Abstract: This paper introduces the structural characteristics and control principles of surface winding and center winding, and provides a detailed analysis of the implementation strategies for constant tension control and taper tension control in center winding.
1 Introduction
Winding is a process in industries such as papermaking, textiles, and printing and dyeing where semi-finished and finished products are wound into various rolls according to a certain pattern. The purpose of winding is to facilitate the storage, transportation, and feeding of the products to the next processing step. Sometimes, rewinding or double-winding is necessary to change the roll capacity, remove defects, and improve quality. To achieve the best winding effect with flush edges and a dense roll, it is essential to select a reasonable, economical, and product-appropriate winding drive control method, taking into account the tension control requirements of the production process and materials, to ensure stable and controllable tension during the winding process.
2. Composition of the winding drive control system
There are two types of winding drive methods: center-driven winding and surface-driven winding. Regardless of the drive method, the control system generally consists of a traction roller drive control system, a winding tension control system, a PLC, and an operator panel. The traction roller drive control system comprises a pair of rubber-coated rollers, a frequency converter, and a variable frequency motor. The linear speed of the traction rollers is the machine speed, which remains constant after being set via the operator panel. The winding tension control system consists of a winding shaft, a frequency converter, a variable frequency motor, and a reducer. For surface-driven winding devices, winding tension control is relatively simple, while for center-driven winding devices, it is more complex.
3 Surface winding tension control system
Surface winding is a passive winding method that uses the frictional force of drive rollers to wind the surface of the material to be wound. The winding control principle is shown in Figure 1. It uses two winding rollers of the same diameter to drive the material to be wound. The drive rollers are controlled by frequency converters U2 and U3, which control a variable frequency motor and a reducer, respectively. By adjusting the density potentiometer R2, a speed difference is created between the surfaces of the two drive rollers, thus establishing the basic tension control mode for surface winding and meeting the density requirements of the wound product. The surface winding device can only provide quasi-constant tension control.
The magnitude of the friction between the drum and the two drive rollers during surface winding is related to the mass of the drum and is not constant. The magnitude of the friction affects the degree of drum slippage. Therefore, when the motor speed is constant, the drum linear speed cannot remain completely constant, especially after the drive roller surface is polished or at the beginning of winding. Due to the relatively small friction between the drum and the drive roller, slippage will occur, and the tension will be difficult to control. This will result in the core material of the drum not being dense. As the drum diameter increases, the drum moves vertically upward, and the weight of the drum also increases. The drum and the drive roller will have a larger friction, and the winding tension will increase. If the speed difference is not properly adjusted, the core material of the drum will be squeezed out, resulting in uneven end faces of the material. To achieve uniform winding tension, a counterweight roller is often added to the winding device. This roller is in close contact with the top of the drum shaft and applies a vertical downward force to the drum shaft using pneumatic or hydraulic means. By automatically adjusting the pressure of the counterweight roller, an ideal frictional force is obtained between the drum and the surfaces of the two drive rollers, ensuring stable tension during the winding process, meeting the requirements for the density and hardness of the roll material, and achieving a good winding effect.
Since surface winding relies on surface friction contact to wind the roll material, there must be sufficient friction between the roll and the drive roller. Therefore, the rubber or turf covering the drive roller needs to be replaced periodically, and the surface cannot be too smooth. However, for some fabrics, surface winding friction can damage the fabric or generate static electricity, so a center-driven winding method must be selected.
4. Center winding tension control system
The center-driven winding system is controlled by a frequency converter, which drives the motor to power the winding shaft. The winding process is completed by the rotational force applied to the winding shaft. The principle of its control system is shown in Figure 2.
In Figure 2, the traction roller drive mechanism uses a speed closed-loop control to regulate the linear speed of the winding process, while the winding shaft uses a tension closed-loop control to drive the winding mechanism to operate according to a preset tension curve, ensuring optimal tension control during the winding process. In the speed closed-loop, the floating roller position detection uses a non-contact variable resistor, ensuring accurate and reliable signal detection. This signal, as an additional speed signal, is superimposed on the main setpoint speed and sent to the AIN terminal of the traction roller inverter 1 to adjust and control the traction roller speed. The tension sensor feeds back the detected tension of the roll material to the analog input terminal of the PLC. Simultaneously, the tension detection signal is superimposed on the speed output signal AOUT of inverter 1 and sent to inverter 2 for PID regulation, achieving closed-loop control of the winding tension.
