Power conversion device, motor control device, and electric power steering device

Abstract

The switch control unit (31, 32) drives the switching element of the first phase at a duty cycle of 100% when the target duty cycle of the first phase, which is the phase with the highest target duty cycle among the multiple phases of the multi-phase inverter (33), is greater than a prescribed first threshold value set to be less than 100%, and drives the switching element of the second phase at a corrected duty cycle obtained by adding and correcting the target duty cycle of the second phase other than the first phase, and the voltage command value limiting unit (45) limits the q-axis voltage command value and the d-axis voltage command value in such a manner that the corrected duty cycle does not exceed the prescribed second threshold value set to be less than 100%.

Description

Technical Field

[0001] The present invention relates to a power conversion device, a motor control device and an electric power steering device. Background Art

[0002] The motor control device described in Patent Document 1 below measures a phase current based on a voltage drop across a shunt resistor connected in series with a switching element of a three-phase inverter.

[0003] Prior art literature

[0004] Patent Literature

[0005] Patent Document 1: Japanese Patent No. 5396948 Summary of the Invention

[0006] Problems to be solved by the invention

[0007] When measuring phase current based on the voltage drop across a shunt resistor connected in series with the switching elements of a multiphase inverter, as the duty cycle of the maximum-duty phase (the phase with the highest duty cycle in PWM control) increases, the on / off switching timing of the switching element of the maximum-duty phase approaches the sampling timing of the voltage across the shunt resistor. Consequently, switching noise may be superimposed on the measured voltage across the shunt resistor, making accurate phase current detection impossible.

[0008] Therefore, in the motor control device described in the above-mentioned patent document 1, when any on-time of the switching element on the low-potential side is shorter than the detection time of the current value, the switching element on the high-potential side of the undetectable current phase corresponding to the switching element is kept on and the switching element on the low-potential side is kept off, and the duty indication value of the phase other than the undetectable current phase is converted to the high-potential side.

[0009] However, if the duty instruction value of a phase other than the current undetectable phase is shifted to the high potential side and the on-period of the low potential switching element is shortened, the phase current flowing through the shunt resistor may not be accurately detected.

[0010] The present invention has been completed in view of the above situation, and its purpose is to improve the measurement accuracy when measuring the phase current based on the voltage drop of the resistance element connected in series with the switching element of the multi-phase inverter when the duty ratio of the phase with the highest duty ratio as the PWM control is high.

[0011] Means for solving problems

[0012] To achieve the above-mentioned object, a power conversion device according to one embodiment of the present invention includes: a multiphase inverter having a series connection of upper arm switching elements and lower arm switching elements for a plurality of phases; a voltage command value generating unit generating a q-axis voltage command value and a d-axis voltage command value for driving the multiphase inverter; a voltage command value limiting unit limiting the q-axis voltage command value and the d-axis voltage command value; a voltage command value converting unit converting the q-axis voltage command value and the d-axis voltage command value limited by the voltage command value limiting unit into multiphase voltage command values; a switching control unit controlling a first switching element and a second switching element by PWM control based on the q-axis voltage command value and the d-axis voltage command value limited by the voltage command value limiting unit, the first switching element being one of the upper arm switching element and the lower arm switching element, and the second switching element being the other of the upper arm switching element and the lower arm switching element other than the first switching element; and a current measuring unit measuring a current flowing through the second switching element based on a voltage drop across a resistance element connected in series with the second switching element.

[0013] The switching control unit sets a target duty ratio based on a target voltage to be applied to a load of the multi-phase inverter. The target duty ratio is a target value of the duty ratio of a period in which the first switching element is turned on, within a PWM period. When the target duty ratio of a first phase, which is a phase having the highest target duty ratio among the multiple phases of the multi-phase inverter, is less than a predetermined first threshold value set to be less than 100%, the switching control unit drives the first switching element of the first phase at the target duty ratio. When the target duty ratio of the first phase is greater than the first threshold value, the switching control unit drives the first switching element of the first phase at a duty ratio of 100% and drives the first switching element of the second phase at a corrected duty ratio obtained by adding a correction to the target duty ratio of a second phase of the multiple phases other than the first phase. The voltage command value limiting unit limits the q-axis voltage command value and the d-axis voltage command value so that the corrected duty ratio does not exceed the predetermined second threshold value set to be less than 100%.

[0014] Effects of the Invention

[0015] According to the present invention, when measuring phase current based on the voltage drop of a resistance element connected in series with the switching elements of a multiphase inverter, measurement accuracy can be improved when the duty ratio of the maximum duty phase, which is the phase with the highest duty ratio in PWM control, is high. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 This is a schematic configuration diagram showing an example of an electric power steering device according to an embodiment.

[0017] Figure 2This is a schematic configuration diagram showing an example of an electronic control unit (ECU).

[0018] Figure 3 This is a block diagram showing an example of the functional configuration of the control operation unit.

[0019] Figure 4 (a) to (e) are schematic timing charts for explaining the switching timing of the switching element and the current measurement timing.

[0020] Figure 5 (a) to (e) are schematic timing charts for explaining the switching timing of the switching element and the current measurement timing.

[0021] Figure 6 This is a schematic diagram explaining how to set the duty ratio.

[0022] Figure 7 (a) to (d) are explanatory diagrams of the duty ratio of each phase and the difference between the duty ratios between the phases.

[0023] Figure 8 (a) and (b) are explanatory diagrams of the effects of the embodiment.

[0024] Figure 9 (a) and (b) are explanatory diagrams of the duty ratio of each phase when the q-axis voltage command value and the d-axis voltage command value are not restricted.

[0025] Figure 10 (a) and (b) are explanatory diagrams of the duty ratio of each phase when the q-axis voltage command value and the d-axis voltage command value are limited.

[0026] Figure 11 This is a flowchart of an example of a current measuring method according to the embodiment.

[0027] Figure 12 This is a schematic structural diagram showing a first modified example of the electric power steering device.

[0028] Figure 13 This is a schematic structural diagram showing a second modified example of the electric power steering device.

[0029] Figure 14 This is a schematic structural diagram showing a third modified example of the electric power steering device. DETAILED DESCRIPTION

[0030] Embodiments of the present invention will be described in detail with reference to the accompanying drawings. The embodiments of the present invention described below illustrate devices and methods for embodying the technical concept of the present invention. However, the technical concept of the present invention is not limited to the structure and arrangement of the components described below. Various modifications can be made to the technical concept of the present invention within the technical scope defined by the claims.

[0031] (structure)

[0032] Figure 1 This is a schematic diagram showing an example of an electric power steering (EPS) system according to an embodiment. A steering shaft (steering shaft, handle shaft) 2 of a steering wheel (steering handle) 1 is connected to steered wheels 8L and 8R via a reduction gear (worm gear) 3 forming a reduction mechanism, universal joints 4a and 4b, a rack and pinion mechanism 5, tie rods 6a and 6b, and finally hub units 7a and 7b.

[0033] The rack and pinion mechanism 5 includes a pinion 5a connected to a pinion shaft to which the steering force from the universal joint 4b is transmitted, and a rack 5b meshing with the pinion 5a. The rack 5b converts the rotational motion transmitted to the pinion 5a into linear motion in the vehicle width direction.

[0034] A torque sensor 10 for detecting a steering torque Th is provided on the steering shaft 2 . Also, a steering angle sensor 14 for detecting a steering angle θh of the steering wheel 1 is provided on the steering shaft 2 .

