Electric machine and method for rotor current estimation of said electric machine

Abstract

The present invention relates to an electric or hybrid vehicle electric machine (1), comprising a rotating part, a transformer (6) and a converter (16) which is operated with phases shifts, the converter (16) comprising a commutation cell (15) comprising a capacitor (17) on an input side of the converter (16) and a plurality of transistors (18) on an output side of the converter (16), the commutation cell (15) comprising a current sensor (S) located between the capacitor (17) and the transistors (18).

Description

[0001] DESCRIPTION

[0002] Title: Electric machine and method for rotor current estimation of said electric machine

[0003] The present invention relates to the fields of electronics and electrical engineering, and more particularly to the field of electric machines.

[0004] Electric machines may be used in electric and hybrid vehicles in order to provide torque to the wheels of the vehicle. The motor can for instance be an electrically excited synchronous machine or EESM. The EESM is an alternative to permanent magnet synchronous machines as the latter require the usage of rare-earth materials.

[0005] In an EESM, energy must be transferred from a non-rotating part, i.e. the stator of the electric machine, to a rotating part, i.e. the rotor. Such power transfer can either be conductive or inductive.

[0006] In the case of a conductive power transfer, electric contact is achieved by pushing brushes of the non-rotating part against slip rings of the rotating part. While the slip ring system of the conductive power transfer allows a direct measurement of the rotor current, which is required for current and torque control, it however causes both mechanical and electric losses and it requires regular maintenance.

[0007] In the case of an inductive power transfer, the power transfer is contactless and uses a rotating high-frequency transformer, which is fed by a phase shift full bridge converter or PSFBC. In this case, a rectifier is connected between the transformer and the rotor winding of the electric machine. There is no mechanical loss in an inductive power transfer and the process is maintenance-free. However, it is highly undesired to place a current sensor on the rotating part for complexity and cost reasons, as this would require a sensor power supply on the rotating part of the machine and a contactless signal transfer to the non-rotating part. As a result, direct measurement of the rotor current is, with today’s existing technology, not possible at acceptable cost.

[0008] In order to accurately determine the rotor current, which is required for efficient operation of the EESM, sensors can be used on the non-rotating part of the inductive power transfer system and a predictive model is provided to estimate the rotor current. In other words, the sensors are positioned on a primary side on the non- rotating part to further estimate what current provided on a secondary side, i.e. on the rotating part.

[0009] It is for instance known to sample the primary side transformer current at a time corresponding to the middle of a positive pulse of the transformer voltage, and then apply a transformer transformation ratio. It is also known to use the converter input current, input voltage, duty cycle, and then apply a transformer transformation ratio.

[0010] However, errors in sampling the primary transformer current of a few hundred nanoseconds can lead to a significant degradation of the measurement. Additionally, when sampling is done in the primary transformer current it is not possible to remove disturbances as filtering is hard to implement.

[0011] The present invention fits into this context by providing a more robust current estimation method, which is less sensitive to timing errors and which is accurate even in dynamic conditions. The invention is also directed to a machine capable of implementing such current estimation method based on a particular location of its current sensor.

[0012] In this context, the present invention is directed to an electric or hybrid vehicle electric drive system comprising an electric machine comprising a rotating part, a transformer and a converter which is operated with phases shifts, the converter comprising a commutation cell comprising a capacitor on an input side of the converter and a plurality of transistors on an output side of the converter, the commutation cell comprising at least one current sensor located between the capacitor and the transistors.

[0013] According to an optional characteristic of the invention, the electric machine according to the invention is an inductive electrically excited synchronous machine or iEESM.

[0014] It comprises, among other components, a rotating part or rotor, a DC / AC converter, a transformer and a rectifier.

[0015] According to an optional characteristic of the invention, the converter is a phase shift full bridge converter or PSFBC.

[0016] In a known embodiment of said converter, the plurality of transistors in the commutation cell are organised in a plurality of switching arms, said switching arms comprising two transistors disposed in series between a positive branch, or top branch, and a negative branch, or bottom branch, of said commutation cell, the common connection point of the two transistors defining a connection point of the second side of the commutation cell, and the capacitor of the commutation cell being connected between the positive and negative branches on the first side of said cell.

[0017] According to an optional characteristic of the invention, the converter is operated with phase shifts with which a duty cycle ratio is associated.

[0018] According to an optional characteristic of the invention, the converter works at a variable PWM frequency. It provides bipolar rectangular voltages which are applied to the transformer, more precisely to a primary side of this transformer. This primary side of the transformer is on a non-rotating part of the electric machine, whereas a secondary side of the transformer is on the rotating part of the electric machine.

[0019] The converter comprises a commutation cell which has a first side, on an input side of the converter, and a second side on an output side of the converter. The commutation cell comprises, among other components, a capacitor and multiple transistors. According to the invention, a current sensor is located on the non-rotating part of the electric drive system. The current sensor is located in the commutation cell between the input side of the converter and the output side of the converter, more precisely between the capacitor and the multiple transistors.

[0020] The current sensor is used to acquire current measurements on the non-rotating part of the electric drive system in order to compare it to an estimate of the current for the rotating part of the electric machine, i.e. its rotor. The positioning of the current sensor within the commutation cell of the converter allows for less sensitive measurements with regard to sensor delay variations. In addition, the current sensor being located between the capacitor and the transistors allows for a precise rotor current estimation in steady state conditions as well as in dynamic conditions.

