Method and control device for operating a brake system

Abstract

To provide a method of actuating a brake system (100) having a pressure reservoir (A) and a return pump (RP) electrically driven.SOLUTION: When an electric current flow (I) for driving a return pump (RP) is lower than the expected current flow, it is recognized that a pressure reservoir (A) is empty.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method and control for operating a braking system. [Background technology]

[0002] The brake system may have a pressure reservoir in which brake fluid can be buffered during braking after it has been released from at least one wheel brake cylinder of the brake system via a discharge valve for the purpose of releasing pressure.

[0003] The pressure reservoir can be emptied through a return pump against a pressure difference between the pressure in the pressure reservoir and the brake pressure in the pressure-actuated part of the brake system, and similarly through a diverter valve when the brake system pressure is lower than the pressure in the pressure reservoir. Summary of the Invention

[0004] Against this background, the proposed approach provides a method and a control device for operating a braking system, and finally a corresponding computer program product and machine-readable storage medium, respectively, according to the independent claims. Preferred developments and improvements of the proposed approach will become apparent from the detailed description and are set out in the dependent claims.

[0005] Advantages of the invention Advantageously, embodiments of the invention make it possible to know the fill level of a pressure reservoir of a brake system, depending on which different actions can be carried out.

[0006] A method is proposed for operating a brake system having a pressure reservoir and an electrically driven return pump, where the pressure reservoir is recognized as empty when the electrical current flow to drive the return pump is less than the expected current flow.

[0007] The ideas relating to the embodiments of the present invention can be considered to be based in particular on the ideas and realizations described below.

[0008] A brake system can be understood to be the braking system of a vehicle. The brake system can be operated by the driver of the vehicle via a brake pedal. The brake system can also be operated by a driver assistance system via a controllable actuator. When the brake pedal or actuator is operated, brake fluid is forced through an intake valve in a valve block of the brake system, generating brake pressure in the wheel brake cylinders of the brake system. This brake pressure presses brake linings against the vehicle's brake discs or brake drums, thereby generating a braking torque at the vehicle's wheels.

[0009] To reduce the braking torque at one wheel, for example to prevent this wheel from locking, the brake pressure in the wheel brake cylinder associated with this wheel can be reduced. For this purpose, the brake fluid can be discharged via the outlet valve of the wheel brake cylinder, which is arranged in the valve block, into the pressure reservoir of the brake system, which is connected to the valve block. For this purpose, the intake valve to the wheel brake cylinder is closed beforehand.

[0010] The pressure reservoir may be a pressure cylinder with a spring-loaded piston that is pressed against a spring by the released brake fluid. The more the piston is pressed against the spring, the higher the pressure required. The pressure reservoir has a limited reservoir volume. To keep the reservoir volume available, the pressure reservoir can be emptied via a return pump of the brake system that is connected to the pressure reservoir.

[0011] The return pump has a design-specific pumping rate, which it pumps as long as brake fluid is present in the pressure reservoir. This requires a pumping power that is essentially dependent on the pressure difference between the pressure reservoir and the brake pressure of the brake system.

[0012] However, when there is no brake fluid to pump, the return pump runs idle and draws in a negative pressure, which can approach a vacuum, during the suction phase. The negative pressure is reduced again during the pumping phase. Without brake fluid to pump, no pump power can be exerted.

[0013] In a rotary pump, the torque for driving the pump is proportional to the currently available pump power. The feed pump can be driven, for example, by an electric motor. The electrical current flow through the electric motor is substantially proportional to the torque and thus to the pump power. By monitoring the current flow, it is possible to monitor whether the return pump is currently pumping brake fluid.

[0014] Furthermore, the pressure reservoir can be recognized as empty when the gradient of the current flow is greater than a threshold gradient. When brake fluid is no longer being pumped, the current flow can decrease rapidly. When the current flow decreases more rapidly than a threshold gradient, it can be recognized that the pressure reservoir is imminent to become completely empty.

[0015] Furthermore, the pressure reservoir can be recognized as empty when the current flow is less than a threshold value. When the pressure reservoir is empty, the current flow only decreases to a base value when the electric motor is operating. To overcome losses such as friction in the return pump, an approximately constant base torque is required. This means that the electric motor provides the base load. The threshold value can be located slightly above this base value.

[0016] When the pressure reservoir is recognized as empty, the return pump can be deactivated, which avoids wear and tear, as well as vacuum noise and vibrations.

