Archive for the 'Project Build Reports' Category

 

A Little Messing with the Modela MDX-20

Mar 31, 2013 in Project Build Reports

My day to day task of what is essentially making sure ducklings don’t fall into storm drains (but in an engineering  instruction capacity) means that my free time is generally more fragmented. Students being about to come in and out of the shop at will means I can be interrupted by questions at any time. So, I’ve taken the past few weeks to fill in some knowledge gaps that I’ve not paid much attention to before, but have always been nagging in the back of my mind otherwise. Since they’re not extensive build projects, I find it easier to fill the voids while supervising the class. These little exercises include more experimentation with CNC subtractive fabrication (read: machining) and making things in CAD that aren’t straight lines for once.

cnc… sort of

One of my darkest and most personal secrets, which I guess I don’t really try to keep but everyone just seems to assume otherwise, is that I don’t know how to CNC things. There, I said it. You can judge me now. By “CNC things” I mean using traditional CNC  3+ axis milling and turning tools. I’ve done some simple “2.5 axis” things using the many EZ-Trak type machines on campus, but haven’t ever gone through the whole design-part-import-into-CAM-software-generate-toolpath-postprocess-zero-the-machine-and-go process once completely. I may or may not have done a few of those things disparately, or taken over for someone / handed a job off to someone else. But never once through.

I think the primary reason behind this is just that I caught onto rapid prototyping machines and processes early on – waterjet cutting and laser cutting in particular – and basically started designing everything around the much easier availability of them at the time. Another turn-off to “independent practice” later on was that the 2.5 and 3 axis machines I am around the most often are also heavily trafficked, and they’re often left in states which were way different than the last time I saw them, or all the tools have changed.

Really what I need to do is just spend some damn time going through the whole process, machining random objects. You learn just by using something so damn often, which is what I did initially with manual machine tools and the waterjet (and then 3D printing through building my own machine). To do this, ideally I’d have a machine which isn’t used often and so I can deteminately track the state of for the first little while.

Or I could start messing with something so simple it doesn’t have any states to mess up. I have one of these little things in the shop:

These contraptions seem to be used frequently by model makers, and they are the choice of machine in the MAS.863 class How to Make a Mess out of Almost Anything, which I helped TA last fall in said shop. They were also the staple tool of this guy, whose site I ran upon a few months ago for the first time, then recently once again, and now view as some sort of god. His Guerrilla Guide to CNC is a helpful read for the uninitiated (while I found the machining knowledge mostly nothing new, it was still an enjoyable read and I consider it an excellent resource if I also get into resin-casting). The most recent time, it was shown to me by one of my friends, upon which I went “Oh! Yeah, I’ve seen that.” and immediately went to hide in a corner afterwards. Reading it thoroughly was pretty much punted me into starting to mess with this bugger.

The best part is that now that the class is not running, the Modela has been sitting mostly idle. In the class, its primary duty was routing small circuit boards from copper-clad PCB stock, and it ran from a Python GUI running a C++ backend (which I am told is new – last year, it was run straight from the command line), which was entirely coded by the professor and his students. But it also has a proprietary Windows software, Modela Player, which is basically a simplified graphical CAM software. Nifty.

Let’s begin. I modeled an appropriate test part in Inventor and exported it as an STL file. Modela Player can import IGES 3d files or STLs. Based on my failure to convince it to read my IGES outputs, it seems to like STLs much better. Hey, it’s like a 3D printer, except it does the opposite of print!

Yeah. Hey, it has flat regions, internal radii, external curves (the o_O is made of filleted cylinders) and some hard to reach inner corners. Perfect! This is what the MP interface looks like. All of the usual CAM-like buttons are there – stock size, faces to machine, and adding machining processes.

For reference, I mostly followed this tutorial. I gave it a read-through, then tried exploring as many of the features as possible without it. Overall, I can say that the software is set up very intuitively and there is a definitely workflow, though the names and labels of functions could have been better translated. Since the software was originally Japanese, I’ll give it a pass for being a little Engrishy.

This is a roughing pass that I generated. MP is interesting in that the feeds and speeds are completely wrapped up in the tool settings. It comes with several built-in tool definitions, and you can add your own. Each tool has a certain feed rate, spindle speed, cut depth, stepover (basically density of those horizontal lines), etc. for each material. So, all you have to do is literally select a tool and indicate your material (which can also  be added custom). I elected to add a 1/16″ carbide ball nose endmill and 1/16″ square ended carbide endmill, using the settings derived from a built-in 1.5mm cutter.

