Design of glue spraying control system based on PLC

现代商业生产流通领域中，产品都离不开包装，而纸箱包装又在该领域中得到最广泛的应用。常见的纸板式装箱机主要有粘箱式和钉箱式两种。粘箱式的工作原理是利用熔胶系统将热熔胶熔解为液体，再通过电磁阀控制喷嘴阀芯，在压缩空气的作用下将熔化了的热熔胶呈条状喷射到纸箱表面，热熔胶冷却后即完成粘合。粘箱式工艺由于不含金属钉而更有利于保证内包装物品的安全性，对于内装软质物体和液态物体就显得更为重要，同时也显著提高了纸箱的强度，有利于被包装物品的长途运输。另外，由于无需除钉，纸箱回收再利用的环保效果明显，该方法中热熔胶封合控制的好坏直接影响到到产品质量、生产效率及生产成本。根据工业现场的应用要求，设计了一种基于PLC的喷胶控制系统，可根据纸箱规格形式的不同，灵活地调整胶体的喷射长度、喷射方式及喷射时间，实现喷点、喷条或喷雾等多种出胶形式。

1 Analysis of hot melt adhesive properties
Ethylene-vinyl acetate copolymer (EVA) is a hot melt adhesive commonly used in spray adhesive systems. It is a solid fusible polymer that does not require solvents and does not contain water. EVA is solid at room temperature. When heated to a certain temperature, it turns into a flowing liquid with a certain viscosity. The liquid is sprayed on the surface of the object and can be bonded and cured in a short time after pressing. Its main components are EVA resin, tackifier and viscosity regulator. As the main component, EVA resin directly determines the properties of hot melt adhesive, such as bonding strength, melting temperature, tensile strength and temperature deformation resistance. Tackifiers are generally polymerized rosin or terpene. Its addition is mainly to prevent the reduction of the bonding force of EVA resin and the penetration force on the surface of the adhered object when the temperature drops. In order to obtain the best combination point between melting temperature, colloid fluidity, wettability, and solidification speed, microcrystalline paraffin or paraffin is often used as a viscosity regulator. The temperature and physical state characteristics of hot melt adhesive are shown in Figure 1.

In Figure 1: 80-135℃ is the softening area. When heated to 80℃, the colloid begins to soften and melt; 135-200℃ is the melting area. At this time, the colloid is hot-melted into a flowable liquid. By applying a pressure of 5×104-1×105Pa to the liquid, the length of the colloid sprayed onto the surface of the adhered object can be controlled. During the entire spraying control process, the heating temperature must not exceed 200℃, otherwise the colloid is in danger of burning. The cooling and curing of EVA hot melt adhesive is usually completed at room temperature. The ideal temperature is 15-26℃, and the humidity should be kept at about 50%.

2 Design of PLC controlled glue spraying system
The main process of the glue-type cardboard packing machine is to spray hot melt glue on the cardboard. The accurate control of parameters such as glue spraying position, glue spraying length and glue spraying time directly affects the quality, appearance and cost control of carton molding. The control principle of the hot melt glue system is shown in Figure 2. The controller determines the temperature and pressure of the hot melt glue through the temperature detector and the pressure detector. Once the working requirements are met, the solenoid valve is driven to change direction, and then the compressed air is sent to the nozzle through the hose, glue dispenser and switch. The molten EVA is sprayed onto the surface of the carton, and then the motor drives the glue spraying head to move, and a certain length of glue strip can be sprayed on the surface of the carton. After the spraying is completed, the solenoid valve is reset, the compressed air is cut off and the glue spraying is terminated. The relevant process control parameters can be set and displayed through the human-machine interface.

2.1 System Hardware Composition
From the perspective of the processing characteristics and cost-effectiveness of glue spraying, the control characteristics of PLC, single-chip microcomputer and industrial computer are comprehensively compared, and the FX1N-40MR PLC of Mitsubishi (MITSUBISHI) of Japan is selected as the controller to realize the collection, conversion and control functions of the field data of the glue spraying system. Mitsubishi FX1N-40MR PLC has the characteristics of compact structure, strong versatility and flexible configuration. It can be used as a separate PLC control machine to realize high-speed discrete control and complex process control on a single platform; it can also be used as an I/O substation to form a distributed large-scale control system with multiple PLCs through the field bus to realize batch operation control of the production line.
The glue spraying control system in this article is applied to the bonding of cartons. It is required to realize accurate continuous and intermittent glue spraying on the three glue-receiving surfaces (two sides and one top surface) of a carton. The I/O allocation of the PLC control system is shown in Table 1. The input points that the system needs to use include the input points for activating glue spraying, the input points that the machine is ready to run, the input points that heating and compressed air are sufficient, and the detection input points for detecting whether the door is opened and the emergency stop is pressed. The output points that the system needs to use include the output points for controlling the switch of the top glue spraying and side glue spraying air valves, the output points for controlling the engagement or separation of the top glue spraying motor clutch, the output points for stopping the machine when heating and cooling or the compressed air pressure is insufficient during operation, and the output points for stopping the machine when the door is accidentally opened or the emergency stop is pressed during operation.

