Method of gear-shifting in a servo-controlled manual gearbox

A method of gear shifting in a servo-controlled gearbox. The method generates an oscillation on angular velocities of primary and secondary shafts of the gearbox, separates the primary shaft from the secondary shaft thus disengaging a first current gear when the oscillation has taken the angular velocity of the primary shaft close to the angular velocity that the primary shaft must assume to engage a second next gear, and connects the primary shaft to the secondary shaft thus engaging the second next gear when the oscillation has led the angular velocity of the secondary shaft to equalize the current angular velocity of the primary shaft multiplied by the transmission ratio of the second next gear.

TECHNICAL FIELD

The present invention relates to method and unit for gear-shifting in a servo-controlled gearbox.

BACKGROUND ART

There is an increasing use of servo-controlled gearboxes, which are structurally similar to manual gearboxes of the traditional type, except for the fact that the clutch pedal and the gear selection lever operated by the user are replaced by corresponding electrical or hydraulic servo-controls. When using a servo-controlled gearbox, the user only needs to send the order to shift up or down to a transmission control unit and the transmission control unit autonomously shifts by acting both on the engine and on the servo-controls associated to clutch and gearbox.

The gear shifting order may be generated either manually, i.e. following a command imparted by the driver, or automatically, i.e. regardless of the action of the driver. When the gear shifting order is generated, the transmission control unit drives the servo-control associated to the clutch to open the clutch so as to mechanically separate a primary shaft of the gearbox from a crankshaft; at the same time, the transmission control unit acts on the engine control unit to temporarily reduce the motive torque output by the engine itself.

Once the transmission control unit has checked opening of the clutch, the transmission control unit drives the servo-controls associated to the clutch to disengage the currently engaged gear; when the transmission control unit has checked gear disengagement, the transmission control unit drives the servo-controls associated to the gearbox to shift the primary shaft with respect to a secondary shaft so as to arrange engagement of the new gear. Once the transmission control unit has checked that the primary shaft has reached the required position with respect to the secondary shaft, the transmission control unit drives the servo-controls associated to the gearbox to engage the new gear.

Finally, when the transmission control unit has checked that the new gear has been engaged, the transmission control unit drives the servo-control associated to the clutch to close the clutch so as to make the primary shaft of the gearbox and the crankshaft reciprocally and angularly integral; at the same time, the transmission control unit acts on the engine control unit to restore the motive torque of the engine itself.

In normal driving conditions, the servo-controlled gearbox is required to rapidly shift gears without triggering abrupt longitudinal acceleration/deceleration on the vehicle, which are annoying for vehicle passengers and subject the vehicle transmission to unnecessary mechanical strain. In sporty driving conditions, the servo-controlled gearbox is required to shift gears as fast as possible without subjecting the transmission to excessive mechanical strain which could lead to damaging the transmission itself.

The currently marketed servo-assisted gearboxes operating according to the above-described method require a time generally from 250 to 600 ms to shift a gear; the time actually employed depends both on the dynamic performance of the gearbox components and on the required level of comfort. A 250 ms gear shift is already very fast; however, it is still relatively slow for sporty driving, especially for competitive track racing.

It is important to observe that the gear must be shifted, compatibly with requirements of comfort and mechanical protection, as rapidly as possible during gear progression, i.e. when shifting from a lower gear to a higher gear, because during gear progression the engine is ‘driving’ to accelerate the vehicle and consequently must be separated from the drive wheels for the shortest possible time; instead, when shifting down, i.e. when shifting from a higher gear to a lower gear, the gear shift may also be slower, because vehicle deceleration is essentially performed by the braking system and the engine does not have an essential role in vehicle dynamics.

It is known that gear shifting time is mainly determined by the new gear synchronisation time, i.e. by the time employed by the synchronisers to adapt the angular velocity of the primary shaft of the gearbox to the angular velocity determined by the new ratio.

In order to reduce the synchronisation time during gear progression, i.e. when shifting from a lower gear to a higher gear, the use of a braking device coupled to the primary shaft of the gearbox has been proposed so as to brake the primary shaft itself and rapidly adapt the angular velocity of the primary shaft to the angular velocity determined by the new ratio. However, this solution is relatively costly and complicated due to the need of arranging and controlling a brake coupled to the primary gearbox shaft.

