After chatting with my engineer friends Pweaver and Ned who are very conveniently my housemates, I'm starting to reconsider the
chocolate printer thing (as discussed in Week 1). It's actually a thermodynamics problem! Can I even get the chocolate to temper and then harden in
time to keep printing, or will I just wind up printing formless chocolatey puddles no matter how elegant my engineering?
Beno from Fab Lab Lima and Fab Lat came over and shared a different idea: making a cheap freeze-drier for smallholder farmers in the Amazon, so they can preserve all the nutrients in their produce as soon as it's picked. We entertained this idea, brainstorming potential designs.
Could we use a series of mason jars in a vacuum chamber?
Basically, I'm developing a swamp cooler conversion kit to turn any old trailer into a climate-controlled, produce transportation truck. The low-power-consumption swamp cooler (also known as an evaporative cooler) technology works by blowing air through a wet sheet of cardboard-and-cellulose material, which takes in heat from the surrounding air as the water evaporates-- thus cooling down the environment while keeping blueberries or asparagus moist (and therefore fresh).
Here's a commercial version from the company Portacool, that isn't built to drive down the highway or specifically designed for produce:
This project will involve the following mechanical components:
I'm running short on time now, so this week will require some very serious mechanical work and a bit more sorting out of the elctronics. I need to get the humidity sensors up-and-running, connected to the web GUI, and also ideally add in some control loops so the pump stops when the humidity increases beyond a certain point.
On the mechanical side, we found a CAD model of a trailer and Abhi has been running some simulations to determine the airflow-- so we know whether funnels will be useful or not, and also how many humidity sensors we should install. We decided on four humidity sensors, or one on each surface of the trailer.
Here's Abhi with the model of airflow through the trailer and the stacked blueberry crates.
We've also got all the dimensions of the blueberry crates that we'll use to test out our system in Peru, so it will be easy to mill out some plywood on the ShopBot to mimic what the final, packed trailer will look like-- so we can actually test out the humidity and airflow in the real system.
To do this week: take apart the Portacool and see how their water pumping system works! Then, build a similar system out of composite materials and piping that won't leak while it's driving down the highway. Also, I need to get my accursed humidity sensor working so I can make all 4 of them and get all the code sorted out! Then I'll CAD some containers to house them...
My humidity sensor is finally working!! Suffice to say I should've paid more attention to the datasheet and double-checked my pull-up resistors, because those are super important for both the SCK and SDA lines. NOTE: UPLOAD FINAL EAGLE DESIGNS
Next step is to figure out how to make 3-4 slave boards, and how to wire those together, and then how to log that data for viewing in the GUI. Turns out that the Softi2C library that I've been using for my ATTiny84 only works for master devices, so I'll need something else for my slave boards! Does this mean that I need a couple different libraries? Or can I figure out the C++ code enough to just write my own? This library looks promising,as does Christiana's example of successfully using ATTinys as both master and slave boards. Most of the other examples online seem to use an Arduino as the master, because it's much more compatible with existing Arduino libraries-- so this makes life more difficult.
Anyway, here's what I need my code to do from the master side:
Include the right ATTiny libraries for software-based I2C, since it doesn't have hard-encoded I2C like Arduino (figure out what these are first!)
Set up the correct SDA/SCL pins for all 3-4 boards
Set up the board addresses to distinguish the slaves from each other
And from the slave side:
Set up correct pins and define slave address
This week I started by dividing up my tasks into manageable chunks, and then writing them onto post-its so I can move them around and enthusiastically cross things off:
I also decided to use the Portacool as is, and just build a modification kit around it for the sake of time. So the first order of business was to get the Portacool into my lab. By some fluke, it fit through the doors with a full couple centimeters of clearance! NOTE TO PROSPECTIVE LARGE PROJECT DEVELOPERS: MAKE SURE THINGS FIT THROUGH YOUR DOORS. This could've been a disaster. Here's Alvaro helping me wheel it into the Legatum Center.
Here's PreserveAir co-founder Mike, with Legatum staff Julie who helped me pick up the unit from the loading dock. (Thanks, guys!) My lab-mates weren't very happy with this monstrosity in the office, so luckily I was successful in begging for a spot in the CBA shop just before the Legatum Center's major holiday party. (Thanks, John!)
Portacool inexplicably sent their manual in Arabic, and promised us a free cap.
Please don't ask why we decided to prototype the whole thing in an full-sized trailer, and where we're parking the full 16ftx8ft trailer. Or how many professors we asked about using their driveways. Or how much clearance the trailer had when pulling into our parking spot at Pika. (Thanks, Pika!) Or whether this Portacool unit will even fit in the trailer. These are questions for another day. Say, Thursday. Instead, let's talk about this lovely 3D model of a trailer, via Kennedy Kangwa on GrabCad, that I'm going to use for modelling how the system fits together!
I haven't actually modelled it yet, though. Here's a vague whiteboard sketch instead, marking out the large mechanical components that I'm building (in green) vs what's coming off-the-shelf.
In particular, here are the mechanical things to build and how I will build them:
Finally, let's make sure that I'm completing all the stages of the final assignment: