Rear wheel steering control system for vehicle

• US 5,103,925 A
• Filed: 08/30/1990
• Issued: 04/14/1992
• Est. Priority Date: 09/04/1989
• Status: Expired due to Fees

• Current Assignee: Nissan Motor Co., Ltd.
• Original Assignee: Nissan Motor Co., Ltd.
• Inventors: Iwata, Toru, Tamura, Minoru, Imaseki, Takashi, Fukuyama, Yuichi
• Primary Examiner(s): Marmor, Charles A.
• Assistant Examiner(s): Kagen, Alan M.
• Application Number:
  US07/574,685
• Time in Patent Office: 593 Days
• Field of Search: 180/140, 180/142, 180/132, 280/91, 364/424.01, 364/424.05
• US Class Current: 180/415
• CPC Class Codes: B60T 2260/022 Rear-wheel steering; Four-wheel steering
• CPC Class Codes: B60T 8/1755 Brake regulation specially adapted to control the stability of the vehicle, e.g. taking into account yaw rate or transverse acceleration in a curve road vehicle drive control systems for control of driving stability otherwise than by controlling a particular sub-unit B60W30/02
• CPC Class Codes: B62D 7/159 characterised by computing methods or stabilisation processes or systems, e.g. responding to yaw rate, lateral wind, load, road condition

Abstract

Steering angle for rear wheels of a vehicle is controlled by determining a rear wheel steering angle in a preselected relation to a steered angle of a steering wheel. A correction factor for the rear wheel steering angle is based on a difference in rotational speed between front and rear wheels induced during turning. The correction factor for rear wheel steering angle is maintained at the value it had prior to braking to cancel the effect of braking slips on the determination of the rear wheel steering angle to improve steering stability.

Claims

1. A rear wheel steering control system for a vehicle comprising:

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 said 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, said third means deriving a rear wheel target steering angle based on the correction value and the steered angle of the front wheels to derive a signal indicative thereof;

fourth means for monitoring braking operation to provide a signal indicative thereof;

fifth means responsive to the signal from said fourth 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; and

sixth means for controlling an actual steering angle of the rear wheels according to the rear wheel target steering angle.

2. A system as set forth in claim 1, wherein said first means includes a sensor for detecting a steered angle θ

of a steering wheel to monitor the steered angle of the front wheels and steering angular velocity θ

of the steering wheel, said third means deriving the rear wheel target steering angle δ

_r according to the following relation,

space="preserve" listing-type="equation">δ

r=KXθ

-TXθ

where

K=K'"'"'-Δ

K

K'"'"'=a coefficient of a reference characteristic for the rear wheel steering variable according to vehicle speed

Δ

K=the correction determined by said third means

T=a coefficient dependent on the vehicle speed.

3. A system as set forth in claim 2, wherein the correction Δ

K is defined by a parameter which increases with increases in the difference in rotational speed Δ

V between the front and rear wheels.

4. A rear wheel steering control system for a vehicle comprising:

first means for detecting a steered angle of front wheels to provide a signal indicative thereof;

second means for monitoring rotational speeds of wheels to determine vehicle speed and a difference in rotational speed between the front and rear wheels caused by driven wheel slippage to a road surface and providing signals indicative thereof;

third means responsive to the signals from said first and second means to determine a rear wheel steering angle in a preselected relation to the steered angle of the front wheels and the vehicle speed;

fourth means responsive to the signal from said second means to determine a correction value for the rear wheel steering angle determined by said third means based on the magnitude of the difference in rotational speeds between the front and rear wheels, said fourth means adjusting the rear wheel steering angle based on the correction value to provide a rear wheel target steering angle and providing a signal indicative thereof;

fifth means for monitoring braking operation to provide a signal indicative thereof;

sixth means responsive to the signal from said fifth means to hold the correction value to a value prior to the braking operation during braking; and

seventh means for controlling an actual steering angle of the rear wheels according to the rear wheel target steering angle.

5. A system as set forth in claim 4, wherein said fifth means is provided with a brake switch lamp response to depression of a brake pedal.

6. A system as set forth in claim 4, wherein said first means includes a sensor for detecting a steered angle θ

of a steering wheel to monitor the steered angle of the front wheels and steering angular velocity θ

of the steering wheel, said fourth means deriving the rear wheel target steering angle δ

_r according to the following relation,

space="preserve" listing-type="equation">δ

r=KXθ

-TXθ

where

K=K'"'"'-Δ

K

K'"'"'=a coefficient of a reference characteristic of rear wheel steering variable according to vehicle speed

Δ

K=the correction determined by said fourth means

T=a coefficient dependent on the vehicle speed.

7. A system as set forth in claim 6, wherein the correction Δ

K is a parameter which increases as there are increases in the difference in rotational speeds Δ

V between the front and rear wheels.

8. A system as set forth in claim 4, wherein said sixth means is responsive to the signal from said fifth means to cancel correcting operation carried out by said fourth means.

