Method for operating an automatic transmission of a motor vehicle

Abstract

A method of operating an automatic transmission of a motor vehicle, in which a hydraulic pump associated with a hydraulic system for the supply of pressure is driven by a drive motor. When the motor vehicle starts, a hydrodynamic starting element forms a driving connection between the drive motor and the automatic transmission. Hydraulic shifting elements (B1, B2, B3, C1, C2) are actuated for engaging gear steps. When the drive motor is started, a shifting element (B1, B2, B3, C1, C2) of the automatic transmission is engaged, and, during the engagement process of the shifting element (B1, B2, B3, C1, C2), a time of a rotational speed variation (n.sub.Ab, n.sub.Tu) of the automatic transmission is determined. With the help of the determined rotational speed variation (n.sub.Ab, n.sub.Tu), a time point is determined at which a pressure present in the hydraulic system reaches or exceeds a target pressure level.

Claims

1. A method of operating an automatic transmission of a motor vehicle, in which a hydraulic pump associated with a hydraulic system for supplying pressure is driven by a drive motor, when the motor vehicle starts, a hydrodynamic starting element forms a driving connection between the drive motor and the automatic transmission, and hydraulic shifting elements are actuated for engaging gear steps, the method comprising: engaging a shifting element of the automatic transmission when the drive motor is started, determining a time of rotational speed variation of the automatic transmission, during an engagement process of the shifting element, and with assistance of the determined rotational speed variation, determining a time point at which a pressure present in the hydraulic system either reaches or exceeds a target pressure level.

2. The method according to claim 1, further comprising recognizing the time point at which the pressure in the hydraulic system either reaches or exceeds the target pressure level by virtue of a discontinuity of the determined rotational speed variation.

3. The method according to claim 1, further comprising using a shifting element of a starting gear of the automatic transmission as the shifting element actuated when the drive motor is started.

4. The method according to claim 1, further comprising determining one of a variation of a turbine rotational speed of the hydrodynamic starting element, a variation of a transmission input rotational speed of the automatic transmission, and a variation of a transmission drive output rotational speed of the automatic transmission as the rotational speed variation.

5. A control unit for an automatic transmission, which is designed to carry out a method of operating an automatic transmission of a motor vehicle, in which a hydraulic pump associated with a hydraulic system for supplying pressure is driven by a drive motor, when the motor vehicle starts, a hydrodynamic starting element forms a driving connection between the drive motor and the automatic transmission, and hydraulic shifting elements are actuated for engaging gear steps, the method comprising: engaging a shifting element of the automatic transmission when the drive motor is started, determining a time of rotational speed variation of the automatic transmission during an engagement process of the shifting element, and with assistance of the determined rotational speed variation, determining a time point at which a pressure present in the hydraulic system either reaches or exceeds a target pressure level.

6. A computer program with program code means being run on a control unit for carrying out the method according to claim 1, wherein the control unit is either a computer or a corresponding computing unit.

7. A computer program product with program code means being stored on a computer-readable data carrier for carrying out the steps according to claim 5, wherein the computer program product is run on a computer.

8. A method of operating an automatic transmission of a motor vehicle, the method comprising: driving a hydraulic pump with a drive motor to pressurize a hydraulic system of the motor vehicle; forming a driving connection between the drive motor and the automatic transmission by engaging a hydrodynamic starting element to start driving the motor vehicle; selectively actuating hydraulic shifting elements of the automatic transmission to engage gear steps in the automatic transmission; engaging a first shifting element of the hydraulic shifting elements of the automatic transmission when the drive motor is started; determining a rotational speed variation of the automatic transmission over a period of time during engaging the first shifting element; and determining a time point at which the pressure in the hydraulic system either reaches or exceeds a target pressure level based on the determined rotational speed variation over the period of time.

9. The method according to claim 8, further comprising detecting a discontinuity of the rotational speed variation over the period of time, and the time point at which the pressure in the hydraulic system either reaches or exceeds the target pressure level is defined as a time at which the discontinuity of the rotational speed variation is detected.

10. The method according to claim 9, further comprising actuating a starting gear of the automatic transmission by engaging the first shifting element of the hydraulic shifting elements.

11. The method according to claim 10, further comprising defining the rotational speed variation as being one of: a variation of a turbine rotational speed of the hydrodynamic starting element, a variation of a drive input rotational speed of the automatic transmission, and a variation of a drive output rotational speed of the automatic transmission.

12. The method according to claim 11, further comprising using a control unit to determine the time point at which the pressure in the hydraulic system either reaches or exceeds the target pressure level.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) To clarify the invention, the description of drawings with example embodiments is attached. The drawings show:

(2) FIG. 1: A conventional drive-train with a planetary automatic transmission and a hydrodynamic torque converter, represented schematically,

(3) FIG. 2: A first diagram showing rotational speed variations for the recognition of a target pressure level in the hydraulic circuit of the automatic transmission, and

(4) FIG. 3: A second diagram showing rotational speed variations for the recognition of a target pressure level in the hydraulic circuit of the automatic transmission.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(5) According to FIG. 1 a conventional drive-train comprises a planetary automatic transmission 1 with an input shaft 17, an output shaft 18 and a hydrodynamic torque converter 20 connected upstream from these. The automatic transmission 1 comprises three planetary gearsets 2, 7, 12 coupled with one another, each consisting of a sun gear 3, 8, 13, a planetary carrier 4, 9, 14 and a ring gear 6, 11, 16 respectively. On the planetary carriers 4, 9, 14 in each case a number of planetary gearwheels 5, 10, 15 are distributed around the circumference and are mounted to rotate, these meshing on one side with the associated sun gear 3, 8, 13 and on the other side with the associated ring gear 6, 11, 16 respectively.

(6) The input shaft 17 of the automatic transmission 1 is rigidly connected to the sun gear 3 of the first planetary gearset 2. The planetary carrier 4 of the first planetary gearset 2 is rigidly connected to ring gear 11 of the second planetary gearset 7, and the planetary carrier 9 of the second planetary gearset 7 is rigidly connected to the ring gear 16 of the third planetary gearset 12. The planetary carrier 14 of the third planetary gearset 12 is rigidly connected to the output shaft 18 of the automatic transmission 1, which in turn is in driving connection with an axle drive of a driven vehicle axle, such as an axle differential or a distributor gearset.

(7) The automatic transmission 1 has five frictionally operating shifting elements, namely two disk clutches C1, C2 and three disk brakes B1, B2, B3, which serve for the shifting of six forward gears and one reversing gear. By closing the first disk clutch C1, the input shaft 17 is connected to the sun gear 8 of the second planetary gearset 7 and to the sun gear 13 of the third planetary gearset 12, By means of the second disk clutch 02, the input shaft 17 can be connected to the planetary carrier 9 of the second planetary gearset 7 and to the ring gear 16 of the third planetary gearset 12.

(8) By closing the first disk brake 31, the ring gear 6 of the first planetary gearset 2 is braked fixed relative to the transmission housing 19. By means of the second disk brake 32, the planetary carrier 4 of the first planetary gearset 2 and the ring gear 11 of the second planetary gearset 7 can be fixed relative to the transmission housing 19. By closing the third disk brake B3, the planetary carrier 9 of the second planetary gearset 7 and the ring gear 16 of the third planetary gearset 12 are held fixed relative to the transmission housing 19.

(9) From the structure of the automatic transmission 1 and the arrangement of the shifting elements C1, C2, B1, B2, B3 it emerges that the engagement of gear steps requires in each case only two shifting elements to be closed, and that to change between two adjacent gear steps, for example for a shift from the first to the second gear step, in each case only one shifting element has to be opened and one other shifting element closed.

(10) On the input side, a hydrodynamic torque converter 20 provided with a bridging clutch 21 is connected upstream from the automatic transmission 1. The torque converter 20 comprises a pump impeller 22, a guide wheel 23 and a turbine wheel 24, which are surrounded by a housing (not shown completely). The pump impeller 22 is rigidly connected to an input shaft 25 which is connected to the driveshaft of a drive motor (not shown), and which can if necessary be connected by way of the bridging clutch 21 and a vibration damper 26 to the input shaft 17 of the automatic transmission 1. The guide wheel 23 is connected by way of an overrunning clutch 27 to a housing component 28, whereby rotation of the guide wheel 23 in the direction opposite to that of the drive motor is prevented. The turbine wheel 24 is connected to the input shaft 17 of the automatic transmission 1.

(11) When there is a large rotational speed difference between the pump impeller 22 and the turbine wheel 24, which happens in particular when the vehicle is at rest, i.e. the turbine wheel 24 is braked and fixed, then if the bridging clutch 21 is open the torque applied at the turbine wheel 24 or the input shaft 17 of the automatic transmission 1 is larger compared with the torque on the pump impeller 22 applied by the drive motor and acts as a so-termed crawling torque. To relieve the load on the wheel brakes of the vehicle concerned, a permanent brake in the form of a primary retarder 29 arranged on the input shaft 17 of the automatic transmission 1 is also provided. Furthermore, the automatic transmission 1 comprises a hydraulic pump (not shown here) which is coupled to the input shaft 17 of the automatic transmission 1 and is driven by the drive motor.

(12) When the drive motor is started, by virtue of a drag torque present in the hydrodynamic torque converter 20 the turbine wheel 24 is accelerated. In turn, by virtue of the drag torques of the disk clutches C1, C2 the turbine wheel 24 of the torque converter 20 accelerates the respective planetary gearsets 2, 7 and 12. In this case the rotational speeds are as yet undefined and result from the combination of drag torque and frictional torque conditions.

(13) According to the present invention, now already at the beginning of the engine starting process a shifting element C1, C2, B1, B2, B3 is actuated. The actuation of the shifting element C1, C2, B1, B2, B3 can take place for example by actuating a proportional magnetic valve, whereby a connection of the shifting element C1, C2, B1, B2, B3 to a pressure circuit of a hydraulic system is formed and a corresponding actuating pressure for the shifting element C1 C2, B1, B2, B3 is produced.

(14) Thereby, a piston of the shifting element C1, C2, B1, B2, B3 is for example pushed against a restoring spring more and more in the direction toward the disk packet. When the air gap of the disk packet has been completely bridged, then at that instant the shifting element C1, C2, B1, B2, B3 suddenly becomes frictionally effective. During this, in the automatic transmission 1 rotating masses are coupled and, depending on the choice of the shift elements C1, C2, B1, B2, B3 to be closed when the engine starts, a corresponding reaction torque can be detected for example at the transmission output or at the transmission input or at the turbine shaft of the torque converter 20. From that time point t.sub.1 onward defined rotational speed and torque conditions exist in the automatic transmission 1 and it can be concluded that the pressure in the hydraulic system has reached or exceeded the target pressure level. A delivery quantity by the hydraulic pump (not shown here) is now substantially higher than the quantity flowing out due to the venting of oil spaces and due to leakage.

(15) The sudden torque increase that takes place during the closing of the shifting element C1, C2, B1, B2, B3 when the mass moments of inertia are coupled can now be observed in a rotational speed signal, which can be registered by means of a rotational speed sensor.

(16) FIG. 2 shows a first diagram with a time variation of a drive output rotational speed n.sub.Ab, a turbine rotational speed n.sub.Tu and an engine rotational speed n.sub.An, with the rotational speed n plotted on the ordinate axis and the time t plotted on the abscissa. The drive motor is started at a time t.sub.0, Since the hydraulic pump is driven by the drive motor, the pressure in the hydraulic circuit begins increasing from that same time point t.sub.0 onward. The torque converter 20 is also driven by the drive motor. Correspondingly, a drive input rotational speed n.sub.An of the pump impeller 22 of the torque converter 20 is produced, which increases until an idling rotational speed of the drive motor is reached. Due to the slip between the pump impeller 22 and the turbine wheel 24, the turbine rotational speed n.sub.Tu, lags behind the drive input rotational speed n.sub.An. Since the vehicle is at rest, the drive output rotational speed n.sub.Ab of the automatic transmission 1 is zero.

(17) At time t.sub.0 the disk brake B3 is now actuated by energizing one of the valves associated with the disk brake B3, whereby a connection is produced between the disk brake B3 and a pressure circuit of a hydraulic system and a corresponding actuation pressure for the disk brake B3 is produced. During this a piston of the disk brake B3 is pushed toward its disk packet. When the air gap of the disk packet has been completely bridged, the disk brake B3 becomes frictionally effective. When the disk brake B3 is frictionally effective all the shafts and planetary gearsets 2, 7, 12 rotating until then are coupled to the transmission housing 19 and thereby braked to a standstill. At the transmission output, a torque peak is produced, which can be recognized in the rotational speed variation of the drive output rotational speed n.sub.Ab as a discontinuity. From that time t.sub.1 onward defined rotational speed and torque conditions prevail in the automatic transmission 1 and it can be concluded that the pressure in the hydraulic system has reached or exceeded the target pressure level.

