Method and arrangement for determining the wear condition of a shifting clutch

This application claims priority from German patent application serial no. 10 2008 043 492.2 filed Nov. 5, 2008.

The invention concerns a method and arrangement for determining the wear condition of a shifting clutch of a motor vehicle, which is provided for interrupting the force flow or assimilating the rotation speed between a drive output shaft of an internal combustion engine and a drivetrain of the motor vehicle.

In motor vehicles a clutch is usually provided between a drive output shaft of the internal combustion engine and the drivetrain, in order to build up or interrupt the force flow from the internal combustion engine without jerkiness during the starting process. In a motor vehicle with a manual shift transmission, for shifting it is necessary to have very rapid and complete interruption of the force flow and the lowest possible mass moments of inertia. These requirements are fulfilled by a shiftable, dry friction clutch consisting of a diaphragm spring pressure plate connected to the driveshaft of the internal combustion engine, a clutch disk that can move axially on the input shaft of the transmission downstream and a disengagement device which, via a release bearing, transfers the release path from the non co-rotating actuation elements of the shifting clutch to the pressure plate. In the non-actuated condition of the shifting clutch the pressure plate is pressed by the diaphragm spring against the clutch plate, producing a frictional connection between the two plates so that both these elements rotate at the same speed. When the shifting clutch is actuated, the disengagement device acts upon the release bearing and presses the diaphragm spring in the direction of the pressure plate. This releases the pressure of the diaphragm spring from the pressure plate and results in disengaging the shifting clutch. Between the fully engaged and fully disengaged positions of the clutch the speed is controlled by the action of one side of the clutch on the other side of the clutch as a function of the pressure force of the diaphragm spring on the pressure plate and, correspondingly, that of the pressure plate against the clutch disk, so that the shifting clutch is acting as a speed converter. In such cases the torque is transmitted unchanged, but during speed conversion the torque transfer takes place with slipping, which leads to heating of the clutch and wear of its lining.

Unless excessive clutch wear is recognized in good time, the motor vehicle may be immobilized since transmission of the force flow from the combustion engine to the drivetrain is no longer possible. For that reason, to an increasing extent clutch wear indicators are provided, by means of which the wear condition of the shifting clutch can be monitored. This enables wear of the shifting clutch to be recognized at an early stage, so that the clutch can be replaced the next time that the motor vehicle is taken to a workshop.

From DE 20 2005 021 024 U1 a shifting clutch for the drivetrain of a motor vehicle is known, in which signal-emitting or signal-generating elements and a sensing element are provided, such that the wear condition of the shifting clutch can be determined from the separation of the signals received by the sensing element. For this purpose one of the signal-emitting elements is arranged on the pressure plate of the shifting clutch, while the respective other signal element is fixed on the housing of the clutch. When the drivetrain is in operation both signal elements sweep across the sensing element. The time separation of the signals then allows a conclusion to be drawn about the position of the pressure plate relative to the housing, and from this the wear can be estimated.

To those familiar with the subject other clutch wear recognition means are known, in which the clutch loading is calculated and the clutch lining wear is estimated on the basis of the loads. Furthermore, there are wear recognition methods that are based on clutch position, which determine the condition of the clutch by comparing the engaged position of the clutch when new, to the current engaged position.

The above methods of the prior art have the disadvantage that the clutch wear cannot be determined as an absolute value. For example, if the control unit is replaced the clutch wear values already determined are lost unless they have been transferred to the newly fitted control unit. Since the process of transferring clutch wear values already recorded into a new control unit can only be controlled with much difficulty in practice, during further operation of the motor vehicle concerned the wear calculations may be wrong and this can lead to replacement of the clutch for no reason, or to immobilization of the vehicle because the clutch has not been replaced in time.

Accordingly, the purpose of the present invention is to provide a method and a device for determining the wear condition of a shifting clutch of a motor vehicle, which enable an absolute determination of the wear of the clutch.

The invention makes use of the technical principle that the wear condition of the shifting clutch can be determined from the parameters of clutch release path and release force in a release bearing of the shifting clutch, and for this purpose a difference between the clutch release path at a first increase of the release force and a reference point represented by a second release force increase during clutch actuation is measured. In this case, to determine the parameter at least one sensor is provided in the area of the release bearing, which in turn is connected to a device for evaluating the parameter it has detected. In the case of a new shifting clutch the difference between the clutch release paths at a first release force increase and at the second increase that represents the reference point has a certain value, which increases progressively with increasing wear. Since both the clutch release path at a first increase of the release force and the release path at the second increase can continually be freshly determined by sensors which detect the parameters of the clutch release path and release force at the release bearing, the wear of the shifting clutch can correspondingly be determined as an absolute value at any time. Consequently, a replacement of the control unit or inaccuracies regarding the fitting position of the clutch are insignificance. In addition, there is no need for additional signaling or scanning elements in the area of the clutch disk or pressure plate.

In a further development of the invention, if there is no reference value and/or after a defined time period, a measurement is made and the current reference value is then stored. This makes it unnecessary to measure the reference value each time the wear condition of the clutch is determined, so that the quantity of data to be processed by the evaluation unit can be kept small. If there is no reference value because the evaluation unit has been replaced, or to equalize the reference value recorded in the evaluation unit, then a new measurement can be made at defined time intervals or in particular situations. By determining the reference value at intervals, calibration errors of the path and force sensors can consequently also be compensated for.

