GB2471989A

GB2471989A - Speed and pressure control of a synchronizer in a gearbox

Info

Publication number: GB2471989A
Application number: GB0907686A
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: 2011-01-26
Also published as: GB0907686D0

Abstract

A method for controlling engagement of a gear in a motor vehicle double-clutch gearbox comprising a synchronizer having a displaceable member or hub 17, 18 which is displaceable between neutral, synchronizing and engaged positions, and a hydraulic actuator 20, 21 for displacing the displaceable member 17, 18. The method comprises the steps of a ) displacing the displaceable member from the neutral to the synchronizing position (tO-t3, fig 3), b) applying a pressure directed towards the engaged position to the displaceable member while in the synchronizing position (t3-t6), and C ) displacing the displaceable member from the synchronizing position to the engaged position (t6-t7). In step a) the speed of the displaceable member is controlled to match a target value (v1) and in step b) the pressure applied to the displaceable member is controlled to match a target value (ps). Also claimed is a motor vehicle gearbox having a controller which controls the speed and pressure applied by the actuator 17, 18 to the displaceable member 17, 18.

Description

Method and Apparatus for Controlling Gear Engagement

Description

The present invention relates to a method for controlling engagement of a gear in a motor vehicle gearbox, in particular in an automated manual transmission, which comprises a synchronizer, the synchronizer having a member which is displaceable between neutral, synchronizing and engaged positions, and an actuator for displacing the displaceable member, and to apparatus for carrying out the method.

A synchronizer of the above-defined type is generally known. Conventionally, it comprises a hub which is fixed to a rotating shaft of the gearbox, a baulk ring, and a gearwheel which is idly mounted on the rotating shaft, and the displaceable member is a shift sleeve which is axially displaceable along the hub. While the shift sleeve is in the neutral position, the gearwheel is free to rotate with respect to the shaft, in the synchronizing position, the shift sleeve presses the baulk ring against the gearwheel and the baulk ring blocks further displacement of the shift sleeve towards the engaged position until the rotation speeds of shaft and gearwheel have become equal. In the engaged position, the shift sleeve is keyed to the gearwheel.

A conventional type of actuator is a hydraulic cylinder which receives pressurized hydraulic fluid from a source, e.g. a pump. When the shift sleeve is moving from the neutral position to the synchronizing position, its speed is governed by the flow rate of the source or, if present, by a throttle installed in a supply line between the source and the actuator. When the baulk ring blocks the shift sleeve in the synchronizing position, the fluid pressure at the actuator becomes equal to the idling pressure delivered by the source. When synchronization is achieved and the shift sleeve is free to advance towards the engaged position, its speed is again governed by the flow rate of the source or, eventually, the throttle.

In an automated gearbox of this type, various problems can arise. On the one hand it is desirable to have a high pump flow rate and idling pressure, in order to achieve fast synchronization. However, a high pressure applied to the actuator will cause the shift sleeve to hit the baulk ring at a high speed when reaching the synchronizing position. If the speed at which the shift sleeve hits the baulk ring is too high, noise is generated, and there is a risk of mating surfaces of the shift sleeve, the baulk ring and the gearwheel being damaged. Excessive pressure between the shift sleeve and the baulk ring during synchronization may cause mating friction surfaces of the baulk ring and the gearwheel to become extremely hot, causing these to wear off prematurely. Noise can be generated for a second time if the shift sleeve reaches the engaged position at a high speed.

Of course, the supply rate of hydraulic fluid to the actuator and the pressure of the hydraulic fluid might be decreased, but this would undesirably increase the time needed for gear shifting.

The object of the present invention is therefore to provide a method and apparatus that facilitate fast and efficient gear shifting but avoid unnecessary noise and wear.

This object is achieved by a method for controlling engagement of a gear in a motor vehicle gearbox comprising a synchronizer which has a member which is displaceable between neutral, synchronizing and engaged positions, and an actuator for displacing the displaceable member, the method comprising the steps of a) displacing the displaceable member from the neutral to the synchronizing position, b) applying a pressure directed towards the engaged position to the displaceable member while in the synchronizing position and c) displacing the displaceable member from the synchronizing position to the engaged position, characterized in that in step a) the speed of the displaceable member is controlled to match a target value and that in step b) the pressure applied to the displaceable member is controlled to match a target value.

By thus using different control parameters for the displacement stage and the synchronizing stage, each of the two stages can be controlled efficiently. In step a) a high pressure can be applied if necessary to reach the desired speed of the displaceable member although the same pressure, if applied during the synchronizing stage, might cause overheating of the synchronizer. Vice versa, the pressure applied in the synchronizing stage can have a value which, if applied in the displacement stage of step a) might cause the shifting process to become intolerably long or might cause the shift sleeve to crash violently into the baulk ring.

