1. Field of the Invention
The invention relates to a method for operating a combined vehicle braking system, in particular for motor vehicles.
2. Description of the Related Art
A combined vehicle braking system for motor vehicles is disclosed. The combined vehicle braking system generally comprises hydraulically actuable wheel brakes on a front axle and electromechanically actuable wheel brakes on a rear axle, wherein the vehicle wheels which are assigned to one axle or to both axles are driven at least intermittently by an electric motor which can be operated as a generator in order to recover braking energy, and in the generator mode a braking force is brought about at the vehicle wheels which are assigned to the drive axle, and wherein a pedal travel sensor determines the driver's braking request and feeds it to an open-loop and closed-loop control unit which carries out a braking force distribution for the hydraulically actuable wheel brakes, the electromechanically actuable wheel brakes and the electric motor which can be operated in the generator mode.
The purpose of such brake systems in motor vehicles is to recover as much of the energy converted during braking, to store it in the vehicle and to re-use it to drive the vehicle. As a result, the consumption of energy by the vehicle overall can be lowered, the efficiency can be increased and the operation can therefore be made more economical. Motor vehicles having a brake system which is configured for regenerative braking generally have for this purpose various types of brakes which are also referred to as brake actuators.
In this context, a pair of hydraulic friction brakes are used for braking the front axle wheels, and a pair of electromechanically actuable friction brakes are used for braking the rear axle wheels, as are known from conventional motor vehicles, along with an electric motor which can be operated as a generator. As much of the entire braking force as possible is harnessed by means of the generator or the electric motor which is in the generator mode. The acquired electrical energy is fed into or fed back into a storage medium such as, for example, an on-board battery, and is re-used to drive the motor vehicle by means of a suitable drive.
In order to brake such a motor vehicle which has an electric motor as a sole drive or additional drive and for recovering braking energy in the generator mode, a further braking torque is applied by the electric motor in addition to the braking torque of the hydraulically actuable and/or electromechanically actuable wheel brakes, which latter braking torque is applied by the driver-actuated brake system. This braking torque of the electric motor arises from the known effect in electric motors which, when they are driven mechanically without electric current being supplied, act as a dynamo or generator and generate electric current. In this context, an opposing torque, which counteracts the mechanical drive and acts in the present case as a braking torque, is produced. The electric motor which is operated as a generator therefore acts as a brake. The entire braking force of the motor vehicle is composed here of the braking force of the hydraulically actuable wheel brakes, the braking force of the electromechanically actuable wheel brakes and the braking force of the electric motor which acts as a generator.
DE 103 19 663 A1 discloses a method for setting the pedal characteristic curve of a hybrid brake system with a changed braking force distribution. In the previously known method for operating a braking system, which method comprises a hydraulic operating brake system and an electric operating brake system with wheel brakes to which brake pressure is applied when a brake pedal is actuated, the control unit controls a brake pressure modulator of the hydraulic operating brake system and the electric operating brake system in such a way that when there is a change in the braking force distribution between the hydraulic and the electric operating brake systems the ratio of the pedal force and/or pedal travel to the overall braking torque of the vehicle remains essentially constant. An electric motor which can be operated as a generator is not provided.
The potential for recovering braking energy is, for a combined vehicle braking system of the generic type specified at the beginning and given a braking force distribution from the front axle to the rear axle of 50% to 50%, correspondingly 50%. However, for the lower deceleration range, a large increase in the proportion of braking force is considered possible and permissible at the rear axle relative to the front axle.
An object of the present invention is therefore to present a method which contributes to improving the energy recovery potential.
This object is achieved in that the braking force distribution is carried out in the range of low vehicle decelerations in such a way that the proportion of braking force at the vehicle wheels of the rear axle is greater than the proportion of braking force at the vehicle wheels of the front axle. In this context there is additionally provision that the proportion of braking force at the vehicle wheels of the driven axle, preferably mainly the rear axle, is generated exclusively, or virtually exclusively, by the electric motor which is in the generator mode.
In order to make the method more specific there is provision that the braking force at the hydraulic wheel brakes is between 0% and 49% of the braking force for the entire motor vehicle, while the braking force which is generated by the electric motor which is in the generator mode is between 51% and 100% of the braking force for the entire motor vehicle.
In contrast, it is provided that the braking force distribution is carried out in the range of relatively high vehicle decelerations in such a way that the proportion of braking force at the vehicle wheels of the rear axle is the same as, virtually the same as or smaller than the proportion of braking force at the vehicle wheels of the front axle.
The range of low vehicle decelerations is below a vehicle deceleration of 0.3 g, while the range of relatively high vehicle decelerations is above a vehicle deceleration of 0.3 g.
