JP2008013007A - Steering system of vehicle - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steering system of a vehicle, which steering system can apply a steering assist force suitable for the steering operation of a steering wheel even when a steering angle detecting means is in trouble. <P>SOLUTION: When the steering angle θ of the steering wheel has exceeded a reference steering angle θa (S103: No), a basic assist current value Ias* is compared with an end current limiting value Iend. If Ias* is larger than Iend, the current limitation is carried out by Iend (S106). If Ias* is smaller than Iend, the current limitation is not carried out (S104). By this method, the current limitation of an electric motor by a small current value can be prevented. In determining Iend, a limitation current lower limit value Ilimit_low is compared with the electric motor actual current I when the steering angle θ has exceeded the reference steering angle θa. If ¾I¾>¾Ilimit_low¾, limit_low is used as Iend. If ¾I¾≤¾Ilimit_low¾, I is used as Iend. The most appropriate end current limitation value Iend can be determined by determining Iend as described above. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a vehicle steering apparatus, and more particularly to a vehicle steering apparatus including an electric power steering apparatus.

  The electric power steering apparatus includes an electric actuator such as an electric motor and a control unit that controls driving of the electric actuator, and uses a driving force generated from the electric actuator as a steering assisting force of the steering operation. It is possible to improve the property. Generally, the driving force of the electric actuator acting as a steering assist force is set to be larger as the steering torque of the steering wheel is larger. The steering torque changes under the influence of an external force such as a self-aligning torque. Since the external force changes according to the steering angle of the steering wheel, the steering torque changes according to the magnitude of the steering angle. In general, the steering torque tends to increase as the steering angle increases. Therefore, as the steering angle increases, the steering assist force also increases, thereby increasing the drive current value for driving the electric actuator.

  By the way, the steering operation of the steering wheel has a limit point (stroke end) for left and right infeed steering. At this stroke end, a mechanical stopper (rack end stopper, etc.) is added to the steering mechanism (rack bar, etc.) connected to the steering mechanism. ) Is prohibited from further steering of the steering wheel. At this time, when the steering wheel has a high steering angular velocity (that is, when the steering wheel is quickly turned), a large rotational torque and a large inertial force are generated in the electric actuator near the stroke end. The impact force at the time of contact increases.

In order to mitigate the impact force, in Patent Document 1, when the steering angle of the steering wheel becomes equal to or larger than a certain steering angle close to the maximum steering angle (steering angle at the stroke end), the current flows to the electric actuator. The current value is determined as the limit current value, and the electric actuator (electric motor) is driven and controlled by the determined limit current value from the constant steering angle to the maximum steering angle. By performing such control, the steering assist force is limited in the steering angle region from the constant steering angle to the maximum steering angle, and the steering wheel turning resistance in this region does not perform this control. Compared to this, it becomes larger and it becomes difficult to turn the steering wheel. As a result, the steering angular velocity is reduced and the rotational torque of the electric actuator is also reduced, so that the impact force at the stroke end can be reduced.
Japanese Patent Laid-Open No. 01-233165

  Since the steering device described in Patent Document 1 determines a region where the electric actuator current is limited based on the steering angle, even if the steering angle detection unit such as the steering angle sensor fails, The current is limited based on the steering angle detected by the steering angle detection means. Therefore, even if the turning position of the steering wheel is close to the neutral position, it may be determined that the steering wheel is close to the stroke end due to erroneous detection due to a failure of the steering angle detecting means. When the drive current of the electric actuator is limited in such a manner, a small current value near the neutral position is set as the limit current value, and thus the steering assist force from the electric actuator is small. For this reason, the turning resistance when steering the steering wheel in a wide steering angle region from the vicinity of the neutral position to the steering limit position becomes large, and there is a possibility that a light steering feeling may be impaired. In particular, if this happens at the time of stationary, the range that can be stationary may be narrowed. Moreover, there is a possibility that the electric power steering device itself may be mistaken for a malfunction.

  Contrary to the above situation, if the steering angle is actually close to the maximum steering angle but the steering angle is still detected as a value much smaller than the maximum steering angle due to erroneous detection, In some cases, the steering mechanism may collide with the mechanical stopper at the stroke end without limiting the driving current of the actuator, so that the current limitation is not substantially performed.

