CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit and priority of German Patent Application No. 10 2008 064 452.8, filed Dec. 22, 2008. The entire disclosure of the above application is incorporated herein by reference in its entirety. 
     FIELD 
     The present disclosure relates to a hydraulic system with a hydraulic fluid, with a work element which can be actuated by the fluid, with a fluid pump for the application of a fluid pressure to the fluid and with a pressure sensor for the measurement of the fluid pressure. 
     BACKGROUND 
     This section provides background information related to the present disclosure which is not necessarily prior art. 
     Such hydraulic systems are generally known and are used, for example, in motor vehicles for the actuation of a friction clutch (e.g. of a transfer case). 
     A problem generally exists in such systems of a temperature-induced and/or age-induced zero point migration of the pressure sensor, also called offset drift, which results in a falsification of the measured results supplied by the pressure sensor. 
     For the first calibration or for the recalibration of the pressure sensor, a pressureless state of the hydraulic system is usually established in that an air bleed valve which separates the hydraulic system from the environment is opened so that the hydraulic system can be set to the environmental pressure. The pressureless state of the hydraulic system (i.e., the status of the hydraulic system at environmental pressure) is then used for the calibration of the pressure sensor (i.e., for the compensation of the sensor offset). The provision of such an air bleed valve means an unwanted apparatus and economic effort. 
     SUMMARY 
     This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features. 
     It is an object of the present disclosure to provide a hydraulic system of the initially named kind which allows the calibration of a pressure sensor without the hydraulic system having to be brought into a pressureless state for this purpose and in particular without an air bleed valve absolutely being required. 
     A hydraulic system in accordance with the present disclosure includes a hydraulic fluid, a work element actuable by the fluid, a fluid pump for the application of a fluid pressure to the fluid and a pressure sensor for the measurement of the fluid pressure. The fluid pump is operable to generate a reference depression in the system. The pressure sensor is in communication with a control unit which is designed to carry out a calibration of the pressure sensor with reference to the reference pressure. 
     In accordance with the invention, the calibration of the pressure sensor therefore does not take place in a pressureless state of the hydraulic system (i.e., with the environmental pressure as the reference), but rather with reference to a reference depression. That depression (sometimes also called a negative pressure) is called the reference depression here which can be achieved as a maximum by the fluid pump at maximum pumping capacity and under normal operating conditions, in particular normal operating conditions in the automotive area. If the pumping capacity of the fluid pump is sufficiently high, the reference pressure can approximate the vapor pressure of the fluid or even correspond to it. The vapor pressure is the pressure at which the hydraulic fluid in the closed system is simultaneously present in the liquid phase and in the gaseous phase. 
     For this purpose, the fluid pump is operable in two directions, namely, on the one hand, in a working or forward direction in which it applies a pressure to the fluid which effects an actuation of the work element and, on the other hand, in the opposite direction which results in the generation of a depression in the hydraulic system due to the closed nature of the hydraulic system with respect to the environment. The depression ultimately being formed in the hydraulic system as a result of the pumping of the fluid pump in the opposite direction is used as the reference pressure for the calibration of the pressure sensor in accordance with the present disclosure. 
     In this respect, the depression which is adopted as a maximum (i.e., the reference depression, in accordance with the vapor pressure of the fluid) is largely independent of the temperature prevailing in the system over a temperature range relevant to automotive applications from approximately −40° C. to +110° C. so that a calibration of the pressure sensor is possible reliably over this temperature range. 
     In the simplest case, the calibration of the pressure sensor is carried out in that the offset of the pressure sensor (i.e., that is the difference between the actual reference depression value and the pressure value measured by the pressure sensor at the reference depression) is determined and is taken into account by a so-called offset shift in the evaluation of the data output by the pressure sensor. 
     Since the calibration of the pressure sensor only requires that the hydraulic system is closed with respect to the environment, the hydraulic system in accordance with the present disclosure generally does not have to have an air bleed valve which allows the establishing of a pressureless state of the system. Such an air bleed valve is also generally not required for the establishing of the fluid pressure required for a desired actuation of the work element since the fluid pressure can be established in a desired manner by a corresponding control of the fluid pump operable in opposite directions. The hydraulic system in accordance with the present disclosure can consequently be equipped without an air bleed valve to establish a pressureless state (i.e., to set ambient pressure in the system). 
     As a result, the hydraulic system in accordance with the present disclosure allows a simple calibration of the pressure sensor, which can be carried out, for example, at regular intervals, without the system having to be brought into a pressureless state for this purpose, whereby a permanently increased measurement precision of the pressure sensor and ultimately a more precisely working total system is achieved. 
     In accordance with an embodiment, the control unit is also provided for the control of the fluid pump. The control unit so-to-say therefore satisfies a dual function in that it not only carries out the calibration of the pressure sensor, but also takes over the control of the fluid pump. By the communication with the pressure sensor, it allows the control unit to control the fluid pump such that a desired fluid pressure is adopted in the hydraulic system. 
