JPH1148997A - Motor-driven power steering device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To further surely suppress instability of a vehicle behavior on a slippery road surface by calculating a value corresponding to horizontal direction grip force that a rear wheel generates at present and relatively reducing steering auxiliary force as the value approaches a capacity limit of a tire. SOLUTION: An auxiliary steering torque command value Ta obtained from speed V and manual steering torque Ts is added to a steering resistance torque command value Tc obtained from steering resistance torque Tcf and steering resistance correction torque Tcr and a motor 4 is controlled. Because a rear wheel becomes a spin tendency when horizontal force utilization factor ξr of the rear wheel exceeds about 1, steering resistance force is generated in a power steering device as the horizontal force utilization factor ξr approaches about 1 and steering auxiliary force is relatively reduced. Thus, an oversteering tendency is eliminated and stability of a vehicle on a slippery road can be improved because a driver recognizes overtuning of a steering wheel and returns a front wheel steering angle.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electric power steering apparatus configured to apply a steering assist force for reducing a steering force to a steering system for giving a steering angle to a front wheel.

[0002]

2. Description of the Related Art Steering resistance (self-aligning torque) from a road surface is small on a road surface (low μ road) such as a snowy road where a friction coefficient between a tire and the road surface is extremely low. There is a problem that a delicate steering is required so as not to cut too much, and a heavy load is imposed on a driver.

In order to improve such inconvenience, a lateral force utilization factor of a steered wheel is calculated based on a friction coefficient between a tire and a road surface and a vehicle speed, and a manual steering system is calculated based on the lateral force utilization value. The present applicant has proposed an electric power steering apparatus in which the applied steering resistance is controlled (Japanese Patent Application No. Hei 8-3).
09496).

[0004]

However, in the case of a front-wheel drive vehicle, when the accelerator is turned off during turning, a so-called tack-in phenomenon occurs in which the lateral force (cornering force) of the front wheels increases and tends to oversteer. However, when it occurs on a road surface with an extremely low friction coefficient such as a snowy road, the lateral force of the rear wheel may be saturated. In other words, the conventional method does not consider the grip condition of the rear wheels, and thus cannot be said to be sufficient for improving the steering response especially on a low μ road.

The present invention has been made to solve such a problem, and a main object of the present invention is to more reliably suppress instability of vehicle behavior on slippery road surfaces. An object of the present invention is to provide an improved electric power steering apparatus.

[0006]

According to the present invention, there is provided a motor for applying power to a steering system for providing a steering angle to front wheels of a vehicle, and a steering assist force generated by the motor. Controlling the electric power steering device having a control means for controlling the rear wheel to calculate a value corresponding to the lateral grip force currently generated by the rear wheel, and as the lateral grip force corresponding value approaches the performance limit of the tire. Control is performed so that the steering assist force is relatively reduced.
According to this, when the rear wheels are likely to deviate from the friction circle, the steering wheel becomes heavy (the force required for steering increases), so that excessive steering can be suppressed.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The construction of the present invention will be described below in detail with reference to embodiments shown in the accompanying drawings.

FIG. 1 shows an example of a front-rear-wheel steering vehicle to which the present invention is applied. On the front wheel 1 side of this vehicle, the rotational force applied to the steering wheel 2 is converted into the axial force of the rack shaft 3 by a pinion (not shown), and the auxiliary axial force by the electric motor 4 is applied to the rack shaft 3. An additional rack / pinion type electric power steering device 5 is provided.

A rear wheel steering device 7 for giving a steering angle to the rear wheel 6 includes:
A steering rod 8 extending in the vehicle width direction is axially driven by an electric motor 9, and its axial force is applied to a knuckle arm 10 supporting left and right rear wheels 6, similarly to the rack shaft 3 on the front wheel 1 side. 11.

The electric power steering device 5 is provided with a steering angle sensor 12 for detecting the steering angle δf of the front wheels 1 from the displacement of the rack shaft 3, and the rear wheel steering device 7 is provided with a displacement of the steering rod 8. , A steering angle sensor 13 for detecting the steering angle δr of the rear wheel 6 is provided. The steering shaft 14 is provided with a steering torque sensor 15 for detecting a manual steering torque Ts applied to the steering wheel 2. Further, a vehicle speed sensor 16 is provided for each wheel, a yaw rate sensor 17 is provided at an appropriate position on the vehicle body, and a brake operation sensor 18 is provided for the brake pedal.

Each of these sensors is electrically connected to a computer unit (ECU) 19 that drives and controls the electric power steering device 5 and the motors 4 and 9 of the rear wheel steering device 7.

