Abstract:
The invention relates to a method for determining the filling quantity in the actuating system of a clutch arrangement which has a forward clutch and a reverse clutch for a respective connection of an input shaft to an output shaft for drive purposes. A first controllable actuating unit is provided for actuating the forward clutch, and a second controllable actuating unit is provided for actuating the reverse clutch.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a national stage application filed under 35 U.S.C. 371 of International Application No. PCT/EP2013/055964 filed Mar. 21, 2013, which claims priority under 35 U.S.C. 119 to German Patent Application No. 102012005765.2 filed Mar. 23, 2012. 
     FIELD OF THE DISCLOSURE 
     The invention relates to a method for determining the filling quantity in the actuating system of a clutch arrangement which has a forward clutch and a reverse clutch for the respective connection of an input shaft to an output shaft for drive purposes wherein a first controllable actuating unit is provided for actuating the forward clutch, and a second controllable actuating unit is provided for actuating the reverse clutch. 
     BACKGROUND 
     In tractors of the John Deere mark gearboxes are used whose forward clutch and reverse clutch of the reversing unit are released during operation of each other clutch so that the actuating unit relevant to the clutch is not only switched without pressure but the hydraulic oil located therein is at least partially emptied. In this situation air is located in the actuating unit which prior to switching the corresponding clutch again by means of the actuating unit has to be removed from same so that a hydraulic pressure can be built up in the actuating unit. This means during operation of the tractor that with each change of driving direction a partial emptying of the actuating unit of the one clutch, and a filling of the actuating unit of the other clutch takes place accordingly. The actual release of the one clutch and switching of the other clutch for the other driving direction is a function of the clutch pedal actuated by the operator so that the release of the clutch pedal controls the ventilation and filling of the actuating unit of the clutch which is to be switched. For the operating comfort which the operator experiences it is on the one hand decisive that the actuating unit is ventilated in the shortest possible, precisely determined, time interval so that from the moment of the complete ventilation and after regulating forwards with a low clutch pressure a deliberate and dosed modulation of the clutch pressure can take place by the operator by means of the clutch pedal. Whereas too long a time interval, within which the actuating unit is first completely ventilated, would leave the operator with the feeling of a sluggish gear change, it is on the other hand decisive for the operating comfort which clutch pressure is set with the optimum possible pedal position for the operator immediately after the ventilation of the actuating unit of the clutch which is to be switched. Too high a clutch pressure is hereby particularly detrimental for the operating comfort since, for the operator, too high a clutch pressure manifests itself as an uncontrollable fierce bite of the clutch arrangement during a change of driving direction. Too low a clutch pressure immediately after the ventilation of the actuating unit would likewise appear to the operator as a sluggish gear change. For the best possible satisfactory operating comfort it is thus a matter of determining this time interval of the ventilation and the adjoining clutch pressure immediately following thereon and matching these with one another. 
     In order to avoid these unacceptable influences on the operating comfort and in view of the fact that each gearbox or each clutch arrangement is subject to tolerances even in high quality production, each gearbox had to run through a calibrating process before starting up a tractor wherein precisely the amount of hydraulic oil which is required for completely ventilating the actuating unit, and precisely the clutch pressure which is applied after ventilation whilst avoiding a fierce bite of the clutch, are determined. Present-day calibrating processes require a load on the output shaft of the clutch arrangement. For certain gearboxes which are installed behind the clutch arrangement, this means that a power flow is switched on from the output shaft of the clutch arrangement via the gearbox and the differential to the drive wheels. In unfavorable cases and those which have to be avoided under all circumstances this power flow can lead to a clear movement of the vehicle during the calibrating process, with the corresponding possibilities of injury in the vicinity of the tractor. 
     SUMMARY 
     Based on this the object of the present invention is to provide a calibrating process which can be carried out with a freely rotating output shaft of the clutch arrangement. 
