Pulse pattern optimisation taking into consideration the maximally achievable switching edges in a microcontroller time slice

Abstract

A method is described for producing a pulse pattern for an inverter for controlling an electric motor by means of pulse-duration modulation. The pulse pattern comprises a plurality of switching edges and is intended to be output by a microcontroller. A limit value for a number of switching edges is specified, wherein the number of switching edges within a sampling period of the microcontroller is limited such that the specified limit value is not exceeded.

Description

[0001] PULSE PATTERN OPTIMIZATION TAKING INTO ACCOUNT THE MAXIMUM

[0002] Realizable switching edges in a microcontroller time slice

[0003] The invention relates to a method for generating a pulse pattern for an inverter for controlling an electric motor using pulse width modulation. The invention also relates to a microcontroller for an inverter.

[0004] Various implementations of pulse width modulation (PDM) are known for controlling electric motors. Using a pulse pattern, an alternating voltage can be generated via an inverter.

[0005] Pulse patterns for setting a high voltage value exhibit a high switching frequency. Since microcontrollers only offer a limited number of switching frequencies (switching edges) per sampling period, special chips, particularly field-programmable gate arrays (FPGAs), have often been used for high-power drives. However, such chips are only suitable to a limited extent for some applications (e.g., in automotive engineering), particularly due to their high cost.

[0006] One object of the invention is to enable a particularly high voltage utilization even when using a microcontroller for pulse duration modulation.

[0007] The object is achieved according to the invention by a method for generating a pulse pattern for an inverter for controlling an electric motor using pulse width modulation. The pulse pattern comprises several switching edges and is intended to be output by a microcontroller. A limit value for the number of switching edges is specified. The number of switching edges within a sampling period of the microcontroller is limited such that the specified limit value is not exceeded.

[0008] This allows the generation of a pulse pattern that is specifically adapted to the properties and, in particular, limitations of the microcontroller. The limitation of the number of switching edges relates specifically to the sampling period of the microcontroller. The inventive solution therefore differs in particular from a merely blanket limitation of the switching frequency. With a blanket limitation of the switching frequency, the number of switching operations would be limited, for example, with respect to one period of the voltage to be generated. However, such a blanket limitation does not imply a direct limitation of the switching edges specifically with respect to the sampling period of a microcontroller.

[0009] In the method according to the invention, a blanket limit on the number of switching operations per period of the voltage to be generated can also be implemented. This blanket limit then represents an additional feature that interacts with the limitation of the switching edges with respect to the microcontroller's sampling period. With a predetermined number of switching operations per period of the voltage to be generated, the limitation of the switching edges per sampling period of the microcontroller is then taken into account as an additional specification.

[0010] The sampling period of the microcontroller is, in particular, a periodically repeated time interval within which the electric motor's current is sampled once. A switching edge of the pulse pattern is, in particular, a transition between two states of the pulse pattern.

[0011] The pulse pattern contains information about the phase angles of the voltage to be generated that trigger a switching edge. The corresponding phase angles are also referred to as switching angles. The voltage to be generated is an alternating voltage, in particular an at least substantially sinusoidal alternating voltage.

[0012] The positions of the switching edges can depend on a modulation index. The modulation index represents, in particular, a ratio between a control voltage and a DC voltage available at the inverter. The control voltage is, in particular, a control value of the voltage to be generated. The modulation index can be normalized to 1, whereby a modulation index of 1 corresponds to a maximum value of the control voltage. The method can comprise generating multiple pulse patterns, each pulse pattern being assigned a specific value of the modulation index. Based on the sampling period of the microcontroller, an angular interval can be determined, whereby the number of switching edges is limited within the angular interval such that the specified limit for the number of switching edges is not exceeded. In this way, the sampling period of the microcontroller can be related to the phase angles of the voltage to be generated.For example, the angle interval may be less than 90, 75, or 45°.

[0013] When determining the angular interval, a specified maximum frequency of a voltage to be generated to control the electric motor can be taken into account. This ensures that the generated pulse pattern for the maximum frequency and all frequencies below it delivers a number of switching edges per sampling period that is below the specified limit.

