basic of dynamic simulation auto desk inventor

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11/29/2006 - 1:00 pm - 2:30 pm Room:Lando - 4303 (MSD Campus

The Basics Of Dynamic Simulation

Dynamic Simulation provides analysis tools for Autodesk Inventor Professional software users thatallow a thorough evaluation of product performance in 3D. Dynamic Simulation provides animation,kinematics evaluation of paths and positions, and dynamic analysis to review timing and determineforces. By enhancing the virtual prototype, Dynamic Simulation streamlines the design process andprovides higher-quality first articles. During this session, well introduce typical design issues that canbe solved with Dynamic Simulation and the basics workflows a) create joint b) define the physical

environment, c) run the simulation, d) analyze the results, and e) export Motion loads to StressAnalysis.

MA23-4

About the Speaker:

Ravi Akella - Autodesk, Inc.

Miles Carlson (Co-Speaker)and

Ravi Akella joined Autodesk in January 2006; he is an MSD Solutions Engineer. Prior to joiningAutodesk, Ravi worked at LMS International where he used engineering analysis tools that work withCATIA V5. There he gained considerable experience in Kinematics and Dynamics, FEA and Solid

Mechanics. He is a member of the Simulation Experts group at the Manufacturing Solutions Division.Ravi presented the simulation capabilities in Inventor R11 to the CAD industry press in March and wasthe primary instructor for Simulation at Autodesks annual training event for reseller engineers. In 2002,Ravi graduated with an MS in Structural Mechanics from The University of Iowa in Iowa City.

BIO TBA

Stay Connect to AU all year at www.autodesk.com/AUOnline


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The Basics of Dynamic Simulation MA 23-4

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What is Simulation?

The Merriam Webster dictionary defines simulation as the imitative representation of the functioning of one systemor process by means of the functioning of another or examinationof a problem often not subject to direct experimentation by means of a simulating device.

In the case of Autodesk Inventor Professional for Simulation, the intent is to imitatively represent the functioning of amechanical device being designed using the 3D CAD tools in Autodesk Inventor by adding aspects of the physicalworld like gravity and friction to the model and applying the numerical methods of Multibody Dynamics.

Brave new world of simulation

We live in a physical world whose laws are described in Newtonian physics. However, in the past 3D CAD modelsexisted in an environment where a feather could move a brick and mechanisms could be driven without concern tothe forces and torques required to drive them in the real world.

In the spirit of functional design, Autodesk Inventor Professional for Simulation R11 allows us to simulate reality.

The key to understanding simulation is the realization that the best investigative models are simplifiedrepresentations of the real thing where the simplifications hinge on valid assumptions.

The French Connection

The Dynamic Simulation functionality in Autodesk Inventor Professional for Simulation R11 is a result ofAutodesk acquiring Solid Dynamics based in Roanne, France an established simulation softwaredeveloper and services provider in August 2005.

The company was founded in 1992 by Fabien Chojnowski and Laurent Chojnowski. The one focus of

this team has been to create the best in class kinematics and dynamics physics simulation softwaretools for engineers and designers.

Typical design issues

Autodesk Inventor Professional for Simulation R11 allows the design engineer to approach a design challengefrom a functional perspective, the form and fit then follow suit. In the absence of proper analysis tools, addressingthese issues will be left to physical prototype testing which is laborious and expensive. Also, testing design variantsis not feasible in most cases.

The typical design problems that arise when creating machine assemblies and mechanisms are as follows:

Detecting collisions during operation

Investigating operating cycle times of the mechanism

Sizing motors, hydraulic elements, springs, and other motive power devices

Ensuring components are strong enough to sustain operating loads


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Detecting collision during operation:

In cases where two moving parts are meant to contact each other, knowing the time and position of thecontact between two moving parts, being able to measure the contact force generated by the impact andthe rebound path and direction of the parts is very important for function oriented design process.

There is also a need in some cases to ensure collision-free operation. Focusing solely on form and fitmakes it very unlikely that contact events during operation will be detected in the design stages.

Investigating operating cycle times of the mechanism:

A lot of machines and mechanical devices are created to perform repetitive tasks that need to be optimizedfor maximum productivity, accuracy and safety of machine operators. Machines that operate too slow ortoo fast are not as productive as those that operate exactly as desired.

Controlling and optimizing operating cycle times is a complicated matter of juggling multiple parameters inthe realm of physical prototype testing.

Sizing motors, hydraulic elements, springs, and other motive power devices:

Anything that moves needs a force or a torque to make it move. In the real world gravity, friction and inertianeed to be overcome by motive power devices in machines. However, the critical question here is therequired force or torque output of these motive devices.

The choice of an inadequate or overtly strong motive device can seriously jeopardize the operatingcharacteristics and structural integrity of the mechanism.

Ensuring components are strong enough to sustain operating loads:

Analyzing the strength of components is a critical part of validating a design. However, the ability toaccurately determine the operating loads and constraints on components is elusive. In the absence ofaccurate operating forces the design engineer is left to leverage his/her prior experience to estimate theapplied loads.

Also, creating designs with overly safe components adds unnecessary bulk to the designs and make thetask of moving the components more resource hungry. Optimized designs that offer adequate strengthand require lesser energy to run are highly desirable.

Dynamic Simulation, How?

At this stage, it is now important to understand how to address these issues using Autodesk Inventor Professionalfor Simulation. Lets proceed to break down the process of a typical Dynamic Simulation study into a series ofsteps.


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Graphical User Interface

But first, lets take a quick look at the graphical user interface of the Dynamic Simulation module inAutodesk Inventor Professional for Simulation

If you are familiar with the user interface of Autodesk Inventor then you will recognize this interfaceinstantly.

To the top left is the Panel Bar with buttons for all the functions, below the Panel Bar is the Browser thatshows all the functional elements that constitute the simulation model.

The Simulation Panel on the bottom left is peculiar to the Dynamic Simulation interface. The SimulationPanel controls the Multibody dynamics solver that actually computes the results of the simulation. The

length of the simulation in terms of time and hence the amount of data points in the results file aredetermined by settings in the Simulation Panel.

Browser

Simulation Panel

Panel Bar


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Creating Bodies... the building blocks

In a design process where form follows function creating the geometry up front is not necessary. Simplelines and arcs can be used to represent components. The geometry could be created after the verifyingthat the model satisfies the design intent.

However, in cases where the geometry is pre-existing Autodesk Inventor Professional for Simulation R11is able to leverage all the work already done and directly pull the mass and center of gravity informationfrom the existing geometry. Also, existing constraints and geometry features like edges and faces candirectly be leveraged to define Dynamic Simulation elements.

In Dynamic Simulation, all the components whose motion is under investigation are considered bodies.The Dynamic Simulation module uses the bodies to convey mass, inertia and centers of gravity informationto the Multibody dynamics solver. The following image shows a list of bodies in a model as seen in thebrowser

Sometimes multiple bodies that are expected to move together can be consolidated into a Welded Groupby multi-selecting the bodies in the browser, then right-clicking to open the contextual menu and picking theWeld Parts option as seen below

Please make note that this functionality is meant only to consolidate bodies in the Dynamic Simulationenvironment and not at all related to the weldments functionality you might otherwise be familiar with in

Autodesk Inventor.


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Creating Joints its all coming together

If you have done assembly modeling in Inventor in the past you are familiar with the idea of constraints.Constraints in assembly modeling determine the relative positions and the available degrees of freedom ofthe components.

Autodesk Inventor Professional for Simulation R11 can leverage pre-existing constraints in an assembledmodel to create joints using the Convert Assembly Constraints option in the Panel Bar as seen below.However, it is important to understand that joints in Dynamic Simulation behave differently from constraintsin the assembly modeling. For example, friction can be added to joints and upon running a simulation theresultant reaction forces in the joint are recorded and available for review and analysis.

At the outset in the Dynamic Simulation environment, all the bodies are assumed to be grounded. This

means that initially there are no available degrees of freedom for the bodies representing the components.As joints are created, degrees of freedom get added to the system and the bodies move from theGrounded group in the browser to the Mobile group.

There are various types of joints available in Autodesk Inventor Professional for Simulation. These jointsare divided into five different groups as follows

Standard Joints: These are the most primary joints used to build up a Dynamic Simulation model.Some examples of standard joints are


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o Revolution joints which restrict two axes belonging to two different bodies to be coincidentand allow rotation about that coincident joint axis but do not allow translation along theaxis.

o Prismatic joints which restrict two lines, axes, edges etc. on two different bodies to becoincident and allow translation along the coincident joint axis but do not allow rotationabout the axis.

Rolling Joints: The properties of these joints set up relationships between the two rollingcomponents or one rolling component and a non-rolling component. Some examples of rolling

joints areo Worm Gear joint which defines the relationship between two bodies representing the

worm and the pinion spur gear. The two axes required to define this joint should beperpendicular.


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o Screw joint which defines the relationship between a bolt and a nut. The relative rotationbetween the two bodies results in the bolt body translating down the coincident joint axistowards or away from the nut body.

