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A speed sensor misses pulses when the target signal falls below what the sensor or the input circuit can resolve — excessive air gap, target runout, very low speed, or a pulse frequency beyond the sensor's switching frequency — and adds pulses when vibration makes the target cross the switching threshold repeatedly or electrical noise is counted as signal. The first safe check is dimensional: measure the actual air gap and its variation across one revolution against the data sheet. Signal measurements near rotating parts and VFD wiring belong to qualified personnel; keep hands, tools and loose clothing clear of rotating machinery at all times.
Missed pulses are a margin problem. Gap too large, target too small, speed too low (insufficient signal amplitude on passive pickups), or tooth frequency above the sensor's switching frequency — all shrink the signal margin until teeth are skipped.
Extra pulses are a threshold or noise problem. Vibration carries the target back and forth across the switching point (double counts per tooth), and VFD/motor-cable noise injected into the signal line is counted as teeth.
Runout makes the gap a moving target. A bent shaft, eccentric gear or loose hub varies the gap every revolution — the worst point of the revolution decides whether pulses are lost, not the average gap.
The PLC is part of the count. Input filters, counter-card maximum frequency and scan behavior can drop or merge pulses even when the sensor is perfect — verify the whole chain before condemning the sensor.
Measure at the extremes. Diagnose at the lowest and highest operating speed, not at a convenient mid-speed; pulse problems live at the edges.
This article covers counting errors between the rotating target and the controller:
Missed pulses — measured speed reads low, drops out at high speed, or reads zero below a threshold speed;
Extra pulses — measured speed reads high, is erratic, or counts while the shaft is visibly stopped (noise);
Intermittent count — correct counts interrupted by bursts of loss or excess, typically correlated with vibration or electrical events.
If the pulses are correct but the speed switch or slip logic trips wrongly during conveyor startup, that is a logic/setting topic — covered in the conveyor startup-trip guide ({{URL_T12}}).
