GB639247A - Improvements in photo-electric follower apparatus - Google Patents

Abstract

639,247. Valve circuits. TREWEEK, K. H., and FORREST, R. H. June 28, 1948, No. 17265. [Class 40 (vi)] [Also in Groups XXXVIII and XL (b)] The deflection of a spot on a C.R.T. screen is photo-electrically controlled to follow the boundaries between areas having contrasting light-reflecting or transmitting properties. A unidirectional follower (Provisional drawing) comprises a transparent film 1 carrying an opaque trace 4, traversed between the screen of a C.R.T. 2 and a photo-cell 3 controlling the Y-plate potentials, i.e. deflection of the spot at right angles to the film traverse. The photocell output is amplified by a normal two-stage R.C. coupled amplifier V1, V2 and fed to a "switching-valve" V3 having a 3 kc. squarewave, with an amplitude of 0 to 40 volts from ground, applied to its suppressor, whereby the output is switched to the screen and anode circuits alternately. These signals are fed after some smoothing to a two-phase D.C. amplifier V4 ... V7 providing a cathodefollower output to the Y-plates, which contains just enough of 3 kc. ripple to cause the spot to vibrate slightly on either side of the line, at the same frequency as the switching in V3. Thus the output of the photo-cell as it views the spot above and below the line 4 is switched respectively to each phase of the cathodefollower amplifier. If the spot appears more on one side of the line than the other the output to the Y-plates will have a D.C. component which shifts the mean spot position behind the line. In an alternative arrangement (complete drawing, not shown), the switching valve is replaced by a ring demodulator driven by a 3 kc. oscillator, which also energizes electromagnetic deflector coils to cause the spot to vibrate (instead of 3 kc. ripple on the deflector plates). To cause the spot to outline an.area, a second demodulator and amplifier is arranged to feed the other set of deflector plates, the second demodulator and a second set of deflector coils being fed from the oscillator through a 90 degree phase shift network. The Specification states that the D.C. "shift" potentials may be used to feed calculating apparatus, control the movement of a stylus or cutter, or a remote C.R.T. indicator ; or, alternatively the light from the C.R.T. spot may be focused on to a surface which reflects light on to the photocell, e.g. in a dynamic balancing machine having a dark spot on a bright surface or vice versa. The Provisional Specification states that the C.R.T. may be replaced by a light beam reflected from the mirror of a galvanometer.