Archive for the 'Project RazEr' Category

 

I accidentally

Mar 24, 2009 in Project Build Reports, Project RazEr

everything.

I am now thoroughly convinced beyond any doubt that power electronics just plain hate me and want to see me suffer.

Last week, the 5v switching regulator on RazEr went out with no explanation. Fortunately, it just stopped switching, and didn’t jam 35 volts through all my control logic like last time.

Today, still depressed from the failure of LOLrioKart and wanting at least one working vehicle, I cracked it open and found out that the (poorly) reflow-soldered output capacitor fell off. After soldering it back on, and still have it be dysfunctional, I made a 30-to-5-volt linear regulator from a 78T05 chained to a 78T12.  One 7805 cannot handle the drop from 30 volts to 5, so the 7812 is an intermediate stage. This is an arrangement that’s worked for me before.

After installation, the headlight, servo driver, and ESC fire right up. I was able to reset my throttle endpoints as usual. Then I pushed on the handlebar throttle, and

FML.

(Oddly enough, the headlight and servo driver  stayed on through the entire ball of fire. That means my regulator works.)

Back to DC motors and relays…

While you were away…

Mar 04, 2009 in Project Build Reports, Project RazEr

I finished the rebuild of Project RazEr. Because the site has been down, I never wrote up any day-by-day build reports, but still took pictures accordingly.

So, here’s an epicly long build report all the way from start to finish. The whole process wasn’t long and drawn out this time, mostly because the fabricated components were conceptually simple. The majority of work was done in a few evenings at MITERS (as usual.)

(more…)

Project RazEr is grounded for maintenance.

Feb 05, 2009 in Project Build Reports, Project RazEr

We apologize for the inconvenience. To me. Because I have to walk now.

Partial electrical system meltdown combined with salty winter slush means that the electrical system has gone haywire.  As in, something is bridging my battery V+ connection, the frame, and the BEC. That’s pretty intense right there – flipping the power switch to OFF does not actually turn the BEC off. Additionally, I ran down my partial charge, and now have no safe means to recharge the cells.

The underside, and part of the inside by the rear wheel, is also pretty well caked in grunge.

So I’m taking the scooter offline for now, and will be going on a fast rebuilding process. The list of updates include

  • One-piece underside attachment that mounts all the batteries and control electronics. On the inside. This means cutting out the bottom of the scooter and using that new underside attachment as a stressed structural element.
  • Waterproof.
  • Minimize custom electronics. Easier on me, more idiot(me)proof. Plus, you can get a servo tester for 5 bucks now..
  • Waterproof.
  • Internal battery balancing. Little dedicated balancers are only a few square inches in area and cost 15 dollars. Well worth the extra cost, in my opinion.
  • Waterproof.
  • All main battery wiring will be heavy gauge, and there will be one charge port.
  • Waterproof.
  • No-through-bolt wheelmotor design. Can actually change tires!
  • Waterproof.
  • Stock LiPoly or LiFePO4 packs, not custom assembled jobbles.
  • Waterproof.
  • External routing of motor wiring. for easier service.
  • Waterproof.
  • Did I mention waterproof?
  • Waterproof.

Lithium battery near-disasters and LOLrioKart shenanigans

Feb 02, 2009 in LOLrio Kart, Project Build Reports, Project RazEr

Lesson for everyone!

1. When you charge your 120 watt-hour lithium ion polymer battery at 6 amps, please make sure your internal charge-balance wiring is not made of 24 gauge wire.

2. If they are, and you should choose to run 6 amps through them, please make sure they are not tensioned against a rough edge in your vehicle’s all-aluminum frame.

3. Should they be so situated, please at least make sure the impending insulation meltdown and dead-shorting of the lithium batteries occurs more than half an inch away from the aforementioned batteries.

If all of these failed to be true, then welcome to my life.

I’m glad that said 24 gauge wire burned through its plastic connector housing before Bad happened.

A few minutes after setting up the charge, I heard my charger beep furiously, indicating a premature charge termination (that’s what she said?).

I turn around and an enormous white smoke cloud is hovering above the scooter back end. Fearing the worst, I grab the thing, bust through the nearest non-emergency door and pitch the whole vehicle into a snow pile.

The heat was intense enough to melt the acrylic connector mounts  and completely vaporize the smaller balancing connectors. The large Deans connectors were fine, because the short occurred through the small wire.  Very fortunately I got it out of there before the ass end of the lithium cells overheated, because angry Li cells are not to be dealt with lightly.

Combine with the very close packing of the cells in the scooter chassis and it could have been.. well, more interesting.

Anyone know what the plastic is that most R/C hobby stuff and electronic casings are made of? Whatever it is, it burns leaving a hideous, acrid, obnoxious smell that can only be described as one part lifelong chain-smoker, one part wet decomposing grass clippings, and one part burnt garlic toast. It also covers the surrounding area in a sticky black oil-like substance.

And it does not ever come out of things. It’s the same stuff which they make power MOSFETs out of, apparently, since those smell just as bad.

The batteries seem to be fine, but the back two cells in the belly pack may have localized thermal damage. Since I don’t like playing lithium polymer games, I might replace those two cells. This is also an opportunity to rethink my battery strategy. The electricals of RazEr are a complete pitch-together hack made of double-sided tape, Goop, zip ties, and heatshrink.

tl;dr use thicker wire 4 batts

LOLrioKart

Over the weekend, I was using my charger to recondition some found SLAs in the great MITERS lead-acid battery pile, before the really dead ones (including 10 car batteries) were sent for disposal.

I remembered I had one of these. And this. Thus, on a whim, we haphazardly taped together a rudimentary electrical system for LOLriokart out of some of those found batteries.

It was just like the first RazEr test run – a knob with no spring return in an awkward position requiring a delicate balance of dexterity and madness to operate. Fortunately, with a 4 wheeled vehicle, no balance was required.

Large model airplane controller and servo tester strapped to the kart for testing...

To my surprise, the sensorless ESC was able to get the kart moving pretty adeptly. I guess that “12mhz CPU” is good for something. (Also, there’s much backlash in the chain drive, so the motor can probably move enough for the ESC to pick up the switching sequence before it hits a load.)

A test video is here.  The 24 volts of SLAs were sagging to under 18 volts loaded.  I estimate the speed at maybe 10MPH, +/- some. Still, in close quarters like the N52 hallway, it was mildly exciting. Obstacles included night janitors, that fire extinguisher, several polished wood and glass art display cabinets, and the MIT Outing Club championship canoe.

And the very well-placed panel of plywood at the end.

See? I promised I’d get the kart moving before February! I just didn’t say how moving!

When the waterjet opens again, and I get a larger sheet of aluminum to finish the battery basket, then the *REAL* fun can begin.

Speaking of the battery basket, here’s the concept.

3D model of the battery basket.

Made of 1/8″ and 1/4″ thick aluminum, it will support the batteries (bounding-box outlines in clear gray) with room to add some shock-absorbing rubber or foam padding. The mounts will clip onto the chassis (bottom halves of the clamp mounts not shown), and be on adjustable-width sliding mounts. The adjustableness compensates for the fact that I don’t actually know how wide the kart, and these compliant mounts allow me to move the batteries slightly if something turns out to be in the way.

I decided to go for the 4-across mounting style just because it leaves more usable (continguous) volume under the basket.

The top plate will be made of whatever nonmetal I find when I cut everything else out. I have it spec’d out as wood, but it could be fiberglass, MDF, Lexan… etc. It will be spring-loaded to the top of the batteries by the corner mounting holes. It will also double as the electronics mount.

Combining the topic of electronics mounting and Conveniently-placed Plywood Planels of Kart-stopping (+1), and continuing my everlasting quest to engineer my way around simple and reliable solutions, I have thinking about giving LOLrioKart power brakes.

Using a beefy servo mounted on the brake mount on each front wheel,  and some interfacing with a foot pedal (you know, like a servo tester, or a microcontroller interface that also handles other vehicle auxilary functions), just use the servos to yank on the levers. With such an interface, I could actually adjust brake balance, timing, bias, and that stuff.

