Final Project

Week 3 thoughts: Let's make a freeze-dryer!

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?

Beno was skeptical, and wisely suggested starting with a single jar before scaling up:

Week 8 thoughts: Let's make an evaporative cooler!

First of all, I'm not giving up on the chocolate printer-- I can instead try convincing my classmates to develop this printer for our machine-building week... Or, my friends at the Lab for Chocolate Science can help me out with this over IAP.

But for my project this term, I want something that involves more building and less tinkering with temperamental tempering. After thinking hard about Beno's project, I ended up chatting with my friend Alvaro who's a fellow in the Legatum Center where I'm doing my RAship. Alvaro has a fascinating, closely-related startup to Beno's: keeping fruits and vegetables cold on smallholder farms in Peru, so they don't rot while they're waiting for transportation. The whole distributed production question is closely related to my own research, and Alvaro needs to design and test out a cooling system next month so the timing is perfect. Also, I've been interested in this challenge for a while; post-harvest food loss is at the top of things that frustrate me about industrialized society (along with water-based sewage systems and corporate taxes on labor). I've been reading Finnish researcher Stephen Fox's writing about moveable microfactories, and the theory behind this idea of distributed food production. I've been interested about food transportation systems for years, and I'm excited by Alvaro's well-researched idea of evaporative cooling rather than full-on refrigeration. As a bonus, my mentor Kipp once challenged me to build a refrigeration system in a fab lab-- so this comes close!

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:

And the following electrical components:

Week 9 thoughts: Still building an evaporative cooler

This week I finally got started on my project, by getting a solid state relay working with an AC fan and my ATTiny44 board. This led to some thoughts about how the electronics will all work out. First I have to decide how to power the system: should I use a wall outlet or a 12V battery? This choice may be determined by what pump and fan I end up using-- and then that plays a role into choosing whether to control the system through a solid state relay (for AC) or an N-Mosfet and/or regulator (for DC). All these electronics choices led me to make a sort of block diagram:

Week 12 thoughts: Built the user interface!

Check out more details here.

Week 13: Figuring out what I'm actually building

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...

Week 13: Programming time...

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

Responsive image

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

Repeat every second or so.

And from the slave side:
Include libraries
Set up correct pins and define slave address

Week 14: Getting serious

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:

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