The invention relates to a braking system according to the generic concept of claim 1 and a corresponding method. 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 versions without travel simulator as known, for example, from WO 2010/088 920 A1 and those with travel simulator as known, for example, from DE 10 2005 018 649 A1 and DE 10 2009 055 721 A1.
 In the above-mentioned brake booster designs, there is a serious safety risk if, during pressure reduction when the pedal force and pedal travel are reduced, the drive of the brake booster jams 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.
 DE 10 2009 043 484 A1 describes a conventional ABS and ESP system, as does DE 103 18 401 A1. Another conventional ABS and ESP system is known from DE 10 2004 050 103 A1.
 DE 10 2006 050 277 A1 describes a genus-forming braking system in which pressure can be released into a reservoir in the event of a fault.
 The task of the invention is to provide an inexpensive and effective measure for this fault case in a brake system.
 The task is solved by the brake system according to claim 1 and by the method according to claim 6.
 The solution to the task is to use an accumulator chamber for pressure reduction in at least one wheel brake. Here, the often already existing accumulator chambers of the conventional ABS or ESP system can be used. In these systems, when the pressure is reduced for ABS control, pressure medium is fed into the accumulator chamber 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 use of an accumulator chamber described above is furthermore applicable to a brake booster with travel simulator and accumulator chamber, as known from DE 10 2009 055 721 A1. Here, too, in the event of a fault, pressure medium can enter the accumulator chamber(s) to reduce the 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 expense for the solution of brake pressure reduction according to the invention is very low in the event of a fault in the drive or transmission of the brake booster. Provided that an accumulator chamber is available, only the software of conventional ABS or EPS systems has to be adapted.
 In the following, the use of at least one storage chamber according to the invention is explained in more detail.
 Showing: EN 10 2010 022 493 B4 2016.11.17 3/5
 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 pressure reduction in the event of a fault in the drive or transmission.
 Fig. 1 shows, from a mechanical point of view, a conventional brake system with ABS system with the known valve circuit and pump, as well as a brake booster 1 for a 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, whereby both pressurized fluid enters the accumulator chamber 8 and is 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 and thus zero pressure is generated, 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 transducer is installed in the wheel circuit, in the event of a fault the exhaust valve 7 is opened until a pressure increase is again desired by greater pedal travel, or the exhaust 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 a high brake pressure also causes high restoring forces on the piston, which counteracts complete blocking.
 Fig. 2 shows a system. This system has a displacement simulator with additional components, which are 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 10 2009 043 484 A1.
 As with the braking system of Fig. 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 case occurs, it is detected by comparing the pedal travel, measured with sensor 3, and the pressure in the master brake 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 DE 10 2010 022 493 B4 2016.11.17 4/5 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 lot 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 Fig. 1 and Fig. 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