2.2 System software design
PLC works in a periodic scanning mode, so the main function of the system is designed as the main program shown in Figure 3. The main program is executed in each scanning cycle, and the top spray glue control module, side spray glue control module, air pressure and heating detection module, emergency stop and safety door switch detection module, etc. are designed as subroutines for calling.

A PLC scanning cycle includes three stages, namely input scanning, program execution and output refresh. First, press the start button of the equipment, the air compressor and the melt glue system work, and the working temperature and pressure are detected by the air pressure relay and the thermostat. When these two parameters reach the set value, the normally open contacts of the air pressure relay and the thermostat are closed, and the initial conditions of the glue spraying work are completed. Otherwise, it is easy to cause no glue spraying or overflow instead of spraying. Then the PLC reads the process parameters such as glue spraying mode, glue spraying time and glue spraying amount input through the control panel, and detects whether the safety door of the glue spraying head is completely closed. Since the temperature of the glue spraying studio reaches 150℃ and works in a spraying mode, a protective safety door of organic glass is installed around the glue spraying head. Under normal working conditions, the safety door is closed, which not only ensures work safety, but also facilitates the observation of the real-time operation status of the equipment. If the safety door is suddenly opened for some reason, the equipment stops urgently. After the safety door is closed normally, press the reset button and the equipment continues to work. After that, the solenoid valve is energized and reversed, the glue spraying head opens, and the melt glue is sprayed out under its own pressure.
The control of the glue spraying length is actually achieved by controlling the motor to drive the glue spraying head to move along the set trajectory. The continuous rotation of the motor and the synchronous glue spraying of the glue spraying head can obtain a specific length of glue strip. By setting the glue spraying time in the PLC, the glue amount can be adjusted arbitrarily within the required range. By controlling the glue spraying head to work intermittently, intermittent glue segments can be obtained on the side of the carton. The opening and closing of the glue spraying head is realized by the reversing of the solenoid valve. When the melt pressure reaches the set value, the solenoid valve is powered on and reversed, and the glue spraying head opens to start spraying glue; when the set time is reached, the solenoid valve loses power and reverses again, and the glue spraying head closes.
2.3 Compensation algorithm for precision control
The solenoid valve is an actuator with a large delay. It takes a certain amount of time to open and close. Although this time is only in milliseconds, and when the movement speed of the glue spraying head is low, the error caused by the delayed opening or closing of the solenoid valve is small. Considering the factor of production efficiency, it is inevitable to increase the movement speed of the glue spraying head, but the error caused by this is also significantly increased. In order to minimize the impact of this error, a mathematical model of error correction compensation is established based on the analysis of the relationship between the movement speed of the glue spray head, the glue spray pressure and the closing characteristics of the solenoid valve, that is, an advance and lag correction value related to the relevant influencing factors is provided to compensate for the position and length errors caused by the delay of the solenoid valve. Since it is impossible to directly abstract the functional relationship between the parameters, the high-order polynomial approximation method is used to construct the corresponding compensation curve as shown in formula (1), and the least squares method is used to fit and solve the polynomial.

Let: x represents the speed of the glue spray head; y represents the corresponding compensation amount, then select m experimental data (xi, yi), where i = 1, 2, ..., n. Use the least squares method to construct the relationship matrix shown in formula (2), solve the linear equations about a0, a1, ..., am, and get the approximate functional relationship between x and y.

Let: ωi=1, that is, take {1, x, ..., xm) as the basis function of the algebraic polynomial for fitting. Considering the balance between accuracy and speed, m=2, n=4 are selected through experimental verification and analysis, and the coefficients a) and a1 are solved by Cramer algorithm to obtain the compensation curve equation:
y=φ(x)=a0+a1x (3)
The error compensation curve equations are calculated for multiple glue spraying control heads respectively, so that the glue box processing control of multiple glue spraying heads at multiple stations can be realized at the same time.

3 System Debugging
The debugging of the PLC control system is divided into two parts: software and hardware. Hardware debugging mainly checks whether the electronic control components work normally and reliably, whether the line connection is correct, and whether the anti-interference measures are reasonable. Software debugging is divided into modules first and then the overall system debugging, gradually analyzing whether the program operation meets the control requirements and eliminating the occurrence of abnormal situations. The actual operation on a carton gluing production line shows that the PLC control glue spraying system meets the requirements of actual production, with high system reliability, easy expansion, convenient maintenance, and strong anti-interference ability.

4 Conclusion
The hot melt glue spray bonding process for carton sealing has strict sequential control requirements. The application of PLC to control the glue spraying process can eliminate the shortcomings of the traditional relay contactor control system to the greatest extent. Through the study of the factors affecting the speed and accuracy of glue spraying, the analysis results of the error compensation model are used for real-time control adjustment, so that the glue spraying control system can perform multi-directional, multi-form, and high-accuracy glue spraying processing on cartons of various specifications and types, showing good flexibility and reliability. Based on the reasonable analysis of the controlled object and taking into account the cost requirements, the system also has good scalability; under the condition of appropriate adjustment of the system hardware and software, the system can be applied to new processing forms and control objects.