I'm looking for a contact-type tachometer for a project where I need to measure the linear speed of a clear plastic film as it passes over a surface. The film will be rolled up onto a take up spool, and the spool is driven by a stepper motor. The goal is to maintain a constant linear speed of the plastic film as it passes over a flat surface.
I've tried some google fu without any luck. All that I encounter diy projects that take a dc motor and fit a wheel onto the end of the shaft. Is there a commercial solution that fits what I'm looking for? I don't think an optical solution is suitable as the film is clear.