Multibody Dynamics (MBD) is a field of mechanics that focuses on systems made of interconnected rigid or flexible bodies. By studying these bodies’ kinematic and dynamic behavior, engineers can anticipate how a system will respond to various operating conditions.
This predictive ability is crucial in guiding design decisions, optimizing performance, and minimizing the need for physical prototypes. One of the most widely used commercial software packages for multibody analysis is Dassault Systèmes SIMULIA Simpack, which specializes in simulating multibody systems across numerous industries.
But… what is Multibody Dynamics (MBD) Simulation?
Multibody Dynamics simulation uses mathematical and computational methods to study systems in which individual bodies may be rigid or flexible. In the latter case, we tend to speak about flexible multibody dynamics. In the latter case, we tend to speak about flexible multibody dynamics. By incorporating flexible bodies into a multibody simulation we can enhance the finite element analysis by detailing the stresses applied on the body during the simulation.
A Software to Simulate Motion
Rigid bodies do not deform when subjected to external forces, while flexible bodies rely on finite element representations to capture deformation modes. The components are connected together by Joints or Forces to form a complete system. The underlying aim is to predict how the entire system moves when subjected to external forces, torques, or imposed motions. Combining equations of motion with real-world constraints makes it possible to model a system holistically and investigate performance under different loading scenarios.
This approach offers substantial benefits, including more realistic simulation of complex assemblies, system-level insight into how component changes affect overall performance, reduced physical prototyping, and improved design optimization.
Introduction to Simpack
Simpack, part of Dassault Systèmes SIMULIA, is a cornerstone tool for modeling and analyzing advanced multibody systems. The software allows engineers to address highly nonlinear and flexible-body problems, simulate transient and steady-state conditions, and use comprehensive post-processing for detailed analyses. Originating from Germany in the late 1980s, Simpack is now used by customers in more than 40 countries. Since its initial development, it has expanded its capabilities to serve numerous industries, proving its versatility through robust solvers and specialized toolkits.
Core Components of a Multibody Dynamics Software like Simpack
In multibody system simulation using a tool like Simpack, users begin by defining or importing model geometry and assigning mass and inertia properties.
Simpack models use “connections” between bodies to define their motion. At the same time, force elements such as springs, dampers, or user-defined actuators can be added to simulate real-world interactions.
Friction models, contact definitions, and advanced interaction (like gears or couplings) are also integrated, ensuring that the system’s equations of motion capture all relevant physics and can provide comprehensive solutions. Simpack then forms the equations of motion to represent the system which are then solved, relying on a suite of solvers that handle both routine and highly nonlinear problems.
Post-processing features help engineers interpret the results, by visualizing time-domain responses, analyzing forces, or generating 3D animations of the system in motion.
But… What Makes Simpack So Specific?
It may not be obvious, but in fact, there are several possible ways to compute the equations of motion. The historical approach for multibody solvers was that the formalism of equation would use an “absolute coordinates” approach. By this, we mean that every body will be represented using six coordinates relative to a fixed “absolute” reference frame. Then, various bodies would be connected by removing degrees of freedom by introducing additional equations (mathematically known as “constraint equations”).
This absolute coordinate approach, which was first conceived by multibody researchers, is not the optimal approach. The idea behind Simpack comes from a different line of thought called “relative coordinates”. Using this approach, the motion of each body is described relative to one of its neighbors, and the coordinates to be solved are the relative positions (and velocities) between the bodies.
Using this approach, each body is given degrees of freedom by adding a kinematic equation and the “constraint equations” can be partially removed. The overall number of equations to be solved tends to be lower, leading to a more straightforward problem from a numerical point of view.
And, guess what… this is the approach used by Simpack 🙂👏
By the way, “absolute coordinates” are also called “maximal coordinates” and “relative coordinates” are often called “minimal coordinates”. The Simpack approach also means that many computer models can be run in realtime, on realtime platforms, without modification.
Modeling and Simulation of Multibody Systems in Simpack
The modeling workflow in Simpack typically begins with the pre-processing stage, where CAD geometry is imported or created, mass properties are assigned, and connections and forces are defined between the bodies. Next, you would select the type of simulation you want to perform, set initial conditions and… let’s go!!
During the simulation, the solver is run to compute system dynamics, and intermediate results may be monitored to adjust settings.
After the solver finishes, post-processing and visualization come into play. Engineers review the time-series data, plot essential quantities such as displacements and forces, and create 3D animations to better understand how the system behaves. A Fast Fourier Transform (FFT) filter can also be applied to convert the outputs into a frequency domain to understand any resonances that exist in the system.
Finally, to iterate on design improvements, Simpack offers DOE tools that allow users to vary geometry and parameters to optimize the design.
Examples of Multibody Dynamics Simulation of Mechanical Systems
Simpack is widely used in automotive industries for tasks like suspension design, chassis modeling, and powertrain analysis. In the rail sector, it addresses bogie-track interactions, ensuring comfort and safety for passengers. Simpack helps evaluate gearbox dynamics and complete wind turbine dynamics in the Wind Energy Industry. Aerospace applications frequently involve analyzing landing gear mechanisms and spacecraft deployment systems. Biomechanical studies benefit from Simpack for modeling human joints and prosthetic designs, while general machinery and robotics also leverage Simpack’s capability to handle complex linkage motions.
Best Practices for Engineers to Effective Multibody Analysis
We typically advise Engineers new to multibody dynamics simulation to consider starting with simple computer models containing only rigid bodies and basic connections. This cautious approach helps verify that everything is set up correctly before introducing flexible bodies, advanced constraints, or intricate force elements.
One should remember that accurate input data is paramount to success. For a typical multibody model, this includes geometric (position of connection points), mass (mass, inertia matrix and Center of gravity location) and non-geometrical (stiffness, advanced model parameters) data. If flexible bodies are involved, you must compute the necessary data from your preferred FEA software using a “substructuring” step.
One thing you will love about Simpack is that you typically do not need to make any solver tuning for accurate results, even for complex computer models, mainly because the flagship solver of Simpack is known for its accuracy, robustness and performance.
One key to any simulation process is a well-thought-out correlation process. This could involve comparing results against test data, simpler analytical models, or known reference values. By refining the model and correlating simulation outcomes with physical measurements, you will enhance the fidelity of the digital representation.
Conclusion
Multibody Dynamics simulation provides a deep understanding of how interconnected mechanical components behave under various conditions. Simpack stands out as a comprehensive software environment that supports detailed modeling, robust solver strategies, and powerful post-processing for a wide array of use cases. In practice, it has proven its value by enabling engineers to optimize designs, reduce reliance on physical prototypes, and streamline product development. Whether in automotive, rail, wind energy, aerospace, or general machinery, Simpack’s capabilities can significantly improve efficiency and innovation by offering high-fidelity simulations that guide informed decision-making.
Frequently Asked Questions on Multibody Simulation
What is an example of a multibody system?
A typical example of a multibody system would be a complete car, with the suspension and chassis. This vehicle can then be analysed for ride comfort and handling.
Can Simpack Be Used for Realtime Simulation?
The Simpack solver is fast, robust, and capable of real-time simulation. Because of the technology used, it is normally possible to run a Simpack model in real time without modification.
What is the multibody dynamics simulation approach in Simpack?
The Simpack approach uses minimal coordinates to form the equations of motion, which reduces the set of equations that need to be solved to represent the multibody system.
Can Simpack be used with Matlab Simulink?
Simpack can be coupled with many third-party products including Matlab Simulink. Simpack fully supports FMU, enabling a Simpack model to be exported and run as a co-simulation within Matlab Simulink. Simpack also allows a Simulink model to be imported into Simpack, extending the functionality of the Simpack model to include complex control systems.
Interested in the latest in simulation? Looking for advice and best practices? Want to discuss simulation with fellow users and Dassault Systèmes experts? The SIMULIA Community is the place to find the latest resources for SIMULIA software and to collaborate with other users. The key that unlocks the door of innovative thinking and knowledge building, the SIMULIA Community provides you with the tools you need to expand your knowledge, whenever and wherever.
Tom Burton is a SIMULIA Industry Process Senior Consultant.