Motor control device, control method, motor module, and electric power steering apparatus

To improve steering feeling felt by a driver, a motor control device includes a processor and a memory which stores a program for controlling an operation of the processor. According to the program, the processor executes: switching from n-phase (n is an integer of three or more) energization control to n−1 phase energization control in response to a switching signal; acquiring a torque command value, an electrical angle of a motor, and an actual current value of the motor; generating a pre-current command value on the basis of the torque command value, the electrical angle of the motor, and the actual current value of the motor which are acquired; generating a current command value by applying dither control to the pre-current command value in a dead point range of an electrical angle range from 0 to 2π; and performing the n−1 phase energization control on the basis of the current command value.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present invention claims priority under 35 U.S.C. § 119 to Japanese Patent Application No. 2020-206252 filed on Dec. 11, 2020, the entire content of which is incorporated herein by reference.

FIELD OF THE INVENTION

The present disclosure relates to a motor control device, a control method, a motor module, and an electric power steering apparatus.

BACKGROUND

A general automobile is mounted with an electric power steering apparatus (EPS) including an electric motor (hereinafter, referred to simply as a “motor”) and a motor control device. The electric power steering apparatus is an apparatus that assists the steering wheel operation of a driver by driving the motor.

There has been developed a technique for assisting the steering wheel operation of the driver by continuing motor driving even in a case where a defect occurs in a part of a motor or an inverter mounted on the electric power steering apparatus. Examples of the defect include disconnection of a winding of the motor or a failure of a switch element included in the inverter. In a case where such a defect only affects a power supply to a specific winding, it is possible to continue the motor driving by continuing to supply power to the remaining normal windings.

Conventionally, an electric power steering apparatus is capable of continuing assistance by normally performing energization control to conduct windings of three phases and performing energization control to energize the remaining normal windings of two phases in a case where an energization failure occurs in any phase of the windings of a motor. In the electric power steering apparatus, a rotation angular velocity of the motor increases by executing an acceleration control in a deceleration section in which a steering speed is decelerated when a generated motor torque falls below an assist force target value. Accordingly, it is possible to suppress non-smoothness felt by the driver in the steering operation in the deceleration section.

In a case where a defect occurs in a motor or an inverter, it is desirable to improve steering feeling felt by a driver when the assistance of the steering wheel operation of the driver is continued.

SUMMARY

In a non-limitative and exemplary embodiment, a control device of the present disclosure is a control device, which is used in an electric power steering apparatus including a motor having n-phase (n is an integer of three or more) windings, for controlling the motor. N-phase energization control of energizing the n-phase windings or n−1 phase energization control of energizing n−1 phase windings is able to be performed. The device includes: a processor; and a memory that stores a program for controlling an operation of the processor. The processor executes, according to the program, switching from the n-phase energization control to the n−1 phase energization control in response to a switching signal, acquiring a torque command value, an electrical angle of the motor, and an actual current value of the motor, generating a pre-current command value on the basis of the torque command value, the electrical angle of the motor, and the actual current value of the motor which are acquired, generating a current command value by applying dither control to the pre-current command value in a dead point range of an electrical angle range from 0 to 2π, and performing the n−1 phase energization control on the basis of the current command value.

In a non-limitative and exemplary embodiment, a motor module of the present disclosure includes a motor and the control device described above.

In a non-limitative and exemplary embodiment, an electric power steering apparatus of the present disclosure includes the motor module described above.

In a non-limitative and exemplary embodiment, a control method of the present disclosure is a control method, which is used in an electric power steering apparatus including a motor having n-phase (n is an integer of three or more) windings, for controlling the motor. N-phase energization control of energizing the n-phase windings or n−1 phase energization control of energizing n−1 phase windings is able to be applied to the motor. The method includes: switching from the n-phase energization control to the n−1 phase energization control in response to a switching signal; acquiring a torque command value, an electrical angle of the motor, and an actual current value of the motor; generating a pre-current command value on the basis of the torque command value, the electrical angle of the motor, and the actual current value of the motor which are acquired; generating a current command value by applying dither control to the pre-current command value in a dead point range of an electrical angle range from 0 to 2π; and performing the n−1 phase energization control on the basis of the current command value.

