Controlling apparatus of steering angle of rear wheels of four-wheel steering vehicle

Abstract: The tuning of the parameters (Kp,Ki,Kd) of a PiD controller for controlling an electric motor (4) of a rear wheel steering device is achieved by first determining the motor transient response to a step input, the motor being disconnected from the steering shaft, and next comparing measured overshoot amount and raising-up time to given values. Further a vibrating state of the electric steering motor under load may be detected by determining that the difference between desired and detected position of the motor repeats positive and negative values more than a predetermined number of times within a preset term. When the unstable vibration of the electric motor is detected a gain to the yaw rate is adjusted.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a controlling apparatus for controlling a steering angle of rear wheels of a four-wheel steering vehicle such as an automobile or the like by use of an electric motor in correspondence to the running state of the vehicle, i.e., a turning angle of a steering wheel, etc. Description of the Prior Art Vigorous research and development has been made for a four-wheel steering system to steer the rear wheels of a vehicle corresponding to the running state, and actually some kinds of the system are put in practical use. In a four-wheel steering system now in practical use, for example, as disclosed in JP-A-1-22675, when the vehicle is running at lower speeds than a predetermined vehicle velocity, a turning ratio between the front and rear wheels is set so that a steering angle of the rear wheels becomes the reverse phase to that of the front wheels. On the other hand, when the vehicle is running at higher speeds than the predetermined vehicle velocity, the turning ratio is set so that the steering angle of the rear wheels becomes the same phase as that of the front wheels. Fig. 3 is a concrete example of the turning ratio set in the prior art four-wheel steering system. A main advantage of the four-wheel steering vehicle is to enforce the cornering force approximately at the same time for the front wheel tires and rear wheel tires. Therefore, the four-wheel steering vehicle is allowed to move in a transverse direction faster than a front-wheel steering vehicle when the operator turns a steering wheel. In other words, if the front and rear wheels are turned in the reverse phase at low speeds and in the same phase at high speeds with an appropriate turning ratio, the vehicle starts turning at an earlier stage, thereby reducing a delay in transverse acceleration after turning of the steering wheel. This realizes, as effects in practical use, a capability of a small turn at low speeds and a capability of an emergency avoidance at high speeds, etc. US-A-4 836 319 defining the closest prior art according to the preamble of claim 1 discloses a steering apparatus for use in a motor vehicle whereby the actual steering angle of a vehicle wheel is controlled to be equal to a target steering angle. The known steering apparatus includes a steering mechanism for regulating a steering angle of the vehicle wheel in accordance with an input signal thereto, a device for determining a target steering angle of the wheel in accordance with the vehicle travelling state, a device for obtaining the difference between the determined target steering angle and the actual steering angle of the wheel, and a device for calculating a steering angular velocity on the basis of the actual steering angle. A control device calculates at least two switching functions on the basis of the obtained difference and the calculated steering angular velocity for obtaining a switching line on the basis of the calculated switching functions. A control signal is generated as the input signal to the steering mechanism so that the obtained difference and the calculated steering angular velocity are respectively converged to zero under restriction of the obtained switching line. However, the characteristic and response of an electric motor are varied with time in the prior art four-wheel steering apparatus of the aforementioned structure, whereby the stability of the rear wheels is undesirably endangered. If the rotation of the electric motor is unstable, with causing vibrations (therefore hindering the electric motor from stopping at a fixed position), the rear wheels become unsteady in their movement, resulting in an increase of power consumption in the electric motor. Considering the yaw rate, if a signal of the yaw rate sensor is to be used for feedback, such delays as a time delay before the yaw rate is generated to the vehicle body after the rear wheels are turned, a response delay of the rear wheels, a time delay of the yaw rate sensor, a time delay for A/D conversion, a phase delay in a low pass filter to reduce noises of the yaw rate sensor, etc. make the movement of the rear wheels unstable, e.g., during the change of a lane at high speeds, thus deteriorating the steering stability. Furthermore, a response speed to a desired steering angle of the rear wheels which angle is determined by the angle of the steering wheel, yaw rate and vehicle velocity, namely, a response' speed of the electric motor mounted in the rear wheels is set always constant in the prior art four-wheel steering apparatus, and aimed for a high speed region requiring a quick response. Therefore, the response speed is kept high even in a region not requiring a quick response, for example, even when the rear wheels remain unturned. As a result, the electric motor consumes unnecessary power. SUMMARY OF THE INVENTION An essential object of the present invention is to provide a controlling apparatus for controlling a steering angle of rear wheels of a four-wheel steering vehicle, which realizes smooth and comfortable running of the vehicle without unstable operation of an electric motor resulting from the unevenness and change with time or vibrations of the characteristic of the electric motor. A further object of the present invention is to provide a controlling apparatus for controlling a steering angle of rear wheels of a four-wheel steering vehicle, whereby a desired steering angle of the rear wheels is obtained stably and a displacement in an advancing direction of the vehicle due to external disturbances such as transverse winds or bad roads, etc. is stably corrected, and which is further characterized in that the steering angle of the rear wheels is properly controlled even when the unstable movement of the rear wheels is detected. A still further object of the present invention is to provide a controlling apparatus for controlling a steering angle of rear wheels of a four-wheel steering vehicle, designed to change a response speed of an electric motor for a desired steering angle of the rear wheels thereby supressing power consumption in the electric motor. In accomplishing the above-described objects, according to the present invention, there is provided a controlling apparatus for controlling the steering angle of the rear wheels of a four-wheel steering vehicle, comprising a vehicle speed sensor for detecting the velocity of the vehicle, a yaw rate sensor for detecting the yaw rate, a steering wheel angle sensor for detecting the turning angle of the steering wheel, a controller for outputting a rear wheel steering angle instructing signal in response to the signals of said sensors, and an electric motor for steering the rear wheels based on said rear wheel steering angle instructing signal, said electric motor being provided with a position detector, said controller being provided with a first interface circuit for obtaining the current position of said electric motor from said position detector, an operating unit for calculating a desired steering angle of said rear wheels from the output value of said sensors and for calculating the deviation between the desired position and current position of said electric motor, and a second interface circuit for feeding a current instruction to a motor driver from said operating unit, characterized in that in said operating unit a value obtained through differential operation of the current position of the electric motor is subtracted from a value obtained by multiplying the sum of a value obtained through proportional operation of the deviation and a value obtained through integral operation of the deviation with a value proportional to the vehicle velocity, so that the resultant value is output as a current instructing value to said motor driver via said second interface circuit. An advantageous embodiment of the invention is defined in claim 2. In the above-described constitution, the response speed of the electric motor is raised as the vehicle velocity is increased. Therefore, power consumption during the low speed running time not requiring a quick response of the rear wheels is reduced. Furthermore, the response speed of the electric motor is made different to the steering wheel angle sensor from that to the yaw rate sensor and the speed in steering the rear wheels in the reverse phase is decreased to the steering wheel angle sensor, thereby reducing power consumption and ensuring smooth generation of the yaw rate. Since the response of the electric motor to the yaw rate sensor in steering the rear wheels in the same phase is maintained high, a feedback loop of a steering system of the rear wheels is stabilized. Errors in an advancing direction or course of the vehicle due to the external disturbances can be corrected stably by steering the rear wheels. These and other objects and features of the present invention will become apparent from the following detailed description taken in conjunction with preferred embodiments thereof with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram of a controlling apparatus for controlling a steering angle of rear wheels of a four-wheel steering vehicle according to a third embodiment of the present invention; Fig. 1(a) is a graph showing the relation between speed and f(V) of Fig. 1; Fig. 2 is a block diagram of a controlling apparatus according to a modified embodiment of Fig. 1; and Fig. 3 is a graph of the ratio of steering angles between the front and rear wheels in a conventional four-wheel steering vehicle. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to a block diagram of Fig. 1 showing a controlling apparatus in a first embodiment of the present invention, right and left rear wheels 1 are turned by an electric motor 4 according to a rear wheel steering angle instructing signal 6 output to a motor driver 3 from a controlling device 2 which controls a steering angle of rear wheels of a four-wheel steering vehicle. A reduction gear 5 is present between the electric motor 4 and rear wheels 1 to amplify a torque. The controlling device 2 is constituted by an operating device 16 which determines a target steering angle of rear wheels to an input value from each of a speed sensor 7, a steering wheel angle sensor 8 and a rear wheel position detector 10 and, also determines the above-referred rear wheel steering angle instructing signal 6 and, interface circuits 12, 14, 15 of the respective sensors. The operating device 16 is comprised of a desired steering angle setting device 22 for setting a desired steering angle of the rear wheels, a comparator 26 and a current instructing value setting device 23 which consists of a comparator 23a and couplers 23b, 23c. Within the controlling device 2 are provided the first interface circuit 14 mounted to the electric motor 4 which converts a value of the position detector 10 to a digital value thereby to obtain the current position of the electric motor 4, steering wheel angle sensor 8, interface circuits 12, 13 for converting a detecting signal of the yaw rate sensor 9 to a digital value, second interface circuit 15 which feeds a current instructing value 6 to the motor driver 3 and operating device 16 which processes a desired steering angle of the rear wheels, the current instructing value 6 and detects a cycle of the speed sensor 7. An algorithm within the operating device 16 will be discussed now. A deviation between a target steering angle of the rear wheels determined by the steering wheel angle, yaw rate and vehicle velocity, i.e., a target steering angle of the electric motor 4 mounted in the rear wheels 1 and, the current position of the electric motor 4 is processed for every sampling cycle. The deviation is integrated for every sampling cycle. In the first place, a value obtained by multiplying the deviation with Kp is added to a value obtained by multiplying an integrating value of the deviation with a coefficient Ki. Then, a value obtained by multiplying a velocity of the electric motor 4 obtained through reduction of the current position and a current position one cycle before with Kd is subtracted from a value obtained by multiplying the above adding value with a function f(V) of the vehicle velocity, which is output