     This is achieved according to the invention by a method according to claim  1  for determining the filling quantity in the actuating system of a clutch arrangement which has a forward clutch and a reverse clutch for a connection of an input shaft to an output shaft for drive purposes wherein a first controllable actuating unit is provided for actuating the forward clutch and a second controllable actuating unit is provided for actuating the reverse clutch. 
     Through this method it is possible in an advantageous manner to dispense with a switched power flow between the output shaft of the clutch arrangement and the drive wheels of an agricultural utility vehicle in which the clutch arrangement is installed. The inventive idea is rather based on connecting the output shaft through one of the clutches for a defined time interval in driving engagement with the input shaft of the clutch arrangement driven by the engine of the agricultural utility vehicle, and on allowing the output shaft to rotate freely afterwards only braked by parasitic loads, in order to connect it again to the input shaft in this condition via the other clutch. 
     Thus on the one hand the case can arise hereby where the output shaft after coupling with the engagement shaft experiences no change in its speed, from which it can be concluded that the corresponding actuating unit has not yet been ventilated and still no clutch pressure was built up. On the other hand the case can arise where the output shaft after coupling with the input shaft undergoes a clear or abrupt change in its speed, from which it can be concluded that the actuating unit is ventilated and in addition a no longer low clutch pressure was already built up which in normal operation would lead to the undesired fierce bite. 
     Preferably a method step is proposed in which during a time interval t 0  an at least partial emptying of the second of the first or second actuating unit takes place. It is hereby ensured that the actuating unit whose filling quantity is to be determined and/or calibrated, is emptied at the start of the proceedings. 
     Preferably the time interval t 0  in dependence on the speed n E  of the input shaft amounts to between 5 and 10 seconds. This means that at low speed of the input shaft the time interval t 0  is selected longer, and vice versa. In each case the desired sufficient emptying of the actuating unit can hereby be guaranteed. 
     The time interval t 1  preferably ends when, by comparing the speeds n E , n A  of the input shaft and the output shaft, an at least approximately slip-free state of the forward clutch or reverse clutch controlled by the first of the first or second actuating unit is detected. A slip-free state means that the input speed has a ratio to the output speed which corresponds precisely to a transmission ratio between the two shafts. The transmission ratio can hereby be different from 1, by way of example if a planetary wheel gear is switched in between the two shafts. 
     The factor X=0.75 and the factor Y=2 preferably applies. A factor X=0.75 is favourable if by way of example the starting speed of the output shaft is 2000 n/min. In a practical implementation of the method this can mean that a typical time span of approximately 500 ms elapses until the speed has dropped to 1500 n/min. A factor Y=2 is favourable since this means a sudden halving of the speed of the output shaft so that an abrupt braking can almost certainly be suggested. 
     The actuating units for filling and emptying preferably each comprise an electrically controllable valve wherein the valve for filling the at least partially emptied second of the first or second actuating unit during the time interval t 3  is opened at least approximately completely. A maximum through-flow through the valve is hereby guaranteed so that the time span during which the valve is opened in order to fill the actuating unit can be kept short. 
     The clutch arrangement preferably comprises a transmission stage which can be locked by means of the reverse clutch relative to a housing of the clutch arrangement, for the reverse driving connection of the input shaft to the output shaft. It is proposed in particular that the transmission stage is designed as a planetary wheel gear. 
     The method preferably comprises the further steps: applying a hydraulic pressure p 2  to the first of the first or second actuating unit over a time interval t 4 , increasing the hydraulic pressure p 2  in the actuating unit over a time interval t 5  and measuring the hydraulic pressure p 2  over the course of the time interval t 5 , storing the mean hydraulic pressure p 2,mittel  when a vibration of the hydraulic pressure p 2  exceeds an amplitude level. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The method according to the invention will now be described below with reference to the drawings. These show: 
         FIG. 1  a longitudinal section through a clutch arrangement; 
         FIG. 2   a  the path of the current for actuating the valve of the actuating unit for the forward clutch; 
         FIG. 2   b  the path of the currents of the actuating units and the speed of the output shaft during implementation of the method according to the invention; and 
         FIG. 3  the path of the clutch pressure of a clutch when determining the kiss-up pressure. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a clutch arrangement  10  for an agricultural utility vehicle, which is designed as a reversing unit  28  and is arranged in a housing  26 . An input shaft  16  and an output shaft  18  are provided which are both designed as hollow shafts. A shaft  30 , which is not described in further detail, runs inside the input shaft  16  and output shaft  18  for driving by way of example the power take-off shaft of the utility vehicle. 
