Archive for November, 2012

 

The DeWut Motor: Next Steps, and Design Files for Making Your Own DeWalt 3-Speed Motor Mount

Nov 26, 2012 in dewut?, Project Build Reports

Alright, with the Long Weekend of Nobody Being Open coming to a close, I’ve pretty much finished the ‘version 1′ of this project and am moving onto another fork of it. As discussed previously, the DeWut is an effort to make the new generation DeWalt 3-speed hammerdrill gearbox actually useful. The ‘old generation’, according to my sources, has totally stopped production, and are getting harder to purchase spares for. DeWalt has been the foundation of so many robots I’m surprised they don’t straight up sponsor events.

The short story is that I am skipping over a waterjet-plate stacking version (… what? Are you out of your mind?!) directly to investigating an all-billet, CNC machined version. That seems a very not-me thing to do, and actually I’m going to build a few of the aforementioned waterjet-compatible mounts for my own amusement, but as a potential product, I am in the process of gathering quotes and potential suppliers.

The long story…

After completing the body of the first version gearbox, I went ahead and carved the future shaft out of some 1566 precision-ground steel stock. Luckily for me, the new generation DeWalt gearbox has a rather normal double-D shaped output adapter, so it was a short mill pass to turn the round shaft into the proper mating shape. Unlike the old style gearbox, which had a…. what the hell is that? Apparently a massive stress riser. This would be only a manufacturability test, because untreated steel is going to be far too weak to handle the torque output of the motor. Based on a few FEA torque simulations, to hold the torque of the motor at stall in low gear at 100 amps, the double-D area is going to see about 180ksi of shear stress.

Ouch. That’s some serious steel – comparatively few alloys can heat treat to the 200ksi+ range and not be brittle as glass, and we all know how my last adventure with “heat treating” went. This is a subject I’m not so familiar with and so will probably have to leave to the professionals when I make these into production parts. For my own amusement, though, I’m probably going to buy a sampler plate of steel rods, make the axles, and try heat treating the ends to various degrees.

I also came up with a new arrangement for the motor’s rear retaining plate. Previously, it only held onto that little nub that the motor’s tailshaft sticks into, and was fairly unstable. A more stable method is enveloping the ‘nub’ and pressing entirely against the motor’s endcap. Unfortunately, the brush holders have conductive crimps that end flush with the endcap surface, but can still short on metal. Hence, a ‘gasket’ of sorts is necessary, shaped to the endcap’s outline, and I’ve modeled that as the little yellow-beige-brown thing to be made out of a thin insulating material.

Additionally, the structure of the Nifty Barrel Shifter retainer has changed. It’s now removable from the side and the idea is to hold onto it with a nut from either side. The bolt slotting narrow a little before opening up to the proper center, so it ‘snaps’ into place and is less likely to fall off. This arrangement makes it easier to change gearing (if needed) but also allows the whole thing to be assembled before picking the gear ratio, since it does not have to be stuffed onto the gearbox while it is being lowered into position.

But the most important addition conducive towards kitting this assembly up is the alignment marks on the side. Starting with 1 little indent slot on the side, just line the plates up in incrementing slot count order. I figure the X shaped motor back end is obvious enough on its own. There’s 9 in the structure, then the gasket, then the motor retainer, for 11 total.

This is what the retaining plate looks like now. Of course, one option to bypass the need of a sketchy nonconductive shim which COULD conceivably become conductive by accident, is to make the thing out of a nonconductive material. Garolite (G10/FR-4 grade) was my prime candidate, but electrical grade fiberglass ought to work too. Anything in 1/4″ short of nonfiberous squishy plastics ought to be stiff enough to hold the motor in place.

Moving the retainer plate onto the motor endcap means the whole assembly becomes basically 6″ long minus the shaft length. A nice, even number to work with!

For the purposes of easy cutting, I arranged these plates into separate 1/4″ and 1/8″ assembly files with little “sprues” in between the plates. This ensures that they stay together and cut as one part, and can be extracted whole.  It’s a little more dangerous than individual parts, but machine time is billable by the second and precious seconds are spent cycling the nozzle, so continuous cutting is marginally cheaper too. For small parts, the sprues prevent them from falling into the tank.

However, I think that’s as far as I’m going to go with this design. Here’s why:

Huh. Well then.

That’s just for the 1/4″ parts! Note that I tiled 4 of the assemblies together, so the real cost is about $38 per set. Add to that the 1/8″ plates (another $20 per set) and the heat treated, custom machined steel shaft (estimating $30+ each), plus all the hardware and the laser-cut gasket. I’m basically looking at a hair under $100 just in cost. And only in quantities 10 and up, assuming I have like $8000 to drop on this right now.

As funny as modern digital manufacturing is, and as great as it is if I really just needed 1 of something, I think I can get a much better price by falling back to good ol’ subtractive machining. Hey, as it turns out, waterjetting in real life is expensive.