MP also comes with a cute graphic visualizer of what your cut will look like. This is a preview of the roughing cut.

I discovered that there’s not really a way to “zero” anything, like on the corner of a stock. It seems like this machine is intended to cut shallow depressions into a block of material that is of indeterminate size, which – go figure – is what you’d do for a molding and casting scenario. The software even has built in draft angle capabilities and “margin” adding – it will automatically add a ring of full depth cut empty space around your parts.

The machine appears to the computer as a printer. The first time I got to this stage, it would output everything at once, but nothing would happen on the machine side. Some investigations concluded that the Windows side drivers were completely messed up. I had to uninstall everything related to the machine (including its strange USB-to-DB25 cable adapter) and reinstall it in the manufacturer’s recommended order before I could get the machine to perk up. This is what the output screen looks like if you have multiple operations involving different tools. You hit Continue and the machine will run its cycle, then pause at the end and move to a tool change location.

Or, at least, it’s supposed to. I tried a few different settings which may or may not have been the tool change location, but none of them were convincing enough for the machine to follow, it seems. While I had instructed it to go to an unmachined location so I can zero off the next tool, it just stopped at the end of its last machining motion on the roughing pass, which happened to be directly over the sinkhole in the middle of the first O.

Gee, thanks. And I have yet to discover how to jog the machine yet, if it even has that function.

Oh well. Onto pictures of the process.

The stock of choice is a little brick of machinable wax left over from the class which I sawed into essentially the right size. You line up the stock visually, on the white grid, and double sided tape is the official work fixturing solution. For the limited capabilities of this machine (which can move at a blistering 15mm/s maximum), that’s perfectly fine. Again, no way to jog and zero or touch off the stock that I’ve noticed.

Z axis zeroing just entails driving the spindle down towards the top of the piece, sticking in the tool, then letting it sort of fall under its own weight onto the material. You’re *supposed* to hit the Tool Up or Tool Down buttons with the tool already in the spindle, but that only increments in 0.1mm as far as I understand.

After the roughing pass completed, I changed the tool in the machine by moving the tool up until it was well clear of the material. Unfortunately, as mentioned before, I have yet to discover how to make it go to my big unmachined margin to the right so I can properly touch off the finishing tool, a 1/16″ ball endmill. So I eyeballed it.

It was pretty good eyeballing – the tool was too low by about 1/2mm. Either way, modeling wax is very, very permissive about how it is to be machined.

The finishing stage took approximately 6 or 8 hours, and ran overnight. The finish in the end was very, very clean. Like so clean. Way better than any 3d print can ever get me, for sure!

I decided to try something a little niftier, closer to a part I’d design and then export as an STL. I downloaded a spiral bevel gear from Thingiverse since it looked pretty machinable and was much more complicated.

This time, the path generation was a little more complicated. There’s no way in the software to say “skip this feature since your tool is too short to machine it”. Instead, you define specific rectangular regions to machine – the default is the whole part – that get pathed independently.

To avoid the through-hole in the middle which would have been too deep for the finishing ball endmill, I therefore had to make 4 rectangular regions which very carefully avoided the hole. There’s no “boolean difference” allowed in this operation.

Here’s the piece during one of the finishing passes. I ran the cutters faster this time, so some of the non-rigidity of the machine is visible in the gear teeth. I also set the margins to be very small and mismeasured my wax chunk, so it machined off most of its own sides anyway. I again couldn’t get the machine to go to a specific spot for a tool change. I must be misinterpreting what it means by “Tool Movement Location”…

I intend to keep experimenting with the machine in the coming days. The machine, sadly, does not talk in G code. Rather, it uses RML, which is Roland’s own little language. There seems to be an avid community of modders that replace the controller inside with a custom board that can interface with commercially available CNC driver software.

I  see how this machine can be very helpful and intuitive for model makers and designers who don’t have an engineering background, and I definitely see how it would be useful in making super fine molds for casting plastic parts. What I’d like to get squared away in the next few days is how to persuade it to go to a known spot for a tool change, something which the Media Lab tutorial I linked to at the start seems to hand wave. Once that’s done, then I will definitely consider trying a few actual molds. Maybe it’s time to stock up on that high density polyurethane board stuff…

However, I definitely should play with the EZ-Traks some more. I think my preferred realm still leans towards using a machine with more cast iron.

 

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 Soft-Launch of DeWut and the Motorama Robot Conflict 2013 Recap

Feb 21, 2013 in Bots, dewut?, Events, Überclocker ADVANCE

So about that Motorama liveblog…

Anyways, now that the event is done and everything has settled into normalcy again, and with the completion of the best user manual / instruction guide I’ve ever made (I think so anyway…), I’d like to make the DeWut publicly available. Get yours today!

At Motorama, 8 of them were run in various fashions. The three in Überclocker, as well as five in a revamped version of Blitz. Five 3-pound motors in a 30 pound bot. That thing was made of motors. Moto was a great durability test of the gearboxes and outputs under various loading conditions. In Clocker, they were indirect driving wheels and gear-driving the fork. In Blitz, however, they were each direct driving a hard rubber wheel. One gearbox grenaded at the event when Blitz took a pneumatic flipper directly to a corner and bounced a few times. Clocker’s fork drive held up great, to my amazement, because there were points during the tournament when I was basically using the fork as a hammer.

DeWut is one of the two love babies I’ve been working on for the past few months (RageBridge being the other…) and it was great to see my parts actually being able to stand up to some use. Speaking of RB itself, I had no issues at all with my two boards on Clocker, but a few other folks were running the beta version and I recovered some of those when they succumbed to strange issues, to be diagnosed. Blitz also lost one production board to suspected thermal overload (from driving two DeWalt motors in parallel with the current limit maxed out) and another due to possibly a metal chip short from drilling the frame. Another bot’s suffered some kind of strange failure where the board itself looks totally fine, powers on fine, but never exits failsafe mode no matter what radio is connected! I’ll diagnose all of those and hopefully find that there’s nothing seriously wrong with my hardware.

Moving, on, here’s what went down at Motorama 2013.

(more…)

Überclocker: T minus 1, the Opponent Threat Assessment

Feb 15, 2013 in Bots, Überclocker ADVANCE

As Motorama 2013 draws closer, and for once in a long time I actually have a robot done and tested, dammit!, I’m going to do something that I have not done since before this site went up in 2007. I call it an “opponent threat assessment”, and it… is pretty much that. It’s me sizing up the other entrants in the class I am competing in, based on their BuildersDB registration into, and thinking about weaknesses and strategies. I used to do this all the time back in my early battling days, but in recent history (some time around late 2006) I pretty much stopped thinking about it.

Well, that’s about when I stopped winning anything too. Hmm…

The cool thing about the OTAs for me, in retrospect, isn’t the planning and strategizing, which is something that is clearly susceptible to some cursory words and scribble without any real thought put into it. For me, when I did this often, the best part was simulating the match in my head – enacting various scenarios and responding to them. I’d started way back in 2001-2002 with pitting my favorite Battlebots (when Battlebots The Show was a thing) against each other. Watching hours upon hours of videos from local and builder-run events, too, back when broadband was still a big deal, also contributed to building up the models. In this way, by the time I got to high school, I’d already developed a fairly good mental ‘physics engine’ of sorts, since I thought about these match scenarios so often. I usually can, with fairly little effort, stare at a mechanism or mechanical implement and understand how it moves and how it would react to loads. It’s like a mental real-time FEA.

That’s one of the skills which I regularly wonder how you teach – my general opinion is that an innate understanding of mechanisms isn’t possible without many years of practice experiencing them. With each working or broken device you build up the physics engine better and patch holes in your reasoning.  That’s how you eventually get to the point of just staring at something intensely and then knowing if it would perform in a given scenario. It’s also how I CAD – staring intensely at the computer screen while I run through maybe dozens of iterations of a design in my head before putting anything down on the screen (which would take much longer, so by the end of the process I’d probably have forgotten why I built a part).

All that is nice, but besides the point of this post. It’s just something that I wanted to get out of the way since in my recent forays into teaching and TA’ing mechanical engineering classes, I’ve realized that most peoples’ grasps of mechanical engineering concepts are superficial and very reliant on “monkey see, monkey do” kind of copying, or even worse (to me, anyway, perhaps not to my more analytically inclined colleagues) on extremely meticulous and detailed theoretical analysis which ignore real-world implications. While any method could provide a path for advancement and evolution, one of my goals while I’m here is to get undergraduate students to take charge of their own learning and build more things so they also build up strong mental analytical engines.

