Problems in using rest_position and position parameters to control rigid body.

[vietlinhnguyen]

Hi,
I am trying to simulate a collision process between a point (Using Rigid3d template with Sphere collision model and RigidMapping) with a soft surface. I am running this code by Sofapython3 and SOFA Windows version. The simulation run well with no pop-up errors. However, I still got a problem that I cannot understand, so hoping that you guys can support me (python scripts and mesh files were attached for easy reference)
tactile.zip

I would like to manipulate the rigid point to get contact with a soft surface by the controller script, here is my way to get this task done:
In the Scene script, I tried to set the constraint for the Rigid Point by two methods as below:
Method 1: PartialFixedConstraint in Z direction to remove the effect of gravity.
Method 2: RestShapeSpringsForceField with BoxROI, stiffness=1e12, angularStiffness=1e12.
I also tried to move the Rigid Point in two ways by the controller script:
• Changing position values with respect to time step (dt) in the Mechanical Object
• Changing rest_position values with respect to time step (dt) in the Mechanical Object.
After these tests, I realized that:
With method 1:
• Changing position/dt: the Rigid point will stop moving when it contacts with the soft surface. Vel value [0, 0, 0, 0, 0, 0]. Force value [0, 0, -98.1, 0, 0, 0].
• Changing rest_pos/dt: the value of rest_pos will change as the script (displayed in the MO) but the Rigid point does not move in the simulation. And the Position values do not change.
With method 2:
• Changing position/dt: the values of position only change a little and stop after a while. I thought that due to the constraint of SpringForceField, the Rigid Point cannot move. In the Vector 1/3 tab of the Mechanical Object, it is shown that there is a high force value in the opposite direction of the expected moving direction. Velocity in the expected moving direction is minor.
• Changing rest_pos/dt: this way brings the most reasonable results. The Rigid Point moves to the contact area and collides with the soft surface. In the tab Vector 1/3 of the MO, the Vel is nearly equal to the ratio of set displacement/time step. The force in moving direction is very high (with e12), I am supposed that the high force value is reasonable because this high force will overcome the stiffness constraint (1e12) to get the Rigid point moved and collided with the soft surface. However, the value of position is not equal to the value of rest_position. When the simulation starts, pos and rest_pos are equal, but after running, rest_pos values always move faster than the pos value.
In conclusion, please help to explain the difference between rest_position and position in my case.

[hugtalbot]

hey @vietlinhnguyen thanks for your question and sorry for keeping you waiting.

have you taken a look a this thread, I think a previous reply might answer your point : #2648 (comment)

can you tell us if it helps?

[vietlinhnguyen]

Hi @hugtalbot ,
Thank you very much for your support. I referred to your code! However, it is really not what I am looking for. It was my fault when I didn’t make clear my questions above. My desired outcome for the simulation is that I would like to get the data of displacements and forces of the soft surface exerted by an external force in a way that I can control the impact on the soft surface.
In order to do that, I built a rigid particle and utilized it to come into contact with the soft surface. With your code, the collision happens but the rigid particle will be pushed inversely as what a collision should be! However, what I am expecting to simulate is slightly different! I want the particle to keep moving forward to the desired point with a constant displacement over time step as the controller script when it gets in contact with the soft surface. I want to manipulate the displacement of the rigid particle even when it collided with the soft surface, the particle must keep moving with a constant displacement! It is like when you use your finger to push at a soft surface, the more you push the more deformable the soft surface will be. That is such I want to simulate. In that way, I can collect the displacement data sets of the soft surface in my desired way.
Thank you in advance.

[hugtalbot]

Hi @vietlinhnguyen

Displacement (from the data field position and reset_position) and force can be accessed in the MechanicalObject.
Have you tried the LinearMovementConstraint to make your particle move. An example LinearMovementConstraint.scn is available here.

Hope this helps

[hugtalbot]

Hi @vietlinhnguyen was my previous advise useful ?