The present invention concerns a method for controlling a clutch unit for a drive train of a motor vehicle, wherein the clutch unit has at a minimum, a wet friction clutch for controllable transmission of a torque from an input element to an output element of the clutch unit, oil for cooling the friction clutch, and an actuator for actuating the friction clutch. The invention also concerns a torque transmission arrangement that has an input element, an output element, a control unit, and a clutch unit of the aforementioned type.
A clutch unit of this nature is used, for example, in a transfer case of a motor vehicle with four-wheel drive for controllable transmission of a drive torque to a primary axle and/or a secondary axle of the motor vehicle. In the case of a so-called “torque on demand” transfer case, the wheels of the primary axle are continuously driven while a portion of the drive torque can be selectively transmitted to the wheels of the secondary axle by means of the aforementioned clutch unit. The transfer case can also be designed as a controllable center differential in which the clutch unit is associated with a differential lock in order to adjust the distribution of the drive torque in the longitudinal direction of the vehicle. A clutch unit of the aforementioned type can also be used in a torque transmission arrangement, which, in a motor vehicle with a continuously driven front axle, permits the transmission of part of the torque to the rear axle, wherein the unit is located on the front axle differential or the rear axle differential, for example. Such different applications and arrangements are known from U.S. Pat. No. 7,111,716 B2, for example.
A clutch unit of the aforementioned type can also act in the transverse direction of the motor vehicle, for example for a differential lock of an axle differential or in a torque superposition arrangement of an axle differential (known as “torque vectoring”). In all of the aforementioned cases, the clutch unit can frictionally connect a rotating input element (e.g., input shaft) and a rotating output element (e.g., output shaft), particularly in order to transmit a drive torque. As an alternative thereto, the clutch unit can be configured as a brake with a stationary input element or a stationary output element, particularly in order to transmit a braking torque.
In the aforementioned applications of the clutch unit, the clutch unit is located after the main transmission of the drive train (e.g., after the manual or automatic transmission or CVT transmission) with respect to the direction of power flow. Normally, the clutch torque—which is to say the torque transmitted by the friction clutch—is variably adjusted as a function of the relevant driving situation. Thus, a change in the torque to be transmitted by the clutch unit takes place in accordance with the requirements of vehicle dynamics, which may depend on such factors as the driving situation or environmental influences (e.g., smooth road surface with slip of the drive wheels occurring). This requires not only controlled engagement of the friction clutch, but frequently also requires a relatively long period of operation with precisely adjusted clutch torque, for which reason the friction clutch usually is designed as a wet plate clutch in the aforementioned applications. Typically, the friction clutch is integrated into a housing, which contains oil for cooling and lubricating the frictional components. For example, an oil sump is provided at the bottom of the housing, whence an oil pump continuously pumps oil during the operation of the clutch and drips it on the friction surfaces. The oil returns to the oil sump from the friction surfaces.
The clutch unit further includes an actuator for actuating the friction clutch. The actuator often has an electric motor attached to the housing of the clutch unit and serves to move the clutch plates into a predetermined engagement position in response to a required torque to be transmitted between the input element and the output element of the clutch unit.
A clutch unit of the aforementioned type and a method for calibrating such a clutch unit are known from WO 2003/025422 A1 (corresponding to U.S. Pat. No. 7,032,733 B2), the content of which is expressly incorporated in the disclosure content of the present application. As is described in greater detail in WO 2003/025422 A1, the setting of a specific desired clutch torque does not necessarily require the provision of direct torque control (with the measured actual clutch torque as the control variable). Instead, the control of the friction clutch can take place by indirect means through controlling the position of the actuator based on an appropriate calibration of the clutch unit. Thus, to set the desired torque to be transmitted, the angle of rotation of the electric motor, for example, or another position variable of the actuator, is employed as a control variable and is set to a value that corresponds to the desired clutch torque. To this end, a clutch torque/actuator position dependence is empirically determined, which is stored as a characteristic curve, for example in the form of a table (lookup table, LUT) or a function (which is to say an algorithm). Using this dependence, the applicable target value of the relevant position variable of the actuator (e.g., angle of rotation) is determined for a specific torque requirement and is regulated.
In this context, the problem arises that the torque actually transmitted at a specific actuator position depends on the coefficient of friction of the clutch plates, and this can change during vehicle operation. This change cannot be taken into account by the aforementioned static relationship between torque requirement and actuator position. Along with the speed difference between the input element and the output element of the clutch unit, the temperature of the clutch plates or that of the oil present on the clutch plates, in particular, affects the coefficient of friction. Since the temperature relationships in the friction clutch can change in accordance with operating conditions, undesirable deviations between the commanded clutch torque (target value) and the clutch torque actually transmitted (actual value) can thus occur. At low temperatures, the magnitude of the coefficient of friction can be so high that the excessive torque can result in damaging stress on transmission components, warping in the drive train, or excessive oil degradation.
The object of the invention is to reduce the deviations between the target value and the actual value of the clutch torque, and thus to increase the accuracy of the clutch control. Furthermore, unacceptably high stress on components of the clutch unit is to be avoided.
This object is attained by a method for controlling a clutch unit with the features from claim 1, and in particular by the following steps:
According to the invention, therefore, a dynamic adaptation of the clutch control takes place as a function of the clutch temperature, which constitutes an important influencing variable with regard to the relationship between the actuator control variable and the torque transmitted. This can be achieved in a simple manner in that a temperature associated with the clutch unit is ascertained on an ongoing basis, and the characteristic curve of the friction clutch, which is customarily stored in a nonvolatile memory, is adapted accordingly. By means of such an adaptation, temperature-induced deviations of the clutch characteristic from the normal behavior can be compensated for, by which means the accuracy of adjustment of the clutch unit is increased. The relationship between clutch temperature and the torque deviation to be compensated can be calculated or determined empirically, and can be present in the form of a lookup table, for example. As a result of adapting the characteristic curve, the clutch control can be corrected rapidly and reliably. No complicated calculations or additional regulating mechanisms are necessary. In particular, the adaptation of the characteristic curve can be performed on a periodic or continuous basis during operation of the clutch.
Preferably, a temperature of the oil is detected for ascertaining the temperature in the friction clutch. Because the oil is directed onto the plates in continuous circulation during operation of the friction clutch, and rapidly flows back into the reservoir of the oil sump after exchanging heat with said plates, the temperature of the oil sump constitutes a representative and easily accessible thermodynamic parameter of the friction clutch. The temperature of the oil not only permits direct inferences concerning its viscosity, which affects the coefficient of friction, but can also serve as a starting point for estimating or calculating other temperatures of the friction clutch, for example the temperature of the clutch plates.
The temperature of the oil can be sensed in a direct and reliable manner by a temperature sensor, located in an oil sump of the clutch unit for example. This suggest itself in particular for clutch units in which such a temperature sensor is provided in any case, for example in order to output a warning signal in the event of impending overheating of the friction clutch. While the frictional clutch plates generally are inaccessible for direct temperature measurement, a temperature sensor can be placed in the oil sump in a simple manner.
The temperature of the oil can also be calculated on the basis of a heat flow model. Such a heat flow model can be based on a calculation of the difference between a thermal input power and a thermal output power of the clutch unit, for example, wherein the clutch torque and the speed of the input element and/or the output element can be taken into account. The advantage of such computationally based temperature determination is that no additional sensors are required. Moreover, various measured quantities already provided as part of vehicle control, such as the speeds of the input element and output element, for example, can be used in advantageous fashion as input parameters for the heat flow model.
Regardless of the method of ascertaining or calculating the oil temperature, the determination of the temperature in the friction clutch can also take into account a heat input that corresponds to the power dissipation generated in the friction clutch. In other words, starting from the oil temperature, the temperature in the friction clutch is inferred by means of the heat input into the friction clutch. By this means, a still better accuracy of the desired temperature compensation can be achieved, and in particular the specific nature of the friction clutch used can be taken into account, for example in order to make a distinction based on whether a plate clutch with steel plates or paper plates is used.
