Abstract:
A system and method for maintaining the operation of an automated transmission ( 16 ), particularly a dual dutch transmission, for a motor vehicle, in case of a failure of an engine speed sensor ( 50 ) or a bus connection ( 44 ) between a first control module ( 42 ) controlling an engine ( 28 ) being connectable to the transmission ( 16 ) through at least one separation clutch ( 12, 149 ), and a second control module ( 34 ) controlling regulating units ( 12, 14 ) for connecting the engine ( 28 ) to the transmission ( 16 ) or to one or more driving wheels ( 34 ) in a force-transmitting manner, wherein gear-shift relevant data, particularly a number of revolutions (n o ) of the engine, is exchanged between the control modules ( 34, 42 ) over the bus connection ( 44 ).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority of the German patent application DE 10 2008 027 675 filed on Jun. 3, 2008 which is fully incorporated by reference herewith. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a system and method for continuing the operation of an automated transmission of a motor vehicle, when a engine speed sensor or a data connection fails in terms of a data bus line between at least two control modules controlling cooperating components of a drive line and exchanging gear-shift relevant data between each other through the bus. 
     RELATED PRIOR ART 
     Different components of conventional automated transmissions of motor vehicles, such as an engine and a transmission, are typically controlled by different control modules, which can also be implemented in terms of a central control device. Particularly, dual clutch transmissions require a corresponding electronic control for ensuring the complex and difficult shifting processes in the two parallel partial transmissions (branches) of the dual clutch transmission, each of which has assigned its own frictional clutch (running dry or wet). There, the input members of the friction clutch are connected to a drive unit, such as to a combustion engine. However, the drive unit can also be an electric motor or a hybrid drive unit. The uneven gears (1, 3, 5, . . . ) are associated with one of the partial transmissions. The even gears (2, 4, 6, . . . ) are associated with the other partial transmission. The transmission of drive power from the drive unit to the driving wheels of a motor vehicle generally happens through one of the two partial transmissions. As a rule, a gear is pre-selected in the respective (inactive) partial transmission, which is not active. Then, a gear shift from the start gear, or source gear, of the active partial transmission to the target gear of the non-active partial transmission can happen by overlapping the actuation of the friction clutches on the input side. This overlapping actuation can then be performed such that any interruption of traction happens during the shifting of gears. 
     The controlling of the friction clutches, required for this purpose, and the engaging and disengaging of gears is caused in an automated manner due to the complexity of the process, namely by means of a superordinated control unit. This control unit selects the torque, which is to be transmitted through the respective friction clutches, by means of an engine control, and selects the transmission ratios, set in the respective partial transmissions, by means of a clutch and transmission control so that these elements are adapted to the current situation of travel (speed of the vehicle, traction operation or overrun, etc.). 
     Typically, during a shifting process the engine control module controls the number of revolutions, or the torque of the motor, such that a gear shift, which does not interrupt the traction, is possible while the traction control module opens and/or closes synchronizer devices of the gears, i.e. if, for example, a source gear is disengaged and a target gear is engaged subsequently. At the same time, conventionally the control modules communicate over a data bus connection such as a Controller Area Network (CAN) bus. The transmission control module typically controls the engine control module during the gear shift. 
     A CAN bus is an asynchronous bus system which was developed for networking control apparatuses in the automobile industry. The CAN bus operates in accordance with the CSMA/CR (Carrier Sense Multiple Access/Collision Resolution) method. A CAN network typically is formed as a line structure. Branch lines are allowed in a limited scope. Further, annular buses as well as star-shaped buses (e.g. central locking) are possible. 
     However, if this bus connection between the control modules fails, e.g. due to a cable break, conventional motor vehicles having automated transmissions may stand still. In order to shift gears, the transmission control module requires at least the information on the number of revolutions of the engine in order to effect, if necessary, the motor control correspondingly (in a reversed direction with respect to the data transmission). This is even more true for dual clutch transmissions. While the separation clutch(s) is/are opened and closed, the engine control module typically controls the number of revolutions of the engine, for example also through a position of a throttle plate. The number of revolutions of the engine, in turn, affects the torque which is transmitted through the separation clutch. Particularly during a start, i.e. if the separation clutch of a motor vehicle is closed, wherein a gear was engaged beforehand during standstill, it is typically necessary to re-adjust the number of revolutions of the engine correspondingly, in order to prevent stalling of the engine. 
     BRIEF SUMMARY OF THE INVENTION 
     Therefore, it is an object of the present invention to provide a system and method for ensuring the continuing operation of an automated transmission during a failure of a data bus connection between control modules being responsible for different components of the drive line of a motor vehicle, which in turn are to be adapted to each other during a gear shift. 
