WO2005063539A1 - Method for adjusting a brake pressure - Google Patents

Abstract

The invention relates to a method for adjusting a brake pressure in the wheel brakes of a brake system, whereby input variables determining the brake pressure in the individual wheel brakes are evaluated and actuating variables of hydraulic valves are fixed. The inventive method is characterized in that a volume flow characteristic line (KV) is stored or is determined and correlates the value of the actuating variable of the hydraulic valve (6) with a volume flow (Q) through the hydraulic valve (6) at a constant differential pressure which is produced upstream and downstream of the hydraulic valve (6). The actuating variable of the hydraulic valve (6) is determined in accordance with the volume flow characteristic line (KV). The inventive method is alternatively characterized in that at least two strategies for adjusting the brake pressure are provided. According to a first strategy, adjustment is carried out in accordance with a volume flow through the hydraulic valve (6) at a presumed constant differential pressure which is produced upstream and downstream of the hydraulic valve (6). According to a second strategy, adjustment is carried out in accordance with a differential pressure upstream and downstream of the hydraulic valve (6).

Description

 Method for adjusting a brake pressure

The invention relates to methods for adjusting a brake pressure in the wheel brakes of a brake system, in which input variables determining the brake pressure in the individual wheel brakes are evaluated and manipulated variables of hydraulic valves are defined.

Vacuum brake boosters require a vacuum supply provided by the motor to support the pedal force to be applied by the driver. Depending on the motor, a condition can be reached even with relatively low pedal forces, where a further increase in the force on the actuating unit is only possible by increasing the pedal force, since the vacuum brake booster has reached the maximum possible support force. The state is called the modulation point of the amplifier. Braking that takes place above the starting point of the vacuum brake booster places high demands on the driver with regard to the pedal force to be applied. For this reason, brake systems (OHB-V) are increasingly being used that support the driver through active hydraulic brake pressure build-up. Brake systems of this type generally have a motor-pump unit and a hydraulic unit, which are regulated by electronic control, such as ESP, ABS, ASR and the like, in accordance with the desired braking force support.

When pressure builds up in the wheel brakes by means of hydraulic support beyond the actuation point of the vacuum brake booster, the () isolation valves, mostly analog solenoid valves, closed between the tandem master cylinder (THZ) and the wheel brakes. The hydraulic pump then delivers brake fluid from the tandem master cylinder (THZ) into the wheel brakes, thus increasing the brake pressure. In the pressure reduction phase, the additional pressure generated via the isolating valves must be comfortably reduced in the tandem master cylinder in terms of pedal feel and braking effect. This usually requires two additional pressure sensors on the wheel brakes, one for each brake circuit, to determine the wheel pressure.

The invention has for its object to provide a method for adjusting a brake pressure in the wheel brakes of a brake system.

According to the invention, this object is achieved in that a volume flow characteristic curve is stored or determined, which correlates the value of the manipulated variable of the hydraulic valve with a volumetric flow through the hydraulic valve at an assumed constant differential pressure formed upstream and downstream of the hydraulic valve, and that the manipulated variable of the Hydraulic valve is determined in accordance with the volume flow characteristic.

Due to the given properties of the valve, depending on the differential pressure and position of the valve with this pressure difference before and after the valve, a volume flow is established. By changing the valve position in an intermediate position, i.e. between the end positions "open" and "closed", the volume flow according to the invention can be changed in a defined manner.

A normally open, electromagnetically actuated valve is advantageously used as the isolation valve. The method is advantageously carried out by a control program with the corresponding program steps in a controller. The regulation according to the volume flow through the hydraulic valve with an assumed constant differential pressure formed upstream and downstream of the hydraulic valve is advantageously carried out for a program run or controller cycle (1 looop).

A map for different differential pressures is advantageously used in correlating the volume flow with the manipulated variable.

According to the invention, the method regulates the brake pressure in the wheel brakes with a volume flow of a pressure fluid (brake fluid) through the hydraulic valve in a range from 5 to 20 cm ³ / sec, preferably about 10 cm ³ / sec, he follows .

According to the invention, this object is also achieved in that at least two strategies for regulating the brake pressure are provided, wherein according to a first strategy the regulation takes place according to a volume flow through the hydraulic valve at an assumed constant differential pressure formed before and after the hydraulic valve, and after a second strategy, the regulation takes place in accordance with a differential pressure upstream and downstream of the hydraulic valve.

It is provided according to the invention that the brake pressure in the wheel brakes is adjusted according to the first strategy according to the method described above.

