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

20-2 Industrial Communication Systems
it to an actuator node, and that actuator node that actuates on the process. At least two feedback
control nodes are needed to have a proper NBC system (as one of those nodes can have two functions,
i.e., a sensor and controller node or a controller and actuator node). In both cases, there is messaging of
sensor to controller or controller to actuator nodes. In general, we should consider that the network is
shared with other nodes, they being feedback control or any other applications. The result is that NBC
systems can contain a large number of interconnected devices (controllers, actuators, sensors) that use
the network for interchanging control messages.
Feedback control imposes several restrictions on the communication networks. The multiple access
schemes of common industrial buses serialize the sending of the messages through the shared media.
As a result, communication delays are created in the feedback control loop. Delays inside a control loop
tend to instabilize the controlled plant. In addition, packets may be lost in a noisy channel, breaking the
control loop path and having also a possible bad consequence on the controlled process. This chapter
reviews the effects on the control side of the communication technology. It also talks about how a proper
use of the communication channel can favor the dynamic operation of the control loop and how the
control law is to be adjusted to take into consideration the communication-imposed constraints.
20.2 Mutual Concepts in Control and in Communications
In a networked control system (NCS), the feedback and the command signals use a communication network
link. When introducing a network in the control loop, some inconveniences such as band-limited
channels, delays, and packet dropouts occur. Such network may also be shared by other applications,
resulting in the channel being inaccessible, as sharing a communication channel imposes a wait time
until the channel is accessible again. From the point of view of control, it represents an additional delay
between the sensor measurement and the control actuation, resulting in a potential loss of control performance.
Classical discrete controller design imposes a periodic sampling scheme, which also imposes
hard real-time operation. The classical definition of hard real-time system is that the actions or tasks
shall be executed within a hard deadline or the result is no longer valid. In a controlled system, the
restriction is more severe, as the sampling and actuation is required to be with (not within) a prescribed
period. As in many cases strictly periodic operation cannot be assured, due to message loss or communication
incurred delays, several schemes can be used.
20.3 architecture of Networked-Based Control
When a traditional feedback control system is closed via a communication channel (such as a network),
which maybe shared with other nodes outside the control system, then the control system is classified as
an NCS or NBC system. All definitions found in literature for NBC have one key feature in common. This
defining feature is that information (reference input, plant output, control input, etc.) is exchanged among
control system components (sensor, controller, actuator, etc.) using a shared network (see Figure 20.1).
Controller-1
Controller-2
Controller-n
Supervisor
Network
Plant-1
Plant-2
Plant-m
Sensor-1
Sensor-2
Sensor-t
FIGURE 20.1
Typical NBC architecture.
© 2011 by Taylor and Francis Group, LLC