Abstract:
At least a method is provided learning a characteristic filling volume of a hydraulic clutch. The method includes, but is not limited to applying a pressure pulse to the hydraulic clutch when the clutch is in a disengaged state and determining an inflection event at an input or at an output of a torque path in which the hydraulic clutch is situated. A characteristic filling volume of the hydraulic clutch is derived from the determined inflection event. Furthermore, a method is provided for learning a characteristic return spring pressure of the hydraulic clutch, for engaging the hydraulic clutch and corresponding devices for carrying out these methods.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to British Patent Application No. 1011510.3, filed Jul. 8, 2010, which is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     The technical field relates to a hydraulic clutch and a method for determining an adaptive clutch fill volume of the hydraulic clutch. 
     BACKGROUND 
     A hydraulic clutch comprises hydraulically actuated friction elements The friction elements generally include a housing, a piston and a clutch apply cavity defined between the housing and one side of the piston. The hydraulic clutch can also include a plurality of alternating metal plates and friction material disks on another side of the piston. Hydraulic oil is pumped into and out of the cavity for causing engagements and disengagements of the hydraulic clutch. In a vehicle power train, a hydraulic clutch may be positioned between a crankshaft and a gearbox for transmitting driving torques from the crankshaft to the gearbox. In the case of an automatic transmission, the gearbox itself may also include hydraulic clutches for actuating the gearwheels and, therefore, for changing gear. 
     For example, US 2009/0105039 A1 discloses a method of operating a powertrain that includes an automatic transmission, an internal combustion engine and an electric machine. The method includes a step of monitoring fluid pressure of a hydraulic clutch by using a pressure control switch. The pressure control switch is connected to a transmission control module for calculating a flow rate and a clutch volume in response to the fluid pressure. Operations of the hydraulic clutch are controlled based upon the clutch fill volume. However, the disclosed method involves expensive components and complicated techniques for its implementation. 
     SUMMARY 
     The present application provides methods and means for deriving a characteristic fill volume of a hydraulic clutch, for deriving a characteristic return spring pressure of the hydraulic clutch and for actuating a hydraulic clutch actuator of the hydraulic clutch, based on the characteristic volume and the characteristic return spring pressure. 
     In particular, the methods and means can be applied advantageously to hydraulic clutches of a planetary gearbox for an automatic transmission. The learning of the fill volume and of the return pressure is achieved by applying pressure pulses to a hydraulic clutch which is not currently engaged and which is essentially empty and by observing inflection events at a torque input or at a torque output of a torque path that comprises the hydraulic clutch. The torque path may comprise, for example, a planetary gearbox, a turbine of a torque converter at the torque input of the planetary gearbox and an output shaft at the torque output of the planetary gearbox. 
     The application discloses a method for learning a characteristic filling volume of a hydraulic clutch, the method comprises iterative steps of applying a pressure pulse to the hydraulic clutch when the clutch is in a disengaged state and determining an inflection event, especially an acceleration of an input shaft or an output shaft at an input or at an output of a torque path which comprises the hydraulic clutch. A characteristic filling volume is derived from the determined inflection event and is stored in a computer readable memory for later use. 
     In the case of an automatic gearbox, the inflection event can be detected at a torque converter of the automatic gearbox, especially at a turbine shaft of the torque converter. It can also be detected at an output shaft of the gearbox. It is advantageous to use an existing velocity or acceleration sensor as inflection sensor. For some automatic gearboxes, such an acceleration sensor is provided at the turbine. 
     The application furthermore discloses a method for learning a characteristic return spring pressure, or, respectively, a return spring force of a hydraulic clutch. The method comprises iterative steps of applying that are performed when the clutch is in a disengaged state. The iterative steps comprise applying a first, a second and a third pressure pulse to the hydraulic clutch. The pressure level of the second pressure pulse is smaller than the pressure level of the first pressure pulse and the pressure level of the third pressure pulse is smaller than the pressure level of the first pressure pulse and greater than the pressure level of the second pressure pulse. 
     While or after the third pressure pulse is applied, an inflection event, especially an acceleration of an input or output shaft, is determined at an input or at an output of a torque path which comprises the hydraulic clutch. The characteristic spring return pressure of the hydraulic clutch is derived from the determined inflection event and the characteristic spring return pressure is stored in a computer readable memory for later use. Preferentially, the height and duration of the first pulse is such that the clutch is close to an engagement after application of the first pulse and the height of the second pressure pulse is approximately at an estimated return spring pressure or slightly below. 
     More specifically, it is disclosed that the step of determining an inflection event comprises a detection whether the hydraulic clutch changes from the disengaged state to an at least partially engaged state. 
     The torque path may comprise one or more other clutches, which are engaged. In this case, the engagement of the hydraulic clutch leads to a tie-up situation which can be detected easily. 
     More specifically, it is disclosed that the hydraulic clutch is essentially empty prior to applying the at least one pressure pulse. In this way, characteristic clutch values such as clutch volume and return spring pressure can be determined more accurately. 
     In particular, the application discloses a method according to the aforementioned which further comprises applying a fast cycle series of pressure pulses with increasing pulse length until the inflection event is detected. The fast cycle series can have a large increment to detect the clutch volume or, respectively, the clutch capacity fast. 
     In addition to the fast cycle series, the method may furthermore comprises a step of applying a slow cycle series of pressure pulses until the inflection event is detected. The slow cycle series comprises pulses with increasing pulse length and a pulse length increment of the slow cycle series is smaller than a pulse length increment of the fast cycle series. In this way, the clutch volume can be determined more accurate after a first estimate has been derived by the fast cycle series. 
     Furthermore, the method may comprise applying a test series of pressure pulses, the pressure pulses having essentially equal lengths and the pulse length being essentially equal to the length of the last pulse of the slow cycle series of pressure pulses. In this way, it can be checked whether the previously derived clutch volume is correct for more accurate determination. If no inflection event is observed during a maximum number of test cycle pulses, the slow cycle may be repeated but with a width of a first pulse which is greater than the width of the previous first pulse of the slow cycle. Especially, the first pulse of the repeated slow cycle can be made essentially equal to the length of the pulse of the test cycle. 
