GB2458497A

GB2458497A - A method of determining the position of a clutch slave cylinder piston

Info

Publication number: GB2458497A
Application number: GB0805215A
Authority: GB
Inventors: Ian Halleron; Themi Petridis
Original assignee: Ford Global Technologies LLC
Current assignee: Ford Global Technologies LLC
Priority date: 2008-03-20
Filing date: 2008-03-20
Publication date: 2009-09-23
Anticipated expiration: 2028-03-20
Also published as: CN101539174B; GB2458497B; DE102009008511B4; CN101539174A; DE102009008511A1; GB0805215D0

Abstract

A method and apparatus for determining the position of a slave cylinder piston 62 connected to a clutch 2 is disclosed in which the position of the slave cylinder piston 62 is measured using a position sensor 63 and the actual displacement of the slave cylinder piston 62 is determined by subtracting a zero value Z from the measured position. The apparatus and method thereby compensates for variations in slave cylinder piston 62 zero position due to clutch wear. The method also compares a current measurement of slave cylinder position with a stored value position and sets the stored value of position to the measured value if the measured value is less than the stored value and retains the stored value if the measured value is greater than the stored value. The apparatus comprises a hydraulic clutch actuation system including a master cylinder 51 controlled by an electronic controller 16 and a mechanical linkage 65 connecting the slave cylinder piston 62 to the clutch 2.

Description

A Method and apparatus for Determining the Position of a Clutch Actuating Piston This invention relates to motor vehicles and in particular to an apparatus and method for determining the position of a clutch actuating piston.

For manual transmission vehicles that are fitted with automated Stop/ Start control of the engine often referred O to as Micro-hybrid vehicles, it is desired to maximise fuel economy by utilising automated engine shut downs and restarts whenever possible. Stop-in-Neutral (SIN) Stop-Start systems are currently employed in the market place but these systems do not maximise fuel economy since many drivers wait in gear in a stationary vehicle. Stop-in-Neutral stops are not invoked under these conditions and a Stop-in-Gear (SIG) strategy is required.

However to employ a Stop-in-Gear (SIG) Stop-Start strategy, it is necessary to ensure that the driveline is disengaged to prevent accidents from occurring or undesirable vehicle motion from taking place. Stop-in-Gear stops and starts would typically be invoked i.f both the clutch and brake pedals were depressed, possibly with the transmission in gear. However to ensure that such a strategy is safe, if a driver-induced restart request is received from, for example, releasing of the brake pedal, the Stop-Start logic must only be allowed to crank the engine if the driveline s fully disengaged as this prevents the vehicle jerking or moving during cranking. If the conditions for an engine shut-down were met (e.g. the vehicle speed is zero and the clutch and brake pedals are pressed) Stop-in-Gear conditions should prevent the engine shutting down if the driveline is riot fully disengaged to ensure that the engine can be restarted.

Furthermore, if the Stop-Start strategy allows system-induced restarts, such as, for example, if the battery requires charging or there is an A/C request, then cranking should only be allowed if the drivelirie is disengaged. This is required to prevent unintended vehicle movement from occurring during a system-induced cranking event which could result in very serious consequences. There is therefore a safety critical requirement for Stop-in-Gear shut down and restarts which is that the driveline must be disengaged.

It is possible to use the position of a slave cylinder piston as a guide to clutch engagement state. However, due to the complexity of the clutch and clutch actuation system, ensuring that the clutch is disengaged is not straightforward and in particular clutch wear will affect the resting or zero position of the slave cylinder piston.

It is an object of this invention to provide a method and apparatus for determining the position of a slave cylinder piston operatively connected to a clutch in a cost effective manner.

According to a first aspect of the invention there is provided a method for determining the position of a slave cylinder piston operatively connected to a clutch, the method comprising measuring the position of the slave cylinder piston, comparing the current measurement of slave cylinder piston position with a stored value from a previous measurement, setting the stored value of position to the current measured value if the measured value is less than the stored value and retaining the stored value if the measured value is not less than the stored value.

Upon initiation of the method, the stored value may be set to a value greater than any possible stored value obtained by measurement of slave cylinder piston position.

The method may further comprise adding a predetermined amount to the stored value every time the clutch is operated and comparing the incremented stored value with the current measurement of slave cylinder position in order to s compensate for auto adjustment of the clutch.

The predetermined amount may be larger than the amount the piston position will he chanqed by auto adjustment of the clutch.

The method may further comprise continuously measuring the position of the slave cylinder position and subtracting the stored value from the measured position of the slave piston to produce a value of actual slave piston displacement.

According to a second aspect of the invention there is provided a clutch system comprising a clutch, a hydraulic clutch actuation system and an electronic controller wherein the hydraulic clutch actuation system includes a slave cylinder having a slave cylinder piston, a mechanical linkage connecting the slave cylinder piston to the clutch, a position sensor to measure the position of the slave cylinder piston and supply a signal indicative of the measurement to the electronic controller wherein the electronic controller is operable to measure the position of the slave cylinder piston, compare the current measurement of slave cylinder piston position with a stored value from a previous measurement, set the stored value of resting position to the current measured value if the measured value is less than the stored value and retain the stored value if the measured value is not less than the stored value.

