JP2539097Y2 - Electric power steering device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial application field) The present invention relates to an electric power steering device in which an electric motor applies a steering assisting force to a steering system of a vehicle, and particularly to a steering assist by the motor. The present invention relates to an electric power steering apparatus in which force acts between a steering gear of a steering system and steered wheels.

(Prior Art) In an automobile, an electric power steering device that generates steering assist torque by an electric motor is often used. An electric power steering device detects a steering torque applied to a steering system from a steering handle and a direction thereof by a steering sensor, controls a motor based on an output signal of the steering sensor, and applies a rotational torque of the motor to the steering system. It has a feature that it can be made compact and that the engine output is not consumed by the power steering device.

As such an electric power steering device,
There are known those that apply a rotational torque of a motor to a steering shaft between a steering handle and a steering gear and those that apply the torque to a steering system between a steering gear and a steered wheel. Among them, a rack and pinion type steering gear is provided as one that applies a steering assist torque between the steering gear and the steered wheels,
There is one in which a motor rotation torque is converted into an axial thrust and acts on a rack shaft of the steering gear. The rack shaft is connected to steered wheels via tie rods.

In the case of an electric power steering device that applies a steering assist torque by a motor to the rack shaft of a rack and pinion steering gear, the steering sensor normally transmits the steering torque transmitted from the steering handle to a pinion gear meshing with the rack shaft. It is to be detected. More specifically, the steering sensor detects torsion occurring on the pinion shaft of the pinion gear.

By the way, in a vehicle equipped with such a type of electric power steering device, a load from a road surface is transmitted to a rack shaft via steered wheels and tie rods during traveling, so that the load differs between left and right steered wheels. And the pinion shaft is twisted. Then, the twist is detected by the steering sensor. Therefore, if the motor is directly controlled by the output signal from the steering sensor,
Even during straight running, the motor is frequently driven by the difference in road load applied to the left and right steered wheels, and the steered wheels are steered repeatedly little by little.

Further, at the time of high-speed running, since the operation angle of the steering wheel is small and the steering force is small, power assist is not required. Rather, in order to prevent the steering wheel from being over-operated, it is required to provide a certain degree of steering reaction force.

Therefore, in general, a control unit that receives an output signal of a steering sensor and generates a command signal to a motor is provided with a dead zone having a predetermined width that does not generate an assist command signal even when a signal from the steering sensor is received, and is detected by the steering sensor. When the applied torque is small, the motor is not driven. The dead zone is relatively wide.

Conventionally, even in the case of an electric power steering apparatus having such a dead zone, the gear ratio of the steering gear has been kept constant throughout.

(Problems to be Solved by the Invention) However, if the gear ratio of the steering gear is constant throughout as described above, the rack shaft is always moved by a fixed amount when the steering handle is rotated by a fixed angle. In this case, the power steering apparatus is provided with the above-mentioned dead zone, and the motor does not operate in a small operating angle range where the steering torque is still within the dead zone at the start of turning of the steering wheel. The movement of the shaft will be performed manually via the pinion gear. Then, the motor connected to the rack shaft is rotated in reverse by the movement of the rack shaft. In addition, since the motor is generally decelerated before applying a torque to the rack shaft, the motor is rotated at an increased speed at that time. When the motor is forcibly rotated at such a speed increase, the inertia force of the motor increases in proportion to the square of the speed increase ratio. Therefore, a large force is required for operating the steering wheel in order to rotate the motor from the stationary state.

On the other hand, when the steering wheel is rotated until the steering wheel exceeds the dead zone, the steering operation force is applied to the steering assist force of the motor, so that the steering wheel operating force is reduced.

Therefore, in a conventional electric power steering apparatus in which a gear ratio of a steering gear is fixed and an assist torque by a motor is applied downstream of the steering gear, as in the related art, the vicinity of a neutral position of a steering handle and both sides thereof are provided. Thus, there is a problem that a difference occurs in the steering wheel operating force and the steering feeling is reduced. In particular, during high-speed traveling, steering near the neutral position is frequently performed, and therefore, an improvement in the steering feeling is desired.

As described above, in the electric power steering apparatus in which the steering assist torque by the motor is applied to the steering system between the steering gear and the steered wheels, a ball screw type steering gear is used instead of the rack and pinion type. Some have been used, but such ones have similar problems.

The present invention has been made in view of such circumstances, and an object of the present invention is to allow a steering assist torque by an electric motor to act on a steering system between a steering gear and a steered wheel via a speed reduction mechanism. In the electric power steering apparatus provided with a dead zone for preventing the generation of the steering assist torque by the motor when the steering torque is small, the steering feeling in the vicinity of the neutral position of the steering wheel is improved. Is to be able to

(Means for Solving the Problems) In order to achieve this object, according to the present invention, the gear ratio of the steering gear near the neutral position of the steering wheel is determined by changing the steering gear when the steering wheel is turned left and right from the neutral position. Gear ratio.

