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Thread: Jumping in to Mech Warfare

  1. Jumping in to Mech Warfare

    Hello all,

    I attended Robogames 2013 and caught some Mech Warfare action. I was instantly hooked. I've been creeping on the forums since trying to learn as much as I can and I'm thinking of getting started with my build in hopes for participating in the future.

    Thinking about a build, I had some questions before I start ordering:

    Would these be viable legs:
    http://www.robotshop.com/productinfo...354&lang=en-US
    I see that these legs use the servos I'm familiar with from RC Car racing. I do have easy access to high-end servos (350+oz/in, 0.13s) of this type that will work.

    Majority of robots that I see in the pictures use servos/actuators of this type:
    http://www.trossenrobotics.com/dynam...-actuator.aspx

    Would a build work in competition using non-dynamixel servos? These are totally new to me. I just want to get a quad walking ASAP.


    Thank you very much.
    -2zon

  2. #2
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    Re: Jumping in to Mech Warfare

    I can think of two competitive mechs that use non-dynamixel hobby-style servos, Bheka and 2nd Amendment. Bheka's servos are more expensive than AX-12s for about the same performance, and 2nd Amendment was just superbly designed and very spartan- I believe Upgrayd used HS-645s for 2nd Amendment. I should note that he's what I would consider a professional builder, so your mileage may vary.

    The biggest question is, how much software experience do you have? If you're pretty well suited in that regard, your hobby servos might work well for you. Otherwise, I would highly suggest going the AX-12/Arbotix route if you're just starting out, if only for the ease of use of NUKE to get your bot walking and the dozens of other competitors using the same hardware as a knowledge base.

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    Re: Jumping in to Mech Warfare

    If you go the hobby servo route, keep the legs as simple as possible. 4 DoF legs are NOT something to start out with. Start out with something like 2 DoF mechanical advantage legs.

  4. Re: Jumping in to Mech Warfare

    Thank you for your responses. The 2DOF route with AX-12 Servos seems most doable for me right now.

    I'm looking to order the PhantomX quad MarkII kit. It looks like this can be built with 2DOF style legs if you just leave out a servo on each leg and some frame parts right?

    I have a question about the ArbotiX RoboController. Would it be able to programmed to take inputs from a 9 Channel RC airplane transmitter and receiver? I would like to use it as the controller.

  5. #5

    Re: Jumping in to Mech Warfare

    I have a question about the ArbotiX RoboController. Would it be able to programmed to take inputs from a 9 Channel RC airplane transmitter and receiver?
    You can probably make a "good enough" decoder of R/C PWM signals, especially if you only use a few channels.

    The main concern is that there is only one hardware capture channel, so you're going to have to build your pulse decoder based on pin change interrupts. Those will give you some jitter.

    With default Arduino code, the jitter can occasionally be pretty bad, because it does a lot of work for the millisecond/microsecond timer with interrupts disabled. If you smooth out the received data, then that's not so bad. Also, if you throw out the Arduino libraries, and write your own straight on top of avr-gcc and avr-libc, you can get better interrupt performance.

    So, the answer is: Yes, the Arbotix can be made to decode those signals. To make it perform well, you have to be reasonably skilled at C programming for the AVR embedded microcontroller.

  6. #6

    Re: Jumping in to Mech Warfare

    Lots of work on the Rx input side using interrupts has been done for the Aeroquad project. They have a couple of different libraries already done for Arduino that could be hacked on pretty easy. They also have a couple of different ways of capturing the pulse stream as one single input for those Rx's that support it. With the spektrum gear for example you can use just the satelite Rx and read all of the positions from it directly using a hardware serial port. If you aren't using the port for an XBee it shouldn't be that hard.

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    Re: Jumping in to Mech Warfare

    As one of the few that use hobby servos, I would add that unless you already have a bunch of them just sitting around, the Dynamixels would probably be the easiest and least expensive way to go. When I built Bheka, I used mostly Hitec HSR-8498's from a Robonova that I scrapped out. When I built Maggot MK3, I used a bunch of HSR-5990/5980's that a generous friend gave me.

    The HSR-5980's run about $110.00 each.

    I went the simple method of control, with a Lynxmotion SSC-32 running sequences. Not very sophisticated to say the least.

    Good luck
    Gary
    Last edited by gdubb2; 05-03-2013 at 10:53 AM. Reason: grammer
    Team Maggot---Mechs. "Bheka" (retired), "Maggot Mk.3(A)"
    " Keep your stick on the ice ".... Red Green

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    Re: Jumping in to Mech Warfare

    A few more comments based on my experience with the Arbotix and Pypose:

    • The Arbotix does not have an embedded radio receiver. However, it does have pins specifically to plug in an XBEE for an easy serial-->XBEE-->XBEE-->serial communication setup. If you buy an RC reciever, you could wire it up to the XBEE pins or to the user defined I/O pins on the arbotix. Then you'll need to do some programming to interpret the signals (as Jwatte and Escott described).
      -------------------------
    • When creating motion sequences in Pypose and generating gates in Pypose/NUKE--The pypose program running on your computer needs to communicate over a serial connection with the pypose sketch running on the arbotix. XBEEs are one way to create this serial connection, if you skip XBEES you'll need another serial connection solution.
      --------------------------
    • XBEEs also have the advantage of 2-way communication. For example, you can have it send back servo temperature to tell if you're overheating. (although this does require more coding on your part)
      -------------------------
    • IMHO, Pypose/NUKE is really not very elegent for doing a 2DOF quad. Currently, NUKE only provides support for 3DOF quads (choice between "lizard" and "mammal" configuration). So if you want NUKE-generated gaits for 2DOF legs, you would need to either program your own 2DOF support into NUKE, or create a 3DOF gait and hack it down. A few other options you have:
      • Use Pypose to capture some basic positions, and then write your own inverse kinematics code.
      • Use Pypose to capture walking and turning pose sequences, and just run those (no inverse kinematics)
      • Keep it 3DOF, and plan on making frame modifications to reduce servo load (such as shortening leg segments, maybe adding springs like Tucson Nomad)


    I'm still a new builder-hopefully the veterans will chime in if I'm off somewhere!

    Lupulus
    Last edited by Lupulus; 05-03-2013 at 11:19 AM.

  9. #9

    Re: Jumping in to Mech Warfare

    Quote Originally Posted by Lupulus View Post
    A few more comments based on my experience with the Arbotix and Pypose:

    • The Arbotix does not have an embedded radio receiver. However, it does have pins specifically to plug in an XBEE for an easy serial-->XBEE-->XBEE-->serial communication setup. If you buy an RC reciever, you could wire it up to the XBEE pins or to the user defined I/O pins on the arbotix. Then you'll need to do some programming to interpret the signals (as Jwatte and Escott described).
      -------------------------
    • When creating motion sequences in Pypose and generating gates in Pypose/NUKE--The pypose program running on your computer needs to communicate over a serial connection with the pypose sketch running on the arbotix. XBEEs are one way to create this serial connection, if you skip XBEES you'll need another serial connection solution.
      --------------------------
    • XBEEs also have the advantage of 2-way communication. For example, you can have it send back servo temperature to tell if you're overheating. (although this does require more coding on your part)
      -------------------------
    • IMHO, Pypose/NUKE is really not very elegent for doing a 2DOF quad. Currently, NUKE only provides support for 3DOF quads (choice between "lizard" and "mammal" configuration). So if you want NUKE-generated gaits for 2DOF legs, you would need to either program your own 2DOF support into NUKE, or create a 3DOF gait and hack it down. A few other options you have:
      • Use Pypose to capture some basic positions, and then write your own inverse kinematics code.
      • Use Pypose to capture walking and turning pose sequences, and just run those (no inverse kinematics)
      • Keep it 3DOF, and plan on making frame modifications to reduce servo load (such as shortening leg segments, maybe adding springs like Tucson Nomad)


    I'm still a new builder-hopefully the veterans will chime in if I'm off somewhere!

    Lupulus
    Wiring an RC receiver to serial pins ONLY works if the Rx speaks some form of serial. Most do not, and all that I know of don't advertise this fact.
    Typical RC Rx's output a PWM pulse on each of several "channels" individual connectors to servos. This is what Jwatte was describing using interrupts to grab. It becomes problematic as you get more than a couple of these channels being read. Lots of jitter is introduced. You can filter and smooth things out, and in controlling something like this, you can quantize pretty well, you just won't have super granularity. For a quad it's not so easy, you need finer control, and fighting the jitter is a big issue. Some Rx's have a summed single wire output of the channels, putting out one pulse for each of the channels in order, then a longer pause pulse, and then repeating. This is pretty much what you'll see on the trainer port of the transmitter as well. This can be read by a single pin on the micro which helps a bit with jitter.
    Without either of these summed options you are left with a micro pin PER channel that you want to read. And at least with the 328p you can't just use whatever pin. I haven't worked a whole lot with the 644 though.
    When I started thinking about building my quadruped I was initially going to use my DX8 to control it. I looked into doing all of these things, and decided that since I'll be using my laptop for looking at video streams it's simpler to use a PS3 controller paired to the laptop. I may do a DIY controller once I have the other stuff working, but for the moment I'll stick to the PS3 controller, the laptop and XBEE.

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    Re: Jumping in to Mech Warfare

    Thanks for the clarifications escott! I was not really familiar with the problem of Rx jitter. The more I know, the happier I am to be using XBEEs...

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