Method for controlling a hybrid vehicle, control device and hybrid vehicle

Abstract

The invention relates to a method for controlling a hybrid vehicle which comprises a mode for series driving, in which a first electric machine can be driven by an internal combustion engine for generating electrical power in order to feed electrical power via a DC link to a second electric machine, which serves as a drive machine for at least one wheel of the hybrid vehicle, the hybrid vehicle further comprising a high-voltage power supply connected to the DC link, a low-voltage power supply and a back-up low-voltage power supply supplied with power via the DC link, wherein the method comprises: identifying a drop in the low-voltage power supply, in the event of which the high-voltage power supply is disconnected from the DC link, the DC link is actively discharged and the power electronics unit is actively shorted, detecting a rotation speed of the second electric machine and, if the rotation speed of the second electric machine is greater than or equal to a specified rotation speed, driving the first electric machine using the internal combustion engine in such a way that at least a specified minimum voltage is applied to the DC link, so that the back-up low-voltage power supply is supplied with power and operates, and maintaining the active short in the power electronics unit.

Description

[0001] Description

[0002] Method for controlling a hybrid vehicle, control device and hybrid vehicle

[0003] The present invention relates to a method for controlling a hybrid vehicle, which comprises a serial drive mode. In the serial drive mode, a first electric machine for generating electrical energy can be driven by an internal combustion engine in order to supply electrical energy via a DC link to a second electric machine, which serves as a drive motor for at least one wheel of the hybrid vehicle. The hybrid vehicle comprises, in particular, a high-voltage power supply connected to the DC link, a low-voltage power supply, and a backup low-voltage power supply supplied via the DC link. Furthermore, the invention relates to a control device that is designed and programmed to carry out the method, and to a hybrid vehicle having the control device.

[0004] A hybrid vehicle that enables a serial hybrid drive operating mode, in which an internal combustion engine acts as a prime mover for a first electric machine for generating electrical energy to use the electrical energy to operate a second electric machine as a prime mover for a transmission input shaft, i.e., for wheels of the hybrid vehicle, is known from DE 10 2017 127 695 A1. The disclosure content thereof is incorporated herein by reference.

[0005] In such a hybrid vehicle, an electric machine can be designed such that a back-EMF (back EMF) generated by the electric machine is higher than a dielectric strength or a permissible collector-emitter voltage of a transistor, e.g., an IGBT, in a power electronics unit of the electric machine, which can lead to problems with high-voltage safety. To prevent the dielectric strength from being exceeded by the back-electromotive force during operation, suitable control mechanisms, such as an adapted field-oriented control or regulation or an active short circuit of the power electronics unit, are implemented.In the event of a fault in which a low-voltage power supply fails, operation of such control mechanisms can no longer be maintained due to a failure of a control device provided for this purpose, so that the generation of a high counter-electromotive force leads to damage to the power electronics unit, in particular to the transistors in the power electronics unit.

[0006] When developing and implementing countermeasures, certain limitations must also be considered. No power supply is available that can be started within a sufficiently short time after a low-voltage power supply failure and a drop in the intermediate circuit voltage below a specified minimum voltage at which the backup low-voltage power supply can no longer operate, to maintain the active short circuit in the power electronics unit to prevent damage to the transistors. Furthermore, space requirements, component availability, and the associated costs must be considered.

[0007] It is therefore an object of the invention to eliminate or at least mitigate these disadvantages. In particular, it is an object of the invention to provide a technology that reliably prevents damage to a power electronics unit, i.e., the transistors used therein, even when a high counter-electromotive force occurs in the event of a low-voltage power supply failure.

[0008] This object is achieved by the subject matter having the features according to the independent claims. Advantageous embodiments are the subject matter of the dependent claims.

[0009] The method according to the invention serves to control a hybrid vehicle comprising a serial drive mode in which a first electric machine for generating electrical energy can be driven by an internal combustion engine to supply electrical energy via a DC link to a second electric machine, which serves as a drive motor for at least one wheel of the hybrid vehicle. The hybrid vehicle further comprises a high-voltage power supply connected to the DC link, a low-voltage power supply, and a backup low-voltage power supply supplied via the DC link.

