Electric control apparatus for four-wheel steering system

Abstract: An electric control apparatus (30) for a four-wheel steering system of an automotive vehicle having a front wheel steering mechanism (10) for a set of dirigible front road wheels (FW1,FW2) and a rear wheel steering mechanism (20) for a set of dirigible rear road wheels (RW1,RW2), which includes a yaw rate sensor (31) arranged to detect an actual yaw rate of the vehicle for producing a detection signal indicative of the actual yaw rate, and a microcomputer (33) programmed to determine a target steering angle of the rear road wheels (RW1,RW2) in proportion to a high-frequency component of the actual yaw rate for producing a control signal indicative of the target steering angle and to apply the control signal to an electrically operated actuator (21) of the rear wheel steering mechanism (20) so that the rear road wheels (RW1,RW2) are steered at the target steering angle.

Description: The present invention relates to a four-wheel steering system for automotive vehicles, more particularly to an electric control apparatus for a rear wheel steering mechanism in the four-wheel steering system for controlling a set of dirigible rear road wheels in accordance with a yaw rate of the vehicle as shown in the European Patent Application 0 150 856. In said document in addition to a conventional steering system for steering a vehicle through the front wheels in accordance with an angular displacement of the steering wheel, the vehicle is equipped with a negative feedback steering control system which steers the vehicle through the front or rear wheels in accordance with an instantaneous yaw rate of the vehicle. The feedback system has a vehicle behavior sensor for sensing the yaw rate, and an control circuit which determines an actual quantity which is equal to the yaw rate or a linear combination of the yaw rate and the time derivative of the yaw rate and a reference quantitiy which is a function of the angular displacement and the vehicle speed, so designed as to be approximately equal to the yaw rate if there is no disturbance. The control circuit produces a feedback signal proportional to a difference between the actual quantity multiplied by a first coefficient and the reference quantity multiplied by a second coefficient. In response to the feedback signal, an actuator corrects the direction of the front or rear wheels in such a direction as to reduce the yaw rate. A lateral acceleration of the vehicle may be used in place of the yaw rate. Disclosed in Japanese Patent Laid-open Publication No. 60-161255 is a further electric control apparatus for a rear wheel steering mechanism which includes a yaw rate sensor arranged to detecte a yaw rate of the vehicle for conducting steerage of a set of dirigible rear road wheels in proportion to the detected yaw rate. Under control of the control apparatus, however, the rear road wheels are steered in the same direction as the front road wheels in steady-state cornering of the vehicle. This decreases the yaw rate gain, resulting in deterioration of the maneuverability of the vehicle. It is, therefore, a primary object of the present invention to provide an electric control apparatus for the rear wheel steering mechanism capable of enhancing the maneuverability of the vehicle in its steady-state cornering without causing any deterioration in the travel stability. According to the present invention, the object is accomplished by providing an electric control apparatus for a four-wheel steering system of an automotive vehicle having a front wheel steering mechanism for a set of dirigible front road wheels and a rear wheel steering mechanism for a set of dirigible rear road wheels, which comprises yaw rate detection means for detecting an actual yaw rate of the vehicle and for producing a detection signal indicative of the actual yaw rate; means responsive to the detection signal for obtaining a high-frequency component of the actual yaw rate; control means for determining a target steering angle of the rear road wheels in proportion to the high-frequency component of the actual yaw rate and for producing a control signal indicative of the target steering angle; and means for applying the control signal to an electrically operated actuator of the rear wheel steering mechanism so that the rear road wheels are steered at the target steering angle. In a preferred embodiment, the means responsive to the detection signal comprises means for calculating a low-frequency component of the actual yaw rate by execution of a low-pass filter calculation and for subtracting the low-frequency component from the actual yaw rate. For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Fig. 1 is a schematic illustration of a four-wheel steering system of an automotive vehicle; Fig. 2 is a flow chart of a control program executed by a microcomputer shown in Fig. 1; Fig. 3 is a graph showing a sudden change of the steering angle of the front road wheels; Fig. 4 is a graph showing a response characteristic of a yaw rate of the vehicle in the sudden change of the steering angle of the front road wheels; Fig. 5 is a graph showing a response characteristic of a slip angle of the vehicle in the sudden change of the steering angle of the front road wheels; Fig. 6 is a graph showing a response characteristic of a lateral acceleration of the vehicle in the sudden chane of the steering angle of the front road wheels; Fig. 7 is a graph showing a response characteristic of a roll angle speed of the vehicle in the sudden change of the steering angle of the front road wheels; Fig. 8 is a graph showing a response characteristic of a roll angle of the vehicle in the sudden change of the steering angle of the