JP6468461B2 - Motor control device - Google Patents

Description

  The present invention relates to a motor control device for PWM driving an electric motor.

When the electric motor is driven by PWM (Pulse Width Modulation), the controllability of the voltage applied to the electric motor is the number of PWM clocks (hereinafter referred to as “PWM count number”) corresponding to one cycle of the PWM signal and the power supply voltage. It depends on. The PWM clock is a reference clock used for generating a PWM signal.
It is assumed that the frequency of the PWM clock of a certain microcomputer is, for example, 10 [MHz]. When the frequency of the PWM signal is set to 10 [kHz], the PWM count number is 10,000,000 × (1 / 10,000) = 1000. The resolution of the voltage applied to the electric motor (applied voltage step size, hereinafter referred to as “voltage resolution Vr”) is expressed as Vr = Vb ÷ Cnt, where the PWM count is Cnt and the power supply voltage is Vb. . Therefore, the voltage resolution Vr when the frequency of the PWM signal is 10 [kHz] is Vb / 1000 [V / LSB]. The smaller the value of the voltage resolution Vr, the finer the step size of the applied voltage and the higher the voltage resolution. In other words, the larger the value of the voltage resolution Vr, the coarser the step size of the applied voltage and the lower the voltage resolution. The resolution of the duty ratio (duty ratio step size) is (1 / Cnt) × 100 [%], and is 0.1 [%]. Therefore, when the frequency of the PWM signal is 10 [kHz], the voltage applied to the electric motor can be set to 72.5 [%] of the power supply voltage, for example.

  When the frequency of the PWM signal is set to 100 [kHz] using this microcomputer, the PWM count number is 10,000,000 × (1 / 100,000) = 100. Therefore, the voltage resolution is Vb / 100 [V / LSB], and the duty ratio resolution is 1 [%]. In this case, the voltage applied to the electric motor cannot be set to 72.5 [%].

JP 2013-192429 A JP-A-10-225132

  An inverter circuit (motor drive circuit) that supplies electric power to the electric motor includes a smoothing capacitor. Since the capacity of the smoothing capacitor can be reduced as the PWM frequency is higher, the smoothing capacitor can be reduced in size by setting the PWM frequency higher. That is, the motor drive circuit can be reduced in size. However, as described above, when the frequency of the PWM signal is increased, the PWM count number is decreased and the voltage resolution is also decreased (coarse). For this reason, when the frequency of the PWM signal is increased, the controllability of the current is also reduced, and noise vibration (NV: Noise Vibration) is deteriorated due to occurrence of current ripple or the like.

  The object of the present invention is to change the PWM frequency in accordance with the motor current within an allowable range from the capacity of the smoothing capacitor in the motor drive circuit. When the motor current is small compared to when the motor current is large An object of the present invention is to provide a motor control device capable of reducing the PWM frequency.

  According to the first aspect of the present invention, there is provided a motor control device (12) for PWM-controlling an electric motor (18) through a motor drive circuit (32) including a smoothing capacitor (62) and a plurality of switching elements (71 to 76). Storage means (40) storing a relationship between a motor current value of the electric motor and a PWM frequency corresponding value that is a value corresponding to the frequency or cycle of the PWM signal, which is allowed from the capacity of the smoothing capacitor; Based on the relation stored in the storage means, the calculation means (43 to 45) for calculating the first duty count number adapted when the PWM frequency is assumed to be a predetermined PWM frequency, the motor current is calculated. An acquisition means (46) for acquiring a corresponding PWM frequency corresponding value, and a first duty count number calculated by the calculation means The conversion means (46) for converting to a second duty count number adapted to the PWM frequency corresponding value acquired by the above, the PWM frequency corresponding value acquired by the acquisition means and the second duty count number obtained by the conversion means And a PWM signal generation means (47) for generating a PWM signal corresponding to the motor control device. In addition, although the alphanumeric character in parentheses represents a corresponding component in an embodiment described later, of course, the scope of the present invention is not limited to the embodiment. The same applies hereinafter.

In this configuration, the PWM frequency can be changed according to the motor current within a range allowed from the capacity of the smoothing capacitor in the motor drive circuit. As a result, the PWM frequency when the motor current is small can be made lower than when the motor current is large.
The invention according to claim 2 is the motor according to claim 1, wherein the PWM frequency corresponding value is a PWM frequency that is a frequency of a PWM signal or a PWM count that is a PWM clock number corresponding to one cycle of the PWM signal. It is a control device.

