GB2458502A

GB2458502A - A Method and Apparatus for Determining the Engagement State of a Clutch

Info

Publication number: GB2458502A
Application number: GB0805223A
Authority: GB
Inventors: Ian Halleron; Themi Petridis; Andrew Anderson
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: CN101539179A; GB0805223D0; DE102009012573A1; CN101539179B; GB2458502B

Abstract

A method and apparatus for determining the engagement state of a clutch 2 is disclosed in which the position of a slave cylinder piston 62 is measured by a sensor 63 and compared with a calculated displacement threshold value XTHRES. If the measured displacement of the slave cylinder piston 62 is less than the displacement threshold value then this is used as an indication that the clutch 2 is engaged and if the measured displacement of the slave cylinder piston 62 is greater than the displacement threshold value XTHRES then this is used as an indication that the clutch 2 is disengaged. This determination of clutch engagement state can be advantageously used in a stop-start system to permit the engine 10 to be started while a transmission 11 is in gear and it can also be used to prevent engine shutdown if the clutch 2 is not fully disengaged.

Description

A Method and apparatus for Determining the Engagement State of a Clutch This invention relates to motor vehicles and in particular to an apparatus and method for determining the engagement state of a clutch selectively supplying drive to a manual transmission.

For manual transmission vehicles that are fitted with automated Stop/ Start control of the engine often referred 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 if 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 is 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 not 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 driveline 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 drivelirie must be disengaged.

However, due to the complexity of the clutch and clutch actuation system, ensuring that the clutch is disengaged is not straightforward and currently there are no robust and cost effective systems available to achieve this.

It is an object of this invention to provide an apparatus and method for determining the engagement state of a clutch in a cost effective manner thereby facilitating the use of a Stop-in-Gear strategy.

According to a first aspect of the invention there is provided a method for determining an engagement state of a clutch actuated by a hydraulic actuator having a moveable piston the method comprising measuring the position of the piston, comparing the measured position to a threshold value and using the result of the comparison as an indication of engagement state.

If the measured position is greater than the threshold value, this may be used as an indication that the clutch is in a disengaged state.

If the measured position is one of the same as the threshold value and less than the threshold value, this may be used as an indication that the clutch is in an engaged state.

The threshold value may be a displacement threshold value arid the method may further comprise determining a fully engaged position of the piston and a measurement of the displacement of the piston from the fully engaged position and comparing the measured piston displacement with the displacement threshold value.

The method may further comprise measuring a temperature and varying the threshold value based upon the measured temperature.

The threshold value may be obtained by calculation.

The threshold value may he obtained by the use of one is or more experimentally derived look-up tables.

According to a second aspect of the invention there is provided an apparatus for determining an engagement state of a clutch actuated by a hydraulic actuator having a moveable piston wherein the apparatus comprises a sensor to measure the position of the piston and provide a signal indicative of the measured position and an electronic controller arranged to receive the signal from the sensor, compare the measured position with a threshold value and use the result of the comparison as an indication of the engagement state of the clutch.

If the measured position is greater than the threshold value, the electronic controller may be operable to use this as an indication that the clutch is in a disengaged state.

If the measured position is one of the same as the threshold value and less than the threshold value, the electronic controller may be operable to use this as an :35 indication that the clutch is in an engaged state.

The threshold value may he a displacement threshold value and the electronic controller may be further operable to determine a fully engaged position of the piston and a measurement of the displacement of the piston from the fully engaged position and compare the measured piston displacement with the displacement threshold value.

The apparatus may further comprises at least one temperature sensor measuring a temperature and the electronic controller may he operable to vary the threshold value based upon the measured temperature.

The threshold value may be obtained by calculation.

The threshold value may be obtained by the use of one or more experimentally derived look-up tables stored in the electronic controller.

The electronic controller may be further operable to use the determination of engagement state to control the operation of one or more systems.

The hydraulic actuator may he a slave cylinder forming part of a clutch hydraulic actuation system.

The invention will now be described by way of example with reference to the accompanying drawing of which:- Fig.l is a schematic representation of a micro-oc 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.l; 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 s 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 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 of a method 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.lO 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.ll 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.1 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 moLor vehicle 5 using a clutch pedal 25.

The transmission 11 has a gear selector (not shown) that is moveable between several positions including at 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.

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 be 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 (icsy-os) and when the ignition switch 17 is in an off' position (KEY-OFF) the engine 10 is not able to run. The ignitJon switch 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 1] 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 but 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 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 be appreciated that a displacement of the clutch pedal in the direction of the arrow clutch pedal travel' on Fig.2 will produce corresponding respective :35 displacements Ditr 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 t is merely necessary to know at this stage that movement of the release bearing 6 in the direction of the arrow D'JL, by the slave cylinder pIston 62 is movement in a clutch disengaging direction and 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 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 drivelirie, 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 () 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 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 used by the electronic controller 16 to control the operation and in particular the automatic stopping and starting of the engine 10. It will also be appreciated that the electronic controller 16 may be formed of several discrete electronic control units which communicate with one anoLher to achieve the required functionality.

During normal engine running, the electronic controller 16 is operable to control the fuel supplied to the engine 10 and 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 arid, 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 be 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.

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 2() 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 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 be disengaged' then the conditions for SIG stop-start operation wi1 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 and 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 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 (XTqr) from the slave piston zero 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 XTHP 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 and 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 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 :35 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 S associated with these sensors 53, 63.

