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Thread: Onyx X build thread

  1. #21

    Re: Onyx X build thread

    Progress is being made.

    The boards came back, and I could Dremel out the bad connection. Unfortunately, I initially created the footprint for the main power screw terminal with too small (default) through holes. I updated the footprint before I sent the boards off, but apparently forgot to merge that update into the actual board -- something KiCad doesn't do on its own. Thus, I couldn't fit the power connector into the holes without shaving down the pins.

    Everything else so far seems to work, so that's good. Except I had assumed 3mm resistors all around, but the parts box only contains 6mm parts for some values I'm using, so I have to inventory those and make a Digi-Key order for the final version.

    Another thing I've noticed is that, when running with 16V supply, the electrolytic capacitors on top of the OpenCM-9.04 run very hot. My guess is that's not actually ripple current, but heat that goes through the vias/leads from the power regulators, which are linear. The maximum input voltage for the regulator is 20V, but the input capacitor is only rated at 16V, and the output is rated at 6V. Given that a 4S battery maxes out at 16.8V, I think that's quite marginal! (Similarly, MX servos are rated for "4S input" but they declare over-voltage if the voltage goes over 16V.)

    The OpenCM draws a fair bit of current as these things go, so using a linear regulator to drop from 16V down to 3.3V (as well as 5V) generates a lot of heat! Maybe it's their way of feeling like MechWarriors :-)

    Options on the table:
    - Let it run like this; heat just makes the electrons flow faster, and nothing should live forever!
    - Drop an adjustable linear regulator into the board, pushing perhaps 7.5V into the OpenCM board ++ pins. (Power for the DXL already runs through fat separate traces.)
    - Drop a switching regulator onto the board, similarly pusing perhaps 7.5V into the OpenCM board, with less wasted current.

    Most of the time was spent clearing up the workspace so I could actually solder up the board, though. After all was said and done, four full trash bags of junk and old finished or failed projects made it towards the landfill and metal recycling (!)

    Click image for larger version. 

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  2. #22

    Re: Onyx X build thread

    This is the current leader among the options. Remains to see if I can find the board space in a good spot.

    https://www.pololu.com/product/2844

  3. #23

    Re: Onyx X build thread

    I verified Xbee connectors work, and gun management works. I can run with or without the IR cycle sensor, and it's obviously better to run with! (But if it dies, it will still fire pellets)

    I structured my Xbee protocol to use very small data packets (5 bytes!) and to send the response with status (if any) right after I get an incoming packet, and so far, that doesn't cause the sluggishness in turning around the direction of the link that I would see before with larger packets and asynchronous sending either way. I may still be able to get telemetry! Yay!

    I fixed the bugs in the board (too small power connector holes; laser 3V connector placed right through main power trace; OpenCM needs a lower voltage in to avoid heating up; 900 MHz Xbee modules do not want an LED on RSSI) and sent off to OSH Park for 2 oz, 0.8mm boards. The thicker copper will have less resistance for the current I draw, although I designed it to allow > 10A everywhere even with 1 oz copper (using top/bottom pours and paying attention there's no tight choke points.) So far, so good!

    Click image for larger version. 

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    And, yes, I think I have all the parts needed to put the full robot together. Just taking it easy, one subsystem at a time.

  4. #24

    Re: Onyx X build thread

    I designed the carrier board for the OpenCM 9.04 because that's what Onyx 7 used.
    It used it, because I wanted to use the Dynamixel library from Robotis.
    However, now that I'm back in the code, I realize that I ditched that library and use my own code, just re-using their low-level functions for switching the TTL bus direction and talking to the USART.
    So... I might just as well have used a Teensy! But too late to change now.

    And, to be fair, using the OpenCM means I don't have to add a 74HCT125 to the board. That's a big win ;-)

  5. #25

    Re: Onyx X build thread

    Still a pile of parts ... most robot time has gone to help my daughter with hers, and re-cutting the femur brackets, because the countersink for the screw heads was 0.3 mm too deep and the bottom of the screws bit into the plastic below the horns when torqued.
    (Using 1mm shorter screws means too little engagement in the horns.)

    At least I have one femur mounted now! I'm halfway done cutting the new tibias, too (with cavity to mount a 1/2" steel ball for a toe.)

    Rev 2 of the PCB is still at OSH Park; it's 2 oz thickness so that'll be good for current.
    Still don't know if double 0.1" delrin plate is stiff enough, or whether I'll need to cut some aluminum in the end. I'm hoping for the Delrin.

    Click image for larger version. 

