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

20-4 Industrial Communication Systems
A 1
S 1
C 1
C 2
S 2
Set point
Status or sensor
measurement
Network
A 2
C M
S 3
C 3
A 3
FIGURE 20.3
Data transfers of hierarchical structure.
Control signal
A 1
C 1
Network
Sensor measurement
S 1
FIGURE 20.4
Data transfers of direct structure.
The second approach of network control is the direct structure, as shown in Figure 20.4. This structure
has sensors, actuators, and a controller of a control loop connected directly to a network. Both the
hierarchical and direct structures have their own pros and cons. Many NCS include features from both
structures, to benefit from their advantages, thus defining the hybrid NCSs.
More details can be found in [1,2].
20.3.1.2.1 Communication Networks
A communication network is the backbone of the NBC. Reliability, security, ease of use, and availability
are the main issues while choosing the communication type. A communication network giving support
to an NBC should have a deterministic end-to-end message response as a principal property. This is the
only way to guarantee the real-time response required by the control algorithm.
The ARPANET developed by Advanced Research Projects Agency Network of the U.S. Department
of Defense in 1969 was the world’s first operational packet switching network and the predecessor of the
global Internet. Later came fieldbus (around since 1988)—an industrial network system for real-time
distributed control. Fieldbus is a generic term, which described a modern industrial digital communications
network intended to replace the existing 4–20.mA analog signal standard. This network is a digital,
bidirectional, multi-drop, serial-bus used to link isolated field devices, especially in the automated
manufacturing environment. Profibus (process fieldbus) is a standard for fieldbus communication in
automation technology and was first promoted in 1989 by BMBF (German department of education and
research) [3].
The basic medium control access scheme is the time division multiple access, where a fraction of the
channel is given to all the nodes. There are several medium access control schemes used by industrial
networks. The two more common are based on a token-passing scheme and a random access scheme.
The token-passing scheme uses a portion of the network bandwidth for transmitting an authorizing
© 2011 by Taylor and Francis Group, LLC