Method for the accelerated determination of a correction value for a pressure sensorless determination of a hydraulic pressure in a hydraulic brake system, as well as hydraulic brake system and use of the brake system

Abstract

Method for the accelerated determination of a correction value for a pressure sensorless determination of a hydraulic pressure in a hydraulic brake system (201), in which an electrically pulsed motor-pump unit (213, 214) delivers hydraulic fluid against the pressure, wherein a pressure-loaded coasting behavior of the motor-pump unit (213, 214) is detected in the form of a generator voltage applied to an electric motor of the motor-pump unit (213, 214) during an electrical pulse pause, and wherein the pressure is determined from the generator voltage by means of a predetermined correlation, characterized in that the correction value is determined using a known valve characteristic.

Description

[0001] The invention relates to a method for the accelerated determination of a correction value for a pressure sensorless determination of a hydraulic pressure in a hydraulic brake system according to the preamble of claim 1 as well as a hydraulic brake system according to the preamble of claim 10 and its use.

[0002] With the overall increasing demands on motor vehicle braking systems, increasingly higher requirements are also being placed on the noise behavior of the braking systems. Control noises from the valves that are perceptible to the driver, for example in autonomous cruise control, pose a particular problem in this regard, as they are perceived as unpleasant and unacceptable by the driver. Furthermore, such noise-causing valve vibrations can also be associated with a longitudinal oscillation of the vehicle speed that is clearly perceptible to the driver. This can only be avoided by the most precise control of the analogue digital valves used. However, in order to be able to control the analogue digital valves precisely with the appropriate current, it is necessary to know not only the opening current characteristic of the valve but also the pressure difference across the valve as precisely as possible.However, in order to ensure cost-effective production of the vehicle brake system, pressure sensors are often omitted, so that the knowledge of the current pre-pressure required for control must be determined using a pre-pressure model.

[0003] In this context, DE 10 2007 019 929 discloses a correction method for calibrating valve control characteristics during control operation. This initially starts with a predefined characteristic, which is then corrected using correction variables determined in a recursive learning process. The correction variables, in turn, are calculated over several learning cycles from pressure build-up times or pressure requirements, thus enabling a gradual adjustment of the predefined control characteristic.

[0004] DE 10 2010 028 083 A1 discloses a method for the individual calibration of a pump motor in a pressure control system of a motor vehicle. After the pressure control system has been manufactured, the pump's output or speed is determined under defined pressure conditions as a function of the electrical control, and the measured data is stored in the pressure control system. Using, for example, a structure-borne sound sensor, the speed is determined during pump operation at a known motor current, and a conclusion is drawn about the pre-pressure based on the measured data stored during the individual calibration.

[0005] WO 2005 / 007475 A1 describes a method that allows the current pre-pressure to be estimated based on the deceleration behavior of a motor-pump unit. An electronic unit electrically controls the motor of the motor-pump unit as needed using pulse-width modulation. During a de-energization phase of the modulation, a generator voltage generated by the motor is tapped. The generator voltage is fed to the electronic unit, which estimates the pre-pressure present in the braking system based on this generator voltage. A speed reduction, detected via the generator voltage and experienced by the motor-pump unit during a de-energization phase due to the pre-pressure load, is used as a benchmark for the pre-pressure.

[0006] DE 10 2007 032 950 A1 describes a method for measuring the pre-pressure at a first analogue, electromagnetically controlled hydraulic valve for sensitively regulating the pressure in a pressure circuit, in which the pre-pressure at the first valve can be adjusted via the delivery capacity of a motor pump unit which is connected to the first valve via a pressure line on the pump outlet side.

[0007] The pre-pressure is to be adjusted by an electronic evaluation of the tappet reaction of the first valve or of another hydraulic valve that is also electromagnetically controlled and connected to the pressure line on the pump outlet side.

[0008] DE 199 46 777 A1 presents yet another method for estimating a pre-pressure prevailing between a master brake cylinder and an inlet valve of a wheel brake cylinder of a motor vehicle brake system, wherein the pre-pressure is estimated taking into account a run-on voltage of a clocked motor of a pump for returning brake fluid from a storage chamber arranged on the output side of the wheel brake cylinder in the master brake cylinder of the brake system and a storage chamber pressure determined in the storage chamber.