For the above-described offsetting of the oil temperature with the heat input, a value corresponding to the oil temperature can be added to or multiplied by a value corresponding to the heat input, for example. The aforementioned heat input can be ascertained as a function of the clutch torque (target value of the torque to be transmitted or actual value of the torque to be transmitted) and a difference between the speed of the input element and the speed of the output element of the clutch unit, for example. In particular, the aforementioned heat input can be determined from a product of the clutch torque and the speed difference, wherein an empirically determined constant that ultimately corresponds to a thermal conductivity coefficient can be taken into account as an additional factor. In addition, a time integral can be formed by means of the aforementioned product of clutch torque and speed difference between input element and output element.
Preferably, a slope and/or an offset are modified for the adaptation of the clutch characteristic curve. In particular, for adapting the characteristic curve, a slope correction value and/or an offset correction value can be determined as a function of the ascertained temperature, wherein a target value of the clutch torque is multiplied by the slope correction value in order to obtain a modified target value of the clutch torque, wherein a temporary target value of the relevant actuator control variable is determined on the basis of the characteristic curve as a function of the modified target value of the clutch torque, and wherein the offset correction value is added to the temporary target value of the actuator control variable in order to determine a target value of the actuator control variable. The stored characteristic curve itself remains unchanged in such a procedure, since only two parameters associated with the characteristic curve are updated. In this way, a complete regeneration of the characteristic curve, with corresponding computational and storage effort, is avoided. The aforementioned slope correction value and the aforementioned offset correction value can be determined empirically and stored in simple lookup tables.
Preferably, for control of the clutch unit, a target value of the relevant actuator control variable is determined as a function of a target value of the clutch torque, and the determined target value of the actuator control variable is regulated, wherein an actual value of the actuator control variable is sensed and is compared with the target value. In general, the actuator control variable is more easily accessible to sensing by measurement means than the clutch torque itself, so that it is easier to provide regulation that relates to the target value and actual value for the selected actuator control variable. The actuator control variable can be an actuator position (in particular, an angle of rotation) or a hydraulic pressure, for example.
The invention also relates to a torque transmission arrangement that has an input element, an output element, a clutch unit, and a control unit, wherein the clutch unit has, at a minimum, a wet friction clutch for controllable transmission of a torque from the input element to the output element, oil for cooling the friction clutch, and an actuator for actuating the friction clutch, wherein the control unit is designed to ascertain the temperature in the friction clutch, adapt, as a function of the ascertained temperature, a characteristic curve of the friction clutch that describes the dependence of a clutch torque on an actuator control variable, and to control the clutch unit in accordance with the characteristic curve by means of the actuator.
The inventive clutch unit or torque transmission arrangement can be used in different configurations in order to transmit a torque along a drive train of a motor vehicle, as was explained at the outset. The invention is explained below with reference to the drawings, in connection with a “torque on demand” transfer case solely by way of example.
Also shown in
The transfer case 15 has a clutch unit 47 with a friction clutch 49 and an actuator 51. The friction clutch 49 has a clutch basket 53 that is attached in a rotationally fixed manner to the input shaft 41 and the first output shaft 43 and that carries multiple clutch plates. The friction clutch 49 also has a rotatably supported clutch hub 55, which likewise carries multiple clutch plates that engage in an alternating arrangement with the plates of the clutch basket 53. The clutch hub 55 is connected in a rotationally fixed manner to an input gear 57 of a chain drive 59. An output gear 61 of the chain drive 59 is connected in a rotationally fixed manner to the second output shaft 45. A gear drive, for example with an intermediate gear between the aforementioned gears 57, 61, may be provided in place of the chain drive 59.
By actuating the actuator 51 in the engagement direction of the friction clutch 49, an increasing fraction of the drive torque introduced into the transfer case 15 through the input shaft 41 can be transmitted to the second output shaft 45.
