One encoder determines the web length in meters at a fixed diameter roller with good traction. The second determines diameter at each revolution of the winding roll. As long as the web is tight and the web path length is not changing, the length of the web at the first encoder matches the length at the winder. Note this method does not depend on the gauge of the web, but only on length per revolution as measured by encoders on two motors.
Lengthfixed = Revwdr * Diamwdr* pi
The advantage of Method 2 is that the pulses from the encoder need not be converted to RPM by differentiation. Positive length measurements are used. Furthermore, the speed feedback in drives is often filtered, making the RPM signal lag the actual RPM. Also Method 2 results in a good diameter reading after only two revolutions of the winding roll. Accuracy is not compromised at low speeds or while the speed is changing.
There are a few compromises when using Method 2 diameter calculation. The first is that the calculation will not work when the line is stopped (the calculation is performed only when a full revolution of the winding roll is complete). This means the diameter is not self starting and the core diameter must be entered prior to starting the winder. Also, special electronics are required to perform calculations on length pulses. This requires a high end drive that can control Position as well as Speed. Alternatively a high speed pulse counter could be used.