     The input shaft  16  can be connected by a forward clutch  12  in driving engagement with the output shaft  18  so that the two shafts  16 ,  18  have the same direction of rotation. The forward clutch  12  is here designed as a multi-plate clutch. The outer plate support  32  is connected to a planetary wheel support  34  of a transmission stage  24  which is designed as a double planetary wheel gearing. The hollow wheel  36  of the double planetary wheel gearing is able to be locked relative to the housing  26  by a reverse clutch  14 , also called a reverse brake. The reverse clutch is likewise designed as a multi-plate clutch. The planetary wheels  38  mounted on the planetary wheel support  34  drive the sun wheel  40  radially on the inside, the sun wheel being in driving connection in suitable manner both with the inner plate support  33  of the forward clutch  12  and also with the output shaft  18 . The input shaft  16  can be connected in reverse driving engagement with the output shaft  18  by the reverse clutch  14  and the transmission stage  24  so that the two shafts  16 ,  18  have opposite directions of rotation. The forward clutch  12  and the reverse clutch  14  are each controllable by the actuating units  20 ,  22 . The actuating units  20 ,  22  each comprise an electromagnetically controllable valve  42 ,  44  through which hydraulic oil which is set under pressure via a hydraulic pump  45 , can be supplied to the relevant pressure chambers  46 ,  48  of the actuating units  20 ,  22 . The actuating units  20 ,  22  each act on the relevant pressure plates  50 ,  52  of the clutches  12 ,  14 . The friction lamellar plates of the clutches  12 ,  14  are biased towards one another via the pressure plates  50 ,  52 .  FIG. 1  does not show, since they are offset relative to the sectional plane, the compression springs which are arranged circumferentially and bias the pressure plates  50 ,  52  away from the friction lamellar plates when there is no hydraulic pressure prevailing in the actuating units  20 ,  22 . 
     During operation of the agricultural utility vehicle the actuating unit  20 ,  22  of the clutch  12 ,  14 , which need not be connected just for moving the vehicle, is emptied at least in part of hydraulic oil in order to ensure a complete release of the relevant clutch  12 ,  14 . This means that the pressure chamber  46 ,  48  of the relevant actuating unit  20 ,  22  is filled at least in part with air. 
     The method according to the invention serves to determine accurately this amount of oil wherein this amount of oil is not a quantity of oil to be determined flat-rate for one type of gear, but rather is to be determined individually for each gear as a result of manufacturing tolerances. With the method according to the invention further method steps can be provided which determine exactly the clutch pressure which has to be set after ventilating the just controlled actuating unit  20 ,  22  so that there is no sluggish shift feeling. This clutch pressure is then correctly set when it is balanced with the resetting forces of the compression springs.  FIG. 2   a  shows diagrammatically and by way of example for the forward clutch  12  the path of the current I FWD  with which the valve  42  is loaded, applied over time. It should be noted that the current I FWD  initially rises for a defined time interval, which can also be called a wake-up pulse duration, to a high level. During this time the valve  42  is fully opened and the actuating unit  20  is ventilated. Following this time interval the current I FWD  is reduced to a level at which the forward clutch  12  already builds up a minimal force connection. The clutch pressure prevailing in this condition can also be called a kiss-up pressure and involves characteristic and measurable vibrations in the oil pressure, as can be seen in  FIG. 3 . 
