So I was reading an article today about a controller called the Darwin that is both inspired by and similar to the Wii Remote. It apparently uses a combination of accelerometers and gyroscopes to detect motion (much like the Wii will soon be able to do, with the MotionPlus attachment).
However, the developer also noted that the controller has a “magnetometer” built in, which he describes as a device that “decides its orientation and tells you its position relative to the Earth’s magnetic field.” Sounds like a glorified compass to my layman ears. He goes on to say that it can “sense the movement of an inch,” and that “[The Wii] lacks magnetometers, so it’s still a system that needs that IR strip to figure out where it is.”
So my question is twofold. First, what the hell is a Magnetometer and how accurate is it? And second, how would its inclusion compare to the Wii Remote’s use of the IR sensor bar (in addition to Wii MotionPlus) when it comes to determining where the controller is in 3-D space?
This patent (US Patent 5526022) probably describes what they’re talking about. The advantage is that it doesn’t require the IR bar for triangulation – it would work anywhere relative to the TV, which is a nice advantage over the Wii.
I’m not an EE, so I can’t answer the question about precision, but I remember in one of my robotics classes I was able to incorporate a simple eight-point compass into a blimp for steering purposes. The device resembled a blue thimble with eight (twelve?) pins sticking out of the bottom for soldering into the circuits. By applying a voltage to one pin and reading the voltage off the other pins, you could read a direction. A more expensive circuit could probably give you better precision, and if each one worked in-plane, you could combine three of them orthogonally and get redundant (more precise) azimuth and elevation info.
Thanks for the link. So I’m curious about its practical application, because to my knowledge, the Wiimote would only use the IR sensor bar for initial calibration and/or when you want to point at the screen (like a cursor, or such). Whereas the Darwin doesn’t use one, but then wouldn’t it not know where the TV is in relation to the player?
I guess the referenced patent may describe the device in use, as its content, timing, and assignee look right.
A magnetometer, in general, is a meter of magnetic field strength. If you can orient the axis of an assembly vertically, and the assembly contains two magnetometers perpendicular to the vertical axis (and preferably to each other too), you can figure out orientation from the two readings using trigonometry. If you can’t count on a vertical orientation you would need three of them.
There are little Hall-effect sensors that cost a buck or so and give you a measure of field strength, as a voltage output. Their sensitivity is too poor to use reliably to sense Earth’s field. If you put “flux concentrators”, or high permeability long bars such as soft iron rods, on either side of them, they can do a good deal better. There are also little magnetoresistive (sp?) sensors that do much better, though they cost more and also require resetting the materials inside with a capacitive discharge pulse, so using them is more complicated.
There’s also the Dinsmore sensor, a nifty little thing. It’s got two Hall-effect sensors at right angles inside, but also has a powerful rare earth magnet floating around them. The magnet aligns itself with Earth’s field, and its own field is easily sensed by the Hall-effect sensors.
Magnetometers respond much faster than you could reorient the device. The little Hall effect sensors have responses in the tens of kilohertz IIRC.
Magnetometers of nice quality could give you better than a degree of accuracy in orienting themselves relative to the magnetic field direction. This direction probably varies by more over time than the accuracy of nice magnetometers. In fact I think they’re used to monitor the field varying. There are much more accurate ways of orienting an instrument (without using manmade external references) - surveying instruments that include north-sensing gyroscopes take, I think, around 20 minutes (of time) to determine north to within a fraction of an arc second (of angle).
Napier’s comment reminds me that when we first moved to our new work building, I wasn’t sure what direction my office faced, so I brought in a compass. Do to stray fields from whatever, it was often well off of the expected direction (like 90 degrees), and varied as I moved from place to place.
I don’t know how much the field would be off from magnetic North, or how much it would vary from place to place in a residence due to stray fields, or if it would matter.