Patent GB-618880-A (GB)
Title: Improvements in or relating to alternating current synchronising apparatus
IPC: H02J3/42
Filed: 1946-05-22
Abstract: 618,880. Automatic control systems. SOC. FRANCAISE RADIO ELECTRIQUE. May 22, 1946, No. 15565. Convention date, June 2, 1945. [Class 38 (iv)] [Also in Group XXXVII] An arrangement for automatically synchronizing two alternating currents comprises a cathode-ray tube, the beam of which is rotated at a rate determined by the frequency of one of the currents and is controlled in accordance with the frequency of the other so that a spot appears periodically on the tube screen at a frequency determined by the latter frequency, an apertured rotatable mark being arranged adjacent said screen with its axis corresponding to the axis of beam rotation and its aperture at a radial distance from the axis substantially equal to the radius of movement of the spot. A photo-cell and optical means for projecting a spot image through said aperture onto said cell are provided. Means controlled by the photo-cell output for alternately reversing the mask, and means, actuated by the maskrotating means, are provided for controlling one of the frequencies so as to obtain equal angular movements of the mask in opposite directions, whereby synchronism is obtained. The output of a slave generator 3 is split into two quadrature components which are respectively applied to mutually perpendicular deflector plates of a cathode-ray tube 1 so as to produce a circular trace on the screen. The control electrode of the tube 1 is normally biassed to cut-off the electron beam but is energized from a master generator 2 so that a spot appears on the screen for a short period during each cycle of the voltage of the master generator, this spot remaining stationary if the slave and master generators are in synchronism, and moving in either direction at a speed proportional to the difference of the frequencies of the generators when they are not synchronized. An opaque circular disc 5 having a aperture 6 is mounted adjacent to the screen of the tube and a reflector 7 is provided to transfer the image of the spot on the screen to a photo-electric cell 8. The disc 5 is driven by a reversible electric motor 12, and, when the spot on the screen is opposite the aperture 6, the cell 8 is illuminated to control, through an amplifier 9, a relay 10 and a device 11 (see Group XXXVII), whereby the motor 12 is reversed.. If the spot on the screen is stationary the disc 5 is rotated alternately in opposite directions through equal angles, but, with movement of the spot, the duration of rotation of the disc in the direction of spot-rotation is greater than that of the opposite sense. Synchronism is obtained by movement of a shaft 15 controlling the generator 3, which is mechanically coupled to the motor 12 through gearing. When the spot on the screen of the tube remains stationary the control 15 turns through small angles in each direction the mean position of the member 15 remaining unaltered. The instantaneous variation of the frequency is very small and may be removed by providing a little play in the control arrangement for the slave generator. Lack of synchronism between the two generators causes the member 15 to take up a new mean position to adjust the generator 3 to synchronize with generator 2.