[0035] Furthermore, a motor 20 that assists the steering force of the steering wheel 1 is connected to the steering shaft 2 via a reduction gear 3. In this specification, an example in which the motor 20 is a three-phase motor is described, but the number of phases of the motor 20 does not need to be three.

[0036] An electronic control unit (ECU) 30 that controls the electric power steering device is supplied with electric power from a battery 13 , and an ignition key signal is inputted via an ignition switch 11 .

[0037] ECU30 calculates the current command value of the auxiliary control instruction based on the steering torque Th detected by the torque sensor 10, the vehicle speed Vh detected by the vehicle speed sensor 12, and the steering angle θh detected by the steering angle sensor 14, and controls the current (U-phase current Iu, V-phase current Iv, W-phase current Iw) supplied to the motor 20 by using the voltage command value obtained by compensating the current command value.

[0038] The steering angle sensor 14 is not essential, and the steering angle θh may be calculated by adding the motor rotation angle θm obtained from the rotation angle sensor 21 that detects the rotation angle of the rotating shaft of the motor 20 and the torsion angle of the torsion bar of the torque sensor 10 .

[0039] Alternatively, the steering angle of the steered wheels 8L, 8R may be used instead of the steering angle θh. For example, the steering angle may be detected by detecting the displacement of the rack 5b.

[0040] The ECU 30 includes, for example, a computer including a processor and peripheral components such as a storage device. The processor may be, for example, a CPU (Central Processing Unit) or an MPU (Micro-Processing Unit).

[0041] The storage device may include any of a semiconductor storage device, a magnetic storage device, and an optical storage device. The storage device may include registers, cache memory, ROM (Read Only Memory) used as a main storage device, and RAM (Random Access Memory) and other memories.

[0042] The functions of the ECU 30 described below are realized by, for example, a processor of the ECU 30 executing a computer program stored in a storage device.

[0043] Alternatively, the ECU 30 may be formed by dedicated hardware for executing each information processing described below.

[0044] For example, the ECU 30 may include a functional logic circuit set in a general-purpose semiconductor integrated circuit. For example, the ECU 30 may include a programmable logic device (PLD) such as a field-programmable gate array (FPGA).

[0045] Figure 2 This is a schematic diagram showing the configuration of an example of an ECU 30 according to an embodiment. ECU 30 includes a control calculation unit 31, a gate drive circuit 32, a multi-phase inverter 33, a current cutoff circuit 34, a cutoff drive circuit 35, a voltage drop measurement unit 36, and a motor speed calculation unit 37. The control calculation unit 31 and the gate drive circuit 32 are examples of the "switch control unit" described in the claims.

[0046] The ECU 30 is connected to a power wiring PW via a connector CNT, which transmits power from the battery 13. Furthermore, signals of the steering torque Th detected by the torque sensor 10, the vehicle speed Vh detected by the vehicle speed sensor 12, and the steering angle θh detected by the steering angle sensor 14 are transmitted to the control calculation unit 31 via the connector CNT.

[0047] The control calculation unit 31 calculates a current command value, serving as a control target value for the drive current of the motor 20, based at least on the steering torque Th. The control calculation unit 31 calculates a voltage command value obtained by applying compensation, etc. to the current command value and generates gate control signals Sgu, Sgv, and Sgw by performing PWM (Pulse Width Modulation) modulation on the voltage command value. Gate control signals Sgu, Sgv, and Sgw are PWM signals that control the drive voltages output from the multi-phase inverter 33 to the U-phase coil 20u, V-phase coil 20v, and W-phase coil 20w of the motor 20.

[0048] The gate drive circuit 32 performs on / off control of the switching elements Qu1, Qv1, Qw1, Qu2, Qv2, and Qw2 of the multiphase inverter 33, described later, based on gate control signals Sgu, Sgv, and Sgw. For example, if the switching elements Qu1, Qv1, Qw1, Qu2, Qv2, and Qw2 are field effect transistors (FETs), gate signals for the FETs are generated based on the gate control signals Sgu, Sgv, and Sgw.

[0049] The multi-phase inverter 33 includes a three-phase bridge connected between a positive-side line and a ground line. The positive-side line is connected to a DC power supply Vdc and is supplied with DC power.

[0050] The three-phase bridge includes switching element pairs formed by connecting switching elements Qu1, Qv1, Qw1 of the upper arms of the U, V, and W phases and switching elements Qu2, Qv2, Qw2 of the lower arms of the U, V, and W phases in series.

[0051] The U-phase current Iu supplied to the U-phase coil 20u of the motor 20 is supplied from the connection point of the switching elements Qu1 and Qu2, the V-phase current Iv supplied to the V-phase coil 20v is supplied from the connection point of the switching elements Qv1 and Qv2, and the W-phase current Iw supplied to the W-phase coil 20w is supplied from the connection point of the switching elements Qw1 and Qw2.

[0052] The U-phase current Iu, the V-phase current Iv, and the W-phase current Iw are supplied to the U-phase coil 20u, the V-phase coil 20v, and the W-phase coil 20w of the motor 20 via the current interruption circuit 34 .

[0053] The current cutoff circuit 34 includes three phase cutoff FETs QAu, QAv, and QAw for cutting off the motor phase current. The control calculation unit 31 outputs a control signal Sm to the cutoff drive circuit 35, which controls the energization and cutoff of the current cutoff circuit 34. Based on the control signal Sm, the cutoff drive circuit 35 outputs gate signals for the phase cutoff FETs QAu to QAw, thereby switching on or off the U-phase current Iu, V-phase current Iv, and W-phase current Iw flowing from the multi-phase inverter 33 to the motor 20.

[0054] A smoothing capacitor Cs is connected in parallel to the multi-phase inverter 33. The smoothing capacitor Cs may be, for example, an electrolytic capacitor. The ECU 30 may include a plurality of smoothing capacitors connected in parallel as the smoothing capacitor Cs.

[0055] Shunt resistors ru, rv, and rw are connected in series between the switching elements Qu2, Qv2, and Qw2 of the lower arms of the U-phase, V-phase, and W-phase and the ground line.

[0056] Voltage drop measurement unit 36 ​​measures the voltage drop across shunt resistors ru, rv, and rw caused by the current flowing through switching elements Qu2, Qv2, and Qw2 of the lower arm. Voltage drop measurement unit 36 ​​outputs the measured values ​​Vud, Vvd, and Vwd of the voltage across shunt resistors ru, rv, and rw to control calculation unit 31.

[0057] The motor rotation speed calculation unit 37 calculates the motor rotation angle θm (for example, motor electrical angle) of the motor 20 based on the detection signal of the rotation angle sensor 21 , and outputs the calculated value to the control calculation unit 31 .

[0058] Figure 3 This is a block diagram showing an example of the functional configuration of the control operation unit 31. The control operation unit 31 includes a current command value operation unit 40, subtractors 41 and 42, a current limiter 43, a proportional-integral (PI) control unit 44, a voltage limiter 45, a two-phase / three-phase converter 46, a dead time compensator 47, a third harmonic compensator 48, adders 49 to 51, a duty converter 52, a PWM controller 53, a current calculator 54, a three-phase / two-phase converter 55, and an angular velocity converter 56, and drives the motor 20 using vector control.

[0059] The current command value calculation unit 40 calculates a q-axis current command value Iq0 and a d-axis current command value Id0 to flow to the motor 20 based on the steering torque Th, the vehicle speed Vh, the motor rotation angle θm of the motor 20 , and the rotation angular velocity ω of the motor 20 .

