JP2005199781A - Power steering device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress generation of radio noise from on-vehicle audio equipment such as a car radio, without taking cost in hardware and software. <P>SOLUTION: This electric power steering device drives an electric motor 5 by a PWM signal according to steering of a steering member 1 to assist the steering. The device is provided with an ON/OFF detection means 20a for detecting ON/OFF of the on-vehicle audio equipment 14, a frequency changing means 20b for setting a PWM frequency to a high frequency side when detecting that the on-vehicle audio equipment 14 is OFF and changing the PWM frequency to a low frequency side when detecting that the equipment 14 is ON, and a PWM signal generation means 24 for generating the PWN signal of the changed PWM frequency. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a power steering apparatus that assists a driver's steering by driving an electric motor with a PWM (pulse width modulation) signal in accordance with an operation of a steering member such as a steering wheel performed by the driver.

  As a vehicle steering device, a so-called electric power steering device that reduces the steering force of a driver by adding a steering assist torque by an electric motor to a steering mechanism during steering of a steering wheel has recently become widespread.

  Among such electric power steering devices, there is one that drives an electric motor by a PWM signal. The device is usually composed of an electric motor, various sensors such as a torque sensor for detecting steering torque and a vehicle speed sensor for detecting the speed (vehicle speed) of the vehicle, and an electronic control unit (ECU). Includes at least a microcomputer, a plurality of sensor signal processing circuits for processing sensor signals from various sensors, a PWM signal generation circuit, and a motor drive circuit including a plurality of power MOSFETs.

  In the electric power steering apparatus described above, the sensor signal of each sensor is processed by the sensor signal processing circuit in the ECU, and then taken into the microcomputer. The microcomputer uses the built-in steering assist program and According to the steering assist data, the motor drive amount necessary to generate the torque suitable for the steering assist is calculated, and a duty ratio control signal corresponding to the drive amount is generated and output to the PWM signal generation circuit. To do. The PWM signal generation circuit generates a PWM signal having a duty ratio corresponding to the duty ratio control signal, applies the PWM signal to the motor drive circuit, and switches each power MOSFET constituting the motor drive circuit to perform an electric motor. Drive.

  In the above case, the PWM signal includes a high frequency component of the Nth order (N times) of the fundamental frequency due to the steep rise and fall waveforms, and when the electric motor is PWM driven by switching the power MOSFET. In addition, high frequency components are emitted with high energy. When the frequency of the radiated Nth-order high-frequency component is close to the channel frequency of the AM broadcast that has been selected and received by the car radio, the Nth-order high-frequency component is mixed into the selected and received AM broadcast, and the high frequency Radio noise corresponding to the deviation between the component and the AM broadcast channel frequency is generated.

  For electromagnetic radiation (EMI) in which radio noise is generated, the pattern shape of power lines and signal lines on a printed circuit board can be changed, and high frequency current suppression components such as ferrite beads and coils can be mounted. These measures are often based on empirical rules, and it is difficult to predict the reduction effect, and the cost increases.

  As a technique for suppressing radio noise in the electric power steering device in terms of software, the electric motor is automatically changed by automatically changing the frequency of the PWM signal in accordance with the channel frequency of the AM broadcast received by the car radio. There has also been proposed a technique that can prevent N-order high-frequency components radiated during PWM driving from being mixed into AM broadcasting and generating noise (see, for example, Patent Document 1).

However, the above-mentioned software countermeasures are complicated in terms of software processing, and it is necessary to add a hardware configuration, which is costly. In addition, when an in-vehicle audio device such as a car radio is not operating and the interior of the vehicle is relatively quiet, it is difficult to effectively suppress the generation of switching operation sound due to the PWM signal.
Japanese Patent Laid-Open No. 10-327597

  Therefore, the present invention can suppress the generation of radio noise from an in-vehicle audio device such as a car radio without cost in terms of hardware and software.