     This object is solved by a method for maintaining the operation of an automated transmission, particularly a dual clutch transmission, of a motor vehicle if an engine speed sensor fails or a bus connection fails between a first control module, which controls an engine connectable to the transmission by means of at least one separation clutch, and a second control module, which controls regulating units (e.g. synchronizer devices and/or separation clutches) for connecting the engine in a force-transmitting manner to the transmission or to one or more driving wheels, wherein the bus connection serves for exchanging gear-shift relevant data, particularly a number of revolutions of the engine, between the control modules, the method comprising the following steps: checking whether a number of revolutions of a shaft, particularly an engine shaft, which is detected by a sensor of the control module dedicated to this purpose, is transmittable from the control module via the bus connection to the other control module; and, if the number of revolutions is not transmittable, performing the following steps: opening all synchronizer devices assigned to the transmission, or to a partial transmission, so that the engine shaft cannot transmit force through a transmission shaft to the driving wheel(s) of the motor vehicle anymore; closing the separation clutch assigned to the transmission, or the partial transmission, comprising the opened synchronizer devices, if the separation clutch is not closed; detecting a number of revolutions of another shaft, particularly the transmission shaft, which can be connected to the shaft via the separation clutch, by means of a sensor dedicated to this purpose; and determining a shift-gear ramp function for one of the control modules, particularly the second control module, based on the detected number of revolutions, particularly from a field of characteristic curves, allowing the performance of a shift gear from a source gear to a target gear. 
     In this manner, it is possible to gain information on the number of revolutions of the engine, although the sensor dedicated to this purpose fails, or the data bus connection between the engine control and the clutch and transmission control fails. According to the prior art, redundant sensors or redundant bus systems are provided for this purpose. This becomes superfluous due to the present invention. Thus, the provision of a second set of components can be omitted, thereby obtaining significant reductions of costs and weight. The motor vehicle is nevertheless operable after the break down of the sensor or the data bus connection, even if in a limited manner only. 
     In the particular case of a dual clutch transmission, the number of revolutions of the engine even can be detected permanently. The clutch and transmission control can retrieve the number of revolutions of the engine at any time, although the bus connection is disturbed, and thus affect the regulating units being responsible for a shifting of gears, such as a separation clutch, in a smart manner in order to shift gears without damaging the transmission or allowing the motor to stall. 
     In accordance with a preferred embodiment the shift-gear ramp function represents one or more parameters selected from a group comprising parameters like: number of revolutions of the engine in a source gear; number of revolution of the engine in a target gear; position of a gas pedal and/or position of a throttle plate, and/or time-dependent displacement path for clutch elements of the separation clutch being assigned to the target gear. 
     The shift-gear ramp function is substantially dependent on the number of revolutions of the engine. The other quantities also affect the shift-gear ramp function. For example, it makes a difference if one shifts up, shifts down or starts. By means of the shift-gear ramp function it is ensured that a gear shift can nevertheless be performed in a safe manner, even if an engine speed sensor failed, and/or even if the bus connection fails. Preferably, the number of revolutions is detected for a predefined period of time, in order to generate a history of the number of revolutions allowing to derive whether or not a gear shift is going to happen soon. 
     Further, it is advantageous if a position of a gas-actuating element can be derived from the detected number of revolutions. 
     A driver&#39;s desire to shift a gear can be derived from the position of the gas-actuating element. If the number of revolutions of the engine constantly increases, it can be assumed that the driver accelerates the vehicle. If the number of revolutions decreases, it can be assumed that the driver has not sufficiently stepped on the accelerator (climbing a hill) or does not even step on the accelerator at all. 
     In accordance with another preferred embodiment, in reaction to a shift-gear command, which is communicated either by the control module or a user of the vehicle, a process for shifting a gear of the transmission is initiated based on the detected number of revolutions of the engine and, if necessary, based on the position of the gas-actuating element, by determining the appropriate shift-gear ramp function 
     Here again, the idea expresses that the motor vehicle is supposed to not break down even if an error occurs. If the automated gear shift can also be actuated on the driver&#39;s demand (e.g. Tiptronic), the driver can nevertheless shift. 
     Further, it is advantageous if: the separation clutch is open; if necessary, a synchronizer device of an engaged source gear is open; an appropriate target gear is engaged by closing an associated synchronizer device and; subsequently, the separation clutch is closed again. 
     In accordance with another preferred embodiment the shift-gear ramp function is used during a time duration between the opening and the repeated closing of the separation clutch for affecting regulating units participating at the process of shifting the gear. 
     The process of shifting the gear represents the critical time duration within which the number of revolutions of the engine can change drastically. In case of a dual clutch transmission, the number of revolutions of the engine is nevertheless provided permanently due to the measurement of the number of revolutions of the inactive partial transmission. With a simple automated transmission the process of shifting a gear is to be performed as fast as possible, since during the gear shift as such no information on the number of revolutions of the engine is present. There is a very short time duration within which any of the separation clutches is closed. 