  The present invention has been made to solve the above-described problem, and is a vehicle steering device that can apply an appropriate steering assist force to the steering operation of the steering wheel even when the steering angle detection means is out of order. It is a technical challenge to provide

  In order to achieve the above object, a feature of the present invention is a vehicle steering apparatus including an actuator that generates a steering assist force for assisting a steering operation of a steering handle, and a control unit that controls driving of the actuator. The control means includes target control amount determination means for determining a target control amount of the actuator so that the actuator generates a predetermined steering assist force, and a steering angle of the steering wheel is equal to or greater than a predetermined reference steering angle. And a control amount limiting means for limiting the control amount of the actuator to a predetermined reference control amount when the target control amount determined by the target control amount determining means is larger than a predetermined reference control amount. It is to have done.

  According to the above invention, when the steering angle becomes equal to or larger than the predetermined reference steering angle, the target control amount of the actuator determined by the target control amount determining means is compared with the reference control amount, and the target control amount is determined as the reference control amount. When the amount is larger than the amount, the control amount is limited to the reference control amount. Therefore, the reference control amount is set to a relatively large value (for example, when the control amount is the current value of the electric motor, the current value sufficiently larger than the current value flowing through the electric motor at the neutral position of the steering wheel) When the target control amount is smaller than the reference control amount, the target control amount determined by the target control amount determining means is used as it is as the control amount of the actuator, thereby preventing the control amount from being limited by a small value. it can. When the target control amount is larger than the reference control amount, the control amount of the actuator is limited to the reference control amount that is set sufficiently large. As described above, according to the present invention, not only the magnitude of the steering angle but also the magnitude of the control amount of the actuator is a target for determining whether or not to limit the steering assist force. Even when the steering wheel is in a proper position, it is possible to apply an appropriate steering assist force to the steering operation of the steering wheel, and to prevent adverse effects on the steering operation such as impairing a light steering feeling due to the limited control amount being too small. Can do.

  In this case, an electric motor is used as the actuator, and the control amount can be a current value flowing through the electric motor. The reference steering angle can be arbitrarily set in the steering angle region from one maximum steering angle of the steering wheel to the other maximum steering angle (lock to lock), but the steering angle is as close to the maximum steering angle as possible. It is good to do. For example, the steering angle can be set to about 90% of the maximum steering angle (when the maximum steering angle is 600 °, the reference steering angle is 540 °). Further, as a means for detecting whether or not the steering angle is equal to or larger than a predetermined reference steering angle, a steering angle detecting means such as a steering angle sensor can be cited, but it does not judge the angle itself, and the steering angle is What is necessary is just to detect whether it is more than the said reference | standard steering angle (for example, detection switch etc.).

  The control means is configured such that an actual control amount of the actuator when a steering angle of the steering wheel becomes the reference steering angle or when a steering angle of the steering wheel becomes equal to or larger than the reference steering angle is a predetermined limit control amount. In the following cases, it is preferable to include a reference control amount setting means for setting the limit control amount to the reference control amount. If the actual control amount of the actuator is small even if the steering angle of the steering wheel becomes large enough to be equal to or exceeding the reference steering angle, the steering angle may be erroneously detected. In the present invention, in this case, the reference control amount is set to a predetermined limit control amount instead of the actual control amount detected at that time. Therefore, after the limit control amount is set to the reference control amount, the control amount is not limited as long as the target control amount does not reach the limit control amount even if the steering angle is further increased. Only after the control amount is reached, the control amount of the actuator is limited to the limit control amount. Therefore, by setting the limit control amount set as the reference control amount to an appropriate value, it is possible to effectively prevent the actuator drive from being limited by a small control amount.

  Since the limit control amount can be a limit value when limiting the current of the actuator, it is preferable that the limit control amount is set to an appropriate value to the extent that the above-described conventional problems can be solved. For example, from experience, the control amount for generating the normal steering assist force required at the reference steering angle can be set as the limit control amount, or the necessary control amount at the reference steering angle can be determined in advance through experiments or the like. It is also possible to acquire and use the acquired control amount as the limit control amount.

