AC current acquisition method

Abstract

The invention relates to an AC acquisition method (1) for an electric machine, comprising:-a step (2) in which the inverter is operated according to a six-step operation, during which a basic period of 2 [pi] is divided into six equal time periods, at most one switching action occurring during each period; a step (8) in which, for each equal time period, a plurality of values of AC from the inverters of the first reference frame are sampled; a step (10) in which the sampled values are rotated from the first reference frame to the second reference frame; a step (12) in which, for each equal time period, the values from the second reference frame are averaged; a step (14) in which the current value is estimated from the average sampled value.

Description

Technical Field

[0001] The present invention relates to the field of electronics and electrical engineering, more specifically to the field of inverters and monitoring of such inverters. Background Art

[0002] An inverter is a power electronic device that is able to generate alternating current (AC) from direct current (DC). An inverter may be integrated into a motor vehicle, in particular a hybrid or electric vehicle, to convert the DC current provided by the battery into AC current to power the motor of the motor vehicle. The motor is then used to provide torque to the wheels of the motor vehicle.

[0003] In order to convert the high-voltage DC current provided by the battery into AC current, the inverter can be indirectly controlled through pulse width modulation (PWM) technology.

[0004] The inverter is operated, for example, in six-step operation, which is particularly effective when the motor is operated in the field weakening region (i.e., at the AC voltage boundary). In this six-step operation, a basic cycle is defined. The basic cycle is divided into sub-cycles or PWM cycles, during which each power switch of the inverter is switched on and off only once to reduce switching losses in the inverter and to achieve the maximum possible base frequency AC output voltage amplitude.

[0005] In other words, during the six-step operation, for each sub-cycle, only one switching bridge of the inverter (here, a three-phase inverter) is operated using the PWM signal. This operation results in the generation of AC voltages in the three phases of the inverter. Since the fundamental frequency of the AC voltage is f0, the PWM signal or f PWM The frequency is equal to 6*f0.

[0006] The amplitude and phase of the fundamental frequency component of the AC current (i.e. the input current of the motor) are decisive for the precise control of the torque at the motor shaft and ultimately at the motor wheels. Such control implies sampling the electric motor current at predetermined times of the PWM cycle in order to obtain the fundamental frequency component of the AC current. In particular, the current ripple caused by the higher harmonics of the pulse width modulated (PWM) AC output voltage and the fundamental frequency component should be suppressed / decoupled.

[0007] It is known to divide the basic period into six sub-periods, to sample a current value per sub-period (ie per PWM period), and then to transform this current value into a rotating reference frame to obtain a direct current value allowing monitoring of the inverter.

[0008] However, in the case of six-step operation, unwanted (6n-1) and (6n+1) harmonics appear wrt the actually measured phase currents. Such harmonics appear as 6n harmonics in a coordinate system that rotates with the fundamental frequency. Since only one value per PWM period is transformed into the rotating coordinate system, there are actually only 6 values ​​per fundamental period, i.e. 6*f0 for the sampling frequency. According to the Nyquist criterion, a correct reconstruction of frequencies above half the sampling frequency, i.e. 3*f0, is not possible. In particular, aliasing effects may occur, which may provide an inappropriate reconstruction of the fundamental frequency components by known acquisition methods, such as 6n-1 harmonics and 6n+1 harmonics aliased to the fundamental frequency. This leads to inaccurate estimates of the AC current and, therefore, to inaccurate estimates of the torque. Summary of the invention

[0009] The present invention accommodates this situation by providing a current acquisition method that eliminates unwanted harmonics by increasing the number of samples per sub-cycle, which are transformed to a rotating coordinate system and averaged in the rotating coordinate system to suppress interfering higher harmonics.

