Model Element

Class Name

Integrator

Description

Integrator defines the simulation control parameters for a time-domain-based nonlinear dynamic analysis.  These parameters control:

Dynamic simulations are performed on systems with one or more degrees of freedom.  The dynamic simulation accounts for all inertia effects, all applied forces, and internal constraints.  This enables you to run accurate system level simulations of complex mechanical systems.

Integrator supports two different categories of equation formulations:

Attribute Summary

Usage

# Integrator using “DSTIFF”

Integrator (type=”DSTIFF”, optional_attributes)

# Integrator using “VSTIFF”, “MSTIFF” or “ABAM”

Integrator (type=”VSTIFF”, optional_attributes)

Integrator (type=”MSTIFF”, optional_attributes)

Integrator (type=”ABAM”,   optional_attributes)

Attributes

Integrator using “DSTIFF” or “GSTIFF”

type

String

Defines the integrator to be used.

Select “DSTIFF”

hmin

Double

Defines the minimum step size the integrator is allowed to take.

The hmin attribute is optional.

When not specified, the default value for hmin is 1.0E-6.

interpolate

Boolean

Specifies whether the integrator uses interpolation for the results at the output steps.

The interpolate attribute is optional.

When not specified, interpolate = True

maxit

Integer

The maximum number of iterations that the corrector is allowed to take to achieve convergence.
 

The default value for MAXIT is 4.  Under no circumstances should this be increased beyond 8.  If the results don’t converge for a particular time step, it may be better to let the integrator fail and try to solve with a smaller time step.

vel_tol_factor

Double

The factor that multiplies integr_tol to yield the error tolerance for velocity states..
 

The default value for vel_tol_factor is 1000.  A good value for vel_tol_factor is the highest frequency in the system that you want to track..

minit

Int

The minimum number of iterations that the corrector is allowed to take before it checks for corrector divergence.
 

The default value for minit is 1.  Under no circumstances should this be increased beyond 3.

dae_index

Int

The index of the DAE formulation. 

Index 3 indicates that position constraints are added to the equations of motion.

Index 1 indicates that position, velocity, and acceleration constraints are added (Stabilized Index 1).

See Comments in XML online help for more detailed information.

The default value for index is 3.

dae_eval_expiry

Int

This is an optional attribute.  It defines the number of integration steps after which the evaluation pattern defined by dae_jacob_eval is ignored, and the default evaluation pattern is to be used.  The default evaluation pattern is automatically determined when a new Jacobian is needed by examining the convergence rate of the corrector.

dae_constr_tol

Double

The tolerance on all algebraic constraint equations that the corrector must satisfy at convergence.

The default value for dae_constr_tol is 1.0E-5.  Decrease this value only if you see a lot of “noise” in the joint or motion reaction forces.  Smaller values for this attribute will decrease the speed of the solution.

pattern

String

This attribute controls the frequency of evaluation of the Jacobian matrix during corrector iterations. The “Kth” entry in the string tell MotionSolve whether to evaluate the Jacobian or not for the “Kth” iteration.

Specify pattern only when the default setting does not work well.  Frequent Jacobian iterations can slow down the simulations dramatically.

When not specified, the integrator examines the rate of convergence of the corrector to automatically determine if a new Jacobian is needed.

Integrator using “VSTIFF”, “ABAM” or “MSTIFF:

type

String

Defines the integrator to be used.

Select “VSTIFF”, “ABAM” or “MSTIFF”

Optional attributes – Available to all variants

error

Double

Represents the maximum absolute error per step that the integrator is allowed in computing the displacement, velocity, and differential equations states.  Differential equation states include those found in DIFF, TFSISO, and the GSE modeling entities.  Since displacement and velocity have different units, they are subject to different error tolerances.

The error attribute is optional.

When not specified, the the default value for error is 1.0E-3.

hmax

Double

Defines the maximum step size the integrator is allowed to take.  You can also use this parameter to control the accuracy of the results.  Generally, a smaller time step leads to more accurate results.  For models with discontinuities, such as contact, a smaller value of HMAX should be specified.

The hmax attribute is optional.

When not specified, the default value for hmax is 0.01.

hinit

Double

Specifies the maximum initial step size.

The hinit attribute is optional.

When not specified, the default value for hinit is 1.0E-8.

kmax

Integer

Specifies the maximum order that the integrator is to take. Higher orders lead to higher accuracy, but lower stability in the numerical method.

For models that contain contact or other discontinuous events, it may beneficial to set KMAX = 2, which prevents the integrator from trying to use a higher order that will normally not succeed.

The kmax attribute is optional.

The table below specifies the maximum order for each integrator.

When not specified, kmax is given the maximum value the integrator can have.

NOTE: kmax ≥ 1

Comments

Example

This example shows the default settings for the Integrator element that uses DSTIFF method.

DSTIFF = Integrator (type="DSTIFF", error=0.001, hinit=1e-008, hmax=0.01, hmin=1e-006,

                    kmax=5)