GB2470013A

GB2470013A - Synchronizer control using estimated heating power released by friction surfaces

Info

Publication number: GB2470013A
Application number: GB0907688A
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: GB0907688D0

Abstract

A motor vehicle double clutch transmission comprises at least one synchronizer (15, 16, fig 1) operated by an actuator (20, 21) which applies a pressing force F to rotating friction surfaces of the synchronizer (15, 16). A transmission controller controls the pressing force F using a method comprising the steps ofa) determining a speed slip ∆ω between the rotating friction surfaces; andb) controlling the pressing force F applied to the rotating friction surfaces based on the slip speed ∆ω. Step b) comprises sub-step b1) which estimates heating power P, Pav; S6, S7 released at the rotating friction surfaces based on the slip speed ∆ω and the pressing force F; and sub-step b2) which compares S8 the heating power P, Pav, S6, S7 to a first threshold Pmaxand reducing S9 the force if the threshold is exceeded. A computer software product having source code means which carries out the method on a programmable processor of the controller that has a computer-readable data carrier.

Description

Method and Apparatus for controling torque transmission

Description

The present invention relates to a method for controlling torque transmission, in particular between mating friction surfaces of a synchronizer, and to apparatus for carrying out the method, such as a transmission controller.

EP 1 505 320 A2 discloses a method for controlling the engagement force of the synchronizers of a dual clutch transmission. In this method, a slip speed, i.e. a difference of rotation speeds of a rotating shaft and a gearwheel to be synchronized to said shaft is determined, and the engagement force applied to the synchronizer is controlled based on this slip speed.

According to this document, the force which must be applied to engage the synchronizer can differ between the various gears of the transmission due to different inertia of rotating components associated with each gear, to the design of the synchronizer system, and to the speed change that must occur during a particular synchronization event. Specifically, this document teaches that the force imparted to the synchronizer should be the higher, the greater the slip speed is.

The document further suggests to sense the temperature of the transmission, since drag forces to which the various moving components of the transmission are subject depend on the temperature of the lubricating fluid in the transmission. The colder this fluid is, the more viscous it tends to be, so that at low transmission temperatures it may be necessary to apply a higher force for displacing the synchronizer than at high temperatures, in order to overcome a deceleration due to drag.

In each synchronization event, heat is released at friction surfaces of a synchronizer. The amount of heat released depends on various factors, such as the speed slip, the inertia of transmission components connected to the surface which is accelerated or decelerated during synchronization, drag, etc. The temperature reached by the friction surfaces in a synchronization event depends on the amount of heat released, the time span in which it is released, the thermal capacity of the friction surfaces, and the thermal conductivity of their environment. Since wear of the friction surfaces increases quickly, the hotter these become, it is desirable to keep the temperature of the friction surfaces low, even in those rare synchronization events in which the initial slip speed is very high.

Since the thermal conductivity of the environment of the friction surfaces has natural limits due to the materials used for building the transmission, the conventional approach is to make the thermal capacity of the transmission high by using large friction surfaces. On the other hand, if the friction surfaces could be made small, a more compact transmission could be designed.

This is desirable not only because the space saved by using a compact transmission can be put to other uses, but also because the ensuing reduction of weight and inertia leads to fuel economies.

In would be desirable, therefore, to have a method of controlling torque transmission between mating rotating friction surfaces, in particular in a synchronizer, in which overheating of friction surfaces can be reliably prevented without requiring the friction surfaces to have large dimensions.

According to the invention, this need is satisfied by a method of controlling torque transmission between mating rotating friction surfaces, comprising the steps of a) determining a slip speed between the rotating friction surfaces; and b) controlling a force applied to the rotating friction surfaces based on said slip speed, in which step b) comprises the sub-steps of bi) estimating the heating power released at the rotating friction surfaces based on said slip speed and said force; and b2) comparing the estimated heating power to a first threshold and reducing the force if the threshold is exceeded.

This first threshold should be selected taking account of the dimensions of the friction surfaces, their materials and the thermal conductivity of their environment so as to ensure that a safe temperature is not exceeded. Such a threshold of the heating power can be determined empirically for a given pair of friction surfaces.

In a first embodiment of the method, the force is set to a predetermined value while the threshold is not exceeded. This embodiment allows for particularly simple controls, but the duration of a synchronization event may vary depending on the initial slip speed.

