Command Element

Description

Specifies options for the transient simulation.

Example

TRA/MSTIFF,ERR=0.001, HINIT=1e-8, HMAX=.001

Format

TRANSIENT/{ABAM, VSTIFF, MSTIFF}

, ERROR = r

[, HINIT = r ]

[, HMAX = r ]

Arguments

Name

Description

{ABAM, VSTIFF, MSTIFF}

Dynamics problems can be classified into two categories: numerically stiff and nonstiff.  Stiff problems are characterized by very large real part of the eigenvalues of the linearized equations of motion at a given configuration.  The corresponding modes are generally over-damped, meaning that any transients die out very quickly.

VSTIFF and MSTIFF are specifically designed for numerically stiff problems.  ABAM is suitable for nonstiff problems.

VSTIFF is the default integrator for all kinds of problems.  This is a good choice because MotionSolve computes the Jacobian analytically, thus minimizing the performance penalty suffered by solvers using numerical Jacobian.  However, for nonstiff problems, ABAM generally performs faster than VSTIFF.

MSTIFF has similar charateristics as VSTIFF, but generally works better for models with fewer degrees of freedom.

ERROR

This represents the maximum error the integrator is allowed to make in computing the displacement and velocity.  MotionSolve monitors errors in both displacement and velocity.  Velocity tolerance is taken to be 1000 times the specified tolerance value.  Displacement tolerance is equal to the specified tolerance value.

Note

ERROR is interpreted in terms of model units.  For example, if you choose SI units, then a tolerance of 1.0e-3 implies an allowable error of 1 millimeter in displacement and 1 meter per second in velocity.

HINIT

Initial step size to be used by the integrator.

HMAX

Maximum step size to be used by the integrator.