HYSTERESIS-BASED ACTUATION OF A VEHICLE DRIVE WITH AN ALTERNATING FREEWHEELING AND SHORT-CIRCUIT STATE IN THE EVENT OF A FAULT

Abstract

A method for actuating a vehicle drive. If a fault occurs in the electric drive or in a component connected thereto, the inverter generates an AKS state in the form of an active short circuit of phase connections of the electric machine, or a freewheeling state of the electric machine. Once the fault has occurred, steps AKS and F are executed alternately. Step AKS: setting the AKS state if a phase current flowing through the phase connections is below a first current limit and changing over to step F if the phase current reaches the first current limit; Step F: setting the freewheeling state if a DC voltage generated by the electric machine is below a first voltage limit and changing over to step AKS if the DC voltage is above the first voltage limit. A changeover from step AKS to step F if the phase current reaches a second current limit below the first current limit. A changeover from step F to step AKS if the DC voltage reaches a second voltage limit below the first voltage limit. A vehicle drive and a corresponding control program are used to execute the method.

Claims

1. A method for actuating a vehicle drive having an electric machine and an inverter which is connected to the electric machine for actuation purposes, wherein, if a fault occurs in the electric drive or in a component connected thereto, the inverter generates an AKS state in the form of an active short circuit of phase connections of the electric machine, or the inverter generates a freewheeling state of the electric machine, wherein, once the fault has occurred, the following steps AKS and F are executed alternately in an intermediate phase: Step AKS: setting the AKS state if a phase current flowing through the phase connections is below a first current limit and changing over to step F if the phase current reaches the first current limit; Step F: setting the freewheeling state if a DC voltage generated by the electric machine is below a first voltage limit and changing over to step AKS if the DC voltage is above the first voltage limit; wherein, furthermore, a changeover is made from step AKS to step F if the phase current reaches a second current limit, which is below the first current limit, and wherein a changeover is made from step F to step AKS if the DC voltage reaches a second voltage limit, which is below the first voltage limit.

2. The method as claimed in claim 1, wherein in a preliminary phase, which begins at or after the occurrence of the fault and takes place before the intermediate phase, an AKS state or a freewheeling state is set until the intermediate phase is reached.

3. The method as claimed in claim 1, wherein in a subsequent phase, which takes place after the intermediate phase, an AKS state or a freewheeling state is set until the end of the subsequent phase.

4. The method as claimed in claim 1, wherein the intermediate phase is ended if the first current limit is not reached in step AKS.

5. The method as claimed in claim 1, wherein if a fault occurs before the intermediate phase in a preliminary phase, an AKS state is set if a rotational speed of the electric machine is below a rotational speed limit and a freewheeling state is set if the rotational speed of the electric machine is above a rotational speed limit.

6. The method as claimed in claim 5, wherein at the beginning of the preliminary phase it is ascertained whether the rotational speed of the electric machine is above or below the rotational speed limit by measuring a rotational speed by a rotational speed sensor and comparing said rotational speed with the rotational speed limit.

7. The method as claimed in claim 5, wherein at the beginning of the preliminary phase it is ascertained whether the rotational speed of the electric machine is above or below the rotational speed limit by setting a freewheeling state at the beginning of the preliminary phase, during which state the DC voltage is measured and is compared with a voltage threshold value, wherein if the voltage threshold value is exceeded a rotational speed above the rotational speed limit is assumed and if the voltage threshold value is fallen below a rotational speed below the rotational speed limit is assumed.

8. The method as claimed in claim 5, wherein at the beginning of the preliminary phase it is ascertained whether the rotational speed of the electric machine is above or below the rotational speed limit by setting an AKS state at the beginning of the preliminary phase, during which state the phase current is measured and is compared with a current threshold value, wherein if the current threshold value is exceeded a rotational speed above the rotational speed limit is assumed and if the current threshold value is fallen below a rotational speed below the rotational speed limit is assumed.

9. An electric vehicle drive having an inverter and an electric machine, wherein provision is made for a control apparatus which is connected to the inverter for actuation purposes and is configured to execute the method as claimed in claim 1.

