  Robust implementable output regulator design approaches are studied for
general linear continuous-time \mbox{systems} with periodically sampled
measurements, consisting of both the regulation errors and extra measurements
that are generally non-vanishing in steady state. A digital regulator is first
developed via the conventional emulation-based approach, rendering the
regulation errors asymptotically bounded with a small sampling period. We then
develop a hybrid design framework by incorporating a generalized hold device,
which transforms the original problem into the problem of designing an output
feedback controller fulfilling two conditions for a discrete-time system. We
show that such a controller can always be obtained by designing a discrete-time
internal model, a discrete-time washout filter, and a discrete-time output
feedback stabilizer. As a result, the regulation errors are shown to be
globally exponentially convergent to zero, while the sampling period is fixed
but can be arbitrarily large. This design framework is further developed for a
multi-rate digital regulator with a large sampling period of the measurements
and a small control execution period.
