JPH04325381A - Rear wheel steering device - Google Patents

Abstract

PURPOSE:To realize the vehicle maneuverability as desired by matching the rear wheel actual steering angle with the target value regardless of the response delay of a rear wheel steering system. CONSTITUTION:A controller 5 calculates the rear wheel steering angle calculation value deltarm in consideration of the response delay characteristic of a rear wheel steering system using a motor 4 as a main element in advance. When the motor 4 receives the command value deltarm and steers rear wheels 1L, 1R, the rear wheel actual steering angle deltar becomes as desired due to the above response delay. This can be realized without reducing the feedback gain or increasing the responsiveness of the rear wheel steering system, thus the disturbance resistance is not sacrificed, and the rear wheel steering system is prevented from being made expensive and large-scaled.

Description

[Detailed description of the invention]

0001

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

0002

2. Description of the Related Art A rear wheel steering device also steers the rear wheels when the front wheels are steered by a steering wheel, and its purpose is to improve the driving performance of a vehicle. and,
Conventionally, the method of giving the rear wheel steering angle
As described in Japanese Patent No. 55, there is a system in which the yaw rate is fed back, the rear wheel steering angle is determined based on this, and the rear wheels are steered to follow the yaw rate. In particular, this publication states that under such a rear wheel steering system, the response delay of the feedback type rear wheel steering system causes signal attenuation in the high steering frequency range, and as a result, the desired vehicle dynamic performance cannot be obtained. To solve the problem, the calculated rear wheel steering angle is corrected by adding the first-order differential value of the yaw rate to be fed back.

0003

[Problems to be Solved by the Invention] However, such conventional rear wheel steering devices determine the target rear wheel steering angle by taking into account the phase delay in the generation of yaw rate, but the actual operation of the rear wheel steering mechanism Since the characteristics (response delay) are not considered, the following problems occur.

That is, FIG. 4 shows the relationship between yaw rate gain (yaw rate generation gain with respect to steering wheel steering angle) and yaw rate phase delay with respect to steering frequency when the steering wheel is steered by ±30 degrees while traveling at 120 km/h. In the figure, α1 and β1 are the yaw rate gain characteristics and phase delay characteristics of a front two-wheel steered vehicle that does not steer the rear wheels, and α2 and β2 are the yaw rate gain characteristics and phase delay characteristics of a four-wheel steered vehicle that steers the rear wheels using the conventional device described above. This is a phase delay characteristic. As shown by α2, β2, in the above conventional device, the resonance point of the yaw rate gain becomes a high steering frequency region by correcting the rear wheel steering angle according to the first-order differential value of the yaw rate, and the phase delay can also be considerably reduced. However, in reality, the rear wheel steering mechanism has a response delay due to friction, etc., and it is difficult to give the rear wheel steering angle as calculated, and the experimental results γ1, ε1 shown in Figure 6
As is clear from the above, the characteristics γ2 and ε2 without response delay
In contrast, a decrease in gain and a phase delay occur from the low steering frequency range. Further, the response delay of the rear wheel steering mechanism is not a first-order delay, but may include a second-order or higher-order delay. In any case, with the conventional device that only corrects the rear wheel steering angle according to the first differential value of the yaw rate without taking into account the operating dynamic characteristics of the rear wheel steering mechanism, it is difficult to achieve the desired vehicle movement. performance cannot be achieved.

[0005] To solve this problem, it is possible to reduce the feedback gain or increase the responsiveness of the rear wheel steering mechanism. However, with the former measure, the original purpose of improving resistance to disturbances in vehicle motion is insufficient, and with the latter measure, the actuator, which is the main part of the rear wheel steering mechanism, becomes extremely expensive and large, and the This is almost impossible due to the available installation space.

The present invention calculates the rear wheel steering angle by calculating the target rear wheel steering angle in advance by considering the response characteristics of the rear wheel steering mechanism, thereby achieving the desired rear wheel steering angle without relying on the above measures. The purpose is to make it possible to achieve the dynamic characteristics of the vehicle.

