RELATED APPLICATION INFORMATION

The present application claims priority to and the benefit of German patent application no. 10 2017 218 669.0, which was filed in Germany on Oct. 19, 2017, the disclosure of which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a method for operating a motor vehicle capable of partly or fully autonomous driving, having a plurality of wheels, a drive system for producing a drive torque at at least one of the wheels, and a brake system for producing at least one holding force for holding at least one of the wheels at a standstill, a rotational speed sensor being allocated to at least one of the wheels of the motor vehicle that produces a respective signal pulse for each of a plurality of positions of angular rotation of the wheel, a specifiable driving maneuver being carried out as a function of the produced signal pulses. In addition, the present invention relates to a device for operating a motor vehicle that is realized as described above.

BACKGROUND INFORMATION

In today's motor vehicles, driving functions are increasingly being used that enable partly or fully autonomous driving operation, intended to relieve the burden on the driver of the motor vehicle. For example, parking assistants are known that carry out a parking process in partly or fully autonomous fashion. Lane keeping assistants and traffic congestion assistants also make driving the motor vehicle easier for the driver.

These functions frequently make use of planning and regulator configurations that are primarily based on speed and acceleration signals, ascertained for example using rotational speed sensors allocated to the wheels. As a rule, such rotational speed sensors supply, for each rotation of a wheel, a specified number of signal pulses (“ticks”), the signal pulses being uniformly distributed over the circumference of the wheel, or over a full rotation of the wheel. Thus, the distance between two adjacent signal pulses on the wheel circumference is for example 1 to 2 cm.

For driving functions at lower speeds, above all for fully automated parking and maneuvering functions, it is difficult to derive the speed and/or acceleration signals from the rotational speed sensors with the required quality, because the number of signal pulses that can be acquired is small. If for example the driving speed is to be determined from the signal pulses of the rotational speed sensors, then a reliable speed value is obtainable only if the traveled path is already long enough that, for example, a braking process would already have to be introduced during a parking process. In this way, travel past the actual destination point could occur. Previously, other sensor configurations were therefore also used to make it possible to reliably track the movement of the motor vehicle.

SUMMARY OF THE INVENTION

The method according to the present invention having the features described herein has the advantage that the signal pulses of the one, or the plurality of, rotational speed sensors are used to carry out a short path driving process. Here the method according to the present invention makes it possible, despite a short driving path and the associated low driving speed, to reliably track the movement of the motor vehicle and thus to safely and reliably carry out the short path driving maneuver. According to the present invention, for this purpose it is provided that, in the case of a short path driving process as a specifiable driving maneuver starting from the stationary state, the braking force is first reduced until the rotational speed sensor produces a first signal pulse, and is then held at least temporarily constant until a specified number of signal pulses has been produced, and subsequently is increased up to the holding force. The present invention thus provides that to carry out a short path driving process the number of signal pulses is used for the actuation of the brake system and thus for the delimitation and setting of the traveled path. To this extent, a short path driving process is understood in particular as a parking maneuver, or entering or leaving a parking place, in which, in particular, only speeds below 20 km/h, in particular below 10 km/h, occur. To carry out the driving process, first the braking force is reduced. As soon as a first signal pulse is produced by a, or the, rotational speed sensor, the current brake force is held constant. Here it is assumed that at this braking force a low travel speed is reached that causes the wheels to roll, but that a further acceleration is at least largely prevented by the brake force held constant. Subsequently, the further signal pulses produced by the rotational speed sensor are counted and, as a function of the counted signal pulses, the driving process is ended by increasing the holding force as soon as a specified number of signal pulses has been produced. Because the number of signal pulses per rotation of a wheel is known, and the distance of the signal pulses from one another is also known, the traveled path can be easily and precisely determined or calculated as a function of the acquired number of signal pulses. The traveled path is thus first converted into the number of signal pulses to be produced as a function of the signal pulses per rotation of a wheel and of the wheel outer diameter, and subsequently, when the specified or calculated number of signal pulses has been reached, the braking force is again increased up to the holding force, which securely prevents rotation of at least one of the wheels, in order to bring the wheels of the vehicle again to a standstill and to end the driving maneuver. In this way, the destination point of the driving maneuver is safely reached.

According to an exemplary embodiment of the present invention, the number of signal pulses is specified as a function of a path to be traveled, as mentioned above. Here, in particular the outer diameter of the wheel, and the number of pulses per rotation of this wheel, are taken into account. In this way, an unambiguous tracking of the movement of the motor vehicle is ensured and the driving maneuver can be carried out precisely.

