While the Kinect has been getting most of the love from the hack-minded gaming community — and Microsoft, smartly, is encouraging innovation by working on a Kinect SDK — it’s neat to see that Sony’s PlayStation Move hasn’t been totally left behind. pabr.org has used the Move to build what it calls a Copernitron, which plays on a keen understanding of physics to measure the Earth’s rotation.
By design the PlayStation Move can track motion, including rotations. Physics says that MEMS gyros must measure rotation, not with respect to the ground, but with respect to an inertial frame of reference (i.e. the distant stars). Therefore the PS Move senses Earth’s rotation, right ?
Yes, but it is optimized for rates of rotation that are suitable for gaming applications (up to about 500 RPM). This is 500 RPM. About 800,000x Earth’s rotation rate.
The gyros have excellent dynamic range (16 bits). Yet Earth’s rotation is only 1/20th of their resolution and 1/100th of RMS noise.
But we can average as many samples as needed to reduce the noise, right ? No, because MEMS gyros suffer from “bias drift”, i.e. slow variations of their DC offset. And this is a good thing: if the PS Move gyros were too accurate, ITAR might classify them as missile components. This is why we can’t have nice motion tracking.
Now introducing the turntable. The turntable modulates the fraction of Earth’s rotation that the gyro senses. Then we demodulate in software and the DC offset is removed by narrowband filtering. This technique is called “homodyning”. It relates to “heterodyning” in radio receivers. The turntable shifts the signal of interest to a region of the spectrum where the noise can be filtered more easily.
We are not done yet: Gyro measurements happen to be influenced by magnetic fields. The two black rings are Helmholtz coils. They counteract Earth’s magnetic field. Each 40 cm diameter coil is 10 turns of copper wire in 25 mm flexible conduit.
Coils off: Note that we have aligned the coil axis with the local geomagnetic field. Coils enabled (450 mA): The compass now spins freely, which confirms that the field is significantly reduced.
One more thing: The turntable must be perfectly aligned (±0.02°) with the horizontal plane and with the gyro axis. Such accuracies can be achieved with laboratory-grade micrometric positioners. Or you can stack strips of paper.