Counting Output on Old Machines: Photo Eyes and Proximity Sensors

Key takeaways

• A count sensor gives Performance and Quality on machines with no usable PLC.
• Photo eyes count products; proximity sensors count metal parts that move once per cycle.
• Mount the sensor where each unit passes once, with a gap between units.
• Pair it with a run signal, such as a current clamp or the stack light, for Availability.
• Check every sensor against a hand count before you trust its numbers.

An old machine with no readable PLC can still give you an output count. A photo eye or a proximity sensor, mounted in the right place, counts units or cycles. With a run signal next to it, that is enough to compute OEE.

Fabrico, OEE and light MES software with AI cameras, connects a machine through its PLC, a Fabrico IoT device or an AI camera. Its site states that "the machine itself needs no PLC". This guide covers the sensor side of that.

Which sensor should count output?

For clear bottles or glossy film, use a sensor model made for clear objects, and test it on the real product. Our guides to photoelectric sensor troubleshooting and proximity sensor troubleshooting cover the common faults.

Where should the count sensor sit?

• Where each unit passes once, in single file, with a gap between units.
• Before the reject point for total count, after it for good count. Two sensors give both.
• Away from splash and wash-down, or in a housing rated for it.
• On a fixed bracket, so a knock does not move it out of line.

On a multi-lane machine, a proximity sensor on the drive often counts cycles. Multiply by the units per cycle to get the total count.

How do you get Availability without a PLC?

A count alone cannot tell a stop from a slow patch. Pair the sensor with a run signal:

• A current clamp on the main motor tells running from idle.
• The stack light, read in parallel, gives running, waiting and fault.
• The count itself, with a rule: no count for longer than a set time means stopped.

Use counts over an interval, not a speed reading at one instant. A speed read at a fixed moment can catch a pause by chance and book a whole interval as a stop.

Worked example: a cartoner with no PLC

Sample line (illustrative numbers, not a customer). An old cartoner has an ideal rate of 60 cartons a minute. One photo eye counts cartons after forming, a second counts them after the reject gate. A current clamp shows when the machine runs. The shift has 480 planned minutes.

OEE = good count x ideal cycle time / planned time = 23,900 / 60 / 480 = 83.0%.

Before trusting that, the crew ran a hand count. In 10 minutes they counted 598 cartons; the first sensor counted 611. The extra 13, or 2.2%, came from cartons touching on the belt. Moving the sensor to a spot where the cartons are spaced fixed it.

How do you check a count sensor?

1. Hand count for 10 minutes and compare with the sensor.
2. Repeat after a changeover, because a new pack size can change what the sensor sees.
3. Clean the lens on a schedule, especially on wet or dusty lines.
4. Filter flicker as a separate step, so one unit is never counted twice.
5. Check the count when the line restarts, when units often bunch up.

Which mistakes make sensor counts wrong?

• One sensor before the reject point, so rejects count as good.
• Touching units counted as one, or one long unit counted twice.
• Reflective packaging fooling a photo eye that was never tested on it.
• Cycles read as units on multi-lane machines.
• A sensor knocked out of line and nobody notices for a week.

For other ways to connect old machines, see our guide to OEE on brownfield and legacy equipment, and OEE monitoring without a PLC.

Where does Fabrico fit?

Fabrico is a computer vision enabled Manufacturing Performance Platform. OEE and light MES come first, on old machines as well as new ones.

• Automatic downtime logging from IoT devices with external sensors, PLC tags or cameras.
• The operator confirms the reason in one tap, because a sensor never knows why a machine stopped.
• OEE live by line and shift, with every loss priced in money.
• A pilot on one line, so the sensors are proven before more machines follow.

Sensor choice and mounting for each plant are agreed with Fabrico's technical team. There is no free plan and there are no tiers: each plant gets a quote. See the product on the computer vision OEE page.

Frequently asked questions

How do you count production on a machine without a PLC?

Mount a photo eye or a proximity sensor where each unit or cycle passes once, and connect it to a small IoT device. Add a run signal, such as a current clamp, to tell running from stopped. Then check the count against a hand count before you use it.

What is the difference between a photo eye and a proximity sensor?

A photo eye uses light and sees the product itself as it passes. An inductive proximity sensor detects metal close by, so it usually counts a machine part that moves once per cycle. The first counts products; the second counts cycles.

Can a photo eye count clear bottles?

Yes, with the right model. A standard retro-reflective sensor can miss clear plastic or glass. Use a sensor made for clear objects, set it up on the real bottles and check it with a hand count during the first runs.

How exact is a sensor count?

As exact as its mounting. A well placed sensor on spaced units can match a hand count closely. Touching units, reflective packs and dirt on the lens all cause errors. Test it by hand when you install it and after each pack change.

Do you need two sensors for OEE?

For full OEE, usually yes: one before the reject point for total count and one after it for good count. With one sensor, take good count from somewhere else, such as a checkweigher's reject counter, so Quality is still measured.

Count your oldest machine first

Bring one machine with no usable PLC. We will show where the sensors go and what its stops cost.

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