Vehicle steering system

The present invention relates to vehicle steering apparatus and process for steering a vehicle with an actuator in response to a driver's steering input.

In a steer-by-wire steering system, a steering gear mechanism for varying a steer angle of steerable wheels of a vehicle is separated from a steering wheel operated by a driver. An angle sensor senses a steering wheel angle with an encoder, and a controller varies the wheel steer angle with an actuator in accordance with the sensed steering wheel angle.

A steering system of such a type of earlier technology is composed of two or more subsystems each including a steering shaft motor for actuating a steering gear mechanism such as a rack and pinion steering mechanism, and a steering controller for controlling the steering shaft motor in accordance with an angular displacement of a steering shaft sensed by an angle sensor, and a desired target steering angle.

In such a steering system, however, each subsystem senses a control quantity, and the control quantity for the steering shaft motor might differ from subsystem to subsystem because of mechanical displacement errors, and electric sensing errors such as sensor errors among the angle sensors. Therefore, the subsystems might drive the respective steering shaft motors inconsistently with different control quantities, and the steering shaft motors might interfere with one another, and might produce torques in opposite directions, so that the drive currents for the steering shaft motors might be increased continuously.

It is, therefore, an object of the present invention to provide vehicle steering apparatus and process for driving a plurality of steering shaft motors with little or no interference.

According to one aspect of the present invention, a vehicle steering system comprises: a steering mechanism including an input section, and a steering section which is mechanically separated from the input section, and arranged to steer a vehicle in accordance with a steering input quantity of the input section; an actuating section to impart an actual torque to the steering mechanism, the actuating section including a plurality of drive units to produce the actual torque; and a controlling section to control the actuating section to achieve a target torque with the drive units, the controlling section including a plurality of control units to control the drive units, respectively, in accordance with a common torque share calculated by one of the control units.

According to another aspect of the invention, a vehicle steering control process for controlling each of steering torques produced with a plurality of drive units, to achieve a target torque in response to a steering input quantity, comprises: a first process element of calculating a common torque share when an assigned role is to serve as a representative unit; a second process element of receiving the common torque share when the assigned role is to serve as a non-representative unit; and a third process element of controlling the steering torque in accordance with the common torque share.

According to still another aspect of the invention, vehicle steering control apparatus for controlling an actual torque for steering a vehicle with a plurality of subsystems to achieve a target torque in response to a driver's steering input quantity, the vehicle steering control apparatus comprises: means for selecting one of the subsystems as a representative unit, and leaving the remainder as a non-representative unit; means for causing the representative unit to calculate a common torque share in accordance with the target torque; means for causing the non-representative unit to receive the common torque share from the representative unit; and means for producing the actual torque in response to the driver's steering input for steering the vehicle, by causing each unit to produce a torque in accordance with the common torque share.

FIG. 1 is a schematic view showing a steering system according to a first embodiment of the present invention.

FIG. 2 is a schematic view showing a steering actuating system of the steering system according to the first embodiment.

FIG. 3 is a flow chart showing a steering shaft angle control process performed by a steering shaft angle controller of each subsystem shown in FIG. 2 .

FIG. 4 is a schematic view of the steering actuating system of FIG. 2 for illustrating the flow of signals in the normal state.

FIG. 5 is a schematic view of the steering actuating system of FIG. 2 for illustrating the flow of signals in the state in which a steering shaft angle controller 20 a is not functioning properly.

FIG. 6 is a schematic view of the steering actuating system of FIG. 2 for illustrating the flow of signals in the state in which a steering shaft motor 11 b is not functioning properly.

FIG. 7 is a schematic view showing a steering actuating system of a steering system according to a second embodiment in which the present invention is applied to a reaction mechanism 50 shown in FIG. 1 .

FIG. 1 schematically shows a vehicle equipped with a steering system according to a first embodiment of the present invention.

A steering mechanism of the vehicle is composed of two separate sections which are mechanically separated from each other. The first section is an input section including a steering wheel 1 , and the second section is a steering (gear) section including a steering gear mechanism 5 (shown in FIG. 2 ). The input section further includes a reaction unit 50 for providing a steering reaction force to steering wheel 1 .

A steering actuating unit or system 10 according to the first embodiment is provided for the steering section ( 5 ) of the steering mechanism. Steering actuating system 10 is mechanically separated from steering wheel 1 . Actuating system 10 of this example is composed of a plurality of subsystems SUBa, SUBb and SUBc which are three in number in this example. Each subsystem includes a drive unit including a steering shaft motor 11 a , 11 b or 11 c , and a control unit for controlling the drive unit. The drive units of the subsystems serve as an actuating section ( 11 a ˜ 11 c ) for imparting an actual torque to steering gear section 5 of the steering mechanism.

As reaction unit 50 , it is possible to employ the construction of a known mechanism for providing a steering reaction. In this example, an upper controller 60 calculates a target reaction torque corresponding to a steering torque in accordance with operating conditions sensed by sensors 90 ; and the reaction unit 50 provides a steering reaction toque corresponding to the calculated target reaction torque, to a steering column shaft 3 of steering wheel 1 , by driving a reaction producing motor. In this example, the target reaction torque is calculated in accordance with a displacement quantity of a steer shaft in actuating unit 10 and a vehicle speed sensed by a vehicle speed sensor included in the sensors 90 . Accordingly, the driver can feel the reaction force corresponding to the steering quantity.

Steering gear mechanism 5 of this example is a rack and pinion steering mechanism as shown in FIG. 2 . Steering actuating unit or system 10 of this example is composed of three subsystems SUBa, SUBb and SUBc each for driving the corresponding one of steering shaft motors 11 a , 11 b and 11 c to drive the pinion of rack and pinion steering mechanism 5 . Subsystems SUBa, SUBb and SUBc are substantially identical to one another in construction, and connected with one another and with upper controller 60 by a communication bus 40 serving as communication line, so that information can be exchanged among the subsystems and upper controller 60 . Each of the subsystems serves as a control unit, and the subsystems and upper controller 60 serves as a controlling section to control the actuating section to achieve a desired target torque with the drive units ( 11 a ˜ 11 c ) of the actuating section.

Subsystem SUBa includes steering shaft motor 11 a for driving steering gear mechanism 5 , a motor driver 12 a for driving the corresponding steering shaft motor 11 a , an angle sensor 13 a for sensing a steering angle and a steering angular speed of a steering shaft member for steering mechanism 5 , a current sensor 14 a for sensing a current supplied to steering shaft motor 11 a , and a steering shaft angle controller 20 a for controlling motor driver 12 a . In accordance with the sensor signal from angle sensor 13 a , a target steering angle command from upper controller 60 , a control angle command from another subsystem, and the sensor signal of current sensor 14 a , the angle controller 20 a calculates a current command for steering shaft motor 11 a , and delivers current corresponding to the current command, to motor driver 12 a . Angle controller 20 a includes an angle control operational section 21 a and a current control operational section 22 a.

Steering motor 11 a is connected with steering gear mechanism 5 through a clutch mechanism capable of disconnecting the drive path between steering motor 11 a and steering gear mechanism 5 . When a malfunction is detected in steering motor 11 a , angle controller 20 a or other component in subsystem SUBa, this control system stops the operation of driving steering motor 11 a , and disconnects steering motor 11 a of subsystem SUBa, from steering gear mechanism 5 by disengaging the clutch mechanism.

Angle control operational section 21 a of angle controller 20 a performs operations by using the steering angle and angular speed sensed by angle sensor 13 a , the target steering angle command of upper controller 60 , the operating states of the other subsystems as to whether the subsystems are in normal operating conditions, and the predetermined order of priority of subsystems SUBa, SUBb and SUBc. If the priority level of its own subsystem SUBa is the highest among all the subsystems, the angle control operational section 21 a calculates a control quantity for the steer angle in accordance with the input information items, and supplies the result of the calculation as the control steering angle command, through communication bus 40 , to the other subsystems.

Current control operational section 22 a performs operations to control the current in accordance with the control steering angle calculated by angle control operational section 21 a or the control steering angle obtained from one of the other subsystems through communication bus 40 , and the sensor signal from current sensor 14 a.

The order of priority is determined, in this example, so that the highest priority is given to the subsystem whose steering angle sensed by its angle sensor is closest to the average among the sensed steering angles of the three subsystems SUBa˜SUBc.