[0014] This is because when the frequency is reduced below the maximum frequency, the specified angular interval is traversed over a longer time interval. This time interval is, in particular, longer than the microcontroller's sampling period. However, due to the limitation related to the angular interval, the number of switching edges does not exceed the specified limit, even for this longer time interval.

[0015] The maximum frequency refers specifically to the fundamental oscillation of the AC voltage to be generated. The specified maximum frequency may, for example, be a maximum desired frequency or a maximum suitable frequency for the electric motor.

[0016] The method can consider different possible positions of the angular interval relative to the phase angle of the voltage to be generated. This can ensure that the number of switching edges does not exceed the specified limit, even if the positioning of the angular intervals relative to the phase is particularly unfavorable in this regard. In particular, it can be ensured that the specified limit is not exceeded, even if the positioning of the angular intervals relative to the phase cannot be controlled. The specified limit for the number of switching edges can be the maximum number of switching edges that the microcontroller can provide per sampling period. This allows the method to be specifically adapted to the limitations of the microcontroller. In particular, an even further limitation that goes beyond the requirements of the microcontroller can be avoided.

[0017] The inverter can be a two-level converter. Two-level converters are also known as two-level converters and are characterized by low complexity and high robustness.

[0018] The method can be designed as an optimization method. This allows an optimized pulse pattern to be generated. In particular, a harmonically optimized pulse pattern can be generated, which results in a reduction of harmonics in the voltage to be generated and the resulting currents. Suitable optimization methods include, for example, the Newton method, the threshold accepting algorithm, or genetic algorithms.

[0019] In addition, a microcontroller for an inverter is provided. The microcontroller stores a pulse pattern generated using one of the previously described methods for generating a pulse pattern. The pulse pattern is intended to be output by the microcontroller. Thus, a microcontroller can be provided with a pulse pattern that is specifically tailored to the characteristics and, in particular, limitations of the microcontroller.

[0020] Furthermore, an inverter system for a motor vehicle is provided. The inverter system comprises a microcontroller configured as described above and an inverter.

[0021] Furthermore, the use of a pulse pattern for controlling an electric motor by means of pulse duration modulation is provided, which has been obtained according to the previously described method for generating a pulse pattern. The aforementioned advantages arise analogously when using the corresponding pulse pattern. Further features and characteristics of the invention will become apparent from the following description of exemplary embodiments, which should not be understood in a limiting sense. The description is made with reference to the drawings, in which:

[0022] Fig. 1 is a schematic flow diagram illustrating a method for generating a pulse pattern according to an embodiment of the invention,

[0023] Fig. 2 is a diagram showing the switching angle and the number of switching edges as a function of a modulation index, with no limitation of the number of switching edges in relation to a sampling period, and

[0024] Fig. 3 is a diagram in which switching angle and number of switching edges are shown as a function of a modulation index, whereby the number of switching edges is limited in relation to a sampling period.

[0025] Fig. 1 is a flowchart illustrating a method according to the invention for generating a pulse pattern for an inverter for controlling an electric motor using pulse width modulation. The inverter is, in particular, a two-stage power converter. The pulse pattern comprises several switching edges and is intended to be output by a microcontroller.

[0026] In step 102, a limit value for the number of switching edges is specified. The specified limit value can be the maximum number of switching edges that the microcontroller can provide per sampling period.

[0027] In a second step 104, an angular interval is determined based on the microcontroller's sampling period. The angular interval serves to limit the number of switching edges within the angular interval such that the predefined limit is not exceeded. When determining the angular interval, a predetermined maximum frequency of a voltage to be generated to control the electric motor can be taken into account. In a further step 106, the pulse pattern is generated, with the number of switching edges within a microcontroller's sampling period being limited such that the predefined limit is not exceeded.

[0028] Method 100 can be configured as an optimization method. This allows an optimized pulse pattern, in particular a harmonically optimized pulse pattern, to be generated.

[0029] If an angular interval was determined in a previous step 104 based on the sampling period, the switching edges can be limited indirectly with respect to the sampling period. Indirect limitation is achieved, in particular, by limiting the number of switching edges within the angular interval such that the specified limit is not exceeded. Different possible positions of the angular interval relative to the phase angle of the voltage to be generated can be considered for generating the pulse pattern.

[0030] The pulse pattern obtained from method 100 can be used to control an electric motor using pulse width modulation. For this purpose, the pulse pattern can be output to an inverter connected to the electric motor.

[0031] Furthermore, the pulse pattern generated by method 100 can be stored in a microcontroller for an inverter. Thus, a microcontroller can be provided in which a corresponding pulse pattern is stored. The pulse pattern is intended to be output by the microcontroller. The microcontroller can form part of an inverter system for a motor vehicle, wherein the inverter system further comprises an inverter.

[0032] Fig. 2 and Fig. 3 each show a diagram in which the switching angle S of a pulse pattern is represented as a function of a modulation index M. In addition, a number of switching edges N is also shown as a function of the modulation index M.

[0033] Since an increased number of switching edges per sampling period occurs at high values ​​of the modulation index, the particularly relevant value range from 0.5 to 1 is shown for M. The modulation index represents, in particular, a ratio between a control voltage and a DC voltage available at the inverter. The control voltage is, in particular, a control value of the voltage to be generated. As is the case in the example shown, the modulation index can be normalized to 1. A modulation index of 1 corresponds to a maximum value of the control voltage.

[0034] The switching angles S are shown for the range between 0 and 90°. Within this range, there are five switching angles, which are shown in the figures as solid curves. The angle specifications refer in particular to the phase angle of the voltage to be generated.

[0035] When determining the switching angles in Fig. 2, no limit was taken into account for the number of switching edges per sampling period of a microcontroller. Only a pulse count was specified, i.e., a number of pulses per period of the voltage to be generated. A pulse corresponds, in particular, to a switching from a first state to a second state and back again. Fig. 2 serves as a comparative example to illustrate the effect shown in Fig. 3.

[0036] To determine the switching angles in Fig. 3, in addition to the pulse number, a limit value for the number of switching edges per sampling period of a microcontroller was also taken into account.

[0037] In this example, a suitable angular interval was determined within which the number of switching edges was limited so that the limit value is not exceeded. A correspondingly suitable angular interval can, for example, be less than 90, 75, or 45°. It can be seen that the switching angles were successfully determined such that the number of switching edges / V is significantly lower than in the example shown in Fig. 2, even at high values ​​of the modulation index M.

Claims

Patent claims 1. A method for generating a pulse pattern for an inverter for controlling an electric motor by means of pulse duration modulation, wherein the pulse pattern comprises a plurality of switching edges and is intended to be output by a microcontroller, wherein a limit value for a number of switching edges is specified, and wherein the number of switching edges within a sampling period of the microcontroller is limited such that the specified limit value is not exceeded.

2. Method according to claim 1, wherein an angular interval is determined based on the sampling period of the microcontroller and wherein the number of switching edges is limited within the angular interval such that the predetermined limit value is not exceeded, in particular wherein the angular interval comprises less than 90°.

3. Method according to claim 2, wherein a predetermined maximum of the frequency of a voltage to be generated to control the electric motor is taken into account in determining the angular interval.

4. The method according to claim 2 or 3, wherein the method takes into account different possible positions of the angular interval relative to the phase angle of the voltage to be generated.

5. Method according to one of the preceding claims, wherein the predetermined limit value is the maximum number of switching edges which the microcontroller can provide per sampling period.

6. Method according to one of the preceding claims, wherein the inverter is a two-stage power converter.

7. Method according to one of the preceding claims, wherein the method is designed as an optimization method.

8. A microcontroller for an inverter, wherein the microcontroller stores a pulse pattern generated by the method according to any one of the preceding claims, and wherein the pulse pattern is intended to be output by the microcontroller.

9. An inverter system for a motor vehicle, the inverter system comprising a microcontroller according to claim 8 and an inverter.

10. Use of the pulse pattern obtained from the method according to one of claims 1 to 7 for controlling an electric motor by means of pulse duration modulation.

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