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

Abstract

When a target duty ratio of a first phase, which is a phase having the highest target duty ratio among a plurality of phases of the multi-phase inverter (33), is greater than a predetermined first threshold value set to be less than 100%, the switching control units (31, 32) drive the switching elements of the first phase at a duty ratio of 100%. The switching element of the second phase is driven at a corrected duty ratio obtained by performing an increase correction on a target duty ratio of the second phase other than the first phase. A voltage command value limiting unit (45) limits the q-axis voltage command value and the d-axis voltage command value such that the corrected duty ratio does not exceed a predetermined second threshold value set to 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 the phase current based on the voltage drop of a shunt resistor connected in series with the switching element of a three-phase inverter.

[0003] Prior Art Documents

[0004] Patent Documents

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

[0006] Problems to be Solved by the Invention

[0007] When measuring the phase current based on the voltage drop of a shunt resistor connected in series with the switching element of a multiphase inverter, as the duty ratio of the phase with the highest duty ratio in PWM control, i.e., the duty ratio of the duty maximum phase, becomes higher, the switching timing of the on / off of the switching element of the duty maximum phase approaches the sampling timing of the voltage across the shunt resistor. Therefore, it is possible that switching noise is superimposed on the measured value of the voltage across the shunt resistor and the phase current cannot be accurately detected.

[0008] Therefore, in the motor control device described in Patent Document 1 above, when the on-time of any of the switching elements 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 non-current-detectable phase corresponding to this 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 non-current-detectable phase is changed to the high-potential side.

[0009] However, when the length of the on-period of the switching element on the low-potential side becomes short due to changing the duty indication value of the phase other than the non-current-detectable phase to the high-potential side, it may not be possible to accurately detect the phase current flowing through the shunt resistor.

[0010] The present invention has been made in view of the above circumstances, and an object thereof is to improve the measurement accuracy in the case where the duty ratio of the duty maximum phase, which is the phase with the highest duty ratio in PWM control, is high when measuring the phase current based on the voltage drop of a resistance element connected in series with the switching element of a multiphase inverter.

[0011] Means for Solving the Problems

[0012] To achieve the above object, a power conversion device according to one aspect of the present invention includes: a multiphase inverter having a series connection of upper-arm switching elements and lower-arm switching elements of a plurality of phases; a voltage command value generation unit that generates a q-axis voltage command value and a d-axis voltage command value for driving the multiphase inverter; a voltage command value limiting unit that limits 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 limiting unit into multiphase voltage command values; a switching control unit that controls 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, where the first switching element is one of the upper-arm switching element and 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 other than the first switching element; and a current measurement unit that measures the current flowing through the second switching element based on the voltage drop of a resistance element connected in series with the second switching element.

[0013] The switching control unit sets a target duty ratio according to a target voltage of a load to be applied to the multiphase inverter. The target duty ratio is a target value of the duty ratio of the period during which the first switching element is turned on in the PWM cycle. When the target duty ratio of the first phase, which is the phase with the highest target duty ratio among the plurality of phases of the multiphase inverter, is equal to or less than a specified first threshold value set to be less than 100%, the switching control unit drives the first switching element of the first phase with 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 with a 100% duty ratio, and drives the first switching element of the second phase, which is a phase other than the first phase among the plurality of phases, with a corrected duty ratio obtained by increasing and correcting the target duty ratio of the second 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 a specified second threshold value set to be less than 100%.

[0014] Advantages of the Invention

[0015] According to the present invention, when measuring the phase current based on the voltage drop of a resistance element connected in series with a switching element of a multiphase inverter, it is possible to improve the measurement accuracy in the case where the duty ratio of the phase with the highest duty ratio as the PWM control is relatively high. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0017] Figure 2It is a schematic structural diagram showing an example of an electronic control unit (ECU: Electronic Control Unit).

[0018] Figure 3 It is a block diagram showing an example of the functional structure of a control arithmetic unit.

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

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

[0021] Figure 6 It is a schematic diagram for explaining a method of setting a duty ratio.

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

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

[0024] Figure 9 (a) and (b) of are explanatory diagrams of the duty ratios of each phase without restricting the q-axis voltage command value and the d-axis voltage command value.

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

[0026] Figure 11 It is a flowchart showing an example of the current measurement method of the embodiment.

[0027] Figure 12 It is a schematic structural diagram showing an example of a first modification of an electric power steering device.

[0028] Figure 13 It is a schematic structural diagram showing an example of a second modification of an electric power steering device.

[0029] Figure 14 It is a schematic structural diagram showing an example of a third modification of an electric power steering device. Detailed implementation mode

[0030] With reference to the accompanying drawings, embodiments of the present invention will be described in detail. In addition, the embodiments of the present invention shown below illustrate devices and methods for embodying the technical idea of the present invention. However, the technical idea of the present invention does not limit the structures, configurations, etc. of the constituent components to the following content. The technical idea of the present invention can be variously modified within the technical scope defined by the claims recited in the claims.

