Method for determining an operating state of frictionally engaged directional clutches of a vehicle drive train

Abstract

Method for determining an operating state of frictionally engaged directional clutches (6, 7) of a vehicle drive train (1) with a prime mover (2), with a transmission (3) and with an output (4) during a reversing process of a vehicle equipped with the vehicle drive train (1), starting from a first direction of travel in the direction of a second direction of travel opposite thereto, during which a first directional clutch (6 or 7) connected to the power flow to represent the first direction of travel is disconnected from the power flow and a second directional clutch (7 or 6) provided to represent the second direction of travel and disconnected from the power flow is connected to the power flow, wherein a profile of at least one operating variable (Δn6actual, Δn7actual, m2actual, n2actual, p5, n4, i3) of the vehicle drive train is monitored during a reversing process, and wherein, depending on threshold values (Δn6, 7desired, Δn6,7sollu,Δn6actual threshold, Δn7actual, Δn7actual1, i3target, i3target, m2actual1 to m2actual5, m2target, m2target, n2actual, n2target, n4target, n4target, p5target, p5target, p5targetso, p5targetsu, p5targetzo, p5targetzu) of the operating variable and / or threshold values of the gradient of the course of the operating variable (Δn6actual, Δn7actual, m2actual, n2actual, p5, n4, i3) of the vehicle drive train (1) within defined monitoring periods, an at least partial load transfer by the directional clutch (6 or 7) to be engaged is determined before a defined time (T3), characterized in that depending on threshold values (Δn6,7sollo, Δn6,7sollu, Δn6istschwell, Δn7schwell, Δn7schwell1, i3sollo, i3sollu, m2istschwell1 to m2istschwell5, m2sollo, m2sollu, n2istschwell, n2sollo, n2sollu, n4sollo, n4sollu, p5sollo, p5sollu, p5sollso, p5sollsu, p5sollzo, p5sollzu) of the operating variable and / or threshold values of the gradient of the operating variable (Δn6ist, Δn7ist, m2ist,n2actual, p5, n4, i3) of the vehicle drive train (1) within defined monitoring periods, an at least partial load transfer by the directional clutch (6 or 7) to be engaged is determined after a further defined time (T3).

Description

[0001] The invention relates to a method for determining an operating state of frictionally engaged directional clutches of a vehicle drive train according to the type defined in more detail in the preamble of patent claim 1.

[0002] In transmissions known from practice and preferably designed as continuously variable power-split transmissions, such as those used in construction machinery or forestry machinery, electro-hydraulically actuated frictional shifting elements are typically used. The shifting elements are filled with hydraulic fluid in the area of the piston chambers and subjected to the appropriate actuation pressure in order to engage them in the power flow of a vehicle drivetrain or to maintain them in the engaged operating state. The fill level of the piston chamber of such a shifting element significantly influences the respective transmission capacity of the shifting element and the torque that can be transmitted via the shifting element.In addition, the actuation sequence of a shift element, starting from an open operating state of the shift element, in which no torque can be transmitted via the shift element and whose transmission capacity is zero, to an operating state in which torque can be transmitted via the shift element in a slipping or slip-free state, affects the closing behavior of the shift element and thus the closing comfort. In turn, the shifting comfort is largely determined by the torque present in the area of a vehicle's output and its progression.

[0003] Since manufacturing tolerances of components used in shift elements and of hydraulic lines produced in series are known to exhibit undesirable variations, calibration must be performed individually for each shift element. This ensures that the respective filling behavior of the shift elements is recognizable and that the shift element is actuated to the required extent to achieve a high level of shifting comfort. The parameters characterizing the respective clutch filling and determined by calibration are stored in the non-volatile memory of a transmission control system and taken into account for the actuation of a shift element during each shift.

[0004] Two parameters in particular are characteristic of the filling behavior of a switching element. The first parameter is the so-called rapid filling time, during which a switching element is subjected to a so-called rapid filling pulse in order to fill the switching element within a short operating time. The piston chamber of a switching element is subjected to a defined rapid filling pressure during the rapid filling time. The rapid filling phase is followed by a so-called filling equalization phase, during which the actuation pressure in the piston chamber is reduced from the rapid filling pressure level to a filling equalization pressure level and maintained for a defined filling equalization time.At the end of the fill-equalization phase, the switching element ideally exists in a defined operating state where its transmission capacity is zero and from which an increase in the actuating force of the switching element in the closing direction results in an immediate increase in its transmission capacity. The second parameter is therefore an IP offset, or fill-equalization pressure offset.

[0005] Continuously variable power split transmissions are known to be designed with so-called directional clutches, which are alternately connected to or disconnected from the power flow of a vehicle drive train in order to be able to represent a forward or reverse direction of travel. For example, if there is a request to change the direction of travel from a forward direction of travel to a reverse direction, the directional clutch currently connected to the power flow is disconnected or switched into an open operating state during a so-called reversing process, while the other directional clutch, which is disconnected during the forward direction of travel, is switched from the open operating state to its closed operating state to represent the change in direction of travel. While the requested change in direction of travel is being carried out orThe requested reversal achieves the newly requested direction of travel via a slipping direction clutch or the reduction of the differential speed in the area of the direction clutch to be closed.

[0006] Unfortunately, underfilled or overfilled directional clutches, or fluctuating clutch pressures, have a negative impact on the speed profile of the output of a vehicle drivetrain and thus on the quality of the reversing process. In addition, there is a risk that underfilled or overfilled directional clutches may be irreversibly damaged during a reversing process, thereby impairing the service life of such directional clutches.

[0007] To avoid the disadvantages described above and those resulting from underfilling or overfilling, filling processes of directional clutches are calibrated. During such a calibration, the rapid filling time of a directional clutch is determined, and a corresponding current-pressure characteristic curve of a pressure regulator assigned to the directional clutch is shifted.

[0008] Since both pressure regulators and adjustment systems intended for the actuation of direction clutches generally do not have optimal pressure control behavior, reversing operations cannot be carried out with the desired continuous sequence, despite the calibration described above, which on the one hand ensures a high reversing quality and on the other hand enables or ensures a low load on the direction clutches.

[0009] From DE 10 2015 225 530 A1 a method for operating a continuously variable transmission of a vehicle with a reversing gear is known.

[0010] Furthermore, DE 195 46 292 A1 discloses a method for the automated coordination of the filling and application process of hydraulically actuated and hydraulically or electronically individually controllable switching elements.

[0011] The present invention is therefore based on the object of providing a method for determining an operating state of frictionally engaged directional clutches of a vehicle drive train, by means of which discontinuous reversing processes can be determined in order to be able to take measures to improve the reversing quality in a simple manner.

[0012] According to the invention, this object is achieved by a method having the features of patent claim 1.

[0013] In the method according to the invention for determining an operating state of frictionally engaged directional clutches of a vehicle drive train with a prime mover, with a transmission and with an output, during a reversing process of a vehicle equipped with the vehicle drive train, starting from a first direction of travel in the direction of a second direction of travel opposite thereto, a first directional clutch connected to the power flow to represent the first direction of travel is disconnected from the power flow and a second directional clutch provided to represent the second direction of travel and disconnected from the power flow is connected to the power flow.

[0014] According to the invention, a profile of at least one operating variable of the vehicle drive train is monitored during a reversing operation. Furthermore, depending on threshold values of the operating variable and / or threshold values of the gradient of the profile of the operating variable of the vehicle drive train, an at least partial load transfer by the directional clutch to be engaged before a defined point in time and an at least partial load transfer by the directional clutch to be engaged after a further defined point in time are determined within defined monitoring periods.

[0015] Using the procedure according to the invention, it is now possible to determine in a simple manner whether the second directional clutch to be engaged was engaged too early or too late in the power flow during the reversing process. Based on this knowledge, measures can be initiated to improve reversing quality and reduce the load on the directional clutches. For example, it is possible to issue a message to the driver of a work machine that, for example, a recalibration of the directional clutches is necessary or that assistance from customer service should be sought if necessary. Additionally or alternatively, it can also be provided that the driver is informed that adapted software with new settings needs to be installed on a vehicle control unit or that operating parameters need to be adapted.

[0016] Generally, when manually adjusting the filling of directional clutches, certain evaluation criteria are used to decide whether the quick-fill time of a directional clutch should be increased or reduced. The filling equalization pressure is calibrated accordingly.

[0017] In contrast, the procedure according to the invention represents an automated detection of a directional coupling that is too full or too empty, which uses suitable logic. Using this suitable logic, an assessment can be carried out on the basis of specific evaluation criteria and defined limit values or threshold values as to whether the directional coupling to be engaged in each case is at least partially engaged too early or too late during an actual reversing maneuver. In this case, the directional coupling to be engaged is available too late with the desired transmission capacity if the filling of the directional coupling to be engaged was not sufficient during a filling phase. If the directional coupling to be engaged is only available with the desired transmission capacity after the further defined point in time, it is also referred to below as being too empty.In contrast, the directional clutch to be engaged is available with excessive transmission capacity during a reversing maneuver before a defined point in time if the filling of the directional clutch to be engaged was too high during the filling phase. In the latter case, the clutch is then also referred to as overfilled.

