In-Situ Ski Angle Measurement

In-Situ Ski Angle Measurement

Postby Holmz » Sun Jan 02, 2011 4:43 pm

I am new here on the forum, but thought I would share where I am at with measuring ski angles, and not sure if anyone is measuring ski angles in-situ.

All data on the graph below is for the right ski only.
The red curve shows the right ski's roll (or tipping) angle.
Negative Roll is a left hand turn with the right skis the outside ski.

Yaw is steering. Looks like I have a high yaw rate as I unload the right ski when transitioning into a right hand turn, and it feels like I am doing an anti-javelin turn as my skis feel a bit splayed out on GS turns.

http://www.flickr.com/photos/32230177@N06/5317415681/

Need to get some video to put the graphs on top of, but there are always more things to do than time available.

Thanks for looking...
Randal
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Re: In-Situ Ski Angle Measurement

Postby HighAngles » Sun Jan 02, 2011 7:15 pm

OK - you've really piqued my interest with this (must be the ski geek in me). How exactly do you collect the data? Is it via video analysis or are you attaching some kind of device to the skis?
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Re: In-Situ Ski Angle Measurement

Postby Holmz » Sun Jan 02, 2011 9:51 pm

HighAngles wrote:OK - you've really piqued my interest with this (must be the ski geek in me). How exactly do you collect the data? Is it via video analysis or are you attaching some kind of device to the skis?


Originally as per the avatar, but now the device is in the boot under the sole plate.
100-Hz rate
Acceleration x 3
Roll, pitch, yaw angles
and R,P,Y-rates

...and GPS at 10-Hz
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Re: In-Situ Ski Angle Measurement

Postby HighAngles » Mon Jan 03, 2011 6:29 am

That is really cool. Where did you get the device? Did you modify it for application in a ski boot? Is it expensive?
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Re: In-Situ Ski Angle Measurement

Postby polecat » Tue Jan 04, 2011 9:31 am

Wow, that's sooooo cool! Thanks for posting it, Holmz!

From the graph, It looks like output from a data acquisition system with an inertial measurement unit. (They're often used for dynamic analysis of motor vehicles by car manufacturers, racing teams, safety testers, etc).

Holmz, how do you set the reference datum? Is it fixed at the beginning of a run? Is the roll angle corrected for the hill's slope (to give tipping angle) or is it relative to inertial space?

Do you have more inputs available, perhaps for strain gauges? It would be really cool to track ski weighting, for/aft balance, etc.



pc.
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Re: In-Situ Ski Angle Measurement

Postby Holmz » Tue Jan 04, 2011 9:56 pm

HighAngles wrote:Where did you get the device?


There are a variety of IMUs at all sorts of cost points, but the long pole in the tent is to log the data. So either you need to carry a computer or some dedicated datalogger, and there are lots available.

HighAngles wrote:Did you modify it for application in a ski boot?


I just took the IMU out of the housing and "dremel tooled" a pocket into the sole plate and added some titanium to carry the foot loads around the IMU. Then added a new LEMO connector out the side of the boot (photo sometime in the future).

HighAngles wrote:Is it expensive?


(ha ha) Nothing is cheap...

polecat wrote:From the graph, It looks like output from a data acquisition system with an inertial measurement unit. (They're often used for dynamic analysis of motor vehicles by car manufacturers, racing teams, safety testers, etc).


Yes exactly. I am using a motorsport datalogger and an IMU at 100-Hz, also GPS at 10-Hz.
One advantage is that the graphing and support is good.
Also .dll can be used to add in more complex math.
Similar issues with orientation and performance are being done at the top level of MotoGP, but that seems pretty closely held info.

polecat wrote:How do you set the reference datum? Is it fixed at the beginning of a run? Is the roll angle corrected for the hill's slope (to give tipping angle) or is it relative to inertial space?


All the angles are ECEF, so it is not corrected to the slope.
Ideally one would use a KALMAN filter to correct everything to the periodic "truth" provided by the GPS.
Recently mentioned by a knowledgeable ski-science guy to use quaternions. And other knowledgeable people in similar fields mentioned similar things. I have a mathematician working on the KALMAN filter this summer (Northern hemisphere summer). I also have a friend who does a lot of small helicopter work looking at conditioning data or putting the KALMAN filter onto a blackfin(R) board and that will dovetail in with his work.
The orientation to the "slope" is a difficult problem and the "slope" is riddled with moguls etc, so at some level you can use pitch and GPS-vertical drop (plus distance) to work out slope and gradient (fall line), but it is not quite the same as a know, fixed, and surveyed surface.
In the end I think one may want to know ski roll angle (edging) relative to the ski's track. i.e. is the ski over-edged (and by how much), or under edged, etc.

polecat wrote:Do you have more inputs available, perhaps for strain gauges? It would be really cool to track ski weighting, for/aft balance, etc.


Yes I have more inputs for foot pressure sensors, but I have not gotten those in as of yet.
Strain gages are also possible, and there are pull-string potentiometers that could be used - but although I have a few I am not sure how they would work in the system. There are 1000 channels possible in the datalogger, so there would likely be enough to go around. A/D converters give a couple dozen channels each.
I think that force transducers between the binding and the ski would also work, but neither shows body position...
They only show effect not cause.

So if one wants to correlate cause with effect then video, body position knowledge, foot-pressure knowledge, etc, are needed to determine what produces the best effect.

Somehow one needs to correlate the effect and cause if the goal is to have causes that produce better effects.
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