Reactive Steering Control System

1. Field of the Invention

The present invention relates to a steering control system for a vehicle equipped with an electric power steering device, and in particular to such a steering control system for a vehicle which is capable of producing a reactive steering torque in response to a detected behavior of the vehicle.

2. Description of the Related Art

Power steering systems for reducing the steering effort required to steer the vehicle are known, and one example of such systems is disclosed in Japanese patent publication (kokoku) No. 50-33584. According to this power steering system, the steering torque applied from a steering wheel is assisted by an output torque of an electric motor whose magnitude primarily depends on the magnitude of the applied manual steering torque. By varying the gain for the detected steering torque applied to the steering wheel by the vehicle operator according to the vehicle speed and the road condition, the output torque of the assisting electric motor is adjusted so that an optimum steering assist torque may be obtained at all times.

When the vehicle encounters a strong crosswind or side wind, or runs into ruts or other irregular road surface features, the vehicle tends to deviate from the intended straight path of travel. When the vehicle travels over a low frictional coefficient road surface, such as a frozen road surface, or the vehicle is traveling at a low speed, the effort required to steer the vehicle significantly diminishes.

According to the conventional power steering system, however, the assisting torque is produced only when the vehicle operator has applied some steering torque to the steering wheel. Therefore, when the vehicle deviates from the intended straight path of travel due to crosswind, the electric motor will not produce any assisting torque by itself. Therefore, to prevent the deviation of the vehicle from the intended path, the vehicle operator must constantly apply a manual steering torque to the steering wheel. According to the conventional steering system, the assisting torque is generally small when the lateral acceleration and the yaw rate of the vehicle are large. Therefore, when the vehicle deviates from an intended path of travel, the steering input necessary to restore the vehicle to the intended path of travel becomes progressively greater as the lateral acceleration and the yaw rate of the vehicle increase.

It was previously proposed to compute a reference behavior of the vehicle in terms of a lateral acceleration and/or a yaw rate for a given steering angle and vehicle speed, detect the actual behavior of the vehicle, and apply a reactive steering torque so as to minimize the deviation of the detected vehicle behavior from the reference vehicle behavior. For details of such control arrangements, reference should be made to U.S. Pat. Nos. 5,528,497, 5,703,775, and 5,774,819 which are assigned to the common assignee, and the contents of these patents are hereby incorporated in this application by reference.

According to such a proposal, it is possible to control the vehicle in a stable fashion even when subjected to various external interferences. Also, the handling of the vehicle on a slippery road surface can be improved by preventing excessive steering. However, such control processes are based on the assumption that the vehicle is traveling forward, and would not work properly if the vehicle is traveling rearward. The vehicle speed sensor is typically unable to distinguish the traveling direction of the vehicle. As can be readily appreciated, the relationship between the steering angle and the resulting yaw rate when the vehicle is traveling rearward is reversed from that when the vehicle is traveling forward. When the vehicle traveling forward is steered to the right, the vehicle turns in clockwise direction as seen from above. However, when the vehicle traveling rearward is steered to the right, the vehicle turns in counter-clockwise direction, as seen from above. Therefore, when a reactive steering assist torque is produced when the vehicle is traveling rearward, it will act in the opposite direction and tends to produce an undesired result. For instance, when the vehicle is traveling on a low-frictional coefficient road surface, the reactive steering assist torque should increase the effort required to steer the vehicle so as to prevent any excessive steering. But, if the vehicle is traveling rearward, the reactive steering assist torque will reduce the effort required to steer the vehicle, and this promotes excessive steering to the discomfort of the vehicle operator.

In view of such problems of the prior art, a primary object of the present invention is to provide a steering system which produces a reactive steering torque in response to a vehicle behavior, and can operate properly without regard to the direction of the movement of the vehicle.

A second object of the present invention is to provide a steering system which would not cause any discomfort to the vehicle operator by ensuring a proper operation under all conditions.

