Patent Description:
A vehicle steering system in which a motor is controlled so as to change a steering angle in accordance with the target route of a vehicle is well known. In a vehicle steering system of this type, distortion in a vehicle steering mechanism may cause a hysteresis between a steering angle, as a rotation angle of a steering shaft, and a turning angle, as a rotation angle of a vehicle turning wheel.

To date, there has been a vehicle steering system in which an element angle calculation means for correcting a hysteresis between a steering angle and a turning angle is provided and in which in the case where it is determined that a hysteresis has occurred between the steering angle and the turning angle, the element angle calculation means provides the hysteresis to a target steering angle so that the hysteresis is corrected (e.g., refer to Patent Document <NUM>).

In addition, to date, there has been an electric power steering apparatus in which a steering component is removed from a dynamic state quantity such as a rotation speed signal by use of a small-amplitude filtration filter for filtering small-amplitude components, and then only a vibration component, which is a component whose amplitude is smaller than the steering component, is accurately extracted so that the vibration component is reduced (e.g., refer to Patent Document <NUM>).

Other vehicle steering systems are known from <CIT> and from <CIT>.

In the conventional vehicle steering system disclosed in Patent Document <NUM>, a hysteresis included in the output of a steering angle sensor is not taken into consideration. Accordingly, in the case where a hysteresis behind a change in a steering angle exists in a detected steering angle to be outputted from a steering angle sensor, the detected steering angle does not change in a hysteresis section, even when the steering angle changes. In particular, because in the case of turn-back steering, a phenomenon that even when a steering angle changes, the detected steering angle does not change in a hysteresis section conspicuously occurs, the detected steering angle suddenly changes at a timing when the steering angle becomes out of the hysteresis section; therefore, feeling of discontinuity occurs during the steering. Moreover, because the steering angle becomes larger than an intended target steering angle by a hysteresis amount, the traveling route of a vehicle may largely deviate from a target route. Moreover, although provided with a technique for decreasing vibration components in an electric signal, the conventional electric power steering apparatus disclosed in Patent Document <NUM> has no technology for correcting the deviation of a detected steering angle.

The present disclosure discloses a technology for solving the foregoing problems in conventional apparatuses; the objective thereof is to provide a vehicle steering system that reduces the effect of a hysteresis included in the output of a vehicle sensor so that accurate and smooth steering can be performed.

Embodiments of the present invention are given in the dependent claims.

There can be obtained a vehicle steering system that reduces the effect of a hysteresis included in the output of a vehicle sensor so that accurate and smooth steering can be performed.

A vehicle steering system disclosed in the present disclosure includes.

<FIG> is an overall configuration diagram of a vehicle steering system according to each of Embodiments <NUM> and <NUM>. In <FIG>, a vehicle steering system <NUM> is provided with a steering mechanism <NUM> for steering a vehicle, a motor <NUM> that makes the steering mechanism <NUM> rotate through the intermediary of a motor-speed reduction gear <NUM>, a steering angle sensor <NUM> that detects a steering angle, which is a rotation angle of the steering mechanism <NUM>, and then outputs a detected steering angle S1, and a steering-angle controller <NUM>. The steering-angle controller <NUM> is provided with a hysteresis estimation unit <NUM> that outputs an after-mentioned hysteresis estimation value H1 corresponding to a deviation between the detected steering angle S1 and a real steering angle, which is an actual steering angle, and a control unit <NUM> that creates a current command value Ic, based on a target steering angle S0 to be outputted from a higher-hierarchy controller <NUM>, the detected steering angle S1, and the hysteresis estimation value H1. The current command value Ic outputted from the control unit <NUM> is transferred to a motor control apparatus (unillustrated) for controlling the motor <NUM>. Based on the current command value Ic, the motor control apparatus controls an inverter apparatus configured with, for example, semiconductor switching devices in such a way that a motor current follows the current command value Ic. The motor control apparatus including the inverter apparatus may be incorporated in the control unit <NUM>.

The control unit <NUM> provides the current command value Ic, created based on a deviation between the target steering angle S0 and the detected steering angle S1, to the motor control apparatus so as to control a motor current Im, and drives the motor <NUM> in such a way that the steering angle follows the target steering angle S0, so that the steering mechanism <NUM> rotates. The detected steering angle S1 to be outputted from the steering angle sensor <NUM> includes a hysteresis behind a change in a real steering angle.

The steering mechanism <NUM> includes a steering wheel <NUM> to be operated by a vehicle driver, a steering shaft <NUM> coupled with the steering wheel <NUM>, a rack-and-pinion gear <NUM> to be driven by the steering shaft <NUM>, and a rack <NUM> that is driven by the rack-and-pinion gear <NUM> so as to turn a pair of turning wheels <NUM>. The foregoing steering angle sensor <NUM> measures a real steering angle, which is a rotation amount of the steering shaft <NUM>, outputs the detected steering angle S1 corresponding to the real steering angle, and then inputs the detected steering angle S1 to the steering-angle controller <NUM>. The higher-hierarchy controller <NUM> calculates a target route of the vehicle, outputs the target steering angle S0 for making the traveling route of the vehicle follow the calculated target route, and then inputs the target steering angle S0 to the steering-angle controller <NUM>.

