JPH04310474A - Motor driven power steering mechanism - Google Patents

Abstract

PURPOSE:To use a motor driven power assisting mechanism in addition to a mechanism capable of changing the transmission ratio of the steering force by driving and controlling these mechanisms to required conditions according to the driving conditions of a vehicle. CONSTITUTION:A variable transmission reatio mechanism 10 which is driven with an electric motor 20 and changes the steered wheel angle relative to the steering wheel angle according to the vehicle speed is added to a steering gear mechanism 5. A power assist mechanism 30 is employed to transmit the torque of the electric motor to part of the steering gear mechanism. Controls 26 and 28 are employed to selectively drive and control both of the mechanisms according to the vehicle speed. The control is usually switched so that the variable steering force transmission mechanism is operated above a specified vehicle speed while the power assist mechanism is operated under the specified vehicle speed. This switching control is made only when the steering wheel is in the neutral position. If the vehicle speed incresese to a high speed during the operation of the power assist mechanism, however, the switch control is not made unless the vehicle speed lowers below the specified value.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to an electric power assist mechanism that uses an electric motor as a steering assist force generating means, and a steering angle that changes the steering angle at the steering wheel side relative to the steering angle at the steering wheel depending on the vehicle speed. By using this together with a variable steering force transmission ratio mechanism, it is possible to improve the steering feel from stationary steering and when driving at low speeds to high speeds, improve the dynamic characteristics of the vehicle, and improve the vehicle's performance. The present invention relates to an electric power steering device that performs steering control according to driving conditions to ensure driving stability.

0002

[Prior Art] Generally, a steering device for a vehicle, etc. converts the rotational motion of the steering wheel during steering into the linear motion of a tie rod through a rack-and-pinion type steering gear mechanism, etc. It is possible to perform steering control to a desired state, but in this case, in a normal steering device, the transmission ratio is set to a constant value by a steering gear mechanism. However, for example, when the vehicle is traveling at high speed, the turning angle of the steered wheels (steering wheel steering angle) with respect to the amount of steering rotation of the steering wheel (steering wheel steering angle) described above is increased, and the steering wheel steering for a constant steering wheel steering angle is increased. Reducing the size of the corners is preferable to ensure driving stability, while at low speeds, it makes the vehicle's behavior more agile, gives a good driving feeling, and makes it easier to park the vehicle in the garage. From the viewpoint of the This was proposed in Japanese Patent No. 58-224852 and the like. In other words, in order to obtain an appropriate steering condition, it is desirable to change the turning angle of the steered wheels relative to the steering wheel operation depending on the driving conditions in order to further improve driving stability. Measures were being taken.

[0003]Furthermore, with such a steering device, steering operations can be performed easily and with a good feeling using the lightest possible steering force, especially when turning stationary while stopped or when steering while driving at low speed. For example, as shown in Japanese Unexamined Patent Application Publication No. 62-31559, it is also desired to have a structure in which, in a conventionally well-known hydraulic power steering device, there is a structure in the middle of the steering gear mechanism of the device. There has also been proposed a structure in which a variable transmission ratio mechanism for steering force is interposed in which the transmission ratio can be varied by an actuator such as a stepping motor. According to such a configuration, the transmission ratio, which is set in consideration of the magnitude of the steering assist force (power assist force) provided by the power steering device, is changed according to the vehicle speed, and thereby the steering angle and the steering angle are changed. The steering angle of the steering wheel can be variably controlled according to the vehicle speed, etc., so that a desired state is achieved, and it is possible to obtain steering control characteristics according to the driving state of the vehicle.

0004

[Problems to be Solved by the Invention] However, in the conventional device described above, a power assist mechanism constituting a power steering device is attached to the steering gear mechanism, and a variable transmission ratio mechanism that responds to vehicle speed etc. is separately provided. is attached, and these mechanisms are configured to be able to selectively operate and control them depending on the vehicle speed. In case,
Switching and controlling both of the above-mentioned mechanisms in conjunction with only the vehicle speed will inevitably cause the driver's steering sensation to suddenly change, which poses a problem in ensuring the running stability of the vehicle. In consideration of these points, it is desired to take some measures that also take into account the running stability of such vehicles and improve the steering characteristics. In particular, when switching between the above-mentioned mechanisms occurs during steering, the magnitude of the steering force changes abruptly, which poses a major practical problem for the driver.

