Steering controller

Provided is a steering controller configured to switch from an interruption state to a transmission state. In a state where power transmission from the steering wheel to steered wheels is interrupted, a maximum value selection processing circuit outputs a maximum value, out of a steered angle and a steering angle, to a limiting reaction force setting processing circuit. When the absolute value of the maximum value has become equal to or larger than a limitation start threshold value, the limiting reaction force setting processing circuit rapidly increases a limiting reaction force. An operation signal generation processing circuit controls a reaction-force motor to achieve a reaction force command value corresponding to the limiting reaction force. When the absolute value of the maximum value has become equal to or larger than an engagement threshold value, a clutch is engaged to transmit reaction force from the steered wheel-side to the steering wheel.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2016-184259 filed on Sep. 21, 2016 including the specification, drawings and abstract, is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a steering controller configured to control a steering system including: a switching device configured to perform switching between a transmission state where power transmission from a steering wheel to steered wheels is allowed and an interruption state where power transmission from the steering wheel to the steered wheels is interrupted; a reaction-force actuator configured to apply a steering reaction force to the steering wheel in the interruption state; and a steered operation actuator configured to steer the steered wheels.

2. Description of the Related Art

For example, Japanese Patent No. 4725132 describes a controller for a steer-by-wire system in which a reaction force is applied by a reaction-force actuator to a steering wheel while a backup clutch (switching device) is disengaged and thus power transmission between the steering wheel and steered wheels is interrupted. When the steered angle of the steered wheels has nearly reached a limit angle, the controller outputs a command to engage the backup clutch and executes a process of progressively increasing the reaction force to be applied by the reaction-force actuator to the steering wheel with the lapse of time until the backup clutch enters an engaged state (power transmission state).

However, engaging the backup clutch in this way may reduce the driving feel. For example, a user may sense, via the steering wheel, that the backup clutch is engaged.

SUMMARY OF THE INVENTION

One object of the invention is to provide a steering controller configured to reduce, as much as possible, the occurrence of switching of a switching device from an interruption state to a transmission state when executing a process for suppressing a steering wheel from being operated such that a steering angle exceeds an upper limit value.

An aspect of the invention relates to a steering controller configured to control a steering system including a switching device configured to perform switching between a transmission state where power transmission from a steering wheel to steered wheels is allowed and an interruption state where power transmission from the steering wheel to the steered wheels is interrupted, a reaction-force actuator configured to apply a steering reaction force to the steering wheel in the interruption state, and a steered operation actuator configured to steer the steered wheels. The steering controller includes: a memory configured to store control software; and a hardware device configured to execute the control software. When an absolute value of a steering angle achieved by an operation of the steering wheel has reached a first threshold value in the interruption state, the steering controller executes a limitation process of operating the reaction-force actuator so as to apply, to the steering wheel, a limiting reaction force that is a reaction force for suppressing the absolute value of the steering angle from further increasing. When the absolute value of the steering angle has become equal to or larger than a second threshold value that is larger than the first threshold value, the steering controller executes a transmission process of operating the switching device so as to execute switching from the interruption state to the transmission state.

According to the above aspect, when the steering angle has reached the first threshold value, the limiting reaction force is applied to the steering wheel to suppress the absolute value of the steering angle from further increasing. This allows a user to sense that the steering angle is near a maximum value, and thus prevents the user from operating the steering wheel so as to further increase the steering angle. If, nevertheless, the user further increases the steering angle and the steering angle has become equal to or larger than the second threshold value, the switching device is switched to the transmission state in order to apply a reaction force from the steered wheel-side to the steering wheel and thereby suppressing further increase in the steering angle. Thus, it is possible to reduce, as much as possible, the occurrence of switching of the switching device from the interruption state to the transmission state when executing a process for suppressing the steering wheel from being operated such that the steering angle exceeds the upper limit value.

DETAILED DESCRIPTION OF EMBODIMENTS

Hereinafter, a steering controller according to a first embodiment of the invention will be described with reference to the accompanying drawings. As illustrated inFIG. 1, in a steering system1according to the present embodiment, a steering wheel10is connected to a reaction-force actuator20configured to apply a reaction force that is a force acting against an operation of the steering wheel10. The reaction-force actuator20includes a steering shaft22fixed to the steering wheel10, a reaction-force-side speed reducer24, a reaction-force motor26provided with a rotary shaft26acoupled to the reaction-force-side speed reducer24, and an inverter28configured to drive the reaction-force motor26. In this case, the reaction-force motor26is a surface permanent magnet synchronous motor (SPMSM).

The steering shaft22can be coupled via a clutch12to a pinion shaft42of a steered operation actuator40. In the present embodiment, the clutch12is a normally-closed clutch that transmits power while no current is applied to the clutch12.

The steered operation actuator40is a dual-pinion steered operation actuator including a first rack-and-pinion mechanism48and a second rack-and-pinion mechanism52, and further includes a steered-side motor56(SPMSM) and an inverter58. The first rack-and-pinion mechanism48includes a rack shaft46and the pinion shaft42that are disposed at a prescribed intersection angle, and first rack teeth46aprovided on the rack shaft46and pinion teeth42aprovided on the pinion shaft42are meshed with each other. Steered wheels30are coupled to respective ends of the rack shaft46via tie-rods.