DETAILED DESCRIPTION

As a result of examination by the present inventors, it has been found that, in a case where a steering angle of a steering wheel is to be maintained in the deceleration section, chattering occurs in a motor current, a motor torque vibrates, and as a result, vibration may occur in the steering wheel. This unintended vibration deteriorates steering feeling felt by a driver. Even when the acceleration control is executed, it is difficult to prevent this deterioration.

The present inventors have found that the deterioration of the steering feeling can be suppressed by applying dither control to calculation of a current command value, and have completed the present invention.

Hereinafter, embodiments of a motor control device, which is mounted on an electric power steering apparatus, a control method, a motor module including the control device, and the electric power steering apparatus including the motor module according to the present disclosure will be described in detail with reference to the accompanying drawings. However, needlessly detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and duplicate description of substantially the same configuration may be omitted. This is to avoid unnecessary redundancy of the following description and to facilitate the understanding of those skilled in the art.

The following embodiments are illustrative, and the motor control device, which is mounted on the electric power steering apparatus, and the control method according to the present disclosure are not limited to the following embodiments. For example, the numerical values, the steps, the order of the steps, and the like illustrated in the following embodiments are only illustrative, and various modifications can be made unless any technical inconsistency occurs. The embodiments described below are illustrative, and various combinations are possible unless any technical inconsistency occurs.

FIG.1is a diagram schematically illustrates a configuration example of an electric power steering apparatus1000according to the present embodiment.

The electric power steering apparatus1000(hereinafter, referred to as an “EPS”) includes a steering system520and an assist torque mechanism540which generates an assist torque. The EPS1000generates the assist torque for assisting the steering torque of the steering system generated when a driver operates a steering wheel. The assist torque reduces an operation load on the driver.

The steering system520includes a steering wheel521, a steering shaft522, universal joints523A and523B, a rotary shaft524, a rack and pinion mechanism525, a rack shaft526, right and left ball joints552A and552B, tie rods527A and527B, knuckles528A and528B, and right and left steered wheels529A and529B, for example.

The assist torque mechanism540includes a steering torque sensor541, a steering angle sensor542, an electronic control unit (ECU)100for automobiles, a motor543, a deceleration gear544, an inverter545, and a torsion bar546, for example. The steering torque sensor541detects a steering torque in the steering system520by detecting the amount of torsion of the torsion bar546. The steering angle sensor542detects a steering angle of the steering wheel. Incidentally, the steering torque may be an estimation value derived from calculation, not a value of the steering torque sensor. The steering angle can also be calculated on the basis of the output value of the angle sensor.

The ECU100generates a motor driving signal on the basis of the detection signals detected by the steering torque sensor541, the steering angle sensor542, a vehicle speed sensor (not illustrated) mounted on a vehicle, or the like, and outputs the motor driving signal to the inverter545. For example, the inverter545converts direct-current power into three-phase alternating-current power having U-phase, V-phase, and W-phase pseudo sine waves in accordance with the motor driving signal and supplies the power to the motor543. The motor543is, for example, a surface permanent-magnet synchronous motor (SPMSM) or a switched reluctance motor (SRM), and is supplied with the three-phase alternating-current power to generate assist torque corresponding to the steering torque. The motor543transmits the generated assist torque to the steering system520via the deceleration gear544. Hereinafter, the ECU100will be referred to as a control device100for the EPS.

The control device100and the motor are modularized and manufactured and sold as a motor module. The motor module includes the motor and the control device100and is suitably used for the EPS. Alternatively, the control device100may be manufactured and sold as a control device for controlling the EPS independently of the motor.

FIG.2is a block diagram illustrating a typical example of a configuration of the control device100according to the present embodiment. The control device100includes a power supply circuit111, an angle sensor112, an input circuit113, a communication I/F114, a drive circuit115, a ROM116, and a processor200, for example. The control device100can be realized as a printed circuit board (PCB) on which these electronic components are implemented.