[0018] In an advantageous variant of the method according to the invention, the directional clutch, which is to be engaged whenever a driver requests a reversing maneuver, is prepared for engagement in the power flow during an adaptable filling phase depending on the current filling level. The filling phase comprises a rapid filling phase and a filling equalization phase. During the rapid filling phase, the directional clutch to be engaged is pre-filled with a rapid filling pressure pulse with a defined rapid filling pressure over a defined rapid filling time. During a subsequent filling equalization phase, the directional clutch to be engaged is subjected to a defined filling equalization pressure, which is lower than the rapid filling pressure, for a filling equalization time.At the end of the filling phase, the respective directional clutch to be engaged is in an operating state in which its transmission capacity is ideally zero and an increase in the actual actuation pressure of the directional clutch to be engaged results in an immediate increase in the transmission capacity of the directional clutch to be engaged.

[0019] Using the last-described variant of the method according to the invention, the directional clutch to be engaged is prepared in a simple manner for engagement in the power flow of the vehicle drivetrain. Starting from the defined operating state, the directional clutch to be engaged can be engaged into the power flow with high precision, and the load transfer by the directional clutch to be engaged can be achieved with minimal control and regulation effort while simultaneously achieving high reversing quality.

[0020] In a variant of the method according to the invention, which can also be implemented with little control and regulation effort, at least partial load transfer by the directional clutch to be engaged before the defined time is determined if a differential speed of the first directional clutch, which has been engaged into the power flow as required, increases to a value greater than a defined threshold during the filling phase of the second directional clutch to be engaged. In addition, the detection of an over-full clutch requires that, during a monitoring period, an increase in the actual torque present in the area of the shift element half of the second directional clutch to be engaged that is operatively connected to the drive engine is additionally determined to be greater than a threshold value or an increase to a predefined threshold value.The monitoring period begins when the defined threshold value of the differential speed of the engaged first direction clutch is reached and ends at the defined time.

[0021] At least partial load transfer by the directional clutch to be engaged before the defined time or an over-engaged directional clutch is detected during a reversing operation of the vehicle if, during a monitoring period, an increase in the actual torque value applied in the area of the switching element half of the second directional clutch to be engaged that is operatively connected to the drive engine is determined, starting from a value during the filling phase of the second directional clutch to be engaged, greater than a threshold value or an increase to a predefined threshold value. The monitoring period begins at the end of the filling phase of the second directional clutch to be engaged and ends at the latest at the defined time,

[0022] Additionally or alternatively, in a further variant of the method according to the invention that can be carried out with little effort, at least partial load transfer by the second directional clutch to be engaged before the defined time or an overly full directional clutch to be engaged is determined if, during a monitoring period, an increase in the actual torque applied in the area of the shift element half of the second directional clutch to be engaged that is operatively connected to the drive engine is determined to be greater than a threshold value, starting from the value of the actual torque applied at the beginning of the monitoring period. The monitoring period begins at a time at which the filling phase of the second directional clutch to be engaged is completed and ends at the latest at the defined time.

[0023] In a further advantageous variant of the method according to the invention, a direction clutch to be fully engaged or an at least partial load transfer by the direction clutch to be engaged before the defined time is determined with little effort if, during the filling phase of the second direction clutch to be engaged, a negative gradient of a profile of the speed of the drive machine is less than a threshold value and, in addition, a decrease in the speed of the drive machine is greater than a threshold value and an actual torque greater than a threshold value present in the area of the switching element half of the second direction clutch to be engaged that is in operative connection with the drive machine is determined.

[0024] An at least partial load transfer by the directional clutch to be engaged after the further defined time or an excessively empty directional clutch to be engaged is determined in a variant of the method according to the invention that can be carried out with little control and regulation effort if a profile of the actual differential speed of the second directional clutch to be engaged deviates from a profile of the target differential speed during a load transfer phase and after the further defined time of the second directional clutch to be engaged by more than a threshold value.

[0025] Alternatively or additionally, in a further variant of the method according to the invention that can be carried out with little effort, at least partial load transfer after the further defined point in time by the directional clutch to be engaged or a second directional clutch to be engaged when empty is detected if, during a monitoring period, a threshold value of the actual torque applied to the shift element half of the second directional clutch to be engaged, which is operatively connected to the drive motor, is not exceeded. However, a second directional clutch to be engaged when empty is detected only if there is a deviation between an actual differential speed and a target differential speed of the second directional clutch to be engaged that is greater than a threshold value.The monitoring period begins at the time at which an increase in the actual transmission capacity of the second direction coupling to be engaged is expected and ends at the defined time.

[0026] A reverse clutch that is engaging too empty is detected when the engine torque is below a defined value at a time when the reverse clutch is expected to transmit torque. In addition, the clutch speed differential must lag behind the target and actual speed.

[0027] This function is deactivated when an increase in engine torque is detected. The engine torque is monitored during the clutch engagement phase until the start of the torque window.

[0028] In a further easily implementable variant of the method according to the invention, a second direction clutch to be engaged that is too empty or an at least partial load transfer by the second direction clutch to be engaged after the further defined time is determined if, during a monitoring period, an increase in an actual torque present in the area of the shift element half of the second direction clutch to be engaged that is operatively connected to the drive motor is determined within a further shorter monitoring period, starting from a value of the actual torque present at the beginning of the further shorter monitoring period, to a threshold value. The monitoring period begins at a time at which an increase in the actual transmission capacity of the second direction clutch to be engaged is expected and ends at the latest when an output speed equal to zero is reached.

[0029] During the reversing process, until the change of direction is reached, the torque currently applied to the switching element half of the second direction clutch to be engaged, which is operatively connected to the drive motor, is determined. To avoid incorrect evaluations of the reversing process, the maximum of this torque is initially determined. The determination of at least partial load transfer after the further defined point in time is omitted if a drop in the torque applied to the switching element half of the second direction clutch to be engaged, which is operatively connected to the drive motor, is greater than a threshold value compared to the maximum.

[0030] If the threshold values are varied depending on the speed of the output and / or a driver-side requirement as to the gradient of the actual curve of the speed of the output with which the reversing process is to be carried out until the vehicle comes to a standstill and also thereafter, the evaluation of the respective reversing process is adapted to the current operating state curve, ie carried out depending on the speed and aggressiveness.

[0031] In a variant of the method according to the invention which can be carried out with low computing power, predefined values of the at least one operating variable are used as threshold values.

[0032] Alternatively, it may also be provided that a sum of an offset value and a value of the operating variables at the beginning of a monitoring period or at least one value of the operating variable before the start of the respective monitoring period is used as threshold values.

[0033] In addition, it is possible to use target curves as threshold values that limit the value ranges of at least one operating variable.

[0034] This makes it possible to evaluate the buildup of the torque of the prime mover applied to the shift element half of the second direction clutch to be engaged, which is operatively connected to the prime mover, a profile of the differential speed of the second direction clutch to be engaged, a profile of the prime mover speed, a profile of the speed of the output of the vehicle drive train, and / or a profile of a high pressure in a high pressure area of a hydrostatic transmission of the vehicle drive train's transmission, or an analogous operating variable. Furthermore, it is possible to perform the evaluation depending on one or more operating variables.In addition, the behavior of the respective directional clutch to be engaged can also be evaluated to a suitable extent depending on a gradient, an increase, a decrease, a current level as well as the general course of the operating variable used or considered for the evaluation within certain phases of a reversing process.

[0035] This makes it possible to detect at least partial load transfer by the second directional clutch to be engaged before the defined time if the torque applied to the switching element half of the second directional clutch to be engaged, which is operatively connected to the drive motor, is above a value range defined by the target curves during a monitoring period. The monitoring period begins with the filling phase of the second directional clutch to be engaged and ends at the defined time.

[0036] In contrast, in a further, easily implemented variant of the method according to the invention, at least partial load transfer by the second directional clutch to be engaged after the defined further point in time is determined if the torque applied to the switching element half of the second directional clutch to be engaged, which is operatively connected to the drive motor, is below the value range defined by the target curves during a monitoring period. The monitoring period begins at a point in time at which an open operating state of the first directional clutch to be disengaged is expected.

[0037] If the differential speed in the range of the second directional clutch to be engaged lies below a differential speed value range defined by the target curves during a monitoring period, a further easily implemented variant of the method according to the invention determines that the second directional clutch to be engaged has at least partially transferred the load before the defined time or that the second directional clutch to be engaged is too fully engaged. The monitoring period begins with the start of the filling phase of the second directional clutch to be engaged and ends at a time at which an increase in the actual transmission capacity of the second directional clutch to be engaged is expected.

[0038] In contrast, in a further advantageous variant of the method according to the invention, at least partial load transfer by the second directional clutch to be engaged after the defined further point in time, or a second directional clutch to be engaged that is too empty, is determined if the differential speed in the range of the second directional clutch to be engaged lies above the value range of the differential speed defined by the target curves during a monitoring period. The monitoring period begins at a point in time at which an open operating state of the first directional clutch to be disengaged and an increase in the actual transmission capacity of the second directional clutch to be engaged are expected.

[0039] If the engine speed lies below a value range of the engine speed defined by the target curves during a monitoring period, a further variant of the method according to the invention, which can be implemented with minimal control and regulation effort, determines that the second directional clutch to be engaged has at least partially taken over the load before the defined time, or that the second directional clutch has been engaged too fully. The monitoring period begins with the start of the filling phase of the second directional clutch to be engaged and ends at a time at which an increase in the actual transmission capacity of the second directional clutch to be engaged is expected.

[0040] If, however, the engine speed in the range of the second directional clutch to be engaged is above the value range of the engine speed defined by the target curves during a monitoring period, a further easily implemented variant of the method according to the invention determines that the second directional clutch to be engaged will at least partially transfer the load after the defined additional point in time. The monitoring period begins at a point in time at which an increase in the actual transmission capacity of the second directional clutch to be engaged is expected.