[0017] When the pressure reservoir is recognized as empty, the opening of the high-pressure switching valve of the brake system can be suspended. When the brake pressure is lower than the pressure in the pressure reservoir, pressure can be released from the pressure reservoir via the high-pressure switching valve. However, if the pressure reservoir is empty before the pressure in the brake system drops, the operation of the high-pressure switching valve is ineffective, which can cause noise and vibration. By suspending the opening, wear can be avoided. In addition to this, noise and vibration can be avoided.

[0018] When the pressure reservoir is recognized as empty, the stiffness adaptation of the brake system can be stopped. In vehicles with an electric motor, the electric motor can be operated as a generator to generate drag torque, thereby recovering electrical energy. However, operation as a generator has limitations, and when the electric motor cannot generate enough drag torque, additional braking torque provided by the brake system is required. Stiffness adaptation of the brake system is necessary to ensure that the mixing of drag torque and braking torque occurs as smoothly as possible. Stiffness adaptation can compensate for changing friction values ​​of, for example, brake discs and / or brake linings, thereby ensuring the desired braking torque is generated.

[0019] When drag torque is mixed or coordinated with braking torque, brake fluid stored in the pressure reservoir is pumped back to the wheel brake cylinder using a return pump to generate braking torque there. If there is not enough brake fluid stored, the pressure reservoir will empty before the desired braking torque is reached. The next time coordination occurs, under the assumption that there is insufficient stiffness, stiffness adaptation will attempt to pump more brake fluid to achieve braking torque. However, the pressure reservoir emptying before the braking torque is reached is not an effect of stiffness. Therefore, stiffness adaptation can be stopped when it is recognized that the pressure reservoir is empty.

[0020] When the pressure reservoir is recognized as empty, the pressure reservoir volume counter can be reset. When the pressure reservoir is recognized as empty, the pressure reservoir calculation model can be zeroed. By defining a reference point, the calculation of the pressure reservoir volume can be performed more accurately.

[0021] The method may be implemented, for example, as software or hardware or as a mixture of software and hardware, for example in a controller.

[0022] The approach proposed herein further provides a control device for operating a braking system configured to perform, control, or otherwise embody the steps of one aspect of the method proposed herein in a corresponding device.

[0023] The control device may be an electrical device having at least one computing unit for processing signals or data, at least one storage unit for saving signals or data, and at least one interface and / or communication interface for reading or outputting data embedded in a communication protocol. The computing unit may be, for example, a signal processor, a so-called system ASIC, or a microcontroller for processing the sensor signals and outputting data signals depending on the sensor signals. The storage unit may be, for example, a flash memory, an EPROM, or a magnetic memory unit. The interface may be configured as a sensor interface for reading sensor signals from sensors and / or as an actuator interface for outputting data and / or control signals to actuators. The communication interface may be configured for wireless and / or wired reading or outputting data. The interface may be, for example, a software module residing alongside other software modules in a microcontroller.

[0024] Also advantageous is a computer program product or computer program having a program code which may be stored on a machine-readable medium or storage medium, such as a semiconductor memory, a hard disk memory or an optical memory, and which is used to perform, implement and / or control the steps of the method according to one of the above-described embodiments, in particular when the program product or program is run on a computer or device.

[0025] It should be noted that while some of the possible features and advantages of the present invention are described herein with respect to different embodiments, those skilled in the art will recognize that the device and method features can be suitably combined, adapted, or interchanged to realize still further embodiments of the present invention.

[0026] Embodiments of the present invention will now be described with reference to the accompanying drawings, but neither the drawings nor the description should be construed as limiting the present invention. [Brief explanation of the drawings]

[0027] [Figure 1] 1 shows a hydraulic design of a brake system with a control device according to an embodiment. [Figure 2] 1 is a diagram illustrating a braking process including deactivation of a return pump according to one embodiment. [Figure 3] 4 is a flowchart illustrating a method of operating a braking system according to one embodiment. [Figure 4] 1 is a diagram illustrating the coordination between brake torque and drag torque of an electric motor of a brake system with empty pressure reservoir recognition according to one embodiment. [Figure 5] 4 is a flowchart of a method of operating a brake system according to another embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0028] The drawings are only schematic and are not to scale, and the same reference numerals represent elements of the same or identical function in the various drawings.

