WO2020043894A1

WO2020043894A1 - A hydraulic control circuit for a continuously variable transmission, a continuously variable transmission and a method for controlling the clamping forces of a continuously variable transmission.

Info

Publication number: WO2020043894A1
Application number: PCT/EP2019/073248
Authority: WO
Inventors: Wouter Benedictus VAN BUUL; Dennis VAN LEEUWEN; Jacobus Hubertus Maria Van Rooij
Original assignee: Punch Powertrain N.V.
Priority date: 2018-08-31
Filing date: 2019-08-30
Publication date: 2020-03-05
Also published as: BE1026582B9; BE1026582A9; BE1026582B1; BE1026582A1; CN112789432A; CN112789432B

Abstract

A hydraulic control circuit (1) to control a CVT (20) that comprises a primary and a secondary pulley (21, 23) mechanically coupled by an endless flexible element (25). The hydraulic control circuit includes a feed pump (10; 10') to supply hydraulic fluid at a first fluid pressure value (P _i ), a bidirectional boost pump (12) coupled via a first and a second hydraulic branch (13, 14) to a respective pulley actuator. The bidirectional boost pump is controllable to displace hydraulic fluid in either direction between the first and the second hydraulic branch. A selection valve (15) controllably couples the feed pump to the first or the second hydraulic branch The hydraulic control circuit controls the selection valve to select the hydraulic branch of the actuator of the pulley that is indicated as the one having the smallest running radius in the prevailing operational state of the CVT.

Description

A hydraulic control circuit for a continuously variable transmission, a continuously variable transmission and a method for controlling the clamping forces of a continuously variable transmission.

BACKGROUND

The present invention relates to a hydraulic control circuit for a continuous variable transmission (also denoted as CVT), a continuously variable transmission and a method for controlling the transmission ratio of a continuously variable transmission and the clamping level for a

transmission element of said continuously variable transmission.

A continuously variable transmission to provide for a continuously variable transmission ratio between an input and an output shaft is described in NL1009954. The continuously variable transmission is provided with a first pulley with an input shaft and a second pulley with an output shaft, a flexible element that mechanically couples the pulleys and a hydraulic control circuit for hydraulic control of the pulleys. A first and a second pump driven by an electric motor pressurize a hydraulic fluid, with which the hydraulic control circuit operates the first and the second pulley in order to enable a slipfree transmission at a desired transmission ratio between the input shaft and the output shaft. The first pump serves as a feed pump and the second pump as a boost pump. A selection valve is provided that responds to the pressures present in the hydraulic branches to the actuators of the pulleys to achieve that the feed pump supplies a flow of hydraulic liquid at a first fluid pressure sufficient to operate the pulley with the lowest required pressure assuring slip-free operation of the CVT. The boost pump receives hydraulic fluid from the feed pump at the first pressure and supplies the received hydraulic fluid at a second pressure to enable control of the other one of the pulleys. Slipping of the flexible element results in a loss of efficiency, and can further lead to wear and breakage of the flexible element. In order to prevent slipping of the flexible element, a fluid pressure exceeding the minimal required fluid pressure with a safety margin is used to control the pulleys. Therewith the clamping force is set to a value that is higher than the clamping force that would be theoretically required for a slip-free operation, so as to take into account physical parameter uncertainties and external disturbances on the variator system. It is a disadvantage of the known transmission that the setting of the selection valve is not reliable when its transmission ratio is within a central range. Hence, in order to avoid the risk of a slipping operation of the variator, the safety margin needs to be relatively high in the central range to take into account this uncertainty. This, however, also reduces the efficiency of the transmission.

SUMMARY

It is an object of the present invention to provide measures that increase the transmission efficiency without increasing the risk of slipping of the flexible element.

To this end, a hydraulic control circuit is provided as claimed in claim 1. In the hydraulic control circuit according to claim 1, the hydraulic pressure with which the pulley actuators are controlled is dependent on which of the pulleys is determined to have the smallest running radius.

This measure is based on the observation that the risk of slip is the highest for the one of the pulleys with the smallest running radius.