[0041] During a reversing operation, a second direction clutch to be engaged too fully or an at least partial load transfer by the second direction clutch to be engaged before the defined time is determined in a further variant of the method according to the invention that can be carried out with little effort if the speed of the output or a gear ratio of a transmission of the vehicle drive train lies below a value range of the speed of the output or the gear ratio of the transmission defined by the target curves during a monitoring period. The monitoring period begins with the start of the filling phase of the second direction clutch to be engaged and ends at a time at which an increase in the actual transmission capacity of the second direction clutch to be engaged is expected.

[0042] In contrast, in a further simple variant of the method according to the invention, at least partial load transfer by the second directional clutch to be engaged after the defined further point in time or a second directional clutch to be engaged that is too empty during a reversing process is determined if the speed of the output or the gear ratio of the transmission lies above a value range of the speed of the output or the gear ratio defined by the target curves during a monitoring period. The monitoring period begins at a point in time at which a filling phase of the second directional clutch to be engaged starts and ends at a point in time at which an increase in the actual transmission capacity of the second directional clutch to be engaged is expected.

[0043] In a further advantageous variant of the method according to the invention, which can be carried out with little control and regulation effort, an at least partial load transfer by the second direction of travel clutch to be engaged or a second direction of travel clutch to be engaged when empty is determined before the defined time if a high pressure in a high pressure range of a hydrostatic transmission of a continuously variable transmission of the vehicle drive train or an operating variable of the vehicle drive train analogous thereto lies above a value range of the high pressure or the operating variable of the vehicle drive train analogous thereto during a monitoring period during overrun or traction operation of the vehicle drive train defined by the target curves.The monitoring period begins at a point in time at which a filling phase of the second direction coupling to be engaged starts and ends at a point in time at which an increase in the actual transmission capacity of the second direction coupling to be engaged is expected.

[0044] If, however, a high pressure in a high pressure range of a hydrostatic transmission of a continuously variable transmission of the vehicle drive train or an analogous operating variable of the vehicle drive train lies below a value range of the high pressure defined by the target curves or the analogous operating variable of the vehicle drive train during a monitoring period, an easily implemented variant of the method according to the invention determines that after the defined further point in time, at least partial load transfer by the second direction clutch to be engaged or that a second direction clutch to be engaged is too empty during a reversing process. The monitoring period begins at a point in time at which a filling phase of the second direction clutch to be engaged starts and ends at a point in time at which an increase in the actual transmission capacity of the second direction clutch to be engaged is expected.

[0045] Additionally, in a further variant of the method according to the invention, the determination of the at least partial load transfer of the second directional clutch to be engaged is carried out before the defined time and after the defined additional time only if various boundary conditions are met. This easily prevents incorrect evaluation of reversing operations.

[0046] In this case, the determination of a direction of travel clutch that is too empty or too full takes place if the operating temperature of the transmission is greater than a threshold value and / or a working hydraulic system or other units of the vehicle that are to be subjected to torque by the drive engine and are connected to the vehicle drive train are deactivated and / or the fill compensation phase of the filling phase of the second direction of travel clutch to be engaged is shorter than a threshold value and / or an unsteady curve of the speed of the output occurs during the reversing process and / or the reversing process takes place with a vehicle longitudinal inclination smaller than a threshold value and / or an actual curve of the high pressure of the hydrostatic drive corresponds to a target curve.

[0047] Both the features specified in the patent claims and the features specified in the following embodiments of the storage device according to the invention are suitable, either individually or in any combination with one another, for further developing the subject matter of the invention.

[0048] Further advantages and advantageous embodiments of the subject matter according to the invention emerge from the patent claims and the exemplary embodiments described in principle below with reference to the drawing.

[0049] It shows: Fig. 1 a highly schematic representation of a vehicle drive train of a work machine; Fig. 2 a simplified representation of a curve of an actuating pressure of a direction clutch to be engaged in a transmission of a vehicle drive train according to Fig. 1 starting from a fully open operating state to a fully closed operating state of the directional coupling to be engaged; Fig. 3 a block diagram of a variant of the method according to the invention; Fig. 4 a range of values, limited by an upper target curve and a lower target curve, of a sensor connected to the drive engine of the vehicle drive train according to Fig. 1 torque applied to the switching element half of the direction of travel clutch to be engaged during a reversing operation; Fig. 5 a Fig. 4 corresponding representation of a value range of a differential speed in the area of the direction of travel clutch to be engaged during a reversing process; Fig. 6 a Fig. 4 corresponding representation of a value range of a speed of the drive machine of the vehicle drive train according to Fig. 1 during a reversing operation; Fig. 7 a Fig. 4 corresponding representation of a value range of a speed of the output of the vehicle drive train according to Fig. 1 or a gear ratio of the vehicle drive train during a reversing operation; Fig. 8 a Fig. 4 corresponding representation of a value range of a high pressure of a variator designed as a hydrostatic transmission of the vehicle drive train according to Fig. 1 during a reversing operation or an analogous operating variable of the vehicle drive train; Fig. 9 several curves of different operating variables of the vehicle drive train according to Fig. 1 over the operating time t during a reversing operation of a vehicle equipped with the vehicle drive train, which occur during an operating state of the vehicle drive train if at least partial load transfer by the directional clutch to be engaged occurs before a defined time; Fig. 10 a Fig. 9 corresponding representation, wherein a detection of whether at least partial load transfer by the directional clutch to be engaged occurs before the defined time is carried out on the basis of the torque curve of the drive motor during a defined monitoring period; Fig. 11 a Fig. 9 corresponding representation during a reversing process, during which a course of the torque of the drive machine is monitored during a defined monitoring period, wherein if the torque increases during the monitoring period to a value greater than a threshold value, at least partial load transfer by the directional clutch to be engaged is determined before the defined time; Fig. 12 a Fig. 9 corresponding representation during a reversing process, wherein a detection of whether at least partial load transfer by the directional clutch to be engaged during the reversing process takes place before the defined time is carried out on the basis of the speed curve of the drive motor; Fig. 13 a Fig. 9 corresponding representation during a reversing process, wherein after a further defined time, at least partial load transfer by the directional clutch to be engaged is determined if a profile of the actual differential speed of the directional clutch to be engaged deviates from a profile of the target differential speed during a load transfer phase and after the further defined time of the directional clutch to be engaged by more than a threshold value; Fig. 14 a Fig. 9 corresponding representation during a reversing process, wherein a direction clutch to be engaged is determined to be empty if an increase in the torque curve of the drive machine during a defined monitoring period is greater than a threshold value and a deviation between the actual differential speed and the target differential speed of the direction clutch to be engaged is determined to be greater than the threshold value; Fig. 15 a Fig. 9 corresponding representation during a reversing process, during which the determination of at least partial load transfer after the further defined time is omitted; Fig. 16 a Fig. 9 corresponding representation during a reversing process, whereby a too empty travel direction clutch to be engaged is detected, since the torque curve of the drive motor increases more than a threshold value during a defined monitoring period within a further shorter monitoring period; and Fig. 17 a Fig. 9 corresponding representation during a reversing process, wherein the determination of an empty direction clutch to be engaged is omitted, since a drop in the torque analogous to the switching element half of the direction clutch to be engaged, which is in operative connection with the drive motor, compared to a maximum of the torque of the drive motor is determined to be greater than a threshold value.

[0050] Fig. Figure 1 shows a highly schematic representation of a vehicle drivetrain 1 of a work machine, such as a wheel loader or a forestry machine. The vehicle drivetrain 1 comprises a prime mover 2, a continuously variable transmission 3, and an output 4. The continuously variable transmission 3 is equipped with a hydrostatic variator 5 and two hydraulically actuated friction-locking directional clutches 6, 7 or multi-disk clutches.

[0051] In the area of the continuously variable transmission 3, several driving ranges or gear ratio ranges can be engaged, within which the gear ratio of the transmission 3 can be continuously varied via the variator 5. The transmission capacity of the two direction clutches 6, 7 is continuously variable. Furthermore, a defined drive rotational movement of the drive motor 2 can be converted by means of the direction clutches 6, 7 into a rotational movement of the transmission output corresponding to the forward direction of the vehicle or into a reverse direction of the vehicle opposite thereto.

[0052] When a request for a change of direction from a current direction of travel to the opposite direction is received, the gear ratio of the line split transmission 3 is first increased or a reciprocal gear ratio value is reduced to implement the requested change of direction or reversing process. In particular, the gear ratio is only reduced when the vehicle is moving or exceeds a dynamic-dependent threshold value. This limits the clutch load. If the speed of the output 4 falls below a defined speed or a corresponding limit value of the vehicle speed, the currently engaged direction clutch 6 or 7 is disengaged and the currently disengaged direction clutch is engaged in a conventional manner.The gear ratio of the power-split transmission 3 is varied during a reversing maneuver until a so-called power reversing threshold of the vehicle speed is reached. Once the power reversing threshold is undershot, the gear ratio of the power-split transmission 3 is kept constant at the value of the gear ratio at the time the power reversing threshold is reached or undershot.

[0053] The ratio of the power split transmission 3 remains constant until the vehicle speed of the vehicle equipped with the vehicle drive train 1 exceeds the power reversing threshold again in the requested direction of travel, i.e., in the forward or reverse direction. The change of direction of travel, after falling below and until subsequently exceeding the power reversing threshold, is carried out solely by reducing a differential speed in the area of the respective directional clutch 6 or 7 to be engaged, while the directional clutch 7 or 6 engaged before the requested reversing process is essentially in the open operating state or with a transmission capacity equal to zero.

