JPS63151578A - Four-wheel steering apparatus for vehicle - Google Patents

Abstract

PURPOSE:To enable the steering ratios to be variably controlled certainly by making corrections for the driving characteristics of an actuator at low temperatures so that they are inhibited from unnecessarily shifting to a fail-safe mode even when a steering ratio changing mechanism is operated at a low temperature. CONSTITUTION:A stepping motor 51 and a solenoid valve 28 are designed in such a manner that their operation is controlled by means of the output from a control unit 100. To the control unit 100 are inputted each of the signals from a vehicle speed sensor 102, a temperature sensor 103 which detects the operating temperature in a steering-ratio changing mechanism, and a steering ratio sensor 101. While the operation of the stepping motor is being controlled, by virtue of the temperature sensor 103 is detected the operating temperature in the steering ratio changing mechanism, and then, the detected temperature is compared with a fixed value at a driving torque correcting unit 105 in the control unit. If the detected temperature is lower than the fixed value, the pulse rate of the driving pulse signals to the stepping motor is corrected so as to become low. Thereby, the output torque of the stepping motor is increased, and then, even if the viscosity of the lubricating oil is high, a reliable operation can be performed.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a four-wheel steering device that steers the front and rear wheels of a vehicle, and particularly relates to a steering ratio change that controls the steering ratio of the front and rear wheels. Concerning countermeasures against malfunction of mechanisms at low temperatures.

(Prior art) In recent years, this type of four-wheel steering system for vehicles has attracted attention as a device that can greatly change the driving characteristics of the vehicle. The steering ratio is controlled to be in the opposite phase and the steering characteristics are changed to oversteer characteristics to improve the turning performance of the vehicle.At the same time, at high vehicle speeds or at small steering angles, the steering ratio is kept in the same phase and the steering characteristics are changed to understeer characteristics. This is to ensure the running stability of the vehicle.

As an example of this four-wheel steering device,
Japanese Patent No. 193770 proposes an actuator in which a stepping motor (pulse motor) is used to variably control the steering ratio of front and rear wheels.

Specifically, the movable member is connected to a rear wheel steering mechanism that steers the rear wheels of the vehicle and is movable in a predetermined movement axis direction, and the movable member swings with a swing center located on the movement axis of the moving member. A swing arm called a swash plate, a connecting member that connects the swing arm and the moving member, and a front wheel steering mechanism that steers the front wheels of the vehicle, and the linking member is connected to the front wheel steering mechanism that steers the front wheels of the vehicle. By providing a rotation imparting arm that rotates around the axis, and changing the inclination angle of the swing center line of the swing arm with respect to the movement axis of the moving member using a stepping motor,
It is designed to change the steering ratio of the front and rear wheels.

(Problems to be Solved by the Invention) Incidentally, the moving parts of the steering ratio changing mechanism that is driven by the stepping motor and changes the steering ratio of the front and rear wheels are coated with sympathetic oil such as grease. When the steering ratio changing mechanism is kept inactive and its operating temperature is low, such as after a vehicle has stopped for a long period of time, the load on the stepping motor increases extremely due to the viscous resistance of the lubricating oil. There is a risk that it will be determined that the engine is running idle. However, when this stepping motor goes out of step,
Normally, in order to improve safety, for example, the steering ratio is forcibly reduced to zero and the vehicle's steering characteristics are maintained at a natural two-wheel steering characteristic, in which case the vehicle enters a fail-safe mode. As the system shifts to fail-safe mode, proper four-wheel steering control cannot be performed after that, and there is room for improvement.

In addition, in order to control the steering ratio to the target value against the viscous resistance of the lubricating oil at low temperatures, a large stepping motor with high output is used so that the load at low temperatures is the rated load. Although it is possible, it is disadvantageous in terms of cost.

Further, even if the steering ratio changing mechanism is driven by an actuator other than a stepping motor, such as a DC motor, the same problem as above occurs.

(Object of the invention) The present invention has been made to solve such problems,
The purpose of this is to correct the drive characteristics of the actuator at low temperatures as described above.
The output torque is increased when the steering ratio change mechanism is operated at a low temperature while using a rotary rotor, so that the steering ratio can be reliably changed without unnecessarily shifting to fail-safe mode even when the operating temperature of the steering ratio change mechanism is low. The goal is to be able to control it.

(Means for solving the problem) In order to achieve this objective, the solving means of the present invention is as follows:
As shown in the figure, as a four-wheel steering device for a vehicle that steers both the front wheels and the rear wheels, there is a steering ratio changing mechanism 32 that changes the steering ratio of the front and rear wheels; an actuator 51 such as a stepping motor that drives the steering wheel; 51 is provided.

Further, a temperature detection means 103 is provided for detecting the operating temperature T of the steering ratio changing mechanism 32, and the output of the temperature detection means 103 is received to drive the actuator 51 when the operating temperature T is lower than a predetermined value. The driving torque correction means 105 is provided for correcting the output characteristics of the steering ratio control means 104 so as to increase the torque.

Increasing the drive torque of the actuator 51, for example in the case of a stepping motor, is achieved by lowering the pulse rate (number of pulses per unit time) of the drive pulse signal output to the motor.

(Function) With the above configuration, in the present invention, the steering ratio control means 104
A target steering ratio for the front and rear wheels is set based on the steering ratio characteristics, and the output signal of the steering ratio control means 104 drives the actuator 51 to control the operation of the steering ratio changing mechanism 32. By operating the ratio changing mechanism 32, the steering ratios of the front and rear wheels are controlled to become the target steering ratio.

In addition, in parallel with such four-wheel steering control, the operating temperature T of the steering ratio changing machine groove 32 is detected by the temperature detecting means 103, and under this operating temperature, the driving torque correcting means 1
05, it is compared with a predetermined value. When the operating temperature is higher than a predetermined value, the steering ratio control means 10
The output characteristics for the actuator 51 are not corrected, but the output characteristics are corrected so that the driving torque of the actuator 51 increases when the temperature is lower than a predetermined value. As a result, even if the viscous resistance of the lubricating oil in the steering ratio changing mechanism 32 increases at low temperatures, the steering ratio can be changed while using the normal small actuator 51 without unnecessarily shifting to fail-safe mode or the like. This means that the target value can be changed reliably.

(Example) Hereinafter, an example of the present invention will be described based on the drawings from FIG. 2 onwards.

In FIG. 2, 1L to 2R are four wheels of the vehicle, and the left and right front wheels 1L and 1R are linked by a front wheel steering mechanism 13, and the left and right rear wheels 2L and 2R are linked by a rear wheel steering mechanism 12. ing.

The front wheel steering mechanism 3 includes a pair of left and right knuckle arms 4L.
, 4R, tie rods 5L, 5R, and a relay rod 6 that connects the left and right tie rods 5L, 5R. A steering wheel 10 is connected to the front wheel steering mechanism 3 via a rack and pinion type steering mechanism 7.
are linked. That is, a rack 8 is formed on the relay rod 6, while a pinion 9 that meshes with the rack 8 is attached to the lower end of the steering shaft 11, to which the steering wheel 10 is connected to the upper end. The left and right front wheels 1L and 1R are steered according to the operation.

On the other hand, similar to the front wheel steering mechanism 3, the rear wheel steering mechanism 12 includes left and right knuckle arms 13L, 13 and tie rods 14L, 14R, and the tie rods 14L, 14R.
A hydraulic power steering mechanism 16 is provided. The power steering mechanism 16 includes a power cylinder 17 that is fixed to the vehicle body and has the relay rod 15 as a piston rod, and inside the power cylinder 17, two hydraulic chambers are formed by a piston 17a that is integrally attached to the relay rod 15. The hydraulic chambers 17b and 17C within the cylinder 17 are respectively connected to the control valve 20 via hydraulic pipes 18 and 19. The control valve 20 also has an oil supply pipe 22 and an oil discharge pipe 2 leading to the reserve tank 21.
Two pipes No. 3 are connected to each other, and the oil supply pipe 22 is provided with a hydraulic pump 24 driven by an on-vehicle engine (not shown). The control valve 20 is a known spool valve type, and includes a cylindrical valve casing 20a integrally attached to the relay rod 15 via a connecting member 25, and a cylindrical valve casing 20a inside the valve casing 2Oa. The relay rod 15 is driven by supplying pressure oil from the hydraulic pump 24 to one hydraulic chamber 17b (17G) of the power cylinder 17 according to the movement of the spool valve (not shown). It assists in power.

In addition, the relay rod 15 is placed in the neutral position (rear wheel 2L) through the piston 17a in the power cylinder 17.
, 2R, a pair of return springs 17d bias the steering angle θR to a position where the steering angle θR is zero.