A vehicle speed sensor300mounted on the vehicle, the steering torque sensor541, and the steering angle sensor542are electrically connected to the processor200. The vehicle speed sensor300, the steering torque sensor541, and the steering angle sensor542transmit a vehicle speed, a steering torque, and a steering angle to the processor200, respectively.

The control device100is electrically connected to the inverter545(seeFIG.1). The control device100controls switching operations of a plurality of switch elements (for example, MOSFETs) included in the inverter545. Specifically, the control device100generates control signals (hereinafter referred to as “gate control signals”) for controlling the switching operations of the respective switch elements and outputs the gate control signals to the inverter545.

The control device100generates a torque command value on the basis of a torsion torque or the like, and controls the torque and rotation speed of the motor543by means of, for example, vector control. The control device100can perform not only the vector control but also other closed-loop control. The rotation speed is expressed as the number of times of rotation (rpm) of a rotor per unit time (for example, one minute) or the number of times of rotation (rps) of the rotor per unit time (for example, one second). The vector control is a method in which current flowing through the motor is separated into a current component that contributes to generation of a torque and a current component that contributes to generation of a magnetic flux, and the current components orthogonal to each other are independently controlled.

The power supply circuit111is connected to an external power supply (not illustrated) and generates DC voltage required for each block in the circuit. The DC voltage to be generated is, for example, 3 V or 5 V.

The angle sensor112is, for example, a resolver or a Hall IC. Alternatively, the angle sensor112is also realized by a combination of an MR sensor having a magnetoresistive (MR) element and a sensor magnet. The angle sensor112detects a rotation angle of the rotor and outputs the rotation angle to the processor200. The control device100may include a speed sensor and an acceleration sensor for detecting the rotation speed and acceleration of the motor instead of the angle sensor112.

The input circuit113receives a motor current value (hereinafter, referred to as an “actual current value”) detected by a current sensor (not illustrated), converts the level of the actual current value into the input level for the processor200as needed, and outputs the actual current value to the processor200. A typical example of the input circuit113is an analog-digital conversion circuit.

The processor200is a semiconductor integrated circuit and is also referred to as a central processing unit (CPU) or a microprocessor. The processor200sequentially executes a computer program which is stored in the ROM116and describes a command set for controlling motor driving, and realizes desired processing. The processor200is widely interpreted as a term including a field programmable gate array (FPGA), an application specific integrated circuit (ASIC), or an application specific standard product (ASSP) mounted with a CPU. The processor200sets a current command value according to the actual current value, the rotation angle of the rotor, and the like, generates a pulse width modulation (PWM) signal, and outputs the PWM signal to the drive circuit115.

The communication I/F114is an input/output interface for transmitting/receiving data in conformity with an in-vehicle control area network (CAN), for example.

The drive circuit115is typically a gate driver (or a pre-driver). The drive circuit115generates a gate control signal in accordance with the PWM signal and gives the gate control signal to the gates of the plurality of switch elements included in the inverter545. When a driving target is a motor which can be driven at low voltage, the gate driver may not always be required. In this case, the functionality of the gate driver may be implemented in the processor200.

The ROM116is electrically connected to the processor200. The ROM116is a writable memory (for example, a PROM), a rewritable memory (for example, a flash memory or an EEPROM), or a read-only memory, for example. The ROM116stores a control program including a command set for causing the processor200to control motor driving. For example, the control program is temporarily expanded to a RAM (not illustrated) at boot time.

FIG.3is a functional block diagram exemplifying functional blocks of processes (or tasks) executed by the processor200of the control device100according to the exemplary embodiment of the present disclosure. The processor200in the exemplary embodiment of the present disclosure may be realized by a plurality of functional blocks including a torque control unit210and a current control unit220.

A torsion torque Ttordetected by the steering torque sensor541is input to the torque control unit210. The torque control unit210generates a torque command value Trefon the basis of the torsion torque Ttor.

