Patent GB-686274-A (GB)
Title: Reading arrangement for photoelectric microscopes
IPC: G01B21/06
Filed: 1950-10-26
Abstract: 686,274. Electric indicating systems. SOC. GENEVOISE D'INSTRUMENTS DE PHYSIQUE. Oct. 26, 1950 [Nov. 11, 1949], No. 26137/50. Class 40 (i) [Also in Groups XXXVI and XL(b)] In a photo-electric microscope, Fig. 1, of the type in which the location of a graduated scale 2 is determined by oscillating an image of the scale over a grid 4 behind which is positioned a photo cell 5, a meter 13 is provided which is connected with a supply of current 18 through a reversing switch 12 operated each time an impulse is produced in the photo cell due to passage of the image of a graduation over a slot in the grid. The image is oscillated sinusoidally so that, if the scale is positioned with a graduation on the optical axis, equal intervals of time occur between successive impulses and current impulses of equal duration and opposite polarity flow alternately through the meter. Under these conditions the pointer remains central. When the scale is displaced either way however, the time intervals between successive pulses become unequal and the pointer assumes a position of equilibrium which is a function of the difference between the durations of flow of current in one or the other direction. If the amplitude of oscillation is large compared with the scale displacements to be measured, a substantially linear relationship exists between the meter reading and the displacement. The reversing switch 12, Fig. 2, comprises two thyratron tubes 16a, 16b the grids of which are biased in opposite phase by alternating potentials derived from transformer T connected to the alternating supply 8 employed to drive the image deflector 6. The photo cell output is applied through amplifier 10 and circuit 11 to the centre tap m of winding 20. During one half cycle, thyratron 16a receives positive bias and the arrival of an impulse from the photo cell is effective to fire the tube. The anode potential immediately falls and a current flows from source 18 through meter 13 in the direction e to r. At the same time condenser 17 charges due to the potential developed across anode load 19a with point B becoming positive with respect to point A. The circuit remains in this condition until during the next half cycle the grid of thyratron 16b is biased positively and allows the succeeding impulse from the. photo cell to fire the tube.. The fall in anode potential lowers the potential at point B and, by virtue of the charge on condenser 17, lowers point A to a potential such that thyratron 16a extinguishes. Point A now returns to the potential of supply 18 and the current through meter 13 reverses, flowing now in the direction r to e. Condenser 17 charges due to the potential drop across anode load 19b with points A becoming positive with respect to point B. At each further oscillation of the scale image, the cycle of operations repeats with the photo cell impulses determining the instants of firing the thyratrons and hence the durations of the current impulses through the meter. A filter 14 may be connected between switch 12 and the meter to eliminate undesired transients. Circuit 11 converts the photo cell pulses into pulses of short duration and may take the form described in Specification 581,930, [Group XL].