Archive for the 'Chibikart' Category

 

The Secret of BurnoutChibi

Mar 27, 2013 in Chibikart, Project Build Reports

Continuing on my last post, I’ll talk a bit about how Chibikart is getting a radical design departure from my usual silly vehicles, and later on some about what the hell I’ve been up to for the past week or so. These days, like last spring, my time during the day is heavily  devoted towards herding 2.007 students in some way, including my own “victory garden” of 2.00gokart students who have seemingly become accustomed (read: spoiled) to work late into the night like I do. I’m clearly being the best of influences for my impressionable undergraduates. With my spare time being more fragmented and less conducive to sustained building, I’ve been revisiting some things which I’ve mentally tucked away for better days.

Anyways, to recap, BurnoutChibi is my proposed refitting of the Chibikart 1 frame, which is currently sitting derelict waiting for motors I will never regain the patience to rewind, to a form which offers some more excitement. The reason for the upgrade is twofold – first, I’d like to bring it up to “Chibi 2″ standards, but more importantly, to up the power and show how an inexpensive sensorless drivetrain should be executed. The last time I showed the design, it was a simple one-stage design that was basically the DPRC with more power, and pneumatic wheels.

As I was designing the drivetrain reduction ratios, I began thinking more and more that having 1 speed, even in an electric vehicle, is just suboptimal. With the power levels that the selected drives were capable of, I could reach about 18mph and smoke some rear tires. But that would be it – the fastest it would ever go would be said 18mph. Yet just power-for-power alone – power dissipated by air resistance at speed versus the mechanical power produced by the motors at 50% load – the drives have enough punch to get me to nearly 40 miles an hour. If I were to gear for that speed, it would accelerate very… gently. And that’s only in the ideal case of the motors being able to sustain the high current draws needed to produce enough torque to get there.  Regular permanent magnet brushless motors are really quite limited in the ranges of speeds and torques they can achieve alone.

I’ve wanted to build a multi-speed vehicle for a while, because it still just seems like a better idea. You can shift the peak power and efficiency speeds around depending on your load requirements. Recalling Ben’s successful über-trike build pushed me even more towards that direction – that thing actually has 8 speeds using a bicycle hub gearset as the transmission. I was interested in even having two – burnout mode, and do-more-interesting-things mode.

It was easy enough for me to cook up a custom “shifter” design, but what better opportunity again to test out new parts on the market? Once again, I turned to Vex Pro for the answer. I’ve been eyeing their “ball shifter” (which sounds somewhat painful) transmissions since they were put up on the website.

Many a drivetrain in FIRST have been built using the classic AndyMark shifter transmissions, ever since the drill transmissions went out of style, and AM was the first place I went to when I was looking for COTS 2-speed options. What I didn’t like about them (both now, and years ago in FIRST) was how huge they were. The large, open frame sheet metal design is easy to manufacture, I’m sure, but there was no way I would have stuffed one onto this frame.

The new VEX transmissions seemed to be better packaged, and used a much more compact shifting mechanism. Inside its hollow shaft is another shaft with a round lobe on it that can slide axially. It pushes out on one of two sets of steel balls seated in the outer shaft, overall resembling a rachet wrench’s ball detent. So, depending on where the lobed shaft is pulled, a different set of balls is pushed outwards, acting like a 3-point spline. The output gears have little cavities that the balls lock into. If the lobe is not present under that set of gears, the rotation of the gear on the shaft will naturally shove the balls back towards the center. Pretty nifty.

Seeing no obvious complaints on the FIRST grapevine about them, I’m assuming they’re working pretty well for the competition and are robust enough to shuffle some 120 pound wimpy robots around. But can they stand up to moving a Chibikart at inadvisable speeds, while transmitting enough torque to break traction on asphalt?