  According to a third aspect of the present invention, there is provided a current detection means (33) for detecting a motor current flowing through the electric motor, and a current command for setting a motor current command value which is a target value of the motor current to be passed through the electric motor. Value setting means (42), wherein the calculation means is adapted to the case where it is assumed that the PWM frequency is the predetermined PWM frequency, and the motor current detection value detected by the current detection means is used as the current detection value. 3. The motor control device according to claim 1, wherein the motor control device is configured to calculate a first duty count number for making the motor current command value set by the command value setting means equal.

According to a fourth aspect of the present invention, the obtaining unit is configured to obtain a PWM frequency corresponding value corresponding to the motor current by regarding the motor current command value set by the current command value setting unit as a motor current. The motor control device according to claim 3.
According to a fifth aspect of the present invention, the acquisition means is a current having a larger absolute value among the motor current command value set by the current command value setting means and the motor current detection value detected by the current detection means. The motor control device according to claim 3, wherein the value is regarded as a motor current, and a PWM frequency corresponding value corresponding to the motor current is acquired.

FIG. 1 is a schematic diagram showing a schematic configuration of an electric power steering apparatus according to an embodiment of the present invention. FIG. 2 is a block diagram showing an electrical configuration of the ECU. FIG. 3 is a schematic diagram schematically showing the configuration of the electric motor. FIG. 4 is an electric circuit diagram schematically showing the configuration of the drive circuit. FIG. 5 is a graph showing a setting example of the assist current value Ia ^{* with} respect to the detected steering torque T. FIG. 6 is a graph showing the relationship between the motor current and the PWM frequency allowed from the capacity of the smoothing capacitor. FIG. 7 is a flowchart for explaining an operation example of the PWM frequency changing unit. FIG. 8 is a flowchart for explaining another operation example of the PWM frequency changing unit. FIG. 9 is a graph showing the relationship between the motor current and the PWM count number allowed from the capacity of the smoothing capacitor.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.
FIG. 1 is a schematic diagram showing a schematic configuration of an electric power steering apparatus according to an embodiment of the present invention.
An electric power steering device (EPS) 1 includes a steering wheel 2 as a steering member for steering a vehicle, and a steering mechanism that steers the steered wheels 3 in conjunction with the rotation of the steering wheel 2. 4 and a steering assist mechanism 5 for assisting the driver's steering. The steering wheel 2 and the steering mechanism 4 are mechanically coupled via a steering shaft 6 and an intermediate shaft 7.

The steering shaft 6 includes an input shaft 8 connected to the steering wheel 2 and an output shaft 9 connected to the intermediate shaft 7. The input shaft 8 and the output shaft 9 are connected via a torsion bar 10 so as to be relatively rotatable.
A torque sensor 11 is disposed in the vicinity of the torsion bar 10. The torque sensor 11 detects the steering torque T applied to the steering wheel 2 based on the relative rotational displacement amount of the input shaft 8 and the output shaft 9. In this embodiment, the steering torque T detected by the torque sensor 11 is detected, for example, as a torque for steering in the right direction as a positive value and a torque for steering in the left direction as a negative value. It is assumed that the magnitude of the steering torque increases as the absolute value thereof is detected.

  The steered mechanism 4 includes a rack and pinion mechanism including a pinion shaft 13 and a rack shaft 14 as a steered shaft. The steered wheel 3 is connected to each end of the rack shaft 14 via a tie rod 15 and a knuckle arm (not shown). The pinion shaft 13 is connected to the intermediate shaft 7. The pinion shaft 13 rotates in conjunction with the steering of the steering wheel 2. A pinion 16 is connected to the tip of the pinion shaft 13 (the lower end in FIG. 1).

  The rack shaft 14 extends linearly along the left-right direction of the automobile. A rack 17 that meshes with the pinion 16 is formed at an intermediate portion in the axial direction of the rack shaft 14. By the pinion 16 and the rack 17, the rotation of the pinion shaft 13 is converted into the axial movement of the rack shaft 14. The steered wheels 3 can be steered by moving the rack shaft 14 in the axial direction.

When the steering wheel 2 is steered (rotated), this rotation is transmitted to the pinion shaft 13 via the steering shaft 6 and the intermediate shaft 7. The rotation of the pinion shaft 13 is converted into an axial movement of the rack shaft 14 by the pinion 16 and the rack 17. Thereby, the steered wheel 3 is steered.
The steering assist mechanism 5 includes an electric motor 18 for assisting steering and a speed reduction mechanism 19 for transmitting the output torque of the electric motor 18 to the steering mechanism 4. The electric motor 18 is provided with a rotation angle sensor 23 made of, for example, a resolver for detecting the rotation angle of the rotor of the electric motor 18. The speed reduction mechanism 19 includes a worm gear mechanism that includes a worm shaft 20 and a worm wheel 21 that meshes with the worm shaft 20.

The worm shaft 20 is rotationally driven by the electric motor 18. Further, the worm wheel 21 is connected to the steering shaft 6 so as to be integrally rotatable. The worm wheel 21 is rotationally driven by the worm shaft 20.
When the worm shaft 20 is rotationally driven by the electric motor 18, the worm wheel 21 is rotationally driven and the steering shaft 6 rotates. The rotation of the steering shaft 6 is transmitted to the pinion shaft 13 via the intermediate shaft 7. The rotation of the pinion shaft 13 is converted into the axial movement of the rack shaft 14. Thereby, the steered wheel 3 is steered. That is, the wheel 3 is steered by rotating the worm shaft 20 by the electric motor 18.

  The vehicle is provided with a vehicle speed sensor 24 for detecting the vehicle speed V. The steering torque T detected by the torque sensor 11, the vehicle speed V detected by the vehicle speed sensor 24, the output signal of the rotation angle sensor 23, and the like are input to an ECU (Electronic Control Unit) 12. The ECU 12 controls the electric motor 18 based on these input signals.

FIG. 2 is a block diagram showing an electrical configuration of the ECU 12.
The ECU 12 includes a microcomputer 31, a motor drive circuit 32 that is controlled by the microcomputer 31 and supplies electric power to the electric motor 18, and a current detection unit 33 that detects a motor current flowing through the electric motor 18.
The electric motor 18 is, for example, a three-phase brushless motor, and includes a rotor 100 as a field and U-phase, V-phase, and W-phase stator windings 101, 102, and 103, as schematically shown in FIG. And a stator 105.

  Three-phase fixed coordinates (UVW coordinate system) are defined in which the U, V, and W axes are taken in the direction of the stator windings 101, 102, and 103 of each phase. Further, a two-phase rotational coordinate system (dq coordinate system) in which the d axis (magnetic pole axis) is taken in the magnetic pole direction of the rotor 100 and the q axis (torque axis) is taken in a direction perpendicular to the d axis in the rotation plane of the rotor 100. The actual rotating coordinate system) is defined. In the dq coordinate system, since only the q-axis current contributes to the torque generation of the rotor 100, the d-axis current may be set to zero and the q-axis current may be controlled according to the desired torque. The rotation angle (electrical angle) θe of the rotor 100 is the rotation angle of the d axis with respect to the U axis. The dq coordinate system is an actual rotation coordinate system according to the rotor rotation angle θe. By using the rotor rotation angle θe, coordinate conversion between the UVW coordinate system and the dq coordinate system can be performed.

FIG. 4 shows the configuration of the motor drive circuit 32.
The motor drive circuit 32 is a three-phase inverter circuit. Motor drive circuit 32 includes a power supply 61, a smoothing capacitor 62, a plurality of switching elements 71 to 76, and a plurality of diode elements 81 to 86. The smoothing capacitor 62 is connected between both terminals of the power supply 61.

  The plurality of switching elements 71 to 76 include a U-phase high-side first switching element 71, a U-phase low-side second switching element 72 connected in series thereto, and a V-phase high-side The third switching element 73, the V-phase low-side fourth switching element 74 connected in series therewith, the W-phase high-side fifth switching element 75, and the series connected thereto. And a W-phase low-side sixth switching element 76. In this embodiment, each of the switching elements 71 to 76 is an n-channel MOSFET. First to sixth diodes 81 to 86 are connected in reverse parallel to the first to sixth switching elements 71 to 76.

  The drains of the first, third and fifth switching elements 71, 73, 75 are connected to the positive terminal of the power supply 61. The sources of the first, third, and fifth switching elements 71, 73, and 75 are connected to the drains of the second, fourth, and sixth switching elements 72, 74, and 76, respectively. The sources of the second, fourth and sixth switching elements 72, 74 and 76 are connected to the negative terminal of the power supply 61.

  A connection point between the first switching element 71 and the second switching element 72 is connected to the U-phase stator winding 101 of the electric motor 18. A connection point between the third switching element 73 and the fourth switching element 74 is connected to the V-phase stator winding 102 of the electric motor 18. A connection point between the fifth switching element 75 and the sixth switching element 76 is connected to the W-phase stator winding 103 of the electric motor 18. The gates of the switching elements 71 to 76 are connected to a PWM control unit 47 described later.

