conveyor — part_count (photo-eye-line-count)
Physics
Section titled “Physics”A through-beam photoelectric sensor pairs an emitter and a receiver on opposite sides of the conveyor so the beam crosses the full belt width at a fixed point. Each part passing that point breaks the beam, and the receiver’s output switches state for the duration of the occlusion; one break-make cycle equals one part. Through-beam (vs. diffuse/retro-reflective) is used because it tolerates dust, oil film, and part color variation on a factory conveyor without a sensitivity retune. Sample rate must resolve the shortest possible occlusion: at a belt speed of 0.5 m/s with parts spaced as close as 50 mm edge-to-edge and 20 mm long, the shortest beam-break is ~40 ms and the shortest gap is ~60 ms; sampling the digital input at 1000 Hz gives >>2x Nyquist margin on the 40 ms pulse and resolves the break/make edges with <1 ms jitter, so two closely spaced parts are never merged into one count.
Sensor
Section titled “Sensor”- Spec: through-beam photoelectric sensor, emitter/receiver pair, 1-10 m range, NPN/PNP digital output
- Signal: counter ({count}), range n/a
Example parts
Section titled “Example parts”- Banner Engineering QS18VP6D through-beam photoelectric pair (18mm barrel, 5 m range, PNP NO)
- SICK WS/WE27-3 through-beam photoelectric pair (30mm barrel, 8 m range, PNP)
- Omron E3Z-T61 through-beam photoelectric pair (dark-on, 4 m range, PNP NO)
Mounting
Section titled “Mounting”Mount the emitter and receiver on rigid brackets on opposite sides of the conveyor at a height that intersects every part’s profile (typically just above the belt surface, or at the part’s narrowest consistent cross-section), with both lenses aligned so the full beam path is unobstructed by the conveyor frame or guide rails. Shim or slot the brackets for alignment and re-check beam alignment after any guide-rail adjustment.
Wiring
Section titled “Wiring”3-wire DC (24 V, 0 V, signal) from the receiver to a digital input / counter channel on the edge box’s I/O module; run emitter and receiver power from the same supply to avoid a ground-potential difference across the belt, and use a shielded cable on runs over a few meters near VFD-driven conveyor motors.
Validation
Section titled “Validation”run 100 parts down the conveyor at normal line speed with a manual tally and compare the detector’s counter delta over the same window
Expected: counted parts within +/-1 of the manual tally when part spacing exceeds the sensor’s response time; closer spacing needs re-verification against the Nyquist margin
Safety notes
Section titled “Safety notes”- de-energize and lock out the conveyor drive (LOTO) before mounting brackets near the belt, pinch points, or rollers
- route and strain-relieve sensor cabling away from the belt and any moving rollers so it cannot be caught and pulled into the drive
- verify sensor supply voltage and output type (NPN/PNP) match the edge I/O module before wiring to avoid back-feeding the input
50–250 USD, difficulty 2/5, ~30 min install downtime.