Abstract:
The present invention relates to a method for measuring a vertical acceleration and a velocity of a semi-active suspension system. Particularly, the present invention provides a method for obtaining a vertical acceleration from vertical accelerations measured from three vertical acceleration sensors of a semi-active suspension system of a vehicle, comprising the steps of: receiving first to third vertical accelerations measured from first to third vertical acceleration sensors; and obtaining a fourth vertical acceleration (Ad) by multiplying the first to third vertical accelerations by correction constants and subsequently summing up them. Therefore, according to the present invention, a fourth vertical acceleration can be obtained by multiplying the three vertical accelerations measured from the three acceleration sensors by the constants for correcting them to accelerations at actually desired damper positions and subsequently summing up them, thereby enabling accurate measurement and correction of the vertical accelerations.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of Invention  
         [0002]     The present invention relates to a semi-active suspension system of a vehicle, and more particularly, to a method for measuring a vertical acceleration of a semi-active suspension system.  
         [0003]     2. Description of the Prior Art  
         [0004]     In general, a semi-active suspension system is an apparatus for improving the driving stability and ride comfort of a vehicle by changing the dynamic characteristics of dampers mounted in a vehicle in real time. When a driver excessively steers a vehicle or the movement of the vehicle is out of its path, load unbalance of the vehicle causes the vehicle to be biased or not to be steerable so that the driver may be in unpredictable danger such as vehicle overturn. However, when a vehicle, which is mounted with the semi-active suspension system, drives on an uneven road, a vertical load at a contact surface of a tire and the road is kept at an appropriate level, so that the stability of the vehicle can be secured in steering, braking and driving. In addition, irregular shocks generated from the road when the vehicle travels are effectively absorbed, so that the ride comfort and driving convenience can be provided to a passenger and a driver.  
         [0005]     Such a semi-active suspension system controls actuators and dampers respectively connected to four road wheels after detecting the driving conditions of the vehicle through vertical acceleration sensors, vehicle velocity sensors, a steering angle sensor, a brake sensor, a throttle position sensor, and the like among a variety of sensors mounted to the vehicle.  
         [0006]     Among them, the vertical acceleration sensors are installed adjacent to four dampers. However, since there is a difference between the practical mounting positions of the acceleration sensors and the installation positions of the dampers, it is difficult to accurately measure the accelerations of the road wheels.  
         [0007]     In a prior art, three vertical accelerations are measured through two vertical acceleration sensors mounted in front of a vehicle body and a vertical acceleration sensor mounted at the rear thereof in the vicinity of the respective dampers, and a fourth vertical acceleration is obtained in such a manner that an ECU receives the three vertical accelerations and calculates the other vertical acceleration using Equation 1 as follows:  
                 a   RL     =       a   RR     +         t   R       t   F       ·     (       a   FL     -     a   FR       )           ,           (   1   )             
 
 where a FL  is a vertical acceleration of a front left side, a FR  is a vertical acceleration of a front right side, a RL  is a vertical acceleration of a rear left side, a RR  is a vertical acceleration of a rear right side, t F  is a front tread, and t R  is a rear tread. 
 
         [0008]     Since it is difficult to accurately obtain a fourth vertical acceleration through calculation using the three vertical accelerations and the threads of the road wheels, there is a problem with accurate determination of the dynamic characteristics of the vehicle.  
         [0009]     Further, in the prior art, there is also a problem in that it is difficult to accurately obtain a velocity even upon calculation thereof from three velocity sensors.  
       SUMMARY OF THE INVENTION  
       [0010]     Accordingly, the present invention is conceived to solve the aforementioned problems in the prior art. An object of the present invention is to provide a method for measuring vertical accelerations of a semi-active suspension system capable of accurately measuring and correcting the vertical accelerations in such a manner that vertical accelerations measured from three acceleration sensors are multiplied by constants for correcting them to accelerations at actually desired positions and are then summed up so as to obtain a fourth vertical acceleration.  
         [0011]     Another object of the present invention is to provide a method for measuring velocities of a semi-active suspension system capable of accurately measuring and correcting the velocities in such a manner that three velocities measured from three velocity sensors are multiplied by constants for correcting them to velocities at actually desired positions and are then summed up so as to obtain a fourth velocity.  
         [0012]     According to an aspect of the present invention for achieving the objects, there is provided a method for obtaining a vertical acceleration from vertical accelerations measured from three vertical acceleration sensors of a semi-active suspension system of a vehicle, comprising the steps of receiving first to third vertical accelerations measured from first to third vertical acceleration sensors; and obtaining a fourth vertical acceleration (Ad) by multiplying the first to third vertical accelerations by correction constants and subsequently summing up them, according to the following equation:
 
 Ad=α×Aα 1+β ×Aα 2+γ ×Aα 3,
 
 where α, β and γ are the correction constants at the position of a damper where a vertical acceleration sensor is not installed, Ad is the fourth vertical acceleration, and Aa1, Aa2 and Aa3 are the first to third vertical accelerations, respectively. 
 