[0054] In order to be able to carry out the reversing process to the desired extent via the direction of travel couplings, the direction of travel coupling, which is switched off from the power flow to represent the current direction of travel, must first be prepared accordingly for switching into the power flow during a filling phase.

[0055] Fig. 2 shows a schematic curve of a desired actuating pressure p6soll or p7soll of the direction clutch 6 or 7 to be connected to the power flow in order to carry out the requested reversing process over the operating time t, starting from a completely switched-off operating state of the direction clutch 6 or 7 up to an operating state completely switched into the power flow.

[0056] In this case, before a time TO, a driver-side request for a change of direction is issued, starting from the current direction of travel of the vehicle equipped with vehicle drive train 1, toward the opposite direction. This request causes the actuation pressure p6soll or p7soll to be suddenly or abruptly increased at time T0 from an actuation pressure p6soll or p7soll equal to zero to a quick-fill pressure level p6sollSF or p7sollSF in order to be able to fill the piston chamber of the directional clutch 6 or 7 with hydraulic fluid within a short operating time.

[0057] Subsequently, the target actuation pressure p6soll or p7soll is left at the rapid filling pressure level p6sollSF or p7sollSF until a time T1, whereby this part of the filling phase is also referred to as the rapid filling phase. From time T1, the target actuation pressure p6soll or p7soll is reduced in a ramp-like manner to a so-called filling equalization pressure level p6sollFA or p7sollFA. In this case, the target actuation pressure p6soll or p7soll reaches the filling equalization pressure level at time T2 and is kept constant or at least approximately constant at this pressure level until a time T3, which represents the end of a so-called filling equalization phase of the filling process of the directional clutch 6 or 7 to be engaged.

[0058] In this case, the rapid filling pressure level p6sollSF or p7sollSF, the filling equalization pressure level p6sollFA or p7sollFA, the duration of the rapid filling phase, and the duration of the filling equalization phase are specified in such a way that the respective directional clutch 6 or 7 to be engaged is in a defined target operating state at time T3. The defined target operating state of the directional clutch 6 or 7 at time T3 is such that the transmission capacity of the respective directional clutch 6 or 7 to be engaged is ideally zero, and an increase in the target actuation pressure p6soll or p7soll causes an immediate increase in the transmission capacity of the respective directional clutch 6 or 7 to be engaged.This means that the respective direction clutch 6 or 7 to be engaged is optimally prepared for engagement in the power flow of the vehicle drive train 1 at time T3 and a reversing operation can be carried out to the desired extent with high reversing quality starting from such an operating state of the respective direction clutch 6 or 7 to be engaged.

[0059] If the aforementioned pressure levels of the rapid filling phase and the filling equalization phase, which characterize the filling phase of the directional clutches 6 and 7, are not determined or specified to the extent required to achieve high reversing quality, the transmission capacity of the respective directional clutch to be engaged either increases before time T3 or the desired immediate increase in transmission capacity after time T3 does not occur, even though the target actuation pressure p6soll or p7soll is increased to the extent shown from time T3. In the latter case, the transmission capacity of the respective directional clutch 6 or 7 to be engaged only increases to an undefined extent at a later point in time following time T3 due to the increase in the target actuation pressure p6soll or p7soll.

[0060] In the event of an increase in the transmission capacity of the respective directional clutch 6 or 7 to be engaged before the defined time T3, the behavior of the respective directional clutch 6 or 7 to be engaged is referred to as being too full, while an undefined increase in the transmission capacity of the respective directional clutch 6 or 7 to be engaged after the defined time T3 is regarded as the behavior of a so-called too empty clutch or directional clutch 6 or 7.

[0061] In this case, the target actuation pressure p6soll or p7soll of the respective directional clutch 6 or 7 to be engaged is increased in a ramp-like manner from time T3 up to a time T4 at which the directional clutch 6 or 7 to be engaged is essentially in a slip-free state, in which the differential speed in the area of the directional clutch 6 or 7 to be engaged is zero. Starting from the slip-free operating state of the directional clutch 6 or 7 to be engaged at time T4, the target actuation pressure p6soll or p7soll is again increased along a pressure ramp to a closing pressure level at which the directional clutch 6 or 7 to be engaged essentially has its full transmission capacity and is fully engaged in the power flow. The pressure ramp ends from time T4 at a time T5, which represents the end of the engagement process of the directional clutch 6 or 7 to be engaged.The gradient of the pressure ramp between times T4 and T5 is significantly greater than the gradient of the pressure ramp between times T3 and T4, since the actuation of the direction clutch 6 or 7 to be engaged between times T3 and T4 must be carried out with greater resolution in order to achieve a high reversing quality than between times T4 and T5.

[0062] In order to be able to carry out reversing operations with a desired high degree of reversing over the entire service life of a working machine equipped with the vehicle drive train 1, the variant of the method according to the invention described in more detail below is carried out during operation of such a working machine. Fig. 3 shows a block diagram of this exemplary variant, in which various operating variables of the vehicle drive train 1, which represent so-called evaluation criteria, are used to assess the filling of the directional clutch 6 or 7 to be engaged, which is responsible for the reversing quality. The evaluation of the filling of the directional clutch 6 or 7 to be engaged takes place in a block B1. During operation of the vehicle drive train 1, block B1 receives various signals of different operating variables of the vehicle drive train 1 as input values, which are generated in the range of signal blocks SB1 to SBx and fed to block B1.

[0063] The evaluation in block B1 is performed depending on the respective application, depending on one of the signals from signal blocks SB1 to SBx, or depending on a combination of several of these signals. One of these evaluation criteria is the actual torque of the drive motor 2 or, generally, an input torque of the power-branching gearbox 3. The gradient of the torque curve of the drive motor 2 or the input torque of the power-branching gearbox 3 can also be considered as a further evaluation criterion. In addition, it is also possible to determine or evaluate an increase or decrease in the torque of the drive motor 2 or the input torque of the power-branching gearbox 3 within a defined time window or within a specific phase of the reversing process.

[0064] Furthermore, it is also possible to evaluate a reversing process or its quality based on a curve of the speed of the drive machine 2, i.e. taking into account the gradient of this curve as well as a level and a time of a speed reduction.

[0065] Furthermore, it can also be provided that the evaluation of the filling of the directional clutch 6 or 7 to be engaged is carried out as a function of the curve of the torque applied to the output 4 or the transmission output torque or also as a curve of a high pressure in the area of the hydrostat 5. Alternatively or additionally to this, depending on the respective application, the evaluation of the filling of the directional clutch 6 or 7 to be engaged is carried out as a function of a curve of the speed of the output 4, a curve of a differential speed of the directional clutch 6 or 7 to be disengaged or also as a function of a differential speed of the directional clutch 6 or 7 to be engaged.In particular, when evaluating the speed curve of the differential speed of the direction clutch 6 or 7 to be engaged, a deviation may also occur between the actual differential speed and the target differential speed, as well as depending on a difference in the gradients of these curves.

[0066] Again additionally or alternatively to this, the evaluation of the filling of the direction of travel clutch 6 or 7 to be engaged can be carried out as a function of a target gear ratio of the variator 5, an actual gear ratio of the variator 5 and as a function of a deviation between these two courses of the gradients of these courses as well as a time offset between the course of the target gear ratio and the course of the actual gear ratio of the variator 5.

[0067] Furthermore, the evaluation of the filling of the directional clutch 6 or 7 to be engaged can be carried out depending on the target and actual curves of the output speed, in particular depending on a deviation, depending on gradients, and also depending on a temporal offset between these curves. In addition or alternatively, it is possible to carry out the evaluation of the clutch filling using a target curve of the reciprocal ratio of the power split transmission 3, an actual curve of the reciprocal ratio, and in particular depending on a deviation between these curves, the gradients of these curves, and a temporal offset between these two curves.

[0068] If the curves of the actual actuating pressures and the target actuating pressures of the two direction of travel clutches 6 and 7 are available during a reversing process, it is again possible to evaluate the filling of the direction of travel clutch 6 or 7 to be engaged and the actuation of the direction of travel clutch 7 or 6 to be disengaged in dependence on them or to assess their quality.

[0069] In addition, it can also be provided that the evaluation of the filling of the clutch 6 or 7 to be engaged is carried out as a function of a torque applied to the switching element half of the direction of travel clutch 6 or 7 to be engaged, which is in operative connection with the drive machine 2, the gradient of this curve or as a function of an increase or a decrease of this curve during a certain phase of the reversing process.

[0070] Furthermore, various signals from different boundary condition blocks RB1 to RBx are considered to ensure the assessability of a current reversing operation. Based on the signals from boundary condition blocks RB1 to RBx, a decision is made with minimal effort as to whether the currently performed reversing operation or the filling of the directional coupling 6 or 7 to be engaged is assessable or not.

[0071] The signals or boundary conditions suitable for this purpose represent, for example, an operating state of a working hydraulic system or other units of the vehicle equipped with the vehicle drive train 1 that load the drive machine 2. For example, no evaluation is carried out in block B1 if it is detected in test block PB based on the signals of the boundary condition blocks RB1 to RBx that the reversing currently being carried out is not suitable for an evaluation of the filling of the directional clutch 6 or 7 to be engaged.