(18) The size of the rotational speed jump in the drive output rotational speed n.sub.Ab detected depends on a characteristic rigidity of the drive-train. Thus, the rigidity of the drive-train can be taken into account appropriately when evaluating the drive output rotational speed n.sub.Ab detected.

(19) After the lapse of the time interval between times t.sub.0 and t.sub.1 the pressure in the hydraulic system has reached the target pressure level and the automatic transmission is then ready to transmit torque. To detect the drive output rotational speed n.sub.Ab, a rotational speed sensor arranged on the output shaft 18 of the automatic transmission 1 is preferably used.

(20) FIG. 3 shows a second diagram with a time variation of a drive output rotational speed n.sub.Ab, a turbine rotational speed n.sub.Tuand an engine rotational speed n.sub.An, with the rotational speed n plotted on the ordinate axis and the time t on the abscissa. In this case too the drive motor is started at a time t.sub.0. Since the hydraulic pump is driven by the drive motor, from that time t.sub.0 onward the pressure in the hydraulic circuit begins increasing. The torque converter 20 is also driven by the drive motor. Accordingly a drive input rotational speed n.sub.An of the pump impeller 22 of the torque converter 20 is produced, which increases until an idling rotational speed of the drive motor is reached. Due to the slip between the pump impeller 22 and the turbine wheel 24, the turbine rotational speed n.sub.Tu lags behind the drive input rotational speed n.sub.An. Since the motor vehicle is at rest, the drive output rotational speed n.sub.Ab of the automatic transmission 1 is zero.

(21) In this case at time t.sub.0 the disk clutch C1 is actuated by energizing one of the valves associated with the disk clutch C1, whereby the disk clutch C1 is connected to a pressure circuit of a hydraulic system and a corresponding actuation pressure for the disk clutch C1 is produced. During this a piston of the disk clutch C1 is pushed toward its disk packet. When the air gap of the disk packet has been completely bridged the disk clutch C1 becomes frictionally effective. When the disk clutch C1 is frictionally effective all the shafts and planetary gearsets 2, 7, 12 rotating until then are coupled to the turbine wheel 24 or turbine shaft and thereby synchronized with the turbine rotational speed existing at the time. At the turbine wheel 24 or turbine shaft this produces a torque peak that can be recognized in the rotational speed variation of the turbine rotational speed n.sub.Tu. During this the turbine rotational speed n.sub.Tu , decreases somewhat for a short time and then increases smoothly again. From that time t.sub.1 onward defined rotational speed and torque conditions exist in the automatic transmission 1 and it can be concluded that the pressure in the hydraulic system has reached the target pressure level.

(22) When the disk clutch C1 becomes frictionally effective, then in an advantageous manner a torque peak at the transmission output can be avoided. Moreover, a discontinuity in the rotational speed variation of the turbine rotational speed n.sub.Tu can be detected more easily than a discontinuity in the rotational speed variation of the drive output rotational speed n.sub.Ab.

(23) After the lapse of the interval between times t.sub.0 and t.sub.1, the pressure in the hydraulic system has reached the target pressure level and the automatic transmission is ready to transmit torque. To detect the turbine rotational speed n.sub.Tu a rotational speed sensor arranged on the turbine shaft or on the input shaft 17 of the automatic transmission 1 is used.

INDEXES

(24) 1 Planetary automatic transmission 2 First planetary gearset 3 Sun gear 4 Planetary carrier 5 Planetary gearwheel 6 Ring gear 7 Second planetary gearset 8 Sun gear 9 Planetary carrier 10 Planetary gearwheel 11 Ring gear 12 Third planetary gearset 13 Sun gear 14 Planetary carrier 15 Planetary gearwheel 16 Ring gear 17 Input shaft 18 Output shaft 20 Torque converter 21 Bridging dutch 22 Pump impeller 23 Guide wheel 24 Turbine wheel 25 Input shaft 26 Vibration damper 27 Overrunning clutch 28 Housing component 29 Primary retarder B1 Shifting element, disk brake B2 Shifting element, disk brake B3 Shifting element, disk brake C1 Shifting element, disk clutch C2 Shifting element, disk clutch n.sub.Ab Drive output rotational speed n.sub.An Drive input rotational speed n.sub.Tu Turbine wheel rotational speed t.sub.0 Engine start time t.sub.1 Time when the target pressure level is reached