Abstract:
A method is provided for calibrating a control algorithin of a clutch control unit of a vehicle. The method includes requesting, clutch disengagement, or engagement, monitoring clutch actuator position, determining a time interval that starts with the clutch disengagement or engagement request and ends when the clutch actuator has reached a predetermined position, and calibrating an estimated time interval of the control algorithm starting with clutch disengagement or engagement bequest and when the clutch actuator has reached a predetermined position based on the determined time interval. A computer program for implementing the method, as well as a vehicle comprising a clutch control unit calibrated according to the method, are also provided.

Description:
BACKGROUND AND SUMMARY 
       [0001]    This disclosure relates to a method for calibrating a control algorithm of a clutch control unit of a vehicle. The disclosure also relates to a vehicle comprising a clutch and a clutch control unit, wherein the control algorithm of the clutch control unit is arranged to be calibrated. The disclosure is advantageous in the field of clutch control systems for all types of vehicles with automatic or semi-automatic transmission. The disclosure also relates to a computer program, a computer program product and a computer system. 
         [0002]    A clutch is a mechanical device used to connect two rotating shafts. When the clutch is engaged, the shafts are locked to each other and torque can be transferred from one shaft to the other. When the clutch is disengaged, the shafts are completely decoupled and no torque is transferred between them. 
         [0003]    Automatic and semi-automatic transmission generally comprises a mechanical clutch, a clutch actuating mechanism and a clutch control unit that controls the torque transmitting capacity of the clutch by means of the clutch actuating mechanism. 
         [0004]    In vehicles, a clutch may be used to control the transfer of torque from the engine to the stepped gear transmission system. Before a gear shift, the clutch has to be disengaged, and after the gear shift reengaged. In order to retain the driving comfort during gear shift, e.g., to avoid jerking and unpleasant sounds from the engine, the clutch torque transmitting capacity has to be smoothly controlled during disengagement and engagement of the clutch. However, after aging and wear of components, the clutch may suffer from disturbances and may not react as when new. 
         [0005]    A known concept for reducing such disturbances is to introduce a process for learning the clutch off point, as described in is described in WO 2010/090196. The clutch off point, also known as touch point, engagement point, slip point or kiss point, is the physical position at which the clutch engages/disengages, i.e. stops/starts to deliver torque from the engine to the transmission. 
         [0006]    However, there are still room for improvements in clutch systems of vehicles with automatic or semi-automatic transmission in order to remove the above mentioned disturbances due to changes in clutch system characteristics as well as adapting clutch systems to the unique characteristics of each individual clutch unit. 
         [0007]    It is desirable to provide a method for calibrating a control algorithm of a clutch control unit where the previously mentioned problem is at least partly avoided. 
         [0008]    This disclosure concerns a method for calibrating a control algorithm of a clutch control unit of a vehicle, in order to adapt the timing of the clutch control unit to the speed of the clutch system. The method comprises: 
         [0009]    requesting clutch disengagement or engagement, 
         [0010]    monitoring clutch actuator position 
         [0011]    determining a time interval that starts with said clutch disengagement or engagement. request and ends when said clutch actuator has reached a predetermined position; and 
         [0012]    calibrating an estimated time interval of the control algorithm, which time interval starts with clutch disengagement or engagement request and ends when the clutch actuator has reached a predetermined position, based on said determined time interval. 
         [0013]    Thus, the method first determines the actual time interval between the request and the clutch reaching the predetermined position. Secondly, the clutch control algorithm is calibrated with this time interval. Thus the control algorithm is adapted to the actual speed of the clutch system, which results in improved timing of the engagement and disengagement of the clutch. The timing of engagement and disengagement of the clutch is crucial in order to smoothly control the torque during a gear shifting process, and thus also crucial to the driving comfort in connection to gear shifting. 
         [0014]    The speed of a specific clutch system depends on the characteristics of the individual components in the system, such as springs, cylinders, control valve and compressed air pressure. Even apparently identical clutch systems from the same manufacturer tend to differ slightly from each other since no individual component is exactly identical to another, for example due to manufacturing variations. Furthermore, the actuation speed of a clutch changes during its lifetime as the components age and wear. 
         [0015]    In a vehicle with a manual transmission system, the experienced driver adapts to the present characteristics of the clutch system and adjusts the clutch movements accordingly in order to achieve proper timing. But in a vehicle with an automatic or semi-automatic transmission, the clutch movement is automatic and has traditionally been based on fixed parameters. If the characteristics of the individual dutch system are not correctly reflected in the fixed parameters, the dutch movements will suffer from had timing, resulting in poor driving comfort. Also an initially perfectly timed fixed parameter clutch system will eventually suffer from bad timing, as the characteristics of its components change due to ageing and wear. Thus, in order to achieve proper timing with an automatic clutch, there is an apparent need to adapt the clutch control algorithm to the actual characteristics of the clutch system. 
         [0016]    The disclosure further concerns a corresponding computer program, a corresponding computer program product, a corresponding computer system for implementing the method, and a corresponding vehicle comprising a clutch and a clutch control unit. 
         [0017]    The time interval determined with this method is defined to end when the clutch actuator has reached a predetermined position. This predetermined position of the clutch actuator may preferably correspond to a predetermined clutch torque transmission capacity. Even more preferably, said predetermined position may correspond to a position where the clutch torque transmission capacity becomes substantially zero during clutch disengagement, i.e. the clutch disengagement point. The clutch disengagement point is of specific interest in clutch control, and thus it is especially useful to know the time it takes to reach it when a request has been made. 
         [0018]    The time interval may advantageously be determined at vehicle standstill in order to eliminate potential disturbance sources that may occur during driving, but the time interval may also be determined during driving of the vehicle. 
         [0019]    A plurality of determined time intervals may be collected, followed by calculating an average time interval based on the plurality of determined time intervals, and calibrating the clutch control unit based on the calculated average time interval. Such an averaging procedure limits the measurement errors and uncertainties in the determination of the time interval 
         [0020]    Preferably, the monitored clutch disengagement or engagement is executed at its currently maximal possible speed, i.e. the clutch actuating mechanism should operate at its maximum speed. The speed of the clutch actuating mechanism obviously affects the determined time interval. Using maxis al possible speed fixes one of the variables that affect the length of said time interval, and the calibration of the clutch control unit is thus rendered easier, 
         [0021]    The clutch actuator may be located at a completely engaged position at time of the clutch disengagement request, and at a predetermined disengaged position at time of the clutch engagement request. The completely engaged position, which may in a normally engaged clutch correspond to a position determined without any influence by the clutch actuator, is usually the starting position of the clutch actuator when a request for clutch disengagement is made. The predetermined disengaged position corresponds in a normally engaged clutch to a position determined by the clutch actuator, and is a position where the clutch exhibits zero torque transmission capacity but not necessarily as far as the clutch plate may travel as this over time would risk permanent deformation of the clutch springs. The predetermined disengaged position is usually the starting position of the clutch actuator when a request for clutch engagement is made. For this reason, the completely engaged position and the predetermined disengaged position of the clutch actuator constitute suitable starting points in determining the time interval used for calibrating the clutch control unit. 
         [0022]    The clutch may be arranged between a propulsion unit and a gear box of the vehicle. 
         [0023]    The vehicle may comprise an automated friction clutch arrangement having a friction clutch, a clutch actuating mechanism and a clutch control unit, wherein the clutch may be disengaged and engaged by the clutch actuating mechanism. 
         [0024]    Further, the clutch actuating mechanism may be powered by pressurised air supplied from a pressurised air storage tank, and in such case the estimated time interval of the control algorithm may be calibrated based also on current gas pressure within the gas storage tank. Incorporating the gas pressure dependence of the clutch actuating mechanism in the clutch control algorithm would be advantageous since the gas pressure affects the speed of the clutch actuating mechanism. The clutch position is consequently a function of both the time elapsed since a clutch engagement/disengagement request and the current gas pressure. 
