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L2-1ANSYS, Inc. Proprietary

2010 ANSYS, Inc. All rights reserved.Release 13.0

December 2010

Introduction to ANSYSRigid Body Dynamics

Customer Training Material

Lecture 2

Rigid Dynamics

Procedure

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Introduction to ANSYS Rigid Body Dynamics



December 2010

Customer Training MaterialSteps in Case setup

A. Add rigid body analysis to project page

B. Geometry definition

C. Material definition

D. ConnectionsE. Meshing

F. Solution settings

G. Boundary condition/Loading

H. Solve

I. Post processing

J. Motion load export

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December 2010

Customer Training MaterialPoints to Remember

Inputs and outputs are forces, moments, displacements, velocities

and accelerations.

All parts are rigid such that there are no stresses and strain results

produced, only forces, moments, displacements, velocities and

accelerations.

The solver is tuned to automatically adjust the time step. Doing it

manually is often inefficient and results in longer run times.

Viscous damping can be taken into account through springs or joints

(bushing, revolute, cylindrical joints)

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December 2010

Customer Training MaterialAnalysis of Multibody Systems

Dynamic Analysis

Study of time-dependent response of the system due to the action of

time-varying loads/boundary conditions.

The time-scale of the loading is such that inertia and/or damping effects

are considered to be important.

Equation of motion to be solved:

( )Ma Cv Ku F t

Inertial Force

Damping Force

Elastic Force

External Force

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December 2010

Customer Training Material

For Rigid Dynamics analysis where all bodies are rigid bodies the

Equation of Motion can be represented as:

Rigid Dynamic Solvers

Explicit 4th and 5th order Runge-Kutta method

If some components are assumed rigid while other components are

assumed flexible then

The inertial force term in the above equation applies to all components

while the damping force and the elastic force terms refer only to the

flexible components

FEA Solvers

Implicit (Newmark method, HHT method)

( )Ma F t

( )Cv KM tua F

Analysis of Multibody Systems

Where the force vector includes Ku +Cv

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December 2010

Customer Training MaterialA. Add Rigid body analysis

From the Toolbox, drag a Rigid Dynamics

template to the Project Schematic.

The analysis tree will be configured for

rigid dynamic analysis.

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December 2010

Customer Training MaterialB. Geometry

Geometry:

Rigid dynamic analysis can handle 2D / 3D bodies.

3D bodies can be shells and/or solids.

Multibody parts (DesignModeler concept) are supported.

All parts are assumed rigid (no stress/strain).

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December 2010

Customer Training MaterialC. Material definition

Solid geometry is used to define the model.

Parts are considered rigid so density is the only required material

property.

To begin a rigid dynamic analysis parts must have their stiffnessbehavior flagged as rigid.

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December 2010

Customer Training MaterialD. Connections

Joints are automatically created as per the proximity of the part.

Unlike flexible analyses, rigid bodies are related to one another and

to ground using joints/springs and/or contacts.

The body is represented by mass elements at the centroid.

Joints are represented by their respective coordinate systems.

Motions at each joint are defined with respect to a local reference

coordinate system associated with each joint.

Mass

Joint

Mass

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December 2010

Customer Training MaterialE. Meshing

Apart from node ( Mass21 element) at the centroid of the

rigid bodies, mesher will create mesh on the

surface/edges of the contact.

Rigid body meshing can be used for 3D contact. Similarly, for 3D models, only the faces of the rigid body in

contact are meshed because the volume mesh is not needed.

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December 2010

Customer Training MaterialF. Solution settings

The Analysis Setting branch contains

the controls for setting up the analysis:

Number of steps

Time step controls

Output controls

Solver tolerances

Note, additional steps are generally used

when loads are applied or removed orwhen load histories undergo abrupt

changes. Output controls allow the

number of solution points to be specified

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December 2010

Customer Training MaterialSolution settings

Rigid dynamic solutions employ explicit time integration, whereas

flexible analyses use implicit integration.

Required time step sizes are dictated by the highest frequencies of

the system.

Auto time stepping is usually recommended since determination of

the optimum time step size is often problematic:

Initial time step: an initial time step that is too large will result in a

message indicating accelerations are too high. If the time step is only

slightly large (or too small) auto time stepping will correct this. Minimum time step: stops solution if the required time step is below this

value.

Maximum time step: places a limit on how large time steps can become in

the auto time stepping routine. Usually used to insure time steps do not

increase to where desired results are stepped over.

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December 2010

Customer Training MaterialG. Boundary condition/Loading

For rigid dynamic analysis, inertial loads (acceleration and/or

gravity) are used.

Joint conditions are used to apply all other loads as constants or

time varying using tabular or functional definitions.

Once inserted each joint in the model is available via the drop down list

in the joint condition details.

A joint can be dragged and dropped from the connections folder to

automatically specify a joint condition.

Remote force

Support: Remote displacement

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December 2010

Customer Training MaterialBoundary condition/Loading

Joint conditions can be defined as

constant, tabular or as a function.

Tabular and functional data can be

input in the details window, the tablein Simulation or in the Engineering

Data application.

Once entered in Simulation the data

can be saved to the Engineering Data

application.

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December 2010

Customer Training MaterialBoundary condition/Loading: Joint Conditions

Position, Velocity, Rotation and Angular velocity tabular joint

conditions are fit using a cubic spline interpolation. In some cases

it may be necessary to add additional steps to resolve differences

between the actual input and the interpolated curve. Note, no slope continuity is enforced between load steps.

Load

Time

Tabular

Points

Interpolated

Curve

Potential additional steps

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December 2010


To Control joints during multiple steps:

Analysis settings details allow the number of steps

to be specified.

Individual joint conditions can then be specified.

A timeline and a table can be displayed for each

joint condition.

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December 2010


Joint conditions can be deactivated or activated from the

timeline/table using a right mouse click.

Place cursor in desired step on the timeline or

in the table and RMB

Note: anywhere within the step is fine

RMB either

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December 2010


Various mathematical functions and arithmetic operators can beused to construct functions using time as an independentvariable.

The function is displayed in function and graphical formats. Help manual link,, can provide information // Mechanical User's

Guide // Features // Applying Loads // Specifying Load Values //Constant Load Expressions about additional functions

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December 2010

Customer Training MaterialBoundary condition/Loading:

Changes in position or angle in a joint condition result in forces and

accelerations in the body which are calculated internally.

Changes in velocities (translational or rotational) result in

accelerations which are calculated internally.

Changes in joint conditions can result in discontinuities in force,

acceleration, velocity or position (see below).

Translation

Velocity

Time Time

VelocityDistance

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December 2010


Discontinuities in forces or accelerations are acceptable and the rigid

solver will generally handle these with no problems.

Positional or angular discontinuities are non physical and will usually

result in no solution or non physical results.

It is recommended that prescribed rotations be limited to 2or less.

Where larger rotations are desired change to angular velocity over

time.

It is generally not recommended to apply multiple joint conditions on

a single joint.

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December 2010

Customer Training MaterialH. Solve

Once setup is complete the solution can be

initiated.

During (and following) the solve the solution

information branch can be highlighted to check

the solver output information.

If warning or error messages are

produced they will be indicated in the

status bar and can be reviewed via

the message window.

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December 2010

Customer Training MaterialI. Postprocessing

Rigid dynamic results are presented in either graphical, tablular or

animation formats.

Only result objects under Deformation and Probe are used.

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December 2010

Customer Training MaterialI. Postprocessing :User defined results

User defined results can be obtained

from worksheet or can be entered

using User defined result tab

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December 2010

Customer Training MaterialPostprocessing

Desired result displays are highlighted in the tree

as with other analysis types.

The timeline displays graphically the same

information as the table.

Note: check boxes above each column control the

graphical content.Toggle

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December 2010


Results can be scoped to entire assemblies

or individual bodies.

For results from a specific time, place the

cursor in the timeline/table and RMB to

retrieve results.

RMB at

specificlocation in

timeline

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December 2010

Customer Training MaterialPostprocessing- Probes

Probes can be inserted into the Solution branch allowing results to

be extracted for specific locations or connections.

Probes are scoped using joints, geometry or coordinate systems.

A shortcut for setting up joint and spring Probes is to drag and drop

the item into the Solution branch.

Drag and Drop

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December 2010

Customer Training MaterialPostprocessing - Joint Output

A variety of joint output is available:

Constraint forces and moments

Stop and lock status

Relative position

Constitutive displacements and rotations

Joint elastic forces, and damping forces

Relative displacement and rotations (cumulative), velocities, and

accelerations

The components of the bases vectors at the two nodes in the deformedconfiguration

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December 2010


Charts allow further refinement to

graphical/tabular data displays.

Select the desired result(s) from the tree to

include in the chart.

Use CTRL or Shift key to make multiple

selections from the tree.

Charts will be inserted in the tree as children of

the Environment.

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December 2010


Chart display for position result.

Note: activate/deactivate chart legend via a RMB.RMB

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December 2010


Chart showing Velocity probe and position result plotted on the same

chart.

Note, the Y axis is normalized when dissimilar results are plotted.

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December 2010


Animation controller:

Animation results may be distributed evenly over the entire result

or actual result points can be used for the animation.

The more frames used the smoother the animation but time to display animation

is increased.

Start/Stop Number of

Frames

Animation

Speed

Results

Distribution

Save .avi

animation file

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December 2010

Customer Training MaterialJ. Export of motion loads as static loads

Export a motion load file containing joint and

inertial loads at a given time

Allows creation of static loads on selected parts,

made flexible

For further detail of this feature download theanimated tutorial available on customer portal

mech-rbd_13.0_l02

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