[0060] On the other hand, the current calculation unit 54 is based on Figure 2The voltage drop measuring unit 36 ​​measures the voltage values ​​Vud, Vvd, and Vwd across the shunt resistors ru, rv, and rw, and calculates the measured values ​​Iud, Ivd, and Iwd of the U-phase current, V-phase current, and W-phase current of the motor 20. The voltage drop measuring unit 36 ​​and the current calculating unit 54 are examples of the "current measuring unit" described in the claims.

[0061] Furthermore, in the phase with the highest duty cycle among the U, V, and W phases, the period during which the switching element of the lower arm is on is short, making it impossible to accurately detect the voltage across the shunt resistor. Therefore, the current calculation unit 54 can measure the phase current of the phase with the highest duty cycle based on the voltage across the shunt resistors of the other two phases other than the phase with the highest duty cycle and Kirchhoff's law (the sum of the U-phase current Iu, the V-phase current Iv, and the W-phase current Iw is 0).

[0062] For example, when the duty cycle of the phase with the highest duty cycle is below a specified value, the phase current of the phase with the highest duty cycle can be measured based on the voltage across the shunt resistor of the phase with the highest duty cycle. When the duty cycle of the phase with the highest duty cycle is greater than a specified value, the phase current of the phase with the highest duty cycle can be measured based on the voltage across the shunt resistors of the other two phases and Kirchhoff's law.

[0063] The three-phase / two-phase conversion unit 55 converts the measured values ​​Iud, Ivd, and Iwd of the U-phase current, the V-phase current, and the W-phase current into a q-axis current iq and a d-axis current id.

[0064] Subtractors 41 and 42 calculate q-axis deviation current Δq0 and d-axis deviation current Δd0 by subtracting the fed-back q-axis current iq and d-axis current id from the q-axis current command value Iq0 and the d-axis current command value Id0, respectively.

[0065] The current limiting unit 43 limits the upper limit values ​​of the q-axis deviation current Δq0 and the d-axis deviation current Δd0. The limited q-axis deviation current Δq and the limited d-axis deviation current Δd are input to the PI control unit 44.

[0066] The PI control unit 44 calculates a basic q-axis voltage command value vq0 and a basic d-axis voltage command value vd0 so that the q-axis deviation current Δq and the d-axis deviation current Δd are zero, respectively.

[0067] For example, the basic q-axis voltage command value vq0 and the basic d-axis voltage command value vd0 may be duty ratios that are ratios of the voltage command values ​​to the power supply voltage. For example, the basic q-axis voltage command value vq0 and the basic d-axis voltage command value vd0 may be zero-center duty ratios that range from -50% to +50% with 0% as the center value.

[0068] The voltage limiter 45 sets the q-axis voltage command value vq and the d-axis voltage command value vd by limiting the basic q-axis voltage command value vq0 and the basic d-axis voltage command value vd0. The function of the voltage limiter 45 will be described in detail later.

[0069] The two-phase / three-phase converter 46 converts the q-axis voltage command value vq and the d-axis voltage command value vd into a first U-phase voltage command value vu1 , a first V-phase voltage command value vv1 , and a first W-phase voltage command value vw1 , respectively.

[0070] The dead time compensating unit 47 outputs a dead time compensation value for compensating for the dead time of the multi-phase inverter 33 to the adders 49 to 51 .

[0071] The third harmonic compensator 48 generates a third harmonic component for improving voltage utilization efficiency and outputs it to the adders 49 to 51. The third harmonic component is a harmonic component having a frequency three times the fundamental frequency of the fundamental U-phase voltage command value vu0, the fundamental V-phase voltage command value vv0, and the fundamental W-phase voltage command value vw0.

[0072] Adders 49 to 51 respectively add the first U-phase voltage command value vu1, the first V-phase voltage command value vv1, and the first W-phase voltage command value vw1 to the dead time compensation value and the third harmonic component to calculate the second U-phase voltage command value vu2, the second V-phase voltage command value vv2, and the second W-phase voltage command value vw2.

[0073] When the maximum voltage command value among the second U-phase voltage command value vu2, the second V-phase voltage command value vv2, and the second W-phase voltage command value vw2 is below the threshold, the duty conversion unit 52 sets the U-phase voltage command value vu, the V-phase voltage command value vv, and the W-phase voltage command value vw based on the second U-phase voltage command value vu2, the second V-phase voltage command value vv2, and the second W-phase voltage command value vw2, and outputs them to the PWM control unit 53.

[0074] On the other hand, when the maximum voltage command value among the voltage command values ​​vu2, vv2, and vw2 is greater than the threshold, the duty conversion unit 52 increases and corrects the U-phase voltage command value vu, the V-phase voltage command value vv, and the W-phase voltage command value vw and outputs them to the PWM control unit 53.

[0075] Furthermore, when the basic q-axis voltage command value vq0, the basic d-axis voltage command value vd0, the first U-phase voltage command value vu1, the first V-phase voltage command value vv1, the first W-phase voltage command value vw1, and the second U-phase voltage command value vu2, the second V-phase voltage command value vv2, and the second W-phase voltage command value vw2 are set to a zero-midpoint duty ratio that varies from -50% to +50% with 0% as the center value, the duty conversion unit 52 corrects the midpoint of the duty ratio and outputs the U-phase voltage command value vu, the V-phase voltage command value vv, and the W-phase voltage command value vw as a duty ratio that varies from 0% to 100% with 50% as the center value. Details of the function of the duty conversion unit 52 will be described later.

[0076] The PWM control unit 53 generates gate control signals Sgu, Sgv, and Sgw by performing PWM modulation on the U-phase voltage command value vu, the V-phase voltage command value vv, and the W-phase voltage command value vw, and outputs the gate control signals Sgu, Sgv, and Sgw to the output terminals. Figure 2 The gate driving circuit 32.

[0077] The angular velocity converter 56 calculates the rotational angular velocity ω of the motor 20 based on the temporal change in the motor rotational angle θm. The motor rotational angle θm and the rotational angular velocity ω are input to the current command value calculation unit 40 and used for vector control.

[0078] (Details of Duty Converter 52)

[0079] The duty conversion unit 52 will be described in detail. First, the relationship between the switching timing of the switching elements Qu1 , Qv1 , Qw1 , Qu2 , Qv2 , and Qw2 of the multi-phase inverter 33 and the current measurement timing will be described.

[0080] In the following description, the period during which the switching elements Qu1, Qv1, and Qw1 of the upper arms of the U-phase, V-phase, and W-phase of the multiphase inverter 33 are turned on is within the PWM period T PWM The duty ratios are recorded as "Du", "Dv" and "Dw" respectively.

[0081] In addition, target values ​​of the duties Du, Dv, and Dw specified by the second U-phase voltage command value vu2, the second V-phase voltage command value vv2, and the second W-phase voltage command value vw2 are described as “target duties Dut, Dvt, Dwt”.

[0082] When the second U-phase voltage command value vu2, the second V-phase voltage command value vv2, and the second W-phase voltage command value vw2 are set to a zero midpoint duty cycle, the midpoint of the duty cycle is corrected to 50%, thereby converting the on-period of the switching element of the upper arm into the PWM period T. PWM The duty cycle value of .

[0083] The phase with the highest target duty cycle among the U, V, and W phases is sometimes described as the "maximum duty phase," the phase with the lowest target duty cycle is sometimes described as the "minimum duty phase," and phases other than the maximum duty phase and the minimum duty phase are sometimes described as "intermediate duty phases." The maximum duty phase is an example of the "first phase" described in the claims, while the intermediate duty phase and the minimum duty phase are examples of the "second phase" described in the claims.

