Application of PLC and AGC system in thickness control of aluminum foil

Abstract: In order to achieve precise control of the thickness of aluminum foil, an AGC system with Siemens S7-400PLC as the core was adopted. At the same time, the fuzzy PID control principle was applied to the thickness control system of aluminum foil in an aluminum plant, and the ideal effect was achieved. The working principle of AGC, system hardware and software design were elaborated in detail. Practice shows that the rolling accuracy of the system has been effectively improved, and the performance indicators meet the needs of production.
Keywords: PLC application; AGC system; fuzzy PID control; control accuracy

At present, the competition of aluminum foil products is becoming increasingly fierce, and the market has higher and higher requirements for the types, quality and precision of aluminum foil, especially for aluminum foil products with a thickness of only tens of microns. In order to be invincible in the market, it is necessary to carry out technological innovation or transformation of the production process of aluminum foil. At present, most of the automatic gauge control (AGC) systems in the aluminum foil rolling industry rely on industrial PCs for control. Since the stability and real-time performance of industrial PCs are not as good as PLCs, this paper adopts the addition of hydraulic control systems and AGC systems with PLC as the core for the cold-rolled aluminum foil production process, which realizes the modularization and networking of the control system, and also greatly improves the control accuracy of the aluminum foil cold rolling mill system.

1 Combined control of AGC system
The purpose of AGC control is to control the thickness of aluminum foil at the exit of the rolling mill within the required target value range as much as possible. Therefore, in order to obtain good control accuracy, the AGC system is equipped with a variety of controllers and compensation links, which are composed of different measuring instruments and sensors. The output value of AGC control is always applied to the inner loop controller of the hydraulic pressing servo mechanism of the cold press system as a compensation value. In the current aluminum foil production process, in order to obtain aluminum foil with more accurate thickness and minimize the errors caused by factors such as billet fluctuations and unstable rolling speed, the AGC system uses a combined control method to keep the thickness error of the aluminum foil within a controllable range.
The specific process of combined control is shown in Figure 1. The gain parameters of the PI regulator are used to control the position and pressure of the roll gap, hydraulic servo cylinder, and ensure that the thickness error of the aluminum foil is within the allowable range. The primary PI adjustment plays the role of feedback control. The controller makes preliminary PI adjustment to the thickness of the aluminum foil within a certain adjustment range. If the thickness of the aluminum foil does not meet the expected accuracy requirements, the AGC system will automatically make a secondary PI adjustment to the thickness of the aluminum foil. The secondary PI adjustment is based on the overflow part of the primary PI adjustment (in the blind spot) as the error signal.

2 Hardware composition of AGC system
As shown in Figure 2, Siemens S7-400 series PLC is used as the core control unit of AGC system. The FM485 function template is used to improve the real-time performance of AGC system, and it also forms FROFIBUS-DP network with the dispersed ET200 communication module, thereby reducing the wiring between the master station and the measuring point. The human-machine interface adopts TP27-6 touch screen produced by Siemens, and the 443-1CPU of S7-400 series is used to complete the communication between the master station and the human-machine interface computer. The displacement signal is measured by MTS absolute value sensor produced in Germany, and the speed measurement of left/right roll machine uses incremental encoder. The absolute value and incremental encoder modules on FM485 function template are used to read the displacement and speed values. Compared with the position inner loop control (APC) of hydraulic pressing servo mechanism, AGC is the outer loop control of aluminum foil thickness. Its output signal is mainly used to correct the roller gap setting value of the position inner loop. Through hydraulic servo drive, the roller moves quickly to achieve the purpose of quickly eliminating thickness error.

The signals involved in the control include analog quantity and switch quantity. The analog quantity signal can speed up the response speed of the AGC system, thereby improving the precision requirements for aluminum foil (μm level). The analog input signal is mainly composed of the displacement, pressure, speed value collected by the sensor and the thickness value measured by the thickness gauge. The analog output signal is composed of the speed adjustment amount and the adjustment amount of the hydraulic press servo. The switch quantity signal is set mainly to facilitate the operator to control the process of rolling aluminum foil through these switches and buttons. The switch input quantity includes the status signal of the thickness gauge and the control signal of the touch screen. The output includes the control of the thickness gauge and the communication signal with other parts of the system.

The control principle block diagram of the AGC system is shown in Figure 3. The control system adopts a double closed-loop control method. The thickness gauge, S7-400 series PLC and rolling mill constitute the outer loop of aluminum foil thickness control, and the outer loop control is completed by the lower computer of the thickness monitoring loop. The inner loop uses two closed loops, namely the displacement sensor loop that controls the servo hydraulic cylinder and the pressure sensor loop of the hydraulic pressing servo machine.