[0021] According to an optional characteristic of the invention, the current sensor of the commutation cell is located on a positive branch of the commutation cell.

[0022] According to an optional characteristic of the invention, the current sensor of the commutation cell is located on a negative branch of the commutation cell. The current sensor of the commutation cell can alternatively be located either on a positive branch or top branch of the commutation cell, or on a negative branch or bottom branch of the commutation cell, without affecting the current measurements. In some embodiments, there could be multiple current sensors in the commutation cell, for instance for safety reasons. For example, there could be at least two current sensors placed on a positive branch of the commutation cell. In another example, there could be at least one current sensor in a positive branch and at least one sensor in a negative branch.

[0023] The present invention is also directed to a method of current estimation for the rotating part of the electric or hybrid vehicle electric machine of the electric drive system as previously described, comprising:

[0024] - a measuring step in which current measurements are taken within at least one PWM half-period by the commutation cell current sensor, the current measurements being taken for a period of time at least as long as a duration of an active pulse of a voltage waveform generated by the converter between the converter and the transformer during the same PWM half-period;

[0025] - an estimation step in which an absolute value of average primary transformer current is estimated based on the current measurements taken during the measuring step;

[0026] - a calculation step in which a rotor current estimate is calculated based on the estimation of the average primary transformer current obtained in the estimation step.

[0027] The current estimation method according to the invention is destined to be used to determine the output current of the rotating part of the electric machine, e.g. the rotor of the motor of the electric or hybrid vehicle. In order to estimate the rotor current, the current estimation method comprises a plurality of steps which are performed consecutively, among which a measuring step, an estimation step and a calculation step.

[0028] During the measuring step, the current sensor located within the converter between the capacitor and the transistors measures the current. Such measurements are taken in at least one active pulse, whether it is a positive pulse and / or a negative pulse.

[0029] The current measurements obtained thanks to the current sensor are then used during the estimation step in order to estimate an absolute value of the average primary transformer current. A rotor current estimate is then calculated thanks to a mathematical formula based on the previously estimated average primary transformer current during an active pulse.

[0030] When all the steps of the current estimation method are completed, the current value for the rotor is known and such current value can be used to control torque provided to the wheels of the electric or hybrid vehicle. Unlike other methods using current measurements taken in the converter input current or in the primary transformer current, the present method is not sensitive to sensor delays and may easily implement filtering to remove possible disturbances. Additionally, the accuracy of the present current estimation method is improved in both steady state conditions and dynamic conditions. Notably, the current sensor being located in the commutation cell makes it possible to cut out only the parts of the primary transformer current waveform which are relevant for the calculation step.

[0031] According to an optional characteristic of the invention, the measurements are obtained by oversampling.

[0032] Using a single measurement for the current value can notably correspond to a singleshot sampling method, whereas using a plurality of measurements can for instance correspond to an oversampling method. The single- shot sampling method is easier to implement while the oversampling method improves accuracy as well as signal quality.

[0033] According to an optional characteristic of the invention, the PWM half-period in which the current measurements are taken comprises at least half of an active pulse of the voltage waveform.

[0034] The target of the measuring step is to obtain measurements which correspond to the average of the primary transformer current during the active pulse.

[0035] According to an optional characteristic of the invention, the PWM half-period starts in the middle of an active pulse of the voltage waveform.

[0036] Each active pulse is thus centered around the start of the PWM half-period. The duty cycle is for instance updated in the middle of the active pulse.

[0037] According to an optional characteristic of the invention, the PWM half-period starts in a zero-voltage output phase in the middle between two consecutive active pulses of the voltage waveform. Each active pulse is thus centered with the middle of a PWM half-period.

[0038] According to an optional characteristic of the invention, during the measuring step, the current measurements are taken during multiple PWM half-periods.

[0039] Using multiple PWM half-periods in which to take the current measurements makes for a more accurate measuring step.

[0040] According to an optional characteristic of the invention, during the measuring step, the current measurements are taken all along the PWM half -period.

[0041] Such current measurements correspond to a first embodiment of the current estimation method according to the invention. Taking current measurements during the complete duration of the PWM half-period allows for a more robust acquisition, as it provides the average value acquired by the commutation cell current sensor over the complete time period. Taking current measurements all along the PWM halfperiod requires specific hardware such as an application- specific integrated circuit (ASIC), a field-programmable gate array (FPGA) or special microcontrollers with advanced ADC (analog-to-digital) peripherals.

[0042] According to an optional characteristic of the invention, during the measuring step, the current measurements are taken for a duration shorter than the PWM half -period.

[0043] Such current measurements correspond to a second embodiment of the current estimation method according to the invention. Taking current measurements for a duration shorter than the PWM half-period is less costly than over the complete PWM half-period. As a result, the second embodiment may be implemented when ASIC or FPGA based acquisitions or solutions with special microcontrollers with advanced ADC peripherals are deemed too expensive.

[0044] According to an optional characteristic of the invention, the current measurements are taken for a duration longer than the active pulse of the voltage waveform.

