JP2722855B2 - Rear wheel steering device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a vehicle rear wheel steering system.

[0002]

2. Description of the Related Art A rear wheel steering device also steers a rear wheel when a front wheel is steered by a steering wheel. The aim is to improve the kinetic performance of a vehicle. And
Conventionally, the method of giving the rear wheel steering angle is disclosed in Japanese Patent Laid-Open No. 60-1612.
As disclosed in Japanese Patent Application Publication No. 55-55, there is an apparatus in which a yaw rate is fed back, a rear wheel steering angle is determined based on the feedback, and the rear wheels are steered to follow the steering angle. In this publication, particularly, under such a rear wheel steering system, a response delay of a feedback type rear wheel steering system causes signal attenuation in a high steering frequency range, and as a result, a desired vehicle motion performance cannot be obtained. In order to solve the problem, the post-computation wheel steering angle is corrected by adding the first derivative of the yaw rate to be fed back.

[0003]

However, such a conventional rear wheel steering apparatus determines a target rear wheel steering angle in consideration of a phase delay of the yaw rate generation. Since the characteristics (response delay) are not taken into account, the following problems occur.

That is, FIG. 4 shows the relationship between the yaw rate gain (gain of the yaw rate with respect to the steering wheel steering angle) and the phase delay of the yaw rate when the steering wheel is steered by ± 30 degrees while traveling at 120 km / h. Where α ₁ and β ₁ are yaw rate gain characteristics and phase delay characteristics of a front two-wheel steered vehicle that does not steer the rear wheels,
α ₂ and β ₂ are a yaw rate gain characteristic and a phase delay characteristic of a four-wheel steering vehicle that steers rear wheels by the above-described conventional device.
As shown by α ₂ and β ₂ , in the conventional device, the yaw rate 1
By correcting the rear wheel steering angle according to the differential value, the resonance point of the yaw rate gain becomes a high steering frequency range, and the phase delay can be considerably reduced. Actually, however, the rear wheel steering mechanism has a response delay due to friction or the like, and it is difficult to provide the rear wheel steering angle as calculated. As is apparent from the experimental results γ ₁ and ε ₁ shown in FIG. With respect to the characteristics γ ₂ and ε _{2 having} no characteristics, a decrease in gain and a phase lag occur from a low steering frequency range. Further, the response delay of the rear wheel steering mechanism may not be a first-order delay but a second-order or higher-order delay. In any case, without considering the operation dynamic characteristics of such a rear wheel steering mechanism at all,
With the above-described conventional device that only corrects the rear wheel steering angle in accordance with the first-order differential value of the yaw rate, it is not possible to achieve the intended kinetic performance of the vehicle.

In order to solve this problem, it is conceivable to reduce the feedback gain or increase the responsiveness of the rear wheel steering mechanism. However, the former countermeasure is not sufficient to improve the disturbance resistance of the vehicle motion, which is the original purpose, and the latter countermeasure requires the actuator, which is the main part of the rear wheel steering mechanism, to be extremely expensive, large, and large. Almost impossible due to the space available.

In the present invention, the rear wheel steering angle is calculated in advance by determining the target rear wheel steering angle in consideration of the response characteristics of the rear wheel steering mechanism, without relying on the above measures. It is an object of the present invention to achieve the kinetic characteristics of a vehicle.

[0007]

SUMMARY OF THE INVENTION For this purpose, the present invention provides a rear wheel steering mechanism which receives a rear wheel steering angle calculation value δ _rm calculated by a rear wheel steering angle calculation means and responds thereto. ,
The rear wheel steering angle actual value [delta] _r is, the feedforward term based on the front wheel steering angle [delta] _f, yaw rate (d / dt) coincides with the sum value of the feedback term based on _{φ δ r = F (S)} δ f + B (S) (d / dt) φ F (S): a feedforward transfer function B (S): a feedback transfer function angular computation means the rear wheel steering angle calculated value [delta] _rm, the transfer function G according to the response characteristic of the rear wheel steering mechanism (S) = δ _r / δ
By adding the _{rm δ rm = {F (S} ) / G (S)} δ f + {B (S) / G (S)} (d / dt) φ or _{δ rm = F (S) δ} f + { B (S) / G (S )} (d / dt) calculated by φ becomes equation is a wheel steering angle calculated value [delta] _rm after finding in this equation that configured to command the rear wheel steering mechanism.

[0008]

The rear wheel steering angle calculating means also uses a transfer function representing a response characteristic of the rear wheel steering mechanism to obtain δ _rm = ｛F (S) / G (S)｝ δ.
_f + ｛B (S) / G (S)｝ (d / dt) φ or _δrm = F (S) δ
_The rear wheel steering angle calculation value _δrm is obtained from _f + ｛B (S) / G (S)｝ (d / dt) φ, and this is commanded to the rear wheel steering mechanism. The mechanism is to steer the rear wheels by receiving this command, the actual rear wheel steering angle [delta] _r by the response delay of the rear-wheel steering mechanism is δ _r = F (S) δ
_f + B (S) (d / dt) φ As intended, it is possible to achieve the intended vehicle dynamics regardless of the response delay of the rear wheel steering mechanism.

Further, since this effect can be obtained without reducing the feedback gain or reducing the response delay of the rear wheel steering mechanism, the adverse effect of these measures, that is, the improvement of the disturbance resistance of the vehicle motion is not achieved. There is no adverse effect that the rear wheel steering actuator becomes sufficient or expensive and large.

[0010]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 shows a system example of a rear wheel steering device of the present invention, wherein 1L and 1R are left and right rear wheels to be steered by the device, respectively. The steering of these rear wheels is performed by a rack and pinion type steering gear 2, and the pinion of the gear is drive-coupled to a motor 4 via a hypoid gear 3. The drive of the motor 4 is controlled by a controller 5, which detects a signal from a sensor 6 for detecting a steering wheel steering angle θ, a signal from a sensor 7 for detecting a vehicle speed V, and a yaw rate (d / dt) φ. Sensor 8
And a signal from a rear wheel steering angle sensor 9 for detecting a pinion rotation angle (rear wheel steering angle δ _r ) from a neutral position of the steering gear 2.

The controller 5 calculates a rear wheel steering angle calculation value δ _rm by a calculation described later based on the input information,
10 supplies a current i corresponding to the calculated value to the motor 4. The motor 4 is driven by the supply current i to steer the rear wheels 1L and 1R via the hypoid gear 3 and the steering gear 2, but the actual rear wheel steering angle δ with respect to the rear wheel steering angle calculation value δ _rm
r is gamma ₁ in many cases 6, occur with a substantially first-order lag as indicated by epsilon _1, gamma ₃ in the figure, first-order lag indicated by _{ε 3 δ r (S) /} δ rm (S ) = K _O / (1 + τ _O S).
By the way, in most cases, τ _O is about 0.05 sec and K _O is 0.7
It is about.

In consideration of such a response delay of the rear wheel steering mechanism, in this embodiment, the rear wheel steering angle calculation value δ _rm is obtained as follows. However, basically, the rear wheel steering angle follows the method given by the sum of the feed forward term according to the front wheel steering angle and the feedback term according to the yaw rate, and the feed forward term alone is sufficient to improve steering stability. Utilize past know-how that can be expected. in this case,
Although the characteristics are improved as shown by α ₃ and β ₃ in FIG. 4, the intended vehicle motion characteristics cannot be achieved as it is due to the response delay of the rear wheel steering mechanism.

Therefore, in this embodiment, the rear wheel steering angle calculation value _δrm is obtained in consideration of the above-mentioned response delay in advance, and the basic concept will be described below. First, when the motion of a vehicle is approximated by a two-wheel model, M ｛(d ² / dt ² ) y + V (d / dt) φ｝ = F ₁ + F ₂ ---- (1) I (d ² / dt ² ) φ = aF ₁ + bF ₂ ---- (2) where F ₁ = C ₁ ｛δ _f -[(d / dt) y + a (d / dt) φ] / V｝ ---- (3 ) F ₂ = C ₂ ｛δ _r -[(d / dt) y-b (d / dt) φ] / V｝ ---- (4) M: Vehicle weight I: Yaw inertia moment of vehicle V: Vehicle speed C _1: front wheel cornering power (two-wheel min) C _2: rear wheel cornering power (two-wheel min) [delta] _f: front wheel steering angle [delta] _r: rear wheel steering angle a: the front axle - the distance between the centers of gravity b: rear wheel - the center of gravity It is known that the distance (d / dt) y: the vehicle lateral speed (d / dt) φ: the yaw rate.