So, as previously mentioned, even though I could pop out a few for my own use, I won’t be able to introduce these as a viable product (in my opinion – feel free to differ). So what’s next?

DeWut? Design Files

I’m going to make everyone else do it. Contained in the above ZIP file is the two tiled parts to be cut (0.25″ and .125″ aluminum) and a 2D drawing of the gasket to be made from something nonconductive, in DXF version 2000. Also included are the original Autodesk Inventor 2012 3d models of the assembly and an exported version in X_T (Parasolid) which can be imported back into individual solids.

The assembly should only fit together one way, basically described in the first post.

There are a few #4-40 holes to tap in order to make blocks from the plates at first.

The mounting holes up front are properly sized to be threaded 1/4″-20, no more than 3/8″ deep.

The shaft solution, however, is left as an exercise to the reader (the shaft model is included).

The model as-cut should fit the new-style DeWalt transmissions (397892 series), possibly with a little stuffing.

There’s no warranty expressed or implied.

Next Stage

Based on my experiences sourcing the hub motor parts for Chibikart, I believe I can get billet mounts for the gearboxes contracted out through a Sketchy Chinese Dude With a CNC Machine for substantially less price. Leaving the thickness quantum domain also means I can make the gearbox mounts better fitting and have multiple threaded mounting holes, etc.

I basically began translating features found in the stacked plates into a single, solid model. There were optimizations for weight reduction made, as well as thicknesses and details changed to fit the manufacturing technology.

This is the end result. Now, it’s not totally free of potential laserjetted parts, because I’m keeping the Nifty Barrel Shifter holder. It will still rest on the tie rods that hold the thing together.

Check out that new motor mount. It’s a 2-piece clamp affair out of necessity because the DeWalt motors don’t face mount to the transmissions, they stick into them a good 1/4″ or so. Hence, the idea is to tighten together the gearbox and shaft portion, then stuff the motor in, then tighten the mounting collar. With a 1/2″ of support on the collar and additionally 3/8″ of support in the aluminum block, I think the motor’s not going to go anywhere short of having the output shaft shoved through it (by which time, terrible things have happened). This obviates the need for a rear retainer.

Overall, I’m looking at 3 aluminum machined parts, 1 heat treated steel part, and a derpy laser-cut thing. Because the NBS holder is not structural, I’m content with making it from Delrin or ABS plastic.

Incidentally, the mounting holes match a Banebots P80, at least on the front. This whole thing is basically a lighter P80 type motor with 3 speeds!

Of course, I’m not leaving the world of 3DRP forever. I wasn’t going to send anything out without a sanity check for the critical mounting dimensions. I turned, appropriately, to a Dimension 1200ES to produce these mostly hollow plastic shell representations of the gearbox. The Dimension printer is much better at making parts ….. on dimension…. than the Makerbot Flock.

Since it cost 3 times more than the aforementioned Makerbot Flock combined, I would expect nothing less…

And hey, everything fits! I verified that the little slots and cutouts to cater to the DeWalt housing were within reason – if they fit in slightly shrinky plastic, then they will be a little looser in aluminum unless Sketchy Chinese Dude with a CNC Machine is really that sketchy. The overall length has remained essentially the same – this was basically a direct plate translation, after all. The Nifty Barrel Shifter holder is not seen in this picture, nor are the #10 cap screws that will keep the two halves firmly locked together.

The next step from here is to pitch all of these parts to my favorite SCDwCNCM as well as mfg.com (my new favorite snack) to compare prices vs. features. At this point in the year, I’m going to consider getting prototypes in before the holiday breaks a total not-happening, so expect some more news on these DeWuts in January.

By the way, this is what is inside an allegedly “empty” Stratasys Dimension ABS build cartridge.

…seriously? That’s like 50 feet. I only feel slightly ripped off.

Luckily, this filament is 1.75mm diameter just like the latest generation Makerbot Flock feed, so I’ve been making heart gears using the Replicator. Productivity!

 

The DeWut? Motor Project

Nov 22, 2012 in dewut?, Project Build Reports, Stuff

With the Thanksgiving holiday weekend in full swing (read: nothing is open) and with RageBridge revision 3 boards still out for manufacturing (read: nothing’s gotten shipped), I’ve been shaking up the old random project idea jar a little for quick, useful things that I can whip out over the next few days. Here’s one of them. It’s been baking in mind actually for a long time (like since Überclocker-original), and I really think it’s long, long overdue to be introduced. I present the DeWut? Motor Project.

This is a “new style” 12-18 volt DeWalt cordless drill gearbox found in their line of 3-speed drills. New, of course meaning a design that’s about 6-7 years old and not actually found in new DeWalt drills as far as I can tell. Correction! These are still used in the 14.4v XRP hammerdrill line (Models 984, 985, and 939) as well as the 18v XRP hammerdrills (models 988 and 989).

