JP3730341B2 - Electric power steering device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an electric power steering device that reduces a steering force by adding an auxiliary steering force by an electric motor to a steering system that gives a steering angle to a steered wheel of a vehicle.
[0002]
[Prior art]
A motor is installed in the steering system that converts the steering angle given to the steering wheel to the steering angle of the steering wheel, and the driver's steering force is reduced by generating an auxiliary steering force in the direction opposite to the road surface reaction force. There is known an electric power steering apparatus configured to do so (see Japanese Patent Application Laid-Open No. 7-132846). In this electric power steering apparatus, the output of the auxiliary steering force generating motor is generally controlled based on the vehicle speed, the rotation angle (steering angle) of the steering shaft, and the manual steering force acting on the pinion of the rack / pinion mechanism. However, when the coefficient of friction between the tire and the road surface is extremely low (hereinafter referred to as the low μ state), such as snowy roads, the steering force of the steering wheel becomes lighter than necessary, and the steering force is almost constant. Or, there was a tendency to decrease. Therefore, in some cases, delicate steering is necessary so as not to turn too much carelessly, and there is a problem that a heavy burden is imposed on the driver.
[0003]
In order to improve such inconvenience, that is, the tendency to overcut the steering wheel in a low μ state, a pseudo steering reaction force determined according to the vehicle speed, the steering angle, and the road surface friction coefficient is In Japanese Patent Application No. 7-337770, the present applicant has proposed a steering device which is added to the auxiliary steering force of a normal power steering device.
[0004]
[Problems to be solved by the invention]
However, in the above-described technique, since the index of the additional reaction force control is exclusively based on the steering angle, a region in which the vehicle exhibits nonlinear characteristics due to the addition of braking force or driving force to reduce cornering force (steering angle and tire) In a region where the relationship with the side slip angle is not in direct proportion), there is a problem that sufficient effects cannot be exhibited.
[0005]
The present invention is intended to solve the problems imposed on the conventional technology and to provide an electric power steering device improved so that the steering wheel can be controlled within the friction circle. It was issued.
[0006]
[Means for Solving the Problems]
In order to achieve such an object, in the present invention, an electric motor for adding a steering force to a steering system for giving a steering angle to a steering wheel of a vehicle, and a steering force detecting means for detecting a manual steering force acting on the steering system. And a control means for generating a control signal for driving the electric motor based on at least a signal from the steering force detection means, to a control means for the electric power steering apparatus, a side slip angle calculation means for calculating a side slip angle of the steered wheels ( Front wheel side slip angle calculation means 2), maximum side slip angle setting means (maximum side slip angle map 6) in which the side slip angle at which the steered wheel generates the maximum side force is preset, and the side slip angle is divided by the maximum side slip angle. a lateral force usage rate calculating means for calculating a lateral force usage rate of road wheels (7), a control signal based on the lateral force utilization value was assumed to provide a correction means to correct (additional reaction force map 8).
[0007]
In this way, by performing the additional reaction force control based on the lateral force utilization rate of the front wheels, it is possible to set the steering reaction force according to the lateral force generated within the friction circle of the tire. Thus, it becomes possible to give an appropriate steering reaction force according to the margin for generating the lateral force, and the above-mentioned problem is solved. In addition, since an additional reaction force is applied so that the front wheel rudder angle remains within the friction circle, it is possible to expect improved steering stability.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the configuration of the present invention will be described in detail with reference to the accompanying drawings.
[0009]
〔overall structure〕
FIG. 1 shows a configuration of a steering force control device of an electric power steering device according to the present invention. The present control device calculates the auxiliary steering torque setting means 1 for setting the auxiliary steering torque command value TA based on the steering torque TS acting on the steering wheel and the vehicle speed V, and the vehicle speed V and front wheel side slip angle calculating means 2. Additional reaction force torque setting means 4 for setting the additional reaction force torque command value TC based on the front wheel side slip angle β and the road surface friction coefficient μ calculated by the road surface friction coefficient calculation means 3; A motor drive control means 5 for controlling the output of the auxiliary torque generating motor M based on the reaction force torque command value TC, and a value TM obtained by adding the additional reaction force torque command value TC to the auxiliary steering torque command value TA. Thus, output control of the electric motor M of a normal power steering apparatus is performed.
[0010]
(Calculation of vehicle side slip angle)
The vehicle body side slip angle β is calculated from the following equation based on the values of the vehicle speed V, the lateral acceleration G, and the yaw rate γ.
[Expression 1]
β = ∫ (γ−G / V) dt
[0011]
[Calculation of front wheel side slip angle]
The front-wheel side slip angle βf is calculated based on the following formula (Sankaidou published as “Automotive Motion”) based on the values of the vehicle speed V, the front-wheel rudder angle δ, the yaw rate γ, the vehicle body side-slip angle β, Control ”(see page 54, equation 3.6).
[Expression 2]
βf = β + γL / V−δ
[0012]
[Correction of actual rudder angle]
The front wheel rudder angle δ is estimated from the steering angle θs of the steering wheel, but is affected by the change in the angle of the arm according to the suspension stroke and the change in the amount of bending of the bush according to the lateral load and the longitudinal load. In order to obtain an accurate actual steering angle value, it is preferable to detect and correct the suspension stroke, the lateral load, and the longitudinal load.
[0013]
[Estimation of road friction coefficient]
First, the tire cornering power Cp is expressed as follows from the FIALA equation (up to the second term).
Cp = K (1−0.0166K / μW)
However, K: Cornering stiffness μ: Road surface friction coefficient W: Ground load
That is, since the cornering power Cp of the tire decreases as the road surface friction coefficient μ decreases (see FIG. 2), in the case of a rack / pinion type steering device, the rack axial reaction force received from the road surface at the same steering angle is the road surface friction. It becomes smaller as the coefficient μ decreases. Therefore, the road surface friction coefficient μ is estimated by actually measuring the front wheel rudder angle and the rack shaft reaction force and comparing the actual rack shaft reaction force with respect to the front wheel rudder angle with a reference rack shaft reaction force set in advance as an internal model. be able to.
[0015]
The actual rack shaft reaction force Frc, that is, the road surface reaction force, can be estimated from the steering torque TS, the motor voltage Vm, and the motor current Im as follows. First, the output shaft torque Tm of the electric motor M in the electric power steering apparatus is given by the following equation.
Tm = Kt · Im−Jm · θm ”
-Cm · θm '± Tf
Where Kt: motor torque constant Im: motor current Jm: moment of inertia θm ′ of motor rotating portion: motor angular velocity θm ”: motor angular acceleration Cm: motor viscosity coefficient Tf: friction torque
If the viscosity term, inertia term, friction term around the steering shaft, and friction term around the motor are minute, if omitted, the balance of forces on the rack shaft is approximately expressed by the following equation.
Fr = Fs + Fm
= TS / rp
+ N (Kt · Im−Jm · θm ″ −Cm · θm ′)
Fr: rack shaft reaction force from the road surface Fs: rack shaft force from the pinion Fm: rack shaft force from the motor TS: steering torque detection value applied to the steering shaft rp: pinion radius N: motor output gear ratio
The motor angular velocity θm ′ is obtained by differentiating the steering wheel steering angle θs or by the following equation from the motor back electromotive force.
θm ′ = (Vm−Im · Rm) / Km
Where Vm: motor voltage Rm: motor resistance Km: induction voltage constant of the motor
Here, strictly speaking, the motor angular velocity θm ′ and the steering wheel angular velocity θs ′ are different, and the motor angular velocity θm ′ is based on the steering wheel angular velocity θs ′ obtained by differentiating the steering wheel steering angle θs. Is obtained from the following equation.
θm ′ = (θs′−TS ′ / Ks) N
Ks: Torque sensor spring constant TS ′: Steering torque differential value
The motor angular acceleration θm ″ can be obtained by differentiating the motor angular velocity θm ′.
[0020]
Next, an internal model serving as a comparison reference for the actual rack shaft reaction force Frc is set as follows.
[0021]
As shown in FIG. 3, the steering angle θs input from the steering wheel is converted into a rack shaft stroke amount via a transmission ratio Np with the pinion. A front wheel side slip angle β is generated according to the stroke amount of the rack shaft. Here, the transfer function Gβ (s) of the front wheel side slip angle β with respect to the stroke amount of the rack shaft changes according to the change of the stability factor accompanying the change of the road surface friction coefficient μ.
[0022]
By applying the cornering power Cp and the trail ζ (caster trail + pneumatic trail) to the front wheel side slip angle β, a moment around the kingpin is obtained. Here, the cornering power Cp and the pneumatic trail vary depending on the road surface friction coefficient μ and the ground load W.
[0023]
A model rack shaft reaction force Frm is obtained by dividing the moment around the kingpin by the distance between the tire rotation center and the rack shaft center, that is, the knuckle arm length rk.
[0024]
From the above, the response of the model rack shaft reaction force Frm to the steering wheel steering angle θs can be replaced with one transfer function Gf (s) derived from the calculation result based on each specification or the identification result from the actual vehicle measurement value. I understand that there is.
