METHOD FOR OPERATING AN ELECTRIC MACHINE, DEVICE FOR OPERATING AN ELECTRIC MACHINE, AND ELECTRIC DRIVE SYSTEM

Abstract

The invention relates to a method for operating an electric machine (2), in particular in a motor vehicle, the machine (2) comprising a rotatably mounted rotor and a motor winding that is electrically connected to an electrical energy store (4) by means of a power electronics (3). In said method, by triggering the power electronics (3), the electric machine (2) is controlled in a field-oriented manner in such a way that the machine (2) generates a specified desired torque (T.sub.target). According to the invention, when it is ascertained that based on a current actual working point (AP1) of the electric machine (2) a specified desired working point (AP2) of the electric machine (2) should be set at least substantially in a time-optimized manner, a predicted pilot control action is specified, and that the desired working point (AP2) is set by triggering the power electronics (3) according to the predicted pilot control action.

Claims

1. A method for operating an electric machine, wherein the machine (2) comprises a rotatably mounted rotor and a motor winding, wherein the motor winding is electrically connected to an electrical energy store (4) by means of a power electronics (3), the method comprising: triggering the power electronics (3) to control the electric machine (2) in a field-oriented manner so that the machine (2) generates a specified desired torque (T.sub.target), wherein when it is determined that based on a current actual working point (AP1) of the electric machine (2) a specified desired working point (AP2) of the electric machine (2) should be set at least substantially in a time-optimized manner, a predicted pilot control action is specified, and the desired working point (AP2) is set by triggering the power electronics (3) according to the predicted pilot control action.

2. The method according to claim 1, wherein the pilot control action is predicted according to the current actual working point (AP1) in the operation of the electric machine (2).

3. The method according to claim 1, wherein the pilot control action is predicted in pilot control action tests and stored in a data memory associated with the machine (2).

4. The method according to claim 1, wherein one pilot control action is respectively predicted for a plurality of potential desired working points (AP2) and/or that one pilot control action is respectively predicted for a plurality of potential actual working points (AP1).

5. The method according to claim 1, wherein a sensor signal of a sensor is compared to a specified threshold value, and that it is ascertained according to the comparison whether the desired working point (AP2) should be set at least substantially in a time-optimized manner.

6. The method according to claim 1, wherein a working point is specified as the desired working point (AP2) in which the machine (2) generates a generative deceleration torque, wherein, upon detection of an emergency braking situation, it is ascertained that the desired working point (AP2) should be set at least substantially in a time-optimized manner, that a working point is specified as the desired working point (AP2) in which the machine (2) generates an acceleration torque, wherein, upon detection of a maximum dynamic default, it is ascertained that the desired working point (AP2) should be set at least substantially in a time-optimized manner.

7. The method according to claim 1, wherein the desired working point (AP2) is specified according to a state of charge of the energy store (4).

8. The method according to claim 1, wherein a desired trajectory (T) for an actual power vector (i.sub.actual,dq) of an electric motor current flowing through the motor winding is ascertained, wherein the desired trajectory (T) extends from the actual working point (AP1) to the desired working point (AP2), and wherein the pilot control action is predicted according to the desired trajectory (T) such that the curve of the actual power vector (i.sub.actual,dq) at least substantially corresponds to the desired trajectory (T) when setting the desired working point (AP2).

9. The method according to claim 1, wherein the desired trajectory (T) is ascertained according to a model of the electric machine (2).

10. The method according to claim 1, wherein a threshold power value (SSW) is specified and that the desired trajectory (T) is ascertained according to the threshold power value (SSW) such that a power value of the actual power vector (i.sub.actual,dq) always falls below the threshold power value (SSW) when setting the desired working point (AP2).

11. The method according to claim 1, wherein a threshold voltage value is specified and that the desired trajectory (T) is ascertained according to the threshold voltage value such that voltage values of electrical clamping voltages of the machine (2) always fall below the threshold voltage value when setting the desired working point (AP2).

12. The method according to claim 8, wherein the desired trajectory (T) is ascertained by a predictive controller (9).

13. A device for operating an electric machine, wherein the machine (2) comprises a rotatably mounted rotor and a motor winding, and wherein the motor winding is electrically connected to an electrical energy store (4) by means of a power electronics (3), wherein a control device (5) is configured to perform the method according to claim 1.

14. The device according to claim 13, wherein the control device (5) comprises a first computing unit (6) and a second computing unit (7), wherein the first computing unit (6) comprises a power controller (8), and wherein the second computing unit (7) comprises a model predictive controller (9).

15. An electrical drive system comprising: an electrical machine (2) having a rotatably mounted rotor and a motor winding, wherein the motor winding is electrically connected to an electrical energy store (4) by means of a power electronics (3), and a control device (5) configured to trigger the power electronics (3) to control the electric machine (2) in a field-oriented manner so that the machine (2) generates a specified desired torque (T.sub.target),wherein when it is determined that based on a current actual working point (AP1) of the electric machine (2) a specified desired working point (AP2) of the electric machine (2) should be set at least substantially in a time-optimized manner, a predicted pilot control action is specified, and the desired working point (AP2) is set by triggering the power electronics (3) according to the predicted pilot control action.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The invention will be explained in more detail in the following with reference to the drawings. Shown are:

[0025] FIG. 1 an electric drive system,

[0026] FIG. 2 a method for operating an electric machine of the drive system,

[0027] FIG. 3 a graph of the setting of a desired working point according to a predicted pilot control action, and

[0028] FIG. 4 a graph of the setting of the desired working point by means of a power controller.

DETAILED DESCRIPTION

[0029] FIG. 1 shows a schematic diagram of an electric drive system 1 of a motor vehicle (not shown in further detail).

[0030] The drive system 1 comprises an electric machine 2. The electric machine 2 comprises a rotatably mounted rotor. The electric machine 2 further comprises a stator winding as a motor winding. The stator winding is arranged in a distributed manner around the rotor such that the rotor is rotatable by a suitable powering of the stator winding. In the present case, the stator winding comprises three phases.

[0031] The drive system 1 also comprises a power electronics 3 having a plurality of switching elements. The stator winding is electrically connected to an electrical energy store 4 of the drive system 1 by the power electronics 3.

[0032] The drive system 1 also comprises a device 10 having a control device 5. In the present case, the control device 5 is a microcontroller 5. The control device 5 is designed to drive the switching elements of the power electronics 3 in order to achieve a desired powering of the phases of the stator winding.

[0033] The control device 5 comprises a first computing unit 6 and a second computing unit 7. The first computing unit 6 comprises a power controller 8. The second computing unit 7 comprises a model predictive controller 9. The control device 5 is designed to determine triggering signals for the switching elements of the power electronics 3 by means of the controllers 8 and 9 and to trigger the switching elements according to the ascertained triggering signals, as will be explained in further detail below in relation to FIG. 2.

[0034] FIG. 2 shows an advantageous method for operating the electric machine 2 in reference to a flow chart.

[0035] In a first step S1, the control device 5 ascertains a desired power vector i.sub.target,dq according to a specified desired torque T.sub.target on the one hand and an actual rotation angle .sub.actual on the other hand. In this case, the desired torque T.sub.target is specified, e.g., according to an actuation of an acceleration pedal of the motor vehicle. The actual rotation angle .sub.actual is sensed, e.g., by a rotation angle sensor associated with the rotor. The desired power vector i.sub.target,dq is a power vector relative to a rotor-fixed coordinate system. The power vector in this case describes the power vector of a torque-forming current i.sub.q on the one hand and the power value of a flow-forming current is on the other hand. The power vector corresponds to a power working point of the electric machine. In this respect, the desired power vector i.sub.target,dq of the electric machine 2 is the desired power working point of the electric machine 2.

[0036] In a second step S2, the control device 5 ascertains a difference between the desired power vector i.sub.target,dq on the one hand and an ascertained actual power vector i.sub.actual,dq on the other hand. For example, the actual power vector i.sub.actual,dq is ascertained according to the actual phase currents flowing through the phases of the motor winding by means of a d/q transformation. The actual power vector i.sub.actual,dq corresponds to an actual power working point of the electric machine 2.

[0037] In a third step S3, the control device 5 ascertains a desired voltage vector U.sub.target,dq relative to the rotor-fixed coordinate system by means of the power controller 8. The desired voltage vector U.sub.target,dq describes clamping voltages to be applied to the phases of the stator winding so that the difference between the desired power vector i.sub.target,dq and the actual power vector i.sub.actual,dq is reduced.

[0038] In a fourth step S4, the control device 5 ascertains triggering signals for the switching elements of the power electronics 3 according to the desired voltage vector U.sub.target,dq.

[0039] In a fifth step S5, the control device 5 controls the switching elements according to the triggering signals ascertained in step S4.

[0040] Steps S1 to S5 are performed continuously during normal operation of the electric machine 2 so that field-oriented control of the electric machine 2 is performed by means of steps S1 to S5.

[0041] In a sixth step S6, a threshold power value for the actual power vector i.sub.actual,dq as well as a threshold voltage for the clamping voltages is specified.

[0042] In a seventh step S7, the control device 5 ascertains a desired trajectory for the actual power vector i.sub.actual,dq by means of the model predictive controller 9, in which case the desired trajectory extends from the current actual working point of the machine 2 to a specified desired working point. In the present case, as a desired working point, the working point of the electric machine 2 is specified in which the machine 2 generates a maximum deceleration torque.

[0043] The model predictive controller 9 in this case ascertains the desired trajectory such that a time-optimized setting of the desired working point starting from the current actual working point is achieved by changing the actual power vector i.sub.actual,dq along the desired trajectory. For this purpose, the model predictive controller 9 ascertains the desired trajectory according to a model of the electric machine 2.

[0044] The model predictive controller 9 also considers the specified threshold power value when determining the desired trajectory. For this purpose, the model predictive controller 9 ascertains the desired trajectory such that the actual power vector i.sub.actual,dq always falls below the threshold power value when setting the desired working point along the desired trajectory.