Wait, can’t you just run some cables? MITERS has bins full of bike brake parts I could just pull.

Yeah, but I’m lazy. I would much rather rebuild the brake mounts to include mounting provisions for a servo, then interface with the brake pedal using a clumsily-built and possibly unreliable electronic interface. It’s all the rage these days, like aluminum billet where a simple clamp-and-weld would have sufficed. Besides, since this is an incredibly bad idea to begin with, I might as well add another layer of bad-idea.

(It is indeed easier, faster, and better to route two Bowden cables – don’t get me wrong.)

Work on LOLrioKart will probably taper off a bit as the semester begins.

Speaking of semester, today is Registration day (as well as Techfair), and I need to wake up before sunset.

One week report!

Dec 09, 2008 in Project Build Reports, Project RazEr

So this project has pretty much moved past the “it works” stage onto the “it’s useful” stage. I think that might make it my first ever constructive build project… as opposed to, you know, destructive. Or just absurd.

Over the past week, I’ve been riding the scooter around to get to class and run errands. It attracts a fair share of stares and questions, since nobody really expects to see a Razor scooter cruising at 10mph. Yes, I mounted the real e-bike throttle onto the handlebars, so it’s actually controllable. No, I have not went back and fixed that terrifically beautiful hack of a signal interface.

I have also been pretty good at “path look-ahead” to avoid potentially lethal potholes, but have yet to attempt a flying leap over the railroad tracks.

The range from a full change has been confirmed to be over 3.5 miles. This is quite in line with the ~4 mile calculation, and seems to include all of the inefficiencies that I did not include in the approximation (rolling resistance, terrain variation, wind resistance, etc). This is perfect. Why? Because it lets me make several cross-campus trips per day – e.g. to get to class and back, to get food, and to get to MITERS and bumble for hours on end. Here’s an example.

Yeah, all the blue lines are on top of eachother. This consists of the following trips from today:

  1. To class in the morning (Ames St. @ Amherst to Mass Ave. @ Memorial Drive)
  2. From class to the Media Lab (Mass Ave @ Mem Drive to Ames St.)
  3. From the Media Lab to more class (Ames St to Mass Ave @ Mem Drive
  4. Class back to dorm, through the north building cluster (Mass Ave. to Ames St. via Stata Center)
  5. Dorm to Student Center for some dinner (Ames St. to Mass Ave)
  6. And back.
  7. Dorm to 7-Eleven (Ames St. to Main @ Portland)
  8. And back again.

That about covers my needs, really. 3.5 miles and back is also enough to make it to area hardware stores, which is of course a priority.

However, I have also been constantly maintaining the motor to keep it running. There’s nothing electrically wrong with it, but the design of the motor and wheel interface is that

1) it tends to force the motor apart, since the inside of the wheel has a chamfer on both sides and I have a matching outward chamfer on the motor endcaps, and

2) the wheel itself tries to torque the endcap tie screws out. How the hell does that happen? I had to cut indents into the tire’s plastic rim to pass those tie screws. They are circular in profile, and my guess is that with every compression cycle of the tire as it rolls on the ground, the indent sort of cams up against the screw and torques out a bit.What then happens is the screw sticks out far enough to bang against the aluminum frame, making a very audible click that tells me to stop immediately and ride unpowered the rest of the way.

This phenomenon has even defeated red Loctite.

I’m working on a design revision of the outer can that doesn’t rely on those screws to keep everything together. Either they will be routed internally, or the can itself will have some other method of fastening things while still allowing the tire to be changed. For now, the stator won’t change, since I can’t find another giant copier to rip another stator out of.

Oh, and I have also let all the Li cells go horribly out of balance. I haven’t made a charge plug for the DB9  charge/balance port – rather I have just been attaching alligator clips from my charger to wire leads shoved into the main + and – pins. Safe, eh?But now I have discovered that the cells have up to a quarter volt disparity, which is too much to let the whole pack charge at once. I’ll get around to making the balancer plug and cycling the cells appropriately.

The bottom line is that IT WORKS and IT’S AWESOME.