  In this case, the reference control amount determining means is configured to determine the actual control amount of the actuator when the steering angle of the steering wheel becomes the reference steering angle or when the steering angle of the steering wheel becomes equal to or larger than the reference steering angle. Is greater than a predetermined limit control amount, the actual control amount may be the reference control amount. If the actual control amount of the actuator exceeds the limit control amount when the steering angle of the steering wheel is equal to or larger than the reference steering angle, the steering angle is considered to have been detected normally. It is done. In the present invention, the actual control amount is set to the reference control amount in such a case. Therefore, after the actual control amount is set as the reference control amount, when the steering angle becomes larger and the target control amount becomes larger than the reference control amount, the actuator control amount is limited to the reference control amount. Thereby, the control amount of the actuator can be limited before the stroke end, and the impact force at the time of the collision between the steering mechanism and the mechanical stopper at the stroke end can be reduced. Note that the actual control amount set as the reference control amount is the reference value from the actual control amount of the actuator when the steering angle of the steering wheel becomes the reference steering angle, or from the state where the steering angle of the steering wheel is equal to or less than the reference steering angle. It is preferable to set the actual control amount of the actuator when the steering angle is equal to or greater than the steering angle.

(First embodiment)
Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is an overall schematic diagram of a vehicle steering apparatus including an electric power steering apparatus which is an apparatus having an assist function with respect to a steering operation by a driver according to an embodiment of the present invention.

  The vehicle steering apparatus includes a steering shaft 12 connected to a steering handle 11 so that an upper end thereof is integrally rotated. A pinion gear 13 is connected to a lower end of the shaft 12 so as to be integrally rotated. The pinion gear 13 meshes with rack teeth formed on the rack bar 14 to constitute a rack and pinion mechanism. Left and right front wheels FW1 and FW2 are steerably connected to both ends of the rack bar 14, and the left and right front wheels FW1 and FW2 are left and right according to the axial displacement of the rack bar 14 as the steering shaft 12 rotates about the axis. Steered to. An electric motor 15 for steering assist is assembled to the rack bar 14. The electric motor 15 generates an assist torque Tas (steering assist force) for assisting the steering operation of the steering handle 11. The assist torque Tas is decelerated (higher output) and converted into a linear driving force by the ball screw mechanism 16 constituting the decelerator. Then, the converted linear driving force is applied to the rack bar 14.

  The operation of the electric motor 15 is controlled by the electric control device 20. The electric control device 20 includes a steering torque sensor 21, a steering angle sensor 22, a vehicle speed sensor 23, and a current sensor 24. The steering torque sensor 21 is assembled to the steering shaft 12 and detects, as the steering torque T, input torque that is input when the driver steers the steering handle 11. The steering torque T represents a positive value of the steering torque detected when the steering handle is rotated in the right direction, and the negative value of the steering torque detected when the steering handle is rotated in the left direction. Expressed by value.

  The steering angle sensor 22 is also assembled with the steering shaft 12 and detects the steering angle θ of the steering handle 11 based on the rotation angle of the steering shaft 12 generated by the steering operation of the steering handle 11. Note that the steering angle θ has a neutral position “0”, the right rotation angle is represented by a positive value, and the left rotation angle is represented by a negative value. The vehicle speed sensor 23 detects and outputs the vehicle speed V. The current sensor 24 detects and outputs the actual current value I flowing through the electric motor 15. The actual current value I represents a current value flowing through the electric motor 15 as a positive value when the electric motor 15 generates a steering assist force so as to rotate the steering handle 11 in the right direction. A current value that flows through the electric motor 15 when the steering assist force is generated so that the steering wheel 15 rotates the steering wheel 11 in the left direction is represented by a negative value.

  Further, the electric control device 20 includes an electronic control unit 25 connected to the steering torque sensor 21, the steering angle sensor 22, the vehicle speed sensor 23, and the current sensor 24. The electronic control unit 25 controls the electric motor 15 via the drive circuit 26 by executing the assist control program shown in FIG. The drive circuit 26 passes an assist current (drive current) Ias specified by the electronic control unit 25 to the electric motor 15.

  Next, the operation of the present embodiment configured as described above will be described. The electronic control unit 25 starts to repeatedly execute the assist control program every predetermined short time by turning on the ignition switch. The execution of the assist control program is started in step S100 of FIG. 2, and in step S101, the steering torque T is input from the steering torque sensor 21, the steering angle θ is input from the steering angle sensor 22, and the vehicle speed V is input from the vehicle speed sensor 23. Next, in step S102, the electronic control unit 25 refers to the assist current table and determines a basic assist current value (target control amount) Ias * corresponding to the input steering torque T and vehicle speed V.