     The method for determining a characteristic return spring pressure of a hydraulic clutch may further more comprise the following steps. If an inflection event is detected in the determination step a return spring pressure is derived from the pressure level of the second pulse. If no inflection event is detected in the determination step the pressure level or, respectively the height of the second pressure pulse is increased and the pressure level of the third pressure pulse is increased as well. The second pulse is applied with the increased pressure level of the second pulse and the third pulse is applied with the increased pressure level of the third pulse. If an inflection event is determined, a characteristic return spring pressure is derived. The steps of applying the second and third pulse with increased pressure levels may be repeated until an inflection event is detected. 
     More specifically, the application discloses a method for determining a characteristic return spring pressure of the hydraulic clutch, wherein the width of the second pressure pulse is greater than the width of the first pressure pulse and the width of the second pressure pulse is greater than the width of the third pressure pulse. 
     The abovementioned methods to derive a characteristic clutch volume and a characteristic return spring pressure are not dependent on pressure measurements at the hydraulic clutch. The characteristic clutch volume and the characteristic return spring pressure can be used in a method for engaging the clutch which does not need pressure measurements at the hydraulic clutch. Therefore, pressure sensors at the hydraulic clutch can be dispensed of That in turn leads to significant improvements regarding cost reduction and serviceability and also to weight reduction and thus to fuel savings. 
     Methods according to the application for determining characteristic clutch values can also be used to compensate for wear and tear of a hydraulic clutch, to indicate a service interval and to indicate specific fault conditions. 
     More specifically, the application also discloses a method for engaging a hydraulic clutch which comprises the following steps. A characteristic clutch volume of the hydraulic clutch is read in from a computer readable memory. A characteristic return spring pressure of the hydraulic clutch is read in from a computer readable memory. For example, the characteristic volume and the characteristic return spring pressure are derived according to one of the abovementioned methods. A filling pressure for the hydraulic clutch is derived, based on the characteristic clutch volume, and the characteristic return spring pressure. A command pressure is derived from the filling pressure and the command pressure is applied to a servo valve of the hydraulic clutch, for example to a VBS (variable bleed solenoid) valve for engaging the hydraulic clutch. 
     The derivation of the filling pressure may furthermore comprise reading in a temperature signal from a temperature sensor in the hydraulic fluid and reading in a speed signal from a rotation speed sensor at the input or at the output of the torque path. An offset pressure is derived by reading out a lookup table based on the temperature signal and the speed signal and the offset pressure is added to the fill pressure to obtain an adapted fill pressure. From the adapted fill pressure, a command pressure is derived and the command pressure is applied to the servo valve of the hydraulic clutch. 
     Furthermore, the application also discloses a computer program product comprising a computer readable code for carrying out one of the aforementioned methods. The computer readable codes can be embedded in the non-volatile memory, an optical storage medium, or other computer readable/writable media, for example in a memory of a microcontroller. For example, the computer program product may be part of the content of an EPROM memory of a microcontroller. In a broader sense “Computer program product” also comprises the device which contains the computer readable code, such as the microcontroller. 
     Moreover, the application discloses a hydraulic clutch assembly which comprises at least one hydraulic clutch, a filling pipeline which is connected to the at least one hydraulic clutch for filling and a clutch fill regulator valve in the filling pipeline for dividing the filling pipeline into an upstream pipeline for receiving hydraulic fluid with a line pressure (Pline) and into a downstream pipeline for connecting the clutch fill regulator valve to the hydraulic clutch. The regulator valve is also referred to as a servo valve. The hydraulic clutch assembly is characterized in that it comprises a control unit. The control unit comprises an output port for an output control signal to the clutch fill regulator valve of the at least one hydraulic clutch and an input port for receiving an input signal from an inflection event sensor in a torque path which comprises the at least one hydraulic clutch. The control unit also comprises a processing unit for determining a characteristic clutch value, such as a clutch volume or a return spring pressure from the output control signal and from the input signal and for storing the characteristic clutch value. 
     More specifically, the application discloses a hydraulic clutch assembly, wherein the downstream pipeline further comprises a downstream orifice for acting with the clutch fill regulator valve to apply a clutch fill pressure (ΔPfill) to the hydraulic clutch. 
     The downstream orifice can divide the downstream pipeline into a regulator downstream pipeline and a clutch downstream pipeline such that an end of the clutch fill regulator valve is connected to the regulator downstream pipeline via a pressure P_valve which is the regulated pressure from a VBS signal. This arrangement allows for a reproducible relationship between command pressure and the clutch fill pressure. Thus, the accuracy of the volume and pressure learning methods is improved. 
     In the above, VBS refers to the type of servo valve or clutch regulator valve used, which is a variable bleed solenoid valve. The downstream orifice is useful for creating stable pressure inside the hydraulic clutch such that the clutch fill regulator valve can exert and regulate pressure pulses of the hydraulic fluid more accurately in a stable and consistent manner. 
     The upstream pipeline can comprise an upstream orifice for controlling fluid pressure received by the clutch fill regulator valve. The upstream orifice and downstream orifice act together to keep the regulator valve fully-filled throughout its operations and for providing precise pressure pulses. 
     Moreover, the application discloses also a gearbox assembly which comprises a planetary gearbox with hydraulic clutches, wherein at least one of the hydraulic clutches is a part of a hydraulic clutch assembly. It is possible to control only part of the clutches with control methods according to the present application and to control the rest of the hydraulic clutches with an existing method. 
     The application further provides a powertrain for a motor vehicle that comprises the hydraulic clutch assembly and a transmission control unit connected to both the clutch fill regulator valve and the hydraulic clutch, for adapting clutch fill volumes. The transmission control unit can be programmed to control the regulator valve and the hydraulic clutch automatically such that a driver of the vehicle is relieved from tedious operations of the hydraulic clutch. Thus, the driver is able to relax and enjoy more on the comfort of driving. 
     According to an embodiment, the fill volume is determined during steady motion state of the vehicle. Thus, the method is not influenced by inputs from clutch interactions or inputs that can change the speed of a turbine shaft or an output shaft during a shift. For example, a clutch torque capacity interaction could lead to a false detection of capacity events. Thus, a better accuracy of the method can be achieved. 
     In an embodiment, the determination of the fluid pressure command comprises the steps of applying pressure pulses of incremental duration to an unused clutch, and detecting if an inflection event is in excess of a given calibration detection threshold. The term “unused clutch” here refers to a clutch that is not used to hold the current gear ratio. This is generally also a state in which the clutch apply cavity is essentially empty from hydraulic fluid. 
