I cringe at the thought of using "rim drive" for something like your application.
Who said anything about 'rim drive'. 'Rim drive' to me implies a capstan-like friction drive. Using the film rewind spool is nothing like that. There's no way it can slip; the film is taped directly to the spool.
the camera has a mechanism designed specifically to draw the film with the correct tension and at a steady rate
The camera has no such mechanism. The sprocket drives in 35mm cameras, and in movie cameras for that matter, are not designed to smoothly pull the film through the gate at a steady rate. They are designed to advance the film to the next frame. I'm not shooting frames, so the current mechanism is worthless to me. Modifying the camera to allow use of the sprocket with continuous film motion would require me to completely gut the internal workings of the camera and make a new drive interface with the film drive sprocket. I might as well make my own camera. It's easier to simply couple to the film rewind spool. This way, I don't even have to modify the camera! The only disadvantage is that the diameter of the spool increases as film winds on, but that is easy to fix in software. The rewind spool speed simply decreases linearly with platform rotation; finding the correct fudge factor is easy. In fact my back-of-the-envelope calculations, measuring the film thickness with calipers, were pretty much right on.
See pictures below.
Schematic of the rewind spool and film:
Picture of the mask in the film gate:
Picture of the coupling I made for the rewind knob:
Picture of the above coupling meshing with the rewind spool:
This works fine; the only problem is the open-loop control with the plain gearmotor, which actually works but only in a fiddly way. I've already taken pictures with this setup folks. I already found the motor I need at Pololu. I'll just buy one of these:http://www.pololu.com/catalog/product/1446
and once I get it reconfigured with that motor motor I'll be rockin'. It's not really a servo motor because I'll have to implement my own PID/servo loop in software, but it's basically what I was trying to do last time with making my own encoder...but hopefully this one will work.