Patent GB-1508035-A (GB)
Title: Detection of surface defects in glass containers
IPC: G01N21/90
Filed: 1975-04-14
Abstract: 1508035 Detecting flaws photo-electrically OWENS-ILLINOIS Inc 14 April 1975 [6 June 1974] 15164/75 Heading G1A Apparatus for detecting flaws in the rim of a glass container comprises; means for rotating the container; a source for illuminating the rim of the container; means receiving reflected light to generate signals having a mean D.C. level and superimposed fluctuations; means for discarding the D.C. component and for separating the fluctuation component into two ranges based on their duration; means for comparing the amplitude of each fluctuation component with at least one predetermined reference level; and logic means for providing a rejection signal responsive to any signal provided by the comparison means. (a) Optics. Figs. 1 and 2. The circular rim 13 Fig. 1 of a glass container 10, which is to serve as a sealing surface, is caused to rotate in the path of a narrow beam from a source 14, reflected light being imaged on a main photo-detector in a receiver 16. The main detector 20, Fig. 2 is mounted behind a slit 22 in a mask 18 arranged to admit light from a portion of the rim 13 extending radially and having a width of 0À003". Four secondary photo-detectors 24-27 mounted behind holes 29-32 in the mask 18, are arranged so that any one or more may signal if the rim 13 is rotating eccentrically. (b) Typical Signals. Fig. 3. The output from the main detector 20 may have one or more of these characteristics:- ' (i) a steady or gently indulating level N; (ii) a narrow dip L due to a line-over-finish defect or an open blister; (iii) a narrow peak C due to a closed blister; (iv) a wide trough U due to an unfilled finish. (c) Amplification. Fig. 4, Fig. 5 (not shown). The main detector signal is applied to a logarithmic amplifier 36 via a preamplifier 34, the combination being corrected for zero drift by a feedback control 60 which is switched to sample and integrate the output of the logarithmic amplifier 36 during intervals between detection cycles, and to hold and apply a correction signal to the preamplifier 34 during detection cycles. A further amplifier 38 provided with a temperaturedependent feedback resistance (84) compensates for thermal variations in a pair of diodes (174, 175) used in the logarithmic amplifier 36. (d) Bandpass Filters. Fig. 4, Fig. 6 (not shown). In bandpass filters 40 the D.C. component is first removed by a CR coupling (160, 162) typically of 75 ms time-constant, a low parallel resistance (164) being switched in during intervals for rapid stabilisation. The A.C. component, after passing through a non-linear low-pass filter (154) to remove noise, is fed in parallel to two high-pass filters of which one (156) passes only the narrow L and C pulses, while the second (158) passes pulses having widths less than and equal to that of the U pulses. The filters (156, 158) include active resistances based on variable-gain amplifiers (182, 194) controlled by a potential from a multiplier 42 in response to signals from units 44 and 46 representing the rim diameter and the speed of rotation. (e) Comparators and Digital Filter. Fig. 4, Fig. 7 (not shown). The L and C pulses on one line (198) are passed in parallel to comparators 40 (202, 203) which are provided with respective negative and positive thresholds and respond respectively to L and to C pulses. The U pulses on another line (200) are passed to a third comparator (204) which emits an output if the amplitude of the pulse exceeds the magnitude of a negative threshold. The U pulses are further analysed by a digital filter 53 controlled by the multiplier 42, for blocking pulses of less than a predetermined width. The input to the digital filter is integrated by an amplifier (220) the rise rate being controlled by a potential (226) from the multiplier 42. Only pulses exceeding the predetermined width will rise sufficiently to pass a threshold set into a comparator (234). (f) Logic and Display. Fig. 4, Fig. 9 (not shown). Signals representing L, C and U defects are applied to a logic unit 50 where any one or more of these signals will, when gated by a timing pulse. Fig. 10 (not shown) at the end of a detection cycle, activate a display unit 52 illuminating a lamp and providing reject output signal for energizing an electro-mechanical reject device. Rejection is inhibited by an absence signal from a bottle presence unit 58 and Fig. 5 (not shown) responsive to loss of D.C. output from the preamplifier 34; or by a signal from an out-of-position unit 54 and Fig. 8 (not shown) responsive to the secondary detectors 24-27; or by a signal from an out-of-calibration unit 66 and Fig. 5 (not shown) responsive to a drift signal from the correction control unit 60.