Dynamical FE simulation
The Meshparts Software was developed to create dynamic simulations of large mechanical assemblies easily and precisely.
The simulation process with mesh parts is typically divided into the following steps:
- Import of the CAD assembly and meshing (automatic)
- Replace modeling-intensive components with FE library components (soon automatic)
- Definition of contacts (automatic)
- modal analysis
- Calculation of the frequency responses
- Coupled controller/structure simulation
Import of the CAD assembly and meshing
We also call this step the top-down approach to FE modeling. The starting point is a complete CAD assembly. The structure of the FE model is predefined by the CAD assembly.
The top-down approach is the classical approach in FE modeling. The Meshparts also allows the bottom-up approach. This creates a FE assembly without the presence of a CAD assembly. First, the lowest assemblies are defined by assembling individual parts directly into Meshparts.
In the following video we demonstrate only the top-down approach in the Meshparts Software.
Observe how a CAD symmetric model structure automatically arises in the model tree. After a CAD import, Meshparts takes over the file and model structure of the CAD assembly one-to-one (symmetrically). For CAD assembly there will be a Meshparts assembly, and for each CAD part there will be a Meshparts part as a separate file model in the same folder as the CAD models.
The symmetry of the file and model structure between CAD and FEA allows a new level of flexibility when doing model changes. In addition, contacts are always stored in the respective subassembly. In assembly-oriented FE models, the complexity of the model plays a socondary role. The user can concentrate and work on smaller subassemblies. When returning to the main assembly, the changes in subassemblies are automatically there.
Replace modeling-intensive components by FE library components
If you simulate larger assemblies in the FEA (for example, machines), many of the installed components will be purchased parts. These components must be simplified by special equivalent models (black box models). Without a library, you must always reproduce the realistic behavior by complex modeling of the components and manually research of the specific catalog data. Examples of purchased parts installed in maschines are:
- Rolling bearings: Axial, radial und tilt stiffness/damping, mass
- Linear guides: Stiffness and Damping Normal, radial, roll, yaw, pitch, mass
- Ball screws: Transmission behavior of rotation to translation, axial and tilt stiffness, mass, damping
- Planetary gears: Transmission behavior of input and output shaft, torsinal and tilt stiffness der of pinion shaft, rotational inertia and total mass, damping
- Servo drives: Torsional stiffness of motor shaft, motor mass, rotational inertia motor shaft, damping of shaft bearings
- Coupplings: Torsional, radial and tilt stiffness, rotational inertia und total mass, torsional damping
- Synchron belts: Längssteifigkeit
- Screws: Pretension by norm or indivudial, streght klasses, variance of the pretension force due to friction effects.
This process is laborious, time intensive, error prone and unnecessarily repetitive. You need expert knowledge in order to correctly represent equivalent models of purchased parts in FEA.
Using ready to use parametric FE models can eliminate the above drawbacks. You no longer model by defining features, but by incorporating components that already contain all the necessary features.
Watch in the next video how standard steel body crosslinked linear guides are replaced by ready-to-use FE simulation models. Watch the tabular properties of the components from minute 1:00. The many numbers at the end of the table represent the nonlinear stiffness curves.
Without contacts, the parts of a FE assembly would fly arround without any interaction with other components. Typically, in large assemblies such as machine tools, so-called bonded "Penalty" contacts are defined.
Penalty bonded contacts behave like glued connections and also work with non-conform FE meshes (nodes are not coincident, interpolation functions can have different polynomial order).
Thousands of contact surfaces often result in large FE assemblies. The FE software takes over the search for congruent surfaces and automatically defines contacts. But how do we know that these automatic contacts are correct? The answer is unfortunately: We generally do not know that.
Examples of geometrically coincident surfaces, but without physical contact, are many.
Take as an example the rail of a linear guide. This has at the bottom a mounting surface and laterally an alignment surface. For the mounting surface we need a contact, for the alignment surface we probably prefer not to define contact.
Now you might think that a FE software would not have a chance to automatically find and define contacts correctly and reliably.
However, the FE models of the Meshparts library contain a definition of all attachment surfaces. In the presence of such library parts in the assembly, contacts are searched only with the designated areas. Wrong definition of contacts with purchased parts is no longer possible.
For the remaining parts, only the less intelligent search for congruent surfaces remains.
The next video demonstrates how to automatically search for contacts in Meshparts. The contact surfaces are marked in red and green. Small squares symbolize the predefined connection points of the purchased parts.
In a modal analysis eigenfrequencies and eigenvectors - Eigenshapes - of the mechanical system are determined. The most important analyses setting is the desired number of eigenfrequencies to be determined. Optionally, you can restrict a frequency range through a lower and/or upper frequency barrier.
Eigenfrequencies are intrinsic properties of systems (not only mechanical but also electrical systems).
The eigenfrequencies of a system depend heavily on the boundary conditions. An aluminum rod clamped at one end, vibrates at a lower eigenfrequency, as if the rod were in free fall or zero gravity. The same rod would vibrate with a higher natural frequency if its two ends were clamped.
To some extent, eigenfrequencies also depend on the (pre)load. For a prestressed string, the axial load is even very important for the eigenfrequencies. In the case of machines, on the other hand, the preload due to its own weight or process forces affects less the eigenfrequencies For example, nonlinear load-dependent stiffness curves of linear guideways and rolling bearings have a certain influence.
Nevertheless, in many cases it is worthwhile to calculate the actual preload through an upstream static analysis. Subsequently, a preloaded modal analysis is carried out, in which case the calculation is carried out at the linearized operating point.
The eigenfrequencies represent potential resonance points, which lead to high-amplitude vibrations when the appropriate force is applied. Excessive vibration of machines leads to poor machining results, to rejects. It is therefore important to know the eigenfrequencies of a system and to avoid any excitation of these. At the same time we also want to know the vibration shapes (the eigenvalues). All this is calculated by a modal analysis.
Calculation of the frequency responses
Unfortunately, a modal analysis can not tell us directly at which amplitude a vibration will take place.
The amplitude of an oscillation depends on the amplitude, direction and the time history of a force excitation.
To get more insight into a system, we analyze the frequency responses of systems.
A frequency response is simply said, the frequency-dependent relationship between the amplitude of two sinusoidal time signals. The signals can be forces, moments, displacements, speeds or accelerations.
In a machine tool, forces or moments are typically applied at the drives or at the Tool Center Point (TCP). Displacements, speeds or accelerations are measured ot the drives, on the direct measuring systems or on the TCP.
The frequency response first shows which of the many eigenfrequencies are relevant for a particular drive or for the machining process. The frequency response is thus initially like a filter for important eigenmodes.
Specialists in control engineering recognize in the frequency response causes for unstable control behavior.
Another interesting field is the evaluation of transverse excitation of feed axes in machines, an undesirable behavior which has to be minimized.
Coupled controller/structure simulation
The frequency responses are for many difficult to interpret. Even experts cannot say on the basis of the frequency responses, how productive a machine or some other system will actually be.
To predict the productivity (e.g. number of manufactured units per year) of a machine, coupled controller/mechanics simulations are recommended.
Optionally, the process (e.g. milling process or turning process) can be taken into account.
However, this type of simulation exceeds the practical limits of FE simulation. We therefore recommend the use of a software such as Matlab/Simulink or Scilab/Xcos.
In Meshparts, you can export a complete FE model as a modal reduced FE model to Matlab or Scilab. At the same time, a block based model for Simulink or Xcos is automatically generated. You can pair this model with another block based model of the controller.