Abstract:
A method for operating a hydraulic system of an automatic transmission, in particular a dual clutch transmission of a vehicle, includes analyzing by a provisioning module of a control device of the automatic transmission whether an intention of a driver to initiate operation of the vehicle exists, wherein the automatic transmission includes a high-pressure circuit which includes a pressure accumulator, at least one clutch, actuators and a charge pump, and when the intention to initiate operation of the vehicle and a requirement to charge the pressure accumulator exist, activating a partial charging operation in which the charge pump is controlled with a charging rotational speed for increasing an actual accumulator pressure of the accumulator to a predetermined switch-off pressure value at which the automatic transmission is ready for operation for a defined period of time.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application claims the priority of German Patent Application, Ser. No. 10 2014 003 083.0, filed Mar. 1, 2014, pursuant to 35 U.S.C. 119(a)-(d), the disclosure of which is incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to a to a method for operating a hydraulic system of an automatic transmission of a motor vehicle. 
         [0003]    The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention. 
         [0004]    In a dual clutch transmission two sub-transmissions enable a fully automatic gear change without interruption of traction. A torque is transmitted via one of two clutches, which connects the two sub-transmissions with the drive. The clutches and the actuators for engaging the gears are hydraulically controllable via a hydraulic system. 
         [0005]    In the hydraulic system known from the state-of-the-art the low-pressure circuit has a cooling pump and the high-pressure circuit has a charge pump with which the pressure accumulator is charged to the required accumulator pressure. The two hydraulic pumps are driven by a common electric motor via a common drive shaft. The electric motor is controlled via a control device. When a requirement to charge the pressure accumulator exists, the electric motor is operated with a charge rotational speed. As an alternative and/or in addition the electric motor is operated with a cooling rotational speed for example when a requirement for cooling exists (i.e., when charging is not required). In addition the high-pressure circuit and low-pressure circuit can be connected via a bypass line with integrated control valve. The control valve can be switched in dependence on the accumulator pressure in the high-pressure circuit without requiring further external energy, i.e., automatically, between a charging position and a non-charging position (i.e., cooling position). In the charging position the hydraulic system operates in the charging mode (i.e., the charge pump is fluidly connected with the high-pressure circuit) at high pump load with correspondingly great actual power consumption. On the other hand in the non-charging position (i.e., cooling position) of the control valve the hydraulic system operates for example in a cooling operation or in another operation, for example a filter cleaning operation. 
         [0006]    For reasons of clarity in the following the term cooling position of the control valve is used in most cases. It is noted that the cooling position and the non-charging position of the control valve are identical. In addition for reasons of clarity the terms cooling operation and cooling rotational speed are used in the following. The cooling operation is only an example for a non-charging operation and hence term cooling operation can be replaced with the more general term non-charging operation. 
         [0007]    In the cooling position of the control valve the charge pump, beside the cooling pump, is also fluidly connected with the low-pressure circuit and decoupled from the high-pressure circuit. In the cooling operation the hydraulic pumps operate—in contrast to the charging operation—at a lower pump load with correspondingly lower actual power consumption. 
         [0008]    In the state-of-the-art the control device can operate in a provisioning mode, in which an intent of the user to start operation of the vehicle is detected via appropriate sensors and/or determined based the occurrence of predefined events. When this is the case it is further tested whether a requirement for charging the pressure accumulator exists. When this criterion is also satisfied a full charge operation is initiated in which the pressure accumulator is filled up to a maximal accumulator pressure. This ensures that the automatic transmission is ready for operation when subsequently initiating operation of the vehicle. The pressure accumulator is therefore always fully charged even when subsequently operation of the vehicle is not initiated but the vehicle is only loaded. 
         [0009]    It would therefore be desirable and advantageous to provide a method for operating a hydraulic system of an automatic transmission, in which unnecessary charging processes up to the complete filling of the pressure accumulator are avoided, the number of charging cycles of the vehicle battery is reduced and generally energy can be saved. 
       SUMMARY OF THE INVENTION 
       [0010]    According to one aspect of the present invention, a. method for operating a hydraulic system of an automatic transmission, in particular a dual clutch transmission of a vehicle, includes analyzing by a provisioning module of a control device of the automatic transmission whether an intention of a driver to initiate operation of the vehicle exists, wherein the automatic transmission includes a high-pressure circuit which includes a pressure accumulator, at least one clutch, actuators and a charge pump; and when the intention to initiate operation of the vehicle and a requirement to charge the pressure accumulator exist, activating a partial charging operation in which the charge pump is controlled with a charging rotational speed for increasing an actual accumulator pressure of the accumulator to a predetermined switch-off pressure value at which the automatic transmission is ready for operation for a defined period of time. In the partial charging operation the charge pump is operated with a charging rotational speed in order to no longer increase the accumulator pressure to a maximal pressure value but rather to a predetermined minimal pressure (i.e., switch-off pressure value) at which the automatic transmission is ready to operate. The above mentioned provisioning module together with the charge pump can be integrated into a regulatory circuit in which the charge pump is operated with the charging rotational speed (control value) based on the desired minimal accumulator pressure (target value) and a determined actual accumulator pressure (actual value). 
         [0011]    According to another advantageous feature of the invention, the provisioning module has a pressure model unit for determining the actual accumulator pressure. In the pressure model unit a pressure model is stored in which the time behavior of the actual accumulator pressure can be modeled for example by an algorithm. A model accumulator pressure is thus generated in the pressure model unit. The latter is used as the basis for the partial charging operation instead of the actual accumulator pressure. In this embodiment therefore neither an accumulator pressure sensor nor any other valve position sensor, with which a requirement for charging the pressure accumulator in the high-pressure circuit can be determined, is required. Such sensors require an increased number of components. In addition the operation of the sensors may be subject to malfunction. 
         [0012]    As mentioned above the partial charging operation is initiated at a start time point when the intent to initiate operation of the vehicle and a requirement to charge the pressure accumulator exist. At this start time point the model accumulator pressure is set to a zero value, at which the model accumulator pressure corresponds to the ambient pressure. When on the other hand it is recognized that no requirement to charge the pressure accumulator exists, i.e., the accumulator pressure is sufficiently great, the provisioning module is deactivated and/or the model accumulator pressure is set to an initial value which is greater than the switch-off pressure value and in particular corresponds to the maximal accumulator pressure. 
         [0013]    In a technical realization, the pressure accumulator can be a piston cylinder unit in which an oil chamber, which is fluidly connected with the hydraulic lines, can be acted upon by means of a pre-tensioned pressure piston. The pretension can be realized via a pressure spring or via a gas pressure. This means that when the oil chamber is filled with hydraulic oil, a hydraulic pressure greater than a pretension pressure exists in the pressure accumulator, which is described below. 
         [0014]    The pressure accumulator can for example be a piston cylinder unit with an oil chamber, which is connected with the hydraulic lines, and a pre-tensioned pressure piston. The pretension is for example achieved by a gas pressure, which impinges on the pressure piston. When the oil chamber is completely empty the pressure piston is pushed with a pretension force against a mechanical stop of the pressure accumulator. This means that for overcoming the pretension force in a filling process, a hydraulic pressure greater than a pretension pressure corresponding to the pretension force must be exerted on the pressure piston. 
         [0015]    Thus in a partially filled state of the pressure accumulator, the hydraulic oil exerts an accumulator pressure on the pressure piston which is greater than the pretension pressure. In the completely empty state the hydraulic lines are not impinged with pressure by means of the pressure accumulator. Rather an ambient pressure prevails in the hydraulic lines. An operational readiness of the automatic transmission is given when all hydraulic lines are filled with hydraulic oil and a hydraulic pressure greater than the pretension pressure prevails in the hydraulic lines. 
         [0016]    The minimum pressure, which forms the target value (i.e., switch-off pressure value), is preferably greater than the pretension pressure by a first pressure difference. When in the partial charging operation the model accumulator pressure exceeds the switch-off pressure, the provisioning module switches off the charge pump. On the other hand when falling below a switch-on pressure value the charge pump is switched on again. The switch-on pressure value is lower than the switch-off pressure value by a second pressure difference. In addition this second pressure difference is preferably smaller than the above-mentioned first pressure difference. This ensures that the partial charging operation is reliably switched on and off above the above-mentioned pretension pressure. 
         [0017]    Due to component tolerances the deviation between the model accumulator pressure and the actual accumulator pressure increases with each switching on and off process and also with increased duration of the partial charge operation, which may result in a faulty control of the charge pump. In order to avoid this the charge pump can be continuously operated with the charging rotational speed independent from the actual calculated model accumulator pressure after a predetermined number of switching on and off processes, for example after three switching on and off processes, which results in an increase of the actual accumulator pressure up to a maximum pressure value. 
         [0018]    In contrast to the above-described partial charging operation, the control device can for example carry out a full charging operation for example when initiating operation of the vehicle and in the presence of a requirement to charge the accumulator. In the full charging operation the charge pump is operated with a charging rotational speed in order to increase the accumulator pressure to a maximal pressure value, which is greater than the switch-off pressure value. 
         [0019]    The provisioning module can deactivate the partial charge operation in the case of pre-defined events for example in the event of an actual start of operation of the vehicle. 
         [0020]    Preferably the requirement for pressure charging can be determined based on the actual power consumption of the electric motor of the charge pump and also on the actual rotational speed of the electric motor. This manner of determining whether a requirement of charging exists is based on the fact that an actual power consumption of the electric motor of the charge pump during a charging operation for charging the pressure accumulator is significantly greater than the actual power consumption of the electric motor in the case of a requirement for cooling the clutches. 
         [0021]    Beside the high-pressure circuit the hydraulic system can have a low-pressure circuit for cooling the clutches. The high-pressure circuit and the low-pressure circuit can respectively have a cooling pump and a charge pump, which can be driven via a common electric motor. The high-pressure circuit and the low-pressure circuit can be connected via a bypass line with integrated control valve. The control valve can be switched in dependence on the accumulator pressure in the high-pressure circuit without requirement of further external energy, which means automatically, between a charging position in which the hydraulic system operates in the charging operation, and a non-charging position (cooling position), in which the hydraulic system is operated in the non-charging operation. The control valve can therefore automatically assume a charging position when the accumulator pressure in the high-pressure circuit falls below a lower threshold value. Vice versa, the control valve can automatically assume its non-charging position (cooling position) when the accumulator pressure in the high-pressure circuit exceeds an upper threshold value. After completed full charging operation the accumulator pressure in the high-pressure circuit corresponds to the upper threshold value. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0022]    Other features and advantages of the present invention will be more readily apparent upon reading the following description of currently preferred exemplified embodiments of the invention with reference to the accompanying drawing, in which: 
           [0023]      FIG. 1  shows a circuit diagram particular clutch transmission for a motor vehicle with seven forward gears and a reverse gear; 
           [0024]      FIG. 2   a  shows a hydraulic system of the dual clutch transmission of  FIG. 1 ; 
           [0025]      FIG. 2   b  shows an example of a pressure accumulator by itself; 
           [0026]      FIG. 3  shows program components in a further circuit diagram, which illustrate the provisioning module; and 
           [0027]      FIGS. 4 to 6  respectively show diagrams illustrating the partial charging operation, the full charging operation, and a termination criterion of the partial charging operation. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0028]    Throughout all the Figures, same or corresponding elements are generally indicated by same reference numerals. These depicted embodiments are to be understood as illustrative of the invention and not as limiting in any way. It should also be understood that the drawings are not necessarily to scale and that the embodiments are sometimes illustrated by graphic symbols, phantom lines, diagrammatic representations and fragmentary views. In certain instances, details which are not necessary for an understanding of the present invention or which render other details difficult to perceive may have been omitted. 