front road wheels; and Fig. 9 is a graph showing a steering angle of the rear road wheels. In Fig. 1 of the drawings, there is schematically illustrated a four-wheel steering system of an automotive vehicle which includes a front wheel steering mechanism 10 for a set of dirigible front road wheels FW1, FW2, a rear wheel steering mechanism 20 for a set of dirigible rear road wheels RW1, RW2, and an electric control apparatus 30 for the rear wheel steering mechanism 20. The front wheel steering mechanism 10 includes a steering shaft 12 provided at its upper end with a steering wheel 11 and connected at its lower end to a rack bar 14 which is axially displaceably mounted within a steering gear box 13. The rack bar 14 is connected at its opposite ends to the front road wheels FW1, FW2 by means of a pair of tie rods 15a, 15b and a pair of knuckle arms 16a, 16b. In operation of the steering wheel 11, the rack bar 14 is axially displaced to steer the front road wheels FW1, FW2, and the displacement amount of rack bar 14 is defined by a predetermined gear ratio at the gear box 13. The rear wheel steering mechanism 20 includes an electrically operated actuator 21 which is arranged to effect axial displacement of a relay rod 22. The relay rod 22 is connected at its opposite ends to the rear road wheels RW1, RW2 by means of a pair of tie rods 23a, 23b and a pair of knuckle arms 24a, 24b to steer the rear road wheels in accordance with axial displacement thereof. The electric control apparatus 30 includes a microcomputer 33 connected to a yaw rate sensor 31 and a rear wheel steering angle sensor 32. The yaw rate sensor 31 is arranged to detect a rotational speed of the vehicle about a vertical axis for producing a detection signal indicative of a yaw rate γ of the vehicle. The rear wheel steering angle sensor 32 is arranged to detect an axial displacement amount of the relay rod 22 for producing a detection signal indicative of a steering angle ϑr of the rear road wheels RW1, RW2. In this embodiment, the yaw rate γ and the rear wheel steering angle ϑr each are represented by a positive value in a rightward direction and by a negative value in a leftward direction. The microcomputer 33 includes a central processing unit or CPU, a read-only memory or ROM, a random access memory or RAM, and an input/output device for I/O. The ROM of computer 33 is arranged to memorize a control program shown by a flow chart in Fig. 2. The computer 33 is connected to a driving circuit 34 which is arranged to activate the actuator 21 in accordance with a control signal applied thereto from the computer 33. Assuming that an ignition switch (not shown) of the vehicle is closed, the computer 33 initiates execution of the control program at step 40 shown in Fig. 2 and repeatedly executes processing at step 41 to 44 for controlling steerage of the rear road wheels RW1, RW2. In such execution of the control program, the CPU of computer 33 reads out at step 41 a yaw rate γ of the vehicle and a steering angle ϑr of the rear road wheels in response to detection signals applied from sensors 31 and 32 and calculates at step 42 a low frequency component γ* of the detected actual yaw rate γ by execution of a low-pass filter calculation or a primary delay calculation described below. γ* = {1/(1 + τ s)} γ where the character τ is a predetermined time constant, and the character s is a laplace operator. After calculation of the low-frequency component γ* of the detected actual yaw rate γ, the CPU of computer 33 calculates at step 43 a target steering angle ϑr* of the rear road wheels in proportion to a difference between the yaw rate γ and its low-frequency component γ* on a basis of the following equation. ϑγ* = K · (γ - γ*) where K is a proportional constant, and the subtraction part represents a high-frequency component of the yaw rate γ. At the following step 44, the CPU of computer 33 produces a control signal indicative of a deviation (ϑr* - ϑr) between the target steering angle and the detected actual steering angle of the rear road wheels. When applied with the control signal from the computer 33, the driving circuit 34 produces a drive signal defined by the control signal therefrom and applies it to the actuator 21. Thus, the actuator 21 is activated to displace the relay rod 22 leftward or rightward in accordance with the deviation between the target steering angle ϑr* and the detected actual steering angle ϑr thereby to steer the rear road wheels RW1, RW2 at the target steering angle ϑr*. Assuming that in such steering operation the vehicle has been applied with crosswind in a transition period for turning, an absolute value |γ - γ*| of the high-frequency component of the yaw rate becomes a large value. In steady-state cornering of the vehicle, however, the absolute value |γ - γ*| is maintained approximately in zero. As a result, the rear road wheels RW1, RW2 are steered in the same direction as the rotational direction of the detected actual yaw rate when applied with the crosswind. This is effective to enhance the travel stability of the vehicle. In the steady-state cornering of the vehicle, the rear road wheels RW1, RW2 are not steered to enhance the maneuverability of the vehicle. Even if the zero point of the yaw rate sensor 31 is drifted by change of the ambient temperature, the high-frequency component of the detected actual yaw rate does not include any drift component therein since the drift will constantly occur. Thus, the steerage of the rear road wheels can be more precisely controlled without any influence of the drift. Since any differential value of the yaw rate γ is not utilized in this embodiment, the steerage of the rear road wheels can be stabilized without any influence caused by noises of the yaw rate sensor 31. For substantiation of the foregoing useful effects. illustrated in Figs. 3 to 8 is a result of simulation of response characteristics of a yaw rate γ, a slip angle β of the vehicle, a lateral acceleration speed Gx, a steering angle ϑr of the rear road wheels, a roll angle speed dϑroll/dt and a roll angle ϑroll in a condition where the steering angle ϑf of the front road wheels has been steppedly changed. In Figs. 4 to 8, a result of the control rule according to the present invention is shown by a solid line, a result of the yaw rate proportional control in the prior art is shown by a broken line, and a result of two wheel steering control is shown by a two-dots and dash line. From the simulation result, the following fact will be understood. 