According to the present invention, these and other objects can be accomplished by providing a vehicle steering system, comprising: a steering shaft having a steering wheel at an end thereof; a steering device attached to the other end of the steering shaft, and powered by an electric motor; a manual steering torque sensor for detecting a manual steering torque applied from the steering wheel; a normal steering assist torque determining unit for determining a normal steering assist torque according to a manual steering torque detected by the manual steering torque sensor; a vehicle behavior sensor for detecting a yaw rate and/or a lateral acceleration of the vehicle; a reactive steering assist torque determining unit for determining a reactive steering assist torque according to a yaw rate and/or a lateral acceleration detected by the vehicle behavior sensor; and a control unit for controlling the electric motor according to outputs from the normal steering assist torque determining unit and the reactive steering assist torque determining unit; wherein the vehicle behavior sensor further comprises a reverse sensor for detecting a traveling direction of the vehicle, the control unit at least preventing a normal output from the reactive steering assist torque determining unit when the traveling direction detected by the reverse sensor indicates a rearward travel.

Thus, the reactive steering assist torque which could impair the behavior of the vehicle is at least prevented or eliminated when the vehicle is detected to be traveling rearward. Preferably, the sense of the reactive steering assist torque is reversed in sense depending on the traveling direction of the vehicle so that the benefits of the reactive steering assist torque control may be made available even when the vehicle is traveling rearward. For favorable control results, the magnitude of the reactive steering assist torque may be either reduced or increased when the vehicle is traveling rearward as compared to that produced when the vehicle is traveling forward at the same speed.

The control unit typically controls the electric motor according to a sum of outputs from the normal steering assist torque determining unit and the reactive steering assist torque determining unit. If desired, the control unit may additionally control the normal steering assist torque determining unit depending on the traveling direction of the vehicle, and it can be readily implemented from the analogy derived from the description on the main concept of the present invention which is set forth hereinafter. Also, the reactive steering assist torque may comprise a first component depending on a steering angular speed, and a second component which depends of the yaw rate and/or lateral acceleration of the vehicle. In this case, the control unit preferably changes only the second component of the reactive steering assist torque depending on the traveling direction of the vehicle detected by the reverse sensor because the first component is not particularly sensitive to the traveling direction of the vehicle. The reverse sensor may be implemented in a number of ways, for instance by detecting the gear position or the shift position of the manual or automatic transmission system. If the vehicle is equipped with a wheel speed sensor which is capable of detecting a rotational direction of the wheel, the need for any special reverse sensor will be eliminated.

Now the present invention is described in the following with reference to the appended drawings, in which:

FIG. 1 is a schematic view of the overall structure of the vehicle steering system embodying the present invention;

FIG. 2 is a circuit block diagram showing the structure of the control unit of the steering system;

FIG. 3 is a flow chart showing a first part of the control process of the steering system;

FIG. 4 is a flow chart showing a second part of the control process of the steering system;

FIG. 5 is a flow chart showing a third part of the control process of the steering system;

FIG. 6 is a block diagram of the control unit including data tables which are used in the control process for the steering system;

FIGS. 7a and 7b are graphs showing the data tables in enlarged scale;

FIG. 8 is a schematic view of a vehicle which has encountered cross wind; and

FIGS. 9a to 9d are diagrams for illustrating the steering torques which are applied to the steering wheel and vary depending on the control mode.

FIG. 1 schematically illustrates the overall structure of a vehicle steering system embodying the present invention. A steering wheel 1 is integrally attached to an upper end of a steering shaft 2, and a pinion 4 is connected to a lower end of the steering shaft 2 via a connecting shaft 3 incorporated with universal joints. The pinion 4 meshes with a rack shaft 8 which extends laterally across the width of the vehicle body, and connected to knuckle arms of right and left front wheels 6 via tie rods 5 at either end thereof. The structure described so far constitutes a known rack and pinion steering mechanism. An electric motor 9 having a hollow output shaft is coaxially disposed in an intermediate part of the rack shaft 8 so as to apply an assist force to the rack shaft 8 and reduce the effort required to steer the vehicle.

The system further comprises a steering angular speed sensor 11 for detecting the angular speed of the steering wheel 1 at the rack and pinion mechanism, a steering torque sensor 12 for detecting the manual steering torque acting upon the pinion 4, a yaw rate sensor 15 for producing a signal corresponding to the yaw rate (yawing angular speed) of the vehicle, a vehicle speed sensor 16 for producing a signal corresponding to a traveling speed of the vehicle, and a steering control unit 17 for controlling the output of the electric motor 9 according to the outputs of these sensors.