         [0029]    Turning now to the drawing, and in particular to  FIG. 1 , there is shown a schematic diagram of a dual-clutch transmission for a motor vehicle with four-wheel-drive. The dual-clutch transmission has seven forward gears (cf. the circled digits  1  to  7 ) and a reverse gear RW. In the following, the dual-clutch transmission is explained to the degree necessary for understanding the invention. The dual-clutch transmission has two input shafts  12 ,  14 , which are arranged coaxial to each other and can be alternately connected with the drive source, for example an internal combustion engine, via two hydraulically actuatable multiple disc clutches K 1 , K 2 . The input shaft  14  is configured as a hollow shaft in which the input shaft  12 , which is configured as solid shaft, is guided. The two input shafts  12 ,  14  drive via gear sets of the forward gears and the reverse gear onto an output shaft  16  which is arranged parallel to the axis, and onto an intermediate shaft  18  which is configured as a hollow shaft. The gear sets of the forward gears  1  to  7  each have fixed gears and movable gears, which can be switched via actuators  22 . The actuators  22  can for example be dual synchronous clutches, which can each switch neighboring movable gears from a neutral position. 
         [0030]      FIG. 2   a  shows the hydraulic system of the dual clutch transmission in a strongly simplified circuit diagram. By means of the hydraulic system the hydraulic cylinders  23  of the clutches K 1 , K 2  and the actuators  22  are actuated. According to  FIG. 2   a  the hydraulic system has a high-pressure circuit H and a low-pressure circuit N. In the high-pressure circuit H the hydraulic cylinders  23  of the clutches K 1 , K 2  switched therein, and the actuators  22  can be impinged with an accumulator pressure p s  via a pressure accumulator  25 , which accumulator pressure is within the range of for example  30  bar. For this a main line  27 , which is connected to the pressure accumulator  25 , is guided to the hydraulic cylinders  23  via not further described partial lines  31 . In the partial lines  31  respective control valves  35  are arranged. The control valves  35  can be controlled via a central control device  39 , in a not shown manner. 
         [0031]    The hydraulic system also has a charge pump  53 , which on the input side is connected with an oil sump  55 . The charge pump  53  can be controlled by the control unit  39  to charge the pressure accumulator  25  via an electric motor  57 . In addition the charge pump  53  together with a cooling pump  59  is arranged on a common drive shaft  60 , which is driven by the electric motor  57 . The cooling pump  59  is on the output side connected with a low-pressure line  61 , which leads to a distribution valve  63 . Depending on the position of the distribution valve  63 , the hydraulic fluid can be conducted to the first and/or second clutch K 1 , K 2  and subsequently back into the oil sump  55  when a cooling requirement exists. 
         [0032]    According to  FIG. 2  the main line  27  of the high-pressure circuit H branches of at a branching site  65  into a bypass line  67 , which is connected with the low-pressure line  61  of the low-pressure circuit N. downstream of the branching site  65  a check valve  69 , which will be described below, is arranged. In addition a control valve  71  is integrated in the bypass line  67 . The control valve  71  can be adjusted depending on the level of the accumulator pressure p s  in the high-pressure circuit H between the charging position L shown in  FIG. 2   a  and a cooling position K. The accumulator pressure p s  in the high-pressure circuit H acts as a control pressure with which the control valve  71  can be adjusted without additional external energy, i.e., automatically. The control valve  71  is configured so that it automatically assumes the charging position L when the accumulator pressure p s  in the high-pressure circuit H for example falls below a lower threshold value for example 25 bar. In addition the control valve  71  is automatically displaced into its cooling position K when the accumulator pressure p s  exceeds an upper threshold value p max  for example 28 bar. 
