Frequently asked questions (FAQ)

Several conceptual and procedural questions are discussed in this section. The information is provided to assist designers interested in using Dynamic Simulation as one of their tools.

When do I use Dynamic Simulation?

Dynamic simulation is useful throughout the design process. It can help refine your design:

• In digital prototyping, make subtle changes that affect the design, and then see the results before you go to physical parts.
• In failure analysis, take existing products and cycle them through simulations. Apply forces the components encounter and find out when and where they fail.

Use simulation and analysis to help you determine optimum shapes for the types of mechanisms you use.

What knowledge is required to use Dynamic Simulation?

Helpful things to know are:

• Assembly constraints position components in relation to one another. When doing a simulation of your assembly it is possible that the assembly constraints can over-constrain the model for simulation purposes. A typical example is an angle constraint placed as a “motor” to cause component motion when driven. Suppress those constraints before entering the Dynamic Simulation environment. Review the assembly constraints to see which ones act as “drivers” for motion, then suppress them. Leave constraints that maintain joint-type relationships unsuppressed, since they are converted to joints.
• Automatically Convert Constraints to Standard Joints, on by default, converts mate and insert constraints into standard joints upon entering the dynamic simulation environment. This function speeds up the process of building a simulation.

  If you are curious about how the constraints contribute to creating a joint, enter the dynamic simulation environment and review the list of automatically created joints. Then, in the Dynamic Simulation Settings dialog box, turn off the automatic constraint conversion option (it removes the automatically created joints) and construct joints manually. You can delete the manual joints and turn the automatic conversion option back on.

  Note: When Automatically Convert Constraints to Standard Joints is checked, the only standard joint available for use is the Spatial joint. You can continue to add non-standard joints.
• When using 2D Contact joints, we recommend increasing the time steps beyond the default 100/sec by 4X - 10X. Based on the number of steps and position of components, a 2D contact can occur between steps. The increased number of steps ensures that the contact occurs during a step.
• Tutorials that ship with Inventor assist with understanding the basics. Go to Help Learning Tools Tutorials Autodesk Inventor Simulation for a list of available simulation tutorials.

How do joints and constraints differ?

Constraints are used in the assembly to place components in relation to one another. Inventor provides these basic constraints, some with modifiers:

• Mate
• Flush
• Angle
• Tangent
• Insert
• Rotation
• Rotation-Translation
• Transitional

Within the Assembly Environment, it is possible to drag parts or to drive a constraint to review motion. Only geometry is respected, and information such as velocity, acceleration, and loads are not available.

In the Dynamic Simulation environment, we use joints to obtain the results. It is possible to define dynamic parameters in joints such as friction, damping, and stiffness. There are standard joints (revolution, prismatic, spherical, and so on) and advanced joints (contact, rolling, sliding, and so on):

Standard joints are built in one of three ways:

• Automatic Joint Conversion (default).
• Manually convert constraints into joints.
• Manually through a series of selections and input.

Advanced joints are built manually through a series of selections and input.

Why do assembly constraints display in the simulation browser?

The simulation browser lists assembly constraints as child nodes so that you can see the constraints that contribute to making that specific joint. Most of the constraint context menu commands are available.

What happens if I edit a constraint? Modifying a contributing constraint has the potential of changing the joint and degrees of freedom.

For example, a revolution joint has two constraints: an axial mate and a face or flush mate for positioning. Notice what happens when one of the constraints is suppressed:

Joint being edited	Action	Resulting joint
	Face or flush mate is suppressed.
	Axial mate is suppressed.

In the browser, the suppressed constraint displays with the component node and removed from the Joint node.

What happens if I turn off automatic constraint conversion?

Turning off the Automatically Convert Constraints to Standard Joints setting removes all joints so that you can manually create the appropriate joints. To create joints manually, use either the Insert Joint or Convert Assembly Constraints command.

Turning the setting back on causes Standard Joints to be calculated and created when you click OK.

Is there a list of joints that result from the constraints?

A list of joints that result from the constraints is in the Help content. See Convert Assembly Constraints for a conversion table.

Is it possible to use subassemblies?

You can use subassemblies. By default, subassemblies are considered rigid bodies. To create joints between subassembly components you must set the assembly to Flexible.

Right-click the assembly and click Flexible.

What causes components to move?

A component moves based on the degree of freedom of the joint and the motion you impose. To impose motion:

1. Right-click a joint and select Body Properties. The Body Properties dialog box displays.
2. Select the appropriate DOF tab.
3. Click the Edit imposed motion icon
4. Check Enable imposed motion and define the motion law

Note: In the simulation construction environment, you can click and drag a component to impose motion temporarily. However, the imposed motion does not adhere to specific settings and direction. Release the mouse button to stop the motion.

Why are all the components in the grounded folder?

In Inventor 2008, when entering Dynamic Simulation, all components were grounded. as is the case when no joints are defined.

You can look at it this way. In the Assembly environment, the first component is grounded by default. Thereafter, all components are unconstrained unless you apply constraints to them.

In Dynamic Simulation, all components are grounded until you define joints for the components. Joints define degrees of freedom. If all the components are ungrounded, then calculating a simulation is extremely time consuming and yields potentially random results.

A component that is grounded in the assembly is grounded when you go into the simulation environment. If you create an assembly using Inventor defaults, the grounded component is the first one placed in the assembly.

In the Dynamic Simulation environment, when the Automatically Convert Constraints option is OFF, all components are placed in the Grounded folder. As you add joints, you define degrees of freedom and it causes a component to move into a mobile group.

When the Automatically Convert Constraints option is ON (the default) components disperse into their Mobile groups. Components can remain in the Grounded folder based on the joints assigned by the automatic constraint conversion engine.

Why is joint friction ignored during an Unknown force simulation?

The Unknown force simulation is a static computation for a succession of positions. There are no velocities in the joints. The friction model of the joint follows a regularized law, depending on the velocity in the degree of freedom (the friction force is equal to 0.0 when the velocity is null). There is no friction in an Unknown force simulation. For the same reason, damping in joints are ignored (depending on velocity). An external load defined with a law based on time, in the Input Grapher, always has the same value for time = 0.0.

Can I use Dynamic Simulation with assemblies and components created with Make Components?

You can analyze assemblies and components created with the Make Components command. Consider the following when you perform dynamic simulations of such models:

• Assembly constraints (Layout constraints and translated constraints), automatically created by Make Components, can result in redundant joints when the assembly is opened in Dynamic Simulation. Accurate motion studies can be conducted when joint redundancies exist; however, a unique solution to the loads at the joints may not be calculated. To obtain a unique solution, perform the simulation with the joint redundancies resolved.
• In the top-down workflow, you control assembly and component changes through your layout. To resolve joint redundancies for assemblies created with Make Components, select Assembly Controls Position 3D in the Layout Constraint context menu. This adds degrees of freedom to your assembly. You can also select 3D Kinematics for translated constraints to add degrees of freedom. If your assembly contains flexible subassemblies, you may need to edit those subassemblies to add more degrees of freedom.
• Assemblies created with Make Components typically have some Layout constraints between the components and the layout part. You may need to turn off the Constrain to layout plane option in the component context menu to add degrees of freedom.
• You can turn off the automatic conversion of constraints to standard joints and manually create the appropriate joints. Keep in mind that the layout part acts as an anchor for assembly components created with the Make Components command. As a result, you may need to create some joints between components and the layout part to keep them positioned properly for Dynamic Simulation.

Can I use Dynamic Simulation with parts made only from sketch geometry?

You can make every part of the mechanism from a sketch. In this case, dynamic simulation sets the mass of the mobile groups to 1 kg and the terms of the inertia matrix diagonal to 0.01 kg.m². Therefore, it is possible to run a simulation to obtain kinematic results. Dynamic results are based on these automatic masses and inertia.

How do 1C and 2C rolling joints differ?

The 1C rolling joint applies only one constraint, rolling without sliding, between the two bodies. The 2C rolling joint applies the same rolling constraint AND a tangency constraint. The 1C rolling joint is used when the two bodies are already tangent due to the geometry. They stay tangent during the simulation due to the construction of the mechanism. The 2C rolling joint is used to maintain the tangency artificially because the construction of the mechanism could allow the two bodies to separate.

Why does the computation time for a simple mass-spring seem lengthy?

To solve the dynamic equations, the DS engine uses an algorithm with automatic time step change. The needed time steps could be few because of mass (M) and stiffness (K) in the mechanism. To ensure a good resolution precision, the time step is equivalent to . When stiffness (K) is high and/or mass (M) is low, the time step is small, resulting in a long computation time. Check mass and stiffness values, a common error is to mix units. For example, it is normal to have a long simulation time if we use 3D contact joints with significant stiffness.

What precautions do I take with an over-constrained mechanism?

An over-constrained mechanism can move, but there are too many loads (forces and moments) to compute in its joints with the hypothesis used by Dynamic Simulation. This situation is caused by not having any gap in joints and rigid parts. The results for positions, velocities, and accelerations are correct, but the solution for loads of the joint is not unique. For example, a four-bar system with only revolution joints is over-constrained. It moves because the rotational axes are perfectly parallel in the model. But it is not possible to give a unique solution to all the loads of the joint. If you change two revolution joints into a cylindrical and a spherical one, the mechanism is not over-constrained. The solution for loads of the joint is now unique.

Note: It corresponds to the real mechanism, which works fine without deformation in parts, even if the revolution axes are not strictly parallel.

Why are there fewer joints available than in Inventor 11?

The Constraint Reduction Engine (CRE) was introduced in Inventor 2008 and subsequent releases include it. The CRE generates standard joints automatically based on assembly constraints. It helps reduce clutter and places the created joints in the Standard Joints folder in the browser.

If you do not want to create Standard Joints automatically, activate the Dynamic Simulation Settings, cancel the Automatically Convert Constraints to Standard Joints selection, and all joints are removed. Then you can manually add the joints you want.

Can I access Dynamic Simulation through the API?

At present, you cannot use the API to drive Dynamic Simulation. We are aware of the request to do so and have logged the request for consideration in a future release.

Can I do static analysis of assemblies?

Dynamic Simulation can compute forces and moments in joints, even when there is no motion. In such a case, the dynamic effects do not exist and dynamic simulation provides the static results.

For example, build a pendulum, block the degree of freedom in the revolution joint and apply an external force at the free extremity. Dynamic Simulation has the force and moment in the joint to equilibrate the external force.

You can also build a "point-plane" joint at the second extremity of the pendulum to lock it, then apply an external force. Dynamic Simulation also has the force and moment in the two joints

Note: In the point-plane example, the point-plane joint is used to create the second support. This joint is sufficient to lock the pendulum and the mechanism is not over-constrained.

Helpful Information

• In Dynamic Simulation, the groups (parts or subassemblies) are rigid. The bending (flexion) and twisting (torsion) of these elements are ignored.
• In this hypothesis, the mechanism can be over-constrained. Dynamic Simulation solves regardless of the condition, but the forces and moments in the joints are only one of several possible solutions, not the unique one.

What actions cause loads exported to FEA to update?

Loads that are identified for exporting to FEA are updated any time that one of the following occurs:

• Check or cancel a time step in the appropriate column.
• Click OK in the Generate Series dialog box.
• Select a part for export.
• Delete a part from the export section.
• Suppress a part in the export section.
• Click OK in the Settings dialog box after you change the Export type (AIP Stress Analysis or ANSYS Workbench).
• Turn Precise Events ON or OFF.
• Validate the “Load-bearing faces selection” dialog box.

How do time steps and images differ when you run a simulation?

Time steps and images are separate outputs from a simulation.

Time steps are the number of steps the software uses to drive the simulation successfully. The software optimizes the number for complex simulations, so the appropriate data is available to the Output Grapher. The number of time steps is always equal to or greater than the number of images you specify. You can go to the Output Grapher and see the time step for the specified increment or an arbitrary one by clicking in the Output Grapher graph window.

"Images" represents the number of images you see when you play back the simulation. You can specify the number you want. The default is 100/second.

Running a 1-second simulation with default settings (Final Time: 1 s, Images: 100) yields 100 images created for playback. One image every 0.01 seconds. The time steps would be 100/second for the simulation. If there was sufficient complexity to require it, the software would increase the number of time steps.

How do I format a text file for a spline?

If you want to use a text file containing tangency points, structure the file as follows:

// comments	You can include one or more lines of commentary in the file. Each line must start with “//”. Comment lines are optional. The value they provide is that you can note the purpose of the spline.
[Tangents] T1 T2	Specify the value of the tangent for the beginning (T1) and ending (T2) points of the sector. These values are shown as the “initial” and “final” slope in the user interface. If no value is supplied, an implied tangent value of 0.0 (horizontal tangent) is assumed. As with comment lines, this line is optional, but as noted, where no tangent value is given some assumptions must be made.
X 1 Y 1	the list of point coordinates, you can list as many points as you need. Specify one point per row.
Example	// // Simulation input spline points // Value: Joint Torque (N mm) // Reference: Time s [Tangents] -3.40775 -5.27803 +0.000 +0.000 +4.313 +1.510 +7.954 -9.756 +1.000 +0.000

Why do some Design Accelerator Spur Gears made prior to Inventor 2009 not get automatically created joints?

Legacy spur gears do not reflect recent improvements therefore it is necessary to update the gear sets to bring those improvements to them. Here’s the list of things to check and do when working with legacy spur gears.

1. Make sure Automatically Convert Constraints to Standard Joints is turned on.
2. Make sure the spur gear assembly is set to Flexible.
3. If the Spur Gears were created in release AIP 2008 or older, then you need to do one of the following:
   • Right-click the Gear Set and click Edit using Design Accelerator. In Design Accelerator, click Calculate, and then click OK. The Gear Set is updated.
   • Right-click the Gear Set and click Component Manual Solve (near bottom of context menu).