The brake system serves to decelerate the motor vehicle and thus it also provides a braking force that is acting on at least one wheel of the motor vehicle. The braking force is applied to the wheel by means of the wheel brake. When the vehicle has several wheels, the brake system pressure is preferably applied to several of these wheels, or to all of these wheels to which brake pressure or actual brake pressure is supplied. In this respect, the brake system is provided as a service brake of motor vehicle, or it forms at least a component of the brake system.
The brake system is preferably provided with a main brake cylinder in which a main brake piston is arranged in a displaceable manner. The main brake piston delimits together with the main brake cylinder the volume of the brake fluid, which is variable, while its amount depends on the position of the main brake system. The main brake piston is coupled to an operating element, which is provided for example in the form of a brake pedal. The driver of the motor vehicle can set a desired braking force by means of the operating element, which is then designated for a predetermined braking force and preferably has a fixed correlation to a predetermined brake pressure.
The brake system is preferably provided as an electro-hydraulic brake system. This means that in at least one operating mode of the brake system, the available volume of the brake fluid does not provide the actual braking pressure immediately upon the actuation of the wheel brake, or provides only a part thereof. Instead, a desired brake pressure is determined during the actuation of the operating element, which can be realized with the aid of at least one sensor that is associated with the operating element and/or with main brake piston and/or with the main brake cylinder and/or with a simulator cylinder at which a simulator piston is arranged or to which it is assigned in a displaceable manner.
The sensor can be for example designed as a displacement sensor or as a pressure sensor. In the first case, the actuating distance of the operating element is determined by means of the sensor, so that the operating element is displaced during its actuation. In addition or as an alternative, it is of course also possible to determine the pressure that is present in the main braking cylinder by means of the sensor. The desired pressure is then determined from the variables that are measured by the sensors, which is to say for example the path and/or the pressure. Subsequently, an actual pressure is applied or set to the wheel brake which corresponds to the desired brake pressure.
The desired brake pressure is provided by the brake pressure source, which is present for example in the form of a pump, in particular a pump that is operated electrically. According to the operating mode described above, the brake system is not connected to the wheel brake, or at least it is not connected directly, or it is fluidly connected. In order to nevertheless provide a haptic feedback for the driver of the motor vehicle during the actuation of the operating element, it is preferred when and optional brake force simulator is associated with the main brake cylinder. This simulator is provided with the simulator piston, which is arranged in a displaceable manner in a simulator cylinder and supported via a spring element on a wall of the simulator cylinder and in this respect subjected to filtering force.
The simulator piston delimits together with the simulator cylinder a simulator cylinder fluid volume, which is variable similarly to the brake fluid volume, wherein the amount of the simulator fluid volume depends on the position of the simulator piston. The simulator fluid volume is fluidically connected with the brake fluid volume. When the operating element is actuated, the brake fluid volume is reduced and the brake fluid that is contained to the brake fluid volume is supplied to the simulator fluid volume. Accordingly, the simulator fluid volume is increased, whereby the simulator piston is deflected in response to the spring force.
Depending on the spring force, which can in turn depend on the deflection of the simulator piston, a counter-force acts in the operating mode described above as a result of the fluidic connection between the simulator fluid volume and the brake fluid volume on the operating element, which is oriented in the opposite direction to the operating force that is applied to the operating element. As a result, the driver receives a haptic signal via the operating element, which is essentially dependent on the deflection of the operating element from its starting position or rest position.
In order to provide a rebound plane in the event of a defect of the brake system, for example in the event of a defect of the brake pressure source, a direct fluidic connection is preferably provided between the main brake cylinder and the wheel brake. The actual pressure can thus be rebuilt in this manner on the wheel brake in case of a defect of the brake system when the operating element is actuated. For this purpose, however, the driver must apply a substantially greater operating force to the operating element than is customary.
The brake pressure source is provided with the pressure cylinder and the pressure piston, wherein the pressure piston is linearly arranged in a displaceable manner in the pressure cylinder. The arrangement of the pressure piston inside the pressure cylinder is accomplished by means of the engine, which is preferably provided as an electric engine. However, it goes without saying that another design of the engine can be also realized. The engine or the drive shaft of the engine is coupled via the transmission to the pressure piston. The transmission in this case serves for converting a rotary movement of the drive shaft to a linear displacement of the pressure piston. The transmission can be thus also referred to a as a translatory gear.
In particular with an autonomous driving operation, which is to a partially autonomous or a fully autonomous driving operation of the motor vehicle, it must be ensured that the actual brake pressure can be built up on the wheel brake by means of the brake pressure source because under some circumstances the driver may not be able to provide the rebound plane described above, for example because he is not on the board of the motor vehicle but instead he may be outside of the vehicle, or he may be present on the board of the vehicle but not in a position in which to actuate the operating element.