     Thus, a series of pressure pulses is applied to an idle clutch that is not used to hold the current gear ratio, wherein the duration of the pressure pulses increases with a subsequent pulse, until the fill pressure reaches a desired level. When the pulse duration is long enough to completely fill the cavity during one pulse, a three-clutch tie-up is caused which can be easily detected. In the tie-up the driving torque prevented from being transferred through the automatic transmission. Such a tie-up is acceptable since, according to the application, the tie-up is being made mild enough that the driver is not affected but, on the other hand, big enough such that it can be detected. 
     At an onset of filling the idle hydraulic clutch, the clutch is not engaged. An inflection event is not detected at the onset. As the hydraulic clutch continues to be filled, the inflection event is detected. If an acceleration of the output shaft of the third clutch is in excess of a calibration detection threshold, it is an indication that the applied pressure and the associated fill time are enough to fill the clutch cavity. Hence, the adaptation method is based on a unique event directly following a pressure application to the empty clutch, specifically for the purpose of producing an adapted detection event. Thus, the clutch fill volume can be learned accurately, rather than making incremental adjustments based on events observed during a change in the gear ratio that could be related to inputs other than a fill time or a fluid pressure command. 
     The inflection event can be a brief change in output shaft or turbine shaft acceleration. Thus an acceleration inflection is used as an indicator that the applied pressure is enough to fill the unused clutch apply cavity. An output shaft or turbine shaft acceleration event and, therefore, the three clutch tie-ups are large enough to make an accurate event detection possible. On the other hand, the three clutch tie-ups are mild enough such that the driver can not detect them, for example by a disturbance in the velocity. 
     According to an embodiment, the applied pressure is recorded when an inflection event is at least once observed in excess of the calibration threshold value, to indicate a fluid pressure command. 
     A detected capacity, responsive to an applied pressure that is large enough to fill the clutch apply cavity when a series of pressure pulses is applied to the clutch during given operating conditions is recorded. Thus, the pressure that applies to the clutch at given pressure and temperature conditions is recorded and, therefore, that value can be used for the calculation of, for example, a fill time for any shift where that clutch is the oncoming hydraulic element, instead of learning a unique volume for each shift. Therefore, a fluid pressure command can be recorded, that is determined without use of signals and values indicating the position of the pressure switches. 
     According to another embodiment, the reactive spring pressure can be determined by applying a clutch pressure profile to the unused clutch, wherein the pressure profile includes slightly under filling the clutch and then holding the clutch at a pressure plateau, and applying a test pulse pressure to the clutch, which increases slightly above the plateau. If an inflection event is observed in excess of the calibration detection threshold value, the pressure plateau level indicates the reactive spring pressure, or the plateau level will be increased. 
     The term “unused clutch” again refers to a clutch that is not used to hold the current gear ratio and, therefore, to a clutch wherein the clutch apply cavity does not contain any hydraulic fluid and is considered empty. 
     Thus, the return spring pressure can be determined in a similar fashion to the pressure command and is close loop controlled. Therefore, the clutch fill volume can be learned accurately, rather than making incremental adjustments based on events observed during a change in the gear ratio that could be related to inputs other than a fill time or a pressure command. Therein, the pressure plateau level and the test pulses increase incrementally in pressure until an inflection event, such as an output or turbine shaft acceleration is observed in excess of the calibration threshold value during the time the responsive test pulse is applied to the clutch. Once the pressure has been incremented such that an inflection event is observed in excess of the calibration threshold value, the clutch is considered completely filled for torque transmission. Therefore, the responsive pressure plateau indicates the reactive return spring pressure. 
     In an embodiment, the reactive return spring pressure is determined after the fluid pressure command is determined. Thus, the actual determined fill time is used to size the pulse applied to the unused clutch. Hence, with the method of the present application, a unit-to-unit variation of the clutch fill volume can be determined, without an event detection of the pressure switches. Therefore, the accuracy of operating the hydraulic clutch according to the method is further enhanced. 
     The flow rate can be calculated from the difference between the fluid pressure command and the reactive return spring pressure. Generally, pressure switches indicate when the clutch regulating valve is in a full-feed position and when the feedback of the clutch pressure on the side of the valve spring of the regulator valve pushes the valve to a regulating position after the clutch is filled. Therefore, it is possible to replace signals and values indicating the position of the regulator valve dependent on an event detection of the pressure switches, by understanding and calculating the difference between the fluid pressure command and the reactive pressure of the return spring, without a loss of accuracy within the calculation of the clutch fill volume. 
     In an embodiment, the calculated clutch fill volume is weighted by a previous pulse cycle clutch fill volume and assigned to an adaptive non-volatile RAM variable used for the clutch volume during a fill phase of a shift. Thus, the clutch volume is weighted in accordance with the previous value of the clutch fill volume and, therefore, compared to the previous value of the clutch fill volume. Herein, the previous pulse cycle clutch fill volume refers to the clutch fill volume calculated within the previous adaptation. Hence, there is a sub-routine for isolating outlier, for example when the difference between the calculated clutch fill volume and the previous pulse adapted clutch fill volume is in excess of a predetermined percentage of the previous pulse adapted clutch fill volume. Therefore, a better accuracy of the current method can be achieved. Without the weighting factors, some of the calculated clutch fill volume values will be correctly identified, and some of the calculated values will be incorrectly identified. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and:  FIG. 1  illustrates a hydraulic clutch that is connected to its filling pipeline; 
         FIG. 2  illustrates the hydraulic clutch and the filling pipeline of a hydraulic clutch assembly; 
         FIG. 3  illustrates a first flow chart on a method for operating the hydraulic clutch based on fill volume adaptation; 
         FIG. 4  illustrates a second flow chart on a method of a first iteration of the fill volume adaptation; 
         FIG. 5  illustrates a third flow chart on a method of a second iteration for the fill volume adaptation; 
         FIG. 6  illustrates a table on relationships of clutches to learn volume versus corresponding gear speeds; 
         FIG. 7  illustrates a method for determining an adaptive clutch fill volume of the hydraulic clutch by using pulse pumping; 
         FIG. 8  illustrates a volume learn pulse strategy corresponding to the method of  FIG. 7 ; 
         FIG. 9  illustrates plotted experimental data on volume learn algorithm action; 
         FIG. 10  illustrates a volume learn pulse algorithm strategy; 
         FIG. 11  illustrates conditions to declare a pulse valid capacity 
         FIG. 12  illustrates a method of learning return spring pressure by multiple pulses of hydraulic oil injection; 
         FIG. 13  a first flow chart on a method for adapting the return spring pressure of the regulator valve; 
         FIG. 14  illustrates a clutch volume adaptation based on fill pulse timemethod; 
         FIG. 15  illustrates clutch volume versus fill pulse conversion; and 
         FIG. 16  illustrates adaptive pulse overfill protection. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is merely exemplary in nature and is not intended to limit application and uses. Furthermore, there is no intention to be bound by any theory presented in the preceding background or summary or the following detailed description 
     In the following description, details are provided to describe the embodiment. It shall be apparent to one skilled in the art that the embodiment may be practised without such details.  FIGS. 1-15  comprise parts with a same reference number. Description of these parts is hereby incorporated by reference. 