Upon initiation of the electronic controller, the :35 electronic controller may be operable to set the stored value to a value greater than any possible stored value obtained by measurement of slave cylinder piston position.

The electronic controller may be further operable to add a predetermined amount to the stored value every time the clutch is operated and compare the incremented stored value with the current measurement of slave cylinder position in order to compensate for auto adjustment of the clutch.

The predetermined amount may be larger than the amount :0 the piston posiLion will be changed by auto adjustment of the clutch.

The electronic controller may he further operable to continuously measure the position of the slave cylinder position and subtract the stored value from the measured position of the slave piston to produce a value of actual slave piston displacement.

The electronic controller may be further operable to use the value of actual slave piston displacement to control one or more systems.

The invention will now be described by way of example with reference to the accompanying drawing of which:- Fig.1 is a schematic representation of a micro-hybrid motor vehicle having a stop-start system; Fig.2 is a schematic diagram of a clutch and a clutch actuation system used in the micro-hybrid vehicle shown in Fig.1; Fig.3 is a high level flow chart showing the actions used for controlling the operation of an internal combustion engine forming part of the vehicle shown in Fig. 1; Fig.4 is a high level flow chart showing a method for providing an indication of the engagement state of the clutch shown in Fig.2; Fig.5 is a flow chart showing a method for validating an output signal from a clutch slave cylinder forming part of the clutch actuation system shown in Fig.2; Fig.6 is a flow chart showing a first embodiment of a method according to this invention for determining a current zero position of a piston of a clutch slave cylinder forming part of the clutch actuation system shown in Fig.2; Fig.7 is a flow chart showing a second embodiment is of a method according to this invention for determining the current zero position of the piston of the clutch slave cylinder; Fig.8 is a flow chart showing a method for determining a threshold displacement required to achieve disengagement of the clutch shown in Fig.2; Fig.9 is a flow chart showing a method for determining the engagement state of the clutch shown in Fig.2; Fig.1O is a diagrammatic representation of the motion of a piston of a clutch slave cylinder forming part of the clutch actuation system shown in Fig.2; Fig.l1 is a table showing hypothetical values obtained during operation of a method as shown in Fig.7; and Fig.12 is a table showing hypothetical values obtained during operation of a method as shown in Fig.6.

With particular reference to Figs.l and 2 there is shown a motor vehicle 5 having an engine 10 driving a multi-speed manual transmission 11. The transmission 11 is driveably connected to the engine 10 by a clutch system 50 which is manually engaged or released by a driver of the motor vehicle 5 using a clutch pedal 25.

The transmission 11 has a gear selector (not shown) that is moveable between several positions Including at ic least one position where a gear forming part of the multi-speed transmission is selected and a neutral position in which no gears of the multi-speed transmission are selected.

When the gear selector is moved to the neutral position the multi-speed transmission 11 is said to be in a neutral state in which drive cannot be transmitted by the multi-speed transmission and when the gear selector is moved to an in gear position the multi-speed transmission 11 is said to be in an in gear state in which drive can be transmitted by the multi-speed transmission. 2C

An engine starter in the form of an integrated starter-generator 13 is driveably connected to the engine 10 and in this case is connected by a flexible drive in the form of a drive belt or chain drive 14 to a crankshaft of the engine 10. The starter-generator 13 is connected to a source of electrical energy in the form of a battery 15 and is used to start the engine 10 and which is recharged by the starter-generator when it is operating as an electrical generator.

It will be appreciated that the starter-generator 13 could he replaced by a starter motor for starting the engine 10.

During starting of the engine 10 the starter-generator 13 drives the crankshaft of the engine 10 and at other times the starter-generator is driven by the engine 10 to generate electrical power.

A driver operable on-off device in the form of a key operable ignition switch 17 is used to control the overall operation of the engine 10. That is to say, when the engine is running the ignition switch 17 is in an on' position (key-on) and when the ignition switch 17 is in an off' position (key-off) the engine 10 is not able to run. The ignition switc.th 17 also includes a third momentary position used to manually start the engine 10. It will be appreciated that other devices may be used to provide this functionality and that the invention is not limited to the use of a key operable ignition switch.

An electronic controller 16 is connected to the starter-generator 13, to the engine 10, to a gear selector sensor 12 used to monitor whether the transmission 11 is in neutral or in gear, to a road speed sensor 21 used to measure the rotational speed of a road wheel 20, to a brake pedal position sensor 24 used to monitor the position of a brake pedal 23, to a clutch master cylinder position sensor 53 used to monitor the position of a master cylinder piston 53 and indirectly the position of the clutch pedal 25 to which the master cylinder piston is mechanically connected, to a clutch slave cylinder position sensor 53 used to monitor the position of a slave cylinder piston 62 and to a throttle position sensor 19 used to monitor the position of an accelerator pedal 18.

The position of the master and slave cylinder pistons 52 and 62 may be measured by the sensors 53, 63 using any of :30 a number of position sensing technologies such as, for example and without limitation, PLCD and Hall effect.

The accelerator pedal 18 provides a driver input of required power output from the engine 10. If the accelerator pedal 18 has been moved from a rest position it is said to be in a pressed position or in a pressed state.

