WO2005047738A1

WO2005047738A1 - Control of the line pressure in a multi-regime continuously variable transmission

Info

Publication number: WO2005047738A1
Application number: PCT/GB2004/004642
Authority: WO
Inventors: John Fuller
Original assignee: Torotrak (Development) Limited
Priority date: 2003-11-07
Filing date: 2004-11-04
Publication date: 2005-05-26
Also published as: GB0326077D0

Abstract

There is provided a method of changing regime in a continuously variable ratio transmission having a variator and a continuously variable ratio transmission having a variator. The continuously variable ratio transmission having a variator (20) and a gearing through which the variator can be coupled between a rotary input and a rotary output of the transmission in either of a low regime and high regime, a clutching (80,82) arrangement for selectively engaging low regime and high regime so that a change of regime can be effected by engaging one regime and disengaging the other, and a hydraulic control comprising a variator control valve arrangement (52,54) for applying an adjustable control pressure to the variator and a hydraulic circuit which communicates with the variator, the transmission being characterised by provision of means for causing pressure in the compliance to be raised prior to engagement of the new regime in a regime change.

Description

DESCRIPTION

CONTROL OF THE LINE PRESSURE IN A MULTI-REGIME CONTINUOUSLY VARIABLE TANSMISSION

The present invention is concerned with regime change in a continuously variable transmission.

In a continuously variable transmission a device referred to as a "variator" provides the required stepless variation of drive ratio and is coupled by gearing between the transmission input (typically the engine driveshaft) and its output (e.g. the driven wheels of a motor vehicle). Typically this gearing serves to expand upon the range of ratios provided by the variator itself through the provision of two or more different "regimes". In a two regime motor vehicle transmission there is (i) a low regime in which the variator' s drive ratio maps onto a low range of transmission ratios (which, as is known in the art, may include reverse and low forward gears as well as "geared neutral", in which the transmission provides infinite reduction so that its output is stationary despite rotation of its input) and (ii) a high regime, in which the variator' s drive ratio maps onto a higher range of forward gears, typically going up to a deep overdrive. Clutches or brakes serve to engage and disengage the two regimes. The shift from one regime to another is typically carried at "synchronous" ratio, which is the ratio at which a change of regime produces no change in the ratio of the transmission as a whole. The principle is very well known and discussed for example in Torotrak's US patent 4464952 which also explains how an epicyclic gear arrangement is used to provide geared neutral operation. Hydraulics are used to control the transmission and are required to provide

for stable control of the variator itself and also to provide rapid pressure response

when necessary, particularly during regime change when pressures applied to both

the regime control clutches and the variator must be adjusted within limited time.

These requirements for stable variator control and rapid pressure response

are potentially conflicting. Analysis of the requirements for stable control of

certain types of variator suggest that the hydraulics should provide both (a)

resistance to fluid flow to and from actuators controlling the variator and (b)

hydraulic compliance, serving to modify the relative phase of flow to/from the

variator actuators and the attendant changes in hydraulic pressure. Such

compliance consumes fluid flow in response to changes in pressure and so reduces the speed of the pressure changes which the circuit can generate.

A pump driven by the engine is typically used to supply the pressurised

fluid required by the hydraulics and it is desirable to minimise the required pump

capacity in order to limit its energy consumption and size. The rate of flow of

fluid required by the hydraulics is highly variable. Events such as regime change

and rapid change of variator ratio require large, albeit transient, rates of flow.

Previous hydraulic control circuits have incorporated an accumulator formed in

known manner by a cylinder connected to the hydraulics and having a spring

biased piston. The accumulator contributed to the available fluid flow when

necessary. A second accumulator contributed, or in some versions simply a

reservoir connected to the hydraulics (utilising compressibility of the fluid)

contributed to the compliance of the hydraulics and so to variator stability. However minimising volume of the transmission is very important, as is reducing its total component count for the sake of economy. Consequently dispensing with one or both of the accumulators is desirable, but this must be done without unacceptably impairing the circuit's pressure response rate.

