Multibody Systems

How it works

How it works

Multibody dynamics (MBD) is used to analyze the motion of complex, mechanical systems. Put simply, it requires 3D CAD geometry as input and delivers the motion and forces acting at the system as output, after the bodies have been connected to each other via constraints (such as joints). The bodies can be assumed to be rigid or flexible. The assumption of rigid bodies is a great simplification of reality, but it has the advantage that it requires relatively short solution times.

If bodies experience major deformations, the assumption of rigid bodies is no longer justified and one or more bodies can be replaced by flexible ones. Due to constantly increasing compute power, MBD and FEA are merging more and more in commercial software packages.

Multibody simulation is also often used to determine loads for FEA simulation.

Our Simulation Capabilities

Why MBD, if Joints have been defined in CAD already?

A question asked many times. The joints were already defined in the CAD system, motion is correct, so everything should be fine, right? A conditional "yes" for kinematics, a very likely "no" for kinetics, i.e. the forces and moments are incorrect. Usually, when creating CAD constraints, no attention is paid to over-definitions, the stiffness of a bearing has not been taken into account, and much more. A properly defined MBD model delivers correct values.

Short Solve Time

In case of rigid bodies the MBD solver requires only very short computing time for dynamic tasks compared to an FEA Analysis.

Different Body Properties

Bodies can be represented as rigid bodies, reduced flexible (CMS) or fully flexible. If, for example, a robot arm is subject to oscillations of the structure due to high accelerations and therefore positioning problems, the components relevant for the rigidity can be switched to flexible. The respective use case and experience determines which bodies are chosen.

Conditional Equations of Motion

It is of great benefit of the software used that equations of motion can also obey conditional rules without involving a control in advance: e.g. "if the z coordinate is 0.2m, then reduce the driving torque by 20 Nm". This allows very quick and smart preliminary design of mechanics.

Extensive Contacts

Additionally to the contacts used in FEA, the software also offers analytical contacts. This makes it possible to achieve short computing times for fundamentally complex contact problems as long as the bodies can be assumed to be rigid. An example is the sliding of a body along a water slide, which simply corresponds to a cylinder-in-cylinder contact. Afterwards forces from such a simulation can be exported to a FEA simulation for strength evaluation of the columns.

Availability of Machine Elements

The software used offers a large number of specific machine element types: chain drive, belt, bearing, gears, ball screw drive, linear guide and more. There are also other special topics, like EHD, simulation of paper strip process and much more, just contact us.

Co-Simulation with System Simulation

The main task here is the integration of the controller. Due to short solution times multibody dynamics simulation is particularly suitable for co-simulation with a system simulation. For example, the complex mechanics of a machine tool can be represented by a multibody simulation, the hydraulic/ electrical/ control technology part by a system simulation.

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