It will be appreciated that the term gear selector sensor is not limited to a sensor that monitors the position of the gear selector but rather is any device which can provide a feedback of whether the transmission 11 is in gear or in neutral and that a gear sensor is not a requirement for a SIG stop-start system.

Similarly, the term brake pedal sensor is not limited to a sensor that monitors the position of the brake pedal hut rather is any device that provides feedback of whether an operator of the motor vehicle 5 has applied pressure to the brake pedal 23 to apply the brakes of the motor vehicle 5. For example the brake pedal sensor could monitor the pressure of the fluid in one or more brake lines. When the is brake pedal 23 has been pressed sufficiently to apply the brakes it is said to be in a pressed state or in a pressed position.

Referring now in particular to Fig. 2 it can be seen that the clutch system 50 comprises of a clutch 2 and a hydraulic actuation system connecting the clutch 2 to the clutch pedal 25. The hydraulic actuation system comprises a mechanical linkage connecting the clutch pedal 25 to the master cylinder piston 52 of a master cylinder 51, a hydraulic connection or conduit 55 connecting an output from the master cylinder 51 to one end of a slave cylinder 61 in which is slidingly mounted the slave cylinder piston 62 and a mechanical linkage 65 from the slave cylinder piston 62 to a release bearing 6 used to selectively release and engage the clutch 2.

It will he appreciated that a displacement of the clutch pedal in the direction of the arrow clutch pedal travel' on Fig.2 will produce corresponding respective displacements D1 and of the master and slave pistons 52 and 62 in a clutch disengaging direction.

The clutch 2 is in this case a push release type of friction clutch and includes a cover and spring assembly 3, a pressure plate 4 and a driven plate 7 interposed between the pressure plate 4 and a flywheel 8 attached to a crankshaft (not shown) of the engine 10. The clutch 2 is conventional in construction and will not be described in detail it is merely necessary to know at this stage that movement of the release bearing 6 in the direction of the arrow DLI, by the slave cylinder piston 62 is movement in a o clutch disengaging direction arid the opposite motion is motion in a clutch engaging direction. At some point during the range of motion of the release bearing 6 the state of the clutch 2 will change from a disengaged state in which substantially no torque can be transmitted by the clutch s from the engine 10 to the transmission 11 to an engaged state where a significant torque can be transmitted. This position of clutch engagement is often referred to as the bite point. The value of torque will vary from vehicle to vehicle depending upon many factors including the mechanical ratio between the engine 10 and the driven wheels (not shown), the friction in the driveline, the friction between the road wheels and the road but in general terms it is a torque that when applied will produce a jerk that is noticeable by a user of the motor vehicle and is typically in the range of 3 to lONm.

The electronic controller 16 receives several signals from the engine 10 including a signal indicative of the rotational speed of the engine 10 from a speed sensor (not 0 shown) and sends signals to the engine used to control shutdown and start-up of the engine 10. In this case the engine 10 is a spark ignited engine 10 and the signals sent from the electronic control unit 16 are used to control a fuel supply system (not shown) for the engine 10 and an :35 ignition system (not shown) for the engine 10. If the engine 10 were to be a diesel engine then only the fuel supply to the engine would be controlled. The electronic -10 -controller 16 may comprise of various components including a central processing unit, memory devices, timers and signal processing devices to convert the signals from the sensors connected to the electronic controller 16 into data which is s used by the electronic controller 16 to control the operation and in particular the automatic stopping and starting of the engine 10. It wiI also be appreciated that the electronic controller 16 may be formed of several discrete electronic control units which communicate with one another to achieve the required functionality.

During normal engine running, the electronic controller 16 is operable to control the fuel supplied to the engine 10 arid to adjust the ignition system so that sparks are .s supplied to the engine 10 from spark plugs at the correct timing to produce the desired engine torque.

The electronic controller 16 controls the operation of the engine 10 which is operable in two modes, a first or stop-start running mode and a second or manual running mode.

The primary factor used to determine whether the engine is operated in the second mode or in the first mode is whether the motor vehicle 5 is moving. If the motor vehicle 5 is moving the engine is operated in the second mode and the engine 10 will be run continuously and, if the motor vehicle 5 is not moving, the engine 10 will be run in the first mode in which automatic stop-start operation of the engine will occur providing other factors as described below indicate that stop-start operation is possible.

In the first or stop-start mode the engine 10 is selectively stopped and started by the electronic controller 16 without driver intervention when one or more predetermined engine stop and start conditions exist. These stop and start conditions are based upon the signals received by the electronic controller 16 from the throttle -11 -sensor 19, the brake sensor 24, the clutch system 50 and the gear selector sensor 12. The position or state of the clutch 2, the accelerator pedal 18, the brake pedal 23 and the transmission 11 are all different motor vehicle variables which can be used to control the operation of the engine 10. It will be appreciated that many other variables can also he used including, hut not limited to, PAS pump operational state, brake vacuum sensor output, stop-start manual inhibit switch.

