Method for freeing a motor vehicle by rocking

Abstract

A method for rocking a motor vehicle free. The vehicle has a drive-train with an automatic drive torque adjusting element, which transmits torque to a wheel, and a rotational speed and direction sensor system for detecting movement of the vehicle such that when the accelerator pedal is biased to a position different from its non-actuated position a desired drive torque can be produced. The torque adjusting element can be controlled based on the accelerator pedal position in relation to the drive torque that can be produced by the driver, and, in a rocking-free process, alternating deflection and release of the accelerator pedal causes the drive torque to fluctuate. With the rotational speed and direction sensor system, the drive torque adjusting element is controlled to suppress drive torque transmission if the travel direction of a current unpowered vehicle movement is not the same as a forthcoming power-driven vehicle movement.

Claims

1. A method of rocking a motor vehicle free, the motor vehicle comprising a drive-train (1) that has an automatically controllable drive torque adjusting element (3) which can transmit drive torque (M) to at least one drivable vehicle wheel (5), and the motor vehicle having a rotational speed and rotational direction sensor system (11) which can detect vehicle movement and travel direction as well as a standstill of the motor vehicle, the method comprising the steps of: producing a desired drive torque when a driver of the motor vehicle deflects an accelerator pedal (8) to an accelerator pedal position different from a non-actuated position of the accelerator pedal; controlling the drive torque adjusting element (3) as a function of the accelerator pedal position in relation to the drive torque (M) that can be produced by the driver; cyclically fluctuating drive torque at the at least one drivable vehicle wheel (5) in a rocking-free situation in which the at least one drivable vehicle wheel (5) has to be moved clear of a depression (6) when the driver alternately deflects and releases the deflection of the accelerator pedal (8); and controlling the drive torque adjusting element (3) with aid of the rotational speed and rotational direction sensor system (11) in such manner that a drive torque input to the drive-train (1), which is specified by the driver by virtue of the accelerator position of the accelerator pedal (8) actuated by the driver, is suppressed so long as the travel direction (10) of a current unpowered vehicle movement is different than the travel direction (7) of a forthcoming power-driven vehicle movement.

2. The method according to claim 1, further comprising the steps of switching the drive torque adjusting element (3) to a no-torque condition, after a driven vehicle movement in a first travel direction (7), either on or shortly after reaching a first travel direction reversal point (B, t.sub.U) to either enable or assist with an unpowered vehicle movement in a second travel direction (10) opposite to the first travel direction (7); and during the unpowered vehicle movement in the second travel direction (10), suppressing a driver requested drive torque acting in the first travel direction (7) until a second travel direction reversal point (C, t.sub.U) is reached at an end of the unpowered vehicle movement in the second travel direction (10).

3. The method according to claim 1, wherein the drive torque adjusting element (3) is either a separator or a starting clutch of the drive-train (1) in a form of a friction clutch, the drive-train (1) comprises a drive engine (2) and an automated transmission (4) which are drivingly connectable with one another by the separator or starting clutch (3), whereby the drive torque (M(t)) is transmittable to the at least one drivable vehicle wheel (5) with which the vehicle became stuck in a depression (6), the method further comprising the steps of: either on or shortly after reaching a first travel direction reversal point (B, t.sub.U) after vehicle movement in a desired travel direction (7) with a starting gear engaged, the separator or starting clutch (3) is either opened or at least adjusted to a target position in which no driving torque (M(t)) is transmittable to either enable or assist an unpowered rolling back of the vehicle in the opposite direction (10); during the unpowered rolling back of the vehicle, if the driver has actuated the accelerator pedal to call for the drive torque (M(t)) acting in the desired travel direction (7), suppressing the drive torque acting in the desired travel direction and maintaining the separator or starting clutch (3) either disengaged or in a no-torque condition until the rolling back movement ends and a second travel direction reversal point (C, t.sub.U) is reached; and either on or shortly after reaching the second travel direction reversal point (C, t.sub.U), either engaging or at least adjusting the separator or starting clutch (3) to a target position in which the drive torque (M(t)) is transmittable to enable a new build-up of drive torque (M(t)) in the desired travel direction (7).

4. The method according to claim 1, further comprising the step of evaluating a rotational speed gradient (dn/dt) determined by the rotational speed and rotational direction sensor system (11), the rotational speed gradient represents a change with time of a travel-direction-relevant rotation speed (n), to recognize a travel direction reversal point (B, C, t.sub.U).

5. The method according to claim 4, further comprising the step of continually determining the rotational speed gradient (dn/dt) so that from the rotational speed gradient (dn(t.sub.0)/dt) at a current time-point (t.sub.0) at which a current rotational speed (n(t.sub.0)) exists which is higher than a specified limit value, a future time-point (t.sub.U,ber)) is calculated, at which the rotation speed (n(t)) will be zero.

