  We study distributed output feedback control for a heterogeneous multi-agent
system (MAS), consisting of N different continuous-time linear dynamical
systems. For achieving output consensus, a virtual reference model is assumed
to generate the desired trajectory that the MAS is required to track and
synchronize. A distinct feature of our results lies in the local optimality and
robustness achieved by our proposed consensus control algorithm. In addition
our study is focused on the case when the available output measurements contain
only relative information from the neighboring agents and reference signal.
Indeed by exploiting properties of strictly positive real (SPR) transfer
matrices, conditions are derived for the existence of distributed output
feedback control protocols, and solutions are proposed to synthesize the
stabilizing and consensus control protocol over a given connected digraph. It
is shown that design techniques based on the LQG, LQG/LTR and H-infinity loop
shaping can all be directly applied to synthesize the consensus output feedback
control protocol, thereby ensuring the local optimality and stability
robustness. Finally the reference trajectory is required to be transmitted to
only one or a few agents and no local reference models are employed in the
feedback controllers thereby eliminating synchronization of the local reference
models. Our results complement the existing ones, and are illustrated by a
numerical example.