In step c), the speed of the displaceable member is preferably controlled again to match a target value.

This target value may be different from the one used in step a).

In step a) the speed target value is preferably reduced as the displaceable member approaches its synchronizing position. In this way, the speed target value can be set very high while the displaceable member is still far off from its synchronizing position, so that the displaceable member can be moved from the neutral position to the synchronizing position in a very short time and still reach the synchronizing position with a speed which is low enough to prevent generation of excessive noise.

The pressure target value is preferably decided anew for each shifting process based on a slip speed, i.e. a difference of rotation speeds between the two rotatable members of the gearbox, which should be measured before the displaceable member reaches its synchronizing position. Since the amount of heat generated in the synchronizer during synchronization depends on the slip speed, overheating can be avoided by appropriately setting the pressure target value while still achieving synchronization in a short time.

Preferably, there is a first slip speed range in which the pressure target value is set to increase with the slip speed, so that the time needed for synchronizing the rotatable members becomes substantially independent of the slip speed.

If the slip speed is extremely high, the amount of heat generated during the synchronization can become so high that overheating might result. tinder these circumstances, it is appropriate to set a pressure target value which decreases with the slip speed. Although the total amount of heat generated at the friction surfaces during a synchronization process is substantially independent of the pressure, the time in which it is generated becomes longer, so that the heat has sufficient time to dissipate into the bulk of the synchronizer, and the surface temperature remains lower.

The heating power generated at the friction surfaces of the synchronizer is proportional to the slip speed and to the pressure to which the friction surfaces are subject. Accordingly, if the pressure is constant, the heating power is highest at the very beginning of the synchronizing stage. In order to achieve synchronization without reaching an excessive heating power, it is useful that at least in a first part of step b) the pressure target value is increased from a low value, so that a pressure which might cause overheating if applied at the beginning of the synchronization stage is reached only when the slip speed has decreased to an uncritical level.

In a second part of step b), it is useful to decrease the pressure target value, so that when synchronization is achieved and the blocking of the displaceable member is released, it will not be accelerated too violently and reach the engaged position with a crash. Preferably, the pressure target value at the end of step b) is the same in all switching processes regardless of the slip speed that reigned at the beginning of the synchronization stage.

The object of the invention is further achieved by a motor vehicle gearbox comprising -at least two rotatable members; -at least one synchronizer for synchronizing said rotatable members, the synchronizer having a member which is displaceable between neutral, synchronizing and engaged positions; $ -an actuator for displacing the displaceable member; -a transmission controller adapted to control the speed of the displaceable member using said actuator while the displaceable member is between its neutral and synchronizing position and to control the pressure applied to the displaceable member while the displaceable member is in its synchronizing position.

The invention may further be embodied by a computer software product having source code means for carrying out the method as described above on a programmable processor of a transmission controller, or by a computer readable data carrier having program instructions recorded on it which enable a programmable processor to carry out the method as described above.

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; Fig. 2 is a diagram of a control valve used in the gearbox of Fig. 1; and Fig. 3 illustrates wave forms of shift sleeve positions, rotation speeds, pressure and shift sleeve displacement speed in the course of a shifting procedure.

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.

Hydraulic circuitry for operating the actuators 20, 21 (and others, not shown), associated to other gearwheels of the gearbox comprises a reservoir 26 for unpressurized hydraulic fluid, a pump 28 which draws fluid from reservoir 26, an accumulator 29 connected to the output of pump 28, control valves 27, each of which has one port connected to the output side of pump 28, another port connected to reservoir 26 and a pressure-controlled port connected to the actuators 20, 21 via way valves 30, 31, 32. Way valve 30 is directly connected to the pressure-controlled ports of control valves 27 and to reservoir 26 and has two positions in which either way valve 31 receives controlled output pressures from control valves 27 and way valve 32 is connected to reservoir 26, or vice versa. Control valves 27 and way valves 30, 31, 32 are controlled by an electronic transmission controller 5.

Way valves 31, 32 are shown in Fig. 1 with two positions each, but in practice the number of positions corresponds to the number of synchronizers associated to gearwheels driven via hollow shaft 3, such as synchronizer 15, whereas the number of positions of way valve 32 corresponds to the number of synchronizers, such as synchronizer 16, associated to gearwheels driven by solid shaft 2.

Fig. 2 schematically illustrates the structure of control valves 27. Within a cylindrical chamber 41 a piston 42 is displaceable by means of a solenoid 43. The piston 42 has end sections 44 and an intermediate section which fill the cross section of chamber 41, and reduced diameter sections 46 between the intermediate section 45 and each end section 44. The reduced diameter sections 46 define cavities 47 within chamber 41, one of which communicates with high-pressure port 49 of the control valve 27, whereas the other communicates with low-pressure port 50. In the configuration shown, the pressure-controlled port 51 is blocked by intermediate section 45. A feedback duct 52 extends from pressure-controlled port 51 to the end of chamber 1 opposite to solenoid 43.

