1 Introduction
Aluminum foil is a primary packaging material in the cigarette packaging industry, composed of aluminum foil and paper laminated together. The aluminum foil used in cigarette boxes needs to be evenly distributed and thinly coated. However, due to its poor flexibility and susceptibility to breakage, it cannot be directly loaded onto the aluminum roll. Instead, it must first be laminated onto a durable plastic film before being bonded to the paper on the roll holder. This results in a finished product wound up on the roll holder consisting of paper, aluminum foil, and plastic film. However, the aluminum foil used in cigarette boxes cannot contain plastic film because it is made of polyvinyl chloride, polyethylene, polypropylene, polystyrene, and other resins, which are difficult to decompose and can be harmful to human health. Therefore, a separate plastic film peeling machine must be connected after the laminating machine to bond the paper and aluminum foil together. Workers must first remove the product from the roll holder and then transport it to the separate peeling machine. This reduces production efficiency, occupies workshop space, and increases the company's costs.
2 System Solution and Strategy
2.1 System Solution
To address the aforementioned issues, we propose the following solution: Add a peeling machine directly after the laminating machine. The sub-finished products exiting the oven pass through the peeling machine, where they are peeled off, allowing the plastic film to be smoothly wound onto the peeling seat for future use. The paper and aluminum foil composite is directly wound onto the winding seat. Furthermore, the control systems of the peeling machine and the laminating machine are integrated into a single unit, thereby saving costs and improving production efficiency. The improved laminating machine's working structure is shown in Figure 1.
Figure 1. Working structure diagram of the improved composite machine
2.2 Control Strategy
The unwinding and rewinding of a laminating machine is a typical time-varying nonlinear system, involving numerous intermediate production stages and inter-unit tension coupling. Therefore, without strict tension control on the production line, issues such as unstable material feeding, jumping, or even material breakage can occur, leading to wrinkles, stretching deformation during lamination, inaccurate printing or ghosting, and uneven winding, all of which negatively impact the quality of cigarette foil products. Thus, achieving constant tension control on the production line has always been a crucial issue for designers.
After the semi-finished products from the oven pass through the peeling machine, the plastic film needs to be evenly wound onto the peeling roller of the peeling seat, while the paper and aluminum foil need to be evenly wound onto the finished product winding seat. This requires that the speeds of the plastic film and the finished product be kept consistent, and that their tension be constant. Therefore, we need to use a high-precision frequency converter and encoder to form a closed-loop control to ensure that the film winding speed is consistent with the finished product winding speed; we also need to use a high-precision tension sensor with strong anti-interference to form a closed-loop control to ensure that the film winding tension of the peeling roller is constant. The working principle is shown in Figure 2.
2.3 Tension Model Analysis
A simplified diagram of the film winding system is shown in Figure 3. Let the plastic film tension be f, the film winding roller diameter be d0, and the linear velocity of the plastic film in the preceding unit m1m2 be v1, while the linear velocity in the following unit n1n2 becomes v2. If v2 > v1, the plastic film is stretched, and the tension f increases; if v2 > v1, the tension f decreases.
In the formula: ε is the elastic modulus of the plastic film; σ is the cross-sectional area of the plastic film; l is the distance between the transmission points; t1 is the start time of the work and t2 is the current time of the work [2].
As can be seen from the above formula, the plastic film is an integral element when it is used as the tension adjustment object. Controlling the tension is actually controlling the difference in linear velocity v2-v1. Therefore, in a sense, the tension control system is actually a linear velocity tracking system. During the start-up process, assuming that v1 is constant, it should always be controlled at v2>v1 so that a certain tension is generated in the plastic film. When the tension reaches a suitable value, the asynchronous motor should be adjusted in time to stabilize v2. In this way, the plastic film will run stably under this tension [3].
To ensure a uniform and smooth roll of the plastic film, we require that the winding linear speed of the plastic film and the system's operating speed be consistent. Let the system's operating speed be v. Therefore, a portion of the control voltage u1 received by the frequency converter can be obtained from the following formula:
u1=kv×v
Where kv is the proportionality coefficient.