An actuating lever 77 or 79 is integrally formed on the bearing ring 63 and the adjusting ring 65, respectively. A roller 81 or 83 is rotatably supported at the free end of each relevant lever 77, 79. By means of the rollers 81, 83, the actuating levers 77, 79 work together with the two end faces 85, 87 of a disk cam 89, which is rotatable relative to an axis C. The end faces 85, 87 are inclined in the circumferential direction relative to a plane perpendicular to the axis C, i.e., the disk cam 89 is wedge-shaped in cross-section. By rotating the disk cam 89, the actuating levers 77, 79 can thus be moved in a scissoring manner in order to rotate the bearing ring 63 and the adjusting ring 65 relative to one another. The disk cam 89 has an integrally formed splined projection 91. By means of said projection, the disk cam 89 can be connected to an electric motor and associated reduction gear (not shown in
In this way, by appropriate control of the aforementioned electric motor the disk cam 89 can be driven into a rotary motion so as to thereby pivot the actuating levers 77, 79 relative to one another. The rotation of the bearing ring 63 and the adjusting ring 65 relative to one another that is produced thereby causes an axial motion of the adjusting ring 65. The pressure ring 73 thus causes an engagement of the friction clutch 49, or—assisted by the spring washer arrangement 75—a disengagement of the friction clutch 49.
It is also evident from
The sensor 107 produces a signal that corresponds to an actuator position value. In the exemplary embodiment shown, this is the actual angular position value α′ of the armature shaft 95. This signal α′ is delivered to a control unit 109 of the transfer case 15. The control unit 109 also receives a torque requirement M, which is to say a target value of the clutch torque, from the vehicle dynamics controller 33 of the motor vehicle (
The control of the clutch unit 47 using the characteristic curve 111 will now be explained in detail with reference to
Adapting the characteristic curve 111 does not necessarily require overwriting the entire stored curve or its associated table. It is sufficient, for example, to multiply each torque requirement M by a temperature-dependent correction value and thereafter determine the target angular position value α using the originally stored characteristic curve 111. This procedure is equivalent to a change in the slope of the characteristic curve, so that this procedure corresponds to the adapted characteristic curve 111′, for example.
Alternatively, it is also possible to first determine the target angular position value α based on the unaltered torque requirement M and the unmodified characteristic curve 111 and to increase or decrease the target angular position value α thus obtained by a temperature-dependent correction value. This procedure is equivalent to a change in the characteristic curve offset, so that this procedure corresponds to the parallel-displaced characteristic curve 111″, for example. In the example shown in
The characteristic curve adaptation can thus be designed in practice such that the characteristic curve 111 that has been stored a single time always remains unchanged, as is illustrated in
In accordance with an advantageous refinement, the desired temperature compensation can additionally take place on the basis of a heat input that corresponds to the power dissipation generated in the friction clutch 49. In this case, the oil temperature T determined by means of the temperature sensor 122 (
An exemplary method for controlling a clutch unit taking into account the temperature T is described with reference to
Insofar as the adaptation of the characteristic curve 111 takes place through the use of correction values as described in connection with
In this way, it is possible, for example, to take into account the fact that the viscosity of the lubricating oil decreases with increasing operating temperature of the clutch unit 47, thus changing the clutch characteristics. As a result of the compensation of the temperature influences, the accuracy of the clutch torque control can be increased. The determined temperature of the oil sump can also be used for additional control tasks as part of vehicle operation, as well. To this end, it can be output to a CAN bus, for example, in order to thus be available to other control units.
While the invention finds especially advantageous application in a transfer case with electromechanical actuation of the friction clutch, the invention is not limited to the above-described exemplary embodiment. Other arrangements in the drive train of a motor vehicle are also possible, as explained at the outset. Furthermore, the actuator 51 can be designed in a different manner than that described above in conjunction with the figures. For example, a different type of reduction gearbox 97 or a different type of deflection device 103 may be provided. In place of the electromechanical actuation of the friction clutch 49 shown, an electromagnetic, hydraulic, or electrohydraulic actuation may also be provided, for instance. In this case a pressure/clutch torque characteristic curve, for example, is adapted as a function of temperature in place of the angular position/clutch torque characteristic curve 111.
Number | Date | Country | Kind |
---|---|---|---|
10 2008 026 554.3 | Jun 2008 | DE | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
---|---|---|---|---|
PCT/EP2009/003728 | 5/26/2009 | WO | 00 | 3/23/2011 |