Method for operating a drive train

A method of operating a drive-train which comprises a drive aggregate, a transmission and a starting clutch connected between the drive aggregate and the transmission such that during rolling and coasting, in a thrust operation, the drive-train is operated in such manner that a target value for the position of the clutch is determined as a function of a transmission input speed. The target value for the position of the clutch is compared with a corresponding actual value and, on the basis of the deviation between the target and actual values, a control parameter is generated for adjusting the clutch. The target value for the position of the clutch, during rolling and coasting, is determined in such manner that for a defined value of the transmission input speed, a target value is determined for the position of the clutch when rolling which is different from that determined when coasting.

This application is a national stage completion of PCT/EP2010/059439 filed Jul. 2, 2010 which claims priority from German Application Serial No. 10 2009 027 564.9 filed Jul. 9, 2009.

FIELD OF THE INVENTION

The invention concerns a method for operating a drive-train.

BACKGROUND OF THE INVENTION

The main components of a drive-train are a drive aggregate and a transmission, the transmission being connected between the drive aggregate and a drive output of the drive-train. The transmission converts rotational speeds and torques and thereby transmits the traction force supplied by the drive aggregate and provides it at the output of the drive-train. The present invention concerns a method for operating a drive-train which, besides the drive aggregate, transmission and output already mentioned, also comprises a starting clutch connected between the drive aggregate and the transmission. The transmission of the drive-train is preferably in the form of an automatic or semi-automatic transmission, in particular an automatic or semi-automatic manual transmission.

From practice it is already known to operate a drive-train for rolling and coasting during thrust operation, in such manner that a target value for the position of the starting clutch connected between the drive aggregate and the transmission is determined as a function of a transmission input speed, then comparing the target value for the position of the starting clutch with a corresponding actual value, so that in a regulator, on the basis of the deviation between the target and the actual values, an adjustment parameter for adjusting the starting clutch is generated, namely in such manner that by virtue of the adjustment of the starting clutch, the actual value approaches the target value. Thrust operation of the drive-train is understood to mean movement, i.e. in this case rolling or coasting, during which an accelerator pedal or gas pedal of the drive-train is not actuated, or whose actuation is smaller than a specified limit value. During rolling or coasting a gear is engaged in the transmission.

In methods known from practice for operating a drive-train in the thrust mode during rolling or coasting, the target value for the position of the starting clutch is preferably determined from a characteristic curve, the same curve being used for rolling and for coasting, so that for a defined value of the transmission input speed in each case the same target value for the position of the starting clutch is determined both for rolling and for coasting. Owing to inertial effects in the control system of the starting clutch, in this case, for example when rolling downhill, the transmission input speed may exceed the speed of the drive aggregate until the starting clutch, which has to be closed farther, has accelerated the drive aggregate to the level of the transmission input speed. Because of this the drive-train becomes somewhat slower, so with decreasing transmission input speed the starting clutch can be opened more easily, in order to be closed again subsequently. In a drive-train prone to oscillations this can lead to resonance effects of the drive-train, since the transmission input speed influences the target value for the position of the starting clutch and is thus regeneratively coupled to the target value for the position of the starting clutch. Such upswings of a drive-train with oscillating movement of the starting clutch impairs the driving comfort and is therefore disadvantageous.

As already mentioned, in the methods known from practice the control parameter for adjusting the starting clutch is determined as a function of the deviation between the target value for the position of the starting clutch and the corresponding actual value, the control parameter typically being an adjustment speed for the starting clutch in its closing or opening direction. This too can result in uncomfortable driving behavior.

As prior art, the documents DE 103 34 930 A1 and DE 10 2007 012 875 A1 can be mentioned, each of which discloses a method for operating a drive-train in the thrust mode.

SUMMARY OF THE INVENTION

Starting from there, the purpose of the present invention is to provide a new type of method for operating a drive-train.

In a first aspect of the inventive method the target value for the position of the starting clutch when rolling and coasting is determined in such manner that with a defined value of the transmission input speed, a target value is determined for the position of the starting clutch during rolling, which is different from the value determined during coasting.

According to this first aspect of the invention, the target value for the position of the starting clutch when rolling and when coasting during thrust operation is determined such that at a given value of the transmission input speed, a different value is determined for rolling from the value determined for coasting. In this way oscillation movement of the starting clutch can be reliably avoided even in drive-trains prone to oscillation. According to the invention, in this way the target value for the position of the starting clutch is associated with a hysteresis, whereby the target value for the position of the starting clutch is made less sensitive to fluctuations of the transmission input speed. This increases the driving comfort.

In a second aspect of the inventive method the control parameter for adjusting the starting clutch is determined on the basis of the relative deviation between the target value for the position of the starting clutch and the actual value of the position of the starting clutch, and additionally on the basis of the absolute actual value of the position of the starting clutch.

According to this second aspect of the invention, the control parameter for adjusting the starting clutch is determined not only on the basis of the relative deviation between the target value for the position of the starting clutch and the corresponding actual value, but rather, additionally on the basis of the absolute actual value of the position of the starting clutch. This aspect of the present invention is based on the recognition that the driving comfort can be increased if, despite having the same relative deviation between the target and actual values of the position of the starting clutch, different control parameters for adjusting the starting clutch are determined as a function of the absolute actual value of the position of the starting clutch.

Preferably, the two aspects according to the invention are applied in combination with one another.

DETAILED DESCRIPTION OF THE DRAWINGS

The present invention relates to a method for operating a drive-train.FIG. 1is a very schematic representation of a drive-train in which the method according to the invention can be used. Thus, the drive-train inFIG. 1has a drive aggregate1and a transmission2, the transmission2being connected between the drive aggregate1and a drive output3of the drive-train.

The transmission2serves to convert rotational speeds and torques so as to deliver the traction force provided by the drive aggregate1to the output3. For the transmission2FIG. 1shows a transmission input speed nGEand a transmission output speed nGA. The transmission2is preferably an automatic or semi-automatic manual transmission.

Between the drive aggregate1and the transmission2, as shown inFIG. 1, is connected a starting clutch4which is preferably a transmission-external starting clutch.

The present invention concerns only such details of a method for operating a drive-train, in which the drive-train is operated in the thrust mode when rolling or coasting. During rolling or coasting, a gear is engaged in the transmission2.

Thrust operation is understood to mean a mode of operation in which, while driving, i.e. in this case while rolling or coasting, an accelerator or gas pedal is not actuated or its actuation is smaller than a specified limit value.

Rolling or coasting in thrust operation with a gear engaged in the transmission2can take place either with or without actuation of a brake pedal.

When rolling or when coasting in thrust operation with a gear engaged in the transmission2, as shown in the block diagram ofFIG. 2a control parameter Y for adjusting the starting clutch4is determined by a regulator5on the basis of a deviation ΔX between a (utilized) target value XSOLLfor, and an actual value XISTof, the position of the starting clutch4. The target value XSOLLfor the position of the starting clutch4is supplied by a target value generating device6.

According to a first aspect of the present invention, the target value XSOLLfor the position of the starting clutch4is determined on the one hand for rolling and on the other hand for coasting, in such manner that with a given value of the transmission input speed nGE, a target value for the position of the starting clutch4during rolling is determined, which is different from that determined during coasting.

Below, this is explained in detail with reference to the diagram shown inFIG. 3, in which the target value XSOLLfor the position of the starting clutch4is plotted as a function of the transmission input speed nGE. A target value XSOLLof 0% corresponds to a fully open starting clutch4. A target value of 100% corresponds to a fully closed starting clutch4.

As shown inFIG. 3, the target value XSOLLfor the position of the starting clutch4, on the one hand during rolling and on the other hand during coasting, is determined on the basis of separate characteristic or performance curves7,8. In the diagram ofFIG. 3, for a specific gear in the transmission2two characteristic curves7and8are plotted, namely a first curve7which applies for rolling in thrust operation and accordingly for a starting clutch4to be closed farther, and a curve8which applies for coasting in thrust operation and thus for a starting clutch4to be opened farther.

FromFIG. 3it follows that with a given transmission input speed nGE, on the basis of the curve7for rolling, a different target value XSOLLwill be obtained than on the basis of the curve8for coasting, this being true for all transmission input speeds nGE, namely between the transmission input speed for which, when rolling and coasting, a fully open starting clutch4(XSOLL=0%) is required, and the transmission input speed for which, when rolling, a fully closed starting clutch4(XSOLL=100%) is required.

As already mentioned, for every possible gear of the transmission2in which rolling and coasting in thrust operation can take place, at least two separate characteristic curves are stored, namely at least one curve for rolling and at least one curve for coasting.

Preferably, for each gear of the transmission there are two characteristic curves for rolling and two for coasting, namely on the one hand curves for rolling and coasting without the actuation of a brake pedal and on the other hand curves for rolling and coasting with actuation of the brake pedal.

Accordingly, it follows from the present invention that when a gear is engaged in the transmission2and the drive-train is operated in the thrust mode during rolling or coasting, the target position XSOLLfor the starting clutch4is determined as a function of the transmission input speed nGE, either from a first characteristic curve which is valid for rolling with increasing speed or increasing transmission input speed of the drive-train and thus a starting clutch4which is to be closed farther, or on the basis of a curve which is valid for coasting with decreasing speed or decreasing transmission input speed of the drive-train and thus a starting clutch4which is to be opened farther.