[0007]

[Means for Solving the Problems] For this purpose, the present invention calculates the rear wheel steering angle δr as the sum of a feedforward term based on the front wheel steering angle δf and a feedback term based on the yaw rate (d/dt)φ. δr = F(S) δf + B(S) (d/dt
)φF(S): Feedforward transfer function B(S)
: In a rear wheel steering device that controls the rear wheel steering angle to a value determined by a feedback transfer function equation, the rear wheel steering angle calculation means converts the rear wheel steering angle calculation value δrm into a transfer function G (relating to the response characteristic of the rear wheel steering mechanism) S) = δr
/δrm, δrm={F(S)/G(S)}δf+
{ B(S) / G(S) } (d/dt)φ or δrm= F(S) δf +{ B(S) / G(
S) }(d/dt)φ, and this calculated value δr
m is configured to command the rear wheel steering mechanism.

[0008]

[Operation] The rear wheel steering angle calculation means also uses a transfer function representing the response characteristics of the rear wheel steering mechanism to calculate δrm={ F(S) /
G(S) }δf +{ B(S) / G(S) }
(d/dt)φ or δrm= F(S) δf +{
B(S) / G(S) }(d/dt)φ calculates the rear wheel steering angle calculation value δrm, and commands this to the rear wheel steering mechanism. Upon receiving this command, the relevant mechanism steers the rear wheels, but due to the response delay of the rear wheel steering mechanism, the actual rear wheel steering angle δr becomes δ
r = F(S) δf + B(S) (d/dt)
The target value expressed by φ is obtained, and the vehicle dynamic performance as desired can be achieved regardless of the response delay of the rear wheel steering mechanism.

Furthermore, this effect can be achieved without reducing the feedback gain or reducing the response delay of the rear wheel steering mechanism, so the adverse effects of these measures, that is, the improvement in the resistance to disturbances of vehicle motion, are avoided. There is no problem that the rear wheel steering actuator becomes expensive and large.

0010

Embodiments Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 shows a system example of the rear wheel steering device of the present invention, and 1L and 1R are the left and right rear wheels to be steered by the device, respectively. Steering of these rear wheels is performed by a rack-and-pinion steering gear 2, whose pinion is drive-coupled to a motor 4 via a hypoid gear 3. Drive control of the motor 4 is performed by a controller 5, and this controller has a steering wheel steering angle θ.
A signal from sensor 6 that detects vehicle speed V, a signal from sensor 7 that detects vehicle speed V, a signal from sensor 8 that detects yaw rate (d/dt)φ, and a pinion rotation angle from the neutral position of steering gear 2 (rear A signal from the rear wheel steering angle sensor 9 that detects the wheel steering angle δr) is input.

Based on this input information, the controller 5 calculates a rear wheel steering angle calculation value δrm by a calculation described later, and the drive circuit 10 supplies the motor 4 with a current i corresponding to this calculation value. The motor 4 is driven by the supplied current i and drives the rear wheels 1L, 1 through the hypoid gear 3 and the steering gear 2.
However, in many cases, the actual rear wheel steering angle δr relative to the rear wheel steering angle calculated value δrm occurs with an approximately first-order lag as shown by γ1 and ε1 in FIG. ，
First-order lag δr (S) /δrm(S) denoted by ε3
Approximately = KO / (1+τOS). Incidentally, in most cases, τO is about 0.05 sec and KO is about 0.7.

Taking into consideration the response delay of the rear wheel steering mechanism in advance, in this example, the rear wheel steering angle calculation value δrm is determined as follows. However, basically, the rear wheel steering angle is given by the sum of a feedforward term corresponding to the front wheel steering angle and a feedback term corresponding to the yaw rate, and the feedforward term alone is sufficient to improve steering stability. Utilize past know-how that can be expected to improve. In this case, although the characteristics are improved as shown by α3 and β3 in FIG. 4, it is still not possible to achieve the desired vehicle motion characteristics due to the response delay of the rear wheel steering mechanism.

