How should Simulation Accuracy settings be modified?

The Simulation Accuracy settings are available through the selection of Accuracy under the World menu. Under normal circumstances, the WM3D user should not worry about adjusting these settings (other than possibly changing the Animation Step), as the default values accommodate most simulations.

Following is a description of each of the settings and how they affect WM3D performance:

Animation Step settings

If your simulation appears too choppy or is animated too fast, decrease the Animation Step to produce a smoother, slower animation.

• _____ s

  This value defines the time between frames of animation. The default value is 0.02 seconds.

• _____ /s

  This value represents the number of animation frames per second of animation time. The default value is 50 frames/second.

Because the two values are inverses, when one value is changed the other will be automatically computed.

Configuration Error settings

The Configuration Error provides an absolute error bound for variable step integration. To increase the accuracy of a simulation being computed with variable step integration, decrease the Configuration Error values. To increase the accuracy of a simulation being computed with fixed step integration, decrease the Integration Step. See How can a simulation be checked for accuracy?

• Position: _____ m

  The default value is 0.01 meters.

• Orientation : _____ deg

  The default value is 1 degree.

See Appendix A.3 "How Working Model 3D Bounds Errors" of the User's Manual for more information. The Configuration Error parameters represent the Absolute Acceptable Error, in the discussion presented in Appendix A.3.

Integrator settings

• Euler (approximate, fast)

  One of the fastest and simplest (but least accurate) of all numerical integration methods. Use only for "quick and dirty" simulations.

• Kutta-Merson (accurate)

  A more complex integration scheme which yields highly accurate results fairly quickly. Use for all simulations where accuracy is important.

See "Integrators" in Appendix A.4 of the User's Manual for more information concerning the differences between Euler and Kutta-Merson integration.

Integration Step settings

• Fixed

  In Fixed mode, the integration time step is locked. The fixed step integrator may be valuable in debugging contact anomalies. See How should collision problems be confronted? for a description.

• Variable

  In Variable mode, the integrator automatically adjusts the time step throughout the simulation to optimize computational performance. The integration step may become smaller than the one specified in the Animation Step but will never become larger. The variable time step integrator should be used for computing most simulations.

• Integration Step: _____ s

  For fixed step integration, this value defines the time step used for numerical integration. For variable step integration, this value determines the maximum time step used for numerical integration.

  The Integration Step is calculated from the Animation Step and Steps Per Frame:

  Integration Step = Animation Step (s) / Steps per Frame

• Steps per Frame: _____

  This value defines the number of integration steps to be used in computing one animation frame. For variable step integration, this will be the minimum number of integration steps used per animation frame.

Other Settings

• Overlap Factor: _____

  This value defines the maximum allowable amount of interpenetration between two colliding bodies. All bodies in WM3D have associated with them a Characteristic Dimension, which is automatically defined to be a fraction of the smallest dimension of a bounding box for that body (see Geometry tab of the Properties window). WM3D will not allow interpenetration between two bodies to exceed the product of the Overlap Factor and the smallest of the two Characteristic Dimensions.

  If a simulation has large bodies that contain small protrusions that are set to collide with other bodies, the Overlap Factor and/or Characteristic Dimension for that body may have to be decreased to accommodate the body detail.

• Assembly Error: _____ m

  The Assembly Error bounds the numerical error when constraints are joined in WM3D (particularly important during assembly of closed-loop mechanisms). If the Assembly Error is changed, existing constraints must be Split (available through right-click menu) and then re-Joined (also available through right-click menu) to be assembled to the new tolerance.

  The Assembly Error is used during the assembly process only (i.e., prior to the simulation). Subsequently, WM3D uses the Configuration Error parameters to monitor errors.

• Significant Digits: _____

  The value given in the Significant Digits box corresponds to the number of digits that are accurate during numerical integration. Increasing this value from its default setting of 5 will have much less affect on simulation accuracy than decreasing the Configuration Error for variable step integration (or decreasing the Integration Step for fixed step integration).

  The Significant Digits value sets the relative error, to 10^-n. See Appendix A.3 "How Working Model 3D Bounds Errors" of the User's Manual for more information.

Warnings

• Inaccurate integration

  See What is meant by the warning "Unable to Achieve Desired Accuracy with variable Time Step Integration"?

• Initial body overlap

  See What is meant by the warning "Initial Body Overlap"?

• Redundant constraints

  See What is meant by the warning "Redundant Constraints"?

• Inconsistent constraints

  See What is meant by the warning "Inconsistent Constraints / Physical Instabilities"?