[0084] Figure 4 (a) and Figure 4 (b) is a schematic timing diagram of the PWM signal for driving the upper arm switching element and the lower arm switching element of the phase with the largest duty cycle, Figure 4 (c) and Figure 4 (d) are schematic timing diagrams of PWM signals for driving the upper arm switching element and the lower arm switching element of the duty middle phase or the duty minimum phase, respectively. Figure 4 (e) is a schematic timing diagram of the output voltage of the AD converter for sampling the voltage across the shunt resistor in the duty middle phase or the duty minimum phase.

[0085] Symbol T PWM Indicates the PWM cycle (i.e. one cycle of the PWM control cycle), symbol T HOFF The symbol T represents the off period when the upper arm switching element is in the off state. LON Indicates the ON period during which the lower arm switching element is in the ON state. When the lower arm switching elements Qu2, Qv2, and Qw2 are connected in series with the shunt resistors ru, rv, and rw (in the case of the so-called "downstream shunt method"), the voltage drop measuring unit 36 ​​measures the voltage drop during the ON period T of the lower arm switching element. LON The middle time point tc (or the off period T of the upper arm switching element) HOFF The voltage across the shunt resistor is sampled at the middle time point tc). The sampling period for sampling the voltage across the shunt resistor may be, for example, a PWM period T PWM Multiples of (n×T PWM )(n is a natural number).

[0086] As a result, as the duty cycle of the maximum duty phase increases, the switching timings thd, thr, tlr, and tld of the switching element of the maximum duty phase approach the sampling timing tc of the voltage across the shunt resistor of the intermediate duty phase and the minimum duty phase. Therefore, switching noise may be superimposed on the measured value of the voltage across the shunt resistor, making it impossible to accurately detect the phase current.

[0087] Therefore, when the target duty ratio of the maximum duty phase is greater than a predetermined first threshold value Dth1 set to be less than 100%, the duty conversion unit 52 sets (converts) the target duty ratio of the maximum duty phase to 100%. In other words, the target duty ratio of the maximum duty phase is increased and corrected to 100%.

[0088] Thus, the switching element of the phase with the largest duty cycle is driven at a duty ratio of 100%. That is, the period during which the upper arm switching element of the phase with the largest duty cycle is turned on is set to 100% during the PWM period T. PWM If the ratio of the duty cycle is set to 100%, the period during which the lower arm switching element of the phase with the maximum duty cycle is turned on is 100% of the PWM cycle T. PWM The ratio of φ is set to 0%, and the upper arm switching element and the lower arm switching element of the phase with the largest duty are driven.

[0089] Figure 5 (a)~ Figure 5 (e) is a schematic timing chart for explaining the switching timing of the switching element and the current measurement timing when the target duty ratio of the maximum duty phase is converted to 100%. Figure 5 (a) and Figure 5 (b) is a schematic timing diagram of the PWM signal for driving the upper arm switching element and the lower arm switching element of the phase with the largest duty cycle, Figure 5 (c) and Figure 5 (d) is a schematic timing diagram of the PWM signal for driving the upper arm switching element and the lower arm switching element of the duty middle phase or the duty minimum phase, respectively. Figure 5 (e) is a schematic timing diagram of the output voltage of the AD converter that samples the voltage across the shunt resistor in the duty middle phase or the duty minimum phase.

[0090] like Figure 5 (a) and Figure 5 As shown in (b), by converting the target duty ratio of the maximum-duty phase to 100%, the switching element in the maximum-duty phase does not switch. Therefore, switching noise is not superimposed on the measured voltage across the shunt resistor in the mid-duty phase and the minimum-duty phase. As a result, the phase currents in the mid-duty phase and the minimum-duty phase can be accurately detected.

[0091] Figure 6 This figure illustrates a method for setting the duty ratio by duty conversion unit 52. Dashed line D1t represents the target duty ratio of the maximum-duty phase among the target duty ratios specified by second U-phase voltage command value vu2, second V-phase voltage command value vv2, and second W-phase voltage command value vw2 input to duty conversion unit 52. Furthermore, single-dot chain line D2t represents the target duty ratio of the intermediate-duty phase or the minimum-duty phase.

[0092] exist Figure 6 In the example, for convenience of explanation, the target duty ratio D1t of the maximum duty phase is gradually increased to 100% over time, while the target duty ratio D2t of the intermediate duty phase or the minimum duty phase is fixed.

[0093] On the other hand, solid line D1 represents the duty ratio of the maximum duty phase among the duty ratios specified by U-phase voltage command value vu, V-phase voltage command value vv, and W-phase voltage command value vw output by duty conversion unit 52. Furthermore, two-dot chain line D2 represents the duty ratio of the intermediate duty phase or the minimum duty phase.

[0094] Duty conversion unit 52 determines whether target duty ratio D1t of the maximum duty phase is less than or equal to first threshold Dth1. If target duty ratio D1t of the maximum duty phase is less than or equal to first threshold Dth1, duty conversion unit 52 does not perform an incremental correction on target duty ratios D1t and D2t. In other words, target duty ratios D1t and D2t are set as they are, respectively, at duty ratios D1 and D2.

[0095] As a result, the switching element in the maximum duty phase is driven at the target duty ratio D1t, and the switching elements in the intermediate duty phase and the minimum duty phase are driven at the target duty ratio D2t.

[0096] On the other hand, if target duty cycle D1t of the maximum-duty phase is greater than first threshold Dth1, duty conversion unit 52 increases and corrects target duty cycle D1t to a duty cycle of 100%. In other words, target duty cycle D1t is corrected to a duty cycle of 100%, and the corrected duty cycle D1 = 100% is output as the voltage command value for the maximum-duty phase. As a result, the switching element of the maximum-duty phase is driven at the corrected duty cycle D1 = 100%.

[0097] Furthermore, when target duty cycle D1t of the maximum duty phase is greater than first threshold value Dth1, duty conversion unit 52 increases and corrects target duty cycles D2t of the intermediate duty phase and the minimum duty phase, thereby outputting corrected duty cycle D2 as voltage command values ​​for the intermediate duty phase and the minimum duty phase. As a result, the switching elements of the intermediate duty phase and the minimum duty phase are driven at the corrected duty cycle D2.

[0098] For example, the duty conversion unit 52 calculates the duty difference ΔD = (100% - D1t) obtained by subtracting the target duty ratio D1t of the maximum duty phase from 100%, sets the sum obtained by adding the target duty ratio D2t of the middle duty phase and the minimum duty phase to the duty difference ΔD as the corrected duty ratio D2 = (D2t + ΔD), and outputs the corrected duty ratio D2 as the voltage command value of the middle duty phase and the minimum duty phase.

[0099] Therefore, when the target duty cycle D1t is greater than the first threshold value Dth1, the correction amount ΔD for correcting the duty cycles of the intermediate duty phase and the minimum duty phase becomes: the smaller the difference obtained by subtracting the target duty cycle D1t of the maximum duty phase from 100% (in other words, the greater the amount by which the target duty cycle D1t exceeds the first threshold value Dth1), the smaller the correction amount.

[0100] As described above, by increasing and correcting the duty ratios of the intermediate and minimum duty phases by the duty difference ΔD as the duty ratio of the maximum duty phase is converted to 100%, it is possible to suppress or prevent fluctuations in U-phase current Iu, V-phase current Iv, and W-phase current Iw caused by the duty ratio conversion. The reasons for this are explained below.

