JP2016086501A - Motor controller and electric power steering device mounting the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a motor controller achieving high output and compaction by switching the modulation frequency of PWM to low frequency at the time of heavy load, thereby reducing the switching loss of PWM control, and to provide an electric power steering device mounting the same.SOLUTION: A motor controller including a control unit for calculating a current command value on the basis of the input torque, and driving a motor by means of a PWM control inverter based on the current command value, and controlling a motor system by means of the control unit further includes a torque determination unit for comparing the input torque with a predetermined torque threshold and outputting a torque determination signal, and a modulation frequency variation unit for varying the frequency of modulation signal of PWM control on the basis of the torque determination signal. The modulation frequency variation unit varies the frequency of modulation signal depending on a torque determination signal having the torque threshold as a displacement value, thus reducing heat loss of the control unit while achieving high output and compaction.SELECTED DRAWING: Figure 4

Description

  In the present invention, the frequency of a modulation signal (carrier) in PWM (pulse width modulation) control of a motor is changed according to input torque (particularly high input), thereby reducing heat loss, increasing output, and reducing size. The present invention relates to a motor control device and an electric power steering device equipped with the motor control device.

  An electric power steering device that is equipped with a motor control device and applies a steering assist force (assist force) to the steering mechanism of a vehicle by the rotational force of the motor is a transmission mechanism such as a gear or a belt that transmits the driving force of the motor via a speed reducer. Thus, a steering assist force is applied to the steering shaft or the rack shaft. Such a conventional electric power steering device (EPS) performs feedback control of the motor current in order to accurately generate the torque of the steering assist force. In feedback control, the motor applied voltage is adjusted so that the difference between the steering assist command value (current command value) and the motor current detection value is small. Generally, the adjustment of the motor applied voltage is a duty of PWM control. It is done by adjusting.

  A general configuration of an electric power steering device (EPS) is described with reference to FIG. The tie rods 6a and 6b are connected to the steered wheels 8L and 8R via the hub units 7a and 7b. The column shaft 2 is provided with a torque sensor 10 that detects the steering torque of the handle 1, and a motor 20 that assists the steering force of the handle 1 is connected to the column shaft 2 via the reduction gear 3. . The control unit (ECU) 30 that controls the electric power steering apparatus is supplied with electric power from the battery 13 and also receives an ignition key signal via the ignition key 11. The control unit 30 calculates the current command value of the assist (steering assist) command based on the steering torque Th detected by the torque sensor 10 and the vehicle speed Vel detected by the vehicle speed sensor 12, and the calculated current command value The current supplied to the motor 20 is controlled by the voltage control value Vref that has been compensated for. The steering angle sensor 14 is not essential and may not be provided, and may be obtained from a rotation sensor connected to the motor 20.

  The control unit 30 is connected to a CAN (Controller Area Network) 40 that transmits and receives various types of vehicle information, and the vehicle speed Vel can also be received from the CAN 40. The control unit 30 can be connected to a non-CAN 41 that transmits / receives communication other than the CAN 40, analog / digital signals, radio waves, and the like.

  In such an electric power steering apparatus, the control unit 30 is mainly composed of a CPU (including an MPU and MCU). General functions executed by a program inside the CPU are shown in FIG. The structure is

  The function and operation of the control unit 30 will be described with reference to FIG. 2. The steering torque Th from the torque sensor 10 and the vehicle speed Vel from the vehicle speed sensor 12 are input to the current command value calculation unit 31, and the current command value calculation unit 31. Calculates a current command value Iref1 using an assist map or the like based on the steering torque Th and the vehicle speed Vel. The calculated current command value Iref1 is added by the adding unit 32A and the compensation signal CM from the compensating unit 34 for improving the characteristics, and the added current command value Iref2 is limited to the maximum value by the current limiting unit 33. The current command value Irefm whose maximum value is limited is input to the subtraction unit 32B and subtracted from the motor current detection value Im.

  The subtraction result I (= Irefm−Im) in the subtraction unit 32B is PI-controlled by the PI control unit 35, and the PI-controlled voltage control value Vref is input to the PWM control unit 36 together with the modulation signal (carrier) CF to set the duty. The motor 20 is PWM driven via the inverter 37 with the PWM signal calculated and duty calculated. The motor current value Im of the motor 20 is detected by the motor current detection means 38, and is input to the subtraction unit 32B and fed back.

