The invention relates to a design method of a line unwinding constant
tension control device in a line winding
system. A
servomotor and a line unwinding wheel are arranged on a frame, the line unwinding wheel is driven by the
servomotor, a
pulley block which comprises two fixed pulleys and a movable
pulley is arranged on the frame behind the line unwinding wheel, the movable
pulley is arranged between the two fixed pulleys and is arranged on a swing rod, the swing rod is articulated with the frame, an air cylinder is articulated on the frame, a
piston rod of the air cylinder is connected with the swing rod in a
driving mode, a tension sensor is arranged on the frame behind the pulley block, lines output by the line unwinding wheel are introduced into the tension sensor by the pulley block, and the lines which pass through the tension sensor are towed to a line winding
host machine. The loading force of the air cylinder has a proportional relation with force and a force arm which act on the movable pulley of the swing rod, and the constant tension is ensured in a small angle. When the swing rod is lower than a balance position, inverse
voltage is output to control the
servomotor to wind the lines; when the swing rod is higher than the balance position, positive
voltage is output to control the servomotor to unwinding the lines; and when used, the line unwinding content
tension control device has higher stability.