Link actuation with a point/frame of a mesh

[bmauze]

Hello everyone,

I am trying to create a soft delta using a SlidingActuator as actuation system. The code i used is inspired from the tripod tutorial. However I do not get how to link the SlidingActuator to the frame associated to my actuation system on the elastic element.
I tried to use a RigidMapping on one side and a RigidRigidMapping on the othersize. But I have succeed to get something that was working. I also tried to use BarycentricMapping but the behavior is very strange . The constraint axis is not the right one (go toward Z while it should be the X one).

So I have two questions:

• If I have a elasticBody element whose some elements are rigidified and linked to a frame. Do you have a idea on how I could link this frame with a SlidingActuator element ?
• When I am using the Barycentric mapping, Why my structure is moving along the constrained wrong direction.

My code and the mesh can be found bellow1:
https://github.com/bmauze/SoftDelta.git

Kinds regards,
Benjamin Mauzé

[EulalieCoevoet]

Hello Benjamin,

1- You can directly add the SlidingActuator in the frames' node. In your case:

    frame = rigidifiedstruct.RigidParts  # your rigidified elements seem to be linked to the frames in this node
    # if you want to control those frames just add the actuators in the corresponding node    
    for i in range(3):
    	frame.addObject('SlidingActuator', name='Act'+str(i+1), template='Rigid3', 
    	                indices=i, direction=[0, 0, 1, 0, 0, 0], 
                        maxForce=forceActuator, minForce=-forceActuator, 
                        maxDispVariation=SetDepAct, maxPositiveDisp=depAct, maxNegativeDisp=depAct)
    # unfortunatly the component SlidingActuator needs the following component to fix the frames in the other directions
    frame.addObject('PartialFixedConstraint', template='Rigid3', indices=[0,1,2], fixedDirections=[1, 1, 0, 1, 1, 1])

Note that we typically use a mapping when the point that we want to control or actuate is not exactly a degree of freedom.
Let's say you have an FEM mesh composed of tetrahedra. If the point you want to control/actuate is not a dof, but is inside a tetrahedron (right case in the image), you'll use a BarycentricMapping to map this point to your FEM mesh.

What this component does is computing the coordinates of this point inside the tetrahedron, and applying the displacements and forces from the point to the tetrahedron and vice-versa.
Similarly with a rigid object you'll use a RigidRigidMapping.

2- I don't know what kind of actuation you'll seek exactly, but if you want your elasticBody to be attached to an articulated chain, I suggest you to have a look at the ArticulatedSystemPlugin. We actually use it in the latest version of the Tripod tutorial, instead of the two components SlidingActuator and PartialFixedConstraint.

[bmauze]

Hello Eulalie,

Thank you for this quick answer and all those details. If I understand well, in your response, the linking between the sliding actuators and the frames comes from the inheritance of the mechanical objects of rigidifiedstruct.RigidParts.
I changed a little bit my code to change manually the actuation system's position with the keyboard. A "problem" appeared. This problem reffers nearly to the same idea behind my previous question. The MechanicalObject freeCenter does not move even if the structure is moving. I thought that the link was made by the MechanicalMatrixMapper but it is not the case (even if it seems to work well for an inverse mode in another program). I tried the getLinkPath() function, but it linked all 4 positions and not only the third one.
Do you have an idea to which direction I should look?

Thank you for the idea with the ArticulatedSystem Plugin. I am currently trying some stuff. Would the simulation work "better" using this new plugin? The actuation system that I want, is just a sliding actuator along a direction (here collinear to the X or Z axis)

[EulalieCoevoet]

No in that case you can stick to using the SlidingActuator :)

Well, if I understood correctly:

• currently all your rigidified elements are mapped to the MechanicalObject in the node RigidParts, which have the four frames
• what you want is to separate them into two groups, one with three of the frames and the other one with the fourth frame (in FreeCenter)?

If you look at the latest version of the Tripod tutorial we use the SubsetMultiMapping to do that. Here we map the four frames into four different nodes, the three ActuatedArms and the FreeCenter:

rigidParts.addObject('SubsetMultiMapping',
                              name="mapping",
                              input=[self.actuatedarms[0].ServoMotor.Articulation.ServoWheel.getLinkPath(),
                                     self.actuatedarms[1].ServoMotor.Articulation.ServoWheel.getLinkPath(),
                                     self.actuatedarms[2].ServoMotor.Articulation.ServoWheel.getLinkPath(),
                                     freeCenter.getLinkPath()],
                              output='@./', indexPairs=[0, 1, 1, 1, 2, 1, 3, 0])

Is this what you wanna do? If yes, may I ask why having them all in the same MechanicalObject bothers you?
In the Tripod for instance we had to couple those frames with different articulations, so it was necessary to separate/map them.
Here if you just want to add a SlidingActuator to three of the frames and an PositionEffector (I guess) to control the fourth point, they can all live in the same MechanicalObject. If that's not what you wanna do just ignore my last comment ^^