When the engine 10 is operating in the second mode it is run continuously so long as the ignition switch 17 remains in the on' position and the engine 10 is stopped and started by manual operation of the ignition switch 17. is

Although the measurement of motor vehicle speed is described above with reference to the use of a road wheel sensor 21 because such sensors are often already present on a motor vehicle as part of a brake anti-lock system it will be appreciated that other suitable means can be used to determine the speed of the motor vehicle 5 such as, for example, a sensor measuring the rotational speed of an output shaft from the transmission 11.

Referring now to Fig.3 there is shown a high level flow chart of the methodology used for determining whether the stop-start or first mode operation is possible.

The method starts at step 30 with the ignition key 17 in the off position and will stay in this state until at step 31 the ignition key 17 is moved to the on position which will start the engine 10 at step 32.

Then at step 33 it is determined whether the conditions OS for stop-start are met. One of these conditions may be whether the vehicle 5 is moving above a predetermined speed -12 -and so far as this invention is concerned it will also include the engagement state of the clutch 2.

Ignoring all of the other conditions that may need to be met, if the state of the clutch 2 is determined to he disengaged' then the conditions for SIG stop-start operation will be met and the method will advance to step 35 where the first mode of operation is selected but if the state of the clutch 2 is determined to be engaged' then the conditions for stop-start operation will not be met and the method will advance to step 34 where the second or normal mode of operation is selected.

After steps 34 arid 35 the method advances to step 36 to determine whether the ignition key 17 is still in the on position. If the key is still on then the method returns to step 33 but if the ignition key is determined to be in the off position the method ends at step 37.

Referring now to Fig.4 there is shown a high level flow chart of a method comprising a number of linked routines required to determine the engagement state of the clutch 2.

The method commences at step 31 when the ignition key 17 is moved to an on position then at step 100 it is determined whether the output from the slave cylinder position sensor 63 can be relied upon.

If it is determined that the slave cylinder position sensor 63 cannot be relied upon then the method advances to step 150 where a flag is set to zero. It will be appreciated that in practice the Flag may be set to Zero upon Key-On in order to provide a consistent start-up condition. The method then advances to step 500 where the state of the flag is communicated to the stop-start control system as an indication of whether the first or second modes of operation needs to be selected. In the example shown, a -13 -flag setting of zero will always result in the stop-start system selecting the second mode of operation. The method then advances to step 600 where it is determined whether the ignition key 17 is still in the on position and, if it is, the method returns to step 100 otherwise the method ends at 1000.

However, if it is determined at step 100 that the slave cylinder position sensor 63 can be relied upon then the o method advances from step 100 to step 200 where a current zero position for the slave piston 62 is determined.

The method then advances to step 300 where a displacement threshold (XTr]p-) from the slave piston zero is position required to ensure disengagement is determined.

Then at step 400 it is determined whether a measured displacement of the slave piston 62 exceeds the displacement threshold XTHPt,; and, if it does, then the flag is set to one otherwise the flag is set to zero.

The method then advances to step 500 where the state of the flag is communicated to the stop-start control system as an indication of whether the first or second modes of operation needs to be selected. In the example shown, a flag setting of zero will always result in the stop-start system selecting the second mode of operation and a flag setting of one will always result in the stop-start system selecting the first mode of operation. The method then advances to step 600 where it is determined whether the ignition key 17 is still in the on position and, if it is, the method returns to step 100 otherwise the method ends at 1000.

It will be appreciated that the opposite flag logic could be used or that some other form of indicator could be used for example the method could GO TO step 34 or step 35 -14 -of the method shown in Fig.3 depending upon whether the test at 400 has been passed or failed and step 150 could be a GO TO step 34' output.

Referring now specifically to Fig.5 there is shown in greater detail the method shown in step 100 on Fig.4.

In summary, the positions of the pistons 52, 62 of the master and slave cylinders 51, 61 of the hydraulic clutch release system are measured using the master arid slave cylinder position sensors 53 and 63, the output signals from these sensors being communicated to the electronic controller 16. The electronic controller 16 is operable to compare the master cylinder piston position as measured by is the master cylinder position sensor 53 with the slave cylinder piston position as measured by the slave cylinder position sensor 63 to provide verification or corroboration of the position of the slave cylinder piston 62.

If the slave cylinder piston 62 is confirmed to be where expected then it is assumed that the output signal from the slave cylinder position sensor 63 is a reliable indication of slave cylinder piston position.

It will be appreciated that upon initial set up of the vehicle 5 the outputs from the two position sensors 53, 63 are calibrated for absolute zero by moving the master and slave pistons 52, 62 to the end of their respective cylinders 51, 61 from which they are displaced when the clutch pedal 25 is pressed or a sensor setting arrangement is used to achieve these baseline values.

Referring back to Fig.5, in step 105 the position of the master cylinder piston 52 is measured using the master cylinder position sensor 53 and then a system temperature is determined in step 115. The system temperature may be obtained from one or more temperature sensors located at -15 -various locations in the clutch actuation system or may be obtained from one or both of the master and slave cylinder position sensors 53 and 63 if a temperature signal is obtainable from the temperature compensation circuits associated with these sensors 53, 63.

Then at step 120 a predicted s]ave piston position is determined by the electronic controller 16 from the signal received from master cylinder position sensor 53.

Two methods of performing this prediction are possible.