  Returning to FIG. 2, the microcomputer 31 includes a CPU and a memory (ROM, RAM, nonvolatile memory 40, etc.), and functions as a plurality of function processing units by executing a predetermined program. Yes. The plurality of function processing units include an assist current value setting unit 41, a current command value setting unit 42, a current deviation calculation unit 43, a PI (proportional integration) control unit 44, a dq / UVW conversion unit 45, A PWM frequency change unit 46, a PWM (Pulse Width Modulation) control unit 47, a UVW / dq conversion unit 48, and a rotation angle calculation unit 49 are included.

The rotation angle calculation unit 49 calculates the rotation angle θe (electrical angle) of the rotor of the electric motor 18 based on the output signal of the rotation angle sensor 23. The rotor rotation angle θe calculated by the rotation angle calculation unit 49 is given to the dq / UVW conversion unit 45 and the UVW / dq conversion unit 48.
The assist current value setting unit 41 sets the assist current value Ia ^* based on the detected steering torque T detected by the torque sensor 11 and the vehicle speed V detected by the vehicle speed sensor 24. A setting example of the assist current value Ia ^{* with} respect to the detected steering torque T is shown in FIG. For the detected steering torque T, for example, the torque for steering in the right direction is a positive value, and the torque for steering in the left direction is a negative value. The assist current value Ia ^* is a positive value when a steering assist force for rightward steering is to be generated from the electric motor 18, and a steering assist force for leftward steering is to be generated from the electric motor 18. Sometimes it is negative. The assist current value Ia ^* is positive for a positive value of the detected steering torque T, and is negative for a negative value of the detected steering torque T.

When the detected steering torque T is a small value (torque dead zone) in the range of -T1 to T1 (for example, T1 = 0.4 N · m), the assist current value Ia ^* is set to zero. When the detected steering torque T is outside the range of -T1 to T1, the assist current value Ia ^* is set so that the absolute value of the assist current value Ia ^* increases as the absolute value of the detected steering torque T increases. The The assist current value Ia ^* is set such that the absolute value thereof decreases as the vehicle speed V detected by the vehicle speed sensor 24 increases. As a result, the steering assist force is increased during low speed travel, and the steering assist force is decreased during high speed travel.

Based on the assist current value Ia ^* set by the assist current value setting unit 41, the current command value setting unit 42 sets a current value to be passed through the coordinate axes of the dq coordinate system as a current command value. Specifically, the current command value setting unit 42 is referred to as a d-axis current command value I _d ^* and a q-axis current command value I _q ^* (hereinafter collectively referred to as “two-phase current command value I _dq ^* ”). ) Is set. More specifically, the current command value setting unit 42 sets the q-axis current command value I _q ^* to the assist current value Ia ^* set by the assist current value setting unit 41, while the d-axis current command value I _d. ^* Is zero. The two-phase current command value I _dq ^* set by the current command value setting unit 42 is given to the current deviation calculation unit 43 and the PWM frequency change unit 46.

The current detection unit 33 detects the U-phase current I _U , the V-phase current I _V and the W-phase current I _W (hereinafter, collectively referred to as “three-phase detection current I _UVW ”) of the electric motor 18. To do. The three-phase detection current I _UVW detected by the current detection unit 33 is given to the UVW / dq conversion unit 48.
The UVW / dq conversion unit 48 converts the three-phase detection current I _UVW in the UVW coordinate system detected by the current detection unit 33 into the two-phase detection currents I _d and I _q (hereinafter, collectively referred to as “two-phase detection”). The coordinates are converted to “current I _dq ”). For this coordinate conversion, the rotor rotation angle θe calculated by the rotation angle calculation unit 49 is used.

The current deviation calculation unit 43 calculates a deviation of the d-axis detection current I _{d from} the d-axis current command value I _d ^* and a deviation of the q-axis detection current I _{q from} the q-axis current command value I _q ^* . These deviations are given to the PI control unit 44.
The PI control unit 44 performs a PI calculation on the current deviation calculated by the current deviation calculation unit 43, so that the two-phase voltage command value V _dq ^* (d-axis voltage command value V _d ^* and q-axis voltage command value V _q ^* ) is generated. The two-phase voltage command value V _dq ^* is given to the dq / UVW converter 45.

The dq / UVW conversion unit 45 converts the two-phase voltage command value V _dq ^* into a duty count number (duty count number for each three-phase voltage) corresponding to the three-phase voltage command value. For this conversion, the rotor rotation angle θe calculated by the rotation angle calculation unit 49 is used. When the PWM reference clock is a PWM clock, the duty count number is the number of PWM clocks corresponding to the pulse width within one cycle of the PWM signal. The duty count number for each three-phase voltage includes a duty count number corresponding to the U-phase voltage command value, a duty count number corresponding to the V-phase voltage command value, and a duty count number corresponding to the W-phase voltage command value.

