State of the art
 Due to the higher functionality and also the lower installation volume, hydraulic brake boosters (BKV) and in particular electromechanical brake boosters will be increasingly used in the future. In the case of brake systems with electromechanical brake boosters, a distinction is made between designs without travel simulator as known, for example, from PCT/EP2009/000694 and those with travel simulator as known, for example, from DE 102005018649 and DE 102009055721.
 From WO 2010/017998 A1 a brake system with adaptively controllable brake lining clearance is known, which comprises storage chambers in which the brake fluid forced out of the wheel brake for adjusting a brake lining clearance is stored. The storage chamber is also used to supply brake fluid to the respective brake circuit.
 From US 2009/0045672 A1, a brake system is known in which the ESP system is used for brake boosting when a fault or failure of the brake booster occurs.
 In above-mentioned brake booster designs, there is a serious safety risk if the brake booster drive jams during pressure reduction when pedal force and pedal travel are reduced, and pressure reduction is no longer possible. This results in the vehicle coming to a standstill on the road at the appropriate pressure level and no longer being able to be moved by the power of the engine. A similar situation can also occur with electric power steering if the motor or transmission jams. To prevent this, elaborate design measures, as well as quality assurance measures, are used to avoid this eventuality.
 The task of the invention is to provide an inexpensive and effective measure for this case of failure in a brake system.
 The solution to the task is to use an accumulator chamber for pressure reduction in at least one wheel brake. The storage chambers of the conventional ABS or ESP system, which often already exist, can be used here, as known, for example, from DE 10 2004 050 103. In these systems, pressure fluid is fed into the accumulator chamber during pressure reduction for ABS control by briefly opening the exhaust valve, thereby preventing the wheel in question from locking. The corresponding volume is pumped back into the brake circuit and master cylinder by the return pump.
 In the described case of a fault with a blocking drive, this path is also used. Provided that the pressure in the brake circuit does not reduce after the pedal force or pedal travel is reduced, which is measured with corresponding sensors, the outlet valve is opened for a longer period of time according to the invention so that the pressure medium enters the accumulator chamber. at the same time, the return pump can also be switched on.
 The high pressure, which is generated by the return pump after the exhaust valve is opened, may well push back the HZ pistons of the brake booster despite the drive being stuck, which can be determined by measuring the pressure or change in position of the drive.
 Depending on the pressure level when the jamming occurs, this solution can reduce the pressure completely or to a low level so that the driver can move the vehicle out of the danger zone at a reduced speed, if necessary.
 The aforementioned use of an accumulator chamber is furthermore applicable to a brake booster with travel simulator and accumulator chamber, as known from DE102009055721 and DE 102008051316. Here, too, in the event of a fault, pressure medium can enter the accumulator chamber(s) to reduce pressure after the upstream solenoid valve is opened, thereby significantly reducing the pressure in the wheel brakes. Fault detection and pressure reduction is particularly successful with this system, since the process can be monitored by system-related and already existing sensors for pedal travel, HZ piston position and pressure.
 Advantageously, the additional expenditure for the solution according to the invention for brake pressure reduction in the event of a fault of the drive or transmission of the brake booster is very low. Provided that a storage chamber is available, only the software, in particular in the case of the system known from DE102009043484, has to be adapted.
 In the following, the use according to the invention of at least one storage chamber for the two brake systems described above is explained in more detail.
 It shows: Fig. 1: Hydraulic brake booster with conventional ABS valve circuit and accumulator chamber for use in the event of a malfunction; Fig. 2: Electromechanical brake booster with accumulator chambers, which are used according to the invention for the event of a malfunction of the drive or transmission for pressure reduction.
 Figure 1 shows a conventional brake system with ABS system with the known valve circuit and pump, as well as a brake booster 1 for one brake circuit, the detailed description of which is omitted.
 The upper part of the figure shows the separately arranged brake booster 1 with tandem master cylinder 4, which is operated by brake pedal 2. Connected to the brake pedal 2 is a pedal travel switch or sensor 3, which indicates the start of braking or is also used for brake boosting. The separate unit, referred to as HCU 5, uses components for pressure modulation of ABS and ESP. Other solenoid valves are used for ESP, e.g., to build up pressure without brake booster activation, which are not shown. One or more pressure transducers 17 are also used for this function. If a wheel enters into greater slip as a result of excessive braking torque, the exhaust valve 7 connected to the brake piston by means of a hydraulic line is opened briefly to reduce pressure, causing both pressurized fluid to enter the accumulator chamber 8 and to be pumped back to the master cylinder 4 by the return pump 6.