[0045] This ensures that the current measurements are robustly corresponding to the average primary transformer current during the active pulse.

[0046] According to an optional characteristic of the invention, the method of current estimation comprises a detection step in which a hardware circuit detects active pulse locations. Such current measurements correspond to a third embodiment of the current estimation method according to the invention, in which a mean value of the current is obtained. This third embodiment includes a hardware circuit which uses the PWM signals or an edge detection circuit to detect the active pulses. This leads to a highly accurate reconstruction of the primary transformer current average during a given active pulse. This third embodiment is accurate in steady state conditions and can be easily implemented.

[0047] According to an optional characteristic of the invention, the measuring step starts at the beginning of a given active pulse and stops at the end of said active pulse.

[0048] In other words, the measurement locations are chosen so that they surround a given pulse of the voltage waveform, with no delay either before or after said pulse.

[0049] According to an optional characteristic of the invention, the current measurements are shifted from a center point of the active pulse.

[0050] For instance, the measuring step may start at the beginning of a given active pulse and stop after the end of said active pulse. In other words, the current measurements take into account the active pulse plus some delays. This accounts for possible sensor delays and / or delays in the PWM generation.

[0051] According to an optional characteristic of the invention, the measuring step starts before a given active pulse in a zero-output voltage phase and stops after the end of said active pulse.

[0052] According to an optional characteristic of the invention, the calculation step takes into account a transformer transformation ratio.

[0053] The transformer transformation ratio is a relation between inductance and mutual inductance of the transformer.

[0054] According to an optional characteristic of the invention, the calculation step takes into account an effective transformation ratio.

[0055] Such effective transformation ratio is necessary if inductance values are not constant but rather current dependent, for instance due to magnetic saturation. According to an optional characteristic of the invention, the method of current estimation comprises a calibration step during which measurements are taken in zero-current conditions.

[0056] In other words, the measurements are taken between active pulses. This calibration step helps learning about any sensor offset, which could result from temperature drifts or drift over life, in order to improve the rotor current estimation accuracy. Such measurements of the sensor offset may be carried out by successive approximation register (SAR) or delta-sigma (DS) analog-to-digital converters (ADC). The use of multiple measurements in zero-current conditions helps improve the quality of the offset learning. The number of such measurements may vary according to the embodiments. The number of measurements can for instance depend on the duty cycle and / or switching frequency of the converter.

[0057] Alternatively, the sensor offset may be learned by interrupting the PWM generation for a predetermined time, or by carrying out measurements when no duty cycle is requested. In any case, the calibration step may occur before the measuring step during a period with no active pulses, e.g. at the start-up of the electronic control unit of the electric machine.

[0058] According to an optional characteristic of the invention, the method of current estimation comprises an averaging step in which the measurements taken in a PWM half-period corresponding to a positive pulse and the measurements taken in a PWM half-period corresponding to a negative pulse are averaged.

[0059] The averaging step helps improve the rotor current estimation accuracy in dynamic conditions.

[0060] According to an optional characteristic of the invention, the method of current estimation comprises a correction step in which the average primary transformer current estimated during the estimation step is corrected with primary transformer current measurements at zero-voltage.

[0061] Compared to the averaging step, the correction step is another way to improve the rotor current estimation accuracy in dynamic conditions. The correction step allows for a correction of the waveform when there is an offset due to a transformer flux imbalance. The primary transformer current measurements at zero-voltage are taken around a given active voltage pulse.

[0062] According to an optional characteristic of the invention, the method of current estimation is combined with a machine model by embedding the machine model in an observer structure.

[0063] The combination of the current estimation method with a machine model improves the accuracy of the rotor current estimation, particularly so in dynamic conditions and when the converter is operated at close to zero duty cycle. The machine model can for instance help reduce deviations generally occurring in dynamic conditions. The observer structure is for instance a Luenberger observer, an extended Luenberger observer, or a Kalman filter.

[0064] Other characteristics, details and advantages of the invention will become clearer on reading the following description, on the one hand, and several examples of realisation given as an indication and without limitation with reference to the schematic drawings annexed, on the other hand, on which:

[0065] [Fig. 1] is a schematic representation of an electric drive system for an electric or hybrid vehicle, comprising an electric machine comprising a rotor, a stator and a transmitter comprising a rectifier and a transformer;

[0066] [Fig. 2] is another schematic representation of part of the electric machine of figure 1, further comprising a converter;

[0067] [Fig. 3] is a schematic representation of a first general embodiment of a current estimation method according to the invention, such method being used to determine a current value for the rotor of figure 1 ;

[0068] [Fig. 4] is a schematic representation of a second general embodiment of the current estimation method according to the invention;

[0069] [Fig. 5] is a schematic representation of a first specific embodiment of the current estimation method according to the invention;

[0070] [Fig. 6] is a schematic representation of a second specific embodiment of the current estimation method according to the invention; [Fig. 7] is a schematic representation of a third specific embodiment of the current estimation method according to the invention.

[0071] The characteristics, variants and different modes of realization of the invention may be associated with each other in various combinations, in so far as they are not incompatible or exclusive with each other. In particular, variants of the invention comprising only a selection of features subsequently described in from the other features described may be imagined, if this selection of features is enough to confer a technical advantage and / or to differentiate the invention from prior art.

