Keywords

1 Introduction

Resistance spot welding (RSW) is a very important material welding technology, which is widely used in various industrial sectors. The assembly method has the characteristics of high production efficiency, low cost, material saving and easy automation [1]. It is widely used in aerospace, electronics, automobile and other industrial fields. Within a short span of time for the nugget formation the weldment material undergoes expanding, yielding and melting process, and diverse disturbing factors and their interactions influence the welding process, so it is difficult to accurately control the quality of spot welding. For these problems above, high frequency inverter spot welding is applied. The inverter spot welding power supply is one of the important directions for the development of resistance spot welding power supply [2].

High power high frequency resistance spot welding is widely used in automobile industry. It is estimated that there needs 4000–6000 resistance welding spots in one car produced [3, 4]. More and more of the resistance welding machine has been in the direction of digital development. It makes the welding quality more reliable on the one hand. On the other hand the application of the human-machine interaction interface makes resistance welding machine been easier and more intuitive to use [5].

In the research field of digital inverter spot welding machine, the man-machine interface is an important part of the welding machine, and the digitalization of interface is very important for the whole spot welding system. The main function of the interaction is to accomplish the input and display of welding parameters of the spot welding control system, which can accomplish energy conversion and technological requirements better [5]. At present in the human-machine interaction system, the main input method of welding parameters is using dial switch, keyboard with LED and LCD. The dial switch fits the condition of less parameters. Now mainly interface use the keyboard with LED or use LCD. There are various problems in these designs. For example, the design above of human-machine interaction interface is single but with complex operation, low portability, long learning process, etc. A software development of the resistance spot welding man-machine interface based on DSP is introduced in literature [3], and the hardware adopted the official development board. The design of the man-machine interactive system based on phase dc spot welding is illustrated in the paper [4], with the keyboard and 8-color LCD.

With the rapid development of chip technology and accuracy improvement of spot welding, the control demand become increasingly complicated, and the user experience become constantly improved. Early embedded control system cannot meet the present higher requirements, which are simpler, more convenient, more beautiful and real-time operated. Now, many companies have designed a series of welders, which are characterized with pure text display and graphical user interface for welding parameters.

2 Hardware Design of Human-Machine Interaction System

2.1 The Overall Structure of High Frequency Inverter Spot Welding System

The spot welding equipment is composed of servo pressurization system, inverter power system, control system and human-machine interface.

The electrode pressure has an important influence on the quality of spot welding. The electric force is usually regulated by the solenoid switch valve and the pressure reducing valve. The servo pressure system, which composed of servo driver and servo motor, can control the pressure of the electrode through the torque of the motor. And the speed of the electrode is controlled by the motor speed. The servo system can ensure the quality of resistance welding.

The power supply system is mainly composed of IGBT inverter, transformer and rectifier. The inverter process of welding machine is as follows: the three-phase or single-phase alternating current are rectified, after filtering, and get a relatively smooth straight. The inverter circuit composed of IGBT inverts the current into alternating current at different frequencies, which is reduced by the transformer. After the pressure, the rectifying filter can obtain a smooth dc output welding current.

2.2 The Structure of Human-Machine Interaction System

Human-machine interaction system consists of man-machine interface and software program. The hardware parts are controlled by the microprocessor, the touch screen and the man-machine interface. The software program adopts modular structure, including communication module, data storage module, monitor and alarm module, etc. Communication module accomplishes the communication between the resistance welder and the touch screen in the system. The data storage module accomplishes to read, record and the store the parameters and the results of welding. Monitor module controls the welding process and gives an alarm with the failure data. The overall structure of human-machine interaction system is shown in Fig. 1.

Fig. 1.
figure 1

General frame of human-machine interaction

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2.3 The Hardware Structure of Human-Machine Interaction System

The core controller of the human-machine interaction interface is PIC32MX764F128L, which is manufactured by Microchip Technology Incorporated. It is a 32-bit microprocessor, and has been enhanced in the image, image processing, peripherals and industrial interface options. The processor has high speed and enough memory, which has very good electrical performance. The MCU core contains [6]:

  • 80 MHz/105DMIPS, 32-bit MIPS M4 K® Core

  • USB 2.0 On-The-Go Peripheral with integrated PHY

  • 10/100 Ethernet MAC with MII/RMII Interfaces

  • 2 x CAN2.0b modules with 1024 buffers

  • 8 Dedicated DMA Channels for USB OTG, Ethernet, and CAN

  • MIPS16e mode for up to 40% smaller code size

  • 32 × 32-bit Core Registers

  • 32 × 32-bit Shadow Registers

  • Fast context switch and interrupt response

  • 512K Flash (plus 12K boot Flash)

  • 64K RAM (can execute from RAM)

  • 8 Channel General Hardware DMA Controller

  • Flash prefetch module with 256 Byte cache

The parameters can meet the design requirements.

The LCD touch screen is mainly used to set welding parameters, choose welding technology and issue welding commands. The touch screen choose XG104STC01N/R series products, which is designed and manufactured by Shenzhen X.R.D LCD Technology Co. Ltd. The screen is carried by SGUS (Super Graphics Utility Software) system, which can easily implement the touch function by SGUS serial intelligent display terminal, such as data text entry, return value of the key, regulation incremental, pop-up menus, etc. It has UART communication, which can support 921600 BPS rate of communication. It can store pictures and fonts, and can be updated via a serial or SD card. It supports UNICODE character display and RTC clock function. The instruction set is simple and easy to develop.

It can also quickly implement display functions such as dial clock, icon variable, art word, curve display, list display, text display, etc.

The communication between the main control chip and the SGUS touch screen is accomplished through RS232 serial port.

2.4 Design Basis of Human-Machine Interaction Interface

The appearance of the man-machine interface is designed according to the welding procedure. The basic welding process of inverter spot welding power source is: the preloading - welding - maintain – stop, four stages. In the welding process, the welding current can be divided into the following phases: gently preheating current-welding current—heat treatment transition—heat treatment current—cold current [7]. The welding time and current are set, regulated and displayed through the man-machine interface. A sequence diagram of spot welding process, which shows the change of welding pressure and current over time is showed in Fig. 2.

Fig. 2.
figure 2

Technics sequence chart of spot welding

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3 Man-Machine Interface Design

The design of the interface is simple by using the touch screen. Considering the actual operation requirements of the industry, the design of interface consists of welding parameters, monitoring parameters, electrode step length, electrode pressure, real-time curve and so on. Only welding parameters and monitoring parameters are described in this paper.

3.1 Welding Parameter Interface Design

The interface of welding parameter is designed according to the welding procedure, which is to set the current and corresponding time of the welding process. The operator can manually enter the required parameters according to the actual situation. This design pattern can ensure the wide technological adaptability of welding process. In a parameterized welding mode, the relevant parameters will be automatically saved after each welding. And next time, the CPU read the last saved welding data first after system initialization so as to achieve the purpose of power-fail protection and welding parameters reproduce last time.

The mode selection of spot welding is also designed in the interface, and the user can choose the mode between single point welding and continuous welding according to different weld assemblies. The entire interface design is shown in the Fig. 3.

Fig. 3.
figure 3

The interface of welding parameter

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3.2 Monitor Parameter Interface Design

The workpieces with different material and thickness determine that the operator needs to adjust the welding current after electing electrode pressure and welding time. Operator should check the weld nugget diameter which is produce by different samples with different welding current, until the solder joint quality fully meets the requirements of the technical conditions. Based on experimental results, users input the upper and lower limit of corresponding welding current, voltage and power, through the touch screen. At the same time, the monitoring module will perform the real-time monitoring, which will generate alarm when detect the abnormal state, to ensure the safety of the welding production. The design is shown in Fig. 4.

Fig. 4.
figure 4

The interface of monitor parameter

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4 Software Design of Human-Machine Interaction System

4.1 Welding Working Process

Following is the working process of the welding machine. The parameters of the man-machine interface are set and sent to the MCU controller before the start of spot welding. After receiving the starting signal, the servo motor will operate according to the spot welding control. The controller will check the mechanical origin when starting the machine and keep each electrode moving from the same position. The electrode drops a certain distance first, and then slowly close to the welding part. This way can realize the soft contact between the workpiece and the electrode.

After the electrode get the suitable place, the motor is controlled. The motor torque controls electrode pre-pressure. The motor torque changes into the pressure after the preloading, and then the welding power supply begins to output current. At the end of welding, the point welding power supply sends an end signal to the MCU. The motor is reversed and the electrode is back to the origin point after the pulse signal controlling. Then the whole system waits for the next instruction controlled by the MCU.

4.2 Software Design of Whole System

Design of the software system adopts hierarchical structured thinking. Modularization of the entire software and simplification of module function, can improve reliability and stability of the software, and facilitate maintenance and debugging. The software mainly completed the following functions [7]:

  1. (a)

    Accomplish the initialization of the system program, and set the storage units to store the welding parameters;

  2. (b)

    Control welding sequence, and set welding current waveform;

  3. (c)

    Communicate with man-machine interface to complete the adjustment and display of welding parameters.

The communication protocol between system and touch screen use asynchronous, full-duplex UART as communication interface. Using 8N1 mode, 8 data bits, n0 check bits, a stop bit, and a start bit are set. The starting bit is the communication standard. Serial port rate is set to 9600 bps. All instructions and data are in hexadecimal format. The data with double-byte or multi-byte, is sent in a high byte and then a low byte.

With the development of each module, a whole function program based on the process, is written to carry out the predetermined function of human-machine interface. The procedure flow is shown in Fig. 5.

Fig. 5.
figure 5

The program flow

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4.3 Software Design of Touch Module

When the touch screen is touched, the jitter must be eliminated first. Then interrupt pin sends interrupt request to the CPU, which will jump into interrupt service program after receiving the request, and shut off external interrupt. Then the CPU collects X, Y coordinate, takes A/D conversion, reads the conversion result, and gets the position coordinates of the touch point. Then the CPU compare the position coordinates with the set key boundary coordinates, and determine the touch key. The flow chart is shown in Fig. 6.

Fig. 6.
figure 6

The touch module flow

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5 Conclusion

A human-machine interaction system of inverter spot welding was introduced based on PIC microprocessor and touch screen control. The system implements the inverter spot welding parameter setting and display of the welding current in different stages, welding mode, monitor mode, selection of welding process and setting. The control system communicates with the man-machine interface. The parameters are sent to the power control system as the welding machine current. It is proved that the interface is simple, clear, easy to operate and high in practical value.