     Therefore, the method of the present invention can be particularly well utilized with dual clutch transmissions, since they allow to tap the number of revolutions of the engines permanently. 
     Additionally, it is advantageous if one retrieves whether an information on the current position of a gas-actuating element in terms of a gas-actuating parameter can be transmitted via the bus connection; wherein, if any information is transmissible, the detected number of revolutions is retrieved, the detected number of revolutions is compared to a threshold value, wherein the threshold value is a predefined number of revolutions of the engine, wherein the predefined number of revolutions corresponds to a predefined position of the gas-actuating element in an actuated state; and, if the detected number of revolutions becomes less than the threshold value, a gas-actuating parameter is set to zero so that a position of the gas-actuating element is represented in an non-actuated position, or, if the detected number of revolutions is greater then the threshold value, the gas-actuating parameter is increased about a value corresponding to the difference. In this manner, it is possible to derive the driver&#39;s desire (acceleration, deceleration, cruising, etc.) also from the detected number of revolutions of the engine. 
     It is clear that the above mentioned features and the ones to follow hereinafter are not only used in the respectively described combinations, but also in other combinations or alone, without departing from the scope of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Embodiments of the invention are depicted in the drawings and will be explained in more detail in the following description. 
         FIG. 1  shows a schematic block diagram of a drive line in accordance with the present invention; 
         FIG. 2  shows a temporal course of the number of revolutions ( FIG. 2A ) and the torques ( FIG. 2B ) during a gear shift without any disturbance, and the temporal course of the number of revolutions ( FIG. 2C ) and the torques ( FIG. 2D ) if a disturbance is present; 
         FIG. 3  shows a flow chart of a method in accordance with the present invention; and 
         FIG. 4  shows a simplified flow chart of another embodiment of the method of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a simplified drive line  10  of a motor vehicle having a dual clutch transmission  16 . 
     The drive line  10  comprises two friction clutches  12 ,  14  which function as separation and starting clutches dedicated to a first partial transmission  18  and a second partial transmission  20 . Here, the first partial transmission  18  exemplarily represents the first, third and fifth gear. Here, the second partial transmission  20  exemplarily represents the second, fourth and sixth gear, respectively. Although here a dual shift transmission is shown, the following explanations and concepts can also be used with single shift gears, if correspondingly adapted. 
     The separation clutches  12 ,  14  are simple friction clutches, each of which can be actuated by means of a clutch actuator which is not shown in greater detail. In a closed position, the first separation clutch  12  connects an engine output shaft  22  in a friction-locked manner to a first transmission input shaft  24 . This also applies to the second transmission line. The second separation clutch  14  connects, in the closed position thereof, the engine output shaft  22  in a friction-locked manner to a second transmission input shaft  26 . The two shafts  22 ,  24  and  22 ,  26 , respectively, are separated from each other in the opened position. During the opening and closing of the separation clutch  12  or  14 , the respective separation clutch runs through a slip phase. On this occasion, two elements of the separation clutch having different numbers of revolutions are engaging with each other in a frictional manner. At the so-called “drag point” (“Schlepppunkt”), the clutch actuator has pushed both of the clutch elements rotating at different numbers of revolutions just so close to each other that a predefined drag torque is transmitted from the engine output shaft  22  to the transmission input shaft(s)  24  and/or  26 . Then, the clutch elements, however, still have different numbers of revolutions. 
     The transmission  16  transmits the power of the engine  28  of the motor vehicle from the respectively selected transmission input shaft  24  or  26 , which is driven by the engine output shaft  22 , to a respective transmission output shaft  30 ,  32 , which are also designated as output shafts  30 ,  32 . The partial transmissions  18 ,  20  respectively drive a plurality of gear sets being not depicted here. Each gear set comprises at least two gear wheels. A shift clutch is arranged between the gear set at the output shafts  30  and  32 , respectively, which is formed as a synchronizer device. The synchronizer device allows, during an engaging phase of a gear, to reduce the different numbers of revolutions between the engine output shaft  22  and the gear wheel to be connected thereto to zero, thereby synchronizing both of the transmission components. The transmitted power is then transmitted to one or more driving wheels  33 . Further details with respect to the arrangement of a single shift transmission can be exemplarily found in DE 10 2008 008 065.9 entitled “Vorrichtung and Verfahren zum Verhindern von Fehlschaltungen in automatischen Getrieben von Kraftfahrzeugen” which was filed by the present applicant on Feb. 1, 2008, and which is fully incorporated by reference herewith. 
     The motor vehicle, which has incorporated the drive line  10 , further comprises at least a clutch and transmission control module  34  as well as an engine control module  42 . 