Furthermore, as described in patent application EP1201483A2, in order to reduce the synchronisation time during gear progression, a method of disengaging gears in a servo-controlled gearbox has been proposed according to which an oscillation is generated on the angular velocity of a primary shaft of the gearbox by abruptly opening the respective clutch, and the gear is disengaged about the maximum amplitude of a first oscillation half-wave, when the oscillation itself has taken the angular velocity of the primary shaft close to the angular velocity that the primary shaft must assume to engage the next gear. However, also the method proposed by patent application EP1201483, while reducing the required shifting time, does not allow to reach the extremely short shifting times required by sporty driving.

DISCLOSURE OF THE INVENTION

It is the object of the present invention to provide a method of gear shifting in a servo-controlled gearbox, which is easy and cost-effective to implement and which, at the same time, is free from the above-described drawbacks and allows for rapid gear shifting.

According to the present invention, a method of gear shifting in a servo-controlled gearbox is provided as recited in the attached claims.

BEST MODE FOR CARRYING OUT THE INVENTION

InFIG. 1, number1indicates a motor vehicle provided with two front wheels2and two rear drive wheels3; an internal combustion engine4, which produces a motive torque which is transmitted to rear drive wheels3by means of a transmission5, is arranged in frontal position. Transmission5comprises a servo-assisted clutch6, which is connected to a bell integral with engine4and adapted to connect crankshaft7of engine4to a propeller shaft8ending in a servo-assisted gearbox9arranged on the rear axle. A self-locking differential10is arranged in cascade to servo-assisted gearbox9, from which differential a pair of drive axles11depart, each of which is integral with a rear drive wheel3. Motor vehicle1comprises a control unit12of engine4, a control unit13of transmission5, and a BUS line14, which is implemented according to the CAN (Car Area Network) protocol and spanned throughout the entire motor vehicle1.

Both control unit12of engine4, and control unit13of transmission5are connected to BUS line14and may therefore communicate with each other by means of messages sent over the BUS line14itself. Furthermore, control unit12of engine4and control unit13of transmission5are directly connected to each other by means of a dedicated electrical synchronisation wire15, which is capable of directly transmitting a binary type signal without the delays introduced by BUS line14from control unit13of transmission5to control unit12of engine4. As shown inFIG. 2, servo-assisted gearbox9comprises a primary shaft16, which revolves at an angular velocity ω1, and a secondary shaft17, which revolves at an angular velocity ω2and transmits motion to the rear drive wheels3by means of differential10and the pair of drive axles11. Servo-assisted gearbox9is operated by a gear engagement/disengagement servo-control18and by a gear selection servo-control19; servo-control18and servo-control19may be either of the electrical type or of the hydraulic type and are driven by control unit13of transmission5.

By interposition of servo-assisted clutch6, primary shaft16is connected to crankshaft7, which is brought into rotation by engine4and rotates at an angular velocity ωm. Servo-assisted clutch6is actuated by a servo-control20, which is preferably of the hydraulic type and is driven by control unit13of transmission5.

In the event of manual control of the transmission, the driver of motor vehicle1will send the gear shift command to control unit13of transmission5in a known way. When shifting from a current gear A to a next gear B, control unit13controls the performance of a series of operations in sequence, each of which must be completed before the next operation can be completed. Generally, the series of operations to be performed to shift from a current gear A to a next gear B comprises:cutting off motive torque output by engine4;opening servo-assisted clutch6by driving servo-control20;disengaging current gear A by driving servo-control18;selecting next gear B by driving servo-control19;engaging next gear B by driving servo-control18;closing servo-assisted clutch6by driving servo-control20; andrestoring motive torque output by engine4.

At the beginning of the gear shift and when servo-assisted clutch6is opened, the motive torque output by engine4must be essentially cancelled out to avoid a rapid, uncontrolled increase of angular velocity (m of crankshaft7; in other words, during gear shift operations, the motive torque generated by engine4is controlled to maintain the angular velocity ° m of engine7equal to required values.

At the end of the gear shift and when servo-assisted clutch6is closed, the motive torque output by engine4must be restored to the same value assumed immediately before the gear shift, so as to avoid discontinuities which tend to generate abrupt longitudinal accelerations/decelerations.

