JP5011875B2 - Electric power steering device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electric power steering apparatus which can suppress peak torque transmitted to a torque transmitting member when reaching a steering limit position without separately adding a steering sensor or the like. <P>SOLUTION: The electric power steering apparatus includes an electric motor 12 for transmitting steering assist torque to a steering mechanism and a motor control part 24 for drive-controlling the electric motor 12 by a pulse-width modulation signal in accordance with a current command value based on steering torque. The motor control part 24 has a drive current detection part 60 for detecting the drive current of the motor, a current change rate detection part 67 for calculating the change rate of the motor drive current detected by the drive current detection part, and a duty ratio limiting part 64 for fixing a duty ratio of the pulse-width modulation signal to a predetermined value for limiting torque transmitted to a torque transmitting member disposed between a steering shaft of the steering mechanism and a steering road wheel when a calculated torque change rate is not lower than a threshold for determining a steering limit position. <P>COPYRIGHT: (C)2008,JPO&amp;INPIT

Description

  The present invention includes a current command value calculation unit that calculates a current command value based on at least a steering torque, an electric motor that applies a steering assist force to a steering mechanism, and a motor control unit that controls the electric motor based on the current command value And an electric power steering apparatus.

2. Description of the Related Art Conventionally, as a steering device, an electric power steering device that applies a steering assist force to a steering mechanism by driving an electric motor in accordance with a steering torque for a driver to steer a steering wheel has been widespread.
In general, in a steering mechanism, if the steering wheel is continuously steered in either the left or right steering direction from the neutral position, when the steering wheel operation amount reaches a maximum steering angle corresponding to the maximum value, the steering mechanism is mechanically stopped. It becomes a steering limit position where it cannot touch any further. Such a steering limit position is referred to as a so-called end pad.

When the steering wheel is operated quickly, that is, when the steering speed is high, the steering assist force generated by the electric power steering device is also increased, and the impact force generated at the time of end contact is increased, resulting in a steering mechanism. The durability of the vehicle may decrease, or the driver may feel uncomfortable during the steering operation.
For this reason, conventionally, as an electric power steering device configured to relieve impact at the time of end contact, an unloader correction unit that reduces and corrects the steering assist torque of the motor when the rudder angle exceeds a predetermined rudder angle near the maximum rudder angle There is known an electric power steering device in which the unloader correction unit increases and corrects the reduction correction amount of the auxiliary steering torque as the steering speed increases (see, for example, Patent Document 1).

In addition, when it is detected that the steered wheel approaches the predetermined maximum steering angle and exceeds the attenuation start steering angle, a damping unit that attenuates the driving force of the electric motor, the load on the steering wheel, and the steering wheel steering There is also known an electric power steering device including an attenuation start steering angle setting unit that sets the attenuation start steering angle according to speed (for example, see Patent Document 2).
However, in the conventional example described in Patent Document 1, the reduction correction amount of the steering assist torque is increased and corrected in the vicinity of the maximum steering angle, so that a sensor for detecting the steering angle is required, When the steering speed is high, there is an unresolved problem that the impact at the time of end contact cannot always be sufficiently mitigated due to the inertia of the electric motor.

  Further, in the conventional example described in Patent Document 2, since the attenuation start steering angle is set according to the load on the steered wheel and the steering speed of the steered wheel, the maximum rudder is obtained when the steering speed is high. By increasing the difference between the angle and the attenuation start steering angle (accelerating the time when the driving force of the electric motor is attenuated), it is possible to prevent a large impact from being generated at the time of end contact due to the motor inertia. However, a sensor for detecting the steering speed of the steered wheel is required, and when the steering wheel is turned back from the vicinity of the maximum steering angle toward the neutral position, the steering assist force is reduced due to the attenuation of the driving force of the electric motor. There is an unsolved problem that the steering feeling becomes worse due to insufficiency.

In order to solve these unsolved problems, it is considered to mechanically reduce the impact of the end pad (see, for example, Patent Document 3). In the invention described in Patent Document 3, the torque acting on the part reaches the predetermined value in the middle of the steering force transmission system from the electric motor to the steered wheel where the torque transmitted from the electric motor acts. In this case, by providing a torque limiter that limits the transmission of the torque, the rotational energy of the electric motor that increases instantaneously when the steering operation of the steering wheel stops suddenly, such as when the steering wheel reaches the steering end. Is limited by a torque limiter to prevent the generation of a large impact force by preventing electric power to the steering wheel side with a torque of a predetermined value or more.
Japanese Patent Laid-Open No. 2001-253356 (first page, FIG. 2, FIG. 7) JP 2001-30933 A (first page, FIG. 2, FIG. 3) JP 2000-335431 A (first page, FIG. 2, FIG. 3)

By the way, as a torque transmission member inserted between the steering shaft and the steering gear among the effects of the impact load when reaching the steering limit position, such as when the tire is in contact with the curb or at the end, The effect on the intermediate shaft is a big problem, and there is a problem that the durability of the intermediate shaft is lowered.
In each of the above conventional examples, the impact can be mitigated when the steering mechanism reaches the steering limit position, such as when the steering mechanism is in contact with the mechanical stopper or when the tire is in contact with the curbstone, etc. In the countermeasures by software as in the first and second conventional examples, since the absolute angle information of the steering wheel is used, a highly accurate rudder angle sensor or an absolute rudder angle estimation function is required. Since the steering angle sensor cannot be used and an expensive steering angle sensor or an absolute steering angle estimation function is required, there is an unsolved problem that the manufacturing cost increases.

Further, as in the third conventional example, in order to mechanically prevent the impact at the end using the torque limiter, it is necessary to incorporate the torque limiter, and the unresolved problem that the manufacturing cost similarly increases. There is.
Therefore, the present invention has been made paying attention to the unsolved problems of the above-described conventional example, and without adding a steering angle sensor, a torque limiter, etc., the tire is brought into contact with the curb stone, etc. An object of the present invention is to provide an electric power steering device that can alleviate an impact force transmitted to a torque transmission member such as an intermediate shaft when the steering limit position is reached.

To achieve the above object, an electric power steering apparatus according to claim 1 includes a steering torque detecting unit for detecting steering torque input to the steering mechanism, based on the steering torque detected by at least the steering torque detector section A current command value calculation unit that calculates a current command value, an electric motor that generates a steering assist torque to be applied to the steering shaft of the steering mechanism, and the electric motor is driven and controlled by a pulse width modulation signal based on the current command value An electric power steering apparatus including a motor control unit, wherein the motor control unit detects a drive current of the electric motor, and a rate of change of the motor drive current detected by the drive current detection unit The current change rate detector that calculates the current change rate and the motor drive current change rate calculated by the current change rate detector When it is equal to or greater than a threshold for determining the limit, it is determined that the duty ratio limiting condition is satisfied, and the duty ratio of the pulse width modulation signal is transmitted to the torque transmission member between the steering shaft and the steered wheels of the steering mechanism. And a duty ratio limiting unit that limits the torque to a predetermined value.

According to a second aspect of the present invention, there is provided the electric power steering apparatus according to the first aspect of the invention, wherein the duty ratio limiting unit includes a motor that has a change rate of the motor driving current equal to or higher than a threshold and is detected by the driving current detecting unit. When the absolute value of the detected drive current is equal to or greater than a predetermined value, it is determined that the duty ratio limiting condition is satisfied and the duty ratio is limited to the predetermined value.

Furthermore, in the electric power steering apparatus according to claim 3, in the invention according to claim 1, the duty ratio limiting unit is configured such that the motor drive current change rate is equal to or higher than a threshold and the motor detected by the drive current detection unit. the absolute value of the drive current detection value is configured to limit the predetermined value the duty ratio is determined as satisfying the duty ratio limiting conditions when predetermined time or longer the state is not less than a predetermined value It is characterized by that.

Still further, in the electric power steering apparatus according to claim 4, in the invention according to any one of claims 1 to 3, the duty ratio limiting unit determines that the duty ratio limiting condition is satisfied, and if the sign of the motor drive current detection value detected by the sign of the rate of change of the motor driving current the drive current detection unit match, that it is configured to limit the duty ratio to the predetermined value It is a feature.

Still further, an electric power steering apparatus according to a fifth aspect of the present invention is the invention according to any one of the first to third aspects, further comprising motor angular velocity detection means for detecting a motor angular velocity of the electric motor, and the duty ratio limiting unit. Is configured to limit the duty ratio to the predetermined value when it is determined that the duty ratio limiting condition is satisfied and the motor angular speed detected by the motor angular speed detecting means is greater than or equal to a predetermined value. It is characterized by being.

According to a sixth aspect of the present invention, there is provided an electric power steering apparatus according to any one of the first to third aspects, further comprising motor angular velocity detecting means for detecting a motor angular velocity of the electric motor, wherein the duty ratio limiting unit is , it is determined that satisfies the duty ratio limit conditions, when the motor angular speed is equal to or larger than the predetermined value and the detected before a predetermined time, the motor angular velocity detection means, so as to limit the duty ratio to the predetermined value It is characterized by being composed.

Furthermore, the electric power steering apparatus according to claim 7, in the invention according to any one of claims 1 to 6, wherein the duty ratio limit unit, a predetermined time after limiting the duty ratio to said predetermined value It is characterized in that the restriction on the duty ratio to the predetermined value is released when it elapses.
Still further, according to an eighth aspect of the present invention, in the electric power steering apparatus according to the seventh aspect, the predetermined time for releasing the restriction of the duty ratio to the predetermined value is set based on a motor angular velocity. Yes.

Further, the electric power steering apparatus according to claim 9, in the invention according to any one of claims 1 to 8, comprising a vehicle speed detecting means for detecting a vehicle speed, the duty ratio limit unit, the steering limit Threshold value setting means for increasing the threshold value to be determined according to the increase in the vehicle speed detected by the vehicle speed detection means is provided.
Furthermore, an electric power steering apparatus according to a tenth aspect is the invention according to any one of the first to ninth aspects, wherein the motor control unit is driven by the pulse width modulation signal to supply a motor driving current to the electric motor. The duty ratio limiting unit controls an electromagnetic brake mode by simultaneously turning on one of the upper arm and the lower arm of the inverter when the duty ratio limiting condition is satisfied. It is characterized by that.

Furthermore, the electric power steering apparatus according to claim 11 is the invention according to any one of claims 1 to 10, the torque transmission member of the steering mechanism has a yoke fabricated by press-molding the steering shaft It is characterized by having an intermediate shaft connected through a joint.
Still further, an electric power steering apparatus according to a twelfth aspect of the invention according to any one of the first to eleventh aspects is characterized in that the steering mechanism includes a buffer material at a rack stroke end of a steering gear. It is said.

According to the present invention, when the rate of change of the motor drive current detected by the current rate of change detection unit is equal to or greater than the threshold for judging the steering limit position, it is determined that the duty ratio limiting condition is satisfied, and the pulse width modulation is performed. Since the duty ratio of the signal is limited to a predetermined value that suppresses the torque transmitted to the torque transmission member between the steering shaft and the steered wheels of the steering mechanism, the intermediate ratio inserted between the steering shaft and the steering gear The steering assist torque generated by the electric motor before excessive torque is transmitted to a torque transmission member such as a shaft can be limited, and without the need for a separate steering angle sensor or torque limiter, The impact force transmitted to the torque transmission member when the steering limit position is reached, such as when it contacts the curb, can be reliably suppressed. The effect is obtained that.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a schematic configuration diagram showing an embodiment of the present invention, where SM is a steering mechanism. This steering mechanism SM has a steering shaft 2 having an input shaft 2a to which a steering force applied from a driver is transmitted to the steering wheel 1 and an output shaft 2b connected to the input shaft 2a via a torsion bar (not shown). It has. The steering shaft 2 is rotatably mounted on the steering column 3, one end of the input shaft 2a is connected to the steering wheel 1, and the other end is connected to a torsion bar (not shown).

  The steering force transmitted to the output shaft 2b is transmitted to the intermediate shaft 5 via the universal joint 4 composed of the two yokes 4a and 4b and the cross connecting portion 4c for connecting them, It is transmitted to the pinion shaft 7 through a universal joint 6 composed of yokes 6a and 6b and a cross connecting portion 6c for connecting them. The steering force transmitted to the pinion shaft 7 is transmitted to the left and right tie rods 9 via the steering gear 8, and steered wheels (not shown) are steered by these tie rods 9.

Here, as shown in FIG. 2, the steering gear 8 is configured in a rack and pinion type having a pinion 8b connected to the pinion shaft 7 and a rack shaft 8c meshing with the pinion 8b in a gear housing 8a. The rotary motion transmitted to the pinion 8b is converted into a straight motion by the rack shaft 8c.
The tie rod 9 is connected to both ends of the rack shaft 8c via ball joints 9a, and the rack shaft 8c is positioned at the steering limit position, that is, the rack, on the inner peripheral surface of the cylindrical portion 8d that covers the rack shaft 8c of the gear housing 8a. When the stroke end is reached, a stopper member 8f is formed on which the buffer member 8e formed on the inner end surface of the ball joint 9a attached to the rack shaft 8c comes into contact.

A steering assist mechanism 10 for transmitting a steering assist force to the output shaft 2b is connected to the output shaft 2b of the steering shaft 2. The steering assist mechanism 10 includes a reduction gear 11 connected to the output shaft 2b, and an electric motor 12 composed of, for example, a brushless motor as an electric motor that generates a steering assist force connected to the reduction gear 11. .
A steering torque sensor 14 is disposed in a housing 13 connected to the steering wheel 1 side of the reduction gear 11. The steering torque sensor 14 detects a steering torque applied to the steering wheel 1 and transmitted to the input shaft 2a. For example, a torsion bar (not shown) in which the steering torque is interposed between the input shaft 2a and the output shaft 2b. The torsional angular displacement is converted into a torsional angular displacement, and this torsional angular displacement is detected by a non-contact magnetic sensor.

The steering torque detection value T output from the steering torque sensor 3 is input to the controller 15 as shown in FIG. In addition to the torque detection value T, the controller 15 detects the vehicle speed detection value V detected by the vehicle speed sensor 16, the motor currents Iu to Iw flowing through the electric motor 12, and the rotation angle sensor 17 constituted by a resolver, an encoder, and the like. The rotation angle θ of the electric motor 12 is also input, and the steering assist torque command value I _M ^* for causing the electric motor 12 to generate a steering assist force corresponding to the input torque detection value T and vehicle speed detection value V is calculated. After performing various compensation processes on the steering assist command value I _M ^* based on the motor angular velocity ω and the motor angular acceleration α calculated based on the rotation angle θ, the phase is converted into a dq axis command value and then two-phase / 3 phase transformation to the three-phase current command value Iu ^* ~Iw ^* is calculated, Fidoba the drive current supplied to the electric motor 12 on the basis of the these three-phase current command value Iu ^* ~Iw ^* and the motor current Iu~Iw And click control process for outputting a motor current Iu, Iv and Iw for driving and controlling the electric motor 12.

