Researchers at the University of New Mexico (UNM) are developing a ZigBee-based wireless RFID sensor system to help scientists better understand the natural environments found within caves and lava tubes, as well as potential changes in Earth’s climate. The technology, the researchers believe, has the potential to be used not only in natural chambers, but also in mines—and possibly even on other planets, such as Mars, where lava tubes are thought to be where life would most likely be found.
Environmental factors outside a cave can affect what occurs underground. Changes in air pressure, caused by weather fluctuations, can create a breathing effect within the caves, as warm and cool air circulates in and out. Much can be understood about the planet, the scientists say, based on the conditions belowground, though little monitoring of conditions within deep or inaccessible caves has been performed to date. Any cave instrumentation employed for such monitoring would need to be deployed in such a manner as to not disturb the cave environment.
The research team first began considering the use of sensors in caves following aboveground studies involving a turbulence sensor (a thermocouple that measures fast fluctuations in temperature). In 2009, the research team—led by Anders Jorgensen, an electrical engineering assistant professor at the university—began planning a system for use in caves, in which a sensor network could track turbulence, temperature, air pressure and humidity, and then transmit that data at regular intervals to a back-end system, where the information could be stored and evaluated.
Later that year, the researchers developed a wireless system that uses 2.4 GHz RFID sensors to collect data regarding conditions within a cave. The system the team built included temperature, air pressure, humidity and turbulence sensors, though the pressure information did not prove useful, Jorgensen says, since the measuring apparatus was not sensitive enough to detect the pressure changes that the team sought to measure. Each sensor was wired to an onboard processor with a battery-powered ZigBee transceiver from Freescale Semiconductor.
During the 2009 deployment, graduate students placed a total of six sensors in natural flat areas near the walls of Junction Cave, in El Malpais National Monument, N.M., located about 90 minutes west of Albuquerque. The sensors were positioned at several elevations within the cave, which was selected, in part, due to its high ceilings. Some sensors were placed close to the floor, while others were located in the middle or near the ceiling of the tunnel. The researchers kept the sensors within the first few hundred feet of the 1,000-foot-long cave, spacing them up to 30 feet apart in order to stay within read range.
Each sensor device (including the ZigBee transceiver) measured approximately 2 inches by 3 inches in size. The RFID technology was part of Freescale Semiconductor’s MC1321x evaluation kit that the university purchased. Each sensor formed a ZigBee mesh network, transmitting data to the next sensor in the mesh until the transmission reached the hub—a small computer running on a Linux system—positioned about 45 meters (147 feet) into the cave, in the center of the sensors, and loaded with RFID software developed by the research team to receive, interpret and manage the data. The sensors measured the humidity, temperature and turbulence at regular intervals, and transmitted that information, along with that sensor unit’s unique ID number. The hub software collected the data, interpreted the measurements and linked them to the sensors and their locations within the cave.
After a 24-hour period, the research team had a collection of data indicating what transpired within the first several hundred feet of the cave in which the system was installed. There was a heavy rain on the night that the system was put in place, but the cave’s interior remained dry. The data indicated that the conditions within the cave changed little during the rainstorm, though there was stronger air turbulence near the roof of the cave overnight, which appeared to coincide with the period of rain. One surprising result, Jorgensen says, was that the mere presence of people appeared to affect the readings. Temperature and turbulence rose when researchers or visitors were in the cave, for example. This type of data is important to scientists, he explains, since there is a concern that a human presence within the caves could compromise their ecosystems.
The study was conducted only for a 24-hour period because the park mandated that the research staff be on-hand at all times that the cave was open to visitors, which required students or faculty members to spend a significant amount of time at the cave. Based on the brief study, however, the researchers found that a wireless sensor system could reliably collect and transmit data within a cave, Jorgensen says. The group has also discussed setting up the system with a cellular connection, so that data could be sent from the hub to a back-end server in real time. That, however, was not accomplished during the pilot.
Subsequently, the group has developed a smaller, more effective system, Jorgensen says, using Texas Instruments‘ eZ430-RF2480 starter kit, thereby making it possible to build the ZigBee-based RFID tags much smaller. The students have found that the new system consumes less power than the previous system, and costs about $25 per sensor node. The nodes have yet to be installed in any caves, and such a project would require additional funding. The researchers found the transmission distance for the new, smaller node to be up to 150 feet with a clear line of sight. The new system’s operational time is expected to be 80 days, collecting a sampling from up to four sensors every minute. One power-saving function is the microcontroller’s ability to collect sensor and transmit data, and to then enter a sleep mode until the next collection.
The UNM researchers are continuing to develop the solution, Jorgensen says, and are working to find a way to ensure that devices could store their data in the event of a network failure. If funding allows, he notes, the group would like to use the new RFID sensors to build a larger network within a cave, and to attempt another evaluation. At present, however, there are no immediate plans to do so.