In accordance with the present invention there is a method of changing regime in a continuously variable ratio transmission having a variator, gearing through which the variator can.be coupled between a rotary input and a rotary output of the transmission in either of a low regime and a high regime, a clutching arrangement for selectively engaging low regime and high regime, and hydraulics comprising a variator control valve arrangement for applying at least one adjustable control pressure to the variator and a hydraulic compliance which communicates with the variator, the method comprising changing the state of the clutching arrangement to engage the new regime and disengage the old regime and being characterised in that it further comprises increasing pressure in the hydraulic compliance prior to engagement of the new regime. The "clutching arrangement" comprises some mechanical means of engaging/disengaging the two regimes but may use some arrangement other than a conventional clutch. For example in some transmissions a brake serving to lock up a component of the epicyclic shunt serves to engage at least one of the regimes. In accordance with a second aspect of the present invention there is a continuously variable ratio transmission having a variator, gearing through which the variator can be coupled between a rotary input and a rotary output of the

transmission in either of a low regime and high regime, a clutching arrangement for selectively engaging low regime and high regime so that a change of regime

can be effected by engaging one regime and disengaging the other, and hydraulics

comprising a variator control valve arrangement for applying at least one adjustable control pressure to the variator and a hydraulic compliance which

communicates with the variator, the transmission being characterised by provision

of means for causing pressure in the compliance to be raised prior to engagement

of the new regime in a regime change.

A specific embodiment of the present invention will now be described, by

way of example only, with reference to the accompanying drawings in which:- Figure 1 is a highly simplified and partly sectional representation of a

rolling-traction variator of a type which is known in itself and is suitable for

implementing the present invention; and

Figure 2 is a diagrammatic representation of a hydraulic circuit embodying aspects of the present invention.

The present invention has been developed in connection with a

transmission using a variator of the toroidal-race rolling-traction type. It is

considered potentially applicable to transmissions using other types of variator.

Nonetheless the toroidal-race variator in question will be briefly described in order

to illustrate certain relevant principles. It comprises a pair of outer discs 12, 13

mounted upon a variator shaft 15 for rotation therewith, and an inner disc 14

which is journalled about the shaft so that it can rotate relative to it. Shaped faces

of the discs seen for example at 17 and 19 define a pair of toroidal cavities 22, 23

and both contain rollers which run on the shaped faces of the discs to transfer drive from the outer discs to the inner or vice versa. To simplify the drawing it

shows only one roller 20 but typically three rollers are provided in each toroidal cavity.

Each roller is mounted in a manner permitting it to move along a

circumferential direction about the shaft axis. The roller is also able to "precess".

That is, the roller's axis is able to rotate, changing the inclination of the roller to

the shaft axis. In the illustrated example the roller is mounted in a carriage 26 coupled by a stem 28 to a piston 30 of an actuator 32. A line 33 from the center of

the piston 30 to the center of the roller 20 constitutes a "precession axis" about

which the whole assembly can rotate. Changes in the inclination of the roller

result in changes in the radii of the paths traced on the inner and outer discs 12, 14 by the roller. Consequently a change in roller inclination is accompanied by a

change in variator drive ratio. Note that the precession axis does not lie precisely in a plane perpendicular

to the disc axis, but is instead angled to this plane. This angle, labeled CA in

Figure 1, is referred to herein as the "castor angle". Furthermore, the action of the

discs 12, 13, 14 upon the roller tends to maintain the rollers at such an inclination

that the roller axis intersects the disc axis. This intersection of the axes can be

maintained, despite movement of the roller along its circumferential path, by

virtue of the castor angle. The result is that translational movement of the roller

about the shaft axis is accompanied by precession of the roller and so by a change

in variator drive ratio. If one neglects slip between the roller and the discs, the

position of the variator roller corresponds to the variator drive ratio. The actuator 32 receives opposed hydraulic fluid pressures through lines

SI, S2 and force applied to the roller by the actuator corresponds to the difference

in pressures in the lines. This pressure difference is the principal control signal

applied to the variator, in this example. The effect of this force is to urge the

roller to move along its circumferential path about the shaft axis. Equivalently

one can say that the actuator exerts a torque about the axis upon the roller. The actuator torque is balanced by torque created by the interaction of the roller with

the discs. The roller exerts a torque T_in upon the input disc 12 and a torque T_out

upon the output disc 14. Correspondingly the discs together exert a torque T_in +

T_0ut upon the roller, about the disc axis. The quantity T_in + T_out (the "reaction torque") is equal to the actuator torque and so directly proportional to the control

signal formed by the aforementioned pressure difference. Hence the control signal

determines the reaction torque created by the variator.

The illustrated variator is of a type referred to as "torque controlled"

because, due to its construction, it serves not to directly control its own drive ratio

but instead to control the torques created at its input and output. It is the reaction

torque which is set by the applied hydraulic pressure difference, as just explained.

Changes of ratio result from the application of T_in and T_out, added to externally

applied torques from engine, brakes, road traction etc., to the inertias referred to

the variator input (which include the inertia of the engine) and its output (which

include vehicle inertia, in a motor vehicle transmission). Variator ratio does not,

therefore, correspond in any direct way to the hydraulic control signal applied to

the variator. There is potential for the variator' s ratio to vary in an oscillatory manner,

which is highly undesirable. One of the reasons for this is that one side of the

variator is coupled to the vehicle wheels through the transmission's driveline

which, inevitably, is of finite stiffness and so is capable of winding up and

unwinding in the manner of a torsional spring, which can drive low frequency

oscillation. The hydraulics controlling the variator are designed to damp variator

oscillation and they can do so because flow to and from the variator through each

of the lines SI and S2 passes through a flow resistance or damper (not seen in

Figure 1 but indicated at FI and F2 in Figure 2) which creates pressure change in

response to flow, and also because the circuit provides hydraulic compliance.

Changes in variator ratio are, as will be understood from the aforegoing,

accompanied by movement of the pistons 30 of the actuators 32. Fluid thereby displaced creates flow through the resistance, which therefore creates a pressure

change. This pressure change created by the flow resistance tends to oppose the

ratio change. Analysis and experiment by the applicant have shown however that

in itself this pressure change does not actually damp oscillatory variator behaviour

unless there is sufficient compliance in the circuit to provide a phase difference

between the change in ratio and the consequent change in pressure. The means used to provide both hydraulic damping and compliance will be described below.

To provide traction between the variator rollers and discs a traction

loading actuator 34 acts upon the outer variator disc 13. Pressure in working

chamber 36 of this actuator urges disc 13 toward the other discs 14, 12 and since

disc 12 is fixed relative to the shaft and so prevented from moving to the left, as viewed, the effect is to bias the discs and the rollers into engagement with each other. In operation the rollers and discs do not actually make contact, being separated by a film of hydraulic fluid maintained between them by continually spraying fluid onto the rollers. Drive is transmitted by virtue of shear within the fluid film. It is necessary to provide a continual flow of fluid to the rollers. Figure 2 illustrates a hydraulic circuit for controlling the transmission. The circuit is driven by a pump 50 formed in the present embodiment as a fixed displacement pump driven from the vehicle's engine, whose output flow thus varies with engine speed. The pressure supplied to the hydraulics is controlled by means of a pilot operated valve 52 which is itself controlled by means of a pilot pressure signal from a solenoid valve 54. This type of two stage valve arrangement is in itself known in the art and makes it possible to specify a relatively small and economical solenoid valve, since this component is not required to handle significant through-flow of fluid. The pilot operated valve 52 through which the supply to the hydraulics flows is a larger unit. Solenoid valve 54 is fed, through line 56, from the downstream side of the pilot operated valve 52. The solenoid's force is opposed by a pilot signal through line 58, representing the solenoid valve's output pressure, and also by a spring. The solenoid valve is controlled by associated electronics. Its output serves as a first pilot signal upon the pilot operated valve and this signal is argumented by a biasing spring but opposed by a second pilot signal taken from the pilot operated valve's output side. Based upon a comparison of these signals the pilot operated valve either leads the pump output to the hydraulics or, through line 60, recirculates it to the pump input. Hence the output of the pilot operated valve varies with the solenoid valve's

output pressure but exceeds it by some offset determined by the spring.

