Archive for January, 2010

 

Cold Arbor Update 7

Jan 28, 2010 in Bots, Cold Arbor, Project Build Reports

Cold Arbor is reaching that point in a build where another day of work will suddenly make a pile of parts appear cohesive and robot-like. Most of the fine details of the two linear actuators have been addressed, and I’m almost ready to move onto making the saw assembly itself.

Additionally, I’ve had the chance to cut out more parts and put the entire frame together.

Pictured here are most of the parts for the swinging saw assembly and the two clamping fingers. These almost go together as-is.

Back frame rail temporarily slipped together. Drive motors mount to the two projections, and the clamp actuator is (mostly) integral to the frame itself.

Here’s the front frame assembly in the “glob on as much brazing alloy as you possibly can” stage of fabrication. I heat up all the requisite areas of the metal and liberally distribute and wet the surface of the aluminum at the tabs with the zinc-aluminum braze.

The interior fillets on this piece are some of the best joints I’ve done so far. I was able to properly fill these joints because of…

…my +1 Frayed-Aircraft-Cable-And-Chunk-Of-Copper-Tubing Brush of Oxide Breaking!

Instead of ordering $9 stainless steel pencil brushes from McMaster, I chopped one up out of some stainless steel aircraft cable and the nearest small tubing I could find. In retrospect, making the handle out of copper, a highly heat-conductive material, was probably a bad move.

The long and thin pseudobristles allowed me to get the brush into narrow corners where my stainless steel toothbrushes could never hope to go.

The stage after globbing is sanding the proverbial daylights out of the part. Using a large and wide belt sander helps establish a new flat surface that somewhat resembles the old. This actually makes the whole part look really nice, almost like it was meant to look like that!

Here’s the backside in the globbing stage.

… and everything put together. Well, kind of squished together for the shot, that is. It looks great, but how well will it perform…

T-nuts installed into the thicker frame components. The waterjet cuts accurate enough such that these things either press in with thumb pressure, or, in the worst case, require a tap from a rubber mallet.

Mocking up the rear actuator. The bottom plate hole pattern mounts a Banebots 20:1 28mm gearmotor.

I bored, drilled and tapped one of the Surplus Center sprockets to fit the 1/2″ ACME leadscrew. The screw itself has two flats to let the set screws grip properly, and an end-threaded hole to act as a physical stop for the clamp.

I ran into a foreseen-but-ignored problem when making the output shaft for the saw actuator. 11 tooth #25 sprockets have a hub that is barely over 0.5″ diameter. The output shaft of a drill gearbox is typically 12mm, or 0.472″. I already sized the output bearings for 12mm.

So that means there’s no way to actually attach the sprocket to the shaft. Too little thickness to set screw or cross-pin, at least that I was comfortable with. While I could have turned the drill shaft down, this required either changing bearings in the actuator body or making some kind of adapting sleeve. I thought the 11 tooth D-bore motor sprocket (from a scooter motor) that I found would save the day, but alas, it was 10mm.

Naturally, I take the solution that would allow me to abuse machine tools: Make the sprocket hub bigger.

Uh oh.

I turned a steel ring that was fitted over the existing sprocket hub. This increased its diameter to around 1″.

Then I welded the ring to the sprocket on the exposed end. While all this was fixtured on the lathe chuck, of course. The sensitive machine surfaces were covered with a welding blanket first.

I could have done this on a less expensive or important fixture, but the steel ring bore was a hair too big to align properly without wobble. I used the machine spindle to correct the wobble, and decided to just weld it right there while everything was still squeezed together.

Mmm… porosity. After depositing the weld, I turned the surfaces clean.

You can tell I didn’t focus very hard on cleaning the sprocket surfaces beforehand. Oh well – this isn’t going into space.

And all ends well.

Not really a pretend-o-bot, but more parts are assembled. I got some 1.5″ long cap screws to close up the gearboxes, so now they are actually complete.  I still need to find a replacement 400-size motor that can stand 18 to 20 volts, however – the stock BB motors are only safe to run up to 12 volts or so.

Things left to do:

  • Finish the saw assembly, including all the random pins that attach things together.
  • Give Deathrunner some windings!
  • Front wheel hubs
  • Waterjet the last of the components – electronics mounting provisions in particular, and the top & bottom plates.
    • Design this stuff first.
  • Panic
  • Panic
  • Panic
  • Panic
    • Panic
  • Panic

Cold Arbor Update 6: More Drivetrain and the Saw Actuator

Jan 24, 2010 in Bots, Cold Arbor, Project Build Reports

Hey, where the heck did January go? There’s only a week left!

