RECUPERATIVE BRAKE ON A VEHICLE

Abstract

The present invention relates to a method for recuperation based braking of a vehicle in which electrical energy generated during a braking process is decreased by operating at lest one second electric machine of the vehicle in a zero slip mode in order to prevent overcharging of a traction battery of the vehicle.

Claims

1. A method for recuperation-based braking of a vehicle, comprising: generating electrical energy with a first electric machine of the vehicle during a braking process; operating with the electrical energy generated by the first electric machine a second electric machine in a zero slip mode in which a rotary field of a stator of the second electric machine synchronously follows a movement of a rotor of the second electric machine, so that no-load losses of the second electric machine are used to decrease the electrical energy generated by the first electric machine to thereby prevent overcharging of a traction battery of the vehicle.

2. A method for recuperation-based braking of a vehicle, comprising: generating electrical energy with a first electric machine of the vehicle during a braking process; as a function of an actual state of charge of a traction battery of the vehicle either operating with the electrical energy generated by the first electric machine a second electric machine in a zero slip mode in which a rotary field of a stator of the second electric machine synchronously follows a movement of a rotor of the second electric machine, so that no-load losses of the second electric machine are used to decrease the electrical energy generated by the first electric machine to thereby prevent overcharging of a traction battery of the vehicle, or transferring the electrical energy to the traction battery of the vehicle.

3. The method of claim 1, further comprising prior to the decreasing step transferring the electrical energy to a buffer storage.

4. The method of claim 3, wherein the buffer storage is configured as a part of the traction battery of the vehicle, said method further comprising dynamically discharging the buffer storage by operating the second electric machine of the vehicle in the zero slip mode when the vehicle is operated in a free mode.

5. The method of claim 3, wherein the second electric machine corresponds to the first electric machine.

6. The method of claim 1, wherein the first electric machine is assigned to a first one of at least two electrically driven axles of the vehicle, and the second electric machine is assigned to a second one of the at least two electrically driven axles of the vehicle.

7. The method of claim 6, wherein the electrical energy generated by the first electric machine is directly decreased by the second electric machine.

8. The method of claim 6, wherein an amount of the electrical energy generated by the first electric machine corresponds exactly to an amount of electrical energy that can subsequently be decreased by the second electric machine.

9. A vehicle comprising: a first electric machine configured to generate electrical energy during a recuperation-supported braking process of the vehicle; a second electric machine; and a control device, said control device being configured, as a function of a state of charge of a traction battery of the vehicle, to operate with the electrical energy generated by the first electric machine the second electric machine in a zero slip mode in which a rotary field of a stator of the second electric machine synchronously follows a movement of a rotor of the second electric machine, so that no-load losses of the second electric machine are used to decrease the electrical energy generated by the first electric machine to thereby prevent overcharging of a traction battery of the vehicle.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0034] Other features and advantages of the present invention will be more readily apparent upon reading the following description of currently preferred exemplified embodiments of the invention with reference to the accompanying drawing, in which

[0035] FIG. 1 shows a schematic representation of a possible embodiment of the disclosed method.

[0036] FIG. 2 shows a schematic representation of a vehicle with a control device configured for implementing the disclosed method.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0037] Throughout all the Figures, same or corresponding elements may generally be indicated by same reference numerals. These depicted embodiments are to be understood as illustrative of the invention and not as limiting in any way. It should also be understood that the figures are not necessarily to scale and that the embodiments are sometimes illustrated by graphic symbols, phantom lines, diagrammatic representations and fragmentary views. In certain instances, details which are not necessary for an understanding of the present invention or which render other details difficult to perceive may have been omitted.

[0038] FIG. 1 shows a flow chart for controlling a recuperation operation of an electric machine of a vehicle.

[0039] For a recuperation-based braking, i.e., a braking in which an electric machine which is operated in a recuperation operation, i.e., an operation in which mechanical energy generated during braking is converted into electrical energy, it is tested first in a step 1 whether the recuperation operation can be released for recuperation-based braking and if a zero slip operation is required, i.e., an operation of the electric machine in a zero slip mode in which no drive torque is generated by the electric machine however electrical energy is decreased. For this it is tested in a step 3 whether a state of charge of a battery such as a traction battery of a vehicle is below a threshold value of for example 98% so that electrical energy generated by the recuperation operation can be transferred to the traction battery. Should the state of charge of the traction battery be below the threshold value of 98% the recuperation operation is released without a zero slip operation of the electric machine as indicated by arrow 5 and correspondingly generated electrical energy is transferred to the traction battery.

[0040] If the state of charge of the traction battery is above the threshold value of 98% as indicated by the arrow 7 the recuperation operation is not permitted according to rule 9. In order to nevertheless release the recuperation operation it first has to be verified in a step 11 whether a respective driver has requested a zero torque, i.e., a coasting.

[0041] If the driver has requested a zero torque as indicated by the arrow 15 and the electric machine is correspondingly available for a zero slip operation the electric machine is switched into the zero slip operation in a step 13 in order to decrease electrical energy from the traction battery and to provide a buffer storage for the electrical energy generated during the recuperation operation. If the driver does not request a zero torque or does not request drive energy from the electric machine it is tested again in a step 3, as indicated by the arrow 17, whether the state of charge of the traction battery is above 98%. As soon as the state of charge of the traction battery falls below 98% the recuperation operation is released without a zero slip operation of the electric machine as indicated by arrow 5. So long as the state of charge of the traction battery is above 98% it is either switched into the zero slip mode or in response to a request by the driver the electric machine is used for generating a torque whereby the traction battery is also partially discharged.

[0042] The zero slip operation can be performed for a predetermined period of time or up to a predetermined threshold value of the state of charge of the traction battery. in any case the zero slip operation ends when the driver requests a torque form the electric machine whereupon the sequence starts again with the testing of the state of charge of the traction battery according to step 3 as indicated by arrow 19. In order to reliably prevent a braking at a state of charge of the traction battery above 98% and to prevent an overcharging of the traction battery as a result of a recuperation operation it can be provided that only a friction brake is used for braking. Hereby however, depending on the strength of a respective braking force requested by a user, an emergency operation can be activated in which independent from the state of charge of the traction battery a recuperation based braking is nevertheless performed.

[0043] FIG. 2 shows a vehicle 20 with a first electric machine 21 and a traction battery 23. In order to be able to decelerate the vehicle 20 at all times with a recuperation-based braking it is required that the traction battery is not charged above a state of charge of 98%.

[0044] In order to prevent the traction battery 23 from being charged beyond the state of charge of 98%, for example during a downhill drive in which electrical energy is constantly supplied to the traction battery 23 due to numerous braking maneuvers, it is provided that the electric machine 21 is always switched into a zero slip mode when the electric machine 21 is not used for generating a drive torque or for a recuperation operation and when for example a friction brake is active without recuperative brake support, in which zero slip mode the electric machine 21 consumes electrical energy but does not generate a torque so that electrical energy is withdrawn from the traction battery 23 and capacity for electrical energy generated in a future braking maneuver is present in a buffer storage provided by the traction battery 23.

[0045] What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims and includes equivalents of the elements recited therein: