The View Up Here

Random scribblings about kites, photography, machining, and anything else

Nighttime KAP – End in Sight?

Posted by Tom Benedict on 16/04/2013

There was a recent post in the KAP Forum that may have answered the last piece to the puzzle of the stabilized KAP rig. But let me back up first:

Some years ago I fell in love with the idea of a KAP rig that could stabilize a camera to the degree that a Cineflex mount does on full-sized aircraft. To give you an idea of what that means, a Swedish film company released a demo reel of their work that began with a frame-filling shot of the full moon, then zoomed out to show that the moon was being videoed from a helicopter that was bopping around in the wind. The moon subtends half a degree in the sky. The full HD frame is just over 1000 pixels high. In the opening shot, the moon was not moving in the frame. Put another way, the Cineflex mount was stabilizing the camera to better than 1/1000 of a degree of rotation, or better than 3.6 arc seconds.

Such stabilization simply doesn’t exist in the world of KAP. But here’s what it would mean if it did:

At the long end of the KAP focal length range, consider a camera with the equivalent of a 35mm lens on a 35mm camera. This is the focal length my A650IS has at its widest focus, and is typical of a crop-sensor camera with a 24mm lens. The horizontal field of view of this setup is 54.5 degrees. Assume the KAPer is of the extreme variety, and that they’re flying a 5DmkIII with its 5760 pixel wide detector. To first order, each pixel covers 0.009 degrees of sky. Mounted in a Cineflex mount, the camera would be stabilized to better than 1/10 of a pixel for the duration of the shot. Considering that Swedish film crew held that shot of the moon for over ten seconds, the “duration of the shot” could be a very long time for an aerial photograph. Long enough to do full night-time aerial photography without a single blurred photo.

So that’s what’s out of reach for KAP. This begs the question: what is in reach for KAP?

Most KAPers, myself included, tend toward very short exposure times. 1/640, 1/1000, 1/2000 second exposures are typical. The reasoning behind that is simple: If your camera is bopping around in the wind, the only way to avoid blurry photos is to make the exposure as short as possible. Much longer than this, and the number of blurry shots per session starts to go up. Go as long as 1/15 second, and the number of non-blurry shots per session gets down in the single digit percentages.

But that’s on an unstabilized rig. Add even the barest attemps at stability, and those numbers improve dramatically. Years ago Scott Armitage developed the GS-1, a gyro-stabilized servo that could be used in a KAP rig. For video, this meant that the horizons stayed level enough that most of the rest of the jitter could be removed by a decent deshaker program. They left enough jitter that still photography suffered, but it was still worlds better than an unstabilized rig.

A while back I started designing a toolkit for experimenting with damped pendulum suspensions. I’ve just about finalized the design for all the parts, and should start cutting metal soon. The finished design looks close to this CAD rendering I posted a while back:

Damped KAP Pendulum - Uncompressed

Or in shrunken form using short spars so all the moving bits can be seen:

Damped KAP Pendulum - Compressed

It uses a long bar to attach to the kite line so there is plenty of purchase to push against in the pan direction. This reduces recoil and oscillation in pan rotations. It uses double-row ball bearings for all pivot joints so there’s no play in any of the axes. This reduces high frequency jitter in all directions. Any or all of the pivoting joints can be damped. This reduces oscillation in the pivoting axes. And I added in a provision to take out the real bugaboo with KAP: roll around the kite line (though I haven’t proved this in the air, so it’s still theory as far as I’m concerned.)

This gets closer to the mark, but it won’t remove everything. Even if the damping on this is carefully tuned, there will be residual oscillation. It’s unavoidable in a passively damped system. But tack active damping on the end of this, and the active system only has to deal with the residuals. It doesn’t have to fight the full motion of the kite, the line, and the rig.

Which brings us back to that post in the forum. RCTimer offers some gimbals for multirotors and RC helicopters, which was part of the discussion in that thread. But they also offer the building blocks for making your own gimbals. Namely, they offer a 2-axis brushless controller board and direct-drive brushless motors for driving a gimbal. Why direct-drive motors? It removes servo backlash from the list of possible sources of high frequency jitter. And why a custom controller rolled around brushless motors? The loop rate of a typical hobby servo is 50Hz, though you can push that higher by using digital servos. The loop rate of a typical brushless gimbal controller is closer to 500-1000Hz. Basic control theory says that if you bump your loop rate by a factor of ten, you also bump the frequency of the events you can sense and correct for by a factor of ten.

The damped pendulum should take out the oscillations above a few Hz, and the brushless gimbal controller should take out the oscillations below 100Hz. Put the two together and you have a system with very little residual left. Good enough to challenge a Cineflex? Probably not. But good enough to do one-second exposures with a KAP rig at night? It might just be possible now.

Unfortunately I used up all of my hobby cash on RC airplanes recently. I won’t be able to pursue this for a while. I have everything to make the pendulum, but I can’t afford the gimbal hardware yet. I’m writing this in the hope that someone decides to take this on and open a new chapter in the history of KAP: the KAP rig that simply doesn’t wobble in the wind.

– Tom


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