Collisions and LinearMovementConstraint

[Francois]

Hi,

I’m trying to create a scene in Python where an instrument enter in collision with another object by following a predetermined path. I am using Discrete Intersections and the LinearMovementConstraint component to control the trajectory of the instrument.

However, the collision response doesn’t work. The intersections seems to be drawn correctly in the GUI but the objects just passe right through each other. Although, when the objects are pulled against each other with the mouse, the collisions work fine.

Any idea why it’s not working? I tried to include a code example within this message but I always get the “*** Forbidden. Message seems to be spam. ***” message when trying to submit.

Thanks you,
Francois

[Hugo]

Hi @fleonard

Could you share your scene with us? (link to a github/wetransfer ..)

Since you are imposing the displacement (using the LinearMovementConstraint), the simulation will make sure this constraint is respected even if the object collides with others. Therefore the result is logical and it is working properly.

To work what you expect, a solution to impose the displacement on the rest_position and to add a RestShapeSpringForceField. But it’s hard to give appropriate advices without seeing your simulation.

Best

Hugo

[Francois]

Hi,

The displacement is only imposed on one object. I don’t understand why the other object doesn’t deform or get pushed by it. I will try the RestShapeSpringForceField to see what it does.

Meanwhile, I’ve put a simplified scene here.

Thank you,

Francois

[Hugo]

Hi @fleonard

I do not advise to use DiscreteIntersection for the detection but rather the LocalMinDistance (or MinProximityIntersection) as shown below:

self.rootNode.createObject("LocalMinDistance", name="Intersection", alarmDistance="0.3", contactDistance="0.1", useLMDFilters="0")

Best

Hugo

[Francois]

Hi @hugo,

Sorry for the late reply. Why do you not advise to use DiscreteIntersection? In my scene, many objects are very close to each other. With proximity intersections, I haven’t been able to set the contact distance small enough so that the objects are not in collision from the start and yet have realistic looking collisions.

Also, how can I impose a displacement on the rest_position?

Best,

Francois

[Hugo]

Hi @fleonard

No prob. LocalMinDistance actually inherits from DiscreteIntersection but has a more complete implementation (e.g. of all intersections between discrete primitives to compute). You can see this here in the API doc.

If your objects are not in contact, you should be able to find a value of alarm and contact distance which satisfies your expectations.

What do you mean by imposing a displacement on the rest_position? Do you want to keep the same position but to have shifted rest_position? or do you want to shift the object at initialization of the scene ?

Hugo

[Francois]

Hi Hugo,

I will try again with LocalMinDistance.

For the rest_position, I was referring to your previous suggestion to impose the displacement on the rest_position and to add a RestShapeSpringForceField, rather then using LinearMovementConstraint. With what component can I do that?

Thanks,

François

[Hugo]

Hi @fleonard

To work with the approach involving the RestShapeSpringForceField, you could simply pass these rest positions to the MechanicalObject at the init (from a loader) like :

<MeshObjLoader name="undeformed" filename="mesh1.obj"/>
<MeshObjLoader name="deformed-targetConfiguration" filename="mesh2.obj"/>
<MechanicalObject name="Dofs" position="@undeformed.position" rest_position="@deformed-targetConfiguration.position" />
<RestShapeSpringForceField />

Best,

Hugo

[Author]

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