That is, the controller 15 calculates the steering assist torque command value I _M ^* based on the steering torque T and the vehicle speed V, and the steering assist torque command value I calculated by the torque command value calculation unit 21. a command value compensating unit 22 that compensates for _M ^*, d-q-axis current command calculating the d-q axis current command value based on the command value compensated torque command value is compensated by the compensation section 22 I _M ^* ' A value calculation unit 23 and a motor current control unit 24 that generates motor currents Iu to Iw based on the command current output from the dq-axis current command value calculation unit 23 are configured.

The steering assist torque command value calculation unit 21 calculates a steering assist torque command value I _M ^* that is a current command value with reference to the steering assist torque command value calculation map shown in FIG. 4 based on the steering torque T and the vehicle speed V. To do.
As shown in FIG. 4, the steering assist torque command value calculation map has a parabolic shape with the steering torque T on the horizontal axis, the steering assist torque command value I _M ^* on the vertical axis, and the vehicle speed V as a parameter. It is composed of a characteristic diagram represented by a curve, and the steering assist torque command value I _M ^* is maintained at “0” when the steering torque T is from “0” to a set value Ts1 in the vicinity thereof, and the steering torque T is When the set value Ts1 is exceeded, initially, the steering assist command value I _M ^* increases relatively gently with respect to the increase in the steering torque T, but when the steering torque T further increases, the steering assist torque command value with respect to the increase. I _M ^* is set so as to increase steeply, and this characteristic curve is set so that the inclination becomes smaller as the vehicle speed increases.

  The command value compensator 22 differentiates the motor rotation angle θ detected by the rotation angle sensor 17 to calculate the motor angular velocity ω, and differentiates the motor angular velocity ω calculated by the angular velocity calculator 31. The angular acceleration calculation unit 32 that calculates the motor angular acceleration α, the convergence compensation unit 33 that compensates for the convergence of the yaw rate based on the motor angular velocity ω calculated by the angular velocity calculation unit 31, and the angular acceleration calculation unit 32 Based on the motor angular acceleration α, the inertia compensator 34 that compensates for the torque equivalent generated by the inertia of the electric motor 12 to prevent the deterioration of the feeling of inertia or control responsiveness, and the self-aligning torque (SAT) are estimated. And at least a SAT estimation feedback unit 35.

  Here, the convergence compensation unit 33 receives the vehicle speed V detected by the vehicle speed sensor 16 and the motor angular velocity ω calculated by the angular velocity calculation unit 31, and the steering wheel 1 shakes to improve the yaw convergence of the vehicle. A convergence compensation value Ic is calculated by multiplying the motor angular velocity ω by a convergence control gain Kv that is changed according to the vehicle speed V so as to apply a brake to the turning operation.

Further, the SAT estimation feedback unit 35 receives the steering torque T, the angular velocity ω, the angular acceleration α, and the steering assist current command value I _M ^* calculated by the steering assist torque command value calculation unit 21, and based on these, the self-aligning is performed. The torque SAT is estimated and calculated. The principle of calculating the self-aligning torque SAT will be described with reference to FIG. 5 showing the state of torque generated between the road surface and the steering. That is, when the driver steers the steering wheel 1, a steering torque T is generated, and the electric motor 12 generates an assist torque Tm according to the steering torque T. As a result, the wheel W is steered and a self-aligning torque SAT is generated as a reaction force. Further, at that time, torque serving as a steering resistance of the steering wheel 1 is generated by the inertia J and friction (static friction) Fr of the electric motor 12. Considering the balance of these forces, the following equation of motion can be obtained:
J ・ α + Fr ・ sign (ω) + SAT = Tm + T (1)
Here, when the above equation (1) is Laplace transformed with the initial value zero and the self-aligning torque SAT is solved, the following equation (2) is obtained.
SAT (s) = Tm (s) + T (s) − J · α (s) + Fr · sign (ω (s)) (2)

As can be seen from the above equation (2), the inertia J and static friction Fr of the electric motor 12 are obtained in advance as constants, so that the self-aligning torque is obtained from the motor angular velocity ω, rotational angular acceleration α, assist torque Tm, and steering torque T. The SAT can be estimated. Here, the assist torque Tm is proportional to the steering assist current command value I _M ^*, to apply a steering assist current command value I _M ^* in place of the assist torque Tm.

Then, the inertia compensation value Ii calculated by the inertia compensation unit 34 and the self-aligning torque SAT calculated by the SAT estimation feedback unit 35 are added by the adder 36, and the addition output of the adder 36 and the convergence compensation unit 33 are added. Is added by the adder 37 to calculate a command compensation value Icom, and this command compensation value Icom is output from the steering assist torque command value calculation unit 21. _The compensated torque command value I _M ^* ′ is calculated by adding to _M ^* by the adder 38, and this compensated torque command value I _M ^* ′ is output to the dq axis current command value calculation unit 23.

Further, the dq axis current command value calculation unit 23 includes a d axis target current calculation unit 51 that calculates a d axis target current Id ^* based on the compensated steering assist torque command value I _M ^* ′ and the motor angular velocity ω. , An induced voltage for calculating a d-axis EMF component e _d (θ) and a q-axis EMF component e _q (θ) of a dq-axis induced voltage model EMF (Electro Magnetic Force) based on the motor rotation angle θ and the motor angular velocity ω. The model calculation unit 52, the d-axis EMF component e _d (θ) and the q-axis EMF component e _q (θ) output from the induced voltage model calculation unit 52, and the d-axis output from the d-axis target current calculation unit 51 From the q-axis target current calculation unit 53 that calculates the q-axis target current Iq ^* based on the target current Id ^* , the compensated steering assist torque command value I _M ^* ′, and the motor angular velocity ω, and the d-axis target current calculation unit 51 the outputted d axis target current Id ^* and Axis target current output from the calculating unit 53 q-axis target current Iq ^* and a 3-phase current command value Iu ^*, and a 2-phase / 3-phase conversion unit 54 for converting the Iv ^* and Iw ^*.

The motor current control unit 24 includes a motor current detection unit 60 that detects motor currents Iu, Iv, and Iw supplied to the phase coils Lu, Lv, and Lw of the electric motor 12, and a dq-axis current command value calculation unit 23. Each phase current deviation is obtained by individually subtracting the motor currents Iu, Iv and Iw detected by the motor current detector 60 from the current command values Iu ^* , Iv ^* and Iw ^* input from the two-phase / three-phase converter 54. Subtractors 61u, 61v, and 61w for obtaining ΔIu, ΔIv, and ΔIw, and a PI current control unit that calculates voltage command values Vu, Vv, and Vw by performing proportional-integral control on the obtained phase current deviations ΔIu, ΔIv, and ΔIw. 62.

Further, the motor current control unit 24 receives the voltage command values Vu, Vv, and Vw output from the PI current control unit 62, performs a duty calculation based on these voltage command values Vu, Vv, and Vw, and performs each phase. The duty ratios Du _B , Dv _B and Dw _B are calculated, and the duty ratios Du _B , Dv _B and Dw _B are limited to a predetermined value, for example, 3%, and the limit duty ratios Du _L , Dv _L and Dw _L are calculated, Duty calculation / limitation calculation units 64u, 64v, and 64w are provided as duty ratio limiting units that select them according to a selection signal SL input from a selection signal forming unit 68 described later.

Here, the duty calculation / limit calculation unit 64u includes a duty ratio calculation unit 64a that calculates a positive / negative duty ratio Du _B based on the voltage command value Vu output from the PI current control unit 62, as shown in FIG. The limiter 64b that limits the duty ratio Du _B calculated by the duty ratio calculation unit 64a to a predetermined value, for example, 3%, the duty ratio Du _B that is output from the duty ratio calculation unit 64a, and the limit duty that is output from the limiter 64b When the ratio Du _L is input, and the selection signal SL to which the ratio Du _L is input is the logical value “0”, the duty ratio Du _B is selected, and when the selection signal SL is the logical value “1”, the limited duty ratio Du _L is selected. And a selection switch section 64c. The other duty calculation / limit calculation units 64v and 64w have the same configuration as the duty calculation / limit calculation unit 64u.

