Auxiliary steering system for wheeled vehicle.

BACKGROUND OF THE INVENTION

0001The present invention relates to an auxiliary steering system for automatically steering a wheeled vehicle so as to improve a cornering characteristic, and a directional stability of the vehicle, in addition to a main steering action commanded by a driver.

0002A turning behavior and steering stability of a wheeled vehicle are usually described by a relationship between a steering input, and a yaw rate or a lateral acceleration of the vehicle. The steering input is a driver's command to steer the vehicle. In general, a steering angle ₀ of a steering wheel is regarded as the steering input. The yaw rate (or yaw angular velocity) is an angular velocity of a rotation of the vehicle about a vertical axis passing through the center of gravity of the vehicle. The lateral acceleration is an acceleration of the center of gravity of the vehicle in a lateral direction of the vehicle.

0003It is desired that a vehicle should be turned to an amount corresponding to the driver's steering input without being affected by a disturbance such as a side wind and a friction coefficient of a road surface, and without delay. The amount of a turn of a vehicle can be described in terms of a ratio (yaw rate gain) of the yaw rate $ (or the lateral acceleration a) to the steering input (the steering angle e of the steering wheel). The delay can be described by a lag (phase lag) of an output of the yaw rate (or the lateral acceleration) with respect to the steering input.

0004In a vehicle having no auxiliary steering control, the yaw rate gain is enhanced at a certain steering frequency, as shown by a curve "a" in Fig. 17, so that the behavior of the vehicle responsive to the steering input is increased abruptly at or near this steering frequency. Furthermore, the phase lag increases rapidly as the steering frequency increases as shown by a curve "a'" in Fig. 17. Therefore, the behavior of the vehicle is retarded with respect to the steering input, so that much skill is required for controlling the vehicle. It is desired that both of the yaw rate gain and the phase lag remain constant over the full range of the steering frequency.

0005Therefore, there has been proposed an auxiliary steering control system, as shown in Fig. 1, which steers the front or rear wheels in proportion to the yaw rate or lateral acceleration of the vehicle in such a negative direction as to reduce the direction change of the vehicle while the front wheels are steered by the steering input in a conventional manner. However, the auxiliary steering control system of this type is still unsatisfactory in that this system cannot fully eliminate the influence of the disturbance without decreasing the yaw rate gain excessively.

SUMMARY OF THE INVENTION

0006It is an object of the present invention to provide an auxiliary steering control system in which a hundred percent of the feedback is obtained to fully eliminate the influence of the disturbance when the steering input is equal to zero, and a desired percent of the feedback is obtained to provide a desired yaw rate gain when the steering wheel is turned.

0007According to the present invention, a vehicle comprises road wheels, a steering wheel, a steering mechanism for steering the vehicle by altering the direction of at least one of the road wheels in accordance with an angular displacement of the steering wheel. The vehicle further comprises a negative feedback system which comprises a steering angle sensor, vehicle behavior sensing means, negative feedback means, and actuating means. The steering angle sensor senses the angular displacement of the steering wheel. The vehicle behavoir sensing means senses at least one variable which is indicative of an actual turning behavoir of the vehicle. The vehicle behavoir sensing means may sense either or both of a yaw rate of the vehicle and a lateral acceleration of the center of gravity of the vehicle. The negative feedback means is connected with the steering angle sensor and the vehicle behavoir sensing means, for producing a negative feedback signal in accordance with a difference between an actual quantity determined in accordance with the variable sensed by the vehicle behavior sensing means and a reference quantity determined in accordance with the angular displacement sensed by the steering angle sensor. The actuating means steers the vehicle by altering the direction of at least one of the road wheels in accordance with the negative feedback signal in such a direction as to reduce the variable.

0008The variable to be sensed by the vehicle behavoir sensing means may be a yaw rate of the vehicle or a lateral acceleration of the center of gravity of the vehicle. Preferably, the vehicle further comprises a vehicle speed sensor for sensing the speed of the vehicle, and the feedback means determines the reference quantity in accordance with the angular displacement sensed by the steering angle sensor and the vehicle speed sensed by the vehicle speed sensor. The actual quantity may be the yaw rate or the lateral acceleration, or a linear combination of the yaw rate and the time derivative of the yaw rate or the lateral acceleration, or a linear combination of the lateral acceleration and the time derivative of the lateral acceleration or the yaw rate. The reference quantity may be a function of the angular displacement of the steering wheel and the vehicle speed, whose value is approximately equal to the actual yaw rate or lateral acceleration if there is no disturbance.

