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Thread: Robust, affordable gun design

  1. #1

    Robust, affordable gun design

    A robust gun design consists of a few things:
    - a gearbox
    - a barrel
    - the spin and gasket for the barrel
    - the hopup for the barrel
    - hardware/brackets to mount it all together
    - some solution for magazines
    - electronics to manage the gun

    Ideally, the electronics are more than just "on/off." Specifically, they should support a variety of available motor voltages (so, PWM,) and some sensor to make sure the breach stays open for long enough for the next BB to fall down.

    I have ideas on all of these, and have started by combining separate sensor and control boards I'm using now into one, that could be screwed into the gun gearbox with shallow screws, to minimize cable length and properly position the sensor on the slide. Here is the first rev of the design for that board. Interface:
    - VCC, digital fire trigger, GND as a three-way connector, doubling the Atmega programming connector
    - optional "ready" indicator on that same connector
    - logic is 3.3V if VCC is 3.3V, and 5.0V if VCC is 5.0V
    - Motor power separate, with screw terminals

    Arduino, Arbotix, CM-9, and most other robo controllers should be able to tell this to fire. The software in the Atmega is intended to drive the motor with PWM, and make a half-second pause with the breech open to allow time for the BB to drop, even if "Fire" goes on.

    Btw:

    Should it be single-fire (requiring "fire" to go low and high again for the next shot) or auto (re-trigger when ready as long as "fire" is high)?

    What duty cycle should the gun control PWM actually use? I'm thinking 3/5, which will drive the Desert Eagle plastic AEG just fine between about 9V and 13V.

    Should there be an option for serial control instead of digital line? It could probably recognize Dynamixel-style packets. Hard-code the ID to 254, and "fire" if the goal position is sent as > 512. Setting the baud rate would be needed, too, and might actually be pretty hard to get robust at 1/2 Mbit, given that the Tiny runs on the internal RC oscillator at 8 MHz, and doesn't have a dedicated USART (will need to decode the packet using the "USI" manual clocking mode.)
    Seems like "high to fire" would be sufficient, although serial mode would allow software-defined tuning of things like rest timing, PWM, etc.


    Looking for feedback on the general approach, as well as feedback on this particular board design.


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  2. #2

    Re: Robust, affordable gun design

    Maybe a jumper, even a solder one to select auto or single? This type of board could see applications other than mech warfare and they might like to be able to pick either one.

  3. #3

    Re: Robust, affordable gun design

    My general thought is that, because there's a programming header, if you want non-stock, then change the firmware :-)

    I was thinking more about "what would it look like if it were a Dynamixel TTL bus device?" The answer is that I'd need a Tiny84a instead of Tiny85, and a crystal for serial bus baud rate stability. And a regulator to run it off servo voltage instead of external logic. All in all, 3" wide instead of 2" if using through-hole components. SMD could make this more elegant, but also pretty much non-DIY for most people.

    This version still has the separate gun power, should you want to use it without DXLs. However, if the idea is to make a starter kit, and that uses AX-12s, the non-DXL parts might be unnecessary.
    Also: All those little diodes, zeners, and resistors that guard against user fumbles add significant surface area :-(

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  4. #4
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    Re: Robust, affordable gun design

    Problem with hard-coding to 254 ID for dynamixel approach is if you have two, nothing works, I think? Though I wonder if this can be worked around simply by not having the attiny ever respond to packets (i.e., I'm not sure of the full set of conditions in which duplicate IDs on a dynamixel bus cases problems).

    I'm surprised the through-hole board sizes are so big. Guess I've had success with 1.4"x1.75" (or so) boards by virtue of skipping a lot of safety bits... Hasn't bitten me yet . I could probably layout a through-hole design in Inkscape, following your schematic and a list of of components.

  5. #5

    Re: Robust, affordable gun design

    The second design would support adjustable id just like a dxl servo.
    The size comes from two things:
    -protection resistors and diodes
    -the board is designed to screw onto the flat top of the aeg with the sensor art the slide
    If "something else" positions the board, about 3/8" width can be saved.

    I'm almost at the point where I'm willing to sacrifice the screw terminal gun power, and just use dxl power. 4A is a little high during start-up, but perhaps an electrolytic will fix that.

  6. #6

    Re: Robust, affordable gun design

    So, if we assume that all mechs will have a Dynamixel-compatible bus (note that you can wire a Hovis or even just a UART as a write-only DXL bus) then we can also draw power from this bus. And if we assume anyone with a serial programmer is not dumb enough to keep the programmer connected while firing the gun, we can remove several protection resistors. We end up with the below design. (To do proper DXL bus, I need a Tiny84a instead of a Tiny85) It's 2.4" times 1.4" where the 0.4" on top are there just to allow screwing into the top of the gun slide to position the sensor robustly. This is for the $12 Desert Eagle plastic AEGs.

    This uses nice, thick traces -- 24 mil for signal, 66 mil for power -- so you might be able to make this using laser toner transfer. Although payint $30 for 3 at OSH Park seems easier :-) I could sell complete kits -- parts, headers, PCB, and pre-programmed MCU -- for perhaps $30.
    We could go smaller by going for SMD devices -- then we could use an 8u2 MCU, which has a real UART, even, as well as USB interface -- but at that point, it's not so much "homebrew friendly" anymore. The software would still be openly programmable, of course.

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  7. #7
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    Re: Robust, affordable gun design

    Quote Originally Posted by jwatte View Post
    So, if we assume that all mechs will have a Dynamixel-compatible bus
    Minor query: While I think the assumption of a Dynamixel-compatible bus is reasonable, what about people using RS-485 as opposed to TTL?

  8. #8
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    Re: Robust, affordable gun design

    Th232, what controller are you using? If you're using an ArbotiX, you should still be able to access the 3-pin dynamixel bus, even with the 4-pin bridge attached.

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    Re: Robust, affordable gun design

    I like the idea of using the dynamixel bus for trigger signal/command and power. Very clean, good idea.

    I'd choose a smaller SMD version at a reasonable cost hit over the DIY through hole version.

  10. #10

    Re: Robust, affordable gun design

    Quote Originally Posted by TXBDan View Post
    I like the idea of using the dynamixel bus for trigger signal/command and power. Very clean, good idea.

    I'd choose a smaller SMD version at a reasonable cost hit over the DIY through hole version.
    Except if it isn't clean, and gun noise gets put back into the dynamixel bus.

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