Abstract:
A motor driven power steering device may include a motor drive unit equipped with a motor moving a rack so as to change steering angle, and a control unit detecting a moving direction of a steering wheel, operating the motor in accordance with the moving direction, and controlling the motor so as to vary the maximum stroke of the rack. The rack stroke may be increased in a predetermined control range of a wheel stroke and the rack stroke may be decreased out of a predetermined control range of a wheel stroke in such a manner that the gap between the wheel and the vehicle body is actively controlled. Further, a separate varying device for limiting the movement of the rack is not included such that the number of components and the manufacturing cost is reduced.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to and the benefit of Korean Patent Application No. 10-2009-0119363 filed in the Korean Intellectual Property Office on Dec. 3, 2009, the entire contents of which application is incorporated herein for all purposes by this reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a motor driven power steering device. More particularly, the present invention relates to a motor driven power steering device that varies the stroke of a rack depending on a driving condition. 
         [0004]    2. Description of Related Art 
         [0005]    Generally, a power steering device for steering a vehicle includes a hydraulic pressure type and a motor driven type, and recently the motor driven type has been gradually substituted for the hydraulic pressure type and the application range is becoming wider from compact to large vehicles. 
         [0006]    However, in a case that the variable rack stroke device is mounted in a motor driven power steering device, an operating apparatus for limiting the rack stroke of the steering device may have to be further mounted, such that the number of related components and the assembly cost thereof increase. 
         [0007]    Further, in a case when the variable rack stroke device is off, there is a problem that the gap between the wheel and the vehicle body excessively decreases while the vehicle is in a full bump condition (vehicle body becomes closer to the ground) or the vehicle is in a full rebound condition (vehicle becomes farther from the ground) 
         [0008]    The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    The present invention has been made in an effort to provide a motor driven power steering device having advantages of reducing assembly cost and the number of components, and varying a rack stroke. 
         [0010]    A motor driven power steering device according to various embodiments of the present invention may include a motor drive unit equipped with a motor moving a rack so as to change steering angle, and a control unit detecting a moving direction of a steering wheel, operating the motor in accordance with the moving direction, and controlling the motor so as to vary the maximum stroke of the rack. 
         [0011]    The control unit may reduce the maximum stroke of the rack by a predetermined amount in a case that an operating switch is on. 
         [0012]    The control unit may vary the maximum stroke of the rack to a predetermined value in a case that an operating switch is off. 
         [0013]    A vehicle height sensor unit may include a vehicle height sensor fixed on a vehicle body, and a link of which a lower end is fixed to a lower arm, and an upper end is connected to the vehicle height sensor, wherein the vehicle height sensor detects movement of the link connected to the lower arm and calculates distance between the vehicle body and the lower arm, and the control unit calculates the wheel stroke amount based on the distance between the vehicle body and the lower arm. 
         [0014]    The maximum stroke of the rack may be increased in a case that the wheel stroke is in a predetermined control range. 
         [0015]    The maximum stroke of the rack may be decreased in a case that the wheel stroke is out of a predetermined control range. 
         [0016]    In a motor driven power steering device according to the present invention as stated above, the motor of the motor drive unit of a steering device is controlled, the rack stroke is increased in a predetermined control range of a wheel stroke, and the rack stroke is decreased out of a predetermined control range of a wheel stroke, in such a manner that the gap between the wheel and the vehicle body is actively controlled. 
         [0017]    Further, a separate varying device for limiting the movement of the rack is not included such that the number of components and the manufacturing cost are reduced. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a schematic diagram of an exemplary motor driven power steering device according to the present invention. 
           [0019]      FIG. 2  is a perspective view of an exemplary motor driven power steering device according to the present invention. 
           [0020]      FIG. 3  is a control flowchart of an exemplary motor driven power steering device according to the present invention. 
           [0021]      FIG. 4  is a table showing operating conditions of an exemplary motor driven power steering device according to the present invention. 
           [0022]      FIG. 5  is a graph showing relations between a vehicle height and a rack stroke in an exemplary motor driven power steering device according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    Reference will now be made in detail to various embodiments of the present invention(s), examples of which are illustrated in the accompanying drawings and described below. While the invention(s) will be described in conjunction with exemplary embodiments, it will be understood that present description is not intended to limit the invention(s) to those exemplary embodiments. On the contrary, the invention(s) is/are intended to cover not only the exemplary embodiments, but also various alternatives, modifications, equivalents and other embodiments, which may be included within the spirit and scope of the invention as defined by the appended claims. 
         [0024]    Referring to  FIG. 1 , motor driven power steering device includes a steering wheel  102 , a steering column  104 , a motor drive unit  100 , a rack  120 , a wheel  106 , a vehicle height sensor unit  130 , an operating switch  110 , and a control unit  140 . 
         [0025]    A driver rotates the steering wheel  102  so as to adjust the moving direction of a vehicle, the torque thereof is transferred to the rack  120  through the steering column  104 , and the steering angle of the wheel  106  is varied depending on the left/right movement of the rack  120 . 
         [0026]    The motor drive unit  100  is mounted at the steering column  104  and moves the rack  120  according to the movement of the steering wheel  102 . The vehicle height sensor unit  130  detects the height variation of the vehicle and transfers the signal thereof to the control unit  140 . 
         [0027]    If the operating switch  110  is on, the maximum value of the rack stroke of the rack  120  is decreased by a predetermined amount. Further, if the operating switch  110  is turned off, the maximum stroke of the rack  120  is variably increased according to a bump or a rebound condition of the vehicle body  220 . 
