| HappySqurriel said: You're wrong about the Wii MotionPlus ... From Wikipedia (with citations): "The device incorporates a dual-axis[13] "tuning fork" angular rate sensor,[14] which can determine rotational motion. The information captured by the angular rate sensor can then be used to distinguish true linear motion from the accelerometer readings.[15] This allows for the capture of more complex movements than possible with the Wii Remote alone" In other words, the device isn't to subtract gravity (which is already possible on the Wiimote) the device is actually a fairly large improvement on the motion sensing capabilities of the Wii |
I don't know your background, but you probably didn't entirely understand what you just quoted. That wikipedia voice supports exactly my point.
Warning: longish physics divulgation follows.
I'll try to explain, and you can google or research real books for the details: there's a general principle in physics (Einstein's equivalence principle) that says that gravitational forces are locally indistinguishable from apparent forces in a non-inertial frame of reference.
A non-inertial frame of reference is basically your Wiimote when you accelerate it, say by shaking it. The apparent forces are the "inertia" forces measured by the accelerometers, like when in your car you brake, the car decelerates and you feel pushed forward. That forces pushing you forward is the apparent force due to the deceleration, and it is proportional to your mass, exactly like the gravitational force. The equivalence principle says that if the car had all windows painted black so that you can't see the outside, you could not say if you felt pushed forward because th car is braking, or pulled forward by a gravitational pull, as if the car was diving nose down.
The inclination relative to vertical of a Wiimote without WM+ can be measured by its accelerometers exactly because of this principle: if I take the Wiimote and put it at rest on a table, the accelerometers inside won't measure 0: one of them will measure a vertical acceleration of 1g, because it can't distinguish between gravity and the force that it would feel in absence of gravity, but being vertically accelerated (see here about accelerometers).
Or looking at it in the other way: if the Wiimote is at rest, I can read its accelerometers to see which way the gravity pulls, ie its orientation from the vertical.
In the same way if I shake the Wiimote horizontally the accelerometers wil measure an acceleration made by the sum of the "real" motion one and the gravity-equivalent one. Follow the link marked [15] in the wikipedia voice, and you'll be directed to a short movie by the IntelliSense guys showing exactly this phenomenon with accelerometers readings.
On the contrary gyroscopes use a very different mechanism to measure orientation changes, thus they can keep a reference direction, and that's useful because this way not only you can measure rotations with a greater precision, but you can know which direction is "down" in real time.
Your wikipedia says: "The information captured by the angular rate sensor can then be used to distinguish true linear motion from the accelerometer readings". Accelerometer readings = a mix (vectorial sum) of the gravity force and the inertial forces due to the motion. Thanks to the gyros we can extract from this mix only the "true linear motion" component, by subtracting a 1g vertical component and the smaller side components due to rotation.
I hope to have clarified more than mystified.
Update: I stand corrected on one point. That is that WM+ has another good use in position tracking besides pointing down, that is to filter out most of the little jitter that comes from your hands slightly shaking. That's another good feature it brings to the table.
