Fail-safe steer angle control system for vehicle.

0001The present invention relates to a steer angle control system for a four wheel steering vehicle, and more specifically to a fail-safe steer angle control system designed to minimize a change in a vehicle steering characteristic when an abnormal condition occurs in either of front and rear wheel steering systems. The following, commonly assigned, U.S. Patent Applications relate to steer angle control systems similar to a steer angle control system of the present invention, but none shows a fail-safe system. <ul id="ul0001" list-style="none"><li>(1) Serial No. 195,085, filed on May 17, 1988.</li><li>(2) Serial No. 195,078, filed on May 17, 1988.</li><li>(3) Serial No. 245,148, filed on September 16, 1988.</li><li>(4) Serial No. 275,061, filed on November 22, 1988.</li><li>(5) Serial No. Unknown, filed on November, 30, 1988, and based on Japanese Application No. 62-299892 (Our ref. U181-88).</li><li>(6) Serial No. Unknown, filed on November 30, 1988, and based on Japanese Applications N0s. 62-299888 and 63-28858 (Our ref. U184-88).</li><li>(7) Serial No. Unknown, filed on November 10, 1988, and based on Japanese Applications Nos. 62-284103 and 62-284104 (Our ref. U186-88).</li><li>(8) Serial No. Unknown, based on Japanese Application No. 62-330281 (Our ref. U193-88).</li></ul>

0002In a vehicle steer angle control system having a front wheel auxiliary steering system and a rear wheel steering system, as disclosed in USP 4,705,131, there is employed a fail-safe system which is arranged to detect an abnormal condition in the front and rear steering systems, and makes the front wheel auxiliary steer angle or the rear wheel steer angle equal to zero by stopping the operation of only the abnormal side.

0003This conventional fail-safe system, however, markedly changes the handling characteristic of the vehicle by reducing the controlled steer angle of the abnormal side to zero, so that it becomes difficult for the driver to control the vehicle.

0004It is an object of the present invention to provide a fail-safe steer angle control system which can perform a fail-safe operation without causing a steep change in vehicle steering characteristic. This is achieved by a system as defined by claim 1. Embodiments of the invention are described with re­ference to the drawings. <ul id="ul0002" list-style="none"><li>Fig. 1 is a schematic view of a vehicle equipped with a fail-safe steer angle control system according to one embodiment of the present invention.</li><li>Fig. 2 is a flowchart showing control operations performed by the control system of Fig. 1.</li><li>Fig. 3 is a block diagram schematically showing a fail-safe steer angle control system of the present invention.</li></ul>

0005According to the present invention, a fail-safe steer angle control system for a vehicle, comprises front control means 301, rear control means 302 and fail-safe means 303, as schematically shown in Fig. 3.

0006The front control means 301 is means for controlling a front wheel steer angle of the vehicle by using a predetermined front control characteristic. The rear control means 302 is means for controlling a rear wheel steer angle of the vehicle by using a predetermined rear control characteristics. The fail-safe means 303 is connected with the front and rear control means. If a malfunction is detected in one of the front and rear control means 301 and 302, the fail-safe means 303 adjusts the control characteristic of the control means 301 or 302 in which no malfunction is detected. Preferably, the fail-safe means 303 is designed to adjust the front control characteristic of the front control means if a malfunction is detected in the rear control means, and to adjust the rear control characteristic of the rear control means if a malfunction is detected in the front control means.

0007The fail-safe steer angle control system may further comprises sensing means 304 for sensing a condition, such as a steering wheel angle, of the vehicle and producing an input signal.

0008One embodiment of the present invention is shown in Figs. 1 and 2.

0009In a vehicle shown in Fig. 1, front wheels 1 are connected with a front steering linkage 3, and rear wheels 2 are connected with a rear steering linkage 4. Therefore, both the front and rear wheels 1 and 2 are steerable. A steering wheel 5 is connected with a steering gear 6. The steering gear 6 of this embodiment has a pinion connected with the steering wheel 5 and a rack provided in the front steering linkage between the left and right front wheels 1.

0010A front wheel auxiliary steering system 8 includes a front wheel auxiliary steering actuator 7 which varies a front wheel steer angle in cooperation with the steering gear 6.

0011A rear wheel steering system 10 includes a rear wheel steering actuator 9 for varying a rear wheel steer angle.

0012Front and rear electromagnetic type proportional control valves 12 and 13 are, respectively connected with the front and rear actuators 7 and 9. Each actuator 7 or 9 is operated by oil pressure supplied from an oil pump 11 through the front or rear control valve 12 or 13.

0013A controller 14 is connected with the front and rear control valves 12 and 13. The controller 14 delivers a front wheel steer angle control signal δ_f and a rear wheel steer angle control signal δ_r, respectively to the front and rear control valves 12 and 13.

0014Steering angle sensor 15 and vehicle speed sensor 18 are connected with the controller 14. The steering angle sensor 15 is provided in a steering shaft, and senses a steering wheel angle. A steering wheel angle signal ϑ produced by the steering angle sensor 15, and a vehicle speed signal V of the vehicle speed sensor 18 are inputted into the controller 14. There are further provided a front wheel steer angle sensor 16 attached to the front actuator 7, and a rear wheel steer angle sensor 17 attached to the rear actuator 9. The front steer angle sensor 16 senses a displacement of a movable element of the actuator 7, and produces a front feedback steer angle signal δ_f′, and the rear steer angle sensor 17 senses a displacement of a movable element of the rear actuator 9, and produces a rear feedback steer angle signal δ_r′. A selector switch 19 is provided near the driver's seat. With the selector switch 19, the driver can select one of a sporty mode and a stable mode.

0015The controller 14 controls the front and rear wheel steer angles by performing a sequence of operations shown in Fig. 2.

0016At a first step 201, the controller 14 reads the steering angle ϑ and the vehicle speed V which are sensed by the sensors 15 and 18.

0017At a next step 202, the controller 14 determines whether each of the front wheel auxiliary steering system (FR) 8 and the rear wheel steering system (RR) 10 is properly functioning. For example, the controller 14 compares the front control signal with the front feedback signal, and produces a front error detection signal representing the existence of an abnormal condition in the front auxiliary steering system FR when a difference between the front control signal and the front feedback signal is greater than a predetermined level. When a difference between the rear control signal and the rear feedback signal is greater than a predetermined level, the controller 14 produces a rear error detection signal to indicate the presence of an abnormal condition in the rear steering system RR.

0018If the front auxiliary steering system FR and the rear steering system RR are both normal, the controller 14 proceeds from the step 202 to a step 203.

0019At the step 203, the controller 14 determines the front wheel auxiliary steer angle δ_f and the rear wheel steer angle δ_r in a normal mode. In this embodiment, the controller 14 calculates the front wheel auxiliary steer angle δ_f and the rear wheel steer angle δ_r in accordance with the following relationships. δ_f = (k_f + T_f·S)ϑ      (1) δ_r = (k_r - T_r·S)ϑ      (2) In these equations, S is a Laplace operator, and k_f, k_r, T_f and T_r are parameters whose whose values are determined by the specifications of the vehicle and the vehicle speed.

0020At a step 204, the controller 14 delivers the front control signal representing the front wheel auxiliary steer angle δ_f to the front control valve 12, and the rear control signal representing the rear wheel steer angle δ_r to the rear control valve 13. As a result, the actual front wheel steer angle is controlled so that the actual front wheel steer angle is held equal to an algebraic sum of a basic front wheel steer angle determined by the steering gear 6 alone, and the front wheel auxiliary steer angle δ_f produced by the actuator 7.

0021If either or both of the front auxiliary steering system FR and the rear steering system RR is not normal, then the controller 14 proceeds from the step 202 to a step 205. At the step 205, the controller 14 turns on an alarm to indicate the occurrence of the abnormal condition.

0022After the step 205, the controller 14 determines, at a step 206, whether the front wheel auxiliary steering system FR is abnormal, and determines, at a step 207, whether the rear wheel steering system is abnormal.

0023If the front wheel auxiliary steering system FR is normal, and the rear wheel steering system RR is abnormal, then the controller 14 proceeds from the step 206 to a step 208. There, the controller 14 makes the rear wheel steer angle δ_r equal to zero, and determines the front wheel auxiliary steer angle δ_f in accordance with; δ_f = (k_f′ + T_f′ S)ϑ by using adjusted parameters k_f′ and T_f′. Then, the controller 14 outputs the thus-determined δ_f and δ_r at the step 204. In this case, therefore, this fail-safe steer angle control system stops the steering operation of the rear wheel steering system, and adjusts the control characteristic of the front wheel auxiliary steering system which is not abnormal.

0024If the rear wheel steering system RR is normal, but the front wheel auxiliary steering system FR is abnormal, then the controller 14 proceeds from the decision step 207 to a step 209. There, the controller 14 makes the front wheel auxiliary steer angle δ_f equal to zero, and determines the rear wheel steer angle δ_r in accordance with; δ_r = (k_r′ - T_r′S)ϑ by using adjusted parameters k_r′ and T_r′. Then, the controller 14 outputs the front and rear control signals at the step 204 in accordance with the results of the step 209. Thus, the fail-safe system puts the front wheel auxiliary steering system FR out of operation, and adjusts the control characteristic of the rear wheel steering system RR.

0025If the front wheel auxiliary steering system FR and the rear wheel steering system RR are both abnormal, then the controller 14 proceeds from the decision step 207 to a step 210, and reduces both of the front wheel auxiliary steer angle δ_f and the rear wheel steer angle δ_r to zero. Therefore, the vehicle is steered only through the steering gear 6 of the front wheels 1.

0026In the above-mentioned control functions (1) and (2), the parameters have the following meanings.

0027k_f: Improvement in vehicle stability.

0028T_f, T_r: Improvement in vehicle response (transient state characteristic). (However, an unnatural feeling is brought about when the difference between T_F and T_r is too much.)

0029k_r: Control of a steady state gain.

0030Therefore, the fail-safe system of this embodiment is arranged to improve the response and the stability of the vehicle by adjusting the steering system on the normal side when the parameters k_f, T_f or k_r, T_r are equal to zero as the result of the detection of an abnormal condition in either the front auxiliary system or the rear steering system.

0031When the front wheel auxiliary steering system becomes abnormal, and k_f is set equal to zero, then this fail-safe system can prevent an undesired decrease of the yaw rate gain of the vehicle by decreasing the parameter k_r. When the front wheel auxiliary steering system becomes abnormal and T_f is set equal to zero, the fail-safe system can ensure the vehicle response characteristic by increasing the parameter T_r.

0032When the rear wheel steering system becomes abnormal and k_r is set equal to zero, then this fail-safe system can prevent the vehicle stability from degrading, by decreasing the parameter k_f. When the rear steering system becomes abnormal and T_r is set equal to zero, this fail-safe system can ensure the steering response by increasing the parameter T_f.

0033At the step 208 of this embodiment, k_f and T_f are both adjusted to k_f′ and T_f′. However, it is optional to adjust only either of k_f and T_f. Similarly, it is optional to adjust only either of k_r and T_r at the step 209.