  Further, the motor current control unit 24 performs a pulse width modulation based on the duty ratio output from the duty calculation / limit calculation units 64u, 64v, and 64w, and outputs a pulse width modulation signal, An inverter 66 that outputs the three-phase motor currents Iu, Iv, and Iw to the electric motor 12 by receiving the selection pulse width modulation signal output from the pulse width modulation unit 65, and the motor current Iu detected by the motor current detection unit 60, A current change rate detection unit 67 that calculates a current change rate ΔIq based on Iv and Iw, and a logical value when the current change rate ΔIq calculated by the current change rate detection unit 67 is less than a threshold value ΔIth that determines a steering limit. A selection signal forming unit 68 that outputs a selection signal SL of “0” and outputs a selection signal SL of a logical value “1” when the selection signal SL is equal to or greater than a threshold value ΔIth. That.

Here, the current change rate detection unit 67 includes a three-phase / two-phase conversion unit 67a that converts the motor currents Iu, Iv, and Iw detected by the motor current detection unit 60 into a d-axis current Id and a q-axis current Iq, It comprises a differentiation circuit 67b for differentiating the q-axis current Iq converted by the three-phase / two-phase conversion section 67a to calculate the current change rate ΔIq.
Further, the selection signal forming unit 68 determines that the current change rate ΔIq calculated by the current change rate detecting unit 67 is such that the rack shaft 8c of the steering gear 8 reaches the rack stroke end or the tire contacts the curb or the like. It is determined whether or not a threshold value ΔIth (for example, 3500 A / sec) or more for determining a current of a large gradient that does not occur in normal steering that occurs when the steering limit position where steering is not possible is satisfied, and ΔIq <ΔIth. When it is determined that the rack stroke end has not been reached, a selection signal SL having a logical value “0” is output. When ΔIq ≧ ΔIth, it is determined that the rack stroke end has been reached and a logical value “1” is output. A selection signal SL is output.

  Here, as shown in FIG. 6, the q-axis current waveform output from the three-phase / two-phase converter 67a when the rack shaft 8c reaches the steering limit position indicates that the rack shaft 8c is at the steering limit position at time t1. As a result, the movement of the rack shaft 8c in the vehicle width direction is stopped, whereby the speed is further reduced via the pinion shaft 7, the universal joint 6, the intermediate shaft 5, the universal joint 4, and the output shaft 2b of the steering shaft 2. Since the rotation of the electric motor 12 is stopped via the gear 11, the motor currents Iu to Iw supplied to the electric motor 12 increase rapidly, and thereby a large inclination such that the q-axis current Iq is not generated by normal steering. (In the example of FIG. 6, about 4000 A / sec), and then the motor currents Iu to Iw are gradually reduced by the operation of the overcurrent protection circuit. It by, q-axis current Iq is decreased gradually. Therefore, the steering limit position can be reliably detected by setting the threshold value ΔIth of the current change rate ΔIq to a predetermined value (for example, about 3500 A / sec).

Next, the operation of the above embodiment will be described.
Now, in order to start traveling of the vehicle, by turning on the ignition switch IG, the controller 15 is powered on and the steering assist control process is started.
Therefore, the steering torque T detected by the steering torque sensor 14, the vehicle speed V detected by the vehicle speed sensor 16, the motor current detection values Iu to Iw detected by the motor current detection units 60 u to 60 w, the motor rotation detected by the rotation angle sensor 17. The angle θ is supplied to the controller 15.

Therefore, the steering assist torque command value calculation unit 21 calculates the steering assist torque command value I _M ^* based on the steering torque T and the vehicle speed V with reference to the steering assist command value calculation map shown in FIG.
On the other hand, the motor rotation angle θ detected by the rotation angle sensor 17 is input to the angular velocity calculation unit 31 to calculate the motor angular velocity ω, and the motor angular velocity ω is input to the angular acceleration calculation unit 32 to calculate the motor angular acceleration α. The

Then, the convergence compensation unit 33 calculates the convergence compensation value Ic based on the motor angular velocity ω, the inertia compensation unit 34 calculates the inertia compensation value Ii based on the motor angular acceleration, and the SAT estimation feedback unit 35 further calculates the motor. The self-aligning torque SAT is calculated based on the angular velocity ω and the motor angular acceleration α, and these are added by the adders 36 and 37 to calculate the command value compensation value Icom. The adder 38 calculates the steering assist torque command value I. _M ^* summed with the compensated steering assist torque command value compensation value I _M ^* 'in is calculated.

Then, the calculated post-compensation steering assist torque command value compensation value I _M ^* ′ is supplied to the dq axis current command value calculation unit 23.
At this time, when the vehicle is stopped and the steering wheel 1 is not steered, the steering torque T detected by the steering torque sensor 14 is “0”, and the vehicle speed V detected by the vehicle speed sensor 16 is also Since it is “0”, the steering assist torque command value I _M ^* calculated by the steering assist torque command value calculation unit 21 is also “0”.

On the other hand, since the electric motor 12 is in a stopped state, the motor currents Iu to Iw detected by the motor current detection unit 60 are also “0”, and the motor currents Iu to Iw are converted by the three-phase / two-phase conversion unit 67a. The q-axis current Iq is also “0”. Therefore, the current change rate ΔIq calculated by the current change rate detection unit 67 is also “0”, which is supplied to the selection signal forming unit 68 and the current change rate ΔIq is less than the threshold value ΔIth. The selection signal SL is output to the duty calculation / limit calculation units 64u to 64w. Therefore, the duty ratios Du _{B to} Dw _B output from the duty ratio calculation unit 64 a are selected by the duty calculation / limitation calculation units 64 u to 64 w, and these duty ratios Du _{B to} Dw _B are input to the pulse width modulation unit 65. Thus, a pulse width modulation signal for driving the gates of the switching elements of the upper arm portion and the lower arm portion constituting the inverter 66 is supplied from the pulse width modulation portion 65 to the inverter 66.

At this time, as described above, the post-compensation steering assist torque command value I _M ^* ′ is “0”, and the post-compensation torque command value I _M ^* ′ is supplied to the dq-axis current command value calculation unit 23. Therefore, the dq-axis current command value calculation unit 23 calculates the command value in the dq-axis coordinate system based on the motor rotation angle θ and the motor angular velocity ω, and the d-axis target current Id ^* and q The axis target current Iq ^* is calculated, and the d-axis target current Id ^* and the q-axis target current Iq ^* are converted into three-phase current command values Iu ^{* to} Iw ^* of “0” by the 2-phase / 3-phase converter 54, respectively ^. And output to the motor current control unit 24.

In the motor current control unit 24, since the motor currents Iu to Iw detected by the motor current detection unit 60 are also “0”, the current deviations ΔIu to ΔIw output from the subtractors 61u to 6w are also “0”. The voltage command values Vu to Vw output from the PI current control unit 62 are also “0”, and the duty ratios Du _{B to} Dw _B output from the duty calculation / limit calculation unit 64 are 0%, and accordingly Since the pulse width modulation signal is supplied from the pulse width modulation unit 65 to the inverter 66, the motor currents Iu to Iw output from the inverter 66 are also "0", and the electric motor 12 continues to be stopped.

