of return exceeds the predefined threshold value. 
     In this way, e.g. in emergency situations, if the gas pedal is returned very quickly the braking process is initiated rapidly and with maximum braking torque, whereby safety can be increased. 
     With one embodiment of the method according to the invention, no braking torque is generated by the regenerative braking device if the speed of return falls below a second predefined threshold value. 
     Thus e.g. in overrun mode of the motor vehicle, i.e. when coasting, a regeneration torque is inhibited, so that the released kinetic energy of the motor vehicle can be used for overcoming a longer coasting distance. Said conversion from one mechanical form of energy to a different mechanical form of energy has particularly high efficiency compared to the conversion of kinetic energy to electrical energy. 
     With one design of the method according to the invention, a braking torque is generated by the regenerative braking device for generating electrical energy if the speed of return is below the second predefined threshold value and an operating element for decelerating the motor vehicle, especially a brake pedal, is operated to brake the motor vehicle. 
     The operation of the brake pedal thus has a higher associated priority compared to gas pedal operation, so that in all cases the total braking effect intended by the driver is generated. 
     The first predefined threshold value can coincide with the second predefined threshold value. According to one embodiment of the method according to the invention, however, the second predefined threshold value is smaller than the first predefined threshold value. 
     In this way the range of gas pedal return speeds is divided into three sub-ranges. Different braking strategies, e.g. a regeneration-optimized and/or deceleration-optimized braking strategy, can be set up using the delineation between the individual ranges. 
     For example, the braking torque varies depending on a position of the operating element for driving the motor vehicle if the speed of return has a value between the first predefined threshold value and the second predefined threshold value. 
     In this way the driver can advantageously control the energy recovery behavior of the motor vehicle according to his wishes without an additional operating element. 
     This aspect is further based on the fact that a significantly higher speed of return of the gas pedal compared to a normal driving operation is assessed as a criterion for the existence of an emergency braking situation. For this purpose the method according to the invention is capable of initiating an automatic braking process at an early stage prior to the operation of the brake pedal. This ensures that in a hazardous situation no time is lost in building up braking torque. 
     With one embodiment of the method according to the invention the first threshold value and/or the second threshold value can especially be adjusted manually by the driver. 
     For example, the predefined braking torque can especially be manually adjusted by the driver. 
     The driver has as a result the option of influencing the driving dynamics state of the vehicle in order thereby to produce a certain driving behavior, e.g. a sporty or comfortable driving behavior. 
     With one design of the method according to the invention, a control device controls the deceleration arrangement depending on a user-specific profile stored in a memory. Thereby the control device is designed to be especially self-learning and the user-specific profile is updated based on control inputs by the driver. 
     Thus a long-term profile can be produced with which driver-specific control can be optimally designed. 
     According to one embodiment of the method according to the invention, surroundings identification is performed for detecting an obstruction in the surroundings of the motor vehicle. Here the predefined braking torque is determined taking into account the results of the surroundings detection. 
     The surroundings detection device can basically be designed as desired. For example, the surroundings detection device can comprise a magnetic sensor, with which a generated magnetic field is changed by an object to be detected and said change is measured. Further, ultrasound-based surroundings detection devices are prior art. For example, an approach of the motor vehicle to an object, e.g. a different motor vehicle, can be detected by the surroundings detection device and an impending collision can be prevented. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       It should be understood that features, properties and advantages of the method according to the invention also apply accordingly to the control device designed according to the invention. 
         FIG. 1  shows a principle circuit diagram of a deceleration arrangement of a motor vehicle for carrying out the method according to the invention; and 
         FIG. 2  shows a flow diagram of a method according to the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a principle circuit diagram of a deceleration arrangement  10  for implementing the method according to the invention. The deceleration arrangement  10  is designed as a regenerative braking system, which also comprises, besides a frictional brake system  14 , an electrical generator  12  and a control unit (inverter controller) for generating electrical energy. The generator  12  can e.g. be operated as a motor for driving the motor vehicle. 
     For example, only the two (not illustrated) front wheels or the front axle of the motor vehicle are/is connected to the generator  12 . The total deceleration of the motor vehicle during a braking process is thereby composed of the deceleration components of the generator  12  and the frictional brake system  14 . 
     The brake pressure required for the friction brakes R 1 , R 2 , R 3 , R 4  is applied by means of a master brake cylinder  16  that is coupled to a brake pedal  20 . For building up the brake pressure a brake medium is forced by the master brake cylinder  16  via suitable hydraulic lines L 1 , L 2 , L 3 , L 4  to the individual friction brakes R 1 , R 2 , R 3 , R 4 . Here each friction brake R 1 , R 2 , R 3 , R 4  is associated with a vehicle wheel. A brake booster  18  is disposed between the brake pedal  20  and the master brake cylinder  16  and boosts a pedal force applied by the driver to the brake pedal  20 , e.g. by means of servo-pneumatically compressed air. 