  The compensation unit 34 adds the detected or estimated self-aligning torque (SAT) to the inertia compensation value 342 by the addition unit 344, and further adds the convergence control value 341 to the addition result by the addition unit 345, and the addition The result is input to the adder 32A as a compensation signal CM to improve the characteristics.

  When the motor 20 is a three-phase motor, the details of the PWM control unit 36 and the inverter 37 are configured as shown in FIG. 3, for example, and the PWM control unit 36 sets the voltage control value Vref for three phases according to a predetermined formula. A duty calculation unit 36A for calculating the PWM duty values D1 to D6, and a gate drive unit 36B for driving the gates of the FETs as drive elements with the PWM duty values D1 to D6 and for turning on / off by compensating for dead time; It consists of For example, a triangular wave modulation signal (carrier) CF is input from the oscillation unit 36C to the duty calculation unit 36A, and the duty calculation unit 36A calculates PWM duty values D1 to D6 in synchronization with the modulation signal CF. The inverter 37 is formed of a three-phase bridge of FETs, and drives the motor 20 by being turned on / off with PWM duty values D1 to D6.

  Note that a motor switch 23 is interposed between the inverter 37 and the motor 20 to cut off the supply of current when the assist control is stopped. The motor 20 may be a brush motor or a brushless motor.

  In such an electric power steering apparatus, in recent years, demands for high output (a current can flow for a long time) and miniaturization (ECU size (heat sink size)) have become strict. As an apparatus that realizes such a requirement, there is a motor control apparatus disclosed in, for example, Japanese Patent Laid-Open No. 9-70195 (Patent Document 1). That is, in the apparatus of Patent Document 1, when the rotation speed of the motor is sufficiently low, the rotation of the motor is considered to be locked by an external force, and the modulation frequency of the PWM signal is switched from the normal 10 kHz to 1.25 kHz. Along with the reduction, the switching frequency of the switching element of the inverter is lowered to reduce the switching loss. For this reason, even if the motor is locked, there is no possibility of sudden heat generation in each switching element of the inverter.

JP-A-9-70195

  However, in the device of Patent Document 1, although the heat generation can be reduced, the frequency to be reduced is too low, that is, it is drastically reduced from 10 kHz to 1.25 kHz (87.5%). coming out. For this reason, it is unreasonable to mount this motor control device on an electric power steering device in which abnormal noise is a major problem. Further, the main purpose of Patent Document 1 is to prevent the motor from being burned out, and it seems that it is not considered to be mounted on the electric power steering apparatus.

  The present invention has been made under the circumstances described above, and an object of the present invention is to switch the frequency of a PWM modulation signal to a low frequency at a low change rate when the load is high (when the input torque is large), and to perform PWM control. It is an object of the present invention to provide a motor control device and an electric power steering device equipped with the motor control device that reduce the switching loss and achieve high output (current can flow for a long time) and miniaturization (ECU size (heat sink size)).

  The present invention relates to a motor control device that includes a control unit that calculates a current command value based on an input torque, drives a motor by a PWM-controlled inverter based on the current command value, and controls a motor system by the control unit, The object of the present invention is to compare the input torque with a predetermined torque threshold value and output a torque determination signal, and to change the frequency of the modulation signal of the PWM control based on the torque determination signal. The modulation frequency variable unit varies the frequency of the modulation signal in accordance with the torque determination signal having the torque threshold as a displacement value, thereby reducing the heat loss of the control unit, increasing the output, and reducing the size. This is achieved by achieving the above.

  Further, the object of the present invention is that the modulation frequency variable unit outputs the modulation signal having a high frequency when the input torque is equal to or less than the torque threshold value, and has a low frequency when the input torque is greater than the torque threshold value. A first oscillator that outputs the modulation signal, or the high frequency is 20 kHz, the low frequency is 15 kHz, or the modulation frequency variable unit oscillates the high frequency modulation signal; A second oscillator that oscillates the low-frequency modulation signal, and a switching unit that switches the high-frequency modulation signal and the low-frequency modulation signal according to the torque determination signal. Effectively achieved.

  The above object is achieved by an electric power steering apparatus equipped with the motor control apparatus and equipped with an ECU that applies an assist force to the steering system of the vehicle based on at least a current command value calculated based on the steering torque.

