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A sensor misses small parts at high speed when any link in the timing chain is slower or larger than the part's passage demands: the beam or spot is wider than the part (diffuse modes) or the part is smaller than the mode can reliably see; the sensor's response time or switching frequency cannot complete one ON/OFF cycle within the part's block time; the PLC input filter or scan time swallows pulses the sensor correctly delivered; or the gap between parts is too short for the chain to reset. The first safe check is arithmetic — compute the part's block time and gap time from its length, the gap and the conveyor speed, and compare against the whole chain: sensor response, input filter, scan. The math is safe; the machine stays running; only verification near the belt is hands-off.
The chain is only as fast as its slowest link. Sensor response time, PLC input filter and scan time add up; a fast sensor behind a slow input still misses parts. Budget the whole chain, not the sensor alone.
Block time and gap time are the two numbers. Part length ÷ speed = how long the sensor must stay ON; gap ÷ speed = how long it has to reset. Both must fit inside the chain's capabilities with margin.
Small parts can be optically invisible before they are temporally missed. A part smaller than the effective beam of a diffuse sensor, or one that fits inside the switching hysteresis zone, never generates a solid signal at any speed — that is a geometry problem, not a speed problem.
Speed problems are repeatable, geometry problems are positional. Misses that scale with speed point to the timing chain; misses that depend on part position or orientation point to optics and geometry.
Faster sensing exists when the chain is the limit. Fiber-optic sensors with fine spots serve very small parts; ToF families with high switching frequencies serve speed — the technology mapping is in the selection guides ({{URL_F01}}, {{URL_E04}}).