[0005]

[Means for Solving the Problems] In order to meet such demands, an electric power steering device according to the present invention includes a steering gear mechanism that is interposed in a part of a steering gear mechanism that transmits steering force from a steering wheel to a steered wheel side. A variable transmission ratio mechanism is provided that is driven and controlled by an electric motor and changes the steering angle on the steering wheel side according to the vehicle speed with respect to the steering angle, and a variable transmission ratio mechanism that transfers the rotational driving force from the electric motor to the steering gear. It is equipped with a power assist mechanism that transmits power to a part of the mechanism, and a control means that selectively drives and controls both of these mechanisms depending on the magnitude of the vehicle speed. The variable transmission ratio mechanism is controlled so that the power assist mechanism is activated when the vehicle speed is below a predetermined speed, and the operation switching between these mechanisms is controlled only when the steering wheel is near the neutral position. It is something. Furthermore, according to the present invention, even if the vehicle speed is high while the power assist mechanism is in operation and the steering wheel is returned to near the neutral position, the operation between these two mechanisms will continue as long as the vehicle speed does not drop below a predetermined value. The configuration is such that switching control cannot be performed.

[0006]

[Operation] According to the present invention, when the vehicle is stopped or traveling at low speed, the electric power assist mechanism exerts the steering assist function, and not only is it possible to perform steering with light and appropriate steering force, but also when traveling at high speed. By stopping the power assist mechanism and controlling the transmission ratio by the variable steering force transmission ratio mechanism, the turning angle of the steering wheels can be maintained at an appropriate level, driving stability can be achieved, and the vehicle can be improved. In response to changes in vehicle speed during steering depending on the driving conditions, switching between the mechanisms described above is prevented.As a result, the steering feeling from stationary steering to high-speed driving is optimized, and the vehicle This makes it possible to improve the dynamic characteristics of the vehicle while ensuring running stability.

[0007]

Embodiment FIGS. 1 to 3 show an embodiment of an electric power steering device according to the present invention.
The schematic structure of the electric power steering device, which is generally designated by the reference numeral 1, will be briefly explained using FIG. An output shaft connected via a planetary gear mechanism 10 as a variable ratio mechanism, 4 a pinion shaft as a steering shaft that is integrally connected to this output shaft 3 by press fitting etc. and forming a steering gear mechanism 5, 6 this This rack has rack teeth 6a that mesh with the pinion 4a of the pinion shaft 4 and constitutes a part of the steering link mechanism.The pinion shaft 4 and the rack 6 form a well-known rack and pinion type steering gear mechanism 5. configured. Here, the details of such a steering gear mechanism 5 are well known, and a specific explanation will be omitted. Also, 7, 7 in the figure
, 7 are bearings that rotatably support the above-mentioned output shaft 3 and pinion shaft 4 within the steering body 8. It goes without saying that such a steering gear mechanism 5 is not limited to such a rack and pinion type.

Reference numeral 10 denotes a planetary gear mechanism serving as a variable transmission ratio mechanism for transmitting steering force from the above-mentioned steering wheel to the steering wheels (not shown), and its sun gear 11 is a drive gear mechanism provided on the input shaft 2. It is provided on a drive shaft 15 having a transmission gear 14 that meshes with a side gear 13 . Reference numeral 16 designates one or more planetary gears disposed around the sun gear 11 in mesh with it, and reference numeral 17 designates an outer ring gear disposed around the sun gear 11 and having an internal tooth portion 17a meshing with the planet gear 16. The outer ring gear 17 has a worm gear 1 on its outer periphery, which is a rotation transmitting means with an electric motor 20, which will be described later.
An external tooth portion 17b that meshes with the gear mechanism 8 is formed and is configured to function as a worm wheel, which allows rotation to be transmitted from the motor side, but does not allow rotation to be transmitted from the planetary gear mechanism 10 side. Transmission gear mechanism 21 with
is configured. Further, reference numeral 19 in the figure is a rotation transmission disk provided on the output shaft 3 side, and the support shaft 16a of the planetary gear 16 is connected to an eccentric position thereof. Note that the members such as the outer ring gear 17 that constitute these planetary gear mechanisms 10 are as follows:
Needless to say, it is rotatably held and disposed at an appropriate position by a bearing or the like. According to such a configuration, the steering force accompanying the steering operation with the steering handle is transmitted from the input shaft 2 to the drive shaft 15 side, and is transmitted to the sun gear 11 of the planetary gear mechanism 10, the planet gears 16, and their supporting shafts. 1
6a to the rotation transmission disk 19, the planetary gear mechanism 10 transmits the rotation to the output shaft 3 with a required transmission ratio, and furthermore, the pinion shaft 4 integrally connected to the shaft.
transmitted to. By transmitting the rotation of the pinion shaft 4 to the rack 6 side, necessary steering control is performed at the steering wheel (not shown).

