This section provides background information related to the present disclosure which is not necessarily prior art.
A clutch unit may be 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 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 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 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, and is attached to the housing of the clutch unit in a thermally conductive way in order to use the housing as a heat sink for the actuator's waste heat. Under certain operating conditions, overheating of the actuator can occur. Consequently, the actuator is typically equipped with a temperature sensor that continuously senses the temperature of the actuator. In this way, clutch operation can be discontinued in the event of impending overheating of the actuator. If the actuator has an electric motor, the temperature sensor can, for example, be attached to the housing of the electric motor or within the same.
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.
For a variety of control tasks relating to operation of the clutch unit, it is necessary to determine the current temperature of the oil located in the clutch housing. To this end, a suitable temperature sensor could be provided in the oil sump, for example. However, an arrangement of this nature is associated with increased effort and additional costs.