Patent GB-1482593-A (GB)
Title: Photodetector compensating circuit
IPC: G01G23/44
Filed: 1974-07-18
Abstract: 1482593 Photo-electric encoders PITNEYBOWES Inc 18 July 1974 [19 July 1973] 31904/74 Heading G1A [Also in Division H3] A photo-electric encoder comprises a reticle 15, Fig. 2; a first photo-detector 40 responsive to radiation modified by the reticle 15; at least a second photo-detector 37 or 39; and a compensatory photo-detector 48, Fig. 5 arranged to bias the first photo-detector 40. The compensatory photo-detector 48, Fig. 5 which may receive radiation from the same source 20 as the first photo-detector 40, may compensate the first photo-detector against variations in radiation intensity and temperature, thereby to stabilize the first photo-detector output as a function of the transition between light and dark areas of the reticle 15.  The first photo-detector 40, Fig. 2 may read the least significant bit 16, 17 of a binary-coded reticle 15, bits of greater significance being read by one of two photo-detector groups 37, 39 selectable in response to the first photo-detector output. The collection-emitter paths of first and compensating phototransistors 40, 48 of similar characteristics are connected in series across a D.C. supply, their common connection being used as a signal output terminal 49. The phototransistors, which may alternatively be photodarlingtons, are mounted on a common heat sink 47 for temperature compensation and are irradiated from a common source 20, the calibration of the first phototransistor 40 being adjustable by means of reference radiation transmitted to the compensating phototransistor 48 by an attenuator 46 which may be an iris diaphragm, screen or neutral filter. The device is used in an arrangement for reading an image of a digital displacement encoder 15, Fig. 2 mounted on a postal weighing machine (12), Fig. 1 (not shown) of the spring-balance type, the image being projected by an adjustable lens (26) and plane mirrors (30, 32, 34) from light transmitted by the encoder (15) from a lamp (20) backed by a curved reflector (22). An array of photodetectors 35, Fig. 2 comprises two parallel rows of detectors 37, 39, a pair comprising one from each row being arranged for reading a respective column of the encoder 15, and a single compensated detector 40 for reading the least significant column 16, 17 and arranged on a line produced midway between the rows 37, 39. The compensated detector 40 is adjusted to switch over precisely at a predetermined transition point between a dark area 16 and a light area 17, and is connected to a multi-plexer 45, Fig. 3 so as to switch the multiplexer input from one to the other of the detector rows 37 and 39, so that, in columns which are due to switch between dark and light, the transistions occur cleanly and simultaneously. In another embodiment Fig. 4, (not shown), the compensated detector (40) and the alternating rows (53, 57) are all arranged in one line. The encoder carries two patterns of columns, identical but staggered in the displacement direction, each pattern arranged to be read by a respective one of rows 53 and 57, as the rows are selectively switched in response to the detector 40.