Rewinder for producing logs of paper material

A rewinder for the production of logs of paper material, comprising a winding station for winding the paper with a first winding roller and a second winding roller, the first winding roller and second winding roller delimiting, with their respective external surfaces, a nip through which a paper web comprising one or more paper plies is fed and intended to be wound in said station to form a log, and a third winding roller which, in relation to a direction from which the web is fed, is positioned downstream of the first two winding rollers,

• wherein the second winding roller is positioned at a lower level than the first winding roller,
• wherein the axes of rotation of the first winding roller, second winding roller and third winding roller are horizontal and parallel to each other, such that they are oriented transversely to the direction from which the web is fed,
• wherein the third winding roller is connected to an actuator which allows it to be moved cyclically from and to the nip so that the position of the winding third roller varies in relation to the other two winding rollers during the production of the logs,
• wherein each of the said winding rollers rotates around its own axis being connected to a corresponding electric motor,
• wherein the rewinder further comprises a detection system with an optical vision system capable to detect, in a succession of predetermined detection times, a succession of diameters assumed in such times by a log being formed in the winding station and a programmable electronic unit connected to said electric motors and to said optical vision system,
• wherein the programmable electronic unit is programmed to compare the measured diameters by the optical vision system with a succession of corresponding diameters of predetermined value and to calculate a sequence of differences between these values,
• wherein said programmable electronic unit determines a parameter related to the trend over time of the values which form said sequence of differences, and
• wherein said programmable electronic unit changes the relative speed of said first and second roll depending on the value of said parameter.