         [0028]    In various embodiments of the present invention, the control unit  140  detects bump or rebound condition of the vehicle body  220  by a signal that is transferred from the vehicle height sensor unit  130  and controls the operation of the motor drive unit  100 . 
         [0029]    Referring to  FIG. 2 , the vehicle height sensor unit  130  includes a sensor  205  and a link  210 . A lower end portion of the link  210  is connected to a lower arm  200 , and an upper end portion of the link  210  is connected to the sensor  205  through a hinge  215 . 
         [0030]    The link  210  rotates the hinge  215  that is mounted on the sensor  205  according to the lower arm  200 , and the sensor  205  detects rotation amount of the hinge  215  and detects the distance between the lower arm  200  and the vehicle body  220  from the rotation amount. 
         [0031]    The control unit  140  calculates the stroke of the wheel  106  based on the distance between the lower arm  200  and the vehicle body  220 . 
         [0032]    In various embodiments of the present invention, the distance between the lower arm  200  and the vehicle body  220  is varied depending on the weight of a load or passengers. Further, the lower arm  200  moves downwards or upwards in accordance with the road condition. 
         [0033]    Here, a condition in which the distance between the lower arm  200  and the vehicle body  220  becomes closer is called “bump”, and a condition in which the distance between the lower arm  200  and the vehicle body  220  becomes longer is called “rebound”. 
         [0034]    In various embodiments of the present invention, the maximum that the rack  120  can move to steer the wheel  106  is varied in the full bump or the full rebound condition that the distance between the lower arm  200  and the vehicle body  220  becomes closer or longer. 
         [0035]    The maximum rack stroke that the rack  120  moves is determined by the gap between the wheel  106  and the components of the vehicle body, and the maximum of the rack stroke can be set at a design or assembly step. 
         [0036]    Referring to  FIG. 3 , the control method includes a zero step S 300 , a first step S 310 , a second step S 320 , a third step S 330 , a fourth step S 340 , a fifth step S 350 , a sixth step S 360 , a seventh step S 370 , and an eighth step S 380 . 
         [0037]    In the zero step S 300 , control is initiated, and in the first step S 310 , it is determined if the operating switch  110  is on or off. 
         [0038]    If the operating switch  110  is on, the motor drive unit  100  operates a motor in the second step S 320  so as to reduce the maximum stroke of the rack  120 . When a chain is wrapped on the wheel  106  so as to prevent slip thereof on a snowy road, the operating switch is turned on. 
         [0039]    In the fourth step S 340 , the distance signal between the vehicle body  220  and the lower arm  200  is measured from the vehicle height sensor unit  130 . In the fifth step S 350 , the moving stroke of the wheel  106  is calculated by the signal transferred from the vehicle height sensor unit  130 . 
         [0040]    If the stroke of the wheel  106  is within a predetermined control range in the sixth step S 360 , the maximum stroke of the rack  120  is increased by a predetermined amount by controlling the motor of the motor drive unit  100  in the seventh step S 370 . 
         [0041]    If it is determined that the stroke of the wheel  106  is out of the predetermined control range in the sixth step S 360 , the maximum stroke of the rack  120  is decreased by a predetermined amount by controlling the motor of the motor drive unit  100  in the eighth step S 380 . Here, the control range of the stroke of the wheel  106  is set beforehand. 
         [0042]    If the engine stops during the third step, the control flow ends. 
         [0043]    Referring to  FIG. 4 , in a case that the condition of the operating switch  110  is off, the stroke of the rack  120  is varied according to the stroke of the wheel  106 , if the stroke of the wheel  106  is within a control range, the stroke of the rack  120  is increased to a predetermined value, and if the stroke of the wheel  106  is out of the control range, the stroke of the rack  120  is decreased to a predetermined value. 
         [0044]    In a case that the condition of the operating switch  110  is on, the stroke of the rack  120  is decreased to a predetermined value regardless of the driving condition. 
         [0045]    Referring to  FIG. 5 , the horizontal axis signifies the stroke of the wheel  106 . That is, it shows the height of the wheel  106 , in a case that it is a negative value, it is considered a rebound condition and the distance between the lower arm and the vehicle body is increased from a standard value, and in a case that it is a positive value, it is considered a bump condition and the distance between the lower arm and the vehicle body is decreased from a standard value. 
         [0046]    Further, the vertical axis signifies a gap between the wheel  106  and a component of the vehicle body, and the minimum value thereof is set to a predetermined value. 
         [0047]    The first line  500  shows a gap between the wheel  106  and the vehicle body  220  depending on the bump or the rebound condition of the vehicle body  220 , in a case that the maximum stroke of the rack  120  is reduced. 
         [0048]    In a case that the operating switch is off, the stroke of the wheel  106  is within a control range from e to d, and the maximum stroke of the rack  120  is increased from the first line  500  to the second line  505 , while a decreased condition is sustained in the other range that is out of the control range. 
         [0049]    In a condition that the operating switch  110  is on, the maximum stroke (movement limit) of the rack  120  sustains the decreased limit. 
         [0050]    In various embodiments of the present invention, the control unit  140  can detect the rotation of the motor of the motor drive unit  100  to detect the stroke of the rack  120 , and can also detect the stroke of the rack  120  from a sensor sensing the position of the rack  120 . 
         [0051]    The foregoing descriptions of specific exemplary embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teachings. The exemplary embodiments were chosen and described in order to explain certain principles of the invention and their practical application, to thereby enable others skilled in the art to make and utilize various exemplary embodiments of the present invention, as well as various alternatives and modifications thereof. It is intended that the scope of the invention be defined by the Claims appended hereto and their equivalents.