Method for calculating the viscosity of a pressure medium in a pressure medium circuit

Abstract

To provide a method for calculating viscosity of a pressure medium in a pressure medium circuit.SOLUTION: A brake circuit of an electronic slip-controllable vehicle brake device includes a pressure supply device that can be loaded via an operation mechanism, a sensor providing an operation signal indicating operation of the operation mechanism, a pressure sensor providing a pressure signal indicating a dominant pressure in the pressure supplier, and an electronic control unit, and further processes the signal in the electronic control unit. The electronic control unit records the operation signal and the pressure signal so as to be time-synchronized with each other, calculates a quotient from the value, and determines viscosity corresponding to a pressure medium by the recorded characteristic curve. An electronic controllable actuator is controlled depending on the calculated value.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a method for calculating the viscosity of a pressure medium in a pressure medium circuit as recited in the preamble of claim 1 . [Background technology]

[0002] In order to make regenerative braking possible with only the smallest possible changes to the hydraulic layout of existing brake systems, regenerative braking systems based on standard ESP brake systems are available on the market. Patent document 1 discloses such a brake system. In this case, during regenerative braking, the brake fluid volume forced into the hydraulic device by the driver from the master brake cylinder is not used to increase the brake pressure in the wheel brakes, but is instead transferred into a pressure medium reservoir by suitable control of the valves of the hydraulic device.

[0003] After the braking operation is terminated, the pressure medium reservoir is drained either passively at low pressure medium viscosities, i.e. without the use of a feed pump, by overflowing a specific valve of the hydraulic system, or actively at high pressure medium viscosities, i.e. using a feed pump and via an alternative flow path. In a retrofittable manner, the drainage gradient, i.e. the amount of pressure medium that can be drained per time unit, is highly dependent on the temperature or the viscosity of the pressure medium and the flow resistance in the flow path.

[0004] High viscosity values ​​can lead to undesirably long extensions of drainage times. However, regenerative braking operations cannot be performed during drainage of the pressure medium reservoir. Furthermore, the regenerative capacity of the brake system is also impaired if the pressure medium reservoir cannot be completely emptied. Ultimately, both effects have a detrimental effect on the energy efficiency of the vehicle drive. Therefore, there is a demand for as rapid and complete drainage of the pressure medium reservoir as possible.

[0005] To shorten the discharge time, a feed pump may be used, as mentioned above. Of course, the operation time of the feed pump must be taken into consideration in the structural design of the feed pump, since it affects the wear characteristics of the feed pump. This increases the structural expense and cost for the feed pump.

[0006] In order to avoid unnecessary control of the feed pump or excessively long operating times and to ensure complete emptying of the pressure medium reservoir, it is necessary to know the viscosity of the pressure medium as well as the filling level of the pressure medium reservoir as accurately as possible. A reliable estimation of the filling level also depends on the viscosity of the pressure medium, since this filling level is not measured, for example, but is indirectly estimated from existing measurement data of the filling level, for example the valve opening time and the pressure applied to the master brake cylinder.

[0007] Conventionally, instead of directly measuring the temperature of the brake fluid, a relatively inaccurate estimation of the temperature or viscosity of the pressure medium is carried out based on the measurement of the existing electrical resistance in the brake system. Moreover, the moisture content of the pressure medium and the type of pressure medium used adversely affect the accuracy of this estimation, so that the feed pump is controlled unnecessarily frequently and is operated unnecessarily long, both of which again affect the structural design and thus the cost of this feed pump. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] DE 19604134 A1 Summary of the Invention Effect of the Invention

[0009] In contrast to this, the method according to the invention for calculating the viscosity of the pressure medium in a pressure medium circuit as specified in the characterizing part of claim 1 has the advantage that a more accurate estimation of the filling level of the pressure medium reservoir and therefore a more accurate determination of the operating requirement or operating time of the feed pump for completely emptying the pressure medium reservoir is possible. Furthermore, due to the adapted pump operation, the regenerative capacity of the brake system can be utilized over a wider operating range, which increases the energy efficiency of the vehicle drive. Furthermore, the overall shorter operating time of the feed pump allows a more cost-effective structural design of the feed pump.