I had my doubts – the ball shifting solution seemed like a great alternative to the AM Shifters’ dog clutches, but I trusted the metal-on-metal pushing of the dog clutches far more. Primarily because the dog clutches transmit torque at a greater radius – such that the stresses in the materials are much lower for the same level of torque transmission; and that the engagement is extremely binary – either engaged, or not. The ball shifters seemed to have a much greater potential of being caught “between gears” if one or more balls don’t release.  But maybe that is just a problem if I push so much power through them the ball detents deform.  I was already aware that shifting under power was basically out of the question – there’s no synchromesh devices on any of these things, and even in a real manual transmission car you wouldn’t hold the throttle down while shifting gears anyway.

So, with this many questions about whether the part would be worth anything, it was clear that I was going to have to get myself a set of Vexboxen. Because if nobody in FIRST is going to break them, I might as well.

Or, hold on… time to get myself another set of Vexboxen. I already have a VersaPlanetary that I’ve dissected and taken pictures of and am still waiting on a reason to do a Beyond Unboxing post on – they’re quire nice pieces of kit.

I downloaded the CAD model of the whole gearbox off the Vex website and immediately started cutting it up. First off, nothing was constrained – the solids just floated around, the constraints being broken by the export to a generic format. So I spent half an hour “reassembling” all the gearbox parts.

Next, I removed everything I didn’t care about – namely, all the hardware and the encoder mounting stuff. And the pneumatic cylinder that is supposed to run the shifting shaft. I was intending to cook up a mechanical linkage of some sort, since I didn’t want an air system on this build, no matter how air actuated brakes would have been.

I replaced the CIM motor with my NTM 5060 model using an adapter plate that will be machined. Both motors have an 8mm shaft, so interfacing with the supplied gears shouldn’t be an issue. I took the opportunity to raid eBay of some 2mm endmills to make the keyway in the NTM motors.

A quick fit test to the frame and… My goodness these things are huge. Luckily, when mounted upside down, the resultant sprocket spacing was acceptable.

Notice how these gearboxes are all designed for 2 CIM motors. I have no qualms, if this experiment is successful, of dropping 4 NTMs on this thing and upping my motor sprocket size for even more ludicrous performance.

I flowed some virtual metal around the mounting points to generate these two-pronged mounting adapters. The big gap in the middle means I can still access all the important motor mounting screws. Else, these gearboxes did not seem to require any other additional mounting – even the Vex website just recommended using their stock L-bracket mount.

The output shaft being a Hex of Convenience and Marketing Exclusivity, I decided to just get a 22 tooth hex-bore sprocket from Vex. Okay, Vex, you win this time.

The final drive ratios for each speed, then, are 18.17:1 in low gear and 8:1 in high, for a resultant speed of only 14mph in low gear and 31mph in high. I would have preferred a speed spread of less than 2.0x with a wider low gear, but hey, that’s what I get for not designing the thing. But the extra-tall low gear will definitely prove my hypothesis that sensorless drives can be successful if they are highly geared.

To actuate the gearbox, I needed a mechanical hookup. I was concerned with how much force the shifter shaft needed to operate – the installed shifting mechanism is a small pneumatic cylinder that is capable of nearly 30 pounds of static pull. Clearly this was not required for operation, because that would be ridiculous. After studying the mechanism in CAD more, I determined that it really should not take much force to actuate if I am not applying power at the same time. The actuation method would just need to be very fast to prevent the balls from skipping slot to slot.

I briefly entertained electronic shifting using big solenoids. The required travel was only 1/2″, which is well within the range of big open frame solenoids I could find for cheap. What drove me away from that was finding said solenoid that had a continuous power dissipation rating. Generally, industrial solenoids are rated for only 10% or 20% duty cycle – in other words, 1 minute on then 9 minutes off is 10% duty cycle. More frequently, the “maximum on time” is also specified, usually also 1 minute. Even though electric shifting would likely have been quicker and only required a button instead of running linkages or cables, I was more into the visceral mechanical solution and not particularly interested in cooking solenoids.

I elected to use a spring-balanced cable sort of mechanism with 10 pounds of return force to shove the shifter back into the starting gear. Luckily for me, the lowest gear was associated with the innermost position of the shifter shaft. So, it was easy to find a spring on McMaster-Carr which qualified for the needed return force at the needed stroke (about 1/2″).