It is provided according to the invention that the brake pressure in the wheel brakes is adjusted with the second strategy with a volume flow of a pressure fluid (brake fluid) through the hydraulic valve in a range from less than 5 to 20 cm ³ / sec, preferably less than about 10 cm ³ / sec.

It is provided according to the invention that a pressure difference characteristic curve is stored or determined for adjusting the brake pressure according to the second strategy, which correlates the value of the manipulated variable of the hydraulic valve with a differential pressure formed upstream and downstream of the hydraulic valve, and that the manipulated variable of the Hydraulic valve is determined in accordance with the pressure difference characteristic.

The manipulated variable is advantageously determined from the pressure difference between the target pressure for the wheel brakes and the pressure of an actuating device (pressure difference of the hydraulic valve). The pressure of

Actuating device (THZ) is determined via the pressure sensor present in a brake circuit in an ESP control and the target pressure in a known model.

It is provided according to the invention that the manipulated variable is an electrical valve current, by means of which the hydraulic valve is controlled in an analog manner.

It is provided according to the invention that, according to the second strategy, the manipulated variable, preferably the valve current, follows the course of the pressure difference characteristic (opening point characteristic) for the purpose of setting a differential pressure. This pressure difference characteristic describes the relationship between the manipulated variable, in particular the valve current (actuating current) and the maximum adjustable differential pressure at the valve. It is provided according to the invention that a third strategy for adjusting the brake pressure is provided, in which the control is carried out according to a step function to the maximum value of the manipulated variable.

It is provided according to the invention that the third strategy regulates the brake pressure in the wheel brakes with a volume flow of a pressure fluid (brake fluid) through the hydraulic valve in a range of greater than 5 to 20 cm ³ / sec, preferably greater than approximately 10 cmVsec, he follows.

It is provided according to the invention that a predetermined variable brake pressure is adjusted, which brings about a hydraulic amplification of the brake pressure applied to the brake system.

A pressure reduction of the brake pressure in the wheel brakes is regulated particularly advantageously with the method according to the invention.

With knowledge of the valve characteristic of a isolating valve, the

Tandem master cylinder pressure and the target pressure for the

Wheel brakes can be applied according to the inventive method

Valve flow is determined and adjusted so that a certain wheel pressure is reached, which is not fallen below, without the need for additional pressure sensors that measure the wheel pressures.

According to the invention, a device for carrying out the method according to the invention is provided, with a hydraulic pump and a preferably similarly controllable hydraulic valve, which device is characterized in that a variable brake pressure is generated by activating the hydraulic pump by means of which a desired hydraulic amplification takes place in each brake circuit of a vehicle brake and that a change in the brake pressure takes place by means of a control of the hydraulic valve according to the inventive method described above.

An embodiment is shown in the drawing and is described in more detail below.

Show it

Fig. 1 shows a brake system with two brake circuits

Fig. 2 shows a volume flow characteristic according to the invention in a schematic representation

Fig. 3 shows a pressure difference characteristic according to the second strategy according to the invention in a schematic representation

Fig. 4 is a schematic representation of the signal curve of the volume flow Q against the brake pressure for the three strategies according to the invention

The one brake circuit shown in FIG. 1 of a brake system for motor vehicles with two brake circuits consists of an actuating unit 1, for example a brake cylinder, with a brake booster 2 which is actuated by a brake pedal 3. A storage container 4 is arranged on the actuation unit 1 and contains a pressure medium volume and is connected to the working chamber of the actuation unit in the brake release position. The illustrated brake circuit has a brake line 5 connected to a working chamber of the actuation unit 1 with a isolating valve 6, which in its Rest position forms an open passage for the brake line 5. The isolation valve 6 is usually actuated electromagnetically.

The brake line 5 branches into two brake lines 8, 9, each of which leads to a wheel brake 10, 11. The brake lines 8, 9 each contain an electromagnetically actuated inlet valve 12, 19, which is open in its rest position and can be switched into a blocking position by energizing the actuating magnet. A check valve 13, which opens in the direction of the brake cylinder 1, is connected in parallel to each inlet valve 12, 19. In parallel to these wheel brake circuits 26, 27, a so-called return circuit is connected, which consists of return lines 15, 32, 33 with a pump 16. The wheel brakes 10, 11 connect via an outlet valve 14, 17 via return lines 32, 33 to the return line 15 and thus to the suction side of the pump 16, the pressure side of which, with the brake pressure line 8, at an intersection point E between the isolating valve 6 and the inlet valves 12 , 19 is connected.

