Search This Blog

Sunday, November 21, 2010


I'll know I'm doing too much stuff when these posts start coming at more than 1 month intervals.

Anyways, time for another ELB update!
I finished cutting and rough sanding the last 40 polycarbonate inserts for my new hubcaps.

It's INCREDIBLY tedious work to polish these things. The above is after cutting, 220 grit, and 400 grit. There's still 600, 1200, 1500, and 3000 (lapping paste) to go. Anyways, this is the point where I got bored and decided to work on something else.

This is my charger: Turnigy 4x6S . It can charge four 6S packs independently. I figured I could use it to charge my 12S ELB pack (just link two of the ports together). But...
Each charging port is NOT independent, despite what you may have heard about it, or may assume about it, or read about in the description ("All ports work independently of each other."). From the outside, all you see is a single input for 12Vish DC and 4 charging ports. Internally, there are 4 separate chargers. Sounds ok, right? However, all of the chargers' inputs (+ and - ) are linked by traces that go all the way around the board. This wouldn't be a problem if they were high quality (isolated input/output) chargers. But this is a Chinese Cheap Charger (CCC). The negative/ground wire is connected straight through from the input to the output (battery ground). This means that all of the outputs are at the same potential, which in turn means you can't link ports and charge anything over 6S. I need 12S. There goes $100. OR:

Doctorbass to the rescue. Check out his hack for this charger...genius. He made it into a single 24S charger by isolating the four chargers by cutting the traces that link them, and then buying 4 small (cheap) switching power supplies, 1 for each charger. His application is a little different than mine, so he did some other cool modifications, but I'll just show how I did it.

Take it apart

Each charger has two holes like this for wires to go into. It's like they were originally going to manufacture it to run off of four independent power supplies (like it should have been), but they instead just added a trace all they way around to save money.

Cut traces on each side of each set of input holes. Dremel worked well.

Cut the bottom traces, too. Be careful not to cut into other traces, just the power traces.

Back side was easier. Could just slit the board without hitting any components.

The four 12V, 5A power supplies. You can get these on eBay for <$40.

Cut the DC plug off the power supplies and string the wires through the wire hole in top plate of the charger. (De-solder the existing input wires first.)
Solder the new inputs to the circuit board.

Yep, definitely isolated now.

Finish soldering all the input wires. Route them as you see fit.

Semi-finished. They all work!

Removed excess wire. That's better.

Double-stick tape/duct tape the power supplies together. I velcroed the charger to the power supplies, too.
Now I have one big block of a 24S, 200W charger. Not too bad for $130 and 2 hours.

In addition to these things, Shane's been helping me calibrate the motor controllers to my motors (well, he's been doing most of it).

Note the old hubcaps with chunks of missing polycarbonate....why I'm making new ones.
I've got updated data for the motors now, too. Before, using a Turnigy controller and running the motors sensorless, we calculated a top speed of about 30mph. Now, using sensored sinusoidal control and about 15 degree advanced timing, the top speed is around 20mph. Not too bad. The really impressive thing is the torque. I'm looking at getting 50lbs of force at the ground..assuming 200lbs of mass (rider+board), that translates into 1/4G of acceleration, which is nuts...especially for something without a seat or handlebars. I probably should have wound the motors a little less for torque and a little more for speed (less windings, or thinner stator, etc), but I think 20mph is plenty. 

That's all for now.

The other Electric Longboard

Well, bad news for the other electric longboard (the one made during the Edgerton Center Summer 2010 Engineering and Design Class).

Remember the eXKateDB (drill battery) hack?:

I shoulda seen it coming, but see those little connectors? They can be plugged in backwards...yeah...bad stuff happened. Turns out the Exkate controller doesn't have reverse polarity protection. I actually found this out the hard way at the end of the class this summer. I plugged the battery in backwards and it fried some components. Luckily, Shane was there to help me fix it. This time (wasn't me) however, more things fried:

Blown up transistor and resistor. Another transistor is fried on the board.

You can see the diode we replaced before. It was fine, but the transistor below it wasn't too happy.

Assuming more stuff wasn't burnt, we probably could have fixed it, but the whole thing was turning into a hack at this point. controller! Brushed Kelly Controller to be specific. It has better specs than the Exkate controller, for a lot less. However, it doesn't have a built in radio system. radio system! A Hobbyking 2 channel 2.4GHz radio was purchased ($17). We'll have to figure out some way to translate the PWM servo signal to a 0-5VDC signal for the Kelly throttle input, but that shouldn't be too difficult. Hmm...what to do with the extra channel.

A new set of Lipos are here (I need to find time to install them :/ ). To prevent what ultimately caused all of this trouble in the first place, a keyed battery selector will also be added. After that, the longboard should pretty much be set and idiot-proof resistant. 

Wednesday, November 10, 2010

About the Edgerton Center at MIT

Check it out.

Saturday, November 6, 2010

Polycarbonate Inserts

Time to suck it up and polish some polycarbonate.
I bought 220, 400, 600, 1200, and 1500 grit sandpaper, as well as some 3000 grit lapping compound, a felt bob, and a cotton bob. With my polishing kit ready, I cut ten pieces of the 1/2" dia. PC and ten of the 1/4" dia. PC with a band-saw. Then I started with the 220 and progressed to the finer grits. To remove the burr raised from sanding, I put each stud in a drill press and filed down the burr. Then I flipped the stud over, and did it again.
For the buffing, I put the felt bob (I found it worked better than the cotton one) in a drill press, applied some of the lapping compound, turned it on, and buffed the ends of the studs. They came out much clearer than before, but still kinda hazy.
So then I took the heat gun and carefully heated up the each end, making sure to take the heat off before the PC bubbled. The final result were ~.45" long PC studs that you can read text through. Then I pressed them into a hubcap with an arbor press. Check it out:

Random sheet of paper I found. You can see the circuit diagram through the PC.

However, it wasn't nearly as easy as I just made it sound. The whole process took 4+ hours (yes, I'm blowing off work). That's just for 1...I still need to do 3 more. Ahhh, not enough time.