So a couple of months ago I went back to Mexicali, Baja California, Mexico. Which is not in SoCal even though most people think that whenever I mention "Baja California" -_-. Anyway, I stayed there for a few days only so I could attend my sister's high school graduation (she's going to Princeton! wooo). During those days, I learned that one of my friends, Juan Bustamante, had a cousin who was throwing away a tiny motorcycle that wasn't working anymore. I went over to see if I could fix it or at least figure out what was wrong. The first things I checked were the battery and the motor. I don't really have any tools back home besides a screwdriver and a wrench so there wasn't much I could do but luckily my Juan had just bought a multimeter so we probed the battery and found that the 18v battery only had 4v across it. Which made sense considering the lead acid battery had not been charged in about four years. To check if the brushed DC motor still worked I had to place enough voltage across it make it overcome its own inertia and have it start spinning. I didn't have a variable power supply so instead we got eight AA batteries and taped them in series to make a 12v "pack". The motor did spin so I decided to keep it for a future project (or maybe just to take it apart and use its stator). After we diagnosed the problem, Juan decided that he was never going to use it anymore so he offered to give it to me. I was unsure about whether or not I should bring the pocket bike with me back to MIT considering that it's more than 2500 miles away and it might be a problem to bring on the plane. So I did a quick Facebook post to see what my friends thought about it.
Clearly, they thought it would be awesome if I brought it and motorized it so I could race against Adrian who built his own electric pocket bike, MilliCycle.
Mount without pocket
The first thing I had to do was figure out how to mount the motor onto the frame. The original motor was a brushed motor and it had its mount holes on the bottom so the frame had a mounting plate for it. I decided to take advantage of this so I took some scrap metal I found in N52-318 (room where the MIT Electric Vehicle Team meets). It was an aluminum C-Channel so my ex-roomie Roberto used the bandsaws and drill presses in D-Lab to help me cut off one of the sides and make mounting holes for both the frame and the motor. We later realized that we did not take into account the aluminum between the motor and the pulley so we had to go down to MITERS to mill a pocket for it.
Motor mount with pocket (it's on the other side)
motor controller attached with more scrap aluminum
I wanted TinyCycle to be way more powerful than RailScooter so I used a motor twice as big as the one on RailScooter. It's a Turnigy SK3 6374 Brushless Outrunner. It's 192kv and has a resistance of .016 ohms. More importantly, it has more power than I really need on such a small vehicle but meh, it should be fun. For the battery pack I soldered two custom 5s2p battery packs in series. This means I have a 33v pack on TinyCycle; much bigger than the 24v battery pack on RailScooter. The motor controller is also different. RailScooter uses a sensored kelly controller whereas TinyCycle uses a sensorless 250watt jasonTroller. The jasonTroller was clearly TinyCycle's limiting factor so I plan to swap it for a more a 500watt jasonTroller. Hopefully that will give me enough power to do wheelies!
bicycle mount in D-Lab
battery pack sketchily mounted with zip-ties.
TinyCycle's Maiden Voyage with the 250w controller
Garage Run with a scooter, two go-karts, a quadrotor, and a tiny motorcycle
So many little EV's!
Garage Run from the Shane's quadrotor's point of view
Cute British Accent Asian Girl
So apparently, tinyCycle is a chick-magnet. A couple of TDC brothers and I were riding it around dorm row last Saturday night around 1am. Which is also the time MIT frat parties are over. Several girls walked over toward us and started asking me about tinyCycle.
First go on the internets and buy some LiFe cells. I specified LiFe cells because those are the ones that I used. You can buy some directly from the A123 website. I used A123 lithium ion cells for my battery packs. They carry 3.3v nominally and 2.2aH. Use a voltmeter to make sure that the voltage across every cell is 3.29v-3.30v. Make sure that all of your battery cells are at the same voltage. Your battery pack is as strong as its weakest cell so if some of your cells have 3.29v and others have 3.3v your battery pack will die when the 3.29v cells die, regardless of whether or not your other cells still have charge.
So many batteries!
Then you should use a hot glue gun to glue together your batteries. I did a 5s2p pack which stands for five in series and 2 in parallel. This means that each battery pack has a total of 16.5v (5x3.3v) and 4.4Ah (2x2.2Ah). I first glued together five pairs of batteries and made sure that they all had the terminals facing the seam direction.
Two cells in parallel
two pairs in parallel
small packs of 2s2p
Now begin gluing the pairs to each other and make sure that their terminals are facing away from each other. This is because we are about to connect the pairs in series. We wil use copper braids to make this connection; I'll explain it later in the tutorial.
You should now be done gluing the cells together and you should be left with 5 pairs of alternating polarities hot glued together.
