The servo web tension control system is mainly used for automatic folding mask machines. According to the loop tension control, a servo tension control system is divided into open loop and closed loop. Open-loop tension control system is composed of a tension sensor, a web tension controller
and a tension indicator, with no tension value feedback. Closed-loop tension control system is composed of a tension sensor, a tension controller, a magnetic powder brake, a magnetic powder clutch, which has a tension value feedback, according to the feedback tension value, adjusting the control, making the result in a right state. The closed-loop control system is more stable. Nowadays, the automatic tension control system is generally adopted, and the generated additional tension will change reversely with the change of unwinding tension, compensating the tension to keep the tension constant.