When the steering wheel 1 is turned to the right (or left) while the electric motor 12 is stopped, the steering torque T corresponding to the steering direction is detected by the steering torque sensor 14, and the steering torque T Is supplied to the controller 15, the steering assist torque command value calculation unit 21 selects the innermost characteristic curve because the vehicle speed V is “0”, and the vehicle speed V increases as the steering torque T increases. A steering assist torque command value I _M ^{* that} is a large value is calculated, and this steering assist command value I _M ^* is output to the adder 38.

Therefore, the adder 38 adds the command compensation value Icom calculated by the command value compensation unit 22 to calculate the post-compensation steering assist torque command value I _M ^* ′, and this post-compensation steering assist torque command value I _M ^* 'Is supplied to the dq-axis current command value calculator 23.
The d-q axis current command value calculation unit 23 calculates the d-axis target current Id ^* and the q-axis target current Iq ^* having values corresponding to the post-compensation torque command value I _M ^* ′. The phase conversion unit 54 converts the current command values into three-phase current command values Iu ^{* to} Iw ^* and outputs them to the motor current control unit 24.

Therefore, in the motor current control unit 24, since the motor currents Iu to Iw detected by the motor current detection unit 60 are “0”, the current deviations ΔIu to ΔIw output from the subtractors 61u to 61w are current command values Iu. ^By supplying ^* to Iw ^* as it is to the PI current control unit 62, PI control processing is performed on the PI current control unit 62, and the voltage command values Vu to Vw are supplied to the duty calculation / limit calculation units 64u to 64w. Is output.

In this state, since the current change rate ΔIq calculated by the current change rate detection unit 67 maintains “0”, the selection signal forming unit 68 continues to output the selection signal SL having the logical value “0”. In the duty calculation / limit calculation units 64u to 64w, the selection switch unit 64c selects the duty ratios Du _{B to} Dw _B corresponding to the voltage command values Vu to Vw output from the duty ratio calculation unit 64a. A pulse width modulation signal corresponding to the duty ratios Du _{B to} Dw _B is supplied from the pulse width modulation unit 65 to the inverter 66. Therefore, the motor currents Iu to Iw are output from the inverter 66 and the electric motor 12 is rotationally driven to generate a steering assist torque corresponding to the steering torque T, which is output via the reduction gear 11 to the output shaft of the steering shaft 2. Since it is transmitted to 2b, steering in a stationary state can be performed lightly.

At this time, the motor currents Iu to Iw rapidly increase. In this case, the current change rate ΔIq is smaller than the current change rate when the steering limit position is reached and smaller than the threshold ΔIth as described above. Therefore, the selection signal forming unit 68 continuously outputs the selection signal SL having the logical value “0”, whereby the voltage output from the duty ratio calculation unit 64a by the selection switch unit 64c of the duty calculation / limit calculation unit 64. The state of selecting the duty ratios Du _{B to} Dw _B corresponding to the command values Vu to Vw is continued.

Thereafter, when the vehicle is started, the vehicle speed V detected by the vehicle speed sensor 16 increases, so that the steering assist torque calculated by the steering assist torque command value calculation unit 21 when the steering wheel 1 is steered during traveling. As the command value is selected in the map of FIG. 4, the outer characteristic curve is selected as the vehicle speed V increases. Therefore, the increase amount of the steering assist torque command value I _M ^* corresponding to the increase of the steering torque T decreases. The steering assist torque generated by the electric motor 12 is also smaller than that at the time of stationary, and the optimum steering assist torque according to the vehicle speed V can be generated.

  By the way, when the steering wheel 1 is steered relatively fast to the steering limit position to the right or left in the extremely low speed traveling state such as the stationary state or the garage, as described above, until the steering limit position is reached, The controller 15 forms motor currents Iu to Iw corresponding to the steering torque T detected by the steering torque sensor 14 at that time, and these are supplied to the electric motor 12 to perform light steering.

At this time, since the steering is performed in the stationary state or the extremely low speed traveling state, the steering torque T is large, and the duty ratio of the pulse width modulation signal output from the pulse width modulation unit 63 is approximately 100%. Yes.
In this state, when the shock absorbing member 8e of the rack shaft 8c reaches the rack stroke end where it contacts the stopper member 8f, or when the tire contacts the curb or the like and reaches the steering limit position, the movement of the rack shaft 8c is stopped. As a result, the rotation of the output shaft 2b of the pinion 8b, the pinion shaft 7, the universal joint 6, the intermediate shaft 5, the universal joint 4, and the steering shaft 2 is stopped, and the electric motor 12 is connected via the reduction gear 11 accordingly. The rotation is also stopped.

  At this time, since the duty ratio of the pulse width modulation signal output from the pulse width modulation unit 63 is close to 100% in the controller 15, the motor currents Iu to Iw output from the inverter 66 increase rapidly. The motor currents Iu to Iw are detected by the motor current detection unit 60 and supplied to the current change rate detection unit 67. For this reason, the q-axis current Iq output from the three-phase / two-phase conversion unit 67a increases steeply as shown in FIG. 6, and the current change rate ΔIq output from the differentiating circuit 67b is equal to or greater than the threshold value ΔIth. Become.

When the current change rate ΔIq that is equal to or greater than the threshold value ΔIth is supplied to the selection signal forming unit 68, the selection signal forming unit 68 generates a selection signal SL having a logical value “1” as a selection switch of the duty calculation / limitation calculation unit 64. Is output to the unit 64c. For this reason, the low duty ratio limit duty ratios Du _{L to} Dw _L output from the limiter 64 b are selected by the selection switch section 64 c and supplied to the pulse width modulation section 65. The pulse width modulation section 65 limits the duty ratio Du. A pulse width modulation signal corresponding to _{L to} Dw _L is supplied to the inverter 66.

  Therefore, the motor currents Iu to Iw output from the inverter 66 are reduced, the steering assist torque generated in the electric motor 12 is reduced, and the peak value of the transmission torque transmitted to the intermediate shaft 5 is shown by the solid line in FIG. As shown by the characteristic curve L1, the torque peak value can be suppressed compared to the characteristic curve L2 shown by the broken line when the duty ratio limit control is not performed, and the durability of the torque transmission member such as the intermediate shaft is improved. Can be made.

Moreover, by calculating the current change rate ΔIq based on the motor currents Iu to Iw detected by the motor current detection unit 60 and comparing the calculation result with the threshold value ΔIth, the end contact state and the state where the tire is in contact with the curb Since the steering limit position is detected, the steering limit position state can be detected in a short time (for example, about 10 msec) after reaching the steering limit position.
Since the steering assist torque generated by the electric motor 12 can be limited in a short time (for example, about 20 msec) from the detection of the steering limit position, the steering limit position is reached when the buffer member 8e is not provided on the rack shaft 8c. It becomes possible to limit the steering assist torque within the time until peak torque is generated in the intermediate shaft 5 (about 30 msec), and the torque generated in the intermediate shaft is reduced to improve the durability of the intermediate shaft 5. be able to. In addition, this effect can be exhibited without providing a separate sensor such as a steering angle sensor.

Further, in the above embodiment, since the buffer member 8e is provided on the rack shaft 8c, the time during which the peak torque is generated in the intermediate shaft 5 by the amount of contraction of the buffer member 8e is delayed when the end of the rack end stroke is reached. The torque generated in the intermediate shaft 5 can be reduced more reliably.
In the above-described embodiment, the case has been described in which the buffer member 8e is provided on the rack shaft 8c, thereby delaying the time from when the end contact is applied to when the peak torque is generated in the intermediate shaft 5, but the present invention is not limited thereto. Instead of pressing the yokes 4a, 4b and 6a, 6b of the universal joints 4 and 6 attached to both ends of the intermediate shaft 5, the rigidity is reduced with respect to the high-strength forged yoke, and the end pads The time from when the peak torque is generated in the intermediate shaft 5 can be lengthened. In this case, since the yoke is press-molded, it can be manufactured at a lower cost than the forged yoke, and the time until peak torque is generated in the intermediate shaft 5 from the time of end contact can be increased. An arithmetic processing device such as a microcomputer having a low level can be applied, and there is an advantage that the cost can be further reduced.