[0010] In this context, the invention relates to a method for collecting alternating current (AC) for an electric or hybrid vehicle motor, the method comprising:

[0011] - a step of operating the inverter for the electric machine according to a six-step operation, during which step at least one basic period of 2π is used, the basic period being divided into six equal time periods, during each of which at most one single switching action occurs;

[0012] - a step of sampling, for each equal time period, a plurality of values ​​of the AC current of the inverter from the first reference frame;

[0013] - a step of rotating the sampled values ​​from the first reference system to the second reference system;

[0014] - a step of averaging, for each equal time period, a value from a plurality of values ​​from the second reference system;

[0015] - A step of estimating the current value based on the average sampled value.

[0016] The implementation of the method starts with a six-step operation of the inverter of the motor, which converts DC into AC. During the six-step operation, a basic cycle is divided into six equal time periods, also called PWM cycles. In other words, each time period corresponds to one sixth of the basic cycle. The six equal time periods, also called sub-cycles, are intervals of the same length of time under steady-state conditions. For example, the basic cycle is 2π and each time period is π / 3.

[0017] The switching action of the inverter corresponds to an open switch of a given phase of the inverter being closed, i.e. switched from "on" to "off", and a closed switch corresponding to the same phase being opened, i.e. switched from "off" to "on". Since there is only one switching action per time period, there are a maximum of six switching actions in a given basic period.

[0018] After the six-step operation, there is a step of sampling the AC value of the inverter, i.e. the input current of the electric motor; more precisely, at least two values ​​are sampled per time period. Thus, there are at least 12 sampled values ​​per basic period. These values ​​are sampled from a first reference system. Each sampled value of the first reference system is then transformed into a second reference system, which is a rotating reference system.

[0019] Once in the second reference frame, the values ​​sampled from the same time period are averaged; in other words, an averaging filter is applied to the samples of each time period. The DC current value corresponding to the frequency component can then be inferred from the averaged sample values.

[0020] The steps of the AC acquisition method are performed in this order; in particular, the rotation of the values ​​from the first reference frame to the second reference frame occurs before the averaging step.

[0021] Since there are at least two sample values ​​per time period, the number of samples is increased compared to prior art methods, which allows a more accurate reconstruction of the frequency components. More specifically, the averaging filter suppresses at least some, ideally most or all, of the unwanted harmonics that are the result of interference or irrelevant frequencies in the signal. The more samples there are, the more accurate the estimate of the frequency components.

[0022] According to an optional feature of the invention, the values ​​are sampled at equidistant intervals.

[0023] In other words, for each time period, the values ​​are sampled at the same place. Sampling may occur, for example, in the middle and at the end of a given time period. Fast oversampling near the middle and / or end of a PWM period beneficially improves signal quality. The difference between two values ​​in the same period is equal to half the period in which the two values ​​were sampled.

[0024] According to an optional feature of the invention, the plurality of values ​​for each equal time period corresponds to two values.

[0025] This corresponds to the first embodiment of the invention, in which two values ​​are sampled for a single time period; therefore, there are 12 samples per fundamental period.

[0026] According to an optional feature, the two values ​​are sampled at first π / 6 intervals of equal time period and at second π / 6 intervals of equal time period, respectively.

[0027] This ensures that in the first embodiment and using equidistant sampling, the two values ​​are sampled at equidistant intervals, since the 2π fundamental period is divided into 12 equal parts resulting in a π / 6 interval.

[0028] According to an optional feature of the invention, the plurality of values ​​per equal time period corresponds to four values.

[0029] This corresponds to a second embodiment of the invention, in which four values ​​are sampled for a single time period; as a result, there are 24 samples per fundamental period. As the number of values ​​sampled per time period increases, the number of unwanted harmonics that are suppressed also increases; thus, as in the case of the first embodiment, for a given time period, more unwanted harmonics are suppressed when there are four samples than when there are two samples.

[0030] According to an optional feature of the invention, the four values ​​are sampled at a first π / 12 interval of equal time period, a second π / 12 interval of equal time period, a third π / 12 interval of equal time period and a fourth π / 12 interval of equal time period respectively.

[0031] Such a π / 12 interval is the result of dividing the basic period of 2π by 4 for every π / 3 equal time period.

[0032] Other embodiments of the present invention may be obtained by selecting any integer number of samples per PWM period which is better than 2, ie 3, 4, 5, 6, ... .

[0033] According to an optional feature of the invention, when a switching action occurs, one of the plurality of values ​​is sampled.

[0034] This corresponds to the aforementioned sampling at the end of a given time period when the switch of a given phase of the inverter switches from "on" to "off" and from "off" to "on".

[0035] According to an optional feature of the invention, the rotation of the sampled values ​​from the first reference frame to the second reference frame depends on an angle of the motor.

[0036] The conversion from the first reference frame to the second reference frame or rotating coordinate system is obtained by a transformation angle consistent with the time point where the sampling is performed. In some cases, the second reference frame is aligned with the flux of the motor rotor; in other words, the rotating coordinate system can be rotor flux oriented. In other cases, as long as the second reference frame rotates at the fundamental angular frequency, it is not aligned with this flux.

[0037] The motor angle is obtained through any angle sensing technique, resolver, tunnel magnetoresistive (TMR) device, encoder, sensorless control or other similar techniques.

[0038] According to an optional feature of the invention, a Park transform is used to rotate the sample values ​​from the first reference system to the second reference system.

[0039] In other words, the Park transformation results in a change of reference frame.

[0040] According to an optional feature of the invention, the sampling in the first reference frame is performed using a sensing method.

[0041] The sensing method may be selected between a two-phase current sensor method, a three-phase current sensor method, a one-shot method, an oversampling method, or any other current sensing equivalent technique.

[0042] According to an optional feature of the invention, the sampling in the first reference frame is performed using an oversampling method.

[0043] For example, using an oversampling method can improve the resulting accuracy of the sampled values ​​by reducing noise. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] According to an optional feature of the invention, the sampling in the first reference frame using the oversampling method uses an acquisition window, which can be fixed for all equal time periods or can be variable for all equal time periods. On the one hand, other features, details and advantages of the invention will become clearer by reading the following description, and on the other hand, with reference to the accompanying drawings, a number of implementation examples are given as indications and not limitations, in which:

[0045] Figure 1 is a flow chart of different steps forming the method for collecting alternating current according to the present invention;

[0046] Figure 2 is a graph showing the single-phase output voltage of a three-phase inverter operating in six steps;

[0047] Figure 3 yes Figure 1 Schematic diagram of the sampling steps of the AC acquisition method. DETAILED DESCRIPTION

[0048] The features, variations and different implementation modes of the present invention can be associated with each other in various combinations, as long as they do not contradict or exclude each other. In particular, it is possible to envision a variant of the present invention that only includes the selection of the features described later from the other features described, if this feature selection is sufficient to confer technical advantages and / or distinguish the present invention from the prior art.

[0049] Like numbers refer to like elements throughout the drawings.

[0050] Figure 1is a flow chart of a plurality of consecutive steps forming an alternating current (AC) acquisition method 1 according to the present invention. This AC acquisition method 1 is used in an electric or hybrid vehicle equipped with an electric machine (e.g., an electric motor). The electric machine is driven by an alternating current generated by an inverter of the electric or hybrid vehicle from a direct current provided by a battery of the vehicle. The inverter here is a three-phase inverter.

[0051] One of the functions of the motor is to provide torque to the wheels of an electric or hybrid vehicle, and the amount of torque delivered to the wheels is controlled by controlling the input current sent to the motor. The AC acquisition method 1 according to the present invention is a way to monitor the inverter based on the AC current value obtained from the inverter.

[0052] Now refer to Figure 1 Flowchart and Figure 2 and 3 The AC acquisition method 1 of the present invention occurs during the six-step operation step 2 of the inverter, which is specifically represented by Figure 2 superior.

[0053] During the six-step operation step 2, the voltage U of the first phase of the three-phase inverter is monitored during the basic period 4 corresponding to 2π pha,U . The basic period 4 is divided into sub-periods or time periods 6; more precisely, the basic period 4 is divided into six time periods 6 of equal length, here six time periods 6 of π / 3. The inverter performs a maximum of six switching actions during the basic period 4, which is equivalent to performing only one switching action during each time period 6. In this way, the closed switch of a given phase of the inverter changes from "on" to open or "off", and the open switch of the phase changes from "off" to closed or "on" only once per time period 6. Note the + phase state, in which the top switch of the phase is operated as "on" and the bottom switch is operated as "off", and the - phase in which the top switch is operated as "off" and the bottom switch is operated as "on". From Figure 2 It can be seen from the graph that due to the switching action from 0 to π / 3 in the first time period 6A, each phase of the three-phase inverter is +-+. More precisely, in the first time period 6A, the top switch of the first phase is operated to be closed, and the bottom switch of the first phase is operated to be opened, so that the first phase is in the + state; the top switch of the second phase is operated to be opened, and the bottom switch of the second phase is operated to be closed, so that the second phase is in the - state; and the top switch of the third phase is operated to be closed, and the bottom switch of the first phase is operated to be opened, so that the third phase is in the + state. As a result, for the first time period 6A, the voltage U of the first phase is pha,U Equal to U DC / 3, the voltage transmitted on the second phase is -2 / 3*UDC , and the voltage of the third phase is equal to U DC / 3, where U DC is the input DC voltage of the three-phase inverter. During the second time period 6B occurring from π / 3 to 2π / 3, the first switch of the third period is open and the second switch of the third phase is closed. The voltage of the first phase is now 2 / 3*U DC , the inverter is in the +-- state. For the third time period 6C occurring from 2π / 3 to π, the top switch of the second phase of the inverter is closed and its bottom switch is disconnected, so that the inverter is in the ++- state. For the fourth time period 6D occurring from π to 4π / 3, the first switch of the first phase of the inverter is closed and its second switch is disconnected, resulting in the inverter being in the -+- state. For the fifth time period 6E occurring from 4π / 3 to 5π / 3, the top switch of the third phase of the inverter is closed and the bottom switch is disconnected to obtain the a-++ state of the inverter, and finally for the sixth time period 6F occurring from 5π / 3 to 2π, the top switch of the second phase is disconnected and the bottom switch is disconnected, so that the inverter is in the --+ state. During these six-step operations, when one half bridge of the inverter is operated at a time, an AC voltage of the fundamental frequency f0 is obtained after six complete time periods 6.

[0054] The inventors have noticed that as part of the six-step operation, during the sampling of the signals controlling the inverter, voltage harmonics appear with significant harmonic content of 6n-1 and 6n+1, where n is an integer. For example, the 5th and 7th harmonics appear as the 6th harmonic in the rotating coordinates, while the 11th and 13th harmonics appear as the 12th harmonic in the rotating coordinates.

[0055] The AC acquisition method solves this unwanted harmonic problem with a sampling step 8 in which a plurality of AC values ​​of the inverter are sampled from each time period 6 of the fundamental period 4. In other words, for each time period 6 there are a plurality of sampled values ​​S. The AC values ​​are sampled more accurately from the first reference frame of the inverter and are sampled at equidistant intervals within a given time period 6.

[0056] The sampling step 8 is performed using any current sensing method. For example, the sampled value S may be extracted using a one-shot method, a two-phase current sensor method, or a three-phase current sensor method.

[0057] In the first embodiment Figure 3In the illustrated embodiment, the plurality of AC values ​​sampled within a given time period 6 corresponds to two values. Thus, there are two sampled values ​​S1, S2 for a first time period 6A, two sampled values ​​S3, S4 for a second time period 6B, two sampled values ​​S5, S6 for a third time period 6C, two sampled values ​​for a fourth time period 6D, two sampled values ​​for a fifth time period 6E, and two sampled values ​​for a sixth time period 6E. Figure 3 Only the sampling values ​​of the first time period 6A to the third time period 6C are shown, but it should be understood that sampling occurs in the fourth, fifth and sixth time periods 6D, 6E, 6F accordingly. Since the sampled values ​​S are collected at equidistant intervals, each value is sampled at π / 6 intervals here.

[0058] In this first embodiment, one of the two sampled values ​​S is sampled when a switching action of the inverter occurs, i.e., at the end of a given time period 6. Thus, the second sampled values ​​S2, S4 and S6 of each time period 6 are extracted when the inverter changes from "on" to "off" or vice versa from "off" to "on". In contrast, the first sampled values ​​S1, S3, S5 of each time period 6 are extracted in the absence of a switching action, more precisely in the middle of a given time period 6.

[0059] The number of sampled values ​​S per time period 6 of the basic period 4 depends on the embodiment. For example, in a second embodiment of the invention not shown here, the plurality of AC values ​​sampled in a given time period 6 corresponds to four values. As a result, there are four sampled values ​​for the first time period 6A, four sampled values ​​for the second time period 6B, four sampled values ​​S5, S for the third time period 6C, four sampled values ​​for the fourth time period 6D, four sampled values ​​for the fifth time period 6E, and four sampled values ​​for the sixth time period 6E. In this second embodiment, each value is sampled at intervals of π / 12.

[0060] Other embodiments may be created by choosing any integer number of equally spaced samples per time period 6 that is better than 2 (3, 4, 5, 6, ...).

[0061] Once the sampling step 8 has been performed and at least two values ​​have been extracted for each time period 6 of the basic cycle, the AC acquisition method 1 comprises a rotation step 10. This rotation step 10 comprises changing the reference system. Thus, the first reference system in which the sampled values ​​S were extracted is transformed into a second reference system. The change in reference system is a result of the rotation. In the present embodiment, each sampled value S is rotated from the first reference system to the second reference system using a Park transformation.

[0062] In the rotation step 10, the Park transform used to rotate the sampled values ​​S from the first reference frame to the second reference frame is a function of the motor angle. The angle of the motor or the transformed angle rotates with a fundamental angular frequency. For example, it is aligned with the flux of the motor rotor. The angle of the motor is detected by any angle sensing technology.

[0063] Once the rotation step 10 has been completed, the AC acquisition method 1 continues with an averaging step 12. This averaging step 12 corresponds to applying an averaging filter to the sampled values ​​S in the second reference frame. In other words, the sampled values ​​are averaged after the Park transformation. The sampled values ​​are averaged more precisely in each time period 6. For example, Figure 1 In the first embodiment, the averaging filter is applied to the two sample values ​​S1, S2 of the first time period 6A, then the averaging filter is applied to the two sample values ​​S3, S4 of the second time period 6B, and so on until the sixth time period 6E of the basic period 4.

[0064] Since the sampling is done at equidistant times within the same sampling period, the resulting values ​​are opposite in positive / negative, so the unwanted harmonics cancel each other out and the averaging filter removes the unwanted harmonics.

[0065] The greater the number of sample values ​​S per time period 6, the greater the number of unwanted harmonics suppressed. Therefore, the number of unwanted harmonics suppressed is higher in the second embodiment than in the first embodiment.

[0066] As previously described, during the six-step operation step 2, the 6n-1 harmonic and the 6n+1 harmonic appear as the 6n harmonic. Thus, the 5th and 7th harmonics appear as the 6th harmonic, while the 11th and 13th harmonics appear as the 12th harmonic, and so on. In the first embodiment, the averaging step 12 applied to the two sampled values ​​S extracted from each time period 6 during the sampling step 8 results in the disappearance of the 6th harmonic, the 18th harmonic, the 30th harmonic and other higher harmonics. When four values ​​are sampled for a given time period 6, as is the case in the second embodiment, the 6th harmonic, the 12th harmonic, the 18th harmonic and other higher harmonics are suppressed. More generally, when M values ​​are sampled for a sampling frequency of 6*M*f0, f0 is the fundamental frequency, the [6*(M-1)*n+6*M*n]*f0 harmonic disappears, while the [6*M+6*M*n] harmonic is still considered, n being a positive integer ranging from 0 to infinity.