[0031] (Structure)

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

[0033] The pinion rack mechanism 5 has a pinion 5a connected to a pinion shaft that receives the steering force from the universal joint 4b and a rack 5b that meshes with the pinion 5a. The rotational motion transmitted to the pinion 5a is converted into a linear motion in the vehicle width direction by the rack 5b.

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

[0035] In addition, a motor 20 for assisting 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 will be described, but the number of phases of the motor 20 may not be three-phase.

[0036] Electric power is supplied from the battery 13 to an electronic control unit (ECU: Electronic Control Unit) 30 that controls the electric power steering device, and an ignition key signal is input via the ignition switch 11.

[0037] The ECU 30 calculates the current command value of the assist control command 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 with a voltage command value obtained by compensating the current command value.

[0038] In addition, the steering angle sensor 14 is not essential. Instead, the motor rotation angle θm obtained from the rotation angle sensor 21 that detects the rotation angle of the rotation shaft of the detection motor 20 can be added to the torsion angle of the torsion bar of the torque sensor 10 to calculate the steering angle θh.

[0039] Alternatively, the steering angles of the steering wheels 8L and 8R can be used instead of the steering angle θh. For example, the steering angle can also be detected by detecting the displacement amount of the rack 5b.

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

[0041] The storage device can be any one of a semiconductor storage device, a magnetic storage device, and an optical storage device. The storage device can include memories such as registers, caches, a ROM (Read Only Memory) used as a main storage device, and a RAM (Random Access Memory).

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

[0043] In addition, the ECU 30 can also be formed by dedicated hardware for performing each information process described below.

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

[0045] Figure 2 It is a schematic structural diagram showing an example of the ECU 30 of the embodiment. The ECU 30 includes a control arithmetic unit 31, a gate drive circuit 32, a multiphase inverter 33, a current cut-off circuit 34, a cut-off drive circuit 35, a voltage drop measurement unit 36, and a motor speed arithmetic unit 37. The control arithmetic 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 via a connector CNT to a power wiring PW that transmits power from the battery 13. Further, 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 via the connector CNT to the control arithmetic unit 31.

[0047] The control arithmetic unit 31 calculates a current command value, which is a control target value of the drive current of the motor 20, at least based on the steering torque Th. The control arithmetic unit 31 calculates a voltage command value obtained by compensating the current command value and the like, and generates gate control signals Sgu, Sgv, and Sgw by performing PWM (Pulse Width Modulation) modulation on the voltage command value. The gate control signals Sgu, Sgv, and Sgw are PWM signals for controlling the drive voltages output from the multiphase 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 the gate control signals Sgu, Sgv, and Sgw. For example, when the switching elements Qu1, Qv1, Qw1, Qu2, Qv2, and Qw2 are field effect transistors (FETs: Field Effect Transistors), gate signals of the field effect transistors are generated based on the gate control signals Sgu, Sgv, and Sgw.

[0049] The multiphase inverter 33 has a three-phase bridge connected between the positive side line and the ground line, and the positive side line is connected to a DC power supply Vdc to be supplied with DC power.

[0050] The three-phase bridge includes pairs of switching elements in which the switching elements Qu1, Qv1, Qw1 of the upper arms of the U-phase, V-phase, and W-phase and the switching elements Qu2, Qv2, Qw2 of the lower arms of the U-phase, V-phase, and W-phase are respectively connected in series with each other.

[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] These U-phase current Iu, V-phase current Iv, and W-phase current Iw are supplied to the U-phase coil 20u, V-phase coil 20v, and W-phase coil 20w of the motor 20 through the current cut-off circuit 34.

[0053] The current cut-off circuit 34 has three phase cut-off FETs QAu, QAv, and QAw for cutting off the phase currents of the motor. The control arithmetic unit 31 outputs a control signal Sm for controlling the energization and cut-off of the current cut-off circuit 34 to the cut-off drive circuit 35. The cut-off drive circuit 35 outputs gate signals of the phase cut-off FETs QAu to QAw according to the control signal Sm, and turns on or off the U-phase current Iu, V-phase current Iv, and W-phase current Iw flowing from the multiphase inverter 33 to the motor 20.

[0054] A smoothing capacitor Cs is connected in parallel to the multiphase inverter 33. The smoothing capacitor Cs can be, for example, an electrolytic capacitor. The ECU 30 may also 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 wire.

[0056] The voltage drop measurement unit 36 measures the voltage drops generated in the shunt resistors ru, rv, and rw due to the currents flowing through the switching elements Qu2, Qv2, and Qw2 of the lower arms. The voltage drop measurement unit 36 outputs the measured values Vud, Vvd, and Vwd of the voltages at both ends of the shunt resistors ru, rv, and rw to the control arithmetic unit 31.

