Superclass of line force elements.
A line force element exerts a force between the origin of frame_a and the origin of frame_b. In subclasses of this class the line force f is given as function of the relative distance s between the origins of the two cut-frames and its first derivative sd. The following parameters are defined:
sEps: When sd , sdd , fa , fb are calculated, a division by
zero takes place, if the distance becomes zero. Therefore, for
this calculation the modified distance smod = max(s, sEps)
is used.
Terminal variables:
f : The scalar value of the line force in [N].
pulling force: f > 0
pressure force: f < 0
s : Distance in [m] (s >= 0).
sd : First derivative of s in [m/s].
sdd : Second derivative of s in [m/s^2].
na(3): Unit vector on the line from the origin of frame_a to
the origin of frame_b, resolved in frame_a.
Note:
Both frames of a force element have always to be connected at
frame_a of a joint, of a body or of the inertial system. It is not possible
to e.g. connect two force elements in series.