DE102011018887A1 - Method for controlling drive train of motor vehicle, involves determining and controlling coupling torque to be transmitted on friction clutch, and determining wheel slip of driven wheels against road corresponding to wheel speed - Google Patents

Abstract

The method involves determining and controlling a coupling torque (MKupp) to be transmitted on the friction clutch (5). A wheel slip of the driven wheels against the road is determined corresponding to the wheel speed (nrad). The coupling torque is adjusted corresponding to a predetermined threshold value of the wheel slip during start-up of the motor vehicle. A wheel slip different at the driven wheels is reduced by an engagement of brake on the fast turning wheel or actuation of a limited slip differential and the coupling torque is adjusted corresponding to the remaining wheel slip.

Description

The invention relates to a method for controlling a drive train of a motor vehicle with an internal combustion engine, a transmission and between these effective, automated friction clutch, a control device for determining and controlling a to be transmitted via the friction clutch torque and a determined based on wheel speeds Radschlupfs of driven wheels the roadway.

Motor vehicles with corresponding drive trains and controls for operating these are known from series applications. Here, the drive train from an internal combustion engine with a crankshaft and a transmission, such as an automatic or manual transmission, dual-clutch transmission, continuously automated adjustable Umschlingungsmittelgetriebe (CVT) and the like is formed, wherein between the crankshaft and the transmission input shaft of the transmission, an automated friction clutch is arranged which is set by a control unit depending on desired or automatically set driving situations, a transferable clutch torque. For example, during an approach of the motor vehicle, an internal combustion engine controlled by the driver's desired torque is transmitted to the driven gear depending on the slip speed of the friction clutch and depending on the gear ratio set therein, a drive torque to the driven wheels. Here, the friction clutch is controlled as quickly as possible in a closed state in which no or a predetermined micro-slip, in which the average engine torque transmitted safely and, for example, torque peaks are reduced by slip, is set.

At maximum friction of the tires of the driven wheels relative to the road while a wheel slip in the range of 10% can be achieved. With correspondingly deteriorated conditions, the wheel slip increases. If the wheel slippage increases, that is, the driven wheels rotate during such approach, for example as a result of too high engine torque with correspondingly reduced adhesion of the wheels on the road surface, the engine torque and the engine speed are reduced in addition to brake interventions on the spinning wheels by means of an engine intervention. The friction clutch is or is closed. The previous systems attempt to regulate this slip with brake and engine interventions. When the friction clutch is closed, it is possible to set a minimum wheel slip above a certain minimum speed, which shows the following relationship: Minimum speed = 90% · Engine speed (approach) [rad / s] · Transmission ratio · Wheel radius [m].

Below this minimum speed, the desired target wheel slip of 10% with closed clutch and previous strategy can not be achieved. The high wheel slip, which occurs below the minimum speed, lowers the maximum transmissible wheel drive force. This makes it impossible to get there in borderline situations, even though it would still be technically feasible. Due to the not optimally adjustable wheel slip at the same time the maximum achievable possible vehicle acceleration is reduced. High wheel slip also reduces the cornering force of the driven wheels. This can cause an avoidable lateral slippage of the driven axle on slippery roads, if the vehicle is not aligned with the slope when approaching. Furthermore, high wheel slip causes unnecessary tire wear. It has also been shown that torque-reducing engine interventions combined with brake interventions with the clutch engaged increase the risk of an engine stalling. Protective measures, such as increasing the engine torque are in contradiction with the known wheel slip control, in which the engine torque is to be reduced. As a result of the changes in the acceleration of the mass of the crankshaft together with the downstream inertia of the flywheel and the like, an engine intervention is imprecise, unresponsive and can be uncomfortable and cause a deteriorated exhaust behavior (ignition interventions). Furthermore, with a low vehicle speed and a closed clutch and a sudden change from a low coefficient of friction of the driven wheels relative to the roadway to a high coefficient of friction, the internal combustion engine or heavy impacts in the drive train may stall.

The object of the invention is therefore to propose a method for controlling a drive train in which the wheel slip is limited to a small extent during startup and at a low wheel slip of the driven wheels, the tires can be operated against the road surface with the highest coefficient of friction.

The object is achieved by a method for controlling a drive train of a motor vehicle with an internal combustion engine, a transmission and an effective between them, automated friction clutch, a control device for determining and controlling a to be transmitted via the friction clutch torque and a determined based on wheel speeds Radschlupfs of driven wheels solved in relation to the road, wherein during a journey of the Motor vehicle clutch torque is adjusted depending on a predetermined limit of Radschlupfs.

According to an advantageous embodiment, it may be provided to regulate at minimum speeds above a predetermined limit by means of an engine intervention and / or a braking intervention on one or more wheels with a wheel slip above a predetermined limit value to a predetermined wheel slip. If increased wheel slip occurs differently on the driven wheels, they can first be regulated by means of a braking intervention or, if appropriate, by means of an existing limited slip differential, by redistributing the drive torque to the driven wheels.

If the predetermined minimum speed is reached or redistribution of the wheel drive torques is not successful in order to limit the wheel slip, the friction clutch can be regulated to a smaller clutch torque depending on the wheel slip. Here, the wheel slip is monitored from the start of a journey or when reaching or falling below the minimum speed, wherein the control of the friction clutch controls the clutch torque so that the average wheel slip, that is, an average of the wheel slip of the driven wheels with respect to a traction with low wheel slip , achieved an optimal range.

This may be increased by an optionally reduced clutch torque in slipping friction clutch, the speed of the engine, so that it may be advantageous after the dynamically fast clutch engagement with a corresponding time delay, for example by actuation of the throttle, an engine intervention of the acceleration due to the acceleration of the masses follow later reacting internal combustion engine. A correspondingly predetermined target rotational speed of the internal combustion engine can be adjusted depending on the transmittable clutch torque corrected via the friction clutch during the clutch engagement to reduce wheel slip. It has proven to be advantageous in this case if, on the one hand, a rotational speed of the crankshaft is adapted to the rotational speed of the transmission input shaft, in order to avoid a high heat input into the friction clutch at high slip, on the other hand a sufficiently high rotational speed of the crankshaft is maintained to ensure that the internal combustion engine in the corrected speed range has sufficient torque reserve to compensate for an increase in the clutch torque can. Here, in particular, the increase in the required engine torque is taken into account, as a result of the proposed method, the coefficient of friction between the tire and the road increases with decreasing wheel slip, so transfer more torque on the tires on the road and thus the vehicle can be accelerated faster given a road condition.

It has been shown here that by means of the slip set in the friction clutch multiplied by the moment of inertia of the crankshaft and the flywheel mounted thereon, a kinetic energy storage can be represented, by means of which the clutch torque can be dynamically adjusted, if necessary, strongly increasing, so that torque requirements sinking wheel slip and thus increasing coefficient of friction reacts quickly and the limited dynamic response of this energy storage can be compensated by the rapid decoupling means of the friction clutch. This can be dispensed with hard engine interventions, such as ignition interventions or switching off the fuel supply ("fuel cut"), which can have a positive effect on the exhaust behavior and ride comfort.

By means of the proposed method, the wheel slippage can be regulated to an optimum range, so that in borderline cases the motor vehicle can still be started, in which exclusively motor engagement and / or braking intervention results in excessive wheel slip and the resulting low coefficient of friction of the tires relative to the roadway would fail. Furthermore, an adjustable clutch slip ensures that braking interventions of traction control devices (TCS) or spontaneously changing roadway properties with a resulting lower coefficient of friction do not pose a risk to the engine stalling. The possible setting of a low wheel slip can be used to reduce or avoid impacts in the drive train if the coefficient of friction changes rapidly, so that increased ride comfort is also achieved here.

Due to the higher and more precise feasibility of clutch engagements with respect to engine interventions, the cornering force of the driven wheels increases when approaching. This can prevent or reduce an avoidable, lateral slippage of the driven axle on a slippery road surface, if the vehicle is not aligned with the slope when approaching. Furthermore, the tires are spared due to the reduced low wheel slip especially when driving at high load on the ground with a relatively high coefficient of friction.