9. A system as set forth in claim 4, wherein said second means includes speed sensors, each of which determines rotational speed of a wheel, to calculate the difference in rotational speeds Δ

V between the front and rear wheels, vehicle speed and a turning radius, said fourth means calculating a component of a difference in rotational speeds Δ

V_R between the front and rear wheels caused by a difference in turning radii therebetween based on the vehicle speed and the turning radius to determine a difference in rotational speeds Δ

V'"'"' given by canceling the difference in rotational speeds Δ

V_R from the difference in rotational speeds Δ

V, said fourth means determining the correction Δ

K based on the magnitude of the difference in rotational speeds Δ

V'"'"'.

10. A system as set forth in claim 9, wherein said first means includes a steering wheel sensor for detecting a steered angle θ

of a steering wheel to monitor the steered angle of the front wheels and steering angular velocity θ

of the steering wheel, said fourth means deriving the rear wheel target steering angle δ

_r according to the following relation,

space="preserve" listing-type="equation">δ

r=KXθ

-TXθ

where

K=K'"'"'-Δ

K

K'"'"'=a coefficient of a reference characteristic of rear wheel steering variable according to vehicle speed

Δ

K=the correction calculated by said fourth means

T=a coefficient dependent on the vehicle speed.

11. A method of controlling rear wheel steering angle of a vehicle having manually controlled front wheel steering and brakes for front and rear wheel sets, comprising (1) determining indications of (a) the difference in rotational speeds of the front and rear wheel sets, (b) the vehicle speed, and (c) steering angle of the front wheel set, (2) determining rear wheel target steering angle in response to functions derived from the determined indications of (b) and (c) as well as a correction factor based on indication (a), and (3) controlling the steering angle of the rear wheel set in response to the target steering angle in substantially a real time basis as determined during step (2) except when the wheel sets are being braked at which time the steering angle is controlled in substantially a real time basis in response to the functions derived from the determined indications of (b) and (c) and the correction factor based on indication (a) as it existed immediately prior to the wheel sets being braked.

12. The method of claim 11 wherein target steering angle is determined as

13. The method of claim 11 wherein the correction factor is determined by determining vehicle turning radius, estimating the rotational speed difference between the front and rear wheel sets in response to determined vehicle turning radius and vehicle speed, determining the deviation of the estimated rotational speed difference from monitored values for the difference between the rotational speeds of the front and rear wheel sets;

and determining the correction factor in response to the determined deviation.

14. The method of claim 13 wherein turning radius is determined from vehicle speed and the deviation of the rotation speed of the wheels of one of the wheel sets.

15. A method of controlling rear wheel steering angle of a vehicle having manually controlled front wheel steering and brakes for front and rear wheel sets, one of said wheel sets being driven by a power plant of the vehicle, the other of said wheel sets being a compliance set, the method comprising (1) normally controlling the rear wheel set steering angle in response to indications of (a) the front wheel set steering angle, (b) vehicle speed and (c) a variable correction value dependent on the difference in rotational speed between the front and rear wheel sets caused by slippage of the driven wheel set, and (2) during braking controlling the rear wheel set steering angle in response to indications of the front wheel steering angle and vehicle speed on a substantially real time basis and the same correction value that was controlling rear wheel steering angle immediately prior to braking.

16. The method of claim 15 wherein the correction factor is determined by determining vehicle turning radius, estimating the rotational speed difference between the front and rear wheel sets in response to determined vehicle turning radius and vehicle speed, determining the deviation of the estimated rotational speed difference from monitored values for the difference between the rotational speeds of the front and rear wheel sets;

and determining the correction factor in response to the determined deviation.

17. A method of controlling a rear wheel steering unit of a vehicle having a front set of wheels, a rear set of wheels and a manually responsive steering controller for the front wheel set, one of said sets of wheels being driven by a power plant of the vehicle and the other wheel set being a compliance set, the method comprising

detecting rotational speeds of the front and back wheel sets,

detecting the front wheel set steering angle,

deriving a first parameter having a value corresponding to wheel slippage of the driven wheel set caused by acceleration during turning and a second parameter representing the difference in turning radii of the front and rear wheel sets in response to the detected rotational speeds of the front and back wheel sets,

normally controlling the rear wheel set steering angle by combining indications of the first and second parameters with the detected front wheel set steering angle so spin or drift out is substantially prevented in the presence of wheel slip of the driven wheel set, the first and second parameters being derived so there is a variable correction value for the rear wheel set steering angle dependent on the difference in rotational speeds between the front and rear wheel sets caused by slippage of the driven wheel set, and

controlling the rear wheel set steering angle during braking in response to indications of the front wheel steering angle and vehicle speed on a substantially real time basis and the same correction value that was controlling rear wheel steering angle immediately prior to braking.

18. A method of controlling rear wheel steering angle of a vehicle having manually controlled front wheel steering and brakes for front and rear wheel sets, one of said wheel sets being driven by a power plant of the vehicle, the other of said wheel sets being a compliance set, the method comprising (1) normally controlling the rear wheel set steering angle in response to indications of (a) a vehicle operating parameter and (b) a variable correction value dependent on slippage of the driven wheel set, and (2) during braking controlling the rear wheel set steering angle in response to indications of the vehicle operating parameter on a substantially real time basis and the same correction value that was controlling rear wheel steering angle immediately prior to braking.