JP2949603B2 - Electric power steering system - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electric power assist mechanism using an electric motor as a steering assist force generating means, and to change a steering angle on a steered wheel with respect to a steering angle of a steering wheel according to a vehicle speed. In combination with the variable transmission ratio mechanism of the steering force to be used, the steering sensation can be improved during stationary operation or during low-speed traveling to high-speed traveling, and the dynamic characteristics of the vehicle can be improved. The present invention relates to an electric power steering device that performs steering control in accordance with running conditions to ensure running stability.

[0002]

2. Description of the Related Art In general, a steering device for a vehicle or the like converts a rotational movement at the time of steering to a steering wheel into a linear movement of a tie rod through a steering gear mechanism of a rack and pinion type or the like, so that steering wheels are formed. The steering control can be performed to a required state. In this case, in a normal steering device, the transmission ratio is set to be constant by the steering gear mechanism. However, for example, when the vehicle is traveling at high speed, the transmission ratio of the steering wheel turning angle (steering wheel steering angle) to the steering rotation amount (steering wheel steering angle) of the steering wheel described above is increased to increase the steering wheel steering angle with respect to a constant steering wheel steering angle. It is preferable to reduce the magnitude of the steering wheel steering angle in order to secure running stability.
From the standpoint of obtaining a good driving feeling by accelerating the behavior of the vehicle or facilitating garage entry, the above-mentioned transmission ratio is made smaller on the contrary, and the steering wheel steering angle for a fixed steering wheel steering angle is increased. It is desired that the steering force transmission ratio variable mechanism having various structures be disclosed in, for example, Japanese Patent Laid-Open No. 58-224.
No. 852, for example. In other words, in order to obtain an appropriate steering state, it is necessary to change the turning angle of the steered wheels with respect to the steering wheel operation according to the driving conditions.
This is desirable for further improving running stability and the like, and measures have been taken for this purpose.

Further, in such a steering device, a steering operation can be performed with a light steering force as light as possible and with a good feeling, especially when the vehicle is stationary or when the vehicle is traveling at a low speed. It is also desired that such a configuration is adopted. For example, as shown in Japanese Patent Application Laid-Open No. Sho 62-31559, in a conventionally known power steering device of a hydraulic type or the like, the power steering device is provided in the middle of a steering gear mechanism of the device. There has also been proposed a configuration in which a transmission ratio variable mechanism of a steering force capable of varying a transmission ratio by an actuator such as a stepping motor is interposed. According to such a configuration, the transmission ratio set in consideration of the magnitude of the steering assist force (power assist force) by the power steering device is changed in accordance with the vehicle speed. The steering wheel steering angle can be variably controlled according to the vehicle speed or the like so as to be in a required state, and it is possible to obtain a steering control characteristic according to the running state of the vehicle.

[0004]

By the way, in the above-mentioned conventional device, a power assist mechanism constituting a power steering device is attached to the steering gear mechanism, and a transmission ratio variable mechanism responsive to a vehicle speed or the like is separately provided. And both of these mechanisms are selectively operated according to the magnitude of the vehicle speed, but the running conditions of the vehicle, that is, the vehicle speed is increased or decreased during steering. In case,
Switching between the two mechanisms described above in conjunction only with the vehicle speed is inevitable for the driver in that the steering sensation changes drastically, and is a problem in securing the running stability of the vehicle. Therefore, it is desired to take such a point into consideration, take into consideration the running stability of such a vehicle, and take some measures to improve the steering characteristics. In particular, if the switching between the mechanisms described above is performed during the steering, the magnitude of the steering force changes abruptly, which poses a serious practical problem for the driver.

[0005]

In order to meet such a demand, an electric power steering apparatus according to the present invention is provided with a part of a steering gear mechanism for transmitting a steering force from a steering wheel to a steered wheel side. A variable transmission ratio mechanism that is driven and controlled by an electric motor that changes the steering angle on the steered wheels side with respect to the steering angle of the steering wheel in accordance with the magnitude of the vehicle speed; A power assist mechanism for transmitting the power to a part of the mechanism, and a control means for selectively controlling the drive of these two mechanisms according to 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 equal to or lower than the predetermined vehicle speed. It is obtained by configured to perform only one time. Further, according to the present invention, even if the vehicle speed is high during the operation of the power assist mechanism and the steering wheel is returned to the vicinity of the neutral position, the operation between these two mechanisms is performed as long as the vehicle speed does not fall below the predetermined vehicle speed. The configuration is such that switching control cannot be performed.

[0006]

According to the present invention, when the vehicle is stopped or running at a low speed, the steering assist function by the electric power assist mechanism is exhibited, so that the steering can be performed with a light and appropriate steering force, and at the time of running at a high speed. By stopping the power assist mechanism and controlling the transmission ratio by the variable steering force transmission ratio mechanism to the required state, the steering angle of the steered wheels can be maintained properly, and running stability can be exhibited. The switching between the mechanisms described above is not performed with respect to the vehicle speed fluctuation during steering depending on the driving conditions, and as a result, the steering feeling from the stationary operation to the high-speed driving state is optimized, and the vehicle The dynamic characteristics can be improved while ensuring the running stability of the vehicle.

[0007]

1 to 3 show an embodiment of an electric power steering apparatus according to the present invention.
The schematic configuration of an electric power steering apparatus generally denoted by reference numeral 1 will be briefly described with reference to FIG. 1. Reference numeral 2 denotes an input shaft connected to a steering handle (not shown); An output shaft 4 connected via a planetary gear mechanism 10 as a variable ratio mechanism is integrally connected to the output shaft 3 by press fitting or the like, and a pinion shaft as a steering shaft constituting a steering gear mechanism 5. A rack that has rack teeth 6a meshing with the pinion 4a of the pinion shaft 4 and constitutes a part of the steering link mechanism. The well-known rack and pinion type steering gear mechanism 5 is formed by the pinion shaft 4 and the rack 6. Be composed. Here, details of such a steering gear mechanism 5 are well known, and a specific description thereof will be omitted. Also, 7,
7, 7 are bearings for rotatably supporting the output shaft 3 and the pinion shaft 4 in the steering body 8. It is needless to say 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 a steering force from the steering wheel to the steered wheels (not shown). The sun gear 11 has a drive gear provided on the input shaft 2. It is provided on a drive shaft 15 having a transmission gear 14 meshing with the side gear 13. Reference numeral 16 denotes one or a plurality of planetary gears arranged around the sun gear 11 in a state meshed with the sun gear 11, and 17 denotes an outer ring gear arranged around the sun gear 11 and having an internal tooth portion 17 a meshed with the planetary gear 16. On the outer peripheral portion of the outer ring gear 17, a worm gear 1 serving as a rotation transmitting unit for an electric motor 20 described later is provided.
The external gear portion 17b meshing with the gear 8 is formed to be a worm wheel, whereby the rotation can be transmitted from the motor side, and the rotation cannot be transmitted from the planetary gear mechanism 10 side. Transmission gear mechanism 21
Is configured. In the drawing, reference numeral 19 denotes a rotation transmitting disk provided on the output shaft 3 side, and a support shaft 16a of the planetary gear 16 is connected to an eccentric position thereof. Note that members such as the outer ring gear 17 and the like that constitute these planetary gear mechanisms 10 include:
Needless to say, it is disposed so as to be rotatable by a bearing or the like at an appropriate position. According to such a configuration, the steering force accompanying the steering operation with the steering wheel is transmitted from the input shaft 2 to the drive shaft 15 side, and the sun gear 11, the planetary gear 16 of the planetary gear mechanism 10, and the support shaft thereof. 1
The power is transmitted to the rotation transmitting disk 19 via 6a, and is transmitted to the output shaft 3 and the pinion shaft 4 integrally connected to the output shaft 3 with a required transmission ratio by the planetary gear mechanism 10. Then, by transmitting the rotation of the pinion shaft 4 to the rack 6 side, required turning control is performed on unillustrated steered wheels.