Though the above explanation is directed to the first subsystem SUBa, the second and third subsystems SUBb and SUBc are substantially identical in construction and operation, to the first subsystem SUBa.

FIG. 3 shows a steering shaft angle control process performed by steering shaft angle controller 20 a according to this embodiment of the invention. The process is the same among the three subsystems SUBa˜SUBc. The following explanation is directed only to subsystem SUBa. Angle controller 20 a is arranged to perform the process at regular intervals of a predetermined cycle time.

At step S 2 , controller 20 a performs a self diagnostic operation. In this example, when an ignition key of the vehicle is inserted, the control system calculates a current command to vary the steering angle by a unit angle, in accordance with a predetermined self-diagnostic steering angle command (set equal to a unit angle, for example), and the steering angle from angle sensor 13 a , and drives the motor 11 a by delivering current corresponding to the calculated current command, to motor driver 12 a . Then, controller 20 a examines the sensor signal of angle sensor 13 a to determine whether the steering operation is performed in conformity with the command. Then, after a start of the steering shaft angle control process, this control system performs a self-diagnostic operation by monitoring a rate of change of the steering shaft angle sensed by angle sensor 13 a , or a change of the steering shaft angle during one cycle, each time the this steering shaft angle control process is performed at regular time intervals of the cycle time.

Then, at step S 4 , controller 20 a checks the result of the self-diagnostic operation and thereby determines whether the subsystem SUBa is normal, or not. When subsystem SUBa is judged to be normal, controller 20 a proceeds to step S 6 , and reads the target steering angle command from upper controller 60 through communication bus 40 . Then, at a next step S 8 , controller 20 a reads information on the operating states of the other subsystems SUBb and SUBc, and examines whether the priority of its own subsystem SUBa is the highest or not, by examining the information on the operating states of the other subsystems, and the preset order of priority. When the preset order of priority is SUBa, SUBb, SUBc, then controller 20 a judges that subsystem SUBa is the highest in the priority. When the preset order of priority is SUBc, SUBa, SUBb, then controller 20 a normally judges that subsystem SUBa is not highest in priority. If, however, subsystem SUBc is considered to be in the abnormal state, from the information on the operating states of the other subsystems, then controller 20 a judges that subsystem SUBa is the highest in priority.

When the judgment is that the priority of subsystem SUBa is the highest, then controller 20 a proceeds from step S 10 to step S 12 . At step S 12 , controller 20 a reads the sensor signal of angle sensor 13 a and transmits the obtained data to the other subsystems SUBb and SUBc.

At step S 14 following step S 12 , controller 20 a calculates a current command I* by performing an angle control calculation. In this example, a steering torque to be provided to the steering mechanism 5 to bring the actual steering angle toward the angle specified as the target steering angle command is calculated in accordance with internal variables successively updated in the current command calculating operation. In this example, the internal variables includes the target steering angle command obtained at step S 6 , the steering angle and steering angular speed obtained at step 512 , and the previous value of the steering current command I*. From the thus-calculated steering torque, controller 20 a calculates the steering current command I* to achieve this steering torque. The angle control calculation is performed at steps S 12 and S 14 .

Then, at step S 16 , controller 20 a counts the subsystems in the normal state in accordance with the operating states of the subsystems ascertained at step S 8 , and determines the number (N) of the subsystems functioning properly in the normal state. At a next step S 18 , controller 20 a determines a divided steering current command Is* for each normal subsystem by dividing the steering current command I* determined at step S 14 , by the number N of the normal subsystems determined at step S 16 . Then, at step S 20 , controller 20 a transmits the divided steering current command Is*, to the other normal subsystems SUBb and SUBc through communication bus 40 . At step S 22 , controller 20 a sets the divided steering current command Is* as the control current command.

When the judgment of step S 10 is that the priority of subsystem SUBa is not highest, then controller 20 a proceeds to step S 24 , and reads the angle data from the other subsystem SUBb or SUBc of the highest priority level.

At step S 26 , controller 20 a calculates the current command I* by performing the angle control calculation like step S 14 . In this example, the steering torque to be provided to the steering mechanism 5 to bring the steering angle toward the angle specified as the target steering angle command is calculated in accordance with the steering angle data obtained from the highest priority subsystem at step S 24 , and the target steering angle command obtained from upper controller 60 at step S 6 . From the thus-calculated steering torque, controller 20 a calculates the steering current command I* to achieve this steering torque. Controller 20 a stores, in a predetermined memory region, the steering current command I* thus calculated in its own subsystem SUBa.

