GB2470016A

GB2470016A - Method of reducing torque shock during a gear shift

Info

Publication number: GB2470016A
Application number: GB0907703A
Authority: GB
Inventors: Lennart Olausson
Original assignee: GM Global Technology Operations LLC
Current assignee: GM Global Technology Operations LLC
Priority date: 2009-05-05
Filing date: 2009-05-05
Publication date: 2010-11-10
Also published as: GB0907703D0; WO2010127742A1

Abstract

A vehicle gearbox comprises a shift fork 22 displaceable between neutral, synchronizing and engaged positions and a double-acting cylinder 20, 21 connected to the shift fork 22 so that a control pressure applied to a first chamber 23 of the double-acting cylinder 20,21 urges the shift fork 22 towards the synchronized position and a counter pressure applied to a second chamber 24 of the double-acting cylinder urges the shift fork 22 towards the neutral position. A method for controlling engagement of a gear in a motor the method comprises the steps of:a) displacing the shift fork 22 from its neutral position to synchronizing position;b) while the shift fork 22 is in its synchronizing position, applying a first level of said control pressure to the first chamber 23 and a positive second level of said counter pressure to the second chamber 24, the first and second pressure levels being set so as to generate a net force towards the engaged position;c) when the shift fork 22 is moving from the synchronizing position to the engaged position, decreasing the control pressure below said counter pressure.

Description

Method and Apparatus for Controlling Gear Engagement Des cript ion The present invention relates to a method for controlling engagement of a gear in a motor vehicle gearbox, in particular an automated manual transmission, in which a shift fork is displaceable between neutral, synchronizing and engaged positions by means of a double-acting cylinder. By applying a control pressure to one or the other of the two chambers of such a cylinder, the shift fork is urged into opposite directions, e.g. from neutral to engaged position or vice versa.

Conventionally the shift fork controls a synchronizer by displacing a shift sleeve thereof along a shaft to which the synchronizer is locked in rotation. In the synchronizing position, the shift fork presses against each other friction surfaces of the synchronizer and of a gearwheel idly mounted on said shaft, so as to equalize their rotation speeds. When the rotation speeds have become equal, a blocking is released, and the synchronizer sleeve is free to advance a little bit more towards the gearwheel, to an engaged position in which the gearwheel is locked in rotation to the shaft.

Synchronizing the gearwheel to the shaft accounts for a major portion of the total time needed for gear shifting. In order to reduce synchronization times, it is conceivable to increase friction by applying an increased control pressure to the first chamber of the double-acting cylinder. By thus pressing together the friction surfaces more strongly, friction is increased, and synchronization is reached quickly. However, as soon as synchronization is achieved, and the shift fork becomes free to advance towards the engaged position, the control pressure causes the shift fork to accelerate towards the engaged position. Of course, the acceleration is the stronger, the higher the control pressure is, and the more abruptly the shift fork is stopped at the engaged position, the louder is the noise it generates.

The path between synchronizing and engaged positions tends to be rather short, in order to reduce as much as possible the time which the shift fork needs for moving along said path. This hardly leaves time for decelerating the shift fork before reaching the engaged position.

One might consider reducing the speed of the shift fork when approaching the engaged position by supplying a counter pressure to the second chamber of the double-acting synchronizer. This approach tends to be unsatisfying, since due to the short path between synchronizing and engaged positions, the time in which the shift fork moves from the synchronizing position to the engaged position is already so short that it is difficult to produce a substantial decelerating counter pressure in the second chamber.

It is an object of the present invention to provide a method for controlling engagement of the gear in a motor vehicle gearbox which allows for fast shifting while not excessively increasing shifting noise.