[0101] Figure 7 The solid line, dotted line, and one-dot chain line in (a) respectively represent waveforms of the duties Du, Dv, and Dw of the U-phase, V-phase, and W-phase when the duty ratio is not changed. Figure 7 The solid line, dotted line and single-point chain line in (b) respectively represent the related duty between the U phase and the V phase when the duty cycle is not changed (i.e., the difference in duty cycle between the U phase and the V phase Du-Dv), the related duty between the V phase and the W phase (Dv-Dw), and the related duty between the W phase and the U phase (Dw-Du).

[0102] on the other hand, Figure 7 The solid line, the dotted line, and the one-dot chain line in (c) respectively represent waveforms of the duty ratios Du, Dv, and Dw of the U-phase, V-phase, and W-phase when the duty ratios are changed. Figure 7 The solid line, dotted line and single-dot chain line in (d) respectively represent the relative duty ratio between the U phase and the V phase (Du-Dv), the relative duty ratio between the V phase and the W phase (Dv-Dw), and the relative duty ratio between the W phase and the U phase (Dw-Du) when the duty ratio is changed. Figure 7 (a) and Figure 7 (b) shows the waveform when the dead time compensation value and the third harmonic component are omitted. Figure 7 (c) and Figure 7 (d) shows a waveform in which the third harmonic component is omitted.

[0103] Depend on Figure 7 (a) and Figure 7 As can be seen from (c), by changing the duty ratio, the waveforms of the duty ratios Du, Dv, and Dw of the U, V, and W phases themselves change. However, as the duty ratio of the maximum duty phase is changed to 100%, the duty ratio of the intermediate duty phase or the minimum duty phase is increased and corrected by the duty difference ΔD. Figure 7 (b) and Figure 7As shown in (d), even if the duty ratio is changed, the difference in duty ratio between the phases does not change.

[0104] As described above, as the duty cycle of the maximum duty phase is changed to 100%, the duty cycles of the middle duty phase and the minimum duty phase are increased and corrected. Therefore, even if the duty cycle is changed, the change of the potential difference between the terminals of the motor 20 can be suppressed or prevented, thereby suppressing or preventing the change of the U-phase current Iu, the V-phase current Iv and the W-phase current Iw.

[0105] Next, the effects of the embodiment will be described. Figure 8 (a) and (b) represent the measured value Iud (solid line) of the U-phase current, the measured value Ivd (dashed line) of the V-phase current, and the measured value Iwd (single-dotted line) of the W-phase current when the duty ratio Du of the U-phase, which is the phase with the largest duty cycle, gradually increases from 80% to 100%. Figure 8 (a) shows the measured value without changing the duty cycle. Figure 8 (b) shows the measured value when the duty ratio is changed. For the sake of convenience, the duty ratio Dv of the V phase and the duty ratio Dw of the W phase are fixed to 80%.

[0106] Without changing the duty cycle ( Figure 8 In (a), as the duty ratio Du of the U-phase (the phase with the maximum duty cycle) approaches 100%, the switching noise caused by the switching operation of the U-phase upper arm switching element Qu1 becomes affected, making it impossible to accurately detect the V-phase current Ivd and the W-phase current Iwd, as indicated by ellipse P1. The U-phase current Iud is calculated from the V-phase current Ivd and the W-phase current Iwd according to Kirchhoff's law. Therefore, if the V-phase current Ivd and the W-phase current Iwd cannot be accurately detected, the U-phase current Iud cannot be accurately detected, as indicated by ellipse P2.

[0107] On the other hand, when the duty cycle is changed ( Figure 8 (b)), even if the U-phase duty ratio Du is close to 100%, as shown in the portion indicated by the ellipse P3, the V-phase current Ivd and the W-phase current Iwd can be accurately detected. As a result, the U-phase current Iud can also be accurately detected.

[0108] (Details of Voltage Limiting Unit 45)

[0109] right Figure 3The details of voltage limiting unit 45 will be described below. When the target duty cycle of the maximum-duty phase exceeds first threshold Dth1, duty conversion unit 52 corrects the target duty cycle D2t of the mid-duty phase by increasing it. As the corrected duty cycle D2 increases, the period during which the switching element of the lower arm is on in the mid-duty phase may become too short. As a result, the voltage across the shunt resistor in the mid-duty phase may not be accurately detected. The mid-duty phase is an example of the "phase with the second-highest target duty cycle" as defined in the claims.

[0110] Figure 9 (a) represents the duty ratios Du, Dv, and Dw of the U-phase, V-phase, and W-phase (i.e., the U-phase voltage command value vu, the V-phase voltage command value vv, and the W-phase voltage command value vw) when the target duty ratio of the maximum duty phase is greater than the first threshold Dth1. Figure 9 (b) means Figure 9 An enlarged view of the portion viewed from the direction A1 in (a).

[0111] Figure 9 The second threshold value Dth2 in (b) is a predetermined threshold value (current detection limit threshold) for accurately detecting the duty cycle of the voltage across the shunt resistor. Second threshold value Dth2 is set to a value less than 100%. When the duty cycle exceeds second threshold value Dth2, the on-period of the lower arm switching element is too short, and the voltage across the shunt resistor may not be accurately detected.

[0112] For example, the second threshold value Dth2 can be set based on the stabilization time required for the voltage across the shunt resistor measured by the voltage drop measuring unit 36 ​​to stabilize after the switching operation of the switching element of the lower arm.

[0113] For example, the second threshold value Dth2 may be set so that the length of the on-period of the switching element of the lower arm of the duty cycle phase is not less than the above-mentioned stabilization time.

[0114] For example, the second threshold Dth2 may be set to a value smaller than the first threshold Dth1.

[0115] Reference Figure 9 In (b), in the portion viewed from arrow B1, the duty ratio Dw of the W-phase, which is the middle duty phase (i.e., the W-phase voltage command value vw), exceeds the second threshold Dth2. For example, by adding the dead time compensation value to the voltage command value, the duty ratio of the middle duty phase is more likely to exceed the second threshold Dth2.

[0116] Therefore, when the target duty ratio of the duty maximum phase is greater than the first threshold value Dth1, the voltage limiting unit 45 limits the basic q-axis voltage command value vq0 and the basic d-axis voltage command value vd0 so that the duty ratio D2 of the duty intermediate phase does not exceed the second threshold value Dth2, thereby setting the q-axis voltage command value vq and the d-axis voltage command value vd.

[0117] For example, the voltage limiting unit 45 is based on the magnitude of the voltage vector of the basic q-axis voltage command value vq0 and the basic d-axis voltage command value vd0.

[0118] [Mathematical formula 1]

[0119]

[0120] Calculate the limit gain G shown in the following formula (1): duty .

[0121] [Mathematical formula 2]

[0122]

[0123] The constant D in the above formula (1) is Ltd The duty cycle limit value is set to a value less than 100%. Ltd The target duty ratio of the duty maximum phase is appropriately set so that the duty ratio D2 of the duty intermediate phase does not exceed the second threshold value Dth2 when the target duty ratio of the duty maximum phase is larger than the first threshold value Dth1.

[0124] In addition, the constant C DT The dead time compensation value C is added to the voltage command value by the dead time compensation unit 47. DT , for example, the PWM period T PWM The result of the division by the dead time (C DT =T PWM / dead time). In addition, there are many cases where the dead time deviates between the design value and the actual value, so an appropriately set value can also be used as the dead time compensation value C DT .

[0125] In addition, the constant The coefficient corresponds to the inverse of the reduction rate of the amplitude of the three-phase voltage command value caused by the superposition of the third harmonic component (amplitude before compensation / amplitude after compensation). The coefficient can also be omitted when the third harmonic component is not superimposed on the three-phase voltage command value.