There are two methods for controlling the center winding tension: constant tension control and taper tension control. In a sense, constant tension control is a special case of taper tension control. Figure 3 shows the taper tension control curve. It can be seen that when the tension setting is 80%, and the taper coefficient HW = 100%, the winding tension curve is A. The tension remains constant during the winding process, which is constant tension control. If the taper coefficient HW = 50%, the tension drops by 50% when the winding reaches the maximum diameter, and the ideal winding curve is B', while the actual winding curve is close to B. If the taper coefficient HW = 0%, the tension will drop to 0 when the drum reaches the maximum diameter (see curves C and C').
When the diameter of the roll changes during constant tension winding, it is essential to ensure that the rotational speed n is inversely proportional to the roll diameter D, and that the change in motor torque M is directly proportional to the roll diameter D. This ensures uniform tightness throughout the roll. When using this method, it is important to note that the torque is at its maximum at the end of winding. The rated torque of the motor should be selected based on the maximum torque and the reduction ratio of the reducer. During winding, the diameter of the roll continuously changes, and the frictional torque Mm on the motor transmission mechanism and the roll support shaft, as well as the dynamic torque Md and linear velocity v required by the winding mechanism during acceleration and deceleration transitions, also change. These changes cause tension fluctuations. To obtain a constant surface tension in the fabric, the electromagnetic torque MD output by the motor must not only ensure the winding torque Mf = F·R(t) but also overcome changes in frictional torque Mm and mechanical inertia. In other words, corresponding dynamic torque compensation, static compensation, and acceleration compensation are required during winding to ensure that the inverter 2 can automatically adapt and follow the changes in motor speed and torque.
Taper tension control maintains a constant torque M during winding. As the roll diameter d gradually increases, the winding angular velocity ω decreases accordingly, and the roll tension F decreases accordingly, achieving a winding effect that is tight inside and loose outside. This is the principle of taper tension control. Taper tension control is essentially the torque control mode of the frequency converter. In winding drive, with a given tension F<sub> set </sub> and a constant linear velocity V<sub> line </sub>, the winding shaft speed n decreases as the roll diameter D increases, and the roll tension decreases accordingly. The torque mode perfectly meets this requirement. In torque control mode, the torque is given, and the torque setpoint of the frequency converter can be calculated based on the set tension F<sub> set </sub> and the roll diameter.
M = F set ×D min /2i
Where: M is the inverter torque setpoint (NM)
F set tension setting value (N)
D min minimum roll diameter
i Mechanical transmission ratio
If the actual torque is lower than the given torque, the rotational speed increases; otherwise, the rotational speed decreases. The speed during the entire winding process fluctuates around the speed corresponding to the given torque.
Because taper tension control relies on motor torque, and during the winding creep stage (the material threading process and the initial winding stage), the material is relaxed, the tension on the reel is very small, and the winding motor's resistance torque is almost zero. If the motor were still in torque control mode, it would need to accelerate instantly to its maximum speed until the material is taut. At this point, the motor speed would be far higher than the ideal speed calculated based on the current linear speed and reel diameter, inevitably causing an impact on the material at the moment of tautness, leading to breakage. Therefore, speed control must be used during the creep stage, which involves adding a small additional setpoint to the ideal speed of the winding motor while limiting the motor's output torque. When the material is relaxed, the motor is in speed control mode, operating at a speed slightly higher than the ideal speed, gradually tauting the material without causing an instantaneous impact. Once the material is taut, due to the torque limiting effect, the winding motor will be unable to reach the set speed, and speed control will saturate and exit. At this point, the controlled torque limit is equivalent to the actual torque of the motor. In taper tension winding control, the torque remains essentially constant, thus preventing damage to the reel.
Figure 4 shows the block diagram of the center winding tension control system. In order to achieve stable winding tension control and improve the dynamic performance of the tension closed-loop control system, the PLC, frequency converter and other components in the control system must have the function of calculating the current winding diameter; dynamic and static friction compensation and inertia compensation functions and parameter signal adaptive capability; and be able to calculate the set value of the winding frequency converter according to the current winding diameter and linear speed, so as to control the winding motor to reach the ideal speed, thereby realizing the gradual tension control or the basic constant tension control.
5. Conclusion
Whether surface winding or center winding, tension control is crucial for product roll formation. Excessive tension can cause fabric stretching, deformation, or even breakage; insufficient tension can result in a loose and incomplete roll; unstable tension can lead to uneven roll tightness, uneven end faces, and delamination. Therefore, to achieve good roll quality, it is essential to improve the tension control system and ensure its stability.
References: Yang Gongyuan, *Application Examples of Commonly Used Frequency Converters*, Electronic Industry Press, October 2006.
About the Author: Feng Jianxiu (1963-) is a senior engineer engaged in the research and practical application of automatic control systems.
Contact Information: Address: Technology Center, Shaanxi Huatai New Materials Co., Ltd., Xingping City, Shaanxi Province
Postal code: 713100
Telephone: 029-38362022
Email: [email protected]
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