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Thread: Supply robot with 8 MX-64T Dynamixels and measure power

  1. Question Supply robot with 8 MX-64T Dynamixels and measure power

    Hi,

    I am currently building a mobile robot with 8 MX-64T Dynamixel motors, a RPi + Sensors and want to supply these with a LiPo battery.

    To power the RPi I want to use a Step-down DC-DC converter like Polulu D24V50F5.

    The motor supply seems to be a harder question. Since each motor pulls a max. stall current of 5.1 A and if I assume that they draw 2 A each under normal operational load, I could use something like a D24V150F12. Also I would need to use a 5S LiPo to get minimal 15 V and maximal 21 V being above 12 V with the dropout voltage of that DC-DC converter.

    I assume that such a solution would have strong ripple noise on the 12 V and heat up the small Polulu board quite a bit (so heat sink needed?). Is it still sufficient to just measure the current and calculate P = I_meas * 12 V? Or do I need to add a LC filter / measure the voltage anyway?

    If I need to measure the voltage anyway, I could supply the motors directly via the LiPo battery and measure the voltage to get the instantenous power... is that a valid approach?

    Or do you guys have a better idea? Own PCB, two step-downs for 4 motors each? How do you power a lot of servos on your robots?

    Best would be a ready to buy solution to fit on a mobile robot with LiPo...

    Thanks for your input already!

  2. #2

    Re: Supply robot with 8 MX-64T Dynamixels and measure power

    You can run the MX-64 with 4S LiPo if you either don't charge the LiPo fully, or use a fat diode in line (to drop out a few tenths of a Volt -- gotta be rated for 8A or more on average, though) or if you turn off the overvoltage warning/error bit.
    Stalling the MX-64 leads them to destroy their motors within a few seconds -- faster than the built-in over-temperature/load-not-reached timeout take to protect them. I wouldn't recommend stalling them, ever, or if there's risk for that, set the "max PWM" to something in the 600-800 range.

    Recommended by Robotis is to use 3S LiPo, though.

    I run my Malum IK robot at 4S, and it works, but I've also destroyed a number of MX-64 motors (requiring replacement from Robotis) before I got to the point where it's reasonably stable. 3S is safer, as you have a second or two more to get out of stall before damage ensues. (The damage is, I think, the motor windings expanding from heat, in a way that doesn't fix itself, and thus the servos become "sticky" and much harder to move by hand when not powered, and this makes them work harder when powered, which adds more heat, which makes then self-destruct in a feedback loop.)

    Ripple noise doesn't matter very much. If you care, stick a 470 uF capacitor right before each of the servos. (This might be a good idea anyway! There's inductance in those 22-gauge 3-wire power cables ...)

    If you continually make all motors work harder than 1A, you're probably in trouble for your design -- Robotis recommends average load of no more than 20% of full stall current. 15A should be sufficient margin for a well-behaved system. (Although you'd need to measure to be sure.) The good thing about using a suitable battery voltage, is that the batteries automatically "sag" a little bit when you load them hard, but they are fairly linear in current demand. Switching regulators ... not as much!

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