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Thread: Hexapod build

  1. Red face Hexapod build

    Hi guys, i am currently working out what i need to build a remote control hexapod. First of all i am not sure what board to use, between an arduino uno or a raspberry pi. I am unsure of the benifits of the raspberry. Also what would i need to control it using say a ps2 controller? It will have a total of 18 servos. Thanks in advance!

  2. #2

    Re: Hexapod build

    What servos?
    The Raspberry Pi cannot drive any servos directly, but is great at driving Dynamixel TTL servos using a USB2AX adapter. (You need to provide power some other way.)
    The Arduino can drive hobby servos, but isn't that great at driving Dynamixels (because of the shared serial port,) but it doesn't actually have enough pins to do all the I/O you need AND drive 18 hobby servos at the same time.

    Easiest is to use a Xbox One or Playstation 4 Bluetooth controller, with a Raspberry Pi 3, or Zero W, which have built-in Bluetooth receivers. If you go Xbox, make sure you get the version of the controller that actually supports Bluetooth. I think the PS/3 controllers are also Bluetooth, if you have one of those around, but check to be sure.

    Another option is to use something like an Xbee to send commands to the walker from a "base station" computer, or even WiFi if you're always in a place where there is WiFi. If you go with a Raspberry Pi and WiFi, then you can also get video from the bot using the Raspberry Pi camera.

  3. Re: Hexapod build

    I am just using sg90 or mg90 servos, and i will be using an ssc32u servo controller for that side of things. So it seems the raspberry has more power and more functions over the arduino say.
    Okay, that was my other option. I can get a usb shield for the arduino and do it that way. Thanks for your help too!

  4. #4

    Re: Hexapod build

    Yes Raspberry PI does have more power than most Arduinos... However it is unclear, how much you need?

    For example there have been several hexapods built in the past with RC servos using the SSC-32 servo controller. Lynxmotion (now part of RobotShop) makes/made several of them, including: http://www.lynxmotion.com/c-117-phoenix.aspx

    With these we have used PS2 based remote controls as well as XBee based controls, like the Arbotix Commander.

    Yes you can also move up to Raspberry PI and PS3 like setup, which then allows you to move up to other things like Wifi/Video... But sometimes it comes at a cost. The cost includes, sometimes it is harder to get smooth motions using something like Linux to talk to the Servo controller as there may be other tasks that are happening in the background, which keeps the processor from precisely controlling when the next step is issued... Also the Linux learning curve.

    Side note: I know that a lot of people have pretty good luck, made robots with cheaper servos like the mg90s, but I have seen others who had issues with reliability and quality... So I always stuck with higher priced Hitec Servos, like 645mg... However now days, I have not done anything with RC servos in a long time.

  5. #5

    Re: Hexapod build

    sometimes it is harder to get smooth motions using something like Linux to talk to the Servo controller as there may be other tasks that are happening in the background
    With a separate servo controller, this is unlikely to actually be a problem. I think the SSC can be told about trajectories, not just "output this PWM," right? (I know the competing Pololu servo controllers can.)
    Also, the modern Raspberry Pi have 4 separate cores, so it's unlikely they will all be busy doing other background stuff. And if you need good timing, Linux has real-time threads (look at SCHED_RR for example) which take priority. On multi-core systems this works great; on single-core systems (which includes the Pi Zero) you can actually lock up the system by using that, though.

    Also the Linux learning curve
    That is true, and I can't help you there :-/
    For what it's worth, the Linux learning curve builds skills that will be helpful in a number of different areas.

    Regarding hobby servos:
    Personally, I'd look at the XL-320 servos from Robotis, and a USB2AX+Pi, or perhaps even a OpenCM 9.04C controller, for a low-cost hexapod.
    The cheap Chinese servos are ... cheap. And the higher-cost Hitec servos are just as high cost, if not more, than the AX12A Robotis servos which are much nicer for walking robots: Greater range of motion, you can send position-and-velocity commands, you can read load/position/voltage/temperature back, and so forth.

  6. #6

    Re: Hexapod build

    @jwatte:
    Yes with SSC-32 doing most of the work, getting smooth movement is not as bad as for example using RPI to control AX servos with the RPI doing the interpolation of the moves.

    But there still is more to deal with about getting precise timing for output for the next move. For example the Phoenix code with the SSC-32 will setup to start the next move at the reasonably exact time the previous move completed. It actually outputs the next commands before the time, except for the terminator which tells the SSC-32 to act. If Ascii input - CR, if Binary input, I believe it was the time portion... With some of the timing issues it has more to do with the Serial latency... Again there are ways to reduce this.

    As for Servos - As I mentioned I have not used RC servos for maybe 5 years now... When I was using RC servos (Again Lynxmotion days), I only had I believe 4 or 5 of the cheap servos. All of them died within a year. Not counting the Orion Robotics modified Hitec servos, I believe I only had 1-2 die in all of the years out of maybe 50-75 servos?

  7. Re: Hexapod build

    Wow, just as i thought i was starting to understand robotics a little, all these new words and abbreviations have come in! But i understand what you are saying. I think i will start with the arduino and just see how i go.
    Thanks again for your input! If i get stuck with anything i know where to ask again.

  8. #8

    Re: Hexapod build

    using RPI to control AX servos with the RPI doing the interpolation of the moves.
    The AX12 can interpolate just fine, because you can tell them both position and velocity, just like the SSC.

    I've found that the scheduling jitter on the Pi is usually less than a millisecond, and much less than that if using a Pi 2/3 (quad-core) and/or using real-time threads.

    @Thomasz Nobody "understands" all of robotics, but the more you work on it, the easier it is to pick up new things and put them in the framework of the things you've already learned. Welcome to the community :-)

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