         [0033]      FIG. 2   b  shows the general construction at the function of the pressure accumulator  25 . Accordingly the pressure regulator  25  is a piston cylinder unit with an oil chamber  26 , which is connected with the hydraulic lines  27 ,  31 , and a pre-tensioned pressure piston  27 . The pretension in this example is achieved by a gas pressure p gas , which acts on the pressure piston  27 . As an alternative the pretension can also be achieved by a spring. When the oil chamber  26  is completely empty the pressure piston  27  (indicated with the dashed line in  FIG. 2   b ) is pushed against a stop  29  of the pressure accumulator  25  with a pretension force F. This means that during a filling process a hydraulic pressure greater than a pretension pressure p v  corresponding to the pretension force F v  prevails for overcoming the pretension force F v . 
         [0034]    In  FIG. 2   b  the pressure accumulator  25  is shown in a partially filled state, in which the hydraulic oil acts on the pressure piston  27  with an accumulator pressure thereby forming a pretension force F v . In the completely empty state the hydraulic lines  27 ,  31  are not impinged with pressure by means of the accumulator  25 . Rather, ambient pressure prevails in the hydraulic lines  27 ,  31 . An operational readiness of the automatic transmission is given when all hydraulic lines  27 ,  31  are filled with hydraulic oil and a hydraulic pressure prevails in the hydraulic lines  27  which is greater than the pretension pressure p v , i.e., by a predetermined pressure difference so that the operational readiness is not immediately lost after switching on the charge pump  53  due to a basic leakage. 
         [0035]    During driving operation pressure losses occur due to actuation of the clutches K 1 , K 2  and the actuators  22 . In addition further pressure losses occur due to the basic leakage in the high-pressure circuit H, which means due to leakages resulting from valve gaps or the like. As a result the accumulator pressure p s  is reduced during the driving operation. In the case that the accumulator pressure p s  falls below the lower threshold value (i.e., a requirement to charge the pressure accumulator exists) the control valve  71  automatically assumes its charging position L ( FIG. 2   a ). When a requirement to charge the pressure accumulator exists the control device  39  controls the electric motor  57  with a charging rotational speed This enables the charge pump  53  to charge the pressure accumulator  25 . In such a charging operation the charge pump  53  operates under great pump load and therefore with a correspondingly great actual power consumption l ist . When the accumulator pressure p s  exceeds the upper threshold value p max  (which means a requirement to charge the pressure accumulator no longer exists) the control valve  71  automatically assumes its cooling position K. In the cooling position K the charge pump  53  delivers hydraulic oil via the now opened bypass line  67  into the low-pressure circuit N. At the same time the high-pressure circuit H is closed pressure-tight via the check valve  69 . Correspondingly the charge pump  53  no longer operates with a high but rather with a reduced pump load and also a correspondingly reduced actual power consumption l ist . 
         [0036]    As mentioned above the control device  39  controls the electric motor  57  with a charge rotational speed when a requirement to charge the pressure accumulator exists. For recognizing such a requirement to charge the pressure accumulator, a pressure sensor in the high-pressure circuit H or a positional sensor in the control valve  71  is not required according to the invention. Instead the control device  39  has an analysis unit  73  ( FIG. 3 ). According to  FIG. 3  the analysis unit  73  is in signal communication with a current-measuring device  75  integrated in the motor control, which current measuring device measures an actual power consumption l ist  of the electric motor  57 . The analysis unit  73  is also in signal communication with a rotational speed sensor  77  which measures an actual rotational speed n ist  of the electric motor  57 . 
         [0037]    According to  FIG. 3  the analysis unit  73  is a component of a provisioning module  40  and is in signal communication with an appropriate sensor system  74  for detecting an intended initiation of operation of the vehicle. The sensor system is not included in the invention. Its construction and its function are therefore not further described. 
         [0038]    When a requirement to charge the pressure accumulator and an intended initiation of operation of the vehicle exist, an activation unit  78  in the provisioning module  40  generates a trigger signal T s  with which a partial charging operation T ( FIG. 4 ) is initiated. In the partial charging operation T ( FIG. 4 ) the charge pump  53  is controlled with a charging rotational speed n L  in order to increase the actual accumulator pressure p ist  to a predetermined switch-off pressure value p aus , at which the automatic transmission is ready for operation for a defined period of time (taking into account the basic leakage). 