Assuming v2 remains constant during winding, the linear velocity v2 = π × d2 × n2. v2 increases proportionally with the increase of the roll diameter d2, and the tension f also increases proportionally. This can easily lead to excessive stretching of the plastic film during winding, resulting in deformation or even breakage. As shown, to maintain constant tension during operation, the film winding linear velocity is not required to be completely consistent with the system operating speed. The other part of the control voltage u2 received by the frequency converter can be obtained from the following formula:
u2=kx×upid
Where kx is the PID output limit ratio and upid is the PID output voltage value[4].
The formula for the roll diameter d2 is as follows:
d2=n×d×2+d0
Where n is the number of counters, d is the thickness of the plastic film, and d0 is the initial roll diameter.
The control voltage accepted by the frequency converter can then be obtained as follows:
u = u1 + u2 = kv × v + kx × upid
3 System Implementation
3.1 System Hardware Configuration
Since the controller of the original composite machine uses Siemens' S7-300PLCCPU313C to complete a relatively complex control, and the TP27 touch screen is used as the human-machine interface, it is convenient to observe the operation of the system in real time and set various parameters. Therefore, we also choose the same model of controller for the subsequent stripping machine, so as to simplify the communication program and improve the reliability of the program. Since the S7-300PLCCPU313C comes with 24DI, 16DO, 5AI and 2AO, and the control I/O of the stripping seat is relatively small, there is no need to add additional input and output modules. In addition, the controller power supply module PS307 is also included. Given that the communication speed requirement of this system is not high and the amount of communication data is not large, we use simple and economical MPI communication to realize the communication between the stripping seat PLC and the original system PLC, and between the stripping seat PLC and the original system touch screen [5]. Its network structure is shown in Figure 4. For the motor that drives the film roll, we choose Siemens' basic asynchronous motor; for the frequency converter, we choose Siemens' general-purpose Micromaster440 frequency converter.
3.2 Software Design
The system was programmed using Siemens' STEP 7 v5.3 , employing both ladder logic (LAD) and statement list (STL) programming languages to handle logic control and numerical processing functions, respectively. The touchscreen was configured using Siemens' ProTool v6.0 . The flowchart of the stripper control system is shown in Figure 5.
Figure 2 Working principle diagram of the stripping seat
Figure 3. Schematic diagram of the film winding system.
Figure 4 System network structure diagram
Figure 5 Program Flowchart
The main software modules of this system include: PID algorithm, tension fine-tuning function, automatic control function, communication function, motor operation function, tension filtering function, and angular velocity voltage function. The PID algorithm and tension filtering function are both placed in the OB35 interrupt function block, allowing real-time acquisition of the PID output voltage value and the actual tension value. For the communication function, we use the built-in system functions SFC67 and SFC68 in STEP 7 V5.3 , making communication more convenient and programming simpler. When calculating the angular velocity voltage of the film roll, the real-time roll diameter of the plastic film is needed; therefore, we use the built-in high-precision counting module SFB47 in STEP 7 V5.3 to count the current number of film rolls and then calculate the current roll diameter.
The PID algorithm is the core of this control system, directly determining its accuracy and even stability. This system employs the widely used and mature integral-separated PID control algorithm. Its control principle is as follows: to reduce the impact of integral compensation on the dynamic performance of the control system, integral compensation needs to be canceled at the beginning of control or during large-scale changes; however, when the error between the actual tension value and the setpoint is less than a certain value, the integral compensation is restored to eliminate steady-state error. The integral-separated algorithm can maintain the integral effect while reducing overshoot, thus improving the performance of the control system.
4. Conclusion
This article summarizes and refines a completed project. Starting with the production process of a wet laminator, it addresses the shortcomings of current laminators in the printing industry and proposes improvements that significantly increase production efficiency and reduce enterprise costs. The schematic diagrams and flowcharts presented are simplifications of the actual system and do not represent its full structure. The system uses a Siemens 7-313C PLC as the controller, achieving coordinated and uniform winding of the plastic film and finished product. The software design employs a modular approach with a clear program structure, facilitating future maintenance and improvements. PID algorithm control is the core of the peeling seat operation and a major factor affecting the flatness of the plastic film. Operational results show that the improved laminator control system operates normally and performs well, enabling uniform and flat winding of the plastic film and finished product. Compared to the previous system, it saves significant time and improves production efficiency.