What I could not do was find one which also worked with the dimensions of their included shifter coupler. The black object in the center of the gearbox between itself and the standoff-mounted plate is my own quickly whipped up coupler design, which let me fit a spring (not shown in image) between it and the rightmost black flanged doobob. This spring will try to keep the gearbox in 1st gear, so long as I am not tugging on the cable to keep it in second gear.

Some cable adjustment will be needed for sure to ensure synchronization of shifting.

On the other end of things, I put together a simple lever using mostly McMasterables. Did you know McMaster sold random knobs and levers? Now you do. Oh, and little ball detents already loaded into threaded bodies. Super simple instant two-click gear shifter!

This and many other reason are why, this coming Dragon*Con, I am hosting a panel session on how to shop on McMaster, among other places.

Time to mince some metal soon! I ordered this stuff last week. I have yet to assemble the gearboxes, but they seem legit. The casing is heavy fiber reinforced nylon (or fiber reinforced something or other), and the gears are… sticky. Seriously, whatever magic coating they put on these things, it straight up sticks to my hand. Repeatedly, even. I can flat-palm a big gear and lift it straight up off the table every time. Hey, aren’t you supposed to make gears slippery?

The picture quality is so lacking compared to my usual ones because after 5 years of nonstop service and over 11,000 pictures, my free-to-me Fuji S9100 has finally bit it. Cause of death? The USB port just straight up fell off inside and likely shorted something. So, back to the phone camera.

Oh, the thing was secondhand, too, so in a prior life it probably took even more pictures. This thing has been such a brick that I am actually eyeing its slightly newer successor, the S100FS, or even the current generation X-S1.

The Boom and Bust Cycle of Chibikart: BurnoutChibi

Mar 18, 2013 in Chibikart, Project Build Reports

All of my projects go through periods of ups and downs. I either end up demoing them too hard, or they get “temporarily” parted out for some valuable component only to never get put back together, or the weather stops being terrible. Given that the winter is letting up and the sun has emerged above the horizon, it’s time to think about the small silly electric rideables once again.

Almost a year after its construction, the story of Chibikart has played out not unlike many Hollywood celebrities and pop icons. After a meteoric rise to fame based on a single hit. which spawned a wannabe or two, Chibikart bungles a live performance but pulls it together just enough for the next show (At the bottom… though I still stand by everything I said in that dribbling polemic). Afterwards, however, Chibikart is never really the same, and sinks into a life of decadence and smoking magnet wire enamel. With my desire to never rewind those damned hub motors again, it seemed like Chibikart was doomed to live under a table forever:


…with his washed up co-stars…

But another season of 2.00silly-rideable-thing dawns, and as such, I must have an Instructor Vehicle. The Instructor Vehicle is totally out of spec and over budget, because Instructor Privilege. I had to decide what to do with Chibikart. It was easy enough to start on another bunny-brained scheme since I have more than enough parts to conceive any rideable, drivable Eldritch abomination, but when I had the semi-working hulk of one of my projects sitting right there taking up space, I found it hard to justify anything else.

So what do I do with it? This was an equally hard decision. I could just rewind the hub motors one last time and be done with it. Chibikart is actually quite fun in hub motor form. I could make, instead, 4 kitmotters to demonstrate the concept in real life even better – one of the ideas on my Infinite Time*Money Back Burner is to make a to-spec Power Racing entry using Kitmotters, since machine time and labor are not budgetable items.

One of my favored ideas from the beginning was to show how to do a sensorless drivetrain “right”. I spent much time in 2.00gokart scaring students away from using R/C style controllers because of their notorious unreliability and lack of current limiting and start-from-zero predictability. Yet at the same time, I wrote an entire section on this in the latest Scooter Power Systems instructable.

