Archive for the 'Project RazEr' Category


Project RazEr: It’s (99%) Legit.

Nov 30, 2008 in Project Build Reports, Project RazEr

Update:Test video! Now it officially exists. Low speed only for now, since I b0rked the controller and stuck it in Sissy Modeâ„¢

After eight months of development, five of which were spent sitting on a shelf at MITERS, Snuffles Reloaded finally moved under its own power. And boy did it move… I have already managed to nearly faceplant once on it.

With the project having entered a state of semi-functionality, and with it standing a decent chance of becoming something worthwhile, I’d like to move onto a less whimsical name. While “Snuffles” was cute, it just doesn’t have that aura of coolness.

Okay, so “RazEr” doesn’t either, and the coolness of a Razor scooter maxed out some time in the year 2000. But see what I did there? It’s an E! Instead of an o! For “ELECTRIC”! And it’s a Razor scooter! Get it?? INNOVATION!

Without further ado, here’s the 99% finished vehicle.

Yes, that IS a small, amorphous blob of wire and circuitry duct-taped to the steering column with a knob on an equally duct-taped stick acting as the speed governor!

No, it’s not spring returned. It’s cruise control for cool. Without caps-lock, and potentially more blood.

I finally mustered up the effort to pull it off the shelf and hook up the last four wires remaining on the thing which had kept it from moving for the past few months. With the help of some friends, we got everything bunched together as a test rig – then decided to call it good and wrapped everything in tape. And good it was. This time, functional cameras are immediately accessible, so expect some “action shots” when the sun comes up.

So why is it “99%” done? Well…. right now, it will probably get me arrested if I take it to the airport (then again, when you’re an MIT student, anything will). Obviously there needs to be some wire cleaning and rerouting. I don’t have a correctly sized box to house the onboard voltage regulator and R/C signal interface yet. I have a proper E-bike throttle that fits over the handlebars. I also need to make a charge-balance plug for the battery pack.

It also needs underglows.

However, it is at least mechanically sound, and I was able to ride it back from MITERS. Therefore, I know the range is at least 3000 feet minus one railroad crossing.

The business end. The relevant power transmission implement – there is only 1 – is housed completely in the rear wheel. It is a 80mm diameter custom-built 3-phase brushless DC motor, conveniently hidden within the confines of a 125mm scooter wheel. Maximum power on it is probably about 1000 watts. I have yet to properly meter it.

Even with no torque advantage (as a direct drive motor), the acceleration is pretty absurd. It’s not quite the neck-snapping and rider-launching takeoff of Snuffles 1, but I do need to hang on pretty hard. It is, however, a controllable launch, and will be even more so when a proper spring-loaded thumb throttle is installed (you know, so I don’t have to hang on with one hand and one leg while twiddling a knob on a stick with the other hand)

The whole vehicle is very low profile. Yes, I managed to drop the height of a Razor scooter – with an add-on battery pack. You think pebbles were bad before…

This is the “larger” flavor from Razor, which has 125mm wheels. With the belly pack, I still have about 1.5 inches of clearance. Not going to be bounding over any curbs or potholes, but it’s enough to get around on bike lanes and sidewalks.

There are four 4.2AH lithium polymer flat cells inside, and four outside. They are run in series for 29.6 volts nominal. The packs are rated for 20C discharge (80 amps) and can burst 30C, so I have plenty of reserve battery power. As for capacity, calculation showed that my optimal range at constant power on flat, frictionless ground in a universe with a constant gravitational field at a constant temperature in air with zero viscosity and me approximated as a point mass… is about 4 miles, no entropy generated. Realistically I expect far less, and need to do a real rangecheck some time soon. Anything above 2 miles is really enough to putz me around campus and into town and back.

With an extension of the belly box, and some custom control electronics, I could conceivably hide all the control stuff inside the tube frame, and only have a throttle cable coming out.

Yeah… that’s about how it is.

And it weighs this much. With better mounting facilities, it should weigh around 12 pounds. That’s a hair under 6 kilograms for you people in metricworld.