  In this embodiment, the duty count for each three-phase voltage obtained by the dq / UVW conversion unit 45 is assumed when the PWM frequency is a preset minimum PWM frequency (the maximum voltage with a preset voltage resolution). This is a duty count number that is suitable for the case of resolution. In other words, the duty count number for each three-phase voltage obtained by the dq / UVW conversion unit 45 is a value obtained by dividing the corresponding three-phase voltage command value by the maximum voltage resolution. The duty count number for each three-phase voltage is given to the PWM frequency changing unit 46.

The PWM frequency changing unit 46 changes the PWM frequency according to the motor current (q-axis current command value I _q ^{* in} this embodiment), and the duty count for each three-phase voltage obtained by the dq / UVW conversion unit 45. The number is converted into a duty count number suitable for the PWM frequency after the change. The PWM frequency after the change and the converted duty count number for each three-phase voltage are given to the PWM control unit 47 after the conversion. Details of the operation of the PWM frequency changing unit 46 will be described later.

  The PWM control unit 47 generates a U-phase PWM signal, a V-phase PWM signal, and a W-phase PWM signal based on the PWM frequency and the duty count number for each three-phase voltage given from the PWM frequency changing unit 46. Specifically, the PWM control unit 47 is configured such that the PWM frequency is the PWM frequency given from the PWM frequency changing unit 46, the duty count number of the U-phase voltage given from the PWM frequency changing unit 46, and the V-phase voltage A U-phase PWM signal, a V-phase PWM signal, and a W-phase PWM signal having duty ratios respectively corresponding to the duty count number and the duty count number of the W-phase voltage are generated. The PWM control unit 47 supplies the U-phase PWM signal, the V-phase PWM signal, and the W-phase PWM signal to the motor drive circuit 32.

The switching elements 71 to 76 in the motor drive circuit 32 are controlled by a PWM signal supplied from the PWM control unit 47, so that a three-phase voltage corresponding to the two-phase voltage command value V _dq ^* is changed for each phase of the electric motor 18. It is applied to the stator windings 101, 102, 103.
The current deviation calculation unit 43 and the PI control unit 44 constitute a current feedback control unit. By the function of the current feedback control means, the motor current flowing through the electric motor 18 is controlled so as to approach the two-phase current command value I _dq ^* set by the current command value setting unit 42.

Hereinafter, the operation of the frequency changing unit 46 will be described in detail.
In order to output an appropriate voltage according to the PWM signal from the motor drive circuit 32, the lower the frequency of the PWM signal (hereinafter referred to as “PWM frequency f _PWM ”) and the greater the motor current, the greater the motor drive circuit 32. The smoothing capacitor 62 requires a large capacity. Therefore, when the capacity of the smoothing capacitor 62 is determined, the minimum value of the PWM frequency f _PWM allowed for each motor current value can be obtained.

  A map storing the relationship between the absolute value of the motor current value (q-axis current value) of the electric motor 18 and the PWM frequency allowed from the capacity of the smoothing capacitor 62 (hereinafter referred to as “motor current-PWM frequency map”). It is created in advance and stored in the nonvolatile memory 40. The motor current-PWM frequency map is created based on, for example, the relationship between the absolute value of the motor current value of the electric motor 18 and the minimum value of the PWM frequency allowed from the capacity of the smoothing capacitor 62. FIG. 6 shows an example of the motor current-PWM frequency map. As shown in FIG. 6, the PWM frequency (minimum value) allowed for each motor current value increases as the motor current increases.

For convenience of explanation, in this embodiment, it is assumed that the frequency f _CL of the PWM clock of the microcomputer 31 is 4 [MHz]. The minimum value of the PWM frequency f _PWM (minimum frequency f _{PWMO of the} PWM signal) is set to 1 [kHz]. The power supply voltage Vb is 40 [V]. In this example, PWM clock number corresponding to one cycle of the PWM signal when the PWM frequency is the lowest value _{f PWMO} (number PWM _{count) C PWMO is, C PWMO = f CL / f} PWMO = 4,000,000 / 1000 = 4,000. The voltage resolution Vr _O if the PWM frequency is the lowest value _{f PWMO becomes Vr O = Vb / C PWMO =} 40 / 4,000 = 0.01 [V / LSB]. Voltage resolution Vr _O if the PWM frequency is the lowest value _{f PWMO} is, the highest voltage resolution becomes (step width of the applied voltage is the finest voltage resolution). The values of the PWM clock frequency f _CL , the PWM signal minimum frequency f _PWMO , the power supply voltage Vb, the PWM count number C _PWMO and the maximum voltage resolution Vr _O when the PWM frequency is the minimum value f _PWMO are stored in advance in the nonvolatile memory 40. Is remembered.

