Steering Control System for Wheeled Vehicle

The present invention relates to a vehicle steering control system for controlling one or more vehicle motion variables so as to achieve prescribed vehicle response characteristics by varying a manipulated quantity of a steering system, and especially to a vehicle steering control system having an internal feedback loop.

The possibility of improvement in steering characteristics of a conventional two wheel steer vehicle (in which only front wheels are steered by angular displacement of a steering wheel) is limited because of the very nature of the two wheel steer vehicle.

A conventional example of a control system for a four wheel steer vehicle is disclosed in "Gakujutsu Koenkai Maezurishu 842058" (pages 307-310) published by Society of Automotive Engineers of Japan in 1984. The control system of this example alters a rear wheel steer angle by sensing an actual yaw rate with a yaw rate sensor and feeding back the sensed value of the yaw rate so as to maintain a steady state side slip angle equal to zero.

However, this system requires a sensor for sensing a vehicle motion variable such as a yaw rate, so that the cost and the accuracy of such a motion variable sensor are troublesome. The disadvantage is further inceased when the system requires a plurality of the vehicle motion variable sensors. Furthermore, noise is superimposed on the sensor signal by vehicle vibrations, and the delay in the feedback loop is increased.

It is an object of the present invention to provide a vehicle control system for controlling a vehicle motion variable which requires no sensor for sensing the controlled vehicle motion variable.

According to the present invention a control system for steering a controlled vehicle having at least one first wheel such as front wheels and at least one second wheel such as rear wheels, comprises a steering input means, actuating means and control means. The steering input means determines a steering input quantity such as a steering wheel angle of the controlled vehicle. The actuating means steers the controlled vehicle by varying a predetermined manipulated variable such as a second wheel steer angle. The control means produces a control signal to command the actuating means in accordance with the steering input quantity. The control means comprises first reference supplying means. actual motion variable estimating means, first comparing means and target manipulated variable determining means. The first reference supplying means determines a desired value of a predetermined first vehicle motion variable in accordance with a first desired vehicle model representing a first desired vehicle. The actual motion estimating means determines an estimated value of the first motion variable corresponding to the steering input quantity in accordance with an actual vehicle model representing the controlled vehicle. The first comparing means compares the desired and estimated values of the first motion variable. The target manipulated variable determining means varies the manipulated variable by producing the control signal representing a target value of the manipulated variable so as to reduce a difference between the desired and estimated values of the first motion variable.

The control system may further comprises means for sensing a vehicle speed of the controlled vehicle. In this case, the first reference supplying means determines the desired value corresponding to the steering input quantity and the vehicle speed, and the actual motion estimating means determines the estimated value corresponding to the steering input quantity and the vehicle speed.

FIG. 1 is a view showing a control system of a first or second embodiment of the present invention.

FIG. 2 is a block diagram showing a control of the first embodiment.

FIG. 3 is a block diagram showing a control of the second embodiment,

FIGS. 4A-4D are graphs showing characteristics of a vehicle controlled by the control system of the second embodiment.

A first embodiment of the present invention is shown in FIGS. 1 and 2.

FIG. 1 shows a controlled vehicle equipped with a control system of the first embodiment. A processor 1 is made up of a microcomputer or other electric circuitry. The processor 1 receives a steering wheel angle .delta..sub.s of a steering wheel 8 sensed by a steering wheel angle sensor 2, and a vehicle speed V of the controlled vehicle sensed by a vehicle speed sensor 3, and outputs a target rear wheel steer angle .delta..sub.R by performing mathematical operations.

Front wheels 9 and 10 of the controlled vehicle of this embodiment are steered through a conventional type mechanical steering linkage 6 so that a front wheel steer angle is made equal to a value corresponding to the angular displacement of the steering wheel 8.

Rear wheels 11 and 12 are steered through a hydraulic steering unit 7, which is controlled by a rear wheel steering regulator 5. The rear wheel steering regulator 5 controls the hydraulic unit 7 by varying a fluid pressure supplied to the unit 7 in accordance with the target rear wheel steer angle .delta..sub.R sent from the processor 1. (The steering regulator 5 and the hydraulic unit 7 are disclosed more in detail in a Japanese patent application No. 59-188153, a prior filed, commonly owned U.S. application Ser. No. 773,650 filed Sept. 9, 1985, U.S. Pat. No. 4,679,809, a West German Offenlegungsshrift No. 35 32 247 published on Mar. 20, 1986, which are all cognate with one another. The disclosures of these documents are herein incorporated by reference.)