The pump 16 is designed as a reciprocating piston pump with a pressure valve (not shown) and a suction valve. On the suction side of the pump 16 there is a low-pressure accumulator 20, consisting of a housing 21 with a spring 22 and a piston 23.

In the connection between the low-pressure accumulator 20 and the pump 16, a preloaded check valve 34 which opens to the pump is inserted.

The suction side of the pump 16 is also connected to the brake cylinder 1 via a suction line 30 with a low pressure damper 18 and a changeover valve 31. In addition to the hydraulic unit 43, the brake force transmission circuit has a device 28 for controlling the brake system. The device is essentially an ESP control unit 45, which has a model 41 for determining the desired brake pressure and a desired volume flow through the isolating valve and a memory 42 for storing the valve characteristics (differential pressure characteristic and volume flow characteristic) for the purpose of controlling the Isolation valve 6 assigned. The pressure sensor 40, which detects the pressure of the actuating unit 1, is arranged in the brake line 5 between the brake cylinder 1 and the changeover valve 31 or the isolating valve 6. Speed sensors assigned to the wheels are denoted by 50, 51. Input variables that are supplied to the ESP control unit 45, such as the signals from the speed sensors, at least one yaw rate sensor, an acceleration sensor or the pressure sensor 40, are designated 55-57 as examples.

The valve characteristics VK and DK (see FIGS. 2 and 3), which are stored in the non-volatile memory 42, can be determined by measuring the valves or by calibrating at the end of the strip. Calibration at the end of the production line of the brake system or the vehicle represents an advantageous variant, since here the complete chain of influencing factors (drivers, coils, valve, etc.) flow into the characteristic curve. These characteristics make it possible to control the pressure generated by actuating the pump 16 in such a way that the desired hydraulic amplification with pressure build-up, pressure maintenance and pressure reduction phases can be set with sufficient accuracy without the driver being able to resolve the difference from the regulated system (with wheel pressure sensors) ,

The brake system works as follows: When braking, for example, the driver increases the brake pressure in the hydraulic unit 43 via the pedal 3 and the actuation unit 1 with the vacuum brake booster 2, without the vehicle being decelerated in accordance with the pedal force. When braking via the pedal, the device 28 evaluates the pressure of the actuating unit 1 determined by the pressure sensor 40 or the brake pressure input into the brake line 5. If the pressure reaches a limit value that describes the actuation pressure of the actuating unit or the actuation point of the vacuum brake booster 2, the transfer takes place from the pneumatic brake force assistance by the vacuum brake booster 2 to the active brake force boosting by the pump 16, in particular according to the relationship

Psoll ⁼ P modulation point "f" factor KX (PTHZ-P modulation point)

with P _solι = set pressure, P _{out euerpun} = pressure at the off control point of the brake booster, P _THZ = pressure of the actuation unit. For this purpose, the (analogized) isolating valves 6 are closed when the changeover valve 31 is open and the hydraulic pump 16 delivers brake fluid from the brake cylinder 1, for example a tandem master cylinder (THZ), into the wheel brakes 10, 11. The inlet valves 12 and 19 are open, respectively Exhaust valves 14 and 17 closed.

In the pressure reduction phase, the additionally generated pressure is comfortably reduced in the brake cylinder 1 via the analogized isolation valves 6 with regard to pedal feel and braking effect according to the method according to the invention. 2 shows a volume flow characteristic curve KV according to the invention. The volume flow characteristic curve KV shows the value of the volume flow through the hydraulic valve (isolating valve) at a constant differential pressure formed upstream and downstream of the hydraulic valve, the pressure difference ΔP ₀ , depending on the manipulated variable, the actuating current I regulated at the hydraulic valve.

For example, for a pressure reduction from a pressure 1 to a pressure 2 via the isolating valve, a known volume of brake fluid must flow through the isolating valve.

According to the third strategy, the isolating valve could be opened completely for a certain time. A differential pressure of 100 bar and a volume flow of 50 cm ³ / sec are then typically established. In the area of normal braking, this would lead to an uncomfortable pedal feel (pedal stroke).

According to the invention, therefore, a defined, constant volume flow, which is less than the maximum possible, is regulated. This is achieved by deliberately opening the valve only partially. For this purpose, in particular an electromagnetic valve is controlled in a quasi "analog" manner by targeted energization below the opening current, which matches a current differential pressure.

