Method for opening a decoupling unit in an electric drive

Abstract

Method for opening a decoupling unit (6) in an electric drive (1) with an electric machine (4), an inverter (3), a control element (2) and a transmission (5), wherein the decoupling unit (6) is mounted between the electric machine (4) and half-shafts (7, 7') to wheels (8) of a vehicle, characterized in that in the event of a system error, the control unit (2) controls the inverter (3) such that the power switches of the inverter (3) are alternately opened and closed in order to apply a torque in the form of a periodic oscillation to the rotor shaft of the electric machine (4) in order to set the positive coupling of the decoupling unit (6) load-free and thus friction-free.

Description

The invention relates to a method for opening a decoupling unit in an electric drive.Prior ArtThe maximum speed that can be achieved with an electric axis depends primarily on the transmission ratio and the maximum rotational speed of the electric motor. The transmission ratio of the electric axle must have a balanced ratio between torque and speed. High torques are necessary for good acceleration and driving dynamics, while an acceptable maximum electrical speed is advantageous for highway driving. Too small a gear ratio results in too large a gear ratio of a large and heavy engine to provide adequate torque. Possible solutions are electric multi-speed transmissions, electric final drives, but the increased complexity, the technical outlay and the costs often prevent implementation.Single-gear electric axles generally have transmission ratios of 8 to 12 and the electric motors used today have maximum rotational speeds between 12000 and 20000 revolutions per minute. This means that hybrid vehicles, which generally drive more quickly, require a system that decouples the electric motor from the drive train in order to prevent over-rotation. A battery electric vehicle having only one primary drive source does not need a decoupling system, since the maximum electrical speed corresponds to the maximum speed of the vehicle. Whereas in the case of battery electric vehicles with 2 drive sources (primary e-axis for the main drive and secondary e-axis for an auxiliary drive), a decoupling system for the unnecessary e-machine is likewise replaced in order to increase the efficiency of the entire drive by decoupling the secondary e-axis if the drive via the primary drive axis is sufficient. When using a dual clutch differential, the implementation of a separation function is very simple. To enable this, both clutches may be fully opened to allow the input shafts to freely rotate.However, a separating system can also be realized in axles with conventional bevel gear differentials. A simple possibility is the use of a claw clutch at the interface between the drive shaft and a side wheel. Since a dog clutch must be synchronized, the engine and drive shaft speeds must be exactly the same to allow for re-engagement during travel. With the aid of the motor power electronics, which know the rotational speed of the electric machine, and the ABS, a control algorithm can be implemented.DE 10 2010 041 581 A1 discloses an electric vehicle having a claw clutch, and a method for opening the claw clutch. The method enables safe and quick opening during a torque-free operating mode of the clutch.DE 10 2022 203 901 B3 discloses a differential with a planetary carrier with a drive gear, at least one planetary gear rotatably mounted in the planetary carrier. The differential has side gears to side shafts of the drive axle of a vehicle which mesh with the planet gears, one side gear being connectable either to the associated side shaft or to the planet carrier by one or two dog clutches on a side shaft, respectively. Between the planetary carrier and the side shaft, an actuator sleeve is installed with an axially effective claw clutch toward the side wheel or toward the planetary carrier and a radially effective sliding toothing toward the side shaft. The actuator sleeve has at one end a claw toothing on the side gear and is axially displaced away from the differential via a deep groove ball bearing which is mounted without play in a centering seat. The planetary carrier is divided and the partial housing is connected with a blocking function to the drive gear exclusively by welding.Many vehicle failures when operating with an electric machine result in a safe state where all switches of the inverter are closed. This state ASC provides a controlled free-wheeling path for the current. ASC on the gate driver is used to short the motor to either the positive or negative DC current by turning on all upper MOSFETs or all lower MOSFETs in a three phase inverter.This state of the excessively high currents in the stator windings of the electric machine leads to potential thermal problems if it is maintained over a longer period of time.In systems with a decoupling unit, a "normally open decoupling unit concept" would fundamentally allow decoupling of the electric machine from the wheels and, in addition, prevent the thermal risk.However, the separating unit in the prior art is not able to open against higher residual torques-max. about 5 Nm-in the drive train.However, there is the requirement that the decoupling unit can always open in the event of a fault in the active short circuit, which means that the decoupling unit must be able to open against significantly higher torques.It is the object of the invention to provide a method for the improved decoupling.DESCRIPTION OF THE INVENTIONThe object is achieved by a method for opening a decoupling unit in an electric drive having an electric machine, an inverter, a control element, a transmission, wherein the decoupling unit is mounted in the transmission between EM and half shafts to wheels of a vehicle, characterized in that, in the event of a failure of the electric machine having an active short circuit, the control unit actuates the inverter in such a way that the circuit breakers are opened and closed alternately in order to apply a moment in the form of a periodic oscillation to the rotor shaft of the electric machines, which torque makes the decoupling unit load-free.The solution according to the invention is to generate a mechanical interference excitation of the rotor, triggered by the power electronics via a separate electrical / electronic circuit which is active parallel to the existing electrical control, while the inverter / processor is no longer active as a result of a system error, whereby a changing torque in the form of an interference excitation is generated in the electrical machine. The system fault is, for example, the failure of the processor.The periodic oscillation of the natural frequency of the rotor is approximated to the electric machine.By briefly opening / closing switches of the power electronics in the inverter, only a braking moment can be generated between the rotor and the stator at any time, but this differs depending on whether the switches are open or closed. Given appropriate actuation of these switches, the aim is to apply a mechanical disturbance excitation to the rotor shaft that exactly coincides with the rotational natural frequency of the oscillation system. This leads to an increase in the oscillation amplitude, which has the aim of generating a zero crossing at the decoupling unit, as a result of which the positive clutch of the decoupling unit can be opened approximately without friction. Without this zero crossing, opening would not be possible, since the applied torque at the decoupling unit would generate too much friction, as a result of which the actuator could not open the clutch.The control unit is used as a separate control unit or integrated control unit in the inverter independently of the other control of the inverter.DESCRIPTION OF THE FIGURESFIG. 1 schematically shows an electric drive, FIG. 2 shows an equivalent circuit diagram of the mechanical oscillation system, FIGS. 3 and 4 show possible characteristics of the vibrational excitation and torque characteristics of the electric machine and of the decoupling unit.FIG. 1 shows an electric drive unit 1 having an electric machine 4. the electric machine 4 is connected to an inverter 3 which in turn has a connection to a control unit 2. The rotor of the electric machine 4 is coupled to a transmission 5, which is connected on the output side to two half shafts 7, 7' of the axle of a vehicle. Wheels 8 of the vehicle are subjected to a torque via the two half shafts 7, 7'. A decoupling unit 6 is located in the power path between the electric machine 4 and the two half shafts 7 and 7'. Ideally, the decoupling unit is integrated in the transmission (5).The control unit 2 is shown as a separate module, but can also be arranged within the inverter 3.FIG. 2 shows schematically in a mechanical analogy how the method according to the invention can compensate a residual torque at the decoupling unit 6.On the right hand side, the inert mass of a wheel is indicated by the reference numeral 8i. This inertial mass 8iis connected to the decoupling unit 6 via the half-shaft 7. The reference numeral 7s indicates the elasticity or rigidity of the half axle 7.The rotor mass behaves as a two-mass spring system with respect to the wheel masses. By exciting the rotor for a very short time t 3 the rotor oscillates and moreover generates a load change at the decoupling unit 6. In this way, sufficient time remains for opening decoupling unit 6 during the load change. The mechanical natural frequency f RES of the oscillation system, shown in the equivalent circuit diagram of FIG. 2, is largely independent of the rotational speed and depends primarily on the masses involved of rotor 8 i, the two wheel masses 4 i, and the stiffness 7 sin between. The stiffnesses are defined primarily by the side shafts, but also by the mounting of the drive system in the vehicle. Viewed in simplified form, this is therefore a two-mass oscillator if the two wheels and side shafts are combined to form a replacement mass or replacement rigidity.The procedure of the method according to the invention is illustrated in two possible embodiments in FIGS. 3 and 4. The "electric torque" of the electric machine 4 (between the rotor and the stator) or the mechanical torque at the decoupling unit is shown over time in the figures.At time t 1 the electric machine 4 delivers an active torque which is positive (=drive) in this example of FIG. 3. At time ts, a system fault occurs, whereby the safety mechanisms in an electric drive put the system in the active short circuit ASC and close the circuit breakers. The torque of the electric machine 4 falls into the negative range, but is still too great for the decoupling unit 6 to open. Here, the control unit 2 starts and starts alternating switching between closed circuit breakers ASC and open circuit breakers ASO. The switching follows the natural frequency f RES with a period duration of t res.The amplitude swings open rapidly, so that already at the time t 22 there is an amplitude which, on the next zero crossing t 3 brings about a cancellation of the residual torque at the decoupling unit. Thus, at time t 3 there is a possibility of opening decoupling unit 6. The "secure state" of the system is then still ensured. The electric machine is subsequently separated from the wheels and would run out / coast safely until standstill.In the example of FIG. 4, one proceeds into the method with a negative torque (braking / recuperation) during the time t 1. Here, the rotor starts to oscillate with one period of the open power switches ASO. Due to the different initial position with the negative torque, the rotor oscillates to an amplitude which at another time t 3 shows a zero crossing of the torque and thus makes the half shaft 7 free of load. The decoupling unit 6 can thus be decoupled.The switching is controlled and not regulated, so that the switching duration must be evaluated empirically in experiments in order to ensure that there is enough time for the interference excitation to generate a zero crossing at the decoupling unit.The disturbance excitation of the rotor is generated by the power electronics of the inverter 3 via a separate control unit 2. The separate control unit 2, which is active in parallel with the existing power electronics and actuation, allows the resonant excitation process to proceed independently of the regular actuation of the electric machine. By briefly opening / closing switches of the power electronics in inverter 3, the rotor mass oscillates and generates the necessary load change at decoupling unit 6, in order to open it in a robust, mechanical manner even in the safe state ASC.

Claims

Method for opening a decoupling unit (6) in an electric drive (1) having an electric machine (4), an inverter (3), a control element (2), a transmission (5), wherein the decoupling unit (6) is mounted between the electric machine (4) and half shafts (7, 7') with respect to wheels (8) of a vehicle, characterized in that, in the event of a system fault, the control unit (2) actuates the inverter (3) in such a way that the circuit breakers of the inverter (3) are opened and closed alternately in order to apply a torque in the form of a periodic oscillation to the rotor shaft of the electric machine (4) in order to set the form-locking clutch of the decoupling unit (6) free of load and thus free of friction.Method according to Claim 1, characterized in that the periodic oscillation is approximated to the rotational natural frequency (f RES) of the rotor of the electric machine (4).Method according to claim 1 or 2, characterised in that an increasing amplitude of the vibration of the rotor is excited.Method according to one of the preceding claims, characterized in that, in the event of a zero crossing of the torque of the oscillation of the rotor, the decoupling unit (6) is opened.Method according to one of the preceding claims, characterized in that the control unit (2) is used as a separate control unit (2) or integrated control unit in the inverter (3) independently of the other control of the inverter (3).

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