         [0013]     According to another aspect of the present invention, there is provided a method for obtaining a velocity from velocities measured from three velocity sensors of a semi-active suspension system of a vehicle, comprising the steps of receiving first to third velocities measured from first to third velocity sensors; and obtaining a fourth velocity (Vd) by multiplying the first to third velocities by correction constants and subsequently summing up them, according to the following equation:
 
 Vd=α×Vα 1+β V×α 2+γ ×Vα 3,
 
 where α, β and γ are the correction constants at the position of a damper where a velocity sensor is not installed, Vd is the fourth velocity, and Va1, Va2 and Va3 are the first to third velocities, respectively.
 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     The above and other objects, features and advantages of the present invention will become apparent from the following description of preferred embodiments given in conjunction with the accompanying drawings, in which:  
         [0015]      FIG. 1  is a view showing positions of four dampers and three vertical acceleration sensors of a vehicle employing a semi-active suspension system;  
         [0016]      FIG. 2  is a block diagram of the semi-active suspension system, illustrating a method for measuring a vertical acceleration according to the present invention;  
         [0017]      FIG. 3  is a view showing positions of the four dampers and three velocity sensors of the vehicle employing the semi-active suspension system; and  
         [0018]      FIG. 4  is a block diagram of a semi-active suspension system, illustrating a method for measuring a velocity according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0019]     Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.  
         [0020]      FIG. 1  is a view showing positions of four dampers and three vertical acceleration sensors of a vehicle employing a semi-active suspension system. Referring to this figure, two vertical acceleration sensors  10  and  12  are mounted adjacent to front dampers  18  and  20  on a vehicle body  1 , respectively, while a vertical acceleration sensor  14  is mounted adjacent to one of rear dampers  22  and  24 . Here, when “X” designates a width direction of the vehicle body and “Y” designates a longitudinal direction thereof, the positions of the two front vertical acceleration sensors  10  and  12  are indicated by coordinates (X 2 , Y 2 ) and (X 1 , Y 1 ), respectively, and the position of the rear vertical acceleration sensor  14  is indicated by coordinates (X 3 , Y 3 ). Reference numerals  1 =FR,  2 =FL,  3 =RR, and  4 =RL, which have not yet been explained, designate the positions of the first damper  18  at the front left side, the second damper  20  at the front right side, the third damper  22  at the rear left side, and the fourth damper  24  at the rear right side, respectively.  
         [0021]     In the present invention, correction constants for a vertical acceleration to be measured at a position where a fourth vertical acceleration sensor would be installed are obtained using the X and Y position coordinate values (for example, X 1 -X 2  and Y 1 -Y 3 ) of the three vertical acceleration sensors  10 ,  12  and  14 .  
         [0022]      FIG. 2  is a block diagram of the semi-active suspension system, illustrating a method for measuring a vertical acceleration according to the present invention. Referring to this figure, the semi-active suspension system comprises the first to third vertical acceleration sensors  10 ,  12  and  14 , an ECU  16 , the first to fourth dampers  18 ,  20 ,  22  and  24 , and first to fourth actuators  26 ,  28 ,  30  and  32 .  
         [0023]     Each of the first to third vertical acceleration sensors  10 ,  12  and  14 , which are installed adjacent to three of the four dampers of the vehicle body, is a sensor for measuring a bounce motion of the vehicle and outputting a vertical acceleration corresponding to the bounce motion of the vehicle body, as a voltage on a gravitational acceleration basis.  
         [0024]     A microprocessor is used for the ECU  16 , and the ECU includes control algorithms for bounce, roll, dive, squat and the like based on sky-hook logic for independently controlling damping forces of the dampers of respective road wheels.  
         [0025]     The first to fourth dampers  18 ,  20 ,  22  and  24  are equipped with variable valves at lateral sides thereof so as to adjust the damping forces for use in controlling the first to fourth actuators  26 ,  28 ,  30  and  32  when the dampers are extended and retracted.  
         [0026]     In the semi-active suspension system configured as above, the method for measuring the vertical acceleration according to the present invention is performed as follows.  
         [0027]     Vertical accelerations Aa1, Aa2 and Aa3 measured by the respective first to third vertical acceleration sensors  10 ,  12  and  14  are input into the ECU  16 .  
         [0028]     The ECU  16  receives the vertical accelerations Aa1, Aa2 and Aa3 measured by the respective first to third vertical acceleration sensors  10 ,  12  and  14  and obtains a fourth vertical acceleration Ad using Equation 2 as follows:
 
 Ad=α×Aα 1+β× Aα 2+γ× Aα 3,  (2)
 
 where α, β and γ are correction constants for the position of the fourth damper where a fourth vertical acceleration sensor is not installed . Ad is the the fourth vertical acceleration, and Aa1, Aa2 and Aa3 are the first to third vertical accelerations, respectively. 
 
         [0029]     Referring to Equation 2, the fourth vertical acceleration Ad at the position of the fourth damper where a vertical acceleration sensor is not installed is obtained by multiplying the vertical accelerations Aa1, Aa2 and Aa3 measured in the first to third vertical acceleration sensors  10 ,  12  and  14  by the corresponding correction constants α, β and γ and subsequently summing up the first to third accelerations that have been multiplied by the correction constants.  
         [0030]     At this time, the correction constants α, β and γ are obtained from the following procedures.  
         [0031]     For example, assuming that a plane equation  P  for the vertical acceleration sensors installed on the vehicle body is Z=AX+BY+C, all the coordinate values of the respective vertical acceleration sensors are included in the equation  P .
 