         [0025]    The clutch actuating mechanism may alternatively be electrically powered, and in such case the estimated time interval of the control algorithm may be calibrated based also on current and/or voltage supplied to the clutch actuating mechanism. Incorporating the voltage dependence of the clutch actuating mechanism in the clutch control algorithm would he advantageous since the voltage affects the speed of the clutch actuating mechanism. 
         [0026]    The estimated time interval of the control algorithm may be calibrated based also on current temperature of the clutch components and/or current ambient temperature of the vehicle and/or clutch actuating mechanism. Thermal expansion and other temperature dependent phenomena in the components may at et the speed of the clutch actuating mechanism and thus the time required to engage or disengage the clutch. Incorporating the temperature dependence of the clutch actuating mechanism in the clutch control algorithm would therefore be advantageous since the temperature affects the speed of the clutch actuating mechanism. 
         [0027]    The clutch actuating mechanism may comprise a directional control valve. The directional control valve governs the clutch actuator, which in turn controls the position of the clutch. 
         [0028]    The friction clutch may be passively engaged by spring pressure, i.e. a so called normally engaged clutch. Being passively engaged by spring pressure means that the clutch is engaged by the force from one or more springs unless the clutch actuator actively exerts a force in a direction opposite to the spring force in order to brine the clutch to a disengaged position. As soon as the clutch actuator force is released, the spring force will return the clutch to its engaged position. 
         [0029]    The opposite construction—a clutch passively disengaged by spring pressure, i.e. a so called normally disengaged clutch—would also he feasible. This means that the clutch is disengaged by spring force unless the clutch actuator actively exerts a force in a direction opposite to the spring force in order to bring the clutch to an engaged position. As soon as the clutch actuator force is released, the spring force will return the clutch to its disengaged position 
         [0030]    The clutch actuator position may be determined by means of a position sensor. A measurement of the clutch actuator position gives an indirect measurement of the clutch position, as the clutch actuator and the clutch are mechanically interconnected. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0031]    In the detailed description of the disclosure given below reference is made to the following figure, in which 
           [0032]      FIG. 1  shows a schematic overview of a drive train of a vehicle, 
           [0033]      FIG. 2  shows a process map for a gear shift process, 
           [0034]      FIG. 3  shows a schematic layout of the clutch and the clutch actuating mechanism, 
           [0035]      FIG. 4  shows a graph of the torque transmitting capability of the clutch as a function of the position of the piston rod of the clutch actuator, 
           [0036]      FIG. 5 a    shows a graph of the clutch position during atypical clutch disengagement process, 
           [0037]      FIG. 5 b    shows a graph of the clutch torque transmitting capacity during a typical clutch disengagement process, 
           [0038]      FIG. 5 c    shows a graph of the torque transmitted by a correct clutch during a typical clutch disengagement process, 
           [0039]      FIG. 5 d    shows a graph of the torque transmitted by a slow clutch duringa typical clutch disengagement process, 
           [0040]      FIG. 5 e    shows a graph of the torque transmitted by a fast clutch during a typical clutch disengagement process, and 
           [0041]      FIG. 6  shows a schematic flow chart for the disclosed method, 
       
    
    
     DETAILED DESCRIPTION 
       [0042]    Various aspects of the disclosure will hereinafter be described in conjunction with the appended drawings to illustrate and not to limit the disclosure, wherein like designations denote like elements, and variations of the aspects are not restricted to the specifically shown embodiment, but are applicable on other variations of the disclosure. 
