ForceField with Python controller

[quentin]

Hi everyone,

I’m trying to make a scene with a force controlled by a python controller file. I want to make a sinus force and I tried to use the ConstantForceField object but it didn’t make the thing I want. It seems that the force is constantly growing and not decrease each period of time like a sinus wave.

I would like to know if it is correct to change with a pythonController the totalForce option of the ConstantForceField component. If not, what is the error ? There is another forcefield simpler to just put a force on points ?

Thanks for your help

[jnbrunet]

Hi Quentin,

Yes, you can use a pythonController and change the value of either the totalForce attribute (vector 1×3) if you want that the force be split among all vertices, the force attribute (vector 1×3) if you want to apply the same force to all vertices, or the forces attribute (vector nx3) to apply different forces to each vertices.

Using a pythonController, simply change the attribute value of the forcefield at each onBeginAnimationStep event (you can find examples in the applications/plugins/SofaPython/examples directory)

Hope that helps.
Jean-Nicolas

[quentin]

Hi Jean-Nicolas,

Thanks for your help ! I changed the string I sent to the totalForce in a vector like that :
self.forceField.findData('totalForce').value = [0, force, 0]

I display the new totalForce on the terminal like that :
print(self.forceField.findData('totalForce').value)

I have the good value as I want with my negative sinus square (I see the force arrow s on the GUI) but the deformation doesn’t seem to follow the force signal. In fact, I have a tube with a spring in the middle and I try to compress it. I block the bottom and apply the force on the top part. The problem is that the force is a sinus and the deformation continues to increase permanently (so a line). Here you can see the force applied in the vector lines and the position of the point just under each vector.

[[0.0, -0.0, 0.0]]
1000.0
[[0.0, -21067.05423352457, 0.0]]
998.210218854
[[0.0, -81309.41177357652, 0.0]]
996.376814199
[[0.0, -172266.21253047822, 0.0]]
993.959163958
[[0.0, -281162.84414120595, 0.0]]
991.292846044
[[0.0, -392705.09831248416, 0.0]]
987.782535008
[[0.0, -491227.196924607, 0.0]]
983.855256778
[[0.0, -562892.0040131591, 0.0]]
979.023956496
[[0.0, -597634.4103943433, 0.0]]
973.678487509
[[0.0, -590574.9483385893, 0.0]]
967.601948838
[[0.0, -542705.0983124842, 0.0]]
961.047161738
[[0.0, -460748.03849369887, 0.0]]
954.060967265
[[0.0, -356214.3943757172, 0.0]]
946.75375456
[[0.0, -243785.6056242822, 0.0]]
939.283539129
[[0.0, -139251.96150630055, 0.0]]
931.704832793
[[0.0, -57294.90168751533, 0.0]]
924.099724048
[[0.0, -9425.051661410507, 0.0]]
916.501371709
[[0.0, -2365.5896056567476, 0.0]]
908.91719647
[[0.0, -37107.99598684137, 0.0]]

[Hugo]

Hi Quentin

Have you tried to use this instead:

forceToSet = self.forceField.findData('totalForce').value
forceToSet[1] = force

or

self.forceField.findData('totalForce').value[1] = force

Let me know how this works
Cheers

Hugo

[quentin]

Hi Hugo,

Thanks for your help ! I tried to directly update the data as you said. The vector is a column one therefore I can access the value like that :
print(self.forceField.findData('totalForce').value[0][1])

Nevertheless, it is impossible to edit it directly, it seems that I have to edit all the vector.

This isn’t working :
self.forceField.findData('totalForce').value[0][1] = force

This is working :
self.forceField.findData('totalForce').value = [0, force, 0]

[Hugo]

Do you mean this is working:
self.forceField.findData('totalForce').value = [0, force, 0] ?

Did you solve your problem then ?

Best
H

[quentin]

Yes I mean it, I see the force arrows as before but I still have the problem of the displacement which isn’t following the force signal. The situation hasn’t changed.

[quentin]

Hi everyone,

I found in the oxygen that the totalForce is the sum of all the forces applied at each point : https://www.sofa-framework.org/api/master/sofa/html/classsofa_1_1component_1_1forcefield_1_1_constant_force_field.html#a7f45cdbc837c9ac22f44763806e0823f

That’s why we have the deformation which is not following the force signal. The problem is when I use the force attribute I have the value of the force applied on each point and not divided by the number of points. Is there another attribute that I didn’t find to apply a force which will not be summed in time to all the points (and not to each point) ?

[quentin]

To resolve my problem, I juste have to see the value of the force really applied on each point. Is it possible to access it with the indices or an option like that ?

[Hugo]

Hi @quentinfrancois0

You have a field “force” in the MechanicalObject summing all forces applied on your object. This can be a state vector to investigate to detect the forces applied on it.
Let me know if you solved your issue.

Best,

Hugo

[quentin]

Hi @Hugo,

Thanks for the help ! The sum that you talk about, is it a temporal sum or just a sum of all the forces at the precise time ?

Thanks for the help

[Hugo]

Hi @quentinfrancois0

the force datafield gathers all forces applied for each time step. The value of the vector is cleared at each new time step.
Best

Hugo

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