Reference numerals 22 and 23 are first and second attached to the input shaft 2 on the steering wheel side and the output shaft 3 on the steered wheel side in the variable steering force transmission ratio mechanism having the above-described configuration. The two steering angle sensors 2
By constantly comparing the outputs from the steering wheels 2 and 23 during steering, it is detected whether the steering request on the steering wheel side and the turning state on the steered wheels are in agreement. Then, in this case, these two sensors 22, 23
When there is a difference in the output from the motor, the motor 20 is driven and its rotational force is transmitted to the outer ring gear 17,
The planetary gear mechanism 10 is used to perform a control operation for eliminating a shift between the input and output shafts 2 and 3. Reference numeral 24 denotes a torque sensor for detecting a steering request transmitted from the steering wheel side by detecting a rotational displacement of the output shaft 3 forming a part of the steering shaft in the steering body 8, and in this embodiment, For example, a case where a magnetostrictive rotation detector as disclosed in Japanese Patent Application Laid-Open No. 2-281115 is used. That is, a strain detecting layer made of a soft magnetic material having a high magnetic permeability is provided on the output shaft 3, and a detecting coil for detecting a change in magnetic permeability due to the strain of the strain detecting layer is provided on the body 8 side so as to surround the periphery. The configuration is Then, an output obtained by the detection coil is sent to a detection circuit 25, and a detection signal is sent to a signal controller 26 which constitutes control means for controlling an electric control system described later. The signal from the first and second steering angle sensors 22 and 23 is also sent to the signal controller 26, and information from the vehicle speed sensor 27 is also input. Here, in the figure, 28 is a power controller connected to the signal controller 26, 29
Is a battery, which is configured to control on / off of the electric motor 20 and the electromagnetic clutches 31 and 32, which will be described later, provided via the electric motor 20 and its rotation transmission system by an electric system control means constituted by these batteries.

Reference numeral 30 denotes a power assist mechanism for transmitting the rotational driving force of the electric motor 20 to the pinion shaft 4 serving as the steering shaft as a steering assist force via a helical gear mechanism or the like. A small gear 35 is fixed to a first motor output shaft 34 connected to one end of the first gear via a first electromagnetic clutch 31 by press-fitting or the like.
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 to transmit the rotational driving force to the pinion shaft 4 via the transmission system including the small gear 35 and the large gear 36, the steering transmitted from the steering wheel side via the planetary gear mechanism 10 is provided. This is to assist the power, thereby reducing the steering operation force of the driver. Reference numeral 37 denotes a second motor output shaft which is connected to the other end of the motor shaft 33 of the electric motor 20 by press-fitting or the like. Bevel gear 3 meshing with a bevel gear 39 provided at an end of the worm gear 18 arranged orthogonally
8 are 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 FIG. 2 is a schematic diagram viewed from the direction of arrow A in FIG.

In this embodiment, in the electric power steering apparatus 1 described above, a part of the steering gear mechanism 5 (on, output shafts 2, 3) for transmitting the steering force from the steering handle to the steered wheels. The planetary gear mechanism 10 as a transmission ratio variable mechanism that changes the steering angle on the steered wheel side with respect to the steering wheel steering angle, and the rotational driving force from the electric motor 20 is applied to the steering gear mechanism 5. One electric motor 20 selectively drives and controls the power assist mechanism 30 transmitting to a part (the end of the pinion shaft 4 on the opposite side to the output shaft 3 side in this embodiment) according to the vehicle speed. It is composed. That is, the electric motor 20 described above is disposed 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. A transmission gear mechanism including a small gear 35 and a large gear 36 of the output shaft 34 is connected to the pinion shaft 4 as a steering shaft, 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,
A worm gear 18 having a bevel gear 39 and an outer ring gear 17 which meshes with the worm gear and forms a worm wheel constituting a transmission gear mechanism 21 with a reverse rotation preventing function, by a transmission ratio in the planetary gear mechanism 10 as a variable transmission ratio mechanism of a steering force. Is configured to be variable. The first and second electromagnetic clutches 31 and 32 have a conventionally well-known structure, and a description of a specific configuration and operation thereof will be omitted.