[0057] The motor speed arithmetic unit 37 calculates the motor rotation angle θm (e.g., motor electrical angle) of the motor 20 based on the detection signal of the rotation angle sensor 21 and outputs it to the control arithmetic unit 31.

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

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

[0060] On the other hand, the current calculation unit 54 is based on Figure 2The measured values Vud, Vvd, and Vwd of the voltages at both ends of the shunt resistors ru, rv, and rw measured by the voltage drop measurement unit 36 are used to calculate 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 measurement unit 36 and the current calculation unit 54 are examples of the "current measurement unit" described in the claims.

[0061] In addition, during the period when the switching element of the lower side arm in the phase with the highest duty ratio among the U-phase, V-phase, and W-phase is turned on, the period is short, so the voltage at both ends of the shunt resistor cannot be accurately detected. Therefore, the current calculation unit 54 can measure the phase current of the phase with the highest duty ratio based on the voltages at both ends of the shunt resistors of the other two phases other than the phase with the highest duty ratio and Kirchhoff's law (the sum of the U-phase current Iu, V-phase current Iv, and W-phase current Iw is 0).

[0062] For example, when the duty ratio of the phase with the highest duty ratio is equal to or less than a specified value, the phase current of the phase with the highest duty ratio can be measured based on the voltage at both ends of the shunt resistor of the phase with the highest duty ratio. When the duty ratio of the phase with the highest duty ratio is greater than the specified value, the phase current of the phase with the highest duty ratio can be measured based on the voltages at both ends of 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, V-phase current, and W-phase current into the q-axis current iq and the d-axis current id.

[0064] The subtractors 41 and 42 calculate the q-axis deviation current Δq0 and the d-axis deviation current Δd0 by subtracting the feedback 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 d-axis deviation current Δd are input to the PI control unit 44.

[0066] The PI control unit 44 calculates the basic q-axis voltage command value vq0 and the basic d-axis voltage command value vd0 that make the q-axis deviation current Δq and the d-axis deviation current Δd equal to 0, respectively.

[0067] For example, the basic q-axis voltage command value vq0 and the basic d-axis voltage command value vd0 can be the duty ratio as the ratio of the voltage command value to the power supply voltage. For example, the basic q-axis voltage command value vq0 and the basic d-axis voltage command value vd0 can be the zero-centered duty ratio that varies in the range from -50% to +50% with 0% as the center value.

[0068] The voltage limiting unit 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. Details of the function of the voltage limiting unit 45 will be described later.

[0069] The two-phase / three-phase conversion unit 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 compensation unit 47 outputs a dead time compensation value for compensating the dead time of the multiphase inverter 33 to the adders 49 to 51.

[0071] The third harmonic compensation unit 48 generates a third harmonic component for improving the voltage utilization rate and outputs it to the adders 49 to 51. The third harmonic component is a harmonic component having a triple frequency of 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] The 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 a second U-phase voltage command value vu2, a second V-phase voltage command value vv2, and a 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 value, 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, respectively, 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 value, the duty conversion unit 52 performs an increase correction 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 them to the PWM control unit 53.

[0075] In addition, 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 zero-centered duty ratios that vary in the range from -50% to +50% with a center value of 0%, the duty conversion unit 52 outputs the U-phase voltage command value vu, the V-phase voltage command value vv, and the W-phase voltage command value vw as duty ratios that vary in the range from 0% to 100% with a center value of 50% by correcting the midpoint of the duty ratio. The detailed 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 them to Figure 2 the gate drive circuit 32.

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

[0078] (Details of the duty conversion unit 52)

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

[0080] In the following description, the duty ratios of the periods 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 in the PWM period T PWM are respectively denoted as "Du", "Dv", and "Dw".

[0081] In addition, the target values of the duty ratios 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 denoted as "target duty ratios 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 zero-centered duty ratios, they are converted into the values of the duty ratios of the periods during which the switching elements of the upper arms are turned on in the PWM period T PWM by correcting the midpoint of the duty ratio to 50%.

[0083] Sometimes, the phase with the highest target duty ratio among the U-phase, V-phase, and W-phase is referred to as the "phase with the largest duty ratio", the phase with the smallest target duty ratio is referred to as the "phase with the smallest duty ratio", and the phases other than the phase with the largest duty ratio and the phase with the smallest duty ratio are referred to as the "phases with intermediate duty ratios". The phase with the largest duty ratio is an example of the "first phase" described in the scope of the claims, and the phases with intermediate duty ratios and the phase with the smallest duty ratio are examples of the "second phase" described in the scope of the claims.

