JPH05213206A - Motor-operated power steering device - Google Patents

Abstract

PURPOSE:To reduce a transmission loss, to reduce capacity of an electric motor, and to increase the degree of freedom of a design. CONSTITUTION:A cylindrical outer rotary shaft 13 rotated by the aid of a brushless electric motor 12 is arranged in a housing 1 for supporting a rack shaft 2. A cylindrical inner rotary shaft 18 is mounted on the inside of the outer rotary shaft 13 so that it is axially movable but prevented from making mutual rotation. A rack shaft 2 is inserted through the inside of the inner rotary shaft 18 and the inner rotary shaft 18 is coupled to the housing 1 through a slip screw 21. The rack shaft 2 is coupled to the inner rotary shaft 18 through a ball screw 25.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electric power steering apparatus for an automobile or the like, and more specifically to moving a rack shaft by a steering force of a driver and an auxiliary steering force of an electric motor according to the steering force. The power steering device

[0002]

2. Description of the Related Art In this type of electric power steering apparatus, it is necessary to reduce the rotation of the electric motor and transmit it to the rack shaft. Conventionally, for example, the rotation of the electric motor is reduced by a gear reduction mechanism such as a worm. After that, it was further transmitted to the rack shaft by a pinion and a rack.

Further, a cylindrical rotary shaft rotated by an electric motor and a ball nut are rotatably supported in a housing supporting the rack shaft, and the rack shaft is passed inside these.
A power steering apparatus has also been proposed in which the rotation of a rotating shaft is transmitted to a ball nut via a planetary gear reduction mechanism, and the rotation of the ball nut is transmitted to a rack shaft via a ball screw mechanism. (See Japanese Unexamined Patent Publication No. 60-154955).

[0004]

However, in the conventional electric power steering apparatus as described above, the rotation of the electric motor is decelerated by the reduction mechanism such as the worm or the planetary gear, so that the transmission loss is very large. Therefore, it is necessary to increase the capacity of the electric motor.

Therefore, the inventors of the present invention provide a cylindrical outer rotating shaft that is rotated by an electric motor in a housing that supports the rack shaft, and the cylindrical inner rotating shaft is axially arranged inside the outer rotating shaft. Are mounted so that they cannot move relative to each other, the rack shaft is passed inside the inner rotating shaft, the inner rotating shaft is connected to the housing through the first ball screw, and the rack shaft is the second ball screw. An electric power steering device connected to an inner rotary shaft via a motor was proposed (see Japanese Patent Application No. 3-184654).

In this electric power steering apparatus, the transmission loss is small and the capacity of the electric motor can be reduced. Further, the reduction ratio can be arbitrarily set by the screw directions of the two ball screws and the screw leads.

However, the lead of the ball screw is restricted by the ball diameter determined by the load capacity, and a ball screw with a very small lead cannot be manufactured.
The degree of freedom in design is relatively low.

The object of the present invention is to solve all the above problems, to reduce the transmission loss, and to reduce the capacity of the electric motor.
Moreover, it is to provide an electric power steering apparatus having a high degree of freedom in design.

[0009]

An electric power steering apparatus according to the present invention is provided with a cylindrical outer rotating shaft which is rotated by an electric motor in a housing which supports a rack shaft.
A cylindrical inner rotary shaft is mounted inside the outer rotary shaft so as to be movable in the axial direction but not rotated relative to each other, and a rack shaft is inserted inside the inner rotary shaft, and the inner rotary shaft is The rack shaft is connected to the inner rotary shaft via the second screw transmission means, and one of the first and second screw transmission means is a ball screw and the other is connected to the housing via the first screw transmission means. It is a sliding screw.

Preferably, the electric motor is a brushless electric motor, and a sensor for detecting the rotational position of the rotor of the electric motor is attached to the electric motor directly or indirectly via a connecting mechanism.

Preferably, the rotor of the electric motor is attached to the outer rotary shaft, and the stator of the electric motor is attached to the housing.