In one particularly advantageous development of the method there is provision that the pedal travel sensor determines the actuation travel of a brake pedal, and the electric motor is driven in the generator mode directly after a driver's braking request has been detected. In this context, there is, in particular, provision that the driving of the electric motor in the generator mode is carried out before the hydraulically actuable wheel brakes make available a braking force. As a result of this measure, when a driver's braking request is detected, braking energy is immediately recovered before all the free travel in the hydraulic brake system is traveled through. The device here has the effect of optimizing the free travel.
These and other aspects of the invention are illustrated in detail by way of the embodiments and are described with respect to the embodiments in the following, making reference to the Figures.
The invention will be explained in more detail below by means of an exemplary embodiment with reference to the appended drawing, in which:
A circuit diagram of the vehicle braking system according to aspects of the invention is illustrated in
As is also apparent from
As is indicated merely schematically in
The driver's braking request is, as already mentioned, sensed by the pedal travel sensor 11 and fed via a signal line 17 to the electronic open-loop and closed-loop control unit 14. The signals of the operator control element 13 of the foundation brake are fed to the open-loop and closed-loop control unit 14. The two decentralized electronic open-loop control units 15 of the electromechanically actuable wheel brakes 2 are also connected to the open-loop and closed-loop control unit 14 via a signal line 17″.
An electric motor 16 which, on the one hand, acts as an independent drive in an electric car or as an additional drive in a vehicle with an internal combustion engine and, on the other hand, is set to recover braking energy in the generator mode, is connected to the open-loop and closed-loop control unit 14 via a further signal line 17′. The electric motor 16 draws its supply voltage in the case of driving via a supply line 18′ from the vehicle's on-board electrical system and feeds electrical energy back into the vehicle's on-board electrical system via the same supply line 18′ in the generator mode. In the generator mode just mentioned, the electric motor 16 acts as a dynamo and generates electric current. In the process an opposing force is produced which acts a further braking torque. The electric motor 16 which is operated as a generator therefore acts as a brake. The entire braking force of the motor vehicle is therefore composed of the braking force of the hydraulically actuable wheel brakes 1, the braking force of the electromechanically actuable wheel brakes 2 and the braking force of the electric motor 16 which acts as a generator. These three braking forces have to be adapted in a suitable way, which is made possible by a suitable braking force distribution, in which case a largely constant relationship exists between the driver's specification through the pedal travel and reaction force of the brake pedal and the front axle brake pressure. Furthermore, a corresponding relationship is assigned to the pedal travel and to the front axle brake pressure according to a fixed algorithm or according to a reproducible algorithm. The rear axle braking force of the electromechanical brakes can then, however, be set freely from zero to a maximum value without a reaction on the brake pedal. As a result, the braking forces from the generator and the rear axle friction brake can be balanced with the same deceleration of the vehicle and without a reaction on the brake pedal.
In the instances of braking force distribution which follow, the braking force at the rear axle, referred to also as RA for short, is always plotted on the ordinate, and the braking force at the front axle, also referred to as FA for short, is always plotted on the abscissa.
In order to bring about a suitable braking force distribution for the three abovementioned braking forces, the previously known braking force distribution of a conventional braking system will first be considered on the basis of
The present invention then provides for the potential for the recovery of braking energy to be increased on the basis of the braking force distribution described in
As is also apparent from
As already described, the pedal travel sensor 11 determines the actuation travel of the brake pedal 3 and detects the driver's request. In this context, it is particularly advantageous that the electric motor 16 in the generator mode is actuated directly after detection of a driver's braking request, as is apparent in
The proposed operator control strategy improves the potential for energy recovery in combined braking systems in hybrid vehicles and virtually the full potential can therefore be utilized with rear axle drive, in a way which is otherwise possible only with special braking systems of the brake-by-wire type. Compared to these braking systems, an optimum braking force distribution can also be set in the case of relatively high deceleration requirements without valve circuits being necessary.
While preferred embodiments of the invention have been described herein, it will be understood that such embodiments are provided by way of example only. Numerous variations, changes and substitutions will occur to those skilled in the art without departing from the spirit of the invention. It is intended that the appended claims cover all such variations as fall within the spirit and scope of the invention.
This application is the U.S. national phase application of PCT International Application No. PCT/EP2007/056355, filed Jun. 26, 2007, which claims priority to German Patent Application No. DE 102006030927.8, filed Jul. 3, 2006 and German Patent Application No. DE 102006055765.4, filed Nov. 25, 2006, the contents of such applications being incorporated by reference herein.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP07/56355 | 6/26/2007 | WO | 00 | 12/29/2008 |