The solenoid 43 applies a force to piston 42 which is proportional to the intensity of a current flowing through solenoid 43. If the force is directed to the right in Fig. 2, the piston is displaced to the right -10 -so that high-pressure port 49 and pressure-controlled port 51 come to communicate. A flow of hydraulic fluid results, and the pressure at pressure-controlled port 51 increases until the pressure communicated to the right-hand end of piston 42 by feedback duct 52 compensates the force of solenoid 43. Conversely, if the solenoid applies a force directed to the left, hydraulic fluid is drained from pressure-controlled port 51 to low-pressure port 50 until the pressure decrease at the right-hand end of piston 42 compensates the force. In this way, a pressure is established at the pressure-controlled port 51 which is a direct function of the current in solenoid 43.

Using a position sensor 53, e.g. a Hall sensor, the position of the piston 42 can be measured. The position is representative of the degree of overlap between the intermediate section 45 and the pressure-controlled port 50 and, hence, of the flow rate of fluid from high-pressure port 49 to pressure-controlled port 51 or from pressure-controlled port 51 to low-pressure port 50. By controlling the solenoid current such that a predetermined piston position is reached, it is possible to control not the pressure at port 51 but the flow rate through control valve 27 to or from actuator 20 or 21.

Since the flow rate is directly proportional to the speed of the actuator, transmission controller 5 can control the advancing speed of shift fork 21 based on data from positioning sensor 53. Alternatively, the advancing speed might be measured and controlled directly using Hall sensor 34.

A typical gear engaging procedure will be described referring to Fig. 3. Fig. 3 comprises four diagrams labelled a) to d). Diagram a) illustrates the displacement of the shift sleeve, diagram b) illustrates the development of rotation speeds of layshaft 14 and of -11 -a gearwheel, e.g. gearwheel 11, which is being synchronized to the layshaft 14, diagram C) shows the development of the pressure applied to shift sleeve 17 by shift fork 22, and diagram d) indicates the speed of displacement of the shift sleeve 17.

The gear engaging procedure starts at time to.

Shift sleeve 17 is in its neutral position, layshaft 14 is rotating at a high speed referred to as C14 in diagram b), whereas gearwheel 11 is going slowly at. The force applied to shift sleeve 17 by shift fork 22 begins to increase gradually, and so does the speed of shift sleeve 17. At a time t1 transmission controller 5 detects that a desired speed v1 of the shift sleeve for the displacement from neutral to synchronizing positions has been reached. It therefore decreases the current in solenoid 43 of control valve 27, and the driving force applied to shift sleeve 17 decreases just so much that its speed is held constant at v1.

When a predetermined time has elapsed since to or ti or, preferably, if transmission controller 5 detects that a predetermined position x short of the synchronizing position has been reached, it ramps down the speed of shift sleeve 17, so that when the synchronizing position is reached at time t3, the shift sleeve has a speed v2 which is substantially less than the constant travelling speed v1 between times t1 und t2.

By setting v2 low enough, a noisy clash of the friction surfaces of baulk ring 19 and gearwheel 11 can be avoided when the shift sleeve 17 reaches the synchronizing position.

While advancing the shift sleeve to the synchronizing position, transmission controller determines the rotation speeds 14, of gearwheel 11 -12 -and layshaft 14. The rotation speed i4 of layshaft 14 is directly proportional to the vehicle speed and is therefore derived from a speedometer signal. The rotation speed of gearwheel 11 is the product of the rotation speed of hollow shaft 3 and a transmission ratio specific to gearwheel 11. Therefore the rotation speeds of all driven gearwheels can be calculated from rotation speeds of solid and hollow shafts 2, 3 measured by associated sensors 33. Such sensors are conventionally connected to transmission controller 5 in order to enable it to select gears appropriately.

The transmission controller 5 decides a desired pressure which is to be applied between the friction surfaces of synchronizer 15 and gearwheel 11 during synchronization. This decision can be carried out e.g. by consulting a lookup table which indicates a desired synchronizing pressure p as a function of rotation speeds, or of the slip speed =1411. The values of p laid down in the lookup table have to meet two criteria. The first is that they should allow synchronization to be achieved in a time span which is substantially independent of the slip speed.co at time t3. This requires the values of Ps to increase with slip speed. On the other hand, p5 must not be so high that at high slip speeds overheating of the friction surfaces can result. Therefore, there is a limit of the slip speed above which the duration of the synchronization must be allowed to increase with time, and in order to keep the temperature of the friction surfaces at a tolerable level, p decreases with the slip speed if the slip speed is above this limit.