[0084] Figure 4 of (a) and Figure 4 of (b) are respectively schematic timing diagrams of the PWM signals for driving the upper-arm switching element and the lower-arm switching element of the phase with the largest duty ratio, Figure 4 of (c) and Figure 4 of (d) are respectively schematic timing diagrams of the PWM signals for driving the upper-arm switching element and the lower-arm switching element of the phase with an intermediate duty ratio or the phase with the smallest duty ratio, Figure 4 of (e) is a schematic timing diagram of the output voltage of the AD converter for sampling the voltage across the shunt resistors of the phase with an intermediate duty ratio or the phase with the smallest duty ratio.

[0085] Symbol T PWM represents the PWM period (i.e., one period of the control period of the PWM control), symbol T HOFF represents the off period during which the upper-arm switching element is in the off state, symbol T LON represents the on period during which the lower-arm switching element is in the on state. In the case where the lower-arm switching elements Qu2, Qv2, Qw2 are connected in series with the shunt resistors ru, rv, rw (the so-called "downstream shunt method" case), the voltage drop measurement unit 36 samples the voltage across the shunt resistors at the mid-time point tc during the on period T LON of the lower-arm switching element (or at the mid-time point tc during the off period T HOFF of the upper-arm switching element). The sampling period for sampling the voltage across the shunt resistors can be, for example, a multiple (n×T PWM ) (n is a natural number) of the PWM period T PWM .

[0086] As a result, as the duty ratio of the phase with the largest duty ratio increases, the switching timings thd, thr, tlr, and tld of the switching elements of the phase with the largest duty ratio approach the sampling timing tc of the voltage across the shunt resistors of the phases with intermediate duty ratios and the phase with the smallest duty ratio. Therefore, it is possible that switching noise overlaps in the measured value of the voltage across the shunt resistors and the phase current cannot be accurately detected.

[0087] Therefore, when the target duty ratio of the maximum-duty phase is greater than a specified first threshold 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%. That is, the target duty ratio of the maximum-duty phase is corrected by increasing it to 100%.

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

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

[0090] As Figure 5 shown in Figure 5 of (a) and

[0091] Figure 6 is an explanatory diagram of the duty ratio setting method of the duty conversion unit 52. The dotted line D1t represents the target duty ratio of the maximum-duty phase in the target duty ratios 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 input to the duty conversion unit 52. In addition, the one-dot chain line D2t represents the target duty ratio of the intermediate-duty phase or the minimum-duty phase.

[0092] InFigure 6 In the example, for the sake of convenience of explanation, a case is shown where the target duty ratio D1t of the maximum-duty phase gradually increases to 100% over time while the target duty ratios D2t of the middle-duty phase or the minimum-duty phase are fixed.

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

[0094] The duty conversion unit 52 determines whether the target duty ratio D1t of the maximum-duty phase is equal to or less than the first threshold value Dth1. When the target duty ratio D1t of the maximum-duty phase is equal to or less than the first threshold value Dth1, the duty conversion unit 52 does not perform an increase correction on the target duty ratios D1t and D2t. That is, the target duty ratios D1t and D2t are directly set as the duty ratios D1 and D2.

[0095] As a result, the switching elements of the maximum-duty phase are driven at the target duty ratio D1t. In addition, the switching elements of the middle-duty phase and the minimum-duty phase are driven at the target duty ratio D2t.

[0096] On the other hand, when the target duty ratio D1t of the maximum-duty phase is greater than the first threshold value Dth1, the duty conversion unit 52 corrects the target duty ratio D1t by increasing it to a duty ratio of 100%. In other words, the target duty ratio D1t is corrected to a duty ratio of 100%, and the corrected duty ratio D1 = 100% is output as the voltage command value of the maximum-duty phase. As a result, the switching elements of the maximum-duty phase are driven at the corrected duty ratio D1 = 100%.

[0097] Furthermore, when the target duty ratio D1t of the maximum-duty phase is greater than the first threshold value Dth1, the duty conversion unit 52 outputs the corrected duty ratio D2 obtained by performing an increase correction on the target duty ratios D2t of the middle-duty phase and the minimum-duty phase as the voltage command values of the middle-duty phase and the minimum-duty phase. As a result, the switching elements of the middle-duty phase and the minimum-duty phase are driven at the corrected duty ratio D2.

[0098] For example, the duty conversion unit 52 calculates a duty difference ΔD = (100% - D1t) obtained by subtracting the target duty ratio D1t of the maximum-duty phase from 100%, and 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 values of the middle-duty phase and the minimum-duty phase.

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

[0100] As described above, by increasing and correcting the duty ratios of the intermediate-duty phase and the minimum-duty phase by the duty difference ΔD while changing the duty ratio of the maximum-duty phase to 100%, it is possible to suppress or prevent fluctuations in the U-phase current Iu, V-phase current Iv, and W-phase current Iw caused by the change in the duty ratio. The reason therefor will be described below.

[0101] Figure 7 The solid line, dashed line, and dotted-dashed line in (a) respectively represent the waveforms of the duty ratios 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, dashed line, and dotted-dashed line in (b) respectively represent the relative duty between the U-phase and the V-phase (i.e., the difference Du - Dv in the duty ratios of the U-phase and the V-phase), the relative duty between the V-phase and the W-phase (Dv - Dw), and the relative duty between the W-phase and the U-phase (Dw - Du) when the duty ratio is not changed.

[0102] On the other hand, Figure 7 The solid line, dashed line, and dotted-dashed line in (c) respectively represent the waveforms of the duty ratios Du, Dv, and Dw of the U-phase, V-phase, and W-phase when the duty ratio is changed. Figure 7 The solid line, dashed line, and dotted-dashed line in (d) respectively represent the relative duty between the U-phase and the V-phase (Du - Dv), the relative duty between the V-phase and the W-phase (Dv - Dw), and the relative duty between the W-phase and the U-phase (Dw - Du) when the duty ratio is changed. For ease of explanation, in Figure 7 (a) and Figure 7 (b), waveforms are shown in a case where the dead time compensation value and the third harmonic component are omitted, and in Figure 7 (c) and Figure 7 (d), waveforms are shown in a case where the third harmonic component is omitted.

[0103] From Figure 7 (a) and Figure 7 (c), it can be seen that by changing the duty ratio, the waveforms of the duty ratios Du, Dv, and Dw of the U-phase, V-phase, and W-phase 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. Thus, as in Figure 7 (b) and Figure 7As shown in (d), even if the duty ratio is changed, the difference in the duty ratios between the phases does not change.

[0104] As described above, as the duty ratio of the phase with the largest duty ratio is changed to 100%, correction for increasing the duty ratios of the phase with the intermediate duty ratio and the phase with the smallest duty ratio is performed. As a result, even if the duty ratio is changed, it is possible to suppress or prevent a change in the potential difference between the terminals of the motor 20, and thus it is possible to suppress or prevent fluctuations in 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) show the measured values of the U-phase current Iud (solid line), the measured values of the V-phase current Ivd (dashed line), and the measured values of the W-phase current Iwd (dash-dotted line) when the duty ratio Du of the U-phase, which is the phase with the largest duty ratio, gradually increases from 80% to 100%. Figure 8 (a) shows the measured values when the duty ratio is not changed. Figure 8 (b) shows the measured values when the duty ratio is changed. For ease of explanation, the duty ratio Dv of the V-phase and the duty ratio Dw of the W-phase are fixed at 80%.

[0106] When the duty ratio is not changed ( Figure 8 (a)), as the duty ratio Du of the U-phase, which is the phase with the largest duty ratio, approaches 100%, due to the influence of switching noise caused by the switching operation of the upper-arm switching element Qu1 of the U-phase, as shown by the portion indicated by the ellipse P1, the V-phase current Ivd and the W-phase current Iwd cannot be accurately detected. 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, when the V-phase current Ivd and the W-phase current Iwd cannot be accurately detected, as shown by the portion indicated by the ellipse P2, the U-phase current Iud cannot be accurately detected.

[0107] On the other hand, when the duty ratio is changed ( Figure 8 (b)), even when the duty ratio Du of the U-phase approaches 100%, as shown by 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 the voltage limiting unit 45)

[0109] Regarding Figure 3The details of the voltage limiting unit 45 will be described. When the target duty ratio of the phase with the maximum duty is greater than the first threshold Dth1, the duty ratio conversion unit 52 performs an increase correction on the target duty ratio D2t of the phase with the intermediate duty. As a result, when the corrected duty ratio D2 becomes large, the period during which the switching element of the lower arm in the phase with the intermediate duty becomes on sometimes becomes too small. As a result, it may not be possible to accurately detect the voltage across the shunt resistor in the phase with the intermediate duty. In addition, the phase with the intermediate duty is an example of the "phase with the second highest target duty ratio" described in the scope of the claims.

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

[0111] Figure 9 The second threshold Dth2 in (b) of is a specified threshold (current detection limit threshold) of the duty ratio at which the voltage across the shunt resistor can be accurately detected. The second threshold Dth2 is set to a value less than 100%. When the duty ratio exceeds the second threshold Dth2, the period during which the switching element of the lower arm is on is too small, and it may not be possible to accurately detect the voltage across the shunt resistor.

[0112] For example, the second threshold Dth2 can be set based on the stabilization time until the measured value of the voltage across the shunt resistor by the voltage drop measurement unit 36 stabilizes after the switching operation of the switching element of the lower arm.

[0113] For example, the second threshold Dth2 can be set in such a way that the length of the on period of the switching element of the lower arm in the phase with the intermediate duty is not less than the above stabilization time.