[0010] The method comprises the following steps: detecting a failure of the low-voltage power supply, in which a separation of the high-voltage power supply from the DC link, an active discharge of the DC link, and an active short circuit in the power electronics unit occur; detecting a speed of the second electrical machine, and if the speed of the second electrical machine is greater than or equal to a predetermined speed, driving the first electrical machine by the internal combustion engine such that at least a predetermined minimum voltage is applied to the DC link, so that the backup low-voltage power supply is supplied and operates, and maintaining the active short circuit in the power electronics unit.

[0011] The first electric motor can be driven by the internal combustion engine in such a way that at least the specified minimum voltage is applied to the DC link until the speed of the second electric motor is less than a speed threshold. Consequently, destruction of the power electronics unit can be reliably prevented.

[0012] The speed threshold can be set to the specified speed. This allows for easy setting of the speed threshold.

[0013] Furthermore, the speed threshold can be set to a speed that is lower than the predefined speed. Consequently, a safety margin can be taken into account, thus reliably preventing damage to the power electronics unit. The method can comprise the following step: after the speed of the second electric machine is lower than the speed threshold, controlling the first electric machine such that a predefined voltage that is greater than the minimum voltage is applied to the DC link. Consequently, control mechanisms can be provided in the hybrid vehicle even after the second electric machine falls below the predefined speed.

[0014] The method may additionally comprise the following step: if the speed of the second electrical machine is lower than the specified speed and a specified time has elapsed since the voltage in the DC link fell below the specified minimum voltage, reconnecting the high-voltage power supply to the DC link so that the backup low-voltage power supply is supplied and operating, and switching the power electronics unit of the second electrical machine into the active short circuit. Consequently, the high-voltage power supply is reconnected to the DC link, and the backup low-voltage power supply can be operated. The power electronics unit can then be switched into the active short circuit so that a counter-induced voltage can be dissipated.

[0015] The specified speed can be set between 9000 and 10500 rpm. Preferably, the specified speed can be set between 9000 and 9500 rpm. Particularly preferably, the specified speed can be set to 9344 rpm. At these speeds, an increase in the counter-electromotive force beyond the dielectric strength of the transistors in the power electronics unit is reliably prevented, even at low temperatures.

[0016] A low-voltage power supply failure can be detected when the voltage falls below a specified threshold. Consequently, low-voltage power supply failure detection can be easily implemented.

[0017] A control device according to the invention is designed and programmed to execute the method according to one of the above aspects. The control device is accordingly designed and programmed to receive corresponding input signals, evaluate and process them, and output corresponding output signals or control signals to the corresponding components of the hybrid vehicle. Consequently, the control device can provide the same advantages as the method according to the invention.

[0018] A hybrid vehicle according to the invention comprises the control device according to the above aspect. Accordingly, the hybrid vehicle can provide the same advantages as the method according to the invention.

[0019] The present invention is described in detail below with reference to the figures. They show:

[0020] Fig. 1 shows a time profile of a speed of an electric machine when a vehicle coasts to a stop without active braking;

[0021] Fig. 2 shows a curve of a counter-electromotive force over the speed of the electric machine;

[0022] Fig. 3 shows a curve of the dielectric strength of a transistor over a temperature;

[0023] Fig. 4 shows time profiles of an intermediate circuit voltage, a voltage of a low-voltage power supply, a voltage of a backup low-voltage power supply, a voltage of an internal low-voltage power supply, a trigger signal for an active short circuit and a trigger signal for an active discharge in the event of a failure of the low-voltage power supply;

[0024] Fig. 5 shows time profiles of the intermediate circuit voltage, the voltage of the low-voltage power supply, the voltage of the backup low-voltage power supply, the voltage of the internal low-voltage power supply, the trigger signal for the active short circuit, and the trigger signal for the active discharge upon failure of the low-voltage power supply and reconnection of a high-voltage power supply to an intermediate circuit; and Fig. 6 shows a flowchart of a method for controlling a hybrid vehicle, including a serial drive mode, according to the present invention.

[0025] The present invention is described below with reference to the figures. However, the following description should not be considered limiting or exhaustive.

