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Thread: Questions about powering

  1. Questions about powering

    Hi, I recall reading something like if two batteries connecting to a system of dynamixels within the same robot are not of the same capacities or at different charge levels, then the dynamixels could be damaged. Something like that. Can anybody please clarify?
    Last edited by Snoopy; 05-24-2020 at 09:35 PM.

  2. #2
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    Re: Questions about powering

    Quote Originally Posted by tician
    Quote Originally Posted by Snoopy
    Hi tician, hope you are well. I recall seeing posts from users having several of their expensive dynamixels damaged. Based on your experience, what do I need to be careful of when using lots of these motors to build robots? For a robot made of 20-30 motors, do you recommend using one or two battery sources? From the forum, it seems that people making hexapods use one battery while Andrew put one battery under each foot of his Giger. Do you know the reason?
    Keep the voltage at or below 12V.

    Never stall the motors ('continuous' torque of the servos is at most ~1/3 of stall torque).

    Never let any joints be driven into interference with any other part of the bot (collisions will stall the motor which causes electrical damage to motor and drivers; collisions will also destroy gears and frames/brackets).

    Splitting each limb onto its own isolated battery pack is not necessarily better than a central power 'hub'. If each limb gets its own pack, then each limb will have a different voltage supply and current capacity as each battery discharges at a different rate due to limb usage/loading. jwatte and others have built large capacity packs out of multiple smaller packs connected in parallel to provide a single supply that gets distributed to each limb via heavy gauge wires. The big issue is making sure the individual packs are matched for capacity and have been charged to the same voltage before carefully connecting them in parallel. Any differences in cell/pack voltages will cause massive current surges when first connecting them in parallel, which will cause sparks and potentially cause a fire.

    IIRC, Giger had one battery under each foot to drop the center of gravity and improve stability - weighted boots instead of heavy backpack. It is less efficient to place the weight in the feet than in the torso - having to lift the weight up on every step versus keeping the higher mounted weight stable as it sways/rocks a bit. If the rest of the robot is sufficiently lightweight compared to the heavy stable feet, then it can make the bot behave as if its feet were literally anchored/strapped to the ground which allows a wider range of motions/poses that are not possible with a more conventional weight distribution (i.e., batteries in chest/torso).
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
    "You have failed me, Brain!"
    gives free advice only on public threads

  3. #3

    Re: Questions about powering

    Also: If you do use multiple batteries, you still have to have a shared ground (negative wire.) Don't forget that!

  4. Re: Questions about powering

    Quote Originally Posted by jwatte View Post
    Also: If you do use multiple batteries, you still have to have a shared ground (negative wire.) Don't forget that!
    Thanks jwatte. Am I correct that I also have to connect the ground of the controller, PC or SBC and the batteries for the dynamixels?

  5. #5

    Re: Questions about powering

    Correct. The system should have a single reference ground, will connected (low impedance.)

  6. Re: Questions about powering

    Thanks. I know the concepts of serial and parallel connection of batteries as shown below.

    However, I have a hard time to visualize the concept when forum members are talking about "parallel connection" of batteries with respect to powering the dynamixels that are serially connected in a chain in an open loop. I worry that I may accidentally damage the dynamixels or controller board. Is there a good picture to show the proper way of connection?

    In the following case:


    where Hub1 and Hub2 are on the opposite sides of a robot. There are two approaches:

    1) Two individually powered Hub1 and Hub2 are connected to each other by a standard robotis cable.
    2) The two individually powered Hub1 and Hub2 are connected to Hub3 which connects to the controller.

    In these two approaches, are Lipo1 and Lipo2 in serial or parallel connection? Since the batteries are not connected one immediately after the other, they don't seem to be connected serially. It looks like an open loop to me in both approaches so the batteries do not seem to be connected in parallel either. Even Lipo1 and Lipo2 are the same product, they could be different due to usage and manufacturing tolerance. Will this cause problems?
    Last edited by Snoopy; 05-26-2020 at 01:29 PM.

  7. #7

    Re: Questions about powering

    Again others can and have answered this a lot better than I can...
    I have not done the multiple batteries in a very long time. Last time I tried it was with RC servos connected up to an SSC-32 controller and had 3 batteries. Two for the Servos (left side different than Right side) and another one different voltage to run the logic portion of the SSC-32 (and the processor board driving it) Note: I am actually slightly lying here, in that I did not actually have the third battery for logic, but instead used a BEC connected to one of the other batteries that converted about 12v down to 5v... Also I did it with different battery chemistry, but the principles are the same.

    As mentioned all of these power setups need a common ground and I would personally not connect the + pins of the batteries to each other in any way (either in series or in parallel)

    So what does that mean?

    Your 1) and 2) can be more or less the same. But in both cases, you cut(remove) the voltage wire from that cable, but leaves you with a common ground and logic pin. Note: this is what logically several adapter boards for PCs do. That is if you look at the USB2AX board that Xevel manufactured, does not connect the Power pin for the 3 pin AX servo connector on the board, as to not feed the higher voltage of the servos back into the processor.

  8. Re: Questions about powering

    Quote Originally Posted by KurtEck View Post
    As mentioned all of these power setups need a common ground and I would personally not connect the + pins of the batteries to each other in any way (either in series or in parallel)
    Thanks KurtEck. Yes, I worry that accidentally I may connect the + pins of the batteries together and damage those expensive motors. So better to use only one battery for all the dynamixels?

  9. #9

    Re: Questions about powering

    Connecting the + pins on the batteries won't damage the motors, but it could damage the batteries, and perhaps the wiring between them, if the batteries are not charged to the same state.

    These things are really basic electrical engineering questions. If you understand Ohm's Law, and understand Kirchoff's Laws, then you would know the answers to these questions yourself. I highly recommend doing some online tutorials on those things; this will make you much more capable and confident as a robot builder!

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