  The assist current table is stored in the ROM of the electronic control unit 25. As shown in FIG. 4, a plurality of basic assist current values Ias * depend on the steering torque T for each of a plurality of representative vehicle speed values. Is remembered. As can be seen from the figure, the basic assist current value Ias * increases nonlinearly as the steering torque T increases in the positive direction, and becomes a constant value when the steering torque T reaches the predetermined steering torque T. Further, it decreases nonlinearly (increases in the negative direction) as the steering torque T increases in the negative direction, and when the steering torque T reaches a predetermined steering torque T, it becomes a constant value. The basic assist current value Ias * increases as the vehicle speed V decreases with respect to the same steering torque T. Instead of using the assist current table, a basic assist current value Ias * that changes according to the steering torque T and the vehicle speed V is defined in advance by a function, and the basic assist current value is calculated using the function. Ias * may be calculated.

  After determining the basic assist current value Ias * in step S102, the electronic control unit 25 determines whether the absolute value | θ | of the steering angle θ is less than the absolute value | θa | of the reference steering angle θa in step S103. judge. The reference steering angle θa is an angle serving as a threshold for determining whether it is necessary to consider limiting the value of the current flowing through the electric motor 15 to a predetermined value. The reference steering angle θa can be arbitrarily determined within a steerable steering angle region of the steering handle, but is preferably set to a steering angle close to the maximum steering angle (steering angle at the stroke end) θmax. For example, when θmax is 600 °, θa can be set around 500 °. If it is determined in step S103 that | θ | is less than | θa |, the process proceeds to step S104. If | θ | is determined to be greater than or equal to | θa |, the process proceeds to step S105.

  When the process proceeds to step S104, it is assumed that | θ | is less than | θa |, and that the steering angle θ is a steering angle away from the maximum steering angle θmax. Therefore, in this case, it is not necessary to consider reducing the impact force when the turning mechanism collides with the mechanical stopper, and therefore it is not necessary to limit the current of the electric motor 15. Therefore, in step S104, the basic assist current value Ias * obtained from the assist current table is set as the assist current Ias as it is. Subsequently, the electronic control unit 25 instructs the drive circuit 26 so that the assist current Ias flows through the electric motor 15 in step S107. As a result, the drive circuit 26 supplies the assist current Ias to the electric motor 15. The electric motor 15 outputs a driving force corresponding to the applied assist current Ias, and this driving force acts as a steering assist force. Then, the process proceeds to step S108 and the execution of this program is terminated.

  On the other hand, if it is determined in step S103 that | θ | is equal to or larger than | θa | and the process proceeds to step S105, it is expected that the steering angle θ is approaching the maximum steering angle θmax. At this time, if the steering angle sensor 22 is normal and the detected steering angle θ is correct, since the steering angle θ is truly approaching the maximum steering angle θmax, the current of the electric motor 15 is limited. It is necessary to mitigate the impact force when the steering mechanism and the mechanical stopper come into contact with each other. However, if the steering angle sensor 22 is malfunctioning and the detected steering angle θ is incorrect, the steering angle θ is close to the maximum steering angle θmax even though the steering angle θ is actually far from the maximum steering angle θmax. It is merely misdetected as a steering angle. In such a case, if the current limitation of the electric motor 15 is performed, a situation occurs in which the current limitation of the electric motor 15 is performed regardless of the steering angle close to the neutral position.

  In the prior art (Patent Document 1), the actual current flowing through the electric motor when the steering angle θ becomes the reference steering angle θa is set as a limiting current value, and this limitation is applied in a steering angle region larger than the reference steering angle θa. The drive of the electric motor is controlled by the current value. When such control is applied in the above situation, a small current value flowing through the electric motor 15 when the steering angle is close to the neutral position is set as the limiting current, and therefore the steering assist force applied from the electric motor 15 is also small. It will be a thing. Furthermore, the current limitation of the electric motor 15 is performed in a wide steering angle region from the steering angle close to the neutral position to the maximum steering angle. Accordingly, since only a small steering assist force can be obtained in a wide steering angle region, there is a case where the assist is insufficient and a burden is imposed on the driver's turning operation, and a comfortable steering feeling may be impaired. In particular, since a large steering assist force is required in the stationary operation, only a small steering assist force can be obtained from the electric motor 15 in the above case. Narrowing can also occur.