[0009] However, the pressure estimation methods known in the prior art have the disadvantage that, without a complex learning process, estimating the pre-pressure with sufficient accuracy is only possible in brake systems with minimal variation in mechanical and hydraulic properties. However, if the brake system is produced in relatively large quantities and possibly even has different suppliers for the required individual components, a comparatively complex learning process is unavoidable according to the state of the art in order to achieve control accuracy sufficient for analog valve control.

[0010] The object of the invention is therefore to propose a method which enables an accelerated determination of a correction value for a pressure sensorless determination of a hydraulic pressure in a hydraulic brake system.

[0011] This object is achieved according to the invention by the method for the accelerated determination of a correction value for a pressure sensorless determination of a hydraulic pressure in a hydraulic brake system according to claim 1.

[0012] The invention relates to a method for the accelerated determination of a correction value for a pressure-sensorless determination of a hydraulic pressure in a hydraulic brake system, in which an electrically cycled motor-pump unit delivers hydraulic fluid against the pressure. During an electrical cycle pause, a pressure-loaded rundown behavior of the motor-pump unit is detected in the form of a generator voltage applied to an electric motor of the motor-pump unit, and the pressure is determined from the generator voltage using a predetermined correlation. The method is characterized in that the correction value is determined using a known valve characteristic.

[0013] This results in the advantage that, using a known valve characteristic of a valve in the brake system, in particular a hydraulic valve in the brake system, the usually lengthy and complex determination of the correction value, which is necessary according to the prior art, can be carried out comparatively easily and quickly. Determining the correction value corresponds to calibrating the motor-pump unit or the generator voltage or the pressure associated with the generator voltage. Since the valve or the hydraulic valve already has a calibrated valve characteristic, the method according to the invention effectively utilizes this valve characteristic of the valve to determine the correction value. A separate and complex learning process for determining the correction value is thus eliminated.

[0014] The correction value is preferably determined during operation of the hydraulic braking system, in particular during ABS control.

[0015] The specified correlation is preferably a characteristic curve, a characteristic map or a mathematical assignment rule which assigns a hydraulic pressure to a generator voltage.

[0016] Preferably, the hydraulic pressure is also provided as a hydraulic pressure prevailing on the outlet side of the motor-pump unit. This pressure is also known as the so-called pre-pressure. The pre-pressure can be measured relatively easily via the motor-pump unit because it acts directly on the pump and thus on the electric motor. Furthermore, it represents the pressure applied to a wheel brake cylinder when the inlet valve is open and thus the respective maximum brake pressure. Knowledge of the pre-pressure is a necessary prerequisite for reliable control of a wide variety of braking interventions.

[0017] Furthermore, various learning methods for valve control are known in the prior art, which enable increasingly better-coordinated valve control over time, as the valve characteristics are known with increasing precision. It is therefore preferable that the method according to the invention be repeated regularly in order to be able to determine the correction value more accurately as the valve characteristics become increasingly precise.

[0018] It is expedient for the valve characteristic to be an opening current characteristic of a hydraulic valve of the braking system, in particular of an inlet valve of a wheel brake cylinder of the braking system. Since the opening current characteristic indicates a relationship between the current supply to the hydraulic valve and the pressure across the hydraulic valve at which the hydraulic valve opens, the correction value can be determined relatively easily from the known current-pressure behavior of the hydraulic valve by comparing it with an observed behavior of the motor-pump unit or the generator voltage or the pressure determined by the correlation.

[0019] Furthermore, it is preferred that the hydraulic pressure at the wheels of a rear axle of a vehicle with at least two axles is controlled analogously and, in particular, that at the wheels of a front axle it is controlled digitally. This results in the development of largely optimal braking performance for the front axle due to the digital control, which, although not very comfortable, is comparatively robust and reliable. Since the rear axle is usually controlled in such a way that largely optimal vehicle stability is achieved, i.e. that wheel slip is kept as low as possible and the possible braking power is not fully utilized anyway, analog control can be carried out here essentially without any negative effects on the braking behavior, even without knowledge of the correction value.Analog control is particularly advantageous when it comes to accessing the opening current characteristic, as the opening current characteristic describes a continuous current-pressure range and thus encompasses a multitude of operating points. Digital control, on the other hand, is limited to two operating points.