FIG. 2 shows a control performed in the processor 1 of the first embodiment.

A reference supplying section 21 determines a desired yaw rate value .phi. corresponding to the steering wheel angle .theta..sub.s and the vehicle speed V by performing mathematical operations according to a predetermined desired vehicle model representing a desired vehicle having desired cornering characteristics.

The desired vehicle model is a simulation model in which desired characteristics and vehicle parameters of a simulated vehicle are set as mathematical relationships (or operational circuits) and constants (or circuit constants). By receiving the steering wheel angle .theta..sub.s and the vehicle speed V, the desired vehicle model provides a value of a vehicle motion variable which would be produced by the desired vehicle. In this embodiment, the section 21 determines the value of the yaw rate of the desired vehicle by using the desired vehicle model. and delivers the thus-determined value as the desired yaw rate value .phi..

The desired yaw rate value .phi. is determined by using the following equations:

where

.delta..sub.F1 =a steer angle of the front wheels of the desired vehicle model (In this embodiment, the desired vehicle is a two-wheel steer vehicle.),

.phi..sub.1 =a yaw rate of the desired vehicle model,

.phi..sub.1 =a yaw acceleration of the desired vehicle model,

y.sub.1 =a side slipping speed of the desired vehicle model,

y.sub.1 =a side slipping acceleration of the desired vehicle model,

.beta..sub.F1 =a side slip angle of the front wheels of the desired vehicle model,

.beta..sub.R1 =a side slip angle of the rear wheels of the desired vehicle model,

C.sub.F1 =a cornering force of each of the front wheels of the desired vehicle model,

C.sub.R1 =a cornering force of each of the rear wheels of the desired vehicle model.

Each of these variables is treated as a variable varying with time in solving these equations.

In this embodiment, the vehicle parameters of the desired vehicle model are as follows:

I.sub.Z1 =a yawing moment of inertia of the desired vehicle model,

M.sub.1 =a vehicle mass of the desired vehicle model,

L.sub.1 =a wheel base of the desired vehicle model,

L.sub.F1 =a distance between a front axle and a center of gravity of the desired vehicle model.

L.sub.R1 =a distance between a rear axle and the center of gravity of the desired vehicle model,

I.sub.K1 =a moment of inertia about a kingpin of the desired vehicle model,

K.sub.S1 =a steering stiffness of the desired vehicle model,

D.sub.K1 =a viscosity coefficient of a steering system of the desired vehicle model,

.xi..sub.1 =a trail of the desired vehicle model,

N.sub.1 =an overall steering (gear) ratio of the desired vehicle model,

K.sub.F1 =a cornering power of each of front wheels of the desired vahicle,

K.sub.R1 =a cornering power of each of rear wheels of the desired vehicle.

Each of the vehicle parameter is treated as a constant in solving the equations (1)-(8).

The desired yaw rate value .phi. determined by the section 21 is inputted to a subtracter 22, which determines a difference (.phi.-.phi.*) between the desired yaw rate value .phi. and an estimated yaw rate value .phi.* determined by an actual motion variable estimating section 24.

The output of the subtracter 22 is inputted to a controlling section 23 in which the difference (.phi.-.phi.*) is multiplied by a constant K. The section 23 sends the product K(.phi.-.phi.*) as the target rear wheel steer angle .delta..sub.R to the rear wheel steering regulator 5.

In response to the signal from the section 23, the steering regulator 5 alters the actual rear wheel steer angle of the controlled vehicle by supplying the fluid pressure proportional to the target rear wheel steer angle .delta..sub.R to the hydraulic unit 7.

The actual motion estimating section 24 receives the target rear wheel steer angle .delta..sub.R from the section 23, estimates a value of the yaw rate of the controlled vehicle corresponding to the target rear wheel steer angle .delta..sub.R by performing mathematical operations. and outputs the result of the mathematical operations as the estimated yaw rate value .phi.*.

The estimated yaw rate value .phi.* is determined in accordance with an actual vehicle model representing the vehicle controlled by the control system of this embodiment. The actual vehicle model is determined by one or more relationships expressing the characteristics of the controlled vehicle, and the vehicle parameters of the controlled vehicle. For example. the vehicle parameters of the controlled vehicle are set equal to values determined when the vehicle is shipped. In this embodiment, the following equations are used for determining the estimated yaw rate value .phi.*.

In the equations (9)-(16):

V*.sub.y =an estimated value of a side slipping speed of the actual vehicle model,

.DELTA.t=a time length of intervals of the operation cycles,

.beta..sub.F =a side slip angle of the front wheels of the actual vehicle model,

.beta..sub.R =a side slip angle of the rear wheels of the actual vehicle model,

C.sub.F =a cornering force of each of the front wheels of the actual vehicle model, and

C.sub.R =a cornering force of each of the rear wheels of the actual vehicle model.

In the equations (9)-(16), the following vehicle parameters are used:

L.sub.F =a distance between a front axle and a center of gravity of the actual vehicle model,

L.sub.R =a distance between a rear axle and the center of gravity of the actual vehicle model,

M=a vehicle mass of the actual vehicle model,

N=an overall steering (gear) ratio of the actual vehicle model,

I.sub.Z =a yawing moment of inertia of the actual vehicle model,

eK.sub.F =a front wheel equivalent cornering power of the actual vehicle model (in which the steering stiffness is taken into account), and

K.sub.R =a rear wheel cornering power of the actual vehicle model.

The estimated yaw rate value .phi.* determined by the section 24 is used as a negative feedback quantity in the proportional control performed by the subtracter 22 and the section 23.

Therefore, this control system continues to steer the rear wheels 11 and 12 by outputting the target rear wheel steer angle until the estimated yaw rate vlaue .phi.* of the controlled vehicle becomes equal to the desired raw rate value .phi.. When the difference (.phi.-.phi.*) is reduced to zero, the control system stops steering the rear wheels. In this way, the control system of this embodiment makes the actual yaw rate of the controlled vehicle equal to the desired yaw rate value .phi., and accordingly makes the dynamic characteristics of the controlled vehicle equal to the characteristics of the desired vehicle.

The control system of this embodiment feeds back the estimated yaw rate of the controlled vehicle in a manner of so-called internal feedback, instead of the actual value sensed by a sensor such as a yaw rate sensor. so that the feedback control of this embodiment is free from noise, and the accuracy in achieving the desired dynamic characteristic, is improved.

In the first embodiment, the yaw rate is employed as a controlled variable. However, it is optional to employ some other vehicle motion variable such as a side slip angle. In the first embodiment, the rear wheel steer angle is employed as a manipulated variable. However, the control system may be arranged to vary the front wheel steer angle of the controlled vehicle, or both of the front wheel steer angle and the rear wheel steer angle.

A second embodiment of the present invention is shown in FIGS. 1 and 3. FIG. 1 is common to both of the first and second embodiments. A control system of the second embodiment comprises processor 1, steering wheel angle sensor 2, vehicle speed sensor 3. rear wheel steering regulator 5 and hydraulic unit 7, as shown in FIG. 1. The control system of the second embodiment is different from the control system of the first embodiment only in control action performed by the processor 1.

FIG. 3 shows the functions performed by the processor 1 of the second embodiment in the form of a block diagram.

A first reference supplying section 121 determines a desired yaw rate value .phi. corresponding to the steering wheel angle .theta..sub.s of the controlled vehicle sensed by the steering wheel angle sensor 2 and the vehicle speed V of the controlled vehicle sensed by the vehicle speed sensor 3, by performing mathematical operations in accordance with a predetermined first desired vehicle model representing a first desired vehicle. The first desired vehicle model is a simulation model in which desired characteristics and desired vehicle parameters are set by mathematical equations (or operational circuits) and constants (or circuit constants). The first desired vehicle model receives the steering wheel angle and the vehicle speed as inputs. and delivers the magnitude of a vehicle motion variable of the first desired vehicle resulting from the steering wheel angle and the vehicle speed. In the second embodiment, the section 121 determines the magnitude of the yaw rate of the first desired vehicle model, and outputs the thus-determined magnitude of the yaw rate as the desired yaw rate value .phi..

The first reference section 121 of the second embodiment has a transfer characteristic (or function) R.sub..phi. (s) expressed as: ##EQU1## In the equation (17), s is a differential operator (Laplace operator or Laplacian), G.sub..phi. (V) is a gain which is a function of the vehicle speed V, and .tau. is a time contant. As known from the equation (17), the desired yaw rate value .phi. is outputted with respect to the steering angle in a manner of a first order lag.

