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Thread: Hexapod Control Help - Mechanical Eng. - little electronics experience

  1. Hexapod Control Help - Mechanical Eng. - little electronics experience

    Hey there,

    Can anyone here help me with an issue iIm running up against?

    I'm building an 18 linear actuator, 8 servo hexapod (LAs for leg joints, servos for rotation of legs and head Pan and Tilt) which will be based on the Takemikazuchi Tank from the Anime Sora-No-Woto

    picture(not mine): http://blog-imgs-46-origin.fc2.com/m...60616429af.jpg

    my current setup looks like this:

    https://www.facebook.com/photo.php?f...type=3&theater

    I have worked with servos before for animatronics but this is the first time i'll be working with Linear Actuators, and more to the point these: http://www.firgelli.com/Uploads/L16_datasheet.pdf

    I was planning to run control for the 18 LAs and 8 servos from a 32 Servo Control Board from Lynxmotion: http://www.lynxmotion.com/p-395-ssc-...ontroller.aspx

    What I'm looking for is any reason why the above plans would completely and utterly fail and if there aren't any reasons then how would I go about with the setup of the LAs for servo based control?

    I'm a mechanical engineer with less than a layperson's understanding of programming or electronics setup so please be gentle haha. (also please forgive me for all the links)

  2. #2

    Re: Hexapod Control Help - Mechanical Eng. - little electronics experience

    The Servo Control Board controls hobby-style PWM-driven servos. Those linear actuators instead need H-bridges, and a controller that can read the position feedback and send the appropriate control to each H-bridge (18 of them.)

    Also, 32 mm per second for the fastest actuator is not particularly fast if you want to create some kind of leverage around rotational joints. Make sure that you understand what kind of speed you will actually need for each joint. Also, a lifetime of 20,000 strokes means 20,000 steps -- and the duty cycle is rated at 20%.

  3. Re: Hexapod Control Help - Mechanical Eng. - little electronics experience

    Quote Originally Posted by jwatte View Post
    The Servo Control Board controls hobby-style PWM-driven servos. Those linear actuators instead need H-bridges, and a controller that can read the position feedback and send the appropriate control to each H-bridge (18 of them.)

    Also, 32 mm per second for the fastest actuator is not particularly fast if you want to create some kind of leverage around rotational joints. Make sure that you understand what kind of speed you will actually need for each joint. Also, a lifetime of 20,000 strokes means 20,000 steps -- and the duty cycle is rated at 20%.
    Thanks for pointing out the issue with the H-bridges. I hadn't noticed they weren't an integrated component like they are on most servos. I have enough ability with electronics to rig up a fix for that but if you happened to know of a control board specifically for H-bridge control of analog motors (if such a thing even exists) I would greatly appreciate that as well.

    As for the speed, it's definitely a concern but i think a manageable one. I've been designing a compromise of having large moment arms for the actuation. A small speed at one end gives me a fairly impressive radial velocity at the long end of the leg. It's also why it has the 3 leg sections (instead of the usual 2) to take the maximum advantage of the slower individual speed to get a larger foot-over-ground speed. Each leg (hardware, actuators, leg components) weighs only about 3/4 to 1 lb depending on the material being used (AL 6061-T6 or Nylon-mix polymer) so even the fastest LA has enough force to lift a complete leg at full extension. I'll have to play around with motion analysis for what the maximum force I can get with the minimum speed I'm willing to accept would be.

    I am still concerned about the life cycle of the LAs but just about every other model I've found has similar stats and the project i'm working towards needs them for the low cross sectional area. If I switch over to servos and scale the legs appropriately then it'll be the size of a washing machine. There are definitely easier ways to do this project with servos, but none I've found would allow me to keep the specs and design I'm going for.

  4. #4

    Re: Hexapod Control Help - Mechanical Eng. - little electronics experience

    even the fastest LA has enough force to lift a complete leg at full extension.
    When the hex has three legs on the ground, the total weight of the hex rests on three legs, and the length of the lever arm is the horizontal distance from ground contact to center of pivot. Make sure you include that in your calculations.

    a control board specifically for H-bridge control of analog motors
    There's no such thing as an analog motor. Perhaps you mean a brushed motor? That's what H-bridges are for (that, and stepper motors, where you need two H-bridges per motor.)
    Just the H-bridge part has tons of options, depending on how much current you need. If the max draw is < 1.5A, then cheap things like http://www.pololu.com/catalog/product/2136 would work fine. If higher current, expect higher expense.
    If you also need the "read the encoder and provide motor control feedback" part of the closed-loop control, then you need something like a Pololu JRK http://www.pololu.com/catalog/category/95 or whatever. (That controller can also accept RC style pulses for control, btw.)
    And, before you ask: yes, you need lots of those!

  5. Re: Hexapod Control Help - Mechanical Eng. - little electronics experience

    hmmm...after your previous post I looked into wiring rigs and was planning on controlling them directly the SSC-32 (http://www.lynxmotion.com/images/html/build136.htm) with an H-Bridge circuit for each LA motor. From the LA spec sheet, pin 3 and 4 should correspond with the VS and Ground pins through the H-Bridge. Then it seems like pin 2 would link with the Pulse pin for position control while pins 1 and 5 would require the two reference voltages stated that I haven't figure out yet. Have I gone wrong in my assessment and if so where?

  6. #6

    Re: Hexapod Control Help - Mechanical Eng. - little electronics experience

    What you're saying doesn't quite match up. Yes, pins 3 and 4 on those actuators is where you hook up the output of the H-bridge.
    The "pulse" output from the SSC32 is not compatible with the "direction" and "enable" (or "Ain" and "Bin") inputs of an H-bridge.
    Also, the SSC32 has no input that can read the analog reference wiper to determine position of the actuators.

  7. Re: Hexapod Control Help - Mechanical Eng. - little electronics experience

    Ah, makes sense. In that case it looks like i'm going to have to use servos because I have nowhere near the funds to get all the peripheral electronics you mentioned. Which unfortunately means I'm about to blow this thing up by about a factor of 1.5 to 2 to get the same dimension relations...

  8. Re: Hexapod Control Help - Mechanical Eng. - little electronics experience

    What may be more annoying is that there are linear servos out there set up for PMW control like these: http://www.servocity.com/html/heavy_...vo__25__l.html

    They're just way too big unfortunately.

  9. #9

    Re: Hexapod Control Help - Mechanical Eng. - little electronics experience

    As a software guy, I feel that kind of pain a lot when doing robotics. In software, actual physical constraints are seldom a real problem. In mechanics, that's the main limiting factor...

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