WO2013024239A1

WO2013024239A1 - Vehicle drivelines including drive clutches

Info

Publication number: WO2013024239A1
Application number: PCT/GB2012/000648
Authority: WO
Inventors: Benjamin Chetwood Struve; Roberto Capolongo
Original assignee: Raicam Clutch Limited
Priority date: 2011-08-18
Filing date: 2012-08-10
Publication date: 2013-02-21
Also published as: EP2745026A1; GB201114197D0

Abstract

A torsional vibration damping arrangement for a vehicle driveline which includes a transmission (13) driven by an engine (12) via a drive clutch (11). The arrangement has a clutch actuator (15) for engaging/disengaging the clutch, an electronic control unit (16) which receives vehicle and clutch operating parameter signals including a signal representative of the speed of rotation of a component of the drive line and issues control signals for the engagement/disengagement of the clutch via the clutch actuator during starting and stopping of the vehicle and during ratio changes in the transmission. The electronic control unit (16) is arranged to control the level of engagement of the clutch (11) to damp torsional vibrations in the drive line by allowing the clutch to slip if the level of torsional acceleration amplitude of the component of the drive line determined from the rotational speed signal exceeds a target level of acceleration amplitude.

Description

VEHICLE DRIVELINES INCLUDING DRIVE CLUTCHES

This invention relates to vehicle drivelines in which a vehicle transmission is driven by an engine via a drive clutch and to arrangements for damping torsional vibrations in such drivelines.

Typically such drivelines have employed mechanical torsional vibration dampers such as complex and expensive twin-mass flywheels or circumferentially acting damping springs provided in the clutch itself.

As such drivelines suffer from noise, vibration and harshness (NVH) over their entire speed of operation mechanical damping arrangements are compromised in order to deliver acceptable damping both in low engine speed idle conditions and in mid and high speed operating conditions.

It is an object of the present invention to provide damping of torsional vibrations in a vehicle driveline which avoids the above problems.

Thus according to the present invention there is provided a torsional vibration damping arrangement for a vehicle driveline comprising a transmission driven by an engine via a drive clutch, the arrangement including :- a clutch actuator for engaging/disengaging the clutch,

an electronic control unit which receives vehicle and clutch operating parameter signals and issues control signals for the

engagement/disengagement of the clutch via the clutch actuator during starting and stopping of the vehicle and during ratio changes in the transmission,

the electronic control unit being arranged to control the level of engagement of the clutch to damp torsional vibrations in the drive line by allowing the clutch to slip if the level of torsional acceleration amplitude of the component of the drive line determined from the rotational speed signal exceeds a target level of acceleration amplitude.

It has been found that using an arrangement in accordance with the present invention the use of a twin-mass flywheel is unnecessary and a cheaper solid flywheel can be employed.

The present invention is applicable to drivelines in which the clutch is manually controlled by a driver-operated clutch actuating pedal to engage/disengage the clutch during starting and stopping of the vehicle and during ratio changes in the transmission.

The present invention can also be used in arrangements where the engagement /disengagement of the clutch are controlled by the electronic control unit in so-called fully or semi-automated manual transmissions.

The electronic control unit compares the target level of acceleration amplitude with the current level of acceleration amplitude and if the target level is exceeded issues an output signal to the clutch actuator to allow the clutch to slip to reduce the level of acceleration amplitude below said target level unless the control unit is in receipt of a signal commanding engagement of the clutch.

The speed sensor may comprise a toothed wheel which rotates with said drive line component and provides a pulse output as each tooth of the wheel passes an associated sensor.

The acceleration of said drive line component may be determined by determining the time period between successive wheel teeth passing the sensor and hence the rotational speed of said component in each successive time interval, the rotational speeds in successive time intervals then being subtracted and divided by the time interval in which this change has occurred to give the current acceleration of said drive line component. The acceleration amplitude value may then be calculated by storing a predetermined number of acceleration values in a memory device such as a numerical array. The maximum and minimum values stored in the array are then selected and the minimum value is subtracted from the maximum value the result giving an indication of the acceleration amplitude. When each acceleration amplitude value has been calculated, the oldest value of acceleration stored in the array is substituted by the current value of acceleration and a new value of acceleration amplitude is calculated etc.