In such a configuration, the steering assist function of the electric power assist mechanism 30 is exerted when the vehicle is stationary or when the vehicle is being steered at a low speed, so that the steering can be performed with a light and appropriate steering force. So that the first
Is turned on, and the motor 20 is driven to rotate in a required direction. At this time, the second electromagnetic clutch 32 constituting the motor transmission system to the planetary gear mechanism 10 is off and its rotation transmission is interrupted. On the other hand, at the time of steering during high-speed running, the power assist mechanism 30 stops its operation by turning off the first electromagnetic clutch 31 and turns on the second electromagnetic clutch 32 to turn on the motor 20.
Are connected to each other, so that the transmission ratio of the steering force variable transmission mechanism can be controlled to a required 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 tooth ratio of the sun gear 11, the planetary gear 16, and the outer ring gear 17, and the outer ring gear 17 is The sun gear 11 and the planetary gear 1 are driven to rotate in a required direction at a required rotational speed.
6, the transmission ratio can be variably controlled with characteristics as shown in FIG. That is, the characteristic is such that the transmission ratio of the steering wheel steering angle to the steering wheel steering angle increases as the transmission ratio decreases during high-speed running and the vehicle speed increases, that is, the magnitude of the steering wheel steering angle for a constant steering wheel steering angle. It is preferable that the electric motor 20 be controlled by the power controller 28 and the signal controller 26 so that the electric motor 20 has a characteristic that becomes smaller as the vehicle speed increases. According to such a configuration, the planetary gear mechanism 10 as a variable steering force transmission ratio mechanism from the steering wheel to the steered wheels.
Power steering mechanism 3
When 0 is used together, the configuration of the entire apparatus can be simplified and the steering feeling can be improved. In particular, according to the present invention, the steering wheel steering angle and the steering wheel steering angle as described above are controlled to required states by changing the vehicle speed, and the power assist mechanism 30 is operated when the vehicle is stopped or running at low speed. By applying the steering assist force, the steering angle of the steered wheels can be properly maintained, the running stability can be exhibited, the steering feeling from stationary to high-speed running can be optimized, and as a result, the dynamic characteristics of the vehicle can be improved. Things.