  Even when the steering angle sensor 22 is normal, for example, when the steering is temporarily stopped during the turning steering and then steered again in the cutting direction, the steering angle θ reaches the reference steering angle θa. The steering torque is not sufficiently large, and the actual current flowing through the electric motor may be smaller than in the case where continuous steering is performed. In this case, this small current value is set as the limit current value, and the subsequent turning steering may become difficult.

  In order to solve the above problem, in the present embodiment, even if it is determined in step S103 that | θ | is equal to or larger than | θa |, current limitation of the electric motor 15 is not performed immediately, but in step S105. Then, it is determined whether the basic assist current value Ias * determined based on the steering torque T and the vehicle speed V is equal to or less than a predetermined end current limit value Iend, and whether or not the current limit is performed based on the determination result. Has been decided. A method for determining the end current limit value Iend will be described later. The end current limit value Iend is determined so as to determine whether or not the steering angle sensor 22 is out of order by comparison in step S103, or the end current value is determined. Thus, even when the steering mechanism collides with the mechanical stopper while the electric motor 15 is being driven, the impact force can be set to a size that does not increase so much. For example, a standard current value flowing through the electric motor 15 when the steering angle θ becomes the reference steering angle θa is investigated in advance, and this current value can be used as the end current limit value Iend.

  When it is determined in step S105 that the basic assist current value Ias * is equal to or less than the end current limit value Iend, the basic assist current value Ias * that is the object of determination is such that the steering angle θ is greater than or equal to the reference steering angle θa. It can be determined that the basic assist current value Ias * determined in this case is a small value. In such a case, there is a possibility that the steering angle sensor 22 is out of order and the steering angle θ is erroneously detected. Even if no erroneous detection is made, even if the turning mechanism collides with the mechanical stopper while the electric motor 15 is driven with such a current value, it is considered that the impact force is not so great. Therefore, in this case, the current limitation of the electric motor 15 is not performed, and the process proceeds to step S104 to set the assist current Ias of the electric motor to the basic assist current value Ias *. Thereafter, the process proceeds to step S107, and the drive circuit 26 is instructed so that the assist current Ias flows through the electric motor 15. As a result, the drive circuit 26 supplies the assist current Ias to the electric motor 15. The electric motor 15 outputs a driving force corresponding to the applied assist current Ias, and this driving force acts as a steering assist force. Then, the process proceeds to step S108 and the execution of this program is terminated.

  On the other hand, if it is determined in step S105 that the basic assist current value Ias * is greater than or equal to the end current limit value Iend, the steering angle sensor 22 operates normally to detect the accurate steering angle θ and is determined. When the electric motor 15 is driven with the basic assist current value Ias *, the impact force is considered to increase when the steering mechanism collides with the mechanical stopper. Accordingly, in this case, the process proceeds to step S106, and the assist current Ias of the electric motor 15 is set to the end current limit value Iend. Thereafter, the process proceeds to step S107, and the drive circuit 26 is instructed so that the assist current Ias (= end current limit value Iend) flows through the electric motor 15. As a result, the drive circuit 26 supplies the assist current Ias to the electric motor 15. The electric motor 15 outputs a driving force corresponding to the applied assist current Ias, and this driving force acts as a steering assist force. As described above, when the process proceeds from step S105 to step S106, the electric motor 15 is driven in a state where the assist current Ias of the electric motor 15 is limited to the end current limit value Iend. Then, the process proceeds to step S108 and the execution of this program is terminated.

  As can be seen from the above description, in the present embodiment, the steering angle θ is equal to or greater than the reference steering angle θa (S103: No), and the basic assist current value Ias * determined by the assist current table is the end current limit value Iend. (S105: No), the assist current Ias of the electric motor 15 is limited to the end current limit value Iend (S106), the steering angle θ is equal to or greater than the reference steering angle θa (S103: No), and the basic When the assist current value Ias * is equal to or less than the end current limit value Iend (S105: Yes), the basic assist current value Ias * is set to the assist current Ias without limiting the current to the electric motor 15 (S104). . For this reason, even if the steering angle θ is larger than the reference steering angle θa, the current limitation of the electric motor 15 is not performed when the assist current is small, so that insufficient assist can be prevented. .

  In the assist control program, the end current limit value Iend used in step S105 and step S106 may be a fixed value determined in advance by experiments or the like, but in this embodiment, an appropriate end current limit value Iend is set according to the situation. This is determined by the end current determination program of FIG. The end current determination program is repeatedly executed by the electronic control unit 25 every predetermined short time. The end current determination program is started in step S200 of FIG. The actual current I from 24 is input. Next, the electronic control unit 25 determines whether or not the absolute value | θ | of the steering angle θ is larger than the absolute value | θa | of the reference steering angle θa in step S202. If it is determined that | θ | is larger than | θa |, the process proceeds to step S203. If it is determined that | θ | is equal to or smaller than | θa |, the process proceeds to step S208.

  If it is determined in step S202 that | θ | is equal to or smaller than | θa | and the process proceeds to step S208, the current steering angle θ is away from the maximum steering angle θmax at the stroke end, and the assist current of the electric motor 15 is calculated. It is considered that the region does not need to be limited to a predetermined current value. Therefore, the end flag EFLG is set to 0 in step S208. Here, the end flag EFLG is a flag indicating that the end current limit value Iend needs to be set to a predetermined current value less than the maximum current value Imax that can flow through the electric motor 15. Subsequently, in step S209, the end current limit value Iend is set to the maximum current value Imax. In step S210, the execution of the end current determination program is terminated. Note that setting the end current limit value Iend to the maximum current value Imax is substantially the same as not having a current limit of the electric motor 15.

  On the other hand, if it is determined in step S202 that | θ | is larger than | θa | and the process proceeds to step S203, the current steering angle θ approaches the maximum steering angle θmax at the stroke end, and the end current limit value Iend is reached. Is set to a predetermined value less than Imax. In this case, it is first determined whether or not the end flag EFLG is 0 in step S203. If the end flag EFLG is 0, the end flag EFLG is set to 1 in the next step S204.

  Next, in step S205, the electronic control unit determines that the absolute value | I | of the current current I of the electric motor 15 detected by the current sensor 24 is larger than the absolute value | Ilimit_low | of the limit current lower limit value Ilimit_low. Determine whether. Here, the limit current lower limit value Ilimit_low is a current value that flows through the electric motor 15 at a minimum even when the electric motor 15 is limited. Accordingly, the size is set such that the light steering feeling is not impaired by the current limitation of the electric motor 15. For example, in this embodiment, Ilimit_low is determined from the graph showing the relationship between the steering angle and the drive current shown in FIG.

  FIG. 6 shows a vehicle equipped with the electric power steering apparatus according to the present embodiment. The steering handle is turned from the neutral position to the maximum right steering angle θmax, and further to the maximum left steering angle −θmax. The relationship between the steering angle θ and the actual current I flowing through the electric motor 15 (steering angle-actual current characteristic line) when returning to the neutral position again is shown, and the curve A in the figure shows the steering angle at the maximum loading. -Actual current characteristic curve, curve B in the figure is the steering angle at the time of minimum loading-actual current characteristic curve. Here, the maximum steering angle θmax is 600 ° (−θmax = −600 °), and the reference steering angle θa serving as a reference for current limitation is 500 ° (−θa = −500 °).

  In the present embodiment, of the two steering angle-actual current characteristic curves, the actual current at the reference steering angle θa and the actual current at the reference steering angle −θa in the characteristic curve B at the minimum load The current value having a larger absolute value is defined as a limit current lower limit value Ilimit_low. In this example, in the characteristic curve B, the actual current at the reference steering angle θa is 43 A, and the actual current at the reference steering angle −θa is −50 A. Therefore, ± Ilimit_low is set to ± 50 A. This setting method is merely an example, and other methods, for example, the actual current at the reference steering angle θa at maximum loading may be Ilimit_low, or when the steering angle is before and after the reference steering angle θa. The actual current value may be Ilimit_low. Alternatively, a value derived from experience may be Ilimit_low.

  In step S205 in FIG. 3, the absolute value | Ilimit_low | of Ilimit_low set as described above is compared with the absolute value | I | of the actual current value I. If it is determined in this step that | I | is greater than | Ilimit_low |, the process proceeds to step S206. If | I | is determined to be equal to or smaller than | Ilimit_low |, the process proceeds to step S207.