[0010] Further advantages or advantageous developments of the method according to the invention are evident from the dependent claims and the following description.

[0011] According to claim 2, the output signal is generated or suppressed depending on a limit value for the viscosity of the electronic control unit, which limit value is determined accordingly, for example depending on the different hydraulic resistances of the different hydraulic devices.

[0012] Apart from the preferred control of the emptying of the pressure medium reservoir by means of a feed pump (claims 3 and 4), the control signal may take into account many other brake control functions by the electronic control unit. In this connection, for example, if an emergency stop is imminent, it must be commanded that the wheel brakes are pre-filled. This filling may also take place by operating the feed pump.

[0013] Basically, the most accurate information possible about the viscosity of the pressure medium is important for all braking operations in which high pressure-development dynamics are important. The brake pressure control in the wheel brakes can also be carried out more precisely if the viscosity of the pressure medium is known, so that, for example, the control times and the necessary current supply to the switching and regulating valves can be adapted accordingly. [Brief description of the drawings]

[0014] [Figure 1] 1 is a hydraulic circuit diagram of a brake system known from the prior art, which forms the basis of the present invention; [Diagram 2] 2 is a flow chart illustrating the sequence of method steps of the method according to the invention; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] The present invention will now be described in detail with reference to the accompanying drawings.

[0016] 1 shows a hydraulic circuit diagram of an electronic slip-controllable braking system 10 of a motor vehicle, known per se. The braking system 10 has a master brake cylinder 14 which can be indirectly actuated by the driver via an actuating mechanism 12, preferably in the form of a brake pedal. A brake booster 16 is arranged between the actuating mechanism 12 and the master brake cylinder 14, which boosts the braking force applied to the actuating mechanism 12. The master brake cylinder 14 serves to detect the driver's braking request and supplies brake pressure to a brake circuit 18; 20 connected to the master brake cylinder 14. The master brake cylinder 14 can therefore be considered as an externally loadable pressure supply device. A hydraulic device 22 is connected to the master brake cylinder 14 for open-loop or closed-loop control of the brake pressure, by which wheel brakes 24 of the braking system 10, which are connected to the hydraulic device 22, are applied.

[0017] The closed-loop control of the brake pressure in the wheel brake 24 by the hydraulic device 22 is effected as a function of the slip ratio prevailing at the wheel of the vehicle which is assigned to the wheel brake 24. This is determined by means of the rotational speed difference between the wheels of the vehicle. Wheel rotational speed sensors 28 which are assigned to the wheels detect the respective rotational speeds and, inter alia, supply this measurement data to an electronic control unit 30 for evaluation.

[0018] The master brake cylinder 14 is designed in a typical two-way manner and therefore supplies two mutually separate brake circuits 18;20 with pressure medium under brake pressure. Each brake circuit 18;20 is equipped with two of the total of four wheel brakes 24 of the braking system 10. Furthermore, each brake circuit 18;20 has a feed pump 34 or a feed pump unit which can be driven by an electric motor 32. Furthermore, a pressure increase valve 36 is connected hydraulically in front of each wheel brake 24 of the brake circuit 18;20 and a pressure reduction valve 38 is connected hydraulically behind each wheel brake 24. The pressure increase valve 36 controls in a typical manner the flow of pressure medium from the master brake cylinder 14 into the respective wheel brake 24 and thereby determines the brake pressure increase, whereas the pressure reduction valve 36 controls the flow of pressure medium from the wheel brake 24 into a return line 40 and thereby determines the brake pressure reduction. A common pressure-reducing valve 38 of each brake circuit 18;20 is downstream of each of them, which first buffers the outflowing pressure medium before it is sucked back in by the corresponding feed pump 34 and returned to the respective brake circuit 18;20. A changeover valve 44 in each brake circuit 18;20 controls the pressure-medium connection between the master brake cylinder 14 and the wheel brakes 24 and thus determines the source of brake pressure (master brake cylinder 14 or feed pump 34) depending on the operating mode of the brake system 10. This changeover valve 44 is arranged directly downstream of the master brake cylinder 14 and upstream of the point where the pressure medium pumped by the feed pump 34 re-enters the brake circuit 18;20. Furthermore, a so-called intake valve 46 is provided, which is arranged in the pressure-medium connection connecting the intake side of the feed pump 34 to the master brake cylinder 14. Via this pressure medium connection, the feed pump 34 can supply pressure medium from the master brake cylinder 14 as required, for example when the pressure medium volume present in the pressure medium reservoir 42 is insufficient for the feed pump 34 to regulate the required brake pressure in the associated brake circuit 18; 20.

