The simplest and cheapest way to check for the closing status of a container, where the closure is metallic, is the inductive-digital. The sensor is then an inductive one, its output an NPN polarity, on-off signal.    The one figured on side shows an example of this basic solution, where the cap sensor is referred to a Trigger photosensor obscured by the container along its passage, before it starts to be detected by the inductive sensor.

  Metal closure inspection, by mean of a proximity sensor, referred to a prior trigger

The crown cork metal closure of this bottle, is a typical application for the inductive presence inspection                   



The system is very cheap but it presents a relevant disadvantage.  It can be detected only:

  • cap presence;
  • cap absence;
  • metal caps (e.g., aluminium, steel or tin).

This meaning that a container capped by, i.e. a cap inclined 45º, then a container surely open, shall be considered correctly closed and passed to the Market.  The signal outcoming by these sensors is normally a quite fluctuating one: impossible to act on the signal duration, when trying to detect mal-positioned closures.   


However simple, this system has however several important advantages:

  • capable to reach also the highest production speed, typically > 140000 cph;
  • economy; a sensor like the one partially visible in the image (Pepperl+Fuchs®) or one of the many equivalent by different Vendors, is very cheap (< 100 $);
  • is always available wherever in the World.

Cans inductive lid inspection. No digital (on-off) inductive lid presence inspection allows to know if a can is closed and if it presents micro-holes, a typical problem affecting this kind of aluminium packaging.  To detect and reject cans whose inner pressure is too low or too high or presenting micro-holes, it is necessary a different technology adopting analog inductive detectors (image credit L’Enquête, JDE Edition Nord-Pas-de-Calais, 2009)

   The Staff in this Heineken brewery is aware that digital (on-off) inductive lid presence inspections do not allow to know if a can is really closed, nor if it presents micro-holes.  Micro-holes are a typical problem affecting this kind of aluminium packaging.  To detect and reject cans whose inner pressure is too low or too high or presenting micro-holes, it is necessary a different technology adopting analog inductive detectors (  L’Enquête/JDE Edition Nord-Pas-de-Calais/2009)

An example of the sensitivity adjustment for an inductive or optic Closure Presence inspection.  Measuring distance parameter has to be set coherently with the shape of what we want to detect and with the particularly geometry of the related state Closure-Detector we’ll create during the measurement.  Upper limit and lower limit for measuring distance are the parameters deciding the cut-off between a Positive (defective) and a Negative (non-defective) Closure.  The signal is a zero volt, logic “1” due to the NPN polarity of the digital Input gating it.  In this case, a signal has been detected along 18 mm in an inspection window 54 mm wide

   The Inductive Closure or Lid Presence Inspection may exist associated to all kinds of electronic inspectors: Full Bottle Inspectors, Empty Bottle Inspectors and Case or Crate Inspectors and Checkers.  In the figure, two different kinds of detectors’ shapes sensible to metal objects over the bottles’ finish and laterally, installed by our staff as part of the InLine™ Empty Bottle Inspector of the GROLSCH® Brewery at Hengelo, The Netherlands

Links to other pages about Closure Inspection:

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