Rear wheel steering control system for vehicle

Abstract: A system for controlling a steering angle for the rear wheels (15a,15b) of a vehicle is provided. This system includes generally a rotational speed sensor (21-24) for detecting rotational speeds of front (6a,6b) and rear wheels (15a,15b) to provide a difference (ΔV) in rotational speeds between the front (6a,6b) and the rear wheels (15a,15b), an angle sensor (25) for sensing a steered angle (ϑ) of a steering wheel (2), a steering controller (20) for determining a rear wheel target steering angle (δr), and an actuator (12) for steering the rear wheels (15a,15b) by the rear wheel target steering angle (δr). The steering controller (20) caculates a component (ΔVR) of a difference in rotational speeds between the front (6a,6b) and rear wheels (15a,15b) caused by a difference in turning tracks therebetween and compensates for this component the difference (ΔV) derived by the rotational speed sensor to provide a correction value (ΔVS',ΔK) for the rotational speeds between the front (6a,6b) and rear wheels (15a,15b) caused by acceleration during turning as a correction for a rear wheel steering angle. The steering controller (20) determines the rear wheel target steering angle (δr) based on the correction (ΔV',ΔK).

Description: The present invention relates generally to a steering control method for a vehicle as claimed in the preamble of claim 1. A method of this kind is already known form DE-A-39 04 920. This known rear wheel steering method for a vehicle using: first means for monitoring a steered angle of front wheels: second means for determining a steered angular velocity of a steering wheel; third means for monitoring rotational speed of front and rear wheels to determine a first difference in rotational speed between the front and rear wheels; fourth means for determining a turning radius of the vehicle during turning to determine a second difference in rotational speed between the front and rear wheels caused by a difference in turning radii of the front and rear wheels; fifth means for determining a first parameter variable in a preselected relation to the steered angle of the front wheels monitored by said first means and a second parameter variable in a preselected relation to the steered angular velocity to provide a rear wheel steering angle based on the first and second parameters, said fifth means determining a third difference in rotational speed between the front and rear wheels caused by wheel slippage relative to a road surface based on the first and second differences to provide a correction value for the rear wheel steering angle based on a value variable in a preselected relation to the third difference; and sixth means for controlling an actual steering angle of the rear wheels according to the rear wheel steering angle. From GB-A-21 73 751 a further rear wheel steering control method for a vehicle is known using: first means for monitoring a steered angle of front wheels to provide a signal indicative thereof; second means for monitoring rotational speed of front and rear wheels to determine a turning radius, said second means determining a value representing a difference in turning tracks between the front and rear wheels based on the turning radius, and a value representing a difference in rotational speed between the front and rear wheels ; third means for determining a correction for a rear wheel steering angle based on the value representing the difference in turning tracks between the front and rear wheels, said third means providing a rear wheel target steering angle based on the correction and the steered angle of the front wheels to provide a signal indicative thereof; and fourth means responsive to the signal from said third means to control an actual steering angle of the rear wheels according to the rear wheel target steering angle. 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. A Japanese Patent First Publication No. 57-60974 discloses another 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 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, a Japanese Patent First Publication No. 62-71761 discloses a rear wheel steering system which controls a steering operation for rear wheels based on a difference in rotation speeds between driven and compliance wheels to prevent the occurence of 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. Detection of the driven wheel slip is effected by determining the difference in rotational speeds between the driven and compliance wheels. As there is a difference in rotational speeds between front and rear wheels induced by a difference in turning radus therebetween in addition to the rotational speed difference due to the driven wheel slip, the rotational speed difference due to the turning radius difference impairs the rear wheel steering control greatly during turns with a minimum turning radius wherein the turning radius difference becomes great. SUMMARY OF THE INVENTION It is accordingly one object of the present invention to provide a rear wheel steering control method which is adapted for compensating for a steering component induced by a difference in turning radius when driven wheel slip is detected to determine actual slip of the driven wheels correctly for providing rear wheel steering angle control according to the degree of slip. In order to solve the object mentioned above, according to the invention said fifth means modifying the steered angle of the front wheels monitored by said first means with said correction value to determine the first parameter, and calculates the rear wheel steering angle δ r according to the following relation: δ r = K x ϑ - T x ϑ where K = K' - ΔK K' = a coefficient of a reference characteristic of rear wheel steering, variable according to the vehicle speed ΔK = the value provided by said fifth means T = a coefficient dependent upon vehicle speed ϑ = the steered angle of the front wheels ϑ ˙ = the steered angle velocity of a steering wheel BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram which shows a steering system incorporated in a front-wheel drive vehicle for carrying out the method of the invention. Fig. 2 is a flowchart which shows a sequence of logical steps carried out by said rear wheel steering control system. Fig. 3 is a table which shows a relation between a turning radius R and vehicle speed V for estimating a difference in rotational speeds between front and rear wheels ΔV R . Fig. 4 is a view which shows variation in the turning center of a vehicle due to vehicle speed. Fig. 5 is a table which shows a relation between a correction ΔK and a difference in rotational speeds between front and rear wheels. DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings, particularly to Fig. 1, a steering control system 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 determine 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 change 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 thereto 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 chambers in the hydraulic cylinder 12, displacing the tie rods 13a and 13b to steer 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, and a steering angle sensor 25 for determining a steered angle ϑ of the steering wheel 2. Theses sensors provide signals indicative of detected parameters to the