BRIEF DESCRIPTION OF THE DRAWINGS

0009<ul id="ul0001" list-style="none"><li>Fig. 1 is a schematic view of a negative feedback system of a conventional type auxiliary steering control system;</li><li>Fig.,2 is a schematic view of a negative feedback system of a first embodiment according to the present invention;</li><li>Fig. 3 is a schematic plan view of one example of a front wheel steering system which can be used in the present invention when the negative feedback steering control is performed by the front wheels;</li><li>Fig. 4 is a schematic plan view of another example of a front wheel steering system which can be used in the present invention when the negative feedback steering control is performed by the front wheels;</li><li>Fig. 5 is a detailed sectional view of a steering gear and an actuator of Fig. 4;</li><li>Fig. 6 is a detailed sectional view of an electromagnetic spool valve used in the example of Fig. 4;</li><li>Figs. 7 to 11 are schematic views of negative feedback systems of five other embodiments of the present invention, respectively;</li><li>Fig. 12 is a schematic view of a negative feedback system of still another embodiment of the present invention in which the negative feedback steering control is performed by the rear wheels;</li><li>Fig. 13 is a schematic plan view of one example of a rear wheel steering system used in the embodiment of Fig. 12;</li><li>Fig. 14 is a schematic plan view of another example of a rear wheel steering system used in the embodiment of Fig. 12;</li><li>Fig. 15 is a schematic elevation as viewed from a line XV-XV in Fig. 14 in a direction shown by arrows;</li><li>Fig. 16 is a detailed sectional view of an actuator shown in Fig. 14; and</li><li>Fig. 17 is a diagram showing frequency characteristics of auxiliary steering control systems.</li></ul>

DETAILED DESCRIPTION OF THE INVENTION

0010Fig. 1 shows the above-mentioned auxiliary steering control system of a conventional type. In this system, front wheels 2 of a vehicle 1 are steered directly by the steering input on one hand. The front wheels 2 are steered, on the other hand, through a negative feedback circuit comprising a vehicle behavior sensor 3, a coefficient setter 4, an amplifier 5 and an actuator 6. The vehicle behavior sensor 3 senses the yaw rate $ or lateral acceleration a of the vehicle, which is sent to the actuator 6 through the coefficient setter 4 and the amplifier 5. The actuator 6 provides a negative feedback input F to a front wheel steering mechanism of the vehicle. The aim of this auxiliary steering control system is to change the yaw rate gain versus steering frequency characteristic from the curve "a" of Fig. 17 toward a flatter curve "b" of a one-dot chain line, and to decrease the phase lag toward a curve "b'" of a one-dot chain line.

0011Even if the steering input is zero, this auxiliary steering control system steers the front wheels in the negative direction to reduce the yaw rate if there is any yaw rate or lateral acceleration caused by a disturbance such as a side wind. In this system, however, the negative feedback input F is determined only by a product of a predetermined coefficient K' and the yaw rate or lateral acceleration. Therefore, if the coefficient K' is great enough to fully eliminate the,adverse influence of the disturbance, the yaw rate gain becomes so small that the driver experiences difficulty to steer the vehicle. In view of this, the coefficient K' is usually set within a range from 0.1 to 0.4. Consequently, this sytem cannot fully eliminate the influence of the disturbance, but this system can correct the vehicle bahavior only to about one third of the full extent (when K' is equal to 0.3).