[0020] It is expedient for the correction value to be a differential pressure between the pressure determined by means of the correlation and a pressure determined by means of the opening current characteristic. The pressure is determined by means of the opening current characteristic, preferably by the opening of the hydraulic valve. When the hydraulic valve opens, i.e. reaches the opening point, the pressure can be determined from the opening current characteristic via the known current supply. The opening of the valve can be detected, for example, by a sudden change in pressure or by means of a known tappet position control system based on a corresponding change in the magnetic resistance of a magnetic circuit of the hydraulic valve. This pressure determined via the hydraulic valve is then compared with the pressure determined by means of the correlation.Since the hydraulic valve is calibrated, it can be assumed that the pressure determined via the hydraulic valve is largely reliable and corresponds to the actual pressure. By determining the differential pressure between the pressure determined by the correlation and the pressure determined by the opening current characteristic, the deviation of the pressure determined by the correlation from the actual pressure is determined. This differential pressure represents the desired correction value.

[0021] In particular, the pressure is determined using the opening current characteristic curve by traversing a current ramp on the hydraulic valve. This represents a simple method for locating the opening point, as a large number of operating points are traversed.

[0022] Furthermore, it is specifically intended that the current ramp be completed in less than 100 ms. This ensures that the method according to the invention determines the required correction value within a comparatively short time, namely 100 ms. Thus, the hydraulic pressure can be determined comparatively accurately immediately thereafter.

[0023] Particularly preferably, a starting value of the current ramp on the opening current characteristic curve corresponds to a pressure that lies between 0 bar and 30 bar above the pressure determined by the correlation. 30 bar corresponds to the typically maximum expected deviation when generating the hydraulic pressure without the correction value. Thus, by traversing the current ramp starting from a starting value 30 bar above the pressure determined by the correlation, the entire expected deviation range can be continuously traversed.

[0024] Furthermore, it is particularly preferred that the current ramp be followed until the hydraulic valve opens, wherein the opening is detected by the onset of a slippage process on a vehicle wheel associated with the hydraulic valve. Since pressure is introduced into the wheel brake cylinder when the hydraulic valve, in particular the inlet valve, opens, a braking effect occurs and, with it, an increase in slippage. The increase in slippage is particularly easy to detect using conventional wheel rotation sensors. Thus, the opening point of the inlet valve can be detected easily and reliably.

[0025] It is advantageous that the correction value is determined exclusively at the vehicle wheels on the rear axle. Since the potential braking power is generally not fully utilized here in favor of essentially optimal vehicle stability, the method according to the invention can be carried out largely without negative effects on the driving behavior or braking behavior. Unless the motor-pump unit is already assigned to a front wheel and a rear wheel together, a correction value determined for a motor-pump unit assigned exclusively to the rear wheels can also be used for a motor-pump unit assigned exclusively to the front wheels, since both motor-pump units of a hydraulic braking system usually exhibit sufficiently similar behavior.

[0026] The invention further relates to a hydraulic brake system, comprising at least one master cylinder for fluid storage, at least one analogue regulating inlet valve for admitting a hydraulic pressure into at least one wheel brake cylinder assigned to a wheel brake and at least one outlet valve for discharging the pressure from the at least one wheel brake cylinder, at least one electrically clocked motor-pump unit and at least one electronic control unit, wherein the motor-pump unit delivers hydraulic fluid against the pressure in accordance with the control unit, wherein the control unit detects a pressure-loaded run-out behavior of the motor-pump unit in the form of a generator voltage applied to an electric motor of the motor-pump unit during an electrical clock pause, and wherein the control unit determines the pressure from the generator voltage by means of a predetermined correlation.The braking system according to the invention is characterized by the fact that it implements the method according to the invention. This results in the advantages already described.

[0027] Furthermore, the invention relates to a use of the braking system according to the invention in a motor vehicle.

[0028] Further preferred embodiments emerge from the subclaims and the following description of an embodiment with reference to figures. It shows Fig. 1 shows, by way of example, a valve flow, a pressure in a wheel brake cylinder, a pre-pressure determined by means of the method according to the invention and an actual pre-pressure and Fig. 2 an exemplary structure of a hydraulic vehicle brake system according to the invention.