As described above, dq / UVW conversion unit 45, (assuming voltage resolution Vr is the highest voltage resolution Vr _O previously set) PWM frequency _{f PWM} is assuming the lowest PWM frequency _{f PWMO} Calculate the duty count for each three-phase voltage that is suitable for. Since the operation of the PWM frequency changing unit 46 with respect to the duty count number of each phase voltage calculated by the dq / UVW conversion unit 45 is the same, the operation of the PWM frequency changing unit 46 with respect to the duty count number of the U phase voltage will be described below.

The duty count number C _DUTYO of the U-phase voltage calculated by the dq / UVW conversion unit 45 is obtained by _changing the U-phase voltage command value corresponding to the two-phase voltage command value V _dq ^* calculated by the PI control unit 44 to V _U ^* . Then, it is expressed by C _DUTYO = V _U ^* / Vr _O. For example, when the U-phase voltage command value V _U ^* corresponding to the two-phase voltage command value V _dq ^* is 20 [V], the duty count number C of the U-phase voltage calculated by the dq / UVW conversion unit 45 _DUTYYO is C _DUTYO = 20 / 0.01 = 2,000. Since the PWM count _{C PWMO} when the voltage resolution is the highest voltage resolution Vr _O is 4,000, the duty ratio when the duty count _{C DUTYO} is 2,000, (2,000 / 4,000 ) × 100 = 50 [%].

FIG. 7 is a flowchart for explaining an operation example of the PWM frequency changing unit 46. The process of FIG. 7 is repeatedly executed at every predetermined calculation cycle.
The PWM frequency changing unit 46 first acquires the q-axis current command value I _q ^* set by the current command value setting unit 42 (step S1). Then, the PWM frequency changing unit 46 regards the acquired q-axis current command value I _q ^* as the motor current, and determines the motor current (q-axis current command value I _q ^* ) from the motor current-PWM frequency map (see FIG. 6). _Is obtained (step S2).

Next, the PWM frequency changing unit 46 calculates the voltage resolution Vr _I corresponding to the PWM frequency f _PWMI acquired in step S2 based on the following equation (1) (step S3).
_{Vr I = Vb ÷ (f CL} / f PWMI) ... (1)
f _CL / f _PWMI is the PWM count number C _PWMI when the PWM frequency is f _PWMI .

For example, when the PWM frequency f _PWMI acquired in step S2 is 5 [kHz], the voltage resolution Vr _I corresponding to the PWM frequency f _PWMI is
Vr _I = 40 ÷ (4,000,000 / 5,000)
= 40 ÷ 800
= 0.05
It becomes.

Then, the PWM frequency changing unit 46 _matches the duty count number C _DUTYO of the U-phase voltage given from the dq / UVW converting unit 45 with the voltage resolution Vr _I calculated in step S3 using the following equation (2). The converted duty count number C _DUTYI is converted (step S4). _{Incidentally, C DUTYO} is duty count _{C DUTYO} the U-phase voltage adapted to assuming that the voltage resolution is the highest voltage resolution Vr _O. Further, C _DUTYI is also a duty count number adapted to the PWM frequency f _PWMI acquired in step S2.

C _DUTYI = C _DUTY ÷ (Vr _I ÷ Vr _O ) (2)
For example, when the duty count number C _DUTY of the U-phase voltage is 2,000 and the voltage resolution Vr _I calculated in step S3 is 0.05, the converted duty count number C _DUTYI is
C _DUTYI = 2,000 / (0.05 / 0.01)
= 400
It becomes.

When the PWM frequency f _PWMI acquired in step S1 is 5 [kHz], the PWM count number C _PWMI is 800. Therefore, the duty ratio when the duty count number C _DUTYI is 400 is (400/800) × 100 = 50 [%], and it can be seen that the duty ratio does not change even if the PWM frequency is changed.
Finally, the PWM frequency changing unit 46 gives the PWM frequency f _PWMI acquired in step S2 and the converted duty count number C _DUTYI obtained in step S4 to the PWM control unit 47 (step S5). Then, the PWM frequency changing unit 46 ends the processing in the current calculation cycle. The PWM controller 47 is a U-phase PWM whose PWM frequency is the PWM frequency f _PWMI given from the PWM frequency _changer 46 and has a duty ratio corresponding to the duty count number _DUTYI given from the PWM frequency _changer 46. Generate a signal.

