# Discussion Forum

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## Moving items relative to each other

Hello,
I am a beginner user, and I was wondering if anyone could help with my problem. I am trying to calculate electrostatic forces between objects, as one is moving relative to another. Must I create a new frame for each step by redrawing the objects with shifted positions or is there a way to automate object displacement? Thank you
Anna

8 Replies Last Post Apr 14, 2013, 1:58 PM EDT

Hi,

It can be helpful if you draw a scheme of your model.
Are you using moving mesh application mode (ALE)? in this case you don't need to care about recreating the frames.
I guess that the motion of your objects is relative to the calculated force?

Cheers,

Thanks for your help! Actually, I'm trying to calculate the electrostatic force in the z direction on an AFM tip (approximating it as a cone) as it is scanned over a sample in the x direction, so the motion is just a constant displacement / frame, and not correlated to the force. I wasn't sure how to do that with ALE, my only idea was to redraw each frame.
Thanks!

OK now I see!!
I think it is much better and easier to do it with ALE, you don't need to redraw you geometry for each displacement.
But it depends on many points:

* If the length of your displacement is higher or no, in order to avoid inverted mesh (but you can use re meshing function-combining Matlab)

* If your model is 2D or 3D, because in 3D is a bit tricky, since that you need lots of memory space if you refine the mesh at the apex of the tip and the motion would be a bit complicated but it's doable!!

Don't hesitate if you need more help

Cheers

Hi Faycal,

I have a very similar question. I am trying to move 2 disks apart each other. i modeled them as squares and i am using axial symetry. when i apply a current to one of them, it should induce currents in the other plate on top and as a result, force it to move upwards. I am trying to use ALE but as the plate moves away, it eats the mesh leaving a white part and does all the calculation as if the plate is not moving. it only shows it top me moving up but calculates every time step on the same position.

Any sugggestion?
thanks

Hi,
This seems quite easy to set, since that the displacement of the plate is not so big "following the intensity of current"!! The important point is that you have to distinguish the boundaries that are expected to move and the rest "non-moving boundaries" This is the key.
it seems from your description that some boundaries are not supposed to move but they do!! or vice versa!!

I have some couple of questions:

- How did you correlate the current and the "force" that has to move the second plate? Do you have a formula for that? is it a function in this form I=f(x)??

- What did you specify on the supposed moving boundaries ?? did you specify a velocity or a displacement?? this should be related to the intensity of the current!! the same question for the boundaries that you are supposing fixed???

I thought that is faster/easier for you to learn via a simple model, which look like yours. axial2D and two parallel plates. So, I set a model that you can find in the attachment.

The upper plate is moving toward the one in the bottom.
You can take a look to the BCs for each boundary and try to make a connection with your displacement/current "condition"
I set a velocity for the moving boundary, you can change of course by a displacement following your model.

I hope it helps,

Cheers

Thank you very much Faycal for your extensive explanation. I appreciate it. you were a great help. have a nice day

Posted: 9 years ago
Dear Sir,
I have a similar problem.
I am a beginner user and I am trying to simulate the movement of a PDMS cube (1 mm thick) in an electric field produced by two surface charges (of opposite signs) placed on the two faces of a ceramics plate (0.5 mm thick), like in a condenser. In the beginning the cube is placed on a glass substrate, 2 mm far from the ceramic plate.
I put air between the glass and the ceramic plate.

I use three application modes: the moving mesh, the plane stress (to consider the cube’s weight) and the electrostatic generalized.

The type-dependent solver doesn’t converge, therefore I use stationary solver (linear).
However some warnings (inverted mesh element near coordinate) appear.
Surely, I did some mistakes!
Please, could you help me to find them?
Melania

P.S. I am using comsol v. 3.5a
I am not able to attach the file. It is not considered an allowed file and I didn't understand why.