At step S 28 , controller 20 a reads, through communication bus 40 , the divided steering current command Is* calculated by the other subsystem of the highest priority, and sets this divided steering current command Is* as the control current command.

After the control current command is determined in this way at step S 22 or S 28 , controller 20 a proceeds to step S 30 , and reads the supply current to steering shaft motor 11 a , sensed by current sensor 14 a . At step S 32 next to S 30 , controller 20 a performs a current control calculation to bring the motor supply current obtained at step S 30 , closer to the control current command, and delivers the drive signal corresponding to the control current command, to motor driver 12 a at step S 34 . Then, controller 20 a terminates this process. The current control calculating operation is constituted by the operations of steps S 30 and S 32 .

In response to the drive signal from controller 20 a , motor driver 12 a drives steering shaft motor 11 a , and hence steering shaft motor 11 a imparts the steering torque to steering gear mechanism 5 .

If subsystem SUBa is not in the normal state, controller 20 a proceeds from step S 4 to step S 40 , and notifies the other subsystems SUBb and SUbc and upper controller 60 through communication bus 40 that subsystem SUBa is not in the normal state. Then, controller 20 a terminates the control process of FIG. 3 .

The control system is operated as follows: Upper controller 60 receives a sensor signal from a steering wheel angle sensor (included in sensors 90 ) for sensing a steering (input) quantity of steering wheel 1 operated by the driver, and calculates the target steering angle command by a known control process in accordance with the sensed steering input quantity. The calculated target steering angle command is delivered to steering actuating unit 10 . Moreover, upper controller 60 calculates a target reaction torque corresponding to the steering torque by a known process in accordance with the steering shaft displacement in steering unit 10 , and the vehicle speed, and drives the reacting producing motor in reaction unit 50 in accordance with the target reaction torque. In response to this, reaction unit 50 imparts a reaction torque corresponding to the steering torque, to the column shaft 3 of steering wheel 1 . The driver can feel the reaction torque corresponding to the steering quantity.

Each of subsystems SUBa, SUBb and SUBc of steering actuating unit 10 performs the steering angle control process. When all the subsystems are functioning properly; each subsystem proceeds from step S 2 , through step S 4 , to step S 6 ; reads the target steering angle command from upper controller 60 at step S 6 ; obtains the information on the operating states of the other subsystems; and recognizes the priority level of its own subsystem at step S 8 by checking the operating states of the other subsystems.

When, for example, the order of priority dictated from upper controller 60 is SUBa, SUBb, SUBc, and all the subsystems are normal, then the subsystem SUBa recognizes its priority to be the highest, the subsystem SUBb recognizes its priority to be the second highest, and the subsystem SUBc recognizes its priority to be the lowest.

In subsystem SUBa of the highest priority level, controller 20 a proceeds from step S 10 to step S 12 , reads the angle data from angle sensor 13 a , and sends the angle data to the other subsystems through communication bus 40 . Subsystem SUBa further proceeds from step S 12 to step S 14 , calculates the steering torque for steering gear mechanism 5 to control the actual steering angle to the target steering command, in accordance with the target steering command supplied from upper controller 60 and the angle data obtained at step S 12 , and calculates the steering current command I* to achieve this steering torque. The steering current command I* calculated by subsystem SUBa is expressed as Ia* with a subscript denoting the subsystem by which the steering current command is calculated. The steering current command I* serves as a control command quantity.

All the subsystems are normal in this example, so that the number N of the normal subsystems is equal to “3” (at S 16 ). Therefore, the divided steering current command Is* per subsystem (corresponding to a common control share) is determined by dividing steering current command Ia* by “3” (at S 18 ), and the divided steering current command Is* is sent to the other subsystems SUBb and SUBc through communication bus 40 (at S 20 ). The divided steering current command Is* calculated by subsystem SUBa is expressed as Isa* with a subscript denoting the subsystem by which the divided steering current command is calculated.