Anyways, work will now continue on dressing up the control system. I’ve officially moved past the “90% Zone of Project-Related Self-Loathing” which occurs with everything I build – I would furiously build it to 90% of the way, then just get sick of it and forget about it for a while. Now that I know it works, I should be a little motivated to push it through.

(read: expect another update in 8 months)

I have kept a detailed build log of this project and its predecessor on this site.

Bot on??

It’s legi…. wait… nevermind.

May 17, 2008 in Project Build Reports, Project RazEr

There seems to be a disturbing trend in all my recent projects of everything progressing smoothly and going right, then the whole project grinding to a halt at the last possible second before completion and being wholly unrecoverable without spending an additional hefty sum of money. Case in point: TB4.5-SP1 for Motorama had essentially half the robot become crippled the day before I had to leave for the tournament.

New case in point: Snuffles Reloaded half-existed for about 30 seconds before the motor controller failed. The failure is undiagnosed at the moment, but probably revolves around blowing a voltage regulator which then shot the controller logic with 35 volts.

Here’s some build pics from over the past few weeks, and the (95%) complete vehicle, along with the half-existance video.

As the mechanicals were nearing completion, I started playing with the electronics. The R/C airplane controller requires a 1 to 2 millisecond long pulse spaced every 20 milliseconds, which is a hobby industry standard for control signals. However, the electric bike throttle I was going to use puts out an analog voltage from 0.8 to 4.2 volts. This is, again, a different industry standard control signal.

I decided to use a microcontroller to translate between the two instead of the awesome ghettomongous discrete-part boardamathinger that I built for the first scooter. A bit of messing around with blinkenlichten on a MITERS STK500 and I was in business.

After playing with the analog-digital converter, I made the output LEDs act as a throttle display meter of sorts, for kicks. Full throttle, all LEDs lit, and they go out in sequence as I let off the throttle. I might actually implement this using a LED bargraph chip in the future.

Next up was assembling the eight giant lithium-polymer cells without killing myself or burning down buildings.

To solder epic batteries, you need an epic soldering iron. Unfortunately, I couldn’t find the epic soldering iron I used at the Media Lab to assemble A123 cells, which are even more epic than my 4AH lipos. So if you do not have an epic soldering iron, you need an epic soldering tip.

A scavenged rod of copper and some lathe time later, and I had an epic soldering tip which fit a Radioshack 40 watt soldering iron, which I do have. The whole thing is 2.5″ long and is .400″ in diameter at the larger half.

The brass one was for testing and practice, since a rod of copper is actually pretty pricey these days to just fuck around with.

The bottom battery pack completed after some careful iron maneuvering. The epic tip made the whole operation touch-and-go, exactly what you want for soldering batteries. You never want to park the iron on a battery cell for more than a second or two.

This was a bit of a risky operation since I was laying the cells face-to-face to conserve wiring volume. Normally, batteries like this are stacked and the cell tabs folded over one another. One wrong move with the massive solid copper tip and I was probably looking at replacing a cell.

After each joint was made (and its balancer lead installed), it was covered in electrical tape. When all the joints were completed, I slammed the whole thing in a tube of giant heatshrink and parked the heat gun over it.

Giant heatshrink should be a primary structural fastener. When it starts tightening down, everything inside sort of scrambles for the lowest volume configuration, and the end result is a very neat package of parts. The pack was embedded into its mount with some also primary-structural double-sided tape.

This pack constitutes cells number 5 through 8. The balancer lead is a standard 3-pin R/C servo plug, which serves the interconnects between cells 5 and 6, 6 and 7, and 7 and 8.

And the test assembly. The connectors fit into the LASER-cut acrylic endcaps as they should, and some CA glue retains them.

Building the internal electronics bay was more interesting, since I had to fit batteries, large power wiring, a controller, a switch, the charging port, and all associated connectors inside. I cut out a rectangular piece of aluminum as the substrate (way to go, conductive mounting surface?). Originally, it was going to be flanged and shaped with a sheet metal machine to accommodate the parts, but I decided to not get fancy and just mount everything with double-sided tape or epoxy.

