Wednesday, August 15, 2012

Monitoring the Charles River

Sophomore Year

My Sensor Buoy Adventures


Every Civil and Environmental Engineering major at MIT takes a lab class their second semester sophomore year loosely titled "1.102: Civil and Environmental Engineering Design II". A more descriptive title, although less likely to attract freshman to the major, would be "1.102: A Crash Course in Everything Course 6".

The class begins with a combination of lectures and labs meant to teach students circuit design and one rather obscure computer program (the name of which I have since forgotten. That kind of obscure). The effectiveness of this teaching method is then tested in the last month by throwing students into a fabrication project which ideally fuses computer, electrical, and structural engineering into a neat, testable package.

When I took the class I chose, along with three other people, to construct a sensor buoy for deployment in the handy river near campus. Of course, the final destination of the buoy was nowhere near as interesting as the capabilities we wanted to implement: thermistors, an anemometer, and wireless data collection so that information could be collected and analyzed in real time.

Spoiler alert: we were getting only a little in over our heads.

But at least we could be in over our heads systematically. The project breakdown was fairly simple:

The Structural Component

We put two people on design and all four on implementation.

The materials were fairly intuitive: PVC pipe framing with foam floatation devices surrounding the joints which contacted the water. The electronics were housed in a large, 6 in. PVC pipe set in the center of the structure.

I drafted some initial sketches of the design using some old sketch-up program I have long forgotten. Back in sophomore year, I was of the "do it reasonably well so it works" project mindset, and cared more about getting the relevant structural aspects documented quickly than making that documentation look pretty.

Hence the following, rather primitive diagrams:
Side view of buoy structure: pink pieces are foam blocks and the grey are PVC piles. Black components represent sensor locations.

Top view of buoy structure

As simplistic as they are, the diagrams served their purpose. Putting the PVC structure together was a fairly simple task. Then we began working with the foam.

The original idea was to cast foam blocks around the joints of the PVC to seal them from the water (more an extra measure--we were waterproofing them as well). However, our attempts to work with hand-poured polyurethane foam involved hastily-designed aluminum foil molds (engineering on the fly!), poor approximations of ratios, early weekend mornings (which may have significantly contributed to the first two), and a nice large stain on the concrete floor of the basement undergraduate lab.

We tried to clean that last one up.

And then we switched to pink foam blocks, cut everything to size, zip-tied it around the joints, and were much happier for it.

The Electrical Component

I have to say, this project began with good intentions. We were rather systematic about it; in the weeks we were wrestling with the exquisite mess pourable foam can make, we were also drafting and soldering a circuit to link the sensors to a microcomputer.

The microcomputer was a bit of a fanciful addition on our part. Apparently the challenge of combining a host of sensors with wireless data collection was not enough for this project; we also needed to toy with a technology that none of us, and none of the professors, understood well.

The microcomputer was found and purchased by something resembling group consensus, steeped in ignorance of what out of its capabilities we actually needed. Of course, the brilliant red board seemed harmless enough at first. When we connected all of the sensors and wireless chips to the microcomputer through our fabricated circuit board, we measured reasonable voltage/current readings and generally saw things run smoothly.

The circuit board was a little thing I found lying in a pile of discarded electronics at MITERs, the student shop of MIT. The rest of the circuit was similarly pieced together from things lying around; true to the MIT spirit, I worked on it mainly on nights and weekends when the student lab was not open.

It was drafted in Eagle, but again the diagram is a painful reminder of my inadequate grasp of circuitry (and, for that matter, Eagle). So you must contend yourself with this snapshot of the final product:

The final circuit board (digital compass for buoy orientation is the small chip on the left). Every component that looks like it is missing was actually housed in a sensor or a microcomputer.