JPH0639263B2 - Vehicle electric power steering device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a motor-driven power steering device for a vehicle, and more particularly to a vehicle speed-sensitive motor-powered steering device for improving steering feeling during ultra-high speed traveling. Regarding the device.

(Prior Art) The yaw acceleration of the vehicle body when the vehicle is steered is strongly influenced by the vehicle speed, and the driver feels that the steering wheel has a poor sense of stability during high-speed traveling. For this reason, even in the case of an electric motor type power steering device in which an electric motor generates a steering assist force, a vehicle speed sensitive type that controls the current supplied to the electric motor according to the vehicle speed has been proposed. Improvement is achieved.

The applicant of the present invention has previously proposed this type of vehicle speed-sensitive motor-driven power steering apparatus. The vehicle speed-sensitive electric motor-powered steering device proposed by the applicant directly contributes to the steering of the steering wheel and the frictional resistance component consumed in the frictional resistance of the steering force transmission system based on the detected steering reaction force. The road surface load component to be calculated is calculated, the road surface load component is corrected to be small at a high speed by the detected vehicle speed to obtain a corrected road surface load component, and the corrected road surface load component and the friction resistance component are added to obtain A current to be supplied to the electric motor is determined based on the target steering assist force, and a steering assist force corresponding to the target steering assist force is generated in the electric motor.

(Problems to be Solved by the Invention) However, in the electric power steering apparatus according to this prior application, the corrected road surface load component is, for example, 0 when the vehicle speed is high.
Even if it is corrected to, the frictional resistance component in the target steering assist force is effectively present, and the electric motor generates a steering assisting force equivalent to the frictional resistance component at least, so that there is no sense of comfort when steering the steering wheel. There was a flaw. As a result, the yaw motion of the vehicle body is strongly influenced by the vehicle speed
It was difficult to obtain a good steering feeling when traveling at a speed of 150 [Km / h] or higher.

The present invention has been made in view of the above problems, and provides an electric motor-driven power steering device that gives a feeling of sitting when steering a steering wheel during ultra-high speed traveling, and improves straight running stability during ultra-high speed traveling. The purpose is to obtain a good steering feeling over the entire vehicle speed range.

(Means for Solving the Problem) As shown in the configuration diagram of FIG. 1, the present invention includes an electric motor for applying a steering assist force to a steering force transmission system, and the electric motor is provided in the steering force transmission system. In a motor-driven power steering apparatus for a vehicle, which is controlled based on the output signals of the steering force detecting means for detecting the steering force of the vehicle and the vehicle speed detecting means for detecting the vehicle speed of the vehicle, the steering force is transmitted based on the output signal of the steering force detecting means Generated by friction of the road force determining system that calculates the road surface load component of the steering assist force necessary to compensate the road surface resistance that the system receives from the road surface, and the friction of the steering force transmission system based on the output signal of the steering force detecting means The frictional resistance determining means for calculating the frictional resistance component of the steering assist force necessary for compensating the influence of the frictional resistance, and the frictional resistance component calculated by the frictional resistance determining means based on the output signal of the vehicle speed detecting means are used in the high vehicle speed range. Correction value calculating means for correcting to obtain a corrected frictional resistance component, and the corrected frictional resistance component calculated by the correction value calculating means and the road surface load component calculated by the road surface load determining means are added to obtain the target. A target steering assist force determining means for determining the steering assist force; and a driving means for driving the steering assist force generated by the electric motor so as to become the target steering assist force based on the output signal of the target steering assist force determining means. Is the gist.

(Operation) According to the electric motor-operated steering apparatus for a vehicle according to the present invention, the road surface load component and the friction resistance component are calculated based on the detected steering force or steering torque, and the friction resistance component is detected as the vehicle speed. The corrected frictional resistance component is calculated to be smaller at high vehicle speed by, and the current to be supplied to the electric motor is determined based on the target steering assist force obtained by adding the corrected frictional resistance component and the road surface load component, A steering assist force equivalent to the target steering assist force is generated in the electric motor. Therefore, at high vehicle speeds, the frictional force of the transmission system can be used to maintain the central position of the steering wheel, the transmission of vibrations from the steering wheels to the steering wheel can be prevented, and the steering wheel can be prevented. It is possible to give a feeling of sitting by making the steering at the beginning of turning heavy and to improve the straight running stability, thereby obtaining a good steering feeling.

Embodiment An embodiment of the present invention will be described below with reference to the drawings.

