Method for operating a brake system, and brake system

1. A method for operating a brake system for motor vehicles, comprising:

a plurality of heel brakes for which wheel-specific setpoint pressures (p n soll ) are specified,

an electrically controllable pressure supply device, which can provide a brake system pressure for actuating the wheel brakes, and

an electrically controllable inlet valve and an electrically controllable outlet valve, the latter being arranged between the wheel brake and a pressure medium reservoir, per wheel brake for setting wheel-specific brake pressures (p n R ), wherein at least one outlet valve, each outlet valve, is analogized or analog-controlled and is controlled by an electric control variable (I n),

wherein at least one valve-specific control characteristic (I Ö (p R ), I s ( p R ), Q(I,p R )) and/or a valve-specific parameter (Q min , p min , Δt Ö ) is determined for the analogized or analog-controlled outlet valve by the brake system, and in that the analogized or analog-controlled outlet valve is controlled in accordance with the valve-specific control characteristic and/or the valve-specific parameter to reduce the brake pressure (p R ) in the wheel brake associated with the outlet valve,

wherein to reduce the brake pressure, the analogized or analog-controlled outlet valve is supplied alternately for a first time period (Δt Ö ) with a value (I open ) of the control variable equal to or above an opening current (I Ö ) of the outlet valve and for a second time period (Δt s ) with a value (I closed ) of the control variable below a closing current (I s ) of the outlet valve, and

wherein a duration of the first time period is calculated based on a valve-specific minimum pressure gradient ({dot over (p)} min ).

2. The method as claimed in claim 1 , wherein the valve-specific control characteristic is a valve-specific opening characteristic curve (I Ö (p R )) of the at least one outlet valve which is determined by the pressure supply device and of a pressure detection device which measures the pressure (P) of the pressure supply device.

3. The method as claimed in claim 1 , wherein the valve-specific control characteristic is a valve-specific closing characteristic curve (I s (p R )) of the at least one outlet valve which is determined by the pressure supply device and of a pressure detection device which measures the pressure (P) of the pressure supply device.

4. The method as claimed in claim 1 , wherein a value is specified for the second time period (Δt s ) and In that the first time period (Δt Ö ) is determined as the valve-specific parameter by the brake system.

5. The method as claimed in claim 1 , wherein the duration of the first time period (Δt Ö ) is also determined in accordance with a setpoint pressure gradient ({dot over (p)} soll ), the setpoint pressure gradient determined from the specified setpoint pressure (p soll ).

6. A method for operating a brake system for motor vehicles, comprising:

a plurality of wheel brakes, for which wheel-specific setpoint pressures (p n soll ) are specified,

an electrically controllable pressure supply device, which can provide a brake system pressure for actuating the wheel brakes, and

an electrically controlled inlet valve and an electrically controllable outlet valve, the latter being arranged between the wheel brake and a pressure medium reservoir, per wheel brake for setting wheel-specific brake pressures (p n R ), wherein at least one outlet valve, each outlet valve, is analogized or analog-controlled and is controlled by an electric control variable (I n ),

wherein at least one valve-specific control characteristic (I Ö (p R ), I s (p R ), Q(I,p R )) and/or a valve-specific parameter (Q min , p min , Δt Ö ) is determined for the analogized or analog-controlled outlet valve by the brake system, and in that the analogized or analog-controlled outlet valve is controlled in accordance with the valve-specific control characteristic and/or the valve-specific parameter to reduce the brake pressure (p R ) in the wheel brake associated with the outlet valve, and

wherein a control characteristic map (Q(I, p R )), which describes the volume flow (Q) through the at least one outlet valve as a function of the control variable (I) of the at least one outlet valve and the brake pressure (p R ) of the wheel brake associated with the at least one outlet valve, is determined by the pressure supply device.

7. The method as claimed in claim 6 , wherein the control characteristic map (Q(I, p R )) is determined by a speed control operation on the pressure supply device.

8. The method as claimed in claim 1 , wherein for different brake pressures (p R ) of the wheel brake associated with the at least one outlet valve, the valve-specific parameter is determined as a valve-specific minimum volume flow (Q min ) or a valve-specific minimum pressure gradient ({dot over (p)} min ) of the at least one outlet valve.

9. The method as claimed in claim 8 , wherein a setpoint pressure gradient ({dot over (p)} soll ) is determined from the current brake pressure (p R ) and a specified setpoint pressure (p soll ) of the wheel brake and is compared with the minimum volume flow (Q min ) or the minimum pressure gradient ({dot over (p)} min ), and in that the at least one outlet valve is controlled in accordance with the comparison, the value of the control variable and/or a duration of the first time period (Δt Ö ), is chosen.

10. A method for operating a brake system for motor vehicles, comprising:

a plurality of wheel brakes, for which wheel-specific setpoint pressures p n soll ) are specified,

an electrically controllable pressure supply device, which can provide a brake system pressure for actuating the wheel brakes, and

an electrically controllable inlet valve and an electrically controllable outlet valve, the latter being arranged between the wheel brake and a pressure medium reservoir, per wheel brake for setting wheel-specific brake pressures (p n R ), wherein at least one outlet valve, each outlet valve, is analogized or analog-controlled and is controlled by an electric control variable (I n ),

wherein at least one valve-specific control characteristic (I Ö (p R ), I s (p R ), Q(I,p R )) and/or a valve-specific parameter (Q min , p min , Δt Ö ) is determined for the analogized or analog-controlled outlet valve by the brake system, and in that the analogized or analog-controlled outlet valve is controlled in accordance with the valve-specific control characteristic and/or the valve-specific parameter to reduce the brake pressure (p R ) in the wheel brake associated with the outlet valve, and

wherein the inlet valve of the one of the plurality of wheel brakes associated with the analogized or analog-controlled outlet valve is controlled in such a way that it is closed but can be overridden, and all the other inlet valves are controlled in such a way that they are closed and cannot be overridden, in order to change the brake pressure (p R ) in the one of the plurality of wheel brakes.

11. The method as claimed in claim 10 , wherein the at least one outlet valve is supplied with a value of the control variable approximately equal to an opening current (I Ö ) or below the opening current (I Ö ) by a specified amount.

12. The method as claimed in claim 11 , wherein the valve-specific parameter is determined as a valve-specific minimum volume flow, and the pressure supply device is controlled in such a way that the volume flow output by the pressure supply device is greater than the valve-specific minimum volume flow (Q min ) of the at least one outlet valve.

13. A brake system for motor vehicles, comprising:

a plurality of wheel brakes,

an electrically controllable pressure supply device, which can provide a brake system pressure for actuating the wheel brakes, wherein the brake system pressure (P) provided can be determined by a pressure detection device,

a brake master cylinder, which can be actuated by a brake pedal and is connected to the wheel brakes in a separable manner,

an electrically controllable inlet valve and an electrically controllable outlet valve, the latter being arranged between the wheel brake and a pressure medium reservoir, per wheel brake, wherein at least one outlet valve is analogized or analog-controlled, and

an electronic control and regulating unit for controlling the electrically controllable pressure supply device, the inlet valves and the outlet valves,

wherein the method as claimed in claim 6 carried out in the electronic control and regulating unit.