[0029] FIG. 1 shows a hydraulic design diagram of a brake system 100 with a control device 102 according to one embodiment. The brake system 100 includes a valve block, a sensor unit 104, and brake devices 106 for each wheel of the vehicle. Here, the sensor unit 104 consists of a brake booster connected to the brake pedal and a master brake cylinder MC. A reservoir 108 for brake fluid or brake fluid is arranged in the master brake cylinder MC. The sensor unit 104 is implemented in a two-circuit configuration. Two pistons are arranged in the master brake cylinder MC, which are simultaneously moved by the brake pedal or the brake booster. Each piston acts on its own master brake cylinder chamber. The first master brake cylinder chamber is fluidly connected to a first brake circuit 110 of the brake system 100. The second master brake cylinder chamber is connected to a second brake circuit 112 of the brake system 100. Two of the brake devices 106 are grouped in each of the brake circuits 110, 112. An X-type brake circuit is shown here. Each of the brake circuits 110, 112 groups together one front wheel V and one diagonally opposite rear wheel H. The first brake circuit 110 thus supplies the brake device 106 for the left front wheel LV and the brake device 106 for the right rear wheel RH. The second brake circuit 112 supplies the brake device 106 for the right front wheel RV and the brake device 106 for the left rear wheel LH.

[0030] Both brake circuits 110, 112 are identical in construction. Therefore, only one of the brake circuits 110, 112 will be described here. Components of the first brake circuit 110 are designated with the numeral 1. Components of the second brake circuit 112 are designated with the numeral 2. Components that are uniquely associated with one wheel are designated with the abbreviation of that wheel.

[0031] The outputs of the master brake cylinder chambers are connected to the inputs of the proportional valve USV and the directional control valve HSV of the valve block. The proportional valve USV can be called a switching valve. The directional control valve HSV can be called a high-pressure switching valve. Both valves are electromagnetically operated. The proportional valve USV is normally open. The directional control valve HSV is normally closed. The directional control valve HSV has only two switching positions: open or closed. The proportional valve USV can have multiple valve positions. The flow cross-section of the proportional valve USV depends on the valve position. In the first brake circuit 110, a pressure sensor 114 is arranged between the output of the first master brake cylinder chamber and the inputs of the proportional valve USV1 and the directional control valve HSV1.

[0032] The output of the proportional valve USV leads to the braking device 106. Between the proportional valves USV and the braking device 106, one further proportional valve EV is arranged in the valve block for each braking device 106. The further proportional valves EV can be called intake valves of the braking device 106. The further proportional valves EV are also electromagnetically operated and are normally open. The further proportional valves EV can be throttled. When the proportional valves USV and the further proportional valves EV are in a de-energized state, a direct fluid connection is established between the master brake cylinder MC and the braking device 106.

[0033] When the brake pedal is operated, brake fluid is forced from the master brake cylinder MC through the proportional valves USV and EV to the braking system 106, which moves brake pistons and brake linings, which are then pressed against the brake discs. The braking system 106 may also include a brake drum. When the brake linings contact the brake discs, brake pressure is generated in the braking system, which generates a braking torque in the braking system. The brake pressure and therefore the braking torque depend on the force exerted on the brake pedal.

[0034] To reduce the braking torque at one wheel, the brake pressure at that wheel can be individually reduced. To do this, each braking device 106 is connected to a separate directional control valve AV of the valve block. The separate directional control valve AV can be called a discharge valve. The separate directional control valve AV is electromagnetically operated and normally closed. The separate directional control valve AV can be throttled. The output of the separate directional control valve AV is connected to a pressure reservoir A. The pressure reservoir A is configured as a cylinder with a spring-loaded piston therein. Brake fluid from the braking device 106 can be discharged through the directional control valve AV into the pressure reservoir A for buffering. When the directional control valve AV opens, the proportional valve EV closes to reduce the pressure.

[0035] All valves in the valve block are controlled by an electrical signal. The directional control valve HSR and the further directional control valve AV each have an open or closed switching position without intermediate positions. The proportional valve USV and the further proportional valve EV also have intermediate positions depending on the value of the controlling electrical signal. The through-flow cross-section of the proportional valve USV and the further proportional valve EV depends on the valve position.

[0036] The pressure reservoir A is connected to the suction side of a return pump RP and to a directional control valve HSV. The pressure side of the return pump RP is connected to the output of a proportional valve USV and the input of another proportional valve EV. The return pump RP is configured to pump brake fluid from the pressure reservoir A back to the system.

[0037] The return pumps RP1, PR2 of both brake circuits 110, 112 are coupled to a common electronically commutated electric motor M, which here is a brushless DC motor. A current sensing device 116 detects at least one resulting current flow I through the windings of the electric motor M and reflects this current flow I in a current signal 118.

[0038] When a voltage signal is applied to the windings of the electric motor M, the rotor and the pump rotors of the associated return pumps RP1, RP2 rotate. The current flow I occurs according to the torque required to rotate the pump rotors. The volume pumped by the return pump RP depends on the pump speed of the pump rotors. The pump speed is a fixed ratio to the speed of the rotors of the electric motor M. In this case, the pump rotors are coupled to the rotors without any intervening gearing, so the pump speed corresponds to the speed of the rotors.

[0039] As long as the return pump RP is able to pump brake fluid, the torque required to rotate the pump rotor depends on the pressure difference between the suction and pressure sides. The current flow I is substantially proportional to the torque.

[0040] When no brake fluid is available to pump, i.e. when the pressure reservoir A is empty, the return pump RP runs idle. Since no volume is being pumped, the torque required to rotate the pump rotor is small, and therefore the current flow I is also small.

[0041] In the proposed approach, the current flow I is monitored in order to monitor the fill level of the pressure reservoir A. To do this, the controller 102 reads the current signal 118.

[0042] As an alternative to the return pump RP, brake fluid can also reach the master brake cylinder MC from the pressure reservoir A via the directional control valve HSV. For this to happen, the brake pedal must be released and the brake pressure in the system must be lower than the pressure stored in the pressure reservoir A.

[0043] 2 shows a diagram illustrating a braking process including deactivation of the return pump according to an embodiment, which can be carried out using a braking system such as that shown in FIG. 1. The diagram shows the time progression of several quantities, one above the other. For this purpose, the progressions are plotted on a graph with time t plotted on the horizontal axis.

[0044] The top graph shows the time course of the current flow value 118, which represents the current flow I through the electric motor. The next graph plots the time course of the brake pressure value 200, which represents the brake pressure p in the master brake cylinder, and the time course of the wheel brake pressure values ​​202, which represent the wheel brake pressure p affected by the ABS of the brake system. Below that are shown the course of the actual value 204 and the estimated value 206 of the volume V stored in the pressure reservoir. Finally, the course of the return pump activation signal 208 is shown.

[0045] When the brake pedal is depressed at the start of the braking process, the brake pressure and wheel brake pressure increase. The brake pressure quickly exceeds the ABS intervention threshold. The wheel brake pressure is reduced by the ABS relative to the brake pressure by periodically opening the outlet valve to release brake fluid into the pressure reservoir. The return pump is also activated at the start of ABS intervention, continuously pumping brake fluid from the pressure reservoir back into the system. This return pumping prevents the pressure reservoir from overfilling. The pressure reservoir therefore empties again quickly at the end of ABS intervention. The return pump can no longer pump brake fluid, and its torque decreases. At the same time, the current flow I also decreases. The decrease in current flow I therefore indicates an empty pressure reservoir. The return pump is no longer needed and is deactivated.

[0046] In one embodiment, any estimated value 206 that does not match the actual value 204 of the stored volume is also reset because the pressure reservoir is now known to be truly empty.

[0047] FIG. 3 shows a flowchart of a method for operating a brake system according to one embodiment. The method may be implemented in a control device, such as the one shown in FIG. 1 by way of example. In this method, in response to ABS control termination 300, the electric motor's power consumption is compared to an expected power consumption in a comparison step 302. When the electric motor's power consumption is significantly lower than the expected power consumption, operation of the return pump is terminated in a termination step 304. In a monitoring step 306, the current flow through the electric motor is monitored. If a drop in current flow through the electric motor is recognized, operation of the return pump is terminated in a termination step 304. In a check step 308, the value of an estimate of the volume stored in the pressure reservoir is checked. When the estimate is zero, operation of the return pump is terminated in a termination step 304.

[0048] In other words, Figures 1 to 3 describe the recognition of an empty pressure reservoir A by the EC motor M in order to improve the NVH (Noise, Vibration, Harshness) behavior when the pressure reservoir is discharged after ABS application.

