The sensor may be a Linear Variable Differential Transducer (LVDT) mounted on the layon roller slide or Rotary Variable Differential Transformer (RVDT)mounted on the layon roller swing point. Older technologies used a string or “yo-yo” pot. As the roll winds larger, the measured distance or angle changes. Roll diameter is then calculated based on the measured signal. Fairly difficult high school geometry may be required in the calculation. Specific calibration procedures are necessary to keep this method accurate.
A strong advantage of this method is that it is self starting at the core and after a splice.
One problem with this measurement of diameter is that during a turret index it is very difficult to maintain contact between the layon roller and the roll. Usually the diameter is frozen prior to a turret index or splice.