Sliding Joints: These joints are used to define sliding motion between two cylinders, a cylinder anda plane, a cylinder and a curve and a point and a curve. Some examples of sliding joints areo Sliding Cylinder and Plane joint, as the name suggests, restricts the outside face of a

cylinder to always contact a plane but allows sliding motion between them.

o Sliding Point and Curve joint, as the name suggests, restricts a point on one body toalways stay coincident with a curve on another body but allows sliding motion betweenthem.


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2D contact Joints: These joints help detect contact between two coplanar 2-dimensional profiles ontwo different bodies. Some examples of 2 D contact joints are

o Point Curve joint detects contact between a point on one body and coplanar curve onanother body.

o Disc Curve joint detects contact between a circular disc on one body and a coplanar curveon another body.

Force Joints: These joints set up force relationships like springs and dampers between bodieswithout affecting the overall degrees of freedom. Also, 3D contact between two bodies can be setup using force joints. Some examples of force joints are


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o Spring/ Damper /Jack force joints, as the name suggests, set up force relationshipsbetween two points on two different bodies governed by specified stiffness, damping andactuator force values.

o 3D contact joints setup full 3D contact force relationships between two bodies. Theamount of contact force produced depends on the specified values of stiffness, damping

and friction values.

One of the important requirements to keep in mind when creating joints in a mechanism model is to avoidredundant constraints. Redundancies are caused when the same degree of freedom is constrained bymore than one joint. The biggest problem arising from redundant constraints is that the reaction forces for a

joint causing the redundancy now has more than one unique numerical solution making the resultsunpredictable. A more detailed explanation of redundant constraints is presenting in Appendix A at the endof this handout document.

Autodesk Inventor Professional for Simulation R11 makes the user aware of redundancies in the modelbut does not altogether stop the user proceeding with modeling. The Repair Redundancies option in thePanel Bar lets the user to come back and fix the model after creating all the joints as seen below


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In Autodesk Inventor Professional for Simulation, joints can also be used to drive the model kinematically.The degrees of freedom associated with a joint can be driven by specifying a constant or varying position,velocity and acceleration using the Imposed Motion option as seen in the image below. The imposedmotion not only drives the bodies but it also provides the force (or torque in case of a rotation) and therequired load for the imposed motion to occur while taking into account the physical parameters like gravity,inertial effects, friction, etc. Those output values are very useful to size motive power devices.


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Creating Forces and Torques time to force the matter or twist it

Once the bodies are created and connected to each other with joints, the user has the option in AutodeskInventor Professional for Simulation R11 to apply forces or torques to the bodies and study themechanisms resultant motion.

The Force and Torque options are available in the Panel Bar for Dynamic Simulation in AutodeskInventor Professional for Simulation R11 as seen below. The forces and torques need a location on a bodythat they are applied on, a direction along which or about which the force and torque is applied and aconstant or varying value for the magnitude of the force or torque. Its also possible to apply the force ortorque as a combination of three orthogonal components.

Creating Traces I know what you did last simulation

The Trace option in the Panel Bar allows the user to pick a point on a body and trace the position, velocityand acceleration of that point and output the values to the results file for the simulation. Traces are a veryuseful tool to study the motion with respect to the global grounded origin or with respect to another body inthe simulation model. Traces can also be used to create function based geometry, for example the trace ofa followers tip can be used to determine the shape of a cam in a cam-follower model.


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Dynamic Part Motion look before you leap

The Dynamic Part Motion functionality exists to allow the user to check the model before running the fullsimulation. It allows a user to click and hold on a body in the model and apply a force to see approximatelyhow the model behaves as a whole.

The magnitude of the force is controlled by how far the mouse cursor is dragged, a coefficient set up toconvert mouse pointer motion to a force value and a user-determined maximum force value. The user alsohas the option to damp the motion of the model by picking no damping, light damping or heavy damping asseen in the following image

Running the Simulation you reap what you sow

Once the model is set up, its time to run the simulation. This is done using the Simulation Panel below thebrowser in the user interface. To run a simulation the user needs to set the Final Time (the length of thesimulation), Time Mode (the number of time steps to record results for during the simulation) and activate


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or deactivate the screen refresh during the simulation. In general, the simulation will solve faster if thescreen refresh is switched off, however, the screen refresh lets the user see if the model is behaving asexpected part way through the simulation.

Once the simulation is running, the background in the browser turns gray to indicate that the model cannotbe modified while the simulation is running without changing the results. Upon completion the simulation, ifthe user saves the assembly the results are written to a file of .iaa format. Those results can then be

imported later in the output grapher to compare 2 simulations. After completing the simulation, it can berewound and played back and looped if needed using the Play current simulation in continuous loopbutton.

If the model needs to be changed, the Activation Construction Mode button can be clicked. Thisreverts back the browser background color to white and lets the user modify the model.

Reviewing results in the output grapher fruits of your labor

The Output Grapher, as the name suggests, is an interface for reviewing, plotting and post processing the

results of a simulation.


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As seen in the image above, the Output Grapher has a browser to the left where the results to be graphedcan be selected, at the top right the selected results are tabulated and at the bottom right the results aregraphed.

The output grapher allows the user to graph the results of a simulation, import and compare the results of

multiple simulations, export the results to an excel file, print out the graphs and finally export the results toFEA analysis in Autodesk Inventor Professional for Simulation. We will discuss the Export to FEAfunctionality in more detail later in this document.

A useful way to review the results of a simulation is to have the output grapher open, have the desiredresults graphed and replaying the simulation from the Simulation Panel. This synchronizes the OutputGrapher and the 3D simulation animation and allows the user to see the behavior of the model whilewatching a cursor track the graphed results.

Creating AVI Animation watch out Pixar

A useful way to present the results of the simulation is to create an animation file. Autodesk InventorProfessional for Simulation R11 allows the user to create an .avi file of the simulation results.

Finding out the Unknown Forcesvoyage into the unknown

Autodesk Inventor Professional for Simulation R11 allows the user to calculate, for a givenposition, the required force, torque or jack to keep a dynamic mechanism in static equilibriumusing the Unknown Force option in the Panel Bar. All the external influences are kept inconsideration during the analysis (gravity, spring, external or joint forces, and so on).

It is possible to study a succession of positions by driving a degree of freedom of one of themechanism joints and by giving the number of desired positions.

As seen below, the Unknown Force requires the user to select force, torque or jack and thendetermine the point and axis along which to determine the Unknown Force. Also the jointsassociated with the body can be driven from their current position to a specified position alongwith a number of steps to determine Unknown Force at intermediate positions.


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This concludes a description of the typical workflows in Dynamic Simulation in Autodesk InventorProfessional for Simulation. Our intent was to cover the basics so that you become familiar with the

technology; if you have further questions, contact Ravi Akella at [email protected]

Dynamic Simulation and FEA

Finite Element Analysis, as you might already be aware, is a method employed to determine thestrength of components when subjected to a given set of boundary conditions and applied forces. Thefollowing is a simplified way to understand the relationship between the Dynamic Simulation and FEA.

In Autodesk Inventor Professional for Simulation R11 the part FEA analysis available assumes linearelastic deformations so its based on Hookes Law. For systems that obey Hooke's law, the extensionproduced is proportional to the load:

Where

xis the deformation,Fis the load or force applied on the part, andkis the stiffness or force constant of the material comprising thepart.


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The force Fin most FEA analyses is an educated guess, which makes the accuracy of the resultingdeformations only as good as the initial guess of the force.

Dynamic Simulation addresses the dependence on educated guesses for applied forces in part FEAanalysis by introducing Newtons second law of motion into the mix.

It has been a common convention to describe Newton's second law in the mathematical formula

WhereFis the resultant Force,

a is the instantaneous acceleration, andm is the mass of the part.

Using Dynamic Simulation, the resultant forces on a part at any instant of time during simulation can bedetermined based on its mass and acceleration at that instant. Hence, Dynamic Simulation now armsus with an accurate value of the force acting on the part for part FEA analysis.

Thanks to the ingenious theories of two knighted 17th century English scientists, Sir Robert Hooke andSir Isaac Newton, Autodesk Inventor Professional for Simulation R11 is able to bring together DynamicSimulation and part FEA analysis.

Export to FEA passing the baton

As discussed in the previous section, the Dynamic Simulation Panel Bar and the Output Grapher have theExport to FEA option. This function passes the resultant forces on a part and the boundary conditions(based on the joints) to the part FEA analysis functionality at a given time step during the simulation.

In order to analyze the part at an instant when the desired force magnitude is maximum, the user needs toright-click on the tabulated force value in the Output Grapher and select the Search Max. option (or SearchMin. if the force values are negative) in order to move the cursor to the time step where the maximumvalue occurs as seen in the following image

Sir Robert Hooke Sir Isaac Newton


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After clicking the Export to FEA in the Dynamic Simulation environment, the user needs to switch to thepart Stress Analysis module in Autodesk Inventor Professional for Simulation R11 and select the materialfor the part, then click the Motion Loads option in the Panel Bar as seen below and then click the Stress

Analysis Update option. Its that easy!


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In Conclusion

We hope that The Basics of Dynamic Simulation session and this document have given you enough insight towant to tackle your design issues with the Dynamic Simulation functionality in Autodesk Inventor Professional forSimulation R11.

We will pat ourselves on the back if we have piqued your interest and you have further questions after trying out thetools on your own. So please feel free to contact us and we will do our best to address your queries.


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APPENDIX A

Redundant constraints and how to avoid them

A mechanism with redundant constraints is analogous to a statically indeterminate problem instatics. Hence, for an overconstrained mechanism, there are infinite solutions for reactionforces.

The concept of redundant constraints might initially seem non-intuitive but with practice anexperienced engineer can learn to avoid creating mechanisms with redundant constraints.

The reasons why redundant constraints may occur in mechanism models in dynamic simulationare as follows:

Joints in dynamic simulation do not account for any clearances. They are kinematicallyperfect joints which is not possible when creating joints in fabricated prototypes in reallife.

The parts or bodies that constitute a mechanism are considered infinitely rigid. In the

real world there is no truly rigid part i.e. all parts are subject to deformation when enoughload is applied.

The following are some basics about degrees of freedom:

Every unconstrained body or part has six degrees of freedom. Three degrees oftranslation i.e. translation along x, y and z axes and three degrees of rotation i.e. rotationabout x, y and z axes.

Joints in dynamic simulation remove some or all of these degrees of freedom from themechanism depending on the type of joint. Some examples are as follows:

o Revolute joint: Allows only rotation about one selected axis. Hence, it removes5 degrees of freedom.

o Prismatic joint: Allows translation along only one axis. Hence, it removes 5degrees of freedom.

o Planar joint: Allows translation in a plane i.e. along two axes. Hence, it removes4 degrees of freedom.

o Spherical joint: Allows rotation about all three axes. Hence, it removes 3degrees of freedom.

A body that is fixed to ground cannot translate along or rotate about any axis. Hence,grounded bodies have zero degrees of freedom. As a general rule, if a degree offreedom on a body is removed by an existing joint, then another joint on the same bodyshould not constrain that same dof.

When the resultant degrees of freedom of a mechanism are less than zero then it is consideredoverconstrained. The following is an example of a mechanism with redundant constraints.


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Consider an in-plane slider crank mechanism as shown in the following image. Whenfabricating such a mechanism it would be intuitive to create three pin (revolute) joints and oneprismatic joint.

The above mechanism has 4 bodies; hence we can calculate the degrees

of freedom (dof) as follows:

Total unconstrained degrees of freedom: +24

(4 bodies multiplied by +6 dof)

Degrees of freedom removed by grounded body: -6

Degrees of freedom removed by 3 revolute joints: -15

(3 joints multiplied by -5 dof)

Degrees of freedom removed by the prismatic joint: -5

Resultant degrees of freedom: -2

Green is grounded

(-6 dof)

Prismatic joint between green and yellow

(-5 dof)

Revolute joint between blue and

green (-5 dof)

Revolute joint between red

and blue

(-5 dof)

Revolute joint between

yellow and red (-5 dof)


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Now consider the same model with joints modified to fix the problem:

Now we can recalculate the degrees of freedom (dof) as follows:

Total unconstrained degrees of freedom: +24

(4 bodies multiplied by +6 dof)

Degrees of freedom removed by the grounded body: -6

Degrees of freedom removed by the revolute joint: -5

Degrees of freedom removed by the cylindrical joint: -4

Degrees of freedom removed by the spherical joint: -3

Degrees of freedom removed by the prismatic joint: -5

Resultant degrees of freedom: +1

Green is

Grounded

(-6 dof)

Revolute joint between blue and

green (-5 dof) Prismatic joint between green and yellow

(-5 dof)

Spherical joint

between yellow and

red (-3 dof)

Cylindrical joint

between red and

blue

(-4 dof)


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It is important to understand that adding a cylindrical joint and a spherical joint does not makethe motion of this mechanism non-planar. All the bodies in this mechanism will still remain in-plane.

This is because adding a cylindrical joint between the blue link and the red link does not allow

translation normal to the plane as the red link is already rigidly constrained in-plane on theother end with a revolute joint to the green link that is fixed to ground.

Similarly, the red link cannot translate out-of-plane because it is connected with a spherical jointto the yellow link that is already rigidly constrained in-plane by a prismatic joint to the greengrounded link. Also, the cylindrical joint between the blue link and the red link rigidlyconstrains any out-of-plane rotations between them.

In conclusion, it is important to realize that in dynamic simulation we use mathematical modelstorepresent reality; not to recreate it.

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basic of dynamic simulation auto desk inventor