In this embodiment, the PWM frequency can be changed according to the motor current (q-axis current command value I _q ^* ) within the allowable range from the capacity of the smoothing capacitor 62 in the motor drive circuit 32. As a result, the PWM frequency when the motor current is small can be made lower than when the motor current is high. Thereby, the current ripple when the motor current is small can be reduced. Thereby, noise vibration when the motor current is small can be reduced.

In the operation example of FIG. 7, the PWM frequency changing unit 46 acquires the PWM frequency f _PWMI corresponding to the q-axis current command value I _q ^* from the motor current-PWM frequency map in step S2. However, in step S2, the PWM frequency changing unit 46 determines the q-axis current command value I _q ^* set by the current command value setting unit 42 and the q-axis detection current I _q calculated by the UVW / dq conversion unit 48. The current value having the larger absolute value may be selected, and the PWM frequency f _PWMI corresponding to the selected current value may be acquired from the motor current-PWM frequency map. That is, the PWM frequency f _PWMI may be obtained by regarding the q-axis current command value I _q ^* and the q-axis detection current I _q as the motor current with the larger absolute value.

FIG. 8 is a flowchart for explaining another operation example of the PWM frequency changing unit 46. The process of FIG. 8 is repeatedly executed at every predetermined calculation cycle. Only the operation of the PWM frequency changing unit 46 with respect to the U-phase voltage duty count will be described.
A map (hereinafter referred to as “motor current-PWM count number map”) that stores the relationship between the absolute value of the motor current value (q-axis current value) of the electric motor 18 and the PWM count number that is allowed from the capacity of the smoothing capacitor 62. ) Is created in advance and stored in the nonvolatile memory 40. The motor current-PWM count number map is created based on, for example, the relationship between the absolute value of the motor current value of the electric motor 18 and the maximum value of the PWM count number that is allowed from the capacity of the smoothing capacitor 62. In the PWM count number C _PWM , if the frequency of the PWM clock is f _CL and the PWM frequency is f _PWM , C _PWM = f _CL / f _PWM , so the motor current-PWM count number map is the motor current in FIG. It can be created from a PWM frequency map. FIG. 9 shows an example of the motor current-PWM count number map. As shown in FIG. 9, the PWM count number (maximum value) allowed for each motor current value decreases as the motor current increases.

For convenience of explanation, in this embodiment, it is assumed that the frequency f _CL of the PWM clock of the microcomputer 31 is 4 [MHz]. Further, the minimum frequency f _PWMO of the PWM signal is _set to 1 [kHz]. The power supply voltage Vb is 40 [V]. In this example, PWM count if the PWM frequency is the lowest value _{f PWMO} (maximum value of the PWM _{count) C PWMO is} a _{f CL / f PWMO = 4,000,000 /} 1,000 = 4,000 Become. The voltage resolution Vr _O if the PWM frequency is the lowest value _{f PWMO is, Vb / Vr O = 40 /} 4,000 = 0.01 [V / LSB] to become. Voltage resolution Vr _O if the PWM frequency is the lowest value _{f PWMO} becomes the maximum voltage resolution. The value of the frequency _{f CL,} the power supply voltage Vb, the number PWM count if the PWM frequency is the lowest value _{f PWMO C PWMO,} maximum voltage resolution and Vr _O of the PWM clock is stored in the nonvolatile memory 40.

Referring to FIG. 8, PWM frequency changing unit 46 first acquires q-axis current command value I _q ^* set by current command value setting unit 42 (step S11). Then, the PWM frequency changing unit 46 regards the acquired q-axis current command value I _q ^* as the motor current, and determines the motor current (q-axis current command value I _q ^* from the motor current-PWM count number map (see FIG. 9). ) _Is acquired (step S12).

Next, the PWM frequency changing unit 46 calculates the duty count number C _DUTYO of the U-phase voltage given from the dq / UVW conversion unit 45 using the following equation (3), the PWM count number C _PWMI acquired in step S12. _Is converted into a duty count number C _DUTYI adapted to (step S13). _{Incidentally, C DUTYO} is duty count _{C DUTYO} the U-phase voltage adapted to assuming that the voltage resolution is the highest voltage resolution Vr _O. C _DUTYI is also a duty count number adapted to the PWM frequency corresponding to the PWM count number C _PWMI acquired in step S12.

C _DUTYI = C _DUTY ÷ (C _PWMO ÷ C _PWMI ) (3)
For example, when the duty count number C _DUTYO of the U-phase voltage is 2,000 and the PWM count number C _PWMI acquired in step S12 is 800, the converted duty count number C _DUTYI is
C _DUTYI = 2,000 / ( _4000/800 )
= 400
It becomes.