At moderate slip speeds, the pressure may be raised to p practically instantaneously at t3, as shown by a dashed line in diagram c). If the slip speed is -1.3 -high, it is preferable that the transmission controller 5 gradually raises the desired pressure in a time interval from t3 to t4. In this way, the heating power at the friction surfaces, being proportional to the product of pressure and slip speed, can be kept at a tolerable level at the beginning of the synchronization process, when the slip speed is highest.

While the synchronization is taking place, the transmission controller 5 continuously compares the slip speed zc to a threshold which is a predetermined function of the synchronization pressure Ps. The threshold.wmjn is predetermined so that if the pressure at the friction surfaces is continuously ramped down from Ps to a desired final pressure f at the end of synchronization, synchronization will be finished within a predetermined time span. Throughout this time span, from t5 to t6, the transmission controller 5 continuously monitors the development of the slip speed and, if necessary, adjusts the pressure ramp so that synchronization will be finished at the programmed instant t6.

At t6 the blocking effect of the baulk ring 19 is released, and the shift sleeve 17 becomes free to advance towards the engaged position, driven by pressure pf. While the shift sleeve 17 advances towards the engaged position, its speed is controlled by transmission controller 5, gradually decreasing it over a major portion of the path from synchronizing to engaged positions, so that when the engaged position is finally reached at time t7 the speed of the shift sleeve 17 has again decreased to a low value similar to v2, so that the shift sleeve and the gearwheel 11 engage without clashing.

List of reference signs 1 input shaft 2 solid shaft 3 hollow shaft 4 double clutch transmission controller 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 control valve 28 pump 29 accumulator way valve 31 way valve 32 way valve 33 sensor 34 Hall sensor 41 chamber 42 piston 43 solenoid 44 end section 45 intermediate section 46 reduced diameter section 47 cavities 48 cavities 49 high-pressure port 50 low-pressure port 51 pressure-controlled port 52 feedback duct 53 position sensor

Claims

Claims 1. A method for controlling engagement of a gear in a motor vehicle gearbox comprising a synchronizer (15, 16), the synchronizer (15, 16) having a member (17) which is displaceable between neutral, synchronizing and engaged positions, and an actuator (20, 21) for displacing the displaceable member (17), the method comprising the steps of a) displacing the displaceable member (17) from the neutral to the synchronizing position (to-t3), b) applying a pressure directed towards the engaged position to the displaceable member (17) while in the synchronizing position (t3-t6), and c) displacing the displaceable member (17) from the synchronizing position to the engaged position (t6-t7), characterized in that in step a) the speed of the displaceable member is controlled to match a target value (v1) and that in step b) the pressure applied to the displaceable member is controlled to match a target value (p5).
2. The method of claim 1, wherein in step c) the speed of the displaceable member is controlled to match a target value (ps).
3. The method of claim 1 or 2, wherein in step a) the speed target value is reduced as the displaceable member approaches its synchronizing position (t2-t3)
4. The method of any of the preceding claims, further comprising the step of determining a slip speed (M) between two rotatable members (11, 14) of the gearbox adapted to be coupled by the synchronizer (15), and setting the pressure target value (ps) depending on the slip speed (M))
5. The method of claim 4, wherein there is a first slip speed range in which the pressure target value is set to increase with the slip speed.
6. The method of claim 4 or 5, wherein there is a second slip speed range in which the pressure target value is set to decrease with the slip speed.
7. The method of claims 5 and 6, wherein the second range is at higher slip speeds than the first range.
8. The method of any of the preceding claims, wherein the pressure target value is increased in at least a first part (t3-t4) of step b)
9. The method of any of the preceding claims, wherein the pressure target value (Ps, pfl is decreased in at least a second part (t5-t6) of step b)
10. The method of claim 9, wherein the pressure target value (pr) at the end (t6) of step b) is independent of the slip speed (&o) at the beginning of step b)
11. A motor vehicle gearbox comprising -at least two rotatable members (11, 14); -at least one synchronizer (15) for synchronizing said rotatable members (11, 14), the synchronizer (15) having a member (17) which is displaceable between neutral, synchronizing and engaged positions, -an actuator (20) for displacing the displaceable member (17); -a transmission controller (5) adapted to control the speed of the displaceable member (17) using said actuator (20) while the displaceable member is between its neutral and synchronizing positions and to control the pressure applied to the displaceable member (17) while the displaceable member (17) is in its synchronizing position.
12. A computer software product having source code means for carrying out the method according to one of claims 1. to 10 on a programmable processor of a transmission controller.
13. A computer-readable data carrier, characterised in that program instructions which enable a programmable processor to carry out the method according to one of claims 1 to 10 are recorded thereon.