     The clutch and transmission control  34  is connected to engine speed sensors  36 ,  38  for detecting the number of revolutions n 1  and n 2 , respectively, of the first transmission input shaft  24  and the second transmission input shaft  26 , respectively. Further, the clutch and transmission control  34  transmits clutch signals over the lines  40 ,  40 ′ to the clutch actuators, which are not depicted here, in order to actuate the separation clutches  12 ,  14  or adjustment elements thereof. 
     The engine control module  42  communicates with the clutch and transmission control module  34  via a data bus connection  44 . It is clear that both the clutch control and the transmission control could be provided respectively with an autonomous module. 
     The engine control module  42  typically detects additionally the position of a gas-actuating element such as the position of a gas pedal  46 , as it is schematically illustrated in  FIG. 1 . The information on the position of the gas pedal  46  is supplied to the engine control  42  in terms of a signal  48 . The engine control  42  receives as another input signal a number of revolutions n e  of the engine output shaft  22  from a speed sensor  50 , detecting the revolutions of the engine output shaft  22 . The number of revolutions n e  of the engine as well as the gas-pedal position signal  48  are typically provided to the control module  34  over the data bus connection  44  such as a CAN bus. Based on this information, shift processes can be initiated and performed. For this purpose, the control module  34  particularly generates engine control commands  52  which typically affects the number of revolutions n e  of the engine  28  by means of the engine control  42  during the shift process. The signal  52  could be used, for example, in order to change the position of a throttle plate. 
     Now, if, for which reason ever, the data bus connection  44  fails, the control module  34  lacks important parameters, particularly the number of revolutions n e  of the engine as well as the position of the gas pedal, in order to perform the gear shift. Similar is true, if an engine speed sensor fails. It is important to know the number of revolutions of the engine n e  as well as, preferably, the position, for example, of the gas pedal; alternatively, it would be sufficient to know the position of the throttle plate. This will be explained in the following with reference to the  FIG. 2 . 
       FIG. 2  shows the temporal course of the numbers of revolutions of the transmission ( FIG. 2A ) and the applied torques ( FIG. 2B ), if the control is disturbed. A disturbance can be caused, for example, by a failure of the bus connection  44  or the failure of the engine speed sensor  50 . The course of the number of revolutions and of the torques, in case of a disturbance, is shown in the  FIGS. 2C and 2D , and designated by “ENGINE CONTROL DISTURBED”. The normal operation is exemplarily designated by “ENGINE CONTROL INTACT”. It is clear that these curves do not only apply if a (CAN) bus fails, but also if the sensor  50  fails. 
       FIG. 2  depicts a gear shift, if it is shifted up with traction. It is clear that the following explanations are applicable analogously for shifting up in an overrun mode, shifting down with traction as well as a shifting down in an overrun mode. Four temporal phases are respectively illustrated, which will be explained in the following in more detail. 
       FIG. 2A  shows the temporal courses of the different numbers of revolutions of the partial transmission shafts as well as the engine shaft  22 . The shaft of the first partial transmission rotates at the number of revolutions n 1 . The shaft of the second partial transmission rotates at the number of revolutions n 2 . The engine rotates at the number of revolution n e . n 1  is depicted by a line consisting of strokes and points. n 2  is depicted by a solid line. n m , is shown as a line of points. The association with the individual components of the drive line happens analogously in the other  FIGS. 2B to 2D . 
     At the beginning of phase  1 , under normal conditions, either the driver or the (superordinated) clutch and transmission control  34  takes the decision, particularly if the sensor  50  functions as well as the data connection  44  is stable, to shift a gear. Subsequently, it is assumed that a gear shift from the third gear to the fourth gear with traction is to be performed. The third gear is the source gear (SG). The fourth gear is the target gear (TG). 
     As can be taken from  FIG. 2B , which is related to  FIG. 3A , the separation clutch  12 , which is associated with the partial transmission  18 , comprising the third gear, is opened at the beginning of the phase  1 . The separation clutch  12  reaches its drag point at the time T 1 . The torque of the first partial transmission  18  decreases from this moment, since the friction-locked connection between the engine output shaft  22  and the first transmission input shaft  24  is disengaged step by step. Also, the torque M ENGINE , which is generated by the engine  28 , decreases from this moment. The torque M DRIVER , which is requested by the driver, remains the same since it is assumed that the driver steps with his foot constantly onto the gas pedal  46 . The torque M 2  transmitted by the second partial transmission  20  is zero, although the fourth gear is already engaged. If the fourth gear is engaged, the second transmission input shaft  26  is connected to the second transmission output shaft  32  in a friction-locked manner. Since the wheels  33  roll, the number of revolutions n 2  of the second partial transmission  20  increases proportionally relative to the number of revolutions of the first partial transmission  18  driving the wheels  33 . It is clear that the fourth gear could also be engaged at a later time only. Then, the curve (solid line) of the number of revolutions n 2  of the second partial transmission  20  would, however, look differently. Further, it is clear that presently the second separation clutch  14  is opened during the first phase. 