2 to 11 show an electric motor type power steering apparatus for a vehicle according to an embodiment of the present invention, FIG. 2 is an overall longitudinal sectional view, and FIG. 3 is a sectional view taken along the line III-III in FIG. Fig. 4, Fig. 4 is a sectional view taken along the line IV-IV in Fig. 2, a sectional view taken along the line V-V in Fig. 5 or Fig. 2, a perspective view in which Fig. 6 is partially taken out, and Fig. 7 are shown. An electric circuit diagram and FIG. 8 are flowcharts.

2 to 6, (11) is a gear case supported by a vehicle body (not shown), and the pinion shaft (12) connected to a running handle (not shown) is substantially cylindrical in the gear case (11). It is supported by a pinion holder (13). The pinion holder (13) is rotatably supported by the gear case (11) via bearings (14) and (15), and the pinion holder (13) has the pinion shaft (12) bearings (16) and (17). It is rotatably inserted through. These pinion shaft (12) and pinion holder (13)
The eccentric center of rotation of the pinion holder (13) causes the pinion holder (13) to rotate in response to the steering reaction force as the pinion shaft (12) swings during steering. As shown in FIG. 3, the pinion holder (13) is biased to the neutral position by the neutral position biasing mechanism (18).

Further, in the gear case (11), a rack shaft (19) connected to a traveling wheel (not shown) via a tie rod or the like is provided on the left side of the drawing via a metal bearing (20) and a pinion shaft (12). ) Is guided by a rack guide mechanism (21) (see FIG. 3) and supported slidably in the axial direction. The rack shaft (19) has rack teeth (19
a) is formed, and the rack teeth (19a) mesh with the pinion gear (12a) formed on the pinion shaft (12). The pinion gear (12a) and the rack teeth (19a) form a rack and pinion type gear mechanism, and transmit the manual steering force applied to the steering wheel to the steering wheels.

In the gear case (11), as shown in FIGS. 3 and 4, a steering torque sensor (22), a steering rotation sensor (23) and a control circuit (24) are arranged around the pinion shaft (12). Further, the unit case (31) accommodating the electric motor drive circuit (30) is fixed below the pinion shaft (12). As shown in FIG. 6, the steering torque sensor (22) and the steering rotation sensor (23) are connected to the control circuit (24) by printed wirings (25) and (26), respectively. 24) is connected to the motor drive circuit (30) by a printed wiring (32). The steering torque sensor (22) is attached to the pinion holder (13).
Of the movable core (27) that is fixed to the case (11) and the movable core (27) that projects from the upper part of the core and rotates integrally with the pinion holder (13).
A differential transformer (81) for detecting the rotational displacement of (7). An AC pulse signal is input from the control circuit (24) to the differential transformer (28), and a signal that represents the rotational displacement of the pinion holder (13), that is, the steering torque as a steering reaction force is output to the control circuit (24). . The steering circuit sensor (23) has a rotating shaft with a gear mechanism (29).
It is composed of a DC generator (which has the same number as the steering rotation sensor) connected to the pinion shaft (12) via the. The direct-current generator (23) has a rotating shaft that rotates integrally with the pinion shaft (12), and outputs a signal indicating the rotating direction and rotating speed of the pinion shaft (12) to the control circuit (24).

