Several drive parameters affect the accuracy of positioning a turret. A previous blog entry discussed the need for better than 1-degree positioning accuracy for many turrets winders.
The most precise and expensive turret drive would be a servo (positioning) drive with an encoder. This would result in a fast index time with very good positioning accuracy (as good as the encoder used (typically 0.1 degrees).
The inclination is to save money with the turret index drive. That may involve the use of a sensorless vector drive with proximity switches to stop the turret on a dime. A few problems present themselves. First, the sensorless drive may have difficulty lifting the wound roll in underwind or holding the wound roll in overwind, especially at low speed. This will compromise positioning accuracy.
The trick in positioning accurately with a low performing drive is to keep the speed slow. By slow, I mean painfully, excruciatingly slow. Generally, the turret rotates at full speed to within 10 degrees of its target position and then slows to creep speed for accurate spotting.
Even at low speed, a timer will be required to stop the turret exactly level. The timer may need a different delay for overwinding and underwind. It may even require a separate delay for very large rolls. Many turret winders use this method.
An encoder can be used to provide excellent positioning accuracy without the expense of a servo (positioning) drive. The position feedback can be wired into a counter card in a PLC. A PLC-based position regulator can adjust speed based on the distance from the setpoint (proportional position regulator). This will not be as fast as the servo drive.
Speed is important as the wound roll runs without nip loading and proper tension regulator during the index cycle. Completing the index in the shortest time possible reduces waste improves product rolls.