Alternatively, exclusive wheel slip limitation may be performed with clutch torque intervention, rather than in conjunction with immediate, less dynamic engine interventions, as far as they are effective, for example, above a minimum speed of the motor vehicle. Here, at the slightest indication of wheel slip during acceleration of the motor vehicle, the active clutch control can be corrected accordingly, while the acceleration of the engine can be unchanged and, for example, depending on the driver's desired torque or other influencing parameters.

According to an advantageous embodiment for carrying out the method, a pre-control torque of the friction clutch is estimated as a function of the wheel slip determined from a comparison of the wheel speeds of the driven and wheel speeds of non-driven wheels. Here, the pilot torque is corrected by means of a Radschlupfreglers with resulting from a comparison of the Sollraddrehzahl and the currently determined wheel speed of the driven wheels cascade structure.

The invention is based on the in the 1 to 3 illustrated embodiments explained in more detail. Showing:

1 a block diagram of a controlled system for wheel slip dependent control of a friction clutch,

2 a characteristic curve for applying a target acceleration of a driven wheel depending on a differential speed of the driven and non-driven wheels in the Regestrecke the 1
and

3 a diagram illustrating the wheel slip control.

1 shows a schematic diagram of the controlled system 1 for controlling the clutch torque MKupp depending on the wheel slip of the driven wheels. This is basically using the feedforward control and the regulator 3 set a desired wheel speed at the driven wheels, at which sets a predetermined wheel slip. For this purpose, the setpoint wheel speed Snrad is first determined for those driven wheels which are to be brought into slip operation. Apart from the drive form, the rear or front wheels may be the driven wheels and complementarily the other wheels may not be driven. From this, the setpoint wheel speed lets you down Snrad = nrad, nd + nOffset (1) nrad, nd the speed of the non-driven wheels and nOffset are a fixed-value offset speed, for example 30 rpm or dependent on an operating point-relevant quantity, for example the wheel speed. From the difference of the Sollraddrehzahl Snrad and the currently measured wheel speed nrad of the driven wheels of the existing slip value is calculated. This is both from the feedforward 2 as well as the regulator 3 used.

The feedforward control 2 uses the Sollraddrehzahl Snrad to after estimating the deductible via the wheels torque M (A) in block 4 by means of a mathematical relationship and further information such as the coefficient of friction RW of the road, the vehicle weight or load state BZ and the curve RW (RS) of the coefficient of friction on the slip in the feedforward control 2 to calculate the pre-control torque M (V), which is sufficient to bring the wheels to spin.

It can be estimated with the following equations (2), (3) the transferable clutch torque MKupp: mFzg · g · cosβ · k · μ <= Mabtrteb / rdyn (2) Mabtrieb = Mkuppigeige eta (3) with the mass mFzg of the motor vehicle, the total translation iges between friction clutch and wheel, the efficiency eta, the dynamic wheel radius rdyn, the proportion k of the respective wheel to the normal force of the motor vehicle, the slip value μ and the slope β of the road. The equations (2) and (3) can be resolved after combination according to the coupling moment MKupp: Mkupp = mFzg · g · cosβ · k · μ · rdyn / (iges · eta) (4)

The mass mFzg and the gradient β can be estimated here, for example, by means of a drive train observer.

The pre-control torque M (V) determined in this way represents a limit torque and would only lead to spinning of the wheels. However, in the above equation (4), the parameters used may be scattered or inaccurate, so that the actual speed curve of the wheels by means of the pilot control torque M (V) of the precontrol 2 sufficiently accurate and determined by the controller 3 is made concrete.

For this purpose, the controller 3 used in cascade, wherein from the Sollraddrehzahl Snrad and the currently measured wheel speed nrad of the driven wheels, the difference nrad_diff is formed: nrad_diff = Snrad - nrad. (5)

The desired acceleration dSnrad of the driven wheels is determined from the difference nrad_diff. For example, mathematical methods such as multiplication are as follows: dSnrad = k · (Snrad - nrad) (6) with the constant k, which is advantageously between 0.5 and 3.

Alternatively, the target acceleration dSnrad can be determined using an interpolation method with the in 2 shown characteristic 20 be determined, which has as input the difference Snrad - nrad. As a result, only small target accelerations dSnrad can advantageously be determined for small deviations. Furthermore, at close to the target wheel speed Snrad values only small changes can be allowed, whereby a damped behavior of the drive train can be achieved. In contrast, by the characteristic 20 disproportionately high setpoint accelerations dSnrad are generated in the case of large deviations, which ultimately ensure that the setpoint wheel speed Snrad is reached quickly.

That from the pilot control 2 and the controller 3 formed clutch torque MKupp serves. the control of the friction clutch 5 that the clutch torque MKupp on the gearbox 6 transmits with the following components such as differential, propeller shafts and the like with the efficiency eta on the wheels.

In the next step, after filtering the wheel speeds, nrad the driven wheels in the filter unit 7 in block 8th the current accelerations dnrad the wheel speeds nrad determined and with the in block 9 determined target accelerations dSnrad compared. The accelerations dnrad determined from the current wheel speeds nrad can be determined, for example, by a power-down method. Alternatively or additionally, in the filter unit 7 adapted Kaimanfilter be provided, which at the same time provide the derivation of the wheel speeds nrad, so that block 8th can be omitted. The difference of the accelerations dnrad and desired accelerations dSnrad become the controller 3 supplied, which may be embodied for example as a PI controller. Alternatively, other controller concepts such as PID controller or the like may be provided. The regulator 3 determines the additive control torque M (R), which is added to the pre-control torque M (V).

The controlled system 1 may be supplemented by a method for controlling the internal combustion engine, in which this is brought to a predetermined operating point, so that they can deliver the respective torque requirements. Such an operating point of the internal combustion engine may be, for example, two thirds of the maximum speed or a maximum engine torque. In order to inform the internal combustion engine of the increased load due to the wheel slip control for keeping a corresponding torque reserve available, a signal which has a leading character is particularly suitable. As favorable, an anticipatory pre-control torque Mvorsteuer determined from the feedforward control and the controller has proved to be advantageous, with the pre-controlled engine torque Mmot, res according to equation (7) too Mmot, res = Mvorsteuer (μ> 0) (7) results, where μ is determined so that the wheel is in the slip mode. This could prevent a collapse of the engine speed.

The wheel speeds to carry out on the controlled system 1 based method can be detected by means of wheel speed sensors. Alternatively or additionally, the control of the controlled system 1 based on converted to transmission input speeds wheel speeds. Here, advantageously only translations and no stiffness or damping of the transmission are taken into account. By using transmission input speeds, the clutch slip can be monitored even better and more directly. For example, this may be necessary to take into account the synchronization of the internal combustion engine and transmission input speed in the engine precontrol due to the inertia of the crankshaft with corresponding masses of the flywheel suitable.

3 shows in the diagram 10 Based on a plurality of partial diagrams recorded synchronously over time, the course of a clutch torque of a friction clutch regulated as a function of slip occurring at the wheels during a full load approach. In detail, show the curve 11 the signal line of the accelerator pedal operated at the full stop at time t1, the curve drawn by the solid line 12 the speed of a driven wheel, the curve shown in dashed line 13 the speed of the internal combustion engine, the curve shown in dashed line 14 the speed of the motor vehicle, the curve shown in dashed line 15 the slip of the friction clutch, the curve shown in a solid line 16 the wheel slip, the curve shown in dashed line 17 the engine torque of the internal combustion engine, the curve shown in a solid line 18 the transmitted via the friction clutch clutch torque and the curve shown in dotted line 19 the wheel torque applied to a driven wheel.

At the time t1, the driver fully depresses the gas pedal and thus signals the driver's intention of a speedy approach. The motor vehicle is still at a standstill and the engine and clutch torque are built.

At time t2, the wheels begin to spin. Increased wheel slip is detected, and the clutch torque is reduced by slipping it while the engine torque is still applied in order to limit wheel slip. From now on is driven with ideal wheel slip. This makes it possible to transmit the maximum possible wheel torque.

At time t3, the speed of the internal combustion engine has reached the predetermined target speed, at which sufficient engine torque can be retrieved. The internal combustion engine is controlled to maintain this speed until clutch slip is reduced or the driver releases the accelerator pedal again.

At time t4, the clutch torque is controlled such that the wheel slip is limited to a predetermined ideal value which can be predetermined as a function of operating parameters. The wave shown at time t6 symbolizes the permanent control of the wheel slip by means of the clutch torque and the control of the speed of the internal combustion engine by means of the engine torque.

At time t5, the clutch slip is almost eliminated. The friction clutch is not completely closed but remains in the slightly slipping state. As a result, the friction clutch retains influence on the wheel torque, this influence is needed to continue to limit the wheel slip.

At time t6 increased wheel slip occurs due to the breakdown of the coefficient of friction between the tire and the road surface. Immediately the clutch torque is lowered to reduce the wheel slip again. The control of the internal combustion engine ensures during wheel slip engagement of the friction clutch with the engine torque reducing engine interventions that the clutch slip is not too large.

At time t7, the coefficient of friction between the tire and the roadway suddenly increases because, for example, the wheel comes on a grippier surface. Here, the wheel adheres and clutch slip builds up. There is only a slight jerk, since only the wheel and the transmission, but not the mass moment of inertia of the internal combustion engine are delayed. At time t8, the friction clutch increases the clutch torque until the wheel is again at the slip limit.

LIST OF REFERENCE NUMBERS

1

controlled system

2

feedforward

3

regulator

4

block

5

friction clutch

6

transmission

7

filter unit

8th

block

9

block

10

diagram

11

Curve

12

Curve

13

Curve

14

Curve

15

Curve

16

Curve

17

Curve

18

Curve

19

Curve

20

curve

BZ

loading condition

dnrad

Acceleration wheel

dSnrad

Target acceleration wheel

M (A)

deductible moment

MKupp

clutch torque

M (R)

controller torque

M (V)

pre-control torque

nrad

Speed wheel

RW

friction

RW (RS)

Course friction coefficient over wheel slip

Snrad

target wheel speed

Claims (8)

Method for controlling a drive train of a motor vehicle with an internal combustion engine, a transmission ( 6 ) and between this effective, automated friction clutch ( 5 ), a control unit for determining and regulating a via the friction clutch ( 5 ) to be transmitted clutch torque (MKupp) and one of wheel speeds (nrad) Radschlupfs determined by driven wheels relative to the roadway, characterized in that during a journey of the motor vehicle, the clutch torque (MKupp) is set depending on a predetermined limit of Radschlupfs. A method according to claim 1, characterized in that on the driven wheels different wheel slip first by means of a braking intervention on the faster rotating wheel or actuation of a limited slip differential and then the clutch torque (MKupp) is set depending on the remaining wheel slip. Method according to claim 1 or 2, characterized in that the limit value consists of a mean wheel slip of the driven wheels is determined. Method according to one of claims 1 to 3, characterized in that at a dependent of the wheel slip setting the clutch torque (MKupp) a speed of the internal combustion engine is limited. Method according to one of claims 1 to 4, characterized in that at a dependent of the wheel slip setting the clutch torque (MKupp) a speed of the internal combustion engine is maintained at a value above a predetermined limit. Method according to one of claims 1 to 5, characterized in that by means of a pilot control ( 2 ) of the clutch torque (MKupp) and subsequent control of the limit of the wheel slip dependent setpoint speed (Snrad) is set. Method according to Claim 6, characterized in that a pilot control torque (M (V)) of the friction clutch ( 5 ) is estimated as a function of the wheel slip determined from a comparison of the wheel speeds (nrad) of the driven and wheel speeds (nrad, nd) of non-driven wheels. A method according to claim 7, characterized in that the pre-control torque (M (V)) by means of a controller ( 3 ) is corrected with a cascade structure resulting from a comparison of the desired wheel speed (Snrad) and the currently determined wheel speed (nrad) of the driven wheels.

Images