[0025]
From the actual rack shaft reaction force value Frc and the model rack shaft reaction force value Frm obtained as described above, the actual and model rack shaft reaction force increase rates with respect to the increase in the steering wheel steering angle θs are obtained (see FIG. 4). Within a rudder angle range in which the vehicle response is linearly approximated, a ratio ΔFrc / ΔFrm between the actual rack shaft reaction force increase rate ΔFrc / Δθs and the model rack shaft reaction force increase rate ΔFrm / Δθs is set in advance. The road surface friction coefficient μ can be estimated with reference to the road surface friction coefficient determination map (see FIG. 5).
[0026]
[Additional reaction torque setting 1]
Next, the function of the additional reaction force torque setting means 4 will be described with reference to FIG. First, from a vehicle speed V and a road surface friction coefficient μ, a maximum side slip angle map 6 obtained in advance by actual vehicle measurement or the like is referred to, and a front wheel side slip angle when generating the maximum tire lateral force, that is, a maximum side slip angle βfmax is obtained. The lateral force utilization factor ξ of the front wheel is calculated from the following equation by the lateral force utilization factor calculating means 7 based on the value and the front wheel side slip angle βf obtained by Equation 2.
ξ = βf / βfmax
[0027]
Next, an additional reaction force torque TC corresponding to the lateral force utilization rate ξ is obtained with reference to a predetermined additional reaction force map 8.
[0028]
It was obtained by subtracting the additional reaction force torque command value TC determined in this way from the auxiliary steering torque command value TA of a normal power steering device determined based on the vehicle speed V and the manual steering torque TS. By controlling the motor M with the control command value TM, the driver is informed by increasing the steering force before the vehicle characteristics reach the limit according to the road surface condition, and the control range of the steering force is expanded. As a result, the electric motor is driven so as to generate an optimal auxiliary steering torque that can prevent the driver from turning at an excessive steering angle not intended by the driver.
[0029]
[Additional reaction torque setting 2]
Next, a second method of the additional reaction force torque setting means 4 will be described with reference to FIG. First, the reaction force control start side slip angle is obtained by subtracting the offset value α1 corresponding to the road surface friction coefficient μ obtained by referring to the preset road surface μ offset map 9 from the front wheel side slip angle value βf obtained in Equation 2. βf0 is set.
[0030]
Next, with reference to a preset reaction force torque base map 10, a base reaction force torque TC0 corresponding to the corrected side slip angle βf0 is obtained.
[0031]
Next, by referring to the vehicle speed offset map 11, an offset value α2 corresponding to the vehicle speed V is obtained, and the base reaction force torque TC0 is corrected with this to obtain a final command value TC of the additional reaction force torque. Similar to the above, this value is added to the normal auxiliary steering torque command value TA to control the drive torque of the electric motor M. According to this, since the calculation of the lateral force utilization factor ξ becomes unnecessary, the reaction force control based on the approximate tire friction circle can be realized more simply than in the first method described above. .
[0032]
【The invention's effect】
As described above, according to the present invention, the reaction force can be applied to the steering force of the driver so that the cornering force of the tire stops within the friction circle of the tire. Therefore, even when a driving force or a braking force is applied, the vehicle Since the change in behavior is suppressed, optimal driving operation support control according to the road surface condition can be realized, the steering stability can be improved, and the driver's burden can be reduced.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram of a control system of a power steering apparatus according to the present invention.
FIG. 2 is a relationship diagram between a cornering power and a road surface friction coefficient.
FIG. 3 is a flowchart related to setting of an internal model.
FIG. 4 is an increase diagram of a vehicle state quantity with respect to a steering angle quantity.
FIG. 5 is a determination map of a road surface friction coefficient.
FIG. 6 is a block diagram of a first embodiment of an additional reaction force setting unit.
FIG. 7 is a block diagram of a second embodiment of the additional reaction force setting means.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Auxiliary steering torque setting means 2 Front wheel side slip angle calculation means 3 Road surface friction coefficient estimation means 4 Additional reaction force torque setting means 5 Motor drive control means 6 Maximum side slip angle map 7 Lateral force utilization rate calculation means 8 Additional reaction force map 9 Road surface μ Offset map 10 Reaction torque base map 11 Vehicle speed offset map

Claims (1)

Based on an electric motor for applying a steering force to a steering system for giving a steering angle to a steering wheel of a vehicle, a steering force detecting means for detecting a manual steering force acting on the steering system, and at least a signal from the steering force detecting means And an electric power steering device having a control means for generating a control signal for driving the electric motor,
The control means includes a side slip angle calculating means for calculating a side slip angle of the steered wheel, a maximum side slip angle setting means for presetting a side slip angle at which the steered wheel generates a maximum lateral force, and the side slip angle is set to the side slip angle. A lateral force utilization factor calculating means for calculating a lateral force utilization factor of the steered wheel by dividing by a maximum side slip angle; and a correcting means for correcting the control signal based on the value of the lateral force utilization factor. An electric power steering device.

Images