wireless ad hoc networking

Fieldbus for Distributed Control Applications 577
PCI architectures are the primary contenders. The most important development
has been the adoption of PMC (PCI Mezzanine Card) by the VMEbus
community. The industrial and real-time computation community will soon
have a wide choice of standardized solutions based upon two important
building blocks: VME and PCI. 5
All such buses were aimed at specific market needs and were relatively
independent from any given chip. The buses are designed mostly for
signal-processing and intensive data-processing applications such as radar,
sonar, imaging, management information system (MIS) servers, transactional
banking applications, Ethernet, integrated service digital network
(ISDN), industrial control where fast data processing is desired, and so
on. 6 The signal-processing architecture is based on digital signal processors
(DSP) boards. Although the DSP performance is very high, the resultant
architectures are often proprietary. At the same time, it is worth mentioning
that most of such bus architectures entertain centralized control implementation.
Although some of the buses are capable of being configured onto
distributed architecture, the overall installation cost seems to be high when
compared with fieldbus-based control implementation.
22.2 Review on Distributed Control
The rapid progress of internationalization following World War II has had an
enormous impact on manufacturing industries as it has become the cornerstone
of many economic activities. During the 1970s most industrial control
systems were relay-based, with nearly a one-to-one I/O ratio. Control systems,
consisting of a group of relays, were capable of controlling machine
tools, conveyor systems, and other mechanical components such as motors,
transportation vehicles, etc. within rugged environments. To increase communication
capabilities, implementation simplicity, cost–performance ratio,
reliability, and capability, PLC-based control systems emerged in 1969. A
PLC is a programmable electronic device that controls machines and processes.
It uses a memory to store instructions and execute specific functions
that include on/off control, timing, counting, sequencing, and arithmetic
and data handling. Functioning as relay replacements, the PLCs are more
reliable than relay-based systems, largely because of the robustness of their
solid-state components. PLCs save installation, troubleshooting, and labor
costs by reducing the wiring and the associated wiring errors. They occupy
less space and their reprogrammability features increased flexibility and
simplicity with regard to changing control schemes. On the other hand,
PLC-based automation was considered a centralized implementation, which
has its own loopholes such as central failure. Consequently, a need to break
down the control process into “cells” was found as another alternative to
improve flexibility (e.g., decentralization of centrally regulated tasks) further.
This in turn led to a more decentralized approach to control solutions.