Servo-controls18,19and20are directly driven by control unit13of transmission5; instead, motive torque variations output by engine4are made by control unit12of engine4following a specific request from control unit13of transmission5. In particular, control unit13of transmission5asks control unit12of engine4to vary the motive torque during gear shifts by means of dedicated electrical synchronisation wire15, which directly connects control unit13of transmission5to control unit12of engine4without the delays introduced by BUS line14.

With particular reference to the time graphs inFIG. 3, the operations required to shift from current gear A to next higher gear B (i.e. presenting a longer transmission ratio τB) are described below; in particular,FIG. 3shows the time evolution of angular velocity ωmof crankshaft7, the time evolution of angular velocity ω1of primary shaft16and the time evolution of angular velocity ω2of secondary shaft17. Gear A presents a transmission ratio τA, while new gear B presents a higher transmission ratio τBwith respect to transmission ratio τAof gear A.

Before the gear shift, when gear A is still engaged, primary shaft16presents an angular velocity ω1equal to angular velocity ωmof crankshaft7, while secondary shaft17presents an angular velocity ω2directly dependent on angular velocity ω1of primary shaft16by means of transmission ratio τAof gear A. Before the gear shift, when gear A is still engaged, angular velocity ωmof crankshaft7(and therefore necessarily also angular velocities ω1and ω2of primary and secondary shafts16and17) increases because motor vehicle1is accelerating.

As shown inFIG. 3, the gear shift starts at an instant t0in which an oscillation on angular velocities ω1and ω2of primary and secondary shafts16and17is generated before disengaging current gear A. The oscillation on angular velocities ω1and ω2of primary and secondary shafts16and17is generated by imposing an abrupt variation, i.e. a step variation, between the torque transmitted by clutch6and the torque load applied to rear drive wheels3; in particular, such variation is considered abrupt if it occurs in a time shorter than the duration of the first fourth of the natural oscillation frequency of the kinematic system to which primary shaft16belongs.

It is important to observe that the whole of primary and secondary shafts16and17of gearbox9, differential10, drive axles11, and rear drive wheels3forms a kinematic system, which is provided with its own inertial mass and its own torsional elasticity (due to the whole of all component deformations in the kinematic system) which is loaded by a torque value equal to the motive torque generated by engine4when motion is transmitted from engine4to rear drive wheels3. Obviously, the motive torque generated by engine4and transmitted by engine4to rear wheels3is contrasted by a torque load applied to rear drive wheels3.

The higher the variation velocity between torque transmitted by clutch6and torque load applied to rear drive wheels3, the higher the amplitude of the oscillation trigged by angular velocities ω1and ω2of primary and secondary shafts16and17; thus, by adjusting the velocity of such variation, it is possible to adjust the amplitude of such variation.

The abrupt variation between the torque transmitted by clutch6and the torque load applied to rear drive wheels3, i.e. the oscillation on angular velocities ω1and ω2of primary and secondary shafts16and17, may be generated either by abruptly opening clutch6and/or by abruptly stalling engine4.

An abrupt opening of clutch6is obtained by actuating clutch6itself by a step command; i.e. clutch6is passed from closed state to open state in the shortest possible time interval compatibly with the physical limits required by the concerned mechanics. An abrupt stalling of engine4(i.e. a sudden cancelling out of motive torque generated by engine4) is obtained by suddenly and instantaneously cutting off fuel injection to the cylinders of engine4; it is important to underline that the instantaneous and temporary stalling of engine4only produces an oscillation on angular velocities ω1and ω2of primary and secondary shafts16and17if clutch6is still closed.

Both the abrupt opening of clutch6, and the abrupt stalling of engine4determine a nearly instantaneously cancelling out of torque applied to primary shaft16and, by effect of the energy stored in the kinematic system elasticity, trigger oscillations of high inertia entity (up to 30-40% of the current values of angular velocities ω1and ω2) which tend to be damped out according to an exponential type law, on angular velocities ω1and ω2of primary and secondary shafts16and17. InFIG. 3, the dotted line shows how the oscillations on angular velocities ω1and ω2of primary and secondary shafts16and17would evolve in time if other factors detailed below did not intervene to stop the natural development of the oscillations themselves. It is important to observe that the oscillation is always triggered with a reduction of angular velocities ω1and ω2, because of the lack of the motive torque which generated system revolution; in other words, the first oscillation half-wave is always negative, i.e. tends to reduce angular speeds ω1and ω2with respect to the values of instant t0.

