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A solar powered ‘Cloud’ instrument for Pachube

02 September 2010

A solar-powered, credit-card sized device is able to deliver its data directly to a targeted website through any commercially available access point or wireless router even in non-optimum lighting conditions

A recent application of the Cores Electronic Tag4M sensor tag proves the ability to run this low-power WiFi sensor tag from solar power and deliver readings to Pachube, which has emerged as one of the leading public web applications that enables people to share real time sensor data from objects, devices and spaces around the World. It is a combination of data-collection hardware and remote analysis/display software that utilises the Instrumentation Cloud, enabling users to read and control a sensor tag from anywhere in the World using any web-enabled device.

In some applications it is desirable to have the Tag4M tag powered by a solar cell in continuous operation. But how can energy be stored during the day to allow for use at night and supply enough current for WiFi communication sessions? In addition, the solar-power scheme’s overall size, complexity and cost must be sufficiently low to make it cost competitive with batteries.

Dr S Folea at the Technical University of Cluj-Napoca, Romania, researched the best way to meet the requirements of a sensor tag located in non-optimum lighting conditions but that nevertheless must provide data 24 hours a day. His design takes into consideration the power needed during the tag’s boot sequence, transmission period, measurement period and receiving period. The peak current required at any time is 210mA and the unit operates from 2V to 3.7Vdc. His final design powers the Tag4M from a solar cell connected to a capacitor array through a buck-boost converter, all augmented with a rechargeable lithium ion accumulator and a charger.

Read data values over the web
This set-up is more than a theoretical concept; it provides real-time feeds of temperature, pressure and humidity data as well as key system parameters on the tag itself, including solar-cell voltage and sleep time, through the Pachube website. While Cores Electronic provides the Tag4M Web Page Instrument (WPI), which allows designers to create their own WPIs that accept and display sensor tag information, public services such as Pachube are becoming increasingly popular as an alternative to developing a WPI.

In order to send sensor data to Pachube, it must be bundled into a script written in EEML (Extended Environments Markup Language), a protocol for sharing sensor data between remote responsive environments, both physical and virtual. In this case, an application running on a local PC reads sensor data from Tag4M over a wireless link, bundles the data into an EEML script in a format Pachube understands and sends it using TCP/IP write commands to the Internet and the Pachube website.

Finally, the Pachube website captures the data and posts it on a feed that is visible to anyone who logs on to that particular web page. In order to eliminate the PC in this process, Cores Electronic is developing a special Tag4M Pachube version with an onboard processor that will create the EEML script and send this feed through an access point directly to the Pachube website for display.

Users anywhere in the World can examine the sensor data and, if the set-up allows it, they are also able to send commands to digital I/O on the sensor tag. In this case, the experiment is running in Cluj-Napoca, Romania, the web page is hosted in London and users with a web browser anywhere in the world can access it.
Because it takes advantage of RFID technology, this small, credit-card sized measurement board (also called a WiFi tag) contains everything needed for true remote standalone operation. It starts with a WiFi radio chip that communicates with a nearby wireless access point, provides measurement circuitry including an A/D and digital I/O, and its CPU controls the operations of those subsystems as well as executing user measurement and control scripts. In addition, extremely low power consumption means that, depending on the frequency of wake-up periods, a tag can operate on a small battery or even from solar power.

Tag4M is supplied and supported by Adept Scientific in the UK, Ireland, Germany, Scandinavia and the Nordic countries.

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