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FEMM 4.2 Electrostatics Tutorial1 David Meeker dmeeker@ieee.org January 25, 2006 1. Introduction Finite Element Method Magnetics (FEMM) is a finite element package for solving 2D planar and axisymmetric problems in electrostatics and in low frequency magnetics. The program runs under runs under Windows 95, 98, ME, NT, 2000 and XP. The program can be obtained via the FEMM home page athttp://femm.foster-miller.com. The package is composed of an interactive shell encompassing graphical pre- and postprocessing; a mesh generator; and various solvers. A powerful scripting language, Lua 4.0, is integrated with the program. Lua allows users to create batch runs, describe geometries parametrically, perform optimizations, etc. Lua is also integrated into every edit box in the program so thatformulas can be entered in lieu of numerical values, if desired. (Detailed information on Lua is available from http://www.lua.org.) There is no hard limit on problem size—maximum problem size is limited by the amount of available memory. Users commonly perform simulations with as many as a million elements. The objective of this document is to get new users “up and running” with the program via a setof step-by-step example electrostatics problems. 2. Basic Introduction: Capacitor with a Square Cross-Section This will take you through a step by step process of analyzing a capacitor with a square crosssection. Users should first refer to the FEMM user’s manual regarding the general interface (i.e. keyboard and mouse controls). This example, as shown below in Figure 1, the outer square has a 4cm size and the inner square has a 2 cm size. The geometry extends for 100 cm in the “into-the-page” direction. The dielectric between the plates is air. We seek to build the problem, analyze it, and determine the capacitance.

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This document is based on the "Introduction to FEA with FEMM" tutorial by Ian Stokes-Rees, TSS (UK) Ltd.

Many thanks to Kostadin Brandisky of the TechnicalUniversity of Sofia for developing the example problems used in this tutorial.

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2 1.5 1 0.5 0 -0.5 -1 -1.5 -2 -2 0 2

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Figure 1: Square Cross-Section Capacitor Because of the symmetry, only one quarter of the device need be modeled. The finished model will look as pictured below in Figure 2 when ready for analysis.

Figure 2: Completed example in the electrostatics preprocessor.

Thesteps required to create this model are as follows: 2.1 Create model Start FEMM by select the "femm 4.0" entry placed in the "femm 4.0" section of your start menu. After the program starts, "File|New" from the main menu. Select "Electrostatics Problem" in the "Create a new problem" dialog that appears. After you hit "OK", a blank electrostatics problem will appear. Select “nodes” from the tool bar(this is the farthest button on the left with a small black box: ) and place 6 nodes for the corner of a box (at, for example, (0,1), (1,1), (1,0) ,(2,0),(2,2) and (0,2)). One can place nodes either by moving the mouse pointer to the desired location and pressing the left mouse button, or by pressing the key and manually entering the point coordinates via a popup dialog. Select “lines” from thetool bar (second button from the left with a blue line: ). To select a node to be the endpoint of a line, click near each desired endpoint with the left mouse button. Select connect the points as pictured in Figure 2 using this mouse endpoint selection approach. 2.2 Add materials to the model Select “Properties|Materials” off of the main menu. In the dialog that appears, click the “Add Material”button. A dialog will pop up with edit boxes for the various material properties. Change the name of the property from “New Material” to “air.” By default the permittivity of a new material is 1, which is what we require for air. Press the “OK” button to complete the creation of the material. 2.3 Define materials for each region ), and place a block label Now click on “Block Labels” (the tool bar...