Based on the detected steering angle S1 obtained from the steering angle sensor <NUM> and the target steering angle S0, the steering-angle controller <NUM> calculates the current command value Ic and then provides the current command value Ic to the motor control apparatus for controlling the motor <NUM>. The motor current Im is controlled by the motor control apparatus so as to follow the current command value Ic and is supplied to the motor <NUM>. The motor <NUM> generates torque corresponding to the supplied motor current Im. The torque generated by the motor <NUM> is transferred to the steering shaft <NUM> through the intermediary of the motor-speed reduction gear <NUM> and is further transferred to the rack <NUM> through the intermediary of the rack-and-pinion gear <NUM>. The rack <NUM> is driven in the axial direction thereof by the rack-and-pinion gear <NUM> so as to turn the pair of the turning wheels <NUM>.

<FIG> is a characteristic chart representing the relationship between the real steering angle and the detected steering angle obtained by the steering angle sensor; a case where a hysteresis is included in the detected steering angle is represented. In <FIG>, the abscissa denotes the real steering angle [deg], and the ordinate denotes the detected steering angle [deg].

As represented in <FIG>, the detected steering angle includes a hysteresis of a dead-zone width 2B behind a change in the real steering angle. Therefore, when the rotation direction of the real steering angle starts to be reversed, the change in the detected steering angle is delayed due to the hysteresis. In the case where in steering-angle control, it is tried to make the target steering angle S0 and the detected steering angle S1 coincide with each other, the deviation between the target steering angle S0 and the detected steering angle S1 increases during the hysteresis section, because the detected steering angle S1 does not change; when the real steering angle falls out of the hysteresis section, the motor is driven based on the increased deviation and hence the real steering angle may suddenly change. Moreover, in a steady state, because the detected steering angle S1 has an error of the hysteresis width 2B with respect to the real steering angle, there may be a probability that the traveling route of the vehicle deviates from the target route. However, in the vehicle steering system according to Embodiment <NUM>, as described later, the steering angle does not suddenly change; thus, the traveling route of the vehicle does not deviate from the target route.

<FIG> is a configuration diagram of a steering-angle controller in the vehicle steering system according to each of Embodiments <NUM> and <NUM>; the configuration of the steering-angle controller <NUM> in which hysteresis compensation is introduced is represented. In <FIG>, the steering-angle controller <NUM> has the hysteresis estimation unit <NUM>, an addition unit <NUM>, a deviation calculation unit <NUM>, and the control unit <NUM>. Based on an input value X1 to be inputted, the hysteresis estimation unit <NUM> calculates and outputs the hysteresis estimation value H1. The addition unit <NUM> adds the detected steering angle S1 detected by the steering angle sensor <NUM> and the hysteresis estimation value H1, and then outputs the addition value, as a compensation detected steering angle S2.

The deviation calculation unit <NUM> subtracts the compensation detected steering angle S2 from the target steering angle S0 inputted from the higher-hierarchy controller <NUM> and then outputs a control deviation ΔS. Based on the inputted control deviation ΔS, the target steering angle S0, and the compensation detected steering angle S2, the control unit <NUM> calculates the current command value Ic and then inputs the current command value Ic to the motor control apparatus for controlling the motor <NUM>. The motor current Im flowing in the motor <NUM> is controlled by the motor control apparatus so as to follow the current command value Ic.

<FIG> is a configuration diagram of a hysteresis estimation unit in the vehicle steering system according to each of Embodiments <NUM> and <NUM>. In <FIG>, the hysteresis estimation unit <NUM> has a steering-angle estimation unit <NUM> and an after-mentioned small-amplitude filtration filter <NUM>. The steering-angle estimation unit <NUM> inputs a steering-angle estimation value S3, calculated based on the input value X1, to the small-amplitude filtration filter <NUM>. The small-amplitude filtration filter <NUM> outputs the hysteresis estimation value H1, based on the steering-angle estimation value S3 inputted from the steering-angle estimation unit <NUM>.

Because having a frequency responsiveness for making the input value X1 approach the real steering angle, the steering-angle estimation unit <NUM> raises the accuracy of hysteresis estimation by the hysteresis estimation unit <NUM>. In Embodiment <NUM>, as the input value X1 to be inputted to the hysteresis estimation unit <NUM>, the target steering angle S0 is utilized. The reason therefor is that in the case where the responsiveness of the steering-angle control is high, it can be assumed that the target steering angle S0 and the real steering angle almost approximate each other. In practice, because the real steering angle follows the target steering angle S0 in a delayed manner, the steering-angle estimation unit <NUM> makes an approximation of the follow-up delay.

Specifically, with regard to the real vehicle, the frequency responsiveness of the steering-angle controller <NUM> is measured so as to obtain the inherent frequency of the steering-angle controller <NUM>; then, the target steering angle S0, as the input value X1, is processed through a lowpass filter having the inherent frequency, so that there is created the steering-angle estimation value S3 having a waveform approximate to the waveform of the real steering angle. Through the result of measurement in the real vehicle, the applicant ascertained that the delay of the steering-angle control is approximately from <NUM> [Hz] to <NUM> [Hz]. Thus, the steering control apparatus according to Embodiment <NUM> is configured in such a way that the steering angle is estimated by use of a lowpass filter having an inherent frequency of approximately from <NUM> [Hz] to <NUM> [Hz], as the steering-angle estimation unit <NUM>. The steering-angle estimation value S3 to be outputted from the steering-angle estimation unit <NUM> is for estimating nothing but the hysteresis of the sensor; therefore, it is not required that the steering-angle estimation value S3 is created with a high estimation accuracy to the extent that the steering-angle estimation value S3 completely coincide with the real steering angle.

Next, the small-amplitude filtration filter <NUM> will be explained. <FIG> is a configuration diagram of a small-amplitude filtration filter in the vehicle steering system according to each of Embodiments <NUM> and <NUM> and in a vehicle steering system according to each of Embodiments <NUM> and <NUM>. A small-amplitude filtration filter is utilized also in foregoing Patent Document <NUM>; however, the application thereof is to extract vibration components in control of an electric power steering apparatus. In Embodiment <NUM> according to the present disclosure, the small-amplitude filtration filter <NUM> is utilized for extracting a hysteresis-related signal from a detected steering angle detected by the steering angle sensor in the steering-angle control.

In <FIG>, the small-amplitude filtration filter <NUM> includes a hysteresis filter <NUM> for performing hysteresis-function processing and a subtractor <NUM>. The hysteresis filter <NUM> applies hysteresis-function processing to an input value X2 and then outputs the processed value, as an output value Z. The hysteresis for applying the hysteresis-function processing to the input value X2 is set in such a way as to have a sensor hysteresis characteristic including a width and a history that are preliminarily measured in the real vehicle.

In Embodiment <NUM>, it is assumed that the hysteresis of the vehicle has the hysteresis characteristic represented in foregoing <FIG>; therefore, it is desirable that a hysteresis characteristic the same as that in <FIG> is provided also to the hysteresis function of the hysteresis filter <NUM>. The subtractor <NUM> outputs, as the hysteresis estimation value H1, a value obtained by subtracting the output value Z of the hysteresis filter <NUM> from the input value X2 of the hysteresis filter <NUM>. In Embodiment <NUM>, as the input value X2 to be inputted to the small-amplitude filtration filter <NUM>, the steering-angle estimation value S3 outputted from the steering-angle estimation unit <NUM> is utilized.

Next, the control unit <NUM> will be explained. <FIG> is a configuration diagram of a control unit in the vehicle steering system according to each of Embodiments <NUM> and <NUM>. In <FIG>, the control unit <NUM> includes a first pseudo-differentiation device <NUM>, a second pseudo-differentiation device <NUM>, a first gain <NUM>, a second gain <NUM>, a third gain <NUM>, a phase compensator <NUM>, an addition unit <NUM>, and a subtraction unit <NUM>. The control unit <NUM> represented in <FIG> basically performs feedback control in such a way as to suppress the foregoing control deviation ΔS.

A target steering angle speed R0 is created from the target steering angle S0 by means of the first pseudo-differentiation device <NUM>; a value obtained by multiplying the target steering angle speed R0 by the first gain <NUM> is inputted to the addition unit <NUM>. A steering-angle-speed command value Rc is created from the control deviation ΔS through processing by the second gain <NUM> and the phase compensator <NUM>; then, the steering-angle-speed command value Rc is inputted to the subtraction unit <NUM>. The second gain <NUM> and the phase compensator <NUM> are designed in such a way as to create the steering-angle-speed command value Rc in which desired stability and trackability to the control deviation ΔS are secured. A detected steering angle speed R1 is created from the compensation detected steering angle S2 through the second pseudo-differentiation device <NUM>; then, the detected steering angle speed R1 is inputted to the subtraction unit <NUM>.

The detected steering angle speed R1 is subtracted from the steering-angle-speed command value Rc by the subtraction unit <NUM>; a value calculated by multiplying the value obtained through the subtraction by the third gain <NUM> is inputted to the addition unit <NUM>. The addition unit <NUM> adds a value obtained by multiplying the foregoing target steering angle speed R0 by the first gain <NUM> and a value obtained by multiplying a value, obtained by subtracting the detected steering angle speed R1 from the steering-angle-speed command value Rc, by the third gain <NUM>, and then outputs the added value, as the current command value Ic.

In order to raise the stability, the control unit <NUM> represented in <FIG> includes a major loop of feedback control utilizing the target steering angle speed R0 obtained from the target steering angle S0 and the steering-angle-speed command value Rc obtained from the control deviation ΔS and a minor loop of feedback control utilizing the detected steering angle speed R1 obtained from the compensation detected steering angle S2 and the steering-angle-speed command value Rc obtained from the control deviation ΔS. In addition, in the control unit <NUM>, there is configured cascade control in which the feedback control utilizing the foregoing minor loop is added to the feedback control utilizing the foregoing major loop.

Because in the control unit <NUM> represented in <FIG>, not the detected steering angle S1 but the compensation detected steering angle S2 to which hysteresis compensation has been provided is utilized, the effect of a hysteresis included in the detected steering angle S1 from the steering angle sensor <NUM> is reduced and hence the steering control can more accurately be performed. Moreover, in order to raise the responsiveness, feed-forward control of the target yaw rate speed YR0 is added.

As described above, Embodiment <NUM> makes it possible that in steering-angle control in a vehicle steering system, the hysteresis in a steering angle sensor is estimated and compensated; thus, smooth steering in which the effect of the hysteresis is reduced can be performed.

