Patent GB-1027457-A (GB)
Title: Improvements relating to two-dimensional photoelectric indicating means
Inventor: CLEMENTS DESMOND REGINALD
IPC: G05D1/12
Filed: 1962-01-30
Abstract: 1,027,457. Photo-electric direction-finding. BRITISH AIRCRAFT CORPORATION (A.T.) Ltd. April 24, 1963 [Jan. 30, 1962], No. 3510/62. Heading H4D. A photo-electric two-dimensional position indicator comprises a group A, B, C, D of photocells arranged side-by-side to form a screen, the screen having a datum point O located in its central region, means for projecting a light beam 17 on to the screen, the axis of the light beam being laterally displaceable with respect to the screen but being arranged to illuminate a part only of the light-sensitive area of each cell when the axis of the beam passes through the datum point O, and electric circuit means connected to the cell outputs and providing an electrical output having at least two components which are respectively dependent upon co-ordinates X, Y of a displacement from the point O of the axis of the light beam. The light-sensitive areas of the cells A . . . D are square and are arranged next to one another to form a square screen, the beam of light 17 being given a square cross-section by passing it through a square-shaped aperture provided in an optical system (Fig. 3, not shown) for projecting the light beam on to the screen. The cells A . . . D are of the silicon-junction type and their outputs Ia, Ib, Ic, Id are alternating currents at a frequency F s  produced by intertupting the beam of light at that frequency. The outputs of the diagonally-opposed cells A, C are connected differentially to opposite ends of the primary P1 of a transformer T1 and, similarly, cells B, D are connected to primary P2 of a transformer T2. The two ends of the secondary S1 are connected, respectively, to the X co-ordinate channel and the Y coordinate channel. A tapping 21 at the midpoint of the secondary S1 is connected to one end of the secondary S2 to form a summing circuit so that the two ends of secondary S1 supply outputs (Ib + Ic)-(Ia + Id) and (Ia + Ib)-(Ic + Id) which are fed, respectively, to gain controlled amplifiers 20 in the X and Y channels. A transformer T3 provides a further summing circuit, the primary P3 being connected to tappings 24, 25 on the primaries P1, P2, the output Ia + Ib + Ic + Id from the secondary S3 being fed to an input of a frequency changer 28 where other output is fed with current at frequency F 0  from an oscillator 29. The output from the secondary S3 also provides a reference current at frequency F s  which is limited to constant amplitude by a limiter 31. The output from changer 28 at a frequency Fp = F o -F s  is fed as a pilot current to the gain-controlled amplifiers 20. In each channel X, Y the output of the amplifiers 20 constitutes superimposed currents at frequencies F s  and Fp but the amplified pilot current at frequency Fp is filtered out by a band-pass filter 32 and rectifier 33, the resultant rectified voltage being used as a feed-back signal to control the gain of the amplifier such that the level of the amplifier pilot current component of the output of the amplifier is maintained substantially constant. Hence the gain of the amplifier 20 in each channel bears a reciprocable relationship to the light signal strength, and the amplified difference current at frequency F s  at the output of each of amplifiers is deprived of fluctuations due to changes in signal strength. The output of each amplifier is fed through a band-pass filter 34 to a phasesensitive rectifier 35 controlled by the reference current at frequency F s  from the limiter 31. The amplitude of the output from rectifier 35 in each channel is thus proportional to the amplitude of the difference current at frequency F s  supplied to the input of the channel, and the polarity of the output depends on the phase of the difference current relatively to that of the reference current. The amplitude and phase of the difference current supplied at frequency F s  to the rectifier 35 in the X channel depends upon the relative magnitudes of the photocell currents (Ib + Ic) and (Ia + Id), and in the Y channel upon the combined currents (In + Ib) and (Ic + Id). Thus the outputs from the channels X, Y are respectively proportional to the displacement of light beam 17 in relation to the datum point O. These outputs may be used as demand signals in a servo system to direct the optical system to track the light source; alternatively, if the light source is mounted on a missile or aircraft the output signals may be supplied to the vehicle as command signals to correct any divergence and cause the vehicle to travel along the optical axis.