As the cost and size of wireless and sensing equipment has dropped, the range and number of applications for wireless sensing has increased proportionally. These include single sensor systems, like the weather balloon and outdoor thermometer, and sensor networks where a cluster of sensor nodes work together to form a distributed sensing and communication network.
If a single-shot or short term measurement is being made, the nodes can be recovered or abandoned once the batteries are exhausted. For ongoing measurements in easily accessable areas, the batteries may be replaced as necessary. However, for long term monitoring in inaccessable areas or hostile environments, the network needs to be able to operate for substantial periods of time (perhaps for the duation of the entire mission) without maintenance. Unless the application can accomodate a large, heavy battery, this means that the node will have to be extremely efficient in its use of power, or will have to have some means of extracting the necessary energy from its environment.
Nodes will be placed at intervals of 50 ft. along the length of each span. Measurements of temperature, vibration, and strain are desired at the location of each node. Sensors for temperature and vibration should be part of the node. Strain will be sensed by an external strain gage. Vibration should be measured by a three-axis accelerometer over an interval of 10 seconds with a bandwidth of 100 Hz, and be represented by the RMS value for each axis. A complete set of measurements should be made once an hour. Data must be available at the collection node within 6 hours of the time it is taken.