[0126] The voltage limiting unit 45 determines the limiting gain G duty Is it greater than "1" (i.e., greater than 100%)?duty When it is “1” or greater, the voltage limiting unit 45 does not limit the basic q-axis voltage command value vq0 and the basic d-axis voltage command value vd0 and outputs them as they are, q-axis voltage command value vq and d-axis voltage command value vd.

[0127] On the other hand, in the limiting gain G duty When the voltage limiting unit 45 is less than "1", the gain (G duty The product of the basic q-axis voltage command value vq0 and the basic d-axis voltage command value vd0 is output as the q-axis voltage command value vq=(G duty / 2)×vq0 and d-axis voltage command value vd=(G duty / 2)×vd0.

[0128] Figure 10 (a) represents the duty ratios Du, Dv, and Dw of the U phase, V phase, and W phase (i.e., the U phase voltage command value vu, the V phase voltage command value vv, and the W phase voltage command value vw) when the basic q-axis voltage command value vq0 and the basic d-axis voltage command value vd0 are limited. Figure 10 (b) means Figure 10 An enlarged view of the portion viewed from the direction A2 in (a).

[0129] In the portion viewed from arrow B2, the duty cycle Dw of the W-phase (i.e., the W-phase voltage command value vw), which serves as the duty-mid phase, is limited to a second threshold value Dth2 or less. This prevents the on-period of the switching element of the lower arm of the duty-mid phase from being too short, preventing accurate detection of the voltage across the shunt resistor rw.

[0130] Furthermore, attention Figure 9 (a) Viewed along line C1. In the view along line C1, the superposition of the third harmonic component causes the duty cycle Dw of the W phase (i.e., the W phase voltage command value vw), the phase with the minimum duty cycle, to approach zero. When the duty cycle approaches zero, the on-period of the upper arm's switching element becomes too short. Even if the duty cycle is slightly greater than zero, current may not flow through the upper arm's switching element.

[0131] In contrast, refer to Figure 10 By limiting the basic q-axis voltage command value vq0 and the basic d-axis voltage command value vd0 as viewed from arrow C2 in (a), the duty ratio of the minimum duty phase can be prevented from being set to a value near "0." Therefore, even if the duty ratio is slightly greater than "0," the problem of current not flowing through the switching element of the upper arm can be avoided.

[0132] (action)

[0133] Figure 11This is a flowchart of an example of a current measuring method according to the embodiment.

[0134] In step S1 , the torque sensor 10 and the vehicle speed sensor 12 detect the steering torque Th of the steering shaft 2 and the vehicle speed Vh.

[0135] In step S2, the current command value calculation unit 40 calculates the q-axis current command value Iq0 and the d-axis current command value Id0 based on the steering torque Th and the vehicle speed Vh. The PI control unit 44 calculates the basic q-axis voltage command value vq0 and the basic d-axis voltage command value vd0 based on the current deviations of the q-axis current iq and the d-axis current id relative to the q-axis current command value Iq0 and the d-axis current command value Id0. The PI control unit 44 calculates the basic q-axis voltage command value vq0 and the basic d-axis voltage command value vd0 as the zero-center duty ratio, which varies from -50% to +50% with 0% as the center value.

[0136] In step S3, the voltage limiter 45 sets the q-axis voltage command value vq and the d-axis voltage command value vd by limiting the basic q-axis voltage command value vq0 and the basic d-axis voltage command value vd0. Specifically, the voltage limiter 45 calculates the limit gain G of the above formula (1): duty , in the limiting gain G duty When it is “1” or greater, the voltage limiting unit 45 does not limit the basic q-axis voltage command value vq0 and the basic d-axis voltage command value vd0 and outputs them as they are, q-axis voltage command value vq and d-axis voltage command value vd.

[0137] On the other hand, in the limiting gain G duty When the voltage limiting unit 45 is less than "1", the gain (G duty The product of the basic q-axis voltage command value vq0 and the basic d-axis voltage command value vd0 is output as the q-axis voltage command value vq=(G duty / 2)×vq0 and d-axis voltage command value vd=(G duty / 2)×vd0.

[0138] In step S4 , the two-phase / three-phase converter 46 converts the q-axis voltage command value vq and the d-axis voltage command value vd into a first U-phase voltage command value vu1 , a first V-phase voltage command value vv1 , and a first W-phase voltage command value vw1 , respectively.

[0139] In step S5, adders 49~51 respectively add the first U-phase voltage command value vu1, the first V-phase voltage command value vv1, and the first W-phase voltage command value vw1 to the dead time compensation value and the third harmonic component to calculate the second U-phase voltage command value vu2, the second V-phase voltage command value vv2, and the second W-phase voltage command value vw2.

[0140] In step S6, the duty conversion unit 52 corrects the midpoint of the duty ratio of the second U-phase voltage command value vu2, the second V-phase voltage command value vv2, and the second W-phase voltage command value vw2 to convert the duty ratio into a duty ratio that varies within a range from 0% to 100% with 50% as the center value.

[0141] In step S7 , the duty conversion unit 52 selects the phase with the maximum duty.

[0142] In step S8, duty conversion unit 52 determines whether target duty ratio D1t of the maximum duty phase is less than or equal to first threshold Dth1. If target duty ratio D1t is less than or equal to first threshold Dth1 (step S8: Yes), the process proceeds to step S9. If target duty ratio D1t is not less than or equal to first threshold Dth1 (step S8: No), the process proceeds to step S10.

[0143] In step S9, the duty conversion unit 52 outputs the second U-phase voltage command value vu2, second V-phase voltage command value vv2, and second W-phase voltage command value vw2, which have undergone midpoint correction in step S6, as the U-phase voltage command value vu, V-phase voltage command value vv, and W-phase voltage command value vw. This drives the switching element in the maximum-duty phase at the target duty cycle D1t. Furthermore, the switching elements in the intermediate-duty phase and the minimum-duty phase are driven at the target duty cycle D2t. The process then proceeds to step S12.

[0144] In step S10, the duty conversion unit 52 converts the target duty ratio D1t to a 100% duty ratio, thereby correcting the duty ratio of the maximum duty phase to 100%. Furthermore, the duty ratios of the intermediate duty phase and the minimum duty phase are corrected to the sum (D2t+ΔD) of the target duty ratio D2t and the duty difference ΔD = (100% - D1t).

[0145] In step S11, duty conversion unit 52 outputs the corrected duty ratios corrected in step S10 as U-phase voltage command value vu, V-phase voltage command value vv, and W-phase voltage command value vw. Consequently, the switching elements in the maximum-duty phase, intermediate-duty phase, and minimum-duty phase are driven at the corrected duty ratios corrected in step S10. The process then proceeds to step S12.

[0146] In step S12, the voltage drop measurement unit 36 ​​and the current calculation unit 54 measure the U-phase current Iu, the V-phase current Iv, and the W-phase current Iw. For example, the current calculation unit 54 may measure the phase currents of the mid-duty phase and the minimum-duty phase based on the voltages across the shunt resistors of the mid-duty phase and the minimum-duty phase, and then determine the phase current of the maximum-duty phase based on these mid-duty phase and minimum-duty phase currents and Kirchhoff's law. The process then terminates.

[0147] (Variation)

[0148] (1) The above embodiment describes a case where shunt resistors ru, rv, and rw are connected in series with the lower-arm switching elements Qu2, Qv2, and Qw2 (a so-called "downstream shunt method"). In this case, the upper-arm switching elements Qu1, Qv1, and Qw1 are examples of the "first switching elements" described in the claims, and the lower-arm switching elements Qu2, Qv2, and Qw2 are examples of the "second switching elements" described in the claims.

