Method and apparatus for controlling a motor vehicle clutch

[0001] The present invention relates to a method and apparatus in the form of a hydraulic unit for actuating a vehicle clutch according to the preamble of Claims 1 and 11.

[0002] Solutions known heretofore for actuating a clutch in an axle drive of a vehicle, in order to compensate for a steering error in certain driving situations, provide for a hydraulic unit that includes a high-pressure storage device, which must be filled up to the maximum clutch pressure in order to be able to engage the clutch or clutches with rapid response in these certain driving conditions. However, a low pressure in the pressure storage device would be adequate for pre-filling a clutch.

[0003] FIG. 1 shows a schematic depiction of such a hydraulic unit, as it is used in the existing art.

[0004] A pump (1) is driven by an (electric) motor 2 and fills the high pressure storage device 3, which is designed as a diaphragm-type accumulator. A sensor 4 measures the pressure. If two clutches 5 and 6 are provided, they are each actuated via a solenoid valve 7 and 8, respectively, and if necessary are supplied with the working fluid from the high pressure storage device 3. The pump 1 is supplied with the working fluid from a tank 9.

[0005] The solenoid valves 7 and 8 are designed for example as pressure-reducing valves. To pre-fill the clutches 5 or 6 it would be adequate to transport the working fluid at a low pressure of for example 5 to 10 bar, in order to fill the lift free-play of the clutches. To engage the clutch 5 or 6 a pressure of about 45 bar would have to be present, without need of a flow volume of working fluid.

[0006] However, the known hydraulic unit uses a large and expensive high-pressure storage device to fill and to engage the clutches 5 or 6.

[0007] If the clutches 5 and 6 are disengaged, the working fluid flows through the solenoid valves 7 and 8 back into the tank 9. The depicted elements of the known hydraulic unit are connected with each other via hydraulic lines or control lines, as shown in FIG. 1.

[0008] By switching the solenoid valves 7 and 8 accordingly, the clutches 5 and 6 are supplied with working fluid from the high pressure storage device 3, so that they are engaged, or the working fluid is transported from the clutches 5 and 6 back into the tank 9, whereupon the clutches 5 or 6 disengage by their own weight or a spring.

[0009] The object of the present invention is therefore to create a device in the form of a hydraulic unit and a method for actuating this device, which enables the filling and engaging of the at least one clutch with simple construction and while maintaining high dynamics (rapid response), while the intent is to dispense with the use of a high-pressure storage device.

[0010] Preferably, in this case two multi-plate clutches provided in a rear axle of a vehicle, i.e. in a rear axle differential, should be engaged hydraulically to redistribute the wheel torques.

[0011] This problem is solved by the features of Claims 1 and 11.

[0012] The method according to the invention for engaging a clutch by means of a device in the form of a hydraulic unit is set forth in Claims 1 through 10.

[0013] Accordingly, the hydraulic unit, which is provided in particular for actuating at least one clutch in a hydraulic circuit for an axle drive of a vehicle, includes a pump which transports a working fluid, a pressure storage device that stores the working fluid under pressure, and at least one solenoid valve which is situated in the infeed of the at least one clutch, the pressure storage device being designed as a low pressure reservoir and being operatively connected with the pump so that depending on the driving situation of the at least one clutch the corresponding requisite working fluid is available at the required pressure.

[0014] The advantage here is that the expensive high pressure tank, as it is used in the existing art, can be saved and replaced by a simple, robust and inexpensive low pressure reservoir. Furthermore, in this embodiment according to the invention the pump can be very small.

[0015] The pump, which is preferably driven by an electric motor, loads the low pressure reservoir up to the pressure that is necessary to fill the clutch, for example 10 bar. However, the pump can be driven equally well by an axle shaft of an electric motor, or of a combustion engine of the motor vehicle. To this end, a pressure limiting valve is preferably built into the hydraulic circuit which limits the pressure accordingly. After the low pressure reservoir is loaded, the pump can be switched off.

[0016] Preferably, a pressure reducing valve is installed ahead of each clutch that is to be operated, so that the pressure from the low pressure reservoir can be directed into the clutch and the latter filled as soon as current is applied to this pressure reducing valve. By preference, the pump is put into operation at the same time, so that directly after the filling of the clutch(es) the pump pressure operates the pressure reducing valve and the clutch engages accordingly.

[0017] In addition, a sequence valve must be provided between the pump and the low pressure reservoir, which always closes far enough so that the pump pressure operates the pressure reducing valve, whereupon the fluid flowing from the sequence valve, preferably an oil, is used to reload the low pressure reservoir.