         [0043]      FIG. 1  shows a schematic overview of an example of a drive train in a vehicle. A clutch  101  is situated between the engine  102  and the gear box  103 , connecting the crankshaft  104  of the engine  102  to the rotatable input shaft  105  of the transmission of the vehicle. An output shaft  111  connects the gearbox with the driving wheels  112  of the vehicle, When the clutch  101  is in an engaged position, it transfers torque between the shafts  104 ;  105 , and when the clutch is in a disengaged position, the shafts  104 ;  105  are decoupled from each other and no torque is transferred between them. The position of the clutch  101  is set by a clutch actuator mechanism  104 . The clutch actuator mechanism  104  is in turn controlled by a clutch control unit  107 . An engine control unit  108  controls the engine  102 . The two control units  107 ;  108  communicate with each other for example via a controller area network (CAN) bus  109 . Alternatively, both the engine and gear box may be controlled by a single control unit. Further, the clutch control unit  107  is connected to the clutch actuator mechanism  106  and the gear box  103  and the engine control unit  108  is connected to the engine  102  via signal cables  110 . 
         [0044]      FIG. 2  shows an exemplary process map for a gear shifting. The first step  201  comprises taking the decision to shift gear. This decision may be taken by the clutch control unit  107 , the engine control unit  108  or another electronic control unit. A typical reason for shifting gear may be to adapt the engine speed to an increased or decreased vehicle speed, or to increase the torque to the wheels when driving up a hill. The second step  202   a ,  202   b  comprises the two simultaneous actions of ramping down the engine torque  202   a  and disengaging the clutch  2021 . The ramp down of the engine torque  202   a  is controlled by the engine control unit  108 , and the disengagement of the clutch is controlled by the clutch control unit  107 . A precise timing of the ramp down of the engine torque and the disengagement of the clutch is required in order to retain the driving comfort. The third step  203  comprises shifting to a new gear. The fourth step  204 ,  204   b  comprises the two simultaneous actions of ramping up the engine torque  204   a  and engaging the clutch  204   b . These actions are controlled by the engine control unit  108  and the clutch control unit  107 , respectively. In the fifth step  205 , the vehicle is running on the new gear. 
         [0045]      FIG. 3  shows a schematic layout of a preferred embodiment of the disclosure. The friction clutch  301  is situated bet peen the engine  302  and the gear box  303 . The friction clutch comprises a first clutch plate  304  which is fixedly mounted to a rotatable crankshaft  305  of the engine  302 , and a second clutch plate  306  which is rotationally fixed but axially movable on a rotatable transmission input shaft  307  connected to the gear box  303 . The clutch is here passively engaged by means of the pressure from at least one spring  308 . The clutch may for example be engaged by means of a plurality of angular spaced apart coil springs, a diaphragm spring or another type of spring. The at least one spring  308  presses the second clutch plate  306  towards the first dutch plate  304 . Unless a force is applied in the opposite direction, the at least one spring  308  will cause the second clutch plate  306  to abut against the first clutch plate  304 , such that torque may be transmitted from one clutch plate to the other clutch plate through surface friction between the faces of the clutch plates. 
         [0046]    The clutch  301  is actuated by a clutch actuating mechanism  309  which is connected to a clutch control unit  310  via signal cables  311 . When the clutch is to be disengaged, the clutch control unit  310  sends a signal to a directional control valve  312 . Upon receipt of the signal, the spool of the directional control valve  312  switches position from a normal position to a working position. In the normal position, air in a working chamber  317  of the clutch actuator is allowed to escape via a control line  318 , the directional control valve  312 , and subsequently out of the system. In the working position, compressed air from a compressed air source  319  is supplied to the piston side of the clutch actuator, thereby feeding compressed air into a single-acting cylinder serving as a clutch actuator  313 . The compressed air presses against a piston rod  314 , forcing the piston rod  314  to move in, as seen in  FIG. 3 , a rightward direction R. A connection member  315  transfers the rightward motion from the piston rod  314  to the second clutch plate  306 , and the second clutch plate  306  is consequently moved away from the first clutch plate  304 . The result is that the clutch becomes partly or completely disengaged, the degree of disengagement depending on how far the second clutch plate  306  is moved away from the first clutch plate  304 . 
         [0047]    Correspondingly, when the clutch is to be engaged, the clutch control unit  310  commands the directional control valve  312  to release compressed air from the clutch actuator  313 . As the pressure from the compressed air is reduced, the piston rod  314  is pressed back into the clutch actuator by an actuator internal spring  320  and/or the spring,  308  of the clutch, the piston rod  314  thus moving in leftward direction L. 
         [0048]    Alternatively, a directional control valve being able to feed compressed air into both ends of a double-acting cylinder serving as clutch actuator  313  may be used. The internal spring  320  would in such case be superfluous, as the piston rod  314  would be forced in leftward direction L by feeding compressed air into the rightward end of the clutch actuator  313  cylinder while allowing air in the working chamber  317  escaping via the control line  318 . In order to force the piston rod  314  in rightward direction R, compressed air is supplied to the working chamber  317  while allowing air to escape from the rightward end of the clutch actuator cylinder via an additional control line (not shown in the figure). 
         [0049]    The position of the second clutch plate  306  along the axis of the transmission shaft  307  is monitored by means of a position sensor  316  that detects the position of the piston rod  314  of the clutch actuator  313  and feeds the information to the electronic control unit  310 . The position of the piston rod  314  is directly linked to the position of the second clutch plate  306  since they are interconnected by a rigid connection member  315 . The position sensor  316  is preferably, but not necessarily, a linear position sensor. Alternatively, the position sensor may monitor the position of the second clutch plate  306  directly, or the position of the connection member  315 , thereby increasing the accuracy of the position detection of the second clutch plate  306 . 
         [0050]    The method of this disclosure concerns calibration of the control algorithm of the clutch control unit  310  in order to achieve improved timing of clutch  301  disengagement and engagement. Factors such as ageing, wear, individual characteristics of the components in the clutch system, pressure of the compressed air source  319 , clutch component temperature and the rapidity of the control valve  312  may influence the timing of clutch  301  engagement and disengagement. 
         [0051]    The method is schematically illustrated by a flow chart in  FIG. 6 . The first step  601  of the method involves requesting clutch disengagement or engagement by sending a corresponding control signal from the clutch control unit  310  to the clutch actuating mechanism  309 . Simultaneously a time measurement is initiated. In a second step  602  the clutch actuator position is more or less continuously monitored by the position sensor  316  sending information to the clutch control unit  310 . In the third step  603  the control unit evaluates whether the clutch actuator  313  has reached a predetermined position or not. Typically, the clutch disengagement point is selected as said predetermined position. The clutch control unit  310  determines that the clutch  301  has become disengaged when the position sensor  316  signals that the piston rod  314  of the clutch actuator  313  has reached the position that corresponds to the second clutch plate  306  reaching the clutch disengagement point, i.e. the position at which the clutch  301  becomes disengaged. The clutch disengagement point and engagement point respectively, as well as the corresponding, clutch actuator positions, are generally regularly determined by the clutch control. unit using any method well-known by the skilled person, such as disclosed in WO12083976 or WO0117815, and are therefore considered known to the control unit. 
         [0052]    If it is determined that the clutch actuator  313  has not yet reached to the predetermined position the control unit continues polling the input signal from the position sensor  316 . Upon determining that the clutch actuator has reached the predetermined position the method proceeds to a fourth step  604  of the method that involves stopping the time measurement and determining the resulting time interval. In a fifth step  605 , the control algorithm of the clutch control unit  310  is then calibrated with the determined time interval, using it as an estimation of how far in advance a request for disengagement has to be made in order for the disengagement to take place at a desired point of time. Since the determined time interval reflects the actual time required from request to disengagement at prevailing clutch system conditions, the calibrated″ clutch control unit  310  will be capable of correct timing of disengagement of the clutch  301 , also in an aged and worn clutch system. 
         [0053]    For improved timing of clutch engagement, the calibration is preferably carried out in a corresponding way; but instead determining the time interval from when a request for engagement is made by the clutch control unit  310  until the second clutch plate  306  reaches the position at which the clutch torque transmission capacity starts to increase during clutch engagement, i.e. at a clutch engagement point. The control algorithm of the clutch control unit  310  is then calibrated with this determined time interval, using it as an estimation of how far in advance a request for clutch engagement has to be made in order for the engagement to take place at a desired point of time. 