Photo shamelessly stolen from the Robot Marketplace (from which you should buy things). These gearboxes have seemingly never found a home in robot drives unlike the “old style” 18 volt drills, partially because they’re so hokey. The “old style” motor bolted to its transmission as a unit – hence making the whole thing relatively easy to mount, and for the longest time, the Team Delta style mount was dominant in the market and used in many different drivetrains. The “new style”, however, is totally dependent on being in the drill in order to be held together and function. Great if you’re DeWalt and aiming to vertically integrate and cost-cut or something, but it makes using these motors a total bitch. Typical solutions have just involved using the gearbox for parts and repackaging the ring gears (Überclocker ran a haphazardly built gearbox for the longest time).

I am probably one of the most persistent preachers for the Church of Power Tool Hacks, and DeWalt is kind of like the robot messiah because their equipment, no matter how hard to use, is generally well engineered. Recently, I’ve pretty much given up hope on the less expensive Harbor Freight class drills for anything bigger than a 12lber – cost cutting and reductions in material durability have rendered them marginally useful even for 30lbers, as I’m finding out in Null Hypothesis. With that option pretty much ruled out for future robots, I’ve been rumbling internally about coming up with a way to use these damn motors and gearboxes.

There’s two major design paths that the process of making the “new style” gearbox useful can take.

  1. As I mentioned before, just extracting the gears and stuffing them into a box of custom design. This is perhaps the most robust, but expensive and up-front engineering intensive cost, because it’s probably going to end up being a pile of CNC machined billets. It would probably be locked to one ratio (out of three) and optimized for space and weight saving. They already make this anyway (see RMP link under gear repackaging).
  2. Using the gearbox as a unit, Team Delta style, including the ability to use the nifty barrel-shifter gear change. The new-style gearbox no longer has any plastic ring gears molded into the casing, which is great. It’s steel rings all the way down, so the casing really just there just to herd the involute-toothed cats. It will be bigger because the case is so damn huge. There can be billetwork involved, but it can also be made from a pile of waterjet-cut plates.

It is probably obvious which way I’m going based on the above. Even if it’s an empty design exercise, I think retaining the ability to switch gears (if needed) is useful, and keeping the gearbox unit intact is also beneficial because you buy them in units, not by the gear, so if one takes damage or wears out another can be swapped in.

Incidentally, the “nifty barrel shifter” seems to have been the downfall of this style of transmission as compared to the newest DW offerings. They seem to have a reputation of falling out of gear or neutral-dropping under power, which probably causes shredding in short order. So whatever I cook up should keep the Nifty Barrel Shifter in place and stop it from moving or expanding. It’s also important to note that these drill transmissions arenot designed for shifting under power like a car transmission would be. They just crash gears into eachother, so shifting while transmitting torque results in sadness. The ability to retain speed selection would only be for switching between projects and duties, in my opinion.

So that’s the brainstorming part. The next step is the stare-really-hard-at-the-gearbox part, where I try to think of where I can use a plate of discrete thickness to retain the gearbox.

I had to start at 3 ends here – the front of the gearbox, the back of the motor, and the middle end, where they need to come together. This picture is already pretty late into the first revision proces.

I’m serious when I say ‘stare at the gearbox’ – I think I’ve managed to just look at the damn thing for 10 minutes straight. It must have appeared very disturbing to an outside viewer. What I’m doing while staring is literally running dozens of different design visualizations, mentally dressing up and stripping the gearbox and motor repeatedly. Consider it using a natural skill for the greater good of robots instead of… other things.

If a potential design vision looks good, I caliper it up and see if it’s even possible given my design constraints: using 0.25″ and 0.125″ plates, and ideally no other weird thicknesses, in as few of them as possible. If it ends up being bogus, I keep staring.

To the uninitiated, it looks like the most unproductive thing on the planet, but neural-net design is definitely a thing, and you can get pretty far with it. It’s also very, very hard to teach because it pretty much relies on I’ve done this before.

That’s pretty much how it’s going to go down. Notice that I didn’t even bother modeling the gearbox itself – I was just plucking dimensions from it as needed.

The constraint for the front end of the gearbox was that there had to be a feature to center it using a convenient circular shoulder, followed by a shaped cutout that prevented it from rotating using the features of the output carrier cap (the weird black endcap on the left in this post’s leading image). X = 0 for this design was actually that black endcap – everything is referenced off it and made to fit it. The octagonal shaped cutout is clearly visible in the pile of plates at the front in the picture above.

It was pretty clear to me that the motor would not be screw-mounted. I don’t even know why it has tapped holes in it – in the native 18v drill, the motor is just mashed a good 1/8″ into the end of the gearbox and retained by an external plastic structure. I had to replicate that somehow, and this explains the strange plate structure in the middle. It contains the weird Ferrari-sides pattern of the ass end of the gearbox (rightmost in the leader image), then bolts to a plate that actually grips the motor by its air cooling vents.

