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Thread: Some love for the Money Pit

  1. #11
    Join Date
    May 2016
    White Plains, NY
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    Re: Some love for the Money Pit

    Holy cow! That thing's a monster! It looks like it could climb Mt. Everest!

    I can see why an AX-12 would have trouble rotating the wheel assembly. You're the one that introduced me to the RoboClaw, so don't blame me if that's my first recommendation! Or a MUCH bigger Dynamixel...

    What's carrying the weight at each wheel? Is there a bearing tucked in there, or is the servo supporting the load?

  2. #12

    Re: Some love for the Money Pit

    There are two RoboClaws on it already :-) (For driving the wheels.)

    It actually does well in most terrains (including, as I found out, climbing chainlink fencing) it's just tall grass that's the problem.
    And, for anyone who has tried to run in tall grass, I bet you can understand why :-)
    I'm actually worried that if I add bigger servos, I'll just start trying to tear the rubber of the wheels off the hubs instead, or bend the 1/8" 6061 aluminum brackets.
    Thus, I'm pretty much settled on finding some less knobbly/mushy wheels, and adding a third axle in the center for carrying 50% of the weight. (And a third RoboClaw for driving it.)

    What's carrying the weight at each wheel?
    There is a thrust bearing between the suspension arm (where the servo is mounted) and the rotating bracket (attached to the AX-12A horn.) Someone had previously made the observation that some common thrust bearing size fit reasonably well around the AX-12A horn, and I machined a race for that into the underside of the arm and overside of the bracket.

    Also, the suspension arms are mounted on 1/4" stainless axles with dual ball bearings, and machined bearing pockets attaching them to the main base plate. Which is overkill no matter how you look at it, and I'll probably just go with oiled-brass bushings for any future version. But it was fun to make :-)

  3. #13
    Join Date
    May 2016
    White Plains, NY
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    Re: Some love for the Money Pit

    Quote Originally Posted by jwatte View Post
    ...Which is overkill no matter how you look at it, and <snip> But it was fun to make :-)
    This is my hobby in a few words: "Overkill", and "But it was fun to make" !!! It is fun!

    And you can't have too many RoboClaws!

  4. Re: Some love for the Money Pit

    I see 4 independent drives could allow you to spin on the spot.
    You could probably gear you ax12s by a factor of almost 2 and still keep your range of movement, say by a timing belt or a simple gear. This would also centralize the motors if you wanted. Although I guess just one more thing to go wrong.

  5. #15

    Re: Some love for the Money Pit

    I see 4 independent drives could allow you to spin on the spot.
    That's right! Turned out the be a crowd pleaser on demo :-)

  6. #16

    Re: Some love for the Money Pit

    And this is why we do simulation before we actually make parts!

  7. #17

    Re: Some love for the Money Pit

    Also, while Inventor was my first love, Fusion 360 is the frikkin' bomb.
    I find it easier to work with for the things I actually need to do, and it has a very good CAM mode for making tool paths.

  8. #18

    Re: Some love for the Money Pit

    The design proceeds apace.

    I received an order of six pavement front wheels for a HPI Baja 5B buggy, 170mm diameter, 65mm width, 24mm hex.
    I machined 24mm hexes (spot on; +0/-20 micron) and they don't fit in the metal inserts for the hexes in the wheels.
    Maybe I will be patient enough to file down 36 flats by 0.1 mm to fit the six hexes. Or maybe I'll try pressing them in. Or maybe I'll just re-cut the hexes.

    More pictures:

    On the previous iteration (four wheels,) I was supposed to use RoboClaw 2x5A controllers, and I had 3 of them (two plus a spare.) There was some problem/software bug with the 25A where it would detect overcurrent when starting up, even though the max stall current of the Pololu 37D motors I'm using is 5A. I had two RoboClaw 30A controllers from the 6-wheel Wild Thumper based Money Pit (1 active, 1 spare) so I put those on, and it worked, and I got a spare (because: always have one spare!)

    I think the design/software problem of overcurrent on start got fixed (I worked with the friendly folks at Ion on this) but not in time for me to design back to the 5A controllers. (Those controllers have now been upgraded to 2x7A, I see.)

    So, I find myself with three 2x5A controllers, and three 2x30A controllers. Whichever I go with, I'll have to get a spare. I'm kind-of leaning towards the 2x5A because it is smaller, and should be sufficient (engineering, not overkiling!) But, on the other hand, this really is the only rover I have running. If I use the 2x30A for this, I don't have any for the Wild Thumper chassis I still have sitting around, but that doesn't REALLY matter. And I know the 2x30A works, and won't be pushed into breaking a sweat, which is important for robustness.

