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Perimeter - which I suppose you are talking about - is a way to control geometry "from the outside". I frequently use this to create flat springs.
When bending a flat spring you know that it keeps its length in the neutral fibre. Simplifying things one can state that a small deformation can be shown as an arc. There is no easy way to establish what radius you need to get a certain amount of bending at a given place. That's where perimeter comes in. Draw an arc for the neutral fibre, change it to construction and draw a parallel contour around it to get a body. Put another construction line under the extremities to dimension the amount of deformation from the straight situation. Make sure that you have geometric constraints, a dimension for the arc radius and a dimension for the "bending". Now highlight the neutral fibre arc and edit it to make it a perimeter. ProE then asks what dimension it can vary to make the perimeter value correct. Point to the radius dimension.
Now you can edit the perimeter (so indicated) and give it the length of the flat spring. Next you can change the amount of bending (the "pushing" line) and ProE will work out a solution where the perimeter stays the same and the curvature of the arc changes to obey the endpoints.
You are not limited to one element. In the above case you could create a longer spring consisting of an arc between the points that get pushed and straight lines beyond that. You can multi-select the entire line-arc-line neutral fibre and make that perimeter.
Some warning :
There is a limit on how little you can bend. Mathematically the radius evolves to infinity when the spring gets straight. No formula likes infinity and ProE gives up long before, because of the limits of the greatest number that can be used.
We had problems in an Intralinink managed environment because of this spring. Finally found out that it was tied to not all workstations having the same build of ProE.
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