Monday, January 10, 2011

Hot n Cold

By now I will be back at Tech. By now I should be starting to go to class. Instead I've been holed up in my residence because GT has shut down due to a flurry of snowfall last evening. It is pretty cold outside, but alas there is work to do: Cake's drive escs are getting too hot!

Seth at the Robot Marketplace recently informed me of a new set of KW motors for the B-series gearmotors. These guys pull more current, but are spec'd at 937 rpms at 12V or so. That is ~300 rpms over the old motors which will work great for my 1.5" drive wheels.

But if you ask me, it still feels slow compared to DDT. So lets take the motor mods a step further.

Tower Hobbies sells paired motors for direct drive foamies. 180 sized, high rpm, high current, and the correct shaft diameter. More importantly, they were on sale for a bulk buy!

The operation was a simple motor switch since the mounting patterns were the same.

Here the motors are installed with the B16 gearboxes. Ignore the new escs for the time being.
Months ago, I also purchased two scorpion mini escs to replace the damaged scorpion HX dual esc. You may recall, I had some SMT components fall off the board and while it has served me well my distrust is well placed. The two scorpion minis side by side fit perfectly horizontal in the electronics bay and open up much more room for the wires and plugs from the drum.

I applied a light application of glue on the edges to hold the two boards together, so that I could use breadboard jumper wires to bridge the main esc terminals like P0W3R busses.

Note how there are two plugs on the esc. One plug is actually for battery, but the other connects the the drum esc. Having the same connect with the same polarity makes for stupid proofing.

The real problem was that the new motors drew more current than the escs could handle. Evaluating my options, I felt that adding heatsinks was a far easier solution than disabling current limiting or upgrading the FETs. I cut some blocks of 1/8" thick copper bar (back when copper was not ass expensive) and tapped some 4-40 threads to facilitate mounting to the FETs.

A light application of silicon based heatsinking compound helps heat transfer between the materials. I also made a note to sand and polish the mating surfaces. Oxidation on the bar left lots of non-conduction crud on the outside.

Finally mounted we discover the robot to be significantly quicker, with extended runtime. I can achieve approximate 3 minutes of vigorous driving before the escs overtemp. This is acceptable.

Here is everything installed in the robot. The first picture of this post shows an unexpected benefit of adding the heatsinks. The board material fits right in between the header pins and the capacitor in my AR6110 receiver for optimal space savings!

Thursday, January 6, 2011

Safety Razor Safety Advisory Report

So it's been a month since I butchered a child's Razor scooter and shoved a electric bike motor system into it. Let's hear some results!

I am able to use it for inter-campus transportation with about 50% capacity drain for a full day.

That's acceptable. Especially since it is probably mostly burst amps from accelerating. Voltage has yet to drop below nominal 22.2 (for a 6-cell lipoly). I still regularly monitor the pack for increased volume, but nothing vile has arisen. The picture above shows post-charge results. We have a 66% drain over an extended run time (I had group projects across campus that evening). You'll have to excuse my camera. No flash superimposed the words "END" and "BAL" to create the illusion of "BAD" on the LCD screen.

Over a period of time, I discovered my ground clearance to be steadily decreasing. My predictions have been reaffirmed. The amount of metal removed from the original frame, combined with the hastily designed frame extensions drastically reduced the integrity of the structure. I disassembled the frame this holiday break and discovered the fracture points to lay just as I had predicted: the region where aluminum and polycarbonate met.

All of this could have been avoided by starting with a single piece aluminum frame extension. Thankfully, I am still a ULI, and speedy metals is having a 10% holiday discount until January 31. Therefore, bring in the upgrades!

From the image, you can see the back end remains virtually the same. I rotated the motor mounting holes so that the centerline from the motor shaft to the wheel deadshaft were in line. Originally, the sprocket would rub against the spacer with left from some interestingly deep erosion marks as illustrated in the picture below. Undamaged spacer included for reference. I hope you know what a hex bolt is supposed to look like.

The majority of the changes were made to the front half. This iteration does away with the original scooter frame. It was far too troublesome modifying the frame to the design and it bore little fruit other than comedic value. This body is serious business. Slots and tabs everywhere. The accessible panel has been moved from the bottom to the top. This is because repeated ground scrapings sheered screw heads which made it a pain to disassemble. Speaking of bottom plates, a UHMW skid bar will be added to take the blunt of aggressive gutters and sidewalk ramps. On top of all that aluminum will most likely be a sheet of 1/16" garolite. Just something to cover the large gaping service channel and be pretty. If it is garo, I might even consider laser engraving a graphic into the center of the deck. Gettin' fancy now...

Of course, this frees up about 2-3" of space in the scooter body, not to mention the extended width and height. This means I can opt for a BIGGER BATTERY or protection to my current pack. Both would be nice, but I may push for the larger battery in preparation for the next advancement in campus transportation: MTD (to be described in detail later).

Lots of great material coming up (but its late, and im lazy).