(X1,Y1,Z1),(X2,Y2,Z2),(X3,Y3,Z3)∈P,  (3)
 
 where A, B and C are constants, which are expressed as the following Equations 4 to 6, respectively.  
             A   =           (       Z   1     -     Z   2       )     ⁢     (       Y   1     -     Y   3       )       -       (       Z   1     -     Z   3       )     ⁢     (       Y   1     -     Y   2       )               (       X   1     -     X   2       )     ⁢     (       Y   1     -     Y   3       )       -       (       X   1     -     X   3       )     ⁢     (       Y   1     -     Y   2       )                   (   4   )               B   =           (       X   1     -     X   2       )     ⁢     (       Z   1     -     Z   3       )       -       (       X   1     -     X   3       )     ⁢     (       Z   1     -     Z   2       )               (       X   1     -     X   2       )     ⁢     (       Y   1     -     Y   3       )       -       (       X   1     -     X   3       )     ⁢     (       Y   1     -     Y   2       )                   (   5   )                     C   =       Z   ⁢           ⁢   1     -     A   ⁢           ⁢   X   ⁢           ⁢   1     -           ⁢     B   ⁢           ⁢   Y   ⁢           ⁢   1                   =       Z   ⁢           ⁢   2     -     A   ⁢           ⁢   X   ⁢           ⁢   2     -     B   ⁢           ⁢   Y   ⁢           ⁢   2                   =       Z   ⁢           ⁢   3     -     A   ⁢           ⁢   X   ⁢           ⁢   3     -     B   ⁢           ⁢   Y   ⁢           ⁢   3                     (   6   )             
 
         [0032]     At this time, assuming that the installation positions of the three vertical acceleration sensors are upper points on the dampers and in a common plane and Y coordinate values of the front first and second vertical acceleration sensors are the same (i.e., Y 1 =Y 2 ), the three vertical acceleration sensors output the accelerations of {umlaut over (z)} 1 , {umlaut over (z)} 2  and {umlaut over (z)} 3 , respectively.  
         [0033]     The plane equation for the first to fourth dampers installed on the vehicle body is considered to be z=Ax+By+C, and is then subjected to second differentiation. At this time, the positions of the dampers are expressed using x and y.  
                 ⅆ           2     ⁢   z         ⅆ     t   2         =           ⅆ           2     ⁢   A         ⅆ     t   2         ⁢   x     +         ⅆ           2     ⁢   B         ⅆ     t   2         ⁢   y     +       ⅆ           2     ⁢   C         ⅆ     t   2                   (   7   )                       ⅆ           2     ⁢   A         ⅆ     t   2         ⁢     (       Z   1     ,     Z   2     ,     Z   3       )       =           ∂   A       ∂     Z   1         ⁢       Z   ¨     1       +         ∂   A       ∂     Z   2         ⁢       Z   ¨     2       +         ∂   A       ∂     Z   3         ⁢       Z   ¨     3           ,     
     ⁢       where   ⁢           ⁢       ∂     Z   i         ∂     t   2           =         Z   ¨     i     .               (   8   )             
 
         [0034]     Since A is a function of Z1, Z2 and Z3 and Z1, Z2, and Z3 are also functions of time, a derivative of A with respect to time is expressed as Equation 8 using a chain rule. Equation 8 can be expressed as Equation 9 by rearranging it according to the accelerations of the respective sensors after expanding Equation 8 with respect to respective coefficients.  
                       ⅆ           ⁢           2     ⁢           ⁢   z         ⅆ           ⁢     t             ⁢   2           =       ⁢         (           ∂   A       ∂     Z   1         ⁢   x     +         ∂   B       ∂     Z   1         ⁢   y     +       ∂   C       ∂     Z   1           )     ⁢       Z   ¨     1       +                     ⁢         (           ∂   A       ∂     Z   2         ⁢   x     +         ∂   B       ∂     Z   2         ⁢   y     +       ∂   C       ∂     Z   2           )     ⁢       Z   ¨     2       +                     ⁢       (           ∂   A       ∂     Z   3         ⁢   x     +         ∂   B       ∂     Z   3         ⁢   y     +       ∂   C       ∂     Z   3           )     ⁢       Z   ¨     3                     (   9   )                   ⅆ           ⁢           2     ⁢           ⁢   z         ⅆ           ⁢     t             ⁢   2           =         α   ⁡     (     x   ,   y     )       ⁢       Z   ¨     1       +       β   ⁡     (     x   ,   y     )       ⁢       Z   ¨     2       +       γ   ⁡     (     x   ,   y     )       ⁢       Z   ¨     3                 (   10   )             
 
         [0035]     Thus, the correction constants α, β and γ at the damper positions (x, y) of the dampers are expressed as the following Equation 11.  
                 α   ⁡     (     x   ,   y     )       =           ∂   A       ∂     Z   1         ⁢   x     +         ∂   B       ∂     Z   1         ⁢   y     +       ∂   C       ∂     Z   1             ⁢     
     ⁢       β   ⁡     (     x   ,   y     )       =           ∂   A       ∂     Z   2         ⁢   x     +         ∂   B       ∂     Z   2         ⁢   y     +       ∂   C       ∂     Z   2             ⁢     
     ⁢       γ   ⁡     (     x   ,   y     )       =           ∂   A       ∂     Z   3         ⁢   x     +         ∂   B       ∂     Z   3         ⁢   y     +       ∂   C       ∂     Z   3             ⁢     
     ⁢         ∂   A       ∂     Z   1         ,         ∂   A       ∂     Z   2         ⁢           ⁢   and   ⁢           ⁢       ∂   A       ∂     Z   3                     (   11   )             
 
 existing in Equation 9 and the correction constants α, β and γ can be obtained as the following Equation 12.  
                   ∂   A       ∂     Z   1         =         Y   2     -     Y   3             (       X   1     -     X   2       )     ⁢     (       Y   1     -     Y   3       )       -       (       X   1     -     X   3       )     ⁢     (       Y   1     -     Y   2       )             ⁢     
     ⁢         ∂   A       ∂     Z   2         =         Y   3     -     Y   1             (       X   1     -     X   2       )     ⁢     (       Y   1     -     Y   3       )       -       (       X   1     -     X   3       )     ⁢     (       Y   1     -     Y   2       )             ⁢     
     ⁢         ∂   A       ∂     Z   3         =         Y   1     -     Y   2             (       X   1     -     X   2       )     ⁢     (       Y   1     -     Y   3       )       -       (       X   1     -     X   3       )     ⁢     (       Y   1     -     Y   2       )                     (   12   )             
 
         [0036]     Further,  
           ∂   B       ∂     Z   1         ,         ∂   B       ∂     Z   2         ⁢           ⁢   and   ⁢           ⁢       ∂   B       ∂     Z   3               
 
 existing in Equation 9 and the correction constants α, β and γ can be obtained as the following Equation 13.  
                   ∂   B       ∂     Z   1         =         X   3     -     X   2             (       X   1     -     X   2       )     ⁢     (       Y   1     -     Y   3       )       -       (       X   1     -     X   3       )     ⁢     (       Y   1     -     Y   2       )             ⁢     
     ⁢         ∂   B       ∂     Z   2         =         X   1     -     X   3             (       X   1     -     X   2       )     ⁢     (       Y   1     -     Y   3       )       -       (       X   1     -     X   3       )     ⁢     (       Y   1     -     Y   2       )             ⁢     
     ⁢         ∂   B       ∂     Z   3         =         X   2     -     X   1             (       X   1     -     X   2       )     ⁢     (       Y   1     -     Y   3       )       -       (       X   1     -     X   3       )     ⁢     (       Y   1     -     Y   2       )                     (   13   )             
 
         [0037]     Furthermore,  
           ∂   C       ∂     Z   1         ,         ∂   C       ∂     Z   2         ⁢           ⁢   and   ⁢           ⁢       ∂   C       ∂     Z   3               
 
 existing in Equation 9 and the correction constants α, β and γ can be obtained as the following Equation 14.  
                   ∂   C       ∂     Z   1         =       1   -         ∂   A       ∂     Z   1         ⁢     X   1       -         ∂   B       ∂     Z   1         ⁢     Y   1         =           X   2     ⁢     Y   3       -       X   3     ⁢     Y   2               (       X   1     -     X   2       )     ⁢     (       Y   1     -     Y   3       )       -       (       X   1     -     X   3       )     ⁢     (       Y   1     -     Y   2       )               ⁢     
     ⁢         ∂   C       ∂     Z   2         =       1   -         ∂   A       ∂     Z   2         ⁢     X   2       -         ∂   B       ∂     Z   2         ⁢     Y   2         =           X   3     ⁢     Y   1       -       X   1     ⁢     Y   3               (       X   1     -     X   2       )     ⁢     (       Y   1     -     Y   3       )       -       (       X   1     -     X   3       )     ⁢     (       Y   1     -     Y   2       )               ⁢     
     ⁢         ∂   C       ∂     Z   3         =       1   -         ∂   A       ∂     Z   3         ⁢     X   3       -         ∂   B       ∂     Z   3         ⁢     Y   3         =           X   1     ⁢     Y   2       -       X   2     ⁢     Y   1               (       X   1     -     X   2       )     ⁢     (       Y   1     -     Y   3       )       -       (       X   1     -     X   3       )     ⁢     (       Y   1     -     Y   2       )                       (   14   )             
 
         [0038]     When the values obtained from Equations 12 to 14 are substituted into Equation 11, the correction constants α, β and γ at the position (x, y) of the fourth damper where a vertical acceleration sensor is not installed can be obtained as Equation 15 from the position coordinate values (X 1 , X 2 , X 3 ) and (Y 1 , Y 2 , Y 3 ) of the three vertical acceleration sensors  10 ,  12  and  14 .  
                     α   ⁡     (     x   ,   y     )       =           (       Y   2     -     Y   3       )     ⁢   x     +       (       X   3     -     X   2       )     ⁢   y     +       X   2     ⁢     Y   3       -       X   3     ⁢     Y   2               (       X   1     -     X   2       )     ⁢     (       Y   1     -     Y   3       )       -       (       X   1     -     X   3       )     ⁢     (       Y   1     -     Y   2       )                         β   ⁡     (     x   ,   y     )       =           (       Y   3     -     Y   1       )     ⁢   x     +       (       X   1     -     X   3       )     ⁢   y     +       X   3     ⁢     Y   1       -       X   1     ⁢     Y   3               (       X   1     -     X   2       )     ⁢     (       Y   1     -     Y   3       )       -       (       X   1     -     X   3       )     ⁢     (       Y   1     -     Y   2       )                         γ   ⁡     (     x   ,   y     )       =           (       Y   1     -     Y   2       )     ⁢   x     +       (       X   2     -     X   1       )     ⁢   y     +       X   1     ⁢     Y   2       -       X   2     ⁢     Y   1               (       X   1     -     X   2       )     ⁢     (       Y   1     -     Y   3       )       -       (       X   1     -     X   3       )     ⁢     (       Y   1     -     Y   2       )                         (   15   )             
 
         [0039]     When the correction constants α, β and γ obtained from Equation 15 and the vertical accelerations Aa1, Aa2 and Aa3 of the first to third vertical acceleration sensors  10 ,  12  and  14  are substituted into Equation 2, the ECU  16  can obtain the vertical acceleration Ad at the position of the fourth damper  24  in which a vertical acceleration sensor is not installed.  
         [0040]     Further, the ECU  16  of the present invention corrects the vertical accelerations of the first to third vertical acceleration sensors  10 ,  12  and  14  to the vertical accelerations at the positions of the first to third dampers  18 ,  20  and  22  using the following Equation 16.
 
 Aα   n [1]=α[1]× A α[1]+β[1]× A α[2]+β[1]× A α[3]
 
 Aα   n [2]=α[2]× A α[1]+β[2]× A α[2]+β[2]× A α[3]
 
 Aα   n [3]=α[3]× A α[1]+β[3]× A α[2]+β[3]× A α[3],  (16)
 
 where Aa n [1], Aa n [2] and Aa n [3] are values corrected to the vertical accelerations at the positions of the first to third dampers. α[1], β[1] and γ[1] are correction constants at the position of the first damper, α[2], β[2] and γ[2] are correction constants at the position of the second damper, and α[3], β[3] and γ[3] are correction constants at the position of the third damper. 
 
         [0041]     For example, as shown in Table 1 below, if the X and Y position coordinate values of the first to third vertical acceleration sensors are given (1500, 1800), (200, 1800) and (1400, 300) and the x and y position coordinate values of the first to fourth dampers are given (1700, 2000), (0, 2000), (1600, 500) and (100, 500), respectively, the correction constants α, β and γ at the positions of the respective dampers are obtained. Then, from the correction constants at the position of the fourth damper and the X and Y position coordinate values of the first to third vertical acceleration sensors, the fourth vertical acceleration, i.e., the vertical acceleration at the position of the fourth damper can be obtained, which is not shown in Table 1.  
                                                                                   TABLE 1                                   1 st     2 nd     3 rd     4 th             Vertical   Vertical   Vertical   Vertical           Accelera-   Accelera-   Accelera-   Accelera-           tion Sensor   tion Sensor   tion Sensor   tion                                    Vertical   X   1500   200   1400           Accelera-   Y   1800   1800   300       tion       Sensor       Damper   x   1700   0   1600   100           y   2000   2000   500   500            α   0.405   0.030   0.276   0.876       β   2.471   2.292   −0.143   1.164       γ   −0.133   −0.133   0.866   0.866                  
 
         [0042]     The ECU  16  of the present invention outputs signals for controlling the damping forces of the first to fourth dampers  18 ,  20 ,  22  and  24  in order to improve the ride comfort of the vehicle according to the values corrected to the vertical accelerations at the positions of the first to fourth dampers using the correction constants α, β and γ at the positions of the respective dampers. Thus, the first to fourth actuators  26 ,  28 ,  30  and  32  are operated with the damping forces.  
         [0043]      FIG. 3  is a view showing the positions of the four dampers and three velocity sensors of the vehicle employing the semi-active suspension system. Referring to  FIG. 3 , two velocity sensors  100  and  102  are mounted adjacent to the front dampers  18  and  20  of the vehicle body  1 , respectively, while a velocity sensor  104  is mounted adjacent to one of the rear dampers  22  and  24 . Here, when “X” designates the width direction of the vehicle body and “Y” designates the longitudinal direction thereof, the positions of the two front velocity sensors  100  and  102  are (X 2 , Y 2 ) and (X 1 , Y 1 ), respectively, and the position of the rear velocity sensor  104  is (X 3 , Y 3 ). Since unexplained reference numerals are the same as those shown in  FIG. 1 , descriptions thereof will be omitted.  
         [0044]     In the present invention, correction constants for a velocity at a position where a fourth velocity sensor would be installed are obtained using the X and Y position coordinate values (for example, X 1 -X 2  and Y 1 -Y 3 ) among the three velocity sensors  100 ,  102  and  104 .  
         [0045]      FIG. 4  is a block diagram of the semi-active suspension system, illustrating a method for measuring a velocity according to the present invention. Referring to this figure, the semi-active suspension system comprises the first to third velocity sensors  100 ,  102  and  104 , the ECU  16 , the first to fourth dampers  18 ,  20 ,  22  and  24 , and the first to fourth actuators  26 ,  28 ,  30  and  32 . Here, the first to third velocity sensors  100 ,  102  and  104  are sensors installed adjacent to three of the four dampers on the vehicle body so as to measure the velocity of the vehicle.  
         [0046]     In the semi-active suspension system configured as above, the method for measuring the velocity according to the present invention is performed as follows. Meanwhile, since a velocity is obtained by integrating an acceleration in the method for measuring the velocity of the present invention, the same equations as those described in the aforementioned method for measuring the acceleration will be used.  
         [0047]     First, velocities Va1, Va2 and Va3 measured by the respective first to third velocity sensors  100 ,  102  and  104  are input into the ECU  16 .  
         [0048]     The ECU  16  receives the velocities Va1, Va2 and Va3 measured by the respective first to third velocity sensors  100 ,  102  and  104  and obtains a fourth velocity Vd using Equation 17 as follows:
 
