User`s guide

Special Situations
When the Z axes of the local and primary reference frames are parallel, roll and yaw produce
the same motion in the same plane, although the two motions may be in different directions.
This is always the case with a four-axis SCARA robot. The system automatically reflects
rotation of the quill in the roll component of a transformation variable, and the yaw
component is forced to 0 degrees. In a SCARA robot equipped with a fifth axis, rotation of the
quill is reflected in the yaw component and motion of a rotating end-effector (sixth axis) is
reflected in the roll component.
Notice in the figure XYZ Elements of a Transformation that the local reference frame points
straight up. This corresponds to a situation where the end of arm tooling points straight back
along the third axis. In a mechanism not equipped with a 360 degree wrist, this is an
impossible position. For a four-axis SCARA, this component must point straight down (pitch =
180 degrees). For a mechanism with a fifth axis, this component must be within the range of
motion of the fifth axis.
NOTE: When thinking about a transformation, remember that the rules of ZYZ' Euler
angles require that the orientation components be applied in order after the local
reference frame has been defined. After calculating the Cartesian components and placing
a local reference frame with x, y, and z axes parallel to the primary reference frame X, Y,
and Z axes, the orientation components are applied in a strict order—yaw is applied first,
then pitch, and, finally, roll.
Location Variables
V+Language User's Guide, v17.0
Page 172