In the above embodiment, when the current change rate ΔIq is equal to or greater than the threshold value ΔIth, the steering torque is determined to be the steering limit position and is output from the duty ratio calculation unit 64a of the duty calculation / limit calculation unit 64. In the above description, the low duty ratio limit duty ratios Du _{L to} Dw _L output from the limiter 64b are selected instead of the duty ratios Du _{B to} Dw _B according to the above. However, the present invention is not limited to this. As shown in FIG. 8, the q-axis current Iq converted by the three-phase / two-phase converter 67a is also input to the selection signal forming unit 68, the current change rate ΔIq is equal to or greater than the threshold value ΔIth, and the q-axis current Iq is predetermined. When the value is equal to or greater than a value (for example, 40 A), the steering limit position detection condition is set. The end contact state can be more accurately detected by setting the steering limit position detection condition when the time of continuing the above for a predetermined time (for example, 10 msec). In this case, during normal steering, the vehicle may be braked on, for example, a Belgian road (cobblestone road) or the like, whereby a large vibration load may be input from the tire. If the vibration load is large, the motor of the electric motor 12 Although the current gradient tends to increase, since a large current does not flow continuously with respect to the vibration load from the tire, erroneous detection of this traveling state can be reliably prevented.

  Further, since the steering limit position occurs only when the steering wheel 1 is steered in the increment direction, it is an increment state in which the sign of the q-axis current Iq coincides with the sign of the current change rate ΔIq. The steering limit position detection condition may be set when the current change rate ΔIq is equal to or greater than the threshold value ΔIth. In this case, the current change rate ΔIq is equal to or greater than the threshold value ΔIth during steering in a direction away from the rack end. In this case, it is possible to reliably prevent erroneous detection.

  Further, in the above-described embodiment, the case where the threshold value ΔIth compared with the current change rate ΔIq in the selection signal forming unit 68 is a fixed value has been described. However, the present invention is not limited to this, and as shown in FIG. The vehicle speed V detected by the vehicle speed sensor 16 is input to the selection signal forming unit 68, and the threshold value setting process shown in FIG. 10 is executed by the selection signal forming unit 68, and the threshold value ΔIth for judging the steering limit is set according to the vehicle speed V. It may be changed.

  That is, the threshold setting process of FIG. 10 is executed as a timer interrupt process at predetermined time intervals (for example, 1 msec). First, in step S31, the vehicle speed V detected by the vehicle speed sensor 16 is read, and then the process proceeds to step S32. The threshold value ΔIth is calculated based on the read vehicle speed V with reference to the threshold value calculation table shown in FIG. 11, and then the process proceeds to step S33, where the calculated threshold value ΔIth can be referred to in the steering limit position detection process described later. After storing in a predetermined storage area such as a random access memory (RAM), the timer interrupt process is terminated and the program returns to the predetermined main program.

Here, the threshold value calculation table of FIG. 11 is set to have a predetermined value ΔIth1 when the vehicle speed V is “0”, and the threshold value ΔIth increases as the vehicle speed V increases.
The processing of FIG. 10 corresponds to threshold setting means.
In this way, by increasing the threshold value ΔIth for determining the steering limit in accordance with the increase in the vehicle speed V, steering to the end contact state does not occur as the vehicle speed increases, so that erroneous detection of the end contact state is ensured. Can be prevented.
In the threshold value setting process of FIG. 10, the case where the threshold value calculation table shown in FIG. 11 is used has been described. However, the present invention is not limited to this, and the characteristic line of FIG. The threshold value ΔIth may be calculated by substituting V. Further, the threshold value is not limited to being set by software, but the threshold value ΔIth corresponding to the vehicle speed V may be set using hardware such as a function generator.

  Further, as shown in FIG. 12, by supplying the motor angular velocity ω calculated by the angular velocity calculating unit 31 in place of the q-axis current Iq to the selection signal forming unit 68, the motor angular velocity ω is equal to or greater than a predetermined value and the current changes. When the rate ΔIq is equal to or greater than the threshold value ΔIth, the steering limit position detection condition may be set. In this case, the motor angular speed ω when the steering limit position is reached as the motor angular speed ω rapidly decreases as shown in FIG. 13, so the motor angular speed immediately before the steering limit position is reached (predetermined time (for example, about 20 msec). Use the previous motor angular speed). Thus, by putting the motor angular velocity ω into the steering limit position detection condition, when the motor angular velocity ω is smaller than a predetermined value, the impact load at the time of end contact is small, and the peak value of the transmission torque transmitted to the intermediate shaft 5 is small. Therefore, there is no need to perform steering assist torque limitation using a small fixed duty ratio, but when the motor angular velocity ω is equal to or greater than a predetermined value, the impact load when reaching the steering limit position becomes large. Since the peak value of the transmission torque transmitted to the intermediate shaft 5 also increases, the steering assist torque is limited to reliably reduce the peak torque transmitted to the intermediate shaft 5.

Furthermore, in the above-described embodiment, the case where the limited duty ratios Du _{L to} Dw _L are selected when the current change rate ΔIq is equal to or greater than the threshold value ΔIth has been described. However, the present invention is not limited to this. The duration time of the ratios Du _{L to} Dw _L is set to a predetermined time (for example, about 20 msec), and after the limited duty ratio state is continued for a predetermined time, the selection switch unit 64c of the duty calculation / limit calculation unit 64 is operated to calculate the duty ratio at normal time. it may be allowed to return to the state of selecting the duty ratio Du _B ~Dw _B in accordance with the voltage command value Vu~Vw output from section 64a. In this case, if the steering assist torque limitation with the long time limit duty ratios Du _{L to} Dw _L is continued, the driver may feel uncomfortable, so the peak of the transmission torque generated in the intermediate shaft 5 is reduced. It is preferable to suppress the uncomfortable feeling given to the driver by continuing the steering assist torque limitation by the limit duty ratios Du _{L to} Dw _L for a sufficiently short time. For this reason, since the impact load is large when the motor angular speed ω is large, the time for continuing the steering assist torque limitation by the limiting duty ratios Du _{L to} Dw _{L is} set to be relatively long. Conversely, when the motor angular speed ω is small, the impact load is Since it is small, it is preferable to set a relatively short time for continuing the steering assist torque limitation with the limit duty ratios Du _{L to} Dw _L.

Furthermore, in the above-described embodiment, the case where the electric motor 12 is controlled by supplying the pulse width modulation signal having the relatively small limit duty ratios Du _{L to} Dw _L to the inverter 66 when the steering limit position is detected has been described. However, the present invention is not limited to this, and the duty ratio of the pulse width modulation signal to the three switching elements constituting the upper arm part (or the lower arm part) of the inverter 66 by the pulse width modulation unit 65 is set to 0%. By fixing the duty ratio of the pulse width modulation signal with respect to the three switching elements constituting the arm part (or upper arm part) to 100%, each coil of the electric motor 12 is made a closed circuit in a short-circuit state, thereby The brake mode can be set so that the inertia force of the rotor of the electric motor 12 is not transmitted to the intermediate shaft 5. .