Finally, the PWM frequency changing unit 46 gives the PWM count number C _PWMI acquired in step S12 and the converted duty count number C _DUTYI obtained in step S13 to the PWM control unit 47 (step S14). Then, the PWM frequency changing unit 46 ends the processing in the current calculation cycle. The PWM control unit 47 is a PWM frequency f _PWMI (= f _CL / C _PWMI ) corresponding to the PWM count number C _PWMI given from the PWM frequency changing unit 46 and given from the PWM frequency changing unit 46. A U-phase PWM signal having a duty ratio corresponding to the duty count number C _DUTYI is generated.

In step S14, the PWM frequency changing unit 46 calculates the PWM frequency f _PWMI (= f _CL / C _PWMI ) corresponding to the PWM count number C _PWMI acquired in step S12 and the converted duty count number obtained in step S13. C _DUTYI may be given to the PWM control unit 47.
In the operation example of FIG. 8, the PWM frequency changing unit 46 acquires the PWM count number C _PWMI corresponding to the q-axis current command value I _q ^* from the motor current-PWM count number map in step S12. However, in step S12, the PWM frequency changing unit 46 determines the q-axis current command value I _q ^* set by the current command value setting unit 42 and the q-axis detection current I _q calculated by the UVW / dq conversion unit 48. The current value having the larger absolute value may be selected, and the PWM count number C _PWMI corresponding to the selected current value may be acquired from the motor current-PWM count number map. That is, the PWM count number C _PWMI may be acquired by regarding the q-axis current command value I _q ^* and the q-axis detection current I _q as the motor current with the larger absolute value.

  The smoothing capacitor 62 in the motor drive circuit 32 may be composed of a plurality of smoothing capacitors connected in parallel. In such a case, if the capacitor capacity is reduced due to a failure of some of the smoothing capacitors, the motor current-PWM frequency map data (or motor current-PWM count number map data) is obtained. What is necessary is just to convert into data suitable for the capacitor capacity after the decrease.

Further, the voltage resolution used when calculating the voltage to be applied to the electric motor 18 may be changed as needed based on the motor current.
In the above embodiment, the case where the present invention is applied to the motor control device of the electric power steering device has been described. However, the present invention can also be applied to a motor control device used other than the electric power steering device.

  In addition, various design changes can be made within the scope of matters described in the claims.

  DESCRIPTION OF SYMBOLS 12 ... ECU, 18 ... Electric power steering apparatus, 32 ... Motor drive circuit, 46 ... PWM frequency change part, 47 ... PWM control part, 62 ... Smoothing capacitor

Claims (5)

A motor control device that PWM-controls an electric motor via a motor drive circuit including a smoothing capacitor and a plurality of switching elements,
Storage means for storing a relationship between a motor current value of the electric motor and a PWM frequency corresponding value that is a value corresponding to the frequency or period of the PWM signal, which is allowed from the capacity of the smoothing capacitor;
A computing means for computing a first duty count number adapted when the PWM frequency is assumed to be a predetermined PWM frequency;
An acquisition means for acquiring a PWM frequency corresponding value corresponding to a motor current based on the relationship stored in the storage means;
Conversion means for converting the first duty count number calculated by the calculation means into a second duty count number adapted to the PWM frequency corresponding value acquired by the acquisition means;
And a PWM signal generation unit configured to generate a PWM signal corresponding to the PWM frequency corresponding value acquired by the acquisition unit and the second duty count obtained by the conversion unit.   The motor control device according to claim 1, wherein the PWM frequency corresponding value is a PWM frequency that is a frequency of a PWM signal or a PWM count that is a PWM clock number corresponding to one cycle of the PWM signal. Current detection means for detecting a motor current flowing in the electric motor;
Current command value setting means for setting a motor current command value, which is a target value of the motor current to be passed through the electric motor,
The calculation means is adapted to the case where the PWM frequency is assumed to be the predetermined PWM frequency, and the motor current command set by the current command value setting means is set to the motor current detection value detected by the current detection means. The motor control device according to claim 1, wherein the motor control device is configured to calculate a first duty count number for equalizing the value.   The said acquisition means considers the motor current command value set by the said current command value setting means as a motor current, and is comprised so that the PWM frequency corresponding | compatible value corresponding to a motor current may be acquired. Motor control device.   The obtaining means regards a current value having a larger absolute value among the motor current command value set by the current command value setting means and the motor current detection value detected by the current detection means as a motor current, The motor control device according to claim 3, wherein the motor control device is configured to acquire a PWM frequency corresponding value corresponding to the motor current.

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