Aerial robot uses wireless link to transfer sensor data

Approximately 70 per cent of solar energy is absorbed by the Earth's atmosphere. As the Earth's surface emits this energy in the form of thermal radiation, the atmosphere naturally captures and recycles a large portion of it, thereby keeping the planet warm. This process is known as the greenhouse effect. Many scientists believe that the greenhouse effect has recently been artificially enhanced by the increased emission of gases that absorb infrared radiation such as carbon dioxide (CO2), methane and nitrous oxide. The increased absorption of thermal radiation may contribute to the Earth's climate change, which has become known as known as global warming.

To gain a better understanding of the impact of the emission of greenhouse gases on the environment, researchers are conducting a study at La Selva Biological Station in the Costa Rican rain forest to measure the exchange of CO2 (also known as the carbon flux) and other materials between the forest floor and the atmosphere. The area under observation lies within a 3900-acre tropical rain forest that averages 13 feet of rainfall per year and is located at the confluence of two major rivers in the Caribbean lowlands of north-eastern Costa Rica.

This area was chosen for observation because rain forests are naturally rich in biodiversity and are carbon sinks, meaning they function in a manner that is opposite of a human lung, absorbing CO2 and releasing oxygen into the environment. Tropical rain forests absorb more CO2 than any other terrestrial ecosystem and affect the climate locally and globally. However, in rain forests, carbon flux is unusually complex because of the multilayered, diverse forest structure.

The 'Gap Theory' is a hypothetical explanation for the complexity of carbon fluxes. It hypothesises that small, open areas in the forest canopy - caused by natural processes such as tree falls - function as chimneys, pulling out CO2 produced by soil respiration and leaking it into the atmosphere at local points. Due to the difficulty in making measurements from multiple points on the forest floor and corresponding points in the canopy, or in a 3D manner, a balanced budget for CO2 fluxes has been historically difficult to measure.

Wireless sensors

The wireless measurement technology deployed in Costa Rica is a networked infomechanical system (NIMS) based on LabVIEW software and CompactRIO hardware. The NIMS application was developed at the University of California Los Angeles (UCLA) by the Center for Embedded Networked Sensing (CENS). CENS develops embedded network sensing systems for critical scientific and social applications. It is a National Science Foundation (NSF) Science & Technology Center with an interdisciplinary and multi-institutional support structure that involves hundreds of faculty engineers, graduate student researchers and undergraduate students from partner institutions throughout California. CENS has received between $4million and $6million in NSF funding per year for the past six years and will continue to receive the organisation's financial support for the next four years.

To increase the accuracy of the measurements being taken and to determine the effects of uneven carbon flux, the researchers developed a mobile, wireless, aerially suspended robotic sensor system that is capable of measuring the transfer of carbon and other materials between the atmosphere and the Earth. There are a wide range of measurements necessary to characterize the carbon flux, including temperature, CO2, humidity, precise 3D wind movement, heat flux, solar radiation and photosynthetic active radiation (PAR).

In the past, acquiring this breadth of measurements required the use of multiple data loggers from different vendors. CENS selected a modular approach using CompactRIO. The CompactRIO platform supports a wide range of measurements using C series modules from National Instruments and third-party vendors. The flexibility of CompactRIO addresses the current measurement needs with a single platform while still leaving room to easily add new measurement modules in the future. The system, called SensorKit, is designed to provide flexibility, ruggedness, mobility, and ease of use by utilising LabVIEW and CompactRIO technology.

Robotic shuttles

Three of the SensorKit systems have been deployed at La Selva Biological Station for the first phase of field trials. The SensorKits are equipped with a variety of instruments, including tools for conducting basic meteorological measurements, sonic anemometers, infrared sensors and radiometers. All of the environmental data necessary to conduct the carbon flux study is acquired through a modular approach. The wireless sensor systems are arranged at points on the forest floor and on aerially suspended robotic shuttles, thereby creating what is thought to be the first environmental monitoring system capable of taking measurements in three dimensions.

In the initial test deployment, the wireless mobile sensing platforms traversed cables along three separate transects of the forest understory. During the deployment, the shuttle stopped at 1m intervals along each transect for 30s to enable the sensors to equilibrate and take the required measurements. Each transect pass required 30minutes and each transect ran for 24hours.

Benefits of LabVIEW

By implementing the system using National Instruments modular hardware and software, we developed a flexible system with the additional communication and configuration advantages of LabVIEW software. CompactRIO was selected as the central measurement unit and the NI Compact FieldPoint network interface with cFP-180x controllers were selected for distributed wireless measurements. The NI Wireless Access Point (WAP-3701) was chosen to transfer data between the distributed sensors, the towers and the canopy floor.

We selected LabVIEW to connect to these distributed wireless measurement platforms and program the embedded CompactRIO processor. Using LabVIEW, we can supply measurements to local researchers in different data formats so that they can perform post-analysis. Because of the flexibility of LabVIEW, we can configure measurement types, select channels, and even add scaling from a laptop connected to the system.

LabVIEW also provides advanced analysis tools for real-time embedded processing to perform local mass flux analysis and post-processing for remote researchers. In addition, LabVIEW is equipped with an HMI, so we can see real-time measurements. Prior to the development of this real-time analysis system, researchers typically spent a long time collecting large amounts of data on-site to bring the information back to their labs for further analysis.

Expansion plans

In conjunction with the system designers at CENS, we plan to expand the system by adding high towers approximately 45m above the forest floor with canopy walkways and increasing the total number of measurement systems. Students from around the world can access the canopy walkways to experience the unique atmosphere and biodiversity of the rain forest canopy.

Additionally, we plan to deliver remote data access through the internet to researchers and students who are not on-site. Using a web browser and the web capabilities of LabVIEW, researchers everywhere will be able to access and download live and archived data for their own analysis.

Performing additional measurements using a 3D measurement system will provide the data needed to validate our 'Gap Theory' hypothesis that carbon transfer occurs unevenly across the rain forest. Gaps in the forest canopy are sources of carbon loss while the canopy is a source of carbon absorption, which increases as the density of canopy vegetation increases. With this research, scientists will better understand the carbon absorption impact of rain forests and potentially calculate the carbon absorption value of an acre of forest, ultimately providing a method of quantifying carbon credits.

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