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Question All about 2DOF pivot points...

Discussion in 'DIY Motion Simulator Building Q&A / FAQ' started by clay_statue, Aug 9, 2021.

  1. clay_statue

    clay_statue Intrigued Dilettante

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    So my understanding that for seat movers, the pivot point is more or less centered under your butt with some users favoring slightly fore or aft of the ol' bunghole. Point being that the bullseye for where it should go is reasonably fixed.

    For a fixed frame scenario I am now a firm believer of 3-points of contact vs 4-points of contact because three-points of contact is the best way to define a plane. I read an excellent comment with youtube videos proving their point about the fidelity of motion being superior with a 3-actuator full frame vs 4-actuator full frame. I'd like to link the source but I cannot post external links yet.

    Now that being said you can accomplish three-points of contact with two actuators and the third point-of-contact being a pivot point.

    Let's assume that you could source two actuators with enough force/speed to move a full-frame. Where should the pivot point go? Instinctively one would want to put the pivot under the center of mass so you are largely balanced upon it. This puts the least amount of load on the actuators allowing them to use their speed to improve responsiveness.

    You can do this gimbal style with two nesting planes pivoting perpendicular to each other, as I have seen some users do roughly like this...

    [​IMG]

    Now the issue strikes me that in a vehicle, the pivot point for pitch/roll isn't underneath the driver! The pivot point for roll is down the centerline of the car... beside the driver. The pivot point for pitch would be just behind the driver's seat.

    [​IMG]


    So for an accurate 2DoF full frame experience of pitch/roll would involve an offset position for the pivot point. The clear x's would be actuators/motors and the brown x would be the pivot. I suppose if the pivot was a third actuator that would give you heave as well, but the purposes of this discussion I'm mostly interested in pitch/roll.

    [​IMG]

    Now excluding concerns about the footprint of the this type of setup, would it actually create more realistic pitch/roll forces upon the driver? Or am I just unnecessarily complicating something that won't necessarily translate into a more immersive user experience?

    Disclaimer: I have zero sim rig experience beyond the most basic of FFB wheels. This is all strictly theory I playing with.

    Attached Files:

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  2. Ads Master

    Ads Master

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  3. Deus Tempestas

    Deus Tempestas New Member

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    This is my thoughts too, in an F1 you straddle the center of the car, but not in a GT or Rally car. I like your tripod idea it would help keep the footprint of the rig manageable.
  4. Joe Cortexian

    Joe Cortexian New Member

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    Interesting idea. You should be able to do everything a corner motor rig can do with this approach.

    I think in addition to the foot print the single point behind the driver is going to be supporting most of the load most of the time. Seems like it will really want to tip. So that would tend to put a lot forces on the back piece.

    There are some pay systems out there that claim correct cog. They are really expensive and they are concerned about moving that point up as well. Sorry no link it’s just a memory.

    I am building a 2 dof rig with the weight mostly resting on the U-joint. If it doesn’t make me hurl I might upgrade. Also having experience in a more traditional setup would help you appreciate what the limitations really are.
  5. noorbeast

    noorbeast VR Tassie Devil Staff Member Moderator Race Director

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    Unnecessarily complicating matters, particularly when a 2DOF is more a cheap entry point to motion simulation.

    The purpose of a motion rig is not to re-create all real world forces, but to exploit your physiological and psychological processing weaknesses to provide adequate cues for your brain to act as though something is real, even though you knows it is not.

    Offsetting the pivot on a traditional 2DOF will result in significant strain on the motors, for likely little or no perceptual gain, which in turn will result in much higher costs for more powerful motors and control hardware to compensate.
    • Agree Agree x 3
  6. SimSpain

    SimSpain New Member

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    Sorry for being so late in the conversation but let me write down my point of view:
    I have been thinking about this because I was involved in a sim some years ago (startup, went down before first prototipe)
    My idea was that this point (in Spain we would call it CIR or Instantaneous Rotation Center) can be moved to whenever you want for a little period of a rotation. Thats what you can achieve with a couple of rods.
    So you can point it over the driver and in a point where, for example while braking, you have a rearwards movement of the seat while rotating to the front, so you have the transient movement right, for that start of the braking, but while you're braking, you still facing forward to feel the frontal Gs.
    I cant explain further before I present this to my boss at my new job in the field, but let me left here this:
    https://en.wikipedia.org/wiki/Instant_centre_of_rotation
    [​IMG]