Control method of an electric motor of an at least partially electrically powered vehicle for an operating situation outside of normal operation

Abstract

A control method for an electric motor of an at least partially electrically driven vehicle is proposed for an operating situation outside of normal operation, wherein if the requested torque of the vehicle is greater than the torque that can be provided by the electric motor for this situation, the electric motor is controlled in such a way that the torque of the electric motor is reduced in a first step until a mechanical preload is reached in the drive train, at which the vehicle is still stationary, and in a second step, after the preload has been reached, the torque of the electric motor is built up again, wherein in the case of a starting situation under load, in a third step the torque of the electric motor is built up in such a way that it exceeds the torque requested by the vehicle and the vehicle is set in motion, or wherein in the case,that a standstill situation on an uphill slope exists in which the vehicle is to be held stationary only by the electric motor, the first and second steps are repeated until a termination criterion is met, on the basis of which the repetition is terminated.

Description

The present invention relates to the field of electromobility, in particular the actuation of an electric machine outside normal operation. The method is used, in particular, in vehicles with an attached load for starting and for holding the vehicle at a standstill only by means of an accelerator pedal.Electric drive systems have an electric motor, also referred to as an electric machine, as the main component. This electric machine serves to drive one or more wheels of a vehicle and is normally designed as a single-stage electric machine. It is generally formed as a permanently excited synchronous machine and is operated with alternating current. Since the energy store of vehicles provides direct current, a further component of electric drives is an inverter, i.e. a DC / AC converter, which converts the direct current provided by the energy store into an alternating current usable by the electric machine. Since the electric machine normally has three phases, (at least) three pairs of switches are also provided in the inverter in order to supply current to the three phases of the electric machine. The switches used in the inverter, as well as other electronics, are largely temperature sensitive power semiconductors. Situations with a high torque demand are particularly critical here, i.e., for example, a stop of the vehicle at a standstill on the hill without actuating the brake, or a start on the hill, in particular with an attached load.Currently, electric drives with single-stage reduction in the drive torque and current during standstill are reduced so that the motor phases and output stage switches of the inverter, which are permanently energized during standstill, do not overheat in the shortest time. This leads to the problem that, in the case of an increased / maximum train load demand, such as in trailer operation, during towing, off-road, or in the standstill held only by the accelerator pedal (upward on an inclination), the breakaway or roll-on torque cannot be exceeded, because, in the case of engine standstill, only two of three motor phases and power semiconductor pairs can be energized and therefore the available torque is reduced by one third.In order to protect the sensitive power electronics from overloading in a situation with a high torque demand, inverters and electric machines are designed to be significantly larger than is necessary for normal operation, which however represents a major portion of the utilization. This leads not only to increased costs, but also to increased weight and thus also to a loss of efficiency.The object of the invention is therefore to provide a method for improved actuation of an electric machine of an at least partially electrically driven vehicle for an operating situation outside normal operation.This object is achieved by the features of the independent claims. Advantageous embodiments are the subject of the dependent claims.A driving method of an electric machine of an at least partially electrically driven vehicle for an operating situation outside normal operation is proposed, wherein if the demanded torque of the vehicle is greater than the torque which can be provided by the electric machine for this situation, the electric machine is driven in such a way that the torque of the electric machine is reduced in terms of amount in a first step until a mechanical prestress is achieved in the drive train, but at which the vehicle is still at a standstill, and in a second step, after the prestress is reached, the torque of the electric machine is built up again, wherein, in the event that a starting situation is under load, the torque of the electric machine is built up in such a way that it exceeds the torque demanded by the vehicle and the vehicle is set in motion in a third step, or wherein, in the case that a standstill situation of an upward gradient is present, in which the vehicle is to be held at standstill only by the electric machine, the first and the second step are repeated until a termination criterion is fulfilled, on the basis of which the repetition is terminated.In one embodiment, an abort criterion is requesting a start from standstill, in which case the torque of the electric machine is built up during the last execution of the second step in such a way that it exceeds the torque requested by the vehicle, and the driving method is then ended. In one embodiment, a termination criterion is an actuation of a brake, wherein the driving method is terminated in this situation. In one embodiment, an abort criterion is a detection of an overload of the e-machine, in which case the driving method is ended.In one embodiment, an alarm is additionally issued in the event of a detection of an overload of the e-machine.In one embodiment, when a predefined minimum rotational speed of the electric machine is exceeded, the driving method is ended starting from this minimum rotational speed being exceeded.In one embodiment, the driving method is activated only when a request for a torque of the E-machine is made, at which more than a predefined percentage of the total torque that can be provided by the E-machine is required.In one embodiment, the predetermined percentage is predetermined as 50% or more, or as 60% or more, or as two-thirds or more of the total torque.In one embodiment, in the event that the first and second steps are repeated, a sinusoidal drive repetition rate is used.Furthermore, a computer program product is provided which carries out the method and can be executed on a control device of an at least partially electrically driven vehicle.Furthermore, a control device is provided in an at least partially electrically driven vehicle, in which the computer program product is implemented.Furthermore, an electric drive, in particular an electric final drive, for a vehicle is provided with at least one transmission device, an electric machine, an inverter circuit that actuates the electric machine, and a control device for actuating the electric machine via the inverter circuit.Further features and advantages of the invention are evident from the following description of exemplary embodiments of the invention, on the basis of the figures of the drawing which shows details according to the invention, and from the claims. The individual features can each be realized individually or severally in any combination in a variant of the invention.Preferred embodiments of the invention are explained in more detail below with reference to the attached drawings. FIG. 1 shows a schematic illustration of the energization of an electric machine with inverter interconnection at standstill according to the prior art. FIG. 2 shows a schematic illustration of the energization of an electric machine with inverter interconnection at standstill according to one embodiment of the present invention. FIG. 3 shows a schematic diagram of the method sequence according to one embodiment of the present invention.In the following descriptions of the figures, identical elements or functions are provided with identical reference numerals.Currently, in the case of at least partially electrically driven vehicles, the electric machine 1 is operated in such a way that, in operating situations in which a rotating field rotational speed DZ EM of the electric machine 1 is below a minimum rotating field number of, for example, 1200 revolutions, only two of the three phases P 1-P 3 are energized, as indicated in FIG. 1 (thick lines indicate the phases used). The waste heat is thus distributed only to two of the three phases P 1-P 3. No commutation takes place, i.e. no change of the phases P 1-P 3 which have to carry the load. In this case (revolutions<minimal number of rotating fields), the torque M EM of the electric machine 1 is smaller than the demanded torque M Fzg of the vehicle: M EM< M Fzg. By distributing the waste heat to only two phases P 1, P 2, an increased probability of failure of the power semiconductors of the inverter 2 in these operating situations and thus an increased risk of failure of the electric machine 1 can be expected. Therefore, the components of the power electronics and the electric machine 1 are currently designed to be oversized for normal operation.In order to solve this problem, a driving method of an electric machine 1 of an at least partially electrically driven vehicle is proposed for precisely these operating situations, in which the following applies: M EM< M Fzg, i.e., for example, for a starting situation under high load or a standstill situation on an upward gradient, such as, for example, on a slope or mountain or a low garage, etc., in which the vehicle is to be held at a standstill only with the accelerator pedal (without using the brake). A starting situation under high load can be, for example, a starting of a train on a slope or mountain, a towing situation or a starting in an off road terrain. The starting situation can be both forward and reverse driving.The object of the invention is to achieve load distribution in situations outside normal operation to all three phases P 1-P 3 of the electric machine 1. Thus, 100% of the total torque and the maximum current of the electric machine 1 that can be taken up can then be used, as shown schematically in FIG. 2 (thick lines indicate the phases used).This is achieved by adjusting the driving method as shown in FIG. 3. Here, a request for forward driving is illustrated. The driving method makes use of the fact that very many mechanical components in the drive train, i.e. between the electric machine 1 and the wheel or wheels of the vehicle to be driven, have play and / or a certain elasticity. Such components in the drive train are, for example, reducer gears, articulated shafts, spline teeth, axle suspension, trailer coupling, flexible axle and motor bearings, plastic / rubber parts, etc. Their play and / or elasticity is used by the drive method to generate a prestress and thus a run-up, as described below.The control of the electric machine 1 in an operating situation in which the demanded torque M Fzg of the vehicle is greater than the torque M EM, that can be provided by the electric machine 1 for this situation (t=1 in FIG. 3 ) is effected in such a way that the torque M EM of the electric machine 1 is first reduced in terms of amount (in the case of torque demand) (time profile t=1 over t=2 to t=3 in FIG. 3 ). When forward running is requested, the torque M EM is reduced as shown in FIG. 3. When reverse travel is requested, the torque M EM is negative and needs to be increased. In both cases, the reduction in the amount of the torque M EM of the E-machine 1 takes place until the rotational direction of the E-machine 1 is reversed, as shown in FIG. 3. A mechanical prestress is thus generated in the components of the drive train which are subject to play and / or elasticity.In any case, the amount of torque M EM of the electric machine 1 is reduced (time profile t=1 over t=2 to t=3) until a mechanical prestress is reached in the drive train, but at which the vehicle is still at a standstill. That is, the wheel speed DZ Rad is at zero revolutions as shown