Referring to FIG. 2, the steering control unit 17 comprises a normal steering assist torque determining unit 17a for computing a normal steering assist torque, and a reactive steering assist torque determining unit 17b for computing a reactive steering assist torque which would be desired for controlling the motion of the vehicle, in particular the lateral and yaw motion of the vehicle. The normal steering assist torque determining unit 17a receives output signals from the steering angular speed sensor 11, the steering torque sensor 12, and the vehicle speed sensor 16 as data for conducting normal steering assist torque control. The reactive steering assist torque control unit 17b receives output signals from the steering angular speed sensor 11, the yaw rate sensor 15, and the vehicle speed sensor 16, and compare these values with reference values so as to determine if and how much the behavior of the vehicle deviates from a reference behavior. When any significant deviation is detected, the reactive steering assist torque determining unit 17b supplies a control signal to the electric motor 9 so as to counteract such a deviation by feedback control. The reactive steering assist torque control unit 17b additionally receives an output signal from a reverse detecting unit 18 which indicates if the vehicle is traveling forward or rearward according to the position of the transmission gears, the position of the shift lever, or other sensors for detecting the traveling direction of the vehicle. The operation of the reactive steering assist torque control unit 17b is modified depending on the output signal from the reverse detecting unit 18, and an appropriate control algorithm is executed in each case as described hereinafter.

The steering control unit 17 also comprises a target current determining unit 17c for determining a target current for the electric motor 9 according to the torque signals supplied from the normal steering assist torque determining unit 17a and the reactive steering assist torque determining unit 17b, and an output current control unit 17d for controlling the electric current actually supplied to the electric motor 9 according to the thus obtained target current. The current control signal from the output current control unit 17d is supplied to a drive circuit 19, and then to the electric motor 9, and a feedback control path is formed between the drive circuit 19 and the output current control unit 17d.

In the reactive steering assist torque determining unit 17b of the steering control unit 17, the process shown in the flow chart of FIG. 3 is repeated. First of all, the signals from the various sensors are read in step 1. A reactive steering assist torque TA is computed in step 2. A target reactive steering assist torque TA is produced in step 3. This is more fully described in the following with reference to the flow charts of FIGS. 4 to 7.

Referring to FIG. 4, step 1 is executed in the following manner. The vehicle speed V (step 11), the steering wheel angular speed .omega. (step 12), and the yaw rate .gamma. (step 13) are read out.

Referring to FIG. 5, step 2 is executed in the following manner. The tables shown in FIGS. 7a and 7b are used for obtaining two components of the reactive steering assist torque, or a damping torque component T1 and a yaw rate torque component T2, in steps 21 and 22. The table shown in FIG. 7a gives the damping torque component T1 for each value of the detected steering angular speed .omega.. This relationship depends on the vehicle speed V. In this embodiment, a plurality of tables are prepared for a number of different speed ranges, and one of such tables is selected for each given vehicle speed. The table shown in FIG. 7b gives the yaw rate torque component T2 for each value of the detected yaw rate .gamma.. Again, a plurality of tables are prepared for a number of different speed ranges, and one of such tables is selected for each given vehicle speed. In either case, as the absolute value of the vehicle speed V increases, each of the tables gives a larger reactive steering assist torque component for a given steering angular speed .omega. or yaw rate .gamma.. The two components T1 and T2 are added in step 23.

In particular, the two tables are each formed so as to give a value of zero when the vehicle speed V is zero. Therefore, when the reverse detecting unit 18 indicates that the vehicle is stationary, no reactive steering assist torque is produced even when the vehicle is moved by outside causes such as when the vehicle is being turned on a turntable or being transported on a car lift or a car carrier. When the reverse detecting unit 18 indicates that the vehicle is traveling rearward, a different set of tables are used for relating the yaw rate with the reactive steering assist torque component T2. When the reverse detecting unit 18 indicates that the vehicle is traveling forward (V.gtoreq.0), the curves indicated by solid lines in FIG. 7b are used. When the reverse detecting unit 18 indicates that the vehicle is traveling rearward (V<0), the curves indicated by dotted lines in FIG. 7b are used. Here, the yaw rate .gamma. is positive when the vehicle is turning clockwise as seen from above, and negative when the vehicle is turning counter-clockwise as seen from above. Similarly, the yaw rate torque component T2 is positive when the torque acts in counter-clockwise direction as seen from above, and negative when the torque acts in clockwise direction as seen from above.

