This paper discusses the notion of operation mode in parallel
manipulators with less than six dof. This notion has been reported
recently in several papers but the physical meaning of an operation
mode is not always clear. Indeed, even if in some cases an operation
mode can be associated with an understandable motion type e.g., the
pure translations or a spherical motion, in some other cases, such an
association is not straightforward. Therefore, the axodes are used in
this paper to characterize any operation mode of lower-mobility
parallel manipulators. A 3-RPS manipulator is used as illustrative
example. This manipulator is special because one can parametrize the
operation modes.