In other words, by means of an analog controllable valve for an essentially constant pressure difference ΔP ₀ , for example approx. 30 bar, which is set at the hydraulic valve at a specific actuating current I, the opening current I ₀ (Io = I (ΔP ₀ )) a relatively constant volume flow Q _c of approximately 5 or 10 cmVsec is set by regulating a working current I _A at the hydraulic valve. By removing the point for the working flow I _A from the opening current I ₀ for the differential pressure of z. B. 30 bar, small characteristic curve errors have no noticeable effects on the control quality. The pressure reduction itself remains very controllable thanks to the reduced volume flow. The pressure reduction is generally very easy to dose and therefore very comfortable for the driver. Because despite the larger volume flow through the hydraulic valve, the isolating valve 6, in a range from 5 to 20 cm ³ / sec, preferably about 10 cm ³ / sec, whereby a new set pressure can be set faster than with strategy 2, discontinuities or Jumps in the pressure reduction can be avoided.

The opening current I ₀ (Io = I (ΔP ₀ )) results from a pressure difference characteristic curve KD. This is shown in FIG. 3. The pressure difference characteristic curve KP represents the valve current I and the corresponding pressure difference ΔP.

However, a map is preferably used for the volume flow characteristic curve KV (see dashed lines Vi and KV ₂ in FIG. 2). Here, the value of the flow rate Q by the hydraulic valve (release valve) at different, is assumed to be constant, formed before and behind the hydraulic valve differential pressure .DELTA.P _0, ι and .DELTA.P _{0, 2,} depending on the manipulated variable, _'which is regulated on the hydraulic valve control current I _o , ι and Io, 2 _f shown.

The pressure difference characteristic curve KD shown in FIG. 3 is also used in accordance with the second strategy according to the invention, wherein adjusting the brake pressure in the wheel brakes with a volume flow of a pressure fluid (brake fluid) through the hydraulic valve in a range from less than 5 to 20 cm ³ / sec, preferably less than about 10 cm ³ / sec. If acceleration of the setting process is necessary or desired, the valve flow I can be modified according to the volume flow characteristic curve VK according to the first strategy (see FIG. 2). As a result, a larger volume flow compared to the second strategy is generated by the isolating valve 6. This means that a rapid opening of the isolation valve 6 is used to respond to rapid pressure reduction requests. Since the print model is no longer based directly on the opening point characteristic KP according to the first strategy, the model errors also become smaller with a longer, slow pressure reduction.

In addition, leaks that have a strong impact on the resulting brake pressure according to the second strategy in the area of the opening point have a less pronounced effect with the first strategy described here.

In addition to the second strategy (brake pressure follows the characteristic curve KP) for small volume flows and the first strategy (regulation of the volume flow, see FIG. 2)) for medium to large volume flows, the third strategy for large volume flows is provided. A maximum volume flow is realized by a jump function to the maximum control current or a "digital control" of the isolating valve 6, with only an "open" or "closed" position without intermediate positions and pressure setting over the duration of the valve energization.

This is shown in Fig. 4, which shows the volume flow Q versus the pressure difference for the three strategies according to the invention. According to strategy 3 (upper curve in FIG. 4), the maximum volume flow Q _{max is} set by a "jump" to a maximum control current. This will advantageously only increase to a certain, maximum value, because a pressure limiting function is preferably provided for the isolating valve, which limits the brake pressure to a maximum possible brake pressure. This ensures that the hydraulic system of the vehicle brake system is not overloaded.

The desired volume flow Q _CO ns _{t is} set according to strategy 1 (middle curve in FIG. 4). This volume flow Q _CO nst is constant even with increasing pressure difference before and after the isolating valve. This is because, according to the invention, the control flow for the isolating valve is regulated according to the characteristic curve KV (FIG. 2).

In strategy 2 (lower curve in FIG. 4), the setting is made in accordance with a pressure difference (characteristic curve KP in FIG. 3). This results in a ramp function in which the volume flow Q _ans t increases with increasing pressure difference before and after the isolating valve. This is because, according to the invention, the control current for the isolating valve is regulated in accordance with a pressure difference to be set.

In principle, all braking processes can be divided into the sections pressure build-up, pressure maintenance and pressure reduction. In principle, the three strategies according to the invention for regulating the brake pressure are provided for each of these three braking cycles. Depending on the situation, the most suitable strategy is selected for each brake circuit. This is particularly intended for brake systems with vehicle dynamics control (ESP), traction control (TCS), active speed and distance control (ACC) or a brake system with active brake boosting by means of a hydraulic pump to increase the brake pressure beyond the control point of a vacuum brake booster or in the event too low a vacuum

Vacuum brake booster (e.g. during a "cold start" of the vehicle).