The motor according to the present embodiment is a three-phase motor in which windings are star-connected to each other. Phase currents flowing through the U phase, the V phase, and the W phase are detected as actual current values Imby the current sensor. The actual current value Im, an electrical angle θmof the motor, and the torque command value Trefare input to the current control unit220. The current control unit220calculates duty command values Dutyu, Dutyv, and Dutywfor respective U, V, and W phases on the basis of the actual current value Im, the electrical angle θmof the motor, and the torque command value Tref, and outputs the calculated values to the drive circuit115. Incidentally, the functions of the torque control unit210and the current control unit220will be described in detail later.

The processing of each functional block is typically described in a computer program in units of software modules and stored in the ROM116. However, in a case of using the FPGA or the like, all or part of these functional blocks can be implemented as hardware accelerators.

In a case in which each functional block is implemented as software (or firmware) in the control device100, a device which executes the software may be the processor200. In one aspect, the motor control device according to the present disclosure includes the processor200and a memory116which stores a program for controlling the operation of the processor200. The processor200executes, according to the program, (1) switching from a three-phase energization control to a two-phase energization control in response to a switching signal, (2) acquiring a torque command value, an electrical angle of the motor, and an actual current value of the motor, (3) generating a pre-current command value on the basis of the acquired torque command value, the electrical angle of the motor, and the actual current value of the motor, (4) generating a current command value by applying dither control to the pre-current command value in a dead point range of an electrical angle range from 0 to 2π, and (5) performing the two-phase energization control on the basis of the current command value.

In a case where each functional block is implemented as software and/or hardware in the control device100, in another aspect, the motor control device100of the present disclosure switches from a three-phase energization control to a two-phase energization control in response to a switching signal output from a failure detection unit224, and performs the two-phase energization control on the basis of a current command value calculated by a current command value calculation unit221included in the current control unit220. The current command value calculation unit221includes a pre-current command value calculation unit221awhich acquires a torque command value, an electrical angle of the motor, and an actual current value of the motor, and generates a pre-current command value on the basis of the torque command value, the electrical angle of the motor, and the actual current value of the motor which are acquired, and a dither control unit221bwhich generates a current command value by applying dither control to the pre-current command value in a dead point range of an electrical angle range from 0 to 2π.

FIG.4is a functional block diagram illustrating a configuration example of the torque control unit210.

In the illustrated example, the torque control unit210includes a responsiveness phase control unit211, a base assist calculation unit212, a stability phase compensation unit213, a torque differential compensation unit214, and an adder215.

The responsiveness phase control unit211adjusts an assist gain within a possible range of a steering frequency when the driver operates the steering wheel, and compensates for the rigidity of the torsion bar. In the present embodiment, an example of the above range is 5 Hz or less. The responsiveness phase control unit211acquires the torsion torque Ttor. The responsiveness phase control unit211generates a responsiveness phase compensation torque Trcby applying a first-order phase compensation to the torsion torque Ttorwhen the steering frequency is 5 Hz or less. The first-order phase compensation is represented by a transfer function of a mathematical expression of Expression 1.

Here, s is a Laplace transformer, f1is a frequency (Hz) for determining the zero point of the transfer function, and f2is a frequency (Hz) for determining the pole of the transfer function. A graph in which the gain (or loop gain) is set as a vertical axis and the logarithm of the frequency is set as a horizontal axis is called a gain diagram. In the gain diagram, the zero point means the intersection of the gain curve and the horizontal axis indicating 0 dB, and the pole means the maximum point of the gain curve. For example, by setting the pole frequency to be higher than the zero point frequency, a phase lead compensation can be applied. The longer the distance between the frequencies is, the larger the amount of phase lead becomes.

The base assist calculation unit212acquires the responsiveness phase compensation torque Trcand a vehicle speed v as input data. The base assist calculation unit212generates a base assist torque Tbaon the basis of the responsiveness phase compensation torque Trcand the vehicle speed v. For example, the base assist calculation unit212may have a lookup table (LUT) which defines a correspondence between the responsiveness phase compensation torque Trc, the vehicle speed v, and the base assist torque Tba. The base assist calculation unit212can determine the base assist torque Tbain the correspondence relationship on the basis of the responsiveness phase compensation torque Trcand the vehicle speed v with reference to the LUT. Further, the base assist calculation unit212can determine a base assist gain k on the basis of a slope defined by a ratio of a variation amount of the base assist torque Tbato a variation amount of the responsiveness phase compensation torque Trc.