In a second, preferred embodiment, the force is controlled so as to achieve a predetermined desired rate of the time derivative of the slip speed while the threshold is not exceeded. This time derivative can be set to be a predetermined fraction of the initial slip speed, so that different synchronization events will take the same time, regardless of the initial slip speed.

In order to provide for a reliable gear engagement at the end of synchronization, it may be provided that in step b2) the force is reduced only if it is above a predetermined minimum required for gear engagement.

Of course, if the force has been reduced in step b2), synchronization will take longer than if it had not. In order to avoid an unnecessary lengthening of the synchronization, the heating power may be compared to a second threshold, and the force increased if the heating power is below the second threshold. This second threshold might be equal to the first, but preferably it is lower in order to establish a hysteresis.

Since it is generally desirable to avoid any unnecessary temperature rise at the friction surfaces, the above-mentioned step of increasing the force may be carried out only in cases where this is necessary to reach a predetermined desired rate of the time derivative of the slip speed.

The above described method is applicable in an automated manual transmission. Such an automated manual transmission may comprise a controller for automatically deciding gear shifts and a user interface which enables a driver to request a gear shift, overriding a decision by the controller. In general, it can be assumed that if the driver decides to override the controller, his request should be observed as quickly as possible. Therefore, it may be appropriate to set the above-mentioned predetermined desired rate higher for a gear shift requested by the driver than for a gear shift decided by the controller. Generally, this will cause the force applied to the friction surfaces to be higher in case of a driver-requested gear shift than in a controller-decided one. Nevertheless, the first threshold (and, if defined, the second) can be the same for both kinds of gear shift, so that in both cases overheating is prevented just as efficiently.

On the other hand, if it is assumed that the driver's shift request is so urgent that a higher than normal temperature can be tolerated at the friction surfaces, the first threshold may be set higher for a gear shift requested by the driver than for a gear shift decided by the controller.

The object of the invention if further achieved by a motor vehicle transmission comprising -at least two rotatable members; -at least one synchronizer for pressing together and synchronizing said rotatable members; -an actuator for applying a pressing force to the synchronizer; and -a transmission controller adapted to control the pressing force of said actuator using the method as described above.

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 an automated manual transmission embodying the present invention; Fig. 2 is a schematic cross section of a pressure controller used in the transmission of Fig. 1; and Fig. 3 is a flowchart of the method of the invention.

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 transmission 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 that mesh with driven -7-.

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 layshaft(s) 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 which are dragged along when shift sleeve 17 is displaced from its neutral position and which have an annular friction surface, and friction surfaces rigidly connected to the adjacent gearwheels, against which the annular friction surface of a baulk ring 19 is pressed when it is dragged along by shift sleeve 17 to the synchronizing position. While the two friction surfaces are not synchronized, the baulk ring 19 blocks further progress of the shift sleeve 17 towards 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 sensor 34 associated to it for monitoring a displacement of its shift fork 22 to the left, towards gearwheel 10 or 12, and to the right, towards gearwheel 11 or 13. 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 transmission) 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 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.

-10 -The control valve of Fig. 2 is advantageous for the present invention in that it allows to control the force applied to the shift sleeve 17 in a simple open control loop. Of course, other types of control valves could also be used, if necessary in combination with a pressure sensor by which the force applied to shift sleeve 17 could be controlled in a closed loop.

Fig. 3 is a flow chart of a procedure carried out by transmission controller 5 at regular time intervals it at instants t. =t-1+At, before and during a synchronization event. The index i corresponds to successive iterations of the procedure and takes values i=0, 1, 2, In a first iteration of the procedure, with i=0, i.e at time t0, immediately when reaching the synchronizing position or while approaching it, transmission controller 5 detects the rotation speed of the gearwheel, say gearwheel 11, which is to be synchronized. To this effect, a rotation sensor 33 is provided at each shaft 2, 3, the rotation speed of shaft 3 is detected in step Si, and the rotation speed w1,i of gearwheel 11 is calculated therefrom using a known transmission ratio r between gearwheels 7 and ii.

Secondly, the rotation speed C14,j of layshaft 14 is detected. It can be calculated directly e.g. from a speedometer signal.

The slip speed i.e. the difference between (14,j and is computed in step S4.

