Seam tracking method improvement

A method for controlling the motion of an electron gun so that the beam generated by said gun follows, at a constant tangential velocity, the seam along which two parts to be welded adjoin, comprising the steps of;

• generating, accelerating and focusing an electron beam;
• directing the beam so that in its at rest position it impinges upon the work at a point close to the said seam;
• deflecting the beam, so that it follows a circular path which crosses the said seam twice during each revolution of said beam, by applying two periodically varying magnetic or electrostatic forces normal to said beam along mutually perpendicular axes, one of said forces directed along one of said axes and varying in proportion to R (sine .omega.t + sine .theta.) and a second of said forces directed along the other of said axes and varying in porportion to R (cosine .omega.t + cosine .theta.);
• generating an electric pulse each time the beam crosses the said seam;
• supplying the said pulse to a bistable electronic circuit so as to produce at the output of said circuit a periodic alternating rectangular voltage wave which alternates between two voltage levels in synchronism with each successive pulse;
• integrating the said rectangular voltage wave, by electronic means, and thus producing a position error signal which represents the deviation of the rest position of the beam from the seam;
• multiplying, by electronic means, the said voltage wave by a periodic voltage varying as the sine .omega.t and simultaneously multiplying, by other electronic means, the said voltage wave by a periodic voltage varying as the cosine .omega.t, so as to obtain separate product voltages which represent respectively the cosine and sine of an angle .theta. formed by the chord between the points of intersection of the rotating beam and the seam, and the first of aforesaid axes;
• multiplying, by electronic means, the said position error signal respectively by separate voltages representing the said sine .theta. and cosine .theta. so as to obtain the position error components with respect to the aforesaid axes;
• multiplying, by electronic means, a voltage which represents a desired tangential velocity for the beam respectively by the separate voltages representing the sine and cosine of the angle .theta. between the seam and the said axis so as to obtain separate voltages representing the components of the said tangential velocity along each of the said axes;
• summing the voltages representing the positional error and the component of the tangential velocity along one of said axes, by electronic means, and summing the voltages representing the positional error and the component of tangential velocity along the second of said axes, by other electronic means, so as to obtain separate control signals and;
• feeding the said signals to a pair of electronic servo systems which drive the gun along the two axes so as to cause the gun to move at a fixed tangential velocity along a path parallel to the said seam.