as the current instructing value 6 to the motor driver 3 via the second interface circuit 15. The function f(V) is a constant proportional to the vehicle velocity. In other words, as the vehicle velocity is increased while the deviation is the same, the current instructing value 6 to the motor driver 3 is increased, thereby enhancing the controlling property of the electric motor 4. Fig. 1(a) is a graph showing the relation between vehicle velocity and function f(V), wherein f(V) is increased in proportion to the vehicle velocity. Fig. 2 is a block diagram of a controlling apparatus according to a modification of the above embodiment. The modified controlling apparatus is fundamentally equal to the apparatus of Fig. 1 in its structure, but is provided with a K1 (same phase) and a K2 (reverse phase) circuits in the f(V) circuit within the operating device 16. Whether the rear wheels are turned in the same phase or reverse phase to the front wheels is detected by a phase detector from a steering wheel angle and a motor (rear wheel) angle. A gain K1 or K2 is multiplied for the same phase time or for the reverse phase time. An algorithm within the operating device 16 will be depicted hereinbelow. A deviation between a target steering angle of the rear wheels determined by the steering wheel angle, yaw rate and vehicle velocity, i.e., a target steering angle of the electric motor 4 mounted in the rear wheels 1 and the current position of the electric motor 4 is processed for every sampling cycle. The deviation is integrated for every sampling cycle. A value obtained by multiplying the deviation with Kp is added to a value obtained by multiplying an integrating value with the coefficient Ki. Thereafter, in the case where the front (not shown) and rear wheels are turned in the same phase, a value obtained by multiplying the velocity of the electric motor obtained through reduction of the current position and a current position one cycle before of the electric motor with Kd is subtracted from a value obtained by multiplying the adding value with K1. On the other hand, if the front and rear wheels are turned in the reverse phase, the value is subtracted from a value obtained by multiplying the adding value with K2. The resultant value is output as the current instructing value 6 to the motor driver 3 via the second interface circuit 15. K1 is set larger than K2. That is, when the rear wheels are turned in the reverse phase to the front wheels in proportion to the size of the steering wheel angle, the electric motor gets delayed in response. If the rear wheels are turned in the same phase as the front wheels in proportion to the size of the yaw rate, the responding capability of the rear wheels is improved. Although the above embodiment employs to control according to a proportional integrating method, the fundamental concept of the above embodiment is made effective in the other controlling methods as well even when the responding capability of the electric motor is changed depending on the vehicle velocity or when the rear wheels are turned in the reverse or same phase to the front wheels. In the above embodiment, since the current instructing value fed to the motor driver is made smaller at a low speed region not requiring a quick response of the rear wheels, the responding speed of the electric motor is decreased to suppress the power consumption. Moreover, since the responding capability of the rear wheels is changed between when the rear wheels are turned in the reverse phase to the front wheels in proportion to the steering wheel angle and when the rear wheels are turned in the same phase as the front wheels in proportion to the yaw rate, the feedback loop for the rear wheel steering system to feed back the yaw rate is retained stable, whereby the advancing direction or course of the rear wheels can be corrected stably.

Claims: A controlling apparatus for controlling the steering angle of the rear wheels (1) of a four-wheel steering vehicle, comprising: a vehicle speed sensor (7) for detecting the velocity of the vehicle; a yaw rate sensor (9) for detecting the yaw rate; a steering wheel angle sensor (8) for detecting the turning angle of the steering wheel; a controller (2) for outputting a rear wheel steering angle instructing signal in response to the signals of said sensors (7, 8, 9); and an electric motor (4) for steering the rear wheels (1) based on said rear wheel steering angle instructing signal, said electric motor (4) being provided with a position detector (10); said controller (2) being provided with a first interface circuit (14) for obtaining the current position of said electric motor (4) from said position detector (10), an operating unit (16) for calculating a desired steering angle of said rear wheels (1) from the output value of said sensors (7, 8, 9) and for calculating the deviation between the desired position and current position of said electric motor (4), and a second interface circuit (15) for feeding a current instruction to a motor driver (3) from said operating unit (16), characterized in that in said operating unit (16) a value obtained through differential operation of the current position of the electric motor (4) is subtracted from a value obtained by multiplying the sum of a value obtained through proportional operation of the deviation and a value obtained through integral operation of the deviation with a value proportional to the vehicle velocity, so that the resultant value is output as a current instructing value (6) to said motor driver (3) via said second interface circuit (15). A controlling apparatus as set forth in claim 1, wherein a deviation between the desired position and current position of said electric motor (4) is processed in the operating unit (16), and a value obtained by differentiating the current position of said electric motor (4) is subtracted from a value obtained by multiplying an adding value of a proportionating value of the deviation and an integrating value of the deviation with a proportionating constant which is different for the reverse phase occasion from for the same phase occasion, and is output as a current instructing value (6) to said electric motor (4) via said second interface circuit (15).

Inventors: FUKUNAGA TAKASHI INOUE MANSHON, SEGAWA AKIYOSHI MEZON MIZUURA, FUKUNAGA, TAKASHI, INOUE MANSHON 4FA, SEGAWA, AKIYOSHI, MEZON MIZUURA TAISHIBASHI 301
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1991
Main classification: B62D7
IPC codes: B62D7