The way I see most people use R/C motors with sensorless ESCs in vehicles is in direct contrast to the guidelines mentioned in the Instructable. They’re generally the biggest (or bigger than necessary) motors you can buy, hooked up to a low or moderate gear ratio calculated from load necessity or cruising speed. While this is perfectly fine for DC motor drivetrain design and also sensored BLDC/AC motors, with R/C controllers it’s just asking for trouble. Huge current bursts will be drawn during the starting regime because the motor might not “start” in the right direction, or if it does, needs to expend tons of power to produce the torque needed to accelerate. More often than not, the first thing that goes is the motor controller, since they’re invariably small and unprotected.  It’s been my assertion for a while that a 50mm class R/C motor (typically 1500 – 2000 watt) is more than enough power, if used correctly, to move a person, but usually I see people gunning for the bigger 63mm and 80mm motors. Hell, that’s even what I run on Melonscooter. And precisely like I warned against, I go through motor controllers like crazy and really shouldn’t try using all 6000+ watts of power – it’s literally enough to get me to highway speeds and I have enough fun already flying by parked cars at 20-25mph.

So, perhaps a more noble goal is to troll everyone by making a properly proportioned sensorless R/C power system with typical hobby parts. I’d pair a 50mm motor with an oversized (for overhead) R/C controller, but instead of gearing sensibly and picking the slowest (highest torque per ampere of current) motor, I’d head the opposite direction: An unreasonably fast motor coupled with a double-digit gear ratio. Remember, R/C controllers love to spin things which are going really fast. In the combat robot world, this technique has a nickname: “Gene-ing” it, based on the habits of one particular builder to overvolt small outrunner motors dramatically and gear them down very low to get tremendous power into his weapons. The whole idea would be to divide down my apparently inertia to the motor so far that even the jerk the controller gives it is enough to move me a little.

I next spent some time on two sites. The first was Hobbyking, shopping for potential motors in the 50 to 60mm range. The second was the still-very-useful Torque and Amp-Hour Calculator whipped up for roughing out Battlebot drivetrains over a decade ago. I was aiming for a few goals, in no particular order:

  • Top speed of about 20mph at my desired system voltage of 37v, to try and conform to at least one of my own rules!
  • Gear ratio in the 10 to 20:1 range. Something easy to accomplish with  two relatively simple stages – such as 3:1 and 4:1.
  • Wheelspin current of about 100 amps or less. This was important – I wanted the wheels to actually break traction at a current which I could find an R/C controller to run ‘continuously’. I mentally derate these things by a factor of 2 or more when juggling numbers. This means that the wheel should never just fully stall out

What this came down to is if I apply a step throttle input to 100%, the motors will have a high enough mechanical advantage to start consistently each time, and as soon as they do, they have enough torque to just break traction.

That‘s my current limit. Doing a burnout. That’s how it should be.

Hence, this project was named BurnoutChibi.

motors

At the end of the night, I arrived upon my component choices:

On the right, the NTM 5060/380kv. This was pretty much the only thing available that satisfied my requirements – any slower and I was going to go back into undergeared startup nightmare world, and the few motors that ranked higher in RPM/V in this size were very expensive (by comparison) “competition” motors. The cost of the motor? $36.

I’ll be amazed if it stays together.

On the left is a Castle Creations Hobbyking “160A HV” controller. Gee, it sure looks an awful lot like an older Castle controller – I’ll call it the Sand Castle!

These things have piqued my curiosity for a while, but I’ve never really bought one to chop up. It turns out they are thermal epoxied together anyway, so it’s not like I can chop it up without permanently breaking something. I blame Apple.

While I could have gone with my usual trusty Sentilons (which I know the operating characteristics of well, have decent startup ability, and enough FETs per bridge to let me sleep at night), I decided to take this opportunity to try something new. If the Sand Castles fail, I have 4 left over Sentilon ESCs from the late and great Deathcopter project. The reason I’m running 37v (10S lithium polymer cells) is because I have two giant 5Ah 10S “sticks” left over from the same project. These Lipos are big enough to beat someone senseless with and probably light them on fire at the same time. You R/C aircraft people are nuts.

Two of those packs ought to be plenty to feed two NTMs flying at top speed. Oh, I’m sorry – did I mention I got 2 sets of the above?

tires and brakes

To dump multiple thousands of mechanical watts onto a Chibikart drivetrain would result in the tiny, stubby caster wheels being ground into a pile of elastomeric powder. I’d need something vulcanized and beefy to stand the increased power, but I also didn’t want to go to 8″ pneumatic tires which are the most common size for small EVs. It just wouldn’t fit the character. Chibikarts have to have small wheels.

