Abstract:
A method and a device for calibrating a wet clutch is provided. The clutch comprises a pump, a piston, a proportional valve, a controller, and a pressure sensor. The method comprises the steps of closing the clutch by sending a pressure profile with fill parameters from the controller to the proportional valve, recording a pressure signal of the hydraulic fluid, comparing the pressure profile with the pressure signal, determining whether at least one defined feature is in the pressure signal that is indicative of errors in the fill parameters, and if said feature is determined in the pressure signal, modification of the pressure profile by changing at least one of the fill parameters.

Description:
RELATED APPLICATIONS 
     The present application claims the benefit to U.S. Provisional Application No. 61/778,723 filed on Mar. 13, 2013, which is incorporated herein in its entirety by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the operation of hydraulic clutches and more specifically to an apparatus and method of learning filling parameters for wet plate clutches. 
     BACKGROUND OF THE INVENTION 
     In the shifting of a stepped ratio transmission, clutches are engaged and disengaged to allow for power transfer through a plurality of different power paths. Typically, when a shift is performed, one clutch is disengaged (also known as an offgoing clutch) by decreasing an oil pressure on a piston of the clutch and another clutch is engaged (also known as an oncoming clutch) by increasing a pressure on a piston of the clutch. During an overlap shift, this process happens simultaneously in a coordinated manner. In a filling phase of a shift, the piston of the ongoing clutch is positioned adjacent a plurality of friction plates by regulating a pressure of the transmission fluid. 
     A positioning of the piston is performed by sending out a pulse width modulated (also known as PWM) signal from a controller to an electroproportional valve. In response to the signal, the electroproportional valve applies a pressure to a piston chamber of the clutch. Depending on a force created by this pressure, a position of the piston can be controlled. Typically, it is desired to position the piston adjacent a set of friction plates as fast as possible while making sure an engagement of the friction plates occurs in a smooth manner. 
     A pressure profile employed by the controller may be dependent on many variables, such as, but not limited to a plurality of mechanical characteristics of the clutch, a temperature of an automatic transmission fluid, a pressure within a fluid conduit, and an amount of air within the fluid conduit. Generally, these variables can be taken into account by scheduling the two parameters with which the pressure profile is parameterized. 
     A problem that remains however, is how to obtain a correct value for each of these parameters. The value should be specific for a transmission and even for an individual clutch. Currently, it is common practice for the filling parameters to be determined through a calibration process. The calibration process is performed following vehicle production and then the calibration process is repeated at fixed intervals based on a number of operating hours of the vehicle. Typically, the process takes place through the following steps. After a predetermined number of operating hours, the controller of the transmission indicates that a recalibration is advised. When the calibration process is started, the controller sends out a number of filling profiles with changing fill parameters to a valve of the transmission. This process is continued until adequate filling is achieved for the corresponding clutch. The timing of a drop in torque converter speed ratio is used as an indicator for a quality of the filling of the clutch. The drop is indicative of torque transfer through the clutch, which is a sign of the piston contacting the set of friction plates. The calibration process is then repeated for each of the remaining clutches. 
     While the calibration process described above is capable of determining the correct filling parameters, it does so only for fixed conditions. The calibration process is performed with a transmission that has been warmed up and a time between fillings is very short in duration. As a result, the parameters that are obtained are in fact only valid in conditions similar to those that were present during the calibration. During actual use of the transmission, artificial and approximate correction factors need to be applied to compensate for such a calibration. The correction factors are not in all cases a good representation of the characteristics of the actual system, which can lead to errors in the filling and consequently, poor shift quality. 
     Further, tolerances on the production process of the components of the transmission are partially responsible for the variability during the filling process. While generally accurate parameters can be obtained by performing a calibration following production, the system also changes as the friction plates wear, an automatic transmission fluid wears out, and a stiffness of a clutch spring deteriorates. The optimal values of these parameters change over time. The current typical calibration process which is used to solve these problems takes a considerable amount of time, and during the calibration process the vehicle cannot be used. As an amount of the time between recalibrations is not based on the actual condition of the transmission, but rather as a fixed number of operating hours, reducing a number of recalibrations is achieved by imposing limitations on the mechanical system. During production, tight tolerances are imposed on both components and assembly of the system. These tolerances, which increase a cost of the system, could be relaxed if a method were available to determine the correct parameters for the filling of a specific clutch and to keep them within acceptable bounds over a lifetime of the clutch. 
