Method for operating two electric drives

Abstract

Method for operating two electric drives (EM1, EM2) which operate on different wheels (12a, 12b) of a motor vehicle (10), wherein, in normal operation, a central control unit (EV-SG) detects a total target torque and divides it into target torques for the two drives (EM1, EM2), wherein each drive (EM1, EM2) is assigned one of two drive control units (LE1, LE2) over which the central control unit (EV-SG) is superordinate, and wherein, in normal operation, the central control unit (EV-SG) communicates the divided target torques to the drive control units (LE1, LE2) so that the latter send corresponding control signals to the electric drives (EM1, EM2) for conversion into actual torques applied by the drives (EM1, EM2), wherein, in an emergency operation in the event of a failure of the central control unit (EV-SG), at least one drive control unit (LE1, LE2) automatically sets an upper limit for the actual torque on at least one wheel (12a, 12b),wherein the upper limit for the actual torque of a first of the two drives (EM1, EM2) is determined as a function of the actual torque of a second of the two drives (EM1, EM2), characterized in that a predetermined torque difference is determined for the two wheels (12a, 12b), which is defined as the difference between the actual torque of the first drive (EM1, EM2) and the actual torque of the second drive (EM1, EM2), wherein the predetermined torque difference is dependent on a speed of the motor vehicle (10) and / or a rotational speed of a shaft or a wheel (12a, 12b) of the motor vehicle (10) and / or on a steering angle set on a steering handle of the motor vehicle (10).

Description

[0001] The invention relates to a method for operating two electric drives that operate on different wheels of a motor vehicle, preferably on wheels on the same axle.

[0002] The assumption here is an arrangement in which each drive is assigned a drive control unit, which supplies the drive with control signals. A central control unit is superordinate to the drive control units, which processes inputs from the driver or specifications from other control units and determines an overall target torque. For example, the torque requested by the driver can be determined based on the position of an accelerator pedal. The central control unit divides the overall target torque into target torques for the individual (both) drives and transmits corresponding signals to the drive control units to communicate the target torques. These can then be implemented by sending control signals to the electric drives to produce the corresponding actual torques generated by the drives.

[0003] In a motor vehicle in which the arrangement described above is implemented, sufficient safety must be provided in the event of a control unit failure. It must be prevented that any situation arises in which the vehicle occupants could be endangered.

[0004] DE 10 2004 006 023 A1 discloses a multi-inverter system for a vehicle. A first and second inverter, respectively, control a first and second motor. Each inverter is assigned a control unit. Correlated clock signals can be generated via a synchronization signal between the control units.

[0005] DE 10 2006 050 555 A1 discloses a method for operating a printing press. The printing press comprises several assemblies, each with at least one drive controlled by a drive controller. Upon actuation of an emergency stop device of at least one assembly, all drives of an assembly are stopped individually.

[0006] DE 698 19 200 T2 describes a method for controlling an AC motor for propelling heavy torpedoes, which comprises two counter-rotating rotors whose concentric shafts are collinear. According to the method, the speed of the rotor of the outer shaft is controlled and the torque of the rotor of the inner shaft is controlled such that the rotor of the inner shaft delivers a torque equivalent to that of the rotor of the outer shaft.

[0007] DE 10 2008 053 ​​113 A1 discloses a drive arrangement with two electric drives for the motorized adjustment of an adjustment element in a motor vehicle. The electric drives are controlled by a central control unit.

[0008] GB 2 463 130 A discloses a torque control system for a vehicle, wherein at least two wheels of the vehicle are each driven by at least one separate electric machine. The torque control system also includes a master controller that monitors the torque of at least one of the electric machines, and upon a determination that the torque provided by one electric machine is being reduced, the torque of at least one other electric machine is adjusted.

[0009] DE 10 2004 058 996 A1 discloses a method for transferring safety-relevant driving functions of a vehicle into a safe state.

[0010] DE 10 2008 002 505 A1 discloses a method for operating an electric drive device of a motor vehicle having at least one electric motor, wherein, upon the occurrence of an error signal, a torque generated by the electric drive device is reduced in a predeterminable manner.

[0011] It is an object of the invention to provide a method for operating a plurality of electric drives in an arrangement of the type described above, which ensures a high level of safety with regard to the failure of components, in particular of the central control unit.

[0012] This problem is solved by a method having the features of claim 1. According to the invention, in the event of a failure of the central control unit, the drive control units (or at least one of them) automatically set an upper limit for the actual torques. Accordingly, they preferably only transmit control signals to the associated electric drives that ensure that the upper limit for the actual torques is not exceeded.

[0013] The invention ensures that the motor vehicle remains stable in the event of a failure of the central control unit and that a particularly high actual torque is not applied to one wheel which differs excessively from the actual torques of the other wheels, so that the vehicle becomes unstable.

[0014] An upper limit for the actual torques can be realized in particular if the target torque is set to low values ​​in emergency operation.

[0015] In a first alternative, which is not encompassed by the invention, the target torque is set to zero in emergency mode. In other words, the motor vehicle is forced to stop. This is a safe condition.

[0016] The upper limit for the actual torque of a first drive is defined as a function of the actual torque of the second drive. This ensures that the difference between the actual torques does not become excessively large and cause the vehicle to become unstable.

[0017] In general, the upper limit for the actual torque of a first drive can be determined based on a predefined torque difference, which is calculated as the difference between the actual torque of the first drive and the actual torque of the second drive. Normally, the torque difference should be zero, but it can be different from zero when cornering or when the wheels are traveling on different surfaces.

[0018] The torque difference depends on a speed of the motor vehicle and / or a speed of rotation on a shaft or wheel of the motor vehicle and / or a steering angle set on a steering handle.

[0019] It goes without saying that the torque difference should generally not be as great when the vehicle is moving at high speed or when the wheels are rotating at high speed as when it is moving at a slower speed.

[0020] In order for a drive control unit to be able to determine the actual torque in relation to other variables, these variables must be known. It is preferred that the arrangement in the motor vehicle include the central control unit and the drive control units being coupled to each other via a communications bus, e.g., a CAN bus ("Controller Area Network"), with each of these control units receiving signals from the other two control units.

[0021] The respective target torque is transmitted from the central control unit to the drive control units via the communication bus. Normally, each drive control unit reports back the actual torque. The torque limits are also specified: There is a lower limit for the actual torque, and there is an upper limit for the actual torque, which must be known to the central control unit. Likewise, the total target torque (sum torque) that must be provided based on a driver's request is also communicated via the communication bus.

[0022] Under the circumstances mentioned, it is possible for a drive control unit to appropriately determine a value for the actual torque of its associated drive based on values ​​communicated by the other drive control unit.

[0023] In principle, a drive control unit can also take on the role of the central control unit and set the upper limit for the actual torque of a drive to which another drive control unit belongs.

[0024] A preferred embodiment of the invention is described below with reference to the drawing, in which Fig. 1 schematically illustrates an arrangement in which the invention can be used, Fig. Figure 2 is a flow chart of steps of the method according to the invention according to two alternatives and Fig. 3 a graph for the limits of a torque difference between two wheels of the motor vehicle from Fig. 1 depending on a speed.

[0025] In a motor vehicle designated as a whole by 10, a first electric motor EM1 drives a first wheel 12a and a second electric motor EM2 drives a second wheel 12b. To operate the electric motors, associated drive control units in the form of power electronics LE1 and LE2 are provided, which supply the electric motors EM1 and EM2 with control signals, in particular with current.

[0026] The drive control units LE1 and LE2 are superordinated to a central control unit EV-SG, and all three control units communicate via a CAN bus (“Controller Area Network” bus).

[0027] During normal operation, the central control unit EV-SG transmits information to the drive control units LE1 and LE2 regarding the target torque to be generated by the electric motors EM1 and EM2.

[0028] In step S10, a drive control unit, e.g., drive control unit LE2, checks whether such a request for the target torque exists. If so, the target torque is adjusted as far as possible by issuing appropriate control signals.

[0029] It is now possible that the central control unit EV-SG fails. In this case, the test question in step S10 is answered with "no." An emergency program is then started in step S14.

[0030] In a first alternative, according to step S16, no more torque is permitted, thus the motor vehicle 10 is brought to a standstill.

[0031] In a second alternative, according to step S16', the actual torque M-Ist-1 is read out by the drive control unit LE2 from the other drive control unit LE1, which communicates this via the CAN bus.

[0032] Subsequently, a rotational speed n at the wheel 12b and the speed v are recorded in a step S18.

[0033] The drive control unit LE2 now sets a target torque for the electric motor EM2 in step S20, whereby the specification is that the actual torque M-Ist-2 at the wheel 12b differs from the actual torque M-Ist-1 by at most a differential torque ΔM(n, v).

[0034] The differential torque ΔM can assume values ​​that lie between curves 14a and 14b: At low speeds n, the actual torque M-actual-2 can be greater than the actual torque M-actual-2 by a value of ΔM-max, and it can also be smaller than the latter by a value of ΔM-min. The possible differences decrease at higher speeds, since otherwise the vehicle would be forced to corner excessively.

[0035] Graphs 14a and 14b are symmetrical to each other with respect to the n-axis. If a specific curve is desired, the axis of symmetry can be shifted and become merely parallel to the n-axis, or the symmetry can be eliminated entirely.

Claims

[1] Method for operating two electric drives (EM1, EM2) which operate on different wheels (12a, 12b) of a motor vehicle (10), wherein in normal operation a central control unit (EV-SG) detects a total target torque and divides it into target torques for the two drives (EM1, EM2), wherein each drive (EM1, EM2) is assigned one of two drive control units (LE1, LE2) over which the central control unit (EV-SG) is superordinate, and wherein in normal operation the central control unit (EV-SG) communicates the divided target torques to the drive control units (LE1, LE2) so that the latter send corresponding control signals to the electric drives (EM1, EM2) for conversion into actual torques applied by the drives (EM1, EM2), wherein at least one drive control unit (LE1, LE2) in an emergency operation in the event of a failure of the central control unit (EV-SG) automatically sets an upper limit for the actual torque on at least one wheel (12a, 12b),wherein the upper limit for the actual torque of a first of the two drives (EM1, EM2) is determined depending on the actual torque of a second of the two drives (EM1, EM2), , characterized by that a predetermined torque difference is determined for the two wheels (12a, 12b), which is defined as the difference between the actual torque of the first drive (EM1, EM2) and the actual torque of the second drive (EM1, EM2), wherein the predetermined torque difference is dependent on a speed of the motor vehicle (10) and / or a rotational speed on a shaft or a wheel (12a, 12b) of the motor vehicle (10) and / or on a steering angle set on a steering handle of the motor vehicle (10). [2] Method according to claim 1, characterized bythat the central control unit (EV-SG) and the drive control units (LE1, LE2) are coupled to one another via a communication bus (CAN), and that each of these control units (LE1, LE2, EV-SG) receives signals emitted by the other two control units (LE1, LE2, EV-SG).

Images