         [0039]    According to the invention the provisioning module  40  controls a charge pump  53  not based on the actual accumulator pressure p ist  but rather based on a pressure model stored in the pressure model unit  79 . In the pressure model the time behavior of the actual accumulator pressure p ist  is modeled by a model accumulator pressure p M . In the time behavior of the model accumulator pressure p M  the control of the charge pump of  53  with the charging rotational of speed n L  as well as a decrease of the accumulator pressure due to a basic leakage pressure loss is taken into account. This model accumulator pressure p M  is the a basis for regulating the partial charging operation T, which enables omitting a sensor-based detection of the actual accumulator pressure. 
         [0040]      FIG. 4  shows diagrams illustrating the partial charging operation T. According to this the partial charging operation T is initiated at a time point t 1 , i.e., after activation of the pressure model by the activation unit  78 . The pressure model is for example stored in the pressure model unit  79  as an algorithm. 
         [0041]    With the start of the partial charging operation T at the time point t 1 , the charge pump  53  is controlled with a charging rotational speed n L , whereby the hydraulic lines  27 ,  31  are filled with hydraulic oil until the pretension pressure p v  is reached at the time point t 2 . After reaching the pretension pressure p v  the oil chamber  26  of the pressure accumulator  25  is filled, i.e., by displacing the pressure piston  27  against the pretension force F v . At the start time point t 1  the model accumulator pressure p M  is also set to a zero value at which the model accumulator pressure p M  corresponds to the ambient pressure. 
         [0042]    At the time point t 3  the model accumulator pressure p M  reaches the switch-off pressure value p aus . When exceeding the switch-off pressure value p aus  the provisioning model  79  switches off the electric motor  57  of the charge pump  53 , that means the charging rotational speed n L  is set to zero and the model accumulator pressure p M  decreases again due to the basic leakage. When on the other hand the model accumulator pressure p M  falls below a switch-on pressure value p ein , the charge pump  53  is switched on again. According to  FIG. 4  the switch-off pressure value p aus  is higher than the pretension pressure p v  by a first pressure difference Δ p1 . In addition the switch-on pressure value p ein  is lower than the switch-off pressure value p aus  by a second pressure difference Δ p2 . The second pressure difference Δ p2  is smaller than the first pressure difference Δ p1 . 
         [0043]    As shown in  FIG. 4  multiple, for example all three, on and off switching processes occur in the partial charging operation T. After a time point t 4  the provisioning model  40  permanently controls the charge pump  53  with the charging rotational speed n L , i.e., independent of the actual calculated model accumulator pressure p M . 
         [0044]    In contrast to the partial charging operation T shown in  FIG. 4 ,  FIG. 5  shows the full charging operation V. The full charging operation V occurs for example when the vehicle is actually operated and also when a requirement to charge the pressure accumulator exists. In contrast to the partial charging operation T, the accumulator pressure p ist(t)  is increased in the full charging operation V to the maximal pressure value p max . 
         [0045]    The partial charging operation T can be deactivated at predetermined events, for example when the vehicle is actually operated or when a requirement to charge the pressure accumulator does not exist, as illustrated in  FIG. 6 . According to  FIG. 6  the partial charging operation T initially starts at a time point t 1 . With a small temporal offset d t  after the time point t 1 , the analysis unit  73  recognizes at the time point t E  that no requirement to charge the pressure accumulator exists, which means that the pressure accumulator  25  is still sufficiently filled with hydraulic oil. At the time point t E  therefore the model pressure p M  is increased to an initial value p init . As a result the charging rotational speed of the charge pump  53  is decreased to zero. 
         [0046]    While the invention has been illustrated and described in connection with currently preferred embodiments shown and described in detail, it is not intended to be limited to the details shown since various modifications and structural changes may be made without departing in any way from the spirit of the present invention. The embodiments were chosen and described in order to best explain the principles of the invention and practical application to thereby enable a person skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. 
         [0047]    What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims and includes equivalents of the elements recited therein: