Brake-by-wire brake systems are being used increasingly widely in motor vehicle technology. Such brake systems often include, in addition to the brake master cylinder actuatable by the vehicle driver, an electrically controllable pressure supply unit by means of which, in the brake-by-wire operating mode, actuation of the wheel brakes takes place either directly or via the brake master cylinder. In order to communicate an agreeable pedal feel to the vehicle driver in the brake-by-wire operating mode, the brake systems usually include a brake pedal feel simulation arrangement which is operatively connected to the brake master cylinder, for example. In order to activate the brake system, a set-point generator which, for example, evaluates the electrical signals from one or more sensors in order to determine the driver's braking request (actuation request), is provided in order to determine a reference value for activating the pressure supply unit. However, in these brake systems the pressure supply unit may be actuated on the basis of electronic signals without active intervention by the driver. These electronic signals may be outputted, for example, by an electronic stability program (ESC) or by a distance control system (Adaptive Cruise Control or ACC), so that the set-point generator determines a reference value for activating the pressure supply unit on the basis of these signals.
A brake system is known from the international patent application WO 2008/025797 A1, in which brake system it is proposed to hold in readiness, in an unpressurized state in the pressure supply unit, the pressure medium required for electrically controlling the pressure introduced into an intermediate chamber used to actuate the brake master cylinder, and to subject said pressure medium to increased pressure as required in order to be able to dispense with complex and energetically unfavorable buffering of hydraulic actuation energy. For this purpose the pressure supply unit is formed by a cylinder-piston arrangement, the piston of which can be actuated by an electromechanical actuator. The brake system includes a pressure sensor for detecting the actual pressure of the pressure supply unit. A control system of the brake system, in particular of the pressure supply unit, is not described.
The brake system described in DE 10 2010 040 097 A1 includes a brake master cylinder and a pressure supply unit formed by a cylinder-piston arrangement, the piston of which can be actuated by an electromechanical actuator. Each wheel brake is hydraulically connectable to the brake master cylinder and to the pressure supply unit. Here, too, pressure sensors for detecting the actual pressure of the pressure supply unit are provided. A method for controlling the pressure supply unit is not described in detail here also.
The unpublished document DE 10 2011 076 675.8 describes a method for controlling an electrohydraulic brake system for motor vehicles having an electronically activatable pressure supply unit which is connected to hydraulically actuatable wheel brakes. The pressure supply unit comprises a cylinder-piston arrangement with a hydraulic pressure chamber the piston of which is displaceable relative to a rest position by an electromechanical actuator. In order to control the pressure supply unit there is provided a control device with a pressure regulator which carries out the control on the basis of a pressure reference value and a pressure actual value of the pressure supply unit. For this purpose the pressure actual value must therefore be determined, for example by means of a pressure sensor as in the above-mentioned brake systems.
It is the object of the present invention to provide a method and a device for controlling an electrohydraulic brake system having an electronically activatable pressure supply unit which make it possible to set a predetermined pressure reference value in the pressure supply unit without the necessity to determine a pressure actual value of the pressure supply unit. In particular, the brake system is to be able to operate without a pressure sensor for measuring the pressure of the pressure supply unit.
This object is achieved according to the invention by a method, a device and by a corresponding brake system as described herein.
The invention is based on the concept of carrying out a position control of the actuator, which control is based on the actuator position actual value and on an actuator position reference value which is determined on the basis of the predetermined pressure reference value.
According to a preferred embodiment of the method according to the invention, an actuator torque actual value of the electromechanical actuator is determined and used for the control.
For simple and rapid determination of the actuator position reference value, the actuator position reference value is preferably determined from the predetermined pressure reference value on the basis of a predetermined pressure/travel characteristic.
In order to take account of the actual situation, the actuator torque actual value is preferably taken into account additionally when determining the actuator position reference value. In this way a second independent input variable, so to speak, is taken into account.