         [0054]    In the examples above, the time interval used in the calibration has been defined to end when the clutch actuator  313  has reached the clutch engagement point and disengagement point respectively. However, the position of the clutch actuator  313  that defines the end point of the time interval could be any predetermined position of the clutch actuator  313 , for example when a 5% torque transataission capacity is provided, or the like. 
         [0055]      FIG. 4  shows a graph of the torque T transmitting capability of the clutch as a function of the position p of the piston rod  314 . At a first position  401  of the piston rod, the second clutch plate  306  completely abuts against the first clutch plate  304 , and the clutch is thus fully engaged and at the maximum of its torque transmitting capability upon feeding the piston side of the clutch actuator  313  with compressed air, the piston rod  314  starts to move in a rightward direction R, away from the first position  410 . Due to play between the piston rod  314  and the second clutch plate  306 , the second clutch plate  306  initially remains unaffected by the displacement of the piston rod  314  and the maximum torque transmitting capability of the clutch is thus initially retained. This is indicated by the torque T curve being flat between the first position  401  and a second position  402  in the graph of  FIG. 4 . As the piston rod  314  reaches the second position  402 , the effect of the play ceases and the second clutch plate  306  starts to follow the movement of the piston rod. As the second clutch plate  306  moves away from the first clutch plate  304 , the torque transmitting capability decreases. At a third position  403  of the piston rod, the second clutch plate  306  has moved, sufficiently far away from the first clutch plate  304  as to completely disengage, and consequently the torque transmitting capability of the clutch is zero. The torque transmitting capability of the clutch remains zero as the piston rod  314  moves from the third position  403  to a fourth position  404  beyond which the piston rod  314  is not able to go. Even though the second clutch plate  306  follows the movement of the piston rod  314  beyond the third position  403 , this does not affect the torque transmitting capability, since the clutch is already fully disengaged. 
         [0056]      FIG. 5 a    shows a graph of the position of the second clutch plate  306  during a typical clutch disengagement process. At position p 1 , the second clutch plate  306  completely abuts against the first clutch plate  304 , and the clutch is fully engaged. Position p 1  corresponds to the first position  401  in  FIG. 4 . Position p 2  is the clutch disengagement point, the position beyond which no torque can be transmitted between the clutch plates  304 ;  306 . Position p 2  corresponds to the third position  403  in  FIG. 4 . The solid line  501  in  FIG. 5 a    represents the position of the second clutch plate  306  in a clutch system with characteristics that are correctly reflected in the control algorithm of the clutch control unit  310 . Upon receiving instructions or self deciding to initiate a gear shifting sequence a control unit, such as the clutch control unit  310  and/or the engine control unit  108  calculates the timing for performing the gear shifting sequence, whereby the timing includes the time point for generating engine torque ramp down instruction and time point for generating clutch disengagement instruction. The timing is calculated taking into account the estimated time period tp cng , from sending the instruction until engine output torque actually reaches zero and the estimated time period tp c i from sending the instruction until the clutch disengagement point is reached, respectively. 
         [0057]    In the example of  FIG. 5A-5E , the clutch disengagement is targeted to occur at time point t 3 . As a result, based on the estimated time period tp cng  engine ramp down is initiated at time t 1 , and based on the estimated time period tp c i clutch disengagement is initiated at time t 2 , in order to accomplish jerk-free disengagement at time t 3 . In this example, the engine ramp down is initiated before the clutch disengagement request, but it could also be the other way around, depending on which of the two processes (engine ramp down or clutch disengagement) that requires the largest amount of time. A clutch system correctly estimated by the control unit reaches disengagement—that is position p 2 —at time t 3 , as illustrated with the solid line  501 . However, if the clutch control unit  31   0  is not correctly calibrated, and the clutch system is slower than estimated by the clutch control unit  31   0 , disengagement will be reached at a later time point t 3   s i 0 w than desired, as illustrated with the dotted line  502 . If the clutch system on the other hand is faster than estimated by the clutch control unit  31   0 , disengagement will be reached already at time point t 3 fast, as illustrated by the dashed line  503 . 
         [0058]      FIG. 5 b    shows a graph of the absolute torque transmitting capacity of three different clutches during a typical clutch disengagement process. The solid line  504  represents a correct clutch with respect to the clutch control algorithm, the dotted line  505  represents a slow clutch, and the dashed line  506  a fast clutch. At the time t 2  of disengagement request, all three clutches are at their maximum torque transmitting capacity. The correct clutch reaches its zero torque transmitting capacity (i,e. disengagement) at the desired time t 3 . However, the fast clutch reaches its zero torque transmitting capacity already at t 3   fas t, and the slow clutch not until at t 3   s i 0 w 
         [0059]    In  FIG. 5 c   , the dotted-dashed line  507  represents the engine torque during the engine torque ramp down which is simultaneous to the clutch disengagement in a gear shifting process (see  FIG. 2 ). The engine torque ramp down starts at time t 1 , and at time t 3  the engine torque passes zero before going negative due to the engine&#39;s internal resistance (i.e. engine braking). The solid line  508  represents the torque transmitted by a correctly timed clutch during the clutch disengagement process. The correct clutch reaches the clutch disengagement point, where it no longer can transmit. torque, at time t 3 , which is exactly the same time as the engine torque also reaches zero. This exact match results in a smoothly transmitted torque throughout the clutch disengagement process, which in turn ensures the driving comfort. 
         [0060]    In  FIG. 5 d    the dotted line  509  shows the torque transmitted when the clutch is slower than estimated by the control unit. The dotted-dashed line  507  still represents the engine torque during ramp down. Here, the slow dutch has not yet reached disengagement when the engine torque passes zero at time t 3 . Hence, the slow clutch will still be capable to transmit torque after time t 3 , which results in negative torque  510  being transferred to the transmission shaft  307  during a short period of time. Transmitting a negative torque to the transmission is equal to engine braking and hence the vehicle will perform an undesired deceleration, which by the driver and passengers often is perceived as an unpleasant jerk A little later, the magnitude of the negative engine torque begins to exceed the torque transmitting capacity of the slow clutch, and the clutch slips  51   1  before reaching zero torque transmitting capacity at t 3   s|ow    
         [0061]    In  FIG. 5 e   , the dashed line  512  shows the torque transmitted when the clutch is faster than estimated by the control unit. The dotted-dashed line  507  still represents the engine torque during ramp down. The fast reduction in torque transmitting capability of the fast clutch leads to the torque transmitting capability falling below the magnitude of the engine torque after a while. At that time, the clutch starts slipping  51   3 . The slipping goes on until time t 3 fast when the fast clutch reaches its disengagement point. Clutch slipping and the fact that the clutch disengagement point is reached before engine output torque has decreased to zero will result in a certain engine speed increase that may, be perceived as uncomfortable by a driver. The sudden decrease in vehicle acceleration may also be negatively perceived by a drier, and clutch slipping will lead to increased wear of the clutch. 
         [0062]    As described in connection to  FIGS. 5 d -5 e   , improperly timed clutch negatively effects the driving comfort, but with the calibration procedure of the disclosed method, the clutch control algorithm may be adapted to clutch system to both slower and faster clutch characteristics, compensating by either an earlier or later directional control valve  312  activation in order to achieve proper timing of clutch disengagement/engagement. 
         [0063]    The method for calibrating the clutch control algorithm has been disclosed in the context of an automatic clutch between an engine and a transmission in a vehicle, but the method is equally applicable to many other applications in vehicles and other machinery. 
         [0064]    Reference signs mentioned in the claims should not be seen as limiting the extent of the matter protected by the claims, and their sole function is to make claims easier to understand. 
         [0065]    As will be realised, the disclosure is capable of modification M various obvious respects, all without departing from the scope of the appended claims. Accordingly, the drawings and the description thereto are to be regarded as illustrative in nature, and not restrictive.