In addition, it may be provided that the time duration beginning with the production of the first signal pulse is acquired and is compared with a specifiable time span, and that when the time duration exceeds the time span the braking force is further reduced. According to this specific embodiment, the braking force is thus held constant after the production or acquisition of the first signal pulse only until the measured time duration since the duration of the first signal pulse exceeds the specified time span. It is then assumed that the braking force is still too great to enable further movement of the motor vehicle. In particular, the braking force is then further reduced until a further signal pulse is produced or acquired. The method then proceeds analogously to the specific embodiment described above, and the brake force is held constant until a total of a specified number of signal pulses has been produced.

In addition, it may be provided that for the short path driving process the drive system is controlled so as to produce a drive torque. In this way, it is ensured that the motor vehicle is set into motion even when it is situated on a horizontal plane. While in principle the driving process can also be realized through the inherent weight of the motor vehicle if the motor vehicle is situated on a corresponding incline, through the production of the drive torque it is ensured that the short path driving process can be carried out independent of the presence or orientation and/or magnitude of an incline.

In particular, it is provided that the drive torque is produced as a function of an incline in the direction of travel of the motor vehicle. The current slope of the roadway on which the motor vehicle is situated is ascertained for example by one or more inclination sensors or the like. As a function of the acquired slope, the drive torque is then produced in such a way that the motor vehicle moves in the desired direction as soon as the braking force has been reduced to a corresponding extent. In particular, the production of the drive torque and the reduction of the braking force are coordinated or matched in such a way that at least one torque equilibrium results at the wheels through which a rolling back of the motor vehicle in the direction opposite the desired direction is avoided. The torques, or the braking force and the drive torque, may be calibrated to one another in such a way that the drive torque is higher than the roll-off moment caused by the slope and the weight of the motor vehicle, so that the motor vehicle moves only in the desired direction of travel when the brake force is reduced.

According to a specific embodiment of the present invention, it is provided that the drive torque is produced before the brake force is reduced. This has the advantage that the motor vehicle is tensioned, or pre-tensioned, while at a standstill, and rolling back of the motor vehicle when the braking force is reduced is reliably prevented. In this way, the robustness of the driving function, or of the short path driving maneuver, is increased. As already mentioned, here as well the drive torque may be set as a function of the current roadway slope in order to reliably prevent the rolling back. In particular, the drive torque is taken back or reduced only upon reaching the last signal pulse of the specified number of signal pulses, so that a uniform driving movement of the motor vehicle up to the reaching of the destination position is insured, and in particular the motor vehicle is prevented from coming to a standstill prematurely, before reaching the target position.

According to an exemplary embodiment of the present invention, it is provided that a plurality of, in particular all, the wheels of the motor vehicle are each assigned a rotational speed sensor, and that the brake force is held constant and/or increased only when a signal pulse has been produced by at least two of the rotational speed sensors. In this way, the robustness of the method is further increased, because for example in the case of a bouncing of a signal pulse, for example if the motor vehicle is moving very slowly and the rotational speed sensor stands at the threshold of a signal pulse for a long time, a beginning movement of the motor vehicle is nonetheless robustly ascertained. In particular, the braking force is held constant or increased only when a first signal pulse has been produced by each of the rotational speed sensors.

In addition, it may be provided that a rotational speed sensor is assigned to each of a plurality, in particular all, of the wheels, and that the drive torque is not reduced until a first signal pulse has been produced by at least two, in particular by all, rotational speed sensors. In this way, it is ensured that at the beginning of the driving maneuver the motor vehicle is set into movement as a whole, and the counting of the signal pulses reliably leads to the specified destination. For the case in which the path to be traveled is not divisible by the signal pulses without a remainder, then in a further specific embodiment a whole-number rounding may be carried out. In particular, the number of signal pulses is rounded down, so that in this way the probability is increased that the traveled path never goes beyond the requested path. In this way, the safety of the driving function is increased because the probability of a collision, and in particular driving past the destination, decreases. The increasing of the braking force may be introduced with the next-to-last signal pulse of the number of signal pulses to be acquired, in order to at least approximately compensate the path additionally traveled during the braking process resulting from the blind travel before the first signal pulse when starting. In this way, the reaching, or traveling, of the requested path length is further improved.

The device according to the present invention having the features described herein is distinguished in that it is specifically set up to carry out the method according to the present invention. In this way, the advantages named above result. Further advantages and features and combinations of features result in particular from the above description and from the further descriptions herein.

In the following, the present invention is explained in more detail on the basis of the drawings.