In foregoing Embodiment <NUM>, in the steering-angle controller <NUM>, as explained in <FIG>, the hysteresis estimation unit <NUM> creates the hysteresis estimation value H1; the addition unit <NUM> adds the detected steering angle S1 and the hysteresis estimation value H1 so as to create the compensation detected steering angle S2; then, the deviation calculation unit <NUM> subtracts the compensation detected steering angle S2 from the target steering angle S0 so as to create the control deviation ΔS. However, as the configuration in which the control deviation ΔS is created by use of the hysteresis estimation value H1, there exist configurations other than the one represented in <FIG>.

<FIG> is a set of explanatory diagrams representing various kinds of configuration examples in each of which a control deviation is created by use of a hysteresis estimation value, in the vehicle steering system according to each of Embodiments <NUM> and <NUM>. The configuration represented in (a) of <FIG> corresponds to the configuration represented in <FIG>; the hysteresis estimation value H1 is added to the detected steering angle S1 so that the compensation detected steering angle S2 is created; then, the compensation detected steering angle S2 is subtracted from the target steering angle S0 so that the control deviation ΔS is created. The reference numeral <NUM> denotes a subtractor.

It may be allowed that as the configuration represented in (b) of <FIG>, which replaces the configuration represented in (a) of <FIG>, the control deviation ΔS is created by subtracting the hysteresis estimation value H1 from a value obtained by subtracting the detected steering angle S1 from the target steering angle S0; alternatively, it may be allowed that as the configuration represented in (c) of <FIG>, the hysteresis estimation value H1 is subtracted from the target steering angle S0 so that a compensation target steering angle S4 is created and then the detected steering angle S1 is subtracted from the compensation target steering angle S4 so that the control deviation ΔS is obtained. The configuration represented in each of (a), (b), and (c) of <FIG> provides the same effect.

Moreover, in Embodiment <NUM>, the steering-angle estimation unit <NUM> obtains the steering-angle estimation value S3 by use of a transfer function; however, it may be allowed that in order to reduce the calculation load, the steering-angle estimation value S3 is obtained by use of a mere gain. Furthermore, it may be allowed that in order to raise the estimation accuracy of the steering-angle estimation unit <NUM>, nonlinear processing such as saturation processing or dead-zone processing is performed.

Still moreover, as the hysteresis filter <NUM> in the small-amplitude filtration filter <NUM>, a filter having an ideal hysteresis characteristic represented in <FIG> is utilized; it is important to make the characteristic of the hysteresis filter <NUM> coincide with the hysteresis characteristic of the steering angle sensor. For example, in the case where as represented in <FIG>, the hysteresis characteristic of the steering angle sensor has a shape in which in comparison with the hysteresis characteristic in <FIG>, the lower-right and upper-left corner portions of the hysteresis loop are suppressed, a hysteresis filter having a hysteresis characteristic coinciding with that particular hysteresis characteristic is utilized. In addition, in the case where as represented in <FIG>, the hysteresis characteristic of the steering angle sensor has a dead zone in the vicinity of the origin, a hysteresis filter having a characteristic coinciding with that particular characteristic is utilized. As described above, the accuracy of the hysteresis estimation value H1 can be raised by making the hysteresis characteristic of the hysteresis filter <NUM> coincide with the hysteresis characteristic of the steering angle sensor.

In the above explanation, it is assumed that a hysteresis exists in the relationship between the steering angle and the detected steering angle; however, the generating factor of the hysteresis is not particularly limited. For example, even when the generation of the hysteresis is caused by the characteristic of the steering angle sensor itself, by a mechanical backlash of the steering mechanism for making the steering angle sensor rotate, or further by both thereof, the vehicle steering system according to Embodiment <NUM> can be utilized and the same effect can be demonstrated.

Next, a vehicle steering system according to Embodiment <NUM> will be explained. In Embodiment <NUM>, as described above, as the input value X1 to be inputted to the hysteresis estimation unit <NUM>, the target steering angle S0 is utilized. However, in the case where the control responsiveness is low or under the environment in which disturbance such as friction exists, the difference between the target steering angle S0 and the steering angle becomes large; therefore, in the configuration according to Embodiment <NUM>, estimation of the hysteresis may become inaccurate. In Embodiment <NUM>, the vehicle steering system is configured in such a way that estimation of a hysteresis is performed by inputting an input value other than a target steering angle to the hysteresis estimation unit <NUM>. Hereinafter, the vehicle steering system according to Embodiment <NUM> will be explained mainly with regard to the difference from the vehicle steering system according to Embodiment <NUM>. <FIG>, <FIG>, <FIG>, <FIG>, <FIG>, and <FIG> are applied also to the vehicle steering system according to Embodiment <NUM>.

The hysteresis estimation unit <NUM> according to Embodiment <NUM> is configured in such a way that the hysteresis estimation value H1 is calculated by use of the detected steering angle S1, as the input value X1 represented in <FIG>. With this configuration, estimation of the hysteresis cannot be performed in a hysteresis section where the detected steering angle S1 does not move; however, in the section out of the hysteresis section, the detected steering angle S1 moves. Accordingly, in the situation where the fluctuation of the steering angle is sufficiently larger than the hysteresis width, the hysteresis can be estimated and hence the hysteresis estimation value H1 can be obtained.