The second rack-and-pinion mechanism52includes the rack shaft46and a pinion shaft50disposed at a prescribed intersection angle, and second rack teeth46bprovided on the rack shaft46and pinion teeth50aprovided on the pinion shaft50are meshed with each other.

The pinion shaft50is connected via a steered-side speed reducer54to a rotary shaft56aof the steered-side motor56. The inverter58is connected to the steered-side motor56. The rack shaft46is housed in a rack housing44.

A spiral cable device60is coupled to the steering wheel10. The spiral cable device60includes a first housing62fixed to the steering wheel10, a second housing64fixed to a vehicle body, a tubular member66that is housed in a space defined by the first housing62and the second housing64and that is fixed to the second housing64, and a spiral cable68wound around the tubular member66. The steering shaft22is inserted in the tubular member66. The spiral cable68is an electric wire that connects a horn70fixed to the steering wheel10and, for example, a battery72fixed to the vehicle body to each other.

A controller80executes control for steering the steered wheels30in response to an operation of the steering wheel10, by operating the steering system1including the reaction-force actuator20and the steered operation actuator40. Specifically, in the present embodiment, a steer-by-wire system is achieved by the reaction-force actuator20and the steered operation actuator40, and the controller80usually executes the control for steering the steered wheels30in response to an operation of the steering wheel10while keeping the clutch12in an interruption state. For this purpose, the controller80acquires a rotation angle θs0of the rotary shaft26aof the reaction-force motor26, which is detected by a steering-side sensor92, and a steering torque Trqs applied to the steering shaft22, which is detected by a torque sensor94. Moreover, the controller80acquires a rotation angle θt0of the rotary shaft56aof the steered-side motor56, which is detected by a steered-side sensor90, and a vehicle speed V detected by a vehicle speed sensor96.

Specifically, the controller80includes a central processing unit (CPU)82and a memory84, and the steered operation actuator40and the reaction-force actuator20are operated as the CPU82executes a program stored in the memory84.

FIG. 2illustrates some of the processes executed by the controller80.FIG. 2illustrates some of the processes that are executed when the CPU82executes programs stored in the memory84, based on the kind of process to be executed.

An integration processing circuit M2converts the rotation angle θs0detected by the steering-side sensor92and the rotation angle θt0detected by the steered-side sensor90into numerical values within an angular range wider than a range of 0° to 360°, thereby obtaining rotation angles θs, θt. Specifically, for example, when the steering wheel10is turned maximally to the right or to the left from a neutral position at which the vehicle travels straight forward, the rotary shaft26arotates beyond 360°. Therefore, for example, when the rotary shaft26arotates twice in a prescribed direction from the state where the steering wheel10is at the neutral position, the integration processing circuit M2sets an output value to 720°. The integration processing circuit M2sets an output value to zero when the steering wheel10is at the neutral position.

A measurement unit setting processing circuit M4multiplies the output value from the steering-side sensor92, which has been subjected to the process by the integration processing circuit M2, by a conversion factor Ks, thereby calculating a steering angle θh, and multiplies the output value from the steered-side sensor90, which has been subjected to the process by the integration processing circuit M2, by a conversion factor Kt, thereby calculating a steered angle θp. In this case, the conversion factor Ks is set based on a ratio of rotation speed between the reaction-force-side speed reducer24and the rotary shaft26aof the reaction-force motor26, and the conversion factor Ks is used to convert an amount of change in the rotation angle θs of the rotary shaft26ainto an amount of turning of the steering wheel10. Thus, the steering angle θh represents a turning angle of the steering wheel10with respect to the neutral position. The conversion factor Kt is a product of a ratio of rotation speed between the steered-side speed reducer54and the rotary shaft56aof the steered-side motor56and a ratio of rotation speed between the pinion shaft50and the pinion shaft42. This conversion factor Kt is used to convert an amount of rotation of the rotary shaft56ainto an amount of turning of the steering wheel10on the assumption that the clutch12is engaged.

In the processes illustrated inFIG. 2, the rotation angles θs, θt, the steering angle θh, and the steered angle θp each take a positive value when the rotation direction is a prescribed direction, whereas the rotation angles θs, θt, the steering angle θh, and the steered angle θp each take a negative value when the rotation direction is a direction opposite to the prescribed direction. Thus, for example, when the rotary shaft26arotates in a direction opposite to the prescribed direction from the state where the steering wheel10is at the neutral position, the integration processing circuit M2outputs a negative output value. However, this is merely an example of control system logics. In particular, in this specification, that the rotation angles θs, θt, the steering angle θh, and the steered angle θp are large means that the amount of change from the neutral position is large. In other words, this means that the absolute value of a parameter that takes a positive value or a negative value as described above is large.

An assist torque setting processing circuit M6sets an assist torque Trqa* based on the steering torque Trqs. The assist torque Trqa* is set to a larger value as the steering torque Trqs is larger. An addition processing circuit M8adds the steering torque Trqs to the assist torque Trqa* and outputs a resultant value.

A reaction force setting processing circuit M10sets a reaction force Fir that is a force acting against turning of the steering wheel10. Specifically, in the reaction force setting processing circuit M10, a base reaction force setting processing circuit M10asets a base reaction force Fib in response to an operation of the steering wheel10, whereas a limiting reaction force setting processing circuit M10bsets a limiting reaction force Fie that is a reaction force acting against an operation of the steering wheel10performed such that the steering angle further approaches an upper limit value when the amount of turning of the steering wheel10has approached an allowable maximum value. Then, in the reaction force setting processing circuit M10, the base reaction force Fib and the limiting reaction force Fie are added together by an addition processing circuit M10c. Then, a resultant value is output, as the reaction force Fir, from the reaction force setting processing circuit M10.

A deviation calculation processing circuit M12outputs a value obtained by subtracting the reaction force Fir from the value output from the addition processing circuit M8. A steering angle command value calculation processing circuit M20sets a steering angle command value θh* based on the value output from the deviation calculation processing circuit M12. The steering angle command value calculation processing circuit M20uses a model equation expressed by Equation (c1) that correlates an output value from the deviation calculation processing circuit M12with the steering angle command value θh*.
Δ=C·θh*′+J·θh*″Equation (c1)

The model expressed by Equation (c1) is a model that defines a relationship between an axial force and a rotation angle of the rack shaft46in a system in which the steering wheel10and the steered wheels30are mechanically coupled to each other. In the Equation (c1), a viscosity coefficient C is obtained by modeling, for example, friction in the steering system, and an inertia coefficient J is obtained by modeling inertia in the steering system. In this case, the viscosity coefficient C and the inertia coefficient J are variably set based on the vehicle speed V.

A steering angle feedback processing circuit M22sets a feedback torque Trqr1* as a manipulated variable that is used to cause the steering angle θh to follow the steering angle command value θh* through feedback control. Specifically, the sum of output values from a proportional element, an integrating element, and a differentiating element based on an input of a value obtained by subtracting the steering angle θh from the steering angle command value θh* is used as the feedback torque Trqr1*.

An addition processing circuit M24outputs the sum of the feedback torque Trqr1* output from the steering angle feedback processing circuit M22and the assist torque Trqa* output from the assist torque setting processing circuit M6, as a torque command value (reaction force command value Trqr*) for the reaction-force motor26.

An operation signal generation processing circuit M26generates an operation signal MSs for the inverter28based on the reaction force command value Trqr*, and outputs the operation signal MSs to the inverter28. This process can be achieved, for example, through known current feedback control in which a q-axis current command value is set based on the reaction force command value Trqr* and a dq-axis voltage command value is set as a manipulated variable used to cause a dq-axis current to follow a command value through feedback control. The d-axis current may be controlled to be zero. However, when the rotation speed of the reaction-force motor26is high, field-weakening control may be executed with the absolute value of the d-axis current set to a value larger than zero. However, the absolute value of the d-axis current may be to a value larger than zero in a low rotation speed range.

Based on the steering angle command value θh*, a steering angle ratio variable setting processing circuit M28sets a target operation angle θa* that is used to variably set a steering angle ratio that is a ratio between the steering angle θh and the steered angle θp. An addition processing circuit M30calculates a steered angle command value θp* by adding the target operation angle θa* to the steering angle command value θh*.

A steered angle feedback processing circuit M32sets a steered operation torque command value Trqt* for a torque generated by the steered-side motor56, as a manipulated variable used to cause the steered angle θp to follow the steered angle command value θp* through feedback control. Specifically, the sum of output values from a proportional element, an integrating element, and a differentiating element based on an input of a value obtained by subtracting the steered angle θp from the steered angle command value θp* is used as the steered operation torque command value Trqt*.

An operation signal generation processing circuit M34generates an operation signal MSt for the inverter58based on the steered operation torque command value Trqt*, and outputs the operation signal MSt to the inverter58. This process can be executed in a manner similar to the manner in which the operation signal generation process is executed by the operation signal generation processing circuit M26.

A maximum value selection processing circuit M36selects a larger value (maximum value θe), in terms of the absolute value, out of the steering angle θh and the steered angle θp, and outputs the maximum value θe. The base reaction force setting processing circuit M10areceives the steered angle command value θp* as an input. On the other hand, the limiting reaction force setting processing circuit M10breceives the maximum value θe as an input, and sets the limiting reaction force Fie based on the maximum value θe. This setting is executed in order to execute control for increasing the force acting against further increase in the absolute value of the steering angle of the steering wheel10, immediately before an end of the rack shaft46comes into contact with the rack housing44due to an axial displacement of the rack shaft46, and immediately before the steering angle of the steering wheel10reaches the upper limit value defined based on the length of the spiral cable68. This setting will be described in detail below.