In addition, for reasons of system security, the strategy with which the isolation valve is to be kept closed in the best possible way is essentially used for closing the isolation valve. This means that the (de-energized open) valve is energized so that it remains securely closed. The effect of a leak is less in this area (the high current supply).

Strategy 1 and 2 offer the advantage that errors in the characteristic curves do not make themselves felt as a blow to the pedal, but are reduced by slow, imperceptible transition processes. The noise level is also greatly reduced. With strategy 1, a relatively large volume flow Q through the isolating valve and thus a relatively rapid change in the wheel brake pressures can also advantageously take place. A desired target pressure in the wheel brakes is thus set in a relatively short time and is nevertheless comfortable and easily controllable.

Claims

claims 1. A method for adjusting a brake pressure in the wheel brakes of a brake system, in which input variables determining the brake pressure in the individual wheel brakes are evaluated and manipulated variables of hydraulic valves are determined, characterized in that a volume flow characteristic curve is stored or ascertained, which is the value of the manipulated variable correlates the hydraulic valve with a volume flow through the hydraulic valve at an assumed constant differential pressure formed upstream and downstream of the hydraulic valve, and that the manipulated variable of the hydraulic valve is determined in accordance with the volume flow characteristic curve. 2. The method according to claim 1, characterized in that the method regulates the brake pressure in the wheel brakes with a volume flow of a pressure fluid (brake fluid) through the hydraulic valve in a range of 5 to 20 cm ³ / sec, preferably about 10 cm ³ / sec. 3. A method for adjusting a brake pressure in the wheel brakes of a brake system, in which input variables determining the brake pressure in the individual wheel brakes are evaluated and manipulated variables of hydraulic valves are defined, characterized in that at least two strategies for adjusting the brake pressure are provided, whereby according to a first strategy the regulation according to a volume flow through the hydraulic valve with an assumed constant, before and after Hydraulic valve formed differential pressure, and according to a second strategy, the regulation takes place in accordance with a differential pressure before and after the hydraulic valve. 4. The method according to claim 3, characterized in that the adjustment of the brake pressure in the wheel brakes according to the ^' first strategy by a method according to claim 1 or 2. 5. The method according to claim 3 or 4, characterized in that with the second strategy, adjusting the brake pressure in the wheel brakes with a volume flow of a pressure fluid (brake fluid) through the hydraulic valve in a range of less than 5 to 20 cm ³ / sec, preferably less approx. 10 cm ³ / sec. 6. The method according to any one of claims 3 'to 5, characterized in that for the adjustment of the brake pressure according to the second strategy, a pressure difference characteristic curve is stored or determined, which forms the value of the manipulated variable of the hydraulic valve with one before and after the hydraulic valve Differential pressure correlates, and that the manipulated variable of the hydraulic valve is determined in accordance with the pressure difference characteristic. 7. The method according to claim 6, characterized in that according to the second strategy, the valve current follows the course of the pressure difference characteristic for the purpose of setting a differential pressure. 8. The method according to any one of claims 3 to 7, characterized in that a third strategy for adjusting the brake pressure is provided, in which the control is carried out according to a step function to the maximum value of the manipulated variable. 9. The method according to claim 8, characterized in that with the third strategy regulating the brake pressure in the wheel brakes with a volume flow of a pressure fluid (brake fluid) through the hydraulic valve in a range of greater than 5 to 20 cm ³ / sec, preferably greater than about 10 cm ³ / sec. 10. The method according to any one of claims 1 to 9, characterized in that the manipulated variable is an electrical valve current, by means of which the hydraulic valve is controlled analogously. 11. The method according to any one of claims 1 to 10, characterized in that the method is used to regulate a predetermined variable brake pressure which brings about a hydraulic amplification of the brake pressure introduced into the brake system. 12. Device for carrying out the method according to one of claims 1 to 11, with a hydraulic pump and a preferably similarly controllable Hydraulikventi1, characterized in that a variable brake pressure is generated by controlling the hydraulic pump, by means of which a desired hydraulic amplification in each Brake circuit of a vehicle brake occurs and that a change in the brake pressure by means of a control of the hydraulic valve according to a method of claims 1 to 11.