The same procedure of soldering and wrapping was done to the 4 internal cells (#1 through #4), with the exception that their balancer leads went straight to the Convenient DB-9 Connector of Cell Balancingâ„¢. An 8 cell pack requires 9 pins to be fully tapped. Guess what has 9 pins?

Three of the DB9 pins went to the rear of this pack, where they met with an R/C servo pigtail which connected the back half of the pack to the balancing port.  Two large power wires also connected to the bottom battery pack, one of which is the 0v  (ground) line, and the other an interconnect between cell 4 and 5.

After everything was assembled, it was time for a test run. Verdict: It moves.

Some shoving and… it fits! To install the thing, I had to take off the folding hinge, cram the assembly through to the back side about halfway, insert the folding hinge nut plate, then slide it in the rest of the way. The nutplate sat snugly above the controller, but not enough as to pinch wiring. This precision engineering part of the build came out (went in?) great.

Before I fitted the internal electronics, I threw together a source of 5 volts for the motor controller. It is an opto-isolated controller, and so needs separate logic and power rails. This was a simple 7805 regulator jammed in the empty space between the controller and switch.

Here’s an assembled-on-the-table test run video (.MOV, 4.8 megs) using the creepy custom throttle interface device. The mechanical noise is from the completely unbolted and unclamped motor and chassis resonating on the table.

Unfortunately, this regulator would ultimately cost me all the work for the past few nights and a good bit of money. Protip: A linear regulator cannot drop such a huge percentage of its input voltage and output any appreciable current. I was most likely hitting the top end rating of the 7805, around 35 volts, and expecting it to output a solid 5 volts with at least 100 or more milliamps of current. Shortly after the video was taken, some things went pop.

My best guess is that the controller logic board was hit with the full 30+ volts of the battery. A switching regulator, or even two stages of linear regulators (inefficient, but hey), even mounted externally, would have prevented this disaster.

Anyways, here’s a pic of the almost-running vehicle.

The ‘empty weight’ is probably around 13 or 14 pounds. Yes, I had to remove the rear brake in order to pass the motor cables – perhaps it would have been better to route them externally. But who needs brakes anyway?!

Regardless, there’s 29.6 volts of 4AH lithium polymer cells, a (former?) 100 amp motor controller, and a very chunky brushless motor shoved into the space of a Razor scooter. I think it’s pretty damn awesome just for that.

I will need to get a new motor controller and devise a new solution to get a stable 5 volts out from the battery pack before the vehicle will run. Seeing as how this will easily cost over $100, it might have to wait a bit. Possibly a long time – we’ll have to see.

Here’s a closer shot of the undercarriage, which houses all the interesting bits.

In the mean time, finals! Bot on, folks, while I attend to these…uhh, pressing matters.

Re(Snuffles Reloaded: Update)

Apr 25, 2008 in Project Build Reports, Project RazEr

There’s plenty in the imaginary part of the update, but you can’t see it anyway.

Hey, this thing looks kind of familiar… It’s the extend-o-pack, with 4 4000mAh lithium polymer cells in place. No, they’re not shorting on eachother, despite the precarious appearance of the tabs.

The Deans connector cutout is a snug fit and should hold a female Deans in place firmly with some CA glue. I need to remember to undersize slots and holes for the LASER cutter by a few thousandths to account for its kerf (since it cuts on the line).

Here’s how it was built.

This is sacrificial vehicle #2, another junked scooter that was sitting around MITERS. I’m not sure if it’s an older generation or what, but there are quite a few structural differences compared to the new A3 model I’m converting. It’s certainly beefier in the folding joint (0.1″ formed steel plate!) and brake area, and there is more material in the chassis.

Efficient re-engineering or corner-cutting?! The world may never know.

The plan is to cut a 12 inch segment out of the chassis and use it as the extend-o-pack body.

