Abstract:
A first rack-and-pinion device is operatively connected to a steering wheel of a motor vehicle, and a second rack-and-pinion device is provided to be operated by a motor. Both racks are operatively connected by a pair of link mechanisms. Each link mechanism is arranged to combine gear ratios of the first and second rack-and-pinion devices to steer front wheels of the vehicle at a resultant gearing ratio.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a steering system for a motor vehicle. 
     The steering system has a reduction gear train for transmitting the rotation of a steering wheel to front wheels of the vehicle. It is preferable to vary the gearing ratio in such a manner that it has a small value on either side of the straight-ahead position of the steering wheel and increases as the steering angle of the steering wheel increases. Such a steering system is called a variable ratio steering system and is disclosed various publications, for example U.S. Pat. No. 3,267,763 and Japanese Utility Model Laid Open No. 59-16269. 
     In the variable ratio steering system, the ratio can not be changed to other ratios than the set values. However, it is desirable for the ratio to be changed in accordance with driving conditions such as vehicle speed, side force exerted on the vehicle and other conditions. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a steering system in which the gearing ratio can be changed in accordance with driving conditions. 
     The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 is a schematic diagram showing a steering system according to the present invention; and 
     FIGS. 2a to 2e are illustrations showing operations of a part of the system. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a steering system comprises a steering wheel 1, steering shaft 2, joint 3, intermediate shaft 4, joint 5 and a first rack-and-pinion device 10. The first rack-and-pinion device has a gear box 6 having a pinion 7. The pinion 7 engages with a first rack 8 in a first rack housing 9. A second rack-and-pinion device 11 is provided in parallel with the first rack-and-pinion device 10. A pinion 12 of the device 11 is engaged with a second rack 13 in a second rack housing 14. The pinion 12 is operatively connected with an electric motor 15. 
     A pair of tie rods 16 connected to the first rack 8 at both ends thereof to form a first tie rod which is pivotally connected to connecting rods 17 by joints 18, respectively. Each tie rod 20 is connected second rack 13 to form a primary tie rod which is connected to the connecting rod 17 through an intermediate rod 21 and joints 22, 23. The primary tie rod and intermediate rods 21 form a second tie rod. A tie rod 24 is connected to the connecting rod 17 through a joint 25 at a position close to the end adjacent the joint 23. The tie rod 24 is operatively connected to a front wheel 26 of a vehicle through a joint 27 and a knuckle arm 28. 
     The system is provided with a steering angle sensor 30, vehicle speed sensor 31 and lateral acceleration sensor 32. 
     The outputs of these sensors are supplied to a control unit 33 for controlling the motor 15. The operation of the motor 15 is controlled by an output signal of the control unit 33 and by a feedback signal from an angular position sensor 34 for detecting an angular output of the motor 15. The control unit 33 is arranged to produce a control signal for operating the motor 15 in accordance with the vehicle speed signal fed from the vehicle speed sensor 31. When the vehicle speed is low, the motor operates to move the second rack 13 at a speed approximately equal to the speed of the first rack 8. As the vehicle speed increases, the speed of the second rack 13 is decreased compared with the speed of the first rack 8. In other words, the ratio of the speed of the second rack to the speed of the first rack is one at a low vehicle speed and approaches zero at a high vehicle speed. 
     If, in a low vehicle speed range lower than 30 Km/h, the ratio of the speed of the second rack to the first rack is set to one, the second rack 13 is moved by the same distance as the first rack 8 as shown in FIG. 2b. Accordingly, the gearing ratio at that case is the same as the ratio (i) of the first rack and pinion device 11. 
     At a high vehicle speed, for example 100 Km/h, the speed ratio is set to zero, the second rack 13 is not moved as shown in FIG. 2d. 
     Assuming the length between the connecting portion P on the connecting rod 17 for the tie rod 24 is 1 and the length of the connecting rod 17 is n (FIG. 2a), the ratio of the displacement of the tie rod 24 to the displacement of the first rack 8 is 1/n. Thus, the resultant gearing ratio of the steering system becomes 1/n .i. 
     At an intermediate speed, for example 60 Km/h, the speed ratio of the second rack is set to a middle value, for example 0.5 as shown in FIG. 2c. The resultant gearing ratio in that case is 0.5 (1+i/n) which is an intermediate value between the ratio i and 1/n .i. 
     In an extreme low speed range, for example below 3 Km/h, the speed ratio of the second rack is set to a value larger than 1 as shown in FIG. 2a. Accordingly, the resultant gearing ratio becomes larger than the ratio i. 
     In an extreme high speed range, for example above 120 Km/h, the speed ratio of the second rack is set to a negative value close to zero as shown in FIG. 2e. The resultant gearing ratio becomes smaller than the ratio 1/n .i. 
     It is preferable that when a vehicle having an understeer characteristic is accelerated during cornering, the steering angle is increased. To meet such a requirement, when the output signal of the vehicle speed sensor 31 increases while the lateral acceleration sensor 32 produces an output which means cornering of the vehicle, the motor 15 is operated to move the second rack 13 to increase the steerig angle. 
     When the vehicle is subjected to a side force by a side wind during driving, the vehicle may become staggered. In such a case, the lateral acceleration sensor produces an output signal because of the staggering. In response to the output signal, the motor 15 is operated to shift the second rack 13 to correct the steering direction. 
     It will be understood that the gearing ratio can be controlled in accordance with the output signal of the steering angle sensor 30 in the same manner as the variable ratio steering system. 
     Although the motor 15 is provided for shifting the second rack 13, other actuators such as a hydraulic cylinder can be used. 
     While the presently preferred embodiments of the present invention have been shown and described, it is to be understood that this disclosure is for the purpose of illustration and that various changes and modifications may be made without departing from the scope of the invention as set forth in the appended claims.