Image Processing Reference
In-Depth Information
increased reliability. his advantage is further accentuated in wireless networks. Another significant
advantage is that networks enable complex distributed control systems to be realized in both horizon-
tal (e.g., peer-to-peer coordinated control among sensors and actuators) and vertical (e.g., machine to
cell to system level control) directions. Other documented advantages of networks include increased
capability for troubleshooting and maintenance, enhanced interchangeability and interoperability of
devices, and improved reconfigurability of control systems [].
With the return-on-investment of control networks clear, the pace of adoption continues to
quicken, with the primary application being supervisory control and data acquisition (SCADA) sys-
tems []. [].These networked SCADA systems often provide a supervisory-level factory-wide solution
for coordination of machine and process diagnostics, along with other factory floor and operations
information. However, networks are being used at all levels of the manufacturing hierarchy, loosely
defined as device, machine, cell, subsystem, system, factory, and enterprise. Within the manufac-
turing domain, the application of networks can be further divided into sub-domains of “control,”
“diagnostics,” and “safety.” Control network operation generally refers to communicating the neces-
sary sensory and actuation information for closed-loop control. The control may be time-critical,
such as at a computer numeric controller (CNC) or servo drive level, or event-based, such as at
a programmable logic controller (PLC) level. In this sub-domain, networks must guarantee a cer-
tain level of response time determinism to be effective. Diagnostics network operation usually refers
to the communication of sensory information as necessary to deduce the health of a tool, prod-
uct, or system; this is differentiated from “network diagnostics,” which refers to deducing the health
of the network [,,,]. Systems diagnostics solutions may “close-the-loop” around the diag-
nostic information to implement control capabilities such as equipment shut-down or continuous
process improvement; however, the performance requirements of the system are driven by the data
collection, and actuation is usually event-based (i.e., not time dependent). An important quality of
diagnostics networks is the ability to communicate large amounts of data, with determinism usually
less important than in control networks. Issues of data compression and security can also play a large
role in diagnostic networks, especially when utilized as a mechanism for communication between
user and vendor to support equipment e-diagnostics [,,]. Safety is the newest of the three
network sub-domains, but is rapidly receiving attention in industry []. Here, network
requirements are often driven by standards, with an emphasis on determinism (guaranteed response
time), network reliability, and capability for self-diagnosis [].
Driven by a desire to minimize cost and maximize interoperability and interchangeability, there
continues to be a movement to try to consolidate around a network technology at different levels of
control and across different application domains. For example, Ethernet, which was widely regarded
as a high level-only communication protocol in the past, is now being utilized as a lower-level control
network. This has enabled capabilities such as web-based “drill-down” (focused data access) to the
sensor level [,]. Also, the debate continues on the consolidation of safety and control on a single
network []. Even more recently wireless technology is being considered as a replacement for wired
networks at all levels, primarily to support diagnostics, but also to support control and even safety
functionality in specific instances [,].
This movement toward consolidation, and indeed the technical selection of networks for a par-
ticular application, revolves around evaluating and balancing Quality of Service (QoS) parameters.
Multiple components (nodes) are vying for a limited network bandwidth, and they must strike a
balance with factors related to the time to deliver information end-to-end between components.
Two parameters that are often involved in this balance are network average speed and determinism;
briefly network speed is a function of the network access time and bit transfer rate, while determinism
is a measure of the ability to communicate data consistently from end-to-end within a guaranteed
time. Note that this QoS issue applies to both wired and wireless network applications; however, with
wireless there must be more focus on external factors that can affect the end-to-end transmission
performance, reliability, and security.
 
Search WWH ::




Custom Search