steering controller 20, which calculates a rear wheel steering angle based on those signals and then calculates a correction for the rear wheel steering angle to provide the signal δr indicative of a corrected rear wheel steering angle. Referring to Fig. 2, a flowchart of a program or sequence of logical steps performed by the steering controller 20 is shown. This program is made once every predetermined cycle. After entering the program, the routine proceeds to step 100 wherein a steered angle ϑ of the steering wheel is input from the steered angle sensor 25 to the steering controller 20. The routine 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 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 The ΔV includes a first component of a difference in rotational speeds between the front and rear wheels (this will be referred as ΔV₁ hereinafter) which corresponds to driven wheel slippage caused by acceleration during turning and a second component of a difference in rotational speeds between the front 6a, 6b and rear wheels 15a, 15b (this will be referred as ΔV₂), caused by a difference in turning tracks therebetween. As mentioned above, this ΔV₂ affects the rear wheel steering control. Accordingly, the embodiment according to the present invention avoids this drawback in such a manner that the component of the ΔV₂ is cancelled to correspond the ΔV to the ΔV₁ for ensuring proper rear wheel steering control. First in step 500, a turning radius R of the vehicle is calculated according to the following equation. R = (t r X V) / V₃ - V₄ where t r is a tread of the vehicle. Subsequently, the routine proceeds to step 600 wherein based on the turning radius R, a difference ΔV R in rotational speeds between the front and rear wheels is estimated which corresponds to ΔV₂. This estimate is accomplished by referring to a table as shown in Fig. 3. Referring to Fig. 3, 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 is shown. 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 variation in the vehicle speed V as shown in Fig. 4 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 ΔV R derived by the curves in Fig. 3 is represented by the following equation: ΔV R = V X ℓ / R² X (m X V² X a / C pr X ℓ - ℓ / 2) where m is vehicle weight, C pr is cornering power of the rear wheels, ℓ is a wheel base, and 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 700 wherein a corrected front-rear wheel rotational speed difference ΔV' is calculated. This calculation is implemented by subtracting the ΔV obtained by step 40 by the ΔV R obtained by step 600 as shown by the following relation: ΔV' = ΔV - ΔV R The routine then proceeds to step 800 wherein a rear wheel target steering angle δ r is mathematically calculated according to the following equation: δ r = 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 ϑ : a steered angle of a steering wheel ϑ ˙ : steering angular velocity of a steering wheel ΔK : a correction for a rear wheel steering angle T : a coefficient variable dependent upon the vehicle speed The above equation (6) is applied to a rear wheel steering control system in which a so-called phase inversion control is performed. In the equation (6), a first term represents proportional members defining a rear wheel steering angle, dependent upon a steered angle (ϑ) of the steering wheel, in the same phase as a steered angle of the front wheels while a second term represents differential members providing a rear wheel steering angle in a phase opposite the front wheel dependent upon steering angular velocity (ϑ ˙) of the steering wheel 2. The equation (6) provides rear wheel steering control such that the rear wheels (15a, 15b) are steered to the opposite phase upon initiation of steering where the steering angle (ϑ) is relatively small and the steering angular velocity is relatively high and steered to the same phase when the steering is held where ϑ is relatively great and ϑ ˙ is being reduced. It will be appreciated that the rear wheels 15a, 15b are controlled to be steered to the opposite phase momentarily at steering initiation (this is also referred as a steering transition period) to improve response of yaw rate and then steered to the same phase as the steering and held to emphasize steering stability. The K of the proportional members (K X ϑ) of the above equation (6) is given by the relation of K = K' - ΔK and ΔK is defined according to a relation between a value of the ΔV' at that time and a preselected correction curve F as shown by a table in Fig. 5. In the correction table, ΔK becomes great as ΔV' or the slip of the driven wheels becomes high. Assuming that the steered angle ϑ is A degrees where the steering wheel is substantially held (ϑ ˙), the proportional member (K X ϑ) of the above equation (6) is K X A degrees and the differential members (T X ϑ ˙) is zero, obtaining the rear wheel steering angle δ r = K X A degrees. If a throttle pedal is depressed suddenly during turning in such a condition, the driven wheels are subject to slip. However, the a difference in the rotational speed ΔV between the driven and compliance wheels at that time includes a difference in the rotational speed (i.e., the previously mentioned ΔV₂) caused by a difference in the turning radii in addition to the slip component of the driven wheels due to the acceleration (i.e., the previously mentioned ΔV₁). Thus, the rear wheel steering angle obtained without modifying the ΔV causes a rear wheel steering angle to be provided which includes a component corresponding to the rotational speed difference due to the turning radii difference. Correction control for the rear wheel steering is sometimes effected even if no slippage of the driven wheels occurs (ΔV₁). Accordingly, the system is operable to detect a turning radius R to estimate the turning speed difference ΔV R corresponding to ΔV₂ and cancels variations in steering properties due to ΔV₂ based on ΔV R . With this control, the correction ΔK utilized in mathematical calculation of the rear wheel steering angle δ r can correspond to a value of the driven wheel slip (ΔV₁) completely, to adjust the proportional members (K X ϑ) correctly using the proper ΔK for providing an optimum rear wheel steering angle corresponding to turning conditions. For instance, in a case of a front-wheel drive vehicle as shown in Fig. 2, reducing a value of the proportional members (K X ϑ) with respect to the same phase steering causes cornering forces of the rear wheels to be decreased, ensuring swinging motion to prevent drift-out from occurring. While the above embodiment is directed to a front-wheel drive vehicle, the present invention may be applicable to a rear-wheel drive vehicle wherein increase in a value of the proportional members (K X ϑ) with respect to the same phase causes cornering forces of the rear wheels to be increased, restricting swinging motion of the vehicle to prevent spin from occurring. As previously mentioned, a feature of the present invention is that 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 (6) 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 rear-wheel drive vehicles and preventing drift-out from occurring in the case of front-wheel drive vehicles. The turning radius may be determined based on a yaw rate or lateral acceleration derived from a lateral acceleration sensor for example.

Claims: A rear wheel steering control method for a vehicle using: first means (25) for monitoring a steered angle (ϑ) of front wheels (6a, 6b); second means (20) for determining a steered angular velocity (ϑ ˙) of a steering wheel (2); third means (21, 22, 23, 24) for monitoring rotational speed of front and rear wheels (6a, 6b, 15a, 15b) to determine a first difference (ΔV) in rotational speed between the front and rear wheels; fourth means (20) for determining a turning radius (R) of the vehicle during turning to determine a second difference (ΔV R ) in rotational speed between the front and rear wheels caused by a difference in turning radii of the front and rear wheels; fifth means (20) for determining a first parameter (K x ϑ) variable in a preselected relation to the steered angle (ϑ) of the front wheels monitored by said first means and a second parameter (T x ϑ ˙) variable in a preselected relation to the steered angular velocity (ϑ ˙) to provide a rear wheel steering angle (δ r ) based on the first and second parameters, said fifth means determining a third difference (ΔV') in rotational speed between the front and rear wheels caused by wheel slippage relative to a road surface based on the first and second differences (ΔV, ΔV R ) to provide a correction value (K) for the rear wheel steering angle based on a value (ΔK) variable in a preselected relation to the third difference (ΔV'), and sixth means for controlling an actual steering angle of the rear wheels according to the rear wheel steering angle (δ r ), characterized in that said fifth means modifying the steered angle (ϑ) of the front wheels (6a, 6b) monitored by said first means with said correction value (K) to determine the first parameter (K x ϑ) and calculates the rear wheel steering angle (δ r ) according to the following relation: δ r = K x ϑ - T x ϑ where K = K' - ΔK K' = a coefficient of a reference characteristic of rear wheel steering. variable according to the vehicle speed ΔK = the value provided by said fifth means T = a coefficient dependent upon vehicle speed The method as set forth in claim 1, characterized in that said third difference (ΔV') is determined by subtracting the second difference (ΔV R ) from the first difference (ΔV). The method as set forth in claim 1 or 2, characterized in that said value (ΔK) increases as the third difference (ΔV') in rotational speed increases. The method as set forth in claim 1, characterized in that said third means includes speed sensors (21, 22, 23, 24) which determine the rotational speeds (V₁, V₂, V₃, V₄) of the wheels respectively to determine vehicle speed (V), said fourth means estimating the second difference (ΔV R ) in the rotational speed between the front and rear wheels based on the vehicle speed (V). The method as set forth in claim 1, characterized in that the speed sensors (21, 22, 23, 24) determine the rotational speeds (V₁, V₂, V₃, V₄) of front and rear wheels respectively, said fourth means calculating the turning radius (R) according to a relation of R = (t r x V / V₃ - V₄) where t r is a tread of the vehicle and the V is the vehicle speed, said fourth means calculating the second difference (ΔV R ) in rotational speed between the front and rear wheels according to the following equation: ΔV R = V x ℓ / R ² x (m x V ² x a / C pr x ℓ - ℓ / 2) where m is vehicle weight, C pr is cornering power of the rear wheels, ℓ is a wheel base, and a is a distance between the center of gravity and a point intermediate the front wheels.

Inventors: FUKUYAMA YUICHI, IMASEKI TAKASHI, IWATA TORU, TAMURA MINORU, FUKUYAMA, YUICHI, IMASEKI, TAKASHI, IWATA, TORU, TAMURA, MINORU
Applicants: NISSAN MOTOR, NISSAN MOTOR CO., LTD.
Application year: 1990
Main classification: B62D7
IPC codes: B62D6, B62D7, B62D105