As a variant example of Embodiment <NUM>, the hysteresis estimation unit <NUM> is configured in such a way that the hysteresis estimation value H1 is calculated by use of an input value based on the yaw rate of a vehicle. Because when a vehicle pivots, a yaw rate occurs, it can be determined that steering has been started, based on the occurrence of the yaw rate. In addition, because the yaw rate is substantially proportional to each of the steering angle and the vehicle speed, the steering angle can be estimated based on the yaw rate and the vehicle speed.

In this case, as the input value X1, an input value based on a value detected by a yaw-rate sensor may be utilized; alternatively, the yaw rate and the hysteresis estimation value H1 may be calculated by use of an input value based on a vehicle condition such as the acceleration, the vehicle speed, or the rotational difference between the right and left wheels.

As an additional variant example of Embodiment <NUM>, the hysteresis estimation value H1 may be calculated by use of a value based on the motor current Im, as the input value X1 to be inputted to the hysteresis estimation unit. In the case where there exists static friction between a road surface and a set of the steering mechanism and the turning wheels and the motor current Im is within a specific value, the static friction is large and hence the steering angle does not move; however, when the motor current Im exceeds the specific value, the steering angle overcomes the static friction and then starts to move. Because it is made possible to determine the occurrence of steering by use of the value of this motor current, as a current threshold value, the estimation of the hysteresis can more accurately be performed.

As described above, the vehicle steering system according to Embodiment <NUM> demonstrates an effect that even in the case where the difference between the target steering angle and the steering angle is large, it is made possible to estimate the hysteresis. In addition, as the input values, either a single kind of signal or two or more signals including a target steering angle may be utilized so as to estimate the hysteresis.

Each of foregoing Embodiments <NUM> and <NUM> is obtained by converting the vehicle steering system, described in one of the items (<NUM>) through (<NUM>) below, into a tangible form.

In each of Embodiments <NUM> and <NUM>, the hysteresis of the detected steering angle S1 in steering-angle control is compensated. In contrast, in Embodiment <NUM>, the hysteresis of a yaw rate in yaw-rate control is compensated. <FIG> is an overall configuration diagram of a vehicle steering system according to Embodiment <NUM>.

In <FIG>, the vehicle steering system according to Embodiment <NUM> performs yaw-rate control, instead of the steering-angle control in the vehicle steering system according to Embodiment <NUM>; based on a detected yaw rate Y1 obtained from a yaw-rate sensor <NUM> and a target yaw rate Y0 created by the higher-hierarchy controller <NUM>, the current command value Ic is created; then, the motor control apparatus is controlled based on this current command value Ic, so that the motor <NUM> is controlled.

In the case where the detected yaw rate Y1 includes a hysteresis, with respect to a yaw rate, that has a characteristic similar to the hysteresis characteristic represented in foregoing <FIG>, <FIG>, or <FIG>, a dead zone due to the hysteresis causes a delay in the detected yaw rate, when the yaw rate of the vehicle changes. In the case where in yaw-rate control, it is tried to make the target yaw rate Y0 and the detected yaw rate Y1 coincide with each other, the deviation between the target yaw rate Y0 and the detected yaw rate Y1 increases during the hysteresis section, because the detected yaw rate Y1 does not change; when the yaw rate falls out of the hysteresis section, the detected yaw rate Y1 is caused to suddenly change. Moreover, in a steady state, because the detected yaw rate Y1 has an error of a width B with respect to the yaw rate, required yaw-rate control cannot be performed. In the vehicle steering system according to Embodiment <NUM>, as described later, the yaw rate does not suddenly change; thus, the traveling route of the vehicle does not deviate from the target route.

In <FIG>, a vehicle steering system <NUM> is provided with the steering mechanism <NUM> for steering a vehicle, the motor <NUM> that makes the steering mechanism <NUM> rotate through the intermediary of the motor-speed reduction gear <NUM>, the yaw-rate sensor <NUM> that detects a yaw rate and then outputs the detected yaw rate Y1, and a yaw rate controller <NUM>. The yaw rate controller <NUM> is provided with a hysteresis estimation unit <NUM> that outputs an after-mentioned hysteresis estimation value H2 corresponding to a deviation between the detected yaw rate Y1 and a real yaw rate, which is an actual yaw rate, and a control unit <NUM> that creates the current command value Ic, based on the target yaw rate Y0 to be outputted from the higher-hierarchy controller <NUM>, the detected yaw rate Y1, and the hysteresis estimation value H2. The current command value Ic outputted from the control unit <NUM> is transferred to a motor control apparatus (unillustrated) for controlling the motor <NUM>. Based on the current command value Ic, the motor control apparatus controls an inverter apparatus configured with, for example, semiconductor switching devices in such a way that a motor current follows the current command value Ic. The motor control apparatus including the inverter apparatus may be incorporated in the control unit <NUM>.

The control unit <NUM> provides the current command value Ic, created based on a deviation between the target yaw rate Y0 and the detected yaw rate Y1, to the motor control apparatus so as to control the motor current Im, and drives the motor <NUM> in such a way that the yaw rate follows the target yaw rate Y0, so that the steering mechanism <NUM> rotates. The detected yaw rate Y1 to be outputted from the yaw-rate sensor <NUM> includes a hysteresis with respect to a real yaw rate.

The steering mechanism <NUM> includes the steering wheel <NUM> to be operated by a vehicle driver, the steering shaft <NUM> coupled with the steering wheel <NUM>, the rack-and-pinion gear <NUM> to be driven by the steering shaft <NUM>, and the rack <NUM> that is driven by the rack-and-pinion gear <NUM> so as to turn a pair of turning wheels <NUM>. The yaw-rate sensor <NUM> measures a real yaw rate of the vehicle, outputs the detected yaw rate Y1 corresponding to the real yaw rate, and inputs the detected yaw rate Y1 to the yaw rate controller <NUM>. The higher-hierarchy controller <NUM> calculates a target route of the vehicle, outputs the target yaw rate Y0 for making the traveling route of the vehicle follow the calculated target route, and then inputs the target yaw rate Y0 to the yaw rate controller <NUM>.

Based on the detected yaw rate Y1 obtained from the yaw-rate sensor <NUM> and the target yaw rate Y0 from the higher-hierarchy controller <NUM>, the yaw rate controller <NUM> calculates the current command value Ic and then provides the current command value Ic to the motor control apparatus (unillustrated) for controlling the motor <NUM>. The motor current Im is controlled by the motor control apparatus so as to follow the current command value Ic and is supplied to the motor <NUM>. The motor <NUM> generates torque corresponding to the supplied motor current Im. The torque generated by the motor <NUM> is transferred to the steering shaft <NUM> through the intermediary of the motor-speed reduction gear <NUM> and is further transferred to the rack <NUM> through the intermediary of the rack-and-pinion gear <NUM>. The rack <NUM> is driven in the axial direction thereof by the rack-and-pinion gear <NUM> so as to turn the pair of the turning wheels <NUM>.

<FIG> is a configuration diagram of a yaw-rate controller in the vehicle steering system according to Embodiment <NUM>; the configuration of the yaw rate controller <NUM> in which hysteresis compensation is introduced is represented. In <FIG>, the yaw rate controller <NUM> has the hysteresis estimation unit <NUM>, the addition unit <NUM>, the deviation calculation unit <NUM>, and the control unit <NUM>. Based on an input value X3 to be inputted, the hysteresis estimation unit <NUM> calculates and outputs the hysteresis estimation value H2. The addition unit <NUM> adds the detected yaw rate Y1 and the hysteresis estimation value H2, and then outputs the addition value, as a compensation detected yaw rate Y2.

The deviation calculation unit <NUM> subtracts the compensation detected yaw rate Y2 from the target yaw rate Y0 and then outputs a control deviation ΔS. Based on the inputted control deviation ΔS, the target yaw rate Y0, and the compensation detected yaw rate Y2, the control unit <NUM> calculates the current command value Ic and then inputs the current command value Ic to the motor control apparatus for controlling the motor <NUM>. The motor current Im flowing in the motor <NUM> is controlled by the motor control apparatus so as to follow the current command value Ic.

<FIG> is a configuration diagram of the hysteresis estimation unit in the vehicle steering system according to Embodiment <NUM>. In <FIG>, the hysteresis estimation unit <NUM> has a yaw-rate estimation unit <NUM> and a small-amplitude filtration filter <NUM>. The yaw-rate estimation unit <NUM> inputs a yaw-rate estimation value Y3, calculated based on the input value X3, to the small-amplitude filtration filter <NUM>. The small-amplitude filtration filter <NUM> outputs the hysteresis estimation value H2, based on the yaw-rate estimation value Y3 inputted from the yaw-rate estimation unit <NUM>.

Because having a frequency responsiveness for making the input value X3 approach the real yaw rate, the yaw-rate estimation unit <NUM> raises the accuracy of hysteresis estimation by the hysteresis estimation unit <NUM>. In Embodiment <NUM>, as the input value X3 to be inputted to the hysteresis estimation unit <NUM>, the target yaw rate Y0 is utilized. The reason therefor is that in the case where the responsiveness of the yaw-rate control is high, it can be assumed that the target yaw rate Y0 and the real yaw rate almost approximate to each other. In practice, because the real yaw rate follows the target yaw rate Y0 in a delayed manner, the yaw-rate estimation unit <NUM> makes an approximation of the follow-up delay.

Specifically, with regard to the real vehicle, the frequency responsiveness of the yaw rate controller <NUM> is measured so as to obtain the inherent frequency of the yaw rate controller <NUM>; then, the target yaw rate Y0, as the input value X3, is processed through a lowpass filter having the inherent frequency, so that there is created the yaw-rate estimation value Y3 having a waveform approximate to the waveform of the real yaw rate. The yaw-rate estimation value Y3 to be outputted from the yaw-rate estimation unit <NUM> is for estimating nothing but the hysteresis of the sensor; therefore, it is not required that the yaw-rate estimation value Y3 is created with a high estimation accuracy to the extent that the yaw-rate estimation value Y3 completely coincide with the real yaw rate. As another example of the input value X3, the detected yaw rate Y1 from the yaw-rate sensor <NUM> may be utilized; alternatively, it may be allowed that the yaw rate is calculated based on a vehicle condition such as the acceleration, the vehicle speed, or the rotational difference between the right and left wheels and then is utilized as the input value.

Next, the control unit <NUM> will be explained. <FIG> is a configuration diagram of a control unit in the vehicle steering system according to each of Embodiments <NUM> and <NUM>.

<FIG> is a configuration diagram of a control unit in the vehicle steering system according to each of Embodiments <NUM> and <NUM>. In <FIG>, the control unit <NUM> includes the first pseudo-differentiation device <NUM>, the second pseudo-differentiation device <NUM>, the first gain <NUM>, the second gain <NUM>, the third gain <NUM>, the phase compensator <NUM>, the addition unit <NUM>, and the subtraction unit <NUM>. The control unit <NUM> represented in <FIG> basically performs feedback control in such a way as to suppress the foregoing control deviation ΔS.

A target yaw rate speed YR0 is created from the target yaw rate Y0 by means of the first pseudo-differentiation device <NUM>; a value obtained by multiplying the target yaw rate speed YR0 by the first gain <NUM> is inputted to the addition unit <NUM>. A yaw-rate-speed command value YRc is created from the control deviation ΔS through processing by the second gain <NUM> and the phase compensator <NUM>; then, the yaw-rate-speed command value YRc is inputted to the subtraction unit <NUM>. The second gain <NUM> and the phase compensator <NUM> are designed in such a way as to create the yaw-rate-speed command value YRc in which desired stability and trackability to the control deviation ΔS are secured. A detected yaw rate speed YR1 is created from the compensation detected yaw rate Y2 through the second pseudo-differentiation device <NUM>; then, the detected yaw rate speed YR1 is inputted to the subtraction unit <NUM>.

The detected yaw rate speed YR1 is subtracted from the yaw-rate-speed command value YRc by the subtraction unit <NUM>; a value calculated by multiplying the value, obtained through the subtraction, by the third gain <NUM> is inputted to the addition unit <NUM>. The addition unit <NUM> adds a value obtained by multiplying the foregoing target yaw rate speed YR0 by the first gain <NUM> and a value obtained by multiplying a value, obtained by subtracting the detected yaw rate speed YR1 from the yaw-rate-speed command value YRc, by the third gain <NUM>, and then outputs the added value, as the current command value Ic.

In order to raise the stability, the control unit <NUM> represented in <FIG> includes a major loop of feedback control utilizing the target yaw rate speed YR0 obtained from the target yaw rate Y0 and the yaw-rate-speed command value YRc obtained from the control deviation ΔS and a minor loop of feedback control utilizing the detected yaw rate speed YR1 obtained from the compensation detected yaw rate Y2 and the yaw-rate-speed command value YRc obtained from the control deviation ΔS. In addition, in the control unit <NUM>, there is configured cascade control in which the feedback control utilizing the foregoing minor loop is added to the feedback control utilizing the foregoing major loop.

Because in the control unit <NUM> represented in <FIG>, not the detected yaw rate Y1 but the compensation detected yaw rate Y2 to which hysteresis compensation has been provided is utilized, the effect of a hysteresis included in the detected yaw rate Y1 from the yaw-rate sensor <NUM> is reduced and hence the steering control can more accurately be performed. Moreover, in order to raise the responsiveness, feed-forward control of the target yaw rate speed YR0 is added.

As described above, Embodiment <NUM> makes it possible that in yaw-rate control in a vehicle steering system, the hysteresis in a yaw-rate sensor is estimated and compensated; thus, smooth steering in which the effect of the hysteresis is reduced can be performed.

In Embodiment <NUM>, in the yaw rate controller <NUM>, as explained in <FIG>, the hysteresis estimation unit <NUM> creates the hysteresis estimation value H2; the addition unit <NUM> adds the detected yaw rate Y1 and the hysteresis estimation value H2 so as to create the compensation detected yaw rate Y2; then, the deviation calculation unit <NUM> subtracts the compensation detected yaw rate Y2 from the target yaw rate Y0 so as to create the control deviation ΔS. However, as the configuration in which the control deviation ΔS is created by use of the hysteresis estimation value H2, there exist configurations other than the one represented in <FIG>.

<FIG> is explanatory diagrams representing various kinds of configuration examples in each of which a control deviation is created by use of a hysteresis estimation value, in the vehicle steering system according to each of Embodiments <NUM> and <NUM>. The configuration represented in (a) of <FIG> corresponds to the configuration represented in <FIG>; the hysteresis estimation value H2 is added to the detected yaw rate Y1 so that the compensation detected yaw rate Y2 is created; then, the compensation detected yaw rate Y2 is subtracted from the target yaw rate Y0 so that the control deviation ΔS is created.

It may be allowed that as the configuration represented in (b) of <FIG>, which replaces the configuration represented in (a) of <FIG>, the control deviation ΔS is created by subtracting the hysteresis estimation value H2 from a value obtained by subtracting the detected yaw rate Y1 from the target yaw rate Y0; alternatively, it may be allowed that as the configuration represented in (c) of <FIG>, the hysteresis estimation value H2 is subtracted from the target yaw rate Y0 so that a compensation target yaw rate Y4 is created and then the detected yaw rate Y1 is subtracted from the compensation target yaw rate Y4 so that the control deviation ΔS is obtained. The configuration represented in each of (a), (b), and (c) of <FIG> provides the same effect.

Moreover, in Embodiment <NUM>, the yaw-rate estimation unit <NUM> obtains the yaw-rate estimation value Y3 by use of a transfer function; however, it may be allowed that in order to reduce the calculation load, the yaw-rate estimation value Y3 is obtained by use of a mere gain. Furthermore, it may be allowed that in order to raise the estimation accuracy of the yaw-rate estimation unit <NUM>, nonlinear processing such as saturation processing or dead-zone processing is performed.