The entire thing has to be retained by tie rods, and that’s what the X plate at the back is for. Because there’s no way to secure the motor to the gearbox (the middle stack of plates is basically alignment-only), I’ll have to crank the whole assembly down using nuts and bolts.

Finally, in the middle, and intended to be adjustable back and forth, is the Nifty Barrel Shifter Holder. The intention is to pick a gear and then lock down the selection with nuts, keeping the plate in one place (and hopefully the NBS too).

That’s about as descriptive as I can get about the design process. Take every word up there and add in 10 seconds of staring at the gearbox between each one and that’s basically how it went down.

Yeah.

Because everything on this design is empirically calipered and eyeballed, I knew it would just have to get iterated until it worked. Sadly, I don’t have enough 1/8″ and 1/4″ aluminum on hand to just keep getting it wrong in metal, so laser-cut-something will need to suffice. Luckily, MDF is not only cheap but also made to exacting specification and tolerances (unlike, say, even plywood). I scrounged several panels of half-used hardboard (masonite, HDF, etc…) that was 1/8″ thick and decided to pile it twice in order to make the 1/4″ parts.

Laser cutting prototypes is a bit dangerous for duplication to metal, because lasers cut on the line and waterjets offset to one side. The advantage of the former is that holes are implicity larger, so you can totally make a part that fits over a protuberance (like a gearbox end) on the laser cutter and then totally have it fail in metal, because the more precise process means that your dimensions, which were wrong but compensated for by the laser kerf, are now totally wrong. I designed in some liberal over-estimates of kerf for these plates, so I was expecting the laser-cut parts to be super loose.

To minimize the kerf effect, I experimented with a few small changes in power setting such that the beam barely broke through the material, leaving the kerf almost zero on one side. A few test cuts later and I was ready to make the real parts. Splitting the 1/4″ thick pieces into two 1/8″ ones also helps with this, because it ensures that there’s at least 2 somewhat on dimension edges in the part.

Here’s a vaguely DeWalt-shaped cavity starting to emerge. The little socket on the right of the circle is designed to compress the torque-limiting clutch to almost all the way locked. The natural movement of the plunger that actuates the torque clutch is a little more than 1/4″, so it won’t compress all the way. If it slips, then I’ll have to add a shim somewhere.

The barrel shifter holder and gearbox stuff in next. Conclusion: Octagonal thing fits perfectly! That, of course, means I need to bring out the dimensions a little if I want the metal version to fit – the hole has to get slightly bigger.

While this assembly was occurring, I was thinking of how I’d write it up for someone else to do it. I’m probably still really good at designing impossible-to-assembly machinery, so if I couldn’t figure out a streamlined way to do this, something has to be changed.

Here’s the Hokey Motor Mount coming together. This part needed a little stuffing, meaning the tab that interfaces with the motor vent is too wide.

As I aligned the plates, I superglued them together to create a more monolithic piece. If the plates were shifting around, I’d have a tougher time confirming that my tolerances were reasonable. These two Dewalt-Shaped Cavities will capture and align the gearbox, and also prevent it from torquing. However, I suspect most of the anti-torque will still come from clamping friction.

Stuff’s starting to get piled together now. Seating the gearbox into the Dewalt-Shaped Cavity means the torque clutch plunger is pushing up on the ring gears. The motor must be clamped down in place to keep the whole thing together, again. Hokeyness.

With the motor in place, things are looking better.

I neglected to put any NBS-securing nuts on because… well.. I was out of 10-24 nuts. Completely. And so was MITERS, and so was the bin of alleged 10-24 hardware upstairs. What.

I could only find six #10-24 locknuts, and 4 of those were going to get reserved for the tie rod at the end. So, in the interest of being able to play around with the different gears, I left the NBS holding plate unsecured.

Whatever, here is the thing. Not bad for a day and some’s design and rapid prototyping work.

The shaft is probably not going to be made right now, because I need to figure out what to make it from first. The Weird Spline Output on DeWalt drill gearboxes has always been a headache for the robot world – usually you’d want a 1/2″ or 5/8″ output shaft to mount a sprocket to, well-supported by bearings. Problem is, that shaft has to get reduced down to a little 8mm flatted shaft in this case, or a very weird ~3/8″ 4-sided spline in the old style gearboxes. Hello, stress riser.

It will most likely be a one-piece machined deal from 1/2″ steel rod, with one heat-treated end for strength. There will be 2 retaining ring grooves on it in order to secure it to the bearings. For the final metal version, I’m thinking of just carving up some O-1 tool steel rod, but if I were to hire this out for machining, I’d probably order a higher strength alloy steel. A popular shaft steel solution for the old style gearboxes was ETD150 or 4140.