Use sandpaper to clean up the terminals
Before we start soldering on the batteries we must clean the terminals using sandpaper. This is to brush away the corrosion on the terminals.
Notice the first pair of batteries have two negative terminals connected with copper braid. This is the pack's main negative terminal.
After sanding the terminals you should solder strands of copper braid in between each battery terminal. I used 12 gauge equivalent grounding braid from mcmaster. Make sure that you are connecting terminals with opposite polarity. The only exceptions are the first and last pair of batteries. Those will be your main positive and main negative terminals. This is the only occasion in which you should have a strand of copper braid connection terminals of the same polarity. To solder this braids you will require two soldering irons: a huge one as wide as your pinky and uhh... a normal sized iron. You'll use the huge iron to solder the negative terminals since they are much larger than the positive ones and therefore have more space for heat to diffuse. For the positive terminals you should just use the normal sized iron because the big one might heat up the terminal too quickly and you could damage the cell. You might want to buy some flux from radioshack or mcmaster to aid you in the soldering although it is not necessary.
Note the thin cables connecting the parallel nodes together.
You should also solder some thin wire in between each parallel node as seen in the picture above. Theoretically, all of the cells should be at the same voltage so there should be no current flowing through these thin wires but if there is a slight discrepancy in voltage across one of these cells. Current can flow from one cell to the other and hopefully equalize the voltage across them.
Finally finished soldering two packs.
Now it's time to make a balance connector. Balancing is very important. It will make your battery pack way more efficient. If your cells are not properly balanced then your pack will die as soon as your first cell dies.
Balance Connector
For the balance connector you will need two things: the connector housing and the crimp connectors. This the most annoying and monotonous part of soldering your battery pack. When choosing a connector housing you gotta make sure that it has the correct number of ports needed for your battery pack. The number you need is one more than the number of cells in series in your pack. So for my 5s2p pack I used a 6-port connector housing.
Balance connector housing
XH Crimp Connector
If you don't have a crimping tool you can just solder the wire onto the crimp connector
If you do decide to solder the wire to the crimp connector you must make sure that you use very little solder. If you use too much solder, when you try to connect your finished balance connector into the female port on the charger you won't be able to plug it in. I messed up my first connector doing this and I've heard that a lot of people make the same mistake their first time. The red wire should go from the "arrow" port on the connector housing to the main positive terminal on the battery pack. The blue wires should each go to a different battery terminal and they must be in order. So the red wire starts in the main positive terminal. The second wire (or the first blue one) should go to the negative terminal opposite the main positive terminal. The third wire (or the second blue one) should go to the positive terminal opposing the terminal you just did, and so on...
Make sure you connect the red wire on the arrow
I realize that the balance wire connections might have been confusing for people so I hope this diagram will help you guys visualize it better. The whole point of these balancing connector is that the charger has to be able to measure the voltage drop across each cell. After you're done you should be able to get a voltmeter and measure each cell's voltage by probing the inputs on the white balance connector housing.
Balance wires and main positive/negative wires complete.
For the main positive/negative wires you must use thicker wires since these wires will be taking on most of the current. I used 10gauge wire I believe. Very important: you should solder a polarized female connector to the end of the power wires. They should be polarized so that nobody can accidentally connect your batteries backward and short them. I like to use XT-60 connectors, but a lot of people use dean's connectors. Personally I think you should just choose one connector and stick with it so that you can use any of your battery packs for any of your electric vehicles. Make sure that the female end of the connector is soldered onto the battery power cables. This will make it harder for your batteries to short accidentally.
Heat shrink the battery pack
After you make sure that all of your balance wires are soldered to the right spots and everything is soldered correctly it's time to insulate the battery pack. There are many ways of doing this. You could do the legit way which would be to get huge heat shrink tubing and a heat gun to insulate the pack. Or you could do it the candace way and just wrap a ton of electrical tape around it.
Done!
Before you actually use the battery pack you must make sure that you use a balance charger to balance the cells. A balance will charge each cell separately to a pre-set voltage. However, after a pack has been balanced it will remain balanced for a while so you can just use a normal charger on it. I own the 1010B iCharger. It's expensive but it's powerful and can charge and balance up to 10s LiFe battery packs. It was recommended by several people who have built many electric vehicles, such as Charles, Shane, and Eli.
Anyway, I hope my tutorial was helpful! Good luck!