Further, in the center of the gear case (11), an electric motor (33) that generates a steering assist force according to the value of the electric current supplied is provided coaxially with the rack shaft (19). The electric motor (33) consists of a field magnet (34) fixed to the inner wall surface of the gear case (11) and a rack shaft.
And a rotor (35) rotatably disposed between the rotor (35) and the (19). The rotor (35) includes a cylindrical shaft (38) that is rotatably supported by bearings (36) and (37) and functions as an output shaft, and a laminated iron core (39) having a skew groove on the outer periphery of the cylindrical shaft (38). ) And the multi-turned armature winding (40) are coaxially and integrally fixed. The armature winding (40) is wired (44) through a commutator (41) fixed to the cylinder shaft (38) and a brush (43) housed in the holder (42) and elastically contacting the commutator (41). ) To the motor drive circuit (30). Furthermore, the gear case (1
In the inside of 1), a reduction mechanism (45) and a ball screw mechanism (46) are arranged on the left side of the electric motor (33) in the figure, and the reduction mechanism (45) and the ball screw mechanism (46) make the electric motor (33 ) Is the rack axis (1
It is connected to 9) so that power can be transmitted. The reduction mechanism (45) includes a drive gear (4) formed on the outer periphery of the cylinder shaft (38) of the electric motor (33).
7) and a driven gear (48) fixed to the end of a screw shaft (49) described later of the ball screw mechanism (46). The ball screw mechanism (46) has a bearing (5
Screw shaft (49) that is rotatable via 0) and (51) and is supported parallel to the rack shaft (19), and a nut member (50) screwed to the screw shaft (49) through a large number of balls. And have. Screw shaft (4
The driven gear (48) is fixed to the motor 9) as described above, and is driven by the electric motor (33) to rotate. The nut member (50) is
As shown in FIG. 5, the rack holder (51) sandwiched between the rack shaft (19) by the bolts (51a) and the rack holder (51) are connected by a pair of bolts (52), and the axial movement of the rack shaft (19) is integrated. Only is allowed. As shown in FIG. 7, a vehicle speed sensor (vehicle speed detecting means) (53) and an alarm lamp (54) are also connected to the control circuit (24). The vehicle speed sensor (53) includes a rotating plate (55) having magnetic poles S and N arranged at equal intervals and fixed to an output shaft of a transmission, and magnetic poles arranged near the rotating plate (55). And a reed switch (56) that operates when S and N approach each other. This vehicle speed sensor (53) uses a pulse signal of a frequency corresponding to the vehicle speed from the reed switch (56) to the control circuit (24).
Output to. The alarm lamp (54) is provided in a portion near the driver's seat where the driver can easily see it.

As shown in FIG. 7, the control circuit (24) includes an interface circuit (57) for the steering torque sensor (22) and a steering rotation sensor.
(23) interface circuit (58), vehicle speed sensor interface circuit (59), alarm lamp (54) drive circuit (6
0), a relay circuit (70) to be described later, a drive circuit (61) for (71), an amplifier circuit (62) for a current detection circuit circuit (72) to be described later, a constant voltage power supply circuit (63), a crystal oscillation circuit ( 64), a microcomputer circuit (65), a booster circuit (66), an output circuit (67) and the like are hybridized. The interface circuit (57) for the steering torque sensor (22) is composed of an oscillation circuit, an AC output circuit, a rectifier circuit, a low-pass filter circuit, etc., and is a differential transformer based on the clock signal a input from the microcomputer circuit (65). Output an AC pulse signal to the device (28),
Further, the output signal of the differential transformer (28) is converted into signals b and c representing the acting direction and the magnitude of the steering torque and output to the microcomputer circuit (65). Similarly, the steering rotation sensor (2
The interface circuit (58) for 3) outputs the signals d and e representing the steering direction and the steering speed to the microcomputer circuit (65).
Interface circuit for the vehicle speed sensor (53) (5
9) is a microcomputer circuit (6
Output to 5). The amplifier circuit (76) outputs to the microcomputer circuit (65) a signal f obtained by amplifying the output signal of the current detection circuit (72) representing the current value of the electric current supplied to the electric motor (33).

The constant voltage power supply circuit (63) is connected to the key switch circuit (7) via the motor drive circuit (30) by the above-mentioned printed wiring (32).
3) and is connected to the battery (75) via the key switch circuit (73) and the fuse circuit (74). This constant voltage power supply circuit (63) is a microcomputer circuit (6
5) Supply constant voltage power to etc. Crystal oscillator circuit (64)
Is a microcomputer circuit that outputs a reference pulse signal.
Output to (65). The microcomputer circuit (65) is R
Each of the sensors (22), (23), (53) and the current detection circuit (72), which is composed of an OM, a RAM, an A / D converter, a CPU, etc., and which is described above according to a control program stored in a ROM
The output signal of the signal is processed to output signals g, h, i, j, k, l, m, o to the drive circuit (61), the output circuit (67), and the drive circuit (60).

The output circuit (67) is supplied with high-voltage power from the booster circuit (66) and outputs a pulse width modulation drive signal (PWM signal) to the motor drive circuit (30) based on the signal input from the microcomputer circuit (65). To do. This output circuit (67) outputs the signal h,
i, j, k are input as direction switching signals, and signals l, m are input as impact coefficient setting signals, and these signals h, i, j, k and signals are input.
A pulse width modulation signal (PWM signal) having an impact coefficient determined as a logical product with l and m is output to the motor drive circuit (30). That is, this output circuit (67) is a motor drive circuit (3
From each of the four terminals connected to (0), a PWM signal having a shock coefficient as a logical product of a signal h and a signal m, a signal i and a signal m, a signal j and a signal l, and a signal k and a signal 1 is output. . The drive circuit (60) is connected to the alarm lamp (54) via the electric motor drive circuit (30) and energizes the alarm lamp (54) according to the signal o input from the microcomputer circuit (65). Similarly, the drive circuit (61) is a motor drive circuit (3
0) is connected to the relay circuit (70) and the electric motor drive circuit (30) and the relay circuit (71) interposed between the battery (75),
The solenoids of the relay circuits (70) and (71) are energized according to the signal g input from the microcomputer circuit (65).