[0029] Fig. 1 shows, by way of example, valve flow 11 of an inlet valve, pressure 12 in a wheel brake cylinder assigned to the inlet valve, pre-pressure 13 determined by means of the method according to the invention, and actual pre-pressure 14 during execution of the method according to the invention. As can be seen, the determined pre-pressure 13 at the beginning of the method still deviates from the actual pre-pressure 14. Initially, pressure 12 in the wheel brake cylinder rises because the motor-pump unit begins to build up pressure and the initially low valve flow 11 cannot yet close the inlet valve. As soon as valve flow 11 increases and the opening gap of the inlet valve decreases accordingly, pressure 12 in the wheel brake cylinder begins to decrease, or initially increases more slowly, because pressure is simultaneously released via the outlet valve. As valve flow 11 later decreases again and the opening gap of the inlet valve thus increases again, pressure 12 in the wheel brake cylinder increases again.At time t0, the wheel rotation sensor detects a sudden increase in slip of the wheel assigned to the wheel brake cylinder, which indicates that the inlet valve is open. The pressure across the inlet valve at time t0 is determined from the known opening current characteristic of the inlet valve and the current present at the inlet valve at the time of opening. This pressure essentially corresponds to the actual pre-pressure. By comparing the pre-pressure determined using the generator voltage with the pre-pressure determined using the valve characteristic, a differential pressure is determined, which represents the desired correction value. From time t0, the correction value is added to the pre-pressure determined using the generator voltage, so that from time t0, essentially the actual pre-pressure can be determined, and calibration is complete.

[0030] Fig. Figure 2 shows an example of a hydraulic brake system 201 according to the invention, which comprises a driver-operated pressure control device 202 consisting of a tandem master cylinder 203, a vacuum booster 204, and a brake pedal 229 arranged thereon. Tandem master cylinder 203 is associated with a pressure sensor 230. Pressure control device 202 is hydraulically coupled to hydraulic pressure control circuits 207 and 208 via hydraulic lines 205 and 206. Hydraulic pressure control circuits 207 and 208 are essentially identical in design and therefore are shown in the Fig.2. Pressure control circuits 207 and 208 are each assigned to the wheel brakes of a brake circuit of a two-axle motor vehicle. Hydraulic pressure control circuits 207 and 208 include changeover valves 209 and 210, isolation valves 211 and 212, electrically timed motor-pump units 213 and 214, normally open inlet valves 215, 216, 217, and 218, and normally closed outlet valves 219, 220, 221, and 222. Furthermore, hydraulic pressure control circuits 207 and 208 include low-pressure accumulators 223 and 224, as well as wheel brake cylinders 225, 226, 227, and 228, each assigned to a vehicle wheel.

[0031] When implementing the method according to the invention in a hydraulic brake system 201, hydraulic pressure is first built up by means of motor-pump units 213 and 214 in accordance with an electronic control unit (not shown). During an electrical cycle pause of motor-pump units 213 and 214, the pressure-loaded run-down behavior of motor-pump units 213 and 214 is recorded in the form of a generator voltage applied to the electric motors of motor-pump units 213 and 214 and evaluated by the control unit. From the generator voltage, the currently prevailing pre-pressure is then determined using a characteristic map digitally stored in the control unit. However, since the motor-pump unit determines the pressure with a random deviation due to unavoidable property variations caused by manufacturing and assembly, it is first necessary to determine a correction value. This is done, for example, by the method according to the invention.The generated pressure is then passed on to wheel brake cylinders 225, 226, 227, and 228 via inlet valves 215, 216, 217, and 218. Wheel rotation sensors (not shown) then control the slip at the vehicle wheels assigned to wheel brake cylinders 225, 226, 227, and 228. The pressure in wheel brake cylinders 225 and 226, which are assigned to the vehicle wheels on the front axle of a two-axle vehicle, is digitally controlled via inlet valves 215 and 216. This results in a brake pressure control that, while not particularly comfortable, is comparatively robust and reliable. The pressure in wheel brake cylinders 227 and 228, which are assigned to the vehicle wheels on the rear axle of the two-axle vehicle, is controlled analogously via inlet valves 217 and 218. The analog control enables continuous running of a known opening current characteristic of intake valves 217 and 218.To accelerate the determination of the correction value, a current ramp is followed, i.e., the current at inlet valves 217 and 218 is reduced within less than 100 ms, starting from a pressure 30 bar above the uncorrected pressure determined using the characteristic map, to such an extent that inlet valves 217 and 218 open. The opening of inlet valves 217 and 218 is detected by a sudden increase in wheel slip of the vehicle's rear axle wheels, as the pressure in wheel brake cylinders 227 and 228 increases and generates a braking effect. Based on the current applied to inlet valves 217 and 218 at the time of the sudden increase in wheel slip, the actual pressure can now be determined from the known opening current characteristic curve(s) of inlet valves 217 and 218.This actual pressure is then compared with the pressure determined by motor-pump units 213 and 214, and the differential pressure is determined as a correction value and stored in the electronic control unit. For future pressure determinations using the generator voltages of motor-pump units 213 and 214, this correction value is added to the pressure determined using the characteristic curve map to always determine the actual prevailing pressure. The process is repeated, for example, at intervals of six months to compensate for gradual changes in the braking system.

