Method for operating a motor vehicle, and motor vehicle

Abstract

A method is indicated for operating a motor vehicle, as well as a motor vehicle, having an electric machine for the positive or negative acceleration of the motor vehicle as well as a mechanical brake device for reducing the vehicle speed of the motor vehicle. In a special brake operation mode, a deceleration demand (V) is detected, and a target rotational speed curve (n.sub.soll) of the electric machine is determined in dependence on the deceleration demand (V). An actual rotational speed (n.sub.ist) of the electric machine for reducing the vehicle speed of the motor vehicle is then adapted to the target rotational speed curve (n.sub.soll) so determined.

Claims

1. A method for operating a motor vehicle, comprising: accelerating positively or negatively, by an electric machine, the motor vehicle; reducing, by a mechanical brake device, the vehicle speed of the motor vehicle; detecting a deceleration demand (V) in a special brake operation mode; determining a target rotational speed curve (n.sub.soll) of the electric machine in dependence on the deceleration demand (V); and adapting an actual rotational speed (n.sub.ist) of the electric machine for reducing the vehicle speed of the motor vehicle to the determined target rotational speed curve (n.sub.soll), wherein upon approaching the actual rotational speed (n.sub.ist) to a rotational speed range, the target rotational speed curve (n.sub.soll) is adapted in an opposite direction of the curve of the actual rotational speed (n.sub.ist).

2. The method as claimed in claim 1, wherein the rotational speed range is assigned to the target rotational speed curve (n.sub.soll), wherein the rotational speed range defines a given distance (n) from the target rotational speed curve (n.sub.soll), and the actual rotational speed (n.sub.ist) is regulated within the rotational speed range.

3. The method as claimed in claim 1, wherein the motor vehicle is braked to a standstill by means of the electric machine in case of a failure of the mechanical brake device.

4. The method as claimed in claim 1, wherein a plurality of target rotational speed curves (n.sub.soll) is stored in a database, and the target rotational speed curve (n.sub.soll) is selected from the plurality of target rotational speed curves (n.sub.soll) and specified based on the deceleration demand (V) as a function of the vehicle speed.

5. The method as claimed in claim 1, wherein an autonomous driving operation is provided for the autonomous control of a driving task of the motor vehicle, and the special brake operation mode occurs, in particular, only during the autonomous driving operation.

6. The method as claimed in claim 1, wherein the special brake operation mode is designed solely for reducing the vehicle speed of the motor vehicle by a value in the range of 10 km/h to 30 km/h.

7. The method as claimed in claim 1, wherein several electric machines are used for the positive or negative acceleration, especially wheel hub motors.

8. The method as claimed in claim 1, wherein the special brake operation mode is activated only in case of a failure of the mechanical brake device.

9. A motor vehicle comprising: an electric machine for the positive or negative acceleration of the motor vehicle and a mechanical brake device for reducing the vehicle speed of the motor vehicle, wherein a control unit is provided for the control of a special brake operation mode and the control unit is designed such that, during operation in the special brake operation mode, a deceleration demand (V) is detected; a target rotational speed curve (n.sub.soll) of the electric machine is determined in dependence on the deceleration demand (V); and an actual rotational speed (n.sub.ist) of the electric machine for reducing the vehicle speed of the motor vehicle is adapted to the target rotational speed curve (n.sub.soll) so determined, wherein upon approaching the actual rotational speed (n.sub.ist) to a rotational speed range, the target rotational speed curve (n.sub.soll) is adapted in an opposite direction of the curve of the actual rotational speed (n.sub.ist).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Exemplary embodiments of the invention shall be explained more closely below on the basis of the figures. The latter show, partly in highly simplified representations:

(2) FIG. 1 a schematic partial diagram of a motor vehicle and

(3) FIG. 2 a rotational speed curve plotted as a function of time.

(4) In the figures, parts with the same effect are represented with the same reference numbers.

DETAILED DESCRIPTION

(5) FIG. 1 shows a rough partial sketch of a motor vehicle 2, especially one axle 4 of the motor vehicle 2. The axle 4 is, for example, a rear axle or a front axle of the motor vehicle 2. The exemplary embodiments and features discussed below may be applied to a rear axle and/or a front axle.

(6) The axle 4 has a wheel 6 at each end. Furthermore, the axle 4 has a differential 8, which is connected by way of a drive shaft 10 to each of the wheels 6. The differential 8 enables a distributing of the torque needed for the propulsion and a different rotational speed for the wheels 6, for example, when the motor vehicle 2 is negotiating a curve.

(7) The motor vehicle 2 in the exemplary embodiment comprises an electric machine 12 for propelling the motor vehicle 2. Thus, the present exemplary embodiment involves a motor vehicle 2 driven purely by an electric motor. Alternatively, the motor vehicle 2 has both an internal combustion engine and an electric machine for its propulsion, i.e., it is designed in the manner of a hybrid vehicle. The electric machine 12 is connected by way of a driven shaft 14 to the differential.

(8) For the reduction of the vehicle speed, the motor vehicle 2 has a mechanical brake device 16. The mechanical brake device 16 in the exemplary embodiment is designed as a conventional disk brake. Hence, the motor vehicle 2 in the exemplary embodiment has two mechanical brake devices 16, i.e., one for each wheel 6.

(9) The motor vehicle 2 in the exemplary embodiment is designed for autonomous driving. That is, the motor vehicle has a control unit 18, by means of which the motor vehicle 2 is steered in instead of a driver. In other words: the actual driver becomes more of a passenger of the motor vehicle 2. The accelerating, braking and steering are taken over by the control unit 18. For example, a deceleration demand V is detected by the control unit 18, i.e., the need to reduce the vehicle speed in a given time, and, on the basis of the deceleration demand V, a brake signal B is generated. The brake signal B is then relayed to the mechanical brake device 16, which exerts a braking torque on the wheels 6 based on the brake signal B.

(10) In the case of a failure of the mechanical brake device 16, i.e., at least a substantial operational impairment, for example, the motor vehicle 2 and especially the control unit 18 are designed to determine, depending on the detected deceleration demand V, a target rotational speed curve n.sub.soll of the electric machine 12, and to generate the necessary braking torque by the electric machine 12. In other words: the electric machine 12 thus serves as a redundancy for the mechanical brake device 16, in order to brake the motor vehicle 2.

(11) Now, for a more precise explanation, the method shall be discussed in further detail in FIG. 2. FIG. 2 sketches the rotational speed n of the electric machine 12 as a function of time t.

(12) As already mentioned, the control unit 18 determines a target rotational speed curve n.sub.soll in dependence on the deceleration demand V. Since the target rotational speed curve n.sub.soll serves for a braking of the motor vehicle, especially down to standstill, it has a declining trend. That is, the target rotational speed decreases in value as the time t progresses. The target rotational speed curve n.sub.soll is associated with a rotational speed range 20. The rotational speed range 20 has a given distance n from the target rotational speed curve n.sub.soll. In other words: the rotational speed range 20 is defined by a maximum rotational speed n.sub.max and by a minimum rotational speed n.sub.min.

(13) In order to reduce the vehicle speed of the motor vehicle 2, an actual rotational speed n.sub.ist of the electric machine 12 is adapted to the target rotational speed curve n.sub.soll. The adaptation is carried out, for example, by an active energizing of the electric machine 12, generating a braking torque.

(14) In order to prevent too much wheel slip occurring at the wheels 6 during the braking by means of the electric machine 12, thereby making the braking process unstable and uncontrollable, the target rotational speed curve n.sub.soll is adapted depending on the wheel slip.

(15) For example, if the actual rotational speed n.sub.ist falls below the rotational speed range 20 and thus the minimum rotational speed n.sub.min (see I), then the braking torque generated by the electric machine 12 is too strong and there is a danger of locking of the wheels 6. In the present instance, the control unit 18 therefore adapts the target rotational speed curve n.sub.soll such that it is displaced in the direction of a higher rotational speed and thus a lower braking torque is generated by the electric machine 12. Hence, the wheel slip that is indirectly correlated with the braking torque is regulated, so that enough wheel slip to brake the motor vehicle 2 is generated during the braking process, but not too much wheel slip, which would result in a locking of the wheels 6 and making the braking process uncontrolled.

(16) Conversely, the above described adaption occurs analogously in the case of exceeding the rotational speed range 20 and thus the maximum rotational speed n.sub.max (see II).

(17) In particular, the above described method is designed for braking the motor vehicle 2 from vehicle speeds with a value of 10 km/h to 30 km/h, for example, during the operation of a parking garage pilot, during which the control unit 18 steers the motor vehicle autonomously through a parking garage.

(18) The individual target rotational speed curves n.sub.soll are stored, for example, in a database 22, and a target rotational speed curve n.sub.soll is selected and specified, for example, by the control unit 18, based on the deceleration demand V as a function of the vehicle speed.

(19) The invention is not limited to the above described exemplary embodiments. Instead, other variants of the invention may also be derived from it by the person skilled in the art, without leaving the subject matter of the invention. In particular, moreover, all individual features described in connection with the exemplary embodiments may be otherwise combined with each other, without leaving the subject matter of the invention.