GB1524564A - Apparatus for detecting the presence of objects in light beams - Google Patents

Abstract

1524564 Light guarding system TELUB AB 16 Sept 1975 [20 Sept 1974] 38096/75 Heading G1A Apparatus for detecting the presence of an object, e.g. object 1, comprises an array 10 of light sources oriented with an array 20 of light sensors, control units 17, 18 arranged to activate the sources 10a-g sequentially and corresponding sensors 20a-g in synchronism therewith, and a signal comparison circuit 22 for providing an output signal on line 16 whenever a light sensor gives no output signal during its activation period, the control units 17, 18 being controlled by clockpulses from a generator 19. The pulses are also fed to the comparison circuit 22, and the control unit 17 is arranged to activate each light source for approximately half the periodic time of the pulse train. In the described embodiment photo-diodes (L10a-b), Fig. 2 (not shown) emitting infrared are controlled by transistors (T10a-b) which are connected to the output lines (173a-p) of a binary-decimal converter (171) in a control unit 17 capable of controlling up to sixteen photo-diodes. A counter (174) receiving pulses from a generator (19) supplies binary information over four lines (172a-b) to converter (171) to cause each output to be activated in turn. An inhibiting input (173q) to the B-D converter receives pulses direct from the generator in order that each diode should be activated for only half the periodic time of the pulse train. Optionally when a count of sixteen is reached a further B-D converter can be activated by means (175) to control a further sixteen diodes. The outputs of eight light sensors comprising photo-transistors (F20a &c.), Fig. 3 (not shown) are connected via high-pass filters (31, 32) to inputs (181a-h) of an analogue connector (181) in control unit (18). A binary counter (184) receiving pulses from the generator 19 controls the analogue connector over three input lines (182a-b) thereby causing each photo-transistor output to be connected in turn to a pulse shaping circuit 21 via an output (181k) of the connector (181). The output of circuit 21 is fed to comparator 22 comprising a counter (186) which provides an output pulse on line 16 whenever no input pulse occurs on line 15. The first counter (184) on reaching a count of eight inhibits connector (181) and activates a further analogue connector (188) controlling eight more photo-transistors. To detect when a required number of objects have not fallen through the light curtain a device 23 provides an output signal when the number of pulses on line 16 over a predetermined time period is less than a number pre-set by circuit 24. The control units may be arranged to activate more than one light source and more than one sensor during one and the same time interval.