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2DOF tilting chair design, where is pivot CofG?

Discussion in 'Motor actuators and drivers' started by steveh2112, Oct 1, 2016.

  1. steveh2112

    steveh2112 Member

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    i'm starting to build a 2DOF tilting chair rig simillar to the DKPro and i'm wondering about where to put the universal pivot joint. i assume of the center of mass of the rig would be best.

    since i'm using oculus (no monitor), me and the chair are probably about 95% of the weight so i assume center of mass in the horizontal plane, which, while in a seated, slightly reclining position with legs stretched out in front of me, is probably around my gentleman's sausage area (to use James May terminology).

    in the vertical plane, i would think the center of mass is perhaps higher up than under my bum. i think if the pivot is under my bum, the rig will be top heavy and it will be hard for the motors to pull me back to level after a full range roll. if the rig is perfectly balanced, i would think pitch and roll would require a lot less energy, which equals more speed for less power.

    what do you think? it obviously easier to put the pivot under the seat then say cut a hole between the thighs and mount a pivot point there. but i wonder is there is a big mechanical advantage to that?

    thx steve

    PS, another idea might be counter weights under the seat, but i've never seen any examples of that.
    Last edited: Oct 1, 2016
  2. Ads Master

    Ads Master

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  3. noorbeast

    noorbeast VR Tassie Devil Staff Member Moderator Race Director

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  4. SeatTime

    SeatTime Well-Known Member

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    In most cars/race cars the CofG is similar to below - point 'D'. The closer you can get to this the more realistic your sim will feel and yes it will take some load off the motors, although adding weight to the moving parts of a sim increases mass and therefore inertia which = bad.

    MHbzjm.jpg

    Unfortunately not so easy to implement fully in a 2DOF sitting on a universal joint.
    Agree with @noorbeast comments for a balanced 2DOF.
  5. steveh2112

    steveh2112 Member

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    interesting pic, thanks. i see the CG is at the front of the lower chest which makes sense. like you said, with a universal joint, there is no practical way to pivot there.

    i'll try mounting the pivot between my thighs anyhow, have it push up through the seat a bit. it will make a bit of a bump in the chair but maybe that will help stabilize me while leaning, a bit like when i'm pushed up against the gas tank on my bike.

    another idea i plan to experiment with is using bungee cords to help pull the rig back to level. i've seen some rigs that maybe use springs for that but i figure bungees are quieter and easier to experiment with.
  6. BlazinH

    BlazinH Well-Known Member

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    I have tried cog at both a D and H on the diagram above. While D is the correct cog H feels better imo. At D you have a reduced sensation in the roll/sway axis. And if your feet rotate the opposite direction because they are below the cog, it feels all wrong.
  7. steveh2112

    steveh2112 Member

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    i was thinking the same, i think by pivoting off COG you'll get more of a sensation of been thrown about. like when you are a kid on the roundabout, you stay in the middle if you're chicken and hang out at the edge for the big thrills.

    i still think i'd like to have the rig roughly symmetrical in terms of angular velocity while traveling from the neutral position to max tilt, and then back. so i'm think bungee or elastic shock cords attached to the back of the seat and to a couple of posts either side of the seat. then when the rig tilts in either axis, the elastic will tend to pull it back. i just got 10m of 5mm shock chord cheap on ebay, i'll try it out
    Last edited: Oct 3, 2016
  8. Archie

    Archie Eternal tinkerer

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    The trouble I found with anything that "pulls back" when doing a tilt is that the other side has to fight the resistance. the net result is you are better off with nothing to assist (the exception being for heave)

    Newton's 3rd Law. The net result is equal.

    [​IMG]
  9. steveh2112

    steveh2112 Member

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    i don't think that's true if i'm using bungee cords, they'll just hang down on the near side, but maybe i misunderstand your point?
  10. eaorobbie

    eaorobbie Well-Known Member SimTools Developer Gold Contributor

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    LOL good to see my pic still floating around.

    On a seatmover the CG is about ya navel when seated in a GT Position. Trick is to have pivot point of the platform as close as possible to the cg or the weight of driver + rig can act as pendulum and almost impossible to hold still or requires a mass torque to catch the moving mass. Plus if connecting rods are under 100-150mm higher than the centre position it requires more than 2 fold the power to keep still or hold an accurate position, hence I use shoulder mounting that greatly amplifies the torque. With 100 kg driver in seat i only need 3kg of force to hold seat level. Same concept of a SIMX unit which only has 5kg of force avail with a scn5.
    Except i got 3 times the speed and 32 kg instead of 5 kg, DC beats the the SCN5 in that class, SCN5 just rule looks and resolution.lol


    With my seat mover my cg is about 200mm above the the centre of the platforms pivot allowing me to use 100% of the speed available at reasonably low amp's, as in normally running Im lucky to pull 6-8 amps off JRKS. But under intense 700 odd mm sec vibrations when going off track or bouncing off walls they can pull over 40 amps. But key is the cg and the geometry of your connecting rods and seat support, where most diy get it wrong, Something learnt over time.
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  11. steveh2112

    steveh2112 Member

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    thanks, can you provide a bit more explanation on this bit 'But key is the cg and the geometry of your connecting rods and seat support, where most diy get it wrong, Something learnt over time'?

    maybe you posted something already?