Boxing clever to attract

01 February 2008

Terry Boughen, managing director of Leuze Mayser, discusses what can be done to overcome difficulties in sensing translucent and clear objects.

For the modern opto-electronic sensor detection of clear glass and PEP bottles is an easy task
For the modern opto-electronic sensor detection of clear glass and PEP bottles is an easy task

The past three months have been challenging for the retail industry, attempting to lure us to spend our money on a whole range of, may I dare say, ‘luxuries’ in the run up to Christmas and the New Year sales. Apart from electrical goods, retailers and manufacturers tempt us to splash out on more expensive perfumes, or a selection of toiletries for our loved ones.

The perfume connoisseur may be looking for a bespoke smell, but for many a large part of the attraction comes by way of the carefully designed packaging, where the psychology of colour meets with the pack designer to create that all compelling look to tempt and tease.

But spare a thought for the production and packaging experts back at the process arm of the operation, where finely tuned fragrances are dispensed into attractive glass or plastic bottles, and soaps wrapped in sparkling foil wrap. Even soft and alcoholic drinks throw out their own challenges by using clear glass and PEP bottles.

Automated high-speed bottling lines require accurate and reliable bottle detection in order for the whole process to run smoothly. For many years mechanical trips and micro-switches operated extremely successfully on glass bottle filling lines. Problems began to arise with the introduction of far lighter and softer plastic bottles, the required contact pressures to trip mechanical devices could rarely be achieved. Also there was a growing need for faster switching signals to support increased operational performance, creating an opening for a different style of detection device – the opto-electronic sensors.

Contact-less detection of white glass or transparent objects with light had always been a real challenge since normally the opaqueness of an object is the basic requirement for using photoelectric sensors. A clear object entering the sensing field is unlikely to be detected. Not to lose a ‘sales opportunity’, opto-electronic sensor manufacturers such as Leuze electronic looked to create a new generation of retro-reflective sensors specifically for the detection of clear objects.

With this style of sensor a reflector plate is mounted directly opposite the transmitting unit, during the detection process, the transmitted light beam passes through the transparent object and is reflected back to the sensor, the received signal is evaluated by the sensors electronics.

A reliable opto-electronic sensor for glass bottles has been created by only allowing light from the reflector to pass through, virtually uninhibited, to the waiting receptor. This is achieved by adjusting the hysteresis range of standard sensors, to around 8%; adding linear polarisers, which help reduce primary and secondary reflections, and introducing circular polarisers, to block light being reflected from the objects surface.

With a traditional sensor there was a likelihood of a double count, particularly with glass bottles. The transmitted light would contact the outside of the glass bottle – this would count as one hit, pass through the open space within the bottle until it came into contact with the inner wall - a second hit, before reaching the reflector and returning back to the transmitter.

Nowadays, the very precise and simplified sensitivity adjustment on modern (photo-electronic) opto-electronic sensors, improvements in detection edge capability, higher resolution and indifference to object size, shape and temperature eliminate this ‘double’ count situation. Another key development that has aided bottling plants is an ability to automatically compensate for lens contamination, any deterioration in the reflected signal strength due to contamination is still sufficient to create a definite ‘object present’ signal for tracking purposes.

This is possible by constantly monitoring the level of the signal received, then comparing it with a reference signal value that had been set during the initial comparison and re-adjusting the transmitting power automatically. The tracking operation – which means constant monitoring and readjusting of the transmitting power – is carried out in each gap of the production flow (when the light path between sensor and reflector is free).

The tracking function is paramount to very high speed processing lines since it guarantees a higher degree of detect-accuracy for glass objects, bringing greater efficiencies (and overall profitability) to any production line on which glass or plastic parts are to be detected.

The technology embodied in the Clear Object Sensor paved the way to achieving outstanding, reliable object detection on automated processing lines where highly reflective foil wraps and carton finishes are to be found. The PRK 25B sensor recently unveiled by Leuze electronic has all these credentials plus an A²LS (active suppression of extraneous light) facility which means outstanding insensitivity to spurious light. If the sensor detects ambient light which is similar in frequency and /or phase of its own sensor signal, the optical sensor actively and automatically changes its signal or phase - as a result, not even the pulsed light from energy-saving lamps or other optical sensors has a negative effect.

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