  A power steering apparatus according to the present invention is a power steering apparatus that assists steering by driving an electric motor with a PWM signal in accordance with steering of a steering member, and includes an on / off detection unit that detects on / off of an in-vehicle audio device, and an in-vehicle audio device. Frequency change means for setting the frequency (PWM frequency) of the PWM signal to the high frequency side when it is detected that the signal is off, and changing the PWM frequency to the low frequency side when it is detected to be on, and the changed PWM PWM signal generating means for generating a PWM signal having a frequency is provided.

  According to the present invention, when the on-off detection means detects that the in-vehicle audio device is off, the frequency changing means changes the PWM frequency to the high frequency side, for example, the high frequency side outside the audible frequency band, It can be driven by an external high PWM frequency PWM signal. As a result, the switching operation sound generated from the motor drive circuit cannot be heard by the driver or the like in the passenger compartment, and the noise is not felt.

  When the on / off detection means detects that the on-vehicle audio device such as a car radio is turned on, the frequency changing means changes the PWM frequency to the low frequency side, for example, the low frequency side of the audible band, and the electric motor is changed to the audible frequency band. It can be driven by a PWM signal having a PWM frequency. At this time, even if a high-frequency component is radiated, since the fundamental frequency of the high-frequency component is an audible band, which is a low frequency, the high-frequency component is lower than the channel frequency of radio waves such as AM broadcasting, and has been selected and received. There is no mixing in the broadcast. As a result, the driver or the like can enjoy broadcasting such as AM without radio noise.

  In this case, although the switching operation sound is generated at the PWM frequency of the audible band, the switching operation sound is erased by sound such as music flowing from the vehicle-mounted audio device, so that it is hardly audible to the driver or the like.

  When the in-vehicle audio equipment is turned on, it is preferable to change the PWM frequency to the low frequency side and then modulate and disperse the PWM frequency to suppress unnecessary high-frequency radiation.

  Further, when the on-vehicle audio device is turned on, it is preferable to remove the high frequency component from the PWM signal in order to reduce the emission of the high frequency component and suppress the generation of radio noise.

  ADVANTAGE OF THE INVENTION According to this invention, generation | occurrence | production of the radio noise from vehicle-mounted audio equipment, such as a car radio, can be suppressed, without incurring cost in hardware and software.

  Hereinafter, an electric power steering apparatus according to the best mode of the present invention will be described with reference to the drawings. FIG. 1 is a schematic view showing the electric power steering apparatus together with a vehicle configuration related thereto.

  This electric power steering apparatus includes a steering shaft 2 having one end fixed to a steering wheel (steering wheel) 1 as steering means for steering, a rack and pinion mechanism connected to the other end of the steering shaft 2, and the like. A mechanism 3, a torque sensor 4 for detecting a steering torque applied to the steering shaft 2 by the operation of the handle 1, and an electric motor 5 for generating a steering assist force for reducing a driver's load due to the steering operation (steering operation). And a reduction gear 6 for transmitting a steering assist force generated by the electric motor 5 to the steering shaft 2 and a power supply from an in-vehicle battery 7 via an ignition switch 8, and a torque sensor 4, a vehicle speed sensor 9 and an engine rotation sensor. Based on the sensor signal from 10, the drive of the electric motor 5 Controlling the and an electronic control unit (ECU) 11.

  When the driver operates the steering wheel 1 in a vehicle equipped with the electric power steering device, the steering torque by the operation is detected by the torque sensor 4, and the detected steering torque and the vehicle speed detected by the vehicle speed sensor 9 are detected. Based on the above, the electric motor 5 is driven by the ECU 11. As a result, the electric motor 5 generates a steering assist force, and this steering assist force is applied to the steering shaft 2 via the reduction gear 6, thereby reducing the burden on the driver due to the steering operation.

  That is, the sum of the steering torque applied by the steering operation and the torque (steering assist torque) by the steering assist force generated by the electric motor 5 is given to the steering mechanism 3 via the steering shaft 2 as an output torque. As a result, when the input shaft of the steering mechanism 3 rotates, the rotation is converted into a reciprocating motion of the rack shaft by the steering mechanism 3. Both ends of the rack shaft are connected to a wheel 13 via a connecting member 12 including a tie rod and a knuckle arm, and the direction of the wheel 13 changes according to the reciprocating motion of the rack shaft.