According to the present invention, in the above-described configuration, the planetary gear mechanism 10 as the steering force transmission ratio variable mechanism is provided.
When selectively driving the power assist mechanism 30 and the power assist mechanism 30 in accordance with the vehicle speed, the planetary gear mechanism 10 is normally controlled to operate 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 performed as follows.
It is configured to perform this only when the steering wheel is near the neutral position. That is, in the above-described structure, the vehicle speed signal is taken into the signal controller 26, and the transmission ratio of the steering force is variably controlled by the planetary gear mechanism 10 when the vehicle speed is equal to or higher than the predetermined vehicle speed. Although the steering assist force is applied, the switching operation between these two mechanisms 10 and 30 is performed only in a limited range where the steering angle of the steering wheel is in a neutral state, that is, the vehicle is in a vicinity of a substantially straight traveling state. The configuration is required to maintain the running stability of the vehicle, and it is desired that the controllers 26 and 28 control the switching operation timing as described above. Further, according to the present invention, even when the vehicle speed becomes high during the operation of the power assist mechanism 30 and the steering wheel is returned to the vicinity of the neutral position, as long as the vehicle speed does not become lower than the predetermined vehicle speed, the two mechanisms 10, 30 are used. It is configured so that the operation switching control between the two cannot be performed. In other words, steering is performed during low-speed running, and power assist is still operating,
When the vehicle is in a high-speed state, control is performed such that switching to the variable transmission ratio mechanism 10 is not performed even if the steering angle of the steering wheel is in the neutral position (straight running state) unless the vehicle speed falls below a certain vehicle speed. It is also necessary. That is, if the power assist is canceled in such a case, the steering force may suddenly increase, 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 of the vehicle and the running stability, etc., a sudden change in the characteristics of the vehicle can be prevented and the steering device can be used. It can exhibit 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
It is determined whether or not it is during the operation of the transmission ratio variable mechanism of the steering force by 0.
Operating state indicator 40 for operating state confirmation to inform the driver
It is practically advantageous to attach the device to a cab or the like. In FIG. 1, reference numeral 41 denotes a lamp for notifying the operating state of the variable mechanism by lighting or blinking, and reference numeral 42 denotes a lamp for notifying the power assist operating state. Further, in the apparatus 1 having the above-described configuration, the electric motor 2 using the DC motor is used.
The electromagnetic clutches 31 and 32 that transmit the rotation from zero to any of the mechanisms 10 and 30 are always in an off state to interrupt the rotation transmission, and are turned on as necessary to perform the rotation transmission. For example, even if both are turned off due to a failure of the electric system or the like, the steering system naturally enters a manual steering state via the planetary gear mechanism 10, and necessary steering can be secured. ing.

It should be noted that the present invention is not limited to the structure of the embodiment described above, and the shape, structure, etc. of each part of the electric power steering device 1 can be freely modified and changed as necessary, and various modifications are possible. An example could 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 steering force transmission ratio variable mechanism are driven by one electric motor 20 is illustrated, the present invention is not limited to this. , 30 can be applied to achieve an effect even when driven by an electric motor provided separately. In addition, as a mechanism for connecting the steering handle to the steering gear mechanism 5 side so that the transmission ratio of the steering force can be changed, the sun gear 11 is transmitted to the handle side and the rotation is transmitted to the output shaft 3 side integrated with the pinion shaft 4 as the steering shaft. The case where 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 the reverse rotation preventing function by the worm gear on the motor 20 side has been described, but the present invention is not limited to this. Instead, as shown in FIG. 4, the outer ring gear 17 is connected to the output shaft 3 side, and meshes with the worm gear 18 on the outer peripheral portion of the rotation transmitting disc 19 connected to the planetary gear 16 side. It is easily understood that even if the external teeth 19a are formed and the disk 19 is used as a worm wheel, the same operation and effect as those of the above-described embodiment can be obtained. Ocean. In short, any configuration may be used as long as the transmission ratio can be variably controlled by rotation control from the motor side when transmitting the steering force from the input shaft 2 to the output shaft 3 side.

Further, in the above-described embodiment, the planetary gear mechanism 10 exemplified as the steering force transmission ratio variable mechanism described above and the motor 20 are replaced with a transmission gear mechanism 21 having a reverse rotation preventing function capable of rotating only from the motor side. Although the worm gear mechanism has been described as an example, the present invention is not limited to this, and it is apparent that a gear mechanism using a hypoid gear or the like may be used. Further, the gear mechanism for transmitting the steering assist force constituting the power assist mechanism 30 from the motor 20 is not limited to the transmission gear mechanism using the small gear 35 and the large gear 36 by the helical gear, but includes a hypoid gear, a spur gear, and the like. A gear mechanism including an off-axis gear such as a helical gear, a planetary gear, a bevel gear, or the like may be used, and various modifications may be considered. Further, in the structure of the embodiment described above,
As the torque sensor 24 for detecting the steering direction, the steering speed, the steering force, and the like with the steering handle on the steering shaft side, a direct extraction type magnetostrictive rotation detector has been exemplified, but the present invention is not limited thereto. For example, it is a matter of course that a detection mechanism that connects the two shafts with a torsion bar and detects the above-described steering request based on the relative rotational displacement may be used.

