STEERING DEVICE

A steering device turns a tire of a vehicle of a steer-by-wire system in which a steering mechanism and a turning mechanism are mechanically separated from each other and is provided with a turning device including a turning actuator and a turning angle control device. The turning actuator turns tires according to an instructed turning angle. The turning angle control device calculates a turning angle command value corresponding to an inputted steering angle signal and generates a signal driving the turning actuator based on that turning angle command value. The turning angle control device applies limit such that the absolute value of a turning angle velocity becomes equal to or below a turning angle velocity limit value set according to a predetermined parameter.

TECHNICAL FIELD

The present disclosure relates to a steering device.

BACKGROUND

A known vehicle includes a steering mechanism and a turning mechanism that are mechanically connected with each other.

SUMMARY

According to an aspect of the present disclosure, a steering device turns a tire of a vehicle of a steer-by-wire system. In the steer-by-wire system, a steering mechanism and a turning mechanism are mechanically separated from each other.

DETAILED DESCRIPTION

Hereinafter, examples of the present disclosure will be described.

According to an example of the present disclosure, a steer-by-wire system includes a steering mechanism and a turning mechanism that are mechanically separated from each other.

In the steer-by-wire system, it is conceivable to generate a turning angle velocity variable. As one example, a turning angle velocity before correction is multiplied by a gain depending on steering angle and vehicle speed to compute an optimum turning angle velocity lower than before correction for optimization of response speed of a turning wheel corresponding to an operating state of a steering wheel.

With a steer-by-wire system, a steering angle ratio that is a ratio of a turning angle to a steering angle can be variably set. However, when at a high steering angle ratio, a steering operation may be performed with the same sensation as at a low steering angle ratio, the tires are turned at a speed beyond imagination. During a turning operation at high steering angle ratio, a vehicle behavior with large roll, yaw, and the like occurs and ride comfort is degraded.

It is notes that, the vehicle of a steer-by-wire system assumed in the present disclosure is not limited to those in which a driver performs driving operation but includes automatic drive vehicles.

According to the present disclosure, a steering device turns a tire of a vehicle of a steer-by-wire system in which a steering mechanism and a turning mechanism are mechanically separated from each other. This steering device, including those applied to an automatic drive vehicle, is provided with, at least, a turning device including a turning actuator and a turning angle control device.

A turning actuator is configured to turn the tire according to an instructed turning angle. A turning angle control device is configured to calculate a turning angle command value, which corresponds to an inputted steering angle signal, and generate a signal to drive the turning actuator based on the turning angle command value. The turning angle control device is configured to apply limit, such that an absolute value of a turning angle velocity becomes equal to or below a turning angle velocity limit value, which is set according to a predetermined parameter.

A steering device applied to a vehicle of a steer-by-wire system in which a driver performs driving operation is further provided with a reaction force device including a reaction force actuator and a reaction force control device. The reaction force actuator imparts reaction force against a driver's steering operation of a steering. The reaction force control device generates a signal driving the reaction force actuator based on a signal from the turning angle control device.

For example, the turning angle control device varies a turning angle velocity limit value according to “a turning angle equivalent value or a steering angle equivalent value, a vehicle behavior, a vehicle speed, a status of turning and returning as “predetermined parameters.” “Turning angle equivalent value” or “steering angle equivalent value” cited here may be respectively a turning angle or a steering angle itself or may be any value correlated with a turning angle or a steering angle. Indicated values in automatic operation are also included in this. “Turning or returning” is not limited to a driver's driving operation and is interpreted so as to expand to a change in a steering direction by an indicated value in automatic operation.

In a steer-by-wire system in which tires are turned in proportion to a steering angle, a vehicle behavior (specifically, a roll angle) occurring during steering operation is in proportion to a yaw angle, a yaw rate, and a time change rate of a tire slip angle. In the present disclosure, therefore, by limiting a turning angle velocity, a vehicle behavior during turning operation can be suppressed to improve ride comfort. Especially, a roll can be suppressed during turning operation at a high steering angle ratio.

Embodiment

A description will be given to an embodiment of a steering device with reference to the drawings. This steering device is a device that turns tires of a vehicle of a steer-by-wire system in which a steering mechanism and a turning mechanism are mechanically separated from each other. The embodiment assumes a steering device applied to a vehicle of a steer-by-wire system in which a driver performs driving operation. As described in the section of other embodiments, this steering device may be applied to an automatic drive vehicle.