Then at step 120 a predicted slave 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 arid 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 ic the vehicle 5 is likely to be 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 he 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 F'ig.5 at step 130 the predicted position (P1rcj)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 (Pr)I5 within predetermined upper and lower tolerance limits.

For example if the predicted position is 15mm and the tolerance limits are + or -0.05mm then the comparison at step 130 would be in the form of:-Is P1- < P1 < P11 ? or applying the prediction and limits given above:-Is 14.95 < P1 < 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 wil.1 be appreciated that, in practice, this comparison may he 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.

-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.

iO 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.

is 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 F'ig.lO 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 for -19 -providing such a minimum 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.lO, is in this case 24mm) . This has the effect of indicating, at key-on before 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 and the minimum displacement position of the piston 62 is used as the engaged position.

Then at step 215 the new measurement of zero offset ZNFW 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 new value of zero offset value ZNW and 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 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 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 not 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 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 be described again in detail regarding these steps.

This second embodiment method 200b is used when the iO 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 is 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 t..hen continues to step 210.

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

Referring first to Fig.11 the tabulated outputs from the various steps of the method 200b can be seen using -22 -hypothetical vales of M=25mm, ZNW = 8.0mm and S = 0.2mm. It will he 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 Lhe purposes of S 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 be 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 ZNW will decrease and so at step 215 the result would be YES and the Z1 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.

-23 -The upper half of the table is identical to that shown in Fig.1l but the lower 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 be appreciated that this error will only be corrected when the next key-on cycle is comnienced.

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 freezing' the ratchet algorithm. When frozen the ratchet mechanism does riot update its currently maintained value and so for iO 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 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 engaqed or disengaged meaning in this context whether or not 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 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 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 o 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 he 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 XTHP: that is used to determine the engagement state of the clutch 2.

The techniques employed in step 320 are manifold but may include using a model of the clutch actuation system to determine the displacement of the slave cylinder piston 62 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 s 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 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 (X7) 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 arid 200b described above with reference to Figs.6 and 7 from the measured value of displacement.

That is to say XCT =X-Z where: -XACT = 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 XIVT 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 XJ(:T > XTflr 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 be engaged.

Fig.10 shows the situation when XT is greater than XTHP 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 an engagement state of a clutch actuated by a hydraulic actuator having a moveable piston the method comprising measuring the position of the piston, comparing the measured position to a threshold value and using the result of the comparison as an indication of engagement state.
2. A method as claimed in claim 1 wherein, if the measured position is greater than the threshold value, using this as an indication that the clutch is in a disengaged state.
3. A method as claimed in claim 1 or in claim 2 wherein if the measured position is one of the same as the threshold value and less than the threshold value, using this as an indication that the clutch is in an engaged state. 2 ()
4. A method as claimed in any of claims 1 to 3 wherein the threshold value is a displacement threshold value and the method further comprises determining a fully engaged position of the piston and a measurement of the displacement of the piston from the fully engaged position and comparing the measured piston displacement with the displacement threshold value.
5. A method as claimed in any of claims 1 to 4 wherein the method further comprises measuring a temperature and varying the threshold value based upon the measured temperature.
6. A method as claimed in any of claims 1 to 5 : wherein the threshold value is obtained by calculation.

-30 -
7. A method as claimed in any of claims 1 to 5 wherein the threshold value is obtained by the use of one or more experimentally derived look-up tables.
8. An apparatus for determining an engagement state of a clutch actuated by a hydraulic actuator having a moveable piston wherein the apparatus comprises a sensor to measure the position of the piston and provide a signal indicative of the measured position and an electronic c controller arranged to receive the signal from the sensor, compare the measured position with a threshold value and use the result of the comparison as an indication of the engagement state of the clutch.
9. An apparatus as claimed in claim 8 wherein, if the measured position is greater than the threshold value, the electronic controller is operable to use this as an indication that the clutch is in a disengaged state.
10. An apparatus as claimed in claim 8 or in claim 9 wherein, if the measured position is one of the same as the threshold value and less than the threshold value, the electronic controller is operable to use this as an indication that the clutch is in an engaged state.
11. An apparatus as claimed in any of claims 8 to 10 wherein the threshold value is a displacement threshold value and the electronic controller is further operable to determine a fully engaged position of the piston and a measurement of the displacement of the piston from the fully engaged position and compare the measured piston displacement with the displacement threshold value.
12. An apparatus as claimed in any of claims 8 to 11 wherein the apparatus further comprises at least one temperature sensor measuring a temperature and the -31 -electronic controller is operable to vary the threshold value based upon the measured temperature.
13. An apparatus as claimed in any of claims 8 to 12 wherein the threshold value is obtained by calculation.
14. An apparatus as claimed n any of claims 8 to 12 wherein the threshold value is obtained by the use of one or more experimentally derived look-up tables stored in the electronic controller.
15. An apparatus as claimed in any of claims 8 to 14 wherein the electronic controller is further operable to use the determination of engagement state to control the operation of one or more systems.
16. An apparatus as claimed in any of claims 8 to 15 wherein the hydraulic actuator is a slave cylinder forming part of a clutch hydraulic actuation system.
17. A method for determining an engagement state of a clutch actuated by a hydraulic actuator having a moveable piston substantially as described herein with reference to the accompanying drawing.
18. An apparatus for determining an engagement state of a clutch actuated by a hydraulic actuator having a moveable piston substantially as described herein with reference to the accompanying drawing.