The disengagement of gear A, i.e. the separation between primary shaft16and secondary shaft17, is performed at instant t1when the oscillation has taken angular velocity ω1of primary shaft16to be close to the angular velocity ω1that the primary shaft16itself must assume in order to engage next gear B. In this way, at the end of gear A disengagement, angular velocity ω1of primary shaft16is already close to the angular velocity ω1that the primary shaft16itself must assume to engage gear B. In particular, in order to maximise the positive effect of reducing angular velocity ω1of primary shaft16, gear A is disengaged about the maximum amplitude of an oscillation half-wave, and in particular about the maximum amplitude of the first oscillation half-wave. By disengaging gear A about the maximum amplitude of the first oscillation half-wave, moreover, the oscillation on angular velocity ω1of primary shaft16is stopped as soon as it arises.

Subsequently, gear B is engaged at instant t2when the oscillation has taken angular velocity ω2of secondary shaft17to be either equal to or essentially equal to current angular velocity ω1of primary shaft16multiplied by transmission ratio τBof gear B. After engaging gear B, primary shaft16and primary shaft17are reciprocally and rigidly connected and angular velocity ω1of primary shaft16presents the same instantaneous trend as angular velocity ω2of secondary shaft17.

After engaging gear B, the oscillation triggered on secondary shaft17rapidly takes angular velocities ω1and ω2of primary and secondary shafts16and17to the value required by the current speed of car1which is reached at instant t3; at this point, i.e. at instant t3in which angular velocities ω1and ω2of primary and secondary shafts16and17equalise the value required by the current speed of car1, clutch6is closed again to make primary shaft16integral to crankshaft7again. Alternatively, to further reduce the gear shifting time, clutch6may be closed in advance also if angular velocities ω1and ω2of primary and secondary shafts16and17have not reached the value determined by the current speed of car1.

During the closing step of clutch6after engaging gear B, angular velocity ωmof crankshaft7is taken to equalise angular velocity ω1of primary shaft16, which angular velocity ω1is required by the speed of car1, because primary shaft16is angularly integral with rear drive wheels3through drive axles11, differential10, secondary shaft17, and the gears of gear B.

The closing of clutch6to mechanically connect primary shaft16to crankshaft7starts at instant t3and ends at instant t4. It is important to observe that essentially already soon after instant t3, clutch6transmits motive torque from crankshaft7to primary shaft16, and therefore to rear drive wheels3; consequently, traction to rear drive wheels3is returned already soon after instant t3and therefore, from a point of view of dynamics of car1, the gear shift ends soon after instant t3also if clutch6is still slipping.

According to the above-described embodiment, the opening of clutch6is provided to separate primary shaft16from crankshaft7; however, according to an alternative embodiment, gear shifts are performed without opening the clutch6(i.e. by always maintaining primary shaft16fastened to crankshaft7) and by generating the abrupt variation between torque transmitted by clutch6and torque load applied to the rear drive wheels3, i.e. by generating the oscillation on angular velocities ω1and ω2of primary and secondary shafts16and17, only by abruptly stalling engine4.

According to a possible embodiment, the speed variation between the torque transmitted by clutch6and the load torque applied to rear drive wheels3may be adjusted to vary the maximum amplitude of the oscillations triggered on angular velocity ω1of primary shaft16according to the deviation between angular velocity ω1of primary shaft16immediately before the gear shift and angular velocity ω1of primary shaft16immediately after the gear shift.

From the above, it is apparent that in order to synchronise new gear B, i.e. to adapt angular velocity ω1of primary shaft16to the angular velocity determined by gear B, the oscillation trigged on both primary shaft16and secondary shaft17is exploited, instead of the synchronisers of gearbox9. In particular, the oscillation triggered on primary shaft16is used to obtain a rapid decrease of angular velocity ω1of primary shaft16and then the oscillation triggered on secondary shaft17is used to obtain a rapid and temporary increase of angular velocity ω2of secondary shaft17so that the angular velocity ω2of secondary shaft17equalises the current angular velocity ω1of primary shaft16multiplied by transmission ratio τBof gear B in order to engage gear B; finally, after engaging gear B, the oscillation triggered on secondary shaft17is used to take angular velocities ω1and ω2of primary and secondary shafts16and17to the value required by the current speed of car1.