1) As shown in Fig. 4, a steady-state yaw rate γ according to the control rule of the present invention becomes approximately equal to that in the two wheel steering and larger than that in the yaw rate proportional control. The convergency of the steady-state yaw rate γ becomes substantially the same as that in the yaw rate proportional control. 2) As shown in Fig. 5, the slip angle β of the vehicle in the steady-state according to the control rule of the present invention becomes approximately equal to that in the two wheel steering and larger than that in the yaw rate proportional control. The convergency of the slip angle β becomes better than that in the yaw rate proportional control. 3) As shown in Fig. 6, the lateral acceleration speed Gx in the steady-state according to the control rule of the present invention becomes approximately equal to that in the two wheel steering and larger than that in the yaw rate proportional control. The convergency of the lateral acceleration speed Gx becomes better than that in the yaw rate proportional control. 4) As shown in Fig. 7, the roll angle speed dϑroll/dt according to the control rule of the present invention becomes approximately equal to that in the yaw rate proportional control and smaller than that in the two wheel steering. The convergency of the roll angle speed becomes better than that in the yaw rate proportional control. 5) As shown in Fig. 8, the roll angle ϑroll in the steady-state according to the control rule of the present invention becomes approximately equal to that in the two wheel steering and larger than that in the yaw rate proportional control. The convergency of the roll angle ϑroll becomes better than that in the yaw rate proportional control. 6) As shown in Fig. 9, the steering angle ϑr of the rear road wheels according to the control rule of the present invention is defined to be smaller than that in the yaw rate proportional control. Thus, it is expected that the control effect of the rear road wheels is maximized at a maximum steering angle of the rear road wheels. Although in the above embodiment, the low-frequency component γ* of the detected actual yaw rate has been calculated by processing of the low-pass filter calculation or primary delay calculation during execution of the control program, the low-frequency component γ* may be obtained by a hardware in such a manner that an output of the yaw rate sensor 31 is applied to the microcomputer 33 through a low-pass filter or a primary delay filter to be replaced with a resultant of calculation at step 42 of the control program, In addition, the high-frequency component γ - γ* of the actual yaw rate may be directly calculated by processing of a high-pass filter calculation of the actual yaw rate. In such a case, the high-pass filter calculation of the actual yaw rate is processed at step 42 of the control program, and a resultant of the calculation is multiplied by the constant coefficient K at step 43. Furthermore, the high-frequency component of the actual yaw rate may be obtained by a hardware in such a manner that an output of the yaw rate sensor 31 is applied to the microcomputer 33 through a high-pass filter to be multiplied by the constant coefficient K at step 43 of the control program.

Claims: An electric control apparatus (30) for a four-wheel steering system of an automotive vehicle having a front wheel steering mechanism (10) for a set of dirigible front road wheels (FW1, FW2) and a rear wheel steering mechanism (20) for a set of dirigible rear road wheels (RW1, RW2), comprising yaw rate detecting means (31) for detecting an actual yaw rate (γ) of the vehicle and for producing a detection signal indicative of the actual yaw rate (γ), control means (33) for determining a target steering angle (ϑγ*) of the rear road wheels (RW1, RW2) and for producing a control signal indicative of the target steering angle (ϑγ*), means (34) for applying the control signal to an electrically operated actuator (21) of said rear wheel steering mechanism (20) so that the rear road wheels (RW1, RW2) are steered at the target angle (ϑγ*), characterised by means responsive to said detection signal indicative of the actual yaw rate (γ) for obtaining a high-frequency component (γ - γ*) of the actual yaw rate whereby the target steering angle (ϑγ*) of the rear road wheels (RW1, RW2) is determined by the control means (33) in proportion to the high-frequency component of the actual yaw rate. An electric control apparatus (30) as recited in Claim 1, wherein said means responsive to the detection signal comprises means for calculating a low-frequency component of the actual yaw rate (γ) by execution of a low-pass filter calculation and for subtracting the low-frequency components from the actual yaw rate (γ).

Inventors: SUGIYAMA MIZUHO, SUGIYAMA, MIZUHO
Applicants: TOYOTA MOTOR CO LTD, TOYOTA JIDOSHA KABUSHIKI KAISHA
Application year: 1993
Main classification: B62D7
IPC codes: B62D7, B62D6, B62D137