In a first option, the master cylinder piston position is used as the input to a polynomial or set of polynomials forming a spline, a discrete filter or a discrete transfer function and the output from the polynomial, spline, filter or transfer function is used as an estimate or prediction of the slave cylinder piston position.

In a second option the master cylinder piston position is used as the input to two look-up tables. The first of these look-up tables generates a value for the corresponding maximum expected slave cylinder piston position and the second of these look-up tables generates a value for the corresponding minimum expected slave cylinder piston position.

Then at step 125 the prediction of slave piston position is corrected for temperature. This is desirable because various factors influence the relationship between the position of the master cylinder piston 52 and the position of the slave cylinder piston 62 but by far the most significant of these factors is temperature which causes expansion and contraction of the hydraulic fluid used to transfer motion and force from the master cylinder 51 to the slave cylinder 61 and expansion/contraction of the pipes used to connect the master and slave cylinders 51 and 61.

-16 -Expansion of these pipes requires extra fluid to fill them which is referred to as the volume loss effect' and is the most significant cause of differences between the master and slave cylinder piston positions.

Because the transfer function or relationship used to produce a prediction of the position of the slave cylinder piston 62 must contain the full range of possible noise factors and the temperature range experienced during use of () the vehicle 5 is likely to he broad, to be robust to the full temperature range requires a broad tolerance band to be applied to prevent the test being failed when in fact the slave cylinder position sensor 63 is working normally. The danger with the use of a broad tolerance band is that a is problem not related to temperature is not detected because it is too small relative to the tolerance band required to account for temperature variations.

Therefore, by including a temperature sensor or temperature model to provide control algorithms with temperature information, the temperature factor can be effectively eliminated thereby allowing a smaller tolerance band to be used to account for other noise factors and thereby increasing the sensitivity of the system to genuine errors in operation of the slave cylinder position sensor 63.

It will be appreciated that in practice steps 120 and may be combined that is to say the transfer function or relationship used to predict the position of the slave cylinder piston 62 will include temperature compensation but are shown separately because it would be possible although not desirable to eliminate steps 115 and 125 and use large tolerance bands to account for temperature variations.

Referring back now to Fig.5 at step 130 the predicted position (P11)of the slave cylinder piston 62 is compared -17 -with the measured position derived from the slave cylinder position sensor 63 in step 110 and at step 135 it is determined as to whether the measured position (P1)is within predetermined upper and lower tolerance limits.

For example if the predicted posiLion is 15mm and the tolerance limits are + or -0.05mm then the comparison at step 130 would he in the form of:-Is P1 i_r iriit < Prd < Pl2r_cI upQr mint or applying the prediction and limits given above:-Is 14.95 < < 15.05 ? If the answer to the test is YES then the method advances to step 140 and, if the answer is NO the method advances to step 145.

It will be appreciated that, in practice, this comparison may be a comparison of digital data or voltage levels and not actual dimensions.

If the method has advanced to step 140 then this indicates that the slave cylinder position sensor 63 has been verified and can be used and so the method then returns to the main method at step 200. Conversely if the method has advanced to step 145 then this indicates that the verification process has been failed and although not shown an error flag could be set. The method then returns to the main method at step 150 indicating that the engine must be operated in the second or normal mode of operation because the output from the slave cylinder position sensor 63 cannot be trusted. :35

-18 -The comparisons described at steps 130 and 135 may be performed on a continuous or repetitive basis after key-on or only when a set of entry conditions are satisfied.

One example of an entry condition is when the master cylinder piston 52 is within a specified part of its range, such as, for example, near the engaged or disengaged end-stops.

Other examples of entry conditions are when the velocity of the master cylinder piston 52 is below a specified threshold or when the velocity of the slave cylinder piston 62 is below a specified threshold.

As the clutch 2 is moved between the fully engaged and fully disengaged positions the piston 62 of the slave cylinder 61 travels through a small distance, typically in the region of 8mm. However, the slave cylinder 61 has a much larger range of possible travel, typically in the region of 24mm and the smaller 8mm range of movement will move within this larger range throughout the life of the clutch as the clutch 2 wears or is replaced (see Figure 10) The effect of clutch wear is to move the resting or zero position of the slave piston 62 when the clutch 2 is fully engaged to the left as viewed on Fig.10 and this movement of zero position needs to be compensated for if measurement errors are to he avoided.

A method is required to determine the minimum position of the resting position of slave cylinder piston 62 thereby removing the effect of this movement of the smaller range within the larger range if the output from the slave cylinder position sensor 63 is to be used to effectively provide an indication of the engagement state of the clutch 2. Fig.6 shows a first embodiment 200a of a method 200 according to this invention for providing such a minimum -19 -zero position thereby taking out or compensating for the effects of clutch wear.

The method 200a starts at step 31 which is when the ignition switch 17 is moved to an on position, the first action taken by the method is to set a current zero offset value Z equal to a maximum zero offset value M. The maximum zero offset value is set to a value equal to or greater than the absolute slave cylinder piston position as measured at the furthest possible point of travel of the slave cylinder piston 62 in the disengaged direction (which, as indicated on Fig.1O, is in this case 24mm) . This has the effect of indicating, at key-on before is the clutch 2 is operated, that the clutch 2 is engaged irrespective of its actual state. This is the preferred condition for safety with a stop-start system as it prevents an unsafe engine start.