    On top, and/or below, this thing I will have to mount:
    - Three RoboClaw controllers (all the motors have sensors)
    - One Jetson TX2 (I don't know whether to use the default development kit board, or the newly-announced Auvidea J90 carrier board, yet)
    - A battery. (I've got two 5000 mAh 3S "square and chubby" packs that will do the trick)
    - Some kind of board that does power control, Robotis TTL bus conversion, and simple I/O (buttons to start/stop) This may have a Teensy on it, for good measure.
    - A GPS, probably. I have a uBlox Neo8m, popular with the drone kids.
    - An IMU, most certainly. I have the BNO55 mini-daugher-board for the Teensy, which I might use.
    - A compass, if the IMU doesn't already have one. I have a couple from old rovers; I2C.
    - Perhaps a fisheye "situational awareness and post-action review" camera (might be 5.8GHz transmitting, might be Raspberry Pi recording based, haven't decided)
    - A brushless gimbal with two 60-90 degree cameras for the real stereo vision/navigation.

    Obviously the last bit is a project in and of itself :-)

    I have an order of some BLDC motors with built-in 12-bit absolute encoders, and the AlexMo controller board, coming through slow boat. I'm going to have to make my own brackets for making that all work. (The idea is that, if that works out, I can then take that same solution and strap it on Onyx X for a stabilized turret platform!)

    I also need to make the shafts and order the bushins and pins and springs from McMaster.

    I have most of the needed aluminum stock in the garage; this will need many separate sessions at the Tech Shop to make the actual parts.

    I haven't yet decided whether to stay with the flat bottom plate, or to just have two one-inch bars/runners along the sides (where the arm brackets and spring brackets screw in) and save some weight by having holes in the center.

    I'm also considering mounting all of the subsystems on their own carriers (probably 2.5mm laser cut Delrin?) that in turn slide into some kind of mount on the chassis. This is good for two reasons:
    - It reduces the risk of accidental shorts
    - It makes it potentially easier to remove bits for service/replacement

    For the removal dream to be a reality, though, I'd have to go with some kind of high end connector system.
    DataMate looks, as we say in NorCal, hella cool! And hella expensive!

    Every vendor has its own options. Some even with flat cables. Hundreds of bucks per foot-long run...
    And so on ...

  9. #19

    Re: Some love for the Money Pit

    If I just use inch wide runners on the outer edges (1/4" thick) for the frame, I can use the space in the center for power bars!
    Two 1/2" x 1/8" runners along the center, with appropriate insulation, could be main switched battery voltage, and ground.
    I just drill and tap a whole wherever I need power.
    Apart from the obvious "this will run into a chainlink fence and short itself out on the first day" challenge, this seems brilliant :-)
    Maybe I could powder coat the bars or something, and scrape it off when I need a contact.
    Or just run thick single-strand wiring and use screw blocks where needed. I can make my own, really.

    Does anyone know of a good compound to use on aluminum/copper interconnects to avoid corrosion?

    (Apparently, the Internet wants to sell me No-Ox-Id A Special. Opinions?)

  10. #20
    Join Date
    Sep 2010
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    Re: Some love for the Money Pit

    Quote Originally Posted by jwatte View Post
    I wonder what will happen if I add stereo input at some point ...

    Right now, the global shutter genlocked camera situation for Jetson TX2 is really bad. As in, there are none supported yet. Something changed in the device tree between TX1 and TX2 so that the cameras painstakingly developed for TX1 all need driver updates :-(
    I was wondering why you were making so much more work for yourself by using only a single camera. One of the many projects I've still not gotten around to writing about is this:
    Click image for larger version. 

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    I've been planning to put it up on tindie, but my photography skills suck something fierce so I've been putting it off. Ten WS8212B LEDs and two RPi-Camera cutouts on a 66mm baseline/diameter aligned to the hours. My little T3-M4 might finally get built after all these years...

    Another project is a 30mm square, 2-stage cycloidal gearbox (19:1 each stage for 361:1 total) powered by a 1306-3100Kv outrunner and a stack of 30mm square PCBs with drivers, sensors, and a Teensy-3.2. Was originally going to try a 20mm square gearbox for a ~100mm cube lightweight mech (Jameson from GITS), but using so many tiny parts led me to settle on just making the bot a bit larger to handle the 30mm gearboxes. Still might be able to get it below 3 lbs. depending on materials. Need to turn on my Windows machine again to turn my 2D gearbox drawings into 3D models with Fusion 360 so I can finally get them printed via 3Dhubs, but can't seem to motivate myself to do that (it's been unused for more than a year now as Autodesk informed me my Fusion license needed yearly renewal).
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
    "You have failed me, Brain!"

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