Subsystem SUBa uses the thus-calculated divided steering current command Isa* as control current command (at S 22 ), and delivers, to motor driver 12 a , the drive signal determined by the current control calculation to control the supply current for steering shaft motor 11 a , sensed by current sensor 14 a , to the control current command set equal to divided steering current command Isa* (at S 30 ˜S 34 ).

On the other hand, subsystem SUBb proceeds from step S 10 to step S 24 since the priority of subsystem SUBb is not the highest. Accordingly, subsystem SUBb obtains the angle data from angle sensor 13 a from subsystem SUBa; calculates the steering current command Ib* in accordance with the thus-obtained angle data; and stores the steering current command Ib* in the predetermined memory region (at S 26 ).

Then, at step S 28 , subsystem SUBb reads the divided steering current command Isa* determined in subsystem SUBa, and sets this command Isa* as control current command of subsystem SUBb. Then, subsystem SUBb reads the current sensed by current sensor 14 b , performs the current control calculation to control the current supplied to steering shaft motor 11 b , sensed by current sensor 14 b , to the control current command set equal to the divided steering current command Isa* calculated by subsystem SUBa, and delivers the drive signal determined by the current control calculation, to motor driver 12 b (at S 30 ˜S 34 ).

Similarly, subsystem SUBc obtains the angle data from angle sensor 13 a from subsystem SUBa of the highest priority; calculates the steering current command Ic* in accordance with the thus-obtained angle data; and stores the steering current command Ic* in the predetermined memory region (at S 24 and S 26 ). Furthermore, subsystem SUBc reads the divided steering current command Isa* determined in subsystem SUBa, and sets this command Isa* as control current command. Then, subsystem SUBc reads the current sensed by current sensor 14 c , performs the current control calculation to control the current supplied to steering shaft motor 11 c , sensed by current sensor 14 c , to the control current command set equal to the divided steering current command Isa* calculated by subsystem SUBa, and delivers the drive signal determined by the current control calculation, to motor driver 12 c (at S 30 ˜S 34 ).

FIG. 4 illustrates the flow of signals among subsystems SUBa, SUBb and SUBc. Subsystem SUBa calculates the divided current command Isa*, and subsystems SUBb and SUBc receive the divided current command Isa* from subsystem SUBa through communication bus 40 . The steering shaft motors 11 a ˜ 11 c are controlled in accordance with the divided current command Isa* and the sensor signals of angle sensors 13 a ˜ 13 c.

Therefore, motor drivers 12 a ˜ 12 c of subsystems SUBa˜SUBc respectively drive the steering shaft motors 11 a ˜ 11 c so that the current supplied to each motor is brought closer to the divided current command Isa*. Therefore, all the motors 11 a ˜ 11 c are so driven as to produce torques of the same magnitude in the same rotational direction.

If a malfunction occurs in the highest priority subsystem SUBa, as shown in FIG. 5 , the subsystem SUBa realizes its own abnormality by the self diagnosis, proceeds from step S 4 to step S 40 , and notifies the other subsystems SUBb and SUBc of the abnormal state of subsystem SUBa, through communication bus 40 . Thereafter, the abnormal subsystem SUBa repeats the operations of steps S 2 , S 4 and S 40 , and holds the steering shaft motor 11 a in an inoperative state without driving the motor 11 a until a judgment of the self diagnosis that subsystem SUBa is normal.

On the other hand, the subsystem SUBb remaining normal proceeds from step S 2 , through step S 4 to step S 6 , reads the target steering angle command from upper controller 60 , and ascertains the priority level of subsystem SUBb at S 8 . In this case, subsystem SUBb recognizes that the priority of subsystem SUBb is the highest, from the information that subsystem SUBa is not normal, and the order of priority preliminarily determined by upper controller 60 .

Therefore, subsystem-SUBb proceeds from step S 8 , through S 10 , to step S 12 ; sends the sensor signal of angle sensor 12 b to the remaining subsystem SUBc; calculates the steering current command Ib* in accordance with the sensor signal of angle sensor 12 b ; calculates the divided steering current command Isb* by dividing the steering current command Ib* by the number (“2”) of the normal subsystems; sets the thus-calculated divided steering current command Isb* as the control current command; and notifies the remaining normal subsystem SUBc of the divided steering current command Isb*. In accordance with the divided steering current command Isb* and the sensor signal from current sensor 14 b , subsystem SUBb performs the current control calculation, and drives steering shaft motor 11 b.

Subsystem SUBc proceeds from step S 10 to step S 24 ; receives the angle data of angle sensor 12 b from the highest priority subsystem SUBb; calculates the steering current command Ic* in accordance with the sensor signal of angle sensor 12 b ; stores the steering current command Ic*; receives the divided current command Isb* from subsystem SUBb; performs the current control calculation in accordance with the divided steering current command Isb* and the sensor signal from current sensor 14 c ; and drives steering shaft motor 11 c.

Therefore, steering shaft motors 11 b and 11 c are controlled, on the basis of the same divided current command Isb*, so as to produce torques of the same magnitude in the same rotational direction. Since subsystem SUBa is not functioning properly, the control system produces the steering torque only with the two motors 11 b and 11 c , excluding the motor 11 a of the abnormal subsystem SUBa. However, the steering current command Ib* is divided by two, and the result is set as the divided current command Isb*. Therefore, the control system can produce the steering torque corresponding to the steering current command Ib* with the two motors 11 b and 11 c , and control the actual steer angle to the target steering angle command properly.

If, for example, a malfunction occurs in steering shaft motor 11 b of subsystem SUBb, as shown in FIG. 6 , while the other subsystems remain normal, then the subsystem SUBb detects the malfunction of motor 11 b by the self diagnosis of step S 2 , proceeds through step S 4 to step S 40 , and notifies the other subsystems, of the abnormal state of subsystem SUBb.

Subsystem SUBa calculates the steering current command Ia* as in the above-mentioned example, in accordance with the sensor signal of angle sensor 13 a and the target steering angle command. In this case, however, the number of the normal subsystems SUBa and SUBc is two, and the divided steering current command Isa* is determined by dividing the steering current command Ia* by two. Subsystem SUBc is notified of the abnormality of subsystem SUBb, but recognizes that the priority of subsystem SUBa is still the highest. Therefore, subsystem SUBc drives steering shaft motor 11 c in accordance with the divided steering current command Isa* sent from subsystem SUBa.

If subsystem SUBa becomes abnormal in addition to subsystem SUBb, then subsystem SUBc judges that its own priority is the highest since the higher rank subsystems SUBa and SUBb are both abnormal, and calculates the steering current command Ic* in accordance with the sensor signal of angle sensor 13 c and the target steering angle command from upper controller 60 . Since the number of the normal subsystem is equal to one, the subsystem SUBc sets the divided steering current command Isc* equal to Ic*, and drives the steering shaft motor 11 c in accordance with the thus-determined divided current command Isc*. In this case, the steering torque is produced only by motor 11 c.

In this way, one of the subsystems acts as a representative subsystem or representative unit, and calculates the divided steering current command Is* as a variable representing the common torque share. The remaining subsystem or subsystems serve as non-representative subsystem or unit, and control the respective steering shaft motors on the basis of divided steering current command Is* determined by the representative unit. Thus, the subsystems can control the respective motors in the same direction with the same torque without undesired interferences.

Even if one of the subsystems becomes abnormal, the subsystems remaining normal can calculate the divided steering current command adequately, and control the motors of the remaining subsystems adequately by dividing the control command quantity by the number of the normal subsystems. In general, actuating system 10 is installed at a location near the exhaust manifold or exhaust pipe, and steering shaft motors 11 a ˜ 11 c are susceptible to adverse influence such as overheating. According to this embodiment, however, the vehicle steering system can continue producing the steering torque properly even if there is some malfunction in one or more of the motors, and by so doing the system can make more effective use of the redundancy.

The steering current command must be calculated by using values of various internal variables used in the previous calculation of the steering current command. However, each of the non-representative subsystems in this embodiment receives the angle data from the representative subsystem of the highest priority, and determines the steering current command I* in accordance with the angle data supplied from the representative unit. Therefore, each non-representative subsystem can start the calculation of the steering current command and the common torque share, quickly and smoothly if the non-representative subsystem is required to take over the role of the representative subsystem. Each non-representative subsystem is arranged to determine the steering current command I* by using the angle data from the representative subsystem. Therefore, the steering current command I* can be performed directly without changing the internal variables.

