In the Linear Glass Returnable Empty Bottle Inspectors, External Sidewall Inspection may be performed adopting one or two CCD-cameras. We’ll treat in the following the application with two CCD-cameras: single-camera applications are not satisfactory, covering only < 80 % of the external surface of the bottles.
External sidewall inspection in linear EBIs, with two cameras
The external sidewall inspection normally consists of two consecutive sidewall inspections on the same bottle. With the bottle rotated in the space between first and the second, by a system of sponge conveyor belts. Their movement is differential, so that bottles are rotated 90º in between infeed and outfeed. The entire 100 % of the external surface results this way visible to the cameras, at least one time. Optic systems starts with strobo flashers or matrices of LEDs, front of whom it exists a polarizer filter, with the purpose to enhance the contrast of the image (S/N ratio), reducing False Positive results (false rejects) and increasing True Positive (hits).
Wavefronts pass thru the glass of the bottles, resulting mainly refracted and diffracted and, in minor extent, reflected. They are later intercepted by 2 systems of 4 mirrors each, set in different positions, so to embrace a wider angle of the external sidewall surface.
Pseudo colours
The figure at left side shows how it looks like a bottle, as seen by 4 different angles by a single CCD-camera. Colours represent the illumination and correspond to grey levels, where black is no illumination and white is saturation.
False colours image of a bottle as seen by a camera by 4 different angles
Figure on right side shows the inspection mask existing in the processed image: the inspection is performed into the inspection areas:
- adiacent areas are grouped in groups, so to detect also defects lying in the middle of two areas;
- grey ranges are grouped (linear combinations) to smooth
- limits are attributed to the number of pixels for each one group of ranges of grey into each one area group.
External sidewall inspection mask. Rectangular areas’ dimensions are inversely proportional to the average defects of the glass itself, like the scuffing due to wearing and to the curvature of the glass, source of relevant refraction and diffraction effects which emulate external defects
As usual, the group of areas whose counters’ limits are exceeded shall be applied the Binary Classification: rejection.
External Sidewall inspection adopts visible light like an instrument. Visible light 100 million times more energetic than Infrared and 100 times less energetic than, i.e. the X-rays used for the Fill Level, Case and Crate Inspections
Inspection of semitransparent defects on the sidewall
All what we said until now is valid for opaque defects. But, trying to detect a transparent or semitransparent defect, like a cigarette foil, adherent to the external internal sidewall, is another matter. The figure below at left side displays the reason. A PET bottle filled with water (and, sugar), reproducing a common soft-drink. Visibly, nearly no contrast due to the transparent plastic foils all around the bottle. You can imagine how worse should be the detection if the bottle should be amber or dark green,
At left side, a common PET bottle filled with water (and, sugar) and right side, a completely different aspect for the same identical bottle. What added such an amount of information to the bottle at left side, to let it appear like at right side ? The correct and deep answer is: photons’ constructive interference (
Carlos López-Mariscal, Naval Research Labs, 2014)
rather than the transparent depicted below. At right side, a completely different aspect for the same identical bottle. What added such an amount of information to the bottle at left side, to let it appear like at right side ? The correct and deep answer is: photons constructive interference. In this particular case, what makes the difference is a couple of polarizer filters, set one before the light get into the sidewall and the second before the light passed through the bottle's sidewall get into the External Sidewall Inspection's camera optics. Components visible here, in another page of this web site. Polarizers are always applied to External Sidewall Inspections’ flashers and camera optics. The same, the application is so intrinsically difficult that they are not enough to assure a high detection ratio for this kind of defects, without an associated high false rejection ratio. The large plastic foil visible in the image below should surely be detected but not one smaller. Say > 10 times smaller. A proper detection for these kinds of defects really implies at least one separate and dedicated External Sidewall Inspection. One with a special Optoelectronics and parametric setup, devoted to transparent foils. Empty Bottle Inspectors equipped this way are rarely encountered.
The reason for an investment in Quality and Safety
The external sidewall inspection is particularly and negatively sensible to:
The differences of colour between bottles add false rejects and reduce the defects' detection ratio
- weared sidewalls, due to scuffing like those visible in the figure at right side, induce false rejects: in this case sidewalls have to be damped with a thin cloud of water micro-drops, before entering the EBI;
- colour differences of the elements of the bottles’ population, like those visible in the figure at right side;
- dimensional differences of the elements of the bottles’ population, like in the figure below where is visible the wide range of mixed bottles sizes and shapes a Bottler considered to be a single format. Visibly different the segments AB and CD;
- mould defects (disalignement) inducing huge variations in the grey ranges of adiacent pixels, like the common longitudinal line visible in low-quality glass bottles;
- knurling marks.
In other words, the bottles' intrinsic Quality defines the lower limit of the EBI performances later observed along years during production. Poor EBI performances, as an example those typically observed in an EBI with very high technical false rejects (False Positives FP due to the weared sidewalls whose wearing is over the EBI reject threshold) in the end affect the entire Beverage Bottling Line performances.
The wide range of bottles sizes and shapes a Bottler considered to be a format the EBI had to check in full Quality. High performances negated by the very economic choice by the same Bottler to leave extremely dispersed the population of bottles
This, mainly because all of the Line’s machines have to run faster to recover that permenent loss of bottles immediately before the Filler Machine. Common examples of this a non-balanced Beverage Bottling Line where the followiing Machines are observed running at production speed higher than their nominal for porcentages like:
- Depalletiser +38 %
- Decrater +34 %
- Bottle Washer +32 %
- Empty Bottle Inspector +30 %
- Electronic Bloc EBI-Filler +30%
To renew the bottles’ population requires an investment. An investment however productive in terms of added Quality, Beverage Safety and Production efficiency. To exchange old bottles, irregular in the dimensions and colour and affected by knurling marks lying in inspection area, with modern bottles studied for electronic inspection allows to increase also the Beverage Bottling Line production efficiency, thanks to the reduction of the EBI rejects.
Coca-Cola® during past decade has introduced new aluminium bottles on side of the traditional glass. Their Fill level is inspected by mean of X-rays
Relevance of glass bottles’ design
Coca-Cola® glass returnable bottles’ design is a particularly complex one, as seen by the point of view of the External Sidewall Inspection of whatever Empty Bottle Inspector. Knurling marks, contours, non-cylindric sidewall, variations of the sidewall thickness and an area, back of the serigraphy, which may only be covered by the Inner Sidewall Inspection. Imagine to have sets of test bottles’ with identically-sized standard defects, for the Coca-Cola® regular returnable bottles above and for the green bottles visible below. The same EBI External Sidewall Inspection applied to Coca-Cola® contours above, shall reject an additional ~2 % with respect to the green bottles below
High quality bottles conceived for electronic inspection of their external sidewalls (figures' credit Owens-Illinois®, Inc.):
- knurling marks and prints out of inspection areas;
- modest variations in the transparency;
- absence of the common effects of defective moulds, like the longitudinal lines;
- strict dimensional tolerances.
Links to other pages:
- EBI Classification
- Linear and Rotary EBIs
- High Frequency residual Liquid control
- IR Residual liquid control
- Base inspection, opaque defects
- Base inspection, transparent defects
- Finish inspection, crown cork
- Finish inspection, broken Thread
- External sidewall inspection, for opaque defects
- Inner sidewall inspection for opaque defects
- Mineral ring inspection
- Scuffing inspection
- Infeed checks
- Colour inspection
- Closure inspection with digital photoscanners
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