The characteristic curves are preferably stored in a transmission control unit of the transmission2, and this in the form of support points, so that if a transmission input speed nGFfalls between two support points, an interpolation is carried out between the target values XSOLLstored for the support points.

During rolling in thrust operation the (rolling) target position XSOLLfor the starting clutch4with increasing transmission input speed or increasing speed of the drive-train is determined along the curve7inFIG. 3in such manner that the starting clutch is closed farther. If now, during rolling, the situation arises that as the target value XSOLLfor the starting clutch4is displaced along the curve7, for example in the direction of the working point P3, the drive-train slows down, then the working point moves along an imaginary line from the working point P3in the direction toward a working point P4, so that as the transmission input speed nGEdecreases the target value XSOLLfor the starting clutch4does not at first change. Only when the working point P4has been reached and the transmission input speed nGEdecreases farther, does the target value XSOLLfor the starting clutch4change along the curve8in the direction toward a starting clutch4which is to be opened farther.

In an analogous manner, if during coasting in thrust operation, the (coasting) target value XSOLLfor the starting clutch4first changes along the curve8, for example starting from a fully closed clutch in the direction toward the working point P1and, for example due to a steeply downhill stretch, the transmission input speed nGEincreases again, the working point moves along an imaginary line to the right in the direction toward the working point P2, and then the target value XSOLLfor the position of the starting clutch4does not change at first, but only when the working point P2has actually been reached and the transmission input speed nGEincreases farther. Only then does the target value XSOLLfor the position of the starting clutch4increase again along the curve7.

In contrast, if the transmission input speed nGEfalls again before the working point P2has been reached, then the target value XSOLLfor the position of the starting clutch4does not change until, with the transmission input speed nGEfalling, the speed has fallen below the value associated with the working point P1.

In the above manner, for the determination of the target value XSOLLfor the position of the starting clutch4, on the basis of the transmission input speed nGEa hysteresis is established so that the target value XSOLLfor the position of the starting clutch4does not react to small speed fluctuations. Thereby, even in a drive-train that is prone to oscillations, an oscillating movement of the starting clutch4can be reliably avoided.

The shape of the characteristic curves, in particular the curves7and8inFIG. 3, can be straight or curved. As already mentioned, for every gear in the transmission a plurality of such curves are preferably stored in a transmission control unit.

With regard to the transmission input speed nGEon the basis of which the target value XSOLLfor the position of the starting clutch4is determined, this can be either a transmission input speed determined by measurement or a calculated transmission input speed, the calculated transmission input speed being determined from a measured transmission output speed and the gear ratio of the transmission2.

As already explained, in the regulator5the control parameter Y for the starting clutch4is determined on the basis of the deviation ΔX between the actual value XISTof the position of the starting clutch4and the target value XSOLLfor the position of the starting clutch4.

According to the invention, in the regulator5the control parameter Y for the starting clutch4is determined not solely on the basis of this relative deviation ΔX, but rather, additionally on the basis of the absolute actual value XISTof the position of the starting clutch4. As the control parameter Y for adjusting the starting clutch4, an adjustment speed is preferably used.

When the absolute actual value of the position of the starting clutch4is close to a fully opened starting clutch, with the same relative deviation ΔX for coasting and thus a starting clutch which is to be opened farther a lower adjustment speed is determined than for rolling and thus a starting clutch which is to be closed farther. On the other hand, if the absolute actual value XISTof the position of the starting clutch4is close to a fully closed starting clutch, then with the same deviation ΔX for rolling and thus a starting clutch which is to be closed farther, a lower adjustment speed is determined than for coasting and thus a starting clutch4which is to be opened farther. This too allows the driving comfort to be increased.

In another advantageous further development it can be provided that if the actual value XISTof the position of the starting clutch4cannot be determined by measurement, a change is made from the above-described regulation of the position of the starting clutch4on the basis of the characteristic curves to a time-controlled operation that depends on speed thresholds. In this case two speed thresholds S1and S2are preferably used, of which the speed threshold S1is higher than the speed threshold S2. In the case when the actual value XISTof the position of the starting clutch4cannot be determined by measurement, if the transmission input speed nGEduring rolling in thrust operation exceeds the higher speed threshold S1, then the starting clutch4is preferably brought to the closed condition for a specified time by controlling the valves provided for actuating the starting clutch.

In contrast, if in this case the transmission input speed nGEduring coasting falls below the lower speed threshold S2, then the starting clutch4is also changed to the open condition in a time-controlled manner, namely again by controlling the corresponding valves for a predefined time.

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