  In the present embodiment, the limit current lower limit value Ilimit_low is determined based on the actual current at the reference steering angle θa as described above. Therefore, in the steering angle region larger than the reference steering angle θa, the actual current I of the electric motor 15 at that time should be larger than Ilimit_low. Therefore, when it is determined in step S205 that | I | is larger than | Ilimit_low |, it is assumed that the steering angle sensor 22 is operating normally and the detected steering angle θ is correct. In this case, the actual current value I actually measured at that time is set as the end current limit value Iend in step S206. In step S210, the end limit current determination program is terminated.

  On the other hand, if it is determined in step S205 that | I | is equal to or smaller than | Ilimit_low |, the actual current I flowing through the electric motor 15 is smaller than the current that should be flowing, so that the steering angle sensor 22 may malfunction. Therefore, it is assumed that the correct steering angle is not detected (in particular, the steering angle is detected to be larger than the actual angle). Alternatively, although the steering angle sensor 22 is operating normally, the driver temporarily stops the steering in the middle of the steering operation (the steering torque becomes zero at this stage, so the assist current also decreases), and then the steering is performed again. Since the steering operation is started, it is assumed that the assist current of the electric motor 15 at the reference steering angle θa is low without being increased. In either case, if the current is limited by the actual current at that time, a great burden is placed on the driver in the subsequent turning operation of the steering wheel. Therefore, in this case, the limit current lower limit value Ilimit_low is set to the end current limit value Iend in step S207. In step S210, the execution of the end restriction determination program is terminated. If it is determined in step S203 that the end flag EFLG is not 0, that is, 1, it can be determined that Iend has already been determined to a predetermined value as described above. Without determining Iend, the process proceeds to step S210 to end the program.

  By executing such an end current determination program, the end current limit value Iend to be compared with the basic assist current value Ias * is determined in the determination in step S105 of FIG. Specifically, the absolute value of the actual current I flowing through the electric motor 15 when the steering angle θ is larger than the reference steering angle θa (step S202: Yes) is smaller than the absolute value of the limit current lower limit value Ilimit_low. At this time (step S205: No), the end current limit value Iend is set to a limit current lower limit value Ilimit_low larger than the actual current I (step S207). When the steering angle θ is larger than the reference steering angle θa (step S202: Yes), the absolute value of the actual current I flowing through the electric motor 15 is equal to or larger than the absolute value of the limit current lower limit value Ilimit_low (step). In S205: Yes, the end current limit value Iend is set to the actual current I which is a current value equal to or greater than the limit current lower limit value Ilimit_low (step S206). That is, the limit current lower limit value Ilimit_low is a minimum current that flows even when the electric motor 15 is current-limited, and the current of the electric motor 15 is not limited by a current value less than this value. Therefore, the electric motor is current-limited with a small current value, so that a light steering feeling is not impaired and an extra burden is not imposed on the driver. Note that the determination in step S202 may be based on the determination when the steering angle θ becomes the reference steering angle θa or when the steering angle θ becomes equal to or greater than the reference steering angle θa. Similarly, the determination in step S205 may be based on when | I | becomes | Ilimit_low | or when | I | becomes greater than or equal to | Ilimit_low |.

  FIG. 5 is a graph showing the relationship between the steering angle θ and the actual current I flowing through the electric motor 15 when the current value of the electric motor 15 is controlled as described above. As can be seen, when the steering angle sensor 22 is normal (“normal” graph), the actual current value I flowing through the electric motor 15 at the reference steering angle θa is larger than the limit current lower limit value Ilimit_low. Yes. For this reason, in the steering angle region larger than the reference steering angle θa, the actual current (Im) at that time is the end current limit value Iend, and the current flowing through the electric motor 15 is limited by this current value.

On the other hand, when the steering angle sensor 22 is out of order, particularly when the steering angle sensor 22 detects a value smaller than the actual steering angle (“abnormality” graph), the electric motor 15 at the reference steering angle θa. The actual current value I flowing through the current becomes smaller than the limit current lower limit value Ilimit_low. In such a case, the end current limit value Iend is set to the limit current lower limit value Ilimit_low. Therefore, even in a steering region where the steering angle θ detected by the steering angle sensor 22 is larger than the reference steering angle θa (for example, the steering angle θ _{1 in the} figure), the basic assist current value Ias * determined based on the basic assist map. Current limit is not performed until the limit current lower limit value Ilimit_low (= Iend). The current limit is performed only when the basic assist current value Ias * reaches the limit current lower limit value Ilimit_low.