6. A method of controlling a drive train of a motor vehicle to rock the motor vehicle free from a position in which at least one wheel of the motor vehicle is caught in a depression, the vehicle having a drive-train comprising a transmission and an automatically controllable clutch, which transmits drive torque to the at least one wheel, and a rotational speed and rotational direction sensor system which detects vehicle movement and a direction of travel of the vehicle, a control system communicates with the drive-train, the rotational speed and rotational direction sensor system, and a pedal deflection sensor which detects deflection of an accelerator pedal, the method comprising the steps of: activating, with the control system, a rocking-free mode when at least one wheel of the motor vehicle is caught in a depression; detecting, with the rotational speed and rotational direction sensor system, the vehicle movement and the direction of travel of the motor vehicle and detecting, with the pedal deflection sensor, the deflection of the accelerator pedal; transmitting, via the drive train, cyclically fluctuating drive torque to the at least one wheel of the motor vehicle when the driver alternately deflects and releases deflection of the accelerator pedal; and controlling the clutch, with the control system, to suppress the transmission of drive torque to the at least one wheel of the motor vehicle so long as the rotational speed and rotational direction sensor system detects that the direction of travel of the motor vehicle, during unpowered vehicle movement, is different than the direction of travel of the motor vehicle of forthcoming power-driven vehicle movement.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) To clarify the invention the description of a drawing of an example embodiment is given below. The drawing shows:

(2) FIG. 1: A schematic representation of a drive-train for carrying out a method for rocking a vehicle free in accordance with the invention

(3) FIG. 2: A variation of rotational speed over time, for pre-calculating a travel direction reversal point during a rocking-free process, and

(4) FIGS. 3a to 3d: Functional variations of vehicle movement during a rocking-free process.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(5) Thus, the drive-train 1 of a motor vehicle shown in FIG. 1 comprises a drive engine 2 in the form of an internal combustion engine, which can be brought into driving connection with an automated transmission 4 by means of a starting clutch 3 (the drive torque adjusting element). A drive output of the transmission 4 is in driving connection with at least one drivable vehicle wheel 5 by way of a differential transmission and driveshafts (not shown).

(6) The internal combustion engine 2, the starting clutch 3 and the transmission 4 are connected to a control unit 9 via sensor and control leads indicated with dotted lines, and can be controlled by means of the unit. In addition an accelerator pedal 8 is indicated schematically, which can be actuated by a driver to control the drive torque. A driver's wish signaled by actuating the accelerator pedal 8 is detected by an accelerator pedal deflection sensor 13 and transmitted to the control unit 9, which carries out the appropriate control and regulation of the internal combustion engine 2, the starting clutch 3 and the transmission 4. In addition a rotational speed and rotational direction sensor system 11 is provided, by means of which the travel direction 7, 10 at the time and the standstill of the vehicle can be determined. This information is also made available to the control unit 9.

(7) The method for rocking a motor vehicle free is explained below with reference to an example scenario. According to this, as shown in FIG. 1 the motor vehicle, with its drive wheel 5, has become stuck in a depression 6 due to a slippery or yielding subsoil, so that the motor vehicle is in a rocking-free situation by which it must be freed from the depression 6 by driving out.

(8) The motor vehicle is first in a position A at the deepest point of the depression 6. The driver wishes to rock free, would like to start, and deflects the accelerator pedal 8 starting from an undeflected position. By means of the internal combustion engine 2, when a starting gear of the transmission 4 is engaged a drive torque is built up in the desired travel direction 7. This travel direction 7 is also the preferred direction for the motor vehicle to leave the depression 6. The motor vehicle moves in the direction toward position B as far as the edge of the depression ahead of it, and comes to a stop at position B due to spinning of the wheels 5 and/or insufficient drive torque. In the present situation this position B constitutes a first reversal point for the vehicle's movement, from which the motor vehicle tends to move in the opposite direction 10, first to position A at the lowest point of the depression 6. At this first travel direction reversal point B the driver releases the deflection of the accelerator pedal 8 in order to allow the motor vehicle to roll back. The rotational speed and rotational direction sensor system 11 indicates to the control unit 9 that the motor vehicle is rolling back, whereupon the unit opens the starting clutch 3 so that the motor vehicle can actually roll back freely.

(9) The motor vehicle now rolls back freely in the second travel direction 10 opposite to the first travel direction 7, during which after passing the position A the kinetic energy of the motor vehicle is converted to potential energy. At the opposite position C the motor vehicle again comes to a stop, since its residual kinetic energy is not yet sufficient for moving all the way through the depression 6. However, unfortunately the driver has been too quick to move the accelerator pedal 8 away from its undeflected position in order to build up again a drive torque in the desired, first travel direction 7. He has wrongly perceived a close approach to the position C, or second reversal point, already as a standstill of the motor vehicle. However, by virtue of the measured values available to it from the rotational speed and rotational direction sensor system 11, the control unit 9 knows the real rolling situation of the motor vehicle, namely that it is still moving. Accordingly the control unit 9 takes no account of the driver's wish for a renewed closing of the starting clutch 3, as signaled by the deflection of the accelerator pedal 8, until the rotational speed and rotational direction sensor system 11 signals that the motor vehicle has actually come to a stop. Only this is interpreted as having reached the second travel direction reversal point C, and only then is the starter clutch 3, actuated by the control unit 9, automatically closed and the motor vehicle driven actively again.