After a trip to the bandsaw, this was what remained. It was much like partitioning a fish for cooking – remove the tail, remove the head…

…and clean the middle. Oddly enough, with the parts that remain, I could make a very innovative vehicle.

Here, the side flanges that used to form the upper deck have been milled off, and in a previous unpictured operation, the mounting ear holes drilled.

It might have been better to mill with each flange facing upwards, since the cutting head is always at the same height as the vise above the table, but the vise might not be aligned with the table axis. I tried centering it in as well as I could, but across 12 inches of travel there was still .003-.005 of deviation, enough to have a flush-cut flange at one end but a very light remnant of it at the other.

Oh well, I’m not that good… yet.

After trimming the flanges off, I milled the remaining channel down to the design height of .606 inches, which is just enough to clear the two cells with some breathing space above. I did this in order to minimize the ground clearance hit – these things aren’t known for their great terrain ability, and I was only going to make it worse by sticking batteries under it.

Here’s one of the LASER cut acrylic endcaps installed. They are retained by some drops of CA wicked into the cracks between acrylic and aluminum and one screw on each side.

The waterjet-cut mounting ears have also been installed. It turns out I was off by exactly 2mm on the width of the channel, so dumping all 4 blocks in the same vise and running an endmill through at 1mm solved the problem and made it a slip fit onto the bottom of the chassis.

The other endcap with connector cutouts! On the right side is the last leg of the height-trimming cut where the milling cutter went Z-axis Tokyo Drift on me and ended up slipping lower. Oops. Crank the drawbar a bit harder next time?

And here it is installed. It actually looks quite elegant, with the exception of the other side where there is a small gap from the milling cutter slipping.

This whole assembly slides onto the chassis tube and the mounting screws grab the small flange on the underside to hold it in place. There’s lots of potential for “slide-on accessories”, actually.

So, with my Maxamps order on the way (two more cells to fill in the insides!) I need to get going and design the internal mounting structure. I suspect that it will also be a “slide-in” thing, but from the front – mounting via the four holes at the front.

Snuffles Reloaded: Update 2F1(a;b;c;z)

Apr 17, 2008 in Project Build Reports, Project RazEr

It’s getting EVEN CLOSER! Holy crap, it might actually MOVE soon! I took the opportunity of a convenient gap in the schedule to get some more work done. The motor and drive (okay, so it’s one unit) is mounted, and pretty much now the only thing missing is electronics. And the battery bay – because despite my best machining abilities, things still take a while.


The motor mounting & structural & electronics bay mounting rails. These are waterjet-cut out of a sheet of 2024 aluminum. In fact, it’s the same sheet I had TB4.5SP1‘s wedges made from.

There are either 2 or 6 mounting points for the assembly, depending on how ambitious I feel. The quarter inch hole in the center of each beam clear 1/4″ screws to mount the motor between two of them (I cut four, in case I somehow fuck up twice). The 3/16″ slot at the front of each beam latch under the bolt which holds the brake fender in place. Overall, the assembly should only need those two (four) mounts, but I also have 4-40-tappable holes in a rectangular arrangement if I feel the need.

The small chunks are the mounting ears I designed for the supplemental battery pack.

Additionally, I GIANT LAZER’d some acrylic endcaps for the supplemental battery pack.


Well, it’s not a particularly structural application, and plus the Media Lab laser cutter does not have a proper ventilation system for handling the cyanide gas that burning polycarbonate can produce. Also, the waterjet would have left a nasty draft and clouded the edges.

So why not. It’ll glow better.

Additional machining was require for each frame rail thing. First, the mounting hole had to be counterbored 1/8″ deep at 1/2″ diameter to accomodate the motor. I could have done without this, but extending the motor shaft gave me a critical few more millimeters to run the wires out without squeezing them. Here, on the right side, the counterbore is “extended” outwards away from the wheel. The three motor wires will sit in the hollow until they pass in front of the wheel.

Compare to the left, which only needs the dimple to seat the motor shaft.

Next, each rail had to get a chamfer machined on its outside-facing edges. The interior of the Razor scooter’s chassis extrusion has a radius of about 2mm. I cannot duplicate this precisely, so a messy one-size-fits-all chamfer takes care of it.

