The main reason to try and create a “middle mile” interconnect (the red bits) is modularity and swappability. I could swap out drive units easily, exchange boards, mix and match undercarriages.
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I do like the idea of each drive unit being a neat 9-pin connector.
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Farthest out of my wheelhouse: rest of the wire harness for peripheral and payload function. There are 8 GPIO pins I can make good use of, but the bus options (CAN, UART, SP1, I2C) are black-magic to me. Where might a camera go?
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I’ll admit the thought crossed my mind since D-subs are the only kind I’ve actually soldered in my brief, unglamorous grad school career in hardware circa 1998. Bulky so easy to solder.
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Replying to @vgr
If you’re proposing a DB9 connector I have to leave.
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For the Gen-Z usb-c kids who’ve never seen this kind of cabling...
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Forking off a subthread on learning to use the Beaglebone Blue, which I suspect will take over this main thread if I let it. Will post main rover shit on this thread, BBBlue-specific stuff on this side thread.
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Off to an inauspicious start with the Beaglebone Blue. The network drivers won’t install on MacOS Catalina. Apparently the packages, HoRNDIS and EnergiaFTDIDrivers (no idea what they do… ELI5?) do shady shit at system level so Catalina sez no.
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Reprinted the ABTF test rig chassis with taller standoffs and now the battery fits and the short cable even reaches the board. But ordered an extension cable too.. this is too awkward.
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As you can see, I had to add like 15mm because I initially planned on flat pouch battery but switched to this cylindrical one. Switch from red to white is because I got a 2 for 1 deal on white PLA on prime day and am saving my red PLA for main rover.
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Half-assed hack of drive and passive wheel struts… measure the wheel/motor subassembly, capture dimensions on rough whiteboard sketch, CAD it up, print… hope to have mechanical assembly for this weekend.
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Can already tell wire harness will be a bitch to manage. Those 6 motor wires are kinda stiff and with wheel pivoting at contact point, wires will swing quite a bit. Enough slack for strain relief, not so much the wires snag.
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Replying to
This whole process is a perfect illustration of the Lindblom "muddling through" process of successive limited comparisons. There's no "root" design, only progressive branching as you gradually lock in one design decision after the other. jstor.org/stable/973677
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This is a classic management paper (also a key reference for the neat little book, Obliquity, by John Kay) that I often recommend to clients, but for people/org management. But it applies to this solo engineering project too.
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Finishing up the mechanical design of ABTF with a pair of designs for a strut cap for motor strut, to hold motor in place. One is a hatch style that will screw on, and should work no problem, the other is a shoe style that I'm hoping will work without screws, interference fit.
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Hmm I suppose I could have printed this as one piece, with just a push-fit hole for the motor 🤔
There would have been an overhang, but I like the idea of a 2-piece assembly, not sure why. But will try single-piece design for NIM.
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Beginning test assembly of Accessory Before the Fact (ABTF). The Bill of Materiels (BoM) even for this simple test rover is 65 distinct parts (+ 4-8 more depending on what interconnect design I settle on to finish the wire harness 😫
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67 [+4-8] actually, miscounted
16 mechanical parts
36 fasteners (12 nut-bolt pairs, 12 screws)
6 motors
1 computer
1 battery
7 interconnect cables (not counting 4 integrated ones)
4-8 connectors, not shown.
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The steel rod at bottom right needs to be hacksawed into 2 axles the size of that toothpick, so that will add 1 more.
At this rate I’m guessing the main Nature is Murder rover will weigh in at ~200 parts. Damn, engineering is complex.
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Discovered an idiotic oversight. There’s no way to attach the servo arm to motor bracket without obscuring the center hole through which you attach servo motor. And if I try to attach motor first, attaching arm is hard but then then motor won’t fit through hole in chassis 😥
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But drill to the rescue! Just drill an access hole in bracket! And of course I mess up a bit. Next bracket, hole will be designed into the print.
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Next, attach arm using 2 tiny self tapping screws. But holding it in place is a challenge in itself. No worries, we use an M2 nut and bolt to hold it in place (the arm/horn center hole is tiny but I have M2s lying around, yay!)
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Now another issue. The self-tapping screw is strong enough to penetrate horn pinhole but apparently not the bracket. It simply pulls up the horn 😡
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Gotta break out the tiny hand drill and create a starter hole. Beginning to appreciate the value of having the right tools at hand. I bought this tiny jewelry hand-drill set a while back precisely for this sort of bs. But again, gonna design the hole into the print next time.
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Managed to strip the head of one screw in self-tapping false-start, despite using small screwdriver. But can tighten it with needle-nose pliers. Lesson: never force tiny parts, they strip easily.
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But… horn finally attached and temp center nut-bolt removed. Whew! Now for the motor.
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Motor attached to chassis and bracket without incident.
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Well, all 4 steering serves and brackets attached. This proto rover will have some major alignment issues.
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Next up, make axles for the 2 passive wheels. Hacksaw time. But first a short recess to go take a consulting call and make some money to pay for all these shady R&D shenanigans.
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The lessons never end. The shaft needs to be 26mm according to the design dimension, but the toothpick piece I used to test the assembly is 27.3mm. Which to use?
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I go with the toothpick. Figure the compliance of strut will create a nice compressive fit. But I forgot steel is much stiffer than wood, so it’s a tighter fit! Still it fits. Btw I’m very impressed with my hacksaw precision. I’ve matched the toothpick exactly 😎
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Second axle I decided to go a little shorter. Note the scratches on the shafts and leftover stock btw… hard to wield a hacksaw precisely without it jumping around a bit. Next time I’ll make a cutting guide or something.
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Passive wheels in!
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Aaaannnd drive motors in as well 😎
The assembly is done! Note holes on drive motor strut fork caps… can add screws there if necessary but interference fit holds motor in place well enough. That was a bitch to get right btw.
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Here’s some posing with steering angles… stylish lil guy eg?
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Let’s add the computer, battery and soft assemble the partial wire harness (battery and servos connected; drive motors not yet… will add 2x6= 12 more wires/4 more cables to this picture).
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Let’s power it up 😎
This is just led blinky running but it’s still neat. I got much further today than I expected. Now for toughest part… rest of wire harness. Unfortunately crimping and/or soldering may be needed and I suck at both.
Then the hellish challenge: programming
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Back to the mechanical design of the main rover... just spent 2 hours figuring out how to get a mcmaster-carr ball joint and some standard nuts and bolts into my assembly, and detected some geometry issues that would have been annoying to discover after printing...
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Finally actually earning the mechanical engineering degree they gave me in 1997. I think the mechanical design is now about 60% done. I'm now aiming to build a model accurate enough to simulate.
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Turnbuckles and bogies need to be added, then the 4 drive units and 2 passive wheels. Another couple of days to rough it out, then 2 more to get all the finicky details doubled checked before I start printing.
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Another good day. Design is finally roughed out. Lots of the dimensions and stuff are tolerances, but now I have something I can refine.
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