  According to the motor control device having the modulation frequency variable function of the present invention, the frequency of the modulation signal (carrier) of PWM control is reduced at a low rate of change (about 25%) according to the input torque (at the time of high input torque), The switching loss of FET at the time of high output can be reduced, and high output and miniaturization can be realized. In the present invention, the frequency of the modulation signal is reduced to about 3/4 and the rate of change is about 25%.

  By mounting the motor control device having the modulation frequency variable function on the electric power steering device, it is possible to increase the output of the ECU and reduce the ECU size. In the electric power steering apparatus, there is no generation of abnormal noise that is particularly strictly required, and there is a great effect.

It is a lineblock diagram showing an outline of an electric power steering device. It is a block diagram which shows the structural example of the control system of an electric power steering apparatus. It is a connection diagram which shows the structural example of a PWM control part and an inverter. It is a block diagram which shows the structural example of this invention. It is a characteristic view which shows the operation principle of this invention. It is a block diagram which shows an example of the oscillation part which concerns on this invention. It is a flowchart which shows the operation example of this invention.

  The present invention has a modulation frequency variable function that changes the frequency of a modulation signal (carrier) of PWM control at a low change rate in accordance with input torque (for example, steering torque). When the input torque becomes greater than or equal to a predetermined threshold value, the modulation signal is switched from a high frequency (for example, 20 kHz) modulation signal to a low frequency (for example, 15 kHz) to reduce the heat loss of the switching element and the ECU. And the like, and a motor control device that achieves high output and miniaturization of the control unit and the like, and an electric power steering device equipped with the motor control device.

  Since the rate of change of the modulation signal is as low as about 25%, no abnormal noise is generated and the control unit is effectively increased in output and size.

  Embodiments of the present invention will be described below with reference to the drawings.

FIG. 4 shows an example of the configuration of the present invention corresponding to FIG. 2. The steering torque Th is input to the torque determination unit 50, and the steering torque Th is set to a predetermined torque threshold T ₀ (for example, 2. 5Nm). A torque determination signal TD that is a comparison result of the torque determination unit 50 is input to the oscillation unit 60, and the oscillation unit 60 outputs a modulation signal CF having a frequency according to the torque determination signal TD. Modulated signal CF, as shown in FIG. 5, for example, in the range steering torque Th is less than the torque threshold value T ₀ is output oscillates at a high frequency of 20 kHz, 15 kHz in the high range of the steering torque Th is higher torque threshold T ₀ It oscillates at a low frequency and is output.

FIG. 6 shows an example of the oscillating unit 60. The first oscillator 61 that oscillates at a high frequency of 20 kHz, the second oscillator 62 that oscillates at a low frequency of 15 kHz, and the contacts a1 and a2 according to the torque determination signal TD. And a switching unit 63 that outputs a modulation signal CF. The torque determination signal TD is “0” when the steering torque Th is smaller than the torque threshold value T ₀ , thereby setting the contact point of the switching unit 63 as a 1 and “1” when the steering torque Th is larger than the torque threshold value T _0. Thus, the contact point of the switching unit 63 is set to a2. That is, the contacts a1 and a2 of the switching unit 63 are switched by the torque determination signal TD.

  Note that the second oscillator 62 that outputs a low-frequency modulation signal can also be configured by a frequency dividing circuit that divides the high frequency from the first oscillator 61 to reduce the frequency.

  The operation related to the present invention will be described with reference to the flowchart of FIG. 7 in the case where the oscillation unit 60 has the configuration of FIG.

  When the operation starts, the oscillator 61 is oscillated at a frequency of 20 kHz, the oscillator 62 is oscillated at a frequency of 15 kHz (step S1), and the contact of the switching unit 63 is connected to “a1” (step S2). In this state, an oscillation signal of 20 kHz from the oscillator 61 is output as the modulation signal CF.

In this state, the torque determination unit 50 reads a steering torque Th (step S3), and the steering torque Th is equal to or a torque threshold _{T 0} or more (step S4), and the steering torque Th is torque threshold _{T 0} or more Is switched from “a1” to “a2” based on the torque determination signal TD (step S10), and the low frequency of 15 kHz from the oscillator 62 is used as the modulation signal CF, whereby PWM control is performed. Driving is performed (step S11).