The electronic control unit may be provided with a signal representative of the current torque output of the engine and the target level of acceleration amplitude may be varied in dependence on engine torque output.

The output signal from the comparison of the current and target acceleration amplitude may have a proportion of the output signal deducted to tend to close the clutch so that the target acceleration amplitude can be attained as the torque output of the engine increases.

The electronic control unit may be arranged to allow the clutch to slip in order to avoid exciting one or more natural frequencies of an associated driveline of the vehicle so as to avoid torsional vibrations in the driveline.

The control unit may receive engine speed signals and does not allow engagement of the clutch until a minimum engine speed has been reached.

During clutch engagement the electronic control unit may ensure that the minimum engine speed is maintained to avoid stalling.

The electronic control unit preferably limits the heat dissipated in the clutch during a clutch engagement by limiting the engine speed and the time during which the clutch is allowed to slip. 4

The electronic control unit may monitor the pedal position and may be arranged to fully engage the clutch if the driver rides the clutch pedal for more than a

predetermined period of time.

The electronic control unit may be arranged to receive one or more vehicle operating parameter signals indicative of a vehicle crash condition and on receiving such a crash signal or signals to disengage the clutch.

The spring means may act on the clutch pedal to provide a pedal effort

characteristic which increases up to a maximum effort at approximately the point of disengagement of the clutch and thereafter reduces.

The electronic control unit preferably ensures that the level of engagement of the clutch is greater than the instantaneous torque provided by the engine.

The invention also provides a clutch monitoring system for predicting the wear of a lining material of a vehicle drive clutch using a numerical model, the system including an electronic control unit which receives signals indicative of clutch input and output speeds, clutch ambient temperature, engine torque, thermal capacity of certain clutch components, heat dissipation rate of these clutch components, and wear rate of the clutch lining material as a function of lining temperature and as a function of the rate of energy dissipation in the lining material; the electronic control unit using combinations of the above parameter signals to predict the level of lining wear which has occurred since the system was last reset and providing an indication to the vehicle driver when predetermined wear conditions have been reached. This numerical model can be used in any vehicle clutch no matter whether it is controlled by a system in accordance with the main claim of this application.

The invention will now be described, by way of example only, with reference to the accompanying drawings in which.- Figure 1 shows diagrammatically a vehicle driveline torsional vibration damping ' arrangement in accordance with the present invention;

Figure 2 shows a typical continuously rising release bearing force/release bearing position curve of a diaphragm spring used to clamp a clutch in the damping arrangement of the present invention and the net clamping force applied to the driven plate;

Figure 3 shows a diagram of a spring arrangement acting on a clutch actuating pedal in the damping arrangement of the present invention;

Figure 4 shows internal details of a spring device used on the clutch pedal;

Figure 5 shows the spring characteristics of the springs operating in the spring arrangement of Figure 4;

Figure 6 shows diagrammatically the overall control strategy of the damping arrangement of the present invention;

Figure 7 shows diagrammatically a pulse period timer block used in the damping arrangement of the present invention;

Figure 8 shows diagrammatically an acceleration amplitude measurement block used in the damping arrangement Of the present invention;

Figure 9 shows a look-up table showing the variation of target acceleration amplitude with engine torque which is used in the damping arrangement of the present invention, and β

Figure 10 shows a variation of the overall control strategy of Figure 6 which may be used in the damping arrangement of the present invention. Referring to the drawings, Figure 1 shows diagrammatically a torsional vibration damping arrangement for a vehicle driveline 0 in which an engine 12 drives a transmission 13 via a drive clutch 11. Transmission 13 in turn drives wheels 14 of the vehicle. The clutch 1 is mounted on a single mass flywheel 11a and has its driven plate (not shown) clamped against flywheel 1 a by a diaphragm spring also not shown. The clutch also has a release means 15 whose operation is controlled by an electronic control unit 16.

