JP2009166633A - Electric power steering device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electric power steering device capable of reducing a load applied to a steering mechanism while reducing discomfort felt by a driver by restricting a duty ratio of a PWM (Pulse Width Modulation) signal. <P>SOLUTION: This electric power steering device is provided with: an assist control part 6 which determines a target assist force according to a steering torque applied to a steering wheel 30 and generating the PWM signal of the duty ratio determined for generating the determined target assist force; and a motor drive circuit 7 which actuates a motor 5 for the steering assist according to the PWM signal generated by the assist control part 6, wherein the duty ratio of the PWM signal determined according to the steering torque is restricted by an upper limit determined according to the battery voltage value detected by a battery sensor 9. The upper limit is so set as to decrease with an increase in the battery voltage value to prevent the moving speed of a rack shaft 10 from getting excessive and reduce the load applied to the steering mechanism 1. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an electric power steering device that assists steering by driving a motor in accordance with an operation of a steering member and applying a rotational force generated by the motor to a steering mechanism.

  An electric power steering device that drives a steering assist motor in response to a rotation operation of a steering member such as a steering wheel and assists the steering by adding the rotation of the motor to a steering mechanism is used to generate a steering torque applied to the steering member. The target assisting force is detected based on the detected steering torque, and the steering assist motor is driven and controlled to generate the determined target assisting force.

  The assist control unit that drives and controls the steering assist motor determines the target assist force as described above, sets the target value of the drive current of the motor, and determines the pulse width modulation signal of the duty ratio determined to obtain the target value. (Hereinafter referred to as a PWM signal) is generated, and this PWM signal is provided to the motor drive circuit. The motor drive circuit includes a plurality of field effect transistors (hereinafter referred to as FETs), and opens and closes these FETs in response to the PWM signal to control the magnitude of the drive current of the motor. The rotational force generated by the motor is controlled.

  In general, an in-vehicle battery is used as a power source for the motor drive circuit. The voltage of the in-vehicle battery varies according to aging and the driving state of the vehicle. For example, in the case of an in-vehicle battery having a standard voltage of 12V, the voltage may drop to about 9V. The in-vehicle device including the electric power steering device is designed so as to ensure a predetermined performance with a fluctuation range of 9 to 12V.

  However, in-vehicle batteries are replaced with aging. Although the voltage before the replacement of the in-vehicle battery is generally lowered, the voltage of the new in-vehicle battery immediately after the replacement is about 16V. Therefore, when the on-board battery is replaced, the voltage difference before and after replacement is large. Therefore, in the case of an electric power steering device, when the voltage supplied to the motor drive circuit changes according to this voltage change, the same PWM The rotational force generated by the motor driven in accordance with the signal is greatly different, and the difference in the steering assist force applied to the steering mechanism becomes significant, which causes a problem that the driver feels uncomfortable.

  In the steering device, the maximum steering angle that can be realized by the operation of the steering member is determined in advance, and the operation of the steering member exceeding the position corresponding to the maximum steering angle (hereinafter referred to as the lock position) is mechanically performed. A locking means for limiting is generally provided. For example, in a rack and pinion type steering mechanism that performs steering by converting rotation of the pinion shaft according to the rotation of the steering member into movement in the axial direction of the rack shaft, the lock shaft can be moved freely. The rack housing that is supported on the rack is provided with a stopper portion, and a part of the rack shaft that has reached the movement position corresponding to the lock position is brought into contact with the stopper portion to mechanically restrain the movement of the rack shaft. Has been.

  For example, immediately after replacing the on-board battery, the voltage supplied to the motor drive circuit becomes high voltage, and the generated force of the motor increases, so the movement speed of the rack shaft becomes excessive, and this excessive movement speed Since the rack shaft hits the stopper portion, the stopper portion and the rack shaft may be deteriorated or damaged.