I decided to try and get some 6″ pneumatic caster wheels. The 6″ pneumatic wheel is something which should exist, but is difficult to find; when it is found, it’s generally expensive and part of a caster already. Harbor Freight doesn’t sell any, and McMaster is generally unhelpful and ambiguous about which one of their 6″ casters is actually pneumatic. For the record, I have a sample of 2717T41, which became the basis for my hunt. I figured if McMaster sold it, there was a cheap and generic verison of it somewhere.

The answer came from Northern Tool, a  Harbor Freight function-alike of somewhat higher market segment. They have a 6″ pneumatic caster wheel replacement for Somewhat Less Price. I went ahead and snagged 4 to see if they were identical.

Also pictured above is a 90mm drum brake I got from Monster Scooter Parts. One of the missions I set out right away to complete was to find a good front wheel brake solution. While both Chibikarts have had rear brakes, this basically precludes the chance of doing any burnout whatsoever. The last time I could do a brake standing burnout was LOLrioKart, and I kind of miss that.

I started out trying to find a small enough disc brake, since I prefer those when I can fit them in. The smallest disc brake setup available stock is 120mm – a bit under 5″, which would have been too close to the ground; I’d be riding on brake rotors if the tire ever went flat or someone bigger than me got on this thing. While I could have just machined a custom brake disc, I decided to be adventurous and see what a cheap scooter drum brake is like. I have plenty of experience with cheap scooter band brakes on front wheels from LOLrioKart’s first front brake attempt, which ended with me discovering what the Capstan Equation meant in real life (Band brakes grip extremely asymmetrically with respect to direction!)

I settled on these and got some samples because I haven’t ever seen them before and was interested in exploring a new part For Make Better Glorious Nation of Silly Go-Karts. It’s my understanding that drum brakes also grip asymmetrically, though to a much lesser degree than band brakes.

As luck would have it, the two spanner wrench holes on the drum line up pretty much perfectly with the hub lug nuts on the wheel. Well then – that pretty much seals the deal.

What I’ll probably do for attachment is make two standoffs that replace the lug nuts with posts I can screw the brake rotor in from the other side with. Two of them will have a pilot “lip” to seat in those holes. I’ll shim up the bore to be concentric, then drill the other two holes using the wheel itself as a template. I’m sure two standoffs might work, but using all four possible locations is just more robust.

This is what the assembly would look like in real life. Instead of using the long built-in torque arm, I’m going to cut it off for compactness and instead mount the brake body itself with a circular bolt pattern. There appears to be enough space between the stamped housing and the brake shoes to drop some #10 or 1/4″ button-head screws into.

I went ahead and made a representative model of this assembly in Inventor. The bore of the brake housing is 14.5mm, so to use with a 1/2″ axle, I’d have to make another spacer with a seating lip.

The next thing to do was to take Chibikart 1′s CAD file and rip everything the hell out of it. Structurewise I was going to turn this into a DPRC, which I engineered a more robust brake pedal for.

I started adding in the new wheels and testing steering geometry, but soon ran into the curious issue of Chibikart being wider than it was long. The pneumatic wheels and brake combination were threatening to push the width out to 30″, because they were just so much wider than the skate wheels!

Some innovative compacting on the steering linkage was going to be needed.

I pushed in the “ears” on the frame a half inch or so, such that the flanged bearings were almost against the 80/20 side rails. Instead of a DPRC-style spindle which is a bolt embedded in a block made of stacked layers, I went for ultimate compactness and designed the spindle blocks to be made from 1″ aluminum square. Like the current arrangement on Chibikart 1, the ‘axle” is a bolt which is tightened into this block.

