Tech Tips: Creating and Analysing CATIA Multibody Models with Dymola

Creating and analyzing multibody models in Dymola using the CATIA Multibody Library is a great way to simulate mechanical assemblies and understand their behavior under different conditions.

In this article, we will learn crucial information on making the most of integrating the CATIA Multibody Library.

What is the CATIA Multibody Library?

The CATIA MultiBody/MultibodyCutJoints library is an essential tool for translating mechanical CATIA assemblies into functional Dymola models.

This is not a library you could import manually; however, it is a library that can be imported whenever you use the “Generate from Kinematics” function or run a Dymola model that requires it.

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This functionality is supported by the Behavior Modeling app; it supports all standard CATIA joints, except for UserDefined constraints. The kinematic representation of a model must be generated from a CATIA mechanism with 0 degrees of freedom with commands.


What is CATIA’s mechanism manager?

The CATIA mechanism manager displays the degree of freedom for each mechanism to help you be sure that every component is fixed which will help prevent the entire assembly from floating in space.

In Dymola, every part and joint from the CATIA assembly is represented, including a global body component with a fixed rotation.

To create a Dymola model from a CATIA mechanism, you must have a mechanism defined with zero degrees of freedom. This is crucial because it ensures that the model behaves predictably in simulations. In CATIA, you’ll use the Mechanism Manager to verify this, looking for mechanisms that are fully constrained except for one controlled degree of freedom.


Breaking Down the Dymola Kinematic Model

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This image represents the standard setup for a Dymola Kinematic model. This setup includes separate models for each part of the assembly and distinct models for the joins, which simulate the mechanical connections between parts. Additionally, a global body component is included, which is anchored with a fixed rotation to the ‘World’ component.

This fixed rotation is a consistent feature across all models and does not require any modification. Within the assembly, the controlled joint is designed to connect with two connectors for operational control, but only one connector is utilized at any given time. This is evident in the model view of the step-pedal mechanism, where a single connector is present.


Joints in Your Model

When it comes to controlling joints, you have three options: none, position, and force. Positional control allows for exact actuation through specific source blocks, while force control responds to external excitations, managed by torque and force source components.


How to Generate Multibody Models in Dymola

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When generating your Dymola model, use the Behavior Modeling app’s “Generate from Kinematics” function. You’ll need to provide a class name, choose a parent class, and select a valid mechanism. Remember, the model will appear in the chosen parent class and under a sub-package called Internal Generated. Edit only the top-level model, not the locked InternalGenerated model.

After generation, the model appears in the selected class and in an internal Generated sub-package. The model InternalGenerated is locked and edited; any changes should be in the top-level model.

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Troubleshooting Common Issues

Despite this process being straightforward, issues can arise. Here are some common problems and solutions:

Incorrect Number of Degrees of Freedom

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This error occurs when your mechanism does not have zero degrees of freedom with commands. The model may still function, but it will not be as accurate and cause large simulation times. To fix this, try proceeding with the following:

Potential Resolution Steps:

  1. Switch to CATIA assembly to view the effected mechanism.
  2. Ensure that the controlled joint is controlled by an angle or displacement in the Mechanism Manager
  3. Fix one of the components (if not already done so) and recheck DOF
  4. View DOF of the entire assembly to see if anything is partially constrained
  5. Recreate mechanism in a subassembly to view DOF



Missing Mass and Inertial Data

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 Sometimes Dymola will not automatically populate mass data. In this case, you may need to try the following:

Potential Resolution Steps:

  1. Measure the Inertial data of each part in the CATIA assembly and try updating and regenerating the Dymola model
  2. Change the material density of the part and update/regenerate Dymola model
  3. If all else fails, input the data manually from Inertial measurements


Cut Joints or Prismatic Loops Errors

These errors often stem from conflicting CATIA joints. Simplifying the model or ensuring sub-assemblies are flexible might resolve these issues.


Best Practices for Successful Integration

  • Always confirm the degree of freedom in CATIA before generating the Dymola model.
  • Do not edit the locked InternalGenerated model directly; any changes should be made to the top-level model.
  • Troubleshoot errors by going back to CATIA and adjusting the model as needed.
  • For missing data, consider manual inputs as a last resort.


Creating and analyzing Multibody models in Dymola via CATIA is a robust process that can streamline simulations and modeling for mechanical systems. By understanding and utilizing the CATIA MultiBody Library properly, you can create detailed and accurate Dymola simulations of your mechanical assemblies. Keep these tips in mind, and you’ll streamline your simulation process, gaining valuable insights into your mechanical designs.