DE10239126A1 - Device for monitoring fluid level and temperature of a brake fluid reservoir, incorporates a thermistor for monitoring the fluid temperature for compensation of the air loading curve for the brake fluid - Google Patents

Abstract

Device for measuring the temperature in a hydraulic fluid reservoir container (4) with a fluid level sensor. The latter comprises two reed contacts (53, 54) arranged one above the other. Each reed contact has a resistor in parallel to it. The lower resistance (55) is a thermistor that can be used to directly measure the temperature in the container. The lower reed contact is continuously open when there is sufficient fluid in the container. An Independent claim is made for a method for monitoring the brake fluid of a motor vehicle brake system. According to the method the characteristic curve of the air loading of the brake fluid is corrected using the temperature measurements provided by the thermistor.

Description

The invention relates to a device for Determination of the fill level of a storage container for Liquids and to determine the temperature of those present Liquid, with at least two in the reservoir superimposed switches are provided, the one mark upper and a lower level and from one Floats are actuated on the liquid, and where an electrical resistor in parallel with each switch is switched so that from the total resistance of the Resistors formed resistance network the level can be determined at least in stages.

Such devices are u. a. at Brake fluid containers used to determine the level. In some use cases, one of which is explained later is, it is necessary to change the temperature of each Knowing liquid. For this, one of the Level measuring device separate temperature sensor brought in. There are thus two sensor devices.

The invention is based on the problem of the device simplify, so that both the Level as well as the temperature can be recorded.

To solve the problem, the invention provides one Device according to the preamble of claim 1 before that the resistor connected in parallel to the lower switch is a temperature dependent resistor and that the lower switch is normally open, so at one sufficiently filled storage container the thermal resistance to Overall resistance contributes to temperature and thus from the total resistance to the temperature of the liquid can be closed.

To be able to carry out a temperature measurement, only a resistance of the level measuring device, the has a fixed value through a resistor with a temperature-dependent value, i.e. a thermal resistance, replaced, the existing power supplies and Evaluation devices continue to be used. The program-technical evaluation of the measured signals is one corresponding program part for calculating the temperature added. The changes in hardware are minimal and can be realized without a high cost.

The evaluation is particularly easy if the switches are electrically connected in series. Then the level is the higher the larger the Total resistance is. However, the maximum should be possible change in resistance of the thermal resistor may be smaller than the resistance of the adjacent switch because otherwise not between an increase in the level and a temperature change can be distinguished.

Particularly suitable switches are reed contacts which plunge vertically into and into the storage container bottom immersed tube are arranged, the Float is guided on the outside of the tube itself is magnetic or a magnet for actuating the Carries reed contacts. Such a device for determination the level is z. B. in DE 198 36 597 A1 described. It is used especially for level measurement Washer fluid and fuel tanks.

Order from the changing value of the total resistance of the Resistor network on a level change or on to be able to conclude a temperature increase is a Measuring device for measuring the voltage drop across the resistor network available, the measured values of an evaluation unit with which the system, which of the with the Device provided storage container is assigned, is controlled depending on the temperature.

Such a device is intended in particular in brake systems with a pedal-operated master brake cylinder, with one from master cylinder pressure prevailing in the master cylinder actuable pedal travel simulator, with a pressure sensor for Detect the master cylinder pressure and one Pedal travel sensor for detecting the pedal travel are used, whereby from the assignment of the master cylinder pressure to Pedal travel in comparison with a characteristic curve on the air pollution the brake fluid and / or a flow resistance is closed. The determination of the air pollution is based on the fact that the in itself almost incompressible Brake fluid through enclosed compressible Air compressible to pressure and thus to one Pedal actuation reacts like a resilient spring.

Knowledge of the air pollution of the brake fluid is necessary for two reasons:
With these brake systems, the type of pedal actuation is used to infer the driver's wish to brake the vehicle with a certain deceleration. When determining the desired deceleration, information about both the pedal travel and the master cylinder pressure is used, and a change in the relationship due to the air pollution can lead to an incorrect assessment of the driver's request.

Furthermore, the air pollution should be relatively accurate be determined so that no false warning is issued becomes.

In addition, cold brake fluid is tougher will, so in narrow channels of the braking system Flow resistances are built up, the quasi as a disturbance of the Relationship between master cylinder pressure and pedal travel Act.

To monitor the procedure described above To improve the brake system, the invention provides that the characteristic curve showing the normal relationship between pedal travel and describes master cylinder pressure using the previously described device for determining the level and the temperature is temperature corrected.

In the following, the Invention will be explained in more detail. To show:

Fig. 1 shows a brake system with a reservoir, which can be equipped with a device according to the invention, and

Fig. 2 is a schematic representation of the device with an electrical circuit diagram of the switches of the device and the resistors connected in parallel.

The electrohydraulic brake system shown in Fig. 1 has a two-circuit master brake cylinder or tandem master brake cylinder 2 which can be actuated by means of an actuating pedal 1 and which cooperates with a pedal travel simulator 3 described in more detail below and in which two pressure chambers which are separate from one another and which are formed with an unpressurized brake fluid are formed receiving pressure medium reservoir 4 are connected. Wheel brakes 9 , 10 assigned to the rear axle, for example, are connected to the first pressure chamber (primary pressure chamber) by means of a lockable first hydraulic line 5 . The line 5 is shut off by means of a first isolating valve 11 , while an electromagnetically actuated, preferably normally open (SO) pressure compensating valve 13 is inserted in a line section 12 connected between the wheel brakes 9 , 10 , which allows brake pressure control to be carried out as required for the wheel.

The second pressure chamber (secondary pressure chamber) of the master brake cylinder 2 , to which a pressure sensor 15 can be connected, can be connected to the other pair of wheel brakes 7 , 8 assigned to the front axle via a second hydraulic line 6 which can be shut off by means of a second separating valve 14 . In a line section 16 connected between the wheel brakes 7 , 8 , an electromagnetically actuated, preferably normally open (SO) pressure compensation valve 19 is again inserted.

As can also be seen in the drawing, a motor-pump unit 20 serving as an auxiliary pressure source is provided with a high-pressure accumulator 21 , which in turn consists of a pump 23 driven by an electric motor 22 and a pressure relief valve 24 connected in parallel with the pump 23 . The high-pressure accumulator 21 , which will be described in more detail below, is provided with an accumulator pressure sensor 25 which is connected to a receiving chamber which receives the pressure medium (the brake fluid) and thus directly measures the accumulator pressure.

A third hydraulic line 26 connects the high-pressure accumulator 21 to the input connections of two electromagnetically controllable, normally closed proportional valves 17 , 18 , which are connected upstream of the wheel brakes 7 and 8 as inlet valves, and to the input connections of two further electromagnetically controllable, normally closed proportional valves 37 , 38 , which are connected upstream of the wheel brakes 10 and 9 as intake valves. In addition, the wheel brakes 7 , 8 are each connected to a fourth hydraulic line via an electromagnetically controllable, normally closed proportional valve or outlet valve 27 , 28, and the wheel brakes 9 , 10 are connected to a fourth hydraulic line via an electromagnetically controllable, normally closed proportional valve or outlet valve 47 , 48 (Return line) 29 connected, which on the other hand is connected to the unpressurized pressure medium reservoir 4 . The hydraulic pressure prevailing in the wheel brakes 7 , 8 is determined with the aid of a pressure sensor 30 , 31 , while the hydraulic pressure prevailing in the wheel brakes 10 , 9 is detected with the aid of a pressure sensor 40 , 41 each.

The common control of the motor-pump unit 20 and the solenoid valves 11 , 13 , 14 , 17 , 18 , 19 , 27 , 28 is used by an electronic control unit 32 which , as input signals, outputs the output signals of a pedal travel sensor 33 which interacts with the actuating pedal 1 and the pressure sensor 15 are supplied, whereby a driver deceleration request detection is made possible. However, other means, for example a force sensor that senses the actuating force on the actuating pedal 1 , can also be used to detect the driver's deceleration request. The output signals of the pressure sensors 30 , 31 , 40 , 41 and the output signals corresponding to the speed of the vehicle from wheel sensors, which are only indicated schematically, are supplied to the electronic control unit 32 as further input variables, the wheel sensors assigned to the wheel brakes 10 , 9 being provided.

The pedal travel simulator 3 consists of a simulator piston 42 which is guided in a bore in the master brake cylinder 2 and which has one end face connected to a simulator chamber 44 connected to the primary pressure chamber via a housing channel 43 and the other end face connected to a pressure medium reservoir via a valve (not shown here) 4 connected additional chamber adjoins. A simulator spring 45 which is supported on the simulator piston 42 is also arranged in this chamber.