in FIG. 3 until the time step t=4. The wheel speed DZ Rad can be monitored with a wheel speed sensor. As shown in FIG. 3, the bias voltage may be held for a period of time (t=2 to t=3) until an end stop of the bias voltage (t=3) is reached. It is important in the reduction of the torque M EM that this does not lead to the wheels of the vehicle turning in the not required direction, but the standstill of the vehicle is maintained, i.e. the end stop of the prestress (t=3) is not exceeded. This can also be monitored by wheel speed sensors.After the prestress, preferably a maximum possible prestress, has been reached, the torque M EM of the electric machine 1 is built up again (as actually requested). As a result of the previous reduction and generation of the prestress, a significantly longer starting travel is present in this case, in which the electric machine 1 can accelerate without load in the idle travel, that is to say without vehicle movement, as is illustrated in FIG. 3 by curve DZ EM( rotational speed E-machine), that is to say starting from t=3.In the driving method, a distinction is made from this point in time t=3 whether a starting situation is present, i.e. it is required that wheel rotational speed DZ Rad >0, i.e. M EM >M Fzg or whether a standstill situation is present, i.e. the vehicle should be held at standstill only with the accelerator pedal, i.e. without a brake, i.e. the wheel rotational speed DZ Rad should remain equal to zero.In the case of a starting situation, the torque M EM of the electric machine 1 is increased until it exceeds the torque M Fzg requested by the vehicle, which is indicated at time step t=4 in FIG. 3. Starting from this time t=4, the idle travel has been used up and the following applies: M EM > M Fzg, that is to say that the breakaway or roll-on torque is exceeded, so that the wheels and thus the vehicle are set in motion, as can be seen in the time steps t=5, t=6, etc., in particular on the basis of the curve of the wheel rotational speed DZ Rad.In the case of a standstill situation, after the prestress has been reached, the torque M EM of the electric machine 1 is built up again, but the torque M Fzg required by the vehicle is prevented from being exceeded. This is done by repeating the step of reducing the torque M EM of the electric machine 1 in terms of amount until the rotational direction of the electric machine is reversed shortly before the torque M Fzg required by the vehicle is reached. When the prestress (preferably the end stop) is reached, the torque M EM of the electric machine 1 is then built up again. This alternating reduction and re-establishment of the torque M EM of the electric machine 1 is repeated until an abort criterion is established.Various termination criteria can be provided in which the repetition of the control method, i.e. the alternating reduction and re-establishment of the torque M EM, is ended.An abort criterion may be that the brake is actuated. In this case, the process is ended.A further termination criterion may be that a starting is desired, i.e. the accelerator pedal is correspondingly actuated in order to transition from the standstill situation to a starting situation. In this case, the process is ended after the torque M EM of the E-machine is allowed to exceed the torque M EM demanded by the vehicle Fzg when the torque M is built up again (as in the first case in the starting situation).A further termination criterion may be that an overload of the electric machine 1 is detected. In this case, the method is ended, preferably immediately, i.e. no matter in which state (time step t) it is currently located. In this case, it can additionally be provided that an alarm is output, for example in the form of an acoustic signal (sound, speech) and / or optical signal (light) in the interior of the vehicle. An alarm can also be output in the form of a warning / warning message, for example as a message to a user via the HMI (human machine interface) of the vehicle, or to a mobile terminal.In principle, the driving method is ended whenever the torque M Fzg demanded by the vehicle is exceeded or is exceeded, or whenever an overload of the electric machine 1 is detected.In the case that the steps of reducing and re-establishing the torque M EM are repeated a number of times, the repetition rate can be effected with sinusoidal actuation. For high-load operation, the described method with the upward and downward swelling pendulum motion can be applied without taking into account acoustics in an overmodulated sinusoidal shape up to trapezoidal or rectangular actuation. This can contribute to minimizing acoustic noise due to the change in the torque M EM.In one embodiment, it can be provided that the method is not executed in each of the described operating situations M EM< M Fzg. Rather, it can be provided that it is carried out when it is sufficient to provide the torque M Fzg required by the vehicle that only two phases P 1, P 2 of the electric machine 1 are operated. This is the case when only a maximum of two thirds of the total torque of the electric machine 1 are required to satisfy the driving request. As a safety buffer, it can be installed that the method is already activated and executed starting from a torque M Fzg demanded by the vehicle of, for example, 50% or 60% of the total torque. In this embodiment, any acoustic noises that may occur due to the change in the torque M EM can be minimized.The driving method is implemented as a computer program on a suitable control device 3 of an at least partially electrically driven vehicle, i.e. both of purely electric vehicles and hybrid-operated vehicles. Even vehicles that use an electric motor only for starting and / or maneuvering can use this method. The control device 3 carries out the method and controls the electric machine accordingly via the inverter circuit. It also receives data from the wheel speed sensor or sensors to determine when a preload end stop is reached and limit the torque reduction accordingly.List of reference charactersM EM Torque of the engine M Fzg Requested torque of the vehicle DZ Rad Wheel speed DZ EM Speed E-engine t Period 1 E-engine 2 Inverters P 1-P 3 Phases

