Have you ever wondered why your SOLIDWORKS Motion simulations sometimes yield unexpected results or fail to solve them altogether? The culprit might be hidden in plain sight! Solving these errors could come down to clearing up redundant mates.
What are redundant mates?
In the world of SOLIDWORKS Motion Simulation, achieving accurate and reliable results hinges on the proper management of mates. Redundant mates occur when multiple mates over-constrain a component which can lead to solver errors as the set of motion equations cannot be solved successfully.
Redundant mates are equivalent to over-defining a model in SOLIDWORKS. For instance, consider a door with two hinges. While one hinge can restrict the vertical translation of the door, the other becomes redundant and leads to potential inaccuracies in the motion analysis. Such redundant mates will cause inaccurate force calculations and unexpected motion behavior.
Identifying and resolving these redundancies is crucial for ensuring that your simulations run smoothly and produce valid results.
Mate Redundancies can lead to:
- Solver Errors: The motion solver may fail to find a solution due to conflicting constraints.
- Inaccurate Forces: Redundant mates can lead to zero or incorrect force calculations at certain mate locations.
- Unexpected Motion: Components may exhibit unintended behavior, such as separation or incorrect movement paths.
SOLIDWORKS assembly with redundant mates
How are redundant mates fixed?
Before solving motion simulations, the solver goes through the process of automatically detecting redundancies in the mechanism. By default, when redundancies are detected in the SOLIDWORKS model the SOLIDWORKS Motion solver will attempt to remove the redundant constraints.
The solver removes redundancies based on the following order:
- Rotational Constraints
- Translational Constraints
- Motion Inputs
At each time step, the solver re-evaluates mate redundancies and automatically removes them if necessary. However, with more complex mechanisms, automatic removal of redundancies does not always work out very well. In those cases, you may notice solid bodies separating, the solver locking up, or incorrect calculation of reaction forces. If this occurs, you will have to add additional information manually.
What are some techniques to resolve mate redundancy?
Systematic Mate definition
The best solution for dealing with redundancies is to create mates in a systematic manner going piece-by-piece with degrees of freedom in perspective.
SOLIDWORKS Motion Simulation Degrees of Freedom
This may also mean redefining the mates in the SOLIDWORKS assembly to simplify the mating scheme and minimize redundancies. Be sure to use as few mates as possible to achieve the desired motion. Dragging or rotating components can be used to check the movement of the assembly while defining mates. MateXpert can be also used to identify and automatically fix simple mate problems. Even if MateXpert cannot fix the problem, it often narrows down the problem by identifying problematic mates.
Rigid Group
Using rigid groups is a fast way to reduce the level of complexity in large assemblies. To create a rigid group, right-click on a single component or group of components in the MotionManager tree and select Add to New Rigid Group. This creates folders of distinct rigid groups containing multiple components that would move together. Rigid groups of components will behave like a single component in motion calculations. The motion and mate definitions between components within a group is ignored. All group components will also contribute to the mass and moment of inertia of the rigid group.
Add to New Rigid Group command
Replace with Bushings
Replace with Bushings will convert redundant mates into bushings, which act as flexible connections, allowing slight movement and reducing solver errors. This option can significantly slow down the simulation time when set at a global level and lead to inaccuracies with an unrealistically stiff mechanism. It is best to specify stiffness parameters for individual mates as needed instead of a generic value.
Specify Stiffness values in a SOLIDWORKS mate
A mate redundancy is not a major issue if you are interested only in kinematic results or when there are no forces or torques that depend upon other joint forces or torques. However, it is always recommended to remove all existing redundancies before running a motion study. Implementing these techniques will lead to more accurate simulations and reliable force calculations in SOLIDWORKS motion studies.
If you want to embrace a streamlined approach to motion analysis and unlock the full potential of SOLIDWORKS Motion Simulation be sure to check out our SOLIDWORKS Training here.
