Rear wheel steering control system for vehicle

Abstract: A system for controlling a steering angle for rear wheels (15a,15b) of a vehicle is provided. This system is adapted for determining a rear wheel steering angle in a preselected relation to a steered angle (ϑ) of a steering wheel (2) and correcting the rear wheel steering angle based on a difference (ΔV) in rotational speed between front (6a,6b) and rear wheels (15a,15b) induced during turning. The rear wheel steering angle is however held without correction when the braking system is operated during turning to cancel the effect of braking slips on the determination of the rear wheel steering angle to improve steering stability.

Description: BACKGROUND OF THE INVENTION 1 Technical Field The present invention relates generally to a steering control system for a vehicle as mentioned in the preamble of claim 1. More particularly, the invention relates to an improved rear wheel steering system for a vehicle which is adapted for detecting slip of driven wheels to determine a rear wheel steering angle based thereon. 2 Background Art It is well known in the art that four-wheel steering (4WS) vehicles are adapted for providing cornering forces on rear wheels during turns without slipping to improve turning stability. The Japanese Patent First Publication No. 57-60974 discloses a rear wheel steering system. This system is operable to detect acceleration or centripetal force acting on a vehicle body during turns to steer rear wheels based thereon to the same phase as a steered angle of front wheels in a case of a front-wheel drive (FWD) vehicle or to a phase opposite the front wheels in a case of a rear-wheel drive (RWD) vehicle for preventing spin of the FWD vehicle from occurring which is caused by driven wheel slippage during turning with acceleration or, in the case of rear-wheel drive vehicles for preventing drift-out from occurring. The above prior art steering system is, however, provided with an acceleration sensor which is installed on a part of the vehicle body to detect acceleration including an unnecessary component induced by vehicle motion such as vertical displacement or pitching due to uneven road or acceleration, resulting in unsuitable rear wheel steering control. For avoiding such drawbacks, the Japanese Patent First Publication No. 62-71761 mentioned in the preamble of claim 1 discloses a rear wheel steering system which controls a steering operation for rear wheels based on a difference in rotational speeds between driven and compliance wheels to prevent spin or drift-out due to driven wheel slippage (i.e. due to rotational speed of the driven wheels being higher than that of the compliance wheels) during turning with acceleration from occurring. However, the prior art rear wheel steering control system is responsive to a difference in rotational speeds between the driven and compliance wheels during turns to correct a steering ratio (i.e., a ratio of the steering angle of the rear wheels to the front wheels) based thereon and controls rear wheel steering according to the corrected steering angle ratio. Thus, this system has a drawback in that there is a tendency for the rear wheel steering control to become unstable when a brake pedal is depressed during turning. Braking forces of the front and rear wheels are normally not constant when a braking operation is made and any of the wheels may sometimes lock early. This results in a difference in rotational speeds between the driven and compliance wheels due to the wheel locking (hereinafter, this difference in rotational speeds due to braking is referred to as braking slips). As a result of braking slips in addition to driven wheel slip being detected as a rotational speed difference between the driven and compliance wheels, a steering ratio set according to such a detected value becomes unsuitable, further reducing steering stability during turns. SUMMARY OF THE INVENTION It is an object of the invention to provide a control system for steering rear wheels which prevents a change in a rear wheel steering angle due to braking slips to ensure steering stability when braking operations are made during turns. In the present system, there is provided a rear wheel steering control system for a vehicle which comprises first means for monitoring a steered angle of front wheels to provide a signal indicative thereof; second means for monitoring rotational speeds of wheels respectively to determine a difference in rotational speed between the front and rear wheels and providing a signal indicative thereof; third means responsive to the signal from the second means to determine a correction value for a rear wheel steering angle based on the magnitude of the difference in rotational speed between the front and rear wheels, the third means providing a rear wheel target steering angle based on the correction value and the steered angle of the front wheels to provide a signal indicative thereof; and fourth means for controlling an actual steering angle of the rear wheels according to the rear wheel target steering angle. According to the invention there is provided fifth means for monitoring braking operation to provide a signal indicative thereof; and sixth means responsive to the signal from the fifth means for stopping the correction value determining operation of said third means to hold the correction value to a value prior to the braking operation. Further advantageous embodiments of the present invention are laid down in the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram which shows a steering system according to the present invention incorporated in a front-wheel drive vehicle. Fig. 2 is a flowchart which shows a sequence of logical steps carried out by a rear wheel steering control system of the invention. Fig. 3 is a flowchart which shows alternate processes for calculating a correction for a rear wheel steering angle carried out by a rear wheel steering control system of the invention. Fig. 4 is a table which shows a relation between a vehicle turning radius R and vehicle speed V for estimating the difference in rotational speeds between