17d has been reduced. In addition, the above hydraulic piping 18.1
9 are connected to a normally closed electromagnetic on-off valve 28 via hydraulic pipes 26 and 27, respectively, and when this electromagnetic on-off valve 28 is opened, both hydraulic chambers 17b of the power cylinder 17,
The return spring 17d uses the same pressure as the oil pressure in 17c.
, 17d positions the piston 17a at a neutral position, and the steering angle θR of the rear wheels 2L and 2R is always set to θR=O, so that the steering characteristic of the vehicle is brought into a two-wheel steering state.

The relay rod 6 of the front wheel steering @@3 is equipped with another rack 2 in addition to the rack 8 that constitutes the steering mechanism 7.
9 is formed, and a pinion 30 attached to the front end of a rotating shaft 31 extending in the longitudinal direction of the vehicle body is engaged with the rack 29, and the rear end of the rotating shaft 31 is connected to the rear wheel steering through a steering ratio changing mechanism 32. It is linked to mechanism 12.

As detailed in FIG. 3, the steering ratio changing mechanism 32 includes:
The control rod 33 has a control rod 33 slidably held on an axis of movement, lh, in the vehicle width direction with respect to the vehicle body, and one end of the control rod 33 is connected to the spool valve of the control valve 20. Further, the steering ratio changing mechanism 32 includes a swinging arm 36 whose base end is swingably supported by a U-shaped holder 34 via a support pin 35, and the holder 34 is connected to a casing ( (not shown) via a support shaft 37 having a rotation axis I22 perpendicular to the movement axis jl of the control rod 33. Support pin 35 of the swing arm 36
is located at the intersection of the two axes fJlt and Q2 and extends in a direction perpendicular to the rotation axis g2, and by rotating the holder 34 around the support shaft 37 (rotation axis J2),
The angle of inclination between the support pin 35 at the tip of the support pin 35 and the movement axis f11 of the control rod 33, that is, the plane in which the swing locus plane of the swing arm 36 centered on the support pin 35 is orthogonal to the movement axis ρ1 (hereinafter referred to as the reference plane The angle of inclination made with respect to the

Further, one end of a connecting rod 39 is connected to the tip of the swing arm 36 via a ball joint 38, and the other end of the connecting rod 39 is connected to the control rod 33 via a ball joint 40. The control rod 33 is connected to the other end, and is configured to displace the control rod 33 in the left-right direction in response to displacement of the tip end of the swing arm 36 in the left-right direction in FIG.

The connecting rod 39 is slidably supported by a rotation imparting arm 41 via a ball joint 42 at a portion thereof close to the ball joint 38 . The rotation imparting arm 41 is provided integrally with a large-diameter bevel gear 44 rotatably supported on the movement axis g] via a support shaft 43, and the bevel gear 44 is attached to the rear of the rotation shaft 31. A bevel gear 45 attached to the end is meshed with the bevel gear 45 to transmit the rotation of the steering wheel 10 to the rotation imparting arm 41. Therefore, the rotation arm 41 and the three connecting rods rotate around the movement axis 111 by an amount corresponding to the rotation angle of the steering wheel 10, and the swing arm 36 swings around the support pin 35 accordingly. When the axis of the pin 35 is aligned with the axis of movement 91 of the control rod 33,
The ball joint 38 at the tip of the swing arm 36 only swings on the reference plane, and the control rod 33 is held stationary. If the swing locus plane deviates from the reference plane, the ball joint 38 will be displaced in the left-right direction in FIG. 3 as the swing arm 36 swings about the pin 35, and this displacement will be caused by The signal is transmitted to the control rod 33 via the rod 39, and the control rod 33 is configured to move along the movement axis βl to operate the spool valve of the control valve 20. That is, even if the swing angle of the swing arm 36 about the axis of the support pin 35 is the same, the displacement of the control rod 33 in the left-right direction is due to the change in the tilt angle of the pin 35, that is, the rotation angle of the holder 34. It changes accordingly. - In order to change the inclination angle of the support pin 35 with respect to the movement axis g1, that is, the inclination angle of the holder 34 with respect to the reference plane, a sector gear 46 as a worm wheel is attached to the support shaft 37 of the holder 34. A worm gear 48 on a rotating shaft 47 is meshed with the worm gear 48 .

