The Basics Of Dynamic Simulation - widom-assoc.comwidom-assoc.com/389_MA23-4.pdf · The Basics Of...

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 that allow 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 determine forces. By enhancing the virtual prototype, Dynamic Simulation streamlines the design process and provides higher-quality first articles. During this session, we’ll introduce typical design issues that can be 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 Stress Analysis.

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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 joining Autodesk, Ravi worked at LMS International where he used engineering analysis tools that work with CATIA 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 was the primary instructor for Simulation at Autodesk’s annual training event for reseller engineers. In 2002, Ravi graduated with an MS in Structural Mechanics from The University of Iowa in Iowa City.

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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 system or process by means of the functioning of another <a computer simulation of an industrial process> or examination of 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 a mechanical device being designed using the 3D CAD tools in Autodesk Inventor by adding aspects of the physical world 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 models existed in an environment where a feather could move a brick and mechanisms could be driven without concern to the 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 simplified representations 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 of Autodesk acquiring Solid Dynamics based in Roanne, France an established simulation software developer 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 software tools for engineers and designers.

Typical design issues… Autodesk Inventor Professional for Simulation R11 allows the design engineer to approach a design challenge from a functional perspective, the form and fit then follow suit. In the absence of proper analysis tools, addressing these issues will be left to physical prototype testing which is laborious and expensive. Also, testing design variants is 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 the contact between two moving parts, being able to measure the contact force generated by the impact and the 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 fit makes 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 optimized for maximum productivity, accuracy and safety of machine operators. Machines that operate too slow or too 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 in the 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 inertia need to be overcome by motive power devices in machines. However, the critical question here is the required force or torque output of these motive devices. The choice of an inadequate or overtly strong motive device can seriously jeopardize the operating characteristics 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 to accurately determine the operating loads and constraints on components is elusive. In the absence of accurate operating forces the design engineer is left to leverage his/her prior experience to estimate the applied loads. Also, creating designs with overly safe components adds unnecessary bulk to the designs and make the task of moving the components more resource hungry. Optimized designs that offer adequate strength and 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 Professional for Simulation. Let’s proceed to break down the process of a typical Dynamic Simulation study into a series of steps.


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Graphical User Interface

But first, let’s take a quick look at the graphical user interface of the Dynamic Simulation module in Autodesk Inventor Professional for Simulation…

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

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

The Simulation Panel on the bottom left is peculiar to the Dynamic Simulation interface. The Simulation Panel 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 are determined 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. Simple lines and arcs can be used to represent components. The geometry could be created after the verifying that the model satisfies the design intent.

However, in cases where the geometry is pre-existing Autodesk Inventor Professional for Simulation R11 is able to leverage all the work already done and directly pull the mass and center of gravity information from the existing geometry. Also, existing constraints and geometry features like edges and faces can directly 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 information to the Multibody dynamics solver. The following image shows a list of bodies in a model as seen in the browser…

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

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


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Creating Joints… it’s 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 of the components. Autodesk Inventor Professional for Simulation R11 can leverage pre-existing constraints in an assembled model 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 constraints in the assembly modeling. For example, friction can be added to joints and upon running a simulation the resultant 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 the ‘Grounded’ group in the browser to the ‘Mobile’ group. There are various types of joints available in Autodesk Inventor Professional for Simulation. These joints are 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 coincident and allow rotation about that coincident joint axis but do not allow translation along the axis.

o Prismatic joints which restrict two lines, axes, edges etc. on two different bodies to be

coincident and allow translation along the coincident joint axis but do not allow rotation about the axis.

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

o 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 be perpendicular.


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

• Sliding Joints: These joints are used to define sliding motion between two cylinders, a cylinder and a plane, a cylinder and a curve and a point and a curve. Some examples of sliding joints are…

o 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 to

always stay coincident with a curve on another body but allows sliding motion between them.


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• 2D contact Joints: These joints help detect contact between two coplanar 2-dimensional profiles on two 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 on another body.

o Disc Curve joint detects contact between a circular disc on one body and a coplanar curve

on another body.

• Force Joints: These joints set up force relationships like springs and dampers between bodies without affecting the overall degrees of freedom. Also, 3D contact between two bodies can be set up 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 relationships between two points on two different bodies governed by specified stiffness, damping and actuator force values.

o 3D contact joints setup full 3D contact force relationships between two bodies. The

amount 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 avoid redundant constraints. Redundancies are caused when the same degree of freedom is constrained by more 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 results unpredictable. A more detailed explanation of redundant constraints is presenting in Appendix A at the end of this handout document. Autodesk Inventor Professional for Simulation R11 makes the user aware of redundancies in the model but does not altogether stop the user proceeding with modeling. The ‘Repair Redundancies’ option in the Panel 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 imposed motion not only drives the bodies but it also provides the force (or torque in case of a rotation) and the required 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 Autodesk Inventor Professional for Simulation R11 to apply forces or torques to the bodies and study the mechanism’s resultant motion. The ‘Force’ and ‘Torque’ options are available in the Panel Bar for Dynamic Simulation in Autodesk Inventor Professional for Simulation R11 as seen below. The forces and torques need a location on a body that they are applied on, a direction along which or about which the force and torque is applied and a constant or varying value for the magnitude of the force or torque. It’s also possible to apply the force or torque 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, velocity and acceleration of that point and output the values to the results file for the simulation. Traces are a very useful tool to study the motion with respect to the global grounded origin or with respect to another body in the simulation model. Traces can also be used to create function based geometry, for example the trace of a follower’s 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 full simulation. It allows a user to click and hold on a body in the model and apply a force to see approximately how 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 to convert mouse pointer motion to a force value and a user-determined maximum force value. The user also has the option to damp the motion of the model by picking no damping, light damping or heavy damping as seen 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 the browser in the user interface. To run a simulation the user needs to set the Final Time (the length of the simulation), 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 the screen refresh is switched off, however, the screen refresh lets the user see if the model is behaving as expected part way through the simulation.

Once the simulation is running, the background in the browser turns gray to indicate that the model cannot be modified while the simulation is running without changing the results. Upon completion the simulation, if the 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 be rewound and played back and looped if needed using the ‘Play current simulation in continuous loop’ button.

If the model needs to be changed, the ‘Activation Construction Mode’ button can be clicked. This reverts 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 graphed can be selected, at the top right the selected results are tabulated and at the bottom right the results are graphed. 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 to FEA analysis in Autodesk Inventor Professional for Simulation. We will discuss the ‘Export to FEA’ functionality 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 desired results graphed and replaying the simulation from the Simulation Panel. This synchronizes the Output Grapher and the 3D simulation animation and allows the user to see the behavior of the model while watching 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 Inventor Professional for Simulation R11 allows the user to create an .avi file of the simulation results.

Finding out the Unknown Forces…voyage into the unknown

Autodesk Inventor Professional for Simulation R11 allows the user to calculate, for a given position, the required force, torque or jack to keep a dynamic mechanism in static equilibrium using the Unknown Force option in the Panel Bar. All the external influences are kept in consideration 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 the mechanism 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 then determine the point and axis along which to determine the Unknown Force. Also the joints associated with the body can be driven from their current position to a specified position along with 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 Inventor Professional 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 the strength of components when subjected to a given set of boundary conditions and applied forces. The following 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 linear elastic deformations so it’s based on Hooke’s Law. For systems that obey Hooke's law, the extension produced is proportional to the load:

Where

x is the deformation, F is the load or force applied on the part, and k is the stiffness or force constant of the material comprising the part.


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The force F in most FEA analyses is an educated guess, which makes the accuracy of the resulting deformations only as good as the initial guess of the force. Dynamic Simulation addresses the dependence on educated guesses for applied forces in part FEA analysis by introducing Newton’s second law of motion into the mix. It has been a common convention to describe Newton's second law in the mathematical formula

Where

F is the resultant Force, a is the instantaneous acceleration, and m is the mass of the part.

Using Dynamic Simulation, the resultant forces on a part at any instant of time during simulation can be determined based on its mass and acceleration at that instant. Hence, Dynamic Simulation now arms us 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 and Sir Isaac Newton, Autodesk Inventor Professional for Simulation R11 is able to bring together Dynamic Simulation 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 the ‘Export 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 to right-click on the tabulated force value in the Output Grapher and select the Search Max. option (or Search Min. if the force values are negative) in order to move the cursor to the time step where the maximum value 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 the part Stress Analysis module in Autodesk Inventor Professional for Simulation R11 and select the material for the part, then click the ‘Motion Loads’ option in the Panel Bar as seen below and then click the ‘Stress Analysis Update’ option. It’s 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 to want to tackle your design issues with the Dynamic Simulation functionality in Autodesk Inventor Professional for Simulation R11. We will pat ourselves on the back if we have piqued your interest and you have further questions after trying out the tools 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 in statics. Hence, for an overconstrained mechanism, there are infinite solutions for reaction forces. The concept of redundant constraints might initially seem non-intuitive but with practice an experienced engineer can learn to avoid creating mechanisms with redundant constraints.

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

• Joints in dynamic simulation do not account for any clearances. They are kinematically perfect joints which is not possible when creating joints in fabricated prototypes in real life.

• 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 enough load is applied.

The following are some basics about degrees of freedom:

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

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

o Revolute joint: Allows only rotation about one selected axis. Hence, it removes 5 degrees of freedom.

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

o Planar joint: Allows translation in a plane i.e. along two axes. Hence, it removes 4 degrees of freedom.

o Spherical joint: Allows rotation about all three axes. Hence, it removes 3 degrees 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 of freedom on a body is removed by an existing joint, then another joint on the same body should not constrain that same dof.

When the resultant degrees of freedom of a mechanism are less than zero then it is considered overconstrained. 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. When fabricating such a mechanism it would be intuitive to create three pin (revolute) joints and one prismatic 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 make the 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 the other 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 joint to the yellow link that is already rigidly constrained in-plane by a prismatic joint to the green grounded link. Also, the cylindrical joint between the blue link and the red link rigidly constrains any out-of-plane rotations between them.

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

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