An electric power steering device has been proposed that focuses on problems caused by fluctuations in the voltage of the vehicle battery (see, for example, Patent Document 1). In the electric power steering device disclosed in Patent Document 1, when the ignition switch is turned on, the PWM gain (= motor applied voltage / duty ratio) is set so as to increase or decrease according to the voltage level of the on-vehicle battery. By defining and changing the proportional gain in the PID control of the current feedback control system according to the determined PWM gain, the product of the PWM gain and the proportional gain is made constant, and the gain of the entire current feedback control system is constant It is supposed to be.
JP-A-8-107696

  In the electric power steering device of Patent Document 1 configured as described above, since the gain of the entire current feedback control system is constant, it is determined according to the steering torque even when the voltage of the in-vehicle battery greatly fluctuates. When the target value of the motor drive current is the same, the rotational force generated by the steering assist motor is equal. As a result, when the voltage of the in-vehicle battery greatly fluctuates, such as immediately after replacing the in-vehicle battery, it is not necessary to give the driver a sense of incongruity due to the fluctuation in the magnitude of the steering assist force, and the rack shaft movement speed is excessive. The deterioration or damage of the stopper portion and the rack shaft as described above can be reduced.

  However, in the electric power steering apparatus of Patent Document 1, since the proportional gain of the PID control is also changed in accordance with the change of the PWM gain, the drive current of the motor depends on the steering torque after the steering torque is applied. The responsiveness until reaching the target value determined in this way, that is, the steering responsiveness changes according to the voltage fluctuation of the in-vehicle battery, especially before and after replacement of the in-vehicle battery in which the voltage difference of the in-vehicle battery becomes significant. Since the change in the steering response is remarkable, there is a possibility that the driver may feel uncomfortable. Further, the gain is set when the ignition switch is turned on, and the steering assist force fluctuates when the voltage of the in-vehicle battery fluctuates during traveling, which may cause the driver to feel uncomfortable.

  The present invention has been made in view of such circumstances, and the steering assist force applied to the steering mechanism is reduced by limiting the duty ratio of the pulse width modulation signal by the upper limit value determined according to the voltage of the vehicle-mounted battery. An object of the present invention is to provide an electric power steering device capable of reducing the load on the steering mechanism while reducing the uncomfortable feeling given to the driver.

  The electric power steering device according to the first aspect of the present invention is a control for determining a target auxiliary force according to a steering torque applied to the steering member and generating a pulse width modulation signal having a duty ratio determined to generate the determined target auxiliary force. And a motor drive circuit for driving a motor for assisting steering in accordance with the pulse width modulation signal generated by the control means, and a voltage for detecting a voltage supplied to the motor drive circuit A detector is provided, and the control means is configured to limit the duty ratio by an upper limit value corresponding to a voltage detected by the voltage detector.

  The electric power steering apparatus according to a second aspect of the present invention is the electric power steering apparatus according to the first aspect, wherein the control means makes the upper limit value small / large according to high / low of the voltage detected by the voltage detector. It is characterized by being.

  The electric power steering apparatus according to a third aspect of the present invention is the electric power steering apparatus according to the first or second aspect, wherein the voltage detector is interposed between a positive electrode and a negative electrode of the in-vehicle battery, and detects an output voltage of the in-vehicle battery. It is characterized by being.

  According to the first invention, the upper limit value is determined based on the detection result of the voltage supplied to the motor drive circuit that drives the steering assist motor, and the duty ratio of the pulse width modulation signal determined according to the steering torque is determined. The steering assist force applied to the steering mechanism is limited by, for example, limiting the duty ratio to be equal to or less than a predetermined value when the voltage of the on-vehicle battery is high. By limiting, the load on the steering mechanism can be reduced. Since the duty ratio is limited only when the duty ratio of the pulse width modulation signal is larger than the upper limit value, the steering response does not change, and the driver feels uncomfortable due to this. I'm sorry.

  According to the second aspect of the invention, the upper limit value of the duty ratio of the pulse width modulation signal is made small / large according to the detected high / low voltage, and the upper limit value of the duty ratio changes continuously according to the voltage. By setting so as to do so, it is possible to prevent the driver from feeling uncomfortable when the voltage of the in-vehicle battery fluctuates during traveling according to the driving state of the vehicle.

  According to the third invention, since the voltage detector is interposed between the positive electrode and the negative electrode of the in-vehicle battery, the duty ratio is appropriately limited without being affected by the harness connecting the in-vehicle battery and the motor drive circuit. Can be implemented.

  Hereinafter, the present invention will be described in detail with reference to the drawings illustrating embodiments thereof. FIG. 1 is a schematic diagram showing a configuration of an electric power steering apparatus according to the present invention including a rack and pinion type steering mechanism.

  A rack and pinion type steering mechanism 1 includes a rack shaft 10 that is supported in an axially movable manner in a rack housing 11 that extends in the left-right direction of a vehicle body (not shown), and crosses the rack housing 11 in the middle. And a pinion shaft 2 rotatably supported inside the pinion housing 20.