FIG.1shows an overall configuration of a steer-by-wire system90. InFIG.1, only a tire99on one side is shown and an illustration of a tire on the opposite side is omitted. A steering device10includes a reaction force device70and a turning device80.

The reaction force device70includes a reaction force actuator78and a reaction force control device75that generates a signal driving the reaction force actuator78and is connected with a steering91via a reaction force reduction gear79and a steering shaft92. The steering91is a means for inputting a steering angle and a steering wheel is typically used but may be in a shape of steering rod or the like. In the steer-by-wire system90, a driver cannot directly sense reaction force to steering. Consequently, the reaction force actuator78rotates the steering91so as to impart reaction force to steering and gives the driver an appropriate steering feeling.

The turning device80includes a turning actuator88and a turning angle control device85that generates a signal driving the turning actuator88. Rotation of the turning actuator88is transmitted from a turning reduction gear89to a tire99via a pinion gear96, a rack shaft97, a tie rod98, and a knuckle arm985. Specifically, rotary motion of the pinion gear96is converted into linear motion of the rack shaft97and the tire99is turned by the tie rod98provided at both ends of the rack shaft97reciprocatively moving the knuckle arm985.

A torque sensor94detects a driver's steering input applied to the steering shaft92based on torsional displacement of a torsion bar. A detection value T_sns of the torque sensor94is inputted to the reaction force control device75.

With respect to a steering angle of the steering91, for example, the CW direction inFIG.1is defined as positive and the CCW direction is defined as negative according to a rotation direction relative to a neutral position of the steering91. The positive or negative of a turning angle of the tire99is defined in correspondence thereto. An angular velocity is defined with the same sign as an angle. When a driver turns the steering91in the CW direction, a detection value T_sns of the torque sensor94is positive.

When the steering91is turned in the CW direction with the reaction force device70, an output torque of the reaction force device70is positive as well. When a driver holds the steering91while an output torque of the reaction force device70is being exerted in the CW direction, it turns out that a torque is applied in the CCW direction; therefore, a detection value T_sns of the torque sensor94is negative.

The reaction force control device75and the turning angle control device85are configured based on a microcomputer and the like and are provided therein with CPU, ROM, RAM, I/O, a bus line connecting these configuration elements, and the like, none of which is illustrated. Each processing by the reaction force control device75and the turning angle control device85may be software processing by CPU executing a previously stored program or may be hardware processing by a dedicated electronic circuit. The reaction force control device75and the turning angle control device85communicate information with each other via such a vehicle network as CAN communication or a dedicated communication line.

A description will be given to a configuration of the steering device10in the steer-by-wire system90with reference toFIG.2. The reaction force device70includes the reaction force control device75, a steering angle sensor76, and the reaction force actuator78. The steering angle sensor76detects a steering angle θr inputted from the steering91. The reaction force control device75generates a reaction force signal driving the reaction force actuator78based on a signal from the turning angle control device85. The reaction force actuator78imparts reaction force to a driver's steering operation of the steering91.

The turning device80includes the turning angle control device85, a turning angle sensor86, and the turning actuator88. The turning angle control device85calculates a turning angle command value θ*t corresponding to an inputted steering angle θr and generates a signal driving the turning actuator88based on that turning angle command value θ*t. The turning actuator88turns tires99according to an instructed turning angle. The turning angle θt is feedback-controlled using the turning angle sensor86. A reaction force of the reaction force actuator78may be computed by current feedback from the turning actuator88in some cases.

The steering device10basically freely controls a turning angle θt according to a steering angle θr and imparts reaction force to the steering91using a value of a current generated at the turning actuator88during turning or the like. In the present specification, a ratio of a turning angle θt to a steering angle θr is defined as “steering angle ratio.” At a high steering angle ratio, a large turning angle is obtained with a small steering angle. In general, in a low-speed region, a steering angle ratio is increased to reduce an amount of steering and in a high-speed region, a steering angle ratio is reduced for vehicle stability.

In the present embodiment, a vehicle speed V detected by a vehicle speed sensor81is inputted to the reaction force control device75and the turning angle control device85. Further, parameters indicating such a vehicle behavior as roll and yaw are inputted from a vehicle behavior detection device82to the turning angle control device85.