In a typical system, release means 15 includes an electric motor whose rotational movement is converted into axial movement of a hydraulic piston in a master cylinder (not shown) by a nut and screw or ball and screw device. This resulting axial movement of the hydraulic piston is used to operate a hydraulic slave cylinder adjacent the clutch which releases the clutch. In a typical van application a conventional brushless DC motor is used which is controlled by ECU 16 which contains the power electronics to commutate the current in its windings.

Alternatively a brushed motor could be used, or a linear electric motor which would generate a translational force instead of a rotational torque.

In an alternative arrangement the release means can be completely electrical with an electric motor operating a ball and screw device which operates directly on the clutch release mechanism without the use of hydraulics.

The control system has a clutch actuating pedal 17 pivoted on a pedal box 8 about an axis 19. Connected with the pedal is a pedal position sensor 20 (e.g. a rotary potentiometer) which provides a signal to an electronic control unit 16 via line 21 indicative of the angle of depression Θ of the pedal 7.

Control unit 16 also receives inputs 22 from other sensors on the vehicle indicative of other vehicle operating parameters such as engine speed, transmission input shaft speed, road speed, selected gear, and temperature from, for example, the vehicle CAN bus. The clutch driven plate, which is clamped against the flywheel 1 1 a by the diaphragm spring, is released by a clutch release bearing (not shown). The diaphragm spring applies an engagement force to the clutch which continuously increases as the clutch is disengaged by the release means 15 and the release bearing giving the continuously rising force/displacement characteristic shown in Figure 2. By using a diaphragm spring with this continuously rising characteristic each level of release bearing force corresponds to a unique release bearing position thus simplifying the control of the clutch by the electronic control unit 16.

The electronic control unit 16 reads the instantaneous torque being generated by the engine. Generally this data would be theoretically calculated, and made available as an output from the standard engine control unit (not shown) which controls the fuelling and timing of the engine and which is broadcast as a message available via the vehicle's CAN bus.

The movement of the clutch pedal 17 is resisted by a two-rate spring device 25 (see Figure 3) which is mounted on the pedal box 18 at 27 and connected with the pedal at 28 by a rod 28a. An over-centre spring 26 also acts on the pedal at 31 and is connected to the pedal box at 29. The spring device 25, shown in more detail in Figure 4, has a housing 40 which contains a first spring 41 of low stiffness (e.g. 9 N/mm) with a high pre-load (e.g. 475 N). This spring is held in place by an end cap 42 retained by a circlip 43. A spring cup 44 is fitted into the end of spring 41. Cup 44 has a flange 44a which contacts the end of spring 41 Inside a spring cup 44 sits a second coil spring 45, which has a higher spring rate than spring 41 (for example 75 N/mm). Rod 28a is provided with flat end 28b which can apply a load to spring 45. Spring 45 has a minimal toad when rod 28a is at rest sufficient to ensure there is no free play in the push rod 28a (e.g. 5 N).

When pedal 7 is pressed, push rod 28a moves into housing 40 and spring 45 is compressed until it generates a force equal to the preload of spring 41 . This gives a force v displacement characteristic for device 25 equal to the stiffness of spring 45 as shown at 25a in Figure 5. Further movement of the pedal compresses both springs 45 and 41 in series and gives a force v stiffness characteristic equal to the series stiffness of the two springs as shown at 25b in Figure 5.

The over-centre spring 26 resists depression of the clutch pedal initially in section 26a and when pedal moves over-centre (i.e. when spring attachment point 31 crosses line 32 which joins pedal pivot axis 9 and point 29) the spring 26 assists further movement of pedal 7 in section 26b of the characteristic. The combined effect of spring device 25 and over-centre spring 26 is shown by characteristic 30 in Figure 5. This characteristic is essentially the same as that experienced by a driver who presses a normal clutch pedal which directly operates a diaphragm spring type clutch (i.e. pedal load increases up to a maximum at approximately the point of disengagement of the clutch and thereafter reduces). The pedal therefore feels "normal" to the vehicle driver despite the fact that there is no direct connection between the pedal 17 and the actuator 15. If desired the pedal position sensor can be a linear sensor built into the spring unit 25.

