Seam tracking method improvement

Abstract

This invention is for a method for controlling the motion of an electron beam gun with respect to a workpiece so that the electron beam will follow the path along the seam between the two parts which are to be welded. The electron beam scans the seam in a circular motion. The center of the circle is offset a distance equal to the radius of the circle ahead of the resting position of the electron beam in the direction of travel, by applying the signals R (sin .omega.t + sin .theta.) and R (cos .omega.t + .theta.) to deflection coils which deflect the beam along two mutually perpendicular axes whose origin is at the resting position of the beam. Control of the motion of the electron beam gun with respect to the workpiece is achieved from signals derived as the seam is scanned by the circular motion of the electron beam in the area including the seam. The signals are in the form of pulses generated by the reduction in secondary and reflected electrons as the electron beam traverses the seam. These pulses bear a phase relationship to a reference sinosoid and are processed in the control so as to provide the sine and cosine of the angle the path takes with respect to a reference axis. From the sine and cosine terms the X and Y motion components are derived and fed to servo operators which drive the gun with respect to the work along the two mutually perpendicular axes so that the gun motion is always tangential to the path at a desired fixed velocity.