According to a preferred development of the method according to the invention, a correction of a (first) actuator position reference value calculated from the pressure reference value is carried out on the basis of the actuator torque actual value. Here, the (first) actuator position reference value is especially preferably calculated solely from the pressure reference value. Adaptation to the actual situation is effected by means of the correction.
First, an actuator torque reference value is preferably determined from the pressure reference value and is then used for the correction of the (first) actuator position reference value.
A correction actuator position value is preferably determined on the basis of the pressure reference value, the actuator torque actual value and an actuator acceleration torque. This correction actuator position value is then especially preferably added to the actuator position reference value calculated from the pressure reference value. This resulting actuator position value is advantageously used as the actuator position reference value in the position control.
It is preferred that the correction actuator position value is formed by integrating a difference which is yielded through subtraction by the actuator torque reference value calculated from the pressure reference value, and the difference from the actuator torque actual value and an actuator acceleration torque.
The method according to the invention is advantageously carried out in a brake system for motor vehicles which can be activated in a so-called brake-by-wire operating mode both by the vehicle driver and also independently of the vehicle driver, is preferably operated in the brake-by-wire operating mode and can be operated in at least one fall-back operating mode in which only operation by the vehicle driver is possible.
An advantage of the invention is that, in order to set the predetermined pressure reference value in the pressure supply unit, it is not necessary to determine a pressure actual value of the pressure supply unit. The control is based only on the signals of the electromechanical actuator, for example the actuator torque and the actuator position, and variables derivable therefrom, which are usually available in any case as a result of activation. The necessary equivalent parameter for the actual pressure information is determined solely on the basis of the actuator signals. A pressure sensor for measuring the pressure of the pressure supply unit can thereby be dispensed with in the brake system, reducing the production costs.
The wheel brake or brakes is/are preferably connected hydraulically to the pressure chamber of the pressure supply unit. In this way pressure medium volume expelled from the pressure chamber is displaced directly into the wheel brake(s), whereby throttling effects through friction or the like are avoided. An electrically activatable inlet valve with which the wheel brake can be isolated hydraulically from the pressure chamber is preferably arranged between one, in particular each, wheel brake and the pressure chamber.
Alternatively, it is preferred that the wheel brake(s) is/are hydraulically connected disconnectably to a hydraulic (outlet) pressure chamber of a brake master cylinder or of a floating piston arrangement, the brake master cylinder piston or the inlet pressure side of the floating piston arrangement being actuatable by the pressure of the pressure chamber of the pressure supply unit. The wheel brake(s) is/are then no longer directly connected hydraulically to the pressure chamber of the pressure supply unit, but is/are actuatable by means of the pressure supply unit.
The, especially each, wheel brake is preferably connectable to a brake fluid reservoir, for example via an electrically activatable outlet valve.
It is also preferred that the hydraulic pressure chamber of the cylinder piston arrangement is connectable to the brake fluid reservoir via at least one electrically activatable valve.
The control device preferably includes a first functional module in which a first actuator position reference value is determined from the pressure reference value (PV,soll) and in which a correction of the first actuator position reference value is carried out on the basis of the actuator torque actual value, in order to take account of operationally induced deviations of the actual interrelationship between pressure and actuator position (pressure/volume interrelationship) from the interrelationship assumed or taken as a basis in the determination. The first actuator position reference value is especially preferably determined from the pressure reference value on the basis of a predetermined pressure-travel characteristic.
The switching states of the valves associated with the wheel brakes, and/or information on whether a pressure control function is active and optionally which pressure control function is active, are preferably supplied additionally to the first functional module. The switching states and/or the information are especially preferably used for determining the first actuator position reference value.
According to a development of the device according to the invention, the control device includes a second functional module in which a correction actuator position value is determined on the basis of the pressure reference value, the actuator torque actual value and an actuator acceleration torque. With the correction actuator position value a motor-signal based correction of the first actuator position reference value is possible.
The invention also relates to a brake system in which a method according to the invention is carried out or which includes a device according to the invention.