In step S5, transmission controller 5 applies a force F to the mating friction surfaces of the synchronizer 11 by supplying associated currents to the -11 -solenoids of control valves 27. In the first iteration of the procedure, Fo may be set to a predetermined default value. Preferably, the amount Fo is chosen by the transmission controller 5 as a function of the slip speed &o computed in step S4 such that if the force having the amount F0 was applied continuously, synchronization would be achieved within a predetermined time after to or after a predetermined number n of iterations of the procedure of Fig. 3, independently of the initial slip speed M0.

In step S6, an estimate of the instantaneous heating power P. released at the friction surfaces is obtained by multiplying the force F and the slip speed In step S7, a quantity representative of average heating power Pav,j is obtained by computing the sum of a predetermined number n of the most recent instantaneous power values P,P-1, ..., ap,i At the beginning of the synchronization, if the procedure of Fig. 3 has not yet been iterated n times, and n instantaneous power values are not yet available, unavailable ones are assumed to be zero.

In an alternative embodiment in which the above assumption that the time zt between successive iterations is constant does not hold, an estimate of the average heating power might be defined as Pt,,, _ fP(t)dt wherein Dt is the observation time interval, and P(t) is a step function which is constant at a power level -12 -measured at the most recent measuring instant until a new power measurement is made.

In a further alternative embodiment, Pav,i iS calculated as follows: P, + cP1; i=O, 1, wherein c is a forgetting factor <1 which ensures that converges if the instantaneous power P is constant, and Pav,-i is assumed to be zero. In a given synchronizing event, c is constant if the time interval Lt between two power measurements is constant, too. Else, it is an increasing function of the time between two power measurements. By a judicious selection of the forgetting factor c, a close correlation between Pav,j and the real temperature of the friction surfaces can be achieved. In order to take account of bulk heating of the synchronizer in previous synchronizing events, c can be defined as a function of energies dissipated in the synchronizer in previous synchronization events and the time elapsed since then.

Step S8 compares the average power Pav,j obtained in step S7 to an upper threshold Pmax. The threshold Pmax is predetermined so that if F&.o equalled Pmax, no overheating of the friction surfaces would result. Accordingly, if the threshold is found to be exceeded, a new value F+1 of the pressing force is decided in step S9, which will be applied at the synchronizer in step S5 of the next iteration of the procedure.

Optionally there may be between steps S8 and S9 a step SB' of comparing the present force value F1 with a predetermined threshold This threshold Fmin corresponds to slightly more than the minimum force required for displacing the shift sleeve 17 into -13 -engagement with gearwheel 11. If the force F was decremented below this threshold Fmi, there is a risk of the shift sleeve not reaching the engaged position.

Therefore, the force reduction of step S9 is carried out only if the current force F1 is high enough to avoid this risk.

It is readily apparent that during the first iterations of the procedure high instantaneous power values P will be accepted without causing the average power to exceed the threshold Pmax. This is acceptable since at the beginning of a synchronization event the friction surfaces can be assumed to be cold, so that there is no risk of overheating during the first iterations, even if the rise in temperature should be steep. Further, since Cj3 decreases as the index I increases during the synchronization, so do P1 and PaV,1.

Therefore, although the heating power released at the beginning of a synchronization may be such that, if released continuously, it would cause the friction surfaces to overheat, the decrease of the slip speed &o may prevent such overheating.

If the upper threshold Pmax was not exceeded in step S8, the average power 9av,i may be compared to a lower threshold Pmjn in step Sb. If it is below this threshold, the pressing force F might well be increased without any risk of overheating. In this case, a new and increased value F1+1 to be set in step S5 of the nest iteration may be decided straight away (S13) Alternatively and as shown in the flowchart, the transmission controller 5 first estimates the time derivative of the slip speed 1th by computing the difference between the present slip speed Awj and the one &Dj-i obtained in the previous iteration of the procedure and dividing it by the time t between two iterations. If -14 -this rate &) is less than a minimum ACA required for finishing the synchronization within a reasonable time, the force F is indeed increased (S13); else it is left unchanged in order to keep the temperature of the friction surfaces as low as reasonable possible.

By repeating the above procedure a large number of times during a synchronization event, the pressing force F applied to the friction surfaces is continuously adjusted in order to achieve fast synchronization without overloading the friction surfaces.

In the above described procedure, all synchronization events can be handled identically, regardless of whether they result from a decision of the transmission controller 5 to shift gears or whether they were triggered by input from the driver, e.g. by operating a dedicated switch for requesting up-or down-shifting, or by stepping hard on the accelerator pedal.

