Method for determining a pressure/volume characteristic curve of a wheel brake

1. A method for determining a pressure/volume characteristic curve (Vi(p)) of one or more of a plurality of wheel brakes in a brake system for a motor vehicle comprising:

an electrically controllable pressure supply device which is connected hydraulically to the plurality of wheel brakes and can output a first pressure medium volume (VDQ) for activating the plurality of wheel brakes, wherein the first pressure medium volume which is output by the pressure supply device can be determined, and

an electrically actuatable inlet valve for each of the plurality of wheel brakes for disconnecting the connection between each wheel brake and the pressure supply device,

wherein the method comprises:

during one or more service braking operations of the motor vehicle occurring when the motor vehicle is in motion, activating at least two different groups (GM) of two or more of the plurality of wheel brakes by the pressure supply device at various pressures;

determining second pressure medium volumes (VDQm) which are output by the pressure supply device in order to activate each of the at least two different groups; and

determining pressure/volume characteristic curves (Vi(p)) of each of the wheel brakes in the at least two different groups during the one or more service braking operations on the basis of the second pressure medium volumes which are output by the pressure supply device to activate the at least two different groups of wheel brakes.

2. The method as claimed in claim 1 , wherein in order to obtain a pressure medium volume (VDQm) which is output by the pressure supply device the pressure supply device is connected to one of the at least two different groups of two or more of the plurality of wheel brakes, while the pressure supply device is disconnected from wheel brake/brakes not included in the one of the at least two different groups, and a pressure buildup in the one of the at least two different groups of wheel brakes is implemented by the pressure supply device.

3. The method as claimed in claim 1 , wherein the at least two different groups (Gm) of wheel brakes are activated in chronological succession by the pressure supply device.

4. The method as claimed in claim 1 , wherein during the one or more service braking operations the at least two different groups (Gm) of wheel brakes are activated with different pressures (pj) by the pressure supply device, wherein the respective pressure of the pressure supply device is measured.

5. The method as claimed in claim 1 , wherein one of at least two different groups of two wheel brakes comprises wheel brakes which are diagonally opposite one another.

6. The method as claimed in claim 1 , wherein one of the at least two different groups of two wheel brakes comprises the two wheel brakes of a front axle.

7. The method as claimed in claim 1 , wherein none of the at least two different groups of wheel brakes comprises at least two rear-axle wheel brakes and just a single front-axle wheel brake.

8. The method as claimed in claim 1 , wherein a pressure (pj) which is built up in a respective one of the at least two different groups (Gm) of activated wheel brakes by the pressure supply device is set in such a way that the braking deceleration (aBrake) requested by a vehicle driver or a braking function is achieved.

9. The method as claimed in claim 1 , wherein said method is carried out exclusively during braking operations at vehicle speeds which are lower than a predefined threshold value.

10. The method as claimed in claim 1 , wherein the activation of the wheel brakes by the pressure supply device for obtaining the pressure/volume characteristic curves (Vi(p)) of the wheel brakes is interrupted as soon as a an anti-lock brake control process or a vehicle movement dynamics control process, is requested.

11. The method as claimed in claim 1 , wherein the number (mmax) of different ones of the at least two different groups (Gm) of wheel brakes is selected to be equal to or larger than the number (n) of wheel brakes.

12. The method as claimed in claim 1 , wherein the capacity (Vi) for each wheel brake is obtained for a predefined or predetermined pressure (pj), from the determined pressure medium volumes on the basis of a linear equation system.

13. The method as claimed in claim 1 , wherein a pressure (pj) which is built up in a respective one of the at least two different groups (Gm) of activated wheel brakes by the pressure supply device is set to achieve an adaptive cruise control function.

14. A brake system for motor vehicles comprising:

an electrically controllable pressure supply device which is connected hydraulically to wheel brakes and can output a pressure medium volume (VDQ) for activating the wheel brakes, wherein the pressure medium volume which is output by the pressure supply device can be determined, and

a master brake cylinder which can be activated by a brake pedal is connected to the wheel brakes in a disconnectable fashion,

an electrically actuable inlet valve for each of the wheel brakes for disconnecting the connection between each wheel brake and the pressure supply device, and

an electronic open-loop and closed-loop control unit for actuating the electrically controllable pressure supply device and the inlet valves,

wherein the electronic open-loop and closed-loop control unit is configured to:

during one or more service braking operations of the motor vehicle occurring when the motor vehicle is in motion, activate at least two different groups (GM) of two or more of the plurality of wheel brakes by the pressure supply device at various pressures;

determine second pressure medium volumes (VDQm) which are output by the pressure supply device in order to activate each of the at least two different groups; and

determine pressure/volume characteristic curves (Vi(p)) of each of the wheel brakes in the at least two different groups during the one or more service braking operations on the basis of the second pressure medium volumes which are output by the pressure supply device to activate the at least two different groups of wheel brakes.