  Both ends of the rack shaft 10 projecting outward from both sides of the rack housing 11 are connected to left and right front wheels 13 and 13 as steering wheels via separate tie rods 12 and 12, respectively. Further, the upper end of the pinion shaft 2 protruding outside from the pinion housing 20 is connected to a steering wheel 30 as a steering member via the steering shaft 3. A pinion (not shown) is integrally formed at a lower portion of the pinion shaft 2 extending inside the pinion housing 20, and the pinion is formed in an intermediate portion of the rack shaft 10 at an intersection with the rack housing 11. Meshed with the rack.

  The steering shaft 3 is rotatably supported inside a cylindrical housing 31 and is attached to the interior of a passenger compartment (not shown) with the front facing down through the housing 31. The pinion shaft 2 is connected to the lower end of the steering shaft 3 that projects downward from the housing 31, and the steering wheel 30 is fixed to the upper end of the steering shaft 3 that projects upward from the housing 31.

  With the above configuration, the steering shaft 3 is rotated by the rotation operation of the steering wheel 30 for steering, and this rotation is transmitted to the pinion shaft 2, and the rotation of the pinion shaft 2 is performed at the meshing portion of the pinion and the rack. The movement of the shaft 10 is converted into the movement in the axial length direction, and such movement of the rack shaft 10 is transmitted to the left and right front wheels 13 and 13 via the separate tie rods 12 and 12 for steering.

  In the middle of the housing 31 that supports the steering shaft 3, a torque sensor 4 that detects a steering torque applied to the steering shaft 3 by a rotation operation of the steering wheel 30 is provided. A steering assist motor 5 is attached below the torque sensor 4.

  The steering assisting motor 5 is attached to the outside of the column housing 31 with its axis substantially orthogonal, and, for example, a worm fixed to an output end extending inside the column housing 31 is externally fixed to the steering shaft 3. The rotation of the motor 5 is decelerated by the worm and the worm wheel and transmitted to the steering shaft 3 to assist the steering performed as described above.

  The steering assisting motor 5 attached in this way is driven in accordance with a control command given from the assist control unit 6 via the motor drive circuit 7. FIG. 2 is a block diagram showing the configuration of the control system of the electric power steering apparatus according to the present invention shown in FIG. A steering torque value T detected by the torque sensor 4 and a vehicle speed value V detected by the vehicle speed sensor 8 are respectively given to the assist control unit 6. In FIG. 2, the assist control unit 6 is represented by functional blocks. The assist control unit 6 includes a CPU, a ROM, and a RAM connected by an internal bus, and the CPU according to the control program stored in the ROM. The ECU is configured to perform the following assist control by operation.

  The steering torque value T is given to the phase compensation unit 61 of the assist control unit 6. In order to stabilize the control system, the phase compensation unit 61 performs, for example, a phase compensation process such as advancing the phase with respect to the steering torque value T given by the torque sensor 4, and performs the phase compensation process. The obtained steering torque value T is given to the target drive current calculator 62.

  Further, the vehicle speed value V is given to the target drive current calculation unit 62. The target drive current calculation unit 62 has an assist control map showing the relationship between the steering torque value T and the target drive current value I, and applies the given steering torque value T and vehicle speed value V to this assist control map. Thus, the target drive current value I is obtained, and the obtained target drive current value I is given to the subtractor 63. As shown in the figure, in the assist control map, when the steering torque value T exceeds a predetermined dead zone, the target drive current value I increases in proportion to the increase in the steering torque value T, and the steering torque value T The target drive current value I is set to be a constant value in a region above the predetermined value. This assist control map is determined for each vehicle speed value V, and is set such that the increase rate of the target drive current value I changes according to the change of the vehicle speed value V.

A subtractor 63, a motor current detection value I _s of the motor current sensor 50 for detecting a current flowing through the motor 5 for steering assistance has output is also provided. Subtractor 63 obtains the deviation between the output value of the target drive current calculation section 62 target drive current value I and the motor current detection value I _s, giving a deviation to the PWM signal generator 64. The PWM signal generation unit 64 performs a PI calculation on the given deviation, generates a PWM signal based on the calculation result, and supplies the PWM signal to the PWM limiting unit 65.

The PWM limiter 65 is also given a battery voltage value V _b detected by the battery sensor 9 provided in the in-vehicle battery 90. In the present embodiment, the battery sensor 9 provided in the in-vehicle battery 90 is also used as a voltage detector that detects the voltage supplied to the motor drive circuit 7. The battery sensor 9 can be directly disposed on the negative electrode of the in-vehicle battery 90, detects the voltage, current and temperature of the in-vehicle battery 90 with high accuracy, and uses these detection values to determine the charge state of the in-vehicle battery 90. The deterioration state can be monitored.

The PWM limiter 65 has a PWM limit map indicating the relationship between the upper limit value _Dmax of the duty ratio of the PWM signal and the battery voltage value _Vb . FIG. 3 is an example of a PWM limit map.

The horizontal axis in FIG. 3 indicates the battery voltage value V _b (V), and the vertical axis indicates the upper limit value D _max (%) of the duty ratio of the PWM signal. As illustrated, the upper limit value D _max of the duty ratio is the battery voltage value V _b is the reference voltage value reference value in the following areas (12V in the figure) (95% in the figure), the battery voltage value V _In a region where _b is greater than or equal to the reference voltage value, the battery voltage value V _b decreases proportionally, and the maximum value D _max of the duty ratio at the maximum battery voltage value V _b (16 V in the figure). Is set to a predetermined value (85% in the figure).

PWM limiting unit 65 obtains the upper limit value D _max of the duty ratio to apply the battery voltage value V _b provided by the battery sensor 9 to the PWM restriction map, a duty ratio of the PWM signal provided by the PWM signal generator 64 when D is greater than the upper limit value D _max gives a PWM signal having a duty ratio of the upper limit value D _max to the motor drive circuit 7. On the other hand, when the duty ratio D of the PWM signal given by the PWM signal generator 64 is smaller than the upper limit value _Dmax , the PWM signal given by the PWM signal generator 64 is given to the motor drive circuit 7 as it is.

  The motor drive circuit 7 opens and closes the FET according to the PWM signal given by the PWM limiter 65 to drive the steering assist motor 5. Note that power for the assist control unit 6 and the steering assist motor 5 is supplied from an in-vehicle battery 90.

FIG. 4 is a flowchart showing the assist control processing procedure of the assist control unit 6. The assist control unit 6 uses the steering torque value T detected by the torque sensor 4, the vehicle speed value V detected by the vehicle speed sensor 8, and the battery voltage value V _b detected by the battery sensor 9 at a predetermined sampling period. Capture (steps S1, 2, 3).

  The assist control unit 6 performs phase compensation processing using the steering torque value T taken in step S1 (step S4). As described above, the phase compensation processing is performed in the phase compensation unit 61.

  Next, the target drive current value I with respect to the steering torque value T subjected to the phase compensation process in step S4 is determined (step S5). As described above, the target drive current value I is calculated by the target drive current calculation unit 62 using the assist control map. When the drive current value of the steering assist motor 5 is matched with the target drive current value I, a steering assist force corresponding to the steering torque applied to the steering wheel 30 is generated.

Next, the assist control portion 6 obtains a deviation between the motor current detection value I _s is the detection value of the current flowing through the target drive current value motor 5 for steering assist and I determined in step S5 (step S6), and A PI calculation is performed on the obtained deviation, and the duty ratio D of the PWM signal is determined based on the calculation result (step S7). Calculating the deviation between the target driving current value I and the motor current detection value I _s as described above is done in the subtractor 63, the determination of the duty ratio D of the PI calculation and the PWM signal is made in the PWM signal generating section 64 .

Next, it is determined whether or not the duty ratio D determined in step S7 is larger than a predetermined upper limit value D _max of the duty ratio of the PWM signal as shown in FIG. 3 (step S8). When the duty ratio D is larger than the upper limit value _Dmax (step S8: YES), the upper limit value _Dmax is set as the duty ratio D of the PWM signal (step S9). On the other hand, when the duty ratio D is equal to or lower than the upper limit value D _max (step S8: NO), the duty ratio D determined in step S7 is used as it is. The restriction of the duty ratio D performed in these steps S8 and S9 is performed by the PWM restriction unit 65 using the PWM restriction map.

  The assist control unit 6 generates a PWM signal having the duty ratio D obtained as described above (step S10), and gives the generated PWM signal to the motor drive circuit 7 (step S11). The motor drive circuit 7 drives the motor 5 by opening and closing the FET based on the PWM signal given by the assist control unit 6.