[0026] In a hybrid vehicle that includes a serial drive mode, a first electric machine for generating electrical energy is driven by an internal combustion engine. The electrical energy generated by the first electric machine is supplied to a second electric machine, which serves as the drive motor for at least one wheel of the hybrid vehicle, via a DC link. Of course, a transmission and / or corresponding connecting power transmission elements can be provided between the second electric machine and the at least one wheel.

[0027] The second electrical machine can be designed such that a generateable back electromotive force (back EMF) is higher than a dielectric strength or a permissible collector-emitter voltage of transistors installed in a power electronics unit (PEU), e.g., in an inverter, of the second electrical machine. To prevent an increase in the back electromotive force beyond the dielectric strength of the transistors, suitable control mechanisms can be implemented during normal operation, such as an adapted field-oriented control or an active short circuit in which the transistors in the power electronics unit are short-circuited to dissipate the back electromotive force.However, a failure of the low-voltage power supply results in such control mechanisms no longer being available due to a failure of a control device responsible for them, causing damage to the power electronics unit, in particular the transistors used therein, when a high counter-electromotive force occurs. The hybrid vehicle further comprises a high-voltage power supply connected to the DC link, in particular a high-voltage battery, which can alternatively or additionally provide electrical energy to drive the second electric machine. Furthermore, electrical energy can be stored in the high-voltage power supply in the event of recuperation by the second electric machine.

[0028] The hybrid vehicle also includes a low-voltage power supply, in particular a low-voltage battery, to provide an internal low-voltage power supply for lower-power consumers, such as the control device.

[0029] In addition, the hybrid vehicle has a backup low-voltage power supply powered by the DC link. This means that the backup low-voltage power supply draws energy from the DC link and converts it into lower-voltage energy. The backup low-voltage power supply is activated in the event of a low-voltage power supply failure and then takes over the internal low-voltage power supply, for example, to power the control device.

[0030] As described, the backup low-voltage power supply is supplied via the DC link and, in the event of a low-voltage power supply failure, takes over the internal low-voltage power supply to enable an active short circuit in the power electronics unit of the second electric machine. However, if the low-voltage power supply fails, the high-voltage power supply is also disconnected from the DC link to prevent uncontrolled feedback. Essentially at the same time, an active discharge of the DC link occurs, so that the DC link voltage in the DC link drops below a specified minimum voltage after some time.Once this minimum voltage is exceeded, the backup low-voltage power supply can no longer be supplied, meaning that an active short circuit in the power electronics unit of the second electric machine and active discharge of the intermediate circuit can no longer be maintained. The following section first describes the relationship between the speed of the second electric machine and the counter-electromotive force generated. Furthermore, the dielectric strength of the transistors installed in the power electronics unit of the second electric machine is explained.

[0031] A failure of the low-voltage power supply can occur while driving at high speed or while driving downhill. If active braking is not applied, the hybrid vehicle will only come to a stop after some time due to friction and air resistance. Fig. 1 shows a speed curve of the second electric motor over time in such a situation. It can be seen that if the low-voltage power supply fails at 0 rpm, the second electric motor rotates at a speed of approximately 13,000 rpm. The hybrid vehicle and the second electric motor only come to a stop after approximately 5 minutes.

[0032] Fig. 2 shows the relationship between the intermediate circuit voltage in the DC link, which results from the counter-electromotive force generated in the second electrical machine, and the speed of the second electrical machine. It can be seen that the intermediate circuit voltage increases approximately linearly with the speed in the speed range under consideration. A DC link voltage of 700 V is achieved at a speed of 9344 rpm and a DC link voltage of 750 V at a speed of 10096 rpm. Furthermore, it was determined that the intermediate circuit voltage increases at a rate of 530 V / ms when the counter-electromotive force is fed back into the DC link.

[0033] Fig. 3 shows a curve of the dielectric strength, or rather a permissible collector-emitter voltage, for the transistors used in the power electronics of the second electric machine over temperature. It can be seen that the dielectric strength remains constant at 750 V at a temperature greater than 25 °C. Below 25 °C, the dielectric strength decreases linearly to a value of 700 V at a temperature of -40 °C. Since such low temperatures must also be considered in the automotive sector, the intermediate circuit voltage generated by the counter-electromotive force must remain below 700 V to prevent damage to the power electronics unit of the second electric machine at all costs.