Further, a bevel gear 49 is attached to the rotating shaft 47,
This bevel gear 49 has a bevel gear 50 attached to the output shaft 51a of a stepping motor 51 as an actuator.
are engaged with each other, and by operating the stepping motor 51 and rotating the sector gear 46, the holder 34
Controls the steering angle θR of the rear wheels 2L, 2R, that is, the steering ratio (rear wheel steering angle θR/front wheel steering angle θF) of the front and rear wheels 1L, 2L (1R, 2R) by changing the inclination angle with respect to the reference plane. For example, if the sector gear 46 is
(At this time, the pole joint 38 at the tip of the swing arm 36 rotates on the reference plane, and the steering angle θR of the rear wheels 2L and 2R becomes 0R.)
= O) when rotating in one direction, the steering ratio of the front and rear wheels IL, 2L is changed to the steering ratio of the rear wheels 2L, 2R, and the front wheels IL, 1R.
On the other hand, when the wheels are rotated in the other direction, the steering ratio is controlled so that the rear wheels 2L and 2R are turned in the opposite direction to the front wheels I.
It is configured to be controlled to be in the same phase and in the same direction as L and 1R.

The support shaft 37 of the holder 34 is provided with a steering ratio sensor 101 comprising a potentiometer that detects the actual steering ratio controlled by the stepping motor 51 based on the rotation angle of the sector gear 46. There is. In addition, stopper members 52 and 53 on the opposite phase side and the same phase side, which are made of pins that restrict the rotation range of the sector gear 46, are attached to the casing that supports the holder 34 on both the left and right sides of the sector gear 46. The control position of the stepping motor 51 when the stepping motor 46 comes into contact with the stopper member 52 on the opposite phase side is set as its initial position. In addition, in Fig. 3, 54 indicates rear wheel steering [@
12 is a rod stopper that restricts the maximum movement range of the relay rod 15.

The stepping motor 51 and the electromagnetic on-off valve 28 are
The control unit 100 includes a vehicle speed sensor 102 that detects the traveling speed of the vehicle (vehicle speed), and the steering ratio changing mechanism 32. The mechanism 3 is attached to the casing of
Detection signals from a temperature sensor 103 that detects the operating temperature T of No. 2 and the steering ratio sensor 101 are inputted.

As shown in FIG. 4, the control unit 100 has, as a signal processing function in its microcomputer, a steering ratio control section 104 that drives and controls the stepping motor 51 and the electromagnetic on-off valve 28, and the steering ratio The driving torque correction section 105 corrects the output characteristics of the control section 104 to the stepping motor 51.

The steering ratio control unit 104 controls the front and rear wheels IL corresponding to the vehicle speed V based on the steering ratio characteristic set in advance according to the vehicle speed
, 2L, and outputs a drive pulse signal as an operation command signal corresponding to the target steering ratio to the stepping motor 51 to drive and control the motor 51. As shown in Figs. 5 and 6, the above steering ratio characteristics change the steering ratios of the front and rear wheels IL and 2L depending on the vehicle speed V, and when the vehicle speed V is low, the turning performance of the vehicle changes. In order to improve the condition, the rear wheels 2L and 2R should be replaced with the front wheels 'IL, I.
The wheels are steered in the opposite direction relative to R, that is, with the opposite phase, and the steering ratio becomes negative (opposite phase). On the other hand, when the vehicle speed reaches a predetermined value, the steering ratio becomes zero, and the front wheels 1L, 'Rear wheel 2 regardless of IR steering.

The steering angle θR of 2R is maintained at θa=O, and the vehicle enters the normal two-wheel steering state. When driving at high speeds, in order to improve the grip of the rear wheels 2L and 2R during cornering and to increase driving stability, the rear wheels 2L and 2R are set to the front wheels 1L and 2R.
It is set so that it is steered in the same direction as lR, that is, in the same phase, and the steering ratio is positive (same phase). In addition, Fig. 5 shows the h characteristics of the rear wheel rudder and angle with respect to the steering wheel steering angle (rotation angle of the steering link wheel 10) when the vehicle speed changes, and Fig. 6 shows the steering ratio characteristics with respect to the vehicle speed at a predetermined steering angle. are shown respectively.