Because I had no dovetail or  real chamfer cutters, I milled these chamfers with a countersink.

That was exciting.

Next, the back end of the chassis extrusion had to be opened up to ~42mm wide, from the 33mm stock. The width of the motor is 40mm, and I did want some play on each side.

And hey, it’s mounted. I had to shave down the waterjet edge draft on the rails before they fit properly, but that was trivial to do on the mill. The motor slips into place and bolts down with no fuss. Best of all, it’s smooth.

Really smooth. I took the thing for a quick test cruise, unpowered of course, down the hallway. It makes the same obnoxious 3-phase PMDC motor sound, amplified n-fold by the waveguide-shaped chassis.

Here, the brake has been removed along with its retaining bolt, which would normally cross the gap and mate with the latch shape in the frame rails.

While the machines were warm, I turned a new standoff & wheelie bar for the back end. I could have just shaved down the stock one a bit, but I left it back at my dorm room.

It took about as long to find a piece of 1/2″ aluminum, shave it down to 11.5mm, drill and thread both ends, and throw it on there, as it would have for me to go back to campus, dig it up, and return, only to throw it on the lathe anyway.

Mine’s shinier!

So here it is, sitting amongst my random stuff pile. I need to design and build the electronics bay (internal) and the supplemental battery pack (external), then wire stuff up. Oh, and order two more 4AH cells from Maxamps.

I did ride it the whole way back (yes, with no brake… who needs ‘em anyway?), and threw some durability tests at it in the form of bunny hopping off sidewalks, wheelieing, skid-stops, and purposefully riding over stone-paved paths and cracks. Nothing has let loose yet.

At this point, I can make a duct tape test rig similar to what I did with Beta 1. Maybe I should do that just so I don’t have to get my hopes up…

Snuffles Reloaded: Update x[n]+2x[n-1]-x[n-2]

Apr 12, 2008 in Project Build Reports, Project RazEr

It’s getting closer.

In the last episode, the can was filled with magnets and the stator was test wound. I settled on 25 turns per tooth, distributed LRK style, as a starting point, and proceeded to wind the whole thing.

And here it is. This took quite a bit of time, as I had to make about 300 little wire loops in total and make sure they’re all aligned and not overlapping as to have the maximum space efficiency.

I initially bolted the stator to the end of a metal flat clamped to a table so I could tug a bit harder. After it took an hour to do half of one phase and after I discovered I could go much faster and pack as well by hand, I ditched it.

One quirk about the LRK scheme is that one phase is wound from the opposite side of the stator from the other two. I neglected that fact, and while it doesn’t impact the performance of the motor in any way, makes my termination slightly inconsistent, resulting in the need to fudge a bit to link the common ends of the windings. So… yeah, start the bB phase from the other side.

From the other side. The terminations exit from 3 holes drilled in the core mount thing, and then through the space between the motor bearing and the shaft flat. I found that this was a better way to do it than running everything through the center of the shaft.

So after the pigtails were soldered on and everything secured by a good helping of epoxy, it was time to drop the stator in. This was not trivial, because I could not hold onto the thing while they were being brought together since the magnetic pull was so strong.

In the end, a giant vise and the quill of a drill press came to the rescue. It may pull hard, but not hard enough to overcome a 50 pound vise.

Completed motor, side one…

Completed motor, side 2.

So now that it was put together, it was time for a test run. Of course, I neglected to bring my stator mounting pole back with me from MITERS, so I bolted it to the nearest object that had a 1/4″ hole in it.

Conveniently, it was my Institvte-provided bedframe. This brings up great visions of motorized furniture, but that’s another project for another day.

Here’s a video of the first firing of the motor. It’s a bit choppy – One of the side plates is not a good press fit, which, along with the poor molding tolerances of the wheel, caused the side place to mount a bit off-axis.