       FIG. 1  illustrates a hydraulic clutch assembly  20  that comprises a hydraulic clutch  22 , a filling pipeline  24  and a clutch fill regulator valve  26 . The hydraulic clutch  22  is connected to a first end  21  of the filling pipeline  24 , whilst the filling pipeline  24  is filled with a hydraulic oil  33 . The regulator valve  26  divides the filling pipeline  24  into an upstream pipeline  28  and a downstream pipeline  30 . In other words, the upstream pipeline  28  and the downstream pipeline  30  are joined together via the regulator valve  26 . The downstream pipeline  30  further connects the regulator valve  26  to the hydraulic clutch  22 . 
     The hydraulic clutch  22  comprises a piston  23 , a return spring  25  and two arrays of rotary friction elements  35 . A first end  29  of the piston is joined to the first end  21  of the filling pipeline  24  such that the piston  23  can be displaced to push the two arrays of rotary friction elements  35  together for torque transmission. A second end  31  of the piston  23  is joined to the return spring  25  such that the first end  29  of the piston  23  is pushed against the first end  21  of the downstream pipeline by spring force. Hence, the piston  23  applies a return spring pressure PRS to the hydraulic oil  33 . 
     In the downstream pipeline  30 , a sharp edged orifice  32  is mounted in a middle position of the downstream pipeline  30  for restricting hydraulic oil  33  flowing in the filling pipeline  24 . The hydraulic oil  33  is also known as a hydraulic fluid that is received at the upstream pipeline  24 . This orifice is known as a downstream orifice  32  which divides the downstream pipeline  30  into a regulator downstream pipeline  42  and a clutch downstream pipeline  44 . The regulator downstream pipeline  42  is provided between the regulator valve  26  and the downstream orifice  32 , whilst the clutch downstream pipeline  44  is provided between the clutch  22  and the downstream orifice  32 . 
     Arrows of  FIG. 1  depict an injection flow path of the hydraulic oil  33  for actuating the hydraulic clutch  22 . The hydraulic oil  33  flows from the upstream pipeline  28 , via the regulator valve  26 , via the downstream pipeline  30  to the hydraulic clutch  22 . The upstream pipeline  28  is under an upstream line pressure Pline. Between the regulator valve  26  and the downstream orifice  32 , the downstream pipeline  30  has a downstream line pressure Pcommand. The downstream line pressure Pcommand determines flow rates of the hydraulic oil  33  that passes through the pipeline  24 . Between the downstream orifice  32  and the hydraulic clutch  22 , the downstream pipeline  30  has a reactive spring pressure PRS, which is related to the fluid pressure in the hydraulic clutch  22 . Both the regulator valve  26  and the hydraulic clutch  22  are connected to a transmission control unit  27  for controlling. The transmission control unit  27  comprises a data processor and a non-volatile memory (NVM) that are connected together for operation. 
       FIG. 2  illustrates the hydraulic clutch  22  and the filling pipeline  24  of the hydraulic clutch assembly  20 . The regulator valve  26  comprises a housing  34 , a valve spring  36 , a core shaft  38  and a plug and retainer  40 . The housing  34  has ports connected to the upstream pipeline  28 , the downstream pipeline  30  and other related hydraulic pipelines. The core shaft  38  has a number of cylinders with various diameters that are coaxially connected and closely enclosed by the housing  34 . A first end  41  of the core shaft  38  is pushed against the valve spring  36 , whilst a second end  43  of the core shaft  38  faces the plug and retainer  40 . 
     At an upstream side, the housing  34  has a port connected to the upstream pipeline  28  and another port connected to the downstream pipeline  30 . An upstream orifice  46 , which is similar to the downstream orifice  32  is installed in the upstream pipeline  28 . At a downstream side, the first end  41  of the regulator valve  26  is connected to the regulator downstream pipeline  42  via a P valve pipeline  48 . The P valve pipeline  48  has a P valve pipeline orifice  50  in the middle. 
     Referring to both  FIG. 1  and  FIG. 2 , the filling pipeline  24  provides a conduit for conveying the hydraulic oil  33  to the clutch  22 . Correspondingly, the regulator valve  26  is used for controlling the flow rate and pressure of the hydraulic oil  33  inside the pipeline  24  by opening, closing, or partially obstructing its passageway. A coil of the plug and retainer  40  can be electrically charged or discharged in order to move the core shaft  38  in its longitudinal axis direction of the core shaft  38  for regulating the hydraulic oil  33  flowing in these hydraulic pipelines. 
       FIG. 3  illustrates a first flow chart on a method  60  for operating the hydraulic clutch  22  based on fill volume adaptation. The flow diagram outlines a series of six sequentially related steps  62 ,  64 ,  66 ,  68 ,  70 ,  72  together with an optional step  74  for operating the hydraulic clutch  22 . The transmission control unit  27 , which is connected to the hydraulic clutch  22 , the hydraulic clutch  22  and other components of an automatic transmission, is programmed to carry out the method  60  automatically. 