The steering ratio control section 104 includes the vehicle speed sensor 1.
02 is input, and this steering ratio control section 1
In 04, the vehicle speed ■ detected by the vehicle speed sensor 102 is checked against the steering ratio characteristics to determine a target steering ratio corresponding to the vehicle speed ■, and the steering ratios of the front and rear wheels 1L and 2L are determined as described above. An operation command signal (pulse signal) is output to the stepping motor 51 so that the target steering ratio is achieved.In other words, the driving of the stepping motor 51 rotates the sector gear 46 of the steering ratio sensor @32. By changing the inclination angle of the holder 34 with respect to the reference plane, the rear wheel 2L
By changing the steering angle θR of , 2R, the front and rear wheels 1L,
The 2L steering ratio is variably controlled to a target value.

The steering ratio control section 104 includes the steering ratio sensor 10.
1 output signal is input as a feedback signal, and the stepping motor 51 is controlled by this feedback signal so that the actual steering ratio matches the target steering ratio. Further, when the actual steering ratio controlled by the stepping motor 51 does not match the target steering ratio, it is assumed that the stepping motor 51 is out of control (idling), and the electromagnetic on-off valve 28 is turned ON. It is configured to maintain the steering ratio of the front and rear wheels 1L and 2L at zero upon operation and shift to a fail-safe mode.

On the other hand, the output signal of the temperature sensor 103 is input to the drive torque correction section 105, and the drive torque correction section 105 compares the operating temperature T of the steering ratio changing mechanism 32 with a predetermined temperature, and is determined to be below a predetermined temperature, the steering ratio is adjusted such that the pulse rate (number of pulses per unit time) of the drive pulse signal for the stepping motor 51 is decreased and the drive torque of the stepping motor 51 is increased. It is configured to correct the output characteristics of the control unit 104.

That is, the operating speed (response speed) of the stepping motor 51 with respect to a change in the pulse rate of the drive pulse signal is
The driving torque changes, and as the pulse rate increases, the operating speed of the motor 51 increases, but the driving torque decreases. As shown in FIG. 7, in normal times when the operating temperature T of the steering ratio changing mechanism 32 is higher than a predetermined value, the operating speed of the stepping motor 51 is increased to improve the responsiveness of the steering control. The pulse rate is set high, and only when the operating temperature of the steering ratio changing mechanism 32 is determined to be a predetermined temperature or lower, the pulse rate is corrected to a lower rate, and the stepping motor 51 is driven by the lower rate. It is designed to increase torque.

Next, the operation of the above embodiment will be explained.

First, when an ignition key switch is turned on to drive a vehicle that is not in use, the initial control position of the stepping motor 51 is determined accordingly. Thereafter, when the vehicle shifts to a running state, the vehicle speed V at that time is detected by the vehicle speed sensor 102, a detection signal is output from the vehicle speed sensor 102 to the control unit 100, and the steering ratio control section 104 of the control unit 100 A target steering ratio corresponding to the vehicle speed V is calculated by comparison with the steering ratio characteristics stored in advance at , and a drive pulse signal corresponding to this target steering ratio is output to the stepping motor 51 . is driven. Driven by the motor 51, the sector gear 46 of the steering ratio changing mechanism 32 rotates, and the tilt of the swing locus plane of the swing arm 36 connected to the sector gear 46 is changed with respect to the reference plane, and this change causes the steering In conjunction with the operation of the wheel 10, that is, the steering of the front wheels 'IL and 'IR, the moving direction and moving distance of the control rod 33 change with respect to the movement of the connecting rod 39, which rotates around the moving axis 91. The power steering system 1 is adjusted so that the rear wheels 2L and 2R achieve the target steering ratio calculated above with respect to the front wheels 1L and 1R according to the movement.
The vehicle is steered while being assisted by the power cylinder 17 of No. 6, thereby controlling the four wheels 1L to 2R of the vehicle so that the steering ratios are in opposite phases at low vehicle speeds and in the same phase at high vehicle speeds. be done.

Further, during control of the stepping motor 51, the temperature sensor 103 detects the operating temperature T of the steering ratio change @@32, and the detected operating temperature T is compared with a predetermined value in the drive torque correction section 105 of the control unit 100. During normal times when the detected operating temperature T is higher than a predetermined value, the pulse rate of the drive pulse signal to the stepping motor 51 is not corrected and is maintained at a high rate. This high pulse rate improves the operating speed of the stepping motor 51 and the responsiveness of the four-wheel steering control.