Anyways, without further ado, here’s the first OTA I’ve written down since maybe 3 or more website iterations ago. The basis is what’s available to me on the BuildersDB for Motorama 2013 in the Sportsman’s class, discussions on the NERC forums with fellow builderrs, videos of the bots in question from events past, or the odd I’ve-fought-this-guy-and-lost experience. This is also assuming the bot doesn’t just fall victim to One Loose Wire syndrome early on…

1. Blitz.

Threat: Moderate

Despite not being pictured on the DB, I know everything about this bot since I’ve pretty much seen it built in front of me. Blitz is in the interesting position of also running RageBridges with DeWut?!s. Why? Because Adam and I are really the people behind the pile of stuff on Equals Zero Designs. The whole damn thing was basically started as an excuse for us to get better parts we couldn’t find elsewhere. Clocker and Blitz are therefore very well matched in speed and tractive force. Blitz’s weapon is a Sewer Snake like dual-hinged flipping arrangement that can throw opponents forward and over (see its first version build midway down). I’d say that Clocker is vulnerable to any attack which can flip it over, not because it’s not invertable, but simply because rolling back on to all 4 wheels takes a precious few seconds. I’d have to avoid being broadsided – a position which Clocker has no defenses against, and I can get continually pushed around in. Because the lifting forks extend out far ahead of the bot and Clocker is known to be very stable even with a 30lb opponent hanging off the fork, a head-on attack might even be my best option. Blitz is fully invertable, but the doubly-hinged weapon would hinder mobility if it’s upside down, a position which I could try and maneuver it into just by using the fork as a flipper. The greatest threat comes from its speed, which is greater than Clocker’s by about 25%, and the widely-placed lifting fingers, which can easily result in a broadside attack if I’m not careful.

2. Diabolical Machine

Threat: Low

DM is a bot I’ve battled before with Clocker in 2010. For this year, the description on the DB reads “Going back to version 4″. Through investigating the builder’s website, “version 4″ is in fact the bot pictured on the DB, and its weapon is a “reciprocating spike”. Besides spikes being actually an ineffective weapon in the combat robot universe, the bot itself is also rather boxy and has no other pushy features like wedges or lifters, and apparently poor inverted performance. I’m anticipating a match filled with much grab-and-go, since its ground clearance also appears rather high. Based on the published build pictures, the drivetrain is not as powerful as Clocker’s, and will probably max out at around 12-15 miles per hour, typical of most cordless drill drivetrains. So short of a spontaneous system failure, I anticipate being able to both outmaneuver and dominate traction. I’m hoping to execute Clocker’s fairly well known spin move with DM if given the chance.

3. Gigarange

Threat: Moderate

Gigarange is a bot I’ve seen in action personally and on video, but haven’t fought. It’s a classic 4 wheel, low profile, 4-bar lifter bot, similar to Test Bot except less wedgy due to the Sportsman’s class rules. Based on the most recent videos of Gigarange at the Franklin Institute event, it’s quick and maneuverable, but I think I have a speed advantage. Its front lifting plate is much narrower than the span of my forks, and the robot is overall boxy and low. I’m fairly certain I can get ahold of it through a frontal attack only. Again, as with all pusher-lifter opponents, I’d want to drive to avoid a broadside attack, but because his lifter is fairly narrow, I may be able to escape from it by rotation – that is, just driving quickly forward and backward if I begin getting pushed sideways. One weakness of Gigarange I’ve observed is that the lifter is fairly slow to act. Hence, again, I may be able to avoid traction breaking using speed alone. The basic strategy would be to attempt to flank to avoid the lifter arm, but if that fails, try attacking full frontal using lift only (to break traction). I’d want to not plant the upper clamp arm on top of its lifter because it can extend with enough force to potentially damage the clamp and actuator (the clamp arm having almost 8:1 leverage on the actuator).

4. Jack Reacher

Threat: High

I’ve been watching the progress of this bot on the NERC forum for months. The bot is one of the few new flywheel powered flipping weapons around, and despite its complexity, the builder is known for reliable designs. Based on test video posted recently, the flipping weapon definitely has enough punch to potentially 360-flip Clocker on a good shot, but more likely, it will just toss me over. I’m rating the bot high in threat just because it can flip and drive reliably (based on my assessment of its drive motor choice and wheel choice/mounting method), which can be bad news for me if I get bowled over and cannot escape in time. Conversely, the complex flywheel machinery may make for vulnerabilities I can exploit by bringing the fork or clamp down on it. The flipper’s geometry is also one I can exploit – instead of “popping out” like many designs, it hinges back such that the majority flipping action occurs at the end of its lifting plate. Hence, if I were risk-oriented, I might actually try grabbing it by the flipper since the actuation motion would try and kick the robot up, rotationally, instead of flinging. My course of action would be to try and bluff the driver into triggering the flipper into an empty shot (e.g. by attacking, but retreating quickly), upon which I would try to either get under the whole bot or attempt to lodge the clamp arm in the flipper. JR can self-right, but with difficulty and only by propelling itself along the ground a few feet based on its test videos, so if I can trap it backwards and upside down in a corner, it will have very little recourse. The worst case failure mode is being flipped upside down, but I hope to be able to recover from the position before JR is able to reload (a process which takes a few seconds as it spools up the flywheel).

5. K-onstant

Threat: High

I’m unfamiliar with both the bot and the builder, and the CAD image posted on the DB is not too helpful. If it’s as described, then I’m going to have to watch out for the spring powered hammer. Clocker’s top armor is definitely a bit deficient, and there are some components sticking up unarmored such as the clamp motor and perhaps the big fork gear itself. Without knowing anything else about how K-onstant drives or loads the hammer, I cannot really make an accurate threat assessment about it. I’m rating it as a high threat because in the event it does work great, I will need to spend most of the match playing defense to avoid the hammer. Against hammer type opponents, I really can only rush them while the hammer is cocking or reloading, hoping ideally for some kind of broadside or up-ending attack with the fork. The best case is trapping them upside-down, with the weapon fired, preferably against a wall, so they have the least chance of being able to self-right

6. Laserbeam Unicorn

Threat: Low

LU is a design I am unfamiliar with, and I don’t know the builder either, but it does have a fairly comprehensive CAD rendering on the DB. The trouble is that I am not very threatened by said drawing. For a lifter, its wheelbase is awfully short and its ground clearance appears limited. The lifter, unlike Gigarange, also does not appear to run the entire longitudinal dimension of the bot, so there is plenty of space for me to plant the clamp onto. The bot seems rather easy to high-center and break traction because of its very short wheelbase compared to bot length. The only issue would be if it were very fast and well driven – but even so, I think I can approach it head on and break its traction with the inner fork tines first (if the bot’s dimensions are roughly what I think they are.

7. Nyx

Threat: High

I think I fought Nyx at least 5 times at Dragon*Con 2012. The match will be completely dependent on driving – the two bots are essentially 1 for 1 in speed. Nyx had a unique ability to wedge itself using its lifting spike between the fork and frame of Clocker and prevent me from lifting, but at the same time trapping himself on the fork. The match will also be dominated by who has better traction as a result. Because of his lifting spike, I can’t approach him head-on like with flat plate bots. Instead, an intricate series of flanking maneuvers (see all Nyx matches in the D*C2012 video) will be needed to get the forks under him. I’m counting on the arena being enclosed this time to hopefully up my unpredictability in maneuvers and intend on using the walls and corners if possible. If I can hook one of his fairly wide and open side rails with the fork, then I have more leverage as a result.  I would have to drive to avoid broadside and rear attacks especially – Nyx has fit very well exactly behind Clocker in the past, and with this build not having changed widths all that much, it will still be a vunerable spot.

8. Palindrome 30

Threat: Low

I watched this bot being built on the NERC forum, and I’m not really sure if the rail of saws will do much damage. Unlike many newbies’ beliefs, saws aren’t that effective in combat because to do damage, you need the opponent to stay still, something which rarely happens. I do expect that Palindrome can do the most “flesh damage” to Clocker, since spinning saws are spinning saws, but unlike many other opponents it has no capability of pushing or wedging. The weapon is also driven by an easily stallable brushless motor and runs in solid bearings, so it could bind very easily. The strategy with Palindrome would just be to grab and go. I do hope to parade him around the arena and mark up the walls or floor.  The bot also has a broadside vulnerability something which I hope to be able to exploit because its speed potential does not appear to be great (using DeWalts in low gear, though with large wheels).

9. Phoenix

Threat: High

Phoenix is a quick and maneuverable flipper bot which I have seen dialed in recently – it was seemingly unreliable in the past, but now it consistently flips 30lb opponents and can also self right handily. Because of the length of its flipping arm ahead of the bot, I’m going to have to avoid any engagement directly, or allow broadsiding. The arm reload cycle does take some time, during which it is raised up, so I could potentially bluff a flip, then attempt to lock the fork under the arm to block him from reloading. Clocker drives much faster than Phoenix, so I should be able to maneuver as needed. Another potential strategy is to keep the fork slightly up, over the height of the body of Phoenix, and attempt to hook his lifting arm at the top where the cylinder attaches. The body of the bot is also short enough to allow a broadside grab. Overall, I’m still rating Phoenix as a high threat because of the potential to flip Clocker over handily if I miss a beat.

10. Such and Such

Threat: Low

Based on the previous version of S&S and recent Facebook photos posted by the builder, S&S is again a “horizontal clamper” – the whole bot expands sideways using a multibar linkage in the center, and can clamp down on you from the side. It then uses dominant traction to corral you around. This year, S&S is actually a “shufflebot”, or a pseudo-walker that uses continuous cam legs, with what appear to be rubber blocks for legs. Walkers are afforded a 50% weight advantage, so S&S may weigh up to 45 pounds. While Clocker could lift it, I’d have to make sure to grab him on a long side (so the bot’s weight is not substantially leveraging more than an average 30lber) but that, of course, risks being grabbed in return. I do think I still have the traction advantage, however, and definitely a speed advantage because of his shuffling nature. As a result, so long as Clocker doesn’t mysteriously fail, I don’t think I can do poorly against S&S so long as I keep driving and avoiding the hug of death.

11. That Robot

Threat: ?!

There’s not enough information on the DB regarding this bot for me to really make a call. Allegedly it has “spinning arms”, which puts it already into the gray zone of Sportsman rules. Clocker is built fairly solidly, so if “spinning arms” does become a real thing, I hope to be able to back into them and stop them. Otherwise, the bot’s design sketch tells me it’s not invertable. I’ll have to wait and see for this one

12. Tyrant

Threat: Low

This year, Tyrant returns with an actual chainsaw attachment. Trouble is, I don’t think it will do that much damage – it’s not geared very highly, and like all saws, will probably bump and skip off a moving opponent. However, in the name of the class, it will put on a GREAT show I’m sure! Another one of those perennial n00b weapon suggestions is a chainsaw, so many people in the audience ought to identify with Tyrant. Because it has no pushing implements and big exposed wheels, I’m going right at him. In fact, I kind of want to try grabbing him by the saw. Tyrant is quick, however, and the chainsaw is sure to win aggression points from the judges and audience, so I’m going to have to control completely (Complete Control style!) or risk losing by decision.

13. Upheaval

Threat: High

Upheaval is the bot which has pretty much won every Sportsman contest there’s ever been. It’s reliable, packs a massive punch, and well-driven. It also has front drive wheels, so it can really just turret around and wait for and of your maneuvers. I fought it in 2010 with Clocker Remix to predictable results. This time around, I should have actually functional drive motors, short of a spontaneous failure (which is always a potential factor). Clocker is now much faster than Upheaval, but his turreting means I’ll have to be clever in my approach. The basic strategy would be much the same as fighting Phoenix or any other flippy bot – try to bluff a flip, then get under him while the arm is reloading. Clocker has many apparently solid spots up front which I could use to my advantage – the fork will tend to slip its clutch if a sudden force is applied, and the springy legs will hopefully live up to their name . Alternatively, as long as I can keep rolling him over (not grabbing), he’d have to waste shots having to self-right, and I could potentially try and trap him against a corner that way, making self-righting impossible. As long as I can keep moving and poking, I should be able to avoid being flipped. The most important part for me would be to never, ever drive across the flipping foot and never engage directly.

Basic strategy for Clocker also goes something like:

  • Try using the fork as a lifter first, to leverage the opponent off the ground, then grab only if needed
  • Don’t body slam people backwards – Clocker may not be able to exit this position, requiring a two-robot unstick pause in the match. Maybe only do it for effect at the very end of a match if needed.
  • Drive slowly and methodically unless I need to run – recently my “stick twitchiness” has gone up due to me being seriously out of practice. I hope my practice driving with Clocker has been able to resolve it
  • Avoid being broadsided at all costs – Clocker Remix had that weakness, and Clocker Advance has the same long flat sides.
  • Drive upside-down to escape a flip if needed, don’t try to self-right on the spot.

Hope this all works out! I’ll be leaving for the tournament in an hour or two, and hopefully tomorrow there will be a live report from the event.