Tidmarsh Sensor Node Platform

Sensor Node PCB Tidmarsh Sensor Node PCB (Rendering)

A significant portion of the real-time data from Tidmarsh comes from the wireless sensor networks that we have installed. The sensor networks are comprised of custom sensor node hardware. This platform combines a standard set of readily available and low-cost sensors with a microcontroller and low-power radio to perform measurement and communication functions. The node is powered by AA batteries and is designed to run for years between battery replacements. The platform and protocols are extensible, allowing additional sensors to be added to the network at specific locations.

Sensors and CPU

The sensor node contains a standard set of sensors. These are parts that could be easily added to the PCB. They form the baseline set of metrics that all of the sensor nodes on the Tidmarsh network can measure.

The sensor node uses an Atmel ATxmega128A4U processor. This 8-bit CPU provides a useful range of communication interfaces and low sleep current with flexible wake-up options. It is running a custom real-time operating system that implements cooperative multi-tasking and scheduled task events. Nodes are field-upgradeable and manageable through an over-the-air update protocol so that new features can be implemented without servicing the nodes.

The sensor node can optionally contain a VS1063 audio codec to record and encode MP3 or Ogg Vorbis audio; however, this functionality is not presently used as the power and bandwith on the node are limited. To stream live audio from Tidmarsh, dedicated audio streaming nodes have been installed instead.

Communications

The sensor nodes contain an AT86RF231 radio and an inverted-F PCB trace antenna. The radio implements the 802.15.4 MAC in hardware. The Tidmarsh network uses a slightly modified version of the Atmel Lightweight Mesh stack. This permits multi-hop routing (using ad-hoc on-demand distance vector routing) without adding significant complexity on top of the 802.15.4 MAC.

Sensor measurements are reported using a compact binary protocol that describes the type and format of each sample. Multiple namespaces for sensor type IDs are supported. A flexible decoder system on the sensor network gateway parses the sensor messages into JSON with full metadata.

Sensor Node in
                the field Sensor node installed in the field.

Power

The sensor nodes are powered by three AA batteries. The quiescent current of the sensor node in sleep mode is less than 20 μA. Depending on the configuration of additional sensors and sampling rates, the batteries will last for several years in the field. The sensor node also contains an option to use a rechargeable lithium ion battery with an integrated solar charger and small external panel for nodes with higher power consumption (such as network repeaters).

Expansion

The backside of the sensor node contains basic expansion capability via several Molex connectors. These connect to GPIO, analog, and serial pins on the MCU.

In the standard configuration, the I²C sensor bus is connected to a waterproof circular connector on the outside of the enclosure. This enables the connection of other devices, such as the analog expansion box.

Deployment

The first Tidmarsh sensor nodes were hand-assembled test prototypes, of which three were constructed and installed in a prototype installation at the Arm. The hardware was then revised and 150 nodes were manufactured. To date, about 80 have been installed at Tidmarsh.

The first production nodes were installed on Cell 3 in February of 2014, initially as a 9-node deployment. By the beginning of 2015, the cell 3 deployment expanded to 64 nodes, also covering part of cell 4. Currently, most of the nodes on cells 3 and 4 have been temporarily removed while the excavators reshape the landscape. They will be reinstalled as soon as possible after the restoration work is completed.

A small second deployment was added during the summer of 2015 at the greenhouse. This deployment currently contains three nodes.

In the fall of 2015, several sensor nodes were installed at a third site, at the former location of Beaver Dam Pond (the impoundment). There are currently 12 nodes installed, with a deployment of 20+ planned for the near future.

Resources and Links

Further documentation and source code for the Tidmarsh sensor node can be found on the project's autogenerated documentation page.

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