[0149] However, the present invention is not limited to the downstream shunt method, and is also applicable to a configuration in which a shunt resistor is connected in series to the upper arm switching elements Qu1 , Qv1 , and Qw1 (a so-called “upstream shunt method” configuration).

[0150] In this case, the upper arm switching elements Qu1 , Qv1 , and Qw1 are examples of the “second switching element” described in the claims, and the lower arm switching elements Qu2 , Qv2 , and Qw2 are examples of the “first switching element” described in the claims.

[0151] In this case, the minimum duty phase (ie, the phase with the minimum target duty ratio) is the period during which the lower arm switching element (first switching element) is turned on within the PWM period T. PWM Hereinafter, the period during which the lower arm switching element is turned on may be set to the PWM period TP. WM The ratio of is recorded as the "lower side duty ratio".

[0152] When the target duty ratio of the minimum duty phase is less than a predetermined threshold value set to a value greater than 0%, the duty conversion unit 52 converts the duty ratio of the minimum duty phase to 0%. Consequently, when the target value of the lower duty ratio in the minimum duty phase is greater than a first threshold value set to a value less than 100%, the target value of the lower duty ratio is converted to 100%. Furthermore, as the duty ratio of the minimum duty phase is converted to 0%, the gate drive circuit 32 corrects the decrease in the duty ratios of the intermediate duty phase and the maximum duty phase.

[0153] (2) While the above description describes an example in which the power conversion device of the present invention is applied to a column-assisted electric power steering device of the so-called upstream assist type, the power conversion device of the present invention can also be applied to a downstream-assisted electric power steering device. Hereinafter, as examples of downstream-assisted electric power steering devices, structural examples in which the power conversion device of the present invention is applied to single-pinion assist type, rack assist type, and double-pinion assist type electric power steering devices will be described.

[0154] In the case of the downstream assist mode, the motor 20, the rotation angle sensor 21, and the ECU 30 may not be separate structures, but may be separate structures. Figures 12 to 14 The dotted line indicates an integrated MCU (Motor Control Unit).

[0155] Figure 12 This figure shows an example of a configuration in which the power conversion device of the present invention is applied to a single-pinion assist electric power steering system. A steering wheel 1 is connected to a universal joint 4a on one side of an intermediate shaft via a steering shaft 2. Furthermore, an input-side shaft 4c of a torsion bar (not shown) is connected to the other universal joint 4b.

[0156] The rack-and-pinion mechanism 5 includes a pinion (pinion gear) 5a, a rack bar (rack) 5b, and a pinion shaft 5c. The input-side shaft 4c is connected to the rack-and-pinion mechanism 5 via a torsion bar (not shown). This torsion bar twists due to the deviation in the rotational angle between the input-side shaft 4c and the rack-and-pinion mechanism 5. The torque sensor 10 electromagnetically measures the torsion angle of the torsion bar as the steering torque Th of the steering wheel 1.

[0157] A motor 20 that assists the steering force of the steering wheel 1 is connected to the pinion shaft 5 c via the reduction gear 3 , and a rotation angle sensor 21 calculates rotation angle information of a motor rotation shaft of the motor 20 .

[0158] (3) Figure 13 This figure shows an example of a configuration in which the power conversion device of the present invention is applied to a rack-assisted electric power steering system. A spiral groove (not shown) is formed on the outer circumference of the rack bar 5b, and a spiral groove (not shown) with the same lead is also formed on the inner circumference of the nut 81. A ball screw is formed by arranging multiple rolling elements in the rolling path formed by these spiral grooves.

[0159] A belt 84 is wound around a drive pulley 82 connected to the rotating shaft 20a of the motor 20 assisting the steering force of the steering wheel 1, and a driven pulley 83 connected to a nut 81. The rotational motion of the rotating shaft 20a is converted into the linear motion of the rack bar 5b. The rotation angle sensor 21 calculates the rotation angle information of the motor rotating shaft of the motor 20.

[0160] (4) Figure 14 The following figure shows an example configuration in which the power conversion device of the present invention is applied to a dual-pinion assist electric power steering system. The dual-pinion assist electric power steering system includes, in addition to the pinion shaft 5c and the pinion 5a, a second pinion shaft 85 and a second pinion 86. The rack bar 5b has first rack teeth (not shown) that mesh with the pinion 5a and second rack teeth (not shown) that mesh with the second pinion 86.

[0161] The motor 20 that assists the steering force of the steering wheel 1 is connected to the second pinion shaft 85 via the reduction gear 3 . The rotation angle sensor 21 similarly calculates rotation angle information of the motor rotation shaft of the motor 20 .

[0162] (Effects of the embodiment)

[0163] (1) An electric power conversion device according to an embodiment includes: a multi-phase inverter having a plurality of phases of upper arm switching elements and lower arm switching elements connected in series; a voltage command value generating unit generating a q-axis voltage command value and a d-axis voltage command value for driving the multi-phase inverter; a voltage command value limiting unit limiting the q-axis voltage command value and the d-axis voltage command value; a voltage command value converting unit converting the q-axis voltage command value and the d-axis voltage command value limited by the voltage command value limiting unit into multi-phase voltage command values; a switching control unit driving a first switching element and a second switching element by PWM control based on the q-axis voltage command value and the d-axis voltage command value limited by the voltage command value limiting unit, wherein the first switching element is a switching element of either the upper arm switching element or the lower arm switching element, and the second switching element is a switching element of the other of the upper arm switching element and the lower arm switching element other than the first switching element; and a current measuring unit measuring a current flowing through the second switching element based on a voltage drop of a resistance element connected in series with the second switching element.

[0164] The switching control unit sets a target duty cycle based on a target voltage to be applied to a load of the multi-phase inverter. The target duty cycle is a target value of the duty cycle of the period during which the first switching element is turned on in the PWM period. When the target duty cycle of the first phase, which is the phase with the highest target duty cycle among the multiple phases of the multi-phase inverter, is below a first threshold value set to be less than 100%, the switching control unit drives the first switching element of the first phase at the target duty cycle. When the target duty cycle of the first phase is greater than the first threshold value, the switching control unit drives the first switching element of the first phase at a duty cycle of 100%, and drives the first switching element of the second phase at a corrected duty cycle obtained by adding and correcting the target duty cycle of a second phase other than the first phase among the multiple phases.

[0165] The voltage command value limiting unit limits the q-axis voltage command value and the d-axis voltage command value so that the corrected duty ratio does not exceed a predetermined second threshold value set to be smaller than 100%.

[0166] This method reduces the superposition of switching noise from the first phase's switching elements when measuring the phase current of the second phase other than the first phase, which has the highest target duty cycle. It also reduces the change in phase current caused by converting the duty cycle of the first phase, which has the highest target duty cycle, to 100%. Furthermore, it prevents inaccurate detection of the second phase's phase current due to the second switching element being on for an excessively short period of time. As a result, it improves phase current measurement accuracy when the duty cycle of the first phase, which has the highest duty cycle in PWM control, is high.

[0167] (2) For example, the second threshold value may be set to a value smaller than the first threshold value. Alternatively, for example, the second threshold value may be set based on the stabilization time from the time the second switching element switches to the time the voltage drop across the resistor detected by the current measuring unit stabilizes. Alternatively, for example, the second threshold value may be set so that the duration of the on-state of the second switching element of the second phase is not less than the stabilization time.

[0168] Thus, the second threshold value can be set so that the on-period of the second switching element of the second phase does not become too short even when the target duty ratio of the second phase is corrected to increase when the duty ratio of the first phase is high.