The hard part was the steering linkage itself. Part of the reason why I made the ‘ears’ so far out in the first place was so I could get enough linkage travel before the ball joint hit the frame. With scooting the steering axis inwards so far, I would only have gotten maybe 15 degrees of wheel travel in either direction. Besides seriously messing with the steering linkage geometry, the other solution was to move the linkage out-of-plane with the frame. I decided to try this first. Basically, the spindle block is solidly connected to the kingpin (which is now a hex head bolt), and on the other side, a big crank arm with a hex bore cut out of it sits like a wrench over this hex head, retained by an axial screw (not shown). This way, the linkage is moved fully under the frame and I can once again have proper steering geometry without  compromising angle.

This configuration might in fact offer too much angle – there’s not a physical hard stop in the system any more, so I might actually add something else later to prevent linkage overextension.

Back on top, I spent a while editing this sheet metal part in-assembly to get the lineup of the cable anchors correct. This plate is essentially an extended version of the “endcap” found on DPRC’s front wheel spindles, with a bolt pattern that engages the spindle block. The mounting bolt pattern for the brake housing, not yet shown, will also be put into this part.

I whipped up a quick 4.16:1 gearbox using VexPro gear models. Vex has gained my interest immensely after debuting their new line of “bigger bot” parts this Spring for the 2013 FIRST season, ostensibly to compete with the likes of AndyMark because FIRST is now big enough to sustain two parts houses. I actually have a VersaPlanetary box bought completely for the purposes of dissection, and intend on writing it up for Beyond Unboxing one of these days – they’re nice, I must say.

The NTM motors have convenient 8mm shafts already, so any gear that can go on a FIRST CIM motor can be fiddled onto this one.

The new rear end of BurnoutChibi nee Chibikart 1 is basically a DPRC back end with a different motor adapter plate. This time, the 4.16:1 gearbox hovers above the main 3:1 chain drive, with the usual chain tensioning method. At this 12.5:1 ratio, the top speed is right at 20mph and the per-wheel burnout current is something like 80 amps. Not bad, but not exactly good either.

So right now, Burnoutchibi is essentially DPRC on some serious roids, with pneumatic tires. By itself, no matter what, it’s not that exciting any more – I’ve built enough things like this already. And by this point, it’s maybe a little hackneyed. The next post will be about what I intend to do about that…

The Pre-Maker Faire Madness of Chibikart

Sep 29, 2012 in Chibikart, D.P.R. Chibikart, Project Build Reports

Along with most of the rest of MITERS, I’ll be party vanning down to the New York Maker Faire on…. well, now. It’s this weekend.

Like last year, I’m hauling an immense pile of MITERS cargo in addition to a few hapless freshmen and sophmores (who I think count as cargo anyway?). Last time, I brought Landbearshark. This time, I’ll be bringing something about equal in mass but a little more fun: Double Chibis! Tagging along also because they fill space efficiently will be RazEr REV2 and Kitmotter Display Stand.

There go any chance of flying down the hillsides at the NY Hall of Science though.

The Chibikarts, unlike most of everything I build, have been working rather reliably. Chibikart 1 suffered 2 broken motors when MITERS used it for Orientation activities – I’m not really sure went on, but the front two motors were just totally unresponsibe – but the controllers were fine. However, Chibikart 1 still worked with the 2 rear motors, so that’s been its demo state for most of this month.

Last week I decided to crack them open in anticipation of repairs for the NYMF.

Well damn. It looks like my somewhat hastily-soldered phase star-point connections exploded. The solder joints became little balls of solder – indicative of a serious current overload or something. Either way, the damage to both of the motors was similar, so I just re-terminated them. I coated the windings in a thick layer of polyurethane varnish that the high-voltage crew at MITERS like to seal their Tesla Coil secondaries with.

A few days ago, Chibikart1 was involved in a…. “filming accident”.

While I was in the middle of the Poorly Coordinated Death Spiral, the right front motor lost power and started smelling real funny. Upon opening the motor again, I discovered that the windings were actually not burnt – but just shorted. As I unwound the stator,huge chunks of the magnet wire insulation were flaking off and coming apart. I was literally pulling lengths of bare wire from the stator.