Using a drill motor with a wheel, I was able to calculate how fast the wheelmotor was rotating at a given drill motor voltage, and correlate it with the AC voltage it was producing at the leads. This gave me a working value for the voltage vonstant Kv, which tells how fast the motor turns given a certain voltage and an unlimited power source.

I worked out the number to be around 65, since it should have been rotating about 306 RPM given my 900RPM drill motor. It was producing 4.7 AC volts at that speed. An actual tachometer will give a more precise reading (and can do one better by factoring in system losses since it’s measuring the motor RPM while driven). Most likely, it’s higher than this, but not by much.

Anyone have a tach I can borrow?

It’s time to address the electrical system. Here’s some 4,000mAh LiPo flat cells from Maxamps, courtesy of the folks at the Media Lab. According to Maxamps, they’re good for at least 80 amps continuously. If I ever draw that much current continuously, I’m making a large smoke cloud with it. I currently (PUN!) have 6 cells for 22.2 volts.

Curiously, Maxamps doesn’t quite explain how 80 amps could be drawn through a little aluminum flashing tab. Yes, one of the tabs is actually aluminum with a copper bit spot-welded to it. Whatever, if it has worked for everyone else…..

Trouble alert.

The 3 cell stack doesn’t fit by about .5 millimeters. I wish I were kidding. However, I knew this would be risky business.

Most LiPo cells of this capacity are around 5.5mm to 6.5mm thick, with the thicker being the higher discharge-rated ones. Maxamps advertised their cells as 6mm thick, which, in a 3-stack, left me less than 1 millimeter of play inside the scooter’s 19mm tall internal channel. The original battery mount was going to be “bottomless”, with the supports for the cells coming only from the sides and the bottom capped by a wide tape, such as fiberglass strapping tape or Kapton tape. My 3S pack with 6mm thick cells (which is not high discharge, unfortunately!) did fit in a test.

However, these cells are actually the 6.5mm type. I won’t blame Maxamps despite the fact that it’s printed right on the cell: “6545135” indicates a 6.5 x 45 x 135mm cell; beecause designing a battery mount to such tight specs is not very wise anyway, considering wires and heat shrink and tolerances have to be accounted for.

The result is that 3 * 6.5 = 19.5 which is more than what a snug fit would allow.

So naturally it’s time to deploy the backup plan.

Here’s a sample section of a Razor scooter chassis. It’s a one-piece extrusion, which is great for mounting stuff to. The part above the flange (which is actually the underside, if you can’t tell from the scuffs) is 63mm side with a 57mm inner width.

Two of the cells stacked give ample clearance for a proper internal mount. Conveniently enough, a regular Razor scooter has enough material between the wheel mounts to hold 4 cells, 2 stacks of 2.

Here’s a rendering of the hypothetical underbody battery pack. The original plan was to use 6 cells mounted inside the body with an optional 6 added on in series on the underside to more run time. However, I have also considered just running one 8S pack split between the internal channel and the external mount.

So the plan is to take the long section of chassis, mill off the flanges and one long side, then make mounting ears that slip onto the underside of my frame and screw in place. I lose about 0.6″ of ground clearance by doing this, which is as low as I want to go anyway. Going this route with only 6 cells isn’t worth the effort, and so I will buy 2 more cells from Maxamps.

The charging port and balancer will probably be located in this secondary channel, since I’m free to machine it however I want.

But wait! Why do all this work to the chassis of another scooter when I could just use plain aluminum channel?!

…because they don’t make 63mm channel, sadly enough, at least not where I can get it readily. The only channel close in dimensions to what I need (that I have found so far) is far enough such that I can’t machine the walls down and maintain structural integrity.

Besides, there’s a junked scooter that’s exactly the size and shape I need.

Hey kids, this is how NOT to wire up lithium batteries!

The first test run of the motor was on 36 volts, off Snuffles I’s pack. I tried a test using only 22.2 volts, and the motor was rather sluggish, and rightfully so. 8S will be advantageous in this situation anyway.

Now that things are actually wired up and moving, it’s time to make the mechanical mounts. I hope that I can get a test drive in before summer…

Bawt on?