Method for activating a parking lock of a motor vehicle

Abstract

A method for actuating a parking lock of a motor vehicle including a piston (10) which is arranged to move axially within a cylinder (9) of a parking lock actuator (8) and is acted upon by pressure in order to disengage the parking lock. A nominal pressure target, for disengaging the parking lock, is continuously increased from a first pressure value (p.sub.start) until the parking lock is either disengaged or until a maximum pressure value (p.sub.max) is reached.

Claims

1. A method for actuating a parking lock of a motor vehicle, the method comprising: pressurizing a piston arranged to move axially within a cylinder of a parking lock actuator to disengage the parking lock; providing a control unit with a plurality of characteristic lines of a nominal pressure target, and each of the plurality of characteristic lines having a different starting pressure value; selecting a characteristic line of the nominal pressure target from the plurality of characteristic lines of the nominal pressure target; acting on the piston with the starting pressure value of the selected characteristic line; and continuously increasing the nominal pressure target according to the selected characteristic line, for disengaging the parking lock, from the starting pressure value of the selected characteristic line either until the parking lock is disengaged or until a maximum pressure value is reached.

2. The method according to claim 1, further comprising increasing the nominal pressure target, starting from the starting pressure value, for disengaging the parking lock either in a ramp-like manner or in steps.

3. A control unit of a motor vehicle, which is designed to carry out the method for actuating the parking lock of the motor vehicle according to claim 1.

4. A computer program product with program code means, stored on a computer-readable data carrier and able to run on the control unit according to claim 3, for carrying out the method for actuating the parking lock of the motor vehicle.

5. A method for actuating a parking lock of a motor vehicle, the method comprising: selecting an initial pressure value of a nominal pressure target from a range of initial pressure values for disengaging the parking lock as a function of a value of torsional moment acting on the parking lock in a currently existing operating situation of the motor vehicle, such that the greater the value of the torsional moment acting on the parking lock is, the greater the initial pressure value from the range of initial pressure values is; pressurizing a piston arranged to move axially within a cylinder of a parking lock actuator to the selected initial pressure value to disengage the parking lock; and increasing the nominal pressure target from the selected initial pressure value either until the parking lock is disengaged or until a maximum pressure value is reached.

6. The method according to claim 5, further comprising selecting the starting pressure value of the nominal pressure target, for disengaging the parking lock, depending on an inclination of either the motor vehicle or of a road on which the motor vehicle is located, such that the starting pressure value selected is greater the greater the inclination is.

7. The method according to claim 5, further comprising selecting the starting pressure value of the nominal pressure target, for disengaging the parking lock, depending on a loading condition of the motor vehicle, such that the starting pressure value selected is greater the greater the loading condition is.

8. The method according to claim 5, further comprising determining the starting pressure value of the nominal pressure target, for disengaging the parking lock, while the vehicle is at rest with the parking lock engaged, based on whether a trailer is either coupled to or detached from the motor vehicle, such that one starting pressure value is determined when the trailer is detached from the motor vehicle, and another relatively higher starting pressure value is determined when the trailer is coupled to the motor vehicle.

9. The method according to claim 5, further comprising providing a plurality of characteristic lines for the nominal pressure target for disengaging the parking lock, and each of the plurality of characteristic lines having a different starting pressure value within the range of the starting pressure values and choosing an appropriate characteristic line based on the currently existing operating situation of the motor vehicle.

10. The method according to claim 5, further comprising, increasing the nominal pressure target from the starting pressure value, for disengaging the parking lock, in either a ramp-like or a stepped manner.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Below, examples of the invention are explained in greater detail with reference to the attached figures, which show:

(2) FIG. 1: A schematic representation of a parking lock of a motor vehicle;

(3) FIG. 2: A first pressure variation for controlling the parking lock; and

(4) FIG. 3: A second pressure variation for controlling the parking lock.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(5) The parking lock for a motor vehicle shown in FIG. 1 comprises a locking pawl 1, which engages in or out of a parking lock wheel (not shown) connected to a drive output (not shown) of a transmission, and a locking element 3 arranged axially movably on a connecting rod 4 to an actuating disk 7 made, for example, in the form of a parking disk, the element being made for example as a locking cone in this case. However, the locking element 3 could also be realized by means of appropriate roller elements. In the locked condition of the parking lock—i.e. when the locking pawl is engaged with interlock in a tooth gap of the parking lock wheel the locking element 3 is clamped between the locking pawl 1 and a guide plate 2 connected to a housing of the transmission (not shown) in order to prevent the locking pawl 1 from being dislodged from a tooth gap of the parking lock wheel. In this case the locking cone 3 is spring-loaded on the connecting rod 4 by a spring element 5, here for example in the form of a spiral spring, and prestressed in the axial direction. For this a stop 6 is arranged on the connecting rod 4, against which the spiral spring 5 is supported axially so that it prestresses the locking cone 3 in the axial direction against the locking pawl 1 and the guide plate 2. When the parking lock is engaged the locking pawl 1 is pushed by the locking element 3 against the parking lock wheel. When the locking pawl 1 encounters a tooth gap, it snaps in and locks the drive output. On the other hand, if the locking pawl 1 encounters a tooth of the parking lock wheel, then it is prestressed by the spring element 5 so that when the drive output rotates, with the help of the prestressing force the locking pawl 1 snaps into the next tooth gap of the parking lock wheel.

(6) The parking disk 7 is connected to a parking lock actuator 8, which as shown in FIG. 1 is in the form of a piston-cylinder unit. To disengage the parking lock the parking lock actuator 8 is acted upon by pressure. In this case the piston-cylinder unit can be in the form of a hydraulic or pneumatic piston-cylinder unit and is accordingly acted upon by a hydraulic or pneumatic pressure medium.

(7) For that purpose, pressure is delivered via a pressure medium delivery line 13 by way of a magnetic valve 12 into a cylinder of the parking lock actuator 8, whereby a piston 10 of the parking lock actuator 8 is moved axially in the cylinder 9 so that the locking element 3 connected via the parking disk 7 and the connecting rod 4 to the piston rod of the parking lock actuator 8, which element is between the locking pawl 1 and the guide plate 2, is pulled out.

(8) To engage the parking lock, by means of the magnetic valve 12 the pressure to the cylinder 9 is cut off and the cylinder is vented via a drainage line 14 into a pressure medium sink 15. The parking lock actuator 8 or the piston 10 thereof is prestressed by means of a spring element in a locking position, so that if the pressure drops the parking lock moves to the engaged position and a transmission drive output shaft is blocked thereby. The spring element for prestressing the parking lock can for example be in the form of a compression spring 11 arranged in the cylinder 9 of the parking lock actuator 8 so that it exerts a corresponding force on the piston 10 of the parking lock actuator 8.

(9) Alternatively or in addition to the spring element 11 shown in FIG. 1, the parking lock can be prestressed by a prestressed torsion spring (not shown) on the parking disk 7, by means of which the piston 10 of the parking lock actuator 8 is moved in the “parking” direction and the parking lock is engaged. Such torsion springs for prestressing a parking lock have long been known.

(10) The magnetic valve 12 is connected to a control unit 16 and is controlled as a function of a selector lever position P, R, N, D. If the selector lever is in the positions R, N or D the magnetic valve is energized and the parking lock actuator 8 is acted upon by pressure, which results in disengagement of the parking lock. On the other hand, in the P position of the selector lever the magnetic valve 12 is not energized and the parking lock is engaged by means of the locking element 3.

(11) In the engaged condition of the parking lock—i.e. when the locking pawl is latched into a tooth gap of the parking lock wheel—the locking element 3 of the parking lock is clamped between the locking pawl 1 and a guide plate 2 of the parking lock, so that a torque applied by the drive wheels on the drive output side to the transmission is supported on the locking pawl 1 and the drive-train is thereby locked.

(12) A torsional moment exerted thereby on the parking lock wheel produces by means of the teeth between the parking lock wheel and the locking pawl 1 a force that presses the locking pawl 1 against the locking element 3. The larger the torsional moment, the larger is the force acting upon the locking element 3 and consequently the larger is the force or the pressure in the parking lock actuator 8 required for disengaging the parking lock.

(13) FIG. 2 shows a first variation of a nominal pressure target for controlling the parking lock when the lock is to be disengaged. If the selector lever is moved away from the selector lever position P to another selector lever position R, N or O, then the magnetic valve 12 is energized by the control unit 16 and pressure is delivered to the parking lock actuator 8 in accordance with a nominal pressure target.

(14) First, the piston 10 of the parking lock actuator 8 is acted upon for a specifiable time t.sub.1 by a first pressure value p.sub.start which is needed in order to reliably disengage the parking lock when the motor vehicle is on level ground and is not loaded. This pressure value is substantially lower than a maximum pressure value p.sub.max and can for example lie within a range between 3 bar and 5 bar. The time t.sub.1 can for example be calculated to allow the pressure according to the nominal pressure target to build up in the cylinder 9 of the parking lock actuator 8. If the pressure bunt up in the cylinder 9 of the parking lock actuator 8 and acting upon the piston 10 is not sufficient to pull out the locking element 3 between the locking pawl 1 and the guide plate 2, then in accordance with the nominal pressure target the pressure in the cylinder 9 is increased in a ramp-like manner during a second time t.sub.2. The pressure ramp can for example be adjusted by means of a gradient target or a target-value specification.

(15) The nominal pressure target is now increased along the pressure ramp until the parking lock has been disengaged or until a maximum pressure value p.sub.max has been reached. The disengagement of the parking lock can be detected by means of a parking lock sensor. The maximum pressure value p.sub.max is preferably calculated in such manner that it suffices to disengage the parking lock when the motor vehicle is fully loaded and is standing on an incline steeper than 15%. For example, the maximum pressure value p.sub.max can be in a range between 15 bar and 18 bar.

(16) FIG. 3 shows a second variation of a nominal pressure target for disengaging the parking lock. If the selector lever is moved away from the selector lever position P to another selector lever position R, N or D, then in this case too the magnetic valve 12 is energized by the control unit 16 and a pressure is delivered to the parking lock actuator 8 in accordance with a nominal pressure target.

(17) In this case, however, the nominal pressure target does not show a ramp-like variation, but rather, the nominal pressure increases stepwise. At first the piston 10 of the parking lock actuator 8 is acted upon by a first pressure value p.sub.start, for a first specifiable time t.sub.3, which pressure is needed for disengaging the parking lock reliably when the motor vehicle is on level ground and is not loaded. This pressure value is substantially lower than a maximum pressure value p.sub.max and can for example be in a range between 3 bar and 5 bar. The time t.sub.3 can for example be calculated to allow the pressure according to the nominal pressure target to build up in the cylinder 9 of the parking lock actuator 8. If the pressure built up in the cylinder 9 of the parking lock actuator 8 and acting upon the piston 10 is not sufficient to pull out the locking element 3 between the locking pawl 1 and the guide plate 2, then in accordance with the nominal pressure target the pressure in the cylinder 9 is increased in steps until the parking lock is disengaged or until a maximum pressure value p.sub.max has been reached. The pressure values p.sub.2, p.sub.n between the first pressure value p.sub.start and the maximum pressure value p.sub.max are in turn maintained for specifiable times t.sub.4, t.sub.5 in order to enable the parking lock to be disengaged at these pressures. The maximum pressure value p.sub.max is preferably calculated such that it suffices to disengage the parking lock when the motor vehicle is fully loaded and standing on an incline steeper than 15%. In this case too the maximum pressure value p.sub.max can for example be in a range between 15 bar and 18 bar.

(18) By virtue of the ramp-like or stepped nominal pressure target the parking lock actuator 8 can be acted upon with a pressure sufficient for disengaging the parking lock, regardless of the incline where the motor vehicle is located and regardless of the loading of the motor vehicle.

INDEXES

(19) 1 Locking pawl

(20) 2 Guide plate

(21) 3 Locking element

(22) 4 Connecting rod

(23) 5 Spring element

(24) 6 Stop

(25) 7 Parking disk

(26) 8 Parking lock actuator

(27) 9 Cylinder

(28) 10 Piston

(29) 11 Spring element

(30) 12 Switching valve

(31) 13 Pressure medium supply line

(32) 14 Drainage line

(33) 15 Pressure medium sink

(34) 16 Control unit