Reference numerals 22 and 23 denote first and second shafts attached to the input shaft 2 on the steering wheel side and the output shaft 3 on the steering wheel side in the steering force transmission ratio variable mechanism configured as described above. These two steering angle sensors 22
, 23 during steering, it is detected whether the steering request on the steering wheel side and the turning state on the steering wheel side match. In this case, if there is a difference in the outputs from these two sensors 22 and 23, the motor 20 is driven and its rotational force is transmitted to the outer ring gear 17, thereby making use of the planetary gear mechanism 10. , a control operation is performed to eliminate misalignment between the output shafts 2 and 3. 24 is a torque sensor for detecting a steering request transmitted from the steering wheel side by detecting the rotational displacement of the output shaft 3 forming a part of the steering shaft within the steering body 8; , a case is illustrated in which a magnetostrictive rotation detector such as that disclosed in, for example, Japanese Unexamined Patent Publication No. 2-281115 is used. That is,
A strain detection layer made of a high magnetic permeability soft magnetic material is provided on the output shaft 3, and a detection coil is provided on the body 8 side surrounding the strain detection layer to detect changes in magnetic permeability due to strain in the strain detection layer. The structure is as follows. The output obtained by this detection coil is sent to a detection circuit 25, and a detection signal is sent to a signal controller 26 constituting a control means for controlling an electrical control system to be described later. Note that signals from the first and second steering angle sensors 22 and 23 described above are also sent to the signal controller 26, and information from the vehicle speed sensor 27 is also input. Here, 28 in the figure is a power controller connected to the signal controller 26;
Reference numeral 29 denotes a battery, and the electric system control means constituted by these drives the electric motor 20 and electromagnetic clutches 31 and 32, which will be described later, and which are provided interposed in the rotation transmission system thereof.
It is configured to control on/off.

Reference numeral 30 denotes a power assist mechanism that transmits the rotational driving force of the electric motor 20 described above to the pinion shaft 4, which is the steering shaft, as a steering assist force via a helical gear mechanism, etc. The motor shaft 33 of the electric motor 20 A small gear 35 is fixedly attached to a first motor output shaft 34 connected to one end via a first electromagnetic clutch 31 by press fitting, etc., and a large gear 3 meshing with this small gear 35.
6 is fixed to the outer end of the pinion shaft 4 by press-fitting or the like. According to such a configuration,
By driving the electric motor 20 and transmitting rotational driving force to the pinion shaft 4 through a transmission system including a small gear 35 and a large gear 36, steering is transmitted from the steering wheel side through the planetary gear mechanism 10. This is to provide power assist to the driver, thereby reducing the steering effort of the helmsman. Further, 37 is a second motor output shaft connected to the other end of the motor shaft 33 of the electric motor 20 by press fitting or the like, and as is clear from FIG. A bevel gear 3 that meshes with a bevel gear 39 provided at the end of the worm gear 18 arranged orthogonally to each other.
8 is provided. And these bevel gears 38,3
9, the rotation from the motor shaft 33 side can be transmitted to the planetary gear mechanism 10 side. Note that this FIG. 2 is a schematic diagram viewed from the direction of arrow A in FIG.

In the electric power steering device 1 as described above, in this embodiment, a part of the steering gear mechanism 5 (input and output shafts 2 and 3) that transmits steering force from the steering wheel to the steering wheel side A planetary gear mechanism 10 is provided as a variable transmission ratio mechanism that is provided between The power assist mechanism 30 that transmits power to a portion (in this embodiment, the end of the pinion shaft 4 opposite to the output shaft 3 side) is selectively driven and controlled by one electric motor 20 according to the vehicle speed. It is composed of That is, the above-mentioned electric motor 20 is arranged in parallel with the steering body 8 constituting the steering gear mechanism 5, and one end of the motor shaft 33 of the electric motor 20 is connected to the first electromagnetic clutch 31 and the first motor. The output shaft 34 is connected to the pinion shaft 4 as a steering shaft by a transmission gear mechanism including a small gear 35 and a large gear 36, and the other end of the motor shaft 33 is connected to the second electromagnetic clutch 32 and the second motor output shaft. 37 bevel gears 38,
The transmission ratio in the planetary gear mechanism 10 as a steering force transmission ratio variable mechanism is controlled by the worm gear 18 having a bevel gear 39 and the outer gear 17 serving as a worm wheel that meshes with the worm gear 18 and constitutes the transmission gear mechanism 21 with a reverse rotation prevention function. is configured to be variable. Note that the first and second electromagnetic clutches 31 and 32 described above have a conventionally known structure, and a detailed explanation of their structure and operation will be omitted here.

[0012] In this configuration, when the vehicle is stationary at a stop or when steering while driving at low speed, the electric power assist mechanism 30 exerts a steering assist function, making it possible to perform light and appropriate steering with appropriate steering force. The first
The electromagnetic clutch 31 is turned on and the motor 20 is rotated in a desired direction. At this time, the second electromagnetic clutch 32, which constitutes the motor transmission system to the planetary gear mechanism 10 described above, is off, and its rotation transmission is interrupted. On the other hand, during steering during high-speed driving, the power assist mechanism 30 stops its operation by turning off the first electromagnetic clutch 31, and by turning on the second electromagnetic clutch 32 to connect the motor 20. The steering force transmission ratio variable mechanism is configured to control the transmission ratio to a desired state. That is, the transmission ratio of the steering force from the input shaft 2 to the output shaft 3 by the planetary gear mechanism 10 is based on the ratio of the number of teeth of the sun gear 11, the planetary gear 16, and the outer ring gear 17, and
By rotating the outer ring gear 17 in a required direction at a required rotation speed by
From the relationship, the transmission ratio can be variably controlled with the characteristics shown in FIG. In other words, when driving at low speeds, the transmission ratio is small, and as the speed increases, the transmission ratio of the steered wheel angle to the steering wheel angle increases. It is preferable that the electric motor 20 is controlled by the power controller 28 and the signal controller 26 so that the electric motor 20 has a characteristic that decreases as the vehicle speed increases. According to such a configuration, the electric power assist mechanism 30 is added to the steering device having the planetary gear mechanism 10 as a variable steering force transmission ratio mechanism from the steering wheel to the steering wheels.
When used in combination with this, it is possible to simplify the overall configuration of the device and improve the steering feel. In particular, according to the present invention, the steering wheel steering angle and the steering wheel steering angle as described above are controlled to desired states by changes in vehicle speed, and the power assist mechanism 30 is operated when the vehicle is stopped or traveling at low speed. By applying steering assist force, the turning angle of the steering wheels can be maintained at an appropriate angle, driving stability can be achieved, and the steering feeling from stationary steering to high-speed driving can be optimized, resulting in improved vehicle dynamic characteristics. It is something.

Now, according to the present invention, in the above-described configuration, the planetary gear mechanism 10 as a variable steering force transmission ratio mechanism
In selectively driving and controlling the power assist mechanism 30 and the power assist mechanism 30 according to the vehicle speed, the planetary gear mechanism 10 is normally operated at a predetermined vehicle speed or higher, and the power assist mechanism 30 is operated at a predetermined vehicle speed or lower. In this case, the operation switching control between these two mechanisms 10 and 30 is
It is configured to perform this operation only when the steering wheel is near the neutral position. That is, in the above-described structure, a vehicle speed signal is input to the signal controller 26, and when the vehicle speed is above a predetermined speed, the transmission ratio of the steering force is variably controlled by the planetary gear mechanism 10, and when the vehicle speed is below the predetermined speed, the transmission ratio of the steering force is variably controlled by the power assist mechanism 30 using the electric motor 20. Although the steering assist force is applied, switching between these two mechanisms 10 and 30 is performed only within a limited range when the steering wheel angle is in a neutral state, that is, when the vehicle is approximately in a straight-ahead state. This configuration is necessary to maintain the running stability of the vehicle, and it is desirable that the controllers 26 and 28 control the switching operation timing as described above. Further, according to the present invention, even if the vehicle speed becomes high while the power assist mechanism 30 is operating and the steering wheel is returned to near the neutral position, as long as the vehicle speed does not drop below a predetermined value, both mechanisms 10, 30 The configuration is such that operation switching control cannot be performed between the two. In other words, if steering is performed while driving at a low speed and the vehicle reaches a high speed with the power assist still operating, even if the steering wheel angle is at the neutral position (straight ahead), the vehicle speed will be reduced to a certain level. It is also necessary to perform control so that switching to the variable transmission ratio mechanism 10 side is not performed unless the following conditions occur. That is, if power assist is canceled in such a case, the steering force may suddenly become heavy, which is not preferable from the viewpoint of running stability. By setting the switching control between the two mechanisms 10 and 30 in consideration of the running conditions and running stability of the vehicle, sudden changes in the characteristics of the vehicle can be prevented, and the system can be used as a steering device. It is possible to demonstrate the performance of

