Study On The Control System Of The Three-phase Secondary Rectification Spot Welding

In modern industry production, the resistance spot welding as an importantwelding technique has been widespread applied to the automobile and the aviationdomain. At present there are three kinds of the common spot welding equipments:single-phase alternating current welder, three-phase secondary rectification welderand inverter based-spot welder. Among these welders three-phase secondaryrectification welder and inverter based-spot welder are the main develops direction ofthe resistance welding machine in the world now. Regarding spot welding controllermainstream control mode remain with single parameter constant control mode such asconstant current, constant voltage, constant power and so on. But these control modescannot guarantee the reliability of the spot welding quality, especially there are notenough research results about three-phase secondary rectification spot weldingmonitoring technique in recent years. This paper takes the three-phase secondaryrectification welder as the control object and designs a single MCU intelligencecontrol system for three-phase secondary rectification welder, which core is AVRseries ATmega16.This paper has established a function about thermal percentage Qn and phaseshifting angle α, which is based on the analysis of the three-phase secondaryrectification welder, it can be used to the thermal assigning and real time adjusting. Bymeans of this foundation, the non-linear eigenvalue Qn(α) has been sought by Cprogram and the chart of Qn-α has been drawn up. From the chart it has been foundthat Qn appears monotonous drop along with the phase shifting angle α's increasing.The curve nearly not be influenced by the change of the power factor when the phaseshifting angle less than 90o (reference voltage is phase voltage UA). This will makethe number of the thermal function tables become less, that is to say, only need onetable.Multi-information fusion technique is a process to treat the multi-information asa whole synthetically. Multi-information fusion attempts take full advantage of theinformation which come from the different sensors have the different characters. Forthe sake of to obtain the uniform explanation or the description, it needs to unite theinformation based on some kind of criterion, these information redundant orcomplementary in space or time. The main target is to educe the more usefulinformation,to obtain the information about the object and the environment as moreas possible by the advantage of the multi-sensor. The heat process of the spot weldinghave some electrical parameters(the voltage between electrodes, the current of thewelding, dynamic resistance) and some mechanical parameters(the displacement ofthe electrode and the speed of the electrode's move). It is multi-information fusion innature when we use these parameters in welding synthetically.The basic thought of the fuzzy control is simulate the men's fuzzy thinking modeso as to control the object. But the men's thinking mode come from the experience, itis hard to exact describe it in mathematic ways. The fuzzy control just use fuzzy judgesentence to achieve logic control to the system. Further more fuzzy control allowinput more than one fuzzy information, it happen to have the same view with themulti-information fusion technique.This paper puts forward a cooperative control strategy that is named PDRCmethod, which combines the constant power control technique with the DRC controltechnique organically by use of the principle of the multi-information fusion controland the fuzzy control arithmetic., that is to say, the constant power control is adoptedbefore the apex of the dynamic resistance curve, which can make up the blind area ofthe DRC control method, can restrain welding splash and can compensate theinfluence of the current shunt at the welding prophase. After that the DRC modelingcontrol method is adopted, which make the nugget molding fast by consolidatedispose the given DRC that make sure the nugget quality by control the droppingscope of the dynamic resistance.This paper applies the PDRC control thought to the hardware system whose maincontrol core is ATmega16 which belongs to the top grade MCU in AVR series. Thehardware system includes the voltage pass zero synchronous detect circuit, the phasequeue identifing circuit, the current sampling circuit, the voltage sampling circuit,thyristor trigging circuit, the electromagnetism valve control circuit, parametersediting and displaiing circuit, the welder states judging circuit, the alarm circuit, theserial communication interface circuit with the upper computer by the criterion of 232,the external watchdog circuit and the foot-press switching circuit.The software is the soul of the control system, and the key technique of thesoftware design is control arithmetic. In consideration of the welding process is a highnonlinear, complicated coupling process with a lot of stochastic factors, and at thesame time in consideration of the process ability of micro controller in real-time, twofuzzy controllers are designed based on the fuzzy control algorithm for the real-timecontrol system.By the steps debugging and the online debugging the hardware circuit function ofthe control system is validated, the software correctness and the reliability of thewhole control system are checked. The PDRC method is only debuged by softwaresimulation because of the time. Thus it needs the further research on the effect of thePDRC in spot welding.In a word, this paper has designed a control system for the three-phase secondaryrectification welder, it supports a lot of control modes which include the constantcurrent control, the constant power control and the PDRC method control, and threeactive statuses such as programming, welding and data reading. Its anticipatedfunction is come true. The thermal function and the program for looking up the tableare built based on the characteristic of the three-phase secondary rectification welder,which can effectively realize welding heat assigning and adjusting on real-time. Thispaper puts forward the cooperative control strategy that is named the PDRC methodand designs the control software by fuzzy control algorithm which profits from theprinciple of the multi-information fusion control. This paper did useful explore for theintelligence control in three-phase secondary rectification welder. This will promotethe technology advancement of the quality control in three-phase secondaryrectification spot welding. This study not only has the important theory significance,but also has the project application value for the further research.