0012One embodiment of the present invention is shown in Fig. 2. A vehicle 1 is steered by means of front wheels 2 in accordance with a steering input corresponding to a driver's steering command. For example, the steering input is a steering angle 0 which is an angular displacement of a steering wheel of the vehicle 1. Supplementarily, a negative feedback control unit 7 steers the vehicle 1 in a negative feedback manner by using an actuator 6. The control unit 7 determines an actual quantity indicative of an actual turning behavior of the vehicle from a predetermined controlled variable such as a yaw rate (yaw angular velocity) of the vehicle, and produce a feedback signal in accordance with a difference between the determined actual quantity and a predetermined reference quantity indicative of a desired reference turning behavior of the vehicle. The control unit 7 has a vehicle behavior sensor for sensing the controlled variable. In this embodiment, the variable is the yaw rate ϕ (ϕ)of the vehicle, and the vehicle behavior sensor is a yaw rate sensor 3' such as a yaw rate gyro. The yaw rate sensor 3' senses the yaw rate φ of the vehicle, and sends a signal indicative of the sensed yaw rate to a first coefficient setter 8. The first coefficient setter 8 determines a product K₁φ of a preset first coefficient K₁ and the sensed yaw rate φ, and sends a signal indicative of the determined product to an operational amplifier 9. In this embodiment, the first coefficient K₁ is equal to 1.0,for example..

0013The operational amplifier 9 of the control unit 7 further receives the steering input (the steering angle θ of the steering wheel), a second coefficient K₂ determined by a second coefficient setter 10, a constant K (stability factor) determined by a constant setter 11, and a vehicle speed V sensed by a vehicle speed sensor 12. In this embodiment, the second coefficient K₂ is equal to 0.7, for example. From these input data, and additional data of a steering gear ratio N and a wheel base ℓ_W which are both inherent to the vehicle, the operational amplifier 9 determines<maths id="math0001" num=""><img file="EP0150856A2_D0001.tif" /></maths>by performing a mathematical operation, and subtracts the determined result from the product K₁.φ. The operational amplifier 9 amplifies a signal indicative of the result<maths id="math0002" num=""><img file="EP0150856A2_D0002.tif" /></maths>of the subtraction, and sends the amplified signal to the actuator 6.

0014In this embodiment, the actual quantity indicative of the actual turning behavior of the vehicle 1 is the yaw rate sensed by the yaw rate sensor 3', and the reference quantity is<maths id="math0003" num=""><img file="EP0150856A2_D0003.tif" /></maths>θ. A feedback gain is determined by a quantity K1 - K₂. The feedback gain is equal to 0.3 when the first coefficient is equal to 1.0 and the second coefficient is equal to 0.7 as in this embodiment.

0015When the steering wheel is not turned, the reference quantity<maths id="math0004" num=""><img file="EP0150856A2_D0004.tif" /></maths>calculated by the operational amplifier 9 is equal to zero because the steering input 6 is equal to zero. If there is no disturbance such as a side wind, and the yaw rate ϕ is equal to zero, then the quantity K₁φ is also zero. Therefore, the operational amplifier 9 sends no signal to the actuator 6, and the actuator 6 produces no feedback input F. In this state, the control unit 7 can prevent.the front wheels from being steered uselessly.

0016If a disturbance arises and causes the yaw rate during the absence of the steering input, then the quantity K₁φ deviates from zero while the reference quantity<maths id="math0005" num=""><img file="EP0150856A2_D0005.tif" /></maths>remains equal to zero. As mentioned above, the first coefficient K₁ is equal to one in this embodiment, so that K₁φ =φ. Therefore, 100 percent of the yaw rate caused by the disturbance is fed back to the actuator 6, which therefore steers the front wheels 2 with the negative feedback input F so as to eliminate the yaw rate caused by the disturbance completely whereas the steering input remains zero.

0017When the steering wheel is turned, the control unit 7 supplies the actuator 6 with the signal indicative of the difference obtained by the subtraction whose minuend is the product of the first coefficient K₁ and the sensed yaw rate φ and whose subtrahend is the product of the second coefficient K₂ and the reference quantity<maths id="math0006" num=""><img file="EP0150856A2_D0006.tif" /></maths>Accordingly, the actuator b produces the negative feedback input F corresponding to the above mentioned difference, and by so doing steers the front wheels 2 secondarily in a return or negative direction opposite to a turn or positive direction commanded by the steering input. In this case, the feedback gain is determined by the difference obtained by subtracting the second coefficient K₂ from the first coefficient K₁. In this embodiment, the feedback gain is normally set equal to 0.3 by setting the first coefficient K₁ equal to 1.0 and the second coefficient K₂ equal to 0.7, so that the desired steering characteristic can be obtained.

