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

Network-Based Control 20-5
message (token) that gives rights to its receiving node to transmit one or more queued messages. The
token is usually sent to an ordered and cycled list of nodes. The result is that the network gives an
ordered access to all the nodes and each node can know in advance the maximum delay a given message
will incur before arriving to its destination.
The random access scheme allows any node to transmit whenever it needs to send a queued message
provided not any other node is sending a message. If there is a message being sent through the network,
the node waits till the end of this message and then it attempts to send its queued message after an
active wait, where a competition between the waiting nodes will give the message access to only one of
them. The result is that a random access scheme is generally nondeterministic.
On top of the access scheme is the end-to-end communication relationship between nodes. The most
common relationships are the master–slave, producer–consumer, and client–server.
• Master–Slave: The network has master nodes and slave nodes. Slave nodes only react to master
nodes commands. Master nodes share the channel access by using a round-robin (i.e., token passing)
scheme. Profibus is an example of this scheme.
• Producer–Consumer: The network has nodes that are producers, consumers, or both. A producer
node when it has a message to send looks for the first opportunity to send it in a broadcast way.
Consumer nodes pick up this message contents as soon as they need it. As an example, Control
Area Network (CAN) uses this paradigm. CAN is one of the accepted fieldbus standards—a
serial, asynchronous, multi-master communication protocol designed for applications needing
high-level data integrity and data rates of up to 1 Mbps.
• Client–Server: In this case, the network has client nodes and server nodes. The client nodes send
requests to the server nodes for services. Based on the best effort, the server provides this service
and notifies the completion. This scheme is not necessarily deterministic as the server node takes
as much time as it needs.
The result of application of those access and relationship schemes is that a given industrial network may
partially use a client–server paradigm and a producer–consumer or master–slave in conjunction with any
of the access schemes. How the relationship is to be operated depends on the transaction to be done. For
example, in an industrial application, during the commissioning phase, a client application installs an
automation program into a Programmable Logic Controller (PLC), which acts as a server, while during
the PLC cyclic operation, it connects to sensors and actuators based on a master–slave scheme.
There is a wide variety of concurring fieldbus standards and, therefore, many times interoperability
becomes an issue. Another communications network used in NCS—Ethernet—has evolved into the
most widely implemented physical and link layer protocol today mainly because of the low cost of the
network components and their backward compatibility with the existing Ethernet infrastructure. Now
we have fast Ethernet (10–100.Mbps) and Gigabit Ethernet (1000.Mbps) [5]. Recently, switched Ethernet
became a very promising alternative for real-time industrial application because it eleminates the collision
in traditional Ethernet [4].
Overall, the choice of network depends upon the desired application. Today, with the help of technologies
like GPS, electronics atlas (Google maps), we are looking at multi-agent traffic control in urban
areas with efficient vehicle communication [7]. Military, surgical, and other emergency medical applications
can use dedicated optical networks to ensure fast-speed and reliable data communication. The
Internet is the most suitable and inexpensive choice for many applications, where the plant and the controller
are far away from each other [6].
20.3.1.2.2 Sampling and Messaging in NBC
Sampling is the activity of measuring the feedback control variable to be sent to the controller. The cyclic
control conceptual scheme follows the order: sampling, control law computation, order to actuator. The
most usual requirement for this sequence is that it has to be executed based on a fixed clock cycle time.
General control law models assume that the time spent between successive samples is a constant, called
© 2011 by Taylor and Francis Group, LLC