Hardware
We designed several custom hardware modules to accomodate the specific needs of LUSTER, including the seven-channel light sensor, energy harvesting module, storage node, and the in-situ user interface node. All these modules were designed to be compatible with MICA2 and MICAz motes by Crossbow Technologies.
Sensor Node
LUSTER needs a large number of light sensors deployed in various configurations. The distance between the light sensors would vary between a centimeter to half a meter. The straightforward solution of having one sensor per node without any wires, but it limited the minimum distance between nodes and demands a large number of Micaz nodes, which leads to large overhead in communication and power consumption.
Therefore an alternative solution was implemented, having a self-contained network node with up to eight sensors attached. The sensor attachment is by cable that is up to0.5 m long. This allows for flexible deployment as well as sensor data aggregation on the network node and less overhead.
Two versions of such a system were implemented: a simple eight-channel sensor node—so called “Medusa” sensor (shown in Figure 6(a)) that can have a variety of sensors attached, and a more sophisticated hybrid node, “Solar Dust,” that also has eight channels which can be multiplexed between energy harvesting and sensing (see Figure 6(b)).
Figure 6: Eight-channel spatially reconfigurable light sensor modules designed for LUSTER.
Storage Node
A module that interfaces a removable SD/MMC flash storage card to a MICAz mote was implemented for innetwork storage (see Figure 7(a)). The card is accessed in SPI mode due to the ease of royalty-free integration with the existing controller on the mote. However, the hardware allows for a proprietary SD interface implementation, providing the implementers have obtained the proper license.
Figure 7: Storage and SeeMote user interface modules.
Validation Node
Deployment time validation tool SeeDTV is enabled by the in-situ user interface module SeeMote (see Figure 7(b)). The module has a 128 x 160 pixel color LCD that is 1.8 inches across—sufficiently large to display information and diagnostics about the deployment, yet small and light enough to fit in a shirt pocket. When deploying a WSN in a remote area, a small, lightweight, energy-efficient device that does not require frequent charging is favorable. SeeMote is such a device and beats its next competitor by several times. We also developed a lightweight GUI library that allows the display of simple text and graphics primitives. This enables SeeMote to be a versatile tool for WSN applications where a user interface is needed with IEEE 802.15.4 communications capabilities.
Packaging
The packaging for the sensor nodes had to be lightweight and compact for easy transportation to the remote site and weather-proof to protect the hardware from damage by corrosive sea salt.
We chose Pelican cases (shown in Figure 8) for packaging since they are easy to operate, have a good reputation of being water-tight and durable, have transparent acrylic covers so we could visually check them without opening if needed, and come in different sizes appropriate to the various sensors. This solution met our expectations.
Figure 8: Packaged “Medusa” sensor module.