FAIL-SAFE VEHICLE CONTROL SYSTEM

Abstract: In a vehicle having a first control system (217) for controlling a steer angle, and a second control system (217a,b) for controlling a condition of the vehicle, such as a driving force distribution in a 4WD system, or a damping characteristic of a suspension system, there is provided a fail-safe control unit. When one of the first and second control systems becomes abnormal, and the other remains normal, the fail-safe control unit adjusts the normal control system so as to maintain or improve a directional stability of the vehicle not­ withstanding the failure of the abnormal system.

Description: BACKGROUND OF THE INVENTION The present invention relates to a system for controlling a vehicle according to the preamble part of claim 1 having a steer angle control system and another vehicle control system, such as a 4WD control system or a suspension characteristic control system, for controlling a condition of the vehicle affecting a handling characteristic of the vehicle. Recently, a lot of publications disclose various 4WS control systems, 4WD control systems and suspension control systems. Some examples about the 4WS control are United State Patent No. 4,705,131, Japanese Patent Provisional Publication No. 60-229873 and Japanese Utility Model Provisional Publication No. 62-23773. Some examples about the 4WD control are United States Patents Nos. 4,757,870; 4,754,834; 4,773,500 and 4,776,424, and "Seigyo Riron no Ohyo to Jidosha no Seigyo"(Application of Control Theory and Control of Automobile), Symposium of Society of Automotive Engineers of Japan, p41 published in 1986. The 4WD system disclosed in the last-named Japanese document is mounted on a NISSAN's CUE-X concept car. The 4WD system of this example is an electronically controlled torque split, full-time 4WD system having a hydraulic multiple disc clutch in a drive path for front wheels. A controller is arranged to control a torque capacity of the multiple disc clutch by varying an applied clutch fluid pressure. A driving force distribution ratio of a front wheel driving force to a rear wheel driving force is approximately Tc : (T-Tc), where Tc is the torque capacity of the multiple disc clutch, and T is a transmission output torque. Therefore, the controller of this example controls the torque capacity Tc to provide an optimum front wheel and rear wheel driving torque distribution in accordance with signals of wheel speed sensors and an accelerator position sensor. Mitsubishi's Galant is one of examples of a vehicle combining the 4WS system and the 4WD system. An example of the suspension control is disclosed in "T12 NISSAN Sahbisu Shuho" (NISSAN Service Bulletin), pages C-7,C-9 and C-10, published by NISSAN Motor Co., Ltd. October, 1985. Some of these control systems are equipped with a fail-safe means for detecting a failure, and electronically or mechanically adjusting an abnormal system in which the failure is detected, so as to maintain the safety of the vehicle. However, the conventional fail-safe control systems are unsatisfactory in that consideration is not given to influence by a failure of one control system on another control system in the same vehicle. In general, a vehicle having a second vehicle control system such as the 4WD control system or the suspension control system, in addition to the 4WS control system, is tuned to have optimum vehicle characteristics when both control systems are normal and functioning properly. Therefore, the vehicle stability becomes worse when one control system is brought to a stop because of its failure. EP-A-0 208 992 discloses a vehicle steering system with a four-wheel steering. The four-wheel steering has a front wheel steering meachanism, a steering wheel for effecting a movement of the front wheel steering mechanism to a desired angle, a rear wheel steering mechanism, a control circuit for effecting a movement of the rear wheel stering mechanism in relation to the steering movement of the front wheels, and a control assembly for compensating the steering angle of the rear wheels in accordance with the drive torque distribution. The ratio of the steering angle of the rear wheels to the steering angle of the front wheels as considered in the same direction of steering is increased in accordance with an increase in the ration of the torque distribution. SUMMARY OF THE INVENTION It is an object of the present invention to provide a fail-safe vehicle control system which can more reliably protect the stability of a vehicle having two different control systems, against failures in the control systems. It is another object of the present invention to provide a vehicle having a steering control system, a second vehicle control system such as a 4WD control system or a suspension control system, and a fail-safe means which can protect the vehicle stability against failures in the control systems by controlling not only the control system which becomes abnormal, but also the control system which remains normal. According to the present invention, the fail-safe vehicle control system is characterised in that the fail-safe means is connected with the first and second control systems, and arranged to adjust the first control system when a failure is detected in the second control system, and to adjust the second control system when a failure is detected in the first control system. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic veiw of a vehicle for showing a first embodiment of the invention. Fig. 2 is a block diagram showing a vehicle control system of the first embodiment. Fig. 3 is a flow chart showing a 4WD control procedure of the first embodiment. Fig. 4 is a flow chart showing a 4WS control procedure of the first embodiment. Fig. 5 is a graph showing steering response characteristics obtained by the control system of the first embodiment. Fig. 6 is a schematic view of a vehicle for showing a second embodiment of the invention. Fig. 7 is a block diagram of a control system of the second embodiment. Fig. 8 is a flow chart showing a suspension control procedure of the second embodiment. Fig. 9 is a flow chart showing a 4WS control procedure of the second embodiment. Figs. 10A and 10B are graphs showing steering response characteristics obtained by the suspension control and the steering control of the second embodiment. Fig. 11 is a block diagram for showing a basic arrangement of a control system of the present invention. DETAILED DESCRIPTION OF THE INVENTION As shown in Fig. 11, a vehicle of the present invention has a steering system 301, a second system 302 such as a 4WD system or a suspension system, a first controlling means 303 for controlling the steering system, a second controlling means 304 for controlling the second system 302, and a fail-safe means 305 for adjusting a first control system constituted by the first controlling means 303 and the steering system 301, and a second control system constituted by the second controlling means 304 and the second system 302 when a failure is detected in the first or second control system. The vehicle may further comprises various sensors 306 for sensing vehicle operating conditions requied in the first and second control systems. A first embodiment of the present invention is shown in Figs. 1-5. A vehicle shown in Fig. 1 has a drive system and a rear wheel auxiliary steering system (which are substantially the same as those shown in Japanese Patent Application No. 63-33892). As shown in Fig. 1, the vehicle has left and right front wheels 1L and 1R, left and right rear wheels 2L and 2R, and a steering wheel 3. The drive system shown in Fig. 1 includes an engine 4, a transmission 5, a rear drive line which always connects the rear wheels 2L and 2R with an output member of the transmission 5, and a front drive line capable of controlling an amount of a driving torque transmitted from the transmission 5 to the front wheels 1L and 1R. The rear drive line includes a rear propeller shaft 6 which is always connected with the transmission 5, and a rear differential 7. The front drive line includes a front drive clutch 8, a front propeller shaft 9, and a front differential 10. The steering system of the vehicle includes a steering gear 11 connected between the steering wheel 3 and the front wheels 1L and 1R so that the front wheels 1L and 1R can be steered by the steering wheel 3 in the conventional manner, and a rear wheel steering actuator 12 for steering the rear wheels 2L and 2R. The rear wheel steering actuator 12 of this embodiment is a spring center type hydraulic actuator. When an oil pressure is supplied to a right chamber 12R of the actuator 12, the actuator 12 steers the rear wheels 2L and 2R to the right through an angle proportional to the oil pressure. When the oil pressure is supplied to a left chamber 12L, the actuator 12 steers the rear wheels 2L and 2R to the left through an angle proportional to the oil pressure. A rear wheel steering control valve 13 of an electromagnetic proportional type is connected with the actuator 12, and arranged to control the oil pressures of the left and right chambers 12L and 12R of the actuator 12. The control valve 13 has four variable orifices 13a, 13b, 13c and 13d which are connected in a bridge circuit as shown in Fig. 1. Four corners of the bridge circuit are connected, respectively, with a pump 14, a reservoir 15, a left oil passage 16 leading to the left chamber 12L of the actuator 12, and a right oil passage 17 leading to the right chamber 12R of the actuator 12. The control valve 13 further includes left and right solenoids 13L and 13R. When both solenoids are off, the opposite orifice pair 13a and 13b, and the other opposite orifice pair 13c and 13d are both fully opened so that both chambers 12L and 12R are put in a non-pressure state (an equilibrium state). When the solenoid 13L or 13R is engergized by a left solenoid current I L or a right solenoid current I R , then the orifice pair 13c and 13d or the orifice pair 13a and 13b is closed to a reduced opening degree corresponding to the magnitude of the solenoid exciting current I L or I R , so that the oil pressure corresponding to the current I L or I R is supplied to the power chamber 12L or 12R of the actuator 12. An electromagnetic proportional pressure regulating valve 18 is connected with the front wheel drive clutch 18, and arranged to control a clutch pressure Pc supplied to the front drive clutch 8. The front drive clutch 8 is arranged to transmit a driving torque corresponding to the clutch pressure Pc, to the front wheels 1L and 1R. The regulating valve 18 normally holds the clutch pressure Pc equal to zero. As a current I C for ergizing a clutch control solenoid 18a of the valve 18 is increased, the clutch pressure regulating valve 18 increases the clutch pressure Pc by admitting an output pressure of the pump 14 to the clutch 8. A controller 19 is provided for