3 Software composition of AGC system
The monitoring software of AGC system is mainly written by WinCCflexible configuration software of TP27-6 touch screen, which can realize the display of aluminum foil thickness, hydraulic pressure, speed of DC speed regulating motor, etc., as well as AGC system parameter setting and alarm functions. For S7-400 series PLC, the software design of AGC system mainly relies on STEP7 programming software. STEP7 is a standard software package for SIMATIC PLC configuration and programming. This software package provides a series of application programs (tools) that can support various stages of automation project creation. With STEP7, system designers can develop PLC control programs that meet actual needs by online diagnosis of PLC hardware status, control of PLC operation status and I/O channel status.
3.1 Principles of system software design
Before software design, it is necessary to consider the function execution process and communication process of AGC. As shown in Figure 4, the basis of system software design mainly consists of three parts.

(1) The touch screen is used to display real-time data, system status and alarm information, as well as the operator's setting and modification of mill parameters.
(2) The sensor and servo system belong to the detection and execution part, which collect various required signals in real time and transmit them to the PLC, and convert the digital signal or analog signal output by the PLC into the operation of the sensor and servo system.
(3) The PLC is the center of data exchange and processing of the entire system. Its main functions are data format conversion, alarm judgment, output display and thickness control. The data signal input from the template must be converted into a unified data format before it can participate in data calculation and display. In addition, a public data area is set up. Whether it is the parameters set by the operator through the touch screen or the parameters collected by the sensor, they must be stored in the public data area. The data area is set to event trigger mode. When the AGC controller or other operations need to read and write data, the data in the data area can be operated after the event is triggered.
3.2 Software Design
The software design flow chart of the system is shown in Figure 5, which can be divided into two parts: automatic operation and manual operation.

Before the aluminum rolling mill works, the cold water pump and heater of the hydraulic station need to be turned on to reduce the oil temperature and the temperature of the uniform emulsion. After automatic or manual operation, zero adjustment and PH curve test must be performed. Zero adjustment is to make the rolls fully contact; PH curve test is to remove the non-linear part of the elastic curve of the rolling mill, eliminate the roll gap error caused by the roll bearing, and avoid the adverse effect of excessive roll gap difference on the aluminum foil line. When performing roll gap zero adjustment and PH curve test, FM485 detects the tension of the aluminum strip through the pressure sensor, so that the hydraulic cylinder works in the rolling force closed-loop control mode. In other cases, the hydraulic cylinder generally works in the position closed-loop control mode. The specific process is as follows: The MTS high-precision displacement sensor detects the position of the hydraulic cylinder, which is obtained by the absolute encoder on the FM485 template, and then after the PI algorithm, the voltage value is output to the servo drive, thereby completing the position closed-loop control of the hydraulic cylinder.

The thickness control process of aluminum foil has time lag, multi-time variability and nonlinearity. To solve this problem, this system adopts fuzzy PID control algorithm. Fuzzy PID control is suitable for systems with multi-variable, nonlinear, multi-disturbance and strong coupling object models that are difficult to establish.
The fuzzy PID controller consists of three main links.
(1) Fuzzification: Fuzzification is the process of converting the determined value of the fuzzy controller input into the corresponding fuzzy language variable value;
(2) Fuzzy reasoning: Fuzzy reasoning includes three components: major premise, minor premise and conclusion. The major premise is a number of multi-dimensional fuzzy conditional statements, which constitute the rule base; the minor premise is a fuzzy judgment sentence, called a fact. Based on the known rule base and input variables, the process of deriving a new fuzzy proposition as a conclusion based on fuzzy transformation is called fuzzy reasoning;
(3) Clarification: Clarification is the process of converting the fuzzy value obtained after fuzzy reasoning into a digital value used for control, which improves the response speed of the system and can obtain higher control accuracy in the shortest time.
The control effect of the fuzzy PID controller is shown in Figure 6. The PID control curve in Figure 6 has almost no overshoot process and quickly and stably reaches the set value thickness of 0. 1 mm, the PID adjustment based on fuzzy control has higher stability than simple PID adjustment, and can solve the time lag problem of the entire AGC system in controlling aluminum foil.

After practical application, the aluminum foil thickness control system based on PLC and AGC has achieved satisfactory results in product production. As shown in Table 1, for aluminum foil with a thickness of 0.1 mm, the proportion of AGC-controlled thickness within the range of 20 μm is 95%, which is much higher than the proportion of 78% for manual rolling. Facts have proved that the fuzzy PID controller adopted in this scheme can fully meet the requirements of aluminum foil thickness control.

4 Conclusion
In actual use, FM458 works stably and reliably, has powerful programming functions and is easy to modify and maintain, and can be well integrated into the S7-400 control system. The thickness of the aluminum foil meets the expected effect under the action of the AGC system based on S7-400PLC. The specific production process verifies that the installation of the AGC system on the aluminum rolling mill not only improves the accuracy of the rolling mill, but also facilitates the manual operation of the on-site operators. The advantages of the AGC system based on S7-400, such as friendly human-machine interface, convenient maintenance, and low cost investment, have produced great social and economic benefits for China's aluminum foil production.