     At time T 2 , the first separation clutch  12  is completely opened. The torque M ENGINE  of the engine drops below zero, since the engine is decelerated due to internal friction. At the time T 2 , the synchronizer device of the third gear is initiated to be opened. As can be seen in  FIG. 2A , the number of revolutions n 1  of the first partial transmission  18  drops to a value at the time T 2 ′, which is greater than zero and corresponds to the number of revolutions of the first transmission output shaft  30 , which is still rotating due to the rolling wheels  33 . It is clear, that this number of revolutions is only designated by n 1  , even in the following, just for the reason of simplicity, although the bus n 1  (only) designates the number of revolutions of the first transmission input shaft  24 . The first transmission output shaft  30  can also optionally cooperate with a speed sensor (not shown in  FIG. 1 ) for allowing at any time determination of the number of revolutions of the third partial transmission  18 . 
     The number of revolutions n e  drops from the time T 2  due to an intervention of the engine control  42 . The second partial transmission  20  (the fourth target gear being already engaged) still rotates at the number of revolutions n 2 , which stays constant due to a lacking supply of torque, if frictional losses are not considered. 
     At the beginning of the third phase, at the time T 3  one starts to close the second separation clutch  14 . A torque is transmitted from the engine  28  through the second separation clutch  14  at the time T 3 , as represented by the increasing torque M 2  of the second partial transmission  20  in  FIG. 2B . 
     The second separation clutch  14  reaches its drag point at the time T 4 . Then, the number of revolutions n e  of the engine and the number of revolutions of the second partial transmission  20  are identical. The shafts  22  and  26  are engaging into each other by means of the second separation clutch  14 . In  FIG. 2A , this is represented in the third phase by the number of revolutions n e  of the engine, being depicted with points, intersecting the solid-lined number of revolutions n 2  of the second partial transmission  20 . The clutch and transmission control  34  regulates the engine  28  from this time T 4  such that the torque M ENGINE  transmitted from the engine to the respective partial transmission increases again until it has reached the torque M DRIVER , desired by the driver, at the time T 5 . The time T 5  can be within the third phase or the fourth phase. Then, the shift gear process is completed. 
     With reference to the  FIGS. 2C and 2D , in the following the temporal course of the numbers of revolutions and the torques, respectively, if a disturbance is present, will be explained exemplarily during a failure of the bus connection  44 . It is clear that these explanations also apply correspondingly to the case when, for example, the speed sensor  50  fails. As above, the third gear is engaged. Actually, now the fourth gear was supposed to be engaged. Since the control module  42  can not be responded by the control module  34  due to the interrupted bus connection  44 , controlling of the engine is not possible during the scheduled gear shift. Therefore, a possible shift strategy will be considered in the following, where only gears of this partial transmission are utilized, which is active at the time of the disturbance. Presently, this is the first partial transmission  18  due to the engaged third gear. Instead of changing into the fourth gear, here it is shifted into the fifth gear, which also belongs to the first partial transmission  18 . It is clear that also different shift strategies can be applied. A shift into the fourth gear is also possible, but will not be described here in detail. 
       FIG. 2C  shows the different numbers of revolutions n 1 , n 2  and n M .  FIG. 2D  shows the different torques M 1 , M 2 , M ENGINE  and M DRIVER . 
     During the first phase the processes run analogously to the processes which are described with reference to the  FIGS. 2A and 2B . However, the following exceptions are to be considered. The number of revolutions of the engine n e  no longer can be regulated by the control module  34  due to the lacking bus connection  44 . Since the driver (cf. M DRIVER  in  FIG. 2D ) uses the gas pedal in a constant manner, the number of revolutions n e  of the engine within the first phase will grow to a certain maximum value n max . At this time, the engine nevertheless transmits only as much torque as desired by the driver, which is regulated by the control module  42 . The number of revolutions n 2  follows the number of revolutions n e  of the engine, since the output shaft  22  is connected, and respectively stays connected, to the input shaft  26  over the separation clutch  14 . 
     The third gear is disengaged (synchronizer is opened) during the second phase of the  FIGS. 2C and 2D , and the fifth gear is engaged, which is represented by a decreasing number of revolutions n 1  which adapts to the number of revolutions n TG  of the target gear. At the beginning of the third phase, one starts to close the first separation clutch  12 . The engine already rotates high; a positive torque is transmitted, whereby the number of revolutions n 1  slightly increases. 
     However, since the control module  34  knows the number of revolutions of the engine via the bypass of the other inactive partial transmission and also has further information with respect to the scheduled gear shift ( 3 - 5 ) as well as to the maximum number of revolutions n max  of the motor  28 , a gear-shift ramp function can be determined such that the number of revolutions n e  of the engine rotates at the time T 4  (reaching the drag point of the first separation clutch  12 ) with the same number of revolutions like the first partial transmission  18  (cf. n 1 ). The course of n e  between T 3  and T 4  in  FIG. 2C  represents a “ramp”, wherein at T 4  the separation clutch is closed and the shafts are synchronized. The position of the intersection point between n e  and n 1  depends on the gears to be engaged and, if necessary, on the driver&#39;s demands (i.e. n e ). 
