How to control the motion with LinearMovementProjectiveConstraint

[Hoangn06]

Hello everyone,

I want ask about how to control the motion with LinearMovementProjectiveConstraint.

1 - When we apply movement with the LinearMovement, are we applying to velocity the DOFs?

2 - I'm so confused with how to control the motion with key times. First i tried to apply the motion at the time 0:

LinearMovementProjectiveConstraint template="Vec3" indices="@BoxROI.indices" keyTimes="0 2 " movements="
-1e-3 0 0 0 0 0
0 0 0 0 0 0"
The object was moved from its original position to the opposite side of the X-axis at the beginning of the simulation, but when I launched the simulation and time began to progress, the object moved in the same direction along the X-axis, even though I had set -1e-3 (which should correspond to the opposite side of the X-axis). What happened in this case? Why did the object move in the another direction from what i had set?

Second tried to apply the motion now at time 2:

LinearMovementProjectiveConstraint template="Vec3" indices="@BoxROI.indices" keyTimes="0 2 " movements="
0 0 0 0 0 0
-1e-3 0 0 0 0 0"

Now with this second try, the object didn't move. Why didn't the object move in this case?

On the third try, I added a null motion at time 4 with:
LinearMovementProjectiveConstraint template="Vec3" indices="@BoxROI.indices" keyTimes="0 2 4 " movements="
0 0 0 0 0 0
-1e-3 0 0 0 0 0
0 0 0 0 0 0"

The object starts from its original position and begins moving downward along the negative X-axis after 2 seconds. Why do I need to add a motion after the 2nd second (in this case is 4th second) for the movement during the 2nd second to be executed?

And how can i make the the object totally stops at the time i want? And how can i change the direction of the movement, for example in the third try, the object is moving downward along the negative X-axis after 2nd second, how can i reverse the movement and make it move along the X-axis from the 4th second? Or make it move along the Y-axis from the 4th second? I've tried to test a lot of movements after but it didn't work and the object continue to move along the negative X-axis.

Thank you everyone and have a great day!

[bakpaul]

Hi

First I'll explain what is a projective constraint. As explained in the documentation here : https://sofa-framework.github.io/doc/simulation-principles/constraint/projective-constraint/ a projective constraint simply project certain dofs of your system. It means that now its position/velocity is not determined by the forces but directly overridden by the value you are providing.

1. Here, the constraint is a positional constraint. Because you are directly imposition the position, the velocity is directly derived from it. So in a way yes, you are imposing the velocity. But there actually exists an object constraining the velocity directly, it is called LinearVelocityProjectiveConstraint
2. At first glance I can see one first mistake you are doing : you templated the object with Vec3 but you are giving velocity vector of size 6. For your last example if you put "0 0 0 -1e-3 0 0 0 0 0" then it should to what you want.

[Hoangn06]

Hello @bakpaul,

Thank you so much for your respond. I dont know why i gave a vector of 6 in the constraint. But it worked for my part. Just one thing i can't do: how to stop the object totally? I tried to put in " 0 0 0 -1e-3 0 0 0 " for 2 key times "0 2" but the object continue to move. I know i can stop the simulation at the final key time but i'm just curious.

[bakpaul]

One trick is to add a last key time that is a repeat of the first one with a time value very high so it is never reached. The one acting on velocity actually has a mechanism 'continueAfterEnd' that does what you would like in your case but it hasn't been coded for the one in potision. We need to refactor this API.

[Hoangn06]

That's a great trick! Thank you @bakpaul