Production line monitoring without PLCs

Yes, you can monitor a whole line without touching its PLCs. Read each step from the outside instead: a laser counter at every step counts the units that pass it, and a vibration sensor on a machine reports when it runs and stops. Compared in sequence, those counts give you line yield, the units lost at each step and the step that stopped first.

This guide is for plant managers and engineers who want line-level numbers before, or instead of, a controller integration project. It works one hypothetical bagging line through from count points to rollout.

What you can measure from outside a machine#

Two signals cover most of what a line manager asks. A sensor on a machine's frame reads its vibration and sound and decides whether it is running or stopped. A laser counter aimed across a conveyor counts every unit that breaks its beam, and the gap between units says whether that point on the line is moving.

The last two rows mark the line between the two approaches. Everything above them comes from outside the machines, and it answers the questions a daily production meeting asks: how much, how fast, how long down and where. Running time and unit counts also feed two of the three OEE factors, availability and performance, which how to calculate OEE works through by hand.

Why line monitoring often starts at the PLC#

A PLC (programmable logic controller) is the computer that runs a machine's sequence. NIST groups industrial control systems under operational technology: programmable systems and devices that interact with the physical environment.^[1] The glossary entry for PLC covers how monitoring sensors relate to one.

The PLC already holds the count, the state and the alarms, so it looks like the obvious source. Getting the data out of every PLC on a line is where the work starts.

Machines on one line speak different protocols#

A line assembled over 20 years can run on several networks at once. HMS Networks' 2026 analysis of newly installed factory network nodes puts PROFINET at 30%, EtherNet/IP at 25%, EtherCAT at 20% and Modbus TCP at 5% within Industrial Ethernet, with older fieldbus networks still at 14% of new nodes.^[2] A bagger from one builder and a case packer from another can sit on two different ones, and a machine from the 1990s may have no network port at all.

Each of those machines then needs its own adapter. AMT's MTConnect use case says it plainly: devices designed without MTConnect in mind “will always require a custom adapter”, and where no data can be extracted from a machine, the standard is impractical.^[3]

Access needs a review first#

OPC UA, a platform-independent framework for industrial data, lets a client read and write data based on access permissions.^[4] A connection that can write to a controller is a security concern for many plants, and IT usually reviews it first. That review is sensible, so put it on the project plan from the first day.

Put a counter at every step#

A count at the end of the line tells you how many good units came off it. It doesn't tell you how many started, or where the rest went. A counter at every step does, as long as you compare each step with the same reference.

The glossary entry for first pass yield uses the same formula.

Comparing every step with Step 1, not with the step before it, keeps the figure cumulative. Step 4's FPY is the share of everything the line started that reached Step 4.

The end-of-line count alone says the shift came up 600 bags short. The step counts say 60% of that loss is the checkweigher, so the first question is whether the bagger's fill weight is drifting, not whether the packer is dropping bags.

The full placement checklist, including clear containers, is in choosing a count point.

Find the step that stopped the line first#

Steps on a line are coupled. When one stops, the steps after it run out of units, and the steps before it fill whatever conveyor sits between them and stop too. Within a few minutes every counter on the line reads stopped, and only one of them caused it.

The timestamps carry the answer. A counted step reads stopped once no unit has passed for longer than its downtime filter, the longest normal gap between units. Date each stop to its last unit, then sort: the earliest stop is the step to walk to first.

In the figure, the metal detector's last bag passes at 10:14:20. The packer infeed runs dry 45 seconds later. The checkweigher and the bagger fill their conveyors and stop at 10:16:10 and 10:16:40.

The bagger operator sees only that the bagger stopped; the sorted timestamps send maintenance to the metal detector.

Set each step's filter from its own normal gaps. A filter shorter than a normal pause, such as a case packer's index between cases, reads every pause as a stop. A filter far past the longest normal gap hides short stops.

For fast continuous production, start at 15 to 45 seconds, as the downtime filter guidance suggests.

Add machine state where a count can't see#

A count says that a point on the line stopped moving. It doesn't say whether a machine with no passing units is running at all. A blow molder, a press feeding the line or a mixer upstream of a filler has a cycle, not a stream of units past one point.

For those, mount a vibration sensor on the machine. It decides running or stopped from the machine's own vibration and sound, and derives cycle counts and cycle times from the trace. For example, a sensor on a 200-ton press feeding a packing line counts strokes, so the press reports as its own step even though nothing passes a beam there.

You don't need a sensor on every machine to start. Counters at each step answer yield, rate and first stop. Add machine sensors where a step keeps stopping and the count can't say why.

When you need controller data after all#

Outside sensors answer what the line did. Some questions need what only the controller knows.

For line yield, rate and the first stop, default to outside sensors. Choose a controller connection when a question depends on what only the control holds: which alarm stopped a filler, which recipe ran during a bad hour, what a sealer's jaw temperature was. Scope it to those machines and those signals, and have IT review the connection first.

On cost, Kentucky's MEP center puts sensor retrofits at a few hundred to a few thousand dollars per machine, with installations usually completed within a few days.^[5] A controller integration is scoped machine by machine, so its cost follows the adapters each machine needs.

The two approaches combine: sensors on every step, controller data from the few machines where it pays off. The sensor-based versus PLC comparison explains MTConnect and OPC UA in plain words.

Monitor a line from outside in IoTFlows#

IoTFlows puts a BeamTracker at each step: an IP67 laser counter with ±2% count accuracy and a range up to 8 m that bolts on in 10 to 20 minutes. Group the steps into a cascade and every step after Step 1 shows its first pass yield, the units lost and its rate difference against Step 1, live.

A SenseAi on a machine adds that machine's running and stopped state from its vibration and sound. Neither device connects to a PLC, and the cascade reference shows how each step reads.

Altium Packaging counts every bottle this way on ten bottling lines in Memphis, at 20 to 140 bottles a minute.

Now we can talk through issues, help our operators and adjust operations in real time. We used to wait days or weeks for reports, then hold meetings and hope the team knew exactly what had happened.

Roll it out on one line#

1. Pick one line with a known loss

   Choose a line whose yield or output your team already argues about, so the first numbers answer a real question.

2. List the steps in order

   Write down every machine a unit passes through, Step 1 first. Step 1 is the reference for every FPY figure on the line.

3. Choose a count point per step

   Place each counter after the step's last reject point, where units run single file and nothing else crosses the beam.

4. Set each downtime filter

   Watch each step's normal unit-to-unit time, then set its filter a little longer than the longest ordinary pause.

5. Validate each count

   Count units by hand at each step over a fixed window and compare. If a count reads exactly double or half, fix the quantity per cycle, as troubleshooting part counts explains.

6. Review the line every day

   Read the lowest-FPY step, the largest loss and the first stop of each long stoppage in the daily production meeting.

Once one line reads correctly, copy its setup to the next. More guides on getting data off machines are under machines and connectivity.