Tension-controlling method for a yarn unwinding from a storage yarn feeder to a textile machine

A tension-controlling method for a yarn unwinding from a yarn feeder to a textile machine, in which the yarn tension is modulated by a yarn-braking device controlled by a control unit which receives signals of measured tension from a tension sensor and compares them with a predetermined reference tension signal in a control loop which generates a braking signal adapted to minimize the difference between them, said control loop being provided with a proportional-integral controller whose error-compensating action is defined by the formula

• Kpe+∫Kie(t)dt
• where Kp is a proportional gain coefficient, Ki is an integral gain coefficient, and e represents the difference between said signal of measured tension and said reference tension signal, wherein it comprises the steps of:
• as long as the reference tension remains unchanged, using a first proportional gain coefficient and a first integral gain coefficient in the control loop causing the control loop to have a relatively slow behaviour in relation to the compensation of the error,
• when a change in the reference tension occurs, from a first value to a second value, switching to a second proportional gain coefficient and a second integral gain coefficient causing the control loop to have a relatively fast behaviour in relation to the compensation of the error, and
• maintaining said second proportional gain coefficient and said second integral gain coefficient as long as the difference between said second value of the reference tension and said measured tension remains greater or equal to a predetermined minimum value, then switching back to said first proportional gain coefficient and said first integral gain coefficient.