Motion Capture - Skier Path

[tap]

So I've been putting some effort into finding a means to track skier path. I first tried latching an accelerometer to myself while skiing, which worked great for acceleration data but not so well for calculating velocity and position data without ridiculous error in the numbers. Next I played around with acoustic location (which I still think has promise), but it required an annoying emitter to be worn by the skier. I read quite a bit about GPS location but, at least for now, I don't think it's quite able to provide clean enough data for what I'm looking to do. Then, primarily from reading about the work Bud Davis was doing, it occurred to me that motion capture was the answer.

 

I took what Bud was doing and moved it a bit further into the digital world. I mapped out some geometrical reference points on a boat, and grabbed a video that was shot from the pylon. Using pixel location I was able to track the skier's center of mass, handle, and front foot. The graphs below are from an open rated mens skier skiing 32 off at 36 mph. The process is fairly manual for now, but seems to provide reasonable results.

 

Pay attention to the labels and units. The first graph is the angular position (in degrees) vs. time. The second graph is angular velocity (in degrees per second) vs. time. The third graph is a bit of a stretch, but I think it is rather informative. It is a graph of the net force on the skier (assuming a 190 lb. skier) in the direction of the center of rotation (the pylon). It is reasonably equivalent to the line load, at least for certain portions of the path. It should be reasonably accurate from center line to apex. From apex to center line I would expect the actual line load to be slightly higher than what's calculated. I could translate the polar coordinates (reference frame of the boat) to Cartesian coordinates (reference frame of the course), but frankly I don't see the point. As far as I'm concerned the angular data is way more interesting (although I reserve the right to change my mind).

 

 

 

 

[Edbrazil]

Congrats on the work. Keep it up. Line load seems a bit low, esp. for 32 off at 36 mph. As in

maybe half of what it likely is.

[AdamCord]

@tap I would say the angular data is much more useful as a tool for computation about speed/load/etc., but from a visual point of view a cartesian graph with respect to the course is more useful for simply understanding skier path. And I would say getting a good understanding of skier path is the hardest thing for people and is probably the most enlightening.

 

From what I understand the load you've calculated is centripetal force, which makes sense from an angular perspective but doesn't take into account drag on the ski. With the pylon as the reference point this won't show up in line load because the direction of travel is tangential, but with respect to the course there is motion in the direction of the rope, correct?

[Than_Bogan]

Great stuff. I thnk angle data is the best starting point and I am aware of some projects to try to get more of it.

 

I'm very intrigued by how close the angle is to a triangle wave. The corresponding path will not look like that of course, but that shape is interesting and (to me) slightly unexpected.

 

But what IS expected is that this food for thought...

 

[lpskier]

On behalf of all the history majors that visit this site, allow me to ask WTF we are talking about?

[ToddL]

@lpskier they are trying to make history in the statistical analysis of slalom.

[Cayman2]

@ipskier, if that's a serious question, then: theyre trying to get an actual, data generated skier path. As of today, there is no easy way to do that, so all the theories proposed on skier technique are just theories. Until we can generate the ideal skier path, the skier actions required to create that path are not understood. That is the reason the accepted "right" way to ski keeps changing.

 

If that's not a serious question, then disregard this post, thread, and focus on how a good pass feels to you. Both are valid approaches to getting better.

[Horton]

@lpskier @ToddL @Cayman2

 

http://media.tumblr.com/fe497dd337d9af8479bb6398b9565d16/tumblr_inline_mg6n5ltl6X1rxe4lt.gif

 

first ever triple Panda

 

[MISkier]

This particular scientific examination of slalom began on another forum. The history of the topic, the theory behind its importance, and its evolution in measurement technique is located there.

[Cayman2]

Triple panda! Yessss!

 

I'd like to thank my teammates, ip and Todd, my parents for believing in me, my sponsors, Zoloft and Tylenol, and most of all horton for this great honor.

[ToddL]

I've gotten 2 panda's in the last 3 days, including this triple panda. Rock on!