Abstract:
A method of determining the coupling moment in a friction coupling with an electro-mechanical actuator which comprises a supporting element axially supported in a housing and a displaceable setting element axially supported on said supporting element, wherein the supporting element is axially supported in the housing via an undisplaceably enclosed hydraulic medium and that the pressure in the hydraulic medium is measured and used by lookup tables of values for the actuator and for the friction coupling for the purpose of calculating the coupling moment in a central ECU.

Description:
TECHNICAL FIELD 
   The invention relates to a method of determining the coupling torque in a friction coupling with an electro-mechanical actuator which comprises a supporting element substantially axially fixed in a housing and an axially displaceable selling element supported on said supporting element. The invention also relates to a friction coupling with an electro-mechanical actuator, more particularly for being used in a lockable differential drive or as a hang-on coupling for an optionally drivable driving axis of a motor vehicle, wherein the actuator comprises a supporting disc axially fixed in a housing and a setting disc which is axially supported on said supporting disc. 
   BACKGROUND 
   Friction couplings for the range of application mentioned here have torque control purposes, i.e. more particularly they serve to control the distribution of torque at two wheels of a driven axle or between two drivable axles. To be able to carry out suitable control processes, the coupling moment transmitted by the coupling has to be known, i.e. it has to be constantly determined by suitable means. In prior art processes of for determining the coupling moment, the values such as speeds, temperatures etc. are measured by sensors, theoretical values (transmission ratios, efficiency etc.) are calculated, and corrected factors (friction coefficients, efficiency, temperature and speed dependencies etc.) as determined by tests are stored. The measured, calculated and interpolated values are evaluated in a computer or processor unit and the correct current for achieving a calculated coupling moment is set at the electric motor of the actuator. There is thus provided an open loop. 
   Because of existing non-linearities and deviations in the coupling behavior and in the behavior of the actuator, the coupling moment values set in this way and thus the respective torque values at the wheel or axles in some cases greatly deviate from the theoretical physical values. The degree of accuracy achieved in this way for setting the coupling moment is sometimes not sufficient. Storing the required evaluation tables in an Electronic Control Unit (ECU) or in a processor is complicated and does not allow the complete physical model of the coupling to be copied sufficiently accurately. For evaluating the measured, calculated and interpolated values ECU storage and computer capacity are used and are time-consuming. Several sensors are required for obtaining the required measured values (speed measurements, temperature sensors, etc.). 
   SUMMARY OF THE INVENTION 
   It is therefore an object of the present invention to provide a process and a friction coupling of the initially mentioned type by which the coupling torque can be controlled in a less complicated and more accurate way. A process is provided in which the supporting element is axially supported in the housing via an undisplaceably enclosed hydraulic medium, and the pressure in the hydraulic medium is measured and used by using value tables for the actuator and the friction coupling for the purpose of calculating the coupling torque in a central ECU. More particularly, the axial force of the actuator is calculated from the measured pressure, using a stored value for the effective face of the supporting element. Furthermore, the method provides that, by using the stored values for the friction value and for the friction faces of the coupling, the coupling moment is calculated from the supporting force of the supporting element and the axial force of the actuator respectively. 
   According to another embodiment, the pressure is controlled in a closed control circuit by setting the actuator to a respective nominal value. This means that instead of an open loop, use is made of a closed loop base on the pressure as the only measured or controlled variable. 
   In this way, the axial setting force is controlled by a closed loop, and for converting the measured pressure into the axial force, only the effective surface of the supporting disc loaded by a hydraulic medium needs to be known and stored; and for calculating the resulting coupling torque, only the friction values of the coupling plates and the mean coupling diameter need to be known and stored in value tables. On the basis of these, the coupling torque required for a certain driving condition is converted by the Electronic Control Unit (ECU) into the nominal value for the axial force of the actuator and the pressure respectively and directly compared with the signal of the pressure sensor by controlling the actuator. The torque calculated on the basis of the axial force can be made available on the vehicle Controller Area Network (CAN) bus. 
   According to a first solution, the inventive friction coupling is characterised in that the supporting disc is provided in the form of an annular piston in an annular chamber filled with a hydraulic medium and that, in the housing, there is arranged a pressure sensor element for measuring the hydraulic pressure in the annular chamber. The pressure sensor element is connected to a branch line leading to the annular chamber, or the pressure sensor element can be directly introduced into the annular chamber. 
   An alternative inventive friction coupling is characterised according to a second solution in that the supporting disc is provided in the form of an annular plunger and that into the housing there is inserted an annular housing with a cover, which annular housing and cover form an annular chamber which is filled with a hydraulic medium and in which there is arranged a pressure sensor element for measuring the hydraulic pressure in the annular chamber, wherein the annular plunger acts on the cover. It is possible for the cover to be provided in the form of a flexible diaphragm closing the annular housing. Alternatively, it is possible for the cover to be displaceable in the annular housing and to be sealed relative to the annular chamber. 
   In any case, the hydraulic medium can be freely selected. However, to avoid any leakage, it is proposed to use a hydraulic medium with a high viscosity, e.g. an oil or gel. For sealing, preference is given to annular seals, but these can be eliminated if the hydraulic medium is formed by an elastic, self-supporting formed member. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the invention are illustrated in the drawings and will be described below. 
       FIG. 1  shows a first embodiment of a multi-plate coupling with an electro-mechanical actuator with hydraulic support. 
       FIG. 2  shows a second embodiment of a multi-plate coupling with an electro-mechanical actuator with hydraulic support. 
       FIG. 3  shows a third embodiment of a multi-plate coupling with an electro-mechanical actuator with hydraulic support. 
       FIG. 4  shows the pressure recording element according to  FIG. 3  in an enlarged form. 
       FIG. 5  shows a fourth embodiment of a multi-plate coupling with an electro-mechanical actuator with hydraulic support. 
   