In the above embodiment, the case where the limit duty ratios Du _{L to} Dw _L are set to 3%, for example, has been described. However, the present invention is not limited to this, and any fixed duty can be set according to the characteristics of the electric motor 12. The ratio can be set.
Furthermore, although the case where the dq axis current command value calculation unit 23 is provided with the two-phase / three-phase conversion unit 54 has been described in the above embodiment, the present invention is not limited to this, and the two-phase / three-phase is provided. The conversion unit 54 is omitted, and a three-phase / two-phase conversion unit is provided on the output side of the motor current detection unit 60 instead of the conversion unit 54 to convert the d-axis current Id and the q-axis current Iq, and the two subtraction units perform d-axis Deviations between the target current Id ^* and the q-axis target current Iq ^* and the d-axis current Id and the q-axis current Iq of the motor may be calculated.

  Furthermore, in the above-described embodiment, the case where the controller 15 is configured by hardware has been described. However, the present invention is not limited to this, and the steering assist torque command value calculation unit 21, command value compensation is applied by applying a microcomputer. Unit 22, dq axis current command value calculation unit 23 and subtractors 61u to 61w of motor control unit 24, PI current control unit 62, duty control / limit control unit 64, pulse width modulation unit 65, current change rate detection unit The functions of 67 and the selection signal forming unit 68 can be processed by software. As processing in this case, the microcomputer may execute the steering assist control processing shown in FIG. 14 and the steering limit position detection processing shown in FIG.

  Here, as shown in FIG. 14, the steering assist control process is executed as a timer interrupt process every predetermined time (for example, 1 msec). First, in step S0, an end point set in a steering limit position detection process described later is set. It is determined whether or not the contact detection flag F is set to “1” indicating that it has detected that the steering limit position has been reached. When the steering limit position detection flag F is reset to “0”, step S1 is performed. Migrate to

In this step S1, the detection values of various sensors such as the steering torque sensor 14, the vehicle speed sensor 16, the rotation angle sensor 17, and the current detection circuit 60 are read, and then the process proceeds to step S2 and described above based on the steering torque T. After calculating the steering assist torque command value I _M ^* with reference to the steering assist torque command value calculation map shown in FIG. 4, the process proceeds to step S3.
In this step S3, the motor rotational angle θ is differentiated to calculate the motor angular velocity ω, then the process proceeds to step S4, the motor angular speed ω is differentiated to calculate the motor angular acceleration α, and then the process proceeds to step S5. Similarly to the convergence compensation unit 33, the motor angular speed ω is multiplied by the compensation coefficient Kv set according to the vehicle speed V to calculate the convergence compensation value Ic, and then the process proceeds to step S6.

In step S6, the inertia compensation value Ii is calculated based on the motor angular acceleration α as in the inertia compensator 34. Then, the process proceeds to step S7, and the motor angular velocity ω and motor angular acceleration are calculated in the same manner as in the SAT estimation feedback unit 35. The self-aligning torque SAT is calculated by performing the above-described calculation of equation (2) based on α.
Next, the process proceeds to step S8, and the post-compensation steering assist torque is obtained by adding the convergence compensation value Ic, inertia compensation value Ii and self-aligning torque SAT calculated in steps S4 to S6 to the steering assist torque command value I _M ^*. The command value I _M ^* ′ is calculated, and then the dq axis command value calculation process similar to the dq axis current command value calculation unit 24 is performed on the steering assist torque command compensation value I _M ^* ′ calculated in step S9. Then, the d-axis target current Id ^* and the q-axis target current Iq ^* are calculated, and then the process proceeds to step S10 to perform a two-phase / three-phase conversion process to calculate motor current command values Iu ^{* to} Iw ^* .

Next, the process proceeds to step S11, where the motor currents Iu to Iw are subtracted from the motor current command values Iu ^{* to} Iw ^* to calculate the current deviations ΔIu to ΔIw. Then, the process proceeds to step S12 and the current deviations ΔIu to ΔIw. The voltage control values Vu to Vw are calculated by performing the PI control process on the motor, and then the process proceeds to step S13 to calculate the duty ratios Du _{B to} Dw _B based on the calculated voltage command values Vu to Vw, and then the pulse width modulation The process is performed to form an inverter gate signal, and then the process proceeds to step S14, where the formed inverter gate signal is output to the inverter 66, and then the steering assist control process is terminated and the process returns to a predetermined main program.

On the other hand, when the result of determination in step S0 is that the steering limit position detection flag F is set to “1”, it is determined that the steering limit position has been reached, the process proceeds to step S15, and the limiter 64b described above is reached. Similarly, after calculating the limit duty ratios Du _{L to} Dw _L set to 3%, for example, pulse width modulation processing is performed to form an inverter gate signal, and then the process proceeds to step S14.

  Further, as shown in FIG. 15, the steering limit position detection process is executed as a timer interrupt process at predetermined time intervals (for example, 1 msec). First, in step S21, the motor current Iu input from the motor current detection unit 60 is executed. ˜Iw is read, and then the process proceeds to step S22, and the read motor currents Iu to Iw are subjected to a three-phase / two-phase conversion process to calculate the q-axis current Iq. Then, the process proceeds to step S23.

  In step S23, the calculated q-axis current Iq is differentiated to calculate a current change rate ΔIq, and then the process proceeds to step S24 to determine whether or not the calculated current change rate ΔIq is equal to or greater than a threshold value ΔIth. When ΔIq ≧ ΔIth, it is determined that the steering limit position is reached, the process proceeds to step S25, the steering limit position detection flag F is set to “1”, the timer interruption process is terminated, and a predetermined main program is entered. When ΔIq <ΔIth, it is determined that the steering limit position is not reached, the process proceeds to step S26, the steering limit position detection flag F is reset to “0”, the timer interruption process is terminated, and a predetermined value is obtained. Return to the main program.

  14 and 15, the process in step S2 in FIG. 14 corresponds to the current command value calculation unit, the process in steps S3 to S14 and the inverter 66 correspond to the motor control unit, and the process in steps S21 to S21 in FIG. The process of S23 corresponds to the current change rate detection unit, and the processes of steps S24 to S26 and the processes of steps S0 and S16 of FIG. 14 correspond to the duty ratio limiting unit.

  The microcomputer performs the steering assist control process of FIG. 14 and the steering limit position detection process of FIG. 15, so that the inverter 66 is brought into contact with the rack 66 when the rack shaft 8 c reaches the rack stroke end as in the above-described embodiment. By limiting the duty ratio of the supplied inverter gate signal to a small value, the motor currents Iu to Iw output from the inverter 66 are reduced to reduce the steering assist torque generated in the electric motor 12 and The peak value of the transmitted torque to be transmitted can be reduced, and the durability of the intermediate shaft 5 can be improved.

Furthermore, in the above embodiment, the case where the present invention is applied to a brushless motor has been described. However, the present invention is not limited to this, and when applied to a motor with a brush, as shown in FIG. Based on the motor current detection value Im output from the motor current detection unit 60 and the motor terminal voltage Vm output from the terminal voltage detection unit 70 in the calculation unit 31, the following equation (3) is calculated to calculate the motor angular velocity ω. At the same time, the dq axis command value calculation unit 23 is omitted, and the compensated torque command value I _M ^* ′ is directly supplied to the motor control unit 24. The motor control unit 24 is further connected to one subtraction unit 61, PI current, respectively. The control unit 62 and the duty calculation / limit calculation unit 64 are configured, and the current change rate detection unit 67 differentiates the motor current Im to calculate the current change rate ΔIdc, thereby forming a selection signal. So as to output a selection signal SL of "0" and "1" depending on whether the current change rate ΔIdc is less than the threshold ΔIth at 68, it may be further modified inverter 66 to the H bridge circuit 71.
ω = (Vm−Im · Rm) / K ₀ (3)
Here, Rm is the motor winding resistance, and K ₀ is the electromotive force constant of the motor.

