Patent GB-1516884-A (GB)
Title: Optical sorting apparatus
IPC: B07C5/342
Filed: 1975-09-11
Abstract: 1516884 Sorting apparatus XELTRON S A 11 Sept 1975 [16 April 1975] 37374/75 Heading G1A Objects are sorted in response to light reflected from the objects and detected photo-electrically using a pair of detectors having differing spectral response characteristics, the detector outputs being integrated and related one with another, a reject signal being produced if the relationship is above a first threshold value, the detector output of one detector also being applied to a comparator circuit to produce a reject signal if the detector output is outside a predetermined range of values. In the arrangement shown, objects such as coffee beans, grain etc, fed from hopper 1 are separated on vibrating tray 3 towards channel 4, Fig. 2, which provides a single file passage of objects through the examination station. This comprises two rings 14, between which are mounted optical fibres, whose ends form an annular array and which convey light reflected from the objects to a pair of detectors, only one shown in Fig. 1. The detectors may be photo-multipliers, silicon or selenium solid state devices e.g. silicon phototransistors. This may be provided by use of coloured filters. The illumination of the examination station is by lamps 6, the fibre ends being shielded by the rings 14 from direct light. Inlet 15 for compressed air prevents deposition of dust. The rejection mechanism 11  comprises a solenoid operated air jet. The detector outputs are amplified at 20 and supplied to integrator circuits 21, 22. The output of one of the detectors is fed to synchronization circuit 23 which ensures that the evaluation of a particular object only occurs when an object is at the examination station. The synchronizing circuit produces signals for enabling comparators 24, 25 and also produces two delayed time displaced pulses for operating the integrating circuits. After a first timed interval the integrating capacitors are read out, through a differencing circuit 26 including a transformer, such that any difference in the accumulated changes of the integrating capacitors provides a signal which is applied to comparator 27 to produce a reject signal via OR gate 28, if the difference signal is above a particular threshold. (In an alternative arrangement, an analog divider may be used instead of a difference circuit.) A second time-delayed pulse from the synchronizing circuit erases the changes in the integrating capacitors before the next object reaches the examination station. If logarithmic function transistors are used in the integrating circuits 21, 22, the relationship between the signals represents the ratio of the integrated detector outputs. The comparators 24, 25 produce an output only if the intensity of  reflected light is respectively below and above, first and second predetermined threshold values. Thus any of the three circuits may provide a reject signal via OR gate 28. Detailed circuitry for the various parts of Fig. 5 is provided in Figs. 6, 7, 8 and 9 (not shown) as are graphs showing the spectral ranges of suitable detectors and the waveform characteristics of various parts of the circuit system. Calibration of the system is carried out using differently coloured electroluminescent diodes.