Tangent and curvature vector definition

@CURVATURE_VECTOR { c1, c2, c3}
@TANGENT_VECTOR { t1, t2, t3}
@COMMENTS {CommentText}


  1. The configuration of the beam is characterized by a curvature vector, c = {c1, c2, c3} and a tangent vector, t = {t1, t2, t3}, which is perpendicular to the beam's cross-section. Two parameters play an important role κ = ||c||, and κ τ = tT c, where κ is the curvature of the beam and τ its twist. Three types of beams can be defined
    1. Straight beams: κ = 0 and τ = 0,
    2. Circular beams: κ ≠ 0 and τ = 0, and
    3. Helicoidal beams: κ ≠ 0 and τ ≠ 0,
    where the latter two categories are also referred to as curved beams.
  2. By default, the curvature and tangent vectors are initialized to a straight-beam configuration: the curvature vector vanishes, c = 0 and the tangent vector is t = {0, 0, 1}.
  3. It is possible to attach comments to the definition of the object; these comments have no effect on its definition.