[0018] In an advantageous design of the invention, the low pressure reservoir is executed as a spring-pressurized reservoir, where a mechanical spring is installed instead of the otherwise normal gas bubble, and thus an especially inexpensive and sturdy pressure storage device is realizable.

[0019] In another advantageous design of the solution according to the invention, either a pressure reducing valve is provided in the supply line for each clutch, or one pressure reducing valve and one selector valve are provided for two clutches.

[0020] It is also advantageous that the solenoid valve is a pressure reducing valve, which dams up the working fluid transported by the pump to the pressure level needed to engage the clutch.

[0021] Preferably each of the two clutches is provided with its own pump, so that the two clutches, executed for example as multi-plate clutches and situated in a rear axle differential of a vehicle, can be actuated or engaged independently of each other.

[0022] A preferred embodiment of the invention provides for the power steering pump of the vehicle to be used as the pump. The existing power steering pump thus serves as the source for the pressurized working fluid. To this end, the outflow pressure of the power steering system is used to fill and engage the clutch(es). In this embodiment, suitable measures must be taken to ensure that the steering function receives higher priority than the clutch function.

[0023] Alternatively, the pump or pumps (if a separate pump is provided for each clutch) can be operated constantly, so that there is no longer a need to provide a pressure storage device in the hydraulic unit.

[0024] An advantage of the solution according to the invention is that the expensive high pressure storage device used in the existing art can be spared.

[0025] The hydraulic unit according to the invention, which is designed in particular to actuate at least one clutch in an axle drive of a vehicle, can have at least one pump which transports a working fluid and at least one solenoid valve which is assigned to the at least one clutch, with the pump transporting the working fluid to the clutch under pressure both to fill the clutch and to engage the clutch.

[0026] The working fluid can be transported constantly by the pump, and can therefore be used not only for the clutch engaging function, but under certain circumstances also for a cooling function, for example to cool a rear axle differential.

[0027] With the present invention, the technical complexity of actuating preferably two hydraulically engageable multi-plate clutches that are to be engaged hydraulically in certain driving situations in a rear axle differential is to be kept to a minimum. A high degree of dynamics is necessary here; an air gap in the clutch must be closed by filling it with the working fluid before the clutch is actually engaged. At least one of the two clutches must be engaged in an extremely short time (under 100 ms). A relatively low pressure of 5 to 10 bar is necessary for filling, whereas a pressure of about 45 bar must be applied for the final engaging of the clutch, although of course practically no flow volume of working fluid is still needed.

[0036] For all embodiments, the same reference labels are used for the same or like-operating components.

[0037] A first preferred embodiment according to the present invention will be described below, with reference to FIG. 2.

[0038] For two clutches 5 and 6, which are installed for example in an axle drive, a rear axle differential of a vehicle to distribute the wheel torque, two pumps 1 are provided which ensure that the clutches 5 and 6 are supplied with working fluid or oil. Each of the clutches 5 and 6 is supplied with oil by a separate pump 1, which are preferably driven by the axle shafts of the rear axle. Optionally, the two pumps 1 may also be driven by electric motors (not shown).

[0039] The two clutches 5 and 6 according to FIG. 2 can be actuated, i.e. engaged, independently of each other, with no high pressure storage device necessary. Instead of two separate pumps 1, an embodiment with only one vane pump can also be realized, in which case each of the two flows from this vane pump then provides the supply of oil for one of the clutches 5 and 6.

[0040] In each hydraulic circle assigned to clutch 5 or 6, when electricity is applied an electrically operated pressure limiting valve 10 or 11 respectively stores up the pump flow volume of working fluid or hydraulic oil to the necessary pressure level so that the particular clutch can be engaged, whereas when no power is applied to the pressure limiting valves 10 and 11 these valves open and divert the working fluid or hydraulic oil into the tank 9.

[0041] Optionally in an emergency, e.g. if one of the pressure limiting valves 10 or 11 fails, even a simple selector valve 12 can shut off the pressure for both clutches 5 and 6. This variant is depicted in the dashed circle in FIG. 2. The working fluid is then diverted from the clutches 5 and 6 through the check valves 13 and the selector valve 12 into the tank 9.

[0042] In the case of this described embodiment with a low pressure reservoir, the pump can be switched off if it is no longer necessary to fill the low pressure reservoir or if the clutch does not need to be engaged. The pump is then only operated when an engaging procedure of the clutch is actually initiated. The clutch will be (pre-)filled from the low pressure reservoir, while preferably the pump starts to run at the same time, in order to accomplish the final engagement of the clutch by means of the pump pressure.

