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Thread: New Heavy

  1. #1
    katch's Avatar
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    Taken the plunge, i know i know, i should probably do a feather first but what the hay its all for fun and if it gets destroyed at least i will find the weak points !

    I have been lurking and learning, asked the odd question, given away 13 electric wheel chairs and motor kits for free to forum members so not totally new.

    I picked up Hardox and Daveimi old shell and sort of merged them !

    Shell - Done ( no paint yet)
    Weapon - 80% done
    Drive Train 60% complete
    Wiring Harness 85% complete
    Wheels - Ready just waiting on motors.
    ESC on its way but harness ready to receive it


    Well within weight limit at 77kg, so Armour will be my last task once weighed...
    ( how is it weighed at a live event & how close do you skirt to 110kg...)

    now my weak point .... i really hate electrics.. but i can do it..

    Can some one translate this for me into suitable battery packs and how many i need based on your experience?

    I am limited to 45v max, but up to 60 amps per channel output.

    Spinner weapon is 17 kilos, was thinking hi amp starter solenoid for it perhaps ?

    i have 4x 350w motors or 2x 800w to choose from.

    ● 6 – 18 cells NiMH or NiCd battery.
    ● 3 – 6 cells LiPo or Li*Ion battery.
    ● 10V – 45V sealed lead acid battery.
    ● 10V – 45V power supply (​ Must be in parallel with a battery with same voltage​


    Opinions and helpful direction welcome.

  2. #2
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    Quote Originally Posted by katch View Post
    Taken the plunge, i know i know, i should probably do a feather first but what the hay its all for fun and if it gets destroyed at least i will find the weak points !

    I have been lurking and learning, asked the odd question, given away 13 electric wheel chairs and motor kits for free to forum members so not totally new.

    I picked up Hardox and Daveimi old shell and sort of merged them !

    Shell - Done ( no paint yet)
    Weapon - 80% done
    Drive Train 60% complete
    Wiring Harness 85% complete
    Wheels - Ready just waiting on motors.
    ESC on its way but harness ready to receive it


    Well within weight limit at 77kg, so Armour will be my last task once weighed...
    ( how is it weighed at a live event & how close do you skirt to 110kg...)

    now my weak point .... i really hate electrics.. but i can do it..

    Can some one translate this for me into suitable battery packs and how many i need based on your experience?

    I am limited to 45v max, but up to 60 amps per channel output.

    Spinner weapon is 17 kilos, was thinking hi amp starter solenoid for it perhaps ?

    i have 4x 350w motors or 2x 800w to choose from.

    ● 6 – 18 cells NiMH or NiCd battery.
    ● 3 – 6 cells LiPo or Li*Ion battery.
    ● 10V – 45V sealed lead acid battery.
    ● 10V – 45V power supply (​ Must be in parallel with a battery with same voltage​


    Opinions and helpful direction welcome.
    I guess the weapon is a spinner?
    Would really help if you could give us the MoI of it (or at least the rough shape and diameter), since weight distribution is at least as, if not even more important than the pure weight to know how it will behave.

  3. #3
    katch's Avatar
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    The Weapon is a 500mm solid hardox Z shape thats been stretched slightly, running on 2x 35mm ID bearings. Its currently 19 grams out of balance, but it will be balanced after the sprockets are fitted to it.

    MOI ?

  4. #4
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    MoI = Moment of Inertia.
    Or: how much do i need to push to get this thing rotating?
    In that, the diameter and mass distribution along this diameter are the factors. For every single bit of mass it's calculated per m*r², so the radius (and weight distribution) is more important than the actual mass to see how fast a spinner will spin up, how much energy it can store etc.
    That's why a spoked disc of the same weight and at the same speed can store more energy than a solid disc or bar, because more of the weight is further away from the axis.
    That's also why i'm always a bit irritated by people giving the weight of the spinner as only factor.

    But apart from having some physics knowledge, i am a noob to robot building, so i'll let the actual answers for other, more experienced guys. Just thought this might be an upcoming question anyway, and wanted to make it easier for those who actually can answer to this thread once they do so

  5. #5
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    Come here often

    Is it running gear for the weapon your stuck with? Are you gonna go with brushless or brushed?

  6. #6
    katch's Avatar
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    Sorted now, got some expert advise and a lot of guidance from a really decent Roboteer !!
    Last edited by katch; 22nd March 2017 at 17:07.

  7. #7
    Eventorizon's Avatar
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    I can guarantee most teams will not have calculated the MOI or inertia of their weapons. Most teams just ball-park their system against previous known setups, both ones that worked and didn't.
    I'd take the mass of your weapon and diameter and find some robots with similar weapons and see what they use as motors. If you can't afford or have the weight for what they use then increase the ratio so it will spin slower and use a smaller motor.

    As for your motors and batteries, they need to be worked out together. You need to guess how much capacity you need and what voltage all your motors run at, be it 24V, 36V or 48V. Again, look at other robot setups, take something that worked and then adjust it to suit your needs.

  8. #8
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    PP3D has a MOI on it's disc of 1.946896539 kgm^2. With a rotational velocity of 2500rpm or 261.7993875 rad/s that results in a kinetic energy of just over 66kJ. Those figures for MOI came from CAD. Of course these are perfect world with no resistance figures. The actual KE is probably a good 10% lower but that doesn't sound as impressive for the cameras.

    If you don't have access to CAD to calculate your MOI you can use the team cosmos calculator to do it for you (with a few assumptions about shapes etc). http://www.teamcosmos.com/ke/ke.shtml

    It is in imperial units but converting your dimensions to inches isn't difficult

  9. #9
    maddox10's Avatar
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    666 hectajoules sounds even "heavier"....

  10. #10
    katch's Avatar
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    With the help of a local bot builder who has competed already, i've picked up some more ideas and removed some poor ideas he shared willingly..

    Ignoring calculations, i'm just not clearly not clever enough for that, i'm going mechanic style, "trial and error".

    Test drive soon i hope ! No weapon, weighted for a battery load test. See how it goes or doesn't..

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