Therefore, in this example, the rear wheel steering angle calculation value δrm is determined by taking the above-mentioned response delay into consideration in advance.The basic concept will be explained below. First, when the motion of the vehicle is approximated by a two-wheel model, M{(d2/dt2)y +V (d/dt)
φ}=F1+F2
-----(1) I(d2/dt2)φ=aF
1 + bF2
-----(2) However, F1 = C1{δf −[ (d /dt)y
+a (d/dt)φ]/V } -----
(3) F2 = C2{δr −[ (d /d
t)y −b (d/dt)φ]/V }
-----(4) M: Vehicle weight I: Vehicle yaw moment of inertia V: Vehicle speed C1: Front wheel cornering power (for 2 wheels) C2: Rear wheel cornering power (for 2 wheels) δf: Front wheel steering angle δr: Rear It is known that wheel steering angle a: distance between front axle and center of gravity b: distance between rear wheel and center of gravity (d/dt) y: vehicle lateral speed (d/dt) φ: yaw rate.

[0014] Here, equations (3) and (4) are replaced by (1), (
2) Substitute into the equation, and further aim to make the yaw rate (d/dt) φ always in the form of a first-order lag (for example, to make it easier to drive stably when changing lanes while driving at high speed). (d/dt)φ={ HO /(1+τS
)}δf -----(
5) However, HO: steady yaw rate gain τ: time constant according to vehicle speed and vehicle specifications, and solving for the rear wheel steering angle δr, the rear wheel steering angle δr to achieve the aim of equation (5) is: δr = {(T1′S +T2′S2) /(1+
τA ´S + τB ´S2) }δf
= F(S)δf
---
-- (6) (However, T1', T2', τA'
, τB' is a constant depending on the vehicle speed and vehicle specifications).

However, actual vehicles cannot be fully approximated by the above two-wheel model, and often deviate from equations (1) to (4). Therefore, the target yaw rate (d/dt)φO is (5
) Corresponding to the formula (d/dt)φO = { HO
/(1+τS)}δf
---(5'), and the actual yaw rate (d
/dt) δr = F(S), taking into account control according to the deviation from φ (feedback constant is G)
δf + G (d/dt)φO − (d/dt)φ
} ----(7) Let us give the rear wheel steering angle δr. In this case, even if the actual vehicle cannot be fully approximated by the two-wheel model, the aim of equation (5) can be achieved.

In reality, as described above with reference to FIG. 6, there is a limit to the responsiveness of the rear wheel steering mechanism (especially the actuator), and it is impossible to give the rear wheel steering angle according to the command value given by equation (7). Therefore, the result is actually (5
) cannot achieve the above aim of Eq.

In this example, the response characteristics of the rear wheel steering mechanism have a time constant τO = 0.05, as shown by γ3 and ε3 in FIG.
First-order lag G(S) in sec, that is, G(S) = KO / (1+τOS)

--- In the case approximated by (8), the rear wheel steering angle calculation value δrm is calculated in advance by taking this into account, and by issuing a command to the rear wheel steering mechanism, the rear wheel steering angle actual value δr is calculated as (7). As a result, the aim of equation (5) can be achieved.

In consideration of this, δr on the left side of equation (7) is replaced with δrm, and on the right side, the reciprocal of equation (8) 1/
The calculation formula for the rear wheel steering angle command value δrm is obtained by multiplying G(S) = (1+τOS)/KO, which is expressed as follows. δrm= F(S)(1 +τO S)δf +(
G/KO) { (d/dt)φO (1+τ
O S) −(d/dt)φ(1+τO
S)} ≒ { (K+T1S+T2S)/
(1+τA S)}δf + (G/K0) {[(1
+ τOS)/ (1+τ
S)]H O δf − [(d/dt)φ+ τO (
d2/dt2 )φ]}Here, considering τO ≒τ,
The above formula is δrm={(K+T1S+T2S)/(1+τA
S)}δf + (G/K O ) { H Oδf
−(d/dt)φ −τO (d2/d
t2)φ}
---(9) becomes.