controlling the exciting currents I L , I R and I C of the solenoids 13L, 13R and 18a. Various sensors are connected with the controller 19. In this embodiment, the sensors are a steering angle sensor 20 for sensing a steering angle ϑ, wheel speed sensors 21L, 21R, 22L and 22R for sensing revolution speeds wf1,wf2,wr1 and wr2 of the wheels 1L, 1R, 2L and 2R, respectively, a vehicle speed sensor 23 for sensing a vehicle speed V, and a throttle opening sensor 24 for sensing a throttle opening degree TH. The steering angle sensor 20 of this embodiment is a sensor for sensing a steering wheel angle ϑ of the steering wheel 3. Output signals of these sensors are inputted to the controller 19. As shown in Fig. 2, the controller 19 of this embodiment has a 4WS control section 19a and a 4WD control section 19b. Fig. 3 shows a driving force distribution control procedure of this embodiment, and Fig. 4 shows a rear wheel steer angle control procedure. The controller 19 periodically repeats each of the control procedures. At a step 51 of Fig. 3, the controller 19 determines a desired front wheel and rear wheel driving force distribution ratio in accordance with a predetermined normal control mode. In this embodiment, the distribution ratio is determined by using the signals of the wheel speed sensors 21L, 21R, 22L and 22R and the throttle opening sensor 24, as in the NISSAN CUE-X concept car (disclosed in Symposium of Society of Automotive Engineers of Japan "Seigyo Riron no Ohyo to Jidosha no Seigyo" (Application of Control Theory and Automobile Control), page 41, 1986). In the present invention, it is possible to employ any one of various front and rear wheel driving force distribution control systems. Some examples are disclosed in United States Patents Nos. 4,757,870; 4,754,834; 4,773,500 and 4,776,424, and United States Applications Serial Nos. 07/127,319; 07/277,746; 07/254,626; 07/254,875; 07/255,820; 07/277,377 and 07/255,939. At a step 52, the controller 19 determines wether a 4WS failure signal is present or not. If it is, then the controller 19 proceeds from the step 52 to a step 53, and from the step 53 to a step 55. At the step 53, the controller 19 corrects the front and rear wheel driving force distribution ratio so as to increase the driving force transmitted to the front wheels relative to the driving force of the rear wheels. If the 4WS failure signal is not present, then the controller 19 proceeds from the step 52 to the step 55 by way of a step 54. At the step 54, the controller 19 sets a correction equal to zero so that the desired front and rear wheel driving force distribution ratio determined at the step 51 remains unchanged. At the step 55, the controller 19 determines whether a failure is present in the sensors and solenoid used in the 4WD control, to determine whether the 4WD control system is functioning properly. If the 4WD control system is not normal, then the controller 19 proceeds from the step 55 to a step 56 to stop the 4WD control, and then to a step 57 to deliver a 4WD failure signal to the 4WS control section 19a. If no failure is detected in the 4WD control system, then the controller 19 proceeds from the step 55 to a step 58. At the step 58, the controller 19 outputs a 4WD control signal to achieve the front and rear wheel driving torque distribution ratio determined at the step 53 or 54. In this embodiment, the 4WD control signal is the solenoid exciting current I C , which is sent to the solenoid 18a of the electromagnetic pressure regulating valve 18 to perform a duty factor control. After the step 57 or 58, the controller 19 returns to the step 51 to repeat a sequence of operations shown in Fig. 3. When the 4WS control system is abnormal, and the 4WD control system is normal, the thus-arranged fail-safe control system of this embodiment increases the driving force distribution ratio which is the ratio of the driving force transmitted to the front wheels of the driving force transmitted to the front wheels so that the understeer tendency is increased and the vehicle stability is improved. The 4 wheel steering control procedure of this embodiment is shown in Fig. 4. At a step 71, the cotnroller 19 reads the current values of the steering wheel angle ϑ and the vehicle speed V. At a step 72, the controller 19 determines values of a rear wheel steering proportional gain (or constant) K and a rear wheel steering derivative gain (or constant) τ(tau) by using the current value of the vehicle speed V. The proportional gain K and the derivative gain τ(tau) are functions of the vehicle speed V which are determined so as to provide the ideal vehicle dynamic characteristics in which the characteristic of the yaw rate gain with respect to the steering frequency is flat so that the yaw rate is produced in proportion to the front wheel steering amount independently of the steering speed without phase delay. Then, the controller 19 determines a rear wheel steering time constant Tr by using the following equation (1); Tr = (τ/K)/A (1) where A is a constant from 2 to 3. At a step 73, the controller 19 determines whether the 4WD failure signal is present or not, to determine whether the 4WD system is abnormal or not. If the 4WD system is abnormal, then the controller 19 proceeds from the step 73 to a step 74. At the step 74, the controller 19 increases the proportional gain K by ΔK (Delta K) (i.e., K = K + ΔK ), or decreases the derivative gain τ(tau) by Δ τ (Delta tau)(i.e., τ = τ - Δ τ ), or decreases the derivative gain τ (tau) to zero(i.e., τ = 0). After the step 74, the controller 19 proceeds to a step 76. If the 4WD system is normal, and hence the answer of the step 73 is negative, then the controller 19 proceeds from the step 73 to a step 75. At the step 75, the contrtoller 19 sets a correction quantity equal to zero, so that the constants K, τ (tau) and Tr remain unchanged. Then, the control is transferred from the step 75 to the step 76. At the step 76, the controller 19 determines a rear wheel steer angle δ r by using the values of the constants K, τ, Tr obtained at the steps 72, 74 and 75. In this embodiment, the rear wheel steer angle is calculated by the following equation (2); where S is a Laplace operator. Instead of the equation (2), it is possible to employ one of various other equations such as; (i) δ r = Kϑ (proportional control); (ii) (iii) δ r = (K - τS)ϑ (first order lead); (iv) where B f , B r , τ f and τ r are functions of the vehicle speed and δ f is a front wheel steer angle (the front and rear wheel steer angle ratio is in the form of first-order/first-order. A U.S. Patent Application Serial No. 07/277,745(filed on November 30, 1988), and a U.S. Patent Application Serial No. 07/314,874 (filed on February 24, 1989)(and a corresponding German Patent Application No. P3905811.5) show equations similar to the above-mentioned equation (2). A U.S. Patent Application Serial No. 07/269,698 (and a corresponding European Patent Application No. 88 118 827.0) show equations similar to the above-mentioned equation of the first order lag. A Japanese Patent Provisional Publication No.60-229873 (laid open in 1985) shows an equation similar to the above-mentioned equation of the first order lead. A Japanese Utility Model Provisional Publication No. 62-23773 (laid open in 1987) shows equations similar to the above-mentioned equation of the first-order/first-order type. At a step 77, the controller 19 determines whether the 4WS system is abnormal or not, by determining whether a failure is detected in the sensors and solenoids used in the 4WS control. If a failure is detected in the 4WS system, then the controller 19 stops the 4WS control at a step 78, and delivers the 4WS failure signal to the 4WD control section 19b. If the 4WS system is normal, the controller 19 performs the 4WS control at a step 80 by outputting a 4WS control signal to achieve the rear wheel steer angle δ r obtained at the step 76. In this embodiment, the controller 19 delivers the exciting currents I L and I R to the solenoids 13L and 13R at the step 80. After the step 79 or 80, the controller 19 returns to the step 71 to repeat the procedure of Fig. 4. When the 4WD control system is abnormal, and the 4WS control system is normal, then the fail-safe control system of this embodiment varies the rear wheel steer angle in such a direction as to increase the amount of the same-direction rear wheel steering by which the rear wheels are steered in the same direction as the steering direction of the front wheels, or to decrease the amount of the opposite-direction rear wheel steering by which the rear wheels are steered in the direction opposite to the steering direction of the front wheels. Therefore, the control system can maintain or improve the vehicle stability by increasing the understeer tendency. The directional stability of a vehicle can be expressed by a static stability dM/dβ (where M is a static restoring yaw moment, and β is an attitude angle)(,as disclosed in "Kiso Jidosha Kogaku (second volume)", pages 29-39, written by KONDO, published June 1979). The static stability is given by; dM/dβ = (l₁T + l₂T)α + C₁l₁ - C₂l₂ = l₂T (3) where T is a total driving force of the vehicle, α (alpha) is a ratio of a front wheel driving force to the total driving force, and l₁,l₂,C₁ and C₂ are constants determined by the specifications of the vehicle. When the stability dM/dβ is greater than zero, the vehicle is directionally stable (This characteristic corresponds to the understeer tendency.). When the static stability dM/dβ is smaller than zero, the vehicle is unstable (the oversteer tendency). Therefore, when the total driving force T is positive, the static stability dM/dβ increases, and the understeer tendency increases with increase of the share α(alpha) of the driving force alloted to the front wheels. In view of this, the control system of this embodiment is arranged to increase the vehicle stability by increasing the share of the driving force alloted to the front wheels when the 4WS system is abnormal. When the 4WD system becomes abnormal, and the clutch is disengaged as in the above-mentioned NISSAN CUE-X, the share of the front wheel driving force is reduced to zero, and the share of the rear wheel driving force is increased to 100 % (two rear wheel drive mode), so that the vehicle stability becomes worse. In contrast to this, the control system of this embodiment is arranged to prevent the vehicle stability from being decreased by the failure of the 4WD system, by utilizing the 4WS control system. When the proportional gain K is increased at the step 74, the steady state gain and phase delay of the yaw rate are lowered from solid line characteristic curves to broken line curves shown in Fig. 5, so that the damping of the yaw rate is improved, and the stability is increased so as to compensate for influence of the failure of the 4WD system. Furthermore, it is possible to improve the vehicle