WiiMote, Motion Plus, Accelerometers, Gyroscopes, Baseball Pitching, and What it All Means

I am working on a much larger post (and page, and even separate website) to detail my work with modeling baseball biomechanics, but I made a post that I want to catalog here on my blog for sharing and archival purposes. This was originally written on a messageboard, so if the formatting is off, I apologize.

Here’s a great video about accelerometers and gyroscopes:


What do I really care about when I’m using the Wii parts? Well, to build a fully functioning Inertial Mass Unit (IMU) to get 1:1 motion capture/control, I need to do what they demonstrate above. However, this is very complicated and requires 6 DOF. The degrees of freedom are:

Moving up and down (heaving)
Moving left and right (swaying)
Moving forward and backward (surging)
Tilting forward and backward (pitching)
Turning left and right (yawing)
Tilting side to side (rolling)

I really only care about what the forearm is doing in relation to the elbow; this eliminates the first three DOF. Fortunately for me, the first 3 DOF are handled by accelerometers and the last 3 DOF are handled by gyroscopes. What matters the most is tracking:

-Humeral internal rotation velocity rate of change (pitch)
-Forearm pronation/supination rate of change (roll)

And to a lesser extent:

-Ulnar/radial degrees of flexion rate of change (yaw)

So the next step is synchronizing what I see on high-speed two-dimensional frontal plane (side view) video and what I get from the gyroscopes. By doing this, I can nearly eliminate the need to have a four or five high-speed camera system that uses Direct Linear Transformation to recreate a three-dimensional model of a pitcher. This is awesome, because DLT is both ****ing ridiculously time intensive as well as somewhat expensive due to the need for 4+ high-speed cameras ($150 each minimum with current consumer technology) and the software to handle it ($50, but it’s very bare bones).

It’s cool to be the guy doing the most to push low-cost / DIY biomechanical analysis of amateur athletics, but it also means I have no peer groups to work with. The Internet helps, but very few people are working with this kind of technology to produce the stuff I want to make. It’s both exciting to be a pioneer in a field and incredibly frustrating because I have no formal education in physics or mechanical engineering, so I need to read pretty much everything I can get my hands on to understand it all.

I’d be remiss if I didn’t mention that it’s a bit terrifying that I could very well be wasting a lot of my time from an application/technology standpoint. If this product is so good (and I believe it is), then it already should exist given that the underlying technologies have been around for some time, though it can be said that it’s only been affordable since the Wii and smartphones have given rise to cheap small consumer electronics for accelerometers and gyroscopes – not very long. But there’s no proven market for what I want to sell, and it will never be huge.

Fortunately, I see this as an awesome opportunity to learn about science and to contribute – however marginally – to the field. Science and technology are two wholly separate disciplines, and as Richard Feynman famously said about his work: “I do things for the pleasure of finding things out.”

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