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class="style6">HELPLINE: </span>(610) 362-1320<br> <span class="style6">WEBSITE: </span><a href="http://www.empf.org">www.empf.org</a><br> <a href="http://www.aciusa.org">www.aciusa.org</a></div></td> </tr> </table> <p>&nbsp;</p> <p><br> <br> </p> <div align="center"> <p align="center" class="style7">Michael D. Frederickson<br> <strong><em>EMPF Director </em></strong><br> <br> Barry Thaler, PhD.<br> bthaler@aciusa.org <br> <strong><em>Empfasis Technical Editor<br> </em></strong><br> Paul Bratt<br> pbratt@aciusa.org <br> <strong><em>Empfasis Editor</em></strong><br> </p> <p align="center" class="style7"><img src="http://www.empf.org/empfasis/archive/images/iab_sm.gif" alt="IAB" width="100" height="52"><br> <span class="style9">Industrial Advisory Board</span><br> <span class="style14"><em>Gerald R. Aschoff</em>, The Boeing Company<br> <em>Dennis M. Kox</em>, Raytheon<br> <em>Gregory X. Krieger</em>, BAE Systems<br> <em>Edward A. Morris</em>, Lockheed Martin<br> <em>Jane Krueger </em>, Rockwell Collins<br> <em>Gary Kirchner</em>, Honeywell<br> <em>Andrew Paradise</em>, Northrop Grumman<br> <em>Richard Kidwell </em>, ITT Industries</span></p> </div> <p align="center" class="style7"><br> </p> </div> </td> </tr> </table> </div></td> <!-- InstanceBeginEditable name="Body1" --><td width="70%" rowspan="2" valign="top"><div class="body_style" id="body_content"> <p align="center"><img src="images/wireless.gif" alt="title"></p> <p class="body_text"><span class="style20">I</span>n order to evaluate the manufacturing technology and understand the structural behavior of a new, high-speed aluminum hulled ship, the U.S. Navy installed a dense system of metal foil strain gauges throughout the experimental littoral combat ship Sea Fighter (FSF-1). To reduce the installation cost and effort, rather than wire each sensor individually, a wireless hull monitoring sensor network was used. But what is a wireless sensor network?</p> <p>A wireless sensor is simply a device that monitors an input and then sends information using either radio waves or infrared light. A garage door opener and a television remote control fall under this broad definition of a wireless sensor. Both monitor their pushbuttons and broadcast a wireless signal when the buttons are pressed. For a wireless sensor s transmission to be effective there must be a receiver within range. A garage door opener remote is ineffective if pressed from more than a few houses away because the power that it transmits is low.</p> <p>Wireless networks are common in everyday life. Anyone that has laptop computers connecting to the internet through a wireless router is working with a wireless network. In a network, multiple sensors (the computers) transmit and receive data through a base station (the router). In a home computer network, each computer must be within range of the router to communicate with the network. This configuration is known as a star network, because all communication radiates out from the router like a star (Figure 3-1). Again, there is a finite distance that any sensor can be from the base station because of the power limitations of the radio frequency (RF) signal. That distance can vary based on the surroundings, physical obstacles, or electronic interference. </p> <p align="center"><img src="images/fig3-1.gif"></p> <p>Another type of network is called a mesh network. In a mesh network, each sensor communicates only with neighboring sensors and those sensors in turn communicate with their neighbors until the data reaches the base station (Figure 3-2). This method is similar to the children's game of "whisper down the lane." The wireless network transmitter/receivers are called "motes." Using a mesh network for data transmission has multiple benefits in the world of high reliability data collection. Some of the benefits are reduced cost of large sensor networks since there is no need to hard wire each sensor, extended transmission range to the base station, lower power consumption, increased reliability of data, and the ability to easily add more sensors to the system at any time.</p> <p align="center"><img src="images/fig3-2.gif"></p> <p>The reduction of the cost of installation of a large data gathering system by using a wireless sensor network over hardwired sensors can be significant. In a hardwired system each sensor needs cabling to transmit the data and possibly a power supply. With a wireless sensor network the installation can be as simple as installing the sensor and turning on the transmitter/receiver. The typical battery life on modern wireless network motes is measured in years. This is significant in that transmitting a radio signal takes a large percentage of the mote s power. Engineers have developed a method of transmitting that reduces the power used by the mote.</p> <p>The method of time division multiple access, or TDMA, allocates a small section of time for each mote to transmit its data to the next mote. Each mote pairing is assigned a time in the system cycle to communicate (Figure 3-3). When mote 1 is scheduled to transmit to mote 2, mote 2 will turn on its receiver and "listen for a communication. If it does not receive communication, it will shut down or "sleep" until the next time it is supposed to receive data from mote 1. If mote 2 receives a transmission from mote 1 it will send back an acknowledgement (Figure 3-4). Each mote will "wake up" at the times it needs to transmit or receive, and "sleep" when there is nothing scheduled. This allows the mote s radios to run on a duty cycle of less than one percent, which extends the battery life into the multiple years range.</p> <p align="center"><img src="images/fig3-3.gif"></p> <p align="center"><img src="images/fig3-4.gif"></p> <p>A basic fact of RF transmission is interference. Interference can come from another RF source transmitting nearby, or a solid body that blocks the radio path between the transmitter and receiver. One method to overcome the changing environment of RF interference is to change channels. The IEEE standard 802.15.4 defines the frequency bandwidths allocated to wireless personal area networks (WPANs) as well as the number channels in each bandwidth. Most of the bandwidths support between 10 and 30 channels, so the ability to channel hop to avoid interference is good. Some of the commercial systems that are gaining acceptance use a pseudo-random channel hopping where the pattern is programmed into all motes. Once the system is synchronized the motes change their transmit and receive channels after each system cycle.</p> <p>To increase the reliability of the data transmission, the motes are programmed to work with a feedback system using acknowledgement of transmissions. In the example shown, mote 2 was expecting to receive a transmission from mote 1 at a certain time. If it does receive a transmission, it will switch from receive mode to transmit mode and send back an acknowledgement to let mote 1 know that the transmission was received. At that point, mote 1 will erase the data that it sent and get ready to acquire data for the next transmission cycle. If mote 1 does not get an acknowledgement of receipt from mote 2, it will try to communicate with a different mote (mote 3) on mote 3 s listening schedule. Mote 1 will try to get its message through to one of the motes that is closer to the base station in this manner until the data is transmitted. In this manner, the data is continually relayed closer to the base station by hopping from one mote to the next. Some commercial systems are transmitting with 99.9995%1 success rate using a combination of channel hopping and multiple path transmission attempts with the acknowledgement of a successful transmission.</p> <p>When wireless signals are broadcast and received in any network, the possibility of data corruption exists. Wireless sensor network engineers have overcome this challenge by encrypting the data in various ways. In a secure wireless sensor network, the acknowledgement of receipt of a data packet is not released until the packet is verified as authentic. Data encryption often relies on including an identifier as to which mote sent the data. If the receiving mote gets an information packet that has different sender identification than the expected transmitting mote, the packet will be disregarded as false data and the information will be disregarded.</p> <p>Most wireless sensor networks are designed so that the addition of new sensors is transparent to the user. As soon as a new mote is activated, it begins to transmit and receive signals that allow it to join the network. The network communication cycle will typically have some time slices allocated for the discovery of new motes. This feature allows the network to self form as new motes are added and self heal if any motes are permanently removed.</p> <p>Wireless sensor network applications are increasing in both the commercial and military arenas. Park services are using wireless sensor networks to monitor both vehicle and foot traffic on park roads and paths. The military benefits from similar applications where seismic and magnetic wireless sensors can monitor the perimeter of a military base or give early warning of enemy troop movements on major roads. Motes can be installed on fleet vehicles and the sensors can transmit GPS position information to allow real time monitoring of the movement and location of each vehicle. Accelerometers and strain gauges can be placed at critical points within the hull of a ship to monitor stress and trigger performance based maintenance.</p> <p>Additional information about the installation and operation of wireless sensor networks, can be found in the articles "Tech Tips: Sensor Installation" and "Energy Harvesting for Wireless Sensor Networks" also appearing in this issue of Empfasis. For answers to your specific questions on wireless sensor networks and their potential applications, please contact the EMPF at 610.362.1320.</p> <p>References</p> <ul style="list-style-type: decimal"> <li>Pister, Kris. "From Smart Dust to Smart Plants: The Evolution of Wireless Sensor Networks." Dust Networks. ISA Expo 2008, 14 Oct. 2008. Web. http://www.wmsmedia.com/Dust5/player.html</li> </ul> <p>General References</p> <ul style="list-style-image: url('images/bullet.gif')" > <li>"Monitoring of a High Speed Naval Vessel using a Wireless Hull Monitoring System." Jerome P. Lynch et.al. Web. http://eil.stanford.edu/publications/kincho_law/IWSHM_Paper_SeaFighter%20(FINAL).pdf</li> <li>"Technical Overview of Time Synchronized Mesh Protocol (TSMP)." Review. 20 June 2006. Web. http://www.dustnetworks.com/cms/sites/default/files/TSMP_Whitepaper.pdf</li> <li>Sikora, Axel. "ZigBee Competitive Technology Analysis." ZigBee Alliance. 6 Nov. 2006. 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