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    I've also worked on the remote control some more. So far, it's looking as if I can actually get a few bytes of telemetry back each frame, without the ongoing stuttering that I've been having previously, by carefully scheduling the on-air packets to never overlap.
    I'm also using "S3B" 900 MHz modules, which claim to go all the way up to 20 kbit/s on air. (The pre-2016 ones only do 10 kbit.) No idea whether that means they are more susceptible to interference.

    Finally, broke down and ordered a FPV-specific display from Amazon. It clearly states that it doesn't go to black/blue screen, so presumably it will display whatever's there even if it's noisy, which was the problem with all the other displays I've tried. (And, thinking back on it, actually was the main unsurmountable obstacle in my mind, keeping me away from re-starting the project earlier.)
    This thing didn't exist two years ago: http://amzn.to/2mGvYkl

  6. #26
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    Re: Onyx X build thread

    Very cool! Nice beefy servos! Won't you have traction problems with a steel ball for a foot?

  7. #27

    Re: Onyx X build thread

    This is actually a re-build of the 2014 Onyx, with some changes.

    The main problem has not been slipping, but instead snagging on carpets and such! When going fast, if a foot gets stuck, it makes a very pretty pirouette, because of the high rotational leverage and low rotational damping of only two ground contacts.

    Now, if the floor is a glossy sheet of Delrin or oil Nylon or something, I'll be in trouble. Better bring some Shoe Goo and sand paper for that eventuality ;-)

    Here's the old version.
    The new version will have only one gun; cutting down on weight!
    It also will have wider clearance between the legs, to make sure there's space for the scoring panels.
    I had to mount them on the head last time, and that made it very top heavy.
    The agility of a 125 mm hip axis baseline was great, but I don't need that much (see "pirouette" above :-) so I'm going with 180mm this time. I'm also going with 100 mm femurs instead of 80 mm, which will give longer stride length, but also will make it wider. Here's hoping the new arena really sticks to the 3 foot lane width!

    Click image for larger version. 

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    Last edited by jwatte; 1 Week Ago at 11:05 AM.

  8. #28

    Re: Onyx X build thread

    I tried to cut the new tibias with room for 1/2" steel balls today. It did NOT go well.

    First, I found a bug in the Mach3 post processor for Fusion 360. It generates code around tool changes that may make a tool crash into the work, because it applies the tool length offset AFTER it already does the first movement to position over the next entry.
    This was my first carbide endmill and work piece lost today.



    Second, the Tormach Tool System uses a special kind of collet that fits in a 3/4" R8 collet, to allow for automatic tool changers, as well as precise positioning relative to the spindle with a second contact surface perpendicular to the table. However, when pushing the mill a little bit (not overloading, just pushing a bit) there's significant risk of pull-out on the collet, seemingly no matter how much you tighten the drawbar.
    If the toolpath goes close to the vise jaws (hardened steel) and the bit is suddenly 1-2mm lower than you previously measured and set it as, sparks will fly.
    This was my second carbide endmill and lost workpiece today.



    At that point, I gave up in disgust and went to work.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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  9. #29
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    Re: Onyx X build thread

    Mostly this hobby is tons of fun. Sometimes it's not...

  10. #30

    Re: Onyx X build thread

    The new boards came back. 2 oz copper, for additional current capacity. This makes the through-holes slightly narrower, which with the default hole sizes in Eagle CAD used to not be a problem. The default hole sizes in KiCad are narrower, and I had to really force some leads through the holes. Even with 1 oz copper, those narrow holes would make a PCB trouble for a through-hole pick-and-place machine, so I don't quite understand why they have it like that?

    Anyway, the new board uses a Pololu 6V switching regulator to power the input of the OpenCM, to avoid the significant heating in the linear regulator they use at 12V (or, worse, 16V.)
    The regulator Robotis uses on the OpenCM is rated at 0.8V drop-out typical, 1.0V max, so 6.0V seemed like the right choice for a 5V rail.
    However, even though the Pololu regulator outputs 6.06V, the OpenCM board does not start up with that voltage. I wonder if this is some code in the firmware/bootloader that attempts to prevent people draining their 2S LiPos, or whether it's some built-in UVLO, or what. I'm going to short the regulator with a 5V Zener for now, and if I spin the board again (I hope not!) I'd go for a 7.5V switching regulator output.

    It's always something.

    In other news, I'm going back to the shop to mill the tibias today, using significantly less aggressive toolpaths (never going above 0.08 HP of the spindle, never deeper than 1/8" with a 3/8" mill, no faster than 280 sfm, and such...)
    Wish me luck!

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