Denis Morozov
Center driven winders and unwinds drive into a constantly changing load as they process and regulate web tension. The changing roll diameter requires the control system must continuously adapt to the correct spindle speed and effectively handle the alterations in roll mass while providing accurate web tension control performance.
Typical internal components of a center wind tension control system can include:
- A diameter calculator
- A closed loop process controller (i.e. PID)
- An inertia calculator
Through these interconnected modules, the tension controller, drive speed, and current controller’s setpoints and performance variables are established. These tension controller components and the process variables they produce are all directly dependent on the accuracy of the actual roll diameter.
The accuracy of the diameter calculator component is highly critical to the system’s performance, to the point that it may be considered the most important element in the control system.
Actual roll diameter can be measured with a sensor or calculated with input values of web and spindle motion. Since measuring roll diameter is normally only suitable for the initial or starting setting, the running diameter is calculated repeatedly during the process. Multiple technologies exist for calculating the running diameter, but modes that utilize actual or derived position changes in the web and spindle can offer greater performance.
Herein, we will discuss tension control concepts, potential methods of diameter calculation, the optimum diameter calculation modes, and the intricate interaction between the diameter calculation and the tension technology controller in regard to performance.
Concepts & Calculations
There are different methods of control that can be applied to center driven winders: torque or speed control, for example. This article will concentrate on the theory of the speed based winder control, although many results regarding diameter calculation methods can be readily applied to torque control winders. In a speed controlled center driven winder, a spindle motor performs two main functions: It matches the linear velocity of the machine and it maintains constant tension in the web.
In order to match constant web velocity (v), spindle angular velocity must decrease as the diameter of the roll increases. The relationship between angular spindle velocity and surface web velocity is shown in Figure 1.
Machine velocity is derived from the nip roll motor that is part of the same drive control system or is measured by an externally mounted encoder. In order to derive correct angular rotation velocity of the spindle (w), a diameter (D) of the roll must be calculated or measured.
Web Tension & Control Methods
In a speed controlled center driven winder, two methods can be used to control tension:
- A load cell can be used to measure web tension. A tension controller alters the velocity of the spindle based on the load cell reading
- A freely moving dancer arm mechanism with preloaded force can be used. Dancer position deviation feedback is used by the position controller to adjust angular velocity of the spindle
Both the position controller for dancer based systems and the tension controller for load cell based systems can be commonly referred to as technology controllers (see Figure 3).
A technology controller is a closed loop control system that applies correction based on the difference between the setpoint and actual values. The setpoint vs. the actual difference is scaled by a factor called proportional gain. An integral and differential component is applied as well. The term PID controller is also commonly used to refer to this calculation.
Measurements
As indicated, the diameter of the roll is needed in order to correctly calculate angular rotational velocity of the spindle to match linear web velocity. Diameter calculation can be accomplished either by direct measurement or by drive system calculation.
Direct diameter measurement (see Figure 4) is performed via an ultrasonic sensor or a lay on roll attached to the measuring device, such as a potentiometer or encoder. Direct measurement methods are subject to mechanical and electrical factors, associated with operation and maintenance of measuring devices and the mechanics involved.
Drive based methods of calculating diameter include material thickness addition, web velocity over winder shaft rpm (v/n), integration of web velocity and position based calculation. With all drive based diameter calculation methods, an initial setting is needed. Initial diameter can be determined one of three ways:
- Direct operator input
- Direct diameter measurement method
- In dancer based systems, a displacement of the dancer vs. rotational angle of the spindle during tension on procedure can be analyzed, as shown in Figure 5
Material thickness addition is potentially the most precise method of diameter calculation. Thickness of the wound material is added to the diameter every revolution.
Furthermore, calculation precision is dependent on three factors:
- Accuracy of the initial diameter
- Accuracy of the specified web thickness
- Web thickness consistency of the material and the ability to build the roll at the specified web thickness. Air trapped between material turns will alter the roll diameter
When calculating web velocity over winder shaft diameter velocity, diameter (D) is determined by using the relationship between surface web velocity (v) and angular velocity of the spindle (w) using formulas outlined in Figure 1. Instantaneous velocity values are used in the calculation.
When calculating the integration of web velocity, the velocity is integrated over time to determine the linear position of the web and angular displacement of the spindle.
Then diameter is calculated based on web distance (S) and angular displacement (θ).
With position based diameter calculation, position is measured directly from spindle and nip roll displacement and diameter is calculated directly using the above diameter calculation. Position reference must be available to perform this calculation.