Image Processing Reference
In-Depth Information
such as sensors and actuators. The controllers used with these devices provide typically on-chip
signal conversion, data and signal processing, and communication functions. The increased func-
tionality, processing, and communication capabilities of controllers have been largely instrumental
in the emergence of a widespread trend for networking of field devices around specialized networks,
frequently referred to as field area networks.
he ield area networks, or fieldbuses [] (fieldbus is, in general, a digital, two-way, multidrop com-
munication link) as commonly referred to, are, in general, the networks connecting field devices such
as sensors and actuators with field controllers (for instance, programmable logic controllers (PLCs)
in industrial automation, or electronic control units (ECUs) in automotive applications), as well as
man-machine interfaces; for instance, dashboard displays in cars.
In general, the benefits of using those specialized networks are numerous, including increased
flexibility attained through the combination of embedded hardware and software, improved system
performance, and ease of system installation, upgrade, and maintenance. Specifically, in automotive
and aircraft applications, for instance, they allow for a replacement of mechanical, hydraulic, and
pneumaticsystemsbymechatronicsystems,wheremechanical or hydraulic components are typically
confined to the end-effectors; just to mention this two different application areas.
Unlike local area networks (LANs), due to the nature of communication requirements imposed by
applications, field area networks, by contrast, tend to have low data rates, small size of data packets,
and typically require real-time capabilities which mandate determinism of data transfer. However,
data rates above Mbit/s, typical of LANs, have become a commonplace in field area networks.
The specialized networks tend to support various communication media like twisted pair cables,
fiber-optic channels, power line communication, radio frequency channels, infrared connections, etc.
Based on the physical media employed by the networks, they can be in general divided into three main
groups, namely, wireline-based networks using media such as twisted pair cables, iber-optic channels
(in hazardous environments like chemical and petrochemical plants), and power lines (in building
automation); wirelss networks supporting radio frequency channels and infrared connections; and
hybrid networks, with wireline extended by wireless links [].
Although the use of wireline-based field area networks is dominant, the wireless technology offers
a range of incentives in a number of application areas. In industrial automation, for instance, wire-
less device (sensor/actuator) networks can provide a support for mobile operation required in case
of mobile robots, monitoring and control of equipment in hazardous and difficult to access envi-
ronments, etc. In a wireless sensor/actuator network, stations may interact with each other on
apeer-to-peerbasis,andwithabasestation.hebasestationmayhaveitstransceiverattached
to a cable of a (wireline) field area network, giving rise to a hybrid wireless-wireline system [].
A separate category is the wireless sensor networks, envisaged to be largely used for monitoring
purposes.
The variety of application domains impose diferent functional and nonfunctional requirements on
to the operation of networked embedded systems. Most of them are required to operate in a reactive
way; for instance, systems used for control purposes. With that comes the requirement for real-time
operation, in which systems are required to respond within a predefined period, mandated by the
dynamics of the process under control. A response, in general, may be periodic to control a spe-
cific physical quantity by regulating dedicated end-effector(s), or aperiodic arising from unscheduled
events such as out-of-bounds state of a physical parameter or any other kind of abnormal conditions.
Broadly speaking, systems which can tolerate a delay in response are called soft real-time systems;
in contrast, hard real-time systems require deterministic response to avoid changes in the system
dynamics which potentially may have negative impact on the process under control, and as a result
may lead to economic losses or cause injury to human operators. Representative examples of sys-
tems imposing hard real-time requirement on their operation are Fly-by-Wire in aircraft control,
Steer-by-Wire in automotive applications, to mention some.