Claims

[1] Method for the accelerated determination of a correction value for a pressure sensor-free determination of a hydraulic pressure in a hydraulic brake system (201), in which an electrically clocked motor-pump unit (213, 214) delivers hydraulic fluid against the pressure, wherein in an electrical clock pause a pressure-loaded run-out behavior of the motor-pump unit (213, 214) is detected in the form of a generator voltage applied to an electric motor of the motor-pump unit (213, 214), and wherein the pressure is determined from the generator voltage by means of a predetermined correlation, characterized by that the correction value is determined using a known valve characteristic. [2] Method according to claim 1, characterized bythat the valve characteristic is an opening current characteristic of a hydraulic valve of the brake system (201), in particular of an inlet valve (215, 216, 217, 218) of a wheel brake cylinder (225, 226, 227, 228) of the brake system (201). [3] Method according to at least one of claims 1 and 2, characterized by that the hydraulic pressure at vehicle wheels of a rear axle of a vehicle with at least two axles is controlled analogously and, in particular, at vehicle wheels of a front axle is controlled digitally. [4] Method according to at least one of claims 1 to 3, characterized by that the correction value is a differential pressure between the pressure determined by means of the correlation and a pressure determined by means of the opening current characteristic. [5] Method according to claim 4, characterized by that the pressure is determined by means of the opening current characteristic by following a current ramp on the hydraulic valve. [6] Method according to claim 5, characterized by that the current ramp is completed in less than 100 ms. [7] Method according to at least one of claims 5 and 6, characterized by that a starting value of the current ramp on the opening current characteristic corresponds to a pressure that is between 0 bar and 30 bar above the pressure determined by means of the correlation. [8] Method according to claim 5, characterized by that the current supply ramp is run until the inlet valve (215, 216, 217, 218) opens, the opening being detected by the onset of a slippage process at a vehicle wheel assigned to the inlet valve (215, 216, 217, 218). [9] Method according to at least one of claims 1 to 8, characterized by that the correction value is determined exclusively at the vehicle wheels of the rear axle. [10] Hydraulic brake system (201), comprising at least one master cylinder (203) for fluid storage, at least one analogue regulating inlet valve (215, 216, 217, 218) for admitting a hydraulic pressure into at least one wheel brake cylinder (225, 226, 227, 228) assigned to a wheel brake and at least one outlet valve (219, 220, 221, 222) for discharging the pressure from the at least one wheel brake cylinder (225, 226, 227, 228), at least one electrically clocked motor-pump unit (213, 214) and at least one electronic control unit, wherein the motor-pump unit (213, 214) delivers hydraulic fluid according to the control unit against the pressure, wherein the control unit in an electrical clock pause a pressure-loaded run-out behavior of the motor-pump unit (213, 214) in the form of a drive unit connected to an electric motor of the motor-pump unit (213,214) applied generator voltage and wherein the control unit determines the pressure from the generator voltage by means of a predetermined correlation, , characterized by that the braking system (201) carries out a method according to at least one of claims 1 to 9. [11] Use of the braking system according to claim 10 in a motor vehicle.

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