     If the data bus connection  44  fails, as in the present case, and, thus, there is no possibility to directly influence the torque delivered by the engine  28 , based on the gear shift ramp function there is the possibility, for example, to affect the torque, which is to be transmitted, by the setting mechanism of the first separation clutch  12 . With other words, this means that the first separation clutch is closed such that the number of revolutions n e  of the engine has suddenly dropped to a value, when the drag point T 4  is reached, which is much lower than the maximum number of revolutions n max . The intersection of the curves n e  and n 1  is effected by the transmission ratio of the target gear and the speed of the motor vehicle. 
     Thus,  FIG. 2C  is different from the  FIG. 2A  only in the course of the number of revolutions n e  of the engine and the number of revolutions n 2  following n M . The  FIG. 2D  is different from  FIG. 2D  in the course of the engine torque. If the engine can not be regulated any longer, the engine torque M ENGINE  being transmitted by the engine stays at the torque M DRIVER  desired by the driver. The engine control  42  cannot be responded by the control  34 . Still, one can shift. The torque M 2  of the second shaft  26  remains unchanged. 
     With reference to  FIG. 3  a method in accordance with the present invention for maintaining the operation of a motor vehicle having an automated transmission will be explained in the following, if, during the operation, a disturbance within the control occurs, which would normally result in a turn-off or termination of the operation of the motor vehicle. 
     In the following, the non-limitative case will be discussed when the bus connection  44  has failed. 
     The method starts in step S 10 . In this context, it is assumed that the bus  44  has failed so that: (optionally) a limited manual operation is possible; the position of the gas pedal  46  is a function of the number of revolutions of the engine; either a starting strategy or a shifting strategy is to be applied; commands from the brake will not be executed during the shift process; and the automated transmission is to be operated in a drive-home mode. These assumptions are exemplarily depicted in box S 12 . 
     In step S 14  it can be checked whether the sensors  36  and  38 —and optionally the sensor  50  as well—do function at all. Provided sensors dedicated to the transmission output shafts  30 ,  32  and a selection lever (SL) are provided, they also can be checked in the step S 14 . If one or some of the sensors do not deliver any information, this can already result in a termination of operation, as depicted in step S 16 . 
     In the following, it is assumed that at least the sensors  36  and  38  of the transmission input shafts  24  and  26  as well as sensors not shown in  FIG. 1 , which are dedicated to the transmission output shafts  30 ,  32  and a selection lever, are operating. 
     In step S 18  it is determined, whether the motor vehicle moves, i.e. if the motor vehicle stands, creeps (very slow ride) or starts. If the motor vehicle drives normally, in step S 20  it is changed to step S 32  which will be explained in the following. 
     If the motor vehicle stands, creeps or starts, all gears are disengaged, i.e. all synchronizer devices of the partial transmissions  18  and  20  are opened so that there is not any friction locked connection between the transmission input and output shafts  24 ,  30  and  26 ,  32 . Subsequently, both separation clutches (SC)  12 ,  14  are closed so that the transmission input shafts can rotate at the number of revolutions of the engine  28 . 
     If the difference of the number of revolutions n 1  of the first transmission input shaft  24  is beneath a predetermined tolerance threshold (n_cal), the method in accordance with the invention continues to step S 32 . However, if there are greater differences in the number of revolutions n 1  and n 2 , it is determined in step  28  that at least one of the sensors  36 ,  38  must to be disturbed. In this case, both separation clutches  12  and  14  are opened in the step S 28 , and the operation of the motor vehicle will be determined in step S 13 . 
     If the method of the present invention arrives at the step S 32 , it is checked in a next step S 34 , whether the selection lever SL is set to reverse. If it is not intended to drive reversely, in step S 36  it is checked whether the motor vehicle is in a parking mode P or in a neutral mode N. 
     If the vehicle is neither parked nor a gear is engaged, in step S 38  it is determined that the selection lever SL is either in the drive position D or in the position N for manually shifting. In this case, it is retrieved in step S 40  whether, for example, an even gear (of the second partial transmission  20 ) is engaged. If an even gear is engaged, it is shifted into an uneven gear in step S 42 . If an uneven gear is already engaged, all even gears are disengaged and the second separation clutch is closed. In this case, the second transmission input shaft  26  rotates at the number of revolutions n 2 , which in turn corresponds to the number of revolutions n_eng of the engine (cf step S 44 ). Then, in step S 46  one continues to the step S 50  “driving”. 
