Instrumenting Manx Shearwater Burrows
I certainly can't complain that I have a boring job. In addition to being responsible for architecting and building some very interesting horizontal software components for computational science investigations, I get to apply my electronics and mechanics skills to creating some interesting special-purpose components. These can range from collaborating on creation of a sub-15-gram solar GPS logger backpack to creating special purpose sensor interfaces to engineering a mechanism to get birds to weigh themselves every time they enter or leave their burrow. Its that last one that I will talk about today.
Among the goals is to try to determine how much food a Manx Shearwater brings home and how much their weight varies over a nesting season by weighing them with gram accuracy each time they enter or leave their burrows. In addition to weight, we need to read the bird's identification (and RFID tag) and sense motion (so that we only power up the very hungry scale and RFID circuits when they are needed). Many of the birds will also be carrying a preliminary version of the GPS backpacks, so we will also know where they have been foraging. In this article, I'll show you how we went about getting the birds to weigh themselves.
This Manx Shearwater research project on Skomer Island (off the western coast of Wales) is led by Prof. Tim Guilford of Oxford and Robin Freeman, a Post-Doc Researcher here at Microsoft Research in Cambridge, UK. Tomek Naumowicz is an endlessly energetic doctoral candidate from Freie Universitat Berlin and Tomek has built the firmware that is being used on the ScatterWeb sensors that monitor each burrow. My contribution has been to design and build some additional circuit cards along with the outdoor housings, the weighing tubes, and a solar-powered WindowsCE + GPRS base station for relaying the data back to the mainland.
As you will see below, the goals of cheap and fast build-out of less than a dozen low-environmental-impact weighing tubes means that the engineering is closer to the Scrapheap Challenge end of the spectrum than the NASA/JPL end.
The central component of the weighing tubes is an aluminum strain gauge harvested from a Tesco kitchen scale. The Tesco scale has 2g resolution from 0-5kg, but through the use of a different A/D converter, different choices for the amplification of the strain gauge signal and a narrower total weight range, we manage to get several times that resolution, down to the sub-gram range.
Out of each scale, we get remove the strain gauge assembly (seen lying on the right half of the dissassembled scale above), and bend the strain gauge base slightly to match the curve of the tube where we will mount it. The left photo below shows gauges before and after bending. The working part of the strain gauges are the aluminum bars, each of which have a patches of material applied which varies in resistance in proportion to the stress on the bar. In the original scales, an amplifier and A/D converter takes the signals from these gauges and amplifies and converts the signals into the weight readouts displayed on the readout. We replaced this with our own amplifier board (green board in the right-hand picture below). Our A/D converter is part of the ScatterWeb board. The amplifier takes the micro-volt changes given off by the strain gauges and boosts it to a 0-2V range which will be interpreted by the ScatterWeb A/D converter a meter or two away.
The strain gauges are mounted in a 5" piece of PVC drainage pipe. They are counter-sunk so that we have room to nest a 4" tube inside this assembly. An earlier version of the weighing setup used a 6" outer tube (which actually has a nearly 6.5" O.D.) but we found that this required too much excavation of the bird's burrows and would have permanently and irreversibly altered the habitat. This led us to this smaller and less-invasive design.
In the right-hand picture above, you can see that the Tesco scales came with two different sized strain gauges. The one on the left came from scales purchased through the Internet. The smaller assembly on the right came from scales purchased in Tesco stores. The inside of the scales had mounting holes for both sizes of strain gauge assembly - we had to accommodate both as well. The aluminum bar in each gauge has one 'free' end and the other end fastened to the white metal base. The important measurement was to make sure that the inner tube would be centered above the free end of the aluminum strain-gauge bar so that the weight would bear directly down on this free end of the bar.
Next, the 4" inner tube was fastened to the strain gauge forming this axial assembly. In the field, this tube was inserted into the burrow entrance and the bird would have to pass through this inner tube on each trip in or out. The axial gap was covered in the field. Field tests indicated that the birds were perfectly comfortable moving through this tube.
The amplifier boards were attached to the inner tube at about the 3:00 position and a small access hole allowed for connection of the cable that would connect the amplifier to the ScatterWeb processor board.
The completed tubes were combined with the ScatterWeb nodes and supporting circuitry and 12V gell batteries and prepared for the field.
The deployment to Skomer Island is just underway now. Robin is making frequent trips to Skomer to deploy and configure hardware. There are lots of unanswered questions about the equipment and its installation. It is unclear what quality of readings we will get from this assembly. Robin did a significant amount of experimentation with weights so that we could calibrate the circuitry. Since the birds are almost continuously moving while being weighed, an actual weight will be a matter of some statistical inference rather than being a straightforward readout. This will probably substantially reduce the effective accuracy of the scales, but hopefully not down to where the data is not meaningful. There are also significant challenges remaining for Tomek as he tweaks the MSP430 code on the ScatterWeb for maximum power savings while juggling the limited interface resources and nearly unlimited need to talk to every interface and storage device all at the same time.
And for me, there is still the matter of the base station. The philosophy we want to support is one of 'Deploy and Depart' research, where you deploy the sensors and then go back to your office to monitor the data stream over a long period of time, with little or no field maintenance. To realize that goal, we need our base station in place. Last year's base station was a partial success and we want to take those lessons and improve on them this year, but the birds wait for no one, and with time running out before hatching, we had to ensure that the data acquisition equipment that I have talked about here, was working first. I will follow up in this blog with more on last year's PC-based base station and this year's solar-powered Windows CE base station.
You will be able to read more about the actual results from this research by tracking the publications of the principal investigators listed in this article.