In one embodiment of the invention the wear condition is measured when the internal combustion engine is started and the shifting clutch is first actuated. This has the advantage that when the motor vehicle is first operated a condition check of the clutch is carried out and consequently a possible failure of the clutch due to excessive wear can be determined before driving begins.

In a further development of the invention, if at least one defined limit of the difference is exceeded a warning is emitted to the driver of the vehicle. This informs the driver that the defined warning threshold has been exceeded, so that he can react to the situation appropriately and plan to make his way to a workshop.

According to another, advantageous version of the invention, when a first limit has been exceeded a warning message of moderate urgency is emitted to the driver of the vehicle, whereas when a second limit is exceeded a warning message of greater urgency is produced. By virtue of such grading of the urgencies the driver of the vehicle can be alerted already very early to the wear condition of the shifting clutch, so that replacement of the clutch can be planned for the next inspection of the motor vehicle at a workshop.

In a further development of the invention, the release force at the release bearing is determined by a pressure and/or force sensor. This enables the release force parameter to be determined reliably. Besides, in some circumstances a sensor already present in the area of the clutch can be used.

In accordance with a further embodiment of the invention the release force can also be calculated. For this, as an example, a known valve control signal in a control unit of the motor vehicle can be used. By means of the valve control signal a corresponding valve for actuating a hydraulic or pneumatic release device is activated, and as a function of the duration of the valve's activation and taking into account certain physical boundary conditions such as piston diameter, valve cross-section, volume flow or density, under- or over-critical pressure ratio, supply pressure and ambient pressure, the clutch release force can be calculated on the basis of the known physical relationships.

In accordance with an advantageous embodiment of the invention, the release force is determined by measuring a pressure of a hydraulic or pneumatic release device. This measure allows the method according to the invention to be also used in the case of shifting clutches with hydraulic or pneumatic release devices. To those familiar with the field, however, it will be clear that depending on the structure of the release device, other ways to determine the release force can be used as well.

Advantageously, the device for evaluating the parameters determined is made in the manner of a control unit. Correspondingly, the said parameters can be evaluated and the wear condition of the clutch accordingly determined centrally in a control unit of the motor vehicle and consequently, depending on the condition determined, an appropriate reaction can also be initiated from that central location.

Below, further invention-improving measures are indicated in more detail together with the description of a preferred example embodiment of the invention with reference to the figures, which show:

FIG. 1 : Very schematic representation of a motor vehicle with a shifting clutch, in which the arrangement and method are use according to the invention; and

FIG. 2 : Diagram showing variations I and II of a release force, in each case plotted against the release path of a shifting clutch.

FIG. 1 shows a schematic representation of a motor vehicle in which a shifting clutch 1 with the method and the arrangement according to the invention for determining the wear condition is provided. This clutch 1 is connected on one side to a driveshaft 2 of an internal combustion engine 3 , while its other half is connected to a drivetrain 4 comprising a transmission, differential, etc. Depending on the disengaging status of the clutch 1 , there is a force flow from the combustion engine 3 to the drivetrain 4 or vice-versa. To actuate the clutch 1 , it is in active connection via a release bearing 5 and an amplifier unit 6 with a clutch pedal 7 , by actuating which a disengaging movement of the clutch 1 —not illustrated here — can be initiated. To determine the parameters of the clutch release path S release and the release force F release in the area of the release bearing 5 , a force sensor 8 and a path sensor 9 are provided. These two sensors 8 and 9 are connected to a control unit 10 of the motor vehicle, in which the wear condition of the clutch 1 is determined with reference to the parameters supplied by the force sensor 8 and the path sensor 9 . In accordance with an advantageous embodiment of the invention, the release force is determined by measuring a pressure of a hydraulic or pneumatic release device.

FIG. 2 shows two variations I and II of the release force parameter F ausrück at the release bearing 5 plotted against the clutch release path parameter s ausrück . Curve 1 shows the release force variation of a new clutch, while curve II shows the release force of an already worn clutch. Both curves I and II show a first increase of the release force F ausrück , represented respectively by the points 11 and 12 , and have a common second increase that forms the reference point 13 . The wear condition of the clutch 1 is now determined in terms of the difference between the clutch release paths s ausrück at which the respective first increase of the release force F ausrück occurs, namely points 11 or 12 , and the reference point 13 . For curve II, typical of a severely worn clutch, the path difference s diff — ausv is substantially larger than the path difference s diff — ausv in curve I. In this way, therefore, i.e. by virtue of the variation of the difference as wear progresses, a conclusion can be drawn about the condition of the shifting clutch 1 at the time. Moreover, since the point corresponding to the first increase of the release force F ausrück , namely 11 or 12 , and the reference point 13 can each be freshly determined by the sensors 8 and 9 at each measurement, an absolute measurement of the wear condition of the clutch 1 can be obtained at any time. In addition, from a defined size of the difference onward the control unit 10 can issue a warning message to the vehicle's driver about the condition of the clutch 1 and thereby alert him to the problem.

Indexes

1 Shifting clutch

2 Driveshaft

3 Internal combustion engine

4 Drivetrain

5 Release bearing

6 Amplifier unit

7 Clutch pedal

8 Force sensor

9 Path sensor

10 Control unit

11 First clutch force increase of a new clutch

12 First clutch force increase of a worn clutch

13 Reference point

I Release force curve of a new clutch

II Release force curve of a worn clutch

F ausrück Release force at the release bearing

S ausrück Clutch release path at the release bearing

S diff — ausv Path difference when the clutch is worn

S diff — ausn Path difference when the clutch is new