Clutch 11 is of the "normally closed" type cover assembly, without self adjustment. The clutch has the greatest pressure plate mass which could be practically designed in order to maximise its ability to absorb and store heat generated by clutch slip, and thereby minimise temperature reached at the slipping surface. The mass of the pressure plate in a typical van type application is of the order of 3.66kg.

In a typical van application of the present invention the ECU 16 receives the clutch pedal position from the sensor 20 fitted to the clutch pedal 17 and a speed signal from a rotating shaft 57 which forms part of the drive line of the vehicle. This speed signal is derived from a sensor 55 which generates an output pulse when each tooth of a toothed wheel 56 passes the sensor. Such sensors are well known and commonly operate by electromagnetic effect. From this sensor signal, the time period between each individual tooth passing the sensor is measured, and based on the number of teeth on the gearwheel 56 the current rotational speed of the shaft and its accelerations can be calculated as explained below. The ECU 6 also receives signals indicative of engine torque (This data is calculated within the Engine Control Module and then broadcast via the vehicle's data network as discussed above).

The control system has an operating algorithm which is designed to control the level of clutch slip so that the engine is isolated from the drive line thus reducing drive line torsional vibration. It has been found that a good level of vibrational isolation can be achieved by controlling the level of clutch slip so that the acceleration amplitude level of the drive line does not exceed a target level.

The algorithm could be used either to control the position of clutch engagement in a clutch system with a mechanical clutch pedal, or could be embedded within the overall control software of an automated or semi-automated transmission or in a double clutch transmission system.

Figure 6 shows a high level diagram of the overall control strategy of a clutch control system in accordance with the present invention in which a clutch demand signal X (which can be either an input value from the clutch pedal sensor 20, or it could be a value generated by the higher level control system of an automated or semi- transmission a double clutch transmission control system) is fed into a comparator 50. For the purposes of this description, it is assumed that a 'high' value (e.g. 1) indicates that the clutch pedal is depressed, or that the system is requiring that the clutch should be 'open', and that a 'low' value (e.g. 0) indicates that the pedal is released, and the requirement is for the clutch to be engaged.

The system also receives speed signal Y from sensor 55 and calculates the current level of acceleration amplitude of shaft 57 in an acceleration amplitude

measurement block 51 as will be described below in relation to Figures 7 and 8.

The first stage of the acceleration amplitude measurement block 51 is the pulse period timer 52 which is shown in Figure 7. The process shown in Figure 7 executes each time a pulse is sensed from the speed sensor 55, and measures the difference in time (dt) between the current pulse and the previous pulse. This data is then used within the acceleration amplitude measurement block 51 shown in Figure 8. This block also executes each time a pulse is sensed from the speed sensor 55.

The difference (dt) in the time period between signal pulses from sensor 55 is fed from the pulse timer 52 into block 60 together with a signal representative of the number of teeth on wheel 56 to provide an output from block 60 which is

representative of the speed of shaft 57 in revs /sec. This signal is converted into a rotational speed in radians /sec in a multiplier 61. The difference (dv) between the current rotational speed and the rotational speed in the previous time interval is calculated in block 63 and since this difference dv was achieved in the time interval dt the level of rotational acceleration can be calculated in block 64.

The acceleration amplitude value may then be calculated by storing a predetermined number of acceleration values in successive time intervals a numerical array 65. The maximum and minimum values stored in the array are then selected and the minimum value is subtracted from the maximum value the result of this subtraction giving an indication of the acceleration amplitude. When each acceleration amplitude value has been calculated, the oldest value of acceleration stored in the array is substituted by the current value of acceleration and a new value of acceleration amplitude is calculated etc.

Within the overall control strategy indicated in Figure 6, the acceleration amplitude value from block 51 is fed as the measured input into a closed loop control loop 53, typically a PID (Proportional Integral Differential) algorithm or similar. This measured input is compared against an acceleration amplitude 'set point' or target value which is stored in look-up tables indicated diagrammatically ay 51a in Figure 6.

When this control strategy is implemented in a motor vehicle, the clutch is controlled as follows. If the transmission shaft acceleration amplitude is found to be above the target value, the output of the control algorithm rises, and a rising clutch control signal Z is sent from the control system to the clutch actuator motor. This rising signal Z causes the clutch to open, and the consequential clutch slip causes the acceleration level of the transmission shaft to reduce until the acceleration amplitude target is met. Conversely, if the transmission shaft acceleration amplitude is found to be below the target value, the output of the control algorithm will fall, causing a falling clutch control signal Z to occur and closure of the clutch. There is no measurement of clutch position itself , it being assumed that the position of the motor and hence the ball screw always has a fixed relationship to the clutch.

If the clutch is already slipping, closure of the clutch will cause the slipping to reduce until the acceleration amplitude target is met, Should the angular acceleration amplitude target measured still be below the target value (in practise this could occur because the angular acceleration excitation from the combustion engine is low, or because the torsional isolation provided by the clutch vibration damper is effective), the output signal from the control algorithm will continue to fall to a value of zero, so that the clutch is fully closed.

In order for the system to allow the clutch to be disengaged and externally controlled during vehicle launch and gear shifting, comparison block 50 is inserted into the control strategy. If the driver presses the clutch pedal fully, the clutch demand signal X will have a higher value than the output signal from the control loop, therefore the higher value is chosen to be passed out of the control system as the clutch control signal Z.

Several desirable refinements could be added to this overall control system:

Firstly, the desirable acceleration amplitude target may not be constant, but may vary according to the operating conditions within the vehicle. In particular, the angular acceleration threshold above which an automotive transmission system starts to emit audible gear rattle has a tendency to be sensitive according to the mean torque which is transmitted through it. When the mean torque has a low value (for instance during the operating condition that the vehicle is idle in a neutral gear), the typical acceleration value at which gear rattle becomes audible may be of the order of 80rad/s2. Conversely, when accelerating at full throttle, the allowable acceleration level may be of the order of 1000rad/s2.

Thus the acceleration amplitude set-point for the control system may be modified according to a 'look-up' table (see Figure 9). The input value for this look-up process is engine torque - an adequate estimation of this value may be obtained by connecting the control system to the CAN Bus of the vehicle, and monitoring the commonly broadcast message for calculated engine torque, which is output from the Engine Control Module.

Secondly, in order to respond to transient changes in torque transmitted through the vehicle's driveline, it may be necessary to improve the layout of the high level control system. In the simple layout, an increase in accelerator opening in the vehicle would lead to an increase in the torque applied by the engine. This would lead to increased slipping speed, and reduced acceleration amplitude level in the transmission shaft. In order to achieve the target acceleration amplitude the control system must reduce the output value of the control loop, in order to reduce the clutch control signal and close the clutch by the appropriate amount to achieve the desired degree of clutch slip. This situation may be improved by utilising the estimated torque signal broadcast by the engine contro) module on the CAN Bus. If multiplied by an appropriate factor "f (see block 54 Figure 10), this signal may be subtracted from the output value of the control loop, thereby ensuring that the output value of the clutch control signal does not need to vary significantly in order to maintain the target acceleration amplitude level.

The effectiveness of subtracting the estimated torque value may be compromised if the estimated signal is broadcast over the CAN Bus infrequently, and the data is consequently 'out of date'. An alternative to subtracting the torque value may be to subtract the value corresponding to accelerator pedal position (again available by monitoring the CAN Bus), or even subtracting a value related to both torque and accelerator position.

The control unit 16 may also employ a numerical clutch wear model. The control unit receives signals indicative of clutch input and output speeds, clutch ambient temperature, engine torque, thermal capacity of certain clutch components, heat dissipation rate of these clutch components, and wear rate of the clutch lining material as a function of lining temperature and as a function of the rate of energy dissipation in the lining material. The electronic control unit uses combinations of the above parameter signals to predict the level of lining wear which has occurred since the system was last reset and provides an indication to the vehicle driver when predetermined wear conditions have been reached.

