Selecting the Right CMS Method for Flexible Body Simulation in MotionView and MotionSolve

Welcome to the Multibody Dynamics for Automotive Applications using MotionView and MotionSolve blog series. In this blog, we will explore how to select the appropriate Component Mode Synthesis (CMS) method for flexible body simulation in multibody dynamics software. Understanding when to use Craig-Bampton, Craig-Chang, or Craig-Bampton Geometric Stiffness methods is essential for engineers working with multibody dynamics for automotive applications. 

Choosing the Right CMS Method 

In multibody dynamics modeling, selecting the right CMS method depends on how the interface nodes are constrained within the vehicle system model. Here’s how each method applies: 

Craig-Bampton Method: 

• Best suited when most of the interface nodes are highly constrained. 
• Commonly used in multibody dynamics automotive applications such as chassis, suspension, and structural components. 

Craig-Chang Method: 

• Ideal when interface nodes are free and have fewer mechanical constraints. 
• Useful for components experiencing independent motion, such as floating engine mounts or isolated subframes. 

Craig-Bampton Geometric Stiffness Method: 

• Required when geometric stiffening effects influence system behavior. 
• Typically used for slender structures, such as helicopter blades, ceiling fans, and rotating shafts. 

Understanding Normal Modes Analysis 

In multibody simulation software, the Craig-Bampton and Craig-Chang methods do not compute normal modes the same way traditional finite element analysis (FEA) does. Instead, they approximate the system response through modal superposition. 

To determine the number of modes needed for a flexible body, consider: 

• Number of interface nodes × Degrees of freedom per node + User-specified modes. 
• Example: If a model has 4 interface nodes, each with 6 degrees of freedom, and 10 free-free eigenmodes are requested, the total number of modes will be 34. 

Handling CMS Failures in MotionView 

A common reason for CMS method failure occurs when rigid spiders (RB3 elements) depend on displacements of outer nodes. The solution is to replace RB3 elements with RB2 elements in OptiStruct, which ensures independent center nodes in the flexible body interface. 

Eigenvalue Solvers in MotionView and MotionSolve 

To perform normal mode analysis in MotionSolve software, two solvers are available: 

Langrange Method: 

• Accurately computes eigenvalues and mode shapes. 
• Best for small to medium-sized models. 
• Slower for large-scale problems with millions of degrees of freedom. 

Automated Multi-Level Substructuring (AMS) Method: 

• Faster for large models since only a portion of the eigenvector is computed. 
• Disk space is greatly reduced, leading to shorter run times. 
• Less accurate than the Langrange method but sufficient for vehicle dynamics simulation. 

By default, MotionSolve and OptiStruct use the Langrange method, but AMS is preferred for high-performance simulations. 

Conclusion 

Selecting the right CMS method in multibody dynamics analysis is crucial for accurate and efficient flexible body simulation. Understanding normal modes, eigenvalue solvers, and interface node constraints will help engineers achieve better results in vehicle dynamics modeling. 

In the next blog, we will discuss how to generate flexible bodies in MotionView using the FlexPrep utility, along with an overview of quasi-static analysis in multibody dynamics simulation. 

This blog is part of our ongoing Multibody Dynamics blog series. If you missed the previous posts, check them out here. 

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