  In the electric power steering apparatus having the above-described configuration, the ECU 11 includes a motor drive circuit that includes a plurality of power MOSFETs and that controls the drive of the electric motor 5 using a PWM signal (PWM drive). In this motor drive circuit, in general, the frequency (PWM frequency) of the PWM signal is set to about 16 kHz, avoiding the audible band, so that noise generated when the power MOSFET is switched by the PWM signal does not cause noise. . However, on the other hand, since this PWM frequency is a high frequency, a high-order high-frequency component is radiated and can be a cause of radio noise from the in-vehicle audio device 14 such as a car radio.

  Therefore, in the present embodiment, the output state of the switch means 15 (which may be installed in the in-vehicle audio device 14 or externally attached) linked to the on / off (ON / OFF) of the in-vehicle audio device 14 is taken into the ECU 11, The microcomputer in the ECU 11 detects the on / off state of the in-vehicle audio device 14 and, as a result of detection, when the in-vehicle audio device 14 is detected to be off, the PWM frequency is set to the high frequency side exceeding the audible band and turned on. When the in-vehicle audio device 14 is listened to, the generation of radio noise can be suppressed by changing the PWM frequency to the low frequency side of the audible band.

  Regarding on / off of the in-vehicle audio device 14, for example, a state in which the car radio power switch is turned off and radio broadcasting cannot be listened to by the car radio is referred to as when the in-vehicle audio device 14 is off, and the car radio power switch is turned on. The state in which the radio broadcast can be listened to by the car radio is referred to as the on-vehicle audio device 14 being on. The switch means 15 may be configured to be interlocked with the power switch of the car radio, and the potential of the contact on the in-vehicle audio device 14 side of the switch means 15 is applied or not applied to the contact on the ECU 11 side via the switch means 15. In the ECU 11, it is preferable that the on / off state of the in-vehicle audio device 14 can be detected from the potential change of the contact on the switch means 15 side.

  The configuration of the ECU 11 will be described in detail with reference to FIG. 2. The ECU 11 is provided corresponding to each of the microcomputer 20, the torque sensor 4, the vehicle speed sensor 9, and the engine rotation sensor 10, and sends sensor signals to the microcomputer 20. Signal processing circuits 21, 22, and 23 as interface means for processing in a form suitable for the above processing, and PWM signal generation for generating a PWM signal Sc having a duty ratio corresponding to the duty ratio control signal Sa from the microcomputer 20 A circuit (PWM signal generation means) 24, a motor drive circuit (motor drive means) 25 comprising a plurality of power MOSFETs, a power supply circuit (power supply means) 26 for supplying power to each circuit in the ECU 11, and PWM signal generation The low frequency component of the PWM signal Sc output from the circuit 24 is passed. A low-pass filter (low-pass means) 27 that blocks the passage of high-frequency components, a side that directly connects the PWM signal generation circuit 24 to the motor drive circuit 25, and an indirect connection to the motor drive circuit 25 via the low-pass filter 27. And a changeover switch (switching means) 28 for switching to the connection side.

  The microcomputer 20 is not particularly shown in hardware, but includes a CPU that controls the control of the electric power steering device, a program memory that contains a control program for the electric power steering device, an assist table memory that will be described later, and the like. And a data memory for storing data necessary for electric power steering control, and the CPU functions as a function processing unit by executing various programs for control and calculation stored in the program memory. .

  The program memory of the microcomputer 20 calculates the duty ratio of the PWM signal Sc based on the sensor signals from the torque sensor 4 and the vehicle speed sensor 9 that are given by the processing of the sensor signal processing circuits 21, 22, and 23, respectively. A calculation program is stored. The CPU of the microcomputer 20 includes calculation means 20e for executing a required calculation in accordance with the calculation program, and the data memory has an assist table memory 20f for storing an assist table necessary for executing the calculation of the calculation means 20e. Prepare. The calculation means 20e generates a duty ratio control signal Sa that determines the duty ratio of the PWM signal Sc of the PWM signal generation circuit 24 in accordance with the calculation program.

