Center of Rotation

Center of Rotation

Center of Rotation

Hey, that's there three times!

Nope, it's not an error. It's because center of rotation is that important.

It's important due to a thing called 'parasitic forces'. When used to simulate g-forces (as with track racing), the machine tilts to the right during left hand turns; gravity pulling your body to the right while the image tells you you're level convinces your brain that you're being pulled to the right by centrifugal force, rather than being physically tilted.

The problem lies in how you get there. As you enter the corner, the car is at neutral lateral acceleration; you turn in and the acceleration progressively increases. So the machine progressively rotates. If you really fling the car into a curve, the machine has to rotate very fast indeed because the car can change acceleration much faster than it can change position — essentially instantly. But the simulator uses position to simulator acceleration, so it has to try to instantly get to a different position.

The upshot of this is that during the beginning of a cornering move, the machine often rotates as fast as it can. During that rotation, your body is subject to various forces due to the physical rotation that it wouldn't be subject to during a pure lateral movement.

When you're building a motion platform, your goal is to minimize those forces — parasitic forces — that tell your brain something's wrong.

That's where center of rotation comes in. Imagine a simple motion platform that only rotates side to side. The center of rotation can be anywhere on a vertical line, above, centered with, or below your body.

Most of our competitors — except those with six axis machines that can raise the center of rotation virtually using real lateral motion — have very low centers of rotation, often a foot below your seat. So, your head is maybe four feet from the center of rotation. During a fast rotation, your head will get flung to the side like a rock on the end of a rope, making you feel like you're flying into space. What's worse, when the center of rotation is below you, that movement is the opposite of the intended force! You're trying to simulate a lateral force pulling you to the left, so you rotate to the left. But in order to do that, the simulator has to fling your whole body to the left at maximum velocity, so while your head in the real car would jerk to the right laterally, your head in the simulator jerks to the left! The result is that with a low center of rotation, every motion you want starts out with a jerk in the opposite direction. Imagine driving your car and turning into a right hand corner, but for the first two tenths of a second you feel like you're being flung to the left.

This is bad. It prevents you from engaging with the simulation, it wastes tons of energy moving a bunch of simulator mass around unnecessarily, and it makes you throw up.

Among our competitors running non-six-axis systems, only SimCraft has a relatively high center of rotation — though lower than that of the 301 and 401 — but they place the pitch center of rotation far forward of the driver's center of mass, resulting in your feeling like you're popping into the air whenever you accelerate.

Our center of rotation is roughly level with your shoulders, and either centered on your shoulders fore/aft or slightly behind your shoulders, depending on seat position.

This means that when you corner, the seat of your pants moves in the right direction, and your head barely moves at all. Your head, and your vestibular system in your inner ears, moves very little during position changes, resulting in minimal parasitic forces and correct parasitic forces down low, where you sit — meaning that you can quite literally drive by the seat of your pants.

Again, I can't emphasize this enough. Machines with centers of rotation below the seat are essentially providing the precise opposite of the intended acceleration during the beginning of every movement.

Machines with centers of rotation below the seat are essentially providing the precise opposite of the intended acceleration during the beginning of every movement.

Put far more succinctly than the graduate thesis above: The geometry of our system allows us flexibility in conveying forces to the driver that match what he would feel in real life, without layering them with opposite forces at the beginning and end of every movement.

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