By the way, there ARE two bearings. They’re flanged FR8 for easy installation (and convenient 1/8″ spacing in between), so this thing should be able to take reasonable sprocket-belt tension loads or directly drive an overhung wheel.

The next step from here is to copy and paste it in aluminum. This thing is going to be chunky and heavy – 2.5″ square, 6.5″ long, and current weighing just under 2.5 pounds. Change the dense wood to aluminum and it will probably be at least 3 pounds. 2.5″ square is essentially the smallest it can get because otherwise I’d be dealing with very small wall thicknesses on the back end of the gearbox (which is already 2.3″ by itself!). No more 2″ bots for me…

Should this experiment turn out well, I’m strongly interested in kitting it up and offering it for sale through the site. One DW motor per side is a good match for RageBridge as Überclocker has confirmed. The plates would be waterjet-machined and shafting sourced appropriately, and the fastening hardware is like $10 on McMaster. Short of that, I’m hoping to convert Null Hypothesis over to quad-DeWaltWut? drive for even more pushybot horsepower, and it will also power the next generation Überclocker.

 

A Little Update on RageBridge v2

Nov 17, 2012 in Motor Controllers, Stuff

In the last episode of RageBridge, I redesigned the logic regulator section of the board in order to make it compatible with a wider range of low input voltages. The current logic power architecture is a single LM2594 buck circuit that supplies the gate drive circuitry with 15 volts, and then a linear regulator to the 5v microcontroller & peripheral logic and receiver power. The issue was that this setup limited the minimum voltage of the input to be around 13 volts – too high for 3S lithium/12 volt systems which are still very common – because the gate drivers shut down at 10 volts. I determined that a better architecture would be to convert down to 5 volts first, then boost from that up to 12 volts. 5-to-12 converters are very common, as are switching buck regulators for 5 volt systems – hell, even the LM2594 is $2 cheaper in small quantity if I only want the fixed 5v form.

Another approach which I settled on after deciding it was at least worth trying is using an integrated buck-boost chip that could handle a very wide input voltage and still give me 15 volts out. This would have necessitated changing the power regulator chain the least, so it was the solution I favored first. After a long period of staring at manufacturer datasheets for relatively simple self-contained buck-boost regulators, I picked the LT3433 to experiment with. It was supposed to be this super magic one-chip solution to the issue, and would allow me to keep the 5v linear regulator (since it could, in principle, generate me 15v from anything between approximately 8 volts to 57).

That’s the board design I sent off to MyroPCB to fabricate. Only 4 this time, though even that was kind of overboard… I figured if it did work out, I could assemble more of them as needed.

2 weeks or so passed, and I received the boards:

Between the last board revision and now, I discovered how to make Eagle ‘pave over’ vias with solder mask. The technique is generally known as ‘tenting’ the via, and its main goal is to prevent exposed vias from wicking solder away from nearby pads (in a via-in-pad situation or via-kinda-awkwardly-hanging-out-near-pad situation if you’re me) or being another source of potential shorts. Turns out there’s a DRC setting for that. Hence, I basically set the Limit under the Masks setting to juuuust bigger than my standard signal via (16 mils). All of the little layer change signal vias were therefore tented, and the whole board just kinda looks better as a result.

I assembled two of the boards for starters. 3 is probably better to reduce the chances of assembly errors and flukes making it through, but I again ran into the trouble of running out of critical components. In this case, it was the 16mhz ceramic resonator and 30A current sensors.

Sadface.

The LT3433 was supposed to make this easy. And as far as I can tell through much testing and component replacing, the chip is working just fine. However, it is not as magic as I had hoped.

The symptom was that while the board by itself worked fine between voltage of about 7 to 55 volts, once I connected a receiver or fan to the thing, I could only get down to about 16 or 15 volts before the gate drives dropped out completely, and the 15v rail began dropping in voltage even up to 22 volts. This was worse than the previous configuration!

I used a Spektrum BR6000 receiver known for being very power-hungry for most of the testing. Maybe it’s not realistic of a receiver these days load-wise (this thing drew, by itself, upwards of 100+mA), but Spektrum receivers are still very common and I’m not sure if they’ve gotten any more efficient. Regardless, if the power supply is unstable supplying just a receiver or fan, it’s definitely not going to work for both at the same time. A Hobbyking receiver, which draws less current (50 or 60mA), fared better.

The picture shows a series resistor I added to the inductor in order to try and measure its current. The 3433′s datasheet is somewhat confusingly written, and I’m thinking I interpreted the load line graphs as a “minimum” load (i.e. load must be this amount or more to retain output stability, because boost converters need that kind of thing). The page of design math also pulled variables out of nowhere (or had other bad notation issues), and I actually skipped over it the first time figuring that the reference circuit would work just fine.

