GB2384038A

GB2384038A - Method for maintaining clutch slip in line with an ideal value

Info

Publication number: GB2384038A
Application number: GB0200661A
Authority: GB
Inventors: Bernhard Boll
Original assignee: LuK Lamellen und Kupplungsbau Beteiligungs KG; LuK Lamellen und Kupplungsbau GmbH
Current assignee: Schaeffler Buehl Verwaltungs GmbH; LuK Lamellen und Kupplungsbau GmbH
Priority date: 2002-01-12
Filing date: 2002-01-12
Publication date: 2003-07-16
Also published as: DE10297708D2; WO2003058085A1; AU2002363835A1; GB0200661D0; EP1468201B1; EP1468201A1; ATE342452T1; DE50208445D1; DE10258312A1; ITMI20030032A1; FR2834761A1

Abstract

A CVT transmission has two pulleys 16,18 with V section grooves 22, which can vary the radius at which a belt 20 engages the grooves 22, to vary the drive ratio. Sensors 30,32,34 may provide details about engine, wheel or pulley speeds. A clutch 28 is automatically controlled to transmit a torque Mk directed from an engine 10, which is a predetermined amount in excess of the engine torque Mm. If engine torque Mm increases rapidly, clutch 28 will slip whilst the torque Mk transmitted by the clutch 28 catches up with Mm, thereby causing a damping effect of shocks in the drive line. The torque Mk is controlled by applying an appropriate thrust to the clutch plates, and will also depend on, eg, the frictional coefficient. A control system is used to compare values gained by the sensors 30,32,34 with ideal values, and the applied thrust is varied accordingly.

Description

CLUTCH CONTROL SYSTEMS The present invention relates to clutch control systems and in particular to a clutch control system for a continuously variable transmission system or for an automated, manual, semi-automatic or automatic transmission system.

With continuously variable transmission systems, a friction clutch is provided between the vehicle engine and the transmission system in order to permit engagement of the continuously variable transmission from rest and disengagement of the continuously variable transmission on coming to rest. Similarly, in automated transmission systems, friction clutches are located between the vehicle engine and a multi-ratio gearbox in order to permit start-up from rest and shifting from one gear ratio to another.

With continuously variable transmission systems or automated transmission systems of this type, the clutch is preferably also controlled so that the torque transmitted thereby is a predetermined amount above the engine torque. In this manner, abrupt increases in engine torque may be damped out by allowing the clutch to slip, thereby avoiding shocks in the driveline.

In order to permit smooth take-up and gear changes and to control the clutch so that the torque transmitted thereby is a predetermined amount above the engine torque, it is necessary to monitor the frictional coefficient of the clutch, so that adjustments may be made to allow for variations therein, due to the operating conditions and wear of the clutch.

The frictional coefficient of the clutch may be determined by. monitoring the amount of slip of the clutch as torque is applied thereto at take-up from rest. Furthermore, with automated manual, semi-automatic and fully automatic transmissions, adaptations for frictional coefficient may also be made upon re-engagement of the clutch after a gear shift, when clutch slip will also occur. However, for continuously variable

transmission systems as the clutch is not disengaged in order to change gear ratio, it will not be possible to adapt the frictional coefficient under such circumstances.

In accordance with the present invention, a method of controlling a clutch which transmits torque between an engine and a transmission system of a motor vehicle, comprises engaging the clutch such that the torque transmitted thereby is a predetermined amount above engine torque and monitoring the amount of slip of the clutch at tip-in when the torque transmitted by the engine is increased rapidly, the amount of slip occurring at tip-in being compared with a model figure representing the ideal slip for the absolute torque, the torque gradient and engine speed and correcting the frictional coefficient of the clutch to equate the ideal slip with the actual slip.

The method of the present invention is particularly suitable for adaptation of the frictional coefficient of clutches used with continuously variable transmission systems, thereby allowing adaptation of the frictional coefficient whenever tip-in occurs and not just at take-up. The method may however also be used with automated manual, semi-automatic or fully automatic transmission systems to allow adaptation of friction coefficient during tip-in as well as at take-up and during gear changes.

The invention is now described, by way of example only, with reference to the accompanying drawings in which :- Figure 1 illustrates diagrammatically a continuously variable transmission system in accordance with the present invention; and Figure 2 is a typical plot of engine and clutch torque for the transmission system illustrated in Fig. 1.

As illustrated in Fig. 1, a typical continuously variable transmission system which connects a vehicle engine 10 to a final drive 12, comprises, a belt drive 14, having a pair or pulleys 16, 18 with a belt 20 running therebetween. The pulleys 16 and 18 have a V-section groove 22 defined between two conical plates 24, the plates 24 being movable axially of one another, so that the radius at which the belt 20 engages the grooves 22 may be varied, to vary the drive ratio. The width of the plates 24 of the pulleys 16 and 18 are adjusted so that as the radius at which belt 20 engages one of the pulleys 16,18 decrease, the radius at which the belt 20 engages the other pulley 18, 16 increases, thereby maintaining the belt 20 under tension.

One pulley 16 of the belt drive 14 is connected to the engine by means of gear train 26 and friction clutch 28, the other pulley 18 being connected to the final drive 18. Speed sensors 30,32 and 34 are provided to measure the speed of the engine 10, pulley 16 and pulley 18 respectively. Alternatively, sensors 32 and 34 may be replaced by a wheel speed sensor, for example as used for a vehicle antilock braking system, the speed of the drive plate from the clutch 28 being calculated from the wheel speed, knowing the drive ratio of the belt drive 14 and gear train 26.

With the transmission system disclosed above, the clutch 28 is normally disengaged when the vehicle is stationary with the engine 10 running. At start-up from rest the clutch 28 is controlled automatically, to take up drive from the engine 10 in controlled manner. Conversely, when coming to rest, clutch 28 will be disengaged.

While the vehicle is in motion the clutch 28 is controlled so that it is capable of transmitting a torque Mk which is a predetermined increment

M in excess of the engine torque Mm, as illustrated in Fig. 2. In this manner, if engine torque M m increases rapidly, clutch 28 will be allowed to slip, while the torque Mk transmitted by the clutch 28 catches up with the engine torque Mm, as illustrated in Fig. 2. This will dampen out shocks in the drive line upon rapid increase in engine torque M m.

The torque Mk transmitted by the clutch 28 is controlled by controlling the position of a clutch actuating means, so that an appropriate thrust is applied to the plates of the clutch. The appropriate position for the clutch actuating mechanism will depend upon the clutch characteristics and will be a function of engine torque, engine speed and the frictional coefficient of the clutch. Knowing the engine torque, engine speed and frictional coefficient, the appropriate clutch position may consequently be calculated.

However, during operation of the clutch, the frictional coefficient may vary due to wear of the clutch plates and the operating conditions, for example, temperature of the clutch. It is consequently necessary to make corrections to the position of the clutch actuating mechanism, in order to take account of variations in the frictional coefficient.

As illustrated in Fig. 2, if the actual frictional coefficient is lower than the model figure, the amount of slip that occurs when the engine torque Mm increases rapidly, will be greater than expected and vice versa, if the torque coefficient is higher the amount of slip that occurs will be diminished. It is consequently possible to make corrections for variations in frictional coefficient when the clutch 28 slips, by comparing the actual slip against an ideal slip based on the absolute torque, torque gradient and engine speed. With automated transmission systems having a multiratio gearbox, this may also be done when starting from rest or while the vehicle is in motion during gear changes. However, with a continuously

variable transmission system there is of course no opportunity to make such corrections on gear changes.

In accordance with the present invention, corrections for the frictional coefficient may also be made during tip-in, where the clutch is permitted to slip when engine torque increases rapidly. While this is particularly suited for continuously variable transmission systems, it also applies to automated transmission systems having a multi-ratio gearbox.

As illustrated in Fig. 2, when a vehicle is travelling along at a substantially constant speed the clutch is engaged to a sufficient extent such that the clutch torque Mk is in excess of the engine torque Mm by a predetermined amount os. If the vehicle throttle is now open at time to to rapidly increase the engine torque Mm, there is a delay before action can be taken to adjust the clutch to increase clutch torque Mk. The clutch torque Mk will consequently lag behind the engine torque Mm. The excess engine torque Mm-Mk will cause the engine to accelerate, hence the engine speed will increase above a clutch driven plate speed and the clutch plates will slip, as illustrated by the cross-hatched area of Fig. 2.

When the clutch is slipping ;

where Jm is the engine's moment of inertia; Mm is the rate of acceleration of the engine; Mm is the engine torque; and Mk is the clutch torque.

The clutch torque Mkfor a given clutch is a function of the position of a clutch actuating mechanism and the frictional coefficient of the clutch

plates. Consequently, it is possible, assuming a nominal frictional coefficient, to construct a dynamic model, from which the ideal amount of slip may be calculated from the absolute torque, torque gradient and engine speed. This calculated ideal slip may be compared with the actual slip which may be measured using the speed of the engine Mm and subtracting the speed Mkof the clutch output shaft, as measured by sensors 30 and 32.

If the actual # slip SAct differs from the ideal slip SMod, then the frictional coefficient may be adjusted accordingly.

Considering Fig. 2; At time to wu = (Ok Assuming that the clutch torque MK remains constant, the speed of the engine MM, at time t1 will have increased by :-

Therefore the new engine speed = ) m + 8t j Jm (Mm - MK) Um The clutch driven speed = (dom Therefore slip = 8tj Jm (Mm - MK) m Therefore the difference in slip between the model and actual

Where MKMod = model clutch torque MKAct = actual clutch torque

For a given position of the clutch MK is however a function of the frictional coefficient .

Therefore bS = (VAct-IlMod) P-Act-iMod = 8S

Therefore the frictional coefficient may be adapted depending on the difference bS between ideal slip and actual slip.

The assumption above that the clutch torque remains constant will however only hold during the delay in actuation of the clutch following an increase in engine torque. In order to take account of the change in clutch torque it is necessary to integrate the amount of slip over a period.

For example, from to to tz, where the engine and clutch output shaft come back into synchronisation. A shorter period, while the clutch is slipping, may however be used.

In order to avoid transient affects, correction of the frictional coefficient is only made if the actual slip differs from the ideal slip by more than a predetermined value.

Corrections to the frictional coefficient may be made on the basis of an appropriate algorithm or by means of look-up tables, held in the software of the control system.

Alternatively, if the difference bS between the actual slip and ideal slip is above the threshold value, then the friction coefficient may be increased by an increment, for example 1 %, if the actual slip is lower than the ideal slip or decreased by an increment, for example 1 %, if the actual slip is higher than the ideal slip. Corrections to the frictional coefficient may be made in this manner until the difference in slip speed 8S is below the threshold value.

The patent claims submitted with the application are proposed formulations without prejudice to the achievement of further patent protection. The applicant reserves the right to submit claims for further combinations of characteristics, previously only disclosed in the description and/or drawings.

References back used in sub-claims refer to the further development of the subject of the main claim by the characteristics of the respective subclaim ; they are not to be understood as a waiver with regard to achieving independent item protection for the combination of characteristics in the related sub-claims.

Since the subject of the sub-claims can form separate and independent inventions with reference to the prior art on the priority date, the applicant reserves the right to make them the subject of independent claims or of division declarations. Furthermore, they may also contain independent inventions which demonstrate a design which is independent of one of the objects of the preceding sub-claims.

The embodiments are not to be considered a restriction of the invention.

Rather, a wide range of amendments and modifications is possible within the scope of the current disclosure, especially those variations, elements and combinations and/or materials which, for example, the expert can learn by combining individual ones together with those in the general

description and embodiments in addition to characteristics and/or elements or process stages described in the claims and contained in the drawings with the aim of solving a task thus leading to a new object or new process stages or sequences of process stages via combinable characteristics, even where they concern manufacturing, testing and work processes.

Claims

CLAIMS 1. A method of controlling a clutch which transmits torque between an engine and a transmission system of a motor vehicle, comprises engaging the clutch such that the torque transmitted thereby is a predetermined amount above engine torque and monitoring the amount of slip of the clutch at tip-in when the torque transmitted by the engine is increased rapidly, the amount of slip occurring at tip-in being compared with a model figure representing the ideal slip for the absolute torque, the torque gradient and engine speed and correcting the frictional coefficient of the clutch to equate the ideal slip with the actual slip.
2. A method according to claim 1 in which the frictional coefficient is corrected as a function of the difference between the model slip and the actual slip using an appropriate algorithm.
3. A method according to claim 1 in which the frictional coefficient is corrected by means of look up tables stored in software programmed into a clutch control system for the clutch.
4. A method according to claim 1 in which the frictional coefficient is corrected incrementally.
5. A method according to any one of the preceding claims in which the frictional coefficient is corrected only if the difference between the model slip and the actual slip is above a predetermined threshold value.
6. A method according to any one of the preceding claims in which the actual slip is measured by means of speed sensors, the speed sensors measuring the engine speed and the clutch driven plate speed.

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7. A method according to claim 6 in which the speed of the clutch driven plate is calculated from a measure of the speed of a driven wheel of the vehicle.
8. A method of controlling a clutch which transmits torque between an engine and a transmission system of a motor vehicle, substantially as described herein, with reference to and as shown in figures 1 and 2 of the accompanying drawings.