However, when the steering ratio changing mechanism 32 is kept in an inactive state and the detected operating temperature T has fallen below a predetermined value due to the vehicle being held in a stopped state for a long time, the stepping The pulse rate of the drive pulse signal for the motor 51 is corrected to be lower, and the output torque of the stepping motor 51 is increased by this lower pulse rate.

Therefore, even if the viscosity of the lubricating oil in the steering ratio changing mechanism 32 is high at low temperatures and the load on the stepping motor 51 is greater than normal due to the increase in viscous resistance, the steering ratio can be turned against the load. The ratio changing mechanism 32 can be operated reliably, thereby ensuring stable four-wheel steering control.

At this time, in order to increase the drive torque of the stepping motor 51 only when the operating temperature T of the steering ratio changing mechanism 32 is low, it is necessary to increase the size of the stepping motor 51 to cope with the load at low temperatures. Instead, a small stepping motor 51 can be used, and costs can be reduced.

In the above embodiment, the case where the steering ratio of the front and rear wheels 1L and 2L of the vehicle is variably controlled according to the vehicle speed has been exemplified. The present invention can also be applied to a four-wheel steering system that is directly driven by a stepping motor according to the steering angle of the steering wheel.

Further, in the above embodiment, the stepping motor 51 was used as the actuator for controlling the steering ratio of the front and rear wheels 1L and 2L, but in this example, the steering ratio is controlled by another actuator such as a DC motor. It can also be applied to a four-wheel steering device. For example, when a DC motor is used, it is possible to increase the driving torque by increasing the voltage applied to the DC motor.

(Effects of the Invention) As explained above, according to the present invention, the target steering ratio of the front and rear wheels is set based on preset steering ratio characteristics, and the steering ratio changing mechanism is actuated by the operation of the actuator. In a four-wheel steering system for a vehicle that controls the steering ratio of the front and rear wheels to the target steering ratio, the operating temperature of the steering ratio changing mechanism is detected, and the operating temperature is lower than a predetermined value. By increasing the drive torque of the actuator when the temperature drops, even if the load on the actuator increases due to an increase in the viscous resistance of the lubricating oil of the steering ratio change mechanism at low temperatures, the large-sized system can handle the large load. The steering ratio changing mechanism can be operated reliably by increasing the drive torque using a small actuator without requiring a second actuator, and it is possible to prevent unnecessary transition to fail-safe mode, etc., and thus the four wheels of the vehicle can be Steering control can be executed stably.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the configuration of the present invention. 2 to 7 show embodiments of the present invention, FIG. 2 is a plan view schematically showing the overall configuration of a four-wheel steering device, and FIG. 3 shows a rear wheel steering mechanism and a steering ratio changing mechanism. A skeleton diagram shown in a perspective view, FIG. 4 is a block diagram showing the configuration of the control unit, FIG. 5 is a characteristic diagram illustrating the characteristics of the rear wheel steering angle with respect to the steering wheel steering angle when the vehicle speed changes, and FIG. FIG. 7 is a characteristic diagram showing the characteristics of the steering ratio with respect to the vehicle speed at the time of steering angle, and FIG. 7 is a characteristic diagram showing the characteristics of the drive torque with respect to the pulse rate of the drive pulse signal output to the stepping motor. IL, 1R...Front wheel, 2L, 2R...Rear wheel, 3...
- Front wheel steering mechanism, 12... Rear wheel steering mechanism, 32...
Steering ratio changing mechanism, 51...stepping motor, 10
0...Control unit, 103...Temperature sensor, 104...Steering ratio control section, 105...Drive torque correction section, T...Operating temperature. Patent applicant Mazda Motor Corporation 5-″=-2 agents Hiroshi 1) -0,・-1- iji, 7°-j

Claims (1)

[Claims] (1) 4 vehicles in which the rear wheels are steered along with the front wheels
The wheel steering device includes a steering ratio changing mechanism that changes the steering ratio of the front and rear wheels, an actuator that drives the steering ratio changing mechanism, and a target steering of the front and rear wheels based on predetermined steering ratio characteristics. a steering ratio control means for setting a steering ratio and controlling the actuator so that the steering ratio of the front and rear wheels becomes the target steering ratio; and a temperature detection means for detecting an operating temperature of the steering ratio changing mechanism. , drive torque correction means that receives the output of the temperature detection means and corrects the output characteristics of the steering ratio control means so as to increase the drive torque of the actuator when the operating temperature of the steering ratio changing mechanism is low, below a predetermined value. A four-wheel steering device for a vehicle, comprising: