butterworth.F

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!||====================================================================
!||    butterworth      ../engine/source/tools/univ/butterworth.F
!||--- called by ------------------------------------------------------
!||    e_period         ../engine/source/general_controls/damping/static.F
!||    sensor_contact   ../engine/source/tools/sensor/sensor_contact.F
!||    sensor_vel       ../engine/source/tools/sensor/sensor_vel.F
!||====================================================================
      SUBROUTINE butterworth (DT,FREQ,X2,X1,X,FX2,FX1,FX)
C-----------------------------------------------
C   I m p l i c i t   T y p e s
C-----------------------------------------------
#include      "implicit_f.inc"
C-----------------------------------------------
C   D u m m y   A r g u m e n t s
C-----------------------------------------------
      my_real, INTENT (IN) :: 
     .   dt,freq,x2,x1,x,fx2,fx1
      my_real, INTENT (OUT) ::
     .   fx
C-----------------------------------------------
C   L o c a l   V a r i a b l e s
C-----------------------------------------------
      INTEGER I
      my_real
     .   dt2,wd,wa,wa2,a0,a1,a2,b1,b2,c1
C=======================================================================
C    INPUT variables:
C    DT   - actual timestep
C    FREQ - filtering frequency
C    X    - current unfiltered variable
C    X1   - unfiltered variable (timestep -1)
C    X2   - unfiltered variable (timestep -2)
C    FX1  - filtered variable (timestep -1)
C    FX2  - filtered variable (timestep -2)
C-----------------------------------------------------------------------
C    OUTPUT variables:
C    FX - current filtered variable
C=======================================================================
      dt2  = dt / two                           
      pi   = atan2(zero,-one)
      wd = sqr2*pi*freq*5.0/3.0  
      wa = sin(wd*dt2) / cos(wd*dt2)                     
      wa2 = wa*wa                                      
      c1 = one + sqr2*wa + wa2                           
      a0 = wa2 / c1                                    
      a1 = two * a0                                   
      a2 = a0                                          
      b1 = -two*(wa2 - one) / c1                       
      b2 = (-one + sqr2*wa - wa2) / c1                   
      fx = a0*x + a1*x1 + a2*x2 + b1*fx1 + b2*fx2           
C---
      RETURN
      END SUBROUTINE butterworth