(Operation) By setting the steering gear to the variable gear ratio and increasing the gear ratio near the neutral position, when the steering wheel is rotated near the neutral position, the steered wheels are steered smaller. Therefore, for example, in the case of using a rack-and-pinion type steering gear, even if the steering handle is rotated relatively large, the rack shaft moves only a little. as a result,
The amount of forced rotation of the motor decreases. That is, when viewed from the pinion gear, the same effect as when the speed increase ratio of the motor is reduced is obtained, and the inertia force of the motor is reduced.

Thus, the steering wheel can be smoothly rotated even near the neutral position. And
If the steering wheel is rotated until it exceeds the dead zone of the power steering device, the steering assist torque by the motor acts, so that the subsequent steering operation is also lightened. Therefore, the steering feeling is improved.

Also, as the steering wheel moves away from the vicinity of the neutral position, the gear ratio of the steering gear decreases,
A small rotation of the steering wheel causes the steered wheels to be largely steered. Therefore, the steering operation does not become busy.

(Example) Hereinafter, an example of the present invention is described using a drawing.

1 shows an embodiment of an electric power steering device according to the present invention. FIG. 1 is a longitudinal sectional view showing a gear box of the power steering device, and FIG. 2 is a control circuit diagram of the power steering device. It is.

As is apparent from FIG. 1, the electric power steering device 1 includes a rack shaft 2 extending in the left-right direction of the vehicle body. The rack shaft 2 is slidably supported in the axial direction by gear box cases 3a and 3b fixed to the vehicle body, and both ends thereof are connected to left and right steered wheels via tie rods. I have.

A rack and pinion type steering gear 4 is provided on the left side of the rack shaft 2 in the drawing. The steering gear 4 includes rack teeth 5 formed on the side surface of the rack shaft 2 along the axial direction, and pinion gears 6 meshing with the rack teeth 5. The pinion gear 6 is fixed to a lower end of a pinion shaft 7 having a torsion bar. The rotation of a steering handle is transmitted to the upper end of the pinion shaft 7 via a steering shaft. Therefore, when the steering wheel is rotated, the rotation is transmitted from the pinion shaft 7 to the pinion gear 6, and a thrust in the axial direction of the rack shaft 2 is applied. At this time, since the road surface load of the steered wheels is acting on the rack shaft 2, the pinion shaft 7
Is twisted by an angle corresponding to the steering torque applied to the steering wheel.

On the outer periphery of the pinion shaft 7, a cylindrical movable core 8 made of a magnetic material is provided. The movable core 8 has an inclined long groove
8a is engaged with the pinion shaft 7 via the pinion shaft 7
Is moved in the axial direction by the torsion. The displacement of the movable core 8 is detected by a differential transformer 9. Thus, the movable core 8 and the differential transformer 9 constitute a torque sensor 10 for detecting a steering torque applied to the steering system.

Further, a rotation sensor 11 for detecting the rotation of the pinion shaft 7 is provided on the outer peripheral portion. The torque sensor 10 and the rotation sensor 11 constitute an intelligent steering sensor 12 that detects the steering state of the vehicle, that is, the steering torque and the steering direction applied to the steering system.

The steering sensor 12 is housed in a shaft case 13 that rotatably supports the pinion shaft 7. The shaft case 13 is fixed to the outside of the end of the gear box case 3a.

A control unit (ECU) 14 to which an output signal of the steering sensor 12 is guided is provided on a side portion of the shaft case 13. A power unit 15 to which a power assist command signal output from the control unit 14 is guided is attached to the gear box case 3a.

A hollow rotor shaft 16 is provided between the gear box cases 3a and 3b.
Is provided coaxially with the rack shaft 2. The rotor shaft 16 is rotatably supported by the gear box cases 3a and 3b. The rack shaft 2 is disposed so that its substantially central portion penetrates through the rotor shaft 16 with an appropriate gap. The motor 17 is driven by the power unit 15.

A drive gear 18 is formed on the outer peripheral surface of the end portion of the rotor shaft 16, and an output gear 19 is meshed with the drive gear 18. The output gear 19 has a diameter larger than that of the drive gear 18, thereby constituting a reduction mechanism. The output gear 19 is fixed to the screw shaft 21 of the ball screw mechanism 20. The nut member 22 of the ball screw mechanism 20 is meshed with the screw shaft 21 via a number of balls.
Thus, when the motor 17 is driven, its rotation is reduced via the drive gear 18 and the output gear 19 and transmitted to the ball screw mechanism 20, and the screw shaft 21 rotates, thereby moving the nut member 22 in the axial direction. It is supposed to.

The screw shaft 21 is arranged parallel to the rack shaft 2. A rack joint 23 fixed to the rack shaft 2 is attached to the nut member 22. Therefore, when the nut member 22 is moved in the axial direction of the screw shaft 21 by the driving force of the motor 17, a thrust in the axial direction is applied to the rack shaft 2. That is, the driving torque of the motor 17 is converted into the thrust of the rack shaft 2.

Thus, when the steering operation is performed, the pinion shaft 7 is twisted by the rotation of the steering handle,
A steering torque is applied to the rack shaft 2 from the pinion gear 6, and the steering torque and the steering direction are determined by a steering sensor.
The motor 17 is driven by the output signal, and a thrust converted from the driving torque of the motor 17 is applied to the rack shaft 2. Therefore, the steering torque from the steering wheel and the steering assist torque generated by the motor 17 act on the rack shaft 2, and the rack shaft 2 moves in the axial direction, and the steered wheels are steered. During this time, the pinion shaft 7 rotates while maintaining a predetermined twist.

As shown in FIG. 2, the control unit 14 includes, in addition to the output signal of the steering sensor 12 including the torque sensor 10 and the rotation sensor 11 described above, a vehicle speed sensor 24 for detecting the speed of the vehicle at that time, An output signal from a current sensor 25 or the like that detects a current flowing through the power unit 15 is input.

The control unit 14 includes an interface circuit 26
And a control circuit 27 including a central processing unit (CPU)
, A failure diagnosis circuit 28, and a power supply circuit (not shown). The interface circuit 26 includes each sensor
The purpose of this is to match the output signals from 10, 11, 24, and 25 with the signals of the control circuit 27. In the control circuit 27, the rotational torque and the rotational direction required for the motor 17 are calculated based on the signals from the sensors 10, 11,. Then, a power assist command signal as shown in FIG. The command signal has a dead zone N having a predetermined width centered on when the steering torque detected by the torque sensor 10 is zero.

The power unit 15 is configured by a bridge circuit using a field-effect transistor, and drives the motor 17 reversibly in response to a command from the control unit 14.

FIG. 4 shows rack teeth 5 constituting the steering gear 4.
2 shows the state of engagement between the gear and the pinion gear 6.

As is clear from this figure, the rack teeth 5 are fine near the center in the axial direction and coarse on both sides. Therefore, when the pinion gear 6 rotates while meshing with the rack teeth 5 near the center thereof, that is, when the steering handle is near the neutral position, the rack shaft 2 is rotated at a constant rotation angle of the steering handle.
Moves small, and when engaged on both sides, the rack shaft 2 moves large. That is, the gear ratio of the steering gear 4 is large near the neutral position of the steering handle, and the gear ratio is reduced when the steering handle is turned left and right from the neutral position.

FIG. 5 is a characteristic diagram showing characteristics of the steering gear 4.

As is clear from this figure, the gear ratio is large near the steering angle of zero, becomes smaller as the vehicle is steered to the left and right, and takes a small constant value when the steering is made larger. Then, the rack gain, that is, the steering wheel 1
Conversely, the amount of movement of the rack shaft 2 per rotation is small when the steering angle is near zero and large on both sides. Therefore,
When the steering wheel is operated by a certain angle, the steered wheels are steered slightly near the neutral position, and largely steered when the steering wheel is turned left and right from the neutral position.

Next, the operation of the electric power steering apparatus 1 configured as described above will be described.

As described above, when the steering wheel is rotated, the rotation is transmitted to the pinion shaft 7 and the pinion shaft 7 is rotated.
Is twisted, and a steering torque corresponding to the twist is applied to the rack shaft 2 from the pinion gear 6. Then, the steering torque and the direction are detected by the steering sensor 12. The output signal of the steering sensor 12 is output to the control unit 1 together with the output signals of the other sensors 24 and 25.
4, the torque and the rotation direction required for the motor 17 are calculated based on those signals. Then, an assist command signal is sent from the control unit 14 to the power unit 15. The power unit 15 supplies a current corresponding to the command signal to the motor 17. As a result, the motor 17 is driven in a predetermined direction with a predetermined torque,
Thrust is applied to the rack shaft 2.

Thus, the rack shaft 2 is driven by the steering torque applied from the steering handle and the steering assist torque generated by the motor 17, and the steered wheels are steered via the tie rods. That is, the steering force is assisted by the motor 17.

During this time, the control unit 14 is provided with a dead zone N in which the assist command signal is not output even when a signal from the steering sensor 12 is received. Does not work. That is, no steering assist torque by the motor 17 is applied. However, since the pinion gear 6 rotates, a thrust is applied to the rack shaft 2 via the rack teeth 5 by the rotation. Therefore, an axial force is applied to the rack joint 23 fixed to the rack shaft 2, and the screw shaft 21 is rotated via the ball screw mechanism 20. Then, the rotation is normally increased in speed via a drive gear 18 and an output gear 19 which constitute a reduction mechanism for transmitting the rotation of the motor 17 to the screw shaft 21 by reducing the rotation of the motor 17 and transmitted to the motor 17 at normal times. The 17 rotor shafts 16 are forcibly rotated.