In normal operation or in the "brake-by-wire" operating mode, the brake system shown in FIG. 1 operates as follows: When the driver steps on the actuation pedal 1 , he feels a path-dependent counterforce, which is predetermined by the defined characteristics of the pedal travel simulator 3 , When a brake request is sensed by means of the pedal travel sensor 33 and / or the pressure sensor 15 , the isolating valves 11 , 14 are closed and the wheel brakes 7 to 10 are separated from the master brake cylinder 2 . A pressure builds up in the master brake cylinder 2 , which results from the actuating force on the actuating pedal 1 . From the signals of the pedal travel sensor 33 and / or the pressure sensor 15 , the driver's braking request is calculated, for example, as a target deceleration or as a target braking force. The individual target wheel brake pressures are formed from this braking request. Depending on the driving condition and slip condition, these pressures are modified and controlled by means of the pressure control valves 17 , 18 , 37 , 38 and 27, 28, 47, 48. In the closed control loop, the current pressures at the wheel pressure sensors are used for the target / actual comparison for each wheel brake 7-10 . At different target pressures in the left and right wheels of a vehicle axle, the pressure compensation valves 13 , 19 are closed and in each wheel brake the predetermined target pressure is adjusted to the target brake pressure by controlling the intake and exhaust valves in the sense of regulating the actual brake pressure. In order to build up pressure on a wheel brake, which is carried out by means of the auxiliary pressure source (motor-pump unit) 20 , the inlet valve is energized until the desired setpoint pressure in the wheel brake develops with the desired dynamics. In this case, pressurized pressure medium flows from the auxiliary pressure source (motor-pump unit) 20, and primarily from the high-pressure accumulator 21 to the wheel brakes. A pressure reduction is accordingly achieved by energizing the outlet valve, the pressure medium flowing back into the pressure medium reservoir 4 via the return line 29 . The pump 23 is activated when the storage pressure in the high-pressure storage device 21 drops below a predetermined value.

Fig. 2 shows a portion of the pressure medium supply container 4. This is provided with a device for determining both the fill level and the temperature of the liquid present there. For this purpose, two switches in the form of reed contacts 53 , 54 are arranged in a dip tube 50 which is closed at the bottom and which is fastened with its upper open end to an upper closure wall 51 of the pressure medium reservoir 4 or is made in one piece therewith. A cover 52 closes the upper opening of the dip tube 50 . The upper reed contact 54 , which marks the maximum fill level, is a break contact and the lower reed contact 53 , which marks the minimum fill level, is a make contact. The two reed contacts are electrically connected in series, with each reed contact 53 , 54 having a resistor 55 , 56 connected in parallel. The upper resistor 56 assigned to the upper reed contact 54 has a fixed value R _const . The lower resistor 55 assigned to the lower reed contact 53 has a temperature-dependent value R _θ and is therefore referred to below as a thermal resistor 55 . It should take the temperature of the brake fluid and is in contact with the inside of the dip tube 50 . But it can also be embedded in the dip tube wall. The thermal resistor 55 can also be located directly in the brake fluid outside the immersion tube 50 , the electrical lines leading to the thermal resistor 55 being led through the wall of the immersion tube 50 so as to be pressure-tight.

The immersion tube 50 projects into the brake fluid in the pressure medium reservoir 4 . An annular float 60 surrounds and is guided by the dip tube 50 . The float 60 is magnetic or carries magnets which actuate the reed contacts 53 , 54 as soon as it comes close to it when carried by the brake fluid.

The feed lines to the network of reed contacts 53 , 54 and resistors 55 , 56 are connected, with the interposition of a series resistor 57, to a resistance measuring device 58 , which forwards the measurement signals to the electronic control unit 32, if necessary in digitized form.

A medium fill level is shown in FIG. 2, so that the float 60 is located between the two reed contacts 53 , 54 . The upper reed contact 54 is thus open and the lower reed contact 55 is closed. The resistance of the network thus has the value R _{tot, norm} = R _V + R _θ .

If the brake fluid level rises, the upper reed contact 54 is opened by the magnetic field of the float 60 , so that the total resistance to R _{tot, max} = R _V + R _const + R _θ .

If the brake fluid level falls below the lower reed contact 53 , this is closed by the magnetic field of the float 60 , so that the total resistance is now R _{tot, min} = R _V + R _const .