     The method  60  starts with a first step  62  of pumping the hydraulic oil  33  into the filling pipeline  24 . The downstream line pressure Pcommand, which represents the fluid pressure value of the hydraulic oil  33  at the regulator downstream pipeline  42 , is determined in a second step  64 . The reactive spring pressure PRS of the return spring  25  is determined at a third step  66 . In a fourth step  68 , the transmission control unit  27  calculates a flow rate Qfill of the hydraulic oil  33  for filling the hydraulic clutch  22 . The transmission control unit  27  also measures a fill time Tp in a fifth step  70 . Using the flow rate Qfill and the fill time Tp (i.e. tp), the transmission control unit  27  calculates a clutch volume Vfill in a sixth step  72 . The method has an optional seventh step  74  in which the hydraulic clutch can actuated for engagement or disengagement. 
     In the above-described method  60 , the step  64  of determining a fluid pressure command value Pcommand and the step  66  of determining a reactive pressure PRS of a return spring  25  can be performed in any order. Also, the step  68  of calculating the flow rate Qfill and the step  70  of measuring a fill time can also be performed in a reverse order. 
       FIG. 4  illustrates a second flow chart on a method  88  of a first iteration of the fill volume adaptation, which corresponds to the method  60  of  FIG. 3 . In a steady state, the hydraulic oil  33  is pulsed  76  into the hydraulic clutch  22  under pressure Ppulse according to a predetermined manner. The steady state or a steady motion state is defined as a cruising vehicle at constant speed without gear shifting of the automatic transmission. The Ppulse is alternatively known as Pcommand. Since both the return spring pressure PRS and the incoming hydraulic oil pressure Ppulse are determined, the transmission control unit  27  can calculate  78  a fluid pressure ΔPorifice at the hydraulic clutch  22  by subtracting the PRS from the Ppulse. In the mean time, the transmission control unit  27  examines  80  a duration taken from a start of pulsing the hydraulic oil  33  to a complete filling of the hydraulic clutch  22 . By doing so, the transmission control unit  27  is able to compute  82  the flow rate Qfill, which further takes the temperature T of the hydraulic oil  33  into account. Accordingly, the clutch fill volume Vfill (i.e., Vclutch or V) can be determined  84  by multiplying the flow rate Qfill with the fill time tp. The transmission control unit  27  stores  86  values of Vfill and Tp into the non-volatile memory after the first iteration of the method  88  according to  FIG. 4 . Details on how Ppulse is applied are explained later. 
     The control unit comprises a predetermined value of the return spring pressure PRS in its memory, which is set, for example, as a factory setting or during a servicing of the vehicle. The preset value can be used to initialize the return pressure learning process or when no valid learned value of PRS is available. 
       FIG. 5  illustrates a third flow chart on a method  90  of a second iteration for the fill volume adaptation. The second iteration is performed based on the stored values Vfill and tp of the first iteration of  FIG. 4 . The second iteration  90  is based on the fluid pressure ΔPorifice at the hydraulic clutch  22 . 
     According to  FIG. 5 , the transmission control unit  27  checks in a first step  92  whether a pump supplies the amount of hydraulic oil  33  at a flow rate Qfill that equals a predetermined value Qmax. If the pump has supplied a sufficient amount of hydraulic oil  33 , the transmission control unit  27  proceeds to a third step  94  in which the transmission control unit  27  further finds a fill pressure ΔPfill based on the Qfill. The clutch fill pressure ΔPfill is determined by subtracting PRS from Pcommand. 
     If the pump has not supplied a sufficient amount of hydraulic oil  33 , the transmission control unit  27  will cause the pump to inject more hydraulic oil  33  into the hydraulic clutch  22  until a flag of maximum clamp is reached (i.e., Qfill=Qmax). Subsequently, the transmission control unit  27  proceeds to the third step  94 . 
     The transmission control unit  27  in a fourth step causes the pump to supply the hydraulic oil  33  at an adapted fluid pressure command value P′command, which is an adapted clutch fill pressure at the regulator downstream pipeline  42 . The adapted fluid pressure command value P′command is obtained by compensating the fill pressure ΔPfill with an offset pressure value Poffset in a fourth step  98 . The Poffset value depends on temperature of the hydraulic oil  33  and the rotary speed ω of the pump. The dependence Poffset (T, ω) may be taken from a stored lookup table which is stored as factory setting and/or updated during a calibration. In a sixth step  102 , the transmission control unit  27  applies the adapted fluid pressure command value P′command to the hydraulic clutch  22  for clutching. In other words, the adapted fluid pressure command value P′command is obtained by weighting the previous fluid fill pressure value Pcommand. 
       FIG. 6  illustrates a table on the suitability of five clutches of a planetary gearbox to apply volume and/or pressure learning according to the application to those clutches. The column headings indicate for which gears a clutch is used and from which variable bleed solenoid valve the clutch is actuated. The row headings indicate the gear. 
     For the specific planetary gearbox to which the table of  FIG. 6  applies, a clutch pulse method to learn volume and or pressure is suitable for the first clutch C 1  when the fifth or sixth gear is engaged, for the second clutch C 2  when the third gear is engaged, for the third clutch C 3  when the fourth or the sixth gear is engaged and for the fourth clutch C 4  when the third, fourth or fifth gear is engaged. 
     In particular, clutch pulsing is not suitable when the clutch is not idle, for example when the clutch is engaged or when it is being purged. Also, the lower gears  1 ,  2  and the reverse gear R are less suitable for clutch pulsing according to the application because a steady state cruising over a longer period of time is unlikely for those gears. For the fifth clutch C 5  a clutch pulsing would be feasible but is not necessary. The fifth clutch is only used for the lock position  1 LCK and for the reverse gear R. 
       FIG. 7  illustrates a method  104  in a chart form for determining an adaptive clutch fill volume Vfill of the hydraulic clutch  22  by using pulse pumping. The method  104  provides further details of the previously described methods  60 ,  88 ,  90  for describing the volume learn pulse strategy. 
       FIG. 7  presents a two dimensional Cartesian coordinate system. Its X-axis denotes pulse counts and, therefore, indicates a number of pressure pulses applied to the hydraulic clutch  22 . Further, its Y-axis denotes pulse duration characterised by seconds. Dots in the chart represents each pulse of the pressure pumping during the entire volume learn pulse process  104 . A zigzag that joins the dots shows a path how the volume learn pulse process  104  is completed. The entire process  104  is divided into four cycles, which consists of a fast cycle  106 , a fast-to-slow cycle  108 , a slow cycle  110  and a check cycle  112  sequentially. In particular, a first fluid fill capacity detection point  113  exists in the fast-to-slow cycle  108 , whilst a second fluid fill capacity detection point  115  exists in the slow cycle  110 .  FIG. 7  is better understood by relating to  FIG. 8 . 