[0043] In the second embodiment according to the present invention, which is depicted in FIG. 3, only one pump 1 is provided, which supplies both clutches 5 and 6 with working fluid through pressure reducing valves 18 and 19 and a back-pressure valve 15.

[0044] In this case pump 1, which again is preferably driven by an axle shaft or an electric motor (not shown), using almost zero pressure in a rest position, conveys the working fluid through back-pressure valve 15 into tank 9, so that a constantly driven pump 1 can be employed.

[0045] According to a further preferred embodiment, not shown, the working fluid that is flowing out of the back-pressure valve 15 may be used for cooling, for example for cooling the rear axle differential.

[0046] When at least one of the two pressure reducing valves 18 or 19 is operated by the control signal of an electronic control unit, the clutch pressure of clutches 5 or 6 that arises, acting through the return with the OR element 17, increases the pressure of the working fluid at the back-pressure valve 15 corresponding to the needed clutch pressure. When the two pressure reducing valves 18 and 19 are operated simultaneously, if this is desired, the greater of the two clutch pressures governs and determines the system pressure.

[0047] When needed, an emergency bleed device 16 similar to that in FIG. 2 can be provided. The OR element 17 then assumes the function of the two check valves 13 provided in FIG. 2. The emergency bleed device 16 is designed in the present embodiment as a 2/2 directional selector valve, and can be implemented as an opener or a closer.

[0048] An especially preferred third embodiment according to the present invention is depicted in FIG. 4.

[0049] A pump 1, which is driven by an electric motor 2, loads the pressure storage device in the form of a low pressure reservoir 20 up to a pressure that is needed to prefill clutches 5 and 6. In practice, this pressure is for example 10 bar. This pressure is limited by the permanently set pressure limiting valve 10 on the low pressure reservoir 20. If the low pressure reservoir 20 is filled or loaded, the pump 1 can be switched off under the control of a pressure sensor 4.

[0050] The storage pressure, i.e. the pressure of the low-pressure reservoir 20, is present at pressure reducing valves 18 and 19 through check valve 13, and can fill the respective clutch 5 or 6 immediately by the electronic control system when current is applied to one of the pressure reducing valves 18 or 19, even while the control unit is restarting the pump 1.

[0051] If the clutch pressure has risen above the storage pressure after the filling phase, in this embodiment the sequence valve 21 always closes far enough so that the pump pressure can operate the pressure reducing valves 18 and 19 and the working fluid flowing from the sequence valve 21 can be used by means of the pressure reducing valve 10 to reload the low pressure reservoir 20.

[0052] Since clutches 5 or 6 are filled from the low pressure reservoir 20, pump 1 can also be very small. Likewise, since the pressure storage device is designed as a low pressure reservoir 20, the latter can be constructed simply, sturdily and inexpensively. For example, a mechanical spring can be used as the storage element instead of the gas bubbles normally employed in diaphragm-type accumulators. The storage device is thus temperature-independent, and suffers no prestress loss as a result of gas diffusion through a membrane.

[0053] If, as a result of the driving situation, the electronic control unit causes one of the clutches 5 or 6 to disengage, for example to compensate for a steering action, the pressure reducing valves 18 or 19 are switched accordingly and the working fluid can flow back into the tank 9.

[0054] In an embodiment that has not yet been depicted graphically, but which is valid for all of the embodiments described so far and is also applicable there, the actuation of two clutches 5 and 6 is achieved by means of only one pressure reducing valve 18 and one simple selector valve 12. That makes it possible to lower the costs by dispensing with another expensive pressure reducing valve. The system switches between the two clutches 5 and 6 with the help of selector valve 12, or in an emergency situation an appropriately designed selector valve 12 can also be used to switch over to the tank 9 in order to disengage either clutch 5 or 6. However, with this non-depicted embodiment the clutches 5 and 6 are operated one after the other, whereas in the embodiments described above the clutches 5 and 6 can work with overlap or can be actuated simultaneously.

[0055] Finally, FIG. 5 depicts another preferred embodiment of the present invention, wherein in this fourth depicted embodiment an already present, constantly driven pump is used as the source of volume flow for the working fluid, for example the steering booster pump or power steering pump 22.

[0056] A power steering system 23 is designed as a circulation system, in which a constant (oil) flow of working fluid is throttled by a steering valve 27 and is applied to one of the two sides of a steering cylinder 26. Since this is a differential cylinder, the outflow pressure plays no role in the steering function, but rather only the built-up differential pressure.