stability by decreasing the derivative gain (tau) (though the steering response gain is somewhat decreased). When the 4WS system becomes abnormal, and the steering mode is reduced to the two wheel steering mode, the understeer tendency is decreased and the steering response gain is enhanced as shown by one dot chain lines in Fig. 5 so that the driver feels an unnatural feeling. The control system of this embodiment can prevent such an unnatural feeling by increasing the share of the driving force alloted to the front wheels. A second embodiment of the present invention is shown in Figs. 6 - 10. A vehicle shown in Fig. 6 has a rear wheel auxiliary steering system and a suspension system (which are substantially the same as those shown in Japanese Patent Application No. 63-22686, and the U.S. Patent Application Serial No. 07/277,745, filed on November 30, 1988). As shown in Fig. 6, the vehicle has front wheels 201L and 201R, rear wheels 202L and 202R, and a steering wheel 203. The steering system of the vehicle includes a steering gear 204 connected between the steering wheel 203 and the front wheels 201L and 201R so that the front wheels are steerable through the steering gear 204 in the conventional manner. In this steering system, a front wheel steer angle δ f is determined by a steering wheel angle ϑ and a steering gear ratio N, as δ f = ϑ/N. A rear suspension system includes left and right transverse links 205L and 205R, and left and right upper arms 206L and 206R. The rear wheels 202L and 202R are connected with a rear suspension member 207 of the vehicle body through the rear suspension system in such a manner that the rear wheels are steerable. An actuator 209 for steering the rear wheels is provided between knuckle arms 208L and 208R of the left and right rear wheels 202L and 202R. Both ends of a piston rod of the rear wheel steering actuator 209 are connected with the knucle arms 208L and 208R through left and right side rods 210L and 210R. The rear wheel steering actuator 209 of this embodiment is a spring center type double acting hydraulic actuator. Left and right power chambers of the actuator 209 are connected with an electromagnetic proportional pressure control valve 212, respectively, through left and right fluid passages 211L and 211R. The control valve 212 is further connected with a pressure supply passage 215 and a drain passage 216 of a pressure source including a pump 213, and a reservoir 214. There is further provided a power steering control valve (P.S. valve). The pressure control valve 212 has left and right solenoids 212L and 212R, and is connected with a controller 217 which controls exciting currents I l and I R of the solenoids 212L and 212R. When the controller 217 stops supplying the currents I L and I R , and holds both solenoids off, the control valve 212 makes the pressures of the left and right power chambers of the actuator 209 equal to each other. Therefore, the actuator 9 is held at an equilibrium state, and performs no steering action. When the left solenoid 212L is energized by the current I L , the control valve 212 increases the pressure of the left power chamber of the actuator 209 by an amount proportional to the magnitude of the exciting current I L . Therefore, the actuator 209 steers the rear wheels 202L and 202R to the left. When the controler 217 energizes the right solenoid 212R by the current I R , then the control valve 212 increases the pressure of the right power chamber of the actuator 209 by an amount proportional to the magnitude of the current I R , and accordingly the actuator 209 steers the rear wheels 202L and 202R to the right. The controller 217 is connected with a steering angle sensor 218 for sensing the steering wheel angle ϑ of the steering wheel 203 and a vehicle speed sensor 219 for sensing a vehicle velocity V of the vehicle. The suspension system of this vehicle has adjustable shock absorbers 220, 221, 222 and 223 which are provided, respectively, for the left and right front wheels 201L and 201R and the left and right rear wheels 202L and 202R. The four wheels 201L, 201R, 202L and 202R are independently attached to the vehicle body. Each shock absorber has a motor 220a, 221a, 222a or 223a for adjusting a suspension damping characteristic, and a shock abosorber sensor 220b, 221b, 222b or 223b, as shown in Fig. 7. The controller 217 is connected with each motor, and arranged to control each of motor driving currents I c1 , I c2 , I c3 and I c4 of the damping characteristic adjusting motors 220a-223a. By controlling the rotation of the adjusting motor, the controller 217 can adjust the damping characteristic, such as a suspension damping force, of each shock absorber, individually. In this embodiment, each shock absorber is adjusted to one of three damping modes, a hard mode (H), a medium mode (M) and a soft mode (S). The controller 217 of this embodiment adjusts the shock absorbers to the H mode when the vehicle velocity becomes low (when the vehicle is at rest, for example), or when the angular speed ϑ of the steering wheel becomes high. When the road condition is bad (when the distance L between the vehicle body and the road surface is outside a predetermined range), then the controller 217 of this embodiment adjusts the front shock absorbers to the M mode, and the rear shock absorbers to the S mode. Each shock absorber sensor is arranged to sense a condition of the associated shock absorber to discriminate the H, M and S damping