Thereafter, the torque determination unit 50 reads a steering torque Th (step S12), the steering torque Th is equal to or a torque threshold _{T 0} or more (step S13), and the steering torque Th is a threshold torque _{T 0} or more In that case, the process returns to step S11 and the above operation is repeated.

In step S13, if the steering torque Th is determined to be smaller than the torque threshold value T _0, it is determined whether further or terminated (step S14), and returns to step S10. If not the end, switching The contact of the unit 63 is switched from “a2” to “a1” (step S10), and the above operation is repeated. If it is determined in step S14 that the process is ended, the process ends.

Further, in step S4, if the steering torque Th is determined to be smaller than the torque threshold value _{T 0} is, to drive the PWM control at a high frequency of 20 kHz (step S5).

  In order to confirm the effect of the present invention, the following tests were conducted.

The calculation conditions of heat loss are as follows: PWM frequency 20kHZ (wavelength 50μs), power supply voltage 14V, average current 30A, FET ON resistance 3mΩ, duty 50%, FET switching time (turn ON / OFF total time) Was 4 μs. And the steady loss Pd_on of FET was calculated by the following formula 1.
(Equation 1)
Pd_on = (average current) ² × FET ON resistance × (ON time × 1/1 / f)
Further, the switching loss Pd_sw of the FET was calculated by the following formula 2.
(Equation 2)
Pd_sw = {(1/6) × power supply voltage × average current}
× (FET switching time × 1/1 / f) × 2
Further, the loss Pd_all of the FET was calculated by the following formula 3.
(Equation 3)
Pd_all = (Pd_on + Pd_sw)

Next, a trial calculation of the case volume was performed. That is, the thermal resistance (case thermal dissipation constant) is used, and the envelope volume and thermal resistance relationship of the aluminum radiator (data: for example, “Transistor Technology”, April 1990 issue and “Electronic Technology”, August 1999 issue. ), The following equation 4 is derived.
(Equation 4)
At 20KHz modulation:
13.9W 6.111511 ° C / W
(150-65) / Pd_all
At 15KHz modulation:
11.1 W 7.65766 ° C./W (thermal resistance)
(150-65) / Pd_all
10 ^{((-0.67474) x LOG10 (volume) +4.024228)} = Thermal resistance

From this, it can be seen that the volume can be considerably reduced by switching and reducing the frequency of the PWM modulation signal.

  In the above description, the frequency of the modulation signal is two types of high and low frequencies, but the modulation frequency of three or more frequencies may be variable.

1 Handle 2 Column shaft (steering shaft, handle shaft)
DESCRIPTION OF SYMBOLS 10 Torque sensor 12 Vehicle speed sensor 13 Battery 20 Motor 30 Control unit (ECU)
31 Current command value calculation unit 35 PI control unit 36 PWM control unit 50 Torque determination unit 60 Oscillation unit 61, 62 Oscillator 63 switching unit

Claims (5)

In a motor control device that calculates a current command value based on an input torque, includes a control unit that drives a motor by an inverter of PWM control based on the current command value, and controls a motor system by the control unit,
A torque determination unit that compares the input torque with a predetermined torque threshold and outputs a torque determination signal; and a modulation frequency variable unit that varies the frequency of the modulation signal of the PWM control based on the torque determination signal;
The modulation frequency variable unit varies the frequency of the modulation signal in accordance with the torque determination signal using the torque threshold as a displacement value, thereby reducing the heat loss of the control unit, increasing the output, and reducing the size. A motor control device. The modulation frequency variable unit outputs the modulation signal having a high frequency when the input torque is equal to or less than the torque threshold, and outputs the modulation signal having a low frequency when the input torque is larger than the torque threshold. The motor control device described in 1. The motor control device according to claim 2, wherein the high frequency is 20 kHz and the low frequency is 15 kHz. The modulation frequency variable unit is
A first oscillator for oscillating the high-frequency modulation signal; a second oscillator for oscillating the low-frequency modulation signal; and the high-frequency modulation signal and the low-frequency modulation signal according to the torque determination signal. The motor control device according to claim 2, further comprising a switching unit that switches between the two. A motor control device according to any one of claims 1 to 4 is mounted, and an ECU is provided that applies an assist force to a steering system of a vehicle based on at least a current command value calculated based on a steering torque. Electric power steering device.

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