     The method according to the invention will now be described below with reference to  FIG. 2   b . For a clutch arrangement  10  which comprises a forward clutch and a reverse clutch  14  the method must be run through individually for each clutch  12 ,  14  wherein only the method for use with the forward clutch  12  will be described below. 
       FIG. 2   b  shows recorded over the time axis first the speed n A  of the output shaft  18 . The current I FWD  and the current I REV  are furthermore recorded over the time axis. These are the currents with which the valves  42 ,  44  of the actuating units  20 ,  22  are each charged for opening. At the start of the process the input shaft  16  is driven at a speed n E  of by way of example 2000 n/min. The constant speed of the input shaft  16  is not shown registered in  FIG. 2   b . As a result of parasitic force engagement effects the output shaft  18  is rotated along at a low speed. The speed of the output shaft  18  is represented in  FIG. 2   b  by the solid line. The actuating unit  20  of the forward clutch  12  is emptied within a time interval t 0  so that air is located in the pressure chamber  46 . 
     During a following time interval t 1  a current I REV  is supplied to the valve  44  of the actuating unit  22  of the reverse clutch  14  so that the valve  44  is opened at least approximately completely and the reverse clutch  14  is charged with an oil pressure p 1 . The current I REV  is represented by the dotted line in  FIG. 2   b . Since here in this case the transmission stage  24  is a reversing unit the output shaft  18  rotates in the opposite direction of rotation to the input shaft  16 . The speed n A  of the output shaft  18  is represented in  FIG. 2   b  by the solid line. The time interval t 1  ends when from a comparison of the speed n E  of the input shaft  16  with the speed n A  of the output shaft  18  an at least approximately slip-free state of the controlled reverse clutch  14  is detected. At the end of the time interval t 1  the speed n A  of the output shaft  18  is measured and stored as a first reference speed n A,Ref1 . 
     Likewise at the end of the time interval t 1  the oil pressure p 1  in the actuating unit  22  of the reverse clutch  14  is released and at least partially emptied so that the output shaft  18  in the following in the widest sense rotates freely or is no longer driven and is only braked by parasitic loads. 
     The speed n A  of the output shaft  18  is measured continuously over a following time interval t 2  wherein the speed n A  of the output shaft  18  is braked as a result of the parasitic loads acting on it. The end of the time interval t 2  is reached when the speed n A  of the output shaft  18  has dropped to 75% of the first reference speed n A,ref1  Depending on the useful area of the method a speed drop limit deviating from 75% can also be expedient. At the end of the time interval t 2  the mean acceleration a mittel  of the output shaft  18  over the time interval t 2  is calculated and stored, wherein this is a time lag taking into account the amount. An empirical value for the length of the time interval t 2  can start from approximately 500 ms. 
     During a subsequent time interval t 3  the valve  42  of the at least partially emptied actuating unit  20  of the forward clutch  12  is charged with a current I FWD  so that the valve  42  opens at least approximately completely and the actuating unit  20  is ventilated and filled with hydraulic oil. The current I FWD  is represented by the chain-dotted line in  FIG. 2   b . The time interval t 3  is the wake-up pulse duration which is to be calibrated so that the length of the time interval t 3  is of decisive importance according to the invention insofar as for the satisfactory operation of the clutch arrangement  10  it is important that the time interval t 3  is to be measured neither too long nor too short. The time interval t 3  is the value which is to be calibrated or set with the method according to the invention. 
     During the initial run-through of the method according to the invention the time interval t 3  is selected so short that at the end the actuating unit  20  has not yet been fully ventilated, that means that there is still some air present in it. In this condition despite rotation of the clutch device  10  no hydraulic pressure will build up in the actuating unit  20  and the forward clutch  12  will furthermore not be active. At the end of the time interval t 3  the speed n A  of the output shaft  18  is measured and stored as a second reference speed n A,Ref2 . The second reference speed n A,Ref2  is then compared with a speed n A,calc  of the output shaft  18  calculated at the end of the time interval t 3  from the first reference speed n A,Ref1  and the mean acceleration a mittel . The state just described, namely where the actuating unit  20  has still not been completely ventilated, is to be detected in that the second reference speed n A,Ref2  is approximately equal to the calculated speed n A,calc  of the output shaft  18 . An additional braking of the output shaft  18  through the forward clutch  12  has not yet taken place since as a result of the incomplete ventilation of the actuating unit  20  this could not actively produce a power flow to the input shaft  16  driven furthermore with the speed n E . 50 ms can be assumed as the empirical value for the time interval t 3  which is the basis of the first run-through of the method. 
     The previous method steps are then implemented again wherein now the actuating unit  20  is filled over a longer time interval t 3,verl . The time difference between the time interval t 3  and the time interval t 3,verl  can amount to 5 ms or 10 ms by way of example. 
     Should the length of the time interval t 3,verl  hereby be just sufficient that the actuating unit  20  is completely ventilated, then as a result of the centrifugal forces arising through the rotation of the clutch arrangement  10  a low oil pressure is set in the actuating unit  20  so that the forward clutch  12  produces a minimal force connection between the input shaft  16  and the output shaft  18  which causes a stronger braking of the output shaft  18  than the parasitic loads. This condition can be detected when comparing the second reference speed n A,ref2  with the calculated speed n A,calc  of the output shaft  18  in that the second reference speed n A,Ref2  is indeed lower than the calculated speed n A,calc , however not considerably lower, but only slightly lower. 
     Should the length of the time interval t 3,verl  be so long however that the actuating unit  20  is completely ventilated and already additionally a low oil pressure was built up in the actuating unit  20  through the influx of hydraulic oil, then a stronger power flow arises in the forward clutch  12  which causes a significant braking of the output shaft  18 . This state can be detected when comparing the second reference speed n A,Ref2  with the calculated speed n A,calc  of the output shaft  18  in that the second reference speed n A,Ref2  is considerably lower than the calculated speed n A,calc . In this case the length of the time interval t 4,verl  is to be reduced. 
     As the last method step this length of the time interval t 3  or t 3,verl , at which the second reference speed n A,Ref2  is slightly lower than the calculated speed n A,calc , is stored. This value of the time interval t 3  is the wake-up pulse duration which is to be determined according to the invention. 
     In order to determine the required clutch pressure and/or the corresponding current I FWD , with which the valve  42  has to be charged after the ventilation of the actuating unit  20 , a hydraulic pressure p 2  is applied to the actuating unit  20  of the forward clutch  12  over a time interval t 4 , as can be seen in  FIG. 3 . The pressure p 2  is to be a low pressure so that the forward clutch  12  still transfers no power flow. The time interval t 4  preferably amounts to some few seconds. 
     During a subsequent time interval t 5  the hydraulic pressure p 2  in the actuating unit  20  is raised by increasing the current I FWD  and the hydraulic pressure p 2  is measured over the path of the time interval t 5 . If a vibration of the hydraulic pressure p 2  exceeds an amplitude height this mean hydraulic pressure p 2,mittel  is stored. This mean hydraulic pressure p 2,mittel  is the kiss-up pressure. The corresponding current I FWD  for this can likewise be stored. This operating state can be detected characteristically by the current flow or by the mean hydraulic pressure or pressure command respectively, and stored as the calibrating value according to the invention. 
     LIST OF REFERENCE NUMERALS 
     
         
           10  Clutch arrangement 
           12  Forward clutch 
           14  Reverse clutch 
           16  Input shaft 
           18  Output shaft 
           20  Actuating unit 
           22  Actuating unit 
           24  Transmission stage 
           26  Housing 
           28  Reversing unit 
           30  Shaft 
           32  Outer plate support 
           33  Inner plate support 
           34  Planetary wheel support 
           36  Hollow wheel 
           38  Planetary wheels 
           40  Sun wheel 
           42  Valve 
           44  Valve 
           45  Hydraulic pump 
           46  Pressure chamber 
           48  Pressure chamber 
           50  Pressure plate 
           52  Pressure plate