In this front-rear-wheel steering vehicle, when the driver steers the steering wheel 2, the rotational motion of the steering wheel 2 is changed to the rack / wheel.
This is converted into a linear motion of the rack shaft 3 by a pinion mechanism, whereby a steering angle is given to the front wheels 1. At the same time, the amount of movement of the rack shaft 3 is input from the steering angle sensor 12 to the ECU 19. Based on the input values of the front wheel steering angle δf, the vehicle speed V, and the yaw rate γ, the optimal steering angle of the rear wheel 6 at that time is determined by the ECU 19, and the rear wheel steering device 7 is accordingly determined.
Is driven to give a steering angle to the rear wheels 6.

FIG. 2 shows the overall configuration of a control system for a front-rear-wheel steering vehicle according to the present invention. This control system is roughly divided into a control system of the power steering device 5 provided on the front wheel 1 side and a control system of a rear wheel steering device 7 provided on the rear wheel 6 side.

The power steering device 5 includes a rack shaft 3
Means 21 for calculating a rack axial force Fr acting on a vehicle, a road friction coefficient calculating means 22 for calculating a road friction coefficient μ, and an equivalent friction circle of the front wheel 1 and the rear wheel 6 based on the road friction coefficient μ. Frictional circle setting means 23 for setting
Front wheel lateral force calculating means 24 for calculating an actual lateral force (cornering force) Fyf of the front wheel 1 based on the rack axial force Fr and the front wheel steering angle δf, and driving obtained from, for example, a relationship between vehicle speed and intake pipe negative pressure. Front and rear force calculating means 25 for calculating the actual front and rear force Fxf of the front wheel 1 based on the force and the braking force obtained from the brake fluid pressure, front wheel equivalent friction circle data, front wheel actual lateral force Fyf, and front wheel actual front and rear force Fxf From the front wheel lateral force utilization ratio calculating means 26 for calculating the current front wheel lateral force utilization ratio ξf, and the steering resistance torque setting means 2 for setting the steering resistance torque command value Tc according to the front wheel lateral force utilization ratio ξf.
7 and the steering shaft 1 via the steering wheel 2 obtained from the output of the steering torque sensor 15 and the vehicle speed V obtained from the output of the vehicle speed sensor 16 on the rear wheel 6 (non-drive wheel) side.
Steering assist torque setting means 28 for setting steering assist torque command value Ta based on manual steering torque Ts acting on actuator 4
And a front wheel motor drive control unit 29 for controlling the output of the motor 4 of the power steering device 5 based on the steering resistance torque command value Tc and the steering assist torque command value Ta.

The steering resistance torque command value Tc to be applied to the motor 4 of the power steering device 5 is a correction value Tcr (R) which is set based on the rear wheel lateral force utilization rate ξr calculated by the rear wheel lateral force utilization rate calculating means described later. (Set by the steering resistance correction torque setting means 38) so that a steering resistance torque that also takes into account the rear wheel lateral force utilization rate Δr can be added.

The rear wheel steering device 7 is a rear wheel basic steering angle setting means for optimally setting the basic steering angle δr of the rear wheel 6 given to the steering rod 8 based on the front wheel steering angle δf, the vehicle speed V and the yaw rate γ. 31, a rod axial force calculating means 32 for calculating an axial force Fl acting on the steering rod 8, and an actual lateral force Fy of the rear wheel 6.
r, rear wheel lateral force calculating means 34 for calculating the actual longitudinal force Fxr of the rear wheel 6 based on the braking force obtained from the brake fluid pressure, equivalent friction circle data,
Rear wheel lateral force utilization calculating means 35 for calculating the current lateral force utilization ξr of the rear wheel 6 from the rear wheel actual lateral force Fyr and the rear wheel actual longitudinal force Fxr, and the front wheel lateral force utilization ξf or the rear wheel The rear wheel correction steering angle setting means 36 for setting the correction steering angle δc of the rear wheel 6 according to the lateral force utilization rate ξr, the rear wheel basic steering angle setting means 31 and the rear wheel correction steering angle setting means 36 And a rear wheel motor drive control means 37 for controlling the motor 9 for driving the steering rod 8 based on the sum of the signals.

Next, a method of calculating the road surface friction coefficient μ will be described. As shown in FIG. 3, the cornering power Cp of the tire decreases as the road surface friction coefficient μ decreases. Therefore, in the case of the rack / pinion type steering device, the rack axial force Fr at the same steering angle is equal to the road surface friction coefficient μ. It becomes smaller as it decreases. Therefore, the road surface friction coefficient μ is obtained by comparing the actual rack axial force Frc with respect to the front wheel steering angle δf and the reference rack axial force Frm preset as an internal model based on the identification results of the design values of the vehicle and the measured values obtained by experiments. Can be estimated.