(10) If now sufficient kinetic energy has been supplied to the motor vehicle and the vehicle has therefore been rocked sufficiently in the longitudinal direction, the speed in the desired travel direction 7 at position B of the original reversal point remains higher than zero and the motor vehicle can move past the edge of the depression 6 to a position D ahead. Otherwise the process is repeated until the motor vehicle breaks free from the depression 6.

(11) FIG. 2 serves to explain the determination according to the invention of the travel direction reversal points B and C by means of a rotational speed gradient evaluation. According to this the rotational speed and rotational direction sensor system 11 continually measures a rotational speed n(t) that depends on the speed of the vehicle. At an assumed starting time t.sub.0 the associated rotational speed is n(t.sub.0), which can still be measured accurately by the rotational speed and rotational direction sensor system 11. The vehicle is moving at a corresponding speed v. However, the vehicle's speed v is already very low, so that the rotational speed and rotational direction sensor system 11 waits a relatively long time for the related pulses of a rotational speed measurement emitter and finally the standstill of the vehicle (rotational speed n=0) would only be determined for a time-point t.sub.U,sen lying far ahead. As shown by FIG. 2, however, the vehicle actually comes to rest already at an earlier time t.sub.U.

(12) In order to be able to determine this actual vehicle standstill more accurately than before, from the time variation of the rotational speed n(t), the rotational speed gradient dn/dt is calculated continuously. With the value of the speed gradient dn(t.sub.0)/dt at the starting time t.sub.0 a later time-point t.sub.U,ber is calculated, at which the vehicle-speed-dependent rotational speed n(t) is expected to become zero. This calculated travel direction reversal point t.sub.U,ber is close to the actual travel direction reversal point t.sub.U at which the vehicle-speed-dependent rotational speed n(t) is zero and the vehicle has actually come to a stop. The pre-calculated travel direction reversal time-point t.sub.U,ber is then used for the control of the starting clutch 3.

(13) According to an embodiment variant of the method the calculation of the travel direction reversal time-point t.sub.U,ber as described does not begin until the measured vehicle-speed-dependent rotational speed n(t) falls below a pre-established lower rotational speed limit value.

(14) FIGS. 3a to 3b show example functional variations of a motor vehicle during a rocking-free process. FIG. 3a shows the time variation of the path s traveled by the vehicle, while FIG. 3b shows the time variation of the vehicle's speed v, to which a rotational speed n(t) corresponds, FIG. 3c shows the time variation of the vehicle's acceleration a, which corresponds to a rotational speed gradient dn/dt, and FIG. 3d shows the time variation of a drive torque M that acts upon the vehicle wheel 5 sitting in the depression 6.

(15) According to these, the vehicle is initially in the depression 6 in a first rocking-free position that corresponds to the so-termed stuck position A shown in FIG. 1. Thereafter the vehicle rocks to and fro between the local reversal point B and the second local reversal point C. At these two reversal points B, C the vehicle's speed v and the vehicle-dependent rotational speed n at the associated reversal point t.sub.U are equal to zero. During a successful rocking-free process the reversal points B, C successively migrate out of the depression 6 until finally the vehicle breaks free out of the depression 6 in the forward direction and passes the so-termed break-free point D (FIG. 3a). During the rocking-free process the vehicle's speed v and the vehicle's acceleration perceptibly increase periodically (FIGS. 3b, 3c). As FIG. 3d shows, during the rocking-free process the drive torque M(t) is periodically switched on and off, meaning that in the forward-travel phases the drive torque M is introduced into the drive-train with the starting clutch 3 closed (driven forward travel) whereas in the unpowered, rolling-back phases the starting clutch 3 is open (rolling back under no drive power).

(16) In FIG. 3b, which shows the time variation of the vehicle's speed v and the vehicle-speed-dependent rotational speed n, areas 12 are highlighted by means of boxes indicated with dotted lines, in which because the rotational speed n is very low a single determination of the reversal time t.sub.U with reference to the rotational speed measurement would be very inaccurate or markedly delayed in time. In these areas 12 the respective reversal point t.sub.U and the direction change of the vehicle wheel 5 can be pre-calculated very accurately as described by means of the evaluation of the rotational speed gradient dn/dt in accordance with FIG. 2.

LIST OF INDEXES

(17) 1 Drive-train 2 Drive engine 3 Drive torque adjusting element, separator or starting clutch 4 Automated transmission 5 Motor vehicle wheel, vehicle wheel 6 Depression 7 First travel direction, desired travel direction 8 Accelerator pedal 9 Control unit 10 Second travel direction, opposite direction 11 Rotational speed and rotational direction sensor system 12 Rotational speed region close to the travel direction reversal time-point t.sub.U 13 Accelerator pedal deflection sensor A Rocking-free position, lowest point of the depression B Rocking-free position, first travel direction reversal point C Rocking-free position, second travel direction reversal point D Rocking-free position, break-free point M Drive torque a Acceleration n Rotational speed s Travel path t Time t.sub.0 Starting time t.sub.U Travel direction reversal time-point t.sub.U,ber Calculated travel direction reversal time-point t.sub.U,sen Measured travel direction reversal time-point dn/dt Rotational speed gradient v Speed