modes. In the embodiment, four road surface sensors 224L 224R and 225L and 225R are provided, respectively, near the left and right front wheels 201L and 201R and the left and right rear wheels 202L and 202R. The road surface sensors of this embodiment are of an ultrasonic type. Each road surface sensor has ultrasonic transmitter and receiver, and measures the distance L between the vehicle body and the road surface by measuring the time interval between transmission of an ultrasonic wave and reception of the reflected wave. The controller 217 is connected with the steering angle sensor 218, the vehicle speed sensor 219, the shock absorber sensors 220b-223b and the road surface sensors 224L, 224R, 225L and 225R. The cotnroller 217 periodically performs a suspension control procedure shown in Fig. 8 for controlling the motor driving currents I c1 , I c2 , I c3 , and I c4 of the shock absorbers, and a rear wheel steering control procedure shown in Fig. 9. As shown in Fig. 7, the controller 217 of this embodiment has a 4WS control section 217a, and a suspension control section 217b. At a first step 251 of the suspension control, the controller 217 uses the steering angle ϑ (the steering angular speed ϑ), the vehicle velocity V, the distances L between the vehicle body and the road surface, and the conditions of the shock absorbers, and determines optimum front and rear suspension damping forces suited to the driving conditions and road conditions. It is possible to employ various suspension control strategies. Some examples are disclosed in United States Patents Nos. 4,616,848, 4,733,883 and 4,717,173. At a second step 252, the controller 217 determines whether a 4WS failure signal is present or not, to determine whether the 4WS system is abnormal or not. If the 4WS failure signal is present, then the controller 217 proceeds from the step 252 to a step 253, and increases the front wheel suspension damping force by one step. For example, the front wheel suspension damping force is increased from the level of the midium damping mode to the level of the hard damping mode, or from the level of the soft mode to the level of the medium mode. Then, the controller 217 proceeds from the step 253 to a step 255. If the 4WS system is normal, then the controller 217 proceeds from the step 252 to the step 255 by way of a step 254. At the step 254, the controller 217 sets a correction quantity equal to zero, so that the front and rear wheel suspension damping forces remain unchanged at values determined at the step 251. At the step 255, the controller 217 determines whether there is a failure in the suspension control system, by detecting a failure in the sensors and actuators used for the suspension control. If the suspension system is abnormal, then the controller 217 proceeds from the step 255 to steps 256 and 257. The controller 217 stops the suspension control at the step 256, and delivers a suspension failure signal to the 4WS control section 219a at the step 257. If the suspension system is normal, then the controller 217 proceeds from the step 255 to a step 258 to output a suspension control signal. In this embodiment, the controller 217 delivers the motor driving currents I c1 , I c2 , I c3 and I c4 , respectively, to the shock absorber adjusting motors 220a - 223a at the step 258. Therefore, each shock absorber is adjusted to the level of the damping force (H or M or S) determined at the steps 251 and 253 or the steps 251 and 254. After the step 257 or 258, the controller 217 returns to the step 251. In this way, this control system makes the front suspension damping characteristic harder when the 4WS system is abnormal, and the suspension system is normal. Therefore, this control system maintains or improves the vehicle stability in spite of the failure of the 4WS system by increasing the understeer tendency by means of the suspension control system. The present invention is applicable to a vehicle having a variable stabilizer bar, or a suspension system adjusting a spring constant. In this case, the fail safe system is arranged to increase the roll stiffness distribution on the front wheels' side when the 4WS system is abnormal and the suspension system is normal. The 4WS control procedure shown in Fig. 9 is almost the same as that of the first embodiment shown in Fig. 4. Steps 271, 272 and 274-280 are substantially the same as the steps 71, 72 and 74-80, respectively. At a step 273 of the second embodiment, the controller 217 checks the suspension failure signal instead of the 4WD failure signal. If the suspension failure signal is present, then the controller 217 proceeds from the step 273 to the step 274. If the suspension system is normal, then the controller 217 proceeds from the step 273 to the step 275. At the step 279, the 4WS failure signal is delivered to the suspension control section 217b. When the suspension control system is abnormal, and the 4WS system is normal, the control system of the second embodiment varies the rear wheel steer angle in such a direction as to increase the amount of the same-direction rear wheel steering, or to decrease the opposite-direction rear wheel steering in the same manner as in the first embodiment. Therefore, the control system of the second embodiment can maintain or improve the vehicle directional stability by increasing the understeer tendency in spite of the failure of the suspension control system. Figs. 10A and 10B show effects of the control