[0034] As can be seen from the diagram in Fig. 2, the speed of the second electric motor must be less than 9344 rpm. However, the diagram in Fig. 1 shows that a period of approximately 1 minute is required for the speed of the second electric motor to decrease to a lower value during coasting without active braking, thus eliminating the risk of damage to the transistors in the power electronics unit.

[0035] Figure 4 illustrates the time profile of the intermediate circuit voltage, a voltage of the low-voltage power supply, a voltage of the backup low-voltage power supply, a voltage of the internal low-voltage power supply, a trigger signal for the active short circuit in the power electronics unit, and a trigger signal for the active discharge of the DC intermediate circuit in the event of a low-voltage power supply failure. As can be seen, the low-voltage power supply begins to drop and, after a while, falls below a specified voltage threshold of 6 V. If this voltage threshold is undershot, a failure of the low-voltage power supply is determined. The voltage threshold for the low-voltage power supply can also be set to a different value between 6 and 9 V.

[0036] Once the voltage of the low-voltage power supply has fallen below the voltage threshold, the backup low-voltage power supply takes over the supply of electrical energy to the internal low-voltage power supply in the hybrid vehicle, so that the internal power supply is operated at the voltage of the backup low-voltage power supply, currently 8.25 V. Of course, the backup low-voltage power supply can also have a different voltage value.

[0037] Essentially at the same time as the voltage falls below the specified threshold, the high-voltage power supply is disconnected from the DC link. In addition, the active short circuit in the power electronics unit of the second electrical machine and the active discharge of the DC link are triggered. The active discharge is carried out for a duration of 2 s, as this time is required for the DC link voltage to drop to the minimum voltage of 60 V. If the DC link voltage falls below this minimum voltage, operation of the backup low-voltage power supply can no longer be maintained. Consequently, the active short circuit in the power electronics unit of the second electrical machine can no longer be maintained due to a failure of the required control device, and the transistors in the power electronics unit are in freewheeling mode.For the same reason, active discharge of the DC link is no longer possible. It should be noted that the minimum voltage can also have a different value, e.g., 50 V.

[0038] If, as shown in the example in Fig. 1, a failure of the low-voltage power supply occurs at a high speed of the second electric machine, the time until the speed falls below the specified speed, below which there is no longer any risk of damage to the power electronics unit, is considerably longer than 2 s. Accordingly, two cases must be distinguished when controlling the hybrid vehicle. In the first case, the speed is lower than the specified speed when the low-voltage power supply of the second electric machine fails, and there is no risk of destruction of the power electronics unit, i.e. the transistors installed therein. In the second case, the speed of the second electric machine is higher than the specified speed when the low-voltage power supply fails, and the power electronics unit, i.e. the transistors, are destroyed.

[0039] First, the case in which the speed is below the specified speed will be described using Fig. 5. According to the scenario described above, the specified speed is set to 9344 rpm to reliably prevent transistor destruction even at a low temperature of -40°C. Fig. 5 shows the time profiles of an intermediate circuit voltage, the voltage of the low-voltage power supply, the voltage of the backup low-voltage power supply, the voltage of the internal low-voltage power supply, the trigger signal for the active short circuit, and the trigger signal for the active discharge.

[0040] At time t1, the hybrid vehicle is first started and the low-voltage power supply is switched on and supplies the internal low-voltage power supply.

[0041] At time t2, the high-voltage power supply is connected to the DC link and the DC link voltage rises to a nominal voltage of 350 V by time t3.

[0042] At time t3, the backup low-voltage power supply supplied from the DC link is initialized to provide a voltage of 8.25 V.

[0043] At time t4, the low-voltage power supply begins to decrease. Consequently, the voltage of the internal low-voltage power supply also decreases accordingly.

[0044] At time t5, the voltage of the low-voltage power supply falls below the specified voltage threshold of 6 V, and the backup low-voltage power supply takes over the supply of power to the internal low-voltage power supply. Consequently, the voltage of the internal low-voltage power supply also drops to the voltage of the backup low-voltage power supply of 8.25 V. At essentially the same time, the high-voltage power supply is disconnected from the DC link, triggering an active short circuit in the power electronics unit and an active discharge of the DC link. Consequently, the DC link voltage decreases.

[0045] At time t6, the voltage of the low-voltage power supply has dropped to 0 V. At time t7, the intermediate circuit voltage has dropped to the minimum voltage of 60 V, and consequently, no more power can be supplied to the internal low-voltage power supply by the backup low-voltage power supply. From this point on, the active short circuit and active discharge can no longer be maintained because a dedicated control device is no longer supplied with power.

[0046] However, since the speed of the second electric machine is below a predetermined speed, the power electronics unit or the transistors installed therein are not destroyed, since the counter electromotive force and thus the intermediate circuit voltage is lower than the dielectric strength of the transistors.

[0047] At time t8, the high-voltage power supply is reconnected to the DC link and the DC link voltage increases accordingly.

[0048] At time t9, the intermediate circuit voltage exceeds 60 V, and consequently, the backup low-voltage power supply can operate again and supply the active short-circuit control device with electrical energy via the internal low-voltage power supply. However, the active discharge is not triggered.

[0049] After the active short circuit has been performed for a specified time, the high-voltage power supply can be disconnected from the DC link again, and the transistors in the power electronics unit can be switched into freewheeling mode. This cycle of active short circuit and freewheeling can also be repeated.

[0050] Since, at a high speed of the second electric machine, the intermediate circuit voltage would rise rapidly at a rate of 530 V / ms after the active short circuit is terminated at time t7, the procedure for reconnecting the high-voltage power supply shown in Fig. 5 cannot be applied to prevent destruction of the power electronics unit of the second electric machine. Consequently, a different procedure is required for a high speed in the event of a failure of the low-voltage power supply. A method that takes this circumstance into account will be described below using the flowchart shown in Fig. 6.

[0051] After the hybrid vehicle has started, a failure of the low-voltage power supply is continuously checked in step S1. As already mentioned, a failure can be detected if the voltage of the low-voltage power supply falls below a specified voltage threshold, e.g., 6 V.

[0052] If it is determined in S1 that there is no failure of the low-voltage power supply (NO in S1), the method proceeds to step S2. Normal operation of the hybrid vehicle then occurs, in which an increase in the counter-electromotive force beyond the dielectric strength is ensured, for example, by control mechanisms, e.g., a suitable field-oriented control or an active short circuit.

[0053] Otherwise (YES in S1), the method proceeds to step S3. In step S3, the high-voltage power supply is disconnected from the DC link. In addition, the active short circuit in the power electronics unit of the second electric machine and the active discharge in the DC link are triggered.

[0054] The method then goes to step S4, in which it is checked whether the speed of the second electric machine is greater than the predetermined speed, e.g. 9344 rpm.

[0055] If the speed of the second electrical machine is lower than the predefined speed (NO in S4), the method proceeds to step S5 and the control described with reference to Fig. 5 is carried out. Otherwise (YES in S4), the method proceeds to step S6, in which the first electrical machine is driven by the internal combustion engine in such a way that at least the predefined minimum voltage is present in the DC link. The predefined minimum voltage corresponds to the DC link voltage, e.g. 60 V, at which the backup low-voltage power supply can operate. Consequently, an active short circuit can be maintained in the power electronics unit of the second electrical machine, thus preventing destruction of the power electronics unit, in particular of the transistors installed therein. However, the first electrical machine can also be driven in such a way that a higher voltage than the minimum voltage is present in the DC link.

[0056] The method then proceeds to step S7, where it is checked whether the speed of the second electric machine has fallen below a speed threshold. The speed threshold can be set to the specified speed, e.g., 9344 rpm, or to a lower speed, e.g., 600 rpm. If the current speed of the second electric machine is greater than the speed threshold (NO in S7), the first electric machine continues to be driven by the internal combustion engine in step S6 such that at least the minimum voltage is present in the DC link.

[0057] Otherwise (YES in S7), the method goes to step S8, in which the first electrical machine is controlled such that a predetermined voltage, e.g. 70 V, which is greater than the minimum voltage, is applied to the DC link.

