analytical surface generation

[lonelytown]

i am trying to generate an analytical surface for contact problem. According to the keyword reference manual, the coordinates of there points, a,b,c, should be provided to establish a local coordinate system. Then, define a curve (straight or curved) in the local x-y plane. The curve on the local x-y plane will extrude along ac direction.

So my question is, does ac have to be perpendicular to ab. It is not clearly stated in the manual. If it has to be perpendicular to ab, actually we have to provide very precise values of a, b, c to guarantee it, which is impossible. If ac doesnot have to be perpendicular to ab, how local z-axis is defined?

To sum up, how the local axes are defined by giving three points?

 

[lonelytown]

The question is in fact that does the local coordinate system of the analytical surface have to be cartesian system.

Any export here having experience upon this can help me out?