In particular in the last case, also referred to as kickdown, it is reasonable to assume that a fast reaction of the transmission controller 5 is required, and in order to increase the shifting speed, a higher temperature of the friction surfaces may be tolerated.

This may be taken account of by the transmission controller 5 e.g. by selecting, when step S5 is executed for the first time, a pressing force F which is higher than normal and would thus lead to faster synchronization than in case of a gear shift decided by the transmission controller 5 itself.

The fact that the present force is set higher in step S5 will cause the temperature of the friction surfaces to increase faster, but they will still not overheat since the temperature limiting mechanism of step -15 -S8 remains active. If a certain increase of wear is judged acceptable in order to further increase the shifting speed in case of a kickdown, the threshold Pmax may be set higher in case of a kick down shift than in case of a shift decided by transmission controller 5.

In case of a shifting process in which initial and final gears differ by two or more, particularly high initial slip speeds ioc occur, and the friction surfaces may heat up considerably. If the driver specifies such a gear shift, according to an advanced embodiment of the invention the transmission controller checks before starting synchronization whether the slip speed is above a predetermined threshold, and if so, replaces the shifting process specified by the driver by a two-fold switching process, from the initial gear to an intermediate gear and from the intermediate gear to the final gear. In this way the slip speed may be maintained below said threshold in both consecutive shifting procedures.

Although the method of the invention was described above referring only to friction surfaces of a synchronizer, a skilled person will readily recognize that it is applicable to other kinds of friction surfaces by which two rotating elements can be coupled, in particular to a clutch located between a combustion engine and a transmission of a motor vehicle.

Claims

-16 -Claims 1. A method of controlling torque transmission between mating rotating friction surfaces, comprising the steps of a) determining a speed slip (Ao) between the rotating friction surfaces; and b) controlling a force (F) applied to the rotating friction surfaces based on said slip speed (M)); characterized in that step b) comprises the sub-steps of bi) estimating the heating power (P, Pay; S6, S7) released at the rotating friction surfaces based on said slip speed (M) and said force (F); and b2) comparing (S8) the heating power (Pay) to a first threshold (Pmax) and reducing (S9) the force if the threshold is exceeded.
2. The method of claim 1 wherein if the threshold (Pmax) is not exceeded, the force is set to a predetermined value.
3. The method of claim 1 wherein if the threshold (Pmax) is not exceeded, the force (F) is controlled to achieve a predetermined desired rate of the time derivative (zth) of the slip speed (ta',).
4. The method of any of the preceding claims, wherein in step b2) the force is reduced (S9) only if it is above a predetermined minimum (Fmin; S8').
5. The method of any of the preceding claims, further comprising the sub-step of b3) comparing (SlO) the heating power to a second threshold (Pmjn) which is lower than the first threshold (Pmax) and increasing the force (Sl3) if -17 -the heating power (Pay) is below said second threshold (Pmjn)
6. The method of claim 5, wherein sub-step b3) is executed only if the time derivative of the slip speed is below a predetermined desired rate (Sil, S12)
7. The method of any of the preceding claims, wherein the friction surfaces are part of a synchronizer (15), in particular a synchronizer (15) of an automated manual transmission.
8. The method of claim 3 or claim 6, wherein the friction surfaces are part of synchronizer (15) of an automated manual transmission which further comprises a controller (5) for automatically deciding gear shifts and a user interface enabling a driver to request a gear shift, and wherein the predetermined desired rate is set higher for a gear shift requested by the driver than for a gear shift decided by the controller (5).
9. The method of claim 8, wherein at least the first threshold (Pmax) is the same for a gear shift requested by the driver and for a gear shift decided by the controller.
10. The method of claim 8, wherein at least the first threshold (Pmax) is set higher for a gear shift requested by the driver than for a gear shift decided by the controller.
11. The method of any of the preceding claims, wherein the heating power (Pay) is estimated by averaging -18 -repeated measurements of slip speed (&o) and force (F).
12. A motor vehicle transmission comprising -at least two rotatable members (11, 14); -at least one synchronizer (15) for pressing together and synchronizing said rotatable members (11, 14); -an actuator (20) for applying a pressing force to the synchronizer (15) ; -a transmission controller (5) adapted to control the pressing force of said actuator (20) using the method of any of the preceding claims.
13. A computer software product having source code means for carrying out the method according to one of claims 1 to 11 on a programmable processor of a transmission controller.
14. 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 11 are recorded thereon.