Optic Closure Presence inspection, 

with analog photosensors


The optic closure inspection, with analog photosensor, is one of the simplest inspections existing.   It adopts an analog photosensor, with a projector irradiating passing closures by the top and a receiver giving out to an operational amplifier a signal whose extension, the duration counted in encoder pulses, is proportional to the reflection.   It only detects closure presence, not its inclination.   Then, a malpositioned closure shall be considered a “good closure” and not rejected.    Refer to the figure below showing an example of the configuration, with one of such sensors lying a few centimeters after, as seen by the incoming PET bottles, the trigger position to which the  closure inspection is geometrically based.   Analog photosensor, geometrically referred to a precedent Trigger.   An inspection window, whose amplitude in millimeters is equal to the closure diameter, is centred around a point corresponding to the sensor’s vertical: the distance-to-trigger of the optic closure inspection, with analog photosensor.   The sensor outfeed signal is a zero volt, logic “1” in the NPN-logic, worldwide traditional inspectors’ standard.    

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  The optic closure inspection and the Trigger to whose position is referred

Measurements' repeatability

The figure above shows the most important condition for repeatability of the measurements, after changeovers.   The illumination spot has always to cross exactly the diagonal of the closure of bottle formats with different diameter. It implies a precise (measured and registered) adjustment of:

  • sensor bridge;


  • conveyor side guides.  
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 If the line is multiformat, it is vital to guarantee the same conveyor side guides’ opening and geometry shall be repeated during change-overs.  The diagonal line of the different formats has always to pass under the sensor illumination spot

The figure below illustrates part of the height adjustments for three inspection bridges vital to assure repeatability to changeover operations, values which have to be programmed for each one format.  

 One of the changeover menus in an FBI. Changeovers imply manual setup to the correct height

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 How it is possible to assure repeatability to a closure inspection born ignoring this relevant requirement

The optic closure inspection with analog sensor must have the possibility to slide precisely to the diagonal line of the bottle format being processed.   If the inspector itself is not fabric-equipped with millimetric reference rulers, they have to be added by Vendor Staff during startup.  They are commercially easily available also stainless steel rulers thinly marked each one-half of millimetre (0.5 mm) with marks whose width is just ~0.15 mm.   They allow changeovers repeatable to 0.25 mm in the horizontal dimension.    We stress that 0.5 mm is much more relevant that the Reader may imagine, to determine the inspection operative performances (false rejects and defects’ hit ratio).   It has not to be forgot that are the false rejects what only limit the guarantees regarding the detection of non-closed bottles.


Figure below illustrates the measurement and comparison process applied to each one  container.  In the example, an inspection window (“distance de mesure”, in the original French language) of 40 mm: it has to be approximately corresponding to closures’ diameter.    22 mm set as sensitivity for the inspection: lower limit of the measurement.   40 mm set as measurements’ allowed maximum (“limite super. de la valeur de mesure”). Measurements out of the allowed range: (22; 40) mm shall be considered defective and rejected. “40” is a maximum adapt to 40 mm diameter closures.

 Optic closure inspection, with analog photosensor.  In this example the measurement (28 millimeters) of the portion of closure by which a reflection has been obtained, is compared by the algorithm with two limits:  22  and 40.  A closure shall be considered “present” because 28 lies in the assigned range

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