     A position of a gas pedal  22  is detected by a gas pedal travel sensor  24  and forwarded to an analysis device  26 . The analysis device  26  is designed to determine the gas pedal return speed from the detected position of the gas pedal  22 . The analysis device  26  is connected on the output side via a data line  27  to a control device  28  for controlling an automatic braking process. The control device  28  can be designed to be self-learning. For this purpose a memory  30  is implemented in writeable form in the control device  28  or outside the same or in one embodiment even outside the vehicle, so that control programs for the control device  28  can be designed and loaded for the individual technical conditions. Here the program comprises self-learning algorithms. The control device  28  controls the deceleration devices  12 ,  14  depending on a user-specific profile stored in the memory  30 . The control device  28  continuously updates the user-specific profile based on an analysis of certain control inputs (e.g. the release of the gas pedal  22 ) by the user. The control device  28  thus detects the behavior of the driver over a long period. 
     A signal for the pedal travel of the brake pedal  20  or a correlated variable is delivered to the control device  28  by means of a data line  31 . The control device  28  can control a switching valve  36  of the brake booster  18  via the control line  33 , so that a brake pressure can be built up independently of the operation of the brake pedal  20 . 
     A driver information device  38 , e.g. a display, provides the driver with information about the settings, e.g. manual settings, about the current status of the recovery performance and information about the operating concept, or information thereof for use for energy-saving driving. Of course the corresponding information is also made available in audible form. 
     Parameters that characterize a driving situation of the motor vehicle, such as e.g. driving dynamics parameters, are detected by means of a sensor system  40  comprising a plurality of sensors. The output signals of the sensor system  40  are transferred to the control device  28  via a communications line  42 . 
     A surroundings detection device  44  is connected via a communications line  46  to the control device  28 . The surroundings detection device  44  comprises e.g. a camera device for generating video signals, which are transmitted via the communications line  46  to the control device  28 . Objects, such as a preceding vehicle or vehicles in front of the vehicle are detected there by means of an image processing unit on the basis of three-dimensional pattern recognition. If the control device  28  detects a critical traffic situation, e.g. an impending collision with another vehicle, it automatically controls the deceleration device  12 ,  14  taking into account the driving situation in order to decelerate the motor vehicle. 
     In a driving mode the analysis device  26  transfers the value of the speed of return V R  of the gas pedal  22  determined by the analysis device  26  to the control device  28  by means of the data line  27 . In the control device  28  the value of the speed of return V R  is compared with an upper threshold value S o  and with a lower threshold value S u . If the value of the speed of return V R  exceeds the upper threshold value S o , the control device  28  controls the generator  12  and thereby causes it to carry out regenerative braking. The control device  28  thereby determines a predefined braking torque B max , which is defined according to the driving situation detected by means of the sensor system  40 . The predefined braking torque B max  can also vary by increasing with time in order to achieve a more comfortable ride. The regenerative braking consists of the released kinetic energy of the vehicle being converted by the generator  12  into electrical energy. The electrical energy built up during regenerative braking is greater the greater the braking torque that has built up. The recovered electrical energy can be used for charging an accumulator or for supplying electrical loads depending on the strategy of the vehicle operation. 
     If the value of the speed of return V R  falls below the lower threshold value S u , the control device  28  causes the application of regenerative power only under the condition that the brake pedal  20  is operated for braking the motor vehicle. In other words, the control device assesses a hesitant release of the gas pedal  22  without a brake pedal operation as a criterion for the desire of the driver to allow the vehicle to coast. 
     If the value of the speed of return V R  falls below the upper threshold value S o  and at the same time exceeds a lower threshold value S u , it can be concluded therefrom that there is neither a desire for greater deceleration nor the desire for coasting of the motor vehicle. Therefore in this case intuitively understandable control of the regenerative torque by the driver is achieved by operating the gas pedal  22 . The control device  28  varies the regeneration torque for this purpose, e.g. depending on a position of the gas pedal  22 , by a fixed specified torque or a using a different strategy. 
     In summary, for the method according to the invention the range of values of the speed of return V R  of the gas pedal  22  is subdivided into three or more sub-ranges, in each of which a different operating concept is implemented, and specifically in a first sub-range with return speed values above the upper threshold value S o  (V R &gt;S o ), a second sub-range in which the return speed values are between the upper threshold value S o  and the lower threshold value S u  (S u &lt;V R &lt;S o ) and in a third sub-range with return speed values below the lower threshold value S u  (V R &lt;S u ). It should be understood that more than two sub-ranges of the return speed values in which different regeneration steps are implemented can also be defined. 
