1. Introduction
With the rapid development of China's economy, the high-speed advancement of automation, computer, and information technologies, and the continuous increase in labor costs, the market demand for high-precision, fully automated equipment is also increasing. Previously, the application of conformal adhesives or paints to printed circuit boards (PCBs) was done manually or semi-automatically. Even when automatic coating was used, the low precision and poor performance resulted in waste and pollution of adhesives or paints. Therefore, we collaborated with relevant equipment manufacturers to develop new automated glue spraying machines. During the equipment development process, our company was responsible for developing the glue spraying machine control system and software, achieving full automation of the glue spraying equipment.
2. Glue spraying machine mechanism
The external structure of the glue spraying machine is shown in Figure 1. The glue spraying power unit uses servo motors to control all four axes (X, Y, Z, U, a total of four axes), as shown in Figure 2. The X and Y axes follow the glue spraying trajectory with planar motion; the Z-axis controls the up-and-down movement of the glue spraying head to easily bypass taller components on the PCB and avoid collisions; the U-axis controls the rotation of different glue spraying heads; and the PCB conveyor belt is controlled by an induction motor.
Figure 2: Schematic diagram of the mechanism space
3. Control system design and configuration
This control system uses Advantech's highly integrated motion controller PEC-3240 as its core control unit. The PEC-3240 uses an Intel Celeron M processor, supports the Windows XP Embedded operating system, and has built-in 4-axis motion control and 32-channel opto-isolated digital I/O. It integrates functions such as computing, control, dual network ports, and serial communication. It is powerful, compact, and very suitable for installation in glue spraying machines.
3.1 The specific hardware configuration is as follows:
As shown in Figure 3, the hardware system architecture diagram reveals that the core of the control system utilizes the PEC-3240, while the display and human-machine interface employ an Advantech FPM-5151G industrial-grade touchscreen monitor. The system uses 7 general-purpose input/output (DI) points, 17 output (DO) points, and the rest are servo-related input/output points. Four Delta servo drives and motors are connected to the motion control card integrated on the PEC-3240 for control. The X and Y axes use 400W motors, the Z axis uses a 200W motor, and the U axis uses a 100W motor. Thanks to servo control technology, this new generation of fully automatic glue spraying machine motion control system offers smoother, more precise, and more stable control, achieving a control accuracy of 0.02mm. One induction motor controls the conveyor belt, offering 8 speed settings. A teach pendant with a character LCD display connects to the PEC-3240's serial port via RS-232 communication, facilitating teaching control of different PCBs. A QR code scanner communicates with the PEC-3240 via USB to automatically identify QR codes on the PCB. This facilitates automatic retrieval of PCB teaching files and automatic switching of the adhesive spraying trajectory, achieving fully automated production and management. All positioning points utilize high-precision proximity sensors or high-precision photoelectric sensors, ensuring the long-term stability and accuracy of the control system. The diagram below illustrates the connection of the configuration components.
Figure 3: Hardware System Architecture Diagram
4. Software design for the glue spraying machine
4.1 Operating System Selection
This motion control system uses Windows XP Embedded as its operating platform and VS2005 C# as its software development platform. This avoids the malfunctions caused by file loss due to improper shutdowns on mechanical equipment when using Windows XP, ensuring the glue spraying machine's motion control system can safely and smoothly execute predetermined program tasks. Furthermore, because the operating system is Windows XP Embedded, applications developed on ordinary PCs can be directly installed on the PEC-3240, reducing the complexity of software development.
4.2 Software Architecture of the Glue Spraying Machine
From an architectural perspective, glue spraying machine software mainly consists of three parts: underlying software, control process implementation, and user interface.
The underlying software defines and configures I/O parameters, including starting speed, acceleration, running speed, and homing mode for the four motion axes, facilitating hardware debugging. It also provides a rich set of functions for upper-level software to use, allowing upper-level software designers to focus on implementing the glue spraying functionality.
The control process layer automatically executes the control process based on the glue spraying trajectory and preset parameter configuration.
The user interface layer is used for human-computer interaction, allowing direct operation of the machine during teaching, editing preset parameters, and manual single-step operation to ensure smooth execution of automatic operation. It also provides prompts for various abnormal conditions to facilitate machine fault diagnosis. Figure 4 shows the main interface of the developed glue spraying machine.