It turns out the current was indeed a maximum limit, meaning with my component choices I could only realistically get 160mA in “bridge mode”. Additionally, the reference design (8-60v to 12v converter) enters bridge mode at 18 volts – with my output voltage requirement being 3 volts higher, I indeed saw the transition happening at around 21 to 22 volts.

I tried other dumb inductor hacks like adding 2 of my 330uH discrete inductors in series (for more inductance), hoping to store a little more energy in that “bridge mode” . Unfortunately the results were not much better, and the reason is because small power inductors of such high inductance values also have very high DC resistance. Hence, the majority of what I gained was lost to I*R drop in the inductor itself. They also heated up quickly, indicating the same.

I ended up playing with the maximum output current design rules and found that yes, more inductance would help, but only if I could keep the DCR very low. In the size of inductor I use, the amount of space available for wire is very limited, and so all of the ones on DigiKey higher than 330uH were also much greater than 1 ohm – a few up to 5 or 6 ohms!

For instance, I can get output currents of 400mA if I had a 1mH (1000uH) inductor, but only if the DC resistance was still under 1 ohm. If I used a known part specification (6 ohms) in the design equations, then the result is actually not much better. I would be burning more watts in that inductor than what would be passed to the board.

“Bridge mode” on this converter is kind of strange. Basically it not only pushes current into one side of the inductor (standard buck converter behavior), but at some point it also starts pulling the other side to 0 volts (boost converter behavior). The input side voltage is the yellow trace and output-side voltage the blue trace – right now, the input is lower than the output, so it is operating solidly in bridge mode.

This is too weird for me, man. I’m not into discontinuous converters like that.

In conclusion: This version of the board is kind of fail. Let me be very clear that the LT3433 is not necessarily a bad thing – I just seem to need more current out of it than what my space-constrained layout, voltage demands, and component choices can sustain. It seems that I definitely need a solid 3 or 400mA+ converter serving the board for worst-case loads of a power-hungry receiver and a fan, and maybe some blinkenlichten.

It’s also not like the board doesn’t work – it functions just fine at voltages above 22 volts (topping out at 50+v with current part selection), or even down to 15-16 volts with suboptimal gate drive performance. But as a result it’s not really an improvement over the original LM2594 input-to-15v conversion. So I’m going to keep these two around anyway in case one of the higher-voltage devices needs one. For instance, Null Hypothesis can run this at the moment because it has a 25.6v nominal systemm.

Therefore, I’m going to quickly whip together a third board revision that retains the minor power and signal layout changes of this revision, but uses the LM2594 again, a circuit that I know and understand well.

But instead of converting to gate drive voltage (15v) first, I’m going to convert to logic level (5 volts) directly, then use a tinyboost like the LT3460, which seems nice and simple, to generate the 15 volt gate drive rail. The 15v will then be only for gate drive – no external tap will be available.

Bucking directly to 5 volts is a little riskier because switching converters will inevitably have more noise than an equivalent linear regulator, and if I am to spec 5v fans to be powered from that , it introduces yet another potential source of noise. I think massive bypass & bus capacitance spamming and 5v TVS diodes will make this a non-issue. I do trust the 2594 to be a stiff 5 volt rail, however. Using the fixed output version instead of the adjustable 2594 will also save on the space taken up by passives.

I don’t have this board designed yet, but hopefully will by the end of the weekend such that I can fire it off ASAP.

The Second Great Awakening of the LandBearShark

Nov 11, 2012 in Land-Bear-Shark, Project Build Reports

Oh, this thing.

LBS is another great example of something I built without much forethought that never quite worked, similar to a certain basket-case go-kart. It has been plagued with reliability problems for its entire life, generally stemming from my inexplicable refusal to use real motor controllers (it’s always the motor controllers) and my insistence on keeping it brushless with R/C type motors. While its initial goals were… somewhat noble, they were pretty much antithesis to what it needed to do in real life, which was to have very high low end torque and fine speed control for scrubbing the tracks during steering, and to push through dirt and snow. To no surprise, the completely open-air electronics deck was not very enthusiastic about doing either of the latter, and the fact that it had always been geared for 20-25mph meant it really didn’t have enough torque to do anything save for drive in a straight line.

Scheduled Plug! I’m still trying to unload stuff. Have a look and see if anything interests you.

The first drivetrain version used rewound C8085-class “melon” motors, hence giving its internal moniker “melon-tank”. Unfortunately as I found out later, the motors were both wound incorrectly and Hall-sensor-appended incorrectly. Hence, this version pretty much never worked at all.  In fact, the most successful rendition of LBS had been its brief DC motor form that I put together afterwards, but even that didn’t resolve the turning issue because of the lack of braking/reversing that the “Beast-it-trollers” featured. So it still couldn’t turn, and one somewhat undergeared CIM motor per side ultimately meant they just overheated and baked. When last winter rolled around, I switched LBS back to a chain-reduced brushless form using (proper) sensored motors. This version has been around the longest and has been consistently working….with the exception that it keeps eating the Hobbyking Car ESCs for reasons unknown. Given that these were never meant to drive something with so much inertia and friction, and don’t have any form of current limiting or control, I am not at all surprised. The motors are also still geared too fast – a top speed of about 18 miles per hour, and I don’t think I have ever stayed on this thing past about 10. After the very mild winter snows melted, I took some parts out of it to let other people borrow for their own projects, and LBS has been living under a table.

Now, with LBS approaching its third Brutal Arctic Winter, it’s time I do something about all of that or close this chapter of my project book forever.

And because I was told that it will actually snow this winter, and with the allure of having a functional offroad/snow vehicle still too strong, it should be clear what path I’m taking!

I’m going to rebuild LBS the way it was meant to be the first time around – bone simple, no frills. It’s going to just be about as smart as one of the Battlebots. Full R/C throttle controls and no more weird sleep-mode contactor closing and opening. One of the causes of my reliability concerns stemmed from the fact that this thing just had too many subsystems being thrown together haphazardly at once.

Here’s the summary of what’s changing:

  • Going back to DC motor drive, using now proven Ragebridges as the drivers. Hopefully, this will be the first test of the version 2 boards.
  • Two CIM motors per side, like the intermediate DC version, geared way down to max out at about 8 miles per hour with much higher torque. My rough calculations show that in perfect traction it will have about 400 pounds of ‘drawbar’ pull. This means it might actually be able to turn in place (skid turn) now!
  • No more weight shift detection, rider switch contactors, tilt steering. Or anything.
  • Actually enclosed electronics and batteries, so going outside in the snow and dirt like it was meant to do doesn’t bury my controllers in grime.
  • Significantly less weight. There was so much wasted metal on LBS that did not need to be there and didn’t contribute to any structural load bearing.

The design has been in on-and-off development for a few weeks now, so most of it’s done already, but I was waiting to make sure I actually started on it before making a vaporware post. Here’s the rundown:

This is what the design looks like as a whole. Notice the much, much lighter aluminum sections and my more extensive use of trussing and triangles instead of big square plates. The aluminum weight on this frame has gone down by about 60% – because previous, every piece was solid aluminum plates!

The new track bogies are also much lighter in weight. Fact is, these things never took any structural loading, so they were entirely for show. There’s no suspension element or frame mounting that is rigidly coupled to the bogie swingarms. As a result, they were made much thinner and simpler. The side which couples to the ‘shocks’ are thicker in section because the loads are transmitted into the shocks and into the main body.

Admittedly, the suspension does absolutely nothing. The mountain bike suspension shock absorbers I got are rated at 700 pounds per inch – in other words, there’s basically no travel or movement at all with 4 of them on there, even if I jump up and down. Hence, I really just think of them as rigid control links in the bogies and not actual suspension elements. They are there to anchor the track axles, closing the structural triangle between themselves, the bogie swingarm, and the central frame.

 

The track pods were totally designed from scratch to be lighter and more elegant, and to minimize the use of 1/4″ aluminum. There’s no more ring of 7 3/8 bolts around the outside of each axle mount, because they were completely useless.

I spent much time trying to play “arrange the motors”, since I wanted to fit 2 motors per side. The track pods were originally designed for an ‘inboard’ drive, or motors mounted away from them driving via a shaft.  Stuffing them into the center cavity between the very short wheelbase tracks along with a method of connecting them to the main body was a bit of a pain, and part of the reason I’m glad this thing doesn’t actually have suspension travel is because if it did, the motors would be bottomed out against the tracks.

I went through several configurations and motor-vs-center-shaft arrangements before setting on this one which kept both motors on one side, hanging off the center shaft in an independent gearbox. The high center shaft meant I didn’t have to use so much metal to join the shaft to the frame since the whole thing can be kept low profile. And, the one-pivot-point mounting meant that keeping tension on the chain would be much easier. The little snail cam thing is designed to keep pressure on that swinging assembly once the chain is installed.

Check out the new drive motor setup – dual CIM motors geared to a common shaft, then chain speed-reduced to the track sprockets.

This thing will need a little explanation. The featureless gray circles are spur gears – for simplicity’s sake, I usually don’t bother modeling the teeth unless I was going to cut those gears out myself. The main spur gear is a hex bore, riding on a 1/2″ hex shaft with bushings inside so it spins on a structural standoff that helps hold the gear case together. A bunch of other hex bore objects are stacked onto the same hex shaft, and the assembly doesn’t have any axial constraining features (snap rings, set screws, shaft collars) save for the 2 end plates and thrust washers. The assembly is hence pretty easy to take apart and put together, and the hex bore transmits torque without the need for a keyway or something.