Provision is made for the wear model to be reset, for example when a new clutch lining is fitted. The control unit compares the available volume of clutch lining material against the calculated sum of the volume of material consumed, and indicates an error state when all of the available friction material has been used.

This numerical clutch wear model may be used in any vehicle clutch no matter whether it is controlled by a system in accordance with the main claim of this application.

The flywheel, clutch and associated driveline are designed so that their natural frequencies of torsional vibration are as low as possible. For example, the torsional stiffness of the clutch driven plate is low (e.g. 25 Nm/degree of rotation) thus reducing the natural frequency of the driveline.

It has been found that by damping the torsional vibrations of the driveline using limited and controlled clutch slip as described above it is possible to use a solid flywheel (thus obviating the need to use expensive twin mass flywheels) and that it is no longer necessary to include idle and creep rattle dampers in the clutch driven plate. By lowering the natural frequencies of the torsional vibrations of the components of the entire driveline the levels of slippage required to damp these vibrations are also lowered so that excessive levels of clutch slip can be avoided thus reducing the heat generated and the clutch can also be kept fully engaged for most of the time.

Typically the clutch is only slipped at or just below the engine speeds at which the natural frequencies are excited.

The control unit 16 can also be programmed to receive signals indicative of the engine speed and, as part of a vehicle launch strategy, not allow the clutch to engage, despite the clutch pedal position, if a minimum engine speed has not been reached in order to avoid stalling and/or vibration of the vehicle. The control system can also slip the clutch if necessary during clutch engagement to ensure that the minimum engine speed is maintained thus further improving the avoidance of stall andJor vibration.

The control unit can also limit the heat generated in the clutch during clutch engagement by limiting engine speed and the time during which the clutch is allowed to slip.

The control unit may also monitor the pedal position signal and if the driver is riding the clutch pedal (i.e. partially depressing the clutch pedal unintentionally) the control unit can be arranged to fully engage the clutch to protect the clutch. This control is implemented by setting a range of angles of depression of the clutch pedal (e.g. 0 to 5 degrees) which is designated as an unintentional depression of the clutch pedal and which is monitored accordingly.

The control unit may also be arranged to receive one or more vehicle operating parameter signals indicative of a vehicle crash condition (e.g. levels of vehicle deceleration, seat belt tensioning and air bag actuating signals or vehicle crash state indications via a data flag carried on the CAN bus) and to disengage the clutch when such crash signals are received. The operation of the pedal 17 to disengage the clutch at any time by the driver is arranged to have priority and always results in immediate disengagement of the clutch for safety reasons.

Although the present invention has been described above in relation to a clutch system which has a driver operated pedal 17 the present invention is also applicable to so-called automated manual transmissions, that is transmissions in which clutch engagement/disengagement for starting and stopping together with clutch disengagement and re-engagement during ratio changes is controlled by an electronic control unit. In such transmissions the decision to change the operative ratio in the transmission may be made totally by the electronic control unit so that the transmission operates like a completely automatic transmission or the decision to change ratio may be made by the vehicle driver when the transmission is referred to as semi-automated. Some transmissions can be selectively operated as fully automated or semi-automated transmission. In all cases the clutch engagement and disengagement is under the control of the electronic control unit 16 and no driver operated clutch pedal 17 is provided.

The torsional vibration damping arrangement of the present invention can also be used in a vehicle driveline in which more than one clutch is used to couple drive or select the operative ratio in the driveline. For example, the arrangement could be used on both clutches in a so-called twin clutch transmission in which the ratios are divided into two groups with one clutch engaging drive via one group of ratios and another clutch engaging drive via the other group of ratios.

