The sensor (laser or ultrasound) is pointed toward the center of the winding roll. As the roll winds larger, the measured distance decreases. Roll diameter is then twice the length between the sensor and the center of the winding roll less the measured distance to the roll surface.
A strong advantage of this method is that it is self starting at the core and after a splice.
Some problems with this direct measurement of diameter are:
- ultrasonic sensors produce a cone of sound waves and are difficult to aim at the center line of the winding roll.
- ultrasonic sensors need to compensate for air temperature variations.
- ultrasonic sensors are very sensitive to loosely wound rolls.
- laser sensors do not see all colors equally well.
- during a turret index, it is very difficult to maintain sensor alignment with the center of the roll. In many cases, diameter is frozen prior to a turret index.