[0072] Like numbers refer to like elements throughout drawings.

[0073] Figures 1 and 2 are schematic representations of an electric drive system comprising an electric machine 1, or part of said electric machine 1. The electric drive system and its electric machine 1 are destined to be mounted in a vehicle such as an electric or hybrid vehicle, where it can be used to provide torque to wheels of the vehicle. Here, the electric machine 1 is an inductive electrically excited synchronous machine, which is also known as its acronym iEESM.

[0074] As can be seen on Figure 1, the electric machine 1 comprises a rotor 2 and a stator 4. In this example, the stator 4 is positioned around the rotor 2, but in other embodiments the rotor 2 could be positioned around the stator 4. In inductive electrically excited synchronous machines such as this electric machine 1, power is transferred to a winding mounted on the rotor 2 using a transformer 6. The transformer 6 is more precisely a rotating high frequency transformer. The transformer 6 comprises a primary side 8 and a secondary side 10, these two sides 8, 10 being different in that the primary side 8 is on a non-rotating part of the electric machine 1, i.e. the stator, whereas the secondary side 10 is on a rotating part of said electric machine 1, i.e. the rotor. On figure 1, a separation between the non-rotating part and the rotating part is illustrated as a dashed line.

[0075] A rectifier 12 is associated with the transformer 6. The rectifier 12 is a rotating rectifier, and as such it is positioned on the secondary side 10 of the transformer 6, i.e. on the rotating part of the electric machine 1. The role of the rectifier 12 is to provide only positive voltage to the rotor 2. This rectifier 12 is, along with the transformer 6, implemented in a transmitter 14, here an inductive transmitter. As is visible on Figure 2, a converter 16 is connected to the primary side 8 of the transformer 6. The converter 16 is a phase shift full bridge converter or PSFBC. It works at a variable pulse width modulation frequency, or PWM frequency. The PWM frequency of the converter 16 can indeed vary in a wide range, for instance from 5 to 100 kHz, depending on the design of the converter 16, the transformer 6 and the required current in the rotor 2.

[0076] The converter 16 comprises a commutation cell 15 which comprises an input capacitor 17 as well as a plurality of transistors on an output side of the converter 16. According to the invention, a current sensor S is placed within the commutation cell 15 of the converter 16. The current sensor S is more precisely located between the input capacitor 17 and the transistors 18. As illustrated on Figure 2, here the current sensor S is shown as being placed on a positive branch of the commutation cell 15. However, in other embodiments the current sensor S could alternatively be positioned on a negative branch of said cell 16.

[0077] The converter 16 is controlled by an electronic control unit of the electric machine such as a microcontroller, a field-programmable gate array (FPGA) or a digital signal processor (DSP) to convert direct current coming from a battery of the vehicle into alternating current in order to provide an appropriate voltage to the primary side 8 of the transformer 6. As shown here, the converter 16 comprises four transistors 18, with two top transistors 18A and two bottom transistors 18B. Each top transistor 18A is associated with a bottom transistor 18B, and form a respective half-bridge, or phase leg. The top 18A and the bottom transistor 18B of a respective half-bridge are controlled inversely. In order to avoid short circuit of the converter input, an additional interlock time is introduced in the command between the top 18A and the bottom 18B transistors of each phase leg. This additional interlock time leads to a delayed switch-on command of the top transistor 18A or the bottom transistor 18B of one half-bridge compared to the switch-off command for the top transistor 18A or the bottom transistor 18B of one half-bridge.

[0078] The converter 16 is operated by phase shifts, with each phase shift being comprised between 0 and 180 °. Additionally, a duty cycle ratio is associated with each phase shift of the converter 16, such duty cycle ratio being comprised between 0 and 1. As an example, a phase shift of 0 ° corresponds to a duty cycle ratio of 0, a phase shift of 90 ° corresponds to a duty cycle ratio of 0.5 and a phase shift of 180 ° corresponds to a duty cycle ratio of 1.

[0079] The converter 16 generates a voltage waveform 20, such waveform 20 being visible on Figures 3 to 7. The voltage waveform 20 can be divided into a plurality of PWM periods 22, with both a positive pulse 24 and a negative pulse 26 occurring in the waveform 20 for each PWM period 22. For a duty cycle ratio equal to 1, positive or negative voltages are applied successively during all given PWM periods, without zero voltage values, each positive or negative voltage being applied during half of the PWM period, whereas for a duty cycle ratio equal to 0.5 for instance, positive and negative voltages are applied for smaller duration during the given PWM period, with zero voltage values provided between them, each positive or negative voltage being applied for half the time of what is done for the duty cycle ratio equal to 1.

[0080] As has been mentioned hereinbefore, the electric machine 1 can be used to provide torque to the wheels of the hybrid or electric vehicle. To control torque, and more generally to ensure an efficient operation of the electric machine 1, it is necessary to accurately determine the current of the rotor 4.

[0081] To this end, a method for rotor current estimation according to the invention can be used. This current estimation method, which covers several embodiments, will now be described in relation with Figures 3 to 7 wherein different waveforms are illustrated. These waveforms correspond to the ideal voltage waveform 20 generated by the converter 16, an idealized input current waveform 28 in the commutation cell 15 of the converter 16, and a sensor signal waveform 30 obtained by the current sensor S. The voltage waveform 20 here comprises a plurality of active pulses, with at least one positive pulse 24 and at least one negative pulse 26.

