One and a Half Million Seconds Off the Grid

My previous post described an initial experiment on collecting data from a solar-powered Sun SPOT. That experiment helped uncover and fix several bugs that caused disruptions in data collection. I repeated the experiment again last month using the red-090113 release of the Sun SPOT SDK to test those fixes.

This time around, the SPOT stayed off the power grid from Feb 12 until Mar 3 -- slightly over 19 days (that's more than 1.6 million seconds) -- reporting light, temperature and other readings every 10 minutes. Unlike last time, there were no occasions when the SPOT failed to enter deep sleep. There was just one disruption and it was caused by a mySQL table crash (still not sure about the cause), but the SPOT stayed up throughout. Its reported up time as well as the time spent in deep and shallow sleep kept rising at a regular clip throughout the experiment (see plot below). Overall, the SPOT spent nearly 95% of the total time in deep sleep, around 4.5% in shallow sleep and about 0.5% in active computation. The large gap from late Feb 22 to noon Feb 23 marks the disruption due to the table crash.

It rained frequently during this experiment (click here for the local weather and look in the "Events" column under "Observations"). The SPOT did not have any special weather-proof casing so I kept it indoors -- by the window in my office -- for the most part and only occasionally took it outdoors when dry. Direct Sun light recharged the SPOT's battery more efficiently. The second plot shows variations in the USB voltage supplied by the solar panel (in light blue), the output voltage measured at the SPOT's battery (in dark blue) and the estimated remaining battery level (in red). Spikes in the light blue plot indicate periods when the solar panel circuitry charged the SPOT's battery and caused an increase in its remaining capacity (red).

The normal discharge rate for the SPOT's built-in battery (rated at 700mAh) was nearly 12% per day and a full charge would have lasted about 8 days. However, the solar panel was able to replenish about 15-18% of the battery whenever it was sunny and I suspect that this figure could be bumped up to 25-30% with some simple optimization of its orientation.

The next plots show changes in light and temperature readings as measured by the built-in sensors on the eDemoBoard. The significant dip in temperature on the early morning of Feb 20 marks the only night the SPOT was left outdoors.

We collected 2559 samples during the experiment when we should have collected 2747; 82 were lost due to the table crash and the rest (106 or 4% of the total) can be attributed to the unreliable nature of the SPOT's UDP-like radiogram communication mechanism. The distribution of lost samples is shown below and does not include losses due to the table crash. It would be interesting to study whether packet loss is correlated to humidity.

The SPOT still had about 10% of its battery remaining when the experiment ended. So what terminated the experiment? Accidental exposure to Dihydrogen Monoxide aka water! On the evening of Mar 3, the SPOT was outdoors when it started to drizzle. I was attending a meeting in a windowless conference room at that time. By the time I became aware of the change in weather outside, it was too late. The rain had caused the SPOT to stop working (see picture below).

Picture of a SPOT with rain drops.

I was able to revive the main board subsequently simply by letting it dry out. The eDemoBoard, however, appears to have been damaged. Nevertheless, it was gratifying to note that it was human error, rather than a software or hardware bug, that brought the experiment to an end this time around.

NOTE: The plots above were generated using Gnuplot. An interactive, and way cooler, version (based on Simile timeplot from MIT) can be accessed here. One can look at individual sample readings by moving the mouse over these "live" plots.


Regarding the effect of humidity on packets, we found that the range of the spots increased with humidity in the rain-forest. We got a range of around 105 Mts. in 75-80% humidity and 1000-1200 Mts. altitude.

Posted by Vasusen Patil on March 19, 2009 at 07:10 PM PDT #

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Vipul Gupta


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