  This paper introduces a packet-based dual-rate control strategy to face
time-varying network-induced delays, packet dropouts and packet disorder in a
Networked Control System. Slow-rate sensing enables to achieve energy saving by
reducing network load. In addition, choosing a slower sensing period than the
longest round-trip time delay can avoid packet disorder. On the other hand, a
slow-rate sensing usually degrades control performance in a conventional
control framework. Therefore, including dual-rate control techniques can be
useful to maintain the desired performance, since the controller is able to
generate a fast-rate control signal from a slow-rate sensing signal. A
dual-rate PID controller is used, which can be split into two parts: a
slow-rate PI controller is located at the remote side (with no permanent
communication to the plant) and a fast-rate PD controller, at the local side
(close to the plant, sensor, and inside the actuator, which can offer a low
computation power). In addition, at the remote side, where a powerful
computation device is located, a prediction stage is included in order to
generate the packet of future, estimated slow-rate control actions to be sent
to the local side. At this side, these actions are converted to fast-rate ones
and used when a packet does not arrive due to the network-induced delay or due
to occurring dropouts. The control proposal is able to reach the nominal
(no-dropout, no-delay) performance despite the existence of time-varying delays
and packet dropouts. Via real-time control for a Cartesian robot, results
clearly reveal the superiority of the control approach compared to a previous
authors\' proposal, where the time-varying delays are faced by means of a gain
scheduling control strategy.
