I worked as Licensed Industrial Electrician (442A) in Canada for over 35 years in industry and have worked on many paper laminating machines that used what is known as a dancer roller. The dancer roller rides on the material being fed into the machine and is connected to a variable resistor or rotary encoder that monitors it's position and determines the roller position. As the material is spooled out the dancer roller will move up and down depending on the slackness or tension the in-feed is feeling. This in turn controlled DC driven unwind motors to ensure that the material is fed in a way that is controlled manner.