     A pressure relief valve can be provided to limit the fluid pressure which can be adopted in the hydraulic system to a maximum possible value. 
     The fluid pump can be any pump which is suitable to work in opposite directions. A gerotor pump can be named purely by way of example in this connection; but other suitable pump types can naturally also be considered. 
     The control unit is preferably designed to operate the fluid pump at a lower pumping capacity during the determination of the reference depression than during the generation of the depression. To establish depression in the system as fast as possible, pumping can thus initially take place at a higher capacity, whereas pumping only takes place with reduced capacity after reaching the reference pressure to maintain the balanced state. It is avoided by the reduction of the pumping capacity that air is sucked into the system unintentionally, e.g. via the pressure relief valve. 
     The control unit is advantageously designed only to determine the reference depression after the end of a preset waiting period, of e.g. approximately 0.2 s, after the generation of the depression. The preset waiting time is in this respect preferably selected so that it is ensured that the measurement of the reference depression and thus ultimately the calibration of the pressure sensor is only carried out after setting a balanced state. 
     Alternatively, the control unit can be designed to determine the reaching of the reference depression by determining a pressure gradient and by comparison of the pressure gradient with a threshold value. In this respect, the balanced state is therefore not assumed after the end of a preset waiting period, but is rather actually determined with reference to the pressure development. As soon as the pressure gradient has exceeded the threshold value, the calibration of the pressure sensor can be carried out. 
     As mentioned, the reference depression is essentially independent of the fluid temperature prevailing in the system over a wide temperature range. Nevertheless, the control unit can be designed to increase the calibration precision to carry out the calibration of the pressure sensor while taking account of the actual fluid temperature. 
     To detect the fluid temperature, a sensor, in particular a sensor arranged in a fluid sump of the system, can be used, for example, which is typically anyway provided. 
     Alternatively or additionally, the control unit can be designed to determine the temperature of the fluid pump and to derive the fluid temperature from this. If the fluid pump includes an electric motor, the temperature of the fluid pump can be determined, for example, from the electrical resistance of the armature windings which is determined during an operating pause of the electric motor. The fluid temperature can also be determined without an additional sensor in this manner. 
     If the fluid temperature is known, the reference depression applicable to the respective fluid temperature can thus be determined from this, for example, with reference to a look-up table, and the pressure sensor calibration can be carried out. 
     In accordance with an embodiment, the work element includes a piston displaceable journalled in a cylinder, in particular for the actuation of a clutch (e.g., of a friction clutch). Specifically, the clutch can be the clutch of a torque transfer device which serves (e.g., in a vehicle with all-wheel drive) to transfer a portion of the driving torque to a secondary drive axle. For example, the torque transfer device can be a transfer case which serves for the distribution of a torque to two output shafts. Alternatively, the clutch can be arranged in a lock of an axle differential or in a torque transfer device effective in the transverse direction (torque vectoring). 
     A further subject of the present disclosure is accordingly a torque transfer device with a drive shaft and an output shaft, with a clutch, in particular a friction clutch, for the transfer of a torque from the drive shaft to the output shaft, and with a hydraulic system for the actuation of the clutch, with the hydraulic system comprising: a hydraulic fluid, a fluid pump for the application of a fluid pressure to the fluid, with the fluid pump being operable to generate a reference depression, and a pressure sensor for the measurement of the fluid pressure which is in communication with a control unit which is designed to carry out a calibration of the pressure sensor with reference to the reference depression. 
     The above-named advantages can be achieved accordingly by the transmission in accordance with the present disclosure and its advantageous embodiments. 
     A further subject matter of the present disclosure is moreover a method for the calibration of a pressure sensor which is provided for the measurement of the fluid pressure of a hydraulic fluid contained in a hydraulic system, in which method a reference depression is produced in the system by means of a pump and a calibration of the pressure sensor is carried out with reference to the reference depression. 
     A calibration of the pressure sensor can be carried out in a simple and reliable manner with the help of the method in accordance with the present disclosure without the hydraulic system having to be brought into a pressureless state for this purpose. It is thus possible also to calibrate pressure systems of those systems which are closed with respect to the environment and do not have air bleed valves which permit a setting of the hydraulic system to environmental pressure. Advantageous embodiments of the method in accordance with the present disclosure result from the above embodiments. 
     Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
    
    
     
       DRAWINGS 
       The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure. 
         FIG. 1  is a schematic representation of a hydraulic system in accordance with the present disclosure; 
         FIG. 2  is an exemplary phase diagram of a hydraulic fluid of the system of  FIG. 1 ; and 
         FIG. 3  is a diagram which shows the reference depression achievable in the system of  FIG. 1  in dependence on the temperature. 
     
    
    
     DETAILED DESCRIPTION 
     Example embodiments will now be described more fully with reference to the accompanying drawings. 
     The hydraulic system shown in  FIG. 1  includes a hydraulic circuit  10  which contains a hydraulic fluid and which is closed with respect to the environment. 
     A fluid pump  14 , e.g. a gerotor pump, is arranged in a first line  12  of the hydraulic circuit  10  and serves to convey hydraulic fluid from a fluid reservoir  16  to a work element  18 . A filter  17  is connected between the fluid pump  14  and the fluid reservoir  16 . 
     The work element  18  includes a piston  22  which is displaceably journalled in a cylinder  20  and which can be provided e.g. for the actuation of a friction clutch of a transfer case. 
     A control unit  24  is connected to a motor  26  of the fluid pump  14  for the control of the fluid pump  14 . The fluid pump  14  can be operated in a first direction (forward or working direction), in which it conveys hydraulic fluid from the fluid reservoir  16  to the work element  18 , and in a second direction (counter-direction), in which it conveys hydraulic fluid from the work element  18  to the fluid reservoir  16 . 
     If the fluid pump  14  is operated in the forward direction, an increased pressure builds up in the section  28  of the first line  12  disposed between the fluid pump  14  and the work element  18 . As a result, the fluid conveyed through the fluid pump  14  to the work element  18  exerts pressure onto the piston  22 , whereby it is displaced in the cylinder  20  (to the right in  FIG. 1 ), e.g. against the restoration force of a spring element. 
     If the increased fluid pressure in the line section  28  exceeds a limit value defined by a pressure relief valve  30 , the pressure relief valve  30  opens and fluid can flow back from the first line  12  via a second line  32  into the fluid reservoir  16  to reduce the fluid pressure. 
     A pressure sensor  34  known per se is provided for the measurement of the fluid pressure in the line section  28 . The pressure sensor  34  is connected to the control unit  24  and is read out by it to enable an operation of the fluid pump  14  such that a desired fluid pressure is adopted in the line section  28  and a desired force is applied to the piston  22  of the work element  18 . 
     The pressure sensor  34  is recalibrated regularly to ensure a sufficient precision of the measurement of the fluid pressure. For this purpose, the fluid pump  14  is operated in the counter-direction so that a depression is adopted in the line section  28 . The depression achievable at a maximum under normal conditions by the fluid pump  14  is called the reference depression. 
     With a sufficiently high pumping capacity of the fluid pump  14 , the reference depression can lie in the range of the vapor pressure of the hydraulic fluid (i.e., in the range of the pressure at which the hydraulic fluid is present simultaneously in the liquid phase and in the gaseous phase). The development of the vapor pressure of the hydraulic fluid is shown by way of example by the curve section A in the phase diagram of  FIG. 2 . 
     In practice, the fluid pump  14  will normally not be dimensioned so thick that the reference depression actually corresponds to the vapor pressure of the hydraulic fluid under operating conditions, in particular operating temperatures, which typically occur e.g. in the automotive area. Nevertheless, a reference depression will be adopted in the line section  28  which, similar to the vapor pressure, is substantially constant over a wide temperature range. 
     In  FIG. 3 , the depression which can be achieved at a maximum at different temperatures (i.e. that is the reference depression) for a hydraulic fluid is shown by way of example. As can be seen from  FIG. 3 , the reference depression has a comparatively small dependence on the temperature over the temperature range in particular relevant in the automotive sector from −40° C. to 100° C. 
     The reference depression is consequently suitable for a use as the reference value in the calibration of the pressure sensor  34 . This applies in particular on a use of the hydraulic system in a motor vehicle since the temperature range shown in  FIG. 3  includes the temperatures which typically occur in an automotive use. 
     As already mentioned, the pressure sensor  34 —controlled by the control unit  34 —is automatically recalibrated at regular time intervals, with an exact observation of the exact time intervals not being necessary. Alternatively or additionally, it is naturally also possible to recalibrate the pressure sensor  34  within the framework of servicing work (e.g., on a visit to the shop). 
     If a calibration of the pressure sensor  34  should be carried out, the control unit  24  outputs a corresponding signal to the motor  26  to operate the fluid pump  14  in the opposite direction. As soon as the fluid pressure detected by the pressure sensor  34  has reached a stable minimum (i.e., as soon as the reference depression has been adopted in the line section  28 ) the pressure sensor  34  is calibrated by the control unit  24  using the reference depression as the reference value. After the end of the calibration procedure, the fluid pump  14  can be stopped or can be operated in the forward direction again. 
     The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the invention, and all such modifications are intended to be included within the scope of the invention.
A hydraulic system is presented with a hydraulic fluid, with a work element actuable by the fluid, with a fluid pump for the application of a fluid pressure to the fluid, with the fluid pump being operable to generate a reference depression in the system, and with a pressure sensor for the measurement of the fluid pressure which is in communication with a control unit which is designed to carry out a calibration of the pressure sensor with reference to the reference depression.