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

[0115] Refer to Figure 9 (b) of, in the portion viewed from B1, the duty ratio Dw (i.e., the W-phase voltage command value vw) of the W-phase as the phase with the intermediate duty exceeds the second threshold Dth2. For example, by adding the dead time compensation value to the voltage command value, the duty ratio of the phase with the intermediate duty easily exceeds the second threshold Dth2.

[0116] Therefore, when the target duty ratio of the phase with the maximum duty is greater than the first threshold Dth1, the voltage limiting unit 45 limits the basic q-axis voltage command value vq0 and the basic d-axis voltage command value vd0 in such a way that the duty ratio D2 of the phase with the intermediate duty does not exceed the second threshold Dth2, and sets 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] to calculate the limiting gain G shown in the following formula (1) duty .

[0121] [Mathematical formula 2]

[0122]

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

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

[0125] In addition, the constant is a coefficient corresponding to the reciprocal 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). When the third harmonic component is not superimposed on the three-phase voltage command value, the coefficient

[0126] The voltage limiting unit 45 determines whether the limiting gain G duty is "1" or more (that is, 100% or more). When the limiting gain Gduty When it is 1 or more, the voltage limiting unit 45 directly outputs the basic q-axis voltage command value vq0 and the basic d-axis voltage command value vd0 without limiting them as the q-axis voltage command value vq and the d-axis voltage command value vd.

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

[0128] Figure 10 (a) of Figure 10 shows the duty ratios Du, Dv, 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, 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) of Figure 10 shows Figure 10 an enlarged view of the part viewed from the A2 direction of (a) of Figure 10 .

[0129] In the part viewed from the B2 direction, the duty ratio Dw of the W-phase as the duty intermediate phase (i.e., the W-phase voltage command value vw) is limited to be equal to or less than the second threshold value Dth2. Therefore, it is possible to prevent the period during which the switching element of the lower arm of the duty intermediate phase is turned on from being too small to accurately detect the voltage across the shunt resistor rw.

[0130] Furthermore, paying attention to Figure 9 the part viewed from the C1 direction of (a) of Figure 10 . In the part viewed from the C1 direction, by superimposing the third harmonic component, the duty ratio Dw of the W-phase as the duty minimum phase (i.e., the W-phase voltage command value vw) approaches "0". When the duty ratio becomes a value near "0", the period during which the switching element of the upper arm is turned on is too small, and even if the duty ratio is slightly larger than "0", there may be a problem that current does not flow through the switching element of the upper arm.

[0131] In contrast, referring to Figure 10 the part viewed from the C2 direction of (a) of Figure 10 , by limiting the basic q-axis voltage command value vq0 and the basic d-axis voltage command value vd0, it is possible to suppress the duty ratio of the duty minimum phase from being set to a value near "0". Therefore, even if the duty ratio is slightly larger than "0", it is possible to avoid the problem that current does not flow through the switching element of the upper arm.

[0132] (Operation)

[0133] Figure 11It is a flowchart of an example of the current measurement method of the embodiment.

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

[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 with respect to the q-axis current command value Iq0 and the d-axis current command value Id0. The PI control unit 44 calculates a zero midpoint duty ratio that varies within a range from -50% to +50% with 0% as the center value as the basic q-axis voltage command value vq0 and the basic d-axis voltage command value vd0.

[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 , when the limit gain G duty is "1" or more, the voltage limiter 45 directly outputs the basic q-axis voltage command value vq0 and the basic d-axis voltage command value vd0 without limiting them as the q-axis voltage command value vq and the d-axis voltage command value vd.

[0137] On the other hand, when the limit gain G duty is less than "1", the voltage limiter 45 outputs the product obtained by multiplying the gain (G duty / 2) by the basic q-axis voltage command value vq0 and the basic d-axis voltage command value vd0 as the q-axis voltage command value vq = (G duty / 2) × vq0 and the d-axis voltage command value vd = (G duty / 2) × vd0.

[0138] In step S4, the two-phase / three-phase conversion unit 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, the adders 49 to 51 add the dead time compensation value and the third harmonic component to 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, respectively, to calculate a second U-phase voltage command value vu2, a second V-phase voltage command value vv2, and a second W-phase voltage command value vw2.

[0140] In step S6, the duty ratio conversion unit 52 converts the duty ratio by correcting the midpoints of the duty ratios 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, so as to obtain a duty ratio that varies within the range from 0% to 100% with 50% as the center value.

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

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

[0143] In step S9, the duty ratio conversion unit 52 outputs 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 whose midpoints are corrected in step S6 as the U-phase voltage command value vu, the V-phase voltage command value vv, and the W-phase voltage command value vw. Thus, the switching elements of the phase with the largest duty ratio are driven at the target duty ratio D1t. In addition, the switching elements of the phase with the intermediate duty ratio and the phase with the smallest duty ratio are driven at the target duty ratio D2t. Then, the process proceeds to step S12.

[0144] In step S10, the duty ratio conversion unit 52 corrects the duty ratio of the phase with the largest duty ratio to 100% by converting the target duty ratio D1t to a duty ratio of 100%. In addition, the duty ratios of the phase with the intermediate duty ratio and the phase with the smallest duty ratio are corrected to the sum (D2t + ΔD) obtained by adding the target duty ratio D2t and the duty ratio difference ΔD = (100% - D1t).

[0145] In step S11, the duty ratio conversion unit 52 outputs the corrected duty ratio corrected in step S10 as the U-phase voltage command value vu, the V-phase voltage command value vv, and the W-phase voltage command value vw. Thus, the switching elements of the phase with the largest duty ratio, the phase with the intermediate duty ratio, and the phase with the smallest duty ratio are driven at the corrected duty ratio corrected in step S10. Then, the process 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 can measure the phase currents of the duty intermediate phase and the duty minimum phase based on the voltages across the shunt resistors of the duty intermediate phase and the duty minimum phase, and measure the phase current of the duty maximum phase based on these phase currents of the duty intermediate phase and the duty minimum phase and Kirchhoff's law. Then, the process ends.

[0147] (Variant example)

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

[0149] However, the present invention is not limited to the downstream shunt method, and can also be applied to a structure in which shunt resistors are connected in series with the upper arm switching elements Qu1, Qv1, and Qw1 (the so-called "upstream shunt method" structure).

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

[0151] In this case, the above-described duty minimum phase (i.e., the phase with the smallest target duty ratio) becomes the phase in which the ratio of the period during which the lower arm switching element (the first switching element) is turned on to the PWM period T PWM is the largest. Hereinafter, the ratio of the period during which the lower arm switching element is turned on to the PWM period TP WM is sometimes referred to as the "lower duty ratio".

[0152] When the target duty ratio of the duty minimum phase is less than a specified threshold value set to be greater than 0%, the duty ratio conversion unit 52 converts the duty ratio of the duty minimum phase to 0%. Thus, when the target value of the lower duty ratio in the duty minimum phase is greater than a first threshold value set to be less than 100%, the target value of the lower duty ratio is converted to 100%. In addition, the gate drive circuit 32 performs a reduction correction on the duty ratios of the duty intermediate phase and the duty maximum phase as the duty ratio of the duty minimum phase is converted to 0%.

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

[0154] In addition, in the case of the downstream assist method, for waterproofing measures, the motor 20, the rotation angle sensor 21, and the ECU 30 may not be a split structure, but may be an integrated structure of an MCU (Motor Control Unit) as shown by the dashed line in Figures 12 - 14 .

[0155] Figure 12 FIG. shows a structural example in which the power conversion device of the present invention is applied to an electric power steering device of a single pinion assist method. The steering wheel 1 is connected to one universal joint 4a of the intermediate shaft via the steering shaft 2. In addition, an input side shaft 4c of a torsion bar (not shown) is connected to the other universal joint 4b.

[0156] The pinion rack mechanism 5 includes a pinion (spur gear) 5a, a rack bar (rack) 5b, and a pinion shaft 5c. The input side shaft 4c is connected to the pinion rack mechanism 5 by a torsion bar (not shown), and the torsion bar is twisted due to the deviation of the rotation angle between the input side shaft 4c and the pinion rack 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] The motor 20 that assists the steering force of the steering wheel 1 is connected to the pinion shaft 5c via the reduction gear 3, and the rotation angle sensor 21 calculates the rotation angle information of the motor rotation shaft of the motor 20.

[0158] (3) Figure 13 FIG. shows a structural example in which the power conversion device of the present invention is applied to an electric power steering device of a rack assist method. A spiral groove (not shown) is formed on the outer peripheral surface of the rack bar 5b, and a spiral groove (not shown) with the same lead is also formed on the inner peripheral surface of the nut 81. A ball screw is formed by arranging a plurality of rolling elements in the rolling path formed by these spiral grooves.

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

[0160] (4) Figure 14 This shows a structural example of applying the power conversion device of the present invention to an electric power steering device with a dual pinion assist system. The electric power steering device with a dual pinion assist system has, 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 a first rack tooth (not shown) meshing with the pinion 5a and a second rack tooth (not shown) meshing with the second pinion 86.

[0161] A motor 20 for assisting the steering force of the steering wheel 1 is connected to the second pinion shaft 85 via a reduction gear 3, and the rotation angle sensor 21 similarly calculates the rotation angle information of the motor rotation shaft of the motor 20.

[0162] (Effects of the Embodiment)

[0163] (1) The power conversion device of the embodiment includes: a multiphase inverter having a series connection of upper-arm switching elements and lower-arm switching elements of a plurality of phases; a voltage command value generation unit that generates a q-axis voltage command value and a d-axis voltage command value for driving the multiphase inverter; a voltage command value limiting unit that limits 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 limiting unit into multiphase voltage command values; a switching control unit that drives the first switching element and the 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, where the first switching element is either an upper-arm switching element or a lower-arm switching element, and the second switching element is the other switching element among the upper-arm switching elements and the lower-arm switching elements except the first switching element; and a current measurement unit that measures the current flowing through the second switching element based on the voltage drop of a resistance element connected in series with the second switching element.

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

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

[0166] Thereby, when measuring the phase current of the second phase other than the first phase with the highest target duty ratio, it is possible to suppress the overlap of the switching noise of the switching element of the first phase. In addition, it is possible to suppress the change in the phase current due to changing the duty ratio of the first phase with the highest target duty ratio to 100%. Also, it is possible to prevent the phase current of the second phase from not being accurately detected because the on period of the second switching element of the second phase is too small. As a result, it is possible to improve the measurement accuracy of the phase current when the duty ratio of the first phase, which is the phase with the highest duty ratio in PWM control, is high.

[0167] (2) For example, the second threshold value can be set to a value smaller than the first threshold value. In addition, for example, the second threshold value can be set according to the stabilization time until the measured value of the voltage drop of the resistance element detected by the current measurement unit stabilizes after the switching operation of the second switching element. In addition, for example, the second threshold value can be set in such a way that the length of the on period of the second switching element of the second phase is not less than the stabilization time.

[0168] Thereby, the second threshold value can be set in such a way that even when the target duty ratio of the second phase is increased and corrected in the case where the duty ratio of the first phase is high, the on period of the second switching element of the second phase will not be too small.

[0169] (3) The voltage command value limiting unit can set a limiting gain based on a ratio obtained by dividing the difference obtained by subtracting the dead time compensation value from a specified upper limit value set to be less than 100% 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] Thereby, the q-axis voltage command value and the d-axis voltage command value can be limited in such a way that the corrected duty ratio of the second phase does not exceed the second threshold value.

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

[0172] Accordingly, it is possible to limit the q-axis voltage command value and the d-axis voltage command value in such a manner 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 in such a manner that the corrected duty ratio in the phase having the second highest target duty ratio among the plurality of phases of the multiphase inverter does not exceed the second threshold value.

[0174] Accordingly, it is possible to limit the q-axis voltage command value and the d-axis voltage command value in such a manner that the period during which the second switching element in the phase having the second highest target duty ratio is turned on is not too small.

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

[0176] Accordingly, it is possible to prevent a change in the phase current caused by changing 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 (auxiliary gear), 5b... Rack bar (rack), 5c... Pinion shaft, 6a, 6b... Tie rod, 7a, 7b... Wheel hub unit, 8L, 8R... Steering wheel, 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 arithmetic unit, 32... Gate drive circuit, 33... Multiphase inverter, 34... Current cut-off circuit, 35... Cut-off drive circuit, 36... Voltage drop measurement unit, 37... Motor speed arithmetic unit, 40... Current command value arithmetic unit, 41, 42... Subtractor, 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... Adder, 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... Driving 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 that limits 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 switch control unit that drives a first switch element and a second switch 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 limiting unit, wherein the first switch element is a switch element of either the upper arm switch element or the lower arm switch element, and the second switch element is a switch element of the other of the upper arm switch element and the lower arm switch element except the first switch element; and a current measuring unit configured to measure 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, The switch control unit sets a target duty ratio according to a target voltage to be applied to a load of the multi-phase inverter, wherein 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 period. When the target duty ratio of a first phase, which is a phase having the highest target duty ratio among a plurality of phases of the multi-phase inverter, is less than or equal to a predetermined first threshold value set to be less than 100%, the switch 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 switch 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 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 that is set to be smaller than 100%.

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

3. The power conversion device according to claim 1, characterized in that: The second threshold value is set based on a stabilization time from when the second switching element performs a switching operation until a measured value of the voltage drop of the resistor element detected by the current measuring unit becomes stable.

4. The power conversion device according to claim 3, characterized in that: 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, characterized in that: 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 q-axis voltage command value and the d-axis voltage command value.

6. The power conversion device according to claim 5, characterized in that: 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 the product of the ratio multiplied by a coefficient corresponding to the rate of reduction of the amplitude of the multi-phase voltage command value caused by the superposition of the third harmonic component, wherein the ratio is obtained by dividing a difference obtained by subtracting a dead time compensation value from a prescribed upper limit value set to be less than 100% by the size of a voltage vector of the q-axis voltage command value and the d-axis voltage command value.

7. The power conversion device according to claim 1, characterized in that: 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.

8. The power conversion device according to claim 1, characterized in that: The switching control unit sets a sum obtained by adding 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.

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

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

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