The above-described methods for shifting from gear A to higher gear B allow to shift gear in a very short time because the synchronisers of gearbox9are not employed and the synchronisation time of new gear B to adapt angular velocity ω1of primary shaft16to angular velocity required by gear B is extremely short.

The above-described methods for shifting from gear A to gear B may be similarly applied also if gear B is lower than gear A; the only difference is that, after the gear shift, primary shaft16must have an angular velocity ω1higher, and not lower, than the situation before the gear shift. Consequently, the oscillation triggered on angular velocities ω1and ω2of primary and secondary shafts16and17must be used to accelerate, and not decelerate, primary shaft16.

According to a preferred embodiment, control unit13of transmission5normally sends a low logical state through dedicated electrical synchronisation wire15and sends a high logical state when the motive torque generated by engine4is to be cut off during gear shifts. In other words, when control unit12of engine4receives a low logical level through dedicated electrical synchronisation wire15, then control unit12of engine4works normally, actuating a motive torque essentially determined according to the commands of the driver; instead, when control unit12of engine4receives a high logical level through dedicated electrical synchronisation wire15, then control unit12of engine4cuts off motive torque and maintains motive torque at essentially zero values until it receives a high logical level through dedicated electrical synchronisation wire15.

Preferably, control unit13of transmission5sends redundant controls to control unit12of engine4, so that control unit12of engine4may monitor the existence of a continuous consistency among received commands; in the event of lack of consistency among received commands, then a fault condition is diagnosed and an emergency operation procedure is activated. In particular, the transmission of a motive torque variation command from control unit13of transmission5to control unit12of engine5requires control unit13of transmission5to be sending the motive torque variation command to control unit12of engine5by means of dedicated electrical synchronisation wire15and at the same time control unit13of transmission5to be sending the same command along with a confirmation message forwarded by the BUS line14to control unit12of engine5. Control unit12of engine5actuates the received motive torque variation command by means of dedicated electrical synchronisation wire15immediately after receiving the command itself, and control unit12of engine4suspends actuation of the motive torque variation command if the confirmation message is not received by the BUS line14within a certain interval of time. In the event of discrepancy between the commands received by means of dedicated electrical synchronisation wire15and the commands received by means of BUS line14, a fault condition of dedicated electrical synchronisation wire15is diagnosed and consequently only the commands sent by means of BUS line14are actuated.

According to a preferred embodiment, in order to cut off motive torque output by engine4during a gear shift, control unit12of engine5only controls the injectors to prevent fuel injection without modifying the throttle position of engine4. Such control method allows to rapidly cut off motive torque and subsequently restore motive torque in an equally rapid way: indeed, it is important to underline that the driving of the throttle of engine4requires the movement of mechanical parts having a relatively high mechanical inertia, while the control of the injectors is performed within a very rapid time.

During the design and tuning step of servo-actuated transmission5, an estimated delay time DT is determined and stored in a memory (not shown) of control unit13of transmission5for each gear shift operation to be performed. The estimated delay time DT for an operation corresponds to the time interval elapsing from the instant in which the actuation of the operation is commanded and the instant in which the operation itself is actually completed.

According to a preferred embodiment, each estimated delay time DT is checked and corrected, if necessary, for each gear shift operation; in other words, at each gear shift operation, the actual value of each estimated delay time DT is determined and if the stored estimated delay time DT is significantly different from the measured delay time DT then the stored estimated delay time DT is corrected by using the measured delay time DT. If the stored estimated delay time DT is significantly different from the measured delay time DT, then the new stored estimated delay time DT is calculated as the (possibly weighed) average between the previously stored estimated delay time DT and the measured delay time DT.

According to a possible embodiment, each envisaged delay time DT could be expressed according to the actual working temperature, i.e. according to the temperature of a cooling liquid of engine4.