[0049] In ESP systems, during ABS operation or individual wheel control (TCS, VDC), brake pressure is released at the wheel by closing the input valve EV and opening the output valve AV. The wheel brake volume of the wheel is transferred into the pressure reservoir A. At the same time, a pump is operated to release the pressure reservoir A in a so-called pressure reservoir release routine. The pressure reservoir release routine is executed until the pressure reservoir A is empty.

[0050] In standard ESP, the pressure reservoir release routine can also be executed by opening the high-pressure switching valve (HSV) when the driver is not operating the brake pedal. The pressure reservoir release routine strategy can select either the HSV or the pump.

[0051] The reservoir volume can be estimated by referencing the dropped pressure and stiffness. However, the model pressure used for this purpose may differ from the actual pressure, and the modeled stiffness has some error. Therefore, the estimated reservoir volume may also differ from the actual volume. To ensure that the pressure reservoir is emptied, two different reservoir volumes are calculated: one based on the nominal value (Vol_Acc) and one based on the worst-case scenario (Vol_Acc_Apparent). Traditionally, the pressure reservoir release routine remains active until the volume reaches zero based on the worst-case scenario. The worst-case scenario takes into account errors in the pV curve and the reaction time of the valve opening and closing. In most cases, the nominal model (Vol_Acc) matches the actual volume, while the worst-case model (Vol_Acc_Apparent) is significantly larger than reality. In terms of safety, such an excessive Vol_Acc_Apparent is not a problem, but in terms of NVH (Noise, Vibration, Harshness) and durability, the proposed approach can improve the switch-off point.

[0052] If Vol_Acc is zero, pump operation can be stopped without further operation until Vol_Acc_Apparent is zero.

[0053] The pressure reservoir release routine by the high pressure switching valve HSV can also deal with excessively long pump operation. However, the high pressure switching valve operation causes valve cracking sounds and also causes loud noises due to the generation of a vacuum between the high pressure switching valve HSV and the pump. Especially when a vacuum is generated between the high pressure switching valve HSV and the pump, such operation sounds are even louder than the pump noise.

[0054] The noise occurs when the driver releases the brake pedal completely after Vol_Acc reaches zero and pump operation stops, and the high pressure switching valve HSV is operated and held open until Vol_Acc_Apparent reaches zero.

[0055] If the HSV is not opened and there is no volume in pressure reservoir A, a vacuum will be created between the HSV and the pump. This vacuum will cause significant noise and vibration when the HSV is subsequently opened.

[0056] In ESPs with electrically commutated motors M, the motor current I is measured across a shunt resistor. As a rule, the power consumption increases with increasing load. When pressure reservoir A is empty, the pump does not transfer volume to the brake circuit, so both the pump load and the power consumption decrease. This makes it possible to recognize an empty pressure reservoir A.

[0057] An empty pressure reservoir A can also be recognized by comparing the measured motor current I with the expected motor current, which then corresponds to the expected motor load and depends mainly on the system pressure. The relationship between power consumption and motor load is known. When the power consumption is very low compared to the expected current for the system pressure, the pressure reservoir A can be considered empty.

[0058] The pump load, based on the system pressure, corresponds to the pressure difference between the intake and discharge of the pump element. The pressure at the intake valve of the pump is the reservoir pressure (2 bar to 6 bar) minus the pressure drop across the check valve (1 bar). The pressure at the discharge valve is the system pressure. When there is no volume in pressure reservoir A, the pump does not transfer volume from pressure reservoir A during the ABS and pressure reservoir release routines, which means that the system pressure does not result in a pump load and the motor power consumption is very low. This difference in pump load (with and without reservoir volume) can be detected by the motor current I.

[0059] An empty pressure reservoir A can be recognized when the motor current I drops suddenly or is very low compared to the motor current expected under the pump load due to the system pressure. When an empty pressure reservoir A is detected via the motor current I, the pump operation for the pressure reservoir release routine can be stopped immediately, and high-pressure switching valve operation is also not required after the driver releases the brake pedal. Because the pump is stopped immediately when an empty pressure reservoir A is detected, further vacuum generation can be prevented or even reduced. In this way, a clear improvement in NVH (Noise, Vibration, Harshness) symptoms can be achieved.

[0060] The approach proposed here is not only applicable to pressure reservoir release routines after ABS, but also to pressure reservoir release routines after regenerative cooperative braking.