Further, in each of the above embodiments, the duty calculation / limit calculation units 64u to 64w and 64 calculate the duty ratio Du _B using the duty ratio calculation unit 64a, and then calculate the limit duty ratio Du _L using the limiter 64b. Although the case where the selection switch unit 64c selects is described, the present invention is not limited to this. The voltage command values Vu to Vw, V ^* are limited to predetermined values by a limiter, and the voltage command values Vu to Vw, V ^* Limit voltage command values Vu _{L to} Vw _L and V ^* _L may be selected by a selection switch unit, and the duty ratios Du to Dw and D may be calculated by the duty ratio calculation unit by selecting the selected voltage value.

1 is a diagram illustrating a schematic configuration of an electric power steering apparatus according to a first embodiment of the present invention. It is the front view which made a part the section which shows the concrete composition of a steering gear. It is a block diagram which shows the specific example of the controller which concerns on this invention. It is a characteristic diagram which shows the steering assist torque command value calculation map which shows the relationship with the steering assist torque command value which uses vehicle speed as a parameter. It is a schematic diagram with which it uses for description of the self-aligning torque. It is a signal waveform diagram showing the q-axis current change when the steering limit is reached. It is a characteristic diagram which shows the torque characteristic transmitted to an intermediate shaft. It is a block diagram which shows other embodiment of this invention. It is a block diagram which shows other embodiment of this invention. 10 is a flowchart illustrating an example of a threshold setting process procedure executed by a selection signal forming unit in FIG. 9. It is a characteristic diagram which shows the threshold value calculation table used with the threshold value setting process of FIG. It is a block diagram which shows other embodiment of this invention. It is a characteristic diagram which shows the q-axis current and the motor angular velocity when the steering limit is reached. It is a flowchart which shows an example of the steering assistance control processing procedure performed with a microcomputer. It is a flowchart which shows an example of the steering limit position detection processing procedure performed with a microcomputer. It is a block diagram which shows embodiment at the time of applying a motor with a brush.

Explanation of symbols

  SM ... steering mechanism, 1 ... steering wheel, 2 ... steering shaft, 2a ... input shaft, 2b ... output shaft, 3 ... steering column, 4, 6 ... universal joint, 5 ... intermediate shaft, 8 ... steering gear, 8a ... gear Housing, 8b ... pinion, 8c ... rack shaft, 8e ... buffer member, 8f ... stopper member, 10 ... steering assist mechanism, 11 ... reduction gear, 12 ... electric motor, 14 ... steering torque sensor, 15 ... control unit, 16 ... Vehicle speed sensor, 17 ... rotation sensor, 21 ... steering assist torque command value calculation unit, 22 ... command value compensation unit, 23 ... dq axis current command value calculation unit, 24 ... motor current control unit, 31 ... angular velocity calculation unit, 32 ... Angular velocity calculation unit, 33 ... Convergence compensation unit, 34 ... Inertia compensation unit, 35 ... SAT estimation feedback unit, 36-38 ... Arithmetic unit 60 ... motor current detection unit 61u to 61w ... subtraction unit 62 ... PI current control unit 63 ... pulse width modulation unit 64 ... fixed duty ratio pulse width modulation unit 65 ... signal selection unit 66 ... inverter , 67 ... Current change rate detection unit, 68 ... Selection signal forming unit

Claims (12)

A steering torque detector for detecting a steering torque input to the steering mechanism, a current command value calculation unit for calculating a current command value based on the steering torque detected by at least the steering torque detecting unit, a steering shaft of the steering mechanism An electric power steering apparatus comprising: an electric motor that generates a steering assist torque to be applied to the motor; and a motor control unit that drives and controls the electric motor based on a pulse width modulation signal based on the current command value,
The motor control unit includes a drive current detection unit that detects a drive current of the electric motor, a current change rate detection unit that calculates a change rate of the motor drive current detected by the drive current detection unit, and the current change rate detection When the rate of change of the motor drive current calculated by the unit is equal to or greater than a threshold for determining the steering limit, it is determined that the duty ratio limiting condition is satisfied, and the duty ratio of the pulse width modulation signal is determined as the steering shaft of the steering mechanism. And a duty ratio limiting unit that limits the torque transmitted to the torque transmission member between the steered wheels to a predetermined value that suppresses the torque. The duty ratio limiting unit satisfies a duty ratio limiting condition when a rate of change of the motor driving current is equal to or greater than a threshold value and an absolute value of a detected value of the motor driving current detected by the driving current detecting unit is equal to or greater than a predetermined value. The electric power steering apparatus according to claim 1, wherein the electric power steering apparatus is configured to limit the duty ratio to the predetermined value based on the determination. The duty ratio limiting unit continues a state where the rate of change of the motor drive current is equal to or greater than a threshold and the absolute value of the detected value of the motor drive current detected by the drive current detection unit is equal to or greater than a predetermined value for a predetermined time or longer. 2. The electric power steering apparatus according to claim 1, wherein the electric power steering apparatus is configured to limit the duty ratio to the predetermined value by determining that the duty ratio limiting condition is satisfied. The duty ratio limit unit, the determines that satisfies the duty ratio limit condition, and the sign of the motor drive current detection value detected by said rate of change of the motor driving current sign the drive current detection unit matches In this case, the electric power steering apparatus according to any one of claims 1 to 3, wherein the duty ratio is configured to be limited to the predetermined value. Motor angular velocity detecting means for detecting a motor angular velocity of the electric motor, wherein the duty ratio limiting unit determines that the duty ratio limiting condition is satisfied, and the motor angular velocity detected by the motor angular speed detecting means is a predetermined value The electric power steering apparatus according to any one of claims 1 to 3, wherein the duty ratio is configured to be limited to the predetermined value in the case described above. Motor angular velocity detecting means for detecting the motor angular velocity of the electric motor, the duty ratio limiting unit determines that the duty ratio limiting condition is satisfied, and the motor detected by the motor angular velocity detecting means a predetermined time before 4. The electric power steering apparatus according to claim 1, wherein the duty ratio is configured to be limited to the predetermined value when the angular velocity is equal to or higher than a predetermined value. 5. The duty ratio limit unit claims, characterized in that it is configured to release the restriction to the predetermined value of the duty ratio when a predetermined time has elapsed since by limiting the duty ratio to said predetermined value The electric power steering apparatus according to any one of 1 to 6 . The electric power steering apparatus according to claim 7 , wherein the predetermined time for releasing the restriction of the duty ratio to the predetermined value is set based on a motor angular velocity. Vehicle speed detecting means for detecting a vehicle speed, and the duty ratio limiting unit includes threshold setting means for increasing a threshold for determining the steering limit according to an increase in the vehicle speed detected by the vehicle speed detecting means. The electric power steering apparatus according to any one of claims 1 to 8 , wherein the electric power steering apparatus is characterized in that: The motor control unit includes an inverter that is driven by the pulse width modulation signal to supply a motor driving current to the electric motor, and the duty ratio limiting unit is configured to perform an upper operation of the inverter when the duty ratio limiting condition is satisfied. arm and an electric power steering apparatus according to any one of claims 1 to 9, characterized in that so as to control the electromagnetic brake mode as simultaneously turned on one of the lower arm. Torque transmitting member of the steering mechanism, according to any one of claims 1 to 10, characterized in that an intermediate shaft connected via a joint having a yoke fabricated by press-molding the steering shaft Electric power steering device. The electric power steering apparatus according to any one of claims 1 to 11 , wherein the steering mechanism includes a buffer material at a rack stroke end of a steering gear.

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