| Narrow gap welding of medium thickness steel has been widely used in oil and gas pipelines,pressure vessels,boilers,shipbuilding,bridges and other industries.Because the thickness of the steel plate is often smaller regarding to the thick plate,lengthening the wire extension and driving it swing or rotating in the groove to ensure complete fusion of both the two walls is normally used.Arc sensor is the first choice because of wire bending and serious magnetic blow in narrow gap welding are taken into account.However,the narrow gap results in a poor stability in welding arc which restricts the application of the arc sensor in the narrow gap seam tracking of medium thickness plates.The aim of this paper is to establish an arc sensing based narrow gap P-GMAW tracking control system for medium and thick plates.First,an all-position welding platform was established,and a signal analysis software for the P-GMAW with swing torch was designed.Based on the analysis of the power supply characteristics of P-GMAW with I/I mode,the law of arc voltage and pulse frequency with the height of welding torch was deduced-neglecting the effect of wire extension on its unit resistance,the change of CTWD is proportional to the change in voltage;the pulse frequency linearly changes with the CTWD in case the wire extension is larger.Then,the arc characteristics were analyzed by high-speed photography and arc signals.(1)The effect mechanism of the groove angle on the arc signal was studied.For the normal V and T groove,the arc is continuously shortened by the side wall during the torch moving toward it,which result in the change of the arc signal.For the narrow groove,the wire is almost parallel to the side wall,the arc signal changing is caused mainly by the magnetic blow.(2)By analyzing the arc voltage and current and their characteristic parameters in narrow gap P-GMAW,it is found that the base current,the base voltage,the pulse voltage,the pulse voltage and the pulse frequency change when the welding gun approaches the side wall.The difference of these characteristic parameters in the left and right side wall can reflect the weld deviation to a certain extent.(3)Necessary conditions for "jumping sidewall" phenomenon occurrence,which was discovered in the process of experiment were analyzed and found that the acute groove angle,the pulse welding current and the distance small enough between the wire and side wall provide sufficient conditions for it,and it is a particular phenomenon in narrow gap P-GMAW welding.When the "jumping sidewall" occurs,the distance from the wire tip to the sidewall is reflected by the base signal,the height of the arc by the pulse signal,and an undercut happens which results in "dig effect".The weld deviation can also be reflected by the "dig effect".By investigating the characteristic parameters that can be used to reflect the weld deviation,it is found that the pulse frequency and the average current in a single pulse cycle has little error and high sensitivity,and they were selected as the characteristic parameter of the arc sensor.The weakening effect of the "dig effect" effect on arc sensing was analyzed,the "slope" method based on the "dig effect " puts forward to obtain the weld deviation,and the control law was established.Finally,the hardware and software system for signal sampling and processing is established,and real-time communication with the swing motor control system is realized.The tracking experiments were completed by the system established and found that the seam tracking accuracy reaches ± 0.5mm when the groove is 5 degrees,and±0.4mm when the groove is 8 degrees. |
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