  As described above, in the vehicle steering apparatus of the present embodiment, when the electric motor 15 is current-limited, a current of at least the limit current lower limit value Ilimit_low flows. Therefore, the malfunction which arises when the limiting current of the electric motor 15 is too low can be prevented. Further, since the end limit current value Iend is determined according to the actual current value flowing through the electric motor 15 when the steering angle θ reaches the reference steering angle θa, an appropriate end limit current value is set according to the situation. can do.

  In the present embodiment, the limit current lower limit value Ilimit_low is applied to the electric motor 15 when the absolute value | θ | of the steering angle θ becomes the absolute value | θa | of the reference steering angle θa, as shown in FIG. It is determined based on the actual current that flows. By determining the limit current lower limit value Ilimit_low in this way, a small current value can be obtained in a wide range (especially in the range of a steering angle θ of −500 ° to about 570 ° as shown in FIG. 6 at the time of minimum loading). Thus, it is possible to effectively prevent the electric motor from being current limited. Further, it is possible to prevent the current from being limited in a wide range of the steering angle θ of about −440 degrees to about 390 degrees not only at the time of minimum loading but also at the time of maximum loading. In this way, it is possible to ensure a wide area where the electric motor is not current limited even if the driving state of the vehicle changes.

1 is an overall schematic diagram of a vehicle steering apparatus according to an embodiment of the present invention. It is a flowchart of the assist control program performed by the electronic control unit of FIG. It is a flowchart of the end current determination program executed by the electronic control unit of FIG. It is a basic assist map showing the relationship between steering torque and basic assist current value. In embodiment of this invention, it is a graph which shows the relationship between a steering angle and the actual electric current which flows into an electric motor. It is a graph which shows the steering angle-actual current characteristic curve for determining limiting current lower limit Ilimit_low.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 ... Steering handle, 15 ... Electric motor (actuator), 20 ... Electric control device, 21 ... Steering torque sensor, 22 ... Steering angle sensor, 23 ... Vehicle speed sensor, 24 ... Current sensor, 25 ... Electronic control unit (control means) , 26 ... drive circuit, Ias * ... basic assist current value (target control amount), Ias ... drive current, Iend ... end current limit value (reference control amount), Ilimit_low ... limit current lower limit value (limit control amount), I ... Actual current value, Imax: Maximum current value, θa: Reference steering angle, θmax: Maximum steering angle

Claims (4)

In a vehicle steering apparatus, comprising: an actuator that generates a steering assist force for assisting a steering operation of the steering handle; and a control unit that controls driving of the actuator.
The control means includes
Target control amount determination means for determining a target control amount of the actuator so that the actuator generates a predetermined steering assist force;
When the steering angle of the steering wheel is equal to or larger than a predetermined reference steering angle and the target control amount determined by the target control amount determining means is larger than the predetermined reference control amount, the control amount of the actuator is set to a predetermined reference amount. A control amount limiting means for limiting the control amount;
A vehicle steering apparatus comprising: The vehicle steering apparatus according to claim 1,
The control means is configured such that an actual control amount of the actuator when a steering angle of the steering wheel becomes the reference steering angle or when a steering angle of the steering wheel becomes equal to or larger than the reference steering angle is a predetermined limit control amount. A vehicle steering apparatus comprising reference control amount setting means for setting the limit control amount to the reference control amount when: The vehicle steering system according to claim 1 or 2,
The reference control amount setting means is configured so that an actual control amount of the actuator when a steering angle of the steering handle becomes the reference steering angle or when a steering angle of the steering handle becomes equal to or larger than the reference steering angle is a predetermined amount. The vehicle steering apparatus, wherein the actual control amount is set to the reference control amount when the control amount is larger than the limit control amount. The vehicle steering apparatus according to any one of claims 1 to 3,
The actuator is an electric motor;
The vehicle steering apparatus according to claim 1, wherein the control amount is a value of a current flowing through the electric motor.

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