I am using DC motors and I am not using any mechanical tension control. The speed of a DC motor is proportional to its voltage. The torque of a DC motor is proportional to its current draw. The current draw of a DC motor can easily be determined via a low ohm sense resistor and an op amp. The current signal is input to your controller and can serve as the input to a PID controller implemented in software. To close the loop, the PID output to the motor is a PWM signal. Since the film is unwinding from a reel of known size, you can adjust the torque based on the calculated leverage of the unwinding film to the center of the reel. I believe this is a very simple and reliable way to maintain constant tension thereby avoiding mechanical dancers and such.