Here, in the electric power steering device 1 as described above, the operation of the device is controlled by the power assist mechanism 3.
0 during PS operation or planetary gear mechanism 1
Whether the steering force transmission ratio variable mechanism is activated by 0,
Operating status indicator 40 for confirming operating status to notify the driver
It is advantageous from a practical point of view to attach this to the driver's cab or the like. In FIG. 1, reference numeral 41 indicates the operating state of the variable mechanism by lighting or flashing, and 42 indicates a power assist operating state. Furthermore, in the device 1 having the above-described configuration, the electric motor 20 is a DC motor.
The electromagnetic clutches 31 and 32, which transmit the rotation from For example, even if the steering system goes off due to a failure in the electrical system, etc., the steering system will naturally enter the manual steering state via the planetary gear mechanism 10, and the necessary steering can be ensured. There is.

It should be noted that the present invention is not limited to the structure of the above-described embodiment, but is free to modify and change the shape, structure, etc. of each part of the electric power steering device 1 as necessary, and various modifications can be made. An example can be considered. For example, in the embodiment described above,
Although the case where the planetary gear mechanism 10 and the power assist mechanism 30 as the variable steering force transmission ratio mechanism are driven by one electric motor 20 has been illustrated, the present invention is not limited to this, and for example, each mechanism 10 , 30 are driven by individually provided electric motors, the present invention can be applied and the effect can be achieved. In addition, as a mechanism for connecting the steering wheel to the steering gear mechanism 5 side in a manner that can vary the transmission ratio of the steering force, a sun gear 11 is attached to the steering wheel side, and rotation is transmitted to the output shaft 3 side that is integrated with the pinion shaft 4 as the steering shaft. Although a case has been described in which the planetary gear 16 is connected via the disk 19 and the outer ring gear 17 is connected to the transmission gear mechanism 21 having a reverse rotation prevention function using a worm gear on the motor 20 side, the present invention is not limited to this. First, as shown in FIG. 4, an outer ring gear 17 is connected to the output shaft 3 side, and meshes with the worm gear 18 on the outer circumference of a rotation transmission disk 19 that is connected to the planetary gear 16 side. It will be easily understood that even if the external toothed portion 19a is formed and the disc 19 is used as a worm wheel, the same effect as in the embodiment described above can be achieved. In short, input shaft 2
In transmitting the steering force from the motor to the output shaft 3 side, any configuration may be used as long as the transmission ratio can be variably controlled by rotation control from the motor side.

Further, in the embodiment described above, the planetary gear mechanism 10 and the motor 20 exemplified as the variable steering force transmission ratio mechanism are replaced by a transmission gear mechanism 21 having a reverse rotation prevention function that allows rotation only from the motor side. Although a worm gear mechanism is illustrated as an example, it is obvious that the present invention is not limited to this, and may be a gear mechanism using, for example, a hypoid gear. Furthermore, the gear mechanism for transmitting steering assist force that constitutes the power assist mechanism 30 from the motor 20 is not limited to a transmission gear mechanism consisting of a small gear 35 and a large gear 36 using helical gears, but also includes hypoid gears, spur gears, etc. A gear mechanism consisting of off-axis gears such as helical gears, planetary gears, bevel gears, etc. may be used, and various modifications may be considered. Further, in the above-described structure of the embodiment, a directly-extractable magnetostrictive rotation detector is used as the torque sensor 24 for detecting the steering direction, steering speed, steering force, etc. at the steering wheel on the steering shaft side. The present invention is not limited to this, and it is of course possible to use a detection mechanism that connects two shafts using a torsion bar and detects the above-mentioned steering request based on their relative rotational displacement.