My first semester sophomore year I decided to buy an arduinomicrocontroller so I could play around with it and learn how to use it. In order to teach myself the basics of the arduino language and microcontroller I gave myself a project and started googling everything I could about it. I wanted to change the color of an RGB LED depending on the music coming out of my iPhone. I used an audio jack and its breakout board that I stole borrowed from 2.s994 (Electronics for Mechanical Engineers). I connected two of these audio jacks so I could listen to my iPhone's music while the arduino sensed it. Before I got my hands on an RGB led I tried to "make" my own RGB LED by sticking a blue, red, and green LED inside a ping pong ball. The ping pong ball would diffuse the different lights from the three LED's. This way I could use PWM to display other colors besides red, green, and blue. Sadly, I don't think I have any pictures of this project since my computer was stolen when I went back home for Christmas break. It was stolen within the first three hours I got home :/ but that's a story for another day.
Throughout that semester I kept nagging my curly-haired Salvadorean roommate Roberto Melendez to help me make an LED matrix. He's a meche but (unlike most meche's) he knows a lot about electrical engineering. The Sunday after the first week of classes I was really bored (I didn't want to start my homework) so I (once again) asked him to help me make an LED matrix. He must have been having a bad day because he yelled at me, "just do it yourself!" So I did. Thanks Roberto!
Ghetto-engineering ftw!
The first thing I had to do was to find a suitable frame for the LED's. I was going to use cardboard at first but I decided it was too frail so instead, I got a hammer and broke open an old speaker we had in our room. I drilled twenty-five holes (mainly because something larger than a 5x5 would end up being way too much soldering) that had a slightly smaller diameter than the LED's diameter. The most time-consuming part of this project was soldering the LED's together. To do this I completely stripped off some 5'' jumper wires and connected the LEDs' cathodes in rows and the anodes in columns. This way I could control each LED independently. For example, if I wanted to light up the center LED I would supply 5 volts on the third cathode and ground the third row, thereby lighting up the (3,3) LED. Problems arose when I tried to light up the 3x3 central square without the center square. In other words, the (2,2), (2,3), (2,4), (3,2), (3,4), (4,2), (4,3), and (4,4) LED's.
Arduino Uno
To bypass this problem I took advantage of the persistence of vision phenomenon. I used an arduino to light up one row at a time very quickly. It "waves" through the five columns so quickly that the human eye can't notice it and it appears to be one 5x5 image.
matrix displaying TDC
Now that I had a working prototype I wanted to build something better. Mainly bigger and prettier. So the week before Science Extravaganza, a statewide educational outreach event that MAES hosts here at MIT, I decided to make a new LED matrix. The architecture lab had given the MAES exec-board temporary access to their laser cutters so that we could laser cut 200 "Aurora Domealis" for the middle-school students coming to Science Extavaganza. Credit for this LED boxes goes to Adrian Tanner; I made very minor changes to his design and added the eagle on the front.
Aurora Domealis
I'll probably write a post about Science Extravaganza later on, but the point of this was that I had one week left before I got my laser-cutter access taken away, so I HAD to taken advantage of this and laser cut something cool. I had also recently learned about t-bolt slots and how useful they are. Both Charles and Candace used them in their 2.007 robot designs, so I thought this would be a good use of the laser cutter.
laser cutted frame
First, I designed the LED matrix frame in solidWorks. I made the top have 64 holes that have diameters that are 1mm less than the LEDs' diameters. One of the sides have two holes so I can connect the arduino to my computer and/or a power supply without unscrewing the hole frame. I was going to add a mount and some holes to screw the arduino on the frame but my laser cutter access could be taken away any day so I decided not to.
LED matrix soldered, time to start coding
I learned several things from making the 5x5 prototype, but the biggest thing I learned was that I needed to figure out an easier way of soldering the LED's together. On the 5x5 I used stripped jumper wires and soldered all the rows close to the frame and all the columns in the air on top of the rows. Soldering the jumper wires in the air made it really annoying and it took me several hours to finish soldering everything together. That was 25 LED's connected directly to the jumper wires. There are two connections per LED which means I had to solder 50 connections. On the 8x8 I'm soldering a resistor to each LED so that means 3 connections per LED making a total of 192 connections. There was no way I was ever going to finish if I tried using jumper wires again to solder all of those LED's. Instead I bought copper tape online and then used electrical tape to insulate the intersections between the rows and the columns. Soldering all the connections still took a really long time but it is nowhere near what it would have taken me to solder all LED's using jumper wires.
Working on the matrix while drinking on the second deck of TDC
Hi Haley :)
Now that the frame was done I just needed to worry about coding the matrix. I couldn't use the same code I used for the 5x5 matrix because the arduino doesn't have enough pins to power every column and row. To get around this I needed shift registers. Perfect excuse to read and teach myself how to use this integrated circuits (IC's). I used two 74HC595 chips which, if connected in series, will take up three pins on the arduino, but allow me to control 16 LED's or in this case, 16 rows and columns of LED's. If you want to learn how to use shift registers I highly recommend that you read this tutorial on the Arduino website. Anyway, it's almost 7am and I should probably start studying for my finals....