FIG. 3illustrates the relationship between an upper limit value θhH of the steering angle θh and an upper limit value θpH of the steered angle θp. As illustrated inFIG. 3, in the present embodiment, the upper limit value θhH of the steering angle θh and the upper limit value θpH of the steered angle θp are almost equal to each other. This relationship is achieved through setting of the measurement units of the steering angle θh and the steered angle θp, which is executed by the measurement unit setting processing circuit M4. Specifically, in the present embodiment, the length of the spiral cable68includes a small margin, so that the steering wheel10can be further turned slightly when the rack shaft46has been displaced in the axial direction to be brought into contact with the rack housing44while the clutch12is engaged. Therefore, the measurement unit setting processing circuit M4sets the steering angle θh to the turning angle of the steering wheel10, and sets the steered angle θp to the turning angle of the steering wheel10on the assumption that the target operation angle θa* is zero, so that the upper limit value θhH of the steering angle θh and the upper limit value θpH of the steered angle θp are almost equal to each other.

In the present embodiment, therefore, a limitation start threshold value θ1is set for each of the steering angle θh and the steered angle θp, and the reaction force applied to the steering wheel10is controlled to be increased before the absolute value of the steering angle θh reaches the upper limit value θhH and before the absolute value of the steered angle θp reaches the upper limit value θpH. The limiting reaction force setting processing circuit M10billustrated inFIG. 2has a map that defines the relationship between the absolute value of the maximum value θe and the limiting reaction force Fie. In this map, the limiting reaction force Fie becomes larger than zero when the absolute value of the maximum value θe becomes equal to or larger than the limitation start threshold value θ1. In particular, the limiting reaction force Fie is set to a value that is large enough to allow the user to sense that it is difficult to operate the steering wheel10so as to further increase the absolute value of the steering angle θh, when the absolute value of the maximum value θe has increased to some extent beyond the limitation start threshold value θ1.FIG. 2illustrates only a case where the limiting reaction force Fie increases as the maximum value θe increases from zero in the prescribed rotation direction. Further, the absolute value of the limiting reaction force Fie increases when the maximum value θe increases in a direction opposite to the prescribed rotation direction. Note that, the limiting reaction force Fie in the process illustrated inFIG. 2takes a negative value when the rotation direction is a direction opposite to the prescribed rotation direction.

In the present embodiment, the controller80executes a process of engaging the clutch12under prescribed conditions while the limiting reaction force Fie is applied to the steering wheel10. The process will be described below.

FIG. 4illustrates the procedures of a process of engaging and disengaging the clutch12. The process illustrated inFIG. 4is achieved when the CPU82executes a program stored in the memory84repeatedly at prescribed time intervals. The following process is executed when the clutch12is not engaged due to, for example, a malfunction of the steering system1. In other words, the process illustrated inFIG. 4is executed on the precondition that the clutch12has been disengaged before the clutch12is engaged in this process. Hereinafter, step numbers will be represented by numbers following the first letter “S.”

In the procedures of the process illustrated inFIG. 4, the CPU82first determines whether a reaction force assist flag F is one (1) (S10). The fact that the reaction force assist flag F is 1 indicates that a logical conjunction of the following logical propositions i), ii) is true: i) the clutch12is switched to an engaged state while a reaction force corresponding to the limiting reaction force Fie is applied to the steering wheel10; and ii) the clutch12has not been disengaged after the engagement of the clutch12. On the other hand, the fact that the reaction force assist flag F is zero indicates that the logical conjunction is false. When the CPU82determines that the reaction force assist flag F is zero (S10: NO), the CPU82then determines whether the absolute value of the maximum value θe is equal to or larger than an engagement threshold value θ2that is larger than the limitation start threshold value θ1(S12). This process is a process of determining whether to switch the clutch12to the engaged state while the limiting reaction force Fie is applied to the steering wheel10. When the CPU82determines that the maximum value θe is equal to or larger than the engagement threshold value θ2(S12: YES), the CPU82then executes a transmission process of engaging the clutch12(S14). Specifically, because the clutch12is a normally-closed clutch, the CPU82stops an operation of applying current to the clutch12. Then, the CPU82sets the reaction force assist flag F to 1 (S16).

On the other hand, when the CPU82determines that the reaction force assist flag F is 1 (S10: YES), the CPU82then determines whether the absolute value of the maximum value θe is equal to or smaller than a disengagement threshold value θ3that is equal to or smaller than the limitation start threshold value θ1(S18). This process is a process of determining whether to switch the clutch12from the engaged state to a disengaged state. When the CPU82determines that the maximum value θe is larger than the disengagement threshold value θ3(S18: NO), the CPU82sets the target operation angle θa* to a fixed value in view of the fact that the clutch12is in the engaged state (S20). This process is a process of stopping the steering angle ratio variable setting process executed by the steering angle ratio variable setting processing circuit M28. Specifically, in this process, for example, a value that is obtained by subtracting the steering angle θh at a timing when the clutch12is switched from the disengaged state to the engaged state from the steered angle θp at the same timing is assigned to the target operation angle θa*.

On the other hand, when the CPU82determines that the maximum value θe is equal to or smaller than the disengagement threshold value θ3(S18: YES), the CPU82then switches the clutch12to the disengaged state (S22). Then, the CPU82sets the reaction force assist flag F to zero (S24).

When the process in S16, S20, or S24is completed or when a negative determination is made in S12, the CPU82ends the procedures of the process illustrated inFIG. 4.

The operation of the present embodiment will be described below. When the maximum value θe has become equal to or larger than the limitation start threshold value θ1, the CPU82causes the reaction-force actuator20to apply a reaction force corresponding to the limiting reaction force Fie to the steering wheel10. As a result, the force acting against an operation of the steering wheel10increases rapidly. This allows the user to sense that the steering angle has approached the limit value.

If the steering wheel10is operated so as to further increase the steering angle despite application of the limiting reaction force Fie to the steering wheel10, the steering torque Trqs exceeds the torque applied to the steering wheel10due to the limiting reaction force Fie. In this case, the CPU82engages the clutch12. As a result, a reaction force corresponding to the rack axial force is applied to the steering wheel10. Although the rack axial force depends on the setting of the suspension geometry, the rack axial force is usually larger when the absolute value of the steered angle θp is large than when the absolute value of the steered angle θp is small. Therefore, even in a case where the steering torque Trqs exceeds a torque that can be generated by the reaction-force actuator20, when the clutch12is engaged to allow the rack axial force to be transmitted to the steering wheel10, it is difficult to further operate the steering wheel10so as to increase the steering angle, depending on the steering torque Trqs. Moreover, when the rack shaft46comes into contact with the rack housing44, the rack shaft46cannot be displaced any more. Therefore, a situation where the steering wheel10is further largely turned can be avoided, and turning of the steering wheel10can be stopped.

Moreover, in the present embodiment, when the absolute value of the steering angle θh has become large, the limiting reaction force Fie is applied to the steering wheel10before the clutch12is engaged. Thus, as the user senses that the limiting reaction force Fie is applied to the steering wheel10, the user is likely to be restrained from operating the steering wheel10so as to further increase the steering angle by applying an excessively large steering torque Trqs to the steering wheel10. Thus, the occurrence of a situation where the clutch12is engaged can be reduced. The limiting reaction force Fie is generated by the reaction-force actuator20under control, so that application of the limiting reaction force Fie is less likely to make an operation of the steering wheel10uncomfortable. In contrast to this, engagement of the clutch12is likely to make an operation of the steering wheel10uncomfortable, due to impact upon engagement of the clutch12or mechanical changes occurring in the steering system1before and after the clutch12is engaged.

The foregoing embodiment produces the following advantageous effects.

(1) The steering wheel10is provided with the spiral cable device60that turns together with the steering wheel10in an integrated manner. In this case, the reliability of the spiral cable68may decrease, if the clutch12is not engaged when the user applies a large torque to the steering wheel10so as to increase the absolute value of the steering angle θh although the maximum value θe has become equal to or larger than the limitation start threshold value θ1and the limiting reaction force Fie is applied to the steering wheel10. In other words, the reliability of the spiral cable68may decrease, because a force that attempts to stretch the spiral cable68is applied thereto even after the spiral cable68has reached its maximum length. For this reason, the process of engaging the clutch12is especially useful.

(2) The clutch12is disengaged when the maximum value θe has become equal to or smaller than the disengagement threshold value θ3while the clutch12is engaged. Further, the disengagement threshold value θ3is set to be equal to or smaller than the limitation start threshold value θ1. Thus, it is possible to reduce the occurrence of a hunting phenomenon in which a process of engaging the clutch12and a process of disengaging the clutch12are repeated.

Next, a second embodiment will be described with reference to the drawings. The differences from the first embodiment will be mainly described below.

FIG. 5illustrates the procedures of a process of engaging and disengaging the clutch12according to the present embodiment. The process illustrated inFIG. 5is achieved when the CPU82executes a program stored in the memory84repeatedly at prescribed time intervals. The process illustrated inFIG. 5substitutes for the process illustrated inFIG. 4. For the sake of convenience, the same steps as those inFIG. 4will be denoted by the same step numbers and the detailed description thereof will be omitted.

In the procedures of the process illustrated inFIG. 5, when the CPU82determines that the reaction force assist flag F is one (1) (S10), the CPU82then determines whether the absolute value of the steering torque Trqs is equal to or smaller than a torque threshold value Trqth (S18a). This process is a process of determining whether to disengage the clutch12. In this case, the torque threshold value Trqth is set to a value smaller than a lower limit value of torque required to further increase the absolute value of the steering angle θh in a range where the maximum value θe is equal to or larger than the limitation start threshold value θ1. In other words, the torque threshold value Trqth is set to a value smaller than the lower limit value of torque at which the absolute value of the steering angle θh can be further increased despite execution of the process of applying the limiting reaction force Fie to the steering wheel10.

When the CPU82determines that the absolute value of the steering torque Trqs is equal to or smaller than the torque threshold value Trqth (S18a: YES), the CPU82disengages the clutch12(S22). On the other hand, when the CPU82determines that the absolute value of the steering torque Trqs is larger than the torque threshold value Trqth (S18a: NO), the CPU82proceeds to S20.