My suspicion is that the urethane varnish damaged the insulation of the wire either by being too tenacious (typical cheap magnet wire with sub-300 celsius insulation rating are coated with polyurethane-based enamels) which caused the insulation to prefer the urethane coat instead of the wire, or the solvent was too strong and dissolved or damaged the insulation chemically.

Bottom line is, don’t seal your motor with urethane if it has wires made of urethane. On a similar note, titanium screws in titanium threads will degenerate into the slightly less useful case of a solid blob of titanium.

What was worse, actually, was that the urethane sealed the whole stator into a solid mass of wires. I could not hope to ever unwind this without baking or chemically destroying the urethane in some way. The magnet wire strands just broke off as I tried to pull on them.

I had to rewind both of the front motors, which didn’t take that long since I was used to it:

To give the wires one more layer of protection, this time I insulated the crossing strands with some Kapton layers.

Completed rewind. I decided to group the star point connections into one termination this time instead of attempting to solder a ring of wire around the outside of the windings. The whole mess was coated in epoxy (like I should have done to start with…), and Chibikart 1 is now kicking again.

Chibikart2/DPRC has received no mechanical mods or upgrades, but I did jump the shunt on the 350W Jasontrollers a bit to give it some more punch. Because of the ~25A constant current limit of the Jasontrollers, DPRC is actually a little anemic despite having higher potential power. To really use those motors, I’d need some sensor boards (hmm, I wonder where I could get some) and use higher-current Kelly controllers.

Come see Chibikart and DPRC (and RazEr & co.) at the MITERS display area in the Hackerspaces area (Zone B) at NYMF!

Oh yes, a preview of things to come:

 

D. P. R. Chibikart Garage Hoonage

Jun 04, 2012 in Chibikart, D.P.R. Chibikart, Project Build Reports

Over the weekend, I took Chibikart (and a few tagalongs) to the Ol’ Silley Vehicule Proving Grounds and took a few metered runs up:

It was actually slower than Chibikart1 by a fair margin, hitting only a 72 second best time, compared to Chibikart 1′s best of 62 seconds. On the whole, though, it was more efficient, consuming only 11Wh of battery during that run. The best product score was 784.8 Wh*s.

Neither result – that it’s slower but more efficient overall – is surprising. First, we already know that hub motors are less efficient than indirect drive systems – they have to pull more current, generally, to perform the same amount of work and being would for high torque also necessarily increases the motor resistance (for the same form factor).

But DPRC is slower because the Turnigy 5065 motors have a much lower torque produced per amp even after accounting for the 2.5:1 geardown between them and the wheels. From my adventure building the new motors, I know their torque constant Kt to be roughly 0.12 Nm/A. For the Turnigy motor, at 236 RPM/V, that translates to a Kt of 0.04 [1] – multiply by the 2.5:1 torque increase of the chain drive that that comes right out to 0.1 Nm/A.

This different alone isn’t enough – Chibikart 1 has four motors, for a grand total (lumped parameter) of 0.48 Nm/A, whereas DPRC only has 2 motors for a total of 0.2 Nm/A. Given that the 350W Jasontroller is safely limited to 25A output in all cases, DPRC can only produce half of the acceleration of Chibikart 1. But most of the garage race is spent flooring it, or at near-constant velocity, so only significant speed changes will contribute to the time. Hence why the discrepancy isn’t, say, 50% slower or something.

I have a feeling that Chibikart 1 on 2 motors will get a much worse result than DPRC – it’s only a ~16% time gain (7/6ths) for half of the available torque!

 

Chibikart: Finishing Touches and More Testing!

Apr 30, 2012 in Chibikart, Project Build Reports

hello internet

I’m actually not sure where the whole “Mariokart” thing that’s going around the various tech websites originated from, but rest assured that Mariokart was not an influencing factor or inspiration for this design. If anything, shouldn’t it have comically large, not comically small wheels to be considered a Mariokart? For something slightly closer to Mariokart, see LOLrioKart, a project of mine from Back In The Day.