[0061] 4 shows a diagram of the coordination of the drag torque 400 of a vehicle's electric drive motor and the brake torque 402 of a vehicle's brake system, with which an empty pressure reservoir is recognized according to one embodiment. The coordination can be performed, for example, using a brake system such as that shown in FIG. 1. The diagram shows the time progression of several quantities, one above the other. For this purpose, these progressions are plotted on a graph with time t plotted on the horizontal axis.

[0062] The top graph shows the time course of the current flow value 118, which represents the current flow I through the electric motor. The next graph plots the time course of the drag torque 400, as well as the target value 404 and the actual brake torque 402. Below that, the time course of the volume V stored in the pressure reservoir 406 is shown.

[0063] To initiate the braking process, the vehicle driver presses the vehicle's brake pedal. The force with which the driver presses the brake pedal is converted into a braking request. Alternatively, the vehicle's driver assistance system can provide the braking request. The braking request is sent to the control of the electric drive motor, which adjusts the drag torque 400 corresponding to the braking request.

[0064] However, when the brake pedal is depressed, brake fluid for the vehicle's friction brakes is also pumped out. To prevent the generation of brake torque 402, the friction brake outlet valve is opened and brake fluid is stored in a pressure reservoir. However, the drive electric motor cannot keep drag torque 400 constant throughout the braking process, so from time t1 onwards, the decreasing drag torque 400 is replaced by an increasing brake torque 402.

[0065] When coordinating, the outlet valve is closed and the return pump is activated at the latest at time t1. Brake fluid is pumped through the return pump from the pressure reservoir back to the friction brake, generating brake pressure. As the brake pressure increases, the required torque of the return pump also increases. In this way, the current flow I through the electric motor of the return pump also increases. The brake torque 402 is increased to the extent that the drag torque 400 is reduced. At time t2, the pressure reservoir is empty. The brake pressure can no longer be increased to the target value 404.

[0066] The emptying of the pressure reservoir results in a reduction in the current flow in the electric motor, through which the emptying is recognized.

[0067] If the pressure reservoir is not empty, the discrepancy between the target value 404 and the braking torque 402 indicates an incorrectly adjusted braking system stiffness, where stiffness represents the ratio of braking pressure to realized braking torque 402. During the next braking process, the stiffness will be changed to better match the target value 404 and realized braking torque 402.

[0068] However, the discrepancy is due to a lack of brake fluid, and therefore adapting the stiffness is not sensible. In the approach proposed here, the stiffness adaptation is forbidden when the pressure reservoir is recognized as empty.

[0069] 5 shows a flowchart of a method for operating a brake system according to another embodiment. In response to terminating coordination 500, a comparison step 502 compares the realized brake pressure to a desired brake pressure. If the realized brake pressure is higher than the desired brake pressure, an increase step 504 increases the stiffness coefficient. If the realized brake pressure is lower than the desired brake pressure, a monitoring step 306 monitors the current flow through the return pump electric motor. If no reduction in current flow is recognized during coordination, a decrease step 506 decreases the stiffness coefficient. However, if a reduction in current flow is recognized in monitoring step 306, the decrease in stiffness coefficient is discontinued in step 508.

[0070] In the embodiment described in Figures 4 and 5, the recognition of an empty pressure reservoir is used to improve the robustness of stiffness learning.

[0071] In highly efficient vehicle ESP (ESPhevX), hydraulically generated brake torque is mixed with regenerative brake torque to achieve constant brake power even during regeneration. For purely regenerative braking, the volume displaced by the driver with the brake pedal is directed into a pressure reservoir to counteract the hydraulic brake torque. A pump is then operated to move the volume back into the circuit when the regenerative brake torque drops and is needed to replace the hydraulic torque with regenerative brake torque.

[0072] The driver brake demand is determined by the stroke of the input rod based on a model of the brake system's S / P characteristic, which is adapted to the real-world characteristics and depends on the pressure difference between the actual pressure after torque coordination and the target pressure.

[0073] When the actual pressure is lower than the target pressure at the end of torque coordination, the S / P characteristic is reduced by a factor.

[0074] Conversely, when the actual pressure is higher than the target pressure, the S / P characteristic is increased by multiplying it by a coefficient, which is increased or decreased depending on the difference to modify the S / P characteristic.

[0075] The P / V curve or S / P characteristic has a large tolerance of + / -30% of the nominal curve.