0018Whether the reference quantity is exactly or approximately equal to<maths id="math0007" num=""><img file="EP0150856A2_D0007.tif" /></maths>e, the result is almost the same. The quantity<maths id="math0008" num=""><img file="EP0150856A2_D0008.tif" /></maths>expresses the vehicle turning behavior which is determined by the steering input and the vehicle speed in a steady state turning movement of the vehicle. When the exact value of<maths id="math0009" num=""><img file="EP0150856A2_D0009.tif" /></maths>is used, the control unit 7 can estimate the disturbance such as a side wind more accurately by comparing the actual quantity and the reference quantity.

0019In this embodiment, the actuator 6 is arranged to actuate a front wheel steering linkage which may be of a type shown in Fig. 3 or a type shown in Fig. 4.

0020In Fig. 3, the right and left front wheels 2 are connected to a vehicle body 17 through knuckle arms 13, side rods 14, a tie rod 15 and a link 16. A steering wheel 18 is connected to a steering gear 19. A swingable arm (pitman arm) 19a of the steering gear 19 is connected to the tie rod 15 through the actuator 6. The actuator 6 is a hydraulic servo actuator which provides the negative feedback input F to the front wheel steering linkage by expanding and contracting in accordance with the feedback signal of the control unit 7. Under command of the feedback signal, the actuator 7 alters the direction of the front wheels 2 in the negative direction which is opposite to the turn direction commanded by the driver through the steering wheel 18 or which is such a direction as to reduce or tend to reduce the yaw rate caused by the disturbance. While there is no feedback steering control action of the actuator 7, the front wheels 2 are steered only by the steering input 0 supplied from the steering wheel 18. In this case, a turning motion is transmitted from the steering wheel 18 through the pitman arm 19a, the actuator 6 held in the deactivated state, the tie rod 15, the side rods 14 and the knuckle arms 13 to the front wheels 2.

0021In the case of Figs. 4 to 6, the actuator 6 is connected with a steering gear of a rack and pinion type. A rack 20 is connected between right and left side rods 14. The rack 20 is engaged with a pinion 21 which is rotated by the steering wheel 18. The rack 20 and the pinion 21 are enclosed in a gear housing 22. In accordance with the steering input 6 provided by the steering wheel 18, the pinion 21 is rotated, the rack 20 is moved longitudinally (toward the right or left front wheel 2), and the front wheels 2 are steered.

0022The gear housing 22 is supported on the vehicle body 17 through rubber bushes 23. The actuator 6 is connected with the gear housing 22 so that the gear housing 22 can be moved relative to the vehicle body toward the right or left front wheel 2 by the actuator 6. The actuator 6 has a hydraulic cylinder having a piston 6a which divides a cylinder bore into two chambers one of which has a port A and the other of which has a port B. The cylinder actuator 6 is disposed between the gear housing 22 and the vehicle body 17. The piston 6a is connected to the gear housing 22 through a piston rod 6b, and the cylinder is connected to the vehicle body 17 through a rod 6c. The actuator 6 is controlled by an electromagnetic spool valve 24. As shown in Fig. 6, the electromagnetic spool valve 24 has two solenoids 24a and 24b, and a spool 24c which is held in a neutral position shown in Fig. 6 by springs when both solenoids are deenergized. The valve 24 is connected with an oil pump 25 and a reservoir 26 by fluid conduits. The valve 24 has a port A' fluidly connected with the port A of the hydraulic cylinder actuator 6, and a port B' fluidly connected with the port B of the cylinder actuator 6.

0023The control unit 7 energizes the solenoid 24a or the solenoid 24b selectively. When the front wheels 2 are turned by the driver in such a left turn direction as to turn the vehicle left or when the yaw rate of the same left turn direction arises during the absence of the steering input of the steering wheel 18, the control unit 7 energizes the solenoid 24b, and thereby moves the spool 24c rightwards in Fig. 6 to supply an oil pressure to the port B'. This oil pressure is introduced into the cylinder actuator 6 through the port B, and moves the piston 6a in a direction shown by an arrow C in Fig. 5. This movement of the piston 6a forces the gear housing 22 to move together with the rack 20 in the same direction by deflecting the rubber bushes 23. Therefore, the front wheels 2 are steered right, that is, in such a negative direction as to return the vehicle toward the straight ahead position or reduce the yaw rate of the vehicle produced during the absence of the driver's steering command. When the front wheels 2 are turned in a right turn direction by the steering wheel 18 or when the yaw rate of the right turn direction arises because of a disturbance, the control unit 7 energizes the solenoid 24a, and steers the front wheels 2 in the negative direction, which is a left direction in this case, to return the front wheels 2 toward the straight ahead position or reduce the yaw rate produced by the disturbance.

