Method for determining a target value of an actuation current

Abstract

Method for determining a target value (i9target(T6), i9target(T8)) of an actuating current (i9) corresponding to a defined operating point of an electro-hydraulically actuated friction-locked switching element (9, 10) of a continuously variable transmission (3), for which the transmission capacity of the switching element (9, 10) is essentially zero and from which an increase in an actuating force results in an immediate increase in the transmission capacity, wherein one half of the switching element is connected to a transmission input (6) and the other half of the switching element (9, 10) can be coupled to a transmission output (4), characterized in that the target value (i9target) of the actuating current (i9) of the switching element (9, 10) which is essentially in the closed operating state,10) when the other half of the switching element is decoupled from the transmission output and the transmission input speed is greater than a defined threshold, the speed difference (Δn) between the speeds of the switching element halves exceeds a predefined limit (Δnlimit1), wherein the target value (i9target(T6)) of the actuation current (i9) at time (T6) when the limit (Δnlimit1) is exceeded is the target value (i9target(T6)) of the actuation current (i9) corresponding to the defined operating point of the switching element (9, 10).

Description

The invention relates to a method for determining a target value of an actuating current corresponding to a defined operating point of an electro-hydraulically actuated friction-locked switching element of a continuously variable transmission according to the type defined in more detail in the preamble of claim 1. In conventional continuously variable transmissions (CVTs) and construction machinery transmissions, electro-hydraulically actuated friction-locked switching elements are typically used. These elements are filled with hydraulic fluid in the piston chambers and pressurized accordingly. The fill level of the piston chamber of such a switching element significantly influences its transmission capacity and the torque it can transmit.In addition, the actuation sequence of a switching element, starting from an open operating state of the switching element, to which essentially no torque can be applied via the switching element and whose transmission capacity is essentially zero, in the direction of an operating state in which torque can be transmitted via the switching element in a slipping state or in a slip-free state, affects the closing behavior of the switching element and thus also the switching comfort, which in turn is largely determined by the torque present in the area of ​​an output of a vehicle or its curve. Since manufacturing tolerances of components used in shift elements, as well as of hydraulic lines manufactured in series, are known to vary undesirably, individual calibration must be performed for each shift element to determine its specific filling behavior and to ensure it can be actuated to the required extent for optimal shifting comfort. The parameters characterizing the respective clutch filling, determined through calibration, are stored in the non-volatile memory of a transmission control unit and are used to actuate each shift element during every gear change. Two parameters are particularly 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 rapid filling pulse to fill it within a short operating time. During this time, the piston chamber of the switching element is pressurized with a defined rapid filling pressure. This rapid filling phase is followed by a so-called filling equalization phase, during which the actuating pressure in the piston chamber is reduced from the level of the rapid filling pressure to a level of a filling equalization pressure (represented by the second parameter) and maintained for a defined filling equalization time.At the end of the filling equalization phase, the switching element is ideally in a defined operating state in which the transmission capability of the switching element is essentially zero and from which an increase in an actuating force of the switching element results in an immediate increase in the transmission capability of the switching element. From DE 10 2011 017 515 A1, a method for determining the flow rate of a friction-fit switching element of an automatic transmission is known. The automatic transmission has a hydrodynamic converter, at which a speed ratio is established, and includes an output shaft. In a control sequence, with the output shaft at a fixed position, the transmission capacity of the respective friction-fit switching element, and thus the speed ratio, is influenced by changing the control current. The flow rate of the friction-fit switching element is determined from the speed ratio profile resulting during the control sequence in conjunction with the profile of the control current. DE 10 2008 000 304 A1 describes a method for operating a valve assembly of a transmission unit of a vehicle powertrain, comprising a valve unit and an electromagnetic actuator that actuates the valve unit. The valve unit allows for the adjustment of a hydraulic pressure value of a hydraulic system of a transmission unit of a vehicle powertrain, depending on a pulse-width modulated current signal applied to the electromagnetic actuator. During dither modulation, the amplitude and / or pulse duration of the pulse-width modulated current signal is periodically varied while maintaining the set pressure value. This imparts an oscillating motion to a longitudinally movable armature and a valve tappet connected to it within the valve assembly.Dither modulation is activated or deactivated depending on the presence of predefined operating states of the vehicle powertrain. A transmission control circuit for controlling a system pressure