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Distance between two plates in a boundary condition
Posted May 18, 2018, 7:46 a.m. EDT 5 Replies
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Dear Comsolers,
using only the Solid Mechanics physics, I try to simulate the mechanical behaviour if a capacitor made out of a rubber-like silicone. The dielectrica as well as the electrodes are soft and compliant.
When a voltage is applied to the electrodes, the dielectrica between the two electrodes gets compressed by a mechanical pressure and the electrodes move closer together.
The electrical field 
causes this mechanical pressure (maxwell stress) 
which can be described by the following equation:
where 
is the vacuum permittivity, 
is the relative static permittivity, 
the applied voltage (a fixed value, say 2.5kV) and 
the distance between the two electrodes.
How can I access the distance between the two electrodes? I would like to use it in order to set up a boundary load condition on the electrode-surfaces using as a pressure. As I define the geometry, the distance is well known at first, but becomes unknown when the capacitor is deformed. Also, the distance has not the same value over the whole geometry, so I need to access a scalar field containig all distances at every node.
I attached a picture of the unformed capacitor, where the elctrodes (blue) are volumes (and not surfaces). I am interested in the distance between the inner surfaces of the two electrodes.
Any help or hint is much appreciated, thanks a lot in advance!
My comsol version: 5.3
P.D.: I don't want to use the Electrostatics module. I think it is unnesessary in this case. Also, I am simply interested in learning how to access the distance d as in my question.
Attachments:
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Hi,
as the distance d extends in z-direction, would d-w be what you need? This can be evaluated on the surfaces.
Cheers Edgar
-------------------Edgar J. Kaiser
emPhys Physical Technology
www.emphys.com
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Dear Edgar, thanks a lot for your reply!
Yes, 
is exactly what I need. To be more precise, I need 
, where 
is the displacment on the upper electrode and 
the displacment of the lower electrode.
How can I evaluate and on the surfaces so it gets updated in every iteration of the solver?
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Freed,
you might use it in a coupling operator, e.g. an average operator that works on the respective boundary.
w1 = aveop1(w) on one electrode w2 = aveop2(w) on the other electrode
Cheers Edgar
-------------------Edgar J. Kaiser
emPhys Physical Technology
www.emphys.com
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Hey Edgar, I'm a beginner in Comsol and almost embarassed to ask, but how do I evaluate w on just one surface and store its values in a variable which I can access somewhere else? I tried it with a surface-probe, but that doesn't seem to work during the calculation (only after the calculation has terminated).
Also, I'd need a point- or knotwise evaluation of w and not an average value. How could I access w pointwise?
Thanks again
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Hi,
You are looking for one of the extrusion coupling operators, under Defintions->Component Couplings.
Regards,
Henrik
Henrik Sönnerlind
COMSOL