wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

20-6 Industrial Communication Systems
the sampling period T, while the time spent between the sampling and the actuation instants is negligible
or constant and known; but the sampling and actuation messages take some time, which depends
on the access scheme and the relationship used, and the control law requires some computation time,
so the above assumption does not hold. This effect becomes more important as the network traffic load
increases.
Considering an NBC scheme (Figure 20.2), in which the control loops consist of plants G, and controllers
C, which exchange through messages the sampled variables y(kT) and the control variables
c(kT), where k is the number of the sample. As soon as the messages are sent through the network, they
experiment a delay, so the controller receives a delayed value of the sampled signal, y(kT + t sc ), while
the actuator receives a delayed value of the control signal, c(kT + t ca ), where t sc and t ca are the time delay
between sensor to controller and controller to actuator, respectively. In order to ensure system performance,
communication-induced time delays have to be considered at the controller design stage.
20.4 Network Effects in Control Performance
The main control performance parameters in a controlled system are stability and the system error. It is
clear that stability must be ensured in all controlled systems. The system error, measured on a time window,
shows the ability of a closed-loop system to recover after a perturbation. In general, we can state that
the smaller the system error, the better the performance.
The performance of an NBC system depends on the control strategy, the message scheduling algorithm,
and the reliability of the communication network. We shall consider the case of limited
communication resources (sensors, controllers, and actuators), which share and compete with other
nodes for accessing the network. The message access scheme and the communication relationship are
used as scheduling strategies to assign network access to nodes. The resulting scheduling will cause a
shortening or increasing of the message latencies, thus influencing system performance.
A delay in the control loop reduces the system performance [8]. As larger is the delay, worse is the performance.
If the delay is known, it can be included into the control design as a parameter and its effect
can be minimized. But in a networked control, the network-induced delay usually varies from actuation
to actuation as the messages are sent only after competing for and gaining access to the network.
The result is that the actual delay varies from cycle to cycle, which not only degrades performance but
can drive the system to instability. In particular, the sensor to controller delay can be time-stamped, so
the controller can know exactly the time instant of the sampled instance, and from there, calculate the
actuation value. But the message from the controller to the actuator can suffer also unpredictable delay
not being possible to introduce it into the control calculation. One possible way to overcome this situation
and obtaining predictability is using the one-shot model [9] where the actuation is synchronized
at the end of the period.
Another important effect in the NCS occurs when a message is lost (there can be several causes for
losing a message). It is clear that losing sampling or control messages may also degrade the performance
of the controlled system. From the control side, there are different strategies for reducing the effects of
lost messages, while from the communication systems side, some kind of redundancy can be used in
high-integrity systems. One of the used methods is including some kind of observer or predictor that
calculates or predicts the lost value [10].
20.5 Design in NBC
Designing an NBC system requires a careful work within the areas of industrial communications, with
reference to the network used and the communication relationship; the control system design areas,
with special emphasis on including the induced network delays into the designed control law; and the realtime
computation areas, as all the control tasks, together with other tasks that possibly use the network, are
to be deployed in computing platforms that shall ensure their continued execution.
© 2011 by Taylor and Francis Group, LLC