[0169] (3) The voltage command value limiting unit may set a limiting gain based on a ratio obtained by subtracting a dead time compensation value from a prescribed upper limit value set to be less than 100% and dividing the difference by the magnitude of the voltage vector of the q-axis voltage command value and the d-axis voltage command value, and limit the q-axis voltage command value and the d-axis voltage command value by multiplying the limiting gain by the q-axis voltage command value and the d-axis voltage command value.

[0170] Thus, the q-axis voltage command value and the d-axis voltage command value can be limited so that the corrected duty ratio of the second phase does not exceed the second threshold value.

[0171] (4) The power conversion device may include a third harmonic compensator that superimposes a third harmonic component on the multi-phase voltage command value. The voltage command value limiting unit may set the limiting gain based on a product of a ratio obtained by multiplying a difference obtained by subtracting a dead time compensation value from a predetermined upper limit value set to be less than 100% by the magnitude of the voltage vector between the q-axis voltage command value and the d-axis voltage command value by a coefficient corresponding to a rate of decrease in the amplitude of the multi-phase voltage command value due to the superimposition of the third harmonic component.

[0172] Thus, the q-axis voltage command value and the d-axis voltage command value can be limited so that the corrected duty ratio of the second phase does not exceed the second threshold value.

[0173] (5) The voltage command value limiting unit may limit the q-axis voltage command value and the d-axis voltage command value so that the corrected duty ratio of the phase having the second highest target duty ratio among the phases of the multi-phase inverter does not exceed the second threshold value.

[0174] Thus, the q-axis voltage command value and the d-axis voltage command value can be limited so that the period during which the second switching element in the phase having the second highest target duty ratio is on does not become too short.

[0175] (6) The switching control unit may set the sum of the difference obtained by subtracting the target duty ratio of the first phase from 100% and the target duty ratio of the second phase as the corrected duty ratio.

[0176] This can prevent a change in the phase current caused by converting the duty ratio of the first phase having the highest target duty ratio to 100%.

[0177] Description of Reference Numerals

[0178] 1…Steering wheel, 2…Steering shaft, 3…Reduction gear, 4a, 4b…Universal joint, 4c…Input-side shaft, 5…Rack-and-pinion mechanism, 5a…Pinion (pinion), 5b…Rack bar (rack), 5c…Pinion shaft, 6a, 6b…Tie rods, 7a, 7b…Hub unit, 8L, 8R…Steering wheels, 10…Torque sensor, 11…Ignition switch, 12…Vehicle speed sensor, 13…Battery, 14…Steering angle sensor, 20…Motor, 20a…Rotating shaft, 20u…U-phase coil, 20v…V-phase coil, 20w…W-phase coil, 21…Rotation angle sensor, 30…Electronic control unit, 31…Control calculation unit, 32…Gate drive circuit, 33…Multi-phase inverter Inverter, 34…current cutoff circuit, 35…cutoff drive circuit, 36…voltage drop measuring unit, 37…motor speed calculation unit, 40…current command value calculation unit, 41, 42…subtractors, 43…current limiting unit, 44…proportional-integral control unit, 45…voltage limiting unit, 46…two-phase / three-phase conversion unit, 47…dead time compensation unit, 48…third harmonic compensation unit, 49, 50, 51…adders, 52…duty conversion unit, 53…PWM control unit, 54…current calculation unit, 55…three-phase / two-phase conversion unit, 56…angular velocity conversion unit, 81…nut, 82…drive pulley, 83…driven pulley, 84…belt, 85…second pinion shaft, 86…second pinion

Claims

1. A power conversion device, characterized in that: The power conversion device comprises: a multiphase inverter having a series connection of upper-side arm switching elements and lower-side arm switching elements of a plurality of phases; a voltage command value generating unit that generates a q-axis voltage command value and a d-axis voltage command value for driving the multi-phase inverter; a voltage command value limiting unit configured to limit the q-axis voltage command value and the d-axis voltage command value; a voltage command value conversion unit that converts the q-axis voltage command value and the d-axis voltage command value limited by the voltage command value limiter into multi-phase voltage command values; a switching control unit that drives a first switching element and a second switching element through PWM control based on the q-axis voltage command value and the d-axis voltage command value limited by the voltage command value limiter, wherein the first switching element is either the upper arm switching element or the lower arm switching element, and the second switching element is the other of the upper arm switching element and the lower arm switching element except the first switching element; and a current measuring unit configured to measure a current flowing through the second switching element based on a voltage drop across a resistance element connected in series with the second switching element; The switching control unit sets a target duty ratio based on a target voltage to be applied to a load of the multi-phase inverter. The target duty ratio is a target value of a duty ratio of a period in which the first switching element is turned on in a PWM cycle. When the target duty ratio of a first phase, which is a phase having the highest target duty ratio among the multiple phases of the multi-phase inverter, is equal to or less than a predetermined first threshold value set to be less than 100%, the switching control unit drives the first switching element of the first phase at the target duty ratio. When the target duty ratio of the first phase is greater than the first threshold value, the switching control unit drives the first switching element of the first phase at the duty ratio of 100%, and drives the first switching element of the second phase at a corrected duty ratio obtained by adding and correcting the target duty ratio of a second phase other than the first phase among the plurality of phases. The voltage command value limiting unit sets a limit gain based on a ratio obtained by dividing a difference obtained by subtracting a dead time compensation value from a predetermined upper limit value set to be less than 100% by a magnitude of a voltage vector of the q-axis voltage command value and the d-axis voltage command value, and The voltage command value limiting unit limits the q-axis voltage command value and the d-axis voltage command value by multiplying the limit gain by the limit gain so that the corrected duty ratio does not exceed a predetermined second threshold value set to be less than 100%.

2. The power conversion device according to claim 1, wherein: The second threshold is set to a value smaller than the first threshold.

3. The power conversion device according to claim 1, wherein: The second threshold value is set based on a stabilization time from the switching operation of the second switching element to the stabilization of the measured value of the voltage drop across the resistance element detected by the current measuring unit.

4. The power conversion device according to claim 3, wherein: The second threshold is set so that the length of the on-period of the second switching element of the second phase is not less than the stabilization time.

5. The power conversion device according to claim 1, wherein: The power conversion device includes a third harmonic compensator that superimposes a third harmonic component on the multi-phase voltage command value. The voltage command value limiting unit sets a limiting gain based on a product of the ratio obtained by multiplying a difference obtained by subtracting a dead time compensation value from a prescribed upper limit value set to be less than 100% by a magnitude of a voltage vector between the q-axis voltage command value and the d-axis voltage command value by a coefficient corresponding to a rate of decrease in the amplitude of the multi-phase voltage command value caused by superposition of the third harmonic component.

6. The power conversion device according to claim 1, wherein: The voltage command value limiting unit limits the q-axis voltage command value and the d-axis voltage command value so that the corrected duty ratio of the phase having the second highest target duty ratio among the phases of the multi-phase inverter does not exceed the second threshold value.

7. The power conversion device according to claim 1, wherein: The switching control unit sets a sum of a difference obtained by subtracting the target duty ratio of the first phase from 100% and the target duty ratio of the second phase as the corrected duty ratio.

8. A motor control device, characterized in that: The motor control device comprises: The power conversion device according to any one of claims 1 to 7; and A controller controls the multi-phase inverter that drives the electric motor based on the measurement result of the current measuring unit.

9. An electric power steering device, characterized in that: The electric power steering device comprises: The motor control device according to claim 8; and a multiphase motor controlled by the motor control device, The electric power steering device applies a steering assist force to a steering system of a vehicle via the multi-phase motor.

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