     Furthermore, only the usage of a single type of transmission fluid is recommended by the manufacturer, as the temperature or viscosity compensation factors are only valid for the recommended type of transmission fluid. Lack of versatility in this respect can increase ownership and maintenance costs of the vehicle. 
     Another problem with the current typical calibration process is that the transmission controller is not aware when a bad shift is performed as a result of unsuitable fill parameters. Even though the mechanical system might have changed considerably, the controller maintains use of the same parameters until the calibration process is initiated manually or the recommended number of operating hours between calibrations is reached. 
     It would be advantageous to develop a device and method for learning filling parameters for a clutch that eliminates a need for transmission calibrations, allows components having greater production tolerances to be used in the clutch, and allows a variety of transmission fluids to be used with the clutch. 
     SUMMARY OF THE INVENTION 
     Presently provided by the invention, a device and method for learning filling parameters for a clutch that eliminates a need for transmission calibrations, allows components having greater production tolerances to be used in the clutch, and allows a variety of transmission fluids to be used with the clutch, has surprisingly been discovered. 
     In one embodiment, the present invention is directed to a method for calibrating a wet clutch. The clutch comprises a pump, a piston, a proportional valve, a controller, and a pressure sensor. The pump provides a housing with a hydraulic fluid. The piston is movably disposed in the housing. The piston is movable between an extended position by a preloaded spring and into a retracted position by applying an engagement pressure on the piston by the hydraulic fluid. In the retracted position, torque is transmittable through the clutch. The proportional valve is disposed between the pump and the housing for regulating a pressure of the hydraulic fluid in the housing. The controller controls the proportional valve. The pressure sensor measures a pressure of the hydraulic fluid in the housing. The method comprises the steps of closing the clutch by sending a pressure profile with fill parameters from the controller to the proportional valve, recording a pressure signal of the hydraulic fluid, comparing the pressure profile with the pressure signal, determining whether at least one defined feature is in the pressure signal that is indicative of errors in the fill parameters, and if said feature is determined in the pressure signal, and modification of the pressure profile by changing at least one of the fill parameters. 
     In another embodiment, the present invention is directed to an apparatus for calibrating a wet clutch. The clutch comprises a pump, a piston, a proportional valve, a controller, a pressure sensor, a recording device, a comparing device, and a determining device. The pump provides a housing with a hydraulic fluid. The piston is movably disposed in the housing. The piston is movable between an extended position by a preloaded spring and into a retracted position by applying an engagement pressure on the piston by the hydraulic fluid. In the retracted position torque is transmittable through the clutch. The proportional valve is disposed between the pump and the housing for regulating a pressure of the hydraulic fluid in the housing. The controller is configured for controlling the proportional valve by sending a pressure profile with fill parameters to the proportional valve. The pressure sensor measures a pressure of the hydraulic fluid in the housing. The recording device records a pressure signal of the hydraulic fluid. The comparing device compares pressure signals. The determining device determines at least one defined feature in a pressure signal that is indicative of errors in the fill parameters. 
     Various aspects of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The above, as well as other advantages of the present invention will become readily apparent to those skilled in the art from the following detailed description when considered in the light of the accompanying drawings in which: 
         FIG. 1  is a graph which illustrates a pressure profile of an engaging hydraulic piston associated with a clutch system; 
         FIG. 2  is a schematic illustration of a clutch system according to the present invention; 
         FIG. 3  is a graph which illustrates a portion of several pressure profiles of an engaging hydraulic piston associated with the clutch system illustrated in  FIG. 2 , the pressure profiles illustrating a feature during a fill time of the clutch system; 
         FIG. 4  is a graph which illustrates a portion of several pressure profiles of an engaging hydraulic piston associated with the clutch system illustrated in  FIG. 2 , the pressure profiles illustrating a feature during a stabilization phase of the clutch system; 
         FIG. 5  is a graph which illustrates a portion of several pressure profiles of an engaging hydraulic piston associated with the clutch system illustrated in  FIG. 2 , the pressure profiles illustrating a feature during a stabilization phase of the clutch system; 
         FIG. 6  is a graph which illustrates a portion of several pressure profiles of an engaging hydraulic piston associated with the clutch system illustrated in  FIG. 2 , the pressure profiles illustrating a feature during a synchronization ramp phase of the clutch system; 
         FIG. 7  is a graph which illustrates a pressure profiles and a plurality of features of the pressure profile of an engaging hydraulic piston associated with the clutch system illustrated in  FIG. 2 ; 
         FIG. 8  is a fill plane which indicates a location of the types of features with respect to the effects of underfill, overfill, overkiss, and underkiss of an engaging hydraulic piston associated with the clutch system illustrated in  FIG. 2 ; and 
         FIG. 9  is a flow chart illustrating a learning algorithm of the clutch system illustrated in  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     It is to be understood that the invention may assume various alternative orientations and step sequences, except where expressly specified to the contrary. It is also to be understood that the specific devices and processes illustrated in the attached drawings, and described in the following specification are simply exemplary embodiments of the inventive concepts defined herein. Hence, specific dimensions, directions or other physical characteristics relating to the embodiments disclosed are not to be considered as limiting, unless expressly stated otherwise. 
     A process of filling of a clutch is commonly characterized by two main parameters, a fill time and a kiss pressure. These parameters are shown on an exemplary pressure profile, which is illustrated in  FIG. 1 . A reference of “T_fill” in  FIG. 1  indicates the fill time of the exemplary pressure profile, and a reference of “P_kiss” indicates the kiss pressure of the exemplary pressure profile. It should be noted that an apparatus and a method described herein is also applicable when more parameters than a fill time and a kiss pressure are analyzed. 
     The fill time indicates a length in time of a pressure profile required to fill a piston chamber with hydraulic oil and to position a piston against a set of friction plates of the clutch. The kiss pressure is a pressure following the fill time for a pressure profile. The kiss pressure indicates a hydraulic force necessary to counteract a spring force once the piston is placed against the set of friction plates. While the kiss pressure changes slowly over time as a plurality of mechanical characteristics of a clutch system including the clutch change, the fill time is dependent on other factors. Primarily, the fill time is function of a temperature of a transmission oil used with the clutch, a pressure signal used with the clutch system, and an amount of time between shifts. 
     The present invention provides a learning algorithm which may be used to determine the fill parameters of fill time and kiss pressure. The learning algorithm determines the fill time and the kiss pressure by evaluating a previous filling process for the clutch and adapting the fill time and the kiss pressure for the next filling process. The learning algorithm may be used during normal operation of a vehicle including the clutch. As a result, a basic implementation of the learning algorithm requires no specialized shifting processes or adaptation of a pressure profile using a controller. By evaluating a previous filling process for the clutch and adapting the fill time and the kiss pressure for the next filling process, the learning algorithm makes use of a last best estimate of the fill parameters. 
     The learning algorithm determines the fill time and the kiss pressure by evaluating the previous filling process, through information collected using at least one sensor. However, a number of sensors available to provide information to the learning algorithm in a transmission including the clutch system is typically limited. Typically, in a production transmission, the controller is able to be provided with information such as a speed signal and the pressure signal, which can be indicative of a quality of the filling process. However, in certain instances, such as during an overlap shift, the speed signal during the filling process is determined by an offgoing clutch. As a result, during the overlap shift, the speed signal provides little information about the filling process. The pressure signal, however, can provide information about the filling process during the overlap shift, for example. 
       FIG. 2  illustrates a clutch system  10  that may be used with the transmission. The clutch system  10  is an electrohydraulically actuated wet multi-plate clutch system. The clutch system  10  is an electrohydraulically actuated wet plate clutch system. The clutch system  10  comprises a sump  12 , a high pressure pump  14 , an electroproportional valve  16 , an accumulator  18 , a piston assembly  20 , a clutch assembly  22 , a controller  24 , and a plurality of fluid conduits  26 . The high pressure pump  14  is in fluid communication with the sump  12  and the electroproportional valve  16 . The piston assembly  20  is in fluid communication with the electroproportional valve  16  and the accumulator  18 . The clutch assembly  22  is disposed adjacent to and may be placed in contact with a portion of the piston assembly  20 . The controller  24  is in communication with the electroproportional valve  16 . 
     The sump  12  is a container in which a hydraulic fluid is stored. The sump  12  is in fluid communication with the high pressure pump  14 . One of the fluid conduits  26  affords fluid communication between the sump  12  and the high pressure pump  14 . A filter  28  forms a portion of the fluid conduit  26  between the sump  12  and the high pressure pump  14 . The sump  12  includes a breather  30 , to facilitate fluid communication between an ambient environment of the clutch system  10  and an interior of the sump  12 . 
     The high pressure pump  14  is a fixed displacement hydraulic pump. The high pressure pump  14  is in fluid communication with the sump  12  and the electroproportional valve  16 . As a non-limiting example, the high pressure pump  14  may generate a pressure of about 20 bar. One of the fluid conduits  26  affords fluid communication between the high pressure pump  14  and the electroproportional valve  16 . A filter  32  forms a portion of the fluid conduit  26  between the high pressure pump  14  and the electroproportional valve  16 . A pressure relief valve  33  is present to limit a pressure difference across the filter  32  created by the high pressure pump  14 , such as if the filter  32  becomes obstructed. Further, it is understood that the high pressure pump  14  may also be in fluid communication with a pressure limiting valve (not shown). The pressure limiting valve limits a pressure within the fluid conduit  26  between the high pressure pump  14  and the electroproportional valve  16 . 
     The electroproportional valve  16  is a hydraulic valve in fluid communication with the high pressure pump  14 , the piston assembly  20 , and the accumulator  18 . The electroproportional valve  16  is in electrical communication with the controller  24 . The electroproportional valve  16  is supplied with a pulse width modulated signal to apply a current to a solenoid  34  forming a portion of the electroproportional valve  16 . Upon receipt of the pulse width modulated signal, the electroproportional valve  16  may be placed in at least a partially open position. In the open position, the electroproportional valve  16  afford fluid communication between the fluid conduit  26  between the high pressure pump  14  and the electroproportional valve  16  and a fluid conduit  26  between the electroproportional valve  16 , the piston assembly  20 , and the accumulator  18 . It is understood that the controller  24  may adjust the pulse width modulated signal to adjust a pressure within the fluid conduit  26  between the electroproportional valve  16 , the piston assembly  20 , and the accumulator  18  by placing the electroproportional valve  16  in at least the partially open position. As shown in  FIG. 2 , the electroproportional valve  16  includes a draining orifice  36 . A flow of hydraulic fluid through the draining orifice  36  is dependent on a pressure within the electroproportional valve  16 , but also a viscosity of the hydraulic fluid and a temperature of the hydraulic fluid. 
     The accumulator  18  is a hydraulic device that dampens rapid changes in pressure (such as pressure drops or pressure peaks) within the fluid conduit  26  between the electroproportional valve  16  and the piston assembly  20 . The accumulator  18  facilitates smooth operation of the clutch assembly  22 . The accumulator  18  is in fluid communication with the piston assembly  20  and the electroproportional valve  16 . As shown in  FIG. 2 , the accumulator  18  includes a draining orifice  38 . A flow of hydraulic fluid through the draining orifice  38  is dependent on a pressure within the fluid conduit  26  between the electroproportional valve  16  and the piston assembly  20 , but also a viscosity of the hydraulic fluid and a temperature of the hydraulic fluid. 
     The piston assembly  20  comprises a housing  40 , a piston  42 , a piston rod  44 , and at least one return spring  46 . The housing  40  is a hollow, cylindrical member in fluid communication with the electroproportional valve  16  through the fluid conduit  26  between the electroproportional valve  16 , the piston assembly  20 , and the accumulator  18 . The piston  42  is a cylindrical member sealingly and slidingly disposed within the housing  40 . The piston rod  44  is an elongate member in driving engagement with the piston  42 . The piston rod  44  is sealingly and slidingly disposed through the housing  40 . The at least one return spring  46  is a biasing member disposed between the piston  42  and the housing  40 . When pressure at or above an engagement threshold is applied to the housing  40  by the electroproportional valve  16 , the pressure within the housing  40  urges the piston  42  and the piston rod  44  towards the clutch assembly  22 , while also compressing the at least one return spring  46 . When pressure at or below a disengagement threshold is present within the housing  40 , the at least one return spring  46  urges the piston  42  and the piston rod  44  into a starting position. As shown in  FIG. 2 , the housing  40  includes a draining orifice  48 . A flow of hydraulic fluid through the draining orifice  48  is dependent on a pressure within the housing  40 , a portion of which may be generated by centripetal forces, but also a viscosity of the hydraulic fluid and a temperature of the hydraulic fluid. 
     The clutch assembly  22  comprises a housing  50 , a first plurality of plates  52 , a second plurality of plates  54 , and a pressure plate  56 . The housing  50  is a hollow member into which a transmission fluid is disposed. The first plurality of plates  52  and the second plurality of plates  54  are rotatingly disposed within the housing  50 . The pressure plate  56  is disposed adjacent the first plurality of plates  52  and the second plurality of plates  54  and may be urged towards the first plurality of plates  52  and the second plurality of plates  54  by the piston rod  44 . The first plurality of plates  52  is interleaved with the second plurality of plates  54 . Within the clutch assembly  22 , an input member (not shown) is drivingly engaged with one of the first plurality of plates  52  and the second plurality of plates  54  and an output member (not shown) is drivingly engaged with a remaining one of the first plurality of plates  52  and the second plurality of plates  54 . A pressure in which the piston rod  44  contacts the pressure plate  56  and where additional pressure would result in at least variable driving engagement between the first plurality of plates  52  and the second plurality of plates  54  is known as a kiss pressure. At pressures greater than the kiss pressure, torque is able to be transferred from the first plurality of plates  52  to the second plurality of plates  54  or from the second plurality of plates  54  to the first plurality of plates, depending on a configuration of the clutch assembly  22 . When pressure at or above the engagement threshold is applied to the housing  40  by the electroproportional valve  16 , the pressure within the housing  40  urges the piston  42  and the piston rod  44  towards the clutch assembly  22 , applying a pressure to the first plurality of plates  52  and the second plurality of plates  54  through the pressure plate  56 . In response to the pressure, the first plurality of plates  52  becomes at least variably drivingly engaged with the second plurality of plates  54 , causing the input member to be at least variably drivingly engaged with the output member. 
     It is understood that the schematic illustration shown in  FIG. 2  is merely exemplary in nature, and that the invention may be adapted for use with any wet plate clutch system. 
     The learning algorithm used with the clutch system  10  performs three steps to revise the fill parameters, fill time and kiss pressure. The three steps are measurement evaluation, feature detection, and implement a corrective action. 
     First, the measurement of the pressure signal from the filling process is evaluated. A goal of the evaluation is to detect a plurality of features that are indicative of errors in the fill parameters used in the previous filling process. The parameters of fill time and kiss pressure can each be either too low or too high. If the fill time is too high, a resulting effect is referred to as overfill. If the fill time is too low, a resulting effect is referred to as underfill. If the kiss pressure at the end of the pressure profile is too low, a resulting effect is referred to as underkiss. If the kiss pressure at the end of the pressure profile is too high, a resulting effect is referred to as overkiss. Because of these effects, four possibilities exist when evaluating the pressure signal. 
     By observing the pressure signal of the filling process with varying fill parameters, deviations in the pressure signal (which may be described as bumps in the signal) can be observed. The deviations in the pressure signal vary in both size and position in the pressure signal. The deviations in the pressure signal may be recognized as a feature of the control signal, which are representative of an error in at least one of the filling parameters. Accordingly, through recognition of features of the pressure signal, information on the error in at least one of the filling parameters can be obtained, and the learning algorithm can implement a corrective action to the control signal. 
     The features of the pressure signal may be categorized. It should be mentioned that different systems will allow for the detection of varying features, and that the examples described below are merely exemplary. The method described however, is generally applicable.  FIGS. 3-6  illustrate experimental data collected that show the features of the pressure signal that may be detected. With regards to detection and quantification of the features shown in  FIGS. 3-6 , the pressure signal is shown in relative time. Relative time is scaled with respect to the fill time. An origin of the horizontal axis shown in  FIGS. 3-6  is located at a beginning of the fill time. 
       FIG. 3  illustrates a feature that occurs near an end of the fill time. The feature can be described as a positive deviation in the pressure signal, and will be referred to as a first feature, and is represented in  FIG. 7-9  as a diamond shape. The first feature primarily occurs in the pressure signal where overfill is present, regardless of whether underkiss, overkiss or a nominal kiss pressure is also present, as shown in  FIG. 8 . Additionally, it has been observed and is shown in the data of  FIG. 3 , that the first feature occurs earlier in time as a severity of overfill is increased, represented by a thickness of a line representing each data set. With regards to the clutch system  10 , this occurs when the piston  42  applies a force to the plates  52 ,  54  through the piston rod  44  and the pressure plate  56  during a fill phase of the housing  40 . If the piston  42  applies a force to the plates  52 ,  54  earlier in the fill phase, and thus at a higher pressure, the first feature is of greater severity. It should be noted however, that the first feature is not only observed where overfill is present. In the pressure signal where severe underfill and underkiss is present, the first feature is also present, as shown in  FIG. 8 . In that case, the learning algorithm is capable of recognizing and disregarding such a condition. Information on a severity of the first feature can also be used to determine the type of filling error. 
       FIG. 4  illustrates a feature that occurs between an end of the fill time and a beginning of a synchronization ramp (shown in  FIG. 1 ), such as during a stabilization phase (also shown in  FIG. 1 ). The feature can be described as a positive deviation in the pressure signal, and will be referred to as a second feature, and is represented in  FIG. 7-9  as a circle shape. The second feature occurs in the pressure signal where underfill and overkiss is present, as shown in  FIG. 8 . With regards to the clutch system  10 , this occurs when the piston  42  applies a force to the plates  52 ,  54  through the piston rod  44  and the pressure plate  56  during the stabilization phase, indicating underfill of the housing  40 . Further, the positive deviation in the pressure signal (representative of the second feature) indicates that the pressure signal is greater than the kiss pressure, indicating overkiss. Additionally, it has been observed and is shown in the data of  FIG. 4 , that the second feature occurs earlier in time as a severity of underfill is increased, represented by a thickness of a line representing each data set. Further, it has also been observed and is shown in the data of  FIG. 5  that the second features occur later in time as a severity of overkiss is increased. As both an amount of overkiss and an amount of underfill have opposite effects on a position of the second feature, it is hard to determine which error is of greater importance. It does appear, however, that an effect of fill time is greater than that of kiss pressure. Accordingly, at least for fill time, the position of the second feature should be considered to determine the correction action. 
       FIG. 6  illustrates a feature that occurs during the synchronization ramp (shown in  FIG. 1 ). The feature can be described as a positive deviation in the pressure signal, and will be referred to as a third feature, and is represented in  FIG. 7-9  as a having an x shape. The third feature can be a result of either underfill or underkiss. The third feature is pronounced in underfill with all kiss scenarios when the stabilization phase (shown in  FIG. 1 ) is short. If the stabilization phase is longer, however, the third feature can be observed in situations with underkiss. In those situations, overkiss or nominal kiss will compensate for the underfill. With regards to the clutch system  10 , this occurs when the piston  42  applies a force to the plates  52 ,  54  through the piston rod  44  and the pressure plate  56  at too high of a pressure. Information on a severity of the third feature can also be used to determine the type of filling error. 
     Another feature, a fourth feature, which is not described in detail or illustrated in  FIGS. 7-9  could be defined as a delay of the response during the synchronization ramp. The fourth feature has the same causes as the third feature, however. In some instances, one of these features is more easily observed than the other. Accordingly, the learning algorithm detects both in parallel. The third feature and the fourth feature are shown in  FIG. 6 . 
     The features described hereinabove are the most important examples of errors which can be detected in the pressure signal of a typical shifting profile. Specific types of errors produce additional features, which are generally more difficult to discern in an automated manner, however, they may be used to augment the learning algorithm. Augment the learning algorithm can allow for a faster convergence rate or a reduced safety factor on the correction step due to higher confidence in the error case. 
     While the features described hereinabove are indicated on an exemplary pressure profile, the methodology is equally applicable to differently shaped pressure profiles used for the actuation of clutches used in shifting procedures. 
