Research On PMIG Arc Welding Inverter Main Control System Based On Fuzzy-PID Control

Arc welding inverter has many advantages such as good electric performance, small size, high productivity, easy automation, intelligent, and so on, which has become the most promising development direction in the filed of welding industry. However, presently in our country, the control systems of arc welding inverters are mostly controlled by analog circuits which have shortcomings of low efficiency, accuracy, and so on. On the contrary, digital arc welding inverter has become advanced welding equipment development direction for future because of the tremendous superiority such as its stability, flexibility, high accuracy, easy control through network, and so on. Domestic digital arc welding inverter research and development is still at an early stage, high-performance digital arc welding inverter mainly relys on imports. Based on digital signal processing technology, the main control system of digital PMIG arc welding inverter were studied in this paper.Through analysis of the own structure and droplet transition characteristic of PMIG arc welding inverter, this paper presents control objectives of system, proposes dual voltage and current closed-loop control programme, and sets up a MCU+DSP dual-microprocessor control hardware platform. Fuzzy controller is used by outer voltage ring, and fuzzy PID controller is used by the inner current ring. This programme has achieved good control results. The main tasks of this paper are:1. Propose system control programme. After the analysis of the droplet transition aspect and wire melting speed aspect of PMIG arc welding inverter, the system control objectives are presented ,which are to ensure droplet "one pulse one drop" transition and wire melting speed matching with feeding speed, at last guarantee the stability of arc length and welding process. The overall system adopts dual voltage and current closed-loop control programme. The outer voltage ring adopts I-I control mode, which guarantees the stability of average welding voltage by adjusting the base time; the inner current ring is to control and achieve precise current pulse waveform of the droplet transfer.2. Set up system hardware platform. TI's DSP TMS320F2812 is chosen to take charge of the real-time welding process control. Freescale's MCU MC9S12XDP512 is chosen and mainly responsible for the overall coordination control and communication interface with other equipments. This paper realized interface circuits of high-speed real-time sampling of voltage and current signals and interface with PWM, also designed LCD display, serial communication and Ethernet interface circuits.3. Study the inner and outer ring controllers of the dual voltage and current closed-loop control system respectively. Through comparison among PID controller, fuzzy controller and fuzzy PID controller, based on respective control characteristics of the inner and outer ring, fuzzy controller is chosen as the outer ring controller, and fuzzy PID controller is chosen as the inner ring controller. Fuzzy control tables about base time increment and PID parameters increments are generated offline.4. Design the software of control system, including software of overall process control and dual closed-loop control. Dual closed-loop control is the most important part, which consists of A/D sampling procedures, D/A output procedures, as well as online look-up table control of outer voltage ring fuzzy controller and inner current ring fuzzy PID controller. Through performance analysis to the dual-loop controller, the voltage outer ring fuzzy controller can inhibit disturbance of arc length, the current inner ring fuzzy PID controller makes the system better dynamic regulate performance through real-time adjustment of PID parameters.5. Tests and experiments on arc welding inverter show this system can achieve control goals and guarantee stability of arc length and welding process.