Thereafter, to avoid any excessive reactive steering assist torque from being produced, it is determined if the target reactive steering assist torque TA has exceeded a maximum value (Tmax) or not. If the target reactive steering assist torque TA exceeds the prescribed maximum value (Tmax), the target reactive steering assist torque TA is set at Tmax. If the target reactive steering assist torque TA is less than the prescribed maximum value (Tmax), it is similarly determined if the target reactive steering assist torque TA falls below a prescribed minimum value (-Tmax) or not, and if the target reactive steering assist torque TA is less (larger in the absolute value) than the prescribed minimum value, the target steering reaction TA is set at -Tmax (step 24).

The control arrangement for step 2 can be represented by the block diagram illustrated in FIG. 6, and the blocks in this block diagram correspond to steps 21 to 24

The thus determined target reactive steering assist torque TA is added to the target normal steering assist torque computed separately, and is converted into a target current value by the output current determining unit 17c which is then supplied to the drive circuit 19.

In this manner, as illustrated in FIG. 8, when the vehicle 20 is subjected to crosswind when travelling a straight ahead course, and has started to deviate from a driving lane 21, the yaw rate .gamma. of the vehicle is detected, and the electric motor is driven in the direction to cancel the yaw rate .gamma. of the vehicle or to restore the vehicle back to the driving lane 21. Likewise, when the vehicle 20 deviates from a straight path because of other external interferences such as irregular road surfaces, ruts, water pools and so on, the electric motor 9 produces a reactive steering assist torque which tends to cancel such a deviation.

Referring to FIGS. 9a to 9d, the torques acting on the steering wheel when the vehicle is traveling rearward are described in the following. FIG. 9a corresponds to a case when the direction of the vehicle travel is disregarded. When the steering wheel is turned right while the vehicle is traveling rearward, the steering wheel receives a normal steering assist torque Ta which opposes a steering reaction from the road surface, and the reactive steering assist torque component (yaw rate component) T2 which also tends to increase the steering angle. Therefore, if the solid line curves in the graph of FIG. 7b for looking up the yaw rate component T2 are used or the sign of the vehicle speed is disregarded, the yaw rate component tends to reduce the effort required to steer the vehicle to the discomfort of the vehicle operator on a slippery road surface, and could otherwise worsen the behavior of the vehicle.

When the sign of the vehicle speed is taken into account, and the broken line curves in the graph of FIG. 7b are used for looking up the yaw rate component T2, the yaw rate component T2 typically opposes the increase in the steering angle, and prevents the excessive reduction in the effort required to steer the vehicle. If the dotted line curves of the graph of FIG. 7b are defined such that the magnitude of the yaw rate component T2 be larger for the given absolute value of the vehicle speed and the given yaw rate than that when the vehicle is traveling forward, it is possible to increase the torque which tends to restore the steering wheel as shown in FIG. 9c, and thereby increase the stability of the vehicle when it is traveling rearward.

As a yet another possible mode of control, it is possible to set the yaw rate component T2 to zero when the vehicle is traveling rearward without regard to the absolute value of the vehicle speed. In this case, as shown in FIG. 9d, there is no yaw rate component T2, and the vehicle will be only as stable as a vehicle not equipped with means for producing a reactive steering assist torque component.

In the above described embodiment, the direction of the vehicle travel was detected from the gear position or the like, but if the speed sensor is capable of indicating the direction of movement as is the case with two-phase vehicle speed sensors, it is possible to indicated the direction of the vehicle travel from the output signal of the vehicle speed sensor.

Thus, according to the present invention, the steering system produces a reactive steering assist torque which tends to cancel any unintended or undesired behavior of the vehicle without regard to the direction of the vehicle travel so that it is possible to execute the vehicle steering control in a stable fashion at all times.

Although the present invention has been described in terms of specific embodiments thereof, it is possible to modify and alter details thereof without departing from the spirit of the present invention.