0024In the embodiment of Fig. 2, there is further provided a gain adjuster 60. The gain adjuster 60 controls the yaw rate gain by varying the first and second coefficients K₁ and K₂ in accordance with any one or more of conditions, such as vehicle speed, force of-side wind, weather condition, road condition, acceleration or deceleration of the vehicle, vehicle weight, weight distribution between front and rear axles, and driver's taste. It is preferable to decrease the yaw rate gain as the vehicle speed increases from the viewpoint of the directional control and stability of the vehicle at high speeds. Therefore, if the gain adjuster 60 employs the vehicle speed, the gain adjuster 60 decreases the yaw rate gain by increasing the first coefficient K₁ or decreasing the second coefficient K₂ or varying both of the first and second coefficients K₁ and K₂ so as to increase the feedback gain K₁-K₂, as the vehicle speed increases. Similarly, the yaw rate gain should be decreased as the environment of the vehicle become worse for the directional control and stability of the vehicle. Therefore, if the environmental conditions are taken into account, the gain adjuster 60 decreases the yaw rate gain by increasing the feedback gain K_l-K₂ when the side wind becomes stronger or when the weather becomes rainy or when the road surface becomes rugged. As the acceleration of the vehicle increases, the tendency to power-slide of the vehicle increase. As the deceleration of the vehicle increases, the tendency to tack-in increases. Therefore, the gain adjuster 60 may be arranged to decrease the yaw rate gain by increasing the feedback gain K_i-K₂ as the acceleration or deceleration increases. As the weight of the vehicle increases, the lateral grip of the tires in the sidewise direction becomes deficient relatively, so that the stability in steering the vehicle becomes worse. Therefore, the gain adjuster 60 may be arranged to decrease the yaw rate gain by increasing the feedback gain K₁-K₂ as the vehicle weight increases. As the weight on the rear wheels increases as compared with the weight on the front wheels, the centrifugal force produced on a turn of the vehicle tends to cause the rear end of the vehicle to slide outwardly. Therefore, the gain adjuster 60 may be arranged to decrease the yaw rate gain by increasing the feedback gain K_i-K₂ as the weight on the rear axle increases. Furthermore, the gain adjuster 60 may be arranged to vary either or both of the first and second coefficients K₁ and K₂ in accordance with driver's manual operation so that the control system can provide the yaw rate gain suitable to the driver's taste. Thus, the gain adjuster 60 controls the yaw rate gain in accordance with any one or more of the vehicle speed, side wind force, weather condition, road surface condition, rate of change of vehicle velocity with respect to time, vehicle weight, weight distribution between front and rear wheels, and driver's taste.

0025A second embodiment of the present invention is shown in Fig. 7, in which the actual behavior of the vehicle is determined by the lateral acceleration a of the vehicle instead of the yaw rate ϕ. The vehicle behavior sensor of this embodiment is a lateral G.sensor 3". The sensor 3" senses the lateral acceleration of the vehicle, and the first coefficient setter 8 provides the operational amplifier 9 with a signal indicative of the product of the sensed lateral acceleration a and a first coefficient K₁. Since the actual behavior is determined by the lateral acceleration, the reference quantity of this embodiment is set equal to -<maths id="math0010" num=""><img file="EP0150856A2_D0010.tif" /></maths>

0026The first and second coefficients K₁ and K₂ and the constant K are set at values adapted to the lateral acceleration instead of the yaw rate.

0027Fig. 8 shows a third embodiment of the present invention in which the actual quantity indicative of the actual behavior of the vehicle is set as being equal to a linear combination of the yaw rate and the time derivative of the yaw rate in order to further improve the cornering characteristic of the vehicle. In the first and second embodiments having no derivative control action, the production of the negative feedback input F is retarded with respect to the detection of the yaw rate or lateral acceleration because of lag of each consituent elements of the control system, and besides, signal damping in unavoidable in a high frequency range. Accordingly, in some cases, the first and second embodiments cannot improve the yaw rate gain characteristic and the phase lag characteristic to a desired extent as shown by curves b and b' in Fig. 17, but only to an insufficient extent as shown by curves c and c' in Fig. 17. The third embodiment is arranged to eliminate lag and damping by adding a rate signal φ which changes speedily in response to a change of the actual behavior of the vehicle. In the system of Fig. 8, the yaw rate ϕ sensed by the yaw rate sensor 3' is inputted directly to the operational amplifier 9 on one hand, and differentiated by a differentiator or differentiating circuit 61 on the other hand. A constant setter 62 receives the time derivative φ from the differentiator 61, and inputs the product k₁φ of the time derivative of ϕ and a predetermined constant k₁ to the operational amplifier 9. The operational amplifier 9 sets the actual quantity equal to the sum φ + k₁φ. Thus, the control system of the third embodiment can provide the desired frequency characteristics as shown by the curves b and b' in Fig. 17.

0028Fig. 9 shows a fourth embodiment of the present invention in which the yaw rate and the time derivative of the yaw rate are obtained in a different way from that of the third embodiment. In the fourth embodiment, a front lateral G sensor 52 is mounted on the vehicle 1 at the front, and a rear lateral G sensor 53 is mounted on the vehicle at the rear.

0029The front and rear lateral G sensors 52 and 53 are at a distance ℓ apart from each other along the longitudinal line of the vehicle 1. The front lateral G sensor 52 senses a lateral acceleration of the front of the vehicle, and the rear lateral G sensor 53 senses a lateral acceleration G₂ of the rear of the vehicle. The time derivative φ of the yaw rate is equal to the quotient obtained by dividing the front lateral acceleration G₁ minus the rear lateral acceleration G₂ by the distance 1. Therefore, a differential amplifier 54 receives the front and rear lateral accelerations G₁ and G₂ sensed, respectively, by the front and rear sensors 52 and 53, and determines<maths id="math0011" num=""><img file="EP0150856A2_D0011.tif" /></maths>

0030The output signal of the differential amplifier 54, indicative of<maths id="math0012" num=""><img file="EP0150856A2_D0012.tif" /></maths>is sent to an integrator or integrating circuit 63, and the constan setter 62. The integrator 63 determines the yaw rate φ by finding the time integral<maths id="math0013" num=""><img file="EP0150856A2_D0013.tif" /></maths>of the input, and then inputs the thus-determined yaw rate φ to the operational amplifier 9. The constant setter 62 multiplies the output of the differential amplifier 54 by a predetermined constant k₁, and-provides the operational amplifier 9 with signal equal to k₁φ. The fourth embodiment can eliminate the necessity of a costly yaw rate gyro by employing the inexpensive lateral G sensors 52 and 53.

0031A fifth embodiment of the present invention is not illustrated. In the fifth embodiment, the actual quantity indicative of the actual behavior of the vehicle is equal to a linear combination of the lateral acceleration a and the time derivative a of the lateral acceleration, while the reference quantity is equal to<maths id="math0014" num=""><img file="EP0150856A2_D0014.tif" /></maths>The control unit-of fifth embodiment has the lateral G sensor for sensing the lateral acceleration of the vehicle, and a differentiator for finding the time derivative of the lateral acceleration. The operational amplifier 9 of the fifth embodiment determines<maths id="math0015" num=""><img file="EP0150856A2_D0015.tif" /></maths>

0032A sixth embodiment of the present invention is shown in Fig. 10. In the sixth embodiment, there are provided the lateral G sensor 3", the yaw rate sensor 3', and the differentiator 61 for finding the time derivative (yaw acceleration) φ of the yaw rate sensed by the yaw rate sensor 3'. The operational amplifier 9 of this embodiment sets the actual quantity equal to a linear combination of the lateral acceleration a and the time derivative of the yaw rate, and produces the feedback signal indicative of<maths id="math0016" num=""><img file="EP0150856A2_D0016.tif" /></maths>

0033In a seventh embodiment of the present invention which is not illustrated, the actual quantity is set equal to a linear combination of the yaw rate and the time derivative a of the lateral acceleration. The reference quantity is equal to<maths id="math0017" num=""><img file="EP0150856A2_D0017.tif" /></maths>

0034The control systems of the fourth, fifth, sixth and seventh embodiments can provide the desired frequency characteristics as in the third embodiment.

0035A eighth embodiment of the present invention is shown in Fig. 11. In the eighth embodiment, the actual quantity is set equal to the linear combination of the lateral acceleration and the time derivative of the yaw rate as in the sixth embodiment of Fig. 10. However, the eighth embodiment uses only one lateral G sensor and requires neither a differentiator nor an integrator, so that the eighth embodiment is advantageous in cost. As shown in Fig. 11, a front lateral G sensor 52 is mounted on the vehicle at the front. The front lateral G sensor 52 is at a distance ℓ' apart from the center of gravity 1a of the vehicle 1 along the longitudinal line of the vehicle. The eighth embodiment utilizes the relationship that a front lateral acceleration G₁ sensed by the front lateral G sensor 52 is equal to a sum a + ℓ'.φ between the lateral acceleration a acting at the center of gravity 1a and a product of the time derivative φ of the yaw rate and the distance ℓ'. In this embodiment, the front lateral G sensor 52 is located at such a position that the distance ℓ' is equal to the constant k₁. Therefore, the constant setter 62 is removed. The front lateral acceleration G₁ sensed by the sensor 52 is directly inputted to the operational amplifier 9, which produces the feedback signal indicative of

0036<maths id="math0018" num=""><img file="EP0150856A2_D0018.tif" /></maths>A ninth embodiment of the present invention is shown in Fig. 12. The control units of the preceding embodiments are arranged to steer the front wheels 2 in the negative direction. The control unit 7 of the ninth embodiment is arranged to steer the rear wheels 27 in such a negative direction as to reduce the yaw rate of the vehicle. That is, if the front wheels 2 are steered by the steering wheel 18, the control unit 7 steers the rear wheels 27 in the same direction as the front wheels 2. The ninth embodiment uses any one of the control units 7 of the preceding embodiments as its control unit 7. The actuator 6 of the ninth embodiment inputs the negative feedback input F in the form of a steering angle -e of the rear wheels 27 into a rear wheel steering mechanism.

0037Fig. 13 shows one example of the actuator 6 used in the ninth embodiment, and Figs. 14 to 16 shows another example. In Fig. 13, the front wheel steering mechanism comprises a steering wheel 18, a steering gear enclosed in a gear housing 22, side rods 14 and knuckle arms 13, but it is not provided with the actuator 6. The front wheel steering mechanims of this embodiment is a conventional one. As shown in Fig. 13, each rear wheel 27 is rotatably supported by a knuckle arm 28. The right and left knuckle arms 28 are connected by a tie rod 29. Thus, the rear wheels 27 are steerable. The actuator 6 of this embodiment comprises a hydraulic cylinder actuator having a piston 6a. The piston 6a of the actuator 6 is fixed to the tie rod 29.

0038A hydraulic system for the actuator 6 has an oil pump 31 driven by an engine 30 mounted on the vehicle. The oil pump 31 sucks oil from an oil reservoir 32 and discharges the oil to an unload valve 33. The unload valve 33 controls the pressure of the oil discharged from the oil pump at a predetermined value, and supplies the pressured oil to an accumulator 34. The oil is supplied from the accumulator 34 to a servo valve 36 through a supply conduit 35. A return conduit 37 conveys an unnecessary oil from the servo valve 36 to the reservoir 32.