10. A vehicle drive control program which is configured to carry out the method as claimed in claim 1.

11. The method as claimed in claim 2, wherein in a subsequent phase, which takes place after the intermediate phase, an AKS state or a freewheeling state is set until the end of the subsequent phase.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] FIGS. 1 and 2 are used to further explain exemplary embodiments according to the procedure described here. In particular, FIG. 2 is used to illustrate the effects of repeatedly changing over between the AKS state and freewheeling state, as also occurs in the procedure according to an aspect of the invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0031] FIG. 1 shows a diagram for illustrating the limits (and threshold values) described here which in particular are used in the method. The phase current I.sub.(U/V/W) is plotted on the x-axis, while the DC voltage on the DC side of the inverter (i.e. the intermediate-circuit voltage) is represented on the y-axis, here illustrated by U.sub.DC. If the DC voltage is below the voltage threshold value UBat(max), a freewheeling state can be set permanently, which state is denoted as 6SO here. If a freewheeling state, in which the DC voltage is below this voltage threshold value, is thus set, a freewheeling state can be set irrespective of the current limits and voltage limits and in particular continues (in contrast to the alternating AKS and freewheeling state according to the method). If the phase current I.sub.(U/V/W) is below the value I.sub.ASC(peak), an AKS state can be set, which state is denoted as 3PS here. This can also then be permanent, in particular irrespective of the voltage limits or current limits described here. If both first limits (current limit, voltage limit) are thus not reached, either an AKS state or a freewheeling state can be set.

[0032] For higher phase currents or DC voltages (greater than U.sub.Bat(max) and I.sub.ASC(peak), respectively), a changeover is made between the freewheeling state and the AKS state depending on voltage limits UG1, UG2 and current limits IG1, IG2. For this purpose, the first current limit IG1 and the second current limit IG2 (which together form a current limit range OC) are plotted in FIG. 1. Furthermore, a first voltage limit UG1 and a second voltage limit UG2, which form the voltage limit range OV, are plotted. Both the current limits IG1, IG2 and the voltage limits UG1, UG2 form (in each case) limits of a hysteresis and together form the limits of the common current-dependent and voltage-dependent hysteresis response.

[0033] If the limits of the illustrated hystereses (Hyst.) are exceeded, this leads to a changeover being made from the AKS state to the freewheeling state, or vice versa. Since the voltage limits UG1, UG2 are relevant in the freewheeling state and the current limits IG2, IG1 are relevant in the AKS state for the switching over, the illustrated voltage and current hystereses can be assigned to a common hysteresis response. In other words, in the AKS state only the current limits IG1, IG2 are relevant for the switching over or for the changing over between the steps AKS and F, and not the voltage limits UG1, UG2. In the same way, in the freewheeling state the current limits IG1, IG2 are not relevant but only the voltage limits UG1, UG2.

[0034] This thus results in a hysteresis range, which is denoted by Hyst. and which leads to the state being changed over when the first limits UG1, IG1 are exceeded, that is to say from the AKS state to the freewheeling state or vice versa, and to the state likewise being changed over when the second limits are fallen below, that is to say the limits UG2 and IG2, respectively.

[0035] It can further be seen from FIG. 1 that, at phase voltages above the first voltage limit UG1, an AKS state is set in order to thus avoid an excessive open-circuit voltage being permanently present at the inverter. The DC voltage in question may be a measured or in particular extrapolated voltage, that is to say an estimated voltage which would occur at the operating parameters in question if a freewheel were to be set.

[0036] In the same way, a freewheel F is set if a (likewise estimated) phase current which would be above the first current limit IG1 would occur. In this case, as illustrated, a freewheeling state F is set. Above these current and voltage limit ranges UV, OC, a state is thus permanently set, in particular a state which is independent of a hysteresis response, that is to say independent of the second limits.

[0037] FIG. 2 illustrates exemplary current and voltage characteristics which illustrate the effect of repeatedly changing over between freewheel and AKS, as also occurs in the procedure according to an aspect of the invention.

[0038] In the upper half of the diagram, the characteristic of a phase voltage is illustrated, here by way of example in a range from 320 V to 540 V. In the lower half of the diagram, the characteristic of a phase current is illustrated in a range from 1200 A to 1200 A. The same time axis (x-axis) is provided for both illustrations, wherein a period of time of between 0 ms and 7 ms is shown here.

[0039] The DC voltage UDC illustrated in the upper half results from the voltage on the DC side of the inverter and in particular from the phase voltages (on the AC side of the inverter) by way of rectification (via the inverter). Generally, instead of the DC voltage, the RMS or peak value of one of the phase voltages (or of all of the phase voltages) can be used as a measure of the DC voltage. The voltage between two phase connections is referred to as the phase voltage.

[0040] The phase currents of the individual phases are referred to as IU, IV and IW. In the diagram, all three phase currents are illustrated but embodiments of the procedure illustrated here can also relate to only one of these phase currents, or an (arithmetic) sum of the phase currents, in particular a sum of the absolute values of the individual phase currents IU, IV and IW.