That means there’s only 3 weeks left until Motorama 2010. Time to pump up the volume on Arbor work. While awaiting more waterjetting time, I finished most of the more complex machined parts for the robot.

I’ve grown into the habit of making my own linear actuators for the robots. Acme nuts and threaded rod are cheap on the surplus market, and I can tune the characteristics of the actuator to suit. I prefer linear actuators and linkages for moving robot assemblies around over direct torque couplings on the end of a gearbox, because it’s a bit easier to make a part strong in tension or compression rather than shear, and the leadscrew isolates the actuator motor from torque shock. Überclocker features a prominent exception to this because the clamp arm needs almost 150 degrees of travel, which is much more difficult to accomplish with a linkage and linear actuators.

Anyway, the saw arm of the robot only needs to travel about 60 degrees, which a single 3 link planar linkage can easily do. A clear image of the actuator is visible in this early design rendering of Arbor. It’s essentially a scaled up version of Clocker’s top clamp arm actuator – a leadscrew nut trapped between two thrust bearings.

To make the actuator body, I needed two big chunks of 2″ x 2.5″ rectangular aluminum bar. I had the bar, but both bandsaws at MITERS were simultaneously down.

Well that sucks. People need to stop accidentally cutting hardened steel on them or something.

The time of day I usually work on these things coincides with absolutely nothing else open on campus that houses a machine tool capable of cutting metals. So, I had to create a hackaround…

Bridgeport lovers and shop instructors avert thine eyes.

Wow, what the hell is that?

It’s a 6 inch milling cutter (with R8 arbor!) that a fellow MITERer picked up to cut a bunch of deep slits in steel. So, I’m not fundamentally doing anything worse, but it’s still one of those exercises that has the potential to destroy property and cause personal injury.

So, with the machine in low gear, a constant stream of Tap Magic, and everything cranked down as tight as I could manage, I plowed the cutter through just under 2 inches of aluminum leaving a roughly .02″ thick edge uncut. This was done mostly to keep the block from being pitched through a window as soon as it fell off the saw.  The whole process took a minute, and…

…left a brilliantly clean finish, almost fly-cutter-like in appearance. To remove the block, I just ripped it off with some vise grips.

Alright, so that was the fun part. Here’s a leap of faith and some finished actuator housings. No other special machining hacks were involved in the making of these parts.

Well, maybe one. Bridgeports have a Z-axis knee handle that can detach from the machine and be stored elsewhere. This is so you don’t accidentally run into it and get OSHA on your case, or move the Z-axis setting. Unfortunately, they have a bad habit of detaching themselves from their drive splines, especially when you’re trying to crank the Z as hard as you can. This has resulted in me clocking myself with the cast aluminum handle in the forehead at least once.

After having the handle fly off too many times, I finally got pissed off enough and put a shaft collar on the handle shaft so the stupid thing doesn’t come off. Ever. I don’t care if Bridgeport made them this way for a reason.

The first tool casualty in a long time comes in the form of me dropping a fully loaded boring head after finishing one of the actuator halves. It landed on the point of the tool, and so the entire tip of the boring bar cracked off.

Sad face. Time to start checking Ebay again?!

Fortunately, I was about to continue with a spare.

There’s something weird about the output gear in this actuator. It’s not a gear. It’s actually a #25 sprocket!

I found out that surplus chain and sprockets can literally be an order of magnitude cheaper than using spur gears. You can’t seem to find industrial 24 or 20 pitch metal spur gears for under $20 to $25 a pop. I was going to have to pay out almost $90 in spur gears alone for the two actuators in the robot.

But short runs of chain can perform the same duties. Surplus Center’s chain and sprocket selection was just too cheap to not explore these options. So, I decided to make the motor-to-leadscrew connection using chain instead. Enough sprockets and chain to build both actuators ran just under $11.

The 14 tooth sprocket shown here is squeeze-fitted onto a 1/2″-10 Acme nut, which, incidentally, is also Surplus Center hardware.There will be an 11 tooth sprocket mounted on the drill motor output shaft.

I heart Surplus Center.

The sprocket was actually once an independent power transmission component. To remove the “sprocket” part, I bored it to death on the the Old Mercedes. One cut at the diameter of the hub, and the outer ring with the teeth just pops off and lands on the tool.

While I had the machines still set up, I popped off these protoforms of the rear drive hubs. The flanges will have a 3 point bolt circle drilled into them later, and two flats will be machined near the retaining ring groove in order to make room for custom D-bore sprockets, just like on Überclocker.

Mounted on their respective gearboxen. The smallnubs will fit into bronze bushings in the side of the robot.

The Scene™ as of yesterday. Lots of things “almost, kind of, sort of” done, but not really.

Time to go catch up on waterjetting so I can continue building…

SEGFAULT Update 8: IT’S ALMOST THEEEERRRRRRREEEE

Jan 22, 2010 in Project Build Reports, Reference Posts, SEGFAULT

Hey Charles, what ever happened to Segfault?

You know, that scooter you were building? With those sensors and shit? That one with the big saw blade…. or was that your robot? Or whatever else you’re building?

It is true that Segfault was sidetracked so I could start on Cold Arbor. I suddenly felt the urge to revisit Segfault again after a lull in the parts shipment schedule for Arbor. I wanted to get at least something done by the end of IAP. It’s probably also wiser to get Segfault out of the way so I can continue work on Arbor with no other project distractions, right up until Motorama 2010.

And so I pulled Segfault out from its corner behind LOLrioKart. In the few weeks that I have neglected it, the MITERS industrial grunge fairy had already deposited a fine layer of her dusts and particles of unknown origin all over the frame. In the last Segfault episode, I finished the wheel hubs and got the vehicle supporting its own weight … to the extent that a Segway-like vehicle can without any electronics, that is. Essentially all the mechanical aspects of the vehicle were done.

Except one.

I had always just hand-waved the steering column’s mechanical detail, telling myself that I’ll take care of it when I get there. After all, how hard can it be to rig a few springs to keep the column centered?

Turns out the answer was very. The control box weighed several pounds, and it was on a lever of nearly 4 feet. So, I couldn’t find springs stiff enough at MITERS to provide a strong enough return force such that the whole handlebar assembly didn’t just become a severely underdamped harmonic oscillator. Design negligence played a part in this, because I didn’t even design in any provisions for mounting springs of any sort – just a hole in a piece of 80/20.

Hello, attention to detail.

Anyway, that was part of the reason for my ditching of Segfault. I moved onto Arbor while solutions to the centering problem slow-cooked to completion. The idea I ended up settling upon was using small gas springs, discovered while virtually rummaging the nether regions of Surplus Center.

I ordered these as a curiosity along with some sprockets and chain for Arbor. They are very basic bidirectionally damped, rod mount gas springs.

Alright, so how was I going to mount these? Because I didn’t design in any mounting provisions for actuators, I had to adapt around what existed in the Segfault frame. I decided to use the gas springs in a triangular arrangement, with the rod ends meeting at a single point on the column and mounted to two points on the longitudinal (front-to-back) rails next to the bearing blocks.

I DOTF’d these mounting “clips” for the piston body end out of aluminum bar. The width of the channel is the width of the longitudinal rails, and the depth just a little more than the thickness. These just slide onto the rails, and the downward force of preloaded gas springs keeps them in place. No further mechanical retention is needed.

Here they are installed. The column mounting point is provided by a shoulder screw and a convenient 80/20 channel nut, so I didn’t have to drill the extrusion.

A better view from the other side. I can’t just drop the gas springs in their fully extended position onto Segfault, because one side of the triangle must necessarily get longer as the column pivots about the base. So they had to be preloaded.

No way was I going to just tighten them down while holding onto them, so I enlisted the help of a bar clamp to squeeze the gas springs a half inch or so.

I needed to cut away a small piece of the top plate in order to clear the new gas springs in both directions of swing. So, feeling the lazy bug, I took a bandsaw to my Shiny Precision Waterjet Machined top plate. It *ALMOST* looks like I designed it that way to begin with!

Almost, meaning “wow, what happened here?”

On the same pass, I opened up the square hole that the column protruded from so I could quickly remove the top plate.

The Continued Tale of the Hardware Balancing Controller

Overall, SEGFAULT’s controller has three major components besides the Class D power amplifier that will drive the motors.

  • The complementary sensor frontend, which reads the angle of tilt of the vehicle
  • The PID compensator block, which attempts to hold the angle signal to zero
  • The steering controller, which creates a speed (voltage) differential from the single output of the PID controller and passes it to the two motor drivers. This differential allows the vehicle to turn.

I’ve already explored #1 pretty thoroughly in updates past. An uncomfortably high portion of this entire venture is me firing into the dark and seeing what happens. I don’t have a solid background in the EE side of things, especially not in the hardware, so the whole thing is one big learning adventure. I think I have gotten the complementary sensor rig working reliably enough to continue.

The next longshot of the project is the steering controller. To steer a Segway-type vehicle, the wheels have to rotate at different speeds. A steering controller would have to take the speed command being sent to the motors and superimpose a differential between the left and right sides.

Let’s hit all of these one step at a time. Here’s the latest wiring nest, labeled for convenience.

A. Version 4 of the Complementary Sensor

Documented in Update 5. It uses a MEMS accelerometer and rate gyroscope in unison to yield a roughly linear function of voltage and tilt angle.

B. Negative-5-voltificalator

A charge pump type circuit to create a -5 volt power supply rail for the op amps.

C. 30kHz Main PWM clock

I made a simple oscillator using a 74HC14 Schmitt Trigger inverter chip. This creates a 30,000Hz triangle wave that is centered roughly around 2.5 volts, with a peak-to-peak of roughly 1 volt. It will be the clock signal from which the motor driver signals are generated.

There’s no need to use such a large chip for exactly 1 function,  so I will probably hand this off to a 555 or something.

D. PID Input Level Shifter

The main control loops on the sensor board use a symmetric +/- 5 volt power supply with 0 volts being “straight up”. To be compatible with the rest of the logic, it needs to be turned into something between 0 and 5 volts, positive. Assuming the signal itself stays within the bounds of +/- 2.5 volts, which it should barring extraordinary circumstances (keeping in mind that the PWM triangle wave will only span 1 to 2 volts total), just adding 2.5 DC volts to the signal will take care of things.

This is done with a simple inverting summation circuit.

The arrangement is unity gain, so it’s just PID_INPUT + voltage at the pot. If the PID input is centered around 0 volts and the pot is set to 2.5, then the new center voltage is 2.5 volts.

But wait… why is that -5 volts going to the potentiometer there? I thought the point was to make the output positive?

Because this arrangement of components creates an inverting summer, if I just feed it + 2.5 volts, then the output will become -(2.5v + PID_INPUT). So the hackaround for this is to feed the circuit -2.5 volts, such that the output is -(-2.5v + PID_INPUT), which is to say, 2.5v – PID_INPUT.

This adds 1 layer of inversion to PID input which I’ll have to compensate for elsewhere. Or perhaps it will cause the system to feature the correct number of inversions again? Who knows…

E. The Differential Signal Generator

This is where the now level-shifted signal is split into left and right channels. With 1 potentiometer (eventually to be mounted to the handlebars), a small voltage is either added or subtracted to the motor command to generate two slightly different, but symmetric commands.

The potentiometer has inputs at +5 and -5 volts, making “center” 0 volts. Using either a unity gain summer or a unity gain subtractor (only slightly more complicated than swapping inputs on the op amp),  the voltage at this pot is superimposed onto the 2.5 volt centered motor command.

Thus the vehicle can spin in place assuming otherwise perfect balance (2.5 volt command), as the left and right channel motor commands would be, for example, 2.6 and 2.4 volts, or vice versa. If the pot is exactly centered… well, nothing happens. 2.5 +/- 0 is still 2.5.

This is why I like having symmetric power supplies. Zero is zero… is zero, which is zero.

F. +2 Comparators of PWM Generation

This is a fairly conventional PWM generator using the intersective method. The aforementioned triangle wave is fed into both comparators, while each individual one receives either the left side or right side signal.

So what the heck is “STP”? Besides “Standard Temperature and Pressure”, it stands for shoot-through protection. H-bridge drives need some form of this to prevent the top and bottom switches in each half from conducting at the same time, which leads to Bad Things happening. This was securely drilled and bolted into my head in 6.131.

Essentially, the STP circuit will force a delay between the top and bottom switches changing states. There are a few ways of doing this in hardware, most of them involving a bunch of inverter gates and diode-shunted low pass filters.

Huh?