The position of the slave cylinder piston 62 is continuously monitored by the slave cylinder position sensor 63 and the minimum zero offset value Z of the slave cylinder piston 62 is stored as indicated at step 210.

It will be appreciated by those skilled in the art that, although it would be possible in step 210 to determine the fully engaged position and then measure the position of the slave cylinder piston 62, in practice the position of the piston 62 is continuously measured arid the minimum :30 displacement position of the piston 62 is used as the engaged position.

Then at step 215 the new measurement of zero offset ZNW is compared with the value of zero offset currently stored in the electronic controller 16.

-20 -If the new value of zero offset Z1 is less then the currently stored value of zero offset Z then the method advances to step 220 otherwise the method advances to step 225.

In step 220 the value of zero offset Z is set to be equal to the now value of zero offset value Z1 arid the method advances to step 230 whereas in step 225 the value of zero offset remains unchanged and so Z is set to equal the 00 existing value of Z and the method then advances to step 230.

At step 230 it is determined whether the ignition switch 17 is still in the on position and, if it is, then i the method reverts to step 210 but if the ignition switch 17 is now off the method ends at step 240.

This method ensures that as the clutch 2 wears the zero offset value Z is adjusted to maintain the true zero value equal to the position of the slave cylinder piston 62 when the clutch 2 is fully engaged and there is no displacement of the clutch pedal 25. If this method is not used then the output from the slave cylinder position sensor 63 would have an increasing error as the clutch 2 wears and would indicate that the slave cylinder piston 62 has riot moved as far as it actually has. This is a problem when the output from the slave cylinder position sensor 63 is used to control other vehicle features such as stop-start based upon clutch engagement state because the position of the slave cylinder piston 62 must be determined to a high degree of accuracy (less than 0.1 mm) in order for the engagement state of the clutch 2 to be accurately and reliably determined.

It will be appreciated that the loop 210, 215, 220, 230, 210 or 210, 215, 225, 230, 210 will be repetitively cycled through so long as the ignition switch 17 remains in the on position.

-21 -Referring now to Fig.7 there is a second embodiment of a method 200 according to this iriventJon for determining a zero offset value for the slave cylinder piston 62 which is identical to that previously described so far as steps 31 to 230 are concerned and so will not he described again in detail regarding these steps.

This second embodiment method 200b is used when the ic release bearing 6 or the clutch 2 includes an auto wear compensation feature to counteract the effect of clutch wear. Such a device operates after a number of clutch operations either when the clutch 2 is in the process of engaging to compensate for the effects of clutch driven plate 7 wear and so the effect, as shown in Fig.10, is to move the zero offset position Z away from the fully engaged position by a predetermined amount such as 0.1mm.

Referring back to Fig.7 from step 230 if the ignition switch 17 is off the method ends at step 240 but if the ignition switch 17 is still on the method advances to step 250 where it is determined whether the clutch 2 has been operated since the last process cycle.

If the clutch has not been operated that is to say it has remained engaged or fully disengaged then the method advances to step 210 but if there has been a clutch operation, that is to say, disengaged and then subsequently engaged then the method advances to step 260 where a small value S is added to the currently stored value of zero offset Z. The method then continues to step 210.

The effect of the perturbation or increment S can best be seen with reference to Figs.11. and 12.

Referring first to Fig.1l the tabulated outputs from the various steps of the method 200b can be seen using -22 -hypothetical vales of M=25mm, Z1 = 8.0mm and S = 0.2mm. It will be appreciated that actual dimensions may not be used to perform this method it may be performed using digital data or a value such as voltage but for the purposes of explanation actual measurement values will be used.

For the upper half of the table the effect of the increment S on the value of zero offset Z can be seen to he negated by the ratchet down algorithm expressed in steps 215 to 230. That is to say Z remains at 8.0 for this range of iterations which is insufficient for any wear to have taken place and for which no self adjustment has taken place by the auto wear compensation mechanism associated with the release bearing 6 or the clutch 2. It will be appreciated that when there is clutch wear the value Z1 will decrease and so at step 215 the result would be YES and the ZNW would be set as the new Z value eq if the new fully engaged position of the slave piston 62 is measured at step 210 as 7.95mm and the current value of Z is 8.0mm then the test at 215 is passed and Z will be set to 7.95mm.

The lower half of the table shows the situation when self adjustment has taken place by the auto wear compensation mechanism associated with the release bearing 6 or the clutch 2. In this case an adjustment of 0.1mm has taken place. The effect of this is that the test at step 215 will be passed because 8.1mm is less than 8.2mm and then at step 220 the zero offset value will be updated to 8.1mm.

The effect of the auto wear adjustment has therefore been compensated for by the method 200b shown in Fig.7.

Referring now to Fig.12 there is shown the situation if the method 200a shown in Fig.6 is used with a clutch 2 or release bearing 6 having wear compensation. :35

-23 -The upper half of the table is identical to that shown in Fig.11 but the Jower half of the table is different because no step increment S is applied with this method.