The stability phase compensation unit213acquires the base assist torque Tbaand the base assist gain k as input data. The stability phase compensation unit213generates a stability phase compensation torque TScon the basis of the base assist torque Tbaand the base assist gain k. The stability phase compensation unit213can apply stability phase compensation to the base assist torque Tbaby using, for example, a stabilization compensator. The stabilization compensator may have a second-order or higher transfer function in which a frequency characteristic is variable in accordance with the base assist gain k. The second-order or higher transfer function is expressed using a responsiveness parameter co and a damping parameter ζ. The second-order or higher transfer function can be expressed by, for example, a mathematical expression of Expression 2. By setting the order number of the transfer function to two, damping can be given to the characteristic of the transfer function. A phase characteristic can be adjusted by changing the damping.

Here, s is a Laplace transformer, ω1is a zero point frequency, ω2is a pole frequency, ζ1is zero point damping, and ζ2is pole damping. In the gain diagram, the zero point means the intersection of the gain curve and the horizontal axis indicating 0 dB, and the pole means the maximum point of the gain curve. The pole frequency ω2is lower than the zero point frequency ω1.

The torque differential compensation unit214calculates a differential compensation torque Tdcon the basis of the temporal change amount of the torsion torque Ttor. The torque differential compensation unit214can calculate the differential compensation torque Tdcon the basis of, for example, a transfer function represented by a mathematical expression of Expression 3. Here, T is a time constant.

The adder215generates the torque command value Trefon the basis of the stability phase compensation torque TScand the differential compensation torque Tdc. More specifically, the adder215adds the differential compensation torque Tdcto the stability phase compensation torque TScto generate the torque command value Tref.

According to the torque control unit210described above, the responsiveness of the motor torque to the torsion torque can be enhanced by applying a torque differential compensation and a phase lead compensation. As a result, a rapid output variation of the motor torque which may occur after a dead point described later is suppressed, and the steering feeling can be improved.

FIG.5is a functional block diagram illustrating a configuration example of the current control unit220.FIG.6is a functional block diagram illustrating a configuration example of the current command value calculation unit221.

In the illustrated example, the current control unit220includes the current command value calculation unit221, a voltage command value calculation unit222, a PWM modulation unit223, and the failure detection unit224. The current control unit220calculates a voltage command value Vrefon the basis of the torque command value Tref, for example, in accordance with vector control. The current control unit220generates a duty command value Duty which is a PWM signal on the basis of the voltage command value Vrefand outputs the duty command value Duty to the drive circuit115.

On the basis of the current command value Iref, the current control unit220performs three-phase energization control to energize windings of three phases in control in a normal state, and performs two-phase energization control to energize windings of two phases out of the three phases in control in an abnormal state.

First, failure detection in the present embodiment will be described.

In the present embodiment, the current control unit220can energize the windings of the motor according to a control mode including control in the normal state and control in the abnormal state. For example, the normal state means a state in which a defect such as disconnection of a winding or an open or short circuit failure of a switch element included in an inverter does not occur. The abnormal state means a state in which the above-described defect occurs.

The current control unit220can perform three-phase energization control to energize the three-phase windings when control in the normal state is selected as the control mode, and can perform two-phase energization control to energize the two-phase windings when control in the abnormal state is selected as the control mode.

The failure detection unit224monitors whether or not there is a winding which cannot be energized among the three-phase windings, and detects a failure of the winding or a switch element included in the inverter. As an example of failure detection, the failure detection unit224can detect a failure of the winding or the switch element for each phase on the basis of each difference between three-phase phase currents Iu, Iv, and Iwand three-phase current command values Iref_u, Iref_v, and Iref_w. Each of the three-phase phase currents can be detected by, for example, a shunt resistor included in a leg of each phase of the inverter. As another example of the failure detection, the failure detection unit224can estimate a current value and specify a failure phase. Alternatively, the failure detection unit224can detect a failure of the switch element by monitoring a drain-source voltage Vdsof the switch element (typically, MOSFET) and comparing a predetermined threshold voltage with Vds. However, the failure detection is not limited to the above methods, and any known method related to the failure detection may be widely used.

