GB2338277A

GB2338277A - Clutch control system.

Info

Publication number: GB2338277A
Application number: GB9902802A
Authority: GB
Inventors: Lakbir Atwal; Glyn David Brian Whitehurst; Diana Katharine Bowker; Charles John Jones
Original assignee: Kongsberg Techmatic UK Ltd; LuK Leamington Ltd
Current assignee: LuK Leamington Ltd
Priority date: 1998-02-19
Filing date: 1999-02-09
Publication date: 1999-12-15
Anticipated expiration: 2019-02-09
Also published as: GB9902802D0; GB2338277A8; GB2338277B; DE19905991A1; FR2774955B1; GB9803436D0; FR2774955A1; BR9900751A

Abstract

A friction clutch control system for a motor vehicle includes an engine speed sensor 24, a vehicle/gearbox input shaft speed sensor 29, a gear ratio selection sensor 23 and parking and foot brake release sensors 26, 27. The control system causes the clutch 11 to partially engage upon release of the brakes when, prior to release of the brakes, the vehicle is substantially at rest with the brakes applied, a gear ratio is selected and the engine speed is below a predetermined value. The system provides 'creep' to assist while starting on a gradient. The clutch is disengaged to reduce wear after 0.5 to 4 seconds from cessation of braking force if torque does not rise above the creep value. The clutch may also disengage after cessation of braking if the accelerator is not depressed and the engine speed falls below a threshold speed.

Description

CLUTCH CONTROL SYSTEMS 2338277 The present invention relates to clutch

control systems and in particular clutch control systems for automatically controlling engagement and disengagement of a friction clutch located in the driveline between a vehicle engine and a gearbox.

With conventional manual transmission systems, the starting of a vehicle presents a coordination problem to the driver, requiring the progressive release of the parking brake at the same time as the engine speed is increased and the clutch is engaged to deliver an increasing drive torque to the transmission. Starting a vehicle of this type on a gradient and preventing the vehicle from rolling down the gradient, commands considerable skill from the driver and for the driver to be familiar with the characteristics of the engine, clutch and parking brake of the vehicle and to take account of the gradient and vehicle loading.

For vehicles with automatic transmissions which use a torque converter interposed between the engine and the automatic gearbox, there is no clutch pedal so the demands on the driver when starting the vehicle on a gradient are less severe. The characteristics of the torque converter are such that at engine idling speed with the vehicle at rest, a small torque is transmitted from the engine to the gearbox when a gear ratio suitable for starting from rest is engaged. This torque, which is ampiif ied by the transmission and delivered to the driving wheels, is sufficient to overcome the static rolling friction of the vehicle. The consequence of this is that when the parking brake is released, the vehicle will be set in motion in the direction determined by the transmission control lever, if the vehicle is on the flat or a slight gradient. This behaviour is referred to as "creep". To set the vehicle in motion on a more severe gradient without rolling back it is sufficient for the driver to accelerate the engine and release the parking brake. Provided that the engine has been accelerated to deliver sufficient torque to overcome the static friction and the gravitational forces, before the brake is released, the vehicle will move off without rolling back. The provision of creep makes the control of starting from rest on the flat or a slight gradient relatively simple for the driver. The vehicle is set in motion by setting the transmission control lever and releasing the brake pedal. The subsequent acceleration is controlled by the throttle pedal.

With transmission systems of the type covered by the present application, for example as disclosed in European Patent Nos. 0038113; 0043660; 0059035 and 0101220, the clutch control system may, for example, include an electronic control unit which controls the initial clutch take-up on starting the vehicle, clutch re-engagement following each ratio change, clutch disengagement to change the operative ratio of the gearbox and clutch disengagement on coming to rest of the vehicle, the level of clutch engagement during take-up or starting of the vehicle being controlled in response to an engine speed error signal derived from a comparison of current engine speed and a reference speed signal generated by the control unit. Transmission systems of this type do not usually provide a creep facility. Whilst a torque convertor is able to deliver a small torque 20 to the vehicle transmission indefinitely while the vehicle remains at rest, an automated friction clutch as used in transmission systems of the type to which the present application applies, is unable to operate similarly without accumulating unacceptable wear.

With vehicles having transmission systems of this type, starting from rest on a gradient, using the brake pedal and optionally the parking brake, the braking action ceases once the driver's foot has been transferred from the brake pedal to the accelerator pedal and the parking brake has been released. There will be a delay whilst the driver's foot is transferred from the brake pedal to accelerator pedal and a further delay after the accelerator pedal is pressed before the engine speed increases and in response to this increasing speed the clutch engages to deliver the torque to the transmission to set the vehicle in motion. During this delay period there will be a tendency for the vehicle to roll down the gradient, unless the driver controls the vehicle motion with the parking brake. There is a reluctance by drivers to use the parking brake under these circumstances because its use involves considerable skill and there is a tendency for the driver to allow the vehicle to roll down the gradient or to race the engine. It would consequently be advantageous in transmission systems of this type, to deliver a level of torque equivalent to the creep torque normally delivered by a torque convertor of an automatic transmission system, when braking torque ceases to be available. Once the clutch has been moved to a position where a small torque is transmitted, only a small further movement of the clutch is required to increase the torque to a level where the vehicle can be set in motion on a gradient.

According to one aspect of the present invention, a clutch control system for a motor vehicle, having an engine, a gearbox having a plurality of drive ratios and a gear selection means, the engine being drivingly connected to the gearbox via a friction clutch; the clutch control system automatically controlling engagement and disengagement of the clutch in synchronisation with actuation of the gear selection means, to alter the gearbox drive ratios, the clutch control system including an engine speed sensor, a vehicle speed sensor, a gear ratio.selection sensor, a brake release sensor to sense the release of a brake of the vehicle and means to partially engage the clutch upon release of the brakes when, prior to release of the brakes, the vehicle is substantially at rest with the brakes applied, a gear ratio is selected, and the engine speed is below a predetermined value.

In accordance with the present invention, upon selection of a gear ratio 11 and release of the brake, the clutch will partially engage and deliver a torque to the driving wheels of the vehicle, which is sufficient to overcome the static rolling friction of the vehicle. This will tend to hold the vehicle on the gradient until the engine is accelerated to increase the engine speed and clutch engagement to a degree where sufficient torque is delivered to the driving wheels of the vehicle, so that the vehicle will move away. With such an arrangement, the ability of the driver to set the vehicle in motion on a gradient is greatly improved by virtue of the creep facility which anticipates the initial action of the take-up control in raising the clutch torque up to the level of the creep torque.

To protect the clutch from excessive wear, the clutch control system preferably returns the clutch to a position where no torque is transmitted if the level of torque determined by the take-up control has not exceeded the creep torque within a specified time, for example four seconds after the braking action ceases.

An embodiment of the invention is now described, by way of example only, with reference to the accompanying drawings which shows, in diagrammatic form, the general layout of clutch control system in accordance with the present invention.

As illustrated in the drawing, an engine 10 is coupled via a friction clutch 11 with a gearbox 12, through a gearbox input shaft 13. In the example described, fuel is supplied to the engine 10 by an electronic or mechanical fuel injection system which may incorporate a throttle valve 15.

The clutch 11 is actuated by a release fork 16 which is operated by a slave cylinder 17. The gearbox 12 is controlled by a gear ratio selector lever 18 which is connected with the gearbox 12 via a selector linkage 19 and which includes a load sensing switch means 20 to detect forces applied to the lever 18 by the driver and produce a signal indicating an intention to change gear. Alternatively, the gearbox 12 may be controlled by an automatic gear selection mechanism.

An electronic control unit 21 controls the actuation of the clutch 11 via an hydraulic control 22 which controls the operation of slave cylinder 17. The electronic control unit 21 receives signals from the gear lever 18, a gear position sensor 23, an engine speed sensor 24, an accelerator position sensor 25, a brake pedal sensor 26, a parking brake sensor 27, a slave cylinder position sensor 28, and a gearbox input shaft speed sensor 29 which gives the speed of the driven plate of the clutch 11. Since t_he speed of the vehicle depends upon the driven plate speed and the gear engaged, the driven plate speed may alternatively be derived from the vehicle speed and the gear engaged.

A buzzer 30 is connected to the control unit 21 to indicate to the driver when certain vehicle operating conditions occur. In addition to or in place of the buzzer a warning light may be used.

As described, for example, in European Patent Specification No.

0038113, the electronic control unit 21 generates a position demand signal for the hydraulic control 22. The electronic control unit 21 also generates and reference signal representative of the desired engine speed during the take-up from rest. This reference signal is compared with the actual engine speed to produce an error signal which determines the position demand for the hydraulic control 22. As the engine speed is raised from below the desired speed to above the desired speed, the clutch position demand progressively moves from disengage to engage.

Part of the increase in engine speed will result in free travel in the clutch 11 being taken up before torque transmission begins. This allows the clutch 11 to be operated with sufficient clearances to eliminate clutch drag, when the vehicle is at rest and the brakes applied.

The clutch control system of the present invention will function in the manner disclosed in European Patent Specification 0038113 to control takeup and release of the clutch 11, to effect changes in the gear selection, when the vehicle is in motion.

