Welcome to Adams/Solver Subroutines - Kxcad.net

Using the DFLAG Variable
Welcome to Adams/Solver Subroutines
The use of the DFLAG variable with a GFOSUB is optional. It merely identifies that GFOSUB is being
called to evaluate a partial derivative. One of the states on which the GFOSUB depends has been
perturbed very slightly. In many situations, it is likely that major calculations in the GFOSUB are
insensitive to small changes in state, and therefore, need not be recalculated.
In such situations, you can structure the GFOSUB not to redo these calculations. For example, assume
you are using a GFOSUB to model tire and terrain interactions. The terrain is modeled as a set of
triangular patches, and the tire forces as a GFOSUB. A labor-intensive calculation in the problem is to
identify the patch that is in contact with the tire. Because pertubations of system states are always small
when DFLAG is true, it is not necessary to recalculate the patch that is in contact with the tire.
Caution: • Euler angles calculated by SYSARY or SYSFNC subroutines may be
discontinuous. To avoid discontinuity, use the RCNVRT utility subroutine to
convert the rotational angles from Euler angles to some other rotational scheme
that does not encounter discontinuity.
FORTRAN - Prototype
• When the iflag argument is true, you must make all the calls to SYSARY and
SYSFNC as are made to compute the component values of the GFORCE
statement. This ensures that Adams/Solver has the proper functional dependencies.
In general, failure to account for dependencies in the GFORCE components can
make it difficult for Adams/Solver to converge to a solution and/or can force
Adams/Solver to take small integration steps. It may increase simulation time
dramatically.
• When the iflag argument is true, SYSARY and SYSFNC return zero values for
system and user-defined variables. Computations that divide by these values will
result in fatal errors. You should check for nonzero values, or ensure the iflag
argument is set to false, before dividing by these values.
A sample structure for GFOSUB is shown next. The comments explain how the subroutine works.
&
SUBROUTINE GFOSUB(ID, TIME, PAR, NPAR, DFLAG,
IFLAG, RESULT)
C
C === Type and dimension statements ===================
C
C Note: For a machine with 60 or more bits per word,
C
C
substitute "REAL" for "DOUBLE PRECISION".
C - External variable definitions -----------
C
INTEGER ID
DOUBLE PRECISION TIME
DOUBLE PRECISION PAR( * )
INTEGER NPAR
LOGICAL DFLAG
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