RE: [femm] Define contour

Hello Roman,

What I do in this situation is to subdivide the airgap between static and moving parts with lines that trace a contour right round the moving part. I can then control the element size within the airgap to ensure that I get an accurate force result. (read the part in the manual that recommends using at least two elements between a line integral and a change in material boundary:- it is important!) See the attached femm model that I used to plot the force on a steel block as it moves into the gap between an electromagnet. (I have also used other lines to subdivide the air into different areas, so that I can control the element size where necessary.)

These lines can then be picked in the post processor. I use the Lua lines below in my post-processor script;

-- obtain forces
clearcontour()
seteditmode("point")
ncpts = getn(cpts.x)
for k = 1,ncpts do
selectpoint(cpts.x[k],cpts.y[k])
end
selectpoint(cpts.x[1],cpts.y[1])

fx, fxi, fy, fyi = lineintegral(3)

The x and y coordinates of the line end points are stored in 'cpts.x' and 'cpts.y' which I define in my preprocessor model script, and read in from a file at the beginning of my post-processor script. You dont actually need to draw lines in your femm model, but you need the points there to use the method described.

Furthermore, I use this line midway in the airgap to help define my element size within the airgap (using the line properties). This way I ensure that I only have really small elements where I need them (this is useful if you have a model with a lot of travel, or a very small airgap). The important obvious thing is to move these contour points with the moving geometry within your Lua pre-processor script (the model attached was designed with static contour lines but this is an exception to what I usually do)

This works well for me, and results agree well with physical systems (to date Femm has predicted forces to within 10-15% of the actual measured forces for 3-4 systems that I have built, which I consider to be excellent for 2D - FEA)

Regards,

Finlay Evans

Finlay Evans
Senior Design Engineer
Group Engineering - Renishaw Plc

-----Original Message-----
From: Roman Beresik [mailto:beresikr@xxxxxxx]
Sent: 24 July 2002 14:26
To: femm@xxxxxxxxxxxxxxx
Subject: [femm] Define contour

Hello femm group

I have a simple simulation of electromagnet. I made program for motion of the core in the electromagnet by the lua script language. I defined segments and nodes selected as a group 1 near to core sufrace. I don't know how to define contour for this group in the post processor for the calculation of line integral. I found only commands which need to define co-ordinates of points for contour. However, the motion is programed by loop. I have a question. How to define the contour automatically for each step of loop ?

Roman Beresik

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