Hi Matt,
The current sensing is a fairly standard technique, I am using “low side current sensing”. I’ve included a link below. Sorry if I wasn’t very descriptive about the torque calculation. Basically you will choose a value for for your tension (or like I did, experiment/tweak until satisfied). It’s actually a force, so let’s call it F. From basic physics, you recall that torque is force times the lever arm (distance). In this case the “lever arm” is the distance from the center of the reel to the film (ie. the radius defined by the film currently on the reel). Since we’re “pulling” the tape off the reel, the force required to maintain a constant tension is proportional to the current radius of the “tape wheel”. When the reel is full, the DC motor requires a bit more torque to maintain tension, but as the reel empties, the “lever arm” (radius of film) slowly decreases and the DC motor torque can lessen (recall that motor torque directly relates to current draw). Every rotation, the radius reduces by the film thickness, until we reach the hub diameter and then we’re done. You could use a hall effect sensor on the reel to count rotations, but you don’t necessarily need to do that since you know the frame rate that you are digitizing at and with a little bit of basic math you could derive the current radius from that. It does mean that full reel diameter and hub diameter ideally are user entered parameters, but you might be fine with some nominal values. One more thing… I’m using salvaged cassette tape drive motors (my telecine is for 8mm). When external resistance is placed on a DC motor, it heats up and may lose some life. In my case these are 12v motors and I “derate” them by powering them at 5V. Plus the fact that I am using PWM to slow them down means that I am not stressing them. I have never noticed any warmth from them or had a problem, but I should mention that as a consideration. It’s rather late as I’m typing this, so if I have been unclear or left off some details, please let me know.