In the electric power steering apparatus according to the present invention configured as described above, the battery voltage value _Vb of the in-vehicle battery 90 supplied to the motor drive circuit 7 that drives the steering assist motor 5 is detected and detected. The upper limit value D _max of the duty ratio of the PWM signal is determined in accordance with the battery voltage value V _b thus determined, and the duty ratio D of the PWM signal determined in accordance with the steering torque is limited by the determined upper limit value D _max . As shown in FIG. 3, the upper limit value D _max is set so as to decrease as the battery voltage value V _b increases in a region where the battery voltage value V _b is equal to or higher than the reference voltage value. Immediately after the replacement, when the vehicle-mounted battery 90 is steered in a high voltage state, the moving speed of the rack shaft 10 is not excessive, and the stopper portion provided in the rack housing 11 and the rack shaft 10 are not moved. Can be reduced. In the present invention, since only the duty ratio D of the PWM signal is limited, the steering response does not change, and it is not necessary to give the driver a sense of discomfort due to this.

In addition, the PWM limit map is set so that the upper limit value D _max of the duty ratio of the PWM signal changes proportionally to small / large according to the high / low of the battery voltage value _Vb , and the vehicle driving state is set. When the voltage of the in-vehicle battery 90 fluctuates during traveling according to the accompanying load change, the driver does not have to feel uncomfortable.

  Furthermore, since the voltage of the in-vehicle battery 90 is directly detected as the voltage supplied to the motor drive circuit 7 and is not affected by the harness connecting the in-vehicle battery 90 and the motor drive circuit 7, the duty ratio is appropriately limited. Can be implemented. Further, since the battery sensor 9 provided in the in-vehicle battery 90 is also used as a voltage detector for detecting the voltage supplied to the motor drive circuit 7, it is not necessary to provide a new voltage detector.

In the above embodiment, as shown in FIG. 3, the duty ratio D of the PWM signal using the PWM limit map indicating the relationship between the upper limit value D _max of the duty ratio of the PWM signal and the battery voltage value V _b. the While seeking upper limit value D _max, not limited thereto, it represents the function whose variable is the upper limit value D _max battery voltage value V _b of the duty ratio, determine the upper limit value D _max of the duty ratio by the function May be. Similarly, the target drive current value I may be obtained by a function instead of a map.

  In the above embodiment, the PWM signal is generated according to the procedure of steps S7 to S10 shown in FIG. 4 by the operation of the CPU according to the control program stored in the ROM of the assist control unit 6. However, the present invention is not limited to this, and a dedicated circuit that performs the procedure of steps S7 to S10 may be provided, and the dedicated circuit may generate the PWM signal.

  In the above embodiment, the battery sensor 9 provided in the in-vehicle battery 90 also serves as a voltage detector that detects the voltage supplied to the motor drive circuit 7. However, the present invention is not limited to this. A voltage detector may be provided between 90 and the motor drive circuit 7.

  Furthermore, it goes without saying that the present invention can be implemented in variously modified forms within the scope of the matters described in the claims.

It is a mimetic diagram showing the composition of the electric power steering device concerning the present invention. It is a block diagram which shows the structure of the control system of the electric power steering apparatus which concerns on this invention. It is an example of a PWM restriction map. It is a flowchart which shows the process sequence of the assist control of an assist control part.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 Steering mechanism, 4 Torque sensor, 5 Steering assistance motor, 6 Assist control part (control means), 7 Motor drive circuit, 9 Battery sensor (voltage detector), 30 Steering wheel (steering member), 90 In-vehicle battery

Claims (3)

  Control means for determining a target assist force according to a steering torque applied to the steering member, generating a pulse width modulation signal having a duty ratio determined to generate the determined target assist force, and a pulse generated by the control means An electric power steering apparatus comprising: a motor drive circuit that drives a steering assist motor in accordance with a width modulation signal; and a voltage detector that detects a voltage supplied to the motor drive circuit. An electric power steering apparatus configured to limit the duty ratio by an upper limit value corresponding to a voltage detected by a voltage detector.   2. The electric power steering apparatus according to claim 1, wherein the control means is configured to make the upper limit value small / large in accordance with high / low of the voltage detected by the voltage detector.   The electric power steering apparatus according to claim 1 or 2, wherein the voltage detector is interposed between a positive electrode and a negative electrode of an in-vehicle battery and configured to detect an output voltage of the in-vehicle battery.

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