[0072] A further boundary condition is the operating temperature of the line branching gearbox 3 or of the respective directional clutch 6 or 7 to be engaged. In this case, an evaluation of the filling of the directional clutch 6 or 7 to be engaged is omitted if the operating temperature of the line branching gearbox 3 or of the directional clutches 6 and 7 is less than a threshold value, preferably less than 50 °C. If a maximum permissible filling equalization time is exceeded, the current reversing is not used to evaluate the filling of the directional clutch 6 or 7 to be engaged. This also applies if a reversing process with a continuous profile of the speed of the output 4 is determined. In contrast, the filling of the directional clutch 6 or 7 to be engaged is evaluated if, for example, a reversing process carried out on a level surface is determined via a vehicle-side inclination sensor.If the high pressure or thrust high pressure curve in the high pressure range of the variator 5 corresponds to an expected target curve, the evaluation of the filling of the directional clutch 6 or 7 to be engaged is permitted. The evaluation result of block B1 is made available to a control unit for further use via an output block AB.

[0073] According to the representations Fig. 4 to Fig. 17 is based on a division of a reversing process into different reversing phases RevStep0 to RevStep5. The first reversing phase RevStep0 begins when the directional clutch 6 or 7 to be engaged is in the Fig. 2 is pre-filled at time TO or the rapid filling phase is started. The first reversing phase RevStep0 ends when the filling process of the directional coupling 6 or 7 to be engaged is completed at time T3. The first reversing phase RevStep0 therefore extends over the rapid filling time and the filling equalization time of the rapid filling phase and the filling equalization phase of the directional coupling 6 or 7 to be engaged and a subsequent holding phase. In this case, filling may be initiated too early. In this case, the coupling is not opened immediately after filling, but is held in its current actuation state. Such a reversal is not evaluated.The period of the first reversing phase RevStep0 varies to the same extent as the quick fill time and the fill equalization time, which is changed depending on the requested aggressiveness with which the reversing is to be carried out.

[0074] A second reversing phase (RevStep1) following the first reversing phase (RevStep0) begins at the end of the first reversing phase (RevStep0), at which point the directional clutch 6 or 7 to be engaged is ideally fully filled and its switching element halves are in contact with each other. The directional clutch 6 or 7 to be engaged is just not transmitting any torque at the time of the changeover between the first reversing phase (RevStep0) and the second reversing phase (RevStep1). Starting from this operating state of the directional clutch 6 or 7 to be engaged, its transmission capacity increases with each further increase in the actuating force to the extent described above.

[0075] In addition, during the second reversing phase RevStep1, the directional clutch 7 or 6 to be switched off is opened or disengaged, whereby the directional clutch 7 or 6 to be switched off has a transmission capacity of zero at the end of the second reversing phase RevStep1. At the same time, in the second reversing phase RevStep1, the torque to be transmitted is increased as calculated by the reversing function. Thus, from a third reversing phase RevStep2 onwards, the clutch to be switched on takes over with the correct torque in order to decelerate the vehicle. The third reversing phase RevStep2, which follows the second reversing phase RevStep1, begins when the directional clutch 6 or 7 to be switched on is in slipping mode and, due to the applied actuating force orthe applied actuating pressure, the differential speed between the switching element halves of the directional clutch 6 or 7 to be engaged is increasingly reduced. The ratio of the line split transmission 3 is changed during the third reversing phase RevStep2 solely by reducing the differential speed of the directional clutch 6 or 7 to be engaged, and the ratio of the variator 5 remains constant.

[0076] When the vehicle comes to a standstill, the third reversing phase (RevStep2) is completed, and the system switches to the fourth reversing phase (RevStep3). During the fourth reversing phase (RevStep3), the vehicle is accelerated in the requested new direction of travel by progressively reducing the differential speed of the directional clutch 6 or 7 to the vehicle speed that the vehicle had at the beginning of the third reversing phase (RevStep2).

[0077] If the vehicle speed reaches the latter value, the fourth reversing phase RevStep3 is completed and the fifth reversing phase RevStep4 is started, whereby the differential speed in the area of the direction clutch 6 or 7 to be engaged is only about 50 to 1001 minutes During the fifth reversing phase (RevStep4), the ratio of the hydrostatic transmission 5 is again changed in order to accelerate the vehicle to the desired extent in the new direction of travel. Once the differential speed in the area of the directional clutch 6 or 7 to be engaged has been completely eliminated, the fifth reversing phase (RevStep4) is also completed, and the sixth reversing phase (RevStep5) is started, during which the vehicle's acceleration is only achieved by adjusting the hydrostatic transmission ratio or by an additional corresponding ratio range change in the power-split transmission 3.

[0078] Fig. Figure 4 shows a range of values for the target drive torque of prime mover 2 over the operating time t during a reversing process, limited by an upper target curve m2sollo and a lower target curve m2sollu. The upper target curve m2sollo only rises to the extent shown at the beginning of the second reversing phase RevStep1 until the transition to the third reversing phase RevStep2 with a greater gradient than during the third reversing phase RevStep2 and the fourth reversing phase RevStep3. The lower target curve m2sollu follows the course of the upper target curve m2sollo with essentially the same gradient.

[0079] If it is determined via the signals of the boundary condition blocks RB1 to RBx and the subsequent test of the test block PB that the Fig. 4 underlying reversing process is suitable for the evaluation of the filling of the directional clutch 6 or 7 to be engaged, a too full directional clutch 6 or 7 to be engaged is determined when values of the drive torque m2 of the drive machine 2 above the upper target curve m2sollo up to a time T6. In contrast, a too empty directional clutch 6 or 7 to be engaged is determined if values of the torque of the drive machine 2 below the lower target curve m2sollu are detected after a time T7.

[0080] Fig. 5 shows a Fig. 4 corresponding representation, whereby the evaluation of the representation according to Fig. 5 underlying reversing is carried out using the differential speed in the area of the direction clutch 6 or 7 to be engaged. In Fig. 5, a value range of the differential speed of the directional clutch 6 or 7 to be engaged during reversing over the operating time t is limited by an upper target curve Δn6,7sollo and a lower target curve Δn6,7sollu. The two target curves Δn6,7sollo and Δn6,7sollu each have an essentially parallel course, with the two target curves Δn6,7sollo and Δn6,7sollu having a nearly constant value up to a time T8 or T9, respectively, and steadily decreasing from times T8 or T9, respectively, with a defined gradient to the extent shown.

[0081] Based on the evaluation of the filling of the directional coupling 6 or 7 to be engaged according to the Fig. According to the procedure shown in Figure 5, a direction clutch that is to be engaged too fully is determined if values of the differential speed Δn6,7 in the area of the direction clutch 6 or 7 to be engaged are detected after a time T11 above the upper target curve Δn6,7sollo.

[0082] Fig. 6 again shows a Fig. 4, in which an evaluation of the filling of the directional clutch 6 or 7 to be engaged is carried out on the basis of the speed n2 of the drive motor 2. This evaluation is in turn based on an upper target curve n2sollo and a lower target curve n2sollu, which limit a value range of the drive motor 2 over the operating time t or during the reversing process. In this case, a directional clutch 6 or 7 to be engaged that is too full is determined if, before a time T12, values of the speed n2 of the drive motor 2 are determined to be below the lower target curve n2sollu. In contrast, a directional clutch 6 or 7 to be engaged that is too empty is determined if, after a time T13, values of the speed n2 of the drive motor 2 are above the upper target curve n2sollo.

[0083] The Fig. The procedure shown in Figure 7 is based on an evaluation of the filling of the directional clutch to be engaged as a function of the speed n4 of the output 4 or a reciprocal gear ratio i3 of the power split gearbox 3. In this case, a value range of the speed n4 or the gear ratio i3 over the operating time t is limited by an upper target curve n4sollo or i3sollo and a lower target curve n4sollu or i3sollu. The target curves nsollo or i3sollo and n4sollu or i3sollu fall during the first three reversing phases RevStep0 to RevStep2 with a constant gradient until the end of the third reversing phase RevStep2 and then rise again with a constant gradient starting from the vehicle's standstill and thus at the beginning of the fourth reversing phase RevStep3. In this case, a directional clutch 6 or 7 to be engaged is determined to be too full if, before a time T14, values of the speed n4 or i3sollo are below the upper target curve n4sollo or i3sollo and a lower target curve n4sollu or i3sollu.the gear ratio i3 is determined below the lower target curve n4sollu or i3sollu. In contrast, a too-empty directional clutch 6 or 7 to be engaged is detected if, before time T14, values of the speed n4 or the gear ratio i3 are detected above the upper target curve n4sollo or i3sollo. After time T14, no further evaluation of the filling of the directional clutch 6 or 7 to be engaged is performed.

[0084] At the Fig. In the representation of a reversing process shown in Figure 8, the filling of the directional clutch to be engaged is evaluated as a function of a high pressure p5 of a high pressure range of the hydrostatic drive 5 of the line branching transmission 3 or an analogous operating variable of the vehicle drive train 1. In this case, a value range of the high pressure p5 or the analogous operating variable is in turn limited by an upper target curve p5sollo and a lower target curve p5sollu. Before a time T15 or during the second reversing phase RevStep1, during which the transmission capacity in the range of the directional clutch 6, 7 to be engaged increases, there is the possibility that the vehicle drive train 1 is in overrun or traction mode. From time T15 onwards, the vehicle drive train 1 transitions to traction mode or remains in traction mode, regardless of whether the vehicle drive train 1 was previously in overrun or traction mode.

[0085] In order to be able to carry out the desired evaluation of the filling of the directional coupling 6 or 7 to be engaged both before and after time T15, the target curves of the high pressure p5sollo and p5sollu have a different level before time T15 with otherwise identical gradients. The upper target curves before time T15 are identified by the reference symbols p5sollzo for traction operation and p5sollso for overrun operation. The lower target curves for traction and overrun operation are each identified by the reference symbols p5sollzu and p5sollsu. If the evaluation of the filling of the directional coupling 6 or 7 to be engaged is carried out according to the Fig. If the procedure shown in Figure 8 is carried out, a clutch that is too full is determined if values of the high pressure p5 are determined above the upper target curves p5sollzo, p5sollso, or p5sollo. In contrast, a direction-of-drive clutch 6 or 7 that is to be engaged is determined to be too empty if values of the high pressure p5 or the analogous operating variable are detected below the lower target curves p5sollzu, p5sollsu, or p5sollu over the operating time t in the area of the vehicle drive train 1.

