Photoelectric Sensors for Bottling, Packaging and Counting Lines

Packaging lines are the densest photoelectric application in any plant. Bottle present, cap present, label present, carton gap, case count, jam detect, accumulation control. A line can carry dozens of sensors and the count at the end has to be right.

The clear container problem

Clear glass and PET are the hardest routine target. The light goes straight through, so a diffuse sensor sees almost nothing come back and a standard retro reflective sensor may see enough through the bottle to think the beam is unbroken.

Two reliable answers:

Through beam across the conveyor. Even a clear bottle refracts and attenuates enough to drop the received signal below threshold if the sensitivity is set correctly. Our D31C through beam parts cover 1 m to 15 m in the M18 body, so a conveyor width is comfortably inside range with signal to spare.

Polarised retro reflective. The corner cube reflector rotates the polarisation of the returning light and the receiver only accepts rotated light. Light passing through a clear bottle is not rotated, so the bottle is detected. This also solves the shiny foil and metallised film case.

A plain, non polarised retro reflective sensor on a clear bottle line will produce intermittent miscounts that look like a mechanical problem for weeks. If you are chasing a count that is slightly wrong, check the sensing mode first.

Counting accuracy

Three things cause count errors, and only one of them is the sensor.

The beam is too wide for the gap. If containers are touching or nearly touching, the sensor must see the gap between them. A beam wider than the gap never goes clear and two bottles count as one. Through beam gives the tightest, best defined beam, which is why it is the counting default. Where the gap is very small, aperture the emitter down.

Bounce and jitter at the threshold. A signal sitting near the switching threshold chatters, producing several counts from one container. Set the sensitivity so there is real margin, and if the PLC allows it, apply a short debounce shorter than the minimum container pitch.

Switching frequency. On a high speed line, check the sensor switching frequency against the container rate. Photoelectric parts are generally fast enough, but at several thousand containers an hour it is worth confirming rather than assuming.

Where each sensor goes

• Infeed presence and accumulation. Retro reflective across the conveyor, mounted at container body height, not neck height, so a tilted container is still seen.
• Cap and closure check. Diffuse at short range looking down, or through beam across at cap height. Confirm against both the tallest and shortest container you run.
• Label present and label gap. A short range diffuse sensor looking at the label edge. Contrast between the label and the container decides whether this works, so test it on your actual product rather than a sample.
• Case and carton counting at the end of line. Through beam, with the beam positioned to see the gap between cases.
• Jam detection. A retro reflective sensor plus a PLC timer. If the beam stays broken longer than a normal container takes to pass, the line is jammed. Cheap, effective, and it catches a jam before it becomes a pileup.

Construction for a wet line

Bottling and food packaging are washed down, so the sensor has to survive the cleaning rather than the process.

Specify IP69K where there is high pressure hot water, IP67 where there is splashing. The difference between the two tests is explained in IP67, IP68 and IP69K, and there is more on construction in washdown sensors for food and beverage.

Two details that matter more on a wet line than a dry one. A water film on the lens scatters light and drops the signal, so mount the sensor so it drains and angle the face slightly downward where you can. And a connector is a joint that has to be cleaned and sealed, so on washdown lines consider a moulded fixed cable, see connector or cable.

Where photoelectric is not the answer

If you are sensing fill level inside an opaque container, or product level in a hopper, that is a capacitive job, because capacitive senses through a non metallic wall and photoelectric cannot see inside.

If you are sensing a metal part, a cap crimping tool or machine position, an inductive sensor is cheaper and more robust. Use photoelectric for product and inductive for machinery, and most lines sort themselves out.

Broader guidance on machine side sensing is in sensors for packaging and pharmaceutical machinery.