In the illustrated example, the number of subsystems (SUBa, SUBb and SUBc) is three. However, the number of subsystems is not limited to three. The number of subsystems may be two or equal to or greater than four. In the illustrated example, each of the angle control calculating sections 21 a ˜ 21 c calculates the steering current command I* representing the desired current, and the common torque share is in the form of the divided steering current command Is*. However, the present invention is not limited to this. Each of angle control calculating sections 21 a ˜ 21 c may be arranged to calculate a torque command representing a desired torque to be supplied to the steering mechanism. In this case, a divided torque command serving as the common torque share is determined by dividing the torque command calculated by the subsystem of the highest priority, by the number of the normal subsystems. Each of current control operating sections 22 a ˜ 22 c controls the corresponding steering shaft motor 11 a , 11 b or 11 c , in accordance with the divided torque command and the current sensed by the current sensor.

FIG. 7 shows a vehicle steering system according to a second embodiment of the present invention. In the second embodiment, a steering actuating unit or system is provided for the reaction unit 50 of the input section including steering wheel 3 . Actuating system 10 of this example is composed of a plurality of subsystems which are three in number in the illustrated example of FIG. 7 . Each subsystem includes a drive unit including a reaction producing motor 11 a ′, 11 b ′ or 11 c ′, and a control unit including a reaction producing controller 20 a ′, 20 b ′ or 20 c ′ for controlling the drive unit.

Each of the reaction producing controllers 20 a ′, 20 b ′ and 20 c ′ is composed of an angle control operational section 21 a ′, 21 b ′ or 21 c ′ and a current control operational section 22 a ′, 22 b ′ or 22 c ′. Angle control operational sections 21 a ′, 21 b ′ and 21 c ′ are arranged to perform an angle control calculation in accordance with information obtained by angle sensors 13 a ′, 13 b ′ and 13 c ′ and a target rotation angle of column shaft 3 of steering wheel 1 corresponding to a target reaction torque calculated by upper controller 60 . In accordance with a current command calculated by the angle control calculation by one of angle control operational sections 21 a ′˜ 21 c ′, and a supply current sensed by one of current sensors 11 a ′˜ 11 c ′, each of current control operational sections 22 a , 22 b and 22 c calculates a drive current to control the actual current for the corresponding one of reaction producing motors 11 a ′, 11 b ′ and 11 c ′ toward the current command. Current corresponding to the thus-determined magnitude is supplied to the corresponding reaction producing motor.

In the second embodiment, too, one of the subsystems serves as a representative unit, and calculates a current command per subsystem. Each subsystem serving as non-representative unit obtains the current command per subsystem from the subsystem of the highest priority, and controls the corresponding reaction producing motor 11 a, 11 b or 11 c.

Steps S 12 ˜S 22 correspond to a process element of calculating a common torque share when an assigned role is to serve as a representative unit. Step S 24 , S 26 and S 28 correspond to a process element of receiving the common torque share when the assigned role is to serve as a non-representative unit. Steps S 30 ˜S 34 correspond to a process element of controlling the steering torque in accordance with the common torque share. Steps S 2 ˜S 10 corresponds to means for selecting one of subsystems as a representative unit, and leaving the remainder as a non-representative unit. Step S 18 corresponds to means for causing the representative unit to calculate a common torque share in accordance with the target torque. Step S 28 corresponds to means for causing the non-representative unit to receive the common torque share from the representative unit. Steps S 30 ˜S 34 correspond to means for producing the actual torque in response to the driver's steering input for steering the vehicle, by causing each unit to produce a torque in accordance with the common torque share. Each of the angle sensors 13 a , 13 b and 13 c (or 13 a ′, 13 b ′ and 13 c ′) serves as an operating condition sensor to sense an operating quantity of the steering mechanism.

This application is based on a prior Japanese Patent Application No. 2002-255072 filed on Aug. 30, 2002. The entire contents of this Japanese Patent Application No. 2002-255072 are hereby incorporated by reference.

Although the invention has been described above by reference to certain embodiments of the invention, the invention is not limited to the embodiments described above. Modifications and variations of the embodiments described above will occur to those skilled in the art in light of the above teachings. The scope of the invention is defined with reference to the following claims.