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Thread: how many dynamixels can an arduino handle?

  1. how many dynamixels can an arduino handle?

    hello, i need some help here. i'm planning to build a snake robot similar to ACMR3 snake robot. the problem is i want to put 10 dynamixel ax-12a. what i want to know is how many arduino i need? is one enough? what came in my mind is arduino uno or arduino mega 2560. i have my consideration in arbotix too.. can somebody help me please?

  2. #2

    Re: how many dynamixels can an arduino handle?

    One Arbotix would be easiest, as it can already talk to the AX-12s.
    One Arduino (Uno or Leonardo or whatever) would also be enough, but you'd need an interface to talk to the Dynamixel bus.
    Also, you probably don't want to try to feed 10 servos from a single daisy-chain strand; you may want to inject power using coarser gauge wire at a few points along that chain.

  3. #3

    Re: how many dynamixels can an arduino handle?

    Our Hexapods(18 servos) all run off an ArbotiX with no problems. I've also seen people use mega to run crawlers, so there shouldn't be a problem if you go that route. But like jwatte mentioned, you'll want to try to avoid a single daisy chain if possible.

    That being said, our PhantomX Reactor used 8 servos on a single chain and we haven't had any problems. Though we do use our own custom cables that are 20AWG (I think the stock ROBOTIS ones are 22AWG iirc)

  4. Re: how many dynamixels can an arduino handle?

    Thank you very much for your answers, so 1 arduino/arbotix is enough then. but is there any problem with the data transfer speed from the arduino to each dynamixels or is it perfectly fine? and for the daisy chain, is it ok to connect all 10 dynamixels data port with daisy chain but each dynamixel has different power source?

  5. #5

    Re: how many dynamixels can an arduino handle?

    Others can answer better than I can, but:

    The Arbotix is the main processor that is used on most of the projects up here including the PhantomX Hexapod, that has 18 servos (or more) on it. On these robots, we typically simply have one power source which often is a 3s Lipo battery. The Arbotix board is setup for this, so you simply have to feed the Lipo into it and then plug in you servos. There are 3 connectors on the board to start your daisy chaining of servos, with all 3 connectors connected up to the same serial port. On the Hexapod, we break up the chains per leg, so we daisy chain from one of the connectors on the Arbotix, to a 6 port hub, where we then plug some of the legs into this hub...

    As for an Arduino, most of them have the exact same processing power as the Arbotix as they are all Atmega processors running at 16mhz. The Arbotix is an Atmega644p, where for example the Arduino Uno is a Atmega328p. The main differences between these two processors, is that the 644p has twice as much memory and more importantly it has a second USART, which on the Arbotix they use to drive the servos. Another standard Arduino that has a second port is the Leonardo, which would also probably be a good choice.
    However again, by itself it has not been adapted to work with the half duplex Dynamixels. Not hard to do, Could go as simple as Arbotix and short the TX and RX to each other and carefully adapt the software, or can use some simple hardware connections, like the: http://www.robotshop.com/productinfo...142&lang=en-US . This adapter also has a connector that allows to to interject the power needed for the servos. If you go with a simple circuit, you can also interject power using something like: http://www.trossenrobotics.com/6-port-ax-mx-power-hub

    One power source can be enough, but if you decide you want more, than get a few of these. Just be careful then on how you connect them up to each other as to isolate the power.

    Kurt

  6. #6

    Re: how many dynamixels can an arduino handle?

    You don't necessarily need a power source for each servo, but in an ideal scenario you want to keep each servo as close to the power supply as possible

    For example, the PhantomX Heaxpaod is wired as so: (Each leg has 3 servos)

    Leg 1
    |
    |
    |
    ArbotiX ----Leg 2
    |
    |
    |
    Hub

    The hub then expands out to the back 4 legs (12 Servos)

    http://www.trossenrobotics.com/images/assembly/phantomX-hexapod/HexPowerWiring2-800.jpg

    Even this wiring could be more balanced (i.e. use 1 hubs per 2 legs) but we've never had any power problems with this wiring on the AX-12As or the AX-18As.

    What you want to try to avoid is
    Arbotix-Leg1-leg2-leg3-leg4-leg5-leg6
    Since all the power for all the legs have to go through a single power wire and a single servo.

    The DYNAMIXELs run best at about 30hz (I've seen people go beyond that, but at some point you'll get erratic behavior). Stay at around 30hz and you're good to go

    tldr: You can run quite a few servos from a single line, but you want to avoid it when practical/possible.
    Last edited by kgranat; 10-25-2013 at 09:32 AM.

  7. Re: how many dynamixels can an arduino handle?

    ok, i think i get it now. Thank you all for answering my question. I read something about the interface for arduino which is texas instrument SN74LS541 from savageelectronics.blogspot.com , is this ic possible too?

  8. #8

    Re: how many dynamixels can an arduino handle?

    One power source can be enough
    Just for clarity: I wasn't suggesting multiple batteries, but I did suggest not sending the power through a single long daisy chain.
    As others have said, a hub in the middle, and one chain per leg (3 servos per chain) works great.
    For a snake-shape robot with 10 servos, you want to run perhaps the first 4 servos off the first chain, and then run a power cable in parallel, that you then tap for the next three servos, and further down for the next three servos.
    Meanwhile, ground and TTL-signal has to run through the entire chain (or be run in parallel with the power as well.)
    The point of this is that the separate power cable can use 16 gauge wiring, which is a lot more forgiving to long power runs than the 22 gauge that ships with Robotis servos.
    Also, the more connectors the power goes through, the more loss there is, and the more danger of overheating the connector there is.

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