The motor drive circuit (30) is a bridge circuit (68) having _four field effect transistors (FETs) Q ₁ , Q ₂ , Q ₃ and Q ₄ .
Relay circuit (70), current detection circuit (72) and fuse circuit
It is composed by hybridizing (69) and so on. Bridge circuit (68)
The drain of the FETs Q _1, Q ₄ is a source to the battery (75) through a fuse circuit (69) is connected to the drain of the FETs Q _2, Q _3, also, FETs Q _2, Q _3, the source is grounded , Source / drain of these FETs Q ₁ , Q ₂ and FE
Motor (33) is connected between the source and drain of TQ _3, Q _4. This bridge circuit (68) consists of each FET Q ₁ ,
_{Q 2, Q 3, FETQ 1} Q from the output circuit to the gate of the ₄ (67) PWM signal is entered, Q ₃ or FETs Q _2, Q ₄ is driven integrally and selectively ON, the energization to the electric motor (33) PWM current
The chopper control is performed according to the impact coefficient of the signal and the energization direction is controlled. The PWM signal of the impact coefficient of the logical product of the signal h and the signal m is input to the gate of the FET Q ₂ , and similarly, the PWM signal of the impact coefficient of the logical product of the signal i and the signal m is the FET Q ₃ and the signal. The PWM signal of the impact coefficient of the logical product of j and the signal l is input to the FET Q ₄ , and the PWM signal of the impact coefficient of the logical product of the signal k and the signal l is input to the FET Q ₁ . The current detection circuit (72) detects the value of the current supplied to the electric motor (33) and outputs a signal representing the current value to the control circuit (24), and the relay circuit
(70) connects / disconnects between the electric motor (33) and the bridge circuit (68) according to the output signal of the control circuit (24). Further, the fuse circuit (69) includes a relay circuit (71) and a fuse circuit (74).
Is connected to the battery (75) through the bridge circuit (68) and the battery (75) to prevent an excessive current from flowing through the bridge circuit (68). Will be described with reference to FIG.

In this electric motor type power steering device, when the ignition key is operated and the key switch (73) is turned on,
The microcomputer circuit (65) executes a series of processes shown in FIG. 8 to control the drive of the electric motor (33).

First, in step P ₁ , the microcomputer circuit
Initialize (63) and erase the data stored in the internal register. Subsequently, in step P ₂ , initial failure diagnosis is performed according to another defined subroutine, and the process proceeds to step P ₃ only when all are functioning normally.

In step P _3, the output signal from the steering torque sensor (22) (b), the failure diagnosis of (c) reading a steering torque sensor in accordance with a subroutine defined in the other in step P ₄ (22). Then, only when it is determined in step P ₄ that the steering torque sensor (22) is functioning normally,
Go to next step P ₅ . In step P ₅ , the signal (c) is subtracted from the signal (b) to generate a signal (T) representing steering torque (hereinafter referred to as steering torque T). This steering torque T is
It takes positive or negative depending on the direction of action, and its absolute value represents the magnitude of torque. Next, in step P _6, whether the steering torque T is positive or negative is determined. If the steering torque T is positive or 0, the flag F for sign determination is set to 0 in step P ₇ , and the steering torque T is negative. sign inversion processing of the steering torque T sets a 1 in step P ₈ and step P ₉ in the flag F (the absolute value) performed as long. Then, step P
_In FIG. ₁₀ , the frictional resistance component DF (T) is searched from the data table 1 shown in FIG. 10 using the steering torque T as an address, and an internal signal (hereinafter, frictional resistance component DF (T)) representing this is searched.
Generated). Similarly, in step P ₁₁ , the steering torque T is calculated from the data table 2 shown in FIG.
The road surface load component DL (T) is searched with the address as an address, and an internal signal representing this is referred to (hereinafter referred to as road surface load component DL (T)).
To generate. The frictional resistance component DF (T) represents the steering assisting force component necessary to eliminate the influences of the frictional resistance of the steering force transmission system and the offset voltage of the circuit, and similarly, the road surface load component DL (T ) Represents the component of the steering assist force necessary to eliminate the influence of road surface resistance on the steering wheel steering.

