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Simulating pull-in with contact pair
Posted 2 avr. 2013, 06:15 UTC−4 MEMS & Nanotechnology, MEMS & Piezoelectric Devices, Structural Mechanics Version 4.2a 6 Replies
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I followed the model name "electrostatically actuated cantilever" in model library to build my own model - an RF MEMS shunt switch with fixed fixed beam. In the "electrostatically actuated cantilever" model, the author finds the pull in voltage by basing on the divergence of the solution. I did not quite satisfy with that since I am also interested in the final geometry when the beam lands on the bottom electrode. I worked for weeks, using contact pair, hoping that the contact will help the beam stop when pull-in happens, longing for a converge solution of that electromechanical problem but did not succeeded.
Is it possible to carry on that way, or should I give up?
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pull-in is mostly a highly non linear or buckling type run-off, you cannot easily ask an iterative solver to converge with such cases, it need some help (=tweaking) to get COMOSLor any other solver programme to follow the solution over the instability.
One way is to attempt a way by enforce a displacement and measure the forces required, or to use time domain, but you needthen to ramp the voltage gently to avoid any oscillation, and then for the contact you will get a lot of ringing if you do not either turn off the inertial terms, or add quite some damping. This requires some tries to give time steps set accordingly to the to come collapse of the beam.
Its better to start with a smpe case and mesh so it solves quickly and hen allows you to adapt the ettings within a limited time frame,then only increase the mesh density and model complexity, else you wait for ages and never really manage to tweak your settings
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Good luck
Ivar
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Thank you for your reply, I'll try with your advice.
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As Ivar points out, this is a 'post-buckling' phenomenon. No stationary solution exists after the pull-in voltage has been reached.
Tracing the solution path with a transient solver is in theory possible, but will in practice be very problematic.
The best way to handle this type of problem is to use the method shown in the Model Library example 'Postbuckling Analysis of a Hinged Cylindrical Shell'.
The method is similar to using what is often called an 'arc-length solver'. You select a parameter which you know will be monotonously increasing as the parameter controlling your problem, and then let e.g. the voltage be controlled by that parameter.
Regards,
Henrik
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It is nice that I have another approach to try.
Regards,
Bao.
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Hi, I have been trying to find out the pull in voltage of a very basic cantilever based MEMS switch. I referred to the tutorial given in MEMS module for resonator pull in voltage calculation and tried to simulate a switch. However, I keep getting errors indication lack of convergence and NaN/Inf not found and so on. Were you successful in finding out the pull in voltage? I would appreciate it if you help me by giving some tips on this issue.
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