Claims

Driving method of an electric machine (1) of an at least partially electrically driven vehicle for an operating situation outside normal operation, wherein if the demanded torque (M Fzg) of the vehicle is greater than the torque (M EM) which can be provided by the electric machine (1) for this situation (t=1), the electric machine (1) is driven in such a way that the torque (M EM) of the electric machine (1) - in a first step (t=1 to t=3) - is reduced in terms of amount until a mechanical prestress is reached in the drive train, at which the vehicle is still at a standstill, however, and - in a second step after the prestress has been reached, the torque (M EM) of the electric machine (1) is built up again (t=3), wherein, in the event that a starting situation under load exists, in a third step, the torque (M EM) of the electric machine (1) is built up in such a way that it exceeds the torque (M Fzg) requested by the vehicle (t=4) and the vehicle is set in motion (t=5, 6...), or wherein, in the event that a standstill situation exists of an upward gradient, in which the vehicle is to be held at a standstill only by the electric machine (1), the first and the second steps are repeated until a termination criterion is fulfilled, on the basis of which the repetition is terminated.Driving method according to claim 1, wherein an abort criterion is one or a combination of: - requesting a start from standstill, in which case the torque (M EM) of the electric machine (1) is built up on the last execution of the second step in such a way that it exceeds the torque (M Fzg) requested by the vehicle, after which the driving method is ended, - actuating a brake, in which situation the driving method is ended, - detecting an overload of the electric machine (1), in which case the driving method is ended.Driving method according to Claim 2, wherein an alarm is additionally issued in the event of a detection of an overload of the electric machine (1).Driving method according to one of the preceding claims, wherein, if a predetermined minimum rotational speed (DZ EM) of the electric machine (1) is exceeded, the driving method is ended starting from this minimum rotational speed (DZ EM) being exceeded.Driving method according to one of the preceding claims, wherein the driving method is activated only when a torque (M EM) of the E-machine (1) is required at which more than a predefined percentage of the total torque which can be provided by the E-machine (1) is required.The driving method according to claim 5, wherein the predetermined percentage is set as 50% or more, or as 60% or more, or as two-thirds or more of the total torque.The driving method according to any one of the preceding claims, wherein in case the first and second steps are repeated, a sinusoidal driving repetition rate is used.Computer program product which carries out the method according to one of the preceding claims and can be executed on a control device (3) of an at least partially electrically driven vehicle.Control device (3) in an at least partially electrically driven vehicle, in which the computer program product according to Claim 8 is implemented.An electric drive, in particular an electric final drive, for a vehicle having at least one transmission device, an electric machine (1), an inverter circuit which actuates the electric machine (1), and a control unit (3) according to Claim 9 for actuating the electric machine (1) via the inverter circuit.

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