Still moreover, as the hysteresis filter <NUM> in the small-amplitude filtration filter <NUM>, a filter having an ideal hysteresis characteristic represented in <FIG> is utilized; it is important to make the characteristic of the hysteresis filter <NUM> coincide with the hysteresis characteristic of the steering angle sensor. For example, in the case where as represented in <FIG>, the hysteresis characteristic of the steering angle sensor has a shape in which in comparison with the hysteresis characteristic in <FIG>, the lower-right and upper-left corner portions of the hysteresis loop are suppressed, a hysteresis filter having a hysteresis characteristic coinciding with that particular hysteresis characteristic is utilized.

In addition, in the case where as represented in <FIG>, the hysteresis characteristic of the yaw-rate sensor has a dead zone in the vicinity of the origin, a hysteresis filter having a characteristic coinciding with that particular characteristic is utilized. As described above, the accuracy of the hysteresis estimation value H2 can be raised by making the hysteresis characteristic of the hysteresis filter <NUM> coincide with the hysteresis characteristic of the yaw-rate sensor.

In the above explanation, it is assumed that a hysteresis exists in the relationship between the yaw rate and the detected yaw rate; however, the generating factor of the hysteresis is not particularly limited. For example, even when the generation of the hysteresis is caused by the characteristic of the yaw-rate sensor itself, by a mechanical backlash of the steering mechanism for making the yaw-rate sensor rotate, or further by both thereof, the vehicle steering system according to Embodiment <NUM> can be utilized and the same effect can be demonstrated.

As described above, the vehicle steering system according to Embodiment <NUM> makes it possible that in yaw-rate control, the hysteresis in a yaw-rate sensor is estimated and compensated; thus, smooth steering in which the effect of the hysteresis is reduced can be performed.

Next, a vehicle steering system according to Embodiment <NUM> will be explained. In Embodiment <NUM>, as described above, as the input value X3 to be inputted to the hysteresis estimation unit <NUM>, the target yaw rate Y0 is utilized. However, in the case where the control responsiveness is low or under the environment in which disturbance such as friction exists, the difference between the target yaw rate Y0 and the yaw rate becomes large; therefore, in the configuration according to Embodiment <NUM>, estimation of the hysteresis may become inaccurate. In Embodiment <NUM>, the vehicle steering system is configured in such a way that estimation of a hysteresis is performed by inputting an input value other than a target yaw rate to the hysteresis estimation unit <NUM>. Hereinafter, the vehicle steering system according to Embodiment <NUM> will be explained mainly with regard to the difference from the vehicle steering system according to Embodiment <NUM>. <FIG>, <FIG>, <FIG>, <FIG>, and <FIG> are applied also to the vehicle steering system according to Embodiment <NUM>.

The hysteresis estimation unit <NUM> according to Embodiment <NUM> is configured in such a way that the hysteresis estimation value H2 is calculated by use of the detected yaw rate Y1, as the input value X3 represented in <FIG>. With this configuration, estimation of the hysteresis cannot be performed in a hysteresis section where the detected yaw rate Y1 does not move; however, in the section out of the hysteresis section, the detected yaw rate Y1 moves. Accordingly, in the situation where the fluctuation of the steering angle is sufficiently larger than the hysteresis width, the hysteresis can be estimated and hence the hysteresis estimation value H2 can be obtained.

As a variant example of Embodiment <NUM>, the hysteresis estimation value H2 may be calculated by use of a value based on the motor current Im, as the input value X3 to be inputted to the hysteresis estimation unit <NUM>. In the case where there exists static friction between a road surface and a set of the steering mechanism and the turning wheels and the motor current Im is within a specific value, the static friction is large and hence the steering angle does not move; however, when the motor current Im exceeds the specific value, the steering angle overcomes the static friction and then starts to move. Because it is made possible to determine the occurrence of steering by use of the value of this motor current, as a current threshold value, the estimation of the hysteresis can more accurately be performed.

As described above, the vehicle steering system according to Embodiment <NUM> demonstrated an effect that even in the case where the difference between the target yaw rate and the yaw rate is large, it is made possible to estimate the hysteresis. In addition, as the input values, either a single kind of signal or two or more signals including a target yaw rate may be utilized so as to estimate the hysteresis.

Although the present application is described above in terms of various exemplary embodiments and implementations, it should be understood that the various features, aspects and functions described in one or more of the individual embodiments are not limited in their applicability to the particular embodiment with which they are described, but instead can be applied, alone or in various combinations to one or more of the embodiments as defined by the appended claims.

Claim 1:
A vehicle steering system comprising:
a steering mechanism (<NUM>) that drives a vehicle;
a motor (<NUM>) that makes the steering mechanism (<NUM>) rotate;
a steering angle sensor (<NUM>) that detects a steering angle, which is a rotation angle of the steering mechanism (<NUM>), and outputs a detected steering angle having a hysteresis with respect to the steering angle;
a hysteresis estimation unit (<NUM>) that estimates a hysteresis estimation value corresponding to a difference between the steering angle and the detected steering angle; and
a control unit (<NUM>) that controls the motor (<NUM>) in such a way as to be equal to a difference between the steering angle and a target steering angle, which is a target value of the steering angle, based on the target steering angle, the detected steering angle, and the hysteresis estimation value;
characterized in that the hysteresis estimation unit (<NUM>) is configured in such a way as to calculate the hysteresis estimation value, based on a yaw rate of the vehicle.