The failure detection is not necessarily performed by the processor200mounted on the ECU (control device100) for controlling the motor, and may be performed by, for example, a processor mounted on another ECU communicably connected to the processor200via the CAN.

The failure detection unit224generates a failure detection signal FD in response to detection of a failure of a phase which cannot be energized. When detecting a failure of the winding or the switch element, the failure detection unit224notifies the current command value calculation unit221of the failure detection signal FD. The current command value calculation unit221receives the failure detection signal FD as a switching signal. For example, when the failure detection signal FD is asserted, the current command value calculation unit221switches the motor control of the control device100from the three-phase energization control to the two-phase energization control in response to the assertion.

For example, when detecting a failure of a high-side switch element included in a U-phase leg of the inverter, the failure detection unit224asserts the failure detection signal FD. This failure is referred to as a U-phase failure. In response to the asserted failure detection signal FD, the current command value calculation unit221switches from the three-phase energization control to two-phase energization control in which the windings of the V and W phases other than the U phase among the three phases are energized. Similarly, when detecting a failure of a high-side switch element included in a V-phase leg of the inverter, for example, the failure detection unit224asserts the failure detection signal FD. This failure is referred to as a V-phase failure. In response to the asserted failure detection signal FD, the current command value calculation unit221switches from the three-phase energization control to two-phase energization control in which the U-phase and W-phase windings other than the V-phase among the three phases are energized. For example, when a failure of a high-side switch element included in a W-phase leg of the inverter is detected, the failure detection unit224asserts the failure detection signal FD. This failure is referred to as a W-phase failure. In response to the asserted failure detection signal FD, the current command value calculation unit221switches from the three-phase energization control to two-phase energization control in which the U-phase and V-phase windings other than the W-phase among the three phases are energized.

In the illustrated example, the current command value calculation unit221includes the pre-current command value calculation unit221aand the dither control unit221b.

The current command value calculation unit221acquires the actual current value Imof the motor including the torque command value Tref, the electrical angle θmof the motor, and the three-phase phase currents Iu, Iv, and Iw. The current command value calculation unit221calculates three-phase current command values Iref_u, Iref_v, and Iref_w, on the basis of the torque command value Tref, the electrical angle θmof the motor, and the three-phase phase currents Iu, Iv, and Iwwhich are acquired.

When the control device100performs the motor control in the normal state, the output of the three-phase motor can be expressed by, for example, a mathematical expression of Expression 4. The phase voltages of the U, V, and W phases are expressed by mathematical expressions of Expressions 5, 6, and 7, respectively. Here, T is a motor torque [Nm], Pn is the number of pole pairs, Ψfis a flux linkage [wb], and ω is an angular velocity [rad/s] of the electrical angle θmof the motor. Ψfis represented by the product (Im·L) of the actual current value Imof the motor and a reactance L of the motor. ω is obtained by time-differentiating θm.
Tω=Pn(iueu+ivev+iwew)  [Expression 4]
eu=Ψfω sin θ  [Expression 5]
ev=Ψfω sin(θ+⅔π)  [Expression 6]
ew=Ψfω sin(θ−2/3π)  [Expression 7]

In the present embodiment, an example of motor control in the abnormal state in which two-phase energization control of energizing the V and W-phase windings is performed assuming that a defect occurs in energization of the U phase among the U, V, and W phases will be described. In this two-phase energization control, each phase current is given by a mathematical expression of Expression 8. i2phaseis a phase current flowing through the U phase and the V phase, and corresponds to the actual current value Imof the motor.
iu=0,iv=−iw=i2phase[Expression 8]

When Expressions 5 to 8 are applied to Expression 4 and organized, a mathematical expression of Expression 9 is obtained. Further, when Expression 9 is organized for the i2phase, a mathematical expression of Expression 10 is obtained. Here, φ is a phase offset [rad]. When a defect occurs in the energization of the U phase, φ=0. When a defect occurs in the energization of the V phase, φ=π/3. When a defect occurs in the energization of the W phase, φ=−π/3.

In the two-phase energization control, the maximum current value limit is set as shown in Expression 11. The phase current is limited to the maximum current value, and the current i2phaseis set as a pre-current command value with respect to the torque command value Tref. More specifically, the U-phase pre-current command value Ipref_uis set to zero. The V-phase pre-current command value Ipref_vis assumed to be the i2phase. The W-phase pre-current command value Ipref_wis set to −i2phase. In this specification, a current command value before dither control described later is applied is referred to as a pre-current command value, and is distinguished from a current command value after dither control is applied.

The pre-current command value calculation unit221acalculates and generates the pre-current command values Ipref_vand Ipref_won the basis of a mathematical expression of Expression 11. The mathematical expression of Expression 11 represents the current i2phaseby using a torque constant Kt[Nm/Arms] represented by a mathematical expression of Expression 12.

The dither control unit221bgenerates a current command value by applying dither control to the pre-current command value in a dead point range of an electrical angle range from 0 to 2π. The dither control unit221bdetermines the pre-current command value as the current command value without applying dither control to the pre-current command value in a range other than the dead point range in the electrical angle range from 0 to 2π.

FIG.7Ais a graph exemplifying a phase current waveform by two-phase energization control at the time of U-phase failure.FIG.7Bis a graph exemplifying a motor torque waveform by two-phase energization control at the time of U-phase failure.FIGS.7A and7Bexemplify waveforms of the phase current and the motor torque to which the maximum current value limit is applied, respectively.

In the three-phase energization control and the two-phase energization control in the present embodiment, the phase current is controlled such that the sum of the phase currents becomes zero. In the two-phase energization control, the current flowing through the U-phase winding is always zero, so that an electrical angle at which the sum of the phase currents flowing through the V-phase and the W-phase is zero is generated. The dead point means this electrical angle. InFIG.7A or7B, the dead point is π/2 or (3/2)π.

The dead point range means a range of the dead point and electrical angles before and after the dead point. In the dead point range, even when the current is commanded, the current cannot flow to the motor, so that the motor torque falls below the torque command value (or a target motor torque). The dead point range includes the range of the electrical angle satisfying a condition of π/4≤θ+φ<(3/4)π or (5/4)π≤θ+φ<(7/4)π. In the present embodiment, in a case where the U phase fails, the phase offset φ is zero. The dead point range corresponds to the electrical angle range of π/4≤θ<(3/4)π or (5/4)π≤θ<(7/4)π.

As exemplified inFIG.7B, the motor output significantly decreases near the dead point. A difference in motor torque between the dead point range and the range other than the dead point increases, and the variation amount of the torsion torque increases. This may be a factor that further deteriorates the steering feeling felt by the driver in a case where the steering angle of the steering wheel is to be maintained near the dead point.

The dither control unit221bcalculates a dither current iDitheron the basis of a mathematical expression of Expression 13, and generates the current command value Irefon the basis of the pre-current command value IPrefand the dither current iDither. Here, ADitheris a dither amplitude, and fDitherfrequency. The dither current iDitheris represented by a periodic current waveform. The dither control unit221bsets the dither current iDitherto zero in a range other than the dead point range as shown in a mathematical expression of Expression 14. Setting the dither current iDitherto zero means that substantially no dither control is applied.

FIG.8is a graph illustrating an example of a dither current waveform in the present embodiment.FIG.9is a graph illustrating a dither current waveform in a comparative example. The dither current according to the comparative example is expressed by a mathematical expression of Expression 15. In the present embodiment, as compared with the dither current according to the comparative example, in the mathematical expression of Expression 13 for giving the dither current, (1) a point of taking the absolute value of the dither current (seeFIG.8) and (2) a point of adding “−1” as a bias to the output of the sin function of the last term of the right side are devised. This devising makes it possible to obtain the effect of the dither control in the dead point range.

