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Thread: Starting a quad build - A couple questions

  1. #11

    Re: Starting a quad build - A couple questions

    The ratings are for "stall torque," and the "continuous torque" is typically 20-25% of the "stall torque" rating.
    This is generally well known among mechanical engineers working with motors, and I think it's also mentioned on the Robotis site.
    But more than one robotics beginner has been fooled by this, it's certainly not obvious until you've learned it!
    Also, the thin wires used for the XL-320 servo connectors may have non-zero resistance when you pull current through them. Measuring the voltage at the connector, you may see a lower voltage under load than you'd see at the source (battery.)

    The standard batteries that come with the Darwin Mini bot (the XL-320 humanoid) are 18650 I think, inside a small plastic shell with some charging circuitry. The OpenCM 9.04 has connectors for two of those, to drive the DXL bus.
    The XL-320 servos don't draw a full amp each, though, so current capacity should be fine, as long as you use quality batteries.

    This should hopefully prevent runaway shooting robots.
    That's a good thing to prevent! A serial number is fine. What I do is cycle once when I receive a "high," and then I wait for a "low" (zero) before allowing the next cycle. It's approximately the same thing, with a single-bit counter :-)
    Most of the time, though, when I've suffered a runaway gun, it's been because of latch-up or other electric problems in the control of the gun MOSFET. The inductive noise from the gun is sometimes high, and a single snubber diode plus capacitor may not be enough (!).

  2. Re: Starting a quad build - A couple questions

    Look at what we have:
    Click image for larger version. 

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    Thanks for your input jwatte about the voltage drop on the cables. We've decided to route all the cables along the top of the leg (as seen in the image to a distribution board near where the leg mounts. In this way each leg only has the voltage drop on the wire leading to it.

    Unfortunately, the leg hasn't "walked" (hopped?) anywhere yet, as we're fluffing around on the software side. We loaded up the "brain" onto the ESP32, and after an evening of playing, it all ran. However, it ran slowly, with the garbage collector was playing havoc with the timings. So it looks like the development will end up being in C. Not that I mind too much, but we'll have to see about getting it to talk to the simulator.

    Another interesting development is on the communications side. As mentioned a couple times in this thread, you guys suggest the Xbee Pro or the 900Mhz variant. Now, our microcontroller (the ESP32) has an inbuilt wifi system, and while we don't want to use wifi as-is due to the difficulty of maintaining a connection in a noisy environment, another interesting opportunity presented itself. The ESP32 supports both packet sniffing and packet injection. After another evening of learning to construct 802.11 packets, we can now send data from one ESP32 to another without needing a wifi connection - it just splats data packets blindly into the ether.
    After about 20m in an office environment with other devices and multiple walls most of the packets are still getting through. We plan to do some more tests before settling on this approach.
    It is still in the wifi frequency bands, but so is the XBee pro. The ESP32 claims to be able to transmit at 100mW, while the Xbee pro 2.4ghz is listed at 63mw. So with luck™ we won't need any extra hardware for communications. Additionally, because it's a wifi radio, it can turn around mid-air without any issues, so we should be able to handle sending telemetry back using the same set of radios.

    The next step is to write the servo protocol interface in C, which shouldn't be too hard as we have a functioning python version...

  3. #13

    Re: Starting a quad build - A couple questions

    Good luck on the "blast it without caring about other transmitters" approach! It's certainly seems to be used by some remote keyboards I could mention ...

  4. Re: Starting a quad build - A couple questions

    We're still waiting on testing the wifi in a crowded environment. We want to use an OLED to display the statistics so we don't get funny looks wandering around a shopping mall with laptops and cables going everywhere. There's a nice ESP32 module with display that we've got two-of on hand, so hopefully this week we'll a minimal interface working.

    In the between time, the code that handles the wifi at the low level has been improved to allow multiple packet types and easy packet decoding. I also discovered how to set the power level, so now we can transmit all the way up to 100mW (limited by local regs to 25mW, but I did do some tests at 100mW and was pleased with the result...). I started looking into FHSS between wifi channels, but will wait for positive results in the crowded-environment tests before I spend that time.

    On the mechanics front, we tested the leg using this contraption:
    Click image for larger version. 

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    We loaded it centrally so the load was (probably) distributed equally on the three wheels and the leg. The leg could sucessfully roll the robot back and forward with 1.5kg mass, and the leg mechanics hit structural issues (the clips came unclipped) at 2kg. This means:
    1) We need to buy some bolts so we aren't relying on the clips
    2) The servos and this servo configuration are adequate for a light mech.

    Time to order another 12 xl-320's. We may change the last joint slightly to allow cleaner leg-lifting over a wider workspace.
    Anyone know how heavy a set of target plates for the light mech class are?


    From the software side, we are hoping to use micropython (as mentioned before). After loading the code from the simulator onto it (and fixing a few compatibility issues) we got a mainloop speed of ~20Hz, and the garbage collector was running all the time. Most of the GC-heavy work was in the packet parsing, so when we change to using the onboard-wifi directly from C, it should be less of a problem. It took a fair while to figure out how to write modules for micropython in C, but it's now figured out, and isn't too hard. Both the interface to the servos and kinematics are now implemented as C modules, but the performance hasn't yet been re-tested (slightly different interfaces). I think performance should be adequate with the communications and kinematics done in C, and the supervisory logic (and gait engine) in python.
    Last edited by sdfgeoff; 1 Week Ago at 01:20 PM.

  5. #15
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    Re: Starting a quad build - A couple questions

    Anyone know how heavy a set of target plates for the light mech class are?
    A complete set of light mech target plates weigh 121 grams. They are half size plates and can be mounted vertical or horizontal with velcro. The full size plates and the half size plates are self powered by a small 3.7V lipo. The normal size plates contain the one battery in the rear plate with the transponder and radio. The half size rear plate only contain the transponder and radio. The battery for the half size plates is mounted at the discretion of the host robot with a pigtail wire connector that connects to the rear plate.

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