[0012]

The rotation of the outer rotary shaft by the electric motor is transmitted to the inner rotary shaft, the inner rotary shaft rotates together with the outer rotary shaft, and at the same time, the first rotary shaft moves the inner rotary shaft in the axial direction. The rotation and the axial movement of the inner rotary shaft are converted into the axial movement of the rack shaft by the second screw transmission means. As a result, the rotation of the electric motor is decelerated and transmitted to the rack shaft.

Since the rotation of the electric motor is reduced by the screw transmission means, the transmission loss is small. Since one of the two screw transmission means is a ball screw, the transmission loss is very small. Further, the movement amount of the rack shaft per unit rotation amount of the outer and inner rotation shafts, that is, the reduction ratio (transmission ratio) can be arbitrarily set by the screw directions and the screw leads of the two screw transmission means. Since one of the two screw transmission means is a ball screw and the other is a slide screw that does not use balls, the lead of the screw can be made small, and the degree of freedom in design is increased.

Since the DC motor used in the conventional electric power steering apparatus has motor characteristics of high speed rotation and low torque, it is necessary to have a large reduction gear ratio, which complicates the reduction gear mechanism. In addition, the adverse effect on the performance such as the steering wheel return is large. When a brushless electric motor is used as the electric motor, the motor characteristics are low rotation and high torque, so that the reduction ratio may be small and the reduction mechanism can be simplified. By combining a brushless motor and a speed reduction mechanism that uses screw transmission means, it is possible to make full use of the low-rotation, high-torque motor characteristics, and the performance such as handle return by reducing the motor inertia is greatly improved. improves.

When the rotor of the electric motor is attached to the outer rotating shaft and the stator of the electric motor is attached to the housing, the outer rotating shaft is directly rotated by the electric motor.
The transmission loss occurs only in the portions of the first and second screw transmission means, and since the transmission loss of the screw transmission means is small, the overall transmission loss is also very small. However, even if the rotation of the electric motor is decelerated by the one-stage gear reduction mechanism to be transmitted to the outer rotary shaft, the transmission loss is generated almost only in this portion, so the overall transmission loss is small.

[0016]

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

FIG. 1 shows a portion of an electric power steering apparatus for an automobile, and FIG. 2 shows details of a main portion thereof. In the following description, the left and right sides of the drawing will be referred to as left and right.

A housing that is long in the left-right direction on the body of the automobile
(1) is fixed, and a rack shaft (2) extending in the left-right direction is supported in the housing (1) so as to be movable in the left-right direction (axial direction) without rotating. housing
An input shaft (4) and an output shaft (5) connected by a torsion bar (not shown) are rotatably supported in a gear box (3) formed near the left end of (1). The output shaft (5) is integrally formed with a pinion (6) that meshes with a rack (not shown) of the rack shaft (2), and the input shaft (4) is connected to a steering wheel (not shown). The gear box (3) is provided with a torque sensor (7) for detecting steering torque from the steering wheel by detecting twist of the torsion bar between the input shaft (4) and the output shaft (5). Rack axle protruding from housing (1)
Tie rods (10) and (11) are connected to both left and right ends of (2) via ball joints (8) and (9).

A cylindrical outer rotating shaft (13) rotated by a brushless electric motor (12) in an intermediate portion of the housing (1).
Are rotatably supported via bearings (14) and (15) so as not to move in the axial direction. The electric motor (12) is composed of a stator (16) fixed in the housing (1) and a rotor (17) provided directly on the outer peripheral portion of the outer rotary shaft (13).

A rotary position sensor (35) using a rotary encoder or the like is mounted at an appropriate position on the housing (1) around the rack shaft (2), and is fixed to the input shaft of this sensor (35). The gear (36) is meshed with the gear (37) fixed to the outer periphery of the right end portion of the outer rotary shaft (13). Then, the rotation of the outer rotating shaft (13) is decelerated by the gears (37) (36) and transmitted to the sensor (35), and the sensor (35) changes the rotational position of the outer rotating shaft (13), that is, the rotor (17). To be detected.

Inside the outer rotating shaft (13), a cylindrical inner rotating shaft (18) is attached via a spline (19) and the like so that they can move in the axial direction but they do not rotate with respect to each other. axis
It is passed inside the (18) via a bush (20) so as to be rotatable and axially movable with respect to each other.

