自动弧焊机控制系统的设计与实现

摘要：本文是对自动氩弧焊机的控制系统进行分析和研发，利用单片机设计一种通用焊接程序控制器，实现对氩弧焊机焊接机头、行走装置和焊接电源的驱动和精准控制。本文通过设计控制时序，为操作者提供良好的人机交互界面，可根据焊接管径、工件厚度、焊接材料等要求设置焊接的距离和速度参数，由电气控制系统进行调节和控制焊接过程。
关键字：焊接程序；单片机；步进电机；

1.引言
随着单片机技术的不断成熟和微电子技术的飞速发展, 近年来单片机技术正逐步进入焊接装备,并因此带来了焊接装备整体水平的大幅度提高。采用单片机系统部分地代替传统的继电器控制系统,不但可以简化操作难度、降低劳动强度、减小控制系统的体积,还能提高设备精度，保证焊接质量。

2. 结构设计
焊机由焊接机头、电气控制箱、行走装置和焊接电源四部分组成，控制箱内主要装有发电机组，整流器，继电器和接触器以及引弧，熄弧等控制系统。工作台的传动方式采用滚珠丝杠螺母传动副，以保证一定的传动精度和平稳性以及结构的紧凑。如图1所示，本控制系统是由单片机控制模块，电机运动模块，弧焊启停模块所组成，以单片机为核心，控制焊接过程的启停和工作台各装置的运动状态。车床x，y,z三坐标轴方向的运动及焊接送丝过程由四xx立的步进电机分别驱动。

3. 硬件设计
在焊接设备中，程序控制是必不可少的控制单元，在此设计成标准的逻辑控制单元。焊接程序控制器的控制对象主要有电磁阀、高频引弧器、焊接电源等。硬件电路由单片机控制电路、电机驱动电路、送丝调速电路、弧焊电源主电路以及隔离、采样、焊接状态识别电路构成。单片机控制部分由 89C51单片机、时序电路等辅助电路、键盘及显示部分、I/O接口及光电隔离电路、控制信号输出电路等组成。其中，逻辑与时序控制由单片机完成，系统的工作程序和控制命令通过键盘操作实现。输入、输出控制采用光电耦合器以提高系统的可靠性。

3.1 运动控制部分
在本系统中运动控制的关键即送丝与各丝杠移动的调节，由四个电机分别驱动X，Y，Z轴方向的丝杠行走以及送丝操作，运动控制部分如果用89C51直接驱动各电机则在软件和硬件实现上都比较困难的，所以选用单片机与步进电机驱动器结合起来控制它们的运动。一个完整的步进电机控制系统应含有步进电机、步进驱动器、直流电源以及控制器 (脉冲源)。本系统中单片机P1口分别向各驱动器提供电机的脉冲和方向信号（其中P1.0，P1.2， P1.4 ，P1.6为时钟脉冲信号，P1.1，P1.3， P1.5 ，P1.7为方向信号），P2.0-P2.3作为选通信号与四个驱动器的使能端相连，用来决定哪个电机处于活动状态。步进电机驱动器的细分功能可以提供较精准的角位移，xx电机的低频振荡。本系统丝杠移动及送丝速度的调节采用开环控制，通过调节焊接速度给定，改变输出脉冲的占空比，即可改变丝杠的行走速度。   
在焊接的过程中需要工件沿着焊缝的方向移动，此时送丝动作和X轴丝杠行走须同步进行。其联合控制的思路是：电机速度比一定，就采用频率{zg}的延时作为基准发脉冲,由此分频输出脉冲。采用T0定时，计算出此基准值对应的初值，事先存于表中，运行联动子程序时，取出赋给T0。X电机脉冲延时值 和Q送丝电机脉冲延时值都以此为基准，在联动时按照一定的比例同步运行，以达到对送丝速度进行控制的同时，调节焊接速度(丝杠行走速度)。需要改变频率时就改变这个时间基准即可。

3.2 弧焊启停控制部分
焊接过程由焊工通过焊炬上的开关或脚踏开关启动，经电气控制系统开启或关闭焊接电流和保护气体。由于焊接电源在整个焊接过程中一直打开，本系统采用继电器按需要控制电极是否通电。一方面使低压的电子电路的控制信号能够控制高压焊接电源电路的执行元件，另一方面又为电子线路的电气电路提供良好的电隔离，以保护电子电路和人身的安全。如图2所示，由单片机的P3.4输出信号控制继电器的开合，以控制焊接电源的启停。

由于单片机在上电后的瞬间，P3口初始状态为高电平， 为了避免在未进入焊接程序时焊机被触发引弧，所以就必须保证继电器在控制端为低电平才吸合。于是硬件上采用两个S8050三极管结合来驱动24V继电器，如图，Q2基极与Q1集电极相联并上拉+5V电压 。选择驱动能力较强的P3口来控制三极管的状态。开机时，单片机初始化后P3.4为高电平，Q1基极被拉高，＋5V电压通过电阻使三极管Q1导通，则其集电极输出低电平，也就是Q2基极输入为低电平，Q2截止，继电器没有电流通过，所以开机后继电器处于释放状态。在进入引弧操作时，程序中会发出一条指令CLR P3.4，输出低电平，Q1的基极就会被拉低到零伏左右，使三极管Q1截至，则Q2基极被＋5V电压拉高，Q2导通，指示灯亮，于是继电器在电压的作用下吸合，焊机即进入正常的起弧与焊接状态。停止焊接时，P3.4再置高，继电器释放，焊枪熄弧。每个继电器都有一个常开转常闭的接点，便于在其他电路中使用，继电器线圈两端反相并联的二极管是起到吸收反向电动势的功能，保护相应的驱动三极管，这种继电器驱动方式硬件结构比较简单。

4.软件设计
控制系统的软件设计是控制系统不可缺少的部分，开发性能良好的控制软件是开发控制系统的重要任务之一，对于改善系统的工作性能，减少系统硬件，降低成本等方而都有重要的意义。
控制系统负责实现工作程序、规范预置、引弧及自动调节等控制功能。根据工作过程的需求，电气控制部分要完成整个焊机的动作程序, 控制机械设备的运动,如图3所示,工作流程依次包括完成水平定位，机臂下降、引弧、预热、焊接(丝杠移动，送焊丝)、计时、熄弧、（停止送丝）、焊枪退出(机臂上升)和程序复位等动作程序。
系统采用模块化方法设计控制软件，整个软件由主程序、焊接控制子程序、中断子程序 ，步进电机驱动子程序、预置子程序、收弧控制子程序构成。其中，焊接控制子程序是本软件结构的主体，按照弧焊要求控制整个焊机工作的流程。如图4所示，上电后复位后，程序开始运行。系统先执行初始化子程序，完成各参数的初始值设定。然后执行键盘扫描程序，检测是否有控制键按下，并进行相应处理。这里可以完成焊接材料选择、焊丝直径选择、焊接模式选择等工作，并确定焊接电流，电压等规范参数。该机的所有参数通过按键和一个编码器调节。当检测到焊接开始信号后，进入引弧程序，该程序内完成焊接气、电、丝的程序控制。电弧引燃后，转入焊接程序。该焊接过程循环进行，直到检测到焊接结束信号，退出焊接状态，转入收弧子程序。{zh1}回到初始待机状态，等待新的开始焊接指令。

5.抗干扰设计
单片机通常对工作的电环境、磁场环境及环境温度较普通电子电路要求更高,而焊接电弧所产生的强磁场、强电场干扰和较高的环境温度对单片机系统的正常和可靠工作构成了严重的威胁。这是一个在工业环境中微机应用普遍面临的技术难题,解决该问题需要从多方面采取措施。合理地在单片机系统、数字电路和模拟电路中进行功能分配，在软件和硬件方面采取有效的抗干扰方式以改善系统工作的可靠性。

6. 结束语
单片机系统功能设计的确定是整个控制系统结构设计的关键问题 。它决定了在整个控制系统中,哪些工作由单片机完成,哪些工作由数字或模拟电路完成。在该设计中, 对设备的功能提出了符合现代焊接装备标准的功能要求,如焊接参数的数字显示、xx预置、慢送丝焊丝位置调整等。这些功能的实现需要单片机技术,同时该技术的采用有效地降低了控制系统的体积。本系统中该控制方法具有控制线路简单，可靠性高的特点；系统编制的软件通过程序优化，指令执行效率高，速度快，既能保证系统控制的精度又能实现多个控制单元协同工作。

Design and Realization of an Automatic Welding Machine
Control System

Abstract: Based on the analysis and research of the automatic argon arc welding control system, this paper has develop a universal program controller for welding procedures,to realize the accurate control for the welding head, walking devices and driver of welding power source. It has provided a friendly man-machine interface for operators through designing the control timing.The distance and speed welding parameters can be set in accordance with the requirements such as the thickness of workpiece and welding material.The electric control system is in charge of regulating and controlling the whole welding process.
Keywords: welding program; SCM(single-chip microcomputer) ; stepper motor;

1. Introduction
With the rapid development of microelectronics technology in recent years, the SCMs is gradually stepping into the field of welding equipment, and has made great improvement over the whole range. The SCM system taking the place of the traditional relay control system can not only simplify the operations, reduce the labor intensity, decrease the system size, but also improve the equipment precision and ensure the welding quality.

2.Structure Design

The welding machine is composed of four parts: welding head, electric control box, walking devices and welding power source.The electric control box is equipped with the generator sets, rectifiers, relays and contactors, as well as the arc ignition and extinction control systems. Workstation employs a ball screw driven rail table to ensure its transmission accuracy, smooth and compact the structure. As shown in Figure 1, the control system are constituted by SCM control module, motor driver module, and arc start-stop modules. Based on the SCM, the system determines the start and stop of the welding process and motion state of all devices on the workstation. Motions of the lathe at the three directions along x, y, z coordinate axis and the feeding process of welding wire are driven by four stepper motors separately.

3.Hardware Design

     Process control is essential to control unit in welding equipment, and here it is designed to be a standard logic control unit. The major objects that mastered by the controller in welding procedure are solenoid valve, high-frequency arc ignition, welding power and so on. The hardware circuit is made up by microcomputer controlling circuit, motor driving circuit, wire-feeding speed regulating circuit, arc welding power source main circuit, as well as the isolating, sampling, welding state identification circuit. Microcomputer system is constituted by 89C51 single-chip microcontroller,auxiliary circuits such as timing circuit and I / O interface circuit, keyboard and display part and the photoelectric isolation circuit, control signal output circuit and other components. Among them, the logic and timing are controlled by the microcontroller , system procedures and control commands are implemented through the keyboard operation input. The input and output circuits are controlled by optocoupler to enhance the system reliability.

3.1 Motion Controlling
In this system the motion controlling centers upon regulating the speed of wire feeding and movement of lead screw in each direction.Four motors separately drive the wire-feeding tape roll and the lead screw move respectively following their directions of X, Y, Z axis. If all the motors are controlled by 89C51 directly,it will rise great difficulty for software and hardware implementation. So the suggested solution is to combine the microcomputer with stepper motor driver and control of their motion. A complete stepper motor control system shall contain the stepper motor, stepper drivers, DC power supply and controller (pulse source). In this system, port P1 of 89C51 provides the pulse and direction signals (P1.0, P1.2, P1.4, P1.6 are the clock pulse signals, while P1.1, P1. 3, P1.5, P1.7 are the direction signals)to the motor drivers for corresponding motors, P2.0-P2.3 act as strobes connected to each driver's enable input and determine which motor is active.
The subdivision function of the stepper motor driver can provide accurate angular displacement and eliminate the low-frequency oscillation.In this system the control of the screw motion and the wire feeding speed is implemented in an open-loop. By adjusting the set welding speed and changing the output pulse duty cycle, the screw's moving speed can be varied if required.    
In the welding process the workpiece need to shift along the welding direction, at this time it is required that the wire feeding and screw of X-axis direction have to work synchronously.The idea of he linkage controlling is: since the motors work at a certain speed ratio, the pulse of highest frequency can be adopted as the reference pulse, and output different pulses by frequency dividing. Timer T0 is working in timing mode.The initial value corresponding to the reference value is calculated and pre-stored in tables of the program, and it will be send to timer T0 when the linkage controlling subprogram is running. The motor X and motor Q , which are in charge of wire feeding, take T0's value as reference pulse time delay and run synchronously following it at certain ratio. The ratio is set before the welding process and if the speed of any motors need to change, workers can adjust the reference value in program.

3.2 Arc start-stop controlling
The welding process is started by the welder pressing the switch button on the torch or stepping on the foot switch, and then the electrical control system will turn on or turn off the welding power and protective gas. Because the welding source is power on in the entire welding process, a relay is used in the system to determine the status the power electrode according to the need. On the one hand, it has made the low-voltage electronic circuits can control the implementation components of high-voltage welding power circuit; on the other hand, it has provided a good electrical isolation to the electronic circuit and ensure the security of both the electronic circuits and physical body. As shown in Figure 2, output signal from P3.4 of the SCM controls the on and off of relays , and determine the welding power supply to start or stop.
Because the initial state of port P3 will be at the high level at the moment after the SCM powers on, in order to avoid the arc welding machine being triggered before the welding procedures, the relay is required to pull in while the control port outputs a low-level signal. So in hardware system, there are two transistor S8050 combined to drive the 24V relay.As Figure 2 shows,the base of Q2 is connected to the collector of Q1 and pull-linked to +5V. Port P3 is chosen to control the transistor state for it has a strong driving capability. After power-on, P3.4 is high after the initialization of SCM. The potential of Q1 base is raised, and voltage +5V makes Q1 break-over through the pull-up resistor, so Q1's collector outputs low level, that is, the input of Q2 base is low level,too.Then Q2 is cut off, there is no current passing through the relay, so the relay is in release state after power-on. When entering the arc ignition operation, the program will send out a command CLR P3.4, and SCM output low level signal, Q1's base is dragged down to the voltage aroud 0V, which makes Q1 cut-off ,and Q2's base will be pulled high by the +5 V voltage. Q2 is turn-on, so relay will pull-in under the action of voltage, and the welding machine is entering the normal arc striking and welding state. When the welding process ends, P3.4 is put high again and the relay release out.

4.Software Design

Software design is an indispensable part of the control system. Developing an efficient control software is one of the most important tasks in the development of control system. It is significant for improving the system performance, reducing system hardware components and saving costs for whole project.
Control system is responsible for functions of implementing the work procedures, presetting parameters and automatically adjusting welding process. In accordance with the working requirements, the electric control part must complete the action procedures of welding machine, and control the motion of mechanical equipment. As shown in Figure 3, the work flow includes horizontal positioning, arm dropping, arc striking, preheating, welding (Leadscrew moving and wire feeding), timing, arc extinguishing, torch quitting(arm rising) and reset actions.
System adopts a modular way to design the control software which is compose of the main program, welding control subroutines, interrupt routines, stepper motor drive subroutine, preset subroutine,and arc striking and extinguishing subroutines.The welding control subroutine is the essence of the software architecture,and decide the the whole work flow of arc welding machine in accordance with the requirements.As shown in Figure 4, the process begins running after the system reset.
The system executes initialization subroutine first to set the initial value of each parameter. Then the keyboard scanner begin to run to detect whether there is control key pressed or not, and respond if need. When start signal for welding is detected, the system enter into the arc striking program and realize the control of gas, electricity, wire for welding. After arc ignition, the system run into the welding process.The welding process will cycle until the end signal has been detected. Then the program withdraw from the welding process and turn into arc extinguishing subroutines. Finally,the system will go back to the initial standby mode, waiting for the new start command for welding.

5.Anti-interference Design

The requirements of SCM system on the work environment, magnetic field environment and the ambient temperature are usually higher than that of an ordinary electronic circuits, however,the interference from strong magnetic field and strong electric field caused by the welding arc and the high ambient temperature will greatly harm the normality and steady work of the SCM system. This is one of the general technology difficulties that often faced by the microcomputer applications in the industrial environment, and this problem should be resolved by different measures from various respects. Appropriately distributing functions in SCM systems, digital circuits and analog circuits, and taking effective anti-interference methods in software and hardware both can improve the system reliability.

6.Conclusions

    The function design of SCM system is the key issue to the entire control system. It determines what should be done by the SCM, and what should be done by the digital or analog circuitry in the whole control system. In the designing procedure, it is required that the functions of the equipment should keep in line with the standards of modern welding equipment,such as the digitalized display of welding parameters, accurate preset and position adjustment for feeding welding wire. The realization of these functions needs SCM technology, meanwhile using the technology will effectively reduce the size of control system.In this system, the control method is of simple circuit and high reliability;the system software is optimized through the preparation process, so the instruction execute efficiently and fast.Thus,it ensures the accuracy of the system control and realize the collaborative work of a number of control units.