[0086] Fig. 9 again shows several curves of various operating variables of the vehicle drive train 1 over the operating time t, which occur during a reversing process starting from a current direction of travel in the direction of a direction opposite to this during a defined operating state curve of the vehicle drive train 1. For the sake of clarity, the following description of Fig. 9 to Fig. 17, the direction clutch 6 is regarded as the direction clutch to be switched off, while the direction clutch 7 represents the direction clutch to be switched on.

[0087] At a time T20, a driver-side request for a change of direction is issued, which is why the status curve RevStep of the reversing process switches from the level of the sixth reversing phase RevStep5 to the level of the first reversing phase RevStep0. At the same time, the curve of the target actuation pressure p7soll of the direction clutch 7 to be engaged increases with a high gradient from time T20. The curve of the actual actuation pressure p7ist follows the curve of the target actuation pressure p7soll due to the hydraulic inertia of the hydraulic system of the line branching gearbox 3 to the extent shown. The target actuation pressure p7soll of the direction clutch 7 to be engaged is abruptly increased to the rapid filling pressure level from time T20 and reduced to the filling compensation level from time T21.At a time T22, the fill compensation phase of the directional coupling 7 to be engaged is completed, which is why the status curve RevStep rises from the level of the first reversing phase RevStep0 to the level of the second reversing phase RevStep1.

[0088] The curve of the target differential speed Δn7soll and the curve of the actual differential speed Δn7ist of the direction clutch 7 to be engaged correspond to the curve shown in Fig. 9, if the transmission capacity in the area of the directional clutch 7 to be engaged is established to the desired extent over the operating time t. At a time T23, a request is made to disengage or release the directional clutch 6 that has been engaged until then, which is why the target actuation pressure p6soll is reduced essentially abruptly at time T23. This leads to the curve of the actual actuation pressure p6ist also falling essentially abruptly with a time delay at a time T24. This in turn leads to the actual differential speed Δn6ist of the directional clutch 6 to be disengaged increasing to the extent shown from a time T25 following time T24.

[0089] A curve of the actual drive torque m2ist of the drive machine 2 has a substantially constant curve up to a time T26 and increases from time T26 onwards due to the increasing transmission capacity of the directional clutch 7 to be engaged, when the load transfer by the directional clutch 7 to be engaged increases from time T26 onwards to an extent required for high reversing quality. In addition, the curve of the actual speed n2ist of the drive machine 2 is also shown over the operating time t, which occurs during a reversing process characterized by high reversing quality.

[0090] At a further time T27 between times T21 and T22, a monitoring period is started due to an event described in more detail below, which ends at a time T28 between times T22 and T26.

[0091] During the monitoring period, it is checked whether the curve of the actual drive torque m2actual of the drive machine 2 rises above a threshold value m2actual threshold. Such a curve of the actual drive torque of the drive machine 2 is Fig. 9 is further identified under the reference symbol m2istV.

[0092] The curve m2istV of the actual drive torque of the prime mover 2 occurs when the directional clutch 7 to be engaged is filled during the filling phase or during the first reversing phase RevStep0 in such a way that the transmission capacity of the directional clutch 7 to be engaged increases accordingly before the transition from the first reversing phase RevStep0 to the second reversing phase RevStep1. In addition, to detect the fully engaged directional clutch 7, the occurrence of a differential speed Δn6ist of the directional clutch 6 to be disengaged is greater than a threshold value Δn6istschwell. This is a prerequisite for the fully evaluated directional clutch 7 to be engaged.

[0093] In this case, this condition is met at time T27, which is why the monitoring period starts at time T27.

[0094] In the representation according to Fig. 10 underlying procedure, a travel direction clutch 7 that is to be engaged too fully is detected during a monitoring period activated at time T22 if the course of the actual drive torque m2istV of the drive machine 2 is greater than a threshold value m2istschwell1.

[0095] Alternatively, it is also possible to use an offset value as the threshold value for evaluating the filling of the directional clutch 7 to be engaged, instead of the threshold value m2actualthreshold1, which represents an absolute value. An offset value as a triggering criterion offers the advantage that, in contrast to a fixed absolute threshold, the detection of an over-full directional clutch 7 to be engaged can be recognized earlier. This is the case, for example, if the initial value of the actual drive torque of the prime mover 2, from which the increase in the actual drive torque m2actual is monitored, is negative during overrun of the vehicle drive train 1 and the increase by the offset threshold value is exceeded earlier than the absolute threshold value m2actualthreshold1.

[0096] In this case, the monitoring period ends at time T29, since at time T29 it is assumed that the direction clutch 7 to be engaged transmits a torque from this point in time that is greater than the monitored threshold value m2actualthreshold1 or that at time T29 an increase in the actual drive torque m2actual is greater than the offset threshold value.

[0097] Another Fig. 9 corresponding illustration shows Fig. 11, whereby a too fully engaged travel direction clutch 7 is detected during a monitoring period activated at time T22 and ending at a time T34 before time T23, if the course of the actual drive torque m2istV exceeds a further threshold value m2istschwell2 within the monitoring period. The further threshold value m2istschwell2, which is smaller than the threshold value m2istschwell1, is monitored over a longer period than is the case with the procedure according to Fig. 10 is the case.

[0098] During the Fig. 12, the evaluation of the filling of the direction clutch 7 to be engaged is carried out as a function of the actual speed n2actual of the drive motor 2 in the manner described in more detail below. A too full direction clutch 7 to be engaged is detected if, during a monitoring process started at the beginning of the first reversing phase RevStep0, it is detected that the curve of the actual speed n2actual of the drive motor 2 drops by at least a threshold value n2actual and with a gradient greater than a threshold value. Such a curve of the actual speed n2actual is shown in Fig. 12 under the reference symbol n2istV between the times T35 and T36.

[0099] In addition, in order to detect a fully engaged travel direction clutch 7, it is necessary that the curve of the actual torque m2actual of the drive motor 2 exceeds a further threshold value m2actualthreshold3. Such a curve of the actual torque m2actualV is shown in Fig. 12 between time points T37 and T38.

[0100] In the case of Fig. 13, a too empty directional clutch 7 to be engaged is detected, since the curve of the actual differential speed Δn7actual lags behind the curve of the target differential speed Δn7target of the directional clutch 7 to be engaged, at least temporarily, by more than a threshold value Δn7threshold. Monitoring to determine whether the directional clutch 7 to be engaged was not sufficiently filled during the filling process starts with the start of the third reversing phase RevStep2 and ends with the sixth reversing phase RevStep5. For this purpose, the curve of the target speed Δn7target of the directional clutch 7 to be engaged is calculated.If the deviation between the actual differential speed Δn7actual and the target speed Δn7target is too large during the load transfer by the directional clutch 7 to be engaged, then the filling of the directional clutch 7 to be engaged is assessed as too low or the directional clutch 7 to be engaged is assessed as too empty.

[0101] In addition, the direction clutch 7 to be engaged is activated in the course of the operating variables of the vehicle drive train 1 during a further reversing process according to Fig. 14 is evaluated as too empty, since the curve of the actual drive torque m2actual of the drive machine 2 during a monitoring period started at time T24 does not exceed a further threshold value m2actualthreshold4 and the curve of the actual differential speed Δn7actual lags behind the curve of the target differential speed Δn7target of the direction clutch 7 to be engaged by more than a threshold value Δn7threshold1. The curve of the actual drive torque m2actual of the drive machine 2, which triggers an evaluation of the direction clutch 7 to be engaged as too empty, is in Fig. 14 under the reference symbol m2istL.

[0102] In the case of Fig. 15 shown curves of the operating variables of the vehicle drive train 1 underlying the reversing process, a determination of a too empty travel direction clutch 6 or 7 to be engaged is omitted, although the Fig. 14, the conditions for determining an empty directional clutch to be engaged are met during the monitoring period activated at time T24 and thereafter. This results from the fact that during a further monitoring period activated at time T22, the curve of the actual drive torque m2istL of the prime mover 2 exceeds a further threshold value m2istschwell5 before a change from the second reversing phase RevStep1 to the third reversing phase RevStep2.

[0103] Based on this event, it is assumed that the directional clutch 6 or 7 to be engaged was filled to the desired extent during the filling phase of the directional clutch 6 or 7 to be engaged, which was completed at time T22. Then, a reversing process with a desired high reversing quality can be implemented with the directional clutch 6 or 7 to be engaged, even if the threshold m2actualthreshold4 is not exceeded by the curve of the actual drive torque m2actualL during the further monitoring period activated at time T24.

[0104] During the Fig. 16, a further monitoring period is activated at time T39 after the expiration of a defined period after time T24. Both the start time and the duration of the further monitoring period can be parameterized depending on the currently requested aggressiveness with which the reversing process is to be carried out, and depending on the actual speed n2actual of the drive motor 2 and the actual speed of the output. In this case, the further monitoring period ends shortly before the transition from the third reversing phase RevStep2 to the fourth reversing phase RevStep3 at time T40.

[0105] If it is determined during the further monitoring period that the curve of the actual drive torque m2actual increases by more than a threshold value within a further defined shorter monitoring period t16, starting from the torque value determined at the beginning of the shorter monitoring period, a direction clutch 6 or 7 to be engaged that is too empty is determined. This in turn results from the fact that such an increase in the curve of the actual drive torque of the drive machine 2 is an indication that the transmission capacity of the direction clutch to be engaged only increases so suddenly in this reversing phase of the reversing process in question, since the curve of the actual actuation pressure of the direction clutch 6 or 7 to be engaged, filled to the requested extent, would have already caused an increase in the transmission capacity of the direction clutch 7 to be engaged at an earlier point in time.

[0106] Fig. 17 again shows a Fig. 9 shows a corresponding representation of various curves of operating variables of the vehicle drive train 1 that occur during a further reversing process. Between times T30 and T31, a curve m2ist17 that deviates from the conventional curve of the actual drive torque m2ist of the drive machine 2 is shown. The curve of the actual drive torque m2ist17 of the drive machine 2 increases more sharply at time T30 than the curve of the actual drive torque m2ist and reaches a maximum at time T32. From time T32, the curve m2ist17 of the actual drive torque decreases to a minimum up to time T33 and then increases again in the direction of the curve of the actual drive torque m2ist up to time T31 with a comparatively high gradient.

[0107] Basically, during the first three reversing phases RevStep0, RevStep1 and RevStep2, the maximum of the actual drive torque curve m2actual or m2actual17 is recorded. In addition, a comparison is made between this determined maximum and the current value of the actual drive torque. If the actual drive torque m2actual or m2actual17 exceeds a defined threshold during the reversing phases RevStep0 to RevStep2 and a drop greater than or equal to a defined offset threshold is determined starting from the determined maximum of the actual drive torque m2actual or m2actual17, then no empty directional clutch 6 or 7 is determined to be engaged. This is also the case although the Fig. 13, Fig. 14 and Fig. 16, the conditions described in more detail for detecting a direction of travel coupling 6 or 7 that is to be engaged too empty are met.

[0108] This procedure provides a simple way of avoiding incorrect detection of a travel direction coupling 6 or 7 that is too empty to be engaged.

[0109] In general, the method according to the invention is also suitable for other friction-locking switching elements, such as multi-disk switching elements or the like, which are involved, for example, in gear ratio changes or are intended to represent a connection between two rotatable components or between a rotatable component and a component fixed to the housing.

Claims

[1] Method for determining an operating state of frictionally engaged directional clutches (6, 7) of a vehicle drive train (1) with a prime mover (2), with a transmission (3) and with an output (4) during a reversing process of a vehicle equipped with the vehicle drive train (1), starting from a first direction of travel in the direction of a second direction of travel opposite thereto, during which a first directional clutch (6 or 7) connected to the power flow to represent the first direction of travel is disconnected from the power flow and a second directional clutch (7 or 6) provided to represent the second direction of travel and disconnected from the power flow is connected to the power flow, wherein a profile of at least one operating variable (Δn6actual, Δn7actual, m2actual, n2actual, p5, n4, i3) of the vehicle drive train is monitored during a reversing process, and wherein, depending on threshold values (Δn6, 7desired, Δn6,7sollu,Δn6actual threshold, Δn7actual threshold, Δn7actual1, i3target, i3target, m2actual threshold1 to m2actual threshold5, m2target, m2target, n2actual, n2target, n4target, n4target, p5target, p5target, p5targetso, p5targetsu, p5targetzo, p5targetzu) of the operating variable and / or threshold values of the gradient of the course of the operating variable (Δn6actual, Δn7actual, m2actual, n2actual, p5, n4, i3) of the vehicle drive train (1) within defined monitoring periods, at least partial load transfer by the directional clutch (6 or 7) to be engaged is determined before a defined time (T3), characterized bythat depending on threshold values (Δn6,7sollo, Δn6,7sollu, Δn6istschwell, Δn7schwell, Δn7schwell1, i3sollo, i3sollu, m2istschwell1 to m2istschwell5, m2sollo, m2sollu, n2istschwell, n2sollo, n2sollu, n4sollo, n4sollu, p5sollo, p5sollu, p5sollso, p5sollsu, p5sollzo, p5sollzu) of the operating variable and / or threshold values of the gradient of the course of the operating variable (Δn6ist, Δn7ist, m2ist, n2ist, p5, n4, i3) of the vehicle drive train (1) within defined monitoring periods, after a further defined time (T3), at least partial load transfer by the to be connected Direction of travel coupling (6 or 7) is determined. [2] Method according to claim 1, characterized bythat the direction of travel clutch (6 or 7) to be engaged in each case when a driver-side request for a reversing operation is present is prepared for engagement in the power flow during an adaptable filling phase depending on a current filling level (BG6,7), wherein the filling phase comprises a rapid filling phase and a filling equalization phase and the direction of travel clutch to be engaged is pre-filled during the rapid filling phase with a rapid filling pressure pulse with a defined rapid filling pressure (p6sollSF, P7sollSF) over a defined rapid filling time and, during a subsequent filling equalization phase, is subjected to a defined filling equalization pressure (p6sollFA, P7sollFA) over a filling equalization time, which is lower than the rapid filling pressure (p6sollSF, P7sollSF), and wherein the direction of travel clutch (6 or 7) to be engaged in each case is in an operating state at the end of the filling phase,in which their transmission capacity is ideally zero and an increase in the actual actuating pressure (p6ist, p7ist) of the direction clutch (6 or 7) to be engaged results in an immediate increase in the transmission capacity of the direction clutch (6 or 7) to be engaged. [3] Method according to claim 1 or 2, characterized bythat an at least partial load transfer by the directional clutch (6 or 7) to be engaged before the defined time (T3) is determined when a differential speed (Δn6ist) of the first directional clutch (6) engaged in the power flow as required increases to a value greater than a defined threshold value (Δn6istschwell) during the filling phase of the second directional clutch (7) to be engaged and during a monitoring period which begins when the defined threshold value (Δn6istschwell) of the differential speed (Δn6ist) of the engaged first directional clutch (6) is reached and ends at a defined time (T28),in addition, an increase in an actual torque (m2actual) present in the area of the switching element half of the second travel direction clutch (7) to be engaged, which is operatively connected to the drive engine, is greater than a threshold value (m2actual threshold) or an increase to a predefined threshold value is determined. [4] Method according to one of claims 1 to 3, characterized bythat an at least partial load transfer by the directional clutch (7) to be engaged before the defined time (T3) is determined if, during a monitoring period which begins at a time (T22) at which a filling phase of the second directional clutch (7) to be engaged has ended and ends at the latest at a defined time (T29), an increase in an actual torque (m2actual) present in the area of the switching element half of the second directional clutch (7) to be engaged, which is operatively connected to the drive machine (2), is greater than a threshold value (m2actualthreshold1), or an increase to a predefined threshold value is determined. [5] Method according to one of claims 1 to 4, characterized bythat an at least partial load transfer by the directional clutch (7) to be engaged before the defined time (T3) is determined if, during a monitoring period which begins at a time (T22) at which the filling phase of the second directional clutch (7) to be engaged has ended and ends at the latest at a defined time (T34), an increase in an actual torque (m2actual) present in the area of the switching element half of the second directional clutch (7) to be engaged, which is in operative connection with the drive machine (2), is determined to be greater than a threshold value (m2actual_threshold2), starting from the value of the actual torque (m2actual) present at the beginning of the monitoring period. [6] Method according to one of claims 1 to 5, characterized bythat an at least partial load transfer by the second direction clutch (7) to be engaged is determined before the defined time (T3) if, during the filling phase of the second direction clutch (7) to be engaged, a negative gradient of a profile of the speed (n2actual) of the drive machine (2) is smaller than a threshold value and, in addition, a decrease in the speed (n2actual) of the drive machine (2) is greater than a threshold value (n2actual threshold) and an actual torque (m2actual) present in the region of the switching element half of the second direction clutch (7) to be engaged, which is operatively connected to the drive machine (2), is greater than a threshold value (m2actual threshold3). [7] Method according to one of claims 1 to 6, characterized bythat an at least partial load transfer by the second direction clutch (7) to be engaged is determined after the further defined time (T3) if a profile of the actual differential speed (Δn7ist) of the second direction clutch (7) to be engaged deviates from a profile of the target differential speed (Δn7soll) during a load transfer phase and after a further defined time of the second direction clutch (7) to be engaged by more than a threshold value. [8] Method according to one of claims 1 to 7, characterized bythat an at least partial load transfer by the directional clutch (7) to be engaged after the further defined time (T3) is determined if, during a monitoring period which begins at a time (T24) at which an increase in the actual transmission capacity of the second directional clutch to be engaged is expected and ends at a defined time, an increase in an actual torque (m2actual) present in the area of the shift element half of the second directional clutch (7) to be engaged, which is operatively connected to the drive machine (2), is determined to be smaller than a threshold value (m2actual_threshold4), starting from the value of the actual torque (m2actual) present at the beginning of the monitoring period, and a deviation between an actual differential speed (Δn7actual) and a target differential speed (Δn7target) of the second directional clutch to be engaged is greater than a threshold value (Δn7threshold1). [9] Method according to one of claims 1 to 8, characterized by that the determination of the at least partial load transfer after the further defined time (T3) is omitted if a torque (m2actual) applied to the switching element half of the second direction clutch (7) to be engaged, which is operatively connected to the drive machine (2), is determined to be greater than a threshold value during a monitoring period which starts at a time (T22) at which the filling phase of the second direction clutch (7) to be engaged has ended and ends at a time at which an actual transmission capacity of the second direction clutch (7) to be engaged is expected to be greater than a threshold value (m2actualthreshold5). [10] Method according to one of claims 1 to 9, characterized bythat an at least partial load transfer by the second directional clutch (7) to be engaged after the further defined time (T3) is determined if, during a monitoring period which begins at a time following a time (T24) at which an increase in the actual transmission capacity of the second directional clutch (7) to be engaged is expected and which ends at the latest when a speed (n4) of the output (4) is reached which is equal to zero, an increase in an actual torque (m2actual) present in the region of the switching element half of the second directional clutch (7) to be engaged, which is in operative connection with the drive machine (2), is determined within a further shorter monitoring period, starting from a value of the actual torque (m2actual) present at the beginning of the further shorter monitoring period, to a threshold value. [11] Method according to one of claims 1 to 10, characterized bythat during the reversing process until the change of direction is reached, the torque (m2ist) currently applied to the switching element half of the second direction clutch (7) to be engaged, which is operatively connected to the drive machine (2), is determined and the maximum of this torque (m2ist) is determined, wherein the determination of the at least partial load transfer after the further defined time (T3) is omitted if a drop in the torque (m2ist) applied to the switching element half of the second direction clutch (7) to be engaged, which is operatively connected to the drive machine (2), is determined to be greater than a threshold value compared to the maximum. [12] Method according to one of claims 1 to 11, characterized bythat the threshold values (Δn6,7sollo, Δn6,7sollu, Δn6istschwell, Δn7schwell, Δn7schwell1, i3sollo, i3sollu, m2istschwell1 to m2istschwell5, m2sollo, m2sollu, n2istschwell, n2sollo, n2sollu, n4sollo, n4sollu, p5sollo, p5sollu, p5sollso, p5sollsu, p5sollzo, p5sollzu) are varied depending on the speed (n4) of the output (4) and / or a driver-side request as to the gradient of the speed (n4) of the output (4) with which the reversing process is to be carried out until the vehicle comes to a standstill and also thereafter. [13] Method according to one of claims 1 to 12, characterized by that predefined values of at least one operating variable (Δn6ist, Δn7ist, m2ist, n2ist) are used as threshold values (Δn6ist, Δn7ist, Δn7ist1, m2ist, m2ist1 to m2ist, n2ist). [14] Method according to one of claims 1 to 13, characterized bythat the sum of offset values and values of the operating variable (Δn6ist, Δn7ist, m2ist, n2ist, p5, n4, i3) at the beginning of a monitoring period or at least one value of the operating variable (Δn6ist, Δn7ist, m2ist, n2ist, p5, n4, i3) before the start of the respective monitoring period are used as threshold values. [15] Method according to one of claims 1 to 14, characterized by that target curves are used as threshold values (Δn6,7sollo, Δn6,7sollu, i3sollo, i3sollu, m2sollo, m2sollu, n2sollo, n2sollu, n4sollo, n4sollu, p5sollo, p5sollu, p5sollso, p5sollsu, p5sollzo, p5sollzu) which limit the value ranges of at least one operating variable (Δn6ist, Δn7ist, m2ist, n2ist, p5, n4, i3). [16] Method according to one of claims 1 to 15, characterized bythat an at least partial load transfer by the second direction clutch (7) to be engaged is determined before the defined time (T3) if the torque (m2actual) applied to the switching element half of the second direction clutch (7) to be engaged, which is operatively connected to the drive machine (2), is above a value range defined by the target curves (m2sollo, m2sollu) during a monitoring period which begins with the filling phase of the second direction clutch to be engaged and which ends at a defined time. [17] Method according to one of claims 1 to 16 characterized bythat an at least partial load transfer by the second direction clutch (7) to be engaged is determined after the defined further time (T3) if the torque (m2actual) applied to the switching element half of the second direction clutch (7) to be engaged, which is in operative connection with the drive machine (2), is below the value range defined by the target curves (m2sollo, m2sollu) during a monitoring period which begins at a time at which an increase in the actual transmission capacity of the second direction clutch (7) to be engaged is expected. [18] Method according to one of claims 1 to 17, characterized bythat an at least partial load transfer by the second direction clutch (7) to be engaged is determined before the defined time (T3) if the differential speed (Δn6,7) in the area of the second direction clutch (6 or 7) to be engaged is below a value range of the differential speed (Δn6,7) defined by the target curves (Δn6,7sollo, Δn6,7sollu) during a monitoring period which begins with the start of the filling phase of the second direction clutch (6 or 7) to be engaged and which ends at a time at which an increase in the actual transmission capacity of the second direction clutch (6 or 7) to be engaged is expected. [19] Method according to one of claims 1 to 18, characterized bythat after the defined further time (T3) at least partial load transfer by the second direction clutch (6 or 7) to be engaged is determined if the differential speed (Δn6,7) in the area of the second direction clutch (6 or 7) to be engaged is above the value range of the differential speed (Δn6,7) defined by the target curves (Δn6,7sollo, Δn6,7sollu) during a monitoring period which begins at a time at which an increase in the actual transmission capacity of the second direction clutch (7 or 6) to be engaged is expected. [20] Method according to one of claims 1 to 19, characterized bythat an at least partial load transfer by the second direction clutch (7) to be engaged is determined before the defined time (T3) if the speed (n2) of the drive machine (2) is below a value range of the speed (n2) of the drive machine (2) defined by the target curves (n2sollo, n2sollu) during a monitoring period which begins with the start of the filling phase of the second direction clutch (7) to be engaged and which ends at a time at which an increase in the actual transmission capacity of the second direction clutch (7) to be engaged is expected. [21] Method according to one of claims 1 to 20, characterized bythat an at least partial load transfer by the second direction clutch (7) to be engaged is determined after the defined further time (T3) if the speed (n2) of the drive machine (2) in the range of the second direction clutch (7) to be engaged is above the value range of the speed (n2) of the drive machine (2) defined by the target curves (n2sollo, n2sollu) during a monitoring period which begins from a time at which an increase in the actual transmission capacity of the second direction clutch (7) to be engaged is expected. [22] Method according to one of claims 1 to 21, characterized bythat an at least partial load transfer by the second direction clutch (7) to be engaged is determined before the defined time (T3) if the speed (n4) of the output (4) or a gear ratio (i3) of the transmission (3) of the vehicle drive train (1) lies below a value range of the speed (n4) of the output (4) or the gear ratio (i3) of the transmission (3) defined by the target curves (n4sollo, n4sollu, i3sollo, i3sollu) during a monitoring period which begins with the start of the filling phase of the second direction clutch (7) to be engaged and which ends at a time at which an increase in the actual transmission capacity of the second direction clutch (7) to be engaged is expected. [23] Method according to one of claims 1 to 22, characterized bythat an at least partial load transfer by the second direction clutch (7) to be engaged is determined after the defined further time (T3) if the speed (n4) of the output or the gear ratio (i3) of the transmission (3) is above a value range of the speed (n4) of the output (4) or the gear ratio (i3) of the transmission (3) defined by the target curves (n4sollo, n4sollu, i3sollo, i3sollu) during a monitoring period which begins at a time at which a filling phase of the second direction clutch (7) to be engaged starts and which ends at a time at which an increase in the actual transmission capacity of the second direction clutch (7) to be engaged is expected. [24] Method according to one of claims 1 to 23, characterized bythat an at least partial load transfer by the second directional clutch (7) to be engaged is determined before the defined time (T3) if a high pressure (p5) in a high pressure range of a hydrostatic transmission (5) of a continuously variable transmission (3) of the vehicle drive train (1) or an operating variable of the vehicle drive train (1) analogous thereto lies above a value range of the high pressure (p5) or the operating variable of the vehicle drive train (1) analogous thereto, during a monitoring period which begins at a time at which a filling phase of the second directional clutch (7) to be engaged starts and which ends at a time at which an increase in the actual transmission capacity of the second directional clutch (7) to be engaged is expected, during overrun or traction operation of the vehicle drive train. [25] Method according to one of claims 1 to 24, characterized by that an at least partial load transfer by the second directional clutch (7) to be engaged is determined after the defined further time (T3) if a high pressure (p5) in a high pressure range of a hydrostat (5) of a continuously variable transmission (3) of the vehicle drive train (1) or an operating variable of the vehicle drive train (1) analogous thereto lies below a value range of the high pressure (p5) or the operating variable of the vehicle drive train analogous thereto, defined by the target curves (p5sollo, p5sollu, p5sollso, p5sollsu, p5sollzo, p5sollzu), during a monitoring period which begins at a time at which a filling phase of the second directional clutch (7) to be engaged starts and which ends at a time at which an increase in the actual transmission capacity of the second directional clutch (7) to be engaged is expected. [26] Method according to one of claims 1 to 25, characterized by that the determination of the at least partial load transfer of the second travel direction coupling (7) to be engaged takes place before the defined time (T3) and after the defined further time (T3) if - an operating temperature of the gearbox (3) is greater than a threshold value and / or - a working hydraulic system or other units of the vehicle equipped with the vehicle drive train (1) to be subjected to torque by the drive engine are deactivated and / or - a residual filling level of the second travel direction clutch (7) to be engaged is less than a threshold value and / or - the filling compensation phase of the filling phase of the second travel direction coupling (7) to be engaged is shorter than a threshold value and / or - an unsteady course of the speed (n4) of the output (4) occurs during the reversing process and / or - the reversing process occurs when the vehicle’s longitudinal inclination is less than a threshold value and / or - an actual curve of the high pressure (p5) of the hydrostat (5) corresponds to a target curve.

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