[0058] The method then proceeds to step S9, where it checks whether the first electric machine is still rotating. If this is the case (YES in S9), the method returns to step S8.

[0059] Otherwise (NO in S9), the method proceeds to S10 and the hybrid vehicle is shut down. This method can thus prevent the destruction of a power electronics unit in a case where the transistors installed therein have a lower dielectric strength than the counter-electromotive force generated by the second electric machine.

[0060] The invention can also be implemented by a control device that is configured and programmed to execute the method. The control device is accordingly configured and programmed to receive corresponding input signals, evaluate and process them, and output corresponding output signals or control signals to the corresponding components of the hybrid vehicle. The control device can be implemented by a dedicated computing unit or by distributed computing units. The control device can then be installed in the hybrid vehicle.

Claims

Claims 1. A method for controlling a hybrid vehicle comprising a serial drive mode in which a first electric machine for generating electrical energy is drivable by an internal combustion engine to supply electrical energy to a second electric machine serving as a drive motor for at least one wheel of the hybrid vehicle via a DC link, wherein the hybrid vehicle further comprises a high-voltage power supply connected to the DC link, a low-voltage power supply, and a backup low-voltage power supply supplied via the DC link, the method comprising the following steps: Detection (S1) of a failure of the low-voltage power supply, in which the high-voltage power supply is separated from the DC link, an active discharge of the DC link and an active short circuit in the power electronics unit occur, Detecting (S4) a rotational speed of the second electrical machine, and if the rotational speed of the second electrical machine is greater than or equal to a predetermined rotational speed (S4: YES), driving (S6) the first electrical machine by the internal combustion engine such that at least a predetermined minimum voltage is applied to the DC intermediate circuit, so that the backup low-voltage energy supply is supplied and operates, and maintaining (S6) the active short circuit in the power electronics unit.

2. The method according to claim 1, wherein the first electric machine is driven by the internal combustion engine such that at least the predetermined minimum voltage is applied to the DC link until the speed of the second electric machine is less than a speed threshold value.

3. The method according to claim 2, wherein the speed threshold is set to the predetermined speed.

4. The method according to claim 2, wherein the speed threshold is set to a speed that is less than the predetermined speed.

5. The method according to any one of claims 2 to 4, comprising the following step: after the rotational speed of the second electrical machine is less than the rotational speed threshold, controlling (S8) the first electrical machine such that a predetermined voltage which is greater than the minimum voltage is applied to the DC link.

6. Method according to one of the preceding steps 1 to 5, comprising the following step: if the speed of the second electrical machine is less than the predetermined speed and a predetermined time has elapsed since the voltage in the DC link has fallen below the predetermined minimum voltage, reconnecting (S5) the high-voltage power supply to the DC link so that the backup low-voltage power supply is supplied and operates, and switching (S5) the power electronics unit of the second electrical machine into the active short circuit.

7. Method according to one of the preceding claims 1 to 6, wherein the predetermined speed is set between 9000 and 10500 rpm, preferably between 9000 and 9500 rpm and particularly preferably at 9344 rpm.

8. Method according to one of the preceding claims 1 to 7, wherein a failure of the low-voltage power supply is detected when a predetermined voltage threshold is undershot.

9. Control device for controlling a hybrid vehicle, which comprises a serial drive mode in which a first electric machine for generating electrical energy is drivable by an internal combustion engine in order to supply electrical energy to a second electric machine, which serves as a drive machine for at least one wheel of the hybrid vehicle, via a DC intermediate circuit, wherein the hybrid vehicle further comprises a high-voltage power supply connected to the DC intermediate circuit, a low-voltage power supply and a backup low-voltage power supply supplied via the DC intermediate circuit, characterized in that the control device is designed and programmed to carry out the method according to one of claims 1 to 7.

10. A hybrid vehicle comprising a serial drive mode in which a first electric machine for generating electrical energy is drivable by an internal combustion engine to supply electrical energy via a DC link to a second electric machine serving as a drive machine for at least one wheel of the hybrid vehicle, the hybrid vehicle further comprising a high-voltage power supply connected to the DC link, a low-voltage power supply, and a backup low-voltage power supply supplied via the DC link, characterized in that it comprises the control device according to claim 9.

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