Figure 4. User interface of the glue spraying machine
4.3 Software Functions and Features of the Glue Spraying Machine
This control system supports self-learning and memory functions for glue spraying trajectory and methods. It intelligently records the trajectory and spraying method after manual instruction, and automatically sprays glue based on the recorded trajectory during subsequent production. The recorded teaching files are generated in G-code format, which can be edited offline or online, allowing for operations such as deleting, inserting, and modifying parameters on specified lines. The system supports teaching not only through application software but also with a dedicated teaching pendant, making the teaching process very simple and quick.
Users can scan the QR code on the PCB with a barcode scanner to automatically retrieve and load files from the teaching file library. After starting automatic operation, the PCB can be automatically sprayed with adhesive, eliminating the hassle of manually searching for the adhesive spraying teaching file library. At the same time, the adhesive spraying process is completely visible, and the progress of PCB adhesive spraying can be intuitively observed from the software's adhesive spraying process trajectory.
During the glue spraying process, the system features continuous interpolation of the spraying trajectory and intelligent forward-looking glue spraying capabilities, preventing uneven glue application and ensuring uniform and rapid spraying. It also incorporates intelligent speed control algorithms based on mixed paths composed of arcs, straight lines, and various complex graphic areas, resulting in smooth, precise, and fast motion. Furthermore, the glue spraying machine is equipped with needle-type dispensing valves, fine conical dispensing valves, and fan-shaped dispensing valves. The control software automatically selects the appropriate valve based on actual conditions, balancing spraying speed and accuracy. Each spraying trajectory can be individually configured with different working speeds, valve numbers, working depths, and glue dispensing/closing delays. The control software automatically performs periodic maintenance based on actual production conditions. For example, it automatically soaks the glue valves during production to effectively prevent clogging and automatically eliminate potential malfunctions. The system also includes a wastewater discharge function to ensure uniform glue application. Automatic soaking, discharge positions, and soaking time intervals can be set according to system recommendations or manually preset.
Furthermore, the control software supports powerful array spraying and area spraying functions, making spraying simpler, faster, and more flexible. The control software can also automatically offset the entire spraying process based on actual conditions, eliminating errors and improving spraying accuracy.
The Z-axis in the control system can automatically rise and fall according to the different heights of the parts on the circuit board. The maximum travel of the Z-axis is 135mm, and the accuracy can reach 0.02mm. It also has a "brake" device that can automatically remember the current position when the control system is powered off, effectively preventing it from sliding down and colliding with the electronic components on the printed circuit board.
The control system can also automatically detect the air supply pressure of pneumatic components during actual production. It can automatically detect and alarm when the air supply pressure is insufficient, thus avoiding some product defects caused by spraying glue.
The control system also possesses comprehensive production management functions, accurately accumulating the number of workpieces currently processed and the total number of processes completed by the machine, as well as precisely calculating the processing time for each product. The core unit of the control system, PEC-3240, is equipped with two Ethernet ports, enabling it to connect to the information management center and upload production information, facilitating the factory's information management.
In addition, the glue spraying machine has an automatic waste gas collection, filtration, and emission device. By integrating with the control system, it can effectively treat polluting gases and eliminate environmental pollution.
5. Reliability and stability assurance measures for the motion control system of the glue spraying machine
Electrically, the control system uses isolation transformers and EMI filters to ensure the reliability of the power supply.
The hardware employs an embedded, highly integrated motion controller as the core control unit, and the display unit uses an Advantech FPM-5151G industrial-grade touchscreen monitor that meets NEMA4/IP65 protection standards. This makes the control system more stable, reliable, and robust compared to the traditional method of using an industrial PC with PCI cards. Because all control components adopt a fanless design and the program is stored on a CF card, electrical maintenance is simplified, ensuring the stability of the control system for long-term operation in harsh production environments.
The printed circuit board (PCB) input, positioning, and output all utilize photoelectric sensors. The mechanical calibration origins of the 4-axis servo system are all high-performance proximity sensors, coupled with automatic calibration control software, ensuring accurate and stable positioning over long-term operation. All I/O ports are opto-isolated with an isolation voltage up to 2500V, giving the control system strong shock resistance.
The application software incorporates various abnormal alarm detection and foolproof designs to effectively prevent production problems caused by continued operation under abnormal conditions and facilitates maintenance.
6. Conclusion
Since its initial application, the control system has been running stably and has demonstrated excellent control performance, significantly improving the production efficiency and quality of PCB adhesive spraying and enhancing the automation level of this type of machine.