Accordingly, the clamping level for this pulley needs to be most accurately controlled. By controlling the hydraulic pressure with which the pulley actuators are controlled in a manner dependent on which of the pulleys is determined to have the smallest running radius, the hydraulic control circuit will by definition supply the actuator of the pulley that is most susceptible to slipping with the fluid pressure of the feed pump, even if different piston surface areas were used to actuate the first and the second pulley. The clamping force exerted with the actuator of the second pulley depends on the clamping force ratio, which needs to be estimated and therewith introduces an additional uncertainty. As this uncertainty relates to the pulley with the larger running radius, which is less critical, this is not a serious issue. Should however the pulley with the smallest radius not be controlled directly by the fluid pressure, then it would be necessary to increase the safety margin, as s result of which on average higher actuation pressures would be required By removal of this uncertainty, the safety margin for the clamping force can be set to a lower value. Accordingly a better transmission efficiency is achieved in that in general lower hydraulic pressures suffice to ensure a slip-free operation of the variator.

In an embodiment the hydraulic control circuit is further configured to take into account an expected state change in selecting the destination for supplying the hydraulic fluid with the first pressure. For example, upon controlling a change of transmission ratio starting from a state wherein the pulleys have an equal running radius, the hydraulic control circuit is arranged to supply hydraulic fluid with the first pressure to the actuator of the pulley which is predicted to have the lowest running radius after said change.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects are described in more detail with reference to the drawings. Therein:

FIG. 1 schematically shows a powertrain;

FIG. 2 shows a first embodiment of a hydraulic control circuit;

FIG. 3 shows a second embodiment of a hydraulic control circuit;

FIG. 4 shows a third embodiment of a hydraulic control circuit; FIG. 5A, 5B shows exemplary elements for use in the hydraulic control circuit.

DESCRIPTION OF EMBODIMENTS

FIG. 1, by way of example, schematically shows a powertrain 2 in a vehicle to transfer power from a power source MOT, such as a combustion engine or an electric motor, to wheels WH of the vehicle. The powertrain includes a continuously variable transmission 20. In the embodiment shown, the powertrain 2 comprises further transmission elements, here a torque converter/lock-up clutch (TC/LUC) TC, a drive-neutral-reverse clutch actuated planetary gear set DNR, a fixed gear FD, and a differential DF.

The torque converter/lock-up clutch TC couples an output shaft of the power source MOT to the drive-neutral-reverse planetary gear set DNR, with a controllable sliprate and torque ratio correlated therewith, i.e. the ratio between the transmitted torque at its output and the torque received at its input from the power source MOT. The DNR planetary gear set is provided to couple the torque converter/lock-up clutch TC to the CVT 20. The DNR clutch is controllable to assume one of a drive mode D corresponding to driving the vehicle in a forward direction, a reverse mode R, wherein the vehicle is driven backward and a neutral mode wherein it keeps the CVT 20 decoupled from the TC/LUC TC. The CVT 20 transmits the power delivered from the power source MOT, through the TC/LUC and the DNR clutch via the fixed gear FD and the differential DF to the wheels WH, at a gear rate that can be selected from a continuous range. In other embodiments of the powertrain, transmission elements may be present in a different order. In still further embodiments one or more of the further transmission elements may be absent, and/or other further transmission elements may be included. Although in this example the CVT is shown as a powertrain its use is not limited to this application. Other applications are conceivable wherein the presence of a continuously variable transmission between an input shaft and an output shaft is advantageous, for instance as a transmission element between the turbine of a windmill and an electric power generator.

A hydraulic control circuit 1 is provided to hydraulically control the setting of the CVT 20 and optional further transmission elements. The hydraulic control circuit 1 on its turn is controlled by control signals from an electronic transmission controller unit TCU 3, for example a general purpose processor, by dedicated hardware, or by a programmable signal processor. An engine control unit ECU to control the power source is further provided which is coupled via a bus, e.g. a CAN bus to the controller TCU.

In analogous to the TCU, also this controller may be for example a general purpose processor, dedicated hardware, or a programmable signal processor. In another embodiment a single control unit may be provided that serves as an ECU and a TCU.

FIG. 2 shows a first embodiment of a hydraulic control circuit 1 for controlling a continuously variable transmission 20. For clarity, FIG.l only shows the CVT 20. Typically, the CVT 20 is part of a powertrain 2, which may also comprise other transmission elements, for example as described with reference to FIG. 1.

