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Thread: First test of robot arm

  1. First test of robot arm

    Here is a video of the first attempt to run a sequence on my robot arm.

    The arm is still a bit of a work-in-progress, and uses a linear actuator to do the heavy lifting, and two Mx28s set up SCARA-style in the x-y directions and two Ax12s for the wrist/gripper.

  2. #2

    Re: First test of robot arm

    Next up: Flip the table! :-)

  3. Re: First test of robot arm

    Here is a bit more info on the design of the arm.

  4. #4

    Re: First test of robot arm

    You mention size and simplicity as reasons for Dynamixels, but there's another one: Direct position feedback!
    (You can get that with steppers but you'd have to add a separate absolute encoder for that.)
    I also like the deflection sensor for the linear actuator!

    Btw: I see that you're using the Dynamixel hubs for extending the wires.
    It turns out that the hubs have a few traces that are very narrow, which may lead to voltage drop under load.
    You may be better off making your own hubs, or getting the Trossen hubs, if that ends up being a problem.
    (Narrow traces act as resistors and thus drop more voltage as the current goes up.)
    Then again, if all you're driving is one AX12 in SCARA style, it might not be enough to matter.

    Anyway, cool project! And conveniently located wooden studs in your workshop ;-)
    Last edited by jwatte; 06-12-2017 at 11:23 AM.

  5. Re: First test of robot arm

    Yeah, the hubs are a bit of a sore point because the exposed soldering on the back, aluminium frame and the Arbotix M serial port did not mix well. I printed backs for them, but too late to save the serial port. But in the end, the Opencm 904 has an extra hardware serial, so it might be the better choice for me anyhow.
    Ideally, I would like to have bought 60cm cables, or some high quality, compact cable joiners. I thought about just soldering two cables together, but I'm not sure if this would affect the 1MBps data line. I also wonder if some kind of "backbone" with cables coming off on t-junctions, rather than daisy-chaining, might be a better arrangement. I have a little voltmeter on the end that shows it dropping from 12.1 to 11.9 under some loads, but then the power supply might not be that great.

    As you say, the Dyanmixels are really convenient, easy to program, all-in-one design and pack quite a punch. I like the torque-control, speed control, temperature monitoring and position monitoring. I don't know if I could get the hand to trace a straight line using two synchronized polar movements. But this is not really a goal for me. I might also look at getting the dead-band smaller and will see how stable that is.

  6. #6

    Re: First test of robot arm

    I thought about just soldering two cables together, but I'm not sure if this would affect the 1MBps data line.
    I think that'll work fine. The cables are not shielded or differential, so a solder joint may actually be more electrically transparent than two plugs.

    It's totally OK to use a single bus and multiple taps for power, that shouldn't cause any problems.

    For TTL, you may be getting weird echo phenomenon on the bus if you multi-tap, although whether this matters at all depends on how strong the transmitter is, how long each "tap" is, and what the input circuitry actually looks like on the servo side. Your runs are a little longer than average; 1 Mbps 5V TTL is very tolerant under a meter in my experience; I have no experience with longer runs.

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