[0082] Figures 3 and 4 correspond respectively to a first general embodiment and a second general embodiment of the current estimation method. In both the first general embodiment and the second general embodiment, the rotor current estimation method may comprise a preliminary step which is a calibration step. This calibration step is used to detect potential offsets in the functioning of the current sensor S. The calibration step is carried out by taking measurements in the voltage waveform 20 in zero-current conditions. The calibration step can for instance be implemented when starting the electronic control unit of the converter 16, i.e. its microcontroller, before active pulses are generated. Alternatively, the calibration step can be carried out while active pulses are generated, the measurements in zero-current conditions then occurring between active pulses. The calibration step may then be carried out concurrently to other steps of the rotor current estimation method.

[0083] The rotor current estimation method comprises a measuring step. During this measuring step, the current sensor S of the commutation cell 15 of the converter 16 takes current measurements for a period of time at least as long as the duration of an active pulse of the voltage waveform 20. In other words, an acquisition period 32 in the voltage waveform 20 is at least as long as an active pulse of said voltage waveform 20. The active pulse is either a positive pulse 24 or a negative pulse 26. It should be noted that such current measurements may be acquired either through single- shot sampling, meaning by sampling a single value, or through oversampling, meaning by sampling a plurality of values on the same active pulse.

[0084] In the general embodiment of Figure 3, i.e. in the first general embodiment, the acquisition period 32 in which the current measurements are taken covers at least one full PWM half-period. There are possibly multiple acquisition periods 32 covering multiple full PWM half-periods. To this end, the converter 16 includes a specific hardware designed to carry out measurements for the entire duration of the PWM half-period. This specific hardware can for instance be an application- specific integrated circuit, a field-programmable gate array or special microcontrollers with advanced ADC (analog-to-digital) peripherals. On the contrary, in the general embodiment of Figure 4, i.e. in the second general embodiment, the acquisition period 32 in which the current measurements are taken covers a period of time which is shorter than a full PWM half-period. The second general embodiment thus does not require any specific hardware designed to carry out measurements for the entire duration of the PWM half-period as was the case for the first general embodiment. Although the current measurements taken by the current sensor S do not cover a full PWM half-period in the second general embodiment, as for the first general embodiment it is also possible to cover multiple PWM half-periods by multiple acquisition periods 32. In any case, in this second general embodiment the acquisition period 32 is taken for a duration longer than a given active pulse. As an example, the acquisition period 32 can cover a period of time which starts slightly before a given active pulse and which ends slightly after said active pulse.

[0085] In the first general embodiment of Figure 3, which represents steady state conditions with a constant duty cycle in the relevant PWM period and the PWM period preceding it, the PWM half-period starts in the middle of the positive pulse 24 and it ends in the middle of the negative pulse 26 of the voltage waveform 20. In case of a constant duty cycle, the PWM half-period is thus aligned with a middle point of the voltage waveform 20 between two consecutive active pulses. As a result, each PWM half-period comprises, in this order, half an active pulse, a zero-current period and half an active pulse. In the second general embodiment of Figure 4, each PWM halfperiod is centered on the middle of one of the active pulses, which means that the PWM half-period starts in the middle between two consecutive active pulses of the voltage waveform 20. The PWM half-period is thus aligned with the center of an active pulse, i.e. it is center aligned. Consequently, each PWM half-period comprises a zero-current period, an entire active pulse and another zero-current period.

[0086] However, there can be some embodiments where there is no zero-current periods within the complete PWM half-period, for instance for embodiments with a duty cycle of 1.

[0087] It should be noted that although hereinbefore the first general embodiment has been described with a PWM half-period aligned with the middle point between two consecutive active pulses and the second general embodiment has been described with a PWM half-period aligned with the center of an active pulse, the rotor current estimation method according to the invention also intends to cover other embodiments where the first general embodiment in which the acquisition period 32 covers at least one full PWM half-period would cover a PWM half-period aligned with the center of an active pulse and the second general embodiment in which the acquisition period 32 covers a period of time which is shorter than a full PWM halfperiod would cover a PWM half-period aligned with the middle point between two consecutive active pulses.

[0088] Additionally, the measuring step of the rotor current estimation method can be implemented differently than what has been described before when it is carried out according to a third general embodiment which is not illustrated here. In this third general embodiment, the measuring step comprises a substep which is a detection step. During this detection step, a hardware circuit of the converter 16 detects active pulse locations in the voltage waveform 20 in order to then carry out the current measurements of the measuring step. The hardware circuit is more precisely used to analyse the output voltage of the converter 16. This analysis comprises a detection of edges of the voltage thanks to a comparator stage. The edges are then used to successively start and reset an average calculation function, which is for instance carried out by an integrator stage. The resulting calculated average signal is then latched and a mean value is provided to the converter.

[0089] In the case of the third general embodiment, the acquisition period 32 can either start at the beginning of a given active pulse and stop at the end of said active pulse, or it can start at the beginning of a given active pulse and stop after the end of said active pulse, the acquisition period 32 thus accounting for a delay after the active pulse.

[0090] For each of the three general embodiments, the rotor current estimation method may comprise an additional averaging step. In said averaging step, the measurements which were taken in an acquisition period 32 covering a positive pulse 24 during the measuring step and the measurements which were taken in an acquisition period 32 covering a negative pulse 26 during the measuring step are averaged together.

[0091] Once the current measurements have been taken, the rotor current estimation method comprises an estimation step. During this estimation step, the current measurements are used to estimate an absolute value of the average primary transformer current. More precisely, the current measurements are used to estimate an absolute value of the average primary transformer current in a given active pulse. Such estimation is based on the following formula: with:

[0092] “ii.abs.PwrBaiCommCei” is the absolute value of the average primary transformer current to be estimated;

[0093] “j ” is an efficiency parameter of the converter 16 and the transformer 6; " ommceii.avg, Half Period" is an average input current of the commutation cell for the considered PWM half-period;

[0094] “(JnaifPeriod” is the duty cycle ratio of the converter 16 for the considered PWM halfperiod.

[0095] The average input current of the commutation cell is calculated according to the following formula:

[0096] In order to implement such integral, the start of the acquisition period 32 is used to define “tstart” and the end of said acquisition period 32 is used to define “tEnd”. The start and end of the acquisition period 32 may differ according to the general embodiment used, as has been detailed hereinbefore.

[0097] The estimate of the primary transformer current may need to be corrected. To this end, a correction step of the rotor current estimation method is performed. The correction step will be further described in relation with Figure 7 hereinafter. This correction step helps avoiding offset due to the transformer 6 flux imbalance. In order to implement the correction step, primary transformer current measurements are taken in the primary transformer current waveform at zero-voltage. More precisely, the zero-voltage measurements are taken so that they surround an active pulse of said primary transformer current waveform. The zero-voltage measurements are either taken at a distance of a half-PWM period or oversampling is performed. Once the zero-voltage measurements are acquired, they are averaged to obtain a correction value. The correction value is then subtracted from the absolute value of the average primary transformer current obtained during the estimation step to obtain a corrected average primary transformer current value.

[0098] Once the absolute value of the average primary transformer current has been estimated and possibly corrected, the rotor current estimation method according to the invention comprises a calculation step. During such calculation step, the result of the estimation step, i.e. the estimate of the average primary transformer current, or when applicable the result of the correction step, i.e. the corrected average primary transformer current value, is used to calculate a rotor current estimate. For this calculation step, a transformer transformation ratio, corresponding to a relation between the inductance of the transformer 6 and its mutual inductance, is considered. The rotor current estimate is more specifically calculated using the following formula: with:

[0099] “ip” is the rotor current value to be estimated;

[0100] “Ln” is the inductance of the transformer 6;

[0101] “M12” is the mutual inductance of the transformer 6; and

[0102] “i i.abs.PwrBaiCommCeii” is the average primary transformer current value obtained during the estimation step.

[0103] Alternatively, the estimation of the rotor current value can be obtained thanks to the following formula:

[0104] Ip ~ li’abs wrBalCommCell with “x” being an effective transformation ratio which can be used instead of the transformer transformation ratio when inductance values are current dependent rather than constant.

[0105] It should be noted that the method of rotor current estimation according to the invention can be combined with a machine model in order to improve the accuracy of the rotor current estimation. Said machine model and the method of rotor current estimation may be used in an observer structure, which is for instance chosen among a Luenberger observer, an extended Luenberger observer, or a Kalman filter.

[0106] Three specific embodiments of the rotor current estimation according to the invention will now be described in relation to Figure 5, Figure 6 and Figure 7, which respectively represent a first specific embodiment, a second specific embodiment and a third specific embodiment. These specific embodiments detail possible acquisition strategies used to implement the measuring step. In these Figures 5 to 7, the voltage waveform 20 generated by the converter 16 is shown in solid lines, the idealized input current waveform 28 in the commutation cell 15 of the converter 16 is shown in dashed lines and the idealized sensor signal waveform 30 obtained by the current sensor S is shown in dotted lines. The PWM frequency within a given evaluation period may be, according to the embodiments, constant or variable. Here, the waveforms 20, 28, 30 are shown for various evaluation periods corresponding to multiples of a PWM period 22. From top to bottom, the waveforms are shown for evaluation periods corresponding to a half PWM period 34, one PWM period 22, 3 / 2 of a PWM period 36 and two PWM periods 38. Figures 5 and 7 also show the waveforms 20, 28, 30 for an evaluation period corresponding to 2,5 PWM periods 40. In these specific embodiments, the evaluation period is measured between two duty cycle updates. As a result, the duty cycle within the evaluation period is here constant. However, there could be other embodiments of the invention where the duty cycle within the evaluation period is variable.