[0067] The AC acquisition method 1 ends with a final estimation step 14 during which the current value is estimated from the previously averaged sampled values ​​S.

[0068] Thus, the present invention encompasses an alternating current acquisition method in which a plurality of sample values ​​are added in order to average them, and by doing so noise in the form of unwanted harmonics in the signal is eliminated, thereby providing a more accurate acquisition.

[0069] Many modifications and other embodiments of the invention set forth herein will occur to those skilled in the art having benefit of the teachings given in the foregoing description and the associated drawings. Therefore, it should be understood that the invention is not limited to the specific embodiments disclosed, and modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are used herein, they are used in a general and descriptive sense only and not for the purpose of limitation.

Claims

1. A method (1) for collecting alternating current (AC) of an electric or hybrid vehicle motor, the method comprising: - step (2), in which the inverter for the electric machine is operated according to a six-step operation, during which at least one basic period (4) of 2π is used, the basic period (4) being divided into six equal time periods (6, 6A, 6B, 6C, 6D, 6E, 6F), during each of which at most one single switching action occurs; - a step (8) in which for each equal time period (6, 6A, 6B, 6C, 6D, 6E, 6F) a plurality of values ​​of the AC current of the inverter from the first reference frame are sampled; - step (10) in which the sampled values ​​(S) are rotated from the first reference system to the second reference system; - a step (12) in which for each equal time period (6, 6A, 6B, 6C, 6D, 6E, 6F), a value (S) from the plurality of values ​​(S) from the second reference system is averaged; - Step (14) in which the current value is estimated based on the average sampled value.

2. The method for collecting alternating current (1) according to the preceding claim, wherein: The values ​​(S) are sampled at equally spaced intervals.

3. The method (1) for collecting alternating current according to any one of the preceding claims, wherein: The multiple values ​​(S) for each equal time period (6, 6A, 6B, 6C, 6D, 6E, 6F) correspond to two values ​​(S1, S2, S3, S4, S5, S6).

4. The method (1) for collecting alternating current according to the preceding claim, wherein: The first value and the second value of the two values ​​(S1, S2, S3, S4, S5, S6) are sampled at a first π / 6 interval of equal time period (6, 6A, 6B, 6C, 6D, 6E, 6F) and a second π / 6 interval of equal time period, respectively.

5. The AC current acquisition method (1) according to any one of claims 1 and 2, wherein: The multiple values ​​(S) for each equal time period (6, 6A, 6B, 6C, 6D, 6E, 6F) correspond to four values.

6. The method (1) for collecting alternating current according to the preceding claim, wherein: The first, second, third and fourth values ​​of the four values ​​are sampled at a first π / 12 interval of equal time period, a second π / 12 interval of equal time period, a third π / 12 interval of equal time period and a fourth π / 12 interval of equal time period, respectively.

7. The method (1) for collecting alternating current according to any one of the preceding claims, wherein: When the switching action occurs, a value among the plurality of values ​​(S) is sampled.

8. The method (1) for collecting alternating current according to any one of the preceding claims, wherein: The rotation of the sampled value (S) from the first reference frame to the second reference frame depends on the angle of the motor.

9. The method (1) for collecting alternating current according to any one of the preceding claims, wherein: The sample values ​​(S) are rotated from the first reference frame to the second reference frame using the Park transform.

10. The method (1) for collecting alternating current according to any one of the preceding claims, wherein: Sampling within the first reference frame is performed using a sensing method.

11. The method (1) for collecting alternating current according to the preceding claim, wherein: Sampling in the first reference frame is performed using an oversampling method.

12. The method (1) for collecting alternating current according to the preceding claim, wherein: The oversampling method uses an acquisition window, which can be fixed for all each equal time period (6, 6A, 6B, 6C, 6D, 6E, 6F) or can be variable for all each equal time period (6, 6A, 6B, 6C, 6D, 6E, 6F).

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