This object is achieved by a method for controlling engagement of a gear in a motor vehicle gearbox comprising a shift fork displaceable between neutral, synchronizing and engaged positions and a double-acting synchronizer connected to the shift fork so that a control pressure applied to a first chamber of the double-acting synchronizer urges the shift fork towards the synchronized position and a counter pressure applied to a second chamber of the double-acting cylinder urges the shift fork towards the neutral position, the method comprising the steps of a) displacing the shift fork from its neutral position to the synchronizing position; b) while the shift fork is in the synchronizing position, applying a first level of said control pressure to the first chamber and a positive second level of said counter pressure to the second chamber, the first and second pressure levels being set so as to generate a net force towards the engaged position; and c) when the shift fork is moving from the synchronizing position to the engaged position, decreasing the control pressure below said counter pressure.

Before the control pressure is decreased, the shift fork is accelerated towards the engaged position by the net force directed towards the engaged position. By decreasing the control pressure below the counter pressure, the movement of the shift fork becomes decelerated, so that when the shift fork reaches the engaged position, its speed is moderate and will not cause substantial noise.

If the control pressure is decreased early during the movement of the shift fork from the synchronizing position to the engaged position, deceleration by the counter pressure can become excessive, causing the shift fork to change direction and move back towards the neutral position, which is clearly not desired. If the control pressure is decreased too late, the shift fork reaches the engaged position at a high speed and noise is created. In order to avoid these two extremes reliably, it is advantageous to monitor the progress of the shift fork on its way from the synchronizing position to the engaged position and to determine an appropriate time for decreasing the control pressure based on said progress.

The above-mentioned step c) of decreasing the control pressure may be followed by a step d) of decreasing also the counter pressure. By doing so in case of need, a situation in which the shift fork moves back towards the neutral position can be reliably avoided.

Between steps c) and d) there can be a phase in which the control and counter pressures are substantially constant.

The time for decreasing the counter pressure should be set such that the shift fork reaches the engaged position with a positive velocity. In order to achieve a substantial noise reduction, this positive velocity should not be more than half of the peak velocity reached by the shift fork on its way from the synchronizing position to the engaged position.

When the progress of the shift fork on its way from the synchronizing position to the engaged position is monitored, as mentioned above, an appropriate time for decreasing said counter pressure can be determined based on said progress, too.

At or after the time of decreasing the counter pressure, the control pressure may be increased again, in order to prevent the shift fork from becoming so slow that the time needed to reach the engaged position would be excessive.

The level to which the control pressure is increased again preferably is below said second level of the counter pressure. Such an increase of the control pressure can be generated easily if the first and second chambers of the double-acting cylinder share a flowpath to a low-pressure reservoir, so that a pressure drop caused along said flowpath by hydraulic fluid escaping from said second chamber causes a pressure increase in the first chamber.

Another object of the invention is a motor vehicle gearbox comprising at least one synchronizer, at least one reservoir for pressurized hydraulic fluid, at least one reservoir for unpressurized hydraulic fluid, a double-acting cylinder having first and second chambers, control valves for selectively connecting said first and second chambers to said reservoirs, a shift fork connected to said double-acting cylinder and displaceable between neutral, synchronizing and engaged positions, and a controller adapted to control said valves based on the above-described method.

The gearbox preferably comprises a pressure regulator, an input of which communicates with the reservoir for pressurized hydraulic fluid, and means for bringing into communication an output of the pressure regulator and the second chamber. In that case, the reservoir for pressurized hydraulic fluid can be held at the control pressure, and the counter pressure is derived from the control pressure by the pressure regulator.

For monitoring the movement of the shift fork, the gearbox preferably comprises a Hall sensor associated to the shift fork for detecting it at an intermediate position between said synchronizing and engaged positions. The Hall sensor can provide an output signal which is representative not only of the presence of the shift fork at the intermediate position but also of its speed.

Further features and advantages of the invention will become apparent from the subsequent description of embodiments thereof referring to the appended drawings.

Fig. 1 is a schematic diagram of the hydraulic circuitry of a gearbox according to the present invention; and Fig. 2 illustrates waveforms of shift fork speed, control pressure and counter pressure in the course of a shifting process.

Fig. 1 illustrates a double-clutch transmission (DCT) which is a preferred field of application of the present invention. It should be understood, though, that the invention is applicable to single clutch transmissions alike.