[0107] In the first specific embodiment of Figure 5, the acquisition period 32 in which the current measurements are taken covers a period of time which is shorter than a full PWM half-period, as was the case in the second general embodiment of Figure 2, but the PWM half-period is aligned with the center of an active pulse as was the case in the first general embodiment of Figure 1. Here, the acquisition period 32 is synchronised to the PWM generation, with both said PWM generation and duty cycle being constant for the duration of the acquisition period 32. As is illustrated on Figure 5, there is no acquisition period 32 for the evaluation period of half a PWM period 32 due to technical limitations of the microcontroller of the converter 16. Here, the acquisition period 32 for the other evaluation periods of Figure 5 is centered, which means that it starts in the middle between two consecutive active pulses of the voltage waveform 20. The duration of the acquisition period 32 may be defined as follows:

[0108] Tacq = (N-l)*TpWM / 2=(N-l) / N*Teval with “Tacq” being the duration of the acquisition period 32;

[0109] “N” being the number of PWM half-periods in the considered evaluation period;

[0110] “TPWM” being the PWM period;

[0111] “Tevai” being the duration of the evaluation period.

[0112] In order to implement the measuring step of the rotor current estimation method, an idealised measurement window 42 is defined. It should be noted that in the specific embodiments of Figures 5 to 7, the evaluation period is shown as containing only one measurement window 42. However, in other embodiments of the rotor current estimation method according to the invention there could be an evaluation period containing multiple measurement windows 42.

[0113] Compared to the acquisition period 32, this measurement window 42 is shifted in order to take into account possible delays. Such delays may for instance correspond to delays in the PWM generation and / or delays due to the current sensor S. The positioning of the current sensor S in the commutation cell 15 according to the invention leads to robust measurements. The shifting of the measurement window 42 compared to the acquisition period 32, while not mandatory thanks to this positioning of current sensor S, is particularly useful in highly dynamic conditions as it allows for optimal results. In other embodiments, for instance in steady state conditions, the measurement window 42 could be aligned with the acquisition period 32.

[0114] Once the current measurements have been taken in the measurement window 42 as has been described in the aforementioned general embodiments, the estimation step of the rotor current estimation method is carried out. The average input current of the commutation cell is calculated according to the following formula, which has been detailed in the estimation step hereinbefore: where:

[0115] " ommceii.avg, Half Period" is the average input current of the commutation cell for the considered acquisition period 42;

[0116] “Tacq” is the duration of the considered acquisition period 32;

[0117] Tacq, start is the beginning of the considered acquisition period 32;

[0118] “Tacq, end” is the end of the considered acquisition period 32.

[0119] The calculated average input current of the commutation cell is then used in the aforementioned formula for calculating the absolute value of primary transformer current, such formula having been described in the estimation step:

[0120] Once the absolute value of primary transformer current is known, it is used in the calculation step to determine the rotor current according to the previously described formula:

[0121] The second specific embodiment is illustrated on Figure 6. It differs from the first specific embodiment in that the PWM generation is center aligned, as here the PWM generation is centered on the start of the evaluation period as was the case in the second general embodiment of Figure 4. On the contrary, the PWM generation was offset in the first specific embodiment, e.g. left aligned or right aligned compared to the start of the evaluation period.

[0122] This second specific embodiment makes it possible, depending on the chosen duty cycle, to completely cover the active pulses in the current waveform over the evaluation period. In this case, the duration of the acquisition period 32 corresponds to the duration of the evaluation period to which two gaps are subtracted, with a first gap before the acquisition period 32 and a second gap after said acquisition period 32. As a result, the formula used to calculate the average input current of the commutation cell is as follows: where:

[0123] “Tevai” is the duration of the considered evaluation period.

[0124] The calculated average input current of the commutation cell can then be used in the formula to determine the absolute value of primary transformer current used during the previously mentioned estimation step and the subsequent calculation step.

[0125] The third specific embodiment will now be described in relation to Figure 7. As was the case in the first specific embodiment, the PWM generation is here offset, e.g. left aligned or right aligned compared to the start of the evaluation period. The third specific embodiment is more precise than the first specific embodiment in that it illustrates the aforementioned correction step. Once the current measurements have been taken in the voltage waveform 20 according to the measuring step and once the estimation step has been performed, the correction step is implemented.

[0126] During this correction step, as described before the primary transformer current measurements are taken in the primary transformer current waveform at zerovoltage. For the evaluation period corresponding to the PWM period 22, a first primary transformer current measurement Ml is taken before the active pulse, which is here a negative pulse 26, and a second primary transformer current measurement M2 is taken after said negative active pulse. The first primary transformer current measurement Ml and the second primary transformer current measurement M2 are taken so that they are at a distance corresponding to a half-PWM period.

[0127] Similarly, for the evaluation period corresponding to two PWM periods 38, the first primary transformer current measurement Ml is taken before the first negative pulse 26 of the acquisition period 32 and the second primary transformer current measurement M2 is taken after said negative pulse 26. In addition, a third primary transformer current measurement M3 is taken after the positive pulse 24 which is in the middle of the acquisition period 32, and a fourth primary transformer current measurement M4 is taken after the second negative pulse 26 of the acquisition period 32. As a result, the first primary transformer current measurement Ml and the second primary transformer current measurement M2 surround the first negative pulse 26 within the acquisition period 32, the second primary transformer current measurement M2 and the third primary transformer current measurement M3 surround the positive pulse 24 in the middle of the acquisition period 32 and the third primary transformer current measurement M3 and the fourth primary transformer current measurement M4 surround the second negative pulse 26 of the acquisition period 32. The same reasoning for sampling primary transformer current measurements may be applied to every evaluation period.