       FIG. 8  illustrates a chart showing how pulsating pumping affects driving torque transmission and volume fill capacity. In the chart, a lateral axis  114  denotes the period T characterised by seconds.  FIG. 8  shows three strings  116 ,  118 ,  120  that extend laterally. The three strings  116 ,  118 ,  120  denote, from top to bottom, acceleration values  116  of the output shaft of the hydraulic clutch, fluid fill volume detections  118  and pumping pulses  120  of hydraulic fluid fills. The fluid fill volume detections  118  are related to fluid pressure fill command value Pcommand or Ppulse. The three strings  116 ,  118 ,  120  present events that occur over the timeline T. 
     The third string  120  is divided into five consecutive segments of cycles of fluid pumping, which consist of an initiation cycle or initiation period  122 , a fast cycle  106 , the fast-to-slow cycle  108 , the slow cycle  110 , the check-cycle  112  and an adapted cycle or adapted period  124 . 
     A starting time “fast init” of the fast cycle  106  at the end of the initiation period  122  is based on a determination of a steady state condition. The steady state condition is determined by conditions such as absence of throttle changes, constant torque, stable road conditions. The road conditions may be inferred from measurements of acceleration sensors, for example. 
     During the fast cycle  106 , the pump propels the hydraulic oil  33  into the empty hydraulic clutch  22 . Internal fluid pressure of the hydraulic clutch  22  is not yet built up. In the fast cycle  106 , the regulator valve  26  provides three pulses of hydraulic oil  33  for filling. The three pulses correspond to three dots in the fast cycle  106  of  FIG. 7 . In the fast-to-slow cycle  108 , the regulator valve  26  offers one pulse of hydraulic oil  33  for filling, which also corresponds to one dot in the slow to fast cycle  108  of  FIG. 7 . In the slow cycle  110 , the regulator valve  26  again provides three pulses of hydraulic oil  33  for filling and they correspond to three dots in the slow cycle  110  of  FIG. 7 . In the check cycle  112 , the regulator valve  26  exerts two pulses of hydraulic oil filling, and the two pulses correspond to the two dots in the check cycle  112  of  FIG. 7 . 
     The second string  118  indicates two fluid pressure command values Pcommand at the two detection points  113 ,  115  of approximately the same magnitude. A first capacity detection flag  113  is raised at an end of the fast cycle  106 , once an input or output acceleration threshold is exceeded. A slow cycle detection flag  115  is raised at an end of the slow cycle  110 , once an input or output acceleration threshold is exceeded. According to the present application, a measurement of the pressure command values Pcommand corresponding to the capacity detection flags  113 ,  115  by inline pressure sensors is not required. Instead, the required pressure values Pcommand are detected by measuring an acceleration of the output shaft of the hydraulic clutch  22  or by measuring an acceleration of a turbine shaft of the hydraulic clutch  22 . 
     The first string  116  plots speeds of the output shaft of the hydraulic clutch  22 . A rotary speed sensor on the output shaft sends signals to the transmission control unit  27  for plotting the speeds of the output shaft. Changes of the speeds are noted as inflection events. The inflection events occur when the required fluid pressure command values Pcommand are reached. In particular, a first inflection event  126  arises at an end of the fast cycle  106  when filling of the hydraulic oil affects rotary speed of the output shaft at the steady state. 
     A second inflection event  128  takes place at an end of the slow cycle  110  when the hydraulic clutch  22  is almost filled. The transmission control unit  27  calculates a clutch fill volume Qfill for adaptation at the second inflection event  128 . A third inflection event  130  occurs at an end of the check cycle  112  where the hydraulic clutch  22  is completely filled with the hydraulic oil  33  at a fluid pressure Pfill according to the applied command pressure value Pcommand. As can be seen in  FIG. 8 , at the first inflection event  126 , a brief period of acceleration in the output shaft or the turbine shaft is observed that are in excess of a calibration detection threshold. This is an indication that an applied pressure is sufficient to fill a cavity of the hydraulic clutch  22 . 
     Referring to both  FIG. 6  and  FIG. 7 , a first series of pressure pulses  132  that occur in the fast cycle  106  have decreasing fill durations sequentially. In contrast, a third series of pressure pulses  134  of the slow cycle  110  have shorter fill durations. In fact, fill durations of the first series  132  are substantially longer than fill durations of subsequent series  133 ,  134 ,  136 . The fill durations reduce significantly in the fast-to-slow cycle  108  in a second series of pulses  133 , as compared to the first series  133 . The second series of pulses  133  is applied after detecting a speed variation of the output shaft of the hydraulic clutch  22  the first time. The third series of pulses  134  is applied in the slow cycle  134 . A fourth series of pressure pulses  136  are applied after observing that the output shaft accelerates a second time. Durations of the third series of pressure pulses are almost constant throughout the check cycle  112 . 
       FIG. 9  illustrates plotted experimental data on volume learn pulse algorithm action, which corresponds to  FIG. 8 . The plotted curves are related to pressure fill cycles  122 ,  106 ,  108 ,  110 ,  112 ,  124 . Among others, a line pressure, an essentially constant vehicle acceleration, pressure values at holding clutches and a tapped pulse pressure are shown. 
     The cycles  122 ,  124  are also referred as fill cycles, although they occur before and after the clutch filling. The curves of  FIG. 9  show a situation in which a clutch capacity is not detected in a check cycle and a slow cycle is repeated 
     After a slow cycle, the hydraulic oil fill capacity is not detected. After an adapted the hydraulic oil fill capacity is detected. At the end of the following check cycle  124 , the hydraulic oil fill capacity is again detected. 
       FIG. 10  illustrates a volume learn pulse algorithm strategy  138 , which is a flow chart showing how the transmission control unit  27  executes the pressure learn pulse algorithm strategy  138 . The transmission control unit  27  is reset at a start state  139 . In the start state  139 , the transmission control unit  27  does not detect any fill capacity because the hydraulic clutch  22  is still empty with no change of output speed on its output shaft. In a first step  140 , the transmission control unit  27  is loaded with previously stored values of clutch fill volumes. 
     In a second step  142 , which occurs in the fast cycle  106 , the regulator valve  26  applies the first series of pulses  132  until the fill capacity is detected. The fill capacity is detected when the output shaft accelerates in its rotary speed as the first inflection event  126 . 
     In a third step  144 , the transmission control unit  27  moves to the fast-to-slow cycle  108 . The fast-to-slow cycle  108  is a transition period between the fast cycle  106  and the slow cycle  110 . 
     In a fourth step  146 , the transmission control unit  27  arrives at the slow cycle  110 . In the slow cycle  110 , the transmission control unit  27  generates pressure pulses one by one at the regulator valve  26  if no clutch fill capacity is detected in a fifth step  148 . However, the transmission control unit  27  causes the regulator valve  26  to move to a next sixth step  150  when the clutch fill capacity is detected at the second inflection event  128 . 
     In the sixth step  150 , the transmission control unit  27  moves to the check cycle  112  for verifying the clutch fill capacity. The verification is completed at the third inflection event  130  and the output shaft of the hydraulic clutch  22  reaches a higher rotation speed of another steady state. 
     As a result, the transmission control unit  27  has learnt another clutch fill volume in a seventh step  152 , known as the clutch volume fill adaptation. Adapted clutch fill volume Vfill, clutch fill pressure ΔPfill and clutch fill command pressure value Pcommand are stored in the non-volatile memory. 
       FIG. 11  illustrates conditions to declare a pulse as a valid capacity, which are based on sensing the inflection events  126 ,  128 ,  130 .  FIG. 11  provides three charts  154 ,  156 ,  158  that share a horizontal axis  114 . The horizontal axis  114  represents time duration in seconds. In a first chart  154 , a first vertical axis  160  indicates Boolean on/off values of a pulse  162  and an observation window  166 . A first pulse  162  of the hydraulic oil  33  which has a pulse length  164  has an associated observation window  166  which has a pulse length  168  for monitoring the inflection events  126 ,  128 ,  130 . The beginning of the observation window  166  lies before the end of the pulse  162 . 
     In a second chart  156 , a torque curve or, respectively a throttle curve  170  shows an output torque of the hydraulic clutch  22  or, respectively an opening of a throttle. A second vertical axis  171  of the second chart  156  represents torque values of the torque curve  170 . The torque curve  170  has a first peak value  172  and a second peak value  174 . The torque trace  170  also shows a beginning torque value  176  before an acceleration event and an end torque value  178  after the acceleration event. According to a first monitoring method, the learning process is determined to be OK if the peak values  172  and  174  of the torque curve or respectively, the throttle curve  170  lie within a predetermined range, which is indicated by dotted horizontal lines. 
     In a third chart  158 , there are two points  180 ,  182  that represent an initial output speed  180  and a predicted output speed  182  at the output shaft of the hydraulic clutch  22 . According to a second monitoring method, the learning process is determined OK if an actual speed of the vehicle lies within a range  184  around the predicted output speed  182 . If the actual speed does not lie within the range  184  it is an indication that the load on the vehicle has changed too much due to, for example, the road slope or brake pedal use and that the result of the learning process has to be discarded. 
       FIG. 12  illustrates a method  186  of learning return spring pressure by multiple pulses of hydraulic oil injection. The method  186  is represented by two charts  187 ,  189 . In a first chart  187 , a horizontal axis  114  of the coordinate system denotes the time period characterized by the dimension in seconds. Further, a vertical axis  190  of the first chart  187  denotes a magnitude of clutch fill pressure. The second chart  189  has a vertical axis  191  which denotes accelerations of the output shaft of the hydraulic clutch  22 . 
     As shown in  FIG. 12 , the regulator valve  26  injects the hydraulic oil  33  into the hydraulic clutch  22  according to a pressure profile  192 . The hydraulic clutch  22  is lightly under-filled at a predetermined pressure  194  and then held at a pressure plateau  196  of a lower pressure. In the mean time, a test pulse pressure  198  is applied whose peak value is slightly higher than the pressure plateau  196 , but lower than the predetermined pressure  194 . When a fourth inflection event  200 , such as the output shaft acceleration  116 , as indicated by the solid line  198 , is observed in excess of the calibration detection threshold, the pressure plateau  196  indicates the reactive pressure PRS of the valve spring  36 . If an inflection is not observed, the method is repeated with successively higher pressure levels  202 ,  204  until an inflection event  200  is detected. 
     After the second fluid fill capacity detection point  115 , the flow rate Qfill is computed  82  (see  FIG. 4 ) based on the difference between the fluid pressure command Ppulse and a reactive pressure of the return spring  202  PRS (see also  FIG. 4 ). 
     As also illustrated again in  FIG. 8 , there is further a fill time measurement. The fill time refers to the period, during which the applied pressure and its associated fill time are sufficient to fill the hydraulic clutch apply cavity, by the determination of the fluid pressure command Pcommand. The calculated flow rate Qfill and the measured fill time tP can be used to calculate a clutch fill volume Vclutch by, for example multiplying the flow rate Qfill with the fill time tP (see  FIG. 4 ). The transmission control unit  27  uses the obtained values Vclutch and tP to operate the automatic transmission, which belongs to a powertrain of a vehicle. The automatic transmission includes the hydraulic clutch assembly  20 . 
       FIG. 13  illustrates a first flow chart showing a method  205  for adapting the return spring pressure PRS  202  of the regulator valve  26 . The method  205  is also known as a return spring pressure adapt algorithm strategy. The method  205  begins with a first step  62  in which the steady state is the start state  62 . In a second step  206 , the transmission control unit  27  undergoes search cycles with regular intervals for checking whether the hydraulic clutch  22  is filled. The hydraulic clutch  22  is considered as filled if two continuous inflection events are detected. The state that the hydraulic clutch  22  is considered filled is alternatively known as fill capacity detected. After confirming the filling of the hydraulic clutch  22 , the transmission control unit  27  proceeds to the check cycle  112 , which is a third step  208  of the method  205 . The fill capacity is verified only if the third inflection event  130  is found. In a fourth step  210 , the return spring pressure PRS is considered to be adapted by the transmission control unit  27  on detecting the third inflection event  130 . 
       FIG. 14  illustrates a method  212  of a clutch volume adaptation based on fill pulse time algorithm. The method  212  begins by obtaining clutch fill pressure values V and tP of a previously learned cycle in a first step  214 . If there is no previously learned fill pressure values V and tP, the transmission control unit  27  obtains the maximum clutch fill volume Vmax(Cx) and the minimum clutch fill volume Vmin(Cx) from the non-volatile memory in a second step  216 . The transmission control unit  27  adapts the clutch fill volume by using either the maximum clutch fill volume Vmax(Cx) or the minimum clutch fill volume Vmin(Cx) for the fast initiation cycle. The selected clutch fill volume is taken as a preceding value Vold in a fourth step  218 . By following through the subsequent fast cycle  106 , the fast-to-slow cycle  108 , the slow cycle  110 , and the check cycle  112 , the transmission control unit  27  learns the adapted clutch fill volume Vclutch (i.e., Vfill or V) in a fourth step  220 . The adapted values of V and tP are stored in the non-volatile memory in a last step  222 . 
     However, in the method of  FIG. 14 , if the transmission control unit  27  finds the clutch fill volume values V and tP from a previously learned cycle, the transmission control unit  27  proceeds to follow through the subsequent cycles  106 ,  108 ,  110 ,  112 ,  124  for finding a new clutch fill volume Vnew. The transmission control unit  27  calculates a relative change of the clutch fill volume Vclutch in a fifth step  224  by taking a ratio of a difference between the new clutch fill volume Vnew and the preceding clutch fill volume Vold over the preceding clutch fill volume Vold. If the change in terms of percentage value is greater than a predetermined value, the new clutch fill volume Vnew is disregarded as an outliner in a sixth step  226 . If the change is less than the predetermined value, an adapted clutch fill volume Vclutch will be calculated by providing an increment ΔV to the preceding clutch fill volume Vold in a seventh step  228 . The increment ΔV is computed based on the new clutch fill volume Vnew and on an adaptive error counter AECv for the fill volume. The adaptive error counter range from a minimum to a maximum value, for example from −7 to +7, wherein a positive value indicates that the adaptive value has been increasing and a negative value indicates that the adaptive value has been decreasing. 
     In the method  212  of clutch fill volume adaptation, shift specific adaptive volumes that are learned based on ratio change event detection will be replaced by one volume learned for each clutch. 
       FIG. 15  illustrates a relation between an applied pressure pulse Pcommand and a corresponding clutch pressure Ppulse. The pressure Pcommand.  FIG. 15  includes a first chart  230  and a second chart  232 . The first chart  230  shows that the clutch pressure Ppulse shows a filling delay as compared to the pulse pressure. The second chart  232  illustrates a method for computing a ramp time tramp for compensating the filling delay. 
     The first chart  230  has a horizontal axis  114  for indicating time T in seconds and a vertical axis  119  for indicating fluid pressure Ppulse in kPa. There are two lines  234 ,  236  in the first chart  230 . A first line  234  represents fluid pressure of injected pressure pulse Ppulse when the regulator valve  26  is open for injecting the hydraulic oil  33  into the hydraulic clutch  22 . The first line  234  includes a portion  238  that shows a pressure ramp up period tramp in the hydraulic clutch  22 . A second line  236  provides an intended pressure pulse profile of the transmission control unit  27 . 
     There are also two lines  236 ,  240  in the second chart  232 . The portion  238  of the first chart  230  is replaced by a straight line  240  in the second chart  232 . A breadth of the straight line  240  in a horizontal direction indicates the time taken for building up pressure inside the hydraulic clutch  22 , which is known as tramp. The duration that the hydraulic clutch  22  is kept at the targeted Pcommand is labelled as period of steady state tss. 
     The conversion from clutch fill volume Qfill to clutch fill volume Vfill is achieved through a series of calculations. Firstly, ΔPfill is obtained by the method of second iteration  90  as in  FIG. 5  based on the input value of Pcommand (i.e., Ppulse). The ΔPfill is alternatively known as ΔPorifice. The method  90  further provides the clutch fill volume Qfill. 
     Accordingly, an adapted clutch fill volume Vclutch (i.e., Vfill) is obtained by following an equation of 
               V   clutch     =         Q   fill     ⁡     (         t   ramp     2     +     t   ss       )       .           
The required fill rate Q_fill, which is multiplied by the corrected pulse time t_ramp+t_ss to obtain the clutch fill volume, is obtained from a look up table based on the filling pressure and on the temperature of the hydraulic fluid.
 
       FIG. 16  illustrates adaptive pulse overfill protection mechanism.  FIG. 16  presents two charts  234 ,  236  that are aligned vertically. The first chart  234  has a horizontal axis T  114  extending laterally, and characterised by seconds as its measurement units. The first chart  234  also has a vertical axis P  190  extending vertically, and characterised by kPa as its measurement units. The first chart  234  depicts a pressure curve  238  in a square form, which has a pulse active window  240 . The pulse active window  240  shows duration  242  and magnitude of a fluid fill pressure Pcommand for injecting fluid into the hydraulic clutch  22 . 
     The second chart  236  also has a horizontal axis T  114  extending laterally. A vertical axis  190  of the second chart  236  indicates an observed pressure P in kPa, which corresponds to the Pcommand. The second chart  236  has two square curves  244 ,  246  that are a detection curve  244  and an observation curve  246  respectively. The detection curve  244  occurs concurrently with the pressure curve  238  under the control of transmission control unit  27  such that a detection window  248  is synchronised with the pulse active window  240 . 
     The observation curve  246  follows immediately after the detection curve  244  and it has an observation window  250 . During the observation window  250 , the transmission control unit  27  observes inflection events  126 ,  128 ,  130  of the automatic transmission for protecting the hydraulic clutch  22  from overfill. A duration  252  of the observation window  250  is shorter that that of the detection window  248 . If an excessive long pulse time is applied to the regulator valve  26  for filling the hydraulic clutch  22 , the automatic transmission will be found tie-up before the observation window  250 . The tie-up event is also realized in the observation window  250 . 
     While at least one exemplary embodiment has been presented in the foregoing summary and detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration in any way. Rather, the foregoing summary and detailed description will provide those skilled in the art with a convenient road map for implementing at least one exemplary embodiment, it being understood that various changes may be made to the functions and arrangement of elements described in an exemplary embodiment without departing from the scope as set forth in the appended claims and their legal equivalents.