However, when starting the vehicle from rest, actuation of the gear ratio selector lever 18 to engage a gear, will cause the clutch control unit 21 via the hydraulic control 22, to disengage the clutch 11. Subsequent release of the brake pedal andlor parking brake, will then cause the clutch control unit 21 via the hydraulic control 22 to partially engage the clutch 11 to apply a sufficient torque to the wheels of the vehicle to overcome the static rolling friction of the vehicle. As the engine speed is raised, the degree to which the clutch is engaged will increase under the normal takeup control of the clutch control system, until it exceeds the partial preengagement of the clutch, so that the torque applied to the wheels is then increased until the vehicle moves away.

To prevent unacceptable clutch wear, partial engagement of the clutch 11 after the cessation of the. braking force, is terminated if the level of torque determined by the normal take-up control of the clutch control system does not exceed the creep torque within a time-out period, typically between half a second and four seconds. Thus, if no attempt is made by the driver to set the vehicle in motion within an allotted time after the braking effort ceases, the assistance provided by the application of creep torque, is terminated..

When the clutch 11 is partially pre-engaged during start-up from rest, the drag of the clutch will cause a reduction in the engine speed. If this reduction is excessive, then the engine is likely to stall. The probability of such an occurrence will depend on the degree to which the clutch is pre engaged and the delay in depression of the accelerator pedal after the cessation of the braking. In order to avoid this problem the clutch control system may furthermore be arranged to immediately disengage the clutch 11 after cessation of the braking action if the accelerator pedal is not depressed and the engine speed drops below a threshold speed.

Preferably said threshold speed will be below the idle speed of the engine when the clutch 11 is fully disengaged.

Various modifications may be made without departing from the invention.

For example, while the above description refers to the clutch control system disclosed in the European Patent Specification 0038113, this invention applies to any transmission system in which drive between an engine and gearbox is controlled by an automatically actuated friction clutch. Also, while in the above embodiment, the clutch is controlled by hydraulic means, any other suitable means may be used. For example the slave cylinder 17 may be driven pneumatically rather than hydraulically.

Alternatively an electrical actuator, for example an electronic motor, which may drive the release fork 16 by a suitable gear system, if required, may be used.

Claims

1. A clutch control system for a motor vehicle having an engine, a gearbox, having a plurality of drive ratios and a gear selection means, the engine being drivingly connected to the gearbox by a friction clutch; the clutch control system automatically controlling engagement and disengagement of the clutch in synchronisation with actuation of the gear selection means, to after the gearbox drive ratios, the clutch control system including an engine speed sensor, a vehicle speed sensor, a gear ratio selection sensor, a brake release sensor to sense the release of a brake of the vehicle and means to partially engage the clutch upon release of the brakes when, prior to release of the brakes, the vehicle is substantially at rest with the brakes applied, a gear ratio is selected, and the engine speed is below a predetermined value.

2. A clutch control system according to claim 1 in which the engine speed sensor generates an engine speed signal variable with the speed of the engine, the control system including a reference signal generator for generating a reference signal, a comparator means for comparing the engine speed signal with the reference signal to produce a command signal, a clutch activation means for controlling the state of engagement of the clutch and a control unit responsive to the command signal to operate the activation means to cause the equalisation of the reference signal and the engine speed signal.

3. A clutch control system according to claim 1 or 2, in which the brake release sensor acts on a brake pedal and/or parking brake lever.

4. A clutch control system according to any one of the preceding claims in which the engagement of the clutch provides a torque sufficient to overcome the rolling friction of the vehicle.

5. A clutch control system according to claim 4 in which the control system takes account of the operative gear ratio to determine the level of clutch engagement required to be transmitted by one or more driving wheels to overcome the rolling friction of the vehicle.

6. A clutch control system according to any one of the preceding claims in which full engagement of the clutch is controlled in accordance with the engine speed, the partial engagement of the clutch after cessation of the braking force being terminated, if the level of torque determined by normal take-up control of the clutch control system does not exceed the torque achieved by partial pre-engagement, within a predetermined timeout period.

7. A clutch control system according to claim 6 in which the timing period is between half a second and four seconds from the cessation of the braking force.

8. A clutch control system according to any one of the preceding claims including a throttle sensor, the control system being arranged to immediately to disengage the clutch after cessation of the braking action, if the accelerator pedal is not depressed and the engine speed drops below a threshold speed.

9. A clutch control system according to claim 8 in which the threshold speed is below the idle speed of the engine when the clutch is fully disengaged.

10. A clutch control system according to any one of the preceding claims in which engagement and disengagement of the clutch is controlled by means of a hydraulic, pneumatic or electrical actuator.

11. A clutch control system substantially as described herein with reference to and as shown in the accompanying drawing.