Deployment
A wireless mesh sensor network consisting of 20 wireless nodes and a base station was deployed at the Gaston plant to perform the targeted monitoring applications. All nodes were programmed to transmit data to the base station at 15-minute intervals. The route update was also set for 15-minute intervals.
Successful Demonstration of Wireless Sensors’ Capabilities
As stated earlier, two sets of low-cost, battery-powered wireless sensors using the IEEE 802.15.4 radios were tested in the fossil power plant demonstration. The first set was off-the-shelf commercial motes that were tested to determine their communication capabilities and battery life. The second set of sensors was based on off-the-shelf commercial motes but had custom software and packaging. The second set was designed for specific monitoring applications on plant components for maintenance purposes.
The communication capabilities were surprisingly good in the test. Even in the heavily congested area under a steam turbine the motes were able to communicate effectively. The mesh network that is part of the IEEE 802.15.4 standard provides multiple potential paths for communication. There were occasions during the test, which lasted several months, when the network altered its configuration, apparently to maintain communication in the face of some disturbance in the radio environment around the motes — a useful feature of mesh networking. The flexibility of the mesh network appears to be an important characteristic when using the low-power radios dictated by battery-operated motes.
A radio frequency site survey was done in the plant before the wireless sensors were installed for testing. The results of the site survey indicated that no other communication sources in the ISM (industrial, scientific, and medical) frequency bands were present. An intermittent broadband noise source was detected, but the source was not identified, and the noise did not appear to interfere with the wireless sensor network.
Battery life of wireless sensors (in this case, motes) is difficult to predict definitively because it depends on the radio environment, which is susceptible to changes in the plant and dependant on the configuration of the network. With two AA alkaline batteries and a data update period of 3-minute intervals, battery life varied from about two months to six months on the first set of motes. On the second set of motes, in which the data update rate was 15 minutes, the projected battery life was about five to six months. These motes were doing more rapid sampling and some on-board processing of the data, which tended to counteract the benefit of the slower update rate. The use of larger batteries such as D cells would increase battery life by approximately five times, and lithium ion batteries would make a further improvement.
The sensors performed well in several different applications, including these:
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Two of the wireless motes were configured to measure temperature using thermocouples. This worked well, with no noticeable noise problems from the low-level analog input system. The thermocouple inputs did not appear to affect battery life compared to that of the motes using a different sensor board.
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Two types of vibration-monitoring motes were developed and tested on a boiler feed pump system. The vibration measurements clearly showed when the equipment was operating.
The Gaston power plant demonstration was a success. Observations from these tests provided valuable insights into the potential of using 2.4 GHz 802.15.4 radio-based low-cost wireless sensor networks for power plant monitoring.
—Cyrus W. Taft, PE (cwtaft@ taftengineering.com) works for Taft Engineering Inc., in Harriman, Tenn. Aaron J. Hussey (ahussey@epri.com) works for Electric Power Research Institute in Charlotte, N.C. Teja Kuruganti (kurugatipv@ornl .gov) works for Oak Ridge National Laboratory in Oak Ridge, Tenn. John N. Sorge, PE (jnsorge@southernco.com) works for Southern Company Services Inc. in Birmingham, Ala. Asis Nasipuri (anasipur@ uncc.edu) works for the University of North Carolina at Charlotte.
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