Claims

1. A torsional vibration damping arrangement for a vehicle driveline comprising a transmission driven by an engine via a drive clutch, the arrangement including:- a clutch actuator for engaging/disengaging the clutch,

an electronic control unit which receives vehicle and clutch operating parameter signals including a signal representative of the speed of rotation of a component of the drive line and issues control signals for the engagement/disengagement of the clutch via the clutch actuator during starting and stopping of the vehicle and during ratio changes in the transmission,

2. A damping arrangement according to claim 1 in which the electronic control unit compares the target level of acceleration amplitude with the current level of acceleration amplitude and if the target level is exceeded issues a n output signal to the clutch actuator to allow the clutch to slip to reduce the level of acceleration amplitude below said target level unless the control unit is in receipt of a signal commanding engagement of the clutch.

3. A damping arrangement according to claim 1 or 2 in which the speed sensor comprises a toothed wheel which rotates with said drive line component and provides a pulse output as each tooth of the wheel passes an associated sensor.

4. A damping arrangement according to claim 3 in which the acceleration amplitude of said drive line component is determined by determining the time period between successive wheel teeth passing the sensor and hence the rotational speed of said component in each successive time interval, the rotational speeds in successive time intervals then being subtracted and divided by the time interval in which this change has occurred to give the current acceleration of said drive line component during the current time interval, the current acceleration amplitude value is then calculated by selecting the maximum and minimum acceleration values from a predetermined number of acceleration values stored in memory and subtracting these maximum and minimum values to give the current acceleration amplitude, a subsequent acceleration amplitude value being calculated by substituting the oldest value of acceleration stored in memory by the current value of acceleration so that a new value of acceleration amplitude can be calculated.

5. A damping arrangement according to any one of claims 1 to 4 in which the electronic control unit is provided with a signal representative of the current torque output of the engine and the target level of acceleration amplitude is varied in dependence on engine torque output.

6. A damping arrangement according to claim 5 in which the output signal from the comparison of the current and target acceleration amplitude has a proportion of the output signal deducted to tend to close the clutch so that the target acceleration amplitude can be attained as the torque output of the engine increases.

7. A damping arrangement according to any one of claims 1 to 6 in which a

driver-operated clutch actuating pedal is provided for engaging/disengaging the clutch during starting and stopping of the vehicle and during ratio changes in the transmission, the position of this pedal providing a further input into the control unit.

8. A damping arrangement according to any one of claims 1 to 6 in which the transmission is of the fully or semi-automated manual type in which all clutch operations are controlled by the electronic control unit.

9. A damping arrangement according to any one of claims 1 to 8 in which the electronic control unit allows the clutch to slip in order to avoid exciting one or more natural frequencies of an associated driveline of the vehicle so as to avoid torsional vibrations in the driveline.

10. A damping arrangement according to any one of claims 1 to 9 in which the control unit receives engine speed signals and does not allow engagement of the clutch until a minimum engine speed has been reached.

1 . A damping arrangement according to claim 10 in which during clutch

engagement the control unit ensures that the minimum engine speed is maintained during engagement.

12. A damping arrangement according to claim 7 in which the pedal position is monitored and the clutch is arranged to be fully engaged by the electronic control unit if the driver rides the clutch pedal for more than a predetermined period of time.

13. A damping arrangement according to any one of claims 1 to 12 in which the control unit receives one or more vehicle operating parameter signals indicative of a vehicle crash condition and on receiving such crash signal or signals disengages the clutch.

14. A damping arrangement according to any one of claims 1 to 13 in which spring means act on the pedal to generate a pedal effort characteristic which increases up to a maximum effort at approximately the point of disengagement of the clutch and thereafter reduces.

15. A damping arrangement according to any one of claims 1 to 14 in which the electronic control unit calculates the temperature of various components of the clutch and estimates a current effective coefficient of friction of the clutch lining material and hence adjusts the engagement force applied to the clutch by the actuator as the clutch lining material temperature rises and falls.

16. A damping arrangement according to any one of claims 1 to 15 in which the electronic control unit limits the heat dissipated in the clutch during a clutch engagement by limiting engine speed and the time during which the clutch is allowed to slip. 7. A twin clutch gearbox in which both clutches are provided with a torsional damping arrangement according to any one of claims 1 to 16.

18. A torsional vibration damping arrangement for a vehicle driveline constructed and arranged substantially as hereinbefore described with reference to and as shown in the accompanying drawings.