 Vd=α×Vα 1+β ×Vα 2+γ ×Vα 3,  (17)
 
 where α, β and γ are correction constants for the position of the fourth damper where a fourth velocity sensor is not installed , Vd is the fourth velocity, and Va1, Va2 and Va3 are the first to third velocities, respectively. 
 
         [0049]     Referring to Equation 17, the fourth velocity Vd at the position of the fourth damper where a velocity sensor is not installed is obtained by multiplying the vertical velocities Va1, Va2 and Va3 measured in the first to third velocity sensors  100 ,  102  and  104  by the corresponding correction constants α, β and γ and subsequently summing up the first to third velocities that have been multiplied by the correction constants.  
         [0050]     The correction constants α, β and γ by which the respective velocities are multiplied are obtained as follows.  
         [0051]     For example, assuming that a plane equation  P  for the velocity sensors installed on the vehicle body is Z=AX+BY+C, all the coordinate values of the respective velocity sensors are included in the equation  P .
 
(X1,Y1,Z1),(X2,Y2,Z2),(X3,Y3,Z3)∈P,  (18)
 
 where A, B and C are constants, which are expressed as the following Equations 19 to 21, respectively.  
             A   =           (       Z   1     -     Z   2       )     ⁢     (       Y   1     -     Y   3       )       -       (       Z   1     -     Z   3       )     ⁢     (       Y   1     -     Y   2       )               (       X   1     -     X   2       )     ⁢     (       Y   1     -     Y   3       )       -       (       X   1     -     X   3       )     ⁢     (       Y   1     -     Y   2       )                   (   19   )               B   =           (       X   1     -     X   2       )     ⁢     (       Z   1     -     Z   3       )       -       (       X   1     -     X   3       )     ⁢     (       Z   1     -     Z   2       )               (       X   1     -     X   2       )     ⁢     (       Y   1     -     Y   3       )       -       (       X   1     -     X   3       )     ⁢     (       Y   1     -     Y   2       )                   (   20   )                     C   =       ⁢       Z   ⁢           ⁢   1     -     A   ⁢           ⁢   X   ⁢           ⁢   1     -     B   ⁢           ⁢   Y   ⁢           ⁢   1                   =       ⁢       Z   ⁢           ⁢   2     -     A   ⁢           ⁢   X   ⁢           ⁢   2     -     B   ⁢           ⁢   Y   ⁢           ⁢   2                   =       ⁢       Z   ⁢           ⁢   3     -     A   ⁢           ⁢   X   ⁢           ⁢   3     -     B   ⁢           ⁢   Y   ⁢           ⁢   3                     (   21   )             
 
         [0052]     At this time, assuming that the installation positions of the three velocity sensors are upper points on the dampers and in a common plane, and Y coordinate values of the front first and second velocity sensors are the same (i.e., Y 1 =Y 2 ), the three velocity sensors output the velocities of {dot over (Z)} 1 , {dot over (Z)} 2  and {dot over (Z)} 3 , respectively.  
         [0053]     The plane equation for the first to fourth dampers installed on the vehicle body is considered to be z=Ax+By+C, and is then subjected to first differentiation. At this time, the positions of the dampers are expressed using x and y.  
                 ⅆ   z       ⅆ   t       =           ⅆ   A       ⅆ   t       ⁢   x     +         ⅆ   B       ⅆ   t       ⁢   y     +       ⅆ   C       ⅆ   t                 (   22   )                       ⅆ   A       ⅆ   t       ⁢     (       Z   1     ,     Z   2     ,     Z   3       )       =         ∂   A       ∂     Z   1         ⁢       Z   .     1     ⁢       ∂   A       ∂     Z   2         ⁢       Z   .     2     ⁢       ∂   A       ∂     Z   3         ⁢       Z   .     3         ,     
     ⁢       where   ⁢           ⁢       ∂     Z   i         ∂   t         =         Z   .     i     .               (   23   )             
 
         [0054]     Since A is a function of Z1, Z2 and Z3 and Z1, Z2, and Z3 are also functions of time, a derivative of A with respect to time is expressed as Equation 23 using a chain rule. That is, Equation 23 can be expressed as Equation 24 by rearranging it according to the velocities of the respective sensors after expanding Equation 23 with respect to respective coefficients.  
                       ⅆ   z       ⅆ   t       =       ⁢         (           ∂   A       ∂           ⁢     Z             ⁢   1           ⁢   x     +         ∂   B       ∂           ⁢     Z             ⁢   1           ⁢   y     +       ∂   C       ∂           ⁢     Z             ⁢   1             )     ⁢       Z   .               ⁢   1         +                     ⁢         (           ∂   A       ∂           ⁢     Z             ⁢   2           ⁢   x     +         ∂   B       ∂           ⁢     Z             ⁢   2           ⁢   y     +       ∂   C       ∂           ⁢     Z             ⁢   2             )     ⁢       Z   .               ⁢   2         +                     ⁢       (           ∂   A       ∂           ⁢     Z             ⁢   3           ⁢   x     +         ∂   A       ∂           ⁢     Z             ⁢   3           ⁢   y     +       ∂   A       ∂           ⁢     Z             ⁢   3             )     ⁢       Z   .               ⁢   3                       (   24   )                   ⅆ   z       ⅆ   t       =         α   ⁡     (     x   ,   y     )       ⁢       Z   .     1       +       β   ⁡     (     x   ,   y     )       ⁢       Z   .     2       +       γ   ⁡     (     x   ,   y     )       ⁢       Z   .     3                 (   25   )             
 
         [0055]     Thus, the correction constants α, β and γ at the damper positions (x, y) of the dampers are expressed as the following Equation 26.  
                       α   ⁡     (     x   ,   y     )       =           ∂   A       ∂     Z   1         ⁢   x     +         ∂   B       ∂     Z   1         ⁢   y     +       ∂   C       ∂     Z   1                         β   ⁡     (     x   ,   y     )       =           ∂   A       ∂     Z   2         ⁢   x     +         ∂   B       ∂     Z   2         ⁢   y     +       ∂   C       ∂     Z   2                         γ   ⁡     (     x   ,   y     )       =           ∂   A       ∂     Z   3         ⁢   x     +         ∂   B       ∂     Z   3         ⁢   y     +       ∂   C       ∂     Z   3                   ⁢     
     ⁢         ∂   A       ∂     Z   1         ,         ∂   A       ∂     Z   2         ⁢           ⁢   and   ⁢           ⁢       ∂   A       ∂     Z   3                     (   26   )             
 
 existing in Equation 26 and the correction constants α, β and γ can be obtained as the following Equation 27.  
                       ∂   A       ∂     Z   1         =         Y   2     -     Y   3             (       X   1     -     X   2       )     ⁢     (       Y   1     -     Y   3       )       -       (       X   1     -     X   3       )     ⁢     (       Y   1     -     Y   2       )                           ∂   A       ∂     Z   2         =         Y   3     -     Y   1             (       X   1     -     X   2       )     ⁢     (       Y   1     -     Y   3       )       -       (       X   1     -     X   3       )     ⁢     (       Y   1     -     Y   2       )                           ∂   A       ∂     Z   3         =         Y   1     -     Y   2             (       X   1     -     X   2       )     ⁢     (       Y   1     -     Y   3       )       -       (       X   1     -     X   3       )     ⁢     (       Y   1     -     Y   2       )                         (   27   )             
 
         [0056]     Further,  
           ∂   B       ∂     Z   1         ,         ∂   B       ∂     Z   2         ⁢           ⁢   and   ⁢           ⁢       ∂   B       ∂     Z   3               
 
 existing in Equation 26 and the correction constants α, β and γ can be obtained as the following Equation 28.  
                       ∂   B       ∂     Z   1         =         X   3     -     X   2             (       X   1     -     X   2       )     ⁢     (       Y   1     -     Y   3       )       -       (       X   1     -     X   3       )     ⁢     (       Y   1     -     Y   2       )                           ∂   B       ∂     Z   2         =         X   1     -     X   3             (       X   1     -     X   2       )     ⁢     (       Y   1     -     Y   3       )       -       (       X   1     -     X   3       )     ⁢     (       Y   1     -     Y   2       )                           ∂   B       ∂     Z   3         =         X   2     -     X   1             (       X   1     -     X   2       )     ⁢     (       Y   1     -     Y   3       )       -       (       X   1     -     X   3       )     ⁢     (       Y   1     -     Y   2       )                         (   28   )             
 
         [0057]     Furthermore,  
           ∂   C       ∂     Z   1         ,         ∂   C       ∂     Z   2         ⁢           ⁢   and   ⁢           ⁢       ∂   C       ∂     Z   3               
 
 existing in Equation 26 and the correction constants α, β and γ can be obtained as the following Equation 29.  
                       ∂   C       ∂     Z   1         =     1   -         ∂   A       ∂     Z   1         ⁢     X   1       -         ∂   B       ∂     Z   1         ⁢     Y   1                     =           X   2     ⁢     Y   3       -       X   3     ⁢     Y   2               (       X   1     -     X   2       )     ⁢     (       Y   1     -     Y   3       )       -       (       X   1     -     X   3       )     ⁢     (       Y   1     -     Y   2       )                           ∂   C       ∂     Z   2         =     1   -         ∂   A       ∂     Z   2         ⁢     X   2       -         ∂   B       ∂     Z   2         ⁢     Y   2                     =           X   3     ⁢     Y   1       -       X   1     ⁢     Y   3               (       X   1     -     X   2       )     ⁢     (       Y   1     -     Y   3       )       -       (       X   1     -     X   3       )     ⁢     (       Y   1     -     Y   2       )                           ∂   C       ∂     Z   3         =     1   -         ∂   A       ∂     Z   3         ⁢     X   3       -         ∂   B       ∂     Z   3         ⁢     Y   3                     =           X   1     ⁢     Y   2       -       X   2     ⁢     Y   1               (       X   1     -     X   2       )     ⁢     (       Y   1     -     Y   3       )       -       (       X   1     -     X   3       )     ⁢     (       Y   1     -     Y   2       )                         (   29   )             
 