Here, equations (3) and (4) are substituted into equations (1) and (2),
Furthermore, in order to make the yaw rate (d / dt) φ always in the form of a first-order delay (for example, to make it easier to drive stably due to a lane change during high-speed driving, etc.), (d / dt) φ = ｛H _O / (1 + τS)｝ δ _f ---- (5) where H _O : steady-state yaw rate gain τ: time constant according to vehicle speed and vehicle specifications, and solving for rear wheel steering angle δ _r , (5) the wheel steering angle [delta] _r after to achieve aim of _{δ r = {(T 1'S} + T 2'S 2) / (1 + τ a'S + τ B'S 2)} δ f = F (S) δ _{f ----} (6) (where T ₁ ′, T ₂ ′, τ _A ′, τ _B ′ are constants according to the vehicle speed and the vehicle specifications).

However, an actual vehicle cannot be approximated by the two-wheel model, and often deviates from the equations (1) to (4). Therefore, the target yaw rate (d / dt) φ _O is set as (d / dt) φ _O = ｛H _O / (1 + τS)｝ δ _f ---- (5 ′) in correspondence with the equation (5), Taking into account the control according to the deviation from the yaw rate (d / dt) φ (the feedback constant is defined as G), δ _r = F (S) δ _f + G (d / dt) φ _O − (d / dt) and providing a rear wheel steering angle [delta] _r in φ} ---- (7). In this case, even if the actual vehicle cannot be fully approximated by the two-wheel model, the aim of Expression (5) can be achieved.

In reality, as described above with reference to FIG. 6, there is a limit to the response of the rear wheel steering mechanism (particularly, the actuator), and it is impossible to provide the rear wheel steering angle according to the command value by the equation (7). Therefore, as a result, the above-mentioned aim of Expression (5) cannot be actually achieved.

In this embodiment, as shown by γ ₃ and ε ₃ in FIG. 6, the response characteristic of the rear wheel steering mechanism has a time constant τ _O = 0.05 sec primary delay G (S), that is, G (S) = K _{O / (1 + τ O S)} ---- per case approximated by (8), determine the rear wheel steering angle calculated value [delta] _rm considering this advance, the rear wheel steering angle by commanding the rear wheel steering mechanism _Assuming that the actual value δr is obtained by equation (7),
As a result, the aim of the equation (5) can be achieved.

In the consideration, the left side δ _{r of the} equation (7)
_Is replaced by _δrm, and the reciprocal 1 / G (S) = (1
+ Τ _O S) / K _O is multiplied to obtain an arithmetic expression for the rear wheel steering angle command value δ _rm , which is expressed as follows. _{δ rm = F (S) (} 1 + τ O S) δ f + (G / K O) {(d / dt) φ O (1 + τ O S) - (d / dt) φ (1 + τ O S)} ≒ { (K + T ₁ S + T ₂ S) / (1 + τ _A S)｝ δ _f + (G / K ₀ ) ｛[(1 + τ _O S) / (1 + τS)] H _O δ _f − [(D / dt) φ + τ _O (d ² / dt ² ) φ]｝ Here, considering τ _O考 え る τ, the above equation becomes δ _rm = ｛(K + T ₁ S + T ₂ S) / ( _{1+ τ a S)} δ f} + (G / K O) {H O δ f - become _{(d / dt) φ -τ O} (d 2 / dt 2) φ} ---- (9).

In FIG. 1, the controller 5 obtains a rear wheel steering angle calculation value δ _rm by the above equation (9), supplies a current i corresponding _thereto to the motor 4 via the drive circuit 10, and outputs the rear wheels 1L, 1R
To steer. This relationship is schematically shown in FIG. 2, and the vehicle produces a desired yaw rate (d / dt) φ at the front and rear wheel steering angles δ _f and δ _r . However, the rear wheel steering mechanism, particularly the motor 4 (actuator), has a response delay due to friction and the like. Since the equation (9) is determined in consideration of this as described above, the actual rear wheel steering angle is (7) a value determined by the equation [delta] _r
As a result, the aim of the equation (5) can be achieved, and the resonance of the yaw rate and the suppression of the phase delay can be realized as is clear from the simulation results indicated by α ₄ and β ₄ in FIG.

Further, since this object is achieved without reducing the feedback gain G or making the rear wheel steering mechanism (motor 4) of high response, the disturbance resistance of the vehicle motion is sacrificed. Also, the motor 4 is not expensive and large.

[0021] Figure 3 shows a control program the controller 5 in FIG. 1 calculates and outputs a rear wheel steering angle command value [delta] _rm, the program is executed every ΔT time (e.g. 0.005 sec). In step 30, the steering wheel steering angle θ, the vehicle speed V, and the yaw rate (d / dt) φ are read, and in step 31, the front wheel steering angle δ _f =
θ / N is calculated, and in steps 32 and 33, the front wheel steering angular velocity (d /
dt) δ _f , front wheel steering angular acceleration (d ² / dt ² ) δ _f and yaw angular acceleration (d ² / dt ² ) φ are calculated. Next Step 34
In steps (36) to (36), the first term on the right side of equation (9), that is, the feedforward term _δrFF, is obtained. Then, in step 37, the feedback gain G, which becomes higher as shown in FIG. In the next step 38, the second right side of equation (9)
Section, i.e. determine the feedback [delta] _rFB section obtains a rear wheel steering angle calculated value [delta] _rm at step 39, and outputs in step 40.

The feedback gain G may be set at step 41 in FIG. 5 instead of being set at step 37 in FIG. That is, in the present example, the feedback gain G is changed according to the steering wheel steering angle θ instead of the vehicle speed V, and the gain G decreases as the θ increases. In this case, disturbance resistance during straight running near θ = 0 is improved. When turning while increasing θ, the rear wheel steering angle is controlled by a sufficiently tuned feedforward term.

[0023]

As described above, the rear wheel steering apparatus of the present invention calculates the target rear wheel steering angle in consideration of the response characteristics of the rear wheel steering mechanism in advance when calculating the target rear wheel steering angle. Since the rear wheel steering angle calculation value is instructed to the rear wheel steering mechanism, the rear wheel steering angle calculation value may deviate from the intended value even due to a response delay of the rear wheel steering mechanism receiving the command of the rear wheel steering angle calculation value. Therefore, the target vehicle motion performance can be reliably achieved. Since this effect can be achieved without reducing the feedback gain or increasing the response of the rear wheel steering mechanism, the disturbance resistance is not sufficiently improved, and the rear wheel steering actuator is expensive and large. There is no adverse effect.

[Brief description of the drawings]

FIG. 1 is a hardware configuration diagram showing an example of a rear wheel steering device of the present invention.

FIG. 2 is a schematic diagram of rear wheel steering control in the same example.

FIG. 3 is a flowchart showing a rear wheel steering angle calculation control program of the controller in the example.

FIG. 4 is a diagram showing an operation characteristic of the example in comparison with an operation characteristic that has conventionally occurred.

FIG. 5 is a flowchart similar to FIG. 3 showing another example of the present invention.

FIG. 6 is a diagram showing a response delay characteristic of a rear wheel steering mechanism.

[Explanation of symbols]

 1L Left rear wheel 1R Right rear wheel 2 Steering gear 3 Hypoid gear 4 Motor 5 Controller 6 Steering angle sensor 7 Vehicle speed sensor 8 Yaw rate sensor 9 Rear wheel steering angle sensor 10 Drive circuit

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code Agency reference number FI Technical display location B62D 137: 00

Claims (1)

(57) [Claims] The rear wheel by wheel steering mechanism after 1. A rear-wheel steering angle calculating means responsive to being commanded wheel steering angle calculated value [delta] _rm After calculating, the rear wheel steering angle actual value [delta] _r is, the front wheel a feed-forward term based on the steering angle [delta] _f, yaw rate (d / dt) coincides with the sum value of the feedback term based on _{φ δ r = F (S)} δ f + B (S) (d / dt) φ F (S): a feedforward transfer function B (S): a feedback transfer function In a rear-wheel steering apparatus aiming at steering, the rear-wheel steering angle calculating means includes a rear-wheel steering angle calculation value δ. _rm the transfer function G according to the response characteristic of the rear wheel steering mechanism (S) = δ _r / δ
By adding the _{rm δ rm = {F (S} ) / G (S)} δ f + {B (S) / G (S)} (d / dt) φ or _{δ rm = F (S) δ} f + { B (S) / G (S)｝ (d / dt) φ The rear wheel steering angle calculation value δ _rm obtained by this equation is commanded to the rear wheel steering mechanism. Rear wheel steering device.