A description will be given to the technical background of the present embodiment. As compared with electric power steering systems in which a steering mechanism and a turning mechanism are mechanically coupled with each other, one of advantages of steer-by-wire systems is that a steering angle ratio can be variably set according to the circumstances. At a high steering angle ratio, a vehicle can be turned up to the maximum turning angle with a small steering angle and a driver can drive without changing the hold of the steering91. As a result, the driver can park a vehicle or make a U-turn with a small steering angle and thus, a steering load is reduced.

A description will be given to that a vehicle behavior may become unstable during turning operation at a high steering angle ratio in some cases with reference toFIG.3andFIG.4.FIG.3shows time changes in turning angle θt, turning angle velocity ωt, and roll angle velocity observed when steering operation is performed from straight-ahead running to U-turn. On the vertical axes, other numeric values than “0” are omitted and the parenthesized units are indicated only for indicating the dimensions of each amount.

When the time is approximated 3.0 seconds, steering operation is started and when the time is approximated 3.8 seconds, the steering operation is terminated. During this period, a turning angle velocity ωt is increased from 0. After the termination of the steering operation, a large roll occurs in the crosswise direction of the vehicle as indicated by the * mark. As described above, during turning operation at a high steering angle ratio, a problem arises. A vehicle behavior with a large roll, yaw, or the like occurs and ride comfort is degraded.

A description will be given to a roll angle φ produced during a left turn and roll moment at that time with reference toFIG.4(Reference: Masato Abe “Automotive Vehicle Dynamics Theory and Application” [Second Edition]). Roll moment is expressed by Formula (1) below. The <1> part on the left side denotes roll stiffness; the <2> part denotes mass eccentricity torque; and the <3> part denotes roll damper. The <4> part on the right side denotes “moment of inertia of roll angle acceleration” and the <5> part denotes “yaw-related moment.”

Roll is influenced by roll stiffness, roll damper, roll angle acceleration to moment of inertia, yaw angle, yaw rate, and a differential value (that is, time change rate) of tire slip angle.

Here, attention should be paid to “yaw-related moment.” At a high steering angle ratio, it is guessed that a degree of increase in turning speed incident to steering speed is increased and yaw angle, yaw rate, and a differential value of tire slip angle have large influence as compared with moment of inertia of roll angle acceleration. In a low-speed region, it is guessed that a yaw angle and a yaw rate are in proportion to a turning angle and a turning angle velocity; therefore, it is assumed that roll is suppressed by limiting a turning angle velocity.

A roll suppression effect is brought about also by adjusting roll stiffness or roll damper instead of yaw angle and yaw rate. JP-5416442B discloses a suspension control device that optimizes response to steering operation from this point of view. However, to vary a parameter of a suspension, four special suspensions are required and increase in cost is incurred. Meanwhile, in a method of limiting a turning angle velocity, control only has to be modified and increase in cost is not incurred.

In the present embodiment, consequently, especially to suppress roll during turning operation at a high steering angle ratio, the turning device80is provided with a block that limits a turning angle velocity of the turning actuator88. A detailed description will be given to a control configuration of the steering device10in the embodiment with reference toFIG.5. “r” is affixed to the symbol of each parameter related to output of the reaction force device70and “t” is affixed to the symbol of each parameter related to output of the turning device80.

It is interpreted that the values of steering angle θr, steering angle velocity ωr, turning angle θr, and like include “equivalent values” obtained by multiplying or dividing a turning angle or an angular velocity of the reaction force actuator78or the turning actuator88by a reduction gear ratio of the reduction gears79,89or the like as appropriate. It is interpreted that “turning torque Tt” directly refers to an output torque of the turning actuator88includes an “equivalent value” of a turning torque command value T*t, a current It passed through the turning actuator88or a current command value I*t, or the like.

The reaction force control device75of the reaction force device70includes a reaction force control unit51, a viscosity control unit52, an inertia control unit53, a return control unit54, a torque deviation calculation unit66, a PID controller67, a current control unit68, and the like. The reaction force control unit51calculates a steering torque command value T*st by increasing or decreasing a turning torque equivalent value Tt depending on a vehicle speed V.

The viscosity control unit52calculates a viscosity command value Tvisc substantially in proportion to a steering angle velocity equivalent value ωr. The “viscosity control unit” may be alternatively designated as “friction control unit.” The inertia control unit53calculates an inertia command value Tinert substantially in proportion to a differential value of a steering angle velocity equivalent value ωr (that is, steering angle acceleration equivalent value). The return control unit54calculates a return command value Tret exerted in a direction in which the steering91is returned to the neutral position based on a steering angle equivalent value θr, a steering angle velocity equivalent value ωr, and a vehicle speed V.