system of the second embodiment. When the front suspension damping force is increased relatively with respect to the rear suspension damping force, a transient wheel load transfer in the front wheels is increased relatively during cornering, and a front wheel cornering force is temporarily decreased, so that the understeer tendency is increased, and the directional stability is improved. Therefore, the control system of the second embodiment increases the understeer tendency by increasing the front suspension damping force, as shown by broken line curves in Fig. 10A, when the 4WS system becomes abnormal. When, for example, the suspension control system becomes abnormal, and unable to increase the damping force during rolling, then the roll angle of the vehicle tends to increase, and the steering characteristic of the vehicle tends to become unstable by the influence of "roll steer" and other factors. In such a case, the control system of the second embodiment can maintain or improve the vehicle stability by adjusting the control characteristic of the 4WS system. When the control system increases the proportional gain K of the rear wheel steering, the steady state gain and the phase delay of the yaw rate are lowered especially in a high speed range from solid line curves to broken line curves shown in Fig. 10B, and accordingly the vehicle stability is increased. When the 4WS system becomes abnormal, and the steering mode is reduced to the two wheel steering mode, the understeer tendency is decreased and the steering response gain of the vehicle is enhanced as shown by one dot chain lines in Fig. 10B so that the driver feels an unnatural feeling. The control system of the second embodiment can prevent such an unnatural feeling by hardening the front suspension.

Claims: A vehicle comprising a first control system comprising -- a first chassis system (301) including a steering system for steering said vehicle and -- a first controlling means (303) for controlling a condition of said first chassis system, and a second control system comprising -- a second chassis system (302) including at least one drive system for driving said vehicle, -- a suspension system for supporting a vehicle body of said vehicle on wheels of said vehicle and -- a second controlling means (304) for controlling a condition of said second chassis system, characterized in that said vehicle further comprises a fail-safe means (305) connected with said first and second control systems for adjusting said first control system when a failure is detected in said second control system and adjusting said second control system when a failure Is detected in said first control system. A vehicle according to Claim 1 wherein said first controlling means (303) is means for controlling a steer angle (ϑ) of said vehicle, and said second controlling means (304) is means for controlling said condition of said second chassis system (302) which affects a steering characteristic of said vehicle, and wherein said fail-safe means (305) adjusts said first controlling means (303) so as to improve a directional stability of said vehicle when a failure is detected in said second control system and said first control system remains normal, and adjusts said second controlling means (304) so as to improve said directional stability when a failure is detected in said first control system and said second control system remains normal. A vehicle according to Claim 2 wherein said second controlling means (304) is means for controlling one of a driving force distribution between front wheels (1L,1R;201L,201R) and rear wheels (2L,2R;202L,202R), and a suspension characteristic. A vehicle according to Claim 3 wherein said drive system is a four wheel drive system capable or varying a driving force distribution ratio which is a ratio of a driving force transmitted to said front wheels (1L,1R;201L,201R), to a driving force transmitted to said rear wheels (2L,2R;202L,202R), and said second controlling means (304) controls said driving force distribution ratio. A vehicle according to Claim 4 wherein said fail-safe means (305) increases said driving force distribution ratio when a failure is detected in said first control system. A vehicle according to Claim 5 wherein said fail-safe means (305) adjusts said first control system so as to decrease a steering response gain of said vehicle when a failure is detected in said second control system. A vehicle according to Claim 6 wherein said steering system is a four wheel steering system, and said first controlling means (303) controls a rear wheel steer angle (ϑ) of said vehicle, and wherein said fail-safe means (305) varies said rear wheel steer angle (ϑ) in such a direction as to increase a same direction rear wheel steering amount by which said rear wheels (2L,2R;202L,202R) of said vehicle are steered in a direction identical to a steering direction of said front wheels (1L,1R;201L,201R) of said vehicle, and to decrease an opposite direction rear wheel steering amount by which said rear wheels are steered in a direction opposite to the steering direction of said front wheels. A vehicle according to Claim 7 wherein said vehicle further comprises sensing means comprising first sensor means (20;218) for sensing a steering wheel angle (ϑ) of said vehicle and second sensor means (23;219) for sensing a vehicle speed (v) of said vehicle, and said first controlling means (303) is connected with said first and second sensor means for producing a rear wheel steer angle control signal which is a predetermined function of said steering wheel angle (ϑ), including parameters dependent on said vehicle speed (v), and said =ail-safe means (305) adjusts said function when a failure is detected in said second control system. A vehicle according to Claim 8 wherein said sensing means further comprises third sensor means (21L,21R,22L,22R) for sensing an operating condition of said vehicle and producing a sensor signal, and said second controlling means (304) is connected with said third sensor means for producing a driving force distribution control signal which is a predetermined control function of said sensor signal of said third sensor means, and said fail-safe means (305) adjusts said control function of said second controlling means so as to increase said driving force transmitted to said front wheels (1L,1R) relative to said driving force transmitted to said rear wheels (2L,2R). A vehicle according to Claim 9 wherein said first controlling means (303)comprises first failure detecting means for detecting a failure in said first control system and producing a first failure signal when a failure is detected in said first control system, and first stopping means for stopping a control action of said first controlling mean (303) and holding said rear wheel steer angle (ϑ) equal to zero when said first failure signal is present, wherein said second controlling means (304) comprises second failure detecting means for detecting a failure in said second control system and producing a second failure signal when a failure is detected in said second control system, and second stopping means for stopping a control action of said second controlling means (304) and holding said driving force distribution ratio at a predetermined fixed value when said second failure signal is present, and wherein said fail-safe means (305) is connected with said first and second failure detecting means for adjusting said first controlling means (303) when said second failure signal is present, and adjusting said second controlling means (304) when said first failure signal is present. A vehicle according to Claim 3 wherein said suspension system is of a type capable of varying said suspension characteristic, and said second controlling means (304) controls said suspension characteristic, and wherein said fail-safe means (305) adjusts said suspension characteristic when a failure is detected in said first control system. A vehicle according to Claim 11 wherein said fail-safe mean (305) adjusts said suspension characteristic of said suspension system so as to increase an understeer tendency or said vehicle. A vehicle according to Claim 12 wherein said fail-safe means (305) adjust said suspension system so as to harden a front suspension of said vehicle relative to a rear suspension. A vehicle according to Claim 12 wherein said suspension system comprises adjustable shock absorbers (220,221,222,223) capable of varying a damping characteristic, and said second controlling means (304) controls said damping characteristics. A vehicle according to Claim 14 wherein said fail-safe means (305) increases a ratio of a damping force of a front suspension to a damping force of a rear suspension when a failure is detected in said first control system, A vehicle according to Claim 15 wherein said fail-safe means (305) adjusts said first control system so as to decrease a steering response gain of said vehicle when a failure is detected in said second control system. A vehicle according to Claim 16 wherein said steering system is a four wheel steering system, and said first controlling means (303) controls a rear wheel steer angle (ϑ) and wherein said fail-safe means (305) varies said rear wheel steer angle (ϑ) in such a direction as to increase a same direction rear wheel steering amount by which said rear wheels (2L,2R;202L,202R) are steered in a direction identical to a steering direction of said front wheels (1L,1R;201L,201R), and to decrease an opposite direction rear wheel steering amount by which said rear wheels are steered in a direction opposite to the steering direction of said front wheels. A vehicle according to Claim 17 wherein said first controlling means (303) comprises first failure detecting means for detecting a failure in said first control system and producing a first failure signal when a failure is detected, and first stopping means for stopping a control action of said first controlling means and holding said rear wheel steer angle (ϑ) equal to zero when said first failure signal is present, wherein said second controlling means (304) comprises second failure detecting means for detecting a failure in said second control system and producing a second failure signal when a failure is detected in said second control system, and second stopping means for stopping a control action of said second controlling means (304) and holding said suspension system at a predetermined state when said second failure signal is present, and wherein said fail-safe means(305) is connected with said first and second failure detecting means for adjusting said first controlling means (303) when said second failure signal is present, and adjusting said second controlling means (304) when said first failure signal is present.

Inventors: KAWAGOE, KENJI
Applicants: NISSAN MOTOR CO., LTD.
Application year: 1989
Main classification: B62D6
IPC codes: B62D7, B60K23, B60G17, F16H61