[0057] The typical working pressures of the steering hydraulics are significantly greater than the pressures to actuate one of the two clutches 5 or 6, so that the tank build-up of the working fluid according to the solution in FIG. 5 has only a slight influence on the design of the power steering system 23.

[0058] The power steering pump 22 transports fluid from the tank 9. A driver of the vehicle inputs the steering impulses through a steering wheel 24, and the steering valve 27 actuates the steering cylinder 26 with working fluid correspondingly, so that the attached wheels 25 are steered accordingly.

[0059] Moreover, as a rule power steering pumps 22 with various pressure ratings are available—depending on the type of engine in the vehicle—so that for coupling with a clutch actuator of the rear axle, as proposed here, a transition to the next larger power steering pump 22 is possible.

[0060] If the outflow stream of the power steering system 23 is dammed up with a pressure limiting valve 10, as depicted in FIG. 3, as part of the prioritizing of the steering function the actuator of the clutch 5 and/or 6 can be targeted simultaneously with the power steering pump 22.

[0061] If the same working fluid or oil is employed in the power steering system 23 to lubricate and cool the clutches 5 and 6 as for the activation, the clutches 5 and 6 can in addition be cooled and lubricated with the drain oil.

[0062] In addition, the engine speed of the combustion engine of the vehicle would even define a certain capacity dependency of the cooling.

[0063] To save on costs, FIG. 6 depicts a fifth preferred embodiment of the solution according to the invention, which is based on the embodiment according to FIG. 4. In this hydraulic unit, again only one pressure reducing valve 18 is employed in combination with a simple selector valve to actuate the two clutches 5 and 6, instead of two pressure reducing valves 18 and 19, while fulfilling the same function. The system switches between the two clutches 5 and 6 by means of the selector valve, or in an emergency situation an appropriately designed selector valve 12 can also be used to switch over to the tank 9 in order to disengage clutches 5 and 6.

[0064] The lift free-play of clutches 5 and 6 is filled with working fluid from the low pressure reservoir 20 while pump 1 starts up and builds up the appropriate pressure through pressure reducing valve 18 for the final engagement of clutch 5 or 6.

[0065] If a signal is given via the electronic controller that clutch 5 or 6 is to be disengaged, as already mentioned at FIG. 4, selector valve 12 is actuated and the working fluid flows back into tank 9 through pressure reducing valve 18.

[0066] Just as in accordance with FIG. 4, in this embodiment as well the dynamics for filling the clutch are retained and the clutch 5 or 6 engages in a time of under 100 ms, in order to be able to compensate for a steering mistake in the shortest time possible. However, an overlap of the engagement procedures of the two clutches 5 and 6 is not possible with this embodiment.

[0067] In FIG. 6, in contrast to FIG. 4, to reduce leakage there are not constantly two pressure reducing valves 18 constantly under pressure. Hence in standby mode the leakage can be cut in half.

[0068] The pressure reducing valve employed in this situation is available for example in cartridge construction form, so that the standby leakage can also be reduced by this construction form.

[0069] If the electric power should fail in an emergency situation, as described earlier, pressure reducing valve 18 will immediate depressurize the clutch pressure. If a pressure reducing valve 18 is jammed, pressure buildup is prevented by the fact that the electric motor 2 for the pump 1 does not start. Due to the leakage at the pressure reducing valve 18, the clutch pressure, if still present, is depressurized to the level of the low pressure reservoir 20.

[0070] The embodiments described above make it possible to actuate a plurality of clutches in a rear axle differential of a vehicle to redistribute the wheel torque without employing a large and expensive diaphragm-type accumulator as high pressure storage device, in order to make available the high pressures of about 45 bar to actuate the clutch(es).

[0071] 1 pump

[0072] 2 electric motor

[0073] 3 high pressure storage device

[0074] 4 manometer (pressure sensor)

[0075] 5 clutch

[0076] 6 clutch

[0077] 7 solenoid valve

[0078] 8 solenoid valve

[0079] 9 tank

[0080] 10 pressure limiting valve

[0081] 11 pressure limiting valve

[0082] 12 selector valve

[0083] 13 check valve

[0084] 14 filter

[0085] 15 back-pressure valve

[0086] 16 emergency bleed device

[0087] 17 OR element

[0088] 18 pressure reducing valve

[0089] 19 pressure reducing valve

[0090] 20 pressure storage device/low pressure reservoir

[0091] 21 sequence valve

[0092] 22 power steering pump

[0093] 23 power steering system

[0094] 24 steering wheel

[0095] 25 wheel

[0096] 26 steering cylinder

[0097] 27 steering valve