       FIG. 8  is a fill plane which indicates a location of the types of features with respect to the effects of underfill, overfill, overkiss, and underkiss. It is understood that the fill plane illustrated in  FIG. 8  is exemplary of a fill plane for a fixed temperature and a fixed time between shifts. It can be appreciated to one skilled in the art, based on the fill plane of  FIG. 8 , that the fill plane may be extended into additional dimensions for other effects. Based on the features described hereinabove and a detection of features in a pressure signal, a region of the fill plane in which a previous filling of the clutch occurred may be identified. Based on a region of the fill plane in which the previous filling of the clutch occurred, a corrective action may be made to improve the filling of the clutch in an iterative manner. 
     In the center of  FIG. 8  an ellipse-shaped area is shown indicating a preferred combination of fill time and kiss pressure that may ensure a smooth engagement of the clutch. In addition, within the ellipse-shaped area a circle shape indicating small overkiss and small underfill is shown. 
     As shown in  FIG. 8  by a size of the symbols used to represent the features, the symbol are representative of a severity of the features of the pressure signal. Based on a severity of the features of the pressure signal, a corrective action applied to the fill time and/or the kiss pressure can be appropriately determined. In  FIG. 8 , not every feature is indicative of a size of a step of a corrective action that may be applied. As a non-limiting example, when the effect of overfill is observed, information is available on the amount of overfill and a presence of overkiss but there is no indication on an amount of overkiss. In such a case, a step in fill time may be determined based on an amount of error, while a step in kiss pressure should be chosen in a more conservative manner, but still in a proper direction to correct the overkiss. 
       FIG. 9  is a flow chart which illustrates the learning algorithm.  FIG. 9  indicates a response which is taken to make an appropriate corrective action based on a presence of the features described hereinabove. A corrective action should only be executed when a feature is present. When multiple features are present, the corrective action having a strongest weight is performed. A correction for a certain error should be scaled with respect to how severe an effect of the observed error is. As non-limiting examples, appropriate measures for a severity of an error can be a timing of the observed feature or a relative height of a deviation in a pressure signal. The scaling depends on an applied pressure profile, a design of the clutch system  10 , and the types of sensors incorporated in the clutch system  10 , as can be appreciated by one skilled in the art. 
     In the event that no features are observed in the pressure signal, the fill time is slightly lowered and the kiss pressure is slightly increased. A combination of a small underfill and a small overkiss is desired, as it ensures a smooth actuation of the clutch assembly  22 . 
     By increasing a length of the stabilization phase, some features may be more easily detected. However, increasing a length of the stabilization phase causes a delay in a shift response, which is typically not preferred in most clutch systems. When shifts are performed manually, such an effect is important, as an expectation of an operator of the clutch is typically that a reaction of the clutch system will occur immediately when the clutch system is provided with the shift signal. When shifts are performed in an automatic manner, a delay in the shift response is of lesser concern, and a longer stabilization phase can be used with the clutch system. When the clutch system includes a shift scheduler to allow for prediction of the shifting, the longer stabilization phase can be used with such a clutch system. 
     It is also within the scope of the present invention close a clutch while another clutch is in operation, thus allowing the pressure signal to be analyzed. Such a procedure allows the clutch system to learn faster, and such a process can be repeated until a good filling is obtained. However, it is understood that such a process should be performed in a manner that does not affect an operating experience. Using such a procedure, the learning algorithm should be implemented carefully, starting with a low fill time and a low kiss pressure. The fill time can be increased until following a long stabilization phase, a slight drop in speed ratio over the clutch can be observed. The fill time can then be slowly increased until the drop in speed ratio occurs at a proper time. 
     The invention according to present disclosure allows for eliminating a need for recalibrations of the clutch system of the transmission. Using the learning algorithm, the fill parameters are always correctly tuned, and a quality of the filling (and thus a quality of the shift) does not deteriorate over time. Furthermore, due to the learning algorithm, the tolerances associated with the components of the clutch system, the production processes used with the components of the clutch system, and an assembly of the clutch system can be increased without detriment. Lastly, as the correct fill parameters for the clutch system are learned for all conditions, a wider range of transmissions fluids can be used with the clutch system. 
     In accordance with the provisions of the patent statutes, the present invention has been described in what is considered to represent its preferred embodiments. However, it should be noted that the invention can be practiced otherwise than as specifically illustrated and described without departing from its spirit or scope.