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In the end, if you can run a simulation at large step-sizes and high order it is guaranteed
that your simulation will finish quickly, and the quality of the results is going to be very
good.
Finally, please note that even when a function looks smooth, what is important is
how many derivatives of the function continue to be smooth. For instance, if a cubic
spline is used to represent a motion then the function will have two continuous
derivatives. The third derivative will be already discontinuous, and therefore GSTIFF
will be prevented to increase the integration order to high values. An easy fix to this
problem is to actually specify the motion as a velocity rather than a position level motion.
This simple change will potentially increase the GSTIFF integration order by one, a very
positive change.
2. Choose the modeling units wisely
A poor choice of modeling units leads in different equations to entries that are
vastly different in magnitude. The end result is that during the solution sequence the
Solver will encounter matrices that have very large condition numbers. If the situation is
not extremely bad, a matrix with large condition number will lead to very poor quality
corrections in the Newton-Raphson algorithm for example (see section 1.1) and therefore
large number of iterations for convergence. If the condition number is very large, the
corrector stage of the solution can fail altogether, or the matrix is identified singular by
the linear sparse solver.
To prevent situations like these:
a. Choose units so that model states (displacements and velocities) assume
reasonable values. For example, choosing "mm" for displacements of a rocket,
which travels thousands of kilometers, is a poor choice.
b. Choose time units appropriate to the phenomena being studied. If duration of
dynamic event time is short, consider using MILLISECONDS units.
3. If it’s possible, avoid dummy parts (any part with zero or very small mass)
Keep in mind these recommendations:
a) Sometimes dummy parts are useful, but if possible, avoid using them.
b) Avoid connecting dummy parts with compliant connections (BEAM, BUSHING,
etc.). If the mass is very small, qualitatively, acceleration = Force/mass = very
large number. Thus, under certain circumstances small masses/moments of
inertia introduce high frequencies and/or jolts into the system, which has
detrimental effects on the solver performance.
c) If you must use dummy parts, then constrain all its degrees of freedom, since with
no degrees of freedom for the dummy part, qualitatively, acceleration=Force/mass
is not an issue.
d) Dummy parts should be massless, 0.0 (or unassigned), and not 1e-20.
4. Keep an eye on the JOINT elements in the model
In most simulations the use of JOINTs in the model is not leading to robustness issues.
However, avoid redundant constraints. ADAMS/Solver tries to eliminate them by
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