Monday, February 25, 2013

Making a home for a sensor node - with a laser.


Sensor nodes need a good home if they're to last any considerable amount of time. And if they're to be deployed outdoors, they need an especially robust home.

This can be complicated. Most sensors need to be exposed to the elements to obtain good readings. But expose these electronics to the elements too much, and you'll break them.

In a pursuit of finding the right balance between price and accuracy for a sensor node for professional and community journalists, I'm fabricating a prototype using open-source hardware and software, 3D printers, and now, lasers. Fortunately the "maker" revolution makes this process more accessible than ever.

Previously I wrote about working with Arduinos and temperature sensors. Continuing on the theme of sensor nodes for journalism, here are some details on the next step in the prototyping process.


Example of a Cotton Region Shelter, from the NOAA Photo Library.

Professional-grade housings for temperature sensors, specifically those used by the National Ocean and Atmospheric Administration (NOAA), are designed to provide as much ventilation as possible while still protecting sensitive equipment.

A classic type of housing is a Cotton Region Shelter (CRS), also known as a Stevenson screen. NOAA has specific rules not just about the dimensions and characteristics of these housings, but also about placement.

For example shelters are to be kept four to six feet from the ground, and at least 100 feet from any paved surface. These guidelines help ensure precision, along with accuracy. For more details about these standards, read the Requirements and Standards for NWS Climate Observations NWSI 10-1302 (pdf).

Specifics will be addressed later, but for now it's important to set the ground rules for how the housing should be designed. Namely, it should be louvered or slotted to allow sufficient air circulation. Otherwise, heat could build around the sensor, much as it did with the previous sensor housing. In a clear plastic container, the first temperature sensor prototype averaged 4.1 degrees Fahrenheit above ambient.

How do you go about designing, prototyping, and manufacturing a precision, custom housing for an Arduino, sensors, and power source? The "maker" revolution supplies some good tools.

The Epilog Helix 350 Laser Engraver. Photo from the Champaign-Urbana Community Fab Lab.
There's few tools out there that can cut as precisely as a laser. They're also handy at making intricate engravings.

The Epilog Helix, made accessible to the public thanks to the Champaign-Urbana Community Fab Lab, is a 50 watt carbon dioxide laser. It will "print" out a properly-formatted PDF document on a variety of materials, including wood, acrylic, glass, and leather.

Interestingly enough, the Epilog actually shows up as any other printer on the computer. To use it, you simply open the PDF of your design in an Adobe Reader and select "print," like any other document. It's only when you select the printer properties that you are exposed to the laser's settings such as power, frequency, and speed.

Any vector image with a line width of 0.001" will be cut out of the material, while raster images will be etched.

Being as precise as they are, laser cutters are well-suited to make interlocking components. When applied to a 3-dimensional perspective, these same technique that allows components to interlock can be applied to make boxes and more advanced shapes. This is called "press-fit."

Press fit boxes, photos published by http://www.flickr.com/photos/sjunnesson/

It's possible to draw these interlocking teeth individually with the right software, but the arduous task of making press fit boxes has been expedited thanks to programmers of open-source software.

There's a website that will spit out a press fit box PDF, given the dimensions for width, height, depth, and material thickness. For this project, I used a box generator app created from Processing (a programming language that forms the root of the Arduino IDE).

Apps to create ready-made press fit boxes are great, but those designs won't have any doors, slots, louvers, or holes to mount circuit boards. For that, a separate drawing program is required.

Inkscape is the preferred open-source drawing program used by the Champaign-Urbana Community Fab Lab. It will output in several formats, including .SVG and .PDF. Measurements can be changed on the fly between inches, millimeters, and pixels. It's also got a handy grid layer, which makes it easy to eyeball your measurements and "snap" your shapes to those measurements.

Template for my outdoor sensor housing, drawn in Inkscape.
A board of birch wood with 1/8" thickness was selected for the sensor housing, but that wasn't used for a mockup. Instead, 1/8" scrap cardboard was acquired, so that all the pieces could be tested without using the more expensive wood.

After some initial difficulty with printing (originally my lines were not 0.001", and the interlocking teeth were not joined into a complete line), the laser cut through the cardboard and produced two of the necessary six sides for the sensor housing.

A cardboard mockup of part of the sensor housing (shelter), with a dust sensor installed.
The remainder of the housing will have to be cut from the cardboard before the project can proceed. But the initial results are promising.

The COSM feed I previously established for sensor journalism has been down for a spell, and will continue to be down until the housing is finished, and the sensors installed within.

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