Next, a third embodiment will be described with reference to the drawings. The differences from the first embodiment will be mainly described below.

In the first embodiment, the rack shaft46is considered to come into contact with the rack housing44before the spiral cable68is fully stretched, when the absolute value of the difference between the steered angle θp and the upper limit value θpH is smaller than the absolute value of the difference between the steering angle θh and the upper limit value θhH at the steering angle ratio at the time when the maximum value θe reaches the engagement threshold value θ2. In that case, the absolute value of the steering angle θh is limited when the rack shaft46comes into contact with the rack housing44, so that a situation where such a large force as to reduce the reliability of the spiral cable68is applied to the spiral cable68is avoided. Nevertheless, if the limiting reaction force Fie keeps being applied to the steering wheel10, the amount of heat generated by the reaction-force motor26may become unnecessarily large.

In the present embodiment, therefore, the process illustrated inFIG. 6is executed instead of the process illustrated inFIG. 4.FIG. 6illustrates the procedures of a process of engaging and disengaging the clutch12according to the present embodiment. The process illustrated inFIG. 6is achieved when the CPU82executes a program stored in the memory84repeatedly at prescribed time intervals. The process illustrated inFIG. 6substitutes for the process illustrated inFIG. 4. For the sake of convenience, the same steps as those inFIG. 4will be denoted by the same step numbers and the detailed description thereof will be omitted.

In the procedures of process illustrated inFIG. 6, when the CPU82determines that the reaction force assist flag F is 1 (S10: YES), then the CPU82then determines whether the absolute value of the difference between the steered angle θp and the upper limit value θpH is equal to or smaller than the absolute value of the difference between the steering angle θh and the upper limit value θhH (S30). This process is a process of determining whether to execute a process of progressively reducing the limiting reaction force Fie. When the CPU82determines that the absolute value of the difference between the steered angle θp and the upper limit value θpH is equal to or smaller than the absolute value of the difference between the steering angle θh and the upper limit value θhH (S30: YES), the CPU82then reduces the absolute value of the limiting reaction force Fie by a prescribed amount ΔFie (S32). The minimum value of the absolute value of the limiting reaction force Fie is set to zero. Therefore, when the absolute value of the limiting reaction force Fie is smaller than the prescribed amount ΔFie, the CPU82sets the limiting reaction force Fie to zero in the process in S32. When the process in S32is completed or when a negative determination is made in S30, the CPU82proceeds to S18.

The operation of the present embodiment will be described below. When the maximum value θe has become equal to or larger than the limitation start threshold value θ1, the CPU82controls the torque generated by the reaction-force motor26so as to achieve the reaction force command value Trqr* that is set based on the limiting reaction force Fie. Then, the CPU82engages the clutch12when the maximum value θe has become equal to or larger than the engagement threshold value θ2. Then, the CPU82progressively reduces the limiting reaction force Fie to zero, on the condition that the CPU82determines that the absolute value of the difference between the steered angle θp and the upper limit value θpH is equal to or smaller than the absolute value of the difference between the steering angle θh and the upper limit value θhH. Thus, the amounts of heat generated by the reaction-force motor26and the inverter28can be reduced. Moreover, in this case, before the spiral cable68is fully stretched, the rack shaft46comes into contact with the rack housing44, which makes it impossible to turn the steering wheel10. Thus, it is possible to reduce the occurrence of a situation where the steering angle θh becomes so large as to reduce the reliability of the spiral cable68, while reducing power consumption of the reaction-force actuator20.

The correspondence relationship between the matters described in the foregoing embodiments and the matters described in claims is as follows.

The clutch12is an example of a switching device. The process executed by the limiting reaction force setting processing circuit M10b, the addition processing circuit M10c, the deviation calculation processing circuit M12, the steering angle command value calculation processing circuit M20, the steering angle feedback processing circuit M22, the addition processing circuit M24, and the operation signal generation processing circuit M26is an example of a limitation process. The process in S14is an example of a transmission process, and the controller80is an example of a steering controller. The steering angle θh at which the maximum value θe reaches the limitation start threshold value θ1is an example of a first threshold value, and the steering angle θh at which the maximum value θe reaches the engagement threshold value θ2is an example of a second threshold value. A value obtained by converting a part of the reaction force command value Trqr*, which contributes to the limiting reaction force Fie, based on a reduction ratio of the reaction-force-side speed reducer24is an example of an absolute value of a limiting reaction force applied to the steering wheel.

The process executed by the steering angle ratio variable setting processing circuit M28is an example of a steering angle ratio variable setting process. The process executed by the integration processing circuit M2and the process executed by the measurement unit setting processing circuit M4are examples of a steering angle acquisition process and a steered angle acquisition process.

The spiral cable device60is an example of a steering-side device, and the battery72is an example of an external device. The process in S30, S32is an example of a progressive reduction process.

At least one of the matters of the foregoing embodiments may be modified as follows.

In the foregoing embodiments, when the clutch12is engaged, the process illustrated inFIG. 2is basically continued while the target operation angle θa* is fixed. However, the operation of the steering system1when the clutch is engaged is not limited to this example. For example, while the reaction force command value Trqr* is set to zero, the steered operation torque command value Trqt* may be set to a value that is obtained by subtracting the limiting reaction force Fie, which is determined based on the steered angle θp through the same process as the process executed by the limiting reaction force setting processing circuit M10b, from a torque determined based on the steering torque Trqs through the same process as the process executed by the assist torque setting processing circuit M6.

For example, when the reaction force command value Trqr* is set to zero, a manipulated variable for the feedback-control of the steered angle θp may be used as the steered operation torque command value Trqt*, instead of the steered operation torque command value Trqt* calculated based on the steering torque Trqs through open-loop control. Specifically, in the same process as the process of setting the steering angle command value θh* inFIG. 2, the steered angle command value θp* as an input into the reaction force setting processing circuit M10may be replaced with the steered angle θp, the steering angle command value θh* as a final output value may be replaced with the steered angle command value θp*, and the steered operation torque command value Trqt* may be set such that the steered angle θp follows the steered angle command value Op* through feedback control. Moreover, for example, the sum of a manipulated variable for the feedback control and a manipulated variable for the open-loop control may be used as the steered operation torque command value Trqt*.

For example, while the steered operation torque command value Trqt* is set to zero, the reaction force command value Trqr* may be set to a value obtained by subtracting the limiting reaction force Fie from the assist torque Trqa*. In this case, the relationship between the steering torque Trqs and the assist torque Trqa* determined by the assist torque setting processing circuit M6may be changed from that before the clutch12is engaged.

For example, a torque value that is obtained by subtracting the limiting reaction force Fie from a torque determined based on the steering torque Trqs by the same process as the process executed by the assist torque setting processing circuit M6may be divided into the reaction force command value Trqr* and the steered operation torque command value Trqt*. Note that, “dividing A into the reaction force command value Trqr* and the steered operation torque command value Trqt*” does not mean that the equation Trqr*+Trqt*=A holds true, but means the following. For example, if A is an amount having the magnitude of a torque of the rotary shaft26a, the sum of the reaction force command value Trqr* and a value that is equivalent to the torque of the rotary shaft26aand that is obtained by converting the steered operation torque command value Trqt* based on the rotation speed ratio between the rotary shaft56aand the rotary shaft26ais A.

In the third embodiment, the limiting reaction force Fie is progressively reduced to zero, but the progressive reduction process is not limited to this example. For example, this process may be a process of progressively reducing the limiting reaction force Fie to a prescribed value that is larger than zero.

As will be described in the paragraphs about the limitation start threshold value, the process in S30may be omitted from the process inFIG. 6, in the case where the spiral cable68is not fully stretched as long as the steered angle is controlled so as to be equal to or smaller than the steered angle threshold value at any steering angle ratio that is set by the steering angle ratio variable setting processing circuit M28.

As for the setting of the steered angle command value and the steering angle command value, instead of the process illustrated inFIG. 2, for example, a steered angle command value calculation processing circuit may be provided. The steered angle command value calculation processing circuit calculates the steered angle command value θp* based on a value output from the deviation calculation processing circuit M12through the same process as the process executed by the steering angle command value calculation processing circuit M20. In this case, a value obtained by subtracting the target operation angle θa* from the steered angle command value θp* may be used as the steering angle command value θh*.

The steering angle command value calculation processing circuit M20may set the steering angle command value θh* according to a model equation expressed by Equation (c2), instead of the model equation expressed by Equation (c1).
Δ=K·θh*+C·θh*′+J·θh*″Equation (c2)

Here, a spring constant K is obtained by modeling an influence of the vehicle, and is determined based on the specifications of the suspension, wheel alignment, and so forth. In the foregoing modified example in which the steered angle command value θp* is set based on the model without using the steering angle command value θh*, the steering angle command value θh* in Equations (c1), (c2) are replaced with the steered angle command value θp*.

The steering angle acquisition process and the steered angle acquisition process are not limited to the processes of acquiring the steered angle θp and the steering angle θh. For example, the steered angle command value θp* may be acquired instead of the steered angle θp. Alternatively, for example, the steering angle command value θh* may be acquired instead of the steering angle θh. Further alternatively, for example, both the steered angle command value θp* and the steering angle command value θh* may be acquired instead of the steered angle θp and the steering angle θh, respectively. However, for example, in a case where a value obtained by adding the target operation angle θa* to the steering angle command value θh* is used as the steered angle command value Op*, it is desirable that, when the reaction force assist flag F is from zero to 1, a value obtained by subtracting the steering angle command value θh* at the timing of the switching from the steered angle command value θp* at the same timing be assigned to the target operation angle θa*. However, the process in S20may be executed instead of this process.

The limitation start threshold value may be set as follows.

(a) In the foregoing embodiments, a pair of parameters that are the steering angle and the steered angle is used as a comparison object to be compared with the limitation start threshold value θ1. However, the comparison object is not limited to this example. For example, in a four-wheel-drive vehicle, the comparison object may be three parameters that are a steered angle of the front wheels, a steered angle of the rear wheels, and a steering angle. In this case, the maximum value θe out of these three parameters may be selected instead of the process executed by the maximum value selection processing circuit M36.

Alternatively, the comparison object may be a single parameter. Specifically, for example, the steered angle may be used as the single parameter, in a case where the spiral cable68has a margin and is not fully stretched as long as the steered angle is controlled so as to be equal to or smaller than the steered angle threshold value at any steering angle ratio that is set by the steering angle ratio variable setting processing circuit M28. Alternatively, for example, the steering angle may be used as the single parameter, in a case where the spiral cable68has no margin and the rack shaft46does not come into contact with the rack housing44as long as the steering angle is controlled so as to be equal to or smaller than the steering angle threshold value at any steering angle ratio that is set by the steering angle ratio variable setting processing circuit M28. However, in order to more reliably prevent a decrease in the reliability of the spiral cable68, it is desirable to adopt such settings that the spiral cable68is not fully stretched as long as the steered angle is controlled so as to be equal to or smaller than the steered angle threshold value at any steering angle ratio that is set by the steering angle ratio variable setting processing circuit M28.

Moreover, the steered angle may be used as the single parameter, if the spiral cable68is not provided, as will be described in the paragraph about the spiral cable device.

(b) The setting of the limitation start threshold value is not limited to the setting in which the steering angle threshold value and the steered angle threshold value are each used as a common threshold value. For example, instead of the limiting reaction force setting processing circuit M10b, a processing circuit configured to set a first limiting reaction force to a large value when the steering angle θh or the steering angle command value θh* has approached the steering angle threshold value, and another processing circuit configured to set a second limiting reaction force to a large value when the steered angle θp or the steered angle command value θp* has approached the steered angle threshold value may be provided. In this case, the sum of the first limiting reaction force and the second limiting reaction force, or the larger one of the first limiting reaction force and the second limiting reaction force may be used as the limiting reaction force Fie. In this case as well, the first threshold value is a smaller one of the steering angle threshold value and the steering angle corresponding to the steered angle threshold value at the present steering angle ratio.

The steering angle feedback processing circuit is not limited to the circuit configured to calculate the feedback torque Trqr1* as the sum of the output values from the proportional element, the integrating element, and the differentiating element based on an input of the value that is obtained by subtracting the steering angle θh from the steering angle command value θh*. For example, the steering angle feedback processing circuit may be a circuit configured to calculate the feedback torque Trqr1* as the sum of the output values from the proportional element and the differentiating element based on the input of the value that is obtained by subtracting the steering angle θh from the steering angle command value θh*.

In the foregoing embodiments, the sum of the feedback torque Trqr1* and the assist torque Trqa* is used as the command value for the reaction-force motor26, but the command value for the reaction-force motor is not limited to this example. For example, the feedback torque Trqr1* may be used as a command value for the reaction-force motor26.

The steered angle feedback processing circuit is not limited to the circuit configured to calculate the feedback manipulated variable (steered operation torque command value Trqt*) as the sum of the output values from the proportional element, the integrating element, and the differentiating element based on the input of the value that is obtained by subtracting the steered angle θp from the steered angle command value θp*. For example, the steered angle feedback processing circuit may be a circuit configured to calculate the feedback manipulated variable as the sum of the output values from the proportional element and the differentiating element based on the input of the value that is obtained by subtracting the steered angle θp from the steered angle command value θp*.

In the steered operation actuator, the steered-side motor56is not limited to an SPMSM, and an IPMSM may be used instead of an SPMSM. The steered-side motor56is not limited to a synchronous motor, and may instead be an induction motor, for example. The steered operation actuator is not limited to a dual pinion-type actuator. For example, a Rack-cross (R) type actuator, a Rack-parallel (R) type actuator, or a rack coaxial type actuator may be used.

The steering controller is not limited to the controller that includes the CPU82and the memory84and executes software processes. For example, the steering controller may include a dedicated hardware circuit (e.g., ASIC) that executes at least some of the processes that are executed by software in the foregoing embodiments. Specifically, the steering controller may have any one of the following configurations (a) to (c): (a) a configuration including a processing device that executes all the processes according to programs, and a memory storing these programs; (b) a configuration including a processing device that executes some of the processes according to the programs, a memory storing these program, and a dedicated hardware circuit that executes the remaining processes; and (c) a configuration including a dedicated hardware circuit that executes all the processes.

The spiral cable device may be a device that receives electric power that is contactlessly supplied thereto from, for example, the battery72serving as a power source. In this case, the spiral cable68is not required, and there is no upper limit value of the steering angle θh attributable to the spiral cable68.

InFIG. 2, the input into the base reaction force setting processing circuit M10amay be the steered angle θp instead of the steered angle command value θp*.

The reaction-force motor26is not limited to an SPMSM, and may instead be an IPMSM. The reaction-force motor26is not limited to a synchronous motor, and may instead be an induction motor, for example. InFIG. 2, it is not absolutely necessary that the steering angle ratio variable setting processing circuit M28is provided.