At this time, the gear ratio of the steering gear 4 is increased near the center of the rack teeth 5, so that the rack shaft 2 moves only a little near the neutral position of the steering handle even by the rotation of the pinion gear 6. Therefore, the motor 17 is also slightly rotated. In other words,
This means that the amount of forced rotation of the motor 17 with respect to the rotation angle of the steering handle is reduced. That is, the same effect as reducing the speed increase ratio of the motor 17 as viewed from the steering wheel is obtained, and the inertia force of the motor 17 is reduced by the square of the speed increase ratio. As a result, the steering wheel operating force near the neutral position of the steering wheel is reduced.

When the steering wheel is further rotated to exceed the dead zone N set in the power steering apparatus 1, an assist command signal is output from the control unit 14, and the steering assist torque by the motor 17
Will be added to Therefore, the subsequent steering operation is performed lightly. Then, at that time, the gear ratio of the steering gear 4 becomes small, so that the steered wheels are largely steered even by a small rotation operation of the steering handle. Therefore, the steering operation does not become busy even at the time of steering at a large angle.

As described above, according to the power steering device 1, even in the vicinity of the neutral position of the steering wheel where the operation assist torque by the motor 17 does not act, the steering operation can be smoothly performed similarly to when the steering assist torque acts. Therefore, the steering feeling is significantly improved. This makes it possible to widen the dead zone N, so that a high steering frequency at a small angle and a high-speed driving that does not require power assist can provide a steering feeling similar to that of manual steering. .

In the above embodiment, the case where the steering gear 4 is of a rack and pinion type including the rack teeth 5 and the pinion gear 6 has been described. However, in the case of an electric power steering apparatus using a ball screw type steering gear. In addition, by increasing the gear ratio of the steering gear in the vicinity of the neutral position of the steering wheel, a similar effect can be obtained.

(Effects of the Invention) As is apparent from the above description, according to the present invention, the electric power steering system in which the steering assist torque by the electric motor is applied to the steering system between the steering gear and the steered wheels via the speed reduction mechanism. In the power steering apparatus of the type, the gear ratio of the steering gear near the neutral position of the steering wheel is set to be larger than the gear ratio of the steering gear when the steering wheel is turned left and right from the neutral position. Also, when the steering wheel is operated from the neutral position, the resistance of the electric motor is reduced, and the steering wheel operation can be performed smoothly. Then, if the steering wheel is rotated to some extent, the steering assist torque by the motor acts, so that the subsequent steering can be performed smoothly. Therefore, it is possible to eliminate the feeling of bitterness at the start of turning the steering wheel, and it is possible to improve the steering feeling.

Further, even in a dead zone region where the power assist does not work near the neutral position of the steering wheel, the steering wheel can be operated lightly, so that the dead zone region can be widened. Therefore, it is possible to obtain a steering feeling similar to that in the case of manual steering during high-speed traveling.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view showing an embodiment of an electric power steering apparatus according to the present invention, in which a gearbox is shown. FIG. 2 is a block diagram showing a control circuit of the power steering apparatus. FIG. 4 is a graph showing characteristics of a power assist command signal output from a control unit of the control circuit. FIG. 4 is a graph showing a change in gear ratio of a rack and pinion type steering gear used in the power steering device. FIG. 5 is a graph showing characteristics of the steering gear. DESCRIPTION OF SYMBOLS 1 ... Electric power steering device, 2 ... Rack shaft 4 ... Steering gear, 5 ... Rack teeth, 6 ... Pinion gear 7 ... Pinion shaft, 10 ... Torque sensor, 11 ... Rotation sensor 12 ... Steering sensor, 14 ... Control unit 15 ... Power unit, 17: Electric motor 18, 19: Drive gear and output gear (reduction mechanism) 20: Ball screw mechanism, N: Dead zone area

Claims (1)

(57) [Scope of request for utility model registration] 1. A steering sensor for detecting a steering torque applied to a steering system from a steering handle, and a steering system from the steering handle to a steered wheel.
An electric motor that applies a steering assist torque through a speed reduction mechanism between a steering gear and a steered wheel; and a control unit that controls the electric motor based on an output signal from the steering sensor. An electric power steering apparatus having a dead zone for not generating the steering assist torque until the steering torque detected by the steering sensor reaches a predetermined value; and the steering in the vicinity of a neutral position of the steering handle. An electric power steering device, wherein the gear ratio of the gear is set to be larger than the gear ratio of the steering gear when the steering wheel is turned left and right from the neutral position.