The actuators used to control the variator rollers are indicated in highly

schematic form in Figure 2 at 62, 62', 62^".. These correspond to the actuator 32 of

Figure 1, each having a piston 64, 64'.. which is coupled to a respective variator

roller, although the rollers are omitted from Figure 2. Pressures in the variator

supply lines SI, S2 leading respectively to opposite sides of the actuators are controlled by means of respective pressure reducing valves VI, V2. These serve

to connect their respective supply lines either to the pressure source or to a

pressure sink 66 (which is in fact the variator's lubrication circuit, as will be

explained below) in dependence upon a comparison between (a) a control input

from a solenoid and (b) a pilot pressure signal taken through a respective feedback

line 68, 70 from the valve's output. Two of the cylinders 62, 62' act as "master" cylinders and hydraulically

limit travel of the variator actuators and rollers - an "end stop" function. Thus

cylinder 62 is the master for supply line VI. During normal operation (away from

the hydraulic end stops) fluid can flow into the supply line through a non return

valve 69 which communicates with end ports 70 in all of the cylinders.

Exhaustion of fluid to reduce the supply line pressure is via master cylinder 62,

the fluid passing into this cylinder via its end port 70 and out through a side port

72. If the piston 64 within cylinder 62 moves far enough to the left to close the

side port 72 then the route for exhaustion of fluid from all of the left hand working

chambers of the actuators is closed. Neglecting leakage, further leftward movement of the actuators is prevented. The second master cylinder 62^' limits rightward actuator motion by controlling flow out of the supply line S2 in similar manner.

Valves 74 serve to release any gas from the hydraulics, by venting to the exterior when pressure demand is low, in order to reduce aeration of the hydraulic fluid which can modify the circuit's pressure response and so affect performance.

A higher pressure wins valve 76 is connected across the supply lines SI, S2. It serves to connect whichever of the supply lines is at any moment at higher pressure to a traction load supply line 78 leading to the traction load actuator, which is schematically indicated in Figure 2 at 34. Connecting the traction load actuator to the higher pressure supply line is in itself known and the function and advantages of such arrangements are explained in several of Torotrak's published patents including EP 0894210. Briefly, however, what the arrangement does is to control the traction load in sympathy with the "reaction force" applied to the variator rollers by the actuators 62. This is desirable in order to minimise energy losses at the roller/disc interface without permitting the roller to lose traction with the disc, leading to a potentially catastrophic level of slip between rollers and

^' discs. The "traction coefficient" at the roller/disc interface can be simply defined as the reaction force divided by the traction load. In the illustrated circuit, if the lower of the two supply line pressure is small enough to be neglected, then both reaction force and traction load are proportional to the higher of the supply line pressures and the traction coefficient is substantially constant.

The illustrated arrangement has high and low regime clutches 80, 82 to respectively engage the high and low regimes of the transmission. Each is governed by a respective clutch valve 84, 86, being connectable through the relevant valve either to the pressure source 50, 52, 54 or to the sinlc 66. The clutch valves are formed as solenoid controlled pressure reducing valves in which the action of the solenoid is opposed by a feedback pilot signal from the valve's output, enabling the solenoid directly to control output pressure. The construction of the .hydraulics having now been described, their operation will now be considered, beginning with the question of variator stability. As noted above, the hydraulics are required to damp oscillatory variator behaviour and can do so by virtue of a combination of hydraulic flow resistance and compliance. Figure 2 shows in schematic form two flow resfrictors FI and F2 through which flow into and out of the respective lines SI and S2 must pass as the pistons 64 (and the variator rollers) move in accordance with changes in variator ratio. Pressure changes created by the restrictors correspond to a modification to the reaction force tending to resist the piston movement. It should be noted that the necessary flow resistance can be provided in various ways. The illustrated restrictors simply comprise a narrowed passage in the supply line. A more sophisticated type of damper is described in Torotrak's pending patent application GB 0317818.3. However pressure reducing valves VI, V2 themselves may contribute sufficient flow resistance making separate components for this function superfluous. The illustrated circuit does not have an accumulator to contribute the required hydraulic compliance. However it has been recognised that the traction load actuator 34 can serve this function, being connected to the higher pressure supply line and capable of absorbing appreciable flow. Hence even without the accumulator the circuit meets the relevant stability criteria. It has also been noted above that the requirements for hydraulic compliance and for rapid hydraulic pressure response are potentially in conflict. The problem is made more acute because the illustrated circuit dispenses with an accumulator, so that the flow required by the circuit (inter alia to effect pressure changes, overcoming the compliance) must not exceed even transiently the capacity of the pump. In this regard, problems arise in particular during the process of regime change when several processes are carried out by the hydraulics :-

i. the clutch corresponding to the new regime is engaged; ii. the pressures in the variator supply lines are modified, in a manner which will be explained below; iii. pressure in the traction load actuator is modified in correspondence

with the pressure changes in the supply lines; and iv. the clutch corresponding to the old regime is released. Engaging the new clutch (step i) requires a significant input of fluid to fill the clutch and take up slack between the clutch plates. Changing the pressure in the

traction load actuator (step iii) also absorbs a significant fluid volume due to the actuator's compliance. Existing strategies for controlling regime change would potentially require more flow than the pump is capable of providing. To explain item ii (the required pressure changes in the SI, S2 supply

lines) in more detail, consider for example what happens as the vehicle is

accelerated from rest. The transmission is initially in low regime at geared

neutral. To create the torque at the driven wheels needed to accelerate the vehicle,

pressure is raised on the SI side of the variator relative to the S2 side. The

variator creates a positive reaction torque and a corresponding positive wheel

torque. As the vehicle accelerates the transmission ratio increases and the variator

correspondingly sweeps through its ratio range from geared neutral until it reaches

synchronous ratio, whereupon regime change is carried out. Upon regime change

the relationship between variator reaction torque and vehicle wheel torque is

changed in both sign and magnitude. After regime change a negative reaction

torque is needed if a positive wheel torque is to be maintained. This requires the

SI pressure to be dropped and the S2 pressure to be raised during regime change. That is, to maintain continuity of wheel torque the direction of action of the

actuators 62 is reversed at regime change. The magnitude of the actuator force

(and so the variator reaction torque) is also required to change, to maintain wheel

torque, because the effective gear ratio between variator and wheels is altered

upon regime change. Hence the new S2 pressure is not the same as the old SI

pressure. After regime change, if the vehicle continues to accelerate and the

transmission ratio continues to rise, the variator is swept back through its ratio

range. The modification to traction load (step iii) corresponding to the change in

magnitude of variator reaction torque must not be subject to appreciable time lag, despite the flow needed to change this pressure, because this would involve the risk of inadequate traction load leading to catastrophic variator traction failure. The inventor has recognised that the above problems can be alleviated by raising pressure in compliant parts of the hydraulics in anticipation of the regime change. In the illustrated circuit the traction load actuator is a major contributor of compliance. Where traction load pressure must increase upon regime change, the necessary pressure increase is carried out before the new regime is engaged. Not only does this reduce the maximum flow requirements during regime change but it also ensures that traction load does not fall unacceptably due to lags in traction load pressure change. Management of the traction load pressure increase is carried out by the transmission's control electronics. It is necessary to predict in advance that regime change is to take place. A very simple strategy would be to raise traction load pressure whenever transmission ratio is sufficiently close to synchronous. More sophisticated strategies additionally take account of factors such as rate of change of transmission ratio. The means for effecting the traction load pressure increase may take a variety of forms. It can be done using the valves VI, V2 which govern the SI, S2 pressures, by increasing the pressure demand to both valves at the same time and by the same amount. Since the variator reaction torque is determined by the pressure difference between the supply lines SI and S2, applying an identical offset to both does not change reaction torque. However pressure in the end load actuator, determined by the higher of the S1/S2 pressures, is increased. A typical sequence of operations approaching the change from low to high regime discussed above, in which SI is initially at higher pressure, would be:-

i. raise both SI and S2 pressures by the offset in anticipation of regime changes, thereby raising the traction load pressure; ii. engage the high regime clutch (in addition to the low regime clutch) as synchronous regime .is reached, briefly locking the transmission at

synchronous regime; iii. set S2 pressure to the level required after regime change (without the offset); iv. drop SI pressure to the level required after regime change, (without

the offset); v. disengage the low regime clutch.

The relative timing of steps iii and iv is chosen to ensure that, as S2

pressure rises and SI pressure falls, the greater of the two pressures is never below

that required to sustain traction loading.

