Foreword
In actual production, if a center winding method is used for winding, the diameter of the winding shaft is constantly changing. The constantly changing winding diameter causes changes in angular velocity, which in turn causes fluctuations in the tension of the material: if the tension is too low, the material will loosen and wrinkle and deviate laterally during winding; if the tension is too high, the material will be overstretched, resulting in tension lines or even longitudinal bulges.
During the winding process, the tension control system is crucial for ensuring production efficiency and winding quality. Tension control generally employs two modes: open-loop and closed-loop. Open-loop control lacks tension detection and feedback mechanisms. While relatively simple in design and structure, it suffers from lower control accuracy and stability. Closed-loop control typically includes a roll diameter detection device and a tension feedback mechanism. It offers strong randomness in control, higher control accuracy, and faster response speed, but its system design is more complex and involves more components, limiting its application on smaller equipment.
Tension control is a crucial aspect of the production process, ensuring product quality and improving production efficiency. This article primarily introduces the control principle of frequency converter winding with tension control. This technology guarantees stability throughout the winding process, preventing excessive tension with small rolls and insufficient tension when starting large rolls. Tension control during winding remains a challenge. Based on an established mathematical model, this article introduces the principle of frequency converter winding, processing data according to a specific control strategy and adjusting the control signal in real time. The PLC calculates the roll diameter, changing the inverter's output frequency to control the motor. For winding, as the roll diameter gradually increases, the torque also increases, and the inverter's output speed decreases accordingly, conforming to the basic principles of winding while maintaining tension control. The system achieves quantitative control of winding tension and automatically tracks and adjusts torque and speed.
1. Working principle and composition of the control system
This paper employs an open-loop tension control system based on a servo system and a PLC system, applied to the winding of 0.1mm grade materials, achieving winding quality equivalent to that of closed-loop control. The servo control system was chosen for its torque control mode, which offers simplicity and high precision in winding. In torque mode, speed control is unnecessary; a speed limit is sufficient to automatically adjust the angular velocity of the winding shaft according to the torque magnitude, achieving constant linear speed winding. Simultaneously, the servo controller's internal torque detection function accurately detects the output current, enabling high-precision torque control. The system's torque, speed commands, and winding radius are calculated internally by the PLC system, further simplifying the system.
The system uses a PLC as the controller and mainly includes: a main unit, digital input/output modules, analog input/output modules, stepper/servo motor control modules, and an LCD color touch screen, etc. Figure 1 shows the hardware composition diagram of the control system.
Figure 1 Hardware composition diagram of the control system
Digital input/output module (including high-speed counting): mainly realizes the input of operation buttons, zero-position detection of retraction rod, counting of photoelectric encoder, and control and status indication of digital input/output.
Analog input module: Used for tension detection. During the winding process, the sensor converts the tension pressure signal into an electrical signal, which is then sent to the PLC analog input module via a tension transmitter for tension control and display.
Analog output module: By detecting the tension and setting the given tension, the PLC generates the current control quantity through PID calculation. The analog output module controls the excitation current of the magnetic powder brake to ensure constant tension; it also controls the speed of the frequency converter to achieve speed control during the winding process, automatic speed reduction, and positioning.
Stepper motor control module: Used to control the stepper motor. The position of the deflection rod is adjusted via the stepper motor according to the set winding diameter. The stepper motor also controls the fine-tuning of the winding diameter for each winding cycle based on the roll thickness.
LCD touch screen: As a human-machine interface, it mainly realizes the input of system control parameters, real-time monitoring of system working process and working status, display of control parameters and system operation functions.
2 Servo System Design
The Mitsubishi MR-J2S servo system has three control modes: position control mode, speed control mode, and torque control mode. This design system adopts the torque control mode.
Speed and torque commands are set on the touchscreen and then transmitted to the PLC. After calculation by the PLC, an analog signal of 0-10V is generated via the A1S68DAV and sent to the servo system. The servo system receives the signal and converts it into speed and torque control signals for the motor, thereby controlling the motor's precise operation. During servo motor operation, the servo motor's rotary encoder (PG) converts the instantaneous speed into a digital signal via the A1S64AD module and inputs it to the PLC. The PLC then calculates the instantaneous winding diameter and adjusts the output torque according to the calculated winding diameter, thus achieving stable and regular tension control. ±8V corresponds to the maximum torque. The output torque corresponding to ±8V input can be changed using the servo system parameter no.26#. For example, no.26=50% means that when the input voltage is ±8V, the corresponding output torque = maximum torque × 50%.
Due to system accuracy limitations, the system cannot accurately set the output torque when the input voltage is below 0.05V. During operation, the motor's forward and reverse rotation can be controlled by setting the polarity of the output voltage.
In servo torque mode, the servo controller only controls the output torque, and the tension is controlled indirectly. Generally, there are three tension curve models: decreasing, increasing, and constant. However, in reality, it is difficult to perfectly match any of these models. Therefore, different winding curves are selected based on different materials and thicknesses, which requires the tension curve to be adjustable.
Since the tension in this system is indirectly controlled by torque, the actual controlled object becomes the control torque. The real-time output torque of the winding motor is generally considered to be expressed by the following formula:
m = mo + mj + mz
In the formula: m—real-time torque; mo is the load torque under no-load conditions; mj is the system damping torque; mz is the increased load inertia torque.
Generally, mj and mo are constants, so what actually changes is the gradually increasing load inertia torque during the winding process. Therefore, the torque algorithm must ensure that the winding output torque changes automatically as the winding diameter increases.
(1) Automatic calculation of roll diameter
Let v be the linear velocity (m/min), d be the diameter of the take-up shaft (mm), n be the rotational speed of the take-up shaft (rpm), nd be the rotational speed of the servo motor (rpm), and i be the transmission ratio.
v=π×d×n=π×d×nd×i
d = v/(π × nd × i) = kv/nd = k × (∫vdt/∫nddt) = k × (linear velocity/angular velocity)
Since the linear velocity is constant, the real-time winding diameter of the winding shaft can be calculated simply by determining the angular velocity of the winding shaft.
(2) Torque Calculation
According to m=(mo+mj)+mz=mo+mj, the value of mz needs to be compensated. Therefore, an increasing (decreasing) coefficient k is set, and a set curve function is selected so that m can change with the radius.
3. Tension Control System Design
This system processes fabric, and the winding tightness and material tension must be maintained during the winding process. Therefore, we need to control the tension during the winding process. The tension measurement principle is shown in Figure 2.
Figure 2. Principle of tension measurement
When the fabric is passed through the three parallel rollers shown in the figure, it is subjected to gravity and the tension of the fabric during winding. The tension of the fabric is twice the tension of the fabric, and the direction is vertically upward. An angle sensor is installed on one side of the roller to convert the tension signal of the fabric into an electrical signal.
The obtained electrical signal is then normalized by the tension transmitter and finally sent to the analog input module. After A/D conversion and digital filtering by the analog input module, it can be sent to the PLC for calculation and processing.
The flowchart of the analog input unit operation process is shown in Figure 3:
Figure 3. Flowchart of analog input unit operation process
Because digital filtering and other processing are required, multiple data acquisitions are necessary during each program run for processing.
After the aforementioned series of processing steps, the tension signal becomes a binary digital quantity, which is then sent to the PLC's PID controller for calculation to obtain the control output value. This value is then converted from digital signal to analog signal using an analog output module. The resulting analog electrical signal is used to drive the magnetic powder brake drive system, thus achieving tension control.
The magnetic powder brake is mounted on the unwinding machine shaft. The magnitude of the resistance torque it generates on the unwinding machine's rotating shaft depends on the magnitude of the braking current applied to its drive system. The larger the braking current, the greater the resistance torque generated, making the unwinding machine less likely to rotate, and consequently increasing the tension on the fabric, and vice versa. In this way, automatic tension control is achieved during the unwinding process of the unwinding machine.
4. Design of winding diameter control system
During the winding process, because the winding material has a certain thickness, the actual winding length will become larger and larger as the number of winding turns increases. This causes the outer circle of fabric to have a larger circumference than the inner circle of fabric, resulting in significant waste when shearing and laying flat. Therefore, in order to avoid this phenomenon, during the winding process, after winding a certain number of turns N (the size of which depends on the thickness of the winding material), the winding diameter should be finely adjusted.
In this control system, the roll material thickness is input via an LCD touchscreen, using the formula:
[Wrapping fine-tuning increment] = Roll thickness * Fine-tuning frequency * 10
The fine-tuning increment is calculated, and then a fine-tuning is performed at regular intervals based on the calculation results.
The system consists of twenty-four parallel cylinders, with the spacing between them controlled by a shrinking rod, which in turn controls the circumference of the fabric during winding. Different angles of the shrinking rod result in different cylinder spacings. By using a servo motor to control the angle of the shrinking rod, the winding circumference can be controlled.
The function of a stepper motor is to convert input electrical pulse signals into step angular displacements. That is, given one pulse signal, the motor rotates by one angle, and its output angular displacement is proportional to the number of input pulses.
In this system, the PLC calculates the fine-tuning frequency and the fine-tuning amount based on the roll thickness input from the LCD touch screen. The PLC's servo motor control module drives the servo motor drive system, causing the servo motor to move at the predetermined frequency and size. This allows for precise fine-tuning of the winding diameter during the winding process.
5. Conclusion
In practical production applications, open-loop control systems based on servo motors and PLCs, because they do not require highly accurate mathematical models, only need to be configured according to load distribution and actual effects. This allows them to be applied to winding materials of various thicknesses and types with excellent results. Touchscreens can control and monitor the process through a menu system. This allows users to input values or control positioning elements by touching pre-configured buttons, and displays the process, equipment, and system on the screen. In addition to displaying process variables (such as output fields, bar graphs, trends, or status), touchscreens can also visualize event and alarm information, facilitating operation and improving interface quality.
