today (aug 25, 2009) we did a number of experiments to determine the rssi from certain distances on all possible power levels on the mote. the first set of experiments were a pager body with a mote inside, usb port sticking straight up, lights facing towards the pole, pager facing straight at other testing mote. powered by usb extension from computer. other mote in purell unit facing straight at pager. these two are set up at a distance of two meters and allowed to gather data for 400 seconds, making 100 data points for each power level. we then moved them to one meter and a half a meter. the three dots for each point are the mean and one standard deviation above and below the mean.
we then turned the pager 90 degrees relative to its original position in the experiment above, to the right, and the purell unit 60 degrees to the left such that the mote and pager had 150 degrees relative offset from above, and redid the same experiment, with all power levels at 2m, 1m, and .5m. this was to see if orientation due to placement on belt or position around the purell unit matters. the control was a test with the purell and mote not turned at 2 meters, while the other readings are with the 150 degree orientation difference at the noted distances.
for our third experiment we moved the pager and purell unit as close as the bases would allow (.25m) and raised the purell to offset the mote by .3m. this is to test how the rssi compares at close ranges: at the front of the cleaning zone, the back of the cleaning zone, and in the “wolfpack.” we introduce the height offset in order to better simulate a hand washing environment: the mote will be on someone’s waist but the hand washing unit will be at a greater height than that.
the purpose of these experiments above are to demonstrate whether we can or cannot use the particular radio built into the telosb to determine who is washing their hands and who is not. we have included rssi data for all possible power levels, about 100 packets each power level in each configuration.
Another great set of experiments. Thanks for these.
Again, we don’t have a discussion section so I’ll try to see if I can interpret the results correctly. The top experiment demonstrates that if the mote is worn on the front of the person and facing the Purell unit, it is pretty easy to distinguish the person’s distance from the Purell with RSSI.
The second experiment demonstrates that if the person is wearing the badge on their hip, on the right side, disregarding the effect of the person’s potential interference with the transmission, it becomes more difficult to separate the person’s distance from the Purell. Some of the experiment is not clear. I’m not sure which direction the Purell was turned — left is ambiguous. Also, it’s not clear what the “control” condition was. We need to clear those points up.
The third experiment, which I think is the most realistic, again indicates that we can get about a 2 standard deviation separation in the RSSI between .25 and .55 distances, which, at least in this lab condition, would allow us to separate the washer from other people near the dispenser in the wolf pack mode.
Comment by gthomas — August 27, 2009 @ 7:30 am
your interpretations are indeed correct. i fixed the post to correct for the ambiguous wording. i hope this is satisfactory.
Comment by E M — August 27, 2009 @ 2:33 pm
Thanks for the clarification. So now I see that the graph in experiment 2 demonstrates that 2m in the ideal condition is approximately equal to (practically indistinguishable from) .5 m in the offset condition representing someone with the mote on their right hip approaching the dispenser 30 degrees from the right when facing the dispenser. That’s a pretty serious limitation, given our paradigm, because if a group of doctors was standing in front of the dispenser (say 1m) and someone tried to “slip in” from the side to wash their hands, it would be hard for the hardware to identify who was washing their hands.
Comment by gthomas — August 28, 2009 @ 10:01 am
[...] We expected to have the inner arc be large circles, indicating strong signal strength close to the dispenser and the outer arc be small circles representing smaller signal strength farther from the dispenser. That’s not clearly the case, although there is a trend. Note however, that not all experiments have turned out this way. Some experiments have shown differences that are more clear. [...]
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