 If a fault now occurs with a blocked drive, the pedal 2 is moved in the direction of the initial position to reduce the pressure. However, due to the blockage, the pressure reduction does not match the pedal position. To re-adjust the relationship between pedal position or pedal travel and set pressure, all outlet valves 7 are opened until the relationship between pedal travel and pressure transmitter 17 is restored. In many cases, the brake pedal 2 is moved to the initial position, which should correspond to zero pressure. If the return pump 6 is now switched on at the same time, its pressure acts against the blocked drive, which in turn can have the effect that the high pressure moves the master cylinder piston against the blocked drive and the pressure is reduced in accordance with the preset value, e.g. pedal travel. In this case, when the brake pedal 2 is in the initial position, the piston is moved over the sniff hole, thus generating zero pressure, which is possible via the pressure transducer in the THZ circuit.
 If the blockage is so severe that the return pump 6 has no effect, it must either be switched off at the maximum permissible pressure, which is measured by the pressure transducer 17, or it blocks and is switched off via the speed or current detection. In this case, the pressure reduction is given by the storage capacity of the storage chamber. If no pressure sensor is installed in the wheel circuit, in the event of a fault the outlet valve 7 is opened until a pressure increase is again desired by greater pedal travel, or the outlet valves 7 remain open for a certain time with "pedal in initial position" so that the storage chamber 8 is filled. Filling is always dependent on the brake pressure applied before blocking. If this is in the partial braking range, the pressure can be reduced to low values without an effective return pump, so that the driver receives a warning indication but can drive to the next parking space without further ado. If the set pressure is high, he can only drive to the next parking space at lower speed with the engine power. He can receive corresponding information in the display. In this way, he can escape the dangerous stopping on the roadway. This case will occur extremely rarely, since high brake pressure also causes high restoring forces on the piston, which counteracts complete blocking.
 Figure 2 shows a system corresponding to DE 102 009 043 484. This system has a travel simulator with additional components, described here only as pedal interface 14. According to the pedal travel preferably measured by means of the sensor 3, the electric motor 11 for brake pressure generation is controlled via the tandem master cylinder (THZ) 4. Storage chambers 8 are inserted in the connecting line from the THZ 4 to the brake caliper 10, which can be filled or emptied via upstream or upstream switchable solenoid valves 15. This can be used, for example, to replenish pressure fluid in the brake circuits for brake lining clearance control or for empty travel control in accordance with DE 102 009 04 484.
 As with the braking system of Figure 1, the storage chambers 8 can also be used in the case of the fault described above. The only difference is that there is no return pump. If the fault occurs, it is detected by comparing the pedal travel, measured with sensor 3, and the pressure in the master cylinder, measured with sensor 9, and by opening the switching valves 15 connected upstream of the storage chambers 8, the pressure reduction is initiated or controlled in both brake circuits in accordance with the pedal travel signal. The fault condition can also be detected or determined by comparing pedal travel, measured with sensor 3, and the measured motor current of the drive.
 Also in this braking system, as described for the braking system of Fig. 1, the pressure reduction into the accumulator chamber 8 depends on the level of the pilot pressure. For smaller values, the pressure reduction is determined by the accumulator pressure characteristic, and for piston accumulators by the effective preload spring, resulting in approx. 3 - 5 bar. This pressure is completely uncritical for driving on to the next parking place due to corresponding indications in the display. On the other hand, at high control pressure, what is said in Fig. 1 applies.
 If, in the brake systems according to Figs. 1 and 2, a renewed pressure build-up takes place during the drive to the parking space, this can take place in both systems by direct action from the brake pedal for the THZ pistons. In this case, pressure fluid can be returned to the brake circuits from the accumulator in the low pressure range by opening valves 7 or 15. Various control options are conceivable for this, which are not described in the variety, since nothing changes in the basic solution.
 As already explained, the additional expenditure in the solution according to the invention for an effective remedy for the critical fault case is very small. List of reference signs
- 1 Brake booster (BKV)
- 2 Brake pedal
- 3 Pedal travel switch / sensor
- 4 Tandem master cylinder (THZ)
- 5 HCU
- 6 Return pump
- 7 A-valve
- 8 Accumulator chamber
- 9 Accumulator chamber switch / sensor
- 10 Brake caliper
- 10a Brake piston
- 11 Electric motor
- 12 Pedal rod
- 13 Rotary encoder
- 14 Accumulator chamber MV
- 15 Pressure sensor