[0019] The valves 36; 38; 44; 46 are designed as electronically controllable valves, in particular as solenoid valves, which can be transferred from a basic position to an operating position. The boost valve 36 is open in the basic position and closed in the operating position, whereas the pressure reducing valve 38 is closed in the basic position and can flow through in the operating position. The electronic control unit 30 detects signals of various sensors 48; 50; 52 which indicate the movement of the actuating mechanism 12, the pressure in one of the brake circuits 18; 20 directly downstream of the master brake cylinder 14 or the brake pressure in the wheel brakes 24 of the brake circuits 18; 20, and evaluates these sensor signals together with the signal of the wheel speed sensor 28 for possible control signals for the valves 36; 38; 44; 46 and for possible control signals for the electric motor 32 for operating the feed pump 34. In this case, the sensors which detect the movement of the actuating mechanism 12 can evaluate the stroke of the actuating mechanism 12, the acceleration of the actuating mechanism 12 or the actuating speed of the actuating mechanism 12 and convert them into corresponding actuating signals.

[0020] As already mentioned at the beginning, the brake system 10 thus designed is installed in a vehicle which is capable of converting the kinetic energy of the vehicle into electrical energy during braking (regenerative braking system). In this case, the adjusted vehicle deceleration no longer has to be brought about by the brake system, but only a correspondingly small hydraulic brake pressure is required to obtain the vehicle deceleration requested by the driver. However, since the driver, as before, sets the braking request by actuating the brake pedal and correspondingly displacing the pressure medium from the master brake cylinder 14 into the hydraulic system 22, the build-up of the hydraulic brake pressure in the wheel brakes 24 must be limited or prevented altogether by corresponding electronic control of the components of the hydraulic system 22. This is achieved in that the pressure medium displaced by the driver is further directed through the open pressure increase valve 36 into the wheel brakes 24 and from the wheel brakes 24 through the likewise open pressure reduction valve 38 into the pressure medium reservoir 42. A corresponding control of the pressure increase valve 36 and the pressure reduction valve 38 is carried out by the electronic control unit 30 during regenerative braking, so that a pressure medium connection is established between the master brake cylinder 14, which would otherwise be blocked, and the pressure medium reservoir 42. As a result, the pressure medium displaced from the master brake cylinder 14 does not contribute at all to the brake pressure buildup in the wheel brakes 24 of the respective brake circuits 18; 20 during regenerative braking, or only reduces the brake pressure buildup.

[0021] After each regenerative braking operation, the pressure medium reservoir 42 must also be emptied in order to restore the full regenerative function of the braking device 10 again for future braking operations. Emptying can be performed with the aid of the feed pump 34 if the viscosity of the pressure medium and / or the filling level of the pressure medium reservoir 42 is too high in order to carry out the evacuation within the desired maximum time interval. In the case of active evacuation, the electronic control unit 30 determines the required operating time of the feed pump 34 until the pressure medium reservoir 42 is completely emptied.

[0022] By means of the method according to the invention, the viscosity of the pressure medium is determined from the actuation signal of a sensor detecting actuation of the brake pedal 12 and from the pressure signal of a pressure sensor 50 detecting the brake pressure in the brake circuit. If the determined viscosity exceeds a limit value stored in the electronic control unit 30, active draining of the pressure medium reservoir 42 is carried out, otherwise draining takes place in a passive manner, i.e. without the involvement of the feed pump 34.

[0023] 2, the method according to the invention consists in that in a first step S1 the actuation signal B and the pressure signal D are recorded time-synchronously with one another by the electronic control unit 30. Then in step S2 the electronic control unit determines the quotient Q from the pair of values. Then in step S3 the prevailing viscosity V is determined by means of a characteristic curve K stored in the electronic control unit 30, which describes the viscosity of the pressure medium as a function of this quotient Q. Said characteristic curve K has been calculated empirically beforehand by a series of experiments with a corresponding hydraulic device 22 and is stored in the electronic control unit 30 in digital form.

