From: Jeff Kreines (email suppressed)
Date: Mon Dec 15 2008 - 13:42:15 PST
Ideally you could lock the projectors together in sync -- most easily
with a timing belt between their inching knobs -- preferably with an
external motor driving both of them -- or if you want to do this
seriously with two motors (selsyns are pretty passe, but steppers
would do) in sync with each other. Keep a loop between both machines
with a weighted roller or two.
Or, you could build a big loop tree with multiple rollers, the bottom
weighted rollers take up slack, and use one fixed and one variable
speed projector to keep the loop big enough. If you have a lot of
film you could have a big loop though that might not work with the
images as you might want them to be close to each other in terms of
sync. (Video makes this easier, but it's video.)
Long ago we had a three-projector interlock rig at MIT that used 3
Hokushin Canary's with selsyn/sync motors, and it worked well -- it
would be simple to reverse one of the machines in that sort of rig by
flipping the motor 180 degrees.
Or if you were electronically/computer savvy, you could run one
projector normally, and run a variable speed projector using a
programmable logic controller that varies the speed of the second
projector with a PID loop (if you have to ask, you don't want to do
it) adjusting speed to maintain loop tension at a certain rate. But
this is a lot of work and I have a feeling you really are looking for
a simple fix. In that case, manually riding the speed control of one
projector to keep the loop size safe is the easy way to go. Second
easiest is the timing belt idea, but that may be tricky in terms of
aligning the projectors.
Good luck, sounds interesting.
Jeff "dealing with PLCs and PIDs reluctantly himself" Kreines
For info on FrameWorks, contact Pip Chodorov at <email suppressed>.