The dither control unit221bgenerates a current command value by subtracting the dither current from the pre-current command value when the pre-current command value is equal to or greater than zero, and generates a current command value by adding the dither current to the pre-current command value when the pre-current command value is less than zero. In other words, the dither control unit221bgenerates the current command value on the basis of a mathematical expression of Expression 17 when the condition shown in Expression 16 is satisfied, and generates the current command value on the basis of a mathematical expression of Expression 18 when the condition shown in Expression 16 is not satisfied.
i2phase≥0  [Expression 16]
i2phase_Dither=i2phase−iDither[Expression 17]
i2phase_Dither=i2phase+iDither[Expression 18]

The dither control unit221boutputs three-phase current command values Iref_u, Iref_v, and Iref_wafter applying the dither control, which are given by a mathematical expression of Expression 19. Incidentally, the U-phase current command value Iref_uat the time of the U-phase failure is zero.
Iref_u=0,Iref_v=i2phase_Dither,Iref_w=−i2phase_Dither[Expression 19]

FIG.10is a graph exemplifying a dither current waveform used for the dither control.FIG.11is a graph exemplifying a phase current waveform after the dither control is applied to a phase current in which chattering occurs.

As described above, the current limitation is applied in the two-phase energization control. Thus, even when the dither current according to the comparative example is applied to the pre-current command value, the suppression of current chattering becomes insufficient, and as a result, there is a problem that the effect of the dither control cannot be obtained. On the other hand, when the dither current according to the present embodiment is applied to the pre-current command value, the current chattering is appropriately suppressed, and as a result, the effect of the dither control can be obtained sufficiently.

FIG.5is referred to again.

The voltage command value calculation unit222acquires the current command values Iref_u, Iref_v, and Iref_w. The voltage command value calculation unit222calculates the voltage command values Vref_u, Vref_v, and Vref_won the basis of the current command values Iref_u, Iref_v, and Iref_w. In the two-phase energization control at the time of the U-phase failure, Vref_uis zero.

The present inventors confirmed the effect obtained by applying the dither control to the pre-current command value by performing actual vehicle measurement. In the actual vehicle measurement, the effect of the dither control was measured when the two-phase energization control was performed, and the application of the dither control was switched from off to on when chattering occurs in the current in the dead point range.

The conditions of the actual vehicle measurement are as follows: (1) torque constant Kt: 0.0452 [Nm/Arms], (2) number of pole pairs Pn: 4, (3) dither amplitude ADither: 2 [Nm], (4) dither frequency fDither: 30 [Hz], and (5) type of motor: brushless motor. The dither amplitude ADitherand the dither frequency fDitherare set as variables, and can be appropriately determined depending on the type of a vehicle or motor on which the EPS is mounted.

FIG.12is a graph illustrating measurement results of a steering angle and a steering torque in a case where the dither control is not applied.FIG.13is a graph illustrating measurement results of the steering angle and the steering torque in a case where the dither control is applied. In the graph, a broken line indicates the steering angle [deg], and a solid line indicates the steering torque [Nm].

Compared with a case where the dither control was not applied, the current chattering was suppressed in a case where the dither control was applied. As a result, it has been found that the vibration of the steering wheel was also suppressed, and specifically, the variation amount of the steering torque decreased by about 5 [Nm].

According to the motor control device100of the present embodiment, it is possible to improve the steering feeling felt by the driver when the energization control of energizing the remaining windings of the normal phase is performed in a case where the energization failure occurs in any phase of the windings of the motor. For example, it is possible to suppress the current chattering in the vicinity of the dead point, which may occur when the two-phase energization control is performed, and to reduce the steering torque variation amount. These effects can contribute to improvement in the safety of a steering wheel operation.

The motor control device or the control method according to the present embodiment can also be used as a control device for a double winding motor capable of performing so-called double inverter drive in which the motor is driven using two inverters. For example, in a case where a U-phase in one of the two inverters fails, it is possible to continuously perform two-phase energization control of energizing V-phase and W-phase windings by using the two inverters.

The embodiments of the present disclosure can be used for a motor control device for controlling an EPS mounted on a vehicle.