[0128] The correction value is then obtained from the primary transformer current measurements. The obtention of said correction value depends on the number of active pulses in the acquisition period 32. When there is only one active pulse in the acquisition period 32, the correction value corresponds to the average of the two primary transformer current measurements surrounding it. For instance, in the case of the evaluation period corresponding to two PWM periods 38, the first primary transformer current measurement Ml and the second primary transformer current measurement M2 surrounding the negative pulse 26 are averaged together.

[0129] In the case of an evaluation period comprising multiple active pulses, for instance a number of n active pulses, the correction value is obtained according to the following formula: primTrafoCurr,n

[0130] The correction value is then subtracted from the absolute value of the average primary transformer current obtained during the estimation step to obtain the corrected average primary transformer current value. As has been described hereinbefore, this corrected average primary transformer current value can then be used during the calculation step in order to obtain the rotor current estimate.

[0131] The present invention thus covers an electric machine of an electric drive system in which a particular position of a current sensor allows for an accurate rotor current estimation in steady state conditions as well as in dynamic conditions, and a rotor current estimation method based on measurements taken by said current sensor.

[0132] Many modifications and other embodiments of the invention set forth herein will come to mind to one skilled in the art to which the invention pertains having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.

Claims

CLAIMS1. An electric or hybrid vehicle electric drive system comprising an electric machine (1) comprising a rotating part, a transformer (6) and a converter (16) which is operated with phases shifts, the converter (16) comprising a commutation cell (15) comprising a capacitor (17) on an input side of the converter (16) and a plurality of transistors (18) on an output side of the converter (16), the commutation cell (15) comprising at least one current sensor (S) located between the capacitor (17) and the transistors (18).

2. The electric or hybrid vehicle electric drive system according to the preceding claim, wherein the current sensor (S) of the commutation cell (15) is located on a positive branch of the commutation cell (15).

3. The electric or hybrid vehicle electric drive system according to claim 1, wherein the current sensor (S) of the commutation cell (15) is located on a negative branch of the commutation cell (15).

4. A method of current estimation for the rotating part of the electric or hybrid vehicle electric machine (1) of the electric drive system according to claims 1 to 3, comprising:- a measuring step in which current measurements are taken within at least one PWM half-period by the commutation cell (15) current sensor (S), the current measurements being taken for a period of time at least as long as a duration of an active pulse of a voltage waveform (20) generated by the converter (16) between the converter (16) and the transformer (6) during the same PWM half-period;- an estimation step in which an absolute value of average primary transformer current is estimated based on the current measurements taken during the measuring step;- a calculation step in which a rotor current estimate is calculated based on the estimation of the average primary current obtained in the estimation step.

5. A method of current estimation according to claim 4, wherein the measurements are obtained by oversampling.

6. A method of current estimation according to any of claims 4 and 5, wherein the PWM half-period in which the current measurements are taken comprises at least half of an active pulse of the voltage waveform (20).

7. A method of current estimation according to the preceding claim, wherein the PWM half-period starts in the middle of an active pulse of the voltage waveform (20).

8. A method of current estimation according to claim 6, wherein the PWM half- period starts in a zero-voltage output phase in the middle between two consecutive active pulses of the voltage waveform (20).

9. A method of current estimation according to any of claims 4 to 8, wherein during the measuring step, the current measurements are taken during multiple PWM half-periods.

10. A method of current estimation according to any of claims 4 to 9, wherein during the measuring step, the current measurements are taken all along the PWM half-period.

11. A method of current estimation according to any of claims 4 to 9, wherein during the measuring step, the current measurements are taken for a duration shorter than the PWM half-period.

12. A method of current estimation according to claim 11, wherein the current measurements are taken for a duration longer than the active pulse of the voltage waveform (20).

13. A method of current estimation according to any of claims 4 to 12, comprising a detection step in which a hardware circuit detects active pulse locations.

14. A method of current estimation according to claim 13, wherein the measuring step starts at the beginning of a given active pulse and stops at the end of said active pulse.

15. A method of current estimation according to claim 13, wherein the current measurements are shifted from a center point of the active pulse.

16. A method of current estimation according to any of claims 1 to 12, wherein the measuring step starts before a given active pulse in a zero-output voltage phase and stops after the end of said active pulse.

17. A method of current estimation according to any of claims 4 to 16, wherein the calculation step takes into account a transformer transformation ratio.

18. A method of current estimation according to any of claims 4 to 17, wherein the calculation step takes into account an effective transformation ratio.

19. A method of current estimation according to any of claims 4 to 18, comprising a calibration step during which measurements are taken in zero-current conditions.

20. A method of current estimation according to any of claims 4 to 19, comprising an averaging step in which the measurements taken in a PWM halfperiod corresponding to a positive pulse and the measurements taken in a PWM half- period corresponding to a negative pulse are averaged.

21. A method of current estimation according to any of claims 4 to 20, comprising a correction step in which correction step in which the average primary transformer current estimated during the estimation step is corrected with primary transformer current measurements (Ml, M2, M3, M4) at zero-voltage.

22. A method of current estimation according to any of claims 4 to 21, being combined with a machine model by embedding the machine model in an observer structure.

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