Develop a custom ForceField

[Michele-Martini]

Hi everyone,
as the title suggests, I am interested in developing a custom ForceField, which should model the effect of liquid onto an underwater robot. I read other sources regarding ForceFields, such as the SOFA documentation and the examples, but there things still unclear to me. For example:

1. since the force exterted by the environment on the robot depends on the robot's surface, how can I retrieve the value of the external surface at run-time given the robot's mesh?
2. I am aware that lots of geometric functionalities are implemented in the form of Topology Algorithms or Modifiers, but I am not sure how to use them in simulation.
3. is it possible to develop the ForceField in Python3 (at first)? Is there some example in Python that I can study before jumping into the C++ development?

Thanks in advance for any help.

[hugtalbot]

Hi Michele,

Nice to hear that you are heading to the implementation of a ForceField, modeling the effect of a liquid environment.
Back some years ago, @alxbilger worked on a similar ForceField maybe he could help or advise.

The API of ForceField is pretty simple, it is described in the abstract class BaseForceField.h.

• as all SOFA Component, a ForceField has an init() function defining how the component inititializes
• an addForce() function computing the explicit force
• an addDForce() function computing the multiplication of a vector (let's note it to be defined) and the force derivative with regards to the degrees of freedom (position), this function therefore computes:
• and two equivalent functions addKToMatrix() now deprecated and buildStiffnessMatrix() which replaces it, both computing the matrix resulting from the force derivative with regards to the degrees of freedom (position):

To answer your points:

1. information from the topology can be retrieved from the TopologyContainer: in C++ it's pretty easy all you need is a pointer towards the BaseTopology of your object, in python I guess it would be similar (@damienmarchal ?)
2. Indeed, you will need them to compute area, volumes of elements as well as adjacency in your FE integration
3. I never did a full FEM FF in python but it is feasible, I would need to dig a bit more in the python API which I am not using on an everyday basis

Cheers

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@Michele-Martini
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@Michele-Martini
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