  That is, the calculation means 20e calculates a target current value to be supplied to the electric motor 5 based on the steering torque information obtained from the sensor signal of the torque sensor 4, the vehicle speed information obtained from the sensor signal of the vehicle speed sensor 9, and the like. To do. The assist table memory 20f stores an assist table indicating the relationship between the target current value to be supplied to the electric motor 5 and the steering torque in order to generate an appropriate steering assist force using the vehicle speed as a parameter. Reference numeral 20e refers to this assist table for setting the target current value. The computing means 20e detects the current actually flowing from the motor drive circuit 25 to the electric motor 5 via the motor current detection circuit 29, and calculates the deviation between the detected current value (detected current value) and the target current value. Calculate. Then, a necessary control calculation such as a proportional integration control calculation is performed so that the target current value flows to the electric motor 5 with this deviation being zero, and a duty ratio control signal Sa is generated in accordance with the calculation result, The duty ratio control signal Sa is output to the PWM signal generation circuit 24.

  The CPU of the microcomputer 20 also functionally responds to the on / off detection means 20a for detecting the on / off state of the in-vehicle audio device 14 based on the potential state of the contact on the ECU 11 side of the switch means 15 and the on / off state of the in-vehicle audio device 14. Frequency changing means 20b for outputting a command to change the PWM frequency to outside the high frequency audible band and to the low frequency audible band, and the changeover switch 28 to the low pass filter 27 side in response to the frequency changing command from the frequency changing means 20b. A waveform blunting command means 20c for giving a command for switching and blunting the waveform of the PWM signal Sc, and a PWM frequency output signal on the high frequency side (for example, 16 kHz) in response to a frequency change command to the high frequency side of the frequency changing means 20b Sb is output, and PWM on the low frequency side (for example, 9 kHz) according to the frequency change command to the low frequency side Frequency output means 20d for outputting a frequency output signal Sb. When outputting the low frequency PWM frequency output signal Sb, the frequency output means 20d modulates the 9 kHz PWM frequency within a range of ± 1 kHz. The PWM frequency output signal Sb from the frequency output means 20d is given to the PWM signal generation circuit 24 as determining the PWM frequency.

  The PWM signal generation circuit 24 can determine the PWM frequency of the PWM signal Sc based on the PWM frequency output signal Sb from the frequency output means 20d of the microcomputer 20 and PWM based on the duty ratio control signal Sa from the calculation means 20e of the microcomputer 20. The duty ratio of the signal Sc can be determined. Therefore, the PWM frequency of the PWM signal Sc from the PWM signal generation circuit 24 is 16 kHz on the high frequency side when the vehicle-mounted audio device 14 is turned off, and the duty ratio corresponds to an appropriate steering assist force. When the device 14 is turned on, it is 9 kHz on the low frequency side (however, this low frequency is frequency-modulated as described later), and its duty ratio corresponds to an appropriate steering assist force.

  When the electric motor 5 is a three-phase brushless motor, the waveform of the PWM signal Sc that drives the brushless motor includes a sine wave, a rectangular wave, and a trapezoidal wave. In this embodiment, the power in the motor drive circuit 25 is In order to reduce the loss of the MOSFET, for example, a rectangular wave is used so that the power MOSFET performs a switching operation in a saturation region.

  Although the PWM signal generation circuit 24 is shown as being externally attached to the microcomputer 20, it is not necessarily required to be externally attached to the microcomputer 20, and may be configured by software inside the microcomputer 20.

  Although the motor drive circuit 25 is not specifically illustrated, for example, if the electric motor 5 is a three-phase brushless motor, two power MOSFETs connected in series between the in-vehicle battery 7 and the ground are connected. The U-phase winding of the brushless motor is connected to the common connection point, the V-phase winding of the brushless motor is connected to the common connection point of the other two power MOSFETs, and the common connection point of the other two power MOSFETs The three-phase inverter circuit configuration is formed by connecting the W-phase winding of the brushless motor to the motor. The motor drive circuit 25 is not limited to the above-described known configuration, but may be any other known configuration as long as the circuit configuration drives the electric motor 5. In addition, the electric motor 5 of the present embodiment is not limited to the brushless motor described above, but may be a motor with a brush or other types of known electric motors.

  The low-pass filter 27 is a filter for removing the high frequency component from the PWM signal Sc whose PWM frequency is set to the low frequency side when the vehicle-mounted audio device 14 is turned on, and blunting the rising and falling of the waveform of the PWM signal Sc. . The low-pass filter 27 is externally attached to the microcomputer 20, but is not necessarily externally attached to the microcomputer 20, and may be configured by software inside the microcomputer 20.

  In response to the presence / absence of a command signal for blunting the waveform of the PWM signal Sc from the waveform blunting command means 20 c of the microcomputer 20, the changeover switch 28 indirectly connects the PWM signal generation circuit 24 via the low-pass filter 27. Thus, the switching movable contact 28a switches between the individual contacts 28b and 28c so as to be connected to the motor drive circuit 25 side or directly to the motor drive circuit 25 side. The changeover switch 28 is externally attached to the microcomputer 20, but is not necessarily externally attached, and may be configured as software inside the microcomputer 20. The changeover switch 28 may be a mechanical type or an electronic type.

  The operation of the ECU 11 having the above configuration will be described with reference to the control program shown in FIG.

  In step S1, the microcomputer 20 detects the on / off state of the in-vehicle audio device 14 periodically or periodically. That is, in step S <b> 1, the microcomputer 20 determines whether or not the in-vehicle audio device 14 is turned off based on a potential change accompanying opening / closing of the switch unit 15. This step S1 constitutes the function of the on / off detection means 20a of the microcomputer 20.

  When the microcomputer 20 determines from the state of the switch means 15 that the in-vehicle audio device 14 is off, the microcomputer 20 proceeds to step S2. In step S2, the microcomputer 20 sets the PWM frequency to a high frequency of 16 kHz outside the audible band, and outputs a PWM frequency output signal Sb corresponding to the PWM frequency. Such a PWM frequency output signal Sb is obtained by dividing the clock pulse of the microcomputer 20 by a predetermined dividing ratio. This step S2 constitutes the function of the frequency changing means 20b of the microcomputer 20 and the function of the frequency output means 20d.

  In the next step S <b> 3, the microcomputer 20 outputs a command for switching the changeover switch 28 to the motor drive circuit 25 side to the changeover switch 28. By this step S3, the switching movable contact 28a of the changeover switch 28 is switched to the individual contact 28b, and the output portion of the PWM signal generating circuit 24 is directly connected to the motor drive circuit 25.

  As described above, the PWM signal generation circuit 24 has an appropriate PWM frequency outside the audible band in accordance with the duty ratio control signal Sa from the computing means 20e of the microcomputer 20 and the PWM frequency output signal Sb from the frequency output means 20d. A PWM signal Sc having a high duty ratio is generated, whereby the motor drive circuit 25 drives the electric motor 5 in accordance with the PWM signal Sc.

  On the other hand, when the microcomputer 20 determines in step S1 that the in-vehicle audio device 14 is not off, that is, is on, from the state of the switch means 15, the process execution procedure proceeds to step S4. In step S4, the microcomputer 20 changes the PWM frequency to a low frequency of 9 kHz and outputs a PWM frequency output signal Sb corresponding to the PWM frequency. This step S4 constitutes the function of the frequency changing means 20b of the microcomputer 20 and the function of the frequency output means 20d.