     If it is determined in the step S 34  that the driver intends to drive reverse, all uneven gears are disengaged, wherein here it is assumed that the reverse gear R is part of the second partial transmission  20 . This happens in step S 52 . During the step S 52 , also the reverse gear is engaged, and the first separation clutch  12  is closed. In this case, the first transmission input shaft  24  rotates at the number of revolutions n 1 , which in turn corresponds to the number of revolutions n_eng of the engine. Subsequently, it is continued at the step S 46  then. 
     If the inquiry of the step S 36  results in that the selection lever is in the position D or N, a gear is selected either by actuating the selection lever or in an automated manner according to the drive-home mode. Subsequently, both separation clutches  12  and  14  (cf step S 54 ) are opened. If these two actions are c, one proceeds to the step S 46 . If it is proceeded to the step S 50 , due to the step S 46 , it is requested at an inquiry S 56 , whether the selection lever is in the position of the reverse gear R, the forward gear D or in a manual mode M. If this is not the case, the method of the present invention returns to the step S 32 . 
     However, if this is the case, it is checked at an inquiry S 58  whether the motor vehicle is to be started. 
     If the vehicle already travels, a gear is selected based on the position of the selection lever or on commands of the control being in the drive-home mode (cf step S 60 ). Subsequently, one shifts, for example, with an interruption of traction as already explained in greater detail in the context of the  FIGS. 2C and 2D . 
     If it is determined in step S 58  that one has to start, the position of the pedal  46  is checked in step S 64 . If the position of the pedal is greater than a preset threshold value, the vehicle is started in step S 66 . Otherwise, the inquiry S 68  checks whether the speed of the vehicle is greater than zero. If the speed of the vehicle is greater than zero, such as when the vehicle rolls down a hill, the control can wait for an appropriate time, in order to engage the selected gear in the step S 60 . 
     The control determines a gear-shift ramp function, considering shifting parameters, in order to adapt to the respective situation and requirement (starting, shifting-up or shifting-down, with traction or overrun, disturbance during travel or starting, or similar). For example, the displacement path of the clutch elements can be influenced by the gear-shift ramp function, dependent on the time, if it is not possible to influence the engine  28  due to the failure of the bus  44 . However, if appropriate precautions have been taken, such as providing a data connection  54  between the control module  34  and the engine  28 , the control  34  can also influence the engine  28  directly. 
     These explanations show how important the knowledge of the number of revolutions of the engine is, as well as preferably the knowledge of the magnitude of a parameter influencing the number of revolutions of the engine, such as the position of the gas pedal or the position of the throttle plate. 
     With reference to  FIG. 4  a simplified embodiment of the method in accordance with the present invention will be explained in an exemplary manner using the drive line  10 , which is illustrated in  FIG. 1 , having an automated dual clutch transmission, if a databus connection  44  between the control modules  42  and  34  fails. 
     In a first step S 100 , one of the control modules responsible for the performance of a gear shift, such as the control module  34 , requests whether the bus connection  44  is interrupted. If it is not interrupted, then the inquiry is repeated within intervals having distances free of choice, preferably briefly before a gear shift is performed. If it is determined that the bus connection  44  is interrupted, then it is preferably determined which partial transmission ( 18  or  20 ) is active or which partial transmission is inactive. In the following it is assumed that at the time of the disturbance of the databus connection  44  the third gear of the first partial transmission  18  is engaged. Thus, the second partial transmission  20  is inactive so that the separation clutch  14  is open. However, the first separation clutch  12  of the first partial transmission  18  is closed. 
     Now, in another step S 114  all synchronizer devices of the inactive partial transmission  20  are open, i.e. the gears  2 ,  4  and  6  are disengaged by means of their respective synchronizer clutches, if not yet happened already. 
     Subsequently, the second separation clutch  14  of the inactive partial transmission  20  is closed in a step S 116 . This is possible, since there is no friction-locked connection between the second transmission input shaft  26  and the second transmission output shaft  32 . The synchronizer devices of the partial transmission  20  are open. The first separation clutch  12  is still closed, and thus effects a friction-locked connection of the first transmission input shaft  24  to the first transmission output shaft  30 . The engine output shaft  22 , the first transmission input shaft  24 , the second transmission input shaft  26  as well as the first transmission output shaft  30  rotate all at the same speed, namely the number of revolutions n e  of the engine  28 . This number of revolutions is measured in another step S 118 . 
     In a step S 120  it is optionally decided whether all gears of the dual clutch transmission are still to be used. 
     In a step S 122  it can be decided that the motor vehicle is to be operated only by means of the gears of the active partial transmission  18 , i.e. the first, third and fifth gear. This is sufficient for driving the motor vehicle at least home or to the next garage. At that time, the comfort of driving can suffer in comparison to an operation without disturbances. However, the motor vehicle does not completely break down due to the breakdown of the databus. 