[0017]

As explained above, according to the electric power steering device according to the present invention, the electric power steering device attached to the steering gear mechanism changes the steered wheel steering angle with respect to the steering wheel steering angle according to the magnitude of the vehicle speed. A variable transmission ratio mechanism that is driven and controlled by a motor, a power assist mechanism that transmits the rotational driving force from the electric motor to a part of the steering gear mechanism, and these are selectively controlled depending on the vehicle speed. The steering power transmission ratio variable mechanism is normally operated at a predetermined vehicle speed or higher, and the power assist mechanism is operated at a predetermined vehicle speed or lower. Even if the vehicle speed becomes high while the power assist mechanism is operating, and the steering wheel is returned to the neutral position, as long as the vehicle speed does not drop below a predetermined value, Since the configuration is configured so that switching control is not performed, despite the simple configuration, when stopped or driving at low speed, the steering assist function by the electric power assist mechanism is activated, and steering with light and appropriate steering force is performed. Not only is this possible, but during high-speed driving, the power assist mechanism is stopped, and the variable steering force transmission ratio mechanism controls the transmission ratio to the required state, thereby maintaining the appropriate turning angle of the steered wheels. In addition to achieving driving stability, this system prevents switching between the above-mentioned mechanisms in response to vehicle speed fluctuations during steering depending on the vehicle's driving conditions, and as a result, the system changes from stationary steering to high-speed driving. This has various excellent effects, such as optimizing the steering feel of the vehicle, thereby ensuring the running stability of the vehicle and improving its dynamic characteristics.

[Brief explanation of drawings]

FIG. 1 is a schematic configuration diagram of the entire device showing an embodiment of an electric power steering device according to the present invention.

FIG. 2 is a schematic enlarged view seen from the direction of arrow A in FIG. 1;

FIG. 3 is a characteristic diagram showing the change characteristics of the transmission ratio according to the vehicle speed by the steering force transmission ratio variable mechanism according to the present invention.

FIG. 4 is a schematic configuration diagram showing another embodiment of the steering force transmission ratio variable mechanism portion in the device of the present invention.

[Explanation of symbols]

1 Electric power steering device 2 Input shaft 3 Output shaft 4 Pinion shaft (steering shaft) 5
Steering gear mechanism 6 Rack 8 Steering body 10 Planetary gear mechanism (variable steering force transmission ratio mechanism) 1
1 Sun gear 13 Drive side gear 14 Transmission gear 15 Drive shaft 16 Planetary gear 16a Support shaft 17 Outer gear (weam wheel) 17a
Internal tooth section 17b External tooth section 18 Worm gear 19 Rotation transmission disk 20 Electric motor 21 Transmission gear mechanism 22 First steering angle sensor 23 Second steering angle sensor 24 Torque sensor 25 Detection circuit 26 Signal controller 27 Vehicle speed sensor 28 Power controller 30 Power assist mechanism 31 Electromagnetic clutch 32 Electromagnetic clutch 33 Motor shaft 34 First motor output shaft 35 Small gear 36 Large gear 37 Second motor output shaft 38 Bevel gear 39 Bevel gear 40 Operating status indicator

Claims (2)

[Claims] Claim 1: A steering gear mechanism that is provided as part of a steering gear mechanism that transmits steering force from a steering wheel to a steering wheel, and is configured to change the steering angle on the steering wheel side in accordance with the vehicle speed with respect to the steering wheel steering angle. a variable rotational transmission ratio mechanism whose drive is controlled by an electric motor; a power assist mechanism which transmits the rotational driving force from the electric motor to a part of the steering gear mechanism; and the variable transmission ratio mechanism and the power assist mechanism. The vehicle is equipped with a control means for selectively controlling the drive according to the magnitude of the vehicle speed, and the control means operates a variable steering force transmission ratio mechanism when the vehicle speed is above a predetermined vehicle speed, and a power assist mechanism when the vehicle speed is below a predetermined vehicle speed. What is claimed is: 1. An electric power steering device characterized in that the electric power steering device is configured such that the operation switching control between these two mechanisms is performed only when the steering wheel is near the neutral position. [Claim 2] A steering wheel is provided as a part of a steering gear mechanism that transmits steering force from a steering wheel to a steering wheel, and is configured to change a steering angle on a steering wheel side in accordance with vehicle speed relative to a steering wheel steering angle. a variable rotational transmission ratio mechanism whose drive is controlled by an electric motor; a power assist mechanism which transmits the rotational driving force from the electric motor to a part of the steering gear mechanism; and the variable transmission ratio mechanism and the power assist mechanism. It is equipped with a control means that selectively controls the drive according to the magnitude of the vehicle speed, and this control means normally controls the steering force transmission ratio variable mechanism at a predetermined vehicle speed or higher, and the power assist mechanism at a predetermined vehicle speed or lower. In addition, even if the vehicle speed is high while the power assist mechanism is in operation and the steering wheel is returned to near the neutral position, the operation between these two mechanisms will continue as long as the vehicle speed does not drop below a predetermined value. An electric power steering device characterized by being configured such that switching control cannot be performed.