Therefore after auto adjustment by the clutch 2 or release bearing 6 of 0.1mm as before the method enters the step 215 with a Z1 value of 8.1mm and, because the Z value has not been incremented up by the value S it remains at 8.0mm, the test at 215 is failed and so the zero offset Z remains at 8.0mm and irrespective of how many auto adjustments take place this will be the case because there is no way for the zero offset Z to be increased only for it to be decreased or kept the same by the algorithm. The effect of this is to produce an increasing error every time auto adjustment by the clutch 2 or release bearing 6 takes place and the slave cylinder position sensor 63 will erroneously indicate that the slave piston 62 is closer to the disengaged position than it really is and so there is a risk that the engagement state of the clutch 2 will be determined to be disengaged when in fact it remains engaged.

It will he appreciated that this error will only be corrected when the next key-on cycle is commenced.

In summary, the ratchet algorithm works as follows, if an absolute slave cylinder piston position is detected which is further in the engaged direction than the currently maintained offset value then the ratchet algorithm replaces the currently maintained zero offset value with the newly measured value. Thus the currently maintained value is always the most engaged position detected in this key-cycle.

The zero offset value maintained by the ratchet can be used as a zero offset' to calculate the relative position of the slave cylinder piston 62 by subtracting the zero offset Z from the absolute slave cylinder piston position as is described in more detail below.

-24 -In some circumstances it may be preferable to prevent the ratchet algorithm from operating. For example, at high engine speeds, distortion of clutch components such as the diaphragm spring may result in slave cylinder piston positions which give an erroneous indication of the fully engaged position of the slave piston 62. Under these or other circumstances, errors may be prevented by freezingt the ratchet algorithm. When frozen the ratchet mechanism does not update its currently maintained value and so for example using the values given above, if an erroneous value of Z = 7.5mm is measured it will have no effect because the zero offset Z will be frozen at 8.0mm.

As discussed above, each time the clutch 2 is disengaged and then engaged the zero offset Z is perturbed or incremented to accommodate any movement of the slave cylinder piston caused by the auto or self adjusting mechanism of the clutch 2. The perturbation is applied by adding a small amount S, typically in the range of 0.1mm and 0.2mm to the zero offset Z, which has the effect of moving the zero point of the relative range of the slave cylinder piston 62 in the disengaged direction with respect to the absolute range. The perturbation is triggered each time the clutch 2 is disengaged and then subsequently engaged but the auto adjusting mechanism does not make an adjustment every time the clutch 2 is disengaged and then engaged and, typically, many kilometers of vehicle travel may elapse between adjustments. However, perturbations made on occasions when no auto adjustment is made are quickly removed by the ratchet algorithm. The magnitude of the perturbations or increment must be selected to be slightly larger than the adjustments made by the auto adjustment mechanism so that adjustments are accommodated within a single perturbation.

In order to provide a cost effective and reliable method for determining the engagement state of the clutch 2 -25 -the inventors have realised that the displacement of the slave cylinder piston 62 from its fully engaged position can be used to provide a value indicative of whether the clutch 2 is engaged or disengaged. The term engaged or disengaged meaning in this context whether or riot a predetermined magnitude of torque is being transmitted by the clutch 2.

The solution presented here determines whether the clutch 2 is in a disengaged state based on the output from in the slave cylinder position sensor 63 which measures the linear position of the slave cylinder piston 62. The method proposed here is to indicate that the clutch 2 is disengaged when the sensed position of the slave cylinder piston 62 moves past a threshold along its travel. This threshold is must be calibrated such that all of the tolerances in the clutch actuation system 50 and the clutch 2 are accounted for and these tolerances include; manufacturing piece to piece variations, assembly variations, wear, environmental conditions such as temperature and sensor accuracy. This ensures that a single clutch disengaged threshold may be calibrated per vehicle line thus avoiding the need for clutch disengagement/ engagement point learning.

Of these variables the most significant is temperature because not only will variations in temperature affect the physical size of components it affects the frictional properties of the clutch 2. A method is therefore shown in Fig.8 which compensates for temperature induced errors thereby improving the accuracy to which the desired displacement threshold can be calculated.

The method 300 commences at step 31 which is a key-on event, the next step is to measure the temperature of the clutch actuation system 50 at one or more key positions.

This may be done by the use of a number of dedicated temperature sensors or maybe achieved by using an output from a temperature compensation circuit of the slave -26 -cylinder position sensor 63. Irrespective of the technique employed this temperature value is used in step 320 to determine a temperature compensated value of displacement threshold XTH: that is used to determine the engagement state of the clutch 2.

The techniques employed n step 320 are manifold hut may include using a model of the clutch actuation system t.o determine the displacement of the slave cylinder piston 62 C) required to ensure that the clutch 2 is disengaged; experimental data stored in one or more look up tables that can be used to determine the displacement of the slave cylinder piston 62 required to ensure that the clutch 2 is disengaged; and estimating the temperature of the clutch 2 and or clutch actuation system based on ambient temperature combined with other sensor signals or information contained within the electronic controller 16 such as engine torque, engine speed, vehicle speed etc. It will be appreciated that step 320 includes at least one algorithm to correlate the position of the slave cylinder piston 62 with the engagement state of the clutch 2 and that this algorithm or algorithms is/ are modified to take into account the measured temperature input from step 310.

