Patent GB-876439-A (GB)
Title: Universal line tracer for a pattern reproducing machine
IPC: B23Q35/128
Filed: 1960-02-04
Abstract: 876,439. Electric copying machines. CINCINNATI MILLING MACHINE CO. Feb. 4, 1960 [March 13, 1959], No. 3988/60. Class 38 (6). [Also in Groups XL(b) and XL(c)] The invention relates to a line tracing device capable of reproducing a shape represented by a pattern line which will follow the shape automatically whether the line is dark on a light background or light on a dark background. The device is of the type in which a light beam is repeatedly scanned across the line and back to provide an output from a photo-electric transducer viewing the scan which consists of pulses so phased that a correction signal can be obtained and used to maintain the scan centred on the line. It is stated that the error signal may be obtained and used in the way described in Specification 769,572. The present invention is concerned with deriving identical series of phased pulsed signals whether the pattern line is dark or light relative to its background. For this purpose a photo-multiplier tube 35 is used to view the scanned pattern and this is operated with equal voltage drops between dynodes 1 to 4 and 6 to 9. The voltage drop between dynodes 4, 5 and 6 is determined by resistors 40 and potentiometer 41 and is arranged so that the drop between 4 and 5 is equal to that between dynodes 5 and 6 and is a half of the voltage drop between adjacent remaining diode pairs. A highfrequency carrier from oscillator 42 is applied to the tube by connection across potentiometer 41. Capacitor 45 holds dynode 5 at A.C. ground potential and the peak-to-peak amplitude of the signal from oscillator 4 is made equal to the D.C. potential between dynodes 4 and 6. The output from anode 37 consists of a high-frequency signal modulated by the light produced variations. Two transformers 52, and 49, which may be separated by a considerable length of screened line 53, couple the output to tuned amplifier 54 feeding the demodulator circuit. This is arranged so that the negative part of the modulation waveform appears across resistor 58 and the positive equivalent across resistor 59. It is necessary to select one of these when a white line on black is traced and the other for a black line on white. Capacitors 70, 71 break the D.C. level of the signal so that the pulses then become merely positive or negative going and limiters 112 and 113 operated at zero bias discriminate against the positive pulses. Positive pulses are fed out by the limiters (equivalent to negative pulses in) and these pass on line 84 or 85 to limiter circuits and thence to selection contacts controlled by relays CR1, CR2. Operation of one or other of these depends on which limiter 76 or 77 is providing an output, the positive pulses being used to cause valve 112 or 113 to conduct and, through the appropriate transformer and bridge rectifier, energize the relay. CR1 is energized when a white line on a black background is being followed. To prevent operation of the relays by spurious signals the bias on the grid of valve 112 is made higher than that on the grid of valve 113 and a normally closed contact of relay CR1 is provided in series with relay CR2. The selection contacts couple the output of the correct channel to a combining circuit consisting of valve 110, both of whose halves are normally biased to cut-off, and transformers 127, 128. The final stages shown consist of a two-stage limiter 132 coupled to output terminal 137 from which the signal required is derived as negative going pulses.