The spur gears are sourced from AndyMark. In fact, this whole damn thing is pretty much an AndyMark-powered FIRST robot. As much as I some times disagree with the philosophy of providing commercial solutions for a high school competition that allegedly encourages students to engineer and design their own robots, AndyMark really does make some neat little shortcut items. AM products are also usually built for the task at hand – which is to say, not things that you need to run for 10,000 hours with absolutely reliability. Robotics competitions are generally fast and brutal, not necessitating long part life, and I do agree with saving a ton of money making gearbox cases from aluminum sheet metal stampings, for instance, over die-case/billet machined stuff.

This means stuff costs less. Seriously, AM is about the cheapest place you can possibly get steel and aluminum spur gears if you don’t mind the limited tooth options. McMaster would have charged me about $30-40 per gear for the set I’m using, and they come with things like hubs that I don’t need. I’m pirating the popular AM method of using hex shafts for everything because it really is convenient. It’s like a real spline shaft, except improv.

In this gearbox, I’m using their 12 tooth pinion for the CIMple Box and a 48 tooth output gear.

The output is a 7 tooth hex-bore sprocket which I will custom-cut from profile.

The electronics box this time is no longer an open-top bucket. Made of 1/8″ and 1/4″ polycarb, it’s fully enclosed on all 6 sides, save for ports for switches and wiring. A fan will blow straight into the two Ragebridges in the back to keep them nice and chilled. The fan will have a foam filter stage in front of it so it doesn’t try to pull in water, and the RB boards will be conformal-coated too for splash protection. The fan will also help keep the electrical box positively pressurized -exhaust vents out the front, so if there is any place gunk could get in it would be through the slits if there was no airflow.

The Big Switch makes a return in the back side, so if this thing does try to get away from me, I wouldn’t have to full-frontal tackle it. At the breakneck speed of 8mph, too, it should be easy to catch.

The main chassis this time is pretty much just a cage with mounting holes that mount the track pods and hang onto the electronics deck, which can be slid into place and locked by the side mounting screws. Exceedingly simple compared to the first time around. I’ve ditched the big crash bar because there’s no more load cells involved, and that thing alone weighs like 10 pounds. The board itself has some little side rails on the bottom so it won’t sit truly flush as shown – I’ll make a little spacing plate if it’s needed.

Overall, the thing loses about 15 pounds compared to what it has been. While I never weighed it with the crashbar assembly on, the bare vehicle weight before was 65 pounds, so it must have been up to 75. What I lost in metal weight and complexities, I gained back some in those damn CIM motors and gearboxes. Steel and DC is heavy!

I do like the new look better, though. The space frame makes it look even more monster truck-y. Here’s to hoping it can actually live up to the hype this time.

And here’s the pile of parts!

Beyond Unboxing: Sensorless Chinese e-Bike Controller Roundup

Nov 05, 2012 in Beyond Unboxing

It’s time for SCIENCE!

As promised in my last post about the Jasontroller’s seemingly language-barrier-impeded sentience (it is screaming to be heard, but we (mostly) cannot hear its 16kHz switching frequency?), I have a pile of different Chinese electric bike controllers purchased from several eBay sellers. They were pretty much all purchased about a month or more ago with the goal of eventually exploring this functionality. This perhaps betrays the planning of this post for a long time.

Update: Check out the “Mini Jasontroller” from eLifeBike!

I’m generally more willing to accept the sketchiness of generic Chinese products than most people. There really is no one company, dealer, seller, or support website for dealing with these controllers. They’re made to fit one or several models of electric bikes and mopeds, one-for-one, no questions asked or answered. All of the information on them on the Internet, in the Western electric vehicle hacker world, is pretty much in the form of blog posts, forums, and wikis. This isn’t something I expect to change any time soon, because there are about as many variants of the controllers as models of derpy electric mopeds. Plenty of companies exist that stand by their products and offer support and usage advice, and they are pretty much the go-to if you need something that will undoubtedly work (though this assumption can be a bit challenging at times…). But  half the fun of projects is the hackery, in my opinion, and it never hurts to expand your resource horizon.

So, this post is not intended as a definitive reference on Chinese e-bike controllers. I think I could test a different one every day for more than a year, guaranteed, and still not be through with all the variants. Some of this info might become totally irrelevant if the company turns over or the seller on eBay disappears. The goal is both more immediate: these are specifications that are known to be good RIGHT NOW, but may not be this time of next year, so buy it now; and more general: to elucidate some of the Chinese controller marketing words such that everyone can look out for future variants and understand what goes inside of one, and why they are not bad, per se (You can’t sell millions of abad thing that people have to use day to day…. or can you?!), but that we, weird brushless scooter and go-kart and e-bike guys, use these devices generally far out of their original design specification.

Which, given how much the same commercial, street-legal electric bikes and things are, is likelyvery very narrow. A recurring trend of the lineup is that they pretty much use the same components in various permutations – things which have stood the test of time and probably stochastic layout design selection. I now present…

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