[0041] Between the times 0 ms and 1.5 ms, FIG. 2 illustrates the current and voltage characteristic during fault-free operation. In FIG. 2, which illustrates a simulation result, a fault is assumed at the time 1.5 ms (frozen PWM, i.e. no clocking pulse width modulation). As a result, the voltage UDC drops in this range, cf. in particular the range FE in which a fault takes place. At the end of the range FE, a short phase in which a freewheel F takes place begins. Setting the freewheel F is the reaction to the fault which occurs at the time 1.5 ms. It can be seen that during this section of time which begins at the end of FE and ends at the start of AKS1, the voltage UDC rises considerably. The voltage UDC corresponds to the DC voltage generated by the electric machine (on the DC side of the inverter). It can further be seen that the currents IU, IV and IW between the phase FE and the phase AKS1, i.e. in the phase of the freewheel F, decrease or are attenuated with respect to their previous characteristic.

[0042] The increase in the DC voltage UDC or the attenuation of the currents IU to IW ends at the beginning of the phase AKS1 which illustrates an AKS state. The transient response illustrated in FIG. 2 of UDC at the beginning of AKS1 is explained by the inductances of the electric machine.

[0043] The AKS state AKS1 extends from about 1.6 m/s to 3.4 m/s. This is directly followed by an intermediate phase T in which the steps AKS and F are executed alternately. In the procedure according to an aspect of the invention, the changeover takes place in accordance with a hysteresis response with two current limits and two voltage limits. In order to simplify the illustrated simulation, FIG. 2 has been based on a periodic (i.e. time-dependent) changing over between AKS and F in the intermediate phase T. This illustrates the effect of repeatedly changing over between AKS and F, as would essentially also occur in the hysteresis-based procedure according to an aspect of the invention for changing over between AKS and F. Although the hysteresis-based alternation of AKS and F according to an aspect of the invention would not lead to exactly periodic changing over, the illustrated simulation does approximate repeatedly changing over between AKS and F precisely enough to roughly illustrate the effects of the intermediate phase according to an aspect of the invention on current and voltage.

[0044] It can be seen that in the intermediate phase T, by virtue of the repeated changing over, the magnetization of the inductances is reduced little by little and that the voltage UDC increases with an increasingly flatter gradient after each changeover to a freewheel. It can therefore be seen that by virtue of the intermediate phase or the changing over between AKS and F, magnetization energy can be reduced in a controlled way and the voltage increase flattens little by little during the changeover to a freewheeling state. Similarly, it can be seen that the currents and also the current surges are likewise limited. This controlled reduction therefore allows the occurrence of a fault in a permanently excited synchronous machine to be handled in a controlled way.

[0045] In contrast to FIG. 1 which is based on a periodic changing over between AKS and F, in a hysteresis-based switching over in the intermediate phase T, the individual spikes in the voltage UDC would be of the same level since an upper voltage limit would define the changeover from F to AKS and would thus define the upper limit for the voltage spikes (cf. section A).

[0046] The voltage spikes in the voltage UDC result exactly at the beginning of the phase AKS2 due to the fact that in the illustrated simulation at the end of the intermediate phase T a freewheeling state has been established for a period of time longer than the duration between a changeover between AKS and F in the intermediate phase T.

[0047] The intermediate phase T is followed by a second phase in which an AKS state exists (the first phase in which an AKS state exists is denoted by AKS1). It can be seen that the DC voltage UDC there behaves essentially constantly after a transient process and that decreasing currents IU to IW result over the course of time.

[0048] Furthermore, FIG. 2 illustrates that there may be a preliminary phase VP (corresponding to the phase AKS1) before the intermediate phase T and that there may be a subsequent phase NP (corresponding to the phase AKS2) after the intermediate phase T. In the preliminary phase VP, the phase AKS1 is preceded by a fault phase FE, wherein a freewheeling phase F is provided between the fault phase FE and the phase AKS1. In other embodiments, only the state AKS1 and not the fault phase FE is included in the preliminary phase VP. The freewheeling phase F, which precedes the phase AKS, may also be included in the preliminary phase VP together with the phase AKS1.

[0049] In the intermediate phase T, it can furthermore be seen that the currents decrease due to the changing over between AKS and F, see reference sign E, wherein in the range C the effects on one of the phase currents is illustrated if AKS and F are set alternately: There, the current is constant except for a ripple, wherein this effect also results when hysteresis-based switching over between AKS and F is made and the lower limit on a current limit is illustrated.