The rack axial force Fr, which balances the steering resistance from the road surface, is determined by the viscosity term around the steering shaft, the inertia term,
Since the friction term and the friction term around the motor 4 are minute, they are omitted. If omitted, it is expressed by the sum of the rack axial force Fp from the steering shaft 14 and the rack axial force Fm from the motor 4, that is, Fr = Fp + Fm. This estimation method will be described with reference to FIG.

First, since the rack axial force Fp from the steering shaft 14 is represented by a value obtained by dividing the steering torque Ts by the pitch circle radius rp of the pinion, that is, Fp = Ts / rp, the pinion axial force calculating means 21a , The output Ts of the steering torque sensor 15 is input.

Next, the rack axial force Fm from the motor 4 is represented by a value obtained by multiplying the output shaft torque Tm of the motor 4 by the motor output gear ratio N, that is, Fm = N · Tm. The value Im and the voltage value Vm are obtained by being input to the motor axial force calculating means 21b.

Here, the output shaft torque Tm of the motor 4 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 of motor rotating part (design value / constant) θm ′: Motor angular velocity θm” : Motor angular acceleration (differential value of motor angular velocity θm ') Cm: Motor viscosity coefficient Tf: Friction torque

The motor angular velocity θm 'is obtained from the motor back electromotive force according to the following equation. θm ′ = (Vm−Im · Rm) / Km where Vm: motor voltage Rm: motor resistance (design value / constant) Km: motor induced voltage constant

The steering shaft 1 obtained as described above
4 and the rack axial force F from the motor 4
In practice, it is preferable to correct the phase shift between Fp and Fm by passing through m through the phase compensation filter 21c.

Actual rack axial force value F obtained as described above
From rc and the preset model rack axial force value Frm, the actual and model rack axial force increase rates with respect to the increase in the front wheel steering angle δf are obtained (see FIG. 5), and the steering angle range in which the vehicle response is linearly approximated. Within the actual rack axial force increase rate ΔF
rc / Δδf and model rack axial force increase rate ΔFrm / Δ
From the ratio ΔFrc / ΔFrm to δf, the road friction coefficient μ can be estimated with reference to a preset road friction coefficient determination map (see FIG. 6).

The maximum grip force Fmax of the tire is given by the product of the coefficient of friction μ between the tire and the road surface and the vertical load W applied to the contact surface of the tire (Fmax = μW). Therefore, if the road surface friction coefficient μ is known, the size of the friction circle is determined based on the basic shape of the friction circle set in advance based on the characteristics of the tire and the wheel load value during turning corrected with the lateral acceleration value. Can be set. If the longitudinal force Fxf is placed on this friction circle, the maximum lateral force Fyfmax at that time is obtained.

Next, a method of estimating the actual lateral force Fyf applied to the contact point of the front wheel 1 will be described with reference to FIG. First, the balance between the actual rack axial force Frc and the actual lateral force Fyf is given by the following equation. Frc · La = Fyf · T · cosδf That is, Fy = Frc · La / T · cosδ where La: distance between the center axes of rack shaft 3 and kingpin shaft K (design value / constant) T: trail δf: front wheel steering Angle (output of steering angle sensor)

Here, the trail T is a caster rail Tc determined by the mechanical setting of the wheel alignment.
This is a value to which a pneumatic trail Tp component that changes according to the vehicle speed V is added.
19, and search based on the vehicle speed V.

From the actual lateral force Fyf obtained in this way and the maximum lateral force Fyfmax obtained from the above-mentioned friction circle, the lateral force utilization calculating means 26 calculates the lateral force utilization ξf of the front wheel 1 from the following equation.
Is calculated. ξf = Fyf / Fyfmax

Next, referring to a map preset in the steering resistance torque setting means 27 and the steering resistance correction torque setting means 38, the steering resistance torque corresponding to the front wheel lateral force utilization factor Δf and the rear wheel lateral force utilization factor Δr is determined. Tcf and the steering resistance correction torque Tcr are obtained.

Note that the rear wheel maximum lateral force Fyrmax and the rear wheel actual lateral force Fyr are also the same as those of the above-described rack axial force.
The basic method is exactly the same as that for the front wheels, only in place of the axial force from the motor 9 acting on the front wheels. The actual rack axial force Frc may be obtained from an output of a load cell provided at an appropriate position in the steering system without using the above calculation.

The steering resistance torque command value Tc, which is the sum of the steering resistance torque Tcf determined in this way and the steering resistance correction torque Tcr, is used as a vehicle speed V in a map preset in the steering assist torque setting means 38. By controlling the motor 4 with a control command value Tm obtained by adding to an auxiliary steering torque command value Ta of a normal power steering device obtained by searching based on the manual steering torque Ts, on a low μ road. The motor 4 generates a pseudo steering resistance torque for preventing the driver from inadvertently performing excessive steering such that the cornering force exceeds the limit (the lateral force utilization factor Δf · Δr is 1 or more). Is driven, that is, the steering wheel 2 is relatively difficult to turn.

When the lateral force utilization ratio r of the rear wheel 6 exceeds 1, the vehicle body turns inward with respect to the tangent to the turning circle, that is, tends to spin. Therefore, in the present invention, as described above, the steering resistance is generated in the power steering device as the lateral force utilization rate Δr of the rear wheel 6 approaches 1, and the steering assist force is relatively reduced. As a result, the driver recognizes that the steering wheel 2 is excessively turned and returns the front wheel steering angle, so that the yawing moment is reduced and the oversteer tendency is eliminated.

In a vehicle in which the rear wheels are also steered, the rear wheel lateral force Fy is determined based on the rear wheel steering angle δr as described above.
r can be calculated, but for vehicles where the rear wheels are not steered,
A load cell is provided at an appropriate position on the support member of the rear wheel to detect the axial load of the rear wheel, that is, the side force, and calculate the vehicle body slip angle β based on the vehicle speed V, the lateral acceleration Gy, and the yaw rate γ by the following equation. Then, the rear wheel lateral force Fyr may be obtained by performing vector calculation from β = β (γ−G / V) dt side force and vehicle body slip angle β.

Although the above description has been made of an example in which the cornering force is used as an index of the lateral grip force on the road surface μ, the allowable value of the lateral acceleration that allows stable turning is determined by the size of the friction circle. The ratio of the lateral acceleration value to the allowable value can also be used.

[0035]

As described above, according to the present invention, the stability of vehicle behavior on a slippery road surface can be further enhanced.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of a mechanical system of a front and rear wheel steering vehicle to which the present invention is applied.

FIG. 2 is a schematic configuration diagram of a control system according to the present invention.

FIG. 3 is a relationship diagram between a cornering power and a road surface friction coefficient.

FIG. 4 is a block diagram of rack axial force calculating means.

FIG. 5 is an increase diagram of a rack axial force with respect to a steering angle.

FIG. 6 is a determination map of a road surface friction coefficient.

FIG. 7 is a block diagram of a lateral force calculating unit.

FIG. 8 is an explanatory diagram relating to lateral force calculation.

[Explanation of symbols]

 Reference Signs List 1 front wheel 2 steering wheel 3 rack shaft 4 electric motor 5 electric power steering device 6 rear wheel 7 rear wheel steering device 8 steering rod 9 electric motor 10 knuckle arm 11 tie rod 12.13 steering angle sensor 14 steering shaft 15 steering torque sensor 16 vehicle speed Sensor 17 Yaw rate sensor 18 Brake operation sensor 19 Computer unit 21 Rack axial force calculating means 22 Road surface friction coefficient calculating means 23 Equivalent friction circle setting means 24 Front wheel lateral force calculating means 25 Front wheel longitudinal force calculating means 26 Front wheel lateral force utilization calculating means 27 Steering resistance torque setting means 28 Steering shaft 29 Front wheel motor drive control means 31 Rear wheel basic steering angle setting means 32 Rod axial force calculation means 33 Rear wheel lateral force calculation means 34 Rear wheel front and rear force calculation means 35 Rear wheel lateral force utilization rate Operator 36 rear wheel correction steering angle setting unit 37 rear wheel motor drive control means 38 steering resistance correction torque setting means 39 trail map

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI B62D 119: 00 137: 00

Claims (2)

[Claims] 1. An electric power steering apparatus comprising: a motor that applies power to a steering system that applies a steering angle to front wheels of a vehicle; and control means that controls a steering assist force generated by the motor. Comprises calculating means for calculating a value corresponding to the lateral grip force currently generated by the rear wheel, and the steering assist force is relatively controlled as the corresponding value of the lateral grip force approaches the performance limit of the tire. An electric power steering apparatus characterized in that the electric power steering apparatus is controlled so as to reduce the power consumption. 2. The electric power steering apparatus according to claim 1, wherein the rear wheels are also mounted on a steered vehicle.