At adders552,553,554, a viscosity command value Tvisc, an inertia command value Tinert, and a return command value Tret are added to a sign inverted value (−T*st) of a steering torque command value T*st in this order. A value obtained after addition by the adder554is outputted as a “target value T**st based on a steering torque command value T*st.”

The torque deviation calculation unit66calculates a torque deviation ΔT of a target value T**st and a detection value T_sns of the torque sensor94. The PID controller67exercises PID control so as to bring a torque deviation ΔT close to 0, that is, such that a detection value T_sns of the torque sensor94follows the target value T**st to compute a current command value *r. The current control unit68controls a current Ir passed through the reaction force actuator78. A steering angle equivalent value θr equivalent to a turning angle of the reaction force actuator78is detected by the steering angle sensor76and is outputted to the return control unit54of the reaction force control device75and the turning angle control device85.

The turning angle control device85of the turning device80includes a steering angle ratio control unit320, a filter33, a turning angle velocity limit value setting unit340, a turning angle velocity limiting unit350, an angle deviation calculation unit36, a PID controller37, a current control unit38, and the like.

The steering angle ratio control unit320computes a steering angle ratio RA that is a ratio of a turning angle θt to a steering angle θr based on a steering angle equivalent value θr and a vehicle speed V and multiplies the steering angle θr by the steering angle ratio RA to calculate a turning angle command value θ*t_0 before limit. A concrete example of steering angle ratio control will be described later with reference toFIG.10andFIG.11. A turning angle command value θ*t_0 before limit is processed by a notch filter avoiding resonance or a filter33comprised of LPF or the like avoiding steep input.

The turning angle velocity limit value setting unit340varies a turning angle velocity limit value ωt_lim according to predetermined parameters. The “predetermined parameters” include a steering angle equivalent value θr or a turning angle equivalent value θt, a vehicle speed V, such a vehicle behavior as yaw and roll, and a status of turning and returning. A concrete example of a turning angle velocity limit value ωt_lim being varied according to each parameter will be described later with reference toFIG.6toFIG.8. Though an illustration of an example of vehicle behavior induction is omitted, real-time control can be exercised by varying a limit value ωt_lim according to a parameter of vehicle behavior.

The turning angle velocity limiting unit350limits a turning angle velocity such that the absolute value of the turning angle velocity becomes equal to a turning angle velocity limit value ωt_lim or below. A concrete example of turning angle command value limit by turning angle velocity limit will be described later with reference toFIG.9. When turning angle velocity limit is applied, as indicated by the bold arrow, a constant of the reaction force control device75may be switched such that reaction force imparted to the reaction force actuator78is increased. As a result, a driver can physically suppress steering speed.

Specifically, at the reaction force control unit51, a constant of reaction force control in proportion to a turning torque equivalent value Tt is switched such that when turning angle velocity limit is applied, reaction force is increased. Or, at the viscosity control unit52and the inertia control unit53, constants of friction control and inertia control basically for building a steering feeling are switched such that when turning angle velocity limit is applied, reaction force is increased. Alternatively, constants may be matched such that reaction force is increased.

The turning angle deviation calculation unit36calculates an angle deviation Δθt of a turning angle command value θ*t and a turning angle feedback value θt. The PID controller37exercises PID control so as to bring an angle deviation Δθt close to 0 and computes a current command value I*t. The current control unit38controls a current It passed through the turning actuator88. A turning angle equivalent value θt equivalent to a turning angle of the turning actuator88is detected by the turning angle sensor86and fed back to the turning angle deviation calculation unit36. A turning torque equivalent value Tt is outputted to the reaction force control device75.

Subsequently, a description will be given to examples of control by each block with reference toFIG.6toFIG.11. With respect to each drawing, a description will be given on assumption that input/output characteristics for parameters are based on a “map” for the sake of convenience but may be based on mathematical calculation.

First, consideration will be given to examples of configurations of the turning angle velocity limit value setting unit340with reference toFIG.6toFIG.8. The turning angle velocity limit value setting unit340in the example shown inFIG.6defines a turning angle velocity limit value ωt_lim relative to the absolute value of a turning angle θt by a steering angle induction map341. For example, in a region where the absolute value of a turning angle θt is ea or below, a limit value ωt_lim is set to a relatively high value ωtH and in a region where the absolute value of a turning angle θt is θβ (>θα) or above, a limit value ωt_lim is set to a relatively low value ωtL. In a region where the absolute value of a turning angle θt is between ea and ep, a limit value ωt_lim is gradually decreased from the high value ωtH to the low value ωtL. As a result, when the absolute value of a turning angle θt is larger than some value, turning with an angular velocity higher than the limit value ωt_lim is prevented.

An input to the steering angle induction map341may be a turning angle detection value θt detected by the turning angle sensor86or may be a turning angle command value θ*t or any other “turning angle equivalent value.” Alternatively, a steering angle θr or a “steering angle equivalent value” before multiplication by a steering angle ratio RA may be taken as an input. Hereafter, every part related to steering angle induction will be similarly interpreted.

By varying a turning angle velocity limit value ωt_lim according to a turning angle equivalent value or a steering angle equivalent value, turning operation can be performed swiftly in a small steering angle range and gently in a large steering angle range. For this reason, influence on a yaw in a small steering angle range where a roll behavior is less prone to occur can be reduced. In the steering angle induction map341shown inFIG.6, two-staged values ωtH, ωtL are taken as a basis and a limit value ωt_lim is linearly varied according to a steering angle. Instead, three or more-staged values may be taken as a basis or a limit value ωt_lim may be curvedly varied according to a steering angle.

In the example shown inFIG.7, a vehicle speed gain map343is used in addition to the same steering angle induction map341as inFIG.6. For example, a vehicle speed gain is 1 in a region equal to vehicle speed Vα or below, is gradually increased from 1 in a region between vehicle speed Vα to vehicle speed Vβ, and is set to INF, a value sufficiently larger than 1, in a region equal to vehicle speed Vβ or above. A multiplier344multiplies a temporary limit value ωt_lim_0 calculated by the steering angle induction map341by a vehicle speed gain to calculate a turning angle velocity limit value ωt_lim. When a vehicle speed gain is a sufficiently large value INF, it is equivalent to that turning angle velocity limit is not subsequently applied.

In a region where a vehicle speed V is high, a steering angle ratio is essentially small; therefore, a delay in turning is increased by additionally applying turning angle velocity limit. Since in a high-speed region, turning operation is not largely performed, turning angle velocity limit is unnecessary. With such a configuration as shown inFIG.7, consequently, a turning angle velocity ωt is limited in a low-speed region and a turning angle velocity ωt is not limited in a high-speed region. As a result, rapid turning operation can be performed in a high-speed region.

The turning angle velocity limit value setting unit340in the example shown inFIG.8includes steering angle induction maps342F,342R for turning and returning, different in steering angle induction characteristics from each other, and a switching device345and varies a turning angle velocity limit value ωt_lim according to a status of turning and returning. A limit value ωt_lim_R of the steering angle induction map342R for returning is set to a smaller value than a limit value ωt_lim_F of the steering angle induction map342F for turning. During turning, energy is accumulated in a spring of a suspension and a vehicle body is prone to more sway during returning than during turning. Therefore, a more stable vehicle behavior is implemented by making a limit value ωt_lim_R for returning smaller than a limit value ωt_lim_F for turning.

The switching device345selects either a limit value ωt_lim_F for turning or a limit value ωt_lim_R for returning according to a signal from the turning/returning determination unit41. For example, the following three methods are present for determining turning and returning: A first method is determination from the signs of a steering angle θr and a steering angle velocity ωr. A second method is determination from the signs of a steering angle velocity ωr and a steering torque in turning and returning during turning (that is, during steering). These methods are used also in electric power steering systems in common.

The third is a method specific to steer-by-wire systems and in this method, attention is paid to “a difference between a reaction force torque Tr outputted from the reaction force actuator78and a detection value T_sns of the torque sensor94” caused by a loss torque of a gear of the reduction gear79. When the steering91is turned by a driver, the absolute value of a detection value T_sns of the torque sensor94is larger than the absolute value of a reaction force torque Tr. When the steering91is returned by the reaction force actuator78, meanwhile, the absolute value of a detection value T_sns of the torque sensor94is smaller than the absolute value of a reaction force torque Tr.

Subsequently, a description will be given to an example of a configuration of the turning angle command value limiting unit350with reference toFIG.9. Delay elements352,355respectively output the previous value of a turning angle command value θ**t after limit to an angular velocity calculator351and an adder354. The angular velocity calculator351calculates a turning angle velocity ωt_0 before limit from a difference between a turning angle command value θ*t_0 before limit and the previous value of a turning angle command value θ**t after limit. An absolute value guard map353guards the absolute value of a turning angle velocity ωt to a turning angle velocity limit value ωt_lim.

The adder354adds a turning angle velocity ωt after limit to the previous value of a turning angle command value θ**t after limit and outputs the current value of the turning angle command value θ**t after limit. A filter may be inserted into a current value output unit to make a change gentle. To mitigate a feeling of wrongness in steering operation in conjunction of turning angle velocity limit, a turning angle velocity limit value ωt_lim may be varied according to a duration for which limit is applied or a steering torque.

A description will be given to an example of a configuration of steering angle ratio control with reference toFIG.10andFIG.11. The turning angle control device85is also capable of limiting a turning angle velocity ωt by varying a steering angle ratio RA according to a steering angle θr. In this case, also with respect to input of steering angle induction, whichever of a steering angle equivalent value or a turning angle equivalent value may be used.

The steering angle ratio control unit320in Example 1 of steering angle ratio control shown inFIG.10includes steering angle induction maps321,322, a vehicle speed gain map325, a multiplier326, an adder327, and a multiplier328. The steering angle induction map321calculates a steering angle induction term RA (θ) corresponding to the absolute value of a steering angle θr. The steering angle induction map322calculates a reference value RA (V)_0 of a vehicle speed induction term corresponding to the absolute value of a steering angle θr. Like the map343inFIG.7, the vehicle speed gain map325calculates a vehicle speed gain corresponding to a vehicle speed V. The multiplier326multiplies a reference value RA (V)_0 of a vehicle speed induction term by a vehicle speed gain to calculate a vehicle speed induction term RA (V).

The adder327adds a steering angle induction term RA (θ) and a vehicle speed induction term RA (V) to calculate a steering angle ratio RA. The multiplier328multiplies a steering angle θr by a steering angle ratio RA to calculate a turning angle command value θ*t_0 before limit.

In Example 1 of steering angle ratio control, a steering angle ratio RA is set small in proximity to the neutral position where the absolute value of a steering angle θr is 0 and a steering angle ratio RA is set large in a region where the absolute value of a steering angle θr is large. In this case, a turning angle velocity ωt is increased in the latter half of turning operation; therefore, turning angle velocity limit at the turning angle velocity limiting unit350is separately required.

The steering angle ratio control unit320in Example 2 of steering angle ratio control shown inFIG.11is different from the configuration inFIG.10only in the characteristics of the steering angle induction maps323,324and is identical in the other respects. In Example 2 of steering angle ratio control, contrary to Example 1 of steering angle ratio control, a steering angle ratio RA is set larger in proximity to the neutral position and a steering angle ratio RA is set small in a region where the absolute value of a steering angle θr is large. In this case, a turning angle velocity ωt is reduced in the latter half of turning operation; therefore, necessity for turning angle velocity limit at the turning angle velocity limiting unit350can be obviated. However, stability is degraded during straight-ahead running.

EFFECTS

According to the present embodiment, as described up to this point, by limiting a turning angle velocity ωt, a vehicle behavior can be suppressed during turning operation and ride comfort can be improved. A roll can be suppressed especially during turning operation at a high steering angle ratio.FIG.12shows a result of a simulation analysis about an influence of roll angle velocity produced by limiting a turning angle velocity ωt. InFIG.12, the broken lines are waveforms before turning angle velocity limit shown inFIG.3and the solid lines are waveforms after turning angle velocity limit.

At time of to after start of steering, a turning angle velocity ωt arrives at a turning angle velocity limit value ωt_lim and application of limit is started. At time of tb, the steering is terminated but a turning angle θt has not arrived at a target value θt_tgt; therefore, output of the turning angle velocity ωt is extended and continued till time of tc. At this time, an integration value S1 of the turning angle velocity ωt reduced by the limit from time of ta to time of tb and an integration value S2 of the turning angle velocity ωt added by the extension from time of tb to time of tc are equal to each other. As a result, at time of tc, a turning angle θt arrives at the target value θt_tgt. A rate of roll produced in the vehicle, occurring in the *-marked parts in the waveform before limit, is reduced by thus limiting a turning angle velocity ωt.

As the result of application of turning angle velocity limit, an angle error can be produced between an expected turning angle in proportion to a primary steering angle θr and an actual turning angle θt and a larger angular deviation can be produced between the neutral position and the end. In the example shown inFIG.12, to compensate an angle error θerr produced at time of tb, steering is further continued till time of tc after a driver's termination of steering operation. As a result, the driver can be given a feeling of wrongness.

Consequently, a description will be given to a working example with reference toFIG.13. In the working example, turning angle velocity limit is applied such that an angle error θerr caused by turning angle velocity limit becomes equal to a predetermined allowable angle error θerr_th or below. The horizontal axis ofFIG.13indicates a “remaining angle θrest” that is the absolute value of a difference between a present turning angle θt or steering angle θr and a critical angle at a corresponding mechanical end. In association with steering, a remaining angle θrest is reduced from the maximum value θN at the neutral position to the value 0 at the end. On the vertical axis ofFIG.13, a turning angle velocity assumed maximum value ωt_max is a turning angle velocity equivalent to the assumed maximum value of a driver's operating speed.

The turning angle control device85reduces a turning angle velocity limit value ωt_lim with reduction in remaining angle θrest according to a remaining angle θrest such that an angle error θerr from the neutral position to the end is constant at an allowable angle error θerr_th. Relation between remaining angle θrest and allowable angle error θerr_th is expressed by Expression (2):

In the expression, a limit index value expressed by “(ωt_max−ωt_lim)/ωt_lim” is more reduced as turning angle limit is laxer in proximity to the neutral position and is more increased as it goes closer to the end and limit becomes stricter. When Expression (2) is organized, Expression (3) is obtained with respect to the turning angle velocity limit value ωt_lim:

When “θrest=θerr_th,” “ωt_lim=ωt_max/2” is derived from Expression (3). That is, an allowable angle error θerr_th is equivalent to a remaining angle θrest obtained when a turning angle velocity limit value ωt_lim is set to (½) of a turning angle velocity assumed maximum value ωt_max.

In this working example, an influence given to a driver by an angle error caused by turning angle velocity limit during turning can be reduced. Therefore, a vehicle behavior suppression effect during turning steering based on turning angle velocity limit and an effect of elimination of a feeling of wrongness due to angular deviation in turning angle can be both favorably achieved. A turning angle velocity limit value ωt_lim need not be calculated by Expression (3) and may be calculated by any other calculation formula, a map, or the like.

Other Embodiment

(a) The steering device10in the above embodiment is assumed to be applied to a vehicle of a steer-by-wire system in which a driver performs driving operation and includes the reaction force device70and the turning device80. This is also the same with vehicles in which manual operation and automatic operation are switchable. When the above embodiment is applied to a vehicle of a steer-by-wire system capable of fully automatic operation, the steering device may be provided only with the turning device80without provision of the reaction force device70.

In this case, the turning device80is capable of exercising the same control as in the above embodiment by inputting a steering angle θr calculated by a control device for automatic operation of the turning device80. Control of switching a constant of the reaction force control device75when turning angle velocity limit is applied as indicated by the bold arrows inFIG.5is unnecessary.

(b) As parameters to be used for setting a turning angle velocity limit value ωt_lim,FIG.6toFIG.8just exemplify a combination of some parameters from among a steering angle equivalent value θr or a turning angle equivalent value θt, a vehicle speed V, a vehicle behavior, and a status of turning and returning. In addition, these parameters can be combined as appropriate. In this case, the influences of individual parameters may be provided with priorities or weighting.

The present disclosure is not limited to such embodiments and can be implemented in various modes without departing from the subject matter thereof.

A control device described in the present disclosure and a technique therefor may be implemented by a dedicated computer provided by configuring a processor and a memory programmed to execute one or more functions crystallized by a computer program. Or, a control device described in the present disclosure and a technique therefor may be implemented by a dedicated computer provided by configuring a processor with one or more dedicated hardware logic circuits. Alternatively, a control device described in the present disclosure and a technique therefor may be implemented by one or more dedicated computers configured of a combination of a processor and a memory programmed to execute one or more functions and a processor configured of one or more hardware logic circuits. A computer program may be stored in a computer-readable non-transitory tangible recording medium as an instruction to be executed by a computer.

The present disclosure has been described in accordance with embodiments but the present disclosure is not limited to those embodiments or structures. The present disclosure also includes various modifications and modifications within an equivalent range. In addition, various combinations and modes and other combinations and modes obtained by adding only one element or more or less element to the combinations and modes are also included in the categories and technical scope of the present disclosure.