In winders and unwinds, the diameter changes whenever the line is running. The drive system needs the diameter for RPM and torque calculations. To save costs, the drive system often calculates diameter using existing instruments necessary for speed control and already paid for. Method 1 diameter calculation involves a calculation based on the ratio of two tachometer RPM feedbacks. This is the most common method of diameter calculation provided by drive system integrators.
One tachometer determines the line speed in MPM. The second determines the RPM of the winding roll. As long as the web is tight and the web path length is not changing, the speed of the web matches the speed at the winder. Note this method does not depend on the gauge of the web, but only on speeds as measured by tachometers on the motors.
RPMweb * Diamweb = RPMwdr * Diamwdr
There are a few compromises when using Method 1 diameter calculation. The first is that the calculation will not work when the line is stopped (division by zero RPM). This means the diameter is not self starting and the core diameter must be entered prior to starting the winder.
Accuracy is decreased below 10% speed since the calculation divides by a low RPM. Accuracy is also decreased during acceleration and deceleration because there is often a filter on both the web speed and winder RPM signals. The filtering may have a different time constant for each signal.