Tham khảo tài liệu 'advanced welding processes episode 2 part 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 274 Advanced Welding Processes CCD CAMERA TRAVEL DIRECTION ELD POOL WINDOW 1 WINDOW 2 Figure Concentric video monitoring system for GTAW. Ultrasonic sensors. Ultrasonic sensors may be used to detect the unfused joint line in the parent plate. The principal problem involved in using these techniques has been the need for intimate contact between the ultrasound transducers and the plate but a recent development 226 using pseudo-immersion probes allows the joint line to be detected to an accuracy of mm with normal plate surface conditions. Chemical composition sensor. A novel approach to seam tracking is the use of a component of the arc spectrum to detect the presence of particular chemical species 227 . The arc emission is collected by a fibre optic and passed to an on-line analysis device which monitors the level of the signal produced by specific element. In a dissimilar metal joint this signal indicates the lateral position of the arc and may be compared with a reference level to generate an error signal for correction of the torch position. In joints of uniform composition tracer elements may be introduced in the form of fusible inserts. Monitoring and Control of Welding Processes 275 b Arc length control Control of arc length is important to ensure consistent heat input constant melting rate stable process performance and adequate shielding. Arc length may be maintained by good joint preparation careful torch positioning or tactile sensors of the type described in section a . Several closed-loop systems have also been used for arc length control. Voltage measurement. In the GTAW process constant-current power sources are normally used and the arc voltage gives a clear indication of arc length at a given current. These systems directly measure arc voltage compare the actual value with a reference and control the torch height to correct the error t The arc voltage depends on current shielding gas workpiece composition polarity and electrode vertex