[0076] Because the pump speed is calculated based on a standard curve, the pump speed for brake systems with relatively high volumetric consumption may be too low compared to the nominal value. This causes a pressure difference between the target pressure and the actual pressure. If the pressure buildup is temporary or if the error in the P / V curve is very large, a low pump speed can result in an actual pressure that is too low, which can lead to incorrect stiffness learning results. This difference is caused by the residual volume in the pressure reservoir because the pump does not pump the entire volume back into the brake circuit. However, conventional stiffness learning logic tries to compensate for the difference by reducing the coefficients. This is actually an incorrect learning, so the same pressure difference occurs during the next torque coordination, which can lead to the stiffness learning logic making an incorrect adaptation again.

[0077] The recognition of an empty pressure reservoir proposed here can prevent erroneous stiffness learning.

[0078] Since the pump load drops sharply when volume is no longer transferred by the pump due to an empty pressure reservoir, two possible causes for the pressure difference between the target pressure and the actual pressure after torque mixing can be distinguished. On the one hand, there may be a lack of available volume, and on the other hand, an inaccurate pressure control may be causing the pressure reservoir to be emptied. When there is a lack of volume availability, the stiffness coefficient can be reduced, reducing the S / P characteristic. When there is an inaccurate pressure control, the stiffness coefficient can be kept unchanged, since it is not affected by the S / P characteristic.

[0079] When there is a pressure difference after torque coordination based on lack of volume availability, the motor current decreases as the pressure reservoir becomes completely empty while reducing the pump load, which can be used to recognize an empty pressure reservoir.

[0080] When there is a pressure difference after torque coordination due to the inaccuracy of pressure control, the motor current does not decrease. In this case, it is preferable not to decrement or increment the stiffness coefficient, because it cannot be determined whether the S / P characteristic matches the actual characteristic.

[0081] An empty pressure reservoir can be detected by a drop in motor current, thereby avoiding a false reduction in the stiffness coefficient.

[0082] Finally, it should be pointed out that the words "comprise" and "include" do not exclude other elements or steps, and the indefinite articles "eine" and "ein" do not exclude a plurality. Claim signs are not to be regarded as limitations. [Explanation of symbols]

[0083] 100 Brake System 102 Control device A Pressure Reservoir RP Return Pump I current flow HSV High Pressure Switching Valve

Claims

1. A method of operating a brake system (100) having a pressure reservoir (A) and an electrically driven return pump (RP), wherein the pressure reservoir (A) is recognized as empty when an electrical current flow (I) for driving the return pump (RP) is lower than an expected current flow; When the pressure reservoir (A) is recognized as empty, the opening of a high pressure switching valve (HSV) of the brake system (100) is suspended; the high pressure switching valve (HSV) is connected at one end to the suction side of the return pump (RP) and the pressure reservoir (A), and at the other end to the output portion of the master brake cylinder chamber, The stiffness adaptation of the braking system (100) is stopped when the pressure reservoir (A) is recognized as empty, The method of claim 1, wherein the stiffness adaptation of the brake system (100) is to match the actual brake torque (402) with a target value (404).

2. 2. The method of claim 1, further comprising determining that the pressure reservoir (A) is empty when the gradient of the current flow (I) is greater than a threshold gradient.

3. 3. The method of claim 1 or 2, further comprising: determining that the pressure reservoir (A) is empty when the current flow (I) is below a threshold value.

4. 4. The method according to claim 1, wherein the return pump (RP) is deactivated when the pressure reservoir (A) is recognized as empty.

5. A method of operating a brake system (100) having a pressure reservoir (A) and an electrically driven return pump (RP), wherein the pressure reservoir (A) is recognized as empty when an electrical current flow (I) for driving the return pump (RP) is lower than an expected current flow; The stiffness adaptation of the braking system (100) is stopped when the pressure reservoir (A) is recognized as empty, The method of claim 1, wherein the stiffness adaptation of the brake system (100) is to match the actual brake torque (402) with a target value (404).

6. 6. The method according to any one of claims 1 to 5, wherein a volume counter of the pressure reservoir (A) is reset when the pressure reservoir (A) is recognized as empty.

7. 10. A control device (102) for operating a brake system (100), the control device (102) being configured to implement, implement and / or control a method according to any one of claims 1 to 6 in a corresponding device.

8. A computer program product set up to perform, embody and / or control the method of any one of claims 1 to 6.

9. A machine-readable memory medium having stored thereon the computer program product of claim 8.

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