A second reference supplying sectron 122 determines a desired side slip angle value .beta. corresponding to the steering wheel angle .theta..sub.s and the vehicle speed V by performing mathematical operations in accordance with a predetermined second desired vehicle model representing a second desired vehicle the second desired vehicle model is also a simulation model.

The second reference section 122 has a transfer characteristic (or function) R.sub..beta. (s) expressed as: ##EQU2## Thus, the desired side slip angle value .beta. is outputted with respect to the steering wheel angle .theta..sub.s in a manner of a second order lag. In the equation (18), G.sub..beta. (V) is a gain which is a function of the vehicle speed V, and .omega. is a natural frequency (or undamped frequency) which is a constant.

The desired yaw rate value .phi. determined by the section 121 is inputted to a first subtracter 123, which compares the desired yaw rate value .phi. and an estimated yaw rate value .phi.* of the controlled vehicle determined by an actual vehicle motion variable estimating section 128, and determines a yaw rate difference (.phi.-.phi.*). The desired side slip angle value .beta. determined by the section 122 is inputted to a second subtracter 124, which compares the desired side slip angle value .beta. and an estimated side slip angle value .beta.* determined by the actual vehicle motion variable estimating section 128, and determines a side slip angle difference (.beta.-.beta.*).

The output of the first subtracter 123 is inputted to a first controlling section 125, which multiplies the yaw rate difference (.phi.-.phi.*) by a predetermined constant -K.sub.1. A second controlling section 126 receives the output of the second subtracter 124, and multiplies the side slip angle difference (.beta.-.beta.*) by a predetermined constant K.sub.2. An adder 127 receives the outputs of the first and second controlling sections 125 and 126, determines the sum of the output of the first controlling section 125 and the output of the second controlling section 126, and delivers the sum as the target rear wheel steer angle value .delta..sub.R to the rear wheel steering regulator 5.

The steering regulator 5 varies the actual rear wheel steer angle of the controlled vehicle by supplying the hydraulic unit 7 with the fluid pressure proportional to the target rear wheel steer angle value .delta..sub.R.

The actual vehicle motion variable estimating section 128 performs mathematical operations to estimate the magnitudes of the yaw rate and side slip angle of the controlled vehicle obtained when the rear wheel steer angle of the controlled vehicle is equal to the target rear wheel steer angle value .delta..sub.R, and outputs the results of the mathematical operations as the estimated yaw rate value .phi.* and the estimated side slip angle value .beta.*.

The estimating section 128 determines the estimated yaw rate value .phi.* and the estimated side slip angle value .beta.* by using an actual vehicle simulation model which is determined by actual vehicle parameters of the controlled vehicle and relationships expressing characteristics of the controlled vehicle. In this embodiment, the estimating section 128 determines .phi.* and .beta.* in accordance with the following equations.

In the equations (19)-(27), the vehicle parameters are:

I.sub.Z =a yawing moment of inertia of the actual vehicle model representing the controlled vehicle,

M=a vehicle mass of the actual vehicle model,

L.sub.F =a distance between a front axle and a center of gravity of the actual vehicle model,

L.sub.R =a distance between a rear axle and the center of gravity of the actual vehicle model,

eK.sub.F =a front equivalent cornering power of the actual vehicle model (in which a steering stiffness is taken into account),

K.sub.R =a rear cornering power of the actual vehicle model,

N=an overall steering (gear) ratio of the actual vehicle model.

The variables are:

.phi.=a yaw acceleration of the actual vehicle model,

.phi.=a yaw rate of the actual vehicle model,

V.sub.y =a side slipping acceleration of the actual vehicle model,

V.sub.y =a side slipping speed of the actual vehicle model,

C.sub.F =a front wheel cornering force of the actual vehicle model,

C.sub.R =a rear wheel cornering force of the actual vehicle model.

The estimated yaw rate value .phi.* determined by the estimating section 128 is treated as a negative feedback quantity in the proportional control of the first subtracter 123 and the first controlling section 125. The estimated side slip angle value .beta.* is treated as a negative feedback quantity in the proportional control of the second subtracter 124 and the second controlling means 126.

