You discover design constraints/needs as you do this. For example, that white connector on the battery is supposed to plug into the 3-pin header on the board near it (this is a Beaglebone Blue robotics board). But it’s too short to come around from the bottom the way I’d planned.
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So either Plan A: add an extension cable (which needs room) to keep the battery on the lower deck or Plan B: put the battery above the board, raising the center of gravity and making the rover more top-heavy than it already is.
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Two more. In assembling-while-designing, I realized my original plan to have a completely covered top deck wouldn't work. The BBBlue antennae need to stick out there, plus need top access to gpio headers to connect stuff on payload deck. So ended up with a nice big hatch hole.
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The second one: while sketching this out, I hadn't factored in that the turnbuckle (red rod a few tweets up) is too long for the differential bar to be on the main body... so now I had to add a tail plane to hold the differential bar (on perseverance this is in the middle)
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Whew, finally figured out how to do mate connectors correctly. Now I have this aluminum strut mated via a slider joint to the rocker hinge
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My rover body is going to end up tiny relative to the undercarriage. This is because my 3d printed part dimensions are limited by my printer's build plate (120mm square, but effective max more like 100mm square)
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Ran out of steam, but printed out a roughly to scale side view so I can sketch to visualize the rest. Not bad.
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Hehe triple-hybrid test assembly: CAD printout, pen lines on paper, some actual components. This could be an art form.
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The first complex thing I ever built was a model kit glider ~1988, that came with 1:1 scale drawings. This now-vintage one. I built it by using drawing itself as the construction template, with nails on a board,. It was all balsa wood cut-and-glue pieces. 1:1 is a great hack!
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One minor problem I have to solve now is to make a passive roller out of this wheel. It has a 3mm d-shaft hole to press fit on the shaft of an N-20 moto, which is how I’ll use it for the 4 drive wheels, front and back, but the middle 2 wheels need to be passive free rollers.
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First thought is to try and drill out the D into a 3mm circular hole, and put a circular shaft into it supported by a fork. But I don’t like destructive mods. Alternative is to use a round shaft with a D-end, and cantilever other end in a sleeve.
There’s just so many details.
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Sketched out wire harness problem. I need to solve for A and B and make 4 of these.
Current solution: A and B both 9-pin DuPont wire-to-wire connector pairs. So would need to cut off the connector that came with the sg90. The red cable needs to route through aluminum struts.
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Thought of using ribbon cable for this (initially thought 18 conductors together for 2 drives on one side). suggested IDC punch down connectors for ribbon end. Not sure what it would mate to on board end… wires soldered onto an IDC header?
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thought this was overkill and just shove 9 separate wires through. He also suggested USB for the red interconnect, though it’s not meant for this use case.
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One interesting thing… the orange stepper female 3-pin futaba connector is what’s needed at the board end. The cable is even long enough and fits through aluminum strut. I may be able to just make A/B a 6-conductor problem instead of 9.
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And for the 4+2 N20 problem, the cables at either end are not long enough but together the should do the trick. Remember this sketch is ~ to scale. If so, no red cable/connectors may be needed at all.
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I could just solder individual wires or use butt connectors like these to connect wires individually.
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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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In main rover plan is to route the bulk of wires through hollow tubes.
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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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