This semester I decided to be a hipster 2.007 and opted out of the mainstream 2.007 robot competition. Instead, I enrolled in the Electric Vehicle (EV) section of 2.007. In this section, led by Charles Guan, Shane Colton, and Eli Davis, each student had to come up with an idea for an electric vehicle. Any electric vehicle, from a go-kart to an electric kid-tricycle. We would then get to design, machine, race, and keep our vehicles! Deciding what vehicle to make was a tough decision between practicality and novelty. But in the end, I decided I would make a scooter because it was something that I could actually use to commute across campus.
$40 A3 scooter from Amazon
The first item I bought was an actual scooter. To make my life easier, I decided to use the scooter's pole and folding mechanism. This meant I had to design and machine a new frame and fork for mine.
Noobishly printed the parts from solidworks
I then bought a large 1/4'' aluminum plate from McMaster-Carr (amazon prime for Meche's) which I used to machine the new fork for my scooter. My initial plan was to print out the shapes from SolidWorks and then trace them on the plate to accurately cut it with a bandsaw. Unfortunately, I somewhere in between the transition from SolidWorks to printer to paper, the ratio between the solidworks sheet and the physical sheet stopped being 1:1. So I ended up with a fork that was larger than I had originally accounted for. Fortunately, I realized this before drilling the holes and assembling the fork.
my decapitated scooter :(
With the drive train done and the fork assembled, I was around a third of the way done with my scooter. But I still had to figure out a way to attach the handle to the frame of the scooter, attach the latch and poly-carbonate cover, and finally, wire the whole thing together.
Playing the sensor game
After finally finishing the mechanical frame, I got to the tedious and annoying find-the-correct-combination-of-wires game. There are 12 combinations: 6 from the three motor wires times 2 from the ABC sensor wires.
the latch works :D
Awesome pcb sensor mounts designed by Charles G.
Charles made our life way easier by designing and ordering some circuit boards that would hold the sensors and make it easier to slide them on the motor in order to find the right position for them. This is important because the position of the sensors will affect the amount the current the motor pulls from the controller.
Finally found the correct combination
I got lucky and found the wire combination pretty quickly but I got stuck a really long while finding the right position for the sensors.
My scooter works!
WUUUUUUWWWWW!!!!! It works!!! Sadly, in an event that reminds me of Charles' reverse polarity battery connection before the 2.007 competition. As I was riding my scooter from N52 towards TDC I decided to go full throttle on Mass Ave and over the railroad tracks.... Needless to say, I flipped over and went flying around 15mph and fell on my left arm. A car, a bicycle, and several bystanders stopped to ask me if I was ok as I stood up, picked up my scooter and the broken fork from the ground and sadly carried my scooter back home :(
This occurred less than twelve hours before the EV Race :(
In memory of the obvious event, I christened my scooter, "RailScooter". :D
Yay! No broken bones.
Things I learned this semester:
Use a mill whenever you can! I had so many problems making a fork for my scooter because I would try to use my calipers to make aligning holes on two pieces and failed every time. The main problem was that I had band-sawed all the pieces for my fork so they were not exactly.. straight. This meant that measuring the proper positions for the holes was very difficult. Finally, I should have used more center point drill bits to make sure that I drilled the holes in the (already inaccurate measured) positions I had chosen.
Very important! Use at least THREE screws (on each side) in the plate that holds the shock absorber. Most of the torque caused by your weight and by going over bumps (and conveniently, railroad tracks) acts on this plate. The problem with screws is that their threads weaken the material and make it very susceptible to shearing. This is why in my new (very hastily built) fork, I made sure that the plate was sitting on the 1/4'' aluminum fork side AND then screwed together. This way the aluminum would have to break before the screws are sheared off. My new fork will probably be waterjetted since I've already made the design in solidworks. It will use t-bolt slots and be substantially prettier and sturdier than my current fork.
In retrospect, I should've done more research on the other scooters that other students had built because, honestly, I did not know what I was doing 80% of the time. Most of my time went into fixing slight misalignments and other errors I made.
Hi my name is Victor Rodriguez and I'm a sophomore at MIT. This semester I took 2.007 and I think the greatest lesson I learned from it is that I like to build things. Unlike most people, I don't know what I want to do with my life. I came to MIT solely on the basis that I like math. Which is also how I decided I wanted to be an engineer. However, once I got to MIT I realized I would have to narrow my interests down to one major. But after taking the GIR's I decided I wanted to be Course 2 or 6 (2 = Mechanical and 6 = Computer Science and Electrical Engineering). So as of now, I'm finding a compromise between the two majors. I'm Course 2a, which is Mechanical Engineering with a concentration in Computer Science.
tl;dr: I'm an MIT meche student who likes buildng things (and sometimes coding).
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