In FIG. 1, the controller 5 is shown in (9) above.
A rear wheel steering angle calculation value δrm is determined by the formula, and a current i corresponding to the calculated value δrm is supplied to the motor 4 via the drive circuit 10 to steer the rear wheels 1L and 1R. This relationship is schematically shown in Figure 2, where the vehicle has front and rear wheel steering angles δf, δr
produces the desired yaw rate (d/dt)φ. However, the rear wheel steering mechanism, especially the motor 4 (actuator)
There is a response delay due to friction, etc., and as Equation (9) was determined taking this into consideration in advance as mentioned above, the actual rear wheel steering angle is the value δr determined by Equation (7), and becomes (5) In addition to achieving the above aim of the formula, α4 and β4 are shown in FIG.
As is clear from the simulation results shown in , it is possible to suppress yaw rate resonance and phase delay.

Moreover, this objective is achieved without reducing the feedback gain G or making the rear wheel steering mechanism (motor 4) highly responsive, so that the resistance to disturbances in vehicle motion is sacrificed. , the motor 4 does not need to be expensive or large.

FIG. 3 shows a control program in which the controller 5 of FIG. 1 calculates and outputs the rear wheel steering angle command value δrm.
This program uses ΔT time (e.g. 0.005 sec
) is executed every time. In step 30, the steering wheel steering angle θ, vehicle speed V, and yaw rate (d/dt) φ are read, and in step 31, the front wheel steering angle δf = θ/N is calculated from θ and the steering gear ratio N. In step 32,
33, the front wheel steering angular velocity (d/dt) δf, the front wheel steering angular acceleration (d2 /dt2) δf, and the yaw angular acceleration (d2
/dt2) Calculate φ. Next steps 34-36
Then, the first term on the right side of equation (9), that is, the feedforward term δrFF is determined, and then in step 37, the feedback gain G, which increases as shown in the figure according to the vehicle speed V, is looked up in the table. In the next step 38 (9)
The second term on the right side of the equation, ie, the feedback δrFB term, is determined, and in step 39 the rear wheel steering angle calculation value δrm is determined, which is output in step 40.

Note that the feedback gain G may be set at step 41 in FIG. 5 instead of the one set at step 37 in FIG. That is, in this example,
It is assumed that 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 this θ increases. In this case, θ=0
Improves resistance to disturbances while driving straight ahead. When turning with a large θ, the rear wheel steering angle is controlled by a sufficiently tuned feedforward term.

[0023]

As described above, the rear wheel steering device of the present invention calculates the target rear wheel steering angle by considering the response characteristics of the rear wheel steering mechanism in advance. Even if there is a delay in the response of the rear wheel steering mechanism that receives the angle calculation value command, the rear wheel steering angle calculation value will not deviate from the target value, and the target vehicle motion performance can be reliably achieved. This effect can be achieved without reducing the feedback gain or increasing the response of the rear wheel steering mechanism, which may result in insufficient improvement in disturbance resistance or the rear wheel steering actuator being expensive and large. It does not cause any harmful effects.

[Brief explanation of 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 same example.

FIG. 4 is a diagram illustrating the operating characteristics of the same example in comparison with the operating characteristics occurring in the past.

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

FIG. 6 is a diagram showing response delay characteristics of the 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

Claims (1)

[Claims] Claim 1: The rear wheel steering angle δr is calculated by combining a feedforward term based on the front wheel steering angle δf and a yaw rate (d/dt
) δr = F(S) δf + B(S) (d/dt
)φF(S): Feedforward transfer function B(S)
: In a rear wheel steering device that controls the rear wheel steering angle to a value determined by a feedback transfer function equation, the rear wheel steering angle calculation means converts the rear wheel steering angle calculation value δrm into a transfer function G (relating to the response characteristic of the rear wheel steering mechanism) S) = δr
/δrm, δrm={F(S)/G(S)}δf+
{ B(S) / G(S) } (d/dt)φ or δrm= F(S) δf +{ B(S) / G(
S) }(d/dt)φ, and this calculated value δr
A rear wheel steering device, characterized in that the rear wheel steering device is configured to command the rear wheel steering mechanism.