Wireless Sensor Networks : Technology, Protocols, and Applications

EXAMPLES OF CATEGORY 1 WSN APPLICATIONS 65
Figure 2.14
Military examples. (Courtesy of Rockwell Scientific.)
Condition-Based Monitoring Again as an illustrative example, Rockwell Scientific
is developing WSNs specifically tailored to the requirements for monitoring
complex machinery and processes. Their WSNs have been deployed on board
U.S. Navy ships as part of a developmental program with the Office of Naval
Research. Exploratory studies have also been done for use of WSNs on aircraft,
rotorcraft, and spacecraft as part of an overall integrated vehicle health management
system. Machinery maintenance has evolved from run to fail (no maintenance)
to scheduled maintenance (e.g., change oil every three months) to
condition-based maintenance (CBM). All three techniques are in current use. The
economic trade-off is between the cost of the CBM equipment and the staffing
resources expended to determine the machine’s health and the cost of unexpected,
as opposed to scheduled, repair and process downtime. With the emphasis of industry
in the last couple of decades on just-in-time processes, unexpected machinery
failure can be costly. The successful application of machinery monitoring programs
can optimize the use of machinery and keep manufacturing costs in check by
making the process more efficient [2.46]. The costs associated with CBM can
be allocated into equipment, installation, and labor costs in collecting and analyzing
the machine health data. WSNs are positioned to minimize all three costs and,
in particular, to eliminate the staffing costs, which often are the largest. With the
continuing advances in data processing hardware and RF transceiver hardware
(cell phone markets drive this), the technology is now becoming available to
install compact monitoring systems on machinery that avoid the installation
expense of data cabling through RF link technology; these systems provide a
mechanism for data acquisition and analysis on the monitoring unit itself. The
primary challenge faced by WSNs for machinery and process monitoring is
related to the quality of the information produced by both the individual sensors
and the distributed sensor network. Nodes located on individual components must
not only be able to provide information on the present state of the component
(e.g., a bearing or gearbox), but also provide an indicator of the remaining useful
life of the component [2.46].
The approach taken at Rockwell Scientific has been to mount two parallel
efforts. Existing diagnostic routines and expert systems are being ported to WSN