provided by a pressure pump in a transmission of a working machine, comprising a pressure regulating valve for adjusting the provided system pressure to a maximum system pressure, an electronic control unit and a pressure limiting valve that can be controlled by the control unit, is known from DE 10 2013 205 358 A1. A pressure regulating valve, as specified in EP 1 229 273 A2, converts a control current into an actuating pressure for a gear actuator. Both the current control current and the actuating pressure are fed back to a controller. The controller converts a setpoint for the actuating pressure into the control current within a specific range of the torque to be transmitted, depending on the current control current, and otherwise depending on the current actuating pressure. The current measuring device for determining the current control current is calibrated to achieve high resolution within this specific range. Within the framework of a method known from DE 10051 537 A1 for the automated determination of the rapid filling time and the filling equalization pressure of switching elements, whose filling and application process can be divided into a rapid filling phase and a filling equalization phase, the rapid filling time and the filling equalization pressure are determined by an indirect measurement in one pass. From DE 100 51 537 A1, a method for the automated determination of the rapid filling time and the filling equalization pressure of switching elements is known, whose filling and application process can be divided into a rapid filling phase and a filling equalization phase. The rapid filling time and the filling equalization pressure are determined by an indirect measurement in a single pass. Unfortunately, the known method is only partially suitable for bringing a switching element into the defined operating state during operation, in which the switching element's transmission capacity is essentially zero and from which an increase in actuating force results in an immediate increase in transmission capacity. This stems from the fact that the electro-hydraulic control system typically used to actuate a switching element exhibits considerable tolerances, particularly in the area of ​​the electrical components or the current setting. Consequently, a setpoint for an actuating current of a switching element required to set the determined filling pressure is highly unlikely to correspond to the current value required to achieve the determined filling pressure. Unlike passenger car transmissions, which are designed with hydrodynamic torque converters and associated converter lock-up clutches, and which, when the converter lock-up clutch is open, allow for a correspondingly smooth or delayed response of the drivetrain to torque changes transmitted within the drivetrain, torque jumps in vehicle drivetrains with continuously variable transmissions, which are correspondingly rigid, inevitably lead to shift jolts. Therefore, inaccurate actuation of an electro-hydraulically actuated friction-locked switching element inevitably impairs the shifting and driving comfort of a construction machine to an undesirable extent. The present invention is therefore based on the objective of providing a method for determining a target value of an actuating current corresponding to the defined operating point of an electro-hydraulically actuated friction-locked switching element of a continuously variable transmission, in order to be able to operate a vehicle equipped with a continuously variable transmission, such as a construction machine, with a desired high level of shifting and driving comfort. According to the invention, this problem is solved by a method having the features of claim 1. In the inventive method for determining a target value of an actuating current corresponding to a defined operating point of an electro-hydraulically actuated friction-locked switching element of a continuously variable transmission, for which the transmission capability of the switching element is essentially zero and from which an increase in an actuating force of the switching element results in an immediate increase in the transmission capability, wherein one half of the switching element is connected to a transmission input and the other half of the switching element can be coupled to a transmission output, the target value of the actuating current of the switching element, which is essentially in the closed operating state, is reduced when the other half of the switching element is decoupled from the transmission output and at a transmission input speed greater than a defined threshold.until a speed difference between the speeds of the switching element halves exceeds a predefined limit, wherein the target value of the actuating current at the time the limit is exceeded is the target value of the actuating current corresponding to the defined operating point of the switching element. The inventive method takes into account the entire actuation chain of a friction-locked switching element of a continuously variable transmission, which includes electrical, hydraulic, and mechanical actuators, when determining the target value of the actuation current of the switching element, corresponding to the defined operating point of the switching element. This means that the target value of the actuation current, corresponding to the filling pressure of the switching element, is precisely determined as a function of the current output of a control unit up to the point of filling the switching element and is used during actuation of the switching element. By precisely determining the relationship between actuation current and actuation pressure, a switching element can be actuated with minimal effort to the extent required for a high level of shifting comfort. The method according to the invention can preferably be used in vehicle drive trains with continuously variable transmissions (CVTs), but it is also possible to use the method according to the invention in vehicle drive trains with transmissions with shift clutches, such as transmissions with a hydrodynamic torque converter, preferably with a closed converter lock-up clutch. The transmission also has a friction-based shifting element. In a simple variant of the method according to the invention, a target value of a hydraulic actuating pressure of the switching element is set in the area of ​​a valve device depending on the target value of the actuating current, which can be applied in the area of ​​a piston chamber of the switching element. If the switching element is first moved from an open operating state, in which the piston chamber is essentially completely emptied, to a closed operating state by guiding the target value of the actuating current and thereby applying a pressure pulse in the area of ​​the piston chamber with a defined pressure level of the actuating pressure and over a defined operating time, the method according to the invention can be carried out within short and reproducible operating times. In one variant of the inventive method, the operating time required to determine the target value of the actuating current corresponding to the defined operating point is reduced by lowering the target value of the actuating current, preferably via a ramp, from the level of the pressure pulse to a level at which the switching element is still in the closed state, before the closed operating state of the switching element is reached, in which the differential speed between the switching element halves is essentially zero, and from which the target value of the actuating current is reduced via a ramp until the speed difference between the speeds of the switching element halves exceeds the predefined limit value. If the target value of the actuating current is set to a level for a predefined period from the time at which the speed difference between the speeds of the switching element halves exceeds the predefined limit, at which the speed difference between the speeds of the switching element halves is greater than an additional limit, which in turn is greater than the predefined limit, it is ensured with little effort that the switching element transitions to its fully open operating state. In a further advantageous embodiment of the method according to the invention, the target value of the actuating current is increased again via a ramp after the predefined period has elapsed, until the speed difference between the speeds of the switching element halves falls below a further predefined limit value, wherein the target value of the actuating current at the time at which the further predefined limit value is undercut is also a target value of the actuating current corresponding to the defined operating point of the switching element. This allows for the determination of an offset value for both branches of the hysteresis curve relating the actuation current and actuation pressure of an electro-hydraulically actuated friction-locked switching element between the actual behavior of the actuation chain of such a switching element and empirically determined so-called ip characteristic curves on the test bench. These values ​​can be directly incorporated into the control system. Alternatively, it is also possible to shift an already defined relationship between the actuation current and the actuation pressure of a switching element to a suitable extent, depending on the values ​​determined during the calibration according to the invention. In a further advantageous variant of the method according to the invention, the target value of the actuating current is reduced to a level at which the switching element transitions into its fully open operating state, after determining the target value of the actuating current corresponding to the defined operating point of the switching element. In a particularly simple variant of the method according to the invention, the rotational speeds of the switching element halves are determined by measurement. If the limit values ​​are determined empirically, the method according to the invention can be carried out with low computing power. In another variant of the method according to the invention, which can also be carried out with low computing power, the switching element is actuated via characteristic maps that depict a relationship between target values ​​of the actuating current, target values ​​of the actuating pressure and an operating temperature of the gearbox, which are determined empirically. If the switching element is actuated via characteristic curves that depict a relationship between target values ​​of the actuating current and target values ​​of the actuating pressure, which are determined empirically, the switching element can also be actuated with minimal effort. In a further variant of the inventive method, the characteristic curves or maps are adapted with minimal effort depending on a deviation between the determined target value of the actuating current corresponding to the defined operating point of the switching element and the target value of the actuating current that can be taken from the maps or curves for the defined operating point of the switching element, whereby the switching element can be actuated to an extent that is adapted, for example, to conditions that change over the service life of a gearbox. The procedure described above can be automated both at the end of a transmission production line and later in the vehicle. Calibration in the vehicle can be automatically activated and performed after defined operating times and / or manually by an operator as needed. Regardless of the method, the current calibration status can be communicated via an internal vehicle CAN bus. Both the features specified in the patent claims and the features specified in the following embodiment of the invention are each suitable, either individually or in any combination, to further develop the invention. Further advantages and beneficial developments of the invention result from the patent claims and the exemplary embodiment described in principle with reference to the drawing. It shows: Fig. 1 a highly schematic block diagram of a vehicle powertrain with a continuously variable power-split transmission; Fig. 2 a gear diagram of a continuously variable power-split transmission; Fig. 3 an actuation chain of one of the switching elements of the transmission according to Fig. 2; and Fig. 4 several curves of various operating state variables of the transmission according to Fig. 2 over time t, which occur during the execution of the procedure according to the invention. Fig. 1 shows a schematic representation of a vehicle drive train 1 with a drive unit 2 and with a continuously variable power-split transmission 3 that can be coupled to it. The drive unit 2 is designed in this case as an internal combustion engine, preferably as a diesel internal combustion engine, and in further embodiments of the vehicle drive train 1 can also be designed as an electric machine or as a combination of an internal combustion engine of any type and an electric machine. On the output side, the gearbox 3 is operatively connected to an output 4, whereby a drive torque provided by the drive unit 2 is converted, depending on the gear ratio set in the gearbox 3, into an output torque in the output 4, representing the corresponding tractive force. In the area between the drive unit 2 and the gearbox 3, a power take-off 5 or a working hydraulic system can be supplied with torque from the drive unit 2. Figure 2 shows a gear diagram of a possible embodiment of the transmission 3 according to Figure 1, which is rotationally fixed to the drive unit 2 in the area of ​​a transmission input shaft or a transmission input 6. The transmission input shaft 6 drives the auxiliary output 5, a further auxiliary output 8, and first switching element halves of friction-fit switching elements 9, 10 via a fixed gear 7 and a fixed gear 8A. The friction-fit switching element 9 is arranged coaxially with the transmission input shaft 6, while the friction-fit switching element 10, or the reverse direction clutch, is positioned on the shaft of the auxiliary output 5, which is arranged coaxially with the transmission input shaft 6. In the closed operating state of the friction-fit switching element 9, or the reverse direction clutch, the transmission is engaged.In the forward direction clutch, the transmission input shaft 6 drives a loose gear 12, which is rotatably mounted on the transmission input shaft 6, via a loose gear 11. The loose gear 12 is rotationally fixed to a planet carrier 13. In the closed operating state of the friction-fit switching element 10, the transmission input shaft 6 drives the loose gear 12 via a loose gear 14. Several double planetary gears 15 are rotatably mounted on the planet carrier 13. The double planetary gears 15 mesh with a first sun gear 16, a second sun gear 17, and a ring gear 18. The first sun gear 16 is rotationally fixed to a shaft 19 of a first hydraulic unit 20 of a hydrostatic unit 21. The ring gear 18 is operatively connected via a fixed gear 22 and a fixed gear 23 to a shaft 24 of a second hydraulic unit 25 of the hydrostatic unit 21. Within the continuously variable power-split transmission 3, several transmission ranges are adjustable, within which the transmission ratio of the transmission 3 can be continuously varied by adjusting the hydrostatic unit 21. Independently of the representation according to Fig. 2, the transmission 3 can be configured as either a primary or a secondary coupled continuously variable power-split transmission, whereby the power splitting can be achieved hydraulically, electrically, or by a combination thereof. A transmission output or transmission output shaft 26 of the transmission 3 can be connected to the second shaft 24 of the hydrostatic unit 21 via a friction-fit switching element 27 arranged coaxially to the transmission output shaft 26 for a first driving range of the transmission 3, a loose gear 28, and a fixed gear 29. Furthermore, the transmission output shaft 26 can be coupled to the second sun gear 17 via a fixed gear 30, a fixed gear 31, and another friction-fit switching element 32 for a second driving range of the transmission 3, as well as a loose gear 33 and a fixed gear 34. The fixed gear 34 is arranged coaxially with the second sun gear 17, while the fixed gear 31, the friction-fit switching element 32 for the second driving range, and the loose gear 33 are arranged coaxially with each other. The fixed gear 30, the friction-locked switching element 27 for the first driving range and the loose gear 28 are in turn provided coaxially to the transmission output shaft 26.In addition, the fixed gear 30 meshes with both the fixed gear 31 and the fixed gear 34 of a shaft 35, which in turn can be connected to the driven vehicle axle or to several driven vehicle axles of the output 4. The direction-of-travel clutches 9 and 10 are designed as wet clutches, which are not only intended to establish the power flow between the drive unit 2 and the output 4, but also simultaneously determine the direction of travel. Due to their capacitive design, the friction-locked switching elements 9 and 10 of the vehicle drivetrain 1, as shown in Fig. 2, can also be used as starting elements. This is the case when a driver, starting from a neutral operating state of the transmission 3, in which the switching elements 27 and 32 are open, selects a direction of travel and simultaneously depresses an accelerator pedal to request a speed. The friction-locked switching elements 9 and 10 are designed in such a way that a change of direction, or a so-called reversing process, is also possible via them from higher speeds in either the forward or reverse direction. During such a reversing process, the vehicle speed is initially reduced from its current speed towards zero. For this purpose, the transmission capability of both the friction-locked switching element 9 and the friction-locked switching element 10 are adjusted accordingly. During the reversing process, the two friction-locked switching elements 9 and 10 operate predominantly in a slipping manner. Once the vehicle speed is essentially zero, the transmission capabilities of the two switching elements 9 and 10 are adjusted so that the vehicle moves forward in the opposite direction to its previous direction of travel until the desired vehicle speed is reached. To enable a start-up process from a standstill and the neutral operating state of the transmission 3 within short operating times and essentially without delay, the shift element 27 of the first gear ratio of the transmission 3 is closed, and, depending on the driver's request for forward or reverse travel, the shift element 9 or the shift element 10 is additionally moved to its closed operating state. During the engagement of the shift element 27 and the shift element 9 or 10, the two hydraulic units 20 and 25 are adjusted via an adjustable yoke 36 such that the desired starting gear ratio is set in the transmission 3. The transmission capacity of the friction-fit shift element 9 or 10 is set to values ​​greater than zero during the setting of the starting gear ratio of the transmission 3 in order to achieve a connection with the vehicle drivetrain 1 as shown in Fig.1. The vehicle being driven is already in motion during a closing process of the friction-locking switching element 9 or 10. Fig. 3 shows part of an electro-hydraulic control device 37, via which, among other things, the friction-fit switching elements 9 and 10 can be actuated to implement the functionalities described above. In this case, to actuate the switching element 9, a setpoint i9 for the actuating current i9 of the switching element 9 is output by an electrical control unit 38 and applied to a valve assembly 39. Thus, an actuating pressure p9, dependent on the setpoint i9 for the actuating current i9, is set in the valve assembly 39 and is applied to the switching element 9 or to a piston chamber of the switching element 9. The valve assembly 39 comprises a proportional pressure regulator 40, the valve spool 41 of which can be displaced by an electromagnet 42 against the spring force of a spring assembly 43. The electromagnet 42 is actuated depending on the setpoint i9 of the actuating current i9. A supply pressure signal p_red or a reducing pressure is present in the area of ​​the proportional pressure regulator 40, which, depending on the setpoint i9 of the actuating current i9, can be applied to a hydraulic amplifier 44 of the valve assembly 39 in the area of ​​a control surface 45 of a valve spool 46 of the hydraulic amplifier 44 at the respective levels required for actuating the switching element 9.Additionally, a supply pressure signal p_sys is present at the hydraulic amplifier 44, which corresponds to a system pressure of a primary pressure circuit of the electro-hydraulic control and regulating device 37 and which, due to the valve amplification present in the area of ​​the hydraulic amplifier 44, can be increased by a factor of 2.7 in the present case as actuation pressure p9 at the switching element 9. Depending on the specific application, it may also be possible for the switching element 9 to be actuated directly, i.e. without the additional hydraulic amplifier, by the system pressure p_sys, if this pressure is sufficiently high. Since all components of the actuation chain or control loop of the switching element 9 shown in Fig. 3, and also the switching element 9 itself, are characterized by manufacturing tolerances or component tolerances, and since the component tolerances in the area of ​​the electrical control unit 38, the proportional pressure regulator 40, the hydraulic amplifier 44, the switching element 9 and the hydraulic lines connecting the hydraulic components are connected in series and have a non-negligible influence on the actuation of the switching element 9, the procedure described in more detail below is carried out in order to be able to actuate the switching element 9 to the desired extent independently of the tolerance chain. When a corresponding request exists to actuate the switching element 9, a corresponding actuation pressure p9 must be applied in the area of ​​the switching element 9. For this purpose, the actuation current i9 corresponding to the requested actuation pressure p9 is selected using so-called ip characteristic curves ipup and ipdown. The ipup characteristic curve represents the empirically determined actuation current-actuation pressure relationship during a change in the operating state of the switching element 9, starting from a completely emptied operating state towards a completely closed operating state of the switching element 9, while the ipdown characteristic curve represents the actuation current-actuation pressure relationship during a change in the operating state of the switching element 9, starting from a completely closed operating state of the switching element 9 towards a completely open operating state.The deviations between the characteristic curve ipup and the characteristic curve ipdown are also referred to as hysteresis, which results from the different behavior of the actuation chain of the switching element 9 shown in Fig. 3 in the latter two different actuation directions of the switching element 9. In order to operate the switching element 9 and also the switching element 10 of the gearbox 3, which is designed as a construction machinery gearbox, to the desired extent, the characteristic curves ipup and ipdown stored in the area of ​​the electrical control unit 38 must be adapted to the actual conditions of the actuation chain. This adaptation is intended to ensure that the electrical control unit 38 outputs the target value i9soll required for setting the respective requested actuation pressure p9, which deviates to a non-negligible extent from the values ​​stored in the form of the characteristic curves ipup and ipdown due to the component tolerances present in the area of ​​the actuation chain. For this purpose, the target value i9 of the actuating current i9 is varied during the calibration procedure over the operating time t in the manner shown in more detail in Fig. 4, in order to determine a target value i9 of the actuating current i9 corresponding to a defined operating state of the switching element 9 or 10. The defined operating point corresponds to an operating state of the switching element 9 or 10 at which the transmission capacity of the switching element 9 or 10 is essentially zero, and from which an increase in an actuating force or actuating pressure p9 results in an immediate increase in the transmission capacity of the switching element 9 or 10. At a time T0, the electrical control unit 38 outputs a target value i9(T0), the level of which corresponds to a so-called diagnostic current level and with which the electrical control loop can be automatically checked with regard to its basic functionality.For example, it is possible to detect the presence of a cable break or other malfunctions interrupting the electrical circuit in the area of ​​the electrical control circuit, since no current flow can be detected in such a case. At time T0, the switching element 9 or the switching element 10 is in a fully open operating state, which is why a speed difference Δn between the speeds of the switching element halves of switching element 9 or 10 has the value Δnauf. The switching element half of switching element 9 or switching element 10, which is connected to the transmission input shaft 6, is driven by the rotational speed of the drive machine 2, which is greater than a predefined threshold during calibration. Simultaneously, switching elements 27 and 32 are open, thus interrupting the connection between the switching element half of switching element 9 or 10 that can be coupled to the output 4 via switching elements 27 and 32, and the output 4. Consequently, the rotational speed of the switching element half of switching element 9 or 10 that can be coupled to the output 4 is essentially zero. At time T1, the target value i9set of the actuating current i9 is abruptly increased to the value i9set(T1) and held at this level until time T2. At time T2, the target value i9set is reduced in a ramp-like manner until time T3. Raising the setpoint i9 between times T1 and T2 causes the switching element 9 or 10 to be subjected to a pressure pulse, which fills the switching element 9 or 10 and moves the open switching element 9 or 10 towards its closed operating state. At time T4, which lies between times T2 and T3, the differential rotational speed Δn drops from the value Δnup to zero, meaning that the switching element 9 or 10 is in a fully closed operating state at time T4. The rotational speeds of the switching element halves of switching element 9 or 10 are determined by measurement. The application of the pressure pulse to the switching element 9 or 10 between times T1 and T2 essentially corresponds to the actuation of the switching element 9 or 10 as provided for in the operation of the transmission 3, where, in a manner known per se, it is filled with a rapid filling pressure during a rapid filling phase and then subjected to a filling equalization pressure during a subsequent filling equalization phase. During the filling equalization phase, all components to be actuated, such as a piston and a multi-plate pack of the switching element 9 or 10, which is designed here as a friction-fit multi-plate clutch, come into contact with one another and are then in gapless operative contact with each other to a extent sufficient for the delay-free actuation of the switching element 9 or 10. Since the basic behavior of switching element 9 or 10 is known, and it is therefore predictable at time T2 that switching element 9 or 10 will reach its closed operating state at time T4, the target value i9 of the actuating current i9 is reduced to the level at time T3 as described above, thus enabling the described procedure to be carried out within short operating times. At the target value i9(T3), switching element 9 or 10 is definitely in a fully closed operating state, which is why the differential rotational speed Δn is still zero at time T3. Starting from time T3, the target value i9 of the actuating current i9 is continuously reduced via a ramp with a significantly smaller gradient than the gradient of the ramp between times T2 and T3.Lowering the target value i9soll causes the speed difference Δn to rise again at time T5 and to exceed a predefined limit Δngrenz1 at time T6. The limit value Δnlimit1 is an empirically determined limit value at which the switching element 9 or 10 exhibits the defined operating point, at which the transmission capacity of the switching element 9 or 10 is essentially zero, and from which an increase in the actuating pressure T9 results in an immediate increase in the transmission capacity of the switching element 9 or 10. Following time T6, the setpoint value i9set(T6) is held essentially constant until a time T7 following time T6. Since the drive speed of the drive machine 2 is set to a constant and the speed of the switching element half of the switching element 9 or 10 that can be coupled to the output 4 is no longer driven by the drive machine 2 at the defined operating point of the switching element 9 or 10, the speed of the switching element half of the switching element 9 or 10 that can be coupled to the output 4 decreases.10 towards zero, which causes an increase in the speed difference Δn. The target value i9soll(T6) determined at time T6 is stored in the electrical control unit 38 as the value to be output as the target value i9soll for setting the filling compensation pressure or the actuation pressure p9 in order to transfer the switching element 9 or 10 to the defined operating point. Additionally, the target value i9soll(T6) is used to adapt the characteristic curve ipdown. Since the actuation pressure p9 of the switching element 9 or 10, at which the switching element 9 or 10 has the defined operating point, is empirically determined and therefore known, the corresponding target value i9soll is first determined using the characteristic curve ipdown and compared to the target value i9soll(T6). Depending on the resulting deviation, the entire characteristic curve ipdown is shifted up or down on the ordinate by the positive or negative deviation and used for the subsequent actuation of the switching element 9 or 10 in the operation of the transmission 3. At time T7, following time T6, which simultaneously marks the end of a waiting period and at which it is reliably assumed that the switching element 9 or 10 is in a fully open operating state, the target value i9 of the actuating current i9 is again ramped up. During the waiting period between times T6 and T7, the speed difference Δn is constantly greater than a minimum threshold, which in turn is greater than the limit value Δnlimit1. The increase in the target value i9 causes the switching element 9 or 10 to be subjected to increasing actuating pressure, thus increasing its transmission capacity. Additionally, the increase in the transmission capacity of the switching element 9 or 10 leads to the speed difference Δn decreasing again after time T7 and falling below a further limit value Δnlimit2 at time T8. At time T8, the defined operating point of the switching element 9 or 10 is reached.The system detects the current i9 at time T8 and stores the target value i9soll(T8) of the actuation current i9 corresponding to the defined operating point of the switching element 9 or 10 in the electronic control unit 38. This target value is used to adapt the characteristic curve ipup. The characteristic curve ipup is adapted to the actual conditions of the actuation chain of switching element 9 or 10 to the same extent as the characteristic curve ipdown, using the target value i9soll(T8) determined at time T8, and is then used in the subsequent operation of the transmission 3. The respective target values ​​are stored in non-volatile memory, i.e., an EPROM or an EEPROM of the electronic control unit 38. Additionally, the setpoint i9 is reduced to the diagnostic current level at time T0, as shown in Fig. 4, thus completing the procedure for determining the setpoint value of the actuating current i9 corresponding to the defined operating point. Due to the reduction of the setpoint value i9 of the actuating current i9, the actuating pressure p9 of the switching element 9 or 10 decreases, and the switching element 9 or 10 is completely emptied. At the same time, the speed difference Δn increases to the opening level Δn after time T8. The procedure described above provides a simple way to consider the entire control loop or actuation chain and its component tolerances when determining the filling pressure of the switching element 9 or 10. Specifically, the procedure determines at which setpoint or current output of the electrical control unit 38 in the area of ​​the switching element 9 or 10 a sufficiently high actuation pressure p9 is present to close a previously open clutch or open a previously closed clutch. A detection function monitors the primary and secondary rotational speeds of the switching element 9 or 10. If the magnitude of the speed difference exceeds the threshold value Δngrenz1 or falls below the threshold value Δngrenz2, the current is no longer sufficient to operate the switching element 9 or 10.to keep 10 closed, or sufficient to close it again, thus finding the so-called slip current of the switching element 9 or 10. Using this method, the offset values ​​for both branches of the hysteresis curve, which includes the characteristic curves ipup and ipdown, can be determined. These offset values ​​can then be directly incorporated into the control unit of the switching element 9 or 10. Alternatively, the aforementioned method allows the empirically determined relationship between actuation current i9 and actuation pressure p9, as established via the characteristic curves ipup and ipdown, to be adapted accordingly using the determined setpoint values ​​i9setpoint(T6) and i9setpoint(T8), respectively, and used for further actuation of the switching element 9 or 10. Reference sign 1 Vehicle drivetrain 2 Drive unit 3 Transmission 4 Output 5 Power take-off 6 Transmission input shaft 7 Fixed gear 8 Additional power take-off 8A Fixed gear 9 Friction-fit switching element 10 Friction-fit switching element 11 Loose gear 12 Loose gear 13 Planet carrier 14 Loose gear 15 Double planet carrier 16 First sun gear 17 Second sun gear 18 Ring gear 19 Shaft 20 First hydraulic unit 21 Hydrostatic unit 22 Fixed gear 23 Fixed gear 24 Shaft 25 Second hydraulic unit 26 Transmission output shaft 27 Friction-fit switching element 28 Loose gear 29 Fixed gear 30 Fixed gear 31 Fixed gear 32 Friction-fit switching element 33 Loose gear 34 Fixed gear 35 Shaft 36 Adjustable yoke 37 Electro-hydraulic control unit 38 Electric control unit 39 Valve assembly 40 Proportional pressure regulator 41 Valve spool of the proportional pressure regulator 42 Electromagnet 43 Spring assembly 44 Hydraulic amplifier 45 Control surface 46 Valve spool Δn Speed ​​difference Δnauf Discrete value of the speed difference Opening levelΔngrenz1 predefined limit of the speed difference Δngrenz2 further defined limit of the speed difference i9 actuation current i9soll target value of the actuation current ipdown characteristic curve ipup characteristic curve p9 actuation pressure p_red supply pressure signal p_sys supply pressure signal, system pressure T0 - T8 discrete time t time

Claims

Method for determining a target value (i9target(T6), i9target(T8)) of an actuating current (i9) corresponding to a defined operating point of an electro-hydraulically actuated friction-locked switching element (9, 10) of a continuously variable transmission (3), for which the transmission capacity of the switching element (9, 10) is essentially zero and from which an increase in an actuating force results in an immediate increase in the transmission capacity, wherein one half of the switching element is connected to a transmission input (6) and the other half of the switching element (9, 10) can be coupled to a transmission output (4), characterized in that the target value (i9target) of the actuating current (i9) of the switching element (9, 10) which is essentially in the closed operating state,10) when the other half of the switching element is decoupled from the transmission output and the transmission input speed is greater than a defined threshold, the speed difference (Δn) between the speeds of the switching element halves exceeds a predefined limit (Δnlimit1), wherein the target value (i9target(T6)) of the actuation current (i9) at time (T6) when the limit (Δnlimit1) is exceeded is the target value (i9target(T6)) of the actuation current (i9) corresponding to the defined operating point of the switching element (9, 10). Method according to claim 1, characterized in that in the area of ​​a valve device (39) a setpoint value of a hydraulic actuating pressure (p9) of the switching element (9, 10) is set depending on the setpoint value (i9set) of the actuating current (i9), which can be applied in the area of ​​a piston chamber of the switching element (9, 10). Method according to claim 2, characterized in that the switching element (9, 10) is first converted from an open operating state, in which the piston chamber is substantially completely emptied, to a closed operating state by guiding the setpoint value (i9 setpoint) of the actuating current (i9) and a resulting application of a pressure pulse in the area of ​​the piston chamber with a defined pressure level of the actuating pressure and over a defined operating time. Method according to claim 3, characterized in that the target value (i9target) of the actuating current (i9) is preferably reduced via a ramp from the level (i9target(T1)) of the pressure pulse towards a level (i9target(T3)) at which the switching element (9, 10) is still in the closed state and from which the target value (i9target) of the actuating current (i9) is reduced via a ramp until the speed difference (Δn) between the speeds of the switching element halves exceeds the predefined limit value (Δnlimit1). Method according to one of claims 1 to 4, characterized in that the setpoint (i9set) of the actuating current (i9) is set to a level for a predefined period from the time (T6) at which the speed difference (Δn) between the speeds of the switching element halves exceeds the predefined limit (Δnlimit1), at which the speed difference (Δn) between the speeds of the switching element halves is greater than an additional limit, which in turn is greater than the predefined limit (Δnlimit1). Method according to claim 5, characterized in that the target value (i9target) of the actuating current (i9) is increased again via a ramp after the predefined period has elapsed until the speed difference (Δn) between the speeds of the switching element halves falls below a further predefined limit value (Δnlimit2), wherein the target value (i9target) of the actuating current (i9) at the time (T8) when the further predefined limit value (Δnlimit2) is undercut is also a target value (i9target) of the actuating current (i9) corresponding to the defined operating point of the switching element (9, 10). Method according to one of claims 1 to 6, characterized in that the target value (i9set) of the actuating current (i9) is reduced to a level (i9set(T0)) at which the switching element (9, 10) transitions to its fully open operating state after determining the target value (i9set) of the actuating current (i9) corresponding to the defined operating point of the switching element (9, 10). Method according to one of claims 1 to 7, characterized in that the rotational speeds of the switching element halves are determined by measurement. Method according to one of claims 1 to 8, characterized in that the limit values ​​(Δnlimit1, Δnlimit2) are determined empirically. Method according to one of claims 1 to 9, characterized in that the switching element is actuated via characteristic maps depicting a relationship between target values ​​of the actuating current, target values ​​of the actuating pressure and an operating temperature of the gearbox, which are determined empirically. Method according to one of claims 1 to 10, characterized in that the switching element (9, 10) is actuated via characteristic curves (ipup, ipdown) which are empirically determined and represent a relationship between target values ​​(i9target) of the actuating current (i9) and target values ​​of the actuating pressure (p9). Method according to claim 10 or 11, characterized in that the characteristic curves (ipup, ipdown) or the characteristic maps are adapted depending on a deviation between the determined target value (i9soll) of the actuating current (i9) corresponding to the defined operating point of the switching element (9, 10) and the target value (i9soll) of the actuating current (i9) obtainable from the characteristic maps or the characteristic curves for the defined operating point of the switching element.

Images