As shown in FIG. 2, the CVT 20 comprises a primary pulley 21 with an input shaft 22, to be driven by a power source. The power source for example a motor, e.g. MOT (FIG. 1) or a windmill turbine. The CVT 20 has a secondary pulley 23 with an output shaft 24 that is to drive a target, e.g. wheels WH (see FIG. 1) of a vehicle or an electric power generator. The CVT 20 further includes an endless, flexible transmission element 25 wrapped around the pulleys 21, 23 that mechanically couples the pulleys 21, 23. The pulleys 21,23 each comprise an axially fixed conical disc 21a resp. 23a and an axially movable conical disc 21b and 23b, the latter discs being axially controllable with a respective hydraulic actuator 26, 27 to set a

transmission ratio between the input shaft 22 and the output shaft 24. Typically, the actuators 26, 27 are positioned in a crosswise position as shown in FIG 2, 3 and 5. The hydraulic control circuit 1 is adapted to control a running radius R21, R23 of each of the pulleys, by driving the respective actuators with a hydraulic fluid at a respective actuation pressure value P26, P27. Therewith a transmission ratio“i” can be set at a desired value

R23/R21.

The hydraulic control circuit includes a feed pump 10 with an output 11 to supply hydraulic fluid at a first fluid pressure value Pi. The hydraulic control circuit also includes a bidirectional boost pump 12, which is coupled via a hydraulic branch 13 to the first hydraulic actuator 26 and via the hydraulic branch 14 to the second hydraulic actuator 27. The bidirectional boost pump 12 is controllable with a control signal S12 from control unit 3 to displace hydraulic fluid in either direction between the and the hydraulic branches 13, 14. The hydraulic control circuit 1 further includes a selection valve 15 which in response to a control signal S15 from the control unit 3 couples the output 11 of the feed pump 10 to the hydraulic branch 13 or to the hydraulic branchl4. In an operational state, the control unit 3 controls the selection valve 15 to select the hydraulic branch that is coupled to the actuator of the pulley that is indicated as the one having the smallest running radius in that operational state.

In the embodiment shown, the running radii R21, R23 of the pulleys 21, 23 are indicated by input signals S21, S23. These input signals may be provided by sensors provided with the pulleys, for example S21, S23 represent inputs signals from sensors 51, 53 (see FIG. 5A) that measure the axial positions of the axially movable conical discs 21b, 23b. The running radii R21, R23 are linearly geometrically coupled to these axial positions. As the pulleys 21, 23 are coupled by the transmission element 25, the positions of the movable conical discs 21b, 23b are geometrically interconnected as well. So, measuring of just one of the axial positions of the conical discs 21b or 23b is sufficient to determine whether the transmission ratio is above or below a ratio i = 1, and therewith sufficient to determine which pulley has the smallest running radius. For example as shown in FIG. 5A, logic element 55 receives the input signal S21 from the first sensor 51, indicative for the axial position of the axially movable conical disc 21b and uses this information to compute the indication I_min, indicating which of the pulleys 21, 23 has the smallest running radius. The second sensor 53 and its output signal are indicated in dashed lines, to clarify that the input signal S21 suffices to determine the indication I_min. Likewise the indication I_min could be computed from the signal of the sensor 53 alone. Nevertheless it may be contemplated to use both sensor signals S21, S23 for example as a way to detect a malfunctioning or to identify a transition region wherein the individual sensor signals S21, S23 would result in mutually different indications for Imin.

As another example, shown in FIG. 5B, S21, S23 represent output signals from rotational speed sensors 61, 63. The pulley with the highest rotational speed is identified by logic unit 65 with signal Imin as the one with the smallest running radius. Equal rotational speed of the pulleys 21,23 will determine the speed ratio i = 1 and will indicate a change of the pulley having the smallest running radius.

Alternatively, sensors may be provided that directly measure the running radius. Still further, the indication may be based on the estimated running radius, depending on the hydraulic pressures with which the actuators are driven. Also various input signals indicative for the running radius may be combined to obtain a resultant indication of the pulley having the smallest running radius. For example a flexible element speed sensor could be used in combination with any pulley speed sensor for determining the running radius.

In the embodiment shown, it is indicated that the primary pulley 21 has the smallest running radius, and the selection valve 15 is accordingly controlled to couple the output 11 of the feed pump 10 to the hydraulic branchl3. Therewith the actuator 26 of the primary pulley 21 is driven with a hydraulic pressure P26, equal to Pi, as supplied by the feed pump 10. With the boost pump 12 controlled to displace the hydraulic fluid in the direction from the hydraulic branch 13 to the hydraulic branch 14 or reversely, the actuator 27 is driven with a pressure P27.

In the embodiment shown the pressure P26, P27 exerted by the hydraulic fluid to the actuators 26, 27 is determined by the pumps 10 and 12. The controller 3 controls these pressures by properly energizing electric motors 10m, 12m, driving the pumps 10, 12, with drive signals S10, S12. To that end the electronic controller 3 may have means for pressure

monitoring, for example a pressure sensor 16 that issues a sensor signal Si₆ indicative for a pressure Pi monitored in branch 19. A pressure control facility is provided in that the electronic controller 3 is capable to control the operational state of the feed pump 10 with control signal S10 to set the pressure Pi at a desired value.

FIG. 3 shows an operational state wherein it is indicated that the secondary pulley 23 has the smallest running radius and the selection valve 15 is accordingly controlled to couple the output 11 of the feed pump 10 to the hydraulic branch 14. Therewith the actuator 27 of the secondary pulley 23 is driven with a hydraulic pressure P27, equal to Pi, as supplied by the feed pump 10. With the boost pump 12 controlled to displace the hydraulic fluid in the direction from the hydraulic branch 14 to the hydraulic branch 13 or reversely, the actuator 26 is driven with a pressure P26.

By controlling the hydraulic pressure for actuation of the pulleys in a manner that depends on which of the pulleys is determined to have the smallest running radius, the hydraulic feed pump 10 directly delivers the actuation pressure to the one of the pulleys that is most susceptible to slippage of the flexible element. Therewith the probability of slipping is mitigated, allowing reduced safety factors or reduced clamping margin on the clamping pressure controlled by the feed pump. Therewith the risk of slipping and wear of the flexible element is substantially reduced and the overall power transmission efficiency of the CVT is increased.

FIG. 4 shows an alternative arrangement. Parts therein

corresponding to those in FIGs. 2 and 3 have the same reference. Contrary to the embodiment shown in FIG. 2, 3, the rotational speed of the feed pump 10’ is not independently controllable. The feed pump 10’ may be driven for example by the power source MOT, that cannot be controlled by the controller 3, for example the engine MOT that drives a vehicle, and which is controlled by a dedicated engine control unit ECU. In the embodiment shown the output 11’ of the feed pump 10’ feeds a branch 19 that supplies an input of the selection valve 15. As in the embodiment of FIG. 2, 3, the electronic controller 3 controls a pressure Pi in the branch 19. Contrary to this previous embodiment however, the electronic controller 3 maintains the hydraulic pressure in branch 19 with a drive signal Si₇ that drives an electronically controllable pressure control valve 17, also denoted as pressure relief valve. The pressure control valve 17 serves as a pressure control facility in this embodiment. In addition an autonomously operating safety valve 18 may be provided that is to bypass a flow of hydraulic fluid in any case when the hydraulic pressure exceeds a safety threshold value.

The feed pump 10’ or other elements of the hydraulic circuit may have outputs to other branches that serve as a supply of oil for the actuation of clutches, and for lubrication and cooling or other hydraulic consumers. It will be appreciated by the person skilled in the art that elements presented in the drawings are merely provided as an exemplary implementation being one of a variety of suitable implementations. For example, the controllable valve 15 could alternatively be provided as a valve that is controllable in three or more discrete stages or one that is proportionally controllable. It could further be contemplated to replace the valve 15 by a pair of

independently controllable valves, one from the output 11 of the pump 10 to hydraulic branch 13 and one from the output 11 of the pump 10 to hydraulic branch 14. Likewise these valves could be simple“open/close” valves, valves that are controllable in three or more discrete stages or proportionally controllable valves.

In the claims the word“comprising” does not exclude other elements or steps, and the indefinite article“a” or“an” does not exclude a plurality. A single component or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. A hydraulic control circuit (1) for controlling a continuously variable transmission (CVT, 20) comprising a primary pulley (21) with an input shaft (22) and a second pulley (23) with an output shaft (24), as well as an endless, flexible transmission element (25) that mechanically couples the pulleys, wherein the pulleys are axially controllable with a respective hydraulic actuator (26, 27) to set a transmission ratio between the input shaft and the output shaft and a clamping level for the transmission element, the hydraulic control circuit being adapted to control a running radius of each of the pulleys, by driving the respective actuators with a hydraulic fluid at a respective actuation pressure value, the hydraulic control circuit including at least a feed pump (10; 10’) with an output (11,

11’) to supply hydraulic fluid at a first fluid pressure value (Pi), a

bidirectional boost pump (12) coupled via a first hydraulic branch (13) to a first one of the hydraulic actuators and via a second hydraulic branch (14) to a second one of the hydraulic actuators, the bidirectional boost pump being controllable to displace hydraulic fluid in either direction between the first and the second hydraulic branch, the hydraulic control circuit further including a selection valve (15) to controllably couple the output of the feed pump to one of the first and the second hydraulic branch, characterized in that in an operational state the selection valve is controlled to select the hydraulic branch coupled to the actuator of the pulley that is indicated as the one having the smallest running radius in that operational state.

2. The hydraulic control circuit according to claim 1, wherein the hydraulic control circuit includes a logic unit (55) that uses an indication (S21) for an axial position of a conical disc (21b) forming part of a pulley (21) to identify the one of the primary pulley (21) and the secondary pulley (22) that has the smallest running radius.

3. The hydraulic control circuit according to claim 1 or 2, wherein the hydraulic control circuit includes a logic unit (65) which identifies the one of the primary pulley and the secondary pulley that has the highest rotational speed as the one with the smallest running radius.

4. The hydraulic control circuit according to claim 1, 2 or 3,

characterized in that a sensor is provided for measuring a fluid pressure provided to the actuator of the pulley with the smallest running radius.

5. The hydraulic control circuit according to one of the previous claims, characterized in that a pressure control facility is provided which is adapted to control a setting for the first fluid pressure based on one or more of: a pressure value indicated by the pressure sensor, and vehicle parameters, such as transmission parameters such as a desired clamping force of the pulleys, belt slip, speed of the endless, flexible element, torque, vehicle speed, transmission ratio, state of the selection valve.

6. The hydraulic control circuit according to any one of the preceding claims, characterized in that the boost pump is arranged to displace a hydraulic fluid between the selected hydraulic branch and other hydraulic branch.

7. The hydraulic control circuit according to any one of the preceding claims, characterized in that the boost pump is arranged to regulate the pressure and volume flow on the basis of a measured current transmission ratio and the desired transmission ratio.

8. The hydraulic control circuit according to any one of the preceding claims, characterized in that upon controlling a change of transmission ration starting from a state wherein the pulleys have an equal running radius, the hydraulic control circuit is arranged to supply hydraulic fluid with the first pressure to the actuator of the pulley which is predicted to have the lowest running radius after said change.

9. The hydraulic control circuit according to any one of the preceding claims, characterized in that the feed pump is driven by an electric motor;

10. The hydraulic control circuit according to any one of the claims 1 to 8, characterized in that the feed pump is driven by a combustion engine;

11. The hydraulic control circuit according to any one of the preceding claims, characterized in that the boost pump is driven by an electric motor;

12. A continuously variable transmission comprising a primary pulley (21) with an input shaft and a second pulley (23) with an output shaft, as well as an endless, flexible transmission element (25) that mechanically couples the pulleys, wherein the first and the second pulley are axially controlled by a respective actuator driven by a hydraulic control circuit according to one of the previous claims.

13. A method for controlling a transmission ratio of a continuously variable transmission, comprising a primary pulley with an input shaft and a second pulley with an output shaft, as well as an endless, flexible transmission element that mechanically couples the pulleys, the method comprising axially setting the pulleys to realize a transmission ratio between the input shaft and the output shaft and a clamping level for the flexible transmission element, the method comprising driving the pulleys by actuation with a hydraulic fluid at a first and a second actuation pressure value respectively, comprising supplying hydraulic fluid at a basic fluid pressure value, pumping hydraulic fluid to increase an actuation of a controllably selected one of the pulleys while decreasing an actuation of the controllably non-selected one of the pulleys, further comprising selectively controlling supplying hydraulic fluid at the basic fluid pressure value to actuate the one of the pulleys that is indicated as the one having the smallest running radius in that operational state.