Patent GB-875953-A (GB)
Title: Improvements in or relating to encoders
IPC: H03M1/00
Filed: 1958-04-28
Abstract: 875,953. Electric selective signalling. BALDWIN PIANO CO. April 28, 1958 [April 29, 1957], No. 13426/58. Class 40 (1). The rotation of a shaft 20, Fig. 1, is converted to a digital signal by a coded disc 24 attached to the shaft and a plurality of photo-cells 26 which receive light from a lamp 28 passing through several tracks on the disc. As shown in Fig. 2 the photo-cells are formed between opposed conductors 52A-52N and 54A-54N deposited on a glass plate 48. A strip 62 of a photo-conductive material bridges the gaps, the separate cells being electrically insulated from each other by dark portions on the coded disc between adjacent tracks. The photoconductive strip is covered with a sealing strip 64 of glass. Conductors 54A-54N are connected to terminal pieces 58A-58N. The photo-cells are scanned either simultaneously or sequentially. Simultaneous scanning (Fig. 5).-The common contact 60 of the photo-cell assembly 26 is connected to a master pulse generator 74. All terminals 58A-58M are connected to amplifiers 80A-80M and terminal 58N is connected to an amplifier 86 which is connected to the amplifiers 80A-80M to compensate for differences of light intensity from the lamp 28. The track on the disc corresponding to the reference photo-cell N is of about half the width of the other tracks so that the output of this photo-cell is about half the maximum output of the others. This output is inverted in amplifier 86 and impressed on the inputs to the other amplifiers 80A-80M so that when the corresponding photo-cell is aligned with a transparent area of coded disc 24 a positive signal is produced and a negative signal is produced when the photo-cell is dark, the polarity of these signals being independent of the intensity of illumination of the photo-cell assembly. The amplifier signals when positive are passed through gates 144A-144M and entered in storage means 82A-82M, each consisting in one form of a transistor multivibrator or alternatively of a condenser storage unit. The gates 144A-144M eliminate spurious pulses occurring at the beginning of a signal by opening at the receipt of a delayed pulse from generator 146. The use of gates 144A-144M may be avoided by using a double pulse generator triggered by the master pulse, the pair of pulses being subtracted from the photo-cell signal to eliminate the beginning and end portions thereof. The middle portion, which remains, is entered into store. The-sequential pulse generator 84 produces between each pulse of the master pulse generator 74 a series of pulses which read the data stored in the stores 82A-82M into a single store 128. Sequential scanning. Fig. 11.-The master pulse generator is connected to a sequential pulse generator 168 which applies pulses in turn to the photo-cells through resistors 172A- 172M. The common contact 60 of the photo-cells is connected to amplifier 174 and the output is passed via sampler 176, amplifier 178, a trigger circuit 180, a mixing circuit 182 and pulse generator to store 186. The sequential pulse generator 168 consists of a series of transistor stages connected in cascade with the base of each capacitively coupled to the collector of the preceding transistor and connected through a capacitor and resistor in series to one of the photo-cells. Photo-cell N is used as before to compensate for changes in light intensity. A sequence of pulses from terminal 170N pass through photo-cell N and are subtracted from the pulses of the other cells. Spurious transients and pulses are eliminated in the sampler 176 which consists of a diode gate enabled in the latter half of the compensated photo-cell signal by a short pulse from terminal 170.0 of the sequential pulse generator 168. The signal passed is amplified in amplifier 178 and used to set trigger circuit 180 which converts the cyclic code of disc 24 to a straight binary-coded representation of the position of shaft 20. This operation is not described. The output of trigger circuit 180 is gated in mixing circuit 182 with the sequence of negative pulses on terminal 170N and the output is applied to pulse generator 184 which sets the storage means 186. This may be a magnetic core matrix or a cathode-ray tube, the data being entered in the proper position by enabling pulses from the sequential pulse generator 168. In another embodiment the storage means is operated directly from the trigger circuit 180. By replacing the sequence pulse generator with a ring counter inside the housing 10 the number of leads passing into or out of the housing is reduced to two.