0039The servo valve 36 is controlled by the control unit 7 which is identical to any one of the control units of the preceding embodiments. When the front wheels 2 are steered right by turning the steering wheel 18 right, or when the yaw rate of the vehicle in the right turn direction is produced by a disturbance in the absence of the steering input, the control unit 7 commands the servo valve 36 to supply the oil pressure to the right cylinder chamber of the hydraulic cylinder, as viewed in Fig. 13. Therefore, the rear wheels 27 are steered in the same direction as the front wheels are steered by the steering wheel 18, that is, the right direction in this example, or in such a direction as to reduce the yaw rate produced by the disturbance. When the front wheels 2 are steered left by turning the steering wheel 18 left or when the leftward yaw rate is produced by a disturbance in the absence of a driver's steering command, the control unit 7 steers the rear wheels 27 in the same direction as the front wheels 2 are turned or in such a direction as to reduce the leftward yaw rate of the vehicle caused by the disturbance.

0040In another example shown in Figs. 14 to 16, each of the rear wheels 27 is rotatably supported by a wheel support member 38. The wheel support member 38 of each rear wheel 27 is supported on the vehicle body 17 by a radius rod 39 in the fore and aft direction of the vehicle, and by two parallel lateral rods 40 and 41 in the lateral direction of the vehicle. As shown in Fig. 15, a strut assembly 42 extends upwardly from the wheel support member 38 to the vehicle body 17. The strut assembly 42 has a suspension spring 43.

0041The actuator 6 of a hydraulic cylinder type is disposed in a middle portion of the lateral rod 40 of each rear wheel 27. As shown in Fig. 16, the actuator 6 has a piston 6a, a piston rod 6b and two fluid chambers separated by the piston 6a. A fluid port A is formed in one fluid chamber and a fluid port B is formed in the other fluid chamber. The lateral rod 40 is divided into an outer portion 40a and an inner portion 40b. The actuator 6 is disposed between the inner and outer portions 40b and 40a. The piston rod 6a and the outer portion 40a of the lateral rod 40 are fixed end to end with the interposition of a circular disc 47 as to form an integral unit. The axes of the piston rod 6b and the outer portion 40a lie on the same straight line. The inner portion 40b of the lateral rod 40 is fixed to the cylinder so as to form an integral unit. Rubber bushes 45 and 46 are disposed on both sides of the disc 44, and enclosed in a tubular case 47 which is fixed to the cylinder so that the rubber bushes 45 and 46 are immovable axially.

0042An outboard end of the outer portion 40a of the lateral rod 40 is connected to the wheel support member 38 through a rubber bush 49 and a pin 48 fixed to the support member 38 as an outboard end of the lateral rod 41. An inboard end of the inner portion 40b of the lateral rod 40 is connected to the vehicle body 17 through a rubber bush 51 and a pin 50 fixed to the vehicle body 17 as an inboard end of the lateral rod 41.

0043The fluid ports A and B of the actuator 6 are fluidly connected, respectively, with ports A' and B' of an electromagnetic spool valve 24 similar to the spool valve of Fig. 6. The valve 24 is controlled by the control unit 7 identical to any one of the control units of the preceding embodiments.

0044When the front wheels 2 are turned right by the steering wheel 18 or when the yaw rate in the same right direction is caused by a disturbance during the absence of the steering wheel movement, the control unit 7 energizes a solenoid 24b of the valve 24 and thereby moves a spool 24c rightwards in Fig. 14. Accordingly, the fluid pressure is supplied to the port B through the port B', and the piston rod 6b is forced to move toward the inboard end of the lateral rod 40 as shown by an arrow in Fig. 16. During this, the piston rod 6b deflects the rubber bush 46 elastically, and the outer portion 40a of the lateral rod 40 moves toward the inboard end together with the piston rod 6b. As a result, the left rear wheel 27 is turned from a position shown by a solid line in Fig. 14 to a position shown by a two-dot chain line. The right rear wheel 27 is steered in the same direction as the left rear wheel 27 by the actuator 6 for the right rear wheel. Thus, the rear wheels 27 are turned in the same direction as the front wheels are turned by the steering wheel 18 or in such a direction as to reduce the yaw rate produced by the disturbance. When the front wheels are turned left by the steering wheel 18 or when the yaw rate in the left turn direction is produced by a disturbance during the absence of the steering wheel movement, the control unit 7 energizes a solenoid 24a of the valve 24 and supplies the fluid pressure to the port A of the actuator 6. Therefore, the rear wheels 27 are turned in the same direction as the front wheels 2 are turned by the steering wheel 18 or in such a direction as to reduce the yaw rate caused by the disturbance.