Magnetic braking apparatus and tension control system using the magnetic braking apparatus

A tension control system for letting off a web wound about a let-off reel at a constant tension, comprising:

• a braking apparatus for applying a braking force to said let-off reel, comprising
• a rotor secured to an input shaft, said input shaft being supported rotatably by a strut,
• a yoke body of a substantially cylindrical shape surrounding said rotor, said yoke body having a recess extending circumferentially through a center portion thereof, said yoke body having front and rear brackets for supporting said yoke body rotatably on a base portion of said rotor,
• an exciting coil accommodated in the recess encircling said rotor,
• said yoke body producing a magnetic attraction force upon excitation of said exciting coil to attract said yoke body to said rotor to apply the braking force to said rotor, and
• a load detector including an arm-shaped strain gauge attached between said yoke body and a stationary object which is stationary with respect to said yoke body, said strain gauge detecting bending of and strain directly produced in said strain gauge due to relative movement between said yoke body and said stationary object and providing an output signal representative of a braking torque applied to said let-off reel as a feedback signal;
• a rotation detector for detecting a number of rotations of said let-off reel and outputting a signal based on the rotations;
• a pulse generator for generating an output signal based on a linear traveling rate of the web;
• a controller means receiving the output signals from said rotation detector and said pulse generator for calculating a roll diameter of said let-off reel based on the received output signals, and for generating a control signal representative of a desired braking torque corresponding to the calculated roll diameter;
• and
• said controller means also for calculating an error signal between the control signal representative of a desired braking torque and the feedback signal, and supplying the error signal to said braking apparatus to excite said exciting coil to achieve the constant tension.

A braking apparatus comprising:

• a housing;
• an input shaft rotatably mounted through the housing;
• means mounted on said housing for braking the input shaft, the means for braking including a movable member which exerts a force on the housing when the means for braking is engaged and the input shaft is rotating;
• an arm-shaped strain gauge attached between the housing and a stationary object which is stationary with respect to said movable member and detecting bending of and strain directly produced in said strain gauge as caused by the rotational force from the means for braking.