Contacts & Collisions
Motion Problems Can Be Tricky...
Written by: Shaun Bentley, Application Engineer
Contact problems can be tricky. If you consider my tips on elastic properties, the contact region, and friction you may be able to get your model moving in the right direction more easily.
There are two elastic property models that will handle calculating normal forces associated with impact: impact and restitution coefficient.
The impact model will cause your models to interact like a beach ball on water. Attempting to push the beach ball into the water will allow the ball to penetrate the plane of the water (see below), but the buoyancy of the beach ball will force it to stay afloat.
In this case the magnitude of this buoyancy effect is similar to the stiffness you enter into the properties below.
The amount of damping you enter will also influence the amount of energy that is lost in the collision. Though it is difficult to come up with accurate values of these properties (many physical and virtual experiments may be needed), many times testing with the defaults and then intuitively scaling up or down by an order of magnitude may be enough depending on the problem you are trying to solve.
In my experience, this method is robust and causes fewer convergence difficulties than the restitution method.
The option to use a restitution coefficient is tempting. It only requires one coefficient to be entered as you see below.
However, I’ve seen this method struggle to converge, likely because it does not allow the models to penetrate one another during impact.
This creates a sharp discontinuity in the model which is one thing we try to avoid, especially when a model struggles to solve. You may have seen the following error in the past:
Will the contact region cause complications? At a glance, the corner contact below looks relatively simple, but on closer inspection, you can see that SOLIDWORKS Motion has a hard time deciding what to do with this interfering corner.
The block jumps around trying to find an equilibrium point due to multiple contact faces coming into play simultaneously. If we instead break up the complex contact into two simple contact regions by adding a chamfer, we get a much smoother result:
A model with friction frequently will take longer to solve since SOLIDWORKS Motion will need to iteratively determine the force that should be acting tangent to the contact elements. This example which includes large friction took approximately 20 minutes to solve:
Its friction-less counterpart took approximately 8 seconds to solve. This represents a time difference of about 150x!
In addition to taking longer to solve, the added calculations can lead to convergence difficulties.
If you are bumping into problems with 3D contact with SOLIDWORKS Motion, first try to simplify your study by removing discontinuities, friction, and complicated contact regions. Once you have a robust model, then add some of these properties in small doses. This should allow you to build a working model.