Just plug this:

$ 1 5.0E-6 0.27182818284590454 46 5.0 50
I 272 288 352 288 0 0.5
w 272 176 352 176 0
r 352 176 416 176 0 1000.0
r 352 288 416 288 0 1000.0
c 416 176 416 288 0 5.0E-8 4.009206304089476
w 416 336 416 288 0
w 352 288 352 336 0
w 352 176 352 128 0
w 416 128 416 176 0
R 176 176 112 176 1 2 1000.0 2.5 2.5 0.0 0.5
w 176 176 176 288 0
I 416 176 496 176 0 0.5
I 416 288 496 288 0 0.5
p 496 176 544 176 0
p 496 288 544 288 0
g 544 176 544 224 0
g 544 288 544 336 0
d 352 128 416 128 1 0.805904783
d 352 336 416 336 1 0.805904783
w 272 176 176 176 0
w 176 288 272 288 0
o 13 1 0 38 8.183476519740355 4.8828125000000005E-155 0 -1
o 14 1 0 38 5.0 9.765625E-5 0 -1

into here. Observe the square wave pattern – never do the top and bottom square waves go HIGH (which would translate to switches turning on) at once!

In the course, I combined the triangle wave generator and STP for a single half bridge (two signals) into one 74HC14. The STP requires 5 inverters and the triangle wave only one, and the 74HC14 is a 6-inverter chip . But, this time, I will need two STP circuits, since I have two sides of the drive to deal with. Nothing special will happen – I’ll stick with what I know for now, and just use two 74HC14s.

Speaking of the two H-bridges, here they are!

A neat little design using the same IXYS half bridge driver chips that I bought for LOLrioKart’s last motor controller.

So how do I control 8 FETs with what amounts to 2 square waves? Locked-antiphase PWM is the secret. Each gate driver controls diagonally opposed FETs instead of two in the same half-bridge. To change the direction of the motor, I merely select the other gate driver. Locked Antiphase PWM is essentially doing this rapid switching of motor direction in complementary duty cycles.

If this complementary duty cycle is symmetric, i.e. 50/50, then the motor will remain still.

If it is off balance, e.g. 75/25 or 25/75, the motor will rotate with half maximum speed.

If the duty cycle is more like 99/1, then the motor will run very near maximum speed.

The final breakdown of the “digital” part of my analog controller is something like:

  • LEFT_STP and RIGHT_STP are “master duty cycle” commands, directly affected by the angle of the vehicle and my steering command input.
  • The STP circuit for LEFT_STP will create an inverted, slightly delayed version of itself. Let’s call this LEFT_STP’. Same goes for the right side.
  • LEFT_STP controls two diagonal legs of the left H-bridge, and LEFT_STP” controls the other two.
  • Hence, the diagonal legs will swap on-times with eachother, rapidly reversing the motor back and forth at 30,000hz.
  • Varying LEFT_STP and RIGHT_STP duty cycles will therefore cause the drive motors to spin.

Now that I’ve written the 1000 words, here’s a VIDEO!!!!

Some things to note about the video:

  • The controller itself was too tied up in wires to actually show on screen.
  • The two square waves are LEFT_STP and RIGHT_STP.
  • They grow and shrink in unison if I rock the controller around.
  • They grow and shrink oppositely if I turn the steering potentiometer.

I haven’t actually implemented the STP circuitry yet, or the actual PID part of the PID controller. Currently, a wire bridges the output of the sensor and the input to the level shifter. I suppose that amounts to a P of 1, I and D of 0.  So what you see on the screen is essentially 1:1 with what’s coming out of the sensors.

Left to go are prototyping the PID stage and the STP circuitry for both sides, then putting the entire mess onto some perf/vector/strip/veroboards! The Inaugural Faceplant draws ever closer!

AAAAAAHHHHHHHHHHHHHH WHAT THE FUCK IS THIS SHIT

Project Investigation: Deathblades

Jan 17, 2010 in Project Build Reports, RazErBlades

With RazEr, LOLrioKart, and my fleet of deathbots, I have on occasion received jibes that my entire engineering career is in fact an elaborate suicide plot.

The reasoning is that since everything I build is by nature harmful to one’s physical health in some way, one of my projects will eventually do me in – especially as I escalate the level of engineering and complexity (and power) involved with each. And because I am purposefully doing so, I MUST be trying to kill myself actively!

Well, sadly enough, I hope I won’t prove all of you right, but check out my latest creation!

Just kidding. I would NEVER build such a thing (blame the other Chinese), nor use it. I mean, come on. It’s gas powered. How lame as shit is that?

I would never stoop that low. But, for the longest time now, I have indeed wanted to build motorized skates. Only problem? I can’t skate.

The only maneuvers I can perform on skates (ice, ball bearings, or otherwise) is “accelerate forward and maintain constant forward velocity”. Note that says nothing about stopping, reversing, or even turning. Like in racing games, that’s what walls and other people are for.

So why on earth do I want to build motorized skates? Because they’re awesome, that’s why. I thought more about this prospective build after I perfected the version 1 wheelmotor. It wasn’t until version 2, though, that the idea was actually within the realm of practicality, for version 1 lacked meaningful amounts of torque.

Version 2 survived for about a year and a half before too many curbhops stripped the case screws and trashed the bearings. I have yet to rebuild it, or for that matter, build any new hub motor, due to other project commitments and the curse of “OK IT WORKS TIME TO MOVE ON”.

Last year, though, I got a free set of Rollerblade (yes, real Rollerblades made by the Rollerblade company) from someone who I presume was going to try and motorize them, but stopped short of actually making modifications.

They look like they’re a decade or two old and are well used, but not beat to shit. Perhaps “well loved” is the better term here. The wheel frame has its fair share of scuffs and scratches, as does the boot, but nothing’s broken. The wheels, though, are nice and cone shaped from what I can only assume is daily recreational use.

I pretty much dropped them off at MITERS immediately afterwards and didn’t look at them again until now. The whole “well, even if I did build them, I’d not be able to actually use them” thing kind of took my mind off the project. But they became background processes. I’d occasionally entertain the thought of actually building them, but wasn’t confident that smaller wheels (in the 70 to 80mm range, as found on most inline skates) could actually move a person. This was before I knew as much about BLDC motors as I do now.

Air Treks

A little known fact about the Version 2 wheelmotor and RazEr is that the single strongest source of attention to the project is the anime and manga community. Why? Because of a semi-popular series called Air Gear, which is a shonen manga and associated anime revolving around (mildly magical) skate-like inventions, which use… of all things, in-wheel something motors. I don’t know if they’re supposed to be electric or black hole powered or what. Apparently, the point of these Air Treks, as they are called, are to make you fly and see cool shit nobody else can see.

Right. But as with any successful series, there are cosplayers and enthusiasts who dress like the characters in the show. Yours Truly has dabbled in this to a degree, and makes a halfway decent L. Out of an almost morbid curiosity, I watched a few episodes of Air Gear.

It’s Dragon Ball Z and Tony Hawk Pro Skater 2′s illegitimate offspring.

Regardless, being constantly bombarded with emails and messages about the secret of Razer, it’s rekindled interest in the project. If I can’t use my own creation effectively, why can’t someone else try? And so I made a few more sports-inclined friends sign their lives away to be my crash test dummies test pilots.

Thus, I made the first forays to investigate the feasibility of hub-motored skates. The project has been temporarily designated Deathblades, because what else can you possibly do on a set of these besides give yourself severe, possibly terminal head trauma?

The Gist

Let’s continue. I excavated (almost literally – it was buried under a year of accumulated cruft) the box of skates from MITERS and started picking them apart.

Luckily for me, the wheel frames come right off. This suddenly just got alot easier. As long as I have the plastic boot with its hardpoints, I can attach fucking jetpacks to the frames if I wan….

… Okay, back up. That’s just a HORRIBLE idea. I’m going to pretend that never happened.

So, the wheels are off, the frames are off, it’s 5 in the morning and I have no sanity to speak of. Let’s start digging for possible parts!

I unearthed all of my Random Small Copier Motors. This is the collective hearts of maybe 5 laser copiers. I have more, but those motors are substantially larger – in fact, a HUGE Xerox motor became the core of Wheelmotor version 2.

All of these things should have stators in the 50 to 55mm diameter range.

I hoped to find at least four matching or quasi-matching ones. Why? Two motors of this size aren’t going to output enough torque to move any meaningful weight around, especially direct drive.

Plus, 4 wheel drive skates. Four wheel drive skates. If I was going to do it, it might as well be good.

I butchered the copier motors and started playing the mix and match game.

The bottom line: I have three stators of identical dimensions, two of the same diameter but identical shorter lengths, and two that match eachother but nothing else, but only by like 1mm difference.

Because I’d have to use different magnet can arrangements if the stator diameters were different, I decided to keep these four around for the time being. They are all identical diameters, but two are 5mm shorter.

This isn’t a dealbreaker, because brushless motors roughly obey a law not unlike a first order Taylor polynomial. With a given motor, you can linearly scale characteristics to a certain point to design another motor. The 5mm short stators will just need some more windings to achieve essentially same torque and back EMF characteristics as the thicker motors, but the overall power output ability will be affected.

These stators are 50.5mm diameter…

…and either 19mm or 14mm long.

Not bad.

I haven’t spec’d out wheel candidates yet, but I presume they will be modified (hollowed out) skate wheels. Wheelmotor Version 2 uses a cored out 125mm skate wheel.

At the point, I don’t expect Deathblade to use 125mm wheels, which seem to be reserved for speedskating, and are also hard to find. The original wheels on the Bravoblade GLX skates are 78mm, and the largest “recreational general purpose” wheel size appears to be 100mm.

Standard Razor scooter wheels are 98mm and seem to feature a moderately large inner diameter, which makes them good candidates. Many people around here own Razors to putz around campus, so I can probably obtain engineering samples from the next floor up or something.

I expect that a 50mm motor will probably have 65mm or larger outer diameter when the can is finished. The 70mm stator in Wheelmotor ver. 2 has a 85mm can OD.

After motor and wheel wanking, I decided to actually measure the mounting provisions on the skates.  The stock wheel frames have a rectangular boss that has a threaded insert inside, which fastens to the boot with a Big Machine Screw.  That’s easy to duplicate. With some caliper tricks, I got the following dimensions:

  • 188mm center to center
  • 24.25 long x 38.5mm wide rectangular boss, 4mm deep
  • 12.5mm height offset between front and rear

Substantially simpler than I had imagined. It’ll not be challenging to machine mounting provisions directly into the replacement wheel frames. The above illustration is for reference only and isn’t part of any design.

So what’s the anticipated final look and layout of Deathblade?

I imagine it looking kind of like aggressive inline skates with a lift kit. I anticipate not using center wheels such that the skates only have 4 wheels and all are driving. The former center wheel volume will be used to house a battery pack and control electronics. The wheelbase will probably be no more than 13 to 14 inches.

Kind of like that, probably not as fancy, and with wires sticking out dangerously everywhere.

Speaking of control  electronics, how the hell do I simultaneously control two foot trolleys of death? Wired is out of the question. Historical motorskate ideas have used a wired speed control with a hand trigger type setup.

I’d have to replicate the function of this hand trigger, but make it control two sides. Luckily, one thing that iRobot taught me this past summer is that XBEE radios are good things. These 2.4Ghz transceiver modules come with a built in Atmel microcontroller running the RF frontend and can be programmed through Digi’s AWESOME HARDWARE CONFIGURATION GUI to perform simple digital IO without writing code.

If there’s anything that I hate doing, it’s writing code.

They can be trained to listen to one, or to eachother. One radio in hand transmitting, and one in each foot taking commands.

For actual motor control, I might start out experimenting with R/C brushless controllers, but ultimately, due to the desire for low speed stability and the need to control torque, I’ll probably try running a variant of Face Vector Modulation.

Lots of speculation. Deathblade work probably will not actually begin for a while, because I’m occupied by Cold Arbor and Segfault until mid February!

Cold Arbor Update 5: Pretend-o-Bot 1

Jan 16, 2010 in Bots, Cold Arbor, Project Build Reports

In the last episode, our heroes were…

… wait, wrong show. Anyways, I said I’d decide between TIG welding and using zinc-aluminum brazing alloy on the frame. I’ve decided to go out on the proverbial limb and put together the entire robot frame using said brazing alloy, due to the fact that getting the MITERS TIG unit up and running in a timely fashion is now impractical.

While other TIG welders exist on campus, I also don’t feel like fucking up someone elses’ machine trying to weld aluminum, something which I have only done on accident with large batteries, and only once with a TIG torch. That was more making a molten puddle of metal than anything else.

I want to investigate how legitimate the alloys are. They’re usually advertised as STRONGER THAN MILD STEEL!!!! or STRONGER THAN THE PARENT METAL!!!! which smell of marketing hype. In other words, if Cold Arbor holds together in the arena, or only falls apart under extraordinary circumstances, then I’ll probably be buying alot more of the stuff.

Alright, here’s Real Production Part #1!

I forgot to take a “before” picture, but imagine the piece with bulging, ugly round corners where I applied the alloy. Result: Not bad for the first shot. There are areas where I didn’t fill as much as I could, and an area where I parked the torch too long – oops.

Aluminum doesn’t change colors before it liquefies, unlike steel. Instead, the surface just becomes a bit darker. Then suddenly your metal starts sagging.

Mild color differences between the 6061 plates and the 95/5 Zn/Al alloy can be seen on the corners.

Another view. You can barely see where the tab-and-slot edges are any more, which is an indication of good fill. Before I applied the alloy, I sanded chamfers into all coincident edges such that each edge was a tiny V joint.

Here’s a shot of the inside edges. Inner corners are the most difficult to do properly, it seems, because you can’t get an abrasive brush into them. I only had “toothbrush” style stainless wire brushes – I’d need a “pencil” brush to get it right.

The brush is what breaks up the surface gunk on aluminum in lieu of a flux. The brazing products are all advertised as being fluxless.

In other words, you apply a large puddle to the workpiece then brush it around to wet the metal.

To hold these T-nutless assemblies together while I held them to the flames, I used center punch dimples to “flare” the tabs inside their slots by punching right on the seam.

This worked amazingly well. Almost to the point where I had a hard time taking the front frame rail assembly apart after discovering I put something in backwards.

Let’s try for something BIG, like the side rails. Click the midsize image to see my unparalleled metal joinery skills up close!

And try not to vomit. The front piece is absolutely horrifying. The problem with using a very temperature-dependent process is that your whole piece practically has to be at the working temperature of the alloy in order for it to flow. Armed with only a propane bottle torch, it was hard for me to keep the alloy molten over distances greater than 3 or 4 inches. By the time I finished one fillet, the other side of the piece has already cooled below 750°F.

Those middle two sections are probably great examples for “How NOT to use Durafix” videos. I could not get in there with a brush at all, so had to resort to prodding the surface with a (melting) rod of alloy, or using a piece of frayed stainless steel aircraft cable. Neither of which worked very well. Thus, the Epic Glob. Areas which I could access with the wire brush, such as the front and back “compartments”, especially on the second piece, were successes.

I am contemplating investing in a cheap toaster oven and modifying it to sustain high temperatures.

Alright, that’s enough for tonight. Both side rails and “intermediate framelets” are complete.

While the aluminum cooled off, I reverted back to working on the drivetrain.

Clockerb0xen cases! They are slightly modified versions of the gearboxes used in Überclocker. They are side mounted, not face mounted. Otherwise, the dimensions are the same – two inches square and about 1.5″ thick.

The stock was roughed out and circular features machine on the lathe, and the mounting holes finished off on the mill.

The nice thing about notebook computers is that they can travel anywhere. The not-so-nice thing about notebook computers is… well, they can travel anywhere.

I got lazy and didn’t print any paper drawings. Instead, I just pulled up the drawing files on the screen while the machine parked on the bandsaw. It’s far enough away from the mill that I’m confident my prized mobile computing implement won’t get showered by slivers of metal.

(Nobody turn the bandsaw on, please…)

Here’s whats inside the gearboxes.

Observe the protruding bearings. This is what happens when you forget that 6901 ball bearings are in fact 6mm wide, not 5. That’s a 6801. The net result is a little bit of bearing overflow – 2mm to be precise, since I used two bearings per gearbox.

A bonehead error but it does not affect the rest of the robot.

The pinions get shoved onto the 750-size drive motors.

I don’t have another set of 14.4 or 18 volt DeWalt motors kicking around, unfortunately, and they’re a bit on the expensive side to actually buy new. I’ll see how far I get with these things.

While the Loctite set on the motors, I turned my attention to the wheels. I noticed McMaster had added some “super soft” rubber wheels to their selection, with a 40A durometer tread. They were cheap, so I snagged 4 for engineering samples.

These things really are soft. Substantially softer than Colsons, and grippy like nothing else I’ve seen. With luck, Arbor will have more pushing power than Clocker.

I had to bore out the axle hole to make the wheels compatible with the robot. This was kind of a hairy operation, because all you have to fixture to is gumball-stiff rubber.

…but it all worked out in the end. The polypropylene hub machines like dense air.

The culmination of all of the day’s effort is PRETEND-O-BOT #1!

Fine, so it’s still more of a pile of parts than anything else. To go: All the wheel hubs, some more missing frame pieces, the structure of the saw itself, all the mechanical parts of the saw, both linear actuators, and the clamp linkage.

And that’s just the mechanicals. One month.