front and rear wheels ΔV R . DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings, particularly to Fig. 1, a steering control system according to the present invention is shown which is incorporated in a front-wheel drive (FWD) vehicle. This system includes generally a front wheel steering unit 1, a rear wheel steering unit 11, and a steering controller 20 operable to detect torque of driven wheels and to adjust a rear wheel target steering angle. The front wheel steering unit 1 includes a steering wheel 2, a rack-and-pinion steering gear 3, relay rods 4a and 4b, and knuckle arms 5a and 5b. The rack-and-pinion steering gear 3 serves to convert rotational motion of the steering wheel 2 into stroke motion to laterally displace the relay rods 4a and 4b for steering front wheels 6a and 6b. Drive shafts 7a and 7b are connected to the front wheels 6a and 6b respectively to transmit drive torque there to provided from an engine 10. Each drive shaft is connected to an output shaft 9a of a transmission 9 through a differential gear 8. The rear wheel steering unit 11 includes a hydraulic cylinder 12 such as a spring centered double acting hydraulic pressure cylinder, tie rods 13a and 13b, and rear knuckle arms 14a and 14b coupling intermediate the tie rods and rear wheels 15a and 15b respectively. The hydraulic cylinder 12 is hydraulically communicated with an electromagnetic proportional pressure control valve (not shown) which is connected to a pressure source (not shown). The control valve is responsive to a rear wheel steering angle signal δr output from the steering controller 20 to provide hydraulic pressure to either of the chambers in the hydraulic cylinder 12, displacing the tie rods 13a and 13b to steer the rear wheels 15a, 15b in a preselected direction. The steering control system further includes a left front wheel rotational speed sensor 21 for determining speed V₁ of the left front wheel, a right front wheel rotational speed sensor 22 for determining speed V₂ of the right front wheel, a left rear wheel rotational speed sensor 23 for determining speed V₃ of the left rear wheel, a right rear wheel rotational speed sensor 24 for determining speed V₄ of the right rear wheel, a steering angle sensor 25 for determining a steered angle ϑ of the steering wheel 2, and a brake sensor 26 for detecting depression or braking operation of a brake pedal (not shown) to provide a braking operation signal S BK to which a brake lamp switch may be applicable (which outputs a 'H' level signal in operation). These sensors provide signals indicative of detected parameters to the steering controller 20, which calculates a rear wheel target steering angle δ r based on those signals. Referring to Fig. 2, a flowchart of a program or sequence of logical steps performed by the steering controller 20 is shown. After entering the program, in step 100 the steering controller 20 reads the steered angle ϑ from the steered angle sensor 25. The routine then proceeds to step 200 wherein the wheel speeds V₁ to V₄ are input from the rotational speed sensors 21 to 24 to the steering controller 20. The routine then proceeds to step 300 wherein vehicle speed V is determined based on the wheel speeds derived in step 200 according to the following equation. V = (V₁ + V₂ + V₃ + V₄) / 4 (1) Subsequently, the routine proceeds to step 400 wherein a difference in rotational speeds ΔV between the front and rear wheels is determined according to the following relation. ΔV = (V₁ + V₂ - V₃ - V₄) / 2 (2) The routine proceeds to step 500 wherein the steering controller reads the braking operation signal S BK output from the braking operation sensor 26. The routine then proceeds to decision step 600 wherein it is determined as to whether the braking operation signal represents a level 'H' or not, or to determine whether a braking system is operating or not. If not, the routine proceeds to step 700 wherein a correction ΔK for calculation of a rear wheel steering angle, as will be described hereinafter in detail, is determined based on the difference in rotational speeds between the front and rear wheels derived in step 400. For instance, a preselected table as shown in step 700 can be used. This table shows a curve defined by a parameter which varies dependent upon variation in the rotational speed difference ΔV or slips of the driven wheels to obtain a greater value of the correction ΔK as the ΔV becomes high. After the correction ΔK is obtained, the routine proceeds to step 800 wherein a rear wheel target steering angle δr (i.e., a corrected angle) is mathematically calculated according to the following equation which includes proportional members (K x ϑ) and differential members (T x ϑ ˙). δr = K X ϑ - T X ϑ (3) where K : K' - ΔK K' : a coefficient of a reference characteristic of rear wheel steering variable according to vehicle speed ϑ : a steered angle of a steering wheel ϑ ˙ : steering angular velocity of a steering wheel T : a coefficient variable dependent upon the vehicle speed The rear wheel target steering angle δr is obtained by subtracting a value (T X ϑ ˙) corresponding to the steering angular velocity ϑ ˙ (dϑ/dt) from a value given by multiplying the steered angle ϑ at that time by the coefficient K. The coefficient K is increased dependent upon increase in the vehicle speed while the coefficient T is decreased dependent upon the increase in the vehicle speed which is defined by measurement based on the type of vehicle. The correction ΔK is used for correcting the K of the proportional members (K X ϑ). For example, if the great ΔK is provided, the proportional member (K X ϑ) is decreased (or becomes minus) to provide a rear wheel steering angle δ r in a phase opposite a steered angle of the front wheels. In a case of a front-wheel drive (FWD) vehicle, an increase in driven wheel slip (increase in the ΔV) during turns causes drift-out which directs the front wheels (i.e., the driven wheels) toward the outside of a turning radius, to cause understeering. Provision of the opposite phase rear wheel steering angle δ r can however improve response of yawing torque to direct a steering characteristic