   DETAILED DESCRIPTION 
     FIGS. 1 to 3  will initially be described jointly to the extent that the identifiable details correspond to one another. A shaft  14  connected to a multi-plate coupling  15  is supported in a multi-part housing  11  via a ball bearing  12  and an axial bearing  13 . The shaft  14  is produced so as to be integral with a hub  16  of the multi-plate coupling, whereas a coupling carrier  17  of the multi-plate coupling is integrally connected to a further hub  19 . The shaft  14  comprises a flange  18  for connecting a first shaft suitable for being flanged on; the second hub  19  comprises a shaft toothing  20  for attaching a second shaft suitable for being plugged in. The multi-plate coupling comprises first coupling plates  22  connected to the hub  16  in a rotationally fast way and second coupling plates  23  which are connected to the carrier  17  and which are arranged so as to alternate in the axial direction. The package comprising the first and second coupling plates  22 ,  23  is supported on a supporting plate  24  secured to the hub  16  and can be axially loaded by a pressure plate  25  which is axially displaceable relative to the hub  16 . Between the coupling plates  22 ,  23  and the pressure plate  25  there is arranged a pair of plate springs  26 ,  27  for returning the pressure plate. The pressure plate  25 , in turn, is displaced via an axial bearing  28  by an axial setting device  29  which can be driven by an electric motor  30 . The drive is effected from the shaft  31  of the electric motor via a reduction stage  32  to the axial setting device  29 . 
   In the embodiment as illustrated, the axial setting device (actuator) comprises a pressure or setting disc  34  rotatingly drivable via a tooth segment  33 , and of an axially supported supporting disc  35  held in the housing  11  in a rotationally fast way. On their end faces facing one another, the discs  34 ,  35  comprise ball grooves  42 ,  43  for balls  45  guided in a cage  44 . The ball grooves are arranged in pairs and extend in the circumferential direction, and they comprise gradients extending in opposite directions and variations in depth. When the disc  34  is rotatingly driven relative to the axially supported and rotationally secured disc  35 , the balls run from deeper ball groove regions to shallower ball groove regions, as a result of which the disc  34  moves away from the disc  35  towards the multi-plate coupling. The coupling package is closed. When the drive rotates in the opposite direction or when the electric motor  30  is current-less, the returning force of the plate springs  26 ,  27  causes the disc  34  to be pressed back and, under the effect of the balls  45  in the ball grooves  42 ,  43 , it is rotated back. 
   In the embodiment according to  FIG. 1 , the supporting disc  35  is provided in the form of an annular piston which is held, so as to be axially free and rotationally secured, in an annular cylindrical chamber  36  filled with a hydraulic medium. The disc  35  is sealed relative to the chamber  36  by sealing rings  52 ,  53  positioned on the piston face. From the chamber  36  there starts a radial bore  37  which is closed by a threaded plug  38 . A transverse bore  39  passing through the radial bore  37  is connected to a pressure sensor element  40  having integrated sensor electronics. The chamber  36 , the radial bore  37  and the transverse bore  39  are completely filled with a hydraulic medium, so that the disc is axially firmly supported mainly by the hydraulic medium. The pressure sensor  40  measures the pressure in the cylindrical chamber  36  and, via a cable  41 , transmits the measured values to an ECU in which the measured pressure is converted in the initially described way into the actually transmissible coupling moment. 
     FIG. 2  shows the supporting disc  35  in the form of an annular piston which is held, to as to be axially free and rotationally secured, in an annular cylindrical chamber  36  which is filled with the hydraulic medium. The disc  35  is sealed relative to the chamber  36  by sealing rings  52 ′,  53 ′ arranged in the chamber  36 . A pressure sensor element  60  introduced into the housing is arranged in the chamber  36 . The pressure sensor element records the pressure in the cylindrical chamber  36  and transmits a pressure signal into a pressure sensor Electronic Control Unit (ECU)  62  which is arranged at the housing and in which, in the initially described way, the measured pressure is converted into the actual coupling moment, with the calculated value, via a bus of the motor vehicle, being made available for further use. 
   In  FIG. 3 , the supporting disc  35  is provided in the form of an annular piston which, so as to be axially free and rotationally secured, is held in an annular cylindrical chamber  36  into which an annular housing  51  filled with a hydraulic medium is inserted without any play. In this embodiment, the disc  35  does not have to be sealed relative to the chamber  36 . A pressure sensor element  60  introduced into the housing is arranged in the annular housing  51 . The pressure sensor element records the pressure in the annular housing  51  and transmits a pressure signal into a pressure sensor Electronic Control Unit (ECU)  62  which is arranged at the housing and in which, in the initially described way, the measured pressure is converted into the actual coupling torque. 
   In  FIG. 4 , the annular housing  51  according to  FIG. 3  is shown in an enlarged form as a detail. It is possible to see an annular housing  51  at whose inner circumference and outer circumference there have been inserted seals  52 ″,  53 ″. At one end face, there is inserted a flat annular cover  54  which is sealingly held by two beadings  55 ,  56  relative to the seals  52 ″,  53 ″. The annular housing  51  is completely filled with a hydraulic medium. At one circumferential place of the housing  51 , there is inserted a pressure sensor element  60  whose attaching end is guided out of the housing through a bore  61 . The cover  54  is provided in the form of an elastic diaphragm or in the form of a displaceable cover which is permanently sealed by the seals  52 ,  53  and which is axially acted upon by the supporting disc. 
   In  FIG. 5 , the supporting disc  35  is provided in the form of an annular piston  35  which, in an axially free and rotationally fastened way, is held in an annular chamber  36  filled with a hydraulic medium. The consistency of the hydraulic medium is such that the disc  35  does not have to be sealed relative to the chamber  36 . For instance, the hydraulic medium can be provided in the form of a two-component gel which is filled into the chamber in a liquid form and then gels. Alternatively, the hydraulic medium can be a prefabricated formed member, e.g. a silicone disc. In this case, too, a pressure sensor element  60  is introduced into the chamber  36 , which records the pressure in the cylindrical chamber  36  and transmits a pressure signal to a pressure sensor electronic system mounted on the housing  11 . For the rest, reference is made to the previous embodiments. 
   LIST OF REFERENCE NUMBERS 
   
       
         11  housing 
         12  ball bearing 
         13  axial bearing 
         14  shaft 
         15  multi-plate coupling 
         16  coupling hub 
         17  coupling carrier 
         18  flange 
         19  hub 
         20  shaft assembly 
       
         21 
       
         22  coupling plates 
         23  coupling plates 
         24  supporting plate 
         25  pressure plate 
         26  plate spring 
         27  plate spring 
         28  axial bearing 
         29  axial setting device (actuator) 
         30  electric motor 
         31  shaft 
         32  reduction stage 
         33  tooth segment 
         34  setting disc 
         35  supporting disc 
         36  annular cylinder 
         37  radial bore 
         38  plug 
         39  transverse bore 
         40  pressure sensor element with integrated electronic system 
         41  cable 
         42  ball groove 
         43  ball groove 
         44  cage 
         45  ball 
         51  annular housing 
         52  seal 
         53  seal 
         54  cover 
         55  beading 
         56  beading 
         60  pressure sensor element 
         61  bore 
         62  pressure sensor electronic system