[0024] Furthermore, a limit value G for the viscosity V is stored in the electronic control unit 30. If this limit value G is exceeded, the pressure medium reservoir 42 is emptied by means of the feed pump 34 in order to maintain a critical emptying period. Accordingly, the electronic control unit 30 compares the calculated viscosity V with the limit value G in step S4 and outputs a control signal A to the electric motor 32 in step S5 to drive the feed pump 34 when the determined pressure medium viscosity V is higher or greater than the stored limit value G. In the opposite case, i.e. below the limit value G, the supply of the control signal A to the electric motor 32 is reduced and the emptying of the pressure medium reservoir 42 is carried out passively.

[0025] For the passive drainage, the pressure medium reservoir 42 is provided with a return element, for example a return spring, which with its spring force loads, for example, a reservoir piston, in such a way that the latter pushes the pressure medium out of the pressure medium reservoir 42. The drainage takes place along a drainage path via a pressure reducing valve 38, which is controlled by the electronic control unit 30 to its open position, i.e., through the wheel brake 24 and is guided back via the intake valve 36, which is in any case open in its basic position, and the likewise passable directional control valve 44, into the master brake cylinder 14 or into a pressure medium reservoir tank 54 connected to this master brake cylinder 14.

[0026] Of course, modifications or additions to the described embodiment are conceivable without departing from the basic idea of ​​the invention. In this connection, it is pointed out that the method is preferably only used for emptying the pressure medium reservoir 42 on the basis of regenerative braking operations. Knowledge of the viscosity of the pressure medium is taken into account by the electronic control unit 30, for example, also for brake pressure regulation or for controlling highly dynamic brake operations. [Explanation of symbols]

[0027] 10 Brake device 12 Operating mechanism 14 Pressure supplier, master brake cylinder 18,20 Pressure medium circuit, brake circuit 30 Electronic Control Unit 34 Feed pump or feed pump unit 42 Pressure medium reservoir 48 Sensors 50 Pressure Sensor A Output Signal B Operation signal D Pressure signal G limit K characteristic curve Q quotient V viscosity

Claims

1. 1. A method for calculating the viscosity (V) of a pressure medium in a pressure medium circuit (18; 20), comprising: The pressure medium circuit (18; 20) is a pressure supply that can be applied via an operating mechanism (12). (14) a sensor (48) for providing an operation signal (B) representing an operation stroke amount of the operation mechanism when the operation mechanism (12) is operated; a pressure sensor (50) for providing a pressure signal (D) representative of a pressure generated by said pressure supply (14) in operation; The method comprises an electronic control unit (30) to which the operating signal (B) and the pressure signal (D) are supplied for further processing, The electronic control unit (30) records the operation signal (B) and the pressure signal (D) in a time-synchronous manner with each other, and calculates a quotient (Q) from the pair of values; determining the viscosity (V) of the pressure medium using at least one characteristic curve (K) stored in the electronic control unit (30) and representing the viscosity (V) depending on the quotient (Q); 13. A method for calculating the viscosity (V) of a pressure medium in a pressure medium circuit, comprising:

2. comparing the calculated viscosity (V) of the pressure medium with a limit value (G) for viscosity stored in the electronic control unit (30); 2. The method of claim 1, further comprising the step of: outputting an output signal (A) from said electronic control unit (30) when said limit value (G) is exceeded.

3. 3. The method according to claim 2, further comprising controlling an electrically-driven feed pump (34) of the pressure medium circuit (18; 20) by means of the output signal (A).

4. 4. The method according to claim 3, characterized in that a pressure medium reservoir (42) of the pressure medium circuit (18; 20), which is filled with pressure medium, is emptied by controlling the electronically activatable feed pump (34).

5. 5. The method according to claim 1, wherein the method is used for controlling the braking operation in a regenerative braking system of a motor vehicle.

6. 2. The method according to claim 1, characterized in that the pressure medium circuit (18; 20) is a brake circuit of an electronic slip-controllable braking system (10) of a motor vehicle.

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