to ''neutral steering'' (or to small oversteering) for emphasizing steering response. In step 600, if a YES answer is obtained, the routine directly proceeds to step 800 passing over step 700 wherein no renewal of the correction ΔK is made. Therefore, the correction value ΔK is held just before braking operation. As a result, even if any of the wheels lock during the braking operation, causing braking slips, no change of the rear wheel steering angle δ r due to the braking slips is made, resulting in improved turning stability. Referring to Fig. 3, an alternate program for calculating the correction ΔK for the rear wheel steering angle is shown. The difference in rotational speeds ΔV between the front and rear wheels obtained by the above equation (2) includes a first component of a difference in rotational speeds between the front and rear wheels (referred to as ΔV₁ hereinafter) which corresponds to slippage of the driven wheels caused by acceleration during turning and a second component representing the difference in rotational speeds between the front and rear wheels (referred to as ΔV₂) caused by a difference in turning tracks therebetween. This ΔV₂ tends to affect the rear wheel steering control greatly when turning with a minimum turning radius wherein a difference in turning tracks between the front and rear wheels becomes great. Accordingly, for avoiding such a drawback, the following processes, as shown in Fig. 3, may be applicable to the calculation for the correction ΔK in place of step 700 as shown in Fig. 2 so that the component of the ΔV₂ is canceled to correspond the ΔV to the ΔV₁ substantially for ensuring the proper rear wheel steering control. If a No answer is obtained in step 600, the routine proceeds to step 720 wherein a turning radius R of the vehicle is calculated according to the following equation. R = (t r X V) / V₃ - V₄ (4) where t r is a tread of the vehicle. Subsequently, the routine proceeds to step 740 wherein, based on the turning radius R, a difference in rotational speeds ΔV R between the front and rear wheels is estimated, the estimated difference corresponds to ΔV₂. To understand how this estimate is determined, reference is made to the table shown in Fig. 4. Referring to Fig. 4, an example for estimating the front-rear wheel rotational speed difference ΔV R according to a relation between the turning radius R and vehicle speed V. In the drawing, the ordinate axis indicates the ΔV R wherein front wheel rotational speed is greater than rear wheel rotational speed in a range above zero (+) while the rear wheel rotational speed is greater than the front wheel rotational speed in a range below zero (-). The axis of abscissa indicates vehicle speed V which increases toward the right. Curves as indicated by solid and broken lines show characteristics with possible minimum turning radius and maximum turning radius respectively. The curves are in a plus range of the ΔV R below the vehicle speed V o while these are in a minus range of the ΔV R above the V o . This is caused by longitudinal displacement of the turning center O of the vehicle dependent upon variations in the vehicle speed V with the result that the front wheel rotational speed increases faster than the rear wheel rotational speed at low speed while it decreases slower at high speeds. The values of ΔV R derived by the curves in Fig. 4 are represented by the following equation. ΔV R = V X ℓ / R² X (m X V² X a / C pr X ℓ - ℓ / 2) (5) where the m is vehicle weight, the C pr is cornering power of the rear wheels, ℓ is a wheel base, and the a is a distance between the center of gravity and a point intermediate the front wheels. After determining the ΔV R , the routine proceeds to step 760 wherein a corrected front-rear wheel rotational speed difference ΔV' is calculated. This calculation is implemented by subtracting ΔV, as obtained by step 400, by ΔV R , as obtained by step 740 as shown by the following relation. ΔV' = ΔV - ΔV R (5) After obtaining the ΔV', the correction ΔK is determined according to the table as shown in step 700 in Fig. 2 (in the table, the ΔV is replaced with the ΔV'). The routine then proceeds to step 800 in Fig. 2. With the additional processes, the rotational speed difference between the front and rear wheels ΔV is subtracted by the rotational speed difference component ΔV R obtained by the turning track difference dependent upon the turning radius R to correct the proportional members (K X ϑ) of the rear wheel steering equation (3) utilizing the resultant value (ΔV'). Thus, the rear wheel steering can be controlled based on the precise amount of driven wheel slip to effectively prevent spin due to acceleration during turning in the case of RWD vehicles and drift-out from occurring in the case of FWD vehicles. As previously mentioned, a feature of the present invention is that calculation of the correction ΔK, for the rear wheel target steering angle, dependent upon the difference in rotational speeds ΔV between the front and rear wheels is made only when the braking system is not operated and a value of the correction ΔK just before the braking operation is held when the braking system is operated during turns. Thus, the present invention avoids a tendency toward steering instability caused by unsuitable rear wheel steering control due to the difference in rotational speeds between the front and rear wheels (i.e., braking slips) generated by lock of a wheel or wheels when the brake pedal is depressed during turns. While the above embodiment utilizes the brake lamp switch as the brake operation sensor, an operation signal of an ABS (anti-skid brake system) or a deceleration signal from a front-rear deceleration sensor may be used to detect braking operation. Additionally, while the above embodiment is directed to a front-wheel drive vehicle, the invention may be applicable to a rear-wheel drive vehicle wherein the K of the above equation (3) is defined by a relation of K' + ΔK to provide a rear wheel steering angle in the same phase as a steered angle of the front wheels when slips of the driven wheels are great so as to prevent spin from occurring.

Claims: A rear wheel steering control system for a vehicle comprising first means (25) for monitoring a steered angle of front wheels (6a,6b) to provide a signal indicative thereof; second means (21-24) for monitoring rotational speeds (V₁-V₄) of wheels respectively to determine a difference (ΔV) in rotational speed between the front (6a,6b) and rear wheels (15a,15b) and providing a signal indicative thereof; third means (20) responsive to the signal from said second means to determine a correction value (ΔK) for a rear wheel steering angle based on the magnitude of the difference (ΔV) in rotational speed between the front (6a,6b) and rear wheels, (15a,15b) said third means providing a rear wheel target steering angle (δr) based on the correction value (ΔK) and the steered angle of the front wheels (6a,6b) to provide a signal indicative thereof; and fourth means for controlling an actual steering angle of the rear wheels (15a,15b) according to the rear wheel target steering angle (δr), characterized by fifth means (26) for monitoring braking operation to provide a signal (S BK ) indicative thereof; and sixth means (20) responsive to the signal from said fifth means (26) for stopping the correction value determining operation of said third means to hold the correction value (ΔK) to a value prior to the braking operation. The system as set forth in claim 1, characterized in that said first means includes a sensor (25) for detecting a steered angle (ϑ) of a steering wheel (2) to monitor the steered angle of the front wheels (6a,6b) and steering angular velocity (ϑ ˙) of the steering wheel (2), said third means provides the rear wheel target steering angle (δ r ) according to the following relation, δr = K X ϑ - T X ϑ where K = K' - ΔK K' = a coefficient of a reference characteristic of rear wheel steering variable according to vehicle speed ΔK = the correction determined by said third means T = a coefficient depending upon the vehicle speed The system as set forth in claim 2, characterized in that the correction (ΔK) is defined by a parameter which increases with increases in the difference (ΔV) in rotational speed ΔV between the front (6a,6b) and rear wheels (15a,15b). The system as set forth in one of the claims 1 to 3, characterized in that the third means (20) is responsive to the signal from said first means to determine a rear wheel steering angle in a preselected relation to the steered angle of the front wheels (6a,6b), said third means is adjusting the rear wheel steering angle based on the correction value (ΔK) to provide a rear wheel target steering angle (δr) and providing a signal indicative thereof. The system as set forth in one of the claims 1 to 4, characterized in that said fifth means (26) is provided with a brake switch lamp responsive to depression of a brake pedal. The system as set forth in one of the claims 1 to 5, characterized in that said second means includes speed sensors (21-24), each of which determines rotational speed of a wheel, to calculate the difference (ΔV) in rotational speeds between the front (6a,6b) and rear wheels (15a,15b), vehicle speed (V) and a turning radius (R), said third means calculating a component (ΔV R ) of a difference in rotational speeds between the front and rear wheels caused by a difference in turning tracks therebetween based on the vehicle speed (V) and the turning radius (R) to determine a difference (ΔV') in rotational speeds given by canceling the difference (ΔV R ) in rotational speeds from the difference (ΔV) in rotational speeds, said third means determining the correction (ΔK) based on the magnitude of the difference (ΔV') in rotational speeds.

Inventors: FUKUYAMA YUICHI, IMASEKI TAKASHI, IWATA TOHRU, TAMURA MINORU, FUKUYAMA, YUICHI, IMASEKI, TAKASHI, IWATA, TOHRU, TAMURA, MINORU
Applicants: NISSAN MOTOR, NISSAN MOTOR CO., LTD.
Application year: 1990
Main classification: B62D7
IPC codes: B62D109, B62D7, B60T8, B62D6, B62D101