Next, read the output signal P from the vehicle speed sensor (53) in step P12, the signal representing the vehicle speed V by the vehicle speed V is calculated from the signal P at following step P ₁₃ (hereinafter, referred to as the vehicle speed V) to produce a . Then, a fault diagnosis of the vehicle speed sensor (53) in accordance with subroutine defined in the other in step P14, the process proceeds to step P ₁₅ only when the vehicle speed sensor (53) is functioning correctly. Step searching speed range frictional resistance correction amount DF (V) in P ₁₅ from the data table 3 shown in FIG. 12 the vehicle speed V as an address signal representative thereof (hereinafter,
High-speed region frictional resistance correction amount DF (V)) is generated.
This high vehicle speed region frictional resistance correction amount DF (V) is a predetermined vehicle speed V (for example, V is 150
It has an effective value in the vehicle speed range above [km / h]).
Subsequently, in step P16, the corrected frictional resistance component DF is obtained by subtracting the high vehicle speed region frictional resistance correction amount DF (T) from the frictional resistance component DF (T). Since the corrected frictional resistance component DF becomes smaller in the high vehicle speed range, the frictional resistance of the transmission system can be utilized at high vehicle speeds by the processing of steps to be described later, and a comfortable feeling is imparted to the steering handle when steering and a good steering feeling is provided. can get. In the next step P ₁₇ , it is determined whether or not the corrected frictional resistance component DF is negative. Only when it is determined that the corrected frictional resistance component DF is negative, the corrected frictional resistance component DF is set to 0 in step P ₁₈ Proceed to P ₁₉ . In step P ₁₉ , the corrected frictional resistance component DF and the road surface load component DL (T) are added to obtain the target steering assist force D (T). Then, in step P ₂₀ , the target steering assist force D
(T) is to determine 0 or not, the process proceeds if the target steering assist force D (T) is a 0 signal l in step P _21, to step 38 to be described later by setting 0 to both m, also, Target steering assist force D (T)
There advance to 0 if not step P _22.

In Step P22 reads the output signals e, f of the steering circuit sensor (23), followed by, in step P _23, a steering in accordance with a subroutine defined in other circuits the sensor (2
Perform failure diagnosis of 3), the process proceeds to Step P ₂₄ only when the steering circuit sensor (23) is functioning correctly. In step P _24, by subtracting the signal f from the signal e and calculates the steering speed N, the steering speed signal representative of the N (hereinafter, referred to as a steering speed N) for generating a. The steering speed N takes a positive or negative value according to the steering direction, and the positive / negative of the steering speed N is the same as the positive / negative of the steering torque described above. Next, in step P _25, to determine the sign of the steering speed N, the flag G for code determined in step P ₂₆ as long steering speed N is positive or 0 0
Is set, and if the steering speed N is negative, step P ₂₇
And in step P ₂₈ , the flag G is set to 1 and the sign reversal process (absolute value conversion) of the steering speed N is performed. Subsequently, in step P ₂₉ , the steering speed correction amount D (N) is searched from the data table 4 shown in FIG. 13 by using the steering speed N as an address, and an internal signal (hereinafter, steering speed correction amount D (N N)) is generated.

Next, signal h in step P ₃₁ as long as the flag F is 1 to determine the value of the flag F in step P _30, i, j, k, respectively
Set 1,0,1,0, and sets the signal h in step P ₃₂ If the flag F is a 0, i, j, k to each 0,1,0,1. In the following step P ₃₃ , it is determined whether or not the flags F and G have the same value. If the flags F and G have the same value, 1 is set to the signal l and the target steering assist is set to the signal m in steps P ₃₄ and P _35. set the force D (T) and the steering speed correction amount D (N) and the added value (D (T) + D ( N)), also the flag F, G are in step P ₃₆ and S P ₃₇ different The steering speed correction amount (1-D (N)) is set in the signal l, and the target steering assist force D (T) is set in the signal m. Then, in the next step P ₃₈ , the signals h, i, j, k, l, m are output, and in step P ₃₉ , failure diagnosis is performed. Therefore, the electric motor (33) is energized with an intermittent current having an impact coefficient corresponding to the values of the signals l and m, and generates a steering assist force corresponding to the impact coefficient to reduce the steering burden on the driver. After that, the series of processes from step P ₃ is repeatedly executed.

As described above, the motor-driven power steering apparatus according to this embodiment obtains the corrected friction resistance component by subtracting the supplemental component of the frictional resistance in the high vehicle speed range searched based on the vehicle speed from the frictional resistance component during traveling at high vehicle speed. , A continuous current having an impact coefficient corresponding to the target steering assist force obtained by adding the frictional resistance component and the road surface load component is supplied to the electric motor (33). Therefore, the electric motor (33)
The steering assist force that is generated becomes small in the high vehicle speed range,
The frictional resistance of the transmission system equipment such as the electric motor (33) gives a feeling of sitting in the steering of the steering wheel, and a good steering feeling can be obtained.

FIG. 9 shows an electric motor type power steering apparatus for a vehicle according to another embodiment of the present invention.

In this embodiment, the mechanism and the electric circuit are the same as the above-mentioned embodiment in the drawing, and only the parts different from the flowchart of FIG. 8 will be described below.

This embodiment executes a series of processes shown in the flowchart of FIG. However, the steps P ₁ in the drawing to the step P ₁₅ Step P ₁₅ from step P ₁ in FIG. 8
The same as above, and from step P ₁₈ to step P
_Since steps ₄₁ to ₄₁ are the same as steps P ₁₆ to P ₃₉ , the description thereof will be omitted.

In Step P ₁₆ , the data table 5 in FIG.
Searching auxiliary coefficient K (F) the vehicle speed V as address, in the next rather step P _17, corrected by multiplying the correction coefficient K (F) to the road load component DL (T). This auxiliary coefficient K
As is clear from FIG. 14, (F) is small in the high vehicle speed range, and the corrected road surface load component DL (T) is also small in the high vehicle speed range. Therefore, a better steering feeling can be obtained in the high vehicle speed range.

(Effects of the Invention) As described above, according to the electric motor-driven power steering apparatus for a vehicle according to the present invention, a feeling of sitting is imparted to steering of the steering wheel during high speed traveling, and straight running stability is improved. A better steering feeling can be obtained.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an electric motor type power steering apparatus for a vehicle according to the present invention. 2 to 8 show an embodiment of an electric motor type power steering apparatus for a vehicle according to the present invention, FIG. 2 is an overall longitudinal sectional view, and FIG. 3 is a sectional view taken along the line III-III in FIG. Fig. 4, Fig. 4 is a sectional view taken along the line IV-IV in Fig. 2, and Fig. 5 is second.
FIG. 6 is a sectional view taken along the line V-V in FIG. 6, FIG. 6 is a perspective view in which essential parts are taken out, FIG. 7 is an electric circuit diagram, and FIG. 8 is a flowchart. FIG. 9 is a flow chart of another embodiment of the electric motor type power steering apparatus for a vehicle according to the present invention. FIG. 10 to FIG. 14 are data tables used for the control processing of each embodiment. 12 …… Pinion shaft, 12a …… Pinion gear 19 …… Rack shaft, 19a …… Rack teeth 22 …… Steering torque sensor (steering force detection means) 23 …… Steering rotation sensor 24 …… Control circuit 30 …… Motor drive Circuit (driving means) 33 …… electric motor 53 …… vehicle speed sensor (vehicle speed detecting means)

Claims (1)

[Claims] 1. A steering force transmission system including an electric motor for applying a steering assist force, the electric motor comprising steering force detection means for detecting a steering force of the steering transmission system and vehicle speed detection means for detecting a vehicle speed of a vehicle. In a motor-driven power steering apparatus for a vehicle controlled based on the output signal of the steering force, a steering assist force necessary for compensating the influence of road resistance received from the road surface by the steering force transmission system based on the output signal of the steering force detection means. And a road surface load determining means for calculating a road surface load component of the steering force detecting means, and a frictional resistance of a steering assist force necessary for compensating an influence of a frictional resistance generated by friction of the steering force transmission system based on an output signal of the steering force detecting means. Frictional resistance determining means for calculating a component, and the frictional resistance component calculated by the frictional resistance determining means based on the output signal of the vehicle speed detecting means is corrected so as to be small in a high vehicle speed range. A correction value calculation means for calculating a resistance component, and a target steering assist force is determined by adding the corrected frictional resistance component calculated by the correction value calculation means and the road surface load component calculated by the road surface load determination means. Target steering assist force determining means, and drive means for driving the steering assist force generated by the electric motor so as to become the target steering assist force based on an output signal of the target steering assist force determining means. Electric power steering device for vehicles.