Another way to effect the required traction load pressure increase prior to

regime change is represented in Figure 2 and uses a switching valve 90 to connect

the traction load actuator 34 to one or other of (a) the higher pressure wins valve

76 and (b) a controllable pressure source, formed in this instance by the main

pressure source 50, 52, 54. Using this arrangement, immediately prior to regime

change the valve 90 is switched to connect the traction load actuator 34 to the

pressure source and isolate it from the variator. During or after regime change valve 90 is switched back to re-connect the actuator to the higher pressure wins

valve and so to the variator.

In order to reduce consumption of fluid flow by whichever of the clutches

is being engaged during regime change, a two stage clutch fill process is

employed. This aspect is not new in itself. Prior to regime change a low "soft

fill" pressure is applied to the new clutch, filling its working chamber and taking

up slack between the clutch plates but permitting the clutch to slip. In this way the

flow needed to subsequently achieve a "hard fill" pressure to positivέly engage the

clutch is reduced.

The valves VI, V2 used to control the variator reaction torque, the valves

80, 82 used to control the clutches and the solenoid valve 54 all have exhaust

passages which are shown in Figure 2 as leading to the pressure sink 66. While they could all be led to the transmission's sump, the present circuit instead makes

use of the exhausted fluid for lubrication. This fluid is thus directed to jets used

to spray fluid onto the variator rollers, and to other parts of the transmissions

requiring lubricant feed.

Claims

1. A method of changing regime in a continuously variable ratio transmission

having a variator, gearing through which the variator can be coupled between a

rotary input and a rotary output of the transmission in either of a low regime and a

high regime, a clutching arrangement for selectively engaging low regime and

high regime, and hydraulics comprising a variator control valve arrangement for

applying at least one adjustable control pressure to the variator and a hydraulic compliance which communicates with the variator, the method comprising

changing the state of the clutching arrangement to engage the new regime and

disengage the old regime and being characterised in that it further comprises increasing pressure in the hydraulic compliance prior to engagement of the new

regime.

2. A method as claimed in claim 1 wherein the variator confrol valve

arrangement serves to apply two opposed adjustable control pressures to the

variator and the method further comprises adjusting the control pressures during

regime change to maintain continuity of transmission output torque.

3. A method as claimed in claim 2 wherein the pressure increase in the hydraulic

compliance is achieved by raising both of the control pressures.

4. A method as claimed in claim 1 or claim 2 comprising isolating the hydraulic

compliance from the variator prior to engagement of the new regime and re¬

connecting the compliance to the variator following regime change.

5. A method as claimed in any preceding claim wherein the compliance is

contributed by an actuator which serves to apply a traction load to enable transfer of drive within the variator.

6. A continuously variable ratio transmission having a variator, gearing through which the variator can be coupled between a rotary input and a rotary output of the transmission in either of a low regime and high regime, a clutching arrangement for selectively engaging low regime and high regime so that a change of regime can be effected by engaging one regime and disengaging the other, and hydraulics comprising a variator control valve arrangement for applying at least one adjustable control pressure to the variator and a hydraulic compliance which communicates with the variator, the transmission being characterised by provision of means for causing pressure in the compliance to be raised prior to engagement of the new regime in a regime change.

7. A continuously variable ratio transmission as claimed in claim 6 wherein the variator control valve arrangement serves to apply two opposed adjustable control pressures to the variator.

8. A continuously variable ratio transmission as claimed in claim 7 further comprising a higher pressure wins valve arrangement which serves to connect the higher of the two control pressures to a traction load actuator of the variator, the traction load actuator forming the aforementioned hydraulic compliance.

9. A continuously variable ratio transmission as claimed in claim 7 or claim 8 wherein the pressure increase in the hydraulic compliance is achieved by raising

both of the control pressures.

10. A continuously variable ratio transmission as claimed in any of claims 6 to 8 further comprising means for isolating the compliance from the variator.

11. A continuously variable ratio transmission as claimed in claim 10 wherein the said means comprise a valve for selectively connecting the compliance to one or other of the variator and a pressure source.

12. A method of operating a continuously variable transmission substantially as herein described with reference to and as illustrated in the accompanying

drawings.

13. A continuously variable transmission substantially as herein described with reference to and as illustrated in the accompanying drawings.