         [0058]     When the values obtained from Equations 27 to 29 are substituted into Equation 26, the correction constants α, β and γ at the position (x, y) of the fourth damper where a velocity sensor is not installed can be obtained as Equation 30 from the position coordinate values (X 1 , X 2 , X 3 ) and (Y 1 , Y 2 , Y 3 ) of the three velocity sensors  100 ,  102  and  104 .  
                 α   ⁡     (     x   ,   y     )       =           (       Y   2     -     Y   3       )     ⁢   x     +       (       X   3     -     X   2       )     ⁢   y     +       X   2     ⁢     Y   3       -       X   3     ⁢     Y   2               (       X   1     -     X   2       )     ⁢     (       Y   1     -     Y   3       )       -       (       X   1     -     X   3       )     ⁢     (       Y   1     -     Y   2       )             ⁢     
     ⁢       β   ⁡     (     x   ,   y     )       =           (       Y   3     -     Y   1       )     ⁢   x     +       (       X   1     -     X   3       )     ⁢   y     +       X   3     ⁢     Y   1       -       X   1     ⁢     Y   3               (       X   1     -     X   2       )     ⁢     (       Y   1     -     Y   3       )       -       (       X   1     -     X   3       )     ⁢     (       Y   1     -     Y   2       )             ⁢     
     ⁢       γ   ⁡     (     x   ,   y     )       =           (       Y   1     -     Y   2       )     ⁢   x     +       (       X   2     -     X   1       )     ⁢   y     +       X   1     ⁢     Y   2       -       X   2     ⁢     Y   1               (       X   1     -     X   2       )     ⁢     (       Y   1     -     Y   3       )       -       (       X   1     -     X   3       )     ⁢     (       Y   1     -     Y   2       )                     (   30   )             
 
         [0059]     When the correction constants α, β and γ obtained from Equation 30 and the velocities Va1, Va2 and Va3 of the first to third velocity sensors  100 ,  102  and  104  are substituted into Equation 17, the ECU  16  can obtain the velocity Vd at the position of the fourth damper  24  where a velocity sensor is not installed.  
         [0060]     Further, the ECU  16  of the present invention corrects the velocities of the first to third velocity sensors  100 ,  102  and  104  to the velocities at the positions of the first to third dampers  18 ,  20  and  22  using the following Equation 31.
 
 Vα   n [1]=α[1]× V α[1]+β[1]× V α[2]+β[1]× V α[3]
 
 Vα   n [2]=α[2]× V α[1]+β[2]× V α[2]+β[2]× V α[3]
 
 Vα   n [3]=α[3]× V α[1]+β[3]× V α[2]+β[3]× V α[3],  (31)
 
 where Va n [1], Va n [2] and Va n [3] are values corrected to the velocities at the positions of the first to third dampers. α[1], β[1] and γ[1] are correction constants at the position of the first damper, α[2], β[2] and γ[2] are correction constants at the position of the second damper, and α[3], β[3] and γ[3] are correction constants at the position of the third damper. 
 
         [0061]     Therefore, when the X and Y position coordinate values of the first to third velocity sensors and the x and y position coordinate values of the first to fourth dampers are given, the correction constants α, β and γ at the positions of the respective dampers are. obtained in the ECU  16  of the present invention. Then, from the correction constants at the position of the fourth damper and the X and Y position coordinate values of the first to third velocity sensors, the fourth velocity, i.e., the velocity at the position of the fourth damper, can be obtained. Further, signals for controlling the damping forces of the first to fourth dampers  18 ,  20 ,  22  and  24  are output in order to improve the ride comfort of the vehicle according to the values corrected to the velocities at the positions of the first to fourth dampers. Thus, the first to fourth actuators  26 ,  28 ,  30  and  32  are operated with the controlled damping forces.  
         [0062]     As described above, according to the present invention, the fourth vertical acceleration can be obtained by multiplying the three vertical accelerations measured from the three acceleration sensors by the constants for correcting them to accelerations at actually desired damper positions and subsequently summing up them. Then, by multiplying the vertical accelerations measured from the three acceleration sensors by the correction constants at the corresponding damper positions, the corrected vertical accelerations can be obtained.  
         [0063]     Thus, according to the present invention, vertical accelerations at the positions of the dampers can be obtained and the vertical accelerations measured from the sensors can be corrected using the correction constants, so that there is an advantage in that the ride comfort of a vehicle can be more correctly controlled using the corrected vertical accelerations.  
         [0064]     In addition, the present invention can be applied to velocity sensors instead of vertical acceleration sensors so as to more correctly control the ride comfort of a vehicle.  
         [0065]     The present invention is not limited to the embodiments described above but may be modified or changed in various manners by those skilled in the art within the scope and the technical spirit of the invention defined by the appended claims.