A part of the inner rotary shaft (18) always protrudes to the left of the outer rotary shaft (13) even if it moves with respect to the outer rotary shaft (13). As described below, the first screw transmission means is connected to the housing (1) through a slide screw (slide screw transmission device) (21). The sliding screw (21) is, for example, a trapezoidal screw, and a trapezoidal screw portion (22) is formed on the outer peripheral surface of the inner rotating shaft (18), and the screw portion (22) is fixed to the housing (1) by a nut ( 23) is screwed on.

The rack shaft (2) is provided with a ball screw (ball screw transmission device) (2
It is connected to the left end of the inner rotary shaft (18) via 5).
That is, a threaded portion (26) is formed on the outer peripheral surface of the rack shaft (2), and this threaded portion (26) is circulated in a large number in a ball nut (27) fixed to the left end of the inner rotating shaft (18). It is screwed through the ball (28).

In this embodiment, the directions of the slide screw (21) and the ball screw (25) are opposite, and the lead of the ball screw (25) is larger than that of the slide screw (21). ..

When the driver rotates the steering wheel, the rotation is transmitted to the pinion (6) via the input shaft (4), the torsion bar and the output shaft (5), and the rotation of the pinion (6) causes the rack shaft ( 2) is moved left and right to steer the wheels. At this time, the electric motor (12) is driven based on the direction and magnitude of the steering torque detected by the torque sensor (7), which moves the rack shaft (2) in the same direction as that by the pinion (6). Be done.

For example, the rack shaft can be operated by operating the handle.
When moving (2) to the left, the rotor (17) of the motor (12)
Is rotated to the right (clockwise) when viewed from the right side of the drawing. As a result, the outer rotary shaft (13) and the inner rotary shaft (18) are also rotated in the same direction by the same amount. Inner rotating shaft (18)
When is rotated in the above direction, the inner screw shaft (18) moves to the left with respect to the housing (1) due to the action of the slide screw (21), and the rack shaft due to the action of the ball screw (25).
(2) moves to the left with respect to the inner rotation axis (18). As a result, the rack shaft (2) moves to the left with respect to the housing (1), and the amount of movement is the inner rotary shaft relative to the housing (1).
It is equal to the sum of the amount of movement of (18) and the amount of movement of the rack shaft (2) with respect to the inner rotating shaft (18). The amount of movement of the inner rotating shaft (18) with respect to the housing (1) is proportional to the amount of rotation of the inner rotating shaft (18) and the lead of the slide screw (21), and ) Is proportional to the amount of rotation of the inner rotating shaft (18) and the lead of the screw of the ball screw (25). Therefore, the amount of movement of the rack shaft (2) with respect to the housing (1) is proportional to the sum of the leads of the sliding ball screw (21) and the ball screw (25) and the amount of rotation of the inner rotating shaft (18).

The same applies when the rack shaft (2) is moved to the right by operating the handle.

When the sliding screw (21) and the ball screw (25) have the same screw direction, the moving direction of the inner rotating shaft (18) with respect to the housing (1) and the rack shaft with respect to the inner rotating shaft (18).
(2) The direction of movement is reversed and therefore the housing
The movement direction of the rack axis (2) with respect to (1) is the inner rotation axis (18)
The movement amount is proportional to the difference between the leads of the slide screw (21) and the ball screw (25) and the amount of rotation of the inner rotation shaft (18).

In this way, the inner rotary shaft by the electric motor (12)
Rack shaft (2) housing per unit rotation of (18)
The amount of movement with respect to (1), that is, the reduction ratio is determined by the leads of the slide screw (21) and the ball screw (25), and thus the reduction ratio can be set arbitrarily by appropriately determining these. In addition, the slide screw (21) is a ball screw (2
The leads can be made smaller than in 5), and therefore the degree of freedom in design is increased.

Since the rotation of the inner rotary shaft (18) by the electric motor (12) is transmitted to the rack shaft (2) by the slide screw (21) having a small transmission loss and the ball screw (25) having a very small transmission loss, The overall transmission loss is very small, so it is possible to reduce the capacity of the electric motor (12).

For example, when the rotation of the electric motor is transmitted to the rack shaft by the worm and the rack and pinion as in the conventional case,
The overall transmission efficiency is about 0.6, and when the rotation of the electric motor is transmitted to the rack shaft by the slide screw and the ball screw as in the above embodiment, the overall transmission efficiency is about 0.8. Therefore, according to the above-described embodiment, the same driving force can be obtained with the electric motor having the capacity of about 3/4 of the conventional one.

Since the brushless electric motor (12) has low rotation speed and high torque as the motor characteristics, the reduction ratio may be small and the speed reduction mechanism can be simplified, and the brushless electric motor (12) and the slide screw (21) and the ball screw can be used. By combining the reduction mechanism using (25), the motor characteristics of low rotation and high torque can be fully utilized, and the performance such as steering wheel return by reducing the motor inertia is greatly improved. Further, since the brushless electric motor (12) is used, a rotational position sensor is required, but since the rotational position sensor (35) is provided around the rack shaft (2), the degree of freedom in mounting is high.

In the above embodiment, the first screw transmission means is the slide screw (21) and the second screw transmission means is the ball screw (25). However, the first screw transmission means is the ball screw and the second screw transmission. The means may be a slide screw.

Further, in the above embodiment, the rotor (17) of the electric motor (12) is attached to the outer rotary shaft (13), and the housing (1)
The stator (16) of the electric motor (12) is attached to the
Since the outer rotary shaft (13) is directly rotated by 2), the transmission loss is the sliding screw (21) and the ball screw (25).
Will occur only in the part of (1) and the transmission loss in these parts will be small, so the overall transmission loss will also be very small. However, even if the rotation of the electric motor (12) is decelerated by the one-stage gear reduction mechanism and transmitted to the outer rotary shaft (13), as described above, the sliding screw (21) and the ball screw (25) The transmission loss is very small, and the transmission loss occurs almost only in this gear reduction mechanism, so the overall transmission loss is small.

[0035]

As described above, according to the electric power transmission apparatus of the present invention, the overall transmission loss is small, and therefore the capacity of the electric motor can be reduced. Also, 2
The amount of rack shaft movement per unit amount of rotation of the electric motor can be set arbitrarily by the screw directions and leads of the two screw transmission means, and one of the two screw transmission means is a ball screw and the other is a slide screw. Therefore, the degree of freedom in design becomes very high.

By using a brushless motor,
The speed reducing mechanism can be simplified and the performance can be improved.

The transmission loss can be further reduced by mounting the rotor of the electric motor on the outer rotary shaft and the stator of the electric motor on the housing.

[Brief description of drawings]

FIG. 1 is a partially cutaway rear view of an electric power steering apparatus according to an embodiment of the present invention.

FIG. 2 is a partially cutaway rear view showing a main part of FIG. 1 in an enlarged manner.

[Explanation of symbols]

 (1) Housing (2) Rack shaft (12) Brushless motor (13) Outer rotary shaft (16) Stator (17) Rotor (18) Inner rotary shaft (21) Slide screw (25) Ball screw (35) Rotation position sensor

Claims (3)

[Claims] 1. A cylindrical outer rotating shaft rotated by an electric motor is provided in a housing supporting a rack shaft, and a cylindrical inner rotating shaft is movable inside the outer rotating shaft in the axial direction. The inner rotary shaft is connected to the housing via the first screw transmission means, and the rack shaft is connected via the second screw transmission means. And an inner rotary shaft, wherein one of the first and second screw transmission means is a ball screw and the other is a slide screw. 2. The electric power according to claim 1, wherein the electric motor is a brushless electric motor, and a sensor for detecting a rotational position of a rotor of the electric motor is attached to the electric motor directly or indirectly via a connecting mechanism. Steering device. 3. The electric power steering apparatus according to claim 1, wherein a rotor of the electric motor is attached to the outer rotary shaft, and a stator of the electric motor is attached to the housing.