Therefore, the target rear wheel steer angle value .delta..sub.R is outputted until the estimated yaw rate value .phi.* becomes equal to the desired yaw rate value .phi. and simultaneously the estimated side slip angle .beta.* becomes equal to the desired side slip angle value .beta.. That is, the rear wheels 11 and 12 are steered until the quantity expressed as -K.sub.1 (.phi.-.phi.*)+K.sub.2 (.beta.-.beta.*) becomes equal to zero.

When the equation -K.sub.1 (.phi.-.phi.*)+Kphd 2(.beta.-.beta.*)=0 is satisfied, the control system of the second embodiment stops steering the rear wheels, so that the actual yaw rate and the actual side slip angle of the controlled vehicle are maintained equal to the desired yaw rate value and the desired side slip angle value .beta., respectively. In this way, the control system makes the characteristics of the controlled vehicle identical to the characteristics of the desired vehicle model or models.

Like the first embodiment, the control system of the second embodiment can provide the feedback vehicle control which is free from noise, and improve the accuracy in achieving the desired characteristics. Furthermore, the control system of the second embodiment can endow the controlled vehicle with more sophisticated characteristics by controlling both of a rotational motion variable (the yaw rate in the second embodiment) and a translational motion variable (the side slip angle in the second embodiment).

The effects of the second embodiment are shown in FIGS. 4A-4D. FIGS. 4B, 4C and 4D shows, respectively, variations of the rear wheel steer angle, yaw rate and side slip angle resulting when the steering wheel angle is changed as shown in FIG. 4A at a high vehicle speed. Solid lines show characteristics of the vehicle controlled by the system of the second embodiment. Two dot chain lines show characteristics of a conventional vehicle in which only front wheels are steered in accordance with the angular displacement of the steering wheel. Broken lines show characteristics of a vehicle in which only the side slip angle is controlled by altering the rear wheel steer angle so as to achieve desired cornering characteristics in a manner similar to the second embodiment. One dot chain lines show characteristics of a vehicle in which only the yaw rate is controlled by altering the rear wheel steer angle.

As evident from FIGS. 4B-4D, in the conventional vehicle, both of the yaw rate and side slip angle is oscillatory and unstable. In the vehicle in which only the side slip angle is controlled, the side slip angle responds speedily, but the yaw rate swings largely. In the vehicle in which only the yaw rate is controlled, the yaw rate is prevented from swinging. but the response of the side slip angle is slow.

In the controlled vehicle of the second embodiment, the response of the side slip angle is speedy, and simultaneously an overshoot of the yaw rate is sufficiently small.

In the second embodiment, it is possible to make the steady state gain of the yaw rate and the steady state gain of the side slip angle equal to each other by determining the gain G.sub..phi. and G.sub..beta. apearing in the equations (17) and (18) so as to satisfy the following equation: ##EQU3##

In the second embodiment, the desired yaw rate value .phi. and the desired side slip angle value .beta. are determined by using two different desired vehicle models. However, it is possible to determine these desired values by using a single desired vehicle model. In this case, the control system uses the single desired vehicle model which includes equations of vehicle motion, and performs mathematical operations to determine the desired values .phi. and .beta. from the steering wheel angle .theta..sub.s and the vehicle speed V (as shown in a copending U.S. patent application Ser. No. 773,622, U.S. Pat. No. 4,679,809, and a West German Offenlegungsschirift No. 35 32 246 published May 20, 1986).

In the second embodiment, each of the first and second controlling sections 125 and 126 performs a proportional control action. Howevers, it is optional to arrange either or both of the first and second controlling sections 125 and 126 so as to have a proportional plus integral control action or a proportional plus integral plus derivative control action.

The control system of the second embodiment may be arranged to vary the front wheel steer angle instead of the rear wheel steer angle, or both of the front wheel and rear wheel steer angles.

As explained above, the present invention makes it possible to control the vehicle response characteristics freely. Furthermore, the feedback control of the present invention requires no sensor for sensing a vehicle motion variable such as a yaw rate sensor. Therefore, the feedback control of the present invention is free from noise attributable to vibrations of the vehicle, and makes a contribution to cost reduction by eliminating the necessity of vehicle motion variable sensors. Besides, the feedback control of the present invention requires no troublesome signal processing circuit for converting the output signal of the motion variable sensor into an appropriate signal form.