Then at step 330 the value of temperature compensated displacement threshold XTHR is stored in the electronic controller 16 for future use. The method 300 then ends at :30 step 340.

Referring now to Figs.4 and 9 there is disclosed a method 400 for determining the engagement state of the clutch 2.

The method 400 commences at step 31 which is a key-on event, then in step 410 the measured displacement (X:,) of -27 -the slave cylinder piston 62 is measured from its absolute zero position using the slave cylinder position sensor 63.

Then in step 420 an actual slave cylinder piston 62 displacement is calculated by subtracting the zero offset Z generated using one of the methods 200a and 20Gb described above with reference to Figs.6 and 7 from the measured value of displacement.

1)) That is to say X7T =X/\-Z where: -XTT = the actual displacement of the slave cylinder piston; = the displacement measured by the slave cylinder position sensor; and Z = zero offset.

The method then advances to step 430 where the actual displacement XT of the slave cylinder piston 62 is compared with the displacement threshold as determined from the method 300 described above and shown in Fig.8.

That is to say the test Is X7(T > XTflpT; is used to determine whether the clutch 2 is engaged or disengaged.

It will be appreciated that in order to account for hysteresis effects there may he two threshold values one for each direction of sensor signal. That is to say, if the signal is increasing one threshold would be used and if the sensor signal is reducing a second threshold would be used.

If the test is passed then the method advances to step 450 and the flag is set to one (1) indicating that the clutch 2 is disengaged but if the test at step 430 is failed -28 -then at step 460 the flag is set to zero (0) indicating that the clutch 2 has been determined to he engaged.

Fig.lO shows the situation when XAT is greater than XTpE; that is to say the actual displacement of the slave cylinder piston 62 is greater than the threshold displacement and the method would therefore determine that the clutch 2 is disengaged.

After steps 450 and 460 the method advances to step 470 and control returns to the main operating routine at step 500 shown in Fig.4 The above-described methods are illustrative examples and the steps therein may, when appropriate, be performed sequentially, synchronously, simultaneously or in a different order depending upon the application.

It will be appreciated by those skilled in the art that although the invention has been described by way of example with reference to one or more embodiments it is not limited to the disclosed embodiments and that one or more modifications to the disclosed embodiments or alternative embodiments could be constructed without departing from the scope of the invention as set out in the appended claims.

Claims

-29 -Claims 1. A method for determining the position of a slave cylinder piston operatively connected to a clutch, the method comprising measuring the position of the slave cylinder piston, comparing the current measurement of slave cylinder piston position with a stored value from a previous measurement, setting the stored value of position to the current measured value if the measured value is less than :0 the stored value and retaining the stored value if the measured value is not less than the stored value.
2. A method as claimed in claim 1 wherein, upon initiation of the method, the stored value is set to a value greater than any possible stored value obtained by measurement of slave cylinder piston position.
3. A method as claimed in claim 1 or in claim 2 wherein the method further comprises adding a predetermined amount to the stored value every time the clutch is operated and comparing the incremented stored value with the current measurement of slave cylinder position in order to compensate for auto adjustment of the clutch.
4. A method as claimed in claim 3 wherein the predetermined amount is larger than the amount the piston position will be changed by auto adjustment of the clutch.
5. A method as claimed in any of claims 1 to 4 o wherein the method further comprises continuously measuring the position of the slave cylinder position and subtracting the stored value from the measured position of the slave piston to produce a value of actual slave piston displacement.
6. A clutch system comprising a clutch, a hydraulic clutch actuation system and an electronic controller wherein -30 -the hydraulic clutch actuation system includes a slave cylinder having a slave cylinder piston, a mechanical linkage connecting the slave cylinder piston to the clutch, a position sensor to measure the position of the slave cylinder piston and supply a signal indicative of the measurement to the electronic controller wherein the electronic controller is operable to measure the position of the slave cylinder piston, compare the current measurement of slave cylinder piston position with a stored value from a o previous measurement, set the stored value of resting position to the current measured value if the measured value is less than the stored value and retain the stored value if the measured value is not less than the stored value.
7. A system as claimed in claim 6 wherein, upon initiation of the electronic controller, the electronic controller is operable to set the stored value to a value greater than any possible stored value obtained by measurement of slave cylinder piston position.
8. A system as claimed in claim 6 or in claim 7 wherein the electronic controller is further operable to add a predetermined amount to the stored value every time the clutch is operated and compare the incremented stored value with the current measurement of slave cylinder position in order to compensate for auto adjustment of the clutch.
9. A system as claimed in claim 8 wherein the predetermined amount is larger than the amount the piston O position will he changed by auto adjustment of the clutch.
10. A system as claimed in any of claims 6 to 9 wherein the electronic controller is further operable to continuously measure the position of the slave cylinder position and subtract the stored value from the measured position of the slave piston to produce a value of actual slave piston displacement.

-31 -
11. A system as claimed in claim 10 wherein the electronic controller is further operable to use the value of actual slave piston displacement to control one or more systems.
12. A method for determining the position of a slave cylinder piston operatively connected to a clutch substantially as described herein with reference to the accompanying drawing.
13. A clutch system substantially as described herein with reference to the accompanying drawing.