System to control the quality of yarn and relative device

System to control the quality of yarn in association with spinning machines or twisting frames, in which the yarn (12) cooperates with a yarn-guide loop (11) fitted solidly to an arm (14) which bears the yarn-guide loop (11) and which is secured to the structure of the machine, the system including at least one piezoelectric sensor (13) solidly fitted to the arm (14) bearing the yarn-guide loop (11), the piezoelectric sensor (13) being associated with a data processing and control means (17), which processes the signal generated by the piezoelectric sensor (13) and generates an electrical signal (16) of a sinusoidal type having an amplitude proportional to the tension of the yarn (12) and a frequency proportional to the speed of rotation of the balloon. Device to control the quality of yarn, which is suitable to be integrated in the system of the claims hereinbefore and comprises an arm (14) secured to the structure of the machine and bearing a yarn-guide loop (11) solidly fixed to the arm (14), the arm (14) being solidly associated lengthwise with a piezoelectric sensor (13).