  In the next step S5, the microcomputer 20 slightly varies the PWM frequency and modulates the PWM frequency by, for example, 9 kHz ± 1 kHz to disperse the high frequency component of the PWM signal Sc. It can be reduced. This is known as SSCG (Spread Spectrum Clock Generator), and is useful as a measure against EMI radiation accompanying the increase in speed and density of electronic devices. The spectrum component of EMI radiation is a high-order component of the fundamental frequency of the PWM signal Sc and indicates a peak value. There is a restriction on EMI, and on the assumption that this restriction is observed, in the present embodiment, as described above, the PWM frequency is slightly changed when the vehicle-mounted audio device 14 is turned on, for example, 9 kHz ± 1 kHz. Thus, the high frequency component of the high frequency component of the PWM signal Sc can be reduced by frequency modulating and dispersing the PWM frequency.

  In step S <b> 6, the microcomputer 20 outputs a command for switching and driving the changeover switch 28 to the low-pass filter 27 side to the changeover switch 28. By this command, the switch 28 is connected to the motor drive circuit 25 via the switching movable contact 28a filter 27. This step S6 constitutes the function of the waveform blunting command means 20c of the microcomputer 20.

  When the PWM signal generation circuit 24 is connected to the low pass filter 27 side, the rise and fall of the waveform of the PWM signal Sc is blunted by the low pass filter 27. In this case, even if the waveform of the PWM signal Sc is dull, since the PWM frequency is low, the switching loss of the power MOSFET of the motor drive circuit 25 is reduced and the heat generation of the power MOSFET is suppressed, while the PWM By removing the high frequency component of the frequency, a noise source for the in-vehicle audio device 14, for example, a car radio is suppressed.

  As described above, when the in-vehicle audio device 14 is turned on, the duty ratio control signal Sa from the computing unit 20e of the microcomputer 20 and the PWM frequency output signal Sb from the frequency output unit 20d are input to the PWM signal generation circuit 24. . The PWM signal generation circuit 24 generates a PWM signal Sc in accordance with both the signals Sa and Sb. This PWM signal Sc is given to the motor drive circuit 25 via the low-pass filter 27, and the electric motor 5 is driven.

  The present invention can also be applied to a hydraulic power steering device that assists steering by driving a pump by an electric motor to supply pressure oil to a hydraulic actuator.

The figure which shows schematically the electric power steering apparatus which concerns on the best form for implementing this invention, and the vehicle structure relevant to it. Control block diagram of electric power steering device Control flow diagram of ECU of electric power steering device

Explanation of symbols

1 Steering wheel (handle)
2 Steering shaft 3 Steering mechanism 4 Torque sensor 5 Electric motor 7 In-vehicle battery 8 Ignition switch 9 Vehicle speed sensor 10 Engine rotation sensor 11 Electronic control unit (ECU)
14 on-vehicle audio equipment 15 switch means 20 microcomputer 20a on / off detection means 20b frequency change means 20c waveform blunt command means 21, 22, 23 sensor signal processing circuit 24 PWM signal generation circuit (PWM signal generation means)
25 Motor drive circuit 26 Power supply circuit 27 Low-pass filter 28 selector switch

Claims (4)

A power steering device that assists steering by driving an electric motor by a PWM signal according to steering of a steering member,
On / off detection means for detecting on / off of the in-vehicle audio device,
A frequency changing means for setting the frequency of the PWM signal (PWM frequency) to the high frequency side when detecting that the in-vehicle audio device is off, and changing the PWM frequency to the low frequency side when detecting that it is on;
PWM signal generating means for generating a PWM signal of the changed PWM frequency,
A power steering apparatus comprising:   The power steering apparatus according to claim 1, wherein the PWM frequency is set to a frequency exceeding an audible band when the on-vehicle audio device is turned off, and the PWM frequency is set to a frequency of the audible band when the on-vehicle audio device is turned on.   The power steering apparatus according to claim 1 or 2, wherein the PWM frequency is frequency-modulated and dispersed when the on-vehicle audio device is turned on.   The power steering apparatus according to any one of claims 1 to 3, wherein a high-frequency component is removed from the PWM signal when the on-vehicle audio device is turned on.

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