     Then, in a step  124  a gear-shift ramp function is determined in case of a gear-shift command, based on the number of revolutions n 2  of the second transmission input shaft  26 , which is preferably detected continuously. The gear-shift ramp function can be converted into a time-dependent displacement command for the clutch actuators (which are not depicted here) of the separation clutch  12  or  14 . Since the data connection is interrupted not only from the engine control module  42  to the clutch transmission control module  34  but also in the inverted direction, it is not possible to influence the number of revolutions of the engine  28  by the aid of the engine control  42  during the shifting process, for example, from the third gear into the fifth gear. However, since the control module  34  knows about the schedules shift from the third gear into the fifth gear, and since the control module  34  also permanently knows the number of revolutions of the engine, because the number of revolutions n 2  of the second transmission input shaft  26  is preferably measured continuously, the control module  34  can vary the shifting process by, for example, opening and closing the separation clutch  12  either fast or slow. Possibly, the driver perceives this in terms of a jolt of the vehicle. However, since the method in accordance with the present invention deals with an emergency, this can be accepted. Otherwise the motor vehicle could not be moved at all. 
     However, if there is a redundant data connection  54 ′ from the clutch and transmission module  34  to the engine control module  42 , then the engine  28  can directly influence the number of revolutions n e  of the engine. Alternatively, the clutch and transmission control module  34  could also be connected, with respect to a regulation, directly to the engine  28  over a line  54 . This option is shown in the other optional method step S 126 . 
     As a result, shifting of a gear is performed (cf step S 128 ). As soon as the gear shift is completed, the method of the present invention returns to the start. 
     With another embodiment of the method of the present invention also both partial transmissions  18  and  20  could still be used, if it is decided in the step S 120  that all gears are still to be used. Then, the control module  34  preferably detects the number of revolutions n 1  or n 2  of the inactive partial transmission, and, by monitoring these numbers of revolutions, particularly the temporal course or the temporal change of these numbers of revolutions, determines events which initiate a gear shift. If the number of revolutions of one of the transmission input shafts  24 ,  26  is changed such that a gear-shift triggering event is present, then the control module  34  determines the associated gear-shift ramp function. If, in the case of the dual transmission, still both partial transmission  18  and  20  are to be used, it can be advantageous if both numbers of revolutions n 1  and n 2  of the transmission input shafts  24 ,  26  are monitored, preferably simultaneously, as well as permanently compared to each other for the purpose of a plausibility check. 
     It is clear that the method of the present invention can also be performed such that it is determined in advance which transmission input shaft is to be used at a higher priority for determining the number of revolutions of the engine. However, the inactive partial transmission is preferably used. Further, it is possible to determine the number of revolutions between both of the partial transmissions in an alternating manner. 
     Due to the so determined numbers of revolutions of the engine it is further possible to obtain information on a gas-actuating element such as the position of the gas pedal  46 . As it is known, the engine control  42  does not supply this information to the control module  34  any longer due to the breakdown of the databus connection  44 . However, the driver&#39;s desire can be simulated by using the determined number of revolutions of the engine. 
     For this purpose, a number of revolutions of the engine in terms of a threshold value is determined in advance, which is initially assigned to a predetermined position of the gas pedal  46 . This threshold value should be greater than the number of revolutions of the engine during idle-speed. Then, if the current number of revolutions of the engine is above this threshold value, this current number of revolutions is assigned to a position of the gas pedal which is greater by the same ratio like the detected number of revolutions is greater than the threshold value. This value, which represents the position of the gas pedal, is reassigned to the threshold value, i.e. the original value for the initial position of the gas pedal is replaced by the value of the new position of the gas pedal. This process is repeated that often, until the number of revolutions of the engine reaches a value which is smaller than the threshold value. In this case, the threshold value is assigned to the pedal position  0 . It is possible to simulate the position, for example, of the gas pedal in this manner. Among other things, this quantity can be used for deciding whether or not a gear is to be shifted. 
     It is clear that the above explanations can also be applied to automated single shift transmission. The number of revolutions of the engine can always be determined by opening all synchronizer devices so that there is no friction-locked connection between the transmission input shaft and the transmission output shaft. Then, if the separation clutch is closed, the transmission input shaft rotates at the same speed as the engine output shaft. Of course, a shifting process, which is regulated in that manner, takes possibly a little longer than a conventional shifting process. Nevertheless, the vehicle is prevented from breaking down due to a failure of the bus connection  44 . 
     With a dual transmission, both partial transmissions can be used, wherein there is no verified information on the number of revolutions of the engine for a brief moment, if both separations clutches are open, so that the separation clutch is operated in terms of an open control loop, in order to transmit a desired torque. The information required for this purpose can be derived from corresponding fields of characteristic lines. However, if only one single partial transmission is used in an emergency instance/drive-home mode a closed control loop is present, because the number of revolutions of the engine can be requested continuously from the inactive partial transmission.