An input shaft 1 of the gearbox comprises two concentrically rotating shaft members, a solid shaft 2 and a hollow shaft 3, both of which carry a clutch plate of double clutch 4. The double clutch 4 is adapted to be selectively engaged in order to transmit engine torque only to solid shaft 2 or only to hollow shaft 3.

Solid and hollow shafts 2, 3 carry a plurality of drive gearwheels 6 to 9 which mesh with driven gearwheels 10 to 13 which are rotatably mounted on a layshaft 14. A second layshaft with more gearwheels meshing with drive gearwheels 6 to 9 can be provided but is not shown in Fig. 1. An output shaft, not shown, carries one or two gearwheels which mesh with pinions, not shown, of the layshafts.

Between driven gearwheel pairs 10, 11 and 12, 13, respectively, synchronizers 15, 16 are provided. The design of the synchronizers 15, 16 is familiar to the man of the art, comprising a shift sleeve 17 which is locked in rotation to a hub 18 on layshaft 14 and is axially displaceable along said layshaft 14 in order to engage one of the adjacent gearwheels 10, 11 or 12, 13 and lock it to the layshaft 14. Baulk rings 19 between the hub 18 and the adjacent gearwheels are dragged along when shift sleeve 17 is displaced from its neutral position. When the shift sleeve 17 reaches a synchronizing position, one of said baulk rings 19 is pressed against a mating friction surface of an adjacent driven gearwheel until that gearwheel is synchronized to the layshaft 14. Only then is a blocking by the baulk ring 19 released, and the shift sleeve 17 is free to move further towards the gearwheel, into an engaged position in which it locks the gearwheel to the layshaft 14.

Each synchronizer 15, 16 has a hydraulic actuator 20, 21 associated to it for displacing a shift fork 22 that engages shift sleeve 17. The actuators 20, 21 are double-acting hydraulic cylinders having first and second chambers 23, 24 at either side of a displaceable piston 25 connected to shift fork 22. Each synchronizer 20, 21 has a Hall sensors 34 associated to it for monitoring a displacement of its shift fork 22 to the left, towards gearwheel 11 or 13, and to the right, towards gearwheel 12 or 14. For detecting displacements into different directions, two magnets are placed on each shift fork 22 or piston rod so that one of them is detected when the shift fork 22 is displaced from neutral to the left, and the other when the shift fork 22 is displaced from neutral to the right. The magnets may differ in field strength and/or orientation, so that from polarity and/or amplitude of the Hall sensor signal the detected magnet can be recognized.

The chambers 23, 24 of each actuator 20, 21 are selectively connected to a control pressure source, a counter pressure source and a low pressure reservoir 26 by a way valve 27. The control pressure source is formed by a pump 28 drawing hydraulic fluid from reservoir 26, eventually in association with an accumulator 29. The control pressure source is a conventional pressure regulator 30 which maintains a predetermined pressure drop between its input connected to an output of way valve 27, and its output connected to reservoir 26.

As shown in Fig. 1, the way valve 27 can assume various positions under the control of an electronic transmission controller 31. To each position is associated one of the six boxes a-f that form the symbol of the way valves 27 in Fig. 1. In the position d illustrated in Fig. 1, the chambers 23, 24 are disconnected both from the pressure sources and from the reservoir 26, so that the associated shift fork 22 is blocked in its position.

In order to displace the shift fork 22 of e.g. actuator 20 from its idle position illustrated in Fig. 1 and referred to as x in the graphs of Fig.2 to an engaged position in which it locks e.g. driven gearwheel 11 to layshaft 14, the shift fork 22 must be displaced to the right by applying a higher pressure to chamber 23 on the left hand side of piston 25 than to chamber 24 on the right hand side. This is done by placing way valve 27 in position a), in which chamber 23 receives hydraulic fluid at the control pressure from pump 28 via a controllable throttle 32. Throttle 32 controls the flow rate of fluid into chamber 23 and, hence, the advancing speed of shift fork 22. The advancement of piston 25 causes the pressure in chamber 24 to rise to a counter pressure which is the pressure level at which pressure regulator 30 opens. Thus fluid from chamber 24 is expelled through pressure regulator 30 to reservoir 26.

Shift fork 22 thus advances at substantially constant speed until, at a location X2, its progress is stopped by baulk ring 19 abutting against a friction cone of gearwheel 11. Pressure levels in chambers 23, 24 are not substantially affected by this.

When synchronization of gearwheel 11 is achieved, baulk ring 19 releases the shift sleeve 17, and shift fork 22 advances further, accelerated by the pressure difference between chambers 23, 24 of actuator 20.

At a location x3 between the synchronizing and engaged positions of the shift fork, transmission controller 31 switches way valve 27 to position b) . In this position, chamber 23 communicates with reservoir 26, and the pressure in chamber 23 decreases quickly. The location X3 may be defined as the location of one of Hall sensors 34 which detects the shift fork 22 passing.

Alternatively, it might be the position the shift fork has reached in a predetermined time after the shift sleeve 17 was released by baulk ring 19. The time at -10 -which the shift sleeve is released might be detected e.g. by a pressure sensor connected to chamber 23, which will detect a short pressure drop when at the moment of release the shift fork starts moving again.

Finally, at position x4, a pressure level is reached in chamber 23 which is less than the counter pressure in chamber 24. From then on, shift fork 22 is decelerated by the pressure difference between the two 0 chambers 23, 24.

When way valve 27 is in position b), the pressure in chamber 23 can even become negative due to a reverse pressure drop of hydraulic fluid sucked from reservoir 26 into chamber 23.

Somewhere between the synchronizing position and the engaged position, the shift fork 22 passes one of Hall sensors 34, and the Hall sensor 34 detects the speed of the shift fork 22. Based on this speed and known pressures in chambers 23, 24, it is possible to predict whether and after what time the shift fork 22 will reach the engaged position in which the shift sleeve 17 locks gearwheel 11 to hub 18. The sensor 34 might be placed between location X4 and the engaged position, in order to be able to detect the effect of the deceleration taking place beyond x4. Based on the detected speed, controller 31 consults a look-up table which indicates, for a given shift fork speed, a delay after which to switch to position c) of way valve 27.

In a particularly simple embodiment, a single Hall sensor 34 is placed at location x3, the passage of the shift fork 22 near this sensor causing the way valve 27 to switch to position b), as mentioned above, and the speed of shift fork 22 determined in this passage is used -11 -by transmission controller 31 to calculate the appropriate time for switching way valve 27 to position c).

In Fig. 5, location X5 corresponds to the instant of switching to position C) . In this position both chambers 23, 24 communicate in parallel with reservoir 26. Hydraulic fluid expelled from chamber 24 is therefore drawn into a chamber 23 via the way valve 27, thus reducing the pressure difference between chambers 23, 24, and, hence, the deceleration of shift fork 22.

By appropriately setting the delays for switching to position c), it can be ensured that the shift fork 22 reaches the engaged position with a speed which is substantially less than its advancing speed between neutral position x0 and synchronizing position x1 or the peak speed at location x4. Since it is possible to reduce the speed in the vicinity of the engaged position to an arbitrary low value by an appropriate choice of locations for switching the way valve 27, high speeds of the shift fork can be output before the shift fork 22 reaches the engaged position, without having to fear that excessive noise is generated. Since high pressures can be applied to chambers 23, 24 in the early stages of the switching process, the shift fork can move fast in the other stages, so that switching can be carried out in a short time.

For synchronizing e.g. gearwheel 11, the way valve 27 associated to synchronizer 15 successively assumes states d), a), b) and c) . Further positions e) and f) are counterparts of states b) and a) in case that gearwheel 12 is synchronized.

-12 -Obviously, the hydraulic circuitry shown in Fig. 1 is only an example of a circuitry appropriate for carrying out the present invention. For a skilled person, other circuitries will be readily conceivable in which both chambers of a double-acting cylinder are pressurized at different positive levels during synchronization and the pressure which drives the progress of the shift fork from the synchronizing to the engaged position is released before the shift fork reaches the engaged position, so that the counter pressure in the other chamber will decelerate the shift fork.