The respective abrupt change in the total resistance R _tot can be interpreted as reaching the lower or the upper fill level.

In the position shown, the total resistance R tot is _norm- dependent, since the thermal resistor 55 is included in the resistance network with its temperature-dependent value R _θ because of the opened lower reed contact 53 . The temperature of the brake fluid in the pressure medium reservoir 4 can therefore be determined by measuring the total resistance.

In order to distinguish the changes in the total resistance caused by a change in the value of the thermal resistance due to temperature fluctuations from changes caused by a change in the fill level, the bandwidth of R _{θ should be} between R _{tot, min} and R _{tot, max} .

For this purpose, the network of reed contacts 53 , 54 , resistors 55 , 56 and series resistor 57 is connected to a voltage source, the voltage drop across the resistor network being measured and converted into a total resistance value R _tot . If the value lies between R _{tot, min} and R _{tot, max} , the measured value can be used to infer the temperature of the thermal resistor 55 and - because of its arrangement in the pressure medium reservoir 4 - the temperature of the brake fluid present there. Since the pressure medium reservoir 4 is arranged on the master brake cylinder 2 (see FIG. 1) and there is only a short pressure medium connection to the primary pressure chamber and to the simulator chamber 44 , it can be assumed in a good approximation that the temperature prevailing there is also detected in this way.

The measured temperature is taken into account when controlling the brake system as follows:
The pressure in the primary pressure chamber, measured with the pressure sensor 15 , and the pedal travel, measured with the pedal travel sensor 33 , are in a specific relationship, which is essentially determined by the spring characteristic of the simulator spring 45 . It is assumed that the brake fluid in the primary pressure chamber is essentially incompressible. In addition, it is assumed that the housing channel 43 forms no flow resistance between the primary pressure chamber and the simulator chamber 44 .

If the brake fluid is enriched with air, the first assumption does not apply. Rather, the pedal travel simulator characteristic is dependent on the temperature, since the compressibility of the air enclosed in the brake fluid is temperature-dependent. In addition, the brake fluid is viscous at low temperatures, so that the housing channel 43 forms a flow resistance between the primary pressure chamber and the simulator chamber 44 . Both effects can be corrected mathematically with knowledge of the temperature of the brake fluid, so that the air enrichment can be determined relatively precisely and the desired deceleration can be determined in an exact manner.

Claims (5)

1.Device for determining the filling level of a storage container for liquids and for determining the temperature of the liquid present there, at least two switches arranged one above the other in the storage container being provided, which mark an upper and a lower filling level and can be actuated by a float on the liquid, and wherein an electrical resistor is connected in parallel to each switch, so that the fill level can be determined at least in steps from the total resistance of the resistor network formed from the resistors, characterized in that the resistor connected in parallel with the lower switch ( 53 ) is a temperature-dependent resistor, and that the lower switch ( 53 ) is normally open, so that with a sufficiently filled storage container, the thermal resistor ( 55 ) contributes to the total resistance in a temperature-dependent manner and thus from the total resistance to the temperature of the liquid can be closed. 2. Device according to claim 1, characterized in that the switches ( 53 , 54 ) are electrically connected in series. 3. Apparatus according to claim 1 or 2, characterized in that the switch reed contacts ( 53 , 54 ) in a vertically immersed in the reservoir ( 4 ) and closed below immersion tube ( 50 ) and that the float ( 60 ) on the outside of the Tube ( 50 ) is guided and is itself magnetic or carries a magnet for actuating the reed contacts ( 53 , 54 ). 4. The device according to claim 1, characterized in that a measuring mechanism ( 58 ) for measuring the voltage drop at the resistance measuring mechanism is present, the measured values being fed to an evaluation unit ( 32 ) with which the system to which the storage container provided with the device is assigned is controlled depending on the temperature. 5. Procedure for monitoring a brake fluid operated brake system with a reservoir for the brake fluid that with a device after is provided with one of the preceding claims a pedal-operated master brake cylinder, with a from the one prevailing in the master brake cylinder Master brake cylinder pressure operated pedal travel simulator, with a Pressure sensor for detecting the master cylinder pressure and a pedal travel sensor for detecting the pedal travel, whereby from the assignment of the master cylinder pressure to the pedal travel in comparison with a characteristic curve on the Air pollution of the brake fluid and / or on one Flow resistance is closed, thereby characterized that the characteristic using the device according to one of the preceding claims is corrected for temperature.