[0045] The model predictive controller 9 also considers the specified threshold voltage value when determining the desired trajectory. For this purpose, the model predictive controller 9 ascertains the desired trajectory such that the clamping voltages always fall below the threshold voltage value when setting the desired working point along the desired trajectory.

[0046] In an eighth step S8, the model predictive controller 9 predicts a pilot control action according to the ascertained desired trajectory. In the present case, the model predictive controller 9 predicts a control sequence comprising a plurality of rotor-fixed coordinate system-based optimized voltage vectors u.sub.opt,dq. If the phases are sequentially applied to electrical clamping voltages according to the voltage vectors u.sub.opt,dq, then the specified desired working point of the machine 2 is set based on the current working point of the machine 2 such that the curve of the power vector i.sub.actual,dq at least substantially corresponds to the desired trajectory.

[0047] Steps S6 to S8 are performed continuously so that a desired trajectory is always ascertained and a pilot control action is predicted for the current working points of the machine 2.

[0048] In a ninth step S9, it is monitored whether the specified desired working point of the electric machine 2 should be set in a time-optimized manner, i.e. as quickly as possible, based on the current actual working point of the electric machine 2. This is the case, for example, when an emergency braking situation is ascertained or is present.

[0049] If it is ascertained in step S9 that the desired working point should be set in a time-optimized manner, then the triggering signals are ascertained in step S4 according to the predicted pilot control action. The consideration of the desired voltage vector U.sub.target,dq is suspended. Accordingly, in step S5, the switching elements are triggered according to triggering signals that were ascertained according to the predicted pilot control action. The specified desired working point is thus set faster than would be the case using field-oriented control. This ultimately reduces the braking distance of the motor vehicle.

[0050] According to a further exemplary embodiment, the pilot control action is predicted in pilot control action tests, i.e. off-line, and stored in a data memory associated with the machine 2. Preferably, a pilot control action is respectively predicted for a plurality of potential actual working points, in which case the pilot control actions are then preferably stored in a characteristic map. If it is ascertained in this case that a specified desired working point should be set starting from a current actual working point, then the corresponding pilot control action is not predicted according to method steps S7 and S8 but rather provided by the data memory.

[0051] FIG. 3 shows a graph of the setting of the specified desired working point AP2 based on the current actual working point AP1 according to the predicted pilot control action.

[0052] In illustration A at left, a power locus curve is shown for this purpose. As can be seen from FIG. 3, the threshold power value SSW in the present case is 400 amperes. The current actual working point AP1 corresponds to an intersection point of a first ISO torque characteristic curve KL1 with the MTPA curve. The specified desired working point AP2 corresponds to an intersection point of a second ISO torque characteristic curve KL2 with the MTPA curve.

[0053] The desired trajectory T follows an ego dynamics of the electric machine 2. The curve V1 of the actual power vector i.sub.actual,dq corresponds to the desired trajectory T when setting the desired working point AP2 according to the pilot control action.

[0054] In illustration B at right, a temporal curve of a torque-forming current i.sub.q and a flow-forming current is when setting the second working point AP2 according to the predicted pilot control action are shown. The torque-forming power i.sub.q corresponds to a first directional component of the power vector i.sub.actual,dq. The flow-forming power is corresponds to a second directional component of the power vector i.sub.actual,dq. A temporal curve of a torque-forming voltage u.sub.q and a flow-forming voltage u.sub.d are also shown. The torque-forming voltage u.sub.q corresponds to a first directional component of the optimized voltage vectors u.sub.opt,dq. The flow-forming voltage u.sub.d corresponds to a second directional component of the optimized voltage vectors u.sub.opt,dq.

[0055] During a first time interval t1, the machine 2 is in the current actual working point AP1. During the first time interval t1, the model predictive controller 9 predicts the pilot control action for the current actual working point AP1.

[0056] At a time t1, it is ascertained that the desired working point AP2 should be set in time-optimized manner. As a result, the switching elements of the power electronics 3 are triggered during a second time interval t2 according to the predicted pilot control action. In the present case, the pilot control action comprises eight optimized voltage vectors u.sub.opt,dq, which are sequentially used as the basis for triggering the switching elements. According to the present example, by triggering the switching elements according to the predicted pilot control action, it is achieved that the second working point AP2 is already set after 800 s, starting from the first working point AP1.

[0057] FIG. 4 shows a graph of the setting of the specified desired working point AP2 starting from the current actual working point AP1 by means of field-oriented control.

[0058] In illustration C at left, a power locus curve is shown for this purpose. As can be seen from FIG. 3, the curve V2 of the actual power vector i.sub.actual,dq differs significantly from the curve V1 shown in illustration A when using field-oriented control.

[0059] In illustration D at right, the temporal curve of the torque-forming current i.sub.q and the temporal curve of the flow-forming current is when setting the desired working point AP2 are shown using field-oriented control. The temporal curve of the torque-forming voltage u.sub.q and the temporal curve of the flow-forming voltage u.sub.d are also shown. As can be seen from illustration D, about 5 ms are required for setting the desired working point AP2 by means of field-oriented control.

[0060] By triggering the switching elements according to the predicted pilot control action, the time required for setting the desired working point AP2 can accordingly be significantly reduced compared to the field-oriented control.