As mentioned above, in order to shift from a current gear to a next gear, a series of operations are performed in sequence, each of which must be completed before the next operation can be completed. With particular reference to the time diagram shown inFIG. 4, control unit13of transmission5commands the actuation of a first operation (OPERATION I) at an instant of time t1; subsequently, control unit13of transmission5estimates an instant of time t2in which the first operation will be actually completed, by adding the estimated delay time DT for the first operation to instant of time t1in which actuation of the first operation was commanded. At this point, control unit13of transmission5determines an instant of time t3in which to command actuation of a next second operation (OPERATION II) by subtracting the estimated delay time DT for the second operation decreased by a safety constant SC, whose function is to avoid overlapping between the actual instant of actuation of the first operation and the instant of actual actuation of the second operation, from instant of time t2.

The above-described control method is applied to all operations which must be performed in sequence to shift a gear; in this way, an interval of time equal to safety constant SC elapses between the instant of time in which an operation is actually completed and the instant of time in which the next operation is actually completed.

According to the above, it is apparent that a previous operation is completed before the next operation is completed, but the actuation of a previous operation may be simultaneous or at least partially simultaneous to the actuation of the next operation. In other words, a previous operation is performed at the same time or nearly at the same time as a next operation, notwithstanding that the previous operation must be completed before completion of the next operation.

When control unit13of transmission5receives the gear shift command, control unit13of transmission5immediately asks control unit12of engine4to cut off motive torque; at this point, control unit13of transmission5defines the instant of time in which the motive torque cut-off operation will be actually completed and therefore determines the instant of time in which to command the opening operation of servo-assisted clutch6so that the opening operation of servo-assisted clutch6is actually completed immediately after (i.e. after a time interval equal to safety constant SC) the actual completion of the motive torque cut-off operation.

By using the instant of time in which the servo-assisted clutch6opening operation will actually be completed, control unit13of transmission5determines the instant of time in which to command actuation of the current gear disengagement operation so that the current gear disengagement operation is actually completed immediately after (i.e. after a time interval equal to safety constant SC) the actual completion of the servo-assisted clutch6opening operation.

By using the instant of time in which the current gear disengagement operation will actually be completed, control unit13of transmission5determines the instant of time in which to command actuation of the next gear selection operation so that the next gear selection operation is actually completed immediately after (i.e. after a time interval equal to safety constant SC) the actual completion of the current gear disengagement operation.

By using the instant of time in which the next gear selection operation will actually be completed, control unit13of transmission5determines the instant of time in which to command actuation of the next gear engagement operation so that the next gear engagement operation is actually completed immediately after (i.e. after a time interval equal to safety constant SC) the actual completion of the next gear selection operation.

By using the instant of time in which the next gear engagement operation will actually be completed, control unit13of transmission5determines the instant of time in which to command actuation of the servo-assisted clutch6closing operation so that the servo-assisted clutch6closing operation is actually completed immediately after (i.e. after a time interval equal to safety constant SC) the actual completion of the next gear engagement operation.

By using the instant of time in which the servo-assisted clutch6closing operation is actually completed, control unit13of transmission5determines the instant of time in which to command actuation of the operation of restoring motive torque output by engine4so that the motive torque restoring operation is actually completed immediately after (i.e. after a time interval equal to safety constant SC) the actual completion of the servo-assisted clutch6closing operation.

According to a preferred embodiment, the value of safety constant SC depends on a driving style selected by the driver of motorcar1; in particular, the sportier the selected driving style, the lower the value of the safety constant (at most, the value of safety constant SC could be cancelled out in the event of competitive track racing).

Furthermore, preferably, each operation is associated to its own safety constant SC, potentially different from the other safety constants SC; in particular, the higher the safety constant SC of an operation, the lower the accuracy with which the delay time DT of the previous operation is calculated.

In the known servo-assisted transmission, before commanding the actuation of an operation, it is necessary to wait for verification of the completion of the previous operation actuation; such verification is provided by specific sensors which determine the state (position and/or speed of revolution) of the servo-assisted components of transmission5. However, by operating in this way, inevitable delays related to the time needed to verify the completion of various operations are introduced. Instead, according to the above-described method, no verification of the completion of the previous operation actuation is performed before commanding the actuation of a next operation, but thanks to the use of estimated delay times DT, an operation is actually completed immediately after (i.e. after a time interval equal to safety constant SC) the actual completion of the previous operation. The various sensors which determine the state (position and/or revolution speed) of the components of servo-assisted transmission5are no longer used to authorise the actuation of the various operations, but are only used to determine the actual value of each estimated delay time DT so as to determine (as previous described) the correctness of the stored estimated delay times DT with a hindsight.