  The presence of faulty or underactuated manipulators can disrupt the
end-effector formation keeping of a team of manipulators. Based on two-link
planar manipulators, we investigate this end-effector formation keeping problem
for mixed fully- and under-actuated manipulators with flexible joints. In this
case, the underactuated manipulators can comprise of active-passive (AP)
manipulators, passive-active (PA) manipulators, or a combination thereof. We
propose distributed control laws for the different types of manipulators to
achieve and maintain the desired formation shape of the end-effectors. It is
achieved by assigning virtual springs to the end-effectors for the
fully-actuated ones and to the virtual end-effectors for the under-actuated
ones. We study further the set of all desired and reachable shapes for the
networked manipulators' end-effectors. Finally, we validate our analysis via
numerical simulations.