List of reference signs 1 input shaft 2 solid shaft 3 hollow shaft 4 double clutch drive gearwheel 6 drive gearwheel 7 drive gearwheel 8 drive gearwheel 9 drive gearwheel driven gearwheel 11 driven gearwheel 12 driven gearwheel 13 driven gearwheel 14 layshaft synchronizer 16 synchronizer 17 shift sleeve 18 hub 19 baulk ring actuator 21 actuator 22 shift fork 23 1St chamber 24 2nd chamber piston 26 reservoir (low pressure) 27 way valve 28 pump 29 accumulator pressure regulator 31 transmission controller 32 throttle 33 throttle 34 Hall sensor

Claims

Claims 1. A method for controlling engagement of a gear in a motor vehicle gearbox comprising a shift fork (22) displaceable between neutral, synchronizing and engaged positions and a double-acting cylinder (20, 21) connected to the shift fork (22) so that a control pressure applied to a first chamber (23) of the double-acting cylinder (20, 21) urges the shift fork (22) towards the synchronized position and a counter pressure applied to a second chamber (24) of the double-acting cylinder urges the shift fork (22) towards the neutral position, the method comprising the steps of a) displacing the shift fork (22) from its neutral position to the synchronizing position; b) while the shift fork (22) is in its synchronizing position, applying a first level of said control pressure to the first chamber (23) and a positive second level of said counter pressure to the second chamber (24), the first and second pressure levels being set so as to generate a net force towards the engaged position; c) when the shift fork (22) is moving from the synchronizing position to the engaged position, decreasing the control pressure below said counter pressure.
2. The method of claim 1, wherein the progress of the shift fork (22) on its way from the synchronizing position to the engaged position is monitored (34), and a time or a location (X3) for decreasing said control pressure is determined based on said progress.
3. The method of claim 1 or 2, wherein in step c) the control pressure is decreased to a negative value.
4. The method of claim 1, 2 or 3, in which step C) is followed by a step d) of decreasing the counter pressure.
5. The method of claim 4, wherein between steps C) and d) there is a phase in which said control and counter pressures are substantially constant.
6. The method of claim 4 or 5, wherein a time or a location for decreasing said counter pressure is set such that the shift fork (22) reaches the engaged position with a positive velocity.
7. The method of claim 6, wherein said positive velocity is not more than half the of peak velocity reached by the shift fork (22) on its way from the synchronizing position to the engaged position.
8. The method of claim 6 or 7, wherein the progress of the shift fork (22) on its way from the synchronizing position to the engaged position is monitored and a time or a location (x5) for decreasing said counter pressure is determined based on said progress.
9. The method of any of claims 4 to 8, wherein at or after the time of decreasing the counter pressure, the control pressure is increased again.
10. The method of claim 9, wherein the control pressure is increased again to a third level which is below said second level of the counter pressure.
11. A motor vehicle gearbox comprising -at least one synchronizer (15; 16); -at least one source (28, 29) for pressurized hydraulic fluid, -at least one reservoir (26) for unpressurized hydraulic fluid, -a double-acting cylinder (20, 21) having first and second chambers (23, 24); -control valves (27) for selectively connecting said first and second chambers (23, 24) to said reservoirs (26); -a shift fork (22) connected to said double-acting cylinder (20, 21) and displaceable between neutral, synchronizing and engaged positions; and a controller adapted to control said valves based on the method of any of the preceding claims.
12. The gearbox of claim 11 further comprising a pressure regulator (30), an input of which communicates with said source (28, 29) for pressurized hydraulic fluid, and means (27, 31) for bringing into communication the pressure regulator (30) and the second chamber (24)
13. The gearbox of claim 11 or 12, wherein first and second chambers (23, 24) share a flowpath to said low pressure reservoir (26).
14. The gearbox of any of claims 11 to 13, wherein a Hall sensor (34) is associated to the shift fork (22) for detecting the shift fork (22) at an intermediate position between said synchronizing and engaged positions.