Kite Aerial Photography
An area with little web coverage is that of Centre of Gravity or balancing your rig. Personally, when I started KAP I had some kite, mechanical and electronics skills, but none in tying all this together, and getting things in balance, where do you start?
The theory is pretty straight-forward. The force exerted by x grams at distance a mm from the red fulcrum must equal the sum of the forces exerted by y and z:
(x.a) = (y.b)+(z.c)
None of this theory helps a lot when you try to bundle together your collection of servos and camera, and try to balance the resulting odd shape, in three dimensions! The rest of this page details how to balance a standard double U KAP rig. Now, every rig is different, each with it's own little personality, so please take the following as it is given, a little help rather than a religious exposition!
First of all, lets get our bearings. We are trying to balance a double U rig like this one. It has a camera (green) which may include some additional metalwork and/or servos for control, various other ancillary equipment (purple), and blue tilt and pan servos. I will use the convention for direction shown in the diagram for the X, Y and Z direction. The method I describe is mostly empirical, you maintain balance as the rig is constructed. To make things easy I try to ensure that the centre of gravity in the Y direction always runs through the center of both U brackets. This is not essential, but it is desirable as it simplifies construction.
The first step is to take the camera, including any metalwork closely associated with it (e.g. servos for shutter release, webcam for viewfinder etc). If necessary use tape to hold it all together in the right place in the absence of the U brackets.
Find the center of gravity for X and Y directions of the camera as follows. Find a convenient point to attach the camera to a kite line. This may be the tripod mount on the base. Don't worry if the camera ends up being upside down, we are not interested in the Z direction at the moment. Fashion a L shaped fixture as shown in the diagram, I used a nylon bolt for the tripod mount, and a small piece of aluminium. Keep the weight of the fixture as low as possible to reduce errors. By hanging the camera from the line, and rotating the L it should be possible to achieve a level camera in the X and Y direction, but not necessarily both at once.
Note the offset from the tripod mount in X and Y when the camera is level as the camera's centre of gravity. If you can't get the camera to level, you'll need a longer L or try a different mounting point. Since the camera is much heavier than the L bracket it probably doesn't make much difference to the balance (<1mm), but for the mathematically challenged you can use the formula at the top of the page to subtract the error caused by the L bracket.
Remember that the balance in some cameras may move about significantly as the
lens focuses or zooms. Also remember to include or exclude the batteries as
Make your bottom U bracket. Make the horizontal wide enough for your camera and other equipment, make the verticals about as high as the camera, we'll come back to them later. Position the camera in the bottom U bracket with the Y in perfect balance along the mid line of the bracket. Don't worry to much about X, but ensure that it is reasonably central.
Now complete the construction of the lower U bracket, EXCEPT
the fixings and holes for the tilt servo. Make sure that all additional elements are
centered along the center line of the bracket, this will ensure you introduce minimal errors
in the Y balance.
Now we'll balance the bottom bracket's Z direction, this is so that balance is maintained as the camera tilts.
Hang the completed bottom bracket from a line as shown right. Adjust the position of the line so that the bracket hangs vertically, you can use a plumb to check this. Mark this point as the pivot point for the bottom bracket. As well as ensuring that the rig stays in balance as the camera is tilted, this also minimises the strain on the tilt servo as it operates.
If the two vertical sides of the bottom bracket are too long, cut of the excess (and save weight). Re-balance after this operation as the reduction will make a very small difference in the pivot point.
Once you have drilled the holes for the pivot of the bottom bracket (remember these should go through the centre line of the bracket), hang the bottom bracket in it's correct orientation by a single line passing through both holes. Double check that the Y direction is still in balance. Make any small adjustments needed to maintain Y balance, e.g. move tie-wraps, reposition ancillary equipment etc.
The bottom bracket is now balanced in both Y and Z directions.
Now fabricate the top bracket including the tilt servo, OMIT the fixings, and don't drill the holes for the pan servo, and connect the completed rig together. Attach the pan servo with tape in roughly the correct position. Balance the rig with a line connected round the top bracket where the connection to the suspension will be fixed. Mark this point as the X balance point which connects to the picovet or suspension mechanism.
Complete the rig by fixing the pan servo, and connection to the suspension. Small errors in X are normally trimmed out by adjusting the position of the batteries etc normally mounted on the top bracket. Errors in Y (and bottom bracket Z) are less of a problem and can be lived with. They tend to affect the tilt of the camera which can be compensated for by the tilt servo. The Z balance of the top bracket is not important.