     By means of an electronic input device  32  the user can make manual adjustments of parameters, such as e.g. the maximum regeneration level or the values for the upper threshold value S o  and/or the lower threshold value S u . The parameter values thus adjusted by the user can for example be displayed on the display of the driver information device  38 . 
       FIG. 2  shows a flow diagram of an embodiment of a method according to the invention. 
     In a first step S 1  a gas pedal return speed V R  is determined by the analysis device  26 . Following step S 1  the method advances to step S 2 . In step S 2  a check is made as to whether the gas pedal return speed V R  is greater than or according to an alternative embodiment less than the specified upper threshold value S o . 
     In the decision step S 2  the method branches depending on whether the result is positive or negative. For a positive result in step S 2  (i.e. the gas pedal return speed exceeds or alternatively is less than the upper threshold value S o ) the method proceeds with step S 3 . In step S 3  a check is made as to whether the brake pedal  20  is being operated. 
     If it was determined in step S 3  that the brake pedal  20  is not being operated, the method changes to step S 4 , otherwise the method terminates at end step S 11 . 
     In order to achieve the maximum possible energy recovery with simultaneous high driving stability according to the respective driving situation, in step S 4  the predefined braking torque B max  for the generator  12  is calculated according to a driving situation, because in non-critical driving situations a higher generator braking torque can be allowed or tolerated than for critical driving situations. The calculation of the predefined braking torque B max  is performed by the control device  28 . For calculation of the predefined braking torque B max  from the current driving situation, one or more of the following parameters can be used and detected by means of the sensor system  40 : The speed of the vehicle, the lateral accelerations of the motor vehicle, the yaw rate of the motor vehicle, the center of gravity position of the motor vehicle, coefficient of friction values of the road surface, coefficient of friction values of the tires, to name just a small selection. In addition properties of the vehicle such as the design of the chassis can be taken into account during the determination of the predefined braking torque B max . Different computer operations known to the person skilled in the art between said parameters or influencing factors are thereby possible for calculation of the predefined braking torque B max . A suitable determination of the predefined braking torque B max  can alternatively also be performed using characteristic curves stored in the memory  30 . With a motor vehicle comprising an electronic stability program (ESP) the signals of an already provided ESP sensor system can be analyzed to assess the current driving situation or driving dynamics. 
     Following step S 4 , i.e. after the calculation of the predefined braking torque B max , the method advances to step S 5 . In step S 5  the control device  28  sends corresponding control signals to the generator  12  for application of the determined predefined braking torque B max  to the wheels or the vehicle axle. In other words the control device  28  passes the calculated predefined braking torque B max  to the generator  12  in step S 5 . 
     Following step S 5  the method is terminated by the end step S 11 . 
     In the event of a negative result in step S 2  the method changes to step S 6 . In step S 6  a check is made as to whether the speed of return V R  is less than the lower threshold value S u . 
     If the check result in step S 6  is negative, it follows therefrom that the current value of the gas pedal return speed V R  is in the range between the lower threshold value S u  and the upper threshold value S o . In this case the method changes to step S 7 . 
     In step S 7  the regenerative power of the generator  12  varies e.g. depending on a position of the gas pedal  22 . For example, a deflection range or displacement travel of the gas pedal  22  is divided into at least two ranges, a lower range and an upper range. In the lower range for a hybrid vehicle with a closed choke flap regenerative or recuperative braking is performed. The regenerative braking torque is controlled here by the pedal displacement. In the upper range the choke flap is controlled depending on the pedal displacement. 
     Following step S 7  the method is terminated in the end step S 11 . 
     If the check result in step S 6  is positive (i.e. the current value of the gas pedal return speed is less than the lower limit value S u ), the method jumps to step S 8 . 
     In step S 8  a check is made as to whether the brake pedal  20  is being operated. If it is determined in step S 8  that the brake pedal  20  is not being operated, the method changes to step S 9 . In step S 9 , coasting of the motor vehicle takes place without the application of a regenerative braking torque until the vehicle comes to rest. 
     Following step S 9  the method is terminated at S 11 . 
     If it is determined in step S 8  that the brake pedal is being operated, the method changes to step S 10 . In step S 10  the deceleration devices  12 ,  14  are controlled, wherein the total deceleration is comprised of the deceleration components of the deceleration device  12 ,  14  and overall fulfills the driver&#39;s braking wishes. 
     Following step S 10  the method is terminated in the end step S 11 .