[0017]

As described above, according to the electric power steering apparatus of the present invention, the electric power steering apparatus is provided with the steering gear mechanism and changes the steering wheel steering angle with respect to the steering wheel steering angle in accordance with 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 selectively controls these drives according to the magnitude of the vehicle speed. Control means for controlling the transmission ratio variable mechanism of the steering force at a predetermined vehicle speed or higher and the power assist mechanism at a predetermined vehicle speed or lower, and controlling the operation switching between the two mechanisms at all times. , Only when the steering wheel is near the neutral position, or when the vehicle speed is high while the power assist mechanism is operating, and the steering wheel is near the neutral position. Even if the vehicle is returned, the switching control is not performed unless the vehicle speed falls below a predetermined vehicle speed. This function not only makes it possible to perform steering with light and appropriate steering force, but also stops the power assist mechanism during high-speed running and controls the transmission ratio by the variable steering force transmission ratio to the required state. By doing so, it is possible to maintain the steering wheel turning angle appropriately and exert running stability, and to prevent the vehicle speed fluctuation during steering due to the running conditions of the vehicle from being switched between the mechanisms described above. As a result, it is possible to optimize the steering feeling from stationary to high-speed driving, thereby improving the dynamic characteristics of the vehicle while ensuring the running stability of the vehicle. There are people excellent effect.

[Brief description of the drawings]

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

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

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

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

[Explanation of symbols]

 Reference Signs List 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 (steering force transmission ratio variable mechanism) 11 sun gear 13 drive side gear 14 transmission Gear 15 Drive shaft 16 Planetary gear 16a Support shaft 17 Outer ring gear (weam wheel) 17a Internal gear 17b External gear 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 operation status indicator

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-62-31559 (JP, A) JP-A-58-224852 (JP, A) JP-A-63-240476 (JP, A) 247973 (JP, A) JP-A-62-238168 (JP, A) JP-A-1-204864 (JP, A) JP-A-2-155871 (JP, A) JP-A-64-32969 (JP, A) (58) Field surveyed (Int. Cl. ⁶ , DB name) B62D ^6/ 00-6/02 B62D 5/04

Claims (2)

(57) [Claims] 1. A steering wheel mechanism for transmitting steering force from a steering wheel to a steered wheel side, wherein the steering angle on the steered wheel side is changed with respect to the steering wheel angle in accordance with the vehicle speed. A variable transmission ratio mechanism that is driven and controlled by an electric motor, a power assist mechanism that transmits rotational driving force from the electric motor to a part of the steering gear mechanism, and a variable transmission ratio mechanism and a power assist mechanism. A control means for selectively controlling the drive according to the magnitude of the vehicle speed is provided. The control means activates the variable transmission ratio mechanism of the steering force at a predetermined vehicle speed or more, and activates the power assist mechanism at a predetermined vehicle speed or less. And the operation switching control between these two mechanisms is performed only when the steering wheel is near the neutral position. Electric power steering system. 2. A steering wheel mechanism for transmitting a steering force from a steering wheel to a steering wheel side, wherein the steering angle on the steering wheel side is changed with respect to the steering wheel angle in accordance with the vehicle speed. A variable transmission ratio mechanism that is driven and controlled by an electric motor, a power assist mechanism that transmits rotational driving force from the electric motor to a part of the steering gear mechanism, and a variable transmission ratio mechanism and a power assist mechanism. Control means for selectively controlling the drive in accordance with the magnitude of the vehicle speed is provided.The control means always 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 to controlling to operate, even if the vehicle speed is high while the power assist mechanism is operating and the steering wheel is returned to near the neutral position, An electric power steering apparatus characterized in that the operation switching control between these two mechanisms cannot be performed unless the vehicle speed becomes lower than the vehicle speed.