Specific offenders: Gizmodo (and Kotaku), Buzzfeed, HuffPo, PCWorld, Daily Mail, Geekologie

Example of interesting coverage with no hackneyed Mariokart jokes: Hackaday, Wired, Redorbit

 

Chibikart, being a relatively quick CAD-to-completion project, is not without its share of random late-night-CADing-induced “This is TOTALLY a great idea!” moments. None of my projects are really complete without one. The first, and most immediately apparent, is this:

Chibikart’s steering linkage is made from two rod ends meeting at right angles. This presented a constraint problem since technically the majority load coming from actuating the steering connecting link (the threaded rod and ball joint) is in torsion. While using a “jam nut” fastening method worked most of the time, if anyone (namely, anyone who wasn’t me and didn’t know of this failure condition) wanged the steering too far, it would twist the black plain rod end. The result was totally changed toe angles and usually the wheels ending up pointed in opposite directions.

And then, for another example, someone running Chibikart into a wall when taking a turn too wide and just straight up shearing off the thing. I was surprised at how soft the steel alloy was. Since I deviated from my usual habit of “buy twice the amount you need” here, I was left with no replacements and also no choice but to quickly re-engineer the thing…

In a move I haven’t actively pulled since the undergrad years of yore (like maybe last year or something), I whipped up two replacements wheel mounting blocks out of solid aluminum billet in about 2 hours. These are essentially identical to the existing blocks except with a .25″ wide “ear” that sticks out the same distance as the rod end linkages did. Much better, more consistent, and it gained me another 2 or 3 degrees of steering travel  due to eliminating the big head of the rod end.

This block is also the proper height for shimming away from the upright arms. Now, none of the components rub on eachother and the steering motion is lighter than ever.

Next order of business was at least getting the rear brake parts installed. Here’s the main body of the thing with the Razor fender mounted. It pivots on a 5mm cap screw which is doubly nutted through the mounting plate. The stock Razor torsion spring keeps it sprung “up”.

It slips onto the existing wheel mounting block like so, and is retained from rotation mostly by bolt clamping pressure but I threw in the set screw locking pin just in case.

And it’s mounted. The brake will not be sticking up this far once it’s ready – I have yet to make the rolley cam thing which will actuate it. In its rest position, the top surface will be approximately horizontal.

Both sides installed. If nothing else, these are pretty neat looking fenders on their own.

Oh, so here’s a picture I forgot about last time: size comparison!

Chibikart was tongue-in-cheek designed to be “exactly the front half of tinykart“. It’s pretty close, I think.

And now, more hoonage.

Chibikart was sent up the de-facto MIT Random Contraption Proving Ground, where it completely defied (again) my expectations. The metric for performance on these tests is minimization of the energy-time product. From start to finish, the watt hours of battery energy consumed and the time taken in seconds is multiplied together. Now, the only physical unit that represents joule seconds is the Planck constant, so it’s essentially just a “score”. However, it’s a very telling score. Divide the energy consumption by the time taken and you have average wattage used in the climb (Joule / second). From the average wattage into the system you can remove estimates of motor losses (such as I^2R loss in the windings) and drivetrain losses to get an idea of the output watts. The game is one of efficiency – doing the most work while wasting the least energy and taking the least time.

Clearly, rider weight matters significantly in such a test since for the most part the You weighs much more than the vehicle.

Anyways, back to Chibikart. I managed a run that was only 62 seconds and 18.6Wh – which is on par with Melonscooter being ridden very fast, but consumed more energy – probably because direct drive magnifies motor imperfections. The total product, 1153 Whs, is actually pretty unique in the range of vehicles that have seen this test so far, and is the second best set of go-kart times (after tinykart, which holds the all-time record so far).

Chibikart pulled 1900W on launch as measured by the wattmeter, and the average draw going up the hill going by